Presenilins: molecular switches between proteolysis and signal transduction

A cell biological perspective on Alzheimer's disease

The amyloid precursor protein and the proteases cleaving this protein are important players in the pathogenesis of Alzheimer's disease via the generation of the amyloid peptide. Physiologically, the amyloid precursor protein is implied in axonal vesicular trafficking and the proteases are implicated in developmentally important signaling pathways, most significantly those involving regulated intramembrane proteolysis or RIP. We discuss the cell biology behind the amyloid and tangle hypothesis for Alzheimer's disease, drawing on the many links to the fields of cell biology and developmental biology that have been established in the recent years.

Amyloid, presenilins, and Alzheimer's disease

The regulated intramembrane proteolysis of the amyloid precursor protein (APP) that results in the generation of a toxic 40 to 42 amino acid fragment, Abeta, and a C-terminal intracellular fragment stands central in the pathogenesis of Alzheimer's disease. The fibrillar Abeta peptide is extracellularly deposited in plaques in the amygdala, the hippocampus, and the neocortex of affected individuals. The APP intracellular fragment binds to transcription factors and is translocated to the nucleus, where it influences transcription. Regulated intramembrane proteolysis of APP is dependent on the activity of a multimeric protein complex of which the essential components are presenilin, nicastrin, PEN-2, and APH-1. Further research into this emerging field of presenilin-dependent APP proteolysis within the plane of the membrane might reveal the necessity of an additional transport step-bringing substrate and enzyme together-before APP can actually be processed.

Enhanced long-term potentiation in the hippocampus of rats expressing mutant presenillin-1 is age related

Electrophysiological recordings were made from Fischer rats engineered to express the human presenilin 1 gene carrying the M146V mutation. Extracellular recordings of field excitatory post-synaptic potential (EPSPs) were made to investigate EPSP properties, paired pulse responses, posttetanic potentiation, and long-term potentiation in the stratum radiatum and dentate gyrus of hippocampal slices maintained in vitro. Transgenic rats aged approximately 6 months showed no differences from their wild-type littermates in any of these properties. However, at 18 months, long-term potentiation in the CA1 was facilitated in the transgenic rats with a different pattern of synaptic enhancement. No changes were observed in paired pulse facilitation (PPF) or post-tetanic potentiation (PPT) and no changes were seen in the dentate gyrus. Field potential amplitudes were significantly greater and PPF was enhanced in the CA1 of all older rats. Intracellular recordings from CA1 pyramidal cells of the older group of rats revealed no differences in the passive or active membrane properties of cells in the two groups, but intracellularly recorded EPSPs were significantly longer.

gamma-Secretase activity requires the presenilin-dependent trafficking of nicastrin through the Golgi apparatus but not its complex glycosylation

Nicastrin and presenilin are two major components of the gamma-secretase complex, which executes the intramembrane proteolysis of type I integral membrane proteins such as the amyloid precursor protein (APP) and Notch. Nicastrin is synthesized in fibroblasts and neurons as an endoglycosidase-H-sensitive glycosylated precursor protein (immature nicastrin) and is then modified by complex glycosylation in the Golgi apparatus and by sialylation in the trans-Golgi network (mature nicastrin). These modifications are not observed with exogenously overexpressed nicastrin. Under normal cell culture conditions, only mature nicastrin is expressed at the cell surface and binds to the presenilin heterodimers. Mature nicastrin has a half-life of more than 24 hours. In the absence of presenilin 1 and 2, nicastrin remains entirely endoglycosidase H sensitive, is retained in the endoplasmic reticulum and is slowly degraded. Single presenilin 1 or presenilin 2 deficiency affects glycosylation of nicastrin to a lesser extent than the combined presenilin deficiencies, suggesting a correlation between either the transport of nicastrin out of the endoplasmic reticulum or the concomitant complex glycosylation of nicastrin, and gamma-secretase activity. However, when complex glycosylation of nicastrin was inhibited using mannosidase I inhibitors, gamma-secretase cleavage of APP or Notch was not inhibited and the immature nicastrin still associates with presenilin and appears at the cell surface. Complex glycosylation of nicastrin is therefore not needed for gamma-secretase activity. Because the trafficking of nicastrin to the Golgi apparatus is dependent on presenilins, our data point to a central role of presenilin in nicastrin maturation/localization, which could help to partially resolve the 'spatial paradox'.

Regulatory region variability in the human presenilin-2 (PSEN2) gene: potential contribution to the gene activity and risk for AD

We have analyzed the 5'-upstream promoter region of the presenilin 2 gene (PSEN2) for regulatory elements and examined Alzheimer disease (AD) patients and non-demented individuals for polymorphisms in the 5' upstream promoter region of the PSEN2 gene. Direct sequencing analysis detected a common single adenine (A) nucleotide deletion polymorphism in the upstream promoter region of the PSEN2 gene. Examination of cohorts of AD patients and age-matched control individuals revealed no statistically significant differences in the frequency of this polymorphism when compared with the total sample of AD patients and control individuals. However, subgroup and regression analysis suggested that the relatively rare -A/-A genotype increases risk of AD among subjects lacking apolipoprotein E (APOE) epsilon4 and among persons ages 65 years and younger. DNA sequence and DNA-protein binding analysis demonstrated that this mutation negates binding with putative repressor transcription factor (TF), interferon regulatory factor 2 (IRF2), in nuclear extracts prepared from the aged human brain neocortex. However this mutation creates a potential regulatory element, C/EBPbeta, that is responsive to pro-inflammatory (PI) induction. The expression activity assay with luciferase reporter gene into normal human neural progenitor cells in primary culture shows that the mutant PSEN2 regulatory region exhibits a 1.8-fold higher level of basal expression and is sensitive to IL-1beta and Abeta42, but that it is synergistically induced 3.2-fold over the wild-type PSEN2 by [IL-1beta+Abeta42]. These results suggest that under Pl and oxygen stress conditions relatively minor variations in PSEN2 promoter DNA sequence structure can enhance PSEN2 gene expression and that consequently these may play a role in the induction and/or proliferation of a Pl response in AD brain.

