Megakaryocyte-specific Profilin1-deficiency alters microtubule stability and causes a Wiskott–Aldrich syndrome-like platelet defect

Prolonged depletion of profilin 1 or F-actin causes an adaptive response in microtubules

In addition to its well-established role in actin assembly, profilin 1 (PFN1) has been shown to bind to tubulin and alter microtubule growth. However, whether PFN1's predominant control over microtubules in cells occurs through direct regulation of tubulin or indirectly through the polymerization of actin has yet to be determined. Here, we manipulated PFN1 expression, actin filament assembly, and actomyosin contractility and showed that reducing any of these parameters for extended periods of time caused an adaptive response in the microtubule cytoskeleton, with the effect being significantly more pronounced in neuronal processes. All the observed changes to microtubules were reversible if actomyosin was restored, arguing that PFN1's regulation of microtubules occurs principally through actin. Moreover, the cytoskeletal modifications resulting from PFN1 depletion in neuronal processes affected microtubule-based transport and mimicked phenotypes that are linked to neurodegenerative disease. This demonstrates how defects in actin can cause compensatory responses in other cytoskeleton components, which in turn significantly alter cellular function.

Profilin affects microtubule dynamics via actin

Profilin binds microtubules in vitro. However, a new study by Vitriol and colleagues (https://doi.org/10.1083/jcb.202309097) now suggests that effects of profilin on microtubule dynamics in cells are indirect and result from its impact on actin dynamics rather than its direct binding to microtubules.

GPIbα-filamin A interaction regulates megakaryocyte localization and budding during platelet biogenesis

ABSTRACT

Glycoprotein Ibα (GPIbα) is expressed on the surface of platelets and megakaryocytes (MKs) and anchored to the membrane skeleton by filamin A (flnA). Although GPIb and flnA have fundamental roles in platelet biogenesis, the nature of this interaction in megakaryocyte biology remains ill-defined. We generated a mouse model expressing either human wild-type (WT) GPIbα (hGPIbαWT) or a flnA-binding mutant (hGPIbαFW) and lacking endogenous mouse GPIbα. Mice expressing the mutant GPIbα transgene exhibited macrothrombocytopenia with preserved GPIb surface expression. Platelet clearance was normal and differentiation of MKs to proplatelets was unimpaired in hGPIbαFW mice. The most striking abnormalities in hGPIbαFW MKs were the defective formation of the demarcation membrane system (DMS) and the redistribution of flnA from the cytoplasm to the peripheral margin of MKs. These abnormalities led to disorganized internal MK membranes and the generation of enlarged megakaryocyte membrane buds. The defective flnA-GPIbα interaction also resulted in misdirected release of buds away from the vasculature into bone marrow interstitium. Restoring the linkage between flnA and GPIbα corrected the flnA redistribution within MKs and DMS ultrastructural defects as well as restored normal bud size and release into sinusoids. These studies define a new mechanism of macrothrombocytopenia resulting from dysregulated MK budding. The link between flnA and GPIbα is not essential for the MK budding process, however, it plays a major role in regulating the structure of the DMS, bud morphogenesis, and the localized release of buds into the circulation.

Phosphoinositides take a central stage in regulating blood platelet production and function

Blood platelets are produced by megakaryocytes through a complex program of differentiation and play a critical role in hemostasis and thrombosis. These anucleate cells are the target of antithrombotic drugs that prevent them from clumping in cardiovascular disease conditions. Platelets also significantly contribute to various aspects of physiopathology, including interorgan communications, healing, inflammation, and thromboinflammation. Their production and activation are strictly regulated by highly elaborated mechanisms. Among them, those involving inositol lipids have drawn the attention of researchers. Phosphoinositides represent the seven combinatorially phosphorylated forms of the inositol head group of inositol lipids. They play a crucial role in regulating intracellular mechanisms, such as signal transduction, actin cytoskeleton rearrangements, and membrane trafficking, either by generating second messengers or by directly binding to specific domains of effector proteins. In this review, we will explore how phosphoinositides are implicated in controlling platelet production by megakaryocytes and in platelet activation processes. We will also discuss the diversity of phosphoinositides in platelets, their role in granule biogenesis and maintenance, as well as in integrin signaling. Finally, we will address the discovery of a novel pool of phosphatidylinositol 3-monophosphate in the outerleaflet of the plasma membrane of human and mouse platelets.

Cytoskeletal adaptation following long-term dysregulation of actomyosin in neuronal processes

Microtubules, intermediate filaments, and actin are cytoskeletal polymer networks found within the cell. While each has unique functions, all the cytoskeletal elements must work together for cellular mechanics to be fully operative. This is achieved through crosstalk mechanisms whereby the different networks influence each other through signaling pathways and direct interactions. Because crosstalk can be complex, it is possible for perturbations in one cytoskeletal element to affect the others in ways that are difficult to predict. Here we investigated how long-term changes to the actin cytoskeleton affect microtubules and intermediate filaments. Reducing F-actin or actomyosin contractility increased acetylated microtubules and intermediate filament expression, with the effect being significantly more pronounced in neuronal processes. Changes to microtubules were completely reversible if F-actin and myosin activity is restored. Moreover, the altered microtubules in neuronal processes resulting from F-actin depletion caused significant changes to microtubule-based transport, mimicking phenotypes that are linked to neurodegenerative disease. Thus, defects in actin dynamics cause a compensatory response in other cytoskeleton components which profoundly alters cellular function.

The WASH-complex subunit Strumpellin regulates integrin αIIbβ3 trafficking in murine platelets

The platelet specific integrin αIIbβ3 mediates platelet adhesion, aggregation and plays a central role in thrombosis and hemostasis. In resting platelets, αIIbβ3 is expressed on the membrane surface and in intracellular compartments. Upon activation, the number of surface-expressed αIIbβ3 is increased by the translocation of internal granule pools to the plasma membrane. The WASH complex is the major endosomal actin polymerization-promoting complex and has been implicated in the generation of actin networks involved in endocytic trafficking of integrins in other cell types. The role of the WASH complex and its subunit Strumpellin in platelet function is still unknown. Here, we report that Strumpellin-deficient murine platelets display an approximately 20% reduction in integrin αIIbβ3 surface expression. While exposure of the internal αIIbβ3 pool after platelet activation was unaffected, the uptake of the αIIbβ3 ligand fibrinogen was delayed. The number of platelet α-granules was slightly but significantly increased in Strumpellin-deficient platelets. Quantitative proteome analysis of isolated αIIbβ3-positive vesicular structures revealed an enrichment of protein markers, which are associated with the endoplasmic reticulum, Golgi complex and early endosomes in Strumpellin-deficient platelets. These results point to a so far unidentified role of the WASH complex subunit Strumpellin in integrin αIIbβ3 trafficking in murine platelets.

