Localization and loss-of-function implicates ciliary proteins in early, cytoplasmic roles in left-right asymmetry

Left-right asymmetry is a crucial feature of the vertebrate body plan. While much molecular detail of this patterning pathway has been uncovered, the embryonic mechanisms of the initiation of asymmetry, and their evolutionary conservation among species, are still not understood. A popular recent model based on data from mouse embryos suggests extracellular movement of determinants by ciliary motion at the gastrulating node as the initial step. An alternative model, driven by findings in the frog and chick embryo, focuses instead on cytoplasmic roles of motor proteins. To begin to test the latter hypothesis, we analyzed the very early embryonic localization of ciliary targets implicated in mouse LR asymmetry. Immunohistochemistry was performed on frog and chick embryos using antibodies that have (KIF3B, Polaris, Polycystin-2, acetylated alpha-tubulin) or have not (LRD, INV, detyrosinated alpha-tubulin) been shown to detect in frog embryos only the target that they detect in mammalian tissue. Immunohistochemistry revealed localization signals for all targets in the cytoplasm of cleavage-stage Xenopus embryos, and in the base of the primitive streak in chick embryos at streak initiation. Importantly, several left-right asymmetries were detected in both species, and the localization signals were dependent on microtubule and actin cytoskeletal organization. Moreover, loss-of-function experiments implicated very early intracellular microtubule-dependent motor protein function as an obligate aspect of oriented LR asymmetry in Xenopus embryos. These data are consistent with cytoplasmic roles for motor proteins in patterning the left-right axis that do not involve ciliary motion.