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Positive Feedback Loop (positive + feedback_loop)
Selected AbstractsThe heel and toe of the cell's foot: A multifaceted approach for understanding the structure and dynamics of focal adhesionsCYTOSKELETON, Issue 11 2009Haguy Wolfenson Abstract Focal adhesions (FAs) are large clusters of transmembrane receptors of the integrin family and a multitude of associated cytoplasmic "plaque" proteins, which connect the extracellular matrix-bound receptors with the actin cytoskeleton. The formation of nearly stationary FAs defines a boundary between the dense and highly dynamic actin network in lamellipodium and the sparser and more diverse cytoskeletal organization in the lamella proper, creating a template for the organization of the entire actin network. The major "mechanical" and "sensory" functions of FAs; namely, the nucleation and regulation of the contractile, myosin-II-containing stress fibers and the mechanosensing of external surfaces depend, to a major extent, on the dynamics of molecular components within FAs. A central element in FA regulation concerns the positive feedback loop, based on the most intriguing feature of FAs; that is, their dependence on mechanical tension developing by the growing stress fibers. FAs grow in response to such tension, and rapidly disassemble upon its relaxation. In this article, we address the mechanistic relationships between the process of FA development, maturation and dissociation and the dynamic molecular events, which take place in different regions of the FA, primarily in the distal end of this structure (the "toe") and the proximal "heel," and discuss the central role of local mechanical forces in orchestrating the complex interplay between FAs and the actin system. Cell Motil. Cytoskeleton 66: 1017,1029, 2009. © 2009 Wiley-Liss, Inc. [source] Indirect effects of algae on coral: algae-mediated, microbe-induced coral mortalityECOLOGY LETTERS, Issue 7 2006Jennifer E. Smith Abstract Declines in coral cover are generally associated with increases in the abundance of fleshy algae. In many cases, it remains unclear whether algae are responsible, directly or indirectly, for coral death or whether they simply settle on dead coral surfaces. Here, we show that algae can indirectly cause coral mortality by enhancing microbial activity via the release of dissolved compounds. When coral and algae were placed in chambers together but separated by a 0.02 ,m filter, corals suffered 100% mortality. With the addition of the broad-spectrum antibiotic ampicillin, mortality was completely prevented. Physiological measurements showed complementary patterns of increasing coral stress with proximity to algae. Our results suggest that as human impacts increase and algae become more abundant on reefs a positive feedback loop may be created whereby compounds released by algae enhance microbial activity on live coral surfaces causing mortality of corals and further algal growth. [source] Modulation by adenosine of both muscarinic M1 -facilitation and M2 -inhibition of [3H]-acetylcholine release from the rat motor nerve terminalsEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 11 2002Laura Oliveira Abstract The crosstalk between adenosine and muscarinic autoreceptors regulating evoked [3H]-acetylcholine ([3H]-ACh) release was investigated on rat phrenic nerve-hemidiaphragm preparations. Motor nerve terminals possess facilitatory M1 and inhibitory M2 autoreceptors that can be activated by McN-A-343 (1,30 µm) and oxotremorine (0.3,100 µm), respectively. The muscarinic receptor antagonist, dicyclomine (3 nm,10 µm), caused a biphasic (inhibitory/facilitatory) effect, indicating that M1 -facilitation prevails during 5 Hz stimulation trains. Concomitant activation of AF,DX 116-sensitive M2 receptors was partially attenuated, as pretreatment with M1 antagonists, muscarinic toxin 7 (MT-7, 0.1 nm) and pirenzepine (1 nm), significantly enhanced inhibition by oxotremorine. Activation of A2A -adenosine receptors with CGS 21680C (2 nm) (i) potentiated oxotremorine inhibition, and (ii) shifted McN-A-343-induced facilitation into a small inhibitory effect. Conversely, the A1 -receptor agonist, R- N6 -phenylisopropyl adenosine (R-PIA, 100 nm), attenuated the inhibitory effect of oxotremorine, without changing facilitation by McN-A-343. Synergism between A2A and M2 receptors is regulated by a reciprocal interaction with facilitatory M1 receptors, which may be prevented by pirenzepine (1 nm). During 50 Hz-bursts, facilitation (M1) of [3H]-ACh release by McN-A-343 disappeared, while the inhibitory (M2) effect of oxotremorine became predominant. This muscarinic shift results from the interplay with A2A receptors, as it was precluded by the selective A2A receptor antagonist, ZM 241385 (10 nm). In conclusion, when the muscarinic M1 positive feedback loop is fully operative, negative regulation of ACh release is mediated by adenosine A1 receptors. During high frequency bursts, tonic activation of A2A receptors promotes M2 autoinhibition by braking the M1 receptor operated