Platelet formation

Arachidonic acid and Docosahexanoic acid enhance platelet formation from human apheresis-derived CD34+ cells

An Aberration in megakaryopoiesis and thrombopoiesis, 2 important processes that maintain hemostasis, leads to thrombocytopenia. Though platelet transfusions are used to treat this condition, blood banks frequently face a shortage of platelets. Therefore, methods to generate platelets on a large scale are strongly desirable. However, to generate megakaryocytes (MKs) and platelets (PLTs) in numbers sufficient for clinical application, it is essential to understand the mechanism of platelet production and explore efficient strategies accordingly. We have earlier reported that the N-6 and N-3 poly-unsaturated fatty acids (PUFAs), Arachidonic acid (AA)/Docosahexanoic acid (DHA) have beneficial effect on the generation of MKs and PLTs from umbilical cord blood derived CD34⁺ cells. Here we tested if a similar effect is observed with peripheral blood derived CD34⁺ cells, which are more commonly used in transplantation settings. We found a significant enhancement in cell numbers, surface marker expression, cellular ploidy and expression of cytoskeletal components during PLT biogenesis in cultures exposed to media containing AA/DHA than control cultures that were not exposed to these PUFAs. The test cells engrafted more efficiently in NOD/SCID mice than control cells. AA/DHA appears to have enhanced MK/PLT generation through upregulation of the NOTCH and AKT pathways. Our data show that PUFAs could be valuable additives in the culture system for large scale production of platelets for clinical applications.

Artificial cells: from basic science to applications

Artificial cells have attracted much attention as substitutes for natural cells. There are many different forms of artificial cells with many different definitions. They can be integral biological cell imitators with cell-like structures and exhibit some of the key characteristics of living cells. Alternatively, they can be engineered materials that only mimic some of the properties of cells, such as surface characteristics, shapes, morphology, or a few specific functions. These artificial cells can have applications in many fields from medicine to environment, and may be useful in constructing the theory of the origin of life. However, even the simplest unicellular organisms are extremely complex and synthesis of living artificial cells from inanimate components seems very daunting. Nevertheless, recent progress in the formulation of artificial cells ranging from simple protocells and synthetic cells to cell-mimic particles, suggests that the construction of living life is now not an unrealistic goal. This review aims to provide a comprehensive summary of the latest developments in the construction and application of artificial cells, as well as highlight the current problems, limitations, challenges and opportunities in this field.

Disassembly of the Dying: Mechanisms and Functions

The disassembly of an apoptotic cell into subcellular fragments, termed apoptotic bodies (ApoBDs), is a hallmark of apoptosis. Although the generation of ApoBDs is generally understood as being stochastic, it is becoming increasingly clear that ApoBD formation is a highly regulated process involving distinct morphological steps and molecular factors. Functionally, ApoBDs could facilitate the efficient clearance of apoptotic material by surrounding phagocytes as well as mediate the transfer of biomolecules including microRNAs and proteins between cells to aid in intercellular communications. Therefore, the formation of ApoBDs is an important process downstream from apoptotic cell death. We discuss here the mechanisms and functions of apoptotic cell disassembly.

Platelet generation in vivo and in vitro

Platelet (PLT) transfusion, which is the primary cell therapy for thrombocytopenia, has been a source of concern in recent years due to its limitations of donor-dependent supply and soaring costs. In vitro platelet generation on an industrial scale is a possible solution requiring exploration. The technology of platelet generation ex vivo has been widely studied across the world, though the mechanisms of physiological thrombopoiesis and platelet biology function in vivo still remain elusive today. Various culture systems have been studied, most of which proved quite inefficient in generating functional platelets ex vivo, so there is still a long way to reach our ultimate goal of generating a fully functional platelet in vitro on an industrial scale. This review integrates the latest research into physiological platelet biogenesis and ex vivo-platelet/megakaryocyte (MK) generation protocols with a focus on the ability to generate PLT/MK in large quantities, summarizes current culture systems based on induced human pluripotent stem cells and adipose-derived stem cells, and discusses significant challenges that must be overcome for these approaches to be perfected.

Cdc42-dependent F-actin dynamics drive structuration of the demarcation membrane system in megakaryocytes

Essentials Information about the formation of the demarcation membrane system (DMS) is still lacking. We investigated the role of the cytoskeleton in DMS structuration in megakaryocytes. Cdc42/Pak-dependent F-actin remodeling regulates DMS organization for proper megakaryopoiesis. These data highlight the mandatory role of F-actin in platelet biogenesis.

SUMMARY

Background Blood platelet biogenesis results from the maturation of megakaryocytes (MKs), which involves the development of a complex demarcation membrane system (DMS). Therefore, MK differentiation is an attractive model for studying membrane remodeling. Objectives We sought to investigate the mechanism of DMS structuration in relationship to the cytoskeleton. Results Using three-dimensional (3D) confocal imaging, we have identified consecutive stages of DMS organization that rely on F-actin dynamics to polarize membranes and nuclei territories. Interestingly, microtubules are not involved in this process. We found that the mechanism underlying F-actin-dependent DMS formation required the activation of the guanosine triphosphate hydrolase Cdc42 and its p21-activated kinase effectors (Pak1/2/3). Förster resonance energy transfer demonstrated that active Cdc42 was associated with endomembrane dynamics throughout terminal maturation. Inhibition of Cdc42 or Pak1/2/3 severely destructured the DMS and blocked proplatelet formation. Even though this process does not require containment within the hematopoietic niche, because DMS structuration was observed upon thrombopoietin-treatment in suspension, integrin outside-in signaling was required for Pak activation and probably resulted from secretion of extracellular matrix by MKs. Conclusions These data indicate a functional link, mandatory for MK differentiation, between actin dynamics, regulated by Cdc42/Pak1/2/3, and DMS maturation.

