Amyloid precursor protein in guinea pigs--complete cDNA sequence and alternative splicing

Nontransgenic Guinea Pig Strains Exhibit Hallmarks of Human Brain Aging and Alzheimer's Disease

Older age is the primary risk factor for most chronic diseases, including Alzheimer's disease (AD). Current preclinical models to study brain aging and AD are mainly transgenic and harbor mutations intended to mirror brain pathologies associated with human brain aging/AD (eg, by increasing production of the amyloid precursor protein, amyloid beta [Aβ], and/or phosphorylated tau, all of which are key pathological mediators of AD). Although these models may provide insight on pathophysiological processes in AD, none completely recapitulate the disease and its strong age-dependence, and there has been limited success in translating preclinical results and treatments to humans. Here, we describe 2 nontransgenic guinea pig (GP) models, a standard PigmEnTed (PET) strain, and lesser-studied Dunkin-Hartley (DH) strain, that may naturally mimic key features of brain aging and AD in humans. We show that brain aging in PET GP is transcriptomically similar to human brain aging, whereas older DH brains are transcriptomically more similar to human AD. Both strains/models also exhibit increased neurofilament light chain (NFL, a marker of neuronal damage) with aging, and DH animals display greater S100 calcium-binding protein B (S100β), ionized calcium-binding adapter molecule 1 (Iba1), and Aβ and phosphorylated tau-which are all important markers of neuroinflammation-associated AD. Collectively, our results suggest that both the PET and DH GP may be useful, nontransgenic models to study brain aging and AD, respectively.

Postnatal Guinea Pig Brain Development, as Revealed by Magnetic Resonance and Diffusion Kurtosis Imaging

This study used in vivo magnetic resonance imaging (MRI) to identify age dependent brain structural characteristics in Dunkin Hartley guinea pigs. Anatomical T₂-weighted images, diffusion kurtosis (DKI) imaging, and T₂ relaxometry measures were acquired from a cohort of male guinea pigs from postnatal day (PND) 18–25 (juvenile) to PND 46–51 (adolescent) and PND 118–123 (young adult). Whole-brain diffusion measures revealed the distinct effects of maturation on the microstructural complexity of the male guinea pig brain. Specifically, fractional anisotropy (FA), as well as mean, axial, and radial kurtosis in the corpus callosum, amygdala, dorsal-ventral striatum, and thalamus significantly increased from PND 18–25 to PND 118–123. Age-related alterations in DKI measures within these brain regions paralleled the overall alterations observed in the whole brain. Age-related changes in FA and kurtosis in the gray matter-dominant parietal cerebral cortex and dorsal hippocampus were less pronounced than in the other brain regions. The regional data analysis revealed that between-age changes of diffusion kurtosis metrics were more pronounced than those observed in diffusion tensor metrics. The age-related anatomical differences reported here may be important determinants of the age-dependent neurobehavior of guinea pigs in different tasks.

Genetic variants associated with neurodegenerative Alzheimer disease in natural models

The use of transgenic models for the study of neurodegenerative diseases has made valuable contributions to the field. However, some important limitations, including protein overexpression and general systemic compensation for the missing genes, has caused researchers to seek natural models that show the main biomarkers of neurodegenerative diseases during aging. Here we review some of these models-most of them rodents, focusing especially on the genetic variations in biomarkers for Alzheimer diseases, in order to explain their relationships with variants associated with the occurrence of the disease in humans.

Aging, cortical injury and Alzheimer's disease-like pathology in the guinea pig brain

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized histopathologically by the abnormal deposition of the proteins amyloid-beta (Aβ) and tau. A major issue for AD research is the lack of an animal model that accurately replicates the human disease, thus making it difficult to investigate potential risk factors for AD such as head injury. Furthermore, as age remains the strongest risk factor for most of the AD cases, transgenic models in which mutant human genes are expressed throughout the life span of the animal provide only limited insight into age-related factors in disease development. Guinea pigs (Cavia porcellus) are of interest in AD research because they have a similar Aβ sequence to humans and thus may present a useful non-transgenic animal model of AD. Brains from guinea pigs aged 3-48 months were examined to determine the presence of age-associated AD-like pathology. In addition, fluid percussion-induced brain injury was performed to characterize mechanisms underlying the association between AD risk and head injury. No statistically significant changes were detected in the overall response to aging, although we did observe some region-specific changes. Diffuse deposits of Aβ were found in the hippocampal region of the oldest animals and alterations in amyloid precursor protein processing and tau immunoreactivity were observed with age. Brain injury resulted in a strong and sustained increase in amyloid precursor protein and tau immunoreactivity without Aβ deposition, over 7 days. Guinea pigs may therefore provide a useful model for investigating the influence of environmental and non-genetic risk factors on the pathogenesis of AD.

