Sox2-mediated conversion of NG2 glia into induced neurons in the injured adult cerebral cortex

Delivery Systems in Neuronal Direct Cell Reprogramming

Neuronal direct cell reprogramming approach allows direct conversion of somatic cells into neurons via forced expression of neuronal cell-lineage transcription factors (TFs). These so-called induced neuronal cells have significant potential as research tools and for therapeutic applications, such as in cell replacement therapy. However, the optimization of TF delivery strategies is crucial to reach clinical practice. In this review, we outlined the currently explored delivery technologies in neuronal direct cell reprogramming and their limitations and advantages. The first employed delivery strategies were mainly integrating viral systems, such as lentiviruses that exert consistently high transgene expression in most cell types. On the other hand, viral systems cause major safety concerns, including the risk for insertional mutagenesis and inflammation. More recently, several safer nonviral delivery systems have been investigated as well; however, these systems generally exert inferior reprogramming efficiency compared with viral systems. Emerging delivery technologies could provide new opportunities in the achievement of safe and effective delivery for neuronal direct cell reprogramming.

Regeneration of retinal ganglion cell-like cells and reconstruction of visual neural circuits in mice with glaucoma

Glaucoma is an irreversible blinding eye disease characterized by apoptosis of mature neurons-retinal ganglion cells (RGCs), visual field defect and vision loss. Regeneration of RGCs and reconstruction of the neural connections between the retina and the brain is considered an effective strategy to promote visual restoration in patients with glaucoma. However, there are currently no effective methods for regenerating RGCs to restore vision in clinical practice. Microglia are a type of glial cells that regulate the immune response in the retina and central nervous system (CNS), whether they have pluripotency and be reversed into RGCs remains unclear and challenging. This study revealed that the ectopic expression of multiple genes (Brn3b, Sox2, Cbln1, and NP1, referred to as BSCN) in microglia can promote their conversion into RGC-like cells by microglia fate lineage tracing in vivo. The regenerated RGC-like cells project axons to the distant brain and reconstruct the visual neural circuit, restoring the impaired vision in adult mice with acute glaucoma induced by retinal ischemia-reperfusion (I/R) injury. Furthermore, the regenerated RGC-like cells could survive stably for up to one year, and the same regeneration strategy was performed in older mice with acute glaucoma, which confirmed the effectiveness of the BSCN reprogramming to regenerate RGC-like cells. In summary, we have identified the microglia as a new type of reprogramming seed cells, and four key genes were found to be involved in regenerating RGC-like cells to restore vision. These findings highlight a new strategy of RGC-like cell regeneration and provide a theoretical basis for treatment of glaucoma in the future.

Hmgb2 improves astrocyte to neuron conversion by increasing the chromatin accessibility of genes associated with neuronal maturation in a proneuronal factor-dependent manner

BACKGROUND

Direct conversion of reactive glial cells to neurons is a promising avenue for neuronal replacement therapies after brain injury or neurodegeneration. The overexpression of neurogenic fate determinants in glial cells results in conversion to neurons. For repair purposes, the conversion should ideally be induced in the pathology-induced neuroinflammatory environment. However, very little is known regarding the influence of the injury-induced neuroinflammatory environment and released growth factors on the direct conversion process.

RESULTS

We establish a new in vitro culture system of postnatal astrocytes without epidermal growth factor that reflects the direct conversion rate in the injured, neuroinflammatory environment in vivo. We demonstrate that the growth factor combination corresponding to the injured environment defines the ability of glia to be directly converted to neurons. Using this culture system, we show that chromatin structural protein high mobility group box 2 (HMGB2) regulates the direct conversion rate downstream of the growth factor combination. We further demonstrate that Hmgb2 cooperates with neurogenic fate determinants, such as Neurog2, in opening chromatin at the loci of genes regulating neuronal maturation and synapse formation. Consequently, early chromatin rearrangements occur during direct fate conversion and are necessary for full fate conversion.

CONCLUSIONS

Our data demonstrate novel growth factor-controlled regulation of gene expression during direct fate conversion. This regulation is crucial for proper maturation of induced neurons and could be targeted to improve the repair process.

Single Cell Deletion of the Transcription Factors Trps1 and Sox9 in Astrocytes Reveals Novel Functions in the Adult Cerebral Cortex

Astrocytes play key roles in brain function, but how these are orchestrated by transcription factors (TFs) in the adult brain and aligned with astrocyte heterogeneity is largely unknown. Here we examined the localization and function of the novel astrocyte TF Trps1 (Transcriptional Repressor GATA Binding 1) and the well-known astrocyte TF Sox9 by Cas9-mediated deletion using Mokola-pseudotyped lentiviral delivery into the adult cerebral cortex. Trps1 and Sox9 levels showed heterogeneity among adult cortical astrocytes, which prompted us to explore the effects of deleting either Sox9 or Trps1 alone or simultaneously at the single-cell (by patch-based single-cell transcriptomics) and tissue levels (by spatial transcriptomics). This revealed TF-specific functions in astrocytes, such as synapse maintenance with the strongest effects on synapse number achieved by Trps1 deletion and a common effect on immune response. In addition, spatial transcriptomics showed non-cell-autonomous effects on the surrounding cells, such as oligodendrocytes and other immune cells with TF-specific differences on the type of immune cells: Trps1 deletion affecting monocytes specifically, while Sox9 deletion acting mostly on microglia and deletion of both TF affecting mostly B cells. Taken together, this study reveals novel roles of Trps1 and Sox9 in adult astrocytes and their communication with other glial and immune cells.

GNAO1 overexpression promotes neural differentiation of glioma stem-like cells and reduces tumorigenicity through TRIM21/CREB/HES1 axis

Inducing tumor cell differentiation is a promising strategy for treating malignant cancers, including glioma, yet the critical regulator(s) underlying glioma cell differentiation is poorly understood. Here, we identify G Protein Subunit Alpha O1 (GNAO1) as a critical regulator of neural differentiation of glioma stem-like cells (GSCs). GNAO1 expression was lower in gliomas than in normal neuronal tissues and high expression of GNAO1 correlated with a better prognosis. GNAO1 overexpression markedly promoted neural differentiation of GSCs, leading to decreased cell proliferation and colony formation. Mechanistically, GNAO1 recruited TRIM21 and facilitated TRIM21-mediated ubiquitination. This ubiquitination resulted in the degradation of CREB and further reduced p300-mediated H3K27ac levels of the HES1 promoter. As a result, GNAO1 overexpression downregulated HES1 expression, which reinforced neuronal differentiation. In addition, knockdown of METTL3, a key writer of the N6-methyladenosine (m6A), enhanced GNAO1 mRNA stability. Treatment with GNAO1 adenovirus increased neuronal differentiation of tumor cells and reduced tumor cell proliferation in orthotopic GSC xenografts and temozolomide further enhanced GNAO1 adenovirus effects, resulting in extended animal survival. Our study presents that engineering GNAO1 overexpression-inducing neural differentiation of GSCs is a potential therapy strategy via synergistic inhibition of malignant proliferation and chemotherapy resistance.

Interplay between pioneer transcription factors and epigenetic modifiers in cell reprogramming

The generation of induced pluripotent stem cells (iPSCs) from differentiated somatic cells by Yamanaka factors, including pioneer transcription factors (TFs), has greatly reshaped our traditional understanding of cell plasticity and demonstrated the remarkable potential of pioneer TFs. In addition to iPSC reprogramming, pioneer TFs are pivotal in direct reprogramming or transdifferentiation where somatic cells are converted into different cell types without passing through a pluripotent state. Pioneer TFs initiate a reprogramming process through chromatin opening, thereby establishing competence for new gene regulatory programs. The action of pioneer TFs is both influenced by and exerts influence on epigenetic regulation. Despite significant advances, many direct reprogramming processes remain inefficient, which limits their reliability for clinical applications. In this review, we discuss the molecular mechanisms underlying pioneer TF-driven reprogramming, with a focus on their interactions with epigenetic modifiers, including Polycomb repressive complexes (PRCs), nucleosome remodeling and deacetylase (NuRD) complexes, and the DNA methylation machinery. A deeper understanding of the dynamic interplay between pioneer TFs and epigenetic modifiers will be essential for advancing reprogramming technologies and unlocking their full clinical potential.

