Specificity and efficiency of reporter expression in adult neural progenitors vary substantially among nestin-CreER(T2) lines

Autocrine VEGF drives neural stem cell proximity to the adult hippocampus vascular niche

The vasculature is a key component of adult brain neural stem cell (NSC) niches. In the adult mammalian hippocampus, NSCs reside in close contact with a dense capillary network. How this niche is maintained is unclear. We recently found that adult hippocampal NSCs express VEGF, a soluble factor with chemoattractive properties for vascular endothelia. Here, we show that global and NSC-specific VEGF loss led to dissociation of NSCs and their intermediate progenitor daughter cells from local vasculature. Surprisingly, though, we found no changes in local vascular density. Instead, we found that NSC-derived VEGF supports maintenance of gene expression programs in NSCs and their progeny related to cell migration and adhesion. In vitro assays revealed that blockade of VEGF receptor 2 impaired NSC motility and adhesion. Our findings suggest that NSCs maintain their own proximity to vasculature via self-stimulated VEGF signaling that supports their motility towards and/or adhesion to local blood vessels.

Tools for Cre-Mediated Conditional Deletion of Floxed Alleles from Developing Cerebellar Purkinje Cells

The Cre-lox system is an indispensable tool in neuroscience research for targeting gene deletions to specific cellular populations. Here we assess the utility of several transgenic Cre lines, along with a viral approach, for targeting cerebellar Purkinje cells (PCs) in mice. Using a combination of a fluorescent reporter line (Ai14) to indicate Cre-mediated recombination and a floxed Dystroglycan line (Dag1flox ), we show that reporter expression does not always align precisely with loss of protein. The commonly used Pcp2Cre line exhibits a gradual mosaic pattern of Cre recombination in PCs from Postnatal Day 7 (P7) to P14, while loss of Dag1 protein is not complete until P30. Ptf1aCre drives recombination in precursor cells that give rise to GABAergic neurons in the embryonic cerebellum, including PCs and molecular layer interneurons. However, due to its transient expression in precursors, Ptf1aCre results in stochastic loss of Dag1 protein in these neurons. NestinCre , which is often described as a "pan-neuronal" Cre line for the central nervous system, does not drive Cre-mediated recombination in PCs. We identify a Calb1Cre line that drives efficient and complete recombination in embryonic PCs, resulting in loss of Dag1 protein before the period of synaptogenesis. AAV8-mediated delivery of Cre at P0 results in gradual transduction of PCs during the second postnatal week, with loss of Dag1 protein not reaching appreciable levels until P35. These results characterize several tools for targeting conditional deletions in cerebellar PCs at different developmental stages and illustrate the importance of validating the loss of protein following recombination.

Tools for Cre-mediated conditional deletion of floxed alleles from developing cerebellar Purkinje cells

The Cre-lox system is an indispensable tool in neuroscience research for targeting gene deletions to specific cellular populations. Here we assess the utility of several transgenic Cre lines, along with a viral approach, for targeting cerebellar Purkinje cells. Using a combination of a fluorescent reporter line (Ai14) to indicate Cre-mediated recombination and a floxed Dystroglycan line (Dag1flox) we show that reporter expression does not always align precisely with loss of protein. The commonly used Pcp2Cre line exhibits a gradual mosaic pattern of Cre recombination in Purkinje cells from P7-P14, while loss of Dag1 protein is not complete until P30. Ptf1aCre drives recombination in precursor cells that give rise to GABAergic neurons in the embryonic cerebellum, including Purkinje cells and molecular layer interneurons. However, due to its transient expression in precursors, Ptf1aCre results in stochastic loss of Dag1 protein in these neurons. NestinCre, which is often described as a "pan-neuronal" Cre line for the central nervous system, does not drive Cre-mediated recombination in Purkinje cells. We identify a Calb1Cre line that drives efficient and complete recombination in embryonic Purkinje cells, resulting in loss of Dag1 protein before the period of synaptogenesis. AAV8-mediated delivery of Cre at P0 results in gradual transduction of Purkinje cells during the second postnatal week, with loss of Dag1 protein not reaching appreciable levels until P35. These results characterize several tools for targeting conditional deletions in cerebellar Purkinje cells at different developmental stages and illustrate the importance of validating the loss of protein following recombination.

Generation of a Dcx-CreERT2 knock-in mouse for genetic manipulation of newborn neurons

A wide variety of CreERT2 driver lines are available for genetic manipulation of adult-born neurons in the mouse brain. These tools have been instrumental in studying fate potential, migration, circuit integration, and morphology of the stem cells supporting lifelong neurogenesis. Despite a wealth of tools, genetic manipulation of adult-born neurons for circuit and behavioral studies has been limited by poor specificity of many driver lines targeting early progenitor cells and by the inaccessibility of lines selective for later stages of neuronal maturation. We sought to address these limitations by creating a new CreERT2 driver line targeted to the endogenous mouse doublecortin locus as a marker of fate-specified neuroblasts and immature neurons. Our new model places a T2A-CreERT2 cassette immediately downstream of the Dcx coding sequence on the X chromosome, allowing expression of both Dcx and CreERT2 proteins in the endogenous spatiotemporal pattern for this gene. We demonstrate that the new mouse line drives expression of a Cre-dependent reporter throughout the brain in neonatal mice and in known neurogenic niches of adult animals. The line has been deposited with the Jackson Laboratory and should provide an accessible tool for studies targeting fate-restricted neuronal precursors.

Ethanol changes Nestin-promoter induced neural stem cells to disturb newborn dendritic spine remodeling in the hippocampus of mice

Adolescent binge drinking leads to long-lasting disorders of the adult central nervous system, particularly aberrant hippocampal neurogenesis. In this study, we applied in vivo fluorescent tracing using NestinCreERT2::Rosa26-tdTomato mice and analyzed the endogenous neurogenesis lineage progression of neural stem cells (NSCs) and dendritic spine formation of newborn neurons in the subgranular zone of the dentate gyrus. We found abnormal orientation of tamoxifen-induced tdTomato+ (tdTom+) NSCs in adult mice 2 months after treatment with EtOH (5.0 g/kg, i.p.) for 7 consecutive days. EtOH markedly inhibited tdTom+ NSCs activation and hippocampal neurogenesis in mouse dentate gyrus from adolescence to adulthood. EtOH (100 mM) also significantly inhibited the proliferation to 39.2% and differentiation of primary NSCs in vitro. Adult mice exposed to EtOH also exhibited marked inhibitions in dendritic spine growth and newborn neuron maturation in the dentate gyrus, which was partially reversed by voluntary running or inhibition of the mammalian target of rapamycin-enhancer of zeste homolog 2 pathway. In vivo tracing revealed that EtOH induced abnormal orientation of tdTom+ NSCs and spatial misposition defects of newborn neurons, thus causing the disturbance of hippocampal neurogenesis and dendritic spine remodeling in mice.

Silencing dentate newborn neurons alters excitatory/inhibitory balance and impairs behavioral inhibition and flexibility

Adult neurogenesis confers the hippocampus with unparalleled neural plasticity, essential for intricate cognitive functions. The specific influence of sparse newborn neurons (NBNs) in modulating neural activities and subsequently steering behavior, however, remains obscure. Using an engineered NBN-tetanus toxin mouse model (NBN-TeTX), we noninvasively silenced NBNs, elucidating their crucial role in impulse inhibition and cognitive flexibility as evidenced through Morris water maze reversal learning and Go/Nogo task in operant learning. Task-based functional MRI (tb-fMRI) paired with operant learning revealed dorsal hippocampal hyperactivation during the Nogo task in male NBN-TeTX mice, suggesting that hippocampal hyperexcitability might underlie the observed behavioral deficits. Additionally, resting-state fMRI (rs-fMRI) exhibited enhanced functional connectivity between the dorsal and ventral dentate gyrus following NBN silencing. Further investigations into the activities of PV+ interneurons and mossy cells highlighted the indispensability of NBNs in maintaining the hippocampal excitation/inhibition balance. Our findings emphasize that the neural plasticity driven by NBNs extensively modulates the hippocampus, sculpting inhibitory control and cognitive flexibility.

