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Alzheimer’s disease (AD) is defined by the presence of abundant amyloid-β (Aβ) and tau neuropathology. While this neuropathology is necessary for AD diagnosis, it is not sufficient for causing cognitive impairment. Up to one third of community dwelling older adults harbor intermediate to high levels of AD neuropathology at death yet demonstrate no significant cognitive impairment. Conversely, there are individuals who exhibit dementia with no gross explanatory neuropathology. In prior studies, synapse loss correlated with cognitive impairment. To understand how synaptic composition changes in relation to neuropathology and cognition, multiplexed liquid chromatography mass-spectrometry was used to quantify enriched synaptic proteins from the parietal association cortex of 100 subjects with contrasting levels of AD pathology and cognitive performance. 123 unique proteins were significantly associated with diagnostic category. Functional analysis showed enrichment of serotonin release and oxidative phosphorylation categories in normal (cognitively unimpaired, low neuropathology) and “resilient” (unimpaired despite AD pathology) individuals. In contrast, frail individuals, (low pathology, impaired cognition) showed a metabolic shift towards glycolysis and increased presence of proteasome subunits.

Senile plaques are a major pathological hallmark of Alzheimer's disease (AD). Compelling evidence suggests that senile plaques lead to structural alterations of neuronal processes and that local toxicity may be mediated by increased oxidative stress. Anti-oxidant therapy can alleviate the neuronal abnormalities in APP mice, but the time-course of this beneficial effect is unknown. We used multiphoton microscopy to assess in vivo the characteristics of antioxidant treatment on senile plaques and neurites in AD model mice (APPswe/PS1dE9). We observed that α-phenyl-N-tert-butyl nitrone (PBN), Ginkgo biloba extract (EGb 761) and Trolox had no effect on the size of existing senile plaques. However, all anti-oxidants had a straightening effect on curved neurites. This effect was detected as soon as 4 days after commencing the treatment, and was maintained after 1 month of daily treatment, with no further increase in the effect. The straightening of neurites persisted 15 days after stopping the treatment. These data indicate that neuronal plasticity is fast and still active in adult animals, and suggest that amelioration of the neuritic distortions associated with senile plaques with antioxidants is both rapid and long lasting.

Amyloid-beta (Abeta) is a major drug target in Alzheimer's disease. Here, we demonstrate that deposited Abeta is SDS insoluble in tgAPP-ArcSwe, a transgenic mouse model harboring the Arctic (E693G) and Swedish (KM670/671NL) APP mutations. Formic acid was needed to extract the majority of deposited Abeta in both tgAPP-ArcSwe and Alzheimer's disease brain, but not in a commonly used type of mouse model with the Swedish mutation alone. Interestingly, the insoluble state of Arctic Abeta was determined early on and did not gradually evolve with time. In tgAPP-ArcSwe, Abeta plaques displayed a patchy morphology with bundles of Abeta fibrils, whereas amyloid cores in tgAPP-Swe were circular with radiating fibrils. Amyloid was more densely stacked in tgAPP-ArcSwe, as demonstrated with a conformation sensitive probe. A reduced increase in plasma Abeta was observed following acute administration of an Abeta antibody in tgAPP-ArcSwe, results that might imply reduced brain to plasma Abeta efflux. TgAPP-ArcSwe, with its insoluble state of deposited Abeta, could serve as a complementary model to better predict the outcome of clinical trials.

The ideal biomarker for Alzheimer's disease (AD) should detect a fundamental feature of neuropathology and be validated in neuropathologically-confirmed cases: it should have a sensitivity >80% for detecting AD and a specificity of >80% for distinguishing other dementias: it should be reliable, reproducible non-invasive, simple to perform, and inexpensive. Recommended steps to establish a biomarker include confirmation by at least two independent studies conducted by qualified investigators with the results published in peer-reviewed journals. Our review of current candidate markers indicates that for suspected early-onset familial AD. it is appropriate to search for mutations in the presenilin 1, presenilin 2, and amyloid precursor protein genes. Individuals with these mutations typically have increased levels of the amyloid Aβ 42 peptide in plasma and decreased levels of APPs in cerebrospinal fluid. In late-onset and sporadic AD. these measures are not useful. but detecting an apolipoprotein E e4 allele can add confidence to the clinical diagnosis. Among the other proposed molecular and biochemical markers for sporadic AD. cerebrospinal fluid assays showing low levels of Aβ 42 and high levels of tau come closest to fulfilling criteria for a useful biomarker.

