Formins: signaling effectors for assembly and polarization of actin filaments

The ABC toxin complex from Yersinia entomophaga can package three different cytotoxic components expressed from distinct genetic loci in an unfolded state: the structures of both shell and cargo

Bacterial ABC toxin complexes (Tcs) comprise three core proteins: TcA, TcB and TcC. The TcA protein forms a pentameric assembly that attaches to the surface of target cells and penetrates the cell membrane. The TcB and TcC proteins assemble as a heterodimeric TcB-TcC subcomplex that makes a hollow shell. This TcB-TcC subcomplex self-cleaves and encapsulates within the shell a cytotoxic `cargo' encoded by the C-terminal region of the TcC protein. Here, we describe the structure of a previously uncharacterized TcC protein from Yersinia entomophaga, encoded by a gene at a distant genomic location from the genes encoding the rest of the toxin complex, in complex with the TcB protein. When encapsulated within the TcB-TcC shell, the C-terminal toxin adopts an unfolded and disordered state, with limited areas of local order stabilized by the chaperone-like inner surface of the shell. We also determined the structure of the toxin cargo alone and show that when not encapsulated within the shell, it adopts an ADP-ribosyltransferase fold most similar to the catalytic domain of the SpvB toxin from Salmonella typhimurium. Our structural analysis points to a likely mechanism whereby the toxin acts directly on actin, modifying it in a way that prevents normal polymerization.

The prognostic value of DAAM2 in lower grade glioma, liver cancer, and breast cancer

PURPOSE

Dishevelled-associated activator of morphogenesis 2 (DAAM2) is a formin protein and has a potential role in the tumor metastasis. The prognostic value of DAAM2 in pan-cancer is investigated in this study.

METHODS

TCGA and GTEx database were downloaded to perform bioinformatics analysis and ROC curves. Then we explored protein-protein interaction and GO-KEGG enrichment to figure out the protein pathways associated with DAAM2 and studied DAAM2-related immune infiltration and methylation. Fifteen pairs of BRCA clinical samples were enrolled to determine the expression and distribution of DAAM2 in BRCA sections by immunohistochemistry. Finally, BRCA cells were transfected with siRNA targeting DAAM2 and subsequently subject to cell proliferation, migration, and invasion assays.

RESULTS

DAAM2 was closely related to the diagnosis and clinical characteristics of lower grade glioma (LGG), liver hepatocellular carcinoma (LIHC), and breast cancer (BRCA). Survival curve analysis demonstrated DAAM2 served as a potential prognostic indicator of LGG and LIHC (P = 0.0029 and P = 0.025, respectively). DAAM2 was mainly participated in signaling pathways mediating cytoskeleton regulation and tumor development. The correlation of DAAM2 with tumor-infiltrating immune cells (TIICs) and methylation levels was conducive to the prediction of novel biomarkers of pan-carcinoma. DAAM2 was highly expressed in BRCA tissues than that in paracancerous tissues. The proliferation, invasion, and migration of BRCA cells were inhibited by DAAM2 siRNA.

CONCLUSION

DAAM2 had a specific value in foretelling the prognosis of LGG, LIHC, and BRCA. High expression level of DAAM2 has longer survival rates in LGG and LIHC. The knockdown of DAAM2 retards the proliferation, invasion, and migration of BRCA cells. This study provides a novel sight of DAAM2 into the exploration of a potential biomarker in pan-cancer.

Dual control of formin-nucleated actin assembly by the chromatin and ER in mouse oocytes

The first asymmetric meiotic cell divisions in mouse oocytes are driven by formin 2 (FMN2)-nucleated actin polymerization around the spindle. In this study, we investigated how FMN2 is recruited to the spindle peripheral ER and how its activity is regulated in mouse meiosis I (MI) oocytes. We show that this process is regulated by the Ran GTPase, a conserved mediator of chromatin signal, and the ER-associated protein VAPA. FMN2 contains a nuclear localization sequence (NLS) within a domain (SLD) previously shown to be required for FMN2 localization to the spindle periphery. FMN2 NLS is bound to the importin α1/β complex, and the disruption of this interaction by RanGTP is required for FMN2 accumulation in the area proximal to the chromatin and the MI spindle. The importin-free FMN2 is then recruited to the surface of ER around the spindle through the binding of the SLD with the ER-membrane protein VAPA. We further show that FMN2 is autoinhibited through an intramolecular interaction between the SLD with the C-terminal formin homology 2 (FH2) domain that nucleates actin filaments. VAPA binding to SLD relieves the autoinhibition of FMN2, leading to localized actin polymerization. This dual control of formin-mediated actin assembly allows actin polymerization to initiate the movement of the meiotic spindle toward the cortex, an essential step in the maturation of the mammalian female gamete.

Diaph3 underlines tumor cell heterogeneity in glioblastoma with implications for treatment modalities resistance

INTRODUCTION

Glioblastoma (GBM) is the most aggressive central nervous system (CNS) tumor with astrocytic differentiation. The growth pattern of GBM mimics that of the precursor cell migration during the fetal development of the brain. Diaphanous homolog (Diaph3) has been established to play a role in both CNS maturation and cancer progression as it is required both for cell migration and division. Furthermore, Diaph3 has been shown to play a role in malignant disease progression through hyperactivation of the EGFR/MEK/ERK in loss of expression and its overexpression correlating to hyperactivity of the mTOR pathway, both of which are with a well-established role in GBM. Herein, we aimed at establishing the diagnostic role of Diaph3 immunohistochemistry expression patterns in GBM and their possible implications for molecular response to different therapies.

MATERIALS AND METHODS

The study utilized a retrospective nonclinical approach. Results of Diaph3 immunohistochemical expression were compared to healthy controls and reactive gliosis and statistically analyzed for correlation with neuroradiological tumor parameters and patient survival.

RESULTS

Healthy controls showed individual weakly positive cells, while reactive gliosis controls showed a strong expression in astrocytic projections. GBM samples showed a heterogeneous positive reaction to Diaph3, mean number of positive cells 62.66%, median 61.5, range 12-96%. Areas of migrating cells showed a strong diffuse cytoplasmic reaction. Cells located in the tumor core and those in areas of submeningeal aggregation had no antibody expression. Statistical analysis revealed no correlation with tumor size or patient survival.

CONCLUSION

The different expression pattern of Diaph3 in healthy controls, reactive gliosis and GBM shows promise as a clinical differentiating marker. Despite Diaph3 expression not correlating with survival and tumor size in GBM, there is an accumulating body of evidence that Diaph3 correlates with mTOR activity and can thus be used as a predictor for response to rapamycin and taxanes, clinical studies of which have shown promising, if mixed results in GBM.

Heterozygous FMN2 missense variant found in a family case of premature ovarian insufficiency

Background

Premature ovarian insufficiency (POI) plagues 1% of women under 40, while quite a few remain an unknown cause. The development of sequencing has helped find pathogenic genes and reveal the relationship between DNA repair and ovarian reserve. Through the exome sequencing, our study targets screening out the possible POI pathogenic gene and variants in a Chinese family and 20 sporadic POI patients, preliminarily exploring the functional impact and finding out potential linkages between the gene and POI.

