Basic Science and Pathogenesis

Background: Activation of the mTOR pathway is pivotal for microglia to induce and sustain neuroprotective functions (Ulland et al., 2017; Wang et al., 2022). mTOR complex 1 (mTORC1) inhibits the translation repressors, eukaryotic translation Initiation Factor 4E (eIF4E)-Binding Proteins (4E-BPs), via phosphorylation, which causes their release from eIF4E to promote mRNA translation (Hay and Sonenberg, 2004). mTORC1 promotes mitochondrial biogenesis via inhibition of 4E-BPs, by preferentially stimulating the translation of mitochondria-related mRNAs (Gandin et al., 2016; Morita et al., 2013). We investigated the mechanisms at the intersection of 4E-BP-dependent translational regulation and metabolism in microglial response to soluble Aβ.

Method: We carried out immunoblot analysis to investigate the phosphorylation status of 4E-BP1, the isoform most abundant in microglia, following exposure to Ab. We manipulated the mTOR pathway by knocking out the downstream effectors, 4E-BPs, to alleviate translation suppression in microglia in vitro and in vivo. We crossed the microglia-specific 4E-BPs knockout mouse with a RiboTag mouse to pull-down ribosome-bound mRNAs, providing a genome-wide pool of actively translating mRNAs in the absence or presence of 4E-BPs. Finally, we examined the relationship between 4E-BP1 levels and neuroinflammation markers in cerebrospinal fluid (CSF) of AD patients.

Result: We showed that 4E-BP1 is inhibited acutely upon exposure to soluble Ab, which is dependent on Spleen Tyrosine Kinase (SYK) activation upstream of mTORC1, but is reduced upon chronic exposure. Furthermore, 4E-BP1 expression is induced during prolonged exposure to Ab. The deletion of 4E-BPs in microglia in vitro leads to an increase in mitochondrial mass and reliance on oxidative phosphorylation while decreasing expression of pro-inflammatory mediators and cell death upon exposure to Ab. We observed that increased levels of 4E-BP1 in the CSF of patients with Aβ pathology are associated with higher neurodegeneration (Nfl) in the presence of microglial activation.

Conclusion: We demonstrate that mTORC1 signaling critically impacts microglia physiology and promotes neuroprotective functions via 4E-BP1 inhibition. 4E-BP1 activity in microglia engenders a dysfunctional or detrimental state that may lead to increased neurodegeneration. Therefore, 4E-BP1 is an attractive target for microglia modulation in AD.