Retinoid-independent motor neurogenesis from human embryonic stem cells reveals a medial columnar ground state

Generation of human iPSC-derived phrenic-like motor neurons to model respiratory motor neuron degeneration in ALS

The fatal motor neuron (MN) disease Amyotrophic Lateral Sclerosis (ALS) is characterized by progressive MN degeneration. Phrenic MNs (phMNs) controlling the activity of the diaphragm are prone to degeneration in ALS, leading to death by respiratory failure. Understanding of the mechanisms of phMN degeneration in ALS is limited, mainly because human experimental models to study phMNs are lacking. Here we describe a method enabling the derivation of phrenic-like MNs from human iPSCs (hiPSC-phMNs) within 30 days. This protocol uses an optimized combination of small molecules followed by cell-sorting based on a cell-surface protein enriched in hiPSC-phMNs, and is highly reproducible using several hiPSC lines. We show further that hiPSC-phMNs harbouring ALS-associated amplification of the C9orf72 gene progressively lose their electrophysiological activity and undergo increased death compared to isogenic controls. These studies establish a previously unavailable protocol to generate human phMNs offering a disease-relevant system to study mechanisms of respiratory MN dysfunction.

Cell Transplantation for Repair of the Spinal Cord and Prospects for Generating Region-Specific Exogenic Neuronal Cells

Spinal cord injury (SCI) is associated with currently irreversible consequences in several functional components of the central nervous system. Despite the severity of injury, there remains no approved treatment to restore function. However, with a growing number of preclinical studies and clinical trials, cell transplantation has gained significant potential as a treatment for SCI. Researchers have identified several cell types as potential candidates for transplantation. To optimize successful functional outcomes after transplantation, one key factor concerns generating neuronal cells with regional and subtype specificity, thus calling on the developmental transcriptome patterning of spinal cord cells. A potential source of spinal cord cells for transplantation is the generation of exogenic neuronal progenitor cells via the emerging technologies of gene editing and blastocyst complementation. This review highlights the use of cell transplantation to treat SCI in the context of relevant developmental gene expression patterns useful for producing regionally specific exogenic spinal cells via in vitro differentiation and blastocyst complementation.

A novel protocol to derive cervical motor neurons from induced pluripotent stem cells for amyotrophic lateral sclerosis

Sporadic amyotrophic lateral sclerosis (sALS) is the majority of ALS, and the lack of appropriate disease models has hindered its research. Induced pluripotent stem cell (iPSC) technology now permits derivation of iPSCs from somatic cells of sALS patients to investigate disease phenotypes and mechanisms. Most existing differentiation protocols are time-consuming or low efficient in generating motor neurons (MNs). Here we report a rapid and simple protocol to differentiate MNs in monolayer culture using small molecules, which led to nearly pure neural stem cells in 6 days, robust OLIG2+ pMNs (73%-91%) in 12 days, enriched CHAT+ cervical spinal MNs (sMNs) (88%-97%) in 18 days, and functionally mature sMNs in 28 days. This simple and reproducible protocol permitted the identification of hyperexcitability phenotypes in our sALS iPSC-derived sMNs, and its application in neurodegenerative diseases should facilitate in vitro disease modeling, drug screening, and the development of cell therapy.

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.

Spinal Cord Organoids to Study Motor Neuron Development and Disease

Motor neuron diseases (MNDs) are a heterogeneous group of disorders that affect the cranial and/or spinal motor neurons (spMNs), spinal sensory neurons and the muscular system. Although they have been investigated for decades, we still lack a comprehensive understanding of the underlying molecular mechanisms; and therefore, efficacious therapies are scarce. Model organisms and relatively simple two-dimensional cell culture systems have been instrumental in our current knowledge of neuromuscular disease pathology; however, in the recent years, human 3D in vitro models have transformed the disease-modeling landscape. While cerebral organoids have been pursued the most, interest in spinal cord organoids (SCOs) is now also increasing. Pluripotent stem cell (PSC)-based protocols to generate SpC-like structures, sometimes including the adjacent mesoderm and derived skeletal muscle, are constantly being refined and applied to study early human neuromuscular development and disease. In this review, we outline the evolution of human PSC-derived models for generating spMN and recapitulating SpC development. We also discuss how these models have been applied to exploring the basis of human neurodevelopmental and neurodegenerative diseases. Finally, we provide an overview of the main challenges to overcome in order to generate more physiologically relevant human SpC models and propose some exciting new perspectives.

Generation of functional posterior spinal motor neurons from hPSCs-derived human spinal cord neural progenitor cells

Spinal motor neurons deficiency results in a series of devastating disorders such as amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA) and spinal cord injury (SCI). These disorders are currently incurable, while human pluripotent stem cells (hPSCs)-derived spinal motor neurons are promising but suffered from inappropriate regional identity and functional immaturity for the study and treatment of posterior spinal cord related injuries. In this study, we have established human spinal cord neural progenitor cells (hSCNPCs) via hPSCs differentiated neuromesodermal progenitors (NMPs) and demonstrated the hSCNPCs can be continuously expanded up to 40 passages. hSCNPCs can be rapidly differentiated into posterior spinal motor neurons with high efficiency. The functional maturity has been examined in detail. Moreover, a co-culture scheme which is compatible for both neural and muscular differentiation is developed to mimic the neuromuscular junction (NMJ) formation in vitro. Together, these studies highlight the potential avenues for generating clinically relevant spinal motor neurons and modeling neuromuscular diseases through our defined hSCNPCs.

Efficient generation of lower induced motor neurons by coupling Ngn2 expression with developmental cues

Human pluripotent stem cells (hPSCs) are a powerful tool for disease modeling of hard-to-access tissues (such as the brain). Current protocols either direct neuronal differentiation with small molecules or use transcription-factor-mediated programming. In this study, we couple overexpression of transcription factor Neurogenin2 (Ngn2) with small molecule patterning to differentiate hPSCs into lower induced motor neurons (liMoNes/liMNs). This approach induces canonical MN markers including MN-specific Hb9/MNX1 in more than 95% of cells. liMNs resemble bona fide hPSC-derived MN, exhibit spontaneous electrical activity, express synaptic markers, and can contact muscle cells in vitro. Pooled, multiplexed single-cell RNA sequencing on 50 hPSC lines reveals reproducible populations of distinct subtypes of cervical and brachial MNs that resemble their in vivo, embryonic counterparts. Combining small molecule patterning with Ngn2 overexpression facilitates high-yield, reproducible production of disease-relevant MN subtypes, which is fundamental in propelling our knowledge of MN biology and its disruption in disease.

