Growth factor treatment promotes mobilization of young but not aged adult subventricular zone precursors in response to demyelination

Neuroblasts contribute to oligodendrocytes generation upon demyelination in the adult mouse brain

After demyelinating insult, the neuronal progenitors of the adult mouse sub-ventricular zone (SVZ) called neuroblasts convert into oligodendrocytes that participate to the remyelination process. We use this rare example of spontaneous fate conversion to identify the molecular mechanisms governing these processes. Using in vivo cell lineage and single cell RNA-sequencing, we demonstrate that SVZ neuroblasts fate conversion proceeds through formation of a non-proliferating transient cellular state co-expressing markers of both neuronal and oligodendrocyte identities. Transition between the two identities starts immediately after demyelination and occurs gradually, by a stepwise upregulation/downregulation of key TFs and chromatin modifiers. Each step of this fate conversion involves fine adjustments of the transcription and translation machineries as well as tight regulation of metabolism and migratory behaviors. Together, these data constitute the first in-depth analysis of a spontaneous cell fate conversion in the adult mammalian CNS.

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NB can contribute to myelin repair by converting into oligodendrocytes

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NB fate conversion occurs gradually, through formation of an intermediate cell type

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NB fate conversion does not involve reversion toward a pluripotent state

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NB fate conversion seems to involve EMT-related mechanisms and metabolic changes

Slit1 Protein Regulates SVZ-Derived Precursor Mobilization in the Adult Demyelinated CNS

Slit1 is a secreted axon guidance molecule, also involved in adult neurogenesis. In physiological conditions, Slit1 loss promotes ectopic dispersal of SVZ-derived neural precursors (SVZ-NPCs) into periventricular structures such as the corpus callosum. Demyelination of the corpus callosum triggers SVZ-NPC migration to ectopic locations and their recruitment by the lesion, suggesting a possible role for Slit1 in SVZ-NPCs ectopic dispersal regulation in pathological conditions. Here, we have investigated the function of Slit1 protein in the recruitment of SVZ-NPCs after CNS demyelination. We find that the dynamics of oligodendrogenesis and temporal profile of developmental myelination in Slit1^–/– mice are similar to Slit1^+/− controls. SVZ micro-dissection and RT-PCR from wild-type mice, show that Slits and Robos are physiologically regulated at the transcriptional level in response to corpus callosum demyelination suggesting their role in the process of SVZ-NPC ectopic migration in demyelinating conditions. Moreover, we find that the number of SVZ-NPCs recruited by the lesion increases in Sli1^–/– mice compared to Slit1^+/− mice, leading to higher numbers of Olig2⁺ cells within the lesion. Time-lapse video-microscopy of immuno-purified NPCs shows that Slit1-deficient cells migrate faster and make more frequent directional changes than control NPCs, supporting a cell-autonomous mechanism of action of Slit1 in NPC migration. In conclusion, while Slit1 does not affect the normal developmental process of oligodendrogenesis and myelination, it regulates adult SVZ-NPC ectopic migration in response to demyelination, and consequently oligodendrocyte renewal within the lesion.

Investigation of Neuregulin-1 and Glial Cell-Derived Neurotrophic Factor in Rodent Astrocytes and Microglia

Growth factors play a crucial role during de- and remyelination of the central nervous system (CNS) due to their neurotrophic functions. We have previously shown that the growth factors neuregulin-1 (Nrg-1) and glial cell-derived neurotrophic factor (Gdnf) are upregulated during the first 2 weeks after induction of toxic demyelination in the CNS. Nevertheless, the factors responsible for Nrg-1/Gdnf upregulation and their effects on glia cells are unknown. We investigated the effect on Nrg-1 and Gdnf expressions after stimulation of primary mouse microglia or astrocytes with various pro- and anti-inflammatory factors. Additionally, primary cells were incubated with NRG-1 and/or GDNF followed by determining the gene expression level of their receptors, chemokines, and other growth factors. We demonstrate that inflammatory stimuli have a distinct impact on the expression of Gdnf, Nrg-1, and their receptors in astrocytes and microglia. In microglia, LPS or simultaneous treatment with IFNγ plus TNFα led to downregulation of Nrg-1, whereas LPS treatment slightly increased Nrg-1 expression in astrocytes. Furthermore, Gdnf was slightly upregulated after TFG-β treatment in microglia, while Gdnf was significantly upregulated after LPS treatment in astrocytes. In contrast, treatment with GDNF or/and NRG-1 did not alter any measured gene expression in microglia or astrocytes. Taken together, our in vitro studies show that Nrg-1, Gdnf, and their receptors are differently regulated in astrocytes and microglia upon inflammatory stimuli. The lack of response of astrocytes and microglia to NRG-1 and GDNF suggests that both factors exert their effects directly on neurons.

Targeting Nicotinamide Phosphoribosyltransferase as a Potential Therapeutic Strategy to Restore Adult Neurogenesis

Adult neurogenesis is the process of generating new neurons throughout life in the olfactory bulb and hippocampus of most mammalian species, which is closely related to aging and disease. Nicotinamide phosphoribosyltransferase (NAMPT), also an adipokine known as visfatin, is the rate-limiting enzyme for mammalian nicotinamide adenine dinucleotide (NAD) salvage synthesis by generating nicotinamide mononucleotide (NMN) from nicotinamide. Recent findings from our laboratory and other laboratories have provided much evidence that NAMPT might serve as a therapeutic target to restore adult neurogenesis. NAMPT-mediated NAD biosynthesis in neural stem/progenitor cells is important for their proliferation, self-renewal, and formation of oligodendrocytes in vivo and in vitro. Therapeutic interventions by the administration of NMN, NAD, or recombinant NAMPT are effective for restoring adult neurogenesis in several neurological diseases. We summarize adult neurogenesis in aging, ischemic stroke, traumatic brain injury, and neurodegenerative disease and review the advances of targeting NAMPT in restoring neurogenesis. Specifically, we provide emphasis on the P7C3 family, a class of proneurogenic compounds that are potential NAMPT activators, which might shed light on future drug development in neurogenesis restoration.

