Cardiac Repair and Regeneration: The Value of Cell Therapies

Login or register to view PDF.
Abstract

Ischaemic heart disease is the predominant contributor to cardiovascular morbidity and mortality; one million myocardial infarctions occur per year in the USA, while more than five million patients suffer from chronic heart failure. Recently, heart failure has been singled out as an epidemic and is a staggering clinical and public health problem associated with significant mortality, morbidity and healthcare expenditures, particularly among those aged ≥65 years. Death rates have improved dramatically over the last four decades, but new approaches are nevertheless urgently needed for those patients who go on to develop ventricular dysfunction and chronic heart failure. Over the past decade, stem cell transplantation has emerged as a promising therapeutic strategy for acute or chronic ischaemic cardiomyopathy. Multiple candidate cell types have been used in preclinical animal models and in humans to repair or regenerate the injured heart, either directly or indirectly (through paracrine effects), including: embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), neonatal cardiomyocytes, skeletal myoblasts (SKMs), endothelial progenitor cells, bone marrow mononuclear cells (BMMNCs), mesenchymal stem cells (MSCs) and, most recently, cardiac stem cells (CSCs). Although no consensus has emerged yet, the ideal cell type for the treatment of heart disease should: (a) improve heart function; (b) create healthy and functional cardiac muscle and vasculature, integrated into the host tissue; (c) be amenable to delivery by minimally invasive clinical methods; (d) be available ‘off the shelf’ as a standardised reagent; (e) be tolerated by the immune system; (f) be safe oncologically, i.e. not create tumours; and (g) circumvent societal ethical concerns. At present, it is not clear whether such a ‘perfect’ stem cell exists; what is apparent, however, is that some cell types are more promising than others. In this brief review, we provide ongoing data on agreement and controversy arising from clinical trials and touch upon the future directions of cell therapy for heart disease.

Disclosure
The authors have no conflicts of interest to declare.
Correspondence
Daniel A Lerman, Royal Infirmary Hospital of Edinburgh (NHS Lothian), University of Edinburgh, Scotland, UK. MRC Centre for Regenerative Medicine and College of Medicine and Veterinary Medicine, University of Edinburgh, Scotland, UK. E: s0978484@staffmail.ed.ac.uk
Received date
27 January 2016
Accepted date
29 February 2016
DOI
http://dx.doi.org/10.15420/ecr.2016:8:1

Acute myocardial infarction (AMI) is still a major public health problem worldwide, causing high rates of morbidity and mortality. In the United States, nearly one million patients suffer from AMI each year.1 In the UK, around 80,000 people died from coronary heart disease (CHD) in 2010.2

The current approach to the treatment of myocardial infarction involves early revascularisation with percutaneous coronary intervention (PCI) or coronary artery bypass grafting (CABG), followed by the medical management of atherosclerotic risk factors, late ventricular remodelling and cardiac arrhythmias.

Improvements in the treatment of AMI, especially use of reperfusion therapy, have led to larger numbers of survivors. In patients who would have survived despite reperfusion therapy, use of this treatment should lead to greater myocardial salvage and a reduced extent of ventricular injury in many. However, others who might not have survived previously may now do so, but with substantial left ventricular damage.3,4 The net consequence of these two opposing effects on the early and later risk of developing heart failure after AMI is uncertain.

Several clinical trials and registries, despite methodological differences, tend to agree that heart failure is a common occurrence after AMI, and there has been concern that an increasing pool of survivors of AMI might fuel an ‘epidemic’ of heart failure.5,6

Patients with chronic heart failure (CHF) have a mortality of 20 % within the first year after diagnosis.2 CHF accounts for roughly 70,000 deaths in the UK each year, corresponding to an average of 190 deaths per day.2

Despite recent advances in medical and device therapy and improvements in care over the past 20 years, the outlook for patients with heart failure remains poor, and survival rates are worse than those for bowel, breast or prostate cancer.7–9 Therefore, any new treatment modality that benefits heart failure patients has the potential to result in a dramatic improvement in health outcomes and substantial cost savings for the community.

Ventricular remodelling after AMI involves replacing a significant amount of cardiomyocyte cell mass with fibrotic tissue, which results in contractile dysfunction. This degenerative process is not always irreversible; depending on the extent of damage and age of the subject, some spontaneous regeneration of the cardiac muscle may occur, which might be of key importance in the next generation of treatment modalities for such a severe and frequently deadly condition. The identity of the stem cells involved in cardiac repair is, however, still uncertain. Several novel treatment strategies are emerging, aiming at each stage of the pathological remodelling process, including stem cell treatments, paracrine signalling, microRNA-modification of key signalling events and tissue engineering. Cardiomyoplasty and stem cell therapy are generating great expectation to treat different types of cardiac diseases, including AMI, refractory angina and CHF. Effective medical treatments of these conditions will produce crucial improvement in overall health outcomes and substantial cost savings for the National Health Service (NHS).

Pathophysiology of Myocardial Infarction

Myocardial ischaemia may result from either a rise in metabolic demand or a reduction of oxygen and nutrient supply to the myocardium. Myocardial infarct occurs if the demand/supply mismatch is enough to trigger cellular necrotic and apoptotic mechanisms within cardiomyocytes. Several conditions are associated with an increased myocardial metabolic demand, such as severe hypertension, severe aortic stenosis, other valvular pathologies and low cardiac output syndromes. Not only do these conditions increase the metabolic demand, but they also have the capability to reduce the coronary perfusion by lowering the mean aortic pressure. Infarction can also be caused by other conditions that are characterised by thromboembolic or atherosclerotic stenosis/occlusion of coronary arteries, leading to ischaemia primarily by decreasing the delivery of oxygen and nutrients to the myocardium.10

Myocardial Repair after Myocardial Infarction

There are several cellular changes that occur in the myocardium following myocardial infarction. During the first 6–12 hours, a process of coagulative necrosis occurs, and the fibres at the periphery of the infarct become elongated and narrowed with signs of vacuolar degeneration. Concomitantly, oedema and neutrophils are observed in the intercellular spaces. This process lasts for 3 to 4 days. Following this stage, the necrotic myocytes are removed by macrophages, which may be actively phagocytic for 7 to 10 days. Finally, granulation tissue with loose collagen fibres and copious capillaries commence the healing and repair processes, in which the necrotic cardiac muscle cells are replaced by a collagen scar.10

