Cortical neurogenesis from pluripotent stem cells: complexity emerging from simplicity

From cells to insights: the power of human pluripotent stem cell-derived cortical interneurons in psychiatric disorder modeling

Psychiatric disorders, such as schizophrenia (SCZ) and autism spectrum disorders (ASD), represent a global health challenge with their poorly understood and complex etiologies. Cortical interneurons (cINs) are the primary inhibitory neurons in the cortex and their subtypes, especially those that are generated from the medial ganglionic emission (MGE) region, have been shown to play an important role in the pathogenesis of these psychiatric disorders. Recent advances in induced pluripotent stem cell (iPSC) technologies provide exciting opportunities to model and study these disorders using human iPSC-derived cINs. In this review, we present a comprehensive overview of various methods employed to generate MGE-type cINs from human iPSCs, which are mainly categorized into induction by signaling molecules vs. direct genetic manipulation. We discuss their advantages, limitations, and potential applications in psychiatric disorder modeling to aid researchers in choosing the appropriate methods based on their research goals. We also provide examples of how these methods have been applied to study the pathogenesis of psychiatric disorders. In addition, we discuss ongoing challenges and future directions in the field. Overall, iPSC-derived cINs provide a powerful tool to model the developmental pathogenesis of psychiatric disorders, thus aiding in uncovering disease mechanisms and potential therapeutic targets. This review article will provide valuable resources for researchers seeking to navigate the complexities of cIN generation methods and their applications in the study of psychiatric disorders.

Vesicular Glutamate Release from Feeder-FreehiPSC-Derived Neurons

Human-induced pluripotent stem cells (hiPSCs) represent one of the main and powerful tools for the in vitro modeling of neurological diseases. Standard hiPSC-based protocols make use of animal-derived feeder systems to better support the neuronal differentiation process. Despite their efficiency, such protocols may not be appropriate to dissect neuronal specific properties or to avoid interspecies contaminations, hindering their future translation into clinical and drug discovery approaches. In this work, we focused on the optimization of a reproducible protocol in feeder-free conditions able to generate functional glutamatergic neurons. This protocol is based on a generation of neuroprecursor cells differentiated into human neurons with the administration in the culture medium of specific neurotrophins in a Geltrex-coated substrate. We confirmed the efficiency of this protocol through molecular analysis (upregulation of neuronal markers and neurotransmitter receptors assessed by gene expression profiling and expression of the neuronal markers at the protein level), morphological analysis, and immunfluorescence detection of pre-synaptic and post-synaptic markers at synaptic boutons. The hiPSC-derived neurons acquired Ca²⁺-dependent glutamate release properties as a hallmark of neuronal maturation. In conclusion, our study describes a new methodological approach to achieve feeder-free neuronal differentiation from hiPSC and adds a new tool for functional characterization of hiPSC-derived neurons.

Cerebral Organoids as an Experimental Platform for Human Neurogenomics

The cerebral cortex forms early in development according to a series of heritable neurodevelopmental instructions. Despite deep evolutionary conservation of the cerebral cortex and its foundational six-layered architecture, significant variations in cortical size and folding can be found across mammals, including a disproportionate expansion of the prefrontal cortex in humans. Yet our mechanistic understanding of neurodevelopmental processes is derived overwhelmingly from rodent models, which fail to capture many human-enriched features of cortical development. With the advent of pluripotent stem cells and technologies for differentiating three-dimensional cultures of neural tissue in vitro, cerebral organoids have emerged as an experimental platform that recapitulates several hallmarks of human brain development. In this review, we discuss the merits and limitations of cerebral organoids as experimental models of the developing human brain. We highlight innovations in technology development that seek to increase its fidelity to brain development in vivo and discuss recent efforts to use cerebral organoids to study regeneration and brain evolution as well as to develop neurological and neuropsychiatric disease models.

