In Vitro Differentiation of Human Neural Progenitor Cells Into Striatal GABAergic Neurons

Profiling transcriptomic responses of human stem cell-derived medium spiny neuron-like cells to exogenous phasic and tonic neurotransmitters

Transcriptomic responses to neurotransmitters contribute to the complex processes driving memory and addiction. Advances in both measurement methods and experimental models continue to improve our understanding of this regulatory layer. Here we focus on the experimental potential of stem cell derived neurons, currently the only ethical model that can be used in reductionist and experimentally perturbable studies of human cells. Prior work has focused on generating distinct cell types from human stem cells, and has also shown their utility in modeling development and cellular phenotypes related to neurodegeneration. Here we seek an understanding of how stem cell derived neural cultures respond to perturbations experienced during development and disease progression. This work profiles transcriptomic responses of human medium spiny neuron-like cells with three specific goals. We first characterize transcriptomic responses to dopamine and dopamine receptor agonists and antagonists presented in dosing patterns mimicking acute, chronic, and withdrawal regimens. We also assess transcriptomic responses to low and persistent tonic levels of dopamine, acetylcholine, and glutamate to better mimic the in vivo environment. Finally, we identify similar and distinct responses between hMSN-like cells derived from H9 and H1 stem cell lines, providing some context for the extent of variability these types of systems will likely pose for experimentalists. The results here suggest future optimizations of human stem cell derived neurons to increase their in vivo relevance and the biological insights that can be garnered from these models.

Directional induction of neural stem cells, a new therapy for neurodegenerative diseases and ischemic stroke

Due to the limited capacity of the adult mammalian brain to self-repair and regenerate, neurological diseases, especially neurodegenerative disorders and stroke, characterized by irreversible cellular damage are often considered as refractory diseases. Neural stem cells (NSCs) play a unique role in the treatment of neurological diseases for their abilities to self-renew and form different neural lineage cells, such as neurons and glial cells. With the increasing understanding of neurodevelopment and advances in stem cell technology, NSCs can be obtained from different sources and directed to differentiate into a specific neural lineage cell phenotype purposefully, making it possible to replace specific cells lost in some neurological diseases, which provides new approaches to treat neurodegenerative diseases as well as stroke. In this review, we outline the advances in generating several neuronal lineage subtypes from different sources of NSCs. We further summarize the therapeutic effects and possible therapeutic mechanisms of these fated specific NSCs in neurological disease models, with special emphasis on Parkinson's disease and ischemic stroke. Finally, from the perspective of clinical translation, we compare the strengths and weaknesses of different sources of NSCs and different methods of directed differentiation, and propose future research directions for directed differentiation of NSCs in regenerative medicine.

Structural Organization of Human Full-Length PAR3 and the aPKC-PAR6 Complex

The tripartite partition defect (PAR) polarity complex, which includes the proteins PAR3, atypical protein kinase C (aPKC), and PAR6, is a major regulator of cellular polarity. It is highly conserved and expressed in various tissues. Its largest component, PAR3, controls protein–protein interactions of the PAR complex with a variety of interaction partners, and PAR3 self-association is critical for the formation of filament-like structures. However, little is known about the structure of the PAR complex. Here, we purified non-filamentous PAR3 and the aPKC–PAR6 complex and characterized them by single-particle electron microscopy (EM). We expressed and purified an oligomerization-deficient form of PAR3, PAR3_V13D,D70K, and the active aPKC–PAR6 dimer. For PAR3, engineering at two positions is sufficient to form stable single particles with a maximum dimension of 20 nm. aPKC–PAR6 forms a complex with a maximum dimension of 13.5 nm that contains single copies of aPKC. Thus, the data present a basis for further high-resolution studies of PAR proteins and PAR complex formation.

Chromatin Structure and Dynamics: Focus on Neuronal Differentiation and Pathological Implication

Chromatin structure is an essential regulator of gene expression. Its state of compaction contributes to the regulation of genetic programs, in particular during differentiation. Epigenetic processes, which include post-translational modifications of histones, DNA methylation and implication of non-coding RNA, are powerful regulators of gene expression. Neurogenesis and neuronal differentiation are spatio-temporally regulated events that allow the formation of the central nervous system components. Here, we review the chromatin structure and post-translational histone modifications associated with neuronal differentiation. Studying the impact of histone modifications on neuronal differentiation improves our understanding of the pathophysiological mechanisms of chromatinopathies and opens up new therapeutic avenues. In addition, we will discuss techniques for the analysis of histone modifications on a genome-wide scale and the pathologies associated with the dysregulation of the epigenetic machinery.

