Cardiovascular changes in calcium-deficient chick embryos

Evolution of a developmental mechanism: Species-specific regulation of the cell cycle and the timing of events during craniofacial osteogenesis

Neural crest mesenchyme (NCM) controls species-specific pattern in the craniofacial skeleton but how this cell population accomplishes such a complex task remains unclear. To elucidate mechanisms through which NCM directs skeletal development and evolution, we made chimeras from quail and duck embryos, which differ markedly in their craniofacial morphology and maturation rates. We show that quail NCM, when transplanted into duck, maintains its faster timetable for development and autonomously executes molecular and cellular programs for the induction, differentiation, and mineralization of bone, including premature expression of osteogenic genes such as Runx2 and Col1a1. In contrast, the duck host systemic environment appears to be relatively permissive and supports osteogenesis independently by providing circulating minerals and a vascular network. Further experiments reveal that NCM establishes the timing of osteogenesis by regulating cell cycle progression in a stage- and species-specific manner. Altering the time-course of D-type cyclin expression mimics chimeras by accelerating expression of Runx2 and Col1a1. We also discover higher endogenous expression of Runx2 in quail coincident with their smaller craniofacial skeletons, and by prematurely over-expressing Runx2 in chick embryos we reduce the overall size of the craniofacial skeleton. Thus, our work indicates that NCM establishes species-specific size in the craniofacial skeleton by controlling cell cycle, Runx2 expression, and the timing of key events during osteogenesis.

Influence of calcium on proliferation and phenotype alteration of cardiomyocyte in vitro

An accelerated weight gain is noted in the heart of Ca-deficient, hypertensive chick embryos maintained in a shell-less culture in vitro. We previously observed that the Ca handling property of cardiomyocytes isolated from the shell-less embryo is altered, i.e., faster Ca uptake, suggesting a requirement for adequate Ca supply and/or proper Ca handling in embryonic cardiac development. In this study, we have examined the function of Ca on cardiomyocytes by analyzing the effects of 1) various Ca concentration in the culture medium (NCa, 1.8 mmol/ L; HCa, 2.8 mmol/L; LCa, 0.9 mmol/L), and 2) various modulators of Ca handling on cell proliferation and phenotype regulation in chick embryonic cardiomyocytes. The analytical parameters included cell number, DNA content, expression of cell cycle-specific and cardiomyocyte-specific proteins, and creatine phosphokinase (CPK) and lactate dehydrogenase (LDH) enzyme activities. Cell number and total DNA were significantly larger (P < 0.01) in LCa cultures compared with those in NCa. The level of LDH was elevated (P < 0.01), but that of CPK was lowered in LCa. Expression of the G1-S-specific protein PCNA was raised, but that of the contractile proteins myosin and tropomyosin was substantially suppressed in LCa; in HCa, the cells did not proliferate as well, whereas the level of contractile proteins was higher. Thapsigargin, a sarcoplasmic reticulum (SR)-specific, Ca-ATPase inhibitor, simulated the effects of LCa by enhancing cell proliferation and lowering the expression of tropomyosin. These results suggest that culturing in low Ca concentration and inhibition of SR Ca pumping enhance myocardial cell proliferation and suppress sarcomeric protein expression, perhaps by inducing cellular de-differentiation. The in vitro effects of medium Ca concentration and Ca handling modulators on cardiomyocytes also suggest that the in vivo cardiomegaly of the SL embryos is a direct result of Ca-deficiency, and that Ca is important in the phenotype regulation of cardiomyocytes.

