The epithelial or endothelial cells that line the human bronchi and the aorta express nicotinic acetylcholine receptors (nAChRs) of alpha3 subtypes. We report here that human bronchial epithelial cells (BEC) and aortic endothelial cells (AEC) express also the nAChR alpha7 subunit, which forms functional nAChRs. Polymerase chain reaction and in situ hybridization experiments detected alpha7 subunit mRNA in cultured human BEC and AEC and in sections of rat trachea. The binding of radiolabeled alpha-bungarotoxin revealed a few thousand binding sites per cell in cultured human BEC and human and bovine AEC. Western blot and immunohistochemistry experiments demonstrated that cultured BEC and AEC express a protein(s) recognized by anti-alpha7 antibodies. Whole-cell patch-clamp studies of cultured human BEC demonstrated the presence of fast-desensitizing currents activated by choline and nicotine that were blocked reversibly by methyllycaconitine (1 nM) and irreversibly by alpha-bungarotoxin (100 nM), consistent with the expression of functional alpha7 nAChRs. In some cells, choline activated also slowly decaying currents, confirming previous reports that BEC express functional alpha3beta4 nAChRs. Exposure of cultured BEC to nicotine (1 microM) for 3 days up-regulated functional alpha7 and alpha3 nAChRs, as indicated by the increased number of cells responding to acetylcholine and choline, with both fast-desensitizing currents, which were blocked irreversibly by alpha-bungarotoxin, and with slowly desensitizing currents, which are alpha-bungarotoxin-insensitive currents. The presence of alpha7 nAChRs in BEC and AEC suggests that some toxic effects of tobacco smoke could be mediated through these nicotine-sensitive receptors.

We demonstrated previously that human skin keratinocytes express acetylcholine receptors (AChRs) sensitive to acetylcholine and nicotine, which regulate cell adhesion and motility. We demonstrate here that human and rodent bronchial epithelial cells (BECs) express AChRs similar to those expressed by keratinocytes and by some neurons. Patch-clamp experiments demonstrated that the BEC AChRs are functional, and they are activated by acetylcholine and nicotine. They are blocked by kappa-bungarotoxin, a specific antagonist of the AChR isotypes expressed by neurons in ganglia. Their ion-gating properties are consistent with those of AChR isotypes expressed in ganglia, formed by alpha3, alpha5, and beta2 or beta4 subunits. Reverse transcription-polymerase chain reaction and in situ hybridization experiments demonstrated the presence in BECs of mRNA transcripts for all those AChR subunits, both in cell cultures and in tissue sections, whereas we could not detect transcripts for the alpha2, alpha4, alpha6, and beta3 AChR subunits. The expression of alpha3 and alpha5 proteins in BEC in vivo was verified by the binding of subunit-specific antibodies to sections of trachea. Mecamylamine and kappa-bungarotoxin, which are cholinergic antagonists able to block the ganglionic alpha3 AChRs, caused a reversible change of the cell shape of cultured, confluent human BECs. This resulted in a reduction of the area covered by the cell and in cell/cell detachment. The presence of AChRs sensitive to nicotine on the lining of the airways raises the possibility that the high concentrations of nicotine resulting from tobacco smoking will cause an abnormal activation, a desensitization, or both of the bronchial AChRs. This may mediate or facilitate some of the toxic effects of cigarette smoking in the respiratory system.