Defective repair of topoisomerase I induced chromosomal damage in Huntington's disease

Topoisomerase1 (TOP1)-mediated chromosomal breaks are endogenous sources of DNA damage that affect neuronal genome stability. Whether TOP1 DNA breaks are sources of genomic instability in Huntington's disease (HD) is unknown. Here, we report defective 53BP1 recruitment in multiple HD cell models, including striatal neurons derived from HD patients. Defective 53BP1 recruitment is due to reduced H2A ubiquitination caused by the limited RNF168 activity. The reduced availability of RNF168 is caused by an increased interaction with p62, a protein involved in selective autophagy. Depletion of p62 or disruption of the interaction between RNAF168 and p62 was sufficient to restore 53BP1 enrichment and subsequent DNA repair in HD models, providing new opportunities for therapeutic interventions. These findings are reminiscent to what was described for p62 accumulation caused by C9orf72 expansion in ALS/FTD and suggest a common mechanism by which protein aggregation perturb DNA repair signaling.

Making neurons, made easy: The use of Neurogenin-2 in neuronal differentiation

Directed neuronal differentiation of human pluripotent stem cells (hPSCs), neural progenitors, or fibroblasts using transcription factors has allowed for the rapid and highly reproducible differentiation of mature and functional neurons. Exogenous expression of the transcription factor Neurogenin-2 (NGN2) has been widely used to generate different populations of neurons, which have been used in neurodevelopment studies, disease modeling, drug screening, and neuronal replacement therapies. Could NGN2 be a “one-glove-fits-all” approach for neuronal differentiations? This review summarizes the cellular roles of NGN2 and describes the applications and limitations of using NGN2 for the rapid and directed differentiation of neurons.

From Progenitors to Progeny: Shaping Striatal Circuit Development and Function

Understanding how neurons of the striatum are formed and integrate into complex synaptic circuits is essential to provide insight into striatal function in health and disease. In this review, we summarize our current understanding of the development of striatal neurons and associated circuits with a focus on their embryonic origin. Specifically, we address the role of distinct types of embryonic progenitors, found in the proliferative zones of the ganglionic eminences in the ventral telencephalon, in the generation of diverse striatal interneurons and projection neurons. Indeed, recent evidence would suggest that embryonic progenitor origin dictates key characteristics of postnatal cells, including their neurochemical content, their location within striatum, and their long-range synaptic inputs. We also integrate recent observations regarding embryonic progenitors in cortical and other regions and discuss how this might inform future research on the ganglionic eminences. Last, we examine how embryonic progenitor dysfunction can alter striatal formation, as exemplified in Huntington's disease and autism spectrum disorder, and how increased understanding of embryonic progenitors can have significant implications for future research directions and the development of improved therapeutic options.SIGNIFICANCE STATEMENT This review highlights recently defined novel roles for embryonic progenitor cells in shaping the functional properties of both projection neurons and interneurons of the striatum. It outlines the developmental mechanisms that guide neuronal development from progenitors in the embryonic ganglionic eminences to progeny in the striatum. Where questions remain open, we integrate observations from cortex and other regions to present possible avenues for future research. Last, we provide a progenitor-centric perspective onto both Huntington's disease and autism spectrum disorder. We suggest that future investigations and manipulations of embryonic progenitor cells in both research and clinical settings will likely require careful consideration of their great intrinsic diversity and neurogenic potential.

Generation of striatal neurons from human induced pluripotent stem cells by controlling extrinsic signals with small molecules

Human induced pluripotent stem cells (hiPSCs) are powerful tools for modeling human brain development and treating neurodegenerative diseases. Here we established a robust protocol with high scalability for generating striatal medium spiny neurons (MSNs) from hiPSCs using small molecules under two- and three-dimensional culture conditions. Using this protocol, GSH2⁺ lateral ganglionic eminence (LGE) progenitors were generated in two-dimensional culture by Sonic hedgehog signaling activation using purmorphamine, WNT signaling inhibition using XAV939, and dual-SMAD inhibition using LDN193189 and A83-01. Quantitative PCR analysis revealed sequential expression of LGE and striatal genes during differentiation. These LGE progenitors subsequently gave rise to DARPP32⁺ MSNs exhibiting spontaneous and evoked monophasic spiking activity. Applying this protocol in three-dimensional culture, we generated striatal neurospheres with gene expression profiles and cell layer organization resembling that of the developing striatum, including distinct ventricular and subventricular zones and DARPP32⁺ neurons at the surface. This protocol provides a useful experimental model for studying striatal development and yields cells potentially applicable for regenerative medicine to treat striatum-related disorders such as Huntington's disease.

