Plasminogen Deficiency Significantly Reduces Vascular Wall Disease in a Murine Model of Type IIa Hypercholesterolemia

The fibrinolytic system has been implicated in the genesis and progression of atherosclerosis. It has been reported that a plasminogen (Pg) deficiency (Plg^−/−) exacerbates the progression of atherosclerosis in Apoe^−/− mice. However, the manner in which Plg functions in a low-density lipoprotein-cholesterol (LDL-C)-driven model has not been evaluated. To characterize the effect of Pg in an LDL-C-driven model, mice with a triple deficiency of the LDL-receptor (LDLr), along with the active component (apobec1) of the apolipoprotein B editosome complex, and Pg (L^−/−/A^−/−/Plg^−/−), were generated. Atherosclerotic plaque formation was severely retarded in the absence of Pg. In vitro studies demonstrated that LDL uptake by macrophages was enhanced by plasmin (Pm), whereas circulating levels of LDL were enhanced, relative to L^−/−/A^−/− mice, and VLDL synthesis was suppressed. These results indicated that clearance of lipoproteins in the absence of LDLr may be regulated by Pg/Pm. Conclusions: The results from this study indicate that Pg exacerbates atherosclerosis in an LDL-C model of atherosclerosis and also plays a role in lipoprotein modification and clearance. Therefore, controlling the Pg system on macrophages to prevent foam cell formation would be a novel therapeutic approach.

Abrogation of plasminogen activator inhibitor-1-vitronectin interaction ameliorates acute kidney injury in murine endotoxemia

Sepsis-induced acute kidney injury (AKI) contributes to the high mortality and morbidity in patients. Although the pathogenesis of AKI during sepsis is poorly understood, it is well accepted that plasminogen activator inhibitor-1 (PAI-1) and vitronectin (Vn) are involved in AKI. However, the functional cooperation between PAI-1 and Vn in septic AKI has not been completely elucidated. To address this issue, mice were utilized lacking either PAI-1 (PAI-1^−/−) or expressing a PAI-1-mutant (PAI-1^R101A/Q123K) in which the interaction between PAI-1 and Vn is abrogated, while other functions of PAI-1 are retained. It was found that both PAI-1^−/− and PAI-1^R101A/Q123K mice are associated with decreased renal dysfunction, apoptosis, inflammation, and ERK activation as compared to wild-type (WT) mice after LPS challenge. Also, PAI-1^−/− mice showed attenuated fibrin deposition in the kidneys. Furthermore, a lack of PAI-1 or PAI-1-Vn interaction was found to be associated with an increase in activated Protein C (aPC) in plasma. These results demonstrate that PAI-1, through its interaction with Vn, exerts multiple deleterious mechanisms to induce AKI. Therefore, targeting of the PAI-1-Vn interaction in kidney represents an appealing therapeutic strategy for the treatment of septic AKI by not only altering the fibrinolytic capacity but also regulating PC activity.

Systemic platelet dysfunction is the result of local dysregulated coagulation and platelet activation in the brain in a rat model of isolated traumatic brain injury

Coagulopathy after severe traumatic brain injury (TBI) has been extensively reported. Clinical studies have identified a strong relationship between diminished platelet-rich thrombus formation, responsiveness to adenosine diphosphate agonism, and severity of TBI. The mechanisms that lead to platelet dysfunction in the acute response to TBI are poorly understood. The development of a rodent model of TBI that mimics the coagulopathy observed clinically has recently been reported. Using immunohistochemical techniques and thromboelastography platelet mapping, the current study demonstrated that the expression of coagulation (tissue factor and fibrin) and platelet activation (P-selectin) markers in the injured brain paralleled the alteration in systemic platelet responsiveness to the agonists, adenosine diphosphate and arachodonic acid. Results of this study demonstrate that local procoagulant changes in the injured brain have profound effects on systemic platelet function.

Early platelet dysfunction in a rodent model of blunt traumatic brain injury reflects the acute traumatic coagulopathy found in humans

Acute coagulopathy is a serious complication of traumatic brain injury (TBI) and is of uncertain etiology because of the complex nature of TBI. However, recent work has shown a correlation between mortality and abnormal hemostasis resulting from early platelet dysfunction. The aim of the current study was to develop and characterize a rodent model of TBI that mimics the human coagulopathic condition so that mechanisms of the early acute coagulopathy in TBI can be more readily assessed. Studies utilizing a highly reproducible constrained blunt-force brain injury in rats demonstrate a strong correlation with important postinjury pathological changes that are observed in human TBI patients, namely, diminished platelet responses to agonists, especially adenosine diphosphate (ADP), and subarachnoid bleeding. Additionally, administration of a direct thrombin inhibitor, preinjury, recovers platelet functionality to ADP stimulation, indicating a direct role for excess thrombin production in TBI-induced early platelet dysfunction.

