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 beta N-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 do-mains stimulates leukocyte transmigration. However, the recombinant (beta 15-66)2 fragment corresponding to a pair of the beta N-domains inhibits NDSK-II-stimulated leukocyte transmigration. To explain this paradox, we hy-pothesized that fibrin beta N-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 pre-pared the dimeric (beta 15-44)2 and (beta 40-66)2 fragments corresponding to the N-and C-terminal regions of the beta N-domains and studied their effect on endothelial permeability and transendothelial migration of leukocytes. The results obtained revealed that (beta 40-66)2 bound to the VLDLR with high affinity and promoted endothelial permeability and leukocyte transmigration while (beta 15-44)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 beta N-domains play a critical role in the inhibitory properties of these domains. Further, the inhibitory properties of the beta N-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.

Automated summary

This study reveals the dual function of the β N-domains of fibrin, with their C-terminal regions promoting leukocyte transmigration and their N-terminal regions inhibiting this process. The (β40-66)2 fragment bound to VLDLR, promoting endothelial permeability and leukocyte transmigration, while the (β15-44)2 fragment did not interact with this receptor and had no effect on leukocyte transmigration.