Isoform-specific knockout ofFE65 leads to impaired learning and memory

FE65 is a multimodular adapter protein that is expressed predominantly in brain. Its C-terminal phosphotyrosine interaction domain (PID) binds to the intracellular tail of the beta-amyloid precursor protein (betaPP), a protein of central importance to the pathogenesis of dementias of the Alzheimer type. To study the physiological functions of FE65, we generated a line of FE65 knockout mice via gene targeting. By Western analysis with a panel of FE65-specific antibodies, we demonstrate that the 97-kDa full-length FE65 (p97) was ablated in the mutant mice, and that a previously undescribed FE65 isoform with apparent molecular mass of 60 kDa (p60) was expressed in both wild-type and mutant mice. p60 had a truncated N-terminus and was likely to be generated through alternative translation. Expressions of the two isoforms appeared to be brain region distinct and age dependent. The p97FE65(-/-) mice were viable and showed no obvious physical impairments or histopathological abnormalities. However, p97FE65(-/-) and p97FE65(+/-) mice exhibited poorer performances than wild-type mice on a passive avoidance task when tested at 14 months (P <.05). p97FE65(-/-) mice at 14 months also exhibited impaired hidden-platform acquisition (P <.05) and a severe reversal-learning deficit (P <.002) but normal visual-platform acquisition in the Morris water maze tests. Probe trials confirmed impairments in p97FE65(-/-) mice in relearning of new spatial information, suggesting a hippocampus-dependent memory-extinction deficit. Reduced secretion of Abeta peptides was observed in primary neuronal cultures of hybrids of p97FE65(-/-)/betaPP transgenic (Tg2576) mice. These studies suggest an important and novel function of FE65 in learning and memory.

Declining expression of neprilysin in Alzheimer disease vasculature: possible involvement in cerebral amyloid angiopathy

Molecular, genetic, and pharmacological studies have shown that neprilysin (also called NEP) catabolizes amyloid beta peptides (A beta) in healthy conditions. However, in Alzheimer disease (AD), A beta accumulates forming senile plaques in brain parenchyma and amyloid deposition around blood vessels. In this study, we tested at cellular level the relationship between neprilysin and A beta in human healthy and AD brain. Our results provided evidence for declining levels of neprilysin in AD brains as compared to healthy controls in parallel with increasing deposition of A beta. In hippocampus of AD individuals we observed a significant down-regulation of neprilysin expression in pyramidal neurons, consistent with the possibility that neprilysin controls the level of A beta accumulation and plaque formation in this area. In the cortex and leptomeninges, neprilysin was expressed in the smooth muscle cells of blood vessels. In sections from AD patients we observed a clear inverse relationship between neprilysin and A beta peptide levels in the vasculature, implicating neprilysin in cerebral amyloid angiopathy.

Gamma-secretase-mediated proteolysis in cell-surface-receptor signalling

Many cell-surface receptors transmit signals to the nucleus through complex protein cascades. By contrast, the Notch signalling pathway uses a relatively direct mechanism, in which the intracellular domain of the receptor is liberated by intramembrane cleavage and translocates to the nucleus. This critical cleavage is mediated by the gamma-secretase complex, and new findings reveal that this mechanism is used by various receptors, although many questions remain about the biochemical details.

Luman, the cellular counterpart of herpes simplex virus VP16, is processed by regulated intramembrane proteolysis

Luman is a human basic leucine zipper transcription factor that, like the herpes simplex virus transcription factor VP16, requires the host cell factor, HCF, for activity. Although both HCF and Luman have been implicated in cell growth, their biological roles have not been clearly defined. Luman conforms to a type II membrane-associated glycoprotein with its carboxyl terminus embedded in cellular membranes and its amino terminus, which contains all its identified functional domains, in the cytoplasm. Here we show that Luman is processed by regulated intramembrane proteolysis (RIP). The site 1 protease (S1P), a Golgi apparatus-resident enzyme responsible for catalyzing the first step in the RIP pathway of the sterol regulatory element binding proteins (SREBPs) and ATF6, may also be involved in the processing of Luman. Thus, processing of Luman was highly stimulated by brefeldin A, a compound that causes the reflux of Golgi apparatus enzymes to the endoplasmic reticulum (ER). In addition, coexpression of Luman with S1P containing a KDEL ER retrieval signal resulted in virtually quantitative cleavage of Luman in the absence of any treatment. Finally, Luman contains a sequence, RQLR, immediately downstream from the transmembrane domain which bears similarity to the consensus S1P cleavage site identified by others. Substitution of arginine residues within this motif abolished S1P cleavage, providing robust evidence that S1P is involved in Luman processing. We observed that following S1P cleavage, the majority of the cleaved Luman was retained in cytoplasmic membranes, indicating that an additional step or enzymes yet to be identified are involved in complete cleavage and release to yield the product which ultimately enters the nuclei of cells.

Selective mutagenesis of lysyl residues leads to a stable and active form of delta 9 stearoyl-CoA desaturase

Stearoyl-CoA desaturase (SCD) is a short-lived integral membrane protein of the endoplasmic reticulum (ER) that catalyzes the insertion of a double bond in the delta 9 position of saturated fatty acids. Its expression has been difficult in heterologous systems. In this study, recombinant adenovirus vector was used to express both wild-type (wt) and engineered forms of rat SCD in human transformed kidney cells. In the engineered form of SCD, lysyl residues at positions 33, 35, and 36 were mutated to alanine (SCD K/A). The recombinant adenovirus also contains a cDNA encoding the green fluorescent protein (GFP). The stable reporter GFP was used to analyze the efficiency of transfection and the stability of expressed SCDs. The wt SCD was unstable upon expression, whereas expression of SCD K/A resulted in the stabilization of the protein. The proteasome inhibitor MG132 did not affect the rapid degradation of expressed wt SCD, implying that proteasome is not involved in this degradation. Functional analysis of microsomes from infected cells expressing SCD K/A resulted in the formation of holoenzyme with desaturase activity. Here we report engineering a stabilized form of a rapidly degraded membrane protein for production of an active mutant form of SCD. The adenovirus transformed cells may provide a model for the study of the effects of positive SCD expression.

