In Vitro Direct Reprogramming of Mouse and Human Astrocytes to Induced Neurons

Direct neuronal reprogramming, rewiring the epigenetic and transcriptional network of a differentiated cell type to neuron, apart from being a very promising approach for the treatment of brain injury and neurodegeneration, also offers a prime opportunity to investigate the molecular underpinnings of neuronal cell fate determination, as the precise molecular mechanisms that establish neuronal fate and diversity at the transcriptional and epigenetic level are incompletely understood. Recent studies from a number of groups, including ours, have shown that astrocytes can be directly reprogrammed into functional neurons in vitro and in vivo following ectopic overexpression of combinations of transcription factors, neurogenic proteins, miRNAs, and small chemical molecules.In this chapter we describe the protocols for in vitro converting primary cortical astrocytes of mouse and human origin to induced neurons, through forced expression of two neurogenic molecules, either each one alone or in combination: the master regulatory bHLH proneural transcription factor NEUROGENIN-2 (NEUROG2) and the neurogenic protein CEND1. Forced expression of each one of the two neurogenic proteins in primary astrocytes via retroviral gene transfer results in their direct conversion to subtype-specific induced neurons, while simultaneous coexpression of both molecules drives them predominantly toward acquisition of a neural precursor cell (NPC) state. Although mouse and human astrocytes exhibit differences in their reprogramming rate and particular characteristics, they can both get efficiently in vitro transdifferentiated to NPCs and induced neurons upon NEUROG2 or/and CEND1 forced expression using the reprogramming protocols described in the chapter, presenting valuable cellular platforms for mechanistic studies and in vivo applications to restore neurodegeneration.

Inhibitor of DNA binding 2 promotes axonal growth through upregulation of Neurogenin2

Manipulation of developmentally regulated genes presents a promising strategy to enhance the intrinsic growth capability of adult neurons. Inhibitor of DNA binding 2 (Id2), a negative regulator of bHLH transcriptional factors, promotes axonal growth after its forced expression in post-mitotic neurons. Neurogenin2 (Ngn2) is a neural specific bHLH factor which controls neuronal fate and drives neuronal differentiation during development. In this study, we investigated the mechanism of Id2 in promoting axonal growth and revealed that Ngn2 contributed to the growth-activating role of Id2 in neurons. Ngn2 expression was upregulated with increased Id2 activity by assessing RNA and protein levels. Forced expression of Id2 or Ngn2 in cortical neurons significantly promoted axonal growth with little effect on dendrites. Furthermore, knockdown of Ngn2 impaired the axonal growth promoting effect of Id2, implying that the effect of Id2 on axonal growth depends on Ngn2. These findings suggest that elevation of neuronal Ngn2 may be a new therapeutic strategy to stimulate axonal regeneration.

Pharmacological Notch pathway inhibition leads to cell cycle arrest and stimulates ascl1 and neurogenin2 genes expression in dental pulp stem cells-derived neurospheres

OBJECTIVE

Human dental pulp-derived stem cells (hDPSCs) are becoming an attractive source for cell-based neurorestorative therapies. As such, it is important to understand the molecular mechanisms that regulate the differentiation of hDPSCs toward the neuronal fate. Notch signaling plays key roles in neural stem/progenitor cells (NS/PCs) maintenance and prevention of their differentiation. The aim of this study was to address the effects of Notch signaling inhibition on neurosphere formation of hDPSCs and neuronal differentiation of hDPSCs-neurospheres.

RESULTS

hDPSCs were isolated from third molar teeth. The cultivated hDPSCs highly expressed CD90 and CD44 and minimally presented CD34 and CD45 surface markers. The osteo/adipogenic differentiation of hDPSCs was documented. hDPSCs were cultured in neural induction medium and N-[N-(3,5-difluorophenacetyl-L-alanyl)]-Sphenylglycine t-butyl ester (DAPT) was applied to impede Notch signaling during transformation into spheres or on the formed neurospheres. Our results showed that the size and number of neurospheres decreased and the expression profile of nestin, sox1 and pax6 genes reduced provided DAPT. Treatment of the formed neurospheres with DAPT resulted in the cleaved Notch1 reduction, G0/G1 arrest and a decline in L-lactate production. DAPT significantly reduced hes1 and hey1 genes, while ascl1 and neurogenin2 expressions augmented. The number of MAP2 positive cells improved in the DAPT-treated group.

CONCLUSIONS

Our findings demonstrated the Notch activity in hDPSCs-neurospheres. DAPT treatment positively regulated proneural genes expression and increased neuronal-like differentiation.

