Neuronal replacement therapy: previous achievements and challenges ahead

Adult Neurogenesis in the Context of Brain Repair and Functional Relevance

Urodeles and some fishes possess a remarkable capacity to regenerate their limbs/fins, a property that correlates with their additional ability to regenerate large areas of the brain and/or produce a variety of new neurons during adulthood. In contrast, neurogenesis in adult mammals is apparently restricted to two main regions, the subventricular zone of lateral ventricles and the subgranular zone of the hippocampus. There, astrocyte-like neural stem cells (NSCs) reside and derive into new neurons. Although it is becoming apparent that other brain regions carry out neurogenesis, in many cases, its functional significance is controversial, particularly, because very few putative NSCs capable of deriving into new neurons have been found. Hence, is renewal of certain neurons a requirement for a healthy brain? Are there specific physiological conditions that stimulate neurogenesis in a particular region? Does the complexity of the brain demand reduced neurogenesis? In this study, we review the production of new neurons in the vertebrate adult brain in the context of a possible functional relevance. In addition, we consider the intrinsic properties of potential cellular sources of new neurons, as well as the contribution of the milieu surrounding them to estimate the reparative capacity of the brain upon injury or a neurodegenerative condition. The conclusion of this review should bring into debate the potential and convenience of promoting neuronal regeneration in the adult human brain.

Transplanted Embryonic Neurons Improve Functional Recovery by Increasing Activity in Injured Cortical Circuits

Transplantation of appropriate neuronal precursors after injury is a promising strategy to reconstruct cortical circuits, but the efficiency of these approaches remains limited. Here, we applied targeted apoptosis to selectively ablate layer II/III pyramidal neurons in the rat juvenile cerebral cortex and attempted to replace lost neurons with their appropriate embryonic precursors by transplantation. We demonstrate that grafted precursors do not migrate to replace lost neurons but form vascularized clusters establishing reciprocal synaptic contacts with host networks and show functional integration. These heterotopic neuronal clusters significantly enhance the activity of the host circuits without causing epileptic seizures and attenuate the apoptotic injury-induced functional deficits in electrophysiological and behavioral tests. Chemogenetic activation of grafted neurons further improved functional recovery, and the persistence of the graft was necessary for maintaining restored functions in adult animals. Thus, implanting neuronal precursors capable to form synaptically integrated neuronal clusters combined with activation-based approaches represents a useful strategy for helping long-term functional recovery following brain injury.

Peptide-Based Scaffolds for the Culture and Transplantation of Human Dopaminergic Neurons

Cell replacement therapy is a promising treatment strategy for Parkinson's disease (PD); however, the poor survival rate of transplanted neurons is a critical barrier to functional recovery. In this study, we used self-assembling peptide nanofiber scaffolds (SAPNS) based on the peptide RADA16-I to support the in vitro maturation and in vivo post-transplantation survival of encapsulated human dopaminergic (DA) neurons derived from induced pluripotent stem cells. Neurons encapsulated within the SAPNS expressed mature neuronal and midbrain DA markers and demonstrated in vitro functional activity similar to neurons cultured in two dimensions. A microfluidic droplet generation method was used to encapsulate cells within monodisperse SAPNS microspheres, which were subsequently used to transplant adherent, functional networks of DA neurons into the striatum of a 6-hydroxydopamine-lesioned PD mouse model. SAPNS microspheres significantly increased the in vivo survival of encapsulated neurons compared with neurons transplanted in suspension, and they enabled significant recovery in motor function compared with control lesioned mice using approximately an order of magnitude fewer neurons than have been previously needed to demonstrate behavioral recovery. These results indicate that such biomaterial scaffolds can be used as neuronal transplantation vehicles to successfully improve the outcome of cell replacement therapies for PD. Impact Statement Transplantation of dopaminergic (DA) neurons holds potential as a treatment for Parkinson's disease (PD), but low survival rates of transplanted neurons is a barrier to successfully improving motor function. In this study, we used hydrogel scaffolds to transplant DA neurons into PD model mice. The hydrogel scaffolds enhanced survival of the transplanted neurons compared with neurons that were transplanted in a conventional manner, and they also improved recovery of motor function by using significantly fewer neurons than have typically been transplanted to see functional benefits. This cell transplantation technology has the capability to improve the outcome of neuron transplantation therapies.

