Alzheimer's disease-related presenilins are thought to be involved in Notch signaling during embryonic development and/or cellular differentiation. Proteins mediating the cellular functions of the presenilins are still unknown. The yeast two-hybrid system was used to identify an interacting armadillo protein, termed p0071, that binds specifically to the hydrophilic loop of presenilin 1. In vivo, the presenilins constitutively undergo proteolytic processing, forming two stable fragments. The C-terminal fragment of presenilin 1 directly binds to p0071. Nine out of 10 armadillo repeats in p0071 are essential for mediating this interaction. Since armadillo proteins, like beta-catenin and APC, are known to participate in cellular signaling, p0071 may function as a mediator of presenilin 1 in signaling events (Stahl, 1999).

Missense substitutions in the presenilin 1 (PS1) and presenilin 2 (PS2) proteins are associated with early-onset familial Alzheimer's disease. Yeast-two-hybrid and coimmunoprecipitation methods were used to show that the large cytoplasmic loop domains of PS1 and PS2 interact specifically with three members of the armadillo protein family, including beta-catenin, p0071, and a novel neuronal-specific armadillo protein--neural plakophilin-related armadillo protein (NPRAP). The PS1:NPRAP interaction occurs between the arm repeats of NPRAP and residues 372-399 at the C-terminal end of the large cytoplasmic loop of PS1. The latter residues contain a single arm-like domain and are highly conserved in the presenilins, suggesting that they form a functional armadillo protein binding site for the presenilins (Levesque, 1999).

The presenilin proteins are components of high-molecular-weight protein complexes in the endoplasmic reticulum and Golgi apparatus that also contain beta-catenin. Presenilin mutations associated with familial Alzheimer disease (but not the non-pathogenic Glu318Gly polymorphism) alter the intracellular trafficking of beta-catenin after activation of the Wnt/beta-catenin signal transduction pathway. As with their effect on betaAPP processing, the effect of PS1 mutations on trafficking of beta-catenin arises from a dominant 'gain of aberrant function' activity. These results indicate that mistrafficking of selected presenilin ligands is a candidate mechanism for the genesis of Alzheimer disease associated with presenilin mutations, and that dysfunction in the presenilin-beta-catenin protein complexes is central to this process (Nishimura, 1999).

Presenilin-1 can associate with members of the catenin family of signalling proteins, but the significance of this association is unknown. Presenilin-1 forms a complex with beta-catenin in vivo that increases beta-catenin stability. Pathogenic mutations in the presenilin-1 gene reduce the ability of presenilin-1 to stabilize beta-catenin, and lead to increased degradation of beta-catenin in the brains of transgenic mice. Moreover, beta-catenin levels are markedly reduced in the brains of Alzheimer's disease patients with presenilin-1 mutations. Loss of beta-catenin signalling increases neuronal vulnerability to apoptosis induced by amyloid-beta protein. Thus, mutations in presenilin-1 may increase neuronal apoptosis by altering the stability of beta-catenin, predisposing individuals to early-onset Alzheimer's disease (Z. Zhang, 1998).

Families bearing mutations in the presenilin-1 (PSI) gene develop Alzheimer's disease (AD). However, the mechanism through which PS1 causes AD is unclear. The co-immunoprecipitation with PS1 in transfected COS-7 cells indicates that PSI directly interacts with endogenous beta-catenin, and the interaction requires residues 322-450 of PSI and 445-676 of beta-catenin. Both proteins are co-localized in the endoplasmic reticulum. Over-expression of PS1 reduces the level of cytoplasmic beta-catenin, and inhibits beta-catenin-T cell factor-regulated transcription. These results indicate that PSI plays a role as inhibitor of the beta-catenin signal, which may be connected with the AD dysfunction (Murayama, 1998).

A screen was carried out for proteins that interact with presenilin (PS) 1, and the full-length cDNA of human delta-catenin, which encodes 1225 amino acids was cloned. Delta-catenin interactes with a hydrophilic loop region in the endoproteolytic C-terminal fragment of PS1, but not with that of PS-2. These results suggest that PS1 and PS2 partly differ in function. PS1 loop fragment containing the pathogenic mutation retains the binding ability. Another armadillo-protein, p0071 interacts with PS1 (Tanahashi, 1999).

