Pluripotent Stem Cells Models for Huntington＇s Disease： Prospects and Challenges

Pluripotent cellular models have shown great promise in the study of a number of neurological disorders.Several advantages of using a stem cell model include the potential for cells to derive disease relevant neuronal cell types,providing a system for researchers to monitor disease progression during neurogenesis,along with serving as a platform for drug discovery.A number of stem cell derived models have been employed to establish in vitro research models of Huntington’s disease that can be used to investigate cellular pathology and screen for drug and cell-based therapies.Although some progress has been made,there are a number of challenges and limitations that must be overcome before the true potential of this research strategy is achieved.In this article we review current stem cell models that have been reported,as well as discuss the issues that impair these studies.We also highlight the prospective application of Huntington’s disease stem cell models in the development of novel therapeutic strategies and advancement of personalized medicine.     

Impact of drug discovery on stem cell biology

Despite the rapid technical progress in pharmaceutical industry in the past decade, it is still a great challenge to find new drugs and the situation seems more and more serious. However, the history of pharmaceutical industry clearly indicated that the significance of drug discovery went far beyond providing new drugs. For instance, drugs or candidates could be used as selective probes to reveal novel cellular mechanisms, which is a fundamental tenet of chemical biology. More interestingly, accumulating evidence indicates that drugs and candidates can find important use in stem cell biology. Not only approved drugs but also undeveloped pharmacological agents could serve as efficient agents to regulate stem cell fate. Moreover, the target and activity knowledge accumulated during the drug discovery process will help select the stem cell fate modulators in a rational manner. As the progress in stem cell biology will bring positive influence to drug discovery, it can be expected that the current drug discovery efforts will finally bear great fruits in the future.     

Large animal models for cardiac stem cell therapies

Cardiovascular disease is the leading cause of death in developed countries and is one of the leading causes of disease burden in developing countries. Therapies have markedly increased survival in several categories of patients, nonetheless mortality still remains high. For this reason high hopes are associated with recent developments in stem cell biology and regenerative medicine that promise to replace damaged or lost cardiac muscle with healthy tissue, and thus to dramatically improve the quality of life and survival in patients with various cardiomyopathies.Much of our insight into the molecular and cellular basis of cardiovascular biology comes from small animal models, particularly mice. However, significant differences exist with regard to several cardiac characteristics when mice are compared with humans. For this reason, large animal models like dog, sheep and pig have a well established role in cardiac research. A distinct characteristic of cardiac stem cells is that they can either be endogenous or derive from outside the heart itself; they can originate as the natural course of their differentiation programme (e.g., embryonic stem cells) or can be the result of specific inductive conditions (e.g., mesenchymal stem cells). In this review we will summarize the current knowledge on the kind of heart-related stem cells currently available in large animal species and their relevance to human studies as pre-clinical models.     

Cardiomyocyte differentiation of pluripotent stem cells and their use as cardiac disease models

More than 10 years after their first isolation, human embryonic stem cells are finally 'coming of age' in research and biotechnology applications as protocols for their differentiation and undifferentiated expansion in culture become robust and scalable, and validated commercial reagents become available. Production of human cardiomyocytes is now feasible on a daily basis for many laboratories with tissue culture expertise. An additional recent surge of interest resulting from the first production of human iPSCs (induced pluripotent stem cells) from somatic cells of patients now makes these technologies of even greater importance since it is likely that (genetic) cardiac disease phenotypes can be captured in the cardiac derivatives of these cells. Although cell therapy based on replacing cardiomyocytes lost or dysfunctional owing to cardiac disease are probably as far away as ever, biotechnology and pharmaceutical applications in safety pharmacology and drug discovery will probably impact this clinical area in the very near future. In the present paper, we review the cutting edge of this exciting area of translational research.     

Bioprocessing of embryonic stem cells for drug discovery

Embryonic stem cells provide a potential resource for research and drug screening. To make such a resource feasible, it is necessary to generate cells of sufficient quality and quantity. The challenge is to expand cell numbers while maintaining the fidelity of phenotype and to control and direct differentiation to produce the cell type of interest in a format that is suitable for drug screening. At present, large-scale culturing of human ES cell lines is problematic and provides substantial challenges. This article provides an overview of current bioprocessing techniques that could be used to generate cells for drug discovery applications. This will generate further technical expertise that can be applied in the production of cells for potential therapeutic applications.     