Fibroblast Growth Factor-2 Enhanced The Recruitment of Progenitor Cells and Myelin Repair in Experimental Demyelination of Rat Hippocampal Formations

OBJECTIVE

Hippocampal insults have been observed in multiple sclerosis (MS) patients. Fibroblast growth factor-2 (FGF2) induces neurogenesis in the hippocampus and en- hances the proliferation, migration and differentiation of oligodendrocyte progenitor cells (OPCs). In the current study, we have investigated the effect of FGF2 on the processes of gliotoxin induced demyelination and subsequent remyelination in the hippocampus.

MATERIALS AND METHODS

In this experimental study adult male Sprague-Dawley rats re- ceived either saline or lysolecithin (LPC) injections to the right hippocampi. Animals re- ceived intraperitoneal (i.p.) injections of FGF2 (5 ng/g) on days 0, 5, 12 and 26 post-LPC. Expressions of myelin basic protein (Mbp) as a marker of myelination, Olig2 as a marker of OPC proliferation, Nestin as a marker of neural progenitor cells, and glial fibrillary acidic protein (Gfap) as a marker of reactive astrocytes were investigated in the right hippocampi by reverse transcriptase-polymerase chain reaction (RT-PCR).

RESULTS

There was reduced Mbp expression at seven days after LPC injection, in- creased expressions of Olig2 and Nestin, and the level of Gfap did not change. FGF2 treatment reversed the expression level of Mbp to the control, significantly enhanced the levels of Olig2 and Nestin, but did not change the level of Gfap. At day-28 post- LPC, the expression level of Mbp was higher than the control in LPC-treated animals that received FGF2. The levels of Olig2, Nestin and Gfap were at the control level in the non-treated LPC group but significantly higher in the FGF2-treated LPC group.

CONCLUSION

FGF2 enhanced hippocampal myelination and potentiated the recruitment of OPCs and neural stem cells (NSCs) to the lesion area. Long-term application of FGF2 might also enhance astrogliosis in the lesion site.

Regulation of subventricular zone-derived cells migration in the adult brain

The subventricular zone of the lateral ventricles (SVZ) is the largest source of neural stem cells (NSCs) in the adult mammalian brain. Newly generated neuroblasts from the SVZ form cellular chains that migrate through the rostral migratory stream (RMS) into the olfactory bulb (OB), where they become mature neurons. Migration through the RMS is a highly regulated process of intrinsic and extrinsic factors, orchestrated to achieve direction and integration of neuroblasts into OB circuitry. These factors include internal cytoskeletal and volume regulators, extracellular matrix proteins, and chemoattractant and chemorepellent proteins. All these molecules direct the cells away from the SVZ, through the RMS, and into the OB guaranteeing their correct integration. Following brain injury, some neuroblasts escape the RMS and migrate into the lesion site to participate in regeneration, a phenomenon that is also observed with brain tumors. This review focuses on factors that regulate the migration of SVZ precursor cells in the healthy and pathologic brain. A better understanding of the factors that control the movement of newly generated cells may be crucial for improving the use of NSC-replacement therapy for specific neurological diseases.

Nogo receptor inhibition enhances functional recovery following lysolecithin-induced demyelination in mouse optic chiasm

BACKGROUND

Inhibitory factors have been implicated in the failure of remyelination in demyelinating diseases. Myelin associated inhibitors act through a common receptor called Nogo receptor (NgR) that plays critical inhibitory roles in CNS plasticity. Here we investigated the effects of abrogating NgR inhibition in a non-immune model of focal demyelination in adult mouse optic chiasm.

METHODOLOGY/PRINCIPAL FINDINGS

A focal area of demyelination was induced in adult mouse optic chiasm by microinjection of lysolecithin. To knock down NgR levels, siRNAs against NgR were intracerebroventricularly administered via a permanent cannula over 14 days, Functional changes were monitored by electrophysiological recording of latency of visual evoked potentials (VEPs). Histological analysis was carried out 3, 7 and 14 days post demyelination lesion. To assess the effect of NgR inhibition on precursor cell repopulation, BrdU was administered to the animals prior to the demyelination induction. Inhibition of NgR significantly restored VEPs responses following optic chiasm demyelination. These findings were confirmed histologically by myelin specific staining. siNgR application resulted in a smaller lesion size compared to control. NgR inhibition significantly increased the numbers of BrdU+/Olig2+ progenitor cells in the lesioned area and in the neurogenic zone of the third ventricle. These progenitor cells (Olig2+ or GFAP+) migrated away from this area as a function of time.

CONCLUSIONS/SIGNIFICANCE

Our results show that inhibition of NgR facilitate myelin repair in the demyelinated chiasm, with enhanced recruitment of proliferating cells to the lesion site. Thus, antagonizing NgR function could have therapeutic potential for demyelinating disorders such as Multiple Sclerosis.

N-cadherin promotes recruitment and migration of neural progenitor cells from the SVZ neural stem cell niche into demyelinated lesions

Discrete cellular microenvironments regulate stem cell pools and their development, as well as function in maintaining tissue homeostasis. Although the signaling elements modulating neural progenitor cells (NPCs) of the adult subventricular zone (SVZ) niche are fairly well understood, the pathways activated following injury and the resulting outcomes, are less clear. In the present study, we used mouse models of demyelination and proteomics analysis to identify molecular cues present in the adult SVZ niche during injury, and analyzed their role on NPCs in the context of promoting myelin repair. Proteomic analysis of SVZ tissue from mice with experimental demyelination identified several proteins that are known to play roles in NPC proliferation, adhesion, and migration. Among the proteins found to be upregulated were members of the N-cadherin signaling pathway. During the onset of demyelination in the subcortical white matter (SCWM), activation of epidermal growth factor receptor (EGFR) signaling in SVZ NPCs stimulates the interaction between N-cadherin and ADAM10. Upon cleavage and activation of N-cadherin signaling by ADAM10, NPCs undergo cytoskeletal rearrangement and polarization, leading to enhanced migration out of the SVZ into demyelinated lesions of the SCWM. Genetically disrupting either EGFR signaling or ADAM10 inhibits this pathway, preventing N-cadherin regulated NPC polarization and migration. Additionally, in vivo experiments using N-cadherin gain- and loss-of-function approaches demonstrated that N-cadherin enhances the recruitment of SVZ NPCs into demyelinated lesions. Our data revealed that EGFR-dependent N-cadherin signaling physically initiated by ADAM10 cleavage is the response of the SVZ niche to promote repair of the injured brain.

