Wild-type neural progenitors divide and differentiate normally in an amyloid-rich environment

Hippocampal neurogenesis and pro-neurogenic therapies for Alzheimer's disease

Adult hippocampal neurogenesis (AHN) facilitates hippocampal circuits plasticity and regulates hippocampus-dependent cognition and emotion. However, AHN malfunction has been widely reported in both human and animal models of Alzheimer's disease (AD), the most common form of dementia in the elderly. Pro-neurogenic therapies including rescuing innate AHN, cell engraftment and glia-neuron reprogramming hold great potential for compensating the neuronal loss and rewiring the degenerated neuronal network in AD, but there are still great challenges to be overcome. This review covers recent advances in unraveling the involvement of AHN in AD and highlights the prospect of emerging pro-neurogenic remedies.

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

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

Treadmill Exercise Ameliorates Adult Hippocampal Neurogenesis Possibly by Adjusting the APP Proteolytic Pathway in APP/PS1 Transgenic Mice

Alzheimer's disease (AD) is a neurodegenerative disorder known to cause cognitive impairment among the elderly worldwide. Although physical exercise-induced adult hippocampal neurogenesis (AHN) improves cognition, understanding its underlying molecular mechanisms requires further investigation using AD mouse models. In this present work, we subjected amyloid precursor protein (APP)/PS1 mice to a 12-week aerobic treadmill exercise to investigate AHN and its potential mechanisms. We divided 3-month-old littermates wild-type and APP/PS1 transgenic male mice into four groups, and the exercise groups performed 12-week treadmill exercise. Next, we evaluated the influence of treadmill exercise on learning and memory capacity, AHN, and APP proteolytic pathway-related factors. As per our results, the treadmill exercise was able to improve the hippocampal microenvironment in APP/PS1 mice probably by regulating various neurotrophic factors and secretases resulting in APP cleavage through a non-amyloidogenic pathway, which seems to further promote new cell proliferation, survival, and differentiation, enhancing hippocampal neurogenesis. All of these effects ameliorate learning and memory capacity. This study provides a theoretical and experimental basis for understanding AHN in an AD mouse model, which is beneficial for preventing and treating AD.

Sustained Hippocampal Neural Plasticity Questions the Reproducibility of an Amyloid-β-Induced Alzheimer's Disease Model

BACKGROUND

The use of Alzheimer's disease (AD) models obtained by intracerebral infusion of amyloid-β (Aβ) has been increasingly reported in recent years. Nonetheless, these models may present important challenges.

OBJECTIVE

We have focused on canonical mechanisms of hippocampal-related neural plasticity to characterize a rat model obtained by an intracerebroventricular (icv) injection of soluble amyloid-β42 (Aβ42).

METHODS

Animal behavior was evaluated in the elevated plus maze, Y-Maze spontaneous or forced alternation, Morris water maze, and open field, starting 2 weeks post-Aβ42 infusion. Hippocampal neurogenesis was assessed 3 weeks after Aβ42 injection. Aβ deposition, tropomyosin receptor kinase B levels, and neuroinflammation were appraised at 3 and 14 days post-Aβ42 administration.

RESULTS

We found that immature neuronal dendritic morphology was abnormally enhanced, but proliferation and neuronal differentiation in the dentate gyrus was conserved one month after Aβ42 injection. Surprisingly, animal behavior did not reveal changes in cognitive performance nor in locomotor and anxious-related activity. Brain-derived neurotrophic factor related-signaling was also unchanged at 3 and 14 days post-Aβ icv injection. Likewise, astrocytic and microglial markers of neuroinflammation in the hippocampus were unaltered in these time points.

CONCLUSION

Taken together, our data emphasize a high variability and lack of behavioral reproducibility associated with these Aβ injection-based models, as well as the need for its further optimization, aiming at addressing the gap between preclinical AD models and the human disorder.

