Alpha-2 macroglobulin in Alzheimer's disease: a marker of neuronal injury through the RCAN1 pathway.

Preclinical changes that precede the onset of symptoms and eventual diagnosis of Alzheimer's disease (AD) are a target for potential preventive interventions. A large body of evidence suggests that inflammation is closely associated with AD pathogenesis and may be a promising target pathway for such interventions. However, little is known about the association between systemic inflammation and preclinical AD pathophysiology. We first examined whether the acute-phase protein, alpha-2 macroglobulin (A2M), a major component of the innate immune system, was associated with cerebrospinal fluid (CSF) markers of neuronal injury in preclinical AD and risk of incident AD in the predictors of cognitive decline among normal individuals (BIOCARD) cohort. We find that A2M concentration in blood is significantly associated with CSF concentrations of the neuronal injury markers, tau and phosphorylated tau, and that higher baseline serum A2M concentration is associated with an almost threefold greater risk of progression to clinical symptoms of AD in men. These findings were replicated in the Alzheimer's Disease Neuroimaging (ADNI) study. Then, utilizing a systems level approach combining large multi-tissue gene expression datasets with mass spectrometry-based proteomic analyses of brain tissue, we identified an A2M gene network that includes regulator of calcineurin (RCAN1), an inhibitor of calcineurin, a well-characterized tau phosphatase. A2M gene and protein expression in the brain were significantly associated with gene and protein expression levels of calcineurin. Collectively these novel findings suggest that A2M is associated with preclinical AD, reflects early neuronal injury in the disease course and may be responsive to tau phosphorylation in the brain through the RCAN1-calcineurin pathway.

Optimum stimulus parameters for lateralized suppression of speech with magnetic brain stimulation.

Rapid-rate transcranial magnetic brain stimulation produces lateralized suppression of speech output over the frontal lobe, consistent with cerebral dominance for language. But the sensitivity of magnetic speech localization has been limited, and reports are imprecise concerning the amount of discomfort involved. Using a focal magnetic coil, we evaluated the effectiveness and pain of stimulation at different intensities, orientations, and repetition rates (2 to 32 Hz) in six normal volunteers. We obtained complete and clearly lateralized speech arrest in all subjects. The best ratio of efficacy to pain occurred using slower repetition rates of 4 to 8 Hz with a horizontal alignment of the induced electric field. Lower stimulation frequency also allowed clearer distinction between speech arrest and dysarthria from tonic contraction of cranial muscles. The relative comfort and safety of stimulation at 4 Hz should allow more widespread use of magnetic speech localization in clinical and research applications.

Differential effects of transforming growth factor-beta(s) and glial cell line-derived neurotrophic factor on gene expression of presenilin-1 in human post-mitotic neurons and astrocytes.

Mutations in the presenilin-1 gene are linked to the majority of early-onset familial Alzheimer's disease cases. We have previously shown that the expression of transforming growth factor-beta is altered in Alzheimer's patients, compared to controls. Here we examine presenilin- expression in human post-mitotic neurons (hNT cells), normal human astrocytes, and human brain tumor cell lines following treatment with three isoforms of transforming growth factor-beta, or glial cell line-derived neurotrophic factor, a member of the transforming growth factor-beta superfamily. As the NT2/D1 teratocarcinoma cell line is treated with retinoic acid to induce differentiation to hNT cells, presenilin-1 messenger RNA expression is dramatically increased. Furthermore, there is a 2-3-fold increase in presenilin-1 messenger RNA expression following treatment of hNT cells with growth factors and similar results are found by Western blotting and with immunohistochemical staining for presenilin-1 protein. However, treatment of normal human astrocytes with cytokines results in minimal changes in presenilin-1 messenger RNA and protein. Interestingly, the expression of presenilin-1 in human U87 MG astrocytoma and human SK-N-SH neuroblastoma cells is only increased when cells are treated with glial cell line-derived neurotrophic factor or transforming growth factor-beta3. These findings suggest that endogenous presenilin-1 gene expression in human neurons can be induced by growth factors present in normal and diseased brain tissue. Cytokines may play a major role in regulating expression of presenilin-1 which may affect its biological actions in physiological and pathological conditions.

A neutral pH silver development method for the visualization of 1-nanometer gold particles in pre-embedding electron microscopic immunocytochemistry.

