Slingshot cofilin phosphatase localization is regulated by receptor tyrosine kinases and regulates cytoskeletal structure in the developing Drosophila eye

Differential gene expression in Chorthippus parallelus (Zetterstedt, 1821) (Orthoptera: Acrididae: Gomphocerinae) induced by Wolbachia infection

Distinct lineages of the grasshopper Chorthippus parallelus (Orthoptera: Acrididae) form well-known hybrid zones (HZs) both in the Pyrenees and the Alps mountain ranges in South Europe. These HZs represent unique experimental systems to identify "key genes" that maintain genetic boundaries between emerging species. The Iberian endemism C. p. erythropus (Cpe) and the subspecies C. p. parallelus (Cpp), widely distributed throughout the rest of Europe, overlap and form the Pyrenean HZ. Both subspecies differ morphologically, as well as in behavioral, mitochondrial, nuclear, and chromosomal traits, and in the strains of the maternally transmitted bacterial endosymbiont Wolbachia infecting them. This results in either unidirectional and bidirectional cytoplasmic incompatibility between both grasshopper subspecies, pointing out that Wolbachia clearly affects gene expression in the infected individuals. Here we explore how Wolbachia may modify the expression of some major genes involved in relevant pathways in Cpp in the Pyrenean HZ. We have analyzed, through molecular biomarkers, the physiological responses in C. parallelus individuals infected by Wolbachia, with particular attention to the energy metabolism, the immune system response, and the reproduction. qPCR was used to evaluate the expression of selected genes in the gonads of infected and uninfected adults of both sexes, since this tissue constitutes the main target of Wolbachia infection. Transcriptional analyses also showed differential sex-dependent responses in most of the analyzed biomarkers in infected and noninfected individuals. We identified for the first time new sensitive biomarkers that might be involved in the reproductive barrier induced by Wolbachia in the hybrid zone.

Alteration of Cytoskeletal Proteins Leads to Retinal Degeneration in Drosophila

The eye holds a special fascination for many neuroscientists because of its meticulously organized structure. Vertebrates typically possess a simple camera-type eye, whereas the compound eye structure is predominantly observed in arthropods including model organism Drosophila melanogaster. Cell shape, cell polarization, and tissue integrity are the cell biological processes crucial for shaping the eye, which directly or indirectly depends on the cytoskeleton. Henceforth the cytoskeleton, specifically actin microfilaments, essentially has a dynamic role in the normal development and growth of eye structure. This review provides insight into the roles played by the actin cytoskeleton during the development and maintenance of the Drosophila eye.

The noncanonical role of the protease cathepsin D as a cofilin phosphatase

Cathepsin D (cathD) is traditionally regarded as a lysosomal protease that degrades substrates in acidic compartments. Here we report cathD plays an unconventional role as a cofilin phosphatase orchestrating actin remodeling. In neutral pH environments, the cathD precursor directly dephosphorylates and activates the actin-severing protein cofilin independent of its proteolytic activity, whereas mature cathD degrades cofilin in acidic pH conditions. During development, cathD complements the canonical cofilin phosphatase slingshot and regulates the morphogenesis of actin-based structures. Moreover, suppression of cathD phosphatase activity leads to defective actin organization and cytokinesis failure. Our findings identify cathD as a dual-function molecule, whose functional switch is regulated by environmental pH and its maturation state, and reveal a novel regulatory role of cathD in actin-based cellular processes.

The Molecular Network of YAP/Yorkie at the Cell Cortex and their Role in Ocular Morphogenesis

During development, the precise control of tissue morphogenesis requires changes in the cell number, size, shape, position, and gene expression, which are driven by both chemical and mechanical cues from the surrounding microenvironment. Such physical and architectural features inform cells about their proliferative and migratory capacity, enabling the formation and maintenance of complex tissue architecture. In polarised epithelia, the apical cell cortex, a thin actomyosin network that lies directly underneath the apical plasma membrane, functions as a platform to facilitate signal transmission between the external environment and downstream signalling pathways. One such signalling pathway culminates in the regulation of YES-associated protein (YAP) and TAZ transcriptional co-activators and their sole Drosophila homolog, Yorkie, to drive proliferation and differentiation. Recent studies have demonstrated that YAP/Yorkie exhibit a distinct function at the apical cell cortex. Here, we review recent efforts to understand the mechanisms that regulate YAP/Yki at the apical cell cortex of epithelial cells and how normal and disturbed YAP-actomyosin networks are involved in eye development and disease.

