Calcineurin signaling in avian cardiovascular development

What chick and mouse models have taught us about the role of the endocardium in congenital heart disease

Specific cell and tissue interactions drive the formation and function of the vertebrate cardiovascular system. Although much attention has been focused on the muscular components of the developing heart, the endocardium plays a key role in the formation of a functioning heart. Endocardial cells exhibit heterogeneity that allows them to participate in events such as the formation of the valves, septation of the outflow tract, and trabeculation. Here we review, the contributions of the endocardium to cardiovascular development and outline useful approaches developed in the chick and mouse that have revealed endocardial cell heterogeneity, the signaling molecules that direct endocardial cell behavior, and how these insights have contributed to our understanding of cardiovascular development and disease.

VEGF and RANKL regulation of NFATc1 in heart valve development

RATIONALE

NFATc1 (nuclear factor of activated T-cells cytoplasmic 1) activity in endocardial cushion (ECC) endothelial cells is required for normal ECC growth and extracellular matrix (ECM) remodeling during heart valve development.

OBJECTIVE

The mechanisms of NFATc1 activation and downstream effects on cell proliferation and ECM-remodeling enzyme gene expression were examined in NFATc1 mutant mice and chick ECC explants.

METHODS AND RESULTS

NFATc1(-/-) mice display reduced proliferation of ECC endothelial and mesenchymal cells at embryonic day 10.5, whereas myocardial cells are unaffected. Vascular endothelial growth factor A (VEGF) activates NFATc1 and promotes ECC cell proliferation via the regulatory phosphatase, calcineurin, and mitogen-activated protein kinase-extracellular signal-regulated kinase 1-extracellular signal-regulated kinase 1/2 (MEK1-ERK1/2)-dependent signaling. As ECCs mature, RANKL (receptor activator of nuclear factor kappaB ligand) and the ECM-remodeling enzyme cathepsin K (CtsK) are expressed by ECC endothelial cells. RANKL inhibits VEGF-induced cell proliferation while causing increased expression of CtsK via calcineurin/NFATc1 and c-Jun N-terminal kinase (JNK)1/2-dependent signaling.

CONCLUSION

These data support a novel mechanism for the transition from ECC growth to remodeling in which NFATc1 promotes a sequential pattern of gene expression via cooperation with ligand-specific cofactors such as MEK1-ERK1/2 or JNK1/2.

Calreticulin signals upstream of calcineurin and MEF2C in a critical Ca(2+)-dependent signaling cascade

We uncovered a new pathway of interplay between calreticulin and myocyte-enhancer factor (MEF) 2C, a cardiac-specific transcription factor. We establish that calreticulin works upstream of calcineurin and MEF2C in a Ca²⁺-dependent signal transduction cascade that links the endoplasmic reticulum and the nucleus during cardiac development. In the absence of calreticulin, translocation of MEF2C to the nucleus is compromised. This defect is reversed by calreticulin itself or by a constitutively active form of calcineurin. Furthermore, we show that expression of the calreticulin gene itself is regulated by MEF2C in vitro and in vivo and that, in turn, increased expression of calreticulin affects MEF2C transcriptional activity. The present findings provide a clear molecular explanation for the embryonic lethality observed in calreticulin-deficient mice and emphasize the importance of calreticulin in the early stages of cardiac development. Our study illustrates the existence of a positive feedback mechanism that ensures an adequate supply of releasable Ca²⁺ is maintained within the cell for activation of calcineurin and, subsequently, for proper functioning of MEF2C.

Regulation of calcineurin through transcriptional induction of the calcineurin A beta promoter in vitro and in vivo

The calcineurin-nuclear factor of activated T cells (NFAT) signaling pathway has been shown to be of critical importance in regulating the growth response of cardiac myocytes. We have previously demonstrated that calcineurin A(beta) (CnA(beta)) mRNA and protein are increased in response to growth stimulation, although the precise regulatory mechanism underlying CnA(beta) upregulation is not clear. Here, we isolated the mouse CnA(beta) promoter and characterized its responsiveness to growth stimuli in vitro and in vivo. A 2.3-kb promoter fragment was strongly activated by phenylephrine and endothelin-1 stimulation and by cotransfection with constitutively active CnA, NFATc4, and GATA4. Using chromatin immunoprecipitation, sequence regions were identified within the 2.3-kb promoter that associated with NFAT and GATA4, as well as with acetylated histone H3, following agonist stimulation. Consistent with the chromatin immunoprecipitation experiments, deletion of the distal half of the CnA(beta) promoter severely reduced NFAT, GATA4, and hypertrophic agonist-mediated activation. To investigate in vivo activity, we generated beta-galactosidase (LacZ) containing transgenic mice under the control of the CnA(beta) 2.3-kb promoter. CnA(beta)-LacZ mice showed expression in the heart that was cyclosporine sensitive, as well as expression in the central nervous system and skeletal muscle from early embryonic stages through adulthood. CnA(beta)-LacZ mice were subjected to cardiac pressure overload stimulation and crossbreeding with mice containing cardiac-specific transgenes for activated calcineurin and NFATc4, which revealed inducible expression in the heart. These results indicate that the CnA(beta) 2.3-kb promoter is specifically activated by hypertrophic stimuli through a positive feedback mechanism involving NFAT and GATA4 transcription factors, suggesting transcriptional induction of CnA(beta) expression as an additional means of regulating calcineurin activity in the heart.

Mechanics and function in heart morphogenesis

For years, biomechanical engineers have studied the physical forces involved in morphogenesis of the heart. In a parallel stream of research, molecular and developmental biologists have sought to identify the molecular pathways responsible for embryonic heart development. Recently, several studies have shown that these two avenues of research should be integrated to explain how genes expressed in the heart regulate early heart function and affect physical morphogenetic steps, as well as to conversely show how early heart function affects the expression of genes required for morphogenesis. This review combines the perspectives of biomechanical engineering and developmental biology to lay out an integrated view of the role of mechanical forces in heart development.

Calcineurin signaling and NFAT activation in cardiovascular and skeletal muscle development

Calcineurin signaling has been implicated in a broad spectrum of developmental processes in a variety of organ systems. Calcineurin is a calmodulin-dependent, calcium-activated protein phosphatase composed of catalytic and regulatory subunits. The serine/threonine-specific phosphatase functions within a signal transduction pathway that regulates gene expression and biological responses in many developmentally important cell types. Calcineurin signaling was first defined in T lymphocytes as a regulator of nuclear factor of activated T cells (NFAT) transcription factor nuclear translocation and activation. Recent studies have demonstrated the vital nature of calcium/calcineurin/NFAT signaling in cardiovascular and skeletal muscle development in vertebrates. Inhibition, mutation, or forced expression of calcineurin pathway genes result in defects or alterations in cardiomyocyte maturation, heart valve formation, vascular development, skeletal muscle differentiation and fiber-type switching, and cardiac and skeletal muscle hypertrophy. Conserved calcineurin genes are found in invertebrates such as Drosophila and Caenorhabditis elegans, and genetic studies have demonstrated specific myogenic functions for the phosphatase in their development. The ability to investigate calcineurin signaling pathways in vertebrates and model genetic organisms provides a great potential to more fully comprehend the functions of calcineurin and its interacting genes in heart, blood vessel, and muscle development.