SEL-10 interacts with presenilin 1, facilitates its ubiquitination,and alters A-beta peptide production

Mutations in the human presenilin genes (PS1 or PS2) have been linked to autosomal dominant, early onset Alzheimer's disease (AD). Presenilins, probably as an essential part of gamma-secretase, modulate gamma-cleavage of the amyloid protein precursor (APP) to the amyloid beta-peptide (Abeta). Mutations in sel-12, a Caenorhabditis elegans presenilin homologue, cause a defect in egg laying that can be suppressed by loss of function mutations in a second gene, SEL-10. SEL-10 protein is a homologue of yeast Cdc4, a member of the SCF (Skp1-Cdc53/CUL1-F-box protein) E2-E3 ubiquitin ligase family. In this study, we show that human SEL-10 interacts with PS1 and enhances PS1 ubiquitination, thus altering cellular levels of unprocessed PS1 and its N- and C-terminal fragments. Co-transfection of sel-10 and APP cDNAs in HEK293 cells leads to an alteration in the metabolism of APP and to an increase in the production of amyloid beta-peptide, the principal component of amyloid plaque in Alzheimer's disease.     

Presenilins, the Endoplasmic Reticulum, and Neuronal Apoptosis in Alzheimer's Disease

Abstract: Many cases of autosomal dominant inherited forms of early-onset Alzheimer's disease are caused by mutations in the genes encoding presenilin-1 (PS-1; chromosome 14) and presenilin-2 (PS-2; chromosome 1). PSs are expressed in neurons throughout the brain wherein they appear to be localized primarily to the endoplasmic reticulum (ER) of cell bodies and dendrities. PS-1 and PS-2 show high homology and are predicted to have eight transmembrane domains with the C terminus, N terminus, and a loop domain all on the cytosolic side of the membrane; an enzymatic cleavage of PSs occurs at a site near the loop domain. The normal function of PSs is unknown, but data suggest roles in membrane trafficking, amyloid precursor protein processing, and regulation of ER calcium homeostasis. Homology of PSs to the C. elegans gene sel-12 , which is involved in Notch signaling, and phenotypic similarities of PS-1 and Notch knockout mice suggest a developmental role for PSs in the nervous system. When expressed in cultured cells and transgenic mice, mutant PSs promote increased production of a long form of amyloid β-peptide (Aβ1-42) that may possess enhanced amyloidogenic and neurotoxic properties. PS mutations sensitize cultured neural cells to apoptosis induced by trophic factor withdrawal, metabolic insults, and amyloid β-peptide. The mechanism responsible for the proapoptotic action of mutant PSs may involve perturbed calcium release from ER stores and increased levels of oxidative stress. Recent studies of apoptosis in many different cell types suggest that ER calcium signaling can modulate apoptosis. The evolving picture of PS roles in neuronal plasticity and Alzheimer's disease is bringing to the forefront the ER, an organelle increasingly recognized as a key regulator of neuronal plasticity and survival.     

PS1 activates PI3K thus inhibiting GSK-3 activity and tau overphosphorylation: effects of FAD mutations

Phosphatidylinositol 3-kinase (PI3K) promotes cell survival and communication by activating its downstream effector Akt kinase. Here we show that PS1, a protein involved in familial Alzheimer's disease (FAD), promotes cell survival by activating the PI3K/Akt cell survival signaling. This function of PS1 is unaffected by gamma-secretase inhibitors. Pharmacological and genetic evidence indicates that PS1 acts upstream of Akt, at or before PI3K kinase. PS1 forms complexes with the p85 subunit of PI3K and promotes cadherin/PI3K association. Furthermore, conditions that inhibit this association prevent the PS1-induced PI3K/Akt activation, indicating that PS1 stimulates PI3K/Akt signaling by promoting cadherin/PI3K association. By activating PI3K/Akt signaling, PS1 promotes phosphorylation/inactivation of glycogen synthase kinase-3 (GSK-3), suppresses GSK-3-dependent phosphorylation of tau at residues overphosphorylated in AD and prevents apoptosis of confluent cells. PS1 FAD mutations inhibit the PS1-dependent PI3K/Akt activation, thus promoting GSK-3 activity and tau overphosphorylation at AD-related residues. Our data raise the possibility that PS1 may prevent development of AD pathology by activating the PI3K/Akt signaling pathway. In contrast, FAD mutations may promote AD pathology by inhibiting this pathway.     

