Гипоксия в множестве склерозов.

Introduction

The etiology of the chronic inflammatory disease multiple sclerosis (MS) is determined by both genetic and environmental factors contributing to specific disease manifestations and unpredictable disease courses [ [1] , [2] , [3] , [4] ]. Symptomatology, disease course, disability worsening and responsiveness to medication are heterogeneous among individuals with MS [ 5 ] and the presentation of symptoms can be variable regarding the localization (anatomical) and time (episodic) of central nervous system (CNS) damage. This is due to interindividual, primarily quantitative differences in concurrent pathophysiological and compensatory processes [ 6 ].

Based on recent findings on the role of ambient hypoxia (reduced oxygen availability in the environment, such as occurs for example at high altitude) in MS, we aim to summarize studies that investigated hypoxia exposure of MS patients and of animal models of MS. We explore the role of tissue and cellular hypoxia, as well as associated vascular pathology and hypoperfusion of the central nervous system (CNS) in MS. In addition, the potential of the application of inspiratory hypoxia in MS will be assessed. Finally, we propose an integrative model that could explain beneficial effects of different types of hypoxia exposures and oxygen supplementation in the context of MS, with potential relevance for many neurological and inflammatory diseases.

Etiology and risk factors of MS

The prevalence (about 120/100,000 in the UK, 35.9/100,000 globally) of MS is rising globally and, like its incidence, it varies strongly, dependent on geographical (possibly also the altitude of residence), demographic and ethnic factors [ 7 , 8 ]. MS is one the most common neurological diseases in young and middle-aged adults and affects about 2–3 times more women than men [ 9 ].

Main genetic modulators of MS are histocompatibility leukocyte antigen (HLA) genes. HLA class II alleles associated especially with CD4 + T-cell activity increase the risk for MS, HLA class I alleles modulating CD8 + T-cell and natural killer cell activity reduce it [ 1 ]. In addition, more than 200 other risk loci have been found to modulate MS risk, however, with low independent contribution [ 1 ]. Among them are vascular endothelial factor (VEGF) [ 10 ], which is upregulated in hypoxia and several mitochondrial gene mutations, as recently reviewed [ 11 ]. An involvement of mitochondrial mutations has initially been suggested after the observation that people with mitochondrial DNA mutations in the context of other diseases can develop MS-like neurological syndromes [ 12 ]. Also, IL-7 receptor alpha polymorphisms have been implicated in the pathogenesis of MS [ 13 ] and hypoxia has been shown to downregulate IL-7 in cellulo [ 14 ], potentially also modulating pathogenesis and disease course.

Among the environmental factors that may trigger, facilitate or aggravate the disease are notable viral infections (herpes viruses), especially by the Epstein–Barr virus, which increased the hazard ratio for MS diagnosis 32fold in a recent prospective, case-controlled study [ 15 ]. Furthermore, nutrition and body composition influence MS risk. Western diets rich in calories, fat and sugar reduce the physiological regenerative capacity from demyelination injuries in MS models [ 16 ] and obesity in humans increases the risk to develop MS [ 1 ]. Moreover, smoking, is an important environmental risk factor for MS [ 9 , 17 ] and an association with unclear causality (confounding and potential reverse causality [ 18 ]) between insufficient sunlight (low ultraviolet radiation exposure) and vitamin D deficiency and MS exists [ 9 ]. The role of environmental hypoxia on MS is complex and the topic of later sections.

Pathology and disease course of MS

The key MS disease mechanism is the infiltration of autoreactive T-lymphocytes into the brain, which leads to inflammatory damage of axonal myelin-sheets and axonal degeneration [ 19 ]. The predominantly inflammatory events of the early relapsing stage are also involved in subsequent neurodegenerative processes, including DNA-damage, demyelination and eventually axonal/synaptic/glial/neuronal degeneration and brain atrophy [ 20 ]. Dysregulated axonal ion-homeostasis and accumulation of oxidative-injury promoting iron are thought to be driving factors of MS pathology [ 20 ]. MS lesions include grey-matter demyelination and white-matter plaques and different types have been described and reviewed [ 20 , 21 ] based on their localization (e.g., leucocortical and subpial lesions) and structural characteristics. In addition, diffuse pathological signs can be found outside of lesion sites and include mitochondrial dysfunction, microglial activation, axonal injuries and atrophy [ 22 ]. While focal white-matter plaques occur early in relapsing-remitting MS, cortical lesions and diffuse pathology are rare in early disease stages but increase with disease progression. Inflammation commonly decreases to levels comparable with age-matched controls at late disease stages [ 23 ]. A strong indication of hypoxia being linked to MS pathology is the more frequent distribution of white matter lesions in relatively lower perfused brain regions [ 24 ].

Imaging signs of MS include plaques/lesions in the brain or spinal cord. Higher levels of oligoclonal immunoglobulins in cerebrospinal fluid than in serum are considered biomarkers of MS. Some criteria (male older patients, primary progressive type, brainstem/cerebellar symptoms) are statistically associated with faster disability, but overall, the disease course of MS is very variable and can be partially modified by life-style factors. Specific lesions detectable by MRI and fulfilling the Okuda criteria [ 25 ] in people without diagnosis of MS are considered a “radiologically isolated syndrome” (RIS). Over 5 years, RIS in about 20–50 % of cases will result in demyelination events and in rare cases may be associated with progressive MS [ 9 ]. Relapses are triggered by acute demyelination events, followed by a stabilization usually after several weeks. While acute demyelination does not immediately cause axonal degeneration, chronic demyelination leads to axonal loss, which in turn does not result in direct neurological deficits due to the innate compensation capacity of the CNS [ 20 ]. However, progressive axonal loss eventually results in irreversible brain damage [ 20 ]. Insufficient remyelination, due to an impaired regeneration capacity, is likely an important factor determining the disease course [ 26 ] and is promoted by ongoing inflammation [ 27 ].

Traditionally, MS has been classified in (1) relapsing-remitting MS (RRMS) and (2) secondary progressive MS (SPMS). In RRMS transient neurological impairments can completely or partly resolve (remission) between acute neurological events (relapses). However, the disease burden increases as the disease advances, by both, relapse associated worsening and progression independent of relapse activity [ 28 ]. SPMS may develop from RRMS and changes into a progressively worsening disease without or only brief phases of remission [ 28 ]. If the disease progressively worsens from disease onset, the disease course is classified as (3) primary progressive MS (PPMS), which is the case in about 10–15 % of patients [ 29 ] ( Fig. 1 ).

Disease courses of multiple sclerosis (MS) and genetic and environmental factors. There are environmental and genetic risk factors associated with MS and its progression. Among the environmental risk factors, hypoxia is not well understood. Several genetic risk factors are related to hypoxia responses, including the gene encoding vascular endothelial growth factor (VEGF) and interleukin 7 receptor A (IL-7RA). Radiologically isolated syndrome (RIS) – as assessed by magnetic resonance imaging (MRI) – and clinically isolated syndrome (CIS) are preclinical manifestations that may be followed by relapses and clinical forms of MS: relapsing-remitting MS (RRMS), secondary progressive MS (SPMS) and primary progressive MS (PPMS).

These definitions of MS disease courses have recently evolved classifying RRMS and SPMS as either active (relapses or MRI activity) or non-active [ 30 ]. If the disability increases after a relapse, the disease course is considered “worsening”, otherwise “non-worsening”. PPMS can be active or inactive (according to MRI activity) and with and without progression. In addition, the importance of the consideration of mechanistic aspects of MS in the definition of progression and the disease course is increasingly highlighted [ 6 ]. These mechanisms notably relate to inflammatory and neurodegenerative processes, as well as to compensatory processes, like neuroplasticity and remyelination [ 6 ].

