Neuropathic pain-induced depressive-like behavior and hippocampal neurogenesis and plasticity are dependent on TNFR1 signaling

Patients suffering from neuropathic pain have a higher incidence of mood disorders such as depression. Increased expression of tumor necrosis factor (TNF) has been reported in neuropathic pain and depressive-like conditions and most of the pro-inflammatory effects of TNF are mediated by the TNF receptor 1 (TNFR1). Here we sought to investigate: (1) the occurrence of depressive-like behavior in chronic neuropathic pain and the associated forms of hippocampal plasticity, and (2) the involvement of TNFR1-mediated TNF signaling as a possible regulator of such events. Neuropathic pain was induced by chronic constriction injury of the sciatic nerve in wild-type and TNFR1^−/− mice. Anhedonia, weight loss and physical state were measured as symptoms of depression. Hippocampal neurogenesis, neuroplasticity, myelin remodeling and TNF/TNFRs expression were analyzed by immunohistochemical analysis and western blot assay.We found that neuropathic pain resulted in the development of depressive symptoms in a time dependent manner and was associated with profound hippocampal alterations such as impaired neurogenesis, reduced expression of neuroplasticity markers and myelin proteins. The onset of depressive-like behavior also coincided with increased hippocampal levels of TNF, and decreased expression of TNF receptor 2 (TNFR2), which were all fully restored after mice spontaneously recovered from pain. Notably, TNFR1^−/− mice did not develop depressive-like symptoms after injury, nor were there changes in hippocampal neurogenesis and plasticity.Our data show that neuropathic pain induces a cluster of depressive-like symptoms and profound hippocampal plasticity that are dependent on TNF signaling through TNFR1.     

Differential regulation of gliogenesis in the context of adult hippocampal neurogenesis in mice

In adult hippocampal neurogenesis, new neurons appear to originate from a cell with astrocytic properties expressing glial fibrillary acidic protein (GFAP). Also, new astrocytes are generated in the adult dentate gyrus. Whereas the putative astrocyte-like progenitor cells are consistently S-100beta-negative, many new astrocytes are S-100beta-positive. Thus, it is unclear whether the GFAP-positive progenitor cells are astrocytes in a general sense or rather neural progenitor cells with certain astrocytic characteristics. We therefore investigated the development of GFAP-expressing cells in the context of adult hippocampal neurogenesis. Proliferating cells could be either GFAP-positive or doublecortin-positive (DCX), but never both, indicating two independent populations of dividing cells in the glial and neuronal lineages. Two distinct populations of cells with astroglial properties were detected-one expressing GFAP, the other co-expressing GFAP and S-100beta. We never found S-100beta-cells to be in S-phase. No overlap between neuronal and glial markers was seen at any time point. Thus, astrogenesis occurred in parallel and to some degree independent of adult neurogenesis. The uninterrupted GFAP expression in this lineage, and neuronal markers in the other lineage, argue against a late common precursor for neurogenesis and gliogenesis in the adult hippocampus. Very few newly generated microglia and no new oligodendrocytes were detected. Environmental enrichment and voluntary wheel running-two experimental paradigms with robust stimulatory effects on adult hippocampal neurogenesis-affected hippocampal astrogenesis differentially: Running, but not enrichment, strongly induced net astrogenesis (GFAP/S-100beta), but also GFAP-positive S-100beta-negative cells, which thus appear to be a transiently amplifiable intermediate population within the glial lineage.     

Vascular niche for adult hippocampal neurogenesis

The thin lamina between the hippocampal hilus and granule cell layer, or subgranule zone (SGZ), is an area of active proliferation within the adult hippocampus known to generate new neurons throughout adult life. Although the neuronal fate of many dividing cells is well documented, little information is available about the phenotypes of cells in S-phase or how the dividing cells might interact with neighboring cells in the process of neurogenesis. Here, we make the unexpected observation that dividing cells are found in dense clusters associated with the vasculature and roughly 37% of all dividing cells are immunoreactive for endothelial markers. Most of the newborn endothelial cells disappear over several weeks, suggesting that neurogenesis is intimately associated with a process of active vascular recruitment and subsequent remodeling. The present data provide the first evidence that adult neurogenesis occurs within an angiogenic niche. This environment may provide a novel interface where mesenchyme-derived cells and circulating factors influence plasticity in the adult central nervous system.     

