Alzheimer's Diseasea Research Articles

Clinical and neuroimaging features of familial Alzheimer's disease.

Subtle phenotypic differences between familial Alzheimer's disease (FAD) pedigrees can be identified which may reflect the genetic and allelic heterogeneity of the disease. Positron emission tomography (PET) of APP mutation and chromosome 14-linked FAD pedigree members reveals biparietal bitemporal hypometabolism. Scanning of asymptomatic at-risk individuals reveals a similar, but quantitatively less severe, pattern of hypometabolism.

The role of heparan sulfate proteoglycans in the pathogenesis of Alzheimer's disease.

The hallmark of Alzheimer's disease (AD) is the deposition of amyloid plaques and neurofibrillary tangles in the brain. The relationship between amyloid deposition and the cognitive deficit is still unclear. The amyloid beta A4 protein is produced by proteolytic cleavage of the amyloid protein precursor (APP). Very little is known about the normal function of APP and the role the protein may play in pathogenesis. Several studies have shown that APP is important for the regulation of neurite outgrowth. Our studies support these findings and indicate that the neurite outgrowth-promoting effects of APP are stimulated by an interaction between APP and specific proteoglycans. Using site-directed mutagenesis, a heparan sulfate binding site which mediates this effect has been mapped to the N-terminus of APP (residues 96-110, HBD-1). A peptide homologous to HBD-1 blocks the trophic effects of APP in cell culture. To purify specific proteoglycans which stimulate the action of APP, an affinity column was constructed using a biotinylated peptide homologous to HBD-1 coupled to streptavidin-agarose. Two proteoglycans were isolated from a crude brain cell conditioned medium by affinity chromatography. The purified proteoglycans bound APP saturably with high affinity and stimulated the action of APP on neurite outgrowth from chick sympathetic neurons. Digestion of the proteoglycan fraction with heparitinase I or chondroitinase ABC demonstrated the presence of two major proteins, a heparan sulfate proteoglycan with a core protein of 63-67 kD molecular mass and a chondroitin sulfate proteoglycan with a core protein of 100-110 kD molecular mass. The results demonstrate that APP binds to at least two proteoglycans and that this interaction may regulate the trophic effects of the protein. The interaction of specific APP-binding proteoglycans with amyloid plaques may disturb the normal function of APP and contribute to the neuritic degeneration that is commonly seen around the amyloid plaque cores.

Apolipoprotein E alleles and brain vascular pathology in Alzheimer's disease.

The presence of apolipoprotein E (ApoE)-E4 allele has been implicated as a risk factor for Alzheimer's disease (AD). We examined the occurrence of ApoE 4 alleles in AD associated with cerebral amyloid angiopathy and other vascular lesions. We found significantly high frequency of the ApoE 4 allele in AD with moderate to severe CAA. The frequency of the allele was also high in AD cases with other vascular lesions such as multiple infarcts and lacunes. As previously reported, we confirm a greater frequency of the ApoE 4 allele in the diffuse Lewy body variant of AD. Our results suggest ApoE 4 allele to be a significant factor in the development of CAA in AD. While this may be related to increased brain amyloid load as a consequence of ApoE genotype, the possibility exists that ApoE may be a specific factor in vascular abnormalities associated with AD.

Apolipoprotein E, plaques, tangles and cholinergic dysfunction in Alzheimer's disease.

Apolipoprotein E is a plasma cholesterol and phospholipid transporter which plays a central role in lipoprotein metabolism in the brain. Apolipoprotein E is a polymorphic protein with three common alleles in the general population, designated epsilon 2, epsilon 3 and epsilon 4 coding for proteins ApoE2, ApoE3 and ApoE4, respectively. Recent findings have demonstrated a significant relationship between the epsilon 4 allele and late onset familial and sporadic Alzheimer's disease. We examined several classical neuropathological hallmarks of Alzheimer's disease to determine whether they might be related to apolipoprotein E genotype: the presence of intracellular neurofibrillary tangles, extracellular senile plaques, and the attenuation of choline acetyltransferase activity. Significant correlations were found between epsilon 4 allele copy number and senile plaque density in the frontal, parietal and fusiform cortical areas. Similarly, significant correlations were also found with increased neurofibrillary tangle number in the frontal and fusiform cortex. Interestingly, there was an inverse correlation between the epsilon 4 allele with temporal cortical choline acetyltransferase activity. To further define the specific function of ApoE4, cultured rat hippocampal neurons were used to investigate interactions involving beta-amyloid protein. In this model, ApoE4 (but not ApoE2) was able to reverse the neuroprotective effects of beta-amyloid. ApoE3 was demonstrated to increase the internalization of beta-amyloid peptide into these neurons. Taken together, these results support the involvement of ApoE4 in the pathogenesis of Alzheimer's disease and also provide some explanations for the possible function of this protein.

