Influence of mechanical activity, adrenergic stimulation, and calcium on the expression of myosin heavy chains in cultivated neonatal cardiomyocytes

Enhancement of Cardiomyogenesis in Murine Stem Cells by Low-Intensity Ultrasound

OBJECTIVES

Low-intensity ultrasound (LIUS) has been shown to enhance bone and cartilage regeneration from stem cells. The ease of its incorporation makes it an attractive mechanical stimulus for not only osteogenesis and chondrogenesis, but also cardiomyogenesis. However, to date, no study has investigated its effects on cardiomyogenesis from embryonic stem cells.

METHODS

In this study, murine embryonic stem cells were differentiated via embryoid body formation and plating, and after 3 days they were subjected to daily 10 minutes of LIUS treatment with various conditions: (1) low-pulsed (21 mW/cm² , 20% duty cycle), (2) low-continuous, (3) high-pulsed (147 mW/cm² , 20% duty cycle), and (4) high-continuous LIUS.

RESULTS

Low-pulsed and high-continuous LIUS had improved beating rates of contractile areas as well as increased late cardiac gene expressions, such as α- and β-myosin heavy chain and cardiac troponin T, showing its benefits on cardiomyocyte differentiation. Meanwhile, an early endodermal marker, α-fetoprotein, was significantly attenuated after LIUS treatments.

CONCLUSIONS

With these observations, it is demonstrated that LIUS simulation could enhance cardiomyogenesis from embryonic stem cells and increase its selectivity toward cardiomyocytes by reducing spontaneous differentiation.

Human embryonic stem cell-derived cardiomyocytes: inducing strategies

Cell-based therapy is emerging as an innovative approach for the treatment of degenerative diseases, and stem cells appear to be an ideal source of cells for this. In cardiology, in particular, human embryonic stem cell (hESC)-derived cardiomyocytes theoretically fulfill most, if not all, of the properties of an ideal donor cell, but several critical obstacles need to be overcome. Many research projects are focusing on set-up strategies for directing hESC differentiation toward the cardiac lineage. It is one of the main difficulties in the search to provide a valuable source of cells to effect regeneration of myocardial tissue in patients with severe heart failure. To date, there are no easy and efficient protocols for the induction of hESC differentiation toward the cardiac lineage. Discovering new molecules or tools capable of doing this is imperative.

Regulation of naturally occurring antisense RNA of myosin heavy chain (MyHC) in neonatal cardiomyocytes

Naturally occurring antisense RNA has been detected for a range of eukaryotic genes. Its abundance compared to levels of its complementary sense mRNA appears to be a factor indicating its possible regulatory function. In previous studies, we detected appreciable levels of antisense RNA against the two isoforms (alpha and beta) of the heavy myosin-chain (MyHC) in the myocardium of rats. If this is to play a significant role in gene expression antisense levels should vary in response to external and internal cellular influences. Recently, a bidirectional promoter located in the alpha/beta MyHC intergenic region was described, which was proposed to regulate coordinated transcription of alpha-MyHC sense and beta-MyHC antisense. To study MyHC antisense regulation in neonatal heart, we investigated cultivated myocytes stimulated with either trijodthyronin (T3) as an inductor of alpha-MyHC or phenylephrine with stimulation of beta-MyHC. RNA-quantification of sense and antisense transcripts of both isoforms was performed by real-time RT-PCR. Stimulation by T3 led to an induction of both sense and antisense of alpha-MyHC and to a decrease of beta-MyHC sense and antisense. Phenylephrine increased sense and antisense beta-MyHC but reduced antisense alpha-MyHC. The sense/antisense of alpha- and beta-MyHC ratio was unchanged compared to control. Results indicate a coregulation of sense and antisense MyHC RNA under stimulation of T3 and phenylephrine in neonatal cardiomyocytes.

