Cytoplasm Of Platelets Research Articles

Natural infection of free-ranging capybaras (Hydrochoerus hydrochaeris) with Anaplasmataecea and Rickettsiaceae bacteria in the Iberá wetlands ecoregion, Argentina.

The current work assessed the infection with Ehrlichia and Anaplasma species, and exposure to Rickettsia spp. in free-ranging capybaras in the Iberá wetlands ecoregion in Argentina. By indirect immunofluorescence assay, 37 out of 51 (73%) capybara sera were seropositive to Rickettsia spp., with 23.5% and 4% samples considered homologous to Rickettsia parkeri and Rickettsia bellii, respectively (or very closely related serotypes). Anaplasmataceae DNA was found to be highly prevalent in capybaras, with 33 out of 62 samples positive for Anaplasma sp. with Ct values of 28.64 ± 0.35 (average ± standard error), and 12 samples positive for Ehrlichia sp. with Ct values of 31.74 ± 0.87. Anaplasma sp. from capybaras was closely related to Anaplasma sp. reported to infect Amblyomma dubitatum in Iberá wetlands and to Anaplasma odocoilei, while the detected Ehrlichia sp. was closely related to "Candidatus Ehrlichia hydrochoerus" previously reported to infect capybaras in Brazil and A. dubitatum in Iberá wetlands. Structures compatible with Anaplasma morulae were observed in the cytoplasm of platelets from Anaplasma-positive capybaras. Our findings show that capybaras from the Iberá wetlands were exposed to Rickettsia species related to R. bellii and to the pathogen R. parkeri, and were infected with "Ca. Ehrlichia hydrochoerus" and a novel Anaplasma species, herein named "Candidatus Anaplasma capybara".

Prior Incubation as a Tool to Overcome Understaining of Leishman Stain: A Cross-sectional Study

Introduction: A well stained peripheral smear is an integral part in the diagnosis and management of many clinicohematologic conditions. Leishman stain is recommended universally by the International Committee for Standardisation in Haematology (ICSH). However, a commonly encountered problem in tropical countries, especially during the rainy season, is under staining by Leishman stain as a result of interference by the atmospheric moisture. Hence, incubation of slides and/or buffer prior to standard Leishman staining protocol could bring out best staining qualities. Aim: To assess the effects of prior incubation of slides and/or buffer on standard Leishman staining technique. Materials and Methods: The cross-sectional study was conducted over a period of 8 weeks in the Central Laboratory, Kempegowda Institute of Medical Sciences, Bangalore in the months of October 2020 to November 2020. Blood samples were received in the central laboratory for routine haematological investigations. Total of 100 samples of left over non haemolysed Dipotassium Ethylenediaminetetraacetic Acid (K2-EDTA) anticoagulated blood irrespective of age, gender and cell counts were included in the study. Slides were stained following prior incubation of slides and/or buffer at 37° C and compared with the standard staining technique. The staining characteristics of nucleus, cytoplasm and granules of White Blood Cells (WBCs), platelets and erythrocytes were observed. One-way Analysis of Variance (ANOVA) test and p-value of <0.05 was considered statistically significant and higher f value (>104.35152) rejected the null hypothesis. Results: Prior incubation of slides and/or buffer resulted in better stained smears which were found to be statistically significant (p-value<0.05) as compared to standard Leishman staining technique. Conclusion: Minor modifications when applied to standard Leishman staining technique such as prior incubation of slides and/ or buffer at 37° Centigrade produced best staining characteristics to overcome the effects of humidity which normally interfere with staining of slides.

Morphofunctional Properties of Human Platelets, Treated with Low Doses of Hydrogen Peroxide

We studied morphofunctional properties of human platelets, treated with low doses of hydrogen peroxide (H2O2). It was shown that platelet incubation with 20 - 75 μM H2O2 during 24 hours at 20°C - 22°C did not cause visible structural deficiencies in cells. Platelets maintained their adhesion activity and velocity of degranulation at 20 μM H2O2. 30 - 60 μM H2O2 dose-dependently reduced lamella’s growth and allowed retaining granule bulk inside cytoplasm of spreading platelets. After 1 - 2 hours, the most effect of platelet granules’ stabilization was registered at samples, previously incubated with 60 μM H2O2. Adrenalin activation led to rapid degranulation of stabilized platelets, whereas lamella’s growth in activated platelets was low or medium.