Platelet production of megakaryocyte: A review with original observations on human in vivo cells and bone marrow

Megakaryocytes (MKs) build characteristic structures to produce platelets in a series of steps. Although mechanisms of demarcation membrane system (DMS) and open canalicular system transformation have been proposed based on experimental studies in recent decades, the related evidence is lacking in human cells in vivo. The present review describes and discusses the development of MKs, transformation of DMS, and the release and maturation of proplatelets based on our observation of human MKs in vivo and bone marrow biopsy by light microscope and transmission electron microscope. Four stages were subdivided from megakaryoblasts to matured cells; presumption of DMS transformation from endoplasmic reticulum and Golgi apparatus were evidenced in contrast to another presumption of DMS transformation from plasma membrane in this review. Effectors of interaction between hematopoietic cells, the sucking and shearing force of sinus blood flow on movement of MKs, and release of proplatelets were emphasized. Additionally, the mechanism of secondary splitting of proplatelets in circulation was demonstrated ultrastructurally. These findings and conceptions might significantly promote our understanding of the mechanism of platelet production in human in vivo cells.

Understanding platelet generation from megakaryocytes: implications for in vitro-derived platelets

Platelets are anucleate cytoplasmic discs derived from megakaryocytes that circulate in the blood and have major roles in hemostasis, thrombosis, inflammation, and vascular biology. Platelet transfusions are required to prevent the potentially life-threatening complications of severe thrombocytopenia seen in a variety of medical settings including cancer therapy, trauma, and sepsis. Platelets used in the clinic are currently donor-derived which is associated with concerns over sufficient availability, quality, and complications due to immunologic and/or infectious issues. To overcome our dependence on donor-derived platelets for transfusion, efforts have been made to generate in vitro-based platelets. Work in this area has advanced our understanding of the complex processes that megakaryocytes must undergo to generate platelets both in vivo and in vitro. This knowledge has also defined the challenges that must be overcome to bring in vitro-based platelet manufacturing to a clinical reality. This review will focus on our understanding of committed megakaryocytes and platelet release in vivo and in vitro, and how this knowledge can guide the development of in vitro-derived platelets for clinical application.

Age and gender effects on 15 platelet phenotypes in a Spanish population

INTRODUCTION

Several studies have analysed the platelet parameters in human blood, nevertheless there are no extensive analyses on the less common platelet phenotypes. The main objective of our study is to evaluate the age and gender effects on 15 platelet phenotypes.

METHODS

We studied 804 individuals, ranging in age from 2 to 93 years, included in the Genetic Analysis of Idiopathic Thrombophilia 2 (GAIT 2) Project. The 15 platelet phenotypes analysed were the platelets counts, platelet volumes, plateletcrits, immature platelet fraction (IPF) and platelet function assay (PFA). A regression-based method was used to evaluate the age and gender effects on these phenotypes.

RESULTS

Our results were consistent with the previously reported results regarding platelet counts and plateletcrit (PCT). They showed a decrease with increasing age. The mean platelet volume (MPV), platelet distribution width (PDW) and platelet-large cell ratio (P-LCR) increased with age, but did not present any gender effect. All the IPF phenotypes increased with age, whereas the PFA phenotypes did not show any relation to age or gender.

DISCUSSION

To sum up, our study provides a comprehensive analysis of the age and gender effects on the platelet phenotypes in a family-base sample. Our results suggest more reasonable age stratification into two distinct groups: childhood, ranging from 2 to 12 years, and the mature group, from 13 to 93 years. Moreover, the PFA phenotypes were maintained constant while the platelet counts, the MPV and IPF levels vary with age.

IL-1α induces thrombopoiesis through megakaryocyte rupture in response to acute platelet needs

An alternative pathway triggering enhanced platelet release from bone marrow megakaryocytes via a rupture-based mechanism is regulated by IL-1α in response to acute platelet requirements.

Intravital visualization of thrombopoiesis revealed that formation of proplatelets, which are cytoplasmic protrusions in bone marrow megakaryocytes (MKs), is dominant in the steady state. However, it was unclear whether this is the only path to platelet biogenesis. We have identified an alternative MK rupture, which entails rapid cytoplasmic fragmentation and release of much larger numbers of platelets, primarily into blood vessels, which is morphologically and temporally different than typical FasL-induced apoptosis. Serum levels of the inflammatory cytokine IL-1α were acutely elevated after platelet loss or administration of an inflammatory stimulus to mice, whereas the MK-regulator thrombopoietin (TPO) was not elevated. Moreover, IL-1α administration rapidly induced MK rupture–dependent thrombopoiesis and increased platelet counts. IL-1α–IL-1R1 signaling activated caspase-3, which reduced plasma membrane stability and appeared to inhibit regulated tubulin expression and proplatelet formation, and ultimately led to MK rupture. Collectively, it appears the balance between TPO and IL-1α determines the MK cellular programming for thrombopoiesis in response to acute and chronic platelet needs.

