Mechanical confinement prevents ectopic platelet release.

Blood platelets are produced by megakaryocytes (MKs), their parent cells, which are in the bone marrow. Once mature, MK pierces through the sinusoid vessel, and the initial protrusion further elongates as proplatelet or buds to release platelets. The mechanisms controlling the decision to initiate proplatelet and platelet formation are unknown. Here, we show that the mechanical properties of the microenvironment prevent proplatelet and platelet release in the marrow stroma while allowing this process in the bloodstream. Loss of marrow confinement following myelosuppression led to inappropriate proplatelet and platelet release into the extravascular space. We further used an inert viscoelastic hydrogel to evaluate the impact of compressive stress. Transcriptional analysis showed that culture in three-dimensional gel induced upregulation of genes related to the Rho-GTPase pathway. We found higher Rho-GTPase activation, myosin light chain phosphorylation and F-actin under mechanical constraints while proplatelet formation was inhibited. The use of latrunculin-A to decrease F-actin promoted microtubule-dependent budding and proplatelet extension inside the gel. Additionally, ex vivo exposure of intact bone marrow to latrunculin-A triggered proplatelet extensions in the interstitial space. In vivo, this confinement-mediated high intracellular tension is responsible for the formation of the peripheral zone, a unique actin-rich structure. Cytoskeleton reorganization induces the disappearance of the peripheral zone upon reaching a liquid milieu to facilitate proplatelet and platelet formation. Hence, our data provide insight into the mechanisms preventing ectopic platelet release in the marrow stroma. Identifying such pathways is especially important for understanding pathologies altering marrow mechanics such as chemotherapy or myelofibrosis.

In Vitro Investigation of the Anti-Fibrotic Effects of 1-Phenyl-2-Pentanol, Identified from Moringa oleifera Lam., on Hepatic Stellate Cells.

Liver fibrosis, characterized by excessive extracellular matrix deposition, is driven by activated hepatic stellate cells (HSCs). Due to the limited availability of anti-fibrotic drugs, the research on therapeutic agents continues. Here we have investigated Moringa oleifera Lam. (MO), known for its various bioactive properties, for anti-fibrotic effects. This study has focused on 1-phenyl-2-pentanol (1-PHE), a compound derived from MO leaves, and its effects on LX-2 human hepatic stellate cell activation. TGF-ß1-stimulated LX-2 cells were treated with MO extract or 1-PHE, and the changes in liver fibrosis markers were assessed at both gene and protein levels. Proteomic analysis and molecular docking were employed to identify potential protein targets and signaling pathways affected by 1-PHE. Treatment with 1-PHE downregulated fibrosis markers, including collagen type I alpha 1 chain (COL1A1), collagen type IV alpha 1 chain (COL4A1), mothers against decapentaplegic homologs 2 and 3 (SMAD2/3), and matrix metalloproteinase-2 (MMP2), and reduced the secretion of matrix metalloproteinase-9 (MMP-9). Proteomic analysis data showed that 1-PHE modulates the Wnt/ß-catenin pathway, providing a possible mechanism for its effects. Our results suggest that 1-PHE inhibits the TGF-ß1 and Wnt/ß-catenin signaling pathways and HSC activation, indicating its potential as an anti-liver-fibrosis agent.

Seeking systems-based facilitators of safety and healthcare resilience: a thematic review of incident reports.

Patient safety incident reports are a key source of safety intelligence. This study aimed to explore whether information contained in such reports can elicit facilitators of safety, including responding, anticipating, monitoring, learning, and other mechanisms by which safety is maintained. The review further explored whether, if found, this information could be used to inform safety interventions. Anonymized incident reports submitted between August and October 2020 were obtained from two large teaching hospitals. The Systems Engineering Initiative for Patient Safety (SEIPS) tool and the resilience potentials (responding, anticipating, monitoring, and learning) frameworks guided thematic analysis. SEIPS was used to explore the components of people, tools, tasks, and environments, as well as the interactions between them, which contribute to safety. The resilience potentials provided insight into healthcare resilience at individual, team, and organizational levels. Sixty incident reports were analysed. These included descriptions of all the SEIPS framework components. People used tools such as electronic prescribing systems to perform tasks within different healthcare environments that facilitated safety. All four resilient capacities were identified, with mostly individuals and teams responding to events; however, monitoring, anticipation, and learning were described for individuals, teams, and organizations. Incident reports contain information about safety practices, much of which is not identified by traditional approaches such as root cause analysis. This information can be used to enhance safety enablers and encourage greater proactive anticipation and system-level learning.

