N-linked glycosylation at Asn3 and the positively charged residues within the amino-terminal domain of the c1 inhibitor are required for interaction of the C1 Inhibitor with Salmonella enterica serovar typhimurium lipopolysaccharide and lipid A

The C1 inhibitor (C1INH), a plasma complement regulatory protein, prevents endotoxin shock, at least partially via the direct interaction of its amino-terminal heavily glycosylated nonserpin region with gram-negative bacterial lipopolysaccharide (LPS). To further characterize the potential LPS-binding site(s) within the amino-terminal domain, mutations were introduced into C1INH at the three N-linked glycosylation sites and at the four positively charged amino acid residues. A mutant in which Asn(3) was replaced with Ala was markedly less effective in its binding to LPS, while substitution of Asn(47) or Asn(59) had little effect on binding. The mutation of C1INH at all four positively charged amino acid residues (Arg(18), Lys(22), Lys(30), and Lys(55)) resulted in near-complete failure to interact with LPS. The C1INH mutants that did not bind to LPS also did not suppress LPS binding or LPS-induced up-regulation of tumor necrosis factor alpha mRNA expression in RAW 264.7 macrophages. In addition, the binding of C1INH mutants to diphosphoryl lipid A was decreased in comparison with that of recombinant wild-type C1INH. Therefore, the interaction of C1INH with gram-negative bacterial LPS is dependent both on the N-linked carbohydrate at Asn(3) and on the positively charged residues within the amino-terminal domain.

A Direct Role for C1 Inhibitor in Regulation of Leukocyte Adhesion

Plasma C1 inhibitor (C1INH) is a natural inhibitor of complement and contact system proteases. Heterozygosity for C1INH deficiency results in hereditary angioedema, which is mediated by bradykinin. Treatment with plasma C1INH is effective not only in patients with hereditary angioedema, but also in a variety of other disease models, in which such therapy is accompanied by diminished neutrophil infiltration. The underlying mechanism has been explained primarily as a result of the inhibition of the complement and contact systems. We have shown that C1INH expresses the sialyl-Lewis(x) tetrasaccharide on its N-linked glycan, via which it binds to E- and P-selectins and interferes with leukocyte-endothelial adhesion in vitro. Here we show that both native C1INH and reactive center cleaved C1INH significantly inhibit selectin-mediated leukocyte adhesion in several in vitro and in vivo models, whereas N-deglycosylated C1INH loses such activities. The data support the hypothesis that C1INH plays a direct role in leukocyte-endothelial cell adhesion, that the activity is mediated by carbohydrate, and that it is independent of protease inhibitory activity. Direct involvement of C1INH in modulation of selectin-mediated cell adhesion may be an important mechanism in the physiologic suppression of inflammation, and may partially explain its utility in therapy of inflammatory diseases.

C1 inhibitor prevents Gram-negative bacterial lipopolysaccharide-induced vascular permeability

Gram-negative bacterial endotoxemia may lead to the pathological increase of vascular permeability with systemic vascular collapse, a vascular leak syndrome, multiple organ failure (MOF), and/or shock. Previous studies demonstrated that C1 inhibitor (C1INH) protects mice from lipopolysaccharide (LPS)-induced lethal septic shock via a direct interaction with LPS. Here, we report that C1INH blocked the LPS-induced increase in transendothelial flux through an endothelial monolayer. In addition, LPS-mediated detachment of cultured endothelial cells was prevented with C1INH. C1INH also inhibited LPS-induced endothelial cell apoptosis as demonstrated by suppression of DNA fragmentation and annexin V expression. As illustrated by laser scanning confocal microscopy, C1INH completely blocked the binding of fluorescein isothiocyanate (FITC)-LPS to human umbilical vein endothelial cells (HUVECs). C1INH protected from localized LPS-induced increased plasma leakage in C57BL/6J mice and in C1INH-deficient mice. Local vascular permeability in response to LPS was increased to a greater extent in C1INH-deficient mice compared with wild-type littermate controls and was reversed by treatment with C1INH. Systemic administration of LPS to mice resulted in increased vascular permeability, which was reduced by C1INH. Therefore, these studies demonstrate that C1INH, in addition to its role in suppression of LPS-mediated macrophage activation, may play an important role in the prevention of LPS-mediated increased vascular permeability, endothelial cell injury, and multiple organ failure.

