Drak Is Required for Actomyosin Organization During Drosophila Cellularization

Alternative splicing-related long noncoding RNA ANRIL facilitates hepatocellular carcinoma by targeting the miR-199a-5p/SRSF1 axis and impacting Anillin

Hepatocellular carcinoma (HCC) is characterized by aberrant alternative splicing (AS), which plays an important part in the pathological process of this disease. However, available reports about genes and mechanisms involved in AS process are limited. Our previous research has identified ANRIL as a long noncoding RNA related to the AS process of HCC. Here, we investigated the exact effect and the mechanism of ANRIL on HCC progress. The ANRIL expression profile was validated using the real-time quantitative polymerase chain reaction assay. The western blot analysis and IHC assay were conducted on candidate targets, including SRSF1 and Anillin. The clinicopathological features of 97 patients were collected and analyzed. Loss-of and gain-of-function experiments were conducted. The dual-luciferase reporter assay was applied to verify the interaction between ANRIL, miR-199a-5p, and SRSF1. Anomalous upregulation of ANRIL in HCC was observed, correlating with worse clinicopathological features of HCC. HCC cell proliferation, mobility, tumorigenesis, and metastasis were impaired by depleting ANRIL. We found that ANRIL acts as a sponger of miRNA-199a-5p, resulting in an elevated level of its target protein SRSF1. The phenotypes induced by ANRIL/miR-199a-5p/SRSF1 alteration are associated with Anillin, a validated HCC promoter. ANRIL is an AS-related lncRNA promoting HCC progress by modulating the miR-199a-5p/SRSF1 axis. The downstream effector of this axis in the development of HCC is Anillin.

Kinesin-1 patterns Par-1 and Rho signaling at the cortex of syncytial embryos of Drosophila

The cell cortex of syncytial Drosophila embryos is patterned into cap and intercap regions by centrosomes, specific sets of proteins that are restricted to their respective regions by unknown mechanisms. Here, we found that Kinesin-1 is required for the restriction of plus- and minus-ends of centrosomal and non-centrosomal microtubules to the cap region, marked by EB1 and Patronin/Shot, respectively. Kinesin-1 also directly or indirectly restricts proteins and Rho signaling to the intercap, including the RhoGEF Pebble, Dia, Myosin II, Capping protein-α, and the polarity protein Par-1. Furthermore, we found that Par-1 is required for cap restriction of Patronin/Shot, and vice versa Patronin, for Par-1 enrichment at the intercap. In summary, our data support a model that Kinesin-1 would mediate the restriction of centrosomal and non-centrosomal microtubules to a region close to the centrosomes and exclude Rho signaling and Par-1. In addition, mutual antagonistic interactions would refine and maintain the boundary between cap and intercap and thus generate a distinct cortical pattern.

FRL and DAAM are required for lateral adhesion of interommatidial cells and patterning of the retinal floor

Optical insulation of the unit eyes (ommatidia) is an important prerequisite of precise sight with compound eyes. Separation of the ommatidia is ensured by pigment cells that organize into a hexagonal lattice in the Drosophila eye, forming thin walls between the facets. Cell adhesion, mediated by apically and latero-basally located junctional complexes, is crucial for stable attachment of these cells to each other and the basal lamina. Whereas former studies have focused on the formation and remodelling of the cellular connections at the apical region, here, we report a specific alteration of the lateral adhesion of the lattice cells, leaving the apical junctions largely unaffected. We found that DAAM and FRL, two formin-type cytoskeleton regulatory proteins, play redundant roles in lateral adhesion of the interommatidial cells and patterning of the retinal floor. We show that formin-dependent cortical actin assembly is crucial for latero-basal sealing of the ommatidial lattice. We expect that the investigation of these previously unreported eye phenotypes will pave the way toward a better understanding of the three-dimensional aspects of compound eye development.