Diagnosis of MS

The diverse symptomatic manifestations of MS include motor symptoms (limb-weakness, paroxysmal symptoms, rubral tremor), fatigue and depression, paresthesia (sensory abnormalities on the skin like numbness, burning, tingling or itching; sensation of vibration in the feet; impaired proprioception of the hands), impaired visual acuity (e.g., double vision, optic neuritis), problems with micturition and - more rarely - cognitive or local cerebral deficits (e.g., aphasia). In addition, atypical presentations (paroxysmal and unusual symptoms) are observed in some cases [ 31 ].

The temporal and anatomical features are the bases for the diagnostic criteria of “dissemination in time and space” [ 32 ]. If at least one clinical event is suggestive for demyelination, “clinically isolated syndrome” (CIS) can be diagnosed based on paraclinical criteria. Characteristic paraclinical findings in MS include T2-hyperintense lesions (T2L) detected by magnetic resonance imaging (MRI) [ 33 ], intrathecal immunoglobulin G (IgG) synthesis assessed by the measurement of intrathecal κ-free light chain [ 34 ] or cerebrospinal fluid-specific oligoclonal bands.

While knowledge of the MS prodrome, availability of biomarkers, diagnostic criteria, treatment options (disease modifying) and recommendations for treatments have substantial improved over the last years [ 9 , [35] , [36] , [37] ], the pathological developments underlying the disease are still not sufficiently understood. MS remains uncurable and for the optimization of personalized treatment strategies, especially for progressive MS, better knowledge of the pathological processes is required. This is further important in light of the increasingly older age of MS patients, requiring better treatment strategies and management of comorbidity [ 9 ]. Aging in general, and senescence of immune and neuronal cells specifically, may aggravate pathological processes in MS and are linked to specific side-effects and attenuated efficacy of treatments [ 38 ].

Animal models of MS

An important rodent (mainly in mice) model of MS is experimental autoimmune encephalomyelitis (EAE). EAE is induced by active immunization with antigens, such as myelin proteins or myelin-derived peptides or injection of activated T cells and activated myelin-specific CD4. This leads to MS-like pathology, including inflammation, axonal demyelination and progressive hindlimb paralysis [ 39 ]. However, other pathological features of MS, e.g., diffuse neurodegenerative pathology, are less readily reproduced in EAE models and the underlying inflammatory reactions in MS and EAE differ, which has to be considered in MS research [ 20 , [39] , [40] , [41] ]. A summary of notable physiological discrepancies between rodents and humans related to oxygen transport, hypoxia and inflammatory responses, as well as important differences between the EAE model and human MS is provided in Table 1 .

Table 1 Relevant physiological species differences between mice and humans and differences between the rodent experimental autoimmune encephalomyelitis (EAE) model and human multiple sclerosis (MS). Table 1 Physiological differences between mice and humans Physiological function Main differences Ref Cardiovascular About 10-time higher heart rate and mass-adjusted cardiac output in mice [ 42 ] Ventilation Higher mass-specific minute ventilation (about 20-fold), greater respiratory frequency in mice [ 42 ] Maximum oxygen uptake Higher mass-specific maximum oxygen uptake but lower range of oxygen consumption related to activity level in mice [ 42 ] Systemic hypoxia responses Increase heart rate in humans, opposite effects in mice, better preservation of oxygen uptake in hypoxia in mice [ 42 ] Inflammatory responses Possibly very different genomic responses in mice and humans [ 43 ] Microglia Unlike in mouse brain, microglia are partially activated even in healthy human brain, more senescent/dysmorphic microglia in human brain [ 41 ] Differences between the EAE model and human MS Ref Focus on a single immune mechamism (autoimmune encephalomyelitis), with CD4 + T cells triggering EAE in mouse models, while diverse pathological mechanisms in MS include an inflammatory reaction dominated by CD8 + T-cells and B-cells [ 40 , 41 ] Differences in gene and protein expression in mouse EAE brain and human MS brain [ 41 ]

Relevant physiological species differences between mice and humans and differences between the rodent experimental autoimmune encephalomyelitis (EAE) model and human multiple sclerosis (MS).

Still, the EAE model has strongly improved our understanding of MS-associated pathology and disease processes, including on the role of hypoxia. Other animal models of MS include the cuprizone or lysolecithin models of demyelinization but only little evidence on the role of oxygen availability is available in these models. Moreover, impairments of antioxidative mechanisms – specifically loss of superoxide dismutase 2 (Sod2) – in adult mice, have been demonstrated to evoke MS-related phenotypes and pathology, including inflammation, demyelination, infiltration of immune cells and motor impairments [ 44 ].

The roles of hypoxia in multiple sclerosis

Reduced local oxygen availability in the CNS due to lower-than-normal partial pressure of oxygen (pO 2 ), appears to be a key event, either as a trigger or consequence of MS [ 21 , 45 , 46 ]. Regions, physiologically or anatomically vulnerable to hypoperfusion, such as watershed areas (i.e., brain regions with low arterial blood supply [ 47 ]), are particularly susceptible to hypoxic/ischemic injuries and hypoperfused brain regions are clearly associated with more MS lesions [ 47 ]. Adaptive immune responses (myelin-reactive T cells, autoantibodies) are commonly viewed as causes of MS-related demyelination and neurodegeneration. However, relatively low levels of infiltration by T cells and the absence of antibody-mediated tissue injury in some patients in early stages of plaque-formation suggest the involvement of other factors in the development of MS pathology [ 48 ]. These early factors may be hypoxia and associated deficits in oxidative stress management and energy metabolism [ 48 ].

The brain relies heavily on oxygen for energy production and thus is highly vulnerable to changes in oxygen. In order to survive transient phases of hypoxia, cells have an elaborate oxygen sensing and hypoxia response system, in which the coordination of hypoxia responses via transcriptional factors such as hypoxia inducible factors (HIFs) or the oxidative stress defense master regulator nuclear factor (erythroid derived 2)-like2 (Nrf2) are central [ 49 ]. HIF-1 and HIF-2 are dimeric transcription factors. In presence of abundant oxygen, the HIF α subunits are continuously hydroxylated by HIF prolyl hydroxylases (PHDs), leading to recognition by the von Hippel Lindau (VHL) tumor suppression protein and subsequent recruitment of an E3 ubiquitin ligase complex and proteasomal degradation [ 50 ]. In hypoxia, the α-subunits accumulate, dimerize with βsubunits (also known as aryl hydrocarbon receptor nuclear translocator protein (ARNT)) and regulate the expression of hundreds of genes. In addition, HIFs are controlled in an oxygen-independent fashion by various other regulators ( Fig. 2 ), including other transcription factors such as Nrf2, the inflammation coordinator nuclear factor-κB (NF-κB), different RNAs or enzymes catalyzing posttranscriptional modifications such as deacetylases of the sirtuin family of proteins [ 51 , 52 ]. Among the most prominent effects of HIF-regulated gene expression are metabolic reprogramming (primarily mediated by HIF-1), vascular remodeling/angiogenesis, regulation of immune function and oxidative stress defense (mediated by HIF-1 and HIF-2) and increasing erythropoietin levels (primarily mediated by HIF-2) [ 53 ]. One protein notably regulated by HIF-1 and importantly contributing to vascular adaptations is VEGF, polymorphisms of which modulate MS susceptibility [ 10 ].

Regulation of hypoxia inducible factors (HIFs). Various transcription factors and posttranslational processes regulate HIF stability and activity. Among them is the oxygen-dependent regulation via HIF prolyl hydroxylases (PHDs), which hydroxylate (OH) HIF α subunits, allowing recognition by the von Hippel Lindau (VHL) tumor suppression protein, polyubiquitination (Ub) and proteasomal degradation under normoxic conditions. In hypoxia, HIF α subunits stabilize, dimerize with β-subunits and regulate transcription of hundreds of genes via binding hypoxia response elements (HRE) and recruiting co-activators such as CREB binding protein (CBP) or p300.