Novel insights into mouse models of ectopic proplatelet release

Mature bone marrow (BM) megakaryocytes (MKs) produce platelets by extending proplatelets into sinusoidal blood vessels. Defects in this process can lead to thrombocytopenia and increased risk of bleeding. Mice lacking the actin-regulatory proteins Profilin 1 (PFN1), Wiskott-Aldrich Syndrome protein (WASp), Actin Related Protein 2/3 complex (Arp2/3), or adhesion and degranulation-promoting adapter protein (ADAP) display thrombocytopenia and ectopic release of (pro)platelet-like particles into the BM compartment, pointing to an important axis of actin-mediated directional proplatelet formation. The mechanism underlying ectopic release in these mice is still not completely understood. However, we hypothesized that similar functional defects account for this observation. We analyzed WASp-, ADAP-, PFN1-, and ARPC2-knockout mice to determine the role of actin reorganization and integrin activation in directional proplatelet formation. ADAP-, ARPC2-, and PFN1-deficient MKs displayed reduced adhesion to collagen, defective F-actin organization, and diminished β1-integrin activation. WASp-deficient MKs showed the strongest reduction in the adhesion assay of collagen and altered F-actin organization with reduced podosome formation. Our results indicate that ADAP, PFN1, WASp, and ARP2/3 are part of the same pathway that regulates polarization processes in MKs and directional proplatelet formation into BM sinusoids.

Targeting of a Conserved Epitope in Mouse and Human GPVI Differently Affects Receptor Function

Glycoprotein (GP) VI is the major platelet collagen receptor and a promising anti-thrombotic target. This was first demonstrated in mice using the rat monoclonal antibody JAQ1, which completely blocks the Collagen-Related Peptide (CRP)-binding site on mouse GPVI and efficiently inhibits mouse platelet adhesion, activation and aggregation on collagen. Here, we show for the first time that JAQ1 cross-reacts with human GPVI (huGPVI), but not with GPVI in other tested species, including rat, rabbit, guinea pig, swine, and dog. We further demonstrate that JAQ1 differently modulates mouse and human GPVI function. Similar to its effects on mouse GPVI (mGPVI), JAQ1 inhibits CRP-induced activation in human platelets, whereas, in stark contrast to mouse GPVI, it does not inhibit the adhesion, activation or aggregate formation of human platelets on collagen, but causes instead an increased response. This effect was also seen with platelets from newly generated human GPVI knockin mice (hGP6^tg/tg). These results indicate that the binding of JAQ1 to a structurally conserved epitope in GPVI differently affects its function in human and mouse platelets.

Impaired microtubule dynamics contribute to microthrombocytopenia in RhoB-deficient mice

RhoB-deficient mice display microthrombocytopenia.

RhoB exerts nonredundant functions in the megakaryocyte lineage compared with RhoA and regulates microtubule dynamics.

Megakaryocytes are large cells in the bone marrow that give rise to blood platelets. Platelet biogenesis involves megakaryocyte maturation, the localization of the mature cells in close proximity to bone marrow sinusoids, and the formation of protrusions, which are elongated and shed within the circulation. Rho GTPases play important roles in platelet biogenesis and function. RhoA-deficient mice display macrothrombocytopenia and a striking mislocalization of megakaryocytes into bone marrow sinusoids and a specific defect in G-protein signaling in platelets. However, the role of the closely related protein RhoB in megakaryocytes or platelets remains unknown. In this study, we show that, in contrast to RhoA deficiency, genetic ablation of RhoB in mice results in microthrombocytopenia (decreased platelet count and size). RhoB-deficient platelets displayed mild functional defects predominantly upon induction of the collagen/glycoprotein VI pathway. Megakaryocyte maturation and localization within the bone marrow, as well as actin dynamics, were not affected in the absence of RhoB. However, in vitro–generated proplatelets revealed pronouncedly impaired microtubule organization. Furthermore, RhoB-deficient platelets and megakaryocytes displayed selective defects in microtubule dynamics/stability, correlating with reduced levels of acetylated α-tubulin. Our findings imply that the reduction of this tubulin posttranslational modification results in impaired microtubule dynamics, which might contribute to microthrombocytopenia in RhoB-deficient mice. Importantly, we demonstrate that RhoA and RhoB are localized differently and have selective, nonredundant functions in the megakaryocyte lineage.

Profilin-1 negatively controls osteoclast migration by suppressing the protrusive structures based on branched actin filaments

BACKGROUND

Profilin-1 (Pfn1), an evolutionarily conserved actin-binding protein, is an important regulator of the cytoskeleton. We previously reported the osteoclast-specific Pfn1-conditional knockout (cKO) mice had postnatal osteolytic phenotype with craniofacial and long-bone deformities associated with increased migration of cultured osteoclasts. We hypothesized the increased cellular processes structured with branched actin filaments may underlies the mechanism of increased bone resorption in these mutant mice.

MATERIALS AND METHODS

The morphological structure and cell migration of the cultured osteoclasts were analyzed using fluorescent microscopy and time-lapse image capturing. Fractional migration distances, as well as the index of protrusive structures (%-PB) that evaluates relative border length of the protrusion were compared between the cells from control and Pfn1-cKO mice.

RESULTS

Time-lapse image analysis showed that %-PB was significantly larger in Pfn1-cKO osteoclasts. In addition, the fractional migration distance was positively correlated with the index. When the branched actin filament organization was suppressed by chemical inhibitors, the osteoclast migration was declined. Importantly, the suppression was more extensive in Pfn1-cKO than in control osteoclasts.

CONCLUSION

Our results indicated the causative involvement of the increased branched actin filament formation at least in part for their excessive migration. Our findings provide a mechanistic rationale for testing novel therapeutic approaches targeting branched actin filaments in osteolytic disorders.

Don't you forget about me(gakaryocytes)

Platelets (small, anucleate cell fragments) derive from large precursor cells, megakaryocytes (MKs), that reside in the bone marrow. MKs emerge from hematopoietic stem cells in a complex differentiation process that involves cytoplasmic maturation, including the formation of the demarcation membrane system, and polyploidization. The main function of MKs is the generation of platelets, which predominantly occurs through the release of long, microtubule-rich proplatelets into vessel sinusoids. However, the idea of a 1-dimensional role of MKs as platelet precursors is currently being questioned because of advances in high-resolution microscopy and single-cell omics. On the one hand, recent findings suggest that proplatelet formation from bone marrow-derived MKs is not the only mechanism of platelet production, but that it may also occur through budding of the plasma membrane and in distant organs such as lung or liver. On the other hand, novel evidence suggests that MKs not only maintain physiological platelet levels but further contribute to bone marrow homeostasis through the release of extracellular vesicles or cytokines, such as transforming growth factor β1 or platelet factor 4. The notion of multitasking MKs was reinforced in recent studies by using single-cell RNA sequencing approaches on MKs derived from adult and fetal bone marrow and lungs, leading to the identification of different MK subsets that appeared to exhibit immunomodulatory or secretory roles. In the following article, novel insights into the mechanisms leading to proplatelet formation in vitro and in vivo will be reviewed and the hypothesis of MKs as immunoregulatory cells will be critically discussed.

