Properties of the demarcation membrane system in living rat megakaryocytes

What It Takes To Be a Platelet: Evolving Concepts in Platelet Production

Platelets are among the most abundant cells within the circulation. Given that the platelet lifespan is 7 to 10 days in humans, a constant production of around 100 billion platelets per day is required. Platelet production from precursor cells called megakaryocytes is one of the most enigmatic processes in human biology. Although it has been studied for over a century, there is still controversy about the exact mechanisms leading to platelet release into circulation. The formation of proplatelet extensions from megakaryocytes into bone marrow sinusoids is the best-described mechanism explaining the origin of blood platelets. However, using powerful imaging techniques, several emerging studies have recently raised challenging questions in the field, suggesting that small platelet-sized structures called buds might also contribute to the circulating platelet pool. How and whether these structures differ from microvesicles or membrane blebs, which have previously been described to be released from megakaryocytes, is still a matter of discussion. In this review, we will summarize what the past and present have revealed about platelet production and whether mature blood platelets might emerge via different mechanisms.

A theory of rapid evolutionary change explaining the de novo appearance of megakaryocytes and platelets in mammals

Platelets are found only in mammals. Uniquely, they have a log Gaussian volume distribution and are produced from megakaryocytes, large cells that have polyploid nuclei. In this Hypothesis, we propose that a possible explanation for the origin of megakaryocytes and platelets is that, ∼220 million years ago, an inheritable change occurred in a mammalian ancestor that caused the haemostatic cell line of the animal to become polyploid. This inheritable change occurred specifically in the genetic programme of the cell lineage from which the haemostatic cell originated and led, because of increase in cell size, to its fragmentation into cytoplasmic particles (platelets) in the pulmonary circulatory system, as found in modern mammals. We hypothesize that these fragments originating from the new large haemostatic polyploid cells proved to be more efficient at stopping bleeding, and, therefore, the progeny of this ancestor prospered through natural selection. We also propose experimental strategies that could provide evidence to support this hypothesis.

Ca2+ waves coordinate purinergic receptor-evoked integrin activation and polarization

Cells sense extracellular nucleotides through the P2Y class of purinergic G protein-coupled receptors (GPCRs), which stimulate integrin activation through signaling events, including intracellular Ca²⁺ mobilization. We investigated the relationship between P2Y-stimulated repetitive Ca²⁺ waves and fibrinogen binding to the platelet integrin α_IIbβ₃ (GPIIb/IIIa) through confocal fluorescence imaging of primary rat megakaryocytes. Costimulation of the receptors P2Y₁ and P2Y₁₂ generated a series of Ca²⁺ transients that each induced a rapid, discrete increase in fibrinogen binding. The peak and net increase of individual fibrinogen binding events correlated with the Ca²⁺ transient amplitude and frequency, respectively. Using BAPTA loading and selective receptor antagonists, we found that Ca²⁺ mobilization downstream of P2Y₁ was essential for ADP-evoked fibrinogen binding, whereas P2Y₁₂ and the kinase PI3K were also required for α_IIbβ₃ activation and enhanced the number of Ca²⁺ transients. ADP-evoked fibrinogen binding was initially uniform over the cell periphery but subsequently redistributed with a polarity that correlated with the direction of the Ca²⁺ waves. Polarization of α_IIbβ₃ may be mediated by the actin cytoskeleton, because surface-bound fibrinogen is highly immobile, and its motility was enhanced by cytoskeletal disruption. In conclusion, spatial and temporal patterns of Ca²⁺ increase enable fine control of α_IIbβ₃ activation after cellular stimulation. P2Y₁-stimulated Ca²⁺ transients coupled to α_IIbβ₃ activation only in the context of P2Y₁₂ coactivation, thereby providing an additional temporal mechanism of synergy between these Gq- and Gi-coupled GPCRs.

Megakaryocyte emperipolesis: a new frontier in cell-in-cell interaction

Histology of bone marrow routinely identifies megakaryocytes that enclose neutrophils and other hematopoietic cells, a phenomenon termed emperipolesis. Preserved across mammalian species and enhanced with systemic inflammation and platelet demand, the nature and significance of emperipolesis remain largely unexplored. Recent advances demonstrate that emperipolesis is in fact a distinct form of cell-in-cell interaction. Following integrin-mediated attachment, megakaryocytes and neutrophils both actively drive entry via cytoskeletal rearrangement. Neutrophils enter a vacuole termed the emperisome which then releases them directly into the megakaryocyte cytoplasm. From this surprising location, neutrophils fuse with the demarcation membrane system to pass membrane to circulating platelets, enhancing the efficiency of thrombocytogenesis. Neutrophils then egress intact, carrying megakaryocyte membrane and potentially other cell components along with them. In this review, we summarize what is known about this intriguing cell-in-cell interaction and discuss potential roles for emperipolesis in megakaryocyte, platelet and neutrophil biology.

Development of a P2X1-eYFP receptor knock-in mouse to track receptors in real time

A P2X1-eYFP knock-in mouse was generated to study receptor expression and mobility in smooth muscle and blood cells. eYFP was added to the C-terminus of the P2X1R and replaced the native P2X1R. Fluorescence corresponding to P2X1-eYFPR was detected in urinary bladder smooth muscle, platelets and megakaryocytes. ATP-evoked currents from wild type and P2X1-eYFP isolated urinary bladder smooth muscle cells had the same peak current amplitude and time-course showing that the eYFP addition had no obvious effect on properties. Fluorescence recovery after photobleaching (FRAP) in bladder smooth muscle cells demonstrated that surface P2X1Rs are mobile and their movement is reduced following cholesterol depletion. Compared to the platelet and megakaryocyte, P2X1-eYFP fluorescence was negligible in red blood cells and the majority of smaller marrow cells. The spatial pattern of P2X1-eYFP fluorescence in the megakaryocyte along with FRAP assessment of mobility suggested that P2X1Rs are expressed extensively throughout the membrane invagination system of this cell type. The current study highlights that the spatiotemporal properties of P2X1R expression can be monitored in real time in smooth muscle cells and megakaryocytes/platelets using the eYFP knock-in mouse model.

