The progress of induced pluripotent stem cells derived from pigs: a mini review of recent advances

Pigs (Sus scrofa) are widely acknowledged as an important large mammalian animal model due to their similarity to human physiology, genetics, and immunology. Leveraging the full potential of this model presents significant opportunities for major advancements in the fields of comparative biology, disease modeling, and regenerative medicine. Thus, the derivation of pluripotent stem cells from this species can offer new tools for disease modeling and serve as a stepping stone to test future autologous or allogeneic cell-based therapies. Over the past few decades, great progress has been made in establishing porcine pluripotent stem cells (pPSCs), including embryonic stem cells (pESCs) derived from pre- and peri-implantation embryos, and porcine induced pluripotent stem cells (piPSCs) using a variety of cellular reprogramming strategies. However, the stabilization of pPSCs was not as straightforward as directly applying the culture conditions developed and optimized for murine or primate PSCs. Therefore, it has historically been challenging to establish stable pPSC lines that could pass stringent pluripotency tests. Here, we review recent advances in the establishment of stable porcine PSCs. We focus on the evolving derivation methods that eventually led to the establishment of pESCs and transgene-free piPSCs, as well as current challenges and opportunities in this rapidly advancing field.

Direct reprogramming of epithelial cell rests of malassez into mesenchymal-like cells by epigenetic agents

The DNA demethylating agent, 5-Azacytidine (5Aza), and histone deacetylase inhibitor, valproic acid (Vpa), can improve the reprogramming efficiencies of pluripotent cells. This study aimed to examine the roles of 5Aza and Vpa in the dedifferentiation of epithelial cell rests of Malassez (ERM) into stem-like cells. Additionally, the ability of stem-like cells to differentiate into mesenchymal cells was evaluated. ERM was cultured in embryonic stem cell medium (ESCM) with 1 µM of 5Aza, or 2 mM of Vpa, or a combination of 5Aza and Vpa. The cells stimulated with both 5Aza and Vpa were named as progenitor-dedifferentiated into stem-like cells (Pro-DSLCs). The Pro-DSLCs cultured in ESCM alone for another week were named as DSLCs. The stem cell markers were significantly higher in the DSLCs than the controls (no additions). The mRNA and protein levels of the endothelial, mesenchymal stem, and osteogenic cell markers were significantly higher in the Pro-DSLCs and DSLCs than the controls. The combination of a demethylating agent and a deacetylated inhibitor induced the dedifferentiation of ERM into DSLCs. The Pro-DSLCs derived from ERM can be directly reprogrammed into mesenchymal-like cells without dedifferentiation into stem-like cells. Isolated ERM treated with epigenetic agents may be used for periodontal regeneration.

Derivation of endothelial cells from porcine induced pluripotent stem cells by optimized single layer culture system

Regenerative therapy holds great promise in the development of cures of some untreatable diseases such as cardiovascular diseases, and pluripotent stem cells (PSCs) including induced PSCs (iPSCs) are the most important regenerative seed cells. Recently, differentiation of human PSCs into functional tissues and cells in vitro has been widely reported. However, although porcine reports are rare they are quite essential, as the pig is an important animal model for the in vitro generation of human organs. In this study, we reprogramed porcine embryonic fibroblasts into porcine iPSCs (piPSCs), and differentiated them into cluster of differentiation 31 (CD31)-positive endothelial cells (ECs) (piPSC-derived ECs, piPS-ECs) using an optimized single-layer culture method. During differentiation, we observed that a combination of GSK3β inhibitor (CHIR99021) and bone morphogenetic protein 4 (BMP4) promoted mesodermal differentiation, resulting in higher proportions of CD31-positive cells than those from separate CHIR99021 or BMP4 treatment. Importantly, the piPS-ECs showed comparable morphological and functional properties to immortalized porcine aortic ECs, which are capable of taking up low-density lipoprotein and forming network structures on Matrigel. Our study, which is the first trial on a species other than human and mouse, has provided an optimized single-layer culture method for obtaining ECs from porcine PSCs. Our approach can be beneficial when evaluating autologous EC transplantation in pig models.

Large Animal Models for the Clinical Application of Human Induced Pluripotent Stem Cells

Induced pluripotent stem cell (iPSC) technology offers a practically infinite and ethically acceptable source to obtain a variety of somatic cells. Coupled with the biotechnologies of cell therapy or tissue engineering, iPSC technology will enormously contribute to human regenerative medicine. Before clinical application, such human iPSC (hiPSC)-based therapies should be assessed using large animal models that more closely match biological or biomechanical properties of human patients. Therefore, it is critical to generate large animal iPSCs, obtain their iPSC-derived somatic cells, and preclinically evaluate their therapeutic efficacy and safety in large animals. During the past decade, the establishment of iPSC lines of a series of large animal species has been documented, and the acquisition and preclinical evaluation of iPSC-derived somatic cells has also been reported. Despite this progress, significant obstacles, such as obtaining or preserving the bona fide pluripotency of large animal iPSCs, have been encountered. Simultaneously, studies of large animal iPSCs have been overlooked in comparison with those of mouse and hiPSCs, and this field deserves more attention and support due to its important preclinical relevance. Herein, this review will focus on the large animal models of pigs, dogs, horses, and sheep/goats, and summarize current progress, challenges, and potential future directions of research on large animal iPSCs.

