Zebrafish are a valuable model for mammalian lipid metabolism; larvae process lipids similarly through the intestine and hepatobiliary system and respond to drugs that block cholesterol synthesis in humans. After ingestion of fluorescently quenched phospholipids, endogenous lipase activity and rapid transport of cleavage products results in intense gall bladder fluorescence. Genetic screening identifies zebrafish mutants, such as fat free, that show normal digestive organ morphology but severely reduced phospholipid and cholesterol processing. Thus, fluorescent lipids provide a sensitive readout of lipid metabolism and are a powerful tool for identifying genes that mediate vertebrate digestive physiology.

The zebrafish fat-free (ffr) mutation was identified in a physiological screen for genes that regulate lipid metabolism. ffr mutant larvae are morphologically indistinguishable from wild-type sibling larvae, but their absorption of fluorescent lipids is severely impaired. Through positional cloning, we have identified a causative mutation in a highly conserved and ubiquitously expressed gene within the ffr locus. The Ffr protein contains a Dor-1 like domain typical of oligomeric Golgi complex (COG) gene, cog8. Golgi complex ultrastructure is disrupted in the ffr digestive tract. Consistent with a possible role in COG-mediated Golgi function, wild-type Ffr-GFP and COG8-mRFP fusion proteins partially colocalize in zebrafish blastomeres. Enterocyte retention of an endosomal lipid marker in ffr larvae support the idea that altered vesicle trafficking contributes to the ffr mutant defect. These data indicate that ffr is required for both Golgi structure and vesicular trafficking, and ultimately lipid transport.

Pseudoxanthoma elasticum (PXE) is caused by mutations in the ABCC6 gene, which encodes a putative efflux pump, ABCC6. The zebrafish (Danio rerio) has two ABCC6-related sequences. To study the function of abcc6 during zebrafish development, the mRNA expression levels were measured using RT-PCR and in situ hybridization. The abcc6a showed a relatively high level of expression at 5 days post-fertilization (dpf) and the expression was specific to the Kupffer’s vesicles. The abcc6b expression was evident at 6 hpf and remained high up to 8 dpf, corresponding to embryonic kidney proximal tubules. Morpholinos were designed to both genes to block translation and to prevent pre-mRNA splicing. Injection of the abcc6a morpholinos into 1–4 cell zebrafish embryos decreased gene expression by 54 to 81%, and induced a phenotype, cardiac edema and curled tail associated with death at around 8 dpf. Microinjecting zebrafish embryos with full-length mouse Abcc6 mRNA together with the morpholino completely rescued this phenotype. No phenotypic changes were observed when the abcc6b gene morpholino was injected to embryos, with knock-down efficiency of 100%. These results suggest that abcc6a is an essential gene for normal zebrafish development and provide novel insight into the function of ABCC6, the gene mutated in PXE.

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