Jenny Hsieh

A small modulatory dsRNA specifies the fate of adult neural stem cells

Discovering the molecular mechanisms that regulate neuron-specific gene expression remains a central challenge for CNS research. Here, we report that small, noncoding double-stranded (ds) RNAs play a critical role in mediating neuronal differentiation. The sequence defined by this dsRNA is NRSE/RE1, which is recognized by NRSF/REST, known primarily as a negative transcriptional regulator that restricts neuronal gene expression to neurons. The NRSE dsRNA can trigger gene expression of neuron-specific genes through interaction with NRSF/REST transcriptional machinery, resulting in the transition from neural stem cells with neuron-specific genes silenced by NRSF/REST into cells with neuronal identity that can express neuronal genes. The mechanism of action appears to be mediated through a dsRNA/protein interaction, rather than through siRNA or miRNA. The discovery of small modulatory dsRNAs (smRNAs) extends the important contribution of noncoding RNAs as key regulators of cell behavior at both transcriptional and posttranscriptional levels.

Wnt-mediated activation of NeuroD1 and retro-elements during adult neurogenesis

In adult hippocampus, new neurons are continuously generated from neural stem cells (NSCs), but the molecular mechanisms regulating adult neurogenesis remain elusive. We found that Wnt signaling, together with the removal of Sox2, triggered the expression of NeuroD1 in mice. This transcriptional regulatory mechanism was dependent on a DNA element containing overlapping Sox2 and T-cell factor/lymphoid enhancer factor (TCF/LEF)-binding sites (Sox/LEF) in the promoter. Notably, Sox/LEF sites were also found in long interspersed nuclear element 1 (LINE-1) elements, consistent with their critical roles in the transition of NSCs to proliferating neuronal progenitors. Our results describe a previously unknown Wnt-mediated regulatory mechanism that simultaneously coordinates activation of NeuroD1 and LINE-1, which is important for adult neurogenesis and survival of neuronal progenitors. Moreover, the discovery that LINE-1 retro-elements embedded in the mammalian genome can function as bi-directional promoters suggests that Sox/LEF regulatory sites may represent a general mechanism, at least in part, for relaying environmental signals to other nearby loci to promote adult hippocampal neurogenesis.

HDAC1 and HDAC2 regulate oligodendrocyte differentiation by disrupting the β-catenin–TCF interaction

Oligodendrocyte development is regulated by the interaction of repressors and activators in a complex transcriptional network. We found that two histone-modifying enzymes, HDAC1 and HDAC2, were required for oligodendrocyte formation. Genetic deletion of both Hdac1 and Hdac2 in oligodendrocyte lineage cells resulted in stabilization and nuclear translocation of β-catenin, which negatively regulates oligodendrocyte development by repressing Olig2 expression. We further identified the oligodendrocyte-restricted transcription factor TCF7L2/TCF4 as a bipartite co-effector of β-catenin for regulating oligodendrocyte differentiation. Targeted disruption of Tcf7l2 in mice led to severe defects in oligodendrocyte maturation, whereas expression of its dominant-repressive form promoted precocious oligodendrocyte specification in developing chick neural tube. Transcriptional co-repressors HDAC1 and HDAC2 compete with β-catenin for TCF7L2 interaction to regulate downstream genes involved in oligodendrocyte differentiation. Thus, crosstalk between HDAC1/2 and the canonical Wnt signaling pathway mediated by TCF7L2 serves as a regulatory mechanism for oligodendrocyte differentiation.

IGF-I instructs multipotent adult neural progenitor cells to become oligodendrocytes.

Adult multipotent neural progenitor cells can differentiate into neurons, astrocytes, and oligodendrocytes in the mammalian central nervous system, but the molecular mechanisms that control their differentiation are not yet well understood. Insulin-like growth factor I (IGF-I) can promote the differentiation of cells already committed to an oligodendroglial lineage during development. However, it is unclear whether IGF-I affects multipotent neural progenitor cells. Here, we show that IGF-I stimulates the differentiation of multipotent adult rat hippocampus-derived neural progenitor cells into oligodendrocytes. Modeling analysis indicates that the actions of IGF-I are instructive. Oligodendrocyte differentiation by IGF-I appears to be mediated through an inhibition of bone morphogenetic protein signaling. Furthermore, overexpression of IGF-I in the hippocampus leads to an increase in oligodendrocyte markers. These data demonstrate the existence of a single molecule, IGF-I, that can influence the fate choice of multipotent adult neural progenitor cells to an oligodendroglial lineage.

Epigenetic Modulation of Seizure-Induced Neurogenesis and Cognitive Decline

The conceptual understanding of hippocampal function has been challenged recently by the finding that new granule cells are born throughout life in the mammalian dentate gyrus (DG). The number of newborn neurons is dynamically regulated by a variety of factors. Kainic acid-induced seizures, a rodent model of human temporal lobe epilepsy, strongly induce the proliferation of DG neurogenic progenitor cells and are also associated with long-term cognitive impairment. We show here that the antiepileptic drug valproic acid (VPA) potently blocked seizure-induced neurogenesis, an effect that appeared to be mainly mediated by inhibiting histone deacetylases (HDAC) and normalizing HDAC-dependent gene expression within the epileptic dentate area. Strikingly, the inhibition of aberrant neurogenesis protected the animals from seizure-induced cognitive impairment in a hippocampus-dependent learning task. We propose that seizure-generated granule cells have the potential to interfere with hippocampal function and contribute to cognitive impairment caused by epileptic activity within the hippocampal circuitry. Furthermore, our data indicate that the effectiveness of VPA as an antiepileptic drug may be partially explained by the HDAC-dependent inhibition of aberrant neurogenesis induced by seizure activity within the adult hippocampus.