Gradient biomimetic platforms for neurogenesis studies

There is a need for the development of new cellular therapies for the treatment of many diseases, with the central nervous system (CNS) currently an area of specific focus. Due to the complexity and delicacy of its biology, there is currently a limited understanding of neurogenesis and consequently a lack of reliable test platforms, resulting in several CNS based diseases having no cure. The ability to differentiate pluripotent stem cells into specific neuronal sub-types may enable scalable manufacture for clinical therapies, with a focus also on the purity and quality of the cell population. This focus is targeted towards an urgent need for the diseases that currently have no cure, e.g. Parkinson's disease. Differentiation studies carried out using traditional 2D cell culture techniques are designed using biological signals and morphogens known to be important for neurogenesis in vivo. However, such studies are limited by their simplistic nature, including a general poor efficiency and reproducibility, high reagent costs and an inability to scale-up the process to a manufacture-wide design for clinical use. Biomimetic approaches to recapitulate a more in vivo-like environment are progressing rapidly within this field, with application of bio(chemical) gradients presented both as 2D surfaces and within a 3D volume. This review focusses on the development and application of these advanced extracellular environments particularly for the neural niche. We emphasise the progress that has been made specifically in the area of stem cell derived neuronal differentiation. Increasing developments in biomaterial approaches to manufacture stem cells will enable the improvement of differentiation protocols, enhancing the efficiency and repeatability of the process with a move towards up-scaling. Progress in this area brings these techniques closer to enabling the development of therapies for the clinic.

Balancing serendipity and reproducibility: Pluripotent stem cells as experimental systems for intellectual and developmental disorders

Reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) and their differentiation into neural lineages is a revolutionary experimental system for studying neurological disorders, including intellectual and developmental disabilities (IDDs). However, issues related to variability and reproducibility have hindered translating preclinical findings into drug discovery. Here, we identify areas for improvement by conducting a comprehensive review of 58 research articles that utilized iPSC-derived neural cells to investigate genetically defined IDDs. Based upon these findings, we propose recommendations for best practices that can be adopted by research scientists as well as journal editors.

Transplanting neural progenitor cells to restore connectivity after spinal cord injury

Spinal cord injury remains a scientific and therapeutic challenge with great cost to individuals and society. The goal of research in this field is to find a means of restoring lost function. Recently we have seen considerable progress in understanding the injury process and the capacity of CNS neurons to regenerate, as well as innovations in stem cell biology. This presents an opportunity to develop effective transplantation strategies to provide new neural cells to promote the formation of new neuronal networks and functional connectivity. Past and ongoing clinical studies have demonstrated the safety of cell therapy, and preclinical research has used models of spinal cord injury to better elucidate the underlying mechanisms through which donor cells interact with the host and thus increase long-term efficacy. While a variety of cell therapies have been explored, we focus here on the use of neural progenitor cells obtained or derived from different sources to promote connectivity in sensory, motor and autonomic systems.

Transplanted Human Induced Pluripotent Stem Cell-Derived Neurons Wire and Fire with Balanced Excitation-Inhibition in Rat Cortex

Highlighted Research Paper:Neurons Derived from Human Induced Pluripotent Stem Cells Integrate into Rat Brain Circuits and Maintain Both Excitatory and Inhibitory Synaptic Activities. Xiling Yin, Jin-Chong Xu, Gun-sik Cho, Chulan Kwon, Ted M. Dawson and Valina L. Dawson.

Identification of FMR1-regulated molecular networks in human neurodevelopment

RNA-binding proteins (RNA-BPs) play critical roles in development and disease to regulate gene expression. However, genome-wide identification of their targets in primary human cells has been challenging. Here, we applied a modified CLIP-seq strategy to identify genome-wide targets of the FMRP translational regulator 1 (FMR1), a brain-enriched RNA-BP, whose deficiency leads to Fragile X Syndrome (FXS), the most prevalent inherited intellectual disability. We identified FMR1 targets in human dorsal and ventral forebrain neural progenitors and excitatory and inhibitory neurons differentiated from human pluripotent stem cells. In parallel, we measured the transcriptomes of the same four cell types upon FMR1 gene deletion. We discovered that FMR1 preferentially binds long transcripts in human neural cells. FMR1 targets include genes unique to human neural cells and associated with clinical phenotypes of FXS and autism. Integrative network analysis using graph diffusion and multitask clustering of FMR1 CLIP-seq and transcriptional targets reveals critical pathways regulated by FMR1 in human neural development. Our results demonstrate that FMR1 regulates a common set of targets among different neural cell types but also operates in a cell type-specific manner targeting distinct sets of genes in human excitatory and inhibitory neural progenitors and neurons. By defining molecular subnetworks and validating specific high-priority genes, we identify novel components of the FMR1 regulation program. Our results provide new insights into gene regulation by a critical neuronal RNA-BP in human neurodevelopment.

