In vivo reprogramming of NG2 glia enables adult neurogenesis and functional recovery following spinal cord injury

Identification of the core regulatory program driving NEUROD1-induced neuronal reprogramming

NEUROD1 (ND1)-induced astrocyte-to-neuron (AtN) conversion shows promise for treating neurological disorders. To gain insight into the molecular mechanisms of neuronal reprogramming, we established an in vitro system using primary cortical astrocyte cultures from postnatal rats and employed single-cell and multiomics sequencing. Our findings indicate that the initial cultures primarily consisted of immature astrocytes (ImAs), with potentially a minor presence of radial glial cells. The ImAs initially went through an intermediate state, activating both astrocyte and neural progenitor genes. Subsequently, they mimic in vivo neurogenesis to acquire mature neuronal characteristics. We show that ND1 acted as a pioneer factor that reshapes the chromatin landscape of astrocytes to that of neurons. This restructuring promotes the expression of neurogenic genes via inducing H3K27ac modification. Through integrative analysis of various ND1-induced neuronal specification systems, we identified 25 ND1 targets, including Hes6, as key regulators. Thus, our work highlights the key role of ND1 and its downstream regulators in neuronal reprogramming.

Effect of microglial Pd1 on glial scar formation after spinal cord injury in mice

The cross talk between microglia and astrocytes following spinal cord injury (SCI) greatly decides the prognosis. However, a comprehensive understanding of the molecular mechanisms by which microglia regulate astrocytic activity post-SCI is lacking. Programmed cell death protein 1 (Pdcd1, Pd1) plays a crucial role in modulating immune responses by exerting suppressive effects on microglia and peripheral immune cells within the central nervous system. Previous studies have shown the involvement of Pd1 in the pathogenesis of SCI; however, the role of microglial Pd1 in astrocytic activation and the following glial scar formation remains elusive. Here, we demonstrated that the pharmacological depletion of microglia using minocycline decreased the expression of tumor necrosis factor-alpha and interleukin-6 while concurrently increasing the expression of interleukin-10 following SCI, thereby facilitating motor function recovery in mice. We observed an increase in Pd1 expression in the injured spinal cord after SCI, with precise localization of Pd1 within microglia. Based on Pd1 knockout (KO) mice, we further revealed that Pd1 deficiency disrupted glial scar formation, leading to increased inflammation, impeded nerve regeneration, enlarged tissue damage, and compromised functional recovery following SCI. In vitro study showed that siRNA-mediated inhibition of Pd1 in microglia followed by lipopolysaccharide treatment significantly inhibited astrocyte migration and upregulated the secretion of tumor necrosis factor-alpha and CXCL9 from microglia, indicating that microglial Pd1 regulates glial scar formation through modulating the inflammatory microenvironment. Our study gains a new mechanistic insight into how microglial Pd1 decides the fate of SCI and promotes microglial Pd1 as a promising therapeutic target for SCI.

Direct reprogramming of fibroblasts into spiral ganglion neurons by defined transcription factors

Degeneration of the cochlear spiral ganglion neurons (SGNs) is one of the major causes of sensorineural hearing loss and significantly impacts the outcomes of cochlear implantation. Functional regeneration of SGNs holds great promise for treating sensorineural hearing loss. In this study, we systematically screened 33 transcriptional regulators implicated in neuronal and SGN fate. Using gene expression array and principal component analyses, we identified a sequential combination of Ascl1, Pou4f1 and Myt1l (APM) in promoting functional reprogramming of SGNs. The neurons induced by APM expressed mature neuronal and SGN lineage-specific markers, displayed mature SGN-like electrophysiological characteristics and exhibited single-cell transcriptomes resembling the endogenous SGNs. Thus, transcription factors APM may serve as novel candidates for direct reprogramming of SGNs and hearing recovery due to SGN damages.

SOX Genes in Spinal Cord Injury: Redefining Neural Stem Cell Regeneration Strategies

The study design is literature review. The sex-determining region Y gene (SRY)-related high mobility group box (HMG)-box (SOX) gene family has primarily been associated with neural development and sex determination and is a key component of human embryonic development. Recent studies on zebrafish models have demonstrated that the unique ability of the latter for central nervous tissue (CNS) repair following injury is largely mediated by SOX genes. Given that efforts aimed at the structural regeneration and functional restoration of neural tissue still represent a major therapeutic challenge in patients suffering CNS injury, these findings have initiated a discussion regarding the development of novel therapeutic strategies for SCI focusing on neural tissue regeneration. Spinal cord injury (SCI), in particular, represents a field that could greatly benefit from studies related to the function of the SOX genes. Neuro-informatics Laboratory, Mayo Clinic, Rochester, MN. A literature review was conducted, with a focus on SOX gene that has been described in the experimental studies of SCI. In this review, the existing evidence linking the SOX gene family to the pathophysiology of SCI is summarized, and future research steps regarding the potential implications of the SOX genes in neurological recovery following SCI are discussed, especially focusing on highlighting potential therapeutic targets. The potential implications of the latter could play a crucial role in future efforts to advance the treatment approaches to SCI.

