Incorporation of a circulating protein into megakaryocyte and platelet granules

Dissecting the roles of dynamin and clathrin in platelet pinocytosis

Platelets endocytose many molecules from their environment. However, this process of pinocytosis in platelets is poorly understood. Key endocytic regulators such as dynamin, clathrin, CDC42 and Arf6 are expressed in platelets but their roles in pinocytosis is not known. Stimulated platelets form two subpopulations of pro-aggregatory and procoagulant platelets. The effect of stimulation on pinocytosis is also poorly understood. In this study, washed human platelets were treated with a range of endocytosis inhibitors and stimulated using different activators. The rate of pinocytosis was assessed using pHrodo green, a pH-sensitive 10 kDa dextran. In unstimulated platelets, pHrodo fluorescence increased over time and accumulated as intracellular puncta indicating constituently active pinocytosis. Stimulated platelets (both pro-aggregatory and procoagulant) had an elevated pinocytosis rate compared to unstimulated platelets. Dynamin inhibition blocked pinocytosis in unstimulated, pro-aggregatory and procoagulant platelets indicating that most platelet pinocytosis is dynamin dependent. Although pinocytosis was clathrin-independent in unstimulated and procoagulant populations, clathrin partially contributed to pinocytosis in pro-aggregatory platelets.

New insights of platelet endocytosis and its implication for platelet function

Endocytosis constitutes a cellular process in which cells selectively encapsulate surface substances into endocytic vesicles, also known as endosomes, thereby modulating their interaction with the environment. Platelets, as pivotal hematologic elements, play a crucial role not only in regulating coagulation and thrombus formation but also in facilitating tumor invasion and metastasis. Functioning as critical components in the circulatory system, platelets can internalize various endosomal compartments, such as surface receptors, extracellular proteins, small molecules, and pathogens, from the extracellular environment through diverse endocytic pathways, including pinocytosis, phagocytosis, and receptor-mediated endocytosis. We summarize recent advancements in platelet endocytosis, encompassing the catalog of cargoes, regulatory mechanisms, and internal trafficking routes. Furthermore, we describe the influence of endocytosis on platelet regulatory functions and related physiological and pathological processes, aiming to offer foundational insights for future research into platelet endocytosis.

Presence and activity of Fibrinogen like protein 2 in platelets

BACKGROUND

Fibrinogen-like protein 2 (FGL2) is a serine protease capable of converting prothrombin into thrombin (i.e., prothrombinase-like activity) while bypassing the classic coagulation cascade. It has been reported to be expressed by mononuclear blood cells and endothelial cells. There are multiple reports that FGL2 supports tumor development and metastasis. However, in the blood, the origin and functional significance of FGL2 has not been established.

OBJECTIVE

To determine if FGL2, a malignancy related enzyme, is present in platelets.

METHODS

Peripheral blood samples were collected in K2 EDTA tubes. Blood cells and platelets were separated and thoroughly washed to produce plasma-free samples. Procoagulant activity was measured in the cell lysates using a thrombin generation test or an adjusted prothrombin time (PT) test in plasma deficient of factor X. The findings were further supported by confocal microscopy, immunoprecipitation, flow cytometry, enzyme-linked immunosorbent assays and specific inhibition assays.

RESULTS

FGL2 protein was readily detected in platelets. Also, despite being expressed by lymphocytes, FGL2 prothrombinase-like activity was solely detected in platelet samples, but not in white blood cell samples. Quiescent platelets were shown to contain the FGL2 protein in an active form. Upon activation, platelets secreted the active FGL2 into the milieu.

CONCLUSIONS

Active FGL2 is found in platelets. This suggests another role for the involvement of platelets in malignancies.

Suppression of fibrin(ogen)-driven pathologies in disease models through controlled knockdown by lipid nanoparticle delivery of siRNA

Fibrinogen plays a pathologic role in multiple diseases. It contributes to thrombosis and modifies inflammatory and immune responses, supported by studies in mice expressing fibrinogen variants with altered function or with a germline fibrinogen deficiency. However, therapeutic strategies to safely and effectively tailor plasma fibrinogen concentration are lacking. Here, we developed a strategy to tune fibrinogen expression by administering lipid nanoparticle (LNP)-encapsulated small interfering RNA (siRNA) targeting the fibrinogen α chain (siFga). Three distinct LNP-siFga reagents reduced both hepatic Fga messenger RNA and fibrinogen levels in platelets and plasma, with plasma levels decreased to 42%, 16%, and 4% of normal within 1 week of administration. Using the most potent siFga, circulating fibrinogen was controllably decreased to 32%, 14%, and 5% of baseline with 0.5, 1.0, and 2.0 mg/kg doses, respectively. Whole blood from mice treated with siFga formed clots with significantly decreased clot strength ex vivo, but siFga treatment did not compromise hemostasis following saphenous vein puncture or tail transection. In an endotoxemia model, siFga suppressed the acute phase response and decreased plasma fibrinogen, D-dimer, and proinflammatory cytokine levels. In a sterile peritonitis model, siFga restored normal macrophage migration in plasminogen-deficient mice. Finally, treatment of mice with siFga decreased the metastatic potential of tumor cells in a manner comparable to that observed in fibrinogen-deficient mice. The results indicate that siFga causes robust and controllable depletion of fibrinogen and provides the proof-of-concept that this strategy can modulate the pleiotropic effects of fibrinogen in relevant disease models.

The ins and outs of endocytic trafficking in platelet functions

PURPOSE OF REVIEW

Although platelet endocytosis has been recognized in granule cargo loading and the trafficking of several platelet surface receptors, its acute physiological relevance is poorly understood as is its mechanism. The present review discusses the current understanding of platelet endocytosis and its implications for platelet function.

RECENT FINDINGS

Recent studies are beginning to identify and define the proteins that mediate platelet endocytosis. These studies have shown that platelets contain different endosomal compartments and may use multiple endocytic routes to take in circulating molecules and surface proteins. The studies have also shown that platelet endocytosis is involved in several aspects of platelet function such as signaling, spreading, and granule cargo loading.