Emerging hiPSC Models for Drug Discovery in Neurodegenerative Diseases

Neurodegenerative diseases affect millions of people worldwide and are characterized by the chronic and progressive deterioration of neural function. Neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD), represent a huge social and economic burden due to increasing prevalence in our aging society, severity of symptoms, and lack of effective disease-modifying therapies. This lack of effective treatments is partly due to a lack of reliable models. Modeling neurodegenerative diseases is difficult because of poor access to human samples (restricted in general to postmortem tissue) and limited knowledge of disease mechanisms in a human context. Animal models play an instrumental role in understanding these diseases but fail to comprehensively represent the full extent of disease due to critical differences between humans and other mammals. The advent of human-induced pluripotent stem cell (hiPSC) technology presents an advantageous system that complements animal models of neurodegenerative diseases. Coupled with advances in gene-editing technologies, hiPSC-derived neural cells from patients and healthy donors now allow disease modeling using human samples that can be used for drug discovery.

Cell Reprogramming to Model Huntington's Disease: A Comprehensive Review

Huntington’s disease (HD) is a neurodegenerative disorder characterized by the progressive decline of motor, cognitive, and psychiatric functions. HD results from an autosomal dominant mutation that causes a trinucleotide CAG repeat expansion and the production of mutant Huntingtin protein (mHTT). This results in the initial selective and progressive loss of medium spiny neurons (MSNs) in the striatum before progressing to involve the whole brain. There are currently no effective treatments to prevent or delay the progression of HD as knowledge into the mechanisms driving the selective degeneration of MSNs has been hindered by a lack of access to live neurons from individuals with HD. The invention of cell reprogramming provides a revolutionary technique for the study, and potential treatment, of neurological conditions. Cell reprogramming technologies allow for the generation of live disease-affected neurons from patients with neurological conditions, becoming a primary technique for modelling these conditions in vitro. The ability to generate HD-affected neurons has widespread applications for investigating the pathogenesis of HD, the identification of new therapeutic targets, and for high-throughput drug screening. Cell reprogramming also offers a potential autologous source of cells for HD cell replacement therapy. This review provides a comprehensive analysis of the use of cell reprogramming to model HD and a discussion on recent advancements in cell reprogramming technologies that will benefit the HD field.

Molecular Components of Store-Operated Calcium Channels in the Regulation of Neural Stem Cell Physiology, Neurogenesis, and the Pathology of Huntington's Disease

One of the major Ca²⁺ signaling pathways is store-operated Ca²⁺ entry (SOCE), which is responsible for Ca²⁺ flow into cells in response to the depletion of endoplasmic reticulum Ca²⁺ stores. SOCE and its molecular components, including stromal interaction molecule proteins, Orai Ca²⁺ channels, and transient receptor potential canonical channels, are involved in the physiology of neural stem cells and play a role in their proliferation, differentiation, and neurogenesis. This suggests that Ca²⁺ signaling is an important player in brain development. Huntington’s disease (HD) is an incurable neurodegenerative disorder that is caused by polyglutamine expansion in the huntingtin (HTT) protein, characterized by the loss of γ-aminobutyric acid (GABA)-ergic medium spiny neurons (MSNs) in the striatum. However, recent research has shown that HD is also a neurodevelopmental disorder and Ca²⁺ signaling is dysregulated in HD. The relationship between HD pathology and elevations of SOCE was demonstrated in different cellular and mouse models of HD and in induced pluripotent stem cell-based GABAergic MSNs from juvenile- and adult-onset HD patient fibroblasts. The present review discusses the role of SOCE in the physiology of neural stem cells and its dysregulation in HD pathology. It has been shown that elevated expression of STIM2 underlying the excessive Ca²⁺ entry through store-operated calcium channels in induced pluripotent stem cell-based MSNs from juvenile-onset HD. In the light of the latest findings regarding the role of Ca²⁺ signaling in HD pathology we also summarize recent progress in the in vitro differentiation of MSNs that derive from different cell sources. We discuss advances in the application of established protocols to obtain MSNs from fetal neural stem cells/progenitor cells, embryonic stem cells, induced pluripotent stem cells, and induced neural stem cells and the application of transdifferentiation. We also present recent progress in establishing HD brain organoids and their potential use for examining HD pathology and its treatment. Moreover, the significance of stem cell therapy to restore normal neural cell function, including Ca²⁺ signaling in the central nervous system in HD patients will be considered. The transplantation of MSNs or their precursors remains a promising treatment strategy for HD.

