The presenilin 1 C92S mutation increases abeta 42 production

Novel insights into presenilin 1 mutation associated with a distinctive dementia phenotype and cotton wool plaques

BACKGROUND

The mutations in the presenilin 1 gene (PSEN1) are the main cause of familial Alzheimer's disease. PSEN1 mutations affect amyloid-beta peptide production, which accumulates in the brain as senile plaque and cotton wool plaques (CWPs) and relates to other neurodegenerative disorders. Here we report the second case of the PSEN1 G266S mutation, which showed distinctive neuropathological features, including abundant CWPs. Lewy body pathology, and altered amyloid-beta production.

METHOD

Using the proband's samples, we performed genetic analysis of the PSEN1, APP, MAPT, and APOE genes, histopathological and immunohistochemical analysis of the brain tissue, and biochemical analysis of Aβ production in COS cells transfected with wild-type or mutant PSEN1.

RESULTS

The patient presented with memory loss, abnormal behavior, and visual hallucinations. Brain scans showed reduced blood flow, mild atrophy, and white matter lesions. Genetic analysis revealed a heterozygous mutation at codon 266 (G266S) of PSEN1 and polymorphism of MAPT (Q230R). The brain had many CWPs, severe cerebral amyloid angiopathy (CAA), senile plaque, Lewy bodies, and neurites. Electron microscopy displayed myelinated fiber degeneration, mitochondrial damage, and amyloid fibrils in the white matter. The production level of Aβ42 in PSEN1 G266S-transfected cells significantly increased.

CONCLUSION

Our findings suggest that the PSEN1 G266S mutation may cause a heterogeneous clinical and pathological phenotype, influenced by other genetic or environmental factors.

Overlapping profiles of Aβ peptides in the Alzheimer's disease and pathological aging brains

INTRODUCTION

A hallmark of Alzheimer's disease (AD) is the presence of senile plaques composed of aggregated amyloid β (Aβ) peptides. Pathological aging (PA) is a postmortem classification that has been used to describe brains with plaque pathology similar in extent to AD, minimal cortical tau pathology, and no accompanying history of cognitive decline in the brain donor prior to death. PA may represent either a prodromal phase of AD, a benign form of Aβ accumulation, or inherent individual resistance to the toxic effects of Aβ accumulation. To attempt to distinguish between these possibilities we have systematically characterized Aβ peptides in a postmortem series of PA, AD and non-demented control (NDC) brains.

METHODS

Aβ was sequentially extracted with tris buffered saline (TBS), radioimmunoprecipitation buffer (RIPA), 2% sodium dodecyl sulfate (SDS) and 70% formic acid (FA) from the pre-frontal cortex of 16 AD, eight PA, and six NDC patients. These extracts were analyzed by 1) a panel of Aβ sandwich ELISAs, 2) immunoprecipitation followed by mass spectrometry (IP/MS) and 3) western blotting. These studies enabled us to asses Aβ levels and solubility, peptide profiles and oligomeric assemblies.

RESULTS

In almost all extracts (TBS, RIPA, 2% SDS and 70% FA) the average levels of Aβ1-40, Aβ1-42, Aβ total, and Aβx-42 were greatest in AD. On average, levels were slightly lower in PA, and there was extensive overlap between Aβ levels in individual PA and AD cases. The profiles of Aβ peptides detected using IP/MS techniques also showed extensive similarity between the PA and AD brain extracts. In select AD brain extracts, we detected more amino-terminally truncated Aβ peptides compared to PA patients, but these peptides represented a minor portion of the Aβ observed. No consistent differences in the Aβ assemblies were observed by western blotting in the PA and AD groups.

CONCLUSIONS

We found extensive overlap with only subtle quantitative differences between Aβ levels, peptide profiles, solubility, and SDS-stable oligomeric assemblies in the PA and AD brains. These cross-sectional data indicate that Aβ accumulation in PA and AD is remarkably similar. Such data would be consistent with PA representing a prodromal stage of AD or a resistance to the toxic effects of Aβ.

When loss is gain: reduced presenilin proteolytic function leads to increased Abeta42/Abeta40. Talking Point on the role of presenilin mutations in Alzheimer disease

More than 100 missense mutations in presenilin 1 and 2 are associated with early-onset dominant Alzheimer disease. These proteins span the membrane several times and are ostensibly the catalytic component of the gamma-secretase complex, which is responsible for producing the amyloid beta-peptide (Abeta) that deposits in the Alzheimer brain. A common outcome of Alzheimer-associated presenilin mutations is an increase in the ratio of the more aggregation-prone 42-residue form of Abeta to the 40-residue variant, which is often referred to as a presenilin 'gain of function'. An apparent paradox is that most of these mutant presenilins have reduced proteolytic efficiency, which forms part of the counter argument that presenilin 'loss of function' can cause the neuronal dysfunction and death that lead to the disease. In this review, a unifying hypothesis is presented that puts forward a biochemical mechanism by which slower less-efficient forms of the protease can result in a greater proportion of 42-residue Abeta.

