Mitochondrial perturbations such as oxidative stress, increased fission/fusion dysfunction, and mitophagy are consistent features of Alzheimer's disease (AD), yet the mechanisms that initiate these ...perturbations are unclear. One potential source for mitochondrial defects could be an imbalance in mitochondrial proteostasis. In this regard, studies indicate that a specialized mitochondrial unfolded protein response (mtUPR) is activated upon the aberrant accumulation of damaged or unfolded proteins in the mitochondrial matrix, resulting in the up-regulation of key genes involved in mitochondrial stabilization. To test whether mtUPR activation occurs in AD, we performed real-time quantitative PCR on postmortem frontal cortex samples from subjects classified as sporadic AD, familial AD linked to presenilin-1 mutations, or cognitively intact controls. Compared to controls, sporadic AD subjects exhibited a significant ~40-60% increase in expression levels of select genes activated by the mtUPR, including mitochondrial chaperones dnaja3, hspd1, and hspe1, mitochondrial proteases clpp and yme1l1, and txn2, a mitochondrial-specific oxidoreductase. Furthermore, levels of all six mtUPR genes were significantly up-regulated by ~70-90% in familial AD compared to controls, and these expression levels were significantly higher compared to sporadic AD. The increase in hspd1 (Hsp60) was validated by western blotting. These data support the concept that both sporadic and familial AD are characterized by mtUPR gene activation. Understanding the physiological consequences of this response may provide subcellular mechanistic clues to selective neuronal vulnerability or endogenous compensatory mechanisms during the progression of AD.
Induction of TLR2 activation depends on its association with the adapter protein MyD88. We have found that TLR2 and MyD88 levels are elevated in the hippocampus and cortex of patients with ...Alzheimer's disease (AD) and in a 5XFAD mouse model of AD. Since there is no specific inhibitor of TLR2, to target induced TLR2 from a therapeutic angle, we engineered a peptide corresponding to the TLR2-interacting domain of MyD88 (TIDM) that binds to the BB loop of only TLR2, and not other TLRs. Interestingly, WT TIDM peptide inhibited microglial activation induced by fibrillar Aβ1-42 and lipoteichoic acid, but not 1-methyl-4-phenylpyridinium, dsRNA, bacterial lipopolysaccharide, flagellin, or CpG DNA. After intranasal administration, WT TIDM peptide reached the hippocampus, reduced hippocampal glial activation, lowered Aβ burden, attenuated neuronal apoptosis, and improved memory and learning in 5XFAD mice. However, WT TIDM peptide was not effective in 5XFAD mice lacking TLR2. In addition to its effects in 5XFAD mice, WT TIDM peptide also suppressed the disease process in mice with experimental allergic encephalomyelitis and collagen-induced arthritis. Therefore, selective targeting of the activated status of 1 component of the innate immune system by WT TIDM peptide may be beneficial in AD as well as other disorders in which TLR2/MyD88 signaling plays a role in disease pathogenesis.
The recent failures of potential disease-modifying drugs for Alzheimer’s disease (AD) may reflect the fact that the enrolled participants in clinical trials are already too advanced to derive a ...clinical benefit. Thus, well-validated biomarkers for the early detection and accurate diagnosis of the preclinical stages of AD will be crucial for therapeutic advancement. The combinatorial use of biomarkers derived from biological fluids, such as cerebrospinal fluid (CSF), with advanced molecular imaging and neuropsychological testing may eventually achieve the diagnostic sensitivity and specificity necessary to identify people in the earliest stages of the disease when drug modification is most likely possible. In this regard, positive amyloid or tau tracer retention on positron emission tomography imaging, low CSF concentrations of the amyloid-β 1-42 peptide, high CSF concentrations in total tau and phospho-tau, mesial temporal lobe atrophy on magnetic resonance imaging, and temporoparietal/precuneus hypometabolism or hypoperfusion on 18F-fluorodeoxyglucose positron emission tomography have all emerged as biomarkers for the progression to AD. However, the ultimate AD biomarker panel will likely involve the inclusion of novel CSF and blood biomarkers more precisely associated with confirmed pathophysiologic mechanisms to improve its reliability for detecting preclinical AD. This review highlights advancements in biological fluid and imaging biomarkers that are moving the field towards achieving the goal of a preclinical detection of AD.
