Platelet membrane abnormality in Alzheimer's disease

Platelets in Amyloidogenic Mice Are Activated and Invade the Brain

Alzheimer’s disease (AD) is a neurodegenerative disease with a complex and not fully understood pathogenesis. Besides brain-intrinsic hallmarks such as abnormal deposition of harmful proteins, i.e., amyloid beta in plaques and hyperphosphorylated Tau in neurofibrillary tangles, blood-derived elements, in particular, platelets have been discussed to be involved in AD pathogenesis. The underlying mechanisms, however, are rather unexplored. Here, we investigate a potential role of platelets in an AD transgenic animal model with severe amyloid plaque formation, the APP-PS1 transgenic mice, and analyzed the presence, spatial location and activation status of platelets within the brain. In APP-PS1 mice, a higher number of platelets were located within the brain parenchyma, i.e., outside the cerebral blood vessels compared to WT controls. Such platelets were activated according to the expression of the platelet activation marker CD62P and to morphological hallmarks such as membrane protrusions. In the brain, platelets were in close contact exclusively with astrocytes suggesting an interaction between these two cell types. In the bloodstream, although the percentage of activated platelets did not differ between transgenic and age-matched control animals, APP-PS1 blood-derived platelets showed remarkable ultrastructural peculiarities in platelet-specific organelles such as the open canalicular system (OCS). This work urges for further investigations on platelets and their yet unknown functional roles in the brain, which might go beyond AD pathogenesis and be relevant for various age-related neurodegenerative diseases.

Platelets, a reliable source for peripheral Alzheimer's disease biomarkers?

Peripheral biomarkers play an indispensable role in quick and reliable diagnoses of any kind of disease. With the population ageing, the number of people suffering from age-related diseases is expected to rise dramatically over the coming decades. In particular, all types of cognitive deficits, such as Alzheimer's disease, will increase. Alzheimer's disease is characterised mainly by coexistence of amyloid plaques and neurofibrillary tangles in brain. Reliable identification of such molecular characteristics antemortem, however, is problematic due to restricted availability of appropriate sample material and definitive diagnosis is only possible postmortem. Currently, the best molecular biomarkers available for antemortem diagnosis originate from cerebrospinal fluid. Though, this is not convenient for routine diagnosis because of the required invasive lumbar puncture. As a consequence, there is a growing demand for additional peripheral biomarkers in a more readily accessible sample material. Blood platelets, due to shared biochemical properties with neurons, can constitute an attractive alternative as discussed here. This review summarises potential platelet Alzheimer's disease biomarkers, their role, implication, and alteration in the disease. For easy comparison of their performance, the Hedge effect size was calculated whenever data were available.

Blood platelets in the progression of Alzheimer's disease

Alzheimer's disease (AD) is characterized by neurotoxic amyloid-ß plaque formation in brain parenchyma and cerebral blood vessels known as cerebral amyloid angiopathy (CAA). Besides CAA, AD is strongly related to vascular diseases such as stroke and atherosclerosis. Cerebrovascular dysfunction occurs in AD patients leading to alterations in blood flow that might play an important role in AD pathology with neuronal loss and memory deficits. Platelets are the major players in hemostasis and thrombosis, but are also involved in neuroinflammatory diseases like AD. For many years, platelets were accepted as peripheral model to study the pathophysiology of AD because platelets display the enzymatic activities to generate amyloid-ß (Aß) peptides. In addition, platelets are considered to be a biomarker for early diagnosis of AD. Effects of Aß peptides on platelets and the impact of platelets in the progression of AD remained, however, ill-defined. The present study explored the cellular mechanisms triggered by Aß in platelets. Treatment of platelets with Aß led to platelet activation and enhanced generation of reactive oxygen species (ROS) and membrane scrambling, suggesting enhanced platelet apoptosis. More important, platelets modulate soluble Aß into fibrillar structures that were absorbed by apoptotic but not vital platelets. This together with enhanced platelet adhesion under flow ex vivo and in vivo and platelet accumulation at amyloid deposits of cerebral vessels of AD transgenic mice suggested that platelets are major contributors of CAA inducing platelet thrombus formation at vascular amyloid plaques leading to vessel occlusion critical for cerebrovascular events like stroke.

Alzheimer disease and platelets: how's that relevant

Alzheimer Disease (AD) is the most common neurodegenerative disorder worldwide, and account for 60% to 70% of all cases of progressive cognitive impairment in elderly patients. At the microscopic level distinctive features of AD are neurons and synapses degeneration, together with extensive amounts of senile plaques and neurofibrillars tangles. The degenerative process probably starts 20-30 years before the clinical onset of the disease. Senile plaques are composed of a central core of amyloid β peptide, Aβ, derived from the metabolism of the larger amyloid precursor protein, APP, which is expressed not only in the brain, but even in non neuronal tissues. More than 30 years ago, some studies reported that human platelets express APP and all the enzymatic activities necessary to process this protein through the same pathways described in the brain. Since then a large number of evidence has been accumulated to suggest that platelets may be a good peripheral model to study the metabolism of APP, and the pathophysiology of the onset of AD. In this review, we will summarize the current knowledge on the involvement of platelets in Alzheimer Disease. Although platelets are generally accepted as a suitable model for AD, the current scientific interest on this model is very high, because many concepts still remain debated and controversial. At the same time, however, these still unsolved divergences mirror a difficulty to establish constant parameters to better defined the role of platelets in AD.

