The population of variable stars in M54 (NGC6715)

We present new B, V and I CCD time-series photometry for 177 variable stars in a 13'X 13' field centered on the globular cluster M54 (lying at the center of the Sagittarius dwarf spheroidal galaxy), 94 of which are newly identified variables. The total sample is composed of 2 anomalous Cepheids, 144 RR Lyrae stars (108 RR0 and 36 RR1), 3 SX Phoenicis, 7 eclipsing binaries (5 W UMA and 2 Algol binaries), 3 variables of uncertain classification and 18 long-period variables. The large majority of the RR Lyrae variables likely belong to M54. Ephemerides are provided for all the observed short-period variables. The pulsational properties of the M54 RR Lyrae variables are close to those of Oosterhoff I clusters, but a significant number of long-period ab type RR Lyrae are present. We use the observed properties of the RR Lyrae to estimate the reddening and the distance modulus of M54, E(B-V)=0.16 +/- 0.02 and (m-M)_0=17.13 +/- 0.11, respectively, in excellent agreement with the most recent estimates. The metallicity has been estimated for a subset of 47 RR Lyrae stars, with especially good quality light curves, from the Fourier parameters of the V light curve. The derived metallicity distribution has a symmetric bell shape, with a mean of =-1.65 and a standard deviation sigma=0.16 dex. Seven stars have been identified as likely belonging to the Sagittarius galaxy, based on their too high or too low metallicity. This evidence, if confirmed, might suggest that old stars in this galaxy span a wide range of metallicities.Comment: 15 pages, 11 figures, accepted for publication by MNRA

Validation of ultrasensitive mutant huntingtin detection in human cerebrospinal fluid by single molecule counting immunoassay

Background: The measurement of disease-relevant biomarkers has become a major component of clinical trial design, but in the absence of rigorous clinical and analytical validation of detection methodology, interpretation of results may be misleading. In Huntington’s disease (HD), measurement of the concentration of mutant huntingtin protein (mHTT) in cerebrospinal fluid (CSF) of patients may serve as both a disease progression biomarker and a pharmacodynamic readout for HTT-lowering therapeutic approaches. We recently published the quantification of mHTT levels in HD patient CSF by a novel ultrasensitive immunoassay-based technology and here analytically validate it for use. / Objective: This work aims to analytically and clinically validate our ultrasensitive assay for mHTT measurement in human HD CSF, for application as a pharmacodynamic biomarker of CNS mHTT lowering in clinical trials. / Methods: The single molecule counting (SMC) assay is an ultrasensitive bead-based immunoassay where upon specific recognition, dye-labeled antibodies are excited by a confocal laser and emit fluorescent light as a readout. The detection of mHTT by this technology was clinically validated following established Food and Drug Administration and European Medicine Agency guidelines. / Results: The SMC assay was demonstrated to be accurate, precise, specific, and reproducible. While no matrix influence was detected, a list of interfering substances was compiled as a guideline for proper collection and storage of patient CSF samples. In addition, a set of recommendations on result interpretation is provided. / Conclusions: This SMC assay is a robust and ultrasensitive method for the relative quantification of mHTT in human CSF

Two distinct sequences of blue straggler stars in the globular cluster M30

Stars in globular clusters are generally believed to have all formed at the same time, early in the Galaxy's history. 'Blue stragglers' are stars massive enough that they should have evolved into white dwarfs long ago. Two possible mechanisms have been proposed for their formation: mass transfer between binary companions and stellar mergers resulting from direct collisions between two stars. Recently, the binary explanation was claimed to be dominant. Here we report that there are two distinct parallel sequences of blue stragglers in M30. This globular cluster is thought to have undergone 'core collapse', during which both the collision rate and the mass transfer activity in binary systems would have been enhanced. We suggest that the two observed sequences arise from the cluster core collapse, with the bluer population arising from direct stellar collisions and the redder one arising from the evolution of close binaries that are probably still experiencing an active phase of mass transfer.Comment: Published on the 24th December 2009 issue of Natur

Blue Straggler Stars in Globular Clusters: a powerful tool to probe the internal dynamical evolution of stellar systems

