The Heart – A Vulnerable Organ That Cannot Be Replaced
The heart is a magnificent biological structure that is the powerhouse to all human life. It is the ultimate driving force like a car engine that allows organisms to keep ticking throughout the day/night cycle. The main function of heart is to pump blood around the body efficiently. Hence, this muscular organ, which is the size of an adult fist, together with the blood vessel network make up the circulatory system that is a vital necessity for maintaining life. A major role of the circulatory system is to supply oxygen and nutrients to the tissues, and remove carbon dioxide and other wastes product that can cause necrosis. The heart is also a vulnerable organ and its ability to pump blood around the body efficiently can be severely compromised as a result of cardiovascular disease (CVD), which are diseases associated with the heart and blood vessels.
In the UK about ‘7 million people are living with CVD’ with a mortality rate of ‘nearly 160,000 deaths each year’, this equates to about ‘26% of all death in the UK’ (British Heart Foundation’s pamphlet). Globally, CVD is still the main cause of ‘morbidity and mortality in many parts of the world’ and this is despite advancements in ‘pharmacological, interventional and surgical therapies’ over the last few decades, Furthermore, current medical and therapeutic treatments can only ‘decelerate, but not prevent, the process of deterioration to end-HF (what is HF)’, where the heart is no longer able to sufficiently pump and maintain blood flow to meet the demand of the body…. . need a linking sentence A number of the challenges researchers face is due to the heart’s ‘limited regenerative capacity’ and therefore has ‘insufficient potential to repair itself’.
Breakthrough in the last few decades on the potential use of stem cell therapy as a hoping option for the future. Stem cell therapy involves the use of stem cells which are undifferentiated cells that are ‘capable of giving rise to all multiple cell types in the body’. The current status in the UK with Stem therapy is still under clinical phase 1 and 2 trials with vast amount of knowledge still to be learned. Current studies involve rat and mice models with very few human trials due to the huge unknown effects of stem cells. These studies show some great promise and in this essay, I will be looking at why the heart cannot regenerate, why embryonic stem cells are not ideal to use, and explore clinical studies into the potential use of induced pluripotent stem cells and bone marrow derived adult stem cells to regenerate the heart. Limited heart repair and regeneration.
Cardiovascular disease presents the heart and rest of body system with detrimental damage that can lead to death. Ischaemic heart disease (IHD) is the most common type of CVD in the UK effecting ‘2. 3 million people’ and ‘responsible for 70,000 deaths each year’ (British Heart Foundation’s pamphlet). IHD consists of a sequence of ‘illnesses ranging from coronary atherosclerosis, myocardial infarction (MI), postinfarct heart failure (HF), and ultimately to end-stage HF’. Ischemic heart failure occurs because the cardiac tissue becomes deprived of oxygen and therefore unable to operate as a pump. The first stage of IHD pathophysiology is ‘coronary atherosclerosis’ where the coronary arteries that supply the cardiac tissue ‘become narrowed by a build-up of atheroma, a fatty material within their walls’. This leads to artery wall degeneration causing ‘endothelial dysfunction and plaque formation’ and the patient usually clinically presents with no symptoms of early IHD. If the plaque is not treated and builds up, this can lead to severe deprivation of oxygen that results in the ‘loss of critical amounts of cardiacmuscle cells (cardiomyocytes)’. This can eventually lead to acute myocardial infarction (AMI) which is commonly caused by a ‘coronary occlusion’ due to ‘thrombus following the rupture of an unstable atherosclerotic plaque’. If the occlusion is not opened immediately the ‘cardiomyocytes’ of the cardiac muscle begin to die and get replaced with ‘fibrous noncontractile scar tissue’ that eventually leads to ‘progressive contractile dysfunction’ and ventricle wall thinning.
There will be ‘overload of blood flow and pressure, ventricular remodelling’ due to ‘viable cardiac cells’ overstretching in order to try and maintain the cardiac output. If this worsens it leads to heart failure and ultimate death. The scar tissue formation is the area of interest in stem cell therapy, as it cannot conduct electrical impulses therefore the heart is unable to reach sufficient ventricular ejection and so the myocardium begins to fail and die.