Task Force Paper On Cerebellar Transplantation: Are We Ready to Treat Cerebellar Disorders with Cell Therapy?

Restoration of damaged central nervous system structures, functional recovery, and prevention of neuronal loss during neurodegenerative diseases are major objectives in cerebellar research. The highly organized anatomical structure of the cerebellum with numerous inputs/outputs, the complexity of cerebellar functions, and the large spectrum of cerebellar ataxias render therapies of cerebellar disorders highly challenging. There are currently several therapeutic approaches including motor rehabilitation, neuroprotective drugs, non-invasive cerebellar stimulation, molecularly based therapy targeting pathogenesis of the disease, and neurotransplantation. We discuss the goals and possible beneficial mechanisms of transplantation therapy for cerebellar damage and its limitations and factors determining outcome.

Transplanted interneurons improve memory precision after traumatic brain injury

Repair of the traumatically injured brain has been envisioned for decades, but regenerating new neurons at the site of brain injury has been challenging. We show GABAergic progenitors, derived from the embryonic medial ganglionic eminence, migrate long distances following transplantation into the hippocampus of adult mice with traumatic brain injury, functionally integrate as mature inhibitory interneurons and restore post-traumatic decreases in synaptic inhibition. Grafted animals had improvements in memory precision that were reversed by chemogenetic silencing of the transplanted neurons and a long-lasting reduction in spontaneous seizures. Our results reveal a striking ability of transplanted interneurons for incorporating into injured brain circuits, and this approach is a powerful therapeutic strategy for correcting post-traumatic memory and seizure disorders.

Inducing Different Neuronal Subtypes from Astrocytes in the Injured Mouse Cerebral Cortex

Astrocytes are particularly promising candidates for reprogramming into neurons, as they maintain some of the original patterning information from their radial glial ancestors. However, to which extent the position of astrocytes influences the fate of reprogrammed neurons remains unknown. To elucidate this, we performed stab wound injury covering an entire neocortical column, including the gray matter (GM) and white matter (WM), and targeted local reactive astrocytes via injecting FLEx switch (Cre-On) adeno-associated viral (AAV) vectors into mGFAP-Cre mice. Single proneural factors were not sufficient for adequate reprogramming, although their combination with the nuclear receptor-related 1 protein (Nurr1) improved reprogramming efficiency. Nurr1 and Neurogenin 2 (Ngn2) resulted in high-efficiency reprogramming of targeted astrocytes into neurons that develop lamina-specific hallmarks, including the appropriate long-distance axonal projections. Surprisingly, in the WM, we did not observe any reprogrammed neurons, thereby unveiling a crucial role of region- and layer-specific differences in astrocyte reprogramming.

Inhibition of Notch1 signaling promotes neuronal differentiation and improves functional recovery in spinal cord injury through suppressing the activation of Ras homolog family member A

Neural stem cells (NSCs) transplantation represents a promising strategy for the repair of injured neurons, since NSCs not only produce multiple neurotrophic growth factors but also differentiate into mature cells to replace damaged cells. Previous studies have shown that Notch signaling pathway had negative effects on neuronal differentiation; however, the precise mechanism remained inadequately understood. This research aimed to investigate whether inhibition of Notch1 signaling promotes neuronal differentiation and improves functional recovery in rat spinal cord injury through suppressing the activation of Ras homolog family member A (RhoA). QPCR, western blot, and immunofluorescence experiments were used to analyze Notch1 signaling pathways, RhoA, Ras homologous -associated coiled-coil containing protein kinase 1 (ROCK1), cleaved caspased-3, and neuronal/astrocytic differentiation markers. The expression of RhoA and ROCK1 was inhibited by lentivirus or specific biochemical inhibitors. In spinal cord injury (SCI), motor function was assessed by hind limbs movements and electrophysiology. Tissue repairing was measured by immunofluorescence, Nissl staining, Fluorogold, HE staining, QPCR, western blot, and magnetic resonance imaging (MRI) experiments. Our results demonstrate that inhibition of Notch1 in NSCs can promote the differentiation of NSCs to neurons. Knockdown of RhoA and inhibition of ROCK1 both can promote neuronal differentiation through inhibiting the activation of Notch1 signaling pathway in NSCs. In SCI, silencing RhoA enhanced neuronal differentiation and improved tissue repairing/functional recovery by inhibiting the activation of Notch1 signaling pathway. Since Notch1 inhibits neuronal differentiation through activating the RhoA/ROCK1 signaling pathway in NSCs, our data suggest that the Notch1/RhoA/ROCK1/Hes1/Hes5 signaling pathway may serve as a novel target for the treatment of SCI.