Cardiac Regeneration and Cell Therapy

The heart, which had been considered as a terminally developed organ with no potential for regeneration in post-natal life, has recently been recognised to possess some intrinsic reparability. Currently, there are two complementary theories about the process of intrinsic repair in the heart after an ischaemic injury: (1) cardiomyocytes re-enter the cell cycle and start the process of proliferation, regeneration and repair of the necrotic tissue;11,12 and (2) certain endogenous cardiac stem cells undergo growth and differentiation, regulated either by secreted inflammatory factors or autocrine regulation.13,14 Both mechanisms may be involved in the process of heart regeneration.15 Currently, the research focus is on how to translate the preclinical cell-based results into effective clinical treatment. In order to repair the human heart, it is crucial to identify the appropriate cell type and optimal route to deliver it. The selected cells should be able to differentiate into mature cardiomyocytes and achieve electrical integration with mechanical coupling. They should also have the capability to repair the heart via paracrine effects. Importantly, delivery of such cells should be done with careful consideration of the risks and benefits to the patient. Possible delivery methods include intravenous, intracoronary or intramyocardial.16 In selecting appropriate cells, one needs to know each cell’s individual potential: its regenerative activity (ESC, iPSC, and endogenous cardiac stem cells), paracrine effects (MSC) and angiogenesis activity (endothelial precursors).

Endogenous Cardiac Progenitors There are three different embryonic cardiac cell precursors: the cardiac mesoderm; the neural crest cells; and the pro-epicardial territory. Each of the original precursors will turn into different cardiac structures, as follows. (1) Cardiac mesoderm becomes endocardial cells, atrial myocytes and ventricular myocytes. (2) Cardiac neural crest becomes aorta smooth muscle cells and autonomic nervous system. (3) Pro-epicardium becomes smooth muscle of coronary arteries, fibroblasts, endothelium of coronary arteries.17–19

Recently, multipotent stem cells were identified in each one of the layers of human blood vessels. Myogenic endothelial cells (MECs) are located in the intima of blood vessels, whereas pericytes and adventitial cells (ACs) are located in the media and adventitia, respectively. MECs and pericytes have the capability to regenerate myofibres in dysfunctional skeletal muscles and to improve cardiac contractility following AMI.19

Recently, cardiogenic progenitor cells (CPCs) were detected in the adult heart. It is still not completely clear whether CPCs originate from the bone marrow, or there are populations of embryonic cells localised in the right atrium and right ventricle. Also, there is still ongoing research to determine the participation of these cells in the physiological turnover of cardiac myocytes and vascular endothelial cells in the absence of myocardial injury.20 CPCs represent 1 % of the total cell population in the heart and are divided into three groups so far identified (c-Kit+, Sca-1+ and ISL-1+ cells) according to the expression of membrane markers.20 c-Kit+ cardiogenic stem cells express pluripotency, clonogenicity and self-renewal capabilities, and differentiate into myogenic, vascular endothelial and smooth muscle lines in vitro. These cells can regenerate the ischaemic myocardium in animal models.21,22 The group of CPCs expressing Sca-1+ interact with a homogeneous cell population in foetal and adult human hearts and show self-renewal properties together with active participation in cell signalling and cell adhesion.23

It is possible to differentiate Sca-1+ CPCs into cardiomyocytes by using 5-azacytidine. 5-Azacytidine is similar to cytidine, a nucleoside found in either DNA or RNA. The mechanism of action of this drug is through inhibition of the enzyme methyltransferase. 5-Azacytidine is incorporated into the structure of DNA and RNA instead of cytidine, inhibiting the synthesis of proteins within the cells.24 Additionally, the activation of extracellular signal-related kinases (ERK) by 5-azacytidine seems to trigger the differentiation of human MSCs into cardiomyocytes in vitro.25 In vitro differentiation to cardiomyocytes appears to involve the receptor for bone morphogenic proteins like BMPR1A.26 Differentiated murine Sca-1+ cells can be detected as mature cardiomyocytes after intravenous transfusion following myocardial ischaemia and necrosis in rats.26

A group of stem cells is found in the hearts of newborn mice, rats and humans. Neonatal mouse hearts have cells that express the transcription factor ISL-1 together with two more factors: Nkx2.5 and GATA4, which are crucial transcription factors that participate actively in the initial stages of cardiogenesis, but don’t express either c-Kit or Sca- 1.26,27 These cells can differentiate into cardiomyocyte phenotypes with intact calcium cycling. They produce action potentials when cultured together with neonatal myocytes.27,28 These findings allow the study of the molecular pathways linked to the differentiation of ISL-1+ cells into the different lineages in either postnatal or embryonic hearts. The limited capacity of human cardiomyocytes to regenerate in vivo is responsible for the development of heart failure after infarction. Understanding the molecular mechanisms involved in the differentiation of the embryonic heart is of crucial importance in the design of effective regenerative stem cell therapies to treat patients with cardiac injury.

Selection of Cell Types

There are two important mechanisms by which stem cells may work. (1) Paracrine effect of the cells: SKMs, BMMNCs and MSCs produce several cytokines and growth factors that increase angiogenesis, reduce apoptosis, decrease fibrosis and induce cardiac regeneration. Ischaemic patients can especially benefit from the paracrine effect, which enhances perfusion.29–31 (2) Trans-differentiation of the stem cells’ phenotypes into cardiomyocytes and replacement of injured cells, increasing the contractility of the injured tissue.

