Lipid Uptake, Metabolism, and Transport in the Larval Zebrafish

Abstract

The developing zebrafish is a well-established model system for studies of energy metabolism, and is amenable to genetic, physiological, and biochemical approaches. For the first 5 days of life, nutrients are absorbed from its endogenous maternally deposited yolk. At 5 days post-fertilization, the yolk is exhausted and the larva has a functional digestive system including intestine, liver, gallbladder, pancreas, and intestinal microbiota. The transparency of the larval zebrafish, and the genetic and physiological similarity of its digestive system to that of mammals make it a promising system in which to address questions of energy homeostasis relevant to human health. For example, apolipoprotein expression and function is similar in zebrafish and mammals, and transgenic animals may be used to examine both the transport of lipid from yolk to body in the embryo, and the trafficking of dietary lipids in the larva. Additionally, despite the identification of many fatty acid and lipid transport proteins expressed by vertebrates, the cell biological processes that mediate the transport of dietary lipids from the intestinal lumen to the interior of enterocytes remain to be elucidated. Genetic tractability and amenability to live imaging and a range of biochemical methods make the larval zebrafish an ideal model in which to address open questions in the field of lipid transport, energy homeostasis, and nutrient metabolism.

Keywords: comparative physiology; enterocytes; lipid metabolism; lipoproteins; zebrafish.

Figure 1

Zebrafish apolipoprotein genes are expressed in the yolk syncytial layer (YSL). The developing zebrafish embryo gradually absorbs lipids from its yolk (a), which is surrounded by the YSL. At 1–5 dpf, the yolk ball is lengthened along the tail of the embryo forming the yolk extension (b). In situ hybridization reveals expression of all 11 zebrafish apolipoprotein genes in the apoB, apoA-IV, apoE, and apoA-I families in the YSL at 1 day post-fertilization. Adapted and reprinted from Miyares et al. (18), and Otis et al. (13), under a CC-BY license.

Figure 2

Acyl-CoA synthetases are expressed in the larval zebrafish yolk syncytial layer (YSL) and intestine. (A) In situ hybridization reveals expression of acsl1b, acsl4b, and acsl5 in the YSL at 24 hpf, and acsl1b in the developing gut at 2 dpf. Adapted from Ref. (17). (B) acsl4a is expressed in the gut and central nervous system (cns) of the 4 dpf larval zebrafish, and in the YSL at 24 hpf and earlier. Reprinted from Ref. (52), Figure S1E in Supplementary Material, under a CC-BY license. (C) fatp4/acsvl4 is expressed in the gut (especially the anterior bulb) of the 5 dpf larval zebrafish. Adapted from Ref. (53).

Figure 3

Zebrafish apolipoprotein genes are expressed in the larval digestive system. In situ hybridization reveals expression of 10 of the 11 zebrafish apolipoprotein genes in the apoB, apoA-IV, apoE, and apoA-I families in the liver and/or intestine of the 6 dpf larva. Dissected intestines probed for apoA-Ia are shown, and the gut of a larva probed for apoA-1b is magnified below the image of the whole larva. L, liver, I, intestine. Adapted and reprinted from Ref. (13), Figures 2–5, under a CC-BY license.

Figure 4

Metabolic labeling with fluorescent fatty acids is performed in the context of zebrafish development, yolk absorption, and dietary lipid metabolism. Fluorescent fatty acids (BODIPY-FL-C12 depicted) are trafficked and metabolized along with native yolk or dietary lipids when delivered to the developing zebrafish by yolk injection or feeding. LD, cytoplasmic lipid droplet, LP, lipoprotein, VLDL, very low-density lipoprotein. Embryo and larva illustrations adapted from Ref. (18).