Impaired basal and running-induced hippocampal neurogenesis coincides with reduced Akt signaling in adult R6/1 HD mice

Huntington's disease (HD) is a fatal neurodegenerative disorder affecting a range of cellular and molecular functions in the brain. Deficits in adult hippocampal neurogenesis (AHN) have been documented in the R6/1 mouse model of HD. Here we examined basal and running-induced neuronal precursor proliferation in adult female and male R6/1 HD mice. We further tested whether sequential delivery of voluntary running followed by environmental enrichment could synergistically enhance functional AHN in female R6/1 HD mice. R6/1 HD mice engaged in significantly reduced levels of voluntary running, with males showing a more severe deficit. Basal neural precursor proliferation in the hippocampal sub-granular zone remained unchanged between female and male R6/1 HD mice and neither sex significantly responded to running-induced proliferation. While discrete provision of running wheels and enriched environments doubled AHN in adult female R6/1 HD mice it did not reflect the significant 3-fold increase in female wildtypes. Nevertheless, triple-label c-Fos/BrdU/NeuN immunofluorescence and confocal microscopy provided evidence that the doubling of AHN in female R6/1 HD mice was functional. Intrinsic cellular dysfunction mediated by protein aggregates containing mutant huntingtin (mHtt) did not appear to coincide with AHN deficits. In the hippocampus of female R6/1 HD mice, proliferating precursors and 6 week old adult-generated neurons were devoid of mHtt immuno-reactive aggregates, as were endothelial, microglial and astroglial cells populating the neurogenic niche. Serum transforming growth factor-β concentrations remained unaltered in female R6/1 HD mice as did the hippocampal levels of proliferating microglia and glial fibrillarly acidic protein expression. Examining the growth hormone/insulin-like growth factor 1 (GH/IGF-1) axis showed no change in base-line serum GH between genotypes. However, despite a reduced distance, acute running increases serum GH in both female wildtype and R6/1 HD mice. Serum IGF-1 levels were increased in female R6/1 HD mice compared to wildtypes during daytime inactive period, while hippocampal levels of the IGF-1 receptor remained unchanged. Running induced Akt phosphorylation in the hippocampus of female wildtype mice, which was not reflected in R6/1 HD mice. Total Akt levels were decreased in the hippocampus of both control and running R6/1 HD mice. Our results show adult-generated hippocampal neurons in female R6/1 HD mice express c-Fos and that running and Akt signaling deficits may mediate reduced basal and running-induced AHN levels.     

Implications of adult hippocampal neurogenesis in antidepressant action

In the dentate gyrus of the hippocampus, cell birth and maturation into neurons, or neurogenesis, occur throughout the lifetime of animals and humans. Multiple factors have been shown to regulate adult neurogenesis, and a number of findings in this field have had a large impact on basic and clinical research in depression. It has been reported that both physical and psychosocial stress paradigms, as well as some animal models of depression, produce a decrease in hippocampal cell proliferation and neurogenesis. Conversely, long-term, but not short-term, treatment with different classes of antidepressant drug increases cell proliferation and neurogenesis. Patients with depressive disorders or post-traumatic stress disorder have reduced hippocampal volume. Given this interaction of stress, depression and neurogenesis, a current hypothesis is that reduced adult hippocampal cell proliferation or neurogenesis may be involved in the pathophysiology of depression and that reversal or prevention of the decrease in neurogenesis may be one way in which the antidepressant drugs exert their effects. Research from this emerging field will further our understanding of the effects of stress and depression on the brain and the mechanism of action of antidepressant drugs.     

