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-CreER(T2)/R26R-YFP/Notch1(loxP/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-CreER(T2)/R26R-YFP/Notch1(w/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

Ascl1 (Mash1) Defines Cells with Long-Term Neurogenic Potential in Subgranular and Subventricular Zones in Adult Mouse Brain

Ascl1 (Mash1) is a bHLH transcription factor essential for neural differentiation during embryogenesis but its role in adult neurogenesis is less clear. Here we show that in the adult brain Ascl1 is dynamically expressed during neurogenesis in the dentate gyrus subgranular zone (SGZ) and more rostral subventricular zone (SVZ). Specifically, we find Ascl1 levels low in SGZ Type-1 cells and SVZ B cells but increasing as the cells transition to intermediate progenitor stages. In vivo genetic lineage tracing with a tamoxifen (TAM) inducible Ascl1CreERT2 knock-in mouse strain shows that Ascl1 lineage cells continuously generate new neurons over extended periods of time. There is a regionally-specific difference in neuron generation, with mice given TAM at postnatal day 50 showing new dentate gyrus neurons through 30 days post-TAM, but showing new olfactory bulb neurons even 180 days post-TAM. These results show that Ascl1 is not restricted to transit amplifying populations but is also found in a subset of neural stem cells with long-term neurogenic potential in the adult brain

Understanding the habenula: A major node in circuits regulating emotion and motivation

Over the last decade, the understanding of the habenula has rapidly advanced from being an understudied brain area with the Latin name ‘habena” meaning “little rein”, to being considered a “major rein” in the control of key monoaminergic brain centers. This ancient brain structure is a strategic node in the information flow from fronto-limbic brain areas to brainstem nuclei. As such, it plays a crucial role in regulating emotional, motivational, and cognitive behaviors and has been implicated in several neuropsychiatric disorders, including depression and addiction. This review will summarize recent findings on the medial (MHb) and lateral (LHb) habenula, their topographical projections, cell types, and functions. Additionally, we will discuss contemporary efforts that have uncovered novel molecular pathways and synaptic mechanisms with a focus on MHb-Interpeduncular nucleus (IPN) synapses. Finally, we will explore the potential interplay between the habenula's cholinergic and non-cholinergic components in coordinating related emotional and motivational behaviors, raising the possibility that these two pathways work together to provide balanced roles in reward prediction and aversion, rather than functioning independently

A subset of Ascl1 lineage cells in adult SVZ have long term self renewing properties in the generation of olfactory bulb neurons.

<p>(A–D) Ascl1 is detected in Nestin::GFP⁺GFAP⁺ cells (B cells) in the SVZ (A–B′) and in Nestin::GFP⁺GFAP⁻ C cells in SVZ (A–B′) and RMS (C,C′) in 8 week old <i>Nestin::GFP</i> transgenic mice. (D) Percentage of Ascl1^High or Ascl1^Low cells that express the markers Nestin::GFP and GFAP (dark shaded bars) or just Nestin::GFP (grey shaded bars) in the RMS and the SVZ. 25 Ascl1⁺ cells per mouse were counted in the RMS; 60 Ascl1⁺ cells per mouse in the SVZ, n = 3 <i>Nestin::GFP</i> mice. (E–E′) mRNA in situ with Ascl1 (E) or Cre (E′) probes in the adult SVZ. (F–T′) Immunofluorescence in <i>Ascl1^CreERT2/+</i>;<i>R26R^YFP/YFP</i> mouse brain sections harvested 7, 30, or 180 days post-TAM demonstrates Ascl1 derived cells along the SVZ-RMS-OB pathway (F–N). 7 days post-TAM most YFP⁺ cells were located in the SVZ, or along the RMS (F–H) and express Sox2 (O–O′) or DCX (P–P′). 30 or 180 days post-TAM YFP⁺ cells mature into neurons in the granule cell layer or the periglomerular layer of the OB (I, L, R–R′, and data not shown). However, many YFP⁺ cells remain as Sox2⁺ or DCX⁺ progenitors in the RMS or SVZ (J–K, M–N, Q–Q′, S–T′). Scale bars  = 50 µm (F–N), 10 µm (O–T′).</p

A subset of Ascl1 lineage cells continue to produce new granule neurons 30 days after initial Ascl1 expression in adult hippocampus.

<p>(A) Targeting strategy for <i>Ascl1^CreERT2/+</i> knock-in mice. (B) Quantification of the percentage of YFP⁺ cells co-labeled with stage-specific markers in hippocampus of adult <i>Ascl1^CreERT2/+;R26R^YFP/YFP</i> mice 7, 30, or 180 days post-TAM. 150–500 YFP⁺ cells per mouse were counted for each marker, n = 2 <i>Ascl1^CreERT2/+;R26R^YFP/YFP</i> mice per time point. (C–F′) 7 days post-TAM YFP⁺ cells co-express GFAP (and have Type-1 morphology), Sox2, or NeuroD1, but not NeuN. (G–J′) 30 days post-TAM YFP⁺ cells overlap with NeuN, but also can co-express GFAP or NeuroD1. (K–N′) 180 days post-TAM a subpopulation of YFP⁺ cells are still Type-1 cells by morphology and express GFAP and Sox2, whereas the majority of YFP⁺ cells express NeuN but not NeuroD1. (O–V) Neurogenesis in the SGZ dramatically decreases between 12 weeks and 34 weeks of age as seen in the decrease in DCX (P,T), NeuroD1 (Q,U) and Ki67 (R,V). Arrowheads indicate the few cells positive for these markers in the 34 week old mice. Notably, Sox2 does not decrease (O,S) so may label quiescent Type-1 like cells. Scale bars  = 50 µm (C,G,K), 10 µm (D–F′, H–J′, I–V).</p

Ascl1 is present in a subpopulation of Type-1 stem cells and Type-2 progenitors in adult hippocampus.