Xenotransplanted Human Cortical Neurons Reveal Species-Specific Development and Functional Integration into Mouse Visual Circuits

How neural circuits develop in the human brain has remained almost impossible to study at the neuronal level. Here, we investigate human cortical neuron development, plasticity, and function using a mouse/human chimera model in which xenotransplanted human cortical pyramidal neurons integrate as single cells into the mouse cortex. Combined neuronal tracing, electrophysiology, and in vivo structural and functional imaging of the transplanted cells reveal a coordinated developmental roadmap recapitulating key milestones of human cortical neuron development. The human neurons display a prolonged developmental timeline, indicating the neuron-intrinsic retention of juvenile properties as an important component of human brain neoteny. Following maturation, human neurons in the visual cortex display tuned, decorrelated responses to visual stimuli, like mouse neurons, demonstrating their capacity for physiological synaptic integration in host cortical circuits. These findings provide new insights into human neuronal development and open novel avenues for the study of human neuronal function and disease.

Video Abstract

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Cell-intrinsic mechanisms of human neoteny in mouse-human chimeric cerebral cortex

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Human neurons show prolonged maturation and single-cell integration in mouse cortex

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Stable dendritic spines and long-term synaptic plasticity in xenotransplanted neurons

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Human neurons show decorrelated activity and tuned responses to visual stimuli

Human Pluripotent Stem-Cell-Derived Cortical Neurons Integrate Functionally into the Lesioned Adult Murine Visual Cortex in an Area-Specific Way

The transplantation of pluripotent stem-cell-derived neurons constitutes a promising avenue for the treatment of several brain diseases. However, their potential for the repair of the cerebral cortex remains unclear, given its complexity and neuronal diversity. Here, we show that human visual cortical cells differentiated from embryonic stem cells can be transplanted and can integrate successfully into the lesioned mouse adult visual cortex. The transplanted human neurons expressed the appropriate repertoire of markers of six cortical layers, projected axons to specific visual cortical targets, and were synaptically active within the adult brain. Moreover, transplant maturation and integration were much less efficient following transplantation into the lesioned motor cortex, as previously observed for transplanted mouse cortical neurons. These data constitute an important milestone for the potential use of human PSC-derived cortical cells for the reassembly of cortical circuits and emphasize the importance of cortical areal identity for successful transplantation.

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Human PSC-derived cortical neurons efficiently integrate into the adult mouse brain

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PSC-derived human neurons reestablish axonal pathways in the lesioned adult cortex

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Restoration of cortical pathways requires a donor and recipient area-identity match

Stem Cells to Inform the Neurobiology of Mental Illness

The inception of human-induced pluripotent stem cell (hiPSCs) technology has provided an exciting platform upon which the modelling and treatment of human neurodevelopmental and neuropsychiatric disorders may be expedited. Although the genetic architecture of these disorders is far more complex than previously imagined, many key loci have at last been identified. This has allowed in vivo and in vitro technologies to be refined to model specific high-penetrant genetic loci involved in both disorders. Animal models of neurodevelopmental disorders, such as schizophrenia and autism spectrum disorders, show limitations in recapitulating the full complexity and heterogeneity of human neurodevelopmental disease states. Indeed, patient-derived hiPSCs offer distinct advantages over classical animal models in the study of human neuropathologies. Here we have discussed the current, relative translational merit of hiPSCs in investigating human neurodevelopmental and neuropsychiatric disorders with a specific emphasis on the utility of such systems to aid in the identification of biomarkers. We have highlighted the promises and pitfalls of reprogramming cell fate for the study of these disorders and provide recommendations for future directions in this field in order to overcome current limitations. Ultimately, this will aid in the development of effective clinical strategies for diverse patient populations affected by these disorders with the aim of also leading to biomarker identification.