Rasa3 deficiency minimally affects thrombopoiesis but promotes severe thrombocytopenia due to integrin-dependent platelet clearance

Platelet homeostasis is dependent on a tight regulation of both platelet production and clearance. The small GTPase Rap1 mediates platelet adhesion and hemostatic plug formation. However, Rap1 signaling is also critical for platelet homeostasis as both Rap1 deficiency and uninhibited Rap1 signaling lead to marked thrombocytopenia in mice. Here, we investigated the mechanism by which deficiency in Rasa3, a critical negative regulator of Rap1, causes macrothrombocytopenia in mice. Despite marked morphological and ultrastructural abnormalities, megakaryocytes in hypomorphic Rasa3hlb/hlb (R3hlb/hlb) or Rasa3-/- mice demonstrated robust proplatelet formation in vivo, suggesting that defective thrombopoiesis is not the main cause of thrombocytopenia. Rather, we observed that R3hlb/hlb platelets became trapped in the spleen marginal zone/red pulp interface, with evidence of platelet phagocytosis by macrophages. Clearance of mutant platelets was also observed in the liver, especially in splenectomized mice. Platelet count and platelet life span in Rasa3-mutant mice were restored by genetic or pharmacological approaches to inhibit the Rap1/talin1/αIIbβ3 integrin axis. A similar pattern of splenic clearance was observed in mice injected with anti-αIIbβ3 but not anti-glycoprotein Ibα platelet-depleting antibodies. In summary, we describe a potentially novel, integrin-based mechanism of platelet clearance that could be critical for our understanding of select inherited and acquired thrombocytopenias.

CRLF3 plays a key role in the final stage of platelet genesis and is a potential therapeutic target for thrombocythemia

The process of platelet production has so far been understood to be a 2-stage process: megakaryocyte maturation from hematopoietic stem cells followed by proplatelet formation, with each phase regulating the peripheral blood platelet count. Proplatelet formation releases into the bloodstream beads-on-a-string preplatelets, which undergo fission into mature platelets. For the first time, we show that preplatelet maturation is a third, tightly regulated, critical process akin to cytokinesis that regulates platelet count. We show that deficiency in cytokine receptor-like factor 3 (CRLF3) in mice leads to an isolated and sustained 25% to 48% reduction in the platelet count without any effect on other blood cell lineages. We show that Crlf3-/- preplatelets have increased microtubule stability, possibly because of increased microtubule glutamylation via the interaction of CRLF3 with key members of the Hippo pathway. Using a mouse model of JAK2 V617F essential thrombocythemia, we show that a lack of CRLF3 leads to long-term lineage-specific normalization of the platelet count. We thereby postulate that targeting CRLF3 has therapeutic potential for treatment of thrombocythemia.

Rac Inhibition Causes Impaired GPVI Signalling in Human Platelets through GPVI Shedding and Reduction in PLCγ2 Phosphorylation

Rac1 is a small Rho GTPase that is activated in platelets upon stimulation with various ligands, including collagen and thrombin, which are ligands for the glycoprotein VI (GPVI) receptor and the protease-activated receptors, respectively. Rac1-deficient murine platelets have impaired lamellipodia formation, aggregation, and reduced PLCγ2 activation, but not phosphorylation. The objective of our study is to investigate the role of Rac1 in GPVI-dependent human platelet activation and downstream signalling. Therefore, we used human platelets stimulated using GPVI agonists (collagen and collagen-related peptide) in the presence of the Rac1-specific inhibitor EHT1864 and analysed platelet activation, aggregation, spreading, protein phosphorylation, and GPVI clustering and shedding. We observed that in human platelets, the inhibition of Rac1 by EHT1864 had no significant effect on GPVI clustering on collagen fibres but decreased the ability of platelets to spread or aggregate in response to GPVI agonists. Additionally, in contrast to what was observed in murine Rac1-deficient platelets, EHT1864 enhanced GPVI shedding in platelets and reduced the phosphorylation levels of PLCγ2 following GPVI activation. In conclusion, Rac1 activity is required for both human and murine platelet activation in response to GPVI-ligands, but Rac1's mode of action differs between the two species.

The tubulin code in platelet biogenesis

Blood platelets are small non-nucleated cellular fragments that prevent and stop hemorrhages. They are produced in the bone marrow by megakaryocytes through megakaryopoiesis. This intricate process involves profound microtubule rearrangements culminating in the formation of a unique circular sub-membranous microtubule array, the marginal band, which supports the typical disc-shaped morphology of platelets. Mechanistically, these processes are thought to be controlled by a specific tubulin code. In this review, we summarize the current knowledge on the key isotypes, notably β1-, α4A- and α8-tubulin, and putative post-translational modifications, involved in platelet and marginal band formation. Additionally, we provide a provisional list of microtubule-associated proteins (MAPs) involved in these processes and a survey of tubulin variants identified in patients presenting defective platelet production. A comprehensive characterization of the platelet tubulin code and the identification of essential MAPs may be expected in the near future to shed new light on a very specialized microtubule assembly process with applications in platelet diseases and transfusion.

In vitro megakaryocyte culture from human bone marrow aspirates as a research and diagnostic tool

Megakaryocytes (MKs) are relatively rare in bone marrow, comprising <0.05% of the nucleated cells, which makes direct isolation from human bone marrow impractical. As such, in vitro expansion of primary MKs from patient samples offers exciting fundamental and clinical opportunities. As most of the developed ex vivo methods require a substantial volume of biomaterial, they are not widely performed on young patients. Here we propose a simple, robust, and adapted method of primary human MK culture from 1 mL of bone marrow aspirate. Our technique uses a small volume of bone marrow per culture, uses straightforward isolation methods, and generates approximately 6 × 10⁵ mature MKs per culture. The relative high cell purity and yield achieved by this technique, combined with efficient use of low volumes of bone marrow, make this approach suitable for diagnostic and basic research of human megakaryopoiesis.

