MicroRNA-Mediated In Vitro and In Vivo Direct Conversion of Astrocytes to Neuroblasts. PLoS One. 2015;10:e0127878. [PMID: 26030913 PMCID: PMC4451260 DOI: 10.1371/journal.pone.0127878]

Delivery Systems in Neuronal Direct Cell Reprogramming

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

Cell reprogramming therapy for Parkinson's disease

Parkinson's disease is typically characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta. Many studies have been performed based on the supplementation of lost dopaminergic neurons to treat Parkinson's disease. The initial strategy for cell replacement therapy used human fetal ventral midbrain and human embryonic stem cells to treat Parkinson's disease, which could substantially alleviate the symptoms of Parkinson's disease in clinical practice. However, ethical issues and tumor formation were limitations of its clinical application. Induced pluripotent stem cells can be acquired without sacrificing human embryos, which eliminates the huge ethical barriers of human stem cell therapy. Another widely considered neuronal regeneration strategy is to directly reprogram fibroblasts and astrocytes into neurons, without the need for intermediate proliferation states, thus avoiding issues of immune rejection and tumor formation. Both induced pluripotent stem cells and direct reprogramming of lineage cells have shown promising results in the treatment of Parkinson's disease. However, there are also ethical concerns and the risk of tumor formation that need to be addressed. This review highlights the current application status of cell reprogramming in the treatment of Parkinson's disease, focusing on the use of induced pluripotent stem cells in cell replacement therapy, including preclinical animal models and progress in clinical research. The review also discusses the advancements in direct reprogramming of lineage cells in the treatment of Parkinson's disease, as well as the controversy surrounding in vivo reprogramming. These findings suggest that cell reprogramming may hold great promise as a potential strategy for treating Parkinson's disease.

Progress of reprogramming astrocytes into neuron

As the main players in the central nervous system (CNS), neurons dominate most life activities. However, after accidental trauma or neurodegenerative diseases, neurons are unable to regenerate themselves. The loss of this important role can seriously affect the quality of life of patients, ranging from movement disorders to disability and even death. There is no suitable treatment to prevent or reverse this process. Therefore, the regeneration of neurons after loss has been a major clinical problem and the key to treatment. Replacing the lost neurons by transdifferentiation of other cells is the only viable approach. Although much progress has been made in stem cell therapy, ethical issues, immune rejection, and limited cell sources still hinder its clinical application. In recent years, somatic cell reprogramming technology has brought a new dawn. Among them, astrocytes, as endogenously abundant cells homologous to neurons, have good potential and application value for reprogramming into neurons, having been reprogrammed into neurons in vitro and in vivo in a variety of ways.

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.

Astrocytes in ischemic stroke: Crosstalk in central nervous system and therapeutic potential

In the central nervous system (CNS), a large group of glial cells called astrocytes play important roles in both physiological and disease conditions. Astrocytes participate in the formation of neurovascular units and interact closely with other cells of the CNS, such as microglia and neurons. Stroke is a global disease with high mortality and disability rate, most of which are ischemic stroke. Significant strides in understanding astrocytes have been made over the past few decades. Astrocytes respond strongly to ischemic stroke through a process known as activation or reactivity. Given the important role played by reactive astrocytes (RAs) in different spatial and temporal aspects of ischemic stroke, there is a growing interest in the potential therapeutic role of astrocytes. Currently, interventions targeting astrocytes, such as mediating astrocyte polarization, reducing edema, regulating glial scar formation, and reprogramming astrocytes, have been proven in modulating the progression of ischemic stroke. The aforementioned potential interventions on astrocytes and the crosstalk between astrocytes and other cells of the CNS will be summarized in this review.

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.

Human Mesenchymal Stem Cell Transplantation Improved Functional Outcomes Following Spinal Cord Injury Concomitantly with Neuroblast Regeneration

Purpose

Spinal cord injury (SCI) is damage to the spinal cord that resulted in irreversible neuronal loss, glial scar formation and axonal injury. Herein, we used the human amniotic fluid mesenchymal stem cells (hAF-MSCs) and their conditioned medium (CM), to investigate their ability in neuroblast and astrocyte production as well as functional recovery following SCI.

Methods

Fifty-four adult rats were randomly divided into nine groups (n=6), included: Control, SCI, (SCI + DMEM), (SCI + CM), (SCI + MSCs), (SCI + Astrocyte), (SCI + Astrocyte + DMEM), (SCI + Astrocyte + CM) and (SCI + Astrocyte + MSCs). Following laminectomy and SCI induction, DMEM, CM, MSCs, and astrocytes were injected. Western blot was performed to explore the levels of the Sox2 protein in the MSCs-CM. The immunofluorescence staining against doublecortin (DCX) and glial fibrillary acidic protein (GFAP) was done. Finally, Basso-Beattie-Brenham (BBB) locomotor test was conducted to assess the neurological outcomes.

Results

Our results showed that the MSCs increased the number of endogenous DCX-positive cells and decreased the number of GFAP-positive cells by mediating juxtacrine and paracrine mechanisms (P<0.001). Transplanted human astrocytes were converted to neuroblasts rather than astrocytes under influence of MSCs and CM in the SCI. Moreover, functional recovery indexes were promoted in those groups that received MSCs and CM.

