Role of stem cells in cardiovascular biology

Tissue-engineered cardiac patch seeded with human induced pluripotent stem cell derived cardiomyocytes promoted the regeneration of host cardiomyocytes in a rat model

Background

Thousands of babies are born with congenital heart defects that require surgical repair involving a prosthetic implant. Lack of growth in prosthetic grafts is especially detrimental in pediatric surgery. Cell seeded biodegradable tissue engineered grafts are a novel solution to this problem. The purpose of the present study is to evaluate the feasibility of seeding human induced pluripotent stem cell derived cardiomyocytes (hiPS-CMs) onto a biodegradable cardiac patch.

Methods

The hiPS-CMs were cultured on a biodegradable patch composed of a polyglycolic acid (PGA) and a 50:50 poly (l-lactic-co-ε-caprolactone) copolymer (PLCL) for 1 week. Male athymic rats were randomly divided into 2 groups of 10 animals each: 1. hiPS-CM seeded group, and 2. Unseeded group. After culture, the cardiac patch was implanted to repair a defect with a diameter of 2 mm created in the right ventricular outflow tract (RVOT) wall. Hearts were explanted at 4 (n = 2), 8 (n = 2), and 16 (n = 6) weeks after patch implantation. Explanted patches were assessed immunohistochemically.

Results

Seeded patch explants did not stain positive for α-actinin (marker of cardiomyocytes) at the 4 week time point, suggesting that the cultured hiPS-CMs evacuated the patch in the early phase of tissue remodeling. However, after 16 weeks implantation, the area fraction of positively stained α-actinin cells was significantly higher in the seeded group than in the unseeded group (Seeded group: 6.1 ± 2.8% vs. Unseeded group: 0.95 ± 0.50%, p = 0.004), suggesting cell seeding promoted regenerative proliferation of host cardiomyocytes.

Conclusions

Seeded hiPS-CMs were not present in the patch after 4 weeks. However, we surmise that they influenced the regeneration of host cardiomyocytes via a paracrine mechanism. Tissue-engineered hiPS-CMs seeded cardiac patches warrant further investigation for use in the repair of congenital heart diseases.

How do resident stem cells repair the damaged myocardium?

It has been a decade since the monumental discovery of resident stem cells in the mammalian heart, and the following studies witnessed the continuous turnover of cardiomyocytes and vascular cells, maintaining the homeostasis of the organ. Recently, the autologous administration of c-kit-positive cardiac stem cells in patients with ischemic heart failure has led to an incredible outcome; the left ventricular ejection fraction of the cell-treated group improved from 30% at the baseline to 38% after one year and to 42% after two years of cell injection. The potential underlying mechanisms, before and after cell infusion, are explored and discussed in this article. Some of them are related to the intrinsic property of the resident stem cells, such as direct differentiation, paracrine action, and immunomodulatory function, whereas others involve environmental factors, leading to cellular reverse remodeling and to the natural selection of “juvenile” cells. It has now been demonstrated that cardiac stem cells for therapeutic purposes can be prepared from tiny biopsied specimens of the failing heart as well as from frozen tissues, which may remarkably expand the repertoire of the strategy against various cardiovascular disorders, including non-ischemic cardiomyopathy and congenital heart diseases. Further translational investigations are needed to explore these possibilities.

Cardiac stem cells: biology and clinical applications

SIGNIFICANCE

Heart disease is the primary cause of death in the industrialized world. Cardiac failure is dictated by an uncompensated reduction in the number of viable and fully functional cardiomyocytes. While current pharmacological therapies alleviate the symptoms associated with cardiac deterioration, heart transplantation remains the only therapy for advanced heart failure. Therefore, there is a pressing need for novel therapeutic modalities. Cell-based therapies involving cardiac stem cells (CSCs) constitute a promising emerging approach for the replenishment of the lost tissue and the restoration of cardiac contractility.

RECENT ADVANCES

CSCs reside in the adult heart and govern myocardial homeostasis and repair after injury by producing new cardiomyocytes and vascular structures. In the last decade, different classes of immature cells expressing distinct stem cell markers have been identified and characterized in terms of their growth properties, differentiation potential, and regenerative ability. Phase I clinical trials, employing autologous CSCs in patients with ischemic cardiomyopathy, are being completed with encouraging results.

