Actin Ring Research Articles

Self-organized spatial targeting of contractile actomyosin rings for synthetic cell division

A key challenge for bottom-up synthetic biology is engineering a minimal module for self-division of synthetic cells. Actin-based cytokinetic rings are considered a promising structure to produce the forces required for the controlled excision of cell-like compartments such as giant unilamellar vesicles (GUVs). Despite prior demonstrations of actin ring targeting to GUV membranes and myosin-induced constriction, large-scale vesicle deformation has been precluded due to the lacking spatial control of these contractile structures. Here we show the combined reconstitution of actomyosin rings and the bacterial MinDE protein system within GUVs. Incorporating this spatial positioning tool, able to induce active transport of membrane-attached diffusible molecules, yields self-organized equatorial assembly of actomyosin rings in vesicles. Remarkably, the synergistic effect of Min oscillations and the contractility of actomyosin bundles induces mid-vesicle deformations and vesicle blebbing. Our system showcases how functional machineries from various organisms may be combined in vitro, leading to the emergence of functionalities towards a synthetic division system.

Geranylgeranyl diphosphate synthase inhibition impairs osteoclast differentiation, morphology and resorptive activity

Abstract Nitrogen bisphosphonates (NBPs), such as zoledronic acid, target the enzyme farnesyl diphosphate synthase (FDPS) in the isoprenoid biosynthetic pathway (IBP), and are the frontline treatment for osteolytic bone diseases. The strong affinity of these agents for bone limits their distribution out of the skeleton. Geranylgeranyl diphosphate synthase (GGDPS) is directly downstream to FDPS in the IBP and novel GGDPS inhibitors such as RAM2061 have been shown to have key drug-like features including prolonged half-life, metabolic stability, and systemic distribution. Furthermore, RAM2061 exerts anti-neoplastic benefits in mouse models of multiple myeloma and Ewing sarcoma. Therefore, we are interested in determining the potential impact of RAM2061 on osteoclast biology and bone remodeling. Studies utilizing undifferentiated RAW264.7 cells demonstrated that treatment with RAM2061 depletes cells of GGPP, impairs protein geranylgeranylation and induces markers of the unfolded protein response pathway and apoptosis. Differentiation of RAW264.7 cells to mature osteoclasts is disrupted by RAM2061, resulting in decreased numbers of mature osteoclasts, altered morphology and decreased tartrate resistant acid phosphatase activity. Treatment of fully differentiated RAW264.7 cells with RAM2061 led to decreased resorptive activity. Confocal microscopy studies revealed that RAM2061 disrupts Cdc42 localization, inhibiting proper actin ring formation in osteoclasts. No significant impact on bone turnover markers or bone histomorphology was observed following a 3-week treatment of CD-1 mice with RAM2061, although decreased numbers of osteoclasts were observed. Liquid chromatography–tandem mass spectrometry studies confirmed accumulation of RAM2061 in bone from the in vivo studies as well as hydroxyapatite binding in vitro. In conclusion, these studies are the first to demonstrate the anti-osteoclastic activity of GGDPS inhibitor treatment and support future studies exploring the therapeutic benefit of this novel therapy in the setting of pathological bone remodeling.

The rat bladder umbrella cell keratin network: Organization, dependence on the plectin cytolinker, and responses to bladder filling.

The keratin cytoskeleton and associated desmosomes contribute to the mechanical stability of epithelial tissues, but their organization in native bladder umbrella cells and their responses to bladder filling are poorly understood. Using whole rat bladders in conjunction with confocal microscopy, super-resolution image processing, three-dimensional image reconstruction, and platinum replica electron microscopy, we identified a cortical cytoskeleton network in umbrella cells that was organized as a dense tile-like mesh comprised of tesserae bordered by cortical actin filaments, filled with keratin filaments, and cross-linked by plectin. Below these tesserae, keratin formed a subapical meshwork and at the cell periphery a band of keratin was linked via plectin to the junction-associated actin ring. Disruption of plectin led to focal keratin network dissolution, loss of the junction-associated keratin, and defects in cell-cell adhesion. During bladder filling, a junction-localized necklace of desmosomes expanded, and a subjacent girded layer formed linking the keratin network to desmosomes, including those at the umbrella cell-intermediate cell interface. Our studies reveal a novel tile- and mesh-like organization of the umbrella cell keratin network that is dependent on plectin, that reorganizes in response to bladder filling, and that likely serves to maintain umbrella cell continuity in the face of mechanical distension.