SUMMARY

Mechanistic studies of platelet endocytosis have shown it to be not only involved in granule cargo loading but also in various other platelet functions important for hemostasis and beyond.

Soluble sortilin is released by activated platelets and its circulating levels are associated with cardiovascular risk factors

OBJECTIVE

Sortilin is involved multilaterally in the development of atherosclerosis. Here, we examine the release of soluble sortilin (sSortilin) from platelets and assess the association between circulating levels of sSoritlin and atherothrombosis such as coronary artery disease (CAD).

METHODS AND RESULTS

sSortilin levels measured in healthy subjects were higher in serum than in plasma (38.4 ± 8.7 vs. 15.8 ± 2.9 ng/mL; p < 0.0001). Platelets were shown to contain both membrane-bound sortilin and its soluble form lacking the cytoplasmic tail. Stimulation of platelet-rich plasma with collagen induced sSortilin release concomitantly with platelet aggregation, and the release was suppressed by aspirin. In clinical evaluation, plasma sSortilin was detected at significantly higher levels in cardiovascular risk patients with hypertension, dyslipidemia, and/or diabetes without CAD (non-CAD, 18.7 ± 3.3 ng/mL) than in patients with CAD under aspirin therapy (17.1 ± 3.6 ng/mL; p < 0.01) or in healthy controls (16.8 ± 2.9 ng/mL; p < 0.01). In these patients, plasma sSortilin levels were significantly correlated with platelet counts (rs = 0.33; p = 0.0085) and showed significant positive associations with cardiovascular risk factors: low-density lipoprotein cholesterol (rs = 0.37; p = 0.0023), triglycerides (rs = 0.28; p = 0.023), and serum uric acid (rs = 0.30; p = 0.017) in non-CAD, and γ-glutamyltransferase (rs = 0.43; p = 0.020) and high-sensitivity C-reactive protein (rs = 0.33, p = 0.0022) in CAD.

CONCLUSION

Elevated plasma sSortilin levels may be associated with in vivo platelet activation and could be a risk factor for atherothrombosis.

Platelet-delivered therapeutics

We have proposed that modified platelets could potentially be used to correct intrinsic platelet defects as well as for targeted delivery of therapeutic molecules to sights of vascular injury. Ectopic expression of proteins within α-granules prior to platelet activation has been achieved for several proteins, including urokinase, factor (F) VIII, and partially for FIX. Potential uses of platelet-directed therapeutics will be discussed, focusing on targeted delivery of urokinase as a thromboprophylactic agent and FVIII for the treatment of hemophilia A patients with intractable inhibitors. This presentation will discuss new strategies that may be useful in the care of patients with vascular injury as well as remaining challenges and limitations of these approaches.

VPS33B regulates protein sorting into and maturation of α-granule progenitor organelles in mouse megakaryocytes

Arthrogryposis, renal dysfunction, and cholestasis (ARC) syndrome is caused by deficiencies in the trafficking proteins VPS33B or VIPAR, and is associated with a bleeding diathesis and a marked reduction in platelet α-granules. We generated a tamoxifen-inducible mouse model of VPS33B deficiency, Vps33b(fl/fl)-ER(T2), and studied the platelet phenotype and α-granule biogenesis. Ultrastructural analysis of Vps33b(fl/fl)-ER(T2) platelets identified a marked reduction in α-granule count and the presence of small granule-like structures in agreement with the platelet phenotype observed in ARC patients. A reduction of ∼65% to 75% was observed in the α-granule proteins von Willebrand factor and P-selectin. Although platelet aggregation responses were not affected, a defect in δ-granule secretion was observed. Under arteriolar shear conditions, Vps33b(fl/fl)-ER(T2) platelets were unable to form stable aggregates, and tail-bleeding measurement revealed a bleeding diathesis. Analysis of bone marrow-derived megakaryocytes (MKs) by conventional and immuno-electron microscopy from Vps33b(fl/fl)-ER(T2) mice revealed a reduction in mature type-II multivesicular bodies (MVB II) and an accumulation of large vacuoles. Proteins that are normally stored in α-granules were underrepresented in MVB II and proplatelet extensions. These results demonstrate that abnormal protein trafficking and impairment in MVB maturation in MKs underlie the α-granule deficiency in Vps33b(fl/fl)-ER(T2) mouse and ARC patients.

Myeloperoxidase induces the priming of platelets

The release of myeloperoxidase (MPO) from polymorphonuclear neutrophils is a hallmark of vascular inflammation and contributes to the pathogenesis of vascular inflammatory processes. However, the effects of MPO on platelets as a contributory mechanism in vascular inflammatory diseases remain unknown. Thus, MPO interaction with platelets and its effects on platelet function were examined. First, dose-dependent binding of MPO (between 1.7 and 13.8nM) to both human and mouse platelets was observed. This was in direct contrast to the absence of MPO in megakaryocytes. MPO was localized both on the surface of and inside platelets. Cytoskeleton inhibition did not prevent MPO localization inside the three-dimensional platelet structure. MPO peroxidase activity was preserved upon the MPO binding to platelets. MPO sequestered in platelets catabolized NO, documented by the decreased production of NO (on average, an approximately 2-fold decrease). MPO treatment did not affect the viability of platelets during short incubations; however, it decreased platelet viability after long-term storage for 7 days (an approximately 2-fold decrease). The activation of platelets by MPO was documented by an MPO-mediated increase in the expression of surface platelet receptors P-selectin and PECAM-1 (of about 5 to 20%) and the increased formation of reactive oxygen species (of about 15 to 200%). However, the activation was only partial, as MPO did not induce the aggregation of platelets nor potentiate platelet response to classical activators. Nor did MPO induce a significant release of the content of granules. The activation of platelets by MPO was connected with increased MPO-treated platelet interaction with polymorphonuclear leukocytes (an approximately 1.2-fold increase) in vitro. In conclusion, it can be suggested that MPO can interact with and activate platelets, which can induce priming of platelets, rather than the classical robust activation of platelets. This can contribute to the development of chronic inflammatory processes in vessels.