Specific ablation of Nampt in adult neural stem cells recapitulates their functional defects during aging

Neural stem/progenitor cell (NSPC) proliferation and self-renewal, as well as insult-induced differentiation, decrease markedly with age. The molecular mechanisms responsible for these declines remain unclear. Here, we show that levels of NAD(+) and nicotinamide phosphoribosyltransferase (Nampt), the rate-limiting enzyme in mammalian NAD(+) biosynthesis, decrease with age in the hippocampus. Ablation of Nampt in adult NSPCs reduced their pool and proliferation in vivo. The decrease in the NSPC pool during aging can be rescued by enhancing hippocampal NAD(+) levels. Nampt is the main source of NSPC NAD(+) levels and required for G1/S progression of the NSPC cell cycle. Nampt is also critical in oligodendrocytic lineage fate decisions through a mechanism mediated redundantly by Sirt1 and Sirt2. Ablation of Nampt in the adult NSPCs in vivo reduced NSPC-mediated oligodendrogenesis upon insult. These phenotypes recapitulate defects in NSPCs during aging, giving rise to the possibility that Nampt-mediated NAD(+) biosynthesis is a mediator of age-associated functional declines in NSPCs.

Ciliary neurotrophic factor controls progenitor migration during remyelination in the adult rodent brain

Ciliary neurotrophic factor (CNTF) has been shown to be expressed after brain lesions and in particular after demyelination. Here, we addressed the role of this cytokine in the regulation of neural progenitor migration in the adult rodent brain. Using an acute model of demyelination, we show that CNTF is strongly re-expressed after lesion and is involved in the postlesional mobilization of endogenous progenitors that participate in the myelin regenerative process. We show that CNTF controls the migration of subventricular zone (SVZ)-derived neural progenitors toward the demyelinated corpus callosum. Furthermore, an ectopic source of CNTF in adult healthy brains changes SVZ-derived neural progenitors' migratory behavior that migrate toward the source by activation of the Janus kinase/signal transducer and activator of transcription 3 (JAK/STAT3) pathway. Using various in vitro assays (Boyden chambers, explants, and video time-lapse imaging), we demonstrate that CNTF controls the directed migration of SVZ-derived progenitors and oligodendrocyte precursors. Altogether, these results demonstrate that in addition to its neuroprotective activity and its role in progenitor survival and maturation, CNTF acts as a chemoattractant and participates in the recruitment of endogenous progenitors during myelin repair.

Signalling pathways that inhibit the capacity of precursor cells for myelin repair

In demyelinating disorders such as Multiple Sclerosis (MS), targets of injury are myelin and oligodendrocytes, leading to severe neurological dysfunction. Regenerative therapies aimed at promoting oligodendrocyte maturation and remyelination are promising strategies for treatment in demyelinating disorders. Endogenous precursor cells or exogenous transplanted cells are potential sources for remyelinating oligodendrocytes in the central nervous system (CNS). Several signalling pathways have been implicated in regulating the capacity of these cell populations for myelin repair. Here, we review neural precursor cells and oligodendrocyte progenitor cells as potential sources for remyelinating oligodendrocytes and evidence for the functional role of key signalling pathways in inhibiting regeneration from these precursor cell populations.

Determinants of central nervous system adult neurogenesis are sex, hormones, mouse strain, age, and brain region

Multiple sclerosis is a sexually dimorphic (SD) disease that causes oligodendrocyte death, but SD of glial cells is poorly studied. Here, we analyze SD of neural progenitors in 6-8 weeks and 6-8 months normal C57BL/6, SJL/J, and BALB/c mice in the subventricular zone (SVZ), dorsolateral horn (DLC), corpus callosum (CC), and parenchyma. With a short 2-h bromodeoxyuridine (BrdU) pulse, no gender and strain differences are present at 6-8 weeks. At 6-8 months, the number of BrdU(+) cells decreases twofold in each sex, strain, and region, indicating that a common aging mechanism regulates BrdU incorporation. Strikingly, 2× more BrdU(+) cells are found in all brain regions in 6-8 months C57BL/6 females versus males, no gender differences in 6-8 months SJL/J, and fewer BrdU(+) cells in females versus males in BALB/cs. The number of BrdU(+) cells modestly fluctuates throughout the estrous cycle in C57BL/6 and SJLs. Castration causes a dramatic increase in BrdU(+) cells in SVZ and DLC. These findings indicate that testosterone is a major regulator of adult neural proliferation. At 6-8 months, the ratio of PDGFRα(+) cells in the CC to BrdU(+) cells in the DLC of both strains, sexes, estrous cycle, and castrated mice was essentially the same, suggesting that BrdU(+) cells in the DLC differentiate into CC oligodendrocytes. The ratio of TUNEL(+) to BrdU(+) cells does not match proliferation, indicating that these events are differentially regulated. Differential regulation of these two processes leads to the variation in glial numbers between gender and strain. Explanations of neural proliferation based upon data from one sex or strain may be very misleading.

Electromagnetic field stimulation potentiates endogenous myelin repair by recruiting subventricular neural stem cells in an experimental model of white matter demyelination

Electromagnetic fields (EMFs) may affect the endogenous neural stem cells within the brain. The aim of this study was to assess the effects of EMFs on the process of toxin-induced demyelination and subsequent remyelination. Demyelination was induced using local injection of lysophosphatidylcholine within the corpus callosum of adult female Sprague-Dawley rats. EMFs (60 Hz; 0.7 mT) were applied for 2 h twice a day for 7, 14, or 28 days postlesion. BrdU labeling and immunostaining against nestin, myelin basic protein (MBP), and BrdU were used for assessing the amount of neural stem cells within the tissue, remyelination patterns, and tracing of proliferating cells, respectively. EMFs significantly reduced the extent of demyelinated area and increased the level of MBP staining within the lesion area on days 14 and 28 postlesion. EMFs also increased the number of BrdU- and nestin-positive cells within the area between SVZ and lesion as observed on days 7 and 14 postlesion. It seems that EMF potentiates proliferation and migration of neural stem cells and enhances the repair of myelin in the context of demyelinating conditions.