Membrane-actin interactions in morphogenesis: Lessons learned from Drosophila cellularization

During morphogenesis, changes in the shapes of individual cells are harnessed to mold an entire tissue. These changes in cell shapes require the coupled remodeling of the plasma membrane and underlying actin cytoskeleton. In this review, we highlight cellularization of the Drosophila embryo as a model system to uncover principles of how membrane and actin dynamics are co-regulated in space and time to drive morphogenesis.

A Comparative Study of the Role of Formins in Drosophila Embryonic Dorsal Closure

Dorsal closure is a late embryogenesis process required to seal the epidermal hole on the dorsal side of the Drosophila embryo. This process involves the coordination of several forces generated in the epidermal cell layer and in the amnioserosa cells, covering the hole. Ultimately, these forces arise due to cytoskeletal rearrangements that induce changes in cell shape and result in tissue movement. While a number of cytoskeleton regulatory proteins have already been linked to dorsal closure, here we expand this list by demonstrating that four of the six Drosophila formin type actin assembly factors are needed to bring about the proper fusion of the epithelia. An analysis of the morphological and dynamic properties of dorsal closure in formin mutants revealed a differential contribution for each formin, although we found evidence for functional redundancies as well. Therefore, we propose that the four formins promote the formation of several, and only partly identical, actin structures each with a specific role in the mechanics of dorsal closure.

Mechanical feedback and robustness of apical constrictions in Drosophila embryo ventral furrow formation

Formation of the ventral furrow in the Drosophila embryo relies on the apical constriction of cells in the ventral region to produce bending forces that drive tissue invagination. In our recent paper we observed that apical constrictions during the initial phase of ventral furrow formation produce elongated patterns of cellular constriction chains prior to invagination and argued that these are indicative of tensile stress feedback. Here, we quantitatively analyze the constriction patterns preceding ventral furrow formation and find that they are consistent with the predictions of our active-granular-fluid model of a monolayer of mechanically coupled stress-sensitive constricting particles. Our model shows that tensile feedback causes constriction chains to develop along underlying precursor tensile stress chains that gradually strengthen with subsequent cellular constrictions. As seen in both our model and available optogenetic experiments, this mechanism allows constriction chains to penetrate or circumvent zones of reduced cell contractility, thus increasing the robustness of ventral furrow formation to spatial variation of cell contractility by rescuing cellular constrictions in the disrupted regions.

The Ringleaders: Understanding the Apicomplexan Basal Complex Through Comparison to Established Contractile Ring Systems

The actomyosin contractile ring is a key feature of eukaryotic cytokinesis, conserved across many eukaryotic kingdoms. Recent research into the cell biology of the divergent eukaryotic clade Apicomplexa has revealed a contractile ring structure required for asexual division in the medically relevant genera Toxoplasma and Plasmodium; however, the structure of the contractile ring, known as the basal complex in these parasites, remains poorly characterized and in the absence of a myosin II homolog, it is unclear how the force required of a cytokinetic contractile ring is generated. Here, we review the literature on the basal complex in Apicomplexans, summarizing what is known about its formation and function, and attempt to provide possible answers to this question and suggest new avenues of study by comparing the Apicomplexan basal complex to well-studied, established cytokinetic contractile rings and their mechanisms in organisms such as S. cerevisiae and D. melanogaster. We also compare the basal complex to structures formed during mitochondrial and plastid division and cytokinetic mechanisms of organisms beyond the Opisthokonts, considering Apicomplexan diversity and divergence.

Spatiotemporal recruitment of RhoGTPase protein GRAF inhibits actomyosin ring constriction in Drosophila cellularization

Actomyosin contractility is regulated by Rho-GTP in cell migration, cytokinesis and morphogenesis in embryo development. Whereas Rho activation by Rho-GTP exchange factor (GEF), RhoGEF2, is well known in actomyosin contractility during cytokinesis at the base of invaginating membranes in Drosophila cellularization, Rho inhibition by RhoGTPase-activating proteins (GAPs) remains to be studied. We have found that the RhoGAP, GRAF, inhibits actomyosin contractility during cellularization. GRAF is enriched at the cleavage furrow tip during actomyosin assembly and initiation of ring constriction. Graf depletion shows increased Rho-GTP, increased Myosin II and ring hyper constriction dependent upon the loss of the RhoGTPase domain. GRAF and RhoGEF2 are present in a balance for appropriate activation of actomyosin ring constriction. RhoGEF2 depletion and abrogation of Myosin II activation in Rho kinase mutants suppress the Graf hyper constriction defect. Therefore, GRAF recruitment restricts Rho-GTP levels in a spatiotemporal manner for inhibiting actomyosin contractility during cellularization.

