Stem Cells as In Vitro Models of Disease

An in vitro heart disease model is a promising model used for identifying the genes responsible for the disease, evaluating the e ﬀ ects of drugs, and regenerative medicine. We were interested in disease models using a patient-induced pluripotent stem (iPS) cell-derived cardiomyocytes because of their similarity to a patient’s tissues. However, as these studies have just begun, we would like to review the literature in this and other related ﬁelds and discuss the path for future models of molecular biology that can help to diagnose and cure diseases, and its involvement in regenerative medicine. The heterogeneity of iPS cells and/or di ﬀ erentiated cardiomyocytes has been recognized as a problem. An in vitro heart disease model should be evaluated using molecular biological analyses, such as mRNA and micro-RNA expression proﬁles and proteomic analysis. Recent studies suggest that extrafollicular dermal melanocyte stem cells (MSCs) persist after birth in the superﬁcial nerve sheath of peripheral nerves and give rise to migratory melanocyte precursors when replacements for epidermal melanocytes are needed on the basal epidermal layer of the skin. If a damaged MSC or melanocyte precursor can be shown to be the primary origin of melanoma, targeted identiﬁcation and eradication of it by antibody-based therapies will be the best method to treat melanoma and a very e ﬀ ective way to prevent its recurrence. Transcription factors and signaling pathways involved in MSC self-renewal, expansion and di ﬀ erentiation are reviewed. A model is presented to show how the detrimental e ﬀ ects of long-term UVA/UVB radiation on DNA and repair mechanisms in MSCs convert them to melanoma stem cells. Zebraﬁsh have many advantages for investigating the role of MSCs in the development of melanoma. The signaling pathways regulating the development of MSCs in zebraﬁsh are very similar to those found in humans and mice. The ability to easily manipulate the MSC population makes zebraﬁsh an excellent model for studying how damage to MSCs may lead to melanoma. Progress in understanding neurodegenerative cell biology in Parkinson’s disease (PD) has been hampered by a lack of predictive and relevant cellular models. In addition, the lack of an adequate in vitro human neuron cell-based model has been an obstacle for the uncover of new drugs for treating PD. The ability to generate induced pluripotent stem cells (iPSCs) from PD patients and a reﬁned capacity to di ﬀ erentiate these iPSCs into DA neurons, the relevant disease cell type, promises a new paradigm in drug development that positions human disease pathophysiology at the core of preclinical drug discovery. Disease models derived from iPSC that manifest cellular disease phenotypes have been established for several monogenic diseases, but iPSC can likewise be used for phenotype-based drug screens in complex diseases for which the underlying genetic mechanism is unknown. Here, we highlight recent advances as well as limitations in the use of iPSC technology for modelling PD “in a dish” and for testing compounds against human disease phenotypes in vitro . We discuss how iPSCs are being exploited to illuminate disease pathophysiology, identify novel drug targets, and enhance the probability of clinical success of new drugs. The most characteristic feature of neuroﬁbromatosis type 1 (NF1) is the development of neuroﬁbromas. It has been suggested that these tumors are caused by somatic inactivation of the wild-type NF1 allele, but the cell that originally su ﬀ ers this mutation remains controversial. Several lines of evidence support the clonal origin of these tumors, and it has been recently suggested that skin-derived precursor cells (SKPs) could be the cell of origin of dermal neuroﬁbromas. Nullizygous ( NF1 − / − ) SKPs do give rise to neuroﬁbromas when transplanted to heterozygous mice. Moreover, a nullizygous population of cells that is S100 β negative is present in human neuroﬁbromas, and NF1 + / − multipotent progenitor cells are seemingly recruited to the tumor. This evidence supports the neuroﬁbroma stem cell hypothesis and a putative involvement of SKPs in the aetiopathogenesis of the disease, suggesting that SKPs could become a valuable tool for the in vitro study α -synuclein is a protein involved in the pathogenesis of several so-called synucleinopathies including Parkinson’s disease. A variety of models have been so far assessed. Human induced pluripotent stem cells provide a patient- and disease-speciﬁc model for in vitro studies, pharmacotoxicological screens, and hope for future cell-based therapies. Initial experimental procedures include the harvest of patients’ material for the reprogramming process, the investigation of the patients genetic background in the cultured cells, and the evaluation of disease-relevant factors/proteins under various cell culture conditions. Endothelial progenitor cells (EPCs) have great clinical value because they can be used as diagnostic biomarkers and as a cellular therapy for promoting vascular repair of ischaemic tissues. However, EPCs also have an additional research value in vascular disease modelling to interrogate human disease mechanisms. The term EPC is used to describe a diverse variety of cells, and we have identiﬁed a speciﬁc EPC subtype called outgrowth endothelial cell (OEC) as the best candidate for vascular disease modelling because of its high-proliferative potential and unambiguous endothelial commitment. OECs are isolated from human blood and can be exposed to pathologic conditions (forward approach) or be isolated from patients (reverse approach) in order to study vascular human disease. The use of OECs for modelling vascular disease will contribute greatly to improving our understanding of endothelial pathogenesis, which will potentially lead to the discovery of novel therapeutic strategies for vascular diseases.