Brain‐derived neurotrophic factor (BDNF) is critical for the function and survival of neurons that degenerate in the late stage of Alzheimer's disease (AD). There are two forms of BDNF, the BDNF ...precursor (proBDNF) and mature BDNF, in human brain. Previous studies have shown that BDNF mRNA and protein, including proBDNF, are dramatically decreased in end‐stage AD brain. To determine whether this BDNF decrease is an early or late event during the progression of cognitive decline, we used western blotting to measure the relative amounts of BDNF proteins in the parietal cortex of subjects clinically classified with no cognitive impairment (NCI), mild cognitive impairment (MCI) or mild to moderate AD. We found that the amount of proBDNF decreased 21 and 30% in MCI and AD groups, respectively, as compared with NCI, consistent with our previous results of a 40% decrease in end‐stage AD. Mature BDNF was reduced 34 and 62% in MCI and AD groups, respectively. Thus, the decrease in mature BDNF and proBDNF precedes the decline in choline acetyltransferase activity which occurs later in AD. Both proBDNF and mature BDNF levels were positively correlated with cognitive measures such as the Global Cognitive Score and the Mini Mental State Examination score. These results demonstrate that the reduction of both forms of BDNF occurs early in the course of AD and correlates with loss of cognitive function, suggesting that proBDNF and BDNF play a role in synaptic loss and cellular dysfunction underlying cognitive impairment in AD.
Cell cycle events play a major role in the loss of neurons in advanced Alzheimer's disease (AD). It is currently unknown, however, whether the same is true for the neuronal losses in early disease ...stages. To explore this issue we analyzed brain autopsy material from individuals clinically categorized with mild cognitive impairment (MCI), many if not most of whom will progress to AD. Immunocytochemistry for three cell cycle-related proteins, proliferating cell nuclear antigen, cyclin D, and cyclin B, was performed on sections from hippocampus, basal nucleus of Meynert, and entorhinal cortex. The results obtained from MCI cases were compared with material from individuals diagnosed with AD and those without cognitive impairment. In both hippocampus and basal nucleus, there was a significant percentage of cell cycle immunopositive neurons in the MCI cases. These percentages were similar to those found in the AD cases but significantly higher than non-cognitively impaired controls. In entorhinal cortex, the density of cell cycle-positive neurons was greater in MCI than in AD. However, we observed large variations in the percentages of immunopositive neurons from individual to individual. These findings lend support to the hypothesis that both the mechanism of cell loss (a cell cycle-induced death) and the rate of cell loss (a slow atrophy over several months) are identical at all stages of the AD disease process. The implication of the findings for human clinical trials is discussed.
J. Neurochem. (2010) 113, 649–660.
Degeneration of locus coeruleus (LC) noradrenergic forebrain projection neurons is an early feature of Alzheimer’s disease. The physiological consequences of this ...phenomenon are unclear, but observations correlating LC neuron loss with increased Alzheimer’s disease pathology in LC projection sites suggest that noradrenaline (NA) is neuroprotective. To investigate this hypothesis, we determined that NA protected both hNT human neuronal cultures and rat primary hippocampal neurons from amyloid‐β (Aβ1–42 and Aβ25–35) toxicity. The noradrenergic co‐transmitter galanin was also effective at preventing Aβ‐induced cell death. NA inhibited Aβ25–35‐mediated increases in intracellular reactive oxygen species, mitochondrial membrane depolarization, and caspase activation in hNT neurons. NA exerted its neuroprotective effects in these cells by stimulating canonical β1 and β2 adrenergic receptor signaling pathways involving the activation of cAMP response element binding protein and the induction of endogenous nerve growth factor (NGF) and brain‐derived neurotrophic factor (BDNF). Treatment with functional blocking antibodies for either NGF or BDNF blocked NA’s protective actions against Aβ1–42 and Aβ25–35 toxicity in primary hippocampal and hNT neurons, respectively. Taken together, these data suggest that the neuroprotective effects of noradrenergic LC afferents result from stimulating neurotrophic NGF and BDNF autocrine or paracrine loops via β adrenoceptor activation of the cAMP response element binding protein pathway.