Physical Activity Ameliorates Impaired Hippocampal Neurogenesis in the Tg4-42 Mouse Model of Alzheimer's Disease

There is growing evidence from epidemiological studies that especially midlife physical activity might exert a positive influence on the risk and progression of Alzheimer’s disease. In this study, the Tg4-42 mouse model of Alzheimer’s disease has been utilized to assess the effect of different housing conditions on structural changes in the hippocampus. Focusing on the dentate gyrus, we demonstrate that 6-month-old Tg4-42 mice have a reduced number of newborn neurons in comparison to age-matched wild-type mice. Housing these mice for 4 months with either unlimited or intermittent access to a running wheel resulted in a significant rescue of dentate gyrus neurogenesis. Although neither dentate gyrus volume nor neuron number could be modified in this Alzheimer’s disease mouse model, unrestricted access to a running wheel significantly increased dentate gyrus volume and granule cell number in wild-type mice.

Interneuron Accumulation of Phosphorylated tau Impairs Adult Hippocampal Neurogenesis by Suppressing GABAergic Transmission

Phospho-tau accumulation and adult hippocampal neurogenesis (AHN) impairment both contribute importantly to the cognitive decline in Alzheimer's disease (AD), but whether and how tau dysregulates AHN in AD remain poorly understood. Here, we found a prominent accumulation of phosphorylated tau in GABAergic interneurons in the dentate gyrus (DG) of AD patients and mice. Specific overexpression of human tau (hTau) in mice DG interneurons induced AHN deficits but increased neural stem cell-derived astrogliosis, associating with a downregulation of GABA and hyperactivation of neighboring excitatory neurons. Chemogenetic inhibition of excitatory neurons or pharmacologically strengthening GABAergic tempos rescued the tau-induced AHN deficits and improved contextual cognition. These findings evidenced that intracellular accumulation of tau in GABAergic interneurons impairs AHN by suppressing GABAergic transmission and disinhibiting neural circuits within the neurogenic niche, suggesting a potential of GABAergic potentiators for pro-neurogenic or cell therapies of AD.

Emergence of synaptic and cognitive impairment in a mature-onset APP mouse model of Alzheimer's disease

The synaptic changes underlying the onset of cognitive impairment in Alzheimer’s disease (AD) are poorly understood. In contrast to the well documented inhibition of long-term potentiation (LTP) in CA3-CA1 synapses by acute Aβ application in adult neurons from rodents, young amyloid precursor protein (APP) transgenic mouse models often, surprisingly, show normal LTP. This suggests that there may be important differences between mature-onset and developmental-onset APP expression/ Aβ accumulation and the ensuing synaptic and behavioural phenotype. Here, in agreement with previous studies, we observed that developmental expression of APP_Sw,Ind (3–4 month old mice from line 102, PLoS Med 2:e355, 2005), resulted in reduced basal synaptic transmission in CA3-CA1 synapses, normal LTP, impaired spatial working memory, but normal spatial reference memory. To analyse early Aβ-mediated synaptic dysfunction and cognitive impairment in a more mature brain, we used controllable mature-onset APP_Sw,Ind expression in line 102 mice. Within 3 weeks of mature-onset APP_Sw,Ind expression and Aβ accumulation, we detected the first synaptic dysfunction: an impairment of LTP in hippocampal CA3-CA1 synapses. Cognitively, at this time point, we observed a deficit in short-term memory. A reduction in basal synaptic strength and deficit in long-term associative spatial memory were only evident following 12 weeks of APP_Sw,Ind expression. Importantly, the plasticity impairment observed after 3 weeks of mature-onset APP expression is reversible. Together, these findings demonstrate important differences between developmental and mature-onset APP expression. Further research targeted at this early stage of synaptic dysfunction could help identify mechanisms to treat cognitive impairment in mild cognitive impairment (MCI) and early AD.