Fluorescent and photoactivable probes in depth-dependent analysis of membranes

This report summarizes our efforts towards depth-dependent analysis of membranes by design of suitable fluorescent and photoactivable lipid probes, which can be incorporated into membranes. The objective of depth-dependent analysis has been two fold, one to obtain information on lipid domains and other on transmembrane domains of membrane-bound proteins. In view of increasing importance of lipid rafts and other localized domain and limited success in case of structure determination of membrane-bound proteins vis-à-vis their soluble counterparts, it is tempting to rapidly attach fluorescent or photoactivable probes to lipids to get a probes where relatively little attention is paid to design of such probes. We have shown here how careful design of such probes is required to immobilize such probes in membranes for effective depth-dependent analysis of membranes. An effective design has become important when identification of putative transmembrane domains predicted primarily from the genome data based on hydropathy plots, often needs confirmation by contemporary methodology.

Protein aggregates and dementia: is there a common toxicity?

This review considers some of the recent advances made in the understanding of the pathogenic proteins known to aggregate and be implicated in neurodegenerative dementing disorders. It concentrates on the two most obvious candidates for the role of toxic protein in Alzheimer's disease (AD)--beta-amyloid peptide and tau--but also considers other proteins in this disorder and in less common but equally devastating diseases.

Pharmacogenomics for the treatment of dementia

Alzheimer's disease (AD) is a genetically complex disorder associated with multiple genetic defects either mutational or of susceptibility. Current AD genetics does not explain in full the etiopathogenesis of AD, suggesting that environmental factors and/or epigenetic phenomena may also contribute to AD pathology and phenotypic expression of dementia. The genomics of AD is still in its infancy, but is helping us to understand novel aspects of the disease including genetic epidemiology, multifactorial risk factors, pathogenic mechanisms associated with genetic networks and genetically-regulated metabolic cascades. AD genomics is also fostering new strategies in pharmacogenomic research and prevention. Functional genomics, proteomics, pharmacogenomics, high-throughput methods, combinatorial chemistry and modern bioinformatics will greatly contribute to accelerating drug development for AD and other complex disorders. The multifactorial genetic dysfunction in AD includes mutational loci (APP, PS1, PS2) and diverse susceptibility loci (APOE, A2M, AACT, LRP1, IL1A, TNF, ACE, BACE, BCHE, CST3, MTHFR, GSK3B, NOS3) distributed across the human genome, probably converging in common pathogenic mechanisms that lead to premature neuronal death. Genomic associations integrate polygenic matrix models to elucidate the genomic organization of AD in comparison to the control population. Using APOE-related monogenic models it has been demonstrated that the therapeutic response to drugs (e.g., cholinesterase inhibitors, non-cholinergic compounds) in AD is genotype-specific. A multifactorial therapy combining three different drugs yielded positive results during 6-12 months in approximately 60% of the patients. With this therapeutic strategy, APOE-4/4 carriers were the worst responders and patients with the APOE-3/4 genotype were the best responders. Other polymorphic variants (PS1, PS2) also influence the therapeutic response to different drugs in AD patients, suggesting that the final pharmacological outcome is the result of multiple genomic interactions, including AD-related genes and genes associated with drug metabolism, disposition, and elimination. The pharmacogenomics of AD may contribute in the future to optimise drug development and therapeutics, increasing efficacy and safety, and reducing side-effects and unnecessary costs.

Drosophila nicastrin is essential for the intramembranous cleavage of notch

The catalytic subunit of gamma-secretase is thought to be Presenilin, which is required for both the cleavage of APP and in the processing of Notch. Presenilin is found in a multisubunit complex that also contains Nicastrin. Nicastrin has been implicated in APP processing, but its role in Notch signaling remains unclear. Here we show that Drosophila Nicastrin is required for Notch signaling, and acts specifically at the S3 cleavage step. Partially processed Notch accumulates apically in nicastrin and presenilin mutant follicle cells. nicastrin and presenilin mutations also disrupt the spectrin cytoskeleton, suggesting that the gamma-secretase complex has another function in Drosophila in addition to its role in processing Notch and APP.

Pro-apoptotic effect of presenilin 2 (PS2) overexpression is associated with down-regulation of Bcl-2 in cultured neurons

Presenilin 2 (PS2) is a polytopic membrane protein that is mutated in some cases of familial Alzheimer's disease (AD). The normal functions of PS2 and its pathogenic role in AD remain unclear. We investigated the biological role of this protein in neurons, using adenovirus-mediated transduction of the PS2 gene into rat primary cortical neurons. Immunocytochemical analyses demonstrated increased PS2 immunoreactivity in most neurons infected with recombinant adenoviruses expressing PS2. Neurons infected with wild-type or mutant (N141I) PS2-expressing adenoviruses showed a significant increase in basal cell death, compared with those infected with control beta-galactosidase-expressing adenovirus. Moreover, PS2 overexpression markedly increased neuronal susceptibility to staurosporine-induced apoptosis. Mutant PS2 was more effective in enhancing apoptosis than its wild-type counterpart. Staurosporine-induced death was significantly inhibited by a specific caspase 3 inhibitor. Western analyses revealed that Bcl-2 protein expression was specifically down-regulated in neurons overexpressing PS2, which temporally corresponded to the accumulation of C- and N-terminal fragments of PS2. Additionally, expression of mutant, but not wild-type PS2, increased the production of beta-amyloid protein (Abeta) 42. These data collectively suggest that the pro-apoptotic effect of PS2 is mediated by down-regulation of Bcl-2. PS2 mutations may increase the susceptibility of neurons to apoptotic stimuli by perturbing the regulation of cell death.

Interaction with telencephalin and the amyloid precursor protein predicts a ring structure for presenilins

The carboxyl terminus of presenilin 1 and 2 (PS1 and PS2) binds to the neuron-specific cell adhesion molecule telencephalin (TLN) in the brain. PS1 deficiency results in the abnormal accumulation of TLN in a yet unidentified intracellular compartment. The first transmembrane domain and carboxyl terminus of PS1 form a binding pocket with the transmembrane domain of TLN. Remarkably, APP binds to the same regions via part of its transmembrane domain encompassing the critical residues mutated in familial Alzheimer's disease. Our data surprisingly indicate a spatial dissociation between the binding site and the proposed catalytic site near the critical aspartates in PSs. They provide important experimental evidence to support a ring structure model for PS.

