The FERM protein Yurt is a negative regulatory component of the Crumbs complex that controls epithelial polarity and apical membrane size

The Crumbs (Crb) complex is a key regulator of epithelial cell architecture where it promotes apical membrane formation. Here, we show that binding of the FERM protein Yurt to the cytoplasmic domain of Crb is part of a negative-feedback loop that regulates Crb activity. Yurt is predominantly a basolateral protein but is recruited by Crb to apical membranes late during epithelial development. Loss of Yurt causes an expansion of the apical membrane in embryonic epithelia and photoreceptor cells similar to Crb overexpression and in contrast to loss of Crb. Analysis of yurt crb double mutants suggests that these genes function in one pathway and that yurt negatively regulates crb. We also show that the mammalian Yurt orthologs YMO1 and EHM2 bind to mammalian Crb proteins. We propose that Yurt is part of an evolutionary conserved negative-feedback mechanism that restricts Crb complex activity in promoting apical membrane formation.     

Copy-number control of the Escherichia coli chromosome: a plasmidologist's view

The homeostatic system that sets the copy number, and corrects over-replication and under-replication, seems to be different for chromosomes and plasmids in bacteria. Whereas plasmid replication is random in time, chromosome replication is tightly coordinated with the cell cycle such that all origins are initiated synchronously at the same cell mass per origin once per cell cycle. In this review, we propose that despite their apparent differences, the copy-number control of the Escherichia coli chromosome is similar to that of plasmids. The basic mechanism that is shared by both systems is negative-feedback control of the availability of a protein or RNA positive initiator. Superimposed on this basic mechanism are at least three systems that secure the synchronous initiation of multiple origins; however, these mechanisms are not essential for maintaining the copy number.     

Cell-free synthesis of encephalomyocarditis virus

We developed a system for complete replication of encephalomyocarditis virus (EMCV) in a test tube by using an in vitro translation extract from Krebs-2 cells. Efficient virus synthesis occurred in a narrow range of Mg(2+) and EMCV RNA concentrations. Excess input RNA impaired RNA replication and virus production but not translation. This suggests the existence of a negative-feedback mechanism for regulation of RNA replication by the viral plus-strand RNA or proteins.     

One neuron-one receptor rule in the mouse olfactory system

Similar to the expression of antigen receptor genes in lymphocytes, the mammalian odorant receptor (OR) genes are expressed in a mutually exclusive and monoallelic manner in olfactory sensory neurons (OSNs). DNA rearrangement has long been regarded as a possible mechanism for the allelic exclusion of the OR genes. However, mice cloned from mature OSN nuclei expressed the full repertoire of ORs, and the possibility of irreversible gene translocation was excluded as a mechanism to activate a single OR gene in each OSN. How is allelic exclusion achieved in the olfactory system? Recent transgenic experiments indicated an inhibitory role of the OR protein in preventing further activation of other OR genes. Stochastic activation of an OR gene and negative-feedback regulation by the OR gene product might ensure the maintenance of the one neuron-one receptor rule in the mammalian olfactory system.     

Tandem adaptation with a common design in Escherichia coli chemotaxis

We analyze a model for motor-level adaptation in Escherichia coli based upon the premise that clockwise (CW) and counter-clockwise (CCW) states have different preferred numbers of FliM subunits. We show that this model provides a simple mechanism for the recently observed motor-level adaptation, and it also explains the long-lasting puzzle on the thresholds observed when tethered cells are used to monitor responses to temporal ramps. We note that the motor-level adaptation has the same negative-feedback network design as the upstream receptor-level adaptation, and the tandem architecture of one control circuit followed by the other mitigates the effects of cell-to-cell variation and broadens the range of stimuli over which cells optimally respond.     

