The presenilin-1 familial Alzheimer disease mutant P117L impairs neurogenesis in the hippocampus of adult mice

Neurogenesis in neurological and psychiatric diseases and brain injury: from bench to bedside

Researchers who have uncovered the presence of stem cells in an adult's central nervous system have not only challenged the dogma that new neurons cannot be generated during adulthood, but also shed light on the etiology and disease mechanisms underlying many neurological and psychiatric disorders. Brain trauma, neurodegenerative diseases, and psychiatric disorders pose enormous burdens at both personal and societal levels. Although medications for these disorders are widely used, the treatment mechanisms underlying the illnesses remain largely elusive. In the past decade, an increasing amount of evidence indicate that adult neurogenesis (i.e. generating new CNS neurons during adulthood) may be involved in the pathology of different CNS disorders, and thus neurogenesis may be a potential target area for treatments. Although new neurons were shown to be a major player in mediating treatment efficacy of neurological and psychotropic drugs on cognitive functions, it is still debatable if the altered production of new neurons can cause the disorders. This review hence seeks to discuss pre and current clinical studies that demonstrate the functional impact adult neurogenesis have on neurological and psychiatric illnesses while examining the related underlying disease mechanisms.

Soyasaponin I improved neuroprotection and regeneration in memory deficient model rats

Soy (Glycine Max Merr, family Leguminosae) has been reported to possess anti-cancer, anti-lipidemic, estrogen-like, and memory-enhancing effects. We investigated the memory-enhancing effects and the underlying mechanisms of soyasaponin I (soya-I), a major constituent of soy. Impaired learning and memory were induced by injecting ibotenic acid into the entorhinal cortex of adult rat brains. The effects of soya-I were evaluated by measuring behavioral tasks and neuronal regeneration of memory-deficient rats. Oral administration of soya-I exhibited significant memory-enhancing effects in the passive avoidance, Y-maze, and Morris water maze tests. Soya-Ι also increased BrdU incorporation into the dentate gyrus and the number of cell types (GAD67, ChAT, and VGluT1) in the hippocampal region of memory-deficient rats, whereas the number of reactive microglia (OX42) decreased. The mechanism underlying memory improvement was assessed by detecting the differentiation and proliferation of neural precursor cells (NPCs) prepared from the embryonic hippocampus (E16) of timed-pregnant Sprague-Dawley rats using immunocytochemical staining and immunoblotting analysis. Addition of soya-Ι in the cultured NPCs significantly elevated the markers for cell proliferation (Ki-67) and neuronal differentiation (NeuN, TUJ1, and MAP2). Finally, soya-I increased neurite lengthening and the number of neurites during the differentiation of NPCs. Soya-Ι may improve hippocampal learning and memory impairment by promoting proliferation and differentiation of NPCs in the hippocampus through facilitation of neuronal regeneration and minimization of neuro-inflammation.

Prolonged running, not fluoxetine treatment, increases neurogenesis, but does not alter neuropathology, in the 3xTg mouse model of Alzheimer's disease

Reductions in adult neurogenesis have been documented in the original 3xTg mouse model of Alzheimer's disease (AD), notably occurring at the same age when spatial memory deficits and amyloid plaque pathology appeared. As this suggested reduced neurogenesis was associated with behavioral deficits, we tested whether activity and pharmacological stimulation could prevent memory deficits and modify neurogenesis and/or neuropathology in the 3xTg model backcrossed to the C57Bl/6 strain. We chronically administered the antidepressant fluoxetine to one group of mice, allowed access to a running wheel in another, and combined both treatments in a third cohort. All treatments lasted for 11 months. The female 3xTg mice failed to exhibit any deficits in spatial learning and memory as measured in the Morris water maze, indicating that when backcrossed to the C57Bl/6 strain, the 3xTg mice lost the behavioral phenotype that was present in the original 3xTg mouse maintained on a hybrid background. Despite this, the backcrossed 3xTg mice expressed prominent intraneuronal amyloid beta (Aβ) levels in the cortex and amygdala, with lower levels in the CA1 area of the hippocampus. In the combined cohort, fluoxetine treatment interfered with exercise and reduced the total distance run. The extent of Aβ neuropathology, the tau accumulations, or BDNF levels, were not altered by prolonged exercise. Thus, neuropathology was present but not paralleled by spatial memory deficits in the backcrossed 3xTg mouse model of AD. Prolonged exercise for 11 months did improve the long-term survival of newborn neurons generated during middle-age, whereas fluoxetine had no effect. We further review and discuss the relevant literature in this respect.

Permeability transition pore-mediated mitochondrial superoxide flashes mediate an early inhibitory effect of amyloid beta1-42 on neural progenitor cell proliferation

Cellular damage by reactive oxygen species and altered neurogenesis are implicated in the etiology of AD and the pathogenic actions of amyloid β-peptide (Aβ); the underlying mechanisms and the early oxidative intracellular events triggered by Aβ are not established. In the present study, we found that mouse embryonic cortical neural progenitor cells exhibit intermittent spontaneous mitochondrial superoxide (SO) flashes that require transient opening of mitochondrial permeability transition pores (mPTPs). The incidence of mitochondria SO flash activity in neural progenitor cells (NPCs) increased during the first 6-24 hours of exposure to aggregating amyloid β-peptide (Aβ1-42), indicating an increase in transient mPTP opening. Subsequently, the SO flash frequency progressively decreased and ceased between 48 and 72 hours of exposure to Aβ1-42, during which time global cellular reactive oxygen species increased, mitochondrial membrane potential decreased, cytochrome C was released from mitochondria and the cells degenerated. Inhibition of mPTPs and selective reduction in mitochondrial SO flashes significantly ameliorated the negative effects of Aβ1-42 on NPC proliferation and survival. Our findings suggest that mPTP-mediated bursts of mitochondrial SO production is a relatively early and pivotal event in the adverse effects of Aβ1-42 on NPCs. If Aβ inhibits NPC proliferation in the brains of AD patients by a similar mechanism, then interventions that inhibit mPTP-mediated superoxide flashes would be expected to protect NPCs against the adverse effects of Aβ.

Endogenous expression of FAD-linked PS1 impairs proliferation, neuronal differentiation and survival of adult hippocampal progenitors

BACKGROUND

Alzheimer's disease (AD) is characterized by progressive memory loss and impaired cognitive function. Early-onset familial forms of the disease (FAD) are caused by inheritance of mutant genes encoding presenilin 1 (PS1) variants. We have demonstrated that prion promoter (PrP)-driven expression of human FAD-linked PS1 variants in mice leads to impairments in environmental enrichment (EE)-induced adult hippocampal neural progenitor cell (AHNPC) proliferation and neuronal differentiation, and have provided evidence that accessory cells in the hippocampal niche expressing PS1 variants may modulate AHNPC phenotypes, in vivo. While of significant interest, these latter studies relied on transgenic mice that express human PS1 variant transgenes ubiquitously and at high levels, and the consequences of wild type or mutant PS1 expressed under physiologically relevant levels on EE-mediated AHNPC phenotypes has not yet been tested.

