Factor XIIa Dose Response Confirmation from Single Well HTS
Factor XII (FXII) is a 80 kDa zymogen found at a concentration of 0.375 uM in plasma, and upon activation by kallikrein at R353, a disulfide-linked two chain molecule called factor XIIa alpha (FXIIa) is generated. FXIIa is also capable of autoactivation by binding to negatively charged surfaces (1). Kallikrein can also cleave other scissile bonds in FXIIa alpha outside of the catalytic domain more ..
BioActive Compounds: 48
Molecular Library Screening Center Network (MLSCN)
Penn Center for Molecular Discovery (PCMD)
Assay Provider: Scott L. Diamond, University of Pennsylvania
MLSCN Grant: X01-MH076406-01
Factor XII (FXII) is a 80 kDa zymogen found at a concentration of 0.375 uM in plasma, and upon activation by kallikrein at R353, a disulfide-linked two chain molecule called factor XIIa alpha (FXIIa) is generated. FXIIa is also capable of autoactivation by binding to negatively charged surfaces (1). Kallikrein can also cleave other scissile bonds in FXIIa alpha outside of the catalytic domain at R334, R343, and R353, generating FXIIa beta, a 30 kDa enzyme that is no longer able to bind to surfaces, and which activates prekallikrein (PK) to kallikrein, using high molecular weight kininogen (HK) as a cofactor (2, 3). FXIIa is irreversibly inhibited by C-1 inhibitor (C1INH), a 105 kDa plasma SERPIN (4-6).
FXII, PK, HK, C1INH, and factor XI (FXI) have been traditionally placed within the intrinsic pathway of blood coagulation. The intrinsic activation pathway was originally named because its major recognized activation of a coagulation protease leading to thrombin formation, FXIIa activation of FXI to factor XIa (FXIa), occurs within, or intrinsic to the bloodstream. FXI was previously removed from this grouping and the remaining proteins are currently regarded as belonging to the contact activation, or Kallikrein-Kinin pathway, which is defined as proteolytic activation events that occur after collagen exposure from the subendothelium to the circulation (9).
FXIIa cleaves an internal R369-I370 bond in each monomer of FXI, yielding the enzyme FXIa (10). FXIa catalyzes FIX to FIXa activation by cleaving two scissile bonds at R145 and R180 (11). The FIXa generated can catalyze FXa formation on the platelet surface with the active cofactor factor VIIIa (FVIIIa), with FVIIIa increasing the Vmax of FX activation by FIXa by 100,000 fold (12). After activation of sufficient levels of FXa by the consolidation pathway, FXa can go on to form a ternary complex with FVa and prothrombin on the platelet surface, to give sufficient levels of thrombin for activation of fibrinogen to fibrin. Formation of this ternary prothrombinase complex in the presence of phospholipids has been shown to increase the rate of prothrombin to thrombin activation by 300,000-fold more than with FXa and prothrombin alone (13).
Thrombin catalyzes activation of fibrinogen to fibrin, cleaving peptides 14-16 amino acids in length, called fibrinopeptides, from the A alpha and B beta subunits of fibrinogen (14, 15). The fibrin monomers produced by thrombin form a noncovalent meshwork with other fibrin molecules to produce a fibrin clot that stabilizes and retracts the initial platelet plug described above. This stable fibrin clot formation stabilizes the primary platelet plug, and reduces the volume of the plug in order to arrest blood loss.
Deficiencies observed in the contact factor pathway (FXII, HK, or PK) have not shown evidence of abnormal bleeding tendencies, though C1INH deficiency causes hereditary angioedema, a swelling of subcutaneous structures, but not abnormal bleeding. The contact factor pathway, while not recognized to play a direct role in the initiation of the coagulation cascade (although FXIIa activates FVII to FVIIa and FXI to FXIa), is believed to play a more important pathological role in stabilization of the fibrin clot, but which can lead to pathological thrombosis and vascular occlusion. This is consistent with mouse models in which FXII deficient mice showed a severe defect in the formation and stabilization of platelet-rich thrombi, which was corrected by adding human FXIIa (16). Deficiency of FXII has been linked to venous and arterial thrombosis as a risk factor for thrombosis, suggesting a role for FXII as a natural anticoagulant (17).
Contact activation components have also been implicated in modulation of complement proteins, monocyte and neutrophil activities, and thrombin-induced platelet activation. This system has also been shown to have counteradhesive properties against platelet and neutrophil adhesion to endothelial cells, as well as modulating angiogenesis, fibrinolysis, and thrombin formation. Thus, biochemical and molecular binding events mediated by these proteins have important physiological effects especially in disease processes, but their roles are very dependent on the given physiological context and hemostatic state.
