Comparison of the sequence of fibrinopeptide A cleavage from fibrinogen fragment E by thrombin, atroxin, or batroxobin

Fibrinogen and fibrin are anti-adhesive for keratinocytes: a mechanism for fibrin eschar slough during wound repair

During cutaneous wound repair the epidermis avoids the fibrin-rich clot; rather it migrates down the collagen-rich dermal wound margin and over fibronectin-rich granulation tissue. The mechanism(s) underlying keratinocyte movement in this precise pathway has not been previously addressed. Here we demonstrate that cultured human keratinocytes do not express functional fibrinogen/fibrin receptors, specifically alpha v beta 3. Biologic modifiers known to induce integrin expression or activation did not induce adhesion to fibrin, fibrinogen, or its fragments. Epidermal explant outgrowth and single epidermal cell migration failed to occur on either fibrin or fibrinogen. Surprisingly, fibrin and fibrinogen mixed at physiologic molar ratios with fibronectin abrogated keratinocyte attachment to fibronectin. Keratinocytes transduced with the beta 3 integrin subunit cDNA, expressed alpha v beta 3 on their surface and attached to and spread on fibrinogen and fibrin. beta-gal cDNA-transduced keratinocytes did not demonstrate this activity. Furthermore, beta 3 cDNA-transduced keratinocyte adhesion to fibrin was inhibited by LM609 monoclonal antibody to alpha v beta 3 in a concentration-dependent fashion. From these data, we conclude that normal human keratinocytes cannot interact with fibrinogen and its derivatives due to the lack of alpha v beta 3. Thus, fibrinogen and fibrin are authentic anti-adhesive for keratinocytes. This may be a fundamental reason why the migrating epidermis dissects the fibrin eschar from wounds.

The fibrin intermediate, its place in the fibrinogen-fibrin transformation

Our preceding study indicated that, in course of coagulation of human fibrinogen by thrombin, substantial production of the fibrin intermediate (alpha-profibrin) lacking only one fibrinopeptide A (FPA) precedes the formation of alpha-fibrin monomer lacking both FPAs. The plateau concentration of alpha-profibrin (20% of initial fibrinogen) appearing in reactions indicated, however, that the second FPA is released four times faster than the first. The study reported here confirms those findings, and provides new insight into the significance of differing rate constants for the production of alpha-profibrin and its conversion to alpha-fibrin. The intermediate could be isolated in a distinct electrophoretic band by electrophoresing partial thrombin digests at high concentrations. Its identity was verified by digesting it with CNBr and by demonstrating that its N-terminal domain, the NDSK fragment, both lacks an FPA and contains an FPA, unlike the NDSKs of the bands from fibrin which contained no FPA or the fibrinogen band that lacked no FPA. The single step isolation also enabled us to confirm the 15-20% plateau level of alpha-profibrin in course of thrombin reactions, well below the 37% maximum that would be expected if release of the first and second FPA proceeded independently with no difference in rate. The 37% maximum is observed in reactions with atroxin, and it is suggested that the abundant production of alpha-profibrin underlies the therapeutic utility of atroxin as a defibrinating agent. Gel chromatography procedures were optimized for isolation of alpha-profibrin/fibrin mixtures free of fibrinogen, the final step of which involves literal use of agarose gel as a filter to remove fibrin aggregates from the fibrinogen free fractions (aggregates are left behind in gel filtration, rather than their moving ahead in gel chromatography). Unlike human fibrinogen, rabbit fibrinogen does not yield much alpha-profibrin in course of its conversion to fibrin, less than 10% as determined by electrophoresis and comparison with abundant production with atroxin. This low production of alpha-profibrin conformed with conclusions from our early studies on the generalized Shwartzman reaction in rabbits, and we now infer that the low production of alpha-profibrin and rapid conversion to fibrin by rabbit fibrinogen underlies the unparalleled susceptibility of these animals toward fibrinoid formation in the generalized Shwartzman reaction.