Cell-specific crosstalk proteomics reveals cathepsin B signaling as a driver of glioblastoma malignancy near the subventricular zone

Glioblastoma (GBM) is the most prevalent and aggressive malignant primary brain tumor. GBM proximal to the lateral ventricles (LVs) is more aggressive, potentially due to subventricular zone (SVZ) contact. Despite this, crosstalk between GBM and neural stem/progenitor cells (NSC/NPCs) is not well understood. Using cell-specific proteomics, we show that LV-proximal GBM prevents neuronal maturation of NSCs through induction of senescence. Additionally, GBM brain tumor initiating cells (BTICs) increase expression of CTSB upon interaction with NPCs. Lentiviral knockdown and recombinant protein experiments reveal both cell-intrinsic and soluble CTSB promote malignancy-associated phenotypes in BTICs. Soluble CTSB stalls neuronal maturation in NPCs while promoting senescence, providing a link between LV-tumor proximity and neurogenesis disruption. Finally, we show LV-proximal CTSB upregulation in patients, showing the relevance of this crosstalk in human GBM biology. These results demonstrate the value of proteomic analysis in tumor microenvironment research and provide direction for new therapeutic strategies in GBM.

Highlights

Periventricular GBM is more malignant and disrupts neurogenesis in a rodent model.Cell-specific proteomics elucidates tumor-promoting crosstalk between GBM and NPCs.NPCs induce upregulated CTSB expression in GBM, promoting tumor progression.GBM stalls neurogenesis and promotes NPC senescence via CTSB.

Neural stem and progenitor cells support and protect adult hippocampal function via vascular endothelial growth factor secretion

Adult neural stem and progenitor cells (NSPCs) reside in the dentate gyrus (DG) of the hippocampus throughout the lifespan of most mammalian species. In addition to generating new neurons, NSPCs may alter their niche via secretion of growth factors and cytokines. We recently showed that adult DG NSPCs secrete vascular endothelial growth factor (VEGF), which is critical for maintaining adult neurogenesis. Here, we asked whether NSPC-derived VEGF alters hippocampal function independent of adult neurogenesis. We found that loss of NSPC-derived VEGF acutely impaired hippocampal memory, caused neuronal hyperexcitability and exacerbated excitotoxic injury. We also found that NSPCs generate substantial proportions of total DG VEGF and VEGF disperses broadly throughout the DG, both of which help explain how this anatomically-restricted cell population could modulate function broadly. These findings suggest that NSPCs actively support and protect DG function via secreted VEGF, thereby providing a non-neurogenic functional dimension to endogenous NSPCs.

Endothelial and Leptin Receptor+ cells promote the maintenance of stem cells and hematopoiesis in early postnatal murine bone marrow

Mammalian hematopoietic stem cells (HSCs) colonize the bone marrow during late fetal development, and this becomes the major site of hematopoiesis after birth. However, little is known about the early postnatal bone marrow niche. We performed single-cell RNA sequencing of mouse bone marrow stromal cells at 4 days, 14 days, and 8 weeks after birth. Leptin-receptor-expressing (LepR⁺) stromal cells and endothelial cells increased in frequency during this period and changed their properties. At all postnatal stages, LepR⁺ cells and endothelial cells expressed the highest stem cell factor (Scf) levels in the bone marrow. LepR⁺ cells expressed the highest Cxcl12 levels. In early postnatal bone marrow, SCF from LepR⁺/Prx1⁺ stromal cells promoted myeloid and erythroid progenitor maintenance, while SCF from endothelial cells promoted HSC maintenance. Membrane-bound SCF in endothelial cells contributed to HSC maintenance. LepR⁺ cells and endothelial cells are thus important niche components in early postnatal bone marrow.

Single-cell transcriptome analysis reveals the immune heterogeneity and the repopulation of microglia by Hif1α in mice after spinal cord injury

Neuroinflammation is regarded as a vital pathological process in spinal cord injury (SCI), which removes damaged tissue, secretes cytokines, and facilitates regeneration. Repopulation of microglia has been shown to favor recovery from SCI. However, the origin and regulatory factors of microglia repopulation after SCI remain unknown. Here, we used single-cell RNA sequencing to portray the dynamic transcriptional landscape of immune cells during the early and late phases of SCI in mice. B cells and migDCs, located in the meninges under physiological conditions, are involved in immune surveillance. Microglia quickly reduced, and peripheral myeloid cells infiltrated three days-post-injury (dpi). At 14 dpi, microglia repopulated, myeloid cells were reduced, and lymphocytes infiltrated. Importantly, genetic lineage tracing of nestin⁺ and Cx3cr1⁺ cells in vivo showed that the repopulation of microglia was derived from residual microglia after SCI. We found that residual microglia regress to a developmental growth state in the early stages after SCI. Hif1α promotes microglial proliferation. Conditional ablation of Hif1α in microglia causes larger lesion sizes, fewer axon fibers, and impaired functional recovery in the late stages after SCI. Our results mapped the immune heterogeneity in SCI and raised the possibility that targeting Hif1α may help in axon regeneration and functional recovery after SCI.

Oral and Injected Tamoxifen Alter Adult Hippocampal Neurogenesis in Female and Male Mice

Inducible Cre recombinase facilitates temporal control of genetic recombination in numerous transgenic model systems, a feature which has made it a popular tool for adult neurogenesis studies. One of the most common forms of inducible Cre, CreER^T2, requires activation by the selective estrogen receptor modulator tamoxifen (TAM) to initiate recombination of LoxP-flanked sequences. To date, most studies deliver TAM via intraperitoneal injection. But the introduction of TAM-infused commercial chows has recently expanded the possible modes of TAM delivery. Despite the widespread use of TAM-inducible genetic models in adult neurogenesis research, the comparative efficiency and off-target effects of TAM administration protocols is surprisingly infrequently studied. Here, we compare a standard, 5 d TAM injection regimen with voluntary consumption of TAM-infused chow. First, we used adult NestinCreER^T2;Rosa-LoxP-STOP-LoxP-EYFP reporter mice to show that two weeks of TAM chow and 5 d of injections led to LoxP recombination in a similar phenotypic population of neural stem and progenitor cells (NSPCs) in the adult dentate gyrus. However, TAM chow resulted in substantially less overall recombination than injections. TAM administration also altered adult neurogenesis, but in different ways depending on administration route: TAM injection disrupted neural progenitor cell proliferation three weeks after TAM, whereas TAM chow increased neuronal differentiation of cells generated during the diet period. These findings provide guidance for selection of TAM administration route and appropriate controls in adult neurogenesis studies using TAM-inducible Cre mice. They also highlight the need for better understanding of off-target effects of TAM in other neurologic processes and organ systems.

