Gpr126 domains control different cellular mechanisms of ventricular chamber development
Authors: Srivastava S , Gunanwan F , Guenther S , Ferrazzi F , Gentile A , Monk K M , Stainier D Y R , Engel F B 
Published: October 2021

Abstract

Introduction
Trabeculation is a crucial process during ventricular chamber development which describes the protrusion of cardiomyocytes into the lumen of the ventricular chamber to form complex muscular structures called trabeculae. Defects in this process results in various human diseases such as left ventricular non compaction cardiomyopathies and other congenital heart defects. Several cellular mechanisms have been identified underlying trabeculation including tension heterogeneity induced cardiomyocyte selection, depolarization and delamination. However, the molecular mechanisms governing trabeculation are still poorly understood.

Purpose
Previously, we have shown that Gpr126 is required for trabeculation and heart development in mice and zebrafish. Gpr126 is an adhesion G-protein coupled receptor which is autoproteolytically cleaved into an N-terminal fragment (NTF) and a C-terminal fragment (CTF). Here, we show that NTF and CTF control different cellular processes during trabeculation.

Methods and results
In-vivo confocal images of hearts of CTF-depleted mutants gpr126st49 (expressing NTF) revealed a multilayered ventricular wall lacking any trabecular projections, which is in contrast to our previous results obtained with morpholinos suggesting that the NTF is sufficient for proper heart development in zebrafish. A molecular characterization of gpr126st49 mutants showed that cardiomyocytes in the multilayer fail to depolarize and relocalize N-cadherin from the lateral to the basal side, indicating that the cardiomyocytes in the multi-layered wall fail to attain a trabecular identity. In addition, these mutants showed significantly upregulated myocardial notch expression, which is known to prevent cardiomyocytes from attaining a trabecular identity. These data suggest that CTF is required for proper formation of trabeculae. We analyzed the full length-depleted mutant gpr126stl47 for trabeculation defects and observed that 17% of gpr126stl47 maternal zygotic mutants exhibited complete absence of trabeculation and 27% hypotrabeculation. Analysis of these mutants revealed that instead of being specifically localized at the junctions, N-cadherin was mainly distributed to the apical and basal side in the compact layer cardiomyocytes. This indicates that the NTF is required for maintaining the cell-cell adhesion in the compact wall. Finally, overexpression of gpr126 in the absence of Erbb2 signaling and blood flow / -or contractility failed to cause multilayering suggesting that Gpr126 is part of the well-established Erbb2 signaling cascade controlling trabeculation.

Conclusion
Collectively, our data support a model with domain-specific functions of Gpr126 in ventricular chamber development, where the NTF of Gpr126 is required for maintaining the compact wall integrity at the onset of trabeculation by maintaining cell-cell junctions, while the CTF helps in providing trabecular identity to cardiomyocytes through modulation of myocardial notch activity.

Funding Acknowledgement
Type of funding sources: Public grant(s) – EU funding. Main funding source(s): DFG