Alzheimer disease: an imbalance of proteolytic regulation?

Insights into the neuropathology of cerebral ischemia and its mechanisms

Cerebral ischemia is a result of insufficient blood flow to the brain. It leads to limited supply of oxygen and other nutrients to meet metabolic demands. These phenomena lead to brain damage. There are two types of cerebral ischemia: focal and global ischemia. This condition has significant impact on patient’s health and health care system requirements. Animal models such as transient occlusion of the middle cerebral artery and permanent occlusion of extracranial vessels have been established to mimic the conditions of the respective type of cerebral ischemia and to further understand pathophysiological mechanisms of these ischemic conditions. It is important to understand the pathophysiology of cerebral ischemia in order to identify therapeutic strategies for prevention and treatment. Here, we review the neuropathologies that are caused by cerebral ischemia and discuss the mechanisms that occur in cerebral ischemia such as reduction of cerebral blood flow, hippocampal damage, white matter lesions, neuronal cell death, cholinergic dysfunction, excitotoxicity, calcium overload, cytotoxic oedema, a decline in adenosine triphosphate (ATP), malfunctioning of Na+/K+-ATPase, and the blood-brain barrier breakdown. Altogether, the information provided can be used to guide therapeutic strategies for cerebral ischemia.

Oxidative Stress in Alzheimer's Disease: Should We Keep Trying Antioxidant Therapies?

The risk of chronic diseases such as Alzheimer's disease is growing as a result of the continuous increasing average life span of the world population, a syndrome characterized by the presence of intraneural neurofibrillary tangles and senile plaques composed mainly by beta-amyloid protein, changes that may cause a number of progressive disorders in the elderly, causing, in its most advanced stage, difficulty in performing normal daily activities, among other manifestations. Therefore, it is important to understand the underlying pathogenic mechanisms of this syndrome. Nevertheless, despite intensive effort to access the physiopathological pathways of the disease, it remains poorly understood. In that context, some hypotheses have arisen, including the recent oxidative stress hypothesis, theory supported by the involvement of oxidative stress in aging, and the vulnerability of neurons to oxidative attack. In the present revision, oxidative changes and redox mechanisms in Alzheimer's disease will be further stressed, as well as the grounds for antioxidant supplementation as adjuvant therapy for the disease will be addressed.

Inflammatory Animal Model for Parkinson's Disease: The Intranigral Injection of LPS Induced the Inflammatory Process along with the Selective Degeneration of Nigrostriatal Dopaminergic Neurons

We have developed an animal model of degeneration of the nigrostriatal dopaminergic neurons, the neuronal system involved in Parkinson's disease (PD). The implication of neuroinflammation on this disease was originally established in 1988, when the presence of activated microglia in the substantia nigra (SN) of parkinsonians was reported by McGeer et al. Neuroinflammation could be involved in the progression of the disease or even has more direct implications. We injected 2 μg of the potent proinflammatory compound lipopolysaccharide (LPS) in different areas of the CNS, finding that SN displayed the highest inflammatory response and that dopaminergic (body) neurons showed a special and specific sensitivity to this process with the induction of selective dopaminergic degeneration. Neurodegeneration is induced by inflammation since it is prevented by anti-inflammatory compounds. The special sensitivity of dopaminergic neurons seems to be related to the endogenous dopaminergic content, since it is overcome by dopamine depletion. Compounds that activate microglia or induce inflammation have similar effects to LPS. This model suggest that inflammation is an important component of the degeneration of the nigrostriatal dopaminergic system, probably also in PD. Anti-inflammatory treatments could be useful to prevent or slow down the rate of dopaminergic degeneration in this disease.

