Amyloid β Is Not the Major Factor Accounting for Impaired Adult Hippocampal Neurogenesis in Mice Overexpressing Amyloid Precursor Protein

The role of exosomes in adult neurogenesis: implications for neurodegenerative diseases

Exosomes are cup-shaped extracellular vesicles with a lipid bilayer that is approximately 30 to 200 nm in thickness. Exosomes are widely distributed in a range of body fluids, including urine, blood, milk, and saliva. Exosomes exert biological function by transporting factors between different cells and by regulating biological pathways in recipient cells. As an important form of intercellular communication, exosomes are increasingly being investigated due to their ability to transfer bioactive molecules such as lipids, proteins, mRNAs, and microRNAs between cells, and because they can regulate physiological and pathological processes in the central nervous system. Adult neurogenesis is a multistage process by which new neurons are generated and migrate to be integrated into existing neuronal circuits. In the adult brain, neurogenesis is mainly localized in two specialized niches: the subventricular zone adjacent to the lateral ventricles and the subgranular zone of the dentate gyrus. An increasing body of evidence indicates that adult neurogenesis is tightly controlled by environmental conditions with the niches. In recent studies, exosomes released from different sources of cells were shown to play an active role in regulating neurogenesis both in vitro and in vivo, thereby participating in the progression of neurodegenerative disorders in patients and in various disease models. Here, we provide a state-of-the-art synopsis of existing research that aimed to identify the diverse components of exosome cargoes and elucidate the therapeutic potential of exosomal contents in the regulation of neurogenesis in several neurodegenerative diseases. We emphasize that exosomal cargoes could serve as a potential biomarker to monitor functional neurogenesis in adults. In addition, exosomes can also be considered as a novel therapeutic approach to treat various neurodegenerative disorders by improving endogenous neurogenesis to mitigate neuronal loss in the central nervous system.

Efr3b is essential for social recognition by modulating the excitability of CA2 pyramidal neurons

CA2 pyramidal neurons (PNs) are associated with social behaviors. The mechanisms, however, remain to be fully investigated. Here, we report that Efr3b, a protein essential for phospholipid metabolism at the plasma membrane, is widely expressed in the brain, especially in the hippocampal CA2/CA3 areas. To assess the functional significance of Efr3b in the brain, we generated Efr3bf/f mice and crossed them with Nestin-cre mice to delete Efr3b specifically in the brain. We find that Efr3b deficiency in the brain leads to deficits of social novelty recognition and hypoexcitability of CA2 PNs. We then knocked down the expression of Efr3b specifically in CA2 PNs of C57BL/6J mice, and our results showed that reducing Efr3b in CA2 PNs also resulted in deficits of social novelty recognition and hypoexcitability of CA2 PNs. More interestingly, restoring the expression of Efr3b in CA2 PNs enhances their excitability and improves social novelty recognition in Efr3b-deficient mice. Furthermore, direct activation of CA2 PNs with chemogenetics improves social behaviors in Efr3b-deficient mice. Together, our data suggest that Efr3b is essential for social novelty by modulating the excitability of CA2 PNs.

Adult neurogenesis research in China

Neural stem cells are multipotent stem cells that generate functional newborn neurons through a process called neurogenesis. Neurogenesis in the adult brain is tightly regulated and plays a pivotal role in the maintenance of brain function. Disruption of adult neurogenesis impairs cognitive function and is correlated with numerous neurologic disorders. Deciphering the mechanisms underlying adult neurogenesis not only advances our understanding of how the brain functions, but also offers new insight into neurologic diseases and potentially contributes to the development of effective treatments. The field of adult neurogenesis is experiencing significant growth in China. Chinese researchers have demonstrated a multitude of factors governing adult neurogenesis and revealed the underlying mechanisms of and correlations between adult neurogenesis and neurologic disorders. Here, we provide an overview of recent advancements in the field of adult neurogenesis due to Chinese scientists.