Expression of human angiotensinogen-renin in rat: effects on transcription and heart function

In double transgenic rats (dTGR) harboring the human angiotensinogen (hAOGEN) and human renin (hREN) genes, we studied cardiac transcript levels of hypertrophy-related, Ca(2+) regulatory, and beta-adrenoceptor-associated proteins. The contractile properties and the cellular signaling of isolated hearts exposed to (-)isoproterenol and/or angiotensin (Ang) I were evaluated. dTGR developed hypertension of 174.1+/- 7.6 versus 109.6 +/- 2.0 mm Hg (P<0.05) in Sprague-Dawley rats and heart hypertrophy. In hearts of dTGR, the transcript levels of ANP, beta-MHC, and alpha-MHC were altered (percentage versus Sprague-Dawley rats, 100%) by 304%, 178%, and 78%, respectively. Transcript levels of L-type Ca(2+) channel, Ca(2+) release channel, SERCA2a, phospholamban, G(i)- and G(s)-proteins were unchanged. Isolated hearts of dTGR indicated higher baseline contractility versus Sprague-Dawley rats. (-)Isoproterenol-modified contractility occurred in both groups; however, the extent (predrug value, 100%) was less in hearts of dTGR versus Sprague-Dawley rats (+dP/dt, 310 +/- 42% versus 534 +/- 63%; P<0.05). Interestingly, (-)isoproterenol shortened the relaxation time by equivalent to 25% in both groups. This finding was reflected by a protein kinase A-related phospholamban phosphorylation. Ang I depressed the heart contractility but did not interact with the protein kinase A pathway. In conclusion, we have found that expression of the hAOGEN-hREN complex in dTGR elicited specific effects on transcripts of ANP and myofibrillar proteins. Although the beta-adrenergically mediated relaxation was not impaired in the hypertrophied hearts, the extent of beta-adrenergic inotropic responsiveness was reduced.

Localized injury in cardiomyocyte network: a new experimental model of ischemia-reperfusion arrhythmias

We developed a new experimental approach to study the effects of local injury in a multicellular preparation and tested the ability of the method to induce reperfusion arrhythmias in cardiomyocyte monolayers. A small region of injury was created using geometrically defined flows of control and ischemia-like solutions. Calcium transients were acquired simultaneously from injured, control, and border zone cells using fluo 4. Superfusion with the injury solution rapidly diminished the amplitude of calcium transients within the injury zone, followed by cessation of cell beating. Reperfusion caused an immediate tachyarrhythmic response in approximately 17% of experiments, with a wave front propagating from a single cell or small cell cluster within the former injury zone. Inclusion of a gap junction uncoupler (1 mM heptanol) in the injury solution narrowed the functional border and sharply increased the number of ectopic foci and the incidence of reperfusion arrhythmias. The model holds a potential to reveal both micro- and macroscopic features of propagation, conduction, and cell coupling in the normal and diseased myocardium and to serve as a new tool to test antiarrhythmic protocols in vitro.

Distant upstream regulatory domains direct high levels of beta -myosin heavy chain gene expression in differentiated embryonic stem cells

Eukaryotic gene transcription takes place in the context of chromatin. In order to study the expression of the beta -myosin heavy chain (MyHC) gene in its appropriate cardiac environment in vitro, embryonic stem cell lines were generated and induced to differentiate into the cardiac lineage. We show that the upstream region of the beta -MyHC gene (-5518 to -2490 relative to the transcriptional start site) directed high levels of transcriptional activity only when stably integrated, but not when expressed extrachromosomally in transient assays. These results are consistent with earlier findings using an in vivo transgenic approach. The expression of beta -MyHC reporter gene constructs was strictly correlated to differentiation status and coincided with the expression of endogenous cardiac marker genes and with morphological differentiation of embryoid bodies in vitro. Using populations of stably transfected cell clones, two domains important for high level expression were identified. The analysis of individual cell clones suggested that the positive regulatory domains act according to the graded model of enhancement. These results show that chromosomal integration is necessary for the appropriate function of the beta -MyHC gene's upstream regulatory region.