Road blocks in making platelets for transfusion

The production of laboratory-generated human platelets is necessary to meet present and future transfusion needs. This manuscript will identify and define the major roadblocks that must be overcome to make human platelet production possible for clinical use, and propose solutions necessary to accelerate development of laboratory-generated human platelets to market.

Anagrelide platelet-lowering effect is due to inhibition of both megakaryocyte maturation and proplatelet formation: insight into potential mechanisms

BACKGROUND AND OBJECTIVES

Anagrelide represents a treatment option for essential thrombocythemia patients. It lowers platelet counts through inhibition of megakaryocyte maturation and polyploidization, although the basis for this effect remains unclear. Based on its rapid onset of action, we assessed whether, besides blocking megakaryopoiesis, anagrelide represses proplatelet formation (PPF) and aimed to clarify the underlying mechanisms.

METHODS AND RESULTS

Exposure of cord blood-derived megakaryocytes to anagrelide during late stages of culture led to a dose- and time-dependent inhibition of PPF and reduced proplatelet complexity, which were independent of the anagrelide-induced effect on megakaryocyte maturation. Whereas anagrelide was shown to phosphorylate cAMP-substrate VASP, two pharmacologic inhibitors of the cAMP pathway were completely unable to revert anagrelide-induced repression in megakaryopoiesis and PPF, suggesting these effects are unrelated to its ability to inhibit phosphodiesterase (PDE) 3. The reduction in thrombopoiesis was not the result of down-regulation of transcription factors which coordinate PPF, while the myosin pathway was identified as a candidate target, as anagrelide was shown to phosphorylate the myosin light chain and the PPF phenotype was partially rescued after inhibition of myosin activity with blebbistatin.

CONCLUSIONS

The platelet-lowering effect of anagrelide results from impaired megakaryocyte maturation and reduced PPF, both of which are deregulated in essential thrombocythemia. These effects seem unrelated to PDE3 inhibition, which is responsible for anagrelide's cardiovascular side-effects and antiplatelet activity. Further work in this field may lead to the potential development of drugs to treat thrombocytosis in myeloproliferative disorders with an improved pharmacologic profile.

New frontiers for platelet CD154

The role of platelets extends beyond hemostasis. The pivotal role of platelets in inflammation has shed new light on the natural history of conditions associated with acute or chronic inflammation. Beyond the preservation of vascular integrity, platelets are essential to tissue homeostasis and platelet-derived products are already used in the clinics. Unanticipated was the role of platelets in the adaptative immune response, allowing a renewed conceptual approach of auto-immune diseases. Platelets are also important players in cancer growth and dissemination. Platelets fulfill most of their functions through the expression of still incompletely characterized membrane-bound or soluble mediators. Among them, CD154 holds a peculiar position, as platelets represent a major source of CD154 and as CD154 contributes to most of these new platelet attributes. Here, we provide an overview of some of the new frontiers that the study of platelet CD154 is opening, in inflammation, tissue homeostasis, immune response, hematopoiesis and cancer.

New explanations for old observations: marginal band coiling during platelet activation

Blood platelets are tiny cell fragments derived from megakaryocytes. Their primary function is to control blood vessel integrity and ensure hemostasis if a vessel wall is damaged. Circulating quiescent platelets have a flat, discoid shape maintained by a circumferential microtubule bundle, called the marginal band (MB). In the case of injury platelets are activated and rapidly adopt a spherical shape due to microtubule motor-induced elongation and subsequent coiling of the MB. Platelet activation and shape change can be transient or become irreversible. This depends on the strength of the activation stimulus, which is translated into a cytoskeletal crosstalk between microtubules, their motors and the actomyosin cortex, ensuring stimulus-response coupling. Following microtubule motor-driven disc-to-sphere transition, a strong stimulus will lead to compression of the sphere through actomyosin cortex contraction. This will concentrate the granules in the center of the platelet and accelerate their exocytosis. Once granules are released, platelets have crossed the point of no return to irreversible activation. This review summarizes the current knowledge of the molecular mechanism leading to platelet shape change, with a special emphasis on microtubules, and refers to previously published observations, which have been essential for generating an integrated view of cytoskeletal rearrangements during platelet activation.

Actin cytoskeletal defects in immunodeficiency

The importance of the cytoskeleton in mounting a successful immune response is evident from the wide range of defects that occur in actin-related primary immunodeficiencies (PIDs). Studies of these PIDs have revealed a pivotal role for the actin cytoskeleton in almost all stages of immune system function, from hematopoiesis and immune cell development, through to recruitment, migration, intercellular and intracellular signaling, and activation of both innate and adaptive immune responses. The major focus of this review is the immune defects that result from mutations in the Wiskott-Aldrich syndrome gene (WAS), which have a broad impact on many different processes and give rise to clinically heterogeneous immunodeficiencies. We also discuss other related genetic defects and the possibility of identifying new genetic causes of cytoskeletal immunodeficiency.