Dynamics of olfactory and hippocampal neurogenesis in adult sheep

Although adult neurogenesis has been conserved in higher vertebrates such as primates and humans, timing of generation, migration, and differentiation of new neurons appears to differ from that in rodents. Sheep could represent an alternative model to studying neurogenesis in primates because they possess a brain as large as a macaque monkey and have a similar life span. By using a marker of cell division, bromodeoxyuridine (BrdU), in combination with several markers, the maturation time of newborn cells in the dentate gyrus (DG) and the main olfactory bulb (MOB) was determined in sheep. In addition, to establish the origin of adult‐born neurons in the MOB, an adeno‐associated virus that infects neural cells in the ovine brain was injected into the subventricular zone (SVZ). A migratory stream was indicated from the SVZ up to the MOB, consisting of neuroblasts that formed chain‐like structures. Results also showed a long neuronal maturation time in both the DG and the MOB, similar to that in primates. The first new neurons were observed at 1 month in the DG and at 3 months in the MOB after BrdU injections. Thus, maturation of adult‐born cells in both the DG and the MOB is much longer than that in rodents and resembles that in nonhuman primates. This study points out the importance of studying the features of adult neurogenesis in models other than rodents, especially for translational research for human cellular therapy. J. Comp. Neurol. 521:169–188, 2013. © 2012 Wiley Periodicals, Inc.     

Notch1信号系统与抑郁模型大鼠海马神经重塑障碍

目的 探讨Notch1信号系统在抑郁海马神经再生障碍中的作用.方法 选择行为学评分相近的68只Sprague-Dawley大鼠,分为对照组、对照+氟西汀组、慢性不可预知温和应激(CUMS)组,CUMS+氟西汀组,每组17只.应用CUMS建立抑郁模型后进行行为学评估;采用免疫组织化学方法检测大鼠海马神经干细胞的增殖和存活;采用实时定量聚合酶链反应和蛋白免疫印迹方法测定Notchl信号通路各个因子(NICD、Hes1、Hes5、Jag1)的基因及蛋白表达水平的改变.结果 (1)干预前,各组体质量、糖水偏好、旷场试验、强迫游泳评分的差异均无统计学意义(P>0.05);干预后,与对照组比较,CUMS组糖水偏好、水平得分和垂直得分降低,漂浮不动时间增加,差异均有统计学意义(P<0.001);与CUMS组比较,CUMS+氟西汀组糖水偏好、水平得分和垂直得分增加,漂浮不动时间降低,差异均有统计学意义(P<0.01);(2)神经干细胞的增殖和存活:与CUMS组(1900.33±104.10)比较,CUMS+氟西汀组(3047.61±158.29)神经干细胞的增殖数显著上升,差异有统计学意义(P<0.01);与CUMS组(1845.33±126.88)比较,CUMS+氟西汀组(2704.21±154.31)神经干细胞的存活数显著上升,差异有统计学意义(P<0.01);(3)海马Notch1信号通路基因和蛋白的表达:CUMS+氟西汀组小鼠抗大鼠Notch1(NICD)mRNA、Hes1 mRNA、Hes5 mRNA、Jag1 mRNA基因表达与CUMS组比较显著上升,差异均有统计学意义(P<0.01);CUMS+氟西汀组NICD、Hes5、Jag1蛋白水平与CUMS组比较显著上升,差异均有统计学意义(P<0.01).结论 Notch1信号系统可能参与慢性应激模型大鼠海马神经再生障碍;氟西汀可能通过上调Notch1信号系统改善海马神经再生,从而缓解大鼠抑郁症状.

Abstract：

Objective To investigate whether the effect of fluoxetine on hippocampal neurogenesis involves Notch1 signaling after chronic stress. Methods Sixty-eight male Sprague-Dawley rats were divided into control group, control + fluoxetine group, depression model group and depression model + fluoxetine group. Chronic unpredictable mild stress (CUMS) was used to make up depression animal model. The function of Notch1 signaling was measured by real-time PCR and western blotting. Simultaneously,hippocampal neurogenesis was monitored by assessing cell proliferation and survival. Results (1) Before starting CUMS protocol, the animals exhibited equivalent weight, sucrose preference, number of squares crossed, number of rearing, and immobility time in behavioral test. Twenty-eight days after CUMS protocol,these parameters were significantly difference in rats exposed to CUMS compared with the controls (sucrose preference, number of squares crossed, number of grooming and rearing, and immobility time, P<0. 01).Administration of fluoxetine was shown to dramatically improve the depression behavior (P<0. 01) .(2) The cell proliferation [(3047. 61 ± 158. 29) vs. (1900. 33 ± 104. 10)] and survive [(2704. 21 ±154. 31) vs. (1845.33 ± 126.88)] were increased after fluoxetine administration in rats with depression (P<0. 01). (3) Fluoxetine increased mRNA expressions of Notch1 signaling components [NICD mRNA (0. 23 ±0. 01) vs. (0. 10 ±0.01), Hes1 mRNA (0. 56 ±0.04) vs. (0. 28 ±0.02), Hes5 mRNA (0. 24 ±0.02) vs. (0.10 ±0.02), Jag1 mRNA (0.82 ±0.06) vs. (0.56 ±0.03)] in the rat hippocampus compared with the CUMS group (P<0. 01 or P<0. 001). Fluoxetine enhanced protein levels of Notch1 signaling components [NICD(1.99 ±0.07) vs. (0.53 ±0.10), Hes5(0. 64 ±0. 04) vs. (0.37 ±0.09),Jag1 (2. 34 ± 0. 13) vs. (0. 68 ± 0. 17)] in the rat hippocampus compared with the CUMS group (P <0.01). Conclusion The up-regulation of the Notch1 pathway with chronic fluoxetine administration might partly contribute to increased neurogenesis in the rat hippocampus with depression.     