Differential requirement of cyclin-dependent kinase 2 for oligodendrocyte progenitor cell proliferation and differentiation

Cyclin-dependent kinases (Cdks) and their cyclin regulatory subunits control cell growth and division. Cdk2-cyclin E complexes, phosphorylating the retinoblastoma protein, drive cells through the G1/S transition into the S phase of the cell cycle. Despite its fundamental role, Cdk2 was found to be indispensable only in specific cell types due to molecular redundancies in its function. Converging studies highlight involvement of Cdk2 and associated cell cycle regulatory proteins in oligodendrocyte progenitor cell proliferation and differentiation. Giving the contribution of this immature cell type to brain plasticity and repair in the adult, this review will explore the requirement of Cdk2 for oligodendrogenesis, oligodendrocyte progenitor cells proliferation and differentiation during physiological and pathological conditions.

Basic fibroblast growth factor potentiates myelin repair following induction of experimental demyelination in adult mouse optic chiasm and nerves

Induction of demyelination in the central nervous system induce the oligodendrocyte progenitors to proliferate, migrate, and differentiate for restoring new myelin sheathes around demyelinated axons. Factors which increase the response of endogenous progenitor cells could be used to improve remyelination. In the current study, the effect of bFGF on lysolecithin-induced demyelination and remyelination processes in mouse optic chiasm and nerves was investigated. Lysolecithin was injected into the optic chiasm of Balb/C mice. Two groups of animals received doses of bFGF (1 or 5 ng/kg i.p.) just before and every 3 days after lysolecithin injection. Delay and amplitude of visual evoked potential (VEP) waves were recorded as indices of axonal demyelination at 7th, 13th, and 28th days post-lesion. Myelin basic protein (MBP) and Olig2 gene expressions were studied as indices of myelination and oligodendrocyte precursors' recruitment into the lesion. Lysolecithin elongated delay of P1 wave and declined the amplitude of P1-N1 wave. Lysolecithin decreased MBP and increased Olig2 expression in different days post-lesion. Lysolecithin-induced changes in VEPs were partially ameliorated by endogenous repair. bFGF reduced the increased delay, increased the reduced amplitude of P1-N1 wave, increased MBP gene expression, and accelerated the increasing pattern of Olig2. bFGF seems to be able to potentiate the endogenous repair mechanisms of myelin. Its effect on demyelination and remyelination processes seems to be mediated by oligodendrocyte progenitor cells and their differentiation to myelinating cells.

Olesoxime accelerates myelination and promotes repair in models of demyelination

OBJECTIVE

Multiple sclerosis is a neurodegenerative disease characterized by episodes of immune attack of oligodendrocytes leading to demyelination and progressive functional deficit. One therapeutic strategy to address disease progression could consist in stimulating the spontaneous regenerative process observed in some patients. Myelin regeneration requires endogenous oligodendrocyte progenitor migration and activation of the myelination program at the lesion site. In this study, we have tested the ability of olesoxime, a neuroprotective and neuroregenerative agent, to promote remyelination in the rodent central nervous system in vivo.

METHODS

The effect of olesoxime on oligodendrocyte progenitor cell (OPC) differentiation and myelin synthesis was tested directly in organotypic slice cultures and OPC-neuron cocultures. Using naive animals and different mouse models of demyelination, we morphologically and functionally assessed the effect of the compound on myelination in vivo.

RESULTS

Olesoxime accelerated oligodendrocyte maturation and enhanced myelination in vitro and in vivo in naive animals during development and also in the adult brain without affecting oligodendrocyte survival or proliferation. In mouse models of demyelination and remyelination, olesoxime favored the repair process, promoting myelin formation with consequent functional improvement.

INTERPRETATION

Our observations support the strategy of promoting oligodendrocyte maturation and myelin synthesis to enhance myelin repair and functional recovery. We also provide proof of concept that olesoxime could be useful for the treatment of demyelinating diseases.

Cdk2 loss accelerates precursor differentiation and remyelination in the adult central nervous system

Cdk2 is not essential for oligodendrocyte maturation and myelination during development, but in response to demyelination, it is required for oligodendrocyte precursor cell proliferation (OPC), and its loss accelerates OPC differentiation and remyelination.

The specific functions of intrinsic regulators of oligodendrocyte progenitor cell (OPC) division are poorly understood. Type 2 cyclin-dependent kinase (Cdk2) controls cell cycle progression of OPCs, but whether it acts during myelination and repair of demyelinating lesions remains unexplored. Here, we took advantage of a viable Cdk2^−/− mutant mouse to investigate the function of this cell cycle regulator in OPC proliferation and differentiation in normal and pathological conditions. During central nervous system (CNS) development, Cdk2 loss does not affect OPC cell cycle, oligodendrocyte cell numbers, or myelination. However, in response to CNS demyelination, it clearly alters adult OPC renewal, cell cycle exit, and differentiation. Importantly, Cdk2 loss accelerates CNS remyelination of demyelinated axons. Thus, Cdk2 is dispensable for myelination but is important for adult OPC renewal, and could be one of the underlying mechanisms that drive adult progenitors to differentiate and thus regenerate myelin.

Cuprizone demyelination of the corpus callosum in mice correlates with altered social interaction and impaired bilateral sensorimotor coordination

For studies of remyelination in demyelinating diseases, the cuprizone model of CC (corpus callosum) demyelination has experimental advantages that include overall size, proximity to neural stem cells of the subventricular zone, and correlation with a lesion predilection site in multiple sclerosis. In addition, cuprizone treatment can be ended to allow more direct analysis of remyelination than with viral or autoimmune models. However, CC demyelination lacks a useful functional correlate in rodents for longitudinal analysis throughout the course of demyelination and remyelination. In the present study, we tested two distinct behavioural measurements in mice fed 0.2% cuprizone. Running on a 'complex' wheel with varied rung intervals requires integration between cerebral hemispheres for rapid bilateral sensorimotor coordination. Maximum running velocity on the 'complex' wheel decreased during acute (6 week) and chronic (12 week) cuprizone demyelination. Running velocity on the complex wheel distinguished treated (for 6 weeks) from non-treated mice, even after a 6-week recovery period for spontaneous remyelination. A second behavioural assessment was a resident-intruder test of social interaction. The frequency of interactive behaviours increased among resident mice after acute or chronic demyelination. Differences in both sensorimotor coordination and social interaction correlated with demonstrated CC demyelination. The wheel assay is applicable for longitudinal studies. The resident-intruder assay provides a complementary assessment of a distinct modality at a specific time point. These behavioural measurements are sufficiently robust for small cohorts as a non-invasive assessment of demyelination to facilitate analysis of subsequent remyelination. These measurements may also identify CC involvement in other mouse models of central nervous system injuries and disorders.