BLOOD PLATELETS AS ACTIVATORS AND REGULATORS OF INFLAMMATORY AND IMMUNE REACTIONS. PART 2. THROMBOCYTES AS PARTICIPANTS OF IMMUNE REACTIONS

Thrombocytes keep a leading role in conjugating thrombosis, inflammation and congenital immune responses. The platelets provide stable adhesion and interaction with immune cells. Activated platelets express CD40L (CD154), a membrane glycoprotein of tumor necrosis factor (TNF) family. Hence, the platelets are the main source of sCD40L in blood plasma. Platelet CD154 may interact with CD40 receptor on endothelial cells, causing an inflammatory response, and enhancing production of immunoglobulins by B-lymphocytes. Membrane and soluble CD154 of platelets combined with other signals can induce maturation and activation of dendritic cells (DC). The platelets possess functional receptors, e.g., TLR2, TLR4, TLR7 and TLR9 they also bear Fc-receptors, including FcγRIIA, FcεRI and FcαRIA. FcγRIIA on platelets mediate protection against bacteria. Cross-linking of FcαRI on platelets results in production of prothrombotic and pro-inflammatory mediators such as tissue factor and IL-1β. Activation of platelets via FcεR1 causes release of chemokine RANTES and serotonin, which contribute to the pro-inflammatory response of other immune cells. Platelets possess receptors for activated complement components and its fragments (CR2, CR3, CR4, C1q, C1 inhibitor and factors D and H). Activated platelets trigger the complement system through the release of protein kinases and ATP, and also by phosphorylation of C3 and C3b. α-granules of platelets contain chemokines which represent the most numerous group of antimicrobial proteins of platelets (kinocidins), and there is an antimicrobial protein of the defensin family – hBD-1 in the cytoplasm of platelets. Ligand and receptor of the TNF superfamily (TRAIL and LIGHT), the SDF-1 chemokine (CXCL12), the IL-1βinterleukins, IL-8 and the soluble IL-6 receptor (sRIL-6) are recognized as platelet products belonging to the family of cytokines and their receptors. The HMGB-1 protein classified as an inflammatory cytokine, is expressed by activated platelets and causes formation of the extracellular traps by neutrophils. Platelets produce numerous growth factors, including EGF-α and EGF-β1, EGF-β2, TGF-α and TGF-β1, TGF-β2, PDGF, HGF, FGF-β, IGF, pro- and antiangiogenic factors, e.g., VEGF-F and angiopoietins Ang-1 and Ang-2. Fulfillment of immune functions by the platelets is carried out by their interaction with leukocytes, which are attracted to the site of infection and inflammation and retained during the development of an “immune thrombus” under conditions of high shear stress. Platelets can not only maintain and guide the immune response, but also initiate these events. They are able to present the antigen in the context of MHC class I molecules, and activate naїve CD8+T lymphocytes. Potential consequences of platelet interaction with neutrophils, monocytes, dendritic cells and lymphocytes are discussed in the review article.

Molecular detection of Anaplasma species in dogs in Colombia.

Anaplasma platys and A. phagocytophilum are tick-borne pathogens that parasitize platelets and neutrophils, respectively, of humans and animals. The former is the etiological agent of canine cyclic thrombocytopenia, while the latter is that of canine granulocytic anaplasmosis. This work involved the detection and identification of Anaplasma species in blood samples from dogs in Colombia, using molecular techniques. Between December 2008 and April 2009, blood samples were drawn from the cephalic vein of 91 dogs in the central-western region of Colombia (cities of Bogota, Villavicencio and Bucaramanga) and stored in tubes containing EDTA. These samples were used in 16S rRNA-Anaplasma spp. nPCR and the preparation of blood smears. One (1.1%) of the 91 sampled dogs showed inclusions suggestive of Anaplasmataceae agents in the cytoplasm of platelets. Based on PCR followed by sequencing and phylogenetic analysis, A. platys and Anaplasma sp. closed related to A. phagocytophilum were detected in two and one dog, respectively. Interestingly, all the samples were negative for specific msp-2-A. phagocytophilum real-time qPCR, suggesting the circulation of an Anaplasma species phylogenetically related to A. phagocytophilum in dogs in the aforementioned region. Hence, Anaplasma spp. circulates among dogs in Colombia, albeit with low frequency. To the best of authors' knowledge, this is the first molecular detection of Anaplasma spp. in dogs in Colombia.