Anillin facilitates cell proliferation and induces tumor growth of hepatocellular carcinoma via miR-138/SOX4 axis regulation

Actin-binding protein Anillin plays a pivotal role in regulating cytokinesis during the cell cycle, and involves in tumorigenesis and progress. However, the exact regulation mechanism of Anillin in human hepatocellular carcinoma (HCC) remains largely unknown. In this study, we examined and verified the anomalous high expression of Anillin in both HCC patients' specimens and HCC cell lines. High expression of Anillin is associated with dismal clinicopathologic features of HCC patients and poor prognosis. We conducted loss-of and gain-of function studies in HCC Hep3B cells. Anillin presented a significantly facilitating effect on cell proliferation in vitro and induced remarkable tumor growth in vivo. We found that the over-expression of Anillin was driven by a potential axis of miR-138/SOX4. Transcription factor SOX4 presented a high expression profile positive correlated with Anillin, and ChIP assay validated the interaction between SOX4 and the specific sequence of the promoter region of Anillin gene. While, we verified miR-138 as an upstream regulator of SOX4, which is abrogated in HCC cells and exerts degenerating effect on SOX4 mRNA. In our conclusion, Anillin facilitates the cell proliferation and enhances tumor growth of HCC, and is modulated by miR-138/SOX4 axis which regulates the transcriptional activity of Anillin. Findings above demonstrate us a probable axis for HCC diagnosis and treatment.

Summary of the main point

Anillin facilitates the cell proliferation and enhances tumor growth in HCC. The transcriptional activity of Anillin is modulated by miR-138/SOX4 axis. Findings above demonstrate us a probable axis for HCC diagnosis and treatment.

F-BAR domain protein Syndapin regulates actomyosin dynamics during apical cap remodeling in syncytial Drosophila embryos

Branched actin networks driven by Arp2/3 interact with actomyosin filaments in processes such as cell migration. Similar interactions occur in the syncytial Drosophila blastoderm embryo where expansion of apical caps by Arp2/3-driven actin polymerization occurs in interphase, and cap buckling at contact edges by Myosin II to form furrows takes place in metaphase. Here, we study the role of Syndapin (Synd), an F-BAR domain-containing protein, in apical cap remodeling prior to furrow extension. We found that depletion of synd resulted in larger apical caps. Super-resolution and TIRF microscopy showed that control embryos had long apical actin protrusions in caps during interphase and short protrusions during metaphase, whereas synd depletion led to formation of sustained long protrusions, even during metaphase. Loss of Arp2/3 function in synd mutants partly reverted defects in apical cap expansion and protrusion remodeling. Myosin II levels were decreased in synd mutants, an observation consistent with the expanded cap phenotype previously reported for Myosin II mutant embryos. We propose that Synd function limits branching activity during cap expansion and affects Myosin II distribution in order to bring about a transition in actin remodeling activity from apical cap expansion to lateral furrow extension.

Genetic interactions between Protein Kinase D and Lobe mutants during eye development of Drosophila melanogaster

Background

In Drosophila, the development of the fly eye involves the activity of several, interconnected pathways that first define the presumptive eye field within the eye anlagen, followed by establishment of the dorso-ventral boundary, and the regulation of growth and apoptosis. In Lobe (L) mutant flies, parts of the eye or even the complete eye are absent because the eye field has not been properly defined. Manifold genetic interactions indicate that L influences the activity of several signalling pathways, resulting in a conversion of eye tissue into epidermis, and in the induction of apoptosis. As information on the molecular nature of the L mutation is lacking, the underlying molecular mechanisms are still an enigma.