Thymosin β4 is essential for thrombus formation by controlling the G-actin/F-actin equilibrium in platelets

Coordinated rearrangements of the actin cytoskeleton are pivotal for platelet biogenesis from megakaryocytes but also orchestrate key functions of peripheral platelets in hemostasis and thrombosis, such as granule release, the formation of filopodia and lamellipodia, or clot retraction. Along with profilin (Pfn) 1, thymosin β4 (encoded by Tmsb4x) is one of the two main G-actin-sequestering proteins within cells of higher eukaryotes, and its intracellular concentration is particularly high in cells that rapidly respond to external signals by increased motility, such as platelets. Here, we analyzed constitutive Tmsb4x knockout (KO) mice to investigate the functional role of the protein in platelet production and function. Thymosin β4 deficiency resulted in a macrothrombocytopenia with only mildly increased platelet volume and an unaltered platelet life span. Megakaryocyte numbers in the bone marrow and spleen were unaltered, however, Tmsb4x KO megakaryocytes showed defective proplatelet formation in vitro and in vivo. Thymosin β4-deficient platelets displayed markedly decreased G-actin levels and concomitantly increased F-actin levels resulting in accelerated spreading on fibrinogen and clot retraction. Moreover, Tmsb4x KO platelets showed activation defects and an impaired immunoreceptor tyrosine-based activation motif (ITAM) signaling downstream of the activating collagen receptor glycoprotein VI. These defects translated into impaired aggregate formation under flow, protection from occlusive arterial thrombus formation in vivo and increased tail bleeding times. In summary, these findings point to a critical role of thymosin β4 for actin dynamics during platelet biogenesis, platelet activation downstream of glycoprotein VI and thrombus stability.

Actin/microtubule crosstalk during platelet biogenesis in mice is critically regulated by Twinfilin1 and Cofilin1

Rearrangements of the microtubule (MT) and actin cytoskeleton are pivotal for platelet biogenesis. Hence, defects in actin- or MT-regulatory proteins are associated with platelet disorders in humans and mice. Previous studies in mice revealed that loss of the actin-depolymerizing factor homology (ADF-H) protein Cofilin1 (Cof1) in megakaryocytes (MKs) results in a moderate macrothrombocytopenia but normal MK numbers, whereas deficiency in another ADF-H protein, Twinfilin1 (Twf1), does not affect platelet production or function. However, recent studies in yeast have indicated a critical synergism between Twf1 and Cof1 in the regulation of actin dynamics. We therefore investigated platelet biogenesis and function in mice lacking both Twf1 and Cof1 in the MK lineage. In contrast to single deficiency in either protein, Twf1/Cof1 double deficiency (DKO) resulted in a severe macrothrombocytopenia and dramatically increased MK numbers in bone marrow and spleen. DKO MKs exhibited defective proplatelet formation in vitro and in vivo as well as impaired spreading and altered assembly of podosome-like structures on collagen and fibrinogen in vitro. These defects were associated with aberrant F-actin accumulation and, remarkably, the formation of hyperstable MT, which appears to be caused by dysregulation of the actin- and MT-binding proteins mDia1 and adenomatous polyposis coli. Surprisingly, the mild functional defects described for Cof1-deficient platelets were only slightly aggravated in DKO platelets suggesting that both proteins are largely dispensable for platelet function in the peripheral blood. In summary, these findings reveal critical redundant functions of Cof1 and Twf1 in ensuring balanced actin/microtubule crosstalk during thrombopoiesis in mice and possibly humans.

Evidence that GPVI is Expressed as a Mixture of Monomers and Dimers, and that the D2 Domain is not Essential for GPVI Activation

Collagen has been proposed to bind to a unique epitope in dimeric glycoprotein VI (GPVI) and the number of GPVI dimers has been reported to increase upon platelet activation. However, in contrast, the crystal structure of GPVI in complex with collagen-related peptide (CRP) showed binding distinct from the site of dimerization. Further fibrinogen has been reported to bind to monomeric but not dimeric GPVI. In the present study, we have used the advanced fluorescence microscopy techniques of single-molecule microscopy, fluorescence correlation spectroscopy (FCS) and bioluminescence resonance energy transfer (BRET), and mutagenesis studies in a transfected cell line model to show that GPVI is expressed as a mixture of monomers and dimers and that dimerization through the D2 domain is not critical for activation. As many of these techniques cannot be applied to platelets to resolve this issue, due to the high density of GPVI and its anucleate nature, we used Förster resonance energy transfer (FRET) to show that endogenous GPVI is at least partially expressed as a dimer on resting and activated platelet membranes. We propose that GPVI may be expressed as a monomer on the cell surface and it forms dimers in the membrane through diffusion, giving rise to a mixture of monomers and dimers. We speculate that the formation of dimers facilitates ligand binding through avidity.

Role of Rho-GTPases in megakaryopoiesis

Megakaryocytes (MKs) are the bone marrow (BM) cells that generate blood platelets by a process that requires: i) polyploidization responsible for the increased MK size and ii) cytoplasmic organization leading to extension of long pseudopods, called proplatelets, through the endothelial barrier to allow platelet release into blood. Low level of localized RHOA activation prevents actomyosin accumulation at the cleavage furrow and participates in MK polyploidization. In the platelet production, RHOA and CDC42 play opposite, but complementary roles. RHOA inhibits both proplatelet formation and MK exit from BM, whereas CDC42 drives the development of the demarcation membranes and MK migration in BM. Moreover, the RhoA or Cdc42 MK specific knock-out in mice and the genetic alterations in their down-stream effectors in human induce a thrombocytopenia demonstrating their key roles in platelet production. A better knowledge of Rho-GTPase signalling is thus necessary to develop therapies for diseases associated with platelet production defects.

Abbreviations: AKT: Protein Kinase BARHGEF2: Rho/Rac Guanine Nucleotide Exchange Factor 2ARP2/3: Actin related protein 2/3BM: Bone marrowCDC42: Cell division control protein 42 homologCFU-MK: Colony-forming-unit megakaryocyteCIP4: Cdc42-interacting protein 4mDIA: DiaphanousDIAPH1; Protein diaphanous homolog 1ECT2: Epithelial Cell Transforming Sequence 2FLNA: Filamin AGAP: GTPase-activating proteins or GTPase-accelerating proteinsGDI: GDP Dissociation InhibitorGEF: Guanine nucleotide exchange factorHDAC: Histone deacetylaseLIMK: LIM KinaseMAL: Megakaryoblastic leukaemiaMARCKS: Myristoylated alanine-rich C-kinase substrateMKL: Megakaryoblastic leukaemiaMLC: Myosin light chainMRTF: Myocardin Related Transcription FactorOTT: One-Twenty Two ProteinPACSIN2: Protein Kinase C And Casein Kinase Substrate In Neurons 2PAK: P21-Activated KinasePDK: Pyruvate Dehydrogenase kinasePI3K: Phosphoinositide 3-kinasePKC: Protein kinase CPTPRJ: Protein tyrosine phosphatase receptor type JRAC: Ras-related C3 botulinum toxin substrate 1RBM15: RNA Binding Motif Protein 15RHO: Ras homologousROCK: Rho-associated protein kinaseSCAR: Suppressor of cAMP receptorSRF: Serum response factorSRC: SarcTAZ: Transcriptional coactivator with PDZ motifTUBB1: Tubulin β1VEGF: Vascular endothelial growth factorWAS: Wiskott Aldrich syndromeWASP: Wiskott Aldrich syndrome proteinWAVE: WASP-family verprolin-homologous proteinWIP: WASP-interacting proteinYAP: Yes-associated protein