The availability of 1-nm gold particles permits the use of a particulate label with standard pre-embedding electron microscopic immunocytochemical techniques. We have employed these particles to localize a synaptic vesicle protein, p65, and a growth-associated protein, GAP-43, in neuron cell cultures. To be detected by standard transmission electron microscopy, these ultra-small gold particles must be enlarged. We have applied a commercially available silver development kit (IntenseM), the method of Danscher, and a neutral pH development procedure which we developed to effect this enlargement. Although IntenseM permits development with good preservation of morphology, it is limited by lack of reproducibility and by variability of final particle size. The method of Danscher provides well-controlled and reproducible enlargement, but is limited with respect to preservation of ultrastructural details. The neutral pH development procedure reproducibly enlarges gold particles with superior preservation of morphology. The use of this development procedure in conjunction with 1-nm gold probes should permit precise ultrastructural localization of a variety of intracellular antigens.

Localization and characterization of speech arrest during transcranial magnetic stimulation.

OBJECTIVE: To determine the anatomic and physiologic localization of speech arrest induced by repetitive transcranial magnetic stimulation (rTMS), and to examine the relationship of speech arrest to language function. METHODS: Ten normal, right-handed volunteers were tested in a battery of language tasks during rTMS. Four underwent mapping of speech arrest on a 1 cm grid over the left frontal region. Compound motor action potentials from the right face and hand were mapped onto the same grid. Mean positions for speech arrest and muscle activation were identified in two subjects on 3-dimensional MRI. RESULTS: All subjects had lateralized arrest of spontaneous speech and reading aloud during rTMS over the left posterior-inferior frontal region. Writing, comprehension, repetition, naming, oral praxis, and singing were relatively spared (P < .05). Stimulation on the right during singing abolished melody in two subjects, but minimally affected speech production. The area of speech arrest overlay the caudal portion of the left precentral gyrus, congruous with the region where stimulation produced movement of the right face. CONCLUSIONS: The site of magnetic speech arrest appears to be the facial motor cortex. Its characteristics differ from those of classic aphasias, and include a prominent dissociation among different types of speech output.

A behavioral task predicts conversion to mild cognitive impairment and Alzheimer's disease.

BACKGROUND/RATIONALE: Currently, we cannot reliably differentiate individuals at risk of cognitive decline, for example, mild cognitive impairment (MCI), Alzheimer's disease (AD), from those individuals who are not at risk. METHODS: A total of 32 participants with MCI and 60 control (CON) participants were tested on an innovative, sensitive behavioral assay, the visual paired comparison (VPC) task using infrared eye tracking. The participants were followed for 3 years after testing. RESULTS: Scores on the VPC task predicted, up to 3 years prior to a change in clinical diagnosis, those patients with MCI who would and who would not progress to AD and CON participants who would and would not progress to MCI. CONCLUSIONS: The present findings show that the VPC task can predict impending cognitive decline. To our knowledge, this is the first behavioral task that can identify CON participants who will develop MCI or patients with MCI who will develop AD within the next few years.

Alterations in cortical thickness and white matter integrity in mild cognitive impairment measured by whole-brain cortical thickness mapping and diffusion tensor imaging.

BACKGROUND AND PURPOSE: Mild cognitive impairment (MCI) is a risk factor for Alzheimer disease and can be difficult to diagnose because of the subtlety of symptoms. This study attempted to examine gray matter (GM) and white matter (WM) changes with cortical thickness analysis and diffusion tensor imaging (DTI) in patients with MCI and demographically matched comparison subjects to test these measurements as possible imaging markers for diagnosis. MATERIALS AND METHODS: Subjects with amnestic MCI (n = 10; age, 72.2 +/- 7.1 years) and normal cognition (n = 10; age, 70.1 +/- 7.7 years) underwent DTI and T1-weighted MR imaging at 3T. Fractional anisotropy (FA), apparent diffusion coefficient (ADC), and cortical thickness were measured and compared between the MCI and control groups. We evaluated the diagnostic accuracy of 2 methods, either in combination or separately, using binary logistic regression and nonparametric statistical analyses for sensitivity, specificity, and accuracy. RESULTS: Decreased FA and increased ADC in WM regions of the frontal and temporal lobes and corpus callosum (CC) were observed in patients with MCI. Cortical thickness was decreased in GM regions of the frontal, temporal, and parietal lobes in patients with MCI. Changes in WM and cortical thickness seemed to be more pronounced in the left hemisphere compared with the right hemisphere. Furthermore, the combination of cortical thickness and DTI measurements in the left temporal areas improved the accuracy of differentiating MCI patients from control subjects compared with either measure alone. CONCLUSIONS: DTI and cortical thickness analyses may both serve as imaging markers to differentiate MCI from normal aging. Combined use of these 2 methods may improve the accuracy of MCI diagnosis.