At the present time, for many human diseases, medical research predominantly relies on having appropriate model systems to study disease states and in order to develop therapies. Nonhuman animal models, particularly transgenics, have allowed us to recreate disease states in vivo. However, there is still a need to have corresponding in vitro model systems to study pathogenesis at the cellular level as well as for fast-tracking discoveries of therapeutic compounds. Ideally, an in vitro disease model would be established from human diseased tissue that can demonstrate relevant degenerative mechanisms. The tremendous advances within the field of stem cell biology over the last decade may now make in vitro models possible for certain diseases that were previously thought unlikely.
This special issue in Stem Cells International is a collection of review articles that describe various ways in which stem cells have be utilized to create in vitro disease models.
The Noble-prize winning discoveries in reprogramming have given scientists the opportunity to generate stem cells from any disease type. Whilst this technology has opened new doors for creating in vitro model systems, challenges still remain including how to differentiate pluripotent stem cells to generate stage-specific (whether progenitor or mature stage) and homogeneous cell types that are relevant to the disease. A significant aspect of that relies on the development of in vitro assays to study the cellular function. N. Kawaguchi et al. highlight these issues with respect to using pluripotent stem cells to model cardiac disease and channelopathies.
Reprogramming technology is highly useful for creating in vitro disease models that are caused by known genetic mutations. The review by L. Linta et al. describes how neurons derived from pluripotent stem cells carrying mutations in alpha-synuclein or LRRK2 genes show pathological characteristics of synucleinopathies. However, for many diseases, the causes are idiopathic and interrogation of specific cell types in isolation may not elucidate other potential environmental triggers. When there are multifactorial causes, such as those for Parkinson's disease as reviewed by P. L. Martínez-Morales and I. Liste, then it is advantageous to establish multiple stem cell lines from different patients, including both idiopathic and specific genetic mutant conditions.
Parallel to the development of reprogramming technologies, the field of stem cell biology has also rapidly expanded through the identification of stem cell niches within most adult tissues. Isolation, maintenance, and expansion of stem cell/progenitor pools within a tissue provide an alterative source of cells that can be used to model disease conditions, particularly of that tissue. The review by R. J. Medina et al. describes how a subpopulation of endothelial progenitor cells isolated from human blood can be used to model vascular disease.
For some diseases, especially certain cancers, pathological mechanisms may begin within the stem cell population, perhaps due to DNA damage. In these scenarios, isolation of tissue-specific stem cells is used to model disease genesis. The review by A. Gutiérrez-Rivera et al. explains how skin-derived precursor cells may be the cells of origin in neurofibromatosis type 1 tumours. J. D. Hoerter et al. also provide a thorough review on how melanomas may arise from damaged extrafollicular melanocyte stem cells.
In summary, this special issue provides a broad spectrum of how stem cells can offer innovative and novel approaches to study disease mechanisms. We would like to thank all of the authors, reviewers, Guest Editors, and Editor-in-Chief for their great contributions and support towards this special issue.