Results

The whole exome sequencing suggested a novel FMN2 heterozygous variant c.1949C > T (p.Ser650Leu) carried by all three patients in a Chinese family and another c.1967G > A(p.Arg656His) variant in a sporadic case. Since no FMN2 missense mutation is reported for causing human POI, we preliminarily assessed p.Ser650Leu variant via cross-species alignment and 3D modeling and found it possibly deleterious. A series of functional evidence was consistent with our hypothesis. We proved the expression of FMN2 in different stages of oocytes and observed a statistical difference of chromosomal breakages between the POI patient carrying p.Arg656His variant and the health control (p = 0.0013). Western Blot also suggested a decrease in FMN2 and P21 in the mutant type and an associated increase in H2AX. The p.Arg656His variant with an extremely low frequency also indicated that the gene FMN2 might play an essential role in the genetic etiology of POI. To the best of our knowledge, this is the first POI report on missense variants of FMN2.

Conclusion

This finding indicates a novel gene possibly related to POI and sheds lights on the study of FMN2.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13048-022-00960-y.

Comprehensive analysis of formin gene family highlights candidate genes related to pollen cytoskeleton and male fertility in wheat (Triticum aestivum L.)

Background

Formin, a highly conserved multi-domain protein, interacts with microfilaments and microtubules. Although specifically expressed formin genes in anthers are potentially significant in research on male sterility and hybrid wheat breeding, similar reports in wheat, especially in thermo-sensitive genic male sterile (TGMS) wheat, remain elusive.

Results

Herein, we systematically characterized the formin genes in TGMS wheat line BS366 named TaFormins (TaFHs) and predicted their functions in inducing stress response. In total, 25 TaFH genes were uncovered, majorly localized in 2A, 2B, and 2D chromosomes. According to the neighbor-joining (NJ) method, all TaFH proteins from wheat and other plants clustered in 6 sub-groups (A-F). The modeled 3D structures of TaFH1-A/B, TaFH2-A/B, TaFH3-A/B and TaFH3-B/D were validated. And different numbers of stress and hormone-responsive regulatory elements in their 1500 base pair promoter regions were contained in the TaFH genes copies. TaFHs had specific temporal and spatial expression characteristics, whereby TaFH1, TaFH4, and TaFH5 were expressed highly in the stamen of BS366. Besides, the accumulation of TaFHs was remarkably lower in a low-temperature sterile condition (Nanyang) than fertile condition (Beijing), particularly at the early stamen development stage. The pollen cytoskeleton of BS366 was abnormal in the three stages under sterile and fertile environments. Furthermore, under different stress levels, TaFHs expression could be induced by drought, salt, abscisic acid (ABA), salicylic acid (SA), methyl jasmonate (MeJA), indole-3-acetic acid (IAA), polyethylene glycol (PEG), and low temperature. Some miRNAs, including miR167, miR1120, and miR172, interacts with TaFH genes; thus, we constructed an interaction network between microRNAs, TaFHs, phytohormone responses, and distribution of cytoskeleton to reveal the regulatory association between upstream genes of TaFH family members and sterile.

Conclusions

Collectively, this comprehensive analysis provides novel insights into TaFHs and miRNA resources for wheat breeding. These findings are, therefore, valuable in understanding the mechanism of TGMS fertility conversion in wheat.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07878-7.

Heterologous Expression and Auto-Activation of Human Pro-Inflammatory Caspase-1 in Saccharomyces cerevisiae and Comparison to Caspase-8

Caspases are a family of cysteine proteases that play an essential role in inflammation, apoptosis, cell death, and development. Here we delve into the effects caused by heterologous expression of human caspase-1 in the yeast Saccharomyces cerevisiae and compare them to those of caspase-8. Overexpression of both caspases in the heterologous model led to their activation and caused mitochondrial hyperpolarization, damage to different organelles, and cell death. All these effects were dependent on their protease activity, and caspase-8 was more aggressive than caspase-1. Growth arrest could be at least partially explained by dysfunction of the actin cytoskeleton as a consequence of the processing of the yeast Bni1 formin, which we identify here as a likely direct substrate of both caspases. Through the modulation of the GAL1 promoter by using different galactose:glucose ratios in the culture medium, we have established a scenario in which caspase-1 is sufficiently expressed to become activated while yeast growth is not impaired. Finally, we used the yeast model to explore the role of death-fold domains (DD) of both caspases in their activity. Peculiarly, the DDs of either caspase showed an opposite involvement in its intrinsic activity, as the deletion of the caspase activation and recruitment domain (CARD) of caspase-1 enhanced its activity, whereas the deletion of the death effector domain (DED) of caspase-8 diminished it. We show that caspase-1 is able to efficiently process its target gasdermin D (GSDMD) when co-expressed in yeast. In sum, we propose that S. cerevisiae provides a manageable tool to explore caspase-1 activity and structure–function relationships.

Sp1-Induced FNBP1 Drives Rigorous 3D Cell Motility in EMT-Type Gastric Cancer Cells

Cancer is heterogeneous among patients, requiring a thorough understanding of molecular subtypes and the establishment of therapeutic strategies based on its behavior. Gastric cancer (GC) is adenocarcinoma with marked heterogeneity leading to different prognoses. As an effort, we previously identified a stem-like subtype, which is prone to metastasis, with the worst prognosis. Here, we propose FNBP1 as a key to high-level cell motility, present only in aggressive GC cells. FNBP1 is also up-regulated in both the GS subtype from the TCGA project and the EMT subtype from the ACRG study, which include high portions of diffuse histologic type. Ablation of FNBP1 in the EMT-type GC cell line brought changes in the cell periphery in transcriptomic analysis. Indeed, loss of FNBP1 resulted in the loss of invasive ability, especially in a three-dimensional culture system. Live imaging indicated active movement of actin in FNBP1-overexpressed cells cultured in an extracellular matrix dome. To find the transcription factor which drives FNBP1 expression in an EMT-type GC cell line, the FNBP1 promoter region and DNA binding motifs were analyzed. Interestingly, the Sp1 motif was abundant in the promoter, and pharmacological inhibition and knockdown of Sp1 down-regulated FNBP1 promoter activity and the transcription level, respectively. Taken together, our results propose Sp1-driven FNBP1 as a key molecule explaining aggressiveness in EMT-type GC cells.

Bidirectional feedback between BCR signaling and actin cytoskeletal dynamics

When B cells are exposed to antigens, they use their B-cell receptors (BCRs) to transduce this external signal into internal signaling cascades and uptake antigen, which activate transcriptional programs. Signaling activation requires complex cytoskeletal remodeling initiated by BCR signaling. The actin cytoskeletal remodeling drives B-cell morphological changes, such as spreading, protrusion, contraction, and endocytosis of antigen by mechanical forces, which in turn affect BCR signaling. Therefore, the relationship between the actin cytoskeleton and BCR signaling is a two-way feedback loop. These morphological changes represent the indirect ways by which the actin cytoskeleton regulates BCR signaling. Recent studies using high spatiotemporal resolution microscopy techniques have revealed that actin also can directly influence BCR signaling. Cortical actin networks directly affect BCR mobility, not only during the resting stage by serving as diffusion barriers, but also at the activation stage by altering BCR diffusivity through enhanced actin flow velocities. Furthermore, the actin cytoskeleton, along with myosin, enables B cells to sense the physical properties of its environment and generate and transmit forces through the BCR. Consequently, the actin cytoskeleton modulates the signaling threshold of BCR to antigenic stimulation. This review discusses the latest research on the relationship between BCR signaling and actin remodeling, and the research techniques. Exploration of the role of actin in BCR signaling will expand fundamental understanding of the relationship between cell signaling and the cytoskeleton and the mechanisms underlying cytoskeleton-related immune disorders and cancer.

