Negative Feedback Control Mechanism Research Articles

Very long-chain fatty acids control peroxisome dynamics via a feedback loop in intestinal stem cells during gut regeneration.

Peroxisome dynamics are crucial for intestinal stem cell (ISC) differentiation and gut regeneration. However, the precise mechanisms that govern peroxisome dynamics within ISCs during gut regeneration remain unknown. Using mouse colitis and Drosophila intestine models, we have identified a negative-feedback control mechanism involving the transcription factors peroxisome proliferator-activated receptors (PPARs) and SOX21. This feedback mechanism effectively regulates peroxisome abundance during gut regeneration. Following gut injury, the released free very long-chain fatty acids (VLCFAs) increase peroxisome abundance by stimulating PPARs-PEX11s signaling. PPARs act to stimulate peroxisome fission and inhibit pexophagy. SOX21, which acts downstream of peroxisomes during ISC differentiation, induces peroxisome elimination through pexophagy while repressing PPAR expression. Hence, PPARs and SOX21 constitute a finely tuned negative-feedback loop that regulates peroxisome dynamics. These findings shed light on the complex molecular mechanisms underlying peroxisome regulation in ISCs, contributing to our understanding of gut renewal and repair.

STAT5-Feedback Controls Distinct Metabolic States for Dynamic Transitions between Cellular Activation and Quiescence in Acute Lymphoblastic Leukemia

Background and significance: B-cell acute lymphoblastic leukemia (B-ALL) is characterized by dynamic shifts between activation and quiescence. These cycles are predicated by fluctuations in oncogenic signaling, which mimic pre-BCR and cytokine receptor signaling, e.g. IL7 receptor signaling and phosphorylation of STAT5. These fluctuations in cytokine receptor and STAT5 signaling are mitigated by powerful feedback regulators, including CISH and SOCS (suppressors of cytokine signaling) family members. While B-ALL cells depend on oncogenic STAT5 signaling, CISH and SOCS2 negative feedback regulators of STAT5 are highly expressed in aggressive B-ALL and high expression levels are strongly linked to poor clinical outcome, positive MRD and increased likelihood of bone marrow relapse in children (COG P9906) and adults (ECOG E2993). To determine whether high levels of CISH and SOCS2 are biomarkers of high oncogenic STAT5 signaling or mechanistically contribute to clonal fitness and adaptability of B-ALL, we performed genetic experiments in mouse models for B-ALL. Here we examined the mechanisms and consequences of active and inactive states of STAT5 by studying constitutively active (CA) and non-phosphorylatable Y694F (YF) mutations of STAT5. Results: Consistent with a central role of STAT5-feedback control in B-ALL survival, inducible ablation of Cishfl/fl resulted in rapid depletion of B-ALL cells. Socs2-/- B-ALL cells rapidly underwent cellular senescence, lacked colony forming and leukemia-initiating capacity in serial transplantation experiments. Likewise direct interference with STAT5-function, by mutations resulting in constitutive activation (STAT5-CA) or an inactive state (STAT5-YF). Consistent with a requirement of adaptive STAT5-feedback control in B-ALL, both Stat5-CA and Stat5-YF resulted in rapid cell death. Induction of Stat5-CA caused accumulation of biomass indicated by their increased cell size, whereas Stat5-YF caused rapid cell shrinkage ( Figure A). To determine whether the mTOR cell growth pathway was responsible for cell death induced by Stat5-CA, we tested whether the genetic deletion of Mtor or pharmacological inhibition of protein synthesis were sufficient to rescue B-ALL cell death. As expected, B-ALL cells retaining intact Mtor rapidly underwent energy crisis and cell death, while genetic deletion of Mtor reversed the toxic effects of Stat5-CA, suggesting that Stat5-CA is associated with excessive protein synthesis and defective ER turnover. Indeed, Stat5-CA increased protein synthesis rate and ER content compared to EV control, and these effects were opposed by Stat5-YF. To compare the metabolic outcomes of Stat5-CA and Stat5-YF, we performed mass spectrometry-based metabolomic analyses ( Figure B). Consistent with the role of mTOR in promoting glycolysis, Stat5-CA increased the content of glycolytic intermediates in B-ALL cells. In contrast, Stat5-YF increased phospholipid intermediates and phosphatidylethanolamine (PtdEtn). Opposed by Myc, Stat5-YF increased the expression levels of Bcl6 and autophagic genes, suggesting that Bcl6-dependent programs promote PtdEtn-LC3B conjugation, thereby autophagosome formation. As expected, microscopic validation with LC3B-GFP reporter revealed that Stat5-YF increased the number of LC3B-GFP puncta, upon pharmacological inhibition of autophagosome-lysosome fusion ( Figure A). Consistent with STAT5-CA promoting Myc- as opposed to STAT5-YF driving Bcl6-dependent programs, a flow cytometry analysis of Myc-eGFP/Bcl6-mCherry dual reporter expression in murine B-ALL cells demonstrated that changes between glycolytic Myc+ and autophagic Bcl6+ states were the direct consequence of Stat5-CA and Stat5-YF induction, respectively. Conclusions: Our results demonstrate feedback regulators of STAT5, including SOCS2 and CISH are not only biomarkers of increased oncogenic STAT5 activity and a more aggressive course of disease linked to poor overall outcomes. Importantly, negative STAT5 feedback control mechanisms are essential to enable nimble adaptations of B-ALL cells in response to fluctuations of oncogenic signaling strength and availability of nutrients during distinct metabolic states of cellular activation (MYC) and quiescence (BCL6).