RESULTS

To assess the impact of mutant PS1 on EE-induced AHNPC phenotypes when expressed under physiological levels, we exposed adult mice that constitutively express the PSEN1 M146V mutation driven by the endogenous PSEN1 promoter (PS1 M146V "knock-in" (KI) mice) to standard or EE-housed conditions. We show that in comparison to wild type PS1 mice, AHNPCs in mice carrying homozygous (PS1M146V/M146V) or heterozygous (PS1M146V/+) M146V mutant alleles fail to exhibit EE-induced proliferation and commitment towards neurogenic lineages. More importantly, we report that the survival of newborn progenitors are diminished in PS1 M146V KI mice exposed to EE-conditions compared to respective EE wild type controls.

CONCLUSIONS

Our findings reveal that expression at physiological levels achieved by a single PS1 M146V allele is sufficient to impair EE-induced AHNPC proliferation, survival and neuronal differentiation, in vivo. These results and our finding that microglia expressing a single PS1 M146V allele impairs the proliferation of wild type AHNPCs in vitro argue that expression of mutant PS1 in the AHNPC niche impairs AHNPCs phenotypes in a dominant, non-cell autonomous manner.

The touchscreen operant platform for testing working memory and pattern separation in rats and mice

The automated touchscreen operant chamber for rats and mice allows for the assessment of multiple cognitive domains within the same testing environment. This protocol presents the location discrimination (LD) task and the trial-unique delayed nonmatching-to-location (TUNL) task, which both assess memory for location. During these tasks, animals are trained to a predefined criterion during ∼20-40 daily sessions. In LD sessions, touching the same location on the screen is rewarded on consecutive trials, followed by a reversal of location-reward contingencies. TUNL, a working memory task, requires animals to 'nonmatch' to a sample location after a delay. In both the LD and TUNL tasks, spatial similarity can be varied, allowing assessment of pattern separation ability, a function that is thought to be performed by the dentate gyrus (DG). These tasks are therefore particularly useful in animal models of hippocampal, and specifically DG, function, but they additionally permit discernment of changes in pattern separation from those in working memory.

Concise review: Can stem cells be used to treat or model Alzheimer's disease?

Alzheimer's disease (AD) is the leading cause of age-related dementia, affecting over 5 million people in the U.S. alone. AD patients suffer from progressive neurodegeneration that gradually impairs their memory, ability to learn, and carry out daily activities. Unfortunately, current therapies for AD are largely palliative and several promising drug candidates have failed in recent clinical trials. There is therefore an urgent need to improve our understanding of AD pathogenesis, create innovative and predictive models, and develop new and effective therapies. In this review, we will discuss the potential of stem cells to aid in these challenging endeavors. Because of the widespread nature of AD pathology, cell-replacement strategies have been viewed as an incredibly challenging and unlikely treatment approach. Yet recent work shows that transplantation of neural stem cells (NSCs) can improve cognition, reduce neuronal loss, and enhance synaptic plasticity in animal models of AD. Interestingly, the mechanisms that mediate these effects appear to involve neuroprotection and trophic support rather than neuronal replacement. Stem cells may also offer a powerful new approach to model and study AD. Patient-derived induced pluripotent stem cells, for example, may help to advance our understanding of disease mechanisms. Likewise, studies of human embryonic and NSCs are helping to decipher the normal functions of AD-related genes; revealing intriguing roles in neural development.

Aβ increases neural stem cell activity in senescence-accelerated SAMP8 mice

Neurogenesis persists in the adult brain as a form of plasticity due to the existence of neural stem cells (NSCs). Alterations in neurogenesis have been found in transgenic Alzheimer's disease (AD) mouse models, but NSC activity and neurogenesis in sporadic AD models remains to be examined. We herein describe a remarkable increase in NSC proliferation in the forebrain of SAMP8, a non-transgenic mouse strain that recapitulates the transition from healthy aging to AD. The increase in proliferation is transient, precedes AD-like symptoms such as amyloid beta 1-42 [Aβ(1-42)] increase or gliosis, and is followed by a steep decline at later stages. Interestingly, in vitro studies indicate that secreted Aβ(1-42) and PI3K signaling may account for the early boost in NSC proliferation. Our results highlight the role of soluble Aβ(1-42) peptide and PI3K in the autocrine regulation of NSCs, and further suggest that over-proliferation of NSCs before the appearance of AD pathology may underlie neurogenic failure during the age-related progression of the disease. These findings have implications for therapeutic approaches based on neurogenesis in AD.

Amyloid β peptides promote autophagy-dependent differentiation of mouse neural stem cells: Aβ-mediated neural differentiation

Although regarded as neurotoxic, amyloid β (Aβ) peptides may also mediate a wide range of nonpathogenic processes. Autophagy has been implicated in Aβ-mediated effects, although its precise function in neural differentiation remains unknown. Here, we addressed the role of different Aβ fragments in neural stem cell (NSC) proliferation and differentiation, and investigated whether autophagy is involved in Aβ-induced alterations of neural fate. Our results demonstrate that neuronal and glial-specific protein markers are significantly induced by both Aβ1-40 and Aβ1-42. However, Aβ1-40 preferentially enhances neurogenesis of NSCs, as determined by βIII-tubulin, NeuN, and MAP2 neuronal marker immunoreactivity, while Aβ1-42 appears to favor gliogenesis. In contrast, Aβ25-35 does not influence NSC fate. The effect of Aβ1-40 on neurogenesis is partially dependent on its role in NSC self-renewal as both S-phase of the cell cycle and BrdU labeling were markedly increased. Nevertheless, Aβ1-40 resulted also in increased Tuj1 promoter activity. Autophagy, assessed by conversion of endogenous LC3-I/II, fluorescence of pGFP-LC3-transfected cells, and Atg9 protein levels, was evident in both Aβ1-40- and Aβ1-42-treated NSCs, independently of reactive oxygen species production and apoptosis. Finally, inhibition of autophagy by pharmacologic means abrogated Aβ-induced lineage-specific protein markers. These results support distinct roles for different Aβ peptides in NSC fate decision and underline the importance of autophagy control of this process.

Mutant presenilin 1 expression in excitatory neurons impairs enrichment-mediated phenotypes of adult hippocampal progenitor cells

Inheritance of mutant presenilin 1 genes (PSEN1) encoding presenilin 1 (PS1)variants causes autosomal dominant forms of familial Alzheimer's disease (FAD). We previously reported that ubiquitous expression of FAD-linked PS1 variants in mice impairs environmental enrichment (EE)-induced proliferation and neuronal commitment of adult hippocampal neural progenitor cells (AHNPCs). Notably, the self-renewal and differentiation properties of cultured AHNPCs expressing either human PS1 wild-type or PS1 variants were identical, suggesting that accessory cells within the hippocampal niche expressing PS1 variants may modulate AHNPC phenotypes in vivo. We now report that nontransgenic mouse AHNPCs transduced with retroviruses harboring cDNAs that encode either human PS1 wild-type or FAD-linked PS1 variants show no differences in EE-mediated proliferation and neuronal differentiation. Moreover, conditional inactivation of a mutant PS1 transgene in type-1 primary progenitor cells failed to rescue impairments of EE-induced proliferation, survival, or neurogenesis. In contrast, conditional inactivation of the mutant PS1 transgene in excitatory neurons of the mouse forebrain largely rescued the deficits in EE-induced proliferation and survival of AHNPCs, but not their differentiation into mature neuronal phenotypes. These results persuasively argue for a noncell autonomous effect of FAD-linked PS1 mutants on EE-mediated adult hippocampal neurogenesis.