counteraction. [source] Elastin-derived peptides: Matrikines critical for glioblastoma cell aggressiveness in a 3-D systemGLIA, Issue 16 2009Bérénice Coquerel Abstract In the most common primary brain tumors, malignant glioma cells invade the extracellular matrix (ECM) and proliferate rapidly in the cerebral tissue, which is mainly composed of hyaluronan (HA) along with the elastin present in the basement membrane of blood vessels. To determine the role of ECM components in the invasive capacity of glioma cell lines, we developed a 3-D cell-culture system, based on a hydrogel in which HA can be coreticulated with kappa-elastin (HA-,E). Using this system, the invasiveness of cells from four glioma cell lines was dramatically increased by the presence of ,E and a related, specific peptide (VGVAPG)3. In addition, MMP-2 secretion increased and MMP-12 synthesis occurred. Extracellular injections of ,E or (VGVAPG)3 provoked a pronounced and dose-dependent increase in [Ca2+]i. ,E significantly enhanced the expression of the genes encoding elastin-receptor and tropoelastin. We propose the existence of a positive feedback loop in which degradation of elastin generates fragments that stimulate synthesis of tropoelastin followed by further degradation as well as migration and proliferation of the very cells responsible for degradation. All steps in this ECM-based loop could be blocked by the addition of either of the EBP antagonists, lactose, and V-14 peptide, suggesting that the loop itself should be considered as a new therapeutic target. © 2009 Wiley-Liss, Inc. [source] The effects of water table draw-down (as a surrogate for climate change) on the hydrology of a fen peatland, CanadaHYDROLOGICAL PROCESSES, Issue 17 2006Peter N. Whittington Abstract Hydrological response to climate change may alter the biogeochemical role that peatlands play in the global climate system, so an understanding of the nature and magnitude of this response is important. In 2002, the water table in a fen peatland near Quebec City was lowered by ,20 cm (Experimental site), and hydrological response was measured compared to Control (no manipulation) and Drained (previously drained c. 1994) sites. Because of the draw-down, the surface in the Experimental pool decreased 5, 15 and 20 cm in the ridge, lawn and mat, respectively, increasing bulk density by ,60% in the Experimental lawn. Hydraulic conductivity (K) generally decreased with depth and from Control (25,125 cm) 10,1 to 10,5 cm s,1 to Experimental (25,125 cm) 10,2 to 10,7 cm s,1 and to Drained (25,75 cm) 10,2 to 10,6 cm s,1. In similar topographic locations (ridge, lawn, mat), K trended Control > Experimental > Drained, usually by an order of magnitude at similar depths in similar topographic locations. Water table fluctuations in the Drained site averaged twice those of the Control site. The water table in the Control lawn remained at a stable depth relative to the surface (,, 1 cm) because the lawn peat floats with changes in water table position. However, the Drained lawn peat was more rigid because of the denser degraded peat, forcing the water to fluctuate relative to the surface and further enhancing peat decay and densification. This provides a positive feedback loop that could intensify further peat degradation, changing the carbon cycling dynamics. Copyright © 2006 John Wiley & Sons, Ltd. [source] ,It Was Such a Handy Term': Management Fashions and Pragmatic Ambiguity*JOURNAL OF MANAGEMENT STUDIES, Issue 6 2006Hélène Giroux abstract This article builds on constructs that authors have labelled strategic ambiguity, interpretative viability, umbrella constructs, and boundary objects, and suggests that these constructs all articulate a central concern for collective action and the role of ambiguity therein. It characterizes as pragmatic ambiguity the condition of admitting more than one course of action, and elucidates and operationalizes this new construct. Drawing on the sociology of translation (Callon, 1986; Latour, 1987),[1] it argues that pragmatic ambiguity is both the result and the resource of a collective process of intéressement occurring during the rise in popularity of a new management approach. Following Benders and van Veen (2001), the article posits that pragmatic ambiguity increases during the rise of a management fashion. It provides empirical evidence in support of this claim by means of a longitudinal analysis of quality management (QM) concepts as articulated by several authors both before and during the Quality Movement of the 1980s and 1990s. The analyses of QM texts show that concepts became vaguer, more ambiguous, and more general as the Quality Movement gained momentum, suggesting the presence of a positive feedback loop between pragmatic ambiguity and popularity. In addition, the data illustrate how pragmatic ambiguity was achieved and sustained textually, and how it was supported by a variety of social, linguistic and rhetorical factors. [source] Hypothalamic-pituitary-adrenal axis activation by experimental periodontal disease in ratsJOURNAL OF PERIODONTAL RESEARCH, Issue 5 2001T. Breivik