The SNARE Protein Syntaxin-1a Plays an Essential Role in Biphasic Exocytosis of the Incretin Hormone Glucagon-Like Peptide 1

Abstract

Exocytosis of the hormone glucagon-like peptide 1 (GLP-1) by the intestinal L cell is essential for the incretin effect after nutrient ingestion and is critical for the actions of dipeptidyl peptidase 4 inhibitors that enhance GLP-1 levels in patients with type 2 diabetes. Two-photon microscopy revealed that exocytosis of GLP-1 is biphasic, with a first peak at 1-6 min and a second peak at 7-12 min after stimulation with forskolin. Approximately 75% of the exocytotic events were represented by compound granule fusion, and the remainder were accounted for by full fusion of single granules under basal and stimulated conditions. The core SNARE protein syntaxin-1a (syn1a) was expressed by murine ileal L cells. At the single L-cell level, first-phase forskolin-induced exocytosis was reduced to basal ( < 0.05) and second-phase exocytosis abolished ( < 0.05) by syn1a knockout. L cells from intestinal-epithelial syn1a-deficient mice demonstrated a 63% reduction in forskolin-induced GLP-1 release in vitro ( < 0.001) and a 23% reduction in oral glucose-stimulated GLP-1 secretion ( < 0.05) in association with impairments in glucose-stimulated insulin release (by 60%; < 0.01) and glucose tolerance (by 20%; < 0.01). The findings identify an exquisite mechanism of metered secretory output that precisely regulates release of the incretin hormone GLP-1 and hence insulin secretion after a meal.

Suppression of circadian secretion of glucagon‐like peptide‐1 by the saturated fatty acid, palmitate

“…To probe for a causative link between Bmal1 expression and GLP‐1 secretion, we generated AMIC cultures, which are enriched in GLP‐1 secreting L‐cells, from Bmal1 wild‐type (WT) and matched C57Bl/6J mice (control) as well as Bmal1 knockout (KO) mice . The known secretagogues, forskolin in combination with 3‐isobutyl‐1‐methylxanthine (F/I), increased GLP‐1 secretion by 0.95‐fold ( P < .001) in control cultures (Figure A). Although no difference in basal GLP‐1 release was observed in KO cultures, secretagogue‐induced secretion was reduced by 0.84‐fold ( P < .001) as compared to control (Figure A).…”

“…However, mitochondrial function, and more specifically, the respiratory chain, has been directly linked to the circadian release of insulin and glucagon by the pancreatic β and α cells, respectively . Finally, ATP may be required for other, known downstream mediators of GLP‐1 secretion, including the SNARE exocytotic machinery, which has also been implicated in the regulation of circadian insulin and glucagon release . Notwithstanding, these findings provide novel insight into the role of the mitochondria in L‐cell function, demonstrating that impairment of mitochondrial function by the saturated fatty acid palmitate results in abrogated GLP‐1 secretion.…”

“…AMIC cultures were generated from the distal 10 cm of the small intestine from age‐matched (7‐ to 11‐week‐old) male and female Bmal1 WT and KO littermates and C57Bl/6 controls, as previously described . In brief, intestinal tissue was dissected for enzymatic digestion to isolate crypts which were plated for 24 hours on Matrigel‐coated 24‐well plates, in quadruplicate wells to generate n = 1 per mouse; 24 hours later (as these cells do not survive long‐term in culture), a 2‐hour secretion assay was conducted using the known secretagogues forskolin in combination with 3‐isobutyl‐1‐methylxanthine (F/I) at concentrations of 50 μmol/L. Active GLP‐1 levels in the media and cells were analysed by ELISA (Millipore, Etobicoke, ON, Canada).…”

Internalization-Dependent Free Fatty Acid Receptor 2 Signaling Is Essential for Propionate-Induced Anorectic Gut Hormone Release

“…More recently, propionate-induced GLP-1 release was also found to be regulated by p38 in chicken intestinal epithelial cells ( Zhang et al., 2019 ), suggesting a conserved role of this kinase in anorectic gut hormone secretion induced by distinct metabolites. How p38 may regulate gut hormone secretion is unknown, although this kinase pathway may phosphorylate components of the SNARE complex such as syntaxin1a, shown recently to be essential for GLP-1 secretion in intestinal L-cells ( Wheeler et al., 2017 ; Campbell et al., 2020 ).…”

Endosomal and Phagosomal SNAREs

“…Stx1A also has secretory roles outside the neurological system, for instance in secretion of the hormone glucagon-like peptide 1 from intestinal enteroendocrine L cells (327). Both Stx1 isoforms have the hallmark topology of the syntaxin subfamily of SNAREs and contain a short NH 2 -terminal regulatory peptide, followed by a regulatory Habcdomain connected to the SNARE motif via a linker and a COOH-terminal transmembrane helix (FIGURE 1).…”