Phosphoproteomic profiling reveals post-translational dysregulation in Huntington's disease patient-derived neurons

Huntington's disease (HD) is a fatal neurodegenerative disorder caused by a CAG repeat expansion in the Huntingtin gene. Although transcriptomic and proteomic changes have been characterized in patient-derived neurons, the contribution of post-translational modifications, such as phosphorylation, remains poorly understood. Here, we present the first phosphoproteomic analysis by mass spectrometry (P-MS) of human induced neurons (iNs) directly reprogrammed from HD patient fibroblasts. We identified 177 phosphopeptides with significantly altered abundance in HD-iNs, mapping to phosphoproteins associated with key signaling pathways known to be affected in HD, such as splicing and autophagy. By integrating P-MS data with previously published proteomic and transcriptomic data from the same donors, we identified distinct subsets of ON-OFF phosphopeptides that exhibited a complete loss of phosphorylation in either HD- or control-iNs, without corresponding changes at the RNA or protein level. An exception was MXRA8, previously described in glial cells as a mediator of blood-brain barrier integrity and astrocyte-mediated neuroinflammation. This protein showed increased protein abundance despite the absence of phosphorylation in HD-iNs, suggesting a compensatory mechanism. In addition, MXRA8 showed altered protein-protein interactions with lysosomal and metabolic regulators in HD-iNs, highlighting its potential role in autophagy impairment as well as in neurovascular dysfunction. These findings uncover a distinct layer of post-translational dysregulation in HD, suggesting that phospho-switch proteins such as MXRA8 may be candidate effectors of pathology, and thus, site-specific phosphorylation loss may contribute to impaired signaling and proteostasis in human HD neurons.