Capacitative calcium entry deficits and elevated luminal calcium content in mutant presenilin-1 knockin mice

Dysregulation of calcium signaling has been causally implicated in brain aging and Alzheimer's disease. Mutations in the presenilin genes (PS1, PS2), the leading cause of autosomal dominant familial Alzheimer's disease (FAD), cause highly specific alterations in intracellular calcium signaling pathways that may contribute to the neurodegenerative and pathological lesions of the disease. To elucidate the cellular mechanisms underlying these disturbances, we studied calcium signaling in fibroblasts isolated from mutant PS1 knockin mice. Mutant PS1 knockin cells exhibited a marked potentiation in the amplitude of calcium transients evoked by agonist stimulation. These cells also showed significant impairments in capacitative calcium entry (CCE, also known as store-operated calcium entry), an important cellular signaling pathway wherein depletion of intracellular calcium stores triggers influx of extracellular calcium into the cytosol. Notably, deficits in CCE were evident after agonist stimulation, but not if intracellular calcium stores were completely depleted with thapsigargin. Treatment with ionomycin and thapsigargin revealed that calcium levels within the ER were significantly increased in mutant PS1 knockin cells. Collectively, our findings suggest that the overfilling of calcium stores represents the fundamental cellular defect underlying the alterations in calcium signaling conferred by presenilin mutations.     

Increased vulnerability of hippocampal neurons from presenilin-1 mutant knock-in mice to amyloid beta-peptide toxicity: central roles of superoxide production and caspase activation

Many cases of early-onset inherited Alzheimer's disease (AD) are caused by mutations in the presenilin-1 (PS1) gene. Overexpression of PS1 mutations in cultured PC12 cells increases their vulnerability to apoptosis-induced trophic factor withdrawal and oxidative insults. We now report that primary hippocampal neurons from PS1 mutant knock-in mice, which express the human PS1M146V mutation at normal levels, exhibit increased vulnerability to amyloid beta-peptide toxicity. The endangering action of mutant PS1 was associated with increased superoxide production, mitochondrial membrane depolarization, and caspase activation. The peroxynitrite-scavenging antioxidant uric acid and the caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone protected hippocampal neurons expressing mutant PS1 against cell death induced by amyloid beta-peptide. Increased oxidative stress may contribute to the pathogenic action of PS1 mutations, and antioxidants may counteract the adverse property of such AD-linked mutations.     

Ca2＋失调与阿尔茨海默病发生发展关系的研究进展

阿尔茨海默病（Alzheimer’s disease,AD）是全球老年人中最常见的神经退行性疾病。在对家族性AD的动物模型和个别AD患者死后脑组织的研究中发现,除了A1342水平升高外,神经元内Ca2＋信号失调,并且Ca2＋信号蛋白表达水平也发生了改变。淀粉样蛋白斑、神经元纤维缠结和神经元丢失是AD的后期标志,它们的共同点可能是破坏神经元钙离子信号。细胞内钙离子水平提高在功能上与淀粉样蛋白斑、早老素突变、tau缠结和突触功能障碍相关,基于这些研究,产生了AD的Ca2＋假说。主要介绍神经元内Ca2＋失调与AD的关系以及钙拮抗剂作为AD的潜在治疗方法。     

Presenilins form ER Ca2+ leak channels, a function disrupted by familial Alzheimer's disease-linked mutations

Alzheimer's disease (AD) is a progressive and irreversible neurodegenerative disorder. Mutations in presenilins 1 and 2 (PS1 and PS2) account for approximately 40% of familial AD (FAD) cases. FAD mutations and genetic deletions of presenilins have been associated with calcium (Ca(2+)) signaling abnormalities. We demonstrate that wild-type presenilins, but not PS1-M146V and PS2-N141I FAD mutants, can form low-conductance divalent-cation-permeable ion channels in planar lipid bilayers. In experiments with PS1/2 double knockout (DKO) mouse embryonic fibroblasts (MEFs), we find that presenilins account for approximately 80% of passive Ca(2+) leak from the endoplasmic reticulum. Deficient Ca(2+) signaling in DKO MEFs can be rescued by expression of wild-type PS1 or PS2 but not by expression of PS1-M146V or PS2-N141I mutants. The ER Ca(2+) leak function of presenilins is independent of their gamma-secretase activity. Our data suggest a Ca(2+) signaling function for presenilins and provide support for the "Ca(2+) hypothesis of AD."     

