Chapter 1: Addressing Challenges to Progress in Human Stem Cell Toxicology Concepts and Practice
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Published:09 Aug 2016
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Special Collection: 2016 ebook collectionSeries: Issues in Toxicology
J. L. Sherley, in Human Stem Cell Toxicology, ed. J. L. Sherley, The Royal Society of Chemistry, 2016, ch. 1, pp. 1-8.
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Although only a subfield of human toxicology, human stem cell toxicology faces greater challenges. Beyond well studied problems in human toxicology like determination of human exposures, causative agents, and toxicity mechanisms, the development of effective human stem cell toxicology concepts and practice will require solutions for two largely unyielding problems that impede progress in all research and applications involving postnatal tissue stem cells. These are (1) identifying tissue stem cells with sufficient specificity to count them; and (2) isolating or producing tissue stem cells in sufficient purity and number for specific assay development. The inherent and unique properties of postnatal tissue stem cells conspire to present these long-standing challenges. Tissue stem cells exist at minute fractions in tissues, and their unique property of asymmetric self-renewal keeps their fraction low while they continuously produce lineage-committed cells that confound their detection, both morphologically and molecularly. The crucial role of stem cells in fetal and postnatal tissue function, health, aging, and disease makes understanding their toxicology an imperative for environmental health science and medicine. Chapter 1 provides an overview of the topics presented in Human Stem Cell Toxicology that begin to illuminate new paths to improved human stem cell toxicology concepts and practice.
1.1 Filling in the Stem Cell Gap in Human Toxicology
One way to gauge the degree of research and practice for a scientific discipline is to evaluate the number of times that, and the period of time over which, its quoted name is found in published scholarly reports. Table 1.1 provides such data from PubMed1 and Google Scholar2 searches for the disciplines of ‘human toxicology’, ‘reproductive toxicology’, ‘stem cell toxicology’ and ‘human stem cell toxicology’. For the term ‘stem cell toxicology’, the results show vastly fewer reports that only appeared in the past several years; and for ‘human stem cell toxicology’, even more dramatically, there are none. The recognition of an immediate need to begin filling in the remarkable gap in knowledge and research disclosed by this analysis was the inspiration for Human Stem Cell Toxicology.
Database search results for fields related to human stem cell toxicology.
Field . | Number of reports . | Report years (total) . |
---|---|---|
PubMed1 | ||
“Human Toxicology” | 1101 | 1960–2016 (56) |
“Reproductive Toxicology” | 1014 | 1980–2016 (36) |
“Stem Cell Toxicology” | 5 | 2015 (1) |
“Human Stem Cell Toxicology” | 0 | N/A |
Google Scholar2 | ||
“Human Toxicology” | 25 400 | 1908–2016 (108) |
“Reproductive Toxicology” | 24 100 | 1909–2016 (107) |
“Stem Cell Toxicology” | 27 | 2010–2016 (6) |
“Human Stem Cell Toxicology” | 0 | N/A |
Field . | Number of reports . | Report years (total) . |
---|---|---|
PubMed1 | ||
“Human Toxicology” | 1101 | 1960–2016 (56) |
“Reproductive Toxicology” | 1014 | 1980–2016 (36) |
“Stem Cell Toxicology” | 5 | 2015 (1) |
“Human Stem Cell Toxicology” | 0 | N/A |
Google Scholar2 | ||
“Human Toxicology” | 25 400 | 1908–2016 (108) |
“Reproductive Toxicology” | 24 100 | 1909–2016 (107) |
“Stem Cell Toxicology” | 27 | 2010–2016 (6) |
“Human Stem Cell Toxicology” | 0 | N/A |
Human Stem Cell Toxicology was developed with the intention of igniting a long overdue effort to set an updated, well-informed foundation for ‘human stem cell toxicology’ as a subfield of human toxicology, which, based on the earliest scholarly reports identified by Google Scholar, is more than a century-old discipline in human science and research. Knowing that the field of stem cell biology is at least five decades from its origins, and human toxicology is a century or more old, may appear to present quite a conundrum as to why stem cell toxicology, and human stem cell toxicology in particular, have such scant and late representation in the scientific literature. However, there is really no puzzle at all. First, as will be the focus later in this opening chapter, the explanation is certainly related to scientific and technical challenges that are unique to postnatal tissue stem cells. However, there are also important features of the history and nature of toxicological sciences that are also likely to have played important roles in the seeming neglect of stem cells in toxicological research.
