Identification of two novel alternatively spliced Neuropilin-1 isoforms

Neuropilin-1 (NRP1) is a coreceptor to a tyrosine kinase receptor for both the vascular endothelial growth factor (VEGF) family and semaphorin (Sema) family members. NRP1 plays versatile roles in angiogenesis, axon guidance, cell survival, migration, and invasion. NRP1 contains three distinct extracellular domains, a1a2, b1b2, and c. We report here the identification of two novel soluble human NRP1 isoforms, which we named s_IIINRP1 and s _IVNRP1. These soluble NRP1 isoforms were generated by alternative splicing of the NRP1 gene, a common regulatory mechanism occurring in cell surface receptor families. Both s_IIINRP1 and s_IVNRP1 contain a1a2 and b1b2 domains, but no c domain, and the rest of the NRP1 sequence. Additionally, s_IIINRP1 is missing 48 amino acids within the C-terminus of the b2 domain. Both s_IIINRP1 and s_IVNRP1 are expressed in human cancerous and normal tissues. These molecules are capable of binding to VEGF₁₆₅ and Sema3A. Furthermore, recombinant s _IIINRP1 and s_IVNRP1 proteins inhibit NRP1-mediated MDA-MB-231 breast cancer cell migration. These results indicate the multiple levels of regulation in NRP1 function and suggest that these two novel NRP1 isoforms are useful antagonists for NRP1-mediated cellular activities.