In vivo reprogramming of astrocytes to neuroblasts in the adult brain. Nat Cell Biol. 2013;15:1164-1175. [PMID: 24056302 DOI: 10.1038/ncb2843]

Striatal astrocytes transdifferentiate into functional mature neurons following ischemic brain injury

To determine whether reactive astrocytes stimulated by brain injury can transdifferentiate into functional new neurons, we labeled these cells by injecting a glial fibrillary acidic protein (GFAP) targeted enhanced green fluorescence protein plasmid (pGfa2-eGFP plasmid) into the striatum of adult rats immediately following a transient middle cerebral artery occlusion (MCAO) and performed immunolabeling with specific neuronal markers to trace the neural fates of eGFP-expressing (GFP(+)) reactive astrocytes. The results showed that a portion of striatal GFP(+) astrocytes could transdifferentiate into immature neurons at 1 week after MCAO and mature neurons at 2 weeks as determined by double staining GFP-expressing cells with βIII-tubulin (GFP(+)-Tuj-1(+)) and microtubule associated protein-2 (GFP(+)-MAP-2(+)), respectively. GFP(+) neurons further expressed choline acetyltransferase, glutamic acid decarboxylase, dopamine receptor D2-like family proteins, and the N-methyl-D-aspartate receptor subunit R2, indicating that astrocyte-derived neurons could develop into cholinergic or GABAergic neurons and express dopamine and glutamate receptors on their membranes. Electron microscopy analysis indicated that GFP(+) neurons could form synapses with other neurons at 13 weeks after MCAO. Electrophysiological recordings revealed that action potentials and active postsynaptic currents could be recorded in the neuron-like GFP(+) cells but not in the astrocyte-like GFP(+) cells, demonstrating that new GFP(+) neurons possessed the capacity to fire action potentials and receive synaptic inputs. These results demonstrated that striatal astrocyte-derived new neurons participate in the rebuilding of functional neural networks, a fundamental basis for brain repair after injury. These results may lead to new therapeutic strategies for enhancing brain repair after ischemic stroke.

How can we exploit the brain's ability to repair itself?

The notion of our brain having a limited repertoire of cells to be used throughout our life has been refuted multiples times, showing systems by which new neurons are generated on demand, accounting for crucial aspects of brain function in a process known as neurogenesis. The potential of neurogenesis to replace lost neurons is enormous and has direct implications on how we understand the brain's response to pathology. But replacing functional neurons is not trivial: an orchestrated sequence of steps is needed to ensure the timed generation of new cells, their migration to the sites of injury and their correct differentiation and integration into the pre-existing circuitry. However, there is evidence of this sequence being effective in replacing certain neuronal populations in brain disease. The perspective of understanding, manipulating and directing the brain's self-repairing responses opens a vast avenue to explore novel therapeutic approaches to replace neuronal loss in neurodegenerative diseases.

Cell replacement therapy for central nervous system diseases

The brain and spinal cord can not replace neurons or supporting glia that are lost through traumatic injury or disease. In pre-clinical studies, however, neural stem and progenitor cell transplants can promote functional recovery. Thus the central nervous system is repair competent but lacks endogenous stem cell resources. To make transplants clinically feasible, this field needs a source of histocompatible, ethically acceptable and non-tumorgenic cells. One strategy to generate patient-specific replacement cells is to reprogram autologous cells such as fibroblasts into pluripotent stem cells which can then be differentiated into the required cell grafts. However, the utility of pluripotent cell derived grafts is limited since they can retain founder cells with intrinsic neoplastic potential. A recent extension of this technology directly reprograms fibroblasts into the final graftable cells without an induced pluripotent stem cell intermediate, avoiding the pluripotent caveat. For both types of reprogramming the conversion efficiency is very low resulting in the need to amplify the cells in culture which can lead to chromosomal instability and neoplasia. Thus to make reprogramming biology clinically feasible, we must improve the efficiency. The ultimate source of replacement cells may reside in directly reprogramming accessible cells within the brain.

In vivo reprogrammed pluripotent stem cells from teratomas share analogous properties with their in vitro counterparts

Recently, induced pluripotent stem cells (iPSCs) have been generated in vivo from reprogrammable mice. These in vivo iPSCs display features of totipotency, i.e., they differentiate into the trophoblast lineage, as well as all 3 germ layers. Here, we developed a new reprogrammable mouse model carrying an Oct4-GFP reporter gene to facilitate the detection of reprogrammed pluripotent stem cells. Without doxycycline administration, some of the reprogrammable mice developed aggressively growing teratomas that contained Oct4-GFP⁺ cells. These teratoma-derived in vivo PSCs were morphologically indistinguishable from ESCs, expressed pluripotency markers, and could differentiate into tissues of all 3 germ layers. However, these in vivo reprogrammed PSCs were more similar to in vitro iPSCs than ESCs and did not contribute to the trophectoderm of the blastocysts after aggregation with 8-cell embryos. Therefore, the ability to differentiate into the trophoblast lineage might not be a unique characteristic of in vivo iPSCs.

CEND1 and NEUROGENIN2 Reprogram Mouse Astrocytes and Embryonic Fibroblasts to Induced Neural Precursors and Differentiated Neurons

Recent studies demonstrate that astroglia from non-neurogenic brain regions can be reprogrammed into functional neurons through forced expression of neurogenic factors. Here we explored the effect of CEND1 and NEUROG2 on reprogramming of mouse cortical astrocytes and embryonic fibroblasts. Forced expression of CEND1, NEUROG2, or both resulted in acquisition of induced neuronal cells expressing subtype-specific markers, while long-term live-cell imaging highlighted the existence of two different modes of neuronal trans-differentiation. Of note, a subpopulation of CEND1 and NEUROG2 double-transduced astrocytes formed spheres exhibiting neural stem cell properties. mRNA and protein expression studies revealed a reciprocal feedback loop existing between the two molecules, while knockdown of endogenous CEND1 demonstrated that it is a key mediator of NEUROG2-driven neuronal reprogramming. Our data suggest that common reprogramming mechanisms exist driving the conversion of lineage-distant somatic cell types to neurons and reveal a critical role for CEND1 in NEUROG2-driven astrocytic reprogramming.