In Vitro Targeting and Imaging of Neurogenic Differentiation in Mouse Bone-Marrow Derived Mesenchymal Stem Cells with Superparamagnetic Iron Oxide Nanoparticles

Spinal cord injuries (SCI) are well thought to be a crucial issue that roots various side effects for a patient during their entire lifetime. Although therapeutical methods to resolve the SCI are limited, stem cell therapy is determined to be a resolving factor since it possesses the ability to induce the neurogenic differentiation and the paracrine effect. However, stem cells are difficult to inject directly into the lesion, so they must be carefully guided through the spinal canal. Therefore, superparamagnetic iron oxide nanoparticles (SPIONs) are introduced as an instigator that makes the cells respond to the applied magnetic field. This study intends to report the synthesis strategy to develop SPIONs that could be used to treat the injury site by an applied magnetic field. SPION-internalized D1 Mesenchymal stem cells (MSCs) are observed consistently using a confocal fluorescence microscope to analyze the toxicity, maintenance, and monitoring points of intracellular SPIONs. The prepared SPIONs are much anticipated to increase the migration efficiency using magnetism, which was not cytotoxic. Hence, the prepared SPIONs can adeptly target the damaged neural tissue to promote tissue regeneration and treat nervous system disorders. This primary study stands as a focal point to solve SCI by stem cell migration effectively.

CXCL12 promotes proliferation of radial glia like cells after traumatic brain injury in rats

To investigate the effect of CXCL12 on regeneration of radial glia like cells after traumatic brain injury (TBI). We randomly divided 48 rats into 4 groups: (1) the sham group, rats were performed craniotomy only, (2) the control group, saline were injected into the ipsilateral cortex after TBI, (3) the CXCL12 group, CXCL12 were injected, and (4) the CXCL12 + AMD3100 group, a mixture of CXCL12 and AMD3100 were injected. Seven days after TBI, the brain tissues were subjected to immunofluorescence double-labeled staining of BrdU/Nestin, BLBP/Nestin, BLBP/Vimentin, BLBP/SOX2, BLBP/CXCR4, BLBP/DCX. Western Blot assay was used to measure the levels of Nestin, BLBP, and Vimentin. Compared with the control group, CXCL12 treatment significantly increased the number of cells stained with BrdU/Nestin, BLBP/Nestin, and BLBP/Vimentin around the injured cortex and corpus callosum areas. CXCL12 + AMD3100 treatment significantly decreased the number of these cells compared with the CXCL12 treatment and control group. The protein levels of Nestin, BLBP, and Vimentin had the same change trends as those of the immunofluorescence staining. The BLBP/Vimentin positive cells presented with the astrocyte pattern around the injured cortex area but with the RGCs pattern around the injured corpus callosum area. The BLBP positive cells also expressed CXCR4 and SOX2. Altogether, CXCL12 promotes the proliferation of neural precursor cells after TBI by combing to its receptor, CXCR4. The proliferating neural precursor cells presents radial glial cell like cells. The RGCs-like cells can differentiate into immature neurons and promote the migration of immature neurons.

Neuronal Stem Cell and Drug Interactions: A Systematic Review and Meta-Analysis: Concise Review

Stem cell therapy is a promising treatment option for neurodegenerative diseases that mostly affect geriatric patients who often suffer from comorbidities requiring multiple medications. However, not much is known about the interactions between stem cells and drugs. Here, we focus on the potential interactions between drugs used to treat the comorbidities or sequelae of neurodegenerative diseases and neuronal stem cells to reveal potential effects on drug safety and efficacy. To determine the potential effects of drugs frequently used in geriatric patients (analgesic, antibiotic, antidepressant, antidiabetic, antihyperlipidemic, and antihypertensive drugs) on neuronal stem cell differentiation and proliferation, we systematically searched PubMed to identify nonreview articles published in English in peer-reviewed journals between January 1, 1991, and June 7, 2018. We identified 5,954 publications, of which 214 were included. Only 62 publications provided the complete data sets required for meta-analysis. We found that antidepressants stimulated neuronal stem cell proliferation but not differentiation under physiologic conditions and increased the proliferation of stem cells in the context of stress. Several other potential interactions were identified, but the limited number of available data sets precludes robust conclusions. Although available data were in most cases insufficient to perform robust meta-analysis, a clear interaction between antidepressants and neuronal stem cells was identified. We reveal other potential interactions requiring further experimental investigation. We recommend that future research addresses such interactions and investigates the best combination of pharmacological interventions and neuronal stem cell treatments for more efficient and safer patient care. Stem Cells Translational Medicine 2019;8:1202-1211.