Using mouse transgenic and human stem cell technologies to model genetic mutations associated with schizophrenia and autism

Solid progress has occurred over the last decade in our understanding of the molecular genetic basis of neurodevelopmental disorders, and of schizophrenia and autism in particular. Although the genetic architecture of both disorders is far more complex than previously imagined, many key loci have at last been identified. This has allowed in vivo and in vitro technologies to be refined to model specific high-penetrant genetic loci involved in both disorders. Using the DISC1/NDE1 and CYFIP1/EIF4E loci as exemplars, we explore the opportunities and challenges of using animal models and human-induced pluripotent stem cell technologies to further understand/treat and potentially reverse the worst consequences of these debilitating disorders.

This article is part of a discussion meeting issue ‘Of mice and mental health: facilitating dialogue between basic and clinical neuroscientists’.

Alzheimer's disease DNA methylome of pyramidal layers in frontal cortex: laser-assisted microdissection study

OBJECTIVE

To study DNA methylation patterns of cortical pyramidal layers susceptible to late-onset Alzheimer's disease (LOAD) neurodegeneration.

METHODS

Laser-assisted microdissection to select pyramidal layers' cells in frontal cortex of 32 human brains (18 LOAD) and Infinium DNA Methylation 450K analysis were performed to find differential methylated positions and regions, in addition to the corresponding gene set functional enrichment analyses.

RESULTS

Differential hypermethylation in several genomic regions and genes mainly in HOXA3, GSTP1, CXXC1-3 and BIN1. The functional enrichment analysis revealed genes significantly related to oxidative-stress and synapsis.

CONCLUSION

The present results indicate the differentially methylated genes related to neural projections, synapsis, oxidative stress and epigenetic regulator genes and represent the first epigenome of cortical pyramidal layers in LOAD.

Generation of a human induced pluripotent stem cell-based model for tauopathies combining three microtubule-associated protein TAU mutations which displays several phenotypes linked to neurodegeneration

INTRODUCTION

Tauopathies are neurodegenerative diseases characterized by TAU protein-related pathology, including frontotemporal dementia and Alzheimer's disease among others. Mutant TAU animal models are available, but none of them faithfully recapitulates human pathology and are not suitable for drug screening.

METHODS

To create a new in vitro tauopathy model, we generated a footprint-free triple MAPT-mutant human induced pluripotent stem cell line (N279K, P301L, and E10+16 mutations) using clustered regularly interspaced short palindromic repeats-FokI and piggyBac transposase technology.

RESULTS

Mutant neurons expressed pathogenic 4R and phosphorylated TAU, endogenously triggered TAU aggregation, and had increased electrophysiological activity. TAU-mutant cells presented deficiencies in neurite outgrowth, aberrant sequence of differentiation to cortical neurons, and a significant activation of stress response pathways. RNA sequencing confirmed stress activation, demonstrated a shift toward GABAergic identity, and an upregulation of neurodegenerative pathways.

DISCUSSION

In summary, we generated a novel in vitro human induced pluripotent stem cell TAU-mutant model displaying neurodegenerative disease phenotypes that could be used for disease modeling and drug screening.

Importin-8 Modulates Division of Apical Progenitors, Dendritogenesis and Tangential Migration During Development of Mouse Cortex

The building of the brain is a multistep process that requires the coordinate expression of thousands of genes and an intense nucleocytoplasmic transport of RNA and proteins. This transport is mediated by karyopherins that comprise importins and exportins. Here, we investigated the role of the ß-importin, importin-8 (IPO8) during mouse cerebral corticogenesis as several of its cargoes have been shown to be essential during this process. First, we showed that Ipo8 mRNA is expressed in mouse brain at various embryonic ages with a clear signal in the sub-ventricular/ventricular zone (SVZ/VZ), the cerebral cortical plate (CP) and the ganglionic eminences. We found that acute knockdown of IPO8 in cortical progenitors reduced both their proliferation and cell cycle exit leading to the increase in apical progenitor pool without influencing the number of basal progenitors (BPs). Projection neurons ultimately reached their appropriate cerebral cortical layer, but their dendritogenesis was specifically affected, resulting in neurons with reduced dendrite complexity. IPO8 knockdown also slowed the migration of cortical interneurons. Together, our data demonstrate that IPO8 contribute to the coordination of several critical steps of cerebral cortex development. These results suggest that the impairment of IPO8 function might be associated with some diseases of neuronal migration defects.

