Presenilin-dependent regulated intramembrane proteolysis and gamma-secretase activity.

Inhibiting the production of amyloid-beta by antagonising gamma-secretase activity is currently being pursued as a therapeutic strategy for Alzheimer's disease (AD). However, early pre-clinical studies have demonstrated that disruption of presenilin-dependent gamma-secretase alters many presenilin-dependent processes, leading to early lethality in several AD model organisms. Subsequently, transgenic animal studies have highlighted several gross developmental side effects arising from presenilin deficiency. Partial knockdown or tissue-specific knockout of presenilins has identified the skin, vascular and immune systems as very sensitive to loss of presenilin functions. A more appreciative understanding of presenilin biology is therefore demanded if gamma-secretase is to be pursued as a therapeutic target. Herein we review the current understanding of gamma-secretase complexes; their regulation, abundance of interacting partners and diversity of substrates. We also discuss regulation of the gamma-secretase complexes, with an emphasis on the functional role of presenilins in cell biology.

Involvement of presenilins in cell-survival signalling pathways.

Familial AD (Alzheimer's disease) is a rare autosomal dominant form of AD, associated with clinical and pathological features similar to those identified in the more prevalent sporadic AD cases. The majority of familial AD cases are caused by mutations in either of the highly homologous PS (presenilins), an essential component of the gamma-secretase enzyme complex, or amyloid precursor protein, a gamma-secretase substrate and the precursor of amyloid beta peptides. The observation that PS are absolutely required for gamma-secretase activity, and parallel studies demonstrating that PS interact with several signalling molecules, modulate their stability or regulate their proteolysis, have led to the suggestion that involvement of PS in additional signalling pathways mediating key cellular functions may contribute to the pathogenesis and progression of neurodegeneration. In this paper, we review PS-regulated molecules, their role in cell signalling and possible involvement in neurodegeneration in patients suffering from AD.

Pathways of apoptosis and importance in development.

The elimination of cells by programmed cell death is a fundamental event in development where multicellular organisms regulate cell numbers or eliminate cells that are functionally redundant or potentially detrimental to the organism. The evolutionary conservation of the biochemical and genetic regulation of programmed cell death across species has allowed the genetic pathways of programmed cell death determined in lower species, such as the nematode Caenorhabditis elegans and the fruitfly Drosophila melanogaster to act as models to delineate the genetics and regulation of cell death in mammalian cells. These studies have identified cell autonomous and non-autonomous mechanisms that regulate of cell death and reveal that developmental cell death can either be a pre-determined cell fate or the consequence of insufficient cell interactions that normally promote cell survival.

Glycogen synthase kinase-3beta regulates presenilin 1 C-terminal fragment levels.

The majority of familial Alzheimer's disease cases have been attributed to mutations in the presenilin 1 (PS1) gene. PS1 is synthesized as an inactive holoprotein that undergoes endoproteolytic processing to generate a functional N- and C-terminal heterodimer (NTF and CTF, respectively). We identified a single residue in PS1, Ser(397), which regulates the CTF levels in a population of dimer that has a rapid turnover. This residue is part of a highly conserved glycogen synthase kinase-3beta (GSK-3beta) consensus phosphorylation site within the loop domain of PS1. Site-directed mutagenesis at the Ser(397) position increased levels of PS1 CTF but not NTF or holoprotein. Similar increases in only CTF levels were seen when cells expressing wild type PS1 were treated with lithium chloride, an inhibitor of GSK-3beta. Both wild type and PS1 S397A CTF displayed a biphasic turnover, reflecting rapidly degraded and stable populations. Rapid turnover was delayed for mutant PS1 S397A, causing increased CTF. These data demonstrate that PS1 NTF.CTF endoproteolytic fragments are generated in excess, that phosphorylation at Ser(397) by GSK-3beta regulates the discard of excess CTF, and that the disposal of surplus NTF is mediated by an independent mechanism. Overall, the results indicate that production of active NTF.CTF dimer is more complex than limited endoproteolysis of PS1 holoprotein and instead involves additional regulatory events.

Substitution of a glycogen synthase kinase-3beta phosphorylation site in presenilin 1 separates presenilin function from beta-catenin signaling.

The majority of cases with early onset familial Alzheimer's disease have been attributed to mutations in the presenilin 1 (PS1) gene. PS1 protein is a component of a high molecular weight membrane-bound complex that also contains beta-catenin. The physiological relevance of the association between PS1 and beta-catenin remains controversial. In this study, we report the identification and functional characterization of a highly conserved glycogen synthase kinase-3beta consensus phosphorylation site within the hydrophilic loop domain of PS1. Site-directed mutagenesis, together with in vitro and in vivo phosphorylation assays, indicates that PS1 residues Ser(353) and Ser(357) are glycogen synthase kinase-3beta targets. Substitution of one or both of these residues greatly reduces the ability of PS1 to associate with beta-catenin. By disrupting this interaction, we demonstrate that the association between PS1 and beta-catenin has no effect on Abeta peptide production, beta-catenin stability, or cellular susceptibility to apoptosis. Significantly, in the absence of PS1/beta-catenin association, we found no alteration in beta-catenin signaling using induction of this pathway by exogenous expression of Wnt-1 or beta-catenin and a Tcf/Lef transcriptional assay. These results argue against a pathologically relevant role for the association between PS1 and beta-catenin in familial Alzheimer's disease.

Communication and documentation before and after laparoscopic surgery.

