Analysis of brain injury following intrahippocampal administration of beta-amyloid in streptozotocin-treated rats

Intrahippocampal administration of A beta(1-40) impairs spatial learning and memory in hyperglycemic mice

Age-related neurodegenerative dementia, particularly Alzheimer's disease (AD), may be exacerbated by several interacting risk factors including genetic predisposition, beta amyloid (A beta) protein accumulation, environmental toxins, head trauma, and abnormal glycolytic metabolism. We examined the spatial learning and memory effects of A beta(1-40) administration on hyperglycemic mice by their performance in the Morris water maze. Chronic hyperglycemia was induced in male C57BL/6J mice to mimic diabetes mellitus by intraperitoneal injection of streptozotocin (STZ), which specifically destroys pancreatic beta-islet cells. Ten days after STZ treatment, intrahippocampal infusion of vehicle, monomer, or oligomer A beta(1-40) was given to these hyperglycemic mice. Our results demonstrate that in comparison with vehicle or monomer A beta(1-40), oligomer A beta(1-40) induced significant deficits of spatial learning and memory in hyperglycemic mice. Apoptotic signals were identified in the CA1 and dentate gyrus of hippocampus in hyperglycemic mice. A beta accumulation, oxidative stress, and apoptosis in the CA1 region were more intensive in hyperglycemic mice than that in normoglycemic mice after acute treatment with oligomer A beta(1-40) peptide treatment. These results indicate that CA1 apoptosis was enhanced by oxidative stress resulting from accumulation of A beta. Considered together, these findings suggest that hyperglycemic mice are more vulnerable to the A beta-induced-oxidative stress than normal subjects. We therefore propose that A beta accumulation would be enhanced by hyperglycemia, and the oxidative stress caused by A beta accumulation would in turn enhance the AD symptoms.

High-cholesterol diets impair short-term retention of memory in alloxan-induced diabetic mice, but not acquisition of memory nor retention of memory in prediabetic mice

Whether high-cholesterol diets (HCD) induce a high incidence of memory deficits in diabetes requires to be established; if so, whether they induce impairments of memory acquired in the pre-diabetic stage as well as in the diabetic stage also needs to be elucidated, and part of the related mechanisms involved in this dysfunction should be determined. The mice were grouped into: normal mice fed normal diets (NN), diabetic mice fed normal diets (DN), normal mice fed HCD (NH), and diabetic mice fed HCD (DH). Animals were subjected to Morris water maze testing: 1) Learning in the pre-diabetic stage and memory retrieval in the diabetic stage; 2) Learning and memory retrieval in the diabetic stage. Following water maze testing, biochemical parameters were estimated in the animals. The results showed that significant impairments of memory retrieval, acquired in the diabetic stage, were observed only in DH group, neither in DN nor NH group in a short term compared with NN group. Biochemical parameters including fasting blood glucose, lipid peroxidation productions and acetylcholinesterase activities in frontal cortex and hippocampus increased more rapidly in DH group than those in the rest. These results indicate that HCD impair the diabetic retention of memory, but neither the diabetic acquisition of memory nor the pre-diabetic retention of memory in diabetic mice in a short term. Controlled HCD may be a strategy to prevent the loss of memory in diabetic individuals after they have acquired new information.

Increased vulnerability of brain mitochondria in diabetic (Goto-Kakizaki) rats with aging and amyloid-beta exposure

This study evaluated the respiratory indexes (respiratory control ratio [RCR] and ADP/O ratio), mitochondrial transmembrane potential (DeltaPsim), repolarization lag phase, repolarization level, ATP/ADP ratio, and induction of the permeability transition pore of brain mitochondria isolated from normal Wistar and GK diabetic rats of different ages (1.5, 12, and 24 months of age). The effect of amyloid beta-peptides, 50 micromol/l Abeta(25-35) or 2 micromol/l Abeta(1-40), on mitochondrial function was also analyzed. Aging of diabetic mice induced a decrease in brain mitochondrial RCR, ADP/O, and ATP/ADP ratios but induced an increase in the repolarization lag phase. Brain mitochondria from older diabetic rats were more prone to the induction of the permeability transition pore, i.e., mitochondria from 24-month-old diabetic rats accumulated much less Ca(2+) (20 micromol/l) than those isolated from 12-month-old rats (50 micromol/l) or 1.5-month-old rats (100 micromol/l). In the presence of 50 micromol/l Abeta(25-35) or 2 micromol/l Abeta(1-40), age-related mitochondrial effects were potentiated. These results indicate that diabetes-related mitochondrial dysfunction is exacerbated by aging and/or by the presence of neurotoxic agents such as amyloid beta-peptides, supporting the idea that diabetes and aging are risk factors for the neurodegeneration induced by these peptides.