Establishment of a Reproducible Ischemic Stroke Model in Nestin-GFP Mice with High Survival Rates

An accumulation of evidence shows that endogenous neural stem/progenitor cells (NSPCs) are activated following brain injury such as that suffered during ischemic stroke. To understand the expression patterns of these cells, researchers have developed mice that express an NSPC marker, Nestin, which is detectable by specific reporters such as green fluorescent protein (GFP), i.e., Nestin-GFP mice. However, the genetic background of most transgenic mice, including Nestin-GFP mice, comes from the C57BL/6 strain. Because mice from this background strain have many cerebral arterial branches and collateral vessels, they are accompanied by several major problems including variable ischemic areas and high mortality when subjected to ischemic stroke by occluding the middle cerebral artery (MCA). In contrast, CB-17 wild-type mice are free from these problems. Therefore, with the aim of overcoming the aforementioned defects, we first crossed Nestin-GFP mice (C57BL/6 background) with CB-17 wild-type mice and then developed Nestin-GFP mice (CB-17 background) by further backcrossing the generated hybrid mice with CB-17 wild-type mice. Subsequently, we investigated the phenotypes of the established Nestin-GFP mice (CB-17 background) following MCA occlusion; these mice had fewer blood vessels around the MCA compared with the number of blood vessels in Nestin-GFP mice (C57BL/6 background). In addition, TTC staining showed that infarcted volume was variable in Nestin-GFP mice (C57BL/6 background) but highly reproducible in Nestin-GFP mice (CB-17 background). In a further investigation of mice survival rates up to 28 days after MCA occlusion, all Nestin-GFP mice (CB-17 background) survived the period, whereas Nestin-GFP mice (C57BL/6 background) frequently died within 1 week and exhibited a higher mortality rate. Immunohistochemistry analysis of Nestin-GFP mice (CB-17 background) showed that GFP⁺ cells were mainly obverted in not only conventional neurogenic areas, including the subventricular zone (SVZ), but also ischemic areas. In vitro, cells isolated from the ischemic areas and the SVZ formed GFP⁺ neurosphere-like cell clusters that gave rise to various neural lineages including neurons, astrocytes, and oligodendrocytes. However, microarray analysis of these cells and genetic mapping experiments by Nestin-CreERT2 Line4 mice crossed with yellow fluorescent protein (YFP) reporter mice (Nestin promoter-driven YFP-expressing mice) indicated that cells with NSPC activities in the ischemic areas and the SVZ had different characteristics and origins. These results show that the expression patterns and fate of GFP⁺ cells with NSPC activities can be precisely investigated over a long period in Nestin-GFP mice (CB-17 background), which is not necessarily possible with Nestin-GFP mice (C57BL/6 background). Thus, Nestin-GFP mice (CB-17 background) could become a useful tool with which to investigate the mechanism of neurogenesis via the aforementioned cells under pathological conditions such as following ischemic stroke.

Sites of Cre-recombinase activity in mouse lines targeting skeletal cells

The Cre/Lox system is a powerful tool in the biologist's toolbox, allowing loss-of-function and gain-of-function studies, as well as lineage tracing, through gene recombination in a tissue-specific and inducible manner. Evidence indicates, however, that Cre transgenic lines have a far more nuanced and broader pattern of Cre activity than initially thought, exhibiting "off-target" activity in tissues/cells other than the ones they were originally designed to target. With the goal of facilitating the comparison and selection of optimal Cre lines to be used for the study of gene function, we have summarized in a single manuscript the major sites and timing of Cre activity of the main Cre lines available to target bone mesenchymal stem cells, chondrocytes, osteoblasts, osteocytes, tenocytes, and osteoclasts, along with their reported sites of "off-target" Cre activity. We also discuss characteristics, advantages, and limitations of these Cre lines for users to avoid common risks related to overinterpretation or misinterpretation based on the assumption of strict cell-type specificity or unaccounted effect of the Cre transgene or Cre inducers. © 2021 American Society for Bone and Mineral Research (ASBMR).

Niches that regulate stem cells and hematopoiesis in adult bone marrow

In mammals, hematopoietic stem cells (HSCs) engage in hematopoiesis throughout adult life within the bone marrow, where they produce the mature cells necessary to maintain blood cell counts and immune function. In the bone marrow and spleen, HSCs are sustained in perivascular niches (microenvironments) associated with sinusoidal blood vessels-specialized veins found only in hematopoietic tissues. Endothelial cells and perivascular leptin receptor⁺ stromal cells produce the known factors required to maintain HSCs and many restricted progenitors in the bone marrow. Various other cells synthesize factors that maintain other restricted progenitors or modulate HSC or niche function. Recent studies identified new markers that resolve some of the heterogeneity among stromal cells and refine the localization of restricted progenitor niches. Other recent studies identified ways in which niches regulate HSC function and hematopoiesis beyond growth factors. We summarize the current understanding of hematopoietic niches, review recent progress, and identify important unresolved questions.

Capicua regulates the development of adult-born neurons in the hippocampus

New neurons continuously arise from neural progenitor cells in the dentate gyrus of the adult hippocampus to support ongoing learning and memory formation. To generate functional adult-born neurons, neural progenitor cells proliferate to expand the precursor cell pool and differentiate into neurons. Newly generated cells then undergo postmitotic maturation to migrate to their final destination and develop elaborate dendritic branching, which allows them to receive input signals. Little is known about factors that regulate neuronal differentiation, migration, and dendrite maturation during adult hippocampal neurogenesis. Here, we show that the transcriptional repressor protein capicua (CIC) exhibits dynamic expression in the adult dentate gyrus. Conditional deletion of Cic from the mouse dentate gyrus compromises the adult neural progenitor cell pool without altering their proliferative potential. We further demonstrate that the loss of Cic impedes neuronal lineage development and disrupts dendritic arborization and migration of adult-born neurons. Our study uncovers a previously unrecognized role of CIC in neurogenesis of the adult dentate gyrus.

Effects of leptin on proliferation of astrocyte- and tanycyte-like neural stem cells in the adult mouse medulla oblongata

Astrocyte- and tanycyte-like neural stem cells (NSCs) were recently detected in the area postrema (AP) and central canal (CC) of the adult medulla oblongata, respectively. The present study aimed to examine dynamical behaviors of the astrocyte- and tanycyte-like NSCs of the mouse medulla oblongata to leptin. The neurosphere assay identified astrocytes in the AP and tanycytes in the CC as NSCs based on their self-renewing neurospherogenic potential. Both NSCs in neurosphere cultures were multipotent cells that generate astrocytes, oligodendrocytes, and neurons. Astrocyte-like NSCs actively proliferated and tanycyte-like NSCs were quiescent under physiologically-relevant in vivo conditions. Chronic leptin treatment promoted proliferation of astrocyte-like NSCs in the AP both in vitro and in vivo. Leptin receptors were expressed in astrocyte-like, but not tanycyte-like NSCs. Food deprivation significantly diminished proliferation of astrocyte-like NSCs. Therefore, the present study indicates that proliferation of astrocyte-like, but not tanycyte-like NSCs is regulated by nutritional conditions.

TAMEP are brain tumor parenchymal cells controlling neoplastic angiogenesis and progression

Aggressive brain tumors like glioblastoma depend on support by their local environment and subsets of tumor parenchymal cells may promote specific phases of disease progression. We investigated the glioblastoma microenvironment with transgenic lineage-tracing models, intravital imaging, single-cell transcriptomics, immunofluorescence analysis as well as histopathology and characterized a previously unacknowledged population of tumor-associated cells with a myeloid-like expression profile (TAMEP) that transiently appeared during glioblastoma growth. TAMEP of mice and humans were identified with specific markers. Notably, TAMEP did not derive from microglia or peripheral monocytes but were generated by a fraction of CNS-resident, SOX2-positive progenitors. Abrogation of this progenitor cell population, by conditional Sox2-knockout, drastically reduced glioblastoma vascularization and size. Hence, TAMEP emerge as a tumor parenchymal component with a strong impact on glioblastoma progression.

