Elucidating the role of Adaptor Protein complex-4 in regulating axonal autophagic and lysosomal pathways

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The autophagic and lysosomal pathways (ALP) clear misfolded proteins and damaged organelles from cells. Their function is therefore particularly critical for long-lived cells such as neurons. Dysfunction in the ALP is associated with various stages of Alzheimer’s disease (AD). This includes the robust accumulation of autophagosomes and lysosome-like organelles in dystrophic axons around extracellular Aβ deposits (Amyloid plaques), which are hallmark pathological features observed in human Alzheimer’s disease brain tissue and recapitulated in transgenic mouse models of Alzheimer’s disease. Amyloid plaque formation has been directly linked to aberrant/amyloidogenic proteolytic processing of amyloid precursor protein (APP) by secretases. However, whether abnormal trafficking and accumulation of these organelles bearing these protein cargoes trigger this critical pathogenic event in Alzheimer’s disease, has not been experimentally addressed. In addition, information on mechanisms and specific molecular components regulating ALP in axons remains limited. Elucidating these mechanisms and identifying molecular components might enable therapeutic modulation of neuronal ALP to reduce amyloid plaque burden and toxic Aβ peptide production in Alzheimer’s disease. To this end, our proposed research seeks to understand a) how the adaptor complex, AP-4, regulates axonal autophagosome and lysosome biogenesis, maturation, and transport; b) how loss of AP-4 contributes to amyloid plaque formation in vivo as well as potentially identify new AP-4 cargo that facilitate optimal retrograde axonal lysosome transport, APOE metabolism, and synaptic activity. Central to these proposed studies is our preliminary data that AP-4 loss causes abnormal accumulation of ALP organelles in axonal swellings reminiscent of AD pathology, including build-up of APP cleaving proteins BACE1 and PSEN2. This and our preliminary data demonstrating reduced AP-4 levels in AD mouse brain and an age-dependent loss of AP-4 in the pre-frontal cortex of even wild type mice lead us to hypothesize that AP-4-dependent axonal autophagosome and lysosome maturation and transport protects neurons from amyloidogenic APP processing and thus, from amyloid plaque development. Additionally, our use of super-resolution microscopy on ALP organelles in axonal swellings as well as proteomics on isolated axons and organelles upon loss of AP-4 complex, will yield novel insight into the kinds of ALP intermediates accumulating under these pathological conditions and their relative contributions to build- up of APP processing machinery. Our proposed efforts to dissect out AP-4 mediated axonal ALP transport and maturation could also lead to new therapeutic opportunities that focus on mobilizing these axonal ALP organelles to limit Aβ production and axonal pathology. Furthermore, new insights into cell biology of neuronal autophagic and lysosomal pathways revealed by our studies could have broader relevance to other neurodegenerative diseases that have a lysosome component to their pathology such as Parkinson’s disease, Hereditary Spastic Paraplegia, and fronto-temporal dementia.