Platelets and infections - complex interactions with bacteria.

Platelets can be considered sentinels of vascular system due to their high number in the circulation and to the range of functional immunoreceptors they express. Platelets express a wide range of potential bacterial receptors, including complement receptors, FcγRII, Toll-like receptors but also integrins conventionally described in the hemostatic response, such as GPIIb–IIIa or GPIb. Bacteria bind these receptors either directly, or indirectly via fibrinogen, fibronectin, the first complement C1q, the von Willebrand Factor, etc. The fate of platelet-bound bacteria is questioned. Several studies reported the ability of activated platelets to internalize bacteria such as Staphylococcus aureus or Porphyromonas gingivalis, though there is no clue on what happens thereafter. Are they sheltered from the immune system in the cytoplasm of platelets or are they lysed? Indeed, while the presence of phagolysosome has not been demonstrated in platelets, they contain antimicrobial peptides that were shown to be efficient on S. aureus. Besides, the fact that bacteria can bind to platelets via receptors involved in hemostasis suggests that they may induce aggregation; this has indeed been described for Streptococcus sanguinis, S. epidermidis, or C. pneumoniae. On the other hand, platelets are able to display an inflammatory response to an infectious triggering. We, and others, have shown that platelet release soluble immunomodulatory factors upon stimulation by bacterial components. Moreover, interactions between bacteria and platelets are not limited to only these two partners. Indeed, platelets are also essential for the formation of neutrophil extracellular traps by neutrophils, resulting in bacterial clearance by trapping bacteria and concentrating antibacterial factors but in enhancing thrombosis. In conclusion, the platelet–bacteria interplay is a complex game; its fine analysis is complicated by the fact that the inflammatory component adds to the aggregation response.

Functional factor XIII-A is exposed on the stimulated platelet surface

AbstractFactor XIII (FXIII) stabilizes thrombi against fibrinolysis by cross-linking α2-antiplasmin (α2AP) to fibrin. Cellular FXIII (FXIII-A) is abundant in platelets, but the extracellular functions of this pool are unclear because it is not released by classical secretion mechanisms. We examined the function of platelet FXIII-A using Chandler model thrombi formed from FXIII-depleted plasma. Platelets stabilized FXIII-depleted thrombi in a transglutaminase-dependent manner. FXIII-A activity on activated platelets was unstable and was rapidly lost over 1 hour. Inhibiting platelet activation abrogated the ability of platelets to stabilize thrombi. Incorporating a neutralizing antibody to α2AP into FXIII-depleted thrombi revealed that the stabilizing effect of platelet FXIII-A on lysis was α2AP dependent. Platelet FXIII-A activity and antigen were associated with the cytoplasm and membrane fraction of unstimulated platelets, and these fractions were functional in stabilizing FXIII-depleted thrombi against lysis. Fluorescence confocal microscopy and flow cytometry revealed exposure of FXIII-A on activated membranes, with maximal signal detected with thrombin and collagen stimulation. FXIII-A was evident in protruding caps on the surface of phosphatidylserine-positive platelets. Our data show a functional role for platelet FXIII-A through exposure on the activated platelet membrane where it exerts antifibrinolytic function by cross-linking α2AP to fibrin.

Kindlins, Integrin Activation and the Regulation of Talin Recruitment to αIIbβ3

Talins and kindlins bind to the integrin β3 cytoplasmic tail and both are required for effective activation of integrin αIIbβ3 and resulting high-affinity ligand binding in platelets. However, binding of the talin head domain alone to β3 is sufficient to activate purified integrin αIIbβ3 in vitro. Since talin is localized to the cytoplasm of unstimulated platelets, its re-localization to the plasma membrane and to the integrin is required for activation. Here we explored the mechanism whereby kindlins function as integrin co-activators. To test whether kindlins regulate talin recruitment to plasma membranes and to αIIbβ3, full-length talin and kindlin recruitment to β3 was studied using a reconstructed CHO cell model system that recapitulates agonist-induced αIIbβ3 activation. Over-expression of kindlin-2, the endogenous kindlin isoform in CHO cells, promoted PAR1-mediated and talin-dependent ligand binding. In contrast, shRNA knockdown of kindlin-2 inhibited ligand binding. However, depletion of kindlin-2 by shRNA did not affect talin recruitment to the plasma membrane, as assessed by sub-cellular fractionation, and neither over-expression of kindlins nor depletion of kindlin-2 affected talin interaction with αIIbβ3 in living cells, as monitored by bimolecular fluorescence complementation. Furthermore, talin failed to promote kindlin-2 association with αIIbβ3 in CHO cells. In addition, purified talin and kindlin-3, the kindlin isoform expressed in platelets, failed to promote each other's binding to the β3 cytoplasmic tail in vitro. Thus, kindlins do not promote initial talin recruitment to αIIbβ3, suggesting that they co-activate integrin through a mechanism independent of recruitment.