beta-Amyloid neurotoxicity is exacerbated during glycolysis inhibition and mitochondrial impairment in the rat hippocampus in vivo and in isolated nerve terminals: implications for Alzheimer's disease

Senile plaques composed mainly by beta-amyloid (Abeta) protein are one of the pathological hallmarks of Alzheimer's disease (AD). In vitro, Abeta and its active fragment 25-35 have been shown either to be directly neurotoxic or to exacerbate the damaging effect of other neurotoxic insults. However, the attempts to replicate Abeta neurotoxicity in vivo have yielded conflicting results. One of the most consistent alterations in AD is a reduced resting glucose utilization. Important evidence suggests that impairment of brain energy metabolism can lead to neuronal damage or facilitate the deleterious effects of some neurotoxic agents. In the present study we have investigated the influence of glycolysis inhibition induced by iodoacetate, and mitochondrial impairment induced by 3-nitropropionic acid (3-NP), in the toxicity of Abeta. We have studied Abeta neurotoxicity during energy deficiency both in vivo in the dentate gyrus of the hippocampal formation and in presynaptic terminals isolated from neocortex and hippocampus. Results show that during metabolic inhibition an enhanced vulnerability of hippocampal neurons to Abeta peptide toxicity occurs, probably resulting from decreased glucose metabolism and mitochondrial ATP production. Synaptosomal response to energy impairment and Abeta toxicity was evaluated by the MTT assay. Results suggest that synapses may be particularly sensitive to metabolic perturbation, which in turn exacerbates Abeta toxicity. The present data provide experimental support to the hypothesis that certain risk factors such as metabolic dysfunction and amyloid accumulation may interact to exacerbate AD, and that metabolic substrates such as pyruvate may play a role as a therapeutic tool.

c-Jun N-terminal kinase (JNK)-interacting protein-1b/islet-brain-1 scaffolds Alzheimer's amyloid precursor protein with JNK

Using a yeast two-hybrid method, we searched for amyloid precursor protein (APP)-interacting molecules by screening mouse and human brain libraries. In addition to known interacting proteins containing a phosphotyrosine-interaction-domain (PID)-Fe65, Fe65L, Fe65L2, X11, and mDab1, we identified, as a novel APP-interacting molecule, a PID-containing isoform of mouse JNK-interacting protein-1 (JIP-1b) and its human homolog IB1, the established scaffold proteins for JNK. The APP amino acids Tyr(682), Asn(684), and Tyr(687) in the G(681)YENPTY(687) region were all essential for APP/JIP-1b interaction, but neither Tyr(653) nor Thr(668) was necessary. APP-interacting ability was specific for this additional isoform containing PID and was shared by both human and mouse homologs. JIP-1b expressed by mammalian cells was efficiently precipitated by the cytoplasmic domain of APP in the extreme Gly(681)-Asn(695) domain-dependent manner. Reciprocally, both full-length wild-type and familial Alzheimer's disease mutant APPs were precipitated by PID-containing JIP constructs. Antibodies raised against the N and C termini of JIP-1b coprecipitated JIP-1b and wild-type or mutant APP in non-neuronal and neuronal cells. Moreover, human JNK1beta1 formed a complex with APP in a JIP-1b-dependent manner. Confocal microscopic examination demonstrated that APP and JIP-1b share similar subcellular localization in transfected cells. These data indicate that JIP-1b/IB1 scaffolds APP with JNK, providing a novel insight into the role of the JNK scaffold protein as an interface of APP with intracellular functional molecules.

Effect of oxidative stress on the uptake of GABA and glutamate in synaptosomes isolated from diabetic rat brain

