Embryonic stem cells expressing expanded CAG repeats undergo aberrant neuronal differentiation and have persistent Oct-4 and REST/NRSF expression

Repressor Element-1 Binding Transcription Factor (REST) as a Possible Epigenetic Regulator of Neurodegeneration and MicroRNA-Based Therapeutic Strategies

Neurodegenerative disorders (NDD) have grabbed significant scientific consideration due to their fast increase in prevalence worldwide. The specific pathophysiology of the disease and the amazing changes in the brain that take place as it advances are still the top issues of contemporary research. Transcription factors play a decisive role in integrating various signal transduction pathways to ensure homeostasis. Disruptions in the regulation of transcription can result in various pathologies, including NDD. Numerous microRNAs and epigenetic transcription factors have emerged as candidates for determining the precise etiology of NDD. Consequently, understanding by what means transcription factors are regulated and how the deregulation of transcription factors contributes to neurological dysfunction is important to the therapeutic targeting of pathways that they modulate. RE1-silencing transcription factor (REST) also named neuron-restrictive silencer factor (NRSF) has been studied in the pathophysiology of NDD. REST was realized to be a part of a neuroprotective element with the ability to be tuned and influenced by numerous microRNAs, such as microRNAs 124, 132, and 9 implicated in NDD. This article looks at the role of REST and the influence of various microRNAs in controlling REST function in the progression of Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD) disease. Furthermore, to therapeutically exploit the possibility of targeting various microRNAs, we bring forth an overview of drug-delivery systems to modulate the microRNAs regulating REST in NDD.

Aberrant Development Corrected in Adult-Onset Huntington's Disease iPSC-Derived Neuronal Cultures via WNT Signaling Modulation

Aberrant neuronal development and the persistence of mitotic cellular populations have been implicated in a multitude of neurological disorders, including Huntington's disease (HD). However, the mechanism underlying this potential pathology remains unclear. We used a modified protocol to differentiate induced pluripotent stem cells (iPSCs) from HD patients and unaffected controls into neuronal cultures enriched for medium spiny neurons, the cell type most affected in HD. We performed single-cell and bulk transcriptomic and epigenomic analyses and demonstrated that a persistent cyclin D1+ neural stem cell (NSC) population is observed selectively in adult-onset HD iPSCs during differentiation. Treatment with a WNT inhibitor abrogates this NSC population while preserving neurons. Taken together, our findings identify a mechanism that may promote aberrant neurodevelopment and adult neurogenesis in adult-onset HD striatal neurons with the potential for therapeutic compensation.

Transcriptional activation of REST by Sp1 in Huntington's disease models

In Huntington's disease (HD), mutant huntingtin (mHtt) disrupts the normal transcriptional program of disease neurons by altering the function of several gene expression regulators such as Sp1. REST (Repressor Element-1 Silencing Transcription Factor), a key regulator of neuronal differentiation, is also aberrantly activated in HD by a mechanism that remains unclear. Here, we show that the level of REST mRNA is increased in HD mice and in NG108 cells differentiated into neuronal-like cells and expressing a toxic mHtt fragment. Using luciferase reporter gene assay, we delimited the REST promoter regions essential for mHtt-mediated REST upregulation and found that they contain Sp factor binding sites. We provide evidence that Sp1 and Sp3 bind REST promoter and interplay to fine-tune REST transcription. In undifferentiated NG108 cells, Sp1 and Sp3 have antagonistic effect, Sp1 acting as an activator and Sp3 as a repressor. Upon neuronal differentiation, we show that the amount and ratio of Sp1/Sp3 proteins decline, as does REST expression, and that the transcriptional role of Sp3 shifts toward a weak activator. Therefore, our results provide new molecular information to the transcriptional regulation of REST during neuronal differentiation. Importantly, specific knockdown of Sp1 abolishes REST upregulation in NG108 neuronal-like cells expressing mHtt. Our data together with earlier reports suggest that mHtt triggers a pathogenic cascade involving Sp1 activation, which leads to REST upregulation and repression of neuronal genes.

Preimplantation genetic diagnosis as a source of human embryonic stem cells for disease research and drug discovery

Embryos surplus to therapeutic requirements following preimplantation genetic diagnosis can be used to derive human embryonic stem cell (hESC) lines carrying mutations significant to human disease. These cells provide a powerful in vitro tool for modelling disease progression in a number of cell types as well as having the potential to revolutionise drug discovery. Robust and reproducible directed differentiation protocols are needed to maximise the potential of these cells. In this review, we explore the current use of hESC and induced pluripotent stem cells in disease-specific research and discuss the use of stem cell technology in drug discovery and toxicity testing.

REST regulates distinct transcriptional networks in embryonic and neural stem cells

The maintenance of pluripotency and specification of cellular lineages during embryonic development are controlled by transcriptional regulatory networks, which coordinate specific sets of genes through both activation and repression. The transcriptional repressor RE1-silencing transcription factor (REST) plays important but distinct regulatory roles in embryonic (ESC) and neural (NSC) stem cells. We investigated how these distinct biological roles are effected at a genomic level. We present integrated, comparative genome- and transcriptome-wide analyses of transcriptional networks governed by REST in mouse ESC and NSC. The REST recruitment profile has dual components: a developmentally independent core that is common to ESC, NSC, and differentiated cells; and a large, ESC-specific set of target genes. In ESC, the REST regulatory network is highly integrated into that of pluripotency factors Oct4-Sox2-Nanog. We propose that an extensive, pluripotency-specific recruitment profile lends REST a key role in the maintenance of the ESC phenotype.