Protein kinase D3 modulates MMP1 and MMP13 expression in human chondrocytes

Many catabolic stimuli, including interleukin-1 (IL-1) in combination with oncostatin M (OSM), promote cartilage breakdown via the induction of collagen-degrading collagenases such as matrix metalloproteinase 1 (MMP1) and MMP13 in human articular chondrocytes. Indeed, joint diseases with an inflammatory component are characterised by excessive extracellular matrix (ECM) catabolism. Importantly, protein kinase C (PKC) signalling has a primary role in cytokine-induced MMP1/13 expression, and is known to regulate cellular functions associated with pathologies involving ECM remodelling. At present, substrates downstream of PKC remain undefined. Herein, we show that both IL-1- and OSM-induced phosphorylation of protein kinase D (PKD) in human chondrocytes is strongly associated with signalling via the atypical PKCι isoform. Consequently, inhibiting PKD activation with a pan-PKD inhibitor significantly reduced the expression of MMP1/13. Specific gene silencing of the PKD isoforms revealed that only PKD3 (PRKD3) depletion mirrored the observed MMP repression, indicative of the pharmacological inhibitor specifically affecting only this isoform. PRKD3 silencing was also shown to reduce serine phosphorylation of signal transducer and activator of transcription 3 (STAT3) as well as phosphorylation of all three mitogen-activated protein kinase groups. This altered signalling following PRKD3 silencing led to a significant reduction in the expression of the activator protein-1 (AP-1) genes FOS and JUN, critical for the induction of many MMPs including MMP1/13. Furthermore, the AP-1 factor activating transcription factor 3 (ATF3) was also reduced concomitant with the observed reduction in MMP13 expression. Taken together, we highlight an important role for PKD3 in the pro-inflammatory signalling that promotes cartilage destruction.

Multi-level control of actin dynamics by protein kinase D

Dynamic actin remodeling is fundamental to processes such as cell motility, vesicle trafficking, and cytokinesis. Protein kinase D (PKD) is a serine-threonine kinase known to be involved in diverse biological functions ranging from vesicle fission at the Golgi complex to regulation of cell motility and invasion. This review addresses the role of PKD in the organization of the actin cytoskeleton with a particular emphasis on the substrates associated with this function. We further highlight the multi-level control of actin dynamics by PKD and suggest that the tight spatio-temporal control of PKD activity is critical for the coordination of directed cell migration.

Physiological functions of protein kinase D in vivo

The cellular functions of the serine/threonine protein kinase D (PKD) have been extensively studied within the last decade and distinct roles such as fission of vesicles at the Golgi compartment, coordination of cell migration and invasion, and regulation of gene transcription have been correlated with this kinase family. Here, we highlight the current state of in vivo studies on PKD function with a focus on animal models and discuss the molecular basis of the observed phenotypic characteristics associated with this kinase family.

AIP1 acts with cofilin to control actin dynamics during epithelial morphogenesis

During epithelial morphogenesis, cells not only maintain tight adhesion for epithelial integrity but also allow dynamic intercellular movement to take place within cell sheets. How these seemingly opposing processes are coordinated is not well understood. Here, we report that the actin disassembly factors AIP1 and cofilin are required for remodeling of adherens junctions (AJs) during ommatidial precluster formation in Drosophila eye epithelium, a highly stereotyped cell rearrangement process which we describe in detail in our live imaging study. AIP1 is enriched together with F-actin in the apical region of preclusters, whereas cofilin displays a diffuse and uniform localization pattern. Cofilin overexpression completely rescues AJ remodeling defects caused by AIP1 loss of function, and cofilin physically interacts with AIP1. Pharmacological reduction of actin turnover results in similar AJ remodeling defects and decreased turnover of E-cadherin, which also results from AIP1 deficiency, whereas an F-actin-destabilizing drug affects AJ maintenance and epithelial integrity. Together with other data on actin polymerization, our results suggest that AIP1 enhances cofilin-mediated actin disassembly in the apical region of precluster cells to promote remodeling of AJs and thus intercellular movement, but also that robust actin polymerization promotes AJ general adhesion and integrity during the remodeling process.

Rapid elevation of Inos and decreases in abundance of other proteins at pupal diapause termination in the flesh fly Sarcophaga crassipalpis

We analyzed changes in brain proteins 24 h after pupal diapause termination in Sarcophaga crassipalpis by a combination of 2-dimensional gel electrophoresis and mass spectrometry. The proteome analysis revealed significant changes in 20 proteins, 11 of which represented >or=2.5-fold changes. Three proteins were present only in the brains of diapausing pupae. Among the most abundant proteins that showed a change, 1 was more abundant, 7 were less abundant, and 2 were absent following diapause termination. The protein that increased in abundance following diapause termination showed highest identity to myo-inositol-1-phosphate synthase (Inos). Proteins that decreased at diapause termination included those showing highest identities to fatty acid binding protein, CG2331-PA, twinstar, catalase, and a histone. Proteins absent at diapause termination included ribosomal protein L17A and one unnamed protein. An increase of Inos protein level was confirmed using Western blot analysis. Attempts to terminate diapause by injection of several Inos-related metabolites failed, thus suggesting that the elevation of Inos at diapause termination is downstream of the physiological regulation that initiates development.