Bone marrow MSCs, adipose-tissue-derived stromal cells and pericytes are known to produce cardio-protective cytokines that could be enhanced by genetic engineering.30–32 These cells also have immunosuppressive properties, which allows their usage as potential allogenic drugs.33 Additionally, the cell factors can induce regeneration from myocardial niches of tissue-resident stem cells. The paracrine effect alone would not be enough to relieve severe heart failure with extended scars as it would require cardiac regeneration to complete the healing process. The cells should be able to contract and coordinate each other through Connexin-43, a protein involved in the myofibrillar coupling structure, thus avoiding lethal arrhythmias.34

Cardiac-committed stem cells could be extracted from endomyocardial biopsies or during CABG, expanded in vitro and reinjected. Current clinical human trials, such as Stem Cell Infusion in Patients with Ischaemic Cardiomyopathy (SCIPIO: cells harvested from right atrial appendage during CABG, which uses c-Kit + CSCs) and Cardiospherederived Autologous SCs to Reverse Ventricular Dysfuntion (CADUCEUS: endomyocardial biopsy, which uses CDCs), have been showing promising results.35,36 In these trials, the cells expanded in vitro are injected into the coronary arteries in the catheterisation laboratory. In contrast, the Autologous Human Cardiac-derived Stem Cell to Treat Ischaemic Cardiomyopathy (ALCADIA) trial involves the delivery of the cells into the myocardium during CABG. Cardiac-derived stem cells are extracted from endomyocardial biopsies, expanded and then delivered to the heart during CABG surgery by intramyocardial injections then a biodegradable gelatin hydrogel sheet containing fibroblast growth factor is implanted on the epicardium.37 The ongoing problem is to clarify the characterisation of the cell phenotypes, as current phenotypic differences could correspond to the same cell in a different stage of development.38

The CADUCEUS trial uses a mixed population of stem cells denominated by cardiospheres of which mesenchymal stem cells are a big proportion.39,40 The SCIPIO trial works with c-Kit cells; whereas ALCADIA uses a mixed population extracted from endomyocardial biopsies and cultured for a month.

Within the pool of pluripotent stem cells, human ESCs could be committed toward cardiac lineage in vitro. These cells were obtained from disposed embryos in the context of assisted fertilisation. Results show good engraftment of differentiated cardiomyocytes, although the risk for teratomas and immune rejection needs further investigation.41–43

Another source of potential cells could be the pool of iPSCs that are selected from the patient’s somatic cells and reprogrammed to embryonic pluripotent status. Because of their oncogenic potential, they still need larger animal trials before they can be introduced to the market.44 MSCs and fibroblasts could be manipulated in vitro towards enhanced cardiopoiesis, thus increasing the intrinsic therapeutic benefit of the treated cells.45–47

Perivascular/Mesenchymal Stem Cells

The pericyte is the second most common cardiac cell type and its participation in cardiac pathophysiology and regenerative medicine is crucial.48 Pericytes are perivascular cells with contractile capability similar to those of the smooth muscle cells that wrap around blood vessels. These cells carry out several functions, including active participation in the development of vessels and their structural maintenance. Additionally, they can communicate with surrounding cells during the angiogenic process, either by direct contact or paracrine signalling.49 New insights into the use of pericyte transfusion as a potential new treatment for AMI showed that there was a significant improvement in the infarcted heart in a mouse model. The effect was achieved through lowering the threshold and the activation of an angiogenic program in the recipient model.50

The identification of pericytes in tissue is a complex process because there is no single reliable marker. Currently, several markers, such as NG2 (neuron-glial antigen 2), α-SMA (alpha smooth muscle actin) and PDGFR-β (beta-type platelet-derived growth factor receptor), are used, each staining a subset of pericytes. Additionally, CD146 stains pericytes and a subset of endothelial cells; CD34 stains endothelial and progenitor cells; and CD31 and CD144 stain mature endothelial cells. Histologically, pericytes are identified as cells positive for CD146 but negative for endothelial markers such as CD34, CD31 or CD144. The NG2 marker is a chondroitin sulphate proteoglycan that can be found on the surface of pericytes and a small subset of glial and endothelial cells and is expressed by SMA-negative pericytes, either in micro-vessels or in the intimal layer of large vessels.51 α-SMA is present in vascular smooth muscle cells and in pericytes. This marker was identified in the microfilament bundles responsible for pericyte contractile functions.52 PDGFR-β is a useful abundant pericyte marker.53 Pericytes from mice that have abnormal PDGFR-β receptors exhibit micro-vascular abnormalities leading to lethal micro-haemorrhages and oedema.54 CD146, also known as Mel-CAM, MUC18, A32 antigen and S-Endo-1, is a specific membrane glycoprotein that can function as a Ca2+ independent cell adhesion molecule with participation in heterophilic activity between cells. CD146 is part of the immunoglobulin gene superfamily.55 CD34 is a trans-membrane protein expressed in either haematopoietic progenitor cells or vascular endothelial cells. In addition, CD34 takes an active part in the regulation of cell migration and differentiation.56

Origin of Potential Stem Cells. Modified from an Original Drawing by Mirko Corselli

Open in new tab
Open ppt

Pericytes have been identified in several human organs, including skeletal muscle, pancreas, adipose tissue and placenta, using markers such as CD146, NG2, and PDGFR-β and absence of haematopoietic, endothelial and myogenic cell markers.57 Recent research demonstrated that human pericytes that are located around capillaries and micro-vessels can produce MSCs while in culture. Additionally, during the process of vascular regeneration and under the effect of growth factors, adventitial cells can undergo a phenotypic trans-differentiation into pericyte-like cells (see Figure 1).58 Furthermore, there is clear evidence that pericytes contribute to cardiac repair by down-regulating immune cells via interaction with immunomodulatory cytokines and growth factors, following pericyte injection into ischaemic tissues.59

Clinical Applications

Current treatment protocols for AMI focus on reducing myocardial necrosis and irreversible damage by improving perfusion to the ischaemic area via medical or mechanical treatment such as CABG or PCI.60–62

