Beta cell primary cilia mediate somatostatin responsiveness via SSTR3

Abstract

Somatostatin is a paracrine modulator of insulin secretion and beta cell function with pleotropic effects on glucose homeostasis. The mechanism of somatostatin-mediated communication between delta and beta cells is not well-understood, which we address in this study via the ciliary somatostatin receptor 3 (SSTR3). Primary cilia are membrane organelles that act as signaling hubs in islets by virtue of their subcellular location and enrichment in signaling proteins such as G-protein coupled receptors (GPCRs). We show that SSTR3, a ciliary GPCR, mediates somatostatin suppression of insulin secretion in mouse islets. Quantitative analysis of calcium flux using a mouse model of genetically encoded beta cell-specific GCaMP6f calcium reporter shows that somatostatin signaling alters beta cell calcium flux after physiologic glucose stimulation, an effect that depends on endogenous SSTR3 expression and the presence of intact primary cilia on beta cells. Comparative in vitro studies using SSTR isoform antagonists demonstrate a role for SSTR3 in mediating somatostatin regulation of insulin secretion in mouse islets. Our findings support a model in which ciliary SSTR3 mediates a distinct pathway of delta-to-beta cell regulatory crosstalk and may serve as a target for paracrine modulation.

Structure, interaction, and nervous connectivity of beta cell primary cilia

“…Beta cell cilia have been shown to be involved in GABA signaling (54). Furthermore, they express the insulin (31), somatostatin (55; 56), and various G protein-coupled receptors (46). If some of these pathways are especially prominent at cilia-islet cell connections remains to be investigated.…”

Keeping pace: the primary cilium as the conducting baton of the islet

“…2 ). Using beta cells lacking cilia or with a defective cilia receptor trafficking machinery, it has been shown that some, if not all, of the effects of both SSTR3 and FFAR4 on insulin secretion modulation are exerted via cilia-localised receptors [ 4 , 6 – 8 , 17 ]. Many receptors or receptor families, for example SSTRs, are also present on the plasma membrane, suggesting that some ligands may trigger parallel canonical and ciliary pathways, which may increase the information content carried by a given ligand.…”

“…The primary cilium is also a unique cAMP compartment that contains both adenylate cyclases (ACs) and phosphodiesterases (PDEs) as well as PKA, and several of these are directly controlled by Ca 2+ /calmodulin, which might enable local crosstalk between the Ca 2+ and cAMP pathways [ 31 ]. Beta cell primary cilia contain both AC3 and AC5/6, and activation of GPR120 can stimulate ciliary cAMP formation [ 4 , 8 , 17 ]. Given the presence of both G s - and G i -coupled receptors on the beta cell cilia (Fig.…”

“…While glucagon retained its ability to enhance insulin secretion from βCKO islets, somatostatin lost the ability to suppress secretion, indicating that beta cell cilia are direct targets of somatostatin action. Consistently, short hairpin RNA (shRNA)-mediated loss of SSTR3, the major ciliary somatostatin receptor, renders somatostatin incapable of modulating cytosolic Ca 2+ signalling and inhibiting insulin secretion [ 17 ]. It has also been shown that insulin receptors dynamically localise to beta cell cilia, and that trafficking to the cilium is important for downstream Rac-alpha serine/threonine-protein kinase (Akt) phosphorylation [ 6 ].…”

“…Recent studies also indicate that the classical hedgehog response can be tuned by the activation of other ciliary receptors, with G i -coupled receptor activation enhancing the response and G s -coupled receptor activation antagonising it [ 46 ]. It therefore seems likely that islet-derived factors, for example somatostatin and GABA [ 4 , 17 ], will crosstalk with the Hh pathway to maintain islet cell identity and functionality, and that disturbances in this regulation, for example changes in cilia structure or composition, will lead to loss of this paracrine control.…”