BACE1 trafficking and degradation in Alzheimer’s disease

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ABSTRACT Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by memory impairments and cognitive deterioration. One of the pathological hallmarks of AD is the presence of neuritic plaques, which consist of a core of aggregate amyloid beta (Aβ) closely associated with dystrophic neurites, activated microglia and reactive astrocytes. β-site amyloid precursor protein cleaving enzyme (BACE1) is the rate- limiting enzyme in the production of Aβ. BACE1 is expressed at high levels in neurons and localizes at the presynaptic terminals. Moreover, BACE1 accumulates in dystrophic presynaptic terminals surrounding Aβ plaques in brains of AD patients and mouse models of AD. However, the specific mechanism(s) of BACE1 accumulation in peri-plaque dystrophic axons remains to be clarified. As changes in BACE1 mRNA levels did not accompany BACE1 protein increases in AD brains in the majority of the studies, post-translation mechanisms are most likely responsible for BACE1 elevation in AD. Our previous studies have shown that BACE1 is degraded via the lysosomal pathway. Furthermore, we found that the clathrin adaptor Golgi-localized γ-ear-containing ARF binding protein 3 (GGA3) regulates BACE1 lysosomal trafficking and degradation. More recently, we have discovered that BACE1 degradation and trafficking is regulated by its ubiquitination. Accordingly, we have identified the endosomal-associated deubiquitinating enzyme USP8 as a negative regulator of BACE1 ubiquitination and degradation. The central hypothesis of this proposal is that GGA3 and USP8 regulate BACE1 axonal trafficking and lysosomal degradation in neurons and their dysfunction results in BACE1 accumulation in pre-synaptic dystrophic neurites observed in AD. In support of this hypothesis, we discovered that levels of GGA3 were decreased and inversely correlated with BACE1 levels in post-mortem AD brains concurrently with caspase-3 activation. We also determined that GGA3 is a caspase 3 substrates and is depleted both in cellular models of apoptosis and in rodent models of stroke and traumatic brain injury. Our new preliminary data demonstrated that GGA3 decreases while BACE1 levels increases with age in a mouse model of AD (5XFAD mice). Furthermore, our recent live-imaging experiments revealed that GGA3 genetic deletion results in the disruption of BACE1 axonal trafficking and its accumulation in swollen axons in cultured hippocampal neurons. Altogether our data indicate that GGA3 depletion is a leading candidate mechanism underlying BACE1 accumulation in pre-synaptic dystrophic neurites in AD. Thus we propose 1) to determine the extent to which AD brain-derived Aβ species and Familial Alzheimer's Disease (FAD-linked) mutations impair axonal trafficking and lysosomal degradation of BACE1 in murine and human iPSC-derived neurons; 2) to determine the extent to which preventing caspase-mediated depletion of GGA3 reduces BACE1 accumulation in peri-plaques dystrophic neurites in vivo; 3) to determine the extent to which depletion of USP8 reduces BACE1 accumulation in dystrophic neuritis and ameliorates Aβ pathology and cognitive deficits in vivo.