Enhancement of motor functional recovery in thoracic spinal cord injury: voluntary wheel running versus forced treadmill exercise

JOURNAL/nrgr/04.03/01300535-202503000-00028/figure1/v/2024-06-17T092413Z/r/image-tiff Spinal cord injury necessitates effective rehabilitation strategies, with exercise therapies showing promise in promoting recovery. This study investigated the impact of rehabilitation exercise on functional recovery and morphological changes following thoracic contusive spinal cord injury. After a 7-day recovery period after spinal cord injury, mice were assigned to either a trained group (10 weeks of voluntary running wheel or forced treadmill exercise) or an untrained group. Bi-weekly assessments revealed that the exercise-trained group, particularly the voluntary wheel exercise subgroup, displayed significantly improved locomotor recovery, more plasticity of dopaminergic and serotonin modulation compared with the untrained group. Additionally, exercise interventions led to gait pattern restoration and enhanced transcranial magnetic motor-evoked potentials. Despite consistent injury areas across groups, exercise training promoted terminal innervation of descending axons. In summary, voluntary wheel exercise shows promise for enhancing outcomes after thoracic contusive spinal cord injury, emphasizing the role of exercise modality in promoting recovery and morphological changes in spinal cord injuries. Our findings will influence future strategies for rehabilitation exercises, restoring functional movement after spinal cord injury.

Single-cell RNA sequencing provides new insights into the interaction between astrocytes and neurons after spinal cord injury in mice

Background

Spinal cord injury (SCI) is a devastating neurological disease in which astrocytes play a central role. Understanding the relationship between different subtypes of astrocytes and neuron subtypes during the progression of SCI is critical to understanding the disease.

Methods and results

In this study, single-cell RNA sequencing (scRNA-seq) was used to analyze the transcriptome data of acute, subacute and intermediate stages of SCI in mice as well as normal tissues. Different subtypes of astrocytes and neuronal cells were identified and their dynamic changes and functionalities during the development of SCI. An intriguing discovery was the identification of a specific subtype of astrocytes characterized by unique expression of Gap43, Vim, Aldoc, and Mt1. This subtype of cells shows similarities in gene expression with neurons and potentially transitioned into neurons during the course of SCI. Furthermore, we have uncovered the important role of the glycolytic pathway in this cellular transformation process. Furthermore, through cellular interaction analysis, we validated pathways (mdk-ptprz1,ptn-ptprz1,ptn-sdc3) associated with the potential conversion of these specific cell subsets into neurons. Finally, these cells were observed by fluorescence microscopy and critical gene expressions were validated by Western blot.

Conclusions

The results of this study not only deepen our understanding of the mechanisms underlying SCI, but also provide new insights and opportunities for the development of novel therapeutic strategies and interventions.

Dynamic regulation of NeuroD1 expression level by a novel viral construct during astrocyte-to-neuron reprogramming

Astrocyte-to-neuron reprogramming presents a viable approach for regenerative medicine. The reprogramming factor NeuroD1 has demonstrated capability of neuronal reprogramming with high efficiency both in culture and in the injured central nervous system. High level of NeuroD1 expression is required to break down the cellular identity barrier for a successful reprogramming, and yet persistence of this high level drives the reprogrammed neurons primarily to glutamatergic subtype. This is consistent with the critical role of NeuroD1 in determination of glutamatergic neuronal lineage during development. However, diversified neuronal subtypes are needed to establish appropriate neuronal connectivity in disease/injury conditions. We reason that continuously high level of NeuroD1 expression forces the reprogrammed neurons into glutamatergic subtype, and that reducing NeuroD1 level after reprogramming may allow generation of neurons with diversified subtypes. For this purpose, we engineered a novel viral expression vector by which NeuroD1 expression can be dynamically regulated during the reprogramming process. Specifically, the target site of a neuron-specific microRNA (miR-124) is incorporated in the expression system. Therefore, this novel construct would still achieve a high NeuroD1 expression level in astrocytes for reprogramming to occur and yet reduce its level in the reprogrammed neurons by suppression of endogenous miR-124. In this study, we demonstrated that this construct elicits a dynamic gene expression pattern with much reduced level of NeuroD1 at later stages of neuronal reprogramming. We also showed that this construct still retains relatively high reprogramming efficiency and can generate mature neurons with an enhanced GABAergic neuronal phenotype.

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.

Recent progress of principal techniques used in the study of Müller glia reprogramming in mice

In zebrafish, Müller glia (MG) cells retain the ability to proliferate and de-differentiate into retinal progenitor-like cells, subsequently differentiating into retinal neurons that can replace those damaged or lost due to retinal injury. In contrast, the reprogramming potential of MG in mammals has been lost, with these cells typically responding to retinal damage through gliosis. Considerable efforts have been dedicated to achieving the reprogramming of MG cells in mammals. Notably, significant advancements have been achieved in reprogramming MG cells in mice employing various methodologies. At the same time, some inevitable challenges have hindered identifying accurate MG cell reprogramming rather than the illusion, let alone improving the reprogramming efficiency and maturity of daughter cells. Recently, several strategies, including lineage tracking, multi-omics techniques, and functional analysis, have been developed to investigate the MG reprogramming process in mice. This review summarizes both the advantages and limitations of these novel strategies for analyzing MG reprogramming in mice, offering insights into enhancing the reliability and efficiency of MG reprogramming.