Progress and challenges in directing the differentiation of human iPSCs into spinal motor neurons

Motor neuron (MN) diseases, including amyotrophic lateral sclerosis, progressive bulbar palsy, primary lateral sclerosis and spinal muscular atrophy, cause progressive paralysis and, in many cases, death. A better understanding of the molecular mechanisms of pathogenesis is urgently needed to identify more effective therapies. However, studying MNs has been extremely difficult because they are inaccessible in the spinal cord. Induced pluripotent stem cells (iPSCs) can generate a theoretically limitless number of MNs from a specific patient, making them powerful tools for studying MN diseases. However, to reach their potential, iPSCs need to be directed to efficiently differentiate into functional MNs. Here, we review the reported differentiation protocols for spinal MNs, including induction with small molecules, expression of lineage-specific transcription factors, 2-dimensional and 3-dimensional cultures, as well as the implementation of microfluidics devices and co-cultures with other cell types, including skeletal muscle. We will summarize the advantages and disadvantages of each strategy. In addition, we will provide insights into how to address some of the remaining challenges, including reproducibly obtaining mature and aged MNs.

Spinal dI4 Interneuron Differentiation From Human Pluripotent Stem Cells

Spinal interneurons (INs) form intricate local networks in the spinal cord and regulate not only the ascending and descending nerve transduction but also the central pattern generator function. They are therefore potential therapeutic targets in spinal cord injury and diseases. In this study, we devised a reproducible protocol to differentiate human pluripotent stem cells (hPSCs) from enriched spinal dI4 inhibitory GABAergic INs. The protocol is designed based on developmental principles and optimized by using small molecules to maximize its reproducibility. The protocol comprises induction of neuroepithelia, patterning of neuroepithelia to dorsal spinal progenitors, expansion of the progenitors in suspension, and finally differentiation into mature neurons. In particular, we employed both morphogen activators and inhibitors to restrict or “squeeze” the progenitor fate during the stage of neural patterning. We use retinoic acid (RA) which ventralizes cells up to the mid-dorsal region, with cyclopamine (CYC), an SHH inhibitor, to antagonize the ventralization effect of RA, yielding highly enriched dI4 progenitors (90% Ptf1a⁺, 90.7% Ascl1⁺). The ability to generate enriched spinal dI4 GABAergicINs will likely facilitate the study of human spinal IN development and regenerative therapies for traumatic injuries and diseases of the spinal cord.

Human stem cell models of neurodegeneration: From basic science of amyotrophic lateral sclerosis to clinical translation

Neurodegenerative diseases are characterized by progressive cell loss leading to disruption of the structure and function of the central nervous system. Amyotrophic lateral sclerosis (ALS) was among the first of these disorders modeled in patient-specific iPSCs, and recent findings have translated into some of the earliest iPSC-inspired clinical trials. Focusing on ALS as an example, we evaluate the status of modeling neurodegenerative diseases using iPSCs, including methods for deriving and using disease-relevant neuronal and glial lineages. We further highlight the remaining challenges in exploiting the full potential of iPSC technology for understanding and potentially treating neurodegenerative diseases such as ALS.

Astrocyte-derived extracellular vesicles enhance the survival and electrophysiological function of human cortical neurons in vitro

Neurons derived from human induced pluripotent stem cells (hiPSCs) are powerful tools for modeling neural pathophysiology and preclinical efficacy/toxicity screening of novel therapeutic compounds. However, human neurons cultured in vitro typically do not fully recapitulate the physiology of the human nervous system, especially in terms of exhibiting morphological maturation, longevity, and electrochemical signaling ability comparable to that of adult human neurons. In this study, we investigated the potential for astrocyte-derived extracellular vesicles (EVs) to modulate survival and electrophysiological function of human neurons in vitro. Specifically, we demonstrate that EVs obtained from human astrocytes promote enhanced single cell electrophysiological function and anti-apoptotic behavior in a homogeneous population of human iPSC-derived cortical neurons. Furthermore, EV-proteomic analysis was performed to identify cargo proteins with the potential to promote the physiological enhancement observed. EV cargos were found to include neuroprotective proteins such as heat shock proteins, alpha-synuclein, and lipoprotein receptor-related protein 1 (LRP1), as well as apolipoprotein E (APOE), which negatively regulates neuronal apoptosis, and a peroxidasin homolog that supports neuronal oxidative stress management. Proteins that positively regulate neuronal excitability and synaptic development were also detected, such as potassium channel tetramerization domain containing 12 (KCTD12), glucose-6- phosphate dehydrogenase (G6PD), kinesin family member 5B (KIF5B), spectrin-alpha non-erythrocytic1 (SPTAN1). The remarkable improvements in electrophysiological function and evident inhibition of apoptotic signaling in cultured neurons exposed to these cargos may hold significance for improving preclinical in vitro screening modalities. In addition, our collected data highlight the potential for EV-based therapeutics as a potential class of future clinical treatment for tackling inveterate central and peripheral neuropathies.

Tissue-Engineered Skeletal Muscle Models to Study Muscle Function, Plasticity, and Disease

Skeletal muscle possesses remarkable plasticity that permits functional adaptations to a wide range of signals such as motor input, exercise, and disease. Small animal models have been pivotal in elucidating the molecular mechanisms regulating skeletal muscle adaptation and plasticity. However, these small animal models fail to accurately model human muscle disease resulting in poor clinical success of therapies. Here, we review the potential of in vitro three-dimensional tissue-engineered skeletal muscle models to study muscle function, plasticity, and disease. First, we discuss the generation and function of in vitro skeletal muscle models. We then discuss the genetic, neural, and hormonal factors regulating skeletal muscle fiber-type in vivo and the ability of current in vitro models to study muscle fiber-type regulation. We also evaluate the potential of these systems to be utilized in a patient-specific manner to accurately model and gain novel insights into diseases such as Duchenne muscular dystrophy (DMD) and volumetric muscle loss. We conclude with a discussion on future developments required for tissue-engineered skeletal muscle models to become more mature, biomimetic, and widely utilized for studying muscle physiology, disease, and clinical use.

Exploring the biological domain of the neural embryonic stem cell test (ESTn): Morphogenetic regulators, Hox genes and cell types, and their usefulness as biomarkers for embryotoxicity screening

Animal-free assessment of compound-induced developmental neurotoxicity will most likely be based on batteries of multiple in vitro tests. The optimal battery is built by combining tests with complementary biological domains that together ideally cover all relevant toxicity pathways. Thus, biological domain definition, i.e. which biological processes and cell types are represented, is an important assay characteristic for determining the place of assays in testing strategies. The murine neural embryonic stem cell test (ESTn) is employed to predict the developmental neurotoxicity of compounds. The aim of this study was to explore the biological domain of ESTn according to three groups of biomarker genes of early (neuro)development: morphogenetic regulators, Hox genes and cell type markers for the ectodermal and neural lineages. These biomarker groups were selected based on their crucial regulatory role in (neuro)development. Analysis of these genes in a series of previously generated whole transcriptome datasets of ESTn showed that at day 7 in culture cell differentiation resembled hindbrain/branchial/thoracic development between E6.5-E12.5 in vivo, with subsequent development into a mixed cell culture containing different neural subtypes, astrocytes and oligodendrocytes by day 13. In addition, the selected biomarkers showed common and distinct responses to compound exposure. Monitoring the biological domain of ESTn through gene expression patterns of morphogenetic regulators, Hox genes and cell type markers proved instrumental in providing mechanistic understanding of compound effects on neural differentiation in ESTn, and can aid in positioning of the test in a battery of complementary in vitro tests in integrated approaches to testing and assessment.

