Targeting the C5-C5aR1 axis: A promising therapeutic strategy for Alzheimer's disease and amyotrophic lateral sclerosis by unlocking neuroprotection.

C5aR1 is a G protein-coupled receptor (GPCR) which is involved in exacerbating neurodegenerative diseases, including Alzheimer's disease (AD) and Amyotrophic Lateral Sclerosis (ALS). This review highlights the critical role of the C5-C5aR1 axis, in the pathogenesis of neurodegenerative diseases such as AD and ALS. In AD and ALS, abnormal protein aggregates activate the complement system (CS), leading to increased production of C5a. C5a activates C5aR1 on microglia, triggering the release of pro-inflammatory cytokines (IL-1β, TNF-α, IL-6) that induce synaptic loss. Concurrently, the C5-C5aR1 axis impairs microglial phagocytic capacity, promoting damage-associated molecular patterns (DAMPs) accumulation and forming a vicious cycle of inflammation and complement activation. Additionally, excessive complement molecule assembles into the terminal complement complex (TCC), which exerts direct neurotoxic effects and drives neuronal apoptosis. Preclinical studies show that C5aR1 antagonists, such as PMX205, mitigate disease progression in AD and ALS animal models by reducing neuroinflammation and preserving synaptic function. These findings underscore the C5-C5aR1 axis as a promising target for neurodegenerative disease therapy and highlight the need for further development of potential antagonists of C5aR1.