Formin proteins in megakaryocytes and platelets::Regulation of actin and microtubule dynamics

The platelet and megakaryocyte cytoskeletons are essential for formation and function of these cells. A dynamic, properly organised tubulin and actin cytoskeleton is critical for the development of the megakaryocyte and the extension of proplatelets. Tubulin in particular plays a pivotal role in the extension of these proplatelets and the release of platelets from them. Tubulin is further required for the maintenance of platelet size, and actin is the driving force for shape change, spreading and platelet contraction during platelet activation. Whilst several key proteins which regulate these cytoskeletons have been described in detail, the formin family of proteins has received less attention. Formins are intriguing as, although they were initially believed to simply be a nucleator of actin polymerisation, increasing evidence shows they are important regulators of the crosstalk between the actin and microtubule cytoskeletons. In this review, we will introduce the formin proteins and consider the recent evidence that they play an important role in platelets and megakaryocytes in mediating both the actin and tubulin cytoskeletons

Mucor circinelloides induces platelet aggregation through integrin αIIbβ3 and FcγRIIA

<p>Thrombosis is a hallmark of the fatal fungal infection mucormycosis. Yet, the platelet activation pathway in response to mucormycetes is unknown. In this study we determined the platelet aggregation potential of <i>Mucor circinelloides</i> (<i>M. circinelloides</i>) NRRL3631, characterized the signaling pathway facilitating aggregation in response to fungal spores, and identified the influence of the spore developmental stage upon platelet aggregation potential. Using impedance and light-transmission aggregometry, we showed that <i>M. circinelloides</i> induced platelet aggregation in whole blood and in platelet-rich plasma, respectively. The formation of large spore-platelet aggregates was confirmed by light-sheet microscopy, which showed spores dispersed throughout the aggregate. Aggregation potential was dependent on the spore’s developmental stage, with the strongest platelet aggregation by spores in mid-germination. Inhibitor studies revealed platelet aggregation was mediated by the low affinity IgG receptor FcγRIIA and integrin αIIbβ3; Src and Syk tyrosine kinase signaling; and the secondary mediators TxA₂ and ADP. Flow cytometry of antibody stained platelets showed that interaction with spores increased expression of platelet surface integrin αIIbβ3 and the platelet activation marker CD62P. Together, this is the first elucidation of the signaling pathways underlying thrombosis formation during a fungal infection, highlighting targets for therapeutic intervention.</p

Loss of mDia1 and Fhod1 impacts platelet formation but not platelet function

An organized and dynamic cytoskeleton is required for platelet formation and function. Formins are a large family of actin regulatory proteins which are also able to regulate microtubule dynamics. There are four formin family members expressed in human and mouse megakaryocytes and platelets. We have previously shown that the actin polymerization activity of formin proteins is required for cytoskeletal dynamics and platelet spreading using a small molecule inhibitor. In the current study, we analyze transgenic mouse models deficient in two of these proteins, mDia1 and Fhod1, along with a model lacking both proteins. We demonstrate that double knockout mice display macrothrombocytopenia which is due to aberrant megakaryocyte function and a small decrease in platelet lifespan. Platelet function is unaffected by the loss of these proteins. This data indicates a critical role for formins in platelet and megakaryocyte function

Salmonella-induced thrombi in mice develop asynchronously in the spleen and liver and are not effective bacterial traps

Thrombosis is a frequent, life-threatening complication of systemic infection, associated with multiple organ damage. We have previously described a novel mechanism of inflammation-driven thrombosis induced by Salmonella Typhimurium infection of mice. Thrombosis in the liver develops 7 days post-infection persisting after the infection resolves, and is monocytic cell-dependent. Unexpectedly, thrombosis was not prominent in the spleen at this time, despite carrying a similar bacterial burden as the liver. In this study, we show that thrombosis does occur in the spleen but with strikingly accelerated kinetics compared to the liver, being evident by 24 h and resolving rapidly thereafter. The distinct kinetics of thrombosis and bacterial burden provide a test of the hypothesis that thrombi form in healthy vessels to trap or remove bacteria from the circulation, often termed immunothrombosis. Remarkably, despite bacteria being detected throughout infected spleens and livers in the early days of infection, immunohistological analysis of tissue sections show that thrombi contain very low numbers of bacteria. In contrast, bacteria are present throughout platelet aggregates induced by Salmonella in vitro. Therefore, we show that thrombosis develops with organ-specific kinetics and challenge the universality of immunothrombosis as a mechanism to capture bacteria in vivo

DC-SCRIPT is a novel regulator of the tumor suppressor gene CDKN2B and induces cell cycle arrest in ERα-positive breast cancer cells

Breast cancer is one of the most common causes of cancer-related deaths in women. The estrogen receptor (ERα) is well known for having growth promoting effects in breast cancer. Recently, we have identified DC-SCRIPT (ZNF366) as a co-suppressor of ERα and as a strong and independent prognostic marker in ESR1 (ERα gene)-positive breast cancer patients. In this study, we further investigated the molecular mechanism on how DC-SCRIPT inhibits breast cancer cell growth. DC-SCRIPT mRNA levels from 190 primary ESR1-positive breast tumors were related to global gene expression, followed by gene ontology and pathway analysis. The effect of DC-SCRIPT on breast cancer cell growth and cell cycle arrest was investigated using novel DC-SCRIPT-inducible MCF7 breast cancer cell lines. Genome-wide expression profiling of DC-SCRIPT-expressing MCF7 cells was performed to investigate the effect of DC-SCRIPT on cell cycle-related gene expression. Findings were validated by real-time PCR in a cohort of 1,132 ESR1-positive breast cancer patients. In the primary ESR1-positive breast tumors, DC-SCRIPT expression negatively correlated with several cell cycle gene ontologies and pathways. DC-SCRIPT expression strongly reduced breast cancer cell growth in vitro, breast tumor growth in vivo, and induced cell cycle arrest. In addition, in the presence of DC-SCRIPT, multiple cell cycles related genes were differentially expressed including the tumor suppressor gene CDKN2B. Moreover, in 1,132 primary ESR1-positive breast tumors, DC-SCRIPT expression also correlated with CDKN2B expression. Collectively, these data show that DC-SCRIPT acts as a novel regulator of CDKN2B and induces cell cycle arrest in ESR1-positive breast cancer cells

Protein expression of tetraspanins on murine DC subsets.

<p>(A) Flow cytometric analysis of total tetraspanin protein expression on DC subsets. Cells from spleen were enriched for DCs as described in the materials and methods. Far left: dot plots of viable CD11c+ cells (CD4+ and CD8α+ DCs) or viable pDCs. CD4+ DCs were identified as CD4+CD11b+, CD8α+ DCs as CD8α+CD11b-, and pDCs as B220+CD11c^int cells. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0184317#pone.0184317.s006" target="_blank">S3 Fig</a> for full gating strategy. Histograms of tetraspanin protein expression on gated cells depicted in the dot plots (black line) and isotype controls (grey line). (B) Average tetraspanin expression of 3 individual mice, geometric mean fluorescence intensity (gMFI) normalized for isotype control binding, each symbol represents one mouse. P-values displayed above each plot represent the result of ANOVA to which multiple testing correction was been applied (n.s. = non-significant). * p<0.05, ** p< 0.01 ***p<0.001 by post-hoc t-testing. See also <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0184317#pone.0184317.s002" target="_blank">S2 Table</a> for full results of the statistical analysis.</p

Tetraspanin protein expression on DC subsets.

<p>Tetraspanin protein expression on DC subsets.</p