Chronic treatment with the GLP1 analogue liraglutide increases cell proliferation and differentiation into neurons in an AD mouse model

Neurogenesis is a life long process, but the rate of cell proliferation and differentiation decreases with age. In Alzheimer's patients, along with age, the presence of Aβ in the brain inhibits this process by reducing stem cell proliferation and cell differentiation. GLP-1 is a growth factor that has neuroprotective properties. GLP1 receptors are present on neuronal progenitor cells, and the GLP-1 analogue liraglutide has been shown to increase cell proliferation in an Alzheimer's disease (AD) mouse model. Here we investigated acute and chronic effects of liraglutide on progenitor cell proliferation, neuroblast differentiation and their subsequent differentiation into neurons in wild type and APP/PS-1 mice at different ages. APP/PS1 and their littermate controls, aged 3, 6, 12, 15 months were injected acutely or chronically with 25 nmol/kg liraglutide. Acute treatment with liraglutide showed an increase in cell proliferation in APP/PS1 mice, but not in controls whereas chronic treatment increased cell proliferation at all ages (BrdU and Ki67 markers). Moreover, numbers of immature neurons (DCX) were increased in both acute and chronic treated animals at all ages. Most newly generated cells differentiated into mature neurons (NeuN marker). A significant increase was observed with chronically treated 6, 12, 15 month APP/PS1 and WT groups. These results demonstrate that liraglutide, which is currently on the market as a treatment for type 2 diabetes (Victoza^TM), increases neurogenesis, which may have beneficial effects in neurodegenerative disorders like AD.

Sex Differences in Presynaptic Density and Neurogenesis in Middle-Aged ApoE4 and ApoE Knockout Mice

Atherosclerosis and apolipoprotein E ε4 (APOE4) genotype are risk factors for Alzheimer's disease (AD) and cardiovascular disease (CVD). Sex differences exist in prevalence and manifestation of both diseases. We investigated sex differences respective to aging, focusing on cognitive parameters in apoE4 and apoE knockout (ko) mouse models of AD and CVD. Presynaptic density and neurogenesis were investigated immunohistochemically in male and female apoE4, apoE ko, and wild-type mice. Middle-aged female apoE4 mice showed decreased presynaptic density in the inner molecular layer of the dentate gyrus of the hippocampus. Middle-aged female apoE ko mice showed a trend towards increased neurogenesis in the hippocampus compared with wild-type mice. No differences in these parameters could be observed in middle-aged male mice. Specific harmful interactions between apoE4 and estrogen could be responsible for decreased presynaptic density in female apoE4 mice. The trend of increased neurogenesis found in female apoE ko mice supports previous studies suggesting that temporarily increased amount of synaptic contacts and/or neurogenesis is a compensatory mechanism for synaptic failure. To our knowledge, no other studies investigating presynaptic density in aging female apoE4 or apoE ko mice are available. Sex-specific differences between APOE genotypes could account for some sex differences in AD and CVD.

The function of BMP4 during neurogenesis in the adult hippocampus in Alzheimer's disease

Alzheimer's disease (AD) is an age-related, progressive and irreversible neurodegenerative disease that results in the loss of selected neurons throughout the basal forebrain, amygdala, hippocampus, and cortical area as well as progressive deficits of cognition and memory. The subgranular zone (SGZ) of the hippocampal dentate gyrus (DG) is one of the regions where adult neurogenesis occurs in mammals, including humans and non-human primates. The new granule cells, which are the primary excitatory neurons in the DG, contribute to the processes of learning and memory. The changes in neurogenesis observed during the initial stages and progression of AD suggest that the modulation of the new production of neurons at neurogenic sites may exert profound effects on hippocampal function. Bone morphogenetic protein-4 (BMP4) and its antagonist Noggin contribute to the modulation of neurogenesis in the adult hippocampus, thereby affecting hippocampal function. This review focuses on the role of BMP4 and Noggin in the control of the stem and precursor cells in the adult hippocampus during AD and their potential as a possible therapeutic strategy for AD sufferers. It is helpful to extend the understanding of the control of stem cells in the normal and diseased hippocampus.

Sox21 promotes hippocampal adult neurogenesis via the transcriptional repression of the Hes5 gene

Despite the importance of the production of new neurons in the adult hippocampus, the transcription network governing this process remains poorly understood. The High Mobility Group (HMG)-box transcription factor, Sox2, and the cell surface activated transcriptional regulator, Notch, play important roles in CNS stem cells. Here, we demonstrate that another member of the SoxB (Sox1/Sox2/Sox3) transcription factor family, Sox21, is also a critical regulator of adult neurogenesis in mouse hippocampus. Loss of Sox21 impaired transition of progenitor cells from type 2a to type 2b, thereby reducing subsequent production of new neurons in the adult dentate gyrus. Analysis of the Sox21 binding sites in neural stem/progenitor cells indicated that the Notch-responsive gene, Hes5, was a target of Sox21. Sox21 repressed Hes5 gene expression at the transcriptional level. Simultaneous overexpression of Hes5 and Sox21 revealed that Hes5 was a downstream effector of Sox21 at the point where the Notch and Sox pathways intersect to control the number of neurons in the adult hippocampus. Therefore, Sox21 controls hippocampal adult neurogenesis via transcriptional repression of the Hes5 gene.

The classification of microglial activation phenotypes on neurodegeneration and regeneration in Alzheimer's disease brain

Alzheimer's disease (AD) is a neurodegenerative disease characterized by progressive decline of cognitive function. There is no therapy that can halt or reverse its progression. Contemporary research suggests that age-dependent neuroinflammatory changes may play a significant role in the decreased neurogenesis and cognitive impairments in AD. The innate immune response is characterized by pro-inflammatory (M1) activation of macrophages and subsequent production of specific cytokines, chemokines, and reactive intermediates, followed by resolution and alternative activation for anti-inflammatory signaling (M2a) and wound healing (M2c). We propose that microglial activation phenotypes are analogous to those of macrophages and that their activation plays a significant role in regulating neurogenesis in the brain. Microglia undergo a switch from an M2- to an M1-skewed activation phenotype during aging. This review will assess the neuroimmunological studies that led to characterization of the different microglial activation states in AD mouse models. It will also discuss the roles of microglial activation on neurogenesis in AD and propose anti-inflammatory molecules as exciting therapeutic targets for research. Molecules such as interleukin-4 and CD200 have proven to be important anti-inflammatory mediators in the regulation of neuroinflammation in the brain, which will be discussed in detail for their therapeutic potential.

Transgenic mouse models of Alzheimer disease: developing a better model as a tool for therapeutic interventions

Alzheimer disease (AD) is the leading cause of dementia among elderly. Currently, no effective treatment is available for AD. Analysis of transgenic mouse models of AD has facilitated our understanding of disease mechanisms and provided valuable tools for evaluating potential therapeutic strategies. In this review, we will discuss the strengths and weaknesses of current mouse models of AD and the contribution towards understanding the pathological mechanisms and developing effective therapies.