Macromolecular selectivity of chick chorioallantoic membrane microvessels during normal angiogenesis and endothelial differentiation

Progressive angiogenesis and endothelial differentiation in the chick chorioallantoic membrane (CAM) serve to accommodate oxygen demands of the growing embryo. The present study evaluated CAM microvascular endothelial permselectivity during the most rapid phase of angiogenesis (day 10) and after initiation of endothelial cytodifferentiation (day 14). Chick embryos were incubated using established shell-less culture techniques for intravital and ultrastructural observations. Systemic microinjections of FITC-dextrans (40, 70 and 150 KDa) provided an index of endothelial permselectivity after 2.5 min and 10 min perfusions. Ultrastructural examinations of the same dextran probes served to detect small, intermittent foci within the perivascular interstitium. Although minor variations of dextran particle distributions around specific segments of the microcirculation were observed ultrastructurally, perivascular accumulation was not sufficient to elicit a detectable fluorescent signal. Thus, substantial accumulation of the graded-dextran series in the perivascular interstitium was not detected. Morphometric analyses of the precapillary, capillary, and postcapillary microvascular segments served to demonstrate a continuous endothelium which displayed cytoplasmic attenuation at day 14. Plasmalemmal vesicles were few and uniform within the microvascular units at day 10. A three-fold increase in vesicle densities characterized the precapillary endothelia at day 14. Average widths of the endothelial junctional clefts were homogeneous within the segmental microvascular endothelia at both days 10 and 14. Junctional cleft lengths were also homogeneous, except the significantly longer capillary endothelial clefts observed at day 10. These results are consistent with the concept that, despite certain differences in segmental vesicle densities and junctional cleft lengths, neovascularization of the CAM is achieved without excessive macromolecular efflux across the microvascular endothelia.

Alterations in cellular calcium handling as a result of systemic calcium deficiency in the developing chick embryo: II. Ventricular myocytes

We have previously shown that cardiovascular anomalies, such as hypertension and tachycardia, develop in Ca(2+)-deficient, shell-less (SL) chick embryos cultured ex ovo, accompanied by elevated circulating catecholamines and higher alpha-adrenergic sensitivity of cardiovascular functions. Results described in the preceding work, using erythrocytes as an experimental system, show that cellular Ca2+ handling properties are also altered as a result of long-term calcium deficiency. To examine the relevance of these findings to cells of the cardiovasculature, we have analyzed and compared the Ca2+ handling characteristics of the heart cells of SL and normal (NL) embryos. For this study, isolated and cultured ventricular myocytes of SL and NL embryos were loaded with Fura-2 via transient membrane damage with glass beads. Compared to Fura-2/AM, bead loading yielded similar values and kinetic profiles of [Ca2+]i-dependent differential fluorescence and, in addition, did not affect cell viability and beating activity. The Fura-2 loaded ventricular myocytes were washed in Ca(2+)-free buffer and then analyzed by ratiometric fluorescence (350 nm/380 nm) microscopy for kinetic changes in [Ca2+]i (R350/380 values) as a function of [Ca2+]o and adrenergic modifiers. At 0.5 and 1.0 mM [Ca2+]o, SL cells showed significantly higher [Ca2+]i, higher beating rates, and faster rate of increase in [Ca2+]i compared to NL cells. At higher [Ca2+]o (3.5 mM), there was no significant difference in [Ca2+]i and beating rate between NL and SL cells. Treatment with norepinephrine (NE; 0.01-1 microM) at 1 mM [Ca2+]o substantially increased [Ca2+]i in both NL and SL cells. In the former, the NE effect was completely inhibited by beta-blockade (1 microM propranolol). In contrast, in SL cells, NE remained effective after beta-blockade, and combined alpha-blockade (1 microM prazosin) and beta-blockade was needed to inhibit completely the NE effect. In both NL and SL cells, treatment with NE substantially increased beating rates in a similar manner. Taken together, these findings suggest that Ca2+ handling and adrenergic regulation of the heart cells are significantly altered in the SL embryos, and that these alterations may be related to the development of impaired cardiovascular functions resulting from systemic Ca2+ deficiency.

