Direct comparison of autologous and allogeneic transplantation of iPSC-derived neural cells in the brain of a non-human primate

Indications and prospects of neural transplantation for chronic neurological diseases

PURPOSE OF REVIEW

The replacement of damaged cells in the central nervous system (CNS) affected by degenerative disorders represents an attractive therapeutic strategy. The advent of stem cell technology may offer the possibility of generating a large number of renewable, specifically differentiated cells to potentially cure large cohorts of patients. In this review, we discuss current knowledge and issues involved in neural cell transplantation. The most important preclinical and clinical results of cellular transplantation applied to Parkinson's, Huntington's disease and amyotrophic lateral sclerosis will be summarized.

RECENT FINDINGS

Cellular transplantation is emerging as a possible therapy for a variety of incurable neurological disorders. The disorders that will primarily take advantage from neural stem cell grafting are those involving a well defined cell population in a restricted area of the CNS. Several clinical trials have been initiated to assess safety and efficacy of different stem cell-derived products, and promising results have been obtained for disorders such as Parkinson's disease. However, several scientific questions remain unanswered. Among these, the impact of the immunological interaction between host and graft in the particular environment of the CNS still requires additional investigations.

SUMMARY

Several chronic neurological disorders appear to be amenable to cell regenerative therapies. However, safety, efficacy and immunological issues will need to be carefully evaluated beforehand.

Transplantation in the nonhuman primate MPTP model of Parkinson's disease: update and perspectives

In order to calibrate stem cell exploitation for cellular therapy in neurodegenerative diseases, fundamental and preclinical research in NHP (nonhuman primate) models is crucial. Indeed, it is consensually recognized that it is not possible to directly extrapolate results obtained in rodent models to human patients. A large diversity of neurological pathologies should benefit from cellular therapy based on neural differentiation of stem cells. In the context of this special issue of Primate Biology on NHP stem cells, we describe past and recent advances on cell replacement in the NHP model of Parkinson's disease (PD). From the different grafting procedures to the various cell types transplanted, we review here diverse approaches for cell-replacement therapy and their related therapeutic potential on behavior and function in the NHP model of PD.

CRISPR/Cas9 Genome-Editing System in Human Stem Cells: Current Status and Future Prospects

Genome-editing involves the insertion, deletion, or replacement of DNA in the genome of a living organism using “molecular scissors.” Traditional genome editing with engineered nucleases for human stem cells is limited by its low efficiency, high cost, and poor specificity. The CRISPR system has recently emerged as a powerful gene manipulation technique with advantages of high editing efficiency and low cost. Although this technique offers huge potential for gene manipulation in various organisms ranging from prokaryotes to higher mammals, there remain many challenges in human stem cell research. In this review, we highlight the basic biology and application of the CRISPR/Cas9 system in current human stem cell research, discuss its advantages and challenges, and debate the future prospects for human stem cells in regenerative medicine.

MHC matching improves engraftment of iPSC-derived neurons in non-human primates

The banking of human leukocyte antigen (HLA)-homozygous-induced pluripotent stem cells (iPSCs) is considered a future clinical strategy for HLA-matched cell transplantation to reduce immunological graft rejection. Here we show the efficacy of major histocompatibility complex (MHC)-matched allogeneic neural cell grafting in the brain, which is considered a less immune-responsive tissue, using iPSCs derived from an MHC homozygous cynomolgus macaque. Positron emission tomography imaging reveals neuroinflammation associated with an immune response against MHC-mismatched grafted cells. Immunohistological analyses reveal that MHC-matching reduces the immune response by suppressing the accumulation of microglia (Iba-1+) and lymphocytes (CD45+) into the grafts. Consequently, MHC-matching increases the survival of grafted dopamine neurons (tyrosine hydroxylase: TH+). The effect of an immunosuppressant, Tacrolimus, is also confirmed in the same experimental setting. Our results demonstrate the rationale for MHC-matching in neural cell grafting to the brain and its feasibility in a clinical setting.

Major histocompatibility complex (MHC) matching improves graft survival rates after organ transplantation. Here the authors show that in macaques, MHC-matched iPSC-derived neurons provide better engraftment in the brain, with a lower immune response and higher survival of the transplanted neurons.

Inherent Immunogenicity or Lack Thereof of Pluripotent Stem Cells: Implications for Cell Replacement Therapy

Donor-specific induced pluripotent stem cells (iPSCs) offer opportunities for personalized cell replacement therapeutic approaches due to their unlimited self-renewal potential and ability to differentiate into different somatic cells. A significant progress has been made toward generating iPSC lines that are free of integrating viral vectors, development of xeno-free culture conditions, and differentiation of pluripotent stem cells (PSCs) into functional somatic cell lineages. Since donor-specific iPSC lines are genetically identical to the individual, they are expected to be immunologically matched and these iPSC lines and their cellular derivatives are not expected to be immunologically rejected. However, studies in mouse models, utilizing rejection of teratomas as a model, have claimed that syngenic iPSC lines, especially the iPSC lines derived with integrating viral vectors, could be inherently immunogenic. This manuscript reviews current understanding of inherent immunogenicity of PSC lines, especially that of the human iPSC lines and their cellular derivatives, and strategies to overcome it.

The Immunogenicity and Immune Tolerance of Pluripotent Stem Cell Derivatives

Human embryonic stem cells (hESCs) can undergo unlimited self-renewal and differentiate into all cell types in human body, and therefore hold great potential for cell therapy of currently incurable diseases including neural degenerative diseases, heart failure, and macular degeneration. This potential is further underscored by the promising safety and efficacy data from the ongoing clinical trials of hESC-based therapy of macular degeneration. However, one main challenge for the clinical application of hESC-based therapy is the allogeneic immune rejection of hESC-derived cells by the recipient. The breakthrough of the technology to generate autologous-induced pluripotent stem cells (iPSCs) by nuclear reprogramming of patient’s somatic cells raised the possibility that autologous iPSC-derived cells can be transplanted into the patients without the concern of immune rejection. However, accumulating data indicate that certain iPSC-derived cells can be immunogenic. In addition, the genomic instability associated with iPSCs raises additional safety concern to use iPSC-derived cells in human cell therapy. In this review, we will discuss the mechanism underlying the immunogenicity of the pluripotent stem cells and recent progress in developing immune tolerance strategies of human pluripotent stem cell (hPSC)-derived allografts. The successful development of safe and effective immune tolerance strategy will greatly facilitate the clinical development of hPSC-based cell therapy.