Near infrared spectroscopy in the diagnosis of Alzheimer's disease.

Near infrared spectroscopy (NIRS) is a new technique that permits noninvasive monitoring of cerebral blood and tissue oxygenation. Recently, we and others have shown that NIRS measurements are sensitive enough to follow changes in cerebral hemoglobin oxygenation due to activation of brain function. Based on these findings we have assessed the influence of aging as well as the influence of neurodegeneration on cerebral hemoglobin oxygenation during mental work. The typical NIRS pattern in young healthy subjects while performing calculation tasks measured in the frontal cortex were increases in oxygenated hemoglobin [HbO2] and total hemoglobin [HbT] while reduced hemoglobin [HbR] decreased. Elderly healthy subjects showed a significant lower mean increase in [HbO2] and [HbT] levels. Regression analysis revealed an age-dependent decline in activation-induced local increase of [HbO2] as well as [HbT]. Furthermore, we monitored changes in cerebral hemoglobin oxygenation in the frontal cortex while patients with probable Alzheimer's disease (AD) performed cognitive tasks. Whereas elderly healthy subjects (as well as patients with major depression, age-associated memory impairments or vascular dementia) again showed clear increases in the local concentrations of [HbO2] and [HbT] during brain activation, AD patients showed significant decreases compared to the baseline levels in both variables that were most pronounced in the parietal cortex. To clarify whether the different patterns in cerebral hemoglobin oxygenation during cognitive activation were due to an altered functional brain organization in AD or to alterations in the cerebrovascular response to neuronal activation, we are currently performing simultaneous NIRS and (015-H20-)PET measurements during performance of a cognitive task (Stroop test). Our finding of a regional reduced oxygen supply during activation of brain function may be of relevance to the development and the time course of neurodegeneration.

Apolipoprotein E. Structure, function, and possible roles in Alzheimer's disease.

Apolipoprotein (apo) E is associated with the two characteristic neuropathologic lesions of Alzheimer's disease--extracellular neuritic plaques representing deposits of amyloid beta (A beta) peptide and intracellular neurofibrillary tangles representing filaments of a microtubule-associated protein called tau. Incubation of the apoE4 isoform with the A beta peptide in vitro results in the formation of a dense, stable network of very long monofibrils, while incubation of apoE3 with the A beta peptide results in the formation of a less dense, less stable network. The more complex nature of the plaques formed with the A beta peptide in the presence of apoE4 in vivo may impair the normal clearance process and enhance plaque formation. Alternatively or additionally, apoE may alter the cytoskeletal structure and function and, under certain conditions, may promote the formation of the neurofibrillary tangles. Our studies have demonstrated that apoE3 and apoE4 exert differential effects on neuronal growth (i.e., neurite extension and branching) in vitro. When combined with a source of lipid, apoE3 stimulated neurite extension in peripheral nervous system neurons (dorsal root ganglia), whereas apoE4 inhibited it. Similar results were obtained with central nervous system neurons (murine neuroblastoma Neuro-2a cells). Addition of free apoE3 or apoE4 without beta-VLDL had no effect on neurite outgrowth. There was also differential accumulation of apoE3 and apoE4 by the neuroblastoma cells: apoE3 accumulated within cell bodies and neurites to a greater extent than apoE4. Thus, apoE3 may facilitate cytoskeletal activity, whereas apoE4 may inhibit it, which would be detrimental during synaptic remodeling.

