A novel species-specific RNA related to alternatively spliced amyloid precursor protein mRNAs

Updates in Alzheimer's disease: from basic research to diagnosis and therapies

Alzheimer's disease (AD) is the most common neurodegenerative disorder, characterized pathologically by extracellular deposition of β-amyloid (Aβ) into senile plaques and intracellular accumulation of hyperphosphorylated tau (pTau) as neurofibrillary tangles. Clinically, AD patients show memory deterioration with varying cognitive dysfunctions. The exact molecular mechanisms underlying AD are still not fully understood, and there are no efficient drugs to stop or reverse the disease progression. In this review, we first provide an update on how the risk factors, including APOE variants, infections and inflammation, contribute to AD; how Aβ and tau become abnormally accumulated and how this accumulation plays a role in AD neurodegeneration. Then we summarize the commonly used experimental models, diagnostic and prediction strategies, and advances in periphery biomarkers from high-risk populations for AD. Finally, we introduce current status of development of disease-modifying drugs, including the newly officially approved Aβ vaccines, as well as novel and promising strategies to target the abnormal pTau. Together, this paper was aimed to update AD research progress from fundamental mechanisms to the clinical diagnosis and therapies.

Genetics of Alzheimer disease

Alzheimer disease (AD) is the most common causes of neurodegenerative disorder in the elderly individuals. Clinically, patients initially present with short-term memory loss, subsequently followed by executive dysfunction, confusion, agitation, and behavioral disturbances. Three causative genes have been associated with autosomal dominant familial AD (APP, PSEN1, and PSEN2) and 1 genetic risk factor (APOEε4 allele). Identification of these genes has led to a number of animal models that have been useful to study the pathogenesis underlying AD. In this article, we provide an overview of the clinical and genetic features of AD.

Amyloid precursor protein mRNA levels in Alzheimer's disease brain

Insoluble beta-amyloid deposits in Alzheimer's disease (AD) brain are proteolytically derived from the membrane bound amyloid precursor protein (APP). The APP gene is differentially spliced to produce isoforms that can be classified into those containing a Kunitz-type serine protease inhibitor domain (K(+), APP(751), APP(770), APRP(365) and APRP(563)), and those without (K(-), APP(695) and APP(714)). Given the hypothesis that Abeta is a result of aberrant catabolism of APP, differential expression of mRNA isoforms containing protease inhibitors might play an active role in the pathology of AD. We took 513 cerebral cortex samples from 90 AD and 81 control brains and quantified the mRNA isoforms of APP with TaqMan real-time RT-PCR. After adjustment for age at death, brain pH and gender we found a change in the ratio of KPI(+) to KPI(-) mRNA isoforms of APP. Three separate probes, designed to recognise only KPI(+) mRNA species, gave increases of between 28% and 50% in AD brains relative to controls (p=0.002). There was no change in the mRNA levels of KPI-(APP 695) (p=0.898). Therefore, whilst KPI-mRNA levels remained stable the KPI(+) species increased specifically in the AD brains.

Protein phosphorylation and APP metabolism

Numerous lines of evidence place signal transduction cascades at the core of many processes having a direct role in neurodegeneration and associated disorders. Key players include neurotransmitters, growth factors, cytokines, hormones, and even binding and targeting proteins. Indeed, abnormal phosphorylation of key control proteins has been detected in many cases and is thought to underlie the associated cellular dysfunctions. Several signaling cascades have been implicated, affecting processes as varied as protein processing, protein expression, and subcellular protein localization, among others. The Alzheimer's amyloid precursor protein (APP) is a phosphoprotein, with well-defined phosphorylation sites but whose function is not clearly understood. The factors and pathways regulating the processing of APP have been particularly elusive, both in normal ageing and the Alzheimer's disease (AD) condition. Not surprisingly, the physiological function(s) of the protein remain(s) to be elucidated, although many hypotheses have been advanced. Nonetheless, considerable data has accumulated over the last decade, placing APP in key positions to be modulated both directly and indirectly by phosphorylation and phosphorylation-dependent events. The pathological end product of APP processing is the main proteinaceous component of the hallmark senile plaques found in the brains of AD patients, that is, a toxic peptide termed Abeta. In this minireview we address the importance of phosphorylation and signal transduction cascades in relation to APP processing and Abeta production. The possible use of the identified molecular alterations as therapeutic targets is also addressed.