Generation of oligodendrocytes and establishment of an all-human myelinating platform from human pluripotent stem cells

Oligodendrocytes (OLs) are responsible for myelin production and metabolic support of neurons. Defects in OLs are crucial in several neurodegenerative diseases including multiple sclerosis (MS) and amyotrophic lateral sclerosis (ALS). This protocol describes a method to generate oligodendrocyte precursor cells (OPCs) from human pluripotent stem cells (hPSCs) in only ~20 d, which can subsequently myelinate neurons, both in vitro and in vivo. To date, OPCs have been derived from eight different hPSC lines including those derived from patients with spontaneous and familial forms of MS and ALS, respectively. hPSCs, fated for 8 d toward neural progenitors, are transduced with an inducible lentiviral vector encoding for SOX10. The addition of doxycycline for 10 d results in >60% of cells being O4-expressing OPCs, of which 20% co-express the mature OL marker myelin basic protein (MBP). The protocol also describes an alternative for viral transduction, by incorporating an inducible SOX10 in the safe harbor locus AAVS1, yielding ~100% pure OPCs. O4⁺ OPCs can be purified and either cryopreserved or used for functional studies. As an example of the type of functional study for which the derived cells could be used, O4⁺ cells can be co-cultured with maturing hPSC-derived neurons in 96/384-well-format plates, allowing the screening of pro-myelinating compounds.

Species-specific pace of development is associated with differences in protein stability

Although many molecular mechanisms controlling developmental processes are evolutionarily conserved, the speed at which the embryo develops can vary substantially between species. For example, the same genetic program, comprising sequential changes in transcriptional states, governs the differentiation of motor neurons in mouse and human, but the tempo at which it operates differs between species. Using in vitro directed differentiation of embryonic stem cells to motor neurons, we show that the program runs more than twice as fast in mouse as in human. This is not due to differences in signaling, nor the genomic sequence of genes or their regulatory elements. Instead, there is an approximately two-fold increase in protein stability and cell cycle duration in human cells compared with mouse cells. This can account for the slower pace of human development and suggests that differences in protein turnover play a role in interspecies differences in developmental tempo.

A Chemically Defined Common Medium for Culture of C2C12 Skeletal Muscle and Human Induced Pluripotent Stem Cell Derived Spinal Spheroids

Introduction

Multicellular platforms and linked multi organ on chip devices are powerful tools for drug discovery, and basic mechanistic studies. Often, a critical constraint is defining a culture medium optimal for all cells present in the system. In this study, we focused on the key cells of the neuromuscular junction i.e., skeletal muscle and motor neurons.

Methods

Formulation of a chemically defined medium for the co-culture of C2C12 skeletal muscle cells and human induced pluripotent stem cell (hiPSC) derived spinal spheroids (SpS) was optimized. C2C12 cells in 10 experimental media conditions and 2 topographies were evaluated over a 14-day maturation period to determine the ideal medium formulation for skeletal muscle tissue development.

Results

During early maturation, overexpression of genes for myogenesis and myopathy was observed for several media conditions, corresponding to muscle delamination and death. Together, we identified 3 media formulations that allowed for more controlled differentiation, healthier muscle tissue, and long-term culture duration. This evidence was then used to select media formulations to culture SpS and subsequently assessed axonal growth. As axonal growth in SpS cultures was comparable in all selected media conditions, our data suggest that the neuronal basal medium with no added supplements is the ideal medium formulation for both cell types.

Conclusions

Optimization using both topographical cues and culture media formulations provides a comprehensive analyses of culture conditions that are vital to future applications for in vitro NMJ models.

Characterization of Human iPSC-derived Spinal Motor Neurons by Single-cell RNA Sequencing

Human induced pluripotent stem cells (iPSCs) offer the opportunity to generate specific cell types from healthy and diseased individuals, allowing the study of mechanisms of early human development, modelling a variety of human diseases, and facilitating the development of new therapeutics. Human iPSC-based applications are often limited by the variability among iPSC lines originating from a single donor, as well as the heterogeneity among specific cell types that can be derived from iPSCs. The ability to deeply phenotype different iPSC-derived cell types is therefore of primary importance to the successful and informative application of this technology. Here we describe a combination of motor neuron (MN) derivation and single-cell RNA sequencing approaches to generate and characterize specific MN subtypes obtained from human iPSCs. Our studies provide evidence for rapid and robust generation of MN progenitor cells that can give rise to a heterogenous population of MNs. Approximately 58% of human iPSC-derived MNs display molecular characteristics of lateral motor column MNs, with a number of molecularly distinct subpopulations present within this MN group. Roughly 19% of induced MNs resemble hypaxial motor column MNs, while ∼6% of induced MNs have features of median motor column MNs. The present study has the potential to improve our understanding of iPSC-derived MN subtype function and dysfunction, possibly leading to improved iPSC-based applications for the study of human MN biology and disease.

Restoring Motor Neurons in Spinal Cord Injury With Induced Pluripotent Stem Cells

Spinal cord injury (SCI) is a devastating neurological disorder that damages motor, sensory, and autonomic pathways. Recent advances in stem cell therapy have allowed for the in vitro generation of motor neurons (MNs) showing electrophysiological and synaptic activity, expression of canonical MN biomarkers, and the ability to graft into spinal lesions. Clinical translation, especially the transplantation of MN precursors in spinal lesions, has thus far been elusive because of stem cell heterogeneity and protocol variability, as well as a hostile microenvironment such as inflammation and scarring, which yield inconsistent pre-clinical results without a consensus best-practice therapeutic strategy. Induced pluripotent stem cells (iPSCs) in particular have lower ethical and immunogenic concerns than other stem cells, which could make them more clinically applicable. In this review, we focus on the differentiation of iPSCs into neural precursors, MN progenitors, mature MNs, and MN subtype fates. Previous reviews have summarized MN development and differentiation, but an up-to-date summary of technological and experimental advances holding promise for bench-to-bedside translation, especially those targeting individual MN subtypes in SCI, is currently lacking. We discuss biological mechanisms of MN lineage, recent experimental protocols and techniques for MN differentiation from iPSCs, and transplantation of neural precursors and MN lineage cells in spinal cord lesions to restore motor function. We emphasize efficient, clinically safe, and personalized strategies for the application of MN and their subtypes as therapy in spinal lesions.

