Trpc1 as the Missing Link Between the Bmp and Ca2+ Signalling Pathways During Neural Specification in Amphibians

Abstract

In amphibians, the inhibition of bone morphogenetic protein (BMP) in the dorsal ectoderm has been proposed to be responsible for the first step of neural specification, called neural induction. We previously demonstrated that in Xenopus laevis embryos, the BMP signalling antagonist, noggin, triggers an influx of Ca2+ through voltage-dependent L-type Ca2+ channels (LTCCs), mainly via CaV1.2, and we showed that this influx constitutes a necessary and sufficient signal for triggering the expression of neural genes. However, the mechanism linking the inhibition of BMP signalling with the activation of LTCCs remained unknown. Here, we demonstrate that the transient receptor potential canonical subfamily member 1, (Trpc1), is an intermediate between BMP receptor type II (BMPRII) and the CaV1.2 channel. We show that noggin induces a physical interaction between BMPRII and Trpc1 channels. This interaction leads to the activation of Trpc1 channels and to an influx of cations, which depolarizes the plasma membrane up to a threshold sufficient to activate Cav1.2. Together, our results demonstrate for the first time that during neural induction, Ca2+ entry through the CaV1.2 channel results from the noggin-induced interaction between Trpc1 and BMPRII.

The Organizer and Its Signaling in Embryonic Development

“…In recent years, many important reports have demonstrated that Ca 2+ is essential for involuting/migrating mesodermal cells [ 71 , 72 ]. However, several reports suggest that Ca 2+ signaling interacts with other pathways and plays an inductive role in neural development in vertebrates; for details, readers may wish to consult the cited articles [ 71 , 73 , 74 , 75 , 76 ].…”

“…Similarly, Nog induces the interaction between Trpc1 (transient receptor potential canonical subfamily member 1, a calcium channel) and BMPRII (BMP receptor type II); this complex allows for the activation of several Ca 2+ channels and cations influx. Increased amounts of intracellular Ca 2+ activate Ca 2+ -mediated signaling pathways and activate neural genes’ expression [ 73 , 76 ]. However, Wnt antagonists are also important to maintain low Wnt activity in the anterior region, inhibit BMP transcription, and establish an endogenous Wnt/β-catenin signaling morphogen gradient along the neural axis, critical for neural development [ 11 ].…”

Transmembrane H+fluxes and the regulation of neural induction inXenopus laevis

“…It has previously been reported from X. laevis embryos that localized, intracellular Ca 2þ transients triggered by planar signals can induce the expression of the Zic3 gene in the dorsal ectoderm during neural induction (Leclerc et al, 2003;Batut et al, 2005;Néant et al, 2019). We propose that an increase in pH i might be associated in some required/synergistic way with these Ca 2þ transients to induce the expression of important early primary genes known to regulate the subsequent expression of other essential neural genes (Nakata et al, 1997;Winata et al, 2013).…”

“…It is not possible at this time to determine if bafilomycin A1 had a direct effect on the signal transduction pathway leading to the expression of Zic3, or if the effect was secondary; for example, resulting from a cessation of the morphogenic movements that contributed to gastrulation. However, it is becoming clear that neural induction is regulated via a complex interplay between cell-to-cell signalling, inductive interactions, morphogenetic movements, and localized gene expression (Keller et al, 1992;Nakata et al, 1997;Muñoz-Sanjuán et al, 2002;Leclerc et al, 2012;Stern, 2006;Néant et al, 2019). It will, therefore, require careful and extended experimentation to decipher exactly where and when the H þ fluxes occur (perhaps at various times and in different intracellular and/or embryonic locations) during the extended process of neural induction and subsequent neurogenesis.…”

“…We suggest that this might, therefore, provide a link between Ca 2þ and H þ being involved in the synergistic regulation of Zic3 expression and subsequent neural induction in the dorsal ectoderm of X. laevis embryos. This adds another layer of complexity to the interaction between the multiple ectodermal and endomesodermal signals (including Chordin, Noggin, BMP4, Wnts, β-catenin, Cerberus, various Ca 2þ channels and H þ ) and gene expression, which combine to regulate the complex process of neural induction in X. laevis (Sater et al, 1994;Baker et al, 1999;Kuroda et al, 2004;Stern, 2006;Moreau et al, 2008Moreau et al, , 2009Moreau et al, , 2020Cho et al, 2014;Néant et al, 2019). Our new data therefore add to the accumulating evidence that suggests that intracellular alkalinization might contribute to establishing anterior neural fate in X. laevis embryos.…”