Epigenetic regulation of the stem cell mitogen Fgf-2 by Mbd1 in adult neural stem/progenitor cells

Whether and how mechanisms intrinsic to stem cells modulate their proliferation and differentiation are two central questions in stem cell biology. Although exogenous basic fibroblast growth factor 2 (FGF-2/Fgf-2) is commonly used to expand adult neural stem/progenitor cells (NSPCs) in vitro, we do not yet understand the functional significance or the molecular regulation of Fgf-2 expressed endogenously by adult NSPCs. We previously demonstrated that methylated CpG binding protein 1 (MBD1/Mbd1) is a transcriptional repressor of Fgf-2 and is enriched in adult brains. Mbd1 deficiency in mice selectively affected adult neurogenesis and the differentiation of NSPCs. Here we show that an Mbd1 and DNA methylation-mediated epigenetic mechanism regulated the expression of stem cell mitogen Fgf-2 in adult NSPCs. Mbd1 bound to the Fgf-2 promoter and regulates its expression in adult NSPCs. In the absence of functional Mbd1, the Fgf-2 promoter was hypomethylated, and treatment with a DNA methylation inhibitor resulted in increased Fgf-2 expression in adult NSPCs. We further demonstrated that both acute knockdown of Mbd1 or overexpression of Fgf-2 in adult NSPCs inhibited their neuronal differentiation, which could be responsible for the neurogenic deficits observed in Mbd1-deficient mice. These data indicate that intrinsic epigenetic mechanisms play critical roles in the regulation of adult NSPC functions.

Myelin repair: the role of stem and precursor cells in multiple sclerosis

Multiple sclerosis is the most common potential cause of neurological disability in young adults. The disease has two distinct clinical phases, each reflecting a dominant role for separate pathological processes: inflammation drives activity during the relapsing-remitting stage and axon degeneration represents the principal substrate of progressive disability. Recent advances in disease-modifying treatments target only the inflammatory process. They are ineffective in the progressive stage, leaving the science of disease progression unsolved. Here, the requirement is for strategies that promote remyelination and prevent axonal loss. Pathological and experimental studies suggest that these processes are tightly linked, and that remyelination or myelin repair will both restore structure and protect axons. This review considers the basic and clinical biology of remyelination and the potential contribution of stem and precursor cells to enhance and supplement spontaneous remyelination.

Cellular and molecular events underlying the dysregulated response of the aged brain to stroke: a mini-review

BACKGROUND

Age-related brain injuries, including stroke, are a major cause of physical and mental disabilities.

OBJECTIVE

Therefore, studying the basic mechanism underlying functional recovery after brain stroke in aged subjects is of considerable clinical interest.

METHODS

This review summarizes the effects of age on recovery after stroke in an animal model, with emphasis on the underlying cellular mechanisms.

RESULTS

Data from our laboratory and elsewhere indicate that, behaviorally, aged rats were more severely impaired by stroke than young rats, and they also showed diminished functional recovery. Infarct volume did not differ significantly between young and aged animals, but critical differences were apparent in the cytological response to stroke, most notably an age-related acceleration in the development of the glial scar. Early infarct in older rats is associated with premature accumulation of BrdU-positive microglia and astrocytes, persistence of activated oligodendrocytes, a high incidence of neuronal degeneration and accelerated apoptosis. In aged rats, neuroepithelial-positive cells were rapidly incorporated into the glial scar, but these neuroepithelial-like cells did not make a significant contribution to neurogenesis in the infarcted cortex in young or aged animals. The response of plasticity-associated proteins like MAP1B, was delayed in aged rats. Tissue recovery was further delayed by an age-related increase in the amount of the neurotoxic C-terminal fragment of the beta-amyloid precursor protein (A-beta) at 2 weeks poststroke.

CONCLUSION

The available evidence indicates that the aged brain has the capability to mount a cytoproliferative response to injury, but the timing of the cellular and genetic response to cerebral insult is dysregulated in aged animals, thereby further compromising functional recovery. Elucidating the molecular basis for this phenomenon in the aging brain could yield novel approaches to neurorestoration in the elderly.

Growth factor regulation of remyelination: behind the growing interest in endogenous cell repair of the CNS

Remyelination facilitates recovery of saltatory conduction along demyelinated axons and may help prevent axon damage in patients with demyelinating diseases, such as multiple sclerosis. The extent of remyelination in multiple sclerosis lesions varies dramatically, indicating a capacity for repair that is not fulfilled in lesions with poor remyelination. In experimental models of demyelinating disease, remyelination is limited by chronic disease that depletes the oligodendrocyte progenitor (OP) population, inhibits OP differentiation into remyelinating oligodendrocytes and/or perturbs cell survival in the lesion environment. Manipulating the activity of growth factor signaling pathways significantly improves the ability of endogenous OP cells to accomplish extensive remyelination. Specifically, growth factors have been identified that can regulate OP proliferation, differentiation and survival in demyelinated lesions. Therefore, growth factors may be key signals for strategies to improve conditions with poor remyelination.