Rapid and efficient differentiation of functional motor neurons from human iPSC for neural injury modelling

Primary rodent neurons and immortalised cell lines have overwhelmingly been used for in vitro studies of traumatic injury to peripheral and central neurons, but have some limitations of physiological accuracy. Motor neurons (MN) derived from human induced pluripotent stem cells (iPSCs) enable the generation of cell models with features relevant to human physiology. To facilitate this, it is desirable that MN protocols both rapidly and efficiently differentiate human iPSCs into electrophysiologically active MNs. In this study, we present a simple, rapid protocol for differentiation of human iPSCs into functional spinal (lower) MNs, involving only adherent culture and use of small molecules for directed differentiation, with the ultimate aim of rapid production of electrophysiologically functional cells for short-term neural injury experiments. We show successful differentiation in two unrelated iPSC lines, by quantifying neural-specific marker expression, and by evaluating cell functionality at different maturation stages by calcium imaging and patch clamping. Differentiated neurons were shown to be electrophysiologically altered by uniaxial mechanical deformation. Spontaneous network activity decreased with applied stretch, indicating aberrant network connectivity. These results demonstrate the feasibility of this rapid, simple protocol for differentiating iPSC-derived MNs, suitable for in vitro neural injury studies focussing on electrophysiological alterations caused by mechanical deformation or trauma.

Priming of the Cells: Hypoxic Preconditioning for Stem Cell Therapy

Objective:

Stem cell-based therapies are promising in regenerative medicine for protecting and repairing damaged brain tissues after injury or in the context of chronic diseases. Hypoxia can induce physiological and pathological responses. A hypoxic insult might act as a double-edged sword, it induces cell death and brain damage, but on the other hand, sublethal hypoxia can trigger an adaptation response called hypoxic preconditioning or hypoxic tolerance that is of immense importance for the survival of cells and tissues.

Data Sources:

This review was based on articles published in PubMed databases up to August 16, 2017, with the following keywords: “stem cells,” “hypoxic preconditioning,” “ischemic preconditioning,” and “cell transplantation.”

Study Selection:

Original articles and critical reviews on the topics were selected.

Results:

Hypoxic preconditioning has been investigated as a primary endogenous protective mechanism and possible treatment against ischemic injuries. Many cellular and molecular mechanisms underlying the protective effects of hypoxic preconditioning have been identified.

Conclusions:

In cell transplantation therapy, hypoxic pretreatment of stem cells and neural progenitors markedly increases the survival and regenerative capabilities of these cells in the host environment, leading to enhanced therapeutic effects in various disease models. Regenerative treatments can mobilize endogenous stem cells for neurogenesis and angiogenesis in the adult brain. Furthermore, transplantation of stem cells/neural progenitors achieves therapeutic benefits via cell replacement and/or increased trophic support. Combinatorial approaches of cell-based therapy with additional strategies such as neuroprotective protocols, anti-inflammatory treatment, and rehabilitation therapy can significantly improve therapeutic benefits. In this review, we will discuss the recent progress regarding cell types and applications in regenerative medicine as well as future applications.

Current Advances and Limitations in Modeling ALS/FTD in a Dish Using Induced Pluripotent Stem Cells

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are two age-dependent multifactorial neurodegenerative disorders, which are typically characterized by the selective death of motor neurons and cerebral cortex neurons, respectively. These two diseases share many clinical, genetic and pathological aspects. During the past decade, cell reprogramming technologies enabled researchers to generate human induced pluripotent stem cells (iPSCs) from somatic cells. This resulted in the unique opportunity to obtain specific neuronal and non-neuronal cell types from patients which could be used for basic research. Moreover, these in vitro models can mimic not only the familial forms of ALS/FTD, but also sporadic cases without known genetic cause. At present, there have been extensive technical advances in the generation of iPSCs, as well as in the differentiation procedures to obtain iPSC-derived motor neurons, cortical neurons and non-neuronal cells. The major challenge at this moment is to determine whether these iPSC-derived cells show relevant phenotypes that recapitulate complex diseases. In this review, we will summarize the work related to iPSC models of ALS and FTD. In addition, we will discuss potential drawbacks and solutions for establishing more trustworthy iPSC models for both ALS and FTD.

Chronically stressed or stress-preconditioned neurons fail to maintain stress granule assembly

Dysregulation of stress granules (SGs) and their resident proteins contributes to pathogenesis of a number of (neuro)degenerative diseases. Phosphorylation of eIF2α is an event integrating different types of cellular stress and it is required for SG assembly. Phosphorylated eIF2α (p-eIF2α) is upregulated in the nervous system in some neurodegenerative conditions. We found that increasing p-eIF2α level by proteasomal inhibition in cultured cells, including mouse and human neurons, before a SG-inducing stress ('stress preconditioning'), limits their ability to maintain SG assembly. This is due to upregulation of PP1 phosphatase regulatory subunits GADD34 and/or CReP in preconditioned cells and early decline of p-eIF2α levels during subsequent acute stress. In two model systems with constitutively upregulated p-eIF2α, mouse embryonic fibroblasts lacking CReP and brain neurons of tau transgenic mice, SG formation was also impaired. Thus, neurons enduring chronic stress or primed by a transient mild stress fail to maintain p-eIF2α levels following subsequent acute stress, which would compromise protective function of SGs. Our findings provide experimental evidence on possible loss of function for SGs in certain neurodegenerative diseases.

Stem cells for spinal cord injury: Strategies to inform differentiation and transplantation

The complex pathology of spinal cord injury (SCI), involving a cascade of secondary events and the formation of inhibitory barriers, hampers regeneration across the lesion site and often results in irreversible loss of motor function. The limited regenerative capacity of endogenous cells after SCI has led to a focus on the development of cell therapies that can confer both neuroprotective and neuroregenerative benefits. Stem cells have emerged as a candidate cell source because of their ability to self-renew and differentiate into a multitude of specialized cell types. While ethical and safety concerns impeded the use of stem cells in the past, advances in isolation and differentiation methods have largely mitigated these issues. A confluence of work in stem cell biology, genetics, and developmental neurobiology has informed the directed differentiation of specific spinal cell types. After transplantation, these stem cell-derived populations can replace lost cells, provide trophic support, remyelinate surviving axons, and form relay circuits that contribute to functional recovery. Further refinement of stem cell differentiation and transplantation methods, including combinatorial strategies that involve biomaterial scaffolds and drug delivery, is critical as stem cell-based treatments enter clinical trials. Biotechnol. Bioeng. 2017;114: 245-259. © 2016 Wiley Periodicals, Inc.

