Therapeutic actions of insulin-like growth factor I on APP/PS2 mice with severe brain amyloidosis.

Transgenic mice expressing mutant forms of both amyloid-beta (Abeta) precursor protein (APP) and presenilin (PS) 2 develop severe brain amyloidosis and cognitive deficits, two pathological hallmarks of Alzheimer's disease (AD). One-year-old APP/PS2 mice with high brain levels of Abeta and abundant Abeta plaques show disturbances in spatial learning and memory. Treatment of these deteriorated mice with a systemic slow-release formulation of insulin-like growth factor I (IGF-I) significantly ameliorated AD-like disturbances. Thus, IGF-I enhanced cognitive performance, decreased brain Abeta load, increased the levels of synaptic proteins, and reduced astrogliosis associated to Abeta plaques. The beneficial effects of IGF-I were associated to a significant increase in brain Abeta complexed to protein carriers such as albumin, apolipoprotein J or transthyretin. Since levels of APP were not modified after IGF-I therapy, and in vitro data showed that IGF-I increases the transport of Abeta/carrier protein complexes through the choroid plexus barrier, it seems that IGF-I favors elimination of Abeta from the brain, supporting a therapeutic use of this growth factor in AD.

Is caspase-3 inhibition a valid therapeutic strategy in cerebral ischemia?

Neurodegenerative diseases are characterized by progressive impairment of brain function as a consequence of ongoing neuronal cell death. Apoptotic mechanisms have been implicated in this process and a major involvement of caspase-3, a typical pro-apoptotic executioner protease, has been claimed. In this review, the role of caspase-3 in neuronal cell loss in animal models of stroke is discussed and critically evaluated. In summary, it is concluded that the biochemical evidence favoring caspase-3 as a therapeutic target in cerebral ischemia is not convincing, and the development of selective caspase-3 inhibitors for the treatment of human stroke must be viewed as high risk.

Amyloidogenic function of the Alzheimer's disease-associated presenilin 1 in the absence of endoproteolysis.

Alzheimer's disease (AD) is characterized by the invariant accumulation of senile plaques predominantly composed of the pathologically relevant 42-amino acid amyloid beta-peptide (Abeta42). The presenilin (PS) proteins play a key role in Abeta generation. FAD-associated mutations in PS1 and PS2 enhance the production of Abeta42, and PS1 is required for physiological Abeta production, since a gene knockout of PS1 and dominant negative mutations of PS1 abolish Abeta generation. PS proteins undergo endoproteolytic processing, and current evidence indicates that fragment formation may be required for the amyloidogenic function of PS. We have now determined the sequence requirements for endoproteolysis of PS1. Mutagenizing amino acids at the previously determined major cleavage site (amino acid 298) had no effect on PS1 endoproteolysis. In contrast, mutations or deletions at the additional cleavage site around amino acid 292 blocked endoproteolysis. The uncleavable PS1 derivatives accumulated as full-length proteins and replaced the endogenous PS1 proteins. In contrast to the previously described aspartate mutations within transmembrane domains 6 and 7, the uncleaved PS1 variants do not act as dominant negative inhibitors of Abeta production. Moreover, when a FAD-associated mutation (M146L) was combined with a mutation blocking endoproteolysis, Abeta42 production still reached pathological levels. These data therefore demonstrate that endoproteolysis of presenilins is not an absolute prerequisite for the amyloidogenic function of PS1. These data also show that accumulation of the PS1 holoprotein is not associated with the pathological activity of PS1 mutations as suggested previously.

The influence of endoproteolytic processing of familial Alzheimer's disease presenilin 2 on abeta42 amyloid peptide formation.