Analysis of neurons created from wild-type and Alzheimer's mutation knock-in embryonic stem cells by a highly efficient differentiation protocol

It is impossible to obtain and amplify live neurons from Alzheimer's disease (AD) patients. To establish the neurons harboring AD abnormality, we constructed mouse embryonic stem (ES) cells, in which the AD-causative V642I mutation was introduced to the endogenous amyloid precursor protein (APP) gene, in combination with a protocol to efficiently differentiate ES cells into postmitotic neurons without using a cell sorter. By this protocol, ES cells differentiated into >90% of the central type of adult postmitotic neurons. Neurons derived from V642I-APP knock-in ES cells were indistinguishable from wild-type ES-derived neurons, as determined by the expression of various markers for neuronal differentiation. Notably, V642I-APP knock-in ES cell-derived neurons exhibited significantly increased secretion of Abeta42 without AD-related hyperphosphorylation of tau, indicating that the direct output of the AD-causative mutation is increased Abeta42 secretion. In this study, we analyze created neurons with wild-type and AD genotypes and propose a new strategy for generating neurons for any dominantly inherited neurodegenerative diseases. The strategy can be applied to create human neurons with AD or any other neurodegenerative disease by using human ES cells.

CTCF is essential for up-regulating expression from the amyloid precursor protein promoter during differentiation of primary hippocampal neurons

The transcriptional mechanism underlying amyloid precursor protein (APP) regulation in primary neurons during development was investigated. We observed an approximately threefold elevation of APP mRNA levels in differentiating rat hippocampal neurons between day 1 and day 7 in culture and in rat brain hippocampi between embryonic day 18 and postnatal day 3. When an APP promoter construct extending to position -2,832 upstream from the main transcriptional start site was transfected into primary rat hippocampal neurons, promoter activity increased from day 1 until reaching a maximum on day 7 in culture. This increase in APP promoter activity was correlated more closely with the time course of expression of the synaptic vesicle protein synaptophysin, an indicator of synaptogenesis, than with neurofilament accumulation, an indicator of neuritogenesis. Transfection of 5' APP promoter deletions and internal block mutations indicated that the CTCF binding domain designated APBbeta was the primary contributor to the increase in APP promoter activity. Furthermore, the binding of transcription factor CTCF to the APBbeta element increased approximately fivefold between day 1 and day 7, whereas the binding of USF to the APBalpha sequence increased only twofold. These results suggest that CTCF is pivotal for the up-regulation of APP expression during synaptogenesis in primary neurons.

Alzheimer's disease as a defect of neuronal autotrophism: a hypothetical analogy with amateur radio operation

Amyloid precursor protein appears to be a signaling protein that plays a role in neuronal autotrophism, indicating integrity of the nerve terminal and synapse. Analogous to amateur radio operation with a damaged antenna in which further attempts to improve signal propagation can result in damage to the radio equipment, abnormal signaling by the secreted amyloid precursor protein stimulates compensatory metabolic activity in the neuron that ultimately leads to paired helical filament formation (neurofibrillary tangles) and further neuronal dysfunction and death.

The role of the amyloid protein precursor (APP) in Alzheimer's disease: does the normal function of APP explain the topography of neurodegeneration?

Alzheimer's disease (AD) is the most common form of dementia in the aged population. Early-onset familial AD (FAD) involves mutations in a gene on chromosome 21 encoding the amyloid protein precursor or on chromosomes 14 or 1 encoding genes known as presenilins. All mutations examined have been found to increase the production of amyloidogenic forms of the amyloid protein (A beta), a 4 kDa peptide derived from APP. Despite the remarkable progress in elucidating the biochemical mechanisms responsible for AD, little is known about the normal function of APP. A model of how APP and A beta are involved in pathogenesis is presented. This model may explain why certain neuronal populations are selectively vulnerable in AD. It is suggested that those neurons which more readily undergo neuritic sprouting and synaptic remodelling are more vulnerable to A beta neurotoxicity.

