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SYNOPSIS

Decline in cognitive functions that accompany aging in dogs may have a biological basis, and many of the disorders associated with aging in canines may be mitigated through dietary modifications that incorporate specific nutraceuticals. Based on previous research and the results of both laboratory and clinical studies – antioxidants may be one class of nutraceutical that provides benefits to aged dogs. Brains of aged dogs accumulate oxidative damage to proteins and lipids, which may lead to dysfunction of neuronal cells. The production of free radicals and lack of increase in compensatory antioxidant enzymes may lead to detrimental modifications to important macromolecules within neurons. Reducing oxidative damage through food ingredients rich in a broad spectrum of antioxidants significantly improves, or slows the decline of, learning and memory in aged dogs. However, determining all effective compounds and combinations, dosage ranges, as well as when to initiate intervention and long term effects constitute gaps in our current knowledge.

Aging and age-related disorders such as Alzheimer’s disease (AD) are usually accompanied by oxidative stress as one of the main mechanisms contributing to neurodegeneration and cognitive decline. Aging canines develop cognitive dysfunction and neuropathology similar to those seen in humans, and the use of antioxidants results in reductions in oxidative damage and in improvement in cognitive function in this canine model of human aging. In the present study, the effect of a long-term treatment with an antioxidant fortified diet and a program of behavioral enrichment on oxidative damage was studied in aged canines. To identify the neurobiological mechanisms underlying these treatment effects, the parietal cortex from 23 beagle dogs (8.1-12.4 years) were treated for 2.8 years in one of four treatment groups: i.e., control food- control behavioral enrichment (CC); control food - behavioral enrichment (CE); antioxidant food-control behavioral enrichment (CA); and enriched environment - antioxidant fortified food (EA). We analyzed the levels of the oxidative stress biomarkers, i.e., protein carbonyls, 3-nitrotyroine (3NT), and the lipid peroxidation product, 4-hydroxynonenal (HNE), and observed a decrease in their levels on all treatments when compared to control, with the most significant effects found in the combined treatment, EA. Since EA treatment was most effective, we also carried out a comparative proteomics study to identify specific brain proteins that were differentially expressed and used a parallel redox proteomics approach to identify specific brain proteins that were less oxidized following EA. The specific protein carbonyl levels of glutamate dehydrogenase [NAD (P)], glyceraldehyde-3-phosphate dehydrogenase (GAPDH), α-enolase, neurofilament triplet L protein, glutathione S-transferase (GST) and fascin actin bundling protein were significantly reduced in brain of EA-treated dogs compared to control. We also observed significant increases in expression of Cu/Zn superoxide dismutase, fructose-bisphosphate aldolase C, creatine kinase, glutamate dehydrogenase and glyceraldehyde-3-phosphate dehydrogenase. The increased expression of these proteins and in particular Cu/Zn SOD correlated with improved cognitive function. In addition, there was a significant increase in the enzymatic activities of glutathione-S-transferase (GST) and total superoxide dismutase (SOD), and significant increase in the protein levels of heme oxygenase (HO-1) in EA treated dogs compared to control. These findings suggest that the combined treatment reduces the levels of oxidative damage and improves the antioxidant reserve systems in the aging canine brain, and may contribute to improvements in learning and memory. These observations provide insights into a possible neurobiological mechanism underlying the effects of the combined treatment. These results support the combination treatments as a possible therapeutic approach that could be translated to the aging human population who are at risk for age-related neurodegenerative disorders, including Alzheimer’s disease.