Alterations in cellular calcium handling as a result of systemic calcium deficiency in the developing chick embryo: I. Erythrocytes

Chick embryos rendered calcium (Ca) deficient by shell-less (SL) culture develop hypertension and tachycardia. Since hypocalcemia is accompanied by hypernatremia systemically but not by lower cellular Ca (Koide and Tuan, 1989), we speculate that cellular Ca handling may be altered in the SL embryo, perhaps involving Na transport. Using erythrocytes (RBC) from day-14 SL and normal (NL) embryos as the experimental cell, cellular Ca handling was studied under varying extracellular osmotic and ionic conditions by analyzing 45Ca uptake and cell volume regulation. Two agents, p-chloromercuriphenylsulfonate (PCM), and inosine/iodoacetamide (INI) were used to treat the RBCs to modify plasma membrane ion permeability and to deplete cellular ATP, respectively. Other cellular functions and activities related to Ca homeostasis, including ATP content and Ca(2+)-ATPase activity, were also analyzed. These analyses showed: (1) in NaCl, Ca uptake was similar in NL and SL cells, except after INI treatment, which resulted in slower Ca uptake by the SL cells, (2) in choline and sucrose, Ca uptake by SL RBCs was higher, (3) Ca uptake by RBCs of both embryos changed depending on the osmotic agent (Na < K < or = choline < sucrose), (4) Ca(2+)-ATPase activity was higher in SL RBC, although there was no change in the size or charge of the enzyme, and (5) in any osmotic agent, cellular Na was significantly lower, whereas cellular K was higher, in SL RBC. Based on these results, three features of RBC Ca handling were apparent: (1) Na-Ca exchange was functional and was more active in SL RBCs, (2) Ca uptake was dependent on the total ionic electrochemical gradient but not on bulk H2O movement, and (3) Ca pumping out capacity was directly correlated with Ca(2+)-ATPase activity. Elevated Ca uptake in sucrose-treated SL RBC is therefore indicative of its greater ion permeability. Taken together, these findings indicate that cellular Ca handling of the RBCs of SL chick embryos is characterized by a more active Na-Ca exchange system, greater ion permeability, and higher Ca pumping out capacity, thereby suggesting an up-regulated Ca handling function in the SL RBCs. The abnormal cellular Ca handling may be a direct result of the systemic Ca deficiency of the SL chick embryo and may be functionally related to its hypertension and tachycardia.

Effect of systemic calcium deficiency on the expression of transforming growth factor-beta in chick embryonic calvaria

The developmental process of intramembranous ossification involves bone formation directly from mesenchymal differentiation without a cartilage intermediate. We have previously observed that systemic calcium deficiency in the developing chick embryo, produced by long-term shell-less culture, results in the appearance of chondrocyte-like cells in the calvarium, a parietal bone which normally develops via intramembranous ossification. This investigation aims to analyze the mechanism underlying this calcium deficiency-related, aberrant appearance of cartilage phenotype in the chick embryonic calvarium. In view of the reported involvement of transforming growth factor beta (TGF-beta) in osteogenesis and chondrogenesis, we have examined and compared here the expression of TGF-beta in the chick embryonic calvaria of normal (in ovo development, NL), shell-less (SL), and calcium-supplemented SL (SL+Ca) embryos. TGF-beta expression was analyzed at the mRNA level by blot and in situ cDNA hybridization, and at the protein level by immunohistochemistry and immunoblotting. The results presented here indicate that: 1) TGF-beta is expressed in the chick embryonic calvarium by both periosteal cells and osteocytes, as revealed by in situ hybridization and immunohistochemistry; 2) TGF-beta expression is significantly increased in SL calvarium compared to NL calvarium, at both protein and mRNA levels; 3) the number of TGF-beta expressing cells increases in the SL calvarium, particularly along the central, subcambial core region of the bone; and 4) exogenous calcium repletion to the SL embryo affects the expression of TGF-beta such that the pattern approaches that in the NL embryo. Taken together, these results indicate that altered TGF-beta expression accompanies the aberrant appearance of cartilage phenotype caused by systemic calcium deficiency. We postulate that normal cellular differentiation along the osteogenic pathway during embryonic intramembranous ossification is crucially dependent on regulated TGF-beta expression.