Release reaction of brain-derived neurotrophic factor (BDNF) through PAR1 activation and its two distinct pools in human platelets

Brain-derived neurotrophic factor (BDNF) is a cytokine that plays important roles in the survival, development, and plasticity of neurons. BDNF is also expressed in peripheral tissues and cells. In this article, we report the BDNF release reaction through thrombin stimulation and its localization in human platelets. Platelets from healthy volunteers were subjected to PAR1-AP or PAR4-AP stimulation. Release of BDNF was measured by ELISA. Localization of BDNF in resting and thrombin-activated platelets was examined by immunoelectron microscopy and sucrose gradient ultracentrifugation following western blotting. BDNF was released dose-dependently with PAR1-AP concentrations with drastic release at low PAR1-AP concentrations and gently release at high PAR1-AP concentrations. Maximum BDNF release was approximately 37% at 132 μM PAR1-AP. In contrast, 3.8% BDNF was released with 1.13 mM PAR4-AP stimulation. In immunoelectron microscopy and sucrose gradient ultracentrifugation analyses, BDNF was detected not only in α-granules but also cytoplasm in of the resting platelets, and it was distributed in the swollen open canalicular system fused to α-granules at 1 min and disappeared at 5 min after stimulation by thrombin. However, BDNF in cytoplasm remained throughout platelet activation. In conclusions, we demonstrate that BDNF is released from platelets through predominately PAR1 regulation. Furthermore, we identified two pools of BDNF in the α-granules and cytoplasm of human platelets, and only BDNF in α-granules is released through platelet activation.

Factor XIII: A Coagulation Factor With Multiple Plasmatic and Cellular Functions

Factor XIII (FXIII) is unique among clotting factors for a number of reasons: 1) it is a protransglutaminase, which becomes activated in the last stage of coagulation; 2) it works on an insoluble substrate; 3) its potentially active subunit is also present in the cytoplasm of platelets, monocytes, monocyte-derived macrophages, dendritic cells, chondrocytes, osteoblasts, and osteocytes; and 4) in addition to its contribution to hemostasis, it has multiple extra- and intracellular functions. This review gives a general overview on the structure and activation of FXIII as well as on the biochemical function and downregulation of activated FXIII with emphasis on new developments in the last decade. New aspects of the traditional functions of FXIII, stabilization of fibrin clot, and protection of fibrin against fibrinolysis are summarized. The role of FXIII in maintaining pregnancy, its contribution to the wound healing process, and its proangiogenic function are reviewed in details. Special attention is given to new, less explored, but promising fields of FXIII research that include inhibition of vascular permeability, cardioprotection, and its role in cartilage and bone development. FXIII is also considered as an intracellular enzyme; a separate section is devoted to its intracellular activation, intracellular action, and involvement in platelet, monocyte/macrophage, and dendritic cell functions.

Phenotypic Correction of Hemophilia A by Ectopic Expression of Activated Factor VII in Platelets

Platelets are receiving much attention as novel target cells to secrete a coagulation factor for hemophilia gene therapy. In order to extend the application of platelet-directed gene therapy, we examined whether ectopic expression of activated factor VII (FVIIa) in platelets would result in an efficient bypass therapy to induce sufficient thrombin generation on platelet surfaces in mice with hemophilia A. Transduction of bone marrow cells with a simian immunodeficiency virus (SIV)-based lentiviral vector harboring the platelet-specific GPIb alpha promoter resulted in efficient transgene expression in platelets. FVIIa antigen was expressed in platelets by this SIV system; FVII transgene products were found to localize in the cytoplasm and translocate toward the sub-membrane zone and cell surface after activation. Although FVII antigen levels in platelets did not reach the therapeutic levels seen with FVIIa infusion therapy, whole-blood coagulation, as assessed by thromboelastography, was significantly improved in mice with hemophilia A. Further, we observed correction of the bleeding phenotype in mice with hemophilia A after transplantation, even in the presence of FVIII-neutralizing antibodies. Our results demonstrate that FVIIa-expressing platelets can strengthen hemostatic function and may be useful in treating hemophilia and other inherited bleeding disorders. These findings are comparable to the proven therapeutic effects of FVIIa infusion.