Alzheimer's disease (AD) is characterized by cognitive impairment, progressive neurodegeneration and formation of amyloid-β (Aβ)-containing plaques and neurofibrillary tangles composed of hyperphosphorylated tau. The neurodegenerative process in AD is initially characterized by synaptic damage accompanied by neuronal loss. In addition, recent evidence suggests that alterations in adult neurogenesis in the hippocampus might play a role. Synaptic loss is one of the strongest correlates to the cognitive impairment in patients with AD. Several lines of investigation support the notion that the synaptic pathology and defective neurogenesis in AD are related to progressive accumulation of Aβ oligomers rather than fibrils. Abnormal accumulation of Aβ resulting in the formation of toxic oligomers is the result of an imbalance between the levels of Aβ production, aggregation and clearance. Aβ oligomers might lead to synaptic damage by forming pore-like structures with channel activity; alterations in glutamate receptors; circuitry hyper-excitability; mitochondrial dysfunction; lysosomal failure and alterations in signaling pathways related to synaptic plasticity, neuronal cell and neurogenesis. A number of signaling proteins, including fyn kinase; glycogen synthase kinase-3β (GSK3β) and cyclin-dependent kinase-5 (CDK5), are involved in the neurodegenerative progression of AD. Therapies for AD might require the development of anti-aggregation compounds, pro-clearance pathways and blockers of hyperactive signaling pathways.

Alzheimer's disease (AD) involves a complex pathological cascade thought to be initially triggered by the accumulation of β-amyloid (Aβ) peptide aggregates or aberrant amyloid precursor protein (APP) processing. Much is known of the factors initiating the disease process decades prior to the onset of cognitive deficits, but an unclear understanding of events immediately preceding and precipitating cognitive decline is a major factor limiting the rapid development of adequate prevention and treatment strategies. Multiple pathways are known to contribute to cognitive deficits by disruption of neuronal signal transduction pathways involved in memory. These pathways are altered by aberrant signaling, inflammation, oxidative damage, tau pathology, neuron loss, and synapse loss. We need to develop stage-specific interventions that not only block causal events in pathogenesis (aberrant tau phosphorylation, Aβ production and accumulation, and oxidative damage), but also address damage from these pathways that will not be reversed by targeting prodromal pathways. This approach would not only focus on blocking early events in pathogenesis, but also adequately correct for loss of synapses, substrates for neuroprotective pathways (e.g., docosahexaenoic acid), defects in energy metabolism, and adverse consequences of inappropriate compensatory responses (aberrant sprouting). Monotherapy targeting early single steps in this complicated cascade may explain disappointments in trials with agents inhibiting production, clearance, or aggregation of the initiating Aβ peptide or its aggregates. Both plaque and tangle pathogenesis have already reached AD levels in the more vulnerable brain regions during the “prodromal” period prior to conversion to “mild cognitive impairment (MCI).” Furthermore, many of the pathological events are no longer proceeding in series, but are going on in parallel. By the MCI stage, we stand a greater chance of success by considering pleiotropic drugs or cocktails that can independently limit the parallel steps of the AD cascade at all stages, but that do not completely inhibit the constitutive normal functions of these pathways. Based on this hypothesis, efforts in our laboratories have focused on the pleiotropic activities of omega-3 fatty acids and the anti-inflammatory, antioxidant, and anti-amyloid activity of curcumin in multiple models that cover many steps of the AD pathogenic cascade (Cole and Frautschy, Alzheimers Dement 2:284–286, 2006).

Vascular dysfunction has a critical role in Alzheimer’s disease (AD). Recent data from brain imaging studies in humans and animal models suggest that cerebrovascular dysfunction may precede cognitive decline and onset of neurodegenerative changes in AD and AD models. Cerebral hypoperfusion and impaired amyloid β-peptide (Aβ) clearance across the blood–brain barrier (BBB) may contribute to the onset and progression of dementia AD type. Decreased cerebral blood flow (CBF) negatively affects the synthesis of proteins required for memory and learning, and may eventually lead to neuritic injury and neuronal death. Impaired clearance of Aβ from the brain by the cells of the neurovascular unit may lead to its accumulation on blood vessels and in brain parenchyma. The accumulation of Aβ on the cerebral blood vessels, known as cerebral amyloid angiopathy (CAA), is associated with cognitive decline and is one of the hallmarks of AD pathology. CAA can severely disrupt the integrity of the blood vessel wall resulting in micro or macro intracerebral bleedings that exacerbates neurodegenerative process and inflammatory response and may lead to hemorrhagic stroke, respectively.Here, we review the role of the neurovascular unit and molecular mechanisms in vascular cells behind AD and CAA pathogenesis. First, we discuss apparent vascular changes, including the cerebral hypoperfusion and vascular degeneration that contribute to different stages of the disease process in AD individuals. We next discuss the role of the low-density lipoprotein receptor related protein-1 (LRP), a key Aβ clearance receptor at the BBB and along the cerebrovascular system, whose expression is suppressed early in AD. We also discuss how brain-derived apolipoprotein E isoforms may influence Aβ clearance across the BBB. We then review the role of two interacting transcription factors, myocardin and serum response factor, in cerebral vascular cells in controlling CBF responses and LRP-mediated Aβ clearance. Finally, we discuss the role of microglia and perivascular macrophages in Aβ clearance from the brain. The data reviewed here support an essential role of neurovascular and BBB mechanisms in contributing to both, onset and progression of AD.