Advanced glycation endproducts and their receptor RAGE in Alzheimer's disease

Alzheimer's disease (AD) is the most common dementing disorder of late life. Although there might be various different triggering events in the early stages of the disease, they seem to converge on a few characteristic final pathways in the late stages, characterized by inflammation and neurodegeneration. In this review, we revisit the hypothesis that advanced glycation endproducts (AGEs) and their receptor RAGE may play an important role in disease pathogenesis. Accumulation of AGEs in cells and tissues is a normal feature of aging, but is accelerated in AD. In AD, AGEs can be detected in pathological deposits such as amyloid plaques and neurofibrillary tangles. AGEs explain many of the neuropathological and biochemical features of AD such as extensive protein crosslinking, glial induction of oxidative stress and neuronal cell death. Oxidative stress and AGEs initiate a positive feedback loop, where normal age-related changes develop into a pathophysiological cascade. RAGE and its decoy receptor soluble RAGE, may contribute to or protect against AD pathogenesis by influencing transport of β-amyloid into the brain or by manipulating inflammatory mechanisms. Targeted pharmacological interventions using AGE-inhibitors, RAGE-antagonists, RAGE-antibodies, soluble RAGE or RAGE signalling inhibitors such as membrane-permeable antioxidants may be promising therapeutic strategies to slow down the progression of AD.

Thrombin induces in vivo degeneration of nigral dopaminergic neurones along with the activation of microglia

Seven days after the injection of different concentrations of thrombin into the nigrostriatal pathway, a strong macrophage/microglial reaction was observed in the substantia nigra (SN), indicated by immunostaining, using OX-42 and OX-6 antibodies, and by the induction of iNOS, IL-1alpha, Il-1beta and TNF-alpha. Moreover, selective damage to dopaminergic neurones was produced after thrombin injection, evidenced by loss of tyrosine hydroxylase immunostaining and tyrosine hydroxylase mRNA-expressing cell bodies, and the unaltered transcription of glutamic acid decarboxylase mRNA in the SN and striatum. These thrombin effects could be produced by its ability to induce the activation of microglia described in in vitro studies, and are in agreement with the effects described for other proinflammatory compounds. Thrombin effects are produced by its biological activity since they almost disappeared when thrombin was heat-inactivated or injected along with its inhibitor alpha-NAPAP. Thrombin is a multi-functional serine protease rapidly produced from prothrombin at the sites of tissue injury, and also upon breakdown of the blood-brain barrier, which strongly suggests it could easily enter into the CNS. These results could have special importance in some degenerative processes of the nigrostriatal dopaminergic system.

Accumulation of C-terminally truncated tau protein associated with vulnerability of the perforant pathway in early stages of neurofibrillary pathology in Alzheimer's disease

Neurofibrillary pathology is a characteristic hallmark of Alzheimer's disease that is closely correlated with cognitive decline. We have analysed the density and distribution of neurofibrillary tangles (NFTs) that are immunoreactive with the monoclonal antibody (mAb) 423 in a prospectively analysed population of Alzheimer's disease (AD) cases and age-matched controls. NFTs were examined in allocortical and isocortical areas and correlated with Braak pathological stage and clinical severity of dementia. The mAb 423 was used as it recognises a C-terminally truncated tau fragment that is a major constituent of NFTs. Our results show that extracellular NFTs and, to a lesser extent, intracellular NFTs, correlated significantly with both Braak stages and the clinical index of severity. Furthermore, a differential distribution of the two types of tangles indicates that layer II of the entorhinal cortex and the transentorhinal area are particularly vulnerable to neurofibrillary degeneration. These areas serve as a point of connection between isocortex and hippocampus. Our findings, therefore, suggest that the perforant pathway may be substantially affected by the accumulation of truncated tau protein in AD and that this represents a neuropathological predictor for the clinical severity of dementia. When neurofibrillary pathology was examined by combined labelling with mAbs 423 and Alz-50 and the dye thiazin red, we were able to demonstrate various stages of tau aggregation. The different stages may represent a sequence of conformational changes that tau proteins undergo during tangle formation in the allocortex during the early development of dementia in AD.

O-Glycosylation in sprouting neurons in Alzheimer disease, indicating reactive plasticity

Reactive plasticity, including axonal and dendritic sprouting and reactive synaptogenesis, has been proposed to contribute to the pathogenesis of several neurological disorders. This work was aimed at identifying the possible role of protein glycosylation in the brain from patients with Alzheimer disease (AD), using lectin histochemistry, as determinants of reactive plasticity. Results indicate an increase in the production of cryptic O-glycosidically linked proteins (NeuAcalpha2,6 Galbeta1,3GalNAcalpha1,0 Ser/Thr or sialyl-T-antigen) in neuritic sprouting in AD brains as determined by positive labeling with Amaranthus leucocarpus (ALL, T-antigen-specific) and Macrobrachium rosenbergii (MRL, specific for NeuAc5,9Ac2) lectins. Immunohistochemistry indicated that lectin staining was specific for the synaptic sprouting process (meganeurites) in AD. These results were confirmed using anti-synaptophysin and anti-GAP 43 antibodies, which recognized meganeurites and dystrophic neurites around amyloid-beta deposits. In normal control brains, labeling with the aforementioned lectins was restricted to microvessels. Control experiments with neuraminidase-treated brain samples revealed positivity to the lectin from Arachis hypogaea (PNA), which is specific for galactose. Our results suggest specific O-glycosylation patterns of proteins closely related to neuronal plasticity in AD.

