Biomedical subjects

Melissa C Colbert

Publications and source records attributed to Melissa C Colbert.

8 recordsLinked to original sources

PubMed2006

Neural crest cells retain multipotential characteristics in the developing valves and label the cardiac conduction system.

Multipotent neural crest cells (NCCs) are a major extracardiac component of cardiovascular development. Although recognized as contributing cells to the arterial valves at early developmental stages, NCC persistence in the valves at later times or in the adult heart is controversial. We analyzed NCC persistence and contributions to both semilunar and atrioventricular (AV) valves in the mature heart. Two NCC-specific promoters driving Cre recombinase, Wnt1-Cre and P0-Cre, were mated with floxed reporter mice, R26R or CAG-CAT-EGFP, to map NCC fate. Hearts were analyzed before aorticopulmonary (AP) septation through adult stages. As previously demonstrated, strong NCC labeling was detected in ventral and dorsal outflow cushions before AP septation. In contrast to previous reports, we found that substantial numbers of labeled cells persisted in the semilunar valves in late fetal, neonatal, and adult hearts. Furthermore, NCCs were also found in the AV valves, almost exclusively in the septal leaflets. NCCs in the AV valves expressed melanocytic and neurogenic markers. However, cells labeled in the proximal cardiac conduction system exhibited neurogenic and gliagenic markers, whereas some NCCs expressed no differentiation specific markers. These results suggest that cardiac NCCs contribute to the mature valves and the cardiac conduction system and retain multipotent characteristics late in development.

Aging↗

PubMed2005

Retinoic acid induces CDK inhibitors and growth arrest specific (Gas) genes in neural crest cells.

Retinoic acid (RA), the active metabolite of vitamin A, regulates cellular growth and differentiation during embryonic development. In excess, this vitamin is also highly teratogenic to animals and humans. The neural crest is particularly sensitive to RA, and high levels adversely affect migration, proliferation and cell death. We investigated potential gene targets of RA associated with neural crest proliferation by determining RA-mediated changes in gene expression over time, using microarrays. Statistical analysis of the top ranked RA-regulated genes identified modest changes in multiple genes previously associated with cell cycle control and proliferation including the cyclin-dependent kinase inhibitors Cdkn1a (p21), Cdkn2b (p15(INK4b)), and Gas3/PMP22. The expression of p21 and p15(INK4b) contribute to decreased proliferation by blocking cell cycle progression at G1-S. This checkpoint is pivotal to decisions regulating proliferation, apoptosis, or differentiation. We have also confirmed the overexpression of Gas3/PMP22 in RA-treated neural crests, which is associated with cytoskeletal changes and increased apoptosis. Our results suggest that increases in multiple components of diverse regulatory pathways have an overall cumulative effect on cellular decisions. This heterogeneity contributes to the pleiotropic effects of RA, specifically those affecting proliferation and cell death.

Animals↗

PubMed2004

Large-scale reprogramming of cranial neural crest gene expression by retinoic acid exposure.

Although retinoic acid (RA), the active form of vitamin A, is required for normal embryonic growth and development, it is also a powerful teratogen. Infants born to mothers exposed to retinoids during pregnancy have a 25-fold increased risk for malformations, nearly exclusively of cranial neural crest-derived tissues. To characterize neural crest cell responses to RA, we exposed murine crest cultures to teratogenic levels of RA and subjected their RNA to microarray-based gene expression profile analysis using Affymetrix MG-U74Av2 GeneChips. RNAs were isolated from independent cultures treated with 10(-6) M RA for 6, 12, 24, or 48 h. Statistical analyses of gene expression profile data facilitated identification of the 205 top-ranked differentially regulated genes whose expression was reproducibly changed by RA over time. Cluster analyses of these genes across the independently treated sample series revealed distinctive kinetic patterns of altered gene expression. The largest group was transiently affected within the first 6 h of exposure, representing early responding genes. Group 2 showed sustained induction by RA over all times, whereas group 3 was characterized by the suppression of a time-dependent expression increase normally seen in untreated cells. Additional patterns demonstrated time-dependent increased or decreased expression among genes not normally regulated to a significant extent. Gene function analysis revealed that more than one-third of all RA-regulated genes were associated with developmental regulation, including both canonical and noncanonical Wnt signaling pathways. Multiple genes associated with cell adhesion and cell cycle regulation, recognized targets for the biological effects of RA, were also affected. Taken together, these results support the hypothesis that the teratogenic effects of RA derive from reprogramming gene expression of a host of genes, which play critical roles during embryonic development regulating pathways that determine subsequent differentiation of cranial neural crest cells.