1.2 Historical Impact of the Hierarchical, Anatomical, Sub-disciplinary Structure of Toxicological Sciences
In the literature search results of Table 1.1, the subfield of reproductive toxicology has a literature representation history that closely follows its parent field, human toxicology. This is not surprising, as the reproductive system is just one of the many tissue and organ systems defined by specific training, research and expertise in human toxicology. ‘Reproductive toxicology’ is considered here, because its usage often applies to studies that might also be considered ‘stem cell toxicology’. Generally, the stem cells of focus have been the germinal stem cells of the ovaries and the testes. To a lesser extent, later on, embryonic stem cells have been considered as models for reproductive toxicology focused on environmental and iatrogenic toxicants of embryonic and fetal development.
The hierarchical, anatomical, sub-disciplinary structure of human toxicological sciences, which is based on population-, tissue-, organ-, cell-, organelle- and molecule-specific organization of toxicants and their mechanisms of action, met a pedagogical impasse with tissue stem cells. Although knowledge of tissue stem cells and their essential roles in tissue function and repair is long-standing, the concept of them as critical targets of toxicants and toxic mechanisms has been largely theoretical in construct. Importantly, this toxicology concept has not been readily approachable experimentally. Even the concept that human carcinogens may act by inducing alterations in tissue stem cells in humans continues to be a point of controversy.3 The essential problem has been the elusive physical nature of postnatal tissue stem cells. The challenge of distinguishing them from other tissue cell types has thwarted the science of toxicology's critical requirement for quantifying effects of toxicants on their biological targets. So, whereas it has been possible to define and establish toxicological disciplines specialized for human populations, human organs, human tissues, and many specifically identifiable other human cell types (e.g. neurons, epidermal cells, endothelial cells, etc.), the same has not been possible for postnatal tissue stem cells.
1.3 Human Stem Cell Toxicology as a Stem Cell Exact Science
The above specification of ‘human stem cell toxicology’ to postnatal tissue stem cells is not a trivial reference. Instead it is a crucial aspect of this new toxicology discipline's foundation. One of the most problematic aspects of the general field of stem cell biology is the inexact language for ‘stem cells’. This problem comes in two forms, technical and conceptual. The technical form of the language problem is an outgrowth of well-informed and well-intended attempts to accurately denote the cellular make-up of stem cell-containing tissue compartments and tissue cell preparations. Because there is often no physical basis for distinguishing postnatal tissue stem cells from local and usually more abundant lineage-committed progenitor cells, which are their progeny, terms like ‘stem/progenitor cells’ are widely used, as in some chapters in this book. This usage is appropriate when the typical uncertainty about stem cell phenotypic identity is an important consideration or qualification. However, this terminology has become so dogmatic in the field of mammalian stem cell biology that it has led to a widely misplaced attitude that even conceptual or theoretical usage of the term ‘stem cell’ without including progenitors is inappropriate. In marked contrast to this scientific piety, discussions involving current ‘stem cell biomarkers’ often completely ignore the stem/progenitor ambiguity and apply and interpret biomarkers, which clearly identify both stem cells and progenitor cells, as if they only identified stem cells. As biomarkers with greater specificity for tissue stem cells become available,4–6 this problem may resolve; but currently it is an important correction required to set the foundation of human stem cell toxicology as an exact science with respect to its definition of ‘stem cell’.
The second conceptual form of the problem is even more fundamental, although not known to many stem cell biologists. Stem cells are often defined as cells that can either self-renew or differentiate. However, this definition is only applicable to three types of cells in vitro: embryonic ‘stem’ cells, induced pluripotent ‘stem’ cells, and cancer cell lines that retain the ability to differentiate, usually into one differentiated cell type. A characteristic property of these cells is that they do not self-renew and differentiate. When placed under conditions that allow differentiation, whether multiple differentiated cell types are produced or only one, no self-renewing cells are maintained. All the ‘stem’ cells, in fact, undergo differentiation. Therefore, fundamentally, these cells are not ‘stem’ cells. They are progenitor cells. Their conditional self-renewal is not a natural state, but one enforced by their in vitro culture conditions. Informatively, epiblast cells, the in vivo originators of embryonic ‘stem’ cells, are also not stem cells in their natural setting, but instead progenitor cells. They undergo differentiation to develop the embryo, but their initial phenotype is lost in the process.
