Alzheimer's disease neuropathology. Current status of interpretation of lesion development

Roles of cholinergic receptors during attentional modulation of cue detection

Basal forebrain corticopetal cholinergic neurons are known to be necessary for normal attentional processing. Alterations of cholinergic system functioning have been associated with several neuropsychiatric diseases, such as Alzheimer’s disease and schizophrenia, in which attentional dysfunction is thought to be a key contributing factor. Loss of cortical cholinergic inputs impairs performance in attention-demanding tasks. Moreover, measures of acetylcholine with microdialysis and, more recently, of choline with enzyme-coated microelectrodes have begun to elucidate the precise cognitive demands that activate the cholinergic system on distinct time scales. However, the receptor actions following acetylcholine release under attentionally-challenging conditions are only beginning to be understood. The present review is designed to summarize the evidence regarding the actions of acetylcholine at muscarinic and nicotinic receptors under cognitively challenging conditions in order to evaluate the functions mediated by these two different cholinergic receptor classes. Moreover, evidence that supports beneficial effects of muscarinic muscarinic-1 receptor agonists and selective nicotinic receptor subtype agonists for cognitive processing will be discussed. Finally, some challenges and limitations of targeting the cholinergic system for treating cognitive deficits along with future research directions will be mentioned. In conclusion, multiple aspects of cholinergic neurotransmission must be considered when attempting to restore function of this neuromodulatory system.

Alzheimer's disease and the Blood-Brain Barrier: Past, Present and Future

Successful prevention and treatment of late-onset Alzheimer’s disease (AD) is a high priority for industrialized societies where the incidence is growing rapidly. Much of the underlying biology leading to AD is unknown, and the more knowledge we gain the more we appreciate the complexities involved. Popular etiologic hypotheses have largely ignored the blood–brain barrier (BBB) as an important factor contributing to the pathologic hallmarks of this most common form of dementia. Evidence identifying BBB dysfunction in AD or patients at risk (i.e., those with mild cognitive impairment) continue to escalate. This review highlights methodological issues facing investigators assessing BBB integrity in living patients while also discussing whether the BBB dysfunction is a cause, effect or epiphenomenon in AD. Rationale for future research pursuits aimed at describing the role of BBB function in AD pathogenesis is also presented.

An in vitro model of inflammatory neurodegeneration and its neuroprotection

Inflammation has been implicated in a variety of acute and chronic neurodegenerative diseases in which the inflammatory processes are considered not only to result from neurodegenerative effects, but also to contribute to these effects. To investigate the primary effect of inflammation on neuronal survival, a co-culture system of neuronal cells (differentiated SH SY5Y human neuroblastoma cells or primary cortical/striatal neurons) and monocytic cells (THP-1) in direct cell-cell contact was set up. After 5 days, THP-1 activation by lipopolysaccharide and phorbol 12-myristate 13-acetate resulted in a significant increase of neuronal cell death compared to co-culture without activation. In neuroprotection studies using this model, ascorbic acid and EDTA demonstrated a highly significant reduction in activated THP-1 induced cell death. Glutathione and NBQX, but not the protease inhibitor, PMSF, and catalase, also significantly reduced this inflammatory neurotoxicity. Indomethacin was protective of the primary cultured neurons but not the SH SY5Y cells. This co-culture of neuronal cells and activated THP-1 provides a useful model for the study of inflammatory mechanisms resulting in neuronal cell death.

Transgenic mice expressing the betaAPP695SWE mutation: effects on exploratory activity, anxiety, and motor coordination

The functional consequences of the betaAPP transgene with the Swedish mutation in mice were assessed in tests of exploratory activity and motor coordination. The betaAPP(695)SWE (Tg2576) transgenic mice are characterized by Abeta plaque formation in the neocortex and hippocampus. By comparison to non-transgenic mice controlled for age and gender, 17-month-old betaAPP(695)SWE transgenic mice displayed impaired spontaneous alternation, increased activity levels in the peripheral part of the open-field, and reduced anxiety in the elevated plus-maze. These results are similar to the loss of inhibitory control observed in some patients with Alzheimer's disease. These measures may be added to cognitive dysfunctions as testing ground for Abeta vaccination and other attempts at experimental therapies.

Role of p38 and p44/42 mitogen-activated protein kinases in microglia

Although microglial cells are thought to play a beneficial role in the regeneration and plasticity of the central nervous system (CNS), recent studies have indicated that at least some molecules released by microglia may be harmful in acute brain insults and neurodegenerative diseases. Therefore, the pathways mediating the synthesis and release of these neurotoxic compounds are of importance. p38 and p44/42 families of mitogen-activated protein kinases (MAPKs) in microglia respond strongly to various extracellular stimuli, such as ATP, thrombin, and beta-amyloid, a peptide thought to be responsible for the neuropathology in Alzheimer's disease. In this review we describe in vivo evidence implicating that p38 and p44/42 MAPKs may play a critical role in harmful microglial activation in acute brain injury, such as stroke, and in more chronic neurodegenerative diseases, such as Alzheimer's disease. We also clarify the extracellular signals responsible for activation of p38 and p44/42 MAPK in microglia and review the responses so far reported to be mediated by these kinases.

