MUSA - Alberta Pharmacy Students' Association

Recommendations

Info

REVIEW with specific functions. 9 Primarily two types of mammalian stem cells are used in myocardial regeneration: embryonic stem cells found in the blastocyst during early embryogenesis, and adult stem cells found in adult tissues acting as progenitor cells. ES cells are pluripotent and can potentially give rise to a number of cell types, they are vital to tissue regeneration therapy, regardless of the field of research. Skeletal myoblasts, on the other hand, are committed progenitor cells of skeletal muscle; they are resistant to ischemia and highly proliferative. 10 These myoblasts, harvested from neonatal and adult animals, have been shown to differentiate into skeletal myotubes and improve left ventricular function following an infarct. 10 These results have been obtained in autologous, syngeneic, allogenic, and xenogenic transplants – largely in mice and rats, but also in swine and canine subjects. 10 Adult bone marrow derived stem cells have also been studied: there are hematopoietic stem cells, endothelial progenitor cells, and mesenchymal stem cells in adult bone marrow. Research has shown that treatment with mesenchymal stem cells (MSCs – precursors to muscle, bone, tendons, and ligaments) improves myocardial function by limiting ventricular remodeling. 11 It was known that intramyocardial injection of Akt-MSCs (mesenchymal stem cells overexpressing the survival gene Akt) restored cardiac function after only 72 hours. 11 Gnecchi et al. hypothesized that, because such a rapid recovery could not be due to differentiation of the donor cells, regeneration was accomplished through the action of factors provided by the MSCs. 12 It was thought that these factors acted in a paracrine fashion to rescue the damaged heart tissue. Gnecchi et al. found that it is possible, in an animal model, to use mesenchymal stem cells to alleviate acute MI by injecting a cell-free supernatant that had been recovered from cultures of mesenchymal stem cells. 12 Adult CD34 + cells, easily obtained from peripheral blood, can trans-differentiate into cardiomyocytes in vivo at the site of injury in mice – yet this is still a work in progress. 13 Lastly, some sources suggest that there are small populations of “resident cardiac stem cells” endogenous to the heart that may serve a minor role in repair. 14 While researchers clearly have many types of stem cells at their disposal for use in cardiovascular therapies, only ES-derived cardiomyocytes and skeletal myoblasts have been able to achieve a proper level of cell survival for complete myocardial regeneration. 7 14 Skeletal Myoblast Transplantation Studies on skeletal myoblasts began with work of Chiu et al., dating back to 1995. His team studied the ability to repair injured myocardium in the presence of skeletal muscle cells, called “satellite cells.” 15 Each skeletal muscle fiber contains a few myogenic satellite cells, which are normally undifferentiated and quiescent. Injury activates these cells, causing them to enter mitosis and restore the functionality of the fiber. 16 Chiu et al. hypothesized that satellite cells, when implanted into injured myocardium and influenced by the cardiac environment, would undergo “milieudependent differentiation.” 15 Chiu et al. conducted two experiments: one in which the histological outcome of implanting skeletal satellite cells into acutely damaged myocardium was observed, and the other in which the presence of satellite cells at the site of implantation was confirmed. 15 Satellite cells were isolated from samples obtained from the tibialis anterior muscle of adult dogs, and then labeled with tritiated thymidine. Following which, the cells were grown in vitro for either 10 days or 3 weeks and implanted into the cryoinjured myocardium of the same animal. A catheter was used to implant the cells into the injured left ventricular free wall, which was acutely damaged by liquid nitrogen. Implant sites were evaluated radiographically to Figure 1. Data comparing the change in slope of PRSW relationship at 3 weeks in cryoinjured myocardium (white) with that in which myoblast transplantation failed (grey) and was successful (black). Courtesy of Nat Med: Taylor, DA. Figure 2. Electron micrograph of the transplanted myoblasts. Intercalated discs (i) connect the myocytes that have been transplanted. Courtesy of Nat Med: Taylor, DA. detect the thymidine labels. The results showed successful transdifferentiation of myoblast satellite cells into cardiomyocytes: new muscle cells in the implant sites histologically mimicked cardiac muscle, including the presence of intercalated discs, which are unique to cardiac muscle fibers. 