Approaches toward reversal of increased vascular permeability in C1 inhibitor deficient mice

C1 inhibitor (C1INH) deficient mice have increased vascular permeability that can be demonstrated by the extravasation of Evans Blue dye. This vascular leak is reversed with protease inhibitors, such as C1INH itself, DX88 (a recombinant variant Kunitz domain plasma kallikrein inhibitor), and the bradykinin receptor type 2 antagonist, Hoe140. The studies described here were undertaken for the following reasons: (1) To provide a more quantitative analysis of the effects of these interventions; (2) to provide data to further test the hypothesis that increased vascular permeability results from contact system activation with kallikrein-mediated release of bradykinin; (3) to test the hypothesis that the amino terminal non-serpin domain of C1INH modulates access to complex proteases, such as kallikrein complexed with high molecular weight kininogen (HK); and (4) to determine whether attenuated androgens or estrogens exert a direct effect on C1INH synthesis. To characterize the differences in these reagents, the dose-response and the rate of reappearance of increased vascular permeability in C1INH(-/-) mice were determined for the following agents: human plasma-derived C1INH, a recombinant Kunitz domain plasma kallikrein inhibitor (DX88), a bradykinin receptor antagonist (Hoe140), and a recombinant C1INH with an amino terminal truncation at amino acid 98 and substitution of the P2 Ala with a Val (Cserp98,A443V). C1INH and Cserp98,A443V were equivalent in activity, which provides further support for the hypothesis that the vascular leak is mediated by bradykinin and suggests that the amino terminal domain neither enhances nor interferes with access to kallikrein within the kallikrein-HK complex. DX88 was effective at very low doses, as was Hoe140. The duration of action of Hoe140 was quite prolonged. The data indicate that, in the mouse, neither danazol nor estrogens have a significant effect on C1INH synthesis.

N-linked glycosylation is required for c1 inhibitor-mediated protection from endotoxin shock in mice

C1 inhibitor (C1INH) prevents endotoxin shock in mice via a direct interaction with lipopolysaccharide (LPS). This interaction requires the heavily glycosylated amino-terminal domain of C1INH. C1INH in which N-linked carbohydrate was removed by using N-glycosidase F was markedly less effective in protecting mice from LPS-induced lethal septic shock. N-deglycosylated C1INH also failed to suppress fluorescein isothiocyanate (FITC)-LPS binding to and LPS-induced tumor necrosis factor alpha mRNA expression by the murine macrophage-like cell line, RAW 264.7, and cells in human whole blood. In an enzyme linked immunosorbent assay, the N-deglycosylated C1INH bound to LPS very poorly. In addition, C1INH was shown to bind to diphosphoryl lipid A (dLPA) but only weakly to monophosphoryl lipid A (mLPA). As with intact LPS, binding of N-deglycosylated C1INH to dLPA and mLPA was diminished in comparison with the native protein. Removal of O-linked carbohydrate had no effect on any of these activities. Neither detoxified LPS, dLPA, nor mLPA had any effect on the rate or extent of C1INH complex formation with C1s or on cleavage of the reactive center loop by trypsin. These data demonstrate that N-linked glycosylation of C1INH is essential to mediate its interaction with the LPA moiety of LPS and to protect mice from endotoxin shock.

C1 inhibitor prevents endotoxin shock via a direct interaction with lipopolysaccharide

C1 inhibitor (C1INH) is beneficial in animal models of endotoxemia and sepsis. However, the mechanism(s) of C1INH protection remain(s) ill-defined. In this study, we demonstrated that both active C1INH and reactive center-cleaved, inactive C1INH protected mice from lethal Gram-negative endotoxemia. Both forms of C1INH blocked the LPS-binding protein-dependent binding of Salmonella typhimurium LPS to the murine macrophage cell line, RAW 264.7, and suppressed LPS-induced TNF-alpha mRNA expression. Inhibition of LPS binding to RAW 264.7 cells was reversed with anti-C1INH Ab and was more efficient when C1INH was incubated first with LPS rather than with the cells. C1INH also suppressed LPS-induced up-regulation of TNF-alpha mRNA in whole human blood. The interaction of C1INH with LPS was directly demonstrated both by ELISA and by nondenaturing PAGE, but deletion of the amino-terminal 97-aa residues abrogated this binding. Therefore, C1INH, in addition to its function as a serine protease inhibitor, has a novel anti-inflammatory function mediated via its heavily glycosylated amino-terminal non-serpin domain.