Accumulation of platelets in the liver may be an important contributory factor to thrombocytopenia and liver fibrosis in chronic hepatitis C

BACKGROUND

Thrombocytopenia is a marked feature of chronic liver disease and cirrhosis. We tried to clarify whether an accumulation of platelets in the liver contributes to thrombocytopenia and liver fibrosis in chronic liver disease.

METHODS

Thirty-eight patients who underwent hepatectomy for hepatocellular carcinoma (HCC) with hepatitis C virus infection were included. The locations of platelets and Kupffer cells and the expression of platelet-derived growth factor (PDGF) receptor-β and smooth muscle actin (SMA) were identified by immunohistochemistry. Perisinusoidal mesenchymal cells that express PDGF receptor-β and SMA were interpreted as transformed hepatic stellate cells (HSCs).

RESULTS

Patients with cirrhosis had a more extensive platelet area in the liver compared to controls (5601 ± 5611 vs. 564 ± 361 μm(2), p = 0.02), although the blood platelet count significantly decreased along with the progression of liver fibrosis. In cirrhotic liver, most platelets were present in the sinusoidal space of the periportal area with inflammation, where HSCs expressing PDGF receptor-β were frequently observed. In addition, the platelet and Kupffer cell areas were significantly smaller in cancerous tissue than those in noncancerous tissues (platelet area: 492 ± 823 vs. 3643 ± 4055 μm(2), p = 0.001; Kupffer cell area: 450 ± 841 vs. 3012 ± 3051 μm(2), p = 0.001).

CONCLUSIONS

The accumulation of platelets in the liver with chronic hepatitis may be involved in thrombocytopenia and liver fibrosis through the activation of HSCs. In addition, our findings also indicate that both platelets and Kupffer cells decrease in HCC tissues.

A combined proteomic and genetic analysis of the highly variable platelet proteome: from plasmatic proteins and SNPs

High biological variation in protein expression represents a major challenge in clinical proteomics. In a study based on 2D-DIGE, we found that the standardised abundance of only a few proteins varied by more than 50%. While some of the highest variable proteins in platelets of 52 healthy elderly were of plasmatic origin, such as albumin or haptoglobin, absence of several other high-abundant plasma proteins strongly suggests that plasma-derived proteins represent an integral part of the platelet proteome. Amongst the highly variable platelet-derived proteins, two spots were both identified as GSTO1 and assigned to either the wild-type or mutant isoform of SNP A140D. Remarkably, when the spots were considered within the respective genotype groups, their CV decreased to about the median variation. Albeit 2D-DIGE allowed correct genotyping, two individuals seemed to be GSTO1*A140 deficient. Probing 2D-Western blots with novel mAb, however, detected A140 protein as additional spot at pH 8.1, caused by the SNPs E155del and E208K. In contrast to previous studies, we show that GSTO1 protein is expressed in vivo, despite the deletion E155. Our data indicate that incorporation of exogenous proteins and genetic polymorphisms of endogenous proteins represent the main source of extreme biological variation in the platelet proteome.

Serglycin proteoglycan deletion in mouse platelets: physiological effects and their implications for platelet contributions to thrombosis, inflammation, atherosclerosis, and metastasis

Serglycin is found in all nucleated hematopoietic cells and platelets, blood vessels, various reproductive and developmental tissues, and in chondrocytes. The serglycin knockout mouse has demonstrated that this proteoglycan is required for proper generation and function of secretory granules in several hematopoietic cells. The effects on platelets are profound, and include diminishing platelet aggregation responses and formation of platelet thrombi. This chapter will review cell-specific aspects of serglycin structure, its gene regulation, cell and tissue localization, and the effects of serglycin deletion on hematopoietic cell granule structure and function. The effects of serglycin knockout on platelets are described and discussed in detail. Rationales for further investigations into the contribution of serglycin to the known roles of platelets in thrombosis, inflammation, atherosclerosis, and tumor metastasis are presented.

Enhanced efficacy of recombinant FVIII in noncovalent complex with PEGylated liposome in hemophilia A mice

Recombinant FVIII formulated in PEG-ylated liposomes (rFVIII-PEG-Lip) was reported to increase the bleed-free days from 7 to 13 days (at 35 IU/kg rFVIII) in severe hemophilia A patients. To understand the underlying mechanism, we sought to recapitulate its efficacy in hemophilia A mice. Animals treated with rFVIII-PEG-Lip achieved approximately 30% higher survival relative to rFVIII after tail vein transection inflicted 24 hours after dosing. The efficacy of rFVIII-PEG-Lip represents an approximately 2.5-fold higher "apparent" FVIII activity, which is not accounted for by its modestly increased (13%) half-life. The enhanced efficacy requires complex formation between rFVIII and PEG-Lip before the administration. Furthermore, PEG-Lip associates with the majority of platelets and monocytes in vivo, and results in increased P-selectin surface expression on platelets in response to collagen. Rotational thromboelastometry (ROTEM) analysis of whole blood from rFVIII-PEG-Lip-treated animals at 5 minutes up to 72 hours after dosing recapitulated the 2- to 3-fold higher apparent FVIII activity. The enhanced procoagulant activity is fully retained in plasma unless microparticles are removed by ultracentrifugation. Taken together, the efficacy of rFVIII-PEG-Lip is mediated mainly by its sensitization of platelets and the generation of procoagulant microparticles that may express sustained high-affinity receptors for FVIII.

