Muscle-derived stem cells for musculoskeletal tissue regeneration and repair

Abstract

Muscle recently has been identified as a good source of adult stem cells that can differentiate into cells of different lineages. The most well-known muscle progenitor cells are satellite cells, which not only contribute to the replenishment of the myogenic cell pool but also can become osteoblasts, adipocytes and chondrocytes. Other populations of stem cells that appear to be distinct from satellite cells also have been discovered recently. Muscle-derived stem cells (MDSCs) can be divided into two major categories based on these cells’ varied abilities to differentiate into myogenic lineages. Interestingly, MDSCs that can differentiate readily into myogenic cells are usually CD45−. In contrast, MDSCs with less myogenic potential are CD45+. Various lines of evidence suggest that different populations of MDSCs are closely related. Furthermore, MDSCs appear to be closely related to endothelial cells or pericytes of the capillaries surrounding myofibers. When used in tissue engineering applications, MDSCs—particularly those genetically engineered to express growth factors—have been demonstrated to possess great potential for the regeneration and repair of muscle, bone and cartilage. Further research is necessary to delineate the relationship between different populations of MDSCs and between MDSCs and other adult stem cells, to investigate their developmental origin, and to determine the regulatory pathways and factors that control stem cell self-renewal, proliferation and differentiation. This knowledge could greatly enhance the usefulness of muscle-derived stem cells, as well as other adult stem cells, for tissue repair and regeneration applications.

Introduction

Muscle could serve as a good source of adult stem cells due to its vast mass and extensive network of capillaries. Muscle-derived stem cells (MDSCs) are defined here as stem cells that normally reside in and can be isolated from skeletal muscles. Research focused on MDSCs has gained momentum in recent years both because of these cells’ possible application in the treatment of muscle diseases, including skeletal and cardiac muscle, and because of their potential in promoting the regeneration and repair of other tissues, such as bone and cartilage. This review summarizes recent progress in the study of MDSCs and their use for the regeneration and repair of muscles, bone and cartilage.

The different populations of muscle-derived progenitor cells appear to exhibit varied degrees of pluripotency. The most well-characterized muscle progenitor cells are satellite cells [1]. In addition to participating in the formation of myofibers, muscle satellite cells also can differentiate into other lineages, such as osteoblastic, adipocytic and chondrogenic cells [2], [3]. Other populations of MDSCs are mostly distinct from satellite cells [4] and can be divided into many different subpopulations by examining the cells’ surface marker profile and ability to undergo myogenic differentiation. MDSCs can be separated into two main subpopulations based on their relationship to hematopoietic cells: CD45+ MDSCs and CD45− MDSCs. CD45+ MDSCs isolated from normal muscle have a very limited myogenic potential in vitro and in vivo, but they possess a high degree of hematopoietic potential [5]. In contrast, CD45− MDSCs readily differentiate into myogenic lineages both in vitro and in vivo [5], [6] and also can differentiate into other lineages, such as blood cells [7].

The different populations of MDSCs are closely related. For example, CD45+ MDSCs, which normally possess low myogenic potential, can undergo active myogenesis during muscle injury by responding to the combined signaling by Wnt 5a, 5b, and 7a [8]. Thus, the differentiation potential of MDSCs may be modified by environmental cues. This study also demonstrated that CD45+ MDSCs are the resident muscle stem cells responsible for the regeneration of injured muscle when the pool of satellite cells has been depleted due to cardiotoxin insult [8]. However, more research is necessary to determine the relative contribution of different populations of muscle progenitor cells, including CD45+ MDSCs, CD45− MDSCs, and satellite cells, during muscle regeneration after common injuries that do not lead to the depletion of a particular group of cells. Nevertheless, this study demonstrated that under the influence of environmental factors (such as Wnt signaling), CD45+ MDSCs can contribute to muscle regeneration. It remains to be seen whether similar pathways also initiate the transdifferentiation of MDSCs into other lineages.

Central to the study of MDSCs is an understanding of the relationship between different populations of MDSCs and their relationship with the stem cells that reside in and can be isolated from other tissues, such as bone marrow. Although it is likely that some bone marrow stem cells can circulate through different tissues and possibly contribute to the stem cell pool of local tissues [9], much evidence indicates that resident stem cells (rather than circulating stem cells) are the cells primarily responsible for the regeneration or repair of local tissues, at least in skeletal muscle [8].