Platelets and cancer: a casual or causal relationship: revisited

Human platelets arise as subcellular fragments of megakaryocytes in bone marrow. The physiologic demand, presence of disease such as cancer, or drug effects can regulate the production circulating platelets. Platelet biology is essential to hemostasis, vascular integrity, angiogenesis, inflammation, innate immunity, wound healing, and cancer biology. The most critical biological platelet response is serving as "First Responders" during the wounding process. The exposure of extracellular matrix proteins and intracellular components occurs after wounding. Numerous platelet receptors recognize matrix proteins that trigger platelet activation, adhesion, aggregation, and stabilization. Once activated, platelets change shape and degranulate to release growth factors and bioactive lipids into the blood stream. This cyclic process recruits and aggregates platelets along with thrombogenesis. This process facilitates wound closure or can recognize circulating pathologic bodies. Cancer cell entry into the blood stream triggers platelet-mediated recognition and is amplified by cell surface receptors, cellular products, extracellular factors, and immune cells. In some cases, these interactions suppress immune recognition and elimination of cancer cells or promote arrest at the endothelium, or entrapment in the microvasculature, and survival. This supports survival and spread of cancer cells and the establishment of secondary lesions to serve as important targets for prevention and therapy.

Thrombocytosis as a prognostic marker in gastrointestinal cancers

Thrombocytosis is an adverse prognostic factor in many types of cancer. These include breast cancer, ovarian and other gynecologic cancers, renal cell carcinoma and lung cancers. In gastrointestinal cancers of various locations and histologic types, thrombocytosis has been reported in general to be associated with adverse clinical outcomes. Platelet count measurement is well standardized and available in every clinical laboratory, making its use as a prognostic marker practical. This paper will discuss the data on the prognostic value of thrombocytosis in gastrointestinal cancers as well as pathogenic aspects of the association that strengthen the case for its use in clinical prognostication.

Biogenesis of the demarcation membrane system (DMS) in megakaryocytes

Using state-of-the-art three-dimensional electron microscopy approaches, we show that the onset of the DMS formation is at the megakaryocyte plasma membrane. A pre-DMS structure is formed in the perinuclear region, through a PM invagination process that resembles cleavage furrow formation.

Modeling stochastic kinetics of molecular machines at multiple levels: from molecules to modules

A molecular machine is either a single macromolecule or a macromolecular complex. In spite of the striking superficial similarities between these natural nanomachines and their man-made macroscopic counterparts, there are crucial differences. Molecular machines in a living cell operate stochastically in an isothermal environment far from thermodynamic equilibrium. In this mini-review we present a catalog of the molecular machines and an inventory of the essential toolbox for theoretically modeling these machines. The tool kits include 1), nonequilibrium statistical-physics techniques for modeling machines and machine-driven processes; and 2), statistical-inference methods for reverse engineering a functional machine from the empirical data. The cell is often likened to a microfactory in which the machineries are organized in modular fashion; each module consists of strongly coupled multiple machines, but different modules interact weakly with each other. This microfactory has its own automated supply chain and delivery system. Buoyed by the success achieved in modeling individual molecular machines, we advocate integration of these models in the near future to develop models of functional modules. A system-level description of the cell from the perspective of molecular machinery (the mechanome) is likely to emerge from further integrations that we envisage here.

Myosin IIA is critical for organelle distribution and F-actin organization in megakaryocytes and platelets

During proplatelet formation, a relatively homogeneous content of organelles is transported from the megakaryocyte (MK) to the nascent platelets along microtubule tracks. We found that platelets from Myh9(-/-) mice and a MYH9-RD patient were heterogeneous in their organelle content (granules and mitochondria). In addition, Myh9(-/-) MKs have an abnormal cytoplasmic clustering of organelles, suggesting that the platelet defect originates in the MKs. Myosin is not involved in the latest stage of organelle traffic along microtubular tracks in the proplatelet shafts as shown by confocal observations of proplatelet buds. By contrast, it is required for the earlier distribution of organelles within the large MK preplatelet fragments shed into the sinusoid circulation before terminal proplatelet remodeling. We show here that F-actin is abnormally clustered in the cytoplasm of Myh9(-/-) MKs and actin polymerization is impaired in platelets. Myosin IIA is required for normal granule motility and positioning within MKs, mechanisms that may be dependent on organelle traveling and tethering onto F-actin cytoskeleton tracks. Altogether, our results indicate that the distribution of organelles within platelets critically depends on a homogeneous organelle distribution within MKs and preplatelet fragments, which requires myosin IIA.

Platelets as immune cells in infectious diseases

Platelets have been shown to cover a broad range of functions. Besides their role in hemostasis, they have immunological functions and thus participate in the interaction between pathogens and host defense. Platelets have a broad repertoire of receptor molecules that enable them to sense invading pathogens and infection-induced inflammation. Consequently, platelets exert antimicrobial effector mechanisms, but also initiate an intense crosstalk with other arms of the innate and adaptive immunity, including neutrophils, monocytes/macrophages, dendritic cells, B cells and T cells. There is a fragile balance between beneficial antimicrobial effects and detrimental reactions that contribute to the pathogenesis, and many pathogens have developed mechanisms to influence these two outcomes. This review aims to highlight aspects of the interaction strategies between platelets and pathogenic bacteria, viruses, fungi and parasites, in addition to the subsequent networking between platelets and other immune cells, and the relevance of these processes for the pathogenesis of infections.