Simple Derivation of Spinal Motor Neurons from ESCs/iPSCs Using Sendai Virus Vectors

Amyotrophic lateral sclerosis (ALS) is a progressive and fatal degenerative disorder of motor neurons (MNs). Embryonic stem cells (ESCs)/induced pluripotent stem cells (iPSCs) now help us to understand the pathomechanisms of ALS via disease modeling. Various methods to differentiate ESCs/iPSCs into MNs by the addition of signaling molecules have been reported. However, classical methods require multiple steps, and newer simple methods using the transduction of transcription factors run the risk of genomic integration of the vector genes. Heterogeneity of the expression levels of the transcription factors also remains an issue. Here we describe a novel approach for differentiating human and mouse ESCs/iPSCs into MNs using a single Sendai virus vector encoding three transcription factors, LIM/homeobox protein 3, neurogenin 2, and islet-1, which are integration free. This single-vector method, generating HB9-positive cells on day 2 from human iPSCs, increases the ratio of MNs to neurons compared to the use of three separate Sendai virus vectors. In addition, the MNs derived via this method from iPSCs of ALS patients and model mice display disease phenotypes. This simple approach significantly reduces the efforts required to generate MNs, and it provides a useful tool for disease modeling.

Creating Interactions between Tissue-Engineered Skeletal Muscle and the Peripheral Nervous System

Effective models of mammalian tissues must allow and encourage physiologically (mimetic) correct interactions between co-cultured cell types in order to produce culture microenvironments as similar as possible to those that would normally occur in vivo. In the case of skeletal muscle, the development of such a culture model, integrating multiple relevant cell types within a biomimetic scaffold, would be of significant benefit for investigations into the development, functional performance, and pathophysiology of skeletal muscle tissue. Although some work has been published regarding the behaviour of in vitro muscle models co-cultured with organotypic slices of CNS tissue or with stem cell-derived neurospheres, little investigation has so far been made regarding the potential to maintain isolated motor neurons within a 3D biomimetic skeletal muscle culture platform. Here, we review the current state of the art for engineering neuromuscular contacts in vitro and provide original data detailing the development of a 3D collagen-based model for the co-culture of primary muscle cells and motor neurons. The devised culture system promotes increased myoblast differentiation, forming arrays of parallel, aligned myotubes on which areas of nerve-muscle contact can be detected by immunostaining for pre- and post-synaptic proteins. Quantitative RT-PCR results indicate that motor neuron presence has a positive effect on myotube maturation, suggesting neural incorporation influences muscle development and maturation in vitro. The importance of this work is discussed in relation to other published neuromuscular co-culture platforms along with possible future directions for the field.

Adaptive regulation of the brain's antioxidant defences by neurons and astrocytes

The human brain generally remains structurally and functionally sound for many decades, despite the post-mitotic and non-regenerative nature of neurons. This is testament to the brain's profound capacity for homeostasis: both neurons and glia have in-built mechanisms that enable them to mount adaptive or protective responses to potentially challenging situations, ensuring that cellular viability and functionality is maintained. The high and variable metabolic and mitochondrial activity of neurons places several demands on the brain, including the task of neutralizing the associated reactive oxygen species (ROS) produced, to limit the accumulation of oxidative damage. Astrocytes play a key role in providing antioxidant support to nearby neurons, and redox regulation of the astrocytic Nrf2 pathway represents a powerful homeostatic regulator of the large cohort of Nrf2-regulated antioxidant genes that they express. In contrast, the Nrf2 pathway is weak in neurons, robbing them of this particular homeostatic device. However, many neuronal antioxidant genes are controlled by synaptic activity, enabling activity-dependent increases in ROS production to be offset by enhanced antioxidant capacity of both glutathione and thioredoxin-peroxiredoxin systems. These distinct homeostatic mechanisms in neurons and astrocytes together combine to promote neuronal resistance to oxidative insults. Future investigations into signaling between distinct cell types within the neuro-glial unit are likely to uncover further mechanisms underlying redox homeostasis in the brain.

Substrate Stiffness Modulates the Maturation of Human Pluripotent Stem-Cell-Derived Hepatocytes

Obtaining functional hepatocytes from human pluripotent stem cells (hPSCs) holds great potential for applications in drug safety testing, as well in the field of regenerative medicine. However, developing functionally mature hPSC-derived hepatocytes (hPSC-Heps) remains a challenge. We hypothesized that the cellular microenvironment plays a vital role in the maturation of immature hepatocytes. In this study, we examined the role of mechanical stiffness, a key component of the cellular microenvironment, in the maturation of hPSC-Heps. We cultured hPSC-Heps on collagen-coated polyacrylamide hydrogels with varying elastic moduli. On softer substrates the hPSC-Heps formed compact colonies while on stiffer substrates they formed a diffuse monolayer. We observed an inverse correlation between albumin production and substrate stiffness. The expression of key cytochrome enzymes, which are expressed at higher levels in the adult liver compared to the fetal liver, also correlated inversely with substrate stiffness, whereas fetal markers such as Cyp3A7 and AFP showed no correlation with stiffness. Culture of hPSC-Heps on soft substrates for 12 days led to 10-30 fold increases in the expression of drug-metabolizing enzymes. These results demonstrate that substrate stiffness similar to that of the liver enables aspects of the maturation of hPSC-Heps.

Modeling ALS with motor neurons derived from human induced pluripotent stem cells

Directing the differentiation of induced pluripotent stem cells into motor neurons has allowed investigators to develop new models of amyotrophic lateral sclerosis (ALS). However, techniques vary between laboratories and the cells do not appear to mature into fully functional adult motor neurons. Here we discuss common developmental principles of both lower and upper motor neuron development that have led to specific derivation techniques. We then suggest how these motor neurons may be matured further either through direct expression or administration of specific factors or coculture approaches with other tissues. Ultimately, through a greater understanding of motor neuron biology, it will be possible to establish more reliable models of ALS. These in turn will have a greater chance of validating new drugs that may be effective for the disease.

Plastin 3 is upregulated in iPSC-derived motoneurons from asymptomatic SMN1-deleted individuals

Spinal muscular atrophy (SMA) is a devastating motoneuron (MN) disorder caused by homozygous loss of SMN1. Rarely, SMN1-deleted individuals are fully asymptomatic despite carrying identical SMN2 copies as their SMA III-affected siblings suggesting protection by genetic modifiers other than SMN2. High plastin 3 (PLS3) expression has previously been found in lymphoblastoid cells but not in fibroblasts of asymptomatic compared to symptomatic siblings. To find out whether PLS3 is also upregulated in MNs of asymptomatic individuals and thus a convincing SMA protective modifier, we generated induced pluripotent stem cells (iPSCs) from fibroblasts of three asymptomatic and three SMA III-affected siblings from two families and compared these to iPSCs from a SMA I patient and control individuals. MNs were differentiated from iPSC-derived small molecule neural precursor cells (smNPCs). All four genotype classes showed similar capacity to differentiate into MNs at day 8. However, SMA I-derived MN survival was significantly decreased while SMA III- and asymptomatic-derived MN survival was moderately reduced compared to controls at day 27. SMN expression levels and concomitant gem numbers broadly matched SMN2 copy number distribution; SMA I presented the lowest levels, whereas SMA III and asymptomatic showed similar levels. In contrast, PLS3 was significantly upregulated in mixed MN cultures from asymptomatic individuals pinpointing a tissue-specific regulation. Evidence for strong PLS3 accumulation in shaft and rim of growth cones in MN cultures from asymptomatic individuals implies an important role in neuromuscular synapse formation and maintenance. These findings provide strong evidence that PLS3 is a genuine SMA protective modifier.

