Lack of association of the cholesterol 24-hydroxylase (CYP46) intron 2 polymorphism with Alzheimer's disease

An association was recently reported between an increased risk of Alzheimer's disease and an intron 2 AA genotype of CYP46, the enzyme hydroxylating cholesterol to 24S-hydroxycholesterol. Moreover, CYP46 AA-carriers were found to have increased levels of amyloid-beta and tau in brain and cerebrospinal fluid. We determined the CYP46 intron 2 genotype in a cohort of 178 AD and 105 non-demented control subjects, but found no significant association with AD for any of the individual genotypes or alleles. Further, in an autopsy confirmed subset of this cohort, the proposed CYP46 risk genotype was not associated with any increase in the brain levels of amyloid-beta40, amyloid-beta42 or in the levels of amyloid plaques and neurofibrillary tangles. Despite growing evidence implicating cholesterol metabolism in AD risk and Abeta generation, our data does not support a robust genetic relationship between the CYP46 intron 2 polymorphism and AD risk or neuropathology.

2-Dialkylamino-6-acylmalononitrile substituted naphthalenes (DDNP analogs): novel diagnostic and therapeutic tools in Alzheimer's disease

This article presents a comprehensive review of the in vitro and in vivo detection of neurofibrillary tangles (NFTs) and beta-amyloid senile plaques (SPs), neuropathological lesions found in the brains of the Alzheimer's disease (AD) patients, using FDDNP and its analogs. FDDNP and its analogs have excellent ability to bind to NFTs and SPs in vitro as shown by binding assays, confocal fluorescence microscopy with stained AD brain tissue and digital autoradiography with [18F]FDDNP. [18F]FDDNP-PET molecular imaging permits detection of these pathologies in living subjects. The discovery of a new binding site to Abeta(1-40) fibrils as a result of FDDNP binding also opens a therapeutic opportunity for early treatment of Alzheimer's disease. FDDNP shares a previously unrecognized common binding site on Abeta(1-40) fibrils and senile plaques with non-steroidal anti-inflammatory drugs (NSAIDs) (e.g., naproxen and ibuprofen). Naproxen, ibuprofen and even FDDNP significantly inhibit aggregation of the Abeta(1-40) peptide in the micromolar range. This new binding site on Abeta(1-40) fibrils also offers a molecular template for design of anti-aggregation drugs without the secondary effects of NSAIDs. Therefore it is anticipated that a new vision for prevention, early diagnosis and treatment of Alzheimer's disease would be rapidly developing.

Anthraquinones inhibit tau aggregation and dissolve Alzheimer's paired helical filaments in vitro and in cells

The abnormal aggregation of tau protein into paired helical filaments (PHFs) is one of the hallmarks of Alzheimer's disease. Aggregation takes place in the cytoplasm and could therefore be cytotoxic for neurons. To find inhibitors of PHF aggregation we screened a library of 200,000 compounds. The hits found in the PHF inhibition assay were also tested for their ability to dissolve preformed PHFs. The results were obtained using a thioflavin S fluorescence assay for the detection and quantification of tau aggregation in solution, a tryptophan fluorescence assay using tryptophan-containing mutants of tau, and confirmed by a pelleting assay and electron microscopy of the products. Here we demonstrate the feasibility of the approach with several compounds from the family of anthraquinones, including emodin, daunorubicin, adriamycin, and others. They were able to inhibit PHF formation with IC50 values of 1-5 microm and to disassemble preformed PHFs at DC50 values of 2-4 microm. The compounds had a similar activity for PHFs made from different tau isoforms and constructs. The compounds did not interfere with the stabilization of microtubules by tau. Tau-inducible neuroblastoma cells showed the formation of tau aggregates and concomitant cytotoxicity, which could be prevented by inhibitors. Thus, small molecule inhibitors could provide a basis for the development of tools for the treatment of tau pathology in AD and other tauopathies.

Neuropathology of primary restless leg syndrome: absence of specific tau- and alpha-synuclein pathology

The neuroanatomical substrate for restless legs syndrome (RLS) is unknown. We identified 4 patients with idiopathic RLS who came to post-mortem examination, where brain and spinal cord tissue were available for neuropathological assessment. Lewy bodies were not identified and alpha-synuclein immunohistochemistry was uniformly negative. Neurofibrillary tangle pathology was variable and nonspecific. These findings suggest that tau- or alpha-synuclein brain pathology is not a component of primary RLS. Although chronic ischemic changes were found in all 4 cases, these were probably incidental. The absence of diagnostic microscopic brain or spinal cord pathology suggests that the pathologic substrate may be neurochemical or receptor based.

