In the natural history of type 2 diabetes, progression of patients to insulin-dependency is linked to pancreatic beta cell dysfunction, endoplasmic reticulum (ER) stress, and ultimately, loss of pancreatic beta cell mass. The ER is a protein folding compartment that serves as the initial launch point in the synthesis of secreted proteins, and secretory protein misfolding has already been found to be linked to the pathogenesis of roughly a hundred distinct diseases. In beta cells, proinsulin synthesis can account for up to 50% of total protein synthesized by the cell, so even fractional proinsulin misfolding can serve as a dominant source of ER stress. Recently, in patients heterozygous for misfolded proinsulin mutants (such patients would be anticipated to have approximately 50% misfolded proinsulin in the ER), the syndrome of Mutant Ins-Gene Induced Diabetes of Youth (MIDY) is thought to trigger diabetes with virtually 100% penetrance, i.e., in every patient acquiring one MIDY mutant allele. I am capitalizing on this observation as a starting point to enable investigation of potential strategies to rescue the phenotype(s) caused by misfolded proinsulin. First, misfolded proinsulins are blocked in their intracellular transport, unable to exit the ER, and it is from thi starting point that they lead to diabetes pathogenesis. If ER exit of these misfolded proinsulins could be induced, then the reduction of downstream ER stress and prevention of beta cell dysfunction might be achievable. In this application, I describe preliminary studies in which I have found that secretion of one of the MIDY mutants, proinsulin-G(B23)V, is actually rescued by co-expression with an increasing concentration of wild- type (WT) proinsulin - even as WT proinsulin is blocked in the ER by an increasing concentration of misfolded MIDY mutant proinsulin! My preliminary data thus far suggest a bi-directional interaction, which I hypothesize is caused by dimerization between mutant and WT proinsulin. The demonstration of dimerization, and the mechanism of the potential rescue of misfolded proinsulin by native WT proinsulin, is the subject of Specific Aim 1 of this proposal. Secondly, I have found that manipulation of the expression level of the ER-oxidoreductin-1 (Ero1?) can also rescue secretion of the MIDY mutant proinsulin-G(B23)V. The mechanism of this potential rescue is similarly the subject of my Specific Aim 2. In summary, both my preliminary findings and my proposed experiments represent a body of work on the hypothesis that accumulation of misfolded proinsulin in the ER of pancreatic beta cells may be an approachable biomedical problem - the treatment of which might ameliorate beta cell dysfunction and beta cell death in diabetes.

Public Health Relevance

Secretory protein misfolding in the endoplasmic reticulum (ER) of pancreatic beta cells, which is linked to the phenomenon of ER stress, has been implicated in the pathogenesis of type 2 diabetes. The major secretory protein in the beta cell ER is proinsulin, and the Aims described herein propose two new approaches to directly rescue misfolded proinsulin in the ER. Thus, the experiments outlined in this proposal serve as initial proof-of- concept studies in the development of novel therapies directed at preserving pancreatic beta cell function in diabetes.

National Institute of Health (NIH)
National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
Individual Predoctoral NRSA for M.D./Ph.D. Fellowships (ADAMHA) (F30)
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Special Emphasis Panel (ZDK1-GRB-R (J1))
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Castle, Arthur
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University of Michigan Ann Arbor
Internal Medicine/Medicine
Schools of Medicine
Ann Arbor
United States
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Liu, Ming; Wright, Jordan; Guo, Huan et al. (2014) Proinsulin entry and transit through the endoplasmic reticulum in pancreatic beta cells. Vitam Horm 95:35-62
Wright, Jordan; Birk, Julia; Haataja, Leena et al. (2013) Endoplasmic reticulum oxidoreductin-1α (Ero1α) improves folding and secretion of mutant proinsulin and limits mutant proinsulin-induced endoplasmic reticulum stress. J Biol Chem 288:31010-8
Haataja, Leena; Snapp, Erik; Wright, Jordan et al. (2013) Proinsulin intermolecular interactions during secretory trafficking in pancreatic β cells. J Biol Chem 288:1896-906
Wright, Jordan; Wang, Xiaofan; Haataja, Leena et al. (2013) Dominant protein interactions that influence the pathogenesis of conformational diseases. J Clin Invest 123:3124-34