Cargo-specific regulation of microglial Aβ phagocytosis by lipid-responsive transcription factor SREBF2

Alzheimer’s disease (AD) is characterized by pathological accumulation of amyloid‑β (Aβ), reflecting an imbalance between peptide production and clearance in the brain [1,2]. Impaired Aβ degradation is a central contributor to disease progression and neurodegeneration [2]. Microglia, the resident immune cells of the central nervous system, are the primary mediators of Aβ phagocytosis and degradation; however, their clearance capacity is strongly shaped by cellular metabolic state and extracellular nutrient availability [3]. Increasing evidence indicates that metabolic reprogramming alters microglial function, yet it remains unclear whether distinct metabolic perturbations differentially regulate Aβ uptake versus intracellular degradation, or whether such regulation can occur in a cargo‑specific manner [1,3]. Clarifying these mechanisms is essential for the development of therapeutic strategies that selectively enhance Aβ clearance while preserving neuronal integrity. We hypothesized that lipid‑dependent metabolic signaling selectively controls cargo‑specific microglial Aβ phagocytosis through the lipid‑responsive transcription factor sterol regulatory element‑binding factor 2 (SREBF2). To test this hypothesis, human HMC3 microglia were subjected to two metabolically distinct starvation paradigms: lipid starvation via medium delipidation and serum starvation. Fluorescently labeled Aβ uptake and degradation were quantified using cell‑based imaging and fluorescence intensity assays to separately assess internalization and intracellular handling. To define the molecular pathways engaged under these conditions, starvation‑induced transcriptional changes were analyzed by bulk RNA sequencing, followed by pathway enrichment analysis with a focus on lipid metabolism, cholesterol biosynthesis, and lysosomal function. Key transcriptional changes were further validated by targeted gene expression analysis to confirm pathway activation. Mechanistic validation was performed using siRNA‑mediated knockdown of SREBF2 to assess its requirement for starvation‑induced Aβ handling. Cargo specificity was evaluated by comparing Aβ uptake with the phagocytosis of neurosynaptosomes derived from healthy synaptic material. Both lipid and serum starvation induced significant transcriptional remodeling of pathways related to cholesterol biosynthesis, sterol metabolism, and SREBF signaling, indicating broad metabolic adaptation. Although Aβ uptake was significantly enhanced under both starvation conditions, efficient and significant degradation of internalized Aβ occurred exclusively during lipid starvation. In contrast, serum starvation significantly impaired intracellular degradation, resulting in significant accumulation of Aβ within microglia. Importantly, knockdown of SREBF2 significantly abolished the lipid starvation–induced enhancement of Aβ phagocytosis, demonstrating that this effect is critically dependent on SREBF2. SREBF2 manipulation did not result in a significant change in neurosynaptosome uptake, indicating that enhanced phagocytosis was selective for pathological Aβ cargo rather than reflecting a generalized increase in phagocytic activity [4,5]. These findings highlight a statistically robust dissociation between uptake and degradation pathways that is significantly governed by lipid availability. In conclusion, these data identify SREBF2‑dependent cholesterol metabolic signaling as a key determinant of efficient microglial Aβ uptake and degradation. Our findings demonstrate that microglial metabolic state selectively regulates pathological cargo handling while sparing healthy synaptic material. By delineating a lipid‑responsive, cargo‑specific clearance mechanism, this study advances current understanding of microglial functional specialization. This work fills an important gap in understanding how lipid metabolism governs cargo specificity in microglia and highlights SREBF2 signaling as a selective target for enhancing microglial Aβ clearance in Alzheimer’s disease.

References

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