The difficulty to model Huntington's disease in vitro using striatal medium spiny neurons differentiated from human induced pluripotent stem cells

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by an expanded polyglutamine repeat in the huntingtin gene. The neuropathology of HD is characterized by the decline of a specific neuronal population within the brain, the striatal medium spiny neurons (MSNs). The origins of this extreme vulnerability remain unknown. Human induced pluripotent stem cell (hiPS cell)-derived MSNs represent a powerful tool to study this genetic disease. However, the differentiation protocols published so far show a high heterogeneity of neuronal populations in vitro. Here, we compared two previously published protocols to obtain hiPS cell-derived striatal neurons from both healthy donors and HD patients. Patch-clamp experiments, immunostaining and RT-qPCR were performed to characterize the neurons in culture. While the neurons were mature enough to fire action potentials, a majority failed to express markers typical for MSNs. Voltage-clamp experiments on voltage-gated sodium (Nav) channels revealed a large variability between the two differentiation protocols. Action potential analysis did not reveal changes induced by the HD mutation. This study attempts to demonstrate the current challenges in reproducing data of previously published differentiation protocols and in generating hiPS cell-derived striatal MSNs to model a genetic neurodegenerative disorder in vitro.

Ouabain-Induced Gene Expression Changes in Human iPSC-Derived Neuron Culture Expressing Dopamine and cAMP-Regulated Phosphoprotein 32 and GABA Receptors

Cardiotonic steroids (CTS) are specific inhibitors and endogenous ligands of a key enzyme in the CNS-the Na⁺, K⁺-ATPase, which maintains and creates an ion gradient on the plasma membrane of neurons. CTS cause the activation of various signaling cascades and changes in gene expression in neurons and other cell types. It is known that intracerebroventricular injection of cardiotonic steroid ouabain causes mania-like behavior in rodents, in part due to activation of dopamine-related signaling cascades in the dopamine and cAMP-regulated phosphoprotein 32 (DARPP-32) expressing medium spiny neurons in the striatum. Dopaminergic projections in the striatum innervate these GABAergic medium spiny neurons. The objective of this study was to assess changes in the expression of all genes in human iPSC-derived expressing DARPP-32 and GABA receptors neurons under the influence of ouabain. We noted a large number of statistically significant upregulated and downregulated genes after a 16-h incubation with non-toxic concentration (30 nM) of ouabain. These changes in the transcriptional activity were accomplished with activation of MAP-kinase ERK1/2 and transcriptional factor cAMP response element-binding protein (CREB). Thus, it can be concluded that 30 nM ouabain incubated for 16 h with human iPSC-derived expressing DARPP-32 and GABA receptors neurons activates genes associated with neuronal maturation and synapse formation, by increasing the expression of genes associated with translation, vesicular transport, and increased electron transport chain function. At the same time, the expression of genes associated with proliferation, migration, and early development of neurons decreases. These data indicate that non-toxic concentrations of ouabain may induce neuronal maturation, neurite growth, and increased synaptogenesis in dopamine-receptive GABAergic neurons, suggesting formation of plasticity and the establishment of new neuronal junctions.

Review: In vitro Cell Platform for Understanding Developmental Toxicity

Developmental toxicity and its affiliation to long-term health, particularly neurodegenerative disease (ND) has attracted significant attentions in recent years. There is, however, a significant gap in current models to track longitudinal changes arising from developmental toxicity. The advent of induced pluripotent stem cell (iPSC) derived neuronal culture has allowed for more complex and functionally active in vitro neuronal models. Coupled with recent progress in the detection of ND biomarkers, we are equipped with promising new tools to understand neurotoxicity arising from developmental exposure. This review provides a brief overview of current progress in neuronal culture derived from iPSC and in ND markers.

