GRIA1 regulates TGN export and secretion of Sonic hedgehog

Sonic hedgehog (Shh) signaling orchestrates diverse developmental processes in metazoans and is implicated in numerous human diseases. While downstream signaling in recipient cells have been extensively characterized, the mechanisms governing secretion of newly synthesized Shh from producer cells remain less well understood. Building on our previous identification of a Surfeit locus protein 4 (SURF4)-to-proteoglycan (PG) relay mechanism that mediates endoplasmic reticulum (ER)-to-Golgi transport of the N-terminal Shh fragment (ShhN), we investigated ShhN export from the trans-Golgi network (TGN). We show that ShhN exits the TGN via a clathrin-dependent secretory pathway. Mechanistic analyses identify the transmembrane protein glutamate receptor 1 (GRIA1) as a key mediator: GRIA1 associates with ShhN in Golgi-derived vesicles, physically interacts with ShhN, colocalizes with ShhN after TGN exit, and is required for efficient TGN export and secretion of ShhN. Notably, the Cardin-Weintraub (CW) motif on ShhN, previously shown to engage SURF4 for ER-to-Golgi trafficking, is also essential for TGN export, and PGs are critical for the GRIA1-ShhN interaction. Furthermore, GRIA1 regulates intracellular trafficking of endogenous full length Shh and modulates Shh pathway activity in Neuro-2a (N2A) cells. Together, these findings identify GRIA1 as an important regulator of Shh TGN export and advance our understanding of the molecular mechanisms that control Shh secretion.