Highlights d Mechanical forces from cardiac contractility positively regulate VCL in vivo d VCL interacts with actin regulators in cardiomyocytes in response to mechanical forces d VCL facilitates cardiomyocyte myofilament maturation through SSH1 and CFL d Zebrafish cfl1 mutants exhibit cardiomyocyte myofilament disruption

Thyroid hormone (TH) signaling promotes tissue maturation and adult organ formation. Developmental transitions alter an organism's metabolic requirements, and it remains unclear how development and metabolic demands are coordinated. We used the zebrafish as a model to test whether and how TH signaling affects pancreatic islet maturation, and consequently glucose homeostasis, during the larval to juvenile transition. We found that exogenous TH precociously activates the β-cell differentiation genes and while downregulating a master regulator of α-cell development and function. Together, these effects induced hypoglycemia, at least in part by increasing and decreasing expression. We visualized TH target tissues using a novel TH-responsive reporter line and found that both α- and β-cells become targets of endogenous TH signaling during the larval-to-juvenile transition. Importantly, endogenous TH is required during this transition for the functional maturation of α- and β-cells in order to maintain glucose homeostasis. Thus, our study sheds new light on the regulation of glucose metabolism during major developmental transitions.

A dense local vascular network is crucial for pancreatic endocrine cells to sense metabolites and secrete hormones, and understanding the interactions between the vasculature and the islets may allow for therapeutic modulation in disease conditions. Using live imaging in two models of vascular disruption in zebrafish, we identified two distinct roles for the pancreatic vasculature. At larval stages, expression of a dominant negative version of Vegfaa (dnVegfaa) in β-cells led to vascular and endocrine cell disruption with a minor impairment in β-cell function. In contrast, expression of a soluble isoform of Vegf receptor 1 (sFlt1) in β-cells blocked the formation of the pancreatic vasculature and drastically stunted glucose response while islet architecture was not affected. Notably, these effects of dnVegfaa or sFlt1 were not observed in animals lacking vegfaa, vegfab, kdrl, kdr, or flt1 function, indicating that they interfere with multiple ligands and/or receptors. In adults, disrupted islet architecture persisted in dnVegfaa expressing animals, while sFlt1 expressing animals displayed large sheets of β-cells along their pancreatic ducts, accompanied by impaired glucose tolerance in both models. Thus, our study reveals novel roles for the vasculature in patterning and function of the islet.

Pathways modulating glucose homeostasis independently of insulin would open new avenues to combat insulin resistance and diabetes. Here, we report the establishment, characterization, and use of a vertebrate ‘insulin-free’ model to identify insulin-independent modulators of glucose metabolism. insulin knockout zebrafish recapitulate core characteristics of diabetes and survive only up to larval stages. Utilizing a highly efficient endoderm transplant technique, we generated viable chimeric adults that provide the large numbers of insulin mutant larvae required for our screening platform. Using glucose as a disease-relevant readout, we screened 2233 molecules and identified three that consistently reduced glucose levels in insulin mutants. Most significantly, we uncovered an insulin-independent beneficial role for androgen receptor antagonism in hyperglycemia, mostly by reducing fasting glucose levels. Our study proposes therapeutic roles for androgen signaling in diabetes and, more broadly, offers a novel in vivo model for rapid screening and decoupling of insulin-dependent and -independent mechanisms.

β-Cell loss and dysfunction play a critical role in the progression of type 1 and type 2 diabetes. Identifying new molecules and/or molecular pathways that improve β-cell function and/or increase β-cell mass should significantly contribute to the development of new therapies for diabetes. Using the zebrafish model, we screened 4,640 small molecules to identify modulators of β-cell function. This in vivo strategy identified 84 stimulators of expression, which simultaneously reduced glucose levels. The promoter activation kinetics for 32 of these stimulators were consistent with a direct mode of action. A subset of stimulators, including the antidiabetic drug pioglitazone, induced the coordinated upregulation of gluconeogenic expression, suggesting functional response to increased insulin action in peripheral tissues. Notably, Kv1.3 inhibitors increased β-cell mass in larval zebrafish and stimulated β-cell function in adult zebrafish and in the streptozotocin-induced hyperglycemic mouse model. In addition, our data indicate that cytoplasmic Kv1.3 regulates β-cell function. Thus, using whole-organism screening, we have identified new small-molecule modulators of β-cell function and glucose metabolism.