RhoA is essential for maintaining normal megakaryocyte ploidy and platelet generation

RhoA plays a multifaceted role in platelet biology. During platelet development, RhoA has been proposed to regulate endomitosis, proplatelet formation, and platelet release, in addition to having a role in platelet activation. These processes were previously studied using pharmacological inhibitors in vitro, which have potential drawbacks, such as non-specific inhibition or incomplete disruption of the intended target proteins. Therefore, we developed a conditional knockout mouse model utilizing the CRE-LOX strategy to ablate RhoA, specifically in megakaryocytes and in platelets to determine its role in platelet development. We demonstrated that deleting RhoA in megakaryocytes in vivo resulted in significant macrothrombocytopenia. RhoA-null megakaryocytes were larger, had higher mean ploidy, and exhibited stiff membranes with micropipette aspiration. However, in contrast to the results observed in experiments relying upon pharmacologic inhibitors, we did not observe any defects in proplatelet formation in megakaryocytes lacking RhoA. Infused RhoA-null megakaryocytes rapidly released platelets, but platelet levels rapidly plummeted within several hours. Our evidence supports the hypothesis that changes in membrane rheology caused infused RhoA-null megakaryocytes to prematurely release aberrant platelets that were unstable. These platelets were cleared quickly from circulation, which led to the macrothrombocytopenia. These observations demonstrate that RhoA is critical for maintaining normal megakaryocyte development and the production of normal platelets.

ROS-mediated platelet generation: a microenvironment-dependent manner for megakaryocyte proliferation, differentiation, and maturation

Platelets have an important role in the body because of their manifold functions in haemostasis, thrombosis, and inflammation. Platelets are produced by megakaryocytes (MKs) that are differentiated from haematopoietic stem cells via several consecutive stages, including MK lineage commitment, MK progenitor proliferation, MK differentiation and maturation, cell apoptosis, and platelet release. During differentiation, the cells migrate from the osteoblastic niche to the vascular niche in the bone marrow, which is accompanied by reactive oxygen species (ROS)-dependent oxidation state changes in the microenvironment, suggesting that ROS can distinctly influence platelet generation and function in a microenvironment-dependent manner. The objective of this review is to reveal the role of ROS in regulating MK proliferation, differentiation, maturation, and platelet activation, thereby providing new insight into the mechanism of platelet generation, which may lead to the development of new therapeutic agents for thrombocytopenia and/or thrombosis.

Platelet aggregation and quality control of platelet concentrates produced in the Amazon Blood Bank

Background

The study of platelet aggregation is essential to assess in vitro platelet function by different platelet activation pathways.

Objective

To assess aggregation and biochemical parameters of random platelet concentrates produced at the Fundação HEMOAM using the quality control tests defined by law.

Methods

Whole blood samples from 80 donors and the respective platelet concentrate units were tested. Platelet concentrates were tested (platelet count, aggregation and pH) on days 1, 3 and 5 of storage. Additionally a leukocyte count was done only on day 1 and microbiological tests on day 5 of storage. Collagen and adenosine diphosphate were used as inducing agonists for platelet aggregation testing.

Results

Donor whole blood had normal aggregation (aggregation with adenosine diphosphate = 67% and with collagen = 78%). The median aggregation in platelet concentrates with adenosine diphosphate was low throughout storage (18% on day 1, 7% on day 3 and 6% on day 5) and the median aggregation with collagen was normal only on day 1 and low thereafter (54.4% on day 1, 20.5% on day 3 and 9% on day 5).

Conclusion

Although the results were within the norms required by law, platelet concentrates had low aggregation rates. We suggest the inclusion of a functional assessment test for the quality control of platelet concentrates for a more effective response to platelet replacement therapy.

The biology of thrombopoietin and thrombopoietin receptor agonists

Thrombopoietin (TPO) is the major physiological regulator of platelet production. TPO binds the TPO receptor, activates JAK and STAT pathways, thus stimulating megakaryocyte growth and platelet production. There is no "sensor" of the platelet count; rather TPO is produced in the liver at a constant rate and cleared by TPO receptors on platelets. TPO levels are inversely proportional to the rate of platelet production. Early recombinant TPO molecules were potent stimulators of platelet production and increased platelets in patients with immune thrombocytopenia, chemotherapy-induced thrombocytopenia, myelodysplastic syndromes and platelet apheresis donors. Neutralizing antibodies formed against one recombinant protein and ended their development. A second generation of TPO receptor agonists, romiplostim and eltrombopag, has been developed. Romiplostim is an IgG heavy chain into which four TPO agonist peptides have been inserted. Eltrombopag is an oral small molecule. These activate the TPO receptor by different mechanisms to increase megakaryocyte growth and platelet production. After administration of either to healthy volunteers, there is a delay of 5 days before the platelet count rises and subsequently reaches a peak after 12-14 days. Both have been highly effective in treating ITP and hepatitis C thrombocytopenia. Studies in a wide variety of other thrombocytopenic conditions are underway.

Sphingosine kinase 2 (Sphk2) regulates platelet biogenesis by providing intracellular sphingosine 1-phosphate (S1P)

Human megakaryocytes (MKs) release trillions of platelets each day into the circulation to maintain normal homeostatic platelet levels. We have previously shown that extracellular sphingosine 1-phosphate (S1P) plays a key role in thrombopoiesis via its receptor S1pr1. In addition to its role as an extracellular mediator, S1P can also function as a second messenger in the intracellular compartment. Although signaling via intracellular S1P is involved in various cellular processes, a role in thrombopoiesis has not been examined. Sphingosine kinases are the key enzymes that produce intracellular S1P. Here we report that sphingosine kinase 2 (Sphk2) is the major messenger RNA species present in MKs. Sphk2 predominantly localizes to the nucleus and is the major source of intracellular S1P in MKs. Loss of Sphk2 significantly reduced intracellular S1P in MKs and downregulated the expression and activity of Src family kinases (SFKs). Loss of Sphk2 and inhibition of SFK activity resulted in defective intravascular proplatelet shedding, the final stage of thrombopoiesis. Correspondingly, mice lacking Sphk2 in the hematopoietic system display thrombocytopenia. Together, our data suggest that Sphk2 provides the source of intracellular S1P that controls thrombopoiesis, which is associated with SFK expression and activity in MKs.

