Loss of medial septum cholinergic neurons in THY-Tau22 mouse model: what links with tau pathology?

Alzheimer's disease (AD) is a neurodegenerative disorder histologically defined by the cerebral accumulation of amyloid deposits and neurofibrillary tangles composed of hyperphosphorylated tau proteins. Loss of basal forebrain cholinergic neurons is another hallmark of the disease thought to contribute to the cognitive dysfunctions. To this date, the mechanisms underlying cholinergic neurons degeneration remain uncertain. The present study aimed to investigate the relationship between neurofibrillary degeneration and cholinergic defects in AD using THY-Tau22 transgenic mouse model exhibiting a major hippocampal AD-like tau pathology and hyperphosphorylated tau species in the septohippocampal pathway. Here, we report that at a time THY-Tau22 mice display strong reference memory alterations, the retrograde transport of fluorogold through the septohippocampal pathway is altered. This impairment is associated with a significant reduction in the number of choline acetyltransferase (ChAT)-immunopositive cholinergic neurons in the medial septum. Analysis of nerve growth factor (NGF) levels supports an accumulation of the mature neurotrophin in the hippocampus of THY-Tau22 mice, consistent with a decrease of its uptake or retrograde transport by cholinergic terminals. Finally, our data strongly support that tau pathology could be instrumental in the cholinergic neuronal loss observed in AD.

Neurotrophic factors in Alzheimer's disease: role of axonal transport.

Neurotrophic factors (NTF) are small, versatile proteins that maintain survival and function to specific neuronal populations. In general, the axonal transport of NTF is important as not all of them are synthesized at the site of its action. Nerve growth factor (NGF), for instance, is produced in the neocortex and the hippocampus and then retrogradely transported to the cholinergic neurons of the basal forebrain. Neurodegenerative dementias like Alzheimer's disease (AD) are linked to deficits in axonal transport. Furthermore, they are also associated with imbalanced distribution and dysregulation of NTF. In particular, brain-derived neurotrophic factor (BDNF) plays a crucial role in cognition, learning and memory formation by modulating synaptic plasticity and is, therefore, a critical molecule in dementia and neurodegenerative diseases. Here, we review the changes of NTF expression and distribution (NGF, BDNF, neurotrophin-3, neurotrophin-4/5 and fibroblast growth factor-2) and their receptors [tropomyosin-related kinase (Trk)A, TrkB, TrkC and p75(NTR)] in AD and AD models. In addition, we focus on the interaction with neuropathological hallmarks Tau/neurofibrillary tangle and amyloid-beta (Abeta)/amyloid plaque pathology and their influence on axonal transport processes in order to unify AD-specific cholinergic degeneration and Tau and Abeta misfolding through NTF pathophysiology.

Neurogenesis and cell cycle-reactivated neuronal death during pathogenic tau aggregation.

The aim of the present study was to investigate the relation between neurogenesis, cell cycle reactivation and neuronal death during tau pathology in a novel tau transgenic mouse line THY-Tau22 with two frontotemporal dementia with parkinsonism linked to chromosome-17 mutations in a human tau isoform. This mouse displays all Alzheimer disease features of neurodegeneration and a broad timely resolution of tau pathology with hyperphosphorylation of tau at younger age (up to 6 months) and abnormal tau phosphorylation and tau aggregation in aged mice (by 10 months). Here, we present a follow-up of cell cycle markers with aging in control and transgenic mice from different ages. We show that there is an increased neurogenesis during tau hyperphosphorylation and cell cycle events during abnormal tau phosphorylation and tau aggregation preceding neuronal death and neurodegeneration. However, besides phosphorylation, other mechanisms including tau mutations and changes in tau expression and/or splicing may be also involved in these mechanisms of cell cycle reactivation. Altogether, these data suggest that cell cycle events in THY-Tau22 are resulting from neurogenesis in young animals and cell death in older ones. It suggests that neuronal cell death in such models is much more complex than believed.

Expression of trkB and trkC receptors and their ligands brain-derived neurotrophic factor and neurotrophin-3 in the murine amygdala.