An update on stem cell biology and engineering for brain development

Two recent technologies, induced-pluripotent stem cells (iPSCs) and direct somatic reprogramming, have shown enormous potential for cell-based therapies against intractable diseases such as those that affect the central nervous system. Already, methods that generate most major cell types of the human brain exist. Whether the cell types are directly reprogrammed from human somatic cells or differentiated from an iPSC intermediate, the overview presented here demonstrates how these protocols vary greatly in their efficiencies, purity and maturation of the resulting cells. Possible solutions including micro-RNA switch technologies that purify target cell types are also outlined. Further, an update on the transition from 2D to 3D cultures and current organoid (mini-brain) cultures are reviewed to give the stem cell and developmental engineering communities an up-to-date account of the progress and future perspectives for modeling of central nervous system disease and brain development in vitro.

Cryopreservation Maintains Functionality of Human iPSC Dopamine Neurons and Rescues Parkinsonian Phenotypes In Vivo

A major challenge for clinical application of pluripotent stem cell therapy for Parkinson's disease (PD) is large-scale manufacturing and cryopreservation of neurons that can be efficiently prepared with minimal manipulation. To address this obstacle, midbrain dopamine neurons were derived from human induced pluripotent stem cells (iPSC-mDA) and cryopreserved in large production lots for biochemical and transplantation studies. Cryopreserved, post-mitotic iPSC-mDA neurons retained high viability with gene, protein, and electrophysiological signatures consistent with midbrain floor-plate lineage. To test therapeutic efficacy, cryopreserved iPSC-mDA neurons were transplanted without subculturing into the 6-OHDA-lesioned rat and MPTP-lesioned non-human-primate models of PD. Grafted neurons retained midbrain lineage with extensive fiber innervation in both rodents and monkeys. Behavioral assessment in 6-OHDA-lesioned rats demonstrated significant reversal in functional deficits up to 6 months post transplantation with reinnervation of the host striatum and no aberrant growth, supporting the translational development of pluripotent cell-based therapies in PD.

•

Cryopreserved human iPSC-mDA neurons retain high viability and midbrain lineage

•

iPSC-mDA neurons secrete dopamine and possess typical electrophysiological parameters

•

Grafted iPSC-mDA neurons survive and innervate rodent and monkey PD models

•

Cryopreserved iPSC-mDA neurons reverse functional motor deficits in 6-OHDA rats

The potential of induced pluripotent stem cells as a tool to study skeletal dysplasias and cartilage-related pathologic conditions

The development of induced pluripotent stem cells (iPSCs) technology has opened up new horizons for development of new research tools especially for skeletal dysplasias, which often lack human disease models. Regenerative medicine and tissue engineering could be the next areas to benefit from refinement of iPSC methods to repair focal cartilage defects, while applications for osteoarthritis (OA) and drug screening have evolved rather slowly. Although the advances in iPSC research of skeletal dysplasias and repair of focal cartilage lesions are not directly relevant to OA, they can be considered to pave the way to future prospects and solutions to OA research, too. The same problems which face the present cell-based treatments of cartilage injuries concern also the iPSC-based ones. However, established iPSC lines, which have no genomic aberrations and which efficiently differentiate into extracellular matrix secreting chondrocytes, could be an invaluable cell source for cell transplantations in the future. The safety issues concerning the recipient risks of teratoma formation and immune response still have to be solved before the potential use of iPSCs in cartilage repair of focal cartilage defects and OA.

Autologous Pluripotent Stem Cell-Derived β-Like Cells for Diabetes Cellular Therapy

Development of stem cell technologies for cell replacement therapy has progressed rapidly in recent years. Diabetes has long been seen as one of the first applications for stem cell-derived cells because of the loss of only a single cell type-the insulin-producing β-cell. Recent reports have detailed strategies that overcome prior hurdles to generate functional β-like cells from human pluripotent stem cells in vitro, including from human induced pluripotent stem cells (hiPSCs). Even with this accomplishment, addressing immunological barriers to transplantation remains a major challenge for the field. The development of clinically relevant hiPSC derivation methods from patients and demonstration that these cells can be differentiated into β-like cells presents a new opportunity to treat diabetes without immunosuppression or immunoprotective encapsulation or with only targeted protection from autoimmunity. This review focuses on the current status in generating and transplanting autologous β-cells for diabetes cell therapy, highlighting the unique advantages and challenges of this approach.

Non-human primate models of PD to test novel therapies

Non-human primate (NHP) models of Parkinson disease show many similarities with the human disease. They are very useful to test novel pharmacotherapies as reviewed here. The various NHP models of this disease are described with their characteristics including the macaque, the marmoset, and the squirrel monkey models. Lesion-induced and genetic models are described. There is no drug to slow, delay, stop, or cure Parkinson disease; available treatments are symptomatic. The dopamine precursor, L-3,4-dihydroxyphenylalanine (L-Dopa) still remains the gold standard symptomatic treatment of Parkinson. However, involuntary movements termed L-Dopa-induced dyskinesias appear in most patients after chronic treatment and may become disabling. Dyskinesias are very difficult to manage and there is only amantadine approved providing only a modest benefit. In this respect, NHP models have been useful to seek new drug targets, since they reproduce motor complications observed in parkinsonian patients. Therapies to treat motor symptoms in NHP models are reviewed with a discussion of their translational value to humans. Disease-modifying treatments tested in NHP are reviewed as well as surgical treatments. Many biochemical changes in the brain of post-mortem Parkinson disease patients with dyskinesias are reviewed and compare well with those observed in NHP models. Non-motor symptoms can be categorized into psychiatric, autonomic, and sensory symptoms. These symptoms are present in most parkinsonian patients and are already installed many years before the pre-motor phase of the disease. The translational usefulness of NHP models of Parkinson is discussed for non-motor symptoms.

The role of nonhuman primate models in the development of cell-based therapies for Parkinson's disease

Through the course of over three decades, nonhuman primate (NHP) studies on cell-based therapies (CBTs) for Parkinson’s disease (PD) have provided insight into the feasibility, safety and efficacy of the approach, methods of cell collection and preparation, cell viability, as well as potential brain targets. Today, NHP research continues to be a vital source of information for improving cell grafts and analyzing how the host affects graft survival, integration and function. Overall, this article aims to discuss the role that NHP models of PD have played in CBT development and highlights specific issues that need to be considered to maximize the value of NHP studies for the successful clinical translation of CBTs.