Mouse models of Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia, affecting 35 million people today. The search for new treatments is made ever more urgent by prospects for increasing prevalence due to population aging. Mouse models are one of the most important research tools for finding new treatments for AD. Here, we review those models. We begin by briefly reviewing the AD genetics on which mouse models are based and then consider the most common mouse models of AD, including mice transgenic for human amyloid precursor protein (hAPP) and beta-amyloid (Aβ), mice expressing mutant presenilin genes, mice modeling tau's role in AD, and apolipoprotein E models. The discussion highlights key features and important differences between these mouse models. We conclude with a discussion about the role of AD mouse models in the translational pipeline.

Neural stem cells: mechanisms and modeling

In the adult brain, neural stem cells have been found in two major niches: the dentate gyrus and the subventricular zone [corrected]. Neurons derived from these stem cells contribute to learning, memory, and the autonomous repair of the brain under pathological conditions. Hence, the physiology of adult neural stem cells has become a significant component of research on synaptic plasticity and neuronal disorders. In addition, the recently developed induced pluripotent stem cell technique provides a powerful tool for researchers engaged in the pathological and pharmacological study of neuronal disorders. In this review, we briefly summarize the research progress in neural stem cells in the adult brain and in the neuropathological application of the induced pluripotent stem cell technique.

The effect of ageing on neurogenesis and oxidative stress in the APP(swe)/PS1(deltaE9) mouse model of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterised by memory loss and impaired cognitive function. One of the hallmarks of AD is the formation of beta amyloid (Aβ) plaques. Aβ has neurodegenerative properties and aggregates in the brain, causing inflammation, oxidative stress and eventually neuronal loss. In AD, adult neurogenesis in the dentate gyrus (DG) of the hippocampus is known to be impaired. We tested how ageing affects neurogenesis and oxidative stress in the commonly used APP(SWE)/PS1(ΔE9) mouse model of AD and their wild type (wt) littermate controls aged 3, 5, 10 and 15months. Progenitor cell proliferation in the DG of APP/PS1 was lower at 3, 5 and 10months compared to controls, while oxidative stress in APP/PS1 mice was increased in the cortex at 3 and 5months of age compared to controls. The numbers of new neurons in the DG were decreased in APP/PS1 mice at 10 and 15months. In APP/PS1 mice, Aβ plaques were evident in the cortex from 3months onward; however these were small and few. Plaque size and number consistently increased with age in APP/PS1 mice. These results show that the damage to the brain occurs already very early in the brain, and although neurogenesis is impaired, it is still active even in late stage AD. Therefore, therapies would have the best effects if started early, but promoting neurogenesis may act in a protective and reconstructive way even in later stages of AD.

Protective effects of a Rhodiola crenulata extract and salidroside on hippocampal neurogenesis against streptozotocin-induced neural injury in the rat

Previously we have demonstrated that a Rhodiola crenulata extract (RCE), containing a potent antioxidant salidroside, promotes neurogenesis in the hippocampus of depressive rats. The current study was designed to further investigate the protective effect of the RCE on neurogenesis in a rat model of Alzheimer's disease (AD) induced by an intracerebroventricular injection of streptozotocin (STZ), and to determine whether this neuroprotective effect is induced by the antioxidative activity of salidroside. Our results showed that pretreatment with the RCE significantly improved the impaired neurogenesis and simultaneously reduced the oxidative stress in the hippocampus of AD rats. In vitro studies revealed that (1) exposure of neural stem cells (NSCs) from the hippocampus to STZ strikingly increased intracellular reactive oxygen species (ROS) levels, induced cell death and perturbed cell proliferation and differentiation, (2) hydrogen peroxide induced similar cellular activities as STZ, (3) pre-incubation of STZ-treated NSCs with catalase, an antioxidant, suppressed all these cellular activities induced by STZ, and (4) likewise, pre-incubation of STZ-treated NSCs with salidroside, also an antioxidant, suppressed all these activities as catalase: reduction of ROS levels and NSC death with simultaneous increases in proliferation and differentiation. Our findings indicated that the RCE improved the impaired hippocampal neurogenesis in the rat model of AD through protecting NSCs by its main ingredient salidroside which scavenged intracellular ROS.

Adult hippocampal neurogenesis and its role in Alzheimer's disease

The hippocampus, a brain area critical for learning and memory, is especially vulnerable to damage at early stages of Alzheimer's disease (AD). Emerging evidence has indicated that altered neurogenesis in the adult hippocampus represents an early critical event in the course of AD. Although causal links have not been established, a variety of key molecules involved in AD pathogenesis have been shown to impact new neuron generation, either positively or negatively. From a functional point of view, hippocampal neurogenesis plays an important role in structural plasticity and network maintenance. Therefore, dysfunctional neurogenesis resulting from early subtle disease manifestations may in turn exacerbate neuronal vulnerability to AD and contribute to memory impairment, whereas enhanced neurogenesis may be a compensatory response and represent an endogenous brain repair mechanism. Here we review recent findings on alterations of neurogenesis associated with pathogenesis of AD, and we discuss the potential of neurogenesis-based diagnostics and therapeutic strategies for AD.

Modeling human neurodegenerative diseases in transgenic systems

Transgenic systems are widely used to study the cellular and molecular basis of human neurodegenerative diseases. A wide variety of model organisms have been utilized, including bacteria (Escherichia coli), plants (Arabidopsis thaliana), nematodes (Caenorhabditis elegans), arthropods (Drosophila melanogaster), fish (zebrafish, Danio rerio), rodents (mouse, Mus musculus and rat, Rattus norvegicus) as well as non-human primates (rhesus monkey, Macaca mulatta). These transgenic systems have enormous value for understanding the pathophysiological basis of these disorders and have, in some cases, been instrumental in the development of therapeutic approaches to treat these conditions. In this review, we discuss the most commonly used model organisms and the methodologies available for the preparation of transgenic organisms. Moreover, we provide selected examples of the use of these technologies for the preparation of transgenic animal models of neurodegenerative diseases, including Alzheimer's disease (AD), frontotemporal lobar degeneration (FTLD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD) and Parkinson's disease (PD) and discuss the application of these technologies to AD as an example of how transgenic modeling has affected the study of human neurodegenerative diseases.

Opportunities and challenges in developing Alzheimer disease therapeutics

Alzheimer disease (AD) is a chronic, progressive disorder with an average disease progression of 7-10 years. However, the histopathological hallmark lesions of this disease, the extracellular Aβ plaques and the intraneuronal neurofibrillary tangles, start as early as childhood in the affected individuals. AD is multifactorial and probably involves many different etiopathogenic mechanisms. Thus, while AD offers a wide window of opportunity that practically includes the whole life span of the affected individuals, and numerous therapeutic targets, the multifactorial nature of this disease also makes the selection of the therapeutic targets an immensely challenging task. In addition to β-amyloidosis and neurofibrillary degeneration, the AD brain also is compromised in its ability to regenerate by enhancing neurogenesis and neuronal plasticity. An increasing number of preclinical studies in transgenic mouse models of AD show that enhancement of neurogenesis and neuronal plasticity can reverse cognitive impairment. Development of both drugs that can inhibit neurodegeneration and drugs that can increase the regenerative capacity of the brain by enhancing neurogenesis and neuronal plasticity are required to control AD.