Results

We have identified Protein Kinase D (PKD) as a strong modifier of the L mutant phenotype. PKD belongs to the PKC/CAMK class of Ser/Thr kinases that have been involved in diverse cellular processes including stress resistance and growth. Despite the many roles of PKD, Drosophila PKD null mutants are without apparent phenotype apart from sensitivity to oxidative stress. Here we report an involvement of PKD in eye development in the sensitized genetic background of Lobe. Absence of PKD strongly enhanced the dominant eye defects of heterozygous L² flies, and decreased their viability. Moreover, eye-specific overexpression of an activated isoform of PKD considerably ameliorated the dominant L² phenotype. This genetic interaction was not allele specific but similarly seen with three additional, weaker L alleles (L¹, L⁵, L^G), demonstrating its specificity.

Conclusions

We propose that PKD-mediated phosphorylation is involved in underlying processes causing the L phenotype, i.e. in the regulation of growth, the epidermal transformation of eye tissue and apoptosis, respectively.

Scribble and Discs-large direct initial assembly and positioning of adherens junctions during the establishment of apical-basal polarity

Apical-basal polarity is a fundamental property of animal tissues. Drosophila embryos provide an outstanding model for defining mechanisms that initiate and maintain polarity. Polarity is initiated during cellularization, when cell-cell adherens junctions are positioned at the future boundary of apical and basolateral domains. Polarity maintenance then involves complementary and antagonistic interplay between apical and basal polarity complexes. The Scribble/Dlg module is well-known for promoting basolateral identity during polarity maintenance. Here, we report a surprising role for Scribble/Dlg in polarity initiation, placing it near the top of the network-positioning adherens junctions. Scribble and Dlg are enriched in nascent adherens junctions, are essential for adherens junction positioning and supermolecular assembly, and also play a role in basal junction assembly. We test the hypotheses for the underlying mechanisms, exploring potential effects on protein trafficking, cytoskeletal polarity or Par-1 localization/function. Our data suggest that the Scribble/Dlg module plays multiple roles in polarity initiation. Different domains of Scribble contribute to these distinct roles. Together, these data reveal novel roles for Scribble/Dlg as master scaffolds regulating assembly of distinct junctional complexes at different times and places.

Protein Kinase D Is Dispensable for Development and Survival of Drosophila melanogaster

Members of the Protein Kinase D (PKD) family are involved in numerous cellular processes in mammals, including cell survival after oxidative stress, polarized transport of Golgi vesicles, as well as cell migration and invasion. PKD proteins belong to the PKC/CAMK class of serine/threonine kinases, and transmit diacylglycerol-regulated signals. Whereas three PKD isoforms are known in mammals, Drosophila melanogaster contains a single PKD homolog. Previous analyses using overexpression and RNAi studies indicated likewise multi-facetted roles for Drosophila PKD, including the regulation of secretory transport and actin-cytoskeletal dynamics. Recently, involvement in growth regulation has been proposed based on the hypomorphic dPKDH allele. We have generated PKD null alleles that are homozygous viable without apparent phenotype. They largely match control flies regarding fertility, developmental timing and weight. Males, but not females, are slightly shorter lived and starvation sensitive. Furthermore, migration of pole cells in embryos and border cells in oocytes appears normal. PKD mutants tolerate heat, cold and osmotic stress like the control but are sensitive to oxidative stress, conforming to the described role for mammalian PKDs. A candidate screen to identify functionally redundant kinases uncovered genetic interactions of PKD with Pkcδ, sqa and Drak mutants, further supporting the role of PKD in oxidative stress response, and suggesting its involvement in starvation induced autophagy and regulation of cytoskeletal dynamics. Overall, PKD appears dispensable for fly development and survival presumably due to redundancy, but influences environmental responses.

