Presenilin 1 mediates protein kinase C dependent alpha-secretase derived amyloid precursor protein secretion and mitogen-activated protein kinase activation in presenilin 1 transfected human embryonic kidney 293 cell

Evidence that neuronal Notch-1 promotes JNK/c-Jun activation and cell death following ischemic stress

Notch signaling is a highly conserved pathway that regulates cell fate decisions during embryonic development. We have recently identified that in ischemic stroke, activity of γ-secretase and the resulting Notch activation may endanger neurons by modulating NF-κB and HIF-1α pathways. Notch signaling can also modulate MAPK-related pathways. However, the role of γ-secretase-mediated Notch signaling in activating MAPK following ischemic stroke has not been investigated. We used control and NICD1-overexpressing HEK and SH-SY5Y cell lines, and inhibitors of γ-secretase and JNK, to explore novel roles of Notch in modulating cell death following ischemic stress in vitro. Our findings indicate that expression of NICD1, JNK/cJun, p38-MAPK and the pro-apoptotic marker, cleaved caspase-3, increased during ischemic conditions. γ-Secretase inhibitors reduced ischemia-induced increase in NICD1 and JNK/p-cJun. Furthermore, NICD overexpression augmented JNK/cJun levels and cell death under these conditions. These results suggest that Notch signaling contributes to the pathogenesis of ischemic stroke, in part by promoting JNK/cJun signaling. These results provide further support for the potential use of γ-secretase inhibitors as therapy for ischemic stroke.

Presenilin acts as a positive regulator of basal level activity of ERK through the Raf-MEK1 signaling pathway

Presenilins (PS) have been reported to be functionally involved in amyloid precursor protein processing, notch receptor signaling, and programmed cell death, or apoptosis. To understand the role of PS1 in the signaling events, we investigated in this study the role of PS1 in the basal level of mitogen-activated protein kinase (MAPK) pathways using PS1(-/-) mouse embryonic fibroblast (MEF) cells from PS1-null mice. Interestingly, the basal level of ERK activity, but not JNK or p38 activity, is lower in PS1(-/-) MEF cells than in PS1(+/+) MEF cells. In PS1(-/-) MEF cells, the basal activities of Raf and MEK, the upstream signaling component of ERK, are also lower than in PS1(+/+) MEF cells. Furthermore, Elk-1 transcription activity also down-regulates in PS1(-/-) MEF cells. Collectively, our data suggest that PS can modulate the basal level of ERK activity through the Raf-MEK-dependent pathway.

Abnormal intracellular trafficking of high affinity nerve growth factor receptor, Trk, in stable transfectants expressing presenilin 1 protein

The pathogenesis of Alzheimer's disease (AD) is now thought to be tightly linked to Abeta deposition and oxidative stress, but it is still unknown how these factors result in neuronal dysfunction and cell death. Mutations of presenilin 1 (PS1) gene are the causative gene for early onset familial AD (FAD) due to the overproduction and deposition of pathogenic Abeta1-42 peptides. We report here the molecular influences of the overexpression of PS1 protein by stable transfection of PS1 cDNA into SH-SY5Y neuroblastoma cells on the function of high affinity nerve growth factor receptor, Trk, that is essential for neuronal survival and differentiation. We examined the sensitivity of these transfectants to oxidative stress and found that mutant (I143T) PS1-expressing clones showed the highest vulnerability to an oxidative stress inducer, hydrogen peroxide treatment compared with that of mock-transfected clones, whereas wild PS1-expressing cells were less vulnerable to the treatment than mutant PS1 transfectants. Because nerve growth factor (NGF) is known to protect neuronal cells from oxidative stress-induced cell death, we examined the NGF-Trk-mediated intracellular signaling pathway in these transfectants. In the wild and mutant PS1 cDNA-transfected cells, NGF did not elicit the autophosphorylation response of Trk, although their basal levels of tyrosine phosphorylation were higher than those of mock-transfected cells. Immunocytochemical and subcellular fractionation studies revealed that most of Trk proteins are abnormally located in the cytoplasm as well as in the nucleus in PS1-overexpressing clones irrespective of wild and mutant forms. These results strongly indicate that the expression level of PS1 protein has a cross talk with the Trk-dependent neuroprotective intracellular signaling pathway.

Mitogen-activated protein kinases and the evolution of Alzheimer's: a revolutionary neurogenetic axis for therapeutic intervention?