Incensole acetate prevents beta-amyloid-induced neurotoxicity in human olfactory bulb neural stem cells

β-Amyloid peptide (Aβ) is a potent neurotoxic protein associated with Alzheimer's disease (AD) which causes oxidative damage to neurons. Incensole acetate (IA) is a major constituent of Boswellia carterii resin, which has anti-inflammatory and protective properties against damage of a large verity of neural subtypes. However, this neuroprotective effect was not studied on human olfactory bulb neural stem cells (hOBNSCs). Herein, we evaluated this effect and studied the underlying mechanisms. Exposure to Aβ_25-35 (5 and 10 μM for 24 h) inhibited proliferation (revealed by downregulation of Nestin and Sox2 gene expression), and induced differentiation (marked by increased expression of the immature neuronal marker Map2 and the astrocyte marker Gfap) of hOBNSCs. However, pre-treatment with IA (100 μM for 4 h) stimulated proliferation and differentiation of neuronal, rather than astrocyte, markers. Moreover, IA pretreatment significantly decreased the Aβ_25-35-induced viability loss, apoptotic rate (revealed by decreased caspase 3 activity and protein expression, downregulated expression of Bax, caspase 8, cyto c, caspase3, and upregulated expression of Bcl2 mRNAs and proteins, in addition to elevated mitochondrial membrane potential and lowered intracellular Ca⁺²). IA reduced Aβ-mediated ROS production (revealed by decreased intracellular ROS and MDA level, and increased SOD, CAT, and GPX contents), and inhibited Aβ-induced inflammation (marked by down-regulated expression of IL1b, TNFa, NfKb, and Cox2 genes). IA also significantly upregulated mRNA and protein expression of Erk1/2 and Nrf2. Notably, IA increased the antioxidant enzyme heme oxygenase-1 (HO-1) expression and this effect was reversed by HO-1 inhibitor zinc protoporphyrin (ZnPP) leading to reduction of the neuroprotective effect of IA against Aβ-induced neurotoxicity. These findings clearly show the ability of IA to initiate proliferation and differentiation of neuronal progenitors in hOBNSCs and induce HO-1 expression, thereby protecting the hOBNSCs cells from Aβ_25-35-induced oxidative cell death. Thus, IA may be applicable as a potential preventive agent for AD by its effect on hOBNSCs and could also be used as an adjuvant to hOBNSCs in cellular therapy of neurodegenerative diseases.

Oligomer-prone E57K-mutant alpha-synuclein exacerbates integration deficit of adult hippocampal newborn neurons in transgenic mice

In the adult mammalian hippocampus, new neurons are constantly added to the dentate gyrus. Adult neurogenesis is impaired in several neurodegenerative mouse models including α-synuclein (a-syn) transgenic mice. Among different a-syn species, a-syn oligomers were reported to be the most toxic species for neurons. Here, we studied the impact of wild-type vs. oligomer-prone a-syn on neurogenesis. We compared the wild-type a-syn transgenic mouse model (Thy1-WTS) to its equivalent transgenic for oligomer-prone E57K-mutant a-syn (Thy1-E57K). Transgenic a-syn was highly expressed within the hippocampus of both models, but was not present within adult neural stem cells and neuroblasts. Proliferation and survival of newly generated neurons were unchanged in both transgenic models. Thy1-WTS showed a minor integration deficit regarding mushroom spine density of newborn neurons, whereas Thy1-E57K exhibited a severe reduction of all spines. We conclude that cell-extrinsic a-syn impairs mushroom spine formation of adult newborn neurons and that oligomer-prone a-syn exacerbates this integration deficit. Moreover, our data suggest that a-syn reduces the survival of newborn neurons by a cell-intrinsic mechanism during the early neuroblast development. The finding of increased spine pathology in Thy1-E57K is a new pathogenic function of oligomeric a-syn and precedes overt neurodegeneration. Thus, it may constitute a readout for therapeutic approaches.

