Fibrinogen αC domain: Its importance in physiopathology

Identification of fibrinogen Aα-chain as an amyloid precursor protein in a case of canine coronary artery amyloidosis

Spontaneous amyloid deposition occurs in the coronary arteries in older dogs, but the pathogenesis, including the precursor protein, is not known. We conducted proteome analysis using liquid chromatography-tandem mass spectrometry to identify the amyloid precursor protein in a 13-y-old male mixed-breed dog with amyloid deposition in intramural coronary arteries, and identified fibrinogen Aα-chain. This form of amyloidosis is characterized by the deposition of amyloid derived from fibrinogen Aα-chains, which has been reported previously only in humans and Japanese squirrels. Within amyloid deposits in the intramural coronary arteries of our canine case, the αC connector region was identified in abundance, whereas in humans and Japanese squirrels, the amyloid arises from the αC domain. Thus, even if amyloid is formed by the same protein, the amyloid-forming peptide and the anatomic site of deposition in our case differed from that in humans and Japanese squirrels.

Unveiling the Architecture of Human Fibrinogen: A Full-Length Structural Model

Fibrinogen is a protein involved in the haemostasis process playing a central role by forming the fibrin clot. An understanding of protein structure is vital to determining biological function. Despite many studies on the fibrin polymerization process, its molecular mechanism remains elusive mainly due to the absence of a full-length three-dimensional model of human fibrinogen. Amino- and carboxyl-terminal regions of the three pairs of chains that form this molecule are missing in the crystallographic structure, being the carboxyl-terminal of the Aα chain the most affected with a section of more than 400 amino acids missing. To have a full structure of the fibrinogen molecule would allow the creation of a model of protofibril, shedding light into the fibrin formation process through computational techniques such as molecular dynamics simulations. Absent regions were explored using homology modelling and coarse-grained molecular dynamics simulations. Later on, the model was refined and stabilized with atomistic molecular dynamic simulations. In the present study, we obtained the first realistic full-length structure of fibrinogen, with features in accordance with previous results obtained by experimental techniques.

Clinical, Laboratory, and Molecular Aspects of Congenital Fibrinogen Disorders

Congenital fibrinogen disorders (CFDs) include afibrinogenemia, hypofibrinogenemia, dysfibrinogenemia, and hypodysfibrinogenemia. The fibrinogen levels, the clinical features, and the genotype define several sub-types, each with specific biological and clinical issues. The diagnosis of CFDs is based on the measurement of activity and antigen fibrinogen levels as well as on the genotype. While relatively easy in quantitative fibrinogen disorders, the diagnosis can be more challenging in qualitative fibrinogen disorders depending on the reagents and methods used, and the underlying fibrinogen variants. Overall, quantitative and qualitative fibrinogen defects lead to a decrease in clottability, and usually in a bleeding tendency. The severity of the bleeding phenotype is moreover related to the concentration of fibrinogen. Paradoxically, patients with CFDs are also at risk of thrombotic events. The impact of the causative mutation on the structure and the fibrinogen level is one of the determinants of the thrombotic profile. Given the major role of fibrinogen in pregnancy, women with CFDs are particularly at risk of obstetrical adverse outcomes. The study of the fibrin clot properties can help to define the impact of fibrinogen disorders on the fibrin network. The development of next generation sequencing now allows the identification of genetic modifiers able to influence the global hemostasis balance in CFDs. Their integration in the assessment of the patient risk on an individual scale is an important step toward precision medicine in patients with such a heterogeneous clinical course.

Exploiting the Molecular Properties of Fibrinogen to Control Bleeding Following Vascular Injury

The plasma protein fibrinogen is critical for haemostasis and wound healing, serving as the structural foundation of the blood clot. Through a complex interaction between coagulation factors, the soluble plasma fibrinogen is converted to insoluble fibrin networks, which form the skeleton of the blood clot, an essential step to limit blood loss after vascular trauma. This review examines the molecular mechanisms by which fibrinogen modulates bleeding, focusing on its interactions with other proteins that maintain fibrin network stability and prevent premature breakdown. Moreover, we also cover the role of fibrinogen in ensuring clot stability through the physiological interaction with platelets. We address the therapeutic applications of fibrinogen across various clinical contexts, including trauma-induced coagulopathy, postpartum haemorrhage, and cardiac surgery. Importantly, a full understanding of protein function will allow the development of new therapeutics to limit blood loss following vascular trauma, which remains a key cause of mortality worldwide. While current management strategies help with blood loss following vascular injury, they are far from perfect and future research should prioritise refining fibrinogen replacement strategies and developing novel agents to stabilise the fibrin network. Exploiting fibrinogen's molecular properties holds significant potential for improving outcomes in trauma care, surgical interventions and obstetric haemorrhage.