It has been suggested that increased oxidative stress might be involved in the pathophysiology of diabetic complications. In this study, we investigated the effect of diabetes on the susceptibility of synaptosomes to oxidative stress (induced by the oxidizing pair ascorbate/Fe(2+)) and on the uptake of the amino acid neurotransmitters gamma-aminobutyric acid (GABA) and glutamate. We found a lower susceptibility of synaptosomes isolated from Goto-Kakizaki (GK) rats, a model of non-insulin-dependent diabetes mellitus, to lipid peroxidation as compared with synaptosomes isolated from Wistar control rats (6.40+/-1.05 and 12.14+/-1.46 nmol thiobarbituric acid reactive substance/mg protein, respectively). The lower susceptibility of GK rat synaptosomes to membrane lipid peroxidation correlates with an increase in synaptosomal vitamin E levels (835+/-58.04 and 624.26+/-50.26 pmol/mg protein in diabetic and normal rats, respectively). In the absence of ascorbate/Fe(2+), no significant differences were observed between the levels of lipid peroxidation of synaptosomes isolated from diabetic and normal rats. Studies of neurotransmitter uptake show that the [(3)H]glutamate uptake was decreased by about 30% in diabetic GK rats as compared with control Wistar rats, whereas the [(3)H]GABA uptake was not significantly different from controls. Under oxidizing conditions, the glutamate uptake in diabetic rats was unaffected, and a decreased GABA uptake (41.39+/-4.41 and 60.96+/-6.4% of control in GK and Wistar rats, respectively) was observed. We conclude that the increased resistance to oxidative stress in GK rat synaptosomes may be due to the increased vitamin E content and that diabetic state and oxidative stress conditions differentially affected the uptake of the neurotransmitters GABA and glutamate.

Neurological changes induced by stress in streptozotocin diabetic rats

Previous studies from our laboratory demonstrated that chronic stress produces molecular, morphological, and ultrastructural changes in the rat hippocampus that are accompanied by cognitive deficits. Glucocorticoid impairment of glucose utilization is proposed as a causative factor involved in stress-induced changes. Current studies have examined the neurological changes induced by stress in rats with a preexisting strain upon their homeostatic load--namely, in streptozotocin (stz)-diabetic rats. Administration of stz (70 mg/kg, i.v.) produced diabetic symptoms such as weight loss, polyuria, polydipsia, hyperglycemia, and neuroendocrine dysfunction. Morphological analysis of hippocampal neurons revealed that diabetes alone produced dendritic atrophy of CA3 pyramidal neurons, an effect potentiated by 7 days of restraint stress. Analysis of genes critical to neuronal homeostasis revealed that glucose transporter 3 (GLUT3) mRNA and protein levels were specifically increased in the hippocampus of diabetic rats, while stress had no effect upon GLUT3 expression. Insulin-like growth factor (IGF) receptor expression was also increased in the hippocampus of diabetic rats subjected to stress. In spite of the activation of these adaptive mechanisms, diabetic rats subjected to stress also had signs of neuronal damage and oxidative damage. Collectively, these results suggest that the hippocampus of diabetic rats is extremely susceptible to additional stressful events, which in turn can lead to irreversible hippocampal damage.

Susceptibility to beta-amyloid-induced toxicity is decreased in goto-kakizaki diabetic rats: involvement of oxidative stress

The response of synaptosomes isolated from Wistar non-diabetic rats and Goto-Kakizaki (GK) diabetic rats to the beta-amyloid fragment Abeta25-35 was compared. The synaptosomal redox activity, evaluated by the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay, was shown to be decreased in GK rats (72.8 +/- 7.45% of MTT reduction). However, the reduction of MTT was decreased in synaptosomes of Wistar rats upon Abeta25-35 treatment (84.47 +/- 3.73%), while in GK rats it was not affected. Abeta25-35 induced lipid peroxidation in synaptosomes of Wistar rats, but not in that of GK rats, leading to an 1.5-fold increase in thiobarbituric acid reactive substances (TBARS) levels. In the absence of Abeta25-35, basal TBARS levels were 1.6-fold higher in GK rats. In the former preparations, the content in vitamin E was also higher (2-fold). A decrease in ATP levels, of about 2-fold, was observed in synaptosomes of Wistar rats treated with Abeta25-35, while no significant changes were observed in synaptosomes of GK rats. No significant differences between the two groups were detected in the basal ATP levels. The extrasynaptosomal accumulation of aspartate and glutamate increased upon Abeta25-35 treatment, only in synaptosomes of Wistar rats (aspartate and glutamate accumulation increased by 1.1-fold and 1.5-fold, respectively), while the accumulation of glycine increased in both Wistar (by 1.8-fold) and GK (by 2.2-fold) rats. No statistical differences in the basal accumulation of aminoacids were observed. These results show that synaptosomes of GK diabetic rats have a lower redox activity, but are less susceptible to the Abeta25-35-induced toxicity. Data also suggest that oxidative stress occurs in this hyperglycemia animal model and that an increase in the antioxidant defense systems may exert protection against toxic insults. This mechanism, occuring in the early phases of diabetes, may correspond to an adaptive response.