Mutant presenilins (PS) contribute to the pathogenesis of familial Alzheimer's disease (FAD) by enhancing the production of Abeta42 from beta-amyloid precursor protein. Presenilins are endoproteolytically processed to N-terminal and C-terminal fragments, which together form a stable 1:1 complex. We have mapped the cleavage site in the PS2 protein by direct sequencing of its C-terminal fragment isolated from mouse liver. Three different N-terminal residues were identified starting at Val-299, Thr-301, and Leu-307 that correspond closely to the previously described N termini of the C-terminal fragment of human PS1. Mutational analysis of the PS2 cleavage site indicates that the principal endoproteolytic cleavage occurs at residues Met-298/Val-299 and that the N terminus is subsequently modified by secondary proteolytic cleavages. We have generated cleavage defective PS2 constructs, which accumulate exclusively as full-length polypeptides in transfected Neuro2a cells. Functional analysis of such cleavage defective PS2 carrying the FAD mutation Asn-141 --> Ile showed that its Abeta42 producing activity was strongly reduced compared with cleavage-competent FAD PS2. In contrast, cleavage defective PS2 was active in rescuing the egg-laying defect of a sel-12 mutant in Caenorhabditis elegans. We conclude that PS2 endoproteolytic cleavage is not an absolute requirement for its activities but may rather selectively enhance or stabilize its functions.

Crystal structure of the novel aspartic proteinase zymogen proplasmepsin II from plasmodium falciparum.

Proplasmepsin II is the zymogen of plasmepsin II, an aspartic proteinase used by Plasmodiumfalciparum to digest hemoglobin during the blood stage of malaria. A large shift between the N-domain and the central and C-domains of proplasmepsin II opens the active site cleft, preventing the formation of a functional aspartic proteinase active site. This mode of inhibition of catalytic activity has not been observed in any other aspartic proteinase zymogen. Instead of occluding a pre-formed active site, as in the gastric aspartic proteinase zymogens, the prosegment of proplasmepsin II interacts extensively with the C-domain and serves as a 'harness' to keep the domains apart. Disruption of key salt bridges at low pH may be important in activation.

Caspase-mediated cleavage is not required for the activity of presenilins in amyloidogenesis and NOTCH signaling.

The Alzheimer's disease (AD) associated presenilin (PS) proteins are proteolytically processed. One of the processing pathways involves cleavage by caspases. Pharmacological inhibition of caspases is currently being discussed as a treatment for a variety of neurodegenerative diseases, including AD. We therefore inhibited caspase mediated processing of PS-1 and PS-2 in cells transfected with wt and mutant PS by mutagenizing the substrate recognition site or by using specific peptide aldehydes known to block caspases. We found that the inhibition of caspase mediated processing of PS proteins does not decrease its amyloidogenic activity. PS cDNA constructs with mutations in the caspase cleavage site are biologically active in Caenorhabditis elegans such as the wt human PS proteins, demonstrating that caspase-mediated cleavage is not required for the physiological PS function in NOTCH signaling.

Alzheimer's disease associated presenilin-1 holoprotein and its 18-20 kDa C-terminal fragment are death substrates for proteases of the caspase family.

Mutations in the presenilin (PS) genes are linked to early onset familial Alzheimer's disease (FAD). PS-1 proteins are proteolytically processed by an unknown protease leading to the formation of two stable fragments of approximately 30 and approximately 20 kDa [Thinakaran, G., et al. (1996) Neuron 17, 181-190]. In addition to the conventional fragments, alternative cleavage products were observed as well. Here we characterize an alternative proteolytic pathway of PS-1 which involves proteases of the caspase superfamily. Caspase mediated cleavage occurs between aspartate 345 and serine 346 C-terminal to the conventional cleavage determined previously [Podlisny, M., et al., (1997) Neurobiol. Dis. 3, 325-337]. Full-length PS-1 can serve as a substrate for caspase-like proteases, as demonstrated by the generation of the alternative C-terminal fragment in cells expressing PS-1 containing the Deltaexon 10 deletion which is known to accumulate as a full-length molecule [Thinakaran, G., et al. (1996)]. By inhibition of de novo protein synthesis in untransfected cells we demonstrate that the conventional C-terminal fragment of PS-1 is a substrate for caspase-like proteases as well. Therefore full-length and the conventional C-terminal fragment of PS-1 can serve as potential death substrates. Due to the fact that very little full-length PS-1 is expressed in vivo, the much more abundant C-terminal fragment and not the full-length precursor is the major in vivo substrate for the alternative cleavage of PS-1 by proteases of the caspase superfamily.

