Cardiomyocyte proliferation refers to the ability of cardiac muscle cells to progress through the cell cycle and continue to divide. Traditionally, cardiomyocytes were believed to have little to no ability to proliferate and regenerate after birth. [1] Although other types of cells, such as gastrointestinal epithelial cells, can proliferate and differentiate throughout life, [2] cardiac tissue contains little intrinsic ability to proliferate, as adult human cells arrest in the cell cycle. [3] However, a recent paradigm shift has occurred. Recent research has demonstrated that human cardiomyocytes do proliferate to a small extent for the first two decades of life. [4] Also, cardiomyocyte proliferation and regeneration has been demonstrated to occur in various neonatal mammals in response to injury in the first week of life. [5] [6] Current research aims to further understand the biological mechanism underlying cardiomyocyte proliferation in hopes to turn this capability back on in adults in order to combat heart disease.
Adult zebrafish have a remarkable ability to completely regenerate cardiac muscle after injury. [7] There are similar genes in zebrafish and humans that control the development of the heart [8] and the phenomenal ability of zebrafish cardiomyocytes to proliferate in response to injury has made it a popular research model. When approximately 20% of the ventricle is resected from adult zebrafish, the cardiac muscle completely regenerates. Injury stimulates a subset of cardiomyocytes in the zebrafish heart that are able to proliferate and dedifferentiate. [9] Cardiomyocytes of zebrafish are mononucleated and diploid. [10]
After cardiomyocyte proliferation and regeneration was demonstrated to occur in zebrafish after resection, various animal models were utilized in order to explore whether mammals also have this innate ability. In 2011, Porrello et al. demonstrated that neonatal mice are able to regenerate heart muscle after resection. [11] Since 2011, many other research groups have explored cardiomyocyte regeneration. The cardiomyocytes of neonatal rats [12] and piglets [13] are also able to undergo proliferation in response to injury during the first week of life.
In 2009, Dr. Jonas Frisén's research group used a technique implementing carbon-dating of cardiomyocytes to propose that human adult cardiomyocytes do proliferate, but at a very slow rate. [14] There have also been case reports that suggest that the cardiomyocytes of newborns are able to proliferate in response to ischemia. [15] [16] A 2013 paper demonstrated that there are a small number of cardiomyocytes in mitosis and cytokinesis in humans up to age 20, with the highest percentage present in infants. [17]
The complete biological mechanism underlying cardiomyocyte proliferation has not been fully elucidated. However, there are various transcription factors and signaling cascades thought to be very important. Cardiomyocytes have been shown to be encouraged to exit the cell cycle then cyclin-dependent kinases are downregulated, or when cell cycle inhibitors are introduced. [18] Many of the signals that a cell receives during phase G1 determine whether the cell will undergo proliferation. Cyclin-dependent kinases and cyclin-dependent kinase inhibitors play key roles in this process. [19] One gene, jumonji (jmj), has been shown to start increasing in its expression in embryonic day 10.5 mice and is proposed to help cease the proliferation of cardiomyocytes by repressing the expression of cyclin D1. [20] Jumonji is believed to recruit G9a and GLP methyltransferases to the cyclin D1 promoter, which are thought to methylate histones H3-H9 and repress cyclin D1 expression. [21]
The transcription factor family E2F are also thought to be very important in regulating cardiomyocyte proliferation. E2F transcription factors influence cellular proliferation and help control apoptosis. When cardiomyocytes were transfected with adenoviruses expressing E2F2, cyclins A and E were upregulated, and cardiomyocytes proliferated. [22]
The cessation of the cardiomyocyte cell cycle is believed to be regulated by transcription factors and cyclin dependent kinase inhibitors, although the exact mechanism remains unclear. [23] One transcription facet that has been shown to be key in his process is Meis1. [24] Meis1 has been shown to be necessary for the activation of the cyclin-dependent kinase inhibitors p15, p16, and p21. Knockout experiments demonstrated that the length of cardiomyocyte proliferation can be extended when Meis1 is deleted in mice. [25] Meis1 has been shown to play a role in the regulation of anaerobic glycolysis. [26] This is particularly interesting because cardiomyocytes undergo a shift in their metabolism during development: cardiomyocytes rely on glycolytic metabolism but switch to relying on oxidative phosphorylation. [27] One research group demonstrated that neonatal transgenic mice deficient in fatty acids had a longer time span in which their cardiomyocytes were able proliferate in response to injury. [28]
