Intramedullary megakaryocytes internalize released platelet factor 4 and store it in alpha granules

The many faces of the megakaryocytes and their biological implications

PURPOSE OF REVIEW

Single-cell RNA sequencing studies have revealed transcriptional heterogeneity within the megakaryocytic lineage and the identified unique subsets. In this review, we discuss the functional and phenotypic plasticity of these subpopulations as well as the impacts on health and disease.

RECENT FINDINGS

Megakaryocytes (MKs) can be transcriptionally categorized into platelet generating, niche supporting, immune, and cycling cells, which are distinguished by their unique gene expression patterns and cellular markers. Additionally, a significant population of these cells has been established to reside in the nonhematopoietic tissues and they display enhanced immune-related characteristics. Combined with the location in which the megakaryocytes exist, these cells can play unique roles dictated by their current environment and biological needs, including responding to changes in pathogen exposure.

SUMMARY

Advances in megakaryocyte research has elucidated the existence of multiple subpopulations of MKs that serve different functions. These subpopulations implicate a greater potential for MKs to be regulators of health and suggest new avenues for treatments and therapies in related diseases.

Megakaryocyte Secreted Factors Regulate Bone Marrow Niche Cells During Skeletal Homeostasis, Aging, and Disease

The bone marrow microenvironment contains a diverse array of cell types under extensive regulatory control and provides for a novel and complex mechanism for bone regulation. Megakaryocytes (MKs) are one such cell type that potentially acts as a master regulator of the bone marrow microenvironment due to its effects on hematopoiesis, osteoblastogenesis, and osteoclastogenesis. While several of these processes are induced/inhibited through MK secreted factors, others are primarily regulated by direct cell-cell contact. Notably, the regulatory effects that MKs exert on these different cell populations has been found to change with aging and disease states. Overall, MKs are a critical component of the bone marrow that should be considered when examining regulation of the skeletal microenvironment. An increased understanding of the role of MKs in these physiological processes may provide insight into novel therapies that can be used to target specific pathways important in hematopoietic and skeletal disorders.

Benzo[a]pyrene Exposure Reduces Cell-Type Diversity and Stimulates Sex-Biased Damage Pathways in End Organs of Lupus-Prone Mice

Studies indicate that genetic factors only account for approximately thirty percent of all autoimmune diseases, while the rest of autoimmune pathogenesis is attributed to environmental factors including toxic chemicals. To understand if and how environmental pollutants trigger autoimmunity, we investigated the effect of benzo[a]pyrene (BaP) exposure on the development of autoimmune phenotypes in the lupus-prone MRL strain. The exposure of MRL mice to BaP over the course of 8 weeks before lupus onset resulted in total body weight loss in males, while marginal changes in anti-dsDNA levels occurred. Multi-organ analyses of BaP-treated and control MRL mice suggested that the kidney is a major organ directly affected by the metabolism of benzene-containing compounds, with increased expression of BaP-target genes including Cyp4b1 and Hao2. Intriguingly, spatial transcriptomic data showed that BaP caused a drastic reduction in cell-type diversity in both the kidneys and spleen of MRL mice. Further analysis of the molecular pathways affected suggested a sex-biased effect of BaP treatment, with the upregulated expression of angiogenesis genes in the lungs and an increased deposition of C3 in the kidneys of male mice. While SLE is more common in women, the disease is more severe in male patients, with an increased risk of disease progression to renal failure and lung cancer. Our results reveal sex-biased molecular pathways stimulated by BaP which may help explain the increased likelihood of end organ damage in males with lupus.

The levels of TGFβ1, VEGF, PDGF-BB, and PF4 in platelet-rich plasma of professional soccer players: a cross-sectional pilot study

Background

Regenerative therapy using platelet-rich plasma (PRP), a rich source of growth factors, has become popular in orthopedic sports medicine. Elite athletes prefer PRP therapy for their injured muscles and tendons primarily to avoid the possible risks of surgical treatment. However, the clinical effectiveness of PRP therapy in elite athletes compared to that in non-athletes remains unknown. Therefore, to investigate the effectiveness of PRP therapy in professional athletes (pro-athletes), we focused on the quality of PRP preparations and compared the levels of bioactive molecules between pro-athletes and non-athletes.

