Expression of amyloid precursor protein, tau and presenilin RNAs in rat hippocampus following deafferentation lesions

The role of astrocytes in amyloid production and Alzheimer's disease

Alzheimer's disease (AD) is marked by the presence of extracellular amyloid beta (Aβ) plaques, intracellular neurofibrillary tangles (NFTs) and gliosis, activated glial cells, in the brain. It is thought that Aβ plaques trigger NFT formation, neuronal cell death, neuroinflammation and gliosis and, ultimately, cognitive impairment. There are increased numbers of reactive astrocytes in AD, which surround amyloid plaques and secrete proinflammatory factors and can phagocytize and break down Aβ. It was thought that neuronal cells were the major source of Aβ. However, mounting evidence suggests that astrocytes may play an additional role in AD by secreting significant quantities of Aβ and contributing to overall amyloid burden in the brain. Astrocytes are the most numerous cell type in the brain, and therefore even minor quantities of amyloid secretion from individual astrocytes could prove to be substantial when taken across the whole brain. Reactive astrocytes have increased levels of the three necessary components for Aβ production: amyloid precursor protein, β-secretase (BACE1) and γ-secretase. The identification of environmental factors, such as neuroinflammation, that promote astrocytic Aβ production, could redefine how we think about developing therapeutics for AD.

Viral gene transfer of APPsα rescues synaptic failure in an Alzheimer's disease mouse model

Alzheimer's disease (AD) is characterized by synaptic failure, dendritic and axonal atrophy, neuronal death and progressive loss of cognitive functions. It is commonly assumed that these deficits arise due to β-amyloid accumulation and plaque deposition. However, increasing evidence indicates that loss of physiological APP functions mediated predominantly by neurotrophic APPsα produced in the non-amyloidogenic α-secretase pathway may contribute to AD pathogenesis. Upregulation of APPsα production via induction of α-secretase might, however, be problematic as this may also affect substrates implicated in tumorigenesis. Here, we used a gene therapy approach to directly overexpress APPsα in the brain using AAV-mediated gene transfer and explored its potential to rescue structural, electrophysiological and behavioral deficits in APP/PS1∆E9 AD model mice. Sustained APPsα overexpression in aged mice with already preexisting pathology and amyloidosis restored synaptic plasticity and partially rescued spine density deficits. Importantly, AAV-APPsα treatment also resulted in a functional rescue of spatial reference memory in the Morris water maze. Moreover, we demonstrate a significant reduction of soluble Aβ species and plaque load. In addition, APPsα induced the recruitment of microglia with a ramified morphology into the vicinity of plaques and upregulated IDE and TREM2 expression suggesting enhanced plaque clearance. Collectively, these data indicate that APPsα can mitigate synaptic and cognitive deficits, despite established pathology. Increasing APPsα may therefore be of therapeutic relevance for AD.

Holo-APP and G-protein-mediated signaling are required for sAPPα-induced activation of the Akt survival pathway

Accumulating evidence indicates that loss of physiologic amyloid precursor protein (APP) function leads to reduced neuronal plasticity, diminished synaptic signaling and enhanced susceptibility of neurons to cellular stress during brain aging. Here we investigated the neuroprotective function of the soluble APP ectodomain sAPPα (soluble APPα), which is generated by cleavage of APP by α-secretase along the non-amyloidogenic pathway. Recombinant sAPPα protected primary hippocampal neurons and SH-SY5Y neuroblastoma cells from cell death induced by trophic factor deprivation. We show that this protective effect is abrogated in neurons from APP-knockout animals and APP-depleted SH-SY5Y cells, but not in APP-like protein 1- and 2- (APLP1 and APLP2) depleted cells, indicating that expression of membrane-bound holo-APP is required for sAPPα-dependent neuroprotection. Trophic factor deprivation diminished the activity of the Akt survival pathway. Strikingly, both recombinant sAPPα and the APP-E1 domain were able to stimulate Akt activity in wild-type (wt) fibroblasts, SH-SY5Y cells and neurons, but failed to rescue in APP-deficient neurons or fibroblasts. The ADAM10 (a disintegrin and metalloproteinase domain-containing protein 10) inhibitor GI254023X exacerbated neuron death in organotypic (hippocampal) slice cultures of wt mice subjected to trophic factor and glucose deprivation. This cell death-enhancing effect of GI254023X could be completely rescued by applying exogenous sAPPα. Interestingly, sAPPα-dependent Akt induction was unaffected in neurons of APP-ΔCT15 mice that lack the C-terminal YENPTY motif of the APP intracellular region. In contrast, sAPPα-dependent rescue of Akt activation was completely abolished in APP mutant cells lacking the G-protein interaction motif located in the APP C-terminus and by blocking G-protein-dependent signaling with pertussis toxin. Collectively, our data provide new mechanistic insights into the physiologic role of APP in antagonizing neurotoxic stress: they suggest that cell surface APP mediates sAPPα-induced neuroprotection via G-protein-coupled activation of the Akt pathway.