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.

Mice expressing nonpolymerizable fibrinogen have reduced arterial and venous thrombosis with preserved hemostasis

ABSTRACT

Elevated circulating fibrinogen levels correlate with increased risk for both cardiovascular and venous thromboembolic diseases. In vitro studies show that formation of a highly dense fibrin matrix is a major determinant of clot structure and stability. Here, we analyzed the impact of nonpolymerizable fibrinogen on arterial and venous thrombosis as well as hemostasis in vivo using FgaEK mice that express normal levels of a fibrinogen that cannot be cleaved by thrombin. In a model of carotid artery thrombosis, FgaWT/EK and FgaEK/EK mice were protected from occlusion with 4% ferric chloride (FeCl3) challenges compared with wild-type (FgaWT/WT) mice, but this protection was lost, with injuries driven by higher concentrations of FeCl3. In contrast, fibrinogen-deficient (Fga-/-) mice showed no evidence of occlusion, even with high-concentration FeCl3 challenge. Fibrinogen-dependent platelet aggregation and intraplatelet fibrinogen content were similar in FgaWT/WT, FgaWT/EK, and FgaEK/EK mice, consistent with preserved fibrinogen-platelet interactions that support arterial thrombosis with severe challenge. In an inferior vena cava stasis model of venous thrombosis, FgaEK/EK mice had near complete protection from thrombus formation. FgaWT/EK mice also displayed reduced thrombus incidence and a significant reduction in thrombus mass relative to FgaWT/WT mice after inferior vena cava stasis, suggesting that partial expression of nonpolymerizable fibrinogen was sufficient for conferring protection. Notably, FgaWT/EK and FgaEK/EK mice had preserved hemostasis in multiple models as well as normal wound healing times after skin incision, unlike Fga-/- mice that displayed significant bleeding and delayed healing. These findings indicate that a nonpolymerizable fibrinogen variant can significantly suppress occlusive thrombosis while preserving hemostatic potential in vivo.

Fibrinogen Bonn (p. Arg510Cys) in the Aα-Chain Is Associated with High Risk of Venous Thrombosis

INTRODUCTION

 Inherited dysfibrinogenemia is a qualitative defect of fibrinogen caused by various mutations among three fibrinogen genes. Dysfibrinogenemia can be associated with an increased risk of thrombosis, bleeding, or both. Here, we report a 36-year-old female with dysfibrinogenemia who experienced two successful pregnancies under thromboprophylaxis after cerebral venous sinus thrombosis (CVST).

PATIENTS AND METHODS

 In addition to plasmatic coagulation tests, fibrinogen genes FGA, FGB, and FGG were screened using direct genomic DNA sequencing. The structural-functional implications of the detected mutation were analyzed in silico.

RESULTS

 Inherited dysfibrinogenemia was diagnosed in an index patient after CVST in a risk situation. Anticoagulation with warfarin was stopped after 12 months when the first pregnancy was planned. Pregnancy and spontaneous delivery (2020) was uncomplicated. A second pregnancy was interrupted because of acute cytomegalovirus infection and the third pregnancy was successful in 2022. Pregnancies were accompanied by thromboprophylaxis with enoxaparin 40 mg once daily until 6 weeks postpartum. Substitution of fibrinogen has not become necessary in the index patient so far. Genetic analysis revealed a novel missense mutation (p. Arg510Cys) in the FGA gene ("fibrinogen Bonn") in the index patient, as well as an asymptomatic sister, and their father who experienced recurrent pulmonary embolism. Surface exposure of wild-type Arg510 suggested the mutated Cys510 to form nonnative disulfide bonds with surface-exposed reactive cysteines from other plasma proteins like albumin leading to formation of aggregates and impaired fibrinolysis.

CONCLUSIONS

 Fibrinogen Bonn might be associated with an increased risk of thrombosis, possibly due to impaired polymerization.