Increasingly, sanctions by Offices of Professional Medical Conduct, functioning at state level, and malpractice suits are being used to discipline physicians. A system of documentation and communication, practiced successfully for more than 30 years, can reduce the risk of such actions. (J Am Assoc Gynecol Laparosc 6(4):379-381, 1999)

Apoptosis induced by Drosophila reaper and grim in a human system. Attenuation by inhibitor of apoptosis proteins (cIAPs).

Previous genetic studies have established Reaper and Grim as central regulators of apoptosis in Drosophila melanogaster. Reaper and Grim induce extensive apoptosis in Drosophila, yet share no homology to known vertebrate proteins. In this study, we show for the first time that ectopic expression of Reaper or Grim induced substantial apoptosis in mammalian cells. Reaper- or Grim-induced apoptosis was inhibited by a broad range of caspase inhibitors and by human inhibitor of apoptosis proteins cIAP1 and cIAP2. Additionally, in vivo binding studies demonstrated that both Reaper and Grim physically interacted with human IAPs through a homologous 15-amino acid N-terminal segment. Deletion of this segment from either Reaper or Grim abolished binding to cIAPs. In vitro binding experiments indicated that Reaper and Grim bound specifically to the BIR domain-containing region of cIAPs as deletion of this region resulted in loss of binding. The physical interaction was further confirmed by immunolocalization. When co-expressed, Reaper or Grim co-localized with cIAP1. However, deletion of the N-terminal 15 amino acids of Reaper or Grim abolished co-localization with cIAP1, suggesting that this homologous region can serve as a protein-protein interacting domain in regulating cell death. Moreover, by virtue of this interaction, we demonstrate that cIAPs can regulate Reaper and Grim by abrogating their ability to activate caspases and thereby inhibit apoptosis. This is the first function attributed to this 15-amino acid N-terminal domain that is the only region having significant homology between these Drosophila death inducers.

RIP2 is a novel NF-kappaB-activating and cell death-inducing kinase.

Through specific interactions with members of the tumor necrosis receptor (TNFR) family, adapter molecules such as the serine/threonine (Ser/Thr) kinase RIP mediate divergent signaling pathways including NF-kappaB activation and cell death. In this study, we have identified and characterized a novel 61-kDa protein kinase related to RIP that is a component of both the TNFR-1 and the CD40 signaling complexes. Receptor interacting protein-2 (RIP2) contains an N-terminal domain with homology to Ser/Thr kinases and a C-terminal caspase activation and recruitment domain (CARD), a homophilic interaction motif that mediates the recruitment of caspase death proteases. Overexpression of RIP2 signaled both NF-kappaB activation and cell death. Mutational analysis revealed the pro-apoptotic function of RIP2 to be restricted to its C-terminal CARD domain, whereas the intact molecule was necessary for NF-kappaB activation. RIP2 interacted with other members of the TNFR-1 signaling complex, including inhibitor of apoptosis protein cIAP1 and with members of the TNFR-associated factor (TRAF) family, specifically TRAF1, TRAF5, and TRAF6, but not with TRAF2, TRAF3, or TRAF4. These TRAF interactions mediate the recruitment of RIP2 to receptor signaling complexes.

Cell shrinkage and apoptosis: a role for potassium and sodium ion efflux.

In this study we have shown that redistribution of the lipid composition of the external and internal leaflets of the PM during apoptosis results in two main alterations in the cell surface, externalisation of PS, and a looser packing of the lipid hydrophobic head groups in the external leaflet. Significantly, neither of these alterations can be directly implicated in the mechanism of apoptotic cell shrinkage, however they do have functions in other phases of the apoptotic process. Progressional studies involving morphological and flow cytometric evaluation, and DNA gel electrophoresis revealed that apoptotic cell shrinkage is associated with a decrease in [Na+]i and [K+]i which occurs after visualisation of chromatin condensation and internucleosomal DNA fragmentation, and prior to apoptotic body formation. When apoptotic cultures were supplemented with inhibitors of the Na+, K+-ATPase pump or the Ca2+-dependent K+ channel, essentially all of the respective Na+ or K+ efflux during apoptosis can be inhibited, suggesting that essentially all of the Na+ and K+ efflux can be ascribed to active pumping via the Na+, K+-ATPase pump and the Ca2+-dependent K+ channel.

Increased resistance to apoptosis associated with HL-60 myeloid differentiation status.

Morphologically HL-60 leukaemia cells largely resemble promyelocytes and can be induced to terminally differentiate in vitro. Upon reaching terminal maturation these cells rapidly undergo apoptosis. Using three chemotherapeutic agents with known apoptosis inducing capability, the susceptibility of RA - and PMA - differentiated cultures was monitored by morphological means and flow cytometry. We observed that as cells with morphological characteristics of mature granulocytes/monocytes became more prominent in the populations, there was an increased resistance to apoptosis. The inhibition of the typical internucleosomal DNA fragmentation was confirmed by agarose gel electrophoresis. However, activation of a CA+/Mg+ independent endonuclease in isolated nuclei was not affected. Flow immunocytometry revealed reduced levels of c-myc and bcl-2 oncoproteins in RA and PMA treated cells. These observations suggest that HL-60 derived granulocytes/monocytes become increasingly resistant to the induction of apoptosis and that this resistance is independent of c-myc and bcl-2 expression. Together these results demonstrate that the phenotypic changes associated with RA and PMA induced differentiation, inhibit a critical step in the progression of apoptosis.