The Goldbeter-Koshland switch in the first-order region and its response to dynamic disorder

In their classical work (Proc. Natl. Acad. Sci. USA, 1981, 78:6840-6844), Goldbeter and Koshland mathematically analyzed a reversible covalent modification system which is highly sensitive to the concentration of effectors. Its signal-response curve appears sigmoidal, constituting a biochemical switch. However, the switch behavior only emerges in the "zero-order region", i.e. when the signal molecule concentration is much lower than that of the substrate it modifies. In this work we showed that the switching behavior can also occur under comparable concentrations of signals and substrates, provided that the signal molecules catalyze the modification reaction in cooperation. We also studied the effect of dynamic disorders on the proposed biochemical switch, in which the enzymatic reaction rates, instead of constant, appear as stochastic functions of time. We showed that the system is robust to dynamic disorder at bulk concentration. But if the dynamic disorder is quasi-static, large fluctuations of the switch response behavior may be observed at low concentrations. Such fluctuation is relevant to many biological functions. It can be reduced by either increasing the conformation interconversion rate of the protein, or correlating the enzymatic reaction rates in the network.Comment: 23 pages, 4 figures, accepted by PLOS ON

Quantitative Analysis of Phase Wave of Gene Expression in the Mammalian Central Circadian Clock Network

BACKGROUND: The suprachiasmatic nucleus (SCN), the master circadian clock, is a heterogeneous oscillator network, yet displays a robust synchronization dynamics. Recent single-cell bioluminescent imaging revealed temporal gradients in circadian clock gene expression in the SCN ex vivo. However, due to technical difficulty in biological approaches to elucidate the entire network structure of the SCN, characteristics of the gradient, which we refer to as phase wave, remain unknown. METHODOLOGY/PRINCIPAL FINDINGS: We implemented new approaches, i.e., quantitative analysis and model simulation to characterize the phase waves in Per2::Luciferase clock reporter gene expression of the rat SCN slice. Our quantitative study demonstrated not only a high degree of synchronization between the neurons and regular occurrence of the phase wave propagation, but also a significant amount of phase fluctuations contained in the wave. In addition, our simulations based on local coupling model suggest that the intercellular coupling strength estimated by the model simulations is significantly higher than the critical value for generating the phase waves. Model simulations also suggest that heterogeneity of the SCN neurons is one of the main factors causing the phase wave fluctuations. Furthermore, robustness of the SCN network against dynamical noise and variation of the natural frequencies inherent in these neurons was quantitatively assessed. CONCLUSIONS/SIGNIFICANCE: To our knowledge, this is the first quantitative evaluation of the phase wave and further characterization of the SCN neuronal network features generating the wave i.e., intercellular synchrony, phase fluctuation, strong local coupling, heterogeneous periodicity and robustness. Our present study provides an approach, which will lead to a comprehensive understanding of mechanistic and/or biological significance of the phase wave in the central circadian oscillatory system

Flavonoids from Pterogyne nitens Inhibit Hepatitis C Virus Entry

Hepatitis C virus (HCV) is one of the leading causes of liver diseases and transplantation worldwide. The current available therapy for HCV infection is based on interferon-α, ribavirin and the new direct-acting antivirals (DAAs), such as NS3 protease and NS5B polymerase inhibitors. However, the high costs of drug design, severe side effects and HCV resistance presented by the existing treatments demonstrate the need for developing more efficient anti-HCV agents. This study aimed to evaluate the antiviral effects of sorbifolin (1) and pedalitin (2), two flavonoids from Pterogyne nitens on the HCV replication cycle. These compounds were investigated for their anti-HCV activities using genotype 2a JFH-1 subgenomic replicons and infectious virus systems. Flavonoids 1 and 2 inhibited virus entry up to 45.0% and 78.7% respectively at non-cytotoxic concentrations. The mechanism of the flavonoid 2 block to virus entry was demonstrated to be by both the direct action on virus particles and the interference on the host cells. Alternatively, the flavonoid 1 activity was restricted to its virucidal effect. Additionally, no inhibitory effects on HCV replication and release were observed by treating cells with these flavonoids. These data are the first description of 1 and 2 possessing in vitro anti-HCV activity

NMDA Receptor Stimulation Induces Reversible Fission of the Neuronal Endoplasmic Reticulum

With few exceptions the endoplasmic reticulum (ER) is considered a continuous system of endomembranes within which proteins and ions can move. We have studied dynamic structural changes of the ER in hippocampal neurons in primary culture and organotypic slices. Fluorescence recovery after photobleaching (FRAP) was used to quantify and model ER structural dynamics. Ultrastructure was assessed by electron microscopy. In live cell imaging experiments we found that, under basal conditions, the ER of neuronal soma and dendrites was continuous. The smooth and uninterrupted appearance of the ER changed dramatically after glutamate stimulation. The ER fragmented into isolated vesicles in a rapid fission reaction that occurred prior to overt signs of neuronal damage. ER fission was found to be independent of ER calcium levels. Apart from glutamate, the calcium ionophore ionomycin was able to induce ER fission. The N-methyl, D-aspartate (NMDA) receptor antagonist MK-801 inhibited ER fission induced by glutamate as well as by ionomycin. Fission was not blocked by either ifenprodil or kinase inhibitors. Interestingly, sub-lethal NMDA receptor stimulation caused rapid ER fission followed by fusion. Hence, ER fission is not strictly associated with cellular damage or death. Our results thus demonstrate that neuronal ER structure is dynamically regulated with important consequences for protein mobility and ER luminal calcium tunneling