Platelet products prepared by different methods of sedimentation undergo platelet activation differently during storage

The activation marker CD40 ligand (CD40L) has recently been demonstrated to be released from the cytoplasm of platelets (PLTs) during storage. CD40L may be associated with some adverse transfusion reactions including febrile responses and transfusion-related acute lung injury. CD62P has been traditionally measured to assess PLT activation. This study compares the surface levels of CD40L and CD62P and accumulation of the soluble forms of these activation markers in the plasma of stored PLTs, prepared by PLT-rich plasma (PRP) or buffy coat (BC) methods and with two apheresis instruments. Individual PLT concentrates (PCs) were prepared in 100 percent plasma from a pool of two ABO-identical whole-blood units. Apheresis PLTs (APs) were prepared in 100 percent plasma using one of two commercially available cell separators (Amicus, Baxter Healthcare; and Trima, Gambro BCT). Surface expression of CD40L and CD62P was measured by flow cytometry, and secretion of soluble CD40L (sCD40L) and soluble CD62 (sCD62) was measured by enzyme-linked immunosorbent assay during 7 days of PLT storage. Secretion of sCD40L was greater in Amicus APs than in Trima APs during the first 3 days of storage. It was also greater in the PRP-PC preparations than in BC-PC preparations through the first day of storage. Surface expression of CD40L was low in all PLT preparations. Secretion of sCD62P was greater in Amicus APs than in Trima APs during the entire storage period and greater in PRP-PC than in BC during the first 5 days of storage. The percentage of CD62P-positive PLTs was greater in Amicus units than Trima units and greater in PRP-PC than BC-PC preparations during the first 5 and 3 days of storage, respectively. The kinetics of the secretion of CD40L are influenced by the method used to prepare PLTs for storage. The patterns for CD40L membrane association and secretion are different than those observed for CD62P during storage.

Characterization of Platelet TFPI.

The accumulation of blood borne tissue factor within a blood clot is thought to be important for normal fibrin generation and may cause propagation of the blood clot into an occlusive thrombus. Tissue factor pathway inhibitor (TFPI), the primary inhibitor of tissue factor activity, is present in platelets where it may have an important role in down regulating the activity of circulating tissue factor. Western blot analysis of gel filtered platelet lysates demonstrates the presence of full-length, 43 kDa TFPI. The lower molecular weight, C-terminally truncated forms of TFPI present in plasma are not present in platelet lysates suggesting that full length TFPI is selectively adsorbed from plasma or that TFPI is synthesized within megakaryocytes. Real time PCR analysis of cDNA produced from highly purified platelet mRNA demonstrates transcripts for both TFPI and TFPI-beta, a truncated form of TFPI produced by alternative splicing. TFPI mRNA is about 3000-fold more abundant than TFPI-beta mRNA. TFPI is not released from platelets following stimulation with the thrombin receptor activating peptide SFLLRN (TRAP) indicating that it is not stored as a soluble alpha-granule protein. Instead, it appears to be associated with platelet membranes. It remains largely insoluble following lysis of platelets by repeated freeze thaw or exposure to hypotonic buffer. There is little to no expression of TFPI on the surface of quiescent platelets as assessed by flow cytometry. TRAP stimulated platelets have surface TFPI that is detected by flow cytometry, but the amount varies between experiments. TFPI is consistently detectable by flow cytometry on the surface of coated-platelets that are produced by stimulation with thrombin/convulxin or thrombin/calcium ionophore. Platelet TFPI activity was measured in eight individuals by determining the rate of factor X activation by factor VIIa/tissue factor. Comparison of quiescent and coated-platelets demonstrates a significant increase in the amount of TFPI activity on the surface the coated-platelets (11.0+/−0.3 vs. 38.7+/−17.2 pmole/10 million platelets; p<0.005), but no significant increase in platelets activated with 20 or 200 micromolar TRAP. Since TFPI associates with the endothelial surface through a GPI-anchor, coated-platelets were treated with phospahtidyl inositol specific phospholipase C (PIPLC) to determine if it would remove TFPI. In experiments where PIPLC removed 56% of the CD59 from the platelet surface, only 22% of the TFPI was removed indicating that if platelet TFPI is GPI-anchored, it is expressed in a manner that is relatively protected from PIPLC. Confocal and electron microscopy studies demonstrate that TFPI is distributed throughout the cytoplasm of quiescent platelets without localization to granules suggesting that it may associate with the platelet cytoskeleton.