The Guinea Pig as a Model for Sporadic Alzheimer's Disease (AD): The Impact of Cholesterol Intake on Expression of AD-Related Genes

We investigated the guinea pig, Cavia porcellus, as a model for Alzheimer’s disease (AD), both in terms of the conservation of genes involved in AD and the regulatory responses of these to a known AD risk factor - high cholesterol intake. Unlike rats and mice, guinea pigs possess an Aβ peptide sequence identical to human Aβ. Consistent with the commonality between cardiovascular and AD risk factors in humans, we saw that a high cholesterol diet leads to up-regulation of BACE1 (β-secretase) transcription and down-regulation of ADAM10 (α-secretase) transcription which should increase release of Aβ from APP. Significantly, guinea pigs possess isoforms of AD-related genes found in humans but not present in mice or rats. For example, we discovered that the truncated PS2V isoform of human PSEN2, that is found at raised levels in AD brains and that increases γ-secretase activity and Aβ synthesis, is not uniquely human or aberrant as previously believed. We show that PS2V formation is up-regulated by hypoxia and a high-cholesterol diet while, consistent with observations in humans, Aβ concentrations are raised in some brain regions but not others. Also like humans, but unlike mice, the guinea pig gene encoding tau, MAPT, encodes isoforms with both three and four microtubule binding domains, and cholesterol alters the ratio of these isoforms. We conclude that AD-related genes are highly conserved and more similar to human than the rat or mouse. Guinea pigs represent a superior rodent model for analysis of the impact of dietary factors such as cholesterol on the regulation of AD-related genes.

The cysteine protease inhibitor, E64d, reduces brain amyloid-β and improves memory deficits in Alzheimer's disease animal models by inhibiting cathepsin B, but not BACE1, β-secretase activity

The cysteine protease cathepsin B is a potential drug target for reducing brain amyloid-β (Aβ) and improving memory in Alzheimer's disease (AD), as reduction of cathepsin B in transgenic mice expressing human wild-type amyloid-β protein precursor (AβPP) results in significantly decreased brain Aβ. Cathepsin B cleaves the wild-type β-secretase site sequence in AβPP to produce Aβ, and cathepsin B inhibitors administered to animal models expressing AβPP containing the wild-type β-secretase site sequence reduce brain Aβ in a manner consistent with β-secretase inhibition. But such inhibitors could act either by direct inhibition of cathepsin B β-secretase activity or by off-target inhibition of the other β-secretase, the aspartyl protease BACE1. To evaluate that issue, we orally administered a cysteine protease inhibitor, E64d, to normal guinea pigs or transgenic mice expressing human AβPP, both of which express the human wild-type β-secretase site sequence. In guinea pigs, oral E64d administration caused a dose-dependent reduction of up to 92% in brain, CSF, and plasma of Aβ40 and Aβ42, a reduction of up to 50% in the C-terminal β-secretase fragment (CTFβ), and a 91% reduction in brain cathepsin B activity, but increased brain BACE1 activity by 20%. In transgenic AD mice, oral E64d administration improved memory deficits and reduced brain Aβ40 and Aβ42, amyloid plaque, brain CTFβ, and brain cathepsin B activity, but increased brain BACE1 activity. We conclude that E64d likely reduces brain Aβ by inhibiting cathepsin B and not BACE1 β-secretase activity and that E64d therefore may have potential for treating AD patients.

Direct exposure of guinea pig CNS to human luteinizing hormone increases cerebrospinal fluid and cerebral beta amyloid levels

BACKGROUND/AIMS

Luteinizing hormone (LH) has been shown to alter the metabolism of beta amyloid (Aβ), a key protein in Alzheimer's disease (AD) pathogenesis. While LH and components required for LH receptor signalling are present in the brain, their role in the CNS remains unclear. In vitro, LH has been shown to facilitate neurosteroid production and alter Aβ metabolism. However, whether LH can directly modulate cerebral Aβ levels in vivo has not previously been studied. In this study, we investigated the effect of chronic administration of LH to the guinea pig CNS on cerebral Aβ levels.

METHODS

Gonadectomised male animals were administered, via cortical placement, either placebo or LH slow-release pellets. At 14 and 28 days after treatment, animals were sacrificed. Brain, plasma and CSF were collected and Aβ levels measured via ELISA. Levels of the Aβ precursor protein (APP) and the neurosteroidogenic enzyme cytochrome P450 side-chain cleavage enzyme (P450scc) were also assayed.

