Current Stem Cell Research & Therapy - Volume 9, Issue 5, 2014

Human embryonic stem cells (ESCs), by virtue of their capability to self-renew and differentiate into a variety of cell types, represent the first type of pluripotent stem cells (PSCs) to be used in clinical transplantation during recent phase-I trials; however, it is still unclear whether hESC-derived tissues can self-organize and form part of the vascularized, functional organ following transplantation. Recently, endothelial cells (ECs) or angiogenic factors such as VEGFA have been demonstrated to support development and regeneration of multiple organ systems, including the heart, pancreas, liver, lung and bone marrow. Therefore, co-transplantation of ECs derived from the same parental PSCs that differentiate into cell types of interest; or overexpression of the inductive angiogenic factors responsible for organ regeneration might be beneficial to support function of hPSC-derived tissues. In this special issue, we discuss how protein kinases (Ng and colleagues); DNA methylation and histone modification (Tsui and colleagues) regulate cellular pluripotency and cell-fate specification of PSCs. In addition, we discuss how ECs and angiogenic factors could contribute to repair and regeneration of organs such as the heart (Yuan and colleagues), the cardiovascular system (Tse and colleagues) and the pancreas (Lui). We also discuss the role of mesenchymal stem cells or paracrine factors secreted by them in tissue repair (Li and colleagues). Lastly, we discuss how to generate self-organized and vascularized tissues derived from PSCs in a 2- or 3-dimensional format by fusing tissue bioengineering approaches with stem cell technology (Chen).

Protein kinases (PKs) mediate the reversible conversion of substrate proteins to phosphorylated forms, a key process in controlling intracellular signaling transduction cascades. Pluripotency is, among others, characterized by specifically expressed PKs forming a highly interconnected regulatory network that culminates in a finely-balanced molecular switch. Current high-throughput phosphoproteomic approaches have shed light on the specific regulatory PKs and their function in controlling pluripotent states. Pluripotent cell-derived endothelial and hematopoietic developments represent an example of the importance of pluripotency in cancer therapeutics and organ regeneration. This review attempts to provide the hitherto known kinome profile and the individual characterization of PK-related pathways that regulate pluripotency. Elucidating the underlying intrinsic and extrinsic signals may improve our understanding of the different pluripotent states, the maintenance or induction of pluripotency, and the ability to tailor lineage differentiation, with a particular focus on endothelial cell differentiation for anti-cancer treatment, cell-based tissue engineering, and regenerative medicine strategies.

Stem cell research has been developing rapidly in diverse areas such as the fields of genetics and molecular biology over the past decades. Genomic studies on both embryonic stem cells (ESCs) and terminally-differentiated cells illustrated that factors apart from their hereditary information disparity are associated with gene expression patterns of ESCs. Therefore, current research is trying to explore the effects of epigenetic processes in stem cell physiology and phenotypic changes. In-depth analyses of the molecular mechanisms underpinning such epigenetic-mediated functions have also been conducted. These findings suggest the importance of understanding the epigenetic influences in stem cell activities. Accordingly this review will describe the regulatory machineries of stem cells development targeting the two epigenetic processes: (1) DNA methylation and (2) histones modification. In addition, up-to-date findings concerning the functional roles of these processes in stem cells homeostasis will be covered.

The heart is the first organ to form during development in vertebrates, and many organs start to develop adjacent to the cardiovascular system. Endothelial cells (ECs) form the inner cell lining of blood vessels and represent the major cell type that interacts with developing organs including the pancreas. ECs receive signals from the developing pancreas to grow and, at the same time, release signals to determine cell-fate specification, morphogenesis and function of the pancreas. In addition to promoting survival of pancreatic islets, in this review, we discuss the role of the vascular niche and angiogenic factors, particularly VEGFA, during pancreatic beta cell development, regeneration and pathophysiological progression of diabetes. Nevertheless, unraveling the molecular signals involved in pancreatic beta cell development and regeneration may shed light into novel drug development to treat diabetes.

Cardiovascular disease is the leading cause of death worldwide. Despite significant progress in understanding of the disease mechanisms, most therapies remain at best palliative. Few therapeutic approaches offer direct tissue repair and regeneration. Cell-based therapy offers a promising approach that involves transplantation of healthy and functional cells to replenish damaged cells and repair injured tissue. Endothelial dysfunction is one of the most important mechanisms of cardiovascular disease, thus endothelial progenitor cells (EPC) and their derivatives have been investigated as a potential source for cell therapy. In pre-clinical and pilot clinical studies, treatment with EPCs or their derivatives as well as their co-transplantation with other cell types has shown some initial promising results. In this review, we will first describe the importance of endothelial cells and EPC homeostasis in the pathophysiology of cardiovascular disease. The potential sources of EPCs, including their isolation and purification, differentiation from pluripotent stem cells and adult stem cells, and trans-differentiation from somatic cells will then be summarized. Lastly, the application of target genome editing tools, such as Zinc Finger Nuclease (ZFN), Transcription Activator Like Effector Nucleases (TALEN) and RNA Guided Endo Nuclease (RGEN) to modify EPCs and their derivatives will be described. These technologies promise to further improve the therapeutic potential of EPCs and their derivatives to treat cardiovascular disease.