Upregulation of APP, ADAM10 and ADAM17 in the denervated mouse dentate gyrus

The disintegrin and metalloproteinases ADAM10 and ADAM17 are regarded as the most important α-secretases involved in the physiological processing of amyloid precursor protein (APP) in brain. Since it has been suggested that processing of APP by α-secretases could be involved in the reorganization of the brain following injury, we studied mRNA expression of the two α-secretases Adam10 and Adam17, the ß-secretase Bace1, and the App-gene family (App, Aplp1, Aplp2) in the dentate gyrus of the mouse following entorhinal denervation. Using laser microdissection, tissue was harvested from the outer molecular layer and the granule cell layer of the denervated dentate gyrus. Expression levels of candidate genes were assessed using Affymetrix GeneChip Mouse Gene 1.0 ST arrays and reverse transcription-quantitative PCR, revealing an upregulation of Adam10 mRNA and Adam17 mRNA in the denervated outer molecular layer and an upregulation of Adam10 mRNA and App mRNA in the dentate granule cell layer. Immunolabeling for ADAM10 or ADAM17 in combination with markers for astro- and microglia revealed an increased labeling of ADAM10 and ADAM17 in the denervated outer molecular layer that was associated with reactive astrocytes but not with microglia. Collectively, these data show that denervation affects the expression level of APP and its two most important α-secretases. This suggests that APP-processing could be shifted towards the non-amyloidogenic pathway in denervated areas of the brain and, thus, towards the formation of neuroprotective APP cleavage products, such as APPsα.

The airbag problem-a potential culprit for bench-to-bedside translational efforts: relevance for Alzheimer's disease

For the last 20 years, the "amyloid cascade hypothesis" has dominated research aimed at understanding, preventing, and curing Alzheimer's disease (AD). During that time researchers have acquired an enormous amount of data and have been successful, more than 300 times, in curing the disease in animal model systems by treatments aimed at clearing amyloid deposits. However, to date similar strategies have not been successful in human AD patients. Hence, before rushing into further clinical trials with compounds that aim at lowering amyloid-beta (Aβ) levels in increasingly younger people, it would be of highest priority to re-assess the initial assumption that accumulation of Aβ in the brain is the primary pathological event driving AD. Here we question this assumption by highlighting experimental evidence in support of the alternative hypothesis suggesting that APP and Aβ are part of a neuronal stress/injury system, which is up-regulated to counteract inflammation/oxidative stress-associated neurodegeneration that could be triggered by a brain injury, chronic infections, or a systemic disease. In AD, this protective program may be overridden by genetic and other risk factors, or its maintenance may become dysregulated during aging. Here, we provide a hypothetical example of a hypothesis-driven correlation between car accidents and airbag release in analogy to the evolution of the amyloid focus and as a way to offer a potential explanation for the failure of the AD field to translate the success of amyloid-related therapeutic strategies in experimental models to the clinic.

Roles of amyloid precursor protein family members in neuroprotection, stress signaling and aging

The roles of amyloid precursor protein (APP) family members in normal brain function are poorly understood. Under physiological conditions the majority of APP appears to be processed along the non-amyloidogenic pathway leading to the formation of the secreted N-terminal APP fragment sAPPα. This cleavage product of APP has been implicated in several physiological processes such as neuroprotection, synaptic plasticity, neurite outgrowth and synaptogenesis. In this review we focus on the role of APP family members in neuroprotection and summarize the cellular and molecular mechanisms which are believed to mediate this effect. We propose that a reduction of APP processing along the non-amyloidogenic pathway during brain aging could result in an enhanced susceptibility of neurons to cellular stress and could contribute to neurodegeneration in Alzheimer's disease.

