Characterization of the Drosophila sphingosine kinases and requirement for Sk2 in normal reproductive function

Sphingosine kinase is a highly conserved enzyme that catalyzes the synthesis of sphingosine 1-phosphate and reduces cellular levels of sphingosine and ceramide. Although ceramide is pro-apoptotic and sphingosine is generally growth-inhibitory, sphingosine 1-phosphate signaling promotes cell proliferation, survival, and migration. Sphingosine kinase is thus in a strategic position to regulate important cell fate decisions which may contribute to normal animal development. To facilitate studies examining the potential role of sphingosine kinase and long chain base metabolism in Drosophila development, we characterized two putative Drosophila sphingosine kinase genes, Sk1 and Sk2. Both genes functionally and biochemically complement a yeast sphingosine kinase mutant, express predominantly cytosolic activities, and are capable of phosphorylating a range of endogenous and non-endogenous sphingoid base substrates. The two genes demonstrate overlapping but distinct temporal and spatial expression patterns in the Drosophila embryo, and timing of expression is consistent with observed changes in long chain base levels throughout development. A null Sk2 transposon insertion mutant demonstrated elevated long chain base levels, impaired flight performance, and diminished ovulation. This is the first reported mutation of a sphingosine kinase in an animal model; the associated phenotypes indicate that Sk1 and Sk2 are not redundant in biological function and that sphingosine kinase is essential for diverse physiological functions in this organism.