From capillaries to cognition: decoding neurovascular unit dysfunction and cerebrovascular contributions in Alzheimer's disease.

Alzheimer's disease (AD), classically defined by abnormal amyloid-β and tau aggregation, is now also understood to profoundly disturb the brain's cerebrovascular system and the integrity of the neurovascular unit (NVU). An ever-growing body of evidence points to NVU dysfunction and neurovascular unit deficits as major contributors to AD pathology. NVU comprises neurons, glial cells, endothelial cells, and pericytes, which together orchestrate cerebral blood flow in the CNS and help maintain the structural and functional integrity of the blood-brain barrier (BBB). Its disruption increases neuronal damage and impairs clearance mechanisms in AD. Interactions between pericytes, astrocytes, smooth muscle cells, neurons, microglia, and NVU endothelial cells support cerebral perfusion, BBB integrity, and metabolic homeostasis. Major pathophysiological events involved in the development of early BBB leakage and neurovascular uncoupling in AD include endothelial impairment, breakdown of tight junctions, thickening of the basement membrane, degeneration of pericytes, and astrogliosis. Activated astrocytes and microglia further exacerbate NVU injury by releasing inflammatory mediators and ROS. Numerous molecular signaling cascades, including PI3K/Akt/mTOR, MAPK/ERK, and NF-κB pathways, are implicated in mechanistic interplay among metabolic perturbations, neuroinflammatory responses, and vascular endothelial damage. Emerging NVU-targeted therapeutic strategies include anti-inflammatory, antioxidant, and vasculoprotective drugs intended to restore BBB integrity, preserving neurovascular coupling, and promoting the removal of amyloid-β. This review synthesizes preclinical and clinical evidence elucidating mechanisms by which capillary-level perturbations lead to cognitive decline, while discussing therapeutic interventions aimed at restoring NVU integrity and preserving BBB architecture. The delineation of cerebral involvement in AD provides support for a potential for timely diagnosis and innovative approaches for treatment.