Thyroid hormone signaling and consequences for cardiac development.

The fetal heart undergoes its own growth and maturation stages all while supplying blood and nutrients to the growing fetus and its organs. Immature contractile cardiomyocytes proliferate to rapidly increase and establish cardiomyocyte endowment in the perinatal period. Maturational changes in cellular maturation, size and biochemical capabilities occur, and require, a changing hormonal environment as the fetus prepares itself for the transition to extrauterine life. Thyroid hormone has long been known to be important for neuronal development, but also for fetal size and survival. Fetal circulating 3,5,3'-triiodothyronine (T3) levels surge near term in mammals and are responsible for maturation of several organ systems, including the heart. Growth factors like insulin-like growth factor-1 stimulate proliferation of fetal cardiomyocytes, while thyroid hormone has been shown to inhibit proliferation and drive maturation of the cells. Several cell signaling pathways appear to be involved in this complicated and coordinated process. The aim of this review was to discuss the foundational studies of thyroid hormone physiology and the mechanisms responsible for its actions as we speculate on potential fetal programming effects for cardiovascular health.

Down-regulation of MEIS1 promotes the maturation of oxidative phosphorylation in perinatal cardiomyocytes.

Fetal cardiomyocytes shift from glycolysis to oxidative phosphorylation around the time of birth. Myeloid ecotropic viral integration site 1 (MEIS1) is a transcription factor that promotes glycolysis in hematopoietic stem cells. We reasoned that MEIS1 could have a similar role in the developing heart. We hypothesized that suppression of MEIS1 expression in fetal sheep cardiomyocytes leads to a metabolic switch as found at birth. Expression of MEIS1 was assayed in left ventricular cardiac tissue and primary cultures of cardiomyocytes from fetal (100- and 135-d gestation, term = 145 d), neonatal, and adult sheep. Cultured cells were treated with short interfering RNA (siRNA) to suppress MEIS1. Oxygen consumption rate was assessed with the Seahorse metabolic flux analyzer, and mitochondrial activity was assessed by staining cells with MitoTracker Orange. Cardiomyocyte respiratory capacity increased with advancing age concurrently with decreased expression of MEIS1. MEIS1 suppression with siRNA increased maximal oxygen consumption in fetal cells but not in postnatal cells. Mitochondrial activity was increased and expression of glycolytic genes decreased when MEIS1 expression was suppressed. Thus, we conclude that MEIS1 is a key regulator of cardiomyocyte metabolism and that the normal down-regulation of MEIS1 with age underlies a gradual switch to oxidative metabolism.-Lindgren, I. M., Drake, R. R., Chattergoon, N. N., Thornburg, K. L. Down-regulation of MEIS1 promotes the maturation of oxidative phosphorylation in perinatal cardiomyocytes.

Thyroid hormone receptor function in maturing ovine cardiomyocytes.

KEY POINTS: Plasma thyroid hormone (tri-iodo-l-thyronine; T3 ) concentrations rise near the end of gestation and is known to inhibit proliferation and stimulate maturation of cardiomyocytes before birth. Thyroid hormone receptors are required for the action of thyroid hormone in fetal cardiomyocytes. Loss of thyroid hormone receptor (TR)α1 abolishes T3 signalling via extracellular signal-related kinase and Akt in fetal cardiomyocytes. The expression of TRα1 and TRß1 in ovine fetal myocardium increases with age, although TRα1 levels always remain higher than those of TRß1. Near term fetal cardiac myocytes are more sensitive than younger myocytes to thyroid receptor blockade by antagonist, NH3, and to the effects of TRα1/α2 short interfering RNA. Although T3 is known to abrogate ovine cardiomyocyte proliferation stimulated by insulin-like growth factor 1, this effect is mediated via the genomic action of thyroid hormone receptors, with little evidence for non-genomic mechanisms. ABSTRACT: We have previously shown that the late-term rise in tri-iodo-l-thyronine (T3 ) in fetal sheep leads to the inhibition of proliferation and promotion of maturation in cardiomyocytes. The present study was designed to determine whether these T3 -induced changes are mediated via thyroid hormone receptors (TRs) or by non-genomic mechanisms. Fetal cardiomyocytes were isolated from 102 ± 3 and 135 ± 1 days of gestational age (dGA) sheep (n = 7 per age; term ∼145 dGA). Cells were treated with T3 (1.5 nm), insulin-like growth factor (IGF)-1 (1 µg mL-1 ) or a combination in the presence of TR antagonist NH3 (100 nm) or following short interfering RNA (siRNA) knockdown of TRα1/α2. Proliferation was quantified by 5-bromo-2'-deoxyuridine (BrdU) uptake (10 µm). Western blots measured protein levels of extracellular signal-related kinase (ERK), Akt, TRα1/ß1 and p21. Age specific levels of TRα1/ß1 were measured in normal hearts from fetuses [95 dGA (n = 8), 135 dGA (n = 7)], neonates (n = 8) and adult ewes (n = 7). TRα1 protein levels were consistently >50% more than TRß1 at each gestational age (P < 0.05). T3 reduced IGF-1 stimulated proliferation by ∼50% in 100 dGA and by ∼75% in 135 dGA cardiomyocytes (P < 0.05). NH3 blocked the T3  + IGF-1 reduction of BrdU uptake without altering the phosphorylation of ERK or Akt at both ages. NH3 did not suppress T3 -induced p21 expression in 100 dGA cardiomyocytes in 135 dGA cardiomyocytes, NH3 alone reduced BrdU uptake (-28%, P < 0.05), as well as T3 -induced p21 (-75%, P < 0.05). In both ages, siRNA knockdown of TRα1/α2 blocked the T3  + IGF-1 reduction of BrdU uptake and dramatically reduced ERK and Akt signalling in 135 dGA cardiomyocytes. In conclusion, TRs are required for normal proliferation and T3 signalling in fetal ovine cardiomyocytes, with the sensitivity to TR blockade being age-dependent.

