Summary The bioactive lipid sphingosine 1-phosphate (S1P) and its G protein-coupled receptors play critical roles in cardiovascular, immune and neural development and function [1–6]. Despite its importance, many questions remain about S1P signaling, including how S1P, which is synthesized intracellularly, is released from cells. Mutations in the zebrafish gene encoding the S1P receptor Miles Apart (Mil)/S1P2 disrupt the formation of the primitive heart tube [5]. We find that mutations of another zebrafish locus, two of hearts (toh), cause phenotypes that are morphologically indistinguishable from those seen in mil/s1p2 mutants. Positional cloning of toh reveals that it encodes a member of the Spinster-like family of putative transmembrane transporters. The biological functions of these proteins are poorly understood, although phenotypes of the Drosophila spinster and zebrafish not really started mutants suggest that these proteins may play a role in lipid trafficking [7, 8]. Through gain- and loss-of-function analyses, we show that toh is required for signaling by S1P2. Further evidence indicates that Toh is involved in the trafficking or cellular release of S1P.

Zebrafish have been increasingly used as a teaching tool to enhance the learning of many biological concepts from genetics, development, and behavior to the understanding of the local watershed. Traditionally, in both research and teaching, zebrafish work has focused on embryonic stages; however, later stages, from larval through adulthood, are increasingly being examined. Defining developmental stages based on age is a problematic way to assess maturity, because many environmental factors, such as temperature, population density, and water quality, impact growth and maturation. Fish length and characterization of key external morphological traits are considered better markers for maturation state. While a number of staging series exist for zebrafish, here we present a simplified normalization table of post-embryonic maturation well suited to both educational and research use. Specifically, we utilize fish size and four easily identified external morphological traits (pigment pattern, tail fin, anal fin, and dorsal fin morphology) to describe three larval stages, a juvenile stage, and an adult stage. These simplified maturation standards will be a useful tool for both educational and research protocols.