Characterization of endogenous APP processing in a cell-free system

We have developed a simple in vitro assay using tissue homogenates that allows detection and characterization of several endogenous proteolytic activities which convert Alzheimer's amyloid precursor protein (APP) to the smaller, carboxy-terminal fragments, postulated to be intermediates in the formation of β-amyloid peptide (Aβ). Incubation at 37°C results in the degradation of transmembrane APP and formation of a mixture of carboxy-terminal containing peptides with mass values of 9-12 kDa. Epitope mapping and electrophoretic comparison with a truncated APP standard showed one of these peptides to contain the entire Aβ sequence. Analysis of pH dependence shows that formation of this carboxy-terminal product as well as another fragment, that is the likely product of 'secretase' activity, requires acidic pH. This suggests that cleavage of full-length APP to secreted forms may take place in an acidic intracellular compartment.

Additive effects of basic fibroblast growth factor and phorbol ester on beta-amyloid precursor protein expression and secretion

Expression of the beta-amyloid precursor protein (beta-APP), a proteoglycan whose proteolytically derived fragments have been implicated in the neuropathology observed in Alzheimer's disease, is regulated by a variety of stimuli including cytokines, phorbol esters, and growth factors. In this study we report the effects of basic fibroblast growth factor (bFGF) and the protein kinase C activator, phorbol 12-myristate 13-acetate (PMA), on beta-APP expression and secretion in SKNMC human neuroblastoma cells. Treatment of the cells with bFGF for 24 h increased APP promoter activity 200%, cell-associated full-length protein 189%, and secreted amino-terminal fragments 192% compared to basal levels. Treatment of the cells with PMA for 24 h also up-regulated APP expression and secretion with increases of 170, 112, and 161% being observed for promoter activity, cell-associated full-length protein, and secreted amino-terminal fragments, respectively. The effects of bFGF and PMA on the expression and secretion of beta-APP were additive and distinct in that: (a) co-treatment of the cells with maximally stimulating doses of bFGF and PMA had an additive effect on both induced full-length protein expression (242%) and secretion of amino-terminal fragments (311%) compared to basal levels; (b) net levels of full-length protein expression and secretion induced by bFGF and PMA differed significantly from each other; and (c) down-regulation of phorbol ester-stimulated protein kinase C by pre-treatment of the cells for 24 h with 1 microM PMA failed to attenuate bFGF-induced transcription or induced secretion of beta-APP.

Two types of amyloid precursor protein (APP) mRNA in rat glioma cell lines: upregulation via a cyclic AMP-dependent pathway

APP is a multifunctional transmembrane glycoprotein and the only known natural source of beta A4 peptide-the major constituent of senile plaques in Alzheimer's disease (AD). The expression and cAMP-dependent regulation of the APP gene were investigated in primary cultures of rat astrocytes and two related glioma cell lines, BT4C and BT4Cn, which exhibit distinct invasive phenotypes. Besides the well-characterized 3.5 kb APP mRNA class, a robust expression of an unusual 2.8 kb APP mRNA class was revealed by Northern blotting in both glioma cell lines, but not in the astrocytes. Low amounts of the 2.8 kb APP mRNA species were also observed in rat liver and occasionally in aged rat brain. The 2.8 kb APP mRNA contained exons 1-18 and may thus be generated by truncation of the 3' untranslated region. For the first time, regulation of the APP gene via a cAMP-dependent mechanism was shown. Exposure to dBcAMP dramatically upregulated the 3.5 and 2.8 kb transcripts in BT4C cells, and, to a lesser extent, in BT4Cn cells where the constitutive expression of the APP gene was much higher. Elucidation of the factors involved in cAMP-dependent induction of APP mRNA in these cells may shed more light on the molecular mechanisms of APP overexpression.