Amyloid beta peptide processing, insulin degrading enzyme, and butyrylcholinesterase

Amyloid beta peptide implicated in Alzheimers disease is cleaved by insulin degrading enzyme (IDE). Abnormal cholinesterases similar to butyrylcholinesterase (BChE) are found in Alzheimer brain. The similarities between IDE and BChE (which is known to have an arylacylamidase and a metallocarboxypeptidase-like activity) such as their zinc metalloenzyme nature, their localization in glia and their ability to bind amyloid peptide in Alzheimers disease raise interesting questions.

The effects of thrombin preconditioning on focal cerebral ischemia in rats

Our previous studies have shown that prior intracerebral infusion of a low dose of thrombin (thrombin preconditioning; TPC) reduces the brain edema that follows a subsequent intracerebral infusion of a high dose of thrombin or an intracerebral hemorrhage. In vitro studies have also demonstrated that low concentrations of thrombin protect neurons and astrocytes from hypoglycemia and oxidative stress-induced damage. This study, therefore, examines the hypothesis that TPC would offer protection from ischemic brain damage in vivo. This was a blinded design study. The rat brain was preconditioned with 1 U thrombin by direct infusion into the left caudate nucleus. Seven days after thrombin pretreatment, permanent middle cerebral artery occlusion (MCAO) was induced. Twenty-four hours post-ischemia, neurological deficit was evaluated and infarction volume, brain water and ion contents were measured. Compared to saline-treated rats, thrombin pretreatment significantly attenuated brain infarction in cortex (90+/-33 vs. 273+/-22 mm(3); P<0.05) and basal ganglia (56+/-17 vs. 119+/-12 mm(3); P<0.05) that followed 24 h of permanent MCAO. TPC also reduced the brain edema in cortex and basal ganglia by 50 and 53% (P<0.05). Neurological deficit was improved in thrombin pretreatment group (P<0.05). These effects of TPC were, in part, prevented by co-injection of hirudin, a thrombin inhibitor, indicating that the protection was indeed thrombin mediated. Cerebral TPC significantly reduces ischemic brain damage, perhaps by activation of the thrombin receptor. This finding provides a new mechanism by which to study ischemic tolerance.

Effect of thrombin inhibition in vascular dementia and silent cerebrovascular disease. An MR spectroscopy study

BACKGROUND AND PURPOSE

Silent cerebrovascular disease (CVD) has been proposed as a predisposing condition for clinically overt stroke and vascular dementia. Recently, we found increased thrombin generation in silent CVD patients. Here, we report the effect of thrombin inhibition using a potent selective thrombin inhibitor on the cerebral metabolism and function in peripheral arterial occlusive disease (PAOD) patients with or without silent CVD.

METHODS

We examined 17 mild chronic PAOD patients, including 2 cases of vascular dementia. We divided the patients into 2 groups: 1 was the advanced CVD group with multiple lacunar infarction and/or advanced periventricular hyperintensity detected by brain MRI (n=12), and the other was the no CVD group that had none of these abnormalities (n=5). We assessed the cerebral biochemical compounds in the deep white matter area and cerebellar hemisphere (8 cm3) by proton MR spectroscopy before and after infusion of argatroban (10 mg/d IV) over 2 hours for 7 days.

RESULTS

The ratio of N-acetylasparate (NAA) to total creatine (Cre) in the deep white matter area was significantly lower in the advanced CVD group than in the no CVD group, whereas there were no significant differences in this ratio in the cerebellar hemisphere between the 2 groups. In the former group, this decreased NAA/Cre ratio significantly increased after argatroban therapy, whereas there was no change in the latter group. The 2 patients with vascular dementia showed clinical improvement with marked increases in the NAA/Cre ratio and mini-mental score.