Abnormal whole blood thrombi in humans with inherited platelet receptor defects

To delineate the critical features of platelets required for formation and stability of thrombi, thromboelastography and platelet aggregation measurements were employed on whole blood of normal patients and of those with Bernard-Soulier Syndrome (BSS) and Glanzmann's Thrombasthenia (GT). We found that separation of platelet activation, as assessed by platelet aggregation, from that needed to form viscoelastic stable whole blood thrombi, occurred. In normal human blood, ristocetin and collagen aggregated platelets, but did not induce strong viscoelastic thrombi. However, ADP, arachidonic acid, thrombin, and protease-activated-receptor-1 and -4 agonists, stimulated both processes. During this study, we identified the genetic basis of a very rare double heterozygous GP1b deficiency in a BSS patient, along with a new homozygous GP1b inactivating mutation in another BSS patient. In BSS whole blood, ADP responsiveness, as measured by thrombus strength, was diminished, while ADP-induced platelet aggregation was normal. Further, the platelets of 3 additional GT patients showed very weak whole blood platelet aggregation toward the above agonists and provided whole blood thrombi of very low viscoelastic strength. These results indicate that measurements of platelet counts and platelet aggregability do not necessarily correlate with generation of stable thrombi, a potentially significant feature in patient clinical outcomes.

Severe deficiency of coagulation Factor VII results in spontaneous cardiac fibrosis in mice

Mice genetically modified to produce low levels (approximately 1% of wild-type) of coagulation FVII presented with echocardiographic evidence of heart abnormalities. Decreases in ventricular size and reductions in systolic and diastolic functions were found, suggestive of a restrictive cardiomyopathy and consistent with an infiltrative myopathic process. Microscopic analysis of mouse hearts showed severe patchy fibrosis in the low-FVII mice. Haemosiderin deposition was discovered in hearts of these mice, along with increases in inflammatory cell number, ultimately resulting in widespread collagen deposition. Significant increases in mRNA levels of TGFbeta, TNFalpha and several matrix metalloproteinases in low-FVII mice, beginning at early ages, supported a state of cardiac remodelling associated with the fibrotic pathology. Mechanistic time-course studies suggested that cardiac fibrosis in low-FVII mice originated from bleeding in heart tissue, resulting in the recruitment of leukocytes, which released inflammatory mediators and induced collagen synthesis and secretion. These events led to necrosis of cardiomyocytes and collagen deposition, characteristics of cardiac fibrosis. The results of this study demonstrated that haemorrhagic and inflammatory responses to a severe FVII deficiency resulted in the development of cardiac fibrosis, observed echocardiographically as a restrictive cardiomyopathy, with compromised ventricular diastolic and systolic functions.

A missense mutation in the sodium phosphate co-transporter Slc34a1 impairs phosphate homeostasis

The sodium phosphate co-transporters Npt2a and Npt2c play important roles in the regulation of phosphate homeostasis. Slc34a1, the gene encoding Npt2a, resides downstream of the gene encoding coagulation factor XII (f12) and was inadvertently modified while generating f12(-/-) mice. In this report, the renal consequences of this modification are described. The combined single allelic mutant Slc34a1m contains two point mutations in exon 13: A499V is located in intracellular loop 5, and V528M is located in transmembrane domain 11. In addition to the expected coagulopathy of the f12(-/-) phenotype, mice homozygous for the double allelic modification (f12(-/-)/slc34a1(m/m)) displayed hypophosphatemia, hypercalcemia, elevated levels of alkaline phosphatase, urolithiasis, and hydronephrosis. Strategic cross-breedings demonstrated that the kidney-related pathology was associated only with autosomal recessive transmission of the slc34a1(m) gene and was not influenced by the simultaneous inactivation of f12. Npt2a[V528M] could be properly expressed in opossum kidney cells, but Npt2a[A499V] could not. These results suggest that a single amino acid substitution in Npt2a can lead to improper translocation of the protein to the cell membrane, disturbance of phosphate homeostasis, and renal calcification. Whether point mutations in the SLC34A1 gene can lead to hypophosphatemia and nephrolithiasis in humans remains unknown.

Focal arterial inflammation is augmented in mice with a deficiency of the protein C gene

Increased risk of thrombosis, with propitious conditions for fibrin deposition, along with upregulation of inflammation, are important factors that enhance plaque formation in atherosclerosis. Evidence supporting the role of anticoagulant protein C (PC) as an inflammatory agent has emerged, supplementing its well-known function as an anticoagulant. Thus, we sought to examine whether a PC deficiency would lead to an enhanced response to an acute arterial hyperplasic challenge. The presentation of early arterial inflammation was studied using a copper/silicone arterial cuff model of accelerated focal neointimal remodeling in mice with a heterozygous total deficiency of PC (PC+/-). Increased inflammation, cell proliferation, cell migration, fibrin elevation, and tissue necrosis were observed in the treated arteries of PC+/- mice, as compared to arteries of equally challenged age- and gender-matched WT mice. These results indicate that PC+/- mice subjected to this challenge displayed enhanced focal arterial inflammation and thrombosis, leading to larger neointimas and subsequent localized occlusion, as compared to their WT counterparts.

