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During inflammation, vascular permeability is increased by various proteolytic events, such as the generation of bradykinin, that augment local tissue responses by enabling tissue penetration of serum proteins, including complement and acute-phase proteins. Proteases also govern inflammatory responses by processing extracellular matrix proteins and soluble bioactive mediators. We quantified changes in the proteome and the nature of protein amino termini (the N-terminome) and the altered abundance of murine proteases and inhibitors during skin inflammation. Through analysis of the N-terminome by iTRAQ-TAILS, we identified cotranslational and posttranslational αN-acetylation motifs, quantitative increases in protein abundance, and qualitative changes in the proteolytic signature during inflammation. Of the proteins identified in normal skin, about half were cleaved, and phorbol ester-induced inflammation increased the proportion of cleaved proteins, including chemokines and complement proteins, that were processed at previously uncharacterized sites. In response to phorbol ester-induced inflammation, mice deficient in matrix metalloproteinase 2 (MMP2) showed reduced accumulation of serum proteins in the skin and exhibited different proteolytic networks from those of wild-type mice. We found that the complement 1 (C1) inhibitor attenuated the increase in serum protein accumulation in inflamed skin. Cleavage and inactivation of the C1 inhibitor by MMP2 increased complement activation and bradykinin generation in wild-type mice, leading to increased vessel permeability during inflammation, which was diminished in Mmp2(-/-) mice. Thus, our systems-level analysis of proteolysis dissected cleavage events associated with skin inflammation and demonstrated that loss of a single protease could perturb the proteolytic signaling network and enhance inflammation.
PURPOSE - Genetic factors influence an individual's risk for developing age-related macular degeneration (AMD), a leading cause of irreversible vision loss. Previous studies investigating the potential association between all AMD subtypes and the SERPING1 gene, which encodes a key regulator of the classic complement pathway, have yielded conflicting results. The purpose of this study is to determine whether variations in SERPING1 are associated with neovascular AMD.
METHODS - A total of 556 patients with neovascular AMD and 256 ethnically matched controls were genotyped for polymorphisms in SERPING1. A tagging single nucleotide polymorphism (tSNP) approach was used to cover the SERPING1 gene plus 2 kb on each side, spanning the promoter and the 3' untranslated regions. Ten SNPs with a minor allele frequency of 0.10 were covered by three tSNPs (rs1005510, rs11603020, rs2511989).
RESULTS - SERPING1 SNPs rs1005510 and rs2511989 were significantly associated with neovascular AMD in our cohort, with rs1005510 conferring an adverse risk effect (OR 1.49, 95% CI 1.18 to 1.88) and rs2511989 conferring a protective effect (OR 0.73, 95% CI 0.59 to 0.90). For both tSNPs, logistic regression of individual genotypes demonstrated statistically significant stepwise changes in the risk of developing AMD. Combined analysis of rs1005510 with variants in CFH and HTRA1 confirmed an independent risk effect. The rs11603020 variant had no effect on AMD susceptibility in this study (OR 0.98, 95% CI 0.78 to 1.24).
CONCLUSIONS - The SERPING1 gene is comprehensively investigated in this study (using three tSNPs), and its genetic variants are evaluated in the largest neovascular AMD cohort to date. The hypothesis that SERPING1 has a modest effect on the risk of neovascular AMD is supported by our results.
The autolysis loops (amino acids 143-154, chymotrypsinogen numbering) of plasma serine proteases play key roles in determining the specificity of protease inhibition by plasma serpins. We studied the importance of four basic residues (Arg-144, Lys-145, Arg-147, and Lys-149) in the autolysis loop of the coagulation protease factor XIa (fXIa) for inhibition by serpins. Recombinant fXIa mutants, in which these residues were replaced individually or in combination with alanine, were prepared. The proteases were compared to wild-type fXIa (fXIa-WT) with respect to their ability to activate factor IX in a plasma clotting assay, to hydrolyze the chromogenic substrate S2366, and to undergo inhibition by the C1-inhibitor (C1-INH), protein Z dependent protease inhibitor (ZPI), antithrombin (AT), and alpha(1)-protease inhibitor (alpha(1)-PI). All mutants exhibited normal activity in plasma and hydrolyzed S2366 with catalytic efficiencies similar to that of fXIa-WT. Inhibition of mutants by C1-INH was increased to varying degrees relative to that of fXIa-WT, with the mutant containing alanine replacements for all four basic residues (fXIa-144-149A) exhibiting an approximately 15-fold higher rate of inhibition. In contrast, the inhibition by ZPI was impaired 2-3-fold for single amino acid substitutions, and fXIa-144-149A was essentially resistant to inhibition by ZPI. Alanine substitution for Arg-147 impaired inhibition by AT approximately 7-fold; however, other substitutions did not affect it or slightly enhanced inhibition. Arg-147 was also required for inhibition by alpha(1)-PI. Cumulatively, the results demonstrate that basic amino acids in the autolysis loop of fXIa are important determinants of serpin specificity.
The glycosaminoglycan heparin enhances several reactions involving coagulation factor XI (FXI) including activation of FXI by factor XIIa, thrombin, and autoactivation; and inactivation of activated FXI (FXIa) by serine protease inhibitors. We examined the effect of heparin on inhibition of FXIa by the inhibitors C1-inhibitor (C1-INH) and antithrombin III (ATIII). Second order rate constants for inhibition in the absence of heparin were 1.57 x 10(3) and 0.91 x 10(3) M-1 s-1 for C1-INH and ATIII, respectively. Therapeutic heparin concentrations (0.1-1.0 units/ml) enhanced inhibition by ATIII 20-55-fold compared with 0.1-7.0-fold for C1-INH. For both inhibitors, the effect of heparin over a wide range of concentrations (10(-1) to 10(5) units/ml) produced bell-shaped curves, demonstrating that inhibition occurs by a template mechanism requiring both inhibitor and protease to bind to heparin. This implies that FXI/XIa contains structural elements that interact with heparin. Human FXI contains a sequence of amino acids (R250-I-K-K-S-K) in the apple 3 domain of the heavy chain that binds heparin (Ho, D., Badellino, K., Baglia, F., and Walsh, P. (1998) J. Biol. Chem. 273, 16382-16390). To determine the importance of this sequence to heparin-mediated reactions, recombinant FXI molecules with alanine substitutions for basic amino acids were expressed in 293 fibroblasts, and tested in heparin-dependent assays. Inhibition of FXIa by ATIII in the presence of heparin was decreased 4-fold by alanine substitution at Lys253 (A253), with smaller effects noted for mutants A255 and A252. FXI undergoes autoactivation to FXIa in the presence of heparin. The rate of autoactivation was decreased substantially for A253 with modest decreases for A255 and A252. Substituting all four charged residues in the sequence resulted in a profound decrease in autoactivation, significantly greater than for any single substitution. Relative affinity for heparin was tested by determining the concentration of NaCl required to elute FXIa from heparin-Sepharose. Wild type FXIa eluted from the column at 320 mM NaCl, whereas FXIa with multiple substitutions (A252-254 or A250-255) eluted at 230 mM NaCl. All proteins with single substitutions in charged amino acids eluted at intermediate NaCl concentrations. The data indicate that FXI/XIa must bind to heparin for optimal inhibition by ATIII and for autoactivation. Lys253 is the most important amino acid involved in binding, and Lys255 and Lys252 also have roles in interactions with heparin.