Hypertension and cognitive performance in African Americans with Alzheimer disease.

The authors examined the relationship between hypertension and cognitive performance in 34 African-American patients with probable Alzheimer disease. Multiple regression analyses indicated that hypertension was associated with poorer overall performance on the Mattis Dementia Rating Scale, particularly the Initiation/Perseveration and Conceptualization subscales, after controlling for gender, age, and education. The findings suggest that African-American patients with hypertension exhibit greater cognitive impairment, possibly reflecting executive dysfunction.

Adrenomedullin: a vasoactive and natriuretic peptide with therapeutic potential.

Adrenomedullin is a potent endogenous vasodilating and natriuretic peptide that is similar in structure to calcitonin gene-related peptide (CGRP). The gene involved in the synthesis of adrenomedullin has been localized to a single locus on chromosome 11, with specific sites on the genome to regulate transcription. Adrenomedullin is normally found in human plasma and in other organs. It is thought that one of the clearance sites for this peptide is in the pulmonary circulation. Endothelial cells are assumed to be one of the major sources of plasma adrenomedullin. Adrenomedullin is an important factor in regulating local and systemic vascular tone, by its activity as an autocrine/paracrine and circulating hormone. Depending on the site of action, adrenomedullin seems to bind to a CGRP receptor and send signals by either cyclic adenosine monophosphate or nitric oxide. From the results of experiments in animals, it has become clear that adrenomedullin's effects are species-specific. However, what is commonly seen with adrenomedullin is peripheral vasodilatation, a positive inotropic action, increased cardiac output, and increased stroke volume. In addition, adrenomedullin has actions in the brain, lungs, and kidneys to regulate regional hemodynamics. With these activities defined, recent studies have suggested a potential therapeutic role for adrenomedullin.

Synaptophysin has a selective distribution in early endosomes of PC12 cells.

We have studied the endocytic pathway in PC12 cells and localized synaptophysin to a subpopulation of early endosomes. Endocytosis was examined by electron microscopy using horseradish peroxidase as an endocytic tracer. Immediately following brief incubations with horseradish peroxidase, label was seen in small vesicles and tubules which appeared to be part of the tubular early endosomal network. A large vacuolar structure, containing a rim of horseradish peroxidase reaction product and an electron-lucent central region, was also labelled at the earliest time point examined. Within 5 min after the horseradish peroxidase pulse, reaction product was seen in multivesicular bodies. After prolonged chase periods, horseradish peroxidase label was lost from the early endosomal structures and accumulated in large dense vesicles containing lamellar stacks of membranes. The observed pattern of horseradish peroxidase distribution is consistent with delivery of the tracer into tubular and vacuolar early endosomes with subsequent movement of horseradish peroxidase out of these compartments and into lysosomes. Examination of synaptophysin distribution by EM immunocytochemistry following incubations with horseradish peroxidase revealed selective immunogold labelling of early endosomal structures. Notably, small vesicular and tubular profiles were frequently double-labelled while vacuolar early endosomes were only rarely labelled for synaptophysin. Immunocytochemical labelling was not observed in multivesicular bodies or large dense vesicles with lamellar stacks. Results of experiments in which endosomal structures were immunoprecipitated with antibody to synaptophysin were consistent with the immunocytochemical findings. Maximal recovery of endocytosed horseradish peroxidase activity was seen immediately following the horseradish peroxidase pulse, and a significant decrease was seen after brief chase periods. These results indicate the presence of synaptophysin in vesicles and tubules of the early endosomal compartment.

Neuronotypic differentiation results in reduced levels and altered distribution of synaptophysin in PC12 cells.

Several synaptic vesicle proteins including synaptophysin and p65/synaptotagmin are expressed by the pheochromocytoma cell line PC12. Stimulation of these cells with nerve growth factor for 7 days induces morphologic neuronotypic differentiation, but the levels of synaptophysin are markedly reduced. Stimulation with cyclic AMP analogs also produces neuronotypic differentiation of PC12 cells, and the degree of morphologic differentiation induced by these agents parallels their ability to effect reduction in synaptophysin levels. By contrast, levels of p65/synaptotagmin are increased following neuronotypic differentiation. The contrasting effects of neuronotypic differentiation on levels of synaptophysin and p65/synaptotagmin indicate potential differences in the regulation of these proteins in PC12 cells. Immunocytochemical labeling of undifferentiated PC12 cells reveals concentrations of synaptophysin in the perinuclear region. After neuronotypic differentiation, there is reduction in perinuclear labeling and concentration of label in swellings along PC12 cell processes. At the ultrastructural level, synaptophysin labeling is found on similar organelles in both undifferentiated and nerve growth factor-stimulated PC12 cells. Although the highest labeling densities were seen on small clear vesicles, specific labeling was also seen on dense core vesicles. The presence of synaptophysin on both small clear vesicles and dense core vesicles indicates potential functional similarities in these vesicle types. The changes in the levels and immunocytochemical distribution of synaptophysin after neuronotypic differentiation suggest possible functional heterogeneity among morphologically similar populations of small clear vesicles.