Neural cell adhesion molecule polysialylation enhances the sensitivity of embryonic stem cell-derived neural precursors to migration guidance cues

The development of stem cell-based neural repair strategies requires detailed knowledge on the interaction of migrating donor cells with the host brain environment. Here we report that overexpression of polysialic acid (PSA), a carbohydrate polymer attached to the neural cell adhesion molecule (NCAM), in embryonic stem (ES) cell-derived glial precursors (ESGPs) strikingly modifies their migration behavior in response to guidance cues. ESGPs transduced with a retrovirus encoding the polysialyltransferase STX exhibit enhanced migration in monolayer cultures and an increased penetration of organotypic slice cultures. Chemotaxis assays show that overexpression of PSA results in an enhanced chemotactic migration toward gradients of a variety of chemoattractants, including fibroblast growth factor 2 (FGF2), platelet-derived growth factor, and brain-derived neurotrophic factor (BDNF), and that this effect is mediated via the phosphatidylinositol 3'-kinase (PI3K) pathway. Moreover, PSA-overexpressing ESGPs also exhibit an enhanced chemotactic response to tissue explants derived from different brain regions. The effect of polysialylation on directional migration is preserved in vivo. Upon transplantation into the adult striatum, PSA-overexpressing but not control cells display a targeted migration toward the subventricular zone. On the basis of these data, we propose that PSA plays a crucial role in modulating the ability of migrating precursor cells to respond to regional guidance cues within the brain tissue. Disclosure of potential conflicts of interest is found at the end of this article.

Radiation Response of Neural Precursor Cells

Considerable data are now available regarding the radiation responsiveness of neural precursor cells that exist in the neurogenic regions of the mammalian forebrain. These cells and their progeny are extremely sensitive to irradiation, undergoing apoptosis after clinically relevant doses that do not result in overt tissue injury. In addition, there is compelling evidence that radiation significantly affects the whole process of neurogenesis and that the sensitivity depends, at least in part, on alterations in the microenvironment within which the precursor cells exist. Although provocative data exist suggesting that inflammation, oxidative stress, or morphologic relations influence neurogenesis, the precise mechanisms involved remain obscure and need to be investigated. Additionally, it is important to try to understand what these findings may mean in the context of radiation paradigms associated with the treatment of intracranial disease. Understanding how neural precursor cells respond to noxious stimuli is likely to lead to new therapeutic approaches that should restore neurogenesis and perhaps improve cognitive performance.

Epigenetic memory loss in aging oligodendrocytes in the corpus callosum

In this study, we address the hypothesis that aging modifies the intrinsic properties of oligodendrocytes, the myelin-forming cells of the brain. According to our model, an "epigenetic memory" is stored in the chromatin of the oligodendrocyte lineage cells and is responsible for the maintenance of a mature phenotype, characterized by low levels of expression of transcriptional inhibitors. We report here an age-related decline of histone deacetylation and methylation, the molecular mechanisms responsible for the establishment and maintenance of this "epigenetic memory" of the differentiated state. We further show that lack of histone methylation and increased acetylation in mature oligodendrocytes are associated with global changes in gene expression, that include the re-expression of bHLH inhibitors (i.e. Hes5 and Id4) and precursor markers (i.e. Sox2). These changes characteristic of the "aging" oligodendrocytes can be recapitulated in vitro, by treating primary oligodendrocyte cultures with histone deacetylase inhibitors. Thus, we conclude that the "epigenetic memory loss" detected in white matter tracts of older mice induces global changes of gene expression that modify the intrinsic properties of aged oligodendrocytes and may functionally modulate the responsiveness of these cells to external stimuli.

Retroviral lineage analysis of fibroblast growth factor receptor signaling in FGF2 inhibition of oligodendrocyte progenitor differentiation

Fibroblast growth factor 2 (FGF2) inhibits oligodendrocyte progenitor cell (OPC) differentiation during development and limits remyelination following chronic demyelination. The current study examines the mechanism underlying this effect of FGF2 expression on OPC differentiation. Retroviral lineage tracing demonstrates a direct in vivo effect of FGF receptor (FGFR) signaling on OPC differentiation. Retrovirus expressing a dominant negative FGFR construct (FGFRdn) and green fluorescent protein (GFP) was injected into the dorsal columns of postnatal day 7 (P7) mice followed by perfusion at P28. Among the GFP-labeled cells, FGFRdn retrovirus generated a higher proportion of oligodendrocytes than did control infections. This result from FGFRdn expression in OPCs was similar to the result obtained in our previous study using control retrovirus in FGF2 null mice. Further, in vitro retroviral siRNA expression distinguishes the function of specific FGFR isoforms in OPC responses to FGF2. FGF2 inhibition of OPC differentiation was effectively blocked by siRNA targeted to FGFR1, but not FGFR2 or FGFR3. We propose a model of direct FGF2 activation of FGFR1 leading to inhibition of OPC differentiation. This signaling pathway may be an important regulator of oligodendrocyte generation during myelination in development and may perturb OPC generation of remyelinating oligodendrocytes in demyelinating disease.

Differences in the early inflammatory responses to toxin-induced demyelination are associated with the age-related decline in CNS remyelination

CNS remyelination occurs more rapidly in young adult rats than in old rats. Since the inflammatory response initiated by demyelination is an important trigger for remyelination, we address whether ageing changes in remyelination are associated with changes in the inflammatory response. Using a toxin model of demyelination, where the inflammatory response largely comprises macrophages, we show that there is a delay in both recruitment and activation of OX-42+ and macrophage scavenger receptor B+ macrophages following demyelination in older rats (10-13 months) compared to young rats (8-10 weeks). This difference is associated with a slower onset of increased expression of several chemokine mRNAs. However, many inflammatory cytokines have similar mRNA expression patterns, with the exception of IL-1beta, IL-6 and TNF-alpha, which have prolonged expression in the older animals. Differences in IL-1beta mRNA expression, a cytokine specifically implicated in CNS remyelination, are not reflected in differences in protein expression detected by immunocytochemistry. These data relate the age-associated delay in remyelination efficiency to changes in the macrophage and inflammatory mediator response to demyelination.