Conclusion

Taken together, our data indicate the MSCs via juxtacrine and paracrine pathways could direct the spinal cord endogenous neural stem cells (NSCs) to the neuroblasts lineage which indicates the capability of the MSCs in the increasing of the number of DCX-positive cells and astrocytes decline.

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.

Astrocytes in Parkinson's Disease: From Role to Possible Intervention

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the loss of dopaminergic neurons. While neuronal dysfunction is central to PD, astrocytes also play important roles, both positive and negative, and such roles have not yet been fully explored. This literature review serves to highlight these roles and how the properties of astrocytes can be used to increase neuron survivability. Astrocytes normally have protective functions, such as releasing neurotrophic factors, metabolizing glutamate, transferring healthy mitochondria to neurons, or maintaining the blood-brain barrier. However, in PD, astrocytes can become dysfunctional and contribute to neurotoxicity, e.g., via impaired glutamate metabolism or the release of inflammatory cytokines. Therefore, astrocytes represent a double-edged sword. Restoring healthy astrocyte function and increasing the beneficial effects of astrocytes represents a promising therapeutic approach. Strategies such as promoting neurotrophin release, preventing harmful astrocyte reactivity, or utilizing regional astrocyte diversity may help restore neuroprotection.

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.

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

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

Astrocyte-Derived Neuronal Transdifferentiation as a Therapy for Ischemic Stroke: Advances and Challenges

After the onset of ischemic stroke, ischemia–hypoxic cascades cause irreversible neuronal death. Neurons are the fundamental structures of the central nervous system, and mature neurons do not renew or multiply after death. Functional and structural recovery from neurological deficits caused by ischemic attack is a huge task. Hence, there remains a need to replace the lost neurons relying on endogenous neurogenesis or exogenous stem cell-based neuronal differentiation. However, the stem cell source difficulty and the risk of immune rejection of the allogeneic stem cells might hinder the wide clinical application of the above therapy. With the advancement of transdifferentiation induction technology, it has been demonstrated that astrocytes can be converted to neurons through ectopic expression or the knockdown of specific components. The progress and problems of astrocyte transdifferentiation will be discussed in this article.

Cellular Reprogramming and Its Potential Application in Alzheimer's Disease

Alzheimer's disease (AD) has become the most common age-related dementia in the world and is currently incurable. Although many efforts have been made, the underlying mechanisms of AD remain unclear. Extracellular amyloid-beta deposition, intracellular tau hyperphosphorylation, neuronal death, glial cell activation, white matter damage, blood-brain barrier disruption, and other mechanisms all take part in this complicated disease, making it difficult to find an effective therapy. In the study of therapeutic methods, how to restore functional neurons and integrate myelin becomes the main point. In recent years, with the improvement and maturity of induced pluripotent stem cell technology and direct cell reprogramming technology, it has become possible to induce non-neuronal cells, such as fibroblasts or glial cells, directly into neuronal cells in vitro and in vivo. Remarkably, the induced neurons are functional and capable of entering the local neural net. These encouraging results provide a potential new approach for AD therapy. In this review, we summarized the characteristics of AD, the reprogramming technique, and the current research on the application of cellular reprogramming in AD. The existing problems regarding cellular reprogramming and its therapeutic potential for AD were also reviewed.

Valproic Acid: A Potential Therapeutic for Spinal Cord Injury

The lack of an effective pharmaceutical agent for spinal cord injury (SCI) is a current problematic situation for clinicians, as the rate of motor vehicle accidents among young adults is on the rise. SCI contributes to the high disability rate. Presently, evidences detailing the precise pathological mechanisms in SCI are limited, compounding to the unavailability of an effective treatment method. Surgery, though not a complete curative method, is useful in managing some of the associated symptoms of secondary SCI. Autophagy and inflammation are contributive factors to both exacerbation and improvement of SCI. The mammalian target of rapamycin (mTOR) signaling pathway is a key player in the regulation of inflammatory response and autophagy. Valproic acid (VPA), a clinically used antiepileptic drug, has been suggested to improve neurological conditions, including SCI. This report reviewed the correlation between mTOR and autophagy, as well as autophagy's role and the therapeutic effects of VPA in SCI. VPA regulates autophagy by potentially inhibiting mTORC1, a complex of mTOR, while also hindering inflammatory response. Conclusively, an effective treatment for SCI could lie in the timely regulation of mTOR signaling pathway, and VPA could be the potential drug that improves SCI owing to its propensity to regulate the mTOR signaling pathway.