Genetic study of familial cases of Alzheimer's disease

A small number (1-5%) of Alzheimer's disease (AD) cases associated with the early-onset form of the disease (EOAD) appears to be transmitted as a pure genetic, autosomal dominant trait. To date, three genes responsible for familial EOAD have been identified in the human genome: amyloid precursor protein (APP), presenilin 1 (PS1), and presenilin 2 (PS2). Mutations in these genes account for a significant fraction (18 to 50%) of familial cases of early onset AD. The mutations affect APP processing causing increased production of the toxic Abeta42 peptide. According to the "amyloid cascade hypothesis", aggregation of the Abeta42 peptide in brain is a primary event in AD pathogenesis. In our study of twenty AD patients with a positive family history of dementia, 15% (3 of 20) of the cases could be explained by coding sequence mutations in the PS1 gene. Although a frequency of PS1 mutations is less than 2% in the whole population of AD patients, their detection has a significant diagnostic value for both genetic counseling and treatment in families with AD.     

Presenilin-2 mutations modulate amplitude and kinetics of inositol 1, 4,5-trisphosphate-mediated calcium signals.

Mutations in the two presenilin genes (PS1, PS2) account for the majority of early-onset familial Alzheimer's disease (FAD) cases. Converging evidence from a variety of experimental systems, including fibroblasts from FAD patients and transgenic animals, indicates that PS1 mutations modulate intracellular calcium signaling pathways. Despite the potential relevance of these changes to the pathogenesis of FAD, a comparable effect for PS2 has not yet been demonstrated experimentally. We examined the effects of wild-type PS2, and both of the identified FAD mutations in PS2, on intracellular calcium signaling in Xenopus oocytes. Inositol 1,4, 5-trisphosphate (IP(3))-evoked calcium signals were significantly potentiated in cells expressing either of the PS2 mutations relative to wild-type PS2-expressing cells and controls. Decay rates of calcium signals were also significantly accelerated in mutant PS2-expressing cells in a manner dependent upon IP(3) concentration. The finding that mutations in both PS1 and PS2 modulate intracellular calcium signaling suggests that these disturbances may represent a common pathogenic mechanism of presenilin-associated FAD.     

Mutant presenilin‐1 deregulated peripheral immunity exacerbates Alzheimer‐like pathology

Mutations in the presenilin‐1 (PS1) gene are independent causes of familial Alzheimer's disease (AD). AD patients have dysregulated immunity, and PS1 mutant mice exhibit abnormal systemic immune responses. To test whether immune function abnormality caused by a mutant human PS1 gene (mhPS1) could modify AD‐like pathology, we reconstituted immune systems of AD model mice carrying a mutant human amyloid precursor protein gene (mhAPP; Tg2576 mice) or both mhAPP and mhPS1 genes (PSAPP mice) with allo‐geneic bone marrow cells. Here, we report a marked reduction in amyloid‐β (Aβ) levels, β‐amyloid plaques and brain inflammatory responses in PSAPP mice following strain‐matched wild‐type PS1 bone marrow reconstitution. These effects occurred with immune switching from pro‐inflammatory T helper (Th) 1 to anti‐inflammatory Th2 immune responses in the periphery and in the brain, which likely instructed microglia to phagocytose and clear Aβ in an ex vivo assay. Conversely, Tg2576 mice displayed accelerated AD‐like pathology when reconstituted with mhPS1 bone marrow. These data show that haematopoietic cells bearing the mhPS1 transgene exacerbate AD‐like pathology, suggesting a novel therapeutic strategy for AD based on targeting PS1 in peripheral immune cells.     

Mechanism of Ca2+ disruption in Alzheimer's disease by presenilin regulation of InsP3 receptor channel gating

Mutations in presenilins (PS) are the major cause of familial Alzheimer's disease (FAD) and have been associated with calcium (Ca2+) signaling abnormalities. Here, we demonstrate that FAD mutant PS1 (M146L)and PS2 (N141I) interact with the inositol 1,4,5-trisphosphate receptor (InsP3R) Ca2+ release channel and exert profound stimulatory effects on its gating activity in response to saturating and suboptimal levels of InsP3. These interactions result in exaggerated cellular Ca2+ signaling in response to agonist stimulation as well as enhanced low-level Ca2+signaling in unstimulated cells. Parallel studies in InsP3R-expressing and -deficient cells revealed that enhanced Ca2+ release from the endoplasmic reticulum as a result of the specific interaction of PS1-M146L with the InsP3R stimulates amyloid beta processing,an important feature of AD pathology. These observations provide molecular insights into the "Ca2+ dysregulation" hypothesis of AD pathogenesis and suggest novel targets for therapeutic intervention.     