N-terminal phosphorylation of xHes1 controls inhibition of primary neurogenesis in Xenopus

The processes of cell proliferation and differentiation are intimately linked during embryogenesis, and the superfamily of (basic) Helix-Loop-Helix (bHLH) transcription factors play critical roles in these events. For example, neuronal differentiation is promoted by class II bHLH proneural proteins such as Ngn2 and Ascl1, while class VI Hes proteins act to restrain differentiation and promote progenitor maintenance. We have previously described multi-site phosphorylation as a key regulator of tissue specific class II bHLH proteins in all three embryonic germ layers, and this enables coordination of differentiation with the cell cycle. Hes1 homologues also show analogous conserved proline directed kinase sites. Here we have used formation of Xenopus primary neurons to investigate the effects of xHes1 multi-site phosphorylation on both endogenous and ectopic proneural protein-induced neurogenesis. We find that xHes1 is phosphorylated in vivo, and preventing phosphorylation on three conserved SP/TP sites in the N terminus of the protein enhances xHes1 protein stability and repressor activity. Mechanistically, compared to wild-type xHes1, phospho-mutant xHes1 exhibits greater repression of Ngn2 transcription as well as producing a greater reduction in Ngn2 protein stability and chromatin binding. We propose that cell cycle dependent phosphorylation of class VI Hes proteins may act alongside similar regulation of class II bHLH proneural proteins to co-ordinate their activity.

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xHes1 is phosphorylated in Xenopus embryos on conserved N terminal SP/TP sites.

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In vitro, xHes1 protein can be phosphorylated by Cyclin-dependent-kinases.

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Under-phosphorylated xHes1 has increased protein stability relative to WT xHes1.

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Under-phosphorylated xHes1 has enhanced inhibitory activity during neurogenesis.

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xHes1 reduces Ngn2 expression, protein stability and chromatin binding.

Interaction between opposing modes of phospho-regulation of the proneural proteins Ascl1 and Ngn2

From the relatively simple nervous system of Drosophila to the elaborate mammalian cortex, neurogenesis requires exceptional spatial and temporal precision to co-ordinate progenitor cell proliferation and subsequent differentiation to a diverse range of neurons and glia. A limited number of transiently expressed proneural basic-helix-loop-helix (bHLH) transcription factors, for example achaete-scute-complex (as-c) and atonal (ato) in Drosophila and the vertebrate homologues Ascl1 and Neurogenin2 (Ngn2), are able to orchestrate the onset of neuronal determination, context-dependent subtype selection and even influence later aspects of neuronal migration and maturation. Within the last decade, two models have emerged to explain how the temporal activity of proneural determination factors is regulated by phosphorylation at distinct sites. One model describes how cell-cycle associated phosphorylation on multiple sites in the N and C termini of vertebrate proneural proteins limits neuronal differentiation in cycling progenitor cells. A second model describes phosphorylation on a single site in the bHLH domain of Drosophila atonal that acts as a binary switch, where phosphorylation terminates proneural activity. Here we combine activating mutations of phosphorylation sites in the N- and C- termini with an inhibitory phospho-mimetic mutation in the bHLH domain of Ascl1 and Ngn2 proteins, and test their functions in vivo using Xenopus embryos to determine which mode of phospho-regulation dominates. Enhancing activity by preventing N- and C terminal phosphorylation cannot overcome the inhibitory effect of mimicking phosphorylation of the bHLH domain. Thus we have established a hierarchy between these two modes of proneural protein control and suggest a model of temporal regulation for proneural protein activity.

Interferon-gamma inhibits the neuronal differentiation of neural progenitor cells by inhibiting the expression of Neurogenin2 via the JAK/STAT1 pathway

Interferon-gamma (IFN-γ) is one of the critical cytokines released by host immune cells upon infection. Despite the important role(s) of IFN-γ in host immune responses, there has been no in vivo study regarding the effects of IFN-γ on brain development, and the results from many in vitro studies are controversial. In this study, the effects of IFN-γ on embryonic neurogenesis were investigated. Treatment of E14.5 mouse neural progenitor cells (NPCs) with IFN-γ resulted in a decrease in the percentage of TuJ1-positive immature neurons but an increase in the percentage of Nestin-positive NPCs. Similar results were obtained in vivo. Treatment of NPCs with a JAK inhibitor or the knockdown of STAT1 expression abrogated the IFN-γ-mediated inhibition of neurogenesis. Interestingly, the expression of one of proneural genes, Neurogenin2 (Neurog2) was dramatically inhibited upon IFN-γ treatment, and cells overexpressing Neurog2 did not respond to IFN-γ. Taken together, our results demonstrate that IFN-γ inhibits neuronal differentiation of NPCs by negatively regulating the expression of Neurog2 via the JAK/STAT1 pathway. Our findings may provide an insight into the role of IFN-γ in the development of embryonic brain.