Furthermore, oxygen metabolism is thought to play a role in cardiomyocyte proliferation. Using a mouse model of myocardial infarction to induce cardiac tissue damage, adult mice exhibited an increase in the proliferation of cardiomyocytes when put in a hypoxic environment. [29] When mice are born, they switch from being in a hypoxic intrauterine environment to an environment rich in oxygen. One research group has shown that oxidative DNA damage to cells and the cellular response to this damage increases in the first week of life, which correlates with the time point when mammalian cardiomyocytes start to lose the ability to regenerate. [30] In the intrauterine environment, cardiomyocytes have limited exposure to oxygen and little damage from reactive oxygen species. At the same time, cardiomyocytes are proliferating in utero. When neonatal mice were exposed to a hypoxic environment after ischemic heart damage, the cardiomyocytes are encouraged to enter mitosis and proliferate. [31]
Another signaling pathway thought to be important for the ability of cardiomyocytes to proliferate is the Hippo pathway, which was previously shown to regulate organ size in a fruit fly model. [32] When key proteins in the Hippo pathway are inactivated in a mouse model, the embryos exhibit cardiomyocyte proliferation and cardiomegaly. Hippo is thought to interact with the Wnt signaling pathway to limit the size of the heart and encourage cessation of cardiomyocyte proliferation. [33]
Heart disease continues to be one of the leading causes of death in the United States. [34] The progression of coronary artery disease can lead to weakened heart muscle and heart failure. If atherosclerosis progresses to the point of occluding a coronary artery, myocardial ischemia and damage can occur, resulting in irreversible cardiomyocyte death. [35] Further understanding of the biological mechanism underlying the cardiomyocyte proliferation that has been demonstrated in adult zebrafish and neonatal mice, rats, and piglets could provide insight into how it may be possible to encourage cardiomyocyte proliferation and heart regeneration in patients with ischemic heart disease or for patients in heart failure. [36]
The G0 phase describes a cellular state outside of the replicative cell cycle. Classically, cells were thought to enter G0 primarily due to environmental factors, like nutrient deprivation, that limited the resources necessary for proliferation. Thus it was thought of as a resting phase. G0 is now known to take different forms and occur for multiple reasons. For example, most adult neuronal cells, among the most metabolically active cells in the body, are fully differentiated and reside in a terminal G0 phase. Neurons reside in this state, not because of stochastic or limited nutrient supply, but as a part of their developmental program.
Myostatin is a protein that in humans is encoded by the MSTN gene. Myostatin is a myokine that is produced and released by myocytes and acts on muscle cells to inhibit muscle growth. Myostatin is a secreted growth differentiation factor that is a member of the TGF beta protein family.
In biology, regeneration is the process of renewal, restoration, and tissue growth that makes genomes, cells, organisms, and ecosystems resilient to natural fluctuations or events that cause disturbance or damage. Every species is capable of regeneration, from bacteria to humans. Regeneration can either be complete where the new tissue is the same as the lost tissue, or incomplete after which the necrotic tissue becomes fibrosis.
The Notch signaling pathway is a highly conserved cell signaling system present in most animals. Mammals possess four different notch receptors, referred to as NOTCH1, NOTCH2, NOTCH3, and NOTCH4. The notch receptor is a single-pass transmembrane receptor protein. It is a hetero-oligomer composed of a large extracellular portion, which associates in a calcium-dependent, non-covalent interaction with a smaller piece of the notch protein composed of a short extracellular region, a single transmembrane-pass, and a small intracellular region.
Myosatellite cells, also known as satellite cells, muscle stem cells or MuSCs, are small multipotent cells with very little cytoplasm found in mature muscle. Satellite cells are precursors to skeletal muscle cells, able to give rise to satellite cells or differentiated skeletal muscle cells. They have the potential to provide additional myonuclei to their parent muscle fiber, or return to a quiescent state. More specifically, upon activation, satellite cells can re-enter the cell cycle to proliferate and differentiate into myoblasts.
p21Cip1, also known as cyclin-dependent kinase inhibitor 1 or CDK-interacting protein 1, is a cyclin-dependent kinase inhibitor (CKI) that is capable of inhibiting all cyclin/CDK complexes, though is primarily associated with inhibition of CDK2. p21 represents a major target of p53 activity and thus is associated with linking DNA damage to cell cycle arrest. This protein is encoded by the CDKN1A gene located on chromosome 6 (6p21.2) in humans.
Estrogen receptor beta (ERβ) also known as NR3A2 is one of two main types of estrogen receptor—a nuclear receptor which is activated by the sex hormone estrogen. In humans ERβ is encoded by the ESR2 gene.
Inhibitor of nuclear factor kappa-B kinase subunit alpha (IKK-α) also known as IKK1 or conserved helix-loop-helix ubiquitous kinase (CHUK) is a protein kinase that in humans is encoded by the CHUK gene. IKK-α is part of the IκB kinase complex that plays an important role in regulating the NF-κB transcription factor. However, IKK-α has many additional cellular targets, and is thought to function independently of the NF-κB pathway to regulate epidermal differentiation.