The two hybrid system and confirmatory co-immunoprecipitations were used to identify a novel catenin, termed delta-catenin, which interacts with PS1 and is principally expressed in brain. The catenins are a gene family related to the Armadillo gene in Drosophila, some of which appear to have dual roles-they are components of cell-cell adherens junctions, and may serve as intermediates in the Wingless (Wg) signaling pathway, which, like Notch/lin-12, is also responsible for a variety of inductive signaling events. In the non-neuronal 293 cell line, PS1 interacts with beta-catenin, the family member with the greatest homology to Armadillo. Wg and Notch interactions are mediated by the Dishevelled gene, which may form a signaling complex with PS1 and Wg pathway intermediates to regulate the function of the Notch/lin-12 gene (Zhou, 1997).

The Alzheimer's disease-linked gene presenilin 1 (PS1) is required for intramembrane proteolysis of APP and Notch. In addition, recent observations strongly implicate PS1 as a negative regulator of the Wnt/ß-catenin signaling pathway, although the mechanism underlying this activity is unknown. Presenilin has been shown to function as a scaffold that rapidly couples ß-catenin phosphorylation through two sequential kinase activities independent of the Wnt-regulated Axin/CK1alpha complex. Thus, presenilin deficiency results in increased ß-catenin stability in vitro and in vivo by disconnecting the stepwise phosphorylation of ß-catenin, both in the presence and absence of Wnt stimulation. These findings highlight an aspect of ß-catenin regulation outside of the canonical Wnt-regulated pathway and a function of presenilin separate from intramembrane proteolysis (Kang, 2002).

E-cadherin controls a wide array of cellular behaviors including cell-cell adhesion, differentiation and tissue development. Presenilin-1 (PS1), a protein involved in Alzheimer's disease, controls a gamma-secretase-like cleavage of E-cadherin. This cleavage is stimulated by apoptosis or calcium influx and occurs between human E-cadherin residues Leu731 and Arg732 at the membrane-cytoplasm interface. The PS1/gamma-secretase system cleaves both the full-length E-cadherin and a transmembrane C-terminal fragment, derived from a metalloproteinase cleavage after the E-cadherin ectodomain residue Pro700. The PS1/gamma-secretase cleavage dissociates E-cadherins, beta-catenin and alpha-catenin from the cytoskeleton, thus promoting disassembly of the E-cadherin-catenin adhesion complex. Furthermore, this cleavage releases the cytoplasmic E-cadherin to the cytosol and increases the levels of soluble beta- and alpha-catenins. Thus, the PS1/gamma-secretase system stimulates disassembly of the E-cadherin-catenin complex and increases the cytosolic pool of beta-catenin, a key regulator of the Wnt signaling pathway (Marambaud, 2002).

The unfolded protein response (UPR) mediates signaling from the endoplasmic reticulum to the nucleus. Presenilin plays an important role in the proteolysis of a protein involved in the UPR, terned Ire1p. Cells respond to the accumulation of unfolded proteins in the lumen of the ER by activating transcription in the nucleus of a set of genes involved in protein folding, such as the molecular chaperones BiP (or GRP78), GRP94, calreticulin, and protein disulfide isomerase. As such, the UPR adjusts the protein folding capacity of the ER according to need. UPR signaling is initiated by Ire1p, a bifunctional ER transmembrane protein with both serine/threonine kinase and endoribonuclease activities. Ire1p is a single spanning ER membrane protein oriented with its N-terminal half inside the ER lumen, and its C-terminal half (which contains both the kinase and nuclease domains) in the cytosol or nucleus. The ER-lumenal portion of Ire1p is thought to function as a sensor domain that detects changes in the concentration of unfolded proteins or unbound chaperones. Activation of Ire1p leads to its phosphorylation and oligomerization, ultimately resulting in the induction of its endoribonuclease activity by unknown means. The substrate of Ire1p is the mRNA encoding the UPR-specific transcription factor Hac1p that binds to the unfolded protein response element (UPRE) in the promoters of direct target genes of the pathway. Upon induction of the UPR, a 252-nucleotide intron present toward the 3' end of HAC1 mRNA is removed, generating the spliced form (HAC1ⁱ mRNA, i = induced). The first catalytic step, cleavage of HAC1 mRNA at both intron-exon junctions, is carried out by Ire1p. The second step, ligation of the two liberated exons, is carried out by tRNA ligase, an enzyme that is shared with the pre-tRNA splicing pathway. Thus, rather than using spliceosomes, the HAC1 intron is removed by a mechanism that resembles pre-tRNA splicing. Both HAC1^u mRNA (u = unspliced, uninduced) and HAC1ⁱ mRNA are exported to the cytosol and become engaged in polyribosomes, but only the spliced form gives rise to Hac1 protein (Niwa, 1999).