Reduction in cholesterol and sialic acid content protects cells from the toxic effects of beta-amyloid peptides

beta-Amyloid (Abeta) is the primary protein component of senile plaques associated with Alzheimer's disease and has been implicated in the neurotoxicity associated with the disease. A variety of evidence points to the importance of Abeta-membrane interactions in the mechanism of Abeta neurotoxicity and indicates that cholesterol and gangliosides are particularly important for Abeta aggregation and binding to membranes. We investigated the effects of cholesterol and sialic acid depletion on Abeta-induced GTPase activity in cells, a step implicated in the mechanism of Abeta toxicity, and Abeta-induced cell toxicity. Cholesterol reduction and depletion of membrane-associated sialic acid residues both significantly reduced the Abeta-induced GTPase activity. In addition, cholesterol and membrane-associated sialic acid residue depletion or inhibition of cholesterol and ganglioside synthesis protected PC12 cells from Abeta-induced toxicity. These results indicate the importance of Abeta-membrane interactions in the mechanism of Abeta toxicity. In addition, these results suggest that control of cellular cholesterol and/or ganglioside content may prove useful in the prevention or treatment of Alzheimer's disease.

The amyloid precursor protein (APP)-cytoplasmic fragment generated by gamma-secretase is rapidly degraded but distributes partially in a nuclear fraction of neurones in culture

The gamma-secretase cleavage is the last step in the generation of the beta-amyloid peptide (Abeta) from the amyloid precursor protein (APP). The Abeta precipitates in the amyloid plaques in the brain of Alzheimer's disease patients. The fate of the intracellular APP carboxy-terminal stub generated together with Abeta has been, in contrast, only poorly documented. The analogies between the processing of APP and other transmembrane proteins like SREBP and Notch suggests that this intracellular fragment could have important signalling functions. We demonstrate here that APP-C59 is rapidly degraded (half-life approximately 5 min) when overexpressed in baby hamster kidney cells or primary cultures of neurones by a mechanism that is not inhibited by endosomal/lysosomal or proteasome inhibitors. Furthermore, APP-C59 binds to the DNA binding protein Fe65, although this does not increase the half-life of APP-C59. Finally, we demonstrate that a fraction of APP-C59 becomes redistributed to the nuclear detergent-insoluble pellet, in which the transcription factor SP1 is also present. Overall our results reinforce the analogy between Notch and APP processing, and suggest that the APP intracellular domain, like the Notch intracellular domain, could have a role in signalling events from the plasma membrane to the nucleus.

The discrepancy between presenilin subcellular localization and gamma-secretase processing of amyloid precursor protein

We investigated the relationship between PS1 and gamma-secretase processing of amyloid precursor protein (APP) in primary cultures of neurons. Increasing the amount of APP at the cell surface or towards endosomes did not significantly affect PS1-dependent gamma-secretase cleavage, although little PS1 is present in those subcellular compartments. In contrast, almost no gamma-secretase processing was observed when holo-APP or APP-C99, a direct substrate for gamma-secretase, were specifically retained in the endoplasmic reticulum (ER) by a double lysine retention motif. Nevertheless, APP-C99-dilysine (KK) colocalized with PS1 in the ER. In contrast, APP-C99 did not colocalize with PS1, but was efficiently processed by PS1-dependent gamma-secretase. APP-C99 resides in a compartment that is negative for ER, intermediate compartment, and Golgi marker proteins. We conclude that gamma-secretase cleavage of APP-C99 occurs in a specialized subcellular compartment where little or no PS1 is detected. This suggests that at least one other factor than PS1, located downstream of the ER, is required for the gamma-cleavage of APP-C99. In agreement, we found that intracellular gamma-secretase processing of APP-C99-KK both at the gamma40 and the gamma42 site could be restored partially after brefeldin A treatment. Our data confirm the "spatial paradox" and raise several questions regarding the PS1 is gamma-secretase hypothesis.

Human cathepsin W, a cysteine protease predominantly expressed in NK cells, is mainly localized in the endoplasmic reticulum

Human cathepsin W (also called lymphopain) is a recently described papain-like cysteine protease of unknown function whose gene expression was found to be restricted to cytotoxic cells. Here we demonstrate that cathepsin W is expressed predominantly in NK cells and, to a lesser extent, in CTLs. Quantitative RT-PCR revealed that NK cells contained approximately 21 times more cathepsin W transcript than CTLs. The predominant expression of cathepsin W in NK cells was further confirmed by Western blot analysis and immunohistochemistry. IL-2-mediated stimulation of NK cells and CTLs revealed a stronger up-regulation of the cathepsin W gene and protein expression in NK cells (7-fold) than in CTLs (2-fold). Transfection experiments of HeLa cells and biochemical analyses revealed that cathepsin W is exclusively "high mannose-type" glycosylated and is mainly targeted to the endoplasmic reticulum (ER). Interestingly, the ER localization of cathepsin W was also found in NK cells, in which colocalization studies revealed an overlapping staining of cathepsin W and Con A, an ER-specific lectin. Furthermore, subcellular fractionation of cathepsin W-expressing cells confirmed the ER localization and showed that cathepsin W is membrane associated. Based on the results of this study, cathepsin W might represent a putative component of the ER-resident proteolytic machinery. The constitutive expression in NK cells and the stronger up-regulation of cathepsin W by IL-2 in NK cells than CTLs suggest that cathepsin W is not just a marker of cytotoxic cells but is, rather, specifically expressed in NK cells.

Linking Notch signaling, chromatin remodeling, and T-cell leukemogenesis

Intercellular communication that controls the developmental fate of multipotent cells is commonly mediated by the Notch family of transmembrane receptors. Specific transmembrane ligands activate Notch receptors on neighboring cells inducing the proteolytic liberation and nuclear translocation of the intracellular domain of Notch (N(IC)). Nuclear N(IC) associates with a transcriptional repressor known as C-promoter binding factor/RBP-J kappa, suppressor of hairless, or LAG-1, converting it from a repressor into an activator. Through physical interactions with chromatin remodeling enzymes and potentially with components of the transcriptional machinery, N(IC) activates target genes that mediate cell fate decisions. As Notch1 is disrupted via a chromosomal translocation in a subset of human T-cell leukemia, leading to a truncated polypeptide resembling N(IC), deregulated chromatin remodeling and transcription may fuel uncontrolled cell proliferation in this hematopoietic malignancy. This review summarizes the mechanics of Notch signaling and focuses on prospective molecular mechanisms for how constitutively active Notch might derail nuclear processes as an initiating step in T-cell leukemogenesis. J. Cell. Biochem. Suppl. 35:46-53, 2000.

