Klaus Fabel

Quiescent and Active Hippocampal Neural Stem Cells with Distinct Morphologies Respond Selectively to Physiological and Pathological Stimuli and Aging

New neurons are generated in the adult hippocampus throughout life by neural stem/progenitor cells (NSCs), and neurogenesis is a plastic process responsive to external stimuli. We show that canonical Notch signaling through RBP-J is required for hippocampal neurogenesis. Notch signaling distinguishes morphologically distinct Sox2(+) NSCs, and within these pools subpopulations can shuttle between mitotically active or quiescent. Radial and horizontal NSCs respond selectively to neurogenic stimuli. Physical exercise activates the quiescent radial population whereas epileptic seizures induce expansion of the horizontal NSC pool. Surprisingly, reduced neurogenesis correlates with a loss of active horizontal NSCs in aged mice rather than a total loss of stem cells, and the transition to a quiescent state is reversible to rejuvenate neurogenesis in the brain. The discovery of multiple NSC populations with Notch dependence but selective responses to stimuli and reversible quiescence has important implications for the mechanisms of adaptive learning and also for regenerative therapy.

Additive effects of physical exercise and environmental enrichment on adult hippocampal neurogenesis in mice.

Voluntary physical exercise (wheel running, RUN) and environmental enrichment both stimulate adult hippocampal neurogenesis but do so by different mechanisms. RUN induces precursor cell proliferation, whereas ENR exerts a survival-promoting effect on newborn cells. In addition, continued RUN prevented the physiologically occurring age-related decline in precursor cell in the dentate gyrus but did not lead to a corresponding increase in net neurogenesis. We hypothesized that in the absence of appropriate cognitive stimuli the potential for neurogenesis could not be realized but that an increased potential by proliferating precursor cells due to RUN could actually lead to more adult neurogenesis if an appropriate survival-promoting stimulus follows the exercise. We thus asked whether a sequential combination of RUN and ENR (RUNENR) would show additive effects that are distinct from the application of either paradigm alone. We found that the effects of 10 days of RUN followed by 35 days of ENR were additive in that the combined stimulation yielded an approximately 30% greater increase in new neurons than either stimulus alone, which also increased neurogenesis. Surprisingly, this result indicates that although overall the amount of proliferating cells in the dentate gyrus is poorly predictive of net adult neurogenesis, an increased neurogenic potential nevertheless provides the basis for a greater efficiency of the same survival-promoting stimulus. We thus propose that physical activity can “prime” the neurogenic region of the dentate gyrus for increased neurogenesis in the case the animal is exposed to an additional cognitive stimulus, here represented by the enrichment paradigm.

The contribution of failing adult hippocampal neurogenesis to psychiatric disorders.

Purpose of review Failing adult neurogenesis is increasingly considered a factor in the pathogenesis and course of psychiatric disorders. The level of evidence in favor of such hypotheses varies, but disturbed cellular plasticity in the hippocampus may be a common aspect of several neuropsychiatric diseases. Recent findings This review covers the literature from mid-2006 to the end of 2007. We discuss studies and theoretical papers dealing with the contribution of adult neurogenesis to dementias and neurodegeneration, major depression, schizophrenia, and alcohol and drug abuse. Of these disorders, most progress has recently been made with schizophrenia for which, in contrast to the other conditions, suggestive genetic evidence exists (e.g. Disc1, Npas3). Summary Failing adult hippocampal neurogenesis may not explain major depression, addiction or schizophrenia, but contributes to the hippocampal aspects of the disease. We propose that the key to a more thorough understanding of this contribution will come from increased knowledge on the functional relevance of new neurons in the hippocampus and better clinical data relating to symptoms possibly related to such function. Research on the molecular basis of adult hippocampal neurogenesis may help to explain how hippocampal aspects of these disorders develop.

Physical Activity and the Regulation of Neurogenesis in the Adult and Aging Brain

The discovery that exercise regulates adult hippocampal neurogenesis, that is, the production of new neurons in the adult brain, was surprising news and changed quite fundamentally our view on how physical activity affects the brain. The everyday experience that not all athletes are necessarily smarter than more sedentary fellows and the scientific insight that adult hippocampal neurogenesis is actually a process that ranges on a very small scale raised important questions on the relevance of this finding. We propose that the exercise-related regulation of adult hippocampal neurogenesis is a qualitative rather than a quantitative event and that it is a particularly prominent and suggestive example of activity-dependent cellular plasticity. For rodents, the animals, in which most of this research has been done, cognition is almost inseparable from locomotion. Physical activity, especially exerted over longer periods of time, might indicate to the brain an increased chance of experience those situations rich in complexity and novelty that presumably benefit from more new neurons. We thus propose that it is not isolated physical activity that is “good for the brain”, but physical activity in the context of cognitive challenges. This would also explain why few new neurons could be beneficial for successful aging. We here review the current stage of the knowledge how this exercise-induced regulation of neurogenesis might work.