Practical whole-tooth restoration utilizing autologous bioengineered tooth germ transplantation in a postnatal canine model

Whole-organ regeneration has great potential for the replacement of dysfunctional organs through the reconstruction of a fully functional bioengineered organ using three-dimensional cell manipulation in vitro. Recently, many basic studies of whole-tooth replacement using three-dimensional cell manipulation have been conducted in a mouse model. Further evidence of the practical application to human medicine is required to demonstrate tooth restoration by reconstructing bioengineered tooth germ using a postnatal large-animal model. Herein, we demonstrate functional tooth restoration through the autologous transplantation of bioengineered tooth germ in a postnatal canine model. The bioengineered tooth, which was reconstructed using permanent tooth germ cells, erupted into the jawbone after autologous transplantation and achieved physiological function equivalent to that of a natural tooth. This study represents a substantial advancement in whole-organ replacement therapy through the transplantation of bioengineered organ germ as a practical model for future clinical regenerative medicine.

Healing of a Large Long-Bone Defect through Serum-Free In Vitro Priming of Human Periosteum-Derived Cells

Clinical translation of cell-based strategies for regenerative medicine demands predictable in vivo performance where the use of sera during in vitro preparation inherently limits the efficacy and reproducibility. Here, we present a bioinspired approach by serum-free pre-conditioning of human periosteum-derived cells, followed by their assembly into microaggregates simultaneously primed with bone morphogenetic protein 2 (BMP-2). Pre-conditioning resulted in a more potent progenitor cell population, while aggregation induced osteochondrogenic differentiation, further enhanced by BMP-2 stimulation. Ectopic implantation displayed a cascade of events that closely resembled the natural endochondral process resulting in bone ossicle formation. Assessment in a critical size long-bone defect in immunodeficient mice demonstrated successful bridging of the defect within 4 weeks, with active contribution of the implanted cells. In short, the presented serum-free process represents a biomimetic strategy, resulting in a cartilage tissue intermediate that, upon implantation, robustly leads to the healing of a large long-bone defect.

•

Serum-free pre-conditioning affects the identity of periosteal progenitor cells

•

A reduced CD105⁺, elevated CD34⁺, and upregulated BMP receptor expression was seen

•

Priming by aggregation and BMP stimulation induced endochondral bone formation

•

Validation in a critical size fracture model confirmed endochondral healing

Ex vivo gene therapy for the treatment of neurological disorders

Ex vivo gene therapy involves the genetic modification of cells outside of the body to produce therapeutic factors and their subsequent transplantation back into patients. Various cell types can be genetically engineered. However, with the explosion in stem cell technologies, neural stem/progenitor cells and mesenchymal stem cells are most often used. The synergy between the effect of the new cell and the additional engineered properties can often provide significant benefits to neurodegenerative changes in the brain. In this review, we cover both preclinical animal studies and clinical human trials that have used ex vivo gene therapy to treat neurological disorders with a focus on Parkinson's disease, Huntington's disease, Alzheimer's disease, ALS, and stroke. We highlight some of the major advances in this field including new autologous sources of pluripotent stem cells, safer ways to introduce therapeutic transgenes, and various methods of gene regulation. We also address some of the remaining hurdles including tunable gene regulation, in vivo cell tracking, and rigorous experimental design. Overall, given the current outcomes from researchers and clinical trials, along with exciting new developments in ex vivo gene and cell therapy, we anticipate that successful treatments for neurological diseases will arise in the near future.

Strategies for bringing stem cell-derived dopamine neurons to the clinic: The Kyoto trial

Concerted efforts are realizing cell-based therapy for Parkinson's disease (PD). In this chapter, I describe efforts at the Center for iPS Cell Research and Application (CiRA), Kyoto University. These efforts use induced pluripotent stem cells (iPSCs) as donor cells. The iPSCs were established as human leukocyte antigen homozygous at CiRA and are intended for allogeneic transplantation. Our manufacturing protocol includes a feeder-free cell culture with laminin fragment LM511-E8 and the sorting of CORIN⁺ cells. Animal experiments, including those with monkey PD models, proved that the grafted cells survive and function as dopaminergic neurons in the brain without forming any tumors. Furthermore, I emphasize that not only the donor cells but also the host brain environment is critical for successful transplantation. To achieve optimization of the host environment, drug administration, gene modification, and rehabilitation are recommended. Based on these results, researchers plan to start a clinical trial at Kyoto University Hospital in the near future.

Development of PET Imaging to Visualize Activated Macrophages Accumulated in the Transplanted iPSc-Derived Cardiac Myocytes of Allogeneic Origin for Detecting the Immune Rejection of Allogeneic Cell Transplants in Mice

Allogeneic transplantation (Tx) of induced pluripotent stem cells (iPSCs) is a promising tissue regeneration therapy. However, this inevitably induces macrophage-mediated immune response against the graft, limiting its therapeutic efficacy. Monitoring the magnitude of the immune response using imaging tools would be useful for prolonging graft survival and increasing the therapy longevity. Minimally invasive quantitative detection of activated macrophages by medical imaging technologies such as positron emission tomography (PET) imaging targets translocator protein (TSPO), which is highly expressed on mitochondrial membrane, especially in activated macrophage. N,N-diethyl-2-[4-(2-fluoroethoxy) phenyl]-5,7-dimethylpyrazolo[1,5-a]pyrimidine-3-acetamide (DPA-714) is known as a TSPO ligand used in clinical settings. We herein hypothesized that immune rejection of the transplanted iPSC-derived cardiomyocytes (iPSC-CMs) of allogeneic origin may be quantitated using 18F-DPA-714-PET imaging study. iPSC-CM cell-sheets of C57BL/6 mice origin were transplanted on the surface of the left ventricle (LV) of C57BL/6 mice as a syngeneic cell-transplant model (syngeneic Tx group), or Balb/c mice as an allogeneic model (allogeneic Tx group). 18F-DPA-714-PET was used to determine the uptake ratio, calculated as the maximum standardized uptake value in the anterior and septal wall of the LV. The uptake ratio was significantly higher in the allogeneic Tx group than in the syngeneic group or the sham group at days 7 and day 10 after the cell transplantation. In addition, the immunochemistry showed significant presence of CD68 and CD3-positive cells at day 7 and 10 in the transplanted graft of the allogeneic Tx group. The expression of TSPO, CD68, IL-1 beta, and MCP-1 was significantly higher in the allogeneic Tx group than in the syngeneic Tx and the sham groups at day 7. The 18F-DPA-714-PET imaging study enabled quantitative visualization of the macrophages-mediated immune rejection of the allogeneic iPSC-cardiac. This imaging tool may enable the understanding and monitoring host-immune response of the host, allogeneic cell transplantation therapy.