Platelet alpha-granules: basic biology and clinical correlates

alpha-Granules are essential to normal platelet activity. These unusual secretory granules derive their cargo from both regulated secretory and endocytotic pathways in megakaryocytes. Rare, inheritable defects of alpha-granule formation in mice and man have enabled identification of proteins that mediate cargo trafficking and alpha-granule formation. In platelets, alpha-granules fuse with the plasma membrane upon activation, releasing their cargo and increasing platelet surface area. The mechanisms that control alpha-granule membrane fusion have begun to be elucidated at the molecular level. SNAREs and SNARE accessory proteins that control alpha-granule secretion have been identified. Proteomic studies demonstrate that hundreds of bioactive proteins are released from alpha-granules. This breadth of proteins implies a versatile functionality. While initially known primarily for their participation in thrombosis and hemostasis, the role of alpha-granules in inflammation, atherosclerosis, antimicrobial host defense, wound healing, angiogenesis, and malignancy has become increasingly appreciated as the function of platelets in the pathophysiology of these processes has been defined. This review will consider the formation, release, and physiologic roles of alpha-granules with special emphasis on work performed over the last decade.

Platelets actively sequester angiogenesis regulators

Clinical trials with antiangiogenic agents have not been able to validate plasma or serum levels of angiogenesis regulators as reliable markers of cancer presence or therapeutic response. We recently reported that platelets contain numerous proteins that regulate angiogenesis. We now show that accumulation of angiogenesis regulators in platelets of animals bearing malignant tumors exceeds significantly their concentration in plasma or serum, as well as their levels in platelets from non-tumor-bearing animals. This process is selective, as platelets do not take up a proportional amount of other plasma proteins (eg, albumin), even though these may be present at higher concentrations. We also find that VEGF-enriched Matrigel pellets implanted subcutaneously into mice or the minute quantities of VEGF secreted by microscopic subcutaneous tumors (0.5-1 mm(3)) result in an elevation of VEGF levels in platelets, without any changes in its plasma levels. The profile of other angiogenesis regulatory proteins (eg, platelet-derived growth factor, basic fibroblast growth factor) sequestered by platelets also reflects the presence of tumors in vivo before they can be macroscopically evident. The ability of platelets to selectively take up angiogenesis regulators in cancer-bearing hosts may have implications for the diagnosis and management of many angiogenesis-related diseases and provide a guide for antiangiogenic therapies.

Oncogenic and angiogenic growth factors accumulate during routine storage of apheresis platelet concentrates

PURPOSE

Platelet concentrates are important for support of patients with malignancies requiring myelotoxic chemotherapy. During storage, 10% to 15% of platelets may become activated resulting in the release of alpha-granules, which contain growth factors. We hypothesize that, during storage, growth factors accumulate in the plasma, specifically platelet-derived growth factor, vascular endothelial growth factor (VEGF), transforming growth factor-beta, and fibroblast growth factor-2, which may adversely affect cancer patients.

EXPERIMENTAL DESIGN

The concentrations of growth factors were measured by ELISA from the plasma of apheresis platelets serially throughout storage (days 1, 3, 5, and 7) and compared with concentrations in fresh plasma from healthy blood donors. Washing was evaluated as a method of growth factor removal, and an in vitro model of platelet transfusion in a patient receiving Bevacizumab (Avastin) using immunoprecipitation was employed to determine if Bevacizumab would be bound by the VEGF in apheresis platelets.

RESULTS

VEGF, platelet-derived growth factor, and transforming growth factor-beta were increased on day 1 versus fresh plasma and throughout storage reaching a relative maximum at outdate (P < 0.01, day 5 or 7). Fibroblast growth factor-2 concentrations were significantly increased on day 7 alone versus day 1 or to fresh plasma (P < 0.01). Washing removed 41 +/- 11% to 56 +/- 2% of the growth factors. Bevacizumab effectively bound the VEGF from apheresis platelets, with significant amounts of VEGF remaining in the supernatant.

CONCLUSIONS

Significant amounts of growth factors are present in apheresis platelets due to the isolation procedures, and these concentrations increase over storage, which may be partially removed by washing. In addition, apheresis platelet transfusion could affect cancer treatment by binding monoclonal antibodies directed against growth factors of tumor origin.

Delivering new insight into the biology of megakaryopoiesis and thrombopoiesis

PURPOSE OF REVIEW

The aim of this review is to explore the state of the art knowledge on the cell biological and molecular pathways that regulate megakaryopoiesis and lead to platelet production.

RECENT FINDINGS

In the last 2 years there has been considerable progress in the elucidation of molecular mechanisms of megakaryocyte development and platelet biogenesis, driven by the application of modern molecular biology approaches to these specialized and unique cells. Studies have for the first time visualized endomitotic spindle dynamics, characterized the maturation of the demarcation membrane system, and delineated the mechanics of organelle transport and microtubule assembly in living megakaryocytes. The role of specific molecules in platelet production has been elucidated in greater detail by combining molecular studies with genetically engineered mice as well as embryonic cell culture systems.

SUMMARY

This review integrates the latest studies of megakaryocyte development into the molecular pathways that regulate megakaryopoiesis and thrombopoiesis. Decoding the pathways of megakaryopoiesis and platelet production should help revolutionize the management of thrombocytopenia and other platelet disorders.

Acquired platelet dysfunction

Acquired platelet dysfunction is encountered frequently in clinical practice. The usual clinical presentation is that of mucosal bleeding, epistaxis, or superficial epidermal bleeds. Often, the dysfunctional platelets are related to a medication or a systemic disorder. Normally, when platelets are exposed to damaged endothelium, they adhere to the exposed basement membrane collagen and change their shape from smooth disks to spheres with pseudopodia. Then, they secrete the contents of their granules, a process referred to as the release reaction. Additional platelets form aggregates on those platelets that have adhered to the vessel wall. As a result, the primary hemostatic plug is formed, and bleeding is arrested. This article reviews the various forms of acquired platelet dysfunction that result in decreased platelet aggregation, adhesion, or secretion.

Evidence that differential packaging of the major platelet granule proteins von Willebrand factor and fibrinogen can support their differential release

BACKGROUND

von Willebrand factor (VWF) and fibrinogen are major storage proteins of platelet alpha-granules. VWF is synthesized by the megakaryocyte, the cell from which platelets bud, and fibrinogen is delivered to alpha-granules by endocytosis.

