title: Munc18-1 and Munc18-2 proteins modulate beta-cell Ca2+ sensitivity and kinetics of insulin exocytosis differently.
abstract: Fast neurotransmission and slower hormone release share the same core fusion machinery consisting of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins. In evoked neurotransmission, interactions between SNAREs and the Munc18-1 protein, a member of the Sec1/Munc18 (SM) protein family, are essential for exocytosis, whereas other SM proteins are dispensable. To address if the exclusivity of Munc18-1 demonstrated in neuroexocytosis also applied to fast insulin secretion, we characterized the presence and function of Munc18-1 and its closest homologue Munc18-2 in β-cell stimulus-secretion coupling. We show that pancreatic β-cells express both Munc18-1 and Munc18-2. The two Munc18 homologues exhibit different subcellular localization, and only Munc18-1 redistributes in response to glucose stimulation. However, both Munc18-1 and Munc18-2 augment glucose-stimulated hormone release. Ramp-like photorelease of caged Ca(2+) and high resolution whole-cell patch clamp recordings show that Munc18-1 and Munc18-2 overexpression shift the Ca(2+) sensitivity of the fastest phase of insulin exocytosis differently. In addition, we reveal that Ca(2+) sensitivity of exocytosis in β-cells depends on the phosphorylation status of the Munc18 proteins. Even though Munc18-1 emerges as the key SM-protein determining the Ca(2+) threshold for triggering secretory activity in a stimulated β-cell, Munc18-2 has the ability to increase Ca(2+) sensitivity and thus mediates the release of fusion-competent granules requiring a lower cytoplasmic-free Ca(2+) concentration, [Ca(2+)](i)(.) Hence, Munc18-1 and Munc18-2 display distinct subcellular compartmentalization and can coordinate the insulin exocytotic process differently as a consequence of the actual [Ca(2+)](i).
authors.affiliation: The Rolf Luft Research Center for Diabetes and Endocrinology, Karolinska Institutet, 17176 Stockholm, Sweden.
authors.forename: Slavena A
authors.initials: SA
authors.lastname: Mandic
authors.forename: Masa
authors.initials: M
authors.lastname: Skelin
authors.forename: Jenny U
authors.initials: JU
authors.lastname: Johansson
authors.forename: Marjan S
authors.initials: MS
authors.lastname: Rupnik
authors.forename: Per-Olof
authors.initials: PO
authors.lastname: Berggren
authors.forename: Christina
authors.initials: C
authors.lastname: Bark
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D045744:Cell Line, Tumor; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology*; D005786:Gene Expression Regulation / Q000187:drug effects; D005947:Glucose / Q000494:pharmacology; D006801:Humans; D007328:Insulin / Q000235:genetics / Q000378:metabolism*; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000166:cytology / Q000378:metabolism*; D008297:Male; D051379:Mice; D051938:Munc18 Proteins / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects / Q000502:physiology; D050600:SNARE Proteins / Q000235:genetics / Q000378:metabolism; D013549:Sweetening Agents / Q000494:pharmacology
chemical_list: D007328:Insulin; D051938:Munc18 Proteins; D050600:SNARE Proteins; C497938:Stxbp1 protein, mouse; C497941:Stxbp2 protein, mouse; D013549:Sweetening Agents; D005947:Glucose; D002118:Calcium