Remodelling of membrane tubules by the actin cytoskeleton

Inside living cells, the remodelling of membrane tubules by actomyosin networks is crucial for processes such as intracellular trafficking or organelle reshaping. In this review, we first present various in vivo situations in which actin affects membrane tubule remodelling, then we recall some results on force production by actin dynamics and on membrane tubules physics. Finally, we show that our knowledge of the underlying mechanisms by which actomyosin dynamics affect tubule morphology has recently been moved forward. This is thanks to in vitro experiments that mimic cellular membranes and actin dynamics and allow deciphering the physics of tubule remodelling in biochemically controlled conditions, and shed new light on tubule shape regulation.

Developmental axon regrowth and primary neuron sprouting utilize distinct actin elongation factors

Intrinsic neurite growth potential is a key determinant of neuronal regeneration efficiency following injury. The stereotypical remodeling of Drosophila γ-neurons includes developmental regrowth of pruned axons to form adult specific connections, thereby offering a unique system to uncover growth potential regulators. Motivated by the dynamic expression in remodeling γ-neurons, we focus here on the role of actin elongation factors as potential regulators of developmental axon regrowth. We found that regrowth in vivo requires the actin elongation factors Ena and profilin, but not the formins that are expressed in γ-neurons. In contrast, primary γ-neuron sprouting in vitro requires profilin and the formin DAAM, but not Ena. Furthermore, we demonstrate that DAAM can compensate for the loss of Ena in vivo. Similarly, DAAM mutants express invariably high levels of Ena in vitro. Thus, we show that different linear actin elongation factors function in distinct contexts even within the same cell type and that they can partially compensate for each other.

Polarisome assembly mediates actin remodeling during polarized yeast and fungal growth

Dynamic assembly and remodeling of actin is critical for many cellular processes during development and stress adaptation. In filamentous fungi and budding yeast, actin cables align in a polarized manner along the mother-to-daughter cell axis, and are essential for the establishment and maintenance of polarity; moreover, they rapidly remodel in response to environmental cues to achieve an optimal system response. A formin at the tip region within a macromolecular complex, called the polarisome, is responsible for driving actin cable polymerization during polarity establishment. This polarisome undergoes dynamic assembly through spatial and temporally regulated interactions between its components. Understanding this process is important to comprehend the tuneable activities of the formin-centered nucleation core, which are regulated through divergent molecular interactions and assembly modes within the polarisome. In this Review, we focus on how intrinsically disordered regions (IDRs) orchestrate the condensation of the polarisome components and the dynamic assembly of the complex. In addition, we address how these components are dynamically distributed in and out of the assembly zone, thereby regulating polarized growth. We also discuss the potential mechanical feedback mechanisms by which the force-induced actin polymerization at the tip of the budding yeast regulates the assembly and function of the polarisome.

Cytoskeletal regulation of dendritic cells: An intricate balance between migration and presentation for tumor therapy

Dendritic cells (DCs) are the main players in many approaches for cancer therapy. The idea with DC tumor therapy is to promote activation of tumor infiltrating cytotoxic T cells that kill tumor cells. This requires that DCs take up tumor Ag and present peptides on MHC class I molecules in a process called cross-presentation. For this process to be efficient, DCs have to migrate to the tumor draining lymph node and there activate the machinery for cross-presentation. In this review, we will discuss recent progress in understanding the role of actin regulators for control of DC migration and Ag presentation. The potential to target actin regulators for better DC-based tumor therapy will also be discussed.

In vivo analysis of formin dynamics in the moss P. patens reveals functional class diversification

Formins are actin regulators critical for diverse processes across eukaryotes. With many formins in plants and animals, it has been challenging to determine formin function in vivo We found that the phylogenetically distinct class I integral membrane formins (denoted For1) from the moss P.patens enrich at sites of membrane turnover, with For1D more tightly associated with the plasma membrane than For1A. To probe formin function, we generated formin-null lines with greatly reduced formin complexity. We found that For1A and For1D help to anchor actin near the cell apex, with For1A contributing to formation of cytosolic actin, while For1D contributes to plasma membrane-associated actin. At the cortex, For1A and For1D localized to motile puncta and differentially impacted actin dynamics. We found that class I cortical formin mobility depended on microtubules and only moderately on actin, whereas class II formin (denoted For2) mobility solely depended on actin. Moreover, cortical For2A tightly correlated with the puncta labeled by the endocytic membrane dye FM4-64, and null mutants in class I formins did not affect uptake of a similar dye, FM1-43, suggesting that class I and II formins are involved in distinct membrane trafficking pathways.

CREPT Promotes Melanoma Progression Through Accelerated Proliferation and Enhanced Migration by RhoA-Mediated Actin Filaments and Focal Adhesion Formation

Melanoma is one of the most aggressive cancers, and patients with distant metastases have dire outcomes. We observed previously that melanoma progression is driven by a high migratory potential of melanoma cells, which survive and proliferate under harsh environmental conditions. In this study, we report that CREPT (cell-cycle related and expression-elevated protein in tumor), an oncoprotein highly expressed in other cancers, is overexpressed in melanoma cells but not melanocytes. Overexpression of CREPT stimulates cell proliferation, migration, and invasion in several melanoma cell lines. Further, we show that CREPT enhances melanoma progression through upregulating and activating Ras homolog family member A (RhoA)-induced actin organization and focal adhesion assembly. Our study reveals a novel role of CREPT in promoting melanoma progression. Targeting CREPT may be a promising strategy for melanoma treatment.

Different formins restrict localization of distinct tropomyosins on dorsal stress fibers in osteosarcoma cells

Formins and tropomyosins (Tpms) are two central components of the microfilaments. Formins are involved in the nucleation and polymerization of actin filaments, and Tpms form along the actin stress fibers to regulate their dynamics. However, the correlation between formins and Tpms remains unclear. Here, we elucidated the function of distinct formins and their specific regulation to the subcellular-localization of Tpm isoforms on dorsal stress fibers in human osteosarcoma cells. Knockdown of individual formin isoform led to varied defects in actin stress fiber network, but did not affect the expression level of other formin isoforms and Tpms. Further investigation showed that different formins regulated distinct Tpm isoforms in decorating dorsal stress fibers. Specifically, DAAM1 and FHOD1 restricted the distal end expression of Tpm3.1; INF2 controlled the approximate localization of Tpm4.2; and Dia1 partially modulated the dorsal localization of Tpm1.6. Taken together, these data provide microscopy experimental evidence that different formins restrict the localization of distinct Tpm isoforms on dorsal actin stress fibers.