Expression of amyloid precursor protein, tau and presenilin RNAs in rat hippocampus following deafferentation lesions

In this study, entorhinal cortex lesions and/or medial septal area cholinergic lesions were used in the rat to mimic some of the principal and earliest affects in Alzheimer's disease, namely hippocampal deafferentation. We wished to test the hypothesis that deafferentation lesions cause changes in the regulation of three proteins that are known to be important in Alzheimer's disease pathology, namely amyloid precursor protein, presenilin and tau. Expression of amyloid precursor protein mRNA was increased in several subfields of hippocampus when examined 1 week after entorhinal cortex lesion, but was reduced, compared to sham operated controls, after medial septal area cholinergic lesions. Cholinergic lesions were combined with entorhinal cortex lesions and produced no change in APP mRNA levels compared to controls. No significant changes were observed in the parietal cortex after entorhinal cortex or cholinergic lesions either alone or in combination. Tau mRNA level in hippocampus was unchanged after lesions. Presenilin-1 mRNA was expressed in the hippocampus at very low levels, and appeared to be increased following entorhinal cortex lesion. Our results support the hypothesis that amyloid precursor protein expression in hippocampal neurons is differentially affected by glutamatergic and cholinergic afferent input, and that presenilin-1, but not tau, may be subject to the same type of control in vivo.

Physiology, phylogeny, and functions of the TRP superfamily of cation channels

The transient receptor potential (TRP) protein superfamily consists of a diverse group of Ca(2+) permeable nonselective cation channels that bear structural similarities to Drosophila TRP. TRP-related proteins play important roles in nonexcitable cells, as demonstrated by the recent finding that a mammalian TRPC protein is expressed in endothelial cells and functions in vasorelaxation. However, an emerging theme is that many TRP-related proteins are expressed predominantly in the nervous system and function in sensory physiology. The TRP superfamily can be divided into six subfamilies, the first of which is composed of the "classical TRPs" (TRPC subfamily). These proteins all share the common features of three to four ankryin repeats, >/=30% amino acid homology over >/=750 amino acids, and a gating mechanism that operates through phospholipase C. Some classical TRPs may be store-operated channels (SOCs), which are activated by release of Ca(2+) from internal stores. The mammalian TRPC proteins are also expressed in the central nervous system, and several are highly enriched in the brain. One TRPC protein has been implicated in the pheromone response. The archetypal TRP, Drosophila TRP, is predominantly expressed in the visual system and is required for phototransduction. Many members of a second subfamily (TRPV) function in sensory physiology. These include VR1 and OSM-9, which respond to heat, osmolarity, odorants, and mechanical stimuli. A third subfamily, TRPN, includes proteins with many ankyrin repeats, one of which, NOMPC, participates in mechanotransduction. Among the members of a fourth subfamily, TRPM, is a putative tumor suppressor termed melastatin, and a bifunctional protein, TRP-PLIK, consisting of a TRPM channel fused to a protein kinase. PKD2 and mucolipidin are the founding members of the TRPP and TRPML subfamilies, respectively. Mutations in PKD2 are responsible for polycystic kidney disease, and mutations in mucolipidin result in a severe neurodegenerative disorder. Recent studies suggest that alterations in the activities of SOC and TRP channels may be at the heart of several additional neurodegenerative diseases. Thus, TRP channels may prove to be important new targets for drug discovery.

Presenilins as therapeutic targets for the treatment of Alzheimer's disease

Studies demonstrating that accumulation and aggregation of the amyloid beta protein (Abeta) within the brain is likely to cause Alzheimer's disease (AD) have provided the rationale for therapeutic strategies aimed at influencing Abeta production, aggregation and clearance. gamma-secretase catalyzes the final cleavage that releases the Abeta from its precursor; therefore, it is a potential therapeutic target for the treatment of AD. Recent data show that the polytopic membrane proteins presenilin 1 and presenilin 2 are either catalytic components or essential co-factors of a membrane-bound proteolytic complex that possesses gamma-secretase activity. Although recent findings demonstrating that gamma-secretase inhibitors bind directly to presenilins (PSs) further support a catalytic role for PSs in gamma-secretase cleavage, additional studies are still needed to clarify the role of PSs in gamma-secretase cleavage and the use of targeting PSs to reduce Abeta production.

Secretases as therapeutic targets for the treatment of Alzheimer's disease

The extracellular deposition of short amyloid peptides in the brain of patients is thought to be a central event in the pathogenesis of Alzheimer's Disease. The generation of the amyloid peptide occurs via a regulated cascade of cleavage events in its precursor protein, A beta PP. At least three enzymes are responsible for A beta PP proteolysis and have been tentatively named alpha-, beta- and gamma-secretases. The recent identification of several of these secretases is a major leap in the understanding how these secretases regulate amyloid peptide formation. Members of the ADAM family of metalloproteases are involved in the non-amyloidogenic alpha-secretase pathway. The amyloidogenic counterpart pathway is initiated by the recently cloned novel aspartate protease named BACE. The available data are conclusive and crown BACE as the long-sought beta-secretase. This enzyme is a prime candidate drug target for the development of therapy aiming to lower the amyloid burden in the disease. Finally, the gamma-secretases are intimately linked to the function of the presenilins. These multi-transmembrane domain proteins remain intriguing study objects. The hypothesis that the presenilins constitute a complete novel type of protease family, and are cleaving A beta PP within the transmembrane region, remains an issue of debate. Several questions remain unanswered and direct proof that they exert catalytic activity is still lacking. The subcellular localization of presenilins in neurons, their integration in functional multiprotein complexes and the recent identification of additional modulators of gamma-secretase, like nicastrin, indicate already that several players are involved. Nevertheless, the rapidly increasing knowledge in this area is already paving the road towards selective inhibitors of this secretase as well. It is hoped that such drugs, possibly in concert with the experimental vaccination therapies that are currently tested, will lead to a cure of this inexorable disease.