About 1 billion cardiomyocytes are lost in heart disease such as myocardial infarction; the regeneration capacity of cardiomyocytes is extremely low and the heart is unable to fix this damage naturally. In the past 20 years, multiple cells, such as skeletal myoblasts, bone marrow-derived cells, cardiac progenitor cells and pluripotent stem cells have been utilized to test the efficacy on heart repair, but these cells show different kinds of drawbacks. Recent studies have revealed that concomitant transplantation of endothelial cells and stem cells can significantly improve the efficacy of cell based heart repair. In this review, we describe the progress on these studies with an emphasis on endothelial cell facilitated stem cell therapy. We also summarize the beneficial mechanisms of endothelial cells in cardiac repair and point out the potential challenges for endothelial cell-facilitated-cell therapy to be adopted for clinical application.

Mesenchymal stem cells (MSCs) are multi-potent cells which have been widely used for tissue regeneration and immunomodulation. The infusion of autologous and allogenic MSCs has been proved to be safe and effective in tissue repair and disease modulation. The inherent homing ability of MSCs ensures the transplanted cells migrating into the damaged tissue areas, but only a small percentage of the transplanted (allogenic) MSCs survive for long. However, the beneficial effects of MSCs transplantation could be noted within 1-2 days that are unlikely due to their proliferation and differentiation. The regulatory roles of MSCs in tissue repair are rather more important than their direct involvement of repair processes. The most important effect of transplanted MSCs is their immunomodulation function through crosstalk with the immune cells or the paracrine actions. The active factor secreted by MSCs may vary in the different disease conditions or tissue niches, and are under dynamic changes in various local environments. To understand and define the MSCs secretion factors in various disease settings could be a future research direction, and the findings could lead to potential new MSCs-based therapeutic products.

Regenerative medicine offers therapeutic approaches to treating non-regenerative diseases such as spinal cord injury and heart disease. Owing to the limited donor tissue available, cell-based therapy using cultured cells with supporting scaffolds has been proposed to rebuild damaged tissue. Early attempts at repairing skin and cartilage achieved significant success thanks to the simplicity of the tissue architecture, which later fueled enthusiasm for applying the same strategy to other types of tissue. However, more complex tissue functions require a more extensive vasculature and heterogeneous cell arrangements, which together constitute a significant hurdle in practical applications. Accordingly, recent years an increased interest has been in the use of decellularized matrices that retain the natural microarchitecture as the scaffold. However, although a number of engineering approaches have been suggested, self-organizing behavior such as cell proliferation, migration, and differentiation may still disorganize and frustrate the artificial attempts. This mini-review first provides examples of the early history of tissue engineering using skin and cartilage as examples, and then elaborates on the key technologies used to fabricate synthetic acellular scaffolds and cell/scaffold constructs with more complicated architectures. It also summarizes the progress achieved in the use of decellularized matrices for cell seeding as well as the recent success seen in self-organizing two- and three-dimensional tissue formation with the aid of biomathematical modeling. The review concludes by proposing the future integration of biomathematics, developmental biology, and engineering in concert with the self-organization approach to tissue regeneration.

Success of stem cell therapies were reported in different medical disciplines, including haematology, rheumatology, orthopaedic surgery, traumatology, and others. Currently, more than 4000 clinical trials using stem cells have been completed or are underway, among which 378 investigated or are at present investigating mesenchymal stromal cells (MSCs). The majority of clinical trials using stem- or progenitor- cells, including hematopoietic stem cells and MSCs, target the immune system. However, therapies based on MSCs are increasingly implemented to treat symptoms in which failure of the resident stem cells in situ, or malfunction of tissues or structures are not associated with immune cells or inflammation, but instead are associated with mechanical or metabolic stress, ageing, developmental or acquired malformations, and other causes. To proceed further in the development of stem cell therapies as a safe and effective treatment for surgical and other medical specialities, the behaviour of MSCs implanted in preclinical models and their impact on the site of application need to be explored in detail. Depending on the pre-clinical model employed, tracking of labelled stem cells in live animals makes an enormous difference for exploration of the mechanisms and kinetics involved in MSC-mediated tissue regeneration. Here we review (pre-)clinically applicable key methods to label human MSCs for short and long-term observations in small and large animal models.