Direct Neural Reprogramming in situ: Existing Approaches and Their Optimization

Direct in situ neuronal reprogramming (transdifferentiation) of glial cells (astrocytes and microglia) has attracted a significant interest as a potential approach for the treatment of a wide range of neurodegenerative diseases and damages of the central nervous system (CNS). The nervous system of higher mammals has a very limited capacity for repair. Disruption of CNS functioning due to traumatic injuries or neurodegenerative processes can significantly affect the quality of patients' life, lead to motor and cognitive impairments, and result in disability and, in some cases, death. Restoration of lost neurons in situ via direct reprogramming of glial cells without the intermediate stage of pluripotency seems to be the most attractive approach from the viewpoint of translational biomedicine. The ability of astroglia to actively proliferate in response to the damage of neural tissue supports the idea that these neuron-like cells, which are already present at the lesion site, are good candidates for transdifferentiation into neurons, considering that the possibility of direct neuronal reprogramming of astrocytes both in vitro and in vivo have demonstrated in many independent studies. Overexpression of proneuronal transcription factors, e.g., neurogenic differentiation factors 1-4 (NeuroD1-4), Neurogenin 2 (NeuroG2), Ascl1 (Achaete-Scute homolog 1), and Dlx2 (distal-less homeobox 2), including pioneer transcription factors that recognize target sequences in the compacted chromatin and activate transcription of silent genes, has already been proven as a potential therapeutic strategy. Other strategies, such as microRNA-mediated suppression of activity of PTB and REST transcription factors and application of small molecules or various biomaterials, are also utilized in neuronal reprogramming. However, the efficiency of direct in situ reprogramming is limited by a number of factors, including cell specificity of transgene delivery systems and promoters, brain regions in which transdifferentiation occurs, factors affecting cell metabolism, microenvironment, etc. Reprogramming in situ, which takes place in the presence of a large number of different cell types, requires monitoring and precise phenotypic characterization of subpopulations of cells undergoing transdifferentiation in order to confirm the reprogramming of the astroglia into neurons and subsequent integration of these neurons into the CNS. Here, we discussed the most efficient strategies of neuronal reprogramming and technologies used to visualize the transdifferentiation process, with special focus on the obstacles to efficient neuronal conversion, as well as approaches to overcome them.

Challenges and Efficacy of Astrocyte-to-Neuron Reprogramming in Spinal Cord Injury: In Vitro Insights and In Vivo Outcomes

Traumatic spinal cord injury (SCI) leads to the disruption of neural pathways, causing loss of neural cells, with subsequent reactive gliosis and tissue scarring that limit endogenous repair. One potential therapeutic strategy to address this is to target reactive scar-forming astrocytes with direct cellular reprogramming to convert them into neurons, by overexpression of neurogenic transcription factors. Here we used lentiviral constructs to overexpress Ascl1 or a combination of microRNAs (miRs) miR124, miR9/9*and NeuroD1 transfected into cultured and in vivo astrocytes. In vitro experiments revealed cortically-derived astrocytes display a higher efficiency (70%) of reprogramming to neurons than spinal cord-derived astrocytes. In a rat cervical SCI model, the same strategy induced only limited reprogramming of astrocytes. Delivery of reprogramming factors did not significantly affect patterns of breathing under baseline and hypoxic conditions, but significant differences in average diaphragm amplitude were seen in the reprogrammed groups during eupneic breathing, hypoxic, and hypercapnic challenges. These results show that while cellular reprogramming can be readily achieved in carefully controlled in vitro conditions, achieving a similar degree of successful reprogramming in vivo is challenging and may require additional steps.

Retrovirus-Mediated Reprogramming of Endogenous Hippocampal Glia into GABAergic Induced Neurons

Lineage reprogramming of glial cells into induced neurons (iNs) has emerged as an innovative strategy to replace neurons lost due to injury or neurological diseases. Here, we describe a step-by-step protocol to induce in vivo conversion of reactive glial cells, proliferating within the injured hippocampus, into mature and functional GABAergic iNs through retrovirus-mediated expression of two neurogenic fate determinants (Ascl1 and Dlx2). We have previously applied this method to study the integration and functional impact of GABAergic iNs in epileptic mice (Lentini et al., Cell Stem Cell 28:2104-2121.e10, 2021). We successfully generated GABAergic iNs that exhibited substantial integration within pathological circuits, leading to a significant reduction in epileptic seizures.

Neurogenic potential of NG2 in neurotrauma: a systematic review

Regenerative approaches towards neuronal loss following traumatic brain or spinal cord injury have long been considered a dogma in neuroscience and remain a cutting-edge area of research. This is reflected in a large disparity between the number of studies investigating primary and secondary injury as therapeutic targets in spinal cord and traumatic brain injuries. Significant advances in biotechnology may have the potential to reshape the current state-of-the-art and bring focus to primary injury neurotrauma research. Recent studies using neural-glial factor/antigen 2 (NG2) cells indicate that they may differentiate into neurons even in the developed brain. As these cells show great potential to play a regenerative role, studies have been conducted to test various manipulations in neurotrauma models aimed at eliciting a neurogenic response from them. In the present study, we systematically reviewed the experimental protocols and findings described in the scientific literature, which were peer-reviewed original research articles (1) describing preclinical experimental studies, (2) investigating NG2 cells, (3) associated with neurogenesis and neurotrauma, and (4) in vitro and/or in vivo, available in PubMed/MEDLINE, Web of Science or SCOPUS, from 1998 to 2022. Here, we have reviewed a total of 1504 papers, and summarized findings that ultimately suggest that NG2 cells possess an inducible neurogenic potential in animal models and in vitro. We also discriminate findings of NG2 neurogenesis promoted by different pharmacological and genetic approaches over functional and biochemical outcomes of traumatic brain injury and spinal cord injury models, and provide mounting evidence for the potential benefits of manipulated NG2 cell ex vivo transplantation in primary injury treatment. These findings indicate the feasibility of NG2 cell neurogenesis strategies and add new players in the development of therapeutic alternatives for neurotrauma.

The Role of Inflammatory Cascade and Reactive Astrogliosis in Glial Scar Formation Post-spinal Cord Injury

Reactive astrogliosis and inflammation are pathologic hallmarks of spinal cord injury. After injury, dysfunction of glial cells (astrocytes) results in glial scar formation, which limits neuronal regeneration. The blood-spinal cord barrier maintains the structural and functional integrity of the spinal cord and does not allow blood vessel components to leak into the spinal cord microenvironment. After the injury, disruption in the spinal cord barrier causes an imbalance of the immunological microenvironment. This triggers the process of neuroinflammation, facilitated by the actions of microglia, neutrophils, glial cells, and cytokines production. Recent work has revealed two phenotypes of astrocytes, A1 and A2, where A2 has a protective type, and A1 releases neurotoxins, further promoting glial scar formation. Here, we first describe the current understanding of the spinal cord microenvironment, both pre-, and post-injury, and the role of different glial cells in the context of spinal cord injury, which forms the essential update on the cellular and molecular events following injury. We aim to explore in-depth signaling pathways and molecular mediators that trigger astrocyte activation and glial scar formation. This review will discuss the activated signaling pathways in astrocytes and other glial cells and their collaborative role in the development of gliosis through inflammatory responses.

In situ chemical reprogramming of astrocytes into neurons: A new hope for the treatment of central neurodegenerative diseases?

Central neurodegenerative disorders (e.g. Alzheimer's disease (AD) and Parkinson's disease (PD)) are tightly associated with extensive neuron loss. Current therapeutic interventions merely mitigate the symptoms of these diseases, falling short of addressing the fundamental issue of neuron loss. Cell reprogramming, involving the transition of a cell from one gene expression profile to another, has made significant strides in the conversion between diverse somatic cell types. This advancement has been facilitated by gene editing techniques or the synergistic application of small molecules, enabling the conversion of glial cells into functional neurons. Despite this progress, the potential for in situ reprogramming of astrocytes in treating neurodegenerative disorders faces challenges such as immune rejection and genotoxicity. A novel avenue emerges through chemical reprogramming of astrocytes utilizing small molecules, circumventing genotoxic effects and unlocking substantial clinical utility. Recent studies have successfully demonstrated the in situ conversion of astrocytes into neurons using small molecules. Nonetheless, these findings have sparked debates, encompassing queries regarding the origin of newborn neurons, pivotal molecular targets, and alterations in metabolic pathways. This review succinctly delineates the background of astrocytes reprogramming, meticulously surveys the principal classes of small molecule combinations employed thus far, and examines the complex signaling pathways they activate. Finally, this article delves into the potential vistas awaiting exploration in the realm of astrocytes chemical reprogramming, heralding a promising future for advancing our understanding and treatment of neurodegenerative diseases.

