Cardiac adaptations to exercise: the role of stem cells and future therapeutic insights

2012/2013 Diogo da Silva Miguel Cardiac adaptations to exercise: the role of stem cells and future therapeutic insights. março, 2013 Mestrado Integrado em Medicina Área: Fisiologia Trabalho efetuado sob a Orientação de: Doutor Paulo Castro Chaves Trabalho organizado de acordo com as normas da revista: Cardiovascular Research Diogo da Silva Miguel Cardiac adaptations to exercise: the role of stem cells and future therapeutic insights. março, 2013 Projeto de Opção do 6º ano - D ECLARAÇÃO DE I NTEGRIDADE Eu, Diogo da Silva Miguel, abaixo assinado, nº mecanográfico 070801255, estudante do 6º ano do Mestrado Integrado em Medicina, na Faculdade de Medicina da Universidade do Porto, declaro ter atuado com absoluta integridade na elaboração deste projeto de opção. Neste sentido, confirmo que NÃO incorri em plágio (ato pelo qual um indivíduo, mesmo por omissão, assume a autoria de um determinado trabalho intelectual, ou partes dele). Mais declaro que todas as frases que retirei de trabalhos anteriores pertencentes a outros autores, foram referenciadas, ou redigidas com novas palavras, tendo colocado, neste caso, a citação da fonte bibliográfica. Faculdade de Medicina da Universidade do Porto, 20/03/2013 Assinatura: Projeto de Opção do 6º ano – D ECLARAÇÃO DE REPRODUÇÃO Nome: Diogo da Silva Miguel Email: [email protected] Título da Monografia: Cardiac adaptations to exercise: the role of stem cells and future therapeutic insights. Orientador: Paulo Manuel Barreiros de Castro Chaves Ano de conclusão: 2013 Designação da área do projeto: Fisiologia É autorizada a reprodução integral desta Monografia para efeitos de investigação e de divulgação pedagógica, em programas e projetos coordenados pela FMUP. Faculdade de Medicina da Universidade do Porto, 20/03/2013 Assinatura: 1 Cardiac adaptations to exercise: the role of stem cells and future therapeutic insights Diogo da Silva Miguel 1 Faculty of Medicine of University of Porto, Physiology and Cardiothoracic Department [email protected] Paulo Castro Chaves MD, PhD Faculty of Medicine of University of Porto, Physiology and Cardiothoracic Department [email protected] Number of words: 5339 1 Corresponding author Faculdade de Medicina da Universidade do Porto, Al. Prof. Hernâni Monteiro, 4200-319 Porto, Portugal Telephone number: +351 22 551 3600 Fax: +351 22 551 3601 E-mail: mimed07[email protected].pt 2 Diogo da Silva Miguel, Paulo Castro Chaves Cardiac adaptations to exercise: the role of stem cells and future therapeutic insights Abstract The heart responds to physical exercise through changes in its morphology known as physiological hypertrophy. Besides cardiomyocyte hypertrophy, there is also an increased cellular proliferation with formation of new myocytes. Altogether, these changes lead to an improvement in cardiac function. The molecular mechanism responsible for the development of physiological hypertrophy is centred in the IGF1R/PI3K/Akt signalling pathway, which modulates the activity of transcription factors involved in protein synthesis and cardiomyocytes proliferation. The endogenous pool of cardiac stem cells is believed to be crucial for the continuous renewal and regenerative potential of cardiac tissue, and has also been positively correlated with physiological hypertrophy. Another system involved in myocardium physiology and pathology is microRNA. The knowledge of the mechanisms underlying the beneficial effects of physical exercise over cardiac function may sustain the development of new therapeutic approaches regarding cardiac pathologies, which represent today an important cause of morbidity and mortality worldwide. Keywords: Exercise, Heart, Hypertrophy, microRNA, Stem cells. 3 Introduction The understanding of heart’s physiology has always been on the top of scientific community’s priorities. Indeed, there is an enormous amount of work available regarding this area of knowledge. The heart responds to increased workload with hypertrophy. According to the nature of the stimulus, the hypertrophy can be either physiological or pathological 1 . The main stimulus that leads to pathological hypertrophy is pressure overload, where the heart responds with hypertrophy of existing myocytes as an attempt to overwhelm the increased load 2 . However, in this setting maladaptive changes, such as apoptosis, necrosis and a shift in gene expression, also occur, which leads to rapid decline in cardiac function, with heart failure as possible outcome 1, 2 . In the opposite side there is the physiological hypertrophy, with physical exercise appearing as the best example of this type of cardiac adaptation. Unlike pressure overload, exercise training increases the blood flow through the heart (volume overload) 1 , which in turn results in lengthening of myocytes and consequent eccentric hypertrophy of the whole organ 2 . The normal structure of the myocardium is kept, the cardiac function remains unchanged or may improve 3 , and the loss of myocytes and increased fibrosis observed in pathological hypertrophy are actually attenuated with exercise training 4 . As noted above, physical exercise is the main trigger of physiological hypertrophy, with all the resulting benefits. Exercise training is today extensively prescribed and is a cheap and effective way for both the prevention and management of heart diseases 3 . Indeed, current evidence points that an improved fitness level is a strong indicator of freedom from all-cause mortality 5 and is enough to decrease the morbidity and mortality linked with cardiac disorders 6 . Besides, physical exercise has been shown to protect the myocardium from age induced apoptosis, fibrosis and senescence 4, 7, 8 . 