https://www.seas.upenn.edu/%7Edischer/pdfs/Cell_on_Gel-CELLpaper.pdf

Matrix elasticity directs stem cell lineage specification.

Adult marrow-derived Mesenchymal Stem Cells (MSCs) are capable of dividing and their progeny are further capable of differentiating into one of several mesenchymal phenotypes such as osteoblasts, chondrocytes, myocytes, marrow stromal cells, tendon-ligament fibroblasts, and adipocytes. In addition, these MSCs secrete a variety of cytokines and growth factors that have both paracrine and autocrine activities. These secreted bioactive factors suppress the local immune system, inhibit fibrosis (scar formation) and apoptosis, enhance angiogenesis, and stimulate mitosis and differentiation of tissue-intrinsic reparative or stem cells. These effects, which are referred to as trophic effects, are distinct from the direct differentiation of MSCs into repair tissue. Several studies which tested the use of MSCs in models of infarct (injured heart), stroke (brain), or meniscus regeneration models are reviewed within the context of MSC-mediated trophic effects in tissue repair.

/pdf/mesenchymal-stem-cells-as-trophic-mediators-1sjekxtzx0.pdf

Mesenchymal stem cells as trophic mediators.

https://www.thelancet.com/pdfs/journals/lancet/PIIS0140-6736(04)16626-9.pdf

Intracoronary autologous bone-marrow cell transfer after myocardial infarction: the BOOST randomised controlled clinical trial

Cardiomyocytes derived from human embryonic stem (hES) cells potentially offer large numbers of cells to facilitate repair of the infarcted heart. However, this approach has been limited by inefficient differentiation of hES cells into cardiomyocytes, insufficient purity of cardiomyocyte preparations and poor survival of hES cell-derived myocytes after transplantation. Seeking to overcome these challenges, we generated highly purified human cardiomyocytes using a readily scalable system for directed differentiation that relies on activin A and BMP4. We then identified a cocktail of pro-survival factors that limits cardiomyocyte death after transplantation. These techniques enabled consistent formation of myocardial grafts in the infarcted rat heart. The engrafted human myocardium attenuated ventricular dilation and preserved regional and global contractile function after myocardial infarction compared with controls receiving noncardiac hES cell derivatives or vehicle. The ability of hES cell-derived cardiomyocytes to partially remuscularize myocardial infarcts and attenuate heart failure encourages their study under conditions that closely match human disease.

https://courses.washington.edu/conj504/readings/murry_reading2.pdf

Cardiomyocytes derived from human embryonic stem cells in pro-survival factors enhance function of infarcted rat hearts

Animal and preliminary human studies of adult cell therapy following acute myocardial infarction have shown an overall improvement of cardiac function. Myocardial and vascular regeneration have been initially proposed as mechanisms of stem cell action. However, in many cases, the frequency of stem cell engraftment and the number of newly generated cardiomyocytes and vascular cells, either by transdifferentiation or cell fusion, appear too low to explain the significant cardiac improvement described. Accordingly, we and others have advanced an alternative hypothesis: the transplanted stem cells release soluble factors that, acting in a paracrine fashion, contribute to cardiac repair and regeneration. Indeed, cytokines and growth factors can induce cytoprotection and neovascularization. It has also been postulated that paracrine factors may mediate endogenous regeneration via activation of resident cardiac stem cells. Furthermore, cardiac remodeling, contractility, and metabolism may also be influenced in a paracrine fashion. This article reviews the potential paracrine mechanisms involved in adult stem cell signaling and therapy.

Paracrine Mechanisms in Adult Stem Cell Signaling and Therapy

The mammalian heart has a very limited regenerative capacity and, hence, heals by scar formation. Recent reports suggest that haematopoietic stem cells can transdifferentiate into unexpected phenotypes such as skeletal muscle, hepatocytes, epithelial cells, neurons, endothelial cells and cardiomyocytes, in response to tissue injury or placement in a new environment. Furthermore, transplanted human hearts contain myocytes derived from extra-cardiac progenitor cells, which may have originated from bone marrow. Although most studies suggest that transdifferentiation is extremely rare under physiological conditions, extensive regeneration of myocardial infarcts was reported recently after direct stem cell injection, prompting several clinical trials. Here, we used both cardiomyocyte-restricted and ubiquitously expressed reporter transgenes to track the fate of haematopoietic stem cells after 145 transplants into normal and injured adult mouse hearts. No transdifferentiation into cardiomyocytes was detectable when using these genetic techniques to follow cell fate, and stem-cell-engrafted hearts showed no overt increase in cardiomyocytes compared to sham-engrafted hearts. These results indicate that haematopoietic stem cells do not readily acquire a cardiac phenotype, and raise a cautionary note for clinical studies of infarct repair.

