Diane C. Lagace

Molecular adaptations underlying susceptibility and resistance to social defeat in brain reward regions.

While stressful life events are an important cause of psychopathology, most individuals exposed to adversity maintain normal psychological functioning. The molecular mechanisms underlying such resilience are poorly understood. Here, we demonstrate that an inbred population of mice subjected to social defeat can be separated into susceptible and unsusceptible subpopulations that differ along several behavioral and physiological domains. By a combination of molecular and electrophysiological techniques, we identify signature adaptations within the mesolimbic dopamine circuit that are uniquely associated with vulnerability or insusceptibility. We show that molecular recapitulations of three prototypical adaptations associated with the unsusceptible phenotype are each sufficient to promote resistant behavior. Our results validate a multidisciplinary approach to examine the neurobiological mechanisms of variations in stress resistance, and illustrate the importance of plasticity within the brains reward circuits in actively maintaining an emotional homeostasis.

Dynamic Contribution of Nestin-Expressing Stem Cells to Adult Neurogenesis

Understanding the fate of adult-generated neurons and the mechanisms that influence them requires consistent labeling and tracking of large numbers of stem cells. We generated a nestin-CreERT2/R26R-yellow fluorescent protein (YFP) mouse to inducibly label nestin-expressing stem cells and their progeny in the adult subventricular zone (SVZ) and subgranular zone (SGZ). Several findings show that the estrogen ligand tamoxifen (TAM) specifically induced recombination in stem cells and their progeny in nestin-CreERT2/R26R-YFP mice: 97% of SGZ stem-like cells (GFAP/Sox2 with radial glial morphology) expressed YFP; YFP+ neurospheres could be generated in vitro after recombination in vivo, and maturing YFP+ progeny were increasingly evident in the olfactory bulb (OB) and dentate gyrus (DG) granule cell layer. Revealing an unexpected regional dissimilarity in adult neurogenesis, YFP+ cells accumulated up to 100 d after TAM in the OB, but in the SGZ, YFP+ cells reached a plateau 30 d after TAM. In addition, most SVZ and SGZ YFP+ cells became neurons, underscoring a link between nestin and neuronal fate. Finally, quantification of YFP+ cells in nestin-CreERT2/R26R-YFP mice allowed us to estimate, for example, that stem cells and their progeny contribute to no more than 1% of the adult DG granule cell layer. In addition to revealing the dynamic contribution of nestin-expressing stem cells to adult neurogenesis, this work highlights the utility of the nestin-CreERT2/R26R-YFP mouse for inducible gene ablation in stem cells and their progeny in vivo in the two major regions of adult neurogenesis.

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.

Adult hippocampal neurogenesis is functionally important for stress-induced social avoidance

The long-term response to chronic stress is variable, with some individuals developing maladaptive functioning, although other “resilient” individuals do not. Stress reduces neurogenesis in the dentate gyrus subgranular zone (SGZ), but it is unknown if stress-induced changes in neurogenesis contribute to individual vulnerability. Using a chronic social defeat stress model, we explored whether the susceptibility to stress-induced social avoidance was related to changes in SGZ proliferation and neurogenesis. Immediately after social defeat, stress-exposed mice (irrespective of whether they displayed social avoidance) had fewer proliferating SGZ cells labeled with the S-phase marker BrdU. The decrease was transient, because BrdU cell numbers were normalized 24 h later. The survival of BrdU cells labeled before defeat stress was also not altered. However, 4 weeks later, mice that displayed social avoidance had more surviving dentate gyrus neurons. Thus, dentate gyrus neurogenesis is increased after social defeat stress selectively in mice that display persistent social avoidance. Supporting a functional role for adult-generated dentate gyrus neurons, ablation of neurogenesis via cranial ray irradiation robustly inhibited social avoidance. These data show that the time window after cessation of stress is a critical period for the establishment of persistent cellular and behavioral responses to stress and that a compensatory enhancement in neurogenesis is related to the long-term individual differences in maladaptive responses to stress.

Ascl1 defines sequentially generated lineage-restricted neuronal and oligodendrocyte precursor cells in the spinal cord

The neural basic helix-loop-helix transcription factor Ascl1 (previously Mash1) is present in ventricular zone cells in restricted domains throughout the developing nervous system. This study uses genetic fate mapping to define the stage and neural lineages in the developing spinal cord that are derived from Ascl1-expressing cells. We find that Ascl1 is present in progenitors to both neurons and oligodendrocytes, but not astrocytes. Temporal control of the fate-mapping paradigm reveals rapid cell-cycle exit and differentiation of Ascl1-expressing cells. At embryonic day 11, Ascl1 identifies neuronal-restricted precursor cells that become dorsal horn neurons in the superficial laminae. By contrast, at embryonic day 16, Ascl1 identifies oligodendrocyte-restricted precursor cells that distribute throughout the spinal cord. These data demonstrate that sequentially generated Ascl1-expressing progenitors give rise first to dorsal horn interneurons and subsequently to late-born oligodendrocytes. Furthermore, Ascl1-null cells in the spinal cord have a diminished capacity to undergo neuronal differentiation, with a subset of these cells retaining characteristics of immature glial cells.

