Reprogramming Glial Cells into Functional Neurons for Neuro-regeneration: Challenges and Promise

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.

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.

Reducing polypyrimidine tract‑binding protein 1 fails to promote neuronal transdifferentiation on HT22 and mouse astrocyte cells under physiological conditions

In contrast to prior findings that have illustrated the conversion of non-neuronal cells into functional neurons through the specific targeting of polypyrimidine tract-binding protein 1 (PTBP1), accumulated evidence suggests the impracticality of inducing neuronal transdifferentiation through suppressing PTBP1 expression in pathological circumstances. Therefore, the present study explored the effect of knocking down PTBP1 under physiological conditions on the transdifferentiation of mouse hippocampal neuron HT22 cells and mouse astrocyte (MA) cells. A total of 20 µM negative control small interfering (si)RNA and siRNA targeting PTBP1 were transfected into HT22 and MA cells using Lipo8000™ for 3 and 5 days, respectively. The expression of early neuronal marker βIII-Tubulin and mature neuronal markers NeuN and microtubule-associated protein 2 (MAP2) were detected using western blotting. In addition, βIII-tubulin, NeuN and MAP2 were labeled with immunofluorescence staining to evaluate neuronal cell differentiation in response to PTBP1 downregulation. Under physiological conditions, no significant changes in the expression of βIII-Tubulin, NeuN and MAP2 were found after 3 and 5 days of knockdown of PTBP1 protein in both HT22 and MA cells. In addition, the immunofluorescence staining results showed no apparent transdifferentiation in maker levels and morphology. The results suggested that the knockdown of PTBP1 failed to induce neuronal differentiation under physiological conditions.

Neuronal Replenishment via Hydrogel-Rationed Delivery of Reprogramming Factors

The central nervous system's limited capacity for regeneration often leads to permanent neuronal loss following injury. Reprogramming resident reactive astrocytes into induced neurons at the site of injury is a promising strategy for neural repair, but challenges persist in stabilizing and accurately targeting viral vectors for transgene expression. In this study, we employed a bioinspired self-assembling peptide (SAP) hydrogel for the precise and controlled release of a hybrid adeno-associated virus (AAV) vector, AAVDJ, carrying the NeuroD1 neural reprogramming transgene. This method effectively mitigates the issues of high viral dosage at the target site, off-target delivery, and immunogenic reactions, enhancing the vector's targeting and reprogramming efficiency. In vitro, this vector successfully induced neuron formation, as confirmed by morphological, histochemical, and electrophysiological analyses. In vivo, SAP-mediated delivery of AAVDJ-NeuroD1 facilitated the trans-differentiation of reactive host astrocytes into induced neurons, concurrently reducing glial scarring. Our findings introduce a safe and effective method for treating central nervous system injuries, marking a significant advancement in regenerative neuroscience.

Amelioration of Motor Performance and Nigrostriatal Dopamine Cell Volume Using a Novel Far-Infrared Ceramic Blanket in an A53T Alpha-Synuclein Transgenic Parkinson's Disease Mouse Model

We had attended a Parkinson's Disease (PD) patient for a non-healing wound who reported a marked decrease in his hand tremor and freezing of gait when his wound was exposed to a ceramic far-field infrared (cFIR) blanket. PD is the most frequent motor disorder and the second most frequent neurodegenerative disease after Alzheimer's Disease (AD). The tremor, rigidity, and slowness of movement associated with Parkinson's disease (PD) affect up to 10 million people throughout the world, and the major contributing factor to the pathogenesis of PD is the accumulation and propagation of pathological α-synuclein (α-Syn) and the death of dopaminergic cells in the Nigrostriatal system. Efforts to slow or stop its spreading have resulted in the development and use of dopaminergic drug replacement therapy. Unfortunately, there is a loss of about 70-80% of substantia nigral dopaminergic neurons in patients by the time they are diagnosed with PD, and various dopaminergic drugs provide only temporary relief of their motor symptoms. There are limitations in treating PD with many conventional medications, necessitating a combination of pharmaceutical and non-pharmacological therapy as an essential adjunct to better address the health and welfare of PD patients. We used male adult A53T alpha-synuclein transgenic mice exposed to a ceramic far-infrared blanket. Motor activity was assessed using the rotarod apparatus, and mouse brains were examined to quantify the fluorescence intensities of the immunostained samples. A53T alpha-synuclein transgenic mice had a significantly shorter time stay on the rotating bar than the wild-type mice (B6C3H). The rotarod performance was significantly improved in A53T alpha-synuclein transgenic mice exposed to cFIR as well as B6C3H healthy wild mice exposed to cFIR. There was a significant statistical and substantive increase in the cellular composition of the Striatum and substantia nigra of cFIR-treated mice. Improvement in motor performance is seen in PD mice and wild mice and is associated with increases in cell volume in the substantia nigra and striatum after treatment.

