Masakazu Namihira

Committed Neuronal Precursors Confer Astrocytic Potential on Residual Neural Precursor Cells

During midgestation, mammalian neural precursor cells (NPCs) differentiate only into neurons. Generation of astrocytes is prevented at this stage, because astrocyte-specific gene promoters are methylated. How the subsequent switch from suppression to expression of astrocytic genes occurs is unknown. We show in this study that Notch ligands are expressed on committed neuronal precursors and young neurons in mid-gestational telencephalon, and that neighboring Notch-activated NPCs acquire the potential to become astrocytes. Activation of the Notch signaling pathway in midgestational NPCs induces expression of the transcription factor nuclear factor I, which binds to astrocytic gene promoters, resulting in demethylation of astrocyte-specific genes. These findings provide a mechanistic explanation for why neurons come first: committed neuronal precursors and young neurons potentiate remaining NPCs to differentiate into the next cell lineage, astrocytes.

Neurons derived from transplanted neural stem cells restore disrupted neuronal circuitry in a mouse model of spinal cord injury

The bodys capacity to restore damaged neural networks in the injured CNS is severely limited. Although various treatment regimens can partially alleviate spinal cord injury (SCI), the mechanisms responsible for symptomatic improvement remain elusive. Here, using a mouse model of SCI, we have shown that transplantation of neural stem cells (NSCs) together with administration of valproic acid (VPA), a known antiepileptic and histone deacetylase inhibitor, dramatically enhanced the restoration of hind limb function. VPA treatment promoted the differentiation of transplanted NSCs into neurons rather than glial cells. Transsynaptic anterograde corticospinal tract tracing revealed that transplant-derived neurons reconstructed broken neuronal circuits, and electron microscopic analysis revealed that the transplant-derived neurons both received and sent synaptic connections to endogenous neurons. Ablation of the transplanted cells abolished the recovery of hind limb motor function, confirming that NSC transplantation directly contributed to restored motor function. These findings raise the possibility that epigenetic status in transplanted NSCs can be manipulated to provide effective treatment for SCI.

Treatment of a Mouse Model of Spinal Cord Injury by Transplantation of Human Induced Pluripotent Stem Cell-Derived Long-Term Self-Renewing Neuroepithelial-Like Stem Cells†‡§

Because of their ability to self‐renew, to differentiate into multiple lineages, and to migrate toward a damaged site, neural stem cells (NSCs), which can be derived from various sources such as fetal tissues and embryonic stem cells, are currently considered to be promising components of cell replacement strategies aimed at treating injuries of the central nervous system, including the spinal cord. Despite their efficiency in promoting functional recovery, these NSCs are not homogeneous and possess variable characteristics depending on their derivation protocols. The advent of induced pluripotent stem (iPS) cells has provided new prospects for regenerative medicine. We used a recently developed robust and stable protocol for the generation of long‐term, self‐renewing, neuroepithelial‐like stem cells from human iPS cells (hiPS‐lt‐NES cells), which can provide a homogeneous and well‐defined population of NSCs for standardized analysis. Here, we show that transplanted hiPS‐lt‐NES cells differentiate into neural lineages in the mouse model of spinal cord injury (SCI) and promote functional recovery of hind limb motor function. Furthermore, using two different neuronal tracers and ablation of the transplanted cells, we revealed that transplanted hiPS‐lt‐NES cell‐derived neurons, together with the surviving endogenous neurons, contributed to restored motor function. Both types of neurons reconstructed the corticospinal tract by forming synaptic connections and integrating neuronal circuits. Our findings indicate that hiPS‐lt‐NES transplantation represents a promising avenue for effective cell‐based treatment of SCI. STEM CELLS2012;30:1163–1173

Astrocyte Differentiation of Neural Precursor Cells is Enhanced by Retinoic Acid Through a Change in Epigenetic Modification

Neurons, astrocytes, and oligodendrocytes—the three major cell types that comprise the central nervous system—are generated from common multipotent neural precursor cells (NPCs). Members of the interleukin‐6 family of cytokines, including leukemia inhibitory factor (LIF), induce astrocyte differentiation of NPCs by activating the transcription factor signal transducer and activator of transcription 3 (STAT3). We show here that retinoic acid (RA) facilitates LIF‐induced astrocyte differentiation of NPCs. RA and LIF synergistically activate the promoter of gfap, which encodes the astrocytic marker glial fibrillary acidic protein, and a putative RA response element in the promoter was found to be critical for this activation. Histone H3 acetylation around the STAT‐binding site in the gfap promoter was increased in NPCs treated with RA, allowing STAT3 to gain access to the promoter more efficiently. These results suggest that RA acts in concert with LIF to induce astrocyte differentiation of NPCs through an epigenetic mechanism that involves cross‐talk between distinct signaling pathways. Stem Cells 2009;27:2744–2752

Periventricular notch activation and asymmetric Ngn2 and Tbr2 expression in pair-generated neocortical daughter cells.

To understand the cellular and molecular mechanisms regulating cytogenesis within the neocortical ventricular zone, we examined at high resolution the spatiotemporal expression patterns of Ngn2 and Tbr2. Individually DiI-labeled daughter cells were tracked from their birth in slice cultures and immunostained for Ngn2 and Tbr2. Both proteins were initially absent from daughter cells during the first 2 h. Ngn2 expression was then initiated asymmetrically in one of the daughter cells, with a bias towards the apical cell, followed by a similar pattern of expression for Tbr2, which we found to be a direct target of Ngn2. How this asymmetric Ngn2 expression is achieved is unclear, but gamma-secretase inhibition at the birth of daughter cells resulted in premature Ngn2 expression, suggesting that Notch signaling in nascent daughter cells suppresses a Ngn2-Tbr2 cascade. Many of the nascent cells exhibited pin-like morphology with a short ventricular process, suggesting periventricular presentation of Notch ligands to these cells.