Adult Hippocampal Neurogenesis in Alzheimer’s Disease: An Overview of Human and Animal Studies with Implications for Therapeutic Perspectives Aimed at Memory Recovery

The mammalian hippocampal dentate gyrus is a niche for adult neurogenesis from neural stem cells. Newborn neurons integrate into existing neuronal networks, where they play a key role in hippocampal functions, including learning and memory. In the ageing brain, neurogenic capability progressively declines while in parallel increases the risk for developing Alzheimer's disease (AD), the main neurodegenerative disorder associated with memory loss. Numerous studies have investigated whether impaired adult neurogenesis contributes to memory decline in AD. Here, we review the literature on adult hippocampal neurogenesis (AHN) and AD by focusing on both human and mouse model studies. First, we describe key steps of AHN, report recent evidence of this phenomenon in humans, and describe the specific contribution of newborn neurons to memory, as evinced by animal studies. Next, we review articles investigating AHN in AD patients and critically examine the discrepancies among different studies over the last two decades. Also, we summarize researches investigating AHN in AD mouse models, and from these studies, we extrapolate the contribution of molecular factors linking AD-related changes to impaired neurogenesis. Lastly, we examine animal studies that link impaired neurogenesis to specific memory dysfunctions in AD and review treatments that have the potential to rescue memory capacities in AD by stimulating AHN.

Implication of Adult Hippocampal Neurogenesis in Alzheimer’s Disease and Potential Therapeutic Approaches

Alzheimer’s disease is the most common neurodegenerative disease, affecting more than 6 million US citizens and representing the most prevalent cause for dementia. Neurogenesis has been repeatedly reported to be impaired in AD mouse models, but the reason for this impairment remains unclear. Several key factors play a crucial role in AD including Aβ accumulation, intracellular neurofibrillary tangles accumulation, and neuronal loss (specifically in the dentate gyrus of the hippocampus). Neurofibrillary tangles have been long associated with the neuronal loss in the dentate gyrus. Of note, Aβ accumulation plays an important role in the impairment of neurogenesis, but recent studies started to shed a light on the role of APP gene expression on the neurogenesis process. In this review, we will discuss the recent approaches to neurogenesis in Alzheimer disease and update the development of therapeutic methods.

Modulating adult neurogenesis affects synaptic plasticity and cognitive functions in mouse models of Alzheimer's disease

New neurons are abnormal in the adult hippocampus of Alzheimer's disease (AD) mouse models. The effects of modulating adult neurogenesis on AD pathogenesis differ from study to study. We reported recently that ablation of adult neural stem cells (aNSCs) was associated with improved memory in AD models. Here, we found that long-term potentiation (LTP) was improved in the hippocampus of APP/PS1 mice after ablation of aNSCs. This effect was confirmed in hAPP-J20 mice, a second AD mouse model. On the other hand, we found that exposure to enriched environment (EE) dramatically increased the number of DCX⁺ neurons, promoted dendritic growth, and affected the location of newborn neurons in the dentate gyrus of APP/PS1 mice, and EE exposure significantly ameliorated memory deficits in APP/PS1 mice. Together, our data suggest that both inhibiting abnormal adult neurogenesis and enhancing healthy adult neurogenesis could be beneficial for AD, and they are not mutually exclusive.

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Ablation of aNSCs improves hippocampal synaptic plasticity in AD mice

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Ablation of aNSCs results in hippocampal remodeling in AD mice

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EE accelerates development of new neurons and improves cognition in AD mice

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Effects of inhibiting and enhancing AHN on AD are not mutually exclusive

microRNA-425 loss mediates amyloid plaque microenvironment heterogeneity and promotes neurodegenerative pathologies