Changes in expression of lymphocyte amyloid precursor protein mRNA isoforms in normal aging and Alzheimer's disease

We measured, by employing a quantitative reverse-transcriptase PCR procedure, the relative (to beta-actin) levels of amyloid precursor protein APP751 and APP770 mRNA isoforms in lymphocytes obtained from 64 cognitively intact subjects ranging in ages from 20 to 91 years and in 19 patients with sporadic Alzheimer's disease. A positive correlation was observed between the relative lymphocyte APP751 mRNA levels and subject age for the cognitively intact cohort. No difference in lymphocyte APP751 mRNA levels was observed between Alzheimer's disease patients and their age-matched controls (> 55 years of age). However, the ratio of lymphocyte APP751:APP770 mRNA levels was significantly lower in Alzheimer's disease subjects compared to the > 55-year-old cohort. This decreased ratio is most likely due to an average 31% increase in the lymphocyte APP770 isoform in Alzheimer's disease patients compared to 12% in the > 55-year-old cognitively intact group. Marked individual differences in amount of APP mRNA isoforms were encountered among all the subject groups and in the < or = 55-year-old cohort, a 10-fold variation in individual APP751 mRNA levels was observed. The relevance of these findings in lymphocytes to the pathogenesis of Alzheimer's disease is discussed.

Genetic dissection of Alzheimer disease, a heterogeneous disorder

The genetics of Alzheimer disease (AD) are complex and not completely understood. Mutations in the amyloid precursor protein gene (APP) can cause early-onset autosomal dominant AD. In vitro studies indicate that cells expressing mutant APPs overproduce pathogenic forms of the A beta peptide, the major component of AD amyloid. However, mutations in the APP gene are responsible for 5% or less of all early-onset familial AD. A locus on chromosome 14 is responsible for AD in other early-onset AD families and represents the most severe form of the disease in terms of age of onset and rate of decline. Attempts to identify the AD3 gene by positional cloning methods are underway. At least one additional early-onset AD locus remains to be located. In late-onset AD, the apolipoprotein E gene allele epsilon 4 is a risk factor for AD. This allele appears to act as a dose-dependent age-of-onset modifier. The epsilon 2 allele of this gene may be protective. Other late-onset susceptibility factors remain to be identified.

Beta APP mRNA transcription is increased in cultured fibroblasts from the familial Alzheimer's disease-1 family

Familial (autosomal dominant) Alzheimer's disease (FAD) is a genetically heterogeneous disorder. Mutations in exons 16 and 17 of the amyloid beta-protein precursor (beta PP) gene currently account for less than 2% of FAD kindreds. No known defect in beta PP quantity, structure, or processing accounts for disease-associated beta-amyloid deposition in the majority of early-onset FAD kindreds. Only two out of a sample of 48 pedigrees (particularly the early onset FAD 4 kindred) contributed noticeably to evidence of linkage at the D21S16/13 and S1/S11 loci in the chromosomal region 21q21 [75]. Many early onset FAD pedigrees (including the FAD 1 and FAD 4 kindreds) show strong evidence of linkage to markers in the chromosome 14q24.3 region. Patients with trisomy 21 (Down's syndrome, DS) virtually always develop a histopathological phenotype indistinguishable from FAD, presumably on the basis of increased beta PP gene dosage and transcription. Whereas no beta PP gene duplication has been found in FAD, other mechanisms that augment beta PP production by effects at the transcriptional level could explain some FAD cases. Here, we report that cultured fibroblasts from affected members of the FAD 1 pedigree show a approximately 1.9 fold increase (P = 0.007) in beta PP mRNA levels compared to unaffected members when the cells are grown under stressed conditions in 0.5% serum. The elevated levels of beta PP mRNA in cells cultured in 0.5% serum also cosegregate with haplotypes in the 14q24.3 region when analyzed by linkage methods (LOD score = 3.26 at theta = 0.001). This is the chromosomal region to which FAD in this family has previously been mapped. As expected, fibroblasts from patients with DS used as a control show a similar beta PP mRNA increase. Fibroblasts from the FAD 4 pedigree did not show this defect under the conditions utilized here. beta PP and A beta protein levels were determined quantitatively after metabolic labeling and immunoprecipitation and found to increase 2.0 and 2.5 fold, respectively, in the fibroblasts from affected FAD 1 members. Finally, transient transfections of a beta PP promoter/chloramphenicol acetyl transferase reporter gene construct demonstrated a approximately 3-4 fold increase in beta PP promoter activity in affected fibroblasts from the FAD 1 but not the FAD 4 pedigree. Taken together, these data raise the possibility that an increase in beta PP transcription may underlie the AD phenotype in at least some of the chromosome 14-linked FAD families.

