Evidence that low molecular fibrinogen (LMW) is formed in man by degradation of high molecular weight fibrinogen (HMW)

Fibrinogen αC-region acts as a functional safety latch: Implications for a fibrin biomechanical behaviour model

Fibrin has unique biomechanical properties which are essential for its role as a scaffold for blood clots. Fibrin is highly extensible and demonstrates significant strain stiffening behaviour, which is essential for stress-distribution in the network. Yet the exact structures of fibrin at the sub-fibre level that contribute to its unique biomechanical characteristic are unknown. Here we show how truncations of the fibrinogen αC-region impact the biomechanical properties of fibrin fibres. Surprisingly, absence of the complete αC-region did not influence the low strain modulus of fibrin fibres but led to premature fibre rupture and decreased extensibility. Intermediate effects were observed with partial deletion of the αC-region, reflected by intermediate rupture stress and toughness. However, overall strain-stiffening behaviour remained even in absence of the αC-region, indicating that strain stiffening is not due to stress being transferred from the αC-region to the protofibril backbone. Upon stress-relaxation, decay constants and their relative contribution to the total relaxation remained similar at all strains, showing that a distinct relaxation process is present until fibre rupture. However, relative contribution of fast relaxation was maximal only in crosslinked fibres if the flexible αC-connector was present. These data show that the αC-region is not the main load-bearing structure within fibrin fibres and point to a critical role for the protofibril backbone instead. We present a revised structural model based on protofibril branching that fully explains the unique biomechanical behaviour of fibrin fibres, while the αC-region primarily acts as a safety latch at the highest of strains. STATEMENT OF SIGNIFICANCE: The findings presented in this paper reveal critically important details about how the molecular structure of fibrin contributes to its unique mechanical properties which are essential to fulfil its function as the scaffold of blood clots. In this work we used engineered proteins with alterations in an important but highly disordered area of the molecule called αC-region and we provide direct evidence for the first time for how the absence of either the globular αC-domain, or the complete αC-region impacts the mechanical behaviour of individual fibrin fibres. Using these results we developed a new structural model of protofibril organisation within fibrin fibres that fully explains their strain stiffening, relatively low modulus and their high, largely variable, extensibility.

Why fibrin biomechanical properties matter for hemostasis and thrombosis

Polymeric fibrin displays unique structural and biomechanical properties that contribute to its essential role of generating blood clots that stem bleeds. The aim of this review is to discuss how the fibrin clot is formed, how protofibrils make up individual fibrin fibers, what the relationship is between the molecular structure and fibrin biomechanical properties, and how fibrin biomechanical properties relate to the risk of thromboembolic disease. Fibrin polymerization is driven by different types of bonds, including knob-hole interactions displaying catch-slip characteristics, and covalent crosslinking of fibrin polypeptides by activated factor XIII. Key biophysical properties of fibrin polymer are its visco-elasticity, extensibility and resistance to rupture. The internal packing of protofibrils within fibers changes fibrin biomechanical behavior. There are several methods to analyze fibrin biomechanical properties at different scales, including AFM force spectroscopy, magnetic or optical tweezers and rheometry, amongst others. Clinically, fibrin biomechanical characteristics are key for the prevention of thromboembolic disorders such as pulmonary embolism. Future studies are needed to address unanswered questions regarding internal molecular structure of the fibrin polymer, the structural and molecular basis of its remarkable mechanical properties and the relationship of fibrin biomechanical characteristics with thromboembolism in patients with deep vein thrombosis and ischemic stroke.

Fibrinogen heterogeneity in horses

Background

Fibrinogen heterogeneity has been observed in humans and can influence fibrinogen measurements when using the modified Clauss assay. We hypothesized that fibrinogen heterogeneity also exists in horses.

Objectives

To determine whether fibrinogen heterogeneity exists in horses.

Animals

Five clinically healthy horses from the university equine teaching herd.

Methods

Presumed fibrinogen was purified from pooled citrated plasma and electrophoresis performed. The purified protein was subjected to Western blotting using sheep antiserum against human fibrinogen, and liquid chromatography‐tandem mass spectrometry (LC‐MS/MS).