Profilin 1 delivery tunes cytoskeletal dynamics toward CNS axon regeneration

After trauma, regeneration of adult CNS axons is abortive, causing devastating neurologic deficits. Despite progress in rehabilitative care, there is no effective treatment that stimulates axonal growth following injury. Using models with different regenerative capacities, followed by gain- and loss-of-function analysis, we identified profilin 1 (Pfn1) as a coordinator of actin and microtubules (MTs), powering axonal growth and regeneration. In growth cones, Pfn1 increased actin retrograde flow, MT growth speed, and invasion of filopodia by MTs, orchestrating cytoskeletal dynamics toward axonal growth. In vitro, active Pfn1 promoted MT growth in a formin-dependent manner, whereas localization of MTs to growth cone filopodia was facilitated by direct MT binding and interaction with formins. In vivo, Pfn1 ablation limited regeneration of growth-competent axons after sciatic nerve and spinal cord injury. Adeno-associated viral (AAV) delivery of constitutively active Pfn1 to rodents promoted axonal regeneration, neuromuscular junction maturation, and functional recovery of injured sciatic nerves, and increased the ability of regenerating axons to penetrate the inhibitory spinal cord glial scar. Thus, we identify Pfn1 as an important regulator of axonal regeneration and suggest that AAV-mediated delivery of constitutively active Pfn1, together with the identification of modulators of Pfn1 activity, should be considered to treat the injured nervous system.

Macrophage galactose lectin is critical for Kupffer cells to clear aged platelets

Every day, megakaryocytes produce billions of platelets that circulate for several days and eventually are cleared by the liver. The exact removal mechanism, however, remains unclear. Loss of sialic acid residues is thought to feature in the aging and clearance of platelets. Using state-of-the-art spinning disk intravital microscopy to delineate the different compartments and cells of the mouse liver, we observed rapid accumulation of desialylated platelets predominantly on Kupffer cells, with only a few on endothelial cells and none on hepatocytes. Kupffer cell depletion prevented the removal of aged platelets from circulation. Ashwell-Morell receptor (AMR) deficiency alone had little effect on platelet uptake. Macrophage galactose lectin (MGL) together with AMR mediated clearance of desialylated or cold-stored platelets by Kupffer cells. Effective clearance is critical, as mice with an aged platelet population displayed a bleeding phenotype. Our data provide evidence that the MGL of Kupffer cells plays a significant role in the removal of desialylated platelets through a collaboration with the AMR, thereby maintaining a healthy and functional platelet compartment.

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.

Profilin choreographs actin and microtubules in cells and cancer

Actin and microtubules play essential roles in aberrant cell processes that define and converge in cancer including: signaling, morphology, motility, and division. Actin and microtubules do not directly interact, however shared regulators coordinate these polymers. While many of the individual proteins important for regulating and choreographing actin and microtubule behaviors have been identified, the way these molecules collaborate or fail in normal or disease contexts is not fully understood. Decades of research focus on Profilin as a signaling molecule, lipid-binding protein, and canonical regulator of actin assembly. Recent reports demonstrate that Profilin also regulates microtubule dynamics and polymerization. Thus, Profilin can coordinate both actin and microtubule polymer systems. Here we reconsider the biochemical and cellular roles for Profilin with a focus on the essential cytoskeletal-based cell processes that go awry in cancer. We also explore how the use of model organisms has helped to elucidate mechanisms that underlie the regulatory essence of Profilin in vivo and in the context of disease.

Intravital imaging of megakaryocytes

The dynamics of platelet formation could only be investigated since the development of two-photon microscopy in combination with suitable fluorescent labeling strategies. In this review paper, we give an overview of recent advances in fluorescence imaging of the bone marrow that have contributed to our understanding of platelet biogenesis during the last decade. We make a brief survey through the perspectives and limitations of today's intravital imaging, but also discuss complementary methods that may help to piece together the puzzle of megakaryopoiesis and platelet formation.

Diagnosis of Inherited Platelet Disorders on a Blood Smear

Inherited platelet disorders (IPDs) are rare diseases featured by low platelet count and defective platelet function. Patients have variable bleeding diathesis and sometimes additional features that can be congenital or acquired. Identification of an IPD is desirable to avoid misdiagnosis of immune thrombocytopenia and the use of improper treatments. Diagnostic tools include platelet function studies and genetic testing. The latter can be challenging as the correlation of its outcomes with phenotype is not easy. The immune-morphological evaluation of blood smears (by light- and immunofluorescence microscopy) represents a reliable method to phenotype subjects with suspected IPD. It is relatively cheap, not excessively time-consuming and applicable to shipped samples. In some forms, it can provide a diagnosis by itself, as for MYH9-RD, or in addition to other first-line tests as aggregometry or flow cytometry. In regard to genetic testing, it can guide specific sequencing. Since only minimal amounts of blood are needed for the preparation of blood smears, it can be used to characterize thrombocytopenia in pediatric patients and even newborns further. In principle, it is based on visualizing alterations in the distribution of proteins, which result from specific genetic mutations by using monoclonal antibodies. It can be applied to identify deficiencies in membrane proteins, disturbed distribution of cytoskeletal proteins, and alpha as well as delta granules. On the other hand, mutations associated with impaired signal transduction are difficult to identify by immunofluorescence of blood smears. This review summarizes technical aspects and the main diagnostic patterns achievable by this method.

Use of electron microscopy to study megakaryocytes

Electron microscopy (EM) has a long history in megakaryocyte (MK) cellular biology. This chapter shows how the electron microscope, since its first appearance almost 90 years ago, has occupied center stage in the studies of MK morphology and function. It describes some of the more productive EM techniques that have shaped our understanding of the physiology of thrombopoiesis. These include the standard transmission and scanning EM techniques as well as the new imaging methods, correlative microscopy and volume EM which provide information on the 3D organization of MKs on different scales: single organelles, whole cells and tissues. For each technique, we list the advantages and limitations, the resolution that can be achieved, the technical difficulties and the applications in MK biology.