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The amyloid precursor protein (APP) is a ubiquitously expressed transmembrane adhesion protein and the progenitor of amyloid-␤ peptides. The major splice isoforms of APP expressed by most tissues contain a Kunitz protease inhibitor domain; secreted APP containing this domain is also known as protease nexin 2 and potently inhibits serine proteases, including trypsin and coagulation factors. The atypical human trypsin isoform mesotrypsin is resistant to inhibition by most protein protease inhibitors and cleaves some inhibitors at a substantially accelerated rate. Here, in a proteomic screen to identify potential physiological substrates of mesotrypsin, we find that APP/protease nexin 2 is selectively cleaved by mesotrypsin within the Kunitz protease inhibitor domain. In studies employing the recombinant Kunitz domain of APP (APPI), we show that mesotrypsin cleaves selectively at the Arg 15 -Ala 16 reactive site bond, with kinetic constants approaching those of other proteases toward highly specific protein substrates. Finally, we show that cleavage of APPI compromises its inhibition of other serine proteases, including cationic trypsin and factor XIa, by 2 orders of magnitude. Because APP/protease nexin 2 and mesotrypsin are coexpressed in a number of tissues, we suggest that processing by mesotrypsin may ablate the protease inhibitory function of APP/protease nexin 2 in vivo and may also modulate other activities of APP/protease nexin 2 that involve the Kunitz domain.Mesotrypsin is a human trypsin encoded by the PRSS3 gene found on chromosome 9p13 (1). Normal expression of PRSS3 is restricted to pancreas, brain, and, to a lesser extent, small intestine and colon (2-4); additionally, PRSS3 appears to be transcriptionally up-regulated with cancer progression in epithelial cancers, including lung (5), colon (6), and prostate. Mesotrypsin exhibits substantially different specificity from other trypsins toward protein substrates. It fails to activate pancreatic zymogens and also shows reduced capacity to degrade trypsinogens (7). Compared with other trypsins, it is significantly compromised in its ability to cleave protease-activated receptors (8 -10). Despite limited activity toward these classic trypsin substrates, mesotrypsin displays enhanced catalytic activity compared with other trypsins in the cleavage of certain specific protein substrates, most notably several canonical protease inhibitors (7, 11).The "canonical" inhibitors of serine proteases, named for a protease-binding loop of highly characteristic backbone conformation (12, 13), fulfill the paradoxical function of binding to a protease in a substrate-like manner yet acting as an inhibitor rather than an ordinary substrate. These inhibitors, representing at least 18 different convergently evolved protein families (14, 15), inhibit their cognate proteases via the "Laskowski mechanism," in which inhibitors act as highly specific, limited proteolysis substrates for target enzymes (14, 16). They bind so as to position a specific peptide bond, the "reactive si...

An important functional property of protein protease inhibitors is their stability to proteolysis. Mesotrypsin is a human trypsin that has been implicated in the proteolytic inactivation of several protein protease inhibitors. We have found that bovine pancreatic trypsin inhibitor (BPTI), a Kunitz protease inhibitor, inhibits mesotrypsin very weakly and is slowly proteolyzed, whereas, despite close sequence and structural homology, the Kunitz protease inhibitor domain of the amyloid precursor protein (APPI) binds to mesotrypsin 100 times more tightly and is cleaved 300 times more rapidly. To define features responsible for these differences, we have assessed the binding and cleavage by mesotrypsin of APPI and BPTI reciprocally mutated at two nonidentical residues that make direct contact with the enzyme. We find that Arg at P 1 (versus Lys) favors both tighter binding and more rapid cleavage, whereas Met (versus Arg) at P 2 favors tighter binding but has minimal effect on cleavage. Surprisingly, we find that the APPI scaffold greatly enhances proteolytic cleavage rates, independently of the binding loop. We draw thermodynamic additivity cycles analyzing the interdependence of P 1 and P 2 substitutions and scaffold differences, finding multiple instances in which the contributions of these features are nonadditive. We also report the crystal structure of the mesotrypsin⅐APPI complex, in which we find that the binding loop of APPI displays evidence of increased mobility compared with BPTI. Our data suggest that the enhanced vulnerability of APPI to mesotrypsin cleavage may derive from sequence differences in the scaffold that propagate increased flexibility and mobility to the binding loop.