Therapeutic effects of combined cell transplantation and locomotor training in rats with brain injury

Cell-based therapies are attracting attention as alternative therapeutic options for brain damage. In this study, we investigated the therapeutic effect of a combined therapy of cell transplantation and locomotor training by evaluating the neuronal connectivity. We transplanted neural cells derived from the frontal cortex of E14.5 GFP-expressing mice into the frontal lobe of 3-week-old rats with brain injury, followed by treadmill training (TMT) for 14 days. In the TMT(-) group, graft-derived neurites were observed only in the striatum and internal capsule. In contrast, in the TMT(+) group, they were observed in the striatum, internal capsule, and the cerebral peduncle and spinal cord. The length of the longest neurite was significantly longer in the TMT(+) group than in the TMT(-) group. In the TMT(+) group, Synaptophysin⁺ vesicles on the neuronal fibers around the ipsilateral red nucleus were found, suggesting that neuronal fibers from the grafted cells formed synapses with the host neurons. A functional analysis of motor recovery using the foot fault test showed that, 1 week after the transplantation, the recovery was significantly better in the cell transplantation and TMT group than the cell transplantation only group. The percentage of cells expressing C-FOS was increased in the grafts in the TMT(+) group. In conclusion, TMT promoted neurite extensions from the grafted neural cells, and the combined therapy of cell transplantation and locomotor training might have the potential to promote the functional recovery of rats with brain injury compared to cell transplantation alone.

Genetic reprogramming of somatic cells into neuroblasts through a co-induction of the doublecortin gene along the Yamanaka factors: A promising approach to model neuroregenerative disorders

Neural stem cell (NSC) mediated adult neurogenesis represents the regenerative plasticity of the brain. The functionality of the neurogenic process appears to be operated by neuroblasts, the multipotent immature neuronal population of the adult brain. While neuroblasts have been realized to play a major role in synaptic remodeling and immunogenicity, neurodegenerative disorders have been characterized by failure in the terminal differentiation, maturation, integration and survival of newborn neuroblasts. Advancement in understanding the impaired neuroregenerative process along the neuropathological conditions has currently been limited by lack of an appropriate experimental model of neuroblasts. The genetic reprogramming of somatic cells into pluripotent state offers a potential strategy for the experimental modeling of brain disorders. Thus, the induced pluripotent stem cell (iPSC) based direct reprogramming of somatic cells into neuroblasts would represent a potential tool to understand the regenerative biology of the adult brain. Therefore, this concise article discusses the significance of iPSCs, the functional roles of neuroblasts in the adult brain and provides a research hypothesis for the direct reprogramming of somatic cells into neuroblasts through the co-induction of a potential proneurogenic marker, the doublecortin (DCX) gene along with the Yamanaka factors. The proposed cellular model of adult neurogenesis may provide us with further insights into neuropathogenesis of many neurodegenerative disorders and will provide a potential experimental platform for diagnostic, drug discovery and regenerative therapeutic strategies.

Protein Kinase C: Targets to Regenerate Brain Injuries?

Acute or chronic injury to the central nervous system (CNS), causes neuronal death and irreversible cognitive deficits or sensory-motor alteration. Despite the capacity of the adult CNS to generate new neurons from neural stem cells (NSC), neuronal replacement following an injury is a restricted process, which does not naturally result in functional regeneration. Therefore, potentiating endogenous neurogenesis is one of the strategies that are currently being under study to regenerate damaged brain tissue. The insignificant neurogenesis that occurs in CNS injuries is a consequence of the gliogenic/non-neurogenic environment that inflammatory signaling molecules create within the injured area. The modification of the extracellular signals to generate a neurogenic environment would facilitate neuronal replacement. However, in order to generate this environment, it is necessary to unearth which molecules promote or impair neurogenesis to introduce the first and/or eliminate the latter. Specific isozymes of the protein kinase C (PKC) family differentially contribute to generate a gliogenic or neurogenic environment in injuries by regulating the ADAM17 mediated release of growth factor receptor ligands. Recent reports describe several non-tumorigenic diterpenes isolated from plants of the Euphorbia genus, which specifically modulate the activity of PKC isozymes promoting neurogenesis. Diterpenes with 12-deoxyphorbol or lathyrane skeleton, increase NPC proliferation in neurogenic niches in the adult mouse brain in a PKCβ dependent manner exerting their effects on transit amplifying cells, whereas PKC inhibition in injuries promotes neurogenesis. Thus, compounds that balance PKC activity in injuries might be of use in the development of new drugs and therapeutic strategies to regenerate brain injuries.

