Proteostatic regulatory mechanisms in the endoplasmic reticulum (ER) such as the ER-associated degradation (ERAD) and unfolded protein response (UPR) systems are commonly dysregulated in neurodegenerative proteinopathies such as Alzheimer?s disease (AD). We identify a new role for the ER-resident component membralin in ERAD function. Interestingly, membralin polymorphisms and splice variation have been implicated in AD, and our results indicate that membralin expression is reduced in AD brain. We also identify nicastrin as a membralin/ERAD substrate, and membralin downregulation results in elevated nicastrin levels, thereby enhancing ?-secretase activity, A? generation and cognitive impairment. Interestingly, we also identify ERAD components in complex with the ?-secretase activating protein (GSAP) by proteomic analysis and demonstrate that membralin and GSAP interact by coimmunoprecipitation. Although GSAP is thought to be pathogenic due to its role in enhancing A? generation, we unexpectedly find that GSAP KO mice show impaired fear memory and synaptic function. Moreover, synaptic response and long-term potentiation in with GSAP deletion is further impaired with A?, thereby implicating a protective ?-secretase-independent role for GSAP in synaptic function. Together, our results indicate that membralin/ERAD and GSAP physically and functionally interact, and that these two components likely support both ?-secretase dependent and independent modes of neuroprotection. Membralin splice variation has been previously characterized in AD, however, how this potentially impacts membralin/ERAD function is unclear. We will therefore characterize expression of long and short membralin isoforms in human AD brain and determine whether C-terminal truncation can impact membralin-associated ERAD function in the short isoform. Given that homozygous membralin deletion is lethal, while heterozygous membralin deletion yields no visible phenotype, it seems likely that membralin haploinsufficiency may induce compensatory changes in the ERAD complex, or associated UPR system to normalize proteostatic function. We will characterize changes in ERAD/UPR systems with membralin downregulation in the absence and presence of A? and determine whether enhancing these compensatory changes can enhance ERAD function. GSAP has been previously known to be processed into N- and C-terminal fragments (NTF/CTFs) through caspase-3 cleavage; we will determine whether full-length GSAP or its cleaved forms are involved in GSAP-dependent synaptic function by overexpression/complementation in GSAP KO mouse hippocampus. Since our previous results indicate that membralin depletion can induce caspase-3 activation, we will determine whether modulating membralin levels can affect GSAP-dependent synaptic function and cognitive behavior. Lastly, we will establish an AD/membralin haploinsufficiency model to determine whether enhancing membralin/ERAD, UPR, or GSAP pathways can ameliorate cognitive, synaptic and pathological AD defects.
We identified an ER resident protein, membralin as a functional component of the ER associated degradation (ERAD) machinery, which is downregulated in AD. We also identified ERAD components in the ?-secretase activating protein (GSAP) interactome, demonstrate interactions between membralin and GSAP, characterize novel functions for ERAD and GSAP, and elucidate their interplay in mediating synaptic activity. The focus of this study is to determine how enhancing proteostatic ERAD and synaptic GSAP function can attenuate cognitive and synaptic impairment in AD.