Altered expression of amyloid beta precursor mRNAs in cerebral vessels, meninges, and choroid plexus in Alzheimer's disease.

Altered tissue-specific processing or production of the amyloid precursor protein (APP) is thought to be central to amyloid deposition in cerebrovascular and the neocortical tissues in Alzheimer's disease (AD). We demonstrate that A beta peptide(s) is readily detectable and increased in cerebral vessels, meninges and choroid plexus obtained at autopsy from AD subjects compared to age-matched controls. Using the reverse transcription (RT)-polymerase chain reaction (PCR), we further found that A beta transcripts encoding the A beta sequence in all forms of APP containing exons 16 and 17 (of APP770) were significantly increased in vessel samples in AD subjects. This was also evident in the neocortical samples and not related to pre-mortem factors or postmortem interval. It is possible that the increased A beta mRNAs reflect enhanced expression of the L-APP isoform (APP770 without exon 15) expressed in leukocytes and glia alike. We also found evidence for changed proportions of APP 770, 756 and 695 mRNAs in cerebral vessel samples from AD subjects compared to controls. Whereas APP770 and APP751, the predominant forms, were significantly decreased, APP695 transcript was increased in vessel samples from AD subjects. Such changes were not evident in neocortical samples from the same subjects. These observations suggest tissue-specific changes in expression of APP isoforms implicating one of the mechanisms for increased accumulation of A beta in cerebrovascular tissues in AD.

Hyperactivation of signal transduction systems in Alzheimer's disease.

A compromise or deregulation in signal transduction cascades could adversely affect cellular functions and possibly contribute to cell death. In recent years, it has become increasingly apparent that pronounced activation of neuronal signal transduction systems is a characteristic of AD brain. There is evidence that signal transduction systems play a role in the formation or development of these pathological features of AD. Aberrant activity and localization of components of signaling mechanisms (growth factors, their receptors, protein kinases, phosphoprotein phosphatases, and phosphoproteins) are closely associated with the intracellular accumulation of PHF, the extracellular deposition of amyloid, and the formation of neuritic plaques in AD brain. In particular, immunohistochemical studies reveal increased levels of neuronal staining for APP, possibly an important growth factor in AD, both in frontal cortex and hippocampus. Anti-APP immunostaining is also associated with the neuritic component of plaques. Additionally, PKC(beta II) immunostaining is increased in the neuronal cell body and neuropil of AD samples, particularly in association with plaques, suggesting a postsynaptic involvement of this enzyme. On the other hand, PKC(beta I) immunostaining is associated with axonal staining particularly in the sprouting neurites of plaques. Sprouting neuritic components of plaques are immunopositive with other growth-associated proteins, such as GAP43, MARCKS, and spectrin. Immunoreactivity of other members of signal transduction systems such as Fos and stathmin are all increased in AD hippocampal neurons. On the other hand, several protein kinases and phosphoproteins were immunolocalized to tangles. Thus, the hyperactivation and dysfunction of signal transduction systems could be involved in the pathogenesis of AD.

Cellular Distribution and Expression of Cortical Acetylcholine Receptors in Aging and Alzheimer's Diseasea

Ligand binding studies show marked reductions of nicotinic, but not of muscarinic binding sites in Alzheimer's disease. Using monoclonal antibodies we studied immunohistochemically the expression of the respective receptor proteins in the frontal cortex of middle-aged (55 +/- 5 yr) controls, age-matched controls (73 +/- 6 yr), and patients with Alzheimer's disease (74 +/- 5 yr). Density of nicotinic cholinoceptive neurons was 8000/mm3 for middle-aged controls and 4000/mm3 for age-matched controls, but only 900/mm3 in Alzheimer's brains (p less than 0.0001). Densities of muscarinic cholinoceptive and of Nissl-stained neurons were not significantly different between the groups, pointing to a selective decrease of nicotinic receptor protein expression in cortical neurons with aging and in Alzheimer's disease.

Essential Fatty Acids in Alzheimer's Diseasea

Concentrations of essential fatty acids (EFAs) in plasma and red blood cell phospholipids were found to be abnormal in patients with Alzheimer's disease. A double-blind, placebo-controlled trial of treatment with EFAs plus appropriate antioxidants was carried out in 36 patients with Alzheimer's disease. After 20 weeks both the EFA and placebo groups had improved, but the degree of improvement was consistently greater in the EFA group.

