Platelet-derived growth factor receptors regulate mesenchymal stem cell fate: implications for neovascularization

Importance of the field: Regulating stem cell contributions to vascularization is a challenging goal, but a fundamental aspect of regenerative medicine. Human mesenchymal stem cells retain considerable potential for adult vascular repair and regeneration therapies. They are readily obtained, rapidly proliferate in culture, display a capacity to differentiate towards endothelial or vascular smooth muscle cells, and play an important role in postnatal neovascularization in various tissue contexts. To therapeutically enhance neovascularization during ischemic disease, or inhibit neovascularization during tumorigenesis, an essential prerequisite is to determine the mechanisms which control the recruitment and differentiation of mesenchymal stem cells towards vascular cells.

Areas covered in this review: In this review, we describe the current understanding of how PDGF receptors contribute prominently to neovascularization, and play a decisive role in modulating mesenchymal stem cell recruitment and differentiation towards vascular cells. We discuss PDGF receptor-based therapeutic strategies to exploit mesenchymal stem cells during vascular repair and regeneration, and to control pathological neovascularization.

Take home message: PDGF receptor signaling is emerging as a critical regulatory mechanism and important therapeutic target, that critically directs the fate of mesenchymal stem cells during postnatal neovascularization.     

Stem cells for tendon tissue engineering and regeneration

Importance of the field: Tendon injuries are common especially in sports activities, but tendon is a unique connective tissue with poor self-repair capability. With advances in stem cell biology, tissue engineering is becoming increasingly powerful for tissue regeneration. Stem cells with capacity of multipotency and self-renewal are an ideal cell source for tissue engineering.

Areas covered in this review: This review focus on discussing the potential strategies including inductive growth factors, bio-scaffolds, mechanical stimulation, genetic modification and co-culture techniques to direct tendon-lineage differentiation of stem cells for complete tendon regeneration. Attempting to use embryonic stem cells as seed cells for tendon tissue engineering have achieved encouraging results. The combination of chemical and physical signals in stem cell microenvironment could be regulated to induce differentiation of the embryonic stem cells into tendon.

What the reader will gain: We summarize fundamental questions, as well as future directions in tendon biology and tissue engineering.

Take home message: Multifaceted technologies are increasingly required to control stem cell differentiation, to develop novel stem cell-based therapy, and, ultimately, to achieve more effective repair or regeneration of injured tendons.     

Epigenetic approaches to regeneration of bone and cartilage from stem cells

Introduction: Embryonic stem cells (ESCs) or adult stem cells, especially mesenchymal stem cells (MSCs), have been intensively studied for skeletal tissue regeneration including bone and cartilage. Epigenetic mechanisms play essential roles in stem cell maintenance and differentiation. However, little is known about the epigenetic regulation of osteogenesis and chondrogenesis of stem cells.

Areas covered: In this review, features of ESCs and adult stem cells, epigenetics and chromatin structure, as well as epigenetic mechanisms, such as chromatin remodeling, DNA methylation and histone modifications, polycomb group (PcG) proteins and microRNAs are described. Epigenetic researches of stem cell are introduced.

Expert opinion: Epigenetic alterations of stem cell during the in vitro differentiation can be controlled for clinical applications. MSCs are effective resources for skeletal tissue regeneration in both undifferentiated and differentiated states. Understanding epigenetic signatures of MSC is crucial to maintain the stemness. In addition, investigation of epigenetic changes in the differentiation of MSCs is very important to develop methods or chemicals to promote efficient differentiation of MSCs. Inhibition of PcG protein enhancer of zeste (Ezh2) a chromatin modifier, could be a promising candidate to improve MSC differentiation by decreasing Ezh2-mediated H3K27me3.     