Functional properties of individual sub-domains of the fibrin(ogen) αC-domains

Background

Fibrinogen is a large polyfunctional plasma protein consisting of a number of structural and functional domains. Among them, two αC-domains, each formed by the amino acid residues Аα392-610, are involved in fibrin polymerization, activation of fibrinolysis, platelet aggregation, and interaction with different cell types. Previous study revealed that each fibrinogen αC-domain consists of the N-terminal and C-terminal sub-domains. The major objections of the present study were to test functional role of these sub-domains in the above mentioned processes.

Methods

To achieve these objections, we used specific proteases to prepare two truncated forms of fibrinogen, fibrinogen desAα505-610 and fibrinogen desAα414-610, missing their N-terminal and both N- and C-terminal sub-domains, respectively.

Results

Our study with these truncated forms using turbidity measurements and electron microscopy revealed that the N- and C-terminal subdomains both contribute to protofibril formation and their lateral aggregation into fibers during fibrin polymerization process. These two sub-domains also contributed to platelet aggregation with the N-terminal sub-domains playing a more significant role in this process. At the same time, the C-terminal sub-domains make the major contribution to the plasminogen activation process. Further, our experiments revealed that the C-terminal sub-domains are involved in endothelial cell viability and migration of cancer cells.

Conclusions

Thus, the results obtained establish the functional role of individual sub-domains of the αC-domains in fibrin polymerization, activation of fibrinolytic system, platelet aggregation, and cellular interactions.

General significance

The present study expands our understanding of the functional role of individual fibrinogen domains and their specific portions in various fibrin(ogen)-dependent processes.

The Story of the Fibrin(ogen) αC-Domains: Evolution of Our View on Their Structure and Interactions

Although much has been established concerning the overall structure and function of fibrinogen, much less has been known about its two αC regions, each consisting of an αC-connector and an αC-domain, but new information has been accumulating. This review summarizes the state of our current knowledge of the structure and interactions of fibrinogen's αC regions. A series of studies with isolated αC regions and their fragments demonstrated that the αC-domain forms compact ordered structures consisting of N- and C-terminal subdomains including β sheets and suggested that the αC-connector has a poly(L-proline) type II structure. Functionally, the αC-domains interact intramolecularly with each other and with the central region of the molecule, first demonstrated by electron microscopy and then quantified by optical trap force spectroscopy. Upon conversion of fibrinogen into fibrin, the αC-domains switch from intra- to intermolecular interactions to form ordered αC polymers. The formation of αC polymers occurs mainly through the homophilic interaction between the N-terminal subdomains; interaction between the C-terminal subdomains and the αC-connectors also contributes to this process. Considerable evidence supports the idea that the αC-regions accelerate fibrin polymerization and affect the final structure of fibrin clots. The interactions between αC-regions are important for the mechanical properties of clots, increasing their stiffness and extensibility. Conversion of fibrinogen into fibrin results in exposure of multiple binding sites in its αC regions, providing interaction of fibrin with different proteins and cell types during hemostasis and wound healing. This heretofore mysterious part of the fibrinogen molecule is finally giving up its secrets.

Congenital (hypo-)dysfibrinogenemia and bleeding: A systematic literature review

Ranging from bleeding to thrombosis, the clinical features of congenital fibrinogen qualitative disorders, including dysfibrinogenemia and hypodysfibrinogenemia, are highly heterogeneous. Although the associations between some specific fibrinogen mutations and the thrombotic phenotypes have been well elucidated, the underlying mechanism between fibrinogen variants and bleeding events remains underestimated. After systematically reviewing the literature of (hypo-)dysfibrinogenemia patients with bleeding phenotypes, we identified several well-characterized bleeding-related fibrinogen variants in those patients. Several possible pathomechanisms are proposed to explain the genotype-phenotype associations: 1, mutations in the NH₂-terminal portion of the Aα chain hamper fibrinogen fitting into the active site cleft of thrombin and drastically slow the conversion of fibrinogen into monomeric fibrin; 2, mutations adding new N-linked glycosylation sites introduce bulky and negatively charged carbohydrate side chains and undermine the alignment of fibrin monomers during polymerization; 3, mutations generating unpaired cysteine form extra disulfide bonds between the abnormal fibrinogen chains and produce highly branched and fragile fibrin networks; 4, truncation mutations in the fibrinogen αC regions impair the lateral fibril aggregation, as well as factor XIII crosslinking, endothelial cell and platelet binding. These established relationships between specific variants and the bleeding tendency will help manage (hypo-)dysfibrinogenemia patients to avoid adverse bleeding outcomes.

