La forme du noyau peut varier significativement au cours du développement ou lors de processus pathologiques en raison des forces mécaniques émanant du microenvironnement ou générées par le cytosquelette. L’impact de la morphologie nucléaire sur la machinerie transcriptionnelle n’est cependant pas connu. En utilisant plusieurs approches afin de manipuler la morphologie nucléaire, nous avons observé que des changements de forme de l’enveloppe nucléaire régulent l’activité de AP1 et TEAD. Nous avons montré que l’aplatissement du noyau augmente la phosphorylation de c-Jun et la translocation de YAP, conduisant à une augmentation de la transcription des gènes cibles de AP1 et TEAD. Nous avons également observé que l’aplatissement du noyau se produit au cours du cycle cellulaire et favorise la prolifération via l’activation de TEAD et AP1 qui stimulent la progression de la phase G1 à la phase S. / .The shape of the cell nucleus can vary considerably during developmental and pathological processes as a consequence of the mechanical forces emanating from the microenvironment or generated by the cytoskeleton. However the impact of nuclear morphology on the transcriptional machinery is not known. Using a combination of tools to manipulate the nuclear morphology, we observed that changes in nuclear shape regulate the activity of AP1 and TEAD. We showed that nuclear flattening increases c-Jun phosphorylation and YAP nuclear translocation, leading to transcriptional induction of AP1 and TEAD-target genes. Surprisingly, we found that nuclear compression is necessary and sufficient to mediate c-Jun and YAP activation in response to cell- generated contractility or cell spreading. We additionally observed that nuclear flattening occurs during the cell cycle and promotes proliferation via TEAD and AP1- dependent G1 to S progression.

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The multifaceted adverse effects of reduced gravity on the skeletal system pose a significant challenge to human spaceflight. There is an interest in investigating any hypothetical differences between partial gravity and microgravity, and in the unmet need to identify countermeasures to both. A hypothesis to be tested is that reduced gravity impairs a variety of osteogenic cell functions, such as proliferation and differentiation, and that these inhibitory effects can be mitigated by nutritional countermeasures or by interrupting signaling pathways that drive undesired osteogenic remodeling. Utilizing the Random Positioning Machine, it is possible to simulate a variety of reduced gravity levels relevant to future manned space missions: Mars, Moon, and Microgravity of the Low Earth Orbit (LEO) environment. In this study, the effects of altered gravity on the physiology and morphology of cultured osteoblasts were investigated, specifically on their proliferation, osteogenic differentiation, and matrix mineralization. In assessing the role of mechanotransduction in microgravity-induced cytoskeletal dysfunction, this thesis also explored whether selective inhibition of specific signaling steps within the Rho-ROCK pathway can be used to modulate the effects of microgravity on osteoblast differentiation and function. Finally, in developing new countermeasures, an investigation was made into the effectiveness of curcumin and carnosic acid, two nutritional antioxidants with pro-osteogenic properties, contrasted with the trace element zinc, as potential alimentary supplements that may mitigate or alleviate the deleterious effects of microgravity. Results showed that short-term (6 days) culture yielded a dose-dependent reduction in proliferation and the enzymatic activity of alkaline phosphatase (ALP), while long-term studies (21 days) showed a distinct dose-dependent inhibition of mineralization. By contrast, expression levels of key osteogenic genes (Alkaline phosphatase, Runt-related Transcription Factor 2, Sparc/osteonectin) exhibited a threshold behavior: gene expression was significantly inhibited when the cells were exposed to Mars-simulating partial gravity, and this was not reduced further when the cells were cultured under simulated Moon or microgravity conditions. My data suggests that impairment of cell function with decreasing simulated gravity levels is graded and that the threshold profile observed for reduced gene expression is distinct from the dose dependence observed for cell proliferation, ALP activity, and mineral deposition. My studies into the gravity-induced re-organization of the cytoskeleton indicate that selective interruption of the Rho-ROCK pathway at ROCK can prevent morphological changes that result in impaired differentiation and mineralization. Further, I found that nutraceuticals partially reversed the inhibitory effects of SMG on ALP activity and promoted osteoblast proliferation and differentiation in the absence of traditional osteogenic media. I further observed a synergistic effect of the intermix of the phytonutrients on ALP activity. Intermixes of phytonutrients may serve as convenient and effective nutritional countermeasures against bone loss in space. / Bioengineering

Formin proteins, important regulators of a cell's actin cytoskeleton, nucleate actin polymerization and promote filament elongation. Actin dynamics are crucial for a form of polarized growth termed tip growth that is performed by cells involved in reproduction and nutrient uptake in plants. Uncovering the molecular basis of how actin associated proteins like formins control actin dynamics is important to gain a fundamental understanding of plant growth mechanisms. In the moss Physcomitrella patens, there are 9 formin genes that group into three distinct classes (I, II and III). From previous work, we suspect that class I formins may play a role in cytokinesis. Thus, I investigated how class I formins localize in tip-growing protonemal cells to gain further insight into their function. To do this, I tagged class I formins with GFP at the endogenous locus and visualized their subcellular localization using confocal microscopy. I found that Formin 1A, 1D, 1E and 1F localize to punctate spots on the plasma membrane and may concentrate at the cell plate during cell division, while 1B localizes to the cytosol. Overall, these data have shown that class I formins may play a role in cell division and potentially in the secretory pathway.

