mTOR signaling in polycystic kidney disease

Polycystic kidney diseases (PKDs) comprise a large group of genetic disorders characterized by formation of cysts in the kidneys and other organs, ultimately leading to end-stage renal disease. Although PKDs can be caused by mutations in different genes, they converge on a set of common molecular mechanisms involved in cystogenesis and ciliary dysfunction, and can be qualified as ciliopathies. Recent advances in understanding the mechanisms regulating disease progression have led to the development of new therapies that are being tested in both preclinical and clinical trials. In this article, we briefly review a network of molecular pathways of cystogenesis that are regulated by ciliary functions. We discuss the mTOR pathway in depth, highlighting recent progress in understanding its role in PKD and the current results of clinical trials.     

Relationship between the structure of amphiphilic copolymers and their ability to disturb lipid bilayers

Nonionic amphiphiles and particularly block copolymers of ethylene oxide and propylene oxide (Pluronics) cause pronounced chemosensitization of tumor cells that exhibit multiple resistance to antineoplastic drugs. This effect is due to inhibition of P-glycoprotein (P-gp) responsible for drug efflux. It was suggested that the inhibition of P-gp might be due to changes in its lipid surrounding. Indeed, high dependence of P-gp activity on the membrane microviscosity was demonstrated [Regev et al. (1999) Eur. J. Biochem. 259, 18-24], suggesting that the ability of Pluronics to affect the P-gp activity is mediated by their effect on the membrane structure. We have found recently that adsorption of Pluronics on lipid bilayers induced considerable disturbance of the lipid packing [Krylova et al. (2003) Chemistry 9, 3930-3936]. In the present paper, we studied 19 amphiphilic copolymers, including newly synthesized hyperbranched polyglycerols, Pluronic and Brij surfactants, for their ability to accelerate flip-flop and permeation of antitumor drug doxorubicin (DOX) in liposomes. It was found that not only bulk hydrophobicity but also the chemical microstructure of the copolymer determines its membrane disturbing ability. Copolymers containing polypropylene oxide caused higher acceleration of flip-flop and DOX permeation than polysurfactants containing aliphatic chains. The effects of copolymers containing hyperbranched polyglycerol "corona" were more pronounced, as compared to the copolymers with linear poly(ethylene oxide) chains, indicating that a bulky hydrophilic block induces additional disturbances in the lipid bilayer. A good correlation between the copolymer flippase activity and a linear combination of copolymer bulk hydrophobicity and the van der Waals volume of its hydrophobic block was found. The relationship between the structure of a copolymer and its ability to disturb lipid membranes presented in this paper may be useful for the design of novel amphiphilic copolymers capable of affecting the activity of membrane transporters in living cells.     

Characterization of a continuous supermacroporous monolithic matrix for chromatographic separation of large bioparticles

A continuous supermacroporous monolithic chromatographic matrix has been characterized using a capillary model, experimental breakthrough curves, and pressure drop experiments. The model describes the convective flow and its dispersive mixing effects, mass transfer resistance, pore size distribution, and the adsorption behavior of the monolithic matrix. It is possible to determine an effective pore size distribution by fitting the capillary model to experimental breakthrough curves and pressure drop experiments. The model is able to describe the flow rate dependence of the experimental breakthrough curves. Mass transport resistance was due to: (i) dispersive mixing effects in the convective flow in the pores; and (ii) slow diffusion in the stagnant film covering the surface within each pore, under adsorption conditions. The monolithic matrix can be described by a very narrow pore size distribution, illustrating one of the advantages of the gel. A broader pore size distribution results in increased band broadening. This can be studied easily using the model developed in this investigation.     

Magnetic resonance imaging of the siliceous skeleton of the demosponge Lubomirskia baicalensis

The skeletal elements (spicules) of the demosponge Lubomirskia baicalensis were analyzed; they are composed of amorphous, non-crystalline silica, and contain in a central axial canal the axial filament which consists of the enzyme silicatein. The axial filament, that orients the spicule in its longitudinal axis exists also in the center of the spines which decorate the spicule. During growth of the sponge, new serially arranged modules which are formed from longitudinally arranged spicule bundles are added at the tip of the branches. X-ray analysis revealed that these serial modules are separated from each other by septate zones (annuli). We describe that the longitudinal bundles of spicules of a new module originate from the apex of the earlier module from where they protrude. A cross section through the oscular/apical-basal axis shows that the bundle rays are organized in a concentric and radiate pattern. High resolution magnetic resonance microimaging studies showed that the silica spheres of the spicules in the cone region contain high amounts of 'mobile' water. We conclude that the radiate accretive growth pattern of sponges is initiated in the apical region (cones) by newly growing spicules which are characterized by high amounts of 'mobile' water; subsequently spicule bundles are formed laterally around the cones.     

Infection by Trypanosoma cruzi Enhances Anion Conductance in Rat Neonatal Ventricular Cardiomyocytes

Recent studies on malaria-infected erythrocytes have shown increased anion channel activity in the host cell membrane, increasing the exchange of solutes between the cytoplasm and exterior. In the present work, we addressed the question of whether another intracellular protozoan parasite, Trypanosoma cruzi, alters membrane transport systems in the host cardiac cell. Neonatal rat cardiomyocytes were cultured and infected with T. cruzi in vitro. Ion currents were measured by patch-clamp technique in the whole-cell configuration. Two small-magnitude instantaneous anion currents, outward- and inward-rectifying, were recorded in all noninfected cardiomyocytes. In addition, ~10% of cardiomyocytes expressed a large anion-preferable, time-dependent current activated at positive membrane potentials. Hypotonic (230?mOsm) treatment resulted in the disappearance of the time-dependent current but provoked a dramatic increase of the instantaneous outward-rectifying one. Both instantaneous currents were suppressed by intracellular Mg(2+). T. cruzi infection did not provoke new anion currents in the host cells but caused an increase of the density of intrinsic swelling-activated outward current, up to twice in heavily infected cells. The occurrence of a time-dependent current dramatically increased in infected cells in the presence of Mg(2+) in the intracellular solution, from ~10 to ~80%, without a significant change of the current density. Our findings represent one further, besides the known Plasmodium falciparum, example of an intracellular parasite which upregulates the anionic currents expressed in the host cell.     

Entropy capacity determines protein folding

Search and study of the general principles that govern kinetics and thermodynamics of protein folding generate a new insight into the factors controlling this process. Here, based on the known experimental data and using theoretical modeling of protein folding, we demonstrate that there exists an optimal relationship between the average conformational entropy and the average energy of contacts per residue-that is, an entropy capacity-for fast protein folding. Statistical analysis of conformational entropy and number of contacts per residue for 5829 protein structures from four general structural classes (all-alpha, all-beta, alpha/beta, alpha+beta) demonstrates that each class of proteins has its own class-specific average number of contacts (class alpha/beta has the largest number of contacts) and average conformational entropy per residue (class all-alpha has the largest number of rotatable angles phi, psi, and chi per residue). These class-specific features determine the folding rates: alpha proteins are the fastest folding proteins, then follow beta and alpha+beta proteins, and finally alpha/beta proteins are the slowest ones. Our result is in agreement with the experimental folding rates for 60 proteins. This suggests that structural and sequence properties are important determinants of protein folding rates.