The new potential cell-based treatments to deal with AMI derive from animal research in which mononuclear cells from bone marrow or peripheral blood were used in cardiac repair.63–68 Ongoing research in cardiac developmental and stem cell biology, as well as recent results from clinical trials SCIPIO and CADUCEUS using resident cardiac stem cells, have improved our understanding of in situ heart stem progenitor cells.69,70 The first non-randomised trials in humans showed that there was an improvement in cardiac function after the infusion of bone marrow stem cells and progenitor stem cells into the myocardium affected by the infarction.71–75 The stem cell types involved in the repair of cardiac tissue were first characterised by Stamm’s group in 2003, when the infusion of CD133+ progenitor cells extracted from haematopoietic tissues were applied into the ischaemic cardiac myocardium. The result of this treatment was an improvement in the general revascularisation process.76 The first randomised multicentre trial in 2009 studied patients with severe left ventricular dysfunction as a consequence of AMI. The patients, who were infused with selected CD34+ and CXCR4+ cells and non-selected mononuclear cells into the lumen of their coronary tree, saw significant improvement in their left ventricular ejection fractionafter 6 months.77 The mechanism of action of such treatment seems to be either an increase in the angiogenesis activity and/or trans-differentiation of the cells into myocytes.78 The paracrine secretion of cytokines and other factors also increases vascular growth, favours cardiac repair and reduces local fibrosis.79 Latest evidence from trials shows that adult bone marrow stem cell treatment significantly improves cardiac function in post MI patients and there is no evidence of any increase in morbidity or mortality in this treated group of patients.80 Research into more effective stem cell treatments allowed the isolation of neonatal and ischaemic myocardial cells expressing the c-Kit, MRD-1, ISL-1 or Sca-1 stem cell markers but no haematopoietic cell markers.26,81 The number of these cells increased after an AMI, suggesting an active role of these cells in cardiac repair.82

Application in Acute Myocardial Infarction with Concurrent CABG

Intra-myocardial injection of BMMNCs during CABG is shown to have improved outcomes compared with those of CABG alone.83 The aim of treating patients with stem cells after or during CABG following an episode of acute MI is to reduce later remodelling, which is known to have a negative impact on long-term outcomes.83 Such treatment is carried out by the interventional cardiologist and consists of delivering BMMNCs into the new coronary bypass graft. Unfortunately, there is still a need for more randomised trials to assess the potential benefits currently observed.83–85 Patients with poor left ventricular function undergoing CABG seem to be better at 6 months post-operative if trans-epicardial injection of CD133+ cells was performed intraoperatively.84,85 These observations likely result from the angiogenic potential of cells rather than cardiomyocyte regeneration, since the CD133+ marker is expressed in the membrane of the endothelial cells. The PRECISE (Percutaneous Robotically-enhanced Coronary Intervention) trials use adipose-tissue-derived cells collected with lipo-aspiration from patients at the time of surgery. These cells are subsequently reinfused into the endocardium of the left ventricle. The final results of this technique are still pending.86

Application in Refractory Angina

A second treatment indication under investigation is for refractory angina (angina caused by coronary insufficiency in the presence of coronary artery disease that cannot be controlled by a combination of medical therapy, angioplasty and coronary bypass surgery), which would involve stem cell treatment alone or complemented with surgery. The aim in this subset of patients would be to use the different cell types as carriers of multiple cytokines and growth factors in order to induce angiogenesis in the affected territory and thus relieve ischaemic symptoms.87,88 Patients with refractory angina are currently under investigation in randomised trials to assess the apparent efficacy of catheter-based endoventricular injection of CD34+ cell progenitors following treatment with granulocyte colony stimulating factor for 5 days in order to induce autologous cell mobilisation.89 Another randomised trial in the population of patients with refractory angina used trans-cathether endomyocardial injections of bone marrow unfractionated derived cells (MNC), which seem to have some efficacy in improving clinical parameters but more data is needed to find significant differences between the study arms.90

Application in Chronic Heart Failure

A third application under research is the treatment of chronic heart failure patients in whom the aim is to regenerate areas of non-contractile myocardial fibrosis to achieve physiological and functional contractility.88 Patients with chronic heart failure were included in the randomised, double-blinded placebo-controlled Myoblast Autologous Grafting trial. In addition to CABG, patients with severe left ventricular function underwent trans-epicardial injections of either autologous SKMs from a skeletal muscle biopsy or placebo injected in and around the scar.91

The preliminary results showed that there was no improvement in the ejection fraction at 6 months, but patients injected with 800 million cells presented a reduction in left ventricular volumes.91 The effects of such treatment on early post-operative rhythm abnormalities and left ventricular remodelling require further investigation. Intracoronary injection of bone-marrow-derived cells with and without CABG was tested in trials, but the results remain inconclusive.92,93 Regarding complications related to the type of cells, ventricular arrhythmia with myoblast implantation is the most worrisome.

Future Prospects

The future of cardiac repair may rely on understanding the intrinsic mechanisms that regulate endogenous mobilisation and or delivery of these cells. Additionally, further studies are needed to develop a deeper understanding of the properties of pericytes and their potential to migrate to different tissues away from their perivascular location and to play an active role in cardiac repair after ischaemia.49 This would involve a more modern interpretation of the pericyte’s role as a cell type involved in reducing the threshold for the activation of an angiogenic program in cardiac repair.50 It has been shown that exogenous administration of MSCs could stimulate cardiac precursors to proliferate and differentiate either by stimulation of the endogenous c-Kit+ CSCs or by improving cardiomyocyte cell cycling.94

Despite advancements in the field of cardiac regenerative biology, the perivascular cell compartment within the myocardium and its regenerative capability have not yet been studied in-depth. Pericytes and perivascular cells have a crucial role in physiological functions, as well as in the development of pathological conditions.95–98 Additionally, the participation of perivascular cells in post-injury tissue fibrosis has been shown in recent studies. The cardiac pericyte is the second most common cardiac cell type, and has started to attract attention in cardiac pathophysiology and regenerative medicine.99 There is ongoing research on the role of pericytes in the activation of endogenous cardiac progenitors during cardiac repair.