Is the somitogenesis clock really cell-autonomous? A coupled-oscillator model of segmentation

A striking pattern of oscillatory gene expression, related to the segmentation process (somitogenesis), has been identified in chick, mouse, and zebrafish embryos. Somitogenesis displays great autonomy, and it is generally assumed in the literature that somitogenesis-related oscillations are cell-autonomous in chick and mouse. We point out in this article that there would be many biological reasons to expect some mechanism of coupling between cellular oscillators, and we present a model with such coupling, but which also has autonomous properties. Previous experiments can be re-interpreted in light of this model, showing that it is possible to reconcile both autonomous and non-autonomous aspects. We also show that experimental data, previously interpreted as supporting a purely negative-feedback model for the mechanism of the oscillations, is in fact more compatible with this new model, which relies essentially on positive feedback.     

Cellular mechanisms underlying the development and expression of individual differences in the hypothalamic-pituitary-adrenal stress response

Several years ago Levine, Denenberg, Ader, and others described the effects of postnatal "handling" on the development of behavioral and endocrine responses to stress. As adults, handled rats exhibited attenuated fearfulness in novel environments and a less pronounced increase in the secretion of the adrenal glucocorticoids in response to a variety of stressors. These findings clearly demonstrated that the development of rudimentary, adaptive responses to stress could be modified by environmental events. We have followed these earlier studies, convinced that this paradigm provides a marvellous opportunity to examine how subtle variations in the early environment alter the development of specific neurochemical systems, leading to stable individual differences in biological responses to stimuli that threaten homeostasis. In this work we have shown how early handling influences the development of certain brain regions that regulate glucocorticoid negative-feedback inhibition over hypothalamic-pituitary-adrenal (HPA) activity. Specifically, handling increases glucocorticoid (type II corticosteroid) receptor density in the hippocampus and frontal cortex, enhancing the sensitivity of these structures to the negative-feedback effects of elevated circulating glucocorticoids, and increasing the efficacy of neural inhibition over ACTH secretion. These effects are reflected in the differential secretory pattern of ACTH and corticosterone in handled and nonhandled animals under conditions of stress. In more recent years, using a hippocampal cell culture system, we have provided evidence for the importance of serotonin-induced changes in cAMP levels in mediating the effect of postnatal handling on hippocampal glucocorticoid receptor density. The results of these studies are consistent with the idea that environmental events in early life can permanently alter glucocorticoid receptor gene expression in the hippocampus, providing evidence for a neural mechanism for the development of individual differences in HPA function.     

Nucleolar Arf sequesters Mdm2 and activates p53

The Ink4/Arf locus encodes two tumour-suppressor proteins, p16Ink4a and p19Arf, that govern the antiproliferative functions of the retinoblastoma and p53 proteins, respectively. Here we show that Arf binds to the product of the Mdm2 gene and sequesters it into the nucleolus, thereby preventing negative-feedback regulation of p53 by Mdm2 and leading to the activation of p53 in the nucleoplasm. Arf and Mdm2 co-localize in the nucleolus in response to activation of the oncoprotein Myc and as mouse fibroblasts undergo replicative senescence. These topological interactions of Arf and Mdm2 point towards a new mechanism for p53 activation.     

Systems-level dissection of the cell-cycle oscillator: bypassing positive feedback produces damped oscillations

The cell-cycle oscillator includes an essential negative-feedback loop: Cdc2 activates the anaphase-promoting complex (APC), which leads to cyclin destruction and Cdc2 inactivation. Under some circumstances, a negative-feedback loop is sufficient to generate sustained oscillations. However, the Cdc2/APC system also includes positive-feedback loops, whose functional importance we now assess. We show that short-circuiting positive feedback makes the oscillations in Cdc2 activity faster, less temporally abrupt, and damped. This compromises the activation of cyclin destruction and interferes with mitotic exit and DNA replication. This work demonstrates a systems-level role for positive-feedback loops in the embryonic cell cycle and provides an example of how oscillations can emerge out of combinations of subcircuits whose individual behaviors are not oscillatory. This work also underscores the fundamental similarity of cell-cycle oscillations in embryos to repetitive action potentials in pacemaker neurons, with both systems relying on a combination of negative and positive-feedback loops.     