Potential Biomarkers of Acute Ischemic Stroke Etiology Revealed by Mass Spectrometry-Based Proteomic Characterization of Formalin-Fixed Paraffin-Embedded Blood Clots

Background and Aims

Besides the crucial role in the treatment of acute ischemic stroke (AIS), mechanical thrombectomy represents a unique opportunity for researchers to study the retrieved clots, with the possibility of unveiling biological patterns linked to stroke pathophysiology and etiology. We aimed to develop a shotgun proteomic approach to study and compare the proteome of formalin-fixed paraffin-embedded (FFPE) cardioembolic and large artery atherosclerotic (LAA) clots.

Methods

We used 16 cardioembolic and 15 LAA FFPE thrombi from 31 AIS patients. The thrombus proteome was analyzed by label-free quantitative liquid chromatography-tandem mass spectrometry (LC-MS/MS). MaxQuant v1.5.2.8 and Perseus v.1.6.15.0 were used for bioinformatics analysis. Protein classes were identified using the PANTHER database and the STRING database was used to predict protein interactions.

Results

We identified 1,581 protein groups as part of the AIS thrombus proteome. Fourteen significantly differentially abundant proteins across the two etiologies were identified. Four proteins involved in the ubiquitin-proteasome pathway, blood coagulation or plasminogen activating cascade were identified as significantly abundant in LAA clots. Ten proteins involved in the ubiquitin proteasome-pathway, cytoskeletal remodeling of platelets, platelet adhesion or blood coagulation were identified as significantly abundant in cardioembolic clots.

Conclusion

Our results outlined a set of 14 proteins for a proof-of-principle characterization of cardioembolic and LAA FFPE clots, advancing the proteome profile of AIS human thrombi and understanding the pathophysiology of ischemic stroke.

Hypofibrinogenemia with preserved hemostasis and protection from thrombosis in mice with an Fga truncation mutation

Genetic variants within the fibrinogen Aα chain encoding the αC-region commonly result in hypodysfibrinogenemia in patients. However, the (patho)physiological consequences and underlying mechanisms of such mutations remain undefined. Here, we generated Fga270 mice carrying a premature termination codon within the Fga gene at residue 271. The Fga270 mutation was compatible with Mendelian inheritance for offspring of heterozygous crosses. Adult Fga270/270 mice were hypofibrinogenemic with ∼10% plasma fibrinogen levels relative to FgaWT/WT mice, linked to 90% reduction in hepatic Fga messenger RNA (mRNA) because of nonsense-mediated decay of the mutant mRNA. Fga270/270 mice had preserved hemostatic potential in vitro and in vivo in models of tail bleeding and laser-induced saphenous vein injury, whereas Fga-/- mice had continuous bleeding. Platelets from FgaWT/WT and Fga270/270 mice displayed comparable initial aggregation following adenosine 5'-diphosphate stimulation, but Fga270/270 platelets quickly disaggregated. Despite ∼10% plasma fibrinogen, the fibrinogen level in Fga270/270 platelets was ∼30% of FgaWT/WT platelets with a compensatory increase in fibronectin. Notably, Fga270/270 mice showed complete protection from thrombosis in the inferior vena cava stasis model. In a model of Staphylococcus aureus peritonitis, Fga270/270 mice supported local, fibrinogen-mediated bacterial clearance and host survival comparable to FgaWT/WT, unlike Fga-/- mice. Decreasing the normal fibrinogen levels to ∼10% with small interfering RNA in mice also provided significant protection from venous thrombosis without compromising hemostatic potential and antimicrobial function. These findings both reveal novel molecular mechanisms underpinning fibrinogen αC-region truncation mutations and highlight the concept that selective fibrinogen reduction may be efficacious for limiting thrombosis while preserving hemostatic and immune protective functions.