Imaging platelet biogenesis in vivo

In this review paper, we give a historical perspective of the development of imaging modalities to visualize platelet biogenesis and how this contributed to our current understanding of megakaryopoiesis and thrombopoiesis. We provide some insight how distinct in vivo and in situ imaging methods, including ultramicrographs, have contributed to the current concepts of platelet formation. The onset of intravital microscopy into the mouse bone marrow has markedly modified and challenged our thinking of platelet biogenesis during the last decade. Finally, we discuss ongoing work, which was presented at the recent International Society on Thrombosis and Haemostasis (ISTH) meeting.

Fluorescence Approaches to Image and Quantify the Demarcation Membrane System in Living Megakaryocytes

The demarcation membrane system (DMS) develops to provide additional surface membrane for the process of platelet production. The DMS is an invagination of the plasma membrane that can extend throughout the extranuclear volume of mature megakaryocytes and its lumen is continuous with the extracellular solution. DMS ultrastructure in fixed samples has been extensively studied using transmission electron microscopy (TEM) and more recently with focused ion beam scanning EM. In addition, whole cell patch clamp membrane capacitance provides a direct measurement of DMS content in living megakaryocytes. However, fluorescence methods to image and quantify the DMS in living megakaryocytes provide several advantages. For example, confocal fluorescence microscopy is easier to use compared to EM or electrophysiological methods and the required equipment is more readily available. In addition, use of living cells avoids artifacts known to occur during the fixation, dehydration, or embedding steps used to prepare EM samples. Here we describe the use of styryl dyes such as FM 1-43 or di-8-ANEPPS and impermeant fluorescent indicators of the extracellular space as simple approaches for imaging and quantification of the DMS.

Importance of environmental stiffness for megakaryocyte differentiation and proplatelet formation

Megakaryocyte (MK) differentiation occurs within the bone marrow (BM), a complex 3-dimensional (3D) environment of low stiffness exerting local external constraints. To evaluate the influence of the 3D mechanical constraints that MKs may encounter in vivo, we differentiated mouse BM progenitors in methylcellulose (MC) hydrogels tuned to mimic BM stiffness. We found that MKs grown in a medium of 30- to 60-Pa stiffness more closely resembled those in the BM in terms of demarcation membrane system (DMS) morphological aspect and exhibited higher ploidy levels, as compared with MKs in liquid culture. Following resuspension in a liquid medium, MC-grown MKs displayed twice as much proplatelet formation as cells grown in liquid culture. Thus, the MC gel, by mimicking external constraints, appeared to positively influence MK differentiation. To determine whether MKs adapt to extracellular stiffness through mechanotransduction involving actomyosin-based modulation of the intracellular tension, myosin-deficient (Myh9^-/-) progenitors were grown in MC gels. Absence of myosin resulted in abnormal cell deformation and strongly decreased proplatelet formation, similarly to features observed for Myh9^-/- MKs differentiated in situ but not in vitro. Moreover, megakaryoblastic leukemia 1 (MKL1), a well-known actor in mechanotransduction, was found to be preferentially relocated within the nucleus of MC-differentiated MKs, whereas its inhibition prevented MC-mediated increased proplatelet formation. Altogether, these data show that a 3D medium mimicking BM stiffness contributes, through the myosin IIA and MKL1 pathways, to a more favorable in vitro environment for MK differentiation, which ultimately translates into increased proplatelet production.

Hyaluronan Depolymerization by Megakaryocyte Hyaluronidase-2 Is Required for Thrombopoiesis

Hyaluronan is the predominant glycosaminoglycan component of the extracellular matrix with an emerging role in hematopoiesis. Modulation of hyaluronan polymer size is responsible for its control over cellular functions, and the balance of hyaluronan synthesis and degradation determines its molecular size. Although two active somatic hyaluronidases are expressed in mammals, only deficiency in hyaluronidase-2 (Hyal-2) results in thrombocytopenia of unknown mechanism. Our results reveal that Hyal-2 knockout mice accumulate hyaluronan within their bone marrow and within megakaryocytes, the cells responsible for platelet generation. Proplatelet formation by Hyal-2 knockout megakaryocytes was disrupted because of abnormal formation of the demarcation membrane system, which was dilated and poorly developed. Importantly, peptide-mediated delivery of exogenous hyaluronidase rescued deficient proplatelet formation in murine and human megakaryocytes lacking Hyal-2. Together, our data uncover a previously unsuspected mechanism of how hyaluronan and Hyal-2 control platelet generation.

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.

Synthesis and dephosphorylation of MARCKS in the late stages of megakaryocyte maturation drive proplatelet formation

Platelets are essential for hemostasis, and thrombocytopenia is a major clinical problem. Megakaryocytes (MKs) generate platelets by extending long processes, proplatelets, into sinusoidal blood vessels. However, very little is known about what regulates proplatelet formation. To uncover which proteins were dynamically changing during this process, we compared the proteome and transcriptome of round vs proplatelet-producing MKs by 2D difference gel electrophoresis (DIGE) and polysome profiling, respectively. Our data revealed a significant increase in a poorly-characterized MK protein, myristoylated alanine-rich C-kinase substrate (MARCKS), which was upregulated 3.4- and 5.7-fold in proplatelet-producing MKs in 2D DIGE and polysome profiling analyses, respectively. MARCKS is a protein kinase C (PKC) substrate that binds PIP2. In MKs, it localized to both the plasma and demarcation membranes. MARCKS inhibition by peptide significantly decreased proplatelet formation 53%. To examine the role of MARCKS in the PKC pathway, we treated MKs with polymethacrylate (PMA), which markedly increased MARCKS phosphorylation while significantly inhibiting proplatelet formation 84%, suggesting that MARCKS phosphorylation reduces proplatelet formation. We hypothesized that MARCKS phosphorylation promotes Arp2/3 phosphorylation, which subsequently downregulates proplatelet formation; both MARCKS and Arp2 were dephosphorylated in MKs making proplatelets, and Arp2 inhibition enhanced proplatelet formation. Finally, we used MARCKS knockout (KO) mice to probe the direct role of MARCKS in proplatelet formation; MARCKS KO MKs displayed significantly decreased proplatelet levels. MARCKS expression and signaling in primary MKs is a novel finding. We propose that MARCKS acts as a "molecular switch," binding to and regulating PIP2 signaling to regulate processes like proplatelet extension (microtubule-driven) vs proplatelet branching (Arp2/3 and actin polymerization-driven).