FGF2 gene transfer restores hippocampal functions in mouse models of Alzheimer's disease and has therapeutic implications for neurocognitive disorders

The adult hippocampus plays a central role in memory formation, synaptic plasticity, and neurogenesis. The subgranular zone of the dentate gyrus contains neural progenitor cells with self-renewal and multilineage potency. Transgene expression of familial Alzheimer's disease-linked mutants of β-amyloid precursor protein (APP) and presenilin-1 leads to a significant inhibition of neurogenesis, which is potentially linked to age-dependent memory loss. To investigate the effect of neurogenesis on cognitive function in a relevant disease model, FGF2 gene is delivered bilaterally to the hippocampi of APP+presenilin-1 bigenic mice via an adenoassociated virus serotype 2/1 hybrid (AAV2/1-FGF2). Animals injected with AAV2/1-FGF2 at a pre- or postsymptomatic stage show significantly improved spatial learning in the radial arm water maze test. A neuropathological investigation demonstrates that AAV2/1-FGF2 injection enhances the number of doublecortin, BrdU/NeuN, and c-fos-positive cells in the dentate gyrus, and the clearance of fibrillar amyloid-β peptide (Aβ) in the hippocampus. AAV2/1-FGF2 injection also enhances long-term potentiation in another APP mouse model (J20) compared with control AAV2/1-GFP-injected littermates. An in vitro study confirmed the enhanced neurogenesis of mouse neural stem cells by direct AAV2/1-FGF2 infection in an Aβ oligomer-sensitive manner. Further, FGF2 enhances Aβ phagocytosis in primary cultured microglia, and reduces Aβ production from primary cultured neurons after AAV2/1-FGF2 infection. Thus, our data indicate that virus-mediated FGF2 gene delivery has potential as an alternative therapy of Alzheimer's disease and possibly other neurocognitive disorders.

The role of mitochondria in brain aging and the effects of melatonin

Melatonin is an endogenous indoleamine present in different tissues, cellular compartments and organelles including mitochondria. When melatonin is administered orally, it is readily available to the brain where it counteracts different processes that occur during aging and age-related neurodegenerative disorders. These aging processes include oxidative stress and oxidative damage, chronic and acute inflammation, mitochondrial dysfunction and loss of neural regeneration. This review summarizes age related changes in the brain and the importance of oxidative/nitrosative stress and mitochondrial dysfunction in brain aging. The data and mechanisms of action of melatonin in relation to aging of the brain are reviewed as well.

The roles of amyloid precursor protein (APP) in neurogenesis: Implications to pathogenesis and therapy of Alzheimer disease

The amyloid-beta (Aβ) peptide is the derivative of amyloid precursor protein (APP) generated through sequential proteolytic processing by β- and γ-secretases. Excessive accumulation of Aβ, the main constituent of amyloid plaques, has been implicated in the etiology of Alzheimer's disease (AD). It was found recently that the impairments of neurogenesis in brain were associated with the pathogenesis of AD. Furthermore recent findings implicated that APP could function to influence proliferation of neural progenitor cells (NPC) and might regulate transcriptional activity of various genes. Studies demonstrated that influence of neurogenesis by APP is conferred differently via its two separate domains, soluble secreted APPs (sAPPs, mainly sAPPα) and APP intracellular domain (AICD). The sAPPα was shown to be neuroprotective and important to neurogenesis, whereas AICD was found to negatively modulate neurogenesis. Furthermore, it was demonstrated recently that microRNA could function to regulate APP expression, APP processing, Aβ accumulation and subsequently influence neurotoxicity and neurogenesis related to APP, which was implicated to AD pathogenesis, especially for sporadic AD. Based on data accumulated, secretase balances were proposed. These secretase balances could influence the downstream balance related to regulation of neurogenesis by AICD and sAPPα as well as balance related to influence of neuron viability by Aβ and sAPPα. Disruption of these secretase balances could be culprits to AD onset.

Neurodegenerative disease and adult neurogenesis

The generation and cell death of newly generated cells have critical roles in brain development and maintenance in the embryonic and adult brain. Alterations in these processes are also seen in neurodegenerative diseases. Genes that are key players in neurodegenerative diseases (α-synuclein, presenilin-1, tau, huntingtin) are also physiologically involved in modulating brain plasticity. Interestingly, in some neurodegenerative diseases, the specific alterations in neurogenic areas such as the dentate gyrus and subventricular zone/olfactory bulb system parallel the early or premotor symptoms that are seen in the early stages of these diseases, such as depression, anxiety or olfactory dysfunction. We will review the modulation of neurogenesis in animal models and human brains of Parkinson's disease, Huntington's disease and Alzheimer's disease.

In vivo monitoring of adult neurogenesis in health and disease

Adult neurogenesis, i.e., the generation of new neurons in the adult brain, presents an enormous potential for regenerative therapies of the central nervous system. While 5-bromo-2'-deoxyuridine labeling and subsequent histology or immunohistochemistry for cell-type-specific markers is still the gold standard in studies of neurogenesis, novel techniques, and tools for in vivo imaging of neurogenesis have been recently developed and successfully applied. Here, we review the latest progress on these developments, in particular in the area of magnetic resonance imaging (MRI) and optical imaging. In vivo in situ labeling of neural progenitor cells (NPCs) with micron-sized iron oxide particles enables longitudinal visualization of endogenous progenitor cell migration by MRI. The possibility of genetic labeling for cellular MRI was demonstrated by using the iron storage protein ferritin as the MR reporter-gene. However, reliable and consistent results using ferritin imaging for monitoring endogenous progenitor cell migration have not yet been reported. In contrast, genetic labeling of NPCs with a fluorescent or bioluminescent reporter has led to the development of some powerful tools for in vivo imaging of neurogenesis. Here, two strategies, i.e., viral labeling of stem/progenitor cells and transgenic approaches, have been used. In addition, the use of specific promoters for neuronal progenitor cells such as doublecortin increases the neurogenesis-specificity of the labeling. Naturally, the ultimate challenge will be to develop neurogenesis imaging methods applicable in humans. Therefore, we certainly need to consider other modalities such as positron emission tomography and proton magnetic resonance spectroscopy ((1)H-MRS), which have already been implemented for both animals and humans. Further improvements of sensitivity and neurogenesis-specificity are nevertheless required for all imaging techniques currently available.

Alzheimer Aβ disrupts the mitotic spindle and directly inhibits mitotic microtubule motors

Chromosome mis-segregation and aneuploidy are greatly induced in Alzheimer's disease and models thereof by mutant forms of the APP and PS proteins and by their product, the Ab peptide. Here we employ human somatic cells and Xenopus egg extracts to show that Aβ impairs the assembly and maintenance of the mitotic spindle. Mechanistically, these defects result from Aβ's inhibition of mitotic motor kinesins, including Eg5, KIF4A and MCAK. In vitro studies show that oligomeric Aβ directly inhibits recombinant MCAK by a noncompetitive mechanism. In contrast, inhibition of Eg5 and KIF4A is competitive with respect to both ATP and microtubules, indicating that Aβ interferes with their interactions with the microtubules of the mitotic spindle. Consistently, increased levels of polymerized microtubules or of the microtubule stabilizing protein Tau significantly decrease the inhibitory effect of Aβ on Eg5 and KIF4A. Together, these results indicate that by disrupting the interaction between specific kinesins and microtubules and by exerting a direct inhibitory effect on the motor activity, excess Ab deregulates the mechanical forces that govern the spindle and thereby leads to the generation of defective mitotic structures. The resulting defect in neurogenesis can account for the over 30% aneuploid/hyperploid, degeneration-prone neurons observed in Alzheimer disease brain. The finding of mitotic motors including Eg5 in mature post-mitotic neurons implies that their inhibition by Ab may also disrupt neuronal function and plasticity.