RESULTS

An increase in CSF Aβ40 levels was observed 28 days following treatment. These CSF data also reflected changes in Aβ40 levels observed in brain homogenates. No change was observed in plasma Aβ40 levels but APP and its C-terminal fragments (APP-CTF) were significantly increased in response to LH exposure. Protein expression of P450scc was increased after 28 days of LH exposure, suggesting activation of the LH receptor.

CONCLUSION

These data indicate that direct exposure of guinea pig CNS to LH results in altered brain Aβ levels, perhaps due to altered APP expression/metabolism.

Genetic cathepsin B deficiency reduces beta-amyloid in transgenic mice expressing human wild-type amyloid precursor protein

Neurotoxic beta-amyloid (Abeta) peptides participate in Alzheimer's disease (AD); therefore, reduction of Abeta generated from APP may provide a therapeutic approach for AD. Gene knockout studies in transgenic mice producing human Abeta may identify targets for reducing Abeta. This study shows that knockout of the cathepsin B gene in mice expressing human wild-type APP (hAPPwt) results in substantial decreases in brain Abeta40 and Abeta42 by 67% and decreases in levels of the C-terminal beta-secretase fragment (CTFbeta) derived from APP. In contrast, knockout of cathepsin B in mice expressing hAPP with the rare Swedish (Swe) and Indiana (Ind) mutations had no effect on Abeta. The difference in reduction of Abeta in hAPPwt mice, but not in hAPPSwe/Ind mice, shows that the transgenic model can affect cathepsin B gene knockout results. Since most AD patients express hAPPwt, these data validate cathepsin B as a target for development of inhibitors to lower Abeta in AD.

Alternative pathways for production of beta-amyloid peptides of Alzheimer's disease

This highlight article describes three Alzheimer's disease (AD) studies presented at the 5th General Meeting of the International Proteolysis Society that address enzymatic mechanisms for producing neurotoxic beta-amyloid (Abeta) peptides. One group described the poor kinetics of BACE 1 for cleaving the wild-type (WT) beta-secretase site of APP found in most AD patients. They showed that cathepsin D displays BACE 1-like specificity and cathepsin D is 280-fold more abundant in human brain than BACE 1. Nevertheless, as BACE 1 and cathepsin D show poor activity towards the WT beta-secretase site, they suggested continuing the search for additional beta-secretase(s). The second group reported cathepsin B as an alternative beta-secretase possessing excellent kinetic efficiency and specificity for the WT beta-secretase site. Significantly, inhibitors of cathepsin B improved memory, with reduced amyloid plaques and decreased Abeta(40/42) in brains of AD animal models expressing amyloid precursor protein containing the WT beta-secretase site. The third group addressed isoaspartate and pyroglutamate (pGlu) posttranslational modifications of Abeta. Results showed that cathepsin B, but not BACE 1, efficiently cleaves the WT beta-secretase isoaspartate site. Furthermore, cyclization of N-terminal Glu by glutaminyl cyclase generates highly amyloidogenic pGluAbeta(3-40/42). These presentations suggest cathepsin B and glutaminyl cyclase as potential new AD therapeutic targets.

Inhibitors of cathepsin B improve memory and reduce beta-amyloid in transgenic Alzheimer disease mice expressing the wild-type, but not the Swedish mutant, beta-secretase site of the amyloid precursor protein

Elucidation of Abeta-lowering agents that inhibit processing of the wild-type (WT) beta-secretase amyloid precursor protein (APP) site, present in most Alzheimer disease (AD) patients, is a logical approach for improving memory deficit in AD. The cysteine protease inhibitors CA074Me and E64d were selected by inhibition of beta-secretase activity in regulated secretory vesicles that produce beta-amyloid (Abeta). The regulated secretory vesicle activity, represented by cathepsin B, selectively cleaves the WT beta-secretase site but not the rare Swedish mutant beta-secretase site. In vivo treatment of London APP mice, expressing the WT beta-secretase site, with these inhibitors resulted in substantial improvement in memory deficit assessed by the Morris water maze test. After inhibitor treatment, the improved memory function was accompanied by reduced amyloid plaque load, decreased Abeta40 and Abeta42, and reduced C-terminal beta-secretase fragment derived from APP by beta-secretase. However, the inhibitors had no effects on any of these parameters in mice expressing the Swedish mutant beta-secretase site of APP. The notable efficacy of these inhibitors to improve memory and reduce Abeta in an AD animal model expressing the WT beta-secretase APP site present in the majority of AD patients provides support for CA074Me and E64d inhibitors as potential AD therapeutic agents.