Human stem cell models of polyglutamine diseases: Sources for disease models and cell therapy

Polyglutamine (polyQ) diseases are a group of neurodegenerative disorders involving expanded CAG repeats in pathogenic genes that are translated into extended polyQ tracts and lead to progressive neuronal degeneration in the affected brain. To date, there is no effective therapy for these diseases. Due to the complex pathologic mechanisms of these diseases, intensive research on the pathogenesis of their progression and potential treatment strategies is being conducted. However, animal models cannot recapitulate all aspects of neuronal degeneration. Pluripotent stem cells (PSCs), such as induced pluripotent stem cells (iPSCs) and embryonic stem cells (ESCs), can be used to study the pathological mechanisms of polyQ diseases, and the ability of autologous stem cell transplantation to treat these diseases. Differentiated PSCs, neuronal precursor cells/neural progenitor cells (NPCs) and mesenchymal stem cells (MSCs) are valuable resources for preclinical and clinical cell transplantation therapies. Here, we discuss diverse stem cell models and their ability to generate neurons involved in polyQ diseases, such as medium spiny neurons (MSNs), cortical neurons, cerebellar Purkinje cells (PCs) and motor neurons. In addition, we discuss potential therapeutic approaches, including stem cell replacement therapy and gene therapy.

Expression of the Neural REST/NRSF-SIN3 Transcriptional Corepressor Complex as a Target for Small-Molecule Inhibitors

The repressor element 1 (RE1) silencing transcription factor/neuron-restrictive silencing factor (REST/NRSF) modulates the expression of genes with RE1/neuron-restrictive silencing element (RE1/NRSE) sites by recruiting the switch independent 3 (SIN3) factor and the REST corepressor (COREST) to its N and C-terminal repressor domain, respectively. Both, SIN3 and COREST assemble into protein complexes that are composed of multiple subunits including a druggable histone deacetylase (HDAC) enzyme. The SIN3 core complex comprises the eponymous proteins SIN3A or SIN3B, the catalytically active proteins HDAC1 or HDAC2, the histone chaperone retinoblastoma-associated protein 46/retinoblastoma-binding protein 7 (RBAP46/RBBP7) or RBAP48/RBBP4, the SIN3-associated protein 30 (SAP30), and the suppressor of defective silencing 3 (SDS3). Here, we overcome a bottleneck limiting the molecular characterization of the REST/NRSF-SIN3 transcriptional corepressor complex. To this end, SIN3 genes were amplified from the complementary DNA of neural stem/progenitor cells, and expressed in a baculovirus/insect cell expression system. We show that the isolates bind to DNA harboring RE1/NRSE sites and demonstrate that the histone deacetylase activity is blocked by small-molecule inhibitors. Thus, our isolates open up for future biomedical research on this critical transcriptional repressor complex and are envisioned as tool for drug testing.

Human Pluripotent Stem Cell-Derived Neurons Are Functionally Mature In Vitro and Integrate into the Mouse Striatum Following Transplantation

Human pluripotent stem cells (hPSCs) are a powerful tool for modelling human development. In recent years, hPSCs have become central in cell-based therapies for neurodegenerative diseases given their potential to replace affected neurons. However, directing hPSCs into specific neuronal types is complex and requires an accurate protocol that mimics endogenous neuronal development. Here we describe step-by-step a fast feeder-free neuronal differentiation protocol to direct hPSCs to mature forebrain neurons in 37 days in vitro (DIV). The protocol is based upon a combination of specific morphogens, trophic and growth factors, ions, neurotransmitters and extracellular matrix elements. A human-induced PSC line (Ctr-Q33) and a human embryonic stem cell line (GEN-Q18) were used to reinforce the potential of the protocol. Neuronal activity was analysed by single-cell calcium imaging. At 8 DIV, we obtained a homogeneous population of hPSC-derived neuroectodermal progenitors which self-arranged in bi-dimensional neural tube-like structures. At 16 DIV, we generated hPSC-derived neural progenitor cells (NPCs) with mostly a subpallial identity along with a subpopulation of pallial NPCs. Terminal in vitro neuronal differentiation was confirmed by the expression of microtubule associated protein 2b (Map 2b) by almost 100% of hPSC-derived neurons and the expression of specific-striatal neuronal markers including GABA, CTIP2 and DARPP-32. HPSC-derived neurons showed mature and functional phenotypes as they expressed synaptic markers, voltage-gated ion channels and neurotransmitter receptors. Neurons displayed diverse spontaneous activity patterns that were classified into three major groups, namely “high”, “intermediate” and “low” firing neurons. Finally, transplantation experiments showed that the NPCs survived and differentiated within mouse striatum for at least 3 months. NPCs integrated host environmental cues and differentiated into striatal medium-sized spiny neurons (MSNs), which successfully integrated into the endogenous circuitry without teratoma formation. Altogether, these findings demonstrate the potential of this robust human neuronal differentiation protocol, which will bring new opportunities for the study of human neurodevelopment and neurodegeneration, and will open new avenues in cell-based therapies, pharmacological studies and alternative in vitro toxicology.