High-resolution bioelectrical imaging of Aβ-induced network dysfunction on CMOS-MEAs for neurotoxicity and rescue studies

Neurotoxicity and the accumulation of extracellular amyloid-beta_1–42 (Aβ) peptides are associated with the development of Alzheimer’s disease (AD) and correlate with neuronal activity and network dysfunctions, ultimately leading to cellular death. However, research on neurodegenerative diseases is hampered by the paucity of reliable readouts and experimental models to study such functional decline from an early onset and to test rescue strategies within networks at cellular resolution. To overcome this important obstacle, we demonstrate a simple yet powerful in vitro AD model based on a rat hippocampal cell culture system that exploits large-scale neuronal recordings from 4096-electrodes on CMOS-chips for electrophysiological quantifications. This model allows us to monitor network activity changes at the cellular level and to uniquely uncover the early activity-dependent deterioration induced by Aβ-neurotoxicity. We also demonstrate the potential of this in vitro model to test a plausible hypothesis underlying the Aβ-neurotoxicity and to assay potential therapeutic approaches. Specifically, by quantifying N-methyl D-aspartate (NMDA) concentration-dependent effects in comparison with low-concentration allogenic-Aβ, we confirm the role of extrasynaptic-NMDA receptors activation that may contribute to Aβ-neurotoxicity. Finally, we assess the potential rescue of neural stem cells (NSCs) and of two pharmacotherapies, memantine and saffron, for reversing Aβ-neurotoxicity and rescuing network-wide firing.

Altered neurogenesis in mouse models of Alzheimer disease

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

Puerarin Ameliorates D-Galactose Induced Enhanced Hippocampal Neurogenesis and Tau Hyperphosphorylation in Rat Brain

Enhanced neurogenesis has been reported in the hippocampus of patients with Alzheimer's disease (AD), the most common neurodegenerative disorder characterized with amyloid-β (Aβ) aggregation, tau hyperphosphorylation, and progressive neuronal loss. Previously we reported that tau phosphorylation played an essential role in adult hippocampal neurogenesis, and activation of glycogen synthase kinase (GSK-3), a crucial tau kinase, could induce increased hippocampal neurogenesis. In the present study, we found that treatment of D-galactose rats with Puerarin could significantly improve behavioral performance and ameliorate the enhanced neurogenesis and microtubule-associated protein tau hyperphosphorylation in the hippocampus of D-galactose rat brains. FGF-2/GSK-3 signaling pathway might be involved in the effects of Puerarin on hippocampal neurogenesis and tau hyperphosphorylation. Our finding provides primary in vivo evidence that Puerarin can attenuate AD-like enhanced hippocampal neurogenesis and tau hyperphosphorylation. Our finding also suggests Puerarin can be served as a treatment for age-related neurodegenerative disorders, such as AD.

Effects of Neurotrophic Support and Amyloid-Targeted Combined Therapy on Adult Hippocampal Neurogenesis in a Transgenic Model of Alzheimer's Disease

Although it is recognized that multi-drug therapies may be necessary to combat AD, there is a paucity of preclinical proof of concept studies. We present a combination treatment paradigm, which temporally affects different aspects of Alzheimer's disease (AD)-like pathology, specifically Aβ-toxicity and neurogenesis. At early stages of AD-like pathology, in TgCRND8 mice, we found that combating Aβ pathology with scyllo-inositol ameliorated deficits in neurogenesis. Older TgCRND8 mice with established amyloid load had decreased progenitor cell proliferation and survival compared to non-transgenic mice, regardless of scyllo-inositol treatment. The prolonged exposure to Aβ-pathology leads to deficits in the neurogenic niche, thus targeting Aβ alone is insufficient to rescue neurogenesis. To support the neurogenic niche, we combined scyllo-inositol treatment with leteprinim potassium (neotrofin), the latter of which stimulates neurotrophin expression. We show that the combination treatment of scyllo-inositol and neotrofin enhances neuronal survival and differentiation. We propose this proof of concept combination therapy of targeting Aβ-pathology and neurotrophin deficits as a potential treatment for AD.

