Signaling Mechanisms Underlying GPM6a–NPTN-Mediated Neurite Inhibition

Neuronal morphogenesis requires coordinated regulation of neurite development and cell-surface signaling. Glycoprotein M6a (GPM6a) and Neuroplastin (NPTN) are neuronal membrane glycoproteins that independently promote neurite outgrowth through Src/MAPK and MAPK/p38 signaling, respectively. Increased p38 and/ or MAPK activity induces the dual-specificity phosphatase 1 (MKP-1), a negative regulator of MAPK signaling. Although GPM6a and NPTN have been implicated in the inhibition of neurite outgrowth, the underlying mechanisms remain unclear. Here, we investigated the extracellular and intracellular mechanisms underlying this effect. Disruption of the GPM6a–NPTN interaction with a peptide targeting the NPTN65-specific Ig1 domain, but not the Ig2–3 domains, restored neurite outgrowth, identifying Ig1 as a critical determinant of the inhibitory phenotype. Pharmacological inhibition of Src, p38 MAPK, or MKP-1 also rescued neurite outgrowth in GPM6a–NPTN-expressing cells, indicating that these pathways contribute to the inhibitory effect. Together, our findings show that the GPM6a–NPTN complex inhibits neurite outgrowth through an extracellular interaction coupled to Src-, p38 MAPK-, and MKP-1-dependent signaling, revealing a mechanism by which neuronal membrane proteins integrate extracellular interactions with intracellular signaling to regulate neuronal morphogenesis.