Dedifferentiation and Proliferation of Mammalian Cardiomyocytes

It has long been thought that mammalian cardiomyocytes are terminally-differentiated and unable to proliferate. However, myocytes in more primitive animals such as zebrafish are able to dedifferentiate and proliferate to regenerate amputated cardiac muscle.Here we test the hypothesis that mature mammalian cardiomyocytes retain substantial cellular plasticity, including the ability to dedifferentiate, proliferate, and acquire progenitor cell phenotypes. Two complementary methods were used: 1) cardiomyocyte purification from rat hearts, and 2) genetic fate mapping in cardiac explants from bi-transgenic mice. Cardiomyocytes isolated from rodent hearts were purified by multiple centrifugation and Percoll gradient separation steps, and the purity verified by immunostaining and RT-PCR. Within days in culture, purified cardiomyocytes lost their characteristic electrophysiological properties and striations, flattened and began to divide, as confirmed by proliferation markers and BrdU incorporation. Many dedifferentiated cardiomyocytes went on to express the stem cell antigen c-kit, and the early cardiac transcription factors GATA4 and Nkx2.5. Underlying these changes, inhibitory cell cycle molecules were suppressed in myocyte-derived cells (MDCs), while microRNAs known to orchestrate proliferation and pluripotency increased dramatically. Some, but not all, MDCs self-organized into spheres and re-differentiated into myocytes and endothelial cells in vitro. Cell fate tracking of cardiomyocytes from 4-OH-Tamoxifen-treated double-transgenic MerCreMer/ZEG mouse hearts revealed that green fluorescent protein (GFP) continues to be expressed in dedifferentiated cardiomyocytes, two-thirds of which were also c-kit(+).Contradicting the prevailing view that they are terminally-differentiated, postnatal mammalian cardiomyocytes are instead capable of substantial plasticity. Dedifferentiation of myocytes facilitates proliferation and confers a degree of stemness, including the expression of c-kit and the capacity for multipotency

Mesenchymal stem cells rescue cardiomyoblasts from cell death in an in vitro ischemia model via direct cell-to-cell connections

<p>Abstract</p> <p>Background</p> <p>Bone marrow derived mesenchymal stem cells (MSCs) are promising candidates for cell based therapies in myocardial infarction. However, the exact underlying cellular mechanisms are still not fully understood. Our aim was to explore the possible role of direct cell-to-cell interaction between ischemic H9c2 cardiomyoblasts and normal MSCs. Using an in vitro ischemia model of 150 minutes of oxygen glucose deprivation we investigated cell viability and cell interactions with confocal microscopy and flow cytometry.</p> <p>Results</p> <p>Our model revealed that adding normal MSCs to the ischemic cell population significantly decreased the ratio of dead H9c2 cells (H9c2 only: 0.85 ± 0.086 vs. H9c2+MSCs: 0.16 ± 0.035). This effect was dependent on direct cell-to-cell contact since co-cultivation with MSCs cultured in cell inserts did not exert the same beneficial effect (ratio of dead H9c2 cells: 0.90 ± 0.055). Confocal microscopy revealed that cardiomyoblasts and MSCs frequently formed 200-500 nm wide intercellular connections and cell fusion rarely occurred between these cells.</p> <p>Conclusion</p> <p>Based on these results we hypothesize that mesenchymal stem cells may reduce the number of dead cardiomyoblasts after ischemic damage via direct cell-to-cell interactions and intercellular tubular connections may play an important role in these processes.</p