This chapter presents an overview of the main observational results obtained to date about Blue Straggler Stars (BSSs) in Galactic Globular Clusters (GCs). The BSS specific frequency, radial distribution, chemical composition and rotational properties are presented and discussed in the framework of using this stellar population as probe of GC internal dynamics. In particular, the shape of the BSS radial distribution has been found to be a powerful tracer of the dynamical age of stellar systems, thus allowing the definition of the first empirical "dynamical clock".Comment: Chapter 5, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G. Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe

Calibration of the pre-main sequence RS Cha binary system

Context: The calibration of binary systems with accurately known masses and/or radii provides powerful tools to test stellar structure and evolution theory and to determine the age and helium content of stars. We study the eclipsing double-lined spectroscopic binary system RS Cha, for which we have accurate observations of the parameters of both stars (masses, radii, luminosities, effective temperatures and metallicity). Aims: We have calculated several sets of stellar models for the components of the RS Cha system, with the aim of reproducing simultaneously the available observational constraints and to estimate the age and initial helium abundance of the system. Methods: Using the CESAM stellar evolution code, we model both components starting from the initial mass and metallicity and adjusting the input parameters and physics in order to satisfy the observational constraints. Results: We find that the observations cannot be reproduced if we assume that the abundance ratios are solar but they are satisfied if carbon and nitrogen are depleted in the RS Cha system with respect to the Sun. This is in accordance with the abundances observed in other young stars. The RS Cha system is in an evolutionary stage at the end of the PMS phase where models are not strongly sensitive to various physical uncertainties. However we show that the oscillations of these two stars, which have been detected, would be able to discriminate between different options in the physical description of this evolutionary phase.Comment: 9 pages, 10 figures, accepted by Astronomy & Astrophysic

Phosphorylation of huntingtin at residue T3 is decreased in Huntington’s disease and modulates mutant huntingtin protein conformation

Posttranslational modifications can have profound effects on the biological and biophysical properties of proteins associated with misfolding and aggregation. However, their detection and quantification in clinical samples and an understanding of the mechanisms underlying the pathological properties of misfolding- and aggregation-prone proteins remain a challenge for diagnostics and therapeutics development. We have applied an ultrasensitive immunoassay platform to develop and validate a quantitative assay for detecting a posttranslational modification (phosphorylation at residue T3) of a protein associated with polyglutamine repeat expansion, namely Huntingtin, and characterized its presence in a variety of preclinical and clinical samples. We find that T3 phosphorylation is greatly reduced in samples from Huntington\u2019s disease models and in Huntington\u2019s disease patients, and we provide evidence that bona-fide T3 phosphorylation alters Huntingtin exon 1 protein conformation and aggregation properties. These findings have significant implications for both mechanisms of disease pathogenesis and the development of therapeutics and diagnostics for Huntington\u2019s disease

Striatal infusion of cholesterol promotes dose-dependent behavioral benefits and exerts disease-modifying effects in Huntington's disease mice

A variety of pathophysiological mechanisms are implicated in Huntington's disease (HD). Among them, reduced cholesterol biosynthesis has been detected in the HD mouse brain from pre-symptomatic stages, leading to diminished cholesterol synthesis, particularly in the striatum. In addition, systemic injection of cholesterol-loaded brain-permeable nanoparticles ameliorates synaptic and cognitive function in a transgenic mouse model of HD. To identify an appropriate treatment regimen and gain mechanistic insights into the beneficial activity of exogenous cholesterol in the HD brain, we employed osmotic mini-pumps to infuse three escalating doses of cholesterol directly into the striatum of HD mice in a continuous and rate-controlled manner. All tested doses prevented cognitive decline, while amelioration of disease-related motor defects was dose-dependent. In parallel, we found morphological and functional recovery of synaptic transmission involving both excitatory and inhibitory synapses of striatal medium spiny neurons. The treatment also enhanced endogenous cholesterol biosynthesis and clearance of mutant Huntingtin aggregates. These results indicate that cholesterol infusion to the striatum can exert a dose-dependent, disease-modifying effect and may be therapeutically relevant in HD

Polyglutamine- and temperature-dependent conformational rigidity in mutant huntingtin revealed by immunoassays and circular dichroism spectroscopy.

