Abeta(1-42) stimulates adult SVZ neurogenesis through the p75 neurotrophin receptor

The intricate interplay between microglia and adult neurogenesis in Alzheimer's disease

Microglia, the resident immune cells of the central nervous system, play a crucial role in regulating adult neurogenesis and contribute significantly to the pathogenesis of Alzheimer's disease (AD). Under physiological conditions, microglia support and modulate neurogenesis through the secretion of neurotrophic factors, phagocytosis of apoptotic cells, and synaptic pruning, thereby promoting the proliferation, differentiation, and survival of neural progenitor cells (NPCs). However, in AD, microglial function becomes dysregulated, leading to chronic neuroinflammation and impaired neurogenesis. This review explores the intricate interplay between microglia and adult neurogenesis in health and AD, synthesizing recent findings to provide a comprehensive overview of the current understanding of microglia-mediated regulation of adult neurogenesis. Furthermore, it highlights the potential of microglia-targeted therapies to modulate neurogenesis and offers insights into potential avenues for developing novel therapeutic interventions.

Amyloid Beta Oligomers Activate Death Receptors and Mitochondria-Mediated Apoptotic Pathways in Cerebral Vascular Smooth Muscle Cells; Protective Effects of Carbonic Anhydrase Inhibitors

Amyloid beta (Aβ) deposition within the brain vasculature is an early hallmark of Alzheimer's disease (AD), which triggers loss of brain vascular smooth muscle cells (BVSMCs) in cerebral arteries, via poorly understood mechanisms, altering cerebral blood flow, brain waste clearance, and promoting cognitive impairment. We have previously shown that, in brain endothelial cells (ECs), vasculotropic Aβ species induce apoptosis through death receptors (DRs) DR4 and DR5 and mitochondria-mediated mechanisms, while FDA-approved carbonic anhydrase inhibitors (CAIs) prevent mitochondria-mediated EC apoptosis in vitro and in vivo. In this study, we analyzed Aβ-induced extrinsic and intrinsic (DR- and mitochondria-mediated) apoptotic pathways in BVSMC, aiming to unveil new therapeutic targets to prevent BVSMC stress and death. We show that both apoptotic pathways are activated in BVSMCs by oligomeric Aβ42 and Aβ40-Q22 (AβQ22) and mitochondrial respiration is severely impaired. Importantly, the CAIs methazolamide (MTZ) and acetazolamide (ATZ) prevent the pro-apoptotic effects in BVSMCs, while reducing caspase 3 activation and Aβ deposition in the arterial walls of TgSwDI animals, a murine model of cerebral amyloid angiopathy (CAA). This study reveals new molecular targets and a promising therapeutic strategy against BVSMC dysfunction in AD, CAA, and ARIA (amyloid-related imaging abnormalities) complications of recently FDA-approved anti-Aβ antibodies.

Amyloid β-based therapy for Alzheimer's disease: challenges, successes and future

Amyloid β protein (Aβ) is the main component of neuritic plaques in Alzheimer's disease (AD), and its accumulation has been considered as the molecular driver of Alzheimer's pathogenesis and progression. Aβ has been the prime target for the development of AD therapy. However, the repeated failures of Aβ-targeted clinical trials have cast considerable doubt on the amyloid cascade hypothesis and whether the development of Alzheimer's drug has followed the correct course. However, the recent successes of Aβ targeted trials have assuaged those doubts. In this review, we discussed the evolution of the amyloid cascade hypothesis over the last 30 years and summarized its application in Alzheimer's diagnosis and modification. In particular, we extensively discussed the pitfalls, promises and important unanswered questions regarding the current anti-Aβ therapy, as well as strategies for further study and development of more feasible Aβ-targeted approaches in the optimization of AD prevention and treatment.

The shift to a proteinopenia paradigm in neurodegeneration

The toxic proteinopathy paradigm has defined neurodegenerative disorders for over a century. This gain-of-function (GOF) framework posited that proteins become toxic when turned into amyloids (pathology), predicting that lowering its levels would translate into clinical benefits. Genetic observations used to support a GOF framework are equally compatible with a loss-of-function (LOF) framework, as the soluble pool of proteins rendered unstable by these mutations (e.g., APP in Alzheimer's disease, SNCA in Parkinson's disease) aggregate, becoming depleted. In this review, we highlight misconceptions that have prevented LOF from gaining currency. Some of these misconceptions include no phenotype in knock-out animals (there is neurodegenerative phenotype in knock-out animals) and high levels of proteins in patients (patients have lower levels of the proteins involved in neurodegeneration than healthy age-matched controls). We also expose the internal contradictions within the GOF framework, namely that (1) pathology can have both pathogenic and protective roles; (2) the neuropathology gold standard for diagnosis can be present in normal individuals and absent in those affected; (3) oligomers are the toxic species even if they are ephemeral and decrease over time. We therefore advocate for a paradigm shift from proteinopathy (GOF) to proteinopenia (LOF) based on the universal depletion of soluble functional proteins in neurodegenerative diseases (low amyloid-β 42 in Alzheimer's disease, low α-synuclein in Parkinson's disease, and low tau in progressive supranuclear palsy) and supported by the confluence of biologic, thermodynamic, and evolutionary principles with proteins having evolved to perform a function, not to become toxic, and where protein depletion is consequential. Such shift to a Proteinopenia paradigm is necessary to examining the safety and efficacy of protein replacement strategies instead of perpetuating a therapeutic paradigm with further antiprotein permutations.

Effects of β-amyloid (1-42) Administration on the Main Neurogenic Niches of the Adult Brain: Amyloid-Induced Neurodegeneration Influences Neurogenesis

Alzheimer's disease (AD) is the most prevalent neurodegenerative disorder and warrants further study as well as timely treatment. Additionally, the mechanisms of the brain's intrinsic defense against chronic injury are not yet fully understood. Herein, we examined the response of the main neurogenic niches to amyloid exposure and the associated changes in structure and synaptic activity. Flow cytometry of Nestin-, Vimentin-, Nestin/Vimentin-, NeuN-, GFAP-, NeuN/GFAP-, NSE-, BrdU-, Wnt-, BrdU/Wnt-, VEGF-, Sox14-, VEGF/Sox14-, Sox10-, Sox2-, Sox10/Sox2-, Bax-, and Bcl-xL-positive cells was performed in the subventricular zone (SVZ), hippocampus, and cerebral cortex of rat brains on 90th day after intracerebroventricular (i.c.v.) single injection of a fraction of β-amyloid (Aβ) (1-42). The relative structural changes in these areas and disruptions to synaptic activity in the entorhinal cortex-hippocampus circuit were also evaluated. Our flow analyses revealed a reduction in the numbers of Nestin-, Vimentin-, and Nestin/Vimentin-positive cells in neurogenic niches and the olfactory bulb. These changes were accompanied by an increased number of BrdU-positive cells in the hippocampus and SVZ. The latter changes were strongly correlated with changes in the numbers of VEGF- and VEGF/Sox14-positive cells. The morphological changes were characterized by significant neural loss, a characteristic shift in entorhinal cortex-hippocampus circuit activity, and decreased spontaneous alternation in a behavioral test. We conclude that although an injection of Aβ (1-42) induced stem cell proliferation and triggered neurogenesis at a certain stage, this process was incomplete and led to neural stem cell immaturity. We propose the idea of enhancing adult neurogenesis as a promising strategy for preventing dementia at healthy elderly people andpeople at high risk for developing AD, or treating patients diagnosed with AD.