Therapeutic potential of perivascular cells from human pluripotent stem cells

Vascularization of injured tissues or artificial grafts is a major challenge in tissue engineering, stimulating a continued search for alternative sources for vasculogenic cells and the development of therapeutic strategies. Human pluripotent stem cells (hPSCs), either embryonic or induced, offer a plentiful platform for the derivation of large numbers of vasculogenic cells, as required for clinical transplantations. Various protocols for generation of vasculogenic smooth muscle cells (SMCs) from hPSCs have been described with considerably different SMC derivatives. In addition, we recently identified hPSC‐derived pericytes, which are similar to their physiological counterparts, exhibiting unique features of blood vessel‐residing perivascular cells, as well as multipotent mesenchymal precursors with therapeutic angiogenic potential. In this review we refer to methodologies for the development of a variety of perivascular cells from hPSCs with respect to developmental induction, differentiation capabilities, potency and their dual function as mesenchymal precursors. The therapeutic effect of hPSC‐derived perivascular cells in experimental models of tissue engineering and regenerative medicine are described and compared to those of their native physiological counterparts. Copyright © 2013 John Wiley & Sons, Ltd.     

Potential benefits of allogeneic bone marrow mesenchymal stem cells for wound healing

Introduction: It is becoming increasingly evident that select adult stem cells have the capacity to participate in repair and regeneration of damaged and/or diseased tissues. Mesenchymal stem cells have been among the most studied adult stem cells for the treatment of a variety of conditions, including wound healing.

Areas covered: Mesenchymal stem cell features potentially beneficial to cutaneous wound healing applications are reviewed.

Expert opinion: Given their potential for in vitro expansion and immune modulatory effects, both autologous and allogeneic mesenchymal stem cells appear to be well suited as wound healing therapies. Allogeneic mesenchymal stem cells derived from young healthy donors could have particular advantage over autologous sources where age and systemic disease can be significant factors.     

The potential of amniotic fluid stem cells for cellular therapy and tissue engineering

Introduction: Foetal cells present in amniotic fluid (AF) have been used for many years to perform prenatal genetic screening. Recent reports suggested that these cells might have additional benefits. AF contains, in addition to committed and differentiated cells, a subpopulation with stem cell characteristics. AF-derived stem cells (AFS) have functions found in mesenchymal stem cells, but in addition, exhibit a potent expansion capacity and plasticity. AFS are able to undergo multi-lineage differentiation and produce progeny indicative of all three germ layers.

Areas covered: The experimental approaches available to isolate AFS and their potential for tissue engineering, the repair of organs through cell replacement and tissue regeneration.

Expert opinion: The deployment of AFS for tissue regeneration offers advantages over the use of embryonic or adult stem cells: i) AF represents a convenient and non-contested source for obtaining stem cells; ii) their derivation is relatively simple and rapid; iii) no feeder layers are required for their cultivation; iv) they display no spontaneous differentiation in culture; and v) their stem cell phenotype is not affected by long-term storage. The application of AFS for tissue replacement therapies in vivo is at a very early stage, but existing studies indicate great potential for clinical use.     

Adipose tissue-derived stem cells in clinical applications

Introduction: In the past decade human adipose tissue has been identified as a source of multipotent stem cells. Adipose tissue derived stem cells (ASCs) are characterised by immunosuppressive properties and low immunogenicity. Therefore, they can be used in regenerative medicine, as well as applied to induce graft tolerance or prevent autoimmunity. ASCs can be easily harvested with low morbidity, which is their main advantage over mesenchymal stem cells (MSCs) derived from other sources.

Areas covered: The review focuses on reported clinical applications of ASCs and discusses technical approaches of their isolation and processing. The differences in phenotype and differentiation preferences between ASCs and other MSCs that may affect the choice of a particular cell type for the future therapy are also described.

Expert opinion: ASCs seem to be the perfect tool for regenerative medicine and immunosuppressive cellular therapies. Nevertheless, there are some tasks that should be addressed by the future studies: i) ASCs require better characterisation; a set of markers determining ASCs should be clearly defined; ii) there is need for more studies on safety of reconstructive therapies with ASCs in cancer patients (e.g., after mastectomy); iii) release criteria should be determined for freshly isolated and ex vivo expanded ASCs designed for clinical applications.     

Potential therapeutic applications of muscle-derived mesenchymal stem and progenitor cells

Importance of the field: Mesenchymal adult stem cells have properties that make them attractive for use in tissue engineering and regenerative medicine. They are inherently plastic, enabling them to differentiate along different lineages, and promote wound healing and regeneration of surrounding tissues by modulating immune and inflammatory responses, promoting angiogenesis and secreting other trophic factors. Unlike embryonic stem cells, clinical uses of mesenchymal stem cells are not encumbered by ethical considerations or legal restrictions.