•

CEND1 reprograms astrocytes and fibroblasts to GABAergic neurons

•

Neurospheres are formed from CEND1⁺ and NEUROG2⁺ cells through the β-catenin pathway

•

CEND1 and NEUROG2 participate in a reciprocal feedback loop leading to neurogenesis

•

CEND1 is a key mediator of NEUROG2 reprogramming function

Neural stem/progenitor cell properties of glial cells in the adult mouse auditory nerve

The auditory nerve is the primary conveyor of hearing information from sensory hair cells to the brain. It has been believed that loss of the auditory nerve is irreversible in the adult mammalian ear, resulting in sensorineural hearing loss. We examined the regenerative potential of the auditory nerve in a mouse model of auditory neuropathy. Following neuronal degeneration, quiescent glial cells converted to an activated state showing a decrease in nuclear chromatin condensation, altered histone deacetylase expression and up-regulation of numerous genes associated with neurogenesis or development. Neurosphere formation assays showed that adult auditory nerves contain neural stem/progenitor cells (NSPs) that were within a Sox2-positive glial population. Production of neurospheres from auditory nerve cells was stimulated by acute neuronal injury and hypoxic conditioning. These results demonstrate that a subset of glial cells in the adult auditory nerve exhibit several characteristics of NSPs and are therefore potential targets for promoting auditory nerve regeneration.

Skin-derived neural precursors competitively generate functional myelin in adult demyelinated mice

Induced pluripotent stem cell-derived (iPS-derived) neural precursor cells may represent the ideal autologous cell source for cell-based therapy to promote remyelination and neuroprotection in myelin diseases. So far, the therapeutic potential of reprogrammed cells has been evaluated in neonatal demyelinating models. However, the repair efficacy and safety of these cells has not been well addressed in the demyelinated adult CNS, which has decreased cell plasticity and scarring. Moreover, it is not clear if these induced pluripotent-derived cells have the same reparative capacity as physiologically committed CNS-derived precursors. Here, we performed a side-by-side comparison of CNS-derived and skin-derived neural precursors in culture and following engraftment in murine models of adult spinal cord demyelination. Grafted induced neural precursors exhibited a high capacity for survival, safe integration, migration, and timely differentiation into mature bona fide oligodendrocytes. Moreover, grafted skin-derived neural precursors generated compact myelin around host axons and restored nodes of Ranvier and conduction velocity as efficiently as CNS-derived precursors while outcompeting endogenous cells. Together, these results provide important insights into the biology of reprogrammed cells in adult demyelinating conditions and support use of these cells for regenerative biomedicine of myelin diseases that affect the adult CNS.

Reawakening the sleeping beauty in the adult brain: neurogenesis from parenchymal glia

Life-long neurogenesis is highly restricted to specialized niches in the adult mammalian brain and therefore the brain's capacity for spontaneous regeneration is extremely limited. However, recent work has demonstrated that under certain circumstances parenchymal astrocytes and NG2 glia can generate neuronal progeny. In the striatum, stroke or excitotoxic lesions can reawaken in astrocytes a latent neurogenic program resulting in the genesis of new neurons. By contrast, in brain areas that fail to mount a neurogenic response following injury, such as the cerebral cortex, forced expression of neurogenic reprogramming factors can lineage convert local glia into induced neurons. Yet, injury-induced and reprogramming-induced neurogenesis exhibit intriguing commonalities, suggesting that they may converge on similar mechanisms.

Human cell dedifferentiation in mesenchymal condensates through controlled autophagy

Tissue and whole organ regeneration is a dramatic biological response to injury that occurs across different plant and animal phyla. It frequently requires the dedifferentiation of mature cells to a condensed mesenchymal blastema, from which replacement tissues develop. Human somatic cells cannot regenerate in this way and differentiation is considered irreversible under normal developmental conditions. Here, we sought to establish in vitro conditions to mimic blastema formation by generating different three-dimensional (3D) condensates of human mesenchymal stromal cells (MSCs). We identified specific 3D growth environments that were sufficient to dedifferentiate aged human MSCs to an early mesendoderm-like state with reversal of age-associated cell hypertrophy and restoration of organized tissue regenerating capacity in vivo. An optimal auophagic response was required to promote cytoplasmic remodeling, mitochondrial regression, and a bioenergetic shift from oxidative phosphorylation to anaerobic metabolism. Our evidence suggests that human cell dedifferentiation can be achieved through autonomously controlled autophagic flux.

Analysis of Mll1 deficiency identifies neurogenic transcriptional modules and Brn4 as a factor for direct astrocyte-to-neuron reprogramming

BACKGROUND

Mixed lineage leukemia-1 (Mll1) epigenetically regulates gene expression patterns that specify cellular identity in both embryonic development and adult stem cell populations. In the adult mouse brain, multipotent neural stem cells (NSCs) in the subventricular zone generate new neurons throughout life, and Mll1 is required for this postnatal neurogenesis but not for glial cell differentiation. Analysis of Mll1-dependent transcription may identify neurogenic genes useful for the direct reprogramming of astrocytes into neurons.

OBJECTIVE

To identify Mll1-dependent transcriptional modules and to determine whether genes in the neurogenic modules can be used to directly reprogram astrocytes into neurons.

METHODS

We performed gene coexpression module analysis on microarray data from differentiating wild-type and Mll1-deleted subventricular zone NSCs. Key developmental regulators belonging to the neurogenic modules were overexpressed in Mll1-deleted cells and cultured cortical astrocytes, and cell phenotypes were analyzed by immunocytochemistry and electrophysiology.

RESULTS

Transcriptional modules that correspond to neurogenesis were identified in wild-type NSCs. Modules related to astrocytes and oligodendrocytes were enriched in Mll1-deleted NSCs, consistent with their gliogenic potential. Overexpression of genes selected from the neurogenic modules enhanced the production of neurons from Mll1-deleted cells, and overexpression of Brn4 (Pou3f4) in nonneurogenic cortical astroglia induced their transdifferentiation into electrophysiologically active neurons.