Different cellular and molecular changes underlie the pathogenesis of Alzheimer's disease (AD). Among these, neuron‐specific dysregulation is a necessary event for accumulation of classic pathologies including amyloid plaques. Here, we show that AD‐associated pathophysiology including neuronal cell death, inflammatory signaling, and endolysosomal dysfunction is spatially colocalized to amyloid plaques in regions with abnormal microRNA‐425 (miR‐425) levels and this change leads to focal brain microenvironment heterogeneity, that is, an amyloid plaque‐associated microenvironment (APAM). APAM consists of multiple specific neurodegenerative signature pathologies associated with senile plaques that contribute to the heterogeneity and complexity of AD. Remarkably, miR‐425, a neuronal‐specific regulator decreased in AD brain, maintains a normal spatial transcriptome within brain neurons. We tested the hypothesis that miR‐425 loss correlates with enhanced levels of mRNA targets downstream, supporting APAM and AD progression. A miR‐425‐deficient mouse model has enhanced APP amyloidogenic processing, neuroinflammation, neuron loss, and cognitive impairment. In the APP/PS1 mouse model, intervening with miR‐425 supplementation ameliorated APAM changes and memory deficits. This study reveals a novel mechanism of dysregulation of spatial transcriptomic changes in AD brain, identifying a probable neuronal‐specific microRNA regulator capable of staving off amyloid pathogenesis. Moreover, our findings provide new insights for developing AD treatment strategies with miRNA oligonucleotide(s).

Overexpression of wild-type human amyloid precursor protein alters GABAergic transmission

The function of the amyloid precursor protein (APP) is not fully understood, but its cleavage product amyloid beta (Aβ) together with neurofibrillary tangles constitute the hallmarks of Alzheimer's disease (AD). Yet, imbalance of excitatory and inhibitory neurotransmission accompanied by loss of synaptic functions, has been reported much earlier and independent of any detectable pathological markers. Recently, soluble APP fragments have been shown to bind to presynaptic GABAB receptors (GABABRs), subsequently decreasing the probability of neurotransmitter release. In this body of work, we were able to show that overexpression of wild-type human APP in mice (hAPPwt) causes early cognitive impairment, neuronal loss, and electrophysiological abnormalities in the absence of amyloid plaques and at very low levels of Aβ. hAPPwt mice exhibited neuronal overexcitation that was evident in EEG and increased long-term potentiation (LTP). Overexpression of hAPPwt did not alter GABAergic/glutamatergic receptor components or GABA production ability. Nonetheless, we detected a decrease of GABA but not glutamate that could be linked to soluble APP fragments, acting on presynaptic GABABRs and subsequently reducing GABA release. By using a specific presynaptic GABABR antagonist, we were able to rescue hyperexcitation in hAPPwt animals. Our results provide evidence that APP plays a crucial role in regulating inhibitory neurotransmission.

ErbB4 mediates amyloid β-induced neurotoxicity through JNK/tau pathway activation: Implications for Alzheimer's disease

Accumulation of amyloid β (Aβ) in the brain is a hallmark of Alzheimer's disease (AD). We previously showed that ErbB4 in parvalbumin (PV)-positive interneurons was associated with Aβ-induced cognitive deficits; however, the underlying mechanism remains undetermined. Here we found that specific deletion of ErbB4 in PV neurons significantly attenuated oligomeric Aβ-induced neuronal toxicity and inhibited Aβ-induced decreases of PSD95 and synaptophysin. Moreover, specific ablation of ErbB4 in PV neurons altered activity-related protein c-Fos and decreased hippocampal PV neurons, especially in the dentate gyrus (DG) of hAPP-J20 mice. Furthermore, c-Jun N-terminal kinase (JNK), a protein downstream of ErbB4, was activated by Aβ but not ErbB4's ligand neuregulin 1 (NRG1) β1, suggesting different downstream pathways for Aβ and NRG1β1. JNK phosphorylation was inhibited by the ErbB4 inhibitor AG1478 and by pretreatment with NRG1β1. More importantly, siRNA knockdown of ErbB4 decreased JNK phosphorylation and expression, tau phosphorylation at Ser396 and Thr 205, and Bax expression. Therefore, ErbB4 might mediate Aβ-induced neuropathology through the JNK/tau pathway and represent a potential therapeutic target in patients with AD.