The Drosophila Afadin and ZO-1 homologues Canoe and Polychaetoid act in parallel to maintain epithelial integrity when challenged by adherens junction remodeling

During morphogenesis, cells must change shape and move without disrupting tissue integrity. This requires cell-cell junctions to allow dynamic remodeling while resisting forces generated by the actomyosin cytoskeleton. Multiple proteins play roles in junctional-cytoskeletal linkage, but the mechanisms by which they act remain unclear. Drosophila Canoe maintains adherens junction-cytoskeletal linkage during gastrulation. Canoe's mammalian homologue Afadin plays similar roles in cultured cells, working in parallel with ZO-1 proteins, particularly at multicellular junctions. We take these insights back to the fly embryo, exploring how cells maintain epithelial integrity when challenged by adherens junction remodeling during germband extension and dorsal closure. We found that Canoe helps cells maintain junctional-cytoskeletal linkage when challenged by the junctional remodeling inherent in mitosis, cell intercalation, and neuroblast invagination or by forces generated by the actomyosin cable at the leading edge. However, even in the absence of Canoe, many cells retain epithelial integrity. This is explained by a parallel role played by the ZO-1 homologue Polychaetoid. In embryos lacking both Canoe and Polychaetoid, cell junctions fail early, with multicellular junctions especially sensitive, leading to widespread loss of epithelial integrity. Our data suggest that Canoe and Polychaetoid stabilize Bazooka/Par3 at cell-cell junctions, helping maintain balanced apical contractility and tissue integrity.

Actomyosin contraction during cellularization is regulated in part by Src64 control of Actin 5C protein levels

Src64 is required for actomyosin contraction during cellularization of the Drosophila embryonic blastoderm. The mechanism of actomyosin ring constriction is poorly understood even though a number of cytoskeletal regulators have been implicated in the assembly, organization, and contraction of these microfilament rings. How these cytoskeletal processes are regulated during development is even less well understood. To investigate the role of Src64 as an upstream regulator of actomyosin contraction, we conducted a proteomics screen to identify proteins whose expression levels are controlled by src64. Global levels of actin are reduced in src64 mutant embryos. Furthermore, we show that reduction of the actin isoform Actin 5C causes defects in actomyosin contraction during cellularization similar to those caused by src64 mutation, indicating that a relatively high level of Actin 5C is required for normal actomyosin contraction and furrow canal structure. However, reduction of Actin 5C levels only slows down actomyosin ring constriction rather than preventing it, suggesting that src64 acts not only to modulate actin levels, but also to regulate the actomyosin cytoskeleton by other means.

Drosophila melanogaster as a Model System for Human Glioblastomas

Glioblastoma multiforme (GBM) is the most common primary malignant adult brain tumor. Genomic amplifications, activating mutations, and overexpression of receptor tyrosine kinases (RTKs) such as EGFR, and genes in core RTK signaling transduction pathways such as PI3K are common in GBM. However, efforts to target these pathways have been largely unsuccessful in the clinic, and the median survival of GBM patients remains poor at 14-15 months. Therefore, to improve patient outcomes, there must be a concerted effort to elucidate the underlying biology involved in GBM tumorigenesis. Drosophila melanogaster has been a highly effective model for furthering our understanding of GBM tumorigenesis due to a number of experimental advantages it has over traditional mouse models. For example, there exists extensive cellular and genetic homology between humans and Drosophila, and 75% of genes associated with human disease have functional fly orthologs. To take advantage of these traits, we developed a Drosophila GBM model with constitutively active variants of EGFR and PI3K that effectively recapitulated key aspects of GBM disease. Researchers have utilized this model in forward genetic screens and have expanded on its functionality to make a number of important discoveries regarding requirements for key components in GBM tumorigenesis, including genes and pathways involved in extracellular matrix signaling, glycolytic metabolism, invasion/migration, stem cell fate and differentiation, and asymmetric cell division. Drosophila will continue to reveal novel biological pathways and mechanisms involved in gliomagenesis, and this knowledge may contribute to the development of effective treatment strategies to improve patient outcomes.