AIM

Considering biosynthetic origins, VWF and fibrinogen might be differentially packaged within platelets. We applied immunofluorescence microscopy to provide whole platelet, global information on the distributions of VWF and fibrinogen.

RESULTS

The distribution of VWF and fibrinogen were characterized in both the resting state and handling activated human platelets. Full cell volume image stacks were collected by spinning-disk confocal microscopy, corrected for a small pixel shift between green and red channels, deconvolved, and visualized in a three-dimensional space. In sum, we found that there was little overlap in the distribution of VWF and fibrinogen in resting state platelets. In an important control, the distributions of green and red secondary antibodies overlapped completely when different color secondary antibodies directed against the same first antibody were used. Moreover, the same result was observed using different first antibodies and switching second antibody color to switch the color of VWF and fibrinogen staining. No accumulation of fibrinogen in late endosomes or lysosomes was detected by co-staining with LAMP2, a late endosome/lysosome membrane protein. Significantly, we found that in handling activated platelets there was differential retention of fibrinogen-positive granules relative to VWF positive granules.

CONCLUSION

Our results indicate that VWF and fibrinogen are differentially packaged in human platelets. Moreover, the results suggest that differential packaging could support differential release of alpha-granule proteins.

Association of Elevated Levels of Vascular Endothelial Growth Factor in Obstructive Sleep Apnea Syndrome with Patient Age rather than with Obstructive Sleep Apnea Syndrome Severity

BACKGROUND

Although certain studies report high levels of vascular endothelial growth factor (VEGF) in obstructive sleep apnea syndrome (OSAS), the effect of systemic hypoxia on circulating VEGF remains controversial.

OBJECTIVES

To study the association of serum VEGF and OSAS in a large group of patients.

METHODS

One hundred patients with OSAS (mean age 58.1+/-12.4 years, mean body mass index 30.6+/-5.4 kg/m2) were tested for serum VEGF levels, and the findings were correlated with the severity of OSAS, as determined by the apnea-hypopnea index (AHI) on the basis of polysomnography and background data.

RESULTS

The mean AHI was 40.0+/-21.2 (range 10-106). Mean minimal oxygen saturation was 80.6+/-11.7% (range 43-98%) and mean time of oxygen saturation under 90% was 50.0+/-75.0 min (range 0-300 min). The mean VEGF level was 445.2+/-289.8 pg/ml in the study group (vs. 280 pg/ml reported in normal controls). The mean platelet count was 233.8+/-64.4 10(3)/ml and the mean VEGF/platelet ratio was 1.95+/-1.40 pg/10(6). There was no association of VEGF or VEGF/platelets with the severity of OSAS. However, both factors showed a significant correlation with patient age (r=0.224, p=0.01 and r=0.425, p=0.01, respectively). Age was the only parameter to significantly predict VEGF and VEGF/platelets on multivariate analysis (R2=0.713, p=0.001 and R2=0.844, p=0.001, respectively).

CONCLUSION

The elevation of serum VEGF in OSAS is not associated with the severity of the disease, but it is associated with patient age. VEGF might be involved in the long-term adaptive mechanism in OSAS, and its age-dependent increase might partly explain the reduced mortality in elderly OSAS patients.

The biogenesis of platelets from megakaryocyte proplatelets

Platelets are formed and released into the bloodstream by precursor cells called megakaryocytes that reside within the bone marrow. The production of platelets by megakaryocytes requires an intricate series of remodeling events that result in the release of thousands of platelets from a single megakaryocyte. Abnormalities in this process can result in clinically significant disorders. Thrombocytopenia (platelet counts less than 150,000/microl) can lead to inadequate clot formation and increased risk of bleeding, while thrombocythemia (platelet counts greater than 600,000/microl) can heighten the risk for thrombotic events, including stroke, peripheral ischemia, and myocardial infarction. This Review will describe the process of platelet assembly in detail and discuss several disorders that affect platelet production.

Megakaryocytes and beyond: the birth of platelets

Megakaryocytes are highly specialized precursor cells that differentiate to produce blood platelets via intermediate cytoplasmic extensions known as proplatelets. Recent advances in the understanding of megakaryocyte differentiation and platelet formation rely on a combination of genetic and cell biological studies with detailed structural analysis of cultured cells. Visualization of sequential steps in endomitosis has expanded our views on how megakaryocytes acquire polyploid DNA content, whereas studies in mouse models of platelet disorders provide clues into transcriptional pathways and those leading to the assembly of platelet-specific secretory granules. The experimental findings forge stronger links between cellular processes and molecular mechanisms, while observation of the underlying morphologic events in beginning to yield insights into the cytoskeletal mechanics of proplatelet formation. Here we review salient aspects of the emerging appreciation of the cellular and molecular basis of thrombopoiesis.

Development of platelet secretory granules

Platelet granule exocytosis plays a critical role in thrombosis and wound healing. Platelets have three major types of secretory granules that are defined by their unique molecular contents, kinetics of exocytosis and morphologies. Although the ontogeny of platelet granules is poorly understood, a convergence of new insights into megakaryocyte development, the molecular mechanisms of vesicle trafficking and the genetic basis of platelet granule defects, is beginning to define the cellular and molecular pathways responsible for platelet granule ontogeny.

Impaired liver function and retroviral activity are risk factors contributing to HIV-associated thrombocytopenia. Swiss HIV Cohort Study

OBJECTIVE

To investigate the relationship between thrombopoetin (TPO) serum levels and HIV-associated thrombocytopenia.

DESIGN AND METHODS

The relationship between TPO levels and severity of HIV-associated thrombocytopenia was investigated. Thirty-eight patients (19 patients with 30-96x10(9) platelets/l and 19 patients with <10x10(9) platelets/l) were matched with 38 HIV-positive non-thrombocytopenic patients (>150x10(9) platelets/l).

