Abstract
3-ketodihydrosphingosine reductase (KDSR) is the key enzyme in the de novo sphingolipid synthesis. We identified a novel missense kdsrI105R mutation in zebrafish that led to a loss of function, and resulted in progression of hepatomegaly to steatosis, then hepatic injury phenotype. Lipidomics analysis of the kdsrI105R mutant revealed compensatory activation of the sphingolipid salvage pathway, resulting in significant accumulation of sphingolipids including ceramides, sphingosine and sphingosine 1-phosphate (S1P). Ultrastructural analysis revealed swollen mitochondria with cristae damage in the kdsrI105R mutant hepatocytes, which can be a cause of hepatic injury in the mutant. We found elevated sphingosine kinase 2 (sphk2) expression in the kdsrI105R mutant. Genetic interaction analysis with the kdsrI105R and the sphk2wc1 mutants showed that sphk2 depletion suppressed liver defects observed in the kdsrI105R mutant, suggesting that liver defects were mediated by S1P accumulation. Further, both oxidative stress and ER stress were completely suppressed by deletion of sphk2 in kdsrI105R mutants, linking these two processes mechanistically to hepatic injury in the kdsrI105R mutants. Importantly, we found that the heterozygous mutation in kdsr induced predisposed liver injury in adult zebrafish. These data point to kdsr as a novel genetic risk factor for hepatic injury.
Highlights
Sphingolipids are essential lipid components of eukaryotic cell membranes and play important roles in membrane trafficking, cell proliferation, differentiation, apoptosis, and cell migration[1,2]
We found that accumulation of ceramides, Sph, and sphingosine 1-phosphate (S1P) resulted from activation of the lysosomal sphingolipid salvage pathway in the kdsrI105R mutant
From the previous forward genetic screening to identify zebrafish mutants with post-developmental liver defects[17], we identified a mutant showing progression of liver defects ranging from hepatomegaly at 6 days post fertilization to steatosis at 7 dpf, and to a more advanced hepatic injury thereafter (Fig. 1A–C)
Summary
Sphingolipids are essential lipid components of eukaryotic cell membranes and play important roles in membrane trafficking, cell proliferation, differentiation, apoptosis, and cell migration[1,2]. We found progression of liver disease phenotype in the kdsr mutant zebrafish and we investigated the mechanism of disease pathogenesis in this paper. Given the well-conserved sphingolipid synthetic pathway in zebrafish and high protein homology with human KDSR, we expect that people who carry KDSR mutations may have liver disease. Zebrafish kdsrI105R mutant that encodes a missense mutation in 3-ketodihydro-sphingosine reductase (kdsr) from a forward genetic screening to identify mutants with post-developmental liver disease[17]. We found that accumulation of ceramides, Sph, and S1P resulted from activation of the lysosomal sphingolipid salvage pathway in the kdsrI105R mutant. Through genetic interaction of kdsr and sphk[2] mutations, we found that sphk2-mediated S1P accumulation is a key factor in both oxidative and ER stress in the kdsrI105R mutant
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