Three-Dimensional Organoid System Transplantation Technologies in Future Treatment of Central Nervous System Diseases

In recent years, scientists have made great achievements in understanding the development of human brain and elucidating critical elements of stepwise spatiotemporal control strategies in neural stem cell specification lineage, which facilitates successful induction of neural organoid in vitro including the cerebral cortex, cerebellar, neural tube, hippocampus cortex, pituitary, and optic cup. Besides, emerging researches on neural organogenesis promote the application of 3D organoid system transplantation in treating central nervous system (CNS) diseases. Present review will categorize current researches on organogenesis into three approaches: (a) stepwise, direct organization of region-specific or population-enriched neural organoid; (b) assemble and direct distinct organ-specific progenitor cells or stem cells to form specific morphogenesis organoid; and (c) assemble embryoid bodies for induction of multilayer organoid. However, the majority of these researches focus on elucidating cellular and molecular mechanisms involving in brain organogenesis or disease development and only a few of them conducted for treating diseases. In this work, we will compare three approaches and also analyze their possible indications for diseases in future treatment on the basis of their distinct characteristics.

New genes for accurate normalization of qRT-PCR results in study of iPS and iPS-derived cells

iPSC-derived cells (from induced pluripotent stem cells) are a useful source that provide a powerful and widely accepted tool for the study of various types of human cells in vitro. Indeed, iPSC-derived cells from patients with hereditary diseases have been shown to reproduce the hallmarks of these diseases in vitro, phenotypes that can then also be manipulated in vitro. Quantitative reverse transcription PCR (qRT-PCR) is often used to characterize the progress of iPSC differentiation, validate mature cell types and to determine levels of pathological markers. Quantitative reverse transcription PCR (qRT-PCR) is used to quantify mRNA levels. This method requires some way of normalizing the data, typically by relating the obtained levels of gene expression to the levels of expression of a "house keeping gene", a gene whose expression is presumed not to change during manipulation of the cells. In the literature, typically only one such reference gene is used and its stability of expression during cell manipulation is not demonstrated. We are not aware of any study systematically looking at the expression of such genes in human iPSC or during their differentiation into neurons. Here we compare the expression of 16 reference genes in iPSC, neural stem cells (NSC) and neurons derived from iPSC. The applications GeNorm and NormFinder were used to identify the most suitable reference genes. We showed that ACTb, C1orf43, PSMB4, GAPDH and HMBS have the most stable expression. The use of these reference genes allows an accurate normalization of qRT-PCR results in all the cell types discussed above. We hope that this report will help to enable the performance of proper qRT-PCR results normalization in studies with iPSC-derived cells and in disease-modeling reports.

Worms on the spectrum - C. elegans models in autism research

The small non-parasitic nematode Caenorhabditis elegans is widely used in neuroscience thanks to its well-understood development and lineage of the nervous system. Furthermore, C. elegans has been used to model many human developmental and neurological conditions to better understand disease mechanisms and identify potential therapeutic strategies. Autism spectrum disorder (ASD) is the most prevalent of all neurodevelopmental disorders, and the C. elegans system may provide opportunities to learn more about this complex disorder. Since basic cell biology and biochemistry of the C. elegans nervous system is generally very similar to mammals, cellular or molecular phenotypes can be investigated, along with a repertoire of behaviours. For instance, worms have contributed greatly to the understanding of mechanisms underlying mutations in genes coding for synaptic proteins such as neuroligin and neurexin. Using worms to model neurodevelopmental disorders like ASD is an emerging topic that harbours great, untapped potential. This review summarizes the numerous contributions of C. elegans to the field of neurodevelopment and introduces the nematode system as a potential research tool to study essential roles of genes associated with ASD.