Detachment of surface membrane invagination systems by cationic amphiphilic drugs

Several cell types develop extensive plasma membrane invaginations to serve a specific physiological function. For example, the megakaryocyte demarcation membrane system (DMS) provides a membrane reserve for platelet production and muscle transverse (T) tubules facilitate excitation:contraction coupling. Using impermeant fluorescent indicators, capacitance measurements and electron microscopy, we show that multiple cationic amphiphilic drugs (CADs) cause complete separation of the DMS from the surface membrane in rat megakaryocytes. This includes the calmodulin inhibitor W-7, the phospholipase-C inhibitor U73122, and anti-psychotic phenothiazines. CADs also caused loss of T tubules in rat cardiac ventricular myocytes and the open canalicular system of human platelets. Anionic amphiphiles, U73343 (a less electrophilic U73122 analogue) and a range of kinase inhibitors were without effect on the DMS. CADs are known to accumulate in the inner leaflet of the cell membrane where they bind to anionic lipids, especially PI(4,5)P2. We therefore propose that surface detachment of membrane invaginations results from an ability of CADs to interfere with PI(4,5)P2 interactions with cytoskeletal or BAR domain proteins. This establishes a detubulating action of a large class of pharmaceutical compounds.

FlnA binding to PACSIN2 F-BAR domain regulates membrane tubulation in megakaryocytes and platelets

Bin-Amphiphysin-Rvs (BAR) and Fes-CIP4 homology BAR (F-BAR) proteins generate tubular membrane invaginations reminiscent of the megakaryocyte (MK) demarcation membrane system (DMS), which provides membranes necessary for future platelets. The F-BAR protein PACSIN2 is one of the most abundant BAR/F-BAR proteins in platelets and the only one reported to interact with the cytoskeletal and scaffold protein filamin A (FlnA), an essential regulator of platelet formation and function. The FlnA-PACSIN2 interaction was therefore investigated in MKs and platelets. PACSIN2 associated with FlnA in human platelets. The interaction required FlnA immunoglobulin-like repeat 20 and the tip of PACSIN2 F-BAR domain and enhanced PACSIN2 F-BAR domain membrane tubulation in vitro. Most human and wild-type mouse platelets had 1 to 2 distinct PACSIN2 foci associated with cell membrane GPIbα, whereas Flna-null platelets had 0 to 4 or more foci. Endogenous PACSIN2 and transfected enhanced green fluorescent protein-PACSIN2 were concentrated in midstage wild-type mouse MKs in a well-defined invagination of the plasma membrane reminiscent of the initiating DMS and dispersed in the absence of FlnA binding. The DMS appeared less well defined, and platelet territories were not readily visualized in Flna-null MKs. We conclude that the FlnA-PACSIN2 interaction regulates membrane tubulation in MKs and platelets and likely contributes to DMS formation.

Physiological significance of delayed rectifier K(+) channels (Kv1.3) expressed in T lymphocytes and their pathological significance in chronic kidney disease

T lymphocytes predominantly express delayed rectifier K(+) channels (Kv1.3) in their plasma membranes. More than 30 years ago, patch-clamp studies revealed that the channels play crucial roles in facilitating the calcium influx necessary to trigger lymphocyte activation and proliferation. In addition to selective channel inhibitors that have been developed, we recently showed physiological evidence that drugs such as nonsteroidal anti-inflammatory drugs, antibiotics, and anti-hypertensives effectively suppress the channel currents in lymphocytes, and thus exert immunosuppressive effects. Using experimental animal models, previous studies revealed the pathological relevance between the expression of ion channels and the progression of renal diseases. As an extension, we recently demonstrated that the overexpression of lymphocyte Kv1.3 channels contributed to the progression of chronic kidney disease (CKD) by promoting cellular proliferation and interstitial fibrosis. Together with our in-vitro results, the studies indicated the therapeutic potency of Kv1.3-channel inhibitors in the treatment or the prevention of CKD.

Systems modeling of Ca(2+) homeostasis and mobilization in platelets mediated by IP3 and store-operated Ca(2+) entry

Resting platelets maintain a stable level of low cytoplasmic calcium ([Ca(2+)]cyt) and high dense tubular system calcium ([Ca(2+)]dts). During thrombosis, activators cause a transient rise in inositol trisphosphate (IP3) to trigger calcium mobilization from stores and elevation of [Ca(2+)]cyt. Another major source of [Ca(2+)]cyt elevation is store-operated calcium entry (SOCE) through plasmalemmal calcium channels that open in response to store depletion as [Ca(2+)]dts drops. A 34-species systems model employed kinetics describing IP3-receptor, DTS-plasmalemma puncta formation, SOCE via assembly of STIM1 and Orai1, and the plasmalemma and sarco/endoplasmic reticulum Ca(2+)-ATPases. Four constraints were imposed: calcium homeostasis before activation; stable in zero extracellular calcium; IP3-activatable; and functional SOCE. Using a Monte Carlo method to sample three unknown parameters and nine initial concentrations in a 12-dimensional space near measured or expected values, we found that model configurations that were responsive to stimuli and demonstrated significant SOCE required high inner membrane electric potential (>-70 mV) and low resting IP3 concentrations. The absence of puncta in resting cells was required to prevent spontaneous store depletion in calcium-free media. Ten-fold increases in IP3 caused saturated calcium mobilization. This systems model represents a critical step in being able to predict platelets' phenotypes during hemostasis or thrombosis.

Dynamin 2-dependent endocytosis is required for normal megakaryocyte development in mice

Dynamins are highly conserved large GTPases (enzymes that hydrolyze guanosine triphosphate) involved in endocytosis and vesicle transport, and mutations in the ubiquitous and housekeeping dynamin 2 (DNM2) have been associated with thrombocytopenia in humans. To determine the role of DNM2 in thrombopoiesis, we generated Dnm2(fl/fl) Pf4-Cre mice specifically lacking DNM2 in the megakaryocyte (MK) lineage. Dnm2(fl/fl) Pf4-Cre mice had severe macrothrombocytopenia with moderately accelerated platelet clearance. Dnm2-null bone marrow MKs had altered demarcation membrane system formation in vivo due to defective endocytic pathway, and fetal liver-derived Dnm2-null MKs formed proplatelets poorly in vitro, showing that DNM2-dependent endocytosis plays a major role in MK membrane formation and thrombopoiesis. Endocytosis of the thrombopoietin receptor Mpl was impaired in Dnm2-null platelets, causing constitutive phosphorylation of the tyrosine kinase JAK2 and elevated circulating thrombopoietin levels. MK-specific DNM2 deletion severely disrupted bone marrow homeostasis, as reflected by marked expansion of hematopoietic stem and progenitor cells, MK hyperplasia, myelofibrosis, and consequent extramedullary hematopoiesis and splenomegaly. Taken together, our data demonstrate that unrestrained MK growth and proliferation results in rapid myelofibrosis and establishes a previously unrecognized role for DNM2-dependent endocytosis in megakaryopoiesis, thrombopoiesis, and bone marrow homeostasis.