Molecular mechanisms of altered adult hippocampal neurogenesis in Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia globally. AD is a progressive neurodegenerative disorder, eventually manifesting as severe cognitive impairment. Adult hippocampal neurogenesis (AHN) occurs throughout adulthood and plays an important role in hippocampus-dependent learning and memory. The stages of AHN, predominantly comprising the proliferation, differentiation, survival, and maturation of newborn neurons, are affected to varying degrees in AD. However, the exact molecular mechanisms remain to be elucidated. Recent evidence suggests that the molecules involved in AD pathology contribute to the compromised AHN in AD. Notably, various interventions may have common signaling pathways that, once identified, could be harnessed to enhance adult neurogenesis. This in turn could putatively rescue cognitive deficits associated with impaired neurogenesis as observed in animal models of AD. In this manuscript, we review the current knowledge concerning AHN under normal physiological and AD pathological conditions and highlight the possible role of specific molecules in AHN alteration in AD. In addition, we summarize in vivo experiments with emphasis on the effect of the activation of certain key signalings on AHN in AD rodent models. We propose that these signaling targets and corresponding interventions should be considered when developing novel therapies for AD.

Does Impairment of Adult Neurogenesis Contribute to Pathophysiology of Alzheimer's Disease? A Still Open Question

Adult hippocampal neurogenesis is a physiological mechanism contributing to hippocampal memory formation. Several studies associated altered hippocampal neurogenesis with aging and Alzheimer's disease (AD). However, whether amyloid-β protein (Aβ)/tau accumulation impairs adult hippocampal neurogenesis and, consequently, the hippocampal circuitry, involved in memory formation, or altered neurogenesis is an epiphenomenon of AD neuropathology contributing negligibly to the AD phenotype, is, especially in humans, still debated. The detrimental effects of Aβ/tau on synaptic function and neuronal viability have been clearly addressed both in in vitro and in vivo experimental models. Until some years ago, studies carried out on in vitro models investigating the action of Aβ/tau on proliferation and differentiation of hippocampal neural stem cells led to contrasting results, mainly due to discrepancies arising from different experimental conditions (e.g., different cellular/animal models, different Aβ and/or tau isoforms, concentrations, and/or aggregation profiles). To date, studies investigating in situ adult hippocampal neurogenesis indicate severe impairment in most of transgenic AD mice; this impairment precedes by several months cognitive dysfunction. Using experimental tools, which only became available in the last few years, research in humans indicated that hippocampal neurogenesis is altered in cognitive declined individuals affected by either mild cognitive impairment or AD as well as in normal cognitive elderly with a significant inverse relationship between the number of newly formed neurons and cognitive impairment. However, despite that such information is available, the question whether impaired neurogenesis contributes to AD pathogenesis or is a mere consequence of Aβ/pTau accumulation is not definitively answered. Herein, we attempted to shed light on this complex and very intriguing topic by reviewing relevant literature on impairment of adult neurogenesis in mouse models of AD and in AD patients analyzing the temporal relationship between the occurrence of altered neurogenesis and the appearance of AD hallmarks and cognitive dysfunctions.

Expression of NLRP3 Inflammasomes in Neurogenic Niche Contributes to the Effect of Spatial Learning in Physiological Conditions but Not in Alzheimer's Type Neurodegeneration

A common feature of neurodegenerative disorders, in particular Alzheimer's disease (AD), is a chronic neuroinflammation associated with aberrant neuroplasticity. Development of neuroinflammation affects efficacy of stem and progenitor cells proliferation, differentiation, migration, and integration of newborn cells into neural circuitry. However, precise mechanisms of neurogenesis alterations in neuroinflammation are not clear yet. It is well established that expression of NLRP3 inflammasomes in glial cells marks neuroinflammatory events, but less is known about contribution of NLRP3 to deregulation of neurogenesis within neurogenic niches and whether neural stem cells (NSCs), neural progenitor cells (NPCs) or immature neuroblasts may express inflammasomes in (patho)physiological conditions. Thus, we studied alterations of neurogenesis in rats with the AD model (intra-hippocampal injection of Aβ1-42). We found that in Aβ-affected brain, number of CD133+ cells was elevated after spatial training in the Morris water maze. The number of PSA-NCAM+ neuroblasts diminished by Aβ injection was completely restored by subsequent spatial learning. Spatial training leads to elevated expression of NLRP3 inflammasomes in the SGZ (subgranular zones): CD133+ and PSA-NCAM+ cells started to express NLRP3 in sham-operated, but not AD rats. Taken together, our data suggest that expression of NLRP3 inflammasomes in CD133+ and PSA-NCAM+ cells may contribute to stimulation of adult neurogenesis in physiological conditions, whereas Alzheimer's type neurodegeneration abolishes stimuli-induced overexpression of NLRP3 within the SGZ neurogenic niche.