Effects of antenatal glucocorticoids on the developing brain

Glucocorticoids (GCs) regulate distinct physiological processes in the developing fetus, in particular accelerating organ maturation that enables the fetus to survive outside the womb. In preterm birth, the developing fetus does not receive sufficient exposure to endogenous GCs in utero for proper organ development predisposing the neonate to complications including intraventricular hemorrhage, respiratory distress syndrome (RDS) and necrotizing enterocolitis (NEC). Synthetic GCs (sGCs) have proven useful in the prevention of these complications since they are able to promote the rapid maturation of underdeveloped organs present in the fetus. While these drugs have proven to be clinically effective in the prevention of IVH, RDS and NEC, they may also trigger adverse developmental side effects. This review will examine the current clinical use of antenatal sGC therapy in preterm birth, their placental metabolism, and their effects on the developing brain.

Building brains in a dish: Prospects for growing cerebral organoids from stem cells

The recent development of organoid techniques, in which embryonic brain-like tissue can be grown from human or mouse stem cells in vitro offers the potential to transform the way in which brain development is studied. In this review, we summarize key aspects of the embryonic development of mammalian forebrains, focussing in particular on the cerebral cortex and highlight significant differences between mouse and primates, including human. We discuss recent work using cerebral organoids that has revealed key similarities and differences between their development and that of the brain in vivo. Finally, we outline the ways in which cerebral organoids can be used in combination with CRISPR/Cas9 genome editing to unravel genetic mechanisms that control embryonic development of the cerebral cortex, how this can help us understand the causes of neurodevelopmental disorders and some of the key challenges which will have to be resolved before organoids can become a mainstream tool to study brain development.

Efficient Generation of Corticofugal Projection Neurons from Human Embryonic Stem Cells

Efforts to study development and function of corticofugal projection neurons (CfuPNs) in the human cerebral cortex for health and disease have been limited by the unavailability of highly enriched CfuPNs. Here, we develop a robust, two-step process for generating CfuPNs from human embryonic stem cells (hESCs): directed induction of neuroepithelial stem cells (NESCs) from hESCs and efficient differentiation of NESCs to about 80% of CfuPNs. NESCs or a NESC faithfully maintain unlimitedly self-renewal and self-organized abilities to develop into miniature neural tube-like structures. NESCs retain a stable propensity toward neuronal differentiation over culture as fate-restricted progenitors of CfuPNs and interneurons. When grafted into mouse brains, NESCs successfully integrate into the host brains, differentiate into CfuPNs and effectively reestablish specific patterns of subcortical projections and synapse structures. Efficient generation of CfuPNs in vitro and in vivo will facilitate human cortex development and offer sufficient CfuPNs for cell therapy.

Review of Brain-Machine Interfaces Used in Neural Prosthetics with New Perspective on Somatosensory Feedback through Method of Signal Breakdown

The brain-machine interface (BMI) used in neural prosthetics involves recording signals from neuron populations, decoding those signals using mathematical modeling algorithms, and translating the intended action into physical limb movement. Recently, somatosensory feedback has become the focus of many research groups given its ability in increased neural control by the patient and to provide a more natural sensation for the prosthetics. This process involves recording data from force sensitive locations on the prosthetics and encoding these signals to be sent to the brain in the form of electrical stimulation. Tactile sensation has been achieved through peripheral nerve stimulation and direct stimulation of the somatosensory cortex using intracortical microstimulation (ICMS). The initial focus of this paper is to review these principles and link them to modern day applications such as restoring limb use to those who lack such control. With regard to how far the research has come, a new perspective for the signal breakdown concludes the paper, offering ideas for more real somatosensory feedback using ICMS to stimulate particular sensations by differentiating touch sensors and filtering data based on unique frequencies.