Presenilin-1 regulates neural progenitor cell differentiation in the adult brain

Presenilin-1 (PS1) is the catalytic core of the aspartyl protease γ-secretase. Previous genetic studies using germ-line deletion of PS1 and conditional knock-out mice demonstrated that PS1 plays an essential role in neuronal differentiation during neural development, but it remained unclear whether PS1 plays a similar role in neurogenesis in the adult brain. Here we show that neural progenitor cells infected with lentiviral vectors-expressing short interfering RNA (siRNA) for the exclusive knockdown of PS1 in the neurogenic microenvironments, exhibit a dramatic enhancement of cell differentiation. Infected cells differentiated into neurons, astrocytes and oligodendrocytes, suggesting that multipotentiality of neural progenitor cells is not affected by reduced levels of PS1. Neurosphere cultures treated with γ-secretase inhibitors exhibit a similar phenotype of enhanced cell differentiation, suggesting that PS1 function in neural progenitor cells is γ-secretase-dependent. Neurospheres infected with lentiviral vectors expressing siRNA for the targeting of PS1 differentiated even in the presence of the proliferation factors epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF), suggesting that PS1 dominates EFG and bFGF signaling pathways. Reduction of PS1 expression in neural progenitor cells was accompanied by a decrease in EGF receptor and β-catenin expression level, suggesting that they are downstream essential transducers of PS1 signaling in adult neural progenitor cells. These findings suggest a physiological role for PS1 in adult neurogenesis, and a potential target for the manipulation of neural progenitor cell differentiation.

Mechanisms mediating brain and cognitive reserve: experience-dependent neuroprotection and functional compensation in animal models of neurodegenerative diseases

'Brain and cognitive reserve' (BCR) refers here to the accumulated neuroprotective reserve and capacity for functional compensation induced by the chronic enhancement of mental and physical activity. BCR is thought to protect against, and compensate for, a range of different neurodegenerative diseases, as well as other neurological and psychiatric disorders. In this review we will discuss BCR, and its potential mechanisms, in neurodegenerative disorders, with a focus on Huntington's disease (HD) and Alzheimer's disease (AD). Epidemiological studies of AD, and other forms of dementia, provided early evidence for BCR. The first evidence for the beneficial effects of enhanced mental and physical activity, and associated mechanistic insights, in an animal model of neurodegenerative disease was provided by experiments using HD transgenic mice. More recently, experiments on animal models of HD, AD and various other brain disorders have suggested potential molecular and cellular mechanisms underpinning BCR. We propose that sophisticated insight into the processes underlying BCR, and identification of key molecules mediating these beneficial effects, will pave the way for therapeutic advances targeting these currently incurable neurodegenerative diseases.

Neurogenesis in the aged and neurodegenerative brain

It has been well established that adult neurogenesis occurs throughout life in the subventricular (SVZ) and subgranular (SGZ) zones. However, the exact role of this type of brain plasticity is not yet clear. Many studies have shown that neurogenesis is involved in learning and memory. This has led to a hypothesis which suggests that impairment in memory during aging and neurodegenerative diseases such as Alzheimer's disease (AD) may involve abnormal neurogenesis. Indeed, during aging, there is an age-related decline in adult neurogenesis. This decline is mostly related to decreased proliferation, associated to decreased stimulation to proliferate in an aging brain. In AD, there is also evidence for decreased neurogenesis, that accompanies the neuronal loss characteristic of the disease. Interestingly in AD, there is increased proliferation, that may be caused by increasing amounts of soluble amyloid ß42-protein (Aβ₄₂). However, most of these new neurons die, and fibrillar Aβ₄₂ seems to be involved in generating an inappropriate environment for these neurons to mature. These findings open prospects for new strategies that can increase neurogenesis in normal or pathological processes in the aging brain, and by that decrease memory deficits.

Neurogenesis in Alzheimer's disease

It is widely acknowledged that neural stem cells generate new neurons through the process of neurogenesis in the adult brain. In mammals, adult neurogenesis occurs in two areas of the CNS: the subventricular zone and the subgranular zone of the dentate gyrus of the hippocampus. The newly generated cells display neuronal morphology, generate action potentials and receive functional synaptic inputs, their properties being equivalent to those of mature neurons. Alzheimer's disease (AD) is the widespread cause of dementia, and is an age-related, progressive and irreversible neurodegenerative disease that results in massive neuronal death and deterioration of cognitive functions. Here, we overview the relations between adult neurogenesis and AD, and try to analyse the controversies in the field. We also summarise recent data obtained in the triple transgenic model of AD that show time- and region-specific impairment of neurogenesis, which may account for the early changes in synaptic plasticity and cognitive impairments that develop prior to gross neurodegenerative alterations and that could underlie new rescue therapies.

Calcium and neurogenesis in Alzheimer's disease

It was evidenced that impairment of calcium homeostasis is a potential mechanism in the development of Alzheimer's disease (AD). It remains, however, unclear how the calcium signaling are associated with in AD progression. Here we review recent studies to discuss the relationship among the signaling of intracellular calcium concentration, neurogenic activity, and AD progression. Analyzing these findings may provide new ideas to improve the neurogenic status in pathological processes in the aging brain.

When neurogenesis encounters aging and disease

In this review, we consider the evidence that a reduction in neurogenesis underlies aging-related cognitive deficits and impairments in disorders such as Alzheimer's disease (AD). The molecular and cellular alterations associated with impaired neurogenesis in the aging brain are discussed. Dysfunction of presenilin-1, misprocessing of amyloid precursor protein and toxic effects of hyperphosphorylated tau and β-amyloid probably contribute to impaired neurogenesis in AD. Because factors such as exercise, environmental enrichment and dietary energy restriction enhance neurogenesis, and protect against age-related cognitive decline and AD, knowledge of the underlying neurogenic signaling pathways could lead to novel therapeutic strategies for preserving brain function. In addition, manipulation of endogenous neural stem cells and stem cell transplantation, as stand-alone or adjunct treatments, seems promising.

Presenilin mouse and zebrafish models for dementia: focus on neurogenesis

Autosomal dominant mutations in the presenilin gene PSEN cause familial Alzheimer's disease (AD), a neurological disorder pathologically characterized by intraneuronal accumulation and extracellular deposition of amyloid-β in plaques and intraneuronal, hyperphosphorylated tau aggregation in neurofibrillary tangles. Presenilins (PS/PSENs) are part of the proteolytic γ-secretase complex, which cleaves substrate proteins within the membrane. Cleavage of the amyloid precursor protein (APP) by γ-secretase releases amyloid-β peptides. Besides its role in the processing of APP and other transmembrane proteins, presenilin plays an important role in neural progenitor cell maintenance and neurogenesis. In this review, we discuss the role of presenilin in relation to neurogenesis and neurodegeneration and review the currently available presenilin animal models. In addition to established mouse models, zebrafish are emerging as an attractive vertebrate model organism to study the role of presenilin during the development of the nervous system and in neurodegenerative disorders involving presenilin. Zebrafish is a suitable model organism for large-scale drug screening, making this a valuable model to identify novel therapeutic targets for AD.