An association between polymorphism of the heme oxygenase-1 and -2 genes and age-related macular degeneration

Iron may be implicated in the generation of oxidative stress by the catalyzing the Haber–Weiss or Fenton reaction. On the other hand, oxidative stress has been implicated in the pathogenesis of age-related macular degeneration (AMD) and heme oxygenase-1 (HO-1), encoded by the HMOX1 gene and heme oxygenase-2 (HO-2), encoded by the HMOX2 gene are important markers of iron-related oxidative stress and its consequences. Therefore, variability of the HMOX1 and HMOX2 genes might be implicated in the pathogenesis of AMD through the modulation of the cellular reaction to oxidative stress. In the present work, we investigated the association between AMD and a G → C transversion at the 19 position in the HMOX1 gene (the 19G>C-HMOX1 polymorphism, rs2071747) and a A → G transition at the −42 + 1444 position in the HMOX2 gene (the −42 + 1444A>G-HMOX2 polymorphism, rs2270363) and its modulation by some environmental factors. 279 patients with AMD and 105 controls were recruited in this study and the polymorphisms were typed by restriction fragment length polymorphism and allele-specific polymerase chain reaction (PCR). We observed an association between the occurrence of dry AMD and the G/A genotype of the −42 + 1444A>G-HMOX2 polymorphism (odds ratio (OR) 2.72), whereas the G/G genotype reduced the risk of dry AMD (OR 0.41). The G/C genotype and the C allele of the 19 G>C-HMOX1 polymorphism and the G/G genotype and the G allele of the −42 + 1444A>G-HMOX2 polymorphism were associated with progression of AMD from dry to wet form (OR 4.83, 5.20, 2.55, 1.69, respectively). On the other hand, the G/G genotype and the G allele of the 19 G>C-HMOX1 polymorphism and the A/G genotype and the A allele of the −42 + 1444A>G-HMOX2 polymorphism protected against AMD progression (OR 0.19, 0.19, 0.34, 0.59, respectively). Therefore, the 19G>C-HMOX1 and the −42 + 1444A>G-HMOX2 polymorphisms may be associated with the occurrence and progression of AMD

Horizontal Transmission of Candida albicans and Evidence of a Vaccine Response in Mice Colonized with the Fungus

Disseminated candidiasis is the third leading nosocomial blood stream infection in the United States and is often fatal. We previously showed that disseminated candidiasis was preventable in normal mice by immunization with either a glycopeptide or a peptide synthetic vaccine, both of which were Candida albicans cell wall derived. A weakness of these studies is that, unlike humans, mice do not have a C. albicans GI flora and they lack Candida serum antibodies. We examined the influence of C. albicans GI tract colonization and serum antibodies on mouse vaccination responses to the peptide, Fba, derived from fructose bisphosphate aldolase which has cytosolic and cell wall distributions in the fungus. We evaluated the effect of live C. albicans in drinking water and antimicrobial agents on establishment of Candida colonization of the mouse GI tract. Body mass, C. albicans in feces, and fungal-specific serum antibodies were monitored longitudinally. Unexpectedly, C. albicans colonization occurred in mice that received only antibiotics in their drinking water, provided that the mice were housed in the same room as intentionally colonized mice. The fungal strain in unintentionally colonized mice appeared identical to the strain used for intentional GI-tract colonization. This is the first report of horizontal transmission and spontaneous C. albicans colonization in mice. Importantly, many Candida-colonized mice developed serum fungal-specific antibodies. Despite the GI-tract colonization and presence of serum antibodies, the animals made antibodies in response to the Fba immunogen. This mouse model has potential for elucidating C. albicans horizontal transmission and for exploring factors that induce host defense against disseminated candidiasis. Furthermore, a combined protracted GI-tract colonization with Candida and the possibility of serum antibody responses to the presence of the fungus makes this an attractive mouse model for testing the efficacy of vaccines designed to prevent human disseminated candidiasis

Enhanced Food Anticipatory Activity Associated with Enhanced Activation of Extrahypothalamic Neural Pathways in Serotonin2C Receptor Null Mutant Mice

The ability to entrain circadian rhythms to food availability is important for survival. Food-entrained circadian rhythms are characterized by increased locomotor activity in anticipation of food availability (food anticipatory activity). However, the molecular components and neural circuitry underlying the regulation of food anticipatory activity remain unclear. Here we show that serotonin2C receptor (5-HT2CR) null mutant mice subjected to a daytime restricted feeding schedule exhibit enhanced food anticipatory activity compared to wild-type littermates, without phenotypic differences in the impact of restricted feeding on food consumption, body weight loss, or blood glucose levels. Moreover, we show that the enhanced food anticipatory activity in 5-HT2CR null mutant mice develops independent of external light cues and persists during two days of total food deprivation, indicating that food anticipatory activity in 5-HT2CR null mutant mice reflects the locomotor output of a food-entrainable oscillator. Whereas restricted feeding induces c-fos expression to a similar extent in hypothalamic nuclei of wild-type and null mutant animals, it produces enhanced expression in the nucleus accumbens and other extrahypothalamic regions of null mutant mice relative to wild-type subjects. These data suggest that 5-HT2CRs gate food anticipatory activity through mechanisms involving extrahypothalamic neural pathways