Nuclear phosphatases and the proteasome in suppression of STAT1 activity in hepatocytes

IFN-gamma induction of C1 inhibitor (C1INH) is mediated by an IFN-gamma-activated sequence (GAS), via binding of signal transducer and activator of transcription 1 (STAT1). These studies focused on the factors responsible for down-regulation of nuclear STAT1 in hepatocytes, the primary site of synthesis of C1INH. The activity of nuclear STAT1 following stimulation with IFN-gamma was sustained with the phosphatase inhibitor, pervanadate, or the proteasome inhibitor, lactacystin. Pervanadate prolonged STAT1 activation and blocked the inactivation of nuclear STAT1. Binding of ubiquitin to phosphorylated STAT1 was detectable in cells treated with lactacystin. Staurosporine only moderately decreased the prolongation of nuclear phosphorylated STAT1 after pretreatment with pervanadate or lactacystin. An antisense mitogen-activated protein kinase phosphatase (MKP-1) oligonucleotide prolonged the accumulation of phosphorylated STAT1. These data are consistent with the hypothesis that down-regulation of IFN-gamma-mediated nuclear STAT1 binding in hepatocytes involves both dephosphorylation by MKP-1 and degradation via proteolysis by the ubiquitin-dependent proteasome pathway.

Increased vascular permeability in C1 inhibitor-deficient mice mediated by the bradykinin type 2 receptor

Heterozygosity for C1 inhibitor (C1INH) deficiency results in hereditary angioedema. Disruption of the C1INH gene by gene trapping enabled the generation of homozygous- and heterozygous-deficient mice. Mating of heterozygous-deficient mice resulted in the expected 1:2:1 ratio of wild-type, heterozygous, and homozygous-deficient offspring. C1INH-deficient mice showed no obvious phenotypic abnormality. However, following injection with Evans blue dye, both homozygous and heterozygous C1INH-deficient mice revealed increased vascular permeability in comparison with wild-type littermates. This increased vascular permeability was reversed by treatment with intravenous human C1INH, with a Kunitz domain plasma kallikrein inhibitor (DX88), and with a bradykinin type 2 receptor (Bk2R) antagonist (Hoe140). In addition, treatment of the C1INH-deficient mice with an angiotensin-converting enzyme inhibitor (captopril) increased the vascular permeability. Mice with deficiency of both C1INH and Bk2R demonstrated diminished vascular permeability in comparison with C1INH-deficient, Bk2R-sufficient mice. These data support the hypothesis that angioedema is mediated by bradykinin via Bk2R.

Arg3.1/Arc mRNA induction by Ca2+ and cAMP requires protein kinase A and mitogen-activated protein kinase/extracellular regulated kinase activation

Long-term potentiation (LTP) is a cellular model for persistent synaptic plasticity in the mammalian brain. Like several forms of memory, long-lasting LTP requires cAMP-mediated activation of protein kinase A (PKA) and is dependent on gene transcription. Consequently, activity-dependent genes such as c-fos that contain cAMP response elements (CREs) in their 5' regulatory region have been studied intensely. More recently, arg3.1/arc became of interest, because after synaptic stimulation, arg3.1/arc mRNA is rapidly induced and distributed to dendritic processes and may be locally translated there to facilitate synapse-specific modifications. However, to date nothing is known about the signaling mechanisms involved in the induction of this gene. Here we report that arg3.1/arc is robustly induced with LTP stimulation even at intensities that are not sufficient to activate c-fos expression. Unlike c-fos, the 5' regulatory region of arg3.1/arc does not contain a CRE consensus sequence and arg3.1/arc is unresponsive to cAMP in NIH3T3 and Neuro2a cells. However, in PC12 cells and primary cultures of hippocampal neurons, arg3.1/arc can be induced by cAMP and calcium. This induction requires the activity of PKA and mitogen-activated protein kinase, suggesting a neuron-specific pathway for the activation of arg3.1/arc expression.

The polo-like protein kinases Fnk and Snk associate with a Ca(2+)- and integrin-binding protein and are regulated dynamically with synaptic plasticity

In order to stabilize changes in synaptic strength, neurons activate a program of gene expression that results in alterations of their molecular composition and structure. Here we demonstrate that Fnk and Snk, two members of the polo family of cell cycle associated kinases, are co-opted by the brain to serve in this program. Stimuli that produce synaptic plasticity, including those that evoke long-term potentiation (LTP), dramatically increase levels of both kinase mRNAs. Induced Fnk and Snk proteins are targeted to the dendrites of activated neurons, suggesting that they mediate phosphorylation of proteins in this compartment. Moreover, a conserved C-terminal domain in these kinases is shown to interact specifically with Cib, a Ca(2+)- and integrin-binding protein. Together, these studies suggest a novel signal transduction mechanism in the stabilization of long-term synaptic plasticity.