Recently, there has been increasing interest in the study of transcription factors and signalling pathways involved in cardiac regeneration. This has triggered the investigation of thymosin β4, which is a protein that can reactivate the cells’ embryonic developmental potential and stimulate epicardial cell trans-differentiation to vascular regeneration.100–103

Conclusions

Autologous cardiac cellular therapies appear to be safe and effective. The future of cardiac repair may rely on understanding the intrinsic mechanisms that regulate endogenous mobilisation and/or delivery of these cells. However, a considerable amount of work is to be performed before cardiomyoplasty (cell therapy of the heart) can be proposed as a routine treatment. The first question is which cells to use, as a variety of embryonic stem cells, reprogrammed adult stem cells, natural adult multi-lineage stem cells and lineage-committed stem cells are presently available. Arguably, the latter endogenous cardiomyogenic stem cells might be the best choice for cardiac repair. Ideally, these cells should be directly stimulated in situ, avoiding extraction, purification, culture and reinjection. It is therefore of uttermost importance to understand the identity and function of the cells that constitute the natural environment of cardiac progenitors and support their quiescence, self-renewal and activation. Additionally, further studies are needed to develop a deeper understanding about the properties of pericytes, as these cells have the potential to migrate to different tissues away from their perivascular location and play an active role in the activation of cardiac repair after ischaemia.49 This would involve a modern interpretation of the pericyte’s role as a cell type involved in reducing the threshold for the activation of an angiogenic program in cardiac repair.50