Newly identified roles for PIEZO1 mechanosensor in controlling normal megakaryocyte development and in primary myelofibrosis.

Mechanisms through which mature megakaryocytes (Mks) and their progenitors sense the bone marrow extracellular matrix to promote lineage differentiation in health and disease are still partially understood. We found PIEZO1, a mechanosensitive cation channel, to be expressed in mouse and human Mks. Human mutations in PIEZO1 have been described to be associated with blood cell disorders. Yet, a role for PIEZO1 in megakaryopoiesis and proplatelet formation has never been investigated. Here, we show that activation of PIEZO1 increases the number of immature Mks in mice, while the number of mature Mks and Mk ploidy level are reduced. Piezo1/2 knockout mice show an increase in Mk size and platelet count, both at basal state and upon marrow regeneration. Similarly, in human samples, PIEZO1 is expressed during megakaryopoiesis. Its activation reduces Mk size, ploidy, maturation, and proplatelet extension. Resulting effects of PIEZO1 activation on Mks resemble the profile in Primary Myelofibrosis (PMF). Intriguingly, Mks derived from Jak2V617F PMF mice show significantly elevated PIEZO1 expression, compared to wild-type controls. Accordingly, Mks isolated from bone marrow aspirates of JAK2V617F PMF patients show increased PIEZO1 expression compared to Essential Thrombocythemia. Most importantly, PIEZO1 expression in bone marrow Mks is inversely correlated with patient platelet count. The ploidy, maturation, and proplatelet formation of Mks from JAK2V617F PMF patients are rescued upon PIEZO1 inhibition. Together, our data suggest that PIEZO1 places a brake on Mk maturation and platelet formation in physiology, and its upregulation in PMF Mks might contribute to aggravating some hallmarks of the disease.

Identifying and mapping measures of medication safety during transfer of care in a digital era: a scoping literature review.

BACKGROUND: Measures to evaluate high-risk medication safety during transfers of care should span different safety dimensions across all components of these transfers and reflect outcomes and opportunities for proactive safety management. OBJECTIVES: To scope measures currently used to evaluate safety interventions targeting insulin, anticoagulants and other high-risk medications during transfers of care and evaluate their comprehensiveness as a portfolio. METHODS: Embase, Medline, Cochrane and CINAHL databases were searched using scoping methodology for studies evaluating the safety of insulin, anticoagulants and other high-risk medications during transfer of care. Measures identified were extracted into a spreadsheet, collated and mapped against three frameworks: (1) 'Key Components of an Ideal Transfer of Care', (2) work systems, processes and outcomes and (3) whether measures captured past harms, events in real time or areas of concern. The potential for digital health systems to support proactive measures was explored. RESULTS: Thirty-five studies were reviewed with 162 measures in use. Once collated, 29 discrete categories of measures were identified. Most were outcome measures such as adverse events. Process measures included communication and issue identification and resolution. Clinic enrolment was the only work system measure. Twenty-four measures captured past harm (eg, adverse events) and six indicated future risk (eg, patient feedback for organisations). Two real-time measures alerted healthcare professionals to risks using digital systems. No measures were of advance care planning or enlisting support. CONCLUSION: The measures identified are insufficient for a comprehensive portfolio to assess safety of key medications during transfer of care. Further measures are required to reflect all components of transfers of care and capture the work system factors contributing to outcomes in order to support proactive intervention to reduce unwanted variation and prevent adverse outcomes. Advances in digital technology and its employment within integrated care provide opportunities for the development of such measures.

Matrix stiffness controls megakaryocyte adhesion, fibronectin fibrillogenesis, and proplatelet formation through Itgß3.