Transcription factors in late megakaryopoiesis and related platelet disorders

Cell type-specific transcription factors regulate the repertoire of genes expressed in a cell and thereby determine its phenotype. The differentiation of megakaryocytes, the platelet progenitors, from hematopoietic stem cells is a well-known process that can be mimicked in culture. However, the efficient formation of platelets in culture remains a challenge. Platelet formation is a complicated process including megakaryocyte maturation, platelet assembly and platelet shedding. We hypothesize that a better understanding of the transcriptional regulation of this process will allow us to influence it such that sufficient numbers of platelets can be produced for clinical applications. After an introduction to gene regulation and platelet formation, this review summarizes the current knowledge of the regulation of platelet formation by the transcription factors EVI1, GATA1, FLI1, NFE2, RUNX1, SRF and its co-factor MKL1, and TAL1. Also covered is how some platelet disorders including myeloproliferative neoplasms, result from disturbances of the transcriptional regulation. These disorders give us invaluable insights into the crucial role these transcription factors play in platelet formation. Finally, there is discussion of how a better understanding of these processes will be needed to allow for efficient production of platelets in vitro.

Can platelet-rich plasma (PRP) improve bone healing? A comparison between the theory and experimental outcomes

The increased concentration of platelets within platelet-rich plasma (PRP) provides a vehicle to deliver supra-physiologic concentrations of growth factors to an injury site, possibly accelerating or otherwise improving connective tissue regeneration. This potential benefit has led to the application of PRP in several applications; however, inconsistent results have limited widespread adoption in bone healing. This review provides a core understanding of the bone healing mechanisms, and corresponds this to the factors present in PRP. In addition, the current state of the art of PRP preparation, the key aspects that may influence its effectiveness, and treatment outcomes as they relate specifically to bone defect healing are presented. Although PRP does have a sound scientific basis, its use for bone healing appears only beneficial when used in combination with osteoconductive scaffolds; however, neither allograft nor autograft appear to be appropriate carriers. Aggressive processing techniques and very high concentrations of PRP may not improve healing outcomes. Moreover, many other variables exist in PRP preparation and use that influence its efficacy; the effect of these variables should be understood when considering PRP use. This review includes the essentials of what has been established, what is currently missing in the literature, and recommendations for future directions.

What we have learned from inherited platelet disorders

Identifying the molecular basis of inherited platelet disorders has contributed to our understanding of normal platelet physiology. Many of these conditions are rare, but close observation of clinical and laboratory phenotype, and subsequent identification of the abnormal protein and mutated gene, have provided us with unique opportunities to examine specific aspects of platelet biogenesis and function. Phenotype-genotype association studies are providing a detailed understanding of the structure and function of platelet membrane receptors, the biogenesis and release of platelet granules, and the assembly of the cytoskeleton. Genetic polymorphisms contributing to decreased or increased platelet adhesion and activation may translate into increased clinical risks for bleeding or thrombosis. More recently, genome wide association studies have identified new genes contributing to the variation in normal platelet function.

Conditional knockout of integrin α2β1 in murine megakaryocytes leads to reduced mean platelet volume

We have engineered a transgenic mouse on a C57BL/6 background that bears a floxed Itga2 gene. The crossing of this mouse strain to transgenic mice expressing Cre recombinase driven by the megakaryocyte (MK)-specific Pf4 promoter permits the conditional knockout of Itga2 in the MK/platelet lineage. Mice lacking MK α2β1 develop normally, are fertile, and like their systemic α2β1 knockout counterparts, exhibit defective adhesion to and aggregation induced by soluble type I collagen and a delayed onset to low dose fibrillar collagen-induced aggregation, results consistent with blockade or loss of platelet α2β1. At the same time, we observed a significant reduction in mean platelet volume, which is consistent with the reported role of α2β1 in MK maturation and proplatelet formation in vivo. This transgenic mouse strain bearing a floxed Itga2 gene will prove valuable to distinguish in vivo the temporal and spatial contributions of α2 integrin in selected cell types.

Aspirin-induced microscopic surface changes stimulate thrombopoiesis in rat megakaryocytes

During the process of thrombopoiesis, invaginations of the plasma membrane occur in megakaryocytes. Since acetylsalicylic acid (aspirin), the most commonly used anti-inflammatory and antiplatelet drug, interacts with the lipid bilayers of the plasma membranes, this drug would affect the process of thrombopoiesis. In the present study, employing a standard patch-clamp whole-cell recording technique, we examined the effects of aspirin on delayed rectifier K(+)-channel (Kv1.3) currents and the membrane capacitance in megakaryocytes. Using confocal imaging of di-8-butyl-amino-naphthyl-ethylene-pyridinium-propyl-sulfonate (di-8-ANEPPS) staining, we also monitored the membrane invaginations in megakaryocytes. Aspirin suppressed both the peak and the pulse-end currents with a significant increase in the membrane capacitance. Massive di-8-ANEPPS staining after treatment with aspirin demonstrated the impaired membrane micro-architecture of megakaryocytes. This study demonstrated for the first time that aspirin induces microscopic surface changes in megakaryocytes. Such surface changes were thought to stimulate thrombopoiesis in megakaryocytes as detected by the increase in the membrane invaginations.