Arp2/3 complex activity enables nuclear YAP for naïve pluripotency of human embryonic stem cells.

Our understanding of the transitions of human embryonic stem cells (hESCs) between distinct stages of pluripotency relies predominantly on regulation by transcriptional and epigenetic programs with limited insight on the role of established morphological changes. We report remodeling of the actin cytoskeleton of hESCs as they transition from primed to naïve pluripotency which includes assembly of a ring of contractile actin filaments encapsulating colonies of naïve hESCs. Activity of the Arp2/3 complex is required for formation of the actin ring, to establish uniform cell mechanics within naïve colonies, to promote nuclear translocation of the Hippo pathway effectors YAP and TAZ, and for effective transition to naïve pluripotency. RNA-sequencing analysis confirms that Arp2/3 complex activity regulates Hippo signaling in hESCs, and impaired naïve pluripotency with inhibited Arp2/3 complex activity is rescued by expressing a constitutively active, nuclear-localized YAP-S127A. Moreover, expression of YAP-S127A partially restores the actin filament fence with Arp2/3 complex inhibition, suggesting that actin filament remodeling is both upstream and downstream of YAP activity. These new findings on the cell biology of hESCs reveal a mechanism for cytoskeletal dynamics coordinating cell mechanics to regulate gene expression and facilitate transitions between pluripotency states.

Crystallographic plane-induced selective mineralization of nanohydroxyapatite on fibrous-grained titanium promotes osteointegration and biocorrosion resistance

The (002) crystallographic plane-oriented hydroxyapatite (HA) and anatase TiO2 enable favorable hydrophilicity, osteogenesis, and biocorrosion resistance. Thus, the crystallographic plane control in HA coating and crystalline phase control in TiO2 is vital to affect the surface and interface bioactivity and biocorrosion resistance of titanium (Ti) implants. However, a corresponding facile and efficient fabrication method is absent to realize the HA(002) mineralization and anatase TiO2 formation on Ti. Herein, we utilized the predominant Ti(0002) plane of the fibrous-grained titanium (FG Ti) to naturally form anatase TiO2 and further achieve a (002) basal plane oriented nanoHA (nHA) film through an in situ mild hydrothermal growth strategy. The formed FG Ti-nHA(002) remarkably improved hydrophilicity, mineralization, and biocorrosion resistance. Moreover, the nHA(002) film reserved the microgroove-like topological structure on FG Ti. It could enhance osteogenic differentiation through promoted contact guidance, showing one order of magnitude higher expression of osteogenic-related genes. On the other hand, the nHA(002) film restrained the osteoclast activity by blocking actin ring formation. Based on these capacities, FG Ti-nHA(002) improved new bone growth and binding strength in rabbit femur implantation, achieving satisfactory osseointegration within 2 weeks.

Irilin D suppresses RANKL-induced osteoclastogenesis and prevents inflammation-induced bone loss by disrupting the NF-κB and MAPK signaling pathways

Excessive activity of osteoclasts(OCs) lead to bone resorption in chronic inflammatory conditions. The use of natural compounds to target OCs offers significant promise in the treatment or prevention of OC-associated diseases. Irilin D (IRD), a natural isoflavone derived from Belamcanda chinensis (L.) DC., has potential effects on OC differentiation both in vitro and in vivo that have yet to be thoroughly explored. In our study, we found that IRD inhibited receptor activator of nuclear factor-κB ligand (RANKL)-induced OC differentiation, actin ring formation, and bone resorption in vitro without compromising cell viability. However, IRD did not exhibit anti-inflammatory effects in lipopolysaccharide (LPS)-stimulated macrophages. Furthermore, IRD reduced LPS-induced inflammatory bone loss by blocking osteoclastogenesis in a mouse model. Mechanistically, IRD disrupted RANKL-induced activation of mitogen-activated protein kinases (MAPKs) and nuclear factor-κB (NF-κB), leading to the inhibition of c-Fos and nuclear factor of activated T cells cytoplasmic 1 (NFATc1) activation. We also demonstrated that IRD inhibited RANKL-induced osteoclastic NFATc1 target genes, including DC-STAMP, ACP5, and CtsK. Our results indicate that IRD mitigates LPS-induced inflammatory bone resorption in mice by inhibiting RANKL-activated MAPKs and NF-κB signaling pathways, suggesting its potential as a natural isoflavone for preventing or treating OC-associated diseases.