Altered processing of a mutant amyloid precursor protein in neuronal and endothelial cells

Altered proteolysis of the amyloid precursor protein (APP) may play an important role in Alzheimer disease (AD). To better understand the role of mutant APP in the pathogenesis of the disease, we stably overexpressed the mutant APP717F approximately twofold vs. the endogenous wild-type gene in several cell types. The processing of APP was examined by Western blot analysis and immunoprecipitation. We observed distinctive patterns of APP metabolites among various cell lines. Neuronal and endothelial cells expressing mutant APP717F generated higher levels of large, potentially amyloidogenic carboxyl terminal fragments, which were enhanced upon treatment of the cells with leupeptin. These results suggest that mutations in the APP gene shift the protein processing towards the amyloidogenic pathway in neuronal and endothelial cells possibly involving the endosomal-lysosomal system.

Expression of amyloid precursor protein (APP) in rat brain and cultured neural cells

Cell type-specific and developmental patterns of APP expression were investigated in rat brain and cultured neural cells. Nearly all astrocytes were APP-positive, whereas only selected population of neurons appeared to express APP. In these neurons, APP immunoreactivity was preferentially restricted to single processes. mRNAs encoding the major APP isoforms, APP695 and APP770, were co-expressed as 3.4-3.6 kb transcripts both in astrocytes and neurons. In addition, an unusual 2.8 kb mRNA size class was revealed in cultured cerebellar granule neurons by means of the probe recognizing APP770 mRNA. Also, for the first time, APP714 mRNA was detected in rat brain by northern blotting. The steady-state levels of these transcripts were increased from birth up to postnatal day 20, whereas no apparent changes were observed after reaching adulthood. These data hint at the involvement of APP in the major morphogenetic events taking place in rat brain during the first three postnatal weeks.

The significance of the Swedish APP670/671 mutation for the development of Alzheimer's disease amyloidosis

Alzheimer's disease is characterized histopathologically by the detection of amyloid plaques and tangles in the brains of its victims. A major component of the amyloid plaques, the beta-amyloid peptide, is a metabolite of a larger transmembrane protein termed the amyloid precursor protein (APP). Several pathogenic mutations causing Alzheimer's disease have been identified in the APP gene, situated on chromosome 21. One of these is a double missense mutation in exon 16 of the APP gene which occurs in a large Swedish pedigree. Studies with transfected cell cultures have shown that cells expressing this double mutation release approximately 7 times more beta-amyloid than their wild type counterparts. Possible mechanisms whereby the APP mutations cause Alzheimer's disease are considered here against the background of current understanding of the physiological role and processing of APP.