Profilin as a dual regulator of actin and microtubule dynamics

Although originally identified as G-actin sequestering proteins, profilins are emerging as critical regulators of actin dynamics, capable of interacting with multiple acting binding proteins, and being able to link membrane lipids to cytoskeleton components. Recently, in addition to its actin, poly-proline, and phosphatidylinositol binding domains, profilin has been shown to contain residues specialized in microtubule binding. Here we will discuss in a critical perspective the emerging body of data supporting that profilins are central mediators of actin microfilament and microtubule interaction. We will also address the unanswered questions in the field, including the nature of the interaction of profilin with microtubules, and its effect on microtubule dynamics. These recent discoveries deepen our understanding on how different cytoskeleton components are integrated within cells.

Coactosin-like 1 integrates signaling critical for shear-dependent thrombus formation in mouse platelets

Platelet aggregate formation is a multistep process involving receptor-mediated, as well as biomechanical, signaling cascades, which are highly dependent on actin dynamics. We have previously shown that actin depolymerizing factor (ADF)/n-cofilin and Twinfilin 2a, members of the ADF homology (ADF-H) protein family, have distinct roles in platelet formation and function. Coactosin-like 1 (Cotl1) is another ADF-H protein that binds actin and was also shown to enhance biosynthesis of pro-inflammatory leukotrienes (LT) in granulocytes. Here, we generated mice lacking Cotl1 in the megakaryocyte lineage (Cotl1^{-/- )} to investigate its role in platelet production and function. Absence of Cotl1 had no impact on platelet counts, platelet activation or cytoskeletal reorganization under static conditions in vitro In contrast, Cotl1 deficiency markedly affected platelet aggregate formation on collagen and adhesion to immobilized von Willebrand factor at high shear rates in vitro, pointing to an impaired function of the platelet mechanoreceptor glycoprotein (GP) Ib. Furthermore, Cotl1 ^-/-platelets exhibited increased deformability at high shear rates, indicating that the GPIb defect may be linked to altered biomechanical properties of the deficient cells. In addition, we found that Cotl1 deficiency markedly affected platelet LT biosynthesis. Strikingly, exogenous LT addition restored defective aggregate formation of Cotl1^-/- platelets at high shear in vitro, indicating a critical role of platelet-derived LT in thrombus formation. In vivo, Cotl1 deficiency translated into prolonged tail bleeding times and protection from occlusive arterial thrombus formation. Together, our results show that Cotl1 in platelets is an integrator of biomechanical and LT signaling in hemostasis and thrombosis.

New insights into cytoskeletal remodeling during platelet production

The past decade has brought unprecedented advances in our understanding of megakaryocyte (MK) biology and platelet production, processes that are strongly dependent on the cytoskeleton. Facilitated by technological innovations, such as new high-resolution imaging techniques (in vitro and in vivo) and lineage-specific gene knockout and reporter mouse strains, we are now able to visualize and characterize the molecular machinery required for MK development and proplatelet formation in live mice. Whole genome and RNA sequencing analysis of patients with rare platelet disorders, combined with targeted genetic interventions in mice, has led to the identification and characterization of numerous new genes important for MK development. Many of the genes important for proplatelet formation code for proteins that control cytoskeletal dynamics in cells, such as Rho GTPases and their downstream targets. In this review, we discuss how the final stages of MK development are controlled by the cellular cytoskeletons, and we compare changes in MK biology observed in patients and mice with mutations in cytoskeleton regulatory genes.

Profilin 1-mediated cytoskeletal rearrangements regulate integrin function in mouse platelets

Profilin 1–mediated cytoskeletal dynamics regulate platelet β1- and β3-integrin function and turnover. Profilin 1 deficiency in platelets impairs hemostasis and results in a marked protection from arterial thrombosis.

Imaging platelet biogenesis in vivo

In this review paper, we give a historical perspective of the development of imaging modalities to visualize platelet biogenesis and how this contributed to our current understanding of megakaryopoiesis and thrombopoiesis. We provide some insight how distinct in vivo and in situ imaging methods, including ultramicrographs, have contributed to the current concepts of platelet formation. The onset of intravital microscopy into the mouse bone marrow has markedly modified and challenged our thinking of platelet biogenesis during the last decade. Finally, we discuss ongoing work, which was presented at the recent International Society on Thrombosis and Haemostasis (ISTH) meeting.

ADAP deficiency impairs megakaryocyte polarization with ectopic proplatelet release and causes microthrombocytopenia

Bone marrow (BM) megakaryocytes (MKs) produce platelets by extending proplatelets into sinusoidal blood vessels. Defects in thrombopoiesis can lead to thrombocytopenia associated with increased bleeding tendency. Recently, the platelet disorder congenital autosomal-recessive small-platelet thrombocytopenia (CARST) was described; it is caused by mutations in the adhesion and degranulation-promoting adaptor protein (ADAP; synonym: FYB, SLAP130/120) gene, and characterized by microthrombocytopenia and bleeding symptoms. In this study, we used constitutive ADAP-deficient mice (Adap^-/- ) as a model to investigate mechanisms underlying the microthrombocytopenia in CARST. We show that Adap^-/- mice display several characteristics of human CARST, with moderate thrombocytopenia and smaller-sized platelets. Adap^-/- platelets had a shorter life span than control platelets, and macrophage depletion, but not splenectomy, increased platelet counts in mutant mice to control levels. Whole-sternum 3-dimensional confocal imaging and intravital 2-photon microscopy revealed altered morphology of ADAP-deficient MKs with signs of fragmentation and ectopic release of (pro)platelet-like particles into the BM compartment. In addition, cultured BM-derived MKs lacking ADAP showed reduced spreading on extracellular matrix proteins as well as activation of β1 integrins, impaired podosome formation, and displayed defective polarization of the demarcation membrane system in vitro. MK-/platelet-specific ADAP-deficient mice (PF4-cre) also produced fewer and smaller-sized platelets and released platelets ectopically. These data demonstrate that the abnormal platelet production in the mutant mice is an MK-intrinsic defect. Taken together, these results point to an as-yet-unidentified role of ADAP in the process of MK polarization and platelet biogenesis.

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.

D-dencichine Regulates Thrombopoiesis by Promoting Megakaryocyte Adhesion, Migration and Proplatelet Formation

Life-threatening chemotherapy-induced thrombocytopenia can increase the risk of bleeding due to a dramatic low platelet count, which may limit or delay treatment schedules in cancer patients. The pressing need for the rapid alleviation of the symptoms of thrombocytopenia has prompted us to search for novel highly effective and safe thrombopoietic agents. Pharmacological investigations have indicated that dencichine can prevent and treat blood loss and increase the number of platelets. On the basis of the neurotoxicity of dencichine, D-dencichine is artificially synthesized in the laboratory. Our initial results showed that D-dencichine had potential to elevate peripheral platelet levels in mice with carboplatin-induced thrombocytopenia. However, the mechanisms of D-dencichine on thrombopoiesis have been poorly understood. In this study, we found that sequential administration of D-dencichine had a distinct ability to elevate numbers of reticulated platelets, and did not alter their clearance. Moreover, we demonstrated that D-dencichine was able to modulate the return of hematopoietic factors to normal levels, including thrombopoietin and IL-6. However, subsequent analysis revealed that D-dencichine treatment had no direct effects on megakaryocytes proliferation, differentiation, and polyploidization. Further in vitro studies, we demonstrated for the first time that D-dencichine significantly stimulated megakaryocyte adhesion, migration, and proplatelet formation in a dose-dependent manner through extracellular regulated protein kinases1/2 (ERK1/2) and v-akt murine thymoma viral oncogene homolog (AKT) signaling pathways. This study sufficiently characterized the role of the effects of D-dencichine treatment on the regulation of thrombopoiesis and provided a promising avenue for CIT treating.