Pluripotent stem cells: the last 10 years

Pluripotent stem cells (PSCs) can differentiate into virtually any cell type in the body, making them attractive for both regenerative medicine and drug discovery. Over the past 10 years, technological advances and innovative platforms have yielded first-in-man PSC-based clinical trials and opened up new approaches for disease modeling and drug development. Induced PSCs have become the foremost alternative to embryonic stem cells and accelerated the development of disease-in-a-dish models. Over the years and with each new discovery, PSCs have proven to be extremely versatile. This review article highlights key advancements in PSC research, from 2006 to 2016, and how they will guide the direction of the field over the next decade.

Beneath the sword of Damocles: regenerative medicine and the shadow of immunogenicity

Few topics in regenerative medicine have inspired such impassioned debate as the immunogenicity of cell types and tissues differentiated from pluripotent stem cells. While early predictions suggested that tissues derived from allogeneic sources may evade immune surveillance altogether, the pendulum has since swung to the opposite extreme, with reports that the ectopic expression of a few developmental antigens may prompt rejection, even of tissues differentiated from autologous cell lines. Here we review the evidence on which these contradictory claims are based in order to reach an objective assessment of the likely magnitude of the immunological challenges ahead. Furthermore, we discuss how the inherent properties of pluripotent stem cells may inform strategies for reducing the impact of immunogenicity on the future ambitions of regenerative medicine.

Neurotrophin Signaling and Stem Cells-Implications for Neurodegenerative Diseases and Stem Cell Therapy

Neurotrophins (NTs) are members of a neuronal growth factor protein family whose action is mediated by the tropomyosin receptor kinase (TRK) receptor family receptors and the p75 NT receptor (p75NTR), a member of the tumor necrosis factor (TNF) receptor family. Although NTs were first discovered in neurons, recent studies have suggested that NTs and their receptors are expressed in various types of stem cells mediating pivotal signaling events in stem cell biology. The concept of stem cell therapy has already attracted much attention as a potential strategy for the treatment of neurodegenerative diseases (NDs). Strikingly, NTs, proNTs, and their receptors are gaining interest as key regulators of stem cells differentiation, survival, self-renewal, plasticity, and migration. In this review, we elaborate the recent progress in understanding of NTs and their action on various stem cells. First, we provide current knowledge of NTs, proNTs, and their receptor isoforms and signaling pathways. Subsequently, we describe recent advances in the understanding of NT activities in various stem cells and their role in NDs, particularly Alzheimer's disease (AD) and Parkinson's disease (PD). Finally, we compile the implications of NTs and stem cells from a clinical perspective and discuss the challenges with regard to transplantation therapy for treatment of AD and PD.

A Comparative Study of Three Different Types of Stem Cells for Treatment of Rat Spinal Cord Injury

Three different sources of human stem cells-bone marrow-derived mesenchymal stem cells (BM-MSCs), neural progenitors (NPs) derived from immortalized spinal fetal cell line (SPC-01), and induced pluripotent stem cells (iPSCs)-were compared in the treatment of a balloon-induced spinal cord compression lesion in rats. One week after lesioning, the rats received either BM-MSCs (intrathecally) or NPs (SPC-01 cells or iPSC-NPs, both intraspinally), or saline. The rats were assessed for their locomotor skills (BBB, flat beam test, and rotarod). Morphometric analyses of spared white and gray matter, axonal sprouting, and glial scar formation, as well as qPCR and Luminex assay, were conducted to detect endogenous gene expression, while inflammatory cytokine levels were performed to evaluate the host tissue response to stem cell therapy. The highest locomotor recovery was observed in iPSC-NP-grafted animals, which also displayed the highest amount of preserved white and gray matter. Grafted iPSC-NPs and SPC-01 cells significantly increased the number of growth-associated protein 43 (GAP43+) axons, reduced astrogliosis, downregulated Casp3 expression, and increased IL-6 and IL-12 levels. hMSCs transiently decreased levels of inflammatory IL-2 and TNF-α. These findings correlate with the short survival of hMSCs, while NPs survived for 2 months and matured slowly into glia- and tissue-specific neuronal precursors. SPC-01 cells differentiated more in astroglial phenotypes with a dense structure of the implant, whereas iPSC-NPs displayed a more neuronal phenotype with a loose structure of the graft. We concluded that the BBB scores of iPSC-NP- and hMSC-injected rats were superior to the SPC-01-treated group. The iPSC-NP treatment of spinal cord injury (SCI) provided the highest recovery of locomotor function due to robust graft survival and its effect on tissue sparing, reduction of glial scarring, and increased axonal sprouting.

Tyrosine hydroxylase (TH), its cofactor tetrahydrobiopterin (BH4), other catecholamine-related enzymes, and their human genes in relation to the drug and gene therapies of Parkinson's disease (PD): historical overview and future prospects

Tyrosine hydroxylase (TH), which was discovered at the National Institutes of Health (NIH) in 1964, is a tetrahydrobiopterin (BH4)-requiring monooxygenase that catalyzes the first and rate-limiting step in the biosynthesis of catecholamines (CAs), such as dopamine, noradrenaline, and adrenaline. Since deficiencies of dopamine and noradrenaline in the brain stem, caused by neurodegeneration of dopamine and noradrenaline neurons, are mainly related to non-motor and motor symptoms of Parkinson's disease (PD), we have studied human CA-synthesizing enzymes [TH; BH4-related enzymes, especially GTP-cyclohydrolase I (GCH1); aromatic L-amino acid decarboxylase (AADC); dopamine β-hydroxylase (DBH); and phenylethanolamine N-methyltransferase (PNMT)] and their genes in relation to PD in postmortem brains from PD patients, patients with CA-related genetic diseases, mice with genetically engineered CA neurons, and animal models of PD. We purified all human CA-synthesizing enzymes, produced their antibodies for immunohistochemistry and immunoassay, and cloned all human genes, especially the human TH gene and the human gene for GCH1, which synthesizes BH4 as a cofactor of TH. This review discusses the historical overview of TH, BH4-, and other CA-related enzymes and their genes in relation to the pathophysiology of PD, the development of drugs, such as L-DOPA, and future prospects for drug and gene therapy for PD, especially the potential of induced pluripotent stem (iPS) cells.