Molecular genetics of Alzheimer's disease: presenilin 1 gene analysis in a cohort of patients from the Poznań region

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by memory loss and personality changes. Pathological hallmarks of AD are: deposition of amyloid plaques and neurofibrillary tangles in the brain, accompanied by neuronal and synaptic loss. The genetic background of AD is heterogeneous and strongly depends on the form of the disease. In most of the families with early-onset AD (EOAD) (10% of the total population of patients), the disease segregates as an autosomal dominant fully penetrant trait. To date, some missense mutations in three genes encoding the amyloid precursor protein, presenilin 1 (PS1) and 2 (PS2) have been found to cause familial EOAD. We screened for mutations in the presenilin genes in a sample of 55 patients with familial or sporadic form of EOAD from the Poznan region. We found 4 missense mutations in the PS1 gene: A246E in exon 7, P267L in exon 8, E318G in exon 9, and L424R in exon 12 among 5 unrelated patients. The frequency of PS1 mutations was 11% (5 of 55) in the whole sample of the patients with EOAD or 50% (3 of 6) if the analysis was restricted to familial cases with a positive history of dementia in the patient's family.     

Increased production of beta-amyloid and vulnerability to endoplasmic reticulum stress by an aberrant spliced form of presenilin 2

An alternative spliced form of the presinilin 2 (PS2) gene (PS2V) lacking exon 5 has previously been reported to be expressed in human brains in sporadic Alzheimer's disease (AD). PS2V encodes the amino-terminal portion of PS2, which contains residues Met1-Leu119 and 5 additional amino acid residues (SSMAG) at its carboxyl terminus. Here we report that PS2V protein impaired the signaling pathway of the unfolded protein response, similarly to familial AD-linked PS1 mutants and caused significant increases in the production of both amyloid beta40 and beta42. Interestingly, PS2V-encoding protein was expressed in neuropathologically affected neurons of the hippocampal CA1 region and temporal cortex in AD patients. These findings suggest that the aberrant splicing of the PS2 gene may be implicated in the neuropathology of sporadic AD.     

Microglial PD‐1 stimulation by astrocytic PD‐L1 suppresses neuroinflammation and Alzheimer’s disease pathology

Chronic neuroinflammation is a pathogenic component of Alzheimer’s disease (AD) that may limit the ability of the brain to clear amyloid deposits and cellular debris. Tight control of the immune system is therefore key to sustain the ability of the brain to repair itself during homeostasis and disease. The immune‐cell checkpoint receptor/ligand pair PD‐1/PD‐L1, known for their inhibitory immune function, is expressed also in the brain. Here, we report upregulated expression of PD‐L1 and PD‐1 in astrocytes and microglia, respectively, surrounding amyloid plaques in AD patients and in the APP/PS1 AD mouse model. We observed juxtamembrane shedding of PD‐L1 from astrocytes, which may mediate ectodomain signaling to PD‐1‐expressing microglia. Deletion of microglial PD‐1 evoked an inflammatory response and compromised amyloid‐β peptide (Aβ) uptake. APP/PS1 mice deficient for PD‐1 exhibited increased deposition of Aβ, reduced microglial Aβ uptake, and decreased expression of the Aβ receptor CD36 on microglia. Therefore, ineffective immune regulation by the PD‐1/PD‐L1 axis contributes to Aβ plaque deposition during chronic neuroinflammation in AD.     

Zebrafish (Danio rerio) presenilin promotes aberrant amyloid beta-peptide production and requires a critical aspartate residue for its function in amyloidogenesis.

Alzheimer's disease (AD) is characterized by the invariable accumulation of senile plaques composed of amyloid beta-peptide (Abeta). Mutations in three genes are known to cause familial Alzheimer's disease (FAD). The mutations occur in the genes encoding the beta-amyloid precursor protein (betaAPP) and presenilin (PS1) and PS2 and cause the increased secretion of the pathologically relevant 42 amino acid Abeta42. We have now cloned the zebrafish (Danio rerio) PS1 homologue (zf-PS1) to study its function in amyloidogenesis and to prove the critical requirement of an unusual aspartate residue within the seventh putative transmembrane domain. In situ hybridization and reverse PCR reveal that zf-PS1 is maternally inherited and ubiquitously expressed during embryogenesis, suggesting an essential housekeeping function. zf-PS1 is proteolytically processed to produce a C-terminal fragment (CTF) of approximately 24 kDa similar to human PS proteins. Surprisingly, wt zf-PS1 promotes aberrant Abeta42 secretion like FAD associated human PS1 mutations. The unexpected pathologic activity of wt zf-PS1 may be due to several amino acid exchanges at positions where FAD-associated mutations have been observed. The amyloidogenic function of zf-PS1 depends on the conserved aspartate residue 374 within the seventh putative transmembrane domain. Mutagenizing this critical aspartate residue abolishes endoproteolysis of zf-PS1 and inhibits Abeta secretion in human cells. Inhibition of Abeta secretion is accompanied by the accumulation of C-terminal fragments of betaAPP, suggesting a defect in gamma-secretase activity. These data provide further evidence that PS proteins are directly involved in the proteolytic cleavage of betaAPP and demonstrate that this function is evolutionarily conserved.     