A Potential Strategy for Treatment of Neurodegenerative Disorders by Regulation of Adult Hippocampal Neurogenesis in Human Brain

Abstract:

Adult hippocampal neurogenesis is a multistage mechanism that continues throughout the lifespan of human and non-human mammals. These adult-born neurons in the central nervous system (CNS) play a significant role in various hippocampus-dependent processes, including learning, mood regulation, pattern recognition, etc. Reduction of adult hippocampal neurogenesis, caused by multiple factors such as neurological disorders and aging, would impair neuronal proliferation and differentiation and result in memory loss. Accumulating studies have indicated that functional neuron impairment could be restored by promoting adult hippocampal neurogenesis. In this review, we summarized the small molecules that could efficiently promote the process of adult neurogenesis, particularly the agents that have the capacity of crossing the blood-brain barrier (BBB), and showed in vivo efficacy in mammalian brains. This may pave the way for the rational design of drugs to treat humnan neurodegenerative disorders in the future.

Fading memories in aging and neurodegeneration: Is p75 neurotrophin receptor a culprit?

Aging and age-related neurodegenerative diseases have become one of the major concerns in modern times as cognitive abilities tend to decline when we get older. It is well known that the main cause of this age-related cognitive deficit is due to aberrant changes in cellular, molecular circuitry and signaling pathways underlying synaptic plasticity and neuronal connections. The p75 neurotrophin receptor (p75^NTR) is one of the important mediators regulating the fate of the neurons in the nervous system. Its importance in neuronal apoptosis is well documented. However, the mechanisms involving the regulation of p75^NTR in synaptic plasticity and cognitive function remain obscure, although cognitive impairment has been associated with a higher expression of p75^NTR in neurons. In this review, we discuss the current understanding of how neurons are influenced by p75^NTR function to maintain normal neuronal synaptic strength and connectivity, particularly to support learning and memory in the hippocampus. We then discuss the age-associated alterations in neurophysiological mechanisms of synaptic plasticity and cognitive function. Furthermore, we also describe current evidence that has begun to elucidate how p75^NTR regulates synaptic changes in aging and age-related neurodegenerative diseases, focusing on the hippocampus. Elucidating the role that p75^NTR signaling plays in regulating synaptic plasticity will contribute to a better understanding of cognitive processes and pathological conditions. This will in turn provide novel approaches to improve therapies for the treatment of neurological diseases in which p75^NTR dysfunction has been demonstrated.

Cooperation of cell adhesion and autophagy in the brain: Functional roles in development and neurodegenerative disease

Cellular adhesive connections directed by the extracellular matrix (ECM) and maintenance of cellular homeostasis by autophagy are seemingly disparate functions that are molecularly intertwined, each regulating the other. This is an emerging field in the brain where the interplay between adhesion and autophagy functions at the intersection of neuroprotection and neurodegeneration. The ECM and adhesion proteins regulate autophagic responses to direct protein clearance and guide regenerative programs that go awry in brain disorders. Concomitantly, autophagic flux acts to regulate adhesion dynamics to mediate neurite outgrowth and synaptic plasticity with functional disruption contributed by neurodegenerative disease. This review highlights the cooperative exchange between cellular adhesion and autophagy in the brain during health and disease. As the mechanistic alliance between adhesion and autophagy has been leveraged therapeutically for metastatic disease, understanding overlapping molecular functions that direct the interplay between adhesion and autophagy might uncover therapeutic strategies to correct or compensate for neurodegeneration.

Diet enriched in omega-3 fatty acids alleviates olfactory system deficits in APOE4 transgenic mice

Olfactory dysfunction is observed in several neurological disorders including Mild Cognitive Impairment (MCI) and Alzheimer disease (AD). These deficits occur early and correlate with global cognitive performance, depression and degeneration of olfactory regions in the brain. Despite extensive human studies, there has been little characterization of the olfactory system in models of AD. In order to determine if olfactory structural and/or molecular phenotypes are observed in a model expressing a genetic risk factor for AD, we assessed the olfactory bulb (OB) in APOE4 transgenic mice. A significant decrease in OB weight was observed at 12 months of age in APOE4 mice concurrent with inflammation and decreased NeuN expression. In order to determine if a diet rich in omega-3s may alleviate the olfactory system phenotypes observed, we assessed WT and APOE4 mice on a docosahexaenoic acid (DHA) diet. APOE4 mice on a DHA diet did not present with atrophy of the OB, and the alterations in NeuN and IBA-1 expression were alleviated. Furthermore, alterations in caspase mRNA and protein expression in the APOE4 OB were not observed with a DHA diet. Similar to the human AD condition, OB atrophy is an early phenotype in the APOE4 mice and concurrent with inflammation. These data support a link between the structural olfactory brain region atrophy and the olfactory dysfunction observed in AD and suggest that inflammation and cell death pathways may contribute to the olfactory deficits observed. Furthermore, the results suggest that diets enriched in DHA may provide benefit to APOE4 allele carriers.

Deconstructing Alzheimer's Disease: How to Bridge the Gap between Experimental Models and the Human Pathology?

Discovered more than a century ago, Alzheimer's disease (AD) is not only still present in our societies but has also become the most common dementia, with 50 million people worldwide affected by the disease. This number is expected to double in the next generation, and no cure is currently available to slow down or stop the disease progression. Recently, some advances were made due to the approval of the aducanumab treatment by the American Food and Drug Administration. The etiology of this human-specific disease remains poorly understood, and the mechanisms of its development have not been completely clarified. Several hypotheses concerning the molecular mechanisms of AD have been proposed, but the existing studies focus primarily on the two main markers of the disease: the amyloid β peptides, whose aggregation in the brain generates amyloid plaques, and the abnormally phosphorylated tau proteins, which are responsible for neurofibrillary tangles. These protein aggregates induce neuroinflammation and neurodegeneration, which, in turn, lead to cognitive and behavioral deficits. The challenge is, therefore, to create models that best reproduce this pathology. This review aims at gathering the different existing AD models developed in vitro, in cellulo, and in vivo. Many models have already been set up, but it is necessary to identify the most relevant ones for our investigations. The purpose of the review is to help researchers to identify the most pertinent disease models, from the most often used to the most recently generated and from simple to complex, explaining their specificities and giving concrete examples.

Adult Hippocampal Neurogenesis in Aging and Alzheimer's Disease

Cognitive deficits associated with Alzheimer's disease (AD) severely impact daily life for the millions of affected individuals. Progressive memory impairment in AD patients is associated with degeneration of the hippocampus. The dentate gyrus of the hippocampus, a region critical for learning and memory functions, is a site of adult neurogenesis in mammals. Recent evidence in humans indicates that hippocampal neurogenesis likely persists throughout life, but declines with age and is strikingly impaired in AD. Our understanding of how neurogenesis supports learning and memory in healthy adults is only beginning to emerge. The extent to which decreased neurogenesis contributes to cognitive decline in aging and AD remains poorly understood. However, studies in rodent models of AD and other neurodegenerative diseases raise the possibility that targeting neurogenesis may ameliorate cognitive dysfunction in AD. Here, we review recent progress in understanding how adult neurogenesis is impacted in the context of aging and AD.

The physiological roles of tau and Aβ: implications for Alzheimer's disease pathology and therapeutics

Tau and amyloid beta (Aβ) are the prime suspects for driving pathology in Alzheimer's disease (AD) and, as such, have become the focus of therapeutic development. Recent research, however, shows that these proteins have been highly conserved throughout evolution and may have crucial, physiological roles. Such functions may be lost during AD progression or be unintentionally disrupted by tau- or Aβ-targeting therapies. Tau has been revealed to be more than a simple stabiliser of microtubules, reported to play a role in a range of biological processes including myelination, glucose metabolism, axonal transport, microtubule dynamics, iron homeostasis, neurogenesis, motor function, learning and memory, neuronal excitability, and DNA protection. Aβ is similarly multifunctional, and is proposed to regulate learning and memory, angiogenesis, neurogenesis, repair leaks in the blood-brain barrier, promote recovery from injury, and act as an antimicrobial peptide and tumour suppressor. This review will discuss potential physiological roles of tau and Aβ, highlighting how changes to these functions may contribute to pathology, as well as the implications for therapeutic development. We propose that a balanced consideration of both the physiological and pathological roles of tau and Aβ will be essential for the design of safe and effective therapeutics.