Partial Cell Fate Transitions to Promote Cardiac Regeneration

Heart disease, including myocardial infarction (MI), remains a leading cause of morbidity and mortality worldwide, necessitating the development of more effective regenerative therapies. Direct reprogramming of cardiomyocyte-like cells from resident fibroblasts offers a promising avenue for myocardial regeneration, but its efficiency and consistency in generating functional cardiomyocytes remain limited. Alternatively, reprogramming induced cardiac progenitor cells (iCPCs) could generate essential cardiac lineages, but existing methods often involve complex procedures. These limitations underscore the need for advanced mechanistic insights and refined reprogramming strategies to improve reparative outcomes in the heart. Partial cellular fate transitions, while still a relatively less well-defined area and primarily explored in longevity and neurobiology, hold remarkable promise for cardiac repair. It enables the reprogramming or rejuvenation of resident cardiac cells into a stem or progenitor-like state with enhanced cardiogenic potential, generating the reparative lineages necessary for comprehensive myocardial recovery while reducing safety risks. As an emerging strategy, partial cellular fate transitions play a pivotal role in reversing myocardial infarction damage and offer substantial potential for therapeutic innovation. This review will summarize current advances in these areas, including recent findings involving two transcription factors that critically regulate stemness and cardiogenesis. It will also explore considerations for further refining these approaches to enhance their therapeutic potential and safety.

Controversies and insights into PTBP1-related astrocyte-neuron transdifferentiation: neuronal regeneration strategies for Parkinson's and Alzheimer's disease

Promising therapeutic strategies are being explored to replace or regenerate the neuronal populations that are lost in patients with neurodegenerative disorders. Several research groups have attempted direct reprogramming of astrocytes into neurons by manipulating the expression of polypyrimidine tract-binding protein 1 (PTBP1) and claimed putative converted neurons to be functional, which led to improved disease outcomes in animal models of several neurodegenerative disorders. However, a few other studies reported data that contradict these claims, raising doubt about whether PTBP1 suppression truly reprograms astrocytes into neurons and the therapeutic potential of this approach. This review discusses recent advances in regenerative therapeutics including stem cell transplantations for central nervous system disorders, with a particular focus on Parkinson's and Alzheimer's diseases. We also provide a perspective on this controversy by considering that astrocyte heterogeneity may be the key to understanding the discrepancy in published studies, and that certain subpopulations of these glial cells may be more readily converted into neurons.

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.

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.

Efficient Dlx2-mediated astrocyte-to-neuron conversion and inhibition of neuroinflammation by NeuroD1

In vivo astrocyte-to-neuron (AtN) conversion induced by overexpression of neural transcriptional factors has great potential for neural regeneration and repair. Here, we demonstrate that a single neural transcriptional factor, Dlx2, converts mouse striatal astrocytes into neurons in a dose-dependent manner. Lineage-tracing studies in Aldh1l1-CreERT2 mice confirm that Dlx2 can convert striatal astrocytes into DARPP32+ and Ctip2+ medium spiny neurons (MSNs). Time-course studies reveal a gradual conversion from astrocytes to neurons in 1 month, with a distinct intermediate state in between astrocytes and neurons. Interestingly, when Dlx2-infected astrocytes start to lose astrocytic markers, the other local astrocytes proliferate to maintain astrocytic levels in the converted areas. Unexpectedly, although Dlx2 efficiently reprograms astrocytes into neurons in the gray matter striatum, it also induces partial reprogramming of astrocytes in the white matter corpus callosum. Such partial reprogramming of white matter astrocytes is associated with neuroinflammation, which can be suppressed by the addition of NeuroD1. Our results highlight the importance of investigating AtN conversion in both the gray matter and white matter to thoroughly evaluate therapeutic potentials. This study also unveils the critical role of anti-inflammation by NeuroD1 during AtN conversion.

Exosomes as promising bioactive materials in the treatment of spinal cord injury

Patients with spinal cord injury (SCI) have permanent devastating motor and sensory disabilities. Secondary SCI is known for its complex progression and presents with sophisticated aberrant inflammation, vascular changes, and secondary cellular dysfunction, which aggravate the primary damage. Since their initial discovery, the potent neuroprotective effects and powerful delivery abilities of exosomes (Exos) have been reported in different research fields, including SCI. In this study, we summarize therapeutic advances related to the application of Exos in preclinical animal studies. Subsequently, we discuss the mechanisms of action of Exos derived from diverse cell types, including neurogenesis, angiogenesis, blood-spinal cord barrier preservation, anti-apoptosis, and anti-inflammatory potential. We also evaluate the relationship between the Exo delivery cargo and signaling pathways. Finally, we discuss the challenges and advantages of using Exos to offer innovative insights regarding the development of efficient clinical strategies for SCI.