The recent identification of Ire1p homologs suggests that at least some aspects of the UPR are conserved in higher eukaryotic cells. In mammals, two Ire1 isoforms have been identified, Ire1alpha and Ire1beta. Overexpression of either isoform is sufficient to induce the UPR, and overexpression of dominant-negative forms blocks the pathway. Sequence comparisons show strong conservation of the C-terminal kinase and endoribonuclease domains among all known Ire1 homologs. In contrast, the ER-lumenal domains are more divergent, even between the two mammalian isoforms. The conservation of the Ire1p kinase and nuclease domains, together with the fact that human Ire1alpha has been shown to cleave the 5' splice junction of yeast HAC1^u mRNA, suggests that a nonconventional splicing event also plays a key role during UPR induction in higher eucaryotes. A basic leucine zipper transcription factor, ATF6, has been identified that is initially synthesized as a transmembrane protein and then becomes proteolyticlly cleaved to release a fragment that participates in transcriptional regulation upon UPR induction. Overexpression of a cytosolic fragment can activate transcription of UPR target genes. In contrast to yeast Hac1p, however, its mRNA is not spliced upon UPR induction (Niwa, 1999).

A remaining enigma in understanding of the UPR concerns the intracellular localization of Ire1p. From its glycosylation pattern, yeast Ire1p is known to reside in the ER membrane and/or inner nuclear membrane, which are continuous around the nuclear pores. The partitioning, if any, of Ire1p between these two membrane domains is unknown, i.e., it has not been determined whether the C-terminal domain (bearing both its kinase and nuclease functions) is cytosolic or nuclear. It is possible, however, that splicing of HAC1 mRNA is a nuclear event because tRNA ligase is localized to the nucleus. Thus, if the C-terminal half of Ire1p is in the cytosol, then activated Ire1p molecules must somehow enter the nucleus to participate in splicing. Ire1p might migrate to the nucleus after its biosynthesis or upon activation of the UPR. There is currently no precedent, however, for a membrane protein with a large cytoplasmic domain to move within the plane of the membrane through nuclear pores. Another possibility is that a fragment of Ire1p is proteolytically severed from the ER membrane upon UPR induction. This fragment could then migrate as a soluble protein into the nucleus to participate in splicing. A precedent for this latter mechanism is found in the pathways controlling sterol biosynthesis and Notch signaling (Niwa, 1999).

Experimental support is provided for the hypothesis that the unusual features of the yeast UPR pathway are conserved. In mammalian cells, the mechanism characterized in yeast is expanded upon to suggest that induction of the UPR involves proteolytic cleavage of Ire1, which allows its cytosolic domains to move into the nucleus, presumably as a prerequisite to participate in RNA splicing. Nuclear localization and induction of the UPR are reduced in cells lacking presenilin-1 (PS1), suggesting PS1 is a new component of the UPR that governs an essential proteolytic step. Yeast HAC1 mRNA is correctly spliced in mammalian cells upon UPR induction and mammalian Ire1 can precisely cleave both splice junctions. Surprisingly, UPR induction leads to proteolytic cleavage of Ire1, releasing fragments containing the kinase and nuclease domains that accumulate in the nucleus. Nuclear localization and UPR induction are reduced in presenilin-1 knockout cells. These results suggest that the salient features of the UPR are conserved among eukaryotic cells and that presenilin-1 controls Ire1 proteolysis in mammalian cells (Niwa, 1999).