Alzheimer's disease (AD) is a complex genetic disorder with no effective treatments. More than 20 common markers have been identified, which are associated with AD. Recently, several rare variants have been identified in Amyloid Precursor Protein (APP), Triggering Receptor Expressed On Myeloid Cells 2 (TREM2) and Unc-5 Netrin Receptor C (UNC5C) that affect risk for AD. Despite the many successes, the genetic architecture of AD remains unsolved. We used Genome-wide Complex Trait Analysis to (1) estimate phenotypic variance explained by genetics; (2) calculate genetic variance explained by known AD single nucleotide polymorphisms (SNPs); and (3) identify the genomic locations of variation that explain the remaining unexplained genetic variance. In total, 53.24% of phenotypic variance is explained by genetics, but known AD SNPs only explain 30.62% of the genetic variance. Of the unexplained genetic variance, approximately 41% is explained by unknown SNPs in regions adjacent to known AD SNPs, and the remaining unexplained genetic variance outside these regions.

Neurons in layer II of the entorhinal cortex consistently develop neurofibrillary tangles in Alzheimer's disease (AD). Experimental neuroanatomical studies have shown that these neurons give rise to the perforant pathway, a major excitatory projection to the hippocampal formation, which terminates in a discrete pattern in the outer portion of the molecular layer of the dentate gyrus. The distribution of two nerve terminal associated proteins, synaptophysin and NT75, was studied in the molecular layer of the dentate gyrus in AD and control cases to determine whether Alzheimer neuronal pathology is associated with loss of synaptic markers. In parallel studies, the effect of ablation of the entorhinal cortex in rats was evaluated. In AD as compared to controls, a decrease in synaptophysin immunostaining was evident in the terminal zone of the perforant pathway. NT75 nerve terminal immunostaining was too weak to interpret in the human hippocampal formation. Both synaptophysin and NT75 immunoreactivity were found in association with some neuritic plaques. In rats, entorhinal lesions resulted in diminished immunoreactivity for both synaptophysin and NT75 in the perforant pathway terminal zone. These results suggest that nerve terminal protein loss is a concomitant feature of neuronal pathology in AD.

Previous studies identified engulfment adapter phosphotyrosine binding (PTB) domain containing 1 (GULP1) as an NPXY-motif interactor of low-density lipoprotein receptor-related protein 1 (LRP1) and suggested a potential relevance in Alzheimer's disease (AD). Since AD associated proteins amyloid-β A4 precursor protein (APP) and LRP1 were shown to interact with the PTB domain of Fe65 and several other adapters via their intracellular NPXY-motifs, we examined a possible interaction of GULP1 PTB domain with the YENPTY-motif of APP. Here we demonstrate that GULP1 is present in human hippocampal and neocortical neurons. Confocal live cell imaging revealed that coexpressed and endogenous GULP1 colocalizes with APP in the Golgi and endoplasmic reticulum. Analysis of the interacting domains by co-immunoprecipitation of point and deletion mutants revealed that the interaction depends on the PTB domain of GULP1 and the YENPTY-motif of APP. Coexpression of GULP1 affected APP cell surface localization and suppressed generation of Aβ40/42 and sAPPα. Taken together, these data identify GULP1 as a novel neuronal APP interacting protein that alters trafficking and processing of APP.

Alpha-synuclein is a major component of Lewy bodies (LBs) in the substantia nigra and cortex in Parkinson's disease (PD) and dementia with Lewy bodies (DLB), and in glial inclusions in multiple systems atrophy (MSA). Mutations in alpha-synuclein have been associated with autosomal dominant forms of PD. We investigated the clinical and neuropathological effects of overexpression of human alpha-synuclein, alpha-synuclein A30P, and alpha-synuclein A53T under the control of the hamster prion protein (PrP) promoter; 5-15x endogenous levels of protein expression were achieved with widespread neuronal, including nigral, transgene expression. High expression of alpha-synuclein A30P in the Tg5093 line was associated with a progressive motor disorder with rigidity, dystonia, gait impairment, and tremor. Histological analysis of this line showed aberrant expression of the protein in cell soma and progressive CNS gliosis, but no discrete Lewy body-like alpha-synuclein inclusions could be identified. Biochemical analysis demonstrated alpha-synuclein fragmentation. Despite strong expression of the transgene in the nigra, there was no specific deterioration of the nigrostriatal dopaminergic system as assessed by quantitation of nigral tyrosine hydroxylase (TH) containing neurons, striatal TH immunoreactivity, dopamine levels, or dopamine receptor number and function. Lower expressing lines had no specific behavioral or histopathological phenotype. Thus, high expression of mutant human alpha-synuclein resulted in a progressive motor and widespread CNS gliotic phenotype independent of dopaminergic dysfunction in the Tg5093 line.

