Elaboration of Cortical Pyramidal Dendrites Requires Dephosphorylation of Signal Regulatory Protein α by Protein Tyrosine Phosphatase δ.

Protein tyrosine phosphatase δ (PTPδ) is involved in Sema3A-induced dendritic elaboration of cortical pyramidal neurons through the activation of Fyn tyrosine kinase. However, the in vivo substrates of PTPδ remain largely unknown. Phosphotyrosine proteome analysis of Ptpδ -/- mice brains from both male and female revealed that signal regulatory protein α (SIRPα) was hyperphosphorylated at the carboxy-terminal Tyr501 residue in the knock-outs. Immunohistochemistry with anti-phospho-Tyr501 SIRPα antibody showed the hyperphosphorylation of SIRPα in various regions including the olfactory nerve layer, cortex, striatum, thalamic reticular, and hypothalamic nuclei in Ptpδ-/- brain sections. These regions partially correlated with PTPδ expression. In the primary cultured wild-type (wt) mouse dorsal root ganglion neurons, Sirpα knockdown or overexpression of a cytoplasmic deleted SIRPα mutant partially blocked the Sema3A-induced growth cone collapse response. However, overexpression of nonphosphorylated mutants of SIRPα did not alter the response. This suggests that SIRPα is involved in the Sema3A-induced collapse response in an independent manner of phosphorylation and/or dephosphorylation. In the primary cultured cortical neurons, Sirpα knockdown or the overexpression of SIRPα-Tyr501Phe mutant attenuated Sema3A-induced dendritic growth. In cultured Sirpα -/- cortical neurons, re-expression of wt SIRPα, but not of SIRPα-Tyr501Phe, restored Sema3A-induced dendritic formation. In vivo analyses revealed ectopic expression of SIRPα-Tyr501Phe in cortical layer II/III pyramidal neurons with misoriented apical dendrites and attenuated basal dendrite arborization. Similar irregular cortical dendrites were observed in Sirpα -/- and Ptpδ -/- brains. Collectively, our results demonstrate that PTPδ regulates the dendritic elaboration of cortical pyramidal neurons through the dephosphorylation of SIRPα.