Novel Approaches Used to Examine and Control Neurogenesis in Parkinson's Disease

Neurogenesis is a key mechanism of brain development and plasticity, which is impaired in chronic neurodegeneration, including Parkinson’s disease. The accumulation of aberrant α-synuclein is one of the features of PD. Being secreted, this protein produces a prominent neurotoxic effect, alters synaptic plasticity, deregulates intercellular communication, and supports the development of neuroinflammation, thereby providing propagation of pathological events leading to the establishment of a PD-specific phenotype. Multidirectional and ambiguous effects of α-synuclein on adult neurogenesis suggest that impaired neurogenesis should be considered as a target for the prevention of cell loss and restoration of neurological functions. Thus, stimulation of endogenous neurogenesis or cell-replacement therapy with stem cell-derived differentiated neurons raises new hopes for the development of effective and safe technologies for treating PD neurodegeneration. Given the rapid development of optogenetics, it is not surprising that this method has already been repeatedly tested in manipulating neurogenesis in vivo and in vitro via targeting stem or progenitor cells. However, niche astrocytes could also serve as promising candidates for controlling neuronal differentiation and improving the functional integration of newly formed neurons within the brain tissue. In this review, we mainly focus on current approaches to assess neurogenesis and prospects in the application of optogenetic protocols to restore the neurogenesis in Parkinson’s disease.

Direct Neuronal Reprogramming: Bridging the Gap Between Basic Science and Clinical Application

Direct neuronal reprogramming is an innovative new technology that involves the conversion of somatic cells to induced neurons (iNs) without passing through a pluripotent state. The capacity to make new neurons in the brain, which previously was not achievable, has created great excitement in the field as it has opened the door for the potential treatment of incurable neurodegenerative diseases and brain injuries such as stroke. These neurological disorders are associated with frank neuronal loss, and as new neurons are not made in most of the adult brain, treatment options are limited. Developmental biologists have paved the way for the field of direct neuronal reprogramming by identifying both intrinsic cues, primarily transcription factors (TFs) and miRNAs, and extrinsic cues, including growth factors and other signaling molecules, that induce neurogenesis and specify neuronal subtype identities in the embryonic brain. The striking observation that postmitotic, terminally differentiated somatic cells can be converted to iNs by mis-expression of TFs or miRNAs involved in neural lineage development, and/or by exposure to growth factors or small molecule cocktails that recapitulate the signaling environment of the developing brain, has opened the door to the rapid expansion of new neuronal reprogramming methodologies. Furthermore, the more recent applications of neuronal lineage conversion strategies that target resident glial cells in situ has expanded the clinical potential of direct neuronal reprogramming techniques. Herein, we present an overview of the history, accomplishments, and therapeutic potential of direct neuronal reprogramming as revealed over the last two decades.

Treating Parkinson's disease by astrocyte reprogramming: Progress and challenges

Parkinson's disease (PD), the second most prevalent neurodegenerative disorder, is typified by both motor and nonmotor symptoms. The current medications provide symptomatic relief but do not stimulate the production of new dopaminergic neurons in the substantia nigra. Astrocyte reprogramming has recently received much attention as an avenue for increasing functional dopaminergic neurons in the mouse PD brain. By targeting a microRNA (miRNA) loop, astrocytes in the mouse brain could be reprogrammed into functional dopaminergic neurons. Such in vivo astrocyte reprogramming in the mouse model of PD has successfully added new dopaminergic neurons to the substantia nigra and increased dopamine levels associated with axonal projections into the striatum. This review deliberates the astrocyte reprogramming methods using specific transcription factors and mRNAs and the progress in generating dopaminergic neurons in vivo. In addition, the translational potential, challenges, and potential risks of astrocyte reprogramming for an enduring alleviation of parkinsonian symptoms are conferred.

The role of microRNAs in multiple sclerosis

Despite numerous studies on multiple sclerosis (MS) and understanding many aspects of this disease, researchers still struggle to find proper biomarkers that facilitate diagnosis; prognosis and monitoring of treatment efficacy in MS. MicroRNAs (miRNAs) are considered as endogenous, comparatively stable and small non-coding RNAs involved in various biological and pathological signaling pathways. Interestingly, miRNAs have been emerged as a potential biomarker for monitoring novel therapies in MS patients. In this review, we described the miRNAs alteration in the MS patients as well as their altered expression in patients under common MS therapies.

Role of miR-302/367 cluster in human physiology and pathophysiology

MicroRNAs (miRNAs) are small non-coding RNAs that negatively regulate target mRNAs at the post-transcriptional level. Increasing evidence shows the involvement of miRNAs in diverse biological processes. miR-302/367 cluster is highly conserved among vertebrates and made up of five members, including miR-367, miR-302a, miR-302b, miR-302c and miR-302d. miR-302/367 cluster plays an important role in cell proliferation, differentiation and reprogramming, affecting the development of tumor, cardiovascular system, nervous system and immune system. In this review, we will summarize the role of miR-302/367 cluster in embryonic stem cells and induced pluripotent stem cells and try to point out its relationship with tumors, cardiovascular system, nervous system and immune system.

Transcription Factor-Based Fate Specification and Forward Programming for Neural Regeneration

Traditionally, in vitro generation of donor cells for brain repair has been dominated by the application of extrinsic growth factors and morphogens. Recent advances in cell engineering strategies such as reprogramming of somatic cells into induced pluripotent stem cells and direct cell fate conversion have impressively demonstrated the feasibility to manipulate cell identities by the overexpression of cell fate-determining transcription factors. These strategies are now increasingly implemented for transcription factor-guided differentiation of neural precursors and forward programming of pluripotent stem cells toward specific neural subtypes. This review covers major achievements, pros and cons, as well as future prospects of transcription factor-based cell fate specification and the applicability of these approaches for the generation of donor cells for brain repair.