Reprogramming astroglia into neurons with hallmarks of fast-spiking parvalbumin-positive interneurons by phospho-site-deficient Ascl1

Cellular reprogramming of mammalian glia to an induced neuronal fate holds the potential for restoring diseased brain circuits. While the proneural factor achaete-scute complex-like 1 (Ascl1) is widely used for neuronal reprogramming, in the early postnatal mouse cortex, Ascl1 fails to induce the glia-to-neuron conversion, instead promoting the proliferation of oligodendrocyte progenitor cells (OPC). Since Ascl1 activity is posttranslationally regulated, here, we investigated the consequences of mutating six serine phospho-acceptor sites to alanine (Ascl1SA6) on lineage reprogramming in vivo. Ascl1SA6 exhibited increased neurogenic activity in the glia of the early postnatal mouse cortex, an effect enhanced by coexpression of B cell lymphoma 2 (Bcl2). Genetic fate-mapping revealed that most induced neurons originated from astrocytes, while only a few derived from OPCs. Many Ascl1SA6/Bcl2-induced neurons expressed parvalbumin and were capable of high-frequency action potential firing. Our study demonstrates the authentic conversion of astroglia into neurons featuring subclass hallmarks of cortical interneurons, advancing our scope of engineering neuronal fates in the brain.

3D model for human glia conversion into subtype-specific neurons, including dopamine neurons

Two-dimensional neuronal cultures have a limited ability to recapitulate the in vivo environment of the brain. Here, we introduce a three-dimensional in vitro model for human glia-to-neuron conversion, surpassing the spatial and temporal constrains of two-dimensional cultures. Focused on direct conversion to induced dopamine neurons (iDANs) relevant to Parkinson disease, the model generates functionally mature iDANs in 2 weeks and allows long-term survival. As proof of concept, we use single-nucleus RNA sequencing and molecular lineage tracing during iDAN generation and find that all glial subtypes generate neurons and that conversion relies on the coordinated expression of three neural conversion factors. We also show the formation of mature and functional iDANs over time. The model facilitates molecular investigations of the conversion process to enhance understanding of conversion outcomes and offers a system for in vitro reprogramming studies aimed at advancing alternative therapeutic strategies in the diseased brain.

Post-transcriptional mechanisms controlling neurogenesis and direct neuronal reprogramming

Neurogenesis is a tightly regulated process in time and space both in the developing embryo and in adult neurogenic niches. A drastic change in the transcriptome and proteome of radial glial cells or neural stem cells towards the neuronal state is achieved due to sophisticated mechanisms of epigenetic, transcriptional, and post-transcriptional regulation. Understanding these neurogenic mechanisms is of major importance, not only for shedding light on very complex and crucial developmental processes, but also for the identification of putative reprogramming factors, that harbor hierarchically central regulatory roles in the course of neurogenesis and bare thus the capacity to drive direct reprogramming towards the neuronal fate. The major transcriptional programs that orchestrate the neurogenic process have been the focus of research for many years and key neurogenic transcription factors, as well as repressor complexes, have been identified and employed in direct reprogramming protocols to convert non-neuronal cells, into functional neurons. The post-transcriptional regulation of gene expression during nervous system development has emerged as another important and intricate regulatory layer, strongly contributing to the complexity of the mechanisms controlling neurogenesis and neuronal function. In particular, recent advances are highlighting the importance of specific RNA binding proteins that control major steps of mRNA life cycle during neurogenesis, such as alternative splicing, polyadenylation, stability, and translation. Apart from the RNA binding proteins, microRNAs, a class of small non-coding RNAs that block the translation of their target mRNAs, have also been shown to play crucial roles in all the stages of the neurogenic process, from neural stem/progenitor cell proliferation, neuronal differentiation and migration, to functional maturation. Here, we provide an overview of the most prominent post-transcriptional mechanisms mediated by RNA binding proteins and microRNAs during the neurogenic process, giving particular emphasis on the interplay of specific RNA binding proteins with neurogenic microRNAs. Taking under consideration that the molecular mechanisms of neurogenesis exert high similarity to the ones driving direct neuronal reprogramming, we also discuss the current advances in in vitro and in vivo direct neuronal reprogramming approaches that have employed microRNAs or RNA binding proteins as reprogramming factors, highlighting the so far known mechanisms of their reprogramming action.

Comparing Viral Vectors and Fate Mapping Approaches for Astrocyte-to-Neuron Reprogramming in the Injured Mouse Cerebral Cortex

Direct neuronal reprogramming is a promising approach to replace neurons lost due to disease via the conversion of endogenous glia reacting to brain injury into neurons. However, it is essential to demonstrate that the newly generated neurons originate from glial cells and/or show that they are not pre-existing endogenous neurons. Here, we use controls for both requirements while comparing two viral vector systems (Mo-MLVs and AAVs) for the expression of the same neurogenic factor, the phosphorylation-resistant form of Neurogenin2. Our results show that Mo-MLVs targeting proliferating glial cells after traumatic brain injury reliably convert astrocytes into neurons, as assessed by genetic fate mapping of astrocytes. Conversely, expressing the same neurogenic factor in a flexed AAV system results in artefactual labelling of endogenous neurons fatemapped by birthdating in development that are negative for the genetic fate mapping marker induced in astrocytes. These results are further corroborated by chronic live in vivo imaging. Taken together, the phosphorylation-resistant form of Neurogenin2 is more efficient in reprogramming reactive glia into neurons than its wildtype counterpart in vivo using retroviral vectors (Mo-MLVs) targeting proliferating glia. Conversely, AAV-mediated expression generates artefacts and is not sufficient to achieve fate conversion.

Two-photon live imaging of direct glia-to-neuron conversion in the mouse cortex

JOURNAL/nrgr/04.03/01300535-202408000-00032/figure1/v/2023-12-16T180322Z/r/image-tiff Over the past decade, a growing number of studies have reported transcription factor-based in situ reprogramming that can directly convert endogenous glial cells into functional neurons as an alternative approach for neuroregeneration in the adult mammalian central nervous system. However, many questions remain regarding how a terminally differentiated glial cell can transform into a delicate neuron that forms part of the intricate brain circuitry. In addition, concerns have recently been raised around the absence of astrocyte-to-neuron conversion in astrocytic lineage-tracing mice. In this study, we employed repetitive two-photon imaging to continuously capture the in situ astrocyte-to-neuron conversion process following ectopic expression of the neural transcription factor NeuroD1 in both proliferating reactive astrocytes and lineage-traced astrocytes in the mouse cortex. Time-lapse imaging over several weeks revealed the step-by-step transition from a typical astrocyte with numerous short, tapered branches to a typical neuron with a few long neurites and dynamic growth cones that actively explored the local environment. In addition, these lineage-converting cells were able to migrate radially or tangentially to relocate to suitable positions. Furthermore, two-photon Ca2+ imaging and patch-clamp recordings confirmed that the newly generated neurons exhibited synchronous calcium signals, repetitive action potentials, and spontaneous synaptic responses, suggesting that they had made functional synaptic connections within local neural circuits. In conclusion, we directly visualized the step-by-step lineage conversion process from astrocytes to functional neurons in vivo and unambiguously demonstrated that adult mammalian brains are highly plastic with respect to their potential for neuroregeneration and neural circuit reconstruction.

Advances in neuronal reprogramming for neurodegenerative diseases: Strategies, controversies, and opportunities

Neuronal death is often observed in central nervous system injuries and neurodegenerative diseases. The mammalian central nervous system manifests limited neuronal regeneration capabilities, and traditional cell therapies are limited in their potential applications due to finite cell sources and immune rejection. Neuronal reprogramming has emerged as a novel technology, in which non-neuronal cells (e.g. glial cells) are transdifferentiated into mature neurons. This process results in relatively minimal immune rejection. The present review discuss the latest progress in this cutting-edge field, including starter cell selection, innovative technical strategies and methods of neuronal reprogramming for neurodegenerative diseases, as well as the potential problems and controversies. The further development of neuronal reprogramming technology may pave the way for novel therapeutic strategies in the treatment of neurodegenerative diseases.