RESULTS

HIV-positive patients with normal platelet counts had a median TPO serum level of 137 pg/ml. Patients with 30-96x10(9) platelets/l had decreased TPO levels with a median of 90 pg/ml (P = 0.016), and were more likely to have elevated serum aspartate-transferase levels (P<0.001) and hepatomegaly by palpation or ultrasound imaging (P = 0.005). The median TPO serum level of HIV-infected patients with severe thrombocytopenia was 110 pg/ml (non-significant). All patients with severe thrombocytopenia were positive for antibodies against hepatitis B virus core antigen, compared with 80% of HIV-infected persons without thrombocytopenia. Patients with severe thrombocytopenia were more likely to have high HIV replication compared to patients with normal platelet counts (P = 0.02), and reduction of plasma HIV-1 RNA levels was associated with increasing platelet counts. Severe thrombocytopenia was not associated with liver disease.

CONCLUSIONS

Liver disease predisposes for low TPO serum levels and mild thrombocytopenia. High retroviral activity predisposes for severe, immune thrombocytopenic purpura-like thrombocytopenia. At least two distinct categories of severe HIV-associated thrombocytopenia exist, one responsive to antiretroviral treatment and one non-responsive to antiretroviral treatment.

Synthesis and Ultrastructural Localization of Protein C Inhibitor in Human Platelets and Megakaryocytes

The occurrence of protein C inhibitor (PCI) in human platelets and megakaryocytes was analyzed. As judged from enzyme-linked immunosorbent assays (ELISAs), PCI was present in platelets at a concentration of 160 ng/2 × 109 cells. Its specific activity was 5 times higher than that of plasma PCI. Consistently, mainly the 57-kD form (active PCI) and some high molecular weight (Mr) forms, but no bands corresponding to cleaved PCI, were detected when platelet lysates were immunoprecipitated with monoclonal anti-PCI-IgG and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting. The localization of PCI in platelets was studied by immunofluorescence histochemistry and immunotransmission electron microscopy: PCI was detected in  granules, in the open canalicular system, and on the plasma membrane. At these sites, colocalization with plasminogen activator inhibitor-1 was seen. Studies were performed to clarify whether platelet PCI is endogenously synthesized or taken up from plasma. Internalization of biotinylated-PCI was analyzed using platelets in suspension and gold-labeled streptavidin for visualization of incorporated biotin. Dose- and time-dependent uptake of PCI was found. PCI mRNA was detected in platelets by reverse transcriptase-polymerase chain reaction (RT-PCR) and Southern blotting, as well as in megakaryocytes by in situ hybridization of human bone marrow cryosections. We therefore conclude that platelets contain a functionally active PCI pool that is derived from both endogenous synthesis as well as internalization.

Synthesis and Ultrastructural Localization of Protein C Inhibitor in Human Platelets and Megakaryocytes

The occurrence of protein C inhibitor (PCI) in human platelets and megakaryocytes was analyzed. As judged from enzyme-linked immunosorbent assays (ELISAs), PCI was present in platelets at a concentration of 160 ng/2 × 109 cells. Its specific activity was 5 times higher than that of plasma PCI. Consistently, mainly the 57-kD form (active PCI) and some high molecular weight (Mr) forms, but no bands corresponding to cleaved PCI, were detected when platelet lysates were immunoprecipitated with monoclonal anti-PCI-IgG and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting. The localization of PCI in platelets was studied by immunofluorescence histochemistry and immunotransmission electron microscopy: PCI was detected in  granules, in the open canalicular system, and on the plasma membrane. At these sites, colocalization with plasminogen activator inhibitor-1 was seen. Studies were performed to clarify whether platelet PCI is endogenously synthesized or taken up from plasma. Internalization of biotinylated-PCI was analyzed using platelets in suspension and gold-labeled streptavidin for visualization of incorporated biotin. Dose- and time-dependent uptake of PCI was found. PCI mRNA was detected in platelets by reverse transcriptase-polymerase chain reaction (RT-PCR) and Southern blotting, as well as in megakaryocytes by in situ hybridization of human bone marrow cryosections. We therefore conclude that platelets contain a functionally active PCI pool that is derived from both endogenous synthesis as well as internalization.

Cell biology of the basophil

The cell biology of basophils, based on published studies spanning 1990-1997, is reviewed. These rarest cells of granulocyte lineages are now available in sufficient numbers for such studies to be done, based on new methods for isolating and purifying the cells from peripheral blood and organ sources and for their derivation in growth factor-containing cultures from their precursors de novo. These studies are dependent on electron microscopy for the accurate identification of basophils, studies which have recently established the presence of basophils in two new species--mice and monkeys. Secretory, endocytotic and storage properties of basophils constitute their mechanistic role(s) in human disease; their role(s) in health is, however, obscure. Development of immunoaffinity and enzyme-affinity ultrastructural labeling techniques to image the Charcot-Leyden crystal protein and histamine in human basophils, coupled with ultrastructural analysis of kinetic samples of cells obtained after stimulation with diverse secretogogues, has provided insight into the role of vesicles in secretory transport mechanisms in human basophils as well as the definition of key ultrastructural phenotypes of secreting basophils.

Normal Platelets and Megakaryocytes Are Produced In Vivo in the Absence of Thrombopoietin

Thrombopoietin (TPO) has been established as the major regulator of megakaryocyte and platelet production. In vitro and in vivo studies have demonstrated that TPO affects both megakaryocyte proliferation and maturation. In vitro, TPO has been reported to be essential for full development of megakaryocytes and platelets. These studies are in contrast to results observed in vivo in mice deficient in the TPO or c-mpl gene (TPO-/- and c-mpl-/-). Both TPO-/- and c-mpl-/- mice exhibit a 90% reduction in megakaryocyte and platelet levels. But even with this small number of circulating platelets, these mice do not have any excessive bleeding. Ultrastructural analysis indicates that platelets and megakaryocytes present in the knockout mice are morphologically normal. Characterization of platelet function shows that platelets from knockout mice are functionally identical to the wild-type platelets as measured by upregulation of 125I-fibrinogen binding to platelets in response to adenosine diphosphate (ADP) stimulation and by platelet attachment to the immobilized extracellular matrix proteins, collagen and von Willebrand factor (vWF). These results demonstrate that in vivo, TPO is required for the control of megakaryocyte and platelet number but not for their maturation. Other factors with megakaryocytopoietic activity may be able to compensate for the maturational role of TPO and lead to the formation of normal megakaryocytes and platelets in TPO-/- and c-mpl-/- mice.