Background

Advanced glycation end-products (AGEs) and their receptor (RAGE) occur in dementia of the Alzheimer's type and diabetic microvascular disease. Accumulation of AGEs relates to risk factors for vascular dementia with ageing, including hypertension and diabetes. Cognitive dysfunction in vascular dementia may relate to microvascular disease resembling that in diabetes. We tested if, among people with cerebrovascular disease, (1) those with dementia have higher levels of neuronal and vascular AGEs and (2) if cognitive dysfunction depends on neuronal and/or vascular AGE levels.

Methods

Brain Sections from 25 cases of the OPTIMA (Oxford Project to Investigate Memory and Ageing) cohort, with varying degrees of cerebrovascular pathology and cognitive dysfunction (but only minimal Alzheimer type pathology) were immunostained for Nε-(carboxymethyl)-lysine (CML), the most abundant AGE. The level of staining in vessels and neurons in the cortex, white matter and basal ganglia was compared to neuropsychological and other clinical measures.

Results

The probability of cortical neurons staining positive for CML was higher in cases with worse cognition (p = 0.01) or a history of hypertension (p = 0.028). Additionally, vascular CML staining related to cognitive impairment (p = 0.02) and a history of diabetes (p = 0.007). Neuronal CML staining in the basal ganglia related to a history of hypertension (p = 0.002).

Conclusion

CML staining in cortical neurons and cerebral vessels is related to the severity of cognitive impairment in people with cerebrovascular disease and only minimal Alzheimer pathology. These findings support the possibility that cerebral accumulation of AGEs may contribute to dementia in people with cerebrovascular disease.

Morphological alterations of mitochondria may play an important role in the pathogenesis of Alzheimer's disease, been associated with oxidative stress and Aβ-peptide-induced toxicity. We proceeded to estimation of mitochondria on electron micrographs of autopsy specimens of Alzheimer's disease. We found substantial morphological and morphometric changes of the mitochondria in the neurons of the hippocampus, the neocortex, the cerebellar cortex, the thalamus, the globus pallidus, the red nucleus, the locus coeruleus, and the climbing fibers. The alterations consisted of considerable changes of the cristae, accumulation of osmiophilic material, and modification of the shape and size. Mitochondrial alterations were prominent in neurons, which showed a depletion of dendritic spines and loss of dendritic branches. Mitochondrial alterations are not related with the accumulation of amyloid deposits, but are prominent whenever fragmentation of the Golgi apparatus exists. Morphometric analysis showed also that mitochondria are significantly reduced in neurons, which demonstrated synaptic pathology.

Alzheimer's disease (AD) is a progressive chronic disorder and is characterized by β-amyloid plaques and angiopathy, tau pathology, neuronal cell death, and inflammatory responses. The reasons for this disease are not known. This review proposes the hypothesis that a chronic mild longlasting cerebrovascular dysfunction could initiate a cascade of events leading to AD. It is suggested that (vascular) risk factors (e.g. hypercholesterolemia, type 2 diabetes, hyperhomocysteinemia) causes either damage of the cerebrovascular system including silent strokes or causes dysregulation of beta-amyloid clearance at the blood-brain barrier resulting in increased brain beta-amyloid. A cascade of subsequent downstream events may lead to disturbed metabolic changes, and neuroinflammation and tau pathology. The role of NGF on the cell death of cholinergic neurons is discussed. Additional risk factors (e.g. acidosis, metals) contribute to plaque development.