BACKGROUND:In Huntington's disease, expansion of a CAG triplet repeat occurs in exon 1 of the huntingtin gene (HTT), resulting in a protein bearing>35 polyglutamine residues whose N-terminal fragments display a high propensity to misfold and aggregate. Recent data demonstrate that polyglutamine expansion results in conformational changes in the huntingtin protein (HTT), which likely influence its biological and biophysical properties. Developing assays to characterize and measure these conformational changes in isolated proteins and biological samples would advance the testing of novel therapeutic approaches aimed at correcting mutant HTT misfolding. Time-resolved Förster energy transfer (TR-FRET)-based assays represent high-throughput, homogeneous, sensitive immunoassays widely employed for the quantification of proteins of interest. TR-FRET is extremely sensitive to small distances and can therefore provide conformational information based on detection of exposure and relative position of epitopes present on the target protein as recognized by selective antibodies. We have previously reported TR-FRET assays to quantify HTT proteins based on the use of antibodies specific for different amino-terminal HTT epitopes. Here, we investigate the possibility of interrogating HTT protein conformation using these assays. METHODOLOGY/PRINCIPAL FINDINGS:By performing TR-FRET measurements on the same samples (purified recombinant proteins or lysates from cells expressing HTT fragments or full length protein) at different temperatures, we have discovered a temperature-dependent, reversible, polyglutamine-dependent conformational change of wild type and expanded mutant HTT proteins. Circular dichroism spectroscopy confirms the temperature and polyglutamine-dependent change in HTT structure, revealing an effect of polyglutamine length and of temperature on the alpha-helical content of the protein. CONCLUSIONS/SIGNIFICANCE:The temperature- and polyglutamine-dependent effects observed with TR-FRET on HTT proteins represent a simple, scalable, quantitative and sensitive assay to identify genetic and pharmacological modulators of mutant HTT conformation, and potentially to assess the relevance of conformational changes during onset and progression of Huntington's disease

Striatal infusion of cholesterol promotes dose-dependent behavioral benefits and exerts disease-modifying effects in Huntington's disease mice

A variety of pathophysiological mechanisms are implicated in Huntington's disease (HD). Among them, reduced cholesterol biosynthesis has been detected in the HD mouse brain from pre-symptomatic stages, leading to diminished cholesterol synthesis, particularly in the striatum. In addition, systemic injection of cholesterol-loaded brain-permeable nanoparticles ameliorates synaptic and cognitive function in a transgenic mouse model of HD. To identify an appropriate treatment regimen and gain mechanistic insights into the beneficial activity of exogenous cholesterol in the HD brain, we employed osmotic mini-pumps to infuse three escalating doses of cholesterol directly into the striatum of HD mice in a continuous and rate-controlled manner. All tested doses prevented cognitive decline, while amelioration of disease-related motor defects was dose-dependent. In parallel, we found morphological and functional recovery of synaptic transmission involving both excitatory and inhibitory synapses of striatal medium spiny neurons. The treatment also enhanced endogenous cholesterol biosynthesis and clearance of mutant Huntingtin aggregates. These results indicate that cholesterol infusion to the striatum can exert a dose-dependent, disease-modifying effect and may be therapeutically relevant in HD

The temperature dependence of the 2B7-MW1 TR-FRET signal is inversely proportional to polyQ length.

<p>A.-B. Fluorescence signals obtained from dilution curves of N548 proteins bearing Q16, Q19, Q25, Q33, Q39 and Q55, tested by TR-FRET (1 ng/µl of 2B7-Tb and 10 ng/µl of MW1-d2) at RT (A) and 4°C (B). C. Ratio between the maximum value of the TR-FRET signal obtained at 4°C and the maximum value of the TR-FRET signal obtained at RT, referred to the curves presented in A and B. D. Ratio obtained and expressed as in C after performing TR-FRET with 2B7-Tb (1 ng/µl) and MW1-d2 (10 ng/µl) antibody combination (as before) or MW1-Tb (1 ng/µl) and 2B7-Alexa647 (10 ng/µl) antibody combination on N548 proteins bearing Q16 and Q55 repeats. In C and D data are represented as mean ± S.D. of three independent experiments; significance was calculated using the one-way ANOVA test and Bonferroni's multiple comparison post-test (** p<0.01 and * p<0.05).</p