The neuropathological diagnosis of Alzheimer's disease currently relies on quantitative or semiquantitative criteria of senile or neuritic plaques that are adjusted for age and for the presence or absence of a clinical history of dementia. Based on clinical-pathological correlation studies, I will argue that neuropathological assessment should stand independently of clinical history and instead should describe brain lesions in the context of the topography and natural history of the disease. Only probabilistic estimates about the presence or absence of dementia can be made from a neuropathological examination, especially in the setting of Alzheimer disease lesions plus other pathological alterations such as Lewy bodies or infarcts. Moreover, I will argue that any neurofibrillary tangles or senile plaques are inherently pathological entities, even if clinically silent and so "incidental" neuropathological findings. Because the intensity and location of neurofibrillary tangles, rather than senile plaques, appears to correlate most closely with clinical symptoms, I suggest using a staging system that highlights this information rather than using absolute numerical cut-offs for diagnostic purposes.

Glycogen synthase kinase (GSK) 3 alpha and 3 beta are two proline-directed serine/ threonine kinases that have been shown in vitro to hyperphosphorylate tau, and therefore, may contribute to neurofibillary tangle (NFT) formation in Alzheimer's disease (AD). We report here that, in the human hippocampal formation of both control and AD individuals, GSK 3 alpha and 3 beta are immunohistochemically localized to neurons within the presubiculumCA1, CA3, and CA4 subfields of the hippocampus, layers IIIIIIV, V, VI of entorhinal cortex, and occasional neurons in layers III, V, and VI of temporal neocortex. By contrast, NFTs occur primarily in CA1. subiculum, layers II and IV of entorhinal cortex, and layers II, III, and V of temporal neocortex. The presubiculum and other subfields are frequently spared. Thus, localization of GSK 3 alpha and GSK 3 beta does not correspond to the expected pattern of neuronal vulnerability to NFT formation in AD. Interpreted within the limitations of immunohistochemical detection, these results argue against a major role of GSK 3 alpha or GSK 3 beta in NFT formation in AD.

While a correlation exists at the regional level between the distribution of neurofibrillary tangles and the predicted sites of brain dysfunction based on clinical and functional neuroimaging studies, the relationship between neurofibrillary tangles and neuronal dysfunction is poorly understood. Using cytochrome oxidase activity as a marker of neuronal functional activity, we found reductions in metabolic activity both in a hippocampal subfield with a high density of neurofibrillary tangles (CA1) as well as in subfields relatively spared (CA3, dentate granule cells). In contrast, we found no reduction in activity in primary visual cortex. Using in situ hybridization, we found a selective reduction in a mitochondrial-encoded cytochrome oxidase mRNA transcript with sparing of a nuclear-encoded transcript. These results suggest that the reduction in cytochrome oxidase activity in Alzheimer's disease brain may be related to an alteration in mitochondrial gene expression. The absence of a direct correlation between structural pathology and cytochrome oxidase activity suggests that neurons that remain structurally intact in Alzheimer's disease may nonetheless undergo substantial changes in metabolic activity.

1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, (MPTP), produces a parkinsonian syndrome both in man and in experimental animals. Its toxicity is mediated by a metabolite, the 1-methyl-4-phenylpyridinium ion (MPP+). When injected into the striatum, MPP+ is accumulated by dopaminergic nerve terminals and retrogradely transported to the substantia nigra pars compacta (SNc) where it causes neuronal degeneration. MPP+ accumulates in mitochondria and blocks complex I of the electron transport chain. A proposed mechanism of neurotoxicity is excitotoxic neuronal degeneration induced by this energy depletion. We examined whether either prior decortication or administration of the N-methyl-D-aspartate (NMDA) receptor antagonist, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) could prevent or diminish the selective nigral neuronal degeneration that follows unilateral intrastriatal injection of MPP+. We quantified the extent of neuronal death in the SNc ipsilateral and contralateral to the injections on Nissl-stained sections with unbiased stereological techniques. One week after injection of MPP+, approximately 75% of the SNc neurons were lost on the side of the injection. The loss was a consequence of the reduction in both SNc volume and neuronal density. Both prior decortication or the administration of MK-801 for 2 days nearly completely prevented MPP(+)-induced neuronal loss in the ipsilateral SNc. These results are consistent with an NMDA receptor mediated excitotoxic mechanism for MPP(+)-induced nigral toxicity.

