Studies on the ultrastructure of fibrin lacking fibrinopeptide B (beta- fibrin)

Abstract

Release of fibrinopeptide B from fibrinogen by copperhead venom procoagulant enzyme results in a form of fibrin (beta-fibrin) with weaker self-aggregation characteristics than the normal product (alpha beta-fibrin) produced by release of fibrinopeptides A (FPA) and B (FPB) by thrombin. We investigated the ultrastructure of these two types of fibrin as well as that of beta-fibrin prepared from fibrinogen Metz (A alpha 16 Arg----Cys), a homozygous dysfibrinogenemic mutant that does not release FPA. At 14 degrees C and physiologic solvent conditions (0.15 mol/L of NaCl, 0.015 mol/L of Tris buffer pH 7.4), the turbidity (350 nm) of rapidly polymerizing alpha beta-fibrin (thrombin 1 to 2 U/mL) plateaued in less than 6 min and formed a “coarse” matrix consisting of anastomosing fiber bundles (mean diameter 92 nm). More slowly polymerizing alpha beta-fibrin (thrombin 0.01 and 0.001 U/mL) surpassed this turbidity after greater than or equal to 60 minutes and concomitantly developed a network of thicker fiber bundles (mean diameters 118 and 186 nm, respectively). Such matrices also contained networks of highly branched, twisting, “fine” fibrils (fiber diameters 7 to 30 nm) that are usually characteristic of matrices formed at high ionic strength and pH. Slowly polymerizing beta-fibrin, like slowly polymerizing alpha beta-fibrin, displayed considerable quantities of fine matrix in addition to an underlying thick cable network (mean fiber diameter 135 nm), whereas rapidly polymerizing beta-fibrin monomer was comprised almost exclusively of wide, poorly anastomosed, striated cables (mean diameter 212 nm). Metz beta-fibrin clots were more fragile than those of normal beta-fibrin and were comprised almost entirely of a fine network. Metz fibrin could be induced, however, to form thick fiber bundles (mean diameter 76 nm) in the presence of albumin at a concentration (500 mumol/L) in the physiologic range and resembled a Metz plasma fibrin clot in that regard. The diminished capacity of Metz beta-fibrin to form thick fiber bundles may be due to impaired use or occupancy of a polymerization site exposed by FPB release. Our results indicate that twisting fibrils are an inherent structural feature of all forms of assembling fibrin, and suggest that mature beta-fibrin or alpha beta-fibrin clots develop from networks of thin fibrils that have the ability to coalesce to form thicker fiber bundles.

The Structure and Biological Features of Fibrinogen and Fibrin

“…The type of network architecture that characterizes the tight fibrin network is entirely consistent with the higher degree of equilateral fibril branching that occurs at low thrombin levels. 38 Such branches enhance clot elasticity.…”

Electrospun fibrinogen: Feasibility as a tissue engineering scaffold in a rat cell culture model

“…41 Perhaps, most important is that the matrix is constructed of fibers with diameters of *80-500 nm, which mimic the fiber diameters of 20-540 nm found in fibrin clots. 42,43 We have previously described how electrospinning can be used to produce highly porous fibrinogen scaffolds with fiber diameters as small as 80 nm 22,44 (Fig. 2).…”

Role of ‘B‐b’ knob‐hole interactions in fibrin binding to adsorbed fibrinogen

“…Thrombin cleaves FpA faster than FpB, so thrombincatalyzed protofibrils are formed predominantly via ÔA-aÕ interactions [13][14][15]. Nevertheless, studies with snake venom enzymes that remove only FpA or principally FpB have demonstrated that fibrin clots can be formed by either ÔA-aÕ or ÔB-bÕ interactions, indicating both interactions can mediate protofibril formation [15][16][17][18][19][20]. Experiments with a variant Correspondence: Oleg V. Gorkun recombinant fibrinogen showed that ÔB-bÕ interactions may play a substantial role in protofibril formation when ÔA-aÕ interactions are weakened [21].…”