PGE2 mediates cytoskeletal rearrangement of hemocytes via Cdc42, a small G protein, to activate actin-remodeling factors in Spodoptera exigua (Lepidoptera: Noctuidae)

Prostaglandin E₂ (PGE₂ ) mediates cellular immune responses in insects by stimulating hemocyte-spreading behavior that is driven by actin remodeling to form filopodial or lamellipodial cytoplasmic extensions. In Spodoptera exigua (Lepidoptera: Noctuidae), Cdc42, a small G protein, played a crucial role in mediating PGE₂ signal on hemocyte-spreading behavior. Hemocyte-spreading behavior requires actin cytoskeletal rearrangement. A plethora of actin-related proteins have been predicted to have functional links with Cdc42. Here, we selected four actin-associated genes (Actin-related protein 2 [Arp2], Profilin, Cofilin, and Fascin) and evaluated their influences on cytoskeletal rearrangement in S. exigua. Bioinformatic analysis confirmed their gene identities. Transcript analysis using reverse-transcription polymerase chain reaction indicated that all four actin-associated genes were expressed in most developmental stages, showing high expression levels in larval hemocytes. RNA interference (RNAi) against these genes was performed by injecting double-stranded RNA (dsRNA) to hemocoel. Under RNAi condition, the hemocyte-spreading behavior was significantly impaired except for dsRNA treatment against Cofilin, an actin-depolymerizing factor. Alteration of cytoskeletal rearrangement appeared to vary after different RNAi treatments. RNAi against Arp2 markedly suppressed lamellipodial extension while RNAi against Profilin or Fascin adversely influenced filopodial extension. RNAi of these actin-associated factors prevented cellular immune responses measured by nodule formation against bacterial challenge. Under RNAi conditions, addition of PGE₂ did not well induce hemocyte-spreading behavior, suggesting that these actin-associated factors might act downstream of the hormone signaling pathway. These results suggest that PGE₂ can mediate hemocyte-spreading behavior via Cdc42 to activate downstream actin polymerization/branching/bundling factors, thus inducing actin cytoskeletal rearrangement.

CCN1 interlinks integrin and hippo pathway to autoregulate tip cell activity

CCN1 (CYR61) stimulates active angiogenesis in various tumours, although the mechanism is largely unknown. Here, we report that CCN1 is a key regulator of endothelial tip cell activity in angiogenesis. Microvessel networks and directional vascular cell migration patterns were deformed in ccn1-knockdown zebrafish embryos. CCN1 activated VEGFR2 and downstream MAPK/PI3K signalling pathways, YAP/TAZ, as well as Rho effector mDia1 to enhance tip cell activity and CCN1 itself. VEGFR2 interacted with integrin αvβ3 through CCN1. Integrin αvβ3 inhibitor repressed tip cell number and sprouting in postnatal retinas from endothelial cell-specific Ccn1 transgenic mice, and allograft tumours in Ccn1 transgenic mice showed hyperactive vascular sprouting. Cancer patients with high CCN1 expression have poor survival outcomes and positive correlation with ITGAV and ITGB3 and high YAP/WWTR1. Thus, our data underscore the positive feedback regulation of tip cells by CCN1 through integrin αvβ3/VEGFR2 and increased YAP/TAZ activity, suggesting a promising therapeutic intervention for pathological angiogenesis.

Autonomous dynamic control of DNA nanostructure self-assembly

Biological cells routinely reconfigure their shape using dynamic signalling and regulatory networks that direct self-assembly processes in time and space, through molecular components that sense, process and transmit information from the environment. A similar strategy could be used to enable life-like behaviours in synthetic materials. Nucleic acid nanotechnology offers a promising route towards this goal through a variety of sensors, logic and dynamic components and self-assembling structures. Here, by harnessing both dynamic and structural DNA nanotechnology, we demonstrate dynamic control of the self-assembly of DNA nanotubes-a well-known class of programmable DNA nanostructures. Nanotube assembly and disassembly is controlled with minimal synthetic gene systems, including an autonomous molecular oscillator. We use a coarse-grained computational model to capture nanotube length distribution dynamics in response to inputs from nucleic acid circuits. We hope that these results may find use for the development of responsive nucleic acid materials, with potential applications in biomaterials science, nanofabrication and drug delivery.

Aspergillus nidulans protein kinase C forms a complex with the formin SepA that is involved in apical growth and septation

The Aspergillus nidulans orthologue of Protein kinase C (PkcA) and the A. nidulans formin SepA participate in polarized growth. PkcA localizes to growing hyphal apices and septation sites, and amino acid sequences within PkcA that are required for PkcA to localize to these sites of cell wall synthesis have been identified. SepA is associated with the contractile actomyosin ring (CAR), and it localizes at hyphal tips in association with the Spitzenkörper (SPK) and as an apical dome. A mutation in the sepA gene (sepA1) renders A. nidulans aseptate at elevated temperature. Progress towards understanding the spatiotemporal relationship between PkcA and SepA during polarized growth is presented here. Fluorescent chimeras of PkcA and SepA strongly overlapped in some hyphal tips in a dome pattern, while other tips displayed SepA SPK and PkcA dome localization within the same tip. At septation sites PkcA and SepA consistently colocalized through late stages of CAR constriction. Bimolecular fluorescence complementation experimental results provide evidence that SepA and PkcA are both present in complexes at both hyphal tip domes and at cortical rings. A Gal4-based yeast two-hybrid analysis confirmed the physical interaction between SepA and PkcA, and indicted that the FH2 domain of SepA is involved in its physical interaction with PkcA. A functional interaction between PkcA and SepA was shown through complementation of the pkcA calC2 mutant's hypersensitivity to cell wall perturbing agents by overexpressed sepA and by the ability of the sepA1 mutation to block PkcA's ability to form cortical rings. Taken together these results suggest that a PkcA/SepA complex is involved in polarized growth. Through experiments using the actin disrupter latrunculin B, evidence is presented suggesting that actin plays a role in the PkcA/SepA complex.

Helical rotation of the diaphanous-related formin mDia1 generates actin filaments resistant to cofilin

The complex interplay between actin regulatory proteins facilitates the formation of diverse cellular actin structures. Formin homology proteins (formins) play an essential role in the formation of actin stress fibers and yeast actin cables, to which the major actin depolymerizing factor cofilin barely associates. In vitro, F-actin decorated with cofilin exhibits a marked increase in the filament twist. On the other hand, a mammalian formin mDia1 rotates along the long-pitch actin helix during processive actin elongation (helical rotation). Helical rotation may impose torsional force on F-actin in the opposite direction of the cofilin-induced twisting. Here, we show that helical rotation of mDia1 converts F-actin resistant to cofilin both in vivo and in vitro. F-actin assembled by mDia1 without rotational freedom became more resistant to the severing and binding activities of cofilin than freely rotatable F-actin. Electron micrographic analysis revealed untwisting of the long-pitch helix of F-actin elongating from mDia1 on tethering of both mDia1 and the pointed end side of the filament. In cells, single molecules of mDia1ΔC63, an activated mutant containing N-terminal regulatory domains, showed tethering to cell structures more frequently than autoinhibited wild-type mDia1 and mDia1 devoid of N-terminal domains. Overexpression of mDia1ΔC63 induced the formation of F-actin, which has prolonged lifetime and accelerates dissociation of cofilin. Helical rotation of formins may thus serve as an F-actin stabilizing mechanism by which a barbed end-bound molecule can enhance the stability of a filament over a long range.