Plakophilin 4 controls the spatio-temporal activity of RhoA at adherens junctions to promote cortical actin ring formation and tissue tension

Plakophilin 4 (PKP4) is a component of cell–cell junctions that regulates intercellular adhesion and Rho-signaling during cytokinesis with an unknown function during epidermal differentiation. Here we show that keratinocytes lacking PKP4 fail to develop a cortical actin ring, preventing adherens junction maturation and generation of tissue tension. Instead, PKP4-depleted cells display increased stress fibers. PKP4-dependent RhoA localization at AJs was required to activate a RhoA-ROCK2-MLCK-MLC2 axis and organize actin into a cortical ring. AJ-associated PKP4 provided a scaffold for the Rho activator ARHGEF2 and the RhoA effectors MLCK and MLC2, facilitating the spatio-temporal activation of RhoA signaling at cell junctions to allow cortical ring formation and actomyosin contraction. In contrast, association of PKP4 with the Rho suppressor ARHGAP23 reduced ARHGAP23 binding to RhoA which prevented RhoA activation in the cytoplasm and stress fiber formation. These data identify PKP4 as an AJ component that transduces mechanical signals into cytoskeletal organization.

Ginsenoside Rb3 alleviates the formation of osteoclasts induced by periodontal ligament fibroblasts in the periodontitis microenvironment through the STAT3 pathway.

This study explores the potential role and mechanism of Ginsenoside Rb3 (Rb3) in modulating osteoclastogenesis induced by human periodontal ligament fibroblasts (hPLFs) within the periodontitis microenvironment. We investigated the anti-inflammatory effects of Rb3 on hPLFs stimulated with Porphyromonas gingivalis lipopolysaccharide (P.g-LPS) utilizing quantitative polymerase chain reaction (qPCR) and enzyme-linked immunosorbent assay techniques. Moreover, the functional role of Rb3 in hPLFs-induced osteoclast formation was assessed by treating human bone marrow-derived macrophages (hBMMs) with conditioned medium from hPLFs, followed by analyses through qPCR, western blot analysis, and staining for tartrate-resistant acid phosphatase (TRAP) and phalloidin. The impact of Rb3 on the activation of the STAT3 signaling pathway was determined via western blot analysis. Results indicated that Rb3 treatment significantly suppressed the upregulation of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6, MCP-1, and IL-18) at both gene and protein levels in hPLFs induced by P.g-LPS. Furthermore, conditioned medium from Rb3 plus P.g-LPS treated hPLFs notably decreased the number of TRAP-positive cells, actin ring formations, and the expression of osteoclast marker genes (including CTSK, NFATC1, and ACP5). Rb3 also inhibited the P.g-LPS-induced activation of the STAT3 pathway, with the activation of STAT3 partially reversing the effects of Rb3 on inflammation and osteoclast differentiation. Collectively, Rb3 ameliorates inflammation in P.g-LPS-stimulated hPLFs and reduces hPLFs-induced osteoclastogenesis by inhibiting the STAT3 signaling pathway, suggesting its potential as a therapeutic agent for periodontitis.

PatternJ: an ImageJ toolset for the automated and quantitative analysis of regular spatial patterns found in sarcomeres, axons, somites, and more.