A fibrinogen deficiency accelerates the initiation of LDL cholesterol-driven atherosclerosis via thrombin generation and platelet activation in genetically predisposed mice

Mice with combined deficiencies of the low-density lipoprotein receptor (LDLR(-/-)) and the catalytic component of an apolipoprotein B-edisome complex (APOBEC1(-/-)) that converts apoB-100 to apoB-48 have been characterized, and this model of LDL cholesterol-driven atherosclerosis was applied to an investigation of the role of fibrinogen (Fg) in the genesis and progression of the plaque. LDLR(-/-)/APOBEC1(-/-)/FG(-/-) (L(-/-)/A(-/-)/FG(-/-)) triple-deficient mice presented more advanced plaque in their aortic trees and aortic sinuses at 24, 36, and 48 weeks of age compared to L(-/-)/A(-/-) mice, a feature that may result from enhanced platelet activation in these former mice. This is supported by the presence of hypercoagulability, increased CD61 and CD62P on resting platelets, and higher plasma soluble P-selectin in L(-/-)/A(-/-)/FG(-/-) mice as compared to L(-/-)/A(-/-), FG(-/-), or wild-type mice. The elevated higher molecular weight forms of von Willebrand factor (VWF) in L(-/-)/A(-/-)/FG(-/-) mice, revealed by increased VWF collagen binding activity, perhaps resulting from down-regulation of its cleaving metalloproteinase, ADAMTS13, further indicates enhanced platelet activation. Thus, the earlier arterial plaque deposition in L(-/-)/A(-/-)/FG(-/-) mice appears to contain a contribution from enhanced levels of thrombin and activated platelets, a synergistic consequence of an Fg deficiency combined with a high LDL cholesterol concentration.

Enhanced in Vitro Proliferation of Aortic Endothelial Cells from Plasminogen Activator Inhibitor-1-deficient Mice

A number of studies have identified a role for plasminogen activator inhibitor-1 (PAI-1) in regulating angiogenesis, although results from these investigations have been controversial. Among key cellular components of an angiogenic vessel are endothelial cells (ECs), which are known to express several components of the fibrinolytic system, including PAI-1. Thus, alterations in expression of this protein may have direct effects on cell functions involved in vascular development. In this study, ECs were isolated from sections of murine arterial trees from wild-type and PAI-1-deficient mice, and low passage (passages 3-4) homogeneous subpopulations of these cells were obtained by immunomagnetic absorption to antibodies against CD105/CD106. The homogeneity of these cells was further assessed by immunohistochemistry and quantitative real-time reverse transcription-PCR analysis of a number of EC markers. Comparative analyses of EC proliferation (one event associated with angiogenesis) in wild-type and PAI-1-deficient ECs demonstrated enhanced rates of cell growth for PAI-1-deficient cells relative to wild-type cells. Additional studies demonstrated similar levels of both vascular endothelial growth factor (VEGF) mRNA and protein and enhanced levels of VEGF receptor-1 (Flt-1) mRNA in PAI-1-deficient cells relative to wild-type cells. Immunohistochemical analyses indicated that phosphorylation of Akt was also enhanced in PAI-1-deficient cells, implicating VEGF-induced cell signaling alterations in PAI-1-deficient cells, the result of which may contribute to alterations in cell proliferation.

Mice with a severe deficiency of the endothelial protein C receptor gene develop, survive, and reproduce normally, and do not present with enhanced arterial thrombosis after challenge

The endothelial cell Protein C receptor (EPCR) functions to enhance activation of anticoagulant Protein C (PC) by the thrombin/ thrombomodulin (Tm) complex on the surface of the endothelium. This overall system functions in anticoagulation, profibrinolytic, and antiinflammatory responses. Mice with a severe targeted deficiency of this receptor have been generated by integration of exogenous DNA elements into the 5'-untranslated region of the EPCR gene. Despite the retention of the entire endogenous EPCR coding sequence in the altered EPCR gene locus, only very low EPCR message contents were detected in mice by quantitative RT-PCR during embryogenesis and up to at least early adulthood. Immunohistochemical analysis of various regions of the arterial tree of mice up to 4 months of age, employing an anti-murine EPCR antibody, confirmed that undetectable levels of this protein were present in arterial regions during these periods. Despite this, these mice are not more prone to arterial thrombosis after challenge in a FeCl3 carotid artery thrombosis model. Small amounts (<10% of wild-type) of this protein were found in other tissues. Matings of mice homozygous for this deficiency led to normal births and survival of the offspring, in contrast to results by others demonstrating early embryonic lethality of a total EPCR deficiency. These data further show that minimal levels of EPCR are able to support male and female virility, as well as embryonic development, birth, and survival to adulthood.