Generation of porcine induced-pluripotent stem cells from Sertoli cells

Induced pluripotent stem cells (iPSCs) are generated by reprogramming of somatic cells using four transcription factors: OCT4, SOX2, KLF-4, and c-MYC (OSKM). However, reprogramming efficiency of iPSCs is currently poor. In this study, we used the Sertoli line as a novel cell source for somatic cell reprogramming. Neonatal testes were collected from 1-week-old piglets. The testes were digested by a two-step enzymatic method to isolate Sertoli cells. The latter were transfected with retroviral vectors expressing OSKM. The Sertoli iPSC-like colonies were subjected to morphological analysis, alkaline phosphatase staining, RT-PCR, G-banding karyotyping, in vitro differentiation, and in vivo differentiation. Primary Sertoli cells had polygon-shaped morphology and manifested phagocytic activity as determined by a fluorescent bead assay. Sertoli cells also expressed the anti-Müllerian hormone protein in the cytoplasm. According to RT-PCR results, these cells expressed Sertoli cell markers (FSHR, KRT18, and GATA6) and endogenous transcription factors genes (KLF4 and c-MYC). A total of 240 colonies (0.3% efficiency) were detected by day 7 after viral transduction of 72500 cells. The Sertoli iPSC-like colonies contained small cells with a high nucleus-to-cytoplasm ratio. These colonies tested positive for alkaline phosphatase staining, expressed endogenous pluripotency genes, and had a normal karyotype. All these cell lines could form in vitro three-dimensional aggregates that represented three germ layers of embryonic-like cells. A total of two cell lines used for in vivo differentiation produced high-efficiency teratoma. In conclusion, Sertoli cells can efficiently serve as a novel cell source for iPSC reprogramming.

Culture of iPSCs Derived Pancreatic β-Like Cells In Vitro Using Decellularized Pancreatic Scaffolds: A Preliminary Trial

Diabetes mellitus is a disease which has affected 415 million patients in 2015. In an effort to replace the significant demands on transplantation and morbidity associated with transplantation, the production of β-like cells differentiated from induced pluripotent stem cells (iPSCs) was evaluated. This approach is associated with promising decellularized scaffolds with natural extracellular matrix (ECM) and ideal cubic environment that will promote cell growth in vivo. Our efforts focused on combining decellularized rat pancreatic scaffolds with mouse GFP⁺-iPSCs-derived pancreatic β-like cells, to evaluate whether decellularized scaffolds could facilitate the growth and function of β-like cells. β-like cells were differentiated from GFP⁺-iPSCs and evaluated via cultivating in the dynamic circulation perfusion device. Our results demonstrated that decellularized pancreatic scaffolds display favorable biochemical properties. Furthermore, not only could the scaffolds support the survival of β-like cells, but they also accelerated the expression of the insulin as compared to plate-based cell culture. In conclusion, these results suggest that decellularized pancreatic scaffolds could provide a suitable platform for cellular activities of β-like cells including survival and insulin secretion. This study provides preliminary support for regenerating insulin-secreting organs from the decellularized scaffolds combined with iPSCs derived β-like cells as a potential clinical application.

Livestock models for exploiting the promise of pluripotent stem cells

Livestock species are widely used as biomedical models. Pigs, in particular, are beginning to have a significant role in regenerative medicine for testing the applicability, success, and safety of grafts derived from induced pluripotent stem cells. Animal testing must always be performed before any clinical trials are performed in humans, and pigs may sometimes be the species of choice because of their physiological and anatomical similarities to humans. Induced pluripotent stem cells (iPSC) have been generated with some success from livestock species by a variety of reprogramming procedures, but authenticated embryonic stem cells (ESC) have not. There are now several studies in which porcine iPSC have been tested for their ability to provide functional grafts in pigs. Pigs have also served as recipients for grafts derived from human iPSC. There have also been recent advances in creating pigs with severe combined immunodeficiency (SCID). Like SCID mice, these pigs are expected to be graft tolerant. Additionally, chimeric, partially humanized pigs could be sources of human organs. Another potential application of pluripotent stem cells from livestock is for the purpose of differentiating the cells into skeletal muscle, which, in turn, could be used either to produce cultured meat or to engraft into damaged muscle. None of these technologies has advanced to a stage that they have become mainstream, however. Despite the value of livestock models in regenerative medicine, only a limited number of institutions are able to use these animals.

Pluripotent Stem Cells from Domesticated Mammals

This review deals with the latest advances in the study of embryonic stem cells (ESC) and induced pluripotent stem cells (iPSC) from domesticated species, with a focus on pigs, cattle, sheep, goats, horses, cats, and dogs. Whereas the derivation of fully pluripotent ESC from these species has proved slow, reprogramming of somatic cells to iPSC has been more straightforward. However, most of these iPSC depend on the continued expression of the introduced transgenes, a major drawback to their utility. The persistent failure in generating ESC and the dependency of iPSC on ectopic genes probably stem from an inability to maintain the stability of the endogenous gene networks necessary to maintain pluripotency. Based on work in humans and rodents, achievement of full pluripotency will likely require fine adjustments in the growth factors and signaling inhibitors provided to the cells. Finally, we discuss the future utility of these cells for biomedical and agricultural purposes.