The Src Family Kinases, Fgr, Fyn, Lck, and Lyn, Colocalize With Coated Membranes in Platelets

Nonreceptor protein tyrosine kinases phosphorylate proteins, thereby activating many intracellular signaling pathways and mediating protein-protein interactions. Protein phosphorylation is regulated in large part by the subcellular localization of these kinases and their respective substrates. Src is the most studied of these kinases, although other members of the Src family have been shown to be important in the differentiation of specific cell types. Src and Src family members are reported to be membrane-associated, but detergent-extraction studies have demonstrated a major difference in the solubility of Src compared with other members of the Src family (Fgr, Fyn, Lck, Lyn, and Yes), suggesting that their subcellular distributions may be different. By immunoelectron microscopy, we demonstrate that, unlike Src, the Src-related kinases are associated with electron-dense cytoplasmic domains and plasma membrane domains that correspond in size and frequency to endocytotic vesicles and coated pits. Clusters of labeling for these kinases also were seen adjacent to granule membranes. These kinases colocalize with the coated vesicle protein, clathrin, confirming their association with this class of endocytotic vesicle. We hypothesize that this vesicular association of Src-related kinases indicates a role for them in the endocytotic vesicle–mediated uptake and trafficking of plasma proteins into platelet granules.

Immunocytochemical colocalization of adhesive proteins with clathrin in human blood platelets: further evidence for coated vesicle-mediated transport of von Willebrand factor, fibrinogen and fibronectin

Coated membranes and vesicles play an important role in receptor-mediated endocytosis and intracellular trafficking in various cell types, and are also present in blood platelets. Platelets take up certain proteins from the blood plasma, such as von Willebrand factor and fibrinogen, and these substances are transferred to storage granules. The receptors for these plasma proteins on the platelet plasma membrane have been well characterized, but morphological evidence for their transport to the storage granules is not yet available. In an attempt to clarify this aspect, we employed postembedding immunocytochemistry on platelets embedded in the acrylic resin LR White. Clathrin as the major coat component of coated vesicles was localized in the cytoplasm, on the plasmic faces of alpha-granules and the open canalicular system, and on the plasmic face of the plasma membrane. Colocalizations of the adhesive proteins, von Willebrand factor, fibrinogen and fibronectin, with clathrin could be observed at the same typical locations as coated vesicles were seen in Araldite-embedded material. These colocalizations have not been reported to date and furnish further evidence for a coated vesicle-mediated transport of blood plasma-derived adhesive proteins from their receptors on the outer plasma membrane to the alpha-granules.

Significance of testing platelet functions in vitro

Platelets respond through discrete receptors to a number of physiological agonists and foreign surfaces with a sequence of measurable responses: shape change, aggregation, secretion and arachidonate liberation. Three secretory responses are distinguished: exocytosis of substances from (1) dense granules, (2) alpha-granules and (3) lysosomes. Free arachidonate, liberated from phospholipids by phospholipase A2, is rapidly converted (by oxygenation) to prostaglandins and thromboxanes which, together with secreted ADP and close cell contact, will cause further platelet activation through 'positive feedback' (autocrine stimulation). Some agonists are classified as 'weak' (ADP, vasopressin, platelet-activating factor [PAF], serotonin) because they depend on autocrine stimulation to promote the full sequence of responses, while others are 'strong' agonists (thrombin, collagen) and activate all responses directly without autocrine stimulation. Adrenaline, long thought to be a platelet agonist per se, most probably acts by amplifying the activation brought about by other, proper, agonists. Such synergistic interaction among agonists is very typical for platelet activation and most likely takes place in vivo. Shape change, aggregation and secretion(s) may be tested by flow cytometry or electron microscopy in vitro under conditions that probably reflect the in vivo situation. However, the aggregation response to weak agonists in vitro is dependent on the extracellular [Ca2+], with biphasic aggregation at the low [Ca2+] present when citrate is used as anticoagulant (or in suspension of washed platelets) but not at the physiological [Ca2+] present in platelet-rich plasma from heparinized blood.

Post-embedding immunocytochemistry for adhesive proteins and clathrin in LR White- and LR Gold-embedded human platelets

Two hydrophilic acrylic resins, LR White and LR Gold, were tested for their suitability for post-embedding immunocytochemistry on human platelets. Both resins gave satisfying results with regard to ultrastructural preservation and retention of antigenicity for von Willebrand factor, fibrinogen, fibronectin and clathrin. Embedding in LR White and polymerization at 50 degrees C without accelerator proved to be the best method because it provided clear negative membrane contrast, good immunocytochemical localization of antigens and the entire procedure required a comparatively short time. Label for von Willebrand factor was found in typically eccentric location in alpha-granules, but also in the form of small caps directly outside the granule membranes, beneath the outer plasma membrane and in the cytoplasm. With the exception of the eccentric location within granules, labelling for fibrinogen and fibronectin revealed very similar results. Label for clathrin appeared in the form of small gold clusters attached to alpha-granules, in the cytoplasm and beneath membranes of the open canalicular system. Together with the appearance of coated vesicles attached to granules and to the membranes of the open canalicular system in araldite-embedded material, these results further strengthen the concept that adhesive proteins are taken up from the plasma and stored in granules. Moreover, the persistence of the clathrin coat on vesicles attached to storage granules may also indicate that receptors for adhesive proteins are recycled.