Molecular aging of tau: disulfide-independent aggregation and non-enzymatic degradation in vitro and in vivo

Smearing from high-molecular-mass regions to low-molecular-mass regions on western blot is the most striking observation of the tau making up paired helical filaments in brain tissues affected by Alzheimer's disease. Because our previous study showed site-specific deamidation/isomerization in the smeared tau in vivo, a feature of protein aging, recombinant tau was subjected to prolonged (up to 90 days) in vitro incubation. Carboxymethylated tau at approximately 50 kDa gradually disappeared and was converted to dimers and to high- and low-molecular-mass smearing. In addition, the same site-specific deamidation/isomerization as previously identified in the smeared tau in vivo emerged. Most importantly, tau was spontaneously degraded, generating fragments that start from bulky residues next to asparaginyl residues. This spontaneous degradation of tau probably represents non-enzymatic cleavage through the formation of succinimide intermediates. Similar degradation products starting from the bulky residues next to asparaginyl residues were found in the smeared tau in vivo partially purified from the homogenates from Alzheimer's disease brains.

Absence of heparan sulfate proteoglycans in Lewy bodies and Lewy neurites in Parkinson's disease brains

alpha-Synuclein is the major constituent of Lewy bodies and Lewy neurites in Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Relatively little is known about the exact mechanism of alpha-synuclein deposition and fibrillization in these alpha-synucleinopathies. In order to better understand the pathogenesis of alpha-synucleinopathies it is important to identify molecules that regulate the fibrillization of alpha-synuclein. Since it has been demonstrated that heparan sulfate proteoglycans (HSPGs) and glycosaminoglycans (GAGs) promote the conversion of non-fibrillar amyloid beta-protein (Abeta) into neurotoxic fibrillar Abeta in Alzheimer's disease, they might also be involved in alpha-synuclein aggregation. It was the aim of our study to examine the distribution pattern of these macromolecules in PD brains and the possible association with Lewy bodies and Lewy neurites. Although HSPGs clearly colocalized with senile plaques, we were unable to identify HSPGs or GAGs in Lewy bodies and Lewy neurites and therefore concluded that it is likely that alpha-synuclein fibrillization and stabilization occurs independently of the presence of HSPGs or GAGs.

Fibrillar amyloid deposition leads to local synaptic abnormalities and breakage of neuronal branches

Amyloid plaques are a hallmark of Alzheimer disease, but their importance in its pathogenesis is controversial. By neuronal labeling and transcranial two-photon imaging, we show in a transgenic mouse model of Alzheimer disease that dendrites passing through or near fibrillar amyloid deposits undergo spine loss and shaft atrophy, and nearby axons develop large varicosities, together leading to neurite breakage and large-scale, permanent disruption of neuronal connections. Thus, fibrillar amyloid deposition is more detrimental to neuronal circuitry than previously thought, underscoring the importance of prevention and early clearance of plaques.

Glucocorticoid hormone (cortisol) affects axonal transport in human cortex neurons but shows resistance in Alzheimer's disease

1 The changes of tissue sensitivity to glucocorticoids are associated with many pathological states including neurological diseases. In the present study, using a novel in vitro post-mortem tracing method on human brain slices, we demonstrated that cortisol, a major glucocorticoid hormone in humans, affected axonal transport both in the cortex neurons in four Alzheimer's disease (AD) patients and four nondemented controls. 2 Cortisol appeared to affect axonal transport of prefrontal cortex (PFC) and temporal cortex (TC) neurons in AD patients and controls in a dose-dependent way at concentrations of 30, 60, 120 and 240 microg dl(-1). 3 Higher doses of cortisol were needed for TC neurons to achieve a similar axonal transport effect as obtained in PFC neurons in AD patients. The maximum effect (Emax) on axonal transport was achieved in PFC slices at relatively low contraction (30-120 microg dl(-1)), while in TC slices, a maximum effect was only reached at relatively high concentrations (120-240 microg dl(-1)). 4 For PFC and TC slices from nondemented aging subjects, lower doses of cortisol (30-60 microg dl(-1)) on axonal transport were sufficient to achieve the maximum effect as compared to those used in AD brain slices, while levels of more than 60 microg dl(-1) of cortisol mostly depressed axonal transport. 5 These results suggest that glucocorticoid resistance, which is thought to contribute to the pathogenesis of a number of common human disorders, may exist in AD brains and play an important role in neuropathological mechanisms and dementia.

