Calreticulin signals upstream of calcineurin and MEF2C in a critical Ca2+-dependent signaling cascade

Abstract

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.

A role for p38α mitogen‐activated protein kinase in embryonic cardiac differentiation

“…However, we cannot exclude the possibility of the existence of two independent pathways, the p38 MAPK and the Ca 2+ -dependent pathway, which would regulate Mef2C activity and localisation. According to this, calreticulin, a Mef2C gene target, involved in the regulation of Mef2C nuclear localisation and activity [28], is not altered in p38a À/À cardiomyocytes, suggesting the presence of a correct supply of Ca 2+ within the cell [24]. Mef2C is a transcription factor expressed early (at E7.5 in mice) during heart development in vertebrates, whose genetic ablation causes morphological and transcriptional abnormalities during early cardiogenesis [4].…”

“…From the results described above, it can be hypothesized that Mef2C would be functionally inactive in p38a À/À cardiomyocytes. Thus, we analysed the expression of Mef2C gene targets ANF, myocardin, calreticulin (crt) and Mlc2v, markers of cardiac differentiation [23][24][25][26], by quantitative RT-PCR in wild-type and p38a À/À cardiomyocytes. As expected (Fig.…”

Transcription factor Mef2c is required for B cell proliferation and survival after antigen receptor stimulation

“…Mef2 factors bind DNA and regulate gene expression through interactions with other factors, including NFAT 16,17 , basic helix-loop-helix transcription factors 20,21 , GATA factors 22 and histone deacetylases 23 . Mef2c (A001503) can repress gene expression by interacting with histone deacetylases and responds to various signaling pathways to activate gene expression after calcium influx, activation of calcineurin and activation of the p38 mitogen-activated protein kinase 16,[24][25][26] . Mef2c is critical in muscle cell development, and deletion of Mef2c causes embryonic death around day 9.5 due to defects in cardiac and vascular development 27-29 . Mef2c also regulates chondrocyte, bone and craniofacial development 29,30 . In examining gene expression in many tissues and immune cells, we found unexpectedly high expression of Mef2c in mature B cells relative to that of other tissues, even skeletal muscle and heart, where Mef2c is known to function 29 .…”

Cardiomyogenesis of embryonic stem cells upon purinergic receptor activation by ADP and ATP

“…scale generation of cardiomyocytes from ES cells is still elusive [15]. Emerging evidence points toward the role of [Ca 2+ ] i as a major second messenger in directing the fate of ES cells into cardiomyocytes, patterning of the heart during early heart development and regulation of cardiac transcription cascades and myofibrillogenesis [16,17]. Intracellular [Ca +2 ] i exerts multiple functions in the process of cardiac cell differentiation and early heart development depending upon the amplitude, space, and time of [Ca 2+ ] i signaling.…”

“…During embryonic heart formation, a couple of cardiogenic factors turn on Ca 2+ -dependent signaling pathways in cardiac progenitor cells. All cardiogenic factors are integrated into transcriptional networks, among which [Ca 2+ ] i plays a principal role [16,17,22]. Purinergic receptors, also known as purinoceptors, belong to the first group of receptors which stimulate intracellular [Ca 2+ ] i signaling during the early time of embryonic development as well as cardiogenesis [23][24][25].…”