Direct reprogramming of terminally differentiated cells into neurons: A novel and promising strategy for Alzheimer's disease treatment

Glial activation is a common pathological process of the central nervous system (CNS) in disorders such as Alzheimer's disease (AD). Several approaches have been used to reduce the number of activated astrocytes and microglia in damaged areas. In recent years, various kinds of fully differentiated cells have been successfully reprogrammed to a desired type of cell in lesion areas. Interestingly, internal glial cells, including astrocytes and NG2 positive cells, were efficiently converted to neuroblasts and neurons by overexpression of some transcription factors (TFs) or microRNAs (miRNAs). Notably, some specific subtypes of neurons have been achieved by in vivo reprogramming and the resulting neurons were successfully integrated into local neuronal circuits. Furthermore, somatic cells from AD patients have been converted to functional neurons. Although direct reprogramming of a patient's own internal cells has revolutionized regenerative medicine, but there are some major obstacles that should be examined before using these induced cells in clinical therapies. In the present review article, we aim to discuss the current studies on in vitro and in vivo reprogramming of somatic cells to neurons using TFs, miRNAs or small molecules in healthy and AD patients.

Adult neurogenesis from reprogrammed astrocytes

The details of adult neurogenesis, including environmental triggers, region specificity, and species homology remain an area of intense investigation. Slowing or halting age-related cognitive dysfunction, or restoring neurons lost to disease or injury represent just a fraction of potential therapeutic applications. New neurons can derive from stem cells, pluripotent neural progenitor cells, or non-neuronal glial cells, such as astrocytes. Astrocytes must be epigenetically “reprogrammed” to become neurons, which can occur both naturally in vivo, and via artificial exogenous treatments. While neural progenitor cells are localized to a few neurogenic zones in the adult brain, astrocytes populate almost every brain structure. In this review, we will summarize recent research into neurogenesis that arises from conversion of post-mitotic astrocytes, detail the genetic and epigenetic pathways that regulate this process, and discuss the possible clinical relevance in supplementing stem-cell neurogenic therapies.

In vivo conversion of rat astrocytes into neuronal cells through neural stem cells in injured spinal cord with a single zinc-finger transcription factor

BACKGROUND

Spinal cord injury (SCI) results in glial scar formation and irreversible neuronal loss, which finally leads to functional impairments and long-term disability. Our previous studies have demonstrated that the ectopic expression of Zfp521 reprograms fibroblasts and astrocytes into induced neural stem cells (iNSCs). However, it remains unclear whether treatment with Zfp521 also affects endogenous astrocytes, thus promoting further functional recovery following SCI.

METHODS

Rat astrocytes were transdifferentiated into neural stem cells in vitro by ZFP521 or Sox2. Then, ZFP521 was applied to the spinal cord injury site of a rat. Transduction, real-time PCR, immunohistofluorescence, and function assessments were performed at 6 weeks post-transduction to evaluate improvement and in vivo lineage reprogramming of astrocytes.

RESULTS

Here, we show that Zfp521 is more efficient in reprogramming cultured astrocytes compared with Sox2. In the injured spinal cord of an adult rat, resident astrocytes can be reprogrammed into neurons through a progenitor stage by Zfp521. Importantly, this treatment improves the functional abilities of the rats as evaluated by the Basso, Beattie, and Bresnahan (BBB) locomotor rating scale and further by calculation of its subscores. There was enhanced locomotor activity in the hind limbs, step length, toe spread, foot length, and paw area. In addition, motor evoked potential recordings demonstrated the functional integrity of the spinal cord.

CONCLUSIONS

These results have indicated that the generation of iNSCs or neurons from endogenous astrocytes by in situ reprogramming might be a potential strategy for SCI repair.

An Easy and Fast Method for Production of Chinese Hamster Ovary Cell Line Expressing and Secreting Human Recombinant Activin A

Objective

Growth factors are key elements of embryonic stem cell (ESC) research. Cell line development in eukaryotes is a time-consuming procedure which usually takes 12-18 months. Here, we report an easy and fast method with which production of Chinese hamster ovary (CHO) cells that express and secrete recombinant Activin A, as a major growth factor in endo/mesoderm differentiation of embryonic stem cells is achieved within 3-4 weeks.

Materials and Methods

In this experimental study, we cloned human Activin A into the pDONR/Zeo gateway entry vector using the BP reaction. Activin A was subcloned next into the pLIX_403 and pLenti6.3/TO/V5-DEST destination vectors by the LR reaction. The result was the production of constructs with which 293T cells were finally transfected for virus production. CHO cells were transduced using viral particles to produce a cell line that secretes the His6- Activin A fusion protein.

Results

We developed a quick protocol which saves up to 3-4 weeks of time for producing recombinant proteins in CHO cells. The recombinant cell line produced 90 mg/L of functional Activin A measured in human ESC line Royan H5 (RH5), during in vitro differentiation into meso-endoderm and definitive endoderm.

Conclusion

Our results showed no significant differences in functionality between commercial Activin A and the one produced using our novel protocol. This approach can be easily used for producing recombinant proteins in CHO.