Controlling the Expression Level of the Neuronal Reprogramming Factors for a Successful Reprogramming Outcome

Neuronal reprogramming is a promising approach for making major advancement in regenerative medicine. Distinct from the approach of induced pluripotent stem cells, neuronal reprogramming converts non-neuronal cells to neurons without going through a primitive stem cell stage. In vivo neuronal reprogramming brings this approach to a higher level by changing the cell fate of glial cells to neurons in neural tissue through overexpressing reprogramming factors. Despite the ongoing debate over the validation and interpretation of newly generated neurons, in vivo neuronal reprogramming is still a feasible approach and has the potential to become clinical treatment with further optimization and refinement. Here, we discuss the major neuronal reprogramming factors (mostly pro-neurogenic transcription factors during development), especially the significance of their expression levels during neurogenesis and the reprogramming process focusing on NeuroD1. In the developing central nervous system, these pro-neurogenic transcription factors usually elicit distinct spatiotemporal expression patterns that are critical to their function in generating mature neurons. We argue that these dynamic expression patterns may be similarly needed in the process of reprogramming adult cells into neurons and further into mature neurons with subtype identities. We also summarize the existing approaches and propose new ones that control gene expression levels for a successful reprogramming outcome.

In vivo neural regeneration via AAV-NeuroD1 gene delivery to astrocytes in neonatal hypoxic-ischemic brain injury

BACKGROUND

Neonatal hypoxic-ischemic brain injury (HIBI) is a significant contributor to neonatal mortality and long-term neurodevelopmental disability, characterized by massive neuronal loss and reactive astrogliosis. Current therapeutic approaches for neonatal HIBI have been limited to general supportive therapy because of the lack of methods to compensate for irreversible neuronal loss. This study aimed to establish a feasible regenerative therapy for neonatal HIBI utilizing in vivo direct neuronal reprogramming technology.

METHODS

Neonatal HIBI was induced in ICR mice at postnatal day 7 by permanent right common carotid artery occlusion and exposure to hypoxia with 8% oxygen and 92% nitrogen for 90 min. Three days after the injury, NeuroD1 was delivered to reactive astrocytes of the injury site using the astrocyte-tropic adeno-associated viral (AAV) vector AAVShH19. AAVShH19 was engineered with the Cre-FLEX system for long-term tracking of infected cells.

RESULTS

AAVShH19-mediated ectopic NeuroD1 expression effectively converted astrocytes into GABAergic neurons, and the converted cells exhibited electrophysiological properties and synaptic transmitters. Additionally, we found that NeuroD1-mediated in vivo direct neuronal reprogramming protected injured host neurons and altered the host environment, i.e., decreased the numbers of activated microglia, reactive astrocytes, and toxic A1-type astrocytes, and decreased the expression of pro-inflammatory factors. Furthermore, NeuroD1-treated mice exhibited significantly improved motor functions.

CONCLUSIONS

This study demonstrates that NeuroD1-mediated in vivo direct neuronal reprogramming technology through AAV gene delivery can be a novel regenerative therapy for neonatal HIBI.

Neuronal conversion from glia to replenish the lost neurons

ABSTRACT

Neuronal injury, aging, and cerebrovascular and neurodegenerative diseases such as cerebral infarction, Alzheimer's disease, Parkinson's disease, frontotemporal dementia, amyotrophic lateral sclerosis, and Huntington's disease are characterized by significant neuronal loss. Unfortunately, the neurons of most mammals including humans do not possess the ability to self-regenerate. Replenishment of lost neurons becomes an appealing therapeutic strategy to reverse the disease phenotype. Transplantation of pluripotent neural stem cells can supplement the missing neurons in the brain, but it carries the risk of causing gene mutation, tumorigenesis, severe inflammation, and obstructive hydrocephalus induced by brain edema. Conversion of neural or non-neural lineage cells into functional neurons is a promising strategy for the diseases involving neuron loss, which may overcome the above-mentioned disadvantages of neural stem cell therapy. Thus far, many strategies to transform astrocytes, fibroblasts, microglia, Müller glia, NG2 cells, and other glial cells to mature and functional neurons, or for the conversion between neuronal subtypes have been developed through the regulation of transcription factors, polypyrimidine tract binding protein 1 (PTBP1), and small chemical molecules or are based on a combination of several factors and the location in the central nervous system. However, some recent papers did not obtain expected results, and discrepancies exist. Therefore, in this review, we discuss the history of neuronal transdifferentiation, summarize the strategies for neuronal replenishment and conversion from glia, especially astrocytes, and point out that biosafety, new strategies, and the accurate origin of the truly converted neurons in vivo should be focused upon in future studies. It also arises the attention of replenishing the lost neurons from glia by gene therapies such as up-regulation of some transcription factors or down-regulation of PTBP1 or drug interference therapies.

Reprogramming of astrocytes and glioma cells into neurons for central nervous system repair and glioblastoma therapy

Central nervous system (CNS) damage is usually irreversible owing to the limited regenerative capability of neurons. Following CNS injury, astrocytes are reactively activated and are the key cells involved in post-injury repair mechanisms. Consequently, research on the reprogramming of reactive astrocytes into neurons could provide new directions for the restoration of neural function after CNS injury and in the promotion of recovery in various neurodegenerative diseases. This review aims to provide an overview of the means through which reactive astrocytes around lesions can be reprogrammed into neurons, to elucidate the intrinsic connection between the two cell types from a neurogenesis perspective, and to summarize what is known about the neurotranscription factors, small-molecule compounds and MicroRNA that play major roles in astrocyte reprogramming. As the malignant proliferation of astrocytes promotes the development of glioblastoma multiforme (GBM), this review also examines the research advances on and the theoretical basis for the reprogramming of GBM cells into neurons and discusses the advantages of such approaches over traditional treatment modalities. This comprehensive review provides new insights into the field of GBM therapy and theoretical insights into the mechanisms of neurological recovery following neurological injury and in GBM treatment.

Small molecules reprogram reactive astrocytes into neuronal cells in the injured adult spinal cord

INTRODUCTION

Ectopic expression of transcription factor-mediated in vivo neuronal reprogramming provides promising strategy to compensate for neuronal loss, while its further clinical application may be hindered by delivery and safety concerns. As a novel and attractive alternative, small molecules may offer a non-viral and non-integrative chemical approach for reprogramming cell fates. Recent definitive evidences have shown that small molecules can convert non-neuronal cells into neurons in vitro. However, whether small molecules alone can induce neuronal reprogramming in vivo remains largely unknown.

OBJECTIVES

To identify chemical compounds that can induce in vivo neuronal reprogramming in the adult spinal cord.

METHODS

Immunocytochemistry, immunohistochemistry, qRT-PCR and fate-mapping are performed to analyze the role of small molecules in reprogramming astrocytes into neuronal cells in vitro and in vivo.

RESULTS

By screening, we identify a chemical cocktail with only two chemical compounds that can directly and rapidly reprogram cultured astrocytes into neuronal cells. Importantly, this chemical cocktail can also successfully trigger neuronal reprogramming in the injured adult spinal cord without introducing exogenous genetic factors. These chemically induced cells showed typical neuronal morphologies and neuron-specific marker expression and could become mature and survive for more than 12 months. Lineage tracing indicated that the chemical compound-converted neuronal cells mainly originated from post-injury spinal reactive astrocytes.

CONCLUSION

Our proof-of-principle study demonstrates that in vivo glia-to-neuron conversion can be manipulated in a chemical compound-based manner. Albeit our current chemical cocktail has a lowreprogramming efficiency, it will bring in vivo cell fate reprogramming closer to clinical application in brain and spinal cord repair. Future studies should focus on further refining our chemical cocktail and reprogramming approach to boost the reprogramming efficiency.