Platelet amyloid precursor protein forms in AD: a peripheral diagnostic tool and a pharmacological target

Alzheimer Disease (AD) is characterized by the progressive deposition of beta-amyloid in the parenchyma and cerebral microvasculature. The beta-amyloid peptide derives from the metabolism of a larger precursor, Amyloid Precursor Protein (APP). This protein is present in central nervous system, but it is also expressed in peripheral tissues such as circulating cells. An alteration of the APP forms pattern in platelets has been recently reported in AD patients when compared to platelets both of control subjects or non AD patients (NADD). The accuracy of the assay to identify AD is high and decreased levels are found throughout the course of AD with a significant association with severity of symptoms. Moreover, a recent study has demonstrated that AD patients on donepezil (5 mg daily) for 4 weeks displayed two-fold increase in their APPr baseline levels up to normal range. Thus, platelet APP ratio (APPr) holds the potential to be a clinical marker, which might be of helpful and adjunctive value in the diagnosis of AD and in tracking the course of illness, also in the early stages when pharmacological treatment has the greatest potential of being effective.

Synthesis and processing of apolipoprotein E in human brain cultures

Apolipoprotein E (apoE) plays a role in the distribution of lipid within many organs and cell types in the human body, including neurons and astrocytes of the central nervous system (CNS). The apoE4 isoform is also a genetic risk factor for late onset Alzheimer's disease (AD). However, the mechanism by which apoE is involved in AD is largely unknown. In order to understand how apoE is involved in the distribution of lipid in the CNS, we sought to investigate not only the origin of intraneuronal apoE, but the pathway by which it is processed once synthesized. We have established that human neurons can synthesize apoE in the presence of astrocytes, and that intracellular neuronal apoE is processed through the rough endoplasmic reticulum, golgi, and CD63-positive lysosomes where it may be stored before secretion. Our results also suggest that apoE synthesis is regulated by a feedback mechanism, controlled by the neuron itself. This regulatory mechanism may be essential to the maintenance of neuronal cholesterol concentrations and in turn membrane stability.

Amyloid beta peptide membrane perturbation is the basis for its biological effects

Experimental studies have indicated that the mechanisms offered for explaining the neurotoxicity of amyloid beta peptide (AbetaP) are diverse, and include altered enzyme activities, disrupted calcium homeostasis, and increased free radical formation. AbetaP appears to interact at the cell membrane with a multitude of receptor sites and also inserts physically into the membrane matrix. This membrane insertion affects the membrane fluidity and potentially influences the function of resident membrane proteins. We propose a unifying hypothesis to explain the experimental observations of the diverse cellular responses to AbetaP. The indiscriminate physical insertion of AbetaP into the cell membrane unspecifically activates a host of membrane processes by perturbation of the membrane proteins. This recurrent activation of membrane processes eventually culminates in neuronal cell death. We recommend that successful therapeutic interventions should be directed at reducing or preventing the interaction of AbetaP with neuronal cell membranes.

Brain endothelial cell enzymes cleave platelet-retained amyloid precursor protein

We have previously demonstrated that thrombin-activated platelets from patients with advanced Alzheimer's disease (AD) retain significantly more surface membrane-bound amyloid precursor protein (mAPP) than platelets from non-demented age-matched individuals (AM). We have studied interactions between these platelets and the cerebrovascular endothelium to which activated platelets adhere in a model system, investigating their involvement in the formation of amyloid beta peptide (Abeta) deposits in AD patients. We report here that there appear to be alpha and beta secretase-like activities in primary human blood brain barrier endothelial cell (BEC) cultures from both AD patients and AM control subjects (AD-BEC and AM-BEC, respectively) as well as a gamma secretase-like activity that appears only in AD-BEC. No such activities were observed in human umbilical vein endothelial cells (HUVECs). Furthermore, there is more penetration of the platelet-released products platelet factor 4 and soluble APP through the BEC layer grown from AD patients than that grown from AM individuals, whereas none penetrate through a HUVEC layer. Thus the interaction between platelets, the APP they have retained or released, and cerebral vascular endothelial cells may be at least partially responsible for amyloidogenic deposits around the cerebral vasculature of AD patients.

Importance of immunological and inflammatory processes in the pathogenesis and therapy of Alzheimer's disease

The contribution of autoimmune processes or inflammatory components in the etiology and pathogenesis of Alzheimer's disease (AD) has been suspected for many years. The presence of antigen-presenting, HLA-DR-positive and other immunoregulatory cells, components of complement, inflammatory cytokines and acute phase reactants have been established in tissue of AD neuropathology. Although these data do not confirm the immune response as a primary cause of AD, they indicate involvement of immune processes at least as a secondary or tertiary reaction to the preexisting pathogen and point out its driving-force role in AD pathogenesis. These processes may contribute to systemic immune response. Thus, experimental and clinical studies indicate impairments in both humoral and cellular immunity in an animal model of AD as well as in AD patients. On the other hand, anti-inflammatory drugs applied for the treatment of some chronic inflammatory diseases have been shown to reduce risk of AD in these patients. Therefore, it seems that anti-inflammatory drugs and other substances which can control the activity of immunocompetent cells and the level of endogenous immune response can be valuable in the treatment of AD patients.