Megakaryocytes (MKs) are the precursor cells of platelets, located in the bone marrow (BM). Once mature, they extend elongated projections named proplatelets through sinusoid vessels, emerging from the marrow stroma into the circulating blood. Not all signals from the microenvironment that regulate proplatelet formation are understood, particularly those from the BM biomechanics. We sought to investigate how MKs perceive and adapt to modifications of the stiffness of their environment. Although the BM is one of the softest tissue of the body, its rigidification results from excess fibronectin (FN), and other matrix protein deposition occur upon myelofibrosis. Here, we have shown that mouse MKs are able to detect the stiffness of a FN-coated substrate and adapt their morphology accordingly. Using a polydimethylsiloxane substrate with stiffness varying from physiological to pathological marrow, we found that a stiff matrix favors spreading, intracellular contractility, and FN fibrils assembly at the expense of proplatelet formation. Itgb3, but not Itgb1, is required for stiffness sensing, whereas both integrins are involved in fibrils assembly. In contrast, soft substrates promote proplatelet formation in an Itgb3-dependent manner, consistent with the ex vivo decrease in proplatelet formation and the in vivo decrease in platelet number in Itgb3-deficient mice. Our findings demonstrate the importance of environmental stiffness for MK functions with potential pathophysiological implications during pathologies that deregulate FN deposition and modulate stiffness in the marrow.

Assessment of Clinical Information Quality in Digital Health Technologies: International eDelphi Study.

BACKGROUND: Digital health technologies (DHTs), such as electronic health records and prescribing systems, are transforming health care delivery around the world. The quality of information in DHTs is key to the quality and safety of care. We developed a novel clinical information quality (CLIQ) framework to assess the quality of clinical information in DHTs. OBJECTIVE: This study explored clinicians' perspectives on the relevance, definition, and assessment of information quality dimensions in the CLIQ framework. METHODS: We used a systematic and iterative eDelphi approach to engage clinicians who had information governance roles or personal interest in information governance; the clinicians were recruited through purposive and snowball sampling techniques. Data were collected using semistructured online questionnaires until consensus was reached on the information quality dimensions in the CLIQ framework. Responses on the relevance of the dimensions were summarized to inform decisions on retention of the dimensions according to prespecified rules. Thematic analysis of the free-text responses was used to revise definitions and the assessment of dimensions. RESULTS: Thirty-five clinicians from 10 countries participated in the study, which was concluded after the second round. Consensus was reached on all dimensions and categories in the CLIQ framework: informativeness (accuracy, completeness, interpretability, plausibility, provenance, and relevance), availability (accessibility, portability, security, and timeliness), and usability (conformance, consistency, and maintainability). A new dimension, searchability, was introduced in the availability category to account for the ease of finding needed information in the DHTs. Certain dimensions were renamed, and some definitions were rephrased to improve clarity. CONCLUSIONS: The CLIQ framework reached a high expert consensus and clarity of language relating to the information quality dimensions. The framework can be used by health care managers and institutions as a pragmatic tool for identifying and forestalling information quality problems that could compromise patient safety and quality of care. INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID): RR2-10.1136/bmjopen-2021-057430.

Megakaryocytes form linear podosomes devoid of digestive properties to remodel medullar matrix.

Bone marrow megakaryocytes (MKs) undergo a maturation involving contacts with the microenvironment before extending proplatelets through sinusoids to deliver platelets in the bloodstream. We demonstrated that MKs assemble linear F-actin-enriched podosomes on collagen I fibers. Microscopy analysis evidenced an inverse correlation between the number of dot-like versus linear podosomes over time. Confocal videomicroscopy confirmed that they derived from each-other. This dynamics was dependent on myosin IIA. Importantly, MKs progenitors expressed the Tks4/5 adaptors, displayed a strong gelatinolytic ability and did not form linear podosomes. While maturing, MKs lost Tks expression together with digestive ability. However, those MKs were still able to remodel the matrix by exerting traction on collagen I fibers through a collaboration between GPVI, ß1 integrin and linear podosomes. Our data demonstrated that a change in structure and composition of podosomes accounted for the shift of function during megakaryopoiesis. These data highlight the fact that members of the invadosome family could correspond to different maturation status of the same entity, to adapt to functional responses required by differentiation stages of the cell that bears them.

