Human adipose cells in vitro are either refractory or responsive to insulin, reflecting host metabolic state.

While intercellular communication processes are frequently characterized by switch-like transitions, the endocrine system, including the adipose tissue response to insulin, has been characterized by graded responses. Yet here individual cells from adipose tissue biopsies are best described by a switch-like transition between the basal and insulin-stimulated states for the trafficking of the glucose transporter GLUT4. Two statistically-defined populations best describe the observed cellular heterogeneity, representing the fractions of refractive and responsive adipose cells.

Trapping by clusters of channels, receptors, and transporters: quantitative description.

Various membrane functional units such as receptors, transporters, and channels, whose action necessarily involves capturing diffusing molecules, are often organized into multimeric complexes forming clusters on the cell and organelle membranes. These functional units themselves are usually oligomers of several integral proteins, which have their own symmetry. Depending on the symmetry, they form clusters on different packing lattices.

Insulin stimulates translocation of human GLUT4 to the membrane in fat bodies of transgenic Drosophila melanogaster.

The fruit fly Drosophila melanogaster is an excellent model system for studies of genes controlling development and disease. However, its applicability to physiological systems is less clear because of metabolic differences between insects and mammals. Insulin signaling has been studied in mammals because of relevance to diabetes and other diseases but there are many parallels between mammalian and insect pathways.

Impaired tethering and fusion of GLUT4 vesicles in insulin-resistant human adipose cells.

Systemic glucose homeostasis is profoundly influenced by adipose cell function. Here we investigated GLUT4 dynamics in living adipose cells from human subjects with varying BMI and insulin sensitivity index (Si) values. Cells were transfected with hemagglutinin (HA)-GLUT4-green fluorescent protein (GFP)/mCherry (red fluorescence), and were imaged live using total internal reflection fluorescence and confocal microscopy.

Insulin regulates Glut4 confinement in plasma membrane clusters in adipose cells.

Insulin-stimulated delivery of glucose transporter-4 (GLUT4) to the plasma membrane (PM) is the hallmark of glucose metabolism. In this study we examined insulin's effects on GLUT4 organization in PM of adipose cells by direct microscopic observation of single monomers tagged with photoswitchable fluorescent protein. In the basal state, after exocytotic delivery only a fraction of GLUT4 is dispersed into the PM as monomers, while most of the GLUT4 stays at the site of fusion and forms elongated clusters (60-240 nm).

Trapping of diffusing particles by clusters of absorbing disks on a reflecting wall with disk centers on sites of a square lattice.

A simple approximate formula is derived for the rate constant that describes steady-state flux of diffusing particles through a cluster of perfectly absorbing disks on the otherwise reflecting flat wall, assuming that the disk centers occupy neighboring sites of a square lattice. A distinctive feature of trapping by a disk cluster is that disks located at the cluster periphery shield the disks in the center of the cluster. This competition of the disks for diffusing particles makes it impossible to find an exact analytical solution for the rate constant in the general case.

Cytoplasmic free Ca2+ is essential for multiple steps in malaria parasite egress from infected erythrocytes.

Background: Egress of Plasmodium falciparum, from erythrocytes at the end of its asexual cycle and subsequent parasite invasion into new host cells, is responsible for parasite dissemination in the human body. The egress pathway is emerging as a coordinated multistep programme that extends in time for tens of minutes, ending with rapid parasite extrusion from erythrocytes. While the Ca2+ regulation of the invasion of P.

Direct chemical evidence for sphingolipid domains in the plasma membranes of fibroblasts.

Sphingolipids play important roles in plasma membrane structure and cell signaling. However, their lateral distribution in the plasma membrane is poorly understood. Here we quantitatively analyzed the sphingolipid organization on the entire dorsal surface of intact cells by mapping the distribution of (15)N-enriched ions from metabolically labeled (15)N-sphingolipids in the plasma membrane, using high-resolution imaging mass spectrometry.

Extracellular annexins and dynamin are important for sequential steps in myoblast fusion.

Myoblast fusion into multinucleated myotubes is a crucial step in skeletal muscle development and regeneration. Here, we accumulated murine myoblasts at the ready-to-fuse stage by blocking formation of early fusion intermediates with lysophosphatidylcholine. Lifting the block allowed us to explore a largely synchronized fusion.

Communication