Fetal calcium regulates branching morphogenesis in the developing human and mouse lung: involvement of voltage-gated calcium channels

Airway branching morphogenesis in utero is essential for optimal postnatal lung function. In the fetus, branching morphogenesis occurs during the pseudoglandular stage (weeks 9–17 of human gestation, embryonic days (E)11.5–16.5 in mouse) in a hypercalcaemic environment (∼1.7 in the fetus vs. ∼1.1–1.3 mM for an adult). Previously we have shown that fetal hypercalcemia exerts an inhibitory brake on branching morphogenesis via the calcium-sensing receptor. In addition, earlier studies have shown that nifedipine, a selective blocker of L-type voltage-gated Ca²⁺ channels (VGCC), inhibits fetal lung growth, suggesting a role for VGCC in lung development. The aim of this work was to investigate the expression of VGCC in the pseudoglandular human and mouse lung, and their role in branching morphogenesis. Expression of L-type (Ca_V1.2 and Ca_V1.3), P/Q type (Ca_V2.1), N-type (Ca_V2.2), R-type (Ca_V2.3), and T-type (Ca_V3.2 and Ca_V3.3) VGCC was investigated in paraffin sections from week 9 human fetal lungs and E12.5 mouse embryos. Here we show, for the first time, that Ca_v1.2 and Ca_v1.3 are expressed in both the smooth muscle and epithelium of the developing human and mouse lung. Additionally, Ca_v2.3 was expressed in the lung epithelium of both species. Incubating E12.5 mouse lung rudiments in the presence of nifedipine doubled the amount of branching, an effect which was partly mimicked by the Ca_v2.3 inhibitor, SNX-482. Direct measurements of changes in epithelial cell membrane potential, using the voltage-sensitive fluorescent dye DiSBAC₂(3), demonstrated that cyclic depolarisations occur within the developing epithelium and coincide with rhythmic occlusions of the lumen, driven by the naturally occurring airway peristalsis. We conclude that VGCC are expressed and functional in the fetal human and mouse lung, where they play a role in branching morphogenesis. Furthermore, rhythmic epithelial depolarisations evoked by airway peristalsis would allow for branching to match growth and distension within the developing lung.

Combined in vivo depletion of glycoprotein VI and C-type lectin-like receptor 2 severely compromises hemostasis and abrogates arterial thrombosis in mice

OBJECTIVE

Platelet inhibition is a major strategy to prevent acute ischemic cardiovascular and cerebrovascular events, which may, however, be associated with an increased bleeding risk. The (hem)immunoreceptor tyrosine activation motif-bearing platelet receptors, glycoprotein VI (GPVI) and C-type lectin-like receptor 2 (CLEC-2), might be promising antithrombotic targets because they can be depleted from circulating platelets by antibody treatment, leading to sustained antithrombotic protection, but only moderately increased bleeding times in mice.

APPROACH AND RESULTS

We investigated whether both (hem)immunoreceptor tyrosine activation motif-bearing receptors can be targeted simultaneously and what the in vivo consequences of such a combined therapeutic GPVI/CLEC-2 deficiency are. We demonstrate that isolated targeting of either GPVI or CLEC-2 in vivo does not affect expression or function of the respective other receptor. Moreover, simultaneous treatment with both antibodies resulted in the sustained loss of both GPVI and CLEC-2, while leaving other activation pathways intact. However, GPVI/CLEC-2-depleted mice displayed a dramatic hemostatic defect and profound impairment of arterial thrombus formation. Furthermore, a strongly diminished hemostatic response could also be reproduced in mice genetically lacking GPVI and CLEC-2.

CONCLUSIONS

These results demonstrate that GPVI and CLEC-2 can be simultaneously downregulated in platelets in vivo and reveal an unexpected functional redundancy of the 2 receptors in hemostasis and thrombosis. These findings may have important implications of the potential use of anti-GPVI and anti-CLEC-2-based agents in the prevention of thrombotic diseases.