First evidence of DAAM1 localization in mouse seminal vesicles and its possible involvement during regulated exocytosis

Dishevelled-associated activator of morphogenesis 1 (DAAM1) is a protein belonging to the formin family, which regulates, together with the small GTPase RhoA, the nucleation and the assembly of actin fibres through Wnt-Dishevelled PCP pathway. Its role has been investigated in essential biological processes, such as cell polarity, movement and adhesion during morphogenesis and organogenesis. In this work, we studied the expression of DAAM1 mRNA and protein by PCR and Western blot analyses and its co-localization with actin in adult mouse seminal vesicles by immunofluorescence. We show that both proteins are cytoplasmic: actin is evident at cell-cell junctions and at cell cortex; DAAM1 had a more diffused localization, but is also prominent at the apical plasmatic membrane of epithelial cells. These findings support our hypothesis of a role of DAAM1 in cytoskeletal rearrangement that occurs during the exocytosis of secretory vesicles, and in particular concerning actin filaments. We were also able to detect DAAM1 and actin association in the smooth muscle cells that surround the epithelium too. In this case, we could only speculate the possible involvement of this formin in muscular cells in the maintenance and the regulation of the contractile structures. The present results strongly suggest that DAAM1 could have a pivotal role in vesicle exocytosis and in the physiology of mouse seminal vesicles.

Evidence for a novel negative-stranded RNA mycovirus isolated from the plant pathogenic fungus Fusarium graminearum

Here we describe a novel (-)ssRNA mycovirus, Fusarium graminearum negative-stranded RNA virus 1 (FgNSRV-1), isolated from Fusarium graminearum strain HN1. The genome of FgNSRV-1 is 9072 nucleotides in length, with five discontinuous but linear ORFs (ORF I-V). Phylogenetic analysis based on entire L polymerase sequences indicated that FgNSRV-1 is related to the (-)ssRNA mycovirus Sclerotinia sclerotiorum negative-stranded RNA virus 1 (SsNSRV-1), and other mycoviruses. Our data suggest that FgNSRV-1 can be classified into the family Mymonaviridae, order Mononegavirales. Putative enveloped virion-like structures with filamentous morphology similar to SsNSRV-1 were observed in virion preparation samples. The L proteins of FgNSRV-1, and other fungal mononegaviruses, were found to be related to L protein-like sequences in some fungal genome, supporting the hypothesis that there is coevolution occurring between mycoviruses and fungi. Besides, clearing the virus from the infected host fungus resulted in no discernable phenotypic change.

Detection of candidate genes for growth and carcass traits using genome-wide association strategy in Chinese Simmental beef cattle

In Chinese beef cattle industry, there are more than 60 million livestock, nearly half of which are Chinese Simmental beef cattle or Simmental crossbreds. Over the past decades, numerous quantitative trait loci for economic traits in cattle have been identified, while few studies for growth and carcass traits have been reported in Simmental beef cattle. In the present study, we conducted genome-wide association study based on BovineHD BeadChip and identified 41, 15, 3, 22 and 16 single-nucleotide polymorphisms significantly associated with average daily gain, liveweight before slaughter, carcass weight, dressing percentage and pure meat percentage respectively. In total, 18 candidate genes were found for growth and carcass traits, and four haplotype blocks for growth and carcass traits were discovered. These findings will facilitate detection of major genes and genetic variants involved in growth and carcass traits of beef cattle in further studies.

Embryonic chirality and the evolution of spiralian left-right asymmetries

The group Spiralia includes species with one of the most significant cases of left–right asymmetries in animals: the coiling of the shell of gastropod molluscs (snails). In this animal group, an early event of embryonic chirality controlled by cytoskeleton dynamics and the subsequent differential activation of the genes nodal and Pitx determine the left–right axis of snails, and thus the direction of coiling of the shell. Despite progressive advances in our understanding of left–right axis specification in molluscs, little is known about left–right development in other spiralian taxa. Here, we identify and characterize the expression of nodal and Pitx orthologues in three different spiralian animals—the brachiopod Novocrania anomala, the annelid Owenia fusiformis and the nemertean Lineus ruber—and demonstrate embryonic chirality in the biradial-cleaving spiralian embryo of the bryozoan Membranipora membranacea. We show asymmetric expression of nodal and Pitx in the brachiopod and annelid, respectively, and symmetric expression of Pitx in the nemertean. Our findings indicate that early embryonic chirality is widespread and independent of the cleavage programme in the Spiralia. Additionally, our study illuminates the evolution of nodal and Pitx signalling by demonstrating embryonic asymmetric expression in lineages without obvious adult left–right asymmetries.

This article is part of the themed issue ‘Provocative questions in left–right asymmetry’.

Plasticity Related Gene 3 (PRG3) overcomes myelin-associated growth inhibition and promotes functional recovery after spinal cord injury

The Plasticity Related Gene family covers five, brain-specific, transmembrane proteins (PRG1-5, also termed LPPR1-5) that operate in neuronal plasticity during development, aging and brain trauma. Here we investigated the role of the PRG family on axonal and filopodia outgrowth. Comparative analysis revealed the strongest outgrowth induced by PRG3 (LPPR1). During development, PRG3 is ubiquitously located at the tip of neuronal processes and at the plasma membrane and declines with age. In utero electroporation of PRG3 induced dendritic protrusions and accelerated spine formations in cortical pyramidal neurons. The neurite growth promoting activity of PRG3 requires RasGRF1 (RasGEF1/Cdc25) mediated downstream signaling. Moreover, in axon collapse assays, PRG3-induced neurites resisted growth inhibitors such as myelin, Nogo-A (Reticulon/RTN-4), thrombin and LPA and impeded the RhoA-Rock-PIP5K induced neurite repulsion. Transgenic adult mice with constitutive PRG3 expression displayed strong axonal sprouting distal to a spinal cord lesion. Moreover, fostered PRG3 expression promoted complex motor-behavioral recovery compared to wild type controls as revealed in the Schnell swim test (SST). Thus, PRG3 emerges as a developmental RasGRF1-dependent conductor of filopodia formation and axonal growth enhancer. PRG3-induced neurites resist brain injury-associated outgrowth inhibitors and contribute to functional recovery after spinal cord lesions. Here, we provide evidence that PRG3 operates as an essential neuronal growth promoter in the nervous system. Maintaining PRG3 expression in aging brain may turn back the developmental clock for neuronal regeneration and plasticity.

Quantitative proteomics of Sf21 cells during Baculovirus infection reveals progressive host proteome changes and its regulation by viral miRNA

System level knowledge of alterations in host is crucial to elucidate the molecular events of viral pathogenesis and to develop strategies to block viral establishment and amplification. Here, we applied quantitative proteomics approach to study global proteome changes in the host; Spodoptera frugiperda upon infection by a baculovirus, Spodoptera litura NPV at two stages i.e. 12 h and 72 h post infection. At 12 hpi, >95% of host proteins remained stable, however at 72 hpi, 52% host proteins exhibited downregulation of 2-fold or more. Functional analysis revealed significant upregulation of transposition and proteasomal machinery while translation, transcription, protein export and oxidative phosphorylation pathways were adversely affected. An assessment of perturbed proteome after viral infection and viral miRNA expression led to the identification of 117 genes that are potential targets of 10 viral miRNAs. Using miRNA mimics, we confirmed the down regulation of 9 host genes. The results comprehensively show dynamics of host responses after viral infection.