Increased expression of the gamma-secretase components presenilin-1 and nicastrin in activated astrocytes and microglia following traumatic brain injury

Gamma-secretase is an aspartyl protease composed of four proteins: presenilin (PS), nicastrin (Nct), APH1, and PEN2. These proteins assemble into a membrane complex that cleaves a variety of substrates within the transmembrane domain. The gamma-secretase cleavage products play an important role in various biological processes such as embryonic development and Alzheimer's disease (AD). The major role of gamma-secretase in brain pathology has been linked to AD and to the production of the amyloid beta-peptide. However, little is known about the possible role of gamma-secretase following acute brain insult. Here we examined by immunostaining the expression patterns of two gamma-secretase components, PS1 and Nct, in three paradigms of brain insult in mice: closed head injury, intracerebroventricular injection of LPS, and brain stabbing. Our results show that in naïve and sham-injured brains expression of PS1 and Nct is restricted mainly to neurons. However, following insult, the expression of both proteins is also observed in nonneuronal cells, consisting of activated astrocytes and microglia. Furthermore, the proteins are coexpressed within the same astrocytes and microglia, implying that these cells exhibit an enhanced gamma-secretase activity following brain damage. In view of the important role played by astrocytes and microglia in brain disorders, our findings suggest that gamma-secretase may participate in brain damage and repair processes by regulating astrocyte and microglia activation and/or function.

Coexisting cholinergic and parahippocampal degeneration: a key to memory loss in dementia and a challenge for transgenic models?

One century after Alzheimer's initial report, a variety of animal models of Alzheimer's disease (AD) are being used to mimic one or more pathological signs viewed as critical for the evolution of cognitive decline in dementia. Among the most common are, (a) traditional lesion models aimed at reproducing the degeneration of one of two key brain regions affected in AD, namely the cholinergic basal forebrain (CBF) and the transentorhinal region, and (b) transgenic mouse models aimed at reproducing AD histopathological hallmarks, namely amyloid plaques and neurofibrillary tangles. These models have provided valuable insights into the development and consequences of the pathology, but they have not consistently reproduced the severity of memory deficits exhibited in AD. The reasons for this lack of correspondence with the severity of expected deficits may include the limited replication of multiple neuropathology in potentially key brain regions. A recent lesion model in the rat found that severe memory impairment was obtained only when the two traditional lesions were combined together (i.e. conjoint CBF and entorhinal cortex lesions), indicative of a dramatic impact on cognitive function when there is coexisting, rather than isolated, damage in these two brain regions. It is proposed that combining AD transgenic mouse models with additional experimental damage to both the CBF and entorhinal regions might provide a unique opportunity to further understand the evolution of the disease and improve treatments of severe cognitive dysfunction in neurodegenerative dementias.

Muscarinic acetylcholine receptor inhibition in transgenic Alzheimer-like Tg2576 mice by scopolamine favours the amyloidogenic route of processing of amyloid precursor protein

The molecular mechanisms of the interrelationship between cholinergic neurotransmission, processing of amyloid precursor protein (APP) and beta-amyloid (Abeta) production in vivo are still less understood. To reveal any effect of cholinergic dysfunction on APP processing in vivo, 11-month-old transgenic Tg2576 mice with Abeta plaque pathology received intraperitoneal injections of scopolamine at a daily dosage of 2mg/kg body weight for 14 days in order to suppress cortical cholinergic transmission by chronic inhibition of muscarinic acetylcholine receptors. Scopolamine treatment of transgenic Tg2576 mice resulted in increased levels of fibrillar Abeta(1-40) and Abeta(1-42), while the soluble, SDS-extractable Abeta level remained unchanged as compared to vehicle-injected Tg2576 mice. alpha-Secretase activity determined in cortical tissue from scopolamine-treated Tg2576 mice was lower by about 30% as compared to that assayed in control mice, while beta-secretase activity and BACE1 protein expression appeared unaffected by scopolamine treatment. The amount of sAPPalpha, the product secreted by alpha-secretase-mediated APP cleavage, and the unprocessed APP were assayed in the soluble and membrane fraction, respectively, of cortical tissue preparations from treated and control mice by Western blotting. Using the anti antibody 6E10 which specifically labels human sAPPalpha and full length APP in transgenic Tg2576, an enhanced APP level was detected in the membrane fraction from treated mice as compared to controls, while in the soluble fraction scopolamine treatment did not affect the protein level of sAPPalpha. These data indicate an accumulation of APP in cortical membrane fraction in scopolamine-treated Tg2576 mice presumably due to the decreased level of alpha-secretase-mediated APP cleavage, and further suggest that chronic suppression of cortical muscarinic cholinergic transmission may alter the balance between alpha- and beta-secretory APP processing by favouring the amyloidogenic route.