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.

Rho kinase inhibition drives megakaryocyte polyploidization and proplatelet formation through MYC and NFE2 downregulation

The processes of megakaryocyte polyploidization and demarcation membrane system (DMS) formation are crucial for platelet production, but the mechanisms controlling these processes are not fully determined. Inhibition of Rho kinase (ROCK) signalling leads to increased polyploidization in umbilical cord blood-derived megakaryocytes. To extend these findings we determined the effect of ROCK inhibition on development of the DMS and on proplatelet formation. The underlying mechanisms were explored by analysing the effect of ROCK inhibition on the expression of MYC and NFE2, which encode two transcription factors critical for megakaryocyte development. ROCK inhibition promoted DMS formation, and increased proplatelet formation and platelet release. Rho kinase inhibition also downregulated MYC and NFE2 expression in mature megakaryocytes, and this down-regulation correlated with increased proplatelet formation. Our findings suggest a model whereby ROCK inhibition drives polyploidization, DMS growth and proplatelet formation late in megakaryocyte maturation through downregulation of MYC and NFE2 expression.

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.

The unique contribution of ion channels to platelet and megakaryocyte function

Ion channels are transmembrane proteins that play ubiquitous roles in cellular homeostasis and activation. In addition to their recognized role in the regulation of ionic permeability and thus membrane potential, some channel proteins possess intrinsic kinase activity, directly interact with integrins or are permeable to molecules up to ≈1000 Da. The small size and anuclear nature of the platelet has often hindered progress in understanding the role of specific ion channels in hemostasis, thrombosis and other platelet-dependent events. However, with the aid of transgenic mice and 'surrogate' patch clamp recordings from primary megakaryocytes, important unique contributions to platelet function have been identified for several classes of ion channel. Examples include ATP-gated P2X1 channels, Orai1 store-operated Ca2+ channels, voltage-gated Kv1.3 channels, AMPA and kainate glutamate receptors and connexin gap junction channels. Furthermore, evidence exists that some ion channels, such as NMDA glutamate receptors, contribute to megakaryocyte development. This review examines the evidence for expression of a range of ion channels in the platelet and its progenitor cell, and highlights the distinct roles that these proteins may play in health and disease.

Suppressive effects of nonsteroidal anti-inflammatory drugs diclofenac sodium, salicylate and indomethacin on delayed rectifier K+-channel currents in murine thymocytes

Lymphocytes predominantly express delayed rectifier K(+)-channels (Kv1.3) in their plasma membranes, and the channels play crucial roles in the lymphocyte activation and proliferation. Since nonsteroidal anti-inflammatory drugs (NSAIDs), the most commonly used analgesic and antipyretic drugs, exert immunomodulatory effects, they would affect the channel currents in lymphocytes. In the present study, employing the standard patch-clamp whole-cell recording technique, we examined the effects of diclofenac sodium, salicylate and indomethacin on the channel currents in murine thymocytes and the membrane capacitance. Diclofenac sodium and salicylate significantly suppressed the pulse-end currents of the channel. However, indomethacin suppressed both the peak and the pulse-end currents with a significant increase in the membrane capacitance. This study demonstrated for the first time that NSAIDs, such as diclofenac sodium, salicylate and indomethacin, exert inhibitory effects on thymocyte Kv1.3-channel currents. The slow inactivation pattern induced by indomethacin was thought to be associated with microscopic changes in the plasma membrane surface detected by the increase in the membrane capacitance.

Megakaryopoiesis and thrombopoiesis: an update on cytokines and lineage surface markers

Megakaryopoiesis is the process by which mature megakaryocytes (MKs) develop from hematopoietic stem cells (HSCs). The biological function of MKs is to produce platelets, which play critical roles in hemostasis and contribute to angiogenesis and wound healing. The generation of platelets from MKs is termed thrombopoiesis. The cytokine thrombopoietin (TPO) is the major regulator of megakaryopoiesis and thrombopoiesis. It binds to its surface receptor, c-Mpl, and acts through multiple downstream signaling pathways, including the PI-3 kinase-Akt, MAPK, and ERK1/ERK2 pathways. However, non-TPO pathways, such as the SDF1/CXCR4 axis, Notch signaling, src family kinases, integrin signaling, and Platelet Factor 4/low-density lipoprotein receptor-related protein 1, have more recently been recognized to influence megakaryopoiesis and thrombopoiesis in vitro and in vivo. In this chapter, we review megakaryopoiesis and thrombopoiesis with emphasis on cell surface marker changes during their differentiation from HSCs, and the classical cytokines that affect these developmental stages. We also discuss non-TPO regulators and their effects on in vitro culture systems.

The sequestration of hydroxyapatite nanoparticles by human monocyte-macrophages in a compartment that allows free diffusion with the extracellular environment

Calcium phosphate and hydroxyapatite nanoparticles are extensively researched for medical applications, including bone implant materials, DNA and SiRNA delivery vectors and slow release vaccines. Elucidating the mechanisms by which cells internalize nanoparticles is fundamental for their long-term exploitation. In this study, we demonstrate that hydrophilic hydroxyapatite nanoparticles are sequestered within a specialized compartment called SCC (surface-connected compartment). This membrane-bound compartment is an elaborate labyrinth-like structure directly connected to the extracellular space. This continuity is demonstrated by in vivo 2-photon microscopy of ionic calcium using both cell-permeable and cell-impermeable dyes and by 3-D reconstructions from serial block-face SEM of fixed cells. Previously, this compartment was thought to be initiated specifically by exposure of macrophages to hydrophobic nanoparticles. However, we show that the SCC can be triggered by a much wider range of nanoparticles. Furthermore, we demonstrate its formation in A549 human lung epithelial cells, which are considerably less phagocytic than macrophages. EDX shows that extensive amounts of hydroxyapatite nanoparticles can be sequestered in this manner. We propose that SCC formation may be a means to remove large amounts of foreign material from the extracellular space, followed by slow degradation, may be to avoid excessive damage to surrounding cells or tissues.