Memory consolidation during sleep and adult hippocampal neurogenesis

In anticipation of the massive burden of neurodegenerative disease within super-aged societies, great efforts have been made to utilize neural stem and progenitor cells for regenerative medicine. The capacity of intrinsic neural stem and progenitor cells to regenerate damaged brain tissue remains unclear, due in part to the lack of knowledge about how these newly born neurons integrate into functional circuitry. As sizable integration of adult-born neurons naturally occurs in the dentate gyrus region of the hippocampus, clarifying the mechanisms of this process could provide insights for applying neural stem and progenitor cells in clinical settings. There is convincing evidence of functional correlations between adult-born neurons and memory consolidation and sleep; therefore, we describe some new advances that were left untouched in our recent review.     

Reduced hippocampal neurogenesis and skill reaching performance in adult Emx1 mutant mice

Mammalian homeobox gene Emx family is involved in the development of the rostral brain. Loss-of-function studies suggest that, despite the agenesis of corpus callosum, the Emx1 mutants display relatively modest defects compared to the Emx2 mutants. However, the role of the Emx1 in neurogenesis and brain function has never been explored. We used unbiased stereology to determine the number of proliferating progenitors and immature neurons in the adult neurogenic zones. Although previous studies have established that the formation of the dentate gyrus (DG) requires Emx2, we found that the adult Emx1 mutants also exhibited a smaller DG, reduced number of proliferating progenitor cells and immature neurons in the DG, in contrast to the indistinguishable level of neurogenesis in the subventricular zone when compared to the wild type mice. In view of the involvement of callosal projection neurons in mediating interhemispheric crosstalk and spatial coupling between the limbs, and the importance of DG in hippocampus-dependent function in learning and memory, we assessed motor and cognitive functions. Emx1 deletion impaired performance on a forelimb skill reaching task and attenuated training induced hippocampal neurogenesis, but it did not affect motor activity or basic motor function as evaluated in the open field, wire hanging and rotor rod tests. Unexpectedly, the adult Emx1 mutant mice did not exhibit impairment in spatial learning and memory in the Barnes maze test. Our data suggest that deletion of the Emx1 gene reduces hippocampal neurogenesis and affects higher motor function that requires extensive learning.     

A developmental perspective on adult hippocampal neurogenesis

The generation of new neurons from neural stem cells (NSCs) throughout adult life in the mammalian brain is a biological process that fascinates scientists for its uniqueness and restorative potential. In the dentate gyrus (DG) of the hippocampus NSCs are able to self-renew and generate new granule cells and astrocytes through a complex and plastic mechanism that can be regulated by endogenous and exogenous cues at different levels. Unexpected recent findings suggest that the population of NSCs is heterogeneous in morphology and behavior. We herein explore the hypothesis that NSC heterogeneity and the neurogenic potential of the DG depends on their developmental origin. We provide an up-to-date picture of the process of neurogenesis in the adult hippocampus with an especial focus on NSCs and outline key unsolved aspects. Further, we discuss the origin of NSCs in the adult DG from a developmental perspective and explore the possibility of NSC heterogeneity being determined from early postnatal periods and being responsible for the neurogenic output of the DG in the long term.     