References
  1. Lloyd-Jones D, Adams R, Carnethon M, et al. Heart disease and stroke statistics – 2009 Update: A Report from the American Heart Association Statistics Committee and Stroke Statistics Subcommittee. Circulation 2009;119:e21–e181.
    Crossref | PubMed
  2. Townsend N, Williams J, Bhatnagar P, et al. Cardiovascular disease statistics, 2014. British Heart Foundation: London.
  3. Guidry UC, Evans JC, Larson MG, et al. Temporal trends in event rates after Q-wave myocardial infarction: the Framingham Heart Study. Circulation 1999;100:2054–9.
    Crossref | PubMed
  4. Steg PG, Dabbous OH, Feldman LJ, et al. Global Registry of Acute Coronary Events Investigators. Determinants and prognostic impact of heart failure complicating acute coronary syndromes: observations from the Global Registry of Acute Coronary Events (GRACE). Circulation 2004;109:494–9.
    Crossref | PubMed
  5. Jhund PS, McMurray JJ. Heart failure after acute myocardial infarction: a lost battle in the war on heart failure? Circulation 2008;118:2019–21.
    Crossref | PubMed
  6. Hellermann JP, Jacobsen SJ, Redfield MM, et al. Heart failure after myocardial infarction: clinical presentation and survival. Eur J Heart Fail 2005;7:119–25.
    Crossref | PubMed
  7. Brenner H, Bouvier AM, Foschi R, et al. Progress in colorectal cancer survival in Europe from the late 1980s to the early 21st century: the EUROCARE study. Int J Cancer 2012;131:1649–58.
    Crossref | PubMed
  8. Coleman MP, Formn D, Bryant H, et al. Cancer survival in Australia, Canada, Denmark, Norway, Sweden, and the UK, 1995–2007 (the International Cancer Benchmarking Partnership): an analysis of population-based cancer registry data. Lancet 2011;377:127–38.
    Crossref | PubMed
  9. Siegel R, DeSantis C, Virgo K, et al. Cancer treatment and survivorship statistics, 2012. CA Cancer J Clin 2012;62: 220–41.
    Crossref | PubMed
  10. Cotran RS, Kumar V, Robbins SL (eds). Robbins Pathologic Basis of Disease. 5th ed. Philadelphia, PA: WB Saunders, 1994.
  11. Bersell K, Arab S, Haring B, et al. Neuregulin1/ErbB4 signaling induced cardiomyocyte proliferation and repair of heart injury. Cell 2009;138:257–70.
    Crossref | PubMed
  12. Kühn B, Del Monte F, Hajjar RJ, et al. Periostin induces proliferation of differentiated cardiomyocytes and promotes cardiac repair. Nat Med 2007;13:962–9.
    Crossref | PubMed
  13. Gnecchi M, He H, Noiseux N, et al. Evidence supporting paracrine hypothesis for Akt-modified mesenchymal stem cell-mediated cardiac protection and functional improvement. FASEB J 2006;20:661–9.
    Crossref | PubMed
  14. Ventura C, Cantoni S, Bianchi F, et al. Hyaluronan mixed esters of butyric and retinoic acid drive cardiac and endothelial fate in term placenta human mesenchymal stem cells and enhance cardiac repair in infarcted rat hearts. J Biol Chem 2007;282:14243–52.
    Crossref | PubMed
  15. Ventura C. Cardiomyocyte proliferation: paving the way for cardiac regenerative medicine without stem cell transplantation. Cardiovasc Res 2010;85:643–4.
    Crossref | PubMed
  16. Bartunek J, Dimmeler S, Drexler H, et al. The consensus of the task force of the European Society of Cardiology concerning the clinical investigation of the use of autologous adult stem cells for repair of the heart. Eur Heart J 2006;27:1338–40.
    Crossref | PubMed
  17. Harvey RP. Patterning the vertebrate heart. Nat Rev Genet 2002;3:544–56.
    Crossref | PubMed
  18. Brand T. Heart development: molecular insights into cardiac specification and early morphogenesis. Dev Biol 2003;258: 1–19.
    Crossref | PubMed
  19. Chen CW, Corselli M, Péault B, et al. Human blood-vesselderived stem cells for tissue repair and regeneration. J Biomed Biotechnol 2012;2012:597439.
    Crossref | PubMed
  20. Henning RJ, Haley JA. Stem cells in cardiac repair. Future Cardiol 2011;7:99–117.
    Crossref | PubMed
  21. Bearzi C, Rota M, Hosoda T, et al. Human cardiac stem cells. Proc Natl Acad Sci U S A 2007;104:14066–73.
    Crossref | PubMed
  22. Beltrami A, Barlucchi I, Torella D, et al. Adult cardiac stem cells are multipotent and support myocardial regeneration. Cell 2003;114:763–76.
    Crossref | PubMed
  23. Van Vliet P, Roccio M, Smits AM, et al. Progenitor cells isolated from the human heart: a potential cell source for regenerative therapy. Neth Heart J 2008;16:163–9.
    Crossref |PubMed
  24. Christman JK. 5-Azacytidine and 5-aza-2’-deoxycytidine as inhibitors of DNA methylation: mechanistic studies and their implications for cancer therapy. Oncogene 2002;21:5483–95.
    Crossref | PubMed
  25. Qian Q, Qian H, Zhang X, et al. 5-Azacytidine induces cardiac differentiation of human umbilical cord-derived mesenchymal stem cells by activating extracellular regulated kinase. Stem Cells Dev 2012;21:67–75.
    Crossref | PubMed
  26. Oh H, Bradfute SB, Gallardo TD, et al. Cardiac progenitor cells from adult myocardium: homing, differentiation, and fusion after infarction. Proc Natl Acad Sci U S A 2003;100:12313–8.
    Crossref | PubMed
  27. Laugwitz KL, Moretti A, Lam J, et al. Postnatal isl1+ cardioblasts enter fully differentiated cardiomyocyte lineages. Nature 2005;433:647–53.
    Crossref | PubMed
  28. Moretti A, Caron L, Nakano A, et al. Multipotent embryonic isl1+ progenitor cells lead to cardiac, smooth muscle, and endothelial cell diversification. Cell 2006;127:1151–65.
    Crossref | PubMed
  29. Perez-Ilzarbe M, Agbulut O, Pelacho B, et al. Characterization of the paracrine effects of human skeletal myoblasts transplanted in infarcted myocardium. Eur J Heart Fail 2008;10:1065–72.
    Crossref | PubMed
  30. Kinnaird T, Stabile E, Burnett MS, et al. Marrow-derived stromal cells express genes encoding a broad spectrum of arteriogenesis cytokines and promote in vitro and in vivo arteriogenesis through paracrine mechanisms. Circ Res 2004;94:678–85.
    Crossref | PubMed
  31. Rehman J, Traktuev D, Li J, et al. Secretion of angiogenic and atiapoptotic factors by hunman adipose stromal cells. Circulation 2004;109:1292–8.
    Crossref | PubMed
  32. Scherschel JA, Soonpaa MH, Srour EF, et al. Adult bone marrow-derived cells do not acquire functional attributes of cardiomyocytes when transplanted into peri-infarct myocardium. Mol Ther 2008;16:1129–37.
    Crossref | PubMed
  33. Bartholomew A, Sturgeon C, Siatskas M, et al. Mesenchymal stem cells suppress lymphocyte proliferation in vitro and prolong skin graft survival in vivo. Exp Hematol 202;30:42–8.
    Crossref | PubMed
  34. Song H, Hwang HJ, Chang W, et al. Cardiomyocytes from phorbol myristate acetate-activated mesenchymal stem cells restore electromechanical function in infarcted rat hearts. Proc Natl Acad Sci U S A 2011;108:296–301.
    Crossref | PubMed