Stem cells, progenitors and myelin repair

Remyelination, the process by which new myelin sheaths are restored to demyelinated axons, represents one of the most compelling examples of adult multipotent progenitor cells contributing to regeneration of the injured central nervous system (CNS). This process can occur with remarkable efficiency in both clinical disease, such as multiple sclerosis, and in experimental models, revealing an impressive ability of the adult CNS to repair itself. However, the inconsistency of remyelination in multiple sclerosis, and the loss of axonal integrity that results from its failure, makes enhancement of remyelination an important therapeutic objective. Identifying potential targets will depend on a detailed understanding of the cellular and molecular mechanisms of remyelination. In this article we address two important issues. First, we consider the nature of the cell or cells that respond to demyelination and generate new oligodendrocytes, identifying current areas of uncertainty and addressing the role of adult CNS stem and progenitor cells. Second, we discuss the concept of adult progenitor activation following demyelination, focusing on the increased expression of (1) olig transcription factors, (2) bone morphogenetic proteins and (3) fyn, a member of the src-family of tyrosine kinases.

Endogenous cell repair of chronic demyelination

In multiple sclerosis lesions, remyelination typically fails with repeated or chronic demyelinating episodes and results in neurologic disability. Acute demyelination models in rodents typically exhibit robust spontaneous remyelination that prevents appropriate evaluation of strategies for improving conditions of insufficient remyelination. In the current study, we used a mouse model of chronic demyelination induced by continuous ingestion of 0.2% cuprizone for 12 weeks. This chronic process depleted the oligodendrocyte progenitor population and impaired oligodendrocyte regeneration. Remyelination remained limited after removal of cuprizone from the diet. Fibroblast growth factor 2 (FGF2) expression was persistently increased in the corpus callosum of chronically demyelinated mice as compared with nonlesioned mice. We used FGF2^−/− mice to determine whether removal of endogenous FGF2 promoted remyelination of chronically demyelinated areas. Wild-type and FGF2^−/− mice exhibited similar demyelination during chronic cuprizone treatment. Importantly, in contrast to wild-type mice, the FGF2^−/− mice spontaneously remyelinated completely during the recovery period after chronic demyelination. Increased remyelination in FGF2^−/− mice correlated with enhanced oligodendroglial regeneration. FGF2 genotype did not alter the density of oligodendrocyte progenitor cells or proliferating cells after chronic demyelination. These findings indicate that attenuating FGF2 created a sufficiently permissive lesion environment for endogenous cells to effectively remyelinate viable axons even after chronic demyelination.

mCD24 regulates proliferation of neuronal committed precursors in the subventricular zone

We previously showed that deletion of the cell surface molecule mCD24 resulted in an increased proliferation in adult subventricular zone (SVZ). Here, we report an increased PSA-NCAM+/TuJ1- population in the mCD24-/- in vivo SVZ as well as in vitro neurospheres. Isolated in vitro, these cells were able to generate neurospheres. Proliferation studies, using BrdU incorporation, showed an increased proliferation in P7 mCD24-/- SVZ and neurospheres. Using electron microscopy, the same cell types were identified in the in vivo SVZ as well as in vitro neurospheres from the WT and mCD24-/- mice. In mixed neurospheres, formed with WT and EGFP/KO cells (enhanced green fluorescent protein mCD24-/-), the WT environment was able to control the proliferation rate of the mCD24-/- cells, but was unable to regulate their differentiation. We concluded that mCD24 acts cell nonautonomously to regulate transit-amplifying cells proliferation and/or differentiation.

Stroke-Induced Neurogenesis in Aged Brain

Background and Purpose— Stroke induced by middle cerebral artery occlusion (MCAO) triggers increased neurogenesis in the damaged striatum and nondamaged hippocampus of young adult rodents. We explored whether stroke influences neurogenesis similarly in the aged brain.

Methods— Young adult (3 months) and old (15 months) rats were subjected to 1 hour of MCAO, and new cells were labeled by intraperitoneal injection of 5-bromo-2′-deoxyuridine 5′-monophosphate (BrdU), a marker for dividing cells, for 2 weeks thereafter. Animals were euthanized at 7 weeks after the insult, and neurogenesis was assessed immunocytochemically with antibodies against BrdU and neuronal markers with epifluorescence or confocal microscopy.

Results— Young and old rats exhibited the same increased numbers of new striatal neurons after stroke, despite basal cell proliferation in the subventricular zone being reduced in the aged brain. In contrast, both the number of stroke-generated granule cells and basal neurogenesis in the dentate subgranular zone were lower in old compared with young animals. Also, the ability of newly formed cells to differentiate into neurons was impaired in the aged dentate gyrus.

Conclusions— Basal neurogenesis is impaired in the subgranular and subventricular zones of aged animals, but both regions react to stroke with increased formation of new neurons. The magnitude of striatal neurogenesis after stroke is similar in young and old animals, indicating that this potential mechanism for self-repair also operates in the aged brain.

Expression of stathmin, a developmentally controlled cytoskeleton-regulating molecule, in demyelinating disorders

Understanding the biological relevance of reexpression of developmental molecules in pathological conditions is crucial for the development of new therapies. In this study, we report the increased expression of stathmin, a developmentally regulated tubulin-binding protein, in the brains of patients with multiple sclerosis (MS). In physiological conditions, stathmin immunoreactivity was observed in polysialic acid-neural cell adhesion molecule-positive migratory progenitors in the subventricular zone, and its expression progressively decreased as the cells matured into oligodendrocytes (OLs). In MS patients, however, stathmin levels were elevated in 2',3'-cyclic nucleotide 3'-phosphodiesterase-positive OLs, in 10 of 10 bioptic samples analyzed. Increased levels of stathmin were confirmed by Western blot analysis of normal-appearing white matter samples from MS brains. In addition, using mass spectrometry, stathmin was identified as the main component of a specific myelin protein fraction consistently increased in MS preparations compared with controls. To test the biological relevance of increased stathmin levels, primary OL progenitors were transfected using a myc-tagged stathmin cDNA and were allowed to differentiate. Consistent with a distinct role played by this molecule in cells of the OL lineage at different developmental stages, transient transfection in progenitors favored the bipolar migratory phenotype but did not affect survival. However, sustained stathmin levels in differentiating OLs, because of overexpression, resulted in enhanced apoptotic susceptibility. We conclude that stathmin expression in demyelinating disorders could have a dual role. On one hand, by favoring the migratory phenotype of progenitors, it may promote myelin repair. On the other hand, stathmin in mature OLs may indicate cell stress and possibly affect survival.

