Intracellular serotonin modulates insulin secretion from pancreatic beta-cells by protein serotonylation.
While serotonin (5-HT) co-localization with insulin in granules of pancreatic beta-cells was demonstrated more than three decades ago, its physiological role in the etiology of diabetes is still unclear. We combined biochemical and electrophysiological analyses of mice selectively deficient in peripheral tryptophan hydroxylase (Tph1-/-) and 5-HT to show that intracellular 5-HT regulates insulin secretion. We found that these mice are diabetic and have an impaired insulin secretion due to the lack of 5-HT in the pancreas. The pharmacological restoration of peripheral 5-HT levels rescued the impaired insulin secretion in vivo. These findings were further evidenced by patch clamp experiments with isolated Tph1-/- beta-cells, which clearly showed that the secretory defect is downstream of Ca(2+)-signaling and can be rescued by direct intracellular application of 5-HT via the clamp pipette. In elucidating the underlying mechanism further, we demonstrate the covalent coupling of 5-HT by transglutaminases during insulin exocytosis to two key players in insulin secretion, the small GTPases Rab3a and Rab27a. This renders them constitutively active in a receptor-independent signaling mechanism we have recently termed serotonylation. Concordantly, an inhibition of such activating serotonylation in beta-cells abates insulin secretion. We also observed inactivation of serotonylated Rab3a by enhanced proteasomal degradation, which is in line with the inactivation of other serotonylated GTPases. Our results demonstrate that 5-HT regulates insulin secretion by serotonylation of GTPases within pancreatic beta-cells and suggest that intracellular 5-HT functions in various microenvironments via this mechanism in concert with the known receptor-mediated signaling.
Paulmann
Nils
N
Department of Human Molecular Genetics, Max Planck Institute for Molecular Genetics, Berlin, Germany.
Grohmann
Maik
M
Voigt
Jörg-Peter
JP
Bert
Bettina
B
Vowinckel
Jakob
J
Bader
Michael
M
Skelin
Masa
M
Jevsek
Marko
M
Fink
Heidrun
H
Rupnik
Marjan
M
Walther
Diego J
DJ
D000818:Animals; D002460:Cell Line; D003920:Diabetes Mellitus / Q000209:etiology / Q000378:metabolism; D006801:Humans; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000378:metabolism*; D042541:Intracellular Space / Q000378:metabolism; D051379:Mice; D018408:Patch-Clamp Techniques; D011499:Protein Processing, Post-Translational; D051381:Rats; D012701:Serotonin / Q000378:metabolism*; D011503:Transglutaminases / Q000378:metabolism; D014365:Tryptophan Hydroxylase / Q000172:deficiency; D020691:rab GTP-Binding Proteins / Q000378:metabolism*; D000075745:rab27 GTP-Binding Proteins; D020826:rab3A GTP-Binding Protein / Q000378:metabolism*
D007328:Insulin; D000075745:rab27 GTP-Binding Proteins; D012701:Serotonin; D014365:Tryptophan Hydroxylase; D011503:Transglutaminases; C512962:Rab27a protein, mouse; D020691:rab GTP-Binding Proteins; D020826:rab3A GTP-Binding Protein