What are embryonic stem cells and why are they not the ideal solution? Embryonic stem cells (ES) are pluripotent cells that have the ability to develop and differentiate into a variety of different body cell types, e. g. ‘cardiomyocytes, endothelial cells, and smooth muscle cells’. The ability to differentiate into different cells, this has allowed ES cells to be a ‘possible source population’ in the production of ‘cardiac repair cells’ for the damaged myocardium. Researchers have carried out studies on ‘murine models’ that showed the injection of cardiomyocytes derived from mouse or humanES cells into the ‘infarcted myocardium’ could ‘survive and improve heart function’. A particular study by ‘Min JY, et al’, where they induced myocardial infraction in two groups of rats via the ‘ligation of the left anterior descending coronary artery’. The study group was treated with ‘cultured ES cells’ by ‘intramyocardial injection’ into the damaged myocardium, whilst the control group was treated with ‘cell-free medium’. The ES cells transplanted into the ‘ischemically-injured myocardium’ rats were able ‘differentiate into normal myocardial cells’. This study demonstrated that this approach of transplanting ES cells into the ‘infracted failing hearts’ of rats were a ‘feasible and novel approach’ to help recover and ‘improve ventricular function’. These differentiated ‘myocardial cells’ were able to stay ‘viable up to four months’, therefore this could suggest them being strong candidate for cardiac regeneration therapy.
Although promising, the use of ES cells has encountered both ethical and safety concerns for use in humans. ES cells are obtained from the destruction of embryo to obtain the ‘inner cell mass’, which raisesconcern of destroying potential life that could have been if not for this method. Also, concerns towards the use of ‘consented’ ‘female donors’ who may be at risk, especially in terms of the procedure to remove the embryo and other side effects. Before human ES cells can be considered as ‘sources to regenerative tissue’, they have to go through ‘rigorous testing and purification procedures’. Researchers have carried out tests to see if ‘putative’ embryonic cells are ‘pluripotent’ by injecting them into ‘immunocompromised mice’ i. e. mice with a failed immune system. . This allowed the cells to have the capability to ‘survive and proliferate’ and ‘form a teratoma, a multi-layered, benign tumor’ which shows these cells have the scope to generate all possible cells from the ‘three germ layers’ of the embryo. Therefore, it is vital that human ES cells undergo ‘tight quality control’ to ensure they can purely differentiate. Otherwise, if ‘injected regenerated cells’ for the transplantation are ‘contaminated with undifferentiated ES cells’, a ‘tumor’ may form which is life threatening to the individual as it can lead to the development of cancer. The current myocardialcells derived from the ES cells are ‘not tissue matched to patients’, therefore the individual will have to be under ‘immunosuppression’ to reduce immunological rejection. Recently, researchers are exploring the use of induced pluripotent stem cells and adult stem cells derived from bone marrow as an alternative ‘to overcome the ethical issues and immune rejection’, as these cells are of ‘autologous origin’ and show great potential in studies.
Potential for heart regeneration using induced pluripotent stem cells Recent research into induced pluripotent stem cells(iPSCs) shows great promise in regenerative medicine. iPSCs are very similar to embryonic stem cells in terms of the benefits they bring, but they also offer great ‘perspectives for personalized medicine’. iPSCs are pluripotent cells that are derived from ‘adult differentiated cells through reprogramming’ with the incorporation of particular transcription factors. Work carried out by ‘Takahashi et al’, discovered that if 4 transcription factors: Sox2, Oct4, c-Myc and Klf4 were all ‘over-expressed’, they had the ability to generate ‘pluripotent stem cells’ from ‘mouse skin fibroblasts’. Research carried by ‘Dr Yamanaka’ in 2007 showed that the over-expression of most of the same transcription factors (‘Oct4, Sox2, Klf4’) in human fibroblasts could also produce iPSCs). These stem cells offered great range of different applications such as ‘autologous cell therapy’, ‘monogenic’ and ‘multigenic’ disease modelling, study of drug interactions and ‘therapeutic screening’.
The majority of iPSCs have been generated in studies so far have been by the ‘integrative delivery system’, where ‘integrating retroviral and lentiviral vectors’ are used to transfer ‘reprograming factors’ into target cells that generate iPSCs from ‘terminally differentiated somatic cells’. The advantage of this type of reprogramming is ease of handling but it is encountered with many drawbacks. The reprogramming process is ‘non-specific’ has poor efficiency of less than ‘0. 01%’ of ‘transduced cells’ becoming iPSCs. In addition to this, there are concerns that the generation of ‘iPSCs with multiple viral transgenes’ can get incorporated randomly into the human genome and formation of ‘partially reprogrammed cells’ due to ‘epigenetic silencing’ of the transgenes. This can result in ‘residual transgene expression’, where the differentiation of the iPSCs dampens down and get suppressed which results in a ‘higher tumorigenic’ risk if they are ‘transplanted into patients’. Therefore, safety concerns present a major challenge with the use of iPSCs in humans to repair heart tissue, so still a lot of learning and trials need to be done to address these concerns.