Role of Microglia in Modulating Adult Neurogenesis in Health and Neurodegeneration

Microglia are the resident immune cells of the brain, constituting the powerhouse of brain innate immunity. They originate from hematopoietic precursors that infiltrate the developing brain during different stages of embryogenesis, acquiring a phenotype characterized by the presence of dense ramifications. Microglial cells play key roles in maintaining brain homeostasis and regulating brain immune responses. They continuously scan and sense the brain environment to detect any occurring changes. Upon detection of a signal related to physiological or pathological processes, the cells are activated and transform to an amoeboid-like phenotype, mounting adequate responses that range from phagocytosis to secretion of inflammatory and trophic factors. The overwhelming evidence suggests that microglia are crucially implicated in influencing neuronal proliferation and differentiation, as well as synaptic connections, and thereby cognitive and behavioral functions. Here, we review the role of microglia in adult neurogenesis under physiological conditions, and how this role is affected in neurodegenerative diseases.

Estradiol Replacement at the Critical Period Protects Hippocampal Neural Stem Cells to Improve Cognition in APP/PS1 Mice

It has been suggested that there is a critical window for estrogen replacement therapy (ERT) in postmenopausal women with Alzheimer’s disease (AD); however, supporting evidence is lacking. To address this issue, we investigated the effective period for estradiol (E2) treatment using a mouse model of AD. Four-month-old female APPswe/PSEN1dE9 (APP/PS1) mice were ovariectomized (OVX) and treated with E2 for 2 months starting at the age of 4 months (early period), 6 months (mid-period), or 8 months (late period). We then evaluated hippocampal neurogenesis, β-amyloid (Aβ) accumulation, telomerase activity, and hippocampal-dependent behavior. Compared to age-matched wild type mice, APP/PS1 mice with intact ovaries showed increased proliferation of hippocampal neural stem cells (NSCs) at 8 months of age and decreased proliferation of NSCs at 10 months of age; meanwhile, Aβ accumulation progressively increased with age, paralleling the reduced survival of immature neurons. OVX-induced depletion of E2 in APP/PS1 mice resulted in elevated Aβ levels accompanied by elevated p75 neurotrophin receptor (p75^NTR) expression and increased NSC proliferation at 6 months of age, which subsequently declined; accelerated reduction of immature neurons starting from 6 months of age, and reduced telomerase activity and worsened memory performance at 10 months of age. Treatment with E2 in the early period post-OVX, rather than in the mid or late period, abrogated these effects, and p75^NTR inhibition reduced the overproliferation of NSCs in 6-month-old OVX-APP/PS1 mice. Thus, E2 deficiency in young APP/PS1 mice exacerbates cognitive deficits and depletes the hippocampal NSC pool in later life; this can be alleviated by E2 treatment in the early period following OVX, which prevents Aβ/p75^NTR-induced NSC overproliferation and preserves telomerase activity.

Contribution of Neurotrophins to the Immune System Regulation and Possible Connection to Alcohol Addiction

The first references to neurotrophic factors date back to the middle of the 20th century when the nerve growth factor (NGF) was first discovered. Later studies delivered a large amount of data on neurotrophic factors. However, many questions regarding neurotrophin signaling still remain unanswered. One of the principal topics in neurotrophin research is their role in the immune system regulation. Another important research question is the possible involvement of neurotrophin signaling in the pathological processes associated with alcoholism. Among known neurotrophins, NT-4 remains the least studied and appears to be involved in alcoholism and chronic stress pathogenesis. In this review we discuss known neurotrophin signaling cascades mediated by different neurotrophin receptors, as well as provide a generalization of the data regarding the influence of neurotrophins NGF, BDNF, and NT-4 on the immune system and their potential contribution to the pathogenesis of alcoholism.

Cell proliferation and neurogenesis alterations in Alzheimer's disease and diabetes mellitus mixed murine models

The classic neuropathological features of Alzheimer's disease (AD) are accompanied by other complications, including alterations in adult cell proliferation and neurogenesis. Moreover recent studies have shown that traditional markers of the neurogenic process, such as doublecortin (DCX), may also be expressed in CD8⁺ T cells and ionized calcium-binding adaptor molecule 1 (Iba1⁺ ) microglia, in the close proximity to senile plaques, increasing the complexity of the condition. Altered glucose tolerance, observed in metabolic alteratioins, may accelerate the neurodegenerative process and interfere with normal adult cell proliferation and neurogenesis. To further explore the role of metabolic disease in AD, we analyzed cell proliferation and neurogenesis using 5'-bromo-2'-deoxyuridine and DCX immunohistochemistry in three different mouse models of AD and metabolic alterations: APP/PS1xdb/db mice, APP/PS1 mice on a long-term high-fat diet, and APP/PS1 mice treated with streptozotozin. As reported previously, an overall reduction in cell proliferation and neurogenesis was observed after streptozotocin administration. In contrast, an increase in cell proliferation and neurogenesis was detected in neurogenic niches in 14- and 26-week-old APP/PS1xdb/db mice, accompanied by a slight increase in cortical cell proliferation. While a similar trend was observed in animals on a high-fat diet, differences were not statistically significant. We observed very few DCX⁺ /CD8⁺ cells and no DCX⁺ /Iba1⁺ cells were observed in the close proximity to senile plaques in any of the groups. Interestingly, metabolic parameters such as body weight and glucose and insulin levels were identified as reliable predictors of cell proliferation and neurogenesis in APP/PS1xdb/db mice. Furthermore, metabolic parameters were also associated with altered Aβ levels in the cortex and hippocampus of APP/PS1xdb/db mice. Altogether, our data suggest that metabolic disease may also interfere with central complications in AD.

Oxidative DNA Damage Signalling in Neural Stem Cells in Alzheimer's Disease

The main pathological symptoms of Alzheimer's disease (AD) are β-amyloid (Aβ) lesions and intracellular neurofibrillary tangles of hyperphosphorylated tau protein. Unfortunately, existing symptomatic therapies targeting Aβ and tau remain ineffective. In addition to these pathogenic factors, oxidative DNA damage is one of the major threats to newborn neurons. It is necessary to consider in detail what causes neurons to be extremely susceptible to oxidative damage, especially in the early stages of development. Accordingly, the regulation of redox status is crucial for the functioning of neural stem cells (NSCs). The redox-dependent balance, of NSC proliferation and differentiation and thus the neurogenesis process, is controlled by a series of signalling pathways. One of the most important signalling pathways activated after oxidative stress is the DNA damage response (DDR). Unfortunately, our understanding of adult neurogenesis in AD is still limited due to the research material used (animal models or post-mortem tissue), providing inconsistent data. Now, thanks to the advances in cellular reprogramming providing patient NSCs, it is possible to fill this gap, which becomes urgent in the light of the potential of their therapeutic use. Therefore, a decent review of redox signalling in NSCs under physiological and pathological conditions is required. At this moment, we attempt to integrate knowledge on the influence of oxidative stress and DDR signalling in NSCs on adult neurogenesis in Alzheimer's disease.