CLEC-2 and Syk in the megakaryocytic/platelet lineage are essential for development

The C-type lectin receptor CLEC-2 signals through a pathway that is critically dependent on the tyrosine kinase Syk. We show that homozygous loss of either protein results in defects in brain vascular and lymphatic development, lung inflation, and perinatal lethality. Furthermore, we find that conditional deletion of Syk in the hematopoietic lineage, or conditional deletion of CLEC-2 or Syk in the megakaryocyte/platelet lineage, also causes defects in brain vascular and lymphatic development, although the mice are viable. In contrast, conditional deletion of Syk in other hematopoietic lineages had no effect on viability or brain vasculature and lymphatic development. We show that platelets, but not platelet releasate, modulate the migration and intercellular adhesion of lymphatic endothelial cells through a pathway that depends on CLEC-2 and Syk. These studies found that megakaryocyte/platelet expression of CLEC-2 and Syk is required for normal brain vasculature and lymphatic development and that platelet CLEC-2 and Syk directly modulate lymphatic endothelial cell behavior in vitro.

Novel regulatory aspects of the extracellular Ca2+-sensing receptor, CaR

The capacity to sense and adapt to changes in environmental cues is of paramount importance for every living organism. From yeast to man, cells must be able to match cellular activities to growth environment and nutrient availability. Key to this process is the development of membrane-bound systems that can detect modifications in the extracellular environment and to translate these into biological responses. Evidence gathered over the last 15 years has demonstrated that many of these cell surface "sensors" belong to the G protein-coupled receptor superfamily. Crucial to our understanding of nutrient sensing in mammalian species has been the identification of the extracellular Ca(2+)/cation-sensing receptor, CaR. CaR was the first ion-sensing molecule identified in man and genetic studies in humans have revealed the importance of the CaR in mineral ion metabolism. Latter, it has become apparent that the CaR also plays an important role outside the Ca(2+) homeostatic system, as an integrator of multiple environmental signals for the regulation of many vital cellular processes, from cell-to-cell communication to secretion and cell survival/cell death. Recently, novel aspects of receptor function reveal an unexpected role for the CaR in the regulation of growth and development in utero.

Regulation of mouse lung development by the extracellular calcium-sensing receptor, CaR

Postnatal lung function is critically dependent upon optimal embryonic lung development. As the free ionized plasma calcium concentration ([Ca(2+)](o)) of the fetus is higher than that of the adult, the process of lung development occurs in a hypercalcaemic environment. In the adult, [Ca(2+)](o) is monitored by the G-protein coupled, extracellular calcium-sensing receptor (CaR), but neither its ontogeny nor its potential role in lung development are known. Here, we demonstrate that CaR is expressed in the mouse lung epithelium, and that its expression is developmentally regulated, with a peak of expression at embryonic day 12.5 (E12.5) and a subsequent decrease by E18, after which the receptor is absent. Experiments carried out using the lung explant culture model in vitro show that lung branching morphogenesis is sensitive to [Ca(2+)](o), being maximal at physiological adult [Ca(2+)](o) (i.e. 1.0-1.3 mM) and lowest at the higher, fetal (i.e. 1.7 mM) [Ca(2+)](o). Administration of the specific CaR positive allosteric modulator, the calcimimetic R-568, mimics the suppressive effects of high [Ca(2+)](o) on branching morphogenesis while both phospholipase C and PI3 kinase inhibition reverse these effects. CaR activation suppresses cell proliferation while it enhances intracellular calcium signalling, lung distension and fluid secretion. Conditions which are restrictive either to branching or to secretion can be rescued by manipulating [Ca(2+)](o) in the culture medium. In conclusion, fetal Ca(2+)(o), acting through a developmentally regulated CaR, is an important extrinsic factor that modulates the intrinsic lung developmental programme. Our observations support a novel role for the CaR in preventing hyperplastic lung disease in utero.