Haematopoietic stem cells do not transdifferentiate into cardiac myocytes in myocardial infarcts

Uniparental parthenotes are considered an unwanted byproduct of in vitro fertilization. In utero parthenote development is severely compromised by defective organogenesis and in particular by defective cardiogenesis. Although developmentally compromised, apparently pluripotent stem cells can be derived from parthenogenetic blastocysts. Here we hypothesized that nonembryonic parthenogenetic stem cells (PSCs) can be directed toward the cardiac lineage and applied to tissue-engineered heart repair. We first confirmed similar fundamental properties in murine PSCs and embryonic stem cells (ESCs), despite notable differences in genetic (allelic variability) and epigenetic (differential imprinting) characteristics. Haploidentity of major histocompatibility complexes (MHCs) in PSCs is particularly attractive for allogeneic cell-based therapies. Accordingly, we confirmed acceptance of PSCs in MHC-matched allotransplantation. Cardiomyocyte derivation from PSCs and ESCs was equally effective. The use of cardiomyocyte-restricted GFP enabled cell sorting and documentation of advanced structural and functional maturation in vitro and in vivo. This included seamless electrical integration of PSC-derived cardiomyocytes into recipient myocardium. Finally, we enriched cardiomyocytes to facilitate engineering of force-generating myocardium and demonstrated the utility of this technique in enhancing regional myocardial function after myocardial infarction. Collectively, our data demonstrate pluripotency, with unrestricted cardiogenicity in PSCs, and introduce this unique cell type as an attractive source for tissue-engineered heart repair.

/pdf/parthenogenetic-stem-cells-for-tissue-engineered-heart-2qqxkxd4m7.pdf

Parthenogenetic stem cells for tissue-engineered heart repair

1. Beat-to-beat performance of myocardium is highly dependent on sarcomere length. The physiological basis for this effect is not well understood but presumably includes alterations in the extent of overlap between thick and thin filaments. Sarcomere length dependence of activation also appears to be involved since length-tension relationships in cardiac muscle are usually steeper than those in skeletal muscle. 2. An explanation recently proposed to account for the difference between length-tension relationships is that the cardiac isoform of troponin C (cTnC) has intrinsic properties that confer greater length-dependent changes in the Ca2+ sensitivity of tension than does skeletal troponin C (sTnC), presumably due to greater length-dependent changes in the Ca(2+)-binding affinity of cTnC. To test this hypothesis, transgenic mice were developed in which fast sTnC was expressed ectopically in the heart. This allowed a comparison of the length dependence of the Ca2+ sensitivity of tension between myocytes having thin filaments that contained either endogenous cTnC or primarily sTnC. 3. In myocytes from both transgenic and normal mice, the Ca2+ sensitivity of tension increased similarly when mean sarcomere length was increased from approximately 1.83 to 2.23 microns. In both cases, the mid-point (pCa50) of the tension-pCa (i.e. -log[Ca2+]) relationship shifted 0.12 +/- 0.01 pCa units (mean +/- S.E.M.) in the direction of lower Ca2+. 4. We conclude that the Ca2+ sensitivity of tension in myocytes changes as a function of sarcomere length but is independent of the isoform of troponin C present in the thin filaments.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1995.sp020573

Length dependence of Ca2+ sensitivity of tension in mouse cardiac myocytes expressing skeletal troponin C.

Cardiomyocyte death is a common endpoint for many forms of cardiovascular disease. Given that adult mammalian cardiomyocytes lack sufficient proliferative capacity to effect regeneration, sustained cardiomyocyte loss culminates with increasing impairment of global myocardial function. In cases of severe myocardial dysfunction, the only therapeutic recourse at the current time is replacement of the diseased heart via transplantation. Several approaches designed to provide either temporary or long-term augmentation of myocardial function have been proposed and/or attempted. These include the use of ventricular assist devices, development of mechanical hearts, cardiomyoplasty, and xenograft transplant.

Mark H. Soonpaa

Papers

Haematopoietic stem cells do not transdifferentiate into cardiac myocytes in myocardial infarcts

Parthenogenetic stem cells for tissue-engineered heart repair

Length dependence of Ca2+ sensitivity of tension in mouse cardiac myocytes expressing skeletal troponin C.

Potential Approaches For Cell-Mediated Myocardial Repair