Animals↗

PubMed2003

Midgestational lethality in mice lacking the parathyroid hormone (PTH)/PTH-related peptide receptor is associated with abrupt cardiomyocyte death.

PTHrP is a key developmental regulatory protein and a potent vasoactive agent. Previous studies have shown that mice lacking either the Pthrp or the PTH type 1 receptor (Pth1r) gene exhibit severe chondrodysplasia. In addition, in most genetic backgrounds, the receptor null mice die prenatally at midgestation, but the cause of death remains elusive. Here we show the loss of the Pth1r gene in C57BL6 mice leads to massive, abrupt cardiomyocyte death and embryonic lethality between embryonic days (E) E11.5 and E12.5. PTH1R mRNA was abundantly expressed in the developing wild-type mouse heart and cardiomyocytes from E11.5 embryos demonstrated acute increases in cAMP and increased Ca(2+)oscillations in response to PTHrP-(1-34)NH(2). Analyses of more than 300 embryos (E8-E14.5) from C57BL6/PTH1R +/- matings showed that PTH1R-/- mice survived until E11 with no obvious defects in any tissue. By E12, only 10% of the PTH1R-/- embryos survived and all PTH1R null mice were dead by E13. Ultrastructural and histological analysis revealed striking mitochondrial abnormalities at E11.5 and precipitous cardiomyocyte death between E12.0 and E12.5, followed by degenerative changes in the liver and massive necrosis of other tissues. No abnormalities were observed in the yolk sac or placenta implicating the heart degeneration as the primary cause of death. Taken together, these findings indicate that the PTH1R is required for the development of normal cardiomyocyte function.

Animals↗

PubMed2002

Mitochondrial death protein Nix is induced in cardiac hypertrophy and triggers apoptotic cardiomyopathy.

Loss of cardiomyocytes through programmed cell death is a key event in the development of heart failure, but the inciting molecular mechanisms are largely unknown. We used microarray analysis to identify a genetic program for myocardial apoptosis in Gq-mediated and pressure-overload cardiac hypertrophy. A critical component of this apoptotic program was Nix/Bnip3L. Nix localized to mitochondria and caused release of cytochrome c, activation of caspase-3 and apoptotic cell death, when expressed in HEK293 fibroblasts. A previously undescribed truncated Nix isoform, termed sNix, was not targeted to mitochondria but heterodimerized with Nix and protected against Nix-mediated apoptosis. Forced in vivo myocardial expression of Nix resulted in apoptotic cardiomyopathy and rapid death. Conversely, sNix protected against apoptotic peripartum cardiomyopathy in G(alpha)q-overexpressors. Thus, Nix/Bnip3L is upregulated in myocardial hypertrophy, and is both necessary and sufficient for Gq-mediated apoptosis of cardiomyocytes and resulting hypertrophy decompensation.

Animals↗

PubMed2002

Remodeling of gap junctions in mouse hearts hypertrophied by forced retinoic acid signaling.