Results

Gel electrophoresis of nonreduced equine purified protein yielded 2 protein bands (approximately 377 and 318 kDa) that corresponded with the molecular weights of human high molecular weight fibrinogen and low molecular weight fibrinogen fractions, respectively. Electrophoretograms of reduced purified protein, Western blots, and LC‐MS/MS supported that the purified nonreduced protein bands were fibrinogen.

Conclusion

Fibrinogen heterogeneity exists in horses.

Morphometric characterization of fibrinogen's αC regions and their role in fibrin self-assembly and molecular organization

The flexible C-terminal parts of fibrinogen's Aα chains named the αC regions have been shown to play a role in fibrin self-assembly, although many aspects of their structure and functions remain unknown. To examine the involvement of the αC regions in the early stages of fibrin formation, we used high-resolution atomic force microscopy to image fibrinogen and oligomeric fibrin. Plasma-purified full-length human fibrinogen or des-αC fibrinogen lacking most of the αC regions, untreated or treated with thrombin, was imaged. Up to 80% of the potentially existing αC regions were visualized and quantified; they were highly heterogeneous in their length and configurations. Conversion of fibrinogen to fibrin was accompanied by an increase in the incidence and length of the αC regions as well as transitions from more compact conformations, such as a globule on a string, to extended and more flexible offshoots. Concurrent dynamic turbidimetry, confocal microscopy, and scanning electron microscopy revealed that trimming of the αC regions slowed down fibrin formation, which correlated with longer protofibrils, thinner fibers, and a denser network. No structural distinctions, except for the incidence of the αC regions, were revealed in the laterally aggregated protofibrils made of the full-length or des-αC fibrinogens, suggesting a pure kinetic effect of the αC regions on the fibrin architecture. This work provides a structural molecular basis for the promoting role of the αC regions in the early stages of fibrin self-assembly and reveals this stage of fibrin formation as a potential therapeutic target to modulate the structure and mechanical properties of blood clots.

Molecular weight fibrinogen variants alter gene expression and functional characteristics of human endothelial cells

BACKGROUND

Fibrin is a temporary matrix that not only seals a wound, but also provides a temporary matrix structure for invading cells during wound healing. Two naturally occurring fibrinogen variants, high molecular weight (HMW) and low molecular weight (LMW) fibrinogen, display different properties in supporting angiogenesis in vivo and in vitro.

OBJECTIVES

This study was aimed at investigating the functional characteristics and molecular mechanisms of human microvascular endothelial cells (HMVECs) cultured on HMW and LMW fibrin matrices.

METHODS AND RESULTS

HMVECs on HMW fibrin matrices showed increased proliferation and tube formation as compared with their counterparts on unfractionated and LMW fibrin. Degradation of HMW fibrin was markedly enhanced by the presence of HMVECs, that of LMW fibrin was enhanced only slightly. However, the expression levels of fibrinolysis-regulating proteins and integrins were similar. Subsequent microarray analysis revealed that the expression of 377 genes differed significantly between HMVECs cultured on HMW fibrin and those cultured on LMW fibrin. Among these genes, UNC5B, DLL4 and the DLL4-Notch downstream targets Hey1, Hey2 and Hes1 showed increased expression in HMVECs on LMW fibrin. However, pharmacologic and genetic (DLL4 small interfering RNA) inhibition of DLL4-Notch signaling blunted rather than enhanced proliferation and tube formation by HMVECs on both fibrin variants.

CONCLUSIONS

Heterogeneity in naturally occurring fibrinogen strongly influences endothelial cell proliferation and tube formation, and causes alterations in gene expression, including that of DLL4-Notch. The higher fibrinolytic sensitivity of HMW fibrin in the presence of HMVECs contributes to increased tube formation. Although the expression of DLL4-Notch was altered, it did not explain the enhanced tube formation in HMW fibrin. This study provides new perspectives for biological and tissue engineering applications.