15 Chiu et al. concluded that the cardiac environment played a role in cell differentiation, possibly through growth factors or other signaling pathways. In 1998, Taylor’s group made use of the cryoinfarction and cell implantation techniques described by Chiu et al. to test whether skeletal myoblast transplantation actually improves myocardial performance. 17 One week following myocardial injury, skeletal myoblasts from the rabbit hindlimb soleus muscle were transplanted into the damaged heart of the same rabbit. Following transplantation, 7 of the 12 rabbits had an improvement in myocardial performance: PRSW slope (an indication of systolic function and contractility) increased 34–400% compared to post-infarct values (Figure 1). 17 Electron microscopy of the implant sites did not show multinucleated skeletal fibers, but rather cells that resembled cardiomyocytes (Figure 2). 17 For the first time in animals, myoblast transplantation into acutely injured hearts was reported to improve cardiac performance in vivo. University of <strong>Alberta</strong> Health Sciences Journal • April 2012 • Volume 7 • Issue 1
Embryonic Stem Cell Transplantation Human ES cells served as another jumpingoff point for producing differentiated cardiomyocytes. Could ES cells normalize cardiac performance by being transplanted into injured myocardium? In 2002, Min’s team set out to answer this question using rat models. ES cells were transfected with green fluorescent protein (GFP) to identify cell survival, and transplanted into male rats after inducing MI by ligating the left anterior descending coronary artery. 18 Hemodynamics and muscle contraction were evaluated both post-MI and after the transplantation. Survival of the transplant cells was confirmed by observing GFP expression. Cardiac α-myosin heavy chain and (α-MHC) and troponin I (cTnI) were identified using specific antibodies. Not only did these cells survive, but they also improved cardiac function. Cardiac muscle wall tension was measured to determine myocyte function at a given ventricular pressure and radius (according to LaPlace’s Law, wall tension (T) is proportional to intraventricular pressure (P) and ventricular radius (r): T ∝ P ∙ r). Wall tension increased ~2-fold in the rats that had undergone transplants. 18 Cell-Based Pacemakers Thus far, efforts in stem cell transplantation have been discussed with the goal of repairing infarcted cardiac tissue. In 2004, Xue et al. approached cardiac therapy from a new angle: would it be possible to coordinate inactive cardiac muscle cells to beat synchronously with pacemakerlike donor cells? Human ES cells were transfected with GFP and transplanted subepicardially into guinea pigs in vivo. 19 After allowing the stem cells differentiate, a beating outgrowth of cardiomyocytes was dissected and checked for GFP expression. The cells were then transplanted onto a layer of quiescent rat cardiomyocytes in vitro, resulting in synchronous contractions at ~49 bpm of GFP-expressing cells and rat cardiomyocytes. 19 Without direct contact between the two cell lines, there was no synchronous beating. It is important to note that, unlike cell-based pacemakers, electronic pacemakers are largely unable to adapt to fluctuating requirements. In addition, sensing and pacing leads may become dislodged or malpositioned, and the pocket in which the electronic pacemaker sits is prone to infection. 20 This research shows that bio-pacemakers are clinically attractive, and may overcome the limitations of the electronic pacemaker. Figure 3. Photograph of a heart at week 9 posttransplantation of the cellular construct. Note the presence of neovascularization into the implanted biograft (B). Courtesy of Circulation: Leor, J. Bioengineered Cardiac Grafts Because it is generally accepted that the myocardium cannot regenerate after injury, much research has gone into replacing damaged muscle. 21 For example, Leor et al. tested whether bioengineering cardiac tissue within three-dimensional (3D) scaffolds would enhance cardiac function after extensive MI. 21 This novel practice involves the use of 3D cross-linked biopolymer, which serves as a support structure upon which functional cells can grow. The structure biodegrades once the cells have formed their own matrix. Rat cardiac cells were isolated and cultured, and seeded in cylindrical scaffolds made of sodium alginate with 100 μm pores. 