Vascular comorbidities and vascular dysfunction are common in MS and cerebral blood flow is reduced in all forms of MS, possibly preceding demyelination, as reviewed previously [ 21 , 54 , 55 ]. Demyelination lesion sites are associated with inflammation and are acutely hypoxic, which has been demonstrated in EAE mice using HIF-immunohistochemistry and pO 2 sensors [ [56] , [57] , [58] ] or susceptibility-weighted imaging (SWI) [ 59 ]. Hypoxia has also been shown in MS patients using frequency domain near-infrared spectroscopy [ [60] , [61] , [62] , [63] ], HIF-immunohistochemistry [ 60 , 64 ] or expression of hypoxia-associated D110 antigen [ [65] , [66] , [67] ] in post-mortem brains. Accordingly, lower oxygenation in the spinal cord by multispectral optoacoustic tomography in EAE model mice [ 68 ] and in the cortex of patients with secondary progressive MS via time-domain near-infrared spectroscopy [ 69 ] have also been reported. Further supporting a role of impaired oxygen supply in MS pathogenesis, small vessel disease is associated with symptoms overlapping with MS, including demyelination and brain atrophy [ 70 ]. In addition, in a subset of MS patients, active lesions resembled hypoxia-like white matter lesions observed in stroke; distal oligodendrogliopathy with relatively preserved myelin sheaths and marked HIF-1α upregulation in oligodendrocytes and other cell types [ 64 ].

In EAE rats, Davies and colleagues demonstrated that the pO 2 decreased during relapse but recovered during remission [ 58 ]. Concomitantly, angiogenic remodeling occurs in EAE [ 71 ] and MS [ 72 , 73 ] and may counteract disease progression [ 57 ]. Accordingly, pharmacological restoration of blood flow in the hypoxic spinal cord of EAE model rats (and another MS model: experimental pattern III demyelinating lesion) via CNS specific vasodilation (with nimodipine) restored spinal oxygenation and improved cerebral blood flow [ 74 ]. Moreover, nimodipine ameliorated neurological and motor function and reduced demyelination [ 74 ]. Conversely, hypoxic damage is linked to mitochondrial dysfunction, oxidative stress (amplified by iron-release, e.g., from oligodendrocytes and myelin during demyelination [ 75 ]) and nitric-oxide-mediated cellular dysfunction. These effects have all been described for MS-related lesions [ [76] , [77] , [78] ], despite enhanced energy demand at such sites [ 79 ]. Dysfunction of mitochondrial respiration (particularly due to complex IV deficits) may be distinctive for a specific pathological pattern described for MS, pattern III, which is characterized by moderate inflammation and the disappearance of myelin proteins in distal oligodendrocyte extensions [ 77 ]. Also, inhibition of respiratory complexes I and III have been described in the motor cortex of MS patients [ 78 ]. An aberrant upregulation of complex IV activity in parallel to elevated levels of mitochondrial stress markers (mtHSP70) and increased mitochondrial content has been reported for axons and astrocytes in MS lesions [ 79 , 80 ]. Other mitochondrial dysfunctions in progressive MS include clonally expanded mitochondrial gene deletions [ 81 ].

The deep and periventricular white matter is particularly vulnerable to hypoxia due to sparse and vulnerable vascular provision and increased risk of hypoperfusion and is typically strongly affected in MS [ 45 ]. In severe/prolonged hypoxic conditions, vulnerable cells, like oligodendrocytes degenerate, causing demyelination [ 45 ]. Metabolic stress conditions – following hypoxia or deprivation of growth factors or glucose – have been shown in cellulo in human oligodendrocytes to promote oligodendrogliopathy comparably to MS [ 82 ]. In these models, reduced overall energy utilization and myelin process withdrawal at relatively preserved cell survival was observed [ 82 ].

Mahad and colleagues [ 20 ] suggested a cascade of inter-related immunological and neurodegenerative diseases underlying MS pathogenesis, in which hypoxia plays a central role. According to this hypothesis, T- and B-cell mediated inflammation following an injury leads to oxidative stress, microglial activation, mitochondrial damage and energy failure, creating conditions of “histotoxic” [ 20 , 64 , 76 ] or “virtual” [ 83 ] hypoxia (i.e., impaired oxygen consumption and ATP-production due to mitochondrial dysfunction, despite normal oxygen availability, and/or increased oxygen demand of demyelinated axons) and “genuine” tissue hypoxia due to impaired blood/oxygen supply (e.g., due to tissue lesions) and/or inflammation. Definitions for terms related to specific hypoxic and hyperoxic conditions are provided in Table 2 . High oxygen demand of infiltrating leukocytes and inflammation-associated enzymes can result in tissue hypoxia and stabilization/activation of HIFs [ 84 ] and has been linked to neurological deficits in EAE rats [ 58 ]. The consequences of histotoxic hypoxia on energy availability and cellular damage (e.g., oxidative damage due to mitochondrial dysfunction) can likely be amplified by reduced pO 2 , as a result of ambient hypoxia or impaired oxygen supply; e.g., vascular damage or respiratory disease. This means that exposure to severe ambient hypoxia in conditions of histotoxic hypoxia may be detrimental and negatively affect the disease course. On the other hand, it also indicates that protection by hypoxia conditioning from subsequent, more severe hypoxia could attenuate pathological processes in MS. Hypoxia conditioning refers to different methods using inspiratory hypoxia and aiming for inducing health- or resistance-promoting cellular and systemic adaptations [ 85 ].

Table 2 Definition of terms describing hypoxic or hyperoxic conditions. Table 2 Term Definition Ambient hypoxia Reduced oxygen availability in the surrounding air, either due to reduced barometric pressure and therefore a reduced partial pressure of oxygen (hypobaric hypoxia) or due to reduced oxygen concentration in the air (normobaric hypoxia). Hypoxemia Reduced oxygen saturation in blood. Genuine tissue hypoxia Reduced oxygen availability in tissues; for example, due to hypoxemia, impaired blood/oxygen supply or inflammation (and related high oxygen demand). Histotoxic or virtual hypoxia Compromised oxygen consumption and ATP-production due to mitochondrial dysfunction. Oxygen availability may be normal. Functional hypoxia Transiently reduced oxygen availability in tissues due to high physiological oxygen demand (e.g., following muscle contraction or neuronal activation). Pathological (tissue) hyperoxia Higher than normal (normoxic) oxygen availability in tissues, for example, due to mitochondrial dysfunction resulting in lower oxygen consumption and therefore accumulation of surplus oxygen.

Definition of terms describing hypoxic or hyperoxic conditions.

Overall, there is strong evidence supporting a role of hypoxia in the development and progression of MS ( Fig. 3 ). However, it remains unclear, whether hypoxia is a causal or contributing/aggravating factor [ 21 ]. It is also possible that hypoxia is a mere consequence of other pathological events (such as inflammation). Cellular responses to hypoxia may even be involved in the induction of molecular defense mechanisms against other pathological events, for example mitochondrial dysfunction, oxidative stress or inflammation.

Hypoxia in multiple sclerosis (MS). Both “real” and “virtual” hypoxia characterize MS pathology and are results of reduced oxygen supply (e.g., hypoperfusion) or increased oxygen demand/mitochondrial damage, respectively (a). Hypoxia is involved in reduced blood-brain-barrier integrity, neuroinflammation and the formation of lesions and demyelination. At the level of the central nervous system (CNS, b) cerebral blood flow and oxygenation of brain and spinal cord are frequently impaired in MS and MS models. The anatomical peculiarities of some CNS areas make them especially vulnerable to hypoxia, and MS lesions preferentially are found in such locations. In addition, some brain regions are thought to link hypoxia and immunological pathologies in MS, among them the choroid plexus. See text for further descriptions.

Studies exploring the influence of environmental hypoxia on multiples sclerosis

To disentangle the contribution of hypoxia to the pathogenesis and symptoms of MS, we searched the literature for interventional studies in MS or models thereof and screened them for indications of how hypoxia interplays with different aspects of MS or MS models.