Mid-gestation ovine cardiomyocytes are vulnerable to mitotic suppression by thyroid hormone.

Circulating fetal 3,3',5-tri-iodo-l-thyronine (T(3) ) is maintained at very low levels until a dramatic prepartum surge. 3,3',5-Tri-iodo-l-thyronine inhibits serum-stimulated proliferation in near-term ovine cardiomyocytes, but it is not known whether midgestation myocytes are also inhibited. Because early cessation of cardiomyocyte mitosis would result in an underendowed heart, we hypothesized that 0.67 gestation (100 of 145 days gestation) ovine cardiomyocytes would be insensitive to suppressive growth effects of T(3) . These younger cardiomyocytes were grown with T(3) in 10% serum-enriched media for 24 hours. Physiological (0.37, 0.75, and 1.5 nmol/L) concentrations of T(3) dramatically suppressed mitotic activity in cardiomyocytes (P < .001). 3,3',5-Tri-iodo-l-thyronine stimulated phosphorylation of extracellular signal-regulated kinase and AKT (also known as Protein Kinase B [PKB]) signaling pathways. Nevertheless, the protein content of the cell cycle suppressor, p21, increased 2-fold (P < .05), and promoter, cyclin D1, decreased by 50%. Contrary to our hypothesis, elevated levels of T(3) powerfully inhibit proliferation of midgestation fetal cardiomyocytes. Thus, midgestation maternal hyperthyroidism might lead to an underendowed fetal myocardium.

Thyroid hormone drives fetal cardiomyocyte maturation.

Tri-iodo-l-thyronine (T(3)) suppresses the proliferation of near-term serum-stimulated fetal ovine cardiomyocytes in vitro. Thus, we hypothesized that T(3) is a major stimulant of cardiomyocyte maturation in vivo. We studied 3 groups of sheep fetuses on gestational days 125-130 (term ∼145 d): a T(3)-infusion group, to mimic fetal term levels (plasma T(3) levels increased from ∼0.1 to ∼1.0 ng/ml; t(1/2)∼24 h); a thyroidectomized group, to produce low thyroid hormone levels; and a vehicle-infusion group, to serve as intact controls. At 130 d of gestation, sections of left ventricular freewall were harvested, and the remaining myocardium was enzymatically dissociated. Proteins involved in cell cycle regulation (p21, cyclin D1), proliferation (ERK), and hypertrophy (mTOR) were measured in left ventricular tissue. Evidence that elevated T(3) augmented the maturation rate of cardiomyocytes included 14% increased width, 31% increase in binucleation, 39% reduction in proliferation, 150% reduction in cyclin D1 protein, and 500% increase in p21 protein. Increased expression of phospho-mTOR, ANP, and SERCA2a also suggests that T(3) promotes maturation and hypertrophy of fetal cardiomyocytes. Thyroidectomized fetuses had reduced cell cycle activity and binucleation. These findings support the hypothesis that T(3) is a prime driver of prenatal cardiomyocyte maturation.