Cysteine protease inhibitors reduce brain beta-amyloid and beta-secretase activity in vivo and are potential Alzheimer's disease therapeutics

Beta-secretase inhibitors that lower brain beta-amyloid peptides (Abeta) are likely to be effective for treating Alzheimer's disease (AD). Irreversible epoxysuccinyl cysteine protease inhibitors are known to reduce brain Abeta and beta-secretase activity in the guinea pig model of human Abeta production. In this study, acetyl-L-leucyl-L-valyl-L-lysinal (Ac-LVK-CHO) is also shown to significantly reduce brain Abeta and beta-secretase activity and brain Abeta in the same model. Ac-LVK-CHO is structurally distinct from the epoxysuccinyl inhibitors and is a reversible cysteine protease inhibitor. The results suggest that cysteine protease inhibitors generally, and reversible cysteine protease inhibitors specifically, have potential for development as AD therapeutics.

Cysteine protease inhibitors effectively reduce in vivo levels of brain beta-amyloid related to Alzheimer's disease

Abnormal accumulation of neurotoxic beta-amyloid peptides (Abeta) in brain represents a key factor in the progression of Alzheimer's disease (AD). Identification of small molecules that effectively reduce brain levels of Abeta is important for development of Abeta-lowering agents for AD. In this study, we demonstrate that in vivo Abeta levels in brain are significantly reduced by the cysteine protease inhibitor E64d and the related CA074Me inhibitor, which inhibits cathepsin B. Direct infusion of these inhibitors into brains of guinea pigs resulted in reduced levels of Abeta by 50-70% after 30 days of treatment. Substantial decreases in Abeta also occurred after only 7 days of inhibitor infusion, with a reduction in both Abeta40 and Abeta42 peptide forms. A prominent decrease in Abeta peptides was observed in brain synaptosomal nerve terminal preparations after CA074Me treatment. Analyses of APP-derived proteolytic fragments showed that CA074Me reduced brain levels of the CTFbeta fragment, and increased amounts of the sAPPalpha fragment. These results suggest that CA074Me inhibits Abeta production by modulating APP processing. Animals appeared healthy after treatment with these inhibitors. These results, showing highly effective in vivo decreases in brain Abeta levels by these cysteine protease inhibitors, indicate the feasibility of using related compounds for lowering Abeta in AD.

Determination of guinea-pig cortical gamma-secretase activity ex vivo following the systemic administration of a gamma-secretase inhibitor

(2S)-2-{[(3,5-Diflurophenyl)acetyl]amino}-N-[(3S)-1-methyl-2-oxo-5-phenyl-2,3-dihydro-1H-1,4-benzodiazepin-3-yl]propanamide (compound E) is a gamma-secretase inhibitor capable of reducing amyloid beta-peptide (1-40) and amyloid beta-peptide (1-42) levels. In this study we investigated the effect of in vivo administration of compound E on guinea-pig plasma, CSF and cortical amyloid beta-peptide (1-40) concentration. Using repeated sampling of CSF, compound E (30 mg/kg p.o.) was shown to cause a time-dependent decrease in CSF amyloid beta-peptide (1-40) levels, which was maximal at 3 h (70% inhibition), compared to baseline controls. After 3 h administration, compound E (3, 10 and 30 mg/kg p.o.), reduced plasma, CSF and DEA-extracted cortical amyloid beta-peptide (1-40) levels by 95, 97 and 99%; 26, 48 and 78%; 32, 33, and 47%, respectively, compared to vehicle control values. In the same animals, compound E (3, 10 and 30 mg/kg p.o.) inhibited cortical gamma-secretase activity, determined ex vivo using the recombinant substrate C100Flag, by 40, 71 and 79% of controls, respectively. These data demonstrate the value of determining not only the extent by which systemic administration of a gamma-secretase inhibitor reduces amyloid beta-peptide, but also the inhibition of brain gamma-secretase activity, as a more direct estimate of enzyme occupancy.

