The mystery of left-right asymmetry - zebrafish has an answer.

Vertebrates appear to have symmetrical left and right halves externally, but in fact internal organs are located at different sides. This phenomenon is called left-right (LR) asymmetry. LR asymmetry is established at the early stage of embryogenesis. In Xenopus lavis, the asymmetrical expression of maternal genes causes cellular polarity and unequal development of left and right sides at the cleavage stage. However, the LR asymmetry has not been documented so early in other animals. In mice and zebrafish, a transient structure called node appears during gastrulation. The node is a vesicle-like structure with a lumen. The lumen has cilia and the cilia beating can result in circulation of lumen fluid. The movement of lumen fluid may thus influence asymmetric LR gene expression, tissue and organ formation in lateral plate mesoderm. Although we have some idea about the function of node, the formation and its regulatory mechanisms are still unclear. The main obstacle is the difficulty to observe dynamic embryonic development of vertebrates, in particular mammals. Fortunately, the development of zebrafish as a model system has greatly enhanced our ability to do so.

Zebrafish is a small size carp. The adult zebrafish is about an inch long. It gains its name by the zebra-like strips on lateral sides and becomes one of the favorite aquarium fishes (Fig. 1) Zebrafish oocytes are matured and spawn in batches through out the year. Under a good laboratory management, zebrafish can lay eggs every 7-10 days. With its small size zebrafish can be kept in larger number per unit space. So under a proper husbandry, scientists may obtain embryos almost everyday for developmental biology research. In addition, zebrafish embryos develop rapidly. Embryos appear like a fish at 24 hours post fertilization. The heart is beating afterward and the rest of other organs also form with 3-5 days. Thus, zebrafish has become one of the best models for organogenesis research. Furthermore, zebrafish generation time is as short as 3 months that is equivalent to mice, but with its large number available, zebrafish has become a vertebrate model of genetic research.

Zebrafish embryos also have a node-like structure called Kupffer’s vesicle (Kv) (Fig. 2). Kv is formed from a group of precursor cells at the dorsal leading edge of ectoderm during gastrulation. The lumen of vesicle also has cilia to control asymmetric LR gene expression, tissue and organ development. As described previously, the regulatory mechanism of node formation is unclear. Little is known for the Kv formation as well. However, a recent study pointed out that the intracellular calcium of Kv precursor cells is higher than that in the rest of embryonic cells. Inhibition of the rise in intracellular calcium in those cells interferes with the Kv formation and subsequent LR asymmetry.

A series of studies done my postdoc Shih-Lei Lai, demonstrated that a lysophophatidic acid (LPA) synthesizing enzyme, autotoxin, is localized in the dorsal shield region, which includes Kv precursor cells. Blocking autotoxin enzyme or gene activities results in defects of Kv formation, asymmetric gene expression, tissue and organ development (Fig. 3). The heart defects are most obvious. We observed that the LR position of heart of heart tube is randomized. It results in malformation of heart, slowdown of heart beats and circulation. In some severe cases, heart beats and circulation are completely absent. Furthermore, we also found that blocking of LPA receptor 3 exhibits similar defects like inhibiting autotoxin! These results suggest that LPA signaling is a key regulator of Kv formation and LR asymmetry. This is an entirely novel discovery. LPA signaling is known for its roles in tumorgenesis and neurogenesis. Its role in early embryonic development is poorly understood. We previously demonstrated LPA signaling is required for the formation of lymphatic vessels. Here we further show that it is essential for determining LR asymmetry and subsequent development.

To enhance the research in biology and biomedical field in our school we have recently built a state-of-the-art zebrafish facility with the supports from the College of Life Science and the Center for Systems Biology (Fig. 4). The facility will serve all researchers who would like to use zebrafish as a model. Please contact Dr. Shyh-Jye Lee (jefflee@ntu.edu.tw) if you have an interest to use this facility.

References:
Lai, S.L., Yao, W.L., Tsao, K.C., Houben, A.J., Albers, H.M., Ovaa, H., Moolenaar, W.H., Lee, S.J., 2012. Autotaxin/Lpar3 signaling regulates Kupffer's vesicle formation and left-right asymmetry in zebrafish. Development 139, 4439-4448. (One of our movies was selected as the only featured movie in the issue.)
Lee, S.J., Chan, T.H., Chen, T.C., Liao, B.K., Hwang, P.P., Lee, H., 2008. LPA1 is essential for lymphatic vessel development in zebrafish. FASEB J 22, 3706-3715.