Distinct functional effects for dynamin 3 during megakaryocytopoiesis

Dynamin 3 (DNM3) is a member of a family of motor proteins that participate in a number of membrane rearrangements such as cytokinesis, budding of transport vesicles, phagocytosis, and cell motility. Recently, DNM3 was implicated as having a role in megakaryocyte (MK) development. To further investigate the functional role of DNM3 during megakaryocytopoiesis, we introduced sequence-specific short hairpin RNAs (shRNAs) into developing MKs. The results showed that knockdown of DNM3 inhibited a stage of MK development that involved progenitor amplification. This was evident by significant decreases in the number of colony forming unit-megakaryocytes, the total number of nucleated cells, and the number of CD41(+) and CD61(+) MKs produced in culture. Using a styrl membrane dye to quantify the demarcation membrane system (DMS) of terminally differentiated MKs, we found that DNM3 co-localized with the DMS and that DNM3 lentiviral shRNAs precluded the formation of the DMS. Knockdown of dynamin 3 in murine MKs also caused a decrease in the number of morphologically large MKs and the overall size of large MKs was decreased relative to controls. MK protein lysates were used in overlay blots to show that both DNM3 and actin bind to nonmuscle myosin IIA (MYH9). Consistent with these observations, immunofluorescence studies of MKs and proplatelet processes showed co-localization of DNM3 with MYH9. Overall, these studies demonstrate that DNM3 not only participates in MK progenitor amplification, but is also involved in cytoplasmic enlargement and the formation of the DMS.

Myosin-II inhibition and soft 2D matrix maximize multinucleation and cellular projections typical of platelet-producing megakaryocytes

Cell division, membrane rigidity, and strong adhesion to a rigid matrix are all promoted by myosin-II, and so multinucleated cells with distended membranes--typical of megakaryocytes (MKs)--seem predictable for low myosin activity in cells on soft matrices. Paradoxically, myosin mutations lead to defects in MKs and platelets. Here, reversible inhibition of myosin-II is sustained over several cell cycles to produce 3- to 10-fold increases in polyploid MK and a number of other cell types. Even brief inhibition generates highly distensible, proplatelet-like projections that fragment readily under shear, as seen in platelet generation from MKs in vivo. The effects are maximized with collagenous matrices that are soft and 2D, like the perivascular niches in marrow rather than 3D or rigid, like bone. Although multinucleation of other primary hematopoietic lineages helps to generalize a failure-to-fission mechanism, lineage-specific signaling with increased polyploidy proves possible and novel with phospho-regulation of myosin-II heavy chain. Label-free mass spectrometry quantitation of the MK proteome uses a unique proportional peak fingerprint (ProPF) analysis to also show upregulation of the cytoskeletal and adhesion machinery critical to platelet function. Myosin-inhibited MKs generate more platelets in vitro and also in vivo from the marrows of xenografted mice, while agonist stimulation activates platelet spreading and integrin αIIbβ3. Myosin-II thus seems a central, matrix-regulated node for MK-poiesis and platelet generation.

The spectrin-based membrane skeleton stabilizes mouse megakaryocyte membrane systems and is essential for proplatelet and platelet formation

Megakaryocytes generate platelets by remodeling their cytoplasm first into proplatelets and then into preplatelets, which undergo fission to generate platelets. Although the functions of microtubules and actin during platelet biogenesis have been defined, the role of the spectrin cytoskeleton is unknown. We investigated the function of the spectrin-based membrane skeleton in proplatelet and platelet production in murine megakaryocytes. Electron microscopy revealed that, like circulating platelets, proplatelets have a dense membrane skeleton, the main fibrous component of which is spectrin. Unlike other cells, megakaryocytes and their progeny express both erythroid and nonerythroid spectrins. Assembly of spectrin into tetramers is required for invaginated membrane system maturation and proplatelet extension, because expression of a spectrin tetramer-disrupting construct in megakaryocytes inhibits both processes. Incorporation of this spectrin-disrupting fragment into a novel permeabilized proplatelet system rapidly destabilizes proplatelets, causing blebbing and swelling. Spectrin tetramers also stabilize the "barbell shapes" of the penultimate stage in platelet production, because addition of the tetramer-disrupting construct converts these barbell shapes to spheres, demonstrating that membrane skeletal continuity maintains the elongated, pre-fission shape. The results of this study provide evidence for a role for spectrin in different steps of megakaryocyte development through its participation in the formation of invaginated membranes and in the maintenance of proplatelet structure.

Compensatory thrombopoietin production from the liver and bone marrow stimulates thrombopoiesis of living rat megakaryocytes in chronic renal failure

BACKGROUND/AIMS

Decreased thrombopoiesis has been ascribed a role in the pathogenesis of uremic bleeding in chronic renal failure (CRF). However, serum thrombopoietin (TPO) levels are usually elevated in CRF patients, suggesting increased thrombopoiesis. The aim of this study was to determine the thrombopoietic activity in CRF.

METHODS

Male Sprague-Dawley rats that underwent 5/6 nephrectomy were used as the model of CRF. Age-matched sham-operated rats were used as controls. Single megakaryocytes were isolated from the rat bone marrow, and their size distribution was examined. Megakaryocyte membrane invaginations were monitored by confocal imaging of di-8-ANEPPS staining, and patch clamp whole-cell recordings of membrane capacitance. TPO gene expression was assessed in various tissues.

RESULTS

Circulating platelet counts and the number of large megakaryocytes were increased in the bone marrow of CRF rats. Massive di-8-ANEPPS staining and increased membrane capacitance in large megakaryocytes demonstrated increased membrane invaginations. Unaffected Kv1.3-channel currents per cell surface area demonstrated unaltered channel densities. TPO transcription was decreased in the renal cortex but increased in the liver and bone marrow of CRF rats.

CONCLUSION

Increased thrombopoiesis in CRF was thought to be a reactive mechanism to platelet dysfunction. Increased TPO production from the liver and bone marrow compensated for decreased production from damaged kidneys.