Guinea pigs as a nontransgenic model for APP processing in vitro and in vivo

Alzheimer's disease (AD) is characterized, amongst others, by the appearance of vascular and parenchymal beta-amyloid deposits in brain. Such aggregates are mainly composed of beta-amyloid peptides, which are derived by proteolytic processing of a larger amyloid precursor protein (APP). APP is highly conserved among mammalian species, but experimental studies in rodents are often hampered by the humble APP-processing in the amyloidogenic pathway and by the inability of rodent beta-amyloid peptides to form higher molecular aggregates such as soluble oligomers and insoluble beta-amyloid plaques. Thus, there is need for in vitro and in vivo model systems that allow identification of factors that increase amyloidogenic APP processing and accelerate beta-amyloid plaque formation and testing the potency of pharmacological manipulations to ameliorate beta-amyloid load in brain. Transgenic mice that overexpress human APP containing AD-associated mutations that favor the amyloidogenic pathway of APP processing represent such a model. However, mutations of the APP gene are not frequent in AD and, therefore, the mechanisms of beta-amyloid plaque formation, the composition of beta-amyloid plaques, and the accompanying tissue response in brain of these animals may be different from that in AD. In contrast, guinea pigs express beta-amyloid peptides of the human sequence and appear to represent a more physiological model to examine the long-term effects of experimental manipulations on APP processing and beta-amyloid plaque formation in vivo. Additionally, APP processing in guinea pig primary neuronal cultures has been shown to be similar to cultures of human origin. In this article we highlight the advantages and limitations of using guinea pigs as experimental models to study APP processing.

Effect of a 5-HT(2C) serotonin agonist, dexnorfenfluramine, on amyloid precursor protein metabolism in guinea pigs

Stimulation of serotonin receptor subtypes 5-HT(2A) or 5-HT(2C) in stably transfected 3T3 cells by dexnorfenfluramine (DEXNOR) or serotonin increases secretion of the APP metabolite APP(s). It is not known whether activation of these receptors can also affect APP metabolism in vivo. We examined the effects of a single intraperitoneal (i.p.) injection of DEXNOR on APP(s) levels in cerebrospinal fluid (CSF) of guinea pigs. These levels were significantly (P<0.05) increased by a single dose of DEXNOR (1-4 mg/kg); those of the APP metabolites Abeta(1-40) and Abeta(1-42) were unaffected. The DEXNOR-induced (1 mg/kg) increases in CSF APP(s) were suppressed by ritanserin (1 mg/kg) but not by ketanserin (2 mg/kg). When given alone, ritanserin did not affect CSF levels of APP(s), Abeta(1-40), or Abeta(1-42). Chronic treatment with DEXNOR for 9 days (1 mg/kg bid, i.p.) increased CSF APP(s) levels, measured 2 h after the last injection (P<0.05), and decreased those of CSF Abeta(1-42) (P<0.05). Neither hippocampal nor cortical levels of the APP holoprotein (APP(h)), nor body weight, were affected by DEXNOR. Chronic administration of mCPP (1-(m-chlorophenyl)piperazine) (2 mg/kg bid, i.p.), a 5-HT(2B/2C) agonist, for 9 days also increased CSF APP(s) levels (P<0.5) when measured 2 h after the drug's last administration; hippocampal and cortical APP(h) levels were unaffected. However, mCPP also caused a significant decrease in body weight gain. These data indicate that the pharmacological activation of 5-HT(2C) receptors can stimulate CSF APP(s) secretion and reduce Abeta production in vivo. Hence 5-HT(2C) receptors, which apparently are localized to the brain, may represent useful targets for the development of treatments for Alzheimer's disease.

Molecular cloning and partial characterization of medaka fish stromelysin-3 and its restricted expression in the oocytes of small growing follicles of the ovary

A cDNA clone (2755-bp) for stromelysin-3 was isolated by screening the cDNA library and by 3'- and 5'-rapid amplification of cDNA ends using ovary RNA of the medaka fish Oryzias latipes. The clone encodes a protein of 492 amino acids. Stromelysin-3 mRNA was detected only in the ovary. In situ hybridization analysis revealed that stromelysin-3 mRNA was localized in the oocyte cytoplasm of small growing follicles. RT-PCR analysis of total RNAs isolated from various-sized follicles and ovulated oocytes was conducted in order to determine the mRNA levels during oocyte growth. The stromelysin-3 mRNA level was the highest in the small follicles, and the mRNA levels decreased as the follicles grew. No significant stromelysin-3 mRNA was detected in the ovulated oocytes or immature ovaries. The fish stromelysin-3 cDNA was expressed in COS-1 cells in order to characterize the intracellular localization of the protein. A 56 kDa protein was synthesized and secreted into the culture medium. The secreted stromelysin-3 exhibited gelatin-degrading activity.