Directed differentiation of functional corticospinal-like neurons from endogenous SOX6+/NG2+ cortical progenitors

Corticospinal neurons (CSN) centrally degenerate in amyotrophic lateral sclerosis (ALS), along with spinal motor neurons, and loss of voluntary motor function in spinal cord injury (SCI) results from damage to CSN axons. For functional regeneration of specifically affected neuronal circuitry in vivo , or for optimally informative disease modeling and/or therapeutic screening in vitro , it is important to reproduce the type or subtype of neurons involved. No such appropriate in vitro models exist with which to investigate CSN selective vulnerability and degeneration in ALS, or to investigate routes to regeneration of CSN circuitry for ALS or SCI, critically limiting the relevance of much research. Here, we identify that the HMG-domain transcription factor Sox6 is expressed by a subset of NG2+ endogenous cortical progenitors in postnatal and adult cortex, and that Sox6 suppresses a latent neurogenic program by repressing inappropriate proneural Neurog2 expression by progenitors. We FACS-purify these genetically accessible progenitors from postnatal mouse cortex and establish a pure culture system to investigate their potential for directed differentiation into CSN. We then employ a multi-component construct with complementary and differentiation-sharpening transcriptional controls (activating Neurog2, Fezf2 , while antagonizing Olig2 with VP16:Olig2 ). We generate corticospinal-like neurons from SOX6+/NG2+ cortical progenitors, and find that these neurons differentiate with remarkable fidelity compared with corticospinal neurons in vivo . They possess appropriate morphological, molecular, transcriptomic, and electrophysiological characteristics, without characteristics of the alternate intracortical or other neuronal subtypes. We identify that these critical specifics of differentiation are not reproduced by commonly employed Neurog2 -driven differentiation. Neurons induced by Neurog2 instead exhibit aberrant multi-axon morphology and express molecular hallmarks of alternate cortical projection subtypes, often in mixed form. Together, this developmentally-based directed differentiation from genetically accessible cortical progenitors sets a precedent and foundation for in vitro mechanistic and therapeutic disease modeling, and toward regenerative neuronal repopulation and circuit repair.

Reactive gliosis in traumatic brain injury: a comprehensive review

Traumatic brain injury (TBI) is one of the most common pathological conditions impacting the central nervous system (CNS). A neurological deficit associated with TBI results from a complex of pathogenetic mechanisms including glutamate excitotoxicity, inflammation, demyelination, programmed cell death, or the development of edema. The critical components contributing to CNS response, damage control, and regeneration after TBI are glial cells-in reaction to tissue damage, their activation, hypertrophy, and proliferation occur, followed by the formation of a glial scar. The glial scar creates a barrier in damaged tissue and helps protect the CNS in the acute phase post-injury. However, this process prevents complete tissue recovery in the late/chronic phase by producing permanent scarring, which significantly impacts brain function. Various glial cell types participate in the scar formation, but this process is mostly attributed to reactive astrocytes and microglia, which play important roles in several brain pathologies. Novel technologies including whole-genome transcriptomic and epigenomic analyses, and unbiased proteomics, show that both astrocytes and microglia represent groups of heterogenic cell subpopulations with different genomic and functional characteristics, that are responsible for their role in neurodegeneration, neuroprotection and regeneration. Depending on the representation of distinct glia subpopulations, the tissue damage as well as the regenerative processes or delayed neurodegeneration after TBI may thus differ in nearby or remote areas or in different brain structures. This review summarizes TBI as a complex process, where the resultant effect is severity-, region- and time-dependent and determined by the model of the CNS injury and the distance of the explored area from the lesion site. Here, we also discuss findings concerning intercellular signaling, long-term impacts of TBI and the possibilities of novel therapeutical approaches. We believe that a comprehensive study with an emphasis on glial cells, involved in tissue post-injury processes, may be helpful for further research of TBI and be the decisive factor when choosing a TBI model.

In Vivo Reprogramming Using Yamanaka Factors in the CNS: A Scoping Review

Central nervous system diseases, particularly neurodegenerative disorders, pose significant challenges in medicine. These conditions, characterized by progressive neuronal loss, have remained largely incurable, exacting a heavy toll on individuals and society. In recent years, in vivo reprogramming using Yamanaka factors has emerged as a promising approach for central nervous system regeneration. This technique involves introducing transcription factors, such as Oct4, Sox2, Klf4, and c-Myc, into adult cells to induce their conversion into neurons. This review summarizes the current state of in vivo reprogramming research in the central nervous system, focusing on the use of Yamanaka factors. In vivo reprogramming using Yamanaka factors has shown promising results in several animal models of central nervous system diseases. Studies have demonstrated that this approach can promote the generation of new neurons, improve functional outcomes, and reduce scar formation. However, there are still several challenges that need to be addressed before this approach can be translated into clinical practice. These challenges include optimizing the efficiency of reprogramming, understanding the cell of origin for each transcription factor, and developing methods for reprogramming in non-subventricular zone areas. Further research is needed to overcome the remaining challenges, but this approach has the potential to revolutionize the way we treat central nervous system disorders.

Partial Reprogramming as a Method for Regenerating Neural Tissues in Aged Organisms

Aging causes numerous age-related diseases, leading the human species to death. Nevertheless, rejuvenating strategies based on cell epigenetic modifications are a possible approach to counteract disease progression while getting old. Cell reprogramming of adult somatic cells toward pluripotency ought to be a promising tool for age-related diseases. However, researchers do not have control over this process as cells lose their fate, and cause potential cancerous cells or unexpected cell phenotypes. Direct and partial reprogramming were introduced in recent years with distinctive applications. Although direct reprogramming makes cells lose their identity, it has various applications in regeneration medicine. Temporary and regulated in vivo overexpression of Yamanaka factors has been shown in several experimental contexts to be achievable and is used to rejuvenate mice models. This regeneration can be accomplished by altering the epigenetic adult cell signature to the signature of a younger cell. The greatest advantage of partial reprogramming is that this method does not allow cells to lose their identity when they are resetting their epigenetic clock. It is a regimen of short-term Oct3/4, Sox2, Klf4, and c-Myc expression in vivo that prevents full reprogramming to the pluripotent state and avoids both tumorigenesis and the presence of unwanted undifferentiated cells. We know that many neurological age-related diseases, such as Alzheimer's disease, stroke, dementia, and Parkinson's disease, are the main cause of death in the last decades of life. Therefore, scientists have a special tendency regarding neuroregeneration methods to increase human life expectancy.

Astrocyte-like subpopulation of NG2 glia in the adult mouse cortex exhibits characteristics of neural progenitor cells

Glial cells expressing neuron-glial antigen 2 (NG2), also known as oligodendrocyte progenitor cells (OPCs), play a critical role in maintaining brain health. However, their ability to differentiate after ischemic injury is poorly understood. The aim of this study was to investigate the properties and functions of NG2 glia in the ischemic brain. Using transgenic mice, we selectively labeled NG2-expressing cells and their progeny in both healthy brain and after focal cerebral ischemia (FCI). Using single-cell RNA sequencing, we classified the labeled glial cells into five distinct subpopulations based on their gene expression patterns. Additionally, we examined the membrane properties of these cells using the patch-clamp technique. Of the identified subpopulations, three were identified as OPCs, whereas the fourth subpopulation had characteristics indicative of cells likely to develop into oligodendrocytes. The fifth subpopulation of NG2 glia showed astrocytic markers and had similarities to neural progenitor cells. Interestingly, this subpopulation was present in both healthy and post-ischemic tissue; however, its gene expression profile changed after ischemia, with increased numbers of genes related to neurogenesis. Immunohistochemical analysis confirmed the temporal expression of neurogenic genes and showed an increased presence of NG2 cells positive for Purkinje cell protein-4 at the periphery of the ischemic lesion 12 days after FCI, as well as NeuN-positive NG2 cells 28 and 60 days after injury. These results suggest the potential development of neuron-like cells arising from NG2 glia in the ischemic tissue. Our study provides insights into the plasticity of NG2 glia and their capacity for neurogenesis after stroke.

Mechanisms of microRNA-132 in central neurodegenerative diseases: A comprehensive review

MicroRNA-132 (miR-132) is a highly conserved molecule that plays a crucial regulatory role in central nervous system (CNS) disorders. The expression levels of miR-132 exhibit variability in various neurological disorders and have been closely linked to disease onset and progression. The expression level of miR-132 in the CNS is regulated by a diverse range of stimuli and signaling pathways, including neuronal migration and integration, dendritic outgrowth, and complexity, synaptogenesis, synaptic plasticity, as well as inflammation and apoptosis activation. The aberrant expression of miR-132 in various central neurodegenerative diseases has garnered widespread attention. Clinical studies have revealed altered miR-132 expression levels in both chronic and acute CNS diseases, positioning miR-132 as a potential biomarker or therapeutic target. An in-depth exploration of miR-132 holds the promise of enhancing our understanding of the mechanisms underlying CNS diseases, thereby offering novel insights and strategies for disease diagnosis and treatment. It is anticipated that this review will assist researchers in recognizing the potential value of miR-132 and in generating innovative ideas for clinical trials related to CNS degenerative diseases.