Regular spatial patterns are ubiquitous forms of organization in nature. In animals, regular patterns can be found from the cellular scale to the tissue scale, and from early stages of development to adulthood. To understand the formation of these patterns, how they assemble and mature, and how they are affected by perturbations, a precise quantitative description of the patterns is essential. However, accessible tools that offer in-depth analysis without the need for computational skills are lacking for biologists. Here, we present PatternJ, a novel toolset to analyze regular one-dimensional patterns precisely and automatically. This toolset, to be used with the popular imaging processing program ImageJ/Fiji, facilitates the extraction of key geometric features within and between pattern repeats in static images and time-lapse series. We validate PatternJ with simulated data and test it on images of sarcomeres from insect muscles and contracting cardiomyocytes, actin rings in neurons, and somites from zebrafish embryos obtained using confocal fluorescence microscopy, STORM, electron microscopy, and brightfield imaging. We show that the toolset delivers subpixel feature extraction reliably even with images of low signal-to-noise ratio. PatternJ's straightforward use and functionalities make it valuable for various scientific fields requiring quantitative one-dimensional pattern analysis, including the sarcomere biology of muscles or the patterning of mammalian axons, speeding up discoveries with the bonus of high reproducibility.

The umbrella cell keratin network: organization as a tile-like mesh, formation of a girded layer in response to bladder filling, and dependence on the plectin cytolinker.

The keratin cytoskeleton and associated desmosomes contribute to the mechanical stability of epithelial tissues, but their organization in bladder umbrella cells and their responses to bladder filling are poorly understood. Using super-resolution confocal microscopy, along with 3D image reconstruction and platinum replica electron microscopy, we observed that the apical keratin network of umbrella cells was organized as a dense tile-like mesh comprised of tesserae bordered on their edges by cortical actin filaments, filled with woven keratin filaments, and crosslinked by plectin. A band of keratin was also observed at the cell periphery that was linked to the junction-associated actin ring by plectin. During bladder filling, the junction-localized desmosomal necklace expanded, and a subjacent girded layer was formed that linked the keratin network to desmosomes, including those at the umbrella cell-intermediate cell interface. Disruption of plectin led to focal keratin network dissolution, loss of the junction-associated band of keratin, perturbation of tight junction continuity, and loss of cell-cell cohesion. Our studies reveal a novel tile-like organization of the umbrella cell keratin cytoskeleton that is dependent on plectin, that reorganizes in response to bladder filling, and that likely serves to maintain umbrella cell continuity in the face of mechanical distension.

Involvement of Siglec-15 in regulating RAP1/RAC signaling in cytoskeletal remodeling in osteoclasts mediated by macrophage colony-stimulating factor

DNAX-associated protein 12 kD size (DAP12) is a dominant immunoreceptor tyrosine-based activation motif (ITAM)-signaling adaptor that activates costimulatory signals essential for osteoclastogenesis. Although several DAP12-associated receptors (DARs) have been identified in osteoclasts, including triggering receptor expressed on myeloid cells 2 (TREM-2), C-type lectin member 5 A (CLEC5A), and sialic acid-binding Ig-like lectin (Siglec)-15, their precise role in the development of osteoclasts and bone remodeling remain poorly understood. In this study, mice deficient in Trem-2, Clec5a, Siglec-15 were generated. In addition, mice double deficient in these DAR genes and FcεRI gamma chain (FcR)γ, an alternative ITAM adaptor to DAP12, were generated. Bone mass analysis was conducted on all mice. Notably, Siglec-15 deficient mice and Siglec-15/FcRγ double deficient mice exhibited mild and severe osteopetrosis respectively. In contrast, other DAR deficient mice showed normal bone phenotype. Likewise, osteoclasts from Siglec-15 deficient mice failed to form an actin ring, suggesting that Siglec-15 promotes bone resorption principally by modulating the cytoskeletal organization of osteoclasts. Furthermore, biochemical analysis revealed that Sigelc-15 activates macrophage colony-stimulating factor (M-CSF)-induced Ras-associated protein-1 (RAP1)/Ras-related C3 botulinum toxin substrate 1 (Rac1) pathway through formation of a complex with p130CAS and CrkII, leading to cytoskeletal remodeling of osteoclasts. Our data provide genetic and biochemical evidence that Siglec-15 facilitates M-CSF-induced cytoskeletal remodeling of the osteoclasts.

