Timothy Huang, Ph.D.

There are two projects that comprise my main focus: 1) SORLA: Sortilin-related receptor with LDLR class A repeats (SOR​LA, SORL1, or LR11) is a genetic risk factor associated with Alzheimer’s disease (AD). Although SOR​LA is known to regulate trafficking of the amyloid β (Aβ) precursor protein to decrease levels of proteotoxic Aβ oligomers, whether SOR​LA can counteract synaptic dysfunction induced by Aβ oligomers remains unclear. Our work indicates that SOR​LA interacts with the EphA4 receptor tyrosine kinase and attenuates ephrinA1 ligand–induced EphA4 clustering and activation to limit downstream effects of EphA4 signaling in neurons. Consistent with these findings, SOR​LA transgenic mice, compared with WT mice, exhibit decreased EphA4 activation and redistribution to postsynaptic densities, with milder deficits in long-term potentiation and memory induced by Aβ oligomers. Importantly, we detected elevated levels of active EphA4 in human AD brains, where EphA4 activation is inversely correlated with SORLA/EphA4 association. These results demonstrate a novel role for SOR​LA as a physiological and pathological EphA4 modulator, which attenuates synaptotoxic EphA4 activation and cognitive impairment associated with Aβ-induced neurodegeneration in AD. 2) TREM2: Although ligands for TREM2 such as ApoE have been previously identified, a definitive mechanism for TREM2 in AD has not been established. We recently determined that TREM2 directly binds Aβ oligomers with nanomolar affinity and R47H mutations attenuate TREM2/Aβ interaction. TREM2 deletion impairs Aβ turnover in primary microglia, and abrogates Aβ clearance in vivo. Aβ also triggers changes in microglial membrane potential which is impaired with TREM2 deletion. Moreover, TREM2 deletion attenuates Aβ-induced microglial morphogenic changes associated with activation, and inhibits Aβ-mediated induction of proinflammatory cytokine expression. Together, these results indicate that TREM2 may have opposing neuroprotective roles in mediating microglial Aβ clearance and turnover, while concurrently transducing potentially neurotoxic Aβ-induced inflammatory signals. To further define a role for TREM2 in Aβ clearance and Aβ-mediated microglial activation/cytokine expression, we plan to exploit use of TREM2 R47H knock-in and TREM2 WT and R47H overexpression mouse models currently housed in our laboratory to determine whether impaired TREM2/Aβ interactions can impair microglial response in the presence of Aβ. This provides essential groundwork in future strategies to optimize neuroprotective TREM2 Aβ clearance while limiting Aβ-induced microglial inflammation.