In vivo findings in rodent models of coagulation in addition to clinical data from patients at risk for pathologic cerebrovascular events have given rise to the recognition of contact activation proteins as fine modulators of blood coagulation whose inhibition does not cause a bleeding diathesis (18). Thus, drug design against FXIIa using the tools of high throughput chemical inhibitor screening may give rise to novel anticoagulant therapies.
HTS was performed on a total of 33,068 compounds of the MLSCN library, 23,017 of which were not in the mixture HTS plates used previously. These compounds were plated as single components of 384 well plates at 0.5 mM stock concentration, and were diluted 50-fold into 10 ul 384 well assay plates (final concentration 10 uM each compound). The assay used to test for percent inhibition was a fluorescence assay utilizing hydrolysis of with Boc-Gln-Gly-Arg-AMC, as first described by Kawabata et al. (19).
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Human plasma factor XIIa alpha was purchased from Enzyme Research Laboratories (Cat # HFXIIa 1212a). Substrate Boc-Gln-Gly-Arg-AMC was from Bachem (Cat #I-1595.0050). Assay buffer consisted of 50 mM Tris, pH 7.4, 150 mM sodium chloride, 0.02% Tween 20. Low-volume 384-well black plates were from Corning (Item #3676).
Factor XIIa alpha (3.5 ug/mL) was incubated with Boc-Gln-Gly-Arg-AMC substrate (15 uM) in 10 uL of assay buffer (see above) for 2 hr at room temperature. HTS was performed using 10 uM compound.
1.Serial dilute single compounds at 50x concentration in DMSO (16 two-fold dilutions from 2.5 mM to 75 nM)
2.Fill low-volume plate with 4 uL water using Multidrop-micro
3.Add 5 uL assay buffer to columns 1 and 23 using Multidrop-384
4.Add 200 nL of compound (in DMSO from step 1) using Evolution pintool
5.Add 1 uL of Boc-Gln-Gly-Arg-AMC substrate (150 uM in 5x assay buffer) using Multidrop-micro
6.Add 5 uL enzyme (7.0 ug/mL in assay buffer) using Multidrop-384
7.Incubate for 2 hr at room temperature
8.Read fluorescence (excitation 355, emission 460) on Envision reader
IC50 plates contained compounds in columns 3-22, controls (enzyme, no compound) in columns 2 and 24, and blanks (no enzyme) in columns 1 and 23. Each column 3-22 contained 16 two-fold dilutions of a single compound, ranging in concentration from 50 uM to 1.5 nM. Percent activity was calculated for each dilution of each compound from the signal in fluorescence units (FU) and the mean of the plate controls and the mean of the plate blanks using the following equation:
% Activity = 100*((signal-blank mean)/(control mean-blank mean))
Dose response curves of percent activity were fit using XLfit equation 205 (four parameter logistic fit with maximum percent activity and minimum percent activity fixed at 100 and 0, respectively).
The activity score reported here is based on follow-up IC50 testing on compounds that showed >30% inhibition in the primary HTS:
IC50 score = sum (IC50 score #1, IC50 score #2, IC50 score #3).
IC50 scores were calculated as follows:
(1) Score = 5.75 x (pIC50-3), where pIC50 = -log(10) of IC50 in mol/L
(2) For IC50 >50 uM (zero in IC50 column), score was calculated from percent activity at maximum concentration tested in assay (50 uM):
Score = [5.75 x (0-3)] + [(100-percent activity at max concentration)/1.75]
Compounds that gave percent inhibition >40 in the primary HTS were judged to be hits and these compounds were selected for follow-up IC50 testing. IC50 values were determined as described in protocol above. A zero in the IC50 field indicates that >50% enzyme activity remained at the maximum compound concentration tested (50 uM), so the IC50 lies outside the range of compound concentrations tested and can be inferred to be >50 uM. The percent activity at the maximum concentration is reported and can be used to estimate the potency of compounds for which the IC50 values were >50 uM.
Activity outcome is reported as follows:
(1) IC50 <50 uM in all three IC50 determinations = active
(2) IC50 <50 uM in only 1 or 2 out of 3 determinations = inconclusive
(3) IC50 >50 uM, percent inhibition 30-50% at 50 uM = inconclusive
(4) IC50 >50 uM, percent inhibition <30% at 50 uM = inactive
This assay was submitted to the PCMD by Scott Diamond, assay development and HTS were conducted by Paul Riley, and data were submitted by Andrew Napper and Paul Riley, all of the University of Pennsylvania.
Categorized Comment - additional comments and annotations
* Activity Concentration. ** Test Concentration.
Data Table (Concise)