BACKGROUND: Beta-MHC-hRARalpha transgenic mice express a constitutively active (truncated) form of the human retinoic acid receptor which triggers development of dilated cardiomyopathy. In those hearts, we studied expression of gap junction proteins in relation to electrical impulse propagation. METHODS AND RESULTS: As compared to wildtype mice, hearts of 4-6 month old mice with 7-12 inserted hRARalpha copies are marked by an increased heart weight/body weight- and heart weight/tibia length ratio. 3-extremity lead ECGs revealed prolongation of the Q-j interval suggesting delayed ventricular activation. Mapping of electrical activity of epi- and endocardial left ventricular free wall revealed activation delay, increased heterogeneity in conduction and regional conduction block. Ventricular tachycardias did not occur spontaneously nor could be induced by ventricular pacing. Immunohistochemical analysis showed profound and heterogeneous redistribution and down-regulation of the gap junction protein connexin43 (Cx43) in the left ventricular free wall. Here, hRARalpha expression induced re-expression of the hypertrophic markers alpha-skeletal actin and beta-MHC, and in 3 out of 10 severely affected mice, re-expression of Cx40. Concomitant with changes in expression/distribution of Cx43, changes in expression and distribution of beta-catenin and N-cadherin (two other intercalated disk associated proteins) were observed. CONCLUSIONS: Beta-MHC-hRARalpha transgenic hearts show heterogeneous re-expression of (early) sarcomeric genes while expression of connexin43, N-cadherin and beta-catenin is down-regulated. We postulate that the resulting aberrant ventricular activation does not trigger development of lethal arrhythmias due to the small size of remaining healthy ventricular tissue where the transgene is not expressed.

Actins↗

PubMed2002

Enhanced myocyte contractility and Ca2+ handling in a calcineurin transgenic model of heart failure.

OBJECTIVE: Impaired myocyte Ca2+ handling is a common characteristic of failing hearts and increases in calcineurin activity, a Ca2+-sensitive phosphatase, have been implicated in heart failure phenotype. Transgenic mice with cardiac-specific expression of an active form of calcineurin display depressed function, hypertrophy and heart failure. We examined whether defects in cardiomyocyte Ca2+ handling properties contribute to the impaired cardiac function in calcineurin transgenic mice. METHODS: The levels of SR Ca2+ handling proteins, SR Ca2+ transport function and cardiomyocyte mechanics, as well as Ca2+ kinetics were examined in mice overexpressing a constitutively active form of calcineurin. RESULTS: Transgenic expression of activated calcineurin catalytic subunit resulted in significant protein increases (66%) in SERCA2 and decreases (35%) in phospholamban, as well as enhanced (approximately 80%) phospholamban phosphorylation. These alterations in the SR Ca2+-transport proteins resulted in increased V(max) and Ca2+-affinity of SERCA2. The myofibrillar Mg-ATPase activity was also significantly increased at pCa>6.0. The enhanced SR Ca2+ handling and Mg-ATPase activity reflected significant elevation in myocyte contractile parameters (3-fold), Ca2+ transient amplitude (1.5-fold) and the rate of Ca2+ signal decay (2-fold). In contrast, in vivo cardiac function assessed by echocardiography, indicated severely depressed contractility in calcineurin hearts. The apparent disparity in contractile properties between the cellular and multicellular preparations may be partially due to tissue remodeling, including interstitial fibrosis and a marked reduction (45%), dephosphorylation (81%) and redistribution of the gap junctional protein connexin-43, which could compromise intercellular communication. CONCLUSION: Despite enhanced SR Ca2+ handling and contractility in myocytes, pathological remodeling and defects in intercellular coupling may underlie contractile dysfunction of the calcineurin hearts.

Animals↗

PubMed2002

Retinoids and cardiovascular developmental defects.

Vitamin A and related retinoids are critical regulators of normal cardiovascular development. Extreme variations in retinoid levels, too little or too much, dramatically alter embryonic morphogenesis that has teratogenic consequences, including effects on the heart and great vessels. Specific cardiovascular targets of retinoid action include effects on the anteroposterior patterning of the early heart, left-right decisions and cardiac situs, endocardial cushion formation, and, in particular, the neural crest. The cardiovascular defects produced are remarkably similar in deficiency and excess, suggesting modulation of common developmental or cellular processes by different levels of retinoids. The isolation of nuclear receptors that mediate retinoid action has led to the identification of some genes directly involved in the regulation of these processes and other gene products that may be affected more indirectly. This review will examine the mechanism of retinoid action, the requirements for vitamin A during normal heart development, and the consequences of nonphysiologic or teratogenic exposure.

Animals↗