Tau is central in the genetic Alzheimer–frontotemporal dementia spectrum

In contrast to the common and genetically complex senile form of Alzheimer's disease (AD), the molecular genetic dissection of inherited presenile dementias has given important mechanistic insights into the pathogenesis of degenerative brain disease. Here, we focus on recent genotype-phenotype correlative studies in presenile AD and the frontotemporal dementia (FTD) complex of disorders. Together, these studies suggest that AD and FTD are linked in a genetic spectrum of presenile degenerative brain disorders in which tau appears to be the central player.

Two domains within the first putative transmembrane domain of presenilin 1 differentially influence presenilinase and gamma-secretase activity

Presenilins (PS) are thought to contain the active site for presenilinase endoproteolysis of PS and gamma-secretase cleavage of substrates. The structural requirements for PS incorporation into the gamma-secretase enzyme complex, complex stability and maturation, and appropriate presenilinase and gamma-secretase activity are poorly understood. We used rescue assays to identify sequences in transmembrane domain one (TM1) of PS1 required to support presenilinase and gamma-secretase activities. Swap mutations identified an N-terminal TM1 domain that is important for gamma-secretase activity only and a C-terminal TM1 domain that is essential for both presenilinase and gamma-secretase activities. Exchange of residues 95-98 of PS1 (sw95-98) completely abolishes both activities while the familial Alzheimer's disease mutation V96F significantly inhibits both activities. Reversion of residue 96 back to valine in the sw95-98 mutant rescues PS function, identifying V96 as the critical residue in this region. The TM1 mutants do not bind to an aspartyl protease transition state analog gamma-secretase inhibitor, indicating a conformational change induced by the mutations that abrogates catalytic activity. TM1 mutant PS1 molecules retain the ability to interact with gamma-secretase substrates and gamma-secretase complex members, although Nicastrin stability is decreased by the presence of these mutants. gamma-Secretase complexes that contain V96F mutant PS1 molecules display a partial loss of function for gamma-secretase that alters the ratio of amyloid-beta peptide species produced, leading to the amyloid-beta peptide aggregation that causes familial Alzheimer's disease.

A new presenilin Alzheimer’s disease case confirms the helical alignment of pathogenic mutations in transmembrane domain 5

In a case of familial early onset Alzheimer's disease, a mutation was detected in exon 7 of the presenilin 1 gene at codon 226 with a resultant amino acid change from leucine (CTC) to arginine (CGC) (L226R). This is a novel finding, yet is consistent with the previously reported mutations at codons 222, 229, 233 and 237 in transmembrane domain 5 which show a helical alignment of mutations in this domain. We conclude that the cause of Alzheimer's disease in this patient is an authentic PS1 gene abnormality responsible for the patient's early onset Alzheimer's disease.

Loss of presenilin function causes impairments of memory and synaptic plasticity followed by age-dependent neurodegeneration

Mutations in presenilins are the major cause of familial Alzheimer's disease, but the pathogenic mechanism by which presenilin mutations cause memory loss and neurodegeneration remains unclear. Here we demonstrate that conditional double knockout mice lacking both presenilins in the postnatal forebrain exhibit impairments in hippocampal memory and synaptic plasticity. These deficits are associated with specific reductions in NMDA receptor-mediated responses and synaptic levels of NMDA receptors and alphaCaMKII. Furthermore, loss of presenilins causes reduced expression of CBP and CREB/CBP target genes, such as c-fos and BDNF. With increasing age, mutant mice develop striking neurodegeneration of the cerebral cortex and worsening impairments of memory and synaptic function. Neurodegeneration is accompanied by increased levels of the Cdk5 activator p25 and hyperphosphorylated tau. These results define essential roles and molecular targets of presenilins in synaptic plasticity, learning and memory, and neuronal survival in the adult cerebral cortex.

