Metabolic insights from zebrafish genetics, physiology, and chemical biology

Abstract

Metabolic diseases-atherosclerotic cardiovascular disease, type 2 diabetes mellitus, obesity, and non-alcoholic fatty liver disease--have reached pandemic proportions. Across gene, cell, organ, organism, and social-environmental scales, fundamental discoveries of the derangements that occur in these diseases are required to develop effective new treatments. Here we will review genetic, physiological, pathological and chemical biological discoveries in the emerging zebrafish model for studying metabolism and metabolic diseases. We present a synthesis of recent studies using forward and reverse genetic tools to make new contributions to our understanding of lipid trafficking, diabetes pathogenesis and complications, and to β-cell biology. The technical and physiological advantages and the pharmacological potential of this organism for discovery and validation of metabolic disease targets are stressed by our summary of recent findings. We conclude by arguing that metabolic research using zebrafish will benefit from adoption of conventional blood and tissue metabolite measurements, employment of modern imaging techniques, and development of more rigorous metabolic flux methods.

Fig. 1

Zebrafish larval anatomy. Within the first week of life, zebrafish adopt a conventional vertebrate body plan. The labeled organs remain visible until the early juvenile period. From Ref. [34]

Fig. 2

Lipid trafficking in enterocytes and hepatocytes relies on the same evolutionarily central machinery. a In the intestine dietary fats are suspended in micelles in the lumen. Cholesterol (C) and plant sterols (PS) are absorbed at the apical surface of enterocytes by NPCL1L1. Plant sterols are immediately excreted back into the lumen by the action of ABCG5/ABCG8 heterodimers. Following hydrolysis of triacylglycerol (TAG), fatty acids (FAs) are transported across the apical surface. Absorbed C and FAs are reassembled in the endoplasmic reticulum (ER) into cholesteryl esters (CE) and TAG by a series of enzymes (none are shown). ER-assembled neutral lipids are packaged with Apolipoprotein B (ApoB) into nascent chylomicrons by Microsomal triglyceride transfer protein (MTP), which then mature as they pass through the Golgi apparatus and are secreted into the basolateral space. In parallel, Apolipoprotein A1 (Apoa1) is release along the basolateral surface, where it combines with ABCA1-transported free C to form nascent HDL particles. b In the liver, MTP packages CE and TAG to make very low-density lipoprotein (VLDL) particles, which are secreted across the basolateral surface into lymphatic vessels. VLDL particles mature in the vasculature into LDL particles that are retrieved by the LDL Receptor (LDLR), which then undergoes endocytosis. Cholesterol is ultimately liberated following degradation of these particles. Free fatty acids are also transported across this surface from the circulation by transporters that are not shown. NPC1L1 and the ABCG5/ABCG8 complex reside on the apical, bile canalicular surface where they serve to retrieve or eliminate (respectively) cholesterol from the bile. Both cell types have the capacity to form cytoplasmic lipid droplets. The molecular cues governing this storage are largely not known. Chylomicrons enter the lymphatic circulation, by-passing perfusion of the liver through the portal circulation. VLDL particles enter the circulation via the hepatic vein

Fig. 3

Factors identified by small molecule screens and dietary interventions in zebrafish that modulate β-cell plasticity. The origin of new β-cells can be roughly classified in differentiation from progenitor cells (pathways marked by red boxes) and in proliferation from existing β-cells (pathways marked by blue boxes). (1) Activators of adenosine signaling enhance proliferation predominantly after tissue damage. (2) The suppression of Notch signaling emerges as a key requirement for differentiation from Notch-responsive cells (NRC) within the intrapancreatic ducts (IPD) towards the endocrine lineage. Pharmacological inhibition of Notch by the γ-secretase inhibitor DAPT is a strong enhancer of secondary islet formation during zebrafish development. Down-regulation of Notch in NRCs precedes new beta cell formation in response to high calorie diet in normal physiology and after beta cell ablation. (3) Lipids and glucose stimulate beta cell differentiation from post-mitotic progenitor cells that are marked by mnx1 or nkx2.2. (4) DSF and MPA induce precocious differentiation of endocrine cells from the IPD without affecting Notch levels. Both compounds induce pax6:GFP positive cells that can give rise to α- and β-cells. The small molecules listed depict the most potent compounds and do not reflect all chemicals found in the different chemical screens. GR glucocorticoid receptor, RA retinoic acid, DSF disulfiram, MPA mycophenolic acid