21 Biograft implantation was performed 7 days post-MI: in each rat, two scaffolds were attached to the scar tissue induced by left main coronary artery blockage. After 9 weeks, the rats were euthanized, and the hearts were examined. Under histology, the scaffolds showed that they had successfully merged with the infarcted area (Figure 3). 21 Control rats were subjected to heart failure as a result of left ventricular remodeling post-MI – but in the biografted rats, there was less ventricular remodeling and deterioration. Biotechnology Builds a Heart Over 1,000 Canadians are waiting for a donor heart. 22 Bioartificial hearts could potentially circumvent this issue, and prevent significant sequelae associated with allogeneic heart transplantation – including long-term immunosuppression, renal failure, and hypertension. 23 Ott et al. describes an attempt to fabricate the construct of an entire heart, complete with vasculature and inner architecture: it involves “decellularizing” whole adult rat hearts using detergents, and then repopulating them with neonatal cardiac cells (Figure 4). 23 This ingenious technique utilizes “nature’s platform” of the heart, rather than attempting to engineer it from scratch. Not only does this bioartifical heart mimic the structure and cellular layout of a true heart – it also functions like one. When stimulating the constructs, acceptable measurements of cardiac function were obtained in 5 out of 8 hearts. 23 It is hypothesized that the bioengineered heart could be used as a full replacement organ in end-stage failure. However, this will only be possible with further organ maturation, reseeding of the heart’s vasculature with endothelial cells, and scaling up of the technology to work with human-sized hearts. Limitations Given the scarcity of donor hearts available to meet transplant needs, these approaches have immense advantages over heart transplants. However, a number of hurdles must be overcome before human ES cells can be used clinically. For instance, ethical issues related to accessing embryos limit scientists’ investigations. Also, human ES cells must go through rigorous testing before the cells can be used as a regenerative therapy. If transplanted regenerative cells are contaminated with undifferentiated ES cells, a tumor could form. 24 It has been suggested that differentiation of ES cells prior to implantation may prevent the formation of cancerous teratomas. 25 Also, in most cell transplant studies, many cells are lost before blood and nutrient supplies are established. 7 Introducing exogenous genes into transplant cells could make them more robust or allow them to release growth factors. For example, induced pluripotent stem cells have been studied by obtaining fibroblasts after genetic reprogramming; however, these have been shown to form teratomas as well. 26 It should also be noted that, in certain animal models, stem cells end up travelling from the heart to other nearby organs only a few hours post-transplant, and in some cases, the use of skeletal myoblasts has caused ventricular tachycardia. 27 Cell-based pacemakers could also lead to arrhythmias if the graft undergoes changes in ion channel expression. 19 Innovative methods have been studied in an attempt to prevent arrhythmias. A recent clinical, the CAuSMIC study, used a novel minimally-invasive catheter system to deliver autologous myoblasts to 23 human subjects with NYHA class II to IV heart failure. 28 This technique resulted in significantly improved heart University of <strong>Alberta</strong> Health Sciences Journal • April 2012 • Volume 7 • Issue 1 15 REVIEW
Page 1 and 2: UAHSJ www.uahsj.ualberta.ca ISSN 17
Page 3 and 4: Contents Editorial Commentary OSCE:
Page 5 and 6: University of Alberta Summer Studen
Page 7 and 8: it has been found that PPAR-γ agon
Page 9 and 10: trioglitazone greatly inhibited inf
Page 11 and 12: types of antipsychotics used in the
Page 13 and 14: antipsychotic. Of these patients, 4
Page 15: 35. Wykoff RF. Delusions of parasit
Page 19 and 20: 13. Yeh ET, Zhang S, Wu HD, Korblin
Page 21 and 22: Give Me Hope | by Vina Nguyen | Med
Page 23 and 24: creative, and actually produce some
Page 25 and 26: References 1. Brett-MacLean PJ, Cav
Page 27 and 28: The arts and Humanities in Medicine
Page 29 and 30: Diverse Classrooms. English Educati
Page 31 and 32: e defined as making judgments about
Page 33 and 34: Figure 1. Dr. Albert Ross Tilley. o
Page 35 and 36: therapy were numerous. Tannic acid
Page 37 and 38: he was acknowledged with Canada and
Page 39 and 40: Thank you The UAHSJ wishes to thank

MUSA - Alberta Pharmacy Students' Association

Create successful ePaper yourself

Delete template?

Save as template?