The neurotrophin brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) and their cognate receptors, trkB and trkC, have a variety of physiological brain functions, ranging from cell survival to mechanisms involved in learning and memory and long-term potentiation (LTP). LTP can be induced in the cortex and hippocampus, as well as within the amygdala. However, the role of neurotrophins in amygdalar LTP is largely unknown. Expression patterns of BDNF and NT-3 and their cognate receptors in the adult mouse amygdala have not been analyzed in detail. We have therefore examined the expression of trkB, trkC, BDNF, and NT-3 mRNA and protein in different amygdalar nuclei as well as in the hippocampal areas CA1-CA3 and the dentate gyrus. The distribution pattern of trkB, trkC, BDNF, and NT-3 mRNA in the murine hippocampus is comparable to that seen in rats. Within most amygdalar nuclei, a moderate BDNF mRNA expression was found; however, BDNF mRNA was virtually absent from the central nucleus. No expression of NT-3 mRNA was found within the amygdala, but trkC mRNA-expressing cells were widely distributed within this brain region. trkB mRNA was strongly expressed in the amygdala. Because trkB is expressed in a full-length and a truncated form (the latter form is also expressed by nonneuronal cells), we also investigated the distribution of full-length trkB mRNA-expressing cells and could demonstrate that this version of trkB receptors is also widely expressed in the amygdala. These results can serve as a basis for studies elucidating the physiological roles of these receptors in the amygdala.

Enhanced apoptosis, oxidative stress and mitochondrial dysfunction in lymphocytes as potential biomarkers for Alzheimer's disease.

Alzheimer's disease (AD) is the most common progressive neurodegenerative disease. Today, AD affects millions of people worldwide and the number of AD cases will increase with increased life expectancy. The AD brain is marked by severe neurodegeneration like the loss of synapses and neurons, atrophy and depletion of neurotransmitter systems in the hippocampus and cerebral cortex. Recent findings suggest that these pathological changes are causally induced by mitochondrial dysfunction, increased oxidative stress and elevated apoptosis. Until now, AD cannot be diagnosed by a valid clinical method or a biomarker before the disease has progressed so far that dementia is present. Furthermore, no valid method is available to determine which patient with mild cognitive impairment (MCI) will progress to AD. Therefore, a correct diagnosis in the early stage of AD is not only of importance considering that early drug treatment is more effective but also that the psychological burden of the patients and relatives could be decreased. In this review, we discuss the potential role of elevated apoptosis, increased oxidative stress and mitochondrial dysfunction as biomarker for AD in a peripheral cell model, the lymphocytes.

Apoptosis of CD4+ T and natural killer cells in Alzheimer's disease.

BACKGROUND: Immunotherapy appears to be a potent treatment against Alzheimer's disease (AD), but the mechanisms underlying neural-immune interaction are still not known. METHODS: Here, we determined cell death and distribution of lymphocyte subsets of peripheral blood mononuclear cells (PBMC) in AD and aging, e.g. T (CD4+ CD3+, CD8+ CD3+), B (CD19+) and NK (CD16++CD56+) cells. RESULTS: Increased apoptosis was found in CD4+ T and NK cells in AD, while in aging all subsets were affected. The expression of anti-apoptotic Bcl2 correlated with observed cell death in T-helper and B cells irrespective of dementia. The levels of Bcl2 in T-cells were significantly increased in mild AD. Apoptosis and Bcl2 levels were also elevated in the APP (751SL)xPS1 (M146L) transgenic mouse model. CONCLUSION: The mechanisms triggering apoptosis and activation of lymphocytes in AD appear therefore to be different than those in immunosenescence and possibly bear an important biomarker to monitor immunotherapy in AD.

Enhanced ROS-generation in lymphocytes from Alzheimer's patients.