The fundamental definition of a stem cell is a cell that can self-renew and produce differentiating cells simultaneously without loss of its stem phenotype for the life of the tissue that it renews. This ability, which is unique to stem cells, is called asymmetric self-renewal.7–10 In marked contrast to progenitor cells, stem cells preserve their stem cell phenotype. Although the mathematical form that achieves this balance continues to be a controversial topic,7,10,11 there is no dispute regarding the preservation of the stemness phenotype being an essential element of stem cell character.12
Three types of human tissue cells have been described that meet the fundamental asymmetric self-renewal definition for stem cells. The first two are well-studied, postnatal tissue-specific stem cells (also called distributed stem cells in Chapter 10) and eukaryotic cancer stem cells (Chapter 11). The third type is a more recently discovered remarkable class of asymmetrically self-renewing stem cells found during the development of fetal organs and tissues, called metakaryotic stem cells (Chapter 9). In each case, these stem cells divide to produce differentiating progeny cells while simultaneously maintaining their stemness properties.
The newly defined discipline of human stem cell toxicology currently encompasses research with pluripotent cell types (e.g. Chapter 6). This inclusion may maintain to the extent that pluripotent cells prove to be able to produce asymmetrically self-renewing tissue-specific stem cells in large numbers. However, the enthusiasm for such studies must always be tempered by the inherent genetic and epigenetic alterations that occur when pluripotent cell types are derived.13 By grounding human stem cell toxicology as an exact stem cell science, defined by investigation of the toxicology of human stem cells that undergo asymmetric self-renewal, the new discipline will also encourage increased precision with these essential concepts in stem cell biology as a whole. Many of the contributed chapters reflect this ideal foundation.
1.4 Health and Medical Applications for Human Stem Cell Toxicological Sciences
Given the ideal foundation in an exact definition for stem cells, human stem cell toxicology has three immediate areas of application that are directly derivative of the involvement of the three identified types of asymmetrically self-renewing stem cells in human health and medicine. As for human toxicology in general, the foremost application is environmental health science. Both fetal metakaryotic stem cells and postnatal homeostatic tissue stem cells figure prominently in this regard. Chapters in this book focus on the importance of these cells as targets for carcinogenic toxicants, and their respective key roles in fetal development and postnatal tissue maturation and aging make them crucial toxicant targets for investigation and elucidation.
The second and third applications reflect a splitting of the personality of toxicology for medicine. On the one hand, a traditional protective toxicology pursuit – understanding and eliminating toxic drug candidates – focused now on postnatal tissue stem cells, is an important need in drug development. Currently, tissue stem cell-toxic drug candidates, which are highly unsafe drugs, are detected by their induction of organ and tissue failure in expensive preclinical animal studies or in even more costly, in both money and human suffering, clinical trials. New, less expensive, cell culture-based assays for detection of tissue stem cell toxicity would accelerate drug discovery and greatly reduce its cost, in terms of both expense and risk to research volunteers and patients. In the case of development of drugs for fetal disorders or that might be taken during pregnancy, this application also applies to metakaryotic stem cells. However, as will be noted below, the biology of metakaryotic stem cells places them in a much greater state of readiness for achieving these advances in drug development applications.
The third application, the development of anti-cancer stem cell drugs, is the atypical face of the toxicology personality split. Instead of seeking to prevent or avoid stem cell toxicity, in the case of new cancer stem cell therapy paradigms, the goal is to discover drugs with cancer stem cell-specific toxicity. By destroying cancer stem cells, which are responsible for the asymmetric self-renewal of tumors, drug developers hope to induce tumor failures, much in the way that agents toxic against normal tissue stem cells induce organ and tissue failure. Cancer stem cell therapeutics faces all the technical problems inherent to investigations of normal postnatal tissue stem cells. In fact, it is likely that many cancer stem cells are derived from mutated variants of tissue stem cells.10 So, no specific quantitative biomarkers exist, and the cells are a small fraction of total tumor cells. The pursuit of anti-cancer stem cell drugs has the added challenge of specific targeting to spare normal tissue cells, and in particular normal tissue stem cells, which are likely to be targeted often by the same agents because of the unique properties they share with cancer stem cells.