Cannabinoid receptor CB1 mediates baseline and activity-induced survival of new neurons in adult hippocampal neurogenesis

BackgroundAdult neurogenesis is a particular example of brain plasticity that is partially modulated by the endocannabinoid system. Whereas the impact of synthetic cannabinoids on the neuronal progenitor cells has been described, there has been lack of information about the action of plant-derived extracts on neurogenesis. Therefore we here focused on the effects of Δ9-tetrahydrocannabinol (THC) and Cannabidiol (CBD) fed to female C57Bl/6 and Nestin-GFP-reporter mice on proliferation and maturation of neuronal progenitor cells and spatial learning performance. In addition we used cannabinoid receptor 1 (CB1) deficient mice and treatment with CB1 antagonist AM251 in Nestin-GFP-reporter mice to investigate the role of the CB1 receptor in adult neurogenesis in detail.ResultsTHC and CBD differed in their effects on spatial learning and adult neurogenesis. CBD did not impair learning but increased adult neurogenesis, whereas THC reduced learning without affecting adult neurogenesis. We found the neurogenic effect of CBD to be dependent on the CB1 receptor, which is expressed over the whole dentate gyrus. Similarly, the neurogenic effect of environmental enrichment and voluntary wheel running depends on the presence of the CB1 receptor. We found that in the absence of CB1 receptors, cell proliferation was increased and neuronal differentiation reduced, which could be related to CB1 receptor mediated signaling in Doublecortin (DCX)-expressing intermediate progenitor cells.ConclusionCB1 affected the stages of adult neurogenesis that involve intermediate highly proliferative progenitor cells and the survival and maturation of new neurons. The pro-neurogenic effects of CBD might explain some of the positive therapeutic features of CBD-based compounds.

Oppositional effects of serotonin receptors 5-HT1a, 2, and 2c in the regulation of adult hippocampal neurogenesis

Serotonin (5-HT) appears to play a major role in controlling adult hippocampal neurogenesis and thereby it is relevant for theories linking failing adult neurogenesis to the pathogenesis of major depression and the mechanisms of action of antidepressants. Serotonergic drugs lacked acute effects on adult neurogenesis in many studies, which suggested a surprisingly long latency phase. Here we report that the selective serotonin reuptake inhibitor fluoxetine, which has no acute effect on precursor cell proliferation, causes the well-described increase in net neurogenesis upon prolonged treatment partly by promoting the survival and maturation of new postmitotic neurons. We hypothesized that this result is the cumulative effect of several 5-HT-dependent events in the course of adult neurogenesis. Thus, we used specific agonists and antagonists to 5-HT1a, 2, and 2c receptor subtypes to analyze their impact on different developmental stages. We found that 5-HT exerts acute and opposing effects on proliferation and survival or differentiation of precursor cells by activating the diverse receptor subtypes on different stages within the neuronal lineage in vivo. This was confirmed in vitro by demonstrating that 5-HT1a receptors are involved in self-renewal of precursor cells, whereas 5-HT2 receptors effect both proliferation and promote neuronal differentiation. We propose that under acute conditions 5-HT2 effects counteract the positive proliferative effect of 5-HT1a receptor activation. However, prolonged 5-HT2c receptor activation fosters an increase in late-stage progenitor cells and early postmitotic neurons, leading to a net increase in adult neurogenesis. Our data indicate that serotonin does not show effect latency in the adult dentate gyrus. Rather, the delayed response to serotonergic drugs with respect to endpoints downstream of the immediate receptor activity is largely due to the initially antagonistic and un-balanced action of different 5-HT receptors.

Synaptic network activity induces neuronal differentiation of adult hippocampal precursor cells through BDNF signaling

Adult hippocampal neurogenesis is regulated by activity. But how do neural precursor cells in the hippocampus respond to surrounding network activity and translate increased neural activity into a developmental program? Here we show that long-term potentiation (LTP)-like synaptic activity within a cellular network of mature hippocampal neurons promotes neuronal differentiation of newly generated cells. In co-cultures of precursor cells with primary hippocampal neurons, LTP-like synaptic plasticity induced by addition of glycine in Mg2+-free media for 5 min, produced synchronous network activity and subsequently increased synaptic strength between neurons. Furthermore, this synchronous network activity led to a significant increase in neuronal differentiation from the co-cultured neural precursor cells. When applied directly to precursor cells, glycine- and Mg2+-free solution did not induce neuronal differentiation. Synaptic plasticity-induced neuronal differentiation of precursor cells was observed in the presence of GABAergic neurotransmission blockers but was dependent on NMDA-mediated Ca2+ influx. Most importantly, neuronal differentiation required the release of brain-derived neurotrophic factor (BDNF) from the underlying substrate hippocampal neurons as well as TrkB receptor phosphorylation in precursor cells. This suggests that activity-dependent stem cell differentiation within the hippocampal network is mediated via synaptically evoked BDNF signaling.

