Fishing for small molecules to treat diabetes, from a beta cell perspective

Abstract

Maintenance of glucose homeostasis necessitates a precise control of insulin secretion. Pancreatic β cells sense fluctuation of blood glucose and secret insulin in response. However, functional β cell mass is decreased in diabetes. Here, to recover functional β cell mass, we use zebrafish as a model to perform in vivo drug discovery.

In paper I, to discover drugs that could expand β cell mass through proliferation, we performed an in vivo screen in zebrafish based on a luminescence ubiquitination-based cell cycle indicator (LUCCI), identified a small molecule called HG-9-91-01, an inhibitor of salt-inducible kinases (SIKs), as a mitogen of mouse and human β cells. Mechanistic studies found HG-9-91-01 induced a transient upregulation of ATF6-dependent unfolded protein response (UPR), which together with other downstream effectors including CRTC1, CRTC2, mTOR and IRE1 led to a mitogenic response in β cells.

In paper II, to discover drugs that could increase β cell mass from other origins, we performed in vivo screening in zebrafish for stimulators that convert glucagon-expressing ⍺ cells, somatostatin-expressing ẟ cells, and elastase-expressing acinar cells to β cells. 4 hits were identified in the screens and shown to promote β cell regeneration as well as reduce glucose in zebrafish with β cell ablation. However, only one hit called A-674563 induced a modest increase in reprogramming of ⍺ cells to β cells in lineage tracing experiments, whereas the other hits failed to promote reprogramming. Spontaneous conversion of ⍺- or ẟ- cells to β cells was rare, and no conversion of acinar cells to β cells was observed during β cell regeneration. Together, reprogramming of other pancreatic cells to β cells is rare and difficult to stimulate by small molecules in zebrafish.

In paper III, a small molecule called Adjudin, identified in paper II, was discovered to promote β cell function in zebrafish. In translational studies using in vitro cultured mouse islets, we found that Adjudin promoted functional maturation of isolated islets from postnatal day 0 (P0) mice, as they gained capability of glucose responsive insulin secretion; Adjudin also improved recovery of islets from a type 2 diabetic mouse model. Moreover, Adjudin stimulated hepatic glucose uptake, an effect we further found largely independent of insulin in zebrafish and validated in primary human hepatocyte (PHH) formed spheroids with insulin resistance. Next, we examined Adjudin in a type 2 diabetic mouse model (db/db mice), observed improved glucose homeostasis in the mice that received Adjudin treatment. Together, Adjudin may serve as a potential therapeutic drug for diabetes.

To summarize, using in vivo drug screening in zebrafish, we identified small molecules that could either expand β cell mass or promote β cell function, such findings may pave the way for future research and development of a novel treatment for diabetes.