Epigenetic regulation of neural cell differentiation plasticity in the adult mammalian brain

Neural stem/progenitor cells (NSCs/NPCs) give rise to neurons, astrocytes, and oligodendrocytes. It has become apparent that intracellular epigenetic modification including DNA methylation, in concert with extracellular cues such as cytokine signaling, is deeply involved in fate specification of NSCs/NPCs by defining cell-type specific gene expression. However, it is still unclear how differentiated neural cells retain their specific attributes by repressing cellular properties characteristic of other lineages. In previous work we have shown that methyl-CpG binding protein transcriptional repressors (MBDs), which are expressed predominantly in neurons in the central nervous system, inhibit astrocyte-specific gene expression by binding to highly methylated regions of their target genes. Here we report that oligodendrocytes, which do not express MBDs, can transdifferentiate into astrocytes both in vitro (cytokine stimulation) and in vivo (ischemic injury) through the activation of the JAK/STAT signaling pathway. These findings suggest that differentiation plasticity in neural cells is regulated by cell-intrinsic epigenetic mechanisms in collaboration with ambient cell-extrinsic cues.

Epigenetic mechanisms regulating fate specification of neural stem cells

Neural stem cells (NSCs) possess the ability to self-renew and to differentiate along neuronal and glial lineages. These processes are defined by the dynamic interplay between extracellular cues including cytokine signalling and intracellular programmes such as epigenetic modification. There is increasing evidence that epigenetic mechanisms involving, for example, changes in DNA methylation, histone modification and non-coding RNA expression are closely associated with fate specification of NSCs. These epigenetic alterations could provide coordinated systems for regulating gene expression at each step of neural cell differentiation. Here we review the roles of epigenetics in neural fate specification in the mammalian central nervous system.

Neuronal differentiation of neural precursor cells is promoted by the methyl-CpG-binding protein MeCP2

Methyl-CpG-binding protein 2 (MeCP2), a methyl-CpG-binding domain protein family member which is expressed predominantly in neurons in the nervous system, acts as a transcriptional repressor by binding to methylated genes, and mutations in mecp2 cause the neurological disorder known as Rett syndrome (RTT). Although MeCP2 has been reported to regulate neuronal maturation rather than fate specification of neural precursor cells (NPCs), we have previously shown that it inhibits astrocyte differentiation of NPCs when ectopically expressed. Here, we show that expression of MeCP2 in NPCs not only suppresses astrocytic differentiation but actually promotes neuronal differentiation, even in the presence of well-known astrocyte-inducing cytokines. This dual function of MeCP2 was abolished by the MEK inhibitor U0126. Moreover, we observed that a truncated form of MeCP2 found in RTT patients fails to promote neuronal differentiation. We further demonstrate that transplanted MeCP2-expressing NPCs differentiate in vivo into neurons in two non-neurogenic regions, striatum and spinal cord. These results suggest a possible therapeutic application for MeCP2 in neurodegenerative diseases and injuries to the central nervous system.

Astrocyte-Specific Genes Are Generally Demethylated in Neural Precursor Cells Prior to Astrocytic Differentiation

Epigenetic changes are thought to lead to alterations in the property of cells, such as differentiation potential. Neural precursor cells (NPCs) differentiate only into neurons in the midgestational brain, yet they become able to generate astrocytes in the late stage of development. This differentiation-potential switch could be explained by epigenetic changes, since the promoters of astrocyte-specific marker genes, glial fibrillary acidic protein (Gfap) and S100β, have been shown to become demethylated in late-stage NPCs prior to the onset of astrocyte differentiation; however, whether demethylation occurs generally in other astrocyctic genes remains unknown. Here we analyzed DNA methylation changes in mouse NPCs between the mid-(E11.5) and late (E14.5) stage of development by a genome-wide DNA methylation profiling method using microarrays and found that many astrocytic genes are demethylated in late-stage NPCs, enabling the cell to become competent to express these genes. Although these genes are already demethylated in late-stage NPCs, they are not expressed until cells differentiate into astrocytes. Thus, late-stage NPCs have epigenetic potential which can be realized in their expression after astrocyte differentiation.

Methyl-CpG binding proteins are involved in restricting differentiation plasticity in neurons.

Neurons and astrocytes are generated from common neural precursors, yet neurogenesis precedes astrocytogenesis, which normally commences at later stages of development. We have previously reported that a particular cytosine residue within a STAT3‐binding site in the astrocyte‐specific marker glial fibrillary acidic protein (GFAP) gene promoter becomes demethylated in neuroepithelial cells as gestation proceeds. This demethylation correlates tightly with the onset of astrocyte differentiation, suggesting that a change in DNA methylation at cell‐type‐specific gene promoters controls the switch from neurogenesis to astrocytogenesis in the developing brain. Here, we show that late‐gestation neuroepithelial cells, which have already lost the methylation in the STAT3‐binding site within the GFAP promoter, can still give rise to neurons and that these neurons do not respond to a STAT3‐activating cytokine to express GFAP. Members of a transcriptional repressor family, the methylated‐CpG binding proteins (MBDs), including MeCP2, are predominantly expressed in neurons, and ectopic MeCP2 expression inhibited astrocyte differentiation of neuroepithelial cells. Moreover, we found that exon 1 of the GFAP gene remains hypermethylated even in neuroepithelial cells at a late developmental stage and in neurons differentiated from such neuroepithelial cells. We further demonstrate that MeCP2 actually binds to the highly methylated exon 1 of the GFAP gene in neurons. These results suggest that region‐specific DNA methylation and MBDs play an important role in the regulation of differentiation plasticity in neurons.