<p>(A–D) Ascl1 is weakly detected in Nestin::GFP⁺GFAP⁺ Type-1 stem cells (arrowhead) or strongly detected in Nestin::GFP⁺GFAP⁻ Type-2 progenitors (arrow) in SGZ of adult <i>Nestin::GFP</i> mice. (E) Percentage of Ascl1^High or Ascl1^Low cells that express the markers Nestin::GFP and GFAP (Type-1) (dark shaded bars) or just the marker Nestin::GFP (Type-2) (grey shaded bars). 50 Ascl1⁺ cells were counted per mouse, n = 3 <i>Nestin::GFP</i> mice. (F) Ascl1 is in Type-1 and early Type-2 cells based on a current model of adult hippocampal neurogenesis <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0018472#pone.0018472-Kempermann1" target="_blank">[14]</a>. Scale bar  = 20 µm.</p

In vivocontribution of nestin- and GLAST-lineage cells to adult hippocampal neurogenesis

Radial glia‐like cells (RGCs) are the hypothesized source of adult hippocampal neurogenesis. However, the current model of hippocampal neurogenesis does not fully incorporate the in vivo heterogeneity of RGCs. In order to better understand the contribution of different RGC subtypes to adult hippocampal neurogenesis, we employed widely used transgenic lines (Nestin‐CreERT2 and GLAST::CreERT2 mice) to explore how RGCs contribute to neurogenesis under basal conditions and after stimulation and depletion of neural progenitor cells. We first used these inducible fate‐tracking transgenic lines to define the similarities and differences in the contribution of nestin‐ and GLAST‐lineage cells to basal long‐term hippocampal neurogenesis. We then explored the ability of nestin‐ and GLAST‐lineage RGCs to contribute to neurogenesis after experimental manipulations that either ablate neurogenesis (i.c.v. application of the anti‐mitotic AraC, cytosine‐β‐D‐arabinofuranoside) or stimulate neurogenesis (wheel running). Interestingly, in both ablation and stimulation experiments, labeled RGCs in GLAST::CreERT2 mice appear to contribute to neurogenesis, whereas RGCs in Nestin‐CreERT2 mice do not. Finally, using NestinGFP reporter mice, we expanded on previous research by showing that not all RGCs in the adult dentate gyrus subgranular zone express nestin, and therefore RGCs are antigenically heterogeneous. These findings are important for the field, as they allow appropriately conservative interpretation of existing and future data that emerge from these inducible transgenic lines. These findings also raise important questions about the differences between transgenic driver lines, the heterogeneity of RGCs, and the potential differences in progenitor cell behavior between transgenic lines. As these findings highlight the possible differences in the contribution of cells to long‐term neurogenesis in vivo, they indicate that the current models of hippocampal neurogenesis should be modified to include RGC lineage heterogeneit

IkB Kinase Regulates Social Defeat Stress-Induced Synaptic and Behavioral Plasticity

The neurobiological underpinnings of mood and anxiety disorders have been linked to the nucleus accumbens (NAc), a region important in processing the rewarding and emotional salience of stimuli. Using chronic social defeat stress, an animal model of mood and anxiety disorders, we investigated whether alterations in synaptic plasticity are responsible for the long-lasting behavioral symptoms induced by this form of stress. We hypothesized that chronic social defeat stress alters synaptic strength or connectivity of medium spiny neurons (MSNs) in the NAc to induce social avoidance. To test this, we analyzed the synaptic profile of MSNs via confocal imaging of Lucifer-yellow-filled cells, ultrastructural analysis of the postsynaptic density, and electrophysiological recordings of miniature EPSCs (mEPSCs) in mice after social defeat. We found that NAc MSNs have more stubby spine structures with smaller postsynaptic densities and an increase in the frequency of mEPSCs after social defeat. In parallel to these structural changes, we observed significant increases in IκB kinase (IKK) in the NAc after social defeat, a molecular pathway that has been shown to regulate neuronal morphology. Indeed, we find using viral-mediated gene transfer of dominant-negative and constitutively active IKK mutants that activation of IKK signaling pathways during social defeat is both necessary and sufficient to induce synaptic alterations and behavioral effects of the stress. These studies establish a causal role for IKK in regulating stress-induced adaptive plasticity and may present a novel target for drug development in the treatment of mood and anxiety disorders in humans.Brain and Behavior Research Foundatio

Hedgehog-interacting protein acts in the habenula to regulate nicotine intake

Hedgehog-interacting protein (HHIP) sequesters Hedgehog ligands to repress Smoothened (SMO)-mediated recruitment of the GLI family of transcription factors. Allelic variation in HHIP confers risk of chronic obstructive pulmonary disease and other smoking-related lung diseases, but underlying mechanisms are unclear. Using single-cell and cell-type-specific translational profiling, we show that HHIP expression is highly enriched in medial habenula (MHb) neurons, particularly MHb cholinergic neurons that regulate aversive behavioral responses to nicotine. HHIP deficiency dysregulated the expression of genes involved in cholinergic signaling in the MHb and disrupted the function of nicotinic acetylcholine receptors (nAChRs) through a PTCH-1/cholesterol-dependent mechanism. Further, CRISPR/Cas9-mediated genomic cleavage of the Hhip gene in MHb neurons enhanced the motivational properties of nicotine in mice. These findings suggest that HHIP influences vulnerability to smoking-related lung diseases in part by regulating the actions of nicotine on habenular aversion circuits.ISSN:0027-8424ISSN:1091-649