PATHOLOGICAL changes associated with Alzheimer's disease (AD) occur in a consistent distribution, severely affecting certain neuronal populations in limbic areas and association cortices. De Lacoste and White (9) argue that the basis of this pattern lies in the connectional relationship among affected neurons. Indeed, in many instances, neurofibrillary tangles occur in projection neurons and senile plaques are found in termination zones. However, there are also counter-examples to this postulate. We suggest that, although disruption of these neural system connections probably underlies cognitive impairment in AD, the basis of selective neuronal vulnerability reflects an interaction between connectivity and specific molecular phenotypes in different brain regions. Whether the development of neurpathological changes associated with AD follows a predictable pattern is a critical issue in understanding the progression of the disease. The model of cortical disconnection and vulnerability due to cortical connectivity, described here by de Lacoste and White (9), proposes that pathological changes in certain brain regions can spread to other regions by well-defined cortical connections. The nature of this spread could be through the transfer of toxins or viruses, or through the disruption of interneuronal connections that normally help maintain neuronal viability. Furthermore, de Lacoste and White identify the interconnected regions of the limbic system and the temporo-occipital cortices as structures suited to test whether the AD-related changes spread from one region of the brain to another. It is important to divide the disconnection~connectivity model of AD into two components. The first is that disruption of connections of neural systems accounts for the clinical manifestations of AD. There are several pieces of data that support this assertion. First, the memory deficit of AD patients could be explained by the disruption of the connections of the hippocampus, amygdala, and entorhinal cortex by the presence of senile plaques (SP's) and neurofibrillary tangles (NFT's). These changes in the medial temporal lobe are the first alterations found in the brains of patients with AD (2,19,20,33) and are consistent with the primacy of memory loss in patients with AD. Second, the distribution and degree of neuropathological changes correlates, at least in some studies, with measures of dementia (2,27,37). Third, the appearance of NFT's and SP's in the visual regions of the occipital lobe correlates with deficits in visuospatial skills in the small subpopulation of AD patients with Balint's syndrome (14). Finally, it seems likely that the correlation between degree of cognitive impairment and the loss of synapses seen in AD reflects a loss of cortico-cortical projections (37). Thus, it appears that disruption of cortical connections can account for the type and severity of the clinical symptoms in AD. The second component of the disconnection/connectivity model is that the pattern of NFT's and SP's in the brains of patients with AD can be explained by the spread of pathological changes through interneuronal connections. One example that supports this concept is the analysis of hippocampal formation damage in AD. NFT's are observed in layer II neurons of the entorhinal cortex whose axons project to the molecular layer of dentate gyrus, an area frequently involved by SP's ( 16,17). SP's also accumulate in specific nuclei in the amygdala that receive hippocampal projections (19,24). NFT's accumulate in high-order association cortices ( 14,25,31), whose projections to other cortices terminate in regions affected with SP's. These observations are consistent with the hypothesis that SP's develop in the terminal zone of neurons that contain NFT's. Alternately, this data could be interpreted to imply that NFI"s form in cells projecting to SP's. However, one can also find instances of areas severely affected by NFI"s whose projection zone is not frequently affected by SP's. For example, subicular/CA 1 hippocampal neurons develop NF'I"s. These neurons project strongly to layer IV of the entorhinal cortex which contains NFT's but little or no amyloid deposition (19). Other apparent discrepancies are found in an analysis of the visual system as described below. Thus, while it may be possible to predict which regions of the brain are affected at different stages of AD, it does not seem possible to predict the pattern of involvement of different brain regions based only on known anatomical connections.

This report summarizes the consensus recommendations of a panel of neuropathologists from the United States and Europe to improve the postmortem diagnostic criteria for Alzheimer’s disease. The recommendations followed from a two-day workshop sponsored by the National Institute on Aging (NIA) and the Ronald and Nancy Reagan Institute of the Alzheimer’s Association to reassess the original NIA criteria for the postmortem diagnosis of Alzheimer’s disease published in 1985 [2] . The consensus recommendations for improving the neuropathological criteria for the postmortem diagnosis of Alzheimer’s disease are reported here, and the “position papers” by members of the Working Group that accompany this report elaborate on the research findings and concepts upon which these recommendations were based. Further, commentaries by other experts in the field also are included here to provide additional perspectives on these recommendations. Finally, it is anticipated that future meetings of the Working Group will reassess these recommendations and the implementation of postmortem diagnostic criteria for Alzheimer’s disease.

The significance of the particular neurons lost in Alzheimer's disease is discussed in the context of their likely anatomic role as projection neurons. The location and function of affected neurons is emphasized, rather than absolute number.