Methods

PRP was prepared from healthy, non-smoking male professional soccer players (pro-athletes) (n = 22) and non-athletes (VEGF: n = 34, others: n = 38). The levels of TGFβ1, PDGF-BB, VEGF, and PF4 were determined using ELISA kits. Polyphosphate was probed with 4’,6-diamidino-2-phenylindole and monitored using a fluorometer. The body composition of the donors was determined using a bathroom weighing scale.

Results

The levels of TGFβ1 and VEGF were significantly lower in pro-athletes than in non-athletes, whereas PF4 levels were significantly higher in pro-athletes. No significant difference was found in PDGF-BB levels between these groups. Biomolecule levels were not correlated with polyphosphate levels.

Conclusion

TGFβ1, VEGF, and PDGF-BB levels in pro-athletes were not higher than those in non-athletes. These findings suggest that growth factor levels in PRP may not be a predominant determinant of the clinical effectiveness of PRP therapy in pro-athletes. Increased PF4 levels in pro-athletes suggest an immunological function of PRP that may positively influence tissue regeneration.

Platelet Membrane: An Outstanding Factor in Cancer Metastasis

In addition to being biological barriers where the internalization or release of biomolecules is decided, cell membranes are contact structures between the interior and exterior of the cell. Here, the processes of cell signaling mediated by receptors, ions, hormones, cytokines, enzymes, growth factors, extracellular matrix (ECM), and vesicles begin. They triggering several responses from the cell membrane that include rearranging its components according to the immediate needs of the cell, for example, in the membrane of platelets, the formation of filopodia and lamellipodia as a tissue repair response. In cancer, the cancer cells must adapt to the new tumor microenvironment (TME) and acquire capacities in the cell membrane to transform their shape, such as in the case of epithelial−mesenchymal transition (EMT) in the metastatic process. The cancer cells must also attract allies in this challenging process, such as platelets, fibroblasts associated with cancer (CAF), stromal cells, adipocytes, and the extracellular matrix itself, which limits tumor growth. The platelets are enucleated cells with fairly interesting growth factors, proangiogenic factors, cytokines, mRNA, and proteins, which support the development of a tumor microenvironment and support the metastatic process. This review will discuss the different actions that platelet membranes and cancer cell membranes carry out during their relationship in the tumor microenvironment and metastasis.

Lin28b regulates age-dependent differences in murine platelet function

Platelets are essential for hemostasis; however, several studies have identified age-dependent differences in platelet function. To better understand the origins of fetal platelet function, we have evaluated the contribution of the fetal-specific RNA binding protein Lin28b in the megakaryocyte/platelet lineage. Because activated fetal platelets have very low levels of P-selectin, we hypothesized that the expression of platelet P-selectin is part of a fetal-specific hematopoietic program conferred by Lin28b. Using the mouse as a model, we find that activated fetal platelets have low levels of P-selectin and do not readily associate with granulocytes in vitro and in vivo, relative to adult controls. Transcriptional analysis revealed high levels of Lin28b and Hmga2 in fetal, but not adult, megakaryocytes. Overexpression of LIN28B in adult mice significantly reduces the expression of P-selectin in platelets, and therefore identifies Lin28b as a negative regulator of P-selectin expression. Transplantation of fetal hematopoietic progenitors resulted in the production of platelets with low levels of P-selectin, suggesting that the developmental regulation of P-selectin is intrinsic and independent of differences between fetal and adult microenvironments. Last, we observe that the upregulation of P-selectin expression occurs postnatally, and the temporal kinetics of this upregulation are recapitulated by transplantation of fetal hematopoietic stem and progenitor cells into adult recipients. Taken together, these studies identify Lin28b as a new intrinsic regulator of fetal platelet function.