Nonhuman primate model in clinical modeling of diseases for stem cell therapy

Nonhuman primates (NHPs) are alike humans in size, behavior, physiology, biochemistry, and immunology. Given close similarities to humans, the NHP model offers exceptional opportunities to understand the biological mechanisms and translational applications with direct relevance to human conditions. Here, we evaluate the opportunities and limitations of NHPs as animal models for translational regenerative medicine. NHP models of human disease propose exceptional opportunities to advance stem cell-based therapy by addressing pertinent translational concerns related to this research. Nonetheless, the value of these primates must be carefully assessed, taking into account the expense of specialized equipment and requirement of highly specialized staff. Well-designed initial fundamental studies in small animal models are essential before translating research into NHP models and eventually into human trials. In addition, we suggest that applying a directed and collaborative approach, as seen in the evolution of stroke NHP models, will greatly benefit the translation of stem cell therapy in other NHP disease models.

Advantages of nonhuman primates as preclinical models for evaluating stem cell-based therapies for Parkinson's disease

The derivation of dopaminergic neurons from induced pluripotent stem cells brings new hope for a patient-specific, stem cell-based replacement therapy to treat Parkinson's disease (PD) and related neurodegenerative diseases; and this novel cell-based approach has already proven effective in animal models. However, there are several aspects of this procedure that have yet to be optimized to the extent required for translation to an optimal cell-based transplantation protocol in humans. These challenges include pinpointing the optimal graft location, appropriately scaling up the graft volume, and minimizing the risk of chronic immune rejection, among others. To advance this procedure to the clinic, it is imperative that a model that accurately and fully recapitulates characteristics most pertinent to a cell-based transplantation to the human brain is used to optimize key technical aspects of the procedure. Nonhuman primates mimic humans in multiple ways including similarities in genomics, neuroanatomy, neurophysiology, immunogenetics, and age-related changes in immune function. These characteristics are critical to the establishment of a relevant model in which to conduct preclinical studies to optimize the efficacy and safety of cell-based therapeutic approaches to the treatment of PD. Here we review previous studies in rodent models, and emphasize additional advantages afforded by nonhuman primate models in general, and the baboon model in particular, for preclinical optimization of cell-based therapeutic approaches to the treatment of PD and other neurodegenerative diseases. We outline current unresolved challenges to the successful application of stem cell therapies in humans and propose that the baboon model in particular affords a number of traits that render it most useful for preclinical studies designed to overcome these challenges.

Stem Cell Therapy for the Central Nervous System in Lysosomal Storage Diseases

Neurological diseases with genetic etiologies result in the loss or dysfunction of neural cells throughout the CNS. At present, few treatment options exist for the majority of neurogenetic diseases. Stem cell transplantation (SCT) into the CNS has the potential to be an effective treatment modality because progenitor cells may replace lost cells in the diseased brain, provide multiple trophic factors, or deliver missing proteins. This review focuses on the use of SCT in lysosomal storage diseases (LSDs), a large group of monogenic disorders with prominent CNS disease. In most patients the CNS disease results in intellectual disability that is refractory to current standard-of-care treatment. A large amount of preclinical work on brain-directed SCT has been performed in rodent LSD models. Cell types that have been used for direct delivery into the CNS include neural stem cells, embryonic and induced pluripotent stem cells, and mesenchymal stem cells. Hematopoietic stem cells have been an effective therapy for the CNS in a few LSDs and may be augmented by overexpression of the missing gene. Current barriers and potential strategies to improve SCT for translation into effective patient therapies are discussed.

Comparative genomics of the human, macaque and mouse major histocompatibility complex

The MHC is a highly polymorphic genomic region that encodes the transplantation and immune regulatory molecules. It receives special attention for genetic investigation because of its important role in the regulation of innate and adaptive immune responses and its strong association with numerous infectious and/or autoimmune diseases. The MHC locus was first discovered in the mouse and for the past 50 years it has been studied most intensively in both mice and humans. However, in recent years the macaque species have emerged as some of the more important and advanced experimental animal models for biomedical research into MHC with important human immunodeficiency virus/simian immunodeficiency virus and transplantation studies undertaken in association with precise MHC genotyping and haplotyping methods using Sanger sequencing and next-generation sequencing. Here, in this special issue on 'Macaque Immunology' we provide a short review of the genomic similarities and differences among the human, macaque and mouse MHC class I and class II regions, with an emphasis on the association of the macaque class I region with MHC polymorphism, haplotype structure and function.

10th anniversary of iPS cells: the challenges that lie ahead

In 2006, induced pluripotent stem (iPS) cells were generated by Yamanaka and Takahashi for the first time from a mouse fibroblast culture by introducing four factors. In the 10 years since then, this breakthrough discovery has been making waves in the fields of biology and medical science. For example, various technologies for generating iPS cells have been developed, and we have cultivated a better understanding of the mechanisms involved in reprogramming. In addition, many researchers have explored the applications of iPS cells, such as drug discovery, the study of disease mechanisms and regenerative medicine, and the development of advanced technologies for the differentiation and qualification of the cells. Furthermore, the concept of iPS cell generation has inspired a number of studies that do not use iPS cells. We herein review and discuss the past, present and future of iPS cells and their related issues.

Differentiation and Characterization of Dopaminergic Neurons From Baboon Induced Pluripotent Stem Cells

: The progressive death of dopamine producing neurons in the substantia nigra pars compacta is the principal cause of symptoms of Parkinson's disease (PD). Stem cells have potential therapeutic use in replacing these cells and restoring function. To facilitate development of this approach, we sought to establish a preclinical model based on a large nonhuman primate for testing the efficacy and safety of stem cell-based transplantation. To this end, we differentiated baboon fibroblast-derived induced pluripotent stem cells (biPSCs) into dopaminergic neurons with the application of specific morphogens and growth factors. We confirmed that biPSC-derived dopaminergic neurons resemble those found in the human midbrain based on cell type-specific expression of dopamine markers TH and GIRK2. Using the reverse transcriptase quantitative polymerase chain reaction, we also showed that biPSC-derived dopaminergic neurons express PAX6, FOXA2, LMX1A, NURR1, and TH genes characteristic of this cell type in vivo. We used perforated patch-clamp electrophysiology to demonstrate that biPSC-derived dopaminergic neurons fired spontaneous rhythmic action potentials and high-frequency action potentials with spike frequency adaption upon injection of depolarizing current. Finally, we showed that biPSC-derived neurons released catecholamines in response to electrical stimulation. These results demonstrate the utility of the baboon model for testing and optimizing the efficacy and safety of stem cell-based therapeutic approaches for the treatment of PD.

