Microglia and Network Redundancy Loss in Early Alzheimer's Disease

Although amyloid-beta plaques and tau tangles are typically used to characterize AD, synapse loss is commonly regarded as the final common pathway leading to cognitive decline. Cognitive symptoms can emerge as synapses and circuits deteriorate, even before significant neuronal loss occurs.

Peer-reviewed studies on synapse loss, microglial phagocytosis, complement signaling, APOE ε4, TREM2, astrocyte-microglia interactions, spatial transcriptomics, and network vulnerability in AD were synthesized through a literature review. Evidence from experimental models, human tissue, cell studies, biomarkers, imaging, and spatial methods was evaluated.

The evidence supports a context-dependent view of glial activity, in which microglia and astrocytes normally protect neural tissue by clearing debris and regulating inflammation, but these same functions can contribute to synaptic vulnerability under AD conditions. Several other mechanisms such as complement signaling, APOE ε4, and TREM2 also contribute to this shift.

Together, these findings suggest that synapse loss contributes to circuit impairment by reducing network redundancy. Synapse loss becomes clinically important when affected connections reduce the ability of memory circuits to use alternative pathways. Therefore, future studies should combine glial-state markers, synaptic biomarkers, spatial mapping, and brain-network measures to test this framework and guide treatments aimed at protecting memory function.