14-3-3 proteins and zeta isoform containing neurofibrillary tangles in patients with Alzheimer's disease

Immunolocalization of 14-3-3 proteins in Alzheimer's disease (AD) brains was investigated using isoform-specific antibodies. Weak granular immunoreactivity of 14-3-3 proteins was found in neuronal cytoplasm in control subjects and AD brains. Both intracellular and extracellular neurofibrillary tangles (NFTs), as well as neuropil thread-like structures, were immunopositive for 14-3-3 proteins. This was corroborated by triple-fluorolabeling method visualizing paired helical filament (PHF) tau and 14-3-3 epitopes in relation to fibrillary state detected by thiazin red. Pretangle neurons (positive for PHF-tau without fibrillary structure detected by thiazin red) only contained fine granular immunoreactivity (IR) of 14-3-3, which was similarly found in unaffected neurons. Granular cytoplasmic IR of 14-3-3 proteins in pretangle neurons was not colocalized to granular tau-like IR, which suggests that participation of 14-3-3 proteins in NFT formation was restricted to its later stages. Its zeta isoform was most prominent in these NFTs, suggesting that this isoform is a major component involved in the formation of NFTs. In contrast, IR of epsilon isoform was found in the neuropil of the hippocampus and that of sigma isoform was localized to granule cells of the dentate gyrus in AD brains, as seen in the age-matched controls. Expression of 14-3-3 proteins were found to be highly variable and dependent on their isoforms, regions and cell types. Molecular, as well as topographical, dissection of 14-3-3 proteins will provide us with an improved understanding of this molecule in normal and pathological conditions.

Enhanced expression of 14-3-3 proteins in reactive astrocytes in Creutzfeldt-Jakob disease brains

14-3-3 proteins have been reported to be detected specifically in the cerebrospinal fluid (CSF) from patients with Creutzfeldt-Jakob disease (CJD). To elucidate the role of 14-3-3 proteins in patients with CJD, we performed immunohistochemical studies on 14-3-3 proteins in autopsied brains from five patients with sporadic CJD (sCJD), three patients with Alzheimer's disease (AD), and seven normal control subjects. Formalin-fixed, paraffin-embedded sections from all cases were immunostained with several types of specific anti-14-3-3 antibodies. In the normal control brains, 14-3-3 immunoreactivity was localized mainly in the neuronal somata and processes; in contrast, glial cells showed no or faint immunoreactivity. In the brains from the patients with AD, 14-3-3 immunoreactivity was observed in the surviving neurons as well as some neurofibrillary tangles. In the brains from the patients with sCJD, 14-3-3 immunoreactivity was well preserved in the remaining neurons. Furthermore, the glial cells, especially the reactive astrocytes, were intensely immunostained in the brains affected by sCJD. Our findings suggest that 14-3-3 proteins may be up-regulated in the glial cells, particularly in reactive astrocytes, and that the enhanced expression of 14-3-3 proteins in these glial elements may be associated with the pathogenesis of sCJD.

MARKing tau for tangles and toxicity

In healthy neurons, tau proteins regulate microtubule function in the axon. In the brains of individuals with Alzheimer's disease, tau is hyperphosphorylated and aggregated into intraneuronal deposits called neurofibrillary tangles (NFTs). Hyperphosporylation dislodges tau from the microtubule surface, potentially resulting in compromised axonal integrity and the accumulation of toxic tau peptides. Recent biochemical and animal model studies have re-evaluated tau phosphorylation and other aspects of neurofibrillar pathology. The results indicate that phosphorylation of tau's microtubule-binding domain by the protein kinase MARK primes tau for hyperphosphorylation by the kinases GSK-3 and Cdk5, which in turn triggers the aggregation of tau into filaments and tangles. Toxic consequences for the neuron might be exacerbated by tangle formation but are already evident during the early steps of the process.