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.

Restorative cell and gene therapies for Parkinson's disease

One of the core pathological features of Parkinson's disease (PD) is the loss of the dopaminergic nigrostriatal pathway which lies at the heart of many of the motor features of this condition as well as some of the cognitive problems. The importance of this pathological event is evident through the clinical benefits that are seen when patients with PD are treated with dopaminergic agents, at least in early-stage disease. However, these agents create problems of their own through stimulation of more intact dopaminergic networks within the central nervous system causing major neuropsychiatric problems including dopamine dysregulation. In addition, over time the nonphysiological stimulation of striatal dopamine receptors by l-dopa containing drugs leads to the genesis of l-dopa-induced dyskinesias that can become very disabling in many cases. As such, there has been much interest in trying to better reconstitute the dopaminergic nigrostriatal pathway using either factors to regrow it, cells to replace it, or gene therapies to restore dopamine transmission in the striatum. In this chapter, we lay out the rationale, history and current status of these different therapies as well as highlighting where the field is heading and what new interventions might come to clinic in the coming years.

Integrative Analysis Reveals the Expression Pattern of SOX9 in Satellite Glial Cells after Sciatic Nerve Injury

BACKGROUND

Several complex cellular and gene regulatory processes are involved in peripheral nerve repair. This study uses bioinformatics to analyze the differentially expressed genes (DEGs) in the satellite glial cells of mice following sciatic nerve injury.

METHODS

R software screens differentially expressed genes, and the WebGestalt functional enrichment analysis tool conducts Gene Ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomics (KEGG) pathway analysis. The Search Tool for the Retrieval of Interacting Genes/Proteins constructs protein interaction networks, and the cytoHubba plug-in in the Cytoscape software predicts core genes. Subsequently, the sciatic nerve injury model of mice was established and the dorsal root ganglion satellite glial cells were isolated and cultured. Satellite glial cells-related markers were verified by immunofluorescence staining. Real-time polymerase chain reaction assay and Western blotting assay were used to detect the mRNA and protein expression of Sox9 in satellite glial cells.

RESULTS

A total of 991 DEGs were screened, of which 383 were upregulated, and 508 were downregulated. The GO analysis revealed the processes of biosynthesis, negative regulation of cell development, PDZ domain binding, and other biological processes were enriched in DEGs. According to the KEGG pathway analysis, DEGs are primarily involved in steroid biosynthesis, hedgehog signaling pathway, terpenoid backbone biosynthesis, American lateral skeleton, and melanoma pathways. According to various cytoHubba algorithms, the common core genes in the protein-protein interaction network are Atf3, Mmp2, and Sox9. Among these, Sox9 was reported to be involved in the central nervous system and the generation and development of astrocytes and could mediate the transformation between neurogenic and glial cells. The experimental results showed that satellite glial cell marker GS were co-labeled with Sox9; stem cell characteristic markers Nestin and p75NTR were labeled satellite glial cells. The mRNA and protein expression of Sox9 in satellite glial cells were increased after sciatic nerve injury.

CONCLUSIONS

In this study, bioinformatics was used to analyze the DEGs of satellite glial cells after sciatic nerve injury, and transcription factors related to satellite glial cells were screened, among which Sox9 may be associated with the fate of satellite glial cells.

Mechanisms Underlying Brain Aging Under Normal and Pathological Conditions

Aging is a major risk factor for many human diseases, including cognitive impairment, which affects a large population of the elderly. In the past few decades, our understanding of the molecular and cellular mechanisms underlying the changes associated with aging and age-related diseases has expanded greatly, shedding light on the potential role of these changes in cognitive impairment. In this article, we review recent advances in understanding of the mechanisms underlying brain aging under normal and pathological conditions, compare their similarities and differences, discuss the causative and adaptive mechanisms of brain aging, and finally attempt to find some rules to guide us on how to promote healthy aging and prevent age-related diseases.

