Amyloid, memory and neurogenesis

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.

Low-Level Laser Therapy to the Bone Marrow Ameliorates Neurodegenerative Disease Progression in a Mouse Model of Alzheimer's Disease: A Minireview

OBJECTIVE

This communication reviews the ability of low-level laser therapy (LLLT) to stimulate mesenchymal stem cells (MSCs) in autologous bone marrow (BM) to enhance the capacity of MSCs to infiltrate the brain, clear β-amyloid, and improve cognition.

BACKGROUND

We recently reported that LLLT applied to the BM enhanced the proliferation of MSCs and their mobilization toward the ischemic heart region, suggesting a possible application of this approach in regenerative medicine and neurodegenerative diseases. It was also shown that circulating monocytes can infiltrate the brain and reduce brain amyloid load in an Alzheimer's disease (AD) mouse model.

METHODS AND RESULTS

MSCs from wild-type mice stimulated with LLLT demonstrated an increased ability to maturate toward a monocyte lineage and to increase phagocytosis of soluble Aβ in vitro. Furthermore, weekly LLLT for 2 months to the BM, starting at 4 months of age (progressive stage of the disease in these 5XFAD transgenic male mice), improved memory and spatial learning, compared to a sham-treated AD mouse model. Histology revealed a significant reduction in Aβ brain burden in the laser-treated mice compared to the nonlaser-treated ones.

CONCLUSIONS

The application of LLLT to the BM is suggested as a therapeutic approach in progressive stages of AD, and its potential role in mediating MSC therapy in brain amyloidogenic disease is implied.

Better Utilization of Mouse Models of Neurodegenerative Diseases in Preclinical Studies: From the Bench to the Clinic

The major symptom of Alzheimer's disease is dementia progressing with age. Its clinical diagnosis is preceded by a long prodromal period of brain pathology that encompasses both formation of extracellular amyloid and intraneuronal tau deposits in the brain and widespread neuronal death. At present, familial cases of dementia provide the most promising foundation for modeling neurodegenerative tauopathies, a group of heterogeneous disorders characterized by prominent intracellular accumulation of hyperphosphorylated tau protein. In this chapter, we describe major behavioral hallmarks of tauopathies, briefly outline the genetics underlying familial cases, and discuss the arising implications for modeling the disease in transgenic mouse systems. The selection of tests performed to evaluate the phenotype of a model should be guided by the key behavioral hallmarks that characterize human disorder and their homology to mouse cognitive systems. We attempt to provide general guidelines and establish criteria for modeling dementia in a mouse; however, interpretations of obtained results should avoid a reductionist "one gene, one disease" explanation of model characteristics. Rather, the focus should be directed to the question of how the mouse genome can cope with the over-expression of the protein coded by transgene(s). While each model is valuable within its own constraints and the experiments performed are guided by specific hypotheses, we seek to expand upon their methodology by offering guidance spanning from issues of mouse husbandry to choices of behavioral tests and routes of drug administration that might increase the external validity of studies and consequently optimize the translational aspect of preclinical research.

Nodakenin Enhances Cognitive Function and Adult Hippocampal Neurogenesis in Mice

In our previous study, we demonstrated that nodakenin, a coumarin compound isolated from Angelica decursiva, ameliorates learning and memory impairments induced by scopolamine. In the present study, we investigated the effects of nodakenin on the cognitive function in the normal naïve mice in a passive avoidance task, and the results showed that nodakenin significantly increased the latency time in normal naïve mice. In addition, sub-chronic administration of nodakenin increased the number of 5-bromo-2-deoxyuridine (BrdU)-positive cells in the hippocampal dentate gyrus (DG) region. The percentage of BrdU and NeuN (neuronal cell marker)-immunopositive cells was also significantly increased by the nodakenin administration. Western blotting results showed that the expression levels of phosphorylated protein kinase B (Akt) and phosphorylated glycogen synthase kinase-3β (GSK-3β) were significantly increased in hippocampal tissue by sub-chronic nodakenin administration. These findings suggest that the sub-chronic administration of nodakenin enhances adult hippocampal neurogenesis in the DG region via Akt-GSK-3β signaling and this increase may be associated with nodakenin's positive effect on cognitive processing.