Direct neuronal conversion of microglia/macrophages reinstates neurological function after stroke

Although generating new neurons in the ischemic injured brain would be an ideal approach to replenish the lost neurons for repairing the damage, the adult mammalian brain retains only limited neurogenic capability. Here, we show that direct conversion of microglia/macrophages into neurons in the brain has great potential as a therapeutic strategy for ischemic brain injury. After transient middle cerebral artery occlusion in adult mice, microglia/macrophages converge at the lesion core of the striatum, where neuronal loss is prominent. Targeted expression of a neurogenic transcription factor, NeuroD1, in microglia/macrophages in the injured striatum enables their conversion into induced neuronal cells that functionally integrate into the existing neuronal circuits. Furthermore, NeuroD1-mediated induced neuronal cell generation significantly improves neurological function in the mouse stroke model, and ablation of these cells abolishes the gained functional recovery. Our findings thus demonstrate that neuronal conversion contributes directly to functional recovery after stroke.

MicroRNA-375 Is Induced during Astrocyte-to-Neuron Reprogramming and Promotes Survival of Reprogrammed Neurons when Overexpressed

While astrocyte-to-neuron (AtN) reprogramming holds great promise in regenerative medicine, the molecular mechanisms that govern this unique biological process remain elusive. To understand the function of miRNAs during the AtN reprogramming process, we performed RNA-seq of both mRNAs and miRNAs on human astrocyte (HA) cultures upon NeuroD1 overexpression. Bioinformatics analyses showed that NeuroD1 not only activated essential neuronal genes to initiate the reprogramming process but also induced miRNA changes in HA. Among the upregulated miRNAs, we identified miR-375 and its targets, neuronal ELAVL genes (nELAVLs), which encode a family of RNA-binding proteins and were also upregulated by NeuroD1. We further showed that manipulating the miR-375 level regulated nELAVLs' expression during NeuroD1-mediated reprogramming. Interestingly, miR-375/nELAVLs were also induced by the reprogramming factors Neurog2 and ASCL1 in HA, suggesting a conserved function to neuronal reprogramming, and by NeuroD1 in the mouse astrocyte culture and spinal cord. Functionally, we showed that miR-375 overexpression improved NeuroD1-mediated reprogramming efficiency by promoting cell survival at early stages in HA and did not appear to compromise the maturation of the reprogrammed neurons. Lastly, overexpression of miR-375-refractory ELAVL4 induced apoptosis and reversed the cell survival-promoting effect of miR-375 during AtN reprogramming. Together, we demonstrated a neuroprotective role of miR-375 during NeuroD1-mediated AtN reprogramming.

Engineered neurogenesis in naïve adult rat cortex by Ngn2-mediated neuronal reprogramming of resident oligodendrocyte progenitor cells

Adult tissue stem cells contribute to tissue homeostasis and repair but the long-lived neurons in the human adult cerebral cortex are not replaced, despite evidence for a limited regenerative response. However, the adult cortex contains a population of proliferating oligodendrocyte progenitor cells (OPCs). We examined the capacity of rat cortical OPCs to be re-specified to a neuronal lineage both in vitro and in vivo. Expressing the developmental transcription factor Neurogenin2 (Ngn2) in OPCs isolated from adult rat cortex resulted in their expression of early neuronal lineage markers and genes while downregulating expression of OPC markers and genes. Ngn2 induced progression through a neuronal lineage to express mature neuronal markers and functional activity as glutamatergic neurons. In vivo retroviral gene delivery of Ngn2 to naive adult rat cortex ensured restricted targeting to proliferating OPCs. Ngn2 expression in OPCs resulted in their lineage re-specification and transition through an immature neuronal morphology into mature pyramidal cortical neurons with spiny dendrites, axons, synaptic contacts, and subtype specification matching local cytoarchitecture. Lineage re-specification of rat cortical OPCs occurred without prior injury, demonstrating these glial progenitor cells need not be put into a reactive state to achieve lineage reprogramming. These results show it may be feasible to precisely engineer additional neurons directly in adult cerebral cortex for experimental study or potentially for therapeutic use to modify dysfunctional or damaged circuitry.

Lmo4 synergizes with Fezf2 to promote direct in vivo reprogramming of upper layer cortical neurons and cortical glia towards deep-layer neuron identities

In vivo direct neuronal reprogramming relies on the implementation of an exogenous transcriptional program allowing to achieve conversion of a particular neuronal or glial cell type towards a new identity. The transcription factor (TF) Fezf2 is known for its role in neuronal subtype specification of deep-layer (DL) subcortical projection neurons. High ectopic Fezf2 expression in mice can convert both upper-layer (UL) and striatal projection neurons into a corticofugal fate, even if at low efficiency. In this study, we show that Fezf2 synergizes with the nuclear co-adaptor Lmo4 to further enhance reprogramming of UL cortical pyramidal neurons into DL corticofugal neurons, at both embryonic and early postnatal stages. Reprogrammed neurons express DL molecular markers and project toward subcerebral targets, including thalamus, cerebral peduncle (CP), and spinal cord (SC). We also show that co-expression of Fezf2 with the reprogramming factors Neurog2 and Bcl2 in early postnatal mouse glia promotes glia-to-neuron conversion with partial hallmarks of DL neurons and with Lmo4 promoting further morphological complexity. These data support a novel role for Lmo4 in synergizing with Fezf2 during direct lineage conversion in vivo.

MicroRNA-375 is induced during astrocyte-to-neuron reprogramming and promotes survival of reprogrammed neurons when overexpressed

While astrocyte-to-neuron (AtN) reprogramming holds great promise in regenerative medicine, the molecular mechanisms that govern this unique biological process remain elusive. MicroRNAs (miRNAs), as post-transcriptional regulators of gene expression, play crucial roles during development and under various pathological conditions. To understand the function of miRNAs during AtN reprogramming process, we performed RNA-seq of both mRNAs and miRNAs on human astrocyte (HA) cultures upon NeuroD1 overexpression. Bioinformatics analyses showed that NeuroD1 not only activates essential neuronal genes to initiate reprogramming process but also induces miRNA changes in HA. Among the upregulated miRNAs, we identified miR-375 and its targets, neuronal ELAVL genes ( nELAVLs ), which encode a family of RNA-binding proteins and are also upregulated by NeuroD1. We further showed that manipulating miR-375 level regulates nELAVLs expression during NeuroD1-mediated reprogramming. Interestingly, miR-375/ nELAVLs are also induced by reprogramming factors Neurog2 and ASCL1 in HA suggesting a conserved function to neuronal reprogramming, and by NeuroD1 in the mouse astrocyte culture and spinal cord. Functionally, we showed that miR-375 overexpression improves NeuroD1-mediated reprogramming efficiency by promoting cell survival at early stages in HA even in cultures treated with the chemotherapy drug Cisplatin. Moreover, miR-375 overexpression doesn't appear to compromise maturation of the reprogrammed neurons in long term HA cultures. Lastly, overexpression of miR-375-refractory ELAVL4 induces apoptosis and reverses the cell survival-promoting effect of miR-375 during AtN reprogramming. Together, we demonstrate a neuro-protective role of miR-375 during NeuroD1-mediated AtN reprogramming and suggest a strategy of combinatory overexpression of NeuroD1 and miR-375 for improving neuronal reprogramming efficiency.

Therapeutic Potential of PTBP1 Inhibition, If Any, Is Not Attributed to Glia-to-Neuron Conversion

A holy grail of regenerative medicine is to replenish the cells that are lost due to disease. The adult mammalian central nervous system (CNS) has, however, largely lost such a regenerative ability. An emerging strategy for the generation of new neurons is through glia-to-neuron (GtN) conversion in vivo, mainly accomplished by the regulation of fate-determining factors. When inhibited, PTBP1, a factor involved in RNA biology, was reported to induce rapid and efficient GtN conversion in multiple regions of the adult CNS. Remarkably, PTBP1 inhibition was also claimed to greatly improve behaviors of mice with neurological diseases or aging. These phenomenal claims, if confirmed, would constitute a significant advancement in regenerative medicine. Unfortunately, neither GtN conversion nor therapeutic potential via PTBP1 inhibition was validated by the results of multiple subsequent replication studies with stringent methods. Here we review these controversial studies and conclude with recommendations for examining GtN conversion in vivo and future investigations of PTBP1.