Research Highlights

► Hypothesis for development of Alzheimers disease. ► Chronic mild vascular risk factors damage brain capillaries. ► Dysregulation of beta-amyloid clearance at the blood-brain barrier. ► Vascular dysfunction causes metabolic disturbances. ► Downstream cascade causes inflammation, oxiditative stress and neurodegeneration.

Neuron loss within the hippocampus and entorhinal cortex occurs as a function of age in humans. We first tested the hypothesis that neuron loss occurs in the aged dog. The total unilateral number of neurons in the canine entorhinal cortex and subdivisions of the hippocampus from the left hemisphere were estimated using the optical fractionator. The brains from 5 old (13.0 – 15.0 years old) and 5 young (3.4 – 4.5 years old) beagle dogs were analyzed. The hilus of the hippocampus showed a significant loss of neurons (~30%) in the aged dog brain compared to young. Differences were not detected in the remaining hippocampal subfields and entorhinal cortex. We further tested the hypothesis that an antioxidant fortified food or behavioral enrichment would reduce the age-related loss of hilar neurons. Behaviorally enriched aged dogs had more neurons in the hilus (~18%) compared to aged controls. These results suggest that the aged canine hippocampus in the left hemisphere shows selective neuron loss and that behavioral enrichment may reduce this loss.

Mitochondria continuously undergo two opposing processes, fission and fusion. The disruption of this dynamic equilibrium may herald cell injury or death and may contribute to developmental and neurodegenerative disorders. Nitric oxide functions as a signaling molecule, but in excess it mediates neuronal injury, in part via mitochondrial fission or fragmentation. However, the underlying mechanism for nitric oxide–induced pathological fission remains unclear. We found that nitric oxide produced in response to β-amyloid protein, thought to be a key mediator of Alzheimer’s disease, triggered mitochondrial fission, synaptic loss, and neuronal damage, in part via S-nitrosylation of dynamin-related protein 1 (forming SNO-Drp1). Preventing nitrosylation of Drp1 by cysteine mutation abrogated these neurotoxic events. SNO-Drp1 is increased in brains of human Alzheimer’s disease patients and may thus contribute to the pathogenesis of neurodegeneration.

The pathological processes of neurodegenerative disorders such as Alzheimer's and Parkinson's diseases engender synaptic and neuronal cell damage. While mild oxidative and nitrosative (nitric oxide (NO)-related) stress mediates normal neuronal signaling, excessive accumulation of these free radicals is linked to neuronal cell injury or death. In neurons, N-methyl--aspartate (NMDA) receptor (NMDAR) activation and subsequent Ca2+ influx can induce the generation of NO via neuronal NO synthase. Emerging evidence has demonstrated that S-nitrosylation, representing covalent reaction of an NO group with a critical protein thiol, mediates the vast majority of NO signaling. Analogous to phosphorylation and other posttranslational modifications, S-nitrosylation can regulate the biological activity of many proteins. Here, we discuss recent studies that implicate neuropathogenic roles of S-nitrosylation in protein misfolding, mitochondrial dysfunction, synaptic injury, and eventual neuronal loss. Among a growing number of S-nitrosylated proteins that contribute to disease pathogenesis, in this review we focus on S-nitrosylated protein-disulfide isomerase (forming SNO-PDI) and dynamin-related protein 1 (forming SNO-Drp1). Furthermore, we describe drugs, such as memantine and newer derivatives of this compound that can prevent both hyperactivation of extrasynaptic NMDARs as well as downstream pathways that lead to nitrosative stress, synaptic damage, and neuronal loss.