Gonadal hormone modulation of hippocampal neurogenesis in the adult

Gonadal hormones modulate neurogenesis in the dentate gyrus (DG) of adult rodents in complex ways. Estradiol, the most potent estrogen, initially enhances and subsequently suppresses cell proliferation in the dentate gryus of adult female rodents. Much less is known about how estradiol modulates neurogenesis in the adult male rodent; however, recent evidence suggests that estradiol may have a moderate effect on cell proliferation but enhances cell survival in the DG of newly synthesized cells but only when estradiol is administered during a specific stage in the cell maturation cycle in the adult male rodent. Testosterone likely plays a role in adult neurogenesis, although there have been no direct studies to address this. However, pilot studies from our laboratory suggest that testosterone up-regulates cell survival but not cell proliferation in the DG of adult male rats. Progesterone appears to attenuate the estradiol-induced enhancement of cell proliferation. Neurosteroids such as allopregnalone decrease neurogenesis in adult rodents, while pregnancy and motherhood differentially regulate adult neurogenesis in the adult female rodent. Very few studies have investigated the effects of gonadal hormones on male rodents; however, studies have indicated that there is a gender difference in the response to hormone-regulated hippocampal neurogenesis in the adult. Clearly, more work needs to be done to elucidate the effects of gonadal hormones on neurogenesis in the DG of both male and female rodents.     

Notch1 signaling,hippocampal neurogenesis and behavioral responses to chronic unpredicted mild stress in adult ischemic rats

Accumulating evidence indicates that the Notch signaling pathway fulfills important roles in ischemia-stimulated neurogenesis, which may be regarded as an etiological factor in post-stroke depression. Here we explored Notch₁ signaling, hippocampal neurogenesis and behavioral responses to chronic unpredicted mild stress (CUMS) in adult ischemic rats. Animals were treated with permanent middle cerebral artery occlusion followed by an 18 day CUMS procedure. Proliferating cells in the hippocampus and their cell fate were investigated on days 19 and 28 after ischemic surgery. Additionally, expression of the Notch₁ intracellular domain (NICD) and its downstream targets Hes1 and Hes5 was examined. A sucrose preference test and forced swim test were used to assess behavioral responses. CUMS produced depressive-like behaviors and decreased the number of proliferating cells on day 19 (both p < 0.001), accompanied by a decreased expression of both Hes1 and Hes5 in the hippocampus of ischemic animals (p < 0.001). On day 28, CUMS resulted in a decreased number of neurogenically-differentiating cells in the subgranular zone (p < 0.001) while permitting differentiation into astrocytes in the hilus (p < 0.05). Hes1 and Hes5 protein expression levels were increased. The expression of the NICD was significantly decreased at both time-points. CUMS led to expression changes in the Notch₁ signaling cascade in ischemic rats, most of which concerned hippocampal neurogenesis. This suggests that variation in Notch₁ activity and subsequent expression of its downstream targets, including Hes1 and Hes5, may, at least in part, contribute to modulation of ischemia-related hippocampal neurogenesis by CUMS.     

Essential role of tau phosphorylation in adult hippocampal neurogenesis

An increased hippocampal neurogenesis has been observed in Alzheimer disease (AD), the most common neurodegenerative disorder characterized with accumulation of β‐amyloid (Aβ) and hyperphosphorylated tau (p‐tau). Studies in transgenic mouse models suggest that the amyloidosis suppresses adult neurogenesis. Although emerging evidence links tau to neurodevelopment, the direct data regarding tau phosphorylation in adult neurogenesis is missing. Here, we found that the immature neurons, identified by doublecortin (DCX) and neurogenic differentiation factor (neuroD), were only immunoreactive to p‐tau but not to the non‐p‐tau in adult rat brain and human patients with AD, and the p‐tau was coexpressed temporally and spatially with DCX and neuroD in the hippocampal dentate gyrus (DG) of the rat brains during postnatal development. A correlative increase of immature neuron markers and tau phosphorylation was induced in rat hippocampal DG by upregulating glycogen synthase kinase‐3 (GSK‐3), a crucial tau kinase, and the increased neurogenesis was due to an enhanced proliferation but not survival or differentiation of the newborn neurons. The hippocampal neurogenesis was severely impaired in tau knockout mice and activation of GSK‐3 in these mice did not rescue the deficits. These results reveal an essential role of tau phosphorylation in adult hippocampal neurogenesis. It suggests that spatial/temporal manipulation of tau phosphorylation may be compensatory for the neuron loss in neurological disorders, including AD. © 2009 Wiley‐Liss, Inc.     