Neural tissue engineering with structured hydrogels in CNS models and therapies

The development of techniques to create and use multiphase microstructured hydrogels (granular hydrogels or microgels) has enabled the generation of cultures with more biologically relevant architecture and use of structured hydrogels is especially pertinent to the development of new types of central nervous system (CNS) culture models and therapies. We review material choice and the customisation of hydrogel structure, as well as the use of hydrogels in developmental models. Combining the use of structured hydrogel techniques with developmentally relevant tissue culture approaches will enable the generation of more relevant models and treatments to repair damaged CNS tissue architecture.

The role of neurogenesis in neurorepair after ischemic stroke

Stroke consists of an abrupt reduction of cerebral blood flow resulting in hypoxia that triggers an excitotoxicity, oxidative stress, and neuroinflammation. After the ischemic process, neural precursor cells present in the subventricular zone of the lateral ventricle and subgranular zone of the dentate gyrus proliferate and migrate towards the lesion, contributing to the brain repair. The neurogenesis is induced by signal transduction pathways, growth factors, attractive factors for neuroblasts, transcription factors, pro and anti-inflammatory mediators and specific neurotransmissions. However, this endogenous neurogenesis occurs slowly and does not allow a complete restoration of brain function. Despite that, understanding the mechanisms of neurogenesis could improve the therapeutic strategies for brain repair. This review presents the current knowledge about brain repair process after stroke and the perspectives regarding the development of promising therapies that aim to improve neurogenesis and its potential to form new neural networks.

Enduring Neuroprotective Effect of Subacute Neural Stem Cell Transplantation After Penetrating TBI

Traumatic brain injury (TBI) is the largest cause of death and disability of persons under 45 years old, worldwide. Independent of the distribution, outcomes such as disability are associated with huge societal costs. The heterogeneity of TBI and its complicated biological response have helped clarify the limitations of current pharmacological approaches to TBI management. Five decades of effort have made some strides in reducing TBI mortality but little progress has been made to mitigate TBI-induced disability. Lessons learned from the failure of numerous randomized clinical trials and the inability to scale up results from single center clinical trials with neuroprotective agents led to the formation of organizations such as the Neurological Emergencies Treatment Trials (NETT) Network, and international collaborative comparative effectiveness research (CER) to re-orient TBI clinical research. With initiatives such as TRACK-TBI, generating rich and comprehensive human datasets with demographic, clinical, genomic, proteomic, imaging, and detailed outcome data across multiple time points has become the focus of the field in the United States (US). In addition, government institutions such as the US Department of Defense are investing in groups such as Operation Brain Trauma Therapy (OBTT), a multicenter, pre-clinical drug-screening consortium to address the barriers in translation. The consensus from such efforts including “The Lancet Neurology Commission” and current literature is that unmitigated cell death processes, incomplete debris clearance, aberrant neurotoxic immune, and glia cell response induce progressive tissue loss and spatiotemporal magnification of primary TBI. Our analysis suggests that the focus of neuroprotection research needs to shift from protecting dying and injured neurons at acute time points to modulating the aberrant glial response in sub-acute and chronic time points. One unexpected agent with neuroprotective properties that shows promise is transplantation of neural stem cells. In this review we present (i) a short survey of TBI epidemiology and summary of current care, (ii) findings of past neuroprotective clinical trials and possible reasons for failure based upon insights from human and preclinical TBI pathophysiology studies, including our group's inflammation-centered approach, (iii) the unmet need of TBI and unproven treatments and lastly, (iv) present evidence to support the rationale for sub-acute neural stem cell therapy to mediate enduring neuroprotection.