While a massive and progressive neuronal loss in specific areas such as the hippocampus and cortex unequivocally underlies cognitive deterioration and memory loss in Alzheimer's disease, noteworthy alterations take place in the neurogenic microenvironments, namely, the subgranule layer of the dentate gyrus and the subventricular zone. Compromised neurogenesis presumably takes place earlier than onset of hallmark lesions or neuronal loss, and may play a role in the initiation and progression of neuropathology in Alzheimer's disease. Neurogenesis in the adult brain is thought to play a role in numerous forms and aspects of learning and memory and contribute to the plasticity of the hippocampus and olfactory system. Misregulated or impaired neurogenesis on the other hand, may compromise plasticity and neuronal function in these areas and exacerbate neuronal vulnerability. Interestingly, increasing evidence suggests that molecular players in Alzheimer's disease, including PS1, APP and its metabolites, play a role in adult neurogenesis. In addition, recent studies suggest that alterations in tau phosphorylation are pronounced in neurogenic areas, and may interfere with the potential central role of tau proteins in neuronal maturation and differentiation. On the other hand, numerous neurogenic players, such as Notch-1, ErbB4 and L1 are substrates of α- β- and γ- secretase that play a major role in Alzheimer's disease. This review will discuss current knowledge concerning alterations of neurogenesis in Alzheimer's disease with specific emphasis on the cross-talk between signaling molecules involved in both processes, and the ways by which familial Alzheimer's disease-linked dysfunction of these signaling molecules affect neurogenesis in the adult brain.

Niemann-Pick type C (NPC) disease is an autosomal recessive neurodegenerative disorder characterized by intracellular accumulation of cholesterol and glycosphingolipids in many tissues including the brain. The disease is caused by mutations of either NPC1 or NPC2 gene and is accompanied by a severe loss of neurons in the cerebellum, but not in the hippocampus. NPC pathology exhibits some similarities with Alzheimer’s disease, including increased levels of amyloid β (Aβ)-related peptides in vulnerable brain regions, but very little is known about the expression of amyloid precursor protein (APP) or APP secretases in NPC disease. In the present study, we evaluated age-related alterations in the level/distribution of APP and its processing enzymes, β- and γ-secretases, in the hippocampus and cerebellum of Npc1−/− mice, a well-established model of NPC pathology. Our results show that levels and expression of APP and β-secretase are elevated in the cerebellum prior to changes in the hippocampus, whereas γ-secretase components are enhanced in both brain regions at the same time in Npc1−/− mice. Interestingly, a subset of reactive astrocytes in Npc1−/− mouse brains expresses high levels of APP as well as β- and γ-secretase components. Additionally, the activity of β-secretase is enhanced in both the hippocampus and cerebellum of Npc1−/− mice at all ages, while the level of C-terminal APP fragments is increased in the cerebellum of 10-week-old Npc1−/− mice. These results, taken together, suggest that increased level and processing of APP may be associated with the development of pathology and/or degenerative events observed in Npc1−/− mouse brains.

The neuropathological hallmarks of Alzheimer disease (AD) include “positive” lesions such as amyloid plaques and cerebral amyloid angiopathy, neurofibrillary tangles, and glial responses, and “negative” lesions such as neuronal and synaptic loss. Despite their inherently cross-sectional nature, postmortem studies have enabled the staging of the progression of both amyloid and tangle pathologies, and, consequently, the development of diagnostic criteria that are now used worldwide. In addition, clinicopathological correlation studies have been crucial to generate hypotheses about the pathophysiology of the disease, by establishing that there is a continuum between “normal” aging and AD dementia, and that the amyloid plaque build-up occurs primarily before the onset of cognitive deficits, while neurofibrillary tangles, neuron loss, and particularly synaptic loss, parallel the progression of cognitive decline. Importantly, these cross-sectional neuropathological data have been largely validated by longitudinal in vivo studies using modern imaging biomarkers such as amyloid PET and volumetric MRI.

Amyloid plaque build-up occurs primarily before the onset of cognitive deficits. Neurofibrillary tangles, neuron loss, and synaptic loss parallel the progression of cognitive decline.