NMDA and benzodiazepine receptors have synergistic and antagonistic effects on precursor cells in adult hippocampal neurogenesis

We studied how the noncompetitive NMDA receptor antagonist MK801 affected different stages of adult hippocampal neurogenesis in mice, and investigated how the activation of benzodiazepine receptors with diazepam interacted with the effects of MK801 on the precursor cells in the adult dentate gyrus. Our findings were: (i) one single MK801 application increased precursor cell proliferation and adult neurogenesis but not gliogenesis 4 weeks later; (ii) the number of label‐retaining precursor cells decreased after MK801 (with P = 0.06); (iii) the pro‐neurogenic effect included increased cell cycle entry of precursor cells as well as completed cell divisions, except in type‐2a cells; (iv) NMDA receptor blockade also increased the number of nestin–GFP‐expressing cells expressing calretinin; (v) diazepam alone had a very similar effect on overall precursor cell proliferation to that of MK801 alone; and (vi) diazepam, when co‐applied with MK801, abolished the suppression of divisions of type‐2a cells induced by MK801 alone, suppressed the MK801‐induced effect on proliferation of type‐2b cells and had no influence on the effects of MK801 on type‐3 cells, but did suppress the increased number of nestin–GFP‐positive cells expressing calretinin. From these results we hypothesize that, depending on the precursor cell stage, NMDA‐dependent neurotransmission has distinct effects that are partly antagonized and partly enhanced by GABAergic input. We propose that NMDA receptor‐dependent signalling maintains the precursor cells in the dentate gyrus while blocking initial stages of development and promoting more advanced stages.

Rostro-caudal gradual loss of cellular diversity within the periventricular regions of the ventricular system.

Neurogenesis occurs constitutively within the periventricular region (PVR) of the lateral ventricles (LV) of the adult mammalian brain. The occurrence of adult neurogenesis within the PVR outside the neurogenic niche of the LV remains controversial, but neural stem cells can be isolated from PVR of the whole ventricular system. The histological basis of this phenomenon including the regional differences of cellular phenotypes within the PVRs is still enigmatic. The occurrence of neurogenesis or manipulable progenitor cells in caudal parts of the adult brain is however one prerequisite for orthotopic regenerative approaches in Parkinsons disease (PD) and other disorders of the midbrain/brainstem. Using quantitative immunohistochemical techniques and electron microscopy, we found a rostro‐caudal gradual loss of cellular diversity within the PVR throughout the whole ventricular axis with loss of transit amplifying epidermal growth factor‐receptor+ type C cells in all parts caudal to the LV, a gradual reduction from rostral to caudal of both stem cells (type B cells or astrocytes) without signs of proliferation outside the PVR of the LV as well as neuroblasts‐like cells (polysialylated neural cell adhesion molecule [PSA‐NCAM]+, but doublecortin negative cells) with a different morphology compared with neuroblasts of the PVR of the LV. Electron microscopy confirmed these immunohistochemical data. The proportion of Nestin+/CD24+ cells and Nestin+/S100β+ ependymal cells were consecutively increased in the PVR from rostral to caudal, and ultrastructural analysis showed a region‐specific morphology with darker cytoplasm with occasional large lipid droplets as well as indented nuclei within the caudal PVRs. The strong correlation of neuroblast‐like cells with the number of neurosphere‐forming cells suggests that a quiescent subtype of PSA‐NCAM+ cells might be a source of neurosphere‐forming cells. We did not find any evidence for neurogenesis or the occurrence of neuroprogenitors within the substantia nigra or other parts of the midbrain/brainstem outside the PVR. Our data provide the histological framework for future studies on orthotopic regenerative approaches in PD by recruiting endogenous predopaminergic progenitors from the midbrain PVR. STEM CELLS 2009;27:928–941

Tis21 Expression Marks Not Only Populations of Neurogenic Precursor Cells but Also New Postmitotic Neurons in Adult Hippocampal Neurogenesis

During embryonic cortical development, expression of Tis21 is associated with cell cycle lengthening and neurogenic divisions of progenitor cells. We here investigated if the expression pattern of Tis21 also correlates with the generation of new neurons in the adult hippocampus. We used Tis21 knock-in mice expressing green fluorescent protein (GFP) and studied Tis21-GFP expression together with markers of adult hippocampal neurogenesis in newly generated cells. We found that Tis21-GFP 1) was absent from the radial glia–like putative stem cells (type-1 cells), 2) first appeared in transient amplifying progenitor cells (type-2 and 3 cells), 3) did not colocalize with markers of early postmitotic maturation stage, 4) was expressed again in maturing neurons, and 5) finally decreased in mature granule cells. Our data show that, in the course of adult neurogenesis, Tis21 is expressed in a phase additional to the one of the embryonic neurogenesis. This additional phase of expression might be associated with a new and different function of Tis21 than during embryonic brain development, where no Tis21 is expressed in mature neurons. We hypothesize that this function is related to the final functional integration of the newborn neurons. Tis21 can thus serve as new marker for key stages of adult neurogenesis.