Organisms respond to inflammatory conditions by mounting a co-ordinated complex series of adaptive responses involving the immune, nervous and endocrine systems that are aimed at restoring the homeostatic balance. We have recently shown in a rat model that inappropriate hypothalamic-pituitary-adrenal (HPA) axis regulation and a subsequent inability to mount a suitable glucocorticoid response to gingival inflammation may influence susceptibility to periodontal disease. This study was designed to investigate whether ligature- and bacterial lipopolysaccharide (LPS)-induced inflammation in the gingival connective tissues may activate this physiological axis, and to further explore the significance of HPA regulation in periodontal disease. Experimental periodontal disease was induced in major histocompability complex (MHC)-identical but HPA low (LEW) and high (F344) responding rat strains. We tested (1) whether ongoing periodontal disease activates the HPA axis as measured by corticosterone levels, and (2) whether genetic differences in HPA regulation modulate periodontal disease progression. In the F344 strain, the periodontal tissue destruction was more severe. This observation was associated with a significant increase of corticosterone levels in F344 rats only. Addition of LPS at the gingival inflammatory site led to a further increase of corticosterone levels and disease severity in F344 rats. These findings illustrate a positive feedback loop between the HPA axis and periodontal disease: the disease activates the HPA axis, and a genetically determined high HPA responsitivity further increases disease susceptibility. [source] Differential Effects of Acute and Chronic Ethanol Exposure on Orexin Expression in the Perifornical Lateral HypothalamusALCOHOLISM, Issue 5 2010Irene Morganstern Background:, Recent reports support the involvement of hypothalamic orexigenic peptides in stimulating ethanol intake. Our previous studies have examined the effects of ethanol on hypothalamic peptide systems of the paraventricular nucleus (PVN) and identified a positive feedback loop in which PVN peptides, such as enkephalin and galanin, stimulate ethanol intake and ethanol, in turn, stimulates the expression of these peptides. Recently, orexin (OX), a peptide produced mainly by cells in the perifornical lateral hypothalamus (PFLH), has been shown to play an important role in mediating the rewarding aspects of ethanol intake. However, there is little evidence showing the effects that ethanol itself may have on the OX peptide system. In order to understand the feedback relationship between ethanol and the OX system, the current investigation was designed to measure OX gene expression in the PFLH following acute as well as chronic ethanol intake. Methods:, In the first experiment, Sprague,Dawley rats were trained to voluntarily consume a 2 or 9% concentration of ethanol, and the expression of OX mRNA in the PFLH was measured using quantitative real-time polymerase chain reaction (qRT-PCR). The second set of experiments tested the impact of acute oral gavage of 0.75 and 2.5 g/kg ethanol solution on OX expression in the PFLH using qRT-PCR, as well as radiolabeled in situ hybridization. Further tests using digoxigenin-labeled in situ hybridization and immunofluorescence histochemistry allowed us to more clearly distinguish the effects of acute ethanol on OX cells in the lateral hypothalamic (LH) versus perifornical (PF) regions. Results:, The results showed chronic consumption of ethanol versus water to dose-dependently reduce OX mRNA in the PFLH, with a larger effect observed in rats consuming 2.5 g/kg/d (,70%) or 1.0 g/kg/d (,50%) compared to animals consuming 0.75 g/kg/d (,40%). In contrast to chronic intake, acute oral ethanol compared to water significantly enhanced OX expression in the PFLH, and this effect occurred at the lower (0.75 g/kg) but not higher (2.5 g/kg) dose of ethanol. Additional analyses of the OX cells in the LH versus PF regions identified the former as the primary site of ethanol's stimulatory effect on the OX system. In the LH but not the PF, acute ethanol increased the density of OX-expressing and OX-immunoreactive neurons. The increase in gene expression was detected only at the lower dose of ethanol (0.75 g/kg), whereas the increase in OX peptide was seen only at the higher dose of ethanol (2.5 g/kg). Conclusion:, These results lead us to propose that OX neurons, while responsive to negative feedback signals from chronic ethanol consumption, are stimulated by acute ethanol administration, most potently in the LH where OX may trigger central reward mechanisms that promote further ethanol consumption. [source] Regional Entrepreneurial Transformation: A Complex Systems PerspectiveJOURNAL OF SMALL BUSINESS MANAGEMENT, Issue 2 2010Andaç T. Ar In this paper, I develop a descriptive process model of regional entrepreneurial transformation based on a conceptualization of regional economies as self-organizing complex systems that consist of interdependent subsystems as represented by heterogeneous actor groups. It is proposed that a change in