Impact of creative art therapy on fatigue and quality of life in patients treated for localized breast cancer: A randomized study.

BACKGROUND: Art therapy (AT) as supportive care may help patients cope with cancer treatments. This non-blinded randomized trial assessed the impact of creative AT on severe fatigue and quality of life (QoL) in localized breast cancer patients undergoing irradiation. MATERIAL AND METHODS: 320 patients were randomized to an AT group (ATG; 8 weekly sessions starting during irradiation) or to a standard group (SG). The primary endpoint was severe global fatigue (Functional Assessment of Chronic Therapy Fatigue subscale score <37) at 1 month post-irradiation. Quality of life (Fact-B), anxiety/depression (Hospital Anxiety and Depression Scale (HADS)) and different dimensions of fatigue 20-item Multidimensional Fatigue Inventory (MFI-20) were assessed at 1, 6 and 12 months post-irradiation. The secondary endpoints, fatigue among patients treated with chemotherapy, QoL (Fact-B), anxiety/depression (HADS) and different dimensions of fatigue (MFI-20) at 1, 6 and 12 months post-irradiation (with post hoc analysis in patients with treated with chemotherapy) were also assessed. RESULTS: 82% of patients completed ≥8 sessions. Severe initial global fatigue was observed in 43% of patients in each group, and among in 64% of patients whose treatment protocol contained chemotherapy. At 1 month post-irradiation, 45% in the ATG and 57% of patients in the SG reported severe global fatigue (p = 0.37); among patients with initial severe mental fatigue (MFF), 79% and 44% had improved MFF (p = 0.007) respectively; similarly 79% and 44% with initial poor motivation had better mental motivation (p = 0.03). At 6 and 12 months, social well-being scores in the ATG were higher (21.3 and 21.4 vs. 19.8 and 19.2, p = 0.05 and p < 0.01) with a significant improvement for patients who had chemotherapy (41% vs. 18%, p = 0.017). A positive association was observed between the number of AT sessions, fatigue and QoL (p < 0.01). CONCLUSION: AT did not significantly improve global severe fatigue among all cancer participants 1 month after radiation therapy, however it had a positive impact on social well-being and may improve MFF and motivation.

Megakaryocyte Culture in 3D Methylcellulose-Based Hydrogel to Improve Cell Maturation and Study the Impact of Stiffness and Confinement.

The 3D environment leading to both confinement and mechanical constraints is increasingly recognized as an important determinant of cell behavior. 3D culture has thus been developed to better approach the in vivo situation. Megakaryocytes differentiate from hematopoietic stem and progenitor cells (HSPCs) in the bone marrow (BM). The BM is one of the softest tissues of the body, confined inside the bone. The bone being poorly extensible at the cell scale, megakaryocytes are concomitantly subjected to a weak stiffness and high confinement. This protocol presents a method for the recovery of mouse lineage negative (Lin-) HSPCs by immuno-magnetic sorting and their differentiation into mature megakaryocytes in a 3D medium composed of methylcellulose. Methylcellulose is non-reactive towards megakaryocytes and its stiffness may be adjusted to that of normal bone marrow or increased to mimic a pathological fibrotic marrow. The process to recover the megakaryocytes for further cell analyses is also detailed in the protocol. Although proplatelet extension is prevented within the 3D milieu, it is described below how to resuspend the megakaryocytes in liquid medium and to quantify their capacity to extend proplatelets. Megakaryocytes grown in 3D hydrogel have a higher capacity to form proplatelets compared to those grown in a liquid milieu. This 3D culture allows i) to differentiate progenitors towards megakaryocytes reaching a higher maturation state, ii) to recapitulate phenotypes that may be observed in vivo but go unnoticed in classical liquid cultures, and iii) to study transduction pathways induced by the mechanical cues provided by a 3D environment.

In Vivo Two-photon Imaging of Megakaryocytes and Proplatelets in the Mouse Skull Bone Marrow.