Efficient reversal of Alzheimer's disease fibril formation and elimination of neurotoxicity by a small molecule

The Abeta1-42 peptide that is overproduced in Alzheimer's disease (AD) from a large precursor protein has a normal amino acid sequence but, when liberated, misfolds at neutral pH to form "protofibrils" and fibrils that are rich in beta-sheets. We find that these protofibrils or fibrils are toxic to certain neuronal cells that carry Ca-permeant alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. Disrupting the structure of the Abeta1-42 fibrils and protofibrils might lead to the discovery of molecules that would be very useful in the treatment of AD. A high-throughput screen of a library of >3,000 small molecules with known "biological activity" was set up to find compounds that efficiently decrease the beta-sheet content of aggregating Abeta1-42. Lead compounds were characterized by using thioflavin T (ThT) as a beta-sheet assay. The most effective of six compounds found was 4,5-dianilinophthalimide (DAPH) under the following conditions: DAPH at low micromolar concentrations abolishes or greatly reduces previously existing fully formed Abeta1-42 fibrils, producing instead amorphous materials without fibrils but apparently containing some protofibrils and smaller forms. Coincubation of the Abeta1-42 peptide with DAPH produces either amorphous materials or empty fields. Coincubation of DAPH and Abeta1-42 greatly reduces the beta-sheet content, as measured with ThT fluorescence, and produces a novel fluorescent complex with ThT. When the Abeta1-42 peptide was coincubated with DAPH at very low micromolar concentrations, the neuronal toxicity mentioned above (Ca(2+) influx) was eliminated. Clearly, DAPH is a promising candidate for AD therapy.

[From empiricism to neuroscience in Alzheimer's disease]

INTRODUCTION AND DEVELOPMENT

It has been almost a hundred years since neurofibrillar degeneration in the neurones of the cerebral cortex was first described by Alois Alzheimer. This finding led to the recognition of a new disease (a presenile dementia unlike senile dementia) using the clinicopathological empirical method employed at that time. Despite Alzheimer's own opinion and the subsequent findings from many neuropathological studies, general acknowledgement of the full implications of this dichotomy has taken decades. All the cases were grouped under the eponym of Alzheimer's disease and it is now being studied with the different modern scientific methods currently available to researchers.

CONCLUSIONS

Alzheimer's disease has given rise to an exceptional associationist movement around the world and has become one of leading social and health problems facing the human race; its research and treatment will require the involvement of national and supranational public authorities.

Neurodegeneration and plasticity

Neurofibrillary degeneration, associated with the formation of paired helical filaments (PHF), is one of the critical neuropathological hallmarks of Alzheimer's disease (AD). Although the microtubule-associated protein tau in a hyperphosphorylated form has been established as primary PHF constituent, the process of tau phosphorylation and its potential link to degeneration is not very well understood, mostly because of the lack of a physiological in vivo model of PHF-like tau phosphorylation. PHF formation in AD follows a hierarchical pattern of development throughout different cortical areas, which closely matches the pattern of neuronal plasticity in the adult brain. Those brain areas are most early and most severely affected which are involved in the regulation of memory, learning, perception, self-awareness, consciousness, and higher brain functions that require a life-long re-fitting of connectivity, a process based on a particularly high degree of plasticity. Failures of synaptic plasticity are, thus, assumed to represent early events in the course of AD that eventually lead to alteration of tau phosphorylation. Recently, we have used the hibernation cycle, a physiological model of adaptation associated with an extraordinary high degree of structural neuronal plasticity, to analyze the potential link between synaptic plasticity, synaptic detachment and the regulation of tau phosphorylation. During torpor, a natural state of hypothermia, synaptic contacts between mossy fibers and hippocampal pyramidal neurons undergo dramatic regressive changes that are fully reversible very rapidly during euthermy. This rapid, reversible, and repeated regression of synaptic and dendritic components on CA3 neurons is associated with a reversible PHF-like phosphorylation of tau at a similar time course. The repeated formation and degradation of PHF-tau might, thus, represent a physiological mechanism not necessarily associated with pathological effects. These findings implicate an essential link between neuronal plasticity and PHF-like phosphorylation of tau, potentially involved in neurofibrillary degeneration.

Spatial and temporal relationships between plaques and tangles in Alzheimer-pathology

One histological hallmark in Alzheimer's disease is the tangle. The other is the plaque. A widely discussed hypothesis is the "amyloid cascade" assuming that tangle formation is a direct consequence of amyloid plaque formation. The aim of this study was to examine plaques and tangles in a highly defined neuronal circuitry in order to determine their detailed spatial and temporal relationships. We investigated serial sections of the whole hippocampal formation of brains with early Braak-stages (0-III) for tangles only, i.e. one case at stage 0, six at stage I, six at stage II, and nine at stage III. Most cases displayed both plaques and tangles. Four cases of stages 0 and I, three cases with stage II, and even one with stage III, however, did not display plaques. In turn, no plaque was found in the absence of tangles. The spatial relationship indicates that plaques lay in the terminal fields of tangle-bearing neurons. Our analysis suggests that tangles either antecede plaques or--less likely--are independently formed.