Factor XIa (FXIa) is a serine protease important for initiating the intrinsic pathway of blood coagulation. Protease nexin 2 (PN2) is a Kunitz-type protease inhibitor secreted by activated platelets and a physiologically important inhibitor of FXIa. Inhibition of FXIa by PN2 requires interactions between the two proteins that are confined to the catalytic domain of the enzyme and the Kunitz protease inhibitor (KPI) domain of PN2. Recombinant PN2KPI and a mutant form of the FXI catalytic domain (FXIac) were expressed in yeast, purified to homogeneity, co-crystallized, and the structure of the complex was solved at 2.6 Å (Protein Data Bank code 1ZJD). In this complex, PN2KPI has a characteristic, disulfide-stabilized double loop structure that fits into the FXIac active site. To determine the contributions of residues within PN2KPI to its inhibitory activity, selected point mutations in PN2KPI Coagulation factor XI (FXI) 4 is a unique homodimeric coagulation protein, present in human plasma at a concentration of ϳ30 nM, that is essential for normal hemostasis, as evidenced by the fact that FXI deficiency is associated with a hemorrhagic disorder (1). The zymogen FXI can bind to receptors (2-4), consisting of the glycoprotein Ib-IX-V complex (5) on the plasma membranes of activated human platelets, where it can be incorporated into platelet membrane microdomains (i.e. lipid rafts) (6) for efficient activation by thrombin or by FXIIa (7-10). The resulting enzyme, FXIa, can then activate the vitamin K-dependent protein, FIX, to initiate the consolidation phase of blood coagulation (1).A variety of important control mechanisms exist for regulating the activity of coagulation proteases in plasma. Several members of the serpin family have been proposed as physiological regulators of FXIa activity in plasma, including C1 inhibitor (11, 12), ␣-1-protease inhibitor (13, 14), antithrombin III (15), ␣-2-antiplasmin (16), and protease nexin 1 (17). However, a platelet secretory protein and member of the class of Kunitz-type inhibitors, protease nexin 2 (PN2), has recently been shown to be a much more potent and physiologically relevant FXIa inhibitor based on detailed kinetics studies (18 -23). PN2 is a ϳ120-kDa isoform of the Alzheimer ␤-amyloid protein precursor (APP) that contains a Kunitz-type serine protease inhibitor (PN2KPI) domain (22). Platelets are an important source of several isoforms of APP, including the APP 751 isoform of PN2 (23, 24). Full-length APP is membraneassociated (25) and is processed by proteases in platelets (26, 27). Upon platelet activation by physiological stimulators, PN2 is secreted from ␣-granules into plasma and inhibits FXIa (22-24), suggesting a role for this protein in blood coagulation. PN2 is a slow, tight binding inhibitor of FXIa with K i of ϳ300 -500 pM (18,20,22). The KPI domain of PN2 is 57 amino acids in length (Glu 289 -Ile 345 in the 751-amino acid isoform of PN2) and is known to contain the entire FXIa inhibitory function of PN2 (19 -21, 28). Similarly, all of the information r...

Activation of factor XI (FXI) by thrombin on stimulated platelets plays a physiological role in hemostasis, providing additional thrombin generation required in cases of severe hemostatic challenge. Using a collection of 53 thrombin mutants, we identified 16 mutants with <50% of the wild-type thrombin FXI-activating activity in the presence of dextran sulfate. These mutants mapped to anion-binding exosite (ABE) I, ABE-II, the Na ؉ -binding site, and the 50-insertion loop. Only the ABE-II mutants showed reduced binding to dextran sulfate-linked agarose. Selected thrombin mutants in ABE-I (R68A, R70A, and R73A), ABE-II (R98A, R245A, and K248A), the 50-insertion loop (W50A), and the Na ؉ -binding site (E229A and R233A) with <10% of the wildtype activity also showed a markedly reduced ability to activate FXI in the presence of stimulated platelets. The ABE-I, 50-insertion loop, and Na ؉ -binding site mutants had impaired binding to FXI, but normal binding to glycocalicin, the soluble form of glycoprotein Ib␣ (GPIb␣). In contrast, the ABE-II mutants were defective in binding to glycocalicin, but displayed normal binding to FXI. Our data support a quaternary complex model of thrombin activation of FXI on stimulated platelets. Thrombin bound to one GPIb␣ molecule, via ABE-II on its posterior surface, is properly oriented for its activation of FXI bound to a neighboring GPI␣ molecule, via ABE-I on its anterior surface. GPIb␣ plays a critical role in the co-localization of thrombin and FXI and the resultant efficient activation of FXI.

The sequence regions of diphtheria toxin (DTX) recognized by CD4+ T cells of seven healthy humans of different major histocompatibility complex haplotypes were identified. Overlapping synthetic peptides, screening the DTX sequence, were used to test in proliferation assays unselected blood CD4+ cells, or DTX-specific CD4+ lines propagated by stimulation with DTX of blood mononuclear cells. Blood CD4+ cells and DTX-specific CD4+ lines gave consistent results. Although each subject had an individual pattern of peptide recognition, six peptide sequences (residues 271-290, 321-340, 331-350, 351-370, 411-430 and 431-450) were recognized by all subjects. In the native DTX molecule, these sequence regions are flanked by sequence loops exposed on the DTX surface. They overlap uncharged segments of the DTX sequence. These structural properties may be general requirements for immunodominance in CD4+ cell sensitization in humans.