Impact of receptor clustering on ligand binding

<p>Abstract</p> <p>Background</p> <p>Cellular response to changes in the concentration of different chemical species in the extracellular medium is induced by ligand binding to dedicated transmembrane receptors. Receptor density, distribution, and clustering may be key spatial features that influence effective and proper physical and biochemical cellular responses to many regulatory signals. Classical equations describing this kind of binding kinetics assume the distributions of interacting species to be homogeneous, neglecting by doing so the impact of clustering. As there is experimental evidence that receptors tend to group in clusters inside membrane domains, we investigated the effects of receptor clustering on cellular receptor ligand binding.</p> <p>Results</p> <p>We implemented a model of receptor binding using a Monte-Carlo algorithm to simulate ligand diffusion and binding. In some simple cases, analytic solutions for binding equilibrium of ligand on clusters of receptors are provided, and supported by simulation results. Our simulations show that the so-called "apparent" affinity of the ligand for the receptor decreases with clustering although the microscopic affinity remains constant.</p> <p>Conclusions</p> <p>Changing membrane receptors clustering could be a simple mechanism that allows cells to change and adapt its affinity/sensitivity toward a given stimulus.</p

IGF1R Signaling in Ewing Sarcoma Is Shaped by Clathrin-/Caveolin-Dependent Endocytosis

Receptor endocytosis is critical for cell signaling. IGF1R mediates an autocrine loop that is de-regulated in Ewing Sarcoma (ES) cells. Here we study the impact of IGF1R internalization, mediated by clathrin and caveolin-1 (CAV1), in ES signaling. We used clathrin and CAV1-siRNA to interfere in clathrin- and caveolin-dependent endocytosis. Chlorpromazine (CPMZ) and methyl-beta-cyclo-dextrin (MCD) were also used in order to inhibit clathrin- and caveolin-dependent endocytosis, respectively. We analyzed IGF1R internalization and co-localization with clathrin and CAV1 upon ligand binding, as well as the status of the IGF1R pathway, cellular proliferation, and the apoptosis of interfered and inhibited ES cells. We performed a high-throughput tyrosine kinase phosphorylation assay to analyze the effects of combining the IGF1R tyrosine kinase inhibitor AEW541 (AEW) with CPMZ or MCD on the intracellular phospho-proteome. We observed that IGF1R is internalized upon ligand binding in ES cells and that this process is dependent on clathrin or CAV1. The blockage of receptor internalization inhibited AKT and MAPK phosphorylation, reducing the proliferative rate of ES cells and increasing the levels of apoptosis. Combination of AEW with CPMZ or MCD largely enhanced these effects. CAV1 and clathrin endocytosis controls IGF1R internalization and signaling and has a profound impact on ES IGF1R-promoted survival signaling. We propose the combination of tyrosine-kinase inhibitors with endocytosis inhibitors as a new therapeutic approach to achieve a stronger degree of receptor inhibition in this, or other neoplasms dependent on IGF1R signaling

Cholesterol Depletion in Adipocytes Causes Caveolae Collapse Concomitant with Proteosomal Degradation of Cavin-2 in a Switch-Like Fashion

Caveolae, little caves of cell surfaces, are enriched in cholesterol, a certain level of which is required for their structural integrity. Here we show in adipocytes that cavin-2, a peripheral membrane protein and one of 3 cavin isoforms present in caveolae from non-muscle tissue, is degraded upon cholesterol depletion in a rapid fashion resulting in collapse of caveolae. We exposed 3T3-L1 adipocytes to the cholesterol depleting agent methyl-β-cyclodextrin, which results in a sudden and extensive degradation of cavin-2 by the proteasome and a concomitant movement of cavin-1 from the plasma membrane to the cytosol along with loss of caveolae. The recovery of cavin-2 at the plasma membrane is cholesterol-dependent and is required for the return of cavin-1 from the cytosol to the cell surface and caveolae restoration. Expression of shRNA directed against cavin-2 also results in a cytosolic distribution of cavin-1 and loss of caveolae. Taken together, these data demonstrate that cavin-2 functions as a cholesterol responsive component of caveolae that is required for cavin-1 localization to the plasma membrane, and caveolae structural integrity

Early antecedents of childhood impulsivity: The role of parent-child interaction, cognitive competence, and temperament

This prospective longitudinal investigation examined early mother-child interaction as a predictor of children's later self-control capabilities. Multimethod assessments of mother-child relationships, primarily focused on observed relationship qualities in the home, were conducted during the first 2 years and related to children's later impulse control capabilities. Child cognitive competence and temperament assessed during the 2nd year were also related to later impulsivity. Follow-up assessments of children's impulsivity were conducted at age 6 ( N= 79), using a variety of laboratory measures. Findings indicated that responsive, cognitively stimulating parenttoddler interactions in the 2nd year modestly predicted later measures of cognitive nonimpulsivity and ability to delay gratification. Security of mother-infant attachment predicted the same outcomes, but only for boys and not for girls. Child cognitive competence in the 2nd year also consistently predicted children's later impulse control capabilities, although this was not true for measures of child temperament. Overall, the findings support a multidimensional and developmental conceptualization of the early antecedents of childhood impulsivity .Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44580/1/10802_2004_Article_BF00916568.pd

Recent island colonization by an introduced shrew in the western mediterranean

Insular ecosystems are sensitive to alien invasive species. The Balearic Islands have been colonized by a large list of invasive species, in most cases due to human activity. White-toothed shrews have been historically recorded in this archipelago and to date four different species have been found. In this study we focus on the white-toothed pygmy shrew (Suncus etruscus) at the island of Mallorca, assessing its current status and identifying both its genetic variation and geographic structure. During the last 50 years this species has colonized a large proportion of the island. Haplotype network analyses support the hypothesis of a unique and recent colonization event from a single but unknown source population. Available genetic information suggests the populations from northeast Iberian Peninsula (Catalonia) and southwest France (Camarge) as the most plausible source of the lineage found in Mallorca. The recent and extensive distribution of Suncus etruscus on this island is characteristic of a fast growing population at an invasive colonization phase, indicating that it has found in Mallorca suitable ecological conditions to thrive