Quo vadis motor neuron disease?

Motor neuron disease (MND), also known as amyotrophic lateral sclerosis, is a relentlessly progressive neurodegenerative condition that is invariably fatal, usually within 3 to 5 years of diagnosis. The aetio-pathogenesis of MND remains unresolved and no effective treatments exist. The only Food and Drug Administration approved disease modifying therapy is riluzole, a glutamate antagonist, which prolongs survival by up to 3 mo. Current management is largely symptomatic/supportive. There is therefore a desperate and unmet clinical need for discovery of disease mechanisms to guide novel therapeutic strategy. In this review, we start by introducing the organizational anatomy of the motor system, before providing a clinical overview of its dysfunction specifically in MND. We then summarize insights gained from pathological, genetic and animal models and conclude by speculating on optimal strategies to drive the step change in discovery, which is so desperately needed in this arena.

Neural Conversion and Patterning of Human Pluripotent Stem Cells: A Developmental Perspective

Since the reprogramming of adult human terminally differentiated somatic cells into induced pluripotent stem cells (hiPSCs) became a reality in 2007, only eight years have passed. Yet over this relatively short period, myriad experiments have revolutionized previous stem cell dogmata. The tremendous promise of hiPSC technology for regenerative medicine has fuelled rising expectations from both the public and scientific communities alike. In order to effectively harness hiPSCs to uncover fundamental mechanisms of disease, it is imperative to first understand the developmental neurobiology underpinning their lineage restriction choices in order to predictably manipulate cell fate to desired derivatives. Significant progress in developmental biology provides an invaluable resource for rationalising directed differentiation of hiPSCs to cellular derivatives of the nervous system. In this paper we begin by reviewing core developmental concepts underlying neural induction in order to provide context for how such insights have guided reductionist in vitro models of neural conversion from hiPSCs. We then discuss early factors relevant in neural patterning, again drawing upon crucial knowledge gained from developmental neurobiological studies. We conclude by discussing open questions relating to these concepts and how their resolution might serve to strengthen the promise of pluripotent stem cells in regenerative medicine.

Generating Diverse Spinal Motor Neuron Subtypes from Human Pluripotent Stem Cells

Resolving the mechanisms underlying human neuronal diversification remains a major challenge in developmental and applied neurobiology. Motor neurons (MNs) represent a diverse pool of neuronal subtypes exhibiting differential vulnerability in different human neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA). The ability to predictably manipulate MN subtype lineage restriction from human pluripotent stem cells (PSCs) will form the essential basis to establishing accurate, clinically relevant in vitro disease models. I first overview motor neuron developmental biology to provide some context for reviewing recent studies interrogating pathways that influence the generation of MN diversity. I conclude that motor neurogenesis from PSCs provides a powerful reductionist model system to gain insight into the developmental logic of MN subtype diversification and serves more broadly as a leading exemplar of potential strategies to resolve the molecular basis of neuronal subclass differentiation within the nervous system. These studies will in turn permit greater mechanistic understanding of differential MN subtype vulnerability using in vitro human disease models.

Retinoic Acid-Mediated Regulation of GLI3 Enables Efficient Motoneuron Derivation from Human ESCs in the Absence of Extrinsic SHH Activation

The derivation of somatic motoneurons (MNs) from ES cells (ESCs) after exposure to sonic hedgehog (SHH) and retinoic acid (RA) is one of the best defined, directed differentiation strategies to specify fate in pluripotent lineages. In mouse ESCs, MN yield is particularly high after RA + SHH treatment, whereas human ESC (hESC) protocols have been generally less efficient. In an effort to optimize yield, we observe that functional MNs can be derived from hESCs at high efficiencies if treated with patterning molecules at very early differentiation steps before neural induction. Remarkably, under these conditions, equal numbers of human MNs were obtained in the presence or absence of SHH exposure. Using pharmacological and genetic strategies, we demonstrate that early RA treatment directs MN differentiation independently of extrinsic SHH activation by suppressing the induction of GLI3. We further demonstrate that neural induction triggers a switch from a poised to an active chromatin state at GLI3. Early RA treatment prevents this switch by direct binding of the RA receptor at the GLI3 promoter. Furthermore, GLI3 knock-out hESCs can bypass the requirement for early RA patterning to yield MNs efficiently. Our data demonstrate that RA-mediated suppression of GLI3 is sufficient to generate MNs in an SHH-independent manner and that temporal changes in exposure to patterning factors such as RA affect chromatin state and competency of hESC-derived lineages to adopt specific neuronal fates. Finally, our work presents a streamlined platform for the highly efficient derivation of human MNs from ESCs and induced pluripotent stem cells.

SIGNIFICANCE STATEMENT

Our study presents a rapid and efficient protocol to generate human motoneurons from embryonic and induced pluripotent stem cells. Surprisingly, and in contrast to previous work, motoneurons are generated in the presence of retinoic acid but in the absence of factors that activate sonic hedgehog signaling. We show that early exposure to retinoic acid modulates the chromatin state of cells to be permissive for motoneuron generation and directly suppresses the induction of GLI3, a negative regulator of SHH signaling. Therefore, our data point to a novel mechanism by which retinoic acid exposure can bypass the requirement for extrinsic SHH treatment during motoneuron induction.

Using human induced pluripotent stem cells to model cerebellar disease: hope and hype

The cerebellum forms a highly ordered and indispensible component of motor function within the adult neuraxis, consisting of several distinct cellular subtypes. Cerebellar disease, through a variety of genetic and acquired causes, results in the loss of function of defined subclasses of neurons, and remains a significant and untreatable health care burden. The scarcity of therapies in this arena can partially be explained by unresolved disease mechanisms due to inaccessibility of human cerebellar neurons in a relevant experimental context where initiating disease mechanisms could be functionally elucidated, or drug screens conducted. In this review we discuss the potential promise of human induced pluripotent stem cells (hiPSCs) for regenerative neurology, with a particular emphasis on in vitro modelling of cerebellar degeneration. We discuss progress made thus far using hiPSC-based models of neurodegeneration, noting the relatively slower pace of discovery made in modelling cerebellar dysfunction. We conclude by speculating how strategies attempting cerebellar differentiation from hiPSCs can be refined to allow the generation of accurate disease models. This in turn will permit a greater understanding of cerebellar pathophysiology to inform mechanistically rationalised therapies, which are desperately needed in this field.