Polypyrimidine tract binding protein knockdown reverses depression-like behaviors and cognition impairment in mice with lesioned cholinergic neurons

Background and objectives

Depression is a common comorbidity of dementia and may be a risk factor for dementia. Accumulating evidence has suggested that the cholinergic system plays a central role in dementia and depression, and the loss of cholinergic neurons is associated with memory decline in aging and Alzheimer's patients. A specific loss of cholinergic neurons in the horizontal limb of the diagonal band of Broca (HDB) is correlated with depression and dysfunction of cognition in mice. In this study, we examined the potential regenerative mechanisms of knockdown the RNA-binding protein polypyrimidine tract binding protein (PTB) in reversing depression-like behaviors and cognition impairment in mice with lesioned cholinergic neurons.

Methods

We lesioned cholinergic neurons in mice induced by injection of 192 IgG-saporin into HDB; then, we injected either antisense oligonucleotides or adeno-associated virus-shRNA (GFAP promoter) into the injured area of HDB to deplete PTB followed by a broad range of methodologies including behavioral examinations, Western blot, RT-qPCR and immunofluorescence.

Results

We found that the conversion of astrocytes to newborn neurons by using antisense oligonucleotides on PTB in vitro, and depletion of PTB using either antisense oligonucleotides or adeno-associated virus-shRNA into the injured area of HDB could specifically transform astrocytes into cholinergic neurons. Meanwhile, knockdown of PTB by both approaches could relieve the depression-like behaviors shown by sucrose preference, forced swimming or tail-suspension tests, and alleviate cognitive impairment such as fear conditioning and novel object recognition in mice with lesioned cholinergic neurons.

Conclusion

These findings suggest that supplementing cholinergic neurons after PTB knockdown may be a promising therapeutic strategy to revert depression-like behaviors and cognitive impairment.

Application and prospects of somatic cell reprogramming technology for spinal cord injury treatment

Spinal cord injury (SCI) is a serious neurological trauma that is challenging to treat. After SCI, many neurons in the injured area die due to necrosis or apoptosis, and astrocytes, oligodendrocytes, microglia and other non-neuronal cells become dysfunctional, hindering the repair of the injured spinal cord. Corrective surgery and biological, physical and pharmacological therapies are commonly used treatment modalities for SCI; however, no current therapeutic strategies can achieve complete recovery. Somatic cell reprogramming is a promising technology that has gradually become a feasible therapeutic approach for repairing the injured spinal cord. This revolutionary technology can reprogram fibroblasts, astrocytes, NG2 cells and neural progenitor cells into neurons or oligodendrocytes for spinal cord repair. In this review, we provide an overview of the transcription factors, genes, microRNAs (miRNAs), small molecules and combinations of these factors that can mediate somatic cell reprogramming to repair the injured spinal cord. Although many challenges and questions related to this technique remain, we believe that the beneficial effect of somatic cell reprogramming provides new ideas for achieving functional recovery after SCI and a direction for the development of treatments for SCI.

Intrinsic heterogeneity in axon regeneration

The nervous system is composed of a variety of neurons and glial cells with different morphology and functions. In the mammalian peripheral nervous system (PNS) or the lower vertebrate central nervous system (CNS), most neurons can regenerate extensively after axotomy, while the neurons in the mammalian CNS possess only limited regenerative ability. This heterogeneity is common within and across species. The studies about the transcriptomes after nerve injury in different animal models have revealed a series of molecular and cellular events that occurred in neurons after axotomy. However, responses of various types of neurons located in different positions of individuals were different remarkably. Thus, researchers aim to find the key factors that are conducive to regeneration, so as to provide the molecular basis for solving the regeneration difficulties after CNS injury. Here we review the heterogeneity of axonal regeneration among different cell subtypes in different animal models or the same organ, emphasizing the importance of comparative studies within and across species.

Ginsenoside Rg1 promotes astrocyte-to-neuron transdifferentiation in rat and its possible mechanism

INTRODUCTION

Neuronal loss caused by spinal cord injury (SCI) usually contributes to irreversible motor dysfunction. Promoting neuronal regeneration and functional recovery is vital to the repair of SCI.

AIMS

Astrocytes, activated by SCI with high proliferative capacity and proximity to neuronal lineage, are considered ideal cells for neuronal regeneration. As previous studies identified several small molecules for the induction of astrocyte-to-neuron, we confirmed that ginsenoside Rg1, a neuroprotective herb, could promote the direct transdifferentiation of astrocyte-to-neuron in rat.

METHODS AND RESULTS

Immunofluorescence staining showed that 26.0 ± 1.5% of the induced cells exhibited less astroglial properties and more neuronal markers with typical neuronal morphologies, reflecting 20.6 ± 0.9% of cholinergic neurons and 22.3 ± 1.9% of dopaminergic neurons. Western blot and qRT-PCR revealed that the induced cells had better antiapoptotic ability and Rg1-promoted neuronal transdifferentiation of reactive astrocytes might take effect through suppressing Notch/Stat3 signal pathway. In vivo, a revised SCI model treated by Rg1 was confirmed with faster functional recovery and less injury lesion cavity.