CONCLUSION

Our results demonstrate that Mll1 is required for the expression of neurogenic but not gliogenic transcriptional modules in a multipotent NSC population and further indicate that specific Mll1-dependent genes may be useful for direct reprogramming strategies.

In Vivo Reprogramming of Striatal NG2 Glia into Functional Neurons that Integrate into Local Host Circuitry

The possibility of directly converting non-neuronal cells into neurons in situ in the brain would open therapeutic avenues aimed at repairing the brain after injury or degenerative disease. We have developed an adeno-associated virus (AAV)-based reporter system that allows selective GFP labeling of reprogrammed neurons. In this system, GFP is turned on only in reprogrammed neurons where it is stable and maintained for long time periods, allowing for histological and functional characterization of mature neurons. When combined with a modified rabies virus-based trans-synaptic tracing methodology, the system allows mapping of 3D circuitry integration into local and distal brain regions and shows that the newly reprogrammed neurons are integrated into host brain.

Applications of Induced Pluripotent Stem Cells in Studying the Neurodegenerative Diseases

Neurodegeneration is the umbrella term for the progressive loss of structure or function of neurons. Incurable neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD) show dramatic rising trends particularly in the advanced age groups. However, the underlying mechanisms are not yet fully elucidated, and to date there are no biomarkers for early detection or effective treatments for the underlying causes of these diseases. Furthermore, due to species variation and differences between animal models (e.g., mouse transgenic and knockout models) of neurodegenerative diseases, substantial debate focuses on whether animal and cell culture disease models can correctly model the condition in human patients. In 2006, Yamanaka of Kyoto University first demonstrated a novel approach for the preparation of induced pluripotent stem cells (iPSCs), which displayed similar pluripotency potential to embryonic stem cells (ESCs). Currently, iPSCs studies are permeating many sectors of disease research. Patient sample-derived iPSCs can be used to construct patient-specific disease models to elucidate the pathogenic mechanisms of disease development and to test new therapeutic strategies. Accordingly, the present review will focus on recent progress in iPSC research in the modeling of neurodegenerative disorders and in the development of novel therapeutic options.

Sensory hair cell development and regeneration: similarities and differences

Sensory hair cells are mechanoreceptors of the auditory and vestibular systems and are crucial for hearing and balance. In adult mammals, auditory hair cells are unable to regenerate, and damage to these cells results in permanent hearing loss. By contrast, hair cells in the chick cochlea and the zebrafish lateral line are able to regenerate, prompting studies into the signaling pathways, morphogen gradients and transcription factors that regulate hair cell development and regeneration in various species. Here, we review these findings and discuss how various signaling pathways and factors function to modulate sensory hair cell development and regeneration. By comparing and contrasting development and regeneration, we also highlight the utility and limitations of using defined developmental cues to drive mammalian hair cell regeneration.

Lost highway(s): barriers to postnatal cortical neurogenesis and implications for brain repair

The genesis of the cerebral cortex is a highly complex and tightly-orchestrated process of cell division, migration, maturation, and integration. Developmental missteps often have catastrophic consequences on cortical function. Further, the cerebral cortex, in which neurogenesis takes place almost exclusively prenatally, has a very poor capacity for replacement of neurons lost to injury or disease. A multitude of factors underlie this deficit, including the depletion of radial glia, the gliogenic switch which mitigates continued neurogenesis, diminished neuronal migratory streams, and inflammatory processes associated with disease. Despite this, there are glimmers of hope that new approaches may allow for more significant cortical repair. Herein, we review corticogenesis from the context of regeneration and detail the strategies to promote neurogenesis, including interneuron transplants and glial reprogramming. Such strategies circumvent the "lost highways" which are critical for cortical development but are absent in the adult. These new approaches may provide for the possibility of meaningful clinical regeneration of elements of cortical circuitry lost to trauma and disease.

Concepts and challenges in cancer risk prediction for the space radiation environment

Cancer is an important long-term risk for astronauts exposed to protons and high-energy charged particles during travel and residence on asteroids, the moon, and other planets. NASA's Biomedical Critical Path Roadmap defines the carcinogenic risks of radiation exposure as one of four type I risks. A type I risk represents a demonstrated, serious problem with no countermeasure concepts, and may be a potential "show-stopper" for long duration spaceflight. Estimating the carcinogenic risks for humans who will be exposed to heavy ions during deep space exploration has very large uncertainties at present. There are no human data that address risk from extended exposure to complex radiation fields. The overarching goal in this area to improve risk modeling is to provide biological insight and mechanistic analysis of radiation quality effects on carcinogenesis. Understanding mechanisms will provide routes to modeling and predicting risk and designing countermeasures. This white paper reviews broad issues related to experimental models and concepts in space radiation carcinogenesis as well as the current state of the field to place into context recent findings and concepts derived from the NASA Space Radiation Program.

Limitations of In Vivo Reprogramming to Dopaminergic Neurons via a Tricistronic Strategy

Parkinson's disease is one of the most common neurodegenerative disorders characterized by cell death of dopaminergic neurons in the substantia nigra. Recent research has focused on cellular replacement through lineage reprogramming as a potential therapeutic strategy. This study sought to use genetics to define somatic cell types in vivo amenable to reprogramming. To stimulate in vivo reprogramming to dopaminergic neurons, we generated a Rosa26 knock-in mouse line conditionally overexpressing Mash1, Lmx1a, and Nurr1. These proteins are characterized by their role in neuronal commitment and development of midbrain dopaminergic neurons and have previously been shown to convert fibroblasts to dopaminergic neurons in vitro. We show that a tricistronic construct containing these transcription factors can reprogram astrocytes and fibroblasts in vitro. However, cassette overexpression triggered cell death in vivo, in part through endoplasmic reticulum stress, while we also detected "uncleaved" forms of the polyprotein, suggesting poor "cleavage" efficiency of the 2A peptides. Based on our results, the cassette overexpression induced apoptosis and precluded reprogramming in our mouse model. Therefore, we suggest that alternatives must be explored to balance construct design with efficacious reprogramming. It is evident that there are still biological obstacles to overcome for in vivo reprogramming to dopaminergic neurons.