Platelets are produced by megakaryocytes, specialized cells located in the bone marrow. The possibility to image megakaryocytes in real time and their native environment was described more than 10 years ago and sheds new light on the process of platelet formation. Megakaryocytes extend elongated protrusions, called proplatelets, through the endothelial lining of sinusoid vessels. This paper presents a protocol to simultaneously image in real time fluorescently labeled megakaryocytes in the skull bone marrow and sinusoid vessels. This technique relies on a minor surgery that keeps the skull intact to limit inflammatory reactions. The mouse head is immobilized with a ring glued to the skull to prevent movements from breathing. Using two-photon microscopy, megakaryocytes can be visualized for up to a few hours, enabling the observation of cell protrusions and proplatelets in the process of elongation inside sinusoid vessels. This allows the quantification of several parameters related to the morphology of the protrusions (width, length, presence of constriction areas) and their elongation behavior (velocity, regularity, or presence of pauses or retraction phases). This technique also allows simultaneous recording of circulating platelets in sinusoid vessels to determine platelet velocity and blood flow direction. This method is particularly useful to study the role of genes of interest in platelet formation using genetically modified mice and is also amenable to pharmacological testing (study the mechanisms, evaluating drugs in the treatment of platelet production disorders). It has become an invaluable tool, especially to complement in vitro studies as it is now known that in vivo and in vitro proplatelet formation rely on different mechanisms. It has been shown, for example, that in vitro microtubules are required for proplatelet elongation per se. However, in vivo, they rather serve as a scaffold, elongation being mainly promoted by blood flow forces.

Proplatelet Formation Dynamics of Mouse Fresh Bone Marrow Explants.

The last stage of megakaryopoiesis leads to cytoplasmic extensions from mature megakaryocytes, the so-called proplatelets. Much has been learned about the proplatelet formation using in vitro-differentiated megakaryocytes; however, there is an increasing evidence that conventional culture systems do not faithfully recapitulate the differentiation/maturation process that takes places inside the bone marrow. In this manuscript, we present an explant method initially described in 1956 by Thiéry and Bessis to visualize megakaryocytes which have matured in their native environment, thus circumventing potential artifacts and misinterpretations. Fresh bone marrows are collected by flushing the femurs of mice, sliced into 0.5 mm cross sections, and placed in an incubation chamber at 37 °C containing a physiological buffer. Megakaryocytes become gradually visible at the explant periphery and are observed up to 6 hours under an inverted microscope coupled to a video camera. Over time, megakaryocytes change their shape, with some cells having a spherical form and others developing thick extensions or extending many thin proplatelets with extensive branching. Both qualitative and quantitative investigations are carried out. This method has the advantage of being simple, reproducible, and fast as numerous megakaryocytes are present, and classically half of them form proplatelets in 6 hours compared to 4 days for cultured mouse megakaryocytes. In addition to the study of mutant mice, an interesting application of this method is the straightforward evaluation of the pharmacological agents on the proplatelet extension process, without interfering with the differentiation process that may occur in cultures.

Ovarian Vein to Portal Vein Reconstruction: Another Option in Liver Transplant With Portal Vein Thrombosis.

Portal vein thrombosis is a common complication in patients with cirrhosis and a challenge for the transplant team. Not so long ago, portal vein thrombosis was considered an absolute contraindication for liver transplant, but improvements in surgical techniques have overcome this problem in many transplant centers around the world. Here, we present the case of a 52-year-old female patient with cirrhosis from a primary biliary cholangitis and a complex portal vein thrombosis. She underwent a deceased donor liver transplant with a Model for End-Stage Liver Disease of 40. The portal thrombosis was handled using a portosystemic shunt from the splenic vein to the left ovarian vein, which was visualized on a computed tomography scan performed as part of the study protocol. The donor was a 52-year-old woman with brain death secondary to a vascular cerebral accident. A caval replacement technique was used with no complications during surgery. For the portal anastomosis, the dilated left ovarian vein was carefully dissected and brought through the lesser sac, behind the stomach, to obtain a suitable length. An end-to-end anastomosis of the graft portal vein to the left ovarian vein was performed with a 6-0 Prolene running suture. An abdominal computed tomography scan was performed 6 months after liver transplant showing patency of portal vein and no anastomotic defects, and after 24 months of follow-up the patient is in good clinical condition with normal laboratory values and Doppler ultrasonography with no vascular anomalies and adequate portal flow. To our knowledge, the use of a spleno-ovarian shunt has not been reported as an alternative for portal reconstruction in a case of thrombosis.