Olfactory Capacities in Aging and Alzheimer's Diseasea

Age-related alterations in the ability to smell are well documented. For example, more than three-fourths of individuals over the age of 80 have major difficulty detecting and identifying odors. Furthermore, olfactory dysfunction is among the first signs of Alzheimer's disease (AD). Indeed, the olfactory pathways of patients with AD evidence disproportionate numbers of neuritic plaques and neurofibrillary tangles relative to other sensory pathways, suggesting that the olfactory system may be the site of first involvement of the AD process. In this article, the literature related to age- and AD-related alterations in olfactory perception has been briefly reviewed, and several current hypotheses regarding the physiologic basis for these changes discussed.

Alterations of phospholipid metabolites in postmortem brain from patients with Alzheimer's disease.

To test the hypothesis that brain cell membranes degenerate in Alzheimer's disease (AD), we measured the levels of phospholipids, their water-soluble metabolites, and glycerophosphocholine (GPC) cholinephosphodiesterase activity in postmortem brain tissue from patients with AD and age-matched controls. We found significantly higher levels of the phospholipid catabolite GPC in AD brain. In contrast, choline and ethanolamine levels were significantly lower in AD, and phospholipid levels were slightly decreased. Furthermore, in AD the activity of the GPC-degrading enzyme GPC cholinephosphodiesterase was unaltered. Our results indicate that membrane phospholipid catabolism is increased in AD brain. Inasmuch as the tissue levels of initial phospholipid precursors were decreased, we suggest that phospholipid turnover is elevated in this neurodegenerative disease.

Mechanisms of Amyloid Deposition in Alzheimer's Diseasea

At the cellular level, Alzheimer's disease (AD) must be the result of neuronal dysfunction and degeneration leading to a reduction in synaptic density. Filamentous deposits of amyloid, which define the disease at the molecular level, occur within perikarya, axons, dendrites, and terminals of neurons as neurofibrillary tangles (NFT), in the extracellular neuropil as amyloid plaques (APC), and around blood vessels as amyloid congophilic angiopathy (ACA). These fibrillar amyloid protein aggregates are also found in the brain of all individuals with Down's syndrome after the age of 30 years. The amyloid deposits apparently occur in the terminal zones of neurons that develop NFT. It is suggested that amyloid deposition is of fundamental significance in AD and that a thorough understanding of amyloid formation will eventually lead to successful therapeutic intervention in AD. As elucidation of the reasons behind amyloid deposition must shed some light on the pathogenesis of AD, we review the current state of knowledge on the nature of the AD amyloid protein, its origin, and its formation. Although there is yet no agreement about the chemical nature of the amyloid protein of NFT, the major constituent of both APC and ACA has been shown to be a 4.5-kD amyloid protein originally termed "beta-protein" or "amyloid A4" which we now denote as "beta A4." Amyloid beta A4 protein is proteolytically derived from a transmembrane protein termed amyloid precursor protein (APP) which is encoded by a widely expressed gene on chromosome 21. Our present results are consistent with the possibility that amyloid formation requires membrane damage or APP molecules that are not or are incorrectly integrated into membranes. To allow the generation of the C-terminus of beta A4, one proteolytic cleavage step has to occur in the sequence that normally forms the transmembrane domain of the APP proteins. This cleavage is crucial for amyloid formation because we could show that the ability of synthetic beta A4 to form amyloid depositions is mainly based on hydrophobic parts of the sequence that have to interact with each other and build up large aggregates under physiologic conditions. Membrane association of APP is expected to interfere with this cleavage and the process of aggregation.

Pathologic Changes in the Olfactory System in Aging and Alzheimer's Diseasea

Pathologic Changes in the Olfactory System in Aging and Alzheimer's Diseasea

Blood‐Brain Barrier Abnormalities in Alzheimer's Diseasea

We studied the cerebral microcirculation and the blood-brain barrier in Alzheimer's disease by assessing the glucose transporter, several functionally important enzymes, and adrenergic receptors in isolated brain microvessels that were obtained at autopsy from subjects with Alzheimer's disease and from age-matched controls. We found a marked decrease of about 50% in the density of the glucose transporter, but increased densities of beta 2- and alpha 2-adrenergic receptors in cerebral microvessels obtained from subjects with Alzheimer's disease. The biologic implications of these findings are discussed.