Areas covered in this review: We discuss skeletal muscle as a source of mesenchymal stem and progenitor cells by reviewing their biology and current applications in tissue engineering and regenerative medicine. This paper covers literature from the last 5 – 10 years.

What the reader will gain: Skeletal muscle is a plentiful source of mesenchymal stem and progenitor cells. This tissue may be obtained via routine biopsy or collection after surgical debridement. We describe the biology of these cells and provide an overview of therapeutic applications currently being developed to take advantage of their regenerative properties.

Take home message: There is potential for stem and progenitor cells derived from skeletal muscle to be incorporated in clinical interventions, either as a cellular therapy to modify the natural history of disease or as a component of engineered tissue constructs that can replace diseased or damaged tissues.     

Self-renewal and cell lineage differentiation strategies in human embryonic stem cells and induced pluripotent stem cells

Introduction: Since the initial discoveries of human embryonic and induced pluripotent stem cells, many strategies have been developed to utilize the potential of these cells for translational research and disease modeling. The success of these aims and the development of future applications in this area will depend on the ability to generate high-quality and large numbers of differentiated cell types that genetically, epigenetically, and functionally mimic the cells found in the body.

Areas covered: In this review, we highlight the current strategies used to maintain stem cell pluripotency (a measure of stem cell quality), as well as provide an overview of the various differentiation strategies being used to generate cells from all three germ lineages. We also discuss the particular considerations that must be addressed when utilizing these cells for translational therapy, and provide an example of a cell type currently used in clinical trials.

Expert opinion: The major challenge in regenerative medicine and disease modeling will be in generating functional cells of sufficient quality that are physiologically and epigenetically similar to the diverse cells that they are modeled after. By meeting these criteria, these differentiated products can be successfully used in disease modeling, drug/toxicology screens, and cellular replacement therapy.     

Skeletal tissue engineering using mesenchymal or embryonic stem cells: clinical and experimental data

Introduction: Mesenchymal stem cells (MSCs) can be obtained from a wide variety of tissues for bone tissue engineering such as bone marrow, adipose, birth-associated, peripheral blood, periosteum, dental and muscle. MSCs from human fetal bone marrow and embryonic stem cells (ESCs) are also promising cell sources.

Areas covered: In vitro, in vivo and clinical evidence was collected using MEDLINE® (1950 to January 2014), EMBASE (1980 to January 2014) and Google Scholar (1980 to January 2014) databases.

Expert opinion: Enhanced results have been found when combining bone marrow–derived mesenchymal stem cells (BMMSCs) with recently developed scaffolds such as glass ceramics and starch-based polymeric scaffolds. Preclinical studies investigating adipose tissue–derived stem cells and umbilical cord tissue–derived stem cells suggest that they are likely to become promising alternatives. Stem cells derived from periosteum and dental tissues such as the periodontal ligament have an osteogenic potential similar to BMMSCs. Stem cells from human fetal bone marrow have demonstrated superior proliferation and osteogenic differentiation than perinatal and postnatal tissues. Despite ethical concerns and potential for teratoma formation, developments have also been made for the use of ESCs in terms of culture and ideal scaffold.     