Steric Effects Induce Geometric Remodeling of Actin Bundles in Filopodia

Filopodia are ubiquitous fingerlike protrusions, spawned by many eukaryotic cells, to probe and interact with their environments. Polymerization dynamics of actin filaments, comprising the structural core of filopodia, largely determine their instantaneous lengths and overall lifetimes. The polymerization reactions at the filopodial tip require transport of G-actin, which enter the filopodial tube from the filopodial base and diffuse toward the filament barbed ends near the tip. Actin filaments are mechanically coupled into a tight bundle by cross-linker proteins. Interestingly, many of these proteins are relatively short, restricting the free diffusion of cytosolic G-actin throughout the bundle and, in particular, its penetration into the bundle core. To investigate the effect of steric restrictions on G-actin diffusion by the porous structure of filopodial actin filament bundle, we used a particle-based stochastic simulation approach. We discovered that excluded volume interactions result in partial and then full collapse of central filaments in the bundle, leading to a hollowed-out structure. The latter may further collapse radially due to the activity of cross-linking proteins, hence producing conical-shaped filament bundles. Interestingly, electron microscopy experiments on mature filopodia indeed frequently reveal actin bundles that are narrow at the tip and wider at the base. Overall, our work demonstrates that excluded volume effects in the context of reaction-diffusion processes in porous networks may lead to unexpected geometric growth patterns and complicated, history-dependent dynamics of intermediate metastable configurations.

TheCellMap.org: A Web-Accessible Database for Visualizing and Mining the Global Yeast Genetic Interaction Network

Providing access to quantitative genomic data is key to ensure large-scale data validation and promote new discoveries. TheCellMap.org serves as a central repository for storing and analyzing quantitative genetic interaction data produced by genome-scale Synthetic Genetic Array (SGA) experiments with the budding yeast Saccharomyces cerevisiae. In particular, TheCellMap.org allows users to easily access, visualize, explore, and functionally annotate genetic interactions, or to extract and reorganize subnetworks, using data-driven network layouts in an intuitive and interactive manner.

Cell-matrix adhesion and cell-cell adhesion differentially control basal myosin oscillation and Drosophila egg chamber elongation

Pulsatile actomyosin contractility, important in tissue morphogenesis, has been studied mainly in apical but less in basal domains. Basal myosin oscillation underlying egg chamber elongation is regulated by both cell–matrix and cell–cell adhesions. However, the mechanism by which these two adhesions govern basal myosin oscillation and tissue elongation is unknown. Here we demonstrate that cell–matrix adhesion positively regulates basal junctional Rho1 activity and medio-basal ROCK and myosin activities, thus strongly controlling tissue elongation. Differently, cell–cell adhesion governs basal myosin oscillation through controlling medio-basal distributions of both ROCK and myosin signals, which are related to the spatial limitations of cell–matrix adhesion and stress fibres. Contrary to cell–matrix adhesion, cell–cell adhesion weakly affects tissue elongation. In vivo optogenetic protein inhibition spatiotemporally confirms the different effects of these two adhesions on basal myosin oscillation. This study highlights the activity and distribution controls of basal myosin contractility mediated by cell–matrix and cell–cell adhesions, respectively, during tissue morphogenesis.

Pulsatile actomyosin contractility during tissue morphogenesis has been mainly studied in apical domains but less is known about the contribution of the basal domain. Here the authors show differential influence of cell-matrix and cell-cell adhesions in regulating oscillations and tissue elongation.

mDia2 and CXCL12/CXCR4 chemokine signaling intersect to drive tumor cell amoeboid morphological transitions

Morphological plasticity in response to environmental cues in migrating cancer cells requires F-actin cytoskeletal rearrangements. Conserved formin family proteins play critical roles in cell shape, tumor cell motility, invasion and metastasis, in part, through assembly of non-branched actin filaments. Diaphanous-related formin-2 (mDia2/Diaph3/Drf3/Dia) regulates mesenchymal-to-amoeboid morphological conversions and non-apoptotic blebbing in tumor cells by interacting with its inhibitor diaphanous-interacting protein (DIP), and disrupting cortical F-actin assembly and bundling. F-actin disruption is initiated by a CXCL12-dependent mechanism. Downstream CXCL12 signaling partners inducing mDia2-dependent amoeboid conversions remain enigmatic. We found in MDA-MB-231 tumor cells CXCL12 induces DIP and mDia2 interaction in blebs, and engages its receptor CXCR4 to induce RhoA-dependent blebbing. mDia2 and CXCR4 associate in blebs upon CXCL12 stimulation. Both CXCR4 and RhoA are required for CXCL12-induced blebbing. Neither CXCR7 nor other Rho GTPases that activate mDia2 are required for CXCL12-induced blebbing. The Rho Guanine Nucleotide Exchange Factor (GEF) Net1 is required for CXCL12-driven RhoA activation and subsequent blebbing. These results reveal CXCL12 signaling, through CXCR4, directs a Net1/RhoA/mDia-dependent signaling hub to drive cytoskeleton rearrangements to regulate morphological plasticity in tumor cells. These signaling hubs may be conserved during normal and cancer cells responding to chemotactic cues.

Mechanisms of Cytokinesis in Basidiomycetous Yeasts

While mechanisms of cytokinesis exhibit considerable plasticity, it is difficult to precisely define the level of conservation of this essential part of cell division in fungi, as majority of our knowledge is based on ascomycetous yeasts. However, in the last decade more details have been uncovered regarding cytokinesis in the second largest fungal phylum, basidiomycetes, specifically in two yeasts, Cryptococcus neoformans and Ustilago maydis. Based on these findings, and current sequenced genomes, we summarize cytokinesis in basidiomycetous yeasts, indicating features that may be unique to this phylum, species-specific characteristics, as well as mechanisms that may be common to all eukaryotes.

Remodeling of the Inner Hair Cell Microtubule Meshwork in a Mouse Model of Auditory Neuropathy AUNA1

Auditory neuropathy 1 (AUNA1) is a form of human deafness resulting from a point mutation in the 5′ untranslated region of the Diaphanous homolog 3 (DIAPH3) gene. Notably, the DIAPH3 mutation leads to the overexpression of the DIAPH3 protein, a formin family member involved in cytoskeleton dynamics. Through study of diap3-overexpressing transgenic (Tg) mice, we examine in further detail the anatomical, functional, and molecular mechanisms underlying AUNA1. We identify diap3 as a component of the hair cells apical pole in wild-type mice. In the diap3-overexpressing Tg mice, which show a progressive threshold shift associated with a defect in inner hair cells (IHCs), the neurotransmitter release and potassium conductances are not affected. Strikingly, the overexpression of diap3 results in a selective and early-onset alteration of the IHC cuticular plate. Molecular dissection of the apical components revealed that the microtubule meshwork first undergoes aberrant targeting into the cuticular plate of Tg IHCs, followed by collapse of the stereociliary bundle, with eventual loss of the IHC capacity to transmit incoming auditory stimuli.