PubMed was searched from inception through the 4th of December 2024 for studies investigating the role of hypoxia on MS. The terms “hypoxia” and “multiple sclerosis” were used to search titles and abstracts for relevant literature. The search yielded 233 results. We selected all original studies reporting the effects of environmental hypoxia and interventions reducing oxygen uptake (1) in people with MS and (2) in rodent models of MS. Additional studies were selected from the reference lists of the identified studies, the authors’ bibliographies and an exploratory literature research in Google Scholar (also using the search terms “hypoxia” and “multiple sclerosis”). Inclusion criteria were: (i) studies on rodent models of MS and/or on people with MS and (ii) exposure to ambient hypoxia. Studies that investigated tissue hypoxia that was not induced through ambient hypoxia were not considered. A flowchart of the selection process is provided in the supplementary materials. The studies were separated in 2 topics that are discussed in the following sections: (3.1) Exploration of the role of hypoxia in preclinical MS models, and (3.2). Hypoxia as a potential treatment strategy in MS patients. Relevant information on the role of hypoxia in MS was extracted from the other results and integrated in other chapters of this review.

Exploration of the role of hypoxia in preclinical multiple sclerosis models

There is a clearer picture from preclinical interventional studies wherein inspired oxygen supply is reduced. The results of such interventions identified in our literature research in the EAE model are summarized in Table 3 .

Table 3 Hypoxia in preclinical multiple sclerosis models. Table 3 MS-Model Intervention Outcomes Ref Level of Evidence EAE mice, MOG 35-55 immunization in C57BL/6 female mice AIH (10 cycles 5 min 11 % oxygen alternating with 5 min 21 % oxygen) or sham AIH improved clinical scores/functional recovery and histopathology compared to controls: markers of myelination, axon protection and oligodendrocyte precursor cell recruitment to demyelinated areas and inflammation were improved [ 86 ] Medium (mouse model, only females, molecular and clinical effects) Relapsing-remitting model of EAE was induced in 10-wk-old female SJL/J mice by immunization with PLP 139–151 After about 10–11 days postimmunization, random assignment to normoxic (control) or chronic hypoxic (10 % oxygen) conditions Improved clinical recovery and reductions in clinical score in hypoxic condition. Attenuated vascular disruption, leukocyte accumulation, and demyelination in hypoxia. Reduced vascular expression of the endothelial activation molecule VCAM-1, increased expression of endothelial tight junction proteins in hypoxia. Better clearance of infiltrated monocytes during the remission phase [ 57 ] Medium (mouse model, only females, molecular and clinical effects) Relapsing-remitting model of EAE was induced in 10-wk-old female SJL/J mice by immunization with PLP 139–151 After immunization, mice were randomly assigned to normoxic (control) or chronic hypoxic (10 % oxygen) conditions Attenuated EAE progression in hypoxia: reduced inflammatory leukocyte infiltration and demyelination. Improved vascular integrity in hypoxia: reduced expression of VCAM-1 and ICAM-1, maintained expression of endothelial tight junction proteins [ 87 ] Medium (mouse model, only females, molecular and clinical effects) MOG 35-55 -induced EAE in female C57BL/6 mice Conditioned medium from human periodontal ligament stem cells cultured under hypoxia (3 % oxygen) or normoxia (20 % oxygen), infused in the tail vein Infused conditioned medium reduced clinical and histologic disease scores, via increased expression of the anti-inflammatory cytokine IL-37 and reduced expression of pro-inflammatory factors, reduced oxidative stress and apoptotic pathways [ 88 ] Low-medium (mouse model, only infused stem cells exposed to hypoxia, molecular and clinical effects) MOG 35-55 -induced EAE in female C57BL/6 mice Normoxic (control) or chronic hypoxic (10 % oxygen) conditions for 3 weeks Hypoxia reduced histological markers of inflammation and EAE onset. Fewer CD 4+ T cells in spinal cords and a delayed Th17-specific cytokine response. Increased IL-10 levels Tregs indicate an anti-inflammatory environment [ 89 ] Medium (mouse model, only females, molecular and clinical effects) EAE mice, MOG 35-55 immunization in C57BL/6 female mice (emulsified with com-plete Freund's adjuvant containing 4 mg/ml M. tuberculosis , sub-cutaneous injection) Normoxic (control) or chronic hypoxic (10 % oxygen) conditions Chronic hypoxia increased vascular density, delayed onset and reduced severity of EAE. Chronic hypoxia after the onset of clinical symptoms, pathology and disease severity was reduced [ 90 ] Medium (mouse model, only females, molecular and clinical effects) Notes. AIH: Acute Intermittent Hypoxia, CD 4+ : cluster of differentiation 4+, EAE: experimental autoimmune encephalomyelitis, MOG: myelin oligodendrocyte glycoprotein, PLP: myelin proteolipid protein, Treg: regulatory T cells, VCAM-1: vascular cell adhesion molecule.

Hypoxia in preclinical multiple sclerosis models.

Notes. AIH: Acute Intermittent Hypoxia, CD 4+ : cluster of differentiation 4+, EAE: experimental autoimmune encephalomyelitis, MOG: myelin oligodendrocyte glycoprotein, PLP: myelin proteolipid protein, Treg: regulatory T cells, VCAM-1: vascular cell adhesion molecule.

A small number of studies using the EAE model with rodents investigated the outcomes of chronic exposure to hypoxia [ 57 , [87] , [88] , [89] , [90] ]. In all of these studies, the mice were kept in an environment of 10 % oxygen for weeks, which has frequently been referred to as mild chronic hypoxia. The findings of these studies suggest that the hypoxia improved various MS-related parameters, including inflammation, neuropathology and clinical scores (see Table 3 ) and indicated that the hypoxia was indeed mild for the mice. However, normobaric oxygen at a concentration of 10 % corresponds to hypobaric hypoxia (as occurs naturally at high altitude) at about 6000 m, which is not considered mild for humans, who stay there for a prolonged time [ 91 ]. In fact, about 5000–5300 m are considered a maximum altitude at which most humans can reside permanently, although many already suffer from chronic mountain sickness at this point [ 92 ]. Mice in that respect seem to be more hypoxia-tolerant to hypoxia than humans, and also to rats [ 93 , 94 ]. Direct translation to humans therefore may not be possible.

Dore-Duffy and colleagues first demonstrated that exposure to chronic, continuous hypoxia delayed disease onset and attenuated symptoms and pathology after EAE induction [ 89 , 90 ]. Those effects were accompanied by better vascularization [ 90 ] and the promotion of an anti-inflammatory environment [ 89 ]. Halder and Mildner later showed similar benefits of the same hypoxic conditions [ 57 , 87 ]. Specifically, chronic hypoxia consistently preserved myelin, and persistently slowed down disease progression and ameliorated clinical recovery and disease severity in a relapsing-remitting model of EAE [ 57 ]. Compared to mice kept in normoxia, these animals exhibited preserved vascular integrity and accelerated monocyte apoptosis in the spinal cord, likely due to increasing HIF-1α levels due to the ambient hypoxia [ 57 ]. Giacoppo and colleagues [ 88 ] showed that even the infusion of hypoxia-conditioned medium from human periodontal ligament stem cells cultured under hypoxia (3 % oxygen) into the tail vein of EAE mice improved clinical scores and attenuated inflammation.

Tokarska and colleagues recently observed pronounced benefits of a related intermittent hypoxia conditioning protocol (acute intermittent hypoxia) in EAE mice with high clinical EAE-scores (above 2, indicating substantial hindlimb paralysis) regarding clinical, histological and inflammation parameters [ 86 ].

Apart from the EAE model, indications of an involvement of oxygen availability and oxygen consumption in MS can also be derived from other rodent models of MS. For example, impaired oxygen utilization has been reported in the cuprizone mouse model of demyelination using near-infrared spectroscopy and MRI analyses [ 95 ]. Moreover, the HIF-activator VCE-004.8 (EHP-101) attenuated inflammation and demyelination, while promoting remyelination in EAE and cuprizone mouse models [ 96 ].