Small molecules combined with collagen hydrogel direct neurogenesis and migration of neural stem cells after spinal cord injury

Complete spinal cord injury (SCI) leads to cell death, interruption of axonal connections and permanent functional impairments. In the development of SCI treatments, cell transplantation combined with biomaterial-growth factor-based therapies have been widely studied. Another avenue worth exploring is the generation of neurons from endogenous neural stem cells (NSCs) or reactive astrocytes activated by SCI. Here, we screened a combination of four small molecules, LDN193189, SB431542, CHIR99021 and P7C3-A20, that can increase neuronal differentiation of mouse and rat spinal cord NSCs. Moreover, the small molecules loaded in an injectable collagen hydrogel induced neurogenesis and inhibited astrogliogenesis of endogenous NSCs in the injury site, which usually differentiate into astrocytes under pathological conditions. Meanwhile, induced neurons migrated into the non-neural lesion core, and genetic fate mapping showed that neurons mainly originated from NSCs in the parenchyma, but not from the central canal of the spinal cord. The neuronal regeneration in the lesion sites resulted in some recovery of locomotion. Our findings indicate that the combined treatment of small molecules and collagen hydrogel is a potential therapeutic strategy for SCI by inducing in situ endogenous NSCs to form neurons and restore damaged functions.

Histological Assessment of Cre-loxP Genetic Recombination in the Aging Subventricular Zone of Nestin-CreERT2/Rosa26YFP Mice

The use of inducible transgenic Nestin-CreER^T2 mice has proved to be an essential research tool for gene targeting and studying the molecular pathways implicated in adult neurogenesis, namely, inside the adult subgranular zone (SGZ) of the dentate gyrus and the adult subventricular zone (SVZ) lining the lateral ventricles. Several lines of Nestin-CreER-expressing mice were generated and used in adult neurogenesis research in the past two decades; however, their suitability for studying neurogenesis in aged mice remains elusive. Here, we assessed the efficiency of Cre-loxP genetic recombination in the aging SVZ using the Nestin-CreER^T2/Rosa26YFP line designed by Lagace et al. (J Neurosci 27(46):12623-12629, 2007). This analysis was performed in 12-month-old (middle-aged) mice and 20-month-old (old) mice compared to 2-month-old (young adult) mice. To evaluate successful recombination, our approach relies on the histological assessment of Cre mRNA level of expression and the YFP reporter gene's expression inside the aging SVZ by combining in situ hybridization and immunohistochemistry. Using co-immunolabeling, this approach also provides the advantage of estimating the percentage of recombined progeny [(GFP+Nestin+)/Nestin+] and the rate of cell proliferation [(GFP+Ki67+)/GFP+] inside the aging SVZ niche.

Poor Concordance of Floxed Sequence Recombination in Single Neural Stem Cells: Implications for Cell Autonomous Studies

To manipulate target gene function in specific adult cell populations, tamoxifen (TAM)-dependent CreER^T2 is widely used to drive inducible, site-specific recombination of loxP flanked sequences. In studies of cell autonomous target gene function, it is common practice to combine these CreER^T2-lox systems with a ubiquitously expressed stop-floxed fluorescent reporter gene to identify single cells supposedly undergoing target gene recombination. Here, we studied the reliability of using Cre-induced recombination of one gene to predict recombination in another gene at the single-cell level in adult hippocampal neural stem and progenitor cells (NSPCs). Using both probabilistic predictions in a generic experimental paradigm, as well as a mouse model with two separate stop-floxed reporters plus a Nestin promoter-driven CreER^T2, we found that, in individual cells, recombination of one gene was a poor predictor of recombination in another. This poor concordance in floxed sequence recombination across genes suggests that use of stop-floxed reporters to investigate cell autonomous gene function may not be universally reliable and could lead to false conclusions.

Off-Target Effects in Transgenic Mice: Characterization of Dopamine Transporter (DAT)-Cre Transgenic Mouse Lines Exposes Multiple Non-Dopaminergic Neuronal Clusters Available for Selective Targeting within Limbic Neurocircuitry

Transgenic mouse lines are instrumental in our attempt to understand brain function. Promoters driving transgenic expression of the gene encoding Cre recombinase are crucial to ensure selectivity in Cre-mediated targeting of floxed alleles using the Cre-Lox system. For the study of dopamine (DA) neurons, promoter sequences driving expression of the Dopamine transporter (Dat) gene are often implemented and several DAT-Cre transgenic mouse lines have been found to faithfully direct Cre activity to DA neurons. While evaluating an established DAT-Cre mouse line, reporter gene expression was unexpectedly identified in cell somas within the amygdala. To indiscriminately explore Cre activity in DAT-Cre transgenic lines, systematic whole-brain analysis of two DAT-Cre mouse lines was performed upon recombination with different types of floxed reporter alleles. Results were compared with data available from the Allen Institute for Brain Science. The results identified restricted DAT-Cre-driven reporter gene expression in cell clusters within several limbic areas, including amygdaloid and mammillary subnuclei, septum and habenula, areas classically associated with glutamatergic and GABAergic neurotransmission. While no Dat gene expression was detected, ample co-localization between DAT-Cre-driven reporter and markers for glutamatergic and GABAergic neurons was found. Upon viral injection of a fluorescent reporter into the amygdala and habenula, distinct projections from non-dopaminergic DAT-Cre neurons could be distinguished. The study demonstrates that DAT-Cre transgenic mice, beyond their usefulness in recombination of floxed alleles in DA neurons, could be implemented as tools to achieve selective targeting in restricted excitatory and inhibitory neuronal populations within the limbic neurocircuitry.

Neural regeneration by regionally induced stem cells within post-stroke brains: Novel therapy perspectives for stroke patients

Ischemic stroke is a critical disease which causes serious neurological functional loss such as paresis. Hope for novel therapies is based on the increasing evidence of the presence of stem cell populations in the central nervous system (CNS) and the development of stem-cell-based therapies for stroke patients. Although mesenchymal stem cells (MSCs) represented initially a promising cell source, only a few transplanted MSCs were present near the injured areas of the CNS. Thus, regional stem cells that are present and/or induced in the CNS may be ideal when considering a treatment following ischemic stroke. In this context, we have recently showed that injury/ischemia-induced neural stem/progenitor cells (iNSPCs) and injury/ischemia-induced multipotent stem cells (iSCs) are present within post-stroke human brains and post-stroke mouse brains. This indicates that iNSPCs/iSCs could be developed for clinical applications treating patients with stroke. The present study introduces the traits of mouse and human iNSPCs, with a focus on the future perspective for CNS regenerative therapies using novel iNSPCs/iSCs.

Resilience Is Associated With Larger Dentate Gyrus, While Suicide Decedents With Major Depressive Disorder Have Fewer Granule Neurons

BACKGROUND

Early life adversity (ELA) increases major depressive disorder (MDD) and suicide risk and potentially affects dentate gyrus (DG) plasticity. We reported smaller DG and fewer granular neurons (GNs) in MDD. ELA effects on DG plasticity in suicide decedents with MDD (MDDSui) and resilient subjects (ELA history without MDD or suicide) are unknown.

METHODS

We quantified neural progenitor cells (NPCs), GNs, glia, and DG volume in whole hippocampus postmortem in four groups of drug-free, neuropathology-free subjects (N = 52 total): psychological autopsy-defined MDDSui and control subjects with and without ELA (before 15 years of age).

RESULTS

ELA was associated with larger DG (p < .0001) and trending fewer NPCs (p = .0190) only in control subjects in whole DG, showing no effect on NPCs and DG volume in MDDSui. ELA exposure was associated with more GNs (p = .0003) and a trend for more glia (p = .0160) in whole DG in MDDSui and control subjects. MDDSui without ELA had fewer anterior and mid DG GNs (p < .0001), fewer anterior DG NPCs (p < .0001), and smaller whole DG volume (p = .0005) compared with control subjects without ELA. In MDDSui, lower Global Assessment Scale score correlated with fewer GNs and smaller DG.