Several stressors fail to reduce adult hippocampal neurogenesis

Neurogenesis in the dentate gyrus of the hippocampus of adult laboratory animals has been widely reported to be vulnerable to many psychological and physical stressors. However, we have found no effects of acute restraint stress, acute or subchronic tailshock stress, or acute, subchronic, or chronic resident-intruder stress on neural progenitor cell (NPC) proliferation, short or long term survival of newborn cells, or brain-derived neurotrophic factor (BDNF) mRNA expression in adult rats. In addition, we did not observe any effect of chronic resident-intruder stress on NPC proliferation in adolescent rats. A selectively bred stress-sensitive line was also found to exhibit no alterations in NPC proliferation following tailshock stress, although this line did exhibit a lower proliferation rate under baseline (unstressed) conditions when compared with non-selected rats. These results challenge the prevailing hypothesis that any stressor of sufficient intensity and duration has a marked negative impact upon the rate of hippocampal neurogenesis, and suggest that some yet unidentified factors related to stress and experimental conditions are crucial in the regulation of neurogenesis.     

Genetic influence on phenotypic differentiation in adult hippocampal neurogenesis

Regulation of adult hippocampal neurogenesis has different regulatory levels, including cell proliferation, survival and differentiation. Cell proliferation and survival are differentially influenced by inheritable traits and the genetic background determines which regulatory levels of adult hippocampal neurogenesis are preferentially involved in a neurogenic response to environmental stimuli. We here compared baseline adult neurogenesis in wild-derived strain Mus spretus and three inbred laboratory strains: A/J, C3H/HeJ and DBA/2J. Proliferation of was similar in the four strains, with the extremes being A/J, which had about 2100+/-570 (mean+/-S.D.) labeled newborn cells per dentate gyrus (after 6 days of bromodeoxyuridine injections), and DBA/2J, which had approximately 1400+/-260. C3H/HeJ had approximately 1500+/-600 and M. spretus had 1550+/-270. Survival of new cells after 4 weeks was 19% in A/J and DBA/2J, and 21% in M. spretus, but 37% in C3H/HeJ. Survival in C3H/HeJ was significantly different from DBA/2. Phenotypic analysis revealed that DBA/2J produced significantly fewer new neurons than A/J and C3H/HeJ (47% vs. 63% and 67%) but significantly more new astrocytes than A/J and C3H/HeJ (28% vs. 9% and 11%). In absolute terms there were 370+/-120 new neurons in C3H/HeJ, 250+/-60 in A/J, 130+/-50 in DBA/2J, and 190+/-130 in M. spretus. Our results indicate that regulation of adult hippocampal neurogenesis affects the level of phenotypic differentiation. At the present time it cannot be determined whether this regulation occurs by influencing cell fate decisions or by promoting selective survival.     

Maternal immune activation primes deficiencies in adult hippocampal neurogenesis

Neurogenesis, the process in which new neurons are generated, occurs throughout life in the mammalian hippocampus. Decreased adult hippocampal neurogenesis (AHN) is a common feature across psychiatric disorders, including schizophrenia, depression- and anxiety-related behaviours, and is highly regulated by environmental influences. Epidemiological studies have consistently implicated maternal immune activation (MIA) during neurodevelopment as a risk factor for psychiatric disorders in adulthood. The extent to which the reduction of hippocampal neurogenesis in adulthood may be driven by early life exposures, such as MIA, is however unclear. We therefore reviewed the literature for evidence of the involvement of MIA in disrupting AHN. Consistent with our hypothesis, data from both in vivo murine and in vitro human models of AHN provide evidence for key roles of specific cytokines induced by MIA in the foetal brain in disrupting hippocampal neural progenitor cell proliferation and differentiation early in development. The precise molecular mechanisms however remain unclear. Nonetheless, these data suggest a potential latent vulnerability mechanism, whereby MIA primes dysfunction in the unique hippocampal pool of neural stem/progenitor cells. This renders offspring potentially more susceptible to additional environmental exposures later in life, such as chronic stress, resulting in the unmasking of psychopathology. We highlight the need for studies to test this hypothesis using validated animal models of MIA, but also to test the relevance of such data for human pathology at a molecular basis through the use of patient-derived induced pluripotent stem cells (hiPSC) differentiated into hippocampal progenitor cells.