Moving stem cells to the clinic: potential and limitations for brain repair

Stem cell-based therapies hold considerable promise for many currently devastating neurological disorders. Substantial progress has been made in the derivation of disease-relevant human donor cell populations. Behavioral data in relevant animal models of disease have demonstrated therapeutic efficacy for several cell-based approaches. Consequently, cGMP grade cell products are currently being developed for first in human clinical trials in select disorders. Despite the therapeutic promise, the presumed mechanism of action of donor cell populations often remains insufficiently validated. It depends greatly on the properties of the transplanted cell type and the underlying host pathology. Several new technologies have become available to probe mechanisms of action in real time and to manipulate in vivo cell function and integration to enhance therapeutic efficacy. Results from such studies generate crucial insight into the nature of brain repair that can be achieved today and push the boundaries of what may be possible in the future.

Regeneration through reprogramming adult cell identity in vivo

The discovery and in vivo application of cell fate reprogramming concepts have jumpstarted new technologies aimed at the functional regeneration of damaged tissues. As most adult organ systems retain only a limited potential for self-regeneration after trauma, the production of fate-specific cells by in vivo transdifferentiation offers a targeted method for tissue bioengineering. Proof-of-principle studies have demonstrated the induction of neural precursor cells, neurons, cardiomyocytes, and insulin-producing β islet cells. Each of these induced cell types survive, mature, and integrate into the local environment in a functionally meaningful manner. Here, we briefly highlight recent advances in the in vivo reprogramming of cell identity and the current challenges that face the clinical relevance of these methods.

Forward engineering neuronal diversity using direct reprogramming

The nervous system is comprised of a vast diversity of distinct neural cell types. Differences between neuronal subtypes drive the assembly of neuronal circuits and underlie the subtype specificity of many neurological diseases. Yet, because neurons are irreversibly post-mitotic and not readily available from patients, it has not been feasible to study specific subtypes of human neurons in larger numbers. A powerful means to study neuronal diversity and neurological disease is to establish methods to produce desired neuronal subtypes in vitro. Traditionally this has been accomplished by treating pluripotent or neural stem cells with growth factors and morphogens that recapitulate exogenous developmental signals. These approaches often require extended periods of culture, which can limit their utility. However, more recently, it has become possible to produce neurons directly from fibroblasts using transcription factors and/or microRNAs. This technique referred to as direct reprogramming or transdifferentiation has proven to be a rapid, robust, and reproducible method to generate mature neurons of many different subtypes from multiple cell sources. Here, we highlight recent advances in generating neurons of specific subtypes using direct reprogramming and outline various scenarios in which induced neurons may be applied to studies of neuronal function and neurological disease.

MicroRNA-Mediated In Vitro and In Vivo Direct Conversion of Astrocytes to Neuroblasts

BACKGROUND

The conversion of astrocytes to neuroblasts holds great promise for treatment of neurodegenerative and traumatic brain diseases.

METHODOLOGY AND PRINCIPAL FINDINGS

Here we have shown that adult human astrocytes could be reprogrammed to neuroblasts by miR-302/367, both in vivo and in vitro. However, the reprogramming of adult mouse astrocytes to neuroblasts required valproic acid (VPA), a histone deacetylase inhibitor. Following induction of astrocytes toward neurons the expression of pluripotency markers were not detected, which suggested direct cell conversion. We did not observed tumor formation during two months follow up.

CONCLUSIONS AND SIGNIFICANCE

These results show that neuroblasts can be generated directly from adult human and mouse astrocytes by miR-302/367-driven induction. This approach seems promising for converting glial scar cells into neuroblasts in a wide range of neurological diseases.

How to make a midbrain dopaminergic neuron

Midbrain dopaminergic (mDA) neuron development has been an intense area of research during recent years. This is due in part to a growing interest in regenerative medicine and the hope that treatment for diseases affecting mDA neurons, such as Parkinson's disease (PD), might be facilitated by a better understanding of how these neurons are specified, differentiated and maintained in vivo. This knowledge might help to instruct efforts to generate mDA neurons in vitro, which holds promise not only for cell replacement therapy, but also for disease modeling and drug discovery. In this Primer, we will focus on recent developments in understanding the molecular mechanisms that regulate the development of mDA neurons in vivo, and how they have been used to generate human mDA neurons in vitro from pluripotent stem cells or from somatic cells via direct reprogramming. Current challenges and future avenues in the development of a regenerative medicine for PD will be identified and discussed.

Epigenetic setting and reprogramming for neural cell fate determination and differentiation

In the mammalian brain, epigenetic mechanisms are clearly involved in the regulation of self-renewal of neural stem cells and the derivation of their descendants, i.e. neurons, astrocytes and oligodendrocytes, according to the developmental timing and the microenvironment, the 'niche'. Interestingly, local epigenetic changes occur, concomitantly with genome-wide level changes, at a set of gene promoter regions for either down- or upregulation of the gene. In addition, intergenic regions also sensitize the availability of epigenetic modifiers, which affects gene expression through a relatively long-range chromatinic interaction with the transcription regulatory machineries including non-coding RNA (ncRNA) such as promoter-associated ncRNA and enhancer ncRNA. We show that such an epigenetic landscape in a neural cell is statically but flexibly formed together with a variable combination of generally and locally acting nuclear molecules including master transcription factors and cell-cycle regulators. We also discuss the possibility that revealing the epigenetic regulation by the local DNA-RNA-protein assemblies would promote methodological innovations, e.g. neural cell reprogramming, engineering and transplantation, to manipulate neuronal and glial cell fates for the purpose of medical use of these cells.

Astrocyte development: A Guide for the Perplexed

Astrocytes are the predominant cell type in the brain and perform key functions vital to CNS physiology, including blood brain barrier formation and maintenance, synaptogenesis, neurotransmission, and metabolic regulation. To fully understand the contributions of astrocytes to brain function, it will be important to bridge the existing gap between development and physiology. In this review, we provide an overview of Astrocyte development, including recent insights into molecular mechanisms of astrocyte specification, regional patterning and proliferation. This developmental perspective is complemented with recent findings that describe the functional maturation of astrocytes and their prospective diversity. Future progress in understanding Astrocyte development will depend on the development of astrocyte- stage specific markers and tools for manipulating astrocytes without affecting neuron production. Ultimately, a mechanistic approach to Astrocyte development will be crucial to developing new treatments for the many neurodevelopmental, neurodegenerative, neuroimmune, and neoplastic diseases involving astrocyte dysfunction.