Modeling Polyglutamine Expansion Diseases with Induced Pluripotent Stem Cells

Polyglutamine expansion disorders, which include Huntington's disease, have expanded CAG repeats that result in polyglutamine expansions in affected proteins. How this specific feature leads to distinct neuropathies in 11 different diseases is a fascinating area of investigation. Most proteins affected by polyglutamine expansions are ubiquitously expressed, yet their mechanisms of selective neurotoxicity are unknown. Induced pluripotent stem cells have emerged as a valuable tool to model diseases, understand molecular mechanisms, and generate relevant human neural and glia subtypes, cocultures, and organoids. Ideally, this tool will generate specific neuronal populations that faithfully recapitulate specific polyglutamine expansion disorder phenotypes and mimic the selective vulnerability of a given disease. Here, we review how induced pluripotent technology is used to understand the effects of the disease-causing polyglutamine protein on cell function, identify new therapeutic targets, and determine how polyglutamine expansion affects human neurodevelopment and disease. We will discuss ongoing challenges and limitations in our use of induced pluripotent stem cells to model polyglutamine expansion diseases.

Identification of protein targets for the antidepressant effects of Kai-Xin-San in Chinese medicine using isobaric tags for relative and absolute quantitation

Kai-Xin-San consists of Ginseng Radix, Polygalae Radix, Acori Tatarinowii Rhizoma, and Poria at a ratio of 3:3:2:2. Kai-Xin-San has been widely used for the treatment of emotional disorders in China. However, no studies have identified the key proteins implicated in response to Kai-Xin-San treatment. In this study, rat models of chronic mild stress were established using different stress methods over 28 days. After 14 days of stress stimulation, rats received daily intragastric administrations of 600 mg/kg Kai-Xin-San. The sucrose preference test was used to determine depression-like behavior in rats, while isobaric tags were used for relative and absolute quantitation-based proteomics to identify altered proteins following Kai-Xin-San treatment. Kai-Xin-San treatment for 2 weeks noticeably improved depression-like behaviors in rats with chronic mild stress. We identified 33 differentially expressed proteins: 7 were upregulated and 26 were downregulated. Functional analysis showed that these differentially expressed proteins participate in synaptic plasticity, neurodevelopment, and neurogenesis. Our results indicate that Kai-Xin-San has an important role in regulating the key node proteins in the synaptic signaling network, and are helpful to better understand the mechanism of the antidepressive effects of Kai-Xin-San and to provide objective theoretical support for its clinical application. The study was approved by the Ethics Committee for Animal Research from the Chinese PLA General Hospital (approval No. X5-2016-07) on March 5, 2016.

Synaptic dysfunction in neurodegenerative and neurodevelopmental diseases: an overview of induced pluripotent stem-cell-based disease models

Synaptic dysfunction in CNS disorders is the outcome of perturbations in physiological synapse structure and function, and can be either the cause or the consequence in specific pathologies. Accumulating data in the field of neuropsychiatric disorders, including autism spectrum disorders, schizophrenia and bipolar disorder, point to a neurodevelopmental origin of these pathologies. Due to a relatively early onset of behavioural and cognitive symptoms, it is generally acknowledged that mental illness initiates at the synapse level. On the other hand, synaptic dysfunction has been considered as an endpoint incident in neurodegenerative diseases, such as Alzheimer's, Parkinson's and Huntington's, mainly due to the considerably later onset of clinical symptoms and progressive appearance of cognitive deficits. This dichotomy has recently been challenged, particularly since the discovery of cell reprogramming technologies and the generation of induced pluripotent stem cells from patient somatic cells. The creation of 'disease-in-a-dish' models for multiple CNS pathologies has revealed unexpected commonalities in the molecular and cellular mechanisms operating in both developmental and degenerative conditions, most of which meet at the synapse level. In this review we discuss synaptic dysfunction in prototype neurodevelopmental and neurodegenerative diseases, emphasizing overlapping features of synaptopathy that have been suggested by studies using induced pluripotent stem-cell-based systems. These valuable disease models have highlighted a potential neurodevelopmental component in classical neurodegenerative diseases that is worth pursuing and investigating further. Moving from demonstration of correlation to understanding mechanistic causality forms the basis for developing novel therapeutics.