4 Until the past decade, it was believed that the heart was a post-mitotic organ, with no regenerative potential. However, an increasing body of evidence supports the existence of a resident population of cardiac stem cells 9-12 . This endogenous pool not only reassures the continuous renewal of cardiac tissue 13 , but also supports some grade of regeneration after an injury 9, 11 . The old paradigm of the heart as a terminally differentiated organ without any renewal potential did not give space for the exploration of regenerative medicine. However, by the light of the evidence accumulated during the past decade some work begun to be done in the field of regenerative medicine applied to the heart. This review aims to look over the mechanisms known to take part in the control of the adaptations of myocardium to physical exercise, and open new perspectives about possible applications of this knowledge in the search of new therapeutic approaches. Macroscopic and functional changes In 1975 Morganroth et al. used echocardiography to describe for the first time the cardiac changes induced by physical exercise, concluding that endurance exercise induced an increase in left ventricle mass and diastolic cavity size, while the wall thickness remained normal (eccentric hypertrophy). On the other hand, athletes that went through a resistance training program showed increased myocardial mass with ventricular wall thickening, but with normal diastolic cavity size (concentric hypertrophy) 14 . This concept has been accepted for decades as the “Morganroth hypothesis”. Indeed, other studies have found an increase in both left ventricular mass and left ventricle end diastolic diameter with endurance exercise 15-17 , though some of them have failed to show any effect of resistance training in cardiac morphology 15, 16 . 5 Adaptations at cellular level Unlike the macroscopic remodelling described in the previous paragraph, due to ethical limitations there is a paucity of studies describing the changes that occur at the cellular level in humans. However, a vast amount of studies using mice have shown that an increase in individual cardiomyocyte dimensions occurs in response to several swimming training programmes 18-20 . That increase comprises both the cardiomyocyte’s long and short axis and mean cell area 21 , and is remarkably related with the exercise’s intensity – high intensity exercise induces bigger changes than moderate intensity 6 . On the other hand, the pattern of cardiomyocyte growth is also determined by the type of physical exercise: while endurance training (swimming, running), and the associated volume overload, prompt myocyte lengthening, resistance training (wrestling, weight lifting), which is associated with an intermittent pressure overload, drives mainly to thickening of myocytes 2 . Until recently, cardiomyocyte hypertrophy was thought to be the only mean by which the heart increased its size, which was in line with the paradigm that heart was a postmitotic organ without any renewal potential. However, nowadays’ current evidence indicates that human myocardium is capable of, at least in part, renovate itself by new cardiomyocyte formation 13 . Actually, if it was not this self-renewing capacity, the heart would lose most of its mass in few decades 11 . Furthermore, increased levels of division markers, along with a rise in the number of cardiomyocytes, has already been documented in mice following the application of endurance exercise protocols 18, 22 . Interestingly, the increase in the number of cardiomyocytes was positively related with the intensity of physical exercise the rats have been exposed to 22 . Therefore, it seems that the regenerative capacity of myocardium is enhanced by physical activity. 12 hepatocyte growth factor (HGF) fostered eCSC activation with consequent cardiac regeneration 40, 44, 45 . Interestingly, this was observed along with increased expression of GATA-4 44 which, as described above, seems to have a role in physiological hypertrophy induced by physical exercise. microRNAs. The discovery of miRNAs has brought a new biological target for future therapies. Actually, today is already possible either to increase the expression, using vectors 46 , or antagonize specific miRNAs using ‘antagomirs’, which are cholesterolassociated RNAs that block the action of miRNAs over their targets 47 . The expression of miRNA-34a, aforementioned to be decreased in the cardiomyocytes of mice expressing caPI3K, has been shown to rise in the ageing heart and following myocardial infarction. Interestingly, using antigomir against miRNA-34a it was possible to observe a drop in cell death and cardiac fibrosis, and consequent improvement in contractile function in mice subjected to myocardial infarction 48 . Another miRNA described above as being negatively correlated with PI3K activity was miRNA-210. By those findings, it would be reasonable to think that a decrease in the expression of this miRNA would be beneficial. However, other studies found that miRNA-210, whose expression is upregulated under hypoxic conditions 49 , is actually beneficial in the setting of myocardial infarction by significantly inducing angiogenesis and decreasing myocytes’ apoptotic rate, thereby contributing for a more favourable ventricular remodelling and cardiac function 46 . Cell therapy. The first attempts to regenerate myocardial tissue have been done through transplant of stem cells from other origins than the heart 50, 51 . Although animal studies have provided exciting results, several clinical trials in humans where already performed and showed only modest, if any, improvement in cardiac function 50 . 