Gpr126 controls distinct cellular mechanisms of heart trabeculation in a domain-specific manner.

Gpr126 is an adhesion G-protein coupled receptor which is autoproteolytically cleaved into an N-terminal fragment (NTF) and a C-terminal fragment (CTF). Previously, we have shown that Gpr126 is required for trabeculation and heart development in mice and zebrafish. Cardiac trabeculation is a crucial process during ventricular chamber development where the cardiomyocytes in the heart wall organize themselves to form two distinct layers, an outer compact wall and an inner trabecular wall. 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. Here, we perform heart specific bulk RNA sequencing of two zebrafish gpr126 mutants, full length-depleted mutant gpr126stl47 and CTF-depleted mutants gpr126st49 to identify alternative splicing isoforms of Gpr126 and if they contribute to trabeculation phenotypes observed in the mutants. Our data utilizing the differential exon usage analysis of the two mutants showed that the mutants do not exhibit modified exon usage. This study along with the full body RNA sequencing analysis of the two mutants suggested that alternative splicing in the heart is not compensating for the predicted non-functional transcripts of the mutants. Utilizing this data, we propose 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.