Aim: Risk alleles for type 2 diabetes (T2D) in the STARD10 locus, impair insulin secretion and are associated with decreased proinsulin:insulin ratios. We have shown that the T2D risk associated with variation at this locus is likely to be mediated through lowered STARD10 expression in the β cell. Here, we investigate the mechanisms by which STARD10 may regulate insulin secretion.
Materials and Methods: A 3-dimensional model of STARD10 was constructed from the structure of STARD2, using the modelling tools Chimera and Modeller. Islets were isolated from StarD10fl/fl-Ins1Cre male mice (βStarD10KO). Electron Microscopy (EM) images were obtained from isolated islets after chemical fixation. Total zinc content was measured by inductively coupled plasma mass spectrometry. Pulse-chase analysis of proinsulin processing was performed using 35S-labelled amino acids. RNA-Seq was done on polyadenylated transcripts selected during the preparation of paired-end, directional RNAseq libraries, sequenced on an Illumina HiSeq 4000 machine.
Results: Molecular modelling indicated that STARD10 binds either phosphatidylcholine or phosphatidylethanolamine. EM analysis of islets from βStarD10KO mice revealed a markedly altered dense core granule appearance, with a dramatic increase (Fold change = 4.3; p<0.001) in "rod like" dense cores, and a ∼2-fold increase in total islet zinc content. Unexpectedly, pulse-chase studies revealed enhanced basal secretion of newly-synthesised proinsulin from βStarD10KO islets. Whilst RNA sequencing identified several dysregulated genes in βStarD10KO islets, STARD10 deletion did not affect prohormone convertase (Pcsk1, Pcsk2), or ZnT8 (Slc30a8) expression.
Conclusion: We identify STARD10 as a critical regulator of insulin granule biogenesis and β cell zinc homeostasis. Our data also suggest that increased β cell Zn2+ secretion in risk allele carriers may decrease the clearance of mature insulin to lower plasma proinsulin:insulin ratio.
G. Carrat: None. E. Haythorne: None. L. Haataja: None. P. Arvan: None. A. Tomas: None. A. Piunti: None. T.J. Pullen: None. E. Georgiadou: None. T. Stylianides: None. V. Salem: None. W. Distaso: None. A. Cakebread: None. D. Hodson: None. A.C. Fung: None. R.B. Sessions: None. F. Alpy: None. A.P. Kong: Advisory Panel; Self; Lilly Diabetes. Research Support; Self; AstraZeneca, Lilly Diabetes. Speaker's Bureau; Self; Abbott. Other Relationship; Self; AstraZeneca, Novartis Pharmaceuticals Corporation, Sanofi. I. Leclerc: None. G.A. Rutter: Consultant; Self; Sun Pharma.
Medical Research Council UK; UK Wellcome Trust; Royal Society; Societe Francophone du Diabete