CONCLUSIONS

Resilience to ELA involves a larger DG, perhaps related to more neurogenesis depleting NPCs, and because mature GNs and glia numbers do not differ in the resilient group, perhaps there are effects on process extension and synaptic load that can be examined in future studies. In MDDSui without ELA, smaller DG volume, with fewer GNs and NPCs, suggests less neurogenesis and/or more apoptosis and dendrite changes.

Dorsal and ventral hippocampal adult-born neurons contribute to context fear memory

The hippocampus contains one of the few neurogenic niches within the adult brain-the subgranular zone of the dentate gyrus. The functional significance of adult-born neurons in this region has been characterized using context fear conditioning, a Pavlovian paradigm in which animals learn to associate a location with danger. Ablation or silencing of adult-born neurons impairs both acquisition and recall of contextual fear conditioning, suggesting that these neurons contribute importantly to hippocampal memory. Lesion studies indicate that CFC depends on neural activity in both the dorsal and ventral hippocampus, subregions with unique extrahippocampal connectivity and behavioral functions. Because most studies of adult neurogenesis have relied on methods that permanently ablate neurogenesis throughout the entire hippocampus, little is known about how the function of adult-born neurons varies along the dorsal-ventral axis. Using a Nestin-CreER^T2 mouse line to target the optogenetic silencer Archaerhodopsin to adult-born neurons, we compared the contribution of dorsal and ventral adult-born neurons to acquisition, recall, and generalization of CFC. Acquisition of CFC was impaired when either dorsal or ventral adult-born neurons were silenced during training. Silencing dorsal or ventral adult-born neurons during test sessions decreased context-evoked freezing but did not impair freezing in a hippocampus-independent tone-shock freezing paradigm. Silencing adult-born neurons modestly reduced generalization of fear. Our data indicate that adult-born neurons in the dorsal and ventral hippocampus contribute to both memory acquisition and recall. The comparatively large behavioral effects of silencing a small number of adult-born neurons suggest that these neurons make a unique and powerful contribution to hippocampal function.

Radial glial cells in the adult dentate gyrus: what are they and where do they come from?

Adult neurogenesis occurs in the dentate gyrus in the mammalian hippocampus. These new neurons arise from neural precursor cells named radial glia-like cells, which are situated in the subgranular zone of the dentate gyrus. Here, we review the emerging topic of precursor heterogeneity in the adult subgranular zone. We also discuss how this heterogeneity may be established during development and focus on the embryonic origin of the dentate gyrus and radial glia-like stem cells. Finally, we discuss recently developed single-cell techniques, which we believe will be critical to comprehensively investigate adult neural stem cell origin and heterogeneity.

Bone Marrow Micro-Environment in Normal and Deranged Hematopoiesis: Opportunities for Regenerative Medicine and Therapies

Various cell types cooperate to create a highly organized and dynamic micro-environmental niche in the bone marrow. Over the past several years, the field has increasingly recognized the critical roles of the interplay between bone marrow environment and hematopoietic cells in normal and deranged hematopoiesis. These advances rely on several new technologies that have allowed us to characterize the identity and roles of these niches in great detail. Here, we review the progress of the last several years, list some of the outstanding questions in the field and propose ways to target the diseased environment to better treat hematologic diseases. Understanding the extrinsic regulation by the niche will help boost hematopoiesis for regenerative medicine. Based on natural development of hematologic malignancies, we propose that combinatory targeting the niche and hematopoietic intrinsic mechanisms in early stages of hematopoietic malignancies may help eliminate minimal residual disease and have the highest efficacy.

An Adeno-Associated Virus-Based Toolkit for Preferential Targeting and Manipulating Quiescent Neural Stem Cells in the Adult Hippocampus

Quiescent neural stem cells (qNSCs) with radial morphology are the only proven source of new neurons in the adult mammalian brain. Our understanding of the roles of newly generated neurons depends on the ability to target and manipulate adult qNSCs. Although various strategies have been developed to target and manipulate adult hippocampal qNSCs, they often suffer from prolonged breeding, low recombination efficiency, and non-specific labeling. Therefore, developing a readily manufactured viral vector that allows flexible packaging and robust expression of various transgenes in qNSCs is a pressing need. Here, we report a recombinant adeno-associated virus serotype 4 (rAAV4)-based toolkit that preferentially targets hippocampal qNSCs and allows for lineage tracing, functional analyses, and activity manipulation of adult qNSCs. Importantly, targeting qNSCs in a non-Cre-dependent fashion opens the possibility for studying qNSCs in less genetically tractable animal species and may have translational impact in gene therapy by preferentially targeting qNSCs.

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rAAV4 vectors preferentially target quiescent NSCs in the adult hippocampus

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rAAV4 vectors with distinct promoters reveal differential selectivity for radial NSCs

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rAAV4 allows for genetic manipulation and lineage tracing of quiescent NSCs

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rAAV4 allows for calcium imaging and activity manipulation of quiescent NSCs

Autophagy and Adult Neurogenesis: Discoveries Made Half a Century Ago Yet in their Infancy of being Connected

Within the brain, the physiological and pathological functions of autophagy in development and throughout the lifespan are being elucidated. This review summarizes recent in vitro and in vivo results that are defining the role of autophagy-related genes during the process of adult neurogenesis. We also discuss the need for future experiments to determine the molecular mechanism and functional significance of autophagy in the different neural stem cell populations and throughout the stages of adult neurogenesis.

Mouse Genetic Analysis of Bone Marrow Stem Cell Niches: Technological Pitfalls, Challenges, and Translational Considerations

The development of mouse genetic tools has made a significant contribution to the understanding of skeletal and hematopoietic stem cell niches in bone marrow (BM). However, many experimental designs (e.g., selections of marker genes, target vector constructions, and choices of reporter murine strains) have unavoidable technological limitations and bias, which lead to experimental discrepancies, data reproducibility issues, and frequent data misinterpretation. Consequently, there are a number of conflicting views relating to fundamental biological questions, including origins and locations of skeletal and hematopoietic stem cells in the BM. In this report, we systematically unravel complicated data interpretations via comprehensive analyses of technological benefits, pitfalls, and challenges in frequently used mouse models and discuss their translational relevance to human stem cell biology. Particularly, we emphasize the important roles of using large human genomic data-informatics in facilitating genetic analyses of mouse models and resolving existing controversies in mouse and human BM stem cell biology.

Ablation of Newly Generated Hippocampal Granule Cells Has Disease-Modifying Effects in Epilepsy

Hippocampal granule cells generated in the weeks before and after an epileptogenic brain injury can integrate abnormally into the dentate gyrus, potentially mediating temporal lobe epileptogenesis. Previous studies have demonstrated that inhibiting granule cell production before an epileptogenic brain insult can mitigate epileptogenesis. Here, we extend upon these findings by ablating newly generated cells after the epileptogenic insult using a conditional, inducible diphtheria-toxin receptor expression strategy in mice. Diphtheria-toxin receptor expression was induced among granule cells born up to 5 weeks before pilocarpine-induced status epilepticus and these cells were then eliminated beginning 3 d after the epileptogenic injury. This treatment produced a 50% reduction in seizure frequency, but also a 20% increase in seizure duration, when the animals were examined 2 months later. These findings provide the first proof-of-concept data demonstrating that granule cell ablation therapy applied at a clinically relevant time point after injury can have disease-modifying effects in epilepsy.

SIGNIFICANCE STATEMENT

These findings support the long-standing hypothesis that newly generated dentate granule cells are pro-epileptogenic and contribute to the occurrence of seizures. This work also provides the first evidence that ablation of newly generated granule cells can be an effective therapy when begun at a clinically relevant time point after an epileptogenic insult. The present study also demonstrates that granule cell ablation, while reducing seizure frequency, paradoxically increases seizure duration. This paradoxical effect may reflect a disruption of homeostatic mechanisms that normally act to reduce seizure duration, but only when seizures occur frequently.