Intraneuronal amyloid precursor protein (APP) and appearance of extracellular beta-amyloid peptide (abeta) in the brain of aging kokanee salmon

Antibodies to human amyloid precursor protein (APP(695)) and beta-amyloid peptide (A beta(1-42)) were used to determine timing of amyloidosis in the brain of kokanee salmon (Oncorhynchus nerka kennerlyi) in one of four reproductive stages: immature (IM), maturing (MA), sexually mature (SM), and spawning (SP), representing a range of aging from somatically mature but sexually immature to spawning and somatic senescence. In IM fish, immunoreactive (ir) intracellular APP occurred in 18 of 23 brain regions. During sexual maturation and aging, the number of neurons expressing APP increased in 11 of these APP-ir regions. A beta-ir was absent in IM fish, present in seven regions in MA fish, moderately abundant in 15 regions in SM fish, and was most abundant in all brain regions of SP fish exhibiting A beta-ir. Intracellular APP-ir was observed in brain regions involved in sensory integration, olfaction, vision, stress responses, reproduction, and coordination. Intra- and extracellular A beta(1-42) immunoreactivity (A beta-ir) was present in all APP-ir regions except the nucleus lateralis tuberis (hypothalamus) and Purkinje cells (cerebellum). APP-ir and A beta deposition increase during aging. APP-ir is present in IM fish; A beta-ir usually appears first in MA or SM fish and increases in SM fish as does APP-ir. Extracellular A beta deposition dramatically increases between SM and SP stages (1-2 weeks) in all fish, indicating an extremely rapid and synchronized process. Rapid senescence observed in pacific salmon could make them a useful model to investigate timing of amyloidosis and neurodegeneration during brain aging.

Expression of cytokine genes and increased nuclear factor-kappa B activity in the brains of scrapie-infected mice

A number of aspects of the pathogenesis of scrapie remain to be elucidated. The cellular and molecular aspects of the neuropathology in scrapie suggest the possibility that the proinflammatory cytokines could act as pathogenic mediators in this neurodegenerative disease. To understand this possibility, we examined the expression of proinflammatory cytokine genes in brains of IM mice-infected with 87V scrapie agent. Additionally, we also analyzed the activity of nuclear factor-kappa B (NF-kappaB), which is the major transcriptional activator for inflammatory cytokines, and formation of reactive oxygen species (ROS) as a common upstream messenger for its activation. The induction of mRNAs of the inflammatory cytokines, IL-1alpha, IL-1beta and TNF-alpha, was detected only in the brains of scrapie-infected mice. The activity of NF-kappaB was significantly increased in the nuclear extracts from brains of the scrapie-infected group and the immunoreactivity of NF-kappaB was increased in the hippocampus and thalamus in the brains of scrapie-infected mice. The NF-kappaB immunoreactivity was observed mainly in GFAP-positive astrocytes and also detected in the PrP-amyloid plaques in the brains of 87V scrapie-infected mice. Gene expression of IL-6 and iNOS, the representative target genes for NF-kappaB activation, were activated only in the infected group. The production of ROS was significantly increased in the brain mitochondrial fractions of scrapie-infected mice. These results suggest that prion accumulation in astrocytes might activate NF-kappaB through the increase of ROS generation, and thus alterations in NF-kappaB-directed gene expression may contribute to both the neurodegeneration and proinflammatory responses which occur in scrapie.

Toxicity of human THP-1 monocytic cells towards neuron-like cells is reduced by non-steroidal anti-inflammatory drugs (NSAIDs)

There is mounting evidence that inflammatory processes, including activation of microglia, are upregulated in Alzheimer's disease. The importance of this phenomenon is indicated by multiple epidemiological studies showing that patients taking non-steroidal anti-inflammatory drugs (NSAIDs) have a substantially reduced prevalence of Alzheimer's disease. The pharmacological actions of anti-inflammatory drugs in brain are still uncertain. As a step towards identifying key pharmacological targets, we developed a neurotoxicity assay based on the property of supernatant media from stimulated human monocytic THP-1 cells to cause human neuroblastoma cell death. Similar neurotoxicity was observed when postmortem human microglia were substituted for THP-1 cells, establishing the validity of the assay for simulating neurotoxicity in human brain. A combination of lipopolysaccharide and interferon-gamma was used to activate the THP-1 cells. NSAIDs were effective in inhibiting neurotoxicity by this assay, while steroidal anti-inflammatories and propentofylline had no effect. The neuroprotective potency of NSAIDs appeared to be unrelated to their selective ability to inhibit cyclooxygenase-1 (COX-1) or cyclooxygenase-2 (COX-2). It is suggested that inhibition of monocyte cytotoxicity might be responsible for the apparent beneficial effects of NSAIDs in Alzheimer's disease.