SIGNIFICANCE

Functional dopamine neurons were produced from baboon induced pluripotent stem cells, and their properties were compared to baboon midbrain cells in vivo. The baboon has advantages as a clinically relevant model in which to optimize the efficacy and safety of stem cell-based therapies for neurodegenerative diseases, such as Parkinson's disease. Baboons possess crucial neuroanatomical and immunological similarities to humans, and baboon pluripotent stem cells can be differentiated into functional neurons that mimic those in the human brain, thus laying the foundation for the utility of the baboon model for evaluating stem cell therapies.

Recent policies that support clinical application of induced pluripotent stem cell-based regenerative therapies

In Japan, a research center network consisting of Kyoto University to provide clinical-grade induced Pluripotent Stem Cells (iPSC) and several major research centers to develop iPSC-based regenerative therapies was formed for the clinical application of iPSCs. This network is under the supervision of a newly formed funding agency, the Japan Agency for Medical Research and Development. In parallel, regulatory authorities of Japan, including the Ministry of Health, Labour and Welfare, and Pharmaceuticals and Medical Devices Agency, are trying to accelerate the development process of regenerative medicine products (RMPs) by several initiatives: 1) introduction of a conditional and time-limited approval scheme only applicable to RMPs under the revised Pharmaceuticals and Medical Devices Act, 2) expansion of a consultation program at the early stage of development, 3) establishment of guidelines to support efficient development and review and 4) enhancement of post-market safety measures such as introduction of patient registries and setting user requirements with cooperation from relevant academic societies and experts. Ultimately, the establishment of a global network among iPSC banks that derives clinical-grade iPSCs from human leukocyte antigens homozygous donors has been proposed. In order to share clinical-grade iPSCs globally and to facilitate global development of iPSC-based RMPs, it will be necessary to promote regulatory harmonization and to establish common standards related to iPSCs and differentiated cells based on scientific evidence.

Tracking cells implanted into cynomolgus monkeys (Macaca fascicularis) using MRI

Regenerative therapy with stem cell transplantation is used to treat various diseases such as coronary syndrome and Buerger’s disease. For instance, stem-cell transplantation into the infarcted myocardium is an innovative and promising strategy for treating heart failure due to ischemic heart disease. Basic studies using small animals have shown that transplanted cells improve blood flow in the infarcted region. Magnetic resonance imaging (MRI) can noninvasively identify and track transplanted cells labeled with superparamagnetic iron oxide (SPIO). Although clinical regenerative therapies have been clinically applied to patients, the fate of implanted cells remains unknown. In addition, follow-up studies have shown that some adverse events can occur after recovery. Therefore, the present study evaluated the ability of MRI using a 3T scanner to track implanted peripheral blood mononuclear cells labeled with SPIO on days 0 and 7 after intramuscular (i.m.) and intravenous (i.v.) injection into a cynomolgus monkey. Labeled cells were visualized at the liver and triceps surae muscle on MR images using T1- and T2-weighted sequences and histologically localized by Prussian blue staining. The transplanted cells were tracked without abnormal clinical manifestations throughout this study. Hence, MRI of cynomolgus monkey transplanted SPIO-labeled cells is a safe and efficient method of tracking labeled cells that could help to determine the mechanisms involved in regenerative therapy.

Estradiol Facilitates Functional Integration of iPSC-Derived Dopaminergic Neurons into Striatal Neuronal Circuits via Activation of Integrin α5β1

For cell transplantation therapy for Parkinson's disease (PD) to be realized, the grafted neurons should be integrated into the host neuronal circuit to restore the lost neuronal function. Here, using wheat-germ agglutinin-based transsynaptic tracing, we show that integrin α5 is selectively expressed in striatal neurons that are innervated by midbrain dopaminergic (DA) neurons. In addition, we found that integrin α5β1 was activated by the administration of estradiol-2-benzoate (E2B) in striatal neurons of adult female rats. Importantly, we observed that the systemic administration of E2B into hemi-parkinsonian rat models facilitates the functional integration of grafted DA neurons derived from human induced pluripotent stem cells into the host striatal neuronal circuit via the activation of integrin α5β1. Finally, methamphetamine-induced abnormal rotation was recovered earlier in E2B-administered rats than in rats that received other regimens. Our results suggest that the simultaneous administration of E2B with stem cell-derived DA progenitors can enhance the efficacy of cell transplantation therapy for PD.

•

Integrin α5 is expressed in striatal neurons innervated by nigral DA neurons

•

Administration of E2B activates integrin α5β1 in the rat striatum

•

E2B facilitates integration of grafted iPSC-derived DA neurons into host striatum

Cell Therapy for Parkinson's Disease

In Parkinson’s disease (PD), dopamine neurons in the substantia nigra are degenerated and lost. Cell therapy for PD replaces the lost dopamine neurons by transplanting donor dopamine neural progenitor cells. Cell therapy for PD has been performed in the clinic since the 1980s and uses donor cells from the mesencephalon of aborted embryos. Regenerative medicine for PD using induced pluripotent stem (iPS) cell technology is drawing attention, because it offers a limitless and more advantageous source of donor cells than aborted embryos.

Tissue Engineering and Regenerative Medicine 2015: A Year in Review

This may be the most exciting time ever for the field of tissue engineering and regenerative medicine (TERM). After decades of progress, it has matured, integrated, and diversified into entirely new areas, and it is starting to make the pivotal shift toward translation. The most exciting science and applications continue to emerge at the boundaries of disciplines, through increasingly effective interactions between stem cell biologists, bioengineers, clinicians, and the commercial sector. In this "Year in Review," we highlight some of the major advances reported over the last year (Summer 2014-Fall 2015). Using a methodology similar to that established in previous years, we identified four areas that generated major progress in the field: (i) pluripotent stem cells, (ii) microtissue platforms for drug testing and disease modeling, (iii) tissue models of cancer, and (iv) whole organ engineering. For each area, we used some of the most impactful articles to illustrate the important concepts and results that advanced the state of the art of TERM. We conclude with reflections on emerging areas and perspectives for future development in the field.