Presenilins are processed by caspase-type proteases.

Presenilin 1 (PS1) and presenilin 2 (PS2) are endoproteolytically processed in vivo and in cell transfectants to yield 27-35-kDa N-terminal and 15-24-kDa C-terminal fragments. We have studied the cleavage of PS1 and PS2 in transiently and stably transfected hamster kidney and mouse and human neuroblastoma cells by immunoblot and pulse-chase experiments. C-terminal fragments were isolated by affinity chromatography and SDS-polyacrylamide gel electrophoresis and sequenced. The processing sites identified in PS1 and PS2 (Asp345/Ser346 and Asp329/Ser330, respectively) are typical for caspase-type proteases. Specific caspase inhibitors and cleavage site mutations confirmed the involvement of caspase(s) in PS1 and PS2 processing in cell transfectants. Fluorescent peptide substrates carrying the PS-identified cleavage sites were hydrolyzed by proteolytic activity from mouse brain. The PS2-derived peptide substrate was also cleaved by recombinant human caspase-3. Additional processing of PS2 by non-caspase-type proteases was also observed.

Pretreatment with a 55-kDa tumor necrosis factor receptor-immunoglobulin fusion protein attenuates activation of coagulation, but not of fibrinolysis, during lethal bacteremia in baboons.

Baboons (Papio anubis) receiving a lethal intravenous infusion with live Escherichia coli were pretreated with either a 55-kDa tumor necrosis factor (TNF) receptor-IgG fusion protein (TNFR55:IgG) (n = 4, 4.6 mg/kg) or placebo (n = 4). Neutralization of TNF activity in TNFR55:IgG-treated animals was associated with a complete prevention of mortality and a strong attenuation of coagulation activation as reflected by the plasma concentrations of thrombin-antithrombin III complexes (P < .05). Activation of fibrinolysis was not influenced by TNFR55:IgG (plasma tissue-type plasminogen activator and plasmin-alpha2-antiplasmin complexes), whereas TNFR55:IgG did inhibit the release of plasminogen activator inhibitor type I (P < .05). Furthermore, TNFR55:IgG inhibited neutrophil degranulation (plasma levels of elastase-alpha1-antitrypsin complexes, P < .05) and modestly reduced release of secretory phospholipase A2. These data suggest that endogenous TNF contributes to activation of coagulation, but not to stimulation of fibrinolysis, during severe bacteremia.

Expression and characterisation of plasmepsin I from Plasmodium falciparum.

Two aspartic proteinases, plasmepsins I and II, are present in the digestive vacuole of the human malarial parasite Plasmodium falciparum and are believed to be essential for parasite degradation of haemoglobin. Here we report the expression and kinetic characterisation of functional recombinant plasmepsin I. In order to generate active plasmepsin I from its precursor, an autocatalytic cleavage site was introduced into the propart of the zymogen by mutation of Lys110P to Val (P indicates a propart residue). Appropriate refolding of the mutated zymogen then permitted pH-dependent autocatalytic processing of the zymogen to the active mature proteinase. A purification scheme was devised that removed aggregated and misfolded protein to yield pure, fully processable, proplasmepsin I. Kinetic constants for two synthetic peptide substrates and four inhibitors were determined for both recombinant plasmepsin I and recombinant plasmepsin II. Plasmepsin I had 5-10-fold lower k(cat)/Km values than plasmepsin II for the peptide substrates, while the aspartic proteinase inhibitors, selected for their ability to inhibit P. falciparum growth, were found to have up to 80-fold lower inhibition constants for plasmepsin I compared to plasmepsin II. The most active plasmepsin I inhibitors were antagonistic to the antimalarial action of chloroquine on cultured parasites. Northern blot analysis of RNA, isolated from specific stages of the erythrocytic cycle of P. falciparum, showed that the proplasmepsin I gene is expressed in the ring stages whereas the proplasmepsin II gene is not transcribed until the later trophozoite stage of parasite growth. The differences in kinetic properties and temporal expression of the two plasmepsins suggest they are not functionally redundant but play distinct roles in the parasite.