Impaired adult neurogenesis is an early event in Alzheimer's disease neurodegeneration, mediated by intracellular Aβ oligomers

Alterations of adult neurogenesis have been reported in several Alzheimer's disease (AD) animal models and human brains, while defects in this process at presymptomatic/early stages of AD have not been explored yet. To address this, we investigated potential neurogenesis defects in Tg2576 transgenic mice at 1.5 months of age, a prodromal asymptomatic age in terms of Aβ accumulation and neurodegeneration. We observe that Tg2576 resident and SVZ-derived adult neural stem cells (aNSCs) proliferate significantly less. Further, they fail to terminally differentiate into mature neurons due to pathological, tau-mediated, and microtubule hyperstabilization. Olfactory bulb neurogenesis is also strongly reduced, confirming the neurogenic defect in vivo. We find that this phenotype depends on the formation and accumulation of intracellular A-beta oligomers (AβOs) in aNSCs. Indeed, impaired neurogenesis of Tg2576 progenitors is remarkably rescued both in vitro and in vivo by the expression of a conformation-specific anti-AβOs intrabody (scFvA13-KDEL), which selectively interferes with the intracellular generation of AβOs in the endoplasmic reticulum (ER). Altogether, our results demonstrate that SVZ neurogenesis is impaired already at a presymptomatic stage of AD and is caused by endogenously generated intracellular AβOs in the ER of aNSCs. From a translational point of view, impaired SVZ neurogenesis may represent a novel biomarker for AD early diagnosis, in association to other biomarkers. Further, this study validates intracellular Aβ oligomers as a promising therapeutic target and prospects anti-AβOs scFvA13-KDEL intrabody as an effective tool for AD treatment.

Early Seizure Activity Accelerates Depletion of Hippocampal Neural Stem Cells and Impairs Spatial Discrimination in an Alzheimer's Disease Model

Adult hippocampal neurogenesis has been reported to be decreased, increased, or not changed in Alzheimer's disease (AD) patients and related transgenic mouse models. These disparate findings may relate to differences in disease stage, or the presence of seizures, which are associated with AD and can stimulate neurogenesis. In this study, we investigate a transgenic mouse model of AD that exhibits seizures similarly to AD patients and find that neurogenesis is increased in early stages of disease, as spontaneous seizures became evident, but is decreased below control levels as seizures recur. Treatment with the antiseizure drug levetiracetam restores neurogenesis and improves performance in a neurogenesis-associated spatial discrimination task. Our results suggest that seizures stimulate, and later accelerate the depletion of, the hippocampal neural stem cell pool. These results have implications for AD as well as any disorder accompanied by recurrent seizures, such as epilepsy.

Roles of Primary Cilia in the Developing Brain

Essential to development, primary cilia are microtubule-based cellular organelles that protrude from the surface of cells. Acting as cellular antenna, primary cilia play central roles in transducing or regulating several signaling pathways, including Sonic hedgehog (Shh) and Wnt signaling. Defects in primary cilia contribute to a group of syndromic disorders known as “ciliopathies” and can adversely affect development of the brain and other essential organs, including the kidneys, eyes, and liver. The molecular mechanisms of how defective primary cilia contribute to neurological defects, however, remain poorly understood. In this mini review, we summarize recent advances in understanding of the interactions between primary cilia and signaling pathways essential to cellular homeostasis and brain development.

Brain-Derived Neurotrophic Factor (BDNF): Novel Insights into Regulation and Genetic Variation

Since its discovery, brain-derived neurotrophic factor (BDNF) has spawned a literature that now spans 35 years of research. While all neurotrophins share considerable overlap in sequence homology and their processing, BDNF has become the most widely studied neurotrophin because of its broad roles in brain homeostasis, health, and disease. Although research on BDNF has produced thousands of articles, there remain numerous long-standing questions on aspects of BDNF molecular biology and signaling. Here we provide a comprehensive review, including both a historical narrative and a forward-looking perspective on advances in the actions of BDNF within the brain. We specifically review BDNF’s gene structure, peptide composition (including domains, posttranslational modifications and putative motif sites), mechanisms of transport, signaling pathway recruitment, and other recent developments including the functional effects of genetic variation and the discovery of a new BDNF prodomain ligand. This body of knowledge illustrates a highly conserved and complex role for BDNF within the brain, that promotes the idea that the neurotrophin biology of BDNF is diverse and that any disease involvement is likely to be equally multifarious.

The Role of Irisin in Alzheimer's Disease

Alzheimer’s disease (AD) is characterized by progressive memory dysfunction, oxidative stress, and presence of senile plaques formed by amyloid beta (Aβ) accumulation in the brain. AD is one of the most important causes of morbidity and mortality worldwide. AD has a variety of risk factors, including environmental factors, metabolic dysfunction, and genetic background. Recent research has highlighted the relationship between AD and systemic metabolic changes such as glucose and lipid imbalance and insulin resistance. Irisin, a myokine closely linked to exercise, has been associated with glucose metabolism, insulin sensitivity, and fat browning. Recent studies have suggested that irisin is involved in the process in central nervous system (CNS) such as neurogenesis and has reported the effects of irisin on AD as one of the neurodegenerative disease. Here, we review the roles of irisin with respect to AD and suggest that irisin highlight therapeutic important roles in AD. Thus, we propose that irisin could be a potential future target for ameliorating AD pathology and preventing AD onset.

BDNF: A Key Factor with Multipotent Impact on Brain Signaling and Synaptic Plasticity

Brain-derived neurotrophic factor (BDNF) is one of the most widely distributed and extensively studied neurotrophins in the mammalian brain. Among its prominent functions, one can mention control of neuronal and glial development, neuroprotection, and modulation of both short- and long-lasting synaptic interactions, which are critical for cognition and memory. A wide spectrum of processes are controlled by BDNF, and the sometimes contradictory effects of its action can be explained based on its specific pattern of synthesis, comprising several intermediate biologically active isoforms that bind to different types of receptor, triggering several signaling pathways. The functions of BDNF must be discussed in close relation to the stage of brain development, the different cellular components of nervous tissue, as well as the molecular mechanisms of signal transduction activated under physiological and pathological conditions. In this review, we briefly summarize the current state of knowledge regarding the impact of BDNF on regulation of neurophysiological processes. The importance of BDNF for future studies aimed at disclosing mechanisms of activation of signaling pathways, neuro- and gliogenesis, as well as synaptic plasticity is highlighted.

Molecular and Cellular Mechanisms of Sporadic Alzheimer's Disease: Studies on Rodent Models in vivo

In this review, recent data are presented on molecular and cellular mechanisms of pathogenesis of the most widespread (about 95%) sporadic forms of Alzheimer's disease obtained on in vivo rodent models. Although none of the available models can fully reproduce the human disease, several key molecular mechanisms (such as dysfunction of neurotransmitter systems, especially of the acetylcholinergic system, β-amyloid toxicity, oxidative stress, neuroinflammation, mitochondrial dysfunction, disturbances in neurotrophic systems) are confirmed with different models. Injection models, olfactory bulbectomy, and senescence accelerated OXYS rats are reviewed in detail. These three approaches to in vivo modeling of sporadic Alzheimer's disease have demonstrated a considerable similarity in molecular and cellular mechanisms of pathology development. Studies on these models provide complementary data, and each model possesses its specific advantages. A general analysis of the data reported for the three models provides a multifaceted and the currently most complete molecular picture of sporadic Alzheimer's disease. This is highly relevant also from the practical viewpoint because it creates a basis for elaboration and preclinical studies of means for treatment of this disease.