Cardiomyocyte enlargement, proliferation and maturation during chronic fetal anaemia in sheep.

Chronic anaemia increases the workload of the growing fetal heart, leading to cardiac enlargement. To determine which cellular process increases cardiac mass, we measured cardiomyocyte sizes, binucleation as an index of terminal differentiation, and tissue volume fractions in hearts from control and anaemic fetal sheep. Fourteen chronically catheterized fetal sheep at 129 days gestation had blood withdrawn for 9 days to cause severe anaemia; 14 control fetuses were of similar age. At postmortem examination, hearts were either enzymatically dissociated or fixed for morphometric analysis. Daily isovolumetric haemorrhage reduced fetal haematocrit from a baseline value of 35% to 15% on the final day (P < 0.001). At the study conclusion, anaemic fetuses had lower arterial pressures than control fetuses (P < 0.05). Heart weights were increased by 39% in anaemic fetuses compared with control hearts (P < 0.0001), although the groups had similar body weights; the heart weight difference was not due to increased ventricular wall water content or disproportionate non-myocyte tissue expansion. Cardiomyocytes from anaemic fetuses tended to be larger than those of control fetuses. There were no statistically significant differences between groups in the cardiomyocyte cell cycle activity. The degree of terminal differentiation was greater in the right ventricle of anaemic compared with control fetuses by 8% (P < 0.05). Anaemia substantially increased heart weight in fetal sheep. The volume proportions of connective and vascular tissue were unchanged. Cardiomyocyte mass expanded by a balanced combination of cellular enlargement, increased terminal differentiation and accelerated proliferation.

Inhibition of vascular smooth muscle cell proliferation in vitro by genetically engineered marrow stromal cells secreting calcitonin gene-related peptide.

Calcitonin gene-related peptide (CGRP) has a beneficial effect in pulmonary hypertension and is a target for cardiovascular gene therapy. Marrow stromal cells (MSCs), also known as mesenchymal stem cells, hold promise for use in adult stem cell-based ex vivo gene therapy. To test the hypothesis that genetically engineered MSCs secreting CGRP can inhibit vascular smooth muscle cell proliferation, rat MSCs were isolated, ex vivo expanded, and transduced with adenovirus containing CGRP. Immunocytochemical analysis demonstrated that wild type rat MSCs express markers specific for stem cells, endothelial cells, and smooth muscle cells including Thy-1, c-Kit, von Willebrand Factor and alpha-smooth muscle actin. Immunocytochemistry confirmed the expression of CGRP by the transduced rat MSCs. The transduced rat MSCs released 10.3+/-1.3 pmol CGRP/1 x 10(6) cells/48 h (mean+/-S.E.M., n=3) into culture medium at MOI 300 and the CGRP-containing culture supernatant from the transduced cells inhibited the proliferation of rat pulmonary artery smooth muscle cells (PASMCs) and rat aortic smooth muscle cells (ASMCs) in culture. Co-culture of the transduced rat MSCs with rat PASMCs or rat ASMCs also inhibited smooth muscle cell proliferation. These findings suggest that this novel adult stem cell-based CGRP gene therapy has potential for the treatment of cardiovascular diseases including pulmonary hypertension.

Rosiglitazone reduces serum homocysteine levels, smooth muscle proliferation, and intimal hyperplasia in Sprague-Dawley rats fed a high methionine diet.

Homocysteine (Hcy) is a metabolite of the essential amino acid methionine. Hyperhomocysteinemia is associated with vascular disease, particularly carotid stenosis. Rosiglitazone, a ligand of the peroxisome proliferator-activated receptor gamma , attenuates balloon catheter-induced carotid intimal hyperplasia in type 2 diabetic rats. We studied 4 groups (n = 7 per group) of adult female Sprague-Dawley rats fed (a) powdered laboratory chow (control), (b) control diet with rosiglitazone (3.0 mg/kg/d), (c) diet containing 1.0% l -methionine, and (d) diet containing methionine and rosiglitazone. After 1 week on high methionine diet, the rats were administered an aqueous preparation of rosiglitazone by oral gavage. One week after initiation of rosiglitazone, balloon catheter injury of the carotid artery was carried out using established methods, and the animals continued on their respective dietary and drug regimens for another 21 days. At the end of the experimental period, blood samples were collected, and carotid arteries and liver were harvested. Serum Hcy increased significantly on methionine diet compared with controls (28.9 +/- 3.2 vs 6.3 +/- 0.04 micromol/L). Development of intimal hyperplasia was 4-fold higher in methionine-fed rats; this augmentation was significantly reduced ( P < .018) in rosiglitazone-treated animals. Rosiglitazone treatment significantly ( P < .001) suppressed Hcy levels and increased the activity of the Hcy metabolizing enzyme, cystathionine-beta-synthase in the liver samples. Hcy (100 micromol/L) produced a 3-fold increase in proliferation of rat aortic vascular smooth muscle cells; this augmentation was inhibited by incorporating rosiglitazone (10 micromol/L). After balloon catheter injury to the carotid artery of animals on a high methionine diet, there was an increase in the rate of development of intimal hyperplasia consistent with the known effects of Hcy. It is demonstrated for the first time that the peroxisome proliferator-activated receptor gamma agonist rosiglitazone can attenuate the Hcy-stimulated increase in the rate of development of intimal hyperplasia indirectly by increasing the rate of catabolism of Hcy by cystathionine-beta-synthase and directly by inhibiting vascular smooth muscle cell proliferation. These findings may have important implications for the prevention of cardiovascular disease and events in patients with hyperhomocysteinemia (HHcy).