The Senescence Hypothesis of Disease Progression in Alzheimer Disease: an Integrated Matrix of Disease Pathways for FAD and SAD

Alzheimer disease (AD) is a progressive, neurodegenerative disease characterised in life by cognitive decline and behavioural symptoms and post-mortem by the neuropathological hallmarks including the microtubule-associated protein tau-reactive tangles and neuritic plaques and amyloid-beta-protein-reactive senile plaques. Greater than 95 % of AD cases are sporadic (SAD) with a late onset and <5 % of AD cases are familial (FAD) with an early onset. FAD is associated with various genetic mutations in the amyloid precursor protein (APP) and the presenilins (PS)1 and PS2. As yet, no disease pathway has been fully accepted and there are no treatments that prevent, stop or reverse the cognitive decline associated with AD. Here, we review and integrate available environmental and genetic evidence associated with all forms of AD. We present the senescence hypothesis of AD progression, suggesting that factors associated with AD can be seen as partial stressors within the matrix of signalling pathways that underlie cell survival and function. Senescence pathways are triggered when stressors exceed the cells ability to compensate for them. The APP proteolytic system has many interactions with pathways involved in programmed senescence and APP proteolysis can both respond to and be driven by senescence-associated signalling. Disease pathways associated with sporadic disease may be different to those involving familial genetic mutations. The interpretation we provide strongly points to senescence as an additional underlying causal process in dementia progression in both SAD and FAD via multiple disease pathways.     

Presenilins: multifunctional proteins involved in Alzheimer's disease pathology

Early-onset aggressive forms of Alzheimer's disease (AD) are of genetic nature and have been linked to inherited mutations located on chromosomes 21, 14, and 1. The gene products of chromosomes 14 and 1, which are responsible for most of these familial forms of the disease (FAD), have been recently identified and referred to as presenilin 1 and 2 (PS1, PS2), respectively. Several lines of evidence derived from neuropathological, cell biology, and transgenesis approaches indicate that PS could interfere with the processing of the beta-amyloid precursor protein (betaAPP). Thus, FAD-linked mutations in PS exacerbate the production of Abeta42, the readily aggregable Abeta species corresponding to one of the main constituents of the senile plaques that invade the cortical areas of affected brains. Recent studies indicate that PS functions could be intimately related with the susceptibility of PS to further processing by caspase-like enzymes and other unknown proteolytic activities. Here I briefly report on the post-translational events undergone by PS and examine recent advances concerning their possible roles in development, cell signaling, and apoptosis. Possible alterations brought by FAD-linked mutations will be discussed.     

Dysfunction of amyloid precursor protein signaling in neurons leads to DNA synthesis and apoptosis

The classic neuropathological diagnostic markers for AD are amyloid plaques and neurofibrillary tangles, but their role in the etiology and progression of the disease remains incompletely defined. Research over the last decade has revealed that cell cycle abnormalities also represent a major neuropathological feature of AD. These abnormalities appear very early in the disease process, prior to the appearance of plaques and tangles; and it has been suggested that neuronal cell cycle regulatory failure may be a significant component of the pathogenesis of AD. The amyloid precursor protein (APP) is most commonly known as the source of the beta-amyloid (Abeta) peptides that accumulate in the brains of patients with AD. However, a large body of work supports the idea that APP is also a signaling receptor. Most recently, it has been shown that familial AD (FAD) mutations in APP or simple overexpression of wild type APP cause dysfunction of APP signaling, resulting in initiation of DNA synthesis in neurons and consequent apoptosis. In this article, we review the evidence that APP has the potential to activate aberrant neuronal cell cycle re-entry in AD, and we describe a signal transduction pathway that may mediate this abnormal activation of the cell cycle.