Cardiomyocytes Derived from MHC-Homozygous Induced Pluripotent Stem Cells Exhibit Reduced Allogeneic Immunogenicity in MHC-Matched Non-human Primates

Induced pluripotent stem cells (iPSCs) can serve as a source of cardiomyocytes (CMs) to treat end-stage heart failure; however, transplantation of genetically dissimilar iPSCs even within species (allogeneic) can induce immune rejection. We hypothesized that this might be limited by matching the major histocompatibility complex (MHC) antigens between the donor and the recipient. We therefore transplanted fluorescence-labeled (GFP) iPSC-CMs donated from a macaque with homozygous MHC haplotypes into the subcutaneous tissue and hearts of macaques having heterozygous MHC haplotypes (MHC-matched; group I) or without identical MHC alleles (group II) in conjunction with immune suppression. Group I displayed a higher GFP intensity and less immune-cell infiltration in the graft than group II. However, MHC-matched transplantation with single or no immune-suppressive drugs still induced a substantial host immune response to the graft. Thus, the immunogenicity of allogeneic iPSC-CMs was reduced by MHC-matched transplantation although a requirement for appropriate immune suppression was retained for successful engraftment.

•

Cardiomyocytes from iPSCs can treat heart disease

•

iPSC-CMs were transplanted into MHC-matched or unmatched cynomolgus macacques

•

Matched iPSC-CM grafts had better survival and less host rejection immune response

•

Immunosuppression was still required for successful allogeneic iPSC-CM engraftment

Modulation of human allogeneic and syngeneic pluripotent stem cells and immunological implications for transplantation

Tissues derived from induced pluripotent stem cells (iPSCs) are a promising source of cells for building various regenerative medicine therapies; from simply transplanting cells to reseeding decellularized organs to reconstructing multicellular tissues. Although reprogramming strategies for producing iPSCs have improved, the clinical use of iPSCs is limited by the presence of unique human leukocyte antigen (HLA) genes, the main immunologic barrier to transplantation. In order to overcome the immunological hurdles associated with allogeneic tissues and organs, the generation of patient-histocompatible iPSCs (autologous or HLA-matched cells) provides an attractive platform for personalized medicine. However, concerns have been raised as to the fitness, safety and immunogenicity of iPSC derivatives because of variable differentiation potential of different lines and the identification of genetic and epigenetic aberrations that can occur during the reprogramming process. In addition, significant cost and regulatory barriers may deter commercialization of patient specific therapies in the short-term. Nonetheless, recent studies provide some evidence of immunological benefit for using autologous iPSCs. Yet, more studies are needed to evaluate the immunogenicity of various autologous and allogeneic human iPSC-derived cell types as well as test various methods to abrogate rejection. Here, we present perspectives of using allogeneic vs. autologous iPSCs for transplantation therapies and the advantages and disadvantages of each related to differentiation potential, immunogenicity, genetic stability and tumorigenicity. We also review the current literature on the immunogenicity of syngeneic iPSCs and discuss evidence that questions the feasibility of HLA-matched iPSC banks. Finally, we will discuss emerging methods of abrogating or reducing host immune responses to PSC derivatives.

5-HT2 receptors mediate functional modulation of GABAa receptors and inhibitory synaptic transmissions in human iPS-derived neurons

Neural progenitors differentiated from induced pluripotent stem cells (iPS) hold potentials for treating neurological diseases. Serotonin has potent effects on neuronal functions through multiple receptors, underlying a variety of neural disorders. Glutamate and GABA receptors have been proven functional in neurons differentiated from iPS, however, little is known about 5-HT receptor-mediated modulation in such neuronal networks. In the present study, human iPS were differentiated into cells possessing featured physiological properties of cortical neurons. Whole-cell patch-clamp recording was used to examine the involvement of 5-HT₂ receptors in functional modulation of GABAergic synaptic transmission. We found that serotonin and DOI (a selective agonist of 5-HT_2A/C receptor) reversibly reduced GABA-activated currents, and this 5-HT_2A/C receptor mediated inhibition required G protein, PLC, PKC, and Ca²⁺ signaling. Serotonin increased the frequency of miniature inhibitory postsynaptic currents (mIPSCs), which could be mimicked by α-methylserotonin, a 5-HT₂ receptor agonist. In contrast, DOI reduced both frequency and amplitude of mIPSCs. These findings suggested that in iPS-derived human neurons serotonin postsynaptically reduced GABAa receptor function through 5-HT_2A/C receptors, but presynaptically other 5-HT₂ receptors counteracted the action of 5-HT_2A/C receptors. Functional expression of serotonin receptors in human iPS-derived neurons provides a pre-requisite for their normal behaviors after grafting.

Comparison of Human Primary with Human iPS Cell-Derived Dopaminergic Neuron Grafts in the Rat Model for Parkinson's Disease

Neuronal degeneration within the substantia nigra and the loss of the dopaminergic nigro-striatal pathway are the major hallmarks of Parkinson's disease (PD). Grafts of foetal ventral mesencephalic (VM) dopaminergic (DA) neurons into the striatum have been shown to be able to restore striatal dopamine levels and to improve overall PD symptoms. However, human foetus-derived cell grafts are not feasible for clinical application. Autologous induced pluripotent stem cell (iPS cell)-derived DA neurons are emerging as an unprecedented alternative. In this review, we summarize and compare the efficacy of human iPS cell-derived DA neuron grafts to restore normal behaviour in a rat model for PD with that of human foetal primary DA neurons. The differences we observed in the efficacy to restore normal function between the 2 types of DA neuron grafts could be ascribed to intrinsic properties of the iPS cell-derived DA neurons that critically affected survival and proper neurite extension in the striatum after implantation.

Translational research on influenza virus infection using a nonhuman primate model

Influenza virus infection is a seasonal infectious disease for humans, whereas it is also a zoonosis that is originally transmitted from animals to humans. Therefore, several animal models are used in research on influenza virus infection. We have used a nonhuman primate (NHP) model to extrapolate pathogenicity of various influenza viruses and efficacy of vaccines and antiviral drugs against the influenza viruses in humans. NHPs have genes, anatomical structure, and immune responses similar to those of humans as compared to other animal models. Using an NHP model, we revealed that the pandemic 2009 influenza A virus caused viral pneumonia as reported in human patients. Thus, it is thought that NHP models can be used to predict replication of emerging viruses in humans. We also examined the pathogenicity of highly pathogenic avian influenza viruses and evaluated a new therapeutic antibody in macaques under an immunocompromised condition. NHP models have provided promising results in research on other infectious diseases including Ebola virus and human/simian immunodeficiency virus infections. Thus, NHPs are important in biomedical research for determining the pathogenesis and for development of treatments, especially when clinical trials are difficult. We summarize the characteristics and advantages of research using NHP models in this review.