Memantine improves spatial learning and memory impairments by regulating NGF signaling in APP/PS1 transgenic mice

Memantine (MEM) is used for improving the cognitive impairments of the patients suffering from Alzheimer's disease (AD) by multiple neuroprotective mechanisms. However, it is still not clear whether nerve growth factor (NGF) signaling is involved in the mechanisms of MEM. The present study investigated the neuroprotective effects of MEM treatment on the cognitive performance and amyloidosis in APP/PS1 transgenic mice, and disclosed the NGF-related mechanism of MEM. We found that MEM treatment improved the cognitive performance by decreasing the escape latency and path length in the navigation test, by shortening the duration in target quadrant and reducing the frequency to pass through the target in probe trial, and by prolonging the latency and decreasing the frequencies of entering the dark compartment in passive avoidance test. The over-expressions of Aβ(1-42) and amyloid precursor protein (APP) were also decreased in the brains of APP/PS1 mice. Interestingly, MEM treatment improved the decreased NGF levels in APP/PS1 mice. Furthermore, NGF/TrkA signaling was activated by increasing the phosphorylation levels of tyrosine kinase (TrkA), proto-oncogene serine/threonine-protein kinase, Raf1 (c-Raf), extracellular regulated protein kinases (ERK)1/2 and cAMP-response element binding protein (CREB) after MEM treatment. Simultaneously, MEM also inhibited NGF/p75(NTR) signaling via decreasing the cleavage substrate of p75(NTR), increasing the JNK2 phosphorylation and decreasing the levels of p53 and cleaved-caspase 3. Therefore, the dual-regulation on NGF signaling was attributed to the improvements of cognitive deficits and Aβ depositions in APP/PS1 mice. In conclusion, MEM treatment activated the NGF/TrkA signaling, and inhibited the p75(NTR) signaling in APP/PS1 mice to ameliorate the behavioral deficits and amyloidosis, indicating that NGF signaling was a new potential target of MEM treatment for AD therapy.

Low-level laser therapy ameliorates disease progression in a mouse model of Alzheimer's disease

Low-level laser therapy (LLLT) has been used to treat inflammation, tissue healing, and repair processes. We recently reported that LLLT to the bone marrow (BM) led to proliferation of mesenchymal stem cells (MSCs) and their homing in the ischemic heart suggesting its role in regenerative medicine. The aim of the present study was to investigate the ability of LLLT to stimulate MSCs of autologous BM in order to affect neurological behavior and β-amyloid burden in progressive stages of Alzheimer's disease (AD) mouse model. MSCs from wild-type mice stimulated with LLLT showed to increase their ability to maturate towards a monocyte lineage and to increase phagocytosis activity towards soluble amyloid beta (Aβ). Furthermore, weekly LLLT to BM of AD mice for 2 months, starting at 4 months of age (progressive stage of AD), improved cognitive capacity and spatial learning, as compared to sham-treated AD mice. Histology revealed a significant reduction in Aβ brain burden. Our results suggest the use of LLLT as a therapeutic application in progressive stages of AD and imply its role in mediating MSC therapy in brain amyloidogenic diseases.

Is neurogenesis relevant in depression and in the mechanism of antidepressant drug action? A critical review

OBJECTIVES

Major depression is a complex disorder that involves genetic, epigenetic and environmental factors in its aetiology. Recent research has suggested that hippocampal neurogenesis may play a role in antidepressant action. However, careful examination of the literature suggests that the complex biological and psychological changes associated with depression cannot be attributed to disturbance in hippocampal neurogenesis alone. While antidepressants may induce hippocampal neurogenesis in non-human primates, there is a paucity of evidence that such effects are sufficient for full therapeutic action in humans.

METHODS

This review examines the literature on neurogenesis and discusses the stress-induced cortisol neurotoxicity and antidepressant-induced neurogenesis rescue model of depression. The disparity between a simple antidepressant-induced neurogenesis rescue model in the hippocampus and the complexity of clinical depression is analyzed through critical evaluation of recent research data.

RESULTS AND CONCLUSIONS

Major depression is a complex brain disorder with multiple symptoms and disturbances reflecting dysfunction in more than one single brain area. Initial research suggesting a model of hippocampal degeneration as basis of depression, and reversal by antidepressants through neurogenesis seems to be over-simplified given the emergence of new data. Synaptogenesis and re-organization or re-integration of new neurons rather than simple addition of new neurons may underlie the role of antidepressant drugs in the reversal of some but not all symptoms in depression. The importance of the neurogenesis hypothesis of depression and antidepressant action lies in stimulating further research into the possible roles played by the new neurons and synapses generated.

Oroxylin A, a flavonoid, stimulates adult neurogenesis in the hippocampal dentate gyrus region of mice

Previously, we reported the cognitive enhancing effects of oroxylin A in unimpaired mice and its memory ameliorating activity in various memory impaired mice. To elucidate the mechanism mediating the cognitive effects of oroxylin A, this study examined the consequences of oroxylin A administration on neurogenesis in the hippocampal dentate gyrus using immunostaining for 5-bromo-2-deoxyuridine (BrdU) incorporation. In addition, we determined whether the new cells adopted a neuronal or glial fate by examining the co-localization of BrdU staining with neuronal or glial markers. Administration of oroxylin A in a dose-dependent and time-dependent manner increased the number of BrdU-incorporating cells. Moreover, the percentage of BrdU-incorporating cells co-localized with neuronal markers, neuronal nuclei, was significantly increased by the oroxylin A administration. These results suggest that the increased neurogenesis induced by the administration of oroxylin A could be, at least in part, associated with its positive effects on cognitive processing.