The role of the presenilin-1 homologue gene sel-12 of Caenorhabditis elegans in apoptotic activities

Many cases of autosomal dominant early onset familial Alzheimer's disease result from mutations in presenilin-1 (PS1). In this study, we examined the role of the PS1 homologue gene sel-12 of Caenorhabditis elegans under oxidative stress and clarified the sel-12-induced apoptosis. A genetic null allele mutant, sel-12(ar171), showed resistance to oxidative stress and prevented mitochondrial dysfunction-induced apoptosis. On the other hand, another allele mutant, sel-12(ar131), that carries a missense mutation showed a proapoptotic activity, which may be the result of a gain of function property. Also, sel-12(ar131)-induced apoptosis was ced-3- and ced-4-dependent. Dantrolene, which specifically inhibits Ca(2+) release from endoplasmic reticulum stores, prevents sel-12(ar131)-induced apoptosis. SEL-12, which is localized in the endoplasmic reticulum, may induce apoptosis through abnormal calcium release from the endoplasmic reticulum. Together, with the previous finding that human PS1 could substitute for SEL-12, these results suggest the similar involvement of PS1-inducing apoptosis under oxidative stress and mitochondrial dysfunction in the Alzheimer's Disease brain.

The metal-binding properties of DREAM: evidence for calcium-mediated changes in DREAM structure

DREAM, an EF-hand protein, associates with and modulates the activity of presenilins and Kv4 potassium channels in neural and cardiac tissues and represses prodynorphin and c-fos gene expression by binding to DNA response elements in these genes. Information concerning the metal-binding properties of DREAM and the consequences of metal binding on protein structure are important in understanding how this protein functions in cells. We now show that DREAM binds 1 mol of calcium/mol of protein with relatively high affinity and another 3 mol of calcium with lower affinity. DREAM binds 1 mol of magnesium/mol of protein. DREAM, pre-loaded with 1 mol of calcium, binds 1 mol of magnesium, thus demonstrating that the magnesium-binding site is distinct from the high affinity calcium-binding site. Analysis of metal binding to mutant DREAM protein constructs localizes the high affinity calcium-binding site and the magnesium-binding site to EF-hands 3 or 4. Binding of calcium but not magnesium changes the conformation, stability, and alpha-helical content of DREAM. Calcium, but not magnesium, reduces the affinity of apo-DREAM for specific DNA response elements in the prodynorphin and c-fos genes. We conclude that DREAM binds calcium and magnesium and that calcium, but not magnesium, modulates DREAM structure and function.

FAD-linked mutations in presenilin 1 alter the length of Abeta peptides derived from betaAPP transmembrane domain mutants

gamma-Secretase is an enzymatic activity responsible for the final cleavage of the amyloid precursor protein leading to the production of the amyloid beta-peptide (Abeta). gamma-Secretase is likely an aspartyl protease, since its activity can be inhibited by both pepstatin and active-site directed aspartyl protease inhibitors. Recent work has indicated that presenilins 1 and 2 may actually be the gamma-secretase enzymes. Presenilin (PS) mutations, which lead to an increase in the production of a longer form of Abeta, are also the most common cause of familial Alzheimer's disease (FAD). Therefore, in an attempt to better characterize the substrate preferences of gamma-secretase, we performed experiments to determine how FAD-linked mutations in PS1 would affect the generation of Abeta peptides from full length precursor substrates that we have previously demonstrated to be proteolytically cleaved at alternative sites and/or by enzymatic activities that are pharmacologically distinct. Presenilin mutations increased the production of Abeta peptides from sites distal to the primary cleavage site ('longer' peptides) and in several cases also decreased production of 'shorter' peptides. These results support a model in which the FAD-linked mutants subtly alter the conformation of the gamma-secretase complex to favor the production of long Abeta.

Metabolism of presenilins

Understanding mechanisms involved in the production of Abeta has long been the central focus of cell biologists engaged in molecular AD research. The discovery of two genes that encode homologous polytopic membrane proteins termed Presenilins (PS), has lead to several exciting recent findings on the proteolytic processes responsible for generating the COOH-terminus of Abeta. What we now know is that PS proteins play an important role in Abeta production and are considered one of the therapeutic targets. Here I have reviewed the vast literature on the biology of PS, especially focusing on PS endoproteolysis and the accumulation of stable PS derivatives that are likely the functional units.

Presenilin mutations line up along transmembrane alpha-helices

Presenilin 1 mutations are the major cause of autosomal dominant Alzheimer's disease. Here we present evidence that pathogenic mutations in putative transmembrane domains 1, 2, 3, 4 and 6 align along helical faces, thus supporting the view that these are indeed transmembrane domains, and suggesting that disruption of the alignment of these domains is responsible for the pathogenicity of the mutations.