Protein Kinase C Inhibition Mediates Neuroblast Enrichment in Mechanical Brain Injuries

Brain injuries of different etiologies lead to irreversible neuronal loss and persisting neuronal deficits. New therapeutic strategies are emerging to compensate neuronal damage upon brain injury. Some of these strategies focus on enhancing endogenous generation of neurons from neural stem cells (NSCs) to substitute the dying neurons. However, the capacity of the injured brain to produce new neurons is limited, especially in cases of extensive injury. This reduced neurogenesis is a consequence of the effect of signaling molecules released in response to inflammation, which act on intracellular pathways, favoring gliogenesis and preventing recruitment of neuroblasts from neurogenic regions. Protein kinase C (PKC) is a family of intracellular kinases involved in several of these gliogenic signaling pathways. The aim of this study was to analyze the role of PKC isozymes in the generation of neurons from neural progenitor cells (NPCs) in vitro and in vivo in brain injuries. PKC inhibition in vitro, in cultures of NPC isolated from the subventricular zone (SVZ) of postnatal mice, leads differentiation towards a neuronal fate. This effect is not mediated by classical or atypical PKC. On the contrary, this effect is mediated by novel PKCε, which is abundantly expressed in NPC cultures under differentiation conditions. PKCε inhibition by siRNA promotes neuronal differentiation and reduces glial cell differentiation. On the contrary, inhibition of PKCθ exerts a small anti-gliogenic effect and reverts the effect of PKCε inhibition on neuronal differentiation when both siRNAs are used in combination. Interestingly, in cortical brain injuries we have found expression of almost all PKC isozymes found in vitro. Inhibition of PKC activity in this type of injuries leads to neuronal production. In conclusion, these findings show an effect of PKCε in the generation of neurons from NPC in vitro, and they highlight the role of PKC isozymes as targets to produce neurons in brain lesions.

Vascularization and Engraftment of Transplanted Human Cerebral Organoids in Mouse Cortex

Neural stem cells (NSCs) hold great promise for neural repair in cases of CNS injury and neurodegeneration; however, conventional cell-based transplant methods face the challenges of poor survival and inadequate neuronal differentiation. Here, we report an alternative, tissue-based transplantation strategy whereby cerebral organoids derived from human pluripotent stem cells (PSCs) were grafted into lesioned mouse cortex. Cerebral organoid transplants exhibited enhanced survival and robust vascularization from host brain as compared to transplants of dissociated neural progenitor cells (NPCs). Engrafted cerebral organoids harbored a large NSC pool and displayed multilineage neurodifferentiation at two and four weeks after grafting. Cerebral organoids therefore represent a promising alternative source to NSCs or fetal tissues for transplantation, as they contain a large set of neuroprogenitors and differentiated neurons in a structured organization. Engrafted cerebral organoids may also offer a unique experimental paradigm for modeling human neurodevelopment and CNS diseases in the context of vascularized cortical tissue.

Transcription factor programming of human ES cells generates functional neurons expressing both upper and deep layer cortical markers

Human neurodegenerative disorders affect specific types of cortical neurons. Efficient protocols for the generation of such neurons for cell replacement, disease modeling and drug screening are highly warranted. Current methods for the production of cortical neurons from human embryonic stem (ES) cells are often time-consuming and inefficient, and the functional properties of the generated cells have been incompletely characterized. Here we have used transcription factor (TF) programming with the aim to induce rapid differentiation of human ES cells to layer-specific cortical neurons (hES-iNs). Three different combinations of TFs, NEUROGENIN 2 (NGN2) only, NGN2 plus Forebrain Embryonic Zinc Finger-Like Protein 2 (FEZF2), and NGN2 plus Special AT-Rich Sequence-Binding Protein 2 (SATB2), were delivered to human ES cells by lentiviral vectors. We observed only subtle differences between the TF combinations, which all gave rise to the formation of pyramidal-shaped cells, morphologically resembling adult human cortical neurons expressing cortical projection neuron (PN) markers and with mature electrophysiological properties. Using ex vivo transplantation to human organotypic cultures, we found that the hES-iNs could integrate into adult human cortical networks. We obtained no evidence that the hES-iNs had acquired a distinct cortical layer phenotype. Instead, our single-cell data showed that the hES-iNs, similar to fetal human cortical neurons, expressed both upper and deep layer cortical neuronal markers. Taken together, our findings provide evidence that TF programming can direct human ES cells towards cortical neurons but that the generated cells are transcriptionally profiled to generate both upper and deep layer cortical neurons. Therefore, most likely additional cues will be needed if these cells should adopt a specific cortical layer and area identity.