Live Cell Imaging of Actin Dynamics in the Filamentous Fungus Aspergillus nidulans

Hyphal cells of filamentous fungi grow at their tips in a method analogous to pollen tube and root hair elongation. This process, generally referred to as tip growth, requires precise regulation of the actin cytoskeleton, and characterizing the various actin structures in these cell types is currently an active area of research. Here, the actin marker Lifeact was used to document actin dynamics in the filamentous fungus Aspergillus nidulans. Contractile double rings were observed at septa, and annular clusters of puncta were seen subtending growing hyphal tips, corresponding to the well-characterized subapical endocytic collar. However, Lifeact also revealed two additional structures. One, an apical array, was dynamic on the face opposite the tip, while a subapical web was dynamic on the apical face and was located several microns behind the growth site. Each was observed turning into the other over time, implying that they could represent different localizations of the same structure, although hyphae with a subapical web grew faster than those exhibiting an apical array. The subapical web has not been documented in any filamentous fungus to date, and is separate from the networks of F-actin seen in other tip-growing organisms surrounding septa or stationary along the plasmalemma.

Slit2N Inhibits Transmission of HIV-1 from Dendritic Cells to T-cells by Modulating Novel Cytoskeletal Elements

Dendritic cells are among the first cells to encounter sexually acquired human immunodeficiency virus (HIV-1), in the mucosa, and they can transmit HIV-1 to CD4(+) T-cells via an infectious synapse. Recent studies reveal that actin-rich membrane extensions establish direct contact between cells at this synapse and facilitate virus transmission. Genesis of these contacts involves signaling through c-Src and Cdc42, which modulate actin polymerization and filopodia formation via the Arp2/3 complex and Diaphanous 2 (Diaph2). We found that Slit2N, a ligand for the Roundabout (Robo) receptors, blocked HIV-1-induced signaling through Arp2/3 and Diaph2, decreased filopodial extensions on dendritic cells, and inhibited cell-to-cell transmission of HIV-1 in a Robo1-dependent manner. Employing proteomic analysis, we identified Flightless-1 as a novel, Robo1-interacting protein. Treatment with shRNAs reduced levels of Flightless-1 and demonstrated its role in efficient cell-to-cell transfer of HIV-1. These results suggest a novel strategy to limit viral infection in the host by targeting the Slit/Robo pathway with modulation of cytoskeletal elements previously unrecognized in HIV-1 transmission.

RhoA-mediated FMNL1 regulates GM130 for actin assembly and phosphorylates MAPK for spindle formation in mouse oocyte meiosis

Formin-like 1 (FMNL1) is a member of Formin family proteins which are the actin nucleators. Although FMNL1 activities have been shown to be essential for cell adhesion, cytokinesis, cell polarization and migration in mitosis, the functional roles of mammalian FMNL1 during oocyte meiosis remain uncertain. In this study, we investigated the functions of FMNL1 in mouse oocytes using specific morpholino (MO) microinjection and live cell imaging. Immunofluorescent staining showed that in addition to its cytoplasmic distribution, FMNL1 was primarily localized at the spindle poles after germinal vesicle breakdown (GVBD). FMNL1 knockdown caused the low rate of polar body extrusion and resulted in large polar bodies. Time-lapse microscopic and immunofluorescence intensity analysis indicated that this might be due to the aberrant actin expression levels. Cortical polarity was disrupted as shown by a loss of actin cap and cortical granule free domain (CGFD) formation, which was confirmed by a failure of meiotic spindle positioning. And this might be the reason for the large polar body formation. Spindle formation was also disrupted, which might be due to the abnormal localization of p-MAPK. These results indicated that FMNL1 affected both actin dynamics and spindle formation for the oocyte polar body extrusion. Moreover, FMNL1 depletion resulted in aberrant localization and expression patterns of a cis-Golgi marker protein, GM130. Finally, we found that the small GTPase RhoA might be the upstream regulator of FMNL1. Taken together, our data indicate that FMNL1 is required for spindle organization and actin assembly through a RhoA-FMNL1-GM130 pathway during mouse oocyte meiosis.

Wattles in goats are associated with the FMN1/GREM1 region on chromosome 10

The presence of congenital appendages (wattles) on the throat of goats is supposed to be under genetic control with a dominant mode of inheritance. Wattles contain a cartilaginous core covered with normal skin resembling early stages of extremities. To map the dominant caprine wattles (W) locus, we collected samples of 174 goats with wattles and 167 goats without wattles from nine different Swiss goat breeds. The samples were genotyped with the 53k goat SNP chip for a subsequent genome-wide association study. We obtained a single strong association signal on chromosome 10 in a region containing functional candidate genes for limb development and outgrowth. We sequenced the whole genomes of an informative family trio containing an offspring without wattles and its heterozygous parents with wattles. In the associated goat chromosome 10 region, a total of 1055 SNPs and short indels perfectly co-segregate with the W allele. None of the variants were perfectly associated with the phenotype after analyzing the genome sequences of eight additional goats. We speculate that the causative mutation is located in one of the numerous gaps in the current version of the goat reference sequence and/or represents a larger structural variant which influences the expression of the FMN1 and/or GREM1 genes. Also, we cannot rule out possible genetic or allelic heterogeneity. Our genetic findings support earlier assumptions that wattles are rudimentary developed extremities.

Rotational movement of formins evaluated by using single-molecule fluorescence polarization

Formin homology proteins (formins) are responsible for the formation of actin structures such as actin stress fibers, actin cables, and cytokinetic contractile rings. Formins are the major actin filament (F-actin) nucleators in the cell. Because formins remain bound to the barbed end after nucleating an actin filament, it was expected that formins might rotate along the double-helical structure of F-actin during processive actin elongation (helical rotation). Here, we describe a method to detect the rotational movement of F-actin elongating from immobilized formins using single-molecule fluorescence polarization (FLP). Tetramethylrhodamine (TMR) attached to Cys-374 of actin emits polarized fluorescence at ≈45° with respect to the filament axis. When the TMR-labeled F-actin laying at 45° in the visual field rotates, the vertical- and horizontal-polarized fluorescence (FLV and FLH, respectively) of TMR alternately become bright. This technique allowed us to demonstrate the helical rotation of mDia1, a mammalian formin. Adenosine triphosphate (ATP) hydrolysis in actin subunits is not required for helical rotation; however, ATP appears to contribute to accelerating actin elongation by mDia1. When helical rotation is limited by trapping both mDia1 and the pointed-end side, the processive filament elongation is blocked. Thus, mDia1 faithfully rotates along the long-pitch helix of F-actin. In this chapter, we introduce the theoretical concept of single-molecule FLP, the optical setup, the preparation of adenosine diphosphate-bound actin, and the procedure to observe the rotational movement of F-actin elongating from immobilized formins.

Fungal morphogenesis

Morphogenesis in fungi is often induced by extracellular factors and executed by fungal genetic factors. Cell surface changes and alterations of the microenvironment often accompany morphogenetic changes in fungi. In this review, we will first discuss the general traits of yeast and hyphal morphotypes and how morphogenesis affects development and adaptation by fungi to their native niches, including host niches. Then we will focus on the molecular machinery responsible for the two most fundamental growth forms, yeast and hyphae. Last, we will describe how fungi incorporate exogenous environmental and host signals together with genetic factors to determine their morphotype and how morphogenesis, in turn, shapes the fungal microenvironment.

