Growth factor-induced signal transduction in adherent mammalian cells is sensitive to gravity.
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Epidermal growth factor (EGF) activates a well-characterized signal transduction cascade in a wide variety of cells. This activation leads to increased cell proliferation in most cell types. Among the early effects evoked by EGF are receptor clustering, cell rounding, and early gene expression. The influence of gravity on EGF-induced EGF receptor clustering and gene expression as well as on actin polymerization and cell rounding have been investigated in adherent A431 epithelial cells with the use of sounding rockets to create microgravity conditions. EGF-induced c-fos and c-jun expression decreased in microgravity. This was caused by alteration of the EGF receptor and protein kinase C-mediated signal transduction pathways. In contrast, neither the binding of EGF to the receptor nor the receptor clustering were changed under microgravity conditions. Because cell morphology was also modulated under microgravity conditions, and the growth factor-induced signal transduction cascades have been demonstrated to be linked to the actin microfilament system, it is tempting to suggest that the actin microfilament system constitutes the gravity-sensitive cell component. (+info)
Microgravity alters protein phosphorylation changes during initiation of sea urchin sperm motility.
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European Space Agency (ESA) studies demonstrated that bull sperm swim with higher velocity in microgravity (microG) than at 1 G. Coupling between protein phosphorylation and sperm motility during activation in microG and at 1 G was examined in the ESA Biorack on two space shuttle missions. Immotile sperm were activated to swim (86-90% motility) at launch +20 h by dilution into artificial seawater (ASW). Parallel ground controls were performed 2 h after the flight experiment. Activation after 0, 30, and 60 s was terminated with electrophoresis sample buffer and samples analyzed for phosphoamino acids by Western blotting. Phosphorylation of a 130-kDa phosphothreonine-containing protein (FP130) occurred three to four times faster in microG than at 1 G. A 32-kDa phosphoserine-containing protein was significantly stimulated at 30 s but returned to 1 G control levels at 60 s. The rate of FP130 phosphorylation in microG was attenuated by D2O, suggesting that changes in water properties participate in altering signal transduction. Changes in FP130 phosphorylation triggered by the egg peptide speract were delayed in microG. These results demonstrate that previously observed effects of microG on sperm motility are coupled to changes in phosphorylation of specific flagellar proteins and that early events of sperm activation and fertilization are altered in microG. (+info)
Mechanotransduction in bone--role of the lacuno-canalicular network.
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The capacity of bone tissue to alter its mass and structure in response to mechanical demands has long been recognized but the cellular mechanisms involved remained poorly understood. Over the last several years significant progress has been made in this field, which we will try to summarize. These studies emphasize the role of osteocytes as the professional mechanosensory cells of bone, and the lacuno-canalicular porosity as the structure that mediates mechanosensing. Strain-derived flow of interstitial fluid through this porosity seems to mechanically activate the osteocytes, as well as ensuring transport of cell signaling molecules and nutrients and waste products. This concept allows an explanation of local bone gain and loss, as well as remodeling in response to fatigue damage, as processes supervised by mechanosensitive osteocytes. (+info)
Electron microscopic analysis of gravisensing Chara rhizoids developed under microgravity conditions.
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Tip-growing, unicellular Chara rhizoids that react gravitropically on Earth developed in microgravity. In microgravity, they grew out from the nodes of the green thallus in random orientation. Development and morphogenesis followed an endogenous program that is not affected by the gravitational field. The cell shape, the polar cytoplasmic organization, and the polar distribution of cell organelles, except for the statoliths, were not different from controls that had grown on earth (ground controls). The ultrastructure of the organelles and the microtubules were well preserved. Microtubules were excluded from the apical zone in both ground controls as well as microgravity-grown rhizoids. The statoliths (vesicles containing BaSO4 crystals in a matrix) in microgravity-grown rhizoids were spread over a larger area (up to 50 microm basal to the tip) than the statoliths of ground controls (10-30 microm). Some statoliths were even located in the subapical zone close to microtubules, which was not observed in ground controls. The crystals in statoliths from microgravity-grown rhizoids appeared more loosely arranged in the vesicle matrix compared with ground controls. The chemical composition of the crystals was identified as BaSO4 by X-ray microanalysis. There is evidence that the amount of BaSO4 in statoliths of rhizoids developed in microgravity is lower than in ground controls, indicating that the gravisensitivity of microgravity-developed rhizoids might be reduced compared with ground controls. Lack of gravity, however, does not affect the process of tip growth and does not inhibit the development of the structures needed for the gravity-sensing machinery. (+info)
Osteoblast fibronectin mRNA, protein synthesis, and matrix are unchanged after exposure to microgravity.