Simple and highly specific targeting of resident microglia with adeno-associated virus

Microglia, as the immune cells of the central nervous system (CNS), play dynamic roles in both healthy and diseased conditions. The ability to genetically target microglia using viruses is crucial for understanding their functions and advancing microglia-based treatments. We here show that resident microglia can be simply and specifically targeted using adeno-associated virus (AAV) vectors containing a 466-bp DNA fragment from the human IBA1 (hIBA1) promoter. This targeting approach is applicable to both resting and reactive microglia. When combining the short hIBA1 promoter with the target sequence of miR124, up to 98% of transduced cells are identified as microglia. Such a simple and highly specific microglia-targeting strategy may be further optimized for research and therapeutics.

Suppression of PTBP1 in hippocampal astrocytes promotes neurogenesis and ameliorates recognition memory in mice with cerebral ischemia

The therapeutic potential of suppressing polypyrimidine tract-binding protein 1 (Ptbp1) messenger RNA by viral transduction in a post-stroke dementia mouse model has not yet been examined. In this study, 3 days after cerebral ischemia, we injected a viral vector cocktail containing adeno-associated virus (AAV)-pGFAP-mCherry and AAV-pGFAP-CasRx (control vector) or a cocktail of AAV-pGFAP-mCherry and AAV-pGFAP-CasRx-SgRNA-(Ptbp1) (1:5, 1.0 × 1011 viral genomes) into post-stroke mice via the tail vein. We observed new mCherry/NeuN double-positive neuron-like cells in the hippocampus 56 days after cerebral ischemia. A portion of mCherry/GFAP double-positive astrocyte-like glia could have been converted into new mCherry/NeuN double-positive neuron-like cells with morphological changes. The new neuronal cells integrated into the dentate gyrus and recognition memory was significantly ameliorated. These results demonstrated that the in vivo conversion of hippocampal astrocyte-like glia into functional new neurons by the suppression of Ptbp1 might be a therapeutic strategy for post-stroke dementia.

Neuronal repair after spinal cord injury by in vivo astrocyte reprogramming mediated by the overexpression of NeuroD1 and Neurogenin-2

BACKGROUND

As a common disabling disease, irreversible neuronal death due to spinal cord injury (SCI) is the root cause of functional impairment; however, the capacity for neuronal regeneration in the developing spinal cord tissue is limited. Therefore, there is an urgent need to investigate how defective neurons can be replenished and functionally integrated by neural regeneration; the reprogramming of intrinsic cells into functional neurons may represent an ideal solution.

METHODS

A mouse model of transection SCI was prepared by forceps clamping, and an adeno-associated virus (AAV) carrying the transcription factors NeuroD1 and Neurogenin-2(Ngn2) was injected in situ into the spinal cord to specifically overexpress these transcription factors in astrocytes close to the injury site. 5-bromo-2´-deoxyuridine (BrdU) was subsequently injected intraperitoneally to continuously track cell regeneration, neuroblasts and immature neurons marker expression, neuronal regeneration, and glial scar regeneration. In addition, immunoprotein blotting was used to measure the levels of transforming growth factor-β (TGF-β) pathway-related protein expression. We also evaluated motor function, sensory function, and the integrity of the blood-spinal cord barrier(BSCB).

RESULTS

The in situ overexpression of NeuroD1 and Ngn2 in the spinal cord was achieved by specific AAV vectors. This intervention led to a significant increase in cell regeneration and the proportion of cells with neuroblasts and immature neurons cell properties at the injury site(p < 0.0001). Immunofluorescence staining identified astrocytes with neuroblasts and immature neurons cell properties at the site of injury while neuronal marker-specific staining revealed an increased number of mature astrocytes at the injury site. Behavioral assessments showed that the intervention did not improve The BMS (Basso mouse scale) score (p = 0.0726) and gait (p > 0.05), although the treated mice had more sensory sensitivity and greater voluntary motor ability in open field than the non-intervention mice. We observed significant repair of the BSCB at the center of the injury site (p < 0.0001) and a significant improvement in glial scar proliferation. Electrophysiological assessments revealed a significant improvement in spinal nerve conduction (p < 0.0001) while immunostaining revealed that the levels of TGF-β protein at the site of injury in the intervention group were lower than control group (p = 0.0034); in addition, P70 s6 and PP2A related to the TGF-β pathway showed ascending trend (p = 0.0036, p = 0.0152 respectively).

CONCLUSIONS

The in situ overexpression of NeuroD1 and Ngn2 in the spinal cord after spinal cord injury can reprogram astrocytes into neurons and significantly enhance cell regeneration at the injury site. The reprogramming of astrocytes can lead to tissue repair, thus improving the reduced threshold and increasing voluntary movements. This strategy can also improve the integrity of the blood-spinal cord barrier and enhance nerve conduction function. However, the simple reprogramming of astrocytes cannot lead to significant improvements in the striding function of the lower limbs.

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.

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.