13 Therefore, new approaches are needed in order to make this type of therapy more effective. One of the first questions to be answered is which type of cells suits better for cellbased therapy. The discovery of a pool of resident cardiac stem cells has opened a new window regarding the regenerative potential of this organ, which can be further explored either as a therapeutic tool or target. Indeed, a phase 1 clinical trial has documented significant improvements in both the ejection fraction and infarct size in patients that received autologous cardiac stem cells therapy compared with controls. Furthermore, the safety of the treatment was absolute, with no adverse effects registered in any of the treated patients 52 . The pool of cardiac stem cells is complex and comprises cells with different capacity to replicate and form mature myocytes and blood vessels. In an attempt to better characterize those different cellular populations, D’Amario et al. concluded that cardiac stem cells expressing IGF-1R were younger and had a better replicative reserve, as opposed with cells expressing IGF-2R and AT1R. Moreover, cells expressing IGF-1R were capable of secreting both IGF-1 and IGF-2, the latter being responsible for the induction of myocyte differentiation. The superiority of IGF-1R expressing cardiac stem cells was further confirmed by the greater degree of cardiac recovery after the transplant of these cells into hearts of rats subjected to myocardial infarction 53 . Besides the choice of the ideal cell, other strategies can be taken to maximize the beneficial effects of cell therapy. Interestingly, the combination of cardiac stem cells administration and nanofibers containing IGF-1 resulted in a higher myocardial recovery after ischemia when compared with each therapy alone 54 . Discussion 14 In the last decade several steps have been taken in order to open new lines of investigation regarding new therapeutic strategies. Maybe the biggest advances have been done in the field of cell therapy, with some clinical trials already in progress. However, as stated above, there is also growing expectation about miRNAs and its manipulation. The central role that the IGF-1R/PI3K/Akt has in the development of physiological hypertrophy has also triggered attempts to modulate this signalling system, namely through the therapy with IGF-1. In fact, the results obtained with IGF-1 administration in animal models of myocardial infarction were encouraging 42, 43 . However, this signalling pathway is present in a wide range of cell types and because of that systemic administration of this growth factor can have diffuse effects and result in the progression of occult neoplasms 31 . Concerning cell therapy, some obstacles have been raised regarding the type of cells that better fulfil the requirements for the success of this therapeutic approach. By one side, the application of cells from other origins than the heart has shown disappointing results. Among the causes advanced for this is the source and inadequate preparation of the cells, as well as the timing and mode of cell delivery. On the other side, it is believed that embryonic stem cells are the ones that have the greatest potential to be successful. However, to the risk of teratoma formation and immunogenic incompatibilities that the use of embryonic stem cells entails, we still have to add the ethical issues 50 . Finally, it appears that autologous cardiac stem cells transplantation may be the best approach, as has been shown by a recent phase 1 clinical trial 52 . Still, more research is needed to find new strategies that can increase the feasibility and effectiveness of cell therapy. 15 The discovery of miRNAs has provided a new overview of cellular regulation mechanisms. In addition, it represents another process where it is possible to act to change the fate of diseased heart. Currently, it is already clear that different miRNAs are important in both the heart development and pathology 55 . Furthermore, some studies performed in animals were successful in improving the pattern of cardiac remodelling after myocardial infarction 46, 48 . Nevertheless, miRNAs represent a class of molecules that exerts a very delicate control of specific genes at a post-transcriptional level 55 . Consequently, it is critical to better understand the function of each specific miRNA in the heart’s physiology and pathology, and only after it will be possible to delineate the best plans to take all the potential from this promising area. Heart diseases, namely those of ischemic cause, are a major source of morbidity and mortality worldwide 51 . Despite the improvement that the development of several classes of drugs have brought to the prognostic of these patients, advances are need so that it can be possible to further decrease the burden of disease of cardiac pathologies. 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