Electrophysiological and gene expression characterization of the ontogeny of nestin-expressing cells in the adult mouse midbrain

The birth of new neurons, or neurogenesis, in the adult midbrain is important for progressing dopamine cell-replacement therapies for Parkinson's disease. Most studies suggest newborn cells remain undifferentiated or differentiate into glia within the adult midbrain. However, some studies suggest nestin+neural precursor cells (NPCs) have a propensity to generate new neurons here. We sought to confirm this by administering tamoxifen to adult NesCreER^T2/R26eYFP transgenic mice, which permanently labelled adult nestin-expressing cells and their progeny with enhanced yellow fluorescent protein (eYFP). eYFP+ midbrain cells were then characterized 1-32weeks later in acutely prepared brain slices using whole-cell patch clamp electrophysiology combined with single-cell RT-qPCR. Most eYFP+ cells exhibited a mature neuronal phenotype with large amplitude fast action potentials (APs), spontaneous post-synaptic currents (sPSCs), and expression of 'mature' neuronal genes (NeuN, Gad1, Gad2 and/or VGLUT2). This was the case even at the earliest time-point following tamoxifen (i.e. 1week). In comparison to neighboring eYFP- (control) cells, eYFP+ cells discharged more APs per unit current injection, and had faster AP time-to-peak, hyperpolarized resting membrane potential, smaller membrane capacitance and shorter duration sPSCs. eYFP+ cells were also differentiated from eYFP- cells by increased expression of 'immature' pro-neuronal genes (Pax6, Ngn2 and/or Msx1). However, further analyses failed to reveal evidence of a place of birth, neuronal differentiation, maturation and integration indicative of classical neurogenesis. Thus our findings do not support the notion that nestin+NPCs in the adult SNc and midbrain generate new neurons via classical neurogenesis. Rather, they raise the possibility that mature neurons express nestin under unknown circumstances, and that this is associated with altered physiology and gene expression.

Adult haematopoietic stem cell niches

Stem cell niches are specialized microenvironments that promote the maintenance of stem cells and regulate their function. Recent advances have improved our understanding of the niches that maintain adult haematopoietic stem cells (HSCs). These advances include new markers for HSCs and niche cells, systematic analyses of the expression patterns of niche factors, genetic tools for functionally identifying niche cells in vivo, and improved imaging techniques. Together, they have shown that HSC niches are perivascular in the bone marrow and spleen. Endothelial cells and mesenchymal stromal cells secrete factors that promote HSC maintenance in these niches, but other cell types also directly or indirectly regulate HSC niches.

USP9X deletion elevates the density of oligodendrocytes within the postnatal dentate gyrus

Neural stem cells (NSCs) within the adult hippocampal dentate gyrus reside in the subgranular zone (SGZ). A dynamic network of signaling mechanisms controls the balance between the maintenance of NSC identity, and their subsequent differentiation into dentate granule neurons. Recently, the ubiquitin-specific protease 9 X-linked (USP9X) was shown to be important for hippocampal morphogenesis, as mice lacking this gene exhibited a higher proportion of proliferating NSCs, yet a decrease in neuronal numbers, within the postnatal dentate gyrus. Here we reveal that Usp9x-deficiency results in the upregulation of numerous oligodendrocytic and myelin-associated genes within the postnatal hippocampus. Moreover, cell counts reveal a significant increase in oligodendrocyte precursor cells and mature oligodendrocytes per unit volume of the mutant dentate gyrus. Collectively, these findings indicate that USP9X may regulate NSC lineage determination within the postnatal SGZ.

Evaluation of Nestin Expression in the Developing and Adult Mouse Inner Ear

Adult stem cells are undifferentiated cells with the capacity to proliferate and form mature tissue-specific cell types. Nestin is an intermediate filament protein used to identify cells with stem cell characteristics. Its expression has been observed in a population of cells in developing and adult cochleae. In vitro studies using rodent cochlear tissue have documented the potential of nestin-expressing cells to proliferate and form hair and supporting cells. In this study, nestin coupled to green fluorescent protein (GFP) transgenic mice were used to provide a more complete characterization of the spatial and temporal expression of nestin in the inner ear, from organogenesis to adulthood. During development, nestin is expressed in the spiral ganglion cell region and in multiple cell types in the organ of Corti, including nascent hair and supporting cells. In adulthood, its expression is reduced but persists in the spiral ganglion, in a cell population medial to and below the inner hair cells, and in Deiters' cells in the cochlear apex. Moreover, nestin-expressing cells can proliferate in restricted regions of the inner ear during development shown by coexpression with Ki67 and MCM2 and by 5-ethynyl-2'-deoxyuridine incorporation. Results suggest that nestin may label progenitor cells during inner ear development and may not be a stem cell marker in the mature organ of Corti; however, nestin-positive cells in the spiral ganglion exhibit some stem cell characteristics. Future studies are necessary to determine if these cells possess any latent stem cell-like qualities that may be targeted as a regenerative approach to treat neuronal forms of hearing loss.

Genetic tools for identifying and manipulating fibroblasts in the mouse

The use of mouse genetic tools to track and manipulate fibroblasts has provided invaluable in vivo information regarding the activities of these cells. Recently, many new mouse strains have been described for the specific purpose of studying fibroblast behavior. Colorimetric reporter mice and lines expressing Cre are available for the study of fibroblasts in the organs prone to fibrosis, including heart, kidney, liver, lung, and skeletal muscle. In this review we summarize the current state of the models that have been used to define tissue resident fibroblast populations. While these complex genetic reagents provide unique insights into the process of fibrosis, they also require a thorough understanding of the caveats and limitations. Here, we discuss the specificity and efficiency of the available genetic models and briefly describe how they have been used to document the mechanisms of fibrosis.

Transgenic mouse models for studying adult neurogenesis

The mammalian hippocampus shows a remarkable capacity for continued neurogenesis throughout life. Newborn neurons, generated by the radial neural stem cells (NSCs), are important for learning and memory as well as mood control. During aging, the number and responses of NSCs to neurogenic stimuli diminish, leading to decreased neurogenesis and age-associated cognitive decline and psychiatric disorders. Thus, adult hippocampal neurogenesis has garnered significant interest because targeting it could be a novel potential therapeutic strategy for these disorders. However, if we are to use neurogenesis to halt or reverse hippocampal-related pathology, we need to understand better the core molecular machinery that governs NSC and their progeny. In this review, we summarize a wide variety of mouse models used in adult neurogenesis field, present their advantages and disadvantages based on specificity and efficiency of labeling of different cell types, and review their contribution to our understanding of the biology and the heterogeneity of different cell types found in adult neurogenic niches.

Activation of local inhibitory circuits in the dentate gyrus by adult-born neurons

Robust incorporation of new principal cells into pre-existing circuitry in the adult mammalian brain is unique to the hippocampal dentate gyrus (DG). We asked if adult-born granule cells (GCs) might act to regulate processing within the DG by modulating the substantially more abundant mature GCs. Optogenetic stimulation of a cohort of young adult-born GCs (0 to 7 weeks post-mitosis) revealed that these cells activate local GABAergic interneurons to evoke strong inhibitory input to mature GCs. Natural manipulation of neurogenesis by aging-to decrease it-and housing in an enriched environment-to increase it-strongly affected the levels of inhibition. We also demonstrated that elevating activity in adult-born GCs in awake behaving animals reduced the overall number of mature GCs activated by exploration. These data suggest that inhibitory modulation of mature GCs may be an important function of adult-born hippocampal neurons. © 2015 Wiley Periodicals, Inc.