Constructing and deconstructing stem cell models of neurological disease

Among the disciplines of medicine, the study of neurological disorders is particularly challenging. The fundamental inaccessibility of the human neural types affected by disease prevents their isolation for in?vitro studies of degenerative mechanisms or for drug screening efforts. However, the ability to reprogram readily accessible tissue from patients into pluripotent stem (iPS) cells may now provide a general solution to this shortage of human neurons. Gradually improving methods for directing the differentiation of patient-specific stem cells has enabled the production of several neural cell types affected by disease. Furthermore, initial studies with stem cell lines derived from individuals with pediatric, monogenic disorders have validated the stem cell approach to disease modeling, allowing relevant neural phenotypes to be observed and studied. Whether iPS cell-derived neurons will always faithfully recapitulate the same degenerative processes observed in patients and serve as platforms for drug discovery relevant to common late-onset diseases remains to be determined.     

Molecular imaging strategies for drug discovery and development

Recent advances in non-invasive molecular imaging provide exciting opportunities for discovery, validation and development of novel therapeutics. As the arsenal of detection devices and strategies, injectable probes, genetically encoded reporters and animal models rapidly expands, molecular imaging is becoming indispensable for drug discovery and development. Not only do such strategies reduce the time, cost and workload associated with conventional destructive end-point assays, but they also enable spatial and temporal monitoring of in vivo gene expression, signaling pathways, biochemical reactions and targets as they relate to the pharmacokinetics and pharmacodynamics of novel drugs.     

Pluripotent stem cell heterogeneity and the evolving role of proteomic technologies in stem cell biology

Stem cells represent obvious choices for regenerative medicine and are invaluable for studies of human development and drug testing. The proteomic landscape of pluripotent stem cells (PSCs), in particular, is not yet clearly defined; consequently, this field of research would greatly benefit from concerted efforts designed to better characterize these cells. In this concise review, we provide an overview of stem cell potency, highlight the types and practical implications of heterogeneity in PSCs and provide a detailed analysis of the current view of the pluripotent proteome in a unique resource for this rapidly evolving field. Our goal in this review is to provide specific insights into the current status of the known proteome of both mouse and human PSCs. This has been accomplished by integrating published data into a unified PSC proteome to facilitate the identification of proteins, which may be informative for the stem cell state as well as to reveal areas where our current view is limited. These analyses provide insight into the challenges faced in the proteomic analysis of PSCs and reveal one area--the cell surface subproteome--that would especially benefit from enhanced research efforts.     

Targeting stem cell signaling pathways for drug discovery: advances in the Notch and Wnt pathways

Signaling pathways transduce extracellular stimuli into cells through molecular cascades to regulate cellular functions.In stem cells,a small number of pathways,notably those of TGF-?/BMP,Hedgehog,Notch,and Wnt,are responsible for the regulation of pluripotency and differentiation.During embryonic development,these pathways govern cell fate specifications as well as the formation of tissues and organs.In adulthood,their normal functions are important for tissue homeostasis and regeneration,whereas aberrations result in diseases,such as cancer and degenerative disorders.In complex biological systems,stem cell signaling pathways work in concert as a network and exhibit crosstalk,such as the negative crosstalk between Wnt and Notch.Over the past decade,genetic and genomic studies have identified a number of potential drug targets that are involved in stem cell signaling pathways.Indeed,discovery of new targets and drugs for these pathways has become one of the most active areas in both the research community and pharmaceutical industry.Remarkable progress has been made and several promising drug candidates have entered into clinical trials.This review focuses on recent advances in the discovery of novel drugs which target the Notch and Wnt pathways.     

Stem cell biology and drug discovery

There are many reasons to be interested in stem cells, one of the most prominent being their potential use in finding better drugs to treat human disease. This article focuses on how this may be implemented. Recent advances in the production of reprogrammed adult cells and their regulated differentiation to disease-relevant cells are presented, and diseases that have been modeled using these methods are discussed. Remaining difficulties are highlighted, as are new therapeutic insights that have emerged.     