The brains of individuals, who are cognitively normal, show age-related changes that include an overall reduction in brain volume and weight, which are associated with gyral atrophy and widening of the sulci of the cerebral cortex, and enlargement of the brain ventricles. These changes are partly the result of nerve cell loss but accurate estimates of neuronal loss are notoriously difficult to make. Microscopically, there are increasing amounts of the age-related pigment, lipofuscin, granulovacuolar degeneration in neurones, Hirano bodies, variable amounts of diffuse deposits of beta-amyloid in the parenchyma, the presence of neurofibrillary tangles mainly confined to the hippocampus and amygdala, and sparse numbers of senile plaques in these brain regions and also in other cortical areas. Of these changes, neurofibrillary tangles and senile plaques are the neuropathological hallmark of Alzheimer's disease in which they are more abundant and widespread. Alzheimer's disease has therefore been regarded as accelerated brain ageing; however, the realization that there is a strong genetic contribution to developing the disease at least implies that it may not be the inevitable, even if frequent, consequence of old age. Understanding the molecular basis of plaque and tangle formation is advancing greatly and is the main focus of research into the cellular and molecular changes observed in the ageing brain.

Thirty two domestic dogs with naturally occurring polyarthritis were investigated to determine the contribution of autoimmunity in the pathological mechanisms. Comparisons were made with canine infective arthritis (12 dogs), osteoarthritis (32), and osteoarthritis secondary to rupture of the cranial cruciate ligament (19). Rheumatoid factors, immune complexes, and complement fixation (C1q binding) were measured in sera and synovial fluids. Compared with normal dogs (32), dogs with rheumatoid arthritis (RA) had increased serum and synovial fluid immune complexes and rheumatoid factors. Increases were generally also seen in dogs with other arthropathies, however. Rheumatoid factors were higher in sera than in synovial fluids. Rheumatoid factors correlated with immune complex levels and complexed rheumatoid factor only in the group of dogs with RA. Both rheumatoid factors and immune complexes may contribute to the pathogenesis of canine RA but are considered to arise as a result of non-specific inflammatory mechanisms in the non-rheumatoid groups.

The present review focuses on the utility of a canine model in evaluating nutritional interventions for age-related cognitive dysfunction. Aged dogs demonstrate progressive cognitive decline with concurrent amyloid-beta pathology that parallels the pathology observed in aging humans. Dogs, therefore, provide a natural model of human pathological aging. We have and are in the process of evaluating several nutritional-based interventions aimed at preventing cognitive decline and brain aging. In a three-year longitudinal study, we examined the effects of a diet enriched with antioxidants and mitochondrial cofactors on several measures of cognition and brain aging. Compared to controls, aged dogs on the enriched diet demonstrated both short- and long-term cognitive benefits, as well decreased deposition of amyloid-beta protein. The diet also reduced behavioral signs associated with canine Cognitive Dysfunction Syndrome when assessed in veterinary clinical trials. We also have preliminary evidence suggesting a beneficial effect of a proprietary blend of docosahexaenoic acid and phospholipids on both cognitive and physiological measures. Collectively, our data indicate (1) that the dog, either in the laboratory or in the clinic, provides an important tool for assessing nutritional interventions and (2) that combination interventions aimed at several mechanisms of pathological aging may prove more effective than single nutritive components in human trials.

The cellular mechanisms underlying neuronal loss and neurodegeneration have been an area of interest in the last decade. Although neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD) and Huntington’s disease (HD) each have distinct clinical symptoms and pathologies, they all share common mechanisms such as protein aggregation, oxidative injury, inflammation, apoptosis and mitochondrial injury that contribute to neuronal loss. Although cerebrovascular disease is due to etiologies quite different from the neurodegenerative disorders, many of the same common disease mechanisms come into play following a stroke. Novel therapies that target each of these mechanisms may be effective in decreasing the risk of disease, abating symptoms or slowing down their progression. While most of these therapies are experimental, and require further investigation, a few seem to offer promise in the near future.

Brain atrophy caused by neuronal loss is a prominent pathological feature of Alzheimer’s disease (AD). Amyloid β (Aβ), the major component of senile plaques, is considered to play a central role in neuronal cell death. In addition to removal of the toxic Aβ, direct suppression of neuronal loss is an essential part of AD treatment; however, no such neuroprotective therapies have been developed. Excess amount of Aβ evokes multiple cytotoxic mechanisms, involving increase of the intracellular Ca2+ level, oxidative stress, and receptor-mediated activation of cell-death cascades. Such diversity in cytotoxic mechanisms induced by Aβ clearly indicates a complex nature of the AD-related neuronal cell death. We have identified a 24-residue peptide, Humanin (HN), which suppresses in vitro neuronal cell death caused by all AD-related insults, including Aβ, so far tested. The anti-AD effect of HN has been further confirmed in vivo using mice with Aβ-induced amnesia. Altogether, such potent neuroprotective activity of HN against AD-relevant cytotoxicity both in vitro and in vivo suggests the potential clinical applications of HN in novel AD therapies aimed at controlling neuronal death.