Targeting therapy for homocysteic acid in the blood represents a potential recovery treatment for cognition in Alzheimer's disease patients

At present, we have no reliable means of recovering cognitive impairment in Alzheimer's disease (AD) patients. We hypothesized that homocysteic acid (HA) in the blood might represent one such pathogen that could be excreted into the urine. Since DHA is known to reduce circulating levels of homocysteine, and since exercise attenuates this effect, it follows that supplementation of the diet with DHA, along with increased levels of physical activity, may help to reduce cognitive impairment in AD patients. Our hypothesis was proven to be correct because memory problems in 3xTg- AD mice (a model for AD in which animals develop amyloid pathology), and in a mouse model of familial AD, were recovered following treatment with an anti-HA antibody and not by amyloid treatment. Interestingly, 3xTg-AD mice with amyloid pathology showed increased levels of HA level. This could perhaps be explained by the fact that amyloid precursor protein and/or presenilin increases calcium influx, which could then increase levels of superoxide and consequently increase levels of HA from homocysteine or methionine. Our hypothesis is also partially supported by an open clinical trial of certain dietary supplements that has shown impressive results. Also there are other treatments hypothesis which would be possible for the effective therapies, such as ribonucleoprotein therapy, a β-secretase inhibitor treatment and the metabolic enhancement treatment.

A Unified Hypothesis of Early- and Late-Onset Alzheimer's Disease Pathogenesis

Early-onset familial Alzheimer's disease (EOFAD) and late-onset sporadic AD (LOSAD) both follow a similar pathological and biochemical course that includes: neuron and synapse loss and dysfunction, microvascular damage, microgliosis, extracellular amyloid-β deposition, tau phosphorylation, formation of intracellular neurofibrillary tangles, endoreduplication and related cell cycle events in affected brain regions. Any mechanistic explanation of AD must accommodate these biochemical and neuropathological features for both forms of the disease. In this insight paper we provide a unifying hypothesis for EOFAD and LOSAD that proposes that the aberrant re-entry of terminally differentiated, post-mitotic neurons into the cell division cycle is a common pathway that explains both early and late-onset forms of AD. Cell cycle abnormalities appear very early in the disease process, prior to the appearance of plaques and tangles, and explain the biochemical (e.g. tau phosphorylation), neuropathological (e.g. neuron hypertrophy; polypoidy) and cognitive changes observed in EOFAD and LOSAD. Genetic mutations in AβPP, PSEN1, and PSEN2 that alter amyloid-β precursor protein and Notch processing drive reactivation of the cell cycle in EOFAD, while age-related reproductive endocrine dyscrasia that upregulates mitogenic TNF signaling and AβPP processing toward the amyloidogenic pathway drives reactivation of the cell cycle in LOSAD. In essence, AβPP and presenilin mutations initiate early, what endocrine dyscrasia initiates later: aberrant cell cycle re-entry of post-mitotic neurons leading to neurodegeneration and cognitive decline in AD. Inhibition of cell cycle re-entry in post-mitotic neurons may be a useful therapeutic strategy to prevent, slow or halt disease progression.

Regulation of global gene expression and cell proliferation by APP

Down syndrome (DS), caused by trisomy of chromosome 21, is one of the most common genetic disorders. Patients with DS display growth retardation and inevitably develop characteristic Alzheimer’s disease (AD) neuropathology, including neurofibrillary tangles and neuritic plaques. The expression of amyloid precursor protein (APP) is increased in both DS and AD patients. To reveal the function of APP and elucidate the pathogenic role of increased APP expression in DS and AD, we performed gene expression profiling using microarray method in human cells overexpressing APP. A set of genes are significantly altered, which are involved in cell cycle, cell proliferation and p53 signaling. We found that overexpression of APP inhibits cell proliferation. Furthermore, we confirmed that the downregulation of two validated genes, PSMA5 and PSMB7, inhibits cell proliferation, suggesting that the downregulation of PSMA5 and PSMB7 is involved in APP-induced cell proliferation impairment. Taken together, this study suggests that APP regulates global gene expression and increased APP expression inhibits cell proliferation. Our study provides a novel insight that APP overexpression may contribute to the growth impairment in DS patients and promote AD pathogenesis by inhibiting cell proliferation including neural stem cell proliferation and neurogenesis.

Regulation of Neurogenesis by Neurotrophins during Adulthood: Expected and Unexpected Roles

The subventricular zone (SVZ) of the anterolateral ventricle and the subgranular zone (SGZ) of the hippocampal dentate gyrus are the two main regions of the adult mammalian brain in which neurogenesis is maintained throughout life. Because alterations in adult neurogenesis appear to be a common hallmark of different neurodegenerative diseases, understanding the molecular mechanisms controlling adult neurogenesis is a focus of active research. Neurotrophic factors are a family of molecules that play critical roles in the survival and differentiation of neurons during development and in the control of neural plasticity in the adult. Several neurotrophins and neurotrophin receptors have been implicated in the regulation of adult neurogenesis at different levels. Here, we review the current understanding of neurotrophin modulation of adult neurogenesis in both the SVZ and SGZ. We compile data supporting a variety of roles for neurotrophins/neurotrophin receptors in different scenarios, including both expected and unexpected functions.

The endocrine dyscrasia that accompanies menopause and andropause induces aberrant cell cycle signaling that triggers re-entry of post-mitotic neurons into the cell cycle, neurodysfunction, neurodegeneration and cognitive disease

This article is part of a Special Issue "SBN 2014". Sex hormones are physiological factors that promote neurogenesis during embryonic and fetal development. During childhood and adulthood these hormones support the maintenance of brain structure and function via neurogenesis and the formation of dendritic spines, axons and synapses required for the capture, processing and retrieval of information (memories). Not surprisingly, changes in these reproductive hormones that occur with menopause and during andropause are strongly correlated with neurodegeneration and cognitive decline. In this connection, much evidence now indicates that Alzheimer's disease (AD) involves aberrant re-entry of post-mitotic neurons into the cell cycle. Cell cycle abnormalities appear very early in the disease, prior to the appearance of plaques and tangles, and explain the biochemical, neuropathological and cognitive changes observed with disease progression. Intriguingly, a recent animal study has demonstrated that induction of adult neurogenesis results in the loss of previously encoded memories while decreasing neurogenesis after memory formation during infancy mitigated forgetting. Here we review the biochemical, epidemiological and clinical evidence that alterations in sex hormone signaling associated with menopause and andropause drive the aberrant re-entry of post-mitotic neurons into an abortive cell cycle that leads to neurite retraction, neuron dysfunction and neuron death. When the reproductive axis is in balance, gonadotropins such as luteinizing hormone (LH), and its fetal homolog, human chorionic gonadotropin (hCG), promote pluripotent human and totipotent murine embryonic stem cell and neuron proliferation. However, strong evidence supports menopausal/andropausal elevations in the LH:sex steroid ratio as driving aberrant mitotic events. These include the upregulation of tumor necrosis factor; amyloid-β precursor protein processing towards the production of mitogenic Aβ; and the activation of Cdk5, a key regulator of cell cycle progression and tau phosphorylation (a cardinal feature of both neurogenesis and neurodegeneration). Cognitive and biochemical studies confirm the negative consequences of a high LH:sex steroid ratio on dendritic spine density and human cognitive performance. Prospective epidemiological and clinical evidence in humans supports the premise that rebalancing the ratio of circulating gonadotropins:sex steroids reduces the incidence of AD. Together, these data support endocrine dyscrasia and the subsequent loss of cell cycle control as an important etiological event in the development of neurodegenerative diseases including AD, stroke and Parkinson's disease.

