MEF2C is required for the normal allocation of cells between the ventricular and sinoatrial precursors of the primary heart field.

Targeted deletion of the mef2c gene results in a small left ventricle and complete loss of the right ventricle (Lin et al. [1997] Science 276:1404-1407). Absence of the right ventricle is from defective differentiation of cells from the secondary heart field. Our studies of the dysmorphogenesis of the left ventricle uncovered morphological and transcriptional abnormalities at the transition from the cardiac crescent to the linear-tube stage heart. Use of the cgata6LacZ transgene demonstrated that lacZ-positive cells, which normally mark the precursors to the atrioventricular canal and adjacent regions of the left ventricle and atria, remain in the sinoatrial region of the mutant. This, along with the absence of a morphologically distinct atrioventricular canal, indicates a misapportioning of cells between the inflow and outflow segments. The underlying genetic program was also affected with altered expression of mlc2a, mlc2v, and irx4 in outflow segment precursors of the primary heart field. In addition, the sinoatrial-enriched transcription factor, tbx5, was ectopically expressed in the primitive ventricle and ventricle-specific splicing of mef2b was lost, suggesting that the mutant ventricle had acquired atrial-specific characteristics. Collectively, these results suggest a fundamental role of MEF2C in ventricular cardiomyocyte differentiation and apportioning of cells between inflow and outflow precursors in the primary heart field.

G Protein-coupled receptor kinase 2 regulator of G protein signaling homology domain binds to both metabotropic glutamate receptor 1a and Galphaq to attenuate signaling.

Heterotrimeric guanine nucleotide-binding (G) protein-coupled receptor kinases (GRKs) are cytosolic proteins that contribute to the adaptation of G protein-coupled receptor signaling. The canonical model for GRK-dependent receptor desensitization involves GRK-mediated receptor phosphorylation to promote the binding of arrestin proteins that sterically block receptor coupling to G proteins. However, GRK-mediated desensitization, in the absence of phosphorylation and arrestin binding, has been reported for metabotropic glutamate receptor 1 (mGluR1) and gamma-aminobutyric acid B receptors. Here we show that GRK2 mutants impaired in Galphaq/11 binding (R106A, D110A, and M114A), bind effectively to mGluR1a, but do not mediate mGluR1a adaptation. Galphaq/11 is immunoprecipitated as a complex with mGluR1a in the absence of agonist, and either agonist treatment or GRK2 overexpression promotes the dissociation of the receptor/Galphaq/11 complex. However, these mGluR1a/Galphaq/11 interactions are not antagonized by the overexpression of either GRK2 mutants defective in Galphaq/11 binding or RGS4. We have also identified a GRK2-D527A mutant that binds Galphaq/11 in an AlF4(-)-dependent manner but is unable to either bind mGluR1a or attenuate mGluR1a signaling. We conclude that the mechanism underlying GRK2 phosphorylation-independent attenuation of mGluR1a signaling is RH domain-dependent, requiring the binding of GRK2 to both Galphaq/11 and mGluR1a. This serves to coordinate GRK2 interactions with Galphaq/11 and to disrupt receptor/Galphaq/11 complexes. Our findings indicate that GRK2 regulates receptor/G protein interactions, in addition to its traditional role as a receptor kinase.