Endogenous presenilin-1 targets to endocytic rather than biosynthetic compartments.

Presenilin-1 (PS1), which is linked to familial Alzheimer's disease, participates in the proteolytic processing of Notch and amyloid-beta precursor protein (APP) by an unknown mechanism. Reports of PS1 localization to the endoplasmic reticulum (ER) and Golgi apparatus have focused attention on the early biosynthetic pathway as the site of PS1 function. However, it is unclear how Notch cleavage and APP processing events which occur at or near the cell surface are influenced by PS1. In contrast to some earlier studies, examination of endogenously expressed PS1 in PC12 cells by subcellular fractionation and immunofluorescence microscopy revealed a distribution distinct from that of ER and Golgi markers. Rather, PS1 colocalized with transferrin receptor, a marker for early endosomes. In addition, electron microscopic examination of intact vesicles immunoisolated with PS1 antibodies allowed visualization of endocytic tracer in endosomes. These findings identify an early endosomal pool of PS1 and suggest alternative mechanisms for PS1 interactions with APP and Notch.

GAP-43 distribution is correlated with development of growth cones and presynaptic terminals.

GAP-43 (F1, B-50, pp46) has been associated with neuronal development and regeneration, but precise localization within neurons is not known. Pre-embedding electron microscopic immunocytochemistry using silver-enhanced 1 nm gold particles was used to localize GAP-43 label in cell cultures of cerebellar neurons. In the plasma membranes of early cultures, high levels of GAP-43 were seen in all parts of the neuron. In older cultures, consistent with previous reports, the first loss of GAP-43 label was seen in the soma and then the axon. Growth cones had high levels of GAP-43 label on the plasma membrane, with increased distribution over unattached relative to attached filopodia. The amount of GAP-43 seen over the plasma membrane of forming presynaptic terminals is lower than over growth cones, indicating a possible correlation between the presence of GAP-43 and the stage of presynaptic terminal development. Intracellular GAP-43 in axons and growth cones was highest in membranes of smooth cisternae. The levels of GAP-43 in smooth cisternae in axons fell by seven days in culture while the levels of GAP-43 in smooth cisternae of growth cones fell at 14 days. When mini-explant cerebellar cultures were examined with light microscopic immunocytochemistry, GAP-43 label of plasma membrane was highest at the periphery of the radial axonal outgrowth, suggesting that addition of GAP-43 to the plasma membrane can occur in the distal axon or at the growth cone.

Redistribution of GAP-43 during growth cone development in vitro; immunocytochemical studies.

The growth-associated protein GAP-43 (B-50, F1, pp46), has been found in elongating axons during development and regeneration, and has also been associated with synaptic plasticity in mature neurons. We have examined the loss of GAP-43 labelling from cerebellar granule cells with immunocytochemical localization of a polyclonal antibody to GAP-43. One day after plating, the plasma membrane of cell bodies, neurites and growth cones were all labelled with anti-GAP-43. By 10 days, most of the cell body labelling was lost, and by 20 days the neuritic and growth cone labelling was greatly reduced. Beginning at six days, anti-GAP-43 labelling of growth cones, which was initially uniform, became clustered. When growth cones were double-labelled with antibodies to GAP-43 and the synaptic vesicle protein, p65, inverse changes in the distribution of label was observed. While growth cone labelling with anti-p65 increased from three to 20 days in culture, GAP-43 label began to be lost from some growth cones by six days and showed continuing decline through 20 days. For individual growth cones, the loss of GAP-43 appeared to parallel the accumulation of p65, and first growth cones to lose GAP-43 appeared to be the first to accumulate p65 label. When cultures were grown on a substrate of basement membrane material, the time frames of neuritic outgrowth, loss of GAP-43 labelling, and increase in p65 labelling were all accelerated. At five days, labelling for GAP-43 was weak and labelling for p65 was strong, in a pattern comparable to that seen in older cultures on a polylysine substrate. These results suggest several conclusions concerning the expression and loss of GAP-43 in cultured cerebellar granule neurons. First, GAP-43 label is initially distributed in all parts of these cells. With increasing time in culture the label is first lost from cell bodies and later from neurites and growth cones. Second, the loss of GAP-43 label from growth cones is correlated with the appearance of the synaptic vesicle protein p65. Finally, in vitro developmental changes in the loss of GAP-43 can be altered by changing the growth substrate.