Ganglioside-Dependent Neural Stem Cell Proliferation in Alzheimer's Disease Model Mice

The aggregation and formation of amyloid plaques by amyloid β-peptides (Aβs) is believed to be one of the pathological hallmarks of Alzheimer's disease (AD). Intriguingly, Aβs have also been shown to possess proliferative effects on neural stem cells (NSCs). Many essential cellular processes in NSCs, such as fate determination and proliferation, are heavily influenced by cell surface glycoconjugates, including gangliosides. It has recently been shown that Aβ1-42 alters several key glycosyltransferases and glycosidases. To further define the effects of Aβs and to clarify the potential mechanisms of action of those peptides on NSCs, NSCs were cultured from embryonic brains of the double-transgenic mouse model of AD [B6C3-Tg(APPswe,PSEN1dE9)85Dbo/J] coexpressing mutants of amyloid precursor protein (APPswe) and presenilin1 (PSEN1dE9). We found that Aβs not only promoted cell proliferation but also altered expression of several key glycogenes for glycoconjugate metabolism, such as sialyltransferases II and III (ST-II & -III) in AD NSCs. In addition, we found upregulation of epidermal growth factor receptor and Notch1 intracellular domain. Moreover, the increased expression of ST-II and -III coincided with the elevated levels of c-series gangliosides (A2B5+ antigens) in AD NSCs. Further, we revealed that epidermal growth factor signaling and gangliosides are necessary components on Aβ-stimulated NSC proliferation. Our present study has thus provided a novel mechanism for the upregulation of c-series ganglioside expression and increases in several NSC markers to account for the proliferative effect of Aβs on NSCs in AD mouse brain. These observations support the potential beneficial effects of Aβs and gangliosides in promoting neurogenesis in AD brain.

Evaluation of APP695 Transgenic Mice Bone Marrow Mesenchymal Stem Cells Neural Differentiation for Transplantation

Objective. Even though there is a therapeutic potential to treat Alzheimer's disease (AD) with neural cell replenishment and replacement, immunological rejections of stem cell transplantation remain a challenging risk. Autologous stem cells from AD patients however may prove to be a promising candidate. Therefore, we studied the neuronal differentiation efficiency of bone marrow mesenchymal stem cells (MSCs) from APP695 transgenic mice, which share features of human AD. Method. Cultured MSCs from APP695 transgenic mice are used; neuronal differentiation was assessed by immunocytochemistry and Western blot. Correlation with Notch signaling was examined. Autophage flux was assessed by western blot analysis. Results. MSCs from APP695 mice have higher neuronal differentiation efficiency than MSCs from wild type mice (WT MSCs). The expression of Notch-1 signaling decreased during the differentiation process. However, autophagy flux, which is essential for neuronal cell survival and neuronal function, was impaired in the neuronally differentiated counterparts of APP695 MSCs (APP695 MSCs–n). Conclusion. These results suggested autologous MSCs of APP690 mice may not be a good candidate for cell transplantation.

The Possible Roles of the Dentate Granule Cell's Leptin and Other Ciliary Receptors in Alzheimer's Neuropathology

Dentate-gyral granule cells in the hippocampus plus dentate gyrus memory-recording/retrieving machine, unlike most other neurons in the brain, are continuously being generated in the adult brain with the important task of separating overlapping patterns of data streaming in from the outside world via the entorhinal cortex. This "adult neurogenesis" is driven by tools in the mature granule cell's cilium. Here we report our discovery of leptin's LepRb receptor in this cilium. In addition, we discuss how ciliary LepRb signaling might be involved with ciliary p75NTR and SSTR3 receptors in adult neurogenesis and memory formation as well as attenuation of Alzheimer's neuropathology by reducing the production of its toxic amyloid-β-derived drivers.

Primary cilia and autophagic dysfunction in Huntington's disease

Huntington's disease (HD) is an inherited, neurodegenerative disorder caused by a single-gene mutation: a CAG expansion in the huntingtin (HTT) gene that results in production of a mutated protein, mutant HTT, with a polyglutamine tail (polyQ-HTT). Although the molecular pathways of polyQ-HTT toxicity are not fully understood, because protein misfolding and aggregation are central features of HD, it has long been suspected that cellular housekeeping processes such as autophagy might be important to disease pathology. Indeed, multiple lines of research have identified abnormal autophagy in HD, characterized generally by increased autophagic induction and inefficient clearance of substrates. To date, the origin of autophagic dysfunction in HD remains unclear and the search for actors involved continues. To that end, recent studies have suggested a bidirectional relationship between autophagy and primary cilia, signaling organelles of most mammalian cells. Interestingly, primary cilia structure is defective in HD, suggesting a potential link between autophagic dysfunction, primary cilia and HD pathogenesis. In addition, because polyQ-HTT also accumulates in primary cilia, the possibility exists that primary cilia might play additional roles in HD: perhaps by disrupting signaling pathways or acting as a reservoir for secretion and propagation of toxic, misfolded polyQ-HTT fragments. Here, we review recent research suggesting potential links between autophagy, primary cilia and HD and speculate on possible pathogenic mechanisms and future directions for the field.

Differential proteomic and behavioral effects of long-term voluntary exercise in wild-type and APP-overexpressing transgenics

Physical exercise may provide protection against the cognitive decline and neuropathology associated with Alzheimer's disease, although the mechanisms are not clear. In the present study, APP/PSEN1 double-transgenic and wild-type mice were allowed unlimited voluntary exercise for 7months. Consistent with previous reports, wheel-running improved cognition in the double-transgenic mice. Interestingly, the average daily distance run was strongly correlated with spatial memory in the water maze in wild-type mice (r(2)=.959), but uncorrelated in transgenics (r(2)=.013). Proteomics analysis showed that sedentary transgenic mice differed significantly from sedentary wild-types with respect to proteins involved in synaptic transmission, cytoskeletal regulation, and neurogenesis. When given an opportunity to exercise, the transgenics' deficiencies in cytoskeletal regulation and neurogenesis largely normalized, but abnormal synaptic proteins did not change. In contrast, exercise enhanced proteins associated with cytoskeletal regulation, oxidative phosphorylation, and synaptic transmission in wild-type mice. Soluble and insoluble Aβ40 and Aβ42 levels were significantly decreased in both cortex and hippocampus of active transgenics, suggesting that this may have played a role in the cognitive improvement in APP/PSEN1 mice. β-secretase was significantly reduced in active APP/PSEN1 mice compared to sedentary controls, suggesting a mechanism for reduced Aβ. Taken together, these data illustrate that exercise improves memory in wild-type and APP-overexpressing mice in fundamentally different ways.

Tailoring the antibody response to aggregated Aß using novel Alzheimer-vaccines

Recent evidence suggests Alzheimer-Disease (AD) to be driven by aggregated Aß. Capitalizing on the mechanism of molecular mimicry and applying several selection layers, we screened peptide libraries for moieties inducing antibodies selectively reacting with Aß-aggregates. The technology identified a pool of peptide candidates; two, AFFITOPES AD01 and AD02, were assessed as vaccination antigens and compared to Aβ1-6, the targeted epitope. When conjugated to Keyhole Limpet Hemocyanin (KLH) and adjuvanted with aluminum, all three peptides induced Aß-targeting antibodies (Abs). In contrast to Aß1-6, AD01- or AD02-induced Abs were characterized by selectivity for aggregated forms of Aß and absence of reactivity with related molecules such as Amyloid Precursor Protein (APP)/ secreted APP-alpha (sAPPa). Administration of AFFITOPE-vaccines to APP-transgenic mice was found to reduce their cerebral amyloid burden, the associated neuropathological alterations and to improve their cognitive functions. Thus, the AFFITOME-technology delivers vaccines capable of inducing a distinct Ab response. Their features may be beneficial to AD-patients, a hypothesis currently tested within a phase-II-study.