CRITICAL ISSUES

Accumulating evidence concerning the role of CSCs in heart regeneration imposes a reconsideration of the mechanisms of cardiac aging and the etiology of heart failure. Deciphering the molecular pathways that prevent activation of CSCs in their environment and understanding the processes that affect CSC survival and regenerative function with cardiac pathologies, commonly accompanied by alterations in redox conditions, are of great clinical importance.

FUTURE DIRECTIONS

Further investigations of CSC biology may be translated into highly effective and novel therapeutic strategies aiming at the enhancement of the endogenous healing capacity of the diseased heart.

IL-12 involvement in myogenic differentiation of C2C12 in vitro

Recently, the extracellular microenvironment has been shown to be critical for the correct differentiation of stem cells to specific tissues. Many factors, including physical (e.g. biomaterial stiffness and topography) and biological (as growth factors, cytokines and chemokines) components, cooperate to create an ideal microenvironment for muscle stem cells, with many of these factors having been widely investigated. We previously demonstrated that the use of non-proliferating muscle-specific and unrelated cells as feeder layers for skeletal muscle progenitor cell differentiation resulted in significant differences in the ability to form myotubes, suggesting the importance of biological factors in myogenic differentiation. In this study, we investigated the biological factors involved in this process, analyzing the expression profile of 84 genes coding for cytokines and chemokines. We successfully identified a novel role for the cytokine IL-12 in the myogenic differentiation of C2C12 mouse skeletal muscle cells. Experiments involving the overexpression or silencing of the IL-12 gene in C2C12 showed that IL-12 enhanced the myogenic differentiation process. Moreover, when IL-12 was overexpressed in non-biologically related feeder cells, the new co-culture system was able to improve myogenic differentiation of C2C12 seeded on top. Although IL-12 is known to be a cytokine involved in inflammatory responses, it also appears to be involved in the myogenic differentiation process, acting as a positive regulator of this mechanism. This fact is expected to prove to be important for the development of functional biomaterials.

Cell therapy for human ischemic heart diseases: critical review and summary of the clinical experiences

A decade ago, stem or progenitor cells held the promise of tissue regeneration in human myocardium, with the expectation that these therapies could rescue ischemic myocyte damage, enhance vascular density and rebuild injured myocardium. The accumulated evidence in 2014 indicates, however, that the therapeutic success of these cells is modest and the tissue regeneration involves much more complex processes than cell-related biologics. As the quest for the ideal cell or combination of cells continues, alternative cell types, such as resident cardiac cells, adipose-derived or phenotypic modified stem or progenitor cells have also been applied, with the objective of increasing both the number and the retention of the reparative cells in the myocardium. Two main delivery routes (intracoronary and percutaneous intramyocardial) of stem cells are currently used preferably for patients with recent acute myocardial infarction or ischemic cardiomyopathy. Other delivery modes, such as surgical or intravenous via peripheral veins or coronary sinus have also been utilized with less success. Due to the difficult recruitment of patients within conceivable timeframe into cardiac regenerative trials, meta-analyses of human cardiac cell-based studies have tried to gather sufficient number of subjects to present a statistical compelling statement, reporting modest success with a mean increase of 0.9-6.1% in left ventricular global ejection fraction. Additionally, nearly half of the long-term studies reported the disappearance of the initial benefit of this treatment. Beside further extensive efforts to increase the efficacy of currently available methods, pre-clinical experiments using new techniques such as tissue engineering or exploiting paracrine effect hold promise to regenerate injured human cardiac tissue.

Cigarette smoke adversely affects functions and cell membrane integrity in c-kit+ cardiac stem cells

Cigarette smoking is a major risk factor for numerous diseases including cardiovascular diseases. Exposure to cigarette smoke (CS) leads to increased cardiovascular risk, myocardial injury, and mortality. Stem cell therapy is one of the promising therapeutic options available to treat myocardial injuries. Understanding the impact of cigarette smoke extract (CSE) on stem cell function would be valuable in determining the risk passed on during transplant. In this study, the impact of CSE on cardiac stem cell (CSC) functions was investigated using c-kit+ rat cardiac stem cells as the experimental model. Here, we hypothesized that CSE attenuates CSC membrane integrity, causes cytotoxicity, and affects many CSC functions via multiple mechanisms including modulation of extracellular stress-regulated kinase (ERK) (44/42) signaling and oxidative stress. The effects of CSE on CSCs were examined in vitro. Based on a published method, CSE was prepared. CSE-induced ERK signaling was detected by western blotting. CSE-induced modulation of catalase activity was also measured. Functional modulations due to CSE were examined via several methods including Apostain, BrdU, and LDH assays. In agreement with the CSE-induced activation of ERK, CSE-induced reduction in viability, migration, and increase in both cytotoxicity and para-cellular permeability were observed in CSCs. These results suggest that CSE impaired CSC responses that contribute to decreased ability of CSC to respond to stress or injury leading to exacerbation of the damage. Our findings will contribute to the understanding of the discipline and might contribute to the development of stem cell therapy approaches in the future.