AAV6 mediated Gsx1 expression in neural stem progenitor cells promotes neurogenesis and restores locomotor function after contusion spinal cord injury

Genomic screened homeobox 1 (Gsx1 or Gsh1) is a neurogenic transcription factor required for the generation of excitatory and inhibitory interneurons during spinal cord development. In the adult, lentivirus (LV) mediated Gsx1 expression promotes neural regeneration and functional locomotor recovery in a mouse model of lateral hemisection spinal cord injury (SCI). The LV delivery method is clinically unsafe due to insertional mutations to the host DNA. In addition, the most common clinical case of SCI is contusion/compression. In this study, we identify that adeno-associated virus serotype 6 (AAV6) preferentially infects neural stem/progenitor cells (NSPCs) in the injured spinal cord. Using a rat model of contusion SCI, we demonstrate that AAV6 mediated Gsx1 expression promotes neurogenesis, increases the number of neuroblasts/immature neurons, restores excitatory/inhibitory neuron balance and serotonergic neuronal activity through the lesion core, and promotes locomotor functional recovery. Our findings support that AAV6 preferentially targets NSPCs for gene delivery and confirmed Gsx1 efficacy in clinically relevant rat model of contusion SCI.

Application of Lineage Tracing in Central Nervous System Development and Regeneration

The central nervous system (CNS) is a complicated neural network. The origin and evolution of functional neurons and glia cells remain unclear, as do the cellular alterations that occur during the course of cerebral disease rehabilitation. Lineage tracing is a valuable method for tracing specific cells and achieving a better understanding of the CNS. Recently, various technological breakthroughs have been made in lineage tracing, such as the application of various combinations of fluorescent reporters and advances in barcode technology. The development of lineage tracing has given us a deeper understanding of the normal physiology of the CNS, especially the pathological processes. In this review, we summarize these advances of lineage tracing and their applications in CNS. We focus on the use of lineage tracing techniques to elucidate the process CNS development and especially the mechanism of injury repair. Deep understanding of the central nervous system will help us to use existing technologies to diagnose and treat diseases.

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.

Oligodendrocyte Progenitors in Glial Scar: A Bet on Remyelination

Oligodendrocyte progenitor cells (OPCs) represent a subtype of glia, giving rise to oligodendrocytes, the myelin-forming cells in the central nervous system (CNS). While OPCs are highly proliferative during development, they become relatively quiescent during adulthood, when their fate is strictly influenced by the extracellular context. In traumatic injuries and chronic neurodegenerative conditions, including those of autoimmune origin, oligodendrocytes undergo apoptosis, and demyelination starts. Adult OPCs become immediately activated; they migrate at the lesion site and proliferate to replenish the damaged area, but their efficiency is hampered by the presence of a glial scar-a barrier mainly formed by reactive astrocytes, microglia and the deposition of inhibitory extracellular matrix components. If, on the one hand, a glial scar limits the lesion spreading, it also blocks tissue regeneration. Therapeutic strategies aimed at reducing astrocyte or microglia activation and shifting them toward a neuroprotective phenotype have been proposed, whereas the role of OPCs has been largely overlooked. In this review, we have considered the glial scar from the perspective of OPCs, analysing their behaviour when lesions originate and exploring the potential therapies aimed at sustaining OPCs to efficiently differentiate and promote remyelination.

Advancements in neuroregenerative and neuroprotective therapies for traumatic spinal cord injury

Traumatic spinal cord injuries (SCIs) continue to be a major healthcare concern, with a rising prevalence worldwide. In response to this growing medical challenge, considerable scientific attention has been devoted to developing neuroprotective and neuroregenerative strategies aimed at improving the prognosis and quality of life for individuals with SCIs. This comprehensive review aims to provide an up-to-date and thorough overview of the latest neuroregenerative and neuroprotective therapies currently under investigation. These strategies encompass a multifaceted approach that include neuropharmacological interventions, cell-based therapies, and other promising strategies such as biomaterial scaffolds and neuro-modulation therapies. In addition, the review discusses the importance of acute clinical management, including the role of hemodynamic management as well as timing and technical aspects of surgery as key factors mitigating the secondary injury following SCI. In conclusion, this review underscores the ongoing scientific efforts to enhance patient outcomes and quality of life, focusing on upcoming strategies for the management of traumatic SCI. Each section provides a working knowledge of the fundamental preclinical and patient trials relevant to clinicians while underscoring the pathophysiologic rationale for the therapies.

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.

Lineage Reprogramming: Genetic, Chemical, and Physical Cues for Cell Fate Conversion with a Focus on Neuronal Direct Reprogramming and Pluripotency Reprogramming

Although lineage reprogramming from one cell type to another is becoming a breakthrough technology for cell-based therapy, several limitations remain to be overcome, including the low conversion efficiency and subtype specificity. To address these, many studies have been conducted using genetics, chemistry, physics, and cell biology to control transcriptional networks, signaling cascades, and epigenetic modifications during reprogramming. Here, we summarize recent advances in cellular reprogramming and discuss future directions.

