TRUNCATED UBAP1 DISRUPTS ENDO-LYSOSOMAL DYNAMICS AND MITOCHONDRIAL FUNCTIONS IN DEEP-LAYER CORTICAL NEURONS

Hereditary spastic paraplegia (HSP) is a group of inheritable neurodegenerative disorders characterized by lower-extremity weakness and spasticity. This manifestation arises from the progressive degeneration of the longest axons in the corticospinal tract - specifically those of upper motor neurons (UMNs) originating from the deep layer of the primary motor cortex. A nonsyndromic HSP subtype, SPG80, is caused by truncating mutations in the UBAP1 (ubiquitin-associated protein 1) gene. Our research team previously established preclinical mouse and cellular models of SPG80 that faithfully recapitulated patients' phenotypes and demonstrated that the mutant UBAP1 protein exerts its pathogenic effects through a dominant-negative mechanism. Nevertheless, the precise pathogenic cascade driving neurodegeneration in SPG80 remains elusive. In the present study, we aim to investigate the cellular consequences of UBAP1 truncation in deep-layer cortical neurons derived from our established SPG80 mouse and cellular models. We observed that neurons expressing truncated UBAP1 exhibited aberrant endo-lysosomal communication, which was characterized by accumulated endosomes and deficient lysosomes. Concomitantly, these neurons exhibited mitochondrial dysfunctions, manifested as impaired motility and functional defects. Notably, these cellular pathological changes were effectively rescued by a mutant-specific gene-silencing therapy we developed. Additionally, pharmacological treatment targeting endosomal dysfunctions also showed potentials for phenotypic amelioration. Overall, our results demonstrate that UBAP1 truncation disrupts the crosstalk between the endo-lysosomal system and mitochondria. This not only provides a mechanistic basis for UBAP1-associated neurodegeneration, but also identifies promising therapeutic targets for SPG80 and potentially a broader spectrum of upper motor neuron disorders.