Effects of dietary marine oils and olive oil on fatty acid composition, platelet membrane fluidity, platelet responses, and serum lipids in healthy humans

The influence of various dietary marine oils and olive oil on fatty acid composition of serum and platelets and effects on platelets and serum lipids were investigated as part of an extensive study of the effects of these oils on parameters associated with cardiovascular/thrombotic diseases. Healthy volunteers (266) consumed 15 mL/d of cod liver oil (CLO); whale blubber oil (refined or unrefined); mixtures of seal blubber oil and CLO; or olive oil/CLO for 12 wk. In the CLO, seal oil/CLO, and whale oil groups, serum levels of eicosapentaenoic acid (EPA) were increased. In platelets, EPA was increased in the CLO, seal/CLO, and olive oil/CLO groups. The localization of n-3 polyunsaturated fatty acids in the triacylglycerols did not seem to influence their absorption. Intake of oleic acid is poorly reflected in serum and platelets. No significant differences in triacylglycerols (TG), total cholesterol, or high density lipoprotein cholesterol were observed, even though TG were reduced in the CLO, CLO/seal oil, and whale oil groups. Mean platelet volume increased significantly in both whale oil groups and the CLO/olive oil group. Platelet count was significantly reduced in the refined whale oil group only. Lipopolysaccharide-stimulated blood tended to generate less thromboxane B2 in CLO, CLO/seal, and CLO/olive groups. The whale oils tended to reduce in vivo release of beta-thromboglobulin. In conclusion, intake of various marine oils causes changes in platelet membranes that are favorably antithrombotic. The combination of CLO and olive oil may produce better effects than these oils given separately. The changes in platelet function are directly associated with alterations of fatty acid composition in platelet membranes.

Moderate and advanced Alzheimer's patients exhibit platelet activation differences

We previously reported that platelets from advanced sporadic Alzheimer's disease (AD) patients exhibit two defects: first, an aberrant signal transduction presenting as a thrombin-induced hyperacidification, which is more severe for donors with the apolipoprotein E4 allele (apoE4), and second, an AD-specific Amyloid Precursor Protein (APP) processing defect that presents as retention of APP on the activated platelets' surface and in independent of the apo E allele. This retention of membrane APP correlates with decreased release of soluble APP. To determine at what stage in the disease progression these defects appear, we performed signal transduction and secretion studies on moderate AD patients. Thrombin-activated platelets from these patients do not exhibit either hyperacidification or APP retention; their APP processing and secretion are normal by Western blotting, suggesting that the two platelet defects appear in the advanced stages of AD.

Lymphocytes and neutrophils as peripheral models to study the effect of beta-amyloid on cellular calcium signalling in Alzheimer's disease

According to the calcium hypothesis of brain aging, disturbances of free intracellular calcium homeostasis ([Ca2+]i) play a key role in pathology of Alzheimer's disease (AD). Recent data from neuronal tissue culture support the contribution of the beta-amyloid peptide (beta A) to neurodegeneration in AD, probably by disruption of the intracellular Ca2+ regulation. On the basis of this premise, we used peripheral blood cells to examine the role of beta A on Ca2+ signalling, not only to obtain an experimental approach to investigate these effects of beta A in man, but also to search for AD-specific alterations of the effects of beta A on Ca2+ signalling. This approach is based on observations indicating that the phytohemagglutinin (PHA)-induced Ca2+ response in circulating human lymphocytes of healthy volunteers is affected by beta A and its fragment 25-35 in a fashion similar to its effects on central neurons, whereas we found no effect of beta A on receptor-activated Ca2+ response in neutrophils. Therefore, we used human blood lymphocytes as peripheral model systems to search directly for AD-related abnormalities of Ca2+ regulation, for alterations of beta A effects on Ca2+ signalling and on membrane fluidity, and for possible changes of potassium channels. In accordance with our data in neutrophils, we were unable to identify any relevant change of the PHA-induced Ca2+ elevations in lymphocytes, which is not supporting the assumption of general alterations of cellular Ca2+ regulation in AD. On the other hand, the amplifying effect of beta A on Ca2+ signalling was significantly reduced in lymphocytes from AD patients. Moreover, Ca2+ responses to beta A25-35 were not different between early- and late-onset AD patients. Our findings indicate that the sensitivity of the lymphocyte for the effects of beta A is reduced in a high percentage of patients with probable or possible AD. As possible explanation we observed a similar reduction of the sensitivity of the lymphocyte membrane for the fluidity-decreasing properties of beta A. Finally, the inhibition of the PHA-induced Ca2+ response by tetraethylammonium (TEA) was lower in the AD group compared to aged controls. This could suggest the presence of a K+ channel dysfunction on AD lymphocytes, as it has been shown on skin fibroblasts of AD patients.