Capacitive-Coupling-Responsive Hydrogel Scaffolds Offering Wireless In Situ Electrical Stimulation Promotes Nerve Regeneration

Electrical stimulation (ES) has shown beneficial effects in repairing injured tissues. However, current ES techniques that use tissue-traversing leads and bulky external power suppliers have significant limitations in translational medicine. Hence, exploring noninvasive in vivo ES to provide controllable electrical cues in tissue engineering is an imminent necessity. Herein, a conductive hydrogel with in situ electrical generation capability as a biodegradable regeneration scaffold and wireless ES platform for spinal cord injury (SCI) repair is demonstrated. When a soft insulated metal plate is placed on top of the injury site as a wireless power transmitter, the conductive hydrogel implanted at the injury site can serve as a wireless power receiver, and the capacitive coupling between the receiver and transmitter can generate an alternating current in the hydrogel scaffold owing to electrostatic induction effect. In a complete transection model of SCI rats, the implanted conductive hydrogels with capacitive-coupling in situ ES enhance functional recovery and neural tissue repair by promoting remyelination, accelerating axon regeneration, and facilitating endogenous neural stem cell differentiation. This facile wireless-powered electroactive-hydrogel strategy thus offers on-demand in vivo ES with an adjustable timeline, duration, and strength and holds great promise in translational medicine.

NG2 glia reprogramming induces robust axonal regeneration after spinal cord injury

Spinal cord injury (SCI) often leads to neuronal loss, axonal degeneration, and behavioral dysfunction. We recently show that in vivo reprogramming of NG2 glia produces new neurons, reduces glial scaring, and ultimately leads to improved function after SCI. By examining endogenous neurons, we here unexpectedly uncover that NG2 glia reprogramming also induces robust axonal regeneration of the corticospinal tract and serotonergic neurons. Such reprogramming-induced axonal regeneration may contribute to the reconstruction of neural networks essential for behavioral recovery.

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.

PD-1 deficiency aggravates spinal cord injury by regulating the reprogramming of NG2 glia and activating the NgR/RhoA/ROCK signaling pathway

Spinal cord injury (SCI) is a devastating disorder and a leading cause of disability in adults worldwide. Multiple studies have reported the upregulation of programmed cell death 1 (PD-1) following SCI. However, the underlying mechanism of PD-1 deficiency in SCI is not well established. Therefore, we aimed to investigate the role and potential mechanism of PD-1 in SCI pathogenesis. PD-1 Knockout (KO) SCI mouse model was established, and PD-1 expression was evaluated in tissue samples by western blot assay. We then used a series of function gain-and-loss assays to determine the role of PD-1 in SCI pathogenesis. Moreover, mechanistic assays were performed to explore the association between PD-1, neuron-glia antigen-2 (NG2) glia cells, and miR-23b-5p and then investigated the involved signaling pathway. Results illustrated that PD-1 deficiency enhanced the inflammatory response, neuron loss, and functional impairment induced by SCI. We found that NG2 glia depletion aggravated inflammation, reduced neural survival, and suppressed locomotor recovery in murine SCI model. Further analysis indicated that NG2+ cells were increased in the spinal cord of SCI mice, and PD-1 deficiency increased the number of NG2+ cells by activating the Nogo receptor/ras homolog family member A/Rho kinase (NgR/RhoA/ROCK) signaling. Mechanistically, miR-23b-5p was identified as the negative regulator of PD-1 in NG2 glia. MiR-23b-5p deficiency reduced the expression of inflammatory cytokines, enhanced neural survival, and promoted locomotor recovery in SCI mice, which was counteracted by PD-1 deficiency. In conclusion, PD-1 deficiency exacerbates SCI in vivo by regulating reprogramming of NG2 glia and activating the NgR/RhoA/ROCK signaling.

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.

Can pluripotent/multipotent stem cells reverse Parkinson's disease progression?

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by continuous and selective degeneration or death of dopamine neurons in the midbrain, leading to dysfunction of the nigrostriatal neural circuits. Current clinical treatments for PD include drug treatment and surgery, which provide short-term relief of symptoms but are associated with many side effects and cannot reverse the progression of PD. Pluripotent/multipotent stem cells possess a self-renewal capacity and the potential to differentiate into dopaminergic neurons. Transplantation of pluripotent/multipotent stem cells or dopaminergic neurons derived from these cells is a promising strategy for the complete repair of damaged neural circuits in PD. This article reviews and summarizes the current preclinical/clinical treatments for PD, their efficacies, and the advantages/disadvantages of various stem cells, including pluripotent and multipotent stem cells, to provide a detailed overview of how these cells can be applied in the treatment of PD, as well as the challenges and bottlenecks that need to be overcome in future translational studies.

A promise for neuronal repair: reprogramming astrocytes into neurons in vivo

Massive loss of neurons following brain injury or disease is the primary cause of central nervous system dysfunction. Recently, much research has been conducted on how to compensate for neuronal loss in damaged parts of the nervous system and thus restore functional connectivity among neurons. Direct somatic cell differentiation into neurons using pro-neural transcription factors, small molecules, or microRNAs, individually or in association, is the most promising form of neural cell replacement therapy available. This method provides a potential remedy for cell loss in a variety of neurodegenerative illnesses, and the development of reprogramming technology has made this method feasible. This article provides a comprehensive review of reprogramming, including the selection and methods of reprogramming starting cell populations as well as the signaling methods involved in this process. Additionally, we thoroughly examine how reprogramming astrocytes into neurons can be applied to treat stroke and other neurodegenerative diseases. Finally, we discuss the challenges of neuronal reprogramming and offer insights about the field.