tipNrich: A Tip-Based N-Terminal Proteome Enrichment Method

The mass spectrometry-based analysis of protein post-translational modifications requires large amounts of sample, complicating the analysis of samples with limited amounts of proteins such as clinical biopsies. Here, we present a tip-based N-terminal analysis method, tipNrich. The entire procedure is processed in a single pipette tip to minimize sample loss, which is so highly optimized to analyze small amounts of proteins, even femtomole-scale of a single protein. With tipNrich, we investigated various single proteins purified from different organisms using a low-resolution mass spectrometer and identified several N-terminal peptides with different Nt-modifications such as ragged N-termini. Furthermore, we applied matrix-assisted laser desorption ionization time-of-flight mass spectrometry to our method for shortening the analysis time. Moreover, we showed that our method could be utilized in disease diagnosis as exemplified by the characterization of wild-type transthyretin amyloidosis patients compared to the healthy individuals based on N-terminome profiling. In summary, tipNrich will satisfy the need of identifying N-terminal peptides even with highly scarce amounts of proteins and of having faster processing time to check the quality of protein products or to characterize N-terminal proteoform-related diseases.

The protective effect of early dietary fat consumption on Alzheimer's disease-related pathology and cognitive function in mice

INTRODUCTION

It has been suggested that obesity may influence Alzheimer's disease (AD) pathogenesis, yet the numerous publications on this topic have inconsistent results and conclusions.

METHODS

Our study examined the effect of varying the timing of high-fat diet (HFD) consumption on AD-related pathology and cognition in transgenic Tg6799 AD mice.

RESULTS

HFD feeding starting at or before 3 months of age, prior to severe AD pathology, had protective effects in AD mice: reduced extracellular amyloid beta (Aβ) deposition, decreased fibrinogen extravasation into the brain parenchyma, and improved cognitive function. However, delaying HFD consumption until 6 months of age, when AD pathology is ubiquitous, reduced these protective effects in AD mice.

DISCUSSION

Overall, we demonstrate that the timeline of HFD consumption may play an important role in how dietary fats affect AD pathogenesis and cognitive function.

Fibrin(ogen) in human disease: both friend and foe

Fibrinogen is an abundant protein synthesized in the liver, present in human blood plasma at concentrations ranging from 1.5-4 g/L in healthy individuals with a normal half-life of 3-5 days. With fibrin, produced by thrombin-mediated cleavage, fibrinogen plays important roles in many physiological processes. Indeed, the formation of a stable blood clot, containing polymerized and cross-linked fibrin, is crucial to prevent blood loss and drive wound healing upon vascular injury. A balance between clotting, notably the conversion of fibrinogen to fibrin, and fibrinolysis, the proteolytic degradation of the fibrin mesh, is essential. Disruption of this equilibrium can cause disease in distinct manners. While some pathological conditions are the consequence of altered levels of fibrinogen, others are related to structural properties of the molecule. The source of fibrinogen expression and the localization of fibrin(ogen) protein also have clinical implications. Low levels of fibrinogen expression have been detected in extra-hepatic tissues, including carcinomas, potentially contributing to disease. Fibrin(ogen) deposits at aberrant sites including the central nervous system or kidney, can also be pathological. In this review, we discuss disorders in which fibrinogen and fibrin are implicated, highlighting mechanisms that may contribute to disease.

Fibrinogen alpha amyloidosis: insights from proteomics

Introduction: Systemic amyloidosis is a diverse group of diseases that, although rare, pose a serious health issue and can lead to organ failure and death. Amyloid typing is essential in determining the causative protein and initiating proper treatment. Mass spectrometry-based proteomics is currently the most sensitive and accurate means of typing amyloid. Areas covered: Amyloidosis can be systemic or localized, acquired or hereditary, and can affect any organ or tissue. Diagnosis requires biopsy, histological analysis, and typing of the causative protein to determine treatment. The kidneys are the most commonly affected organ in systemic disease. Fibrinogen alpha chain amyloidosis (AFib) is the most prevalent form of hereditary renal amyloidosis. Select mutations in the fibrinogen Aα (FGA) gene lead to AFib. Expert commentary: Mass spectrometry is currently the most specific and sensitive method for amyloid typing. Identification of the mutated fibrinogen alpha chain can be difficult in the case of 'private' frameshift mutations, which dramatically change the sequences of the expressed fibrinogen alpha chain. A combination of expert pathologist review, mass spectrometry, and gene sequencing can allow for confident diagnosis and determination of the fibrinogen alpha chain mutated sequence.