Peripheral antioxidant enzyme activities and selenium in elderly subjects and in dementia of Alzheimer's type--place of the extracellular glutathione peroxidase

Defenses against free radical damage were determined in red blood cells and plasma from 40 patients with dementia of the Alzheimer-type (DAT) and 34 aged control subjects with normal cognitive function. No crude significant difference in erythrocyte copper-zinc superoxide dismutase (E-CuZnSOD), seleno-dependent glutathione peroxidase (E-GSH-Px), glutathione reductase (E-GSSG-RD) activities, and selenium (Se) concentration was found between DAT cases and control subjects. The peroxidation products evaluated in plasma by the thiobarbituric-reactive material (TBARS) were at the same level in the DAT group as compared to controls. In the DAT group, plasma GSH-Px (P-GSH-Px) activity and plasma Se (P-Se) were negatively correlated with age (r = -0.58; p < 0.001 and r = -0.63; p < 0.001 respectively). Moreover, erythrocyte GSH-Px activity and Se were also negatively correlated with age (r = -0.40; p < 0.01 and r = -0.46; p < 0.01, respectively). No significant correlation with age was observed in the controls. When controlling for age, a significant increase for P-GSH-Px activity and P-Se was observed in DAT patients as compared to controls. These significant differences mostly appeared in DAT subjects under 80 years. Some correlations were only observed in the DAT group such as P-GSH-Px and E-GSH-Px (r = +0.68; p < 0.001); P-GSH-Px and E-Se (r = +0.79; p < 0.001). Correlations between P-GSH-Px and P-Se, E-GSH-Px and P-Se, and P-Se with E-Se are greater in the DAT group (r = +0.84; p < 0.001; r = +0.76; p < 0.001 and r = 0.75; p < 0.001) than in the control group (r = 0.54, pI < 0.01; r = 0.43, p < 0.01 and r = +0.34, p < 0.05 respectively). The fact that first -- a significant increase in P-GSH-Px and P-Se, second -- some modifications in the relationships between antioxidant parameters, and third -- age-dependent decreases of glutathione-peroxidase activities and their cofactor, appeared only in the DAT group suggest that DAT is associated with an oxidative stress due to an imbalance between reactive oxygen species and the peripheral antioxidant opposing forces.

No evidence for systemic oxidant stress in Parkinson's or Alzheimer's disease

Oxidant stress secondary to dopamine metabolism has been proposed as a pathogenic factor in the development of Parkinson's disease. Biochemical abnormalities extending beyond the central nervous system have been identified in patients with this condition. Previous investigators have found abnormally elevated concentrations of the lipid peroxidation product, malondialdehyde, in the plasma and serum of patients with Parkinson's disease. We attempted to replicate these findings but controlled for other factors that could influence malondialdehyde levels. We detected no significant elevations in mean serum malondialdehyde concentrations in either levodopa-treated or untreated patients with Parkinson's disease, compared to normal controls; similarly, no elevation was found in a group of patients with dementia of Alzheimer's type. On the other hand, a group of subjects with diabetes mellitus but no neurodegenerative disease had significantly elevated mean serum malondialdehyde levels, consistent with previous studies of diabetic patients. Autoxidation is one of the two major routes by which dopamine and dopa metabolism may generate oxygen free radicals. We analyzed the autoxidation product of dopa, 5-S-cysteinyl-dopa, in the plasma of these same groups of patients with neurodegenerative disease and normal controls; no significant differences were identified. Serum concentrations of two other antioxidant substances, alpha-tocopherol and uric acid, were also statistically similar in these groups. In conclusion, analysis of several blood products relevant to oxidant stress, including malondialdehyde, 5-S-cysteinyl-dopa, alpha-tocopherol, and uric acid, failed to distinguish patients with Parkinson's disease or dementia of Alzheimer's type from controls.

Decreased methionine adenosyltransferase activity in erythrocytes of patients with dementia disorders

ATP:1-methionine S-adenosyltransferase (EC 2.5.1.6, MAT) activity was analyzed in erythrocytes from nine patients with a clinical diagnosis of probable Alzheimer's disease (Pro.AD), four with possible Alzheimer's disease (Pos.AD), three with mild cognitive dysfunction (MCD) and two with dementia of vascular origin (VD), and 10 age-matched control subjects. Significantly lower kinetic parameters (Vmax and Km towards methionine) for MAT were observed in all the dementia cases. In the subgroup of Pro.AD patients who also had low plasma levels of vitamin B12 (B12), the reduction in MAT Km was significantly correlated with an increase in the serum levels of homocysteine, while no such correlation was observed in all the other dementia groups. Treatment for 6 months of this subgroup of Pro.AD patients with B12 (1 mg x 7 days + 1 mg/week, i.m.), S-adenosylmethionine (SAM, 200 mg twice daily, p.o.) and folate (2.5 mg every 2 days, p.o.) caused a significant decrease in homocysteine in parallel with a significant increase in Km for MAT. These findings support the hypothesis that aberrations in the B12 dependent transmethylation reactions might be involved in the pathogenesis of dementia, and suggest that the evaluation of erythrocyte MAT activity may be a useful marker for the detection of such an aberration.