INTRODUCTION: Reactive oxygen species (ROS) have been implicated in neurodegeneration and seem to be involved in the physiology and pathophysiology of several diseases, including normal aging and Alzheimer's disease (AD). Enhanced ROS production in aging or AD is not restricted to the brain, but can also been seen in several peripheral tissues. The objective of the present study was to evaluate whether the mechanisms involved in the generation of oxidative stress in normal senescence and Alzheimer's disease are identical or not. METHODS: We analysed intracellular basal levels of ROS in lymphocytes from AD patients and healthy young and aged not-demented subjects as well as ROS levels following stimulation with d-ribose and staurosporine in all three groups. ROS levels were measured by flow cytometry using the intracellular fluorescence dye dihydrorhodamine123 (DHR123). RESULTS: Our study shows that AD lymphocytes have increased basal levels of ROS, low susceptibility to ROS stimulation by 2-deoxy- D-ribose (dRib) and an increased response to staurosporine when compared with age-matched controls. DISCUSSION: The data suggest that the defect(s) responsible for enhanced ROS production in AD may involve different or additional biological pathways than those involved in enhanced ROS generation during aging.

Effects of EGb 761 Ginkgo biloba extract on mitochondrial function and oxidative stress.

As major sources of reactive oxygen species (ROS), mitochondrial structures are exposed to high concentrations of ROS and may therefore be particularly susceptible to oxidative damage. Mitochondrial damage could play a pivotal role in the cell death decision. A decrease in mitochondrial energy charge and redox state, loss of transmembrane potential (depolarization), mitochondrial respiratory chain impairment, and release of substances such as calcium and cytochrome c all contribute to apoptosis. These mitochondrial abnormalities may constitute a part of the spectrum of chronic oxidative stress in Alzheimer's disease. Accumulation of amyloid beta (Abeta) in form of senile plaques is also thought to play a central role in the pathogenesis of Alzheimer's disease mediated by oxidative stress. In addition, increasing evidence shows that Abeta generates free radicals in vitro, which mediate the toxicity of this peptide. In our study, PC12 cells were used to examine the protective features of EGb 761(definition see editorial) on mitochondria stressed with hydrogen peroxide and antimycin, an inhibitor of complex III. In addition, we investigated the efficacy of EGb 761 in Abeta-induced MTT reduction in PC12 cells. Moreover, we examined the effects of EGb 761 on ROS levels and ROS-induced apoptosis in lymphocytes from aged mice after in vivo administration. Here, we will report that EGb 761 was able to protect mitochondria from the attack of hydrogen peroxide, antimycin and Abeta. Furthermore, EGb 761 reduced ROS levels and ROS-induced apoptosis in lymphocytes from aged mice treated orally with EGb 761 for 2 weeks. Our data further emphasize neuroprotective properties of EGb 761, such as protection against Abeta-toxicity, and antiapoptotic properties, which are probably due to its preventive effects on mitochondria.

Age-related increase of oxidative stress-induced apoptosis in mice prevention by Ginkgo biloba extract (EGb761).

Enhanced apoptosis and elevated levels of reactive oxygen species (ROS) play a major role in aging. In addition, several neurodegenerative diseases are associated with increased oxidative stress and apoptosis in neuronal tissue. Antioxidative treatment has neuro-protective effects. The aim of the present study was to evaluate changes of susceptibility to apoptotic cell death by oxidative stress in aging and its inhibition by the antioxidant Ginkgo biloba extract EGb761. We investigated basal and ROS-induced levels of apoptotic lymphocytes derived from the spleen in young (3 months) and old (24 months) mice. ROS were induced by 2-deoxy-D-ribose (dRib) that depletes the intracellular pool of reduced glutathione. Lymphocytes from aged mice accumulate apoptotic cells to a significantly higher extent under basal conditions compared to cells from young mice. Treatment with dRib enhanced this difference, implicating a higher sensitivity to ROS in aging. Apoptosis can be reduced in vitro by treatment with EGb761. In addition, mice were treated daily with 100 mg/kg EGb761 per os over a period of two weeks. ROS-induced apoptosis was significantly reduced in the EGb761 group. Interestingly, this effect seemed to be more pronounced in old mice.

Alzheimer's disease-like alterations in peripheral cells from presenilin-1 transgenic mice.