Of social molecules: The interactive assembly of ASH1 mRNA-transport complexes in yeast

Asymmetric, motor-protein dependent transport of mRNAs and subsequent localized translation is an important mechanism of gene regulation. Due to the high complexity of such motile particles, our mechanistic understanding of mRNA localization is limited. Over the last two decades, ASH1 mRNA localization in budding yeast has served as comparably simple and accessible model system. Recent advances have helped to draw an increasingly clear picture on the molecular mechanisms governing ASH1 mRNA localization from its co-transcriptional birth to its delivery at the site of destination. These new insights help to better understand the requirement of initial nuclear mRNPs, the molecular basis of specific mRNA-cargo recognition via cis-acting RNA elements, the different stages of RNP biogenesis and reorganization, as well as activation of the motile activity upon cargo binding. We discuss these aspects in context of published findings from other model organisms.

Actinous enigma or enigmatic actin: Folding, structure, and functions of the most abundant eukaryotic protein

Being the most abundant protein of the eukaryotic cell, actin continues to keep its secrets for more than 60 years. Everything about this protein, its structure, functions, and folding, is mysteriously counterintuitive, and this review represents an attempt to solve some of the riddles and conundrums commonly found in the field of actin research. In fact, actin is a promiscuous binder with a wide spectrum of biological activities. It can exist in at least three structural forms, globular, fibrillar, and inactive (G-, F-, and I-actin, respectively). G-actin represents a thermodynamically instable, quasi-stationary state, which is formed in vivo as a result of the energy-intensive, complex posttranslational folding events controlled and driven by cellular folding machinery. The G-actin structure is dependent on the ATP and Mg²⁺ binding (which in vitro is typically substituted by Ca²⁺) and protein is easily converted to the I-actin by the removal of metal ions and by action of various denaturing agents (pH, temperature, and chemical denaturants). I-actin cannot be converted back to the G-form. Foldable and “natively folded” forms of actin are always involved in interactions either with the specific protein partners, such as Hsp70 chaperone, prefoldin, and the CCT chaperonin during the actin folding in vivo or with Mg²⁺ and ATP as it takes place in the G-form. We emphasize that the solutions for the mysteries of actin multifunctionality, multistructurality, and trapped unfolding can be found in the quasi-stationary nature of this enigmatic protein, which clearly possesses many features attributed to both globular and intrinsically disordered proteins.

A cooperative jack model of random coil-to-elongation transition of the FH1 domain by profilin binding explains formin motor behavior in actin polymerization

Filopodia are essential for the development of neuronal growth cones, cell polarity and cell migration. Their protrusions are powered by the polymerization of actin filaments linked to the plasma membrane, catalyzed by formin proteins. The acceleration of polymerization depends on the number of profilin-actins binding with the formin-FH1 domain. Biophysical characterization of the disordered formin-FH1 domain remains a challenge. We analyzed the conformational distribution of the diaphanous-related formin mDia1-FH1 bound with one to six profilins. We found a coil-to-elongation transition in the FH1 domain. We propose a cooperative "jack" model for the Formin-Homology-1 (FH1) domain of formins stacked by profilin-actins.

Formin 1 and filamin B physically interact to coordinate chondrocyte proliferation and differentiation in the growth plate

Filamin B (FlnB) is an actin-binding protein thought to transduce signals from various membrane receptors and intracellular proteins onto the actin cytoskeleton. Formin1 (Fmn1) is an actin-nucleating protein, implicated in actin assembly and intracellular signaling. Human mutations in FLNB cause several skeletal disorders associated with dwarfism and early bone fusion. Mouse mutations in Fmn1 cause aberrant fusion of carpal digits. We report here that FlnB and Fmn1 physically interact, are co-expressed in chondrocytes in the growth plate and share overlapping expression in the cell cytoplasm and nucleus. Loss of FlnB leads to a dramatic decrease in Fmn1 expression at the hypertrophic-to-ossification border. Loss of Fmn1-FlnB in mice leads to a more severe reduction in body size, weight and growth plate length, than observed in mice following knockout of either gene alone. Shortening of the long bone is associated with a decrease in chondrocyte proliferation and an overall delay in ossification in the double-knockout mice. In contrast to FlnB null, Fmn1 loss results in a decrease in the width of the prehypertrophic zone. Loss of both proteins, however, causes an overall decrease in the width of the proliferation zone and an increase in the differentiated hypertrophic zone. The current findings suggest that Fmn1 and FlnB have shared and independent functions. FlnB loss promotes prehypertrophic differentiation whereas Fmn1 leads to a delay. Both proteins, however, regulate chondrocyte proliferation, and FlnB may regulate Fmn1 function at the hypertrophic-to-ossification border, thereby explaining the overall delay in ossification.

Roles of ion transport in control of cell motility

Cell motility is an essential feature of life. It is essential for reproduction, propagation, embryonic development, and healing processes such as wound closure and a successful immune defense. If out of control, cell motility can become life-threatening as, for example, in metastasis or autoimmune diseases. Regardless of whether ciliary/flagellar or amoeboid movement, controlled motility always requires a concerted action of ion channels and transporters, cytoskeletal elements, and signaling cascades. Ion transport across the plasma membrane contributes to cell motility by affecting the membrane potential and voltage-sensitive ion channels, by inducing local volume changes with the help of aquaporins and by modulating cytosolic Ca(2+) and H(+) concentrations. Voltage-sensitive ion channels serve as voltage detectors in electric fields thus enabling galvanotaxis; local swelling facilitates the outgrowth of protrusions at the leading edge while local shrinkage accompanies the retraction of the cell rear; the cytosolic Ca(2+) concentration exerts its main effect on cytoskeletal dynamics via motor proteins such as myosin or dynein; and both, the intracellular and the extracellular H(+) concentration modulate cell migration and adhesion by tuning the activity of enzymes and signaling molecules in the cytosol as well as the activation state of adhesion molecules at the cell surface. In addition to the actual process of ion transport, both, channels and transporters contribute to cell migration by being part of focal adhesion complexes and/or physically interacting with components of the cytoskeleton. The present article provides an overview of how the numerous ion-transport mechanisms contribute to the various modes of cell motility.

Novel small-molecule compounds that affect cellular morphogenesis in yeast and mammalian cells

Drugs affecting cellular morphological changes leading to tumor cell migration and invasion are desirable for cancer therapy. In the present study, we screened for small-molecule compounds that affect the cellular morphology of both unicellular yeast and mammalian HEK293 cells to identify drug candidates. The yeast formin protein Bni1 and Src homology 3 (SH3)-pleckstrin homology (PH) domain protein Boi1, which are required for proper morphogenesis, cause growth defects when overexpressed in yeast. Using this system, we screened a chemical library consisting of ～8000 compounds to identify drug candidates that suppress these growth defects. None of the screened compounds induced morphological changes in vegetatively growing yeast cells, but several compounds had inhibitory effects on pheromone-induced projection formation and actin localization, suggesting that these compounds affected a specific stage of morphogenesis. Five of the compounds also induced morphological changes in mammalian HEK293 cells. Among the identified compounds, BTB03156, 2-[(4-chlorophenyl)sulfonyl]-1-methyl-3,5-dinitrobenzene, and BTB02467, 1-[(4-chlorophenyl)sulfonyl]-2-nitro-4-(trifluoromethyl)benzene, although they have similar structures, displayed differing effects on the yeast growth defects caused by latrunculin A, an actin polymerization inhibitor. The chemical library compounds identified using this in vivo screening approach are simple, cell-permeable molecules, and therefore may be useful in the development of therapeutic drugs for cancer metastasis and other actin-related diseases.