Intracellular Ca(2+) pools in neuronal signalling

Different intracellular pools participate in generating Ca(2+) signals in neuronal cells and in shaping their spatio-temporal patterns. They include the endoplasmic reticulum (endowed with different classes of Ca(2+) channels, with distinct functional properties and highly defined expression patterns in the brain), the Golgi apparatus, and the mitochondria. The release of Ca(2+) from intracellular pools plays an important role in controlling processes such as neurite outgrowth, synaptic plasticity, secretion and neurodegeneration.

Membrane-bound transcription factors: regulated release by RIP or RUP

Regulated nuclear transport of transcription factors from cytoplasmic pools is a major route by which eukaryotes control gene expression. Exquisite examples are transcription factors that are kept in a dormant state in the cytosol by membrane anchors; such proteins are released from membranes by proteolytic cleavage, which enables these transcription factors to enter the nucleus. Cleavage can be mediated either by regulated intramembrane proteolysis (RIP) catalysed by specific membrane-bound proteases or by regulated ubiquitin/proteasome-dependent processing (RUP). In both cases processing can be controlled by cues that originate at or in the vicinity of the membrane.

A pharmacogenomic approach to Alzheimer's disease

Single nucleotide polymorphisms (susceptibility genetics) and genomic point mutations (mendelian genetics) can be used in Alzheimer's disease (AD) for diagnostic, predictive and therapeutic purposes. Using a matrix genetic model, including APOE, PS1 and PS2 allelic variants, we have studied the distribution of 36 different genotypes in the AD population (N= 479) and the genotype-related cognitive response to a multifactorial therapy in AD patients with mild-to-moderate dementia. The 10 most frequent AD genotypes are the following: 1) E33P112P2 + (17.75%), 2) E33P112P2- (15.55%), 3) E33P111P2+ (10.85%), 4) E34P112P2+ (9.60%), 5) E34P112P2- (7.56%), 6) E33P111P2- (7.10%), 7) E34P111P2+ (4.80%), 8) E33P122P2+ (4.38%), 9) E34P111P2- (4.18%), and 10) E34P122P2+ (3.55%). APOE-4/4-related genotypes represent less than 3% in the following order: E44P112P2 + > E44P111P2+ = E44P111P2- > E44P112P2+ > E44P122P2+ = E44P122P2. Multifactorial therapy with CDP-choline (1,000 mg/day) + piracetam (2,400 mg/day) + anapsos (360 mg/day) did improve mental performance during the first 6-15 months in a genotype-specific fashion. The best responders in the APOE series were patients with APOE-3/4 genotype (r= +0.013), while the worst responders were APOE-4/4 patients (r= -0.93). PS1-related genotypes responded in a similar manner; and patients with a defective PS2 gene exon 5 (PS2+) always showed a poorer therapeutic response than PS2- patients. All these data suggest that the therapeutic outcome in AD exhibits a genotype-specific pattern, and that a pharmacogenomic approach to AD might be a valuable strategy for drug development and monitoring.

Characterization and developmental expression of the amphioxus homolog of Notch (AmphiNotch): evolutionary conservation of multiple expression domains in amphioxus and vertebrates

Notch encodes a transmembrane protein that functions in intercellular signaling. Although there is one Notch gene in Drosophila, vertebrates have three or more with overlapping patterns of embryonic expression. We cloned the entire 7575-bp coding region of an amphioxus Notch gene (AmphiNotch), encoding 2524 amino acids, and obtained the exon/intron organization from a genomic cosmid clone. Southern blot and PCR data indicate that AmphiNotch is the only Notch gene in amphioxus. AmphiNotch, like Drosophila Notch and vertebrate Notch1 and Notch2, has 36 EGF repeats, 3 Notch/lin-12 repeats, a transmembrane region, and 6 ankyrin repeats. Phylogenetic analysis places it at the base of all the vertebrate genes, suggesting it is similar to the ancestral gene from which the vertebrate Notch family genes evolved. AmphiNotch is expressed in all three embryonic germ layers in spatiotemporal patterns strikingly similar to those of all the vertebrate homologs combined. In the developing nerve cord, AmphiNotch is first expressed in the posteriormost part of the neural plate, then it becomes more broadly expressed and later is localized dorsally in the anteriormost part of the nerve cord corresponding to the diencephalon. In late embryos and larvae, AmphiNotch is also expressed in parts of the pharyngeal endoderm, in the anterior gut diverticulum, and, like AmphiPax2/5/8, in the rudiment of Hatschek's kidney. A comparison with Notch1 and Pax5 and Pax8 expression in the embryonic mouse kidney helps support homology of the amphioxus and vertebrate kidneys. AmphiNotch is also an early marker for presumptive mesoderm, transcripts first being detectable at the gastrula stage in a ring of mesendoderm just inside the blastopore and subsequently in the posterior mesoderm, notochord, and somites. As in sea urchins and vertebrates, these domains of AmphiNotch expression overlap with those of several Wnt genes and brachyury. These relationships suggest that amphioxus shares with other deuterostomes a common mechanism for patterning along the anterior/posterior axis involving a posterior signaling center in which the Notch and Wnt pathways and brachyury interact.

Altered apolipoprotein D expression in the brain of patients with Alzheimer disease

The etiology of late-onset Alzheimer disease is poorly understood. Predisposing factors such as the apolipoprotein E4 allele, as well as protective factors (e.g., antioxidants) have been proposed to play a role in the disease's process. A search for predisposing factors contributing to sporadic late-onset Alzheimer disease was initiated using the differential display technique. RNA expression profiles of the entorhinal cortex and the cerebellum of Alzheimer-diseased and normal patients were compared. The entorhinal cortex is the first brain region to accumulate neurofibrillary tangles during disease progression, whereas the cerebellum is spared. In the Alzheimer cases of this study, one signal showing preferential expression in the entorhinal cortex corresponded to the apolipoprotein D gene. This preferential expression might be genuine at the RNA level as suggested by the in situ hybridization method used. In addition, immunohistochemical experiments showed higher percentages of Apolipoprotein D reactive pyramidal neurons in the entorhinal cortex and region 1 of Ammon's horn in diseased patients. This increase correlated with the number of neurofibrillary tangles in Alzheimer as well as in normal patients. Colocalization of Apolipoprotein D proteins and neurofibrillary tangles in the same neuron was rare. Thus, these results suggest that in Alzheimer disease and aging, apolipoprotein D gene expression is increased in stressed cortical neurons before they possibly accumulate neurofibrillary tangles.