Fibrin structure and wound healing

Fibrinogen and fibrin play an important role in blood clotting, fibrinolysis, cellular and matrix interactions, inflammation, wound healing, angiogenesis, and neoplasia. The contribution of fibrin(ogen) to these processes largely depends not only on the characteristics of the fibrin(ogen) itself, but also on interactions between specific-binding sites on fibrin(ogen), pro-enzymes, clotting factors, enzyme inhibitors, and cell receptors. In this review, the molecular and cellular biology of fibrin(ogen) is reviewed in the context of cutaneous wound repair. The outcome of wound healing depends largely on the fibrin structure, such as the thickness of the fibers, the number of branch points, the porosity, and the permeability. The binding of fibrin(ogen) to hemostasis proteins and platelets as well as to several different cells such as endothelial cells, smooth muscle cells, fibroblasts, leukocytes, and keratinocytes is indispensable during the process of wound repair. High-molecular-weight and low-molecular-weight fibrinogen, two naturally occurring variants of fibrin, are important determinants of angiogenesis and differ in their cell growth stimulation, clotting rate, and fibrin polymerization characteristics. Fibrin sealants have been investigated as matrices to promote wound healing. These sealants may also be an ideal delivery vehicle to deliver extra cells for the treatment of chronic wounds.

Fibrinogen heterogeneity: inherited and noninherited

PURPOSE OF REVIEW

Many noninherited or inherited variations have been described in fibrinogen and they may affect the different functions of fibrinogen.

RECENT FINDINGS

A number of the acquired variations in fibrinogen affect the properties of the fibrinogen molecule, such as the conversion rate to fibrin and/or the characteristics of the fibrin clot. Also, genetic polymorphisms are known that can affect the function of the fibrinogen molecule. In addition, some other genetic variants are associated with plasma levels of fibrinogen and with the increase of fibrinogen levels during an acute-phase reaction.

SUMMARY

In this review the authors discuss the noninherited and inherited variations of fibrinogen and the clinical implications (e.g. when determining the risk of cardiovascular disease).

Fibrinogen and high molecular weight fibrinogen during and after normal pregnancy

INTRODUCTION

Pregnancy has recently been described as a generalized intravascular inflammatory response to the conceptus. Total fibrinogen concentrations increase during pregnancy. The percentage high molecular weight fibrinogen (HMW-Fg) of the concentration total fibrinogen is known to increase during acute-phase conditions like inflammation. Therefore, we investigated whether the percentage high molecular weight fibrinogen increases during normal pregnancy.

MATERIALS AND METHODS

Eighteen healthy nulliparous women with uncomplicated pregnancies with normal course and outcome participated in this study. Five blood samples were drawn from every woman in the gestational age periods 9 to 16, 17 to 24, 25 to 33 and 34 to 42 weeks and at 12 to 20 weeks after delivery. Total fibrinogen concentrations were determined according to Clauss and the percentage high molecular weight fibrinogen was assessed by SDS-electrophoresis and densitometry after isolation of fibrinogen by precipitation. One-way analysis of variance (ANOVA) was used to evaluate differences between gestational age periods and correlation coefficients were calculated by Pearson's method.

RESULTS

Total fibrinogen concentrations increased with advancing gestational age and decreased after delivery. The percentage high molecular weight fibrinogen of the total fibrinogen remained unaltered during and after pregnancy.

CONCLUSIONS

During normal pregnancy, there is an increase of total fibrinogen concentrations with advancing gestational age, without a rise in percentage high molecular weight fibrinogen. After delivery, the total fibrinogen returns to baseline concentrations.

Increased high molecular weight fibrinogen in pre-eclampsia

INTRODUCTION

The major coagulation protein fibrinogen (Fg) is a heterogeneous protein with three main fractions: high molecular weight fibrinogen (HMW-Fg), low molecular weight fibrinogen (LMW-Fg) and low molecular weight' fibrinogen. The clottability of high molecular weight fibrinogen is highest as compared to the other fractions. Pre-eclampsia is associated with a state of hypercoagulability, and with an increase of fibrinogen concentration. The aim of the present study was to examine if the increased total fibrinogen plasma concentration in patients with pre-eclampsia is associated with a change in distribution of the main fibrinogen fractions.