Potential beta PP-processing proteinase activities from Alzheimer's and control brain tissues

Fluorogenic peptide substrates designed to encompass the reported alpha-secretory and amyloidogenic cleavage sites of the amyloid-beta precursor protein (beta PP) were used to analyze proteinase activities in brain extracts from control patients and those with Alzheimer's disease (AD). Activity against the secretory substrate at pH 7.5 in control and AD brains produced a major endopeptidase cleavage at the Lys687-Leu688 bond (beta PP770 numbering), consistent with the beta PP secretase cleavage. Activity in control brains against the amyloidogenic substrate at pH 7.5 produced one cleavage at the Ala673-Glu674 bond, two residues C-terminal to the amyloidogenic Met-Asp site. However, in three of four AD brains, the major cleavage was at the Asp-Ala bond, one residue from the amyloidogenic site. Both endopeptidase and carboxypeptidase activities in AD brains were lower than in control brains. Proteinase activities against the secretory substrate had a major optimum at pH 3.0-4.0 and another at pH 6.0-7.5. Proteinase activities against the amyloidogenic substrate had a major optimum at or below pH 3.0 and another at pH 6.0. Using both substrates, activities at low pH were higher in AD-brains than in controls, while at pH above 6.5, activities in control brains were higher than in AD. These results indicate that the levels of proteolytic enzymes in AD brains are altered relative to controls.

Alzheimer's disease and soluble A beta

The discovery of soluble amyloid beta (sA beta) suggests that the role of amyloid in Alzheimer's disease (AD) is similar to the previously studied systemic amyloidoses and alters the notion that membrane damage is the initial event in AD. The disease state is characterized by the abnormal accumulation of a normal degradative peptide, which becomes resistant to further proteolysis due to a conformational change. Mutations in the beta PP gene have been found in a very small percentage of AD cases; hence other factors, both genetic and environmental, need to be identified. Priority needs to be given to detailed studies of the structural differences between sA beta and the A beta in amyloid deposits. This will help uncover the determining factors governing the aggregation of sA beta. These structural alterations may be critical for the possible toxic effects A beta and/or associated proteins (molecular chaperones, e.g., apolipoprotein E) have on brain cell function.

Unifying features of systemic and cerebral amyloidosis

Amyloidosis is a generic term for a group of clinically and biochemically diverse diseases that are characterized by the deposition of an insoluble fibrillar protein in the extracellular space. Over 16 biochemically distinct amyloids are known. Despite this diversity, all amyloids have a particular ultrastructural and tinctorial appearance, a beta-pleated sheet structure, and are codeposited with a group of amyloid-associated proteins. The most common amyloidosis is Alzheimer's disease (AD), where A beta is the main component of the amyloid. Recently it has been found that A beta exists as a normal soluble protein (sA beta) in biological fluids. This links AD more closely to some of the systemic amyloidoses, where the amyloid precursor is found in the circulation normally. Numerous mutations have been found in the A beta precursor (beta PP) gene, associated with familial AD. Many mutations are also found in some of the hereditary systemic amyloidoses. For example, over 40 mutations in the transthyretin (TTR) gene are associated with amyloid. However, both A beta and TTR related amyloid deposition can occur with no mutation. The pathogenesis of amyloid is complex, and appears to be associated with genetic and environmental risk factors that can be similar in the systemic and cerebral amyloidoses.

Isolation and characterization of APLP2 encoding a homologue of the Alzheimer's associated amyloid beta protein precursor

Familial Alzheimer's disease (FAD) is a genetically heterogeneous disorder that includes a rare early-onset form linked to mutations in the amyloid b protein precursor (APP) gene. Clues to the function of APP derive from the recent finding that it is a member of a highly conserved protein family that includes the mammalian amyloid precursor-like protein (APLP1) gene which maps to the same general region of human chromosome 19 linked to late-onset FAD. Here we report the isolation of the human APLP2 gene. We show that APLP2 is a close relative of APP and exhibits a very similar pattern of expression in the brain and throughout the body. Like APP, APLP2 contains a cytoplasmic domain predicted to couple with the GTP-binding protein G(o) indicating that it may be an additional cell surface activator of this G protein.