Cardiac telocytes — their junctions and functional implications

Telocytes (TCs) form a cardiac network of interstitial cells. Our previous studies have shown that TCs are involved in heterocellular contacts with cardiomyocytes and cardiac stem/progenitor cells. In addition, TCs frequently establish ‘stromal synapses’ with several types of immunoreactive cells in various organs (www.telocytes.com). Using electron microscopy (EM) and electron microscope tomography (ET), we further investigated the interstitial cell network of TCs and found that TCs form ‘atypical’ junctions with virtually all types of cells in the human heart. EM and ET showed different junction types connecting TCs in a network (puncta adhaerentia minima, processus adhaerentes and manubria adhaerentia). The connections between TCs and cardiomyocytes are ‘dot’ junctions with nanocontacts or asymmetric junctions. Junctions between stem cells and TCs are either ‘stromal synapses’ or adhaerens junctions. An unexpected finding was that TCs have direct cell–cell (nano)contacts with Schwann cells, endothelial cells and pericytes. Therefore, ultrastructural analysis proved that the cardiac TC network could integrate the overall ‘information’ from vascular system (endothelial cells and pericytes), nervous system (Schwann cells), immune system (macrophages, mast cells), interstitium (fibroblasts, extracellular matrix), stem cells/progenitors and working cardiomyocytes. Generally, heterocellular contacts occur by means of minute junctions (point contacts, nanocontacts and planar contacts) and the mean intermembrane distance is within the macromolecular interaction range (10–30 nm). In conclusion, TCs make a network in the myocardial interstitium, which is involved in the long-distance intercellular signaling coordination. This integrated interstitial system appears to be composed of large homotropic zones (TC–TC junctions) and limited (distinct) heterotropic zones (heterocellular junctions of TCs)

Is Homosexuality a Paraphilia? The Evidence For and Against

Whether homosexuality should be described as one among many paraphilic sexual interests or an altogether different dimension of sexual interest has long been discussed in terms of its political and social implications. The present article examined the question instead by comparing the major correlates and other features of homosexuality and of the paraphilias, including prevalence, sex ratio, onset and course, fraternal birth order, physical height, handedness, IQ and cognitive neuropsychological profile, and neuroanatomy. Although those literatures remain underdeveloped, the existing findings thus far suggest that homosexuality has a pattern of correlates largely, but not entirely, distinct from that identified among the paraphilias. At least, if homosexuality were deemed a paraphilia, it would be relatively unique among them, taxonometrically speaking

Mathematical simulations of sphingosine-1-phosphate actions on mammalian ventricular myofibroblasts and myocytes

Mathematical modeling has been used to explore the consequences of the actions of sphingosine-1-phosphate (S-1-P) within the ventricular myocardium. Electrophysiological data obtained from rabbit cultured myofibroblasts (Chilton et al. 2007) provided the basis for modifications of our model of electrotonic coupling between ventricular myocytes and fibroblasts (MacCannell et al. 2007). Specifically, an in silico fibroblast/myocyte hybrid model was modified to account for the electrophysiological properties that are characteristic of the myofibroblast (the wound healing phenotype of the fibroblast). In addition, equations describing an S-1-P-induced current that can be activated in the myofibroblast were added. \ud \ud The sets of simulations that constitute this paper demonstrate that S-1-P can cause a significant depolarization of the resting membrane potential in both the myofibroblast and myocyte. When the myocyte to fibroblast coupling ratio is 1:1, this concentration-dependent effect is due to ligand-gated current in the myofibroblast depolrizing the myocyte through heterotypic connexin-mediated intercellular junctions. In addition to changing the resting potential in the myocyte, the S-1-P induced current resulted in significant changes in action potential waveform.\ud \ud A second set of simulations was done for the purpose of exploring the effects of S-1-P on myocytes that have some of the main electrophysiological properties of those from the failing heart. In these computations, the ten Tusscher model of the human ventricular myocyte was modified by reducing parameters as follows: cell capacitance, inward rectifier K+ current, delayed-rectifier K+ currents (IKs and IKr), and transient outward K+ current. In combination, these changes (each of which is associated with heart failure), resulted in prolongation of action potential duration. Simulations of electrotonic coupling between this 'failing' myocyte and myofibroblasts demonstrated that the resting potential and APD in the failing myocyte is more susceptible to modulation by electrotonic influences from S-1-P-stimulated myofibroblasts when a 'failing' electrophysiological phenotype is mimicked.\ud \ud In summary, our simulations draw attention to important effects of S-1-P on the ventricular myocardium even when this paracrine substance actos only on the fibroblast cell population. These cell-specific S-1-P effects alter the myocyte action potential via electrotonic coupling with myocytes. It is apparent that myofibroblasts can have significant effects on myocyte action potentials; and these effects would be expected to be more pronounced in the presence of ligand-gated effects on the myofibroblast. The general setting that we have attempted to replicate with this first order model has some similarities to acute or sterile inflammation in the myocardium wherein S-1-P concentrations in the interstitium are relatively high