Cdk5 is essential for adult hippocampal neurogenesis

The molecular factors regulating adult neurogenesis must be understood to harness the therapeutic potential of neuronal stem cells. Although cyclin-dependent kinase 5 (Cdk5) plays a critical role in embryonic corticogenesis, its function in adult neurogenesis is unknown. Here, we assessed the role of Cdk5 in the generation of dentate gyrus (DG) granule cell neurons in adult mice. Cre recombinase-mediated conditional knockout (KO) of Cdk5 from stem cells and their progeny in the DG subgranular zone (SGZ) prevented maturation of new neurons. In addition, selective KO of Cdk5 from mature neurons throughout the hippocampus reduced the number of immature neurons. Furthermore, Cdk5 gene deletion specifically from DG granule neurons via viral-mediated gene transfer also resulted in fewer immature neurons. In each case, the total number of proliferating cells was unaffected, indicating that Cdk5 is necessary for progression of adult-generated neurons to maturity. This role for Cdk5 in neurogenesis was activating-cofactor specific, as p35 KO but not p39 KO mice also had fewer immature neurons. Thus, Cdk5 has an essential role in the survival, but not proliferation, of adult-generated hippocampal neurons through both cell-intrinsic and cell-extrinsic mechanisms.

Focal cerebral ischemia induces a multilineage cytogenic response from adult subventricular zone that is predominantly gliogenic

The purpose of this study was to ascertain the relative contribution of neural stem/progenitor cells (NSPCs) of the subventricular zone (SVZ) to lineages that repopulate the injured striatum following focal ischemia. We utilized a tamoxifen‐inducible Cre/loxP system under control of the nestin promoter, which provides permanent YFP labeling of multipotent nestin+ SVZ‐NSPCs prior to ischemic injury and continued YFP expression in all subsequent progeny following stroke. YFP reporter expression was induced in adult male nestin‐CreERT2:R26R‐YFP mice by tamoxifen administration (180 mg kg−1, daily for 5 days). Fourteen days later, mice were subjected to 60‐min transient middle cerebral artery occlusion (MCAO) and sacrificed at 2 days, 2 weeks, or 6 weeks post‐MCAO for phenotypic fate mapping of YFP+ cells using lineage‐specific markers. Migration of YFP+ cells from SVZ into the injured striatal parenchyma was apparent at 2 and 6 weeks, but not 2 days, post‐MCAO. At 2 weeks post‐MCAO, the average percent distribution of YFP+ cells within the injured striatal parenchyma was as follows: 10% Dcx+ neuroblasts, 15–20% oligodendrocyte progenitors, 59% GFAP+ astrocytes, and only rare NeuN+ postmitotic neurons. A similar phenotypic distribution was observed at 6 weeks, except for an increased average percentage of YFP+ cells that expressed Dcx+ (20%) or NeuN (5%). YFP+ cells did not express endothelial markers, but displayed unique anatomical relationships with striatal vasculature. These results indicate that nestin+ NSPCs within the SVZ mount a multilineage response to stroke that includes a gliogenic component more predominant than previously appreciated.

Juvenile Administration of Methylphenidate Attenuates Adult Hippocampal Neurogenesis

BACKGROUND The neural consequences of early-life exposure to methylphenidate (MPH; Ritalin) are of great interest given the widespread, and sometimes inappropriate, use in children. Here we examine the impact of juvenile MPH exposure on adult hippocampal neurogenesis. METHODS Rats received MPH (2.0 mg/kg, intraperitoneal, twice daily) or saline (SAL) during preadolescence (postnatal days 20-35). Hippocampal cell proliferation (Experiment 1), neurogenesis (Experiment 2), and stress-induced changes in cell proliferation (Experiment 3) were assessed at several developmental stages including adulthood. RESULTS Juvenile exposure to MPH did not alter proliferation at any developmental time point relative to control rats; however, exposure to MPH significantly decreased the long-term survival of newborn cells in adult rats, particularly in the temporal hippocampus. Although MPH-treated rats had higher levels of corticosterone after restraint stress, they did not show the expected greater decrease in hippocampal cell proliferation relative to control animals. CONCLUSIONS Early-life exposure to MPH inhibits the survival of adult-generated neurons in the temporal hippocampus and may reduce progenitor sensitivity to corticosterone-induced decreases in proliferation. These findings suggest that decreased adult neurogenesis is an enduring consequence of early-life exposure to MPH and are discussed for their relevance to humans.

Making a neuron: Cdk5 in embryonic and adult neurogenesis

Cyclin-dependent kinase 5 (Cdk5) has been implicated in the migration, maturation and survival of neurons born during embryonic development. New evidence suggests that Cdk5 has comparable but also distinct functions in adult neurogenesis. Here we summarize accumulating evidence on the role of Cdk5 in regulation of the cell cycle, migration, survival, maturation and neuronal integration. We specifically highlight the many similarities and few tantalizing differences in the roles of Cdk5 in the embryonic and adult brain. We discuss the signaling pathways that might contribute to Cdk5 action in regulating embryonic and adult neurogenesis, highlighting future research directions that will help to clarify the mechanisms underlying lifelong neurogenesis in the mammalian brain.

In vivo contribution 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 heterogeneity.