Many cases of early-onset inherited Alzheimer's disease (AD) are caused by mutations in the presenilin-1 (PS1) gene. Expression of PS1 mutations in cell culture systems and in primary neurons from transgenic mice increases their vulnerability to cell death. Interestingly, enhanced vulnerability to cell death has also been demonstrated for peripheral lymphocytes from AD patients. We now report that lymphocytes from PS1 mutant transgenic mice show a similar hypersensitivity to cell death as do peripheral cells from AD patients and several cell culture systems expressing PS1 mutations. The cell death-enhancing action of mutant PS1 was associated with increased production of reactive oxygen species and altered calcium regulation, but not with changes of mitochondrial cytochrome c. Our study further emphasizes the pathogenic role of mutant PS1 and may provide the fundamental basis for new efforts to close the gap between studies using neuronal cell lines transfected with mutant PS1, neurons from transgenic animals, and peripheral cells from AD patients.

Age-related changes of apoptotic cell death in human lymphocytes.

Apoptosis seems to be involved in immunosenescence associated with aging. Moreover, in lymphocytes (PBL) of patients with Alzheimer's disease, an increased susceptibility to the apoptotic pathway has been described possibly due to impaired protection of oxidative stress. Accordingly, it seemed to be of particular interest to investigate the contribution of normal aging to the susceptibility from human lymphocytes to programmed cell death. We could show that PBL from elderly individuals (>60 years) accumulate apoptosing cells to a significant higher extent in spontaneous and activation-induced cell death compared to younger controls (<35 years). Treatment with the oxidative stressor 2-deoxy-D-ribose or with agonistic-CD95-antibody pronounced this effect even more implicating a higher sensitivity to reactive oxygen species and a higher functional CD95 expression, respectively. In addition, expression of the activation markers HLA-DR and CD95 was significantly increased in CD3+-cells of aged subjects, while expression of CD25 did not seem to be affected by age. Expression of Bcl-2 was increased in aging and correlated with the number of apoptotic cells.

Reduced antioxidant enzyme activity in brains of mice transgenic for human presenilin-1 with single or multiple mutations.

Alzheimer's disease-related mutations in the presenilin-1 gene (PS1) are leading to an elevated production of neurotoxic beta-amyloid 1-42 and may additionally enhance oxidative stress. Here, we provide in vivo evidence indicating that brains of transgenic mice expressing different human Alzheimer-linked PS1 mutations exhibit a reduced activity of two antioxidant enzymes. For this purpose, mice transgenic for human PS1 and for single and multiple PS1 mutations were generated. Mice with multiple PS1 mutations showed a significantly decreased activity of the antioxidant enzymes Cu/Zn superoxide dismutase and glutathione reductase already at an age of 3-4 months. As expected, this effect was less pronounced for the mice with a single PS1 mutation. By contrast, animals bearing normal human PS1 showed significantly elevated enzyme activities relative to non-transgenic littermate controls.

In vivo manipulation of interleukin-2 expression by a retroviral tetracycline (tet)-regulated system.

We have used the tetracycline (tet)-regulated system as described previously to evaluate the applicability of controlled gene expression in cancer gene therapy. As a model gene, we used the human interleukin-2 (IL-2) gene, which has been placed under the transcriptional control of the tetO/promoter. Human melanoma cells were transduced by two modified retroviral tet vectors containing the transactivator regulatory unit and the IL-2 gene driven by the tetO/promoter, respectively. In the absence of tet, IL-2 expression in the target cells was stable over several months. IL-2 production was in the range of 40 U/10(6) cells/24 hours. A fine tuning of IL-2 expression could be achieved by culturing the transduced cells with increasing doses of tet, whereby a concentration of 500 ng/mL tet in the culture medium abrogated IL-2 expression. Most importantly for clinical application, IL-2 expression by the transduced melanoma cells could also be regulated in vivo. When nu/nu mice were inoculated with the transduced tumor cells, they failed to develop tumors. Instead, the inhibition of IL-2 expression in the transduced tumor cells by oral administration of tet led to subcutaneous tumor growth; this growth rate was comparable with the growth rate of subcutaneously inoculated untransduced parental cells. The finding demonstrates the applicability of the tet-regulated system in cancer gene therapy.