Tumor necrosis factor-alpha induces activation of coagulation and fibrinolysis in baboons through an exclusive effect on the p55 receptor.

Tumor necrosis factor-alpha (TNF-alpha) can bind to two distinct transmembrane receptors, the p55 and p75 TNF receptors. We compared the capability of two mutant TNF proteins with exclusive affinity for the p55 or p75 TNF receptor with that of wild type TNF, to activate the hemostatic mechanism in baboons. Both activation of the coagulation system, monitored by the plasma levels of thrombin-antithrombin III complexes, and activation of the fibrinolytic system (plasma levels of tissue-type plasminogen activator, and plasminogen activator inhibitor type I), were of similar magnitude after intravenous injection of wild type TNF or the TNF mutant with affinity only for the p55 receptor. Likewise, wild type TNF and the TNF p55 specific mutant were equally potent in inducing neutrophil degranulation (plasma levels of elastase-alpha 1-antitrypsin complexes). Wild type TNF tended to be a more potent inducer of secretory phospholipase A2 release than the p55 specific TNF mutant. Administration of the TNF mutant binding only to the p75 receptor did not induce any of these responses. We conclude that TNF-Induced stimulation of coagulation, fibrinolysis, neutrophil degranulation, and release of secretory phospholipase A2 are predominantly mediated by the p55 TNF receptor.

A human tumor necrosis factor p75 receptor agonist stimulates in vitro T cell proliferation but does not produce inflammation or shock in the baboon.

Tumor necrosis factor (TNF) is a potentially useful adjunct to anticancer therapies. However, the clinical utility of TNF has been limited by generalized toxicity and hypotension. Recently, studies have begun to dissect the individual proinflammatory and immunologic responses that result from TNF binding to its two cellular receptors, p55 and p75, in an attempt to develop TNF receptor agonists with reduced systemic toxicity. To evaluate a p75 receptor selective TNF mutant (p75TNF), TNF and p75TNF were administered to healthy anesthetized baboons. Intravenous infusion of the p75TNF produced none of the hemodynamic changes seen after the infusion of TNF. Infusion of p75TNF also failed to induce the plasma appearance of interleukins 6 and 8. However, p75TNF enhanced in vitro baboon thymocyte proliferation to concanavalin A, and infusion of p75TNF resulted in increased soluble p55 and p75 receptor plasma concentrations. Local skin necrosis and tissue neutrophil infiltration were seen after subcutaneous injections of TNF and p55TNF. Subcutaneous injection of p75TNF did not result in skin necrosis but did result in a modest dermal infiltration of lymphocytes and macrophages. The findings suggest that p75TNF may stimulate T cell proliferation without the systemic and local toxicity seen with TNF.

Protection against lethal Escherichia coli bacteremia in baboons (Papio anubis) by pretreatment with a 55-kDa TNF receptor (CD120a)-Ig fusion protein, Ro 45-2081.

Fusion proteins of the human 55-kDa TNF receptor extracellular domain with hinge and C2/C3 constant domains of human IgG1 or IgG3 heavy chains were tested in a primate sepsis model. Twenty-four baboons received 4.6, or 0.2 mg/kg of TNFR5-G1,3, or placebo, before the administration of a lethal dose of live Escherichia coli. Treatment with TNFR5-G1,3 decreased 5-day mortality from 88% in the placebo group to 12% in the TNFR5-G1,3-treated animals (p < 0.01 by Fisher's exact test). Treatments with TNR5-G1 and TNFR5-G3 in doses from 0.2 to 4.6 mg/kg were efficacious. Free plasma TNF was neutralized by all treatments, but inactive TNF/TNFR5-G1,3 complexes remained in circulation for prolonged periods. TNFR5-1,3 treatments attenuated the hemodynamic disturbances, reduced fluid requirements, and decreased the systemic IL-1 beta, IL-6, and IL-8 responses. In addition, TNFR5-G1,3 treatment shortened the granulocytopenia and reduced the loss of cellular TNF receptors from granulocytes. The decrease in fibrinogen concentrations and increase in prothrombin and partial thromboplastin times were significantly attenuated by TNFR5-G1,3 treatment. TNFR5-G1,3 treatment markedly attenuated the rise in plasma lactate concentration. Histologic studies of TNFR5-G1,3 revealed dose-dependent protection against tissue injury by Escherichia coli administration.