Dynamic nature of the p75 neurotrophin receptor in response to injury and disease

Neurotrophins and their respective tropomyosin related kinase (Trk) receptors (TrkA, TrkB, and TrkC) and the p75 neurotrophin receptor (p75(NTR)) play a fundamental role in the development and maintenance of the nervous system making them important targets for treatment of neurodegenerative diseases. Whereas Trk receptors are directly activated by specific neurotrophins, the p75(NTR) is a multifunctional receptor that exerts its effects via heterodimeric interactions with TrkA, TrkB, TrkC, sortilin or the Nogo receptor to regulate a wide array of cellular functions. By partnering with different receptors the p75(NTR) regulates binding of mature versus pro-neurotrophins and activation of different signaling pathways with outcomes ranging from growth and survival to cell death. While the developmental downregulation of the p75(NTR) has raised questions regarding its role in the mature nervous system, recent data have revealed widespread expression of low levels, a role in synaptic plasticity and adult neurogenesis and upregulation in response to injury or disease. Studies are needed to better understand these processes, particularly in the damaged nervous system, but will be complicated by expression of p75(NTR) on immune cells including macrophages and microglia that are intimately involved in disease and repair processes. Recent approaches that regulate p75(NTR) function with small non-peptide ligands have demonstrated potent neuroprotection in models of injury and neurodegenerative diseases that highlight the importance of the p75(NTR) as a therapeutic target. Future studies hold the promise of revealing a wealth of information on the multifaceted actions of the p75(NTR) that will inform the design of new neurotrophin-based therapies.

A small molecule p75(NTR) ligand protects neurogenesis after traumatic brain injury

The p75 neurotrophin receptor (p75(NTR)) influences the proliferation, survival, and differentiation of neuronal precursors and its expression is induced in injured brain, where it regulates cell survival. Here, we test the hypotheses that pharmacologic modulation of p75(NTR) signaling will promote neural progenitor survival and proliferation, and improve outcomes of traumatic brain injury (TBI). LM11A-31, an orally available, blood-brain barrier-permeant small-molecule p75(NTR) signaling modulator, significantly increased proliferation and survival, and decreased JNK phosphorylation, in hippocampal neural stem/progenitor cells in culture expressing wild-type p75(NTR), but had no effect on cells expressing a mutant neurotrophin-unresponsive form of the receptor. The compound also enhanced the production of mature neurons from adult hippocampal neural progenitors in vitro. In vivo, intranasal administration of LM11A-31 decreased postinjury hippocampal and cortical neuronal death, neural progenitor cell death, gliogenesis, and microglial activation, and enhanced long-term hippocampal neurogenesis and reversed spatial memory impairments. LM11A-31 diminished the postinjury increase of SOX2-expressing early progenitor cells, but protected and increased the proliferation of endogenous polysialylated-neural cell adhesion molecule positive intermediate progenitors, and restored the long-term production of mature granule neurons. These findings suggest that modulation of p75(NTR) actions using small molecules such as LM11A-31 may constitute a potent therapeutic strategy for TBI.

Olfactory Dysfunction in the Elderly: Basic Circuitry and Alterations with Normal Aging and Alzheimer's Disease

Preclinical detection of Alzheimer disease is critical to determining at-risk individuals in order to improve patient and caregiver planning for their futures and to identify individuals likely to benefit from treatment as advances in therapeutics develop over time. Identification of olfactory dysfunction at the preclinical and early stages of the disease is a potentially useful method to accomplish these goals. We first review basic olfactory circuitry. We then evaluate the evidence of pathophysiological change in the olfactory processing pathways during aging and Alzheimer disease in both human and animal models. We also review olfactory behavioral studies during these processes in both types of models. In doing so, we suggest hypotheses about the localization and mechanisms of olfactory dysfunction and identify important avenues for future work.

Insights into the physiological function of the β-amyloid precursor protein: beyond Alzheimer's disease

The β-amyloid precursor protein (APP) has been extensively studied for its role as the precursor of the β-amyloid protein (Aβ) of Alzheimer's disease. However, the normal function of APP remains largely unknown. This article reviews studies on the structure, expression and post-translational processing of APP, as well as studies on the effects of APP in vitro and in vivo. We conclude that the published data provide strong evidence that APP has a trophic function. APP is likely to be involved in neural stem cell development, neuronal survival, neurite outgrowth and neurorepair. However, the mechanisms by which APP exerts its actions remain to be elucidated. The available evidence suggests that APP interacts both intracellularly and extracellularly to regulate various signal transduction mechanisms.

This article reviews studies on the structure, expression and post-translational processing of β-amyloid precursor protein (APP), as well as studies on the effects of APP in vitro and in vivo. We conclude that the published data provide strong evidence that APP has a trophic function. APP is likely to be involved in neural stem cell development, neuronal survival, neurite outgrowth and neurorepair. However, the mechanisms by which APP exerts its actions remain to be elucidated. The available evidence suggests that APP interacts both intracellularly and extracellularly to regulate various signal transduction mechanisms.

Implications of p75NTR for dentate gyrus morphology and hippocampus-related behavior revisited

The pan-neurotrophin receptor p75NTR is expressed in the adult brain in a discrete pattern. Although numerous studies have addressed its implications for hippocampal functions, the generated sets of data are surprisingly conflicting. We have therefore set out to re-investigate the impact of a deletion of the full-length p75NTR receptor on several parameters of the dentate gyrus (DG), including neurogenesis and hippocampus-related behavior by using p75NTR(ExIII) knockout mice. Moreover, we investigated further parameters of the DG (cholinergic innervation, dendritic spines). In addition, we analyzed on the morphological level the impact of aging by comparing adult and aged p75NTR(ExIII) mice and their age-matched littermates. Adult (4-6 months old), but not aged (20 months old), p75NTR(ExIII) knockout mice display an enhanced volume of the DG. However, adult neurogenesis within the adult DG was unaffected in both adult and aged p75NTR(ExIII) knockout mice. We could further demonstrate that the change in the volume of the DG was accompanied by an increased cholinergic innervation and increased spine densities of granule cells in adult, but not aged p75NTR deficient mice. These morphological changes in the adult p75NTR deficient mice were accompanied by specific alterations in their behavior, including altered behavior in the Morris water maze test, indicating impairments in spatial memory retention.

Amyloid β-peptide 1-42 modulates the proliferation of mouse neural stem cells: upregulation of fucosyltransferase IX and notch signaling

Amyloid β-peptides (Aβs) aggregate to form amyloid plaques, also known as senile plaques, which are a major pathological hallmark of Alzheimer's disease (AD). Aβs are reported to possess proliferation effects on neural stem cells (NSCs); however, this effect remains controversial. Thus, clarification of their physiological function is an important topic. We have systematically evaluated the effects of several putative bioactive Aβs (Aβ1-40, Aβ1-42, and Aβ25-35) on NSC proliferation. Treatment of NSCs with Aβ1-42 significantly increased the number of those cells (149 ± 10 %). This was not observed with Aβ1-40 which did not have any effects on the proliferative property of NSC. Aβ25-35, on the other hand, exhibited inhibitory effects on cellular proliferation. Since cell surface glycoconjugates, such as glycolipids, glycoproteins, and proteoglycans, are known to be important for maintaining cell fate determination, including cellular proliferation, in NSCs and they undergo dramatic changes during differentiation, we examined the effect of Aβs on a number of key glycoconjugate metabolizing enzymes. Significantly, we found for the first time that Aβ1-42 altered the expression of several key glycosyltransferases and glycosidases, including fucosyltransferase IX (FUT9), sialyltransferase III (ST-III), glucosylceramide ceramidase (GLCC), and mitochondrial sialidase (Neu4). FUT9 is a key enzyme for the synthesis of the Lewis X carbohydrate epitope, which is known to be expressed in stem cells. Aβ1-42 also stimulated the Notch1 intracellular domain (NICD) by upregulation of the expression of Musashi-1 and the paired box protein, Pax6. Thus, Aβ1-42 upregulates NSC proliferation by modulating the expression of several glycogenes involved in Notch signaling.