Hepatic perivascular mesenchymal stem cells with myogenic properties

Pericytes are multipotent mesenchymal stem cells located on the walls of blood vessels in various organs and are characterized as CD146⁺ cells. In this study, we first immunohistochemically detected pericytes in the perivascular regions of liver from two mouse genotypes, namely wild‐type (WT) and myostatin null (Mstn^?/?). We further isolated pericytes using sorting as CD146⁺ CD34^? CD56^? CD45^? cells. The main finding of this study involves the contrasting fibrogenic vs. myogenic behaviour of liver pericytes from WT and Mstn^?/? mice, respectively. Sorted CD146⁺ liver pericytes (WT and Mstn^?/?) expressed PDGFRβ, NG2, vimentin, adult stem cell markers CD73, CD105, CD44 and could be readily differentiated into adipogenic, osteogenic and chondrogenic lineages. Furthermore, these CD146⁺ cells from WT and Mstn^?/? livers did not express myostatin, in contrast to the total liver tissue of WT. The absence of αSMA and GFAP made these cells easily distinguishable from hepatic stellate cells. When subjected to standard myogenic differentiation with low serum the CD146⁺ cells from WT liver differentiated into myofibroblasts (fibrogenic) and the CD146⁺ cells from Mstn^?/? liver differentiated into multinucleated myotubes (myogenic). Finally, we transplanted CD146⁺ pericytes into tibialis anterior muscle of dystrophic mice and established the generation of novel myofibres, thereby proving their cell therapy potential. The liver tissue microenvironment with myostatin in WT and the absence of myostatin in Mstn^?/? conditions might exert a paracrine effect in determining the fate of pericyte‐like cells in the liver.     

Potential application of adipose-derived stem cells and their secretory factors to skin: discussion from both clinical and industrial viewpoints

Importance of the field: Adipose tissue is one of the richest sources of mesenchymal stem cells. Even more interesting is the fact that adipose-derived stem cells (ASCs) show an outstanding ability to regenerate damaged skin. Thus, ASCs are a popular and feasible treatment in clinical dermatology.

Areas covered in this review: This review discusses the potential applications of ASCs and conditioned medium of ASC (ASC-CM) to skin, and briefly touches on the mechanisms by which ASCs promote skin regeneration.

What the reader will gain: Clinically, processed lipo-aspirated (PLA) cells are commonly used for treatment of aged skin; however, the use of PLA cells for cosmetic purposes is not convenient, because PLA cells are prepared from patients. Alternatively, cosmetics that contain ASC-CM can be pre-made from healthy volunteers such that they are immediately available for clinical treatment of aged skin. Cell-based therapies are adequate for improvement of wrinkles or for soft tissue augmentation, whereas ASC-CM has merit for amelioration of skin tone. When culturing ASCs for the production of cosmetic raw materials, hypoxic culture conditions and transduction of specific genes into ASCs may increase the regenerative protein content of the conditioned medium.

Take home message: Application of ASCs and ASC-CM to dermatology shows promising results for skin regeneration.     

Dental stem cells in tooth regeneration and repair in the future

Introduction: Human dental stem cells can be obtained from postnatal teeth, extracted wisdom teeth or exfoliated deciduous teeth. Due to their differentiation potential, these mesenchymal stem cells are promising for tooth repair. Therefore, the development of dental tissue regeneration represents a suitable but challenging, target for dental stem cell therapies.

Areas covered: In this review, the authors provide an overview of human dental stem cells and their properties for regeneration medicine. Numerous preclinical studies have shown that dental stem cells improve bone augmentation and healing of periodontal diseases. Clinical trials are ongoing to validate the clinical feasibility of these approaches. Dental stem cells are also important for basic research.

Expert opinion: Dental stem cells offer numerous advantages for tooth repair and regeneration. Data obtained from different studies are encouraging. In the next few years, investigations on dental stem cells in basic research, pre-clinical research and clinical studies will pave the way to optimizing patient-tailored treatments for repair and regeneration of dental tissues.     

Current advances in cell therapy strategies for muscular dystrophies

Introduction: Muscular dystrophies are a heterogeneous group of genetic diseases characterized by muscle weakness, wasting and degeneration. Cell therapy consists of delivering myogenic precursor cells to damaged tissue for the complementation of missing proteins and/or the regeneration of new muscle fibres.

Areas covered: We focus on human candidate cells described so far (myoblasts, mesoangioblasts, pericytes, myoendothelial cells, CD133⁺ cells, aldehyde-dehydrogenase-positive cells, mesenchymal stem cells, embryonic stem cells, induced pluripotent stem cells), gene-based strategies developed to modify cells prior to injection, animal models (dystrophic and/or immunodeficient) used for pre-clinical studies, and clinical trials that have been performed using cell therapy strategies. The approaches are reviewed in terms of feasibility, hurdles, potential solutions and/or research areas from where the solution may come and potential application in terms of types of dystrophies and targets.