MATERIAL AND METHODS

Plasma was collected from 14 patients with pre-eclampsia and from 14 healthy pregnant matched controls. Total fibrinogen concentrations were determined according to Clauss. The percentage high molecular weight fibrinogen was assessed by SDS-electrophoresis and densitometry after isolation of fibrinogen by precipitation. The study groups were compared by the Mann-Whitney U-test.

RESULTS

The median (range) total fibrinogen concentration in the pre-eclampsia group was 5.04 (3.25-6.51) g/l and in the control group 4.19 (3.61-5.38) g/l (p<0.05). The median (range) percentage high molecular weight fibrinogen was 76.5 (69.6-84.0)% and 73.0 (69.0-78.9)% in the pre-eclampsia and control group, respectively (p<0.05).

CONCLUSIONS

In pre-eclampsia, the concentration of total fibrinogen is increased and the percentage high molecular weight fibrinogen is also slightly higher than in normal pregnancy. These results may be a reflection of the exaggerated inflammatory response, and subsequent endothelial activation, which are currently believed to be the key pathophysiological mechanisms in pre-eclampsia.

Effect of genetic background and diet on plasma fibrinogen in mice. Possible relation with susceptibility to atherosclerosis

Many epidemiological studies suggest that elevated plasma fibrinogen concentrations form one of the most important independent risk factors in blood for cardiovascular disease and particularly atherosclerosis in humans. To clarify the effect of genetic factors, diets and their interactions on plasma fibrinogen concentrations, we examined plasma fibrinogen levels in four strains of mice, which differ in their susceptibility to cholesterol-induced atherosclerosis. When maintained on basal diet, two strains 129/J and C3H/HeJ exhibited a significantly higher plasma fibrinogen concentration (2.1 and 1.9 mg/ml) than C57BL/6J and BALB/C strains (1.5 and 1.4 mg/ml). The strongest and most rapid (1 week) increase of plasma fibrinogen (by all semi-synthetic diets) is observed in C57BL/6J mice, which are known to be highly susceptible to diet-induced atherosclerosis. After a period of 8 weeks an increase in plasma fibrinogen of approximately 30-50% was observed in all strains on all semi-synthetic diets. Remarkably, no increase was observed in the fibrinogen Aalpha- Bbeta- and gamma-chain mRNA levels in the liver on the same diets. These mRNA levels were even decreased by approximately 20-50% in all strains on an extremely atherogenic diet. It was found that: genetic background determines the plasma fibrinogen levels on basal diet; plasma fibrinogen levels are altered by diet; the extent of these changes depends on the genetic background: surprisingly, this increase of fibrinogen in plasma is independent of transcription; the diet-induced increase of fibrinogen was very fast in the very highly atherosclerosis-susceptible strain C57BL/6J having a low basal fibrinogen level, and very slow in the atherosclerosis-resistant strain C3H/HeJ having a high basal fibrinogen level. It might be concluded that it is the kinetics of the response of fibrinogen to diet rather than the actual level, which relates to atherosclerosis susceptibility.

Fibrinogen fractions in the third trimester of pregnancy and in puerperium

The concentration of fibrinogen, its fractions, and the concentration of C-reactive protein were determined in 45 healthy pregnant women before and after vaginal (27) or caesarean section 18 delivery. The control group consisted of 33 blood donors. In pregnancy, increased concentration of total fibrinogen and its fractions and a normal concentration of C-reactive protein were noted. Three days after vaginal delivery the concentration of total and high molecular weight fibrinogen fraction decreased slightly and the ratio of high to low molecular weight fibrinogen increased. After caesarean section both total and high molecular weight fibrinogen and the ratio of high to low molecular weight fibrinogen increased. The C-reactive protein concentration increased after either type of delivery. The degree of augmentation, however, was ten times as strong after caesarean section. In all women at the end of puerperium the concentrations of the compounds studied returned to normal values. The results suggest that the mechanisms leading to the increase of fibrinogen during pregnancy and after delivery are different. The increase of all fibrinogen fractions in pregnancy may depend first of all on hormones, whereas the increased proportion of high molecular weight to low molecular weight fibrinogen after delivery depends on the acute phase reaction. The degree of this reaction depends on the type of delivery.

