The extracellular Ca2+‐sensing receptor branches out – a new role in lung morphogenesis

Abstract

The lung develops from a pouch of pharyngeal tissue which gives rise to the highly branched airways of the fully mature adult lung by undergoing a tightly controlled pattern of repeated budding and branching (Warburton, 2008). Moreover, throughout this developmental process, liquid is secreted into the lumen of the developing lung via a mechanism dependent upon electrogenic, transepithelial anion transport (Olver & Strang, 1974) and, since this liquid is secreted against the resistance provided by the larynx, it establishes a distending pressure (2–3 Torr) that is crucial for lung proper morphogenesis (Olver & Strang, 1974) and clinical conditions which disrupt this pressure (e.g. oligohydramnios, congenital diaphragmatic hernia) lead to lung hypoplasia and a reduction in the number of airway generations. It is therefore clear that lung morphogenesis is a complex process dependent upon a finely controlled developmental programme. Whilst the factors which control this process have yet to be fully understood, in this issue of The Journal of Physiology, Finney et al. (2008) publish work that has revealed a highly novel and unexpected aspect to this mechanism. The starting point for this new work came from the observation that the concentration of Ca2+ in fetal plasma (∼1.7 mm) is slightly greater than that measured in adult plasma (1.0–1.3 mm). Whilst this difference is relatively modest, it is large enough to cause differential activation of extracellular Ca2+ sensing receptors (CaR), G protein-coupled receptors that are known to play a central role in Ca2+ homeostasis. Furthermore, inactivating mutations in the genes encoding these receptors were already known to be associated with interstitial lung disease (Auwerx et al. 1985; Hu et al. 2004), but the mechanisms underlying this effect were completely unknown and earlier studies had failed to provide any evidence of CaR expression in adult lung tissues. These authors therefore took a new approach by exploring the possibility that these receptors may be expressed by fetal lung tissues. Whilst these imunohistochemical studies clearly revealed epithelial expression of CaR in developing mouse lung, the expression of these receptors peaked at embryonic day 12.5 (E12.5) and then declined so that no expression could be detected after E18. This result was highly significant since it implied that CaR was expressed at a time when the lung was undergoing branching morphogenesis and airway expansion, and subsequent experiments therefore explored the effects of Ca2+ upon these processes. Morphometric studies of explanted mouse lung buds showed clearly that lowering Ca2+ to adult levels (1.2 mm) stimulated airway branching suggesting that the relatively high levels of Ca2+ found in fetal plasma could exert inhibitory control over this process. Interestingly, this inhibitory effect of high Ca2+ was mimicked by a calcimimetic drug (R-568) that is known to activate the CaR, whilst pharmacological inhibition of downstream CaR effectors (PLC and PI3K) blocked the suppressive effect of high Ca2+. Interestingly, these studies also showed that lung explants which had been incubated in high Ca2+ became more distended than did tissues maintained at 1.2 mm Ca2+, and this suggested that exposure to fetal Ca2+ levels might also stimulate the secretion of fluid into the developing airways, and parallel electrophysiological measurements provided further evidence of this by showing that exposure to fetal Ca2+ also caused a clear hyperpolarization of the transepithelial potential difference. Fetal mouse lung tissues thus appear to express a functional CaR that allows the relatively high levels of Ca2+ found in fetal plasma to exert inhibitory control over lung branching morphogenesis and to stimulate lung expansion by increasing the secretion of fluid into the lumen of the developing lungs. Whilst it is now well documented that CaR plays a central role in Ca2+ homeostasis, these new data establish an entirely novel role for these receptors in the control of lung morphogenesis. These new observations therefore provide a possible physiological basis for the previously reported association between CaR mutation and lung disease (Auwerx et al. 1985; Hu et al. 2004) and also raise the possibility of treating hypoplastic/hyperplastic lung disease using drugs which modify the CaR function.