Ablating Adult Neural Stem Cells Improves Synaptic and Cognitive Functions in Alzheimer Models

Adult neurogenesis is impaired in the hippocampus of patients with Alzheimer disease (AD) as well as AD models. However, it is far from clear how modulating adult neurogenesis affects AD neuropathology. We confirm that adult hippocampal neurogenesis is impaired in two AD models. Surprisingly, however, cognitive functions are improved in AD models after ablating adult neural stem cells (aNSCs). Ablation of aNSCs does not affect the levels of amyloid β but restores the normal synaptic transmission in the dentate gyrus (DG) granule cells of AD models. Furthermore, calbindin depletion in the DG of AD mice is ameliorated after aNSC ablation, and knocking down calbindin abolishes the effects of aNSC ablation on synaptic and cognitive functions of AD mice. Together, our data suggest that cognitive functions of AD mice are improved after aNSC ablation, which is associated with the restoration of synaptic transmission in the DG granule cells with calbindin as an important mediator.

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Adult hippocampal neurogenesis was impaired in two AD models

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Cognitive functions were improved in AD models after ablation of aNSCs

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Ablating aNSCs restored the normal synaptic transmission in the DG granule cells of AD models

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Altered expression of calbindin mediated the effects of ablating aNSCs on synaptic and cognitive functions in AD mice

Hippocampal Neurogenesis Is Enhanced in Adult Tau Deficient Mice

Tau dysfunction is common in several neurodegenerative diseases including Alzheimer’s disease (AD) and frontotemporal dementia (FTD). Affective symptoms have often been associated with aberrant tau pathology and are commonly comorbid in patients with tauopathies, indicating a connection between tau functioning and mechanisms of depression. The current study investigated depression-like behavior in Mapt^−/− mice, which contain a targeted deletion of the gene coding for tau. We show that 6-month Mapt^−/− mice are resistant to depressive behaviors, as evidenced by decreased immobility time in the forced swim and tail suspension tests, as well as increased escape behavior in a learned helplessness task. Since depression has also been linked to deficient adult neurogenesis, we measured neurogenesis in the hippocampal dentate gyrus and subventricular zone using 5-bromo-2-deoxyuridine (BrdU) labeling. We found that neurogenesis is increased in the dentate gyrus of 14-month-old Mapt^−/− brains compared to wild type, providing a potential mechanism for their behavioral phenotypes. In addition to the hippocampus, an upregulation of proteins involved in neurogenesis was observed in the frontal cortex and amygdala of the Mapt^−/− mice using proteomic mass spectrometry. All together, these findings suggest that tau may have a role in the depressive symptoms observed in many neurodegenerative diseases and identify tau as a potential molecular target for treating depression.