The results summarized in Table 3 indicate that both continuous and intermittent hypoxia exposures can improve outcomes in animal models of MS. A discussion of these different types of hypoxia exposure is provided in section 6.1. In humans, intermittent hypoxia exposures are more feasible.

Hypoxia as a potential treatment strategy in MS patients

There are differences of MS-models like EAE with human MS [ 20 , 39 , 40 ], and perhaps different tolerance to hypoxia between rodents and humans [ 93 , 94 ], that put the translation of findings of hypoxia benefits obtained from animal models into humans in question. Therefore, direct application in patients are of interest and a few such studies are available. All identified studies of our literature research reporting hypoxia interventions in MS patients are summarized in Table 4 .

Table 4 Hypoxia interventions in multiple sclerosis (MS) patients. Table 4 Population and study design Intervention Outcomes Ref Level of Evidence Randomized, blinded, placebo-controlled, crossover study in 10 patients (1 woman) with relapsing-remitting MS AIH (15 brief exposures to FiO2 = 9 %, alternating with normoxia) or sham AIH improved cognitive processing speed and motor capacity (plantarflexion and dorsiflexion ankle torque) [ 97 ] Low - medium (preprint, low N, clear effects) Case report: one female MS patient (EDSS score 3.0) Single bout of AIH (FiO2 = 9 %) or sham Upper extremity function (shoulder abduction and wrist extension strength) were improved after AIH [ 98 ] Low (case study) Crossover trial in 13 relapsing-remitting MS patients (42.3 ± 9.6 years old, 7 women, EDSS 2.8 ± 1.5) One set of vibration training (12 sets of 1-min isometric squats) at FiO2 = 15 % and 20,9 % (1 week separated) Improved rate of force development in the legs in the hypoxic condition [ 99 ] Low-medium (small N, heterogeneous population) Randomized, single blind, parallel-group study in 34 relapsing-remitting MS patients (EDSS 3.0[0–4]): Normoxia group: 51 ± 10 years old, hypoxia group: 49 ± 9 years old, 22 women) Treadmill training, 3 times/week for 1 h over 4 weeks, either in hypoxia FiO2 = 15 % (N = 17, 11 women) or normoxia FiO2 = 21 % (N = 17, 11 women) Increased fat oxidation only in hypoxia group, improved Th17/Treg profile only in normoxia group, fitness levels and mood were improved in both groups [ 100 ] Low (small N (pilot study), low hypoxic dose: clear exercise effect, less clear hypoxia effect) Randomized prospective longitudinal pilot study in 32 MS patients (43,4 ± 7 years old, 10 women) Endurance training (cycling) at FiO2 = 15.5 % (N = 13) or 20,9 % (N = 8), 6 days/week for 2 weeks (45 min per session) compared to normal rehabilitation (N = 11) Fatigue scores improved in all groups but faster in the exercise groups, spasticity (MSSS-88) improved only in the exercise groups (normoxia and hypoxia), walking (6-min walking test) improved only in the hypoxic exercise group [ 101 , 102 ] Medium – high (pilot-study, synergistic exercise and hypoxia effects) Feasibility study in 12 MS patients (45.5 ± 10.6 years old, 6 women) Sprint interval training (3 sessions of 5 × 10 s cycling sprints at FiO2 = 16.3 % for three weeks (in total 7 trainings) in addition to standard therapy. Repeated sprint protocol under hypoxia demonstrated to be feasible in MS patients. Improvements in peak power output, 6-min walking test, and patient reported outcome measurement questionnaire (PROMIS) [ 103 ] Low - medium (only abstract, clear improvements, no exercise control) Notes: AIH: Acute Intermittent Hypoxia, EDSS: Expanded Disability Status Scale, MSSS-88: Multiple Sclerosis Spasticity Scale, Th17: T cells that produce cytokines including IL-17, Treg: regulatory T cells.

Hypoxia interventions in multiple sclerosis (MS) patients.

Notes: AIH: Acute Intermittent Hypoxia, EDSS: Expanded Disability Status Scale, MSSS-88: Multiple Sclerosis Spasticity Scale, Th17: T cells that produce cytokines including IL-17, Treg: regulatory T cells.

Results of hypoxia exposure in combination with aerobic exercise in MS patients have first been published in 2014 [ 101 , 102 ]. The MS patients performed cycling exercise over 2 weeks in either mild hypoxia (FiO2 = 15.5 %, corresponding to about 2500 m of altitude) or normoxia. Those who exercised in hypoxia benefited more in terms of performance and spasticity than those who exercised in normoxia. Another study investigated MS patients performing treadmill training over 4 weeks either in mild hypoxia (FiO2 = 15 %) or normoxia [ 100 ]. In that study, the group exercising in hypoxia showed metabolic improvements (fatty acid oxidation) but benefitted less than the normoxia group in terms of inflammation. A third study combining hypoxia with exercise analysed the effects of a single session of vibration training with and without hypoxia and found improved lower limb force development in the hypoxia condition [ 99 ].

Therapeutic acute intermittent hypoxia protocols have been reported in two – at the time of writing of the present review – preliminary publications, an abstract [ 98 ] and a preprint manuscript [ 97 ]. In one of these studies upper extremity function could be improved by acute intermittent hypoxia [ 98 ], in the other cognitive and motor improvements were observed [ 97 ]. In a feasibility study, MS patients performed high intensity exercises (repeated cycle sprints) under hypoxia (FiO2 = 16.3 %) in addition to their standard rehabilitation program. This improved physical performance and subjective symptom ratings after the 3 weeks lasting intervention [ 103 ].

Similar protocols of intermittent hypoxia conditioning, often combined with exercise/motorcognitive challenges or hyperoxic periods, have been applied or are currently investigated in a great number of diseases, ranging from cardiovascular and metabolic to neuropsychiatric diseases [ 85 ]. Notable applications related to the human CNS include the improvement of mild cognitive impairment and normal-age-related cognition [ 104 ], depression [ 105 ] and Parkinson's disease [ 106 , 107 ].

It is not yet clear, how these intermittent hypoxia conditioning protocols improve neuropsychiatric conditions, in particular MS. However, therapeutic acute intermittent hypoxia is thought to act primarily via improving synaptic plasticity thereby contributing to improved respiratory and motor functions [ 108 ]. Hypoxia conditioning in general has been proposed to improve the function of the respiratory, cardiovascular and immune system, upregulate anti-oxidative capacity and resilience of cells and improve metabolism and oxygen delivery systems among others [ 85 ]. For MS it appears plausible that in particular improved cellular resilience, oxygen supply and anti-inflammatory mechanisms play a role. MS-associated mitochondrial dysfunctions [ 11 ] may also be partially amenable by hypoxia conditioning [ 85 ].

The few available studies in humans and the described difficulties in translating data from hypoxia exposures in animal models of MS to patients make the detailed exploration of mechanistic, efficiency and safety features of hypoxia conditioning crucial challenges for future research. A “translational roadmap” for research of therapeutic acute intermittent hypoxia has recently been presented specifically for spinal cord injury and neuromuscular disease and is also relevant for MS [ 108 ].

Potential mechanisms of hypoxia-mediated benefits in multiple sclerosis

Specific patterns of MS lesions have been suggested to be generated as a result of potential endogenous hypoxia (pre-)conditioning effects and protect the affected regions from subsequent damage. An expressional pattern (mainly in oligodendrocytes) reminiscent of tissue preconditioning has been described at the borders of active lesions in Baló's concentric sclerosis [ 109 , 110 ], a disease likely sharing pathogenetic mechanisms with MS [ 111 ] and pattern III lesions in MS [ 112 ]. Among the involved proteins were HIF-1α and heat-shock protein 70 [ 109 ]. Together with the evidence of the previous sections, this indicates that hypoxia conditioning could be an effective strategy in MS. It has the potential to improve inflammation, pathology and symptomatology and possibly modify the disease course. Some hypotheses how this may be explained mechanistically is described in the following sections.