GSK3 beta forms a tetrameric complex with endogenous PS1-CTF/NTF and beta-catenin. Effects of the D257/D385A and FAD-linked mutations

We have previously shown that the endogenous C-terminal fragment of presenilin 1 co-immunoprecipitates with endogenous beta-catenin. Since PS1 has been suggested to be involved in beta-catenin stabilization, we further investigated whether GSK3 beta, responsible for beta-catenin phosphorylation and degradation, is part of the PS1/beta-catenin complex. In naïve H4 and CHO cells, PS1 co-immunoprecipitated with both endogenous beta-catenin and GSK3 beta. In addition, GSK3 beta endogenously binds to the PS1-CTF/NTF complex and beta-catenin in naïve CHO cells. GSK3 beta also co-immunoprecipitated with PS1 full length in CHO cell lines overexpressing PS1 wild type. Given that it has been recently shown that PS1 mutations of aspartate 257 or 385 result in prevention of PS1 endoproteolysis and inhibition of gamma-secretase activity, we also tested whether PS1 endoproteolysis is required for beta-catenin/GSK3 beta/PS1 binding and whether PS1 FAD-linked mutations affect GSK3 beta recruitment in the PS1/beta-catenin complex. GSK3 beta was detected in PS1 immunoprecipitates from H4 cell lines overexpressing PS1 wild type, delta E10, A286E, L246V and in CHO cell lines overexpressing aspartate or M146L mutations. The latter data show that the absence of PS1 endoproteolysis (D257A/D385A and delta E10) or the presence of PS1-FAD mutations does not interfere with beta-catenin/GSK3 beta/PS1 complex formation.

Overexpression of presenilin-2 enhances apoptotic death of cultured cortical neurons

Presenilin-2 (PS2) is a gene of unknown function linked with some forms of familial Alzheimer's disease. To investigate the biological role of PS2 in neurons, we overexpressed PS2 in primary cortical neurons using recombinant adenoviral vectors. Western blot and immunohistochemical analyses showed enhanced expression of PS2 proteins in infected neurons after infection of recombinant adenoviruses containing the human wild-type or mutant PS2 gene. Neuronal survival was decreased by approximately 30% in cultures infected with adenovirus expressing either wild-type or mutant PS2, as compared with those infected with adenovirus expressing the LacZ gene. Fragmented nuclei were frequently observed in dying neurons. These data suggest that apoptotic death of cultured cortical neurons is enhanced by PS2 overexpression.

Presenilin function in APP processing

Familial Alzheimer's disease (FAD) is now linked to at least three genes encoding the amyloid precursor protein (APP) on chromosome 21, and presenilin 1 and 2 on chromosome 14 and 1, respectively. FAD cases in whom presenilin mutations occur are more frequent than those with APP mutations. However, altogether they only account for approximately 0.1% of all the people suffering from Alzheimer's disease (AD), and the causes of the remaining 99.9% of the sporadic form of AD or senile dementia remain unknown. Since FAD presents with the same neuropathological features as sporadic AD, i.e., cognitive impairments and the amyloid plaques and tangles in the brain, our working hypothesis is that similar molecular pathogenic mechanisms underly both sporadic and familial AD. It follows that APP and the presenilins must be key players in the disease. Detailed knowledge about the cell biology of these proteins will be a rich source of insight into the pathology of AD, but will also shed light on the fundamental neurobiology of these proteins.

Protease inhibitors divert amyloid precursor protein to the secretory pathway

Addition of cysteine protease inhibitors to cells expressing amyloid precursor protein (APP) resulted in a >2-fold increase in appearance of the secreted extracellular domain of APP in the media. This was accounted for by increased flux of APP into the secretory pathway since protease inhibitors also caused a twofold increase in newly translated, incompletely glycosylated APP detected by pulse-labeling. These results show that a portion of newly translated APP molecules are normally rapidly degraded by cysteine protease(s) but can enter the secretory pathway when degradation is inhibited. Newly translated APP molecules are thus still competent for posttranslational processing in distal cellular compartments. Their degradation thus may not result from misfolding but merely susceptibility to an endoplasmic reticulum localized cysteine protease.

JNK activation is associated with intracellular beta-amyloid accumulation

c-Jun has been implicated in the pathogenesis of Alzheimer's disease (AD), but the upstream cascade leading to c-Jun activation in AD is not known. Activation of c-Jun N-terminal kinase (JNK) is obviously a candidate for the upstream event. We tested this possibility focusing on PS1-linked AD. First, we observed that JNK is actually activated in cerebral neurons of PS1-linked AD patients, using immunohistochemistry and Western blot analyses with anti-activated JNK antibodies. We analyzed the relationship between beta-amyloid (beta A) and JNK activation by using aged transgenic mice overexpressing mutant (M146L) PS1 and human AD brains. The mice showed no neuronal loss but a very few diffuse beta A deposits, corresponding to the early stage of PS1-linked AD brain. Some neurons were reactive for anti-beta A antibodies in the cerebral cortex. Interestingly, JNK activation was observed in neurons showing intracellular beta A immunoreactivity in transgenic mice. Association between intracellular beta A and JNK activation was confirmed in cortical neurons of sporadic and PS1-linked AD patients. Furthermore, introduction of beta A peptides into the primary culture cortical neurons induced JNK activation and cell death. Collectively, these results suggested that intracellular beta A accumulation might trigger JNK activation leading to neuronal death.

Genomic structure, expression pattern, and chromosomal localization of the human calsenilin gene: no association between an exonic polymorphism and Alzheimer's disease

Calsenilin is a recently-identified member of the neuronal calcium sensor family. Like other members of this family, it is found in the brain and binds calcium. Calsenilin was discovered by virtue of its interaction with both presenilin-1 and -2, proteins that are involved in the etiology of Alzheimer's disease. Because calsenilin may play a role in Alzheimer's disease and other disease with alterations in calcium homeostasis, we characterized the human gene. The gene, which we localized to chromosome 2, extends over a region of at least 74 kb and includes nine exons. Interestingly, the ninth exon of calsenilin contains a highly polymorphic CA repeat, adjacent to the stop codon. In a study of Alzheimer patients and their unaffected siblings, there was no evidence of association of AD with any calsenilin allele. This CA repeat will be useful for linkage and linkage disequilibrium studies to determine whether calsenilin variants contribute to risk in other diseases.