Conversion of adult human fibroblasts into neural precursor cells using chemically modified mRNA

Direct reprogramming offers a unique approach by which to generate neural lineages for the study and treatment of neurological disorders. Our objective is to develop a clinically viable reprogramming strategy to generate neural precursor cells for the treatment of neurological disorders through cell replacement therapy. We initially developed a method for directly generating neural precursor cells (iNPs) from adult human fibroblasts by transient expression of the neural transcription factors, SOX2 and PAX6 using plasmid DNA. This study advances these findings by examining the use of chemically modified mRNA (cmRNA) for direct-to-iNP reprogramming. Chemically modified mRNA has the benefit of being extremely stable and non-immunogenic, offering a clinically suitable gene delivery system. The use of SOX2 and PAX6 cmRNA resulted in high co-transfection efficiency and cell viability compared with plasmid transfection. Neural positioning and fate determinant genes were observed throughout reprogramming with ion channel and synaptic marker genes detected during differentiation. Differentiation of cmRNA-derived iNPs generated immature GABAergic or glutamatergic neuronal phenotypes in conjunction with astrocytes. This represents the first time a cmRNA approach has been used to directly reprogram adult human fibroblasts to iNPs, potentially providing an efficient system by which to generate human neurons for both research and clinical application.

Human cellular models of medium spiny neuron development and Huntington disease

The loss of gamma-aminobutyric acid (GABA)-ergic medium spiny neurons (MSNs) in the striatum is the hallmark of Huntington disease (HD), an incurable neurodegenerative disorder characterized by progressive motor, psychiatric, and cognitive symptoms. Transplantation of MSNs or their precursors represents a promising treatment strategy for HD. In initial clinical trials in which HD patients received fetal neurografts directly into the striatum without a pretransplant cell-differentiation step, some patients exhibited temporary benefits. Meanwhile, major challenges related to graft overgrowth, insufficient survival of grafted cells, and limited availability of donated fetal tissue remain. Thus, the development of approaches that allow modeling of MSN differentiation and HD development in cell culture platforms may improve our understanding of HD and translate, ultimately, into HD treatment options. Here, recent advances in the in vitro differentiation of MSNs derived from fetal neural stem cells/progenitor cells (NSCs/NPCs), embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), and induced NSCs (iNSCs) as well as advances in direct transdifferentiation are reviewed. Progress in non-allele specific and allele specific gene editing of HTT is presented as well. Cell characterization approaches involving phenotyping as well as in vitro and in vivo functional assays are also discussed.

Induced Pluripotent Stem Cells in Huntington's Disease Research: Progress and Opportunity

Induced pluripotent stem cells (iPSCs) derived from controls and patients can act as a starting point for in vitro differentiation into human brain cells for discovery of novel targets and treatments for human disease without the same ethical limitations posed by embryonic stem cells. Numerous groups have successfully produced and characterized Huntington’s disease (HD) iPSCs with different CAG repeat lengths, including cells from patients with one or two HD alleles. HD iPSCs and the neural cell types derived from them recapitulate some disease phenotypes found in both human patients and animal models. Although these discoveries are encouraging, the use of iPSCs for cutting edge and reproducible research has been limited due to some of the inherent problems with cell lines and the technological differences in the way laboratories use them. The goal of this review is to summarize the current state of the HD iPSC field, and to highlight some of the issues that need to be addressed to maximize their potential as research tools.

Stem cell transplantation for Huntington's diseases

Therapeutic approaches based on stem cells have received considerable attention as potential treatments for Huntington's disease (HD), which is a fatal, inherited neurodegenerative disorder, caused by progressive loss of GABAergic medium spiny neurons (MSNs) in the striatum of the forebrain. Transplantation of stem cells or their derivatives in animal models of HD, efficiently improved functions by replacing the damaged or lost neurons. In particular, neural stem cells (NSCs) for HD treatments have been developed from various sources, such as the brain itself, the pluripotent stem cells (PSCs), and the somatic cells of the HD patients. However, the brain-derived NSCs are difficult to obtain, and the PSCs have to be differentiated into a population of the desired neuronal cells that may cause a risk of tumor formation after transplantation. In contrast, induced NSCs, derived from somatic cells as a new stem cell source for transplantation, are less likely to form tumors. Given that the stem cell transplantation strategy for treatment of HD, as a genetic disease, is to replace the dysfunctional or lost neurons, the correction of mutant genes containing the expanded CAG repeats is essential. In this review, we will describe the methods for obtaining the optimal NSCs for transplantation-based HD treatment and the differentiation conditions for the functional GABAergic MSNs as therapeutic cells. Also, we will discuss the valuable gene correction of the disease stem cells by the CRISPR/Cas9 system for HD treatment.