Expression profiling in laser-microdissected hippocampal subregions in rat brain reveals large subregion-specific differences in expression

Expression profiling in the hippocampus is hampered by its cellular heterogeneity. The aim of this study was to evaluate the feasibility of using laser-microdissected hippocampal subregions for expression profiling to improve detection of transcripts with a subregion-specific expression. Cornu ammonis (CA)3 and dentate gyrus (DG) subregions were isolated from rat brain slices using laser microdissection, subjected to two rounds of linear amplification and hybridized to rat GeneChips containing approximately 8000 transcripts (RG_U34A; Affymetrix). Analysis of the data using significance analysis of microarrays revealed 724 genes with a significant difference in expression between CA3 and DG with a false discovery rate of 2.1%, of which 264 had higher expression in DG and 460 in CA3. Several transcripts with known differential expression between the subregions were included in the dataset, as well as numerous novel mRNAs and expressed sequence tags. Sorting of the differentially expressed genes according to gene ontology classification revealed that genes involved in glycolysis and general metabolism, neurogenesis and cell adhesion were consistently expressed at higher levels in CA3. Conversely, a large cluster of genes involved in protein biosynthesis were expressed at higher levels in DG. In situ hybridization was used to validate differential expression of a selection of genes. The results of this study demonstrate that the combination of laser microdissection and GeneChip technology is both technically feasible and very promising. Besides providing an extensive inventory of genes showing differential expression between CA3 and DG, this study supports the idea that profiling in hippocampal subregions should improve detection of genes with a subregion-specific expression or regulation.

Changes in hippocampal SNAP-25 expression following afferent lesions

Reductions in SNAP-25 immunohistochemistry were found after removing the glutamatergic and cholinergic inputs to the rat hippocampus. SNAP-25 levels were normalised by 1 month after afferent lesions. Surprisingly, a superimposed cholinergic lesions did not affect the return to normal SNAP-25 levels after a long-term entorhinal cortex lesion. It is concluded that changes in SNAP-25 may represent early markers of synaptic loss following afferent lesions to the hippocampus.

Alzheimer's disease proteins in cerebellar and hippocampal synapses during postnatal development and aging of the rat

Alzheimer's dementia may be considered a synaptic disease of central neurons: the loss of synapses, reflected by early cognitive impairments, precedes the appearance of extra cellular focal deposits of beta-amyloid peptide in the brain of patients. Distinct immunocytochemical patterns of amyloid precursor proteins (APPs) have previously been demonstrated in the synapses by ultrastructural analysis in the cerebellum and hippocampus of adult rats and mice. Now we show that during postnatal development and during aging in these structures, the immunocytochemical expression of APPs increases in the synapses in parallel with the known up-regulation of total APPs brain levels. Interestingly, as shown previously in the adult rodents, the presenilins (PSs) 1 and 2, which intervene in APPs metabolism, exhibit a synaptic distribution pattern similar to that of APPs with parallel quantitative changes throughout life. In the brain tissue, single and double immunocytochemistry at the ultrastructural level shows co-localisation of APPs and PSs in axonal and dendritic synaptic compartments during postnatal synaptogenesis, adulthood and aging. In addition, double-labelling immunocytofluorescence detects these proteins close to synaptophysin at the growth cones of developing cultured neurons. Thusly, the brain expression of APPs and PSs appears to be regulated synchronously during lifespan in the synaptic compartments where the proteins are colocated. This suggests that PS-dependent processing of important synaptic proteins such as APPs could intervene in age-induced adjustments of synaptic relationships between specific types of neurons.

Functional recovery of cholinergic basal forebrain neurons under disease conditions: old problems, new solutions?

