An Early Function of Polycystin-2 for Left-Right Organizer Induction in Xenopus

Abstract

SummaryNodal signaling controls asymmetric organ placement during vertebrate embryogenesis. Nodal is induced by a leftward fluid flow at the ciliated left-right organizer (LRO). The mechanism of flow sensing, however, has remained elusive. pkd2 encodes the calcium channel Polycystin-2, which is required for kidney development and laterality, and may act in flow perception. Here, we have studied the role of Polycystin-2 in Xenopus and show that pkd2 is indispensable for left-right (LR) asymmetry. Knockdown of pkd2 prevented left-asymmetric nodal cascade induction in the lateral plate mesoderm. Defects were due to failure of LRO specification, morphogenesis, and, consequently, absence of leftward flow. Polycystin-2 synergizes with the unconventional nodal-type signaling molecule Xnr3 to induce the LRO precursor tissue before gastrulation, upstream of symmetry breakage. Our data uncover an unknown function of pkd2 in LR axis formation, which we propose represents an ancient role of Polycystin-2 during LRO induction in lower vertebrates.

A dual function of FGF signaling inXenopusleft-right axis formation

“…This reasoning was further supported by our recent demonstration that knockdown of the Fgfr1 ligand nodal3.1 (nodal homolog 3, gene 1; previously known as xnr3) in the SM inhibited foxj1 induction (Vick et al, 2018;Yokota et al, 2003). In agreement with this notion, embryos treated with SU5402 at early gastrula (stage 10.5) showed a significant reduction of foxj1 in the SM, confirming a role of Nodal3.1/FGF in setting-up a functional GRP ( Fig.…”

“…Like FGF, pkd2 is required in a dual way during symmetry breakage in Xenopus (Vick et al, 2018). In a first stage, it cooperates with the FGF ligand nodal3.1 to induce foxj1 in the SM.…”

“…Later, it is required for somitic GRP cells and initiation of nodal1/dand5 (Vick et al, 2018). As pkd2 encodes the calcium channel Polycystin-2 (Busch et al, 2017;Koulen et al, 2002), we reasoned that it could interact synergistically with the FGF/Ca 2+ -pathway.…”

Bicc1anddicerregulate left-right patterning through post-transcriptional control of the Nodal-inhibitordand5

“…Upon co-injection of full-length pkd2 mRNA, reporter activity was further repressed to less than 20% ( Figure 5C). Because pkd2 is maternally expressed in animal tissue, like dand5 45 , we tested this interaction further by coinjecting pkd2 MO, the specificity of which we showed previously 27, 45 . Loss of pkd2 partially rescued bicc1-mediated repression of the Luciferase dand5 reporter ( Figure 5C), establishing a firm link between bicc1, pkd2 and post-transcriptional regulation of dand5.…”

“…Finally, we wondered whether pkd2, one of two published active components in the flow sensor 16,44 , acted in the same pathway as well. Our recent demonstration of an earlier (likely maternal) Pkd2 function in the specification and morphogenesis of the LRO prevented us from investigating this question in the context of LR axis formation in the embryo itself 45 . In zebrafish, however, zygotic pkd2 mutant embryos have strongly delayed induction of Nodal, but show normal KV ciliation and morphology 46,47 , suggesting a role for TRPP2 in flow sensing.…”