2-O, 3-O desulfated heparin mitigates murine chemotherapy- and radiation-induced thrombocytopenia

Thrombocytopenia is a significant complication of chemotherapy and radiation therapy. Platelet factor 4 (PF4; CXCL4) is a negative paracrine of megakaryopoiesis. We have shown that PF4 levels are inversely related to steady-state platelet counts, and to the duration and severity of chemotherapy- and radiation-induced thrombocytopenia (CIT and RIT, respectively). Murine studies suggest that blocking the effect of PF4 improves megakaryopoiesis, raising nadir platelet counts and shortening the time to platelet count recovery. We examined the ability of 2-O, 3-O desulfated heparin (ODSH), a heparin variant with little anticoagulant effects, to neutralize PF4's effects on megakaryopoiesis. Using megakaryocyte colony assays and liquid cultures, we show that ODSH restored megakaryocyte proliferation in PF4-treated Cxcl4^-/- murine and human CD34⁺-derived megakaryocyte cultures (17.4% megakaryocyte colonies, P < .01 compared with PF4). In murine CIT and RIT models, ODSH, started 24 hours after injury, was examined for the effect on hematopoietic recovery demonstrating higher platelet count nadirs (9% ± 5% treated vs 4% ± 4% control) and significantly improved survival in treated animals (73% treated vs 36% control survival). Treatment with ODSH was able to reduce intramedullary free PF4 concentrations by immunohistochemical analysis. In summary, ODSH mitigated CIT and RIT in mice by neutralizing the intramedullary negative paracrine PF4. ODSH, already in clinical trials in humans as an adjuvant to chemotherapy, may be an important, clinically relevant therapeutic for CIT and RIT.

Megakaryocytes as immune cells

Platelets play well-recognized roles in inflammation, but their cell of origin-the megakaryocyte-is not typically considered an immune lineage. Megakaryocytes are large polyploid cells most commonly identified in bone marrow. Egress via sinusoids enables migration to the pulmonary capillary bed, where elaboration of platelets can continue. Beyond receptors involved in hemostasis and thrombosis, megakaryocytes express receptors that confer immune sensing capacity, including TLRs and Fc-γ receptors. They control the proliferation of hematopoietic cells, facilitate neutrophil egress from marrow, possess the capacity to cross-present antigen, and can promote systemic inflammation through microparticles rich in IL-1. Megakaryocytes internalize other hematopoietic lineages, especially neutrophils, in an intriguing cell-in-cell interaction termed emperipolesis. Together, these observations implicate megakaryocytes as direct participants in inflammation and immunity.

Phosphatidylinositol transfer proteins regulate megakaryocyte TGF-β1 secretion and hematopoiesis in mice

We hypothesized that megakaryocyte (MK) phosphoinositide signaling mediated by phosphatidylinositol transfer proteins (PITPs) contributes to hematopoietic stem cell (HSC) and hematopoietic progenitor cell (HPC) regulation. Conditional knockout mice lacking PITPs specifically in MKs and platelets (pitpα^-/- and pitpα^-/-/β^-/-) bone marrow (BM) manifested decreased numbers of HSCs, MK-erythrocyte progenitors, and cycling HPCs. Further, pitpα^-/-/β^-/- BM had significantly reduced engrafting capability in competitive transplantation and limiting dilution analysis. Conditioned media (CM) from cultured pitpα^-/- and pitpα^-/-/β^-/- BM MKs contained higher levels of transforming growth factor β1 (TGF-β1) and interleukin-4 (IL-4), among other myelosuppressive cytokines, than wild-type BM MKs. Correspondingly, BM flush fluid from pitpα^-/- and pitpα^-/-/β^-/- mice had higher concentrations of TGF-β1. CM from pitpα^-/- and pitpα^-/-/β^-/- MKs significantly suppressed HPC colony formation, which was completely extinguished in vitro by neutralizing anti-TGF-β antibody, and treatment of pitpα^-/-/β^-/- mice in vivo with anti-TGF-β antibodies completely reverted their defects in BM HSC and HPC numbers. TGF-β and IL-4 synergized to inhibit HPC colony formation in vitro. Electron microscopy analysis of pitpα^-/-/β^-/- MKs revealed ultrastructural defects with depleted α-granules and large, misshaped multivesicular bodies. Von Willebrand factor and thrombospondin-1, like TGF-β, are stored in MK α-granules and were also elevated in CM of cultured pitpα^-/-/β^-/- MKs. Altogether, these data show that ablating PITPs in MKs indirectly dysregulates hematopoiesis in the BM by disrupting α-granule physiology and secretion of TGF-β1.

