Mitochondria-Endoplasmic Reticulum Contact Sites (MERCS/MAM): Multi-Layered Upstream Regulators of Microglial...

Alzheimer's disease (AD) is an age-dependent neurodegenerative syndrome in which microglial senescence bridges biological ageing and Aβ-tau pathology. Senescent microglia undergo permanent cell-cycle arrest, upregulate p16^INK4a and SA-β-gal, and secrete a pro-inflammatory senescence-associated secretory phenotype (SASP) that sustains chronic neuroinflammation. Mitochondria-endoplasmic reticulum contact sites (MERCS) are dynamic physical junctions between the endoplasmic reticulum (ER) and mitochondria, whereas mitochondria-associated membranes (MAM) are biochemically enriched ER subdomains located at MERCS. MERCS coordinate calcium shuttling, mitochondrial dynamics, lipid trafficking and the unfolded protein response (UPR); however, whether their dysfunction drives microglial senescence in vivo remains largely untested.

This review critically integrates five MERCS-linked signalling axes that are hypothesised to drive microglial senescence, namely calcium overload, mitochondrial fission-fusion imbalance, inflammatory amplification, lipid dysregulation and unresolved ER stress. We apply a standardized four-tier evidence-grading framework to stratify causal evidence, systematically distinguish dystrophic, disease-associated (DAM) and bona-fide senescent microglia on the basis of transcriptomic and proteomic data from AD models and human tissues, and address underexplored dimensions including APOE/TREM2 crosstalk, mitophagy, epigenetic regulation, peripheral inflammation and senolytic combinations. We stress that inflammatory activation, oxidative stress and mitochondrial dysfunction represent common cellular stress responses that cannot independently define bona-fide microglial senescence. Because more than 90% of current mechanistic evidence is derived from non-microglial models, MERCS dysfunction in microglia remains a compelling hypothesis that requires rigorous in-vivo validation. Notably, MERCS dysfunction and AD-relevant APOE/TREM2 signalling engage in mutually modulatory crosstalk rather than a simple linear upstream-downstream hierarchy.

Translational development faces several barriers, including poor blood-brain barrier (BBB) penetration, off-target neurotoxicity, uncharacterised long-term safety and limited generalisability from APP/PS1 models lacking tauopathy. All MERCS-targeted and senolytic combinatorial strategies discussed herein represent prospective pre-clinical research directions rather than mature clinical therapeutic approaches. We propose an experimental roadmap based on conditional knockouts, intravital imaging and bidirectional interventions. Overall, MERCS dysfunction is proposed as a candidate multimodal upstream hub for calcium, mitochondrial, inflammatory, lipid and ER-stress signalling, and rigorous in-vivo validation together with microglia-selective delivery platforms is essential for therapeutic translation.