CONCLUSIONS

These results suggest that increased thrombin generation may have some pathophysiological roles in developing vascular dementia and its chronic predisposing conditions. Thrombin inhibition may break this vicious cycle and lead to clinical improvement.

Abnormal localization of iron regulatory protein in Alzheimer's disease

A role for altered iron metabolism in the pathogenesis of Alzheimer's disease has been suggested by several reports associating the cardinal neuropathologic lesions with markers of free radical-induced damage and redox-active iron. We hypothesized that the abnormal distribution of iron in Alzheimer brain might result from alterations in iron regulatory proteins (IRP) such as IRP-1 and IRP-2, the main control elements of cellular iron homeostasis. Here, we report that while IRP-1 is present at similar levels in both Alzheimer and control brain tissue, IRP-2 shows striking differences and is associated with intraneuronal lesions, including neurofibrillary tangles, senile plaque neurites and neuropil threads. Since IRP-2 colocalizes with redox-active iron, our results suggest that alterations in IRP-2 might be directly linked to impaired iron homeostasis in Alzheimer's disease.

Advanced glycation endproducts in ageing and Alzheimer's disease

Accumulation of advanced glycation endproducts (AGE) in the brain is a feature of ageing and degeneration, especially in Alzheimer's disease (AD). Increased AGE levels explain many of the neuropathological and biochemical features of AD such as extensive protein crosslinking (beta-amyloid and MAP-tau), oxidative stress and neuronal cell death. Oxidative stress and AGEs initiate a positive feedback loop, where normal age-related changes develop into a pathophysiological cascade. Combined intervention using antioxidants, metal chelators, anti-inflammatory drugs and AGE-inhibitors may be a promising neuroprotective strategy.

Trypsin interaction with the senile plaques of Alzheimer disease is mediated by beta-protein precursor

In this study we demonstrate by in situ binding that trypsin interacts with the senile plaques found in Alzheimer disease. Characterization of various potential trypsin binding proteins shows that trypsin binding is mediated by beta-protein precursor (beta PP)-the progenitor of amyloid-beta in senile plaques. Using specific antisera against various proteins to sterically block trypsin blocking, we found that only those antibodies raised against proteins or peptides containing the Kunitz protease inhibitor domain were able to abolish binding. By analogy with other protease/inhibitor interactions, we speculate that the binding of trypsin to beta PP could involve concomitant beta PP cleavage. Therefore, beta PP in protecting against potentially damaging proteolysis could simultaneously liberate beta PP fragments or intermediate precursors of amyloid-beta deposits.

Extracellular neurofibrillary tangles reflect neuronal loss and provide further evidence of extensive protein cross-linking in Alzheimer disease

In this report we quantitatively assess the numbers of intracellular and extracellular neurofibrillary tangles (NFT) in the brains of a series of individuals with Alzheimer's disease and of controls and correlate these with neuronal loss. Our data indicate that in some cases, NFT are not removed from the brain throughout the disease process. This finding, together with our previous demonstration of carbonyl-related modifications in NFT, provides additional evidence that the protein constituents of NFT are resistant to proteolytic removal, possibly as a result of extensive cross-links. Additionally, correlation between the number of NFT and neuronal loss indicates that there are at least two distinct mechanisms responsible for neuronal death in Alzheimer's disease that are directly and indirectly related to the presence of neurofibrillary pathology.

Radical AGEing in Alzheimer's disease

The pathological presentation of Alzheimer's disease, the leading cause of senile dementia, involves regionalized neuronal death and an accumulation of intracellular and extracellular filamentous protein aggregates that form lesions termed neurofibrillary tangles and senile plaques, respectively. Several independent parameters have been suggested as the primary factor that is responsible for this pathogenesis, including apolipoprotein epsilon genotype, hyperphosphorylation of cytoskeletal proteins, or metabolism of amyloid beta. However, at present, no one theory explains adequately the host of complex biochemical and pathological facets of the disease. Recent findings suggest that age-related increases in oxidative stress and protein glycation either individually, or more probably in a synergistic manner, could, exclusive of the other theories or in concert with them, account for all aspects of Alzheimer's disease.