Engineering ex vivo-expanded marrow stromal cells to secrete calcitonin gene-related peptide using adenoviral vector.

Calcitonin gene-related peptide (CGRP) is a target for cardiovascular gene therapy. Marrow stromal cells (MSCs) hold promise for use in adult stem cell-based cell and gene therapy. To determine the feasibility of adenoviral-mediated CGRP gene transfer into ex vivo-expanded MSCs, rat MSCs were isolated, ex vivo expanded, and transduced with adenoviruses. Adprepro-CGRP and AdntlacZ, adenoviral vectors containing prepro-CGRP or nuclear-targeted beta-galactosidase reporter gene ntlacZ under the control of Rous sarcoma virus promoter, were used. In this study, it can be shown that transduction efficiency of adenoviral-mediated gene transfer into ex vivo-expanded MSCs is dose dependent, transgene expression persists for more than 21 days in culture, and adenoviral transduction does not alter the proliferation or viability of MSCs. Transduced MSCs retain multipotentiality and transgene expression after cell differentiation. The expression and secretion of CGRP by Adprepro- CGRP-transduced MSCs was confirmed by Western blot analysis and enzyme immunoassay. The secretion of CGRP by Adprepro-CGRP-transduced MSCs is dose dependent, and the transduced cells release as much as 9.5 +/- 0.4 pmol CGRP/1 x 10(6) cells/48 hours (mean +/- standard error of mean, n = 3) into culture medium at a multiplicity of infection of 300. Furthermore, culture supernatant from Adprepro-CGRP-transduced MSCs increases intracellular cyclic AMP levels in pulmonary artery smooth muscle cells in culture. These findings suggest that replication-deficient recombinant adenovirus can be used to gene engineer ex vivo-expanded MSCs and that high-level secretion of biologically active CGRP can be achieved, underscoring the clinical potential of using this novel adult stem cell-based cell and gene therapy strategy for the treatment of cardiovascular diseases.

Adenoviral gene transfer of eNOS: high-level expression in ex vivo expanded marrow stromal cells.

Endothelial nitric oxide synthase (eNOS) is an attractive target for cardiovascular gene therapy. Marrow stromal cells (MSCs), also known as mesenchymal stem cells, hold great promise for use in adult stem cell-based cell and gene therapy. To determine the feasibility of adenoviral-mediated eNOS gene transfer into ex vivo expanded MSCs, rat MSCs (rMSCs) were isolated, expanded ex vivo, and transduced with Ad5RSVeNOS, an adenoviral vector containing the eNOS gene under the control of the Rous sarcoma virus promoter. The presence of eNOS protein in Ad5RSVeNOS-transduced rMSCs was confirmed by immunohistochemical and Western blot analysis. Transduction efficiency was dose dependent, and eNOS transgene expression in rMSCs persisted for > or =21 days in culture. The rMSCs retained multipotential differentiation capability after adenoviral-mediated eNOS gene transfer. Furthermore, intracavernosal injection of Ad5RSVeNOS-transduced rMSCs increased the expression of eNOS in the corpus cavernosum, and stem cells were identified within corporal sinusoids. These findings demonstrate that replication-deficient recombinant adenovirus can be used to engineer ex vivo expanded rMSCs and that high-level eNOS transgene expression can be achieved, pointing out the clinical potential of using this novel adult stem cell-based gene therapy method for the treatment of cardiovascular diseases.