Isolated adult rat myocytes were subjected to 180 min of metabolic inhibition or incubated in ischaemic pellets, in the presence and absence of 10 μm okadaic acid (OA) or calyculin A (CL-A). Contracture and viability was determined by light microscopic analysis of trypan blue-stained preparations and ATP levels by HPLC. Osmotic fragility was assessed by brief hypotonic swelling of cells in 170 or 85 mOsm media prior to determination of viability. Neither drug significantly affected the relatively rapid rates of contracture of myocytes during metabolic inhibition, and both afforded significant protection from development of trypan blue permeability and osmotic fragility. Both OA and CL-A significantly accelerated the rates of contracture and ATP depletion of myocytes during ischaemic incubations. Despite an enhanced rate of ATP depletion, which would be expected to accelerate development of injury, neither drug accelerated development of loss of viability or development of osmotic fragility as measured by 170 mOsm swelling. Mathematical compensation for different rates of ATP depletion confirmed that a protective effect of the drugs, during ischaemic incubation, was masked by their enhancement of the rate of injury, following swelling at 170 mOsm. When the effects of CL-A on ischaemic cells were examined at 85 mOsm, a more stringent test for osmotic fragility, protection was found without compensation for differing rates of ATP depletion. A dose/response curve for CL-A showed some effect at 100 nm and a nearly full effect during metabolic inhibition at 1 μm concentrations. It is concluded that protein phosphatase inhibitors reduce the rates of development of osmotic fragility of metabolically inhibited cells and reduces the rate of injury relative to the rate of ATP depletion of ischaemic cardiomyocytes. Phosphorylation mechanisms may be important to development of irreversible myocardial cell injury.

Immunofluorescence and quantitative flow cytometry was used to determine if alterations in cytoskeletal proteins (vinculin and tubulin) occur during metabolic inhibition and ischemic incubation of isolated adult rat cardiomyocytes. Effects of cell shape changes on fluorescence, were controlled for by the contractile inhibitor, butanedione monoxime (BDM) and gated analysis. Flow cytometry differentiated rod- and round-shaped myocytes on the basis of forward and side scattering. Severe contracture of metabolically inhibited (iodoacetic acid and amytal) myocytes caused an artefactual increase in fluorescence intensity and a redistribution of tubulin into microblebs on the cell surface, which tended to mask specific losses of fluorescence. Fluorescence microscopy showed that round cells stained intensely for vinculin, but not for tubulin and that vinculin redistributed into coarse patches between 60 and 90 min, times which corresponded to small rebounds of fluorescence. With gated analysis, to exclude severely contracted round and squared cells, and with BDM inhibition of contracture, both metabolically inhibited and ischemic pelleted myocytes showed an early decrease in specific immunofluorescence staining for tubulin and vinculin, which preceded loss of cell viability, as determined by trypan blue staining. In both ischemic and metabolically inhibited cells, decreases of vinculin fluorescence preceded or coincided with increasing osmotic fragility. It is concluded that early cytoskeletal alterations of vinculin in ischemic and anoxic injury correlate with the development of osmotic fragility and irreversible myocyte injury.

Indiana University-Purdue University Indianapolis (IUPUI) / Knowledge of the molecular and cellular mechanisms of ischemic injury is necessary for understanding acute kidney injury and devising optimal treatment regimens. The cortical actin cytoskeleton in the proximal tubule epithelial cells of the kidney nephron, playing an important role in both the establishment and maintenance of cell polarity, is drastically disrupted by the onset of ischemia. We found that in LLC-PK cells (a porcine kidney proximal tubule epithelial cell line), cortactin, an important regulator of actin assembly and organization, translocated from the cell cortex to the cytoplasmic regions upon ischemia/ATP-depletion. Meanwhile both the tyrosine phosphorylation level of cortactin and cortactin’s interaction with either F-actin or the actin nucleator Arp2/3 complex were down-regulated upon ischemia/ATP-depletion or inhibition of Src kinase activity. These results suggest that tyrosine phosphorylation plays an important role in regulating cortactin’s cellular function and localization in the scenario of kidney ischemia. The Rho GTPase signaling pathways is also a critical mediator of the effects of ATP depletion and ischemia on the actin cytoskeleton, but the mechanism by which ATP depletion leads to altered RhoA and Rac1 activity is unknown. We propose that ischemia and ATP depletion result in activation of AMP-activated protein kinase (AMPK) and that this affects Rho GTPase activity and cytoskeletal organization (possibly via TSC1/2 complex and/or mTOR complex). We found that AMPK was rapidly activated (≤5 minutes) by ATP depletion in S3 epithelial cells derived from the proximal tubule in mouse kidney, and there was a corresponding decrease in RhoA and Rac1 activity. During graded ATP-depletion, we found intermediate levels of AMPK activity at the intermediate ATP levels, and that the activity of RhoA and Rac1 activity correlated inversely with the activity of AMPK. Activation of AMPK using two different drugs suppressed RhoA activity, and also led to morphological changes of stress fibers. In addition, the inhibition of AMPK activation partially rescued the disruption of stress fibers caused by ATP-depletion. This evidence supports our hypothesis that the activation of AMPK is upstream of the signaling pathways that eventually lead to RhoA inactivation and cytoskeletal dysregulation during ATP-depletion.