Peripheral markers and diagnostic criteria in Alzheimer's disease: critical evaluations

This review analyzes recent developments in diagnostic criteria and peripheral markers used clinically in the definitive diagnosis of Alzheimer's disease (AD), comparing past and current views, together with a discussion of their shortcoming and difficulties of implementation. Consideration is given to studies on the presence of amyloid substances outside the central nervous system: in cerebrospinal fluid, in plasma, in primary cultures, and in continuous cultures of cell lines of neuronal and glial origin. We discuss alterations of cholinesterases and noradrenaline in red blood cells (RBC) in AD and, with relation to the infectious theory, the presence of spirochaetes in patients. The activities of the enzymes leading to the formation of amyloid substances and those reflecting more general alterations of metabolic processes are considered, both in respect to their role in the pathogenesis of the neurodegenerative disorders of AD and of their potential use as markers. Enzymatic changes have been studied comparing AD patients with non AD controls as well as with AD relatives: proteases and their inhibitors; plasminogen activators; transketolases; increases in the activity of Cu-Zn superoxide dismutase in AD patients' RBC, serum, fibroblasts and cortical neurons, pointing to alterations in oxidative processes; and apolipoprotein E epsilon 4 allele, linked to late-onset AD and familial cases. This review presents reasons why the involvement of peripheral markers in AD should advance from hypothesis to accepted fact.

Platelet morphology in Parkinson's disease: an electron microscopic study

There are no peripheral diagnostic markers for Parkinson's disease (PD). However, recent studies of platelets in PD patients indicate that mitochondrial and monoamine oxidase function may be abnormal. This investigation examines platelets in PD from a morphological standpoint utilizing transmission electron microscopy (EM). Fourteen PD patients (seven treated, seven untreated) and seven age matched controls had platelets separated from other blood components, fixed in a standardized fashion and examined by EM. Platelets (in the activated form because they were collected in glass tubes) were evaluated at magnifications of 15,000x and 40,000x. Abnormalities observed in treated and untreated PD patients included the presence of numerous large intracytoplasmic vacuoles formed from the open canalicular system. Morphometric examination performed at 40,000x magnification indicated that the mean area of vacuoles and the cytoplasmic volume percent of platelets occupied by vacuoles were significantly greater in PD (p < 0.05) than controls. However, differences observed between treated and untreated PD groups suggest that these changes could be caused by the disease or the treatment or both. No abnormalities were found in relation to mitochondria, storage granules and glycogen. From EM assessment, we conclude that platelets in PD are morphologically abnormal.

Platelet membrane fluidity and plasma malondialdehyde levels in Alzheimer's demented patients with and without family history of dementia

Platelet membrane fluidity (PMF) was measured with three different fluorescent probes, 1,6-diphenyl-1,3,5-hexatriene (DPH), 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH), 3-(p-phenyl-1,3,5-hexatrienyl)phenyl-propionic acid (DPH-PA), which labeled different parts of the bilayer (the hydrophobic core and the positively and negatively charged regions, respectively) in Alzheimer's disease (AD) patients with and without a family history of dementia, and in a control group. In support of earlier findings in the literature, significantly increased PMF was found by the application of DPH in both groups with AD. The use of the fluorescence probe TMA-DPH, however, revealed no differences between the groups. In contrast, significant rigidification was observed with DPH-PA, but only in the AD group with a positive family history of dementia. The plasma malondialdehyde levels appeared to be similar in each group. Our findings are discussed in light of the controversies regarding the value of PMF measurements in AD.

Platelet phosphatidylinositol kinase activity is not altered in Alzheimer disease

We previously reported a specific decline in phosphatidylinositol (PI) kinase activity in the neocortex of patients with Alzheimer disease (AD) as compared to controls, whereas phosphatidylinositol phosphate (PIP) kinase activity appeared not to be affected (Jolles et al., 1992). In search of a possible systemic effect of AD, in the present study we investigated phosphoinositide kinase activity in platelets from patients with AD and from control subjects. The study was based on the notion that disease-specific abnormalities in the brain could be reflected in blood platelets. PI kinase activity was studied in platelet homogenates and in a salt-solubilized protein fraction of platelets, because of the difference in subcellular localization of the different types of PI kinases. In addition, NADH cytochrome-C reductase was measured in platelet homogenates as a marker for the endoplasmic reticulum, to detect a possible proliferation of the endoplasmic reticulum. AD patients and normal elderly controls showed no difference in PI kinase activity in either enzyme fraction. Furthermore, NADH cytochrome-C reductase activity and the protein/phospholipid ratio per 10(6) platelets were the same for AD patients and controls. This was taken as an indication that platelets in AD patients do not show proliferation of intracellular membranes.

Evidence for a membrane lipid defect in Alzheimer disease

We have previously shown that cells normally maintain their lipid metabolic pools at a critical composition, appropriate for spontaneous assembly of a stable membrane bilayer at their physiological temperature. When disease affects membrane lipids such that the new composition will only form a stable bilayer at a critical temperature (T*), which differs from the physiological value, membrane destabilization and hence cellular damage will necessarily ensue. We have previously tested this pathogenetic mechanism in metachromatic leukodystrophy, a disorder with a known primary lipid metabolic defect. In the present study, we found T* for cerebral cortex lipids from three Alzheimer disease (AD) patients ranged between 19 and 28 degrees C, independent of membrane protein composition. Control cortex lipids yielded a normal value for T* of 37 degrees C. Thus, one possible mechanism for neurodegeneration in AD is membrane destabilization secondary to a lipid compositional aberration, which shifts T* away from 37 degrees C. This lipid defect is brain region-specific as cerebellar lipids from the AD patients gave a normal value for T*. Studies aimed at delineating the nature of the biochemical anomaly are in progress.