Morphometric analysis of spread platelets identifies integrin αIIbβ3-specific contractile phenotype

Haemostatic platelet function is intimately linked to cellular mechanics and cytoskeletal morphology. How cytoskeletal reorganizations give rise to a highly contractile phenotype that is necessary for clot contraction remains poorly understood. To elucidate this process in vitro, we developed a morphometric screen to quantify the spatial organization of actin fibres and vinculin adhesion sites in single spread platelets. Platelets from healthy donors predominantly adopted a bipolar morphology on fibrinogen and fibronectin, whereas distinguishable, more isotropic phenotypes on collagen type I or laminin. Specific integrin α_IIbβ₃ inhibitors induced an isotropic cytoskeletal organization in a dose-dependent manner. The same trend was observed with decreasing matrix stiffness. Circular F-actin arrangements in platelets from a patient with type II Glanzmann thrombasthenia (GT) were consistent with the residual activity of a small number of α_IIbβ₃ integrins. Cytoskeletal morphologies in vitro thus inform about platelet adhesion receptor identity and functionality, and integrin α_IIbβ₃ mechanotransduction fundamentally determines the adoption of a bipolar phenotype associated with contraction. Super-resolution microscopy and electron microscopies further confirmed the stress fibre-like contractile actin architecture. For the first time, our assay allows the unbiased and quantitative assessment of platelet morphologies and could help to identify defective platelet behaviour contributing to elusive bleeding phenotypes.

Method for the simulation of blood platelet shape and its evolution during activation

We present a simple physically based quantitative model of blood platelet shape and its evolution during agonist-induced activation. The model is based on the consideration of two major cytoskeletal elements: the marginal band of microtubules and the submembrane cortex. Mathematically, we consider the problem of minimization of surface area constrained to confine the marginal band and a certain cellular volume. For resting platelets, the marginal band appears as a peripheral ring, allowing for the analytical solution of the minimization problem. Upon activation, the marginal band coils out of plane and forms 3D convoluted structure. We show that its shape is well approximated by an overcurved circle, a mathematical concept of closed curve with constant excessive curvature. Possible mechanisms leading to such marginal band coiling are discussed, resulting in simple parametric expression for the marginal band shape during platelet activation. The excessive curvature of marginal band is a convenient state variable which tracks the progress of activation. The cell surface is determined using numerical optimization. The shapes are strictly mathematically defined by only three parameters and show good agreement with literature data. They can be utilized in simulation of platelets interaction with different physical fields, e.g. for the description of hydrodynamic and mechanical properties of platelets, leading to better understanding of platelets margination and adhesion and thrombus formation in blood flow. It would also facilitate precise characterization of platelets in clinical diagnosis, where a novel optical model is needed for the correct solution of inverse light-scattering problem.

GPVI signaling is compromised in newly formed platelets after acute thrombocytopenia in mice

At sites of vascular injury, exposed subendothelial collagens trigger platelet activation and thrombus formation by interacting with the immunoreceptor tyrosine-based activation motif (ITAM)-coupled glycoprotein VI (GPVI) on the platelet surface. Platelets are derived from the cytoplasm of megakaryocytes (MKs), which extend large proplatelets into bone marrow (BM) sinusoids that are then released into the bloodstream, where final platelet sizing and maturation occurs. The mechanisms that prevent activation of MKs and forming proplatelets in the collagen-rich BM environment remain largely elusive. Here, we demonstrate that newly formed young platelets (NFYPs) released after antibody-mediated thrombocytopenia in mice display a severe and highly selective signaling defect downstream of GPVI resulting in impaired collagen-dependent activation and thrombus formation in vitro and in vivo. The diminished GPVI signaling in NFYPs is linked to reduced phosphorylation of key downstream signaling proteins, including Syk, LAT, and phospholipase Cγ2, whereas the G protein-coupled receptor and C-type lectin-like receptor 2 signaling pathways remained unaffected. This GPVI signaling defect was overcome once the platelet counts were restored to normal in the circulation. Overall, these results indicate that the GPVI-ITAM signaling machinery in NFYPs after antibody-mediated thrombocytopenia only becomes fully functional in the blood circulation.

Autonomous role of Wiskott-Aldrich syndrome platelet deficiency in inducing autoimmunity and inflammation

BACKGROUND

Wiskott-Aldrich syndrome (WAS) is an X-linked immunodeficiency characterized by eczema, infections, and susceptibility to autoimmunity and malignancies. Thrombocytopenia is a constant finding, but its pathogenesis remains elusive.

OBJECTIVE

To dissect the basis of the WAS platelet defect, we used a novel conditional mouse model (CoWas) lacking Wiskott-Aldrich syndrome protein (WASp) only in the megakaryocytic lineage in the presence of a normal immunologic environment, and in parallel we analyzed samples obtained from patients with WAS.

METHODS

Phenotypic and functional characterization of megakaryocytes and platelets in mutant CoWas mice and patients with WAS with and without autoantibodies was performed. Platelet antigen expression was examined through a protein expression profile and cluster proteomic interaction network. Platelet immunogenicity was tested by using ELISAs and B-cell and platelet cocultures.

RESULTS

CoWas mice showed increased megakaryocyte numbers and normal thrombopoiesis in vitro, but WASp-deficient platelets had short lifespan and high expression of activation markers. Proteomic analysis identified signatures compatible with defects in cytoskeletal reorganization and metabolism yet surprisingly increased antigen-processing capabilities. In addition, WASp-deficient platelets expressed high levels of surface and soluble CD40 ligand and were capable of inducing B-cell activation in vitro. WASp-deficient platelets were highly immunostimulatory in mice and triggered the generation of antibodies specific for WASp-deficient platelets, even in the context of a normal immune system. Patients with WAS also showed platelet hyperactivation and increased plasma soluble CD40 ligand levels correlating with the presence of autoantibodies.

CONCLUSION

Overall, these findings suggest that intrinsic defects in WASp-deficient platelets decrease their lifespan and dysregulate immune responses, corroborating the role of platelets as modulators of inflammation and immunity.