The Active Gate Drive Based on Negative Feedback Mechanism for Fast Switching and Crosstalk Suppression of SiC Devices

This article introduces the negative feedback into the gate drive. It proposes a negative feedback active gate drive (NFAGD) for silicon carbide devices to fully utilize their potential of high switching-speed capability in a phase-leg configuration. An auxiliary P-channel <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> is introduced to construct a negative feedback control mechanism. Due to the negative feedback mechanism, the proposed drive can automatically attenuate the disturbance from the complementary device of the phase-leg. Compared to conventional gate drives, the proposed drive can suppress the crosstalk without influencing the switching speed, thus enables coordinated optimization of gate voltage stability and switching behavior. This article analyzes the operation principle and then recommend the fundamental design principle for the proposed NFAGD. Finally, experiment results demonstrate the effectiveness of the proposed NFAGD.

Neutrophils: Amoeboid Migration and Swarming Dynamics in Tissues.

Neutrophils are key cells of our innate immune response with essential roles for eliminating bacteria and fungi from tissues. They are also the prototype of an amoeboid migrating leukocyte. As one of the first blood-recruited immune cell types during inflammation and infection, these cells can invade almost any tissue compartment. Once in the tissue, neutrophils undergo rapid shape changes and migrate at speeds higher than most other immune cells. They move in a substrate-independent manner in interstitial spaces and do not follow predetermined tissue paths. Instead, neutrophil navigation is largely shaped by the chemokine and chemoattractant milieu around them. This highlights the decisive role of attractant-sensing G-protein coupled receptors (GPCRs) and downstream molecular pathways for controlling amoeboid neutrophil movement in tissues. A diverse repertoire of cell-surface expressed GPCRs makes neutrophils the perfect sentinel cell type to sense and detect danger-associated signals released from wounds, inflamed interstitium, dying cells, complement factors or directly from tissue-invading microbes. Moreover, neutrophils release attractants themselves, which allows communication and coordination between individual cells of a neutrophil population. GPCR-mediated positive feedback mechanisms were shown to underlie neutrophil swarming, a population response that amplifies the recruitment of amoeboid migrating neutrophils to sites of tissue injury and infection. Here we discuss recent findings and current concepts that counteract excessive neutrophil accumulation and swarm formation. In particular, we will focus on negative feedback control mechanisms that terminate neutrophil swarming to maintain the delicate balance between tissue surveillance, host protection and tissue destruction.

In silico Analysis of Single Nucleotide Polymorphism in INHA Gene of Sheep and Goats