Clinical neuropathologic studies suggest that the selective vulnerability of hippocampal CA1 pyramidal projection neurons plays a key role in the onset of cognitive impairment during the early phases ...of Alzheimer's disease (AD). Disruption of this neuronal population likely affects hippocampal pre- and postsynaptic efficacy underlying episodic memory circuits. Therefore, identifying perturbations in the expression of synaptic gene products within CA1 neurons prior to frank AD is crucial for the development of disease modifying therapies. Here we used custom-designed microarrays to examine progressive alterations in synaptic gene expression within CA1 neurons in cases harvested from the Rush Religious Orders Study who died with a clinical diagnosis of no cognitive impairment (NCI), mild cognitive impairment (MCI, a putative prodromal AD stage), or mild/moderate AD. Quantitative analysis revealed that 21 out of 28 different transcripts encoding regulators of synaptic function were significantly downregulated (1.4–1.8 fold) in CA1 neurons in MCI and AD compared to NCI, whereas synaptic transcript levels were not significantly different between MCI and AD. The downregulated transcripts encoded regulators of presynaptic vesicle trafficking, including synaptophysin and synaptogyrin, regulators of vesicle docking and fusion/release, such as synaptotagmin and syntaxin 1, and regulators of glutamatergic postsynaptic function, including PSD-95 and synaptopodin. Clinical pathologic correlation analysis revealed that downregulation of these synaptic markers was strongly associated with poorer antemortem cognitive status and postmortem AD pathological criteria such as Braak stage, NIA-Reagan, and CERAD diagnosis. In contrast to the widespread loss of synaptic gene expression observed in CA1 neurons in MCI, transcripts encoding β-amyloid precursor protein (APP), APP family members, and regulators of APP metabolism were not differentially regulated in CA1 neurons across the clinical diagnostic groups. Taken together, these data suggest that CA1 synaptic gene dysregulation occurs early in the cascade of pathogenic molecular events prior to the onset of AD, which may form the basis for novel pharmacological treatment approaches for this dementing disorder.
This article is part of a Special Issue entitled ‘Neurodegenerative Disorders’.
•Hippocampal CA1 neurons display widespread synaptic gene downregulation in MCI and AD.•Genes encoding APP and APP family members were unaffected in MCI and AD.•Synaptic mRNA levels correlate with poorer cognition and increased AD pathology.•CA1 synaptic dysregulation is a very early pathogenic event in AD.
Alzheimer's disease (AD) is a uniquely human brain disorder characterized by the accumulation of amyloid-beta protein (Aβ) into extracellular plaques, neurofibrillary tangles (NFT) made from ...intracellular, abnormally phosphorylated tau, and selective neuronal loss. We analyzed a large group of aged chimpanzees (n = 20, age 37–62 years) for evidence of Aβ and tau lesions in brain regions affected by AD in humans. Aβ was observed in plaques and blood vessels, and tau lesions were found in the form of pretangles, NFT, and tau-immunoreactive neuritic clusters. Aβ deposition was higher in vessels than in plaques and correlated with increases in tau lesions, suggesting that amyloid build-up in the brain's microvasculature precedes plaque formation in chimpanzees. Age was correlated to greater volumes of Aβ plaques and vessels. Tangle pathology was observed in individuals that exhibited plaques and moderate or severe cerebral amyloid angiopathy, a condition in which amyloid accumulates in the brain's vasculature. Amyloid and tau pathology in aged chimpanzees suggests these AD lesions are not specific to the human brain.
One of the major neuropathological findings in the brains of individuals with Alzheimer's disease (AD) is a loss of synaptic contacts in both the neocortex and hippocampus. Here we report, for the ...first time, an estimate of the total number of synapses in the outer molecular layer (OML) of the human dentate gyrus, in individuals with early Alzheimer's disease (eAD), mild cognitive impairment (MCI), or no cognitive impairment (NCI). An unbiased stereologic sampling scheme coupled with transmission electron microscopy to directly visualize synaptic contacts, was used to estimate the total number of synapses in short postmortem autopsy tissue. Individuals with eAD had significantly fewer synapses than the other two diagnostic groups. Seventy-five percent of the individuals with MCI had synaptic values that were lower than the NCI group mean. The number of synapses showed a significant correlation with the subject's Mini-Mental State score and with cognitive tests involving delayed recall. Synaptic loss showed no relationship to Braak stage or to apoE genotype. The volume of the OML was significantly reduced in eAD compared to the other two diagnositic groups that were not different from each other. These data suggest that a loss of afferents from the entorhinal cortex underlie the synapse loss seen in eAD. This study supports the concept that synapse loss is an early event in the disease process and suggests that MCI may be a transition stage between eAD and NCI with synaptic loss a structural correlate involved in cognitive decline.
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