Alternative splicing of neurexins: a role for neuronal polypyrimidine tract binding protein.

Neurexins are polymorphic synaptic membrane proteins generated by alternative splicing of transcripts from six promoters in three genes (two promoters in each gene) at five canonical sites. Neurexins and factors regulating their alternative splicing may orchestrate coordinated presynaptic and postsynaptic development. Bioinformatic analysis revealed several sequence elements that could be involved in the regulation of alternative splicing at splice site 4 (SS#4); among them elements suspected as intronic binding sites of polypyrimidine tract binding protein (PTB), and its neuron specific analog nPTB. The role of nPTB in neurexin-2alpha and beta SS#4 splicing were studied using small interfering RNA (siRNA) to silence nPTB expression. Transformed rat retinal ganglion (RGC-5) cells expressed nPTB, PTB and neurexin-2alpha (mostly exon 20 excluded form) but not neurexin-2beta. Mouse spinal-cord glioma hybrid (NSC-34) cells expressed nPTB, PTB and both neurexin-2alpha and beta (mostly exon 20 included form). nPTB-siRNA inhibited nPTB but not PTB expression and significantly enhanced neurexin-2alpha SS# 4 exon-excluded transcript in both cell types. Neurexin-2beta SS#4 splicing was not affected by nPTB-siRNA. These results show a role for nPTB in neurexin-2alpha alternative splicing. The differential enhancement of SS# 4 exon exclusion in neurexin-2alpha suggests that the effects of nPTB are mediated via additional site(s) present in neurexin-2alpha but not in the shorter, beta form.