Inhibin A (INHA) a member of the transforming growth factor-β (TGF-β) family has been implicated in the negative feedback control mechanism of the pituitary follicle stimulating hormone (FSH). Inhibin has been reported to be associated with litter size, milk yield, fertility and reproductive traits in ruminants.&#x0D; A total of ten amino acid sequences (four sheep and six goats) were downloaded from the National Centre for Biotechnology Information database (http://www.ncbi.nlm.nih.gov/snp). The amino acid sequence alignment was carried out using ClustalW algorithm of Molecular Evolutionary Genetic Analysis software version 6.0. The functional effects of eighteen (18) amino acid substitutions of INHA gene in each of sheep and goats were predicted computationally using Polyphen-2, PROVEAN and SIFT algorithms while INHA gene functions and interactions with associated genes were investigated using GeneMANIA. Variants that were consensually predicted to be deleterious by the three algorithms utilised were referred to as ‘Cmutant’ and ‘Dmutant’ in sheep and goats, respectively. The MutPred was further used to determine the tolerance degree for each amino acid substitution of both the ‘Cmutant’ and ‘Dmutant’.&#x0D; GeneMANIA revealed 20 genes that co-localised, co-expressed with, play functionally similar role or has physical or genetic interaction with INHA gene. Out of the studied eighteen (18) amino acid substitutions; there was a consensus by SIFT, PROVEAN and PolyPhen-2 algorithms in the prediction of variants C99R, C264D, L237T and F14C as being deleterious in sheep and variants D77F, L190K, R223D, L240N, C322T and P326C in goats. In goats, MutPred revealed that variants R223D and D77F were not harmful while, L190K, R223D, L240N and P326C mutations were observed to be highly harmful. In sheep, the ‘Dmutants’ (C99R, C264D, F14C and L237T) were predicted to be highly harmful. The obtained findings would be useful in planning research aiming at exploring the association between INHA gene variants and economically important traits of small ruminants.

Alteration of Carotenoid Metabolic Machinery by β-Carotene Biofortification in Rice Grains

To increase nutritional values as dietary sources of provitamin A and health-promoting antioxidants in rice grains, carotenoids have been biofortified in a Golden Ricelike variety, the stPAC (stPsy-2A-stCrtI) rice. In both of non-transgenic (NT) and stPAC seeds, total chlorophylls and carotenoids were gradually decreased during seed development while de novo biosynthesized carotenoids being comprised mainly of β-carotene, lutein and zeaxanthin were accumulated from early stage of 10 DAF and peaked at 20 DAF in stPAC seeds. The de novo production of carotenoids coincided with the high levels of transgene expression driven by the rice globulin gene promoter. Interestingly, expression levels of endogenous carotenoid metabolic pathway genes were the highest at 30 DAF in NT seeds whereas they were generally down-regulated in stPAC seeds, suggesting a negative feedback control mechanism of carotenoid metabolism by enhanced carotenoid production. The transgenic protein levels in stPAC seeds were not changed much during seed storage for up to 5 years, while carotenoid contents in the seeds were decreasing after 1 year of storage. The decrease in carotenoid contents was restored when the transgenic plants re-grown, supporting the reliability of transgenic pathways for carotenoid biofortification in rice grains. Thus, our results showed that transgene-driven biofortification of carotenoids was made and maintained over several transgenic generations with a possible negative feed-back control of endogenous carotenoid metabolism during seed development.

Simulation of glucose regulating mechanism with an agent-based software engineering tool

This study provides a detailed explanation of a regulating mechanism of the blood glucose levels by an agent-based software engineering tool. Repast Simphony which is used in implementation of this study is an agent-based software engineering tool based on the object-oriented programming using Java language. Agent-based modeling and simulation is a computational methodology for simulating and exploring phenomena that includes a large set of active components represented by agents. The agents are main components situated in space and time of agent-based simulation environment. In this study, we present hormonal regulation of blood glucose levels by our improved agent-based control mechanism. Hormonal regulation of blood glucose levels is an important process to maintain homeostasis inside the human body. We offer a negative feedback control mechanism with agent-based modeling approach to regulate the secretion of insulin hormone which is responsible for increasing the blood glucose levels. The negative feedback control mechanism run by three main agents that interact with each other to perform their local actions in the simulation environment. The result of this study shows the local behavior of the agents in the negative feedback loop and illustrates how to balance the blood glucose levels. Finally, this study which is thought a potential implementation of agent-based modeling and simulation may contribute to the exploration of other homeostatic control systems inside the human body.

EZH2 induces the expression of miR-1301 as a negative feedback control mechanism in triple negative breast cancer.