Altered levels and splicing of the amyloid precursor protein in the adult rat hippocampus after treatment with DMSO or retinoic acid

Alzheimer's disease and cognitive impairment in rats has been associated with an increase in the percentage of amyloid precursor protein (APP) containing the KPI domain. It has recently been reported that retinoic acid (RA) is capable of increasing the levels and altering the splicing ratio of APP in cultured SH-SY5Y cells. The effects of peripherally administered RA (64 or 640 micrograms/kg; i.p.; q.d.) on the abundance of APP, the ratio of the three major isoforms, and the relative abundance of nerve growth factor (NGF), brain derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) were determined by rtPCR in the hippocampus of aged rats. Corresponding changes in choline acetyltransferase (ChAT) activity were also measured. Vehicle (DMSO) treated rats exhibited a 2 x (P < 0.01) increase in total APP and an 8 x (P < 0.001) decrease in the cyclophilin transcript. In addition, DMSO increased the percentage of APP 695 from 89% in saline treated rats to 94%. Treatment of RA in DMSO decreased the accumulation of total APP relative to cyclophilin at both the low (6.4 x; P < 0.01) and high (8 x; P < 0.05) dosages when compared to DMSO treated rats. Furthermore, the level of APP-695 decreased to 82% with low dosage of RA and 75% at high dosage of the total APP transcripts. No significant change in either NGF, NT-3, or BDNF transcripts were observed following low or high dosage RA administration relative to cyclophilin RNA nor was a change in ChAT activity detected at either of the dosages tested.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular specificity and regional distribution of amyloid beta protein precursor alternative transcripts are unaltered in Alzheimer hippocampal formation

Studies comparing the expression of the amyloid beta protein precursor (APP) gene in brains of patients with Alzheimer's disease (AD) and control individuals have resulted in contradictory findings indicative of selective reductions and relative increases of APP alternative transcripts in AD brain. It has been suggested that changes in APP expression in relation to AD neuropathology may represent highly specific and localized events involving only select populations of cells in particular brain regions. For example, reported AD-related alterations in ratios of APP alternative transcripts could be attributed to changes in expression of APP in specific neuronal subpopulations, and/or reactive astrocytes and/or microglia. To address this question, we have employed in situ hybridization using biotinylated oligonucleotide probes designed to localize specific APP mRNA transcripts in the hippocampal formation of AD patients and age-matched controls since this method allows a clear distinction of the classes of neurons and glia containing a particular message.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic heterogeneity of gene defects responsible for familial Alzheimer disease

Inherited Alzheimer's disease is a genetically heterogeneous disorder that involves gene defects on at least five chromosomal loci. Three of these loci have been found by genetic linkage studies to reside on chromosomes 21, 19, and 14. On chromosomes 21, the gene encoding the precursor protein of Alzheimer-associated amyloid (APP) has been shown to contain several mutations in exons 16 and 17 which account for roughly 2-3% of familial Alzheimer's disease (FAD). The other loci include what appears to be a susceptibility gene on chromosome 19 associated with late-onset (> 65 years) FAD, and a major early-onset FAD gene defect on the long arm of chromosome 14. In other early- and late-onset FAD kindreds, the gene defects involved do not appear to be linked to any of these three loci, indicating the existence of additional and as of yet unlocalized FAD genes. This review provides a historical perspective of the search for FAD gene defects and summarizes the progress made in world-wide attempts to isolate and characterize the genes responsible for this disorder.

Altered genetic response to beta-adrenergic receptor activation in late passage C6 glioma cells

Previous studies have demonstrated variability in the phenotype of rat C6 glioma cells. In the present study, we compared morphology, growth rate, and beta-adrenergic regulation of gene expression in early (P39-47) and late (P55-90) passage C6 cells. Morphological changes were observed in five independently derived, late passage populations. In four of the five, the untreated cells were more polygonal than the fibroblast-like parental cells, and only a small fraction exhibited process outgrowth after dbcAMP treatment. Untreated cells from the fifth late passage population had longer cytoplasmic processes than parental cells and responded to dbcAMP with further process outgrowth. All late passage populations had shorter generation times than the parental cells. In early passage cells, treatment with the beta-adrenergic agonist, isoproterenol (IPR), resulted in an increase in c-fos mRNA and a decrease in c-jun mRNA (Gu-bits RM, Yu H: J Neurosci Res, 30:625-630, 1991). Both of these immediate early gene responses were irreversibly lost between P50 and P55. Additional differences in basal or IPR-induced mRNA levels were observed for beta-APP, GFAP, NGF, and PPE, but not for a number of other mRNAs. These results are discussed in relationship to previously described differences in the ability of early and late passage C6 cells to accumulate cAMP (Mallorga P, et al.: Biochim Biophys Acta 678:221-229, 1981).