  35. Narazaki G, Uosaki H, Teranishi M, et al. Directed and systematic differentiation of cardiovascular cells from mouse induced pluripotent stem cells. Circulation 2008;118: 498–506.
    Crossref | PubMed
  36. Zhang J, Wilson GF, Soerens AG, et al. Functional cardiomyocytes derived from human induced pluripotent stem cells. Circ Res 2009;104:e30–41.
    Crossref | PubMed
  37. Yacoub MH, Terrovitis J. CADUCEUS, SCIPIO, ALCADIA: Cell therapy trials using cardiac-derived cells for patients with post myocardial infarction LV dysfunction, still evolving. Glob Cardiol Sci Pract 2013;2013:5–8.
    Crossref | PubMed
  38. Bollini S, Smart N, Riley PR. Resident cardiac progenitor cells: at the heart of regeneration. J Mol Cell Cardiol 2011;50: 296–303.
    Crossref | PubMed
  39. Davis DR, Kizana E, Terrovitis J, et al. Isolation and expansion of functionally competent cardiac progenitor cells directly from heart biopsies. J Mol Cell Cardiol 2010;49:312–21.
    Crossref | PubMed
  40. Mishra R, Vijayan K, Colletti EJ, et al. Characterization and functionality of cardiac progenitor cells in congenital heart patients. Circulation 2011;123:364–73.
    Crossref | PubMed
  41. Li Z, Lee A, Huang M, et al. Imaging survival and function of transplanted cardiac resident stem cells. J Am Coll Cardiol 2009;53:1229–40.
    Crossref | PubMed
  42. Caspi O, Huber I, Kehat I, et al. Transplantation of human embryonic stem cell-derived cardiomyocytes improves myocardial performance in infarcted rat hearts. J Am Coll Cardiol 2007;50:1884–93.
    Crossref | PubMed
  43. Blin G, Nury D, Stefanovic S, et al. A purified population of multipotent cardiovascular progenitors derived from primate pluripotent stem cells engrafts in postmyocardial infarcted nonhuman primates. J Clin Invest 2010;120:1125–39.
    Crossref | PubMed
  44. Pera MF. Stem cells: the dark side of induced pluripotency. Nature 2011;471:46–7.
    Crossref | PubMed
  45. Behfar A, Yamada S, Crespo-Diaz R, et al. Guided cardiopoiesis enhances therapeutic benefit of bone marrow human mesenchymal stem cells in chronic myocardial infarction. J Am Coll Cardiol 2010;56:721–34.
    Crossref | PubMed
  46. Hahn JY, Cho HJ, Kang HJ, et al. Pre-treatment of mesenchymal stem cells with a combination of growth factors enhances gap junction formation, cytoprotective effect on cardiomyocytes, and therapeutic efficacy for myocardial infarction. J Am Coll Cardiol 2008;51:933–43.
    Crossref | PubMed
  47. Ieda, M, Fu JD, Delgado-Olquin P, et al. Direct reprogramming of fibroblasts into functional cardiomyocytes by defined factors. Cell 2010;142:375–86.
    Crossref | PubMed
  48. Nees S, Weiss D, Juchem G. Focus on cardiac pericytes. Pflugers Arch 2013;465:779–87.
    Crossref | PubMed
  49. Bergers G, Song S. The role of pericytes in blood vessel formation and maintenance. Neuro Oncol 2005;7:452–64.
    Crossref | PubMed
  50. Katare R, Riu F, Mitchel K, et al. Transplantation of human pericyte progenitor cells Improves the repair of Infarcted Heart through activation of an angiogenic program Involving Micro-RNA-132. Circ Res 2011;109:894–906.
    Crossref | PubMed
  51. Ozerdem U, Grako KA, Dahlin-Huppe K, et al. NG2 proteoglycan is expressed exclusively by mural cells during vascular morphogenesis. Dev Dyn 2001;222:218–27.
    Crossref | PubMed
  52. Skalli O, Pelte MF, Peclet MC, et al. Alpha-smooth muscle actin, a differentiation marker of smooth muscle cells, is present in microfilamentous bundles of pericytes. J Histochem Cytochem 1989;37:315–21.
    Crossref | PubMed
  53. Winkler EA, Bell RD, Zlokovic BV. Pericyte-specific expression of PDGF beta receptor in mouse models with normal and deficient PDGF beta receptor signalling. Mol Neurodegener 2010;5:32.
    Crossref | PubMed
  54. Abramsson A, Lindblom P, Betsholtz C. Endothelial and nonendothelial sources of PDGF-B regulate pericyte recruitment and influence vascular pattern formation in tumors. J Clin Invest 2003;112:1142–51.
    Crossref | PubMed
  55. Shih IM. The role of CD146 (Mel-CAM) in biology and pathology. J Pathol 1999;189:4–11.
    Crossref | PubMed
  56. Ling G, Finger E, Gutierrez-Ramos JC. Expression of CD34 in endothelial cells, hematopoietic progenitors and nervous cells in fetal and adult mouse tissues. Eur J Immunol 1995;25:1508–16.
    Crossref | PubMed
  57. Crisan M, Yap S, Casteilla L, et al. A perivascular origin for mesenchymal stem cells in multiple human organs. Cell Stem Cell 2008;3:229–30.
    Crossref | PubMed
  58. Corselli M, Chen CW, Sun B, et al. The tunica adventitia of human arteries and veins as a source of mesenchymal stem cells. Stem Cells Dev 2012;21:1299–308.
    Crossref | PubMed
  59. Kovac A, Erickson MA, Banks WA. Brain microvascular pericytes are immunoactive in culture: cytokine, chemokine, nitric oxide, and LRP-1 expression in response to lipopolysaccharide. J Neuroinflammation 2011;8:139.
    Crossref | PubMed
  60. Rathore SS, Gersh BJ, Weinfurt KP, et al. The role of reperfusion therapy in paced patients with acute myocardial infarction. Am Heart J 2001;142:516–9.
    Crossref | PubMed
  61. Ryan TJ. Percutaneous coronary intervention in st-elevation myocardial infarction. Curr Cardiol Rep 2001;3:273–9.
    Crossref | PubMed
  62. Siddiqui MA, Tandon N, Mosley L, et al. Interventional therapy for acute myocardial infarction. J La State Med Soc 2001;153:292–9.
    PubMed
  63. Deb A, Wang S, Skelding KA, et al. Bone marrow-derived cardiomyocytes are present in adult human heart: A study of gender-mismatched bone marrow transplantation patients. Circulation 2003;107:1247–9.
    Crossref | PubMed
  64. Jackson KA, Majka SM, Wang H, et al. Regeneration of ischemic cardiac muscle and vascular endothelium by adult stem cells. Journal of Clinical Investigation 2001;107:1395–402.
    Crossref | PubMed
  65. Orlic D, Kajstura J, Chimenti S, et al. Bone marrow cells regenerate infarcted myocardium. Nature 2001;410:701–5.
    Crossref | PubMed
  66. Orlic D, Kajstura J, Chimenti S, et al. Mobilized bone marrow cells repair the infarcted heart, improving function and survival. Proc Natl Acad Sci U S A 2001;98:10344–9.
    Crossref | PubMed
  67. Toma C, Pittenger MF, Cahill KS, et al. Human mesenchymal stem cells differentiate to a cardiomyocyte phenotype in the adult murine heart. Circulation 2002;105:93–8.
    Crossref | PubMed
  68. Yoon YS, Wecker A, Heyd L, et al. Clonally expanded novel multipotent stem cells from human bone marrow regenerate myocardium after myocardial infarction. J Clin Invest 2005;115:326–38.
    Crossref | PubMed
  69. Chien KR, Domian IJ, Parker KK. Cardiogenesis and the complex biology of regenerative cardiovascular medicine. Science 2008;322:1494–7.
    Crossref | PubMed