GluN2B-Containg NMDA Receptors on Adult-Born Granule Cells Contribute to the Antidepressant Action of Fluoxetine

Ablation of adult neurogenesis in mice has revealed that young adult-born granule cells (abGCs) are required for some of the behavioral responses to antidepressants (ADs), yet the mechanism by which abGCs contribute to AD action remains unknown. During their maturation process, these immature neurons exhibit unique properties that could underlie their ability to influence behavioral output. In particular, abGCs in the DG exhibit a period of heightened plasticity 4–6 weeks after birth that is mediated by GluN2B-expressing NMDA receptors. The functional contribution of this critical window to AD responsiveness is unclear. Here, we determined the behavioral and neurogenic responses to the AD fluoxetine (FLX) in mice lacking GluN2B-containing NMDA receptors in abGCs. We found that these mice exhibited an attenuated response to FLX in a neurogenesis-dependent behavioral assay of FLX action, while neurogenesis-independent behaviors were unaffected by GluN2B deletion. In addition, deletion of GluN2B attenuated FLX-induced increases in dendritic complexity of abGCs suggesting that the blunted behavioral efficacy of FLX may be caused by impaired differentiation of young abGCs.

Usp9x-deficiency disrupts the morphological development of the postnatal hippocampal dentate gyrus

Within the adult mammalian brain, neurogenesis persists within two main discrete locations, the subventricular zone lining the lateral ventricles, and the hippocampal dentate gyrus. Neurogenesis within the adult dentate gyrus contributes to learning and memory, and deficiencies in neurogenesis have been linked to cognitive decline. Neural stem cells within the adult dentate gyrus reside within the subgranular zone (SGZ), and proteins intrinsic to stem cells, and factors within the niche microenvironment, are critical determinants for development and maintenance of this structure. Our understanding of the repertoire of these factors, however, remains limited. The deubiquitylating enzyme USP9X has recently emerged as a mediator of neural stem cell identity. Furthermore, mice lacking Usp9x exhibit a striking reduction in the overall size of the adult dentate gyrus. Here we reveal that the development of the postnatal SGZ is abnormal in mice lacking Usp9x. Usp9x conditional knockout mice exhibit a smaller hippocampus and shortened dentate gyrus blades from as early as P7. Moreover, the analysis of cellular populations within the dentate gyrus revealed reduced stem cell, neuroblast and neuronal numbers and abnormal neuroblast morphology. Collectively, these findings highlight the critical role played by USP9X in the normal morphological development of the postnatal dentate gyrus.

Low excitatory innervation balances high intrinsic excitability of immature dentate neurons

Persistent neurogenesis in the dentate gyrus produces immature neurons with high intrinsic excitability and low levels of inhibition that are predicted to be more broadly responsive to afferent activity than mature neurons. Mounting evidence suggests that these immature neurons are necessary for generating distinct neural representations of similar contexts, but it is unclear how broadly responsive neurons help distinguish between similar patterns of afferent activity. Here we show that stimulation of the entorhinal cortex in mouse brain slices paradoxically generates spiking of mature neurons in the absence of immature neuron spiking. Immature neurons with high intrinsic excitability fail to spike due to insufficient excitatory drive that results from low innervation rather than silent synapses or low release probability. Our results suggest that low synaptic connectivity prevents immature neurons from responding broadly to cortical activity, potentially enabling excitable immature neurons to contribute to sparse and orthogonal dentate representations.

Immature dentate gyrus neurons are highly excitable and are thought to be more responsive to afferent activity than mature neurons. Here, the authors find stimulation of the entorhinal cortex paradoxically generates spiking in mature rather than immature neurons due to low synaptic connectivity of immature cells.

Conditional Disabled-1 Deletion in Mice Alters Hippocampal Neurogenesis and Reduces Seizure Threshold

Many animal models of temporal lobe epilepsy (TLE) exhibit altered neurogenesis arising from progenitors within the dentate gyrus subgranular zone (SGZ). Aberrant integration of new neurons into the existing circuit is thought to contribute to epileptogenesis. In particular, adult-born neurons that exhibit ectopic migration and hilar basal dendrites (HBDs) are suggested to be pro-epileptogenic. Loss of reelin signaling may contribute to these morphological changes in patients with epilepsy. We previously demonstrated that conditional deletion of the reelin adaptor protein, disabled-1 (Dab1), from postnatal mouse SGZ progenitors generated dentate granule cells (DGCs) with abnormal dendritic development and ectopic placement. To determine whether the early postnatal loss of reelin signaling is epileptogenic, we conditionally deleted Dab1 in neural progenitors and their progeny on postnatal days 7–8 and performed chronic video-EEG recordings 8–10 weeks later. Dab1-deficient mice did not have spontaneous seizures but exhibited interictal epileptiform abnormalities and a significantly reduced latency to pilocarpine-induced status epilepticus. After chemoconvulsant treatment, over 90% of mice deficient for Dab1 developed generalized motor convulsions with tonic-clonic movements, rearing, and falling compared to <20% of wild-type mice. Recombination efficiency, measured by Cre reporter expression, inversely correlated with time to the first sustained seizure. These pro-epileptogenic changes were associated with decreased neurogenesis and increased numbers of hilar ectopic DGCs. Interestingly, neurons co-expressing the Cre reporter comprised a fraction of these hilar ectopic DGCs cells, suggesting a non-cell autonomous effect for the loss of reelin signaling. We also noted a dispersion of the CA1 pyramidal layer, likely due to hypomorphic effects of the conditional Dab1 allele, but this abnormality did not correlate with seizure susceptibility. These findings suggest that the misplacement or reduction of postnatally-generated DGCs contributes to aberrant circuit development and hyperexcitability, but aberrant neurogenesis after conditional Dab1 deletion alone is not sufficient to produce spontaneous seizures.

Role of the Retinoblastoma protein, Rb, during adult neurogenesis in the olfactory bulb

Adult neural stem cells (aNSCs) are relatively quiescent populations that give rise to distinct neuronal subtypes throughout life, yet, at a very low rate and restricted differentiation potential. Thus, identifying the molecular mechanisms that control their cellular expansion is critical for regeneration after brain injury. Loss of the Retinoblastoma protein, Rb, leads to several defects in cell cycle as well as neuronal differentiation and migration during brain development. Here, we investigated the role of Rb during adult neurogenesis in the olfactory bulb (OB) by inducing its temporal deletion in aNSCs and progenitors. Loss of Rb was associated with increased proliferation of adult progenitors in the subventricular zone (SVZ) and the rostral migratory stream (RMS) but did not alter self-renewal of aNSCs or neuroblasts subsequent migration and terminal differentiation. Hence, one month after their birth, Rb-null neuroblasts were able to differentiate into distinct subtypes of GABAergic OB interneurons but were gradually lost after 3 months. Similarly, Rb controlled aNSCs/progenitors proliferation in vitro without affecting their differentiation capacity. This enhanced SVZ/OB neurogenesis associated with loss of Rb was only transient and negatively affected by increased apoptosis indicating a critical requirement for Rb in the long-term survival of adult-born OB interneurons.