Resident MDSCs in muscle appear to be associated closely with the vasculature—specifically, the capillaries surrounding myofibers. According to the histological findings generated by our previous studies [6], [10], although some of the MDSCs were located beneath the basal lamina of myofibers, the area in which satellite cells normally reside, other MDSCs were found in the areas that normally are also occupied by capillaries running along the myofibers. Furthermore, both capillary endothelial cells and 60% of MDSCs are CD34+ and Scal+, although some MDSCs also are CD34− and Scal+ [6]. The Scal+/CD34+ MDSCs have been shown to restore dystrophin expression after intraarterial infusion to mdx mice, which model Duchenne muscular dystrophy [DMD; [11]].

Another of our recent studies has shown that CD34+ MDSCs are significantly better than CD34− MDSCs in restoring dystrophin expression, due mainly to the higher rate of CD34+ MDSC proliferation after intramuscular implantation [12]. This finding suggests that some MDSCs may be subpopulations of capillary endothelial cells or pericytes. This hypothesis is supported by a recent study that demonstrated the existence of myogenic-endothelial progenitor cells that are CD34+ and CD45− [13]. These cells differentiated into both vascular endothelial cells and skeletal muscle fibers after implantation into muscle [13]. Furthermore, myogenic cells expressing both myogenic and endothelial markers have been isolated from the aortas of mouse embryos [14], [15] and post natal mice [15]. These mesoangioblasts are also capable of regenerating skeletal muscle following intraaterial delivery [15].

Collectively, MDSCs may be subpopulations of the endothelial cells or pericytes associated with the muscle capillaries. Similarly, the so-called side population stem cells (SP cells) observed in a variety of tissues [16] may actually represent a population of pericytes that reside along the vasculature. Pericytes located along the vasculature originally were described as a population of osteoprogenitor cells because of the pericytes’ participation in bone regeneration after injury [17], their ability to undergo osteogenic differentiation in vitro [18], [19], [20] and in vivo [20], and their cell marker profile [21]. Not coincidentally, the MDSCs we have identified also readily undergo osteogenic differentiation after treatment with BMP2 or BMP4 [10], [22] (Fig. 1).

Several lines of evidence indicate a common origin of the different populations of adult stem cells: (1) the plasticity of adult stem cells; (2) the widespread presence of stem cells in a variety of tissues; and (3) the ability of these stem cells to undergo hematopoietic differentiation. Although this common origin could be attributed to the presence of circulating stem cells, a common developmental origin is more likely.

Pericytes could be the common origin of the adult stem cells found in different tissues. In muscle, some pericytes (e.g. CD45− MDSCs) may be more committed to myogenic differentiation, while others (e.g. CD45+ MDSCs) may be less so, unless stimulated by signaling molecules such as Wnt. Less committed MDSCs may retain more of their hematopoietic potential [5], a property shared by stem cells isolated from other tissues [16]. However, compared to hematopoietic stem cells isolated from bone marrow, MDSCs’ ability to differentiate into blood cells is much more limited [23]. Thus, it is likely that pericytes may contribute to the stem cell pool of all types of tissues, but most of the pericytes located in a particular tissue may already be imprinted to differentiate into the lineages of the tissues in which they reside. Alternatively, the microenvironments of each tissue could affect the differentiation potential of some populations of pericytes, and thereby lead to the existence of both tissue-specific and tissue non-specific stem cells in all the tissues containing vasculature [24]. Notably, the distinction between these two populations of stem cells is not permanent; tissue non-specific stem cells could become more tissue-specific under the influence of environmental factors.

One of the important issues in the study of MDSCs is the identification of factors that regulate MDSC self-renewal and differentiation. In recent studies we have identified several growth factors, including fibroblast growth factor-2 (FGF-2), epithelial growth factor (EGF), insulin-like growth factor-1 (IGF-1), and stem cell factor (SCF), that can enhance the in vitro proliferation of MDSCs through shortening the cell cycle and activating quiescent cells [25]. Recently, Wnt signaling has been shown to improve both cell growth and the self-renewal ability of hematopoietic stem cells [26], [27]. In light of the possible common origin of adult stem cells, it is likely that some of the regulatory pathways identified in other stem cells, especially hematopoietic stem cells, may also operate in MDSCs. It is feasible that some types of Wnt proteins enhance the myogenic potential of MDSCs, while others promote the self-renewal of these cells.