Azoles activate type I and type II programmed cell death pathways in crop pathogenic fungi

Triazoles are widely used to control pathogenic fungi. They inhibit the ergosterol biosynthetic pathway, but the precise mechanisms leading to fungicidal activities in many fungal pathogens are poorly understood. Here, we elucidate the mode of action of epoxiconazole and metconazole in the wheat pathogen Zymoseptoria tritici and the rice blast fungus Magnaporthe oryzae. We show that both azoles have fungicidal activity and reduce fluidity, but not integrity, of the plasma membrane. This impairs localisation of Cdc15-like F-BAR proteins, resulting in defective actin ring assembly and incomplete septation. However, mutant studies and pharmacological experiments in vitro and in planta show that azole lethality is due to a combination of reactive oxygen species-induced apoptosis and macroautophagy. Simultaneous inhibition of both programmed cell death pathways abolishes azole-induced cell death. Other classes of ergosterol biosynthesis inhibitors also induce apoptosis and macroautophagy, suggesting that activation of these two cell death pathways is a hallmark of ergosterol synthesis-targeting fungicides. This knowledge will inform future crop protection strategies.

Myosin-1C augments endothelial secretion of von Willebrand factor by linking contractile actomyosin machinery to the plasma membrane

AbstractBlood endothelial cells control the hemostatic and inflammatory response by secreting von Willebrand factor (VWF) and P-selectin from storage organelles called Weibel-Palade bodies (WPBs). Actin-associated motor proteins regulate this secretory pathway at multiple points. Before fusion, myosin Va forms a complex that anchors WPBs to peripheral actin structures, allowing for the maturation of content. After fusion, an actomyosin ring/coat is recruited and compresses the WPB to forcibly expel the largest VWF multimers. Here, we provide, to our knowledge, the first evidence for the involvement of class I myosins during regulated VWF secretion. We show that the unconventional myosin-1C (Myo1c) is recruited after fusion via its pleckstrin homology domain in an actin-independent process. This provides a link between the actin ring and phosphatidylinositol 4,5-bisphosphate (PIP2) at the membrane of the fused organelle and is necessary to ensure maximal VWF secretion. This is an active process requiring Myo1c ATPase activity because inhibition of class I myosins using the inhibitor pentachloropseudilin or expression of an ATPase-deficient Myo1c rigor mutant perturbs the expulsion of VWF and alters the kinetics of the exocytic actin ring. These data offer a novel insight into the control of an essential physiological process and provide a new way in which it can be regulated.

Two Septin complexes mediate actin dynamics during cell wound repair

Cells have robust wound repair systems to prevent further damage or infection and to quickly restore cell cortex integrity when exposed to mechanical and chemical stress. Actomyosin ring formation and contraction at the wound edge are major events during closure of the plasma membrane and underlying cytoskeleton during cell wound repair. Here, we show that all five Drosophila Septins are required for efficient cell wound repair. Based on their different recruitment patterns and knockdown/mutant phenotypes, two distinct Septin complexes, Sep1/Sep2/Pnut and Sep4/Sep5/Pnut, are assembled to regulate actin ring assembly, contraction, and remodeling during the repair process. Intriguingly, we find that these two Septin complexes have different F-actin bending activities. In addition, we find that Anillin regulates the recruitment of only one of two Septin complexes upon wounding. Our results demonstrate that two functionally distinct Septin complexes work side by side to discretely regulate actomyosin ring dynamics during cell wound repair.

Chordoma cells possess bone-dissolving activity at the bone invasion front.

Chordomas are malignant tumors that destroy bones, compress surrounding nerve tissues and exhibit phenotypes that recapitulate notochordal differentiation in the axial skeleton. Chordomas recur frequently, as they resist radio-chemotherapy and are difficult to completely resect, leading to repeated bone destruction and local expansion via unknown mechanisms. Here, using chordoma specimens and JHC7 chordoma cells, we asked whether chordoma cells possess bone-dissolving activity. CT imaging and histological analysis were performed to evaluate the structure and mineral density of chordoma-invaded bone and osteolytic marker expression. JHC7 cells were subjected to immunocytochemistry, imaging of cell fusion, calcium dynamics and acidic vacuoles, and bone lysis assays. In patients, we found that the skull base invaded by chordoma was highly porous, showed low mineral density and contained brachyury-positive chordoma cells and conventional osteoclasts both expressing the osteolytic markers tartrate-resistant acid phosphatase (TRAP) and collagenases. JHC7 cells expressed TRAP and cathepsin K, became multinucleated via cell-cell fusion, showed spontaneous calcium oscillation, and were partly responsive to the osteoclastogenic cytokine RANKL. JHC7 cells exhibited large acidic vacuoles, and nonregulatory bone degradation without forming actin rings. Finally, bone-derived factors, calcium ions, TGF-β1, and IGF-1 enhanced JHC7 cell proliferation. In chordoma, we propose that in addition to conventional bone resorption by osteoclasts, chordoma cells possess bone-dissolving activity at the tumor-bone boundary. Furthermore, bone destruction and tumor expansion may occur in a positive feedback loop.