These results suggest that misfolding of proteins in the ER leads to increased proteolysis of Ire1 by a PS1-dependent pathway. This raises the question whether other substrates of this proteolytic system, such as APP and/or Notch, are also cleaved at an increased rate when the UPR is induced. Interestingly, it has been suggested that ATF6 is also synthesized as an ER transmembrane protein that becomes cleaved and enters the nucleus upon induction of the UPR. These observations suggest that the processing of ATF6 and Ire1 could be coordinated, possibly both being carried out by gamma-secretase (gamma-secretase is known to cleave in the middle of the transmembrane domain of amyloid precursor protein APP). This putative cross-talk could be explained by either of the two pathways. Ire1-dependent activation of gamma-secretase might directly lead to increased processing of other substrates, including ATF6. Alternatively, activation of Ire1 might lead to the formation of protein complexes in the membrane that, in addition to Ire1, include other proteins which then also become substrates for cleavage by a constitutively active gamma-secretase. Thus, it is possible that APP cleavage is activated following UPR induction; ER stress may therefore lead to an increased production of Abeta₄₂, in turn leading to increased amyloid deposits characteristic of Alzheimer's disease. Environmental exposure to agents that cause ER protein misfolding or inherited mutations that predispose an individual to protein folding defects in the ER could therefore cause or accelerate the rate of onset of Alzheimer's disease. Conversely, cells bearing PS1 mutations are hypersensitive to UPR-inducing agents. In the broadest sense, it is intriguing that Alzheimer's disease involves the extracytosolic deposit of aggregated (misfolded?) Abeta₄₂ and that the UPR regulates the cell's extracytosolic protein folding capacity. Thus, the observation that PS1 plays a role in the UPR suggests a potential link between a disease that results from deposits of aberrant protein and a system that monitors their proper maturation. In this light, it is particularly interesting that levels of the molecular chaperone BiP are reduced in the brain of Alzheimer's disease patients. The potential connection between the UPR and the generation of amyloid deposits in Alzheimer's disease raises new possibilities for understanding and modifying the pathogenesis of this disease (Niwa, 1999).

Missense mutations in the human presenilin-1 (PS1) gene, which is found on chromosome 14, cause early-onset familial Alzheimer's disease (FAD). FAD-linked PS1 variants alter proteolytic processing of the amyloid precursor protein and cause an increase in vulnerability to apoptosis induced by various cell stresses. However, the mechanisms responsible for these phenomena are not clear. Mutations in PS1 affect the unfolded-protein response (UPR), which responds to the increased amount of unfolded proteins that accumulate in the endoplasmic reticulum (ER) under conditions that cause ER stress. PS1 mutations also lead to decreased expression of GRP78/Bip, a molecular chaperone, present in the ER, that can enable protein folding. Interestingly, GRP78 levels are reduced in the brains of Alzheimer's disease patients. The downregulation of UPR signaling by PS1 mutations is caused by disturbed function of IRE1, which is the proximal sensor of conditions in the ER lumen. Overexpression of GRP78 in neuroblastoma cells bearing PS1 mutants almost completely restores resistance to ER stress to the level of cells expressing wild-type PS1. These results show that mutations in PS1 may increase vulnerability to ER stress by altering the UPR signaling pathway (Katayama, 1999).

