Structure and function of fibrinogen BβN-domains

Formation and elimination of soluble fibrin and D-dimer in the bloodstream

Soluble fibrin is composed mainly of desA fibrin and fibrinogen oligomers consisting of fewer than 16 monomers partially cross-linked by factor XIIIa. Soluble fibrin cannot stimulate Glu-plasminogen activation by tissue plasminogen activator (t-PA); therefore, it may not be a direct predecessor of D-dimer. However, within the microcirculatory system, soluble fibrin oligomers may form microclots. Fibrin microclots stimulate Glu-plasminogen activation by t-PA, a process resulting in the formation of Glu-plasmin. Glu-plasmin dissolves the microclots, forming D-dimer. In normal and pathological blood plasma samples, soluble fibrin levels are substantially higher than those of D-dimer. Their concentrations in the plasma are also regulated by transendothelial transfer, absorption by blood macrophages, and binding and internalization with low-density lipoprotein receptors of the cells of the reticuloendothelial system. Therefore, the exact mechanisms of fibrin clots formation and elimination in normal and pathological conditions remain unclear. In this study, we reviewed findings on the molecular mechanisms of the formation and dissolution of fibrin clots, fibrin-dependent activation of Glu-plasminogen by t-PA, and blood plasma behavior in the microcirculatory system. Finally, we proposed a model that explains the relations of D-dimer and soluble fibrin underlying the common and separate mechanisms of their formation and elimination.

Current View on the Molecular Mechanisms Underlying Fibrin(ogen)-Dependent Inflammation

Numerous studies have revealed the involvement of fibrinogen in the inflammatory response. To explain the molecular mechanisms underlying fibrinogen-dependent inflammation, two bridging mechanisms have been proposed in which fibrin(ogen) bridges leukocytes to endothelial cells. The first mechanism suggests that bridging occurs via the interaction of fibrinogen with the leukocyte receptor Mac-1 and the endothelial receptor ICAM-1 (intercellular adhesion molecule-1), which promotes leukocyte transmigration and enhances inflammation. The second mechanism includes bridging of leukocytes to the endothelium by fibrin degradation product E1 fragment through its interaction with leukocyte receptor CD11c and endothelial VE-cadherin to promote leukocyte transmigration. The role of E1 in promoting inflammation is inhibited by the fibrin-derived β15-42 fragment, and this has been suggested to result from its ability to compete for the E1-VE-cadherin interaction and to trigger signaling pathways through the src kinase Fyn. Our recent study revealed that the β15-42 fragment is ineffective in inhibiting the E1- or fibrin-VE-cadherin interaction, leaving the proposed signaling mechanism as the only viable explanation for the inhibitory function of β15-42. We have discovered that fibrin interacts with the very-low-density lipoprotein (VLDL) receptor, and this interaction triggers a signaling pathway that promotes leukocyte transmigration through inhibition of the src kinase Fyn. This pathway is inhibited by another pathway induced by the interaction of β15-42 with a putative endothelial receptor. In this review, we briefly describe the previously proposed molecular mechanisms underlying fibrin-dependent inflammation and their advantages/disadvantages and summarize our recent studies of the novel VLDL receptor-dependent pathway of leukocyte transmigration which plays an important role in fibrin-dependent inflammation.

Dual functions of the fibrin βN-domains in the VLDL receptor-dependent pathway of transendothelial migration of leukocytes