Suppression of bone resorption by Mori Radicis Cortex through NFATc1 and c-Fos signaling-mediated inhibition of osteoclast differentiation.

Mori Radicis Cortex (MRC) is the root bark of the mulberry family as Morus alba L. In Korea, it is known as "Sangbaegpi". Although MRC has demonstrated anti-inflammatory and antioxidant effects, its specific mechanisms of action and impact on osteoporosis remain poorly understood. To investigate the antiosteoporosis effect of MRC, we examined the level of osteoclast differentiation inhibition in receptor activator of nuclear factor kappa-Β ligand (RANKL)-induced-RAW 264.7 cells and animal models of ovariectomy (OVX) with MRC. Serum analysis in OVX animals was investigated by enzyme-linked immunosorbent assay (ELISA), and bone density analysis was confirmed by micro-computed tomography (micro-CT). The expression analysis of nuclear factor of activated T cells 1 (NFATc1) was confirmed by immunohistochemistry (IHC) in femur tissue. In addition, osteoclast differentiation inhibition was measured using tartrate-resistant acid phosphatase (TRAP). mRNA analysis was performed using reverse transcription-polymerase chain reaction (RT-PCR), and the protein expression analysis was investigated by western blot. Micro-CT analysis showed that MRC effectively inhibited bone loss in the OVX-induced rat model. MRC also inhibited the expression of alkaline phosphatase (ALP) and TRAP in serum. Histological analysis showed that MRC treatment increased bone density and IHC analysis showed that MRC significantly inhibited the expression of NFATc1. In RANKL-induced-RAW 264.7 cells, MRC significantly reduced TRAP activity and actin ring formation. In addition, MRC significantly inhibited the expression of NFATc1 and c-Fos, and suppressed the mRNA expression. Based on micro-CT, serum and histological analysis, MRC effectively inhibited bone loss in an OVX-induced rat model. In addition, MRC treatment suppressed the expression of osteoclast differentiation, fusion, and bone resorption markers through inhibition of NFATc1/c-Fos expression in RANKL-induced RAW 264.7 cells, ultimately resulting in a decrease in osteoclast activity. These results demonstrate that MRC is effective in preventing bone loss through inhibiting osteoclast differentiation and activity.

In vitro evaluation of bone cell response to novel 3D-printable nanocomposite biomaterials for bone reconstruction.

Critically-sized segmental bone defects represent significant challenges requiring grafts for reconstruction. 3D-printed synthetic bone grafts are viable alternatives to structural allografts if engineered to provide appropriate mechanical performance and osteoblast/osteoclast cell responses. Novel 3D-printable nanocomposites containing acrylated epoxidized soybean oil (AESO) or methacrylated AESO (mAESO), polyethylene glycol diacrylate, and nanohydroxyapatite (nHA) were produced using masked stereolithography. The effects of volume fraction of nHA and methacrylation of AESO on interactions of differentiated MC3T3-E1 osteoblast (dMC3T3-OB) and differentiated RAW264.7 osteoclast cells with 3D-printed nanocomposites were evaluated in vitro and compared with a control biomaterial, hydroxyapatite (HA). Higher nHA content and methacrylation significantly improved the mechanical properties. All nanocomposites supported dMC3T3-OB cells' adhesion and proliferation. Higher amounts of nHA enhanced cell adhesion and proliferation. mAESO in the nanocomposites resulted in greater adhesion, proliferation, and activity at day 7 compared with AESO nanocomposites. Excellent osteoclast-like cells survival, defined actin rings, and large multinucleated cells were only observed on the high nHA fraction (30%) mAESO nanocomposite and the HA control. Thus, mAESO-based nanocomposites containing higher amounts of nHA have better interactions with osteoblast-like and osteoclast-like cells, comparable with HA controls, making them a potential future alternative graft material for bone defect repair.