Breast cancer is one of the most common malignancies in women. ERα, PR, and HER2 triple negative breast cancer (TNBC) is the current research focus because of the lack of effective targeted therapies. In our study, lentivirus systems were used to overexpress EZH2 and miR-1301 in TNBC cell lines. Western blot analysis and RT-qPCR were used to detect the protein and microRNA levels. The TCGA and Kaplan Meier plotter databases were used to analyze the EZH2 and miR-1301 expression levels in breast cancer. The effect of miR-1301 overexpression on cell proliferation, migration and colony formation were determined by using the sulforhodamine B (SRB) assay, wound healing assay and colony formation assay, respectively. Furthermore, an xenograft mouse model was used to investigate the function of miR-1301 overexpression in vivo. Finally, dual luciferase reporter assay was used to verify the binding site of EZH2 and miR-1301. We found that EZH2 induced the expression of miR-1301 in two TNBC cell lines, HCC1937 and HCC1806. Overexpression of miR-1301 suppressed TNBC cell proliferation, migration and colony formation, as well as the xenograft tumor growth in immunodeficient mice. Interestingly, miR-1301 inhibited the expression of EZH2 by binding to the 3'-UTR of EZH2 gene. These data suggest that EZH2 induces the expression of miR-1301 as a negative feedback control mechanism in TNBC.

Agent-based modeling and simulation of blood vessels in the cardiovascular system

The purpose of this study is to develop a model to simulate the behavior of the human cardiovascular system for use in medical education. The proposed model ensures that the output of the system is accurately represented in both equilibrium conditions and imbalance conditions including in the presence of adaptive agents. In this study, field experts develop an agent-based blood vessel model, i.e., a submodel for the stated purpose. In the proposed blood vessel model, vessels are represented by agents whereas blood flow is represented by the interaction between agents. Adaptive behavior shown by vessels in terms of resistance to the blood flow is defined by the agents’ properties, which are used as the basis for calculating and graphically representing the physical parameters of blood flow, specifically blood pressure, blood flow velocity, and the resistance of the vessel. The adaptation of the vessel agents is supported by a case study, which demonstrates the adaptive behavior of the blood vessel agents through a negative feedback control mechanism. The blood vessel model proposed is flexible in nature such that it can be adapted to account for the behavior of the vessel sections in any vascular structure.

Mitogen-activated protein kinase phosphatase-1: a critical phosphatase manipulating mitogen-activated protein kinase signaling in cardiovascular disease (review).

Mitogen-activated protein kinase (MAPK) cascades are important players in the overall representation of cellular signal transduction pathways, and the deregulation of MAPKs is involved in a variety of diseases. The activation of MAPK signals occurs through phosphorylation by MAPK kinases at conserved threonine and tyrosine (Thr-Xaa-Tyr) residues. The mitogen-activated protein kinase phosphatases (MKPs) are a major part of the dual-specificity family of phosphatases and specifically inactivate MAPKs by dephosphorylating both phosphotyrosine and phosphoserine/phosphothreonine residues within the one substrate. MAPKs binding to MKPs can enhance MKP stability and activity, providing an important negative-feedback control mechanism that limits the MAPK cascades. In recent years, accumulating and compelling evidence from studies mainly employing cultured cells and mouse models has suggested that the archetypal MKP family member, MKP-1, plays a pivotal role in cardiovascular disease as a major negative modulator of MAPK signaling pathways. In the present review, we summarize the current knowledge on the pathological properties and the regulation of MKP-1 in cardiovascular disease, which may provide valuable therapeutic options.

Calcium-dependent inactivation of calcium channels in the medial striatum increases at eye opening.

Influx of calcium through voltage-gated calcium channels (VGCCs) is essential for striatal function and plasticity. VGCCs expressed in striatal neurons have varying kinetics, voltage dependences, and densities resulting in heterogeneous subcellular calcium dynamics. One factor that determines the calcium dynamics in striatal medium spiny neurons is inactivation of VGCCs. Aside from voltage-dependent inactivation, VGCCs undergo calcium-dependent inactivation (CDI): inactivating in response to an influx of calcium. CDI is a negative feedback control mechanism; however, its contribution to striatal neuron function is unknown. Furthermore, although the density of VGCC expression changes with development, it is unclear whether CDI changes with development. Because calcium influx through L-type calcium channels is required for striatal synaptic depression, a change in CDI could contribute to age-dependent changes in striatal synaptic plasticity. Here we use whole cell voltage clamp to characterize CDI over developmental stages and across striatal regions. We find that CDI increases at the age of eye opening in the medial striatum but not the lateral striatum. The developmental increase in CDI mostly involves L-type channels, although calcium influx through non-L-type channels contributes to the CDI in both age groups. Agents that enhance protein kinase A (PKA) phosphorylation of calcium channels reduce the magnitude of CDI after eye opening, suggesting that the developmental increase in CDI may be related to a reduction in the phosphorylation state of the L-type calcium channel. These results are the first to show that modifications in striatal neuron properties correlate with changes to sensory input.