Time-dependent reversal of long-term potentiation in area CA1 of the freely moving rat induced by theta pulse stimulation

Previous studies in slices have shown that low-frequency stimulation at 5 Hz, i.e., theta pulse stimulation (TPS), completely reverses long-term potentiation (LTP) in area CA1 when delivered within 1-2 min after induction but produces progressively less depotentiation at longer delays, until it has no longer any impact at 30 min after induction. The present study examined whether LTP in the freely moving rat exhibits a similar time-dependent susceptibility to reversal. Adult male Long-Evans rats with bilateral stimulating electrodes activating collateral/commissural projections to area CA1 were used. A 1 min episode of TPS, ineffective when applied to naive pathways, was found to permanently erase LTP when delivered to the test pathway either 30 sec or 15 min after induction. Administered at a delay of 30 min, however, the same treatment no longer had any impact on established LTP. Additional experiments examined the ability of shorter TPS episodes to erase LTP and found that a 30 sec treatment was effective at 30 sec but not 15 min after induction. When the duration of TPS was further reduced to 15 sec, a reversal was no longer obtained at any delay. These results provide the first demonstration that the limited vulnerability of LTP to reversal by TPS, originally observed in vitro, also holds true for LTP in the awake animal and occurs along the same time frame, supporting the notion that LTP stabilization mechanisms take less than 30 min to be complete.

Pim kinase expression is induced by LTP stimulation and required for the consolidation of enduring LTP

In animals and several cellular models of synaptic plasticity, long-lasting changes in synaptic strength are dependent on gene transcription and translation. Here we demonstrate that Pim-1, a serine/threonine kinase closely related to Pim-2 and Pim-3, is induced in hippocampus in response to stimuli that evoke long-term potentiation (LTP). Mice deficient for Pim-1 show normal synaptic transmission and short-term plasticity. However, they fail to consolidate enduring LTP even though Pim-2 and Pim-3 are constitutively expressed in the hippocampus and Pim-3 expression is similarly induced by synaptic activity. Thus, expression of Pim-1 is required for LTP. Its level of expression and, consequently, its capacity to phosphorylate target proteins in dendritic and nuclear compartments of stimulated neurons might be a determining factor for the establishment of long-lasting changes in synaptic strength.

GABAB receptor antagonism: facilitatory effects on memory parallel those on LTP induced by TBS but not HFS

The present experiments used CGP 35348, a selective GABAB receptor antagonist with a significantly higher affinity for post- versus presynaptic receptors, to dissociate the role of antagonist concentration versus stimulation mode in determining whether GABAB receptor blockade facilitates or suppresses long-term potentiation (LTP). The antagonist was applied by pressure ejection to one of two recording sites in area CA1 of hippocampal slices before LTP was induced at both sites with either theta burst or high-frequency stimulation (TBS or HFS). TBS produced a dose-dependent facilitation of potentiation that turned into depression at the highest concentration tested, a result reflecting the dose-dependent balance between the drug's postsynaptic disinhibitory effect and its action on presynaptic autoreceptors regulating the release of GABA. In contrast, HFS-induced LTP increased monotonically with drug concentration, suggesting that blockade of postsynaptic GABAB receptors is the only factor contributing to HFS-induced LTP. To test the relevance of the two sets of LTP results, we performed behavioral studies examining the effect of different dosages of antagonist on spatial retention and found that memory was enhanced at intermediate dosages but not at very low and high concentrations, reminiscent of the bell-shaped dose-response curve obtained for TBS-induced LTP. These findings are consistent with the notion that LTP induced by electrical stimulation modeled after endogenous theta-modulated activity patterns bears more relevance to behavior than does potentiation induced by arbitrary tetanic trains.

Studies on long-term depression in area CA1 of the anesthetized and freely moving rat

Homosynaptic long-term depression (LTD) is reported to occur in field CA1 of hippocampal slices collected from immature brains. Because the effect has been postulated to be a memory storage mechanism, it is of interest to test for its presence in adult, awake animals. Unfortunately, not only has hippocampal LTD proved difficult to obtain reliably in vivo, but the few successful studies vary with respect to protocols and evidence that the depression is input-specific. The present study tested for input-specific (homosynaptic) LTD in field CA1 after application of various stimulation protocols to the Schaffer collateral/commissural projections in freely moving, adult rats. The results indicate that although low-frequency trains do induce decrements in synaptic transmission lasting for hours to several days, the success rate of eliciting input-specific LTD in the awake rat is very modest compared with the ease with which stable potentiation is obtained in the same synapses. Moreover, it is questionable that the effective protocols represent patterns of activity likely to occur during behavior. The stronger the afferent activation during low-frequency stimulation, the greater was the probability of eliciting LTD accompanied by persistent heterosynaptic depression. Clear evidence for the occurrence of LTD, irrespective of stimulation protocol and current intensity, could not be obtained in rats under barbiturate anesthesia. In all, the results do not accord with the suggestion that LTD occurs routinely in the hippocampus in vivo as part of memory encoding.