Astrocyte-to-neuron reprogramming and crosstalk in the treatment of Parkinson's disease

Parkinson's disease (PD) is currently the fastest growing disabling neurological disorder worldwide, with motor and non-motor symptoms being its main clinical manifestations. The primary pathological features include a reduction in the number of dopaminergic neurons in the substantia nigra and decrease in dopamine levels in the nigrostriatal pathway. Existing treatments only alleviate clinical symptoms and do not stop disease progression; slowing down the loss of dopaminergic neurons and stimulating their regeneration are emerging therapies. Preclinical studies have demonstrated that transplantation of dopamine cells generated from human embryonic or induced pluripotent stem cells can restore the loss of dopamine. However, the application of cell transplantation is limited owing to ethical controversies and the restricted source of cells. Until recently, the reprogramming of astrocytes to replenish lost dopaminergic neurons has provided a promising alternative therapy for PD. In addition, repair of mitochondrial perturbations, clearance of damaged mitochondria in astrocytes, and control of astrocyte inflammation may be extensively neuroprotective and beneficial against chronic neuroinflammation in PD. Therefore, this review primarily focuses on the progress and remaining issues in astrocyte reprogramming using transcription factors (TFs) and miRNAs, as well as exploring possible new targets for treating PD by repairing astrocytic mitochondria and reducing astrocytic inflammation.

Endothelial cell direct reprogramming: Past, present, and future

Ischemic cardiovascular disease still remains as a leading cause of morbidity and mortality despite various medical, surgical, and interventional therapy. As such, cell therapy has emerged as an attractive option because it tackles underlying problem of the diseases by inducing neovascularization in ischemic tissue. After overall failure of adult stem or progenitor cells, studies attempted to generate endothelial cells (ECs) from pluripotent stem cells (PSCs). While endothelial cells (ECs) differentiated from PSCs successfully induced vascular regeneration, differentiating volatility and tumorigenic potential is a concern for their clinical applications. Alternatively, direct reprogramming strategies employ lineage-specific factors to change cell fate without achieving pluripotency. ECs have been successfully reprogrammed via ectopic expression of transcription factors (TFs) from endothelial lineage. The reprogrammed ECs induced neovascularization in vitro and in vivo and thus demonstrated their therapeutic value in animal models of vascular insufficiency. Methods of delivering reprogramming factors include lentiviral or retroviral vectors and more clinically relevant, non-integrative adenoviral and episomal vectors. Most studies made use of fibroblast as a source cell for reprogramming, but reprogrammability of other clinically relevant source cell types has to be evaluated. Specific mechanisms and small molecules that are involved in the aforementioned processes tackles challenges associated with direct reprogramming efficiency and maintenance of reprogrammed EC characteristics. After all, this review provides summary of past and contemporary methods of direct endothelial reprogramming and discusses the future direction to overcome these challenges to acquire clinically applicable reprogrammed ECs.

Evaluation of dihydrotestosterone and dihydroprogesterone levels and gene expression of genes involved in neurosteroidogenesis in the SH-SY5Y Alzheimer disease cell model

Introduction

Alzheimer's disease (AD) is the most common form of dementia worldwide. This study investigated the effects of lipopolysaccharide on neurosteroidogenesis and its relationship to growth and differentiation using SH-SY5Y cells.

Methods

In this study, we used the MTT assay to assess the impact of LPS on SH-SY5Y cell viability. We also evaluated apoptotic effects using FITC Annexin V staining to detect phosphatidylserine in the cell membrane. To identify gene expression related to human neurogenesis, we utilized the RT² Profiler TM PCR array human neurogenesis PAHS-404Z.

Results

Our study found that LPS had an IC50 level of 0.25 μg/mL on the SH-SY5Y cell line after 48 h. We observed Aβ deposition in SH-SY5Y cells treated with LPS, and a decrease in DHT and DHP levels in the cells. Our analysis showed that the total rate of apoptosis varied with LPS dilution: 4.6% at 0.1 μg/mL, 10.5% at 10 μg/mL, and 44.1% at 50 μg/mL. We also observed an increase in the expression of several genes involved in human neurogenesis, including ASCL1, BCL2, BDNF, CDK5R1, CDK5RAP2, CREB1, DRD2, HES1, HEYL, NOTCH1, STAT3, and TGFB1, after treatment with LPS at 10 μg/mL and 50 μg/mL. LPS at 50 μg/mL increased the expression of FLNA and NEUROG2, as well as the other genes mentioned.

Conclusion

Our study showed that LPS treatment altered the expression of human neurogenesis genes and decreased DHT and DHP levels in SH-SY5Y cells. These findings suggest that targeting LPS, DHT, and DHP could be potential therapeutic strategies to treat AD or improve its symptoms.

Strategies and mechanisms of neuronal reprogramming

Traumatic injury and neurodegenerative diseases of the central nervous system (CNS) are difficult to treat due to the poorly regenerative nature of neurons. Engrafting neural stem cells into the CNS is a classic approach for neuroregeneration. Despite great advances, stem cell therapy still faces the challenges of overcoming immunorejection and achieving functional integration. Neuronal reprogramming, a recent innovation, converts endogenous non-neuronal cells (e.g., glial cells) into mature neurons in the adult mammalian CNS. In this review, we summarize the progress of neuronal reprogramming research, mainly focusing on strategies and mechanisms of reprogramming. Furthermore, we highlight the advantages of neuronal reprogramming and outline related challenges. Although the significant development has been made in this field, several findings are controversial. Even so, neuronal reprogramming, especially in vivo reprogramming, is expected to become an effective treatment for CNS neurodegenerative diseases.

The cortical NG2-glia response to traumatic brain injury

Traumatic brain injury (TBI) is a significant worldwide cause of morbidity and mortality. A chronic neurologic disease bearing the moniker of "the silent epidemic," TBI currently has no targeted therapies to ameliorate cellular loss or enhance functional recovery. Compared with those of astrocytes, microglia, and peripheral immune cells, the functions and mechanisms of NG2-glia following TBI are far less understood, despite NG2-glia comprising the largest population of regenerative cells in the mature cortex. Here, we synthesize the results from multiple rodent models of TBI, with a focus on cortical NG2-glia proliferation and lineage potential, and propose future avenues for glia researchers to address this unique cell type in TBI. As the molecular mechanisms that regulate NG2-glia regenerative potential are uncovered, we posit that future therapeutic strategies may exploit cortical NG2-glia to augment local cellular recovery following TBI.

In vivo astrocyte-to-neuron reprogramming for central nervous system regeneration: a narrative review

The inability of damaged neurons to regenerate within the mature central nervous system (CNS) is a significant neuroscientific challenge. Astrocytes are an essential component of the CNS and participate in many physiological processes including blood-brain barrier formation, axon growth regulation, neuronal support, and higher cognitive functions such as memory. Recent reprogramming studies have confirmed that astrocytes in the mature CNS can be transformed into functional neurons. Building on in vitro work, many studies have demonstrated that astrocytes can be transformed into neurons in different disease models to replace damaged or lost cells. However, many findings in this field are controversial, as the source of new neurons has been questioned. This review summarizes progress in reprogramming astrocytes into neurons in vivo in animal models of spinal cord injury, brain injury, Huntington's disease, Parkinson's disease, Alzheimer's disease, and other neurodegenerative conditions.

Stem cell programming - prospects for perinatal medicine

Recreating human cell and organ systems in vitro has tremendous potential for disease modeling, drug discovery and regenerative medicine. The aim of this short overview is to recapitulate the impressive progress that has been made in the fast-developing field of cell programming during the past years, to illuminate the advantages and limitations of the various cell programming technologies for addressing nervous system disorders and to gauge their impact for perinatal medicine.