Control of Potassium: Distribution, Excretion, and Extracellular Concentration

Abstract Download Free Sample Several physiological mechanisms act to regulate renal potassium excretion and distribution. Together they comprise an integrated control system that may be envisioned as being made up of several interacting negative feedback control mechanisms, all affecting removal of potassium from the extracellular fluid by renal excretion or transfer across cell membranes into or out of the intracellular compartment. The intent of this presentation is to describe the mechanisms of potassium regulation, beginning in Chapter 1 with an overview of the system, Chapter 2 with mechanisms that determine movement of potassium between the extracellular and intracellular compartments, in Chapter 3 focusing on renal tubular transport systems, and in Chapter 4 presenting the mechanisms of control of aldosterone secretion. Greatest emphasis will be on describing the integrated functions of the components, in Chapter 5, as they operate together in a system to regulate extracellular potassium concentra...

A patient-specific model of the negative-feedback control of the hypothalamus-pituitary-thyroid (HPT) axis in autoimmune (Hashimoto's) thyroiditis

The purpose of modelling the negative-feedback control mechanism of the hypothalamus-pituitary-thyroid (HPT) axis in autoimmune (Hashimoto's) thyroiditis is to describe the clinical course of euthyroidism, subclinical hypothyroidism and overt hypothyroidism for patients. Thyroid hormone thyroxine (T4) and triiodothyronine (T3) levels are controlled by negative-feedback control through thyroid-stimulating hormone (TSH). T4, like other hormones, can be bound or unbound; the unbound T4 (FT4) is used as a marker for hypothyroidism. Autoimmune thyroiditis is a disease in which the thyroid-infiltrating lymphocytes attack autoantigens in follicle cells, destroying them over a long time. To describe the operation of the feedback control, we developed a mathematical model involving four clinical variables: TSH, FT4, anti-thyroid peroxidase antibodies and the thyroid gland's functional size. The first three variables are regularly measured while the last variable is determined through relationships between the other three variables. The problem of two different time scales for circulating hormones and thyroid damage is addressed using singular perturbation theory. Analysis of the mathematical model establishes stability and conditions under which the diseased state can maintain the slow movement toward diseased state equilibrium. Although we have used four variables in modelling the feedback control through the HPT axis, the predicted clinical course given any set of parameters is shown to depend on the steady-state levels of TSH and FT4. This observation makes possible the development of the clinical charts based only on the levels of TSH, time and potential steady-state values. To validate the model predictions, a dataset obtained from a Sicilian adult population has been employed.

A role for inactivation of the Foxo1 transcription factor in the differentiation of T follicular helper cells (P1131)

Abstract We have previously shown that mice with a T cell-specific deletion of Foxo1 develop autoimmunity due to reduced Foxp3+ regulatory T cell development and function. These mice also have an increased percentage and absolute number of CXCR5+PD1+ T follicular helper (TfH) cells, a T cell subset that is able to help B cells, and this may help to explain how high levels of auto-antibodies develop in mice in which Foxo1 is specifically deleted from T cells. However, it was unclear if the large number of TfH cells is due to a cell intrinsic role for Foxo1 in restricting TfH differentiation or more indirectly the result of inflammatory autoimmunity. We now report that activated Foxo1-deficient CD4 T cells more readily differentiate into TfH cells. This increased differentiation into TfH cells occurs despite reduced expression of ICOS, a key molecule leading to TfH differentiation, in the absence of Foxo1. Furthermore, Foxo1-deficient CD4 T cells can differentiate into TfH cells in the absence of ICOS signaling. Additionally, ICOS stimulation rapidly decreases Foxo1 protein levels in activated CD4 T cells suggesting a negative feedback control mechanism. In conclusion, our data strongly suggest that ICOS signaling serves to inactivate Foxo1 in the differentiation of TfH.