Thrombocytogenesis by megakaryocyte; Interpretation by protoplatelet hypothesis

Serial transmission electron microscopy of human megakaryocytes (MKs) revealed their polyploidization and gradual maturation through consecutive transition in characteristics of various organelles and others. At the beginning of differentiation, MK with ploidy 32N, e.g., has 16 centrosomes in the cell center surrounded by 32N nucleus. Each bundle of microtubules (MTs) emanated from the respective centrosome supports and organizes 16 equally volumed cytoplasmic compartments which together compose one single 32N MK. During the differentiation, single centriole separated from the centriole pair, i.e., centrosome, migrates to the most periphery of the cell through MT bundle, corresponding to a half of the interphase array originated from one centrosome, supporting one "putative cytoplasmic compartment" (PCC). Platelet demarcation membrane (DM) is constructed on the boundary surface between neighbouring PCCs. Matured PCC, composing of a tandem array of platelet territories covered by a sheet of DM is designated as protoplatelet. Eventually, the rupture of MK results in release of platelets from protoplatelets. (Communicated by Tadamitsu Kishimoto, M.J.A.).

Direct voltage control of endogenous lysophosphatidic acid G-protein-coupled receptors in Xenopus oocytes

Lysophosphatidic acid (LPA) G-protein-coupled receptors (GPCRs) play important roles in a variety of physiological and pathophysiological processes, including cell proliferation, angiogenesis, central nervous system development and carcinogenesis. Whilst many ion channels and transporters are recognized to be controlled by a change in cell membrane potential, little is known about the voltage dependence of other proteins involved in cell signalling. Here, we show that the InsP(3)-mediated Ca(2+) response stimulated by the endogenous LPA GPCR in Xenopus oocytes is potentiated by membrane depolarization. Depolarization was able to repetitively stimulate transient [Ca(2+)](i) increases after the initial agonist-evoked response. In addition, the initial rate and amplitude of the LPA-dependent Ca(2+) response were significantly modulated by the steady holding potential over the physiological range, such that the response to LPA was potentiated at depolarized potentials and inhibited at hyperpolarized potentials. Enhancement of LPA receptor-evoked Ca(2+) mobilization by membrane depolarization was observed over a wide range of agonist concentrations. Importantly, the amplitude of the depolarization-evoked intracellular Ca(2+) increase displayed an inverse relationship with agonist concentration such that the greatest effect of voltage was observed at near-threshold levels of agonist. Voltage-dependent Ca(2+) release was not induced by direct elevation of InsP(3) or by activation of heterotrimeric G-proteins in the absence of agonist, indicating that the LPA GPCR itself represents the primary site of action of membrane voltage. This novel modulation of LPA signalling by membrane potential may have important consequences for control of Ca(2+) signals both in excitable and non-excitable tissues.

Kv1.3 is the exclusive voltage-gated K+ channel of platelets and megakaryocytes: roles in membrane potential, Ca2+ signalling and platelet count

A delayed rectifier voltage-gated K⁺ channel (Kv) represents the largest ionic conductance of platelets and megakaryocytes, but is undefined at the molecular level. Quantitative RT-PCR of all known Kv α and ancillary subunits showed that only Kv1.3 (KCNA3) is substantially expressed in human platelets. Furthermore, megakaryocytes from Kv1.3^−/− mice or from wild-type mice exposed to the Kv1.3 blocker margatoxin completely lacked Kv currents and displayed substantially depolarised resting membrane potentials. In human platelets, margatoxin reduced the P2X₁- and thromboxaneA₂ receptor-evoked [Ca²⁺]_i increases and delayed the onset of store-operated Ca²⁺ influx. Megakaryocyte development was normal in Kv1.3^−/− mice, but the platelet count was increased, consistent with a role of Kv1.3 in apoptosis or decreased platelet activation. We conclude that Kv1.3 forms the Kv channel of the platelet and megakaryocyte, which sets the resting membrane potential, regulates agonist-evoked Ca²⁺ increases and influences circulating platelet numbers.

The evolution of platelet-directed pharmacotherapy

The evolution of platelet directed pharmacotherapy in the prevention and treatment of patients with thrombotic disorders is based soundly on a rapidly expanding knowledge of platelet biology. Traditionally viewed, throughout most of its relatively brief history in medicine, as an anucleate, passive contributor to hemostasis, a more contemporary perspective acknowledges platelets as complex, multidimensional cells that participate actively in coagulation, vascular repair, angiogenesis and thrombosis within the micro and the macro-circulatory systems. Herein, we consider platelet-directed pharmacotherapy from these fundamental, biology-based exemplars--megakaryocytes, signal transduction and the platelet--coagulation protease interface. We also highlight the emerging biopharmacology platform of oligonucleotide platelet adhesion antagonists and their complementary antidotes.

Platelet production by megakaryocytes: protoplatelet theory justifies cytoplasmic fragmentation model

The maturation of megakaryocytes (MKs) leading to platelet production is carefully reviewed. For instance, when MK with ploidy 16N enters the maturation stage, eight centrosomes are clustered in the cell center surrounded by 16N nucleus. Each bundle of microtubules (MTs) emanated from the respective centrosome supports and organize eight equally volumed cytoplasmic compartments which together compose one single 16N MK. Then, the following three processes take place in parallel until the complete maturation of MKs Virchow's Arch. 1906;186:55-63. Two centrioles, composing centrosome, are separated and each one with pericentriolar material migrates to just beneath the plasma membrane through the MT bundle [corresponding to a half of the interphase array, originated from one centrosome, supporting one "putative cytoplasmic compartment "(PCC)] (Blood. 2005;106:4066-75). Platelet specific granules and other cellular components, newly formed in the central field of the cell, are transported along the MTs and many platelet territories, future platelets, are elaborated as a tandem array from the center to periphery in each PCC [3]. All the important membranes including plasma membrane and platelet demarcation membrane (DM) are synthesized de novo and those are transported as membrane vesicles (MVs) from Golgi body along the MTs. MVs arranged on the boundary surface between neighboring PCCs undergo fusion and fission to yield a paired membrane. Further connection with the external milieu results in the completion of DM system. The PCC covered by a sheet of DM is designated as protoplatelet. Excessive production of the MVs, most probably intervenes between the respective protoplatelets. Eventually, the matured MK ruptures as a whole, resulting in release of platelets from protoplatelets and many of MVs, though the mechanism is not fully elucidated yet.

A role for membrane potential in regulating GPCRs?

