Calcium-mediated calpain activation and microtubule dissociation in cell model of hereditary sensory neuropathy type-1 expressing V144D SPTLC1 mutation

Hereditary sensory neuropathy type 1A (HSN1A) is an autosomal, dominantly inherited peripheral neuropathy caused by mutations in serine palmitoyl transferase long chain 1 (SPTLC1), involved in the de novo synthesis of sphingolipids. We have previously reported calcium imbalance, as well as mitochondrial and ER stress in bothHSN1 patient lymphoblasts and a transiently transfected cell model. In this study, we investigated the role of theCa2+-activated protease calpain in destabilizing the cell cytoskeleton, by examining calpain activity in SH-SY5Y cells overexpressing the V144D mutant and changes in microtubule-associated proteins (MAP). Intra-mitochondrial Ca2+was found to be significantly depleted and cytoplasmic Ca2+increased in the V144D mutant. Subsequently, calpain and proteasome activity were increased and calpain substrates, microtubule associated proteins MAP2, and tau were significantly reduced in the microtubule fraction of the mutant. Significant changes were also found in motor protein dynein and KIF2A detected in the microtubule fraction of cells overexpressing the V144D mutation. There was also a reduction in anterograde and retrograde mitochondrial transport velocities in the V144D mutant. These findings strongly implicate cytoskeletal aberration caused byCa2+dysregulation and subsequent loss of microtubule transport functions as the cause of axonal dying back that is characteristic of HSN1.