The regulation of presenilin-1 by nerve growth factor.

Presenilin-1 (PS1) protein concentration is linked to neuronal development and to the pathogenesis of Alzheimer's disease, yet little is known about the biological factors and mechanisms that control cellular levels of PS1 protein. As PS1 levels are highest in the developing brain, we tested whether neurotrophin-induced differentiation influences PS1 expression using neuronotypic pheochromocytoma (PC12) cells. Treatment of PC12 cells with nerve growth factor (NGF) caused approximately 60-75% increases in the steady-state levels of endogenous PS1 N- and C-terminal fragments. PS1 protein accumulation was dose-responsive to NGF and required the presence of the TrkA NGF receptor tyrosine kinase. NGF also induced PS1 fragment accumulation in cultured explants of rat dorsal root ganglia. Quantitative northern blot analysis using PC12 cultures indicated that NGF did not increase steady-state PS1 mRNA levels. However, pulse-chase experiments indicated that NGF slowed the degradation rate of endogenous PS1 fragments, increasing the half-life from t(1/2) @22.5 to @25.0 h. This increase in half-life was insufficient to account for the approximately 60-75% increase in PS1 fragment levels measured in NGF-treated cells. Thus, NGF may regulate PS1 protein concentration in NGF-responsive cells by a complex mechanism that increases PS1 fragment production independent of holoprotein synthesis.

Rab5-dependent trafficking of the m4 muscarinic acetylcholine receptor to the plasma membrane, early endosomes, and multivesicular bodies.

The m4 subtype of muscarinic acetylcholine receptor regulates many physiological processes and is a novel therapeutic target for neurologic and psychiatric disorders. However, little is known about m4 regulation because of the lack of pharmacologically selective ligands. A crucial component of G protein-coupled receptor regulation is intracellular trafficking. We thus used subtype-specific antibodies and quantitative immunocytochemistry to characterize the intracellular trafficking of m4. We show that following carbachol stimulation, m4 co-localizes with transferrin, and the selective marker of early endosomes, EEA1. In addition, m4 intracellular localization depends on Rab5 activity. The dominant negative Rab5S34N inhibits m4 endocytosis initially following carbachol stimulation, and reduces the size of m4 containing vesicles. The constitutively active Rab5Q79L enhances m4 intracellular distribution, even in unstimulated cells. Rab5Q79L also produces strikingly enlarged vacuoles, which by electron microscopy contain internal vesicles, suggesting that they are multivesicular bodies. m4 localizes both to the perimeter and interior of these vacuoles. In contrast, transferrin localizes only to the vacuole perimeter, demonstrating divergence of m4 trafficking from the pathway followed by constitutively endocytosed transferrin. We thus suggest a novel model by which multivesicular bodies sort G protein-coupled receptors from a transferrin-positive recycling pathway to a nonrecycling, possibly degradative pathway.

Subcellular localization of presenilins: association with a unique membrane pool in cultured cells.

We have investigated the subcellular distribution of presenilin-1 (PS1) and presenilin-2 (PS2) in a variety of mammalian cell lines. In Iodixanol-based density gradients, PS1 derivatives show a biphasic distribution, cofractionating with membranes containing ER-resident proteins and an additional population of membranes with low buoyant density that do not contain markers of the Golgi complex, ERGIC, COP II vesicles, ER exit compartment, COP II receptor, Golgi SNARE, trans-Golgi network, caveolar membranes, or endocytic vesicles. Confocal immunofluorescence and immunoelectron microscopy studies fully supported the fractionation studies. These data suggest that PS1 fragments accumulate in a unique subcompartment(s) of the ER or ER to Golgi trafficking intermediates. Interestingly, the FAD-linked PS1 variants show a marked redistribution toward the heavier region of the gradient. Finally, and in contrast to PS1, PS2 fragments are detected preponderantly in more densely sedimenting membranes, suggesting that the subcellular compartments in which these molecules accumulate are distinct.

Multiple effects of aspartate mutant presenilin 1 on the processing and trafficking of amyloid precursor protein.