Immune Cell Response to Negative Feedback Effect on HIV

Alteration in host-virus interaction dynamics during long-term infection by HIV necessitates consideration of inverse relationship between high viral load and density of CTL response leading to dis-regulation of host immunity system. Mathematical modeling introducing negative feedback control mechanism helps in establishment of threshold condition for disease eradication as well as necessary conditions for existence of different equilibria depending on values of basic reproduction ratio. Moreover, IL-2 adjuvenated HAART therapy has been found to be highly cost-effective in recovery of the immunity status in the present mathematical model using negative feedback effect. KeywordsHIV; CD4T Cells; CTL; Negative Feedback Control; HAART; IL-2

Abstract 2103: High resolution melting analysis: a sensitive screening method for the detection of MDM2 promotor SNP309

Abstract Background: Murine double minutes-2 (MDM2) protein plays a key role in physiological processes like growth arrest, senescence and apoptosis. MDM2 negatively regulates key proteins like p53 by inhibiting its transcriptional activity and promoting its proteosomal degradation through ubiquitinylation. In turn, the MDM2 expression is regulated in part by a p53-responsive promotor. This negative feedback control mechanism assures that both p53 and MDM2 are kept at very low levels in proliferating cells. However, in about half of all human tumors, the normal regulation of p53 might be disrupted through direct overexpression of MDM2 caused by e.g. MDM2 gene amplification or a T to G substitution (SNP309) in the promotor region of MDM2. High resolution melting analysis (HRMA) provides a valid approach to efficiently detect DNA mutations. The current study aimed at validating and implementing HRMA for screening of colorectal cancer patients (CRC) to detect MDM2 promotor SNP309. Pyrosequencing was used to confirm and characterize HRMA results. Materials and Methods: First, HRMA sensitivity was established using a cell line and a FFPE (formalin-fixed paraffin embedded) dilution model. Then, the sensitivity of pyrosequencing to detect MDM2 promotor SNP309 was evaluated using the same dilution models. Next, HRMA was validated on 10 cell lines. Since the MDM2 SNP309 status for these cell lines was unavailable, all HRMA results were confirmed and characterized by pyrosequencing. Results: The cell line dilution model revealed a detection limit of 6% while the detection limit in a background of FFPE wild-type DNA was found to be 3%. The detection limit of pyrosequencing to analyze SNP309 in a background of wild-type cell line DNA seems to be between 20% and 15%. In a background of FFPE wild-type DNA, the detection limit seems to be 3%. HRMA revealed abnormal melting patterns in 5/10 cell lines and pyrosequencing confirmed the presence of 2 homozygous (G/G) and 3 heterozygous (G/T) genotypes. Preliminary results on FFPE material from CRC patient samples showed abnormal melting patterns in 6/13 samples. These are currently being confirmed using pyrosequencing. Discussion: HRMA was found to be a fast, efficient, sensitive and reproducible screening method for MDM2 promotor SNP309 detection with a detection limit between 3% and 6%, which is higher than that of conventional sequencing methods. Preliminary results indicate that HRMA can be used as a screening method for MDM2 SNP309 by which DNA from FFPE tissues can be tested. Currently, a cohort of CRC patients is being tested for the presence of SNP309 using HRMA and pyrosequencing. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2103. doi:1538-7445.AM2012-2103

Adenine nucleotide control of coronary blood flow during exercise

The adenine nucleotide hypothesis postulates that the ATP released from red blood cells is broken down to ADP and AMP in coronary capillaries and that ATP, ADP, and AMP act on purinergic receptors on the surface of capillary endothelial cells. Purinergic receptor activation initiates a retrograde conducted vasodilator signal to the upstream arteriole that controls coronary blood flow in a negative feedback manner. A previous study (M. Farias 3rd, M. W. Gorman, M. V. Savage, and E. O. Feigl, Am J Physiol Heart Circ Physiol 288: H1586-H1590, 2005) demonstrated that coronary venous plasma ATP concentration increased during exercise and correlated with coronary blood flow. The present experiments test the adenine nucleotide hypothesis by examining the balance between oxygen delivery (via coronary blood flow) and myocardial oxygen consumption during exercise before and after purinergic receptor blockade. Dogs (n = 7) were chronically instrumented with catheters in the aorta and coronary sinus and a flow transducer around the circumflex coronary artery. During control treadmill exercise, myocardial oxygen consumption increased and the balance between oxygen delivery and myocardial oxygen consumption fell as indicated by a declining coronary venous oxygen tension. Blockade of P1 and P2Y(1) purinergic receptors combined with inhibition of nitric oxide synthesis significantly decreased the balance between oxygen delivery and myocardial oxygen consumption compared with control. The results support the hypothesis that ATP and its breakdown products ADP and AMP are part of a negative feedback control mechanism that matches coronary blood flow to myocardial oxygen consumption at rest and during exercise.