Bilateral injections of amyloid-beta 25-35 into the amygdala of young Fischer rats: behavioral, neurochemical, and time dependent histopathological effects

To examine the time course of the histopathological effects of bilateral injections of amyloid-beta 25-35 (A beta) and to determine if these effects are associated with a reduction in choline acetyltransferase activity and behavioral impairments, we injected A beta (5.0 nmol) into the amygdala of young male Fischer rats. Control rats received vehicle infusions. For histological analysis, animals were sacrificed at 8, 32, 64, 96, and 128 days postoperatively (n = 21-33 per timepoint). A beta induced neuronal tau-2 staining in the right, but not the left amygdala and hippocampus. A beta also induced reactive astrocytosis and neuronal shrinkage within the right hippocampus and amygdala, respectively. As with tau-2, these same brain regions within the left hemisphere in the A beta-treated rats were significantly less affected. In addition, A beta appeared to induce microglial and neuronal interleukin-1beta staining. The histopathological effects of A beta peaked at 32 days postoperatively but were not associated with a reduction in amygdaloid choline acetyltransferase activity. In a separate experiment, behavioral effects of bilateral intra-amygdaloid injections of A beta were analyzed at 34-52 days postoperatively. In an open field test, the treatment groups differed only in the numbers of rears emitted (p = 0.016). There was no effect of A beta in the Morris water maze or in the acquisition and retention of a one-way conditioned avoidance response. These data suggest a laterality in the histopathological effects of A beta and that the effects of single injections are in part transient. These findings also suggest a direct association between plaque and tangle formation in Alzheimer's disease, and support the use of this rat model to screen drugs that may alter the initial pathological events associated with Alzheimer's disease, that occur before the manifestations of extensive behavioral impairments become evident.

Permeability and residual plasma volume of human, Dutch variant, and rat amyloid beta-protein 1-40 at the blood-brain barrier

The permeability of normal human, the human Dutch variant, and the rat A beta 1-40 proteins at the blood-brain barrier (BBB) was determined in the normal adult rat by quantifying the permeability coefficient-surface area (PS) product for each protein after correction for the residual plasma volume (Vp) occupied by the protein in the blood vessels of different brain regions. The PS for normal and Dutch A beta ranged from 13 x 10(-6) to 22 x 10(-6) ml/g/s in different brain regions, which is 130 to 220 times greater than albumin. These high PS values compare to that of insulin, whose uptake is decidedly by a receptor-mediated transport process, and suggest a similar mechanism for A beta. Remarkably, the PS for rat A beta was 4 times higher and ranged from 54 x 10(-6) to 82 x 10(-6) ml/g/s for different brain regions, suggesting a distinctive species specificity. While the Vp values of human and rat A beta were comparable, the Dutch variant was 2 to 3 times higher, indicating adherence to the vessel walls in different brain regions, consistent with the heavy A beta deposition that has been described in intracerebral vessel walls with this variant. The high PS values observed for A beta at the BBB suggest that sources outside the nervous system could contribute, at least in part, to the cerebral A beta deposits seen in Alzheimer's disease. SDS-PAGE of 125I-labeled human A beta after 60 min of uptake revealed intact protein in plasma and in different brain regions. In addition, 125I-labeled human A beta binding to a protein of 67,000 in both plasma and brain tissue regions was observed with SDS-PAGE. This protein was tentatively identified as albumin, and it was not detectable in the brain regions of animals that had undergone intracardiac perfusion; hence, a portion of A beta binds tightly to and is likely transported by albumin in plasma. The absence of this A beta-albumin complex in brain regions after perfusion and the low permeability of albumin at the BBB imply that A beta itself is efficiently transported at the BBB to account for the high PS values, although presentation of A beta to the capillary endothelial cell by albumin or other plasma proteins cannot be excluded.

Local and distant histopathological effects of unilateral amyloid-beta 25-35 injections into the amygdala of young F344 rats

To determine if amyloid-beta (A beta) induces tau-immunoreactivity (IR) and reactive astrocytosis in vivo, we injected A beta 25-35 (5.0 nmol) into the right amygdala of rats. At 8 days postinjection, the peptide induced tau-2 IR in neuronal cell bodies and processes ipsilaterally in the amygdala, cingulate cortex, and hippocampus. At 32 days postinjection, the intensity of tau-2 IR was greater than at 8 days in the amygdala and hippocampus, but not in the cingulate cortex. Induction of Alz-50 IR also was progressive but the morphology and distribution was different from tau-2 IR. Beaded fibers with occasional neuronal perikarya were visualized with Alz-50, and the IR was primarily observed in the ipsilateral amygdala. In addition, amygdaloid injections of A beta 25-35 induced reactive astrocytosis, particularly in the ipsilateral hippocampus at 32 days postoperatively. To our knowledge, this is the first study to show that in vivo injections of A beta 25-35 induce progressive transsynaptic cytoskeletal and astrogliotic reactions, that gradually spread from the area of injection to brain regions that have prominent efferent connections with that area. These findings also suggest a direct association between plaque and tangle formation in Alzheimer's disease.

