The lung megakaryocytes and pulmonary fibrosis in systemic sclerosis

Platelets, extracellular vesicles and coagulation in pulmonary arterial hypertension

Pulmonary arterial hypertension is a rare disease of the pulmonary vasculature, characterised pathologically by proliferation, remodelling and thrombosis in situ. Unfortunately, existing therapeutic interventions do not reverse these findings and the disease continues to result in significant morbidity and premature mortality. A number of haematological derangements have been described in pulmonary arterial hypertension which may provide insights into the pathobiology of the disease and opportunities to explore new therapeutic pathways. These include quantitative and qualitative platelet abnormalities, such as thrombocytopaenia, increased mean platelet volume and altered platelet bioenergetics. Furthermore, a hypercoagulable state and aberrant negative regulatory pathways can be observed, which could contribute to thrombosis in situ in distal pulmonary arteries and arterioles. Finally, there is increasing interest in the role of extracellular vesicle autocrine and paracrine signalling in pulmonary arterial hypertension, and their potential utility as biomarkers and novel therapeutic targets. This review focuses on the potential role of platelets, extracellular vesicles and coagulation pathways in the pathobiology of pulmonary arterial hypertension. We highlight important unanswered clinical questions and the implications of these observations for future research and pulmonary arterial hypertension-directed therapies.

Hemodialysis exacerbates proteolytic imbalance and pro-fibrotic platelet dysfunction

Multi-organ fibrosis among end stage renal disease (ESRD) patients cannot be explained by uremia alone. Despite mitigation of thrombosis during hemodialysis (HD), subsequent platelet dysfunction and tissue dysregulation are less understood. We comprehensively profiled plasma and platelets from ESRD patients before and after HD to examine HD-modulation of platelets beyond thrombotic activation. Basal plasma levels of proteolytic regulators and fibrotic factors were elevated in ESRD patients compared to healthy controls, with isoform-specific changes during HD. Platelet lysate (PL) RNA transcripts for growth and coagulative factors were elevated post-HD, with upregulation correlated to HD vintage. Platelet secretome correlations to plasma factors reveal acutely induced pro-fibrotic platelet phenotypes in ESRD patients during HD characterized by preferentially enhanced proteolytic enzyme translation and secretion, platelet contribution to inflammatory response, and increasing platelet dysfunction with blood flow rate (BFR) and Vintage. Compensatory mechanisms of increased platelet growth factor synthesis with acute plasma matrix metalloproteinase (MMP) and tissue inhibitor of MMPs (TIMP) increases show short-term mode-switching between dialysis sessions leading to long-term pro-fibrotic bias. Chronic pro-fibrotic adaptation of platelet synthesis were observed through changes in differential secretory kinetics of heterogenous granule subtypes. We conclude that chronic and acute platelet responses to HD contribute to a pro-fibrotic milieu in ESRD.

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 participate in the occurrence of bleomycin-induced pulmonary fibrosis

Pulmonary fibrosis is characterized by the remodeling of fibrotic tissue and collagen deposition, which mainly results from aberrant fibroblasts proliferation and trans-differentiation to myofibroblasts. Patients with chronic myelogenous leukemia, myeloproliferative disorder, and scleroderma with pulmonary fibrosis complications show megakaryocyte infiltration in the lung. In this study, we demonstrated that the number of CD41⁺ megakaryocytes increased in bleomycin (BLM)-induced lung fibrosis tissues through the Chemokine (CXCmotif) ligand 12/Chemokine receptor 4 (CXCL12/CXCR4) axis. Pharmacological inhibition of the CXCL12/CXCR4 axis with WZ811 prevented migration of CD41⁺ megakaryocytes induced by BLM-injured lung tissue ex vivo and in vivo. In addition, WZ811 significantly attenuated lung fibrosis after BLM challenge. Moreover, megakaryocytes directly promoted fibroblast proliferation and trans-differentiation to myofibroblasts. We conclude that thrombopoietin (TPO) activated megakaryocytes through transforming growth factor β (TGF-β) pathway to promote fibroblast proliferation and trans-differentiation to myofibroblasts, which is abolished by treatment with selective TGF-βR-1/ALK5 inhibitors. Therefore, CD41⁺ megakaryocytes migrate to injured lung tissue partially through the CXCL12/CXCR4 axis to promote the proliferation and trans-differentiation of fibroblasts through direct contact and the TGF-β1 pathway.