Intrastriatal transforming growth factor alpha delivery to a model of Parkinson's disease induces proliferation and migration of endogenous adult neural progenitor cells without differentiation into dopaminergic neurons

We examined the cell proliferative, neurogenic, and behavioral effects of transforming growth factor alpha (TGFalpha) in a 6-OHDA Parkinson's disease model when compared with naive rats. Intrastriatal TGFalpha infusion induced significant proliferation, hyperplastic nodules, and substantial migratory waves of nestin-positive progenitor cells from the adult subventricular zone (SVZ) of dopamine-denervated rats. Interestingly, SVZ cells in naive rats displayed proliferation but minimal migration in response to the TGFalpha infusion. The cells in the expanded SVZ accumulated cytoplasmic beta-catenin, indicating activation of classical Wnt signaling. However, no evidence of any neuronal differentiation was found of these recruited progenitor cells anywhere examined in the brain. Consequently, no evidence of dopaminergic (DA) neurogenesis was found in the striatum or substantia nigra in any experimental group, and amphetamine-induced behavioral rotations did not improve. In summary, the cells in the TGFalpha-induced migratory cellular wave remain undifferentiated and do not differentiate into midbrain-like DA neurons.

Primary central white matter hypoplasia of the neocortex

Leukodystrophies, or diseases of the white matter, represent acute or ongoing damage to the oligodendrocytes of the central nervous system. Early childhood white matter disease is most commonly observed after hypoxic ischemic insults, with acute magnetic resonance imaging changes followed by atrophy or periventricular leukomalacia. Dysmyelination occurring in the setting of inborn errors of metabolism is characterized by progressive changes with high signal intensity in white matter on magnetic resonance imaging. This report presents a patient whose magnetic resonance imaging scans demonstrated hypoplasia of myelin in the telencephalon, without clinical or magnetic resonance imaging evidence of inflammatory dysmyelination. Clinical features included intractable seizures, severe hypotonia, and dysmorphic facial features coupled with a static failure to gain developmental milestones. Together, the clinical and magnetic resonance imaging findings are evidence of a primary failure of myelination in the neocortex.

Thyroid hormone administration enhances remyelination in chronic demyelinating inflammatory disease

Chronic disabilities in multiple sclerosis are believed to be due to neuron damage and degeneration, which follow remyelination failure. Due to the presence of numerous oligodendrocyte precursors inside demyelination plaques, one reason for demyelination failure could be the inability of oligodendrocyte precursor cells to turn into myelinating oligodendrocytes. In this study, we show that thyroid hormone enhances and accelerates remyelination in an experimental model of chronic demyelination, i.e., experimental allergic encephalomyelitis in congenic female Dark Agouti rats immunized with complete guinea pig spinal cord. Thyroid hormone, when administered during the acute phase of the disease, increases expression of platelet-derived growth factor alpha receptor, restores normal levels of myelin basic protein mRNA and protein, and allows an early and morphologically competent reassembly of myelin sheaths. Moreover, thyroid hormone exerts a neuroprotective effect with respect to axonal pathology.

Endogenous adult neural stem cells: Limits and potential to repair the injured central nervous system

Mitotic activity persists in various regions of the adult mammal CNS. While evidences of neurogenesis appeared, many studies focused on the features of the adult stem cells from germinative areas such as the subventricular zone of the lateral ventricles, the dentate gyrus of the hippocampus, the cortex, the fourth ventricle and the central canal of the spinal cord. In the present paper, we review the potentialities of the adult germinative areas in terms of proliferation, migration and differentiation in non pathological situation and in response to different type of CNS injury. Adult endogenous stem cells are activated in response to various injuries but their capacities to migrate and to undergo either neurogenesis or gliogenesis differ according to the lesion-type and the germinative zone from which they arise. Different works demonstrated that epigenic factors such as growth factors can enhance the repair potential of the adult stem cells. Reactivation and mobilization of endogenous stem cells as well as demonstration of their long-term survival and functionality appear to be interesting strategies to investigate in order to promote endogenous repair of the adult CNS.

Osteopontin is upregulated during in vivo demyelination and remyelination and enhances myelin formation in vitro

We have used in vitro oligodendrocyte differentiation and the in vivo remyelination model, the cuprizone model, to identify genes regulating oligodendrocyte function and remyelination. One of the genes we identified, osteopontin (opn), is a secreted glycoprotein with cytokine-like, chemotactic, and anti-apoptotic properties that contains an Arg-Gly-Asp (RGD) cell adhesion motif-mediating interactions with several integrins. Both microglia and astrocytes in demyelinating brain regions of cuprizone-fed mice expressed OPN protein. Recombinant OPN protein produced in a baculovirus expression system induced proliferation of both the rat CG-4 and the mouse Oli-neu oligodendrocyte precursor (OLP)-like cell lines in a dose-dependent manner. In addition, recombinant OPN treatment stimulated both myelin basic protein (MBP) synthesis and myelin sheath formation in mixed cortical cultures from embryonic mouse brain, an in vitro primary culture model of myelination. Interestingly, myelinating mixed cultures prepared from OPN(-/-) mice contained significantly less MBP compared to wild-type cultures after 17 days in culture. We propose that in the central nervous system, OPN may act as a novel regulator of myelination and remyelination.

Stem cell biology of the central nervous system

Neural stem cells (NSCs) are multipotential progenitor cells that have self-renewal activities. A single NSC is capable of generating various kinds of cells within the central nervous system (CNS), including neurons, astrocytes, and oligodendrocytes. Because of these characteristics, there is increasing interest in NSCs and neural progenitor cells from the aspects of both basic developmental biology and therapeutic applications to the damaged brain. This special issue, dedicated to understanding the nature of the NSCs present in the CNS, presents an introduction to several avenues of research that may lead to feasible strategies for manipulating cells in situ to treat the damaged brain. The topics covered by these studies include the extracellular factors and signal transduction cascades involved in the differentiation and maintenance of NSCs, the population dynamics and locations of NSCs in embryonic and adult brains, prospective identification and isolation of NSCs, the induction of NSCs to adopt particular neuronal phenotypes, and their transplantation into the damaged CNS.