Serotonin type 6 receptor antagonist attenuates the impairment of long-term potentiation and memory induced by Abeta

Alzheimer's disease (AD) is the most common cause of dementia, characterized by memory impairment and synaptic loss. Long-term potentiation (LTP), a type of synaptic plasticity, is impaired during AD. Serotonin type 6 receptor (5-HT6R) inactivation is proposed as a therapeutic target for AD. This study examined the effects of chronic administration of the 5-HT6R antagonist, SB-258585, on cognitive, memory, and hippocampal plasticity in a rat model of AD. Abeta neurotoxicity was induced in rats using Aβ (1.35 pmol intracerebroventricular [ICV] injection). The following groups were formed: control sustained surgery and saline-treated, Aβ+saline (1 μL ICV for 30 days), and Aβ+SB-258585 (0.024 mg/kg, ICV for 30 days). The learning and memory were tested using the novel object recognition and passive avoidance tests. Next, anesthetized rats were placed in a stereotaxic apparatus. The population spike (PS) amplitude and the slope of the excitatory postsynaptic potentials (fEPSPs) of the LTP were measured following high-frequency stimulation in the dentate gyrus. The Aβ injection reduced step-through latency in the passive avoidance test and decreased the discrimination index in the novel object test. Aβ diminished both the amplitude of hippocampal neuron population spikes and the slope of excitatory postsynaptic potentials, compared to the control group. The administration of SB-258585 in rats receiving Aβ attenuated the Aβ-induced deficits in cognition, memory, and LTP in comparison with the Aβ group. It can be concluded that chronic treatment with SB-258585 antagonist can prevent Aβ-related deficiencies in learning and memory performance by improving neuronal plasticity. SB-258585 can prevent the progression of AD.

Ablation of ErbB4 in parvalbumin-positive interneurons inhibits adult hippocampal neurogenesis through down-regulating BDNF/TrkB expression

Parvalbumin (PV) positive interneurons in the subgranular zone (SGZ) can regulate adult hippocampal neurogenesis. ErbB4 is mainly expressed in PV neurons in the hippocampus and is crucial for keeping normal function of PV neurons. However, whether ErbB4 in PV interneurons affects the adult hippocampal neurogenesis remains unknown. In the present study, we deleted ErbB4 specifically in PV neurons by crossing PV-Cre mice with ErbB4^f/f mice. Results of BrdU labeling and NeuN staining revealed that the proliferation of neural progenitors was increased but the survival and maturation of newborn neurons were decreased in the hippocampus of mice after deleting ErbB4 in PV neurons, suggesting that ErbB4 in PV neurons is closely associated with the process of adult hippocampal neurogenesis. Interestingly, the expression of brain-derived neurotrophic factor (BDNF) and its receptor, tropomyosin-related kinase B (TrkB), was significantly decreased in the hippocampus of ErbB4-deleted mice. Together, our data suggested that ErbB4 in PV neurons might modulate adult hippocampal neurogenesis by affecting BDNF-TrkB signaling pathway.

Amyloid β Peptide Compromises Neural Stem Cell Fate by Irreversibly Disturbing Mitochondrial Oxidative State and Blocking Mitochondrial Biogenesis and Dynamics

Alzheimer's disease (AD) is the most common neurodegenerative disease and is characterized by the accumulation of amyloid β peptide (Aβ). Although most AD mouse models present a decline in neurogenesis, they express mutated genes which regulate neurogenesis per se and are not present in most AD patients, thus masking the real impact of Aβ on adult neurogenesis. Mitochondrion, a well-known target of Aβ in neurons, is a main regulator of neural stem cell (NSC) fate. Here, we aimed to investigate the impact of Aβ on NSC mitochondria and cell fate decisions, namely whether and how Aβ affects neurogenesis. NSC fate and mitochondrial parameters, including biogenesis, dynamics, and oxidative stress, were evaluated. Our results showed that Aβ impaired NSC viability and proliferation and indirectly blocked neurogenic differentiation, by disrupting mitochondrial signaling of self-renewing NSCs. Importantly, Aβ decreased ATP levels, generated oxidative stress, and affected the radical scavenger system through SOD2 and SIRT3. Aβ also reduced mtDNA and mitochondrial biogenesis proteins, such as Tfam, PGC-1α, and NRF1, and inhibited activation of PGC-1α-positive regulator CREB. Moreover, Aβ triggered mitochondrial fragmentation in self-renewing NSCs and reduced mitochondrial fusion proteins, such as Mfn2 and ERRα. Notably, Aβ compromised NSC commitment and survival by irreversibly impairing mitochondria and thwarting any neurogenic rescue through mitochondrial biogenesis, dynamics, or radical scavenger system. Altogether, this study brings new perspective to rethink the molecular targets relevant for endogenous NSC-based strategies in AD.