Expert opinion: Cell therapy for muscular dystrophies should be put in the context of which dystrophy or muscle group is targeted, what tools are available at hand, but even more importantly what can cell therapy bring as compared with and/or in combination with other therapeutic strategies. The solution will probably be the right dosage of these combinations adapted to each dystrophy, or even to each type of mutation within a dystrophy.     

Potential of mesenchymal stem cells in gene therapy approaches for inherited and acquired diseases

The intriguing biology of stem cells and their vast clinical potential is emerging rapidly for gene therapy. Bone marrow stem cells, including the pluripotent haematopoietic stem cells (HSCs), mesenchymal stem cells (MSCs) and possibly the multipotent adherent progenitor cells (MAPCs), are being considered as potential targets for cell and gene therapy-based approaches against a variety of different diseases. The MSCs from bone marrow are a promising target population as they are capable of differentiating along multiple lineages and, at least in vitro, have significant expansion capability. The apparently high self-renewal potential makes them strong candidates for delivering genes and restoring organ systems function. However, the high proliferative potential of MSCs, now presumed to be self-renewal, may be more apparent than real. Although expanded MSCs have great proliferation and differentiation potential in vitro, there are limitations with the biology of these cells in vivo. So far, expanded MSCs have failed to induce durable therapeutic effects expected from a true self-renewing stem cell population. The loss of in vivo self-renewal may be due to the extensive expansion of MSCs in existing in vitro expansion systems, suggesting that the original stem cell population and/or properties may no longer exist. Rather, the expanded population may indeed be heterogeneous and represents several generations of different types of mesenchymal cell progeny that have retained a limited proliferation potential and responsiveness for terminal differentiation and maturation along mesenchymal and non-mesenchymal lineages. Novel technology that allows MSCs to maintain their stem cell function in vivo is critical for distinguishing the elusive stem cell from its progenitor cell populations. The ultimate dream is to use MSCs in various forms of cellular therapies, as well as genetic tools that can be used to better understand the mechanisms leading to repair and regeneration of damaged or diseased tissues and organs.     

Toward the use of endometrial and menstrual blood mesenchymal stem cells for cell-based therapies

Introduction: Bone marrow is a widely used source of mesenchymal stem cells (MSCs) for cell-based therapies. Recently, endometrium – the highly regenerative lining of the uterus – and menstrual blood have been identified as more accessible sources of MSCs. These uterine MSCs include two related cell types: endometrial MSCs (eMSCs) and endometrial regenerative cells (ERCs).

Areas covered: The properties of eMSCs and ERCs and their application in preclinical in vitro and in vivo studies for pelvic organ prolapse, heart disorders and ischemic conditions are reviewed. Details of the first clinical Phase I and Phase II studies will be provided.

Expert opinion: The authors report that eMSCs and ERCs are a readily available source of adult stem cells. Both eMSCs and ERCs fulfill the key MSC criteria and have been successfully used in preclinical models to treat various diseases. Data on clinical trials are sparse. More research is needed to determine the mechanism of action of eMSCs and ERCs in these regenerative medicine models and to determine the long-term benefits and any adverse effects after their administration.     

Corneal epithelial stem cells for corneal injury

Introduction: Ocular surface diseases with limbal insufficiency represent a therapeutic challenge for restoring vision. This corneal deficiency includes both classical ocular diseases (as chemical burns) and rare ocular diseases (as congenital aniridia and ocular cicatricial pemphigoid).

Areas covered: Our understanding of limbal epithelial stem cells (LESCs) has increased the potential for treatment options. Pharmacological treatment strategies (as regenerating agent ophthalmic solutions) and especially surgical treatment strategies are available. Isolated LESCs can be produced by limbal primary cultures obtained from explants or cell suspensions. We review the latest cornea surgery techniques.

Expert opinion: The adjunction of human limbal mesenchymal cells as a support for limbal stem cell primary cultures appears to be of great interest. Recently, human-induced pluripotent stem cells have allowed the generation of minicorneal organoids. This potential means of creating a three-dimensional cornea with in vitro maturation opens up important research areas for corneal regeneration therapy.