Down-Regulation of Myogenin Can Reverse Terminal Muscle Cell Differentiation

Certain higher vertebrates developed the ability to reverse muscle cell differentiation (dedifferentiation) as an additional mechanism to regenerate muscle. Mammals, on the other hand, show limited ability to reverse muscle cell differentiation. Myogenic Regulatory Factors (MRFs), MyoD, myogenin, Myf5 and Myf6 are basic-helix-loop-helix (bHLH) transcription factors essential towards the regulation of myogenesis

Genetic background determines response to hemostasis and thrombosis

BACKGROUND: Thrombosis is the fatal and disabling consequence of cardiovascular diseases, the leading cause of mortality and morbidity in Western countries. Two inbred mouse strains, C57BL/6J and A/J, have marked differences in susceptibility to obesity, atherosclerosis, and vessel remodeling. However, it is unclear how these diverse genetic backgrounds influence pathways known to regulate thrombosis and hemostasis. The objective of this study was to evaluate thrombosis and hemostasis in these two inbred strains and determine the phenotypic response of A/J chromosomes in the C57BL/6J background. METHODS: A/J and C57Bl/6J mice were evaluated for differences in thrombosis and hemostasis. A thrombus was induced in the carotid artery by application of the exposed carotid to ferric chloride and blood flow measured until the vessel occluded. Bleeding and rebleeding times, as surrogate markers for thrombosis and hemostasis, were determined after clipping the tail and placing in warm saline. Twenty-one chromosome substitution strains, A/J chromosomes in a C57BL/6J background, were screened for response to the tail bleeding assay. RESULTS: Thrombus occlusion time was markedly decreased in the A/J mice compared to C57BL/6J mice. Tail bleeding time was similar in the two strains, but rebleeding time was markedly increased in the A/J mice compared to C57BL/6J mice. Coagulation times and tail morphology were similar, but tail collagen content was higher in A/J than C57BL/6J mice. Three chromosome substitution strains, B6-Chr5(A/J), B6-Chr11(A/J), and B6-Chr17(A/J), were identified with increased rebleeding time, a phenotype similar to A/J mice. Mice heterosomic for chromosomes 5 or 17 had rebleeding times similar to C57BL/6J mice, but when these two chromosome substitution strains, B6-Chr5(A/J )and B6-Chr17(A/J), were crossed, the A/J phenotype was restored in these doubly heterosomic progeny. CONCLUSION: These results indicate that susceptibility to arterial thrombosis and haemostasis is remarkably different in C57BL/and A/J mice. Three A/J chromosome substitution strains were identified that expressed a phenotype similar to A/J for rebleeding, the C57Bl/6J background could modify the A/J phenotype, and the combination of two A/J QTL could restore the phenotype. The diverse genetic backgrounds and differences in response to vascular injury induced thrombosis and the tail bleeding assay, suggest the potential for identifying novel genetic determinants of thrombotic risk

Computational Models of the Notch Network Elucidate Mechanisms of Context-dependent Signaling

The Notch signaling pathway controls numerous cell fate decisions during development and adulthood through diverse mechanisms. Thus, whereas it functions as an oscillator during somitogenesis, it can mediate an all-or-none cell fate switch to influence pattern formation in various tissues during development. Furthermore, while in some contexts continuous Notch signaling is required, in others a transient Notch signal is sufficient to influence cell fate decisions. However, the signaling mechanisms that underlie these diverse behaviors in different cellular contexts have not been understood. Notch1 along with two downstream transcription factors hes1 and RBP-Jk forms an intricate network of positive and negative feedback loops, and we have implemented a systems biology approach to computationally study this gene regulation network. Our results indicate that the system exhibits bistability and is capable of switching states at a critical level of Notch signaling initiated by its ligand Delta in a particular range of parameter values. In this mode, transient activation of Delta is also capable of inducing prolonged high expression of Hes1, mimicking the “ON” state depending on the intensity and duration of the signal. Furthermore, this system is highly sensitive to certain model parameters and can transition from functioning as a bistable switch to an oscillator by tuning a single parameter value. This parameter, the transcriptional repression constant of hes1, can thus qualitatively govern the behavior of the signaling network. In addition, we find that the system is able to dampen and reduce the effects of biological noise that arise from stochastic effects in gene expression for systems that respond quickly to Notch signaling

Noise Contributions in an Inducible Genetic Switch: A Whole-Cell Simulation Study

Stochastic expression of genes produces heterogeneity in clonal populations of bacteria under identical conditions. We analyze and compare the behavior of the inducible lac genetic switch using well-stirred and spatially resolved simulations for Escherichia coli cells modeled under fast and slow-growth conditions. Our new kinetic model describing the switching of the lac operon from one phenotype to the other incorporates parameters obtained from recently published in vivo single-molecule fluorescence experiments along with in vitro rate constants. For the well-stirred system, investigation of the intrinsic noise in the circuit as a function of the inducer concentration and in the presence/absence of the feedback mechanism reveals that the noise peaks near the switching threshold. Applying maximum likelihood estimation, we show that the analytic two-state model of gene expression can be used to extract stochastic rates from the simulation data. The simulations also provide mRNA–protein probability landscapes, which demonstrate that switching is the result of crossing both mRNA and protein thresholds. Using cryoelectron tomography of an E. coli cell and data from proteomics studies, we construct spatial in vivo models of cells and quantify the noise contributions and effects on repressor rebinding due to cell structure and crowding in the cytoplasm. Compared to systems without spatial heterogeneity, the model for the fast-growth cells predicts a slight decrease in the overall noise and an increase in the repressors rebinding rate due to anomalous subdiffusion. The tomograms for E. coli grown under slow-growth conditions identify the positions of the ribosomes and the condensed nucleoid. The smaller slow-growth cells have increased mRNA localization and a larger internal inducer concentration, leading to a significant decrease in the lifetime of the repressor–operator complex and an increase in the frequency of transcriptional bursts

Arthritogenicity of collagen type II is increased by chlorination

SUMMARY: During inflammation, activated neutrophils, monocytes and macrophages produce and release myeloperoxidase (MPO). MPO converts hydrogen peroxide to hypochlorous acid, a highly reactive and oxidizing agent. Proteins subjected to hypochlorous acid become chlorinated. We analysed how chlorination of the cartilage antigen collagen type II (CII) affects its immunogenic and arthritogenic properties by studying immune responses to chlorinated CII in comparison to immune responses to CII and by studying the development of arthritis in rats immunized with CII–Cl. CII–Cl immunization of LEW.1AV1 rats caused a 100% incidence of arthritis with a mean maximum score of 9·2 (maximal score possible 16). The same dose of non-chlorinated CII did not induce arthritis at all. Rats immunized with CII–Cl developed high anti-CII–Cl IgG titres and also developed IgG antibodies recognizing the non-chlorinated form of CII. Analysis of cytokine mRNA expression in lymph nodes 10 days after immunzation revealed an increased expression of interferon (IFN)-γ mRNA and interleukin (IL)-1β mRNA in CII–Cl-immunized rats compared to CII-immunized rats. Thus, chlorination of CII increased its immunogenicity as well as its arthritogenicity. As neutrophils, monocytes and macrophages are abundant cells in arthritic joints of patients with rheumatoid arthritis, chlorination might be a mechanism by which immunoreactivity to CII is induced and by which chronic joint inflammation is supported