Copper-zinc superoxide dismutase activity in red blood cells and serum in demented patients and in aging

The activity of the enzyme copper-zinc superoxide dismutase (Cu-Zn SOD) has been investigated in serum and red blood cells (RBC) homogenate obtained from demented patients with associated vascular lesions (VD), demented patients with probable Alzheimer's disease (DAT) and healthy controls (CG) of the same age. The increase in SOD activity was statistically significant (P < 0.01) in RBCs homogenate of DAT and VD patients, when compared to controls, but no differences appear between the two diseases groups. Additionally, a statistically significant increase in SOD activity (P < 0.01) in DAT patients above 70 years as compared to those 50-70 years old, and a relation between SOD and age were found. No changes in SOD activity with age in healthy controls nor in vascular dementia group were detected. A statistically significant increase in Circulating SOD activity (P < 0.01) was observed in vascular patients compared to controls. The observed increase in DAT Circulating SOD activity (against CG) was not significant. The increased levels of Cu-Zn SOD, probably represent a general alteration of the oxidative processes characteristic of these dementias and suggest that the enzyme might be used as a marker.

Are thrombocyte membranes altered in Alzheimer's disease? A morphometric and biochemical study

Morphometric analysis of thrombocytes from patients with Alzheimer's disease, from patients with multi-infarct dementia, and from young and age-matched healthy control donors, did not reveal any Alzheimer-related increase in internal membranes. Biochemical analysis showed a reduced cholesterol content of thrombocyte membrane preparations from Alzheimer patients relative to age-matched controls, but not relative to multi-infarct dementia patients. Overall distribution of protein kinase C activity (PKC) between cytosol and membrane, in resting as well as in activated thrombocytes from Alzheimer patients, was similar to that in the control groups. However, both Alzheimer and multi-infarct dementia patients had lower cytosolic levels of basal kinase and PKC activities than age-matched controls, while only Alzheimer patients had lower cytoskeletal PKC activity than controls.

The inhibitory effect of 9-amino-1,2,3,4-tetrahydroacridine (THA) on platelet function

Tetrahydroacridine (THA), or Cognex, is currently awaiting FDA approval for the treatment of Alzheimer's disease. In addition to reports indicating that THA improves the symptoms of patients with Alzheimer's disease, we have found that THA possesses potent antiplatelet activity. THA produced dose-dependent inhibition of human platelet aggregation induced by collagen, ADP, A23187, and phorbol ester. THA, when added to activated platelets, dispersed the platelet aggregates. We have also examined the effects of THA on intracellular Ca++ mobilization, ATP release, and production of cyclic AMP.

Platelet protease nexin-2/amyloid beta-protein precursor. Possible pathologic and physiologic functions

The amyloid beta-protein and its parent protein, amyloid beta-protein precursor (APP), are major constituents of neuritic plaques and cerebrovascular deposits in Alzheimer's disease and Down's syndrome. We reported that the protease inhibitor protease nexin-2 (PN-2) is the secreted form of APP that contains the Kunitz protease inhibitor domain. Previous studies suggested that circulating forms of PN-2/APP exist. Recently, we reported that PN-2/APP is a platelet alpha granule protein and is secreted upon platelet activation. Subsequent studies revealed that platelets are the major circulating repository for PN-2/APP and may contribute to its deposition in Alzheimer's disease. Protease inhibition measurements demonstrated that PN-2/APP is a potent inhibitor of certain serine proteases, particularly intrinsic blood coagulation factor XIa. Together, these findings indicate that PN-2/APP regulates blood coagulation, and possibly other proteolytic events, at sites of vascular injury.

Intracellular membranes are more fluid in platelets of Alzheimer's disease patients

We studied 12 patients with probable Alzheimer's disease versus age- and sex-matched healthy control subjects. Platelets were subfractionated into intracellular membranes and plasma membranes, and steady-state anisotropy of diphenylhexatriene was measured on the preparations as an index of membrane fluidity. Fluidity was higher in intracellular membranes from platelets of Alzheimer's patients compared to controls (P = 0.016). However, no difference was observed in purified plasma membrane's fluidity from the same patients versus controls. Neither the platelet counts, platelet volumes, percent of larger platelets, nor the amount of internal membrane protein per platelet were different between groups. There was no correlation between intracellular membrane anisotropy and severity of dementia as measured on the Mini-Mental State Exam. The results extend previous studies suggesting that there is an intracellular membrane alteration in platelets in Alzheimer's disease.

Reduced natural killer cell activity in patients with dementia of the Alzheimer type

We examined the natural killer (NK) cell activity in 50 patients with dementia of the Alzheimer type (DAT) and 37 age-matched normal controls. The NK cell activity in DAT was significantly lower than in the normal controls. The NK cell activity induced by either interferon-alpha (IFN-alpha) or interleukin-2 (IL-2) in DAT was also significantly lower than in the normal controls. There were no significant differences in the level of serum IL-2 and IFN-alpha between the two groups. As regards NK cell subsets, two-color flow cytometric analysis showed no significant differences between the percentages of Leu-11+ Leu-7- cells, Leu-11+ Leu-7+ cells and Leu-11- Leu-7+ cells in the two groups. Our results suggest that NK cells in DAT may have functional abnormalities and may provide important clues to fundamental cellular and molecular aberrations in DAT.