Interplay of hypoxia and immune-pathways/inflammation

Hypoxia and inflammation are highly interrelated, with systemic hypoxia leading to increased circulating proinflammatory cytokines [ 113 ]. On the molecular level, HIFs and the negative HIF regulators PHDs are involved both in the regulation of hypoxia responses and inflammation, partly via oxygen-level-dependent regulation of major inflammatory pathways like NF-κB [ 114 ]. Because hypoxia can lead to inflammation and inflammation can cause hypoxia, this interaction represents a potential vicious cycle, possibly driving MS pathogenesis and progression. The relevance of this hypoxia-inflammation cycle in the context of MS has been recently reviewed [ 115 ]. The mechanisms by which hypoxia may benefit MS also include improved vascular integrity and density, which have been hypothesized to improve clearance of disease-progress associated substances (e.g., myelin breakdown products and pro-inflammatory cytokine) and thereby attenuate inflammation [ 57 ], similar to regular exercise [ 116 ]. In addition, attenuation of immune cell infiltration in the spinal cord, e.g., by hypoxia-enhanced apoptotic clearance of monocytes [ 57 ], may underly hypoxia-benefits in MS. HIF-1 activation in hypoxia has been linked to apoptosis induction (e.g., Refs. [ 117 , 118 ]), which can further promote the clearance of damaged or dangerous cells (such as over-activated monocytes) in mild hypoxia but these effects may be detrimental in severe hypoxia.

Hypoxia conditioning, energy metabolism and oxidative stress

The hypo-oxygenation in the MS-afflicted CNS is expected to compromise the oxidative metabolism, especially of the cells’ main oxygen consuming organelles, the mitochondria. In EAE models, mitochondrial damage is an early pathological feature and manifests in functional and morphological alterations [ 119 ] that change according to disease stage and severity [ 120 ]. Oxidative and nitrative stress result in oxidative phosphorylation dysfunction, reduced ATP levels and morphological abnormalities of mitochondria and has been found to precede infiltration of inflammatory cells [ 121 ]. Therefore mitochondrial dysfunction – maybe as a result of hypoxia – is thought to represent an early event in the pathogenesis of EAE and may trigger a cascade of further pathological events [ 121 ]. Davis and colleagues confirmed a correlation of hypoxia (visualized with an immunohistochemical and an oxygen-sensitive optical probe) with neurological deficits in EAE model rats [ 58 ]. They demonstrated a reversal of neurological deficits by provision of increased oxygen-concentrations [ 58 ].

MS lesions are often characterized by high complex IV activity, increased mitochondrial mass and reduced mitochondrial mobility [ 77 , 79 , 122 ]. Moreover, the aberrant mitochondrial activity is associated with excessive oxidative stress and increased expression of mitochondrial stress markers, such as heat shock protein 70 [ 79 , 121 , 123 ]. Despite the increased mitochondrial mass at MS lesions, mitochondrial biogenesis may be impaired, as suggested by reduced mRNA levels of the transcription factor peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), a mitochondrial biogenesis master regulator [ 79 , 124 ].

In postmortem motor cortex, reduced mitochondrial gene expression has been found specifically in neurons [ 78 ]. The authors of this study hypothesized that reduced ATP-production in demyelinated motor neuron axons leads to axonal degeneration and neurological impairments [ 78 ]. Mahad and colleagues found hypoxia-like tissue injuries and mitochondrial respiratory system deficits primarily in pattern III MS lesions in postmortem brain [ 77 ]. They also discovered an absence of immunoreactivity of the complex IV subunit COX I specifically in oligodendrocytes, hypertrophied astrocytes and axons [ 77 ]. The exposure to cerebrospinal fluid from MS patients has been shown to induce mitochondrial structural and functional changes in previously healthy neurons as well [ 125 ]. In this study, increased levels of ceramides (a class of sphingolipids, some types of which are associated with mitochondrial damage [ 126 , 127 ]) were found in MS cerebrospinal fluid and proposed to be the main factors leading to the observed mitochondrial alterations [ 125 ]. Ceramides and sphingolipid metabolism are known to be highly sensitive to hypoxic stress and high levels of long-chain ceramides can induce mitochondrial dysfunction [ 128 ].

While mitochondrial dysfunction as a result of changed oxygen availability has been notoriously difficult to monitor in the living human brain, novel techniques increasingly allow monitoring of mitochondrial changes in vivo, for example in the anterior visual system, as recently reviewed [ 129 ]. This metabolically highly active system is affected in almost all MS-cases. In addition, biomarkers in the cerebrospinal fluid or blood can provide information on changes in energy metabolism in MS patients. For example, ATP metabolites in the cerebrospinal fluid of MS patients support ATP deficiencies [ 130 ]. Moreover, serum lactate levels have been increased in MS patients and more in progressive forms than in those with a relapsing-remitting disease course [ 131 ]. These results may indicate mitochondrial dysfunction [ 131 ] but could also mean a compensatory switch to increased glycolysis (and associated higher lactate production) due to hypoxia induced and HIF-mediated metabolic reprogramming.

Axons are highly vulnerable to mitochondrial dysfunction, due to structural limitations of mitochondrial networks (filamentous distribution as opposed to networks in soma, high energy demand, e.g., for maintenance of membrane potential to enable action potential transmission and the transport of molecules within the axon). Demyelination further increases axonal susceptibility to injuries by exposing them to increased environmental risks [ 20 ].

While insufficiently explored in the context of MS, hypoxia conditioning is thought to improve energy metabolism and especially mitochondrial function [ 132 ]. HIFs again are major mediators of these outcomes and for example coordinate the switch to anaerobic energy metabolism pathways, the finetuning of the mitochondrial respiratory system to hypoxic conditions, improved antioxidative capacities and mitochondrial quality control. Via the upregulation of gene-expression for glycolytic enzymes and glucose transporters, HIFs facilitate the utilization of glycolytic ATP-production [ 133 ]. HIFs also upregulate pyruvate dehydrogenase kinase 1 [ 134 , 135 ], shunting pyruvate away from the tricarboxylic acid, again promoting glycolysis. In addition, HIFs downregulate subunits of mitochondrial complex I and coordinate a shift in the composition of complex IV, in order to reduced cellular respiration or make it more effective, respectively [ 49 ]. In combination with improved antioxidant capacities by HIF-activation, this optimization of cellular respiration results in lower levels of oxidative stress [ 51 ]. Furthermore, via the promotion of mitophagy-regulating proteins like BCL2 Interacting Protein 3 (BNIP-3) [ 136 ], HIFs can improve the clearance of dysfunctional mitochondria from cells, reducing oxidative stress and inflammation.

Taken together, the potential of hypoxia conditioning to improve mitochondrial function and energy metabolism in general could underly the beneficial effects of hypoxia exposure in MS but requires mechanistic confirmation.

Hypoxia conditioning and oxygen transport

Physiological responses to hypoxia include several mechanisms leading to improved oxygen supply, importantly mediated via HIFs [ 49 ] that lead to beneficial effects in EAE models [ 137 ]. Those include the upregulation of VEGF and associated neoangiogenesis and vascular remodeling. Increased angiogenesis in the CNS is a characteristic of MS [ 138 ] and likely linked to tissue hypoxia. In addition, hypoxia (via HIF) leads to an increased release of erythropoietin, long known for its general neuroprotective properties [ 139 ] that may also be beneficial in MS. In acute EAE in rats, erythropoietin delayed disease onset, attenuated inflammation and improved clinical scores [ [140] , [141] , [142] , [143] , [144] , [145] ]. In an exploratory open-label study (phase I/IIa), high doses of recombinant erythropoietin (48000 IU) had long-term beneficial effects on progressive MS patients (disease duration >9.2 years) regarding expanded disability status scale (EDSS) assessment and cognition [ 146 ]. While these findings were subsequently confirmed [ 147 , 148 ], more recent results did not demonstrate efficiency of this treatment in MS. In a double-blind, placebo-controlled phase 2 trial, 52 patients with progressive MS received recombinant erythropoietin (48,000 IU) or placebo (intravenously 17 times during 24 weeks). No improvements of primary (composite score of maximum gait distance, hand dexterity, and cognition) and secondary (clinical and imaging parameters) outcomes following EPO treatment were found in this study [ 149 ]. These discrepancies may be explained by various methodological problems in the latter study as highlighted in a response to this study [ 150 ].