Experimental Advances Towards Neural Regeneration from Induced Stem Cells to Direct In Vivo Reprogramming

Neuronal loss is a common substrate of many neurological diseases that still lack effective treatments and highly burden lives of affected individuals. The discovery of self-renewing stem cells within the central nervous system (CNS) has opened the doors to the possibility of using the plasticity of CNS as a potential strategy for the development of regenerative therapies after injuries. The role of neural progenitor cells appears to be crucial, but insufficient in reparative processes after damage. In addition, the mechanisms that regulate these events are still largely unknown. Stem cell-based therapeutic approaches have primarily focused on the use of either induced pluripotent stem cells or induced neural stem cells as sources for cell transplantation. More recently, in vivo direct reprogramming of endogenous CNS cells into multipotent neural stem/progenitor cells has been proposed as an alternative strategy that could overcome the limits connected with both the invasiveness of exogenous cell transplantation and the technical issues of in vitro reprogramming (i.e., the time requested and the limited available amount of directly induced neuronal cells). In this review, we aim to highlight the recent studies on in vivo direct reprogramming, focusing on astrocytes conversion to neurons or to neural stem/precursors cells, in the perspective of future therapeutic purposes for neurological disorders.

Probing disorders of the nervous system using reprogramming approaches

The groundbreaking technologies of induced pluripotency and lineage conversion have generated a genuine opportunity to address fundamental aspects of the diseases that affect the nervous system. These approaches have granted us unrestricted access to the brain and spinal cord of patients and have allowed for the study of disease in the context of human cells, expressing physiological levels of proteins and under each patient's unique genetic constellation. Along with this unprecedented opportunity have come significant challenges, particularly in relation to patient variability, experimental design and data interpretation. Nevertheless, significant progress has been achieved over the past few years both in our ability to create the various neural subtypes that comprise the nervous system and in our efforts to develop cellular models of disease that recapitulate clinical findings identified in patients. In this Review, we present tables listing the various human neural cell types that can be generated and the neurological disease modeling studies that have been reported, describe the current state of the field, highlight important breakthroughs and discuss the next steps and future challenges.

In vivo reprogramming for tissue repair

Vital organs such as the pancreas and the brain lack the capacity for effective regeneration. To overcome this limitation, an emerging strategy consists of converting resident tissue-specific cells into the cell types that are lost due to disease by a process called in vivo lineage reprogramming. Here we discuss recent breakthroughs in regenerating pancreatic β-cells and neurons from various cell types, and highlight fundamental challenges that need to be overcome for the translation of in vivo lineage reprogramming into therapy.

SOX2 reprograms resident astrocytes into neural progenitors in the adult brain

Glial cells can be in vivo reprogrammed into functional neurons in the adult CNS; however, the process by which this reprogramming occurs is unclear. Here, we show that a distinct cellular sequence is involved in SOX2-driven in situ conversion of adult astrocytes to neurons. This includes ASCL1⁺ neural progenitors and DCX⁺ adult neuroblasts (iANBs) as intermediates. Importantly, ASCL1 is required, but not sufficient, for the robust generation of iANBs in the adult striatum. These progenitor-derived iANBs predominantly give rise to calretinin⁺ interneurons when supplied with neurotrophic factors or the small-molecule valproic acid. Patch-clamp recordings from the induced neurons reveal subtype heterogeneity, though all are functionally mature, fire repetitive action potentials, and receive synaptic inputs. Together, these results show that SOX2-mediated in vivo reprogramming of astrocytes to neurons passes through proliferative intermediate progenitors, which may be exploited for regenerative medicine.

•

SOX2 induces ASCL1-positive neural progenitors in the adult mouse brain

•

Ascl1 in resident astrocytes is required for SOX2-mediated in vivo reprogramming

•

Induced ASCL1-positive neural progenitors generate mature calretinin neurons

Direct reprogramming of induced neural progenitors: a new promising strategy for AD treatment

Alzheimer's disease (AD) is a prominent form of dementia, characterized by aggregation of the amyloid β-peptide (Aβ) plaques and neurofibrillary tangles, loss of synapses and neurons, and degeneration of cognitive functions. Currently, although a variety of medications can relieve some of the symptoms, there is no cure for AD. Recent breakthroughs in the stem cell field provide promising strategies for AD treatment. Stem cells including embryonic stem cells (ESCs), neural stem cells (NSCs), mesenchymal stem cells (MSCs), and induced pluripotent stem cells (iPSCs) are potentials for AD treatment. However, the limitation of cell sources, safety issues, and ethical issues restrict their applications in AD. Recently, the direct reprogramming of induced neural progenitor cells (iNPCs) has shed light on the treatment of AD. In this review, we will discuss the latest progress, challenges, and potential applications of direct reprogramming in AD treatment.

Direct lineage conversion of astrocytes to induced neural stem cells or neurons

Since the generation of induced pluripotent stem cells in 2006, cellular reprogramming has attracted increasing attention as a revolutionary strategy for cell replacement therapy. Recent advances have revealed that somatic cells can be directly converted into other mature cell types, which eliminates the risk of neoplasia and the generation of undesired cell types. Astrocytes become reactive and undergo proliferation, which hampers axon regeneration following injury, stroke, and neurodegenerative diseases. An emerging technique to directly reprogram astrocytes into induced neural stem cells (iNSCs) and induced neurons (iNs) by neural fate determinants brings potential hope to cell replacement therapy for the above neurological problems. Here, we discuss the development of direct reprogramming of various cell types into iNs and iNSCs, then detail astrocyte-derived iNSCs and iNs in vivo and in vitro. Finally, we highlight the unsolved challenges and opportunities for improvement.