Presenilin I interaction with cytoskeleton and association with actin filaments

Presenilin I (PSI) has been shown to interact with microfilament-associated proteins of the filamin family. Here, we investigated a possible association of PSI with the cytoskeleton. Immunoblotting of detergent-insoluble fractions of rat brain homogenate revealed enrichment of neuron-specific 36 and 14 kDa proteolytic fragments of PSI, whereas 30 and 20 kDa fragments were found in the detergent-soluble fraction. Specific severing of microfilaments with gelsolin in the detergent-insoluble pellet and subsequent centrifugation led to the detection of both actin and PSI fragments in the supernatant. In addition, in vitro translated PSI cosedimented with actin filaments. Our findings provide biochemical evidence for the association of PSI fragments with actin filaments.

Presenilin-mediated modulation of capacitative calcium entry

We studied a novel function of the presenilins (PS1 and PS2) in governing capacitative calcium entry (CCE), a refilling mechanism for depleted intracellular calcium stores. Abrogation of functional PS1, by either knocking out PS1 or expressing inactive PS1, markedly potentiated CCE, suggesting a role for PS1 in the modulation of CCE. In contrast, familial Alzheimer's disease (FAD)-linked mutant PS1 or PS2 significantly attenuated CCE and store depletion-activated currents. While inhibition of CCE selectively increased the amyloidogenic amyloid beta peptide (Abeta42), increased accumulation of the peptide had no effect on CCE. Thus, reduced CCE is most likely an early cellular event leading to increased Abeta42 generation associated with FAD mutant presenilins. Our data indicate that the CCE pathway is a novel therapeutic target for Alzheimer's disease.

Left-right asymmetry in C. elegans intestine organogenesis involves a LIN-12/Notch signaling pathway

The C. elegans intestine is a simple tube consisting of a monolayer of epithelial cells. During embryogenesis, cells in the anterior of the intestinal primordium undergo reproducible movements that lead to an invariant, asymmetrical ‘twist’ in the intestine. We have analyzed the development of twist to determine how left-right and anterior-posterior asymmetries are generated within the intestinal primordium. The twist requires the LIN-12/Notch-like signaling pathway of C. elegans. All cells within the intestinal primordium initially express LIN-12, a receptor related to Notch; however, only cells in the left half of the primordium contact external, nonintestinal cells that express LAG-2, a ligand related to delta. LIN-12 and LAG-2 mediated interactions result in the left primordial cells expressing lower levels of LIN-12 than the right primordial cells. We propose that this asymmetrical pattern of LIN-12 expression is the basis for asymmetry in later cell-cell interactions within the primordium that lead directly to intestinal twist. Like the interactions that initially establish LIN-12 asymmetry, the later interactions are mediated by LIN-12. The later interactions, however, involve a different ligand related to delta, called APX-1. We show that the anterior-posterior asymmetry in intestinal twist involves the kinase LIT-1, which is part of a signaling pathway in early embryogenesis that generates anterior-posterior differences between sister cells.

Misfolding of membrane proteins in health and disease: the lady or the tiger?

Protein misfolding is increasingly recognized as a factor in many diseases, including cystic fibrosis, Parkinson's, Alzheimer's and atherosclerosis. Many proteins involved in misfolding-based pathologies are membrane-associated, such that the bilayer may play roles in normal and aberrant folding. It can be argued that the in vivo partitioning of eukaryotic membrane proteins between folding and misfolding pathways is under kinetic control. Moreover, the balance between these pathways can be surprisingly delicate.

Therapeutic approaches to Alzheimer's disease

Several recent advances have provided new insights and possibilities in defining therapeutic targets for Alzheimer's disease. Of particular importance is the identification of the beta-secretase enzyme and the demonstration that immunization of a transgenic mouse model of Alzheimer's disease with Abeta(1-42) peptide can prevent or alleviate neuropathological features of the disease.

Alzheimer's disease: A hypothesis on pathogenesis

Alzheimer's disease (AD) is the major cause of dementia. It is a systemic disorder whose major manifestations are in the brain. AD cases can be categorized into two groups on the basis of the age of onset-before or after about age 60. The majority of cases, 90-95 percent, are in the late onset category. Early onset cases are largely, if not all, familial (FAD). These are caused by mutations in the genes for the amyloid precursor protein (APP), presenilin 1 (PS1), and presenilin 2 (PS2). In contrast late onset cases are mainly sporadic. The disorder is characterized by intraneuronal fibrillary tangles, plaques, and cell loss. The brain lesions in both early and late-onset AD are the same, and in the same distribution pattern, as those seen in individuals with Down's syndrome (DS) and in smaller numbers in normal older individuals. Extensive studies of AD have yet to result in a generally accepted hypothesis on the pathogenesis of the disorder. Major emphasis has been placed on the role of amyloid, the neurotoxin formed by the action of free radicals on preamyloid. The observation that AD lesions are frequently present in normal older individuals prompted the hypothesis that AD is the result of faster than normal aging of the neurons associated with it. This hypothesis provides plausible explanations for FAD and AD. FAD is associated with mutations in APP, PS1, and PS2. These substances, along with their normal counterparts, undergo proteolytic processing in the endoplasmic reticulum (ER). The mutated compounds, aside from increasing the ratio of βA42 to βA40, may down-regulate the calcium buffering activity of the ER in a manner akin to one or more of the many compounds known to do so. Decreases in the ER calcium pool would cause compensatory increases in other calcium pools, particularly in mitochondria. Increases in mitochondrial calcium levels are associated with enhanced formation of superoxide radical formation, and hence of the rate of aging. SAD may be caused by nuclear and/or mitochondrial DNA mutations beginning early in life that enhance mitochondrial superoxide radical formation in the neurons associated with the disorder. The above explanations for FAD and AD are suggestive of measures to prevent and for treatment.