beta-Amyloid toxicity in organotypic hippocampal cultures: protection by EUK-8, a synthetic catalytic free radical scavenger

Oxygen free radicals have been proposed to mediate amyloid peptide (beta-AP)-induced neurotoxicity. To test this hypothesis, we evaluated the effects of EUK-8, a synthetic catalytic superoxide and hydrogen peroxide scavenger, on neuronal injury produced by beta-AP in organotypic hippocampal slice cultures. Cultures of equivalent postnatal day 35 (defined as mature) and 14 (defined as immature) were exposed to various concentrations of beta-AP (1-42 or 1-40) in the absence or presence of 25 microM EUK-8 for up to 72 hours. Neuronal injury was assessed by lactate dehydrogenase release and semiquantitative analysis of propidium iodide uptake at various times after the initiation of beta-AP exposure. Free radical production was inferred from the relative increase in dichlorofluorescein fluorescence, and the degree of lipid peroxidation was determined by assaying thiobarbituric acid-reactive substances. Treatment of mature cultures with beta-AP (50-250 microg/ml) in serum-free conditions resulted in a reproducible pattern of damage, causing a time-dependent increase in neuronal injury accompanied with formation of reactive oxygen species. However, immature cultures were entirely resistant to beta-AP-induced neurotoxicity and also demonstrated no dichlorofluorescein fluorescence or increased lipid peroxidation after beta-AP treatment. Moreover, mature slices exposed to beta-AP in the presence of 25 microM EUK-8 were significantly protected from beta-AP-induced neurotoxicity. EUK-8 also completely blocked beta-AP-induced free radical accumulation and lipid peroxidation. These results not only support a role for oxygen free radicals in beta-AP toxicity but also highlight the therapeutic potential of synthetic radical scavengers in Alzheimer disease.

Glucose regulation and cognitive functions: relation to Alzheimer's disease and diabetes

Glucose has been found to improve memory in animals and humans. Animal research has revealed that glucose may improve memory through a facilitation of acetylcholine (ACh) synthesis and release in the brain. This glucose-related memory improvement has prompted research in elderly humans. These studies have shown that the memory-improving action of glucose depends on each individuals' blood glucose regulation. Based on these data, researchers have evaluated the effect of glucose on memory in patients with Alzheimer's disease (AD). Results demonstrated that glucose could improve memory in a subset of patients that had abnormalities in their blood glucose regulation. Interestingly, these alterations in blood glucose regulation were believed to depend on the severity of the disease process. Another line of investigation has focused on alterations in brain glucose metabolism. Both animal models and studies with Type II diabetic elderly patients have shown that altered glucose regulation impairs learning and memory processes. It is possible that in AD patients, hyperglycemia exerts a deleterious effect by potentiating the neuronal death produced by other pathological processes taking place such as amyloid deposition. Based on these data, it appears important to find the prevalence of altered glucoregulation at various stages of AD. Secondly, it may be of interest to determine prospectively whether altered glucoregulation is linked to a faster progression of the disease. Finally, if such a relationship is observed, the next logical step would be to determine whether AD patients could benefit from treatments aimed at normalizing blood glucose regulation and improving insulin sensitivity.

Desensitization of the neuronal insulin receptor: a new approach in the etiopathogenesis of late-onset sporadic dementia of the Alzheimer type (SDAT)?

Even in its incipient stage, late-onset sporadic dementia of the Alzheimer type (SDAT) is characterized by an abnormal reduction in brain glucose consumption and energy formation. Gathering evidence indicates that cerebral glucose metabolism is controlled by brain insulin/insulin receptors. This led us to hypothesize that the abnormal reduction in glucose utilization found in Alzheimer brains is preceded by a desensitization of cerebral insulin receptors which might be due to enhanced levels of stress factors such as cortisol and catecholamines. The hypothesis is supported by clinical findings of an abnormal response to the oral glucose tolerance test in AD patients. Furthermore, experimental desensitization of the cerebral insulin receptor resulted in both cognitive deficits and metabolic abnormalities in cerebral oxidative glucose metabolism resembling those described in incipient late-onset SDAT. Glucose is the major source of energy in the CNS, and any impairment in cerebral glucose oxidation can be expected to result in deficits in both acetylcholine synthesis and ATP formation, which might contribute to altered APP processing and enhanced susceptibility to neurotoxicity.