Effect of cell cycle on the regulation of the cell surface and secreted forms of type I and type II human tumor necrosis factor receptors.

The cell cycle has been shown to regulate the biological effects of human tumor necrosis factor (TNF), but to what extent that regulation is due to the modulation of TNF receptors is not clear. In the present report we investigated the effect of the cell cycle on the expression of surface and soluble TNF receptors in human histiocytic lymphoma U-937. Exposure to hydroxyurea, thymidine, etoposide, bisbensimide, and demecolcine lead to accumulation of cells primarily in G1/S, S, S/G2/M, G2/M, and M stages of the cell cycle, respectively. While no significant change in TNF receptors occurred in cells arrested in G1/S or S/G2 stages, about a 50% decrease was observed in cells at M phase of the cycle. Scatchard analysis showed a reduction in receptor number rather than affinity. In contrast, cells arrested at S phase (thymidine) showed an 80% increase in receptor number. The decrease in the TNF receptors was not due to changes in cell size or protein synthesis. The increase in receptors, however, correlated with an increase in total protein synthesis (to 3.8-fold of the control levels). A proportional change was observed in the p60 and p80 forms of the TNF receptors. A decrease in the surface receptors in cells arrested in M phase correlated with an increase in the amount of soluble receptors. The cellular response to TNF increased to 8- and 2-fold in cells arrested in G1 and S phase, respectively; but cells at G2/M phase showed about 6-fold decrease in response. In conclusion, our results demonstrate that the cell cycle plays an important role in regulation of cell-surface and soluble TNF receptors and also in the modulation of cellular response.

Protective effect of 55- but not 75-kD soluble tumor necrosis factor receptor-immunoglobulin G fusion proteins in an animal model of gram-negative sepsis.

The aim of this study was to compare the ability of both a 55- and 75-kD soluble tumor necrosis factor receptor immunoglobulin G fusion protein (sTNFR-IgG) in protecting against death in a murine model of gram-negative sepsis. Pretreatment with 250 micrograms of the p75 construct delayed but did not avert death in this model, reducing peak bioactive TNF-alpha levels after infection from 76.4 ng ml-1 in control mice to 4.7 ng ml-1 in the treated group (p < 0.05, two-sample t test). However, these low levels of bioactive TNF-alpha persisted in the p75 fusion protein-treated animals compared with the controls and were sufficient to mediate delayed death. In contrast, pretreatment with 200 micrograms of the p55 sTNFR-IgG gave excellent protection against death with complete neutralization of circulating TNF. Studies of the binding of TNF-alpha with the soluble TNFR fusion proteins showed that the p75 fusion construct exchanges bound TNF-alpha about 50-100-fold faster than the p55 fusion protein. Thus, although both fusion proteins in equilibrium bind TNF-alpha with high affinity, the TNF-alpha p55 fusion protein complex is kinetically more stable than the p75 fusion construct, which thus acts as a TNF carrier. The persistent release of TNF-alpha from the p75 fusion construct limits its therapeutic effect in this model of sepsis.

Differential responses of fibroblasts from wild-type and TNF-R55-deficient mice to mouse and human TNF-alpha activation.