Current problem associated with the use of synthetic derived matrixes for heart tissue engineering is poor ‘biocompatibility’ (A study carried out by ‘Ottel al’ involved the ‘reseeding of neonatalrat cardiomyocytes’ into the decellularized heart of rats to produce ‘bioartificial’ hearts. The great positive of the use of decellularizedhearts is that they allow the ‘3D architecture’ and the natural extra-cellular matrix to be maintained in comparison to the use of other synthetic matrixes. This is of crucial importance in order to reform the muscular heart tissue and associated vascular components. However, the possibility of human tissue engineering was left largely untouched due to limitations in the ‘availability of human’ cardiomyocytes until now with great discovery of iPSCs showing the ‘feasibility’ to generate ‘unlimited CMs from humans’.
A trial Lu et al. , 2013was carried out where human heart tissue was engineered by ‘repopulating whole decellularized mouse hearts with human iPS cell-derived multipotential cardiovascular progenitors (MCPs)’. Stage 1 of the trial was to generate MCPs from ‘Y1-iPSCs’ using a ‘serum-free protocol’ that consists of the use of ‘different growth factors’ from the ‘early cardiovascular development’. From the Y1-iPSCs a wide variety of components were generated such as embryoid bodies (EB), bone morphogenetic proteins e. g. BMP4, and lots of growth factors e. g. bFGF, VEGF, DKKI etc. During stage 2, the embryoid bodies began to separate into individual cells by day 6 and under ‘fluorescence-activated cell sorting analysis’ approximately ‘63%’ of cells were ‘human iPSC-derived MCPs’. These results suggest that these ‘exogenous growth factors’ were able to determine the ‘cardiovascular lineage’ parameters from the human iPSC-derived MCPs. The final stage of the trial involved the use of VEGF, DKKI and bFGF growth factors for the perfusion of for the ‘MCP-repopulated mouse decellularized hearts’ to upregulate cardiac muscle reconstruction and regrowth of endothelial tissue. After 7 days of perfusion, in the extra-cellular matrix (ECM) approximately ‘10-15%’ of the ‘repopulated cells’ were still present and had a stable interaction with the EC. After 20 days, ‘electrophysiological analysis’ was carried out which showed some good signs as ‘90%’ of the engineered heart components displayed ‘spontaneous contractions’ whereas there was failure to beat for ‘10%’ due to poor cell retention or perfusion contamination. However, problems were noted from CaiT mapping which was able to discover anatomical blocked zones where uncoupling tissues were found. In addition, there were ‘abnormal electrograms’ which indicated poor electrical impulse conduction because of the ‘lack of gap junctions’ and low number of cardiomyocytes). Therefore, this was a major issue noted in this trial as the ‘engineered heart tissue’ and constructs displayed poor blood pumping and were ‘not fully synchronized’. This trial showed the potential of iPSCs but there still is a longway to go in terms of studies and learning before considering them as a viable possibility to test on humans.
Potential for heart regeneration using adult bone marrow derived stem cells Adult bone marrow derived stem cells (BMSCs) are multipotent stem cells such as hematopoietic, mesenchymal, endothelial stem cells, skeletal myoblasts and ‘cardiac stems cells present in adult tissues’. The BMSCs can be obtained easily via ‘bone marrow aspiration’ or ‘by isolation from peripheral blood’, where the peripheral blood has undergone assortment with ‘stem cell factor (SCF) and granulocyte colony-stimulating factor (G-CSF)’ cytokines. Currently, BMSCs are under great research because of their many advantages such as being from an ‘autologous origin’, ‘reduced immunogenicity’, ease of extraction and safety. However, these cells are multipotent and differentiate into a wide range of cells, therefore they carry the ‘potential risk’ of forming ‘bone cartilage’ or ‘adipose tissue’ if transplanted into the heart. A study carried out with animal models reported that the transplantation of some ‘unselected bone marrow cells’ and mesenchymal stem cells caused ‘intramyocardial calcification’ and lead to the formation of bone in the infarcted hearts. Even though there are a few shortcomings, BMSCs research is a growing promise with a lot of studies being undertaken.