Platelets in lung biology

Platelets and the lungs have an intimate relationship. Platelets are anucleate mammalian blood cells that continuously circulate through pulmonary vessels and that have major effector activities in hemostasis and inflammation. The lungs are reservoirs for megakaryocytes, the requisite precursor cell in thrombopoiesis, which is the intricate process by which platelets are generated. Platelets contribute to basal barrier integrity of the alveolar capillaries, which selectively restricts the transfer of water, proteins, and red blood cells out of the vessels. Platelets also contribute to pulmonary vascular repair. Although platelets bolster hemostatic and inflammatory defense of the healthy lung, experimental evidence and clinical evidence indicate that these blood cells are effectors of injury in a variety of pulmonary disorders and syndromes. Newly discovered biological capacities of platelets are being explored in the context of lung defense, disease, and remodeling.

Inherited thrombocytopenias: the evolving spectrum

The chapter of inherited thrombocytopenias has expanded greatly over the last decade and many "new" forms deriving from mutations in "new" genes have been identified. Nevertheless, nearly half of patients remain without a definite diagnosis because their illnesses have not yet been described. The diagnostic approach to these diseases can still take advantage of the algorithm proposed by the Italian Platelet Study Group in 2003, although an update is required to include the recently described disorders. So far, transfusions of platelet concentrates have represented the main tool for preventing or treating bleedings, while haematopoietic stem cell transplantation has been reserved for patients with very severe forms. However, recent disclosure that an oral thrombopoietin mimetic is effective in increasing platelet count in patients with MYH9-related thrombocytopenia opened new therapeutic perspectives. This review summarizes the general aspects of inherited thrombocytopenias and describes in more detail MYH9-related diseases (encompassing four thrombocytopenias previously recognized as separate diseases) and the recently described ANKRD26-related thrombocytopenia, which are among the most frequent forms of inherited thrombocytopenia.

Why does my patient have thrombocytopenia?

Thrombocytopenia, usually defined as a platelet count of less than 150,000/μL, is a common reason for a hematology consult in both the inpatient and outpatient setting. In most patients, the cause of the thrombocytopenia can be identified and treated. This article reviews the clinical approach to the patient with thrombocytopenia, the mechanisms that underlie it, and the laboratory tests available to investigate it. A practical approach to the investigation and management of thrombocytopenia in the clinical settings commonly encountered by the hematology consultant is then described.

High-content live-cell imaging assay used to establish mechanism of trastuzumab emtansine (T-DM1)--mediated inhibition of platelet production

Proplatelet production represents a terminal stage of megakaryocyte development during which long, branching processes composed of platelet-sized swellings are extended and released into the surrounding culture. Whereas the cytoskeletal mechanics driving these transformations have been the focus of many studies, significant limitations in our ability to quantify the rate and extent of proplatelet production have restricted the field to qualitative analyses of a limited number of cells over short intervals. A novel high-content, quantitative, live-cell imaging assay using the IncuCyte system (Essen BioScience) was therefore developed to measure the rate and extent of megakaryocyte maturation and proplatelet production under live culture conditions for extended periods of time. As proof of concept, we used this system in the present study to establish a mechanism by which trastuzumab emtansine (T-DM1), an Ab-drug conjugate currently in clinical development for cancer, affects platelet production. High-content analysis of primary cell cultures revealed that T-DM1 is taken up by mouse megakaryocytes, inhibits megakaryocyte differentiation, and disrupts proplatelet formation by inducing abnormal tubulin organization and suppressing microtubule dynamic instability. Defining the pathways by which therapeutics such as T-DM1 affect megakaryocyte differentiation and proplatelet production may yield strategies to manage drug-induced thrombocytopenias.

BAR proteins in cancer and blood disorders

Remodeling of the membrane and cytoskeleton is involved in a wide range of normal and pathologic cellular function. These are complex, highly-coordinated biochemical and biophysical processes involving dozens of proteins. Serving as a scaffold for a variety of proteins and possessing a domain that interacts with plasma membranes, the BAR family of proteins contribute to a range of cellular functions characterized by membrane and cytoskeletal remodeling. There are several subgroups of BAR proteins: BAR, N-BAR, I-BAR, and F-BAR. They differ in their ability to induce angles of membrane curvature and in their recruitment of effector proteins. Evidence is accumulating that BAR proteins contribute to cancer cell invasion, T cell trafficking, phagocytosis, and platelet production. In this review, we discuss the physiological function of BAR proteins and discuss how they contribute to blood and cancer disorders.