Enhanced dentate gyrus synaptic plasticity but reduced neurogenesis in a mouse model of amyloidosis

Long-term potentiation (LTP) and neurogenesis in the dentate gyrus (DG) are correlated forms of hippocampal plasticity which share, under physiological conditions, common regulatory mechanisms. In Alzheimer's disease (AD), their alterations are potentially associated with the early cellular pathology and cognitive decline. We analyzed DG LTP and neurogenesis in B6.152H mice, an amyloid precursor protein and presenilin 2 double-transgenic mouse model of amyloidosis and observed that DG LTP was strongly enhanced before and after amyloid plaque formation. Whereas proliferation of DG neuronal progenitor cells was unchanged, survival of newborn neurons was strongly decreased already before plaque formation. As similar alteration of neurogenesis was observed in PS2APP mice without changes in DG LTP (Richards et al. 2003), this study suggests that enhanced synaptic plasticity did not rescue impaired neurogenesis, and supports decreased survival of newborn neurons as an early event associated with AD detectable even before plaque formation.

Hippocampal neurogenesis as a target for the treatment of mental illness: a critical evaluation

Over one-quarter of adult Americans are diagnosed with a mental illness like Major Depressive Disorder (MDD), Post-Traumatic Stress Disorder (PTSD), schizophrenia, and Alzheimer's Disease. In addition to the exceptional personal burden these disorders exert on patients and their families, they also have enormous cost to society. Although existing pharmacological and psychosocial treatments alleviate symptoms in many patients, the comorbidity, severity, and intractable nature of mental disorders strongly underscore the need for novel strategies. As the hippocampus is a site of structural and functional pathology in most mental illnesses, a hippocampal-based treatment approach has been proposed to counteract the cognitive deficits and mood dysregulation that are hallmarks of psychiatric disorders. In particular, preclinical and clinical research suggests that hippocampal neurogenesis, the generation of new neurons in the adult dentate gyrus, may be harnessed to treat mental illness. There are obvious applications and allures of this approach; for example, perhaps stimulating hippocampal neurogenesis would reverse the overt and noncontroversial hippocampal atrophy and functional deficits observed in Alzheimer's Disease and schizophrenia, or the more controversial hippocampal deficits seen in MDD and PTSD. However, critical examination suggests that neurogenesis may only correlate with mental illness and treatment, suggesting targeting neurogenesis alone is not a sufficient treatment strategy. Here we review the classic and causative links between adult hippocampal neurogenesis and mental disorders, and provide a critical evaluation of how (and if) our basic knowledge of new neurons in the adult hippocampus might eventually help combat or even prevent mental illness.

Inhibition of PI3K-Akt signaling blocks exercise-mediated enhancement of adult neurogenesis and synaptic plasticity in the dentate gyrus

Background

Physical exercise has been shown to increase adult neurogenesis in the dentate gyrus and enhances synaptic plasticity. The antiapoptotic kinase, Akt has also been shown to be phosphorylated following voluntary exercise; however, it remains unknown whether the PI3K-Akt signaling pathway is involved in exercise-induced neurogenesis and the associated facilitation of synaptic plasticity in the dentate gyrus.

Methodology/Principal Findings

To gain insight into the potential role of this signaling pathway in exercise-induced neurogenesis and LTP in the dentate gyrus rats were infused with the PI3K inhibitor, LY294002 or vehicle control solution (icv) via osmotic minipumps and exercised in a running wheel for 10 days. Newborn cells in the dentate gyrus were date-labelled with BrdU on the last 3 days of exercise. Then, they were either returned to the home cage for 2 weeks to assess exercise-induced LTP and neurogenesis in the dentate gyrus, or were killed on the last day of exercise to assess proliferation and activation of the PI3K-Akt cascade using western blotting.

Conclusions/Significance

Exercise increases cell proliferation and promotes survival of adult-born neurons in the dentate gyrus. Immediately after exercise, we found that Akt and three downstream targets, BAD, GSK3β and FOXO1 were activated. LY294002 blocked exercise-induced phosphorylation of Akt and downstream target proteins. This had no effect on exercise-induced cell proliferation, but it abolished most of the beneficial effect of exercise on the survival of newly generated dentate gyrus neurons and prevented exercise-induced increase in dentate gyrus LTP. These results suggest that activation of the PI3 kinase-Akt signaling pathway plays a significant role via an antiapoptotic function in promoting survival of newly formed granule cells generated during exercise and the associated increase in synaptic plasticity in the dentate gyrus.