The recently discovered metakaryotic stem cells provide a new paradigm that sets the standard for achievement by all future investigations in human stem cell toxicology. Unlike postnatal tissue stem cells and previously described eukaryotic cancer stem cells, metakaryotic stem cells, both normal and tumor-derived, are specifically and directly identifiable and quantifiable based on their morphology and molecular expression (Chapter 9). Because of their unique forms of amitotic cell division and DNA replication by a RNA:DNA hybrid intermediate that are not shared by other cell types, metakaryotic stem cells are physically and molecularly distinctive. Given these ideal properties, it seems very likely that many future standard analysis paradigms in human stem cell toxicology will be developed first in investigations of the toxicology of metakaryotic stem cells, both normal ones in fetal development and cancerous ones in fetal and postnatal tumors. Whether there is a developmental lineage relationship between metakaryotic stem cells and homeostatic postnatal tissue stem cells or eukaryotic cancer stem cells is presently unclear. However, if such lineage connections exist, continued investigations of metakaryotic stem cell biology and toxicology may reduce some of the current seemingly insurmountable barriers to toxicological analyses of the other two human stem cell types.
1.5 Introducing the Future Diverse Impacts of Human Stem Cell Toxicology
Beyond setting a foundation of stem cell exactness to the new field of human stem cell toxicology and highlighting the challenges of identifying stem cell toxicants and their mechanism of action against human stem cells specifically, this inaugural volume begins the introduction to the diverse aspects of human stem cell toxicological science, including development of new technologies for improving stem cell toxicant detection and addressing the problems of specific stem cell detection; investigations for toxicant targets that impact stem cell function indirectly; and examples of the investigation of the effects of previously well-studied human toxicants on tissue stem cells.
As might be expected, the category with the largest number of chapters contributed (Chapters 2, 3, 5, 9, 10) provides treatments of the progress and challenges in developing high-throughput screens for stem cell toxicants in both environmental health science and drug development arenas. In most cases, developing better assays with greater stem cell definition is the main objective of these presentations; whereas for others, the main focus is an advance in human stem cell toxicology that required innovation in assay technology as well.
In one case, the reported advance is an integration of a new stem cell toxicology concept with a new technological development (Chapter 10). The new concept, ‘kinetotoxicity’, is conceptually similar to the long-standing colony forming unit (CFU) approach to quantifying toxicant effects on elusive human hematopoietic stem cells (HSCs; Chapters 2 and 7), but technically distinct. The presence of stem cells is detected indirectly by their cellular output. In the case of the CFU approach, the scored output, morphologically differentiated cell colonies, provides a largely qualitative assessment of stem cell number and is limited to HSCs. Kinetotoxicity is a quantitative measure of a toxicant's interference with stem cell asymmetric self-renewal, a kinetics output, and a highly associated stem cell process, immortal strand co-segregation (ISC) that can be scored molecularly. These advances for postnatal tissue stem cells have the potential to provide the specific and quantitative capabilities of metakaryotic stem cells.
Although stem cells are typically sequestered in access-limited micro-anatomical niches, these are islands with weather and shores. There are many aspects of stem cell biology besides the autonomous processes of stem cells (i.e. metabolism, cell division, mutagenesis, etc.) that determine their function. Human Stem Cell Toxicology includes chapters on ‘stem cell differentiation’ (Chapter 5), catecholamine regulation of stem cell mobilization (Chapter 4), and stem cell epigenetics as targets for stem cell toxicants (Chapter 8). This is but a very short list of important stem cell interactions and regulation that toxicants might disrupt to cause stem cell toxicity. However, these few examples serve to confirm that a basic axiom of general human toxicology also applies to human stem cell toxicology. For example, just as some human toxicants require metabolism by the liver before being activated to affect a different organ site, stem cell-toxic mechanisms may involve sites of action distal to the impacted stem cells.
A corollary of the distal action axiom is that investigations of the mechanisms of stem cell toxicants can lead to increased understanding of the function and regulation of one of the most elusive cell types in the body. In keeping with this concept, some chapters in Human Stem Cell Toxicology consider the responses of stem cells to well-studied environmental toxicants like pesticides (Chapter 7) and even household medicines (Chapter 9). From these inaugural contributions to establish the discipline, as well as other chapters in this volume, it is certain that human stem cell toxicology promises many intriguing revelations in the future that will greatly impact human health and medicine.
I thank Professor Diana Anderson, of the University of Bradford, UK and The Royal Society for Chemistry, for her vision of the timeliness of this volume and her gracious invitation to me to serve as the editor for its completion. In addition to my co-authors for their seminal contributions, I also wish to thank the unseen members of the international human toxicology community who expresssed genuine enthusiasm for the project and recommended ideal authors, both of which were crucial elements in its successful completion.