New role for endothelin signalling during development.

Endothelin-1 (ET-1) is a 21-amino acid peptide produced by the endothelium and other cell types with a variety of physiological functions. Its original description dates back to 1980s, and it is distinguished by a very potent vasoconstrictor activity (Yanagisawa et al. 1988, Braunwald et al. 2001). Two other isoforms of endothelin have been discovered (ET-2 and ET-3), but endothelium produces only ET-1. Synthesis of ET-1 is complex, starting with a large precursor molecule, pre-proendothelin, which is processed to ‘big endothelin’ and finally converted by the action of endothelin-converting enzyme (ECE) to the fully active ET-1. In vascular tissues, ET-1 is synthesized in endothelium and secreted on the abluminal side of the vascular wall, acting as an autocrine or paracrine substance. In the heart, ET-1 is also synthesized by cardiac interstitial cells. Although active ET is present in circulation and its levels are increased in patients with heart failure, the half-life of ET is only a few minutes because of rapid clearance by the lung; therefore, ET is an autocrine or paracrine cytokine, not a hormone. ET-mediated vasoconstriction has been implicated in several human cardiovascular disorders, such as pulmonary hypertension or congestive heart failure (Braunwald et al. 2001). At the cellular level, it interacts with other substances influencing vascular tonus, most notably the nitric oxide, also produced by the endothelial cells. Its effects are mediated by two kinds of receptors, ETA and ETB. ETA stimulation leads to proliferation of smooth muscle cells in the vessel wall and their contraction; ETB seems to be involved in contraction of the pulmonary vasculature. Inhibitors of this signalling pathway, such as bosentan or tezosentan, have emerged as a new class of drugs for the treatment of hypertension and heart failure in adult patients. Endothelin signalling mediated via ETA receptor is essential for normal embryonic patterning in both avians (Kempf et al. 1998) and mammals (Kurihara et al. 1994, 1995, 1997, Yanagisawa et al. 1998a). In both classes of vertebrates, targeting of this pathway leads to perturbation of craniofacial neural crest derivatives and also cardiovascular defects. Ventricular septal defects were noted in 48% of ET-1-null mice (Kurihara et al. 1995), and this frequency increased up to 90% with an additional ETA antagonist treatment. Cardiovascular defects in this latter subgroup included reductions in trabeculation and loss of muscle and fibrous tissues at the crest of the interventricular septum. Similar basal/crestal defects to the interventricular septum have been noted in double knockouts in mice of genes encoding ECE-1 and ECE2, the enzymes responsible for the activation of bigET (Yanagisawa et al. 1998b). While these reports pointed to a primary myocardial defect in response to loss of ET-1 signalling function, no morphological or functional analyses were undertaken specifically on conduction tissues. Exogenous ET activity can induce conduction cell differentiation in the chick embryonic heart in vitro and in vivo (Gourdie et al. 1998, Takebayashi-Suzuki et al. 2000, Gourdie & Watanabe 2004). Mechanoinduced upregulation in ET signalling has been correlated with precocious conduction system development in vivo (Reckova et al. 2003, Hall et al. 2004). However, it remains unclear whether ET signalling has an influence on the development or function of the mammalian conduction system, although there are some in vitro data supporting the notion that this might be the case (Gassanov et al. 2004). ET signalling appears to be developmentally regulated and is critically dependent on the presence of functional ET receptors (Gourdie et al. 1998). This seems to serve to restrict its effects to precisely defined windows for the induction of conduction system differentiation. Similarly, ETA-mediated vasoconstriction is not present during the early postnatal period in mice (Kuwaki et al. 2002), while this pathway is of considerable importance in the adult. Present study by Karppinen et al. (2013) explores further the function of ET receptors in control of heart rate during development. They build upon earlier observations linking the ET-1 signalling via inositol triphosphate pathway on calcium oscillations and pacemaking and take on to investigate these effects in vitro and in vivo during mouse development. Their main finding is that ET-1 serves as a stabilizer of heart rhythm via control of calcium leak through the inositol triphosphate receptors, which occurs through ETB, rather than ETA. This is significant especially during the early stages of development, where If alone is not sufficient for maintaining rhythmic membrane voltage oscillations. The authors nicely correlate these findings with quantitative measurements of both ETA