[Inducing brain regeneration from within: in vivo reprogramming of endogenous somatic cells into neurons]

In order to overcome the quasi-total inability of the mammalian central nervous system to regenerate in response to injuries, and in parallel to the studies dedicated to prevent neuronal loss under these circumstances, alternative approaches based on the programming of pluripotent cells or the reprogramming of somatic cells into neurons have recently emerged. These uniquely combine growing knowledge of the mechanisms that underlie neurogenesis and neuronal specification during development to the most recent findings of the molecular and epigenetic mechanisms that govern the acquisition and maintenance of cellular identity. Here, we discuss the possibility to instruct the regeneration of the central nervous system from within for therapeutic purposes, in light of the recent works reporting on the generation of neurons by direct conversion of various cerebral cell types in vitro and in vivo.

Striatal astrocytes produce neuroblasts in an excitotoxic model of Huntington's disease

In the adult brain, subsets of astrocytic cells residing in well-defined neurogenic niches constitutively generate neurons throughout life. Brain lesions can stimulate neurogenesis in otherwise non-neurogenic regions, but whether local astrocytic cells generate neurons in these conditions is unresolved. Here, through genetic and viral lineage tracing in mice, we demonstrate that striatal astrocytes become neurogenic following an acute excitotoxic lesion. Similar to astrocytes of adult germinal niches, these activated parenchymal progenitors express nestin and generate neurons through the formation of transit amplifying progenitors. These results shed new light on the neurogenic potential of the adult brain parenchyma.

Involvement of calpains in adult neurogenesis: implications for stroke

Calpains are ubiquitous proteases involved in cell proliferation, adhesion and motility. In the brain, calpains have been associated with neuronal damage in both acute and neurodegenerative disorders, but their physiological function in the nervous system remains elusive. During brain ischemia, there is a large increase in the levels of intracellular calcium, leading to the activation of calpains. Inhibition of these proteases has been shown to reduce neuronal death in a variety of stroke models. On the other hand, after stroke, neural stem cells (NSC) increase their proliferation and newly formed neuroblasts migrate towards the site of injury. However, the process of forming new neurons after injury is not efficient and finding ways to improve it may help with recovery after lesion. Understanding the role of calpains in the process of neurogenesis may therefore open a new window for the treatment of stroke. We investigated the involvement of calpains in NSC proliferation and neuroblast migration in two highly neurogenic regions in the mouse brain, the dentate gyrus (DG) and the subventricular zone (SVZ). We used mice that lack calpastatin, the endogenous calpain inhibitor, and calpains were also modulated directly, using calpeptin, a pharmacological calpain inhibitor. Calpastatin deletion impaired both NSC proliferation and neuroblast migration. Calpain inhibition increased NSC proliferation, migration speed and migration distance in cells from the SVZ. Overall, our work suggests that calpains are important for neurogenesis and encourages further research on their neurogenic role. Prospective therapies targeting calpain activity may improve the formation of new neurons following stroke, in addition to affording neuroprotection.

Potential of neural stem cell-based therapies for Alzheimer's disease

Alzheimer's disease (AD), known to be a leading cause of dementia that causes heavy social and financial burdens worldwide, is characterized by progressive loss of neurons and synaptic connectivity after depositions of amyloid-β (Aβ) protein. Current therapies for AD patients can only alleviate symptoms but cannot deter the neural degeneration, thus providing no long-term recovery. Neural stem cells (NSCs), capable of self-renewal and of differentiation into functional neurons and glia, have been shown to repair damaged networks and reverse memory and learning deficits in animal studies, providing new hope for curing AD patients by cell transplantation. Under AD pathology, the microenvironment also undergoes great alterations that affect the propagation of NSCs and subsequent therapeutic efficiency, calling for measures to improve the hostile environment for cell transplantation. This article reviews the therapeutic potential of both endogenous and exogenous NSCs in the treatment of AD and the challenges to application of stem cells in AD treatment, particularly those from the microenvironmental alterations, in the hope of providing more information for future research in exploiting stem cell-based therapies for AD. © 2015 Wiley Periodicals, Inc.

Turning reactive glia into functional neurons in the brain

Following brain injury, reactive glial cells can create scars that inhibit neural repair responses. In this issue of Cell Stem Cell, Guo et al. report that overexpression of NeuroD1 in vivo can directly reprogram reactive glial cells into glutamatergic and GABAergic neurons that integrate into the host's neural circuitry.

Pericytes at the intersection between tissue regeneration and pathology

Perivascular multipotent cells, pericytes, contribute to the generation and repair of various tissues in response to injury. They are heterogeneous in their morphology, distribution, origin and markers, and elucidating their molecular and cellular differences may inform novel treatments for disorders in which tissue regeneration is either impaired or excessive. Moreover, these discoveries offer novel cellular targets for therapeutic approaches to many diseases. This review discusses recent studies that support the concept that pericyte subtypes play a distinctive role in myogenesis, neurogenesis, adipogenesis, fibrogenesis and angiogenesis.

Direct conversion of fibroblasts into functional astrocytes by defined transcription factors

Direct cell reprogramming enables direct conversion of fibroblasts into functional neurons and oligodendrocytes using a minimal set of cell-lineage-specific transcription factors. This approach is rapid and simple, generating the cell types of interest in one step. However, it remains unknown whether this technology can be applied to convert fibroblasts into astrocytes, the third neural lineage. Astrocytes play crucial roles in neuronal homeostasis, and their dysfunctions contribute to the origin and progression of multiple human diseases. Herein, we carried out a screening using several transcription factors involved in defining the astroglial cell fate and identified NFIA, NFIB, and SOX9 to be sufficient to convert with high efficiency embryonic and postnatal mouse fibroblasts into astrocytes (iAstrocytes). We proved both by gene-expression profiling and functional tests that iAstrocytes are comparable to native brain astrocytes. This protocol can be then employed to generate functional iAstrocytes for a wide range of experimental applications.