Postoperative plasma interleukin-6 in patients with renal cancer correlates with C-reactive protein but not with total fibrinogen or with high molecular weight fibrinogen fraction

The concentration of fibrinogen (Fb) and its fractions, the levels of interleukin-6 (I1-6), C-reactive protein (CRP), and immunoglobulin G (IgG) were determined in 38 patients operated on because of renal cancer. The increased Fb and I1-6 concentrations were found in approximately one-half of the patients with malignancy. The relations among the high molecular weight (HMW) and two low molecular weight (LMW and LMW') fibrinogen fractions in these patients before surgery did not differ from the corresponding relations in normal subjects. The levels of all (except IgG) compounds studied increased after surgery and the peak of I1-6 was observed on the first postoperative day but that of CRP on the third day. The concentrations of total Fb and of its HMW fraction were the highest also on the third postoperative day and this was in contrast with the decline of low molecular weight fractions at the same time. These variations of estimated variables can be regarded as being relevant to the acute phase response. We have noted a correlation between the maximal concentrations of I1-6 and CRP, but not between the corresponding concentrations of Il-6 and total Fb or HMW Fb; this may suggest that the concentration of Fb is also under the control of a factor other than I1-6.

Changes in Fibrinogen After rt-PA Administration

Among patients participating in the TIMI-II protocol, there was a variability in the fibrinolytic re sponse to recombinant tissue plasminogen activator (rt- PA). A cohort of 20 TIMI-II patients was selected for detailed study because their responses to rt-PA varied widely in the degree of fibrin(ogen)olysis. Patient plasmas were analyzed by immunoblotting for changes in fibrino gen and plasminogen. Measurements of fibrinogen, fibrin ogen degradation product (FDP), D-dimer, Bβ 1-42, plas minogen, and t-PA were also correlated. Three patterns of response to rt-PA were identified: Group A ( n = 4) had fibrinogenolysis without fibrinolysis; Group B ( n = 11) had fibrinolysis and mild fibrinogenolysis; and Group C ( n = 5) had fibrinolysis with intense fibrinogenolysis. Group C patients also demonstrated qualitative changes in high- molecular-weight (HMW) and low-molecular-weight (LMW) fibrinogens, whereas Group A and B patients demonstrated only mild alterations in fibrinogen compo sition. Plasmin-inhibitor complexes were identified in all three groups. All patients had both plasmin-α2-anti plasmin and plasmin-α2-macroglobulin complexes at the 50-min time point. The concentration of pretreatment plasminogen correlated with the degree of fibrinogenoly sis.

Interactions of human blood platelets with three circulating plasma fibrinogens of different molecular weights

The ability of three naturally occurring human fibrinogen species, HMW, LMW and LMW', to support ADP-induced platelet aggregation was investigated and compared to their ability to bind to gel-filtered platelets. Whereas HMW had intact subunit chains, LMW and LMW' had defined lesions in the C-terminal part of one (LMW) or both (LMW') of the A alpha-chains. The ADP-induced aggregation of gel-filtered platelets in the presence of LMW was about 75 per cent of that obtained with HMW (0.2 mumol/l of fibrinogen and 10 mumol/l of ADP). A mixture of equal amounts of LMW and LMW' showed around 50% decrease in aggregation. Compared to these difference in aggregation co-factor function, direct binding to gel-filtered platelets was less affected by the A alpha-chain degradation. However, a difference between LMW, LMW' and HMW binding was significant when the fibrinogens labelled with two different isotopes of iodine were present simultaneously. These results demonstrate that naturally occurring plasma fibrinogens differ in their interactions with platelets. As the HMW/LMW ratio changes during the acute phase, this may be of physiological significance.