Knowledge of proteins with which the presenilins interact should lead to a better understanding of presenilin function in normal and disease states. A calcium-binding protein, calmyrin, has been identified that interacts preferentially with presenilin 2 (PS2). Calmyrin is myristoylated, membrane-associated, and colocalizes with PS2 when the two proteins are overexpressed in HeLa cells. Calmyrin accumulates in the nucleus and cytoplasm, but When coexpressed with PS2 these two proteins colocalize at the ER. Yeast two-hybrid liquid assays, affinity chromatography, and coimmunoprecipitation experiments confirm binding between PS2 and calmyrin. Two lines of evidence favor the PS2-loop region as the critical site of calmyrin interaction: reduced in vivo colocalization when calmyrin is coexpressed with a loop-deficient PS2 construct and increased yeast liquid culture binding of calmyrin to the PS2-loop rather than the PS2- COOH-terminal domain. Deletion analysis indicates that calmyrin binding is mediated primarily by the NH2-terminal 31 amino acids of the PS2-loop. Remarkably, despite only a three-amino acid difference, the comparable loop region of PS1 interactes with less than one-tenth the strength in similar yeast two-hybrid assays. The loop region is a site associated with several PS-processing phenomena, including proteolytic cleavage, caspase cleavage, as well as abnormal splicing. Apart from calmyrin, several other proteins have been found to interact with the PS-loop, namely, gamma-catenin, filamin, calselinin, mu-calpain, and armadillo protein p007. The data showing that minor (single amino acid) alterations in the loop sequence can produce dramatic changes in protein binding not only has implications in terms of calmyrin function, but may also have important consequences for the other processing events and binding partners associated with this region. Therefore, it is not surprising that many FAD mutations map to the PS1 loop. Functionally, calmyrin and PS2 increase cell death when cotransfected into HeLa cells. These results allude to several provocative possibilities for a dynamic role of calmyrin in signaling, cell death, and AD (Stabler, 1999).

Most early-onset familial Alzheimer's disease cases are caused by mutations in the highly related genes presenilin 1 (PS1) and presenilin 2 (PS2). Presenilin mutations produce increases in beta-amyloid (Abeta) formation and apoptosis in many experimental systems. A cDNA (ALG-3) encoding the last 103 amino acids of PS2 has been identified as a potent inhibitor of apoptosis. Using this PS2 domain in the yeast two-hybrid system, a neuronal protein has been identified that binds calcium and presenilin, which has been called calsenilin. Calsenilin interacts with both PS1 and PS2 in cultured cells, and can regulate the levels of a proteolytic product of PS2. Thus, calsenilin may mediate the effects of wild-type and mutant presenilins on apoptosis and on Abeta formation. Further characterization of calsenilin may lead to an understanding of the normal role of the presenilins and of the role of the presenilins in Alzheimer's disease (Buxbaum, 1998).

A screened for proteins that interact with PS2 to understand its pathological and physiological functions. Using the PS2 loop domain as the bait, the yeast two-hybrid system was used for screening, and mu-calpain was identified as a PS2 binding protein. In COS-1 cells, the interaction of PS2 with mu-calpain was confirmed by immunoprecipitation. These results suggest that PS2 and mu-calpain interact with each other, and might regulate each other's functions (Shinozaki, 1998).

Although structural features indicate that the presenilins are membrane proteins, their function(s) is unknown. The presenilins have been localized to the nuclear membrane, its associated interphase kinetochores, and the centrosomes-all subcellular structures involved in cell cycle regulation and mitosis. The colocalization of the presenilins with kinetochores on the nucleoplasmic surface of the inner nuclear membrane, together with other results, suggests that they may play a role in chromosome organization and segregation, perhaps as kinetochore binding proteins/receptors. A pathogenic pathway for familial Alzheimer's disease is discussed in which defective presenilin function causes chromosome missegregation during mitosis, resulting in apoptosis and/or trisomy 21 mosaicism (J. Li, 1997).

A novel function of the presenilins (PS1 and PS2) in governing capacitative calcium entry (CCE), a refilling mechanism for depleted intracellular calcium stores, is reported. Abrogation of functional PS1, by either knocking out PS1 or expressing inactive PS1, markedly potentiates CCE, suggesting a role for PS1 in the modulation of CCE. In contrast, familial Alzheimer's disease (FAD)-linked mutant PS1 or PS2 significantly attenuates CCE and store depletion-activated currents. While inhibition of CCE selectively increases the amyloidogenic amyloid ß peptide (Aß42), increased accumulation of the peptide has no effect on CCE. Thus, reduced CCE is most likely an early cellular event leading to increased Aß42 generation associated with FAD mutant presenilins. These data indicate that the CCE pathway is a novel therapeutic target for Alzheimer's disease (Yoo, 2000).

