A novel SRPK signalling pathway to microtubules is implicated in neurological disorders

mMAP1S S786 was identified as a potential noncanonical substrate of SRPK through a global phosphoproteomic screen conducted in our lab. Given the separate associations of dysregulated SRPK phosphorylation and aberrant MAP1S function in neurological disorders, we investigated the possibility of SRPK-MAP1S signalling in neuronal development. In Chapter 3, I identified multiple SRPK phosphorylation sites on MAP1S, both in vitro and in vivo, using Mass Spectrometry (MS), aligned with known SRPK phosphorylation motifs. Identified sites cluster around the microtubule-binding domain and a Calpain-mediated proteolytic site, suggesting a regulatory role of SRPK phosphorylation in MAP1S processing and microtubule interactions. In Chapter 4, studies using Map1s-/- mESCs models indicated that MAP1S deficiency slightly reduced cell proliferation and impaired differentiation into mature neurons, though it did not impact cell morphology or pluripotency. MAP1S did not associate with microtubules in mESCs but colocalized with them in the neurites and cell bodies of mESC-derived neurons, implying a specialized role in microtubule stability in neural cells. In Chapter 5, I demonstrated that SRPK phosphorylation facilitates Calpain-mediated proteolysis of MAP1S into heavy and light chains with cleaved light chains showing increased SRPK phosphorylation at S786. MAP1S proteolysis is upregulated during neuronal differentiation from human iPSCs, suggesting that proteolysis may play a regulatory role in MAP1S function during neuronal development. SRPK phosphorylation was also found to negatively regulate MAP1S-microtubule association in vitro. These data indicate that MAP1S processing is essential for its roles in neural development, such as its association with microtubules in neurons. SRPK phosphorylation plays a key role in modulating both the proteolytic processing and microtubule binding of MAP1S. These findings underscore the importance of SRPK in regulating the roles of MAP1S during neuronal development and suggest that dysregulation of this pathway may contribute to the pathogenesis of neurological disorders.