Direct Conversion of Human Urine Cells to Neurons by Small Molecules

Transdifferentiation of other cell type into human neuronal cells (hNCs) provides a platform for neural disease modeling, drug screening and potential cell-based therapies. Among all of the cell donor sources, human urine cells (hUCs) are convenient to obtain without invasive harvest procedure. Here, we report a novel approach for the transdifferentiation of hUCs into hNCs. Our study demonstrated that a combination of seven small molecules (CAYTFVB) cocktail induced transdifferentiation of hUCs into hNCs. These chemical-induced neuronal cells (CiNCs) exhibited typical neuron-like morphology and expressed mature neuronal markers. The neuronal-like morphology revealed in day 1, and the Tuj1-positive CiNCs reached to about 58% in day 5 and 38.36% Tuj1+/MAP2+ double positive cells in day 12. Partial electrophysiological properties of CiNCs was obtained using patch clamp. Most of the CiNCs generated using our protocol were glutamatergic neuron populations, whereas motor neurons, GABAergic or dopaminergic neurons were merely detected. hUCs derived from different donors were converted into CiNCs in this work. This method may provide a feasible and noninvasive approach for reprogramming hNCs from hUCs for disease models and drug screening.

Evolving principles underlying neural lineage conversion and their relevance for biomedical translation

Scientific and technological advances of the past decade have shed light on the mechanisms underlying cell fate acquisition, including its transcriptional and epigenetic regulation during embryonic development. This knowledge has enabled us to purposefully engineer cell fates in vitro by manipulating expression levels of lineage-instructing transcription factors. Here, we review the state of the art in the cell programming field with a focus on the derivation of neural cells. We reflect on what we know about the mechanisms underlying fate changes in general and on the degree of epigenetic remodeling conveyed by the distinct reprogramming and direct conversion strategies available. Moreover, we discuss the implications of residual epigenetic memory for biomedical applications such as disease modeling and neuroregeneration. Finally, we cover recent developments approaching cell fate conversion in the living brain and define questions which need to be addressed before cell programming can become an integral part of translational medicine.

Generation of neural stem cells from adult astrocytes by using a single reprogramming factor

Generating neural stem cells (NSCs) from astroglia as an abundant cell type in the mammalian brain has a promising outlook to be used in cell-replacement therapy for treatment of neurodegenerative disorders and neuronal trauma. However, little is known about a single reprogramming factor that may lead to the generation of induced NSCs (iNSCs) from adult brain-derived astrocytes in the absence of extrinsic inductive signals. Here, we show that zinc-finger nuclear protein Zfp521 alone is sufficient for converting the adult mouse brain-derived astrocytes into iNSCs. In vitro, Zfp521-iNSCs demonstrated long-term self-renewal and multipotency and expressed related markers. Moreover, single-seeded iNSCs were able to produce NSC colonies. These results suggest that application of Zfp521 to generate iNSCs could be regarded as a new approach for conversion of resident astrocytes into iNSCs in cell therapy for in vivo treatment of neural injuries.

In vivo conversion of astrocytes to oligodendrocyte lineage cells in adult mice demyelinated brains by Sox2

Sox2 transcription factor has been frequently used for reprograming starting cells to neural progenitor/stem cells. In vivo administration of Sox2 in the adult mouse brain reprogrammed the transduced astrocytes to neurons. In searching for adequate cell source for repairing the myelin insults, here, we studied the possible conversion of astrocytes to oligodendrocyte lineage cells by Sox2, while an extensive demyelination exists in animal brain. Lentiviral particles expressing Sox2-GFP were injected into the corpora callosa of animals fed with cuprizone diet for 12 weeks. Transduced cells were mainly astrocytes that changed their fate to oligodendrocyte lineage cells by time. For further conformation astrocytes received the vector in culture and then transplanted to the animal brains. Tracing the fate of transplanted cells showed their conversion to oligodendrocyte lineage cells. In vitro transduced cell were also maintained in the oligodendrocyte progenitor cell (OPC) induction medium. Produced OPC-like cells were positive for specific markers. This study provides a new strategy for endogenous production of myelinating cells. After optimizing the experimental conditions for safety and feasibility, this approach may contribute into future cell based therapies in patients with white matter insults as like as those with multiple sclerosis.

Trichostatin A Promotes the Conversion of Astrocytes to Oligodendrocyte Progenitors in a Defined Culture Medium

The generation of oligodendrocyte progenitor cells (OPCs) offers tremendous opportunities for cell replacement therapy in demyelinating diseases such as multiple sclerosis (MS) and spinal cord injury. Recently, the prospect of reprogramming terminally differentiated adult cells towards another mature somatic cell or progenitor cells without an intermediate pluripotent state has been of interest. Trichostatin A is a histone deacetylase inhibitor which opens the chromatin and facilitates the transcription of silence genes. In this study, we have treated human astrocytes line U87 and primary culture of mouse astrocytes with TSA for 12 h, prior their transfer to OPC induction medium. Then we evaluated the morphology and the fate of the treated astrocytes at post-treatment days. Both cell lines acquired OPC morphology and expressed OPC specific markers. Following transfer to differentiation medium, U87-derived iOPCs differentiated to oligodendrocyte like cells and expressed PLP as a mature oligodendrocyte marker. Our results introduced TSA as an inducer for production of OPCs from astrocytes and could be considered a potential way for the treatment of demyelinating diseases.