Stress induces aggregation of RNA-binding proteins to form inclusions, termed stress granules (SGs). Recent evidence suggests that SG proteins also colocalize with neuropathological structures, but whether this occurs in Alzheimer’s disease is unknown. We examined the relationship between SG proteins and neuropathology in brain tissue from P301L Tau transgenic mice, as well as in cases of Alzheimer’s disease and FTDP-17. The pattern of SG pathology differs dramatically based on the RNA-binding protein examined. SGs positive for T-cell intracellular antigen-1 (TIA-1) or tristetraprolin (TTP) initially do not colocalize with tau pathology, but then merge with tau inclusions as disease severity increases. In contrast, G3BP (ras GAP-binding protein) identifies a novel type of molecular pathology that shows increasing accumulation in neurons with increasing disease severity, but often is not associated with classic markers of tau pathology. TIA-1 and TTP both bind phospho-tau, and TIA-1 overexpression induces formation of inclusions containing phospho-tau. These data suggest that SG formation might stimulate tau pathophysiology. Thus, study of RNA-binding proteins and SG biology highlights novel pathways interacting with the pathophysiology of AD, providing potentially new avenues for identifying diseased neurons and potentially novel mechanisms regulating tau biology.

Alzheimer’s disease (AD) neuropathology is characterized by loss of synapses and neurons, neuritic plaques consisting of β-amyloid (Aβ) peptides, and neurofibrillary tangles consisting of intracellular aggregates of hyperphosphorylated tau protein in susceptible brain regions. Aβ oligomers trigger a cascade of pathogenic events including tau hyperphosphorylation and aggregation, inflammatory reactions, and excitotoxicity that contribute to the progression of AD. The molecular chaperone Hsp90 facilitates the folding of newly synthesized and denatured proteins and is believed to play a role in neurodegenerative disorders in which the defining pathology results in misfolded proteins and the accumulation of protein aggregates. Some agents that inhibit Hsp90 protect neurons against Aβ toxicity and tau aggregation, and assays for rapidly screening potential Hsp90 inhibitors are of interest. We used the release of the soluble cytosolic enzyme lactate dehydrogenase (LDH) as an indicator of the loss of cell membrane integrity and cytotoxicity resulting from exposure to Aβ peptides to evaluate the neuroprotective properties of novel novobiocin analogues and established Hsp90 inhibitors. Compounds were assessed for potency in protecting proliferating and differentiated SH-SY5Y neuronal cells against Aβ-induced cell death; the potential of each agent alone was also determined. The data indicated that several of the compounds decreased Aβ toxicity even at low nanomolar concentrations and, unexpectedly, were more potent in protecting the undifferentiated cells against Aβ. The novobiocin analogues alone were not toxic even up to 10 μM concentrations whereas GDA and the parent compound, novobiocin, were toxic at 1 and 10 μM, respectively. The results suggest that novobiocin analogues may provide novel leads for the development of neuroprotective drugs.

Aged dogs and humans share complex cognitive and pathological responses to aging. Specifically, dogs develop Alzheimer’s Disease (AD) like beta-amyloid (Aβ) that are associated with cognitive deficits. Currently, therapeutic approaches to prevent AD are targeted towards reduced production, aggregation and increased clearance of Aβ. The current review discusses cognition and neuropathology of the aging canine model and how it has and continues to be useful in further understanding the safety and efficacy of potential AD prevention therapies targeting Aβ.