Neurod1 is essential for the survival and maturation of adult-born neurons

The transcriptional program that controls adult neurogenesis is unknown. We generated mice with an inducible stem cell–specific deletion of Neurod1, resulting in substantially fewer newborn neurons in the hippocampus and olfactory bulb. Thus, Neurod1 is cell-intrinsically required for the survival and maturation of adult-born neurons.

Discovery of a Proneurogenic, Neuroprotective Chemical

An in vivo screen was performed in search of chemicals capable of enhancing neuron formation in the hippocampus of adult mice. Eight of 1000 small molecules tested enhanced neuron formation in the subgranular zone of the dentate gyrus. Among these was an aminopropyl carbazole, designated P7C3, endowed with favorable pharmacological properties. In vivo studies gave evidence that P7C3 exerts its proneurogenic activity by protecting newborn neurons from apoptosis. Mice missing the gene encoding neuronal PAS domain protein 3 (NPAS3) are devoid of hippocampal neurogenesis and display malformation and electrophysiological dysfunction of the dentate gyrus. Prolonged administration of P7C3 to npas3(-/-) mice corrected these deficits by normalizing levels of apoptosis of newborn hippocampal neurons. Prolonged administration of P7C3 to aged rats also enhanced neurogenesis in the dentate gyrus, impeded neuron death, and preserved cognitive capacity as a function of terminal aging. PAPERCLIP:

Histone deacetylases 1 and 2 control the progression of neural precursors to neurons during brain development

The molecular mechanism by which neural progenitor cells commit to a specified lineage of the central nervous system remains unknown. We show that HDAC1 and HDAC2 redundantly control neuronal development and are required for neuronal specification. Mice lacking HDAC1 or HDAC2 in neuronal precursors show no overt histoarchitectural phenotypes, whereas deletion of both HDAC1 and HDAC2 in developing neurons results in severe hippocampal abnormalities, absence of cerebellar foliation, disorganization of cortical neurons, and lethality by postnatal day 7. These abnormalities in brain formation can be attributed to a failure of neuronal precursors to differentiate into mature neurons and to excessive cell death. These results reveal redundant and essential roles for HDAC1 and HDAC2 in the progression of neuronal precursors to mature neurons in vivo.

Notch1 Is Required for Maintenance of the Reservoir of Adult Hippocampal Stem Cells

Notch1 regulates neural stem cell (NSC) number during development, but its role in adult neurogenesis is unclear. We generated nestin-CreERT2/R26R-YFP/Notch1loxP/loxP [Notch1inducible knock-out (iKO)] mice to allow tamoxifen (TAM)-inducible elimination of Notch1 and concomitant expression of yellow fluorescent protein (YFP) in nestin-expressing Type-1 NSCs and their progeny in the adult hippocampal subgranular zone (SGZ). Consistent with previous research, YFP+ cells in all stages of neurogenesis were evident in the subgranular zone (SGZ) of wild-type (WT) mice (nestin-CreERT2/R26R-YFP/Notch1w/w) after tamoxifen (post-TAM), producing adult-generated YFP+ dentate gyrus neurons. Compared with WT littermates, Notch1 iKO mice had similar numbers of total SGZ YFP+ cells 13 and 30 d post-TAM but had significantly fewer SGZ YFP+ cells 60 and 90 d post-TAM. Significantly fewer YFP+ Type-1 NSCs and transiently amplifying progenitors (TAPs) resulted in generation of fewer YFP+ granule neurons in Notch1 iKO mice. Strikingly, 30 d of running rescued this deficit, as the total YFP+ cell number in Notch iKO mice was equivalent to WT levels. This was even more notable given the persistent deficits in the Type-1 NSC and TAP reservoirs. Our data show that Notch1 signaling is required to maintain a reservoir of undifferentiated cells and ensure continuity of adult hippocampal neurogenesis, but that alternative Notch- and Type-1 NSC-independent pathways compensate in response to physical activity. These data shed light on the complex relationship between Type-1 NSCs, adult neurogenesis, the neurogenic niche, and environmental stimuli.

The oligodendrocyte-specific G-protein coupled receptor GPR17 is a cell-intrinsic timer of myelination

The basic helix-loop-helix transcription factor Olig1 promotes oligodendrocyte maturation and is required for myelin repair. We characterized an Olig1-regulated G protein–coupled receptor, GPR17, whose function is to oppose the action of Olig1. Gpr17 was restricted to oligodendrocyte lineage cells, but was downregulated during the peak period of myelination and in adulthood. Transgenic mice with sustained Gpr17 expression in oligodendrocytes exhibited stereotypic features of myelinating disorders in the CNS. Gpr17 overexpression inhibited oligodendrocyte differentiation and maturation both in vivo and in vitro. Conversely, Gpr17 knockout mice showed early onset of oligodendrocyte myelination. The opposing action of Gpr17 on oligodendrocyte maturation reflects, at least partially, upregulation and nuclear translocation of the potent oligodendrocyte differentiation inhibitors ID2/4. Collectively, these findings suggest that GPR17 orchestrates the transition between immature and myelinating oligodendrocytes via an ID protein–mediated negative regulation and may serve as a potential therapeutic target for CNS myelin repair.