A ‘touch’ of the White platelet syndrome

Investigations into structural defects in platelets from a large family with the White platelet syndrome (WPS) separated the members into three groups. The first group of 22 members was the subject of our first report (White JG, Key NS, King RA, Vercellotti GM. The white platelet syndrome: A new autosomal dominant platelet disorder. Platelets 2004;15:173–184). A third group of 13 members had no abnormalities of platelet ultrastructure. The second group of 17 members, the focus of the present study, had a ‘touch’ of the WPS. Platelet counts, mean platelet volumes (MPVs) and platelet responses to aggregating agents were normal in ‘touch’ patients in contrast to platelets of those with the full WPS in whom these parameters were abnormal. Up to 13% of the full WPS platelets contained large, fully developed Golgi complexes, up to seven in number, extruding innumerable vesicles from the trans-Golgi face and filling the cytoplasm of many platelets. Many Golgi complexes had centrioles associated with them. ‘Touch’ platelets had one or two Golgi complexes of intermediate size in 3–5% of their platelets. Golgi vesicles were uncommon and centrioles absent. Gray platelets and hypogranular cells were infrequent in patients with a ‘touch’ of the WPS, whereas up to 44% of the platelets from those with the WPS were gray or hypogranular. Elements of the dense tubular system were prominent in full WPS platelets, together with their formation into areas of cytoplasmic sequestration and autodigestion. These features were absent in ‘touch’ platelets. As commonly observed in full WPS platelets, mitochondria were larger and more numerous than alpha granules in some ‘touch’ cells. Both ‘touch’ and full WPS platelets frequently contained giant and rod-shaped granules. Dense bodies, however, were normal in size and number in ‘touch’ platelets, and half normal size in full WPS platelets. The separation of ultrastructural abnormalities in the two varieties of the WPS suggests that genetic defects involve more than a single gene and the genes are variable in their penetrance. Genetic studies to determine if this is the case are currently in progress.

Spliced in the Cytoplasm

STKE In nucleated cells, noncoding introns are removed from pre-messenger RNA (pre-mRNA) transcripts by the spliceosome (a nuclear complex of proteins and RNAs) before export of the mRNA from the nucleus. Thus, one would not expect that platelets—anucleate blood cells that bud from megakaryocytes—would contain spliceosome components or pre-mRNAs. Nevertheless, Denis et al. show that components of the spliceosome are present in the cytoplasm of human megakaryocytes and also in circulating platelets. Interleukin-1β (IL-1β) pre-mRNA is present in the cytoplasm of quiescent platelets, whereas platelets that had been activated by adhesion to fibrinogen in the presence of thrombin contain mature IL-1β mRNA and protein. Moreover, IL-1β pre-mRNA could be converted into mature mRNA by a platelet extract. Thus, pre-mRNA splicing is a key regulatory point for cytokine production during platelet activation. — EMA Cell 122 , 379 (2005).