Lrig1 controls intestinal stem-cell homeostasis by negative regulation of ErbB signalling

Maintenance of adult tissues is carried out by stem cells and is sustained throughout life in a highly ordered manner. Homeostasis within the stem-cell compartment is governed by positive- and negative-feedback regulation of instructive extrinsic and intrinsic signals. ErbB signalling is a prerequisite for maintenance of the intestinal epithelium following injury and tumour formation. As ErbB-family ligands and receptors are highly expressed within the stem-cell niche, we hypothesize that strong endogenous regulators must control the pathway in the stem-cell compartment. Here we show that Lrig1, a negative-feedback regulator of the ErbB receptor family, is highly expressed by intestinal stem cells and controls the size of the intestinal stem-cell niche by regulating the amplitude of growth-factor signalling. Intestinal stem-cell maintenance has so far been attributed to a combination of Wnt and Notch activation and Bmpr inhibition. Our findings reveal ErbB activation as a strong inductive signal for stem-cell proliferation. This has implications for our understanding of ErbB signalling in tissue development and maintenance and the progression of malignant disease.     

Therapeutic Effects of Suppressors of Cytokine Signaling in Diabetic Nephropathy

Inflammation is an important pathophysiological mechanism in diabetic nephropathy (DN). Tubular epithelial cell-myofibroblast transdifferentiation (TEMT), which can be induced by many cytokines, is an important event in DN. Oncostatin M (OSM), an inflammatory cytokine, can induce TEMT in vitro. The suppressors of cytokine signaling (SOCS) proteins are negative-feedback regulators of cytokine signaling. The purpose of this study was to investigate the role of SOCS in DN. The results demonstrated that overexpression of SOCS ameliorated proteinuria, the expression of α-SMA and OSM in tubular epithelial cells, and interstitial extracellular matrix accumulation in the renal tissue of CD-1 mice. In addition, our previous studies indicated that OSM induced TEMT by activating the JAK/STAT pathway, which could be inhibited by SOCS. These results indicate that overexpression of SOCS has a therapeutic effect in DN.     

Functional significance of the negative-feedback regulation of ATP release via pannexin-1 hemichannels under ischemic stress in astrocytes

The opening of pannexin-1 (Px1) hemichannels is regulated by the activity of P2X(7) receptors (P2X(7)Rs). At present, however, little is known about how extracellular ATP-sensitive P2X(7)Rs regulates the opening and closure of Px1 hemichannels. Several lines of evidence suggest that P2X(7)Rs are activated under pathological conditions such as ischemia, resulting in the opening of Px1 hemichannels responsible for the massive influx of Ca(2+) from the extracellular space and the release of ATP from the cytoplasm, leading to cell death. Here we show in cultured astrocytes that the suppression of the activity of P2X(7)Rs during simulated ischemia (oxygen/glucose deprivation, OGD) resulted in the opening of Px1 hemichannels, leading to the enhanced release of ATP. In addition, the suppression of the activity of P2X(7)Rs during OGD resulted in a significant increase in astrocytic damage. Both the P2X(7)Rs suppression-induced enhancement of the release of ATP and cell damage were reversed by co-treatment with blockers of Px1 hemichannels, suggesting that suppression of the activity of PX(7)Rs resulted in the opening of Px1 hemichannels. All these findings suggested the existence of a negative-feedback loop regulating the release of ATP via Px1 hemichannels; ATP-induced suppression of ATP release. The present study indicates that ATP, released through Px1 hemichannels, activates P2X(7)Rs, resulting in the closure of Px1 hemichannels during ischemia. This negative-feedback mechanism, suppressing the loss of cellular ATP and Ca(2+) influx, might contribute to the survival of astrocytes under ischemic stress.     

Second messenger signalling in olfaction.