Protective effect of S14G-humanin against beta-amyloid induced LTP inhibition in mouse hippocampal slices

Synaptic dysfunction induced by amyloid-beta protein (Abeta) has been shown to play a critical role in cognitive deficits of Alzheimer's disease (AD). Currently, however there is no clinical causative therapy for the disease. S14G-humanin (HNG) is best known for its strong neuroprotective ability against AD-related insults in vitro, and several in vivo studies have shown its effectiveness in ameliorating the cognitive impairment, but the precise mechanism of HNG on neuroprotection still remains to be elucidated. The present study examined the effects of HNG on Abeta-induced inhibition of hippocampal long-term potentiation (LTP) in mouse hippocampal slices. The results disclosed that soluble Abeta(25-35) significantly inhibited the induction of early-phase LTP (E-LTP) and late-phase LTP (L-LTP) in the hippocampal CA1 region without affecting the basal synaptic transmission, while HNG significantly ameliorated such inhibition of E-LTP and L-LTP in a dose-dependent manner. In addition, the reduction of phosphorylated CREB trigged by Abeta(25-35) was restored by HNG during L-LTP induction, possibly attributing to the improvement of the L-LTP inhibition. Collectively, our findings add to the evidence that soluble Abeta-induced LTP inhibition may represent an early pathological event of AD, and demonstrate for the first time that HNG may improve LTP inhibition by subneurotoxic concentration of soluble Abeta, suggesting that HNG may have therapeutic potential for Abeta-induced synaptic dysfunction closely associated with cognitive deficits in the early stage of AD.

Growth hormone signaling and hippocampal neurogenesis: insights from genetic models

Adult hippocampal neurogenesis (AHN) is modulated by a variety of factors through effects on the proliferation-differentiation-survival regulatory axis. We have employed growth hormone receptor knockout (GH-R-/-) and suppressor of cytokine signaling-2 transgenic (SOCS-2 Tg) mice as models of altered GH-signaling to assess their affects on basal and exercised-induced hippocampal neurogenesis. Assessment of proliferation 24-h after 7-days of bromodeoxyuridine (BrdU) labeling with or without voluntary running showed that the density of BrdU(+) cells in the subgranular zone remained unchanged between genotypes in control housing, while running induced significant increases in BrdU-labeled cells in WT, GH-R-/-, and SOCS-2 Tg mice. The proportion of BrdU/doublecortin and BrdU/S100beta cells did not vary between genotype or running conditions at this time-point. Assessment of cell survival 28-days after BrdU labeling showed that SOCS-2 Tg animals had significantly higher BrdU(+) cell densities in the granule cell layer compared to WT and GH-R-/- animals in control housing and after voluntary running. There were no differences in cell survival between WT and GH-R-/- mice with or without running. Mature phenotype analysis showed similar proportions of BrdU/NeuN and BrdU/S100beta in all groups. While SOCS-2 Tg mice had similar social interaction behaviors and sensorimotor gating, they appeared to be less anxious with heightened basal locomotor activity and showed enhanced performance in the Morris watermaze test. Overall, our data indicated that mice over-expressing SOCS-2 showed increased survival of neurons generated during AHN, which correlated with improved performance in a hippocampal-dependent cognitive task. Furthermore, voluntary running increased AHN in WT, SOCS-2 Tg, and serum-IGF-1-deficient GH-R-/- mice.

The contribution of failing adult hippocampal neurogenesis to psychiatric disorders

PURPOSE OF REVIEW

Failing adult neurogenesis is increasingly considered a factor in the pathogenesis and course of psychiatric disorders. The level of evidence in favor of such hypotheses varies, but disturbed cellular plasticity in the hippocampus may be a common aspect of several neuropsychiatric diseases.

RECENT FINDINGS

This review covers the literature from mid-2006 to the end of 2007. We discuss studies and theoretical papers dealing with the contribution of adult neurogenesis to dementias and neurodegeneration, major depression, schizophrenia, and alcohol and drug abuse. Of these disorders, most progress has recently been made with schizophrenia for which, in contrast to the other conditions, suggestive genetic evidence exists (e.g. Disc1, Npas3).

SUMMARY

Failing adult hippocampal neurogenesis may not explain major depression, addiction or schizophrenia, but contributes to the hippocampal aspects of the disease. We propose that the key to a more thorough understanding of this contribution will come from increased knowledge on the functional relevance of new neurons in the hippocampus and better clinical data relating to symptoms possibly related to such function. Research on the molecular basis of adult hippocampal neurogenesis may help to explain how hippocampal aspects of these disorders develop.