Macrophage-megakaryocyte interaction in bone marrow after high-dose intravenous immunoglobulin therapy

Localization of IgG in bone marrow after high-dose intravenous immunoglobulin therapy (IVIG) was investigated via light and electron microscopy. IgG was incorporated into the cytoplasm of various types of bone marrow cells of all the three lineages, particularly in reticulum cells, fat cells, and megakaryocytes. In addition, both reticulum cells and fat cells showed many elongated cytoplasmic protrusions, which were in contact with the various types of blood cells, especially the megakaryocytes. A filamentous structure was also seen near the point of contact between the cells. Ultrastructural changes of macrophages in bone marrow after IVIG suggest that these cells were activated by IVIG.

Platelet activation releases megakaryocyte-synthesized apolipoprotein J, a highly abundant protein in atheromatous lesions

Apolipoprotein J (apoJ) is an abundant glycoprotein in many biological fluids, and its constitutive high level synthesis is characteristic of many epithelial cells exposed to harsh fluids such as urine, bile, and gastric secretions. In addition, dramatic induction of apoJ occurs in cells surrounding several kinds of pathological lesions. Because platelets and circulating inflammatory cells represent critical elements in numerous pathological processes, we evaluated bone marrow cells for the presence of apoJ. Based upon messenger RNA in situ hybridization and immunofluorescent protein detection, high-level apoJ gene expression and protein accumulation occurred exclusively in mature megakaryocytes. Our results indicate that apoJ is stored in platelet granules and is released into extracellular fluid following platelet activation. Because atheromatous plaque development involves platelet aggregation and activation, we looked for and found abundant apoJ protein in advanced human atheromatous lesions. Thus, platelet sequestration and activation may lead to the rapid deployment of apoJ into sites of vascular injury. We hypothesize that platelet-derived apoJ participates in both short-term wound repair processes and chronic pathogenic processes at vascular interfaces.

Kistrin, an integrin antagonist, blocks endocytosis of fibrinogen into guinea pig megakaryocyte and platelet alpha-granules

Recent data indicate that megakaryocyte/platelet alpha-granule fibrinogen is endocytosed from plasma. Because fibrinogen is the major platelet protein present in high concentrations in alpha-granules, fibrinogen uptake into alpha-granules may occur via specific receptors. In that cells of the megakaryocyte/platelet lineage contain two integrins--alpha IIb beta 3 (GP IIb-IIIa) and the vitronectin receptor (alpha v beta 3)--that can bind fibrinogen, one or both of these receptors may mediate the endocytic uptake of fibrinogen. To test this hypothesis, we examined the effect of Kistrin, an RGD-containing protein purified from the venom of Agkistrodon rhodostoma that inhibits fibrinogen binding to human platelet receptors, on endocytosis of fibrinogen by megakaryocytes and platelets. Continuous intravenous infusion of kistrin into guinea pigs (200 micrograms/h) over a 24-h period inhibited collagen-induced platelet aggregation. When biotinylated fibrinogen was injected intravenously into animals receiving Kistrin, megakaryocytes failed to endocytose the labeled fibrinogen. Endocytosis of fibrinogen into platelets was also inhibited in these animals. In contrast, platelets and megakaryocytes obtained from sham-infused control animals contained the injected biotinylated fibrinogen. We conclude that, in addition to the well-known extracellular function of cell adhesion, integrins can also act as receptors that mediate endocytosis of exogenous proteins and incorporate them into regulated secretory granules.

Platelets contribute to circulating levels of bone sialoprotein in human

Bone sialoprotein (BSP) is a major bone-related protein. Although a few other tissues contain trace amounts of BSP message, bone cells and bone matrix are the major sources of BSP, suggesting that this protein could be a potential marker of bone metabolism. Purified bovine BSP showed a 70% homology of its first 13 amino acid N-terminal sequence with human BSP and was used to raise antibodies in rabbit and to develop a specific radioimmunoassay (RIA). Using this RIA, we have shown that BSP is present in serum with values in the range of 10-30 ngEq/ml in the serum of normal adults. Values obtained in plasma prepared without platelet activation are about one-half of those in matched sera, suggesting that BSP present in serum is in part derived from platelets during the activation process. Using Western blot and RIA techniques, we confirmed that platelets contain immunoreactive BSP and that the protein is released after thrombin stimulation of these cells. In addition to BSP, platelets contain a 45 kD immunoreactive material that has not been precisely identified. Available evidence indicates that this material is not osteonectin or osteopontin and that it may be a BSP-like protein rather than a degradation product of BSP. Platelets from a patient having a gray platelet syndrome, characterized by a deficiency in platelet alpha-granules and in the alpha-granule secretory proteins, did not show any deficiency of BSP, suggesting that immunoreactive BSP present in platelets is not endogenously synthesized by megakaryocytes but rather originates from plasma by endocytosis.

In vivo defibrination results in markedly decreased amounts of fibrinogen in rat megakaryocytes and platelets

Recently evidence was provided for a pathway whereby circulating fibrinogen enters megakaryocyte granules by an endocytic mechanism. Synthesis of fibrinogen by megakaryocytes has been reported. To determine the relationship between plasma fibrinogen and alpha-granule fibrinogen in megakaryocytes and platelets, the fibrinogen content of these cells was studied in rats defibrinated by use of Ancrod, a thrombinlike enzyme purified from the venom of Agkistrodon rhodostoma. Unlike thrombin, Ancrod does not induce platelet secretion. Rats were injected with Ancrod (50 units/kilogram body weight) at 8-hour intervals for 5 days. There were no significant changes in platelet counts. Blood from the treated rats failed to clot, and plasma fibrinogen levels were less than 15 mg/dl. Bone marrow from defibrinated rats and untreated control rats was stained immunohistochemically for fibrinogen and two other alpha-granule proteins, albumin and platelet factor 4 (PF4), in plastic-embedded sections. The presence of these three proteins in platelets was detected by Western blots. Only trace amounts of fibrinogen were detected in megakaryocytes and platelets from defibrinated rats, but fibrinogen in control megakaryocytes and platelets was readily demonstrated. However defibrinated and control rats did not differ in albumin and PF4 content in megakaryocytes and platelets. It is concluded that a major portion of rat platelet fibrinogen is derived from plasma by endocytosis by megakaryocytes.