Fibrinogen stabilizes placental-maternal attachment during embryonic development in the mouse

In humans, maternal fibrinogen (Fg) is required to support pregnancies by maintaining hemostatic balance and stabilizing uteroplacental attachment at the fibrinoid layer found at the fetal-maternal junction. To examine relationships between low Fg levels and early fetal loss, a genetic model of afibrinogenemia was developed. Pregnant mice homozygous for a deletion of the Fg-gamma chain, which results in a total Fg deficiency state (FG(-/-)), aborted the fetuses at the equivalent gestational stage seen in humans. Results obtained from timed matings of FG(-/-) mice showed that vaginal bleeding was initiated as early as embryonic day (E)6 to 7, a critical stage for maternal-fetal vascular development. The condition of afibrinogenemia retarded embryo-placental development, and consistently led to abortion and maternal death at E9.75. Lack of Fg did not alter the extent or distribution pattern of other putative factors of embryo-placental attachment, including laminin, fibronectin, and Factor XIII, indicating that the presence of fibrin(ogen) is required to confer sufficient stability at the placental-decidual interface. The results of these studies demonstrate that maternal Fg plays a critical role in maintenance of pregnancy in mice, both by supporting proper development of fetal-maternal vascular communication and stabilization of embryo implantation.

The characterization of mice with a targeted combined deficiency of protein c and factor XI

Activated protein C functions directly as an anticoagulant and indirectly as a profibrinolytic enzyme. To determine whether the fibrin deposition previously observed in PC(-/-) murine embryos and neonates was mediated through the FXI pathway, PC(+/-)/FXI(-/-) mice were generated and crossbred to produce double-deficient progeny (PC(-/-)/FXI(-/-)). PC(-/-)/FXI(-/-) mice survived the early lethality observed in the PC(-/-)/FXI(+/+) neonates, with the oldest PC(-/-)/FXI(-/-) animal living to 3 months of age. However, the majority of these animals was sedentary and significantly growth-retarded. On sacrifice or natural death, all of these PC(-/-)/FXI(-/-) mice demonstrated massive systemic fibrin deposition with concomitant hemorrhage and fibrosis, as confirmed through histological analyses. Several of these animals also presented with enlarged lymph nodes and extensive lymphatic fluid in the thoracic cavity. Thus, although a number of the PC(-/-)/FXI(-/-) mice survived the lethal perinatal coagulopathy seen in the PC(-/-) neonates, they nonetheless succumbed to overwhelming thrombotic disease in later life. This combined deficiency state provided the first clear indication that the course of a severe thrombotic disorder could be manipulated by blocking the intrinsic pathway and provided the first opportunity to study a total protein C deficiency in an adult animal.

Remodeling of the vessel wall after copper-induced injury is highly attenuated in mice with a total deficiency of plasminogen activator inhibitor-1

Clinical studies have indicated that high plasma levels of fibrinogen, or decreased fibrinolytic potential, are conducive to an increased risk of cardiovascular disease. Other investigations have shown that insoluble fibrin promotes atherosclerotic lesion formation by affecting smooth muscle cell proliferation, collagen deposition, and cholesterol accumulation. To directly assess the physiological impact of an imbalanced fibrinolytic system on both early and late stages of this disease, mice deficient for plasminogen activator inhibitor-1 (PAI-1(-/-)) were used in a model of vascular injury/repair, and the resulting phenotype compared to that of wild-type (WT) mice. A copper-induced arterial injury was found to generate a lesion with characteristics similar to many of the clinical features of atherosclerosis. Fibrin deposition in the injured arterial wall at early (7 days) and late (21 days) times after copper cuff placement was prevalent in WT mice, but was greatly diminished in PAI-1(-/-) mice. A multilayered neointima with enhanced collagen deposition was evident at day 21 in WT mice. In contrast, only diffuse fibrin was identified in the adventitial compartments of arteries from PAI-1(-/-) mice, with no evidence of a neointima. Neovascularization was observed in the adventitia and was more extensive in WT arteries, relative to PAI-1(-/-) arteries. Additionally, enhanced PAI-1 expression and fat deposition were seen only in the arterial walls of WT mice. The results of this study emphasize the involvement of the fibrinolytic system in vascular repair processes after injury and indicate that alterations in the fibrinolytic balance in the vessel wall have a profound effect on the development and progression of vascular lesion formation.