Kinetic trapping organizes actin filaments within liquid-like protein droplets

Several actin-binding proteins (ABPs) phase separate to form condensates capable of curating the actin network shapes. Here, we use computational modeling to understand the principles of actin network organization within VASP condensate droplets. Our simulations reveal that the different actin shapes, namely shells, rings, and mixture states are highly dependent on the kinetics of VASP-actin interactions, suggesting that they arise from kinetic trapping. Specifically, we show that reducing the residence time of VASP on actin filaments reduces degree of bundling, thereby promoting assembly of shells rather than rings. We validate the model predictions experimentally using a VASP-mutant with decreased bundling capability. Finally, we investigate the ring opening within deformed droplets and found that the sphere-to-ellipsoid transition is favored under a wide range of filament lengths while the ellipsoid-to-rod transition is only permitted when filaments have a specific range of lengths. Our findings highlight key mechanisms of actin organization within phase-separated ABPs.

Hem1 is essential for ruffled border formation in osteoclasts and efficient bone resorption

Bone resorption is highly dependent on the dynamic rearrangement of the osteoclast actin cytoskeleton to allow formation of actin rings and a functional ruffled border. Hem1 is a hematopoietic-specific subunit of the WAVE-complex which regulates actin polymerization and is crucial for lamellipodia formation in hematopoietic cell types. However, its role in osteoclast differentiation and function is still unknown. Here, we show that although the absence of Hem1 promotes osteoclastogenesis, the ability of Hem1-/- osteoclasts to degrade bone was severely impaired. Global as well as osteoclast-specific deletion of Hem1 in vivo revealed increased femoral trabecular bone mass despite elevated numbers of osteoclasts in vivo. We found that the resorption defect derived from the morphological distortion of the actin-rich sealing zone and ruffled border deformation in Hem1-deficient osteoclasts leading to impaired vesicle transport and increased intracellular acidification. Collectively, our data identify Hem1 as a yet unknown key player in bone remodeling by regulating ruffled border formation and consequently the resorptive capacity of osteoclasts.

Effects of rpl1001 Gene Deletion on Cell Division of Fission Yeast and Its Molecular Mechanism.

The rpl1001 gene encodes 60S ribosomal protein L10, which is involved in intracellular protein synthesis and cell growth. However, it is not yet known whether it is involved in the regulation of cell mitosis dynamics. This study focuses on the growth, spore production, cell morphology, the dynamics of microtubules, chromosomes, actin, myosin, and mitochondria of fission yeast (Schizosaccharomyces pombe) to investigate the impact of rpl1001 deletion on cell mitosis. RNA-Seq and bioinformatics analyses were also used to reveal key genes, such as hsp16, mfm1 and isp3, and proteasome pathways. The results showed that rpl1001 deletion resulted in slow cell growth, abnormal spore production, altered cell morphology, and abnormal microtubule number and length during interphase. The cell dynamics of the rpl1001Δ strain showed that the formation of a monopolar spindle leads to abnormal chromosome segregation with increased rate of spindle elongation in anaphase of mitosis, decreased total time of division, prolonged formation time of actin and myosin loops, and increased expression of mitochondrial proteins. Analysis of the RNA-Seq sequencing results showed that the proteasome pathway, up-regulation of isp3, and down-regulation of mfm1 and mfm2 in the rpl1001Δ strain were the main factors underpinning the increased number of spore production. Also, in the rpl1001Δ strain, down-regulation of dis1 caused the abnormal microtubule and chromosome dynamics, and down-regulation of hsp16 and pgk1 were the key genes affecting the delay of actin ring and myosin ring formation. This study reveals the effect and molecular mechanism of rpl1001 gene deletion on cell division, which provides the scientific basis for further clarifying the function of the Rpl1001 protein in cell division.