A New Paradigm in Spinal Cord Injury Therapy: from Cell-free Treatment to Engineering Modifications

Spinal cord injury (SCI) is an intractable and poorly prognostic neurological disease, and current treatments are still unable to cure it completely and avoid sequelae. Extracellular vesicles (EVs), as important carriers of intercellular communication and pharmacological effects, are considered to be the most promising candidates for SCI therapy because of their low toxicity and immunogenicity, their ability to encapsulate endogenous bioactive molecules (e.g., proteins, lipids, and nucleic acids), and their ability to cross the blood-brain/cerebrospinal barriers. However, poor targeting, low retention rate, and limited therapeutic efficacy of natural EVs have bottlenecked EVs-based SCI therapy. A new paradigm for SCI treatment will be provided by engineering modified EVs. Furthermore, our limited understanding of the role of EVs in SCI pathology hinders the rational design of novel EVbased therapeutic approaches. In this study, we review the pathophysiology after SCI, especially the multicellular EVs-mediated crosstalk; briefly describe the shift from cellular to cell-free therapies for SCI treatment; discuss and analyze the issues related to the route and dose of EVs administration; summarize and present the common strategies for EVs drug loading in the treatment of SCI and point out the shortcomings of these drug loading methods; finally, we analyze and highlight the feasibility and advantages of bio-scaffold-encapsulated EVs for SCI treatment, providing scalable insights into cell-free therapy for SCI.

CREB1 Facilitates GABAergic Neural Differentiation of Human Mesenchymal Stem Cells through BRN2 for Pain Alleviation and Locomotion Recovery after Spinal Cord Injury

The transplantation of GABAergic neuron cells has been reported to alleviate nerve pain and improve motor function after spinal cord injury (SCI). However, human mesenchymal stem cell (hMSC) differentiation into GABAergic neuron cells in a sufficient quantity remains to be accomplished. From a database screening, cAMP-responsive element-binding protein 1 (CREB1) was chosen as a potential modulator due to its critical role in the protein-protein interaction of genes related to GABAergic neural differentiation. Here, CREB1 was overexpressed in transfected hMSCs, where CREB1 could induce differentiation into GABAergic neuron cells with an upregulation of Map2 and GAD1 by 2- and 3.4-fold, respectively. Additionally, GABAergic neural differentiation was enhanced, while Notch signaling was inhibited, and BRN2 transcriptional activation played an important role in neuronal maturation. Moreover, transfected hMSCs injected into immunocompromised mice caused by CsA exhibited the neuronal markers Tuj1 and Map2 via the intraspinal route, suggesting an improvement in survival and neural differentiation. Significantly, improvement in both BMS scores (6.2 ± 1.30 vs. 4 ± 0) and thermal hyperalgesia latency (7.74 ± 2.36 s vs. 4.52 ± 0.39 s) was seen compared with the SCI naïve treatment at 4 weeks post-transplantation. Our study demonstrates that CREB1 is crucial in generating induced GABAergic neuron cells (iGNs) originating from hMSCs. Transplanting iGNs to injured spinal cord provides a promising strategy for alleviating neuropathic pain and locomotion recovery after SCI.

Simple and Highly Specific Targeting of Resident Microglia with Adeno-Associated Virus

Microglia, as the immune cells of the central nervous system (CNS), play dynamic roles in both health and diseased conditions. The ability to genetically target microglia using viruses is crucial for understanding their functions and advancing microglia-based treatments. We here show that resident microglia can be simply and specifically targeted using adeno-associated virus (AAV) vectors containing a 466-bp DNA fragment from the human IBA1 (hIBA1) promoter. This targeting approach is applicable to both resting and reactive microglia. When combining the short hIBA1 promoter with the target sequence of miR124, up to 95% of transduced cells are identified as microglia. Such a simple and highly specific microglia-targeting strategy may be further optimized for research and therapeutics.

Vasculature cells control neuroglial co-localization and synaptic connection in a central nervous system tissue mimic system

Despite the development of neural tissue differentiation methods using a wide variety of stem cells and compartments, there is no standardized strategy for establishing synapses. As the neuronal network is developed in parallel with blood vessel angiogenesis in the central nervous system (CNS) from the embryonic period, we examined neuron-astrocyte-vasculature interactions to understand the effect of the vasculature on the development and stabilization of neurological morphogenesis. We generated a cellular co-culture module targeting the CNS that was embedded in a collagen-based extracellular matrix (ECM) gel. Our neuron-astrocyte-vascular complex module identified the neurological co-localization effect by endothelial cells, as well as the pericyte-induced improvement of synaptic connections. Furthermore, it was suggested that the PDGF, BDNF, IGF, and WNT/BMP pathways were upregulated in synaptic connections enhanced conditions, which are composed of neurexin. These results suggest that the integrity of the vasculature cells in the CNS is important for the establishment of neuronal networks and for synapse connection.