The possible impact of human embryonic stem cells on safety pharmacological and toxicological assessments in drug discovery and drug development

The successful establishment of human embryonic stem cell (hESC) lines has raised high expectation for their future applications. The major focus of hESC research has been on their potential use in replacement therapies. However, the most immediate application of hESCs may be in establishment of humanised in vitro tests, which have potential to reduce problems of interspecies variations in safety assessments. Improved prediction of human hazard would increase patient safety and reduce the number of laboratory animals needed for toxicological and safety pharmacological testing, leading to improved efficiency of drug discovery and development in term of cost and time. The current review describes some of the newest research programmes on the use of hESCs for safety evaluations of conventional drugs. It provides an overview of the possible impact of hESCs and their derivates on regulatory drug safety assessments and discusses the potential effects on the product pipeline organisation. The review additionally summarizes initiatives in establishing quality criteria for hESC expansion and differentiation. Such criteria are necessary in order to achieve high standardisation and throughput of pharmacological and toxicological tests. Finally, it will discuss the actions needed to scientifically prove the relevance and reliability of safety tests based on hESCs.     

Emerging models and paradigms for stem cell ageing

Ageing is accompanied by a progressive decline in stem cell function, resulting in less effective tissue homeostasis and repair. Here we discuss emerging invertebrate models that provide insights into molecular pathways of age-related stem cell dysfunction in mammals, and we present various paradigms of how stem cell functionality changes with age, including impaired self-renewal and aberrant differentiation potential.     

Redox platforms in cancer drug discovery and development

Redox homeostasis is frequently dysregulated in human disease, particularly cancer. Recent and ongoing efforts seek to validate and extend this platform for the discovery/development of anticancer drugs. As the primary source of cellular redox buffer, thiols (in particular glutathione) have been therapeutically targeted in cancer treatment, myeloproliferation, hematopoietic progenitor cell mobilization and immune response. A number of 'redox modulating' drugs have been, or are, under development and the pipeline seems viable. Moreover, S-glutathionylation is a protein post-translational modification that influences a number of critical cell pathways and in the medium term, defining the 'glutathionome' has the possibility to provide opportunities for target identification for therapeutic intervention perhaps with a relevance that parallels ongoing efforts with the kinome.     

Using photolabile ligands in drug discovery and development

Photoactivatable ligands are important tools used in drug discovery and drug development. These ligands enable researchers to identify the targets of drugs, to determine the affinity and selectivity of the drug-target interaction, and to identify the binding site on the target. Examples are presented from three fundamentally different approaches: (1) photoaffinity labeling of target macromolecules; (2) photoactivation and release of 'caged ligands'; and (3) photoimmobilization of ligands onto surfaces.     

Extinction models for cancer stem cell therapy

Cells with stem cell-like properties are now viewed as initiating and sustaining many cancers. This suggests that cancer can be cured by driving these cancer stem cells to extinction. The problem with this strategy is that ordinary stem cells are apt to be killed in the process. This paper sets bounds on the killing differential (difference between death rates of cancer stem cells and normal stem cells) that must exist for the survival of an adequate number of normal stem cells. Our main tools are birth-death Markov chains in continuous time. In this framework, we investigate the extinction times of cancer stem cells and normal stem cells. Application of extreme value theory from mathematical statistics yields an accurate asymptotic distribution and corresponding moments for both extinction times. We compare these distributions for the two cell populations as a function of the killing rates. Perhaps a more telling comparison involves the number of normal stem cells N_H at the extinction time of the cancer stem cells. Conditioning on the asymptotic time to extinction of the cancer stem cells allows us to calculate the asymptotic mean and variance of N_H. The full distribution of N_H can be retrieved by the finite Fourier transform and, in some parameter regimes, by an eigenfunction expansion. Finally, we discuss the impact of quiescence (the resting state) on stem cell dynamics. Quiescence can act as a sanctuary for cancer stem cells and imperils the proposed therapy. We approach the complication of quiescence via multitype branching process models and stochastic simulation. Improvements to the τ-leaping method of stochastic simulation make it a versatile tool in this context. We conclude that the proposed therapy must target quiescent cancer stem cells as well as actively dividing cancer stem cells. The current cancer models demonstrate the virtue of attacking the same quantitative questions from a variety of modeling, mathematical, and computational perspectives.