Development is punctuated by morphogenetic rearrangements of epithelial tissues, including complete detachment of individual motile cells during epithelial-mesenchymal transition (EMT). Dramatic actin rearrangements occur as cell-cell junctions are dismantled and cells become independently motile during EMT. Characterizing dynamic actin rearrangements and identifying actin machinery driving these rearrangements is essential for understanding basic mechanisms of cell-cell junction remodeling; yet, neither the precise series of actin rearrangements at cell-cell junctions that accompany EMT, nor the machinery that controls actin rearrangement during EMT, have been identified. This work represents a detailed study of junctional actin reorganization in cells undergoing EMT, identifies actin regulatory proteins that control this actin reorganization, and defines the specific function of one regulatory protein, zyxin, in EMT. Using immunofluorescence and live cell imaging of HGF induced scattering of MDCK cells, dynamic actin rearrangement events occurring during EMT are characterized. Junctional actin characteristic of cell-cell adherent cells is rearranged into contractile medial actin networks linked to the junctional membrane in the initial steps of cell scattering. This actin rearrangement is accompanied by dynamic redistribution of specific actin regulatory proteins, namely α-actinin and zyxin-VASP complexes. α-Actinin mediates higher order structure of junctional actin. Zyxin-VASP complexes mediate linkage of dynamic medial actin networks to adherens junction membranes. Zyxin regulation of actin-membrane linkage controls whether cell migration during EMT occurs independently in solitary cells or is coordinated through tissues. The functional analysis employed here uses novel, quantitative methods that define specific cellular EMT ‘phenotypes’ to reveal the precise role of zyxin in EMT. Constitutive active zyxin mutants exhibit persistent actin-membrane linkages and a scattering phenotype in which cells migrate without loss of cell-cell adhesion. Zyxin is proposed to regulate EMT progression by regulating disruption or maintenance of actin membrane linkages at cell-cell junctions. Zyxin alters the ability of cells to fully detach and migrate independently during EMT and may be an important regulator of morphogenetic plasticity.

Epithelial morphogenesis is contingent upon cell shape changes. Cell shape changes are the driving force for the metamorphosis of the adult Drosophila leg from the leg imaginal disc precursor. Genetic analysis has identified several Drosophila genes involved in regulating cell shape changes during leg disc morphogenesis. These include members of the RhoA signaling pathway and the product of the Stubble-stubbloid (Sb-sbd) locus, a transmembrane serine protease. Mutations in the Sb-sbd gene interact genetically with the members of the RhoA signaling pathway, however the nature of the relationship between Sb-sbd serine protease activity and RhoA signaling is not understood. To identify additional components of the RhoA signaling pathway that may help us to understand the role of the Sb-sbd protease in RhoA signaling the Drosophila genome was systematically scanned for genes that interact with Sb-sbd and RhoA mutations using deletions/deficiencies of specified regions of each chromosome. A total of 201 deficiencies uncovering approximately 84.9-91% of the euchromatic genome and spanning the X, second, and third chromosoms were tested. Of the 201 deficiencies tested, five putative interacting genetic regions and one gene within these deficiencies were identified. The candidate gene Eip78C encodes a nuclear steroid hormone receptor previously identified as having an important role in metamorphosis.

<p>The actin cytoskeleton is involved in many essential cellular events such as mitosis, cell migration, control of epithelial barrier function, and adherence of immune cells. Given the essential roles of the actin cytoskeleton, it is not surprising that it is a common target for bacterial virulence factors. <em>Legionella pneumophila</em>, the causative agent of Legionnaire’s pneumonia, establishes a replicative compartment using effectors secreted by its Dot/Icm secretion system. At least four Dot/Icm effectors, VipA, Ceg14, LegK2 and RavK have been shown to modulate the host actin cytoskeleton. Here, we identified RavJ (lpg0944) as an additional effector that interferes with the actin cytoskeleton in mammalian cells. We demonstrated that RavJ is a transglutaminase that functions to induce crosslink between actin and members of the Motin protein family, leading to inhibition of the binding between actin and ADF/cofilin. We also found that LegL1 (lpg0945) is a metaeffector of RavJ, which suppresses the transglutaminase activity of RavJ by blocking its enzymatic domain. </p>