Pharmacologic reversal of neurogenic and neuroplastic abnormalities and cognitive impairments without affecting Aβ and tau pathologies in 3xTg-AD mice

In addition to the occurrence of numerous neurofibrillary tangles and Aβ plaques, neurogenesis and neuronal plasticity are markedly altered in Alzheimer disease (AD). Although the most popular therapeutic approach has been to inhibit neurodegeneration, another is to promote neurogenesis and neuronal plasticity by utilizing the regenerative capacity of the brain. Here we show that, in a transgenic mouse model of AD, 3xTg-AD mice, there was a marked deficit in neurogenesis and neuroplasticity, which occurred before the formation of any neurofibrillary tangles or Aβ plaques and was associated with cognitive impairment. Furthermore, peripheral administration of Peptide 6, an 11-mer, which makes an active region of ciliary neurotrophic factor (CNTF, amino acid residues 146-156), restored cognition by enhancing neurogenesis and neuronal plasticity in these mice. Although this treatment had no detectable effect on Aβ and tau pathologies in 9-month animals, it enhanced neurogenesis in dentate gyrus, reduced ectopic birth in the granular cell layer, and increased neuronal plasticity in the hippocampus and cerebral cortex. These findings, for the first time, demonstrate the possibility of therapeutic treatment of AD and related disorders by peripheral administration of a peptide corresponding to a biologically active region of CNTF.

Impaired neurogenesis is an early event in the etiology of familial Alzheimer's disease in transgenic mice

Formation of new neurons in the adult brain takes place in the subventricular zone and in the subgranule layer of the dentate gyrus throughout life. Neurogenesis is thought to play a role in hippocampus- and olfaction-dependent learning and memory. However, whether impairments in neurogenesis take place in learning and memory disorders, such as Alzheimer's disease, is yet to be established. Importantly, it remains to be elucidated whether neurogenic impairments play a role in the course of the disease or are the result of extensive neuropathology. We now report that transgenic mice harboring familial Alzheimer's disease-linked mutant APPswe/PS1DeltaE9 exhibit severe impairments in neurogenesis that are evident as early as 2 months of age. These mice exhibit a significant reduction in the proliferation of neural progenitor cells and their neuronal differentiation. Interestingly, levels of hyperphosphorylated tau, the cytotoxic precursor of the Alzheimer's disease hallmark neurofibrillary tangles, are particularly high in the neurogenic niches. Isolation of neural progenitor cells in culture reveals that APPswe/PS1DeltaE9-expressing neurospheres exhibit impaired proliferation and tau hyperphosphorylation compared with wildtype neurospheres isolated from nontransgenic littermates. This study suggests that impaired neurogenesis is an early critical event in the course of Alzheimer's disease that may underlie memory impairments, at least in part, and exacerbate neuronal vulnerability in the hippocampal formation and olfaction circuits. Furthermore, impaired neurogenesis is the result of both intrinsic pathology in neural progenitor cells and extrinsic neuropathology in the neurogenic niches. Finally, hyperphosphorylation of the microtubule-associated protein tau, a critical player in cell proliferation, neuronal maturation, and axonal transport, is a major contributor to impaired neurogenesis in Alzheimer's disease.

Presenilin 1 mutants impair the self-renewal and differentiation of adult murine subventricular zone-neuronal progenitors via cell-autonomous mechanisms involving notch signaling

The vast majority of pedigrees with familial Alzheimer's disease (FAD) are caused by inheritance of mutations in the PSEN1 1 gene. While genetic ablation studies have revealed a role for presenilin 1 (PS1) in embryonic neurogenesis, little information has emerged regarding the potential effects of FAD-linked PS1 variants on proliferation, self-renewal and differentiation, key events that control cell fate commitment of adult brain neural progenitors (NPCs). We used adult brain subventricular zone (SVZ)-derived NPC cultures transduced with recombinant lentivirus as a means to investigate the effects of various PS1 mutants on self-renewal and differentiation properties. We now show that viral expression of several PS1 mutants in NPCs leads to impaired self-renewal and altered differentiation toward neuronal lineage, in vitro. In line with these observations, diminished constitutive proliferation and steady-state SVZ progenitor pool size was observed in vivo in transgenic mice expressing the PS1DeltaE9 variant. Moreover, NPC cultures established from the SVZ of adult mice expressing PS1DeltaE9 exhibit reduced self-renewal capacity and premature exit toward neuronal fates. To these findings, we show that both the levels of endogenous Notch/CBF-1-transcriptional activity and transcripts encoding Notch target genes are diminished in SVZ NPCs expressing PS1DeltaE9. The deficits in self-renewal and multipotency are restored by expression of Notch1-ICD or a downstream target of the Notch pathway, Hes1. Hence, we argue that a partial reduction in PS-dependent gamma-secretase processing of the Notch, at least in part, accounts for the impairments observed in SVZ NPCs expressing the FAD-linked PS1DeltaE9 variant.

Transgenic mouse models of Alzheimer's disease

Alzheimer's disease is the most common cause of senile dementia in the United States and Europe. At present, there is no effective treatment. Given the disease's prevalence and poor prognosis, the development of animal models has been a high research priority. Transgenic modeling has been pursued on the basis of the amyloid hypothesis and has taken advantage of mutations in the amyloid precursor protein and the presenilins that cause familial forms of Alzheimer's disease. Modeling has been most aggressively pursued in mice, for which the techniques of genetic modification are well developed. Transgenic mouse models now exist that mimic a range of Alzheimer's disease-related pathologies. Although none of the models fully replicates the human disease, the models have contributed significant insights into the pathophysiology of beta-amyloid toxicity, particularly with respect to the effects of different beta-amyloid species and the possible pathogenic role of beta-amyloid oligomers. They have also been widely used in the preclinical testing of potential therapeutic modalities and have played a pivotal role in the development of immunotherapies for Alzheimer's disease that are currently in clinical trials. These models will, without a doubt, continue to play central roles in preclinical testing and be used as tools for developing insights into the biological basis of Alzheimer's disease.

Molecular mechanisms of neurodegeneration in Alzheimer's disease

Alzheimer's disease (AD) is characterized by cognitive impairment, progressive neurodegeneration and formation of amyloid-beta (Abeta)-containing plaques and neurofibrillary tangles composed of hyperphosphorylated tau. The neurodegenerative process in AD is initially characterized by synaptic damage accompanied by neuronal loss. In addition, recent evidence suggests that alterations in adult neurogenesis in the hippocampus might play a role. Synaptic loss is one of the strongest correlates to the cognitive impairment in patients with AD. Several lines of investigation support the notion that the synaptic pathology and defective neurogenesis in AD are related to progressive accumulation of Abeta oligomers rather than fibrils. Abnormal accumulation of Abeta resulting in the formation of toxic oligomers is the result of an imbalance between the levels of Abeta production, aggregation and clearance. Abeta oligomers might lead to synaptic damage by forming pore-like structures with channel activity; alterations in glutamate receptors; circuitry hyper-excitability; mitochondrial dysfunction; lysosomal failure and alterations in signaling pathways related to synaptic plasticity, neuronal cell and neurogenesis. A number of signaling proteins, including fyn kinase; glycogen synthase kinase-3beta (GSK3beta) and cyclin-dependent kinase-5 (CDK5), are involved in the neurodegenerative progression of AD. Therapies for AD might require the development of anti-aggregation compounds, pro-clearance pathways and blockers of hyperactive signaling pathways.