Generation and isolation of oligodendrocyte progenitor cells from human pluripotent stem cells

In the CNS, oligodendrocytes act as the myelinating cells. Oligodendrocytes have been identified to be key players in several neurodegenerative disorders. This protocol describes a robust, fast and reproducible differentiation protocol to generate human oligodendrocytes from pluripotent stem cells (PSCs) using a chemically defined, growth factor-rich medium. Within 8 d, PSCs differentiate into paired box 6-positive (PAX6(+)) neural stem cells, which give rise to OLIG2(+) progenitors by day 12. Oligodendrocyte lineage transcription factor 2-positive (OLIG2(+)) cells begin to express the transcription factor NKX2.2 around day 18, followed by SRY-box 10 (SOX10) around day 40. Oligodendrocyte progenitor cells (OPCs) that are positive for the cell surface antigen recognized by the O4 antibody (O4(+)) appear around day 50 and reach, on average, 43% of the cell population after 75 d of differentiation. O4(+) OPCs can be isolated by cell sorting for myelination studies, or they can be terminally differentiated to myelin basic protein-positive (MBP(+)) oligodendrocytes. This protocol also describes an alternative strategy for markedly reducing the length and the costs of the differentiation and generating ∼30% O4(+) cells after only 55 d of culture.

Moving stem cells to the clinic: potential and limitations for brain repair

Stem cell-based therapies hold considerable promise for many currently devastating neurological disorders. Substantial progress has been made in the derivation of disease-relevant human donor cell populations. Behavioral data in relevant animal models of disease have demonstrated therapeutic efficacy for several cell-based approaches. Consequently, cGMP grade cell products are currently being developed for first in human clinical trials in select disorders. Despite the therapeutic promise, the presumed mechanism of action of donor cell populations often remains insufficiently validated. It depends greatly on the properties of the transplanted cell type and the underlying host pathology. Several new technologies have become available to probe mechanisms of action in real time and to manipulate in vivo cell function and integration to enhance therapeutic efficacy. Results from such studies generate crucial insight into the nature of brain repair that can be achieved today and push the boundaries of what may be possible in the future.

Foxp1-mediated programming of limb-innervating motor neurons from mouse and human embryonic stem cells

Spinal motor neurons (MNs) control diverse motor tasks including respiration, posture and locomotion that are disrupted by neurodegenerative diseases such as amyotrophic lateral sclerosis and spinal muscular atrophy. Methods directing MN differentiation from stem cells have been developed to enable disease modelling in vitro. However, most protocols produce only a limited subset of endogenous MN subtypes. Here we demonstrate that limb-innervating lateral motor column (LMC) MNs can be efficiently generated from mouse and human embryonic stem cells through manipulation of the transcription factor Foxp1. Foxp1-programmed MNs exhibit features of medial and lateral LMC MNs including expression of specific motor pool markers and axon guidance receptors. Importantly, they preferentially project axons towards limb muscle explants in vitro and distal limb muscles in vivo upon transplantation-hallmarks of bona fide LMC MNs. These results present an effective approach for generating specific MN populations from stem cells for studying MN development and disease.

Deterministic HOX patterning in human pluripotent stem cell-derived neuroectoderm

Colinear HOX expression during hindbrain and spinal cord development diversifies and assigns regional neural phenotypes to discrete rhombomeric and vertebral domains. Despite the precision of HOX patterning in vivo, in vitro approaches for differentiating human pluripotent stem cells (hPSCs) to posterior neural fates coarsely pattern HOX expression thereby generating cultures broadly specified to hindbrain or spinal cord regions. Here, we demonstrate that successive activation of fibroblast growth factor, Wnt/β-catenin, and growth differentiation factor signaling during hPSC differentiation generates stable, homogenous SOX2⁺/Brachyury⁺ neuromesoderm that exhibits progressive, full colinear HOX activation over 7 days. Switching to retinoic acid treatment at any point during this process halts colinear HOX activation and transitions the neuromesoderm into SOX2⁺/PAX6⁺ neuroectoderm with predictable, discrete HOX gene/protein profiles that can be further differentiated into region-specific cells, e.g., motor neurons. This fully defined approach significantly expands capabilities to derive regional neural phenotypes from diverse hindbrain and spinal cord domains.

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Deterministic HOX expression in hPSC-derived neuromesoderm progenitors (NMPs)

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Wnt/β-catenin, FGF, and GDF signaling regulate HOX activation in NMPs

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Retinoic acid (RA) transitions NMPs to neuroectoderm and halts HOX activation

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Neural cells can be patterned to any rostrocaudal hindbrain or spinal cord domain

Defined human pluripotent stem cell culture enables highly efficient neuroepithelium derivation without small molecule inhibitors

The embryonic neuroepithelium gives rise to the entire central nervous system in vivo, making it an important tissue for developmental studies and a prospective cell source for regenerative applications. Current protocols for deriving homogenous neuroepithelial cultures from human pluripotent stem cells (hPSCs) consist of either embryoid body-mediated neuralization followed by a manual isolation step or adherent differentiation using small molecule inhibitors. Here, we report that hPSCs maintained under chemically defined, feeder-independent, and xeno-free conditions can be directly differentiated into pure neuroepithelial cultures ([mt]90% Pax6(+)/N-cadherin(+) with widespread rosette formation) within 6 days under adherent conditions, without small molecule inhibitors, and using only minimalistic medium consisting of Dulbecco's modified Eagle's medium/F-12, sodium bicarbonate, selenium, ascorbic acid, transferrin, and insulin (i.e., E6 medium). Furthermore, we provide evidence that the defined culture conditions enable this high level of neural conversion in contrast to hPSCs maintained on mouse embryonic fibroblasts (MEFs). In addition, hPSCs previously maintained on MEFs could be rapidly converted to a neural compliant state upon transfer to these defined conditions while still maintaining their ability to generate all three germ layers. Overall, this fully defined and scalable protocol should be broadly useful for generating therapeutic neural cells for regenerative applications.

Potential for pharmacological manipulation of human embryonic stem cells

The therapeutic potential of human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) is vast, allowing disease modelling, drug discovery and testing and perhaps most importantly regenerative therapies. However, problems abound; techniques for cultivating self-renewing hESCs tend to give a heterogeneous population of self-renewing and partially differentiated cells and general include animal-derived products that can be cost-prohibitive for large-scale production, and effective lineage-specific differentiation protocols also still remain relatively undefined and are inefficient at producing large amounts of cells for therapeutic use. Furthermore, the mechanisms and signalling pathways that mediate pluripotency and differentiation are still to be fully appreciated. However, over the recent years, the development/discovery of a range of effective small molecule inhibitors/activators has had a huge impact in hESC biology. Large-scale screening techniques, coupled with greater knowledge of the pathways involved, have generated pharmacological agents that can boost hESC pluripotency/self-renewal and survival and has greatly increased the efficiency of various differentiation protocols, while also aiding the delineation of several important signalling pathways. Within this review, we hope to describe the current uses of small molecule inhibitors/activators in hESC biology and their potential uses in the future.