CONCLUSION

In summary, our study provided a novel strategy of direct transdifferentiation of endogenous rat reactive astrocytes into neurons with Rg1 and promotion of neuronal regeneration after SCI.

IRES-mediated Wnt2 translation in apoptotic neurons triggers astrocyte dedifferentiation

Reactive astrogliosis usually bears some properties of neural progenitors. How injury triggers astrocyte dedifferentiation remains largely unclear. Here, we report that ischemia induces rapid up-regulation of Wnt2 protein in apoptotic neurons and activation of canonical Wnt signaling in reactive astrocytes in mice, primates and human. Local delivery of Wnt2 shRNA abolished the dedifferentiation of astrocytes while over-expressing Wnt2 promoted progenitor marker expression and neurogenesis. Both the activation of Wnt signaling and dedifferentiation of astrocytes was compromised in ischemic caspase-3^−/− cortex. Over-expressing stabilized β-catenin not only facilitated neurogenesis but also promoted functional recovery in ischemic caspase-3^−/− mice. Further analysis showed that apoptotic neurons up-regulated Wnt2 protein via internal ribosome entry site (IRES)-mediated translation. Knocking down death associated protein 5 (DAP5), a key protein in IRES-mediated protein translation, significantly diminished Wnt activation and astrocyte dedifferentiation. Our data demonstrated an apoptosis-initiated Wnt-activating mechanism which triggers astrocytic dedifferentiation and facilitates neuronal regeneration.

Regenerative Strategies for Retinal Neurons: Novel Insights in Non-Mammalian Model Organisms

A detailed knowledge of the status of the retina in neurodegenerative conditions is a crucial point for the development of therapeutics in retinal pathologies and to translate eye research to CNS disease. In this context, manipulating signaling pathways that lead to neuronal regeneration offers an excellent opportunity to substitute damaged cells and, thus, restore the tissue functionality. Alternative systems and methods are increasingly being considered to replace/reduce in vivo approaches in the study of retina pathophysiology. Herein, we present recent data obtained from the zebrafish (Danio rerio) and the fruit fly Drosophila melanogaster that bring promising advantages into studying and modeling, at a preclinical level, neurodegeneration and regenerative approaches in retinal diseases. Indeed, the regenerative ability of vertebrate model zebrafish is particularly appealing. In addition, the fruit fly is ideal for regenerative studies due to its high degree of conservation with vertebrates and the broad spectrum of genetic variants achievable. Furthermore, a large part of the drosophila brain is dedicated to sight, thus offering the possibility of studying common mechanisms of the visual system and the brain at once. The knowledge acquired from these alternative models may help to investigate specific well-conserved factors of interest in human neuroregeneration after injuries or during pathologies.

Current Approaches and Molecular Mechanisms for Directly Reprogramming Fibroblasts Into Neurons and Dopamine Neurons

Parkinson's disease is mainly caused by specific degeneration of dopaminergic neurons (DA neurons) in the substantia nigra of the middle brain. Over the past two decades, transplantation of neural stem cells (NSCs) from fetal brain-derived neural stem cells (fNSCs), human embryonic stem cells (hESCs), and induced pluripotent stem cells (iPSCs) has been shown to improve the symptoms of motor dysfunction in Parkinson's disease (PD) animal models and PD patients significantly. However, there are ethical concerns with fNSCs and hESCs and there is an issue of rejection by the immune system, and the iPSCs may involve tumorigenicity caused by the integration of the transgenes. Recent studies have shown that somatic fibroblasts can be directly reprogrammed to NSCs, neurons, and specific dopamine neurons. Directly induced neurons (iN) or induced DA neurons (iDANs) from somatic fibroblasts have several advantages over iPSC cells. The neurons produced by direct transdifferentiation do not pass through a pluripotent state. Therefore, direct reprogramming can generate patient-specific cells, and it can overcome the safety problems of rejection by the immune system and teratoma formation related to hESCs and iPSCs. However, there are some critical issues such as the low efficiency of direct reprogramming, biological functions, and risks from the directly converted neurons, which hinder their clinical applications. Here, the recent progress in methods, mechanisms, and future challenges of directly reprogramming somatic fibroblasts into neurons or dopamine neurons were summarized to speed up the clinical translation of these directly converted neural cells to treat PD and other neurodegenerative diseases.