Sphingosine kinase catalyses the formation of S1P from sphingosine. Since introduction of Spns2 increases the export of S1P from the cells, Spns2 functions as a S1P transporter. S1P signalling mediated by Mil/S1P₂ regulates the migration of cardiac progenitors.

Image courtesy of Dr Atsuo Kawahara, National Cardiovascular Center Research Institute, Suita, Osaka, Japan.

Sphingosine-1-phosphate (S1P) is a secreted signalling sphingolipid known to play a major role in vascular development. However, the way that S1P secretion is regulated during embryogenesis is poorly understood. In Science, Kawahara et al. now report that S1P export — which affects the migration of myocardial precursors — requires the multipass transmembrane protein Spns2.

In a screen for regulators of zebrafish heart development the authors isolated a recessive mutant (ko157) that displayed two hearts — a condition known as cardia bifida. During zebrafish embryogenesis, myocardial precursors migrate from both sides of the anterior lateral plate mesoderm towards the midline to form the heart tube. In ko157 mutants, these precursor cells failed to migrate and instead differentiated into two chambers at lateral positions.

Genetic mapping of the ko157 mutation revealed that the mutant allele contained a mutation in the spns2 gene, which encodes a sphingolipid transporter. Furthermore, knockdown and rescue experiments confirmed that the functional impairment in ko157 mutants was due to the mutation in Spns2.

The defects seen in ko157 mutants resembled those seen in S1P receptor-2 (mil/S1P2) mutants, and the authors could demonstrate a genetic interaction between mil/S1P2 and spns2. Furthermore, injection of S1P into spns2^ko157 mutants rescued the cardia bifida phenotype, whereas injection of spns2 mRNA failed to rescue the mutant phenotype due to a lack of mil/S1P2 suggesting that Spns2 functions upstream of Mil/S1P₂.

The authors went on to test whether Spns2 functions as a S1P transporter by using Chinese hamster ovary cells lacking S1P export activity. Expression of a sphingosine kinase that induces S1P synthesis resulted in the accumulation of intracellular S1P but concomitant expression of Spns2 rescued S1P export, showing that S1P secretion depends on Spns2.

During early embryogenesis, spns2 was found to be strongly expressed in the yolk syncitial layer — an extraembryonic membrane surrounding the yolk — just below the developing myocardial precursors. Interestingly, spns2 knockdown in the yolk syncitial layer induced cardia bifida suggesting Spns2 contributes to the migration of myocardial precursors.

This study shows that Spns2 functions as an S1P transporter whose activity in the yolk syncitial layer, presumably mediated by the Mil/S1P₂ pathway, is required for myocardial precursor cell migration. However, the mechanisms underlying S1P-dependent migration remain to be fully elucidated.

Author: Kim Baumann - Copyright © 2009 Nature Publishing Group, a division of MacMillan Publishers Limited; used with permission