Morphological characterization of Thioflavin-S-positive amyloid plaques in transgenic Alzheimer mice and effect of passive Abeta immunotherapy on their clearance

Transgenic mice mimicking certain features of Alzheimer's disease (AD)-pathology, namely amyloid plaques and neurofibrillary tangles, have been developed in an effort to better understand the mechanism leading to the formation of these characteristic cerebral lesions. More recently, these animal models have been widely used to investigate emergent therapies aimed at the reduction of the cerebral amyloid load. Several studies have shown that immunotherapy targeting the amyloid peptide (Abeta) is efficacious at clearing the amyloid plaques or preventing their formation, and at reducing the memory/behavior impairment observed in these animals. In AD, different types of plaques likely have different pathogenic significance, and further characterization of plaque pathology in the PDAPP transgenic mice would enhance the evaluation of potential therapeutics. In the present study, a morphological classification of amyloid plaques present in the brains of PDAPP mice was established by using Thioflavin-S staining. Neuritic dystrophy associated with amyloid plaques was also investigated. Finally, the efficacy of passive immunization with anti-Abeta antibodies on the clearance of Thio-S positive amyloid plaques was studied. Our results show that distinct morphological types of plaques are differentially cleared depending upon the isotype of the antibody.

Luigi Amaducci memorial award winner's paper 2003. A neurologist's view of Alzheimer's disease and dementia

Senile dementia was the third most common admission diagnosis for New York psychiatric hospitals at the start of the twentieth century and the distinction between vascular and senile dementia was understood by psychiatrists even then. The term Alzheimer's disease (AD) was originally introduced to distinguish a pre-senile dementia from the common general paresis, but Alzheimer raised the possibility that pre-senile AD might not be distinguishable in clinical or histological terms from senile dementia. By the late 1970s it had become clear that the most common disorder producing dementia in elderly people was clinically and pathologically identical to pre-senile AD. AD is malignant, reducing remaining life expectancy by almost half and raising the risk of death over five years threefold (cancer raises it fourfold). Synapse loss associated with beta amyloid oligomers is a strong determinant of cognitive decline in patients with AD. Tau-containing neurofibrillary tangles usefully track disease severity. Unmodifiable risk factors include mutations in three genes which affect the production or metabolism of beta amyloid, the risk factor gene for Apolipoprotein [see symol in text]4 and female gender. The overriding risk factor is age, the prevalence of AD doubling with every five years of age until 90. Low education, head injury and low folate levels are examples of potentially modifiable risk factors. Since a delay of onset of five years would halve the number of patients with the disease, clinical trials for such putative protective factors as estrogens, folic acid, vitamin E, statins, and NSAIDs have begun. Cognitive and leisure activity may be protective against the development of AD but any protective function can only be confirmed by clinical trials.

Amyloid‐induced neurofibrillary tangle formation in Alzheimer's disease: insight from transgenic mouse and tissue‐culture models

Of all forms of dementia, Alzheimer's disease is the most prevalent. It is histopathologically characterized by beta-amyloid-containing plaques, tau-containing neurofibrillary tangles, reduced synaptic density and neuronal loss in selected brain areas. For the rare familial forms of Alzheimer's disease, pathogenic mutations have been identified in both the gene encoding the precursor of the Abeta peptide, APP, itself and in the presenilin genes which encode part of the APP-protease complex. For the more frequent sporadic forms of Alzheimer's disease, the pathogenic trigger has not been unambiguously identified. Whether Abeta is again the main cause remains to be heavily discussed. In a related disorder termed frontotemporal dementia, which is characterized by tangles in the absence of beta-amyloid deposition, mutations have been identified in tau which also lead to neurodegeneration and dementia. For Alzheimer's disease the existence of familial forms lead to the proposition of the amyloid cascade hypothesis, which claims that beta-amyloid causes or enhances the tangle pathology. In this review, we describe tau transgenic mouse models in which aspects of the tau-associated pathology, including tangle formation, has been achieved. Moreover, tau transgenic mouse and tissue-culture models were used to test the amyloid cascade hypothesis. In addition, we discuss alternative hypotheses to explain the sporadic forms. The animal and tissue-culture models will provide insight into the underlying biochemical mechanisms of tau aggregation and nerve cell degeneration. These mechanisms may be partially shared between sporadic Alzheimer's disease, the familial forms and frontotemporal dementia. Eventually, Alzheimer's disease may be redefined based on biochemical events rather than phenotype.