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The well-defined osteoblast line, MC3T3-E1 was used to examine fibronectin (FN) mRNA levels, protein synthesis, and extracellular FN matrix accumulation after growth activation in spaceflight. These osteoblasts produce FN extracellular matrix (ECM) known to regulate adhesion, differentiation, and function in adherent cells. Changes in bone ECM and osteoblast cell shape occur in spaceflight. To determine whether altered FN matrix is a factor in causing these changes in spaceflight, quiescent osteoblasts were launched into microgravity and were then sera activated with and without a 1-gravity field. Synthesis of FN mRNA, protein, and matrix were measured after activation in microgravity. FN mRNA synthesis is significantly reduced in microgravity (0-G) when compared to ground (GR) osteoblasts flown in a centrifuge simulating earth's gravity (1-G) field 2.5 h after activation. However, 27.5 h after activation there were no significant differences in mRNA synthesis. A small but significant reduction of FN protein was found in the 0-G samples 2.5 h after activation. Total FN protein 27.5 h after activation showed no significant difference between any of the gravity conditions, however, there was a fourfold increase in absolute amount of protein synthesized during the incubation. Using immunofluorescence, we found no significant differences in the amount or in the orientation of the FN matrix after 27.5 h in microgravity. These results demonstrate that FN is made by sera-activated osteoblasts even during exposure to microgravity. These data also suggest that after a total period of 43 h of spaceflight FN transcription, translation, or altered matrix assembly is not responsible for the altered cell shape or altered matrix formation of osteoblasts. (+info)
The effect of microgravity on morphology and gene expression of osteoblasts in vitro.
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The mass and architecture of the skeletal system adapt, to some extent, to their mechanical environment. A site-specific bone loss of 1-2% is observed in astronauts and in-flight animals after 1 month of spaceflight. Biochemical data of astronauts and histomorphometric analysis of rat bones show that the change in bone mass is a result of decreased bone formation in association with normal (or increased) bone resorption. The changes in bone formation appear to be due in part to decreased osteoblast differentiation, matrix maturation, and mineralization. Recent data show that spaceflight alters the mRNA level for several bone-specific proteins in rat bone, suggesting that the characteristics of osteoblasts are altered during spaceflight. A possible underlying mechanism is that osteoblasts themselves are sensitive to altered gravity levels as suggested by several studies investigating the effect of microgravity on osteoblasts in vitro. Changes in cell and nuclear morphology were observed as well as alterations in the expression of growth factors (interleukin-6 and insulin-like growth factor binding proteins) and matrix proteins (collagen type I and osteocalcin). Taken together, this altered cellular function in combination with differences in local or systemic factors may mediate the effects of spaceflight on bone physiology. (+info)
Chromosome mechanics of fungi under spaceflight conditions--tetrad analysis of two-factor crosses between spore color mutants of Sordaria macrospora.
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Spore color mutants of the fungus Sordaria macrospora Auersw. were crossed under spaceflight conditions on the space shuttle to MIR mission S/MM 05 (STS-81). The arrangement of spores of different colors in the asci allowed conclusions on the influence of spaceflight conditions on sexual recombination in fungi. Experiments on a 1-g centrifuge in space and in parallel on the ground were used for controls. The samples were analyzed microscopically on their return to earth. Each fruiting body was assessed separately. Statistical analysis of the data showed a significant increase in gene recombination frequencies caused by the heavy ion particle stream in space radiation. The lack of gravity did not influence crossing-over frequencies. Hyphae of the flown samples were assessed for DNA strand breaks. No increase in damage was found compared with the ground samples. It was shown that S. macrospora is able to repair radiation-induced DNA strand breaks within hours. (+info)
Implications for interrelationships between nuclear architecture and control of gene expression under microgravity conditions.
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Components of nuclear architecture are functionally interrelated with control of gene expression. There is growing appreciation that multiple levels of nuclear organization integrate the regulatory cues that support activation and suppression of genes as well as the processing of gene transcripts. The linear representation of genes and promoter elements provide the potential for responsiveness to physiological regulatory signals. Parameters of chromatin structure and nucleosome organization support synergism between activities at independent regulatory sequences and render promoter elements accessible or refractory to transcription factors. Association of genes, transcription factors, and the machinery for transcript processing with the nuclear matrix facilitates fidelity of gene expression within the three-dimensional context of nuclear architecture. Mechanisms must be defined that couple nuclear morphology with enzymatic parameters of gene expression. The recent characterization of factors that mediate chromatin remodeling and identification of intranuclear targeting signals that direct transcription factors to subnuclear domains where gene expression occurs link genetic and structural components of transcriptional control. Nuclear reorganization and aberrant intranuclear trafficking of transcription factors for developmental and tissue-specific control occurs in tumor cells and in neurological disorders. Compromises in nuclear structure-function interrelationships can occur as a consequence of microgravity-mediated perturbations in cellular architecture. (+info)