Alzheimer's disease (AD) is a neurogenetic condition that affects the processes via which the brain functions. Major observable hallmarks of AD are accumulated clusters of proteins in the brain. These clusters, termed neurofibrillary tangles (NFT), resemble pairs of threads wound around each other in a helix fashion accumulating within neurons. These tangles consist of a protein called Tau, which binds to tubulin, thus forming microtubules. Unlike NFTs, deposits of amyloid precursor protein (beta-APP) gather in the spaces between nerve cells. The nearby neurons often look swollen and deformed, and the clusters of protein are usually accompanied by reactive inflammatory cells, microglia, which are part of the brain's immune system responsible for degrading and removing damaged neurons or plaques. Since phosphorylation/dephosphorylation mechanisms are crucial in the regulation of Tau and beta-APP, a superfamily of mitogen-activated protein kinases (MAPKs) has recently emerged as key regulators of the formation of plagues, eventually leading to dementia and AD. The complex molecular interactions between MAPKs and proteins (plagues) associated with the evolution of AD form a cornerstone in the knowledge of a still burgeoning field of neurodegenerative diseases and ageing. This review overviews current understanding of the molecular pathways related to MAPKs and their role in the development of AD and, possibly, dementia. MAPKs, therefore, may constitute a neurogenetic, therapeutic target for the diagnosis and evolution of a preventative medical strategy for early detection, and likely treatment, of Alzheimer's.

Alternative splicing in the N-terminus of Alzheimer's presenilin 1

Presenilin 1 (PS1) is mutated in the majority of familial cases of Alzheimer disease (AD). Although it is clear that PS1 is involved in the processing of the amyloid precursor protein (APP), the exact function of PS1 is still elusive. Human presenilin 1 (PS1) is alternatively spliced, resulting in the presence or absence of a four-amino acid motif, VRSQ, in the PS1 N-terminus. In human tissues, both isoforms are expressed. Here we report that mouse and rat only express the longer PS1 isoform. The presence of this motif introduces a potential phosphorylation site for protein kinase C. Because the splice occurs in the region of PS1 that we have previously shown to bind to rabGDI, this might provide a regulatory mechanism for this interaction. Our data show that the -VRSQ isoform binds rabGDI, but the +VRSQ does not. Moreover, mutation of the putatively phosphorylated threonine in PS1 disrupts the binding to rabGDI, showing its importance for the interaction. To our knowledge this is the first study showing a functional difference between PS1 splice variants. The possible consequences for APP processing and the pathogenesis of AD are discussed.

Stimulation of β-amyloid precursor protein α-processing by phorbol ester involves calcium and calpain activation

Normal processing of Alzheimer's beta-amyloid precursor protein (APP) is markedly stimulated by phorbol esters, but the underlying mechanisms have yet to be fully understood. In this study, we observed that: (a) Phorbol 12,13-dibutyrate (PDBu)-stimulated APP secretion in cultured SH-SY5Y neuroblastoma and fibroblast cells was blocked by EGTA and calpain inhibitors in a concentration-dependent manner, but not by other protease inhibitors. (b) Secretion of fibronectin, another secretory protein tested for comparison, was enhanced by PDBu, but insensitive to calpain inhibitors. (c) PDBu stimulated intracellular calpain activity as measured by the hydrolysis of a fluorogenic calpain substrate. (d) PDBu also induced rapid proteolysis of two endogenous substrates of calpains, i.e., tau and microtubule-associated protein-2 (MAP-2) and the proteolysis was blocked by EGTA and calpain inhibitors. Taken together, these results suggest that stimulation of APP alpha-processing by PDBu is through a mechanism that involves the activation of Ca(2+) and, most notably, calpain. The implications of the findings are discussed in relation to the regulatory mechanism of APP alpha-processing.

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.

Presenilin 1 regulates pharmacologically distinct gamma -secretase activities. Implications for the role of presenilin in gamma -secretase cleavage

Presenilins (PSs) are polytopic membrane proteins that have been implicated as potential therapeutic targets in Alzheimer's disease because of their role in regulating the gamma-secretase cleavage that generates the amyloid beta protein (Abeta). It is not clear how PSs regulate gamma-secretase cleavage, but there is evidence that PSs could be either essential cofactors in the gamma-secretase cleavage, gamma-secretase themselves, or regulators of intracellular trafficking that indirectly influence gamma-secretase cleavage. Using presenilin 1 (PS1) mutants that inhibit Abeta production in conjunction with transmembrane domain mutants of the amyloid protein precursor that are cleaved by pharmacologically distinct gamma-secretases, we show that PS1 regulates multiple pharmacologically distinct gamma-secretase activities as well as inducible alpha-secretase activity. It is likely that PS1 acts indirectly to regulate these activities (as in a trafficking or chaperone role), because these data indicate that for PS1 to be gamma-secretase it must either have multiple active sites or exist in a variety of catalytically active forms that are altered to an equivalent extent by the mutations we have studied.