The role of the two TNF receptor types, TNF-R55 and TNF-R75, was studied on mouse fibroblasts, taking advantage of TNF-R55-deficient mice generated by gene targeting (Tnfr1 degree-mice), and selectivity of human TNF-alpha for mouse TNF-R55. Radioligand binding assays showed that both TNF receptors were expressed on wild-type mouse fibroblasts, whereas normal levels of TNF-R75 were expressed on mouse fibroblasts isolated from Tnfr1 degree-mice. It was found that TNF-R55 controlled four major TNF-induced fibroblast functions: (1) adhesion to leukocyte cell lines as well as ICAM-1, VCAM-1, CD44, and MHC class I up-regulation; (2) secretion of other cytokines as demonstrated by stimulated IL-6 and granulocyte-macrophage-CSF releases; (3) cell proliferation; and (4) NF-kappa B activation. Stimulation through TNF-R75, in TNF-R55-deficient fibroblasts, did not have any effect in these functions. In general, mouse TNF-alpha (recognizing both mouse TNF receptors) had a higher sp. act. than human TNF-alpha (recognizing only mouse TNF-R55) in wild-type fibroblasts, whereas both mouse and human TNF-alpha had similar cytotoxic activities in WEHI 164 cells.

Tumor necrosis factor-alpha inhibits stem cell factor-induced proliferation of human bone marrow progenitor cells in vitro. Role of p55 and p75 tumor necrosis factor receptors.

Stem cell factor (SCF), a key regulator of hematopoiesis, potently synergizes with a number of hematopoietic growth factors. However, little is known about growth factors capable of inhibiting the actions of SCF. TNF-alpha has been shown to act as a bidirectional regulator of myeloid cell proliferation and differentiation. This study was designed to examine interactions between TNF-alpha and SCF. Here, we demonstrate that TNF-alpha potently and directly inhibits SCF-stimulated proliferation of CD34+ hematopoietic progenitor cells. Furthermore, TNF-alpha blocked all colony formation stimulated by SCF in combination with granulocyte colony-stimulating factor (CSF) or CSF-1. The synergistic effect of SCF observed in combination with GM-CSF or IL-3 was also inhibited by TNF-alpha, resulting in colony numbers similar to those obtained in the absence of SCF. These effects of TNF-alpha were mediated through the p55 TNF receptor, whereas little or no inhibition was signaled through the p75 TNF receptor. Finally, TNF-alpha downregulated c-kit cell-surface expression on CD34+ bone marrow cells, and this was predominantly a p55 TNF receptor-mediated event as well.

Bifunctional effects of tumor necrosis factor alpha (TNF alpha) on the growth of mature and primitive human hematopoietic progenitor cells: involvement of p55 and p75 TNF receptors.

Tumor necrosis factor alpha (TNF alpha) has previously been reported to have both inhibitory and stimulatory effects on hematopoietic progenitor cells. Specifically, TNF alpha has been proposed to stimulate early hematopoiesis in humans. In the present study we show that TNF alpha, in a dose-dependent fashion, can potently inhibit the growth of primitive high proliferative potential colony-forming cells (HPP-CFCs) stimulated by multiple cytokine combinations. Using agonistic antibodies to the p55 and p75 TNF receptors or TNF alpha mutants specific for either of the two TNF receptors, we show that both receptors can mediate this inhibition. In contrast, the potent stimulation of interleukin-3 (IL-3) plus granulocyte-macrophage colony-stimulating factor (GM-CSF) induced HPP-CFC colony formation observed at low concentrations of TNF alpha (2 ng/mL) was only a p55-mediated event. Moreover, the stimulatory effects of TNF alpha on GM-CSF or IL-3-induced colony formation, as well as the inhibition of G-CSF-induced colony growth, were also exclusively signaled through the p55 TNF receptor. Taken together, our results suggest that the inhibitory effects of TNF alpha on primitive bone marrow progenitor cells are mediated through both p55 and p75 TNF receptors, whereas the p55 receptor exclusively mediates the bidirectional effects on more mature, single factor-responsive bone marrow progenitor cells as well as stimulation of IL-3 plus GM-CSF-induced HPP-CFC colony growth.