Crystalizing our view of the contact system

How does the contact activation system (CAS) assemble on cellular surfaces? Although researchers have been studying the contact factors prekallikrein (PK), high-molecular-weight kininogen (HK), and factor XII (FXII), and their respective roles in inflammation, immunity, and coagulation for over half a century, we now get some clarity on a quaternary structure for this fascinating multienzyme complex. In this issue of Blood, Kaira and colleagues describe the first crystal structure of an FXII domain in complex with a putative receptor, and they propose a model by which this binding protein, the globular complement C1q receptor (gC1qR), can act as a chaperone to cluster contact factors together prior to initiating factor XI (FXI)-dependent blood coagulation and inflammatory bradykinin (BK) liberation.¹ 

Interest in the CAS has expanded significantly in recent years as contact factors are now being explored as therapeutic targets for thrombotic and inflammatory conditions. Indeed, inhibitors of and/or deficiencies in PK, HK, and FXII have been shown to limit experimental thrombosis without increased bleeding in animal models,²  whereas early human studies have indicated that targeting FXI is antithrombotic with no signs yet of significant hemostatic compromise.³  FXII gain-of-function mutations have also been described that trigger the rare inflammatory disease hereditary angioedema with normal C1 inhibitor.⁴  Certainly, the myriad of biologic activities associated with the CAS provide opportunities for new approaches to reduce thrombosis and inflammation in diverse disease states.

Given its numerous functions in a growing list of disease settings, gC1qR has also become a potential target for therapeutic development. gC1qR is a ubiquitously expressed, highly anionic multifunctional protein that plays an important role in infection, inflammation, as well as cancer progression and metastasis.^5,6  gC1qR has been shown to bind an array of proteins on the cell surface, in plasma, and on pathogenic microbes, whereas the plasma proteins that bind to gC1qR are primarily those of the CAS, as well as fibrinogen, thrombin, and multimeric vitronectin.^7,8  This suggests that gC1qR may be involved in fibrin formation, particularly at sites of immune injury and/or inflammation. In order to further delineate specific downstream activities, though, we must develop a better understanding of the structure-function relationship that allows for rational design of allosteric modulators.

The findings by Kaira et al provide a framework for a potential path forward and also offers insight into how the CAS may interact with other receptors and surface initiators to promote coagulation and inflammation (see figure). Using crystallography, surface plasmon resonance, and plasma-based coagulation assays, the authors determined specific residues and cofactors that are necessary for the assembly of HK/FXII/gC1qR complex, while also showing that gC1qR promotes FXII-dependent coagulation. Gel filtration experiments reveal for the first time that simultaneous HK and FXII binding occurs as part of this assembly with gC1qR clustering FXII and HK into a higher-order ternary complex. Mutagenesis studies also identify a critical component of the gC1qR trimer that suggests steric occlusion as the mechanism for HK asymmetric binding.

Exposure of blood to various negatively charged substances or artificial surfaces triggers proteolytic CAS activation that leads to thrombin generation, fibrin formation, and inflammatory BK liberation. Typically, coagulation events are depicted as a sequential cascade of enzymatic steps and pathways. The study presented here illustrates that higher-order quaternary strictures of multienzyme complexes are likely to play a central role in enhancing the catalytic efficiency of many coagulation factor interactions. Indeed, since FXI is a homolog of PK that also circulates in complex with HK,⁹  and FXI has been shown to reciprocally activate FXII,¹⁰  the proposed model may directly link CAS-initiated FXI activation and subsequent coagulation. Although this paper ends a longstanding debate on how contact factors are assembled with respect to gC1qR, additional studies are needed to examine the relative importance of other putative receptors/cofactors, such as the urokinase receptor or cytokeratin 1, in CAS modulation.