Zebrafish as a Model Animal for Studying Lysophosphatidic Acid Signaling

Lysophosphatidic acid (LPA) is a second-generation lysophospholipid mediator that exerts multiple biological functions through its own cognate receptors. LPA is produced by specific enzymatic reactions and activates receptors with similar structures (Edg receptors and P2Y receptors), which results in a variety of actions from embryonic blood vessel formation to development of cancer. LPA-related genes are highly conserved in vertebrates. In the zebrafish genome, three LPA-producing enzymes and nine LPA receptors are present. In vitro experiments have shown that LPA-related genes in zebrafish are conserved biochemically. LPA-related genes can be up- and downregulated by injecting morpholino antisense oligonucleotides (MOs) specific to LPA-related genes or mRNAs in zebrafish embryos. Such tools help to assess the biological function of these genes. For example, knockdown of the LPA-produced enzyme autotaxin (ATX) in zebrafish embryos resulted in malformation of embryonic blood vessel formation, which has also been observed in embryos from ATX-knockout mice. Simultaneous inactivation of multiple genes is possible by injecting more than one MO in zebrafish embryos, which makes it possible to identify the LPA receptors responsible for embryonic blood vessel formation. Gene functions can be also eliminated in zebrafish embryos by pharmacological tools such as enzyme inhibitors or receptor antagonists. Interestingly, overexpression of ATX in zebrafish embryos resulted in duplication of the heart (two-heart phenotype) and the phenotype was canceled by treating the embryos with LPA receptor antagonists. The zebrafish system is a powerful tool not only for identification of gene functions but also for development of drugs against enzymes and receptors.