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

Adult hippocampal neurogenesis was impaired in several Alzheimer's disease models overexpressing mutant human amyloid precursor protein (hAPP). However, the effects of wild-type hAPP on adult neurogenesis and whether the impaired adult hippocampal neurogenesis was caused by amyloid β (Aβ) or APP remained unclear. Here, we found that neurogenesis was impaired in the dentate gyrus (DG) of adult mice overexpressing wild-type hAPP (hAPP-I5) compared with controls. However, the adult hippocampal neurogenesis was more severely impaired in hAPP-I5 than that in hAPP-J20 mice, which express similar levels of hAPP mRNA but much higher levels of Aβ. Furthermore, reducing Aβ levels did not affect the number of doublecortin-positive cells in the DG of hAPP-J20 mice. Our results suggested that hAPP was more likely an important factor inhibiting adult neurogenesis, and Aβ was not the major factor affecting neurogenesis in the adult hippocampus of hAPP mice.

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Overexpression of wild-type hAPP impairs adult hippocampal neurogenesis

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Adult hippocampal neurogenesis was more severely impaired in hAPP-I5 than in hAPP-J20

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Reducing Aβ levels did not affect adult neurogenesis in hAPP-J20 mice

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Aβ is not the major factor accounting for impaired adult neurogenesis in hAPP mice

Sex-related dimorphism in dentate gyrus atrophy and behavioral phenotypes in an inducible tTa:APPsi transgenic model of Alzheimer's disease

Sex differences are a well-known phenomenon in Alzheimer's disease (AD), with women having a higher risk for AD than men. Many AD mouse models display a similar sex-dependent pattern, with females showing earlier cognitive deficits and more severe neuropathology than males. However, whether those differences are relevant to human disease is unclear. Here we show that in AD mouse models that overexpress amyloid precursor protein (APP) under control of the prion protein promoter (PrP), female transgenic mice have higher APP expression than males, complicating interpretations of the role of sex-related factors in such models. By contrast, in a tTa:APPsi model, in which APP expression is driven by the tetracycline transactivator (tTa) from the CaMKIIα promoter, there are no sex-related differences in expression or processing of APP. In addition, the levels of Aβ dimers and tetramers, as well as Aβ peptide accumulation, are similar between sexes. Behavioral testing demonstrated that both male and female tTa:APPsi mice develop age-dependent deficits in spatial recognition memory and conditional freezing to context. These cognitive deficits were accompanied by habituation-associated hyperlocomotion and startle hyper-reactivity. Significant sex-related dimorphisms were observed, due to females showing earlier onsets of the deficits in conditioned freezing and hyperlocomotion. In addition, tTa:APPsi males but not females demonstrated a lack of novelty-induced activation. Both males and females showed atrophy of the dentate gyrus (DG) of the dorsal hippocampus, associated with widening of the pyramidal layer of the CA1 area in both sexes. Ventral DG was preserved. Sex-related differences were limited to the DG, with females showing more advanced degeneration than males. Collectively, our data show that the tTa:APPsi model is characterized by a lack of sex-related differences in APP expression, making this model useful in deciphering the mechanisms of sex differences in AD pathogenesis. Sex-related dimorphisms observed in this model under conditions of equal APP expression between sexes suggest a higher sensitivity of females to the effects of APP and/or Aβ production.

Neural stem/progenitor cells in Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease and a worldwide health challenge. Different therapeutic approaches are being developed to reverse or slow the loss of affected neurons. Another plausible therapeutic way that may complement the studies is to increase the survival of existing neurons by mobilizing the existing neural stem/progenitor cells (NSPCs) - i.e. "induce their plasticity" - to regenerate lost neurons despite the existing pathology and unfavorable environment. However, there is controversy about how NSPCs are affected by the unfavorable toxic environment during AD. In this review, we will discuss the use of stem cells in neurodegenerative diseases and in particular how NSPCs affect the AD pathology and how neurodegeneration affects NSPCs. In the end of this review, we will discuss how zebrafish as a useful model organism with extensive regenerative ability in the brain might help to address the molecular programs needed for NSPCs to respond to neurodegeneration by enhanced neurogenesis.

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.