Cytoskeletal-based mechanisms differently regulate in vivo and in vitro proplatelet formation.

Platelets are produced by bone marrow megakaryocytes through cytoplasmic protrusions, named native proplatelets (nPPT), into blood vessels. Proplatelets also refer to protrusions observed in megakaryocyte culture (cPPT) that are morphologically different. Contrary to cPPT, the mechanisms of nPPT formation are poorly understood. We show here in living mice that nPPT elongation is in equilibrium between protrusive and retraction forces mediated by myosin-IIA. We also found, using WT and ß1-tubulin-deficient mice, that microtubule behavior differs between cPPT and nPPT, being absolutely required in vitro, while less critical in vivo. Remarkably, microtubule depolymerization in myosin-deficient mice did not affect nPPT elongation. We then calculated that blood Stokes'forces may be sufficient to promote nPPT extension, independently of myosin and microtubules. Together, we propose a new mechanism for nPPT extension that might explain contradictions between severely affected cPPT production and moderate platelet count defects in some patients and animal models.

Blood platelet formation at a glance.

The main function of blood platelets is to ensure hemostasis and prevent hemorrhages. The 1011 platelets needed daily are produced in a well-orchestrated process. However, this process is not yet fully understood and in vitro platelet production is still inefficient. Platelets are produced in the bone marrow by megakaryocytes, highly specialized precursor cells that extend cytoplasmic projections called proplatelets (PPTs) through the endothelial barrier of sinusoid vessels. In this Cell Science at a Glance article and the accompanying poster we discuss the mechanisms and pathways involved in megakaryopoiesis and platelet formation processes. We especially address the - still underestimated - role of the microenvironment of the bone marrow, and present recent findings on how PPT extension in vivo differs from that in vitro and entails different mechanisms. Finally, we recapitulate old but recently revisited evidence that - although bone marrow does produce megakaryocytes and PPTs - remodeling and the release of bona fide platelets, mainly occur in the downstream microcirculation.

Megakaryocytes use in vivo podosome-like structures working collectively to penetrate the endothelial barrier of bone marrow sinusoids.

BACKGROUND: Blood platelets are anucleate cell fragments that prevent bleeding and minimize blood vessel injury. They are formed from the cytoplasm of megakaryocytes located in the bone marrow. For successful platelet production, megakaryocyte fragments must pass through the sinusoid endothelial barrier by a cell biology process unique to these giant cells as compared with erythrocytes and leukocytes. Currently, the mechanisms by which megakaryocytes interact and progress through the endothelial cells are not understood, resulting in a significant gap in our knowledge of platelet production. OBJECTIVE: The aim of this study was to investigate how megakaryocytes interact and progress through the endothelial cells of mouse bone marrow sinusoids. METHODS: We used a combination of fluorescence, electron, and three-dimensional microscopy to characterize the cellular events between megakaryocytes and endothelial cells. RESULTS: We identified protrusive, F-actin-based podosome-like structures, called in vivo-MK podosomes, which initiate the formation of pores through endothelial cells. These structures present a collective and spatial organization through their interconnection via a contractile network of actomyosin, essential to regulate the endothelial openings. This ensures proper passage of megakaryocyte-derived processes into the blood circulation to promote thrombopoiesis. CONCLUSION: This study provides novel insight into the in vivo function of podosomes of megakaryocytes with critical importance to platelet production.

Use of Iliac Allograft from Cadaveric Donor as a Rescue Technique in Living Donor Kidney Transplant: Two Case Reports.