Lewy body-related alpha-synucleinopathy in aging

To clarify the significance of Lewy body (LB)-related alpha-synucleinopathy in aging, we investigated the incidence of LBs in 1,241 consecutive autopsy cases (663 males and 578 females). LB pathology was identified histologically in sections stained with hematoxylin and eosin and with anti-ubiquitin and anti-alpha-synuclein antibodies. Cases without LBs were classified as LB stage 0 (987 cases). Cases with LBs were classified as follows: LB stage I = incidental LBs (149 cases); LB stage II = LB-related degeneration without attributable clinical symptoms (47 cases); LB stage III = Parkinson disease without dementia (10 cases); LB stage IV = dementia with Lewy bodies (DLB) transitional (limbic) form (25 cases); and LB stage V = DLB neocortical form (23 cases). The average age at death was greater for those cases with LBs. There were no gender differences in the LB pathology. G842A polymorphism in the paraoxonase I gene was associated with men in LB stage II or above and suggests a gender-specific risk factor. LB stage V had higher stages of neurofibrillary tangle and senile plaque involvement and also had a higher frequency of apolipoprotein E epsilon4. Our findings indicate that LBs are associated with cognitive decline, either independently or synergistically with neurofibrillary tangles and senile plaques.

Active caspase-6 and caspase-6-cleaved tau in neuropil threads, neuritic plaques, and neurofibrillary tangles of Alzheimer's disease

Previously, we have shown that caspase-6 but not caspase-3 is activated by serum deprivation and induces a protracted cell death in primary cultures of human neurons (LeBlanc AC, Liu H, Goodyer C, Bergeron C, Hammond J: Caspase-6 role in apoptosis of human neurons, amyloidogenesis and Alzheimer's disease. J Biol Chem 1999, 274:23426-23436 and Zhang Y, Goodyer C, LeBlanc A: Selective and protracted apoptosis in human primary neurons microinjected with active caspase-3, -6, -7, and -8. J Neurosci 2000, 20:8384-8389). Here, we show with neoepitope antibodies that the p20 subunit of active caspase-6 increases twofold to threefold in the affected temporal and frontal cortex but not in the unaffected cerebellum of Alzheimer's disease brains and is present in neurofibrillary tangles, neuropil threads, and the neuritic plaques. Furthermore, a neoepitope antibody to caspase-6-cleaved Tau strongly detects intracellular tangles, extracellular tangles, pretangles, neuropil threads, and neuritic plaques. Immunoreactivity with both antibodies in pretangles indicates that the caspase-6 is active early in the pathogenesis of Alzheimer's disease. In contrast to the nuclear and cytosolic localization of active caspase-6 in apoptotic neurons of fetal and adult ischemic brains, the active caspase-6 in Alzheimer's disease brains is sequestered into the tangles or neurites. The localization of active caspase-6 may strongly jeopardize the structural integrity of the neuronal cytoskeletal system leading to inescapable neuronal dysfunction and eventual cell death in Alzheimer's disease neurons. Our results suggest that active caspase-6 is strongly implicated in human neuronal degeneration and apoptosis.

Infiltration of the brain by pathogens causes Alzheimer's disease

Despite very numerous studies on Alzheimer's disease (AD), especially on amyloid plaques and neurofibrillary tangles, little information has been obtained thus on the causes of the disease. Evidence is described here that implicates firstly herpes simplex virus type 1 (HSV1) as a strong risk factor when it is present in brain of carriers of the type 4 allele of the gene for apolipoprotein E (APOE-4). Indirect support comes from studies indicating the role of APOE in several diverse diseases of known pathogen cause. A second putative risk factor is the bacterium, Chlamydia pneumoniae. This pathogen has been identified and localized in AD brain. Current studies aimed at "proof of principle" address the entry of the organism into the CNS, the neuroinflammatory response to the organism, and the role that the organism plays in triggering AD pathology. An infection-based animal model demonstrates that following intranasal inoculation of BALB/c mice with C. pneumoniae, amyloid plaques/deposits consistent with those observed in the AD brain develop, thus implicating this infection in the etiology of AD.