Recognition of the involvement of cholinergic neurons in the modulation of cognitive functions and their severe dysfunction in neurodegenerative disorders, such as Alzheimer's disease, initiated immense research efforts aimed at unveiling the anatomical organization and cellular characteristics of the basal forebrain (BFB) cholinergic system. Concomitant with our unfolding knowledge about the structural and functional complexity of the BFB cholinergic projection system, multiple pharmacological strategies were introduced to rescue cholinergic nerve cells from noxious attacks; however, a therapeutic breakthrough is still awaited. In this review, we collected recent findings that significantly contributed to our better understanding of cholinergic functions under disease conditions, and to the design of effective means to restore lost or damaged cholinergic functions. To this end, we first provide a brief survey of the neuroanatomical organization of BFB nuclei with emphasis on major evolutionary differences among mammalian species, in particular rodents and primates, and discuss limitations of the translation of experimental data to human therapeutic applications. Subsequently, we summarize the involvement of cholinergic dysfunction in the pathogenesis of severe neurological conditions, including stroke, traumatic brain injury, virus encephalitis and Alzheimer's disease, and emphasize the critical role of pro-inflammatory cytokines as common mediators of cholinergic neuronal damage. Moreover, we review leading functional concepts on the limited recovery of cholinergic neurons and their impaired plastic re-modeling, as well as on the hampered interplay of the ascending cholinergic and monoaminergic projection systems under neurodegenerative conditions. In addition, recent advances in the dynamic labeling of living cholinergic neurons by fluorochromated antibodies, referred to as in vivo labeling, and novel neuroimaging approaches as potential diagnostic tools of progressive cholinergic decline are surveyed. Finally, the potential of cell replacement strategies using embryonic and adult stem cells, and multipotent neural progenitors, as a means to recover damaged cholinergic functions, is discussed.

Zinc takes the center stage: its paradoxical role in Alzheimer's disease

There is compelling evidence that the etiology of Alzheimer's disease (AD) involves characteristic amyloid-beta (Abeta) deposition, oxidative stress, and anomalous metal-Abeta protein interaction. New studies have implicated redox active metals such as copper, iron, and zinc as key mediating factors in the pathophysiology of Alzheimer's disease. There is also evidence that drugs with metal chelating properties could produce a significant reversal of amyloid-beta plaque deposition in vitro and in vivo. This paper reviews current observations on the etiologic role of zinc in AD. We also discuss the interactions of zinc and copper with Abeta, a factor that purportedly facilitates disease processes. Finally, we review the protective role of zinc against Abeta cytotoxicity and hypothesize how the apparent effect of zinc on AD pathology may be paradoxical, The Zinc Paradox. Indeed, complex pathologic stressors inherent to the Alzheimer's diseased brain dictate whether or not zinc will be neuroprotective or neurodegenerative. Further research on the zinc paradox in AD is needed in order to elucidate the exact role zinc plays in AD pathogenesis.

Fimbria-fornix lesion does not affect APP levels and amyloid deposition in the hippocampus of APP+PS1 double transgenic mice

The deposition of amyloid beta peptides (Abeta) and cholinergic dysfunction are two characteristic features of Alzheimer's disease. Several studies have suggested that a compromised cholinergic transmission can increase the amount of amyloid precursor protein (APP) in the denervated cortex (or hippocampus); however, whether this will increase Abeta production is unknown. To investigate the relation between cholinergic neurotransmission and APP metabolism, and the possible role of cholinergic dysfunction in the development of amyloid neuropathology, we lesioned the fimbria-fornix pathway in APP+PS1 double transgenic mice, at 5 and 7 months of age. Three months and 11 months postlesion, the mice were sacrificed for biochemical and histopathological analyses. The fimbria-fornix transection resulted in a substantial depletion of cholinergic markers in the hippocampus at both time points. Three months postlesion, hippocampal APP and Abeta levels were not significantly changed. At 11 months postlesion, the fimbria-fornix lesion did not result in an alteration in either the hippocampal Abeta levels or the extent of Abeta deposition, as assessed by amyloid plaque counts and image analysis of Abeta load in the 18-month-old APP+PS1 mice. Our findings indicate that APP metabolism in mice may be dissociated from cholinergic neurotransmission rather than related as previously suggested in other mammalian species.

Transgenic mice expressing the human presenilin 1 gene demonstrate enhanced hippocampal reorganization following entorhinal cortex lesions

We have examined the effects of the presence of the mutated human presenilin 1 gene (M146L; hps1*) on lesion-induced sprouting in the hippocampus of the mouse (C57/CBA). The entorhinal cortex was unilaterally lesioned with ibotenic acid in adult, male mice. Four weeks later the subsequent axonal sprouting in the dentate gyrus was analysed, by measuring the density of the synaptophysin immunocytochemical staining in the termination area of the entorhinal cortex axons. The data demonstrate that mice expressing either the human presenilin 1 gene (hps1) or the hps1* gene display a significantly increased density of immunocytochemical staining for synaptophysin, indicative of axonal sprouting, compared to the control mice. No (or a very small) sprouting response is observed in mice expressing the normal mouse ps1 gene. Taken together, these data indicate that the presence of a human ps1 gene, normal or with an Alzheimer's disease mutation, leads to enhanced plasticity in the mouse brain.