It is suggested that autosomal dominant FAD mutant presenilins exert a gain of function by downregulating CCE while increasing IP₃-mediated release from the ER store, leading to diminished luminal Ca²⁺ concentration ([Ca²⁺]_ER). It is interesting to note that changes in [Ca²⁺]_ER influence a number of cellular functions, including chaperone activities and gene expression. Therefore, it is tempting to speculate that reduced CCE may also be an upstream event leading to other molecular phenotypes associated with FAD mutant presenilins, including altered unfolded protein response. Interestingly, in transgenic mice harboring spinocerebellar ataxia type 1 (SCA1) mutant gene products, TRP3, SERCA2, and IP₃-R (all components of CCE), are specifically downregulated. This suggests the potential contribution of CCE dysregulation in other neurodegenerative diseases in addition to AD. CCE involves direct physical interaction between the ER and plasma membrane constituents. According to this conformational coupling mechanism, a conformational change of the IP₃ receptor (IP₃-R) upon agonist stimulation and subsequent release of Ca²⁺ leads to the formation of a molecular complex containing IP₃-R bound to molecular constituents in the plasma membrane harboring CCE channels. This then allows extracellular Ca²⁺ to replenish the ER store. It has been postulated that the presenilins modulate the gamma-secretase activity via few possible mechanisms: the presenilins might be the gamma-secretases themselves, and serve as essential cofactors for the gamma-secretase action, or regulate intracellular trafficking of a putative gamma-secretase to the target site where relevant substrates are localized. Given a role for presenilins in governing CCE, the presenilins may also modulate proteolytic processing of APP and Notch at or near the cell surface at sites of ER-plasma membrane coupling. It is conceivable that the presenilins may regulate or directly mediate the cleavage of protein(s) involved in modulating CCE. In any event, a gain in the biological activity of the presenilins, owing to autosomal dominant FAD mutations, may attenuate CCE while increasing gamma-secretase activity. Further experimentation will be necessary to elucidate this connection. Finally, augmentation of CCE, through the identification of agonists of plasma membrane store-operated Ca²⁺ channels (e.g., TRP or as yet undiscovered CCE channels) that mediate CCE, could potentially be employed to reduce PS-associated gamma-secretase activity, and the generation of Aß as a novel therapeutic means for preventing or treating AD (Yoo, 2000).

Mutations in the highly homologous presenilin genes encoding presenilin-1 and presenilin-2 (PS1 and PS2) are linked to early-onset Alzheimer's disease (AD). However, apart from a role in early development, neither the normal function of the presenilins nor the mechanisms by which mutant proteins cause AD are well understood. The properties are described of a novel human interactor of the presenilins named ubiquilin. Yeast two-hybrid (Y2H) interaction, glutathione S-transferase pull-down experiments, and colocalization of the proteins expressed in vivo, together with coimmunoprecipitation and cell fractionation studies, provide compelling evidence that ubiquilin interacts with both PS1 and PS2. Ubiquilin is noteworthy since it contains multiple ubiquitin-related domains typically thought to be involved in targeting proteins for degradation. However, ubiquilin promotes presenilin protein accumulation. Pulse-labeling experiments indicate that ubiquilin facilitates increased presenilin synthesis without substantially changing presenilin protein half-life. Immunohistochemistry of human brain tissue with ubiquilin-specific antibodies reveal prominent staining of neurons. Moreover, the anti-ubiquilin antibodies robustly stain neurofibrillary tangles and Lewy bodies in AD and Parkinson's disease affected brains, respectively. These results indicate that ubiquilin may be an important modulator of presenilin protein accumulation and that ubiquilin protein is associated with neuropathological neurofibrillary tangles and Lewy body inclusions in diseased brain (Mah, 2000).