Modeling the autistic cell: iPSCs recapitulate developmental principles of syndromic and nonsyndromic ASD

The opportunity to model autism spectrum disorders (ASD) through generation of patient-derived induced pluripotent stem cells (iPSCs) is currently an emerging topic. Wide-scale research of altered brain circuits in syndromic ASD, including Rett Syndrome, Fragile X Syndrome, Angelman's Syndrome and sporadic Schizophrenia, was made possible through animal models. However, possibly due to species differences, and to the possible contribution of epigenetics in the pathophysiology of these diseases, animal models fail to recapitulate many aspects of ASD. With the advent of iPSCs technology, 3D cultures of patient-derived cells are being used to study complex neuronal phenotypes, including both syndromic and nonsyndromic ASD. Here, we review recent advances in using iPSCs to study various aspects of the ASD neuropathology, with emphasis on the efforts to create in vitro model systems for syndromic and nonsyndromic ASD. We summarize the main cellular activity phenotypes and aberrant genetic interaction networks that were found in iPSC-derived neurons of syndromic and nonsyndromic autistic patients.

Is this a brain which I see before me? Modeling human neural development with pluripotent stem cells

The human brain is arguably the most complex structure among living organisms. However, the specific mechanisms leading to this complexity remain incompletely understood, primarily because of the poor experimental accessibility of the human embryonic brain. Over recent years, technologies based on pluripotent stem cells (PSCs) have been developed to generate neural cells of various types. While the translational potential of PSC technologies for disease modeling and/or cell replacement therapies is usually put forward as a rationale for their utility, they are also opening novel windows for direct observation and experimentation of the basic mechanisms of human brain development. PSC-based studies have revealed that a number of cardinal features of neural ontogenesis are remarkably conserved in human models, which can be studied in a reductionist fashion. They have also revealed species-specific features, which constitute attractive lines of investigation to elucidate the mechanisms underlying the development of the human brain, and its link with evolution.

A Brain in a Dish

Ohio State University researchers have made a leap forward in disease research by creating an eraser sized human "brain" in a petri dish1. Although lacking a circulatory system their brain model includes spinal cord, cortex, midbrain, brain stem, and even the beginnings of an eye- aiding in the effectiveness of research on complex neurological disease. To create their new brain model, the researchers converted adult skin cells into pluripotent stem cells, which afforded the opportunity to build the multiple nervous cell types required for such a complex system. Having this tissue model will assist researchers in developing new disease models, and thus, facilitate the development of novel clinical interventions.

Area-specific reestablishment of damaged circuits in the adult cerebral cortex by cortical neurons derived from mouse embryonic stem cells

Pluripotent stem-cell-derived neurons constitute an attractive source for replacement therapies, but their utility remains unclear for cortical diseases. Here, we show that neurons of visual cortex identity, differentiated in vitro from mouse embryonic stem cells (ESCs), can be transplanted successfully following a lesion of the adult mouse visual cortex. Reestablishment of the damaged pathways included long-range and reciprocal axonal projections and synaptic connections with targets of the damaged cortex. Electrophysiological recordings revealed that some grafted neurons were functional and responsive to visual stimuli. No significant integration was observed following grafting of the same neurons in motor cortex, or transplantation of embryonic motor cortex in visual cortex, indicating that successful transplantation required a match in the areal identity of grafted and lesioned neurons. These findings demonstrate that transplantation of mouse ESC-derived neurons of appropriate cortical areal identity can contribute to the reconstruction of an adult damaged cortical circuit.

[Inducing brain regeneration from within: in vivo reprogramming of endogenous somatic cells into neurons]

In order to overcome the quasi-total inability of the mammalian central nervous system to regenerate in response to injuries, and in parallel to the studies dedicated to prevent neuronal loss under these circumstances, alternative approaches based on the programming of pluripotent cells or the reprogramming of somatic cells into neurons have recently emerged. These uniquely combine growing knowledge of the mechanisms that underlie neurogenesis and neuronal specification during development to the most recent findings of the molecular and epigenetic mechanisms that govern the acquisition and maintenance of cellular identity. Here, we discuss the possibility to instruct the regeneration of the central nervous system from within for therapeutic purposes, in light of the recent works reporting on the generation of neurons by direct conversion of various cerebral cell types in vitro and in vivo.