A study by ‘Orlic and colleagues’ explored the use of ‘hematopoietic stem cells’ for heart tissue regeneration. They use a mice model where MI was induced into the mice by ‘tying up’ the main left coronary artery. The researchers then selectively extracted a group of ‘adult primitive bone marrow cells’ that had the potential to differentiate into wide range of cell types (National Institutes of health (2001)). These cells were then implanted via a ‘direct injection’ into the infarcted ventricular wall and ultimately resulted in the production of ‘new cardiomyocytes, vascular endothelium, and smooth muscle cells’. Approximately ‘70%’ of the infracted ventricle was replaced by new myocardium and the ‘survival rates’ were higher in mice that received the primitive adult stemcells over the control group. The cytokines SCF and G-CSF were used to help mobilize the ‘primitive cells’ into the ‘myocardial infraction’ of the mice. These cytokines were used 5 days before and 3 days after the ‘coronary artery ligation’, and compared with the saline-injected control group at ’27 days post infract’, there was drop in ‘mortality, infract size, magnitude of cavity dilation and diastolic stress’ (Mathur et al. , 2008). Despite these positive findings, other clinical trials that have been done were unable to show that ‘cardiomyocyte transdifferentiation’ using hematopoietic stem cells could improve the ‘cardiac function’ of heat attack animal models and several trials have shown ‘insignificant’ gains with respect to ventricular ejection.
In the UK, Barts Health NHS Trust have supported the REGENERATE-AMI clinical trial. The aim of this trial was to find out if ‘early autologous bone marrow cell infusion’ in patients had effects on the left ventricle function after an AMI if it is ‘delivered within 24hrs of successful reperfusion therapy’. ‘Earlier time points’ had not been considered to be evaluated before this trial as there were concerns regarding safety in pre-clinical studies where ‘3-5h post-reperfusion’ was used’. Therefore 24-hour time frame was set to see if this cell therapy method is feasible and can be conducted alongside ‘standard hospital care’ for AMI patients. This trial was a randomized double blind and placebo trial which consisted of 100 patients equally divided into a 1:1 ratio to either an ‘intracoronary infusion of BMC or placebo’ group within the 24-hour period after ‘successful primary percutaneous intervention (PPCI)’. Individuals were selected on the requirements they had ‘anterior AMI and significant regional wall motion abnormality’. The primary endpoint of the trial was determined by ‘cardiac imaging’ which suggested there to be ‘6%’ change between the baseline and 1 year later left ventricular ejection fraction (LVEF). The CMR revealed that although the in-between group difference for the BMC group showed a ‘5. 1% increase in LVEF’ and the placebo group with ‘2. 8%’ increase, the actual ‘between group difference’ was very small only favoring the BMC group by ‘2. 2%’ was considered to be insignificant and failed to meet the primary endpoint.
However, other data collected from the trial showed that the BMC group was linked with a ‘reduction in infarct size’ and ‘increased myocardial salvage’. There were limitations to note from this trial including the small sample size could have impacted on not achieving the predicted LVEF or not. The use of ‘angiographic’ imaging instead of ‘3-D cross-sectional imaging’, which was not possible to do within 24 hours may have resulted in higher ‘ejection fractions’ at baseline which resulted in fewer participants enrolled with ‘significantly impaired LVEF’. Therefore, further trials are needed in order to increase insight into all dynamics of this technique and currently a massive trial is being carried out by Barts NHS Trust hospital called the ‘BAMI Trial’ which consists of 3000 individuals with ‘all-cause mortality at 5 years as the primary end point’.
Conclusion
In the UK, CVD is still the main contributor to high mortality and morbidity. Although huge advancements have been made in terms of pharmacological and surgical treatments to help sustain life, these only slow down individual’s progression towards end stage heart failure. Our only current viable option is cardiac transplantation but this faces challenges with regards to tissue rejection, limited organ donors and life-long immunosuppressive therapies. Stem cell therapy has opened the door to cardiac regeneration as a hopeful possibility for the future. iPSCs and BMSCs overcome the ethical conflicts encountered by the use of embryonic stem cells as they are obtained from autologous origin.
iPSCs show great signs in tissue engineering in regenerating some forms of heart contraction but have issues regarding safety such as high tumorigenic risk due to interaction with human genome. The use of BMSCs in the REGENERATIVE trial showed there was no significant improvement in LVEF, but did show improvements in reduced infract size and increased myocardial salvage. There are concerns as BMSCs are multipotent, they could lead to the formation of other cells in the mouse heart such as cells that form bone. At the current stage, we are far behind from making either stem cell therapy as a viable option for standard hospital care. But I do believe that further trials and studies are needed to widen our insight into all unknown aspects, feasibility, and safety before being used on humans on a large scale to see the therapeutic effect.
At the moment, we are at the early stages but if this is a success, I feel stem cell therapy could help improve the quality of life of many patients and help reduce the pressures on the NHS in relation to transplantation waiting list and financial costs.