BACKGROUND: A short right renal vein (RRV) remains a challenge for renal transplant surgery, especially in the living donor. Different techniques exist to obtain an RRV with a suitable length in cadaveric donor; however, in living donors the options are limited. MATERIAL AND METHODS: We present 2 living kidney transplants in which we obtained a very short RRV, making the implantation very difficult. We describe our technique to overcome this problem by using cadaveric iliac vessels retrieved from previous cadaveric donations and preserved at 4°C in histidine-tryptophan-ketoglutarate (HTK) solution, without intraoperative or postoperative complications. We complied with the Helsinki Congress and the Istanbul Declaration regarding the donor source. RESULTS: In both cases, kidney grafts had optimal primary function, with good creatinine clearance after transplant and good patency of vascular anastomosis by Doppler ultrasounds. CONCLUSIONS: We believe the use of cadaveric vessel grafts in living donor kidney transplant is a valuable resource as a rescue tool in emergency situations like the ones being presented in this article in order to avoid discarding a kidney graft with damage or short vessels. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Combined deficiency of RAB32 and RAB38 in the mouse mimics Hermansky-Pudlak syndrome and critically impairs thrombosis.

The biogenesis of lysosome related organelles is defective in Hermansky-Pudlak syndrome (HPS), a disorder characterized by oculocutaneous albinism and platelet dense granule (DG) defects. The first animal model of HPS was the fawn-hooded rat, harboring a spontaneous mutation inactivating the small guanosine triphosphatase Rab38 This leads to coat color dilution associated with the absence of DGs and lung morphological defects. Another RAB38 mutant, the cht mouse, has normal DGs, which has raised controversy about the role of RAB38 in DG biogenesis. We show here that murine and human, but not rat, platelets also express the closely related RAB32. To elucidate the parts played by RAB32 and RAB38 in the biogenesis of DGs in vivo and their effects on platelet functions, we generated mice inactivated for Rab32, Rab38, and both genes. Single Rab38 inactivation mimicked cht mice, whereas single Rab32 inactivation had no effect in DGs, coat color, or lung morphology. By contrast, Rab32/38 double inactivation mimicked severe HPS, with strong coat and eye pigment dilution, some enlarged lung multilamellar bodies associated with a decrease in the number of DGs. These organelles were morphologically abnormal, decreased in number, and devoid of 5-hydroxytryptamine content. In line with the storage pool defect, platelet activation was affected, resulting in severely impaired thrombus growth and prolongation of the bleeding time. Overall, our study demonstrates the absence of impact of RAB38 or RAB32 single deficiency in platelet biogenesis and function resulting from full redundancy, and characterized a new mouse model mimicking HPS devoid of DG content.

Clot Contraction Drives the Translocation of Procoagulant Platelets to Thrombus Surface.

Objective- After activation at the site of vascular injury, platelets differentiate into 2 subpopulations, exhibiting either proaggregatory or procoagulant phenotype. Although the functional role of proaggregatory platelets is well established, the physiological significance of procoagulant platelets, the dynamics of their formation, and spatial distribution in thrombus remain elusive. Approach and Results- Using transmission electron microscopy and fluorescence microscopy of arterial thrombi formed in vivo after ferric chloride-induced injury of carotid artery or mechanical injury of abdominal aorta in mice, we demonstrate that procoagulant platelets are located at the periphery of the formed thrombi. Real-time cell tracking during thrombus formation ex vivo revealed that procoagulant platelets originate from different locations within the thrombus and subsequently translocate towards its periphery. Such redistribution of procoagulant platelets was followed by generation of fibrin at thrombus surface. Using in silico model, we show that the outward translocation of procoagulant platelets can be driven by the contraction of the forming thrombi, which mechanically expels these nonaggregating cells to thrombus periphery. In line with the suggested mechanism, procoagulant platelets failed to translocate and remained inside the thrombi formed ex vivo in blood derived from nonmuscle myosin ( MYH9)-deficient mice. Ring-like distribution of procoagulant platelets and fibrin around the thrombus observed with blood of humans and wild-type mice was not present in thrombi of MYH9-knockout mice, confirming a major role of thrombus contraction in this phenomenon. Conclusions- Contraction of arterial thrombus is responsible for the mechanical extrusion of procoagulant platelets to its periphery, leading to heterogeneous structure of thrombus exterior.