G-protein-coupled receptors (GPCRs) have ubiquitous roles in transducing extracellular signals into cellular responses. Therefore, the concept that members of this superfamily of surface proteins are directly modulated by changes in membrane voltage could have widespread consequences for cell signalling. Although several studies have indicated that GPCRs can be voltage dependent, particularly P2Y(1) receptors in the non-excitable megakaryocyte, the evidence has been mostly indirect. Recent work on muscarinic receptors has stimulated substantial interest in this field by reporting the first voltage-dependent charge movements for a GPCR. An underlying mechanism is proposed whereby a voltage-induced conformational change in the receptor alters its ability to couple to the G protein and thereby influences its affinity for an agonist. We discuss the strength of the evidence behind this hypothesis and include suggestions for future work. We also describe other examples in which direct voltage control of GPCRs can account for effects of membrane potential on downstream signals and highlight the possible physiological consequences of this phenomenon.

Plant sterols cause macrothrombocytopenia in a mouse model of sitosterolemia

Mutations in either ABCG5 or ABCG8 cause sitosterolemia, an inborn error of metabolism characterized by high plasma plant sterol concentrations. Recently, macrothrombocytopenia was described in a number of sitosterolemia patients, linking hematological dysfunction to disturbed sterol metabolism. Here, we demonstrate that macrothrombocytopenia is an intrinsic feature of murine sitosterolemia. Abcg5-deficient (Abcg5(-/-)) mice showed a 68% reduction in platelet count, and platelets were enlarged compared with wild-type controls. Macrothrombocytopenia was not due to decreased numbers of megakaryocytes or their progenitors, but defective megakaryocyte development with deterioration of the demarcation membrane system was evident. Lethally irradiated wild-type mice transplanted with bone marrow from Abcg5(-/-) mice displayed normal platelets, whereas Abcg5(-/-) mice transplanted with wild-type bone marrow still showed macrothrombocytopenia. Treatment with the sterol absorption inhibitor ezetimibe rapidly reversed macrothrombocytopenia in Abcg5(-/-) mice concomitant with a strong decrease in plasma plant sterols. Thus, accumulation of plant sterols is responsible for development of macrothrombocytopenia in sitosterolemia, and blocking intestinal plant sterol absorption provides an effective means of treatment.

Molecular and electrophysiological characterization of transient receptor potential ion channels in the primary murine megakaryocyte

The molecular identity of platelet Ca(2+) entry pathways is controversial. Furthermore, the extent to which Ca(2+)-permeable ion channels are functional in these tiny, anucleate cells is difficult to assess by direct electrophysiological measurements. Recent work has highlighted how the primary megakaryocyte represents a bona fide surrogate for studies of platelet signalling, including patch clamp recordings of ionic conductances. We have now screened for all known members of the transient receptor potential (TRP) family of non-selective cation channels in murine megakaryocytes following individual selection of these rare marrow cells using glass micropipettes. RT-PCR detected messages for TRPC6 and TRPC1, which have been reported in platelets and megakaryocytic cell lines, and TRPM1, TRPM2 and TRPM7, which to date have not been demonstrated in cells of megakaryocytic/platelet lineage. Electrophysiological recordings demonstrated the presence of functional TRPM7, a constitutively active cation channel sensitive to intracellular Mg(2+), and TRPM2, an ADP-ribose-dependent cation channel activated by oxidative stress. In addition, the electrophysiological and pharmacological properties of the non-selective cation channels stimulated by the physiological agonist ADP are consistent with a major role for TRPC6 in this G-protein-coupled receptor-dependent Ca(2+) influx pathway. This study defines for the first time the principal TRP channels within the primary megakaryocyte, which represent candidates for Ca(2+) influx pathways activated by a diverse range of stimuli in the platelet and megakaryocyte.

Characterization of the megakaryocyte demarcation membrane system and its role in thrombopoiesis

To produce blood platelets, megakaryocytes elaborate proplatelets, accompanied by expansion of membrane surface area and dramatic cytoskeletal rearrangements. The invaginated demarcation membrane system (DMS), a hallmark of mature cells, has been proposed as the source of proplatelet membranes. By direct visualization of labeled DMS, we demonstrate that this is indeed the case. Late in megakaryocyte ontogeny, the DMS gets loaded with PI-4,5-P(2), a phospholipid that is confined to plasma membranes in other cells. Appearance of PI-4,5-P(2) in the DMS occurs in proximity to PI-5-P-4-kinase alpha (PIP4Kalpha), and short hairpin (sh) RNA-mediated loss of PIP4Kalpha impairs both DMS development and expansion of megakaryocyte size. Thus, PI-4,5-P(2) is a marker and possibly essential component of internal membranes. PI-4,5-P(2) is known to promote actin polymerization by activating Rho-like GTPases and Wiskott-Aldrich syndrome (WASp) family proteins. Indeed, PI-4,5-P(2) in the megakaryocyte DMS associates with filamentous actin. Expression of a dominant-negative N-WASp fragment or pharmacologic inhibition of actin polymerization causes similar arrests in proplatelet formation, acting at a step beyond expansion of the DMS and cell mass. These observations collectively suggest a signaling pathway wherein PI-4,5-P(2) might facilitate DMS development and local assembly of actin fibers in preparation for platelet biogenesis.

Emerging roles for P2X1 receptors in platelet activation

The platelet surface membrane possesses three P2 receptors activated by extracellular adenosine nucleotides; one member of the ionotropic receptor family (P2X(1)) and two members of the G-protein-coupled receptor family (P2Y(1) and P2Y(12)). P2Y(1) and P2Y(12) receptors have firmly established roles in platelet activation during thrombosis and haemostasis, whereas the importance of the P2X(1) receptor has been more controversial. However, recent studies have demonstrated that P2X(1) receptors can generate significant functional platelet responses alone and in synergy with other receptor pathways. In addition, studies in transgenic animals indicate an important role for P2X(1) receptors in platelet activation, particularly under conditions of shear stress and thus during arterial thrombosis. This review discusses the background behind discovery of P2X(1) receptors in platelets and their precursor cell, the megakaryocyte, and how signalling via these ion channels may participate in platelet activation.