The role of extracellular matrix stiffness in megakaryocyte and platelet development and function

The extracellular matrix (ECM) is a key acellular structure in constant remodeling to provide tissue cohesion and rigidity. Deregulation of the balance between matrix deposition, degradation, and crosslinking results in fibrosis. Bone marrow fibrosis (BMF) is associated with several malignant and nonmalignant pathologies severely affecting blood cell production. BMF results from abnormal deposition of collagen fibers and enhanced lysyl oxidase-mediated ECM crosslinking within the marrow, thereby increasing marrow stiffness. Bone marrow stiffness has been recently recognized as an important regulator of blood cell development, notably by modifying the fate and differentiation process of hematopoietic or mesenchymal stem cells. This review surveys the different components of the ECM and their influence on stem cell development, with a focus on the impact of the ECM composition and stiffness on the megakaryocytic lineage in health and disease. Megakaryocyte maturation and the biogenesis of their progeny, the platelets, are thought to respond to environmental mechanical forces through a number of mechanosensors, including integrins and mechanosensitive ion channels, reviewed here.

Central Nodes in Protein Interaction Networks Drive Critical Functions in Transforming Growth Factor Beta-1 Stimulated Kidney Cells

Objective

Despite the huge efforts, chronic kidney disease (CKD) remains as an unsolved problem in medicine. Many studies have shown a central role for transforming growth factor beta-1 (TGFβ-1) and its downstream signaling cascades in the pathogenesis of CKD. In this study, we have reanalyzed a microarray dataset to recognize critical signaling pathways controlled by TGFβ-1.

Materials and Methods

This study is a bioinformatics reanalysis for a microarray data. The GSE23338 dataset was downloaded from the gene expression omnibus (GEO) database which assesses the mRNA expression profile of TGFβ-1 treated human kidney cells after 24 and 48 hours incubation. The protein interaction networks for differentially expressed (DE) genes in both time points were constructed and enriched. In addition, by network topology analysis, genes with high centrality were identified and then pathway enrichment analysis was performed with either the total network genes or with the central nodes.

Results

We found 110 and 170 genes differentially expressed in the time points 24 and 48 hours, respectively. As the genes in each time point had few interactions, the networks were enriched by adding previously known genes interacting with the differentially expressed ones. In terms of degree, betweenness, and closeness centrality parameters 62 and 60 nodes were considered to be central in the enriched networks of 24 hours and 48 hours treatment, respectively. Pathway enrichment analysis with the central nodes was more informative than those with all network nodes or even initial DE genes, revealing key signaling pathways.

Conclusion

We here introduced a method for the analysis of microarray data that integrates the expression pattern of genes with their topological properties in protein interaction networks. This holistic novel approach allows extracting knowledge from raw bulk omics data.

Regulation of fas/fas ligand-mediated apoptosis by nuclear factor of activated T cells in megakaryocytes

Signal transduction pathways in megakaryocytes, a rare population of bone marrow cells, are poorly understood. We have previously shown that the calcineurin-dependent transcription factor Nuclear Factor of Activated T cells (NFAT) is expressed in megakaryocytes and is required for the transcription of specific megakaryocytic genes. The biological role of NFAT in megakaryocytes, however, is unknown. Here we show that activation of the calcineurin/NFAT pathway in megakaryocytes forces the cells to go into apoptosis. Calcineurin/NFAT activation in megakaryocytes leads to membrane expression of Fas ligand (FASLG), a pro-apoptotic member of the tumour necrosis factor superfamily. Expression of FASLG was augmented in cells stably overexpressing NFATC2 and suppressed in cells either pretreated with the calcineurin inhibitor ciclosporin A (CsA) or expressing the specific peptide inhibitor of NFAT, VIVIT. In cocultures with Fas-expressing Jurkat T cells, the presence of activated megakaryocytic cells, but not of unstimulated cells or cells stimulated in the presence of CsA, significantly induced apoptosis in Jurkat cells in a Fas/FASLG- and NFAT-dependent manner. These results represent the first evidence for a biological function of the calcineurin/NFAT pathway in megakaryocytes, and suggest that the biological role of megakaryocytes may include the induction of apoptosis in bystander cells.