PS1 deficiency and expression of PS1 with substitutions of two conserved transmembrane aspartate residues ("PS1 aspartate variants") leads to the reduction of Abeta peptide secretion and the accumulation of amyloid precursor protein (APP) C-terminal fragments. To define the nature of the "dominant negative" effect of the PS1 aspartate variants, we stably expressed PS1 harboring aspartate to alanine substitutions at codons 257 (D257A) or 385 (D385A), singly or in combination (D257A/D385A), in mouse neuroblastoma, N2a cells. Expression of the PS1 aspartate variants resulted in marked accumulation of intracellular and cell surface APP C-terminal fragments. While expression of the D385A PS1 variant reduced the levels of secreted Abeta peptides, we now show that neither the PS1 D257A nor D257A/D385A variants impair Abeta production. Surprisingly, the stability of both immature and mature forms of APP is dramatically elevated in cells expressing PS1 aspartate variants, commensurate with an increase in the cell surface levels of APP. These findings lead us to conclude that the stability and trafficking of APP can be profoundly modulated by coexpression of PS1 with mutations at aspartate 257 and aspartate 385.

Light and electron microscopic localization of presenilin-1 in primate brain.

Several genes have been implicated in the pathogenesis of early-onset familial Alzheimer's disease. A majority of the autosomal dominant cases are linked to recently identified mutations in the presenilin-1 gene on chromosome 14. The native presenilin-1 protein in primates has not been well characterized, and its precise localization is unknown. We have studied the native presenilin-1 protein in monkey brain and peripheral tissues by using a monoclonal antibody specific for the N-terminal domain of human presenilin-1. Western blots detect polypeptide species of approximately 49 and approximately 32 kDa from COS-7 and PC12 cells transfected with full-length human presenilin-1 cDNA and from in vitro translations of the normal human presenilin-1 mRNA. A 32 kDa polypeptide is detected in monkey peripheral tissues, with the highest expression in testis and lung. In all brain regions the 32 kDa band is the predominant form of presenilin-1, and it is found in particulate subfractions. Light microscopic immunocytochemistry reveals presenilin-1 staining in all brain regions, with the strongest labeling in neurons and neuropil. In addition, weaker immunoreactivity is also present in glia and blood vessels. Neuronal staining shows significant variability, with particularly intense labeling of certain cell types, including large neocortical and hippocampal pyramidal neurons, magnocellular basal forebrain neurons, brainstem motoneurons, and some populations of interneurons. By electron microscopic immunocytochemistry, highly selective presenilin-1 staining is seen on the cytoplasmic surfaces of membranous organelles, which suggest localization to the endoplasmic reticulum-Golgi intermediate compartment, a subdomain of the endoplasmic reticulum, and some coated transport vesicles.

Presenilin-1 protein expression in familial and sporadic Alzheimer's disease.

Mutations of the presenilin PS1 and PS2 genes are closely linked to aggressive forms of early-onset (< 60 years) familial Alzheimer's disease. A highly specific monoclonal antibody was developed to identify and characterize the native PS1 protein. Western blot analyses revealed a predominant 32-kd immunoreactive polypeptide in a variety of samples, including PC12 cells transfected with human PS1 complementary DNA, brain biopsy specimens from demented patients, and postmortem samples of frontal neocortex from early-onset familial Alzheimer's disease cases (PS1 and PS2), late-onset sporadic Alzheimer's disease cases, and cases of other degenerative disorders. This truncated polypeptide contains the N-terminus of PS1 and appeared unchanged across cases. In 2 early-onset cases linked to missense mutations in the PS1 gene, a PS1 immunoreactive protein (approximately 49 kd) accumulated in the frontal cortex. This protein was similar in size to full-length PS1 protein present in transfected cells overexpressing PS1 complementary DNA, and in lymphocytes from an affected individual with a deletion of exon 9 of the PS1 gene, suggesting that mutations of the PS1 gene peturb the endoproteolytic processing of the protein. Immunohistochemical studies of control brains revealed that PS1 is expressed primarily in neurons, with the protein localized in the soma and dendritic processes. In contrast, PS1 showed striking localization to the neuropathology in early-onset familial Alzheimer's disease and sporadic Alzheimers' disease cases. PS1 immunoreactivity was present in the neuritic component of senile plaques as well as in neurofibrillary tangles. Localization of PS1 immunoreactivity in familial and sporadic Alzheimer's disease suggests that genetically heterogeneous forms of the disease share a common pathophysiology involving PS1 protein.