Adipose-Derived Stem Cells Expressing the Neurogenin-2 Promote Functional Recovery After Spinal Cord Injury in Rat

Neurogenin2 (Ngn2) is a proneural gene that directs neuronal differentiation of progenitor cells during development. This study aimed to investigate whether the use of adipose-derived stem cells (ADSCs) over-expressing the Ngn2 transgene (Ngn2-ADSCs) could display the characteristics of neurogenic cells and improve functional recovery in an experimental rat model of SCI. ADSCs from rats were cultured and purified in vitro, followed by genetically modified with the Ngn2 gene. Forty-eight adult female Sprague-Dawley rats were randomly assigned to three groups: the control, ADSCs, and Ngn2-ADSCs groups. The hind-limb motor function of all rats was recorded using the Basso, Beattie, and Bresnahan locomotor rating scale for 8 weeks. Moreover, hematoxylineosin staining and immunohistochemistry were also performed. After neural induction, positive expression rate of NeuN in Ngn2-ADSCs group was upon 90 %. Following transplantation, a great number of ADSCs was found around the center of the injury spinal cord at 1 and 4 weeks, which improved retention of tissue at the lesion site. Ngn2-ADSCs differentiated into neurons, indicated by the expression of neuronal markers, NeuN and Tuj1. Additionally, transplantation of Ngn2-ADSCs upregulated the trophic factors (brain-derived neurotrophic factor and vascular endothelial growth factor), and inhibited the glial scar formation, which was indicated by immunohistochemistry with glial fibrillary acidic protein. Finally, Ngn2-ADSCs-treated animals showed the highest functional recovery among the three groups. These findings suggest that transplantation of Ngn2-overexpressed ADSCs promote the functional recovery from SCI, and improve the local microenvironment of injured cord in a more efficient way than that with ADSCs alone.

Exploring the potential relationship between Notch pathway genes expression and their promoter methylation in mice hippocampal neurogenesis

The Notch pathway is a highly conserved pathway that regulates hippocampal neurogenesis during embryonic development and adulthood. It has become apparent that intracellular epigenetic modification including DNA methylation is deeply involved in fate specification of neural stem cells (NSCs). However, it is still unclear whether the Notch pathway regulates hippocampal neurogenesis by changing the Notch genes' DNA methylation status. Here, we present the evidence from DNA methylation profiling of Notch1, Hes1 and Ngn2 promoters during neurogenesis in the dentate gyrus (DG) of postnatal, adult and traumatic brains. We observed the expression of Notch1, Hes1 and Ngn2 in hippocampal DG with qPCR, Western blot and immunofluorescence staining. In addition, we investigated the methylation status of Notch pathway genes using the bisulfite sequencing PCR (BSP) method. The number of Notch1 or Hes1 (+) and BrdU (+) cells decreased in the subgranular zone (SGZ) of the DG in the hippocampus following TBI. Nevertheless, the number of Ngn2-positive cells in the DG of injured mice was markedly higher than in the DG of non-TBI mice. Accordingly, the DNA methylation level of the three gene promoters changed with their expression in the DG. These findings suggest that the strict spatio-temporal expression of Notch effector genes plays an important role during hippocampal neurogenesis and suggests the possibility that Notch1, Hes1 and Ngn2 were regulated by changing some specific CpG sites of their promoters to further orchestrate neurogenesis in vivo.

Neurogenin2-d4Venus and Gadd45g-d4Venus transgenic mice: visualizing mitotic and migratory behaviors of cells committed to the neuronal lineage in the developing mammalian brain

To achieve highly sensitive and comprehensive assessment of the morphology and dynamics of cells committed to the neuronal lineage in mammalian brain primordia, we generated two transgenic mouse lines expressing a destabilized (d4) Venus controlled by regulatory elements of the Neurogenin2 (Neurog2) or Gadd45g gene. In mid-embryonic neocortical walls, expression of Neurog2-d4Venus mostly overlapped with that of Neurog2 protein, with a slightly (1 h) delayed onset. Although Neurog2-d4Venus and Gadd45g-d4Venus mice exhibited very similar labeling patterns in the ventricular zone (VZ), in Gadd45g-d4Venus mice cells could be visualized in more basal areas containing fully differentiated neurons, where Neurog2-d4Venus fluorescence was absent. Time-lapse monitoring revealed that most d4Venus⁺ cells in the VZ had processes extending to the apical surface; many of these cells eventually retracted their apical process and migrated basally to the subventricular zone, where neurons, as well as the intermediate neurogenic progenitors that undergo terminal neuron-producing division, could be live-monitored by d4Venus fluorescence. Some d4Venus⁺ VZ cells instead underwent nuclear migration to the apical surface, where they divided to generate two d4Venus⁺ daughter cells, suggesting that the symmetric terminal division that gives rise to neuron pairs at the apical surface can be reliably live-monitored. Similar lineage-committed cells were observed in other developing neural regions including retina, spinal cord, and cerebellum, as well as in regions of the peripheral nervous system such as dorsal root ganglia. These mouse lines will be useful for elucidating the cellular and molecular mechanisms underlying development of the mammalian nervous system.