Platelet-inspired medicine for tumor therapy

As the number of patients with tumor increases dramatically recent years, traditional therapies expose more and more problems which can even lead to death. Many researchers and clinicians quest for an efficient drug delivery system to deal with tumor as a result. With the researches further develop, we find that platelet can interact with tumor cells through a variety of ways. So it can be used as a carrier broadly to deliver different anti-tumor drugs for tumor treatment. In the present review, we summarize the interaction of tumor cells and platelet. At the same time, we focus on recent progress on the application of platelet drug-loaded system in the anti-tumor prospects.

Platelet-Specific Chemokines Contribute to the Pathogenesis of Acute Lung Injury

Platelets and neutrophils contribute to the development of acute lung injury (ALI). However, the mechanism by which platelets make this contribution is incompletely understood. We investigated whether the two most abundant platelet chemokines, CXCL7, which induces neutrophil chemotaxis and activation, and CXCL4, which does neither, mediate ALI through complementary pathogenic pathways. To examine the role of platelet-derived chemokines in the pathogenesis of ALI using Cxcl7^-/- and Cxcl4^-/- knockout mice and mice that express human CXCL7 or CXCL4, we measured levels of chemokines in these mice. ALI was then induced by acid aspiration, and the severity of injury was evaluated by histology and by the presence of neutrophils and protein in the bronchoalveolar lavage fluid. Pulmonary vascular permeability was studied in vivo by measuring extravasation of fluorescently labeled dextran. Murine CXCL7, both recombinant and native protein released from platelets, can be N-terminally processed by cathepsin G to yield a biologically active CXCL7 fragment. Although Cxcl7^-/- mice are protected from lung injury through the preservation of endothelial/epithelial barrier function combined with impaired neutrophils transmigration, Cxcl4^-/- mice are protected through improved barrier function without affecting neutrophils transmigration to the airways. Sensitivity to ALI is restored by transgenic expression of CXCL7 or CXCL4. Platelet-derived CXCL7 and CXCL4 contribute to the pathogenesis of ALI through complementary effects on neutrophil chemotaxis and through activation and vascular permeability.

Platelet Factor 4 Inhibits and Enhances HIV-1 Infection in a Concentration-Dependent Manner by Modulating Viral Attachment

Platelet factor 4 (PF4) has been recently shown to inhibit infection by a broad range of human immunodeficiency virus type 1 (HIV-1) isolates in vitro. We found that the inhibitory effects of PF4 are limited to a defined concentration range where PF4 exists largely in a monomeric state. Under these conditions, PF4 bound the HIV-1 envelope protein and inhibited HIV-1 attachment to the cell surface. However, as concentrations increased to the point where PF4 exists largely in tetrameric or higher-order forms, viral infection in vitro was enhanced. Enhancement could be inhibited by mutations in PF4 that shift the oligomeric equilibrium toward the monomeric state, or by using soluble glycosaminoglycans (GAGs) to which tetrameric PF4 avidly binds. We conclude that at physiologically relevant concentrations, oligomeric PF4 enhances infection by HIV-1 by interacting with the viral envelope protein as well as cell surface GAGs, enhancing virus attachment to the cell surface. This effect was not specific to HIV-1, as enhancement was seen with some but not all other viruses tested. The biphasic effects of PF4 on HIV-1 infection suggest that native PF4 will not be a useful antiviral agent and that PF4 could contribute to the hematologic abnormalities commonly seen in HIV-infected individuals by enhancing virus infection in the bone marrow.