Drak/STK17A Drives Neoplastic Glial Proliferation through Modulation of MRLC Signaling

Glioblastoma (GBM) and lower grade gliomas (LGG) are the most common primary malignant brain tumors and are resistant to current therapies. Genomic analyses reveal that signature genetic lesions in GBM and LGG include copy gain and amplification of chromosome 7, amplification, mutation, and overexpression of receptor tyrosine kinases (RTK) such as EGFR, and activating mutations in components of the PI3K pathway. In Drosophila melanogaster, constitutive co-activation of RTK and PI3K signaling in glial progenitor cells recapitulates key features of human gliomas. Here we use this Drosophila glioma model to identify death-associated protein kinase (Drak), a cytoplasmic serine/threonine kinase orthologous to the human kinase STK17A, as a downstream effector of EGFR and PI3K signaling pathways. Drak was necessary for glial neoplasia, but not for normal glial proliferation and development, and Drak cooperated with EGFR to promote glial cell transformation. Drak phosphorylated Sqh, the Drosophila ortholog of nonmuscle myosin regulatory light chain (MRLC), which was necessary for transformation. Moreover, Anillin, which is a binding partner of phosphorylated Sqh, was upregulated in a Drak-dependent manner in mitotic cells and colocalized with phosphorylated Sqh in neoplastic cells undergoing mitosis and cytokinesis, consistent with their known roles in nonmuscle myosin-dependent cytokinesis. These functional relationships were conserved in human GBM. Our results indicate that Drak/STK17A, its substrate Sqh/MRLC, and the effector Anillin/ANLN regulate mitosis and cytokinesis in gliomas. This pathway may provide a new therapeutic target for gliomas.Significance: These findings reveal new insights into differential regulation of cell proliferation in malignant brain tumors, which will have a broader impact on research regarding mechanisms of oncogene cooperation and dependencies in cancer.See related commentary by Lathia, p. 1036.

Dynamics of cortical domains in early Drosophila development

Underlying the plasma membrane of eukaryotic cells is an actin cortex that includes actin filaments and associated proteins. A special feature of all polarized and epithelial cells are cortical domains, each of which is characterized by specific sets of proteins. Typically, an epithelial cell contains apical, subapical, lateral and basal domains. The domain-specific protein sets contain evolutionarily conserved proteins, as well as cell-type-specific factors. Among the conserved proteins are, the Par proteins, Crumbs complex and the lateral proteins Scribbled and Discs large 1. Organization of the plasma membrane into cortical domains is dynamic and depends on cell type, differentiation and developmental stage. The dynamics of cortical organization is strikingly visible in early Drosophila embryos, which increase the number of distinct cortical domains from one, during the pre-blastoderm stage, to two in syncytial blastoderm embryos, before finally acquiring the four domains that are typical for epithelial cells during cellularization. In this Review, we will describe the dynamics of cortical organization in early Drosophila embryos and discuss the processes and mechanisms underlying cortical remodeling.

Diverse integrin adhesion stoichiometries caused by varied actomyosin activity

Cells in an organism are subjected to numerous sources of external and internal forces, and are able to sense and respond to these forces. Integrin-mediated adhesion links the extracellular matrix outside cells to the cytoskeleton inside, and participates in sensing, transmitting and responding to forces. While integrin adhesion rapidly adapts to changes in forces in isolated migrating cells, it is not known whether similar or more complex responses occur within intact, developing tissues. Here, we studied changes in integrin adhesion composition upon different contractility conditions in Drosophila embryonic muscles. We discovered that all integrin adhesion components tested were still present at muscle attachment sites (MASs) when either cytoplasmic or muscle myosin II was genetically removed, suggesting a primary role of a developmental programme in the initial assembly of integrin adhesions. Contractility does, however, increase the levels of integrin adhesion components, suggesting a mechanism to balance the strength of muscle attachment to the force of muscle contraction. Perturbing contractility in distinct ways, by genetic removal of either cytoplasmic or muscle myosin II or eliminating muscle innervation, each caused unique alterations to the stoichiometry at MASs. This suggests that different integrin-associated proteins are added to counteract different kinds of force increase.