Perspectives: Cardiomyocytes from Skeletal Muscle Stem Cells for Cardiac Repair

Heart failure is a major contributor to mortality in the United States. Many cardiomyocytes (CM) die following myocardial insult, and the post-natal mammalian heart has very limited regenerative capacity. Heart organ transplantation is a final therapeutic option to prevent patient death when heart failure is imminent. However, donor organ availability cannot completely meet current demands, and long-term prognosis remains unsatisfactory. Cellular cardiomyoplasty has emerged as one potential option to reverse maladaptive remodeling and restore contractile function. It involves the transplantation of cells into the heart to repair damaged myocardium. Choosing the right cell type remains a subject of debate. Using fetal CMs or embryonic stem (ES) cells would be undesirable from an ethical standpoint because they require the destruction of embryos and fetuses. While induced pluripotent stem cells (iPSc) can generate CMs with high purity, and direct CM induction from fibroblasts can produce CMs without reprogramming to pluripotency, their phenotype remains immature. Genome modification also poses a risk of oncogenic transformation in vivo [1,2]. Use of autologous adult stem cells overcomes the risks of tumor formation and other safety factors involved in iPSc generation or direct CM induction from fibroblasts. Various types of adult stem/progenitor cells have been tested for cardiac repair including skeletal myoblasts, bone marrow derived stem cells (BMSC), adipose stem cells (ASC), and more recently, cardiac stem cells (CSC). While many of these stem cell types have shown functional improvements in animal and clinical models, these benefits were thought to be primarily due to paracrine mechanisms that stimulate angiogenesis and attenuate fibrosis [3]. Few BMSCs and ASCs differentiate into CMs. CSCs are promising, but their isolation remains a challenge, and CSC pools are often depleted after myocardial injury [4]. In human trials, only high-dose skeletal myoblast transplantation has been shown to produce contractile tissue in vivo with functional improvement in some human patients. However, high incidence of ventricular arrhythmias is a major issue for the use of skeletal myoblasts for cardiac repair. The cause of ventricular arrhythmia following skeletal myoblast transplantation is lack of electrical coupling between donor myoblasts and surrounding myocardium [5]. Despite this early limitation, skeletal myoblasts remain one of the few widely investigated cell types which can generate contractile force to support a failing heart. Adult skeletal muscle contains various stem/progenitor cell populations aside from skeletal myoblasts (satellite cells). Skeletal muscle stem cells (skmSC) are multipotent cells that retain their myogenic heritage but have greater multilineage potential, being able to generate other tissue types including, bone, cartilage, and fat [6]. They are isolated from muscle biopsies by a modified pre-plate method. Since cardiac and skeletal muscle both arise from mesoderm, and cardiac and skeletal muscle share major transcription factor and sarcomere proteins during development [7], it has been theorized that skmSCs can differentiate into CM-like cells, which would overcome the limitations of skeletal myoblast transplantation. Our research has focused on determining the methods by which skmSCs can differentiate into CMs. We first reported that rat skmSCs cultured in a 3D environment with collagen and matrix factors differentiated into functional CM-like cells that beat spontaneously, generated contractile force, expressed cardiac-specific genes/proteins including connexin-43 gap junctions, displayed cardiac-like intracellular calcium transients, and responded to isoproterenol [8]. Recently, we reported that human skmSCs cultured as a 3D engineered tissue also beat spontaneously, generated force, expressed cardiac-specific genes/proteins, displayed cardiac-like intracellular calcium transients, and responded to isoproterenol [9]. However, they retained properties of skeletal muscle including expression of MyoD, myogenin, and fast skeletal MHC. In addition, their electrical coupling remained immature. Interestingly, iPSc derived CMs also expressed these skeletal muscle genes/proteins, supporting the idea of an overlapping biochemical signature between immature cardiac and skeletal muscle [9].