Combination of drug and stem cells neurotherapy: Potential interventions in neurotrauma and traumatic brain injury

Traumatic brain injury (TBI) has been recognized as one of the major public health issues that leads to devastating neurological disability. As a consequence of primary and secondary injury phases, neuronal loss following brain trauma leads to pathophysiological alterations on the molecular and cellular levels that severely impact the neuropsycho-behavioral and motor outcomes. Thus, to mitigate the neuropathological sequelae post-TBI such as cerebral edema, inflammation and neural degeneration, several neurotherapeutic options have been investigated including drug intervention, stem cell use and combinational therapies. These treatments aim to ameliorate cellular degeneration, motor decline, cognitive and behavioral deficits. Recently, the use of neural stem cells (NSCs) coupled with selective drug therapy has emerged as an alternative treatment option for neural regeneration and behavioral rehabilitation post-neural injury. Given their neuroprotective abilities, NSC-based neurotherapy has been widely investigated and well-reported in numerous disease models, notably in trauma studies. In this review, we will elaborate on current updates in cell replacement therapy in the area of neurotrauma. In addition, we will discuss novel combination drug therapy treatments that have been investigated in conjunction with stem cells to overcome the limitations associated with stem cell transplantation. Understanding the regenerative capacities of stem cell and drug combination therapy will help improve functional recovery and brain repair post-TBI. This article is part of the Special Issue entitled "Novel Treatments for Traumatic Brain Injury".

Interactions Between Neural Progenitor Cells and Microglia in the Subventricular Zone: Physiological Implications in the Neurogenic Niche and After Implantation in the Injured Brain

The adult subventricular zone (SVZ) of the mammalian brain contains neural progenitor cells (NPCs) that continuously produce neuroblasts throughout life. These neuroblasts migrate towards the olfactory bulb where they differentiate into local interneurons. The neurogenic niche of the SVZ includes, in addition to NPCs and neuroblasts, astrocytes, ependymal cells, blood vessels and the molecules released by these cell types. In the last few years, microglial cells have also been included as a key component of the SVZ neurogenic niche. Microglia in the SVZ display unique phenotypic features, and are more densely populated and activated than in non-neurogenic regions. In this article we will review literature reporting microglia-NPC interactions in the SVZ and the role of this bilateral communication in microglial function and in NPC biology. This interaction can take place through the release of soluble factors, extracellular vesicles or gap junctional communication. In addition, as NPCs are used for cell replacement therapies, they can establish therapeutically relevant crosstalks with host microglia which will also be summarized throughout the article.

The neurofilament derived-peptide NFL-TBS.40-63 enters in-vitro in human neural stem cells and increases their differentiation

Regenerative medicine is a promising approach to treat neurodegenerative diseases by replacing degenerating cells like neurons or oligodendrocytes. Targeting human neural stem cells directly in the brain is a big challenge in such a strategy. The neurofilament derived NFL-TBS.40-63 peptide has recently been introduced as a novel tool to target neural stem cells. Previous studies showed that this peptide can be internalized by rat neural stem cells in vitro and in vivo, which coincided with lower proliferation and self-renewal capacity and increase of differentiation. In this study, we analyzed the uptake and potential effects of the NFL-TBS.40-63 peptide on human neural stem cells isolated from human fetuses. We showed that the peptide inhibits proliferation and the ability to produce neurospheres in vitro, which is consistent with an increase in cell adhesion and differentiation. These results confirm that the peptide could be a promising molecule to target and manipulate human neural stem cells and thus could serve as a strategic tool for regenerative medicine.

Kuhnian revolutions in neuroscience: the role of tool development

The terms "paradigm" and "paradigm shift" originated in "The Structure of Scientific Revolutions" by Thomas Kuhn. A paradigm can be defined as the generally accepted concepts and practices of a field, and a paradigm shift its replacement in a scientific revolution. A paradigm shift results from a crisis caused by anomalies in a paradigm that reduce its usefulness to a field. Claims of paradigm shifts and revolutions are made frequently in the neurosciences. In this article I will consider neuroscience paradigms, and the claim that new tools and techniques rather than crises have driven paradigm shifts. I will argue that tool development has played a minor role in neuroscience revolutions.