Regulation of Somatostatin Receptor 2 Trafficking by C-Tail Motifs and the Retromer

Abstract

The G _i-coupled somatostatin receptor 2 (SST ₂) is a G protein–coupled receptor (GPCR) that mediates many of somatostatin’s neuroendocrine actions. Upon stimulation, SST ₂ is rapidly internalized and transported to early endosomes before being recycled to the plasma membrane. However, little is known about the intracellular itinerary of SST ₂ after it moves to the early endosomal compartment or the cytoplasmic proteins that regulate its trafficking. As postsynaptic density protein/discs large 1/zonula occludens-1 (PDZ) domain interactions often regulate the trafficking and signaling potential of GPCRs, we examined the role of the SST ₂ PDZ ligand and additional C-terminal residues in controlling its intracellular trafficking. We determined that SST ₂ can recycle to the plasma membrane via multiple pathways, including a LAMP1/Rab7-positive late endosome to the trans-Golgi network (TGN) pathway. Trafficking from the late endosome to the TGN is often regulated by the retromer complex of endosomal coat proteins, and disrupting the retromer components sorting nexins 1/2 inhibits the budding of SST ₂ from late endosomes. Moreover, trafficking through the late endosomal/TGN pathway is dependent on an intact PDZ ligand and C-terminal tail, as truncating either the 3 or 10 C-terminal amino acids of SST ₂ alters the pathway through which it recycles to the plasma membrane. Moreover, addition of these amino acids to a heterologous receptor is sufficient to redirect it from a degradation pathway to a recycling itinerary. Our results demonstrate that endosomal trafficking of SST ₂ is dependent on numerous regulatory mechanisms controlled by its C terminus and the retromer machinery.