An Overview of Direct Somatic Reprogramming: The Ins and Outs of iPSCs

Stem cells are classified into embryonic stem cells and adult stem cells. An evolving alternative to conventional stem cell therapies is induced pluripotent stem cells (iPSCs), which have a multi-lineage potential comparable to conventionally acquired embryonic stem cells with the additional benefits of being less immunoreactive and avoiding many of the ethical concerns raised with the use of embryonic material. The ability to generate iPSCs from somatic cells provides tremendous promise for regenerative medicine. The breakthrough of iPSCs has raised the possibility that patient-specific iPSCs can provide autologous cells for cell therapy without the concern for immune rejection. iPSCs are also relevant tools for modeling human diseases and drugs screening. However, there are still several hurdles to overcome before iPSCs can be used for translational purposes. Here, we review the recent advances in somatic reprogramming and the challenges that must be overcome to move this strategy closer to clinical application.

Mitochondrial resetting and metabolic reprogramming in induced pluripotent stem cells and mitochondrial disease modeling

BACKGROUND

Nuclear reprogramming with pluripotency factors enables somatic cells to gain the properties of embryonic stem cells. Mitochondrial resetting and metabolic reprogramming are suggested to be key early events in the induction of human skin fibroblasts to induced pluripotent stem cells (iPSCs).

SCOPE OF REVIEW

We review recent advances in the study of the molecular basis for mitochondrial resetting and metabolic reprogramming in the regulation of the formation of iPSCs. In particular, the recent progress in using iPSCs for mitochondrial disease modeling was discussed.

MAJOR CONCLUSIONS

iPSCs rely on glycolysis rather than oxidative phosphorylation as a major supply of energy. Mitochondrial resetting and metabolic reprogramming thus play crucial roles in the process of generation of iPSCs from somatic cells.

GENERAL SIGNIFICANCE

Neurons, myocytes, and cardiomyocytes are cells containing abundant mitochondria in the human body, which can be differentiated from iPSCs or trans-differentiated from fibroblasts. Generating these cells from iPSCs derived from skin fibroblasts of patients with mitochondrial diseases or by trans-differentiation with cell-specific transcription factors will provide valuable insights into the role of mitochondrial DNA heteroplasmy in mitochondrial disease modeling and serves as a novel platform for screening of drugs to treat patients with mitochondrial diseases.

Understanding Stem Cell Immunogenicity in Therapeutic Applications

Stem cells and their differentiated progeny offer great hope for treating disease by providing an unlimited source of cells for repairing or replacing damaged tissue. Initial studies suggested that, unlike 'normal' transplants, specific characteristics of stem cells enabled them to avoid immune attack. However, recent findings have revealed that the immunogenicity of stem cells may have been underestimated. Here, we review the current understanding of the mechanisms of immune recognition associated with stem cell immunogenicity, and discuss the relevance of reprogramming and differentiation strategies used to generate cells or tissue from stem cells for implantation in eliciting an immune response. We examine the effectiveness of current strategies for minimising immune attack in light of our experience in the transplantation field and, in this context, outline important challenges moving forward.

Banking on iPSC--is it doable and is it worthwhile

The discovery of induced pluripotent stem cells (iPSCs) and concurrent development of protocols for their cell-type specific differentiation have revolutionized studies of diseases and raised the possibility that personalized medicine may be achievable. Realizing the full potential of iPSC will require addressing the challenges inherent in obtaining appropriate cells for millions of individuals while meeting the regulatory requirements of delivering therapy and keeping costs affordable. Critical to making PSC based cell therapy widely accessible is determining which mode of cell collection, storage and distribution, will work. In this manuscript we suggest that moderate sized bank where a diverse set of lines carrying different combinations of commonly present HLA alleles are banked and differentiated cells are made available to matched recipients as need dictates may be a solution. We discuss the issues related to developing such a bank and how it could be constructed and propose a bank of selected HLA phenotypes from carefully screened healthy individuals as a solution to delivering personalized medicine.

Aberrant epigenome in iPSC-derived dopaminergic neurons from Parkinson's disease patients

The epigenomic landscape of Parkinson's disease (PD) remains unknown. We performed a genomewide DNA methylation and a transcriptome studies in induced pluripotent stem cell (iPSC)-derived dopaminergic neurons (DAn) generated by cell reprogramming of somatic skin cells from patients with monogenic LRRK2-associated PD (L2PD) or sporadic PD (sPD), and healthy subjects. We observed extensive DNA methylation changes in PD DAn, and of RNA expression, which were common in L2PD and sPD. No significant methylation differences were present in parental skin cells, undifferentiated iPSCs nor iPSC-derived neural cultures not-enriched-in-DAn. These findings suggest the presence of molecular defects in PD somatic cells which manifest only upon differentiation into the DAn cells targeted in PD. The methylation profile from PD DAn, but not from controls, resembled that of neural cultures not-enriched-in-DAn indicating a failure to fully acquire the epigenetic identity own to healthy DAn in PD. The PD-associated hypermethylation was prominent in gene regulatory regions such as enhancers and was related to the RNA and/or protein downregulation of a network of transcription factors relevant to PD (FOXA1, NR3C1, HNF4A, and FOSL2). Using a patient-specific iPSC-based DAn model, our study provides the first evidence that epigenetic deregulation is associated with monogenic and sporadic PD.

Prospects and Challenges of Induced Pluripotent Stem Cells in Equine Health

Pluripotent stem cells (PSCs) hold, through the capacity to differentiate into virtually all body cell types, unprecedented promise for human and animal medicine. PSCs are naturally found in the early embryo, and in rodents and humans they can be robustly harvested and grown in culture in the form of embryonic stem cells (ESCs); however, the availability of ESCs from horses is limited. ES-like cells named induced pluripotent stem cells (iPSCs) can be derived in vitro by transcription factor-mediated reprogramming of adult cells. As such, iPSCs can be generated in a patient-specific manner providing unmatched potential for tissue transplantation and in vitro disease modeling. In humans, clinical trials using iPSC-derived cells are already taking place and the use of in vitro iPSC models has identified novel mechanisms of disease and therapeutic targets. Although to a more limited extent, iPSCs have also been generated from horses, a species in which, after humans, these cells are likely to hold the greatest potential in regenerative medicine. Before a clinical use can be envisioned, however, significant challenges will need to be addressed in relation to the robust derivation, long-term culture, differentiation, and clinical safety of equine iPSCs. Toward this objective, recent studies have reported significant improvement in culture conditions and the successful derivation for the first time of functional cell types from equine iPSCs. Given the wide range of exciting applications they could have, it is hoped future research will make the biomedical promise of iPSCs a reality not only for humans but also horses.