Increased and pathologic emperipolesis of neutrophils within megakaryocytes associated with marrow fibrosis in GATA-1low mice

Deletion of megakaryocytic-specific regulatory sequences of GATA-1 (Gata1tm2Sho or GATA-1low mutation) results in severe thrombocytopenia, because of defective thrombocytopoiesis, and myelofibrosis. As documented here, the GATA-1low mutation blocks megakaryocytic maturation between stage I and II, resulting in accumulation of defective megakaryocytes (MKs) in the tissues of GATA-1low mice. The block in maturation includes failure to properly organize α granules because von Willebrand factor is barely detectable in mutant MKs, and P-selectin, although normally expressed, is found frequently associated with the demarcation membrane system (DMS) instead of within granules. Conversely, both von Willebrand factor and P-selectin are barely detectable in GATA-1low platelets. Mutant MKs are surrounded by numerous myeloperoxidase-positive neutrophils, some of which appear in the process to establish contact with MKs by fusing their membrane with those of the DMS. As a result, 16% (in spleen) to 34% (in marrow) of GATA-1low MKs contain 1 to 3 neutrophils embedded in a vacuolated cytoplasm. The neutrophil-embedded GATA-1low MKs have morphologic features (high electron density and negativity to TUNEL staining) compatible with those of cells dying from para-apoptosis. We suggest that such an increased and pathologic neutrophil emperipolesis may represent one of the mechanisms leading to myelofibrosis by releasing fibrogenic MK cytokines and neutrophil proteases in the microenvironment.

The interpretation of current-clamp recordings in the cell-attached patch-clamp configuration

In these experiments we have investigated the feasibility and accuracy of recording steady-state and dynamic changes in transmembrane potential noninvasively across an intact cell-attached patch using the current-clamp mode of a conventional patch-clamp amplifier. Using an equivalent circuit mimicking simultaneous whole-cell voltage-clamp and cell-attached current-clamp recordings we have defined both mathematically and experimentally the relationship between the membrane patch resistance, the seal resistance, and the fraction of the whole-cell potential recorded across an intact membrane patch. This analysis revealed a steep increase in the accuracy of recording of steady-state membrane potential as the seal/membrane ratio increases from 0. The recording accuracy approaches 100% as the seal/membrane ratio approaches infinity. Membrane potential measurements across intact cell-attached patches in rat basophilic leukemia cells and rat megakaryocytes revealed a surprisingly high degree of accuracy and demonstrated the ability of this noninvasive technique to follow dynamic changes in potential in nonexcitable cells.

Discrete but simultaneous release of adenine nucleotides and serotonin from mouse megakaryocytes as detected with patch- and carbon-fiber electrodes

Using patch- and carbon-fiber electrodes, we studied release phenomena of adenine nucleotides and serotonin from megakaryocytes isolated from the bone marrow of the mouse. Megakaryocytes express ionotropic purinergic receptors on their surfaces. Under the condition of whole cell recording, the cells showed spikelike spontaneous inward currents. The spontaneous currents were carried by cations and had amplitudes of 30-800 pA at -43 mV and durations of 0.1-0.3 s. Pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS; 100 microM) and suramin (100 microM), purinoceptor-blocking agents, depressed the currents reversibly. It is thought that the receptor involved was the P2X1 subtype on the cell and that the currents were due to activation of the P2X1 receptor by adenine nucleotides released from the cell. The currents showed a skewed amplitude distribution, suggesting variation of vesicular contents and/or distinct localization or varied density of receptors on the cell. Frequency of the spontaneous inward currents was enhanced by external application of platelet-activating substances, thrombin (0.4 U/ml), phorbol ester (100 nM), and ADP (2 microM), at low concentrations. With a carbon-fiber electrode, which can detect oxidizable substances including serotonin, spikelike oxidation currents from the external surface of the megakaryocyte were detected. The frequency of the oxidation currents increased remarkably after the application of thrombin (10 U/ml). The majority of the oxidation currents coincided with the rising phase of the whole cell currents, suggesting corelease of serotonin and adenine nucleotide from the same vesicle. We concluded that megakaryocytes store adenine nucleotides and serotonin in the same vesicle and release them simultaneously in a discrete manner.

Sensitivity limits for voltage control of P2Y receptor-evoked Ca2+ mobilization in the rat megakaryocyte

G-protein-coupled receptor signalling has been suggested to be voltage dependent in a number of cell types; however, the limits of sensitivity of this potentially important phenomenon are unknown. Using the non-excitable rat megakaryocyte as a model system, we now show that P2Y receptor-evoked Ca2+ mobilization is controlled by membrane voltage in a graded and bipolar manner without evidence for a discrete threshold potential. Throughout the range of potentials studied, the peak increase in intracellular Ca2+ concentration ([Ca2+]i) in response to depolarization was always larger than the maximal reduction in [Ca2+]i following an equivalent amplitude hyperpolarization. Significant [Ca2+]i increases were observed in response to small amplitude (< 5 mV, 5 s duration) or short duration (25 ms, 135 mV) depolarizations. Individual cardiac action potential waveforms were also able to repeatedly potentiate P2Y receptor-evoked Ca2+ release and the response to trains of normally paced stimuli fused to generate prolonged [Ca2+]i increases. Furthermore, elevation of the temperature to physiological levels (36 degrees C) resulted in a more sustained depolarization-evoked Ca2+ increase compared with more transient or oscillatory responses at 20-24 degrees C. The ability of signalling via a G-protein-coupled receptor to be potentiated by action potential waveforms and small amplitude depolarizations has broad implications in excitable and non-excitable tissues.

Thrombopoietin Enhances Rapid Current Responses Mediated by P2X1 Receptors on Megakaryocytic Cells in Culture

The effect of thrombopoietin (TPO), a magakaryocytopoietic cytokine, on the functional maturation of megakaryocytes was studied by using cell culture and patch-clamp techniques focusing on purinergic 2X(1) (P2X(1))-receptors, which are expressed specifically on platelets and their progenitors. Meg-01 cells, one of the typical human megakaryocytic cell lines, were cultured and studied by using a whole-cell patch electrode. In control cells cultured in RPMI1640 medium, an application of adenosine nucleotide (ADP, 40 microM) evoked transient inward currents with amplitudes of 45 +/- 19 pA (at -43 mV). Based on kinetic, ionic, and pharmacological properties as well as on previously reported findings, these currents were thought to be mediated by P2X(1) receptors. When Meg-01 cells were cultured for 7-9 d in a medium to which the differentiation-inducing agent phorbol ester (PMA; 10 nM) or TPO (100 ng/ml) had been added, the responses of the cells to ADP increased to about 150% of the control with PMA and to about 200% of the control value with TPO. A combination of the two agents enhanced the response of the cells to ADP to about 570% of the control value. These results suggest that phorbol ester and TPO cause cellular differentiation of Meg-01 cells and enhance the level of expression of P2X(1)-receptors on cell membranes in a synergetic manner. The effect of TPO on the induction of P2X(1)-receptors on mouse megakaryocytes in culture was more obvious.