Hypoxia also induced adaptive changes in the microvasculature, affecting cerebrovascular adaptations and cerebral blood flow. This involves the inter-cellular coordination, partly via prostaglandin signaling, of endothelial cells and pericytes, which is required to maintain vascular homeostasis [ 151 ]. Cerebrovascular reactivity is essential for the regulation of blood-flow and thus oxygen distribution in the brain and has been shown to be impaired in MS [ 152 ]. The induction of cerebrovascular reactivity, vasodilation and stimulation of cerebral blood flow by acute and intermittent hypoxia [ 153 ] might be one particularly important mechanism involved in the positive effects of hypoxia conditioning in MS. Hypoxia-induced HIF-activation results in vascular remodeling that improves oxygen supply [ 154 ] and has been shown throughout the CNS in mice exposed to chronic hypoxia (14 day at an oxygen concentration of 8 % [ 155 ], as well as in the spinal cord of presymptomatic EAE mice [ 71 ]. In addition, intermittent hypoxia conditioning (3 × 4 min at an oxygen saturation of 80 % interspersed with 4 min normoxia) has been shown to protect the endothelium from subsequent ischemia-reperfusion insults [ 156 , 157 ]. Hypoxia also regulates cerebral blood flow acutely and can improve oxygen supply under hypoxic conditions [ 158 ]. Importantly, while moderate and transient hypoxia have been suggested to protect cerebrovascular function [ 159 ], severe hypoxia results in reduced cerebral blood flow [ 160 , 161 ] and can damage the blood-brain barrier [ 162 ]. In MS, it has been shown that proinflammatory cytokines, such as IL-1B are involved in increasing the blood-brain barrier permeability, via induction of HIF-1 and VEGF [ 163 ]. Accordingly, hypoxia conditioning (10 cycles per day of 13 % oxygen for 5 min intervals interspersed with 5 min normoxia for 2 weeks) has been shown to improve microvascular angiogenesis and cerebral microcirculation and to protect in a stroke model in mice, while chronic intermittent hypoxia (12 h continuous 13 % oxygen per day for 2 weeks) yielded none of these benefits [ 164 ]. Neurological impairment was observed only at much lower oxygen levels (7 %) [ 164 ]. These authors suggest that the therapeutic window for intermittent hypoxia treatments is between about FiO 2  = 10–15 % for short hypoxic cycles, with lower doses (FiO 2 >15 %) being not sufficient to induce vascular benefits and too high doses (FiO 2 <10 %) being dangerous for the CNS [ 165 ]. Finally, reduced cerebral blood flow is a frequent condition in MS and pharmacologically improving cerebral blood flow has early been described as a rapid modality to relieve MS symptoms [ 166 , 167 ].

In conclusion, severe hypoxia may promote MS-pathology, whereas controlled hypoxia could protect from future hypoxic insults.

Ambiguities and debates in hypoxia therapy for multiple sclerosis

Differences between intermittent and continuous hypoxia

As outlined, both intermittent and continuous hypoxia can be both detrimental or beneficial for the brain, depending on individual vulnerabilities and the hypoxic dose. However, these 2 modalities are characterized by different adaptive outcomes, risks and their practicality for clinical applications vary.

While continuous prolonged hypoxia can lead to high-altitude illnesses, such as acute mountain sickness and high-altitude cerebral or pulmonary edema and continuous chronic hypoxia can result in chronic mountain sickness, severe chronic intermittent hypoxia can be caused by obstructive sleep apnea (OSA). Oxidative stress [ 168 ], mitochondrial dysfunction [ 169 ], aberrant activation of inducible NO synthase (iNOS) [ 170 ] and resultant inflammatory processes are involved in the pathogenesis of OSA and related diseases [ 171 ]. Sleep disorders and sleep-disordered breathing are common in MS [ 172 ]. Recently hypoxia associated with sleep disturbances (e.g., sleep apnea) have been shown to aggravate cognitive deficits in MS [ 173 ].

On the other hand, mild intermittent hypoxia (therapeutic acute intermittent hypoxia or intermittent hypoxia conditioning), while also resulting in the production of (lower levels of) ROS and nitric oxide [ 174 , 175 ] is met by cells with the induction of protective mechanisms, largely orchestrated by HIFs and leading to enhanced antioxidant and energy-metabolism preserving defense mechanisms protecting cells [ 176 ].

In intermittent hypoxia conditioning, more moderate ROS and NO formation [ 174 , 175 ] than in severe or chronic hypoxia forms have been found to evoke favorable adaptations, including upregulation of antioxidant defense systems, increased mitochondrial resilience and efficiency and HIF-activated gene expression, which collectively oppose development of pathogenic cellular environments [ 85 ].

In terms of efficiency to induce adaptations, intermittent hypoxia may be superior, if total duration of exposure is compared. This has recently been confirmed by Tobin et al. [ 177 ], who observed increased hematocrit/hemoglobin content by 12–15 % in older adults after intermittent (85 % peripheral oxygen saturation) but not continuous hypoxia.

It is noteworthy that very beneficial effects of similar chronic and continuous exposure to hypoxia as used in EAE mice (see Table 3 : [ 57 , [87] , [88] , [89] , [90] ]) has been shown to be highly beneficial in mice with impaired function of complex I of the mitochondrial respiratory system. Mice with such deficits serve as models for Leigh disease and Friedreich ataxia and in those models, hypoxia (mostly 11 % oxygen) greatly improved neurological deficits and prolonged lifespan [ [178] , [179] , [180] , [181] , [182] , [183] ], as recently reviewed [ 184 ]. The mechanism appears to be largely independent of HIFs, but the hypoxia exposure has been demonstrated to normalize detrimental hyperoxia, which is thought to be a consequence of the pronounced mitochondrial dysfunction and associated reduced utilization of oxygen transported to the affected cells [ 179 ]. The same protocol even proved beneficial in a mouse model of accelerated aging (Ercc1 Δ/- mouse model), in which it prolonged lifespan [ 185 ]. Since mitochondrial dysfunction is a prominent feature of MS and EAE, the benefits of chronic hypoxia might be similar, possibly also reducing pathological local hyperoxia in these animals. Since also hyperoxic treatments have been shown to be beneficial in EAE models [ 58 , 186 ], this seems unlikely. However, in a cuprizone mouse model of demyelination also increased cerebral tissue oxygenation probably as a consequence of impaired oxygen consumption has been reported [ 95 ].

For most applications, including the treatment of different conditions in humans, intermittent hypoxia exposures are today considered superior to chronic continuous exposures [ 85 ]. Acute intermittent hypoxia is a heavily investigated type of intermittent hypoxia conditioning with great promises for applications in incomplete spinal cord injury, neuromuscular disease, stroke and others [ 108 ]. Several studies investigating acute intermittent hypoxia in the context of MS are ongoing, but with the exception of the study of Tokarska and colleagues (see Table 3 ) none on EAE have yet been published [ 86 ]. However, preliminary results on MS patients are promising (see Table 4 [ 97 , 98 ]).