Innate regeneration in the aging heart: healing from within

The concept of the heart as a terminally differentiated organ incapable of replacing damaged myocytes has been at the center of cardiovascular research and therapeutic development for the past 50 years. The progressive decline in myocyte number as a function of age and the formation of scarred tissue after myocardial infarction have been interpreted as irrefutable proofs of the postmitotic characteristic of the heart. However, emerging evidence supports a more dynamic view of the heart in which cell death and renewal are vital components of the remodeling process that governs cardiac homeostasis, aging, and disease. The identification of dividing myocytes in the adult and senescent heart raises the important question concerning the origin of these newly formed cells. In vitro and in vivo findings strongly suggest that replicating myocytes derive from lineage determination of resident primitive cells, supporting the notion that cardiomyogenesis is controlled by activation and differentiation of a stem cell compartment. It is the current view that the myocardium is an organ permissive of tissue regeneration mediated by exogenous and endogenous progenitor cells.

Mitochondria play a central role in nonischemic cardiomyocyte necrosis: common to acute and chronic stressor states

The survival of cardiomyocytes must be ensured as the myocardium adjusts to a myriad of competing physiological and pathophysiological demands. A significant loss of these contractile cells, together with their replacement by stiff fibrillar collagen in the form of fibrous tissue accounts for a transition from a usually efficient muscular pump into one that is failing. Cellular and subcellular mechanisms involved in the pathogenic origins of cardiomyocyte cell death have long been of interest. This includes programmed molecular pathways to either necrosis or apoptosis, which are initiated from ischemic or nonischemic origins. Herein, we focus on the central role played by a mitochondriocentric signal-transducer-effector pathway to nonischemic cardiomyocyte necrosis, which is common to acute and chronic stressor states. We begin by building upon the hypothesis advanced by Albrecht Fleckenstein and coworkers some 40 years ago based on the importance of calcitropic hormone-mediated intracellular Ca(2+) overloading, which predominantly involves subsarcolemmal mitochondria and is the signal to pathway activation. Other pathway components, which came to be recognized in subsequent years, include the induction of oxidative stress and opening of the mitochondrial inner membrane permeability transition pore. The ensuing loss of cardiomyocytes and consequent replacement fibrosis, or scarring, represents a disease of adaptation and a classic example of when homeostasis begets dyshomeostasis.

C-kit-positive cardiac stem cells and myocardial regeneration

Two distinct subpopulations of endogenous cardiac stem cells (CSCs) are identified in the adult heart; myogenic CSCs (mCSCs), which are characterized by the presence of c-kit receptor, reside in the myocyte niche surrounded by matured cardiomyocytes, whereas vasculogenic CSCs (vCSCs), which express c-kit as well as KDR, are stored in the vascular niche found in the vessel wall. They both possess the fundamental properties of stem cells: self -renewing, clonogenic, and multipotent. While mCSCs preferentially differentiate into myocyte lineage in vitro, vCSCs tend to commit to vascular endothelial or smooth muscle lineages upon stimulation. Intramyocardial injection of human mCSCs to the region bordering infarct of immunosuppressed animals induces cardiac regeneration involving intensive muscle and vessel formation. On the other hand, the administration of human vCSCs near the critical stenosis created in the dog coronary artery results in the formation of large conductive vessels, i.e. "biological coronary bypass", coupled with the improved tissue perfusion. A clinical trial utilizing autologous c-kit-positive CSCs on patients with chronic ischemic heart failure was launched recently. Upon coronary artery bypass graft (CABG) surgery, a small portion of the right atrial appendage was obtained and used for isolation and expansion of CSCs. Intracoronary CSC implantation was performed 4 months after the surgery, and the symptom and the cardiac function were followed. Although the study is still ongoing, no adverse event due to cell infusion has been reported in any of the cell-treated patients, and the initial outcome is very promising. Disease-based customized cell therapy employing various combinations of autologous mCSCs and vCSCs would become available in the near future.