The primary reactions of the chemo-electrical signal transduction pathway in olfactory receptor neurons are mediated by two alternative second messengers, cAMP and inositol 1,4,5-trisphosphate. The rapid and transient intracellular signalling is terminated by the action of negative-feedback loops which uncouple the reaction cascades (desensitization). Recent evidence suggests that secondary reactions in olfaction (adaptation) may also be controlled by second messengers.     

Estradiol treatment increases feeding-induced c-Fos expression in the brains of ovariectomized rats

The steroid hormone estradiol decreases meal size by increasing the potency of negative-feedback signals involved in meal termination. We used c-Fos immunohistochemistry, a marker of neuronal activation, to investigate the hypothesis that estradiol modulates the processing of feeding-induced negative-feedback signals within the nucleus of the solitary tract (NTS), the first central relay of the neuronal network controlling food intake, and within other brain regions related to the control of food intake. Chow-fed, ovariectomized rats were injected subcutaneously with 10 microg 17-beta estradiol benzoate or sesame oil vehicle on 2 consecutive days. Forty-eight hours after the second injections, 0, 5, or 10 ml of a familiar sweet milk diet were presented for 20 min at dark onset. Rats were perfused 100 min later, and brain tissue was collected and processed for c-Fos-like immunoreactivity. Feeding increased the number of c-Fos-positive cells in the NTS, the paraventricular nucleus of the hypothalamus (PVN), and the central nucleus of the amygdala (CeA) in oil-treated rats. Estradiol treatment further increased this response in the caudal, subpostremal, and intermediate NTS, which process negative-feedback satiation signals, but not in the rostral NTS, which processes positive-feedback gustatory signals controlling meal size. Estradiol treatment also increased feeding-induced c-Fos in the PVN and CeA. These results indicate that modest amounts of food increase neuronal activity within brain regions implicated in the control of meal size in ovariectomized rats and that estradiol treatment selectively increases this activation. They also suggest that estradiol decreases meal size by increasing feeding-related neuronal activity in multiple regions of the distributed neural network controlling meal size.     

Wnt3a plays a major role in the segmentation clock controlling somitogenesis

The vertebral column derives from somites generated by segmentation of presomitic mesoderm (PSM). Somitogenesis involves a molecular oscillator, the segmentation clock, controlling periodic Notch signaling in the PSM. Here, we establish a novel link between Wnt/beta-catenin signaling and the segmentation clock. Axin2, a negative regulator of the Wnt pathway, is directly controlled by Wnt/beta-catenin and shows oscillating expression in the PSM, even when Notch signaling is impaired, alternating with Lfng expression. Moreover, Wnt3a is required for oscillating Notch signaling activity in the PSM. We propose that the segmentation clock is established by Wnt/beta-catenin signaling via a negative-feedback mechanism and that Wnt3a controls the segmentation process in vertebrates.     

SOCS2 can enhance interleukin-2 (IL-2) and IL-3 signaling by accelerating SOCS3 degradation

Cytokine responses can be regulated by a family of proteins termed suppressors of cytokine signaling (SOCS) which can inhibit the JAK/STAT pathway in a classical negative-feedback manner. While the SOCS are thought to target signaling intermediates for degradation, relatively little is known about how their turnover is regulated. Unlike other SOCS family members, we find that SOCS2 can enhance interleukin-2 (IL-2)- and IL-3-induced STAT phosphorylation following and potentiate proliferation in response to cytokine stimulation. As a clear mechanism for these effects, we demonstrate that expression of SOCS2 results in marked proteasome-dependent reduction of SOCS3 and SOCS1 protein expression. Furthermore, we provide evidence that this degradation is dependent on the presence of an intact SOCS box and that the loss of SOCS3 is enhanced by coexpression of elongin B/C. This suggests that SOCS2 can bind to SOCS3 and elongin B/C to form an E3 ligase complex resulting in the degradation of SOCS3. Therefore, SOCS2 can enhance cytokine responses by accelerating proteasome-dependent turnover of SOCS3, suggesting a mechanism for the gigantism observed in SOCS2 transgenic mice.