Platelet membrane fluidity in Alzheimer's disease and multi-infarct dementia: a spin label study

The membrane fluidity of platelets isolated from 15 patients with probable Alzheimer's disease (AD), 11 patients with probable multi-infarct dementia (MID), and 7 neurologically healthy controls was studied by electron spin resonance (ESR) spectroscopy employing spin label techniques. Spin label I(12,3) probed the shallow site (hydrophilic region) and spin label I(5, 10) the deeper site (hydrophobic region) of the platelet membrane. With both probes, a significant increase in membrane fluidity was observed in patients with AD and MID, as compared to age-matched controls. However, there were no significant differences in fluidity between AD and MID patients. Our results suggest an increased platelet membrane fluidity in dementias, but the change seems not to be specific to AD.

Altered membrane anisotropy gradients of plasma membranes of living peripheral blood leukocytes in aging and Alzheimer's disease

Several reports have suggested that membrane rigidity, a term that refers to the relative motion of membrane constituents, is decreased in Alzheimer's Disease. Accordingly, a series of fluorescent membrane probes was used to evaluate the rigidity from the surface to the center of the outer hemi-leaflet of the plasma membrane of living neutrophils, monocytes and lymphocytes. Anisotropy, a parameter which increases with increasing membrane rigidity, was calculated from flow cytometric measurements of vertically and horizontally polarized components of the fluorescence emission of the probes. These preliminary experiments suggest that whereas membrane rigidity in certain regions of the plasma membrane of peripheral blood leukocytes is increased as expected in elderly controls, it is decreased in Alzheimer's disease.

Autopsy samples of Alzheimer's cortex show increased peroxidation in vitro

The formation of thiobarbituric acid-reactive products was measured as an index of peroxidation by oxygen free radicals in homogenates of frontal cortex and cerebellum from brains taken at autopsy and verified histologically as being Alzheimer's (n = 6) or normal (n = 6). Compared with controls, basal peroxidation is significantly higher in Alzheimer's cortex, and this difference is also evident in the presence of exogenous iron. Peroxidation in cerebellum and levels of total glutathione, RNA, and DNA in cortex and cerebellum do not differ significantly between Alzheimer's brain and controls. Iron-induced peroxidation in cortex is reduced by the lazaroid U-74500A, with calculated IC50 values that are significantly higher in Alzheimer's samples (10 microM) than in controls (2.5 microM). These observations suggest that cerebral cortex from Alzheimer's patients differs from controls with respect to in vitro peroxidation.

Increased platelet 3H-imipramine binding and monoamine oxidase B activity in Alzheimer's disease

Several biochemical abnormalities in peripheral tissues have been reported in Alzheimer's disease (AD). With this in mind we studied platelet monoamine oxidase B (MAO B) activity and 3H-imipramine (IMI) binding in both AD patients and healthy subjects and found a significantly higher level of platelet MAO B activity and 3H-IMI Bmax values in the AD patients. In view of the part that MAO B plays in metabolizing serotonin (5HT) and of the relationship which exists between 3H-IMI binding and 5HT uptake, our results would suggest that with AD there occurs a complex dysfunction in the 5HT system, at least at a peripheral level.

SOD-1 activity and platelet membrane fluidity in Alzheimer's disease

An early-onset, familial form of Alzheimer's disease (AD) has been reported to be linked to a locus on the long arm of chromosome 21 (21q21). Furthermore, duplications in the vicinity of this locus involving the beta-amyloid gene and the proto-oncogene ets-2 have been reported in association with AD. The structural gene for Cu,Zn superoxide dismutase, SOD-1, is located between the beta-amyloid gene and ets-2. For this reason and because SOD-1 is a plausible candidate for a gene that might influence the fluidity of cellular membranes, we determined whether or not the subtype of AD with increased platelet membrane fluidity was associated with an increase in Cu,Zn superoxide dismutase activity.

Aluminum-induced neurotoxicity: alterations in membrane function at the blood-brain barrier

Aluminum is established as a neurotoxin, although the basis for its toxicity is unknown. It recently has been shown to alter the function of the blood-brain barrier (BBB), which regulates exchanges between the central nervous system (CNS) and peripheral circulation. The BBB owes its unique properties to the integrity of the cell membranes that comprise it. Aluminum affects some of the membrane-like functions of the BBB. It increases the rate of transmembrane diffusion and selectively changes saturable transport systems without disrupting the integrity of the membranes or altering CNS hemodynamics. Such alterations in the access to the brain of nutrients, hormones, toxins, and drugs could be the basis of CNS dysfunction. Aluminum is capable of altering membrane function at the BBB; many of its effects on the CNS as well as peripheral tissues can be explained by its actions as a membrane toxin.