SPECT in Early‐ and Late‐Onset Alzheimer's Diseasea

We assessed with SPECT the influence of age of onset in Alzheimer's disease in patients who were matched for severity of disease with early and late onset of the disease. Twenty-six patients, severity-matched according to scores on Mini-Mental State Examination, were compared to nine controls. Comparison of the temporoparietal cerebellar ratio (TPC) revealed a statistically significantly lower ratio in the early-onset group compared to the late-onset group of patients. These results are in agreement with other imaging, structural, and neurochemical studies, supporting the view of heterogeneity of Alzheimer's disease according to age of onset.

Cytochrome oxidase deficiency in Alzheimer's disease.

Alzheimer's disease (AD) is a degenerative neurologic disorder that may be familial but is usually sporadic and not easily analyzable in terms of conventional Mendelian genetics. The mitochondrial electron transport chain contains 13 proteins that are encoded by mitochondrial genes rather than nuclear (chromosomal) genes. Disorders resulting from heteroplasmic mutations of mitochondrial genes may appear to be sporadic rather than familial. We evaluated electron transport chain activity in platelet mitochondria prepared from patients with AD and found a specific defect in cytochrome oxidase in five of six patients studied. The mitochondrial genome may play a role in the pathogenesis of AD.

Retinal Degeneration in the Macula of Patients with Alzheimer's Diseasea

Recent reports (Hinton et al. 1986; Blanks et al. 1989) document the involvement of the retina in the constellation of neurodegenerative changes present in Alzheimer's disease (AD). These studies demonstrate the degeneration of large numbers of optic nerve axons and loss of retinal ganglion cells (RGCs) in patients with AD, but the quantitative changes in the retina of patients with AD compared with age-matched controls have not been examined. An important question is whether the lesion affects the macula, the area of highest visual acuity and the region of the greatest density of cone photoreceptor cells and RGCs. Additionally, it is unknown if patients with AD have a uniform thinning of cells in the ganglion cell layer (GCL) or if there is a differential loss of the medium- to large-sized cells, as suggested earlier (Bassi et al. 1987) and documented histopathologically in some areas of the central nervous system of patients with AD (Kemper 1984). If patients with AD were to show a differential loss of large versus small RGCs with characteristic differences in density, distribution, central projections, and physiologic properties (see review by Rowe and Stone 1977), then a loss of the visual functions normally ascribed to these classes of mammalian RGCs might be expected. This quantitative study of the retinal lesions in the macula of patients with AD provides important data on the progression of the disease and may eventually be the basis for diagnostic procedures for assessing the severity of AD.

Analysis of Monoamines in the Cerebrospinal Fluid of Chinese Patients with Alzheimer's Diseasea

We have established HPLC assay conditions that could measure the levels of 14 monoamines and their metabolites simultaneously. Monoamine levels in cerebrospinal fluids of 12 Chinese patients with Alzheimer's disease were compared with those in samples from patients with benign prostate hyperplasia. Of the 14 monoamines and metabolites, only three were found to be present in all samples. Although the levels of 5-hydroxy-3-indoleacetic acid (HIAA) and homovanillic acid (HVA) in cerebrospinal fluid of patients with Alzheimer's disease were lower, and the levels of 3-methoxy-4-hydroxyphenylglycol (MHPG) higher, as compared to control patients, no significant differences were found between these two groups.

Book Reviews

Book reviewed in this article:B. Cohen & I. Bodis‐Wollner Vision and the Brain: The Organization of the Central Visual SystemR. J. Wurtman, S. Corkin, J. H. Growdon & E. Ritter‐Walker (Eds) Advances in Neurology. Volume 51. Alzheimer's DiseaseA. J. Barkovich (with contributions by V. H. Halbach, M. S. B. Edwards & T. P. Naidich) Pediatric Neuroimaging