11-P004 Left–right mutant medaka identifies a candidate sensor of leftward flow

Internal organs in vertebrates show distinctive left–right asymmetry. To establish left–right patterning, creation of a leftward flow plays a critical role. However, how this flow function or how this flow is sensed is still debated, because a flow sensor itself has not been identified yet. Here we show a medaka mutant abecobe (abc), isolated by our recent ENU-mediated mutagenesis screening with medaka (Japanese killifish). abc displays a reversal of left–right polarity in organ positioning in the half of the homozygotes. The asymmetric expression of the sided genes, southpaw, lefty and charon, is randomized in abc, indicating that abc acts upstream of these early asymmetric genes. The leftward flow generated by rotating cilia in Kupffer’s vesicle (KV), the first sign of symmetry breaking, is, however, normal in abc. By positional cloning, we have identified abc as the medaka homolog of human PKD1L1, which encodes a transmembrane protein of unknown function and has homology with polycystins. Polycystin-1 is thought to act as a mechanosensor to detect fluid flow in the kidney. The expression of pkd1l1 is confined to KV. This candidate gene was also found to be mutated in the other abc allele. These results suggest that the abc gene is crucially involved in the formation of left–right asymmetry downstream of leftward flow, and may function as a sensor of the leftward flow. We believe that further analysis of Abc function will provide insights into a molecular link between leftward flow and the asymmetric gene expression.