Stress-Induced Anxiety- and Depressive-Like Phenotype Associated with Transient Reduction in Neurogenesis in Adult Nestin-CreERT2/Diphtheria Toxin Fragment A Transgenic Mice

Depression and anxiety involve hippocampal dysfunction, but the specific relationship between these mood disorders and adult hippocampal dentate gyrus neurogenesis remains unclear. In both humans with MDD and rodent models of depression, administration of antidepressants increases DG progenitor and granule cell number, yet rodents with induced ablation of DG neurogenesis typically do not demonstrate depressive- or anxiety-like behaviors. The conflicting data may be explained by the varied duration and degree to which adult neurogenesis is reduced in different rodent neurogenesis ablation models. In order to test this hypothesis we examined how a transient–rather than permanent–inducible reduction in neurogenesis would alter depressive- and anxiety-like behaviors. Transgenic Nestin-CreER^T2/floxed diphtheria toxin fragment A (DTA) mice (Cre+DTA+) and littermates (Cre+DTA-; control) were given tamoxifen (TAM) to induce recombination and decrease nestin-expressing stem cells and their progeny. The decreased neurogenesis was transient: 12 days post-TAM Cre+DTA+ mice had fewer DG proliferating Ki67+ cells and fewer DCX+ neuroblasts/immature neurons relative to control, but 30 days post-TAM Cre+DTA+ mice had the same DCX+ cell number as control. This ability of DG neurogenesis to recover after partial ablation also correlated with changes in behavior. Relative to control, Cre+DTA+ mice tested between 12–30 days post-TAM displayed indices of a stress-induced anxiety phenotype–longer latency to consume highly palatable food in the unfamiliar cage in the novelty-induced hypophagia test, and a depression phenotype–longer time of immobility in the tail suspension test, but Cre+DTA+ mice tested after 30 days post-TAM did not. These findings suggest a functional association between adult neurogenesis and stress induced anxiety- and depressive-like behaviors, where induced reduction in DCX+ cells at the time of behavioral testing is coupled with stress-induced anxiety and a depressive phenotype, and recovery of DCX+ cell number corresponds to normalization of these behaviors.

Long-Term Fate Mapping Using Conditional Lentiviral Vectors Reveals a Continuous Contribution of Radial Glia-Like Cells to Adult Hippocampal Neurogenesis in Mice

Newborn neurons are generated throughout life in two neurogenic regions, the subventricular zone and the hippocampal dentate gyrus. Stimulation of adult neurogenesis is considered as an attractive endogenous repair mechanism to treat different neurological disorders. Although tremendous progress has been made in our understanding of adult hippocampal neurogenesis, important questions remain unanswered, regarding the identity and the behavior of neural stem cells in the dentate gyrus. We previously showed that conditional Cre-Flex lentiviral vectors can be used to label neural stem cells in the subventricular zone and to track the migration of their progeny with non-invasive bioluminescence imaging. Here, we applied these Cre-Flex lentiviral vectors to study neurogenesis in the dentate gyrus with bioluminescence imaging and histological techniques. Stereotactic injection of the Cre-Flex vectors into the dentate gyrus of transgenic Nestin-Cre mice resulted in specific labeling of the nestin-positive neural stem cells. The labeled cell population could be detected with bioluminescence imaging until 9 months post injection, but no significant increase in the number of labeled cells over time was observed with this imaging technique. Nevertheless, the specific labeling of the nestin-positive neural stem cells, combined with histological analysis at different time points, allowed detailed analysis of their neurogenic potential. This long-term fate mapping revealed that a stable pool of labeled nestin-positive neural stem cells continuously contributes to the generation of newborn neurons in the mouse brain until 9 months post injection. In conclusion, the Cre-Flex technology is a valuable tool to address remaining questions regarding neural stem cell identity and behavior in the dentate gyrus.

Chromatin Remodeling Factor Brg1 Supports the Early Maintenance and Late Responsiveness of Nestin-Lineage Adult Neural Stem and Progenitor Cells

Insights from embryonic development suggest chromatin remodeling is important in adult neural stem cells (aNSCs) maintenance and self-renewal, but this concept has not been fully explored in the adult brain. To assess the role of chromatin remodeling in adult neurogenesis, we inducibly deleted Brg1--the core subunit of SWI/SNF-like Brg1/Brm-associated factor chromatin remodeling complexes--in nestin-expressing aNSCs and their progeny in vivo and in culture. This resulted in abnormal adult neurogenesis in the hippocampus, which initially reduced hippocampal aNSCs and progenitor maintenance, and later reduced its responsiveness to physiological stimulation. Mechanistically, deletion of Brg1 appeared to impair cell cycle progression, which is partially due to elevated p53 pathway and p21 expression. Knockdown of p53 rescued the neurosphere growth defects caused by Brg1 deletion. Our results show that epigenetic chromatin remodeling (via a Brg1 and p53/p21-dependent process) determines the aNSCs and progenitor maintenance and responsiveness of neurogenesis.

Adult Neural Stem Cells from the Subventricular Zone Give Rise to Reactive Astrocytes in the Cortex after Stroke

Reactive astrocytes (RAs) have been reported to convert to multipotent neural stem cells (NSCs) capable of neurosphere (NS) formation and multilineage differentiation in vitro. Using genetic tagging, we determined that subventricular zone (SVZ) NSCs give rise to NSs derived from the stroke-injured cortex. We demonstrate that these cells can be isolated from the cortex in two different models of stroke and from different stroke-lesioned cortical regions. Interestingly, SVZ NSCs give rise to a subpopulation of RAs in the cortex that contribute to astrogliosis and scar formation. Last, we show that these SVZ derived RAs can be converted to neurons in vivo by forced expression of Ascl1. Identifying the contribution of cells originating from the SVZ to injury repair has implications for neural regeneration strategies.

Increasing Adult Hippocampal Neurogenesis is Sufficient to Reduce Anxiety and Depression-Like Behaviors

Adult hippocampal neurogenesis is increased by antidepressants, and is required for some of their behavioral effects. However, it remains unclear whether expanding the population of adult-born neurons is sufficient to affect anxiety and depression-related behavior. Here, we use an inducible transgenic mouse model in which the pro-apoptotic gene Bax is deleted from neural stem cells and their progeny in the adult brain, and thereby increases adult neurogenesis. We find no effects on baseline anxiety and depression-related behavior; however, we find that increasing adult neurogenesis is sufficient to reduce anxiety and depression-related behaviors in mice treated chronically with corticosterone (CORT), a mouse model of stress. Thus, neurogenesis differentially affects behavior under baseline conditions and in a model of chronic stress. Moreover, we find no effect of increased adult hippocampal neurogenesis on hypothalamic-pituitary-adrenal (HPA) axis regulation, either at baseline or following chronic CORT administration, suggesting that increasing adult hippocampal neurogenesis can affect anxiety and depression-related behavior through a mechanism independent of the HPA axis. The use of future techniques to specifically inhibit BAX in the hippocampus could be used to augment adult neurogenesis, and may therefore represent a novel strategy to promote antidepressant-like behavioral effects.

Purification of neural precursor cells reveals the presence of distinct, stimulus-specific subpopulations of quiescent precursors in the adult mouse hippocampus

The activity of neural precursor cells in the adult hippocampus is regulated by various stimuli; however, whether these stimuli regulate the same or different precursor populations remains unknown. Here, we developed a novel cell-sorting protocol that allows the purification to homogeneity of neurosphere-forming neural precursors from the adult mouse hippocampus and examined the responsiveness of individual precursors to various stimuli using a clonal assay. We show that within the Hes5-GFP(+)/Nestin-GFP(+)/EGFR(+) cell population, which comprises the majority of neurosphere-forming precursors, there are two distinct subpopulations of quiescent precursor cells, one directly activated by high-KCl depolarization, and the other activated by norepinephrine (NE). We then demonstrate that these two populations are differentially distributed along the septotemporal axis of the hippocampus, and show that the NE-responsive precursors are selectively regulated by GABA, whereas the KCl-responsive precursors are selectively modulated by corticosterone. Finally, based on RNAseq analysis by deep sequencing, we show that the progeny generated by activating NE-responsive versus KCl-responsive quiescent precursors are molecularly different. These results demonstrate that the adult hippocampus contains phenotypically similar but stimulus-specific populations of quiescent precursors, which may give rise to neural progeny with different functional capacity.