Amyloid-β(1-42) oligomer accelerates senescence in adult hippocampal neural stem/progenitor cells via formylpeptide receptor 2

The failure of adult hippocampal neurogenesis is increasingly considered as an important factor in the pathological correlates for memory decline in Alzheimer's disease (AD). Loss of adult-born neurons and abnormalities of neural stem/progenitor cells (NSPCs) within the dentate gyrus (DG) of adult hippocampus might contribute to this process. In this study, we showed that amyloid-β_1–42 (Aβ₄₂) oligomer triggers senescent phenotype of NSPCs in vitro. Oligomerized Aβ₄₂ induced the production of senescence-associated biomarkers p16 and senescence-associated β-galactosidase (SA-β-gal) in adult mouse hippocampal NSPCs, as well as inhibited cells proliferation and differentiation. In the DG of amyloid precursor protein/presenilin1 (APP/PS1) transgenic mice, the number of senescent NSPCs was significantly increased and senescence-associated protein p16 was upregulated. Formylpeptide receptor 2 (FPR2), one of Aβ₄₂ functional receptors, may be involved in NSPCs senescence. The FPR2 antagonist WRW4 significantly inhibited NSPCs senescence induced by Aβ₄₂. In addition, the activation of p38 mitogen-activated protein kinase (MAPK) in response to the accumulation of reactive oxygen species (ROS) was involved in NSPCs senescence induced by Aβ₄₂. WRW4 inhibited the accumulation of ROS and the activation of p38 MAPK in NSPCs. Our data suggest that Aβ₄₂ accelerates NSPCs senescence via FPR2-dependent activation of its downstream ROS-p38 MAPK signaling, which limits the function of NSPCs and contributes to failure of neurogenesis. This is the first demonstration of NSPCs senescence response to Aβ₄₂.

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.

Aβ40 modulates GABA(A) receptor α6 subunit expression and rat cerebellar granule neuron maturation through the ERK/mTOR pathway

In addition to their neurotoxic role in Alzheimer's disease (AD), β-amyloid peptides (Aβs) are also known to play physiological roles. Here, we show that recombinant Aβ40 significantly increased the outward current of the GABA(A) receptor containing (GABA(A)α6) in rat cerebellar granule neurons (CGNs). The Aβ40-mediated increase in GABA(A)α6 current was mediated by an increase in GABA(A)α6 protein expression at the translational rather than the transcriptional level. The exposure of CGNs to Aβ40 markedly induced the phosphorylation of ERK (pERK) and mammalian target of rapamycin (pmTOR). The increase in GABA(A)α6 current and expression was attenuated by specific inhibitors of ERK or mTOR, suggesting that the ERK and mTOR signaling pathways are required for the effect of Aβ40 on GABA(A)α6 current and expression in CGNs. A pharmacological blockade of the p75 neurotrophin receptor (p75(NTR)), but not the insulin or α7-nAChR receptors, abrogated the effect of Aβ40 on GABA(A)α6 protein expression and current. Furthermore, the expression of GABA(A)α6 was lower in CGNs from APP(-/-) mice than in CGNs from wild-type mice. Moreover, the internal granule layer (IGL) in APP(-/-) mice was thinner than the IGL in wild-type mice. The injection of Aβ40 into the cerebellum reversed this effect, and the application of p75(NTR) blocking antibody abolished the effects of Aβ40 on cerebellum morphology in APP(-/-) mice. Our results suggest that low concentrations of Aβ40 play a role in regulating CGN maturation through p75(NTR).

Effects of amyloid β-peptides and gangliosides on mouse neural stem cells

The interaction of amyloid β-proteins (Aβs) with membrane lipids has been postulated as an early event in Aβ fibril formation in Alzheimer's disease. We evaluated the effects of several putative bioactive Aβs and gangliosides on neural stem cells (NSCs) isolated from embryonic mouse brains or the subventricular zone of adult mouse brains. Incubation of the isolated NSCs with soluble Aβ1-40 alone did not cause any change in the number of NSCs, but soluble Aβ1-42 increased their number. Aggregated Aβ1-40 and Aβ1-42 increased the number of NSCs but soluble and aggregated Aβ25-35 decreased the number. Soluble Aβ1-40 and Aβ1-42 did not affect the number of apoptotic cells but aggregated Aβ1-40 and Aβ1-42 did. When NSCs were treated with a combination of GM1 or GD3 and soluble Aβ1-42, cell proliferation was enhanced, indicating that both GM1 and GD3 as well as Aβs are involved in promoting cell proliferation and survival of NSCs. These observations suggest the potential of beneficial effects of using gangliosides and Aβs for promoting NSC proliferation.

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.

Rapid β-amyloid deposition and cognitive impairment after cholinergic denervation in APP/PS1 mice

Although extensive evidence supports the role of β-amyloid (Aβ) in Alzheimer disease (AD), the neurotoxic mechanisms underlying AD pathogenesis are not understood. On the other hand, neuronal loss is the pathologic feature that best correlates with cognitive impairment. We hypothesized that cholinergic neurodegeneration may lead to Aβ deposition and tested this by inducing selective cholinergic lesions in APPswe/PS1dE9 mice with murine p75 saporin (mu p75-SAP). Intracerebroventricular lesions that removed approximately 50% of cholinergic innervation to the cortex and hippocampus were induced in animals with incipient (∼3 months) and marked (∼7 months of age) Aβ deposition. Cranial windows were implanted, and Aβ deposition was monitored in vivo using multiphoton microscopy. Deposition of Aβ was increased as soon as 7 days after the lesion, and this effect was maintained up to 3 months later. Postmortem studies using immunohistochemistry with an anti-Aβ antibody corroborated these findings in both cerebral cortex and hippocampus. Tau phosphorylation was also significantly increased after the lesions. Cholinergic denervation resulted in early memory impairment at 3 months of age that worsened with age (∼7 months); there was a synergistic effect between cholinergic denervation and the presence of APP/PS1 transgenes. Altogether, our data suggest that cholinergic denervation may trigger Aβ deposition and synergistically contribute to cognitive impairment in AD patients.

The path from the choroid plexus to the subventricular zone: go with the flow!

In adult mammals, under physiological conditions, neurogenesis, the process of generating new functional neurons from precursor cells, occurs mainly in two brain areas: the subgranular zone in the dentate gyrus of the hippocampus, and the subventricular zone (SVZ) lining the walls of the brain lateral ventricles. Taking into account the location of the SVZ and the cytoarchitecture of this periventricular neural progenitor cell niche, namely the fact that the slow dividing primary progenitor cells (type B cells) of the SVZ extend an apical primary cilium toward the brain ventricular space which is filled with cerebrospinal fluid (CSF), it becomes likely that the composition of the CSF can modulate both self-renewal, proliferation and differentiation of SVZ neural stem cells. The major site of CSF synthesis is the choroid plexus (CP); quite surprisingly, however, it is still largely unknown the contribution of molecules specifically secreted by the adult CP as modulators of the SVZ adult neurogenesis. This is even more relevant in light of recent evidence showing the ability of the CP to adapt its transcriptome and secretome to various physiologic and pathologic stimuli. By giving particular emphasizes to growth factors and axonal guidance molecules we will illustrate how CP-born molecules might play an important role in the SVZ niche cell population dynamics.

Chemokine CXCL12 in neurodegenerative diseases: an SOS signal for stem cell-based repair

The dynamic relation between stem cells and their niche governs self-renewal and progenitor cell deployment. The chemokine CXCL12 (C-X-C motif ligand 12) and its signaling receptor CXCR4 (C-X-C motif receptor 4) represent an important pathway that regulates homing and maintenance of stem cells in neural niches. Neural stem cells (NSCs) reside in specific niches where communication with blood vessels is regulated by CXCL12. In neurodegenerative diseases and brain tumors, reactive vasculature forms in response to diseased tissues to create new niches that secrete CXCL12, enhancing the recruitment of neural progenitor cells (NPCs) to lesion sites via long-range migration. These observations suggest that the CXCL12-CXCR4 axis maintains NSCs and serves as an emergent salvage signal for initiating endogenous stem cell-based tissue repair.