Optical Clearing of Murine Bones to Study Megakaryocytes in Intact Bone Marrow Using Light-Sheet Fluorescence Microscopy

In mammals, the differentiation and maturation of megakaryocytes (MKs) occurs in the bone marrow (BM). The three-dimensional environment influences megakaryopoiesis and platelet release. Thus, imaging MKs within the intact BM is important to understand megakaryopoiesis. Here, we present an optical clearing protocol for intact bones and the subsequent microscopic analysis including image processing to quantitatively assess MK size and distribution. This technique overcomes the limitations of classical sectioning methods as the entire bone can be imaged.

Targeting platelet receptors in thrombotic and thrombo-inflammatory disorders

Platelet activation at sites of vascular injury is critical for the formation of a hemostatic plug which limits excessive blood loss, but also represents a major pathomechanism of ischemic cardio- and cerebrovascular diseases. Although currently available antiplatelet therapies have proved beneficial in preventing the recurrence of vascular events, their adverse effects on primary hemostasis emphasize the necessity to identify and characterize novel pharmacological targets for platelet inhibition. Increasing experimental evidence has suggested that several major platelet surface receptors which regulate initial steps of platelet adhesion and activation may become promising new targets for anti-platelet drugs due to their involvement in thrombotic and thrombo-inflammatory signaling cascades.

This review summarizes recent developments in understanding the function of glycoprotein (GP)Ib, GPVI and the C-type lectin-like receptor 2 (CLEC-2) in hemostasis, arterial thrombosis and thrombo-inflammation and will discuss the suitability of the receptors as novel targets to treat these diseases in humans.

Profilin Directly Promotes Microtubule Growth through Residues Mutated in Amyotrophic Lateral Sclerosis

Profilin is an abundant actin monomer-binding protein with critical actin regulatory roles in vivo [1, 2]. However, profilin also influences microtubule dynamics in cells, which may be mediated in part through its interactions with formins that in turn bind microtubules [3, 4]. Specific residues on human profilin-1 (PFN1) are mutated in patients with amyotrophic lateral sclerosis (ALS) [5, 6]. However, the observation that some ALS-linked PFN1 mutants fail to alter cellular actin organization or dynamics [5-8] or in vitro actin-monomer affinity [9] has been perplexing, given that profilin is best understood as an actin regulator. Here, we investigated direct effects of profilin on microtubule dynamics and whether ALS-linked mutations in PFN1 disrupt such functions. We found that human, fly, and yeast profilin homologs all directly enhance microtubule growth rate by several-fold in vitro. Microtubule stimulatory effects were unaffected by mutations in the canonical actin- or poly-proline-binding sites of profilin. Instead, microtubule activities depended on specific surface residues on profilin mutated in ALS patients. Furthermore, microtubule effects were attenuated by increasing concentrations of actin monomers, suggesting competition between actin and microtubules for binding profilin. Consistent with these biochemical observations, a 2-fold increase in the expression level of wild-type PFN1, but not the ALS-linked PFN1 mutants, increased microtubule growth rates in cells. Together, these results demonstrate that profilin directly enhances the growth rate of microtubules. They further suggest that ALS-linked mutations in PFN1 may perturb cellular microtubule dynamics and/or the coordination between the actin and microtubule cytoskeletons, leading to motor neuron degeneration.

Whole Wiskott‑Aldrich syndrome protein gene deletion identified by high throughput sequencing

Wiskott‑Aldrich syndrome (WAS) is a rare X‑linked recessive immunodeficiency disorder, characterized by thrombocytopenia, small platelets, eczema and recurrent infections associated with increased risk of autoimmunity and malignancy disorders. Mutations in the WAS protein (WASP) gene are responsible for WAS. To date, WASP mutations, including missense/nonsense, splicing, small deletions, small insertions, gross deletions, and gross insertions have been identified in patients with WAS. In addition, WASP‑interacting proteins are suspected in patients with clinical features of WAS, in whom the WASP gene sequence and mRNA levels are normal. The present study aimed to investigate the application of next generation sequencing in definitive diagnosis and clinical therapy for WAS. A 5 month‑old child with WAS who displayed symptoms of thrombocytopenia was examined. Whole exome sequence analysis of genomic DNA showed that the coverage and depth of WASP were extremely low. Quantitative polymerase chain reaction indicated total WASP gene deletion in the proband. In conclusion, high throughput sequencing is useful for the verification of WAS on the genetic profile, and has implications for family planning guidance and establishment of clinical programs.

Deletion of the Arp2/3 complex in megakaryocytes leads to microthrombocytopenia in mice

Actin reorganization regulates key processes in platelet activation. Here we examined the role of the Arp2/3 complex, an essential component in actin filament branching, in platelet function. The Arpc2 gene, encoding the p34 subunit of the Arp2/3 complex, was deleted in the megakaryocyte lineage (Arpc2^fl/flPF4-Cre). Deletion of the Arp2/3 complex resulted in marked microthrombocytopenia in mice, caused by premature platelet release into the bone marrow compartment and impaired platelet survival in circulation. Arpc2^fl/flPF4-Cre platelets exhibited alterations in their actin cytoskeleton and their peripheral microtubule coil. Thrombocytopenia was alleviated following clodronate liposome-induced macrophage depletion in Arpc2^fl/flPF4-Cre mice. Arpc2^fl/flPF4-Cre platelets failed to spread and showed a mild defect in integrin activation and aggregation. However, no significant differences in hemostasis or thrombosis were observed between Arpc2^fl/flPF4-Cre and control mice. Thus, Arp2/3 is critical for platelet homeostasis but plays only a minor role for vascular hemostasis.

Thrombopoiesis is spatially regulated by the bone marrow vasculature

In mammals, megakaryocytes (MKs) in the bone marrow (BM) produce blood platelets, required for hemostasis and thrombosis. MKs originate from hematopoietic stem cells and are thought to migrate from an endosteal niche towards the vascular sinusoids during their maturation. Through imaging of MKs in the intact BM, here we show that MKs can be found within the entire BM, without a bias towards bone-distant regions. By combining in vivo two-photon microscopy and in situ light-sheet fluorescence microscopy with computational simulations, we reveal surprisingly slow MK migration, limited intervascular space, and a vessel-biased MK pool. These data challenge the current thrombopoiesis model of MK migration and support a modified model, where MKs at sinusoids are replenished by sinusoidal precursors rather than cells from a distant periostic niche. As MKs do not need to migrate to reach the vessel, therapies to increase MK numbers might be sufficient to raise platelet counts.Megakaryocyte maturation is thought to occur as the cells migrate from a vessel-distant (endosteal) niche to the vessel within the bone. Here, the authors show that megakaryocytes represent largely sessile cells in close contact with the vasculature and homogeneously distributed in the bone marrow.