Increased adult neurogenesis associated with reactive astrocytosis occurs prior to neuron loss in a mouse model of neurodegenerative disease

AIMS

This study was to investigate whether cell proliferation and adult neurogenesis are affected at early neurodegenerative stage when neuron loss has not begun to display.

METHODS AND RESULTS

Forebrain-specific nicastrin (NCT) conditional knockout (cKO) mice were generated by crossing NCT^f/f with CaMKIIα-Cre Tg mice. BrdU was used as a lineage tracer to label proliferating neural progenitor cells (NPCs). Immunohistochemistry (IHC) on BrdU indicated that the total number of BrdU positive (+) cells was increased in NCT cKO mice. IHC on doublecortin (DCX) showed that the total number of DCX+ cells was also increased in NCT cKO mice. NCT cKO mice displayed significant astrogliosis as well. However, NCT cKO mice at 3 months did not show significant neuronal death or synaptic loss.

CONCLUSIONS

NCT-dependent γ-secretase activity plays an important role in cell proliferation and immature neuron generation. Enhanced neurogenesis and astrogliosis may be early cellular events prior to the occurrence of neuronal death in neurodegenerative disease.

Depletion of adult neurogenesis exacerbates cognitive deficits in Alzheimer's disease by compromising hippocampal inhibition

Background

The molecular mechanism underlying progressive memory loss in Alzheimer’s disease is poorly understood. Neurogenesis in the adult hippocampus is a dynamic process that continuously changes the dentate gyrus and is important for hippocampal plasticity, learning and memory. However, whether impairments in neurogenesis affect the hippocampal circuitry in a way that leads to memory deficits characteristic of Alzheimer’s disease is unknown. Controversial results in that regard were reported in transgenic mouse models of amyloidosis.

Methods

Here, we conditionally ablated adult neurogenesis in APPswe/PS1ΔE9 mice by crossing these with mice expressing nestin-driven thymidine kinase (δ-HSV-TK).

Results

These animals show impairment in performance in contextual conditioning and pattern separation tasks following depletion of neurogenesis. Importantly, these deficits were not observed in age-matched APPswe/PS1ΔE9 or δ-HSV-TK mice alone. Furthermore, we show that cognitive deficits were accompanied by the upregulation of hyperphosphorylated tau in the hippocampus and in immature neurons specifically. Interestingly, we observed upregulation of the immediate early gene Zif268 (Egr-1) in the dentate gyrus, CA1 and CA3 regions of the hippocampus following learning in the neurogenesis-depleted δ-HSV-TK mice. This may suggest overactivation of hippocampal neurons in these areas following depletion of neurogenesis.

Conclusions

These results imply that neurogenesis plays an important role in the regulation of inhibitory circuitry of the hippocampus. This study suggests that deficits in adult neurogenesis may contribute to cognitive impairments, tau hyperphosphorylation in new neurons and compromised hippocampal circuitry in Alzheimer’s disease.

Electronic supplementary material

The online version of this article (10.1186/s13024-017-0207-7) contains supplementary material, which is available to authorized users.

Altered neurogenesis in mouse models of Alzheimer disease

Amyloid-β (Aβ) peptides, as well as a variety of other protein fragments, are derived from proteolytical cleavage of the amyloid precursor protein (APP) and have been demonstrated to play a key role in the pathological changes underlying Alzheimer disease (AD). In AD mouse models, altered neurogenesis has been repeatedly reported to be associated with further AD-typical pathological hallmarks such as extracellular plaque deposition, behavioral deficits or neuroinflammation. While a toxic role of Aβ in neurodegeneration and impaired neuronal progenitor proliferation is likely and well-accepted, recent findings also suggest an important influence of APP-derived proteolitical fragments like the APP intracellular domain (AICD), as well as of APP itself.