In vivo conversion of astrocytes to oligodendrocyte lineage cells using chemicals: targeting gliosis for myelin repair

AIM

It would be clinically ideal to target astrocytes in vivo for conversion into oligodendrocyte lineage cells to reduce astrogliosis and generate new myelinating cells.

MATERIALS & METHODS

Here, we prepared a GFP-labeled human astrocyte cell line, treated with epigenetic modifiers trichostatin A or 5-azacytidine and transplanted them into cuprizone-induced demyelinated mice brains. The fate of the transplanted astrocytes was studied at days 7, 14 and 28 post-transplantation.

RESULTS

GFP⁺ astrocytes were reduced over time, whereas the GFP⁺ oligodendrocyte lineage cells were found on days 14 and 28. Nontreated astrocytes did not convert to myelinating cells following transplantation. Cell conversion was proved in vitro by maintaining the treated cells in oligodendrocyte progenitor cell medium.

CONCLUSION

These findings seem promising for the application of epigenetic modifiers, especially their targeted delivery to glial scars to treat demyelinating diseases.

In vivo conversion of astrocytes into oligodendrocyte lineage cells with transcription factor Sox10; Promise for myelin repair in multiple sclerosis

Recent studies demonstrate that astroglial cells can be directly converted into functional neurons or oligodendrocytes. Here, we report that a single transcription factor Sox10 could reprogram astrocytes into oligodendrocyte-like cells, in vivo. For transdifferentiation, Sox10-GFP expressing viral particles were injected into cuprizone-induced demyelinated mice brains after which we assessed for the presence of specific oligodendrocyte lineage cell markers by immunohistofluorescence (IHF). As control, another group of demyelinated mice received GFP expressing viral particles. After 3 weeks, the majority of transduced (GFP+) cells in animals which received control vector were astrocytes, while in animals which received Sox10-GFP vector, the main population of GFP+ cells were positive for oligodendrocyte lineage markers. We also extracted primary astrocytes from mouse pups and purified them. Primary astrocytes were transduced in vitro and then transplanted into demyelinated brains for later fate mapping. After three weeks, in vitro transduced and then transplanted astrocytes showed oligodendrocyte progenitor and mature oligodendrocyte markers. Further confirmation was done by transduction of astrocytes with lentiviral particles that expressed Sox10 and GFP and their culture in the oligodendrocyte progenitor medium. The induced cells expressed oligodendrocyte progenitor cells (iOPCs) markers. Our findings showed the feasibility of reprogramming of astrocytes into oligodendrocyte-like cells in vivo, by using a single transcription factor, Sox10. This finding suggested a master regulatory role for Sox10 which enabled astrocytes to change their fate to OPC-like cells and establish an oligodendroglial phenotype. We hope this approach lead to effective myelin repair in patients suffering from myelination deficit.

Directed glial differentiation and transdifferentiation for neural tissue regeneration

Glial cells which are indispensable for the central nervous system development and functioning, are proven to be vulnerable to a harmful influence of pathological cues and tissue misbalance. However, they are also highly sensitive to both in vitro and in vivo modulation of their commitment, differentiation, activity and even the fate-switch by different types of bioactive molecules. Since glial cells (comprising macroglia and microglia) are an abundant and heterogeneous population of neural cells, which are almost uniformly distributed in the brain and the spinal cord parenchyma, they all create a natural endogenous reservoir of cells for potential neurogenerative processes required to be initiated in response to pathophysiological cues present in the local tissue microenvironment. The past decade of intensive investigation on a spontaneous and enforced conversion of glial fate into either alternative glial (for instance from oligodendrocytes to astrocytes) or neuronal phenotypes, has considerably extended our appreciation of glial involvement in restoring the nervous tissue cytoarchitecture and its proper functions. The most effective modulators of reprogramming processes have been identified and tested in a series of pre-clinical experiments. A list of bioactive compounds which are potent in guiding in vivo cell fate conversion and driving cell differentiation includes a selection of transcription factors, microRNAs, small molecules, exosomes, morphogens and trophic factors, which are helpful in boosting the enforced neuro-or gliogenesis and promoting the subsequent cell maturation into desired phenotypes. Herein, an issue of their utility for a directed glial differentiation and transdifferentiation is discussed in the context of elaborating future therapeutic options aimed at restoring the diseased nervous tissue.

Reprogramming of somatic cells to induced neural stem cells

Recent investigations have demonstrated that defined sets of exogenous factors (chemical and/or biochemical) can convert human and mouse somatic cells into induced neural stem cells (iNSCs). Considering the self-renewal and multi-potential differentiation capabilities of iNSCs, generation of these cells has considerably enhanced cell therapy for treatment of neurodegenerative disorders. These cells can also serve as models for investigation of the mechanism(s) underlying neurodegenerative diseases and as an asset in drug discovery. Meanwhile, using the process of direct conversion/transdifferentiation, by bypassing pluripotent state and consequently reducing tumorigenesis and genetic instability risks, establishment of several desired cells are feasible. In this review, we describe the pros and cons of different methods employed to directly reprogram somatic cells to iNSCs along with the progress of iNSCs applications and the future challenges.