•

NFIA, NFIB, and SOX9 reprogram fibroblasts into induced astrocytes (iAstrocytes)

•

iAstrocytes reprogramming induces a global change in gene-expression profiling

•

iAstrocytes are functionally comparable to native astrocytes

•

NFIA, NFIB, and SOX9 induce an astrocytic phenotype in human fibroblasts

How to make neurons--thoughts on the molecular logic of neurogenesis in the central nervous system

Neuronal differentiation relies on a set of interconnected molecular events to achieve the differentiation of pan-neuronal hallmarks, together with neuronal subtype-specific features. Here, we propose a conceptual framework for these events, based on recent findings. This framework encompasses a dimension in time during development, progressing from early master regulators to later expressed effector genes and terminal selector genes. As a horizontal intersection, we propose the action of permissive fate determinants that are critical in allowing progression through the above transcriptional phases. Typically, these are widely expressed and often interact with the chromatin remodeling machinery. We conclude by discussing this model in the context of the direct fate conversion of various somatic cells into neurons.

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

The adult cerebral cortex lacks the capacity to replace degenerated neurons following traumatic injury. Conversion of nonneuronal cells into induced neurons has been proposed as an innovative strategy toward brain repair. Here, we show that retrovirus-mediated expression of the transcription factors Sox2 and Ascl1, but strikingly also Sox2 alone, can induce the conversion of genetically fate-mapped NG2 glia into induced doublecortin (DCX)⁺ neurons in the adult mouse cerebral cortex following stab wound injury in vivo. In contrast, lentiviral expression of Sox2 in the unlesioned cortex failed to convert oligodendroglial and astroglial cells into DCX⁺ cells. Neurons induced following injury mature morphologically and some acquire NeuN while losing DCX. Patch-clamp recording of slices containing Sox2- and/or Ascl1-transduced cells revealed that a substantial fraction of these cells receive synaptic inputs from neurons neighboring the injury site. Thus, NG2 glia represent a potential target for reprogramming strategies toward cortical repair.

•

Sox2 or Sox2/Ascl1 can convert glia into induced DCX⁺ neurons in the injured cortex

•

Sox10-iCreER^T2-mediated fate mapping shows that induced neurons are NG2 glia derived

•

Induced neurons receive (or retain) synapses from preexisting neurons

•

Without prior injury, Sox2 does not convert cortical macroglia into neurons

Cellular reprogramming for understanding and treating human disease

In the last two decades we have witnessed a paradigm shift in our understanding of cells so radical that it has rewritten the rules of biology. The study of cellular reprogramming has gone from little more than a hypothesis, to applied bioengineering, with the creation of a variety of important cell types. By way of metaphor, we can compare the discovery of reprogramming with the archeological discovery of the Rosetta stone. This stone slab made possible the initial decipherment of Egyptian hieroglyphics because it allowed us to see this language in a way that was previously impossible. We propose that cellular reprogramming will have an equally profound impact on understanding and curing human disease, because it allows us to perceive and study molecular biological processes such as differentiation, epigenetics, and chromatin in ways that were likewise previously impossible. Stem cells could be called “cellular Rosetta stones” because they allow also us to perceive the connections between development, disease, cancer, aging, and regeneration in novel ways. Here we present a comprehensive historical review of stem cells and cellular reprogramming, and illustrate the developing synergy between many previously unconnected fields. We show how stem cells can be used to create in vitro models of human disease and provide examples of how reprogramming is being used to study and treat such diverse diseases as cancer, aging, and accelerated aging syndromes, infectious diseases such as AIDS, and epigenetic diseases such as polycystic ovary syndrome. While the technology of reprogramming is being developed and refined there have also been significant ongoing developments in other complementary technologies such as gene editing, progenitor cell production, and tissue engineering. These technologies are the foundations of what is becoming a fully-functional field of regenerative medicine and are converging to a point that will allow us to treat almost any disease.

Finding degrees of separation: experimental approaches for astroglial and oligodendroglial cell isolation and genetic targeting

The study of CNS glial cell function requires experimental methods to detect, purify, and manipulate each cell population with fidelity and specificity. With the identification and cloning of cell- and stage-specific markers, glial cell analysis techniques have grown beyond physical methods of tissue dissociation and cell culture, and become highly specific with immunoselection of cell cultures in vitro and genetic targeting in vivo. The unique plasticity of glial cells offers the potential for cell replacement therapies in neurological disease that utilize neural cells derived from transplanted neural stem and progenitor cells. In this mini-review, we outline general physical and genetic approaches for macroglial cell generation. We summarize cell culture methods to obtain astrocytes and oligodendrocytes and their precursors, from developing and adult tissue, as well as approaches to obtain human neural progenitor cells through the establishment of stem cells. We discuss popular targeting rodent strains designed for cell-specific detection, selection and manipulation of neuroglial cell progenitors and their committed progeny. Based on shared markers between astrocytes and stem cells, we discuss genetically modified mouse strains with overlapping expression, and highlight SOX-expressing strains available for targeting of stem and progenitor cell populations. We also include recently established mouse strains for detection, and tag-assisted RNA and miRNA analysis. This discussion aims to provide a brief overview of the rapidly expanding collection of experimental approaches and genetic resources for the isolation and targeting of macroglial cells, their sources, progeny and gene products to facilitate our understanding of their properties and potential application in pathology.

Reprogramming the fate of human glioma cells to impede brain tumor development

Malignant gliomas, the most common solid tumors in the central nervous system, are essentially incurable due to their rapid growth and very invasive nature. One potential approach to eradicating glioma cells is to force these cells to undergo terminal differentiation and, in the process, to irreversible postmitotic arrest. Here, we show that neurogenin 2 (NGN2, also known as NEUROG2) synergizes with sex-determining region Y-box 11 (SOX11) to very efficiently convert human glioma cells to terminally differentiated neuron-like cells in both cell culture and adult mouse brains. These cells exhibit neuronal morphology, marker expression, and electrophysiological properties. The conversion process is accompanied by cell cycle exit, which dramatically inhibits glioma cell proliferation and tumor development after orthotopic transplantation. Most importantly, intracranial injection of NGN2- and SOX11-expressing virus into the tumor mass also curtails glioma growth and significantly improves survival of tumor-bearing mice. Taken together, this study shows a simple and highly efficient strategy for reprogramming malignant glioma cells into postmitotic cells, which might be a promising therapeutic approach for brain tumors.