Spliced in the Cytoplasm

Denis et al. made the unexpected discovery that precursor mRNAs (pre-mRNAs) and functional spliceosomes are found in the cytoplasm of platelets and, further, that splicing of this cytoplasmic pre-RNA is regulated by signals deriving from platelet activation. In nucleated cells, noncoding introns are removed from newly transcribed pre-mRNA by the spliceosome (a nuclear complex containing small nuclear ribonucleoproteins and auxiliary proteins), creating translatable mRNA, which may then be exported from the nucleus. Thus, one would not expect that platelets, anucleate blood cells that bud from megakaryocytes, would contain spliceosome components or pre-mRNAs. Denis et al. found, however, that components of the spliceosome (both proteins and small nuclear splicing RNAs) were present not only in the cytoplasm of human megakaryocytes but also in circulating platelets. Interleukin-1β (IL-1β) pre-mRNA was present in the cytoplasm of quiescent platelets (and in the cytoplasm of megakaryocytes extending the proplatelets that give rise to platelets), whereas platelets activated by adhesion to fibrinogen in the presence of thrombin contained mature IL-1β mRNA and protein. Indeed, reverse transcription polymerase chain reaction (RT-PCR) revealed a time-dependent decrease in IL-1β pre-mRNA and an increase in IL-1β mRNA after platelet activation. Moreover, IL-1β pre-mRNA that was transcribed in vitro was spliced into mRNA by a fraction of platelet extract. Thus, pre-mRNA splicing not only takes place in the cytoplasm of platelets but also provides a mechanism for regulating cytokine production after platelet activation. Meshorer and Misteli provide context in a Preview and speculate on the possibility that pre-mRNA splicing could occur in the cytoplasm of other cell types. M. M. Denis, N. D. Tolley, M. Bunting, H. Schwertz, H. Jiang, S. Lindemann, C. C. Yost, F. J. Rubner, K. H. Albertine, K. J. Swoboda, C. M. Fratto, E. Tolley, L. W. Kraiss, T. M. McIntyre, G. A. Zimmerman, A. S. Weyrich, Escaping the nuclear confines: Signal-dependent pre-mRNA splicing in anucleate platelets. Cell 122 , 379-391 (2005). [PubMed] E. Meshorer, T. Misteli, Splicing misplaced. Cell 122 , 317-318 (2005). [PubMed]

Splicing Misplaced

Newly synthesized transcripts are usually spliced during transcription or immediately thereafter. So pre-mRNA splicing has been presumed to occur exclusively in the cell nucleus. In this issue of Cell, Denis et al. (2005) now report the presence of functional spliceosomes and signal-dependent pre-mRNA splicing in the cytoplasm of platelets.

Medich giant platelet disorder: a unique α granule deficiency I. Structural abnormalities

Human platelet granule deficiency disorders include the gray platelet syndrome (GPS), α δ storage pool deficiency, the Hermansky–Pudlak syndrome and the White platelet syndrome. The present study describes a patient with a lifelong history of easy bleeding, thrombocytopenia and giant platelets. Her cells were found to have normal numbers of dense bodies, but a markedly decreased number of α granules. Many platelets had no α granules and resembled the gray platelets of patients with GPS. However, the empty vacuoles without granule contents that fill the cytoplasm of GPS platelets were not present in significant numbers in her platelets. In addition to the decrease in α granules the patients platelets contained membranous inclusions resembling cigars or scrolls. Usually, only one scroll open at each end was present, but many platelets contained two and some as many as five. Freeze-fracture revealed an absence of intramembranous particles in many layers of the scrolls. They occur in no other human platelet disorder, but are common in platelets from the Wistar–Furth rat. Thus, the patient is a unique variant of human platelet granule deficiency disorders unlike any described previously.

Stabilization of membranes in human platelets freeze-dried with trehalose

Human blood platelets are normally stored in blood banks for 3–5 days, after which they are discarded. We have launched an effort at developing means for preserving the platelets for long term storage. In previous studies we have shown that trehalose can be used to preserve biological membranes and proteins during drying and have provided evidence concerning the mechanism. A myth has grown up about special properties of trehalose, which we discuss here and clarify some of what is fact and what is misconception. We have found a simple way of introducing this sugar into the cytoplasm of platelets and have successfully freeze-dried the trehalose-loaded platelets, with very promising results. We present evidence that membrane microdomains are maintained intact in the platelets freeze-dried with trehalose. Finally, we propose a possible mechanism by which the microdomains are preserved.

Cytoskeletal Proteins and Platelet Signaling

The actin filament network fills the cytoplasm of unstimulated platelets and connects with a submembranous latticework of short cross-linked actin filaments, known as the membrane skeleton. One function of the cytoskeleton is to direct the contours of the membrane in the unstimulated platelet and the rapid changes in shape in the activated platelet. Activation-induced changes result from events such as phosphorylation or calpain-induced cleavage of cytoskeletal proteins. The specific reorganizations depend upon the combination of signals to which platelets are exposed. A second function of the cytoskeleton is to bind other cellular components; it binds signaling molecules, localizing them to specific cellular locations; it binds the plasma membrane regulating properties of the membrane, maintaining microdomains in the membrane, or regulating activities of membrane proteins. In this way, the cytoskeleton plays a critical role in regulation of spatial organizations and, thus, in the integration of cellular activities.