Cerebral hypometabolism and amyloid accumulation are principal neuropathological manifestations of Alzheimer’s disease (AD). Whether and how brain/neuronal activity might modulate certain pathological process of AD are interesting topics of recent clinical and basic research in the field, and may be of potential medical relevance in regard to both the disease etiology and intervention. Using the Tg2576 transgenic mouse model of AD, this study characterized a promotive effect of neuronal hypoactivity associated with functional deprivation on amyloid plaque pathogenesis in the olfactory pathway. Unilateral naris-occlusion caused BACE1 elevation in neuronal terminals in the deprived relative to the non-deprived bulb and piriform cortex in young adult mice. In parallel with the overall age-related plaque development in the forebrain, locally-increased BACE1 immunoreactivity co-occurred with amyloid deposition first in the piriform cortex then within the bulb, more prominent on the deprived relative to the non-deprived side. Biochemical analyses confirmed elevated BACE1 protein levels, enzymatic activity and products in the deprived relative to non-deprived bulbs. Plaque-associated BACE1 immunoreactivity in the bulb and piriform cortex was localized preferentially to swollen/sprouting glutamatergic axonal terminals, with Aβ immunoreactivity occurred inside as well as around these terminals. Together, these findings suggest that functional deprivation or neuronal hypoactivity facilitates amyloid plaque formation in the forebrain in a transgenic model of AD, which operates synergistically with age effect. The data also implicate an intrinsic association of amyloid accumulation and plaque formation with progressive axonal pathology.

Alzheimer’s disease (AD) is characterized by progressive neurodegeneration and cerebral accumulation of the β-amyloid peptide (Aβ), but it is unknown what makes neurons susceptible to degeneration. We report that the TGF-β type II receptor (TβRII) is mainly expressed by neurons, and that TβRII levels are reduced in human AD brain and correlate with pathological hallmarks of the disease. Reducing neuronal TGF-β signaling in mice resulted in age-dependent neurodegeneration and promoted Aβ accumulation and dendritic loss in a mouse model of AD. In cultured cells, reduced TGF-β signaling caused neuronal degeneration and resulted in increased levels of secreted Aβ and β-secretase–cleaved soluble amyloid precursor protein. These results show that reduced neuronal TGF-β signaling increases age-dependent neurodegeneration and AD-like disease in vivo. Increasing neuronal TGF-β signaling may thus reduce neurodegeneration and be beneficial in AD.

Loss of synapses and synaptic damage are the best correlates of cognitive decline identified in patients with Alzheimer’s disease (AD), and mitochondrial oxidative damage and synaptic pathology have been identified as early events in the progression of AD. The progressive accumulation of amyloid beta (Aβ) in synapses and synaptic mitochondria are hypothesized to cause synaptic degeneration and cognitive decline in patients with AD. However, the precise mechanistic link between Aβ and mitochondria is not well understood. The purpose of this study was to better understand the effects of Aβ on mitochondrial axonal transport and synaptic alterations in AD. Using mouse hippocampal neurons and Aβ25-35 peptide, we studied axonal transport of mitochondria, including mitochondrial motility, mitochondrial length and size, mitochondrial index per neurite, and synaptic alterations of the hippocampal neurons. In the PBS-treated neurons, 36.4 ± 4.7% of the observed mitochondria were motile, with 21.0 ± 1.3% moving anterograde and 15.4 ± 3.4% moving retrograde and the average speed of movement was 12.1 ± 1.8 μm/min. In contrast, in the Aβ-treated neurons, the number of motile mitochondria were significantly less, at 20.4 ± 2.6% (P<0.032), as were those moving anterograde (10.1 ± 2.6%, P<0.016) relative to PBS-treated neurons, suggesting that the Aβ25-35 peptide impairs axonal transport of mitochondria in AD neurons. In the Aβ-treated neurons, the average speed of motile mitochondria was also less, at 10.9 ±1.9 μm/min, and mitochondrial length was significantly decreased. Further, synaptic immunoreactivity was also significantly less in the Aβ-treated neurons relative to the PBS-treated neurons, indicating that Aβ affects synaptic viability. These findings suggest that, in neurons affected by AD, Aβ is toxic, impairs mitochondrial movements, reduces mitochondrial length, and causes synaptic degeneration.