Reduction of cortical amyloid beta levels in guinea pig brain after systemic administration of physostigmine

Overproduction of the peptide amyloid beta (Abeta) is thought to be a critical pathogenetic event in Alzheimer's disease (AD). Decreasing A production may therefore slow or halt the progression of AD. In vitro work has indicated that cholinergic muscarinic receptor agonists may reduce cellular production of Abeta. Here we show that systemic administration of physostigmine, an acetylcholinesterase inhibitor, lowers Abeta levels in vivo. Guinea pigs treated for 10 days with s.c. physostigmine had levels of cortical AbetaN-40 and N-42 which were 57% and 72%, respectively, of those in control animals. Levels of cortical beta-amyloid precursor protein were not significantly affected by drug treatment. These results suggest that cholinergic therapy may affect the course of AD by limiting Abeta accumulation.

Developmentally induced microencephalopathy in guinea pigs--embryonic glial cell activation marks selective neuronal death

We have recently shown that in utero treatment of guinea pigs with the DNA methylating substance methylazoxymethanol acetate (MAM) on gestation day (GD) 24 results in neocortical microencephalopathy, increased protein kinase C activity and altered processing of the amyloid precursor protein in neocortex of the offsprings. In order to identify the primary neuronal lesions produced by MAM-treatment, we mapped the 5-bromo-2'-deoxyuridine (BrdU)-incorporation in dividing neurons on GD 24 and we followed the effects of MAM-treatment on GD 24 on embryonic immediate early gene expression and on glial cell activation. BrdU injected on GD 24 labeled many neurons of the ventricular zone and of the intermediate zone but only scattered neurons of the cortical plate. When time-mated guinea pigs were injected intraperitoneally with MAM on GD 24, we observed the activation of microglial cells in the ventricular/intermediate zone and the appearence of astrocytes between the intermediate zone and the cortical plate, 48 h after intoxification. The activation of glial cells was accompanied by the neuronal expression of c-Fos but not of c-Jun in the ventricular/intermediate zone. Based on our observations on BrdU-incorporation and on the morphological outcome of MAM treatment in the juvenile guinea pig, our data presented here indicate that selective neurodegeneration during development induces the activation of both phagocytotic microglial cells and of astrocytes which might trophically support damaged neurons surviving this lesion procedure.

Protein kinase Calpha and beta1 isoforms are regulators of alpha-secretory proteolytic processing of amyloid precursor protein in vivo

We have recently shown that in utero treatment of guinea pigs with the DNA methylating substance methylazoxymethanol acetate (MAM) results in neocortical microencephalopathy, increased protein kinase C (PKC) activity and altered processing of the amyloid precursor protein (APP) in neocortex of offspring. Here we show that PKCalpha and PKCbeta1 are the key regulators of alpha-secretory APP processing in guinea pig neocortex under these experimental conditions in vivo. This conclusion is based on the selective translocation of PKCalpha and PKCbeta1 isoforms to the cell membrane in MAM-treated guinea pigs, as revealed by Western blot analysis and by immunocytochemistry. Additionally, we observed that [3H]phorbol ester binding to protein kinase C increased by 38% and enhanced basal PKC activity by 58% in the neocortex of microencephalic guinea pigs. Inhibition of PKCalpha/PKCbeta1 by Gö6976 abolished this difference, suggesting that constitutive overactivation of these PKC isoforms accounts for the increase in total PKC activity. We also observed a strong positive correlation between levels of alpha-secretase-processed APP and PKC activity in the neocortex of individual animals, providing further evidence for a significant role of classical PKC isoforms in nonamyloidogenic APP processing.

Ovariectomy and 17beta-estradiol modulate the levels of Alzheimer's amyloid beta peptides in brain

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by accumulation of aggregated forms of the 40- and 42-amino acid Abeta peptides (Abeta40 and Abeta42). Estrogen replacement therapy (ERT) in postmenopausal women is associated with decreased risk for AD and/or delay in disease onset. The mechanism by which estrogen exerts this neuroprotective effect is elusive. 17beta-estradiol (E2) was shown to reduce the release of Abeta peptides by primary neuronal cultures of murine and human origin. To test whether estrogen can modulate the metabolism of Abeta peptides in vivo, four experimental sets of guinea pigs were used: intact animals, ovariectomized animals, and ovariectomized animals that received E2 at two different doses. Ovariectomy was associated with a 1.5-fold average increase in total brain Abeta levels as compared to intact controls. E2 treatment significantly reversed the ovariectomy-induced increase in brain Abeta levels. The high-dose E2 treatment did not lead to further decrease in brain Abeta beyond the one observed with the low-dose E2 treatment. Our results infer that cessation of ovarian estrogen production in postmenopausal women might facilitate Abeta deposition by increasing the local concentrations of Abeta40 and Abeta42 peptides in brain and suggest that modulation of Abeta metabolism may be one of the ways by which ERT prevents and/or delays the onset of AD in postmenopausal women.