Gray-matter degeneration in presenile Alzheimer's disease

Previous comparisons between presenile Alzheimer's disease (AD) and senile dementia of the Alzheimer type (SDAT) did not control for disease severity and duration. In the current study, 18 patients with each diagnosis were matched for disease duration, cognitive dysfunction, and behavioral symptoms (using the modified Mini-Mental Status [mMMS] examination and the Blessed Dementia Rating Scale [BDRS] ). Regional cerebral blood flow (rCBF) was quantified by the 133xenon inhalation technique, and several indices of tissue perfusion were examined. The two variables of primary interest were relative gray-matter relative weight (35% in presenile patients versus 39% in senile patients and healthy control subjects, p = 0.006), with neither perfusion nor disease severity differences between the two dementia samples. This loss of gray matter was significantly related to both severity and duration of disease in the patients with presenile AD, but not in patients with SDAT. These findings lend support ot previous suggestions of greater degenerative process in presenile AD and confirm the need to examine and control age of onset in future investigations of AD. Further, correlation analysis suggests greater proportion of common variance among clinical and physiological indices in presenile AD.

Endoplasmic reticulum abnormality in Alzheimer's disease: selective alteration in platelet NADH-cytochrome c reductase activity

Previous evidence suggests that the increase in platelet membrane fluidity associated with a subgroup of patients with Alzheimer's disease results from the accumulation of internal membrane. The specific activities of enzyme markers for selective cell membrane compartments were compared in platelets from subgroups of demented patients with normal or increased fluidity as well as from normal control subjects. A statistically significant change in enzyme activity was observed only for antimycin A-insensitive NADH-cytochrome reductase, a selective marker for smooth endoplasmic reticulum (SER) in platelets. This reduction was limited to the subgroup of demented patients who had increased platelet membrane fluidity, and therefore is not a nonspecific concomitant of neurodegeneration, medication exposure, or chronic illness in general. Since the platelet membrane alteration associated with Alzheimer's disease results from the inheritance of a single major locus, these results suggest that a defect in SER function may exist in brain cells as well as peripheral cells that express this genotype.

Cognitive function and platelet membrane fluidity in Alzheimer's disease

Increased platelet membrane fluidity, as reflected by a decrease in the fluorescence anisotropy of diphenylhexatriene in labeled membranes, identifies a clinically distinct subgroup of approximately 50% of patients at our center who meet NINCDS-ADRDA clinical criteria for Alzheimer's disease. In the current study, we compared the cognitive impairments of patients in this subgroup to those observed in the residual subgroup of patients with Alzheimer's disease who had normal platelet membrane fluidity. No significant differences in the number or distribution of deficits in six cognitive domains were observed between the two subgroups. However, in the subgroup with increased platelet membrane fluidity, there were significantly more patients who exhibited dissociation of deficits on tests related to left and right parietal lobe function than in the residual subgroup. Moreover, the cases with dissociation of deficits consisted almost entirely of patients with deficits on tests reflecting left parietal lobe function and no deficit on tests of right parietal lobe function.

Longitudinal study of platelet membrane fluidity in Alzheimer's disease

Increased platelet membrane fluidity identifies a prominent subgroup of patients with Alzheimer's disease who exhibit distinct clinical features. In the current longitudinal study, the stability of this membrane characteristic was determined for 15 patients with Alzheimer's disease and 10 healthy elderly controls over a 1-year follow-up period.

Monozygotic twins concordant for probable Alzheimer disease and increased platelet membrane fluidity

This report describes monozygotic twins who were concordant for probable Alzheimer disease, as defined by currently-accepted clinical criteria. Monozygosity was established by blood typing. Their ages of symptomatic onset were 57 and 66 yr, and the times from onset to institutionalization were 8 and 2 yr, respectively. These results suggest that age at onset and rate of progression are clinical features that can be affected by random processes or exposure to environmental factors. The platelet membrane fluidity of both twins was abnormally increased, and the respective values were identical within experimental limits. This result is consistent with published data suggesting that increased platelet membrane fluidity is associated with a clinically distinct subtype of Alzheimer disease and that this platelet membrane characteristic may be genetically determined.

Platelet membrane fluidity and treatment response in cognitively-impaired, depressed elderly: initial results

A prospective study of response to treatment with nortriptyline was carried out in a group of 14 inpatients with mixed symptoms of depression and dementia. Platelet membrane fluidity, as reflected by the fluorescence anisotropy of DPH labeled membranes, was determined for all 14 drug-free subjects prior to treatment by staff who were blind to clinical and demographic data. Patients who exhibited worsening of their cognitive impairment exhibited "increased" platelet membrane fluidity, an abnormally found in approximately 50% of patients with Alzheimer's disease.

Family study of platelet membrane fluidity in Alzheimer's disease

The fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene in labeled platelet membranes, an index of membrane fluidity, identifies a prominent subgroup of patients with Alzheimer's disease who manifest distinct clinical features. In a family study, the prevalence of this platelet membrane abnormality was 3.2 to 11.5 times higher in asymptomatic, first-degree relatives of probands with Alzheimer's disease than in neurologically healthy control subjects chosen without regard to family history of dementia. The pattern of the platelet membrane abnormality within families was consistent with that of a fully penetrant autosomal dominant trait. Thus, this abnormality of platelet membranes may be an inherited factor that is related to the development of Alzheimer's disease.

Platelet membrane fluidity identifies a clinical subtype of Alzheimer's disease

1. The fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) in labeled platelet membranes, an index of membrane fluidity, identifies a prominent subgroup (approx. 50%) of patients with Alzheimer's disease who manifest distinct clinical features. 2. We review an integrated series of studies that explore both the clinical significance of this finding and the biological basis for the platelet membrane alteration.