Protein kinase C epsilon type (PKCε) is an enzyme that in humans is encoded by the PRKCE gene. PKCε is an isoform of the large PKC family of protein kinases that play many roles in different tissues. In cardiac muscle cells, PKCε regulates muscle contraction through its actions at sarcomeric proteins, and PKCε modulates cardiac cell metabolism through its actions at mitochondria. PKCε is clinically significant in that it is a central player in cardioprotection against ischemic injury and in the development of cardiac hypertrophy.
Transcription factor GATA-4 is a protein that in humans is encoded by the GATA4 gene.
Cyclin-A2 is a protein that in humans is encoded by the CCNA2 gene. It is one of the two types of cyclin A: cyclin A1 is expressed during meiosis and embryogenesis while cyclin A2 is expressed in the mitotic division of somatic cells.
Serine/threonine kinase 11 (STK11) also known as liver kinase B1 (LKB1) or renal carcinoma antigen NY-REN-19 is a protein kinase that in humans is encoded by the STK11 gene.
Protein Jumonji is a protein that in humans is encoded by the JARID2 gene. JARID2 is a member of the alpha-ketoglutarate-dependent hydroxylase superfamily.
The retinoblastoma protein is a tumor suppressor protein that is dysfunctional in several major cancers. One function of pRb is to prevent excessive cell growth by inhibiting cell cycle progression until a cell is ready to divide. When the cell is ready to divide, pRb is phosphorylated, inactivating it, and the cell cycle is allowed to progress. It is also a recruiter of several chromatin remodeling enzymes such as methylases and acetylases.
Cellular cardiomyoplasty, or cell-based cardiac repair, is a new potential therapeutic modality in which progenitor cells are used to repair regions of damaged or necrotic myocardium. The ability of transplanted progenitor cells to improve function within the failing heart has been shown in experimental animal models and in some human clinical trials. In November 2011, a large group of collaborators at Minneapolis Heart Institute Foundation at Abbott Northwestern found no significant difference in left ventricular ejection fraction (LVEF) or other markers, between a group of patients treated with cellular cardiomyoplasty and a group of control patients. In this study, all patients were post MI, post percutaneous coronary intervention (PCI) and that infusion of progenitor cells occurred 2–3 weeks after intervention. In a study that is currently underway, however, more positive results were being reported: In the SCIPIO trial, patients treated with autologous cardiac stem cells post MI have been reported to be showing statistically significant increases in LVEF and reduction in infarct size over the control group at four months after implant. Positive results at the one-year mark are even more pronounced. Yet the SCIPIO trial "was recently called into question". Harvard University is "now investigating the integrity of some of the data". The Lancet recently published a non-specific ‘Expression of concern’ about the paper. Subsequently, another preclinical study also raised doubts on the rationale behind using this special kind of cell, as it was found that the special cells only have a minimal ability in generating new cardiomyocytes. Some specialists therefore now raise concerns to continue.
Retinal regeneration refers to the restoration of vision in vertebrates that have suffered retinal lesions or retinal degeneration.
Human engineered cardiac tissues (hECTs) are derived by experimental manipulation of pluripotent stem cells, such as human embryonic stem cells (hESCs) and, more recently, human induced pluripotent stem cells (hiPSCs) to differentiate into human cardiomyocytes. Interest in these bioengineered cardiac tissues has risen due to their potential use in cardiovascular research and clinical therapies. These tissues provide a unique in vitro model to study cardiac physiology with a species-specific advantage over cultured animal cells in experimental studies. hECTs also have therapeutic potential for in vivo regeneration of heart muscle. hECTs provide a valuable resource to reproduce the normal development of human heart tissue, understand the development of human cardiovascular disease (CVD), and may lead to engineered tissue-based therapies for CVD patients.
Regeneration in humans is the regrowth of lost tissues or organs in response to injury. This is in contrast to wound healing, or partial regeneration, which involves closing up the injury site with some gradation of scar tissue. Some tissues such as skin, the vas deferens, and large organs including the liver can regrow quite readily, while others have been thought to have little or no capacity for regeneration following an injury.
Kenneth D. Poss is an American biologist and currently James B. Duke Professor of Cell Biology and director of the Regeneration Next Initiative at the Duke University School of Medicine.
Dedifferentiation is a transient process by which cells become less specialized and return to an earlier cell state within the same lineage. This suggests an increase in cell potency, meaning that, following dedifferentiation, a cell may possess the ability to re-differentiate into more cell types than it did prior to dedifferentiation. This is in contrast to differentiation, where differences in gene expression, morphology, or physiology arise in a cell, making its function increasingly specialized.