Constitutive overactivation of protein kinase C in guinea pig brain increases alpha-secretory APP processing without decreasing beta-amyloid generation

Whilst it is generally accepted that the activation of protein kinase C (PKC) increases amyloid precursor protein (APP) secretion in vitro, the role of PKC in the regulation of APP processing and beta-amyloid generation in vivo is still not well understood. In order to address this question, we established the animal model of neocortical microencephalopathy in guinea pigs caused by in utero treatment with methylazoxymethanol acetate, a DNA-methylating substance that eliminates proliferating cells of neuroepithelial origin. The induction of this neocortical malformation is accompanied by constitutive overactivation of PKC in the neocortex of the offspring. In the cortical and hippocampal tissues of juvenile microencephalic guinea pigs (postnatal day 30), we observed significant increases in basal (by 58% and 74%, respectively,) and phorbol ester-stimulated PKC enzyme activity (by 47% and 71%) as compared to age-matched control animals. In the same cortical/hippocampal preparations of methylazoxymethanol-treated animals, there was increased alpha-secretion of APP by 35% and 30% as measured by Western blot analysis using the antibody 6E10, whilst total APP secretion as well as APP mRNA expression remained unaltered. This upregulation of APP alpha-secretion was limited to brain areas that displayed elevated PKC activity. However, constitutive overactivation of neocortical PKC did not affect the generation of beta-amyloid peptides 1-40 or 1-42 as measured by ELISA, suggesting that only the alpha-secretase pathway of APP processing is affected by chronic PKC overactivation in vivo.

Guinea pig D-amino-acid oxidase cDNA and phylogenetic position

The nucleotide sequence of cDNA that encodes guinea pig D-amino-acid oxidase (DAO) was determined. The cDNA consisted of 1,399 nucleotides and a poly(A) tail. The cDNA encodes 347 amino acid residues. In contrast to the hamster, rat, and mouse DAOs, guinea pig DAO had the 25th amino acid residue. The homology in amino acid sequences between the guinea pig DAO and the rodent DAOs was not high in comparison to the homology in amino acid sequences between the guinea pig DAO and DAOs of humans, pigs and rabbits. The phylogenetic position of the guinea pig varied depending on the source of sequences (amino acids or nucleotides) and the methods of phylogenetic tree construction. These results suggest that the guinea pig is not a simple rodent.

Ovariectomy and 17beta-estradiol modulate the levels of Alzheimer's amyloid beta peptides in brain

OBJECTIVE

To test whether female gonadal hormone status and estrogen modulate the metabolism of Abeta peptides in vivo.

BACKGROUND

AD is a neurodegenerative disorder characterized by accumulation of aggregated forms of the 40- and 42-amino acid Abeta peptides (Abeta40 and Abeta42). Estrogen replacement therapy in postmenopausal women is associated with decreased risk for AD or delay in disease onset or both. The mechanism by which estrogen exerts this neuroprotective effect is elusive. 17beta-estradiol (E2) was shown to reduce the release of Abeta peptides by primary neuronal cultures of murine and human origin.

METHODS

For this purpose, four experimental sets of guinea pigs were used: intact animals, ovariectomized animals (ovx), and ovariectomized animals that received E2 at two different doses (ovx+low-dose E2 and ovx+high-dose E2). Brain Abeta40 and Abeta42 levels were assessed using Abeta40 and Abeta42-specific ELISA assays.

RESULTS

Prolonged ovariectomy resulted in uterine atrophy and decreased serum E2 levels and was associated with a pronounced increase in brain Abeta levels. Total brain Abeta in the ovx animals was increased by 1. 5-fold on average as compared to intact controls. E2 treatment of ovariectomized animals led to uterine hypertrophy and a dose-dependent increase in serum E2 levels. In addition, both doses of E2 significantly reversed the ovariectomy-induced increase in brain Abeta levels. The high-dose E2 treatment did not lead to a further decrease in brain Abeta beyond that observed with the low-dose E2 treatment.

CONCLUSIONS

Our results infer that cessation of ovarian estrogen production in postmenopausal women might facilitate Abeta deposition by increasing the local concentrations of Abeta40 and Abeta42 peptides in brain. In addition, our finding that E2 treatment is associated with diminution of brain Abeta levels suggests that modulation of Abeta metabolism may be one of the ways by which estrogen replacement therapy prevents or delays the onset of AD or both in postmenopausal women.