Panobinostat (LBH589)-induced acetylation of tubulin impairs megakaryocyte maturation and platelet formation

Drug-induced thrombocytopenia often results from dysregulation of normal megakaryocytopoiesis. In this study, we investigated the mechanisms responsible for thrombocytopenia associated with the use of Panobinostat (LBH589), a histone deacetylase inhibitor with promising anti-cancer activities. The effects of LBH589 were tested on the cellular and molecular aspects of megakaryocytopoiesis by utilizing an ex vivo system in which mature megakaryocytes (MK) and platelets were generated from human primary CD34(+) cells. We demonstrated that LBH589 did not affect MK proliferation or lineage commitment but inhibited MK maturation and platelet formation. Although LBH589 treatment of primary MK resulted in hyperacetylation of histones, it did not interfere with the expression of genes that play important roles during megakaryocytopoiesis. Instead, we found that LBH589 induced post-translational modifications of tubulin, a nonhistone protein that is the major component of the microtubule cytoskeleton. We then demonstrated that LBH589 treatment induced hyperacetylation of tubulin and alteration of microtubule dynamics and organization required for proper MK maturation and platelet formation. This study provides new insights into the mechanisms underlying LBH589-induced thrombocytopenia and provides a rationale for using tubulin as a target for selective histone deacetylase inhibitor therapies to treat thrombocytosis in patients with myeloproliferative neoplasms.

Hirudin and heparin enable efficient megakaryocyte differentiation of mouse bone marrow progenitors

Hematopoietic progenitors from murine fetal liver efficiently differentiate in culture into proplatelet-producing megakaryocytes and have proved valuable to study platelet biogenesis. In contrast, megakaryocyte maturation is far less efficient in cultured bone marrow progenitors, which hampers studies in adult animals. It is shown here that addition of hirudin to media containing thrombopoietin and serum yielded a proportion of proplatelet-forming megakaryocytes similar to that in fetal liver cultures (approximately 50%) with well developed extensions and increased the release of platelet particles in the media. The effect of hirudin was maximal at 100 U/ml, and was more pronounced when it was added in the early stages of differentiation. Hirugen, which targets the thrombin anion binding exosite I, and argatroban, a selective active site blocker, also promoted proplatelet formation albeit less efficiently than hirudin. Heparin, an indirect thrombin blocker, and OTR1500, a stable heparin-like synthetic glycosaminoglycan generated proplatelets at levels comparable to hirudin. Heparin with low affinity for antithrombin was equally as effective as standard heparin, which indicates antithrombin independent effects. Use of hirudin and heparin compounds should lead to improved culture conditions and facilitate studies of platelet biogenesis in adult mice.

Mouse models of MYH9-related disease: mutations in nonmuscle myosin II-A

We have generated 3 mouse lines, each with a different mutation in the nonmuscle myosin II-A gene, Myh9 (R702C, D1424N, and E1841K). Each line develops MYH9-related disease similar to that found in human patients. R702C mutant human cDNA fused with green fluorescent protein was introduced into the first coding exon of Myh9, and D1424N and E1841K mutations were introduced directly into the corresponding exons. Homozygous R702C mice die at embryonic day 10.5-11.5, whereas homozygous D1424N and E1841K mice are viable. All heterozygous and homozygous mutant mice show macrothrombocytopenia with prolonged bleeding times, a defect in clot retraction, and increased extramedullary megakaryocytes. Studies of cultured megakaryocytes and live-cell imaging of megakaryocytes in the BM show that heterozygous R702C megakaryocytes form fewer and shorter proplatelets with less branching and larger buds. The results indicate that disrupted proplatelet formation contributes to the macrothrombocytopenia in mice and most probably in humans. We also observed premature cataract formation, kidney abnormalities, including albuminuria, focal segmental glomerulosclerosis and progressive kidney disease, and mild hearing loss. Our results show that heterozygous mice with mutations in the myosin motor or filament-forming domain manifest similar hematologic, eye, and kidney phenotypes to humans with MYH9-related disease.

Novel loci involved in platelet function and platelet count identified by a genome-wide study performed in children

BACKGROUND

Genome-wide association studies are currently identifying new loci with potential roles in thrombosis and hemostasis: these loci include novel polymorphisms associated with platelet function traits and count. However, no genome-wide study performed on children has been reported to date, in spite of the potential that these subjects have in genetic studies, when compared to adults, given the minimal degree of confounders, i.e., acquired and environmental factors, such as smoking, physical activity, diet, and drug or hormone intake, which are particularly important in platelet function.

DESIGN AND METHODS

To identify new genetic variants involved in platelet reactivity and count, we performed a genome-wide association study on 75 children (8.5±1.8 years) using the Illumina Sentrix Human CNV370-Quad BeadChip containing 320,610 single nucleotide polymorphisms. Functional analyses included assessment of platelet aggregation and granule secretion triggered by different agonists (arachidonic acid, collagen, epinephrine, ADP), as well as platelet count. Associations were selected based on statistical significance and physiological relevance for a subsequent replication study in a similar sample of 286 children.

RESULTS

We confirmed previously established associations with plasma levels of factors XII, VII and VIII as well as associations with platelet responses to ADP. Additionally, we identified 82 associations with platelet reactivity and count with a P value less than 10(-5). From the associations selected for further replication, we validated two single nucleotide polymorphisms with mildly increased platelet reactivity (rs4366150 and rs1787566) on the LPAR1 and MYO5B genes, encoding lisophosphatidic acid receptor-1 and myosin VB, respectively; and rs1937970, located on the NRG3 gene coding neuroregulin-3, associated with platelet count.

CONCLUSIONS

Our genome-wide association study performed in children, followed by a validation analysis, led us to the identification of new genes potentially relevant in platelet function and biogenesis.