Elimination of Negative Feedback Control Mechanisms Along the Insulin Signaling Pathway Improves β-Cell Function Under Stress

OBJECTIVECellular stress and proinflammatory cytokines induce phosphorylation of insulin receptor substrate (IRS) proteins at Ser sites that inhibit insulin and IGF-1 signaling. Here, we examined the role of Ser phosphorylation of IRS-2 in mediating the inhibitory effects of proinflammatory cytokines and cellular stress on β-cell function.RESEARCH DESIGN AND METHODSFive potential inhibitory Ser sites located proximally to the P-Tyr binding domain of IRS-2 were mutated to Ala. These IRS-2 mutants, denoted IRS-25A, and their wild-type controls (IRS-2WT) were introduced into adenoviral constructs that were infected into Min6 cells or into cultured murine islets.RESULTSWhen expressed in cultured mouse islets, IRS-25A was better than IRS-2WT in protecting β-cells from apoptosis induced by a combination of IL-1β, IFN-γ, TNF-α, and Fas ligand. Cytokine-treated islets expressing IRS25A secreted significantly more insulin in response to glucose than did islets expressing IRS-2WT. This could be attributed to the higher transcription of Pdx1 in cytokine-treated islets that expressed IRS-25A. Accordingly, transplantation of 200 islets expressing IRS25A into STZ-induced diabetic mice restored their ability to respond to a glucose load similar to naïve mice. In contrast, mice transplanted with islets expressing IRS2WT maintained sustained hyperglycemia 3 days after transplantation.CONCLUSIONSElimination of a physiological negative feedback control mechanism along the insulin-signaling pathway that involves Ser/Thr phosphorylation of IRS-2 affords protection against the adverse effects of proinflammatory cytokines and improves β-cell function under stress. Genetic approaches that promote IRS25A expression in pancreatic β-cells, therefore, could be considered a rational treatment against β-cell failure after islet transplantation.

Autoregulatory systems controlling translation factor expression: Thermostat-like control of translational accuracy

In both prokaryotes and eukaryotes, the expression of a large number of genes is controlled by negative feedback, in some cases operating at the level of translation of the mRNA transcript. Of particular interest are those cases where the proteins concerned have cell-wide function in recognizing a particular codon or RNA sequence. Examples include the bacterial translation termination release factor RF2, initiation factor IF3, and eukaryote poly(A) binding protein. The regulatory loops that control their synthesis establish a negative feedback control mechanism based upon that protein's RNA sequence recognition function in translation (for example, stop codon recognition) without compromising the accurate recognition of that codon, or sequence during general, cell-wide translation. Here, the bacterial release factor RF2 and initiation factor IF3 negative feedback loops are reviewed and compared with similar negative feedback loops that regulate the levels of the eukaryote release factor, eRF1, established artificially by mutation. The control properties of such negative feedback loops are discussed as well as their evolution. The role of negative feedback to control translation factor expression is considered in the context of a growing body of evidence that both IF3 and RF2 can play a role in stimulating stalled ribosomes to abandon translation in response to amino acid starvation. Here, we make the case that negative feedback control serves primarily to limit the overexpression of these translation factors, preventing the loss of fitness resulting from an unregulated increase in the frequency of ribosome drop-off.

Specificity and Robustness of the Mammalian MAPK-IEG Network

The mitogen-activated protein kinase cascade is a conserved signal transduction pathway found in organisms of complexity spanning from yeast to humans. In many mammalian tissue types, this pathway can correctly transduce signals from different extracellular messengers, leading to specific and often mutually exclusive cellular responses. The transduced signal is tuned by a complicated set of positive and negative feedback control mechanisms and fed into a downstream gene expression network. This network, based on the immediate early gene system, has two possible, mutually exclusive outcomes. Using a mathematical model, we study how different stimuli lead to different temporal signal structure. Further, we investigate how each of the feedback controls contributes to the overall specificity of the gene expression output, and hypothesize that the complicated nature of the mammalian mitogen-activated protein kinase pathway results in a system able to robustly identify and transduce the proper signal without investing in two completely separate signal cascades. Finally, we quantify the role of the RKIP protein in shaping the signal, and propose a novel mechanism of its involvement in cancer metastasis.