Abstract
Objective Risk alleles for type 2 diabetes at the STARD10 locus are associated with lowered STARD10 expression in the β-cell, impaired glucose-induced insulin secretion and decreased circulating proinsulin:insulin ratios. Although likely to serve as a mediator of intracellular lipid transfer, the identity of the transported lipids, and thus the pathways through which STARD10 regulates β-cell function, are not understood. The aim of this study was to identify the lipids transported and affected by STARD10 in the β-cell and its effect on proinsulin processing and insulin granule biogenesis and maturation.
Methods We used isolated islets from mice deleted selectively in the β-cell for Stard10 (βStarD10KO) and performed electron microscopy, pulse-chase, RNA sequencing and lipidomic analyses. Proteomic analysis of STARD10 binding partners was executed in INS1 (832/13) cell line. X-ray crystallography followed by molecular docking and lipid overlay assay were performed on purified STARD10 protein.
Results βStarD10KO islets had a sharply altered dense core granule appearance, with a dramatic increase in the number of “rod-like” dense cores. Correspondingly, basal secretion of proinsulin was increased. Amongst the differentially expressed genes in βStarD10KO islets, expression of the phosphoinositide binding proteins Pirt and Synaptotagmin 1 were decreased while lipidomic analysis demonstrated changes in phosphatidyl inositol levels. The inositol lipid kinase PIP4K2C was also identified as a STARD10 binding partner. STARD10 bound to inositides phosphorylated at the 3’ position and solution of the crystal structure of STARD10 to 2.3 Å resolution revealed a binding pocket capable of accommodating polyphosphoinositides.
Conclusion Our data indicate that STARD10 binds to, and may transport, phosphatidylinositides, influencing membrane lipid composition, insulin granule biosynthesis and insulin processing.
Footnotes
The author name Andreas Mueller has been corrected and his ORCID ID updated
Abbreviations
- BUDE
- Bristol University Docking Engine
- GFP
- Green Fluorescent Protein
- GWAS
- Genome Wide Association Study
- KATP
- ATP-sensitive K+ channels
- HRP
- Horseradish Peroxidase
- Kir6.2
- Kcnj11: Potassium Inwardly Rectifying Channel Subfamily J Member 11
- KOMP
- NIH Knock-Out Mouse Project
- ICP/MS
- Inductively coupled plasma mass spectrometry
- IMPC
- International Mouse Phenotyping Consortium
- LPC
- Lysophosphatidylcholine
- MBP
- Maltose Binding Protein
- PC
- Phosphatidylcholine
- PE
- Phosphatidylethanolamine
- PI
- Phosphatidylinositol
- PI(3)P
- Phosphatidylinositol 3-phosphate
- PI(4,5)P2
- PIP2, Phosphatidylinositol 4,5-bisphosphate
- PI(5)P
- Phosphatidylinositol 5-phosphate
- PIP
- phosphatidylinositol phosphate, phosphatidylinositide
- Pip4k2c
- phosphatidylinositol 5-phosphate 4-kinase type-2 gamma
- Pirt
- phosphoinositide-interacting regulator of transient receptor potential channels
- RNAseq
- RNA sequencing
- Ptbp1
- Polypyrimidine tract-binding protein 1
- Slc30a8
- ZnT8, Solute Carrier Family 30 Member 8
- SNARE
- Soluble N-ethylmaleimide sensitive factor Attachment protein Receptor
- STARD10
- StAR Related Lipid Transfer Domain Containing 10
- Syt1
- Synaptotagmin 1
- Sytl4
- synaptotagmin-like 4, granuphilin
- TMT
- Tandem Mass Tag
- TRP
- Transient receptor potential