Efficient generation of myelinating oligodendrocytes from primary progressive multiple sclerosis patients by induced pluripotent stem cells

Multiple sclerosis (MS) is a chronic demyelinating disease of unknown etiology that affects the CNS. While current therapies are primarily directed against the immune system, the new challenge is to address progressive MS with remyelinating and neuroprotective strategies. Here, we develop a highly reproducible protocol to efficiently derive oligodendrocyte progenitor cells (OPCs) and mature oligodendrocytes from induced pluripotent stem cells (iPSCs). Key elements of our protocol include adherent cultures, dual SMAD inhibition, and addition of retinoids from the beginning of differentiation, which lead to increased yields of OLIG2 progenitors and high numbers of OPCs within 75 days. Furthermore, we show the generation of viral and integration-free iPSCs from primary progressive MS (PPMS) patients and their efficient differentiation to oligodendrocytes. PPMS OPCs are functional, as demonstrated by in vivo myelination in the shiverer mouse. These results provide encouraging advances toward the development of autologous cell therapies using iPSCs.

Hippo/YAP-mediated rigidity-dependent motor neuron differentiation of human pluripotent stem cells

Our understanding of the intrinsic mechanosensitive properties of human pluripotent stem cells (hPSCs), in particular the effects that the physical microenvironment has on their differentiation, remains elusive. Here, we show that neural induction and caudalization of hPSCs can be accelerated by using a synthetic microengineered substrate system consisting of poly(dimethylsiloxane) micropost arrays (PMAs) with tunable mechanical rigidities. The purity and yield of functional motor neurons derived from hPSCs within 23 days of culture using soft PMAs were improved more than fourfold and tenfold, respectively, compared with coverslips or rigid PMAs. Mechanistic studies revealed a multi-targeted mechanotransductive process involving Smad phosphorylation and nucleocytoplasmic shuttling, regulated by rigidity-dependent Hippo/YAP activities and actomyosin cytoskeleton integrity and contractility. Our findings suggest that substrate rigidity is an important biophysical cue influencing neural induction and subtype specification, and that microengineered substrates can thus serve as a promising platform for large-scale culture of hPSCs.

How to make spinal motor neurons

All muscle movements, including breathing, walking, and fine motor skills rely on the function of the spinal motor neuron to transmit signals from the brain to individual muscle groups. Loss of spinal motor neuron function underlies several neurological disorders for which treatment has been hampered by the inability to obtain sufficient quantities of primary motor neurons to perform mechanistic studies or drug screens. Progress towards overcoming this challenge has been achieved through the synthesis of developmental biology paradigms and advances in stem cell and reprogramming technology, which allow the production of motor neurons in vitro. In this Primer, we discuss how the logic of spinal motor neuron development has been applied to allow generation of motor neurons either from pluripotent stem cells by directed differentiation and transcriptional programming, or from somatic cells by direct lineage conversion. Finally, we discuss methods to evaluate the molecular and functional properties of motor neurons generated through each of these techniques.

Trophic and proliferative effects of Shh on motor neurons in embryonic spinal cord culture from wildtype and G93A SOD1 mice

Background

The developmental morphogen sonic hedgehog (Shh) may continue to play a trophic role in the support of terminally-differentiated motor neurons, of potential relevance to motor neuron disease. In addition, it may support the proliferation and differentiation of endogenous stem cells along motor neuronal lineages. As such, we have examined the trophic and proliferative effects of Shh supplementation or Shh antagonism in embryonic spinal cord cell cultures derived from wildtype or G93A SOD1 mice, a mouse model of amyotrophic lateral sclerosis.

Results

Shh supported survival, and stimulated growth of motor neurons, neurite outgrowth, and neurosphere formation in primary culture derived from both G93A SOD1 and WT mice. Shh increased the percentage of ciliated motor neurons, especially in G93A SOD1 culture. Shh-treated cultures showed increased neuronal proliferation compared to controls and especially cyclopamine treated cultures, from G93A SOD1 and WT mice. Moreover, Shh enhanced cell survival and differentiation of motor neuron precursors in WT culture.

Conclusions

Shh is neurotrophic to motor neurons and has mitogenic effects in WT and mSOD1 G93A culture in vitro.

Human stem cell-derived astrocytes and their application to studying Nrf2-mediated neuroprotective pathways and therapeutics in neurodegeneration

Glia, including astrocytes, are increasingly at the forefront of neurodegenerative research for their role in the modulation of neuronal function and survival. Improved understanding of underlying disease mechanisms, including the role of the cellular environment in neurodegeneration, is central to therapeutic development for these currently untreatable diseases. In these endeavours, experimental models that more closely reproduce the human condition have the potential to facilitate the transition between experimental studies in model organisms and patient trials. In this review we discuss the growing role of astrocytes in neurodegenerative diseases, and how astrocytes generated from human pluripotent stem cells represent a useful tool for analyzing astrocytic signalling and influence on neuronal function.

Adult bone marrow neural crest stem cells and mesenchymal stem cells are not able to replace lost neurons in acute MPTP-lesioned mice

Adult bone marrow stroma contains multipotent stem cells (BMSC) that are a mixed population of mesenchymal and neural-crest derived stem cells. Both cells are endowed with in vitro multi-lineage differentiation abilities, then constituting an attractive and easy-available source of material for cell therapy in neurological disorders. Whereas the in vivo integration and differentiation of BMSC in neurons into the central nervous system is currently matter of debate, we report here that once injected into the striatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice, pure populations of either bone marrow neural crest stem cells (NCSC) or mesenchymal stem cells (MSC) survived only transiently into the lesioned brain. Moreover, they do not migrate through the brain tissue, neither modify their initial phenotype, while no recovery of the dopaminergic system integrity was observed. Consequently, we tend to conclude that MSC/NCSC are not able to replace lost neurons in acute MPTP-lesioned dopaminergic system through a suitable integration and/or differentiation process. Altogether with recent data, it appears that neuroprotective, neurotrophic and anti-inflammatory features characterizing BMSC are of greater interest as regards CNS lesions management.

Dopamine from the brain promotes spinal motor neuron generation during development and adult regeneration

Coordinated development of brain stem and spinal target neurons is pivotal for the emergence of a precisely functioning locomotor system. Signals that match the development of these far-apart regions of the central nervous system may be redeployed during spinal cord regeneration. Here we show that descending dopaminergic projections from the brain promote motor neuron generation at the expense of V2 interneurons in the developing zebrafish spinal cord by activating the D4a receptor, which acts on the hedgehog pathway. Inhibiting this essential signal during early neurogenesis leads to a long-lasting reduction of motor neuron numbers and impaired motor responses of free-swimming larvae. Importantly, during successful spinal cord regeneration in adult zebrafish, endogenous dopamine promotes generation of spinal motor neurons, and dopamine agonists augment this process. Hence, we describe a supraspinal control mechanism for the development and regeneration of specific spinal cell types that uses dopamine as a signal.