Guinea-pig primary cell cultures provide a model to study expression and amyloidogenic processing of endogenous amyloid precursor protein

Until now guinea-pigs have been rarely used to investigate formation and deposition of Alzheimer's disease-associated amyloid beta peptides despite the sequence identity of human and guinea-pig amyloid beta peptides being known, and the overall similarity of human and guinea-pig amyloid precursor protein. We now describe a primary cell culture system of mixed fetal guinea-pig brain cells, which we have applied to characterize endogenous amyloid precursor protein processing and amyloid beta formation. These cell cultures were established at embryonic day 24 of guinea-pigs after comparison of selected stages of guinea-pig ontogenetic development with the known ontogeny of rats, and were characterized by immunocytochemical detection of neuronal and glial marker proteins. Amyloid precursor protein expression, processing and amyloid beta formation increased in parallel with cellular maturation during cultivation and reached a stable phase after approximately 14 days in vitro therefore providing a suitable time for analysis. Aged cultures display strong neuronal amyloid precursor protein immunoreactivity and an altered profile of amyloid precursor protein isoform messenger RNA expression due to glial proliferation as single neurons were shown to retain their typical pattern of amyloid precursor protein expression. We show that amyloid precursor protein in guinea-pig cells is processed by different protease activities which most likely represent alpha- and beta-secretase, leading to the generation of soluble amyloid precursor protein derivatives. Furthermore, endogenous amyloid precursor protein processing leads to production of substantial amounts of amyloid beta-peptides which accumulate in conditioned culture medium. Amyloid beta was readily detectable by western blot analysis and was shown to consist of approximately 80-90% amyloid beta(1-40). We suggest that primary guinea-pig cell cultures provide a valuable tool in amyloid research that resembles amyloid precursor protein processing under physiological concentrations and, therefore, the situation in humans more closely than current rodent models. It should be especially useful in screening experiments for secretase inhibiting compounds.

Phylogenetic diversity of the expression of the microtubule-associated protein tau: implications for neurodegenerative disorders

The microtubule-associated protein tau regulates the dynamic stability of the neuronal cytoskeleton by interacting with microtubules. It is encoded by a single gene, but expressed in a variety of isoforms due to differential RNA splicing. Six isoforms can be found in the human central nervous system. These isoforms differ in their ability to promote the assembly of microtubules as well as in their capacity to stabilize existing microtubule structures. Furthermore, some of the isoforms of tau are specifically involved in the pathogenesis of neurodegenerative disorders. Thus, splicing of tau might critically influence the physiological functions of tau protein as well as the pathogenesis of neurodegenerative diseases with tauopathy. The present study addresses the differential expression of the six isoforms of tau in the central nervous system of 12 mammalian species including Homo sapiens. The occurrence of each of the six tau isoforms was highly variable. However, species that were phylogenetically related expressed a similar pattern of tau isoforms. These results suggest a phylogenetic descent of splicing paradigms, which can be matched with known phylogenetic concepts based on morphological and molecular genetical studies. Especially, the unique expression pattern of tau isoforms in the human central nervous system implicates a possible link to the particular vulnerability of humans to neurodegenerative disorders with tauopathy, namely Alzheimer's disease, frontotemporal dementia and Pick's disease.

Cholinergic immunolesions by 192IgG-saporin--useful tool to simulate pathogenic aspects of Alzheimer's disease

Alzheimer's disease, the most common cause of senile dementia, is characterized by intracellular formation of neurofibrillary tangles, extracellular deposits of beta amyloid as well as cerebrovascular amyloid accumulation and a profound loss of cholinergic neurons within the nucleus basalis Meynert with alterations in cortical neurotransmitter receptor densities. The use of the cholinergic immunotoxin 192IgG-saporin allows for the first time study of the impact of cortical cholinergic deafferentation on cortical neurotransmission, learning, and memory without direct effects on other neuronal systems. This model also allows the elucidation of contributions of cholinergic mechanisms to the establishment of other pathological features of Alzheimer's disease. The findings discussed here demonstrate that cholinergic immunolesions by 192IgG-saporin induce highly specific, permanent cortical cholinergic hypoactivity and alterations in cortical neurotransmitter densities comparable to those described for Alzheimer's disease. The induced cortical cholinergic deficit also leads to cortical/hippocampal neurotrophin accumulation and reduced amyloid precursor protein (APP) secretion, possibly reflecting the lack of stimulation of postsynaptic M1/M3 muscarinic receptors coupled to protein kinase C. This immunolesion model should prove useful to test therapeutic strategies based on stimulation of cortical cholinergic neurotransmission or amelioration of pathogenic aspects of cholinergic degeneration in the basal forebrain. Application of the model to animal species that can develop beta-amyloid plaques could provide information about the contribution of cholinergic function to amyloidogenic APP processing.