Synergistic anti-cancer effects of epigenetic drugs on medulloblastoma cells

PURPOSE

Medulloblastomas are aggressive brain malignancies. While considerable progress has been made in the treatment of medulloblastoma patients with respect to overall survival, these patients are still at risk of developing neurologic and cognitive deficits as a result of anti-cancer therapies. It is hypothesized that targeted molecular therapies represent a better treatment option for medulloblastoma patients. Therefore, the aim of the present study was to test a panel of epigenetic drugs for their effect on medulloblastoma cells under mild hypoxic conditions that reflect the physiological concentrations of oxygen in the brain.

METHODS

Protein levels of histone deacetylase 1 (HDAC1) and DNA methyltransferase 1 (DNMT1) in medulloblastoma-derived cells (Daoy and D283 Med), as well as in developing and differentiated brain cells, were determined and compared. Class I and II histone deacetylase inhibitors (HDACi) and a DNMT inhibitor, 5-aza-2'-deoxycytidine (5-aza-dC), were applied to Daoy and D283 Med cells, and their effects were studied using viability, apoptosis and cancer sphere assays.

RESULTS

We found that in HDAC1 and DNMT1 overexpressing medulloblastoma-derived cells, cell death was induced under various epigenetic drug conditions tested. At low HDACi concentrations, however, a pro-proliferative effect was observed. Parthenolide, a drug that affects cancer stem cells, was found to be efficient in inducing cell death in both cell lines tested. In contrast, we found that Daoy cells were more resistant to 5-aza-dC than D283 Med cells. When suberoylanilide hydroxamic acid (SAHA) and parthenolide were individually applied to both cell lines in combination with 5-aza-dC, a synergistic effect on cell survival was observed.

CONCLUSIONS

Our current results suggest that the application of HDACi in combination with drugs that target DNMT may represent a promising option for the treatment of medulloblastoma.

Use of human stem cells in Huntington disease modeling and translational research

Huntington disease (HD) is a devastating neurological disorder caused by an extended CAG repeat in exon 1 of the gene that encodes the huntingtin (HTT) protein. HD pathology involves a loss of striatal medium spiny neurons (MSNs) and progressive neurodegeneration affects the striatum and other brain regions. Because HTT is involved in multiple cellular processes, the molecular mechanisms of HD pathogenesis should be investigated on multiple levels. On the cellular level, in vitro stem cell models, such as induced pluripotent stem cells (iPSCs) derived from HD patients and HD embryonic stem cells (ESCs), have yielded progress. Approaches to differentiate functional MSNs from ESCs, iPSCs, and neural stem/progenitor cells (NSCs/NPCs) have been established, enabling MSN differentiation to be studied and disease phenotypes to be recapitulated. Isolation of target stem cells and precursor cells may also provide a resource for grafting. In animal models, transplantation of striatal precursors differentiated in vitro to the striatum has been reported to improve disease phenotype. Initial clinical trials examining intrastriatal transplantation of fetal neural tissue suggest a more favorable clinical course in a subset of HD patients, though shortcomings persist. Here, we review recent advances in the development of cellular HD models and approaches aimed at cell regeneration with human stem cells. We also describe how genome editing tools could be used to correct the HTT mutation in patient-specific stem cells. Finally, we discuss the potential and the remaining challenges of stem cell-based approaches in HD research and therapy development.

iPSC-based drug screening for Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder, caused by an expansion of the CAG repeat in exon 1 of the huntingtin gene. The disease generally manifests in middle age with both physical and mental symptoms. There are no effective treatments or cures and death usually occurs 10-20 years after initial symptoms. Since the original identification of the Huntington disease associated gene, in 1993, a variety of models have been created and used to advance our understanding of HD. The most recent advances have utilized stem cell models derived from HD-patient induced pluripotent stem cells (iPSCs) offering a variety of screening and model options that were not previously available. The discovery and advancement of technology to make human iPSCs has allowed for a more thorough characterization of human HD on a cellular and developmental level. The interaction between the genome editing and the stem cell fields promises to further expand the variety of HD cellular models available for researchers. In this review, we will discuss the history of Huntington's disease models, common screening assays, currently available models and future directions for modeling HD using iPSCs-derived from HD patients. This article is part of a Special Issue entitled SI: PSC and the brain.