Our previous studies revealed that fibrin interacts with the VLDL receptor (VLDLR) through a pair of its βN-domains and this interaction promotes transendothelial migration of leukocytes and, thereby, inflammation. In agreement, the NDSK-II fragment representing the central part of the fibrin molecule and containing these domains stimulates leukocyte transmigration. However, the recombinant (β15-66)2 fragment corresponding to a pair of the βN-domains inhibits NDSK-II-stimulated leukocyte transmigration. To explain this paradox, we hypothesized that fibrin βN-domains have dual function in fibrin-dependent inflammation, namely, their C-terminal regions containing the VLDLR-binding sites promote leukocyte transmigration while their N-terminal regions are responsible for inhibition of this process. To test this hypothesis and to further clarify the molecular mechanisms underlying fibrin-induced VLDLR-dependent pathway of leukocyte transmigration and its inhibition, we prepared the dimeric (β15-44)2 and (β40-66)2 fragments corresponding to the N- and C-terminal regions of the βN-domains and studied their effect on endothelial permeability and transendothelial migration of leukocytes. The results obtained revealed that (β40-66)2 bound to the VLDLR with high affinity and promoted endothelial permeability and leukocyte transmigration while (β15-44)2 did not interact with this receptor and had no effect on leukocyte transmigration, in agreement with our hypothesis. We also found that the first three N-terminal residues of the βN-domains play a critical role in the inhibitory properties of these domains. Further, the inhibitory properties of the βN-domains were expressed only upon their isolation from the fibrin molecule. The question of whether their inhibitory function may play a role in fibrin remains to be addressed.

LIMITED PROTEOLYSIS OF FIBRINOGEN αC-REGION REVEALS ITS STRUCTURE

Aim. The purpose of our study was to compare hydrolytic action of proteases from Gloydius halys halys, Agkistrodon contortrix contortrix and Calloselasma rhodostoma rhodostoma snake venoms and from Bacillus thuringiensis вар. israelensis IMV B-7465 culture medium on αC-regions of fibrinogen molecule. Methods. Products of hydrolysis were characterized by SDS-PAGE under reducing conditions with following Western-Blot using the mouse monoclonal 1-5А (anti-Aα509-610) and ІІ-5С (anti-Aα20-78) antibody. MALDI-TOF analysis of fibrinogen hydrolysis products was performed using a Voyager-DE. Results. Combination of SDS-PAGE, FPLC and MALDI-TOF analysis enabled to detect the peptide bonds cleaved by studied proteases. In particular proteases from Gloydius halys halys and Agkistrodon contortrix contortrix snake venoms cleaved peptide bond Aα413-414. Action of protease from Calloselasma rhodostoma rhodostoma on fibrinogen led to the formation of hydrolytic product generated from C-terminal portion of Aα-chain that corresponded to fragments generated by enzymes from two other snakes. On the other hand protease from Bacillus thuringiensis вар. israelensis IMV B-7465 culture medium cleaved peptide bond Aα504-505. Conclusions. Use of limited proteolysis technique as the source of additional information for computer modeling allowed us to propose an improved model of 3D-structure of fibrinogen αC-regions. This model takes into account the behavior of αC-regions in the physiological condition and contributes to the general knowledge about fibrinogen structure.

Structural Basis for the Interaction of Fibrin with the Very Low-Density Lipoprotein Receptor Revealed by NMR and Site-Directed Mutagenesis

Interaction of fibrin with the very low-density lipoprotein receptor (VLDLR) promotes transendothelial migration of leukocytes and thereby inflammation. To establish the structural basis for this interaction, we have previously localized the VLDLR-binding site to fibrin βN-domains including fibrin β chain sequence 15-64 and determined the NMR solution structure of the VLDLR(2-4) fragment containing fibrin-binding CR domains 2-4 of VLDLR. In this study, we identified amino acid residues in VLDLR and the βN-domains that are involved in the interaction using NMR and site-directed mutagenesis. The results obtained revealed that Lys47 and Lys53 of the second and third positively charged clusters of the βN-domain, respectively, interact with Trp20 and Asp25 of the CR2 domain and Trp63 and Glu68 of the CR3 domain, respectively. This finding indicates that Lys residues of the βN-domain interact with the Lys-binding site of the CR domains in a manner proposed earlier for the interaction of other members of the LDL receptor family with their ligands. In addition, Gly15 of the βN-domain and its first positively charged cluster contribute to the high-affinity interaction with VLDLR. Molecular modeling based on the results obtained and analysis of the previously published structures of such domains complexed with RAP and HRV2 allowed us to propose a model of interaction of fibrin βN-domains with the fibrin-binding CR domains of the VLDL receptor.