Grk5l Controls Heart Development by Limiting mTOR Signaling during Symmetry Breaking

The correct asymmetric placement of inner organs is termed situs solitus and is determined early during development. Failure in symmetry breaking results in conditions ranging from randomized organ arrangement to a complete mirror image, often accompanied by severe congenital heart defects (CHDs). We found that the zebrafish homolog of mammalian G protein-coupled receptor kinase 5 (GRK5) employs noncanonical, receptor-independent functions to secure symmetry breaking. Knockdown of GRK5's closest homolog in zebrafish embryos, Grk5l, is sufficient to randomize cardiac looping and left-right asymmetry. Mechanistically, we found that loss of GRK5 increases mammalian target of rapamycin complex 1 (mTORC1) activity. This causes elongation of motile cilia in the organ of laterality, a consequence that is known to be sufficient to trigger aberrant organ arrangement. By fine-tuning mTORC1, GRK5 thus serves an unanticipated function during early development, besides its well-characterized role in the adult heart. These findings could implicate GRK5 as a susceptibility allele for certain cases of CHD. [Display omitted] •GRK5 guides symmetry breaking during early embryonic development•GRK5 modulates cilia length in the organ of laterality by negatively regulating mTOR•GRK5 regulates heart development G protein-coupled receptor kinase 5 (GRK5) is a key regulator of cardiac physiology. So far, however, the enzyme has been thought to act solely in the adult heart. Philipp and colleagues report that GRK5 influences the heart very early in life. Using zebrafish as a model, they found that GRK5 helps to establish left-right asymmetry by fine-tuning mammalian target of rapamycin (mTOR) and modulating cilia length. By doing so, GRK5 ensures proper heart positioning and shaping well before the heart becomes functional.