Hypoxia or hyperoxia

Seemingly in opposition to the reported beneficial effects of hypoxia/HIF-activation, also normobaric oxygen treatment (80–95 % oxygen, 1 h up to 7 continuous days) has been suggested to be beneficial for MS, since it prevented demyelination [ 187 ] and improved neurological function in EAE rats [ 58 ]. Similarly, breathing 95 % oxygen for 72 h (or prophylactically) attenuated the integrated stress response in oligodendrocytes and reduced demyelination and oligodendrocyte loss in EAE model mice [ 186 ]. This treatment did not improve T-cell infiltration and cytokine expression in the spinal cord [ 186 ]. It is important to emphasize that acute benefits of oxygen therapy in the settings of acute lesions may not translate in benefits regarding long-term disease development in progressive MS, where preexisting mitochondrial damage may prevents the therapeutic advantages of additional oxygen [ 188 ].

The history of hyperbaric oxygen therapy in MS has recently been summarized [ 21 ]. As argued by Halder and Milner [ 21 , 57 ], increasing oxygen supply may be acutely and transiently beneficial by supporting cellular functioning in hypoxia (e.g., by maintaining oxidative energy metabolism), while hypoxia induces responses that equip cells to better tolerate future hypoxic conditions. Increasing oxygen levels therefore may be more effective to immediately counteract hypoxia but hypoxia could be more efficient for prevention or long-term protection. The combination of hypoxia and hyperoxia, as applied in several protocols targeting neurological diseases [ 176 , 189 ], is a promising new development from this perspective.

Beside changes of the ambient oxygen concentration, pharmacological modulation of HIFs, which are at the core of molecular hypoxia responses [ 176 ], has been suggested as potential strategy in MS and has been tested in MS models. For example, the HIF-stabilizing cannabinoid derivative VCE-004.8 attenuated the expression of pro-inflammatory factors, immune cell infiltration, demyelination and axonal damage in 2 rodent models of MS, EAE and Theiler's virus-induced encephalopathy (TMEV) [ 190 ]. The drug methylprednisolone, used for treating MS, has also been shown to stabilize HIF and increase erythropoietin levels [ 191 ]. Similarly, erythropoietin upregulation via HIF-1 and HIF-2 stabilizing recombinant lactoferrin and increased Nrf2 activity were also protective in EAE rats, reducing symptoms, disease duration, mortality and clinical scores [ 192 ]. Surprisingly, dimethyl fumarate, an FDA-approved first-line for MS with anti-oxidant and anti-inflammatory effects of the class of fumaric acid esters [ 193 ], has been reported to be a negative regulator of HIF (by promoting its proteasomal, but VHL-independent degradation) [ 194 ]. However, dimethyl fumarate also promotes the activity of another hypoxia-induced transcription factor with strong anti-oxidative capacities, Nrf2 [ 193 , 195 ], which is also involved in protection of the blood-brain barrier integrity [ 196 ]. Recently dimethyl fumarate has gained substantial attention for its capacity to delay conversion of RIS to MS [ 197 ]. Another study reported an increased HIF-1 activity after the treatment of EAE model mice with IL-17 resulting in increased inflammatory responses [ 198 ]. Astrocyte-specific knockdown of IL-17RA and HIF-1α attenuated pro-inflammatory cytokine levels in EAE brain, and improved pathology and clinical scores [ 198 ]. An earlier study, which also reported substantially increased HIF-1α levels in astrocytes and myeloid cells in EAE-mouse spinal cord, showed that cell-specific knock-out of only HIF-1α or conditional knock-out of the negative HIF-1 regulator VHL did not modulate the course of the disease [ 199 ].

Similar to results of the modulation of environmental oxygen levels, these results show that both the stabilization and reduction of HIFs can be beneficial in MS models. These apparently conflicting findings may be reconciled considering the fluctuations in disability and inflammation characteristic for MS. It is possible that curtailing HIF-activities in periods of strong hypoxic load (usually during periods of strong inflammation) is beneficial, while the activation of HIF in during remission may be beneficial by promoting vascularization, antioxidant and anti-inflammatory environments similarly to what has been shown in the studies on hypoxia therapies shown in Table 3 .

How both hyperoxia and hypoxia may benefit MS pathology remains to be reconciled but a theoretical model is proposed in Fig. 4 .

A model of differential effects of hyperoxia and hypoxia-interventions on multiple sclerosis features. Oxygen supplementation, intermittent and chronic hypoxia probably improve pathological tissue hypoxia and associated metabolic consequences by different ways (a). While hyperoxia and chronic continuous hypoxia probably are effective mainly during the intervention (blue bars), intermittent hypoxia approaches are thought to induce protective adaptations to hypoxia that may persist for some time after cessation of the intervention. A theoretical model about how the modulation of hypoxia inducible factor (HIF) pathways may be beneficial in both directions (up- and downregulation of HIF activity) is shown in panels b–j. Indicated are hypoxia responses (including HIF-activities) over time. The hypothetical lower (black) dashed line indicates the threshold that has to be reached for hypoxia responses to induce robust hypoxia adaptations (black bent arrows), while the upper (red) dashed line indicates a threshold for pathological/damaging hypoxia responses. A normal hypoxia response is characterized by low baseline levels and a robust response to a hypoxic stimulus (b). High baseline levels (c) of hypoxia-responses (including HIF abundance) are characteristic for tissue hypoxia (e.g., tumor hypoxia) and have been reported for some aging tissues – this may be also the case in some parts of the CNS, e.g., during periods with high inflammation or due to mitochondrial dysfunction, in MS. Low baseline levels of hypoxia responses and insufficient responses following a hypoxic stimulus to induce adaptations (d) may be the result of artificial oxygenation, e.g., hyperoxic treatments, or impaired/downregulated HIF-pathways (h). Upregulation of the hypoxia response can be detrimental, if the hypoxia response is already efficient (e) or if baselines are high already (f). It can be beneficial, if the hypoxia response is weak (g). HIF inhibition can be beneficial, if the baseline hypoxia response is high (i) and can further deteriorate an already weak hypoxia response (j).

Conclusions

While the precise roles of hypoxia in MS remain poorly understood, it has become clear that tissue hypoxia is a prominent feature in MS brain and spinal cord. Reports on beneficial effects of both hyperoxia and hypoxia may appear conflicting and the reasons for these apparent discrepancies have to be assessed in future work. However, we think that these results can still be reconciled theoretically ( Fig. 4 ). Major determinants of whether hypoxia is effective, benign or detrimental are the hypoxic dose (duration and severity), the mode/patterns of exposure (continuous versus intermittent) and combination with other conditions affecting oxygen availability. Among those conditions are intense exercise and cognitive efforts, which increase oxygen demand, and conditions or periods of reduced ventilation, such as during sleep or specific pathological conditions. This warrants the investigation of the influence of other conditions associated with hypoxia with MS symptomatology and progression. Moreover, the potentials of the combination of exercise (and/or cognitive activity) with mild hypoxic exposures (as occur for example at moderate altitudes) to modulate beneficial adaptations to oxygen in our opinion deserve particular attention in future studies on MS.

How cell-type specific involvements of hypoxia responses and intercell-communication are related to MS pathology and progression remain important questions for basic science, while the optimization of therapeutic hypoxia exposure is the major challenge for clinical applications.

CRediT authorship contribution statement

Johannes Burtscher: Writing – review & editing, Writing – original draft, Visualization, Supervision, Project administration, Methodology, Conceptualization. Robert W. Motl: Writing – review & editing. Klaus Berek: Writing – review & editing. Hannelore Ehrenreich: Writing – review & editing. Martin Kopp: Writing – review & editing. Erich Hohenauer: Writing – review & editing, Conceptualization.

Declaration of competing interest

The authors declare no conflicts of interest related to the topic of this review.

Footnotes

Supplementary data to this article can be found online at https://doi.org/10.1016/j.redox.2025.103666 .

Supplementary data

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References

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Data Availability Statement

No data was used for the research described in the article.