a region's entrepreneurial profile requires shifts in collectively shared subsystem-level schemata that cause actors to behave in ways that prevent emergent entrepreneurship. The model highlights four factors that contribute to the initiation of a positive feedback loop through which simultaneous schemata changes and consequently a regional entrepreneurial shift occur. Theoretical and policy implications are discussed. [source] NPH4/ARF7 and ARF19 promote leaf expansion and auxin-induced lateral root formationTHE PLANT JOURNAL, Issue 1 2005Jill C. Wilmoth Summary Auxin response factors (ARFs) bind auxin response promoter elements and mediate transcriptional responses to auxin. Five of the 22 ARF genes in Arabidopsis thaliana encode ARFs with glutamine-rich middle domains. Four of these can activate transcription and have been ascribed developmental functions. We show that ARF19, the fifth Q-rich ARF, also activates transcription. Mutations in ARF19 have little effect on their own, but in combination with mutations in NPH4/ARF7, encoding the most closely related ARF, they cause several phenotypes including a drastic decrease in lateral and adventitious root formation and a decrease in leaf cell expansion. These results indicate that auxin induces lateral roots and leaf expansion by activating NPH4/ARF7 and ARF19. Auxin induces the ARF19 gene, and NPH4/ARF7 and ARF19 together are required for expression of one of the arf19 mutant alleles, suggesting that a positive feedback loop regulates leaf expansion and/or lateral root induction. [source] Regulation of uterine function by cytokines in cows: Possible actions of tumor necrosis factor-,, interleukin-1, and interferon-,ANIMAL SCIENCE JOURNAL, Issue 3 2006Kiyoshi OKUDA ABSTRACT When animals do not become pregnant, regression of the corpus luteum (CL) is essential for normal cyclicity because it allows the development of a new ovulatory follicle. Luteal regression is caused by a pulsatile release of prostaglandin (PG) F2, from the uterus in the late luteal phase in most mammals including cattle. Although it has been proposed in ruminants that pulsatile PGF2, secretion is generated by a positive feedback loop between luteal and/or hypophyseal oxytocin and uterine PGF2,, the bovine endometrium may possess other mechanisms for initiation of luteolytic PGF2, secretion. There is increasing evidence that several cytokines mainly produced by immune cells modulate CL and uterine function in many species. Tumor necrosis factor-, (TNF-,) stimulates PGF2, output from bovine endometrium not only at the follicular phase but also at the late luteal phase. Administration of TNF-, at a high concentration prolongs luteal lifespan, whereas administration of a low concentration of TNF-, accelerates luteal regression in cows. The data obtained from the authors' previous in vitro and in vivo studies strongly suggest that TNF-, is a crucial factor in regulating luteolysis in cows. The authors' recent study has shown that interleukin-1, mediates PG secretion from bovine endometrium as a local regulator. Furthermore, interferon-, (IFN-,) suppresses the action of TNF-, on PGF2, synthesis by the bovine endometrium in vitro, suggesting that IFN-, plays a luteoprotective role by inhibiting TNF-,-induced PGF2, production in early pregnancy. The purpose of the present review is to summarize current understanding of the endocrine mechanisms that regulate uterine function by cytokines during the estrous cycle and early pregnancy in cows. [source] Mechanotransduction in endothelial cell migrationJOURNAL OF CELLULAR BIOCHEMISTRY, Issue 6 2005Song Li Abstract The migration of endothelial cells (ECs) plays an important role in vascular remodeling and regeneration. EC migration can be regulated by different mechanisms such as chemotaxis, haptotaxis, and mechanotaxis. This review will focus on fluid shear stress-induced mechanotransduction during EC migration. EC migration and mechanotransduction can be modulated by cytoskeleton, cell surface receptors such as integrins and proteoglycans, the chemical and physical properties of extracellular matrix (ECM) and cell,cell adhesions. The shear stress applied on the luminal surface of ECs can be sensed by cell membrane and associated receptor and transmitted throughout the cell to cell,ECM adhesions and cell,cell adhesions. As a result, shear stress induces directional migration of ECs by promoting lamellipodial protrusion and the formation of focal adhesions (FAs) at the front in the flow direction and the disassembly of FAs at the rear. Persistent EC migration in the flow direction can be driven by polarized activation of signaling molecules and the positive feedback loops constituted by Rho GTPases, cytoskeleton, and FAs at the leading edge. Furthermore, shear stress-induced EC migration can overcome the haptotaxis of ECs. Given the hemodynamic environment of the vascular system, mechanotransduction during EC migration has a significant impact on vascular development, angiogenesis, and vascular wound healing. J. Cell. Biochem. © 2005 Wiley-Liss, Inc. [source] | ||||||||||||||||