Project Summary
Diabetes is among the fastest growing health challenges of the 21st century, affecting >30 million people, with
≥80,000 deaths annually, and involving ~15% of U.S. national health expenditures. Type 2 diabetes (T2D) is
the most common form of diabetes, which is linked to an insufficient amount of circulating insulin because of
the body’s insensitivity to the hormone. Maintenance of the insulin storage pool requires synthesis of ~6000
proinsulin (PI) molecules/ß-cell/second, each delivered to the endoplasmic reticulum (ER) for folding. Even
more molecules are needed in states of insulin resistance. Significantly, we discovered that PI enters aberrant
disulfide-linked intermolecular complexes, even in healthy (human and murine) islets. Under conditions that
demand increased insulin production (even prediabetes), these complexes dramatically increase, thus limiting
insulin production. We show new key evidence that these aberrantly folded PI complexes can be resolved to
monomeric PI within the ER. We recently elucidated the first map of the human PI interactome identifying PI
folding modifiers. The most significant PI interactor in human islets is the ER chaperone BiP and we present
new evidence (both gain of function, and loss of function) that this interaction, supported by BiP co-
chaperones, is absolutely required for productive proinsulin folding (and limiting misfolding), leading to
successful anterograde transport. For our studies we generated a novel BiP-tagged mouse that can for the
first time identify fundamental steps in PI folding essential for insulin production. Moreover, we show that
increased expression of BiP and its co-chaperone P58IPK dramatically reduces accumulation of the high
molecular weight PI complexes. Thus, our discoveries open the possibility that pharmacologic intervention may
improve chaperone-dependent PI folding, and this may attenuate T2D. As we begin to elucidate the human PI
folding pathway, we are developing parallel animal models to determine how PI folds/misfolds. Here we
propose to: 1) Mechanistically dissect how BiP and additional PI interactors in the ER orchestrate successful PI
folding and determine which step(s) of PI folding go awry in T2D; 2) Identify how the PI interactome changes
in human T2D; determine the function of altered PI interactions in islets from patients with T2D; and utilize
novel assays to measure productive PI folding/trafficking in ß-cells. We will integrate physiologic studies of
human islets with novel genetic and biochemical approaches to generate a comprehensive understanding of
how PI folding homeostasis impacts ß-cell function in health and disease. We believe that this hypothesis is a
high-impact idea essential to the mission of the NIDDK, and we now bring tools, assays, and approaches that
are not currently available anywhere else.