It will be interesting to determine the precise mechanism by which ubiquilin induces increased presenilin protein synthesis. Ubiquilin could increase presenilin synthesis by simply increasing presenilin transcription, increasing presenilin translation, or facilitating correct polypeptide folding, maturation, and intracellular targeting of the polytopic transmembrane presenilin protein. The possibility that ubiquilin may act as a molecular chaperone is especially intriguing. Studies of the Xenopus ubiquilin homologue, XDRP1, have suggested that ubiquilin can act posttranscriptionally like a molecular chaperone and prevent degradation of in vitro translated cyclin A protein. Chap1 (ubiquilin 2) has been shown to bind Stch, an Hsp70-like protein. In turn, many heat-shock proteins have been shown to function as molecular chaperones, preventing protein aggregation and protein degradation. Recent evidence has linked ubiquilin proteins to the proteasome. Meanwhile, the 19S regulatory subunit of the 26S proteasome (the degradation complex for ubiquitin-tagged proteins) has been shown to possess protein-unfolding activity. Indirect evidence that ubiquilin may aid in presenilin protein folding or targeting comes from the observation that the presenilin construct PS2(DeltaLC), with deletions of both the loop and COOH-terminal ubiquilin-interaction sites, frequently accumulates into large cytoplasmic aggregates. In contrast, presenilin molecules containing ubiquilin interaction sites, rarely form large protein aggregates. Finally, the presenilins have themselves been linked to molecular chaperones of the ER, that are involved in the unfolded-protein response. Another mechanism by which ubiquilin might increase presenilin accumulation is to alter presenilin degradation rates, especially those of the ubiquitinated forms of presenilins. In fact, evidence has been found that overexpression of human ubiquilin proteins, hPLIC-1 (ubiquilin 1) and hPLIC-2 (ubiquilin 2), interfers with ubiquitin-dependent degradation of p53 and IkBalpha. Although no significant change in the turnover rate of the major 54-kD PS2 polypeptide species (corresponding to full-length PS2) has been found, the possibility that certain ubiquitinated forms of presenilins may have altered turnover rates cannot be excluded. It will be important in future studies to determine if ubiquilin is involved in ubiquitin-dependent degradation of presenilins (Mah, 2000 and references therein).

The influence of presenilins on the genetic cascades that control neuronal differentiation have been examined in Xenopus embryos. Resembling sonic hedgehog (shh) overexpression, presenilin mRNA injection reduces the number of N-tubulin plus primary neurons and modulates Gli3 and Zic2 according to their roles in activating and repressing primary neurogenesis, respectively. Presenilin increases shh expression within its normal domain, mainly in the floor plate, whereas an antisense X-presenilin-alpha morpholino oligonucleotide reduces shh expression. Both shh and presenilin promote cell proliferation and apoptosis, but the effects of shh are widely distributed, while those resulting from presenilin injection coincide with the range of shh signaling. It is suggested that presenilin may modulate primary neurogenesis, proliferation, and apoptosis in the neural plate, through the enhancement of shh signaling (Paganelli, 2001).

Presenilin (PS) genes linked to early-onset familial Alzheimer's disease encode polytopic membrane proteins that are presumed to constitute the catalytic subunit of gamma-secretase, forming a high molecular weight complex with other proteins. During attempts to identify binding partners of PS2, CALP (calsenilin-like protein)/KChIP4, a novel member of calsenilin/KChIP protein family that interacts with the C-terminal region of PS, was cloned. Upon co-expression in cultured cells, CALP directly binds to and co-localizes with PS2 in endoplasmic reticulum. Overexpression of CALP does not affect the metabolism or stability of PS complex, and gamma-cleavage of betaAPP or Notch site 3 cleavage was not altered. However, co-expression of CALP and a voltage-gated potassium channel subunit Kv4.2 reconstitutes the features of A-type K(+) currents and CALP directly binds Kv4.2, indicating that CALP functions as KChIPs that are known as components of native Kv4 channel complex. Taken together, CALP/KChIP4 is a novel EF-hand protein interacting with PS as well as with Kv4 that may modulate functions of a subset of membrane proteins in brain (Morohashi, 2002).

Mutations in presenilins (PS) are the major cause of familial Alzheimer's disease (FAD) and have been associated with calcium (Ca2+) signaling abnormalities. 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 (Cheung, 2008).