Beta-secretase processing of the beta-amyloid precursor protein in transgenic mice is efficient in neurons but inefficient in astrocytes

Alzheimer's disease is characterized by the extracellular deposition of beta-amyloid peptide (Abeta) in cerebral plaques and evidence is accumulating that amyloid is neurotoxic. Abeta is derived from the beta-amyloid precursor protein (APP). Proteolytic processing of APP by the enzyme, beta-secretase, produces the N terminus of Abeta, and releases a secreted ectodomain of APP (beta-s-APP). To develop animal models for measuring beta-secretase activity in specific brain cells in vivo, we have targeted the expression of the full-length human APP to either neurons or astrocytes in transgenic mice using the neuron- specific enolase (NSE) promoter or a modified glial fibrillary acidic protein (GFAP) gene, respectively. The APP cDNAs expressed were mutated (KM to NL at 670/671) to encode amino acid substitutions that enhance amyloidogenic processing in vitro. Western analyses revealed abundant production of beta-s-APP in the brains of NSE-APP mice and enzyme-linked immunosorbent assay analyses showed production of Abeta in fetal primary mixed brain cultures and brain homogenates from these transgenic animals. Because the NSE promoter drives expression primarily in neurons, this provides in vivo evidence that the beta-secretase cleavage necessary for generation of beta-s-APP and Abeta is efficiently performed in neurons. In contrast, only little beta-s-APP was detected in brain homogenates of GFAP-APP mice, indicating that astrocytes show very little beta-secretase activity in vivo. This provides strong in vivo evidence that the major source of Abeta in brain is from neurons and not from astrocytes.

Expression of Kunitz protease inhibitor--containing forms of amyloid beta-protein precursor within vascular thrombi

BACKGROUND

The presence of patent neovessels within vascular occlusions in chronic thromboembolic pulmonary hypertension suggests that local mechanisms exist to regulate the coagulation system. This study investigated the expression of a potent inhibitor of Factor IXa and Factor XIa (ie, protease nexin-2/ amyloid beta-protein precursor, A beta PP) in the organized vascular occlusions harvested from patients with this disease.

METHODS AND RESULTS

Immunohistochemical analysis revealed intense immunoreactivity for A beta PP in the single layer of cells that line the neovessels. A positive signal was also detected by in situ hybridization analysis with the use of a 35S-UTP-labeled antisense riboprobe that recognizes the various alternatively spliced mRNA forms of this molecule. To identify the forms of A beta PP produced within the thrombi, total RNA was extracted from the thrombi, reverse transcribed, and subjected to amplification with the use of the polymerase chain reaction (PCR) and primers that flank the region encoding the alternatively spliced 56-amino acid Kunitz-type protease inhibitor (KPI) domain. The major PCR products consisted of 255 bp and 312 bp and corresponded to transcripts encoding this domain (ie, A beta PP751 and A beta PP770). In situ hybridization analysis with the use of a 35S-UTP-labeled antisense riboprobe complementary to the region encoding the KPI domain confirmed the presence of these mRNA species in nucleated cells lining the neovessels.

CONCLUSIONS

The expression of KPI-containing isoforms of A beta PP in thrombus endothelial cells may represent one mechanism utilized in this disease to shift the local hemostatic balance and preserve regional vessel patency.

Diagnosis of Alzheimer's disease: defining genetic profiles (genotype vs phenotype)

The early identification of Alzheimer's disease (AD) requires a multifactorial diagnostic approach. Components of a comprehensive assessment include a clinical examination, neuropsychological and psychometric evaluations, biochemical tests, cardiovascular and radiological examinations, neuroimaging, brain bioelectrical activity mapping, an evaluation of brain hemodynamics, and assessment of biological markers. Genetic testing should be incorporated into the diagnostic protocol only for research purposes and risk evaluation. The genetic characterization of familial AD genotypes (FAD-21, FAD-14, FAD-1, FAD-19, APP-21m) can help define the phenotypic profiles of AD clinical subtypes. The correlation of genotypic and phenotypic profiles might have a predictive value in terms of AD diagnosis and therapeutic responses to particular drugs. However, available genetic markers (APP-21m, APO-E) are not yet conclusive for diagnostic purposes. In contrast, AD-related Apo-E genotypes appear to correlate with defined AD phenotypes. The presence of an Apo-E4 allele seems to represent an important risk factor for dementia.

A decrease in serum sialyltransferase levels in Alzheimer's disease

A reliable antemortem serum marker for Alzheimer's disease (AD) would be of great importance for the early detection and subsequent therapeutic management of the disease. We have noted a significant decrease in serum levels of the soluble form of the sialyltransferase enzyme in a group of AD patients when compared with both age-matched elderly (over 60 years) and young (under 60 years) controls. In a population of Down's syndrome patients, who develop AD pathology with increasing age, there was an age-related decrease in serum sialyltransferase activity in patients from 20 to 60 years to approach enzyme levels similar to those observed in the AD group. This significant decrease in serum sialyltransferase levels observed may both prove a useful peripheral early biochemical marker of neurodegeneration and provide an indication of the underlying cellular events that occur during the process of nerve cell death in AD.