Up-Regulation of Astrocytic Fgfr4 Expression in Adult Mice after Spinal Cord Injury

Spinal cord injury (SCI) leads to persistent neurological deficits without available curative treatment. After SCI astrocytes within the lesion vicinity become reactive, these undergo major morphological, and molecular transformations. Previously, we reported that following SCI, over 10% of resident astrocytes surrounding the lesion spontaneously transdifferentiate towards a neuronal phenotype. Moreover, this conversion is associated with an increased expression of fibroblast growth factor receptor 4 (Fgfr4), a neural stem cell marker, in astrocytes. Here, we evaluate the therapeutic potential of gene therapy upon Fgfr4 over-expression in mature astrocytes following SCI in adult mice. We found that Fgfr4 over-expression in astrocytes immediately after SCI improves motor function recovery; however, it may display sexual dimorphism. Improved functional recovery is associated with a decrease in spinal cord lesion volume and reduced glial reactivity. Cell-specific transcriptomic profiling revealed concomitant downregulation of Notch signaling, and up-regulation of neurogenic pathways in converting astrocytes. Our findings suggest that gene therapy targeting Fgfr4 over-expression in astrocytes after injury is a feasible therapeutic approach to improve recovery following traumatism of the spinal cord. Moreover, we stress that a sex-dependent response to astrocytic modulation should be considered for the development of effective translational strategies in other neurological disorders.

Inducing Neural Regeneration from Glia Using Proneural bHLH Transcription Factors

Endogenous regeneration strategies to replace lost neurons hold great promise for treating neurodegenerative disorders. In the majority of cases, neural regeneration is induced by converting resident glial cells into neurogenic precursors. This review will outline how proneural bHLH transcription factors can be used to reprogram glia in the brain and retina into a source for new neurons.

Astrocyte contribution to dysfunction, risk and progression in neurodegenerative disorders

There is increasing appreciation that non-neuronal cells contribute to the initiation, progression and pathology of diverse neurodegenerative disorders. This Review focuses on the role of astrocytes in disorders including Alzheimer disease, Parkinson disease, Huntington disease and amyotrophic lateral sclerosis. The important roles astrocytes have in supporting neuronal function in the healthy brain are considered, along with studies that have demonstrated how the physiological properties of astrocytes are altered in neurodegenerative disorders and may explain their contribution to neurodegeneration. Further, the question of whether in neurodegenerative disorders with specific genetic mutations these mutations directly impact on astrocyte function, and may suggest a driving role for astrocytes in disease initiation, is discussed. A summary of how astrocyte transcriptomic and proteomic signatures are altered during the progression of neurodegenerative disorders and may relate to functional changes is provided. Given the central role of astrocytes in neurodegenerative disorders, potential strategies to target these cells for future therapeutic avenues are discussed.

Neurogenesis potential of oligodendrocyte precursor cells from oligospheres and injured spinal cord

Severe traumatic spinal cord injury (SCI) leads to long-lasting oligodendrocyte death and extensive demyelination in the lesion area. Oligodendrocyte progenitor cells (OPCs) are the reservoir of new mature oligodendrocytes during damaged myelin regeneration, which also have latent potential for neurogenic regeneration and oligospheres formation. Whether oligospheres derived OPCs can differentiate into neurons and the neurogenesis potential of OPCs after SCI remains unclear. In this study, primary OPCs cultures were used to generate oligospheres and detect the differentiation and neurogenesis potential of oligospheres. In vivo, SCI models of juvenile and adult mice were constructed. Combining the single-cell RNA sequencing (scRNA-seq), bulk RNA sequencing (RNA-seq), bioinformatics analysis, immunofluorescence staining, and molecular experiment, we investigated the neurogenesis potential and mechanisms of OPCs in vitro and vivo. We found that OPCs differentiation and oligodendrocyte morphology were significantly different between brain and spinal cord. Intriguingly, we identify a previously undescribed findings that OPCs were involved in oligospheres formation which could further differentiate into neuron-like cells. We also firstly detected the intermediate states of oligodendrocytes and neurons during oligospheres differentiation. Furthermore, we found that OPCs were significantly activated after SCI. Combining scRNA-seq and bulk RNA-seq data from injured spinal cord, we confirmed the neurogenesis potential of OPCs and the activation of endoplasmic reticulum stress after SCI. Inhibition of endoplasmic reticulum stress could effectively attenuate OPCs death. Additionally, we also found that endoplasmic reticulum may regulate the stemness and differentiation of oligospheres. These findings revealed the neurogenesis potential of OPCs from oligospheres and injured spinal cord, which may provide a new source and a potential target for spinal cord repair.

Enhanced proliferation of oligodendrocyte progenitor cells following retrovirus mediated Achaete-scute complex-like 1 overexpression in the postnatal cerebral cortex in vivo

The proneural transcription factor Achaete-scute complex-like 1 (Ascl1) is a major regulator of neural fate decisions, implicated both in neurogenesis and oligodendrogliogenesis. Focusing on its neurogenic activity, Ascl1 has been widely used to reprogram non-neuronal cells into induced neurons. In vitro, Ascl1 induces efficient reprogramming of proliferative astroglia from the early postnatal cerebral cortex into interneuron-like cells. Here, we examined whether Ascl1 can similarly induce neuronal reprogramming of glia undergoing proliferation in the postnatal mouse cerebral cortex in vivo. Toward this goal, we targeted cortical glia during the peak of proliferative expansion (i.e., postnatal day 5) by injecting a retrovirus encoding for Ascl1 into the mouse cerebral cortex. In contrast to the efficient reprogramming observed in vitro, in vivo Ascl1-transduced glial cells were converted into doublecortin-immunoreactive neurons only with very low efficiency. However, we noted a drastic shift in the relative number of retrovirus-transduced Sox10-positive oligodendrocyte progenitor cells (OPCs) as compared to glial fibrillary acidic protein (GFAP)-positive astrocytes. Genetic fate mapping demonstrated that this increase in OPCs was not due to Ascl1-mediated astrocyte-to-OPC fate conversion. Rather, EdU incorporation experiments revealed that Ascl1 caused a selective increase in proliferative activity of OPCs, but not astrocytes. Our data indicate that rather than inducing neuronal reprogramming of glia in the early postnatal cortex, Ascl1 is a selective enhancer of OPC proliferation.

Reprogramming Müller glia to regenerate ganglion-like cells in adult mouse retina with developmental transcription factors

Many neurodegenerative diseases cause degeneration of specific types of neurons. For example, glaucoma leads to death of retinal ganglion cells, leaving other neurons intact. Neurons are not regenerated in the adult mammalian central nervous system. However, in nonmammalian vertebrates, glial cells spontaneously reprogram into neural progenitors and replace neurons after injury. We have recently developed strategies to stimulate regeneration of functional neurons in the adult mouse retina by overexpressing the proneural factor Ascl1 in Müller glia. Here, we test additional transcription factors (TFs) for their ability to direct regeneration to particular types of retinal neurons. We engineered mice to express different combinations of TFs in Müller glia, including Ascl1, Pou4f2, Islet1, and Atoh1. Using immunohistochemistry, single-cell RNA sequencing, single-cell assay for transposase-accessible chromatin sequencing, and electrophysiology, we find that retinal ganglion-like cells can be regenerated in the damaged adult mouse retina in vivo with targeted overexpression of developmental retinal ganglion cell TFs.

Cell Reprogramming for Regeneration and Repair of the Nervous System

A persistent barrier to the cure and treatment of neurological diseases is the limited ability of the central and peripheral nervous systems to undergo neuroregeneration and repair. Recent efforts have turned to regeneration of various cell types through cellular reprogramming of native cells as a promising therapy to replenish lost or diminished cell populations in various neurological diseases. This review provides an in-depth analysis of the current viral vectors, genes of interest, and target cellular populations that have been studied, as well as the challenges and future directions of these novel therapies. Furthermore, the mechanisms by which cellular reprogramming could be optimized as treatment in neurological diseases and a review of the most recent cellular reprogramming in vitro and in vivo studies will also be discussed.

The Notch Signaling Pathway Regulates Differentiation of NG2 Cells into Oligodendrocytes in Demyelinating Diseases

NG2 cells are highly proliferative glial cells that can self-renew or differentiate into oligodendrocytes, promoting remyelination. Following demyelination, the proliferative and differentiation potentials of NG2 cells increase rapidly, enhancing their differentiation into functional myelinating cells. Levels of the transcription factors Olig1 and Olig2 increase during the differentiation of NG2 cells and play important roles in the development and repair of oligodendrocytes. However, the ability to generate new oligodendrocytes is hampered by injury-related factors (e.g., myelin fragments, Wnt and Notch signaling components), leading to failed differentiation and maturation of NG2 cells into oligodendrocytes. Here, we review Notch signaling as a negative regulator of oligodendrocyte differentiation and discuss the extracellular ligands, intracellular pathways, and key transcription factors involved.