  70. Makkar RR, Smith RR, Cheng K, et al. Intracoronary cardiosphere-derived cells for heart regeneration after myocardial infarction (CADUCEUS): a prospective, randomised phase 1 trial. Lancet 2012;379:895–904.
    Crossref | PubMed
  71. Assmus B, Schachinger V, Teupe C, et al. Transplantation of progenitor cells and regeneration enhancement in acute myocardial infarction (TOPCARE-AMI). Circulation 2002;106:3009–17.
    Crossref | PubMed
  72. Fernandez-Aviles F, San Roman JA, Garcia-Frade J, et al. Experimental and clinical regenerative capability of human bone marrow cells after myocardial infarction. Circulation 2004;95:742–8.
    Crossref | PubMed
  73. Meyer GP, Wollert KC, Drexler H. Stem cell therapy: a new perspective in the treatment of patients with acute myocardial infarction. Eur J Med Res 2006;11:439–46.
    PubMed
  74. Strauer BE, Brehm M, Zeus T, et al. Repair of infarcted myocardium by autologous intracoronary mononuclear bone marrow cell transplantation in humans. Circulation 2002;106:1913–8.
    Crossref | PubMed
  75. Tse HF, Kwong YL, Chan JK, et al. Angiogenesis in ischaemic myocardium by intramyocardial autologous bone marrow mononuclear cell implantation. Lancet 2003;361:47–9.
    Crossref | PubMed
  76. Stamm C, Westphal B, Kleine HD, et al. Autologous bone-marrow stem-cell transplantation for myocardial regeneration. Lancet 2003;361:45–6.
    Crossref | PubMed
  77. Tendera M, Wojakowski W, Ruzylllo W, et al. Intracoronary infusion of bone marrow-derived selected CD34+CXCR4+ cells and non-selected mononuclear cells in patients with acute STEMI and reduced left ventricular ejection fraction: results of randomized, multicentre Myocardial Regeneration Infarction (REGENT) Trial. Eur Heart J 2009;30:1313–21.
    Crossref | PubMed
  78. Leri A, Kajstura J, Anversa P, et al. Myocardial regeneration and stem cell repair. Curr Probl Cardiol 2009;33:91–153.
    Crossref | PubMed
  79. Bartunek J, Vanderheyden M, Hill J, et al. Cells as biologics for cardiac repair in ischaemic heart failure. Heart 2010;96:792– 800.
    Crossref | PubMed
  80. Clifford DM, Fisher SA, Brunskill SJ, et al. Stem cell treatment for acute myocardial infarction. Cochrane Database Syst Rev 2012;2:CD006536.
    Crossref | PubMed
  81. Urbanek K, Torella D, Sheikh F, et al. Myocardial regeneration by activation of multipotent cardiac stem cells in ischemic heart failure. Proc Natl Acad Sci U S A 2005;102:8692–7.
    Crossref | PubMed
  82. Beltrami AP, Urbanek K, Kajstura J, et al. Evidence that human cardiac myocytes divide after myocardial infarction. N Engl J Med 2001;344:1750–7.
    Crossref | PubMed
  83. Janssens S. Stem cells in the treatment of heart disease. Annu Rev Med 2010;61:287–8.
    Crossref | PubMed
  84. Zhao Q, Sun Y, Xia L, et al. Randomized study of mononuclear bone marrow cell transplantation in patients with coronary surgery. Ann Thorac Surg 2008;86:1833–40.
    Crossref | PubMed
  85. Akar AR, Durdu S, Arat M, et al. Five years follow up after transepicardial implantation of autologous bone marrow mononuclear cells to ungraftable coronary territories for patients with ischaemic cardiomyopathy. Eur J Cardiothoracic Surg 2009;36:633–43.
    Crossref | PubMed
  86. Sridhar P, Hedrick M, Baker T, et al. Adipose-derived regenerative cells for the treatment of patients with nonrevascularisable ischaemic cardiomyopathy – The PRECISE Trial. Intervent Cardiol Rev 2012;7:77–80.
    Crossref
  87. Wollert KC, Drexler H. Cell therapy for the treatment of coronary heart disease: a critical appraisal. Nat Rev Cardiol 2010;7: 204–15.
    Crossref | PubMed
  88. Vrijsen KR, Chamuleau SA, Noort WA, et al. Stem cell therapy for end-stage heart failure: indispensable role for the cell? Curr Opin Organ Transplant 2009;14:560–5.
    Crossref | PubMed
  89. Losordo DW, Schatz RA, White CJ, et al. Intramyocardial transplantation of autologous CD34+ stem cells for intractable angina: a phase I/IIa double blind, randomized controlled trial.Circulation 2007;115:3165–72.
    Crossref | PubMed
  90. Van Ramshorst J, Bax JJ, Beeres SL, et al. Intramyocardial bone marrow cell injection for chronic myocardial ischaemia: a randomized controlled trial. JAMA 2009;301:1997–2004.
    Crossref | PubMed
  91. Menache P, Alfieri O,Janssens S, et al. The myoblast autologus grafting in ischaemic cardiomyopathy (magic) trial. First randomized placebo-controlled study of myoblast transplantation. Circulation 2008;117:1189–200.
    Crossref | PubMed
  92. Strauer BE, Brehm M, Zeus T, et al. Regeneration of human infarcted heart muscle by intracoronary autologous bone marrow cell transplantation in chronic coronary artery disease: the IACS Study. J Am Coll Cardiol 2005;46:1651–8.
    Crossref | PubMed
  93. Fischer-Rasokat U, Assmus B, Seeger FH, et al. A pilot trial to assess potential effects of selective intracoronary bone marrow-derived progenitor cell infusion in patients with nonischaemic dilated cardiomyopathy: final 1-year results of the TOPCARE-DCM trial. Circ Heart Fail 2009;2:417–23.
    Crossref | PubMed
  94. Hatzistergos KE, Quevedo H, Oskouei BN, et al. Bone marrow mesenchymal stem cells stimulate cardiac stem cell proliferation and differentiation. Circ Res 2010;107:913–22.
    Crossref | PubMed
  95. Dellavalle A, Maroli G, Covarello D, et al. Pericytes resident in postnatal skeletal muscle differentiate into muscle fibers and generate satellite cells. Nat Commun 2011;2:499.
    Crossref | PubMed
  96. Tang W, Zeve D, Suh JM, et al. White fat progenitor cells reside in the adipose vasculature. Science 2008;322:583–6.
    Crossref | PubMed
  97. Armulik A, Genové G, Mäe M, et al. Pericytes regulate the blood–brain barrier. Nature 2010;468:557–61.
    Crossref | PubMed
  98. Tang Z, Wang A, Yuan F, et al. Differentiation of multipotent vascular stem cells contributes to vascular diseases. Nat Commun 2012;3:875.
    Crossref | PubMed
  99. Nees S, Weiss DR, Juchem G. Focus on cardiac pericytes. Pflugers Archiv 2013;465:779–87.
    Crossref | PubMed
  100. Bollini Sl, Vieira JM, Howard S, et al. Re-activated adult epicardial progenitor cells are a heterogeneous population molecularly distinct from their embryonic counterparts. Stem Cells Dev 2014;23:1719–30.
    Crossref | PubMed
  101. Crockford D, Turjman N, Allan C, et al. Thymosin beta4: structure, function, and biological properties supporting current and future clinical applications. Ann N Y Acad Sci 2010;1194:179–89.
    Crossref | PubMed
  102. Gomez-Marquez J, Dosil M, Segade F, et al. Thymosin-beta 4 gene. Preliminary characterization and expression in tissues, thymic cells, and lymphocytes. J Immunol 1989;143:2740–4.
    PubMed
  103. Rui Ll, Yu N, Hong L, et al. Extending the time window of mammalian heart regeneration by thymosin beta 4. J Cell Mol Med 2014;18:2417–24.
    Crossref | PubMed