Understanding the role of BORCS5 in neuronal lysosomal function and neurodegeneration - Project summary A central role for lysosomal dysfunction has been highlighted for neurodegenerative disease mechanisms across multiple genetic forms of movement disorders linked to degeneration of dopaminergic (DA) neurons, including Parkinson disease (PD) and dystonia. We recently identified pathogenic mutations in BORCS5 causing a form of early-onset neurodegeneration with severe parkinsonism and dystonia. BORCS5 encodes a subunit of the BLOC1-related complex (BORC), which is currently known to be involved in anterograde movement of lysosomes. However, whether BORCS5 mutations contribute to neurodegeneration through disruptions of lysosomal trafficking or other mechanisms is not known. Physiologically, most functionally mature lysosomes are concentrated in the neuronal body and whether the presence of active lysosomes is required for normal neuronal homeostasis in distal axons is debated. Surprisingly, while BORCS5 complete knock-out (KO) caused the expected reduction in centrifugal lysosomal movements, BORCS5-patients’ cells carrying the disease– causing mutation R95Q showed impaired lysosomal maturation and defects in autophagic flux, but not the anticipated abnormalities in lysosomal cellular distribution. These findings suggest that BORCS5 is involved in additional essential aspects of lysosomal biology and I hypothesize that dysfunction of these novel functions of BORCS5 is central for disease pathogenesis in patients with BORCS5 mutations. Thus, I propose to investigate the molecular mechanisms underlying BORCS5-related neurodegeneration using long-term cultures of human iPSC-derived DA neurons on micropatterned substrates to facilitate the study of lysosomal dynamics through high spatial and temporal resolution live cell microscopy, coupled with lysosomal function assays and assessment of human specific DA neuron pathological phenotypes, which we have recently identified as an important contributor to progressive lysosomal dysfunction and degeneration in human DA neurons. In aim 1, I will investigate which aspects of BORC5 biology that are implicated in lysosomal function and autophagy are disrupted by disease-causing mutations in DA neurons. In aim 2, I will explore how these disrupted mechanisms contribute to downstream pathogenic events implicated in DA neuron degeneration and explore the therapeutic potential of BORCS5 genetic manipulation in neurons from patients with genetic forms of PD and dystonia featuring lysosomal dysfunction. Characterizing additional rare genetic causes of lysosomal dysfunction is crucial for recognizing clinically relevant modifiers of lysosomal activity. Ultimately, this project will help understand the complex interplay between lysosomal positioning and lysosomal activity in human DA neurons and potentially identify novel therapeutic angles for targeting lysosomal dysfunction in neurodegeneration. Furthermore, this proposed research and career development plan will provide me with essential new training in experimental techniques that will facilitate my transition to independent researcher, investigating the genetics and molecular mechanisms of lysosomal dysfunction in degenerative movement disorders, including dystonia and PD.