Embryonic stem cells have the unique and defining property of pluripotency: the ability to differentiate into all cell types. Key transcription factors form interconnected gene regulatory networks that control pluripotency and differentiation. Recently, the transcriptional repressor RE1-silencing transcription factor (REST) was implicated in the maintenance of pluripotency. This was surprising, given that REST has long been known as an essential regulator of neurodevelopment. How does REST regulate pluripotency? Does REST have distinct cohorts of binding sites and target genes in different developmental contexts? To address these questions, we made whole-genome maps of REST binding sites in two mouse stem cell types: embryonic (ESC) and neural (NSC) stem cells. These data were compared with each other and with gene expression data from cells in which REST activity was inhibited. The target genes were almost completely distinct in the two cell types. Surprisingly, we found that REST recruitment has two approximately equal components: common sites across all cells and an ESC-specific component. These pluripotency-associated sites are enriched for particular classes of genes, including those mediating the Wnt signaling pathway, which is an essential regulator of pluripotency.

Whole-genome mapping of the essential transcriptional repressor REST reveals distinct binding profiles and diverse roles in embryonic and neural stem cells.

Neur-ons and neur-offs: regulators of neural induction in vertebrate embryos and embryonic stem cells

Although the spatial and temporal orchestration of early vertebrate embryogenesis is missing from cell culture systems, recent work suggests that many of the same signals affecting neural induction in vertebrate embryos also regulate embryonic stem (ES) cell neurogenesis. One key regulatory mechanism involved in both in vivo and in vitro neural induction is the inhibition of bone morphogenetic protein (BMP) signals. Wnts and Fibroblast Growth Factors represent additional regulatory influences, which may affect the adoption of neural fates through both BMP-dependent and BMP-independent mechanisms. Insights into neural induction in vivo help to guide paradigms for promoting neural differentiation by ES cells. Conversely, insights into the mechanisms by which ES cells adopt neural fates may provide an improved understanding of neural induction in the early embryo.

Merging fields: stem cells in neurogenesis, transplantation, and disease modeling

Traditionally, applied stem cell research has been segregating into strategies aiming at endogenous repair and cell transplantation. Recent advances in both fields have unraveled unexpected potential for synergy between these disparate fields. The increasing dissection of the step-wise integration of adult-born neurons into an established brain circuitry provides a highly informative blueprint for the functional incorporation of grafted neurons into a host brain. On the other hand, in vitro recapitulation of developmental differentiation cascades permits the de novo generation of various neural cell types from pluripotent embryonic stem (ES) cells. Advanced tools in stem cell engineering enable not only genetic selection and instruction of disease-specific donor cells for neural replacement but also the exploitation of stem cells as transgenic cellular model systems for human diseases. In a comparative approach we here illuminate the functional integration of neurons derived from endogenous and transplanted stem cells, the evolving technologies for advanced stem cell engineering and the impact of cloned and mutated stem cells on disease modeling.

Polyglutamine expansion causes neurodegeneration by altering the neuronal differentiation program

Huntington's disease (HD) and spinocerebellar ataxia type 7 (SCA7) belong to a group of inherited neurodegenerative diseases caused by polyglutamine (polyQ) expansion in corresponding proteins. Transcriptional alteration is a unifying feature of polyQ disorders; however, the relationship between polyQ-induced gene expression deregulation and degenerative processes remains unclear. R6/2 and R7E mouse models of HD and SCA7, respectively, present a comparable retinal degeneration characterized by progressive reduction of electroretinograph activity and important morphological changes of rod photoreceptors. The retina, which is a simple central nervous system tissue, allows correlating functional, morphological and molecular defects. Taking advantage of comparing polyQ-induced degeneration in two retina models, we combined gene expression profiling and molecular biology techniques to decipher the molecular pathways underlying polyQ expansion toxicity. We show that R7E and R6/2 retinal phenotype strongly correlates with loss of expression of a large cohort of genes specifically involved in phototransduction function and morphogenesis of differentiated rod photoreceptors. Accordingly, three key transcription factors (Nrl, Crx and Nr2e3) controlling rod differentiation genes, hence expression of photoreceptor specific traits, are down-regulated. Interestingly, other transcription factors known to cause inhibitory effects on photoreceptor differentiation when mis-expressed, such as Stat3, are aberrantly re-activated. Thus, our results suggest that independently from the protein context, polyQ expansion overrides the control of neuronal differentiation and maintenance, thereby causing dysfunction and degeneration.

Embryonic stem cell models of CAG repeat disease

Nine neurodegenerative disorders are caused by CAG/polyglutamine (polyQ) repeat expansions. The underlying molecular mechanisms responsible for disease specific neurodegeneration remain elusive. In vivo and in vitro models utilizing rodent tissues, immortalized human cell lines, and human post mortem samples have provided insight into disease mechanisms. Concern that cellular and molecular processes observed in these models may not faithfully reproduce human diseases or be useful to identify compounds of therapeutic utility has driven development of new disease models. In addition to their therapeutic potential, stem cells represent a renewable source of tissue that can be directed into neurons and glia and can be used to study neurodegenerative cascades from their inception. Neuronally differentiated human stem cells containing CAG repeat expansions have the potential to accurately replicate human CAG repeat diseases and may be a faithful predictor of which compounds will be of human benefit. As a first step in development of this type of model, we developed murine embryonic stem cell models to study the mechanisms of polyQ tract induced neuronal degeneration.