Tissue engineering and regenerative medicine: a year in review

It is an exciting time to be involved in tissue engineering and regenerative medicine (TERM) research. Despite its relative youth, the field is expanding fast and breaking new ground in both the laboratory and clinically. In this "Year in Review," we highlight some of the high-impact advances in the field. Building upon last year's article, we have identified the recent "hot topics" and the key publications pertaining to these themes as well as ideas that have high potential to direct the field. Based on a modified methodology grounded on last year's approach, we have identified and summarized some of the most impactful publications in five main themes: (1) pluripotent stem cells: efforts and hurdles to translation, (2) tissue engineering: complex scaffolds and advanced materials, (3) directing the cell phenotype: growth factor and biomolecule presentation, (4) characterization: imaging and beyond, and (5) translation: preclinical to clinical. We have complemented our review of the research directions highlighted within these trend-setting studies with a discussion of additional articles along the same themes that have recently been published and have yet to surface in citation analyses. We conclude with a discussion of some really interesting studies that provide a glimpse of the high potential for innovation of TERM research.

Reshaping the chromatin landscape after spinal cord injury

The pathophysiology underlying spinal cord injury is complex. Mechanistic understanding of the adaptive responses to injury is critical for targeted therapy aimed at reestablishing lost connections between proximal and distal neurons. After injury, cell-type specific gene transcription programs govern distinct cellular behaviors, and chromatin regulators play a central role in shaping the chromatin landscape to adjust transcriptional profiles in a context-dependent manner. In this review, we summarize recent progress on the pleiotropic roles of chromatin regulators in mediating the diverse adaptive behaviors of neurons and glial cells after spinal cord injury, and wherever possible, discuss the underlying mechanisms and genomic targets. We specifically draw attention to the perspective that takes into consideration the impact of epigenetic modulation on axon growth potential, together with its effect on wound-healing properties of glial cells. Epigenetic modulation of chromatin state represents an emerging therapeutic direction to promote neural repair and axon regeneration after spinal cord injury.

Differentiated type II pneumocytes can be reprogrammed by ectopic Sox2 expression

The adult lung contains several distinct stem cells, although their properties and full potential are still being sorted out. We previously showed that ectopic Sox2 expression in the developing lung manipulated the fate of differentiating cells. Here, we addressed the question whether fully differentiated cells could be redirected towards another cell type. Therefore, we used transgenic mice to express an inducible Sox2 construct in type II pneumocytes, which are situated in the distal, respiratory areas of the lung. Within three days after the induction of the transgene, the type II cells start to proliferate and form clusters of cuboidal cells. Prolonged Sox2 expression resulted in the reversal of the type II cell towards a more embryonic, precursor-like cell, being positive for the stem cell markers Sca1 and Ssea1. Moreover, the cells started to co-express Spc and Cc10, characteristics of bronchioalveolar stem cells. We demonstrated that Sox2 directly regulates the expression of Sca1. Subsequently, these cells expressed Trp63, a marker for basal cells of the trachea. So, we show that the expression of one transcription factor in fully differentiated, distal lung cells changes their fate towards proximal cells through intermediate cell types. This may have implications for regenerative medicine, and repair of diseased and damaged lungs.

Glial cells as progenitors and stem cells: new roles in the healthy and diseased brain

The diverse functions of glial cells prompt the question to which extent specific subtypes may be devoted to a specific function. We discuss this by reviewing one of the most recently discovered roles of glial cells, their function as neural stem cells (NSCs) and progenitor cells. First we give an overview of glial stem and progenitor cells during development; these are the radial glial cells that act as NSCs and other glial progenitors, highlighting the distinction between the lineage of cells in vivo and their potential when exposed to a different environment, e.g., in vitro. We then proceed to the adult stage and discuss the glial cells that continue to act as NSCs across vertebrates and others that are more lineage-restricted, such as the adult NG2-glia, the most frequent progenitor type in the adult mammalian brain, that remain within the oligodendrocyte lineage. Upon certain injury conditions, a distinct subset of quiescent astrocytes reactivates proliferation and a larger potential, clearly demonstrating the concept of heterogeneity with distinct subtypes of, e.g., astrocytes or NG2-glia performing rather different roles after brain injury. These new insights not only highlight the importance of glial cells for brain repair but also their great potential in various aspects of regeneration.

Prospects for engineering neurons from local neocortical cell populations as cell-mediated therapy for neurological disorders

There is little cell replacement following neurological injury, limiting the regenerative response of the CNS. Progress in understanding the biology of neural stem cells has raised interest in using stem cells for replacing neurons lost to injury or to disease. Stem cell therapy may also have a role in rebuilding deficient neural circuitry underlying mood disorders, epilepsy, and pain modulation among other roles. In vitro expansion of stem cells with directed differentiation prior to transplantation is one approach to stem cell therapy. Emerging evidence suggests that it may be possible to convert in vivo endogenous neural cells to a neuronal fate directly, providing an alternative strategy for stem cell therapy to the CNS. This review assesses the evidence for engineering a subtype-specific neuronal fate of endogenous neural cells in the cerebral cortex as a function of initial cell lineage, reactive response to injury, conversion factors, and environmental context. We conclude with a discussion of some of the challenges that must be overcome to move this alternative in vivo engineered conversion process toward becoming a viable therapeutic option.

Restricted nature of adult neural stem cells: re-evaluation of their potential for brain repair

Neural stem cells (NSCs) in the walls of the lateral ventricles continue to produce new neurons and oligodendrocytes throughout life. The identification of NSCs, long-range neuronal migration, and the integration of new neurons into fully formed mature neural circuits-all in the juvenile or adult brain-has dramatically changed concepts in neurodevelopment and suggests new strategies for brain repair. Yet, the latter has to be seen in perspective: NSCs in the adult are heterogeneous and highly regionally specified; young neurons derived from these primary progenitors migrate and integrate in specific brain regions. Neurogenesis appears to have a function in brain plasticity rather than brain repair. If similar processes could be induced in regions of the brain that are normally not a target of new neurons, therapeutic neuronal replacement may one day reinstate neural circuit plasticity and possibly repair broken neural circuits.