Improving Efficiency of Direct Pro-Neural Reprogramming: Much-Needed Aid for Neuroregeneration in Spinal Cord Injury

Spinal cord injury (SCI) is a medical condition affecting ~2.5-4 million people worldwide. The conventional therapy for SCI fails to restore the lost spinal cord functions; thus, novel therapies are needed. Recent breakthroughs in stem cell biology and cell reprogramming revolutionized the field. Of them, the use of neural progenitor cells (NPCs) directly reprogrammed from non-neuronal somatic cells without transitioning through a pluripotent state is a particularly attractive strategy. This allows to "scale up" NPCs in vitro and, via their transplantation to the lesion area, partially compensate for the limited regenerative plasticity of the adult spinal cord in humans. As recently demonstrated in non-human primates, implanted NPCs contribute to the functional improvement of the spinal cord after injury, and works in other animal models of SCI also confirm their therapeutic value. However, direct reprogramming still remains a challenge in many aspects; one of them is low efficiency, which prevents it from finding its place in clinics yet. In this review, we describe new insights that recent works brought to the field, such as novel targets (mitochondria, nucleoli, G-quadruplexes, and others), tools, and approaches (mechanotransduction and electrical stimulation) for direct pro-neural reprogramming, including potential ones yet to be tested.

In vivo cell fate reprogramming for spinal cord repair

Spinal cord injury (SCI) can lead to the loss of motor, sensory, or autonomic function due to neuronal death. Unfortunately, the adult mammalian spinal cord has limited intrinsic regenerative capacity, making it difficult to rebuild the neural circuits necessary for functional recovery. However, recent evidence suggests that in vivo fate reprogramming of resident cells that are normally non-neurogenic can generate new neurons. This process also improves the pathological microenvironment, and the new neurons can integrate into the local neural network, resulting in better functional outcomes in SCI animal models. In this concise review, we focus on recent advances while also discussing the challenges, pitfalls, and opportunities in the field of in vivo cell fate reprogramming for spinal cord repair.

A cutting-edge strategy for spinal cord injury treatment: resident cellular transdifferentiation

Spinal cord injury causes varying degrees of motor and sensory function loss. However, there are no effective treatments for spinal cord repair following an injury. Moreover, significant preclinical advances in bioengineering and regenerative medicine have not yet been translated into effective clinical therapies. The spinal cord's poor regenerative capacity makes repairing damaged and lost neurons a critical treatment step. Reprogramming-based neuronal transdifferentiation has recently shown great potential in repair and plasticity, as it can convert mature somatic cells into functional neurons for spinal cord injury repair in vitro and in vivo, effectively halting the progression of spinal cord injury and promoting functional improvement. However, the mechanisms of the neuronal transdifferentiation and the induced neuronal subtypes are not yet well understood. This review analyzes the mechanisms of resident cellular transdifferentiation based on a review of the relevant recent literature, describes different molecular approaches to obtain different neuronal subtypes, discusses the current challenges and improvement methods, and provides new ideas for exploring therapeutic approaches for spinal cord injury.

Three-dimensional nanofibrous sponges with aligned architecture and controlled hierarchy regulate neural stem cell fate for spinal cord regeneration

Background: Spinal cord injury (SCI) induces neuronal death and disrupts the nerve fiber bundles, which leads to severe neurological dysfunction and even permanent paralysis. A strategy combining biomimetic nanomaterial scaffolds with neural stem cell (NSC) transplantation holds promise for SCI treatment. Methods: Innovative three-dimensional (3D) nanofibrous sponges (NSs) were designed and developed by a combination of directional electrospinning and subsequent gas-foaming treatment. Immunofluorescence, mRNA sequencing, magnetic resonance imaging, electrophysiological analysis, and behavioral tests were used to investigate the in vitro and in vivo regenerative effects of the 3D NSs. Results: The generated 3D NSs exhibited uniaxially aligned nano-architecture and highly controllable hierarchical structure with super-high porosity (99%), outstanding hydrophilicity, and reasonable mechanical performance. They facilitated cell infiltration, induced cell alignment, promoted neuronal differentiation of NSCs, and enhanced their maturation mediated through cellular adhesion molecule pathways. In vivo, the NSC-seeded 3D NSs efficiently promoted axon reinnervation and remyelination in a rat SCI model, with new "neural relays" developing across the lesion gap. These histological changes were associated with regain of function, including increasing the neurological motor scores of SCI rats, from approximately 2 to 16 (out of 21), and decreasing the sensing time in the tape test from 140 s to 36 s. Additionally, the scaffolds led to restoration of ascending and descending electrophysiological signalling. Conclusion: The as-fabricated 3D NSs effectively regulate NSC fates, and an advanced combination of 3D NS design and transplanted NSCs enables their use as an ideal tissue-engineered scaffold for SCI repair.