Allopregnanolone reverses neurogenic and cognitive deficits in mouse model of Alzheimer's disease

Our previous analyses showed that allopregnanolone (APalpha) significantly increased proliferation of rodent and human neural progenitor cells in vitro. In this study, we investigated the efficacy of APalpha to promote neurogenesis in the hippocampal subgranular zone (SGZ), to reverse learning and memory deficits in 3-month-old male triple transgenic mouse model of Alzheimer's (3xTgAD) and the correlation between APalpha-induced neural progenitor cell survival and memory function in 3xTgAD mice. Neural progenitor cell proliferation was determined by unbiased stereological analysis of BrdU incorporation and survival determined by FACS for BrdU+ cells. Learning and memory function was assessed using the hippocampal-dependent trace eye-blink conditioning paradigm. At 3 months, basal level of BrdU+ cells in the SGZ of 3xTgAD mice was significantly lower relative to non-Tg mice, despite the lack of evident AD pathology. APalpha significantly increased, in a dose-dependent manner, BrdU+ cells in SGZ in 3xTgAD mice and restored SGZ proliferation to normal magnitude. As with the deficit in proliferation, 3xTgAD mice exhibited deficits in learning and memory. APalpha reversed the cognitive deficits to restore learning and memory performance to the level of normal non-Tg mice. In 3xTgAD mice, APalpha-induced survival of neural progenitors was significantly correlated with APalpha-induced memory performance. These findings suggest that early neurogenic deficits, which were evident before immunodetectable Abeta, may contribute to the cognitive phenotype of AD, and that APalpha could serve as a regenerative therapeutic to prevent or delay neurogenic and cognitive deficits associated with mild cognitive impairment and Alzheimer's disease.

DHEA prevents Aβ25-35-impaired survival of newborn neurons in the dentate gyrus through a modulation of PI3K-Akt-mTOR signaling

Infusion (i.c.v.) of beta-amyloid 25-35 (Abeta(25-35)) stimulates proliferation of progenitor cells in the hippocampal dentate gyrus (DG) of adult male mice, but a large population of the newborn cells will die in the 2nd week after birth, a critical period for neurite growth. Neurosteroid dehydroepiandrosterone (DHEA) has been demonstrated to promote neurite growth. Herein, we report that the DHEA-treatment on 6-12 days after BrdU-injection (BrdU-D(6-12)) dose-dependently attenuates the loss of newborn neurons induced by Abeta(25-35)-infusion. The DHEA-neuroprotection was blocked by the sigma(1) receptor antagonist NE100 and mimicked by the sigma(1) receptor agonist PRE084 when administered on BrdU-D(6-12). The DHEA-action was sensitive to the PI3K inhibitor LY294002 and the mammalian target of rapamycin (mTOR) inhibitor rapamycin. The Abeta(25-35)-infusion decreased the levels of Akt, mTOR and p70S6k phosphorylation, which could be rescued by DHEA-treatment in a sigma(1) receptor-dependent manner. Furthermore, the Abeta(25-35)-infusion led to a decrease in the dendritic density and length of doublecortin positive cells in the DG, which also was improved by the DHEA-treatment on BrdU-D(6-12). These findings suggest that DHEA prevents the Abeta(25-35)-impaired survival and dendritic growth of newborn neurons through a sigma(1) receptor-mediated modulation of PI3K-Akt-mTOR-p70S6k signaling.

Neurogenesis in mouse models of Alzheimer's disease

The brains of the adult mouse and human possess neural stem cells (NSCs) that retain the capacity to generate new neurons through the process of neurogenesis. They share the same anatomical locations of stem cell niches in the brain, as well as the prominent feature of rostral migratory stream formed by neuroblasts migrating from the lateral ventricles towards the olfactory bulb. Therefore the mouse possesses some fundamental features that may qualify it as a relevant model for adult human neurogenesis. Adult born young hippocampal neurons in the mouse display the unique property of enhanced plasticity, and can integrate physically and functionally into existing neural circuits in the brain. Such crucial properties of neurogenesis may at least partially underlie the improved learning and memory functions observed in the mouse when hippocampal neurogenesis is augmented, leading to the suggestion that neurogenesis induction may be a novel therapeutic approach for diseases with cognitive impairments such as Alzheimer's disease (AD). Research towards this goal has benefited significantly from the use of AD mouse models to facilitate the understanding in the impact of AD pathology on neurogenesis. The present article reviews the growing body of controversial data on altered neurogenesis in mouse models of AD and attempts to assess their relative relevance to humans.

Alzheimer Abeta peptide induces chromosome mis-segregation and aneuploidy, including trisomy 21: requirement for tau and APP

Chromosome aneuploidy, especially trisomy 21, arises in both familial and sporadic Alzheimer's disease. Expression of FAD genes or exposure to Aβ peptide induces aneuploidy in tg-mice and cultured cells. The requirement for GSK-3β, calpain, and Tau in Aβ-induced chromosome mis-segregation points to MT dysfunction as contributing to AD pathogenesis.

Both sporadic and familial Alzheimer's disease (AD) patients exhibit increased chromosome aneuploidy, particularly trisomy 21, in neurons and other cells. Significantly, trisomy 21/Down syndrome patients develop early onset AD pathology. We investigated the mechanism underlying mosaic chromosome aneuploidy in AD and report that FAD mutations in the Alzheimer Amyloid Precursor Protein gene, APP, induce chromosome mis-segregation and aneuploidy in transgenic mice and in transfected cells. Furthermore, adding synthetic Aβ peptide, the pathogenic product of APP, to cultured cells causes rapid and robust chromosome mis-segregation leading to aneuploid, including trisomy 21, daughters, which is prevented by LiCl addition or Ca²⁺ chelation and is replicated in tau KO cells, implicating GSK-3β, calpain, and Tau-dependent microtubule transport in the aneugenic activity of Aβ. Furthermore, APP KO cells are resistant to the aneugenic activity of Aβ, as they have been shown previously to be resistant to Aβ-induced tau phosphorylation and cell toxicity. These results indicate that Aβ-induced microtubule dysfunction leads to aneuploid neurons and may thereby contribute to the pathogenesis of AD.

Nurturing brain plasticity: impact of environmental enrichment

Environmental enrichment (EE) is known to profoundly affect the central nervous system (CNS) at the functional, anatomical and molecular level, both during the critical period and during adulthood. Recent studies focusing on the visual system have shown that these effects are associated with the recruitment of previously unsuspected neural plasticity processes. At early stages of brain development, EE triggers a marked acceleration in the maturation of the visual system, with maternal behaviour acting as a fundamental mediator of the enriched experience in both the foetus and the newborn. In adult brain, EE enhances plasticity in the cerebral cortex, allowing the recovery of visual functions in amblyopic animals. The molecular substrate of the effects of EE on brain plasticity is multi-factorial, with reduced intracerebral inhibition, enhanced neurotrophin expression and epigenetic changes at the level of chromatin structure. These findings shed new light on the potential of EE as a non-invasive strategy to ameliorate deficits in the development of the CNS and to treat neurological disorders.