Human brain beta A4 amyloid protein precursor of Alzheimer's disease: purification and partial characterization

The major component of the amyloid deposition that characterizes Alzheimer's disease is the 4-kDa beta A4 protein, which is derived from a much larger amyloid protein precursor (APP). A procedure for the complete purification of APP from human brain is described. The same amino terminal sequence of APP was found in two patients with Alzheimer's disease and one control subject. Two major forms of APP were identified in human brain with apparent molecular masses of 100-110 kDa and 120-130 kDa. Soluble and membrane fractions of brain contained nearly equal amounts of APP in both humans and rats. Immunoprecipitation with carboxyl terminus-directed antibodies indicates that the soluble forms of APP are truncated. Carboxyl terminus truncation of membrane-associated forms of human brain APP was also found to occur during postmortem autolysis. The availability of purified human brain APP will facilitate the investigation of its normal function and the events that lead to its abnormal cleavage in patients with Alzheimer's disease.

Molecular biology of Alzheimer's amyloid--Dutch variant

Hereditary cerebral hemorrhage with amyloidosis, Dutch type (HCHWA-D) (or familial cerebral amyloid angiopathy) and familial Alzheimer's disease (FAD) share several properties. Both are autosomal dominant forms of cerebral amyloidosis characterized by beta-amyloid (A beta) deposition. In HCHWA-D the A beta is predominantly found in blood vessels and in early parenchymal plaques, whereas in AD parenchymal A beta deposits in the form of senile plaques and neurofibrillary tangles are a more prominent finding. Point mutations in the amyloid precursor protein (APP) have recently been described, in both conditions. A G to C transversion at codon 618 (extracellular portion of APP695), producing a single amino acid substitution of glutamine instead of glutamine acid, occurs in HCHWA-D; whereas mutations at codon 642 in the intramembrane region of APP695 (phenylalanine, isoleucine, or glycine instead of valine) are associated with early onset FAD. This suggests that the site of particular mutations in the APP gene and the type of amino acid substitution in the APP holoprotein are more important in determining clinicopathological phenotype and age at which A beta is deposited. Thus FAD and HCHWA-D can be regarded as two sides of the same coin.

Beta A4 amyloid protein and its precursor in Alzheimer's disease

The beta A4 amyloid protein is now understood to play a pivotal role in the development of Alzheimer's disease. This protein is generated by the abnormal processing of the amyloid protein precursor, a large membrane glycoprotein. Insights into the mechanisms of this abnormal processing will give information relevant to the design of new therapeutic strategies for Alzheimer's disease.

Quantitative measurement of alternatively spliced amyloid precursor protein mRNA expression in Alzheimer's disease and normal brain by S1 nuclease protection analysis

We have used an S1 nuclease protection strategy to measure alternatively spliced amyloid precursor protein (APP) mRNAs associated with Alzheimer's disease (AD) to determine whether the expression of either one or more of the transcripts correlate with observed amyloid plaque pathology. Comparison of AD with normal cortex reveals that increasing plaque density parallels an increase in the fraction of APP-695 and a corresponding decrease in APP-770 and 751 mRNA fractions. A specific increase of APP-695, the protease inhibitor-lacking APP RNA form, in those brain regions most involved with amyloid plaque formation, suggests that an imbalance in the protease inhibitor is potentially significant in the disease. These data are consistent with cellular/tissue region-specific regulation of alternative splicing accounting for AD-related changes in the expression of APP mRNA forms.