Cofilin/ADF is required for retinal elongation and morphogenesis of the Drosophila rhabdomere

Drosophila photoreceptors undergo marked changes in their morphology during pupal development. These changes include a five-fold elongation of the retinal cell body and the morphogenesis of the rhabdomere, the light sensing structure of the cell. Here we show that twinstar (tsr), which encodes Drosophila cofilin/ADF (actin-depolymerizing factor), is required for both of these processes. In tsr mutants, the retina is shorter than normal, the result of a lack of retinal elongation. In addition, in a strong tsr mutant, the rhabdomere structure is disorganized and the microvilli are short and occasionally unraveled. In an intermediate tsr mutant, the rhabdomeres are not disorganized but have a wider than normal structure. The adherens junctions connecting photoreceptor cells to each other are also found to be wider than normal. We propose, and provide data supporting, that these wide rhabdomeres and adherens junctions are secondary events caused by the inhibition of retinal elongation. These results provide insight into the functions of the actin cytoskeleton during morphogenesis of the Drosophila eye.

The cofilin pathway in breast cancer invasion and metastasis

Recent evidence indicates that metastatic capacity is an inherent feature of breast tumours and not a rare, late acquired event. This has led to new models of metastasis. The interpretation of expression-profiling data in the context of these new models has identified the cofilin pathway as a major determinant of metastasis. Recent studies indicate that the overall activity of the cofilin pathway, and not that of any single gene within the pathway, determines the invasive and metastatic phenotype of tumour cells. These results predict that inhibitors directed at the output of the cofilin pathway will have therapeutic benefit in combating metastasis.

The Cdi/TESK1 kinase is required for Sevenless signaling and epithelial organization in the Drosophila eye

How cellular behaviors such as cell-to-cell communication, epithelial organization and cell shape reorganization are coordinated during development is poorly understood. The developing Drosophila eye offers an ideal model system to study these processes. Localized actin polymerization is required to constrict the apical surface of epithelial cells of the eye imaginal disc to maintain the refined arrangement of the developing ommatidia. The identity of each photoreceptor cell within the epithelium is determined by cell-to-cell contacts involving signal transduction events. The R7 photoreceptor cell requires the activity of the Sevenless RTK to adopt a proper cell fate. We performed an EP screen for negative regulators of this inductive process, and we identified the serine/threonine kinase Center divider (cdi) as a suppressor of the phenotype caused by an activated Sevenless receptor. Cdi is homologous to the human testis-specific kinase 1 (TESK1), a member of the LIM kinases involved in cytoskeleton control through ADF/cofilin phosphorylation. We have analyzed the effects of gain- and loss-of-function of cdi and found alterations in actin organization and in the adherens junctions proteins DE-cadherin and beta-catenin, as well as in Sevenless apical localization. Interference with the function of the ADF/cofilin phosphatase Slingshot (ssh), which antagonizes Cdi, also results in a suppression of signaling triggered by the Sevenless RTK. Our results reveal a critical interplay between the localization of molecules involved in epithelial organization and signal transduction.

The Drosophila p21 activated kinase Mbt regulates the actin cytoskeleton and adherens junctions to control photoreceptor cell morphogenesis

The p21 activated kinase (Pak) family of protein kinases are involved in many cellular functions like re-organisation of the cytoskeleton, transcriptional control, cell division, and survival. These pleiotropic actions are reflected in a plethora of known interacting proteins and phosphorylation substrates. Yet, the integration of a single Pak protein into signalling pathways controlling a particular developmental process are less well studied. For two of the three known Pak proteins in Drosophila melanogaster, D-Pak and Mbt, distinct functions during eye development have been established. In this study we undertook a genetic approach to identify proteins acting in the Mbt signalling pathway during photoreceptor cell morphogenesis. The genetic screen identified the actin depolymerisation factor Twinstar/Cofilin as one target of Mbt signalling. Twinstar/Cofilin becomes phosphorylated upon activation of Mbt. However, biochemical and genetic experiments question the role of the LIM domain protein kinase (Limk) as a major link between Mbt and Twinstar/Cofilin as it has been suggested for other PAK proteins. Constitutive activation of Mbt not only disturbs the actin cytoskeleton but also affects adherens junction organisation indicating a requirement of the protein in cell adhesion dependent processes during photoreceptor cell differentiation.