Enhanced neurogenesis in degenerated hippocampi following pretreatment with miR-302/367 expressing lentiviral vector in mice

Astrogliosis is the main landmark of neurodegenerative diseases. In vivo reprogramming of reactive astrocytes to functional neurons opened a new horizon in regenerative medicine. However there is little evidence that show possible application of in vivo reprogramming approaches for enhancement of neurogenesis. Cluster miR-302/367 showed high capability in cell reprogramming. Here we show that application of lentiviral particles expressing cluster miR-302/367 along with systemic valproate (VPA) enhanced the capability of mice brains for neurogenesis in CA3 area following kainic acid (KA) induced hippocampal neurodegeneration. Following pretreatment with miR-302/367 expressing viral particles and VPA, transduced cells showed neuroblast and mature neuron markers when neuronal loss was induced by KA. Comparing the neuron counts in CA3 region also showed that neurogenesis was increased in CA3 region in animals which were pretreated with miR-302/367 vector and VPA, only in injected side of the brain. Our data suggest that targeted application of miR-302/367 expressing vector may enhance the capacity of hippocampus and other brain structures for regeneration following neuronal loss.

Attempts to Overcome Remyelination Failure: Toward Opening New Therapeutic Avenues for Multiple Sclerosis

Multiple sclerosis (MS) is a chronic immune-mediated disorder of the central nervous system that results in destruction of the myelin sheath wrapped around the axons and eventual axon degeneration. The disease is pathologically heterogeneous; however, perhaps its most frustrating aspect is the lack of efficient regenerative response for remyelination. Current treatment strategies are based on anti-inflammatory or immunomodulatory medications that have the potential to reduce the numbers of newly evolving lesions. However, therapies are still required that can repair already damaged myelin for which current treatments are not effective. A prerequisite for the development of such new treatments is understanding the reasons for insufficient endogenous repair. This review briefly summarizes the currently suggested causes of remyelination failure in MS and possible solutions.

MicroRNA-Directed Neuronal Reprogramming as a Therapeutic Strategy for Neurological Diseases

The loss of neurons due to injury and disease results in a wide spectrum of highly disabling neurological and neurodegenerative conditions, given the apparent limited capacity of endogenous repair of the adult central nervous system (CNS). Therefore, it is important to develop technologies that can promote de novo neural stem cell and neuron generation. Current insights in CNS development and cellular reprogramming have provided the knowledge to finely modulate lineage-restricted transcription factors and microRNAs (miRNA) to elicit correct neurogenesis. Here, we discuss the current knowledge on the direct reprogramming of somatic non-neuronal cells into neural stem cells or subtype specific neurons in vitro and in vivo focusing on miRNA driven reprogramming. miRNA can allow rapid and efficient direct phenotype conversion by modulating gene networks active during development, which promote global shifts in the epigenetic landscape pivoting cell fate decisions. Furthermore, we critically present state-of-the-art and recent advances on miRNA therapeutics that can be applied to the diseased CNS. Together, the advances in our understanding of miRNA role in CNS development and disease, recent progress in miRNA-based therapeutic strategies, and innovative drug delivery methods create novel perspectives for meaningful therapies for neurodegenerative disorders.

The essentiality of non-coding RNAs in cell reprogramming

In mammals, short (mi-) and long non-coding (lnc) RNAs are immensely abundant and they are proving to be more functional than ever before. Particularly in cell reprogramming, non-coding RNAs are essential to establish the pluripotent network and are indispensable to reprogram somatic cells to pluripotency. Through systematic screening and mechanistic studies, diverse functional features of both miRNA and lncRNAs have emerged as either scaffolds, inhibitors, or co-activators, necessary to orchestrate the intricacy of gene regulation. Furthermore, the collective characterizations of both miRNA and lncRNA reveal their interdependency (e.g. sequestering the function of the other) to modulate cell reprogramming. This review broadly explores the regulatory processes of cell reprogramming - with key functional examples in neuronal and cardiac differentiations - in the context of both short and long non-coding RNAs.

Adeno-Associated Viral Vectors Serotype 8 for Cell-Specific Delivery of Therapeutic Genes in the Central Nervous System

Adeno-associated viruses (AAVs) have become highly promising tools for research and clinical applications in the central nervous system (CNS). However, specific delivery of genes to the cell type of interest is essential for the success of gene therapy and therefore a correct selection of the promoter plays a very important role. Here, AAV8 vectors carrying enhanced green fluorescent protein (eGFP) as reporter gene under the transcriptional control of different CNS-specific promoters were used and compared with a strong ubiquitous promoter. Since one of the main limitations of AAV-mediated gene delivery lies in its restricted cloning capacity, we focused our work on small-sized promoters. We tested the transduction efficacy and specificity of each vector after stereotactic injection into the mouse striatum. Three glia-specific AAV vectors were generated using two truncated forms of the human promoter for glial fibrillar acidic protein (GFAP) as well as a truncated form of the murine GFAP promoter. All three vectors resulted in predominantly glial expression; however we also observed eGFP expression in other cell-types such as oligodendrocytes, but never in neurons. In addition, robust and neuron-specific eGFP expression was observed using the minimal promoters for the neural protein BM88 and the neuronal nicotinic receptor β2 (CHRNB2). In summary, we developed a set of AAV vectors designed for specific expression in cells of the CNS using minimal promoters to drive gene expression when the size of the therapeutic gene matters.

Differentiation of Spermatogonia Stem Cells into Functional Mature Neurons Characterized with Differential Gene Expression

Transplantation of embryonic stem cells (ESCs) is a promising therapeutic approach for the treatment of neurodegenerative diseases. However, ESCs are not usable clinically due to immunological and ethical limitations. The identification of an alternative safe cell source opens novel options via autologous transplantation in neuro-regeneration circumventing these problems. Here, we examined the neurogenic capacity of embryonic stem-like cells (ES-like cells) derived from the testis using neural growth factor inducers and utilized them to generate functional mature neurons. The neuronal differentiation of ES-like cells is induced in three stages. Stage 1 is related to embryoid body (EB) formation. To induce neuroprogenitor cells, EBs were cultured in the presence of retinoic acid, N₂ supplement and fibroblast growth factor followed by culturing in a neurobasal medium containing B₂₇, N₂ supplements for additional 10 days, to allow the maturation and development of neuronal progenitor cells. The neurogenic differentiation was confirmed by immunostaining for markers of mature neurons. The differentiated neurons were positive for Tuj1 and Tau1. Real-time PCR dates indicated the expression of Nestin and Neuro D (neuroprogenitor markers) in induced cells at the second stage of the differentiation protocol. The differentiated mature neurons exhibited the specific neuron markers Map2 and β-tubulin. The functional maturity of neurons was confirmed by an electrophysiological analysis of passive and active neural membrane properties. These findings indicated a differentiation capacity of ES-like cells derived from the testis to functionally mature neurons, which proposes them as a novel cell source for neuroregenerative medicine.

Chemical transdifferentiation: closer to regenerative medicine

Cell transdifferentiation, which directly switches one type of differentiated cells into another cell type, is more advantageous than cell reprogramming to generate pluripotent cells and differentiate them into functional cells. This process is crucial in regenerative medicine. However, the cell-converting strategies, which mainly depend on the virus-mediated expression of exogenous genes, have clinical safety concerns. Small molecules with compelling advantages are a potential alternative in manipulating cell fate conversion. In this review, we briefly retrospect the nature of cell transdifferentiation and summarize the current developments in the research of small molecules in promoting cell conversion. Particularly, we focus on the complete chemical compound-induced cell transdifferentiation, which is closer to the clinical translation in cell therapy. Despite these achievements, the mechanisms underpinning chemical transdifferentiation remain largely unknown. More importantly, identifying drugs that induce resident cell conversion in vivo to repair damaged tissue remains to be the end-goal in current regenerative medicine.

Generation of diverse neural cell types through direct conversion

A characteristic of neurological disorders is the loss of critical populations of cells that the body is unable to replace, thus there has been much interest in identifying methods of generating clinically relevant numbers of cells to replace those that have been damaged or lost. The process of neural direct conversion, in which cells of one lineage are converted into cells of a neural lineage without first inducing pluripotency, shows great potential, with evidence of the generation of a range of functional neural cell types both in vitro and in vivo, through viral and non-viral delivery of exogenous factors, as well as chemical induction methods. Induced neural cells have been proposed as an attractive alternative to neural cells derived from embryonic or induced pluripotent stem cells, with prospective roles in the investigation of neurological disorders, including neurodegenerative disease modelling, drug screening, and cellular replacement for regenerative medicine applications, however further investigations into improving the efficacy and safety of these methods need to be performed before neural direct conversion becomes a clinically viable option. In this review, we describe the generation of diverse neural cell types via direct conversion of somatic cells, with comparison against stem cell-based approaches, as well as discussion of their potential research and clinical applications.

New approaches for direct conversion of patient fibroblasts into neural cells

Recent landmark studies have demonstrated the production of disease-relevant human cell types by two different methods; differentiation of stem cells using external morphogens or lineage conversion using genetic factors. Directed differentiation changes embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs) into a desired cell type by providing developmental cues in an in vitro environment. Direct reprogramming is achieved by the introduction of exogenous lineage specific transcription factors to convert any somatic cell type into another, thereby bypassing an intermediate pluripotent stage. A variety of somatic cell types such as blood, keratinocytes and fibroblasts can be used to derive iPSC cells. However, the process is time consuming,laborious, expensive and gives rise to cells with reported epigenetic heterogeneity even amongst different iPSC lines from same patient which could propagate phenotypic variability. A major concern with the use of pluripotent cells as starting material for cell replacement therapy is their incomplete differentiation and their propensity to form tumors following transplantation. In comparison, transcription factor mediated reprogramming offers a direct route to target cell types. This could allow for rapid comparison of large cohorts of patient and control samples at a given time for disease modeling. Additionally, transcription factors that drive maturation may yield more functionally mature cells than directed differentiation. Several studies have demonstrated the feasibility of generating of cell types such as cardiomyocytes, hepatocytes, and neurons from fibroblasts. Here, we will discuss recent advances and key challenges regarding direct reprogramming of somatic cell types into diverse neural cells. This article is part of a Special Issue entitled SI: Exploiting human neurons.