Fibrinogen present in EDTA--anticoagulated plasma stimulates the tissue-type plasminogen activator-catalysed conversion of plasminogen to plasmin

Determination of anti-Müllerian hormone concentrations in blood as a tool to select Holstein donor cows for embryo production: from the laboratory to the farm

High between-animal variability in the number of embryos produced by multiple ovulation and embryo transfer (MOET) and ovum pick-up and in vitro production (OPU–IVP) methods remains a major limit to the development of embryo biotechnologies in cattle. The measurement of anti-Müllerian hormone (AMH) endocrine concentrations in cows can help to predict their follicular and ovulatory responses to gonadotrophin treatment. The present study aimed to provide practical information for a simple prognostic method based on AMH measurement in Holstein cows. Accurate AMH concentrations could be measured with ELISA in blood or plasma. In cows undergoing repeated OPU protocols over 1 year, the AMH concentrations measured in plasma samples collected before each gonadotrophin treatment were found to be highly repeatable and were tightly correlated with follicular responses. From data obtained at both an experimental station and farm settings, it was possible to propose AMH cut-off values to identify low-responding cows. Gonadotrophin-stimulated cows producing fewer than 15 large follicles at oestrus and fewer than 10 embryos in MOET protocols could be discarded efficiently with plasma AMH concentrations below 87 and 74 pg mL–1, respectively. In conclusion, we propose a prognostic method based on a single AMH measurement to improve the results of embryo biotechnologies.

Effects of histidine-proline-rich glycoprotein on plasminogen activation in solution and on surfaces

Histidine-proline-rich glycoprotein (HPRG) has long been known to associate with plasminogen (Plg) in solution, but the consequences of this interaction have not been defined. Here we show that HPRG adsorbed to a glycosaminoglycan (GAG) surface also binds Plg with a Kd value of 0.7 micromol/l. Moreover, we present evidence that HPRG acts as a modulator of the activation of Plg by tissue-type Plg activator. Specifically, Plg complexed with HPRG on a GAG surface is more readily activated by tissue-type Plg activator than free Plg, with a 10-fold difference in apparent catalytic efficiency (kcat/Km). HPRG also augments the increase in Plg activation caused by fibrinogen fragments either in solution or on GAG surfaces. In contrast, HPRG abrogates the stimulatory effects of fibrinogen on Plg activation in solution. These observations demonstrate that HPRG can act as either a positive or negative effector of Plg activation in vitro and may serve as a modulator of fibrinolysis in vivo.

Fibrin-mediated plasminogen activation

Fibrin, but not fibrinogen, enhances the rate of activation of plasminogen by tissue type plasminogen activator (t-PA). Studies with enzymatic and chemical fragments of fibrinogen showed that several sites in fibrinogen are involved in this rate enhancement; these are, A alpha 148-160 (located in CNBr fragment FCB-2), and FCB-5 (a CNBr fragment comprising gamma 312-324), and recently discovered sites in the fibrinogen alpha C domains. All these sites are buried in fibrinogen, but exposed in fibrin and some fibrinogen fragments. For the first two of these, located in the D-domains, this was shown by the fact that monoclonal antibodies against A alpha 148-160 and gamma 312-324 bind to fibrin and rate enhancing fibrin(ogen) fragments, but not to fibrinogen. Direct binding studies indicate that at physiological concentrations plasminogen binds to FCB-2, and t-PA to FCB-5. More detailed studies have demonstrated the importance of residues A alpha-157 and A alpha-152, and that the minimum stretch with rate enhancing properties is A alpha 154-159. The sites in the alpha C domains await further identification. With the recently reported three-dimensional structure of fragments D and D-dimer it is now possible to explain these findings at the molecular level. Molecular calculations and experimental data show that the site A alpha 148-160 in fibrinogen is covered among others by a part of the A alpha chain (A alpha 166-195) that forms an alpha-helix, and by a globular domain formed by the beta-chain. On fibrin formation, the last two may move away, and give access to A alpha 148-160. It is conceivable that in the alpha C domain sites are involved in the early phases of fibrinolysis. The site A alpha 148-160 and that in FCB-5 may be more important at later stages. It is also clear that fibrin polymerization is important. This polymerization has probably several effects: exposure of the rate enhancing sites; mutual positioning of the t-PA and plasminogen binding sites; a concentrating effect of t-PA and plasminogen on the fibrin surface; effects on the kinetic properties of t-PA and plasminogen. These effects together explain the rate enhancement.

Characterization of the interaction of recombinant apolipoprotein(a) with modified fibrinogen surfaces and fibrin clots

Elevated levels of lipoprotein(a) [Lp(a)] in plasma are a significant risk factor for the development of atherosclerotic disease, a property which may arise from the ability of this lipoprotein to inhibit fibrinolysis. In the present study we have quantitated the binding of recombinant forms of apolipoprotein(a) [17K and 12K r-apo(a); containing 8 and 3 copies, respectively, of the major repeat kringle sequence (kringle IV type 2)] to modified fibrinogen surfaces. Iodinated 17K and 12K r-apo(a) bound to immobilized thrombin-modified fibrinogen (i.e., fibrin) surfaces with similar affinities (Kd~ 1.2 - 1.6 µM). The total concentration of binding sites (Bmax) present on the fibrin surface was ~4-fold greater for the 12K than for the 17K (Bmaxvalues of 0.81 ± 0.09 nM, and 0.20 ± 0.01 nM respectively), suggesting that the total binding capacity on fibrin surfaces is reduced for larger apolipoprotein(a) (apo(a)) species. Interestingly, binding of apo(a) to intact fibrin was not detected as assessed by measurement of intrinsic fluorescence of free apo(a) present in the supernatants of sedimented fibrin clots. In other experiments, the total concentration apo(a) binding sites available on plasmin-modified fibrinogen surfaces was shown to be 13.5-fold higher than the number of sites available on unmodified fibrin surfaces (Bmaxvalues of 2.7 ± 0.3 nM and 0.20 ± 0.01 nM respectively) while the affinity of apo(a) for these surfaces was similar. The increase in Bmaxwas correlated with plasmin-mediated exposure of C-terminal lysines since treatment of plasmin-modified fibrinogen surfaces with carboxypeptidase B produced a significant decrease in total binding signal as detected by ELISA (enzyme linked immunosorbent assay). Taken together, these data suggest that apo(a) binds to fibrin with poor affinity (low µM) and that the total concentration of apo(a) binding sites available on modified-fibrinogen surfaces is affected by both apo(a) isoform size and by the increased availability of C-terminal lysines on plasmin-degraded fibrinogen surfaces. However, the low affinity of apo(a) for fibrin indicates that Lp(a) may inhibit fibrinolysis through a mechanism distinct from binding to fibrin, such as binding to plasminogen.Key words: fibrinolysis, lipoprotein(a), plasminogen activation.

D-dimer specific monoclonal antibodies react with fibrinogen aggregates

Human fibrinogen exposed to 46.5 degrees C was subjected to gel permeation chromatography. The protein eluted in two distinct peaks. The first peak appeared in the void volume containing soluble fibrinogen aggregates, while the other peak represented monomeric fibrinogen. In contrast to the monomeric peak material, the aggregate fraction reacted with a panel of monoclonal antibodies specific for fragment D-dimer using an ELISA system. Edman degradation showed that both the aggregate and the monomeric fractions were devoid of soluble fibrin, and immunoblots of SDS-PAG electrophoretic profiles disclosed no sign of stabilized high molecular weight derivatives. We have previously shown that the aggregate fraction of similarly treated fibrinogen, in contrast to the monomeric fraction, stimulates the t-PA catalyzed conversion of plasminogen to plasmin and concomitantly exposes the sequences Aalpha-(148-160) and gamma-(312-324) involved in t-PA stimulation. Our present and previous findings suggest that soluble fibrinogen aggregates possess a fibrin-like structure, and that fibrin or fibrinogen polymer formation is a prerequisite for the enhancing effect on t-PA-mediated plasminogen to plasmin conversion which is seen even with the polymers in the soluble state.

Stimulating effect on tissue-type plasminogen activator--a new and sensitive indicator of denatured fibrinogen

A high clottability and a short thrombin clotting time have routinely been considered as evidence of genuineness of the fibrinogen molecule. Since denatured fibrinogen stimulates the t-PA-catalysed conversion of plasminogen to plasmin, it was of interest to study the sensitivity of t-PA-stimulation as evidence of fibrinogen denaturation. Therefore, fibrinogen was intentionally exposed to various denaturating conditions (freeze-drying, heating, EDTA, alkali), and the clottability, the thrombin clotting time and the t-PA-stimulating effect were recorded. We found that the clottability was a poor indicator of fibrinogen denaturation, whereas the t-PA-stimulating effect could detect even mild fibrinogen denaturation. The thrombin clotting time was shortened after freeze-drying or heating at 47 degrees C, in spite of what might have been expected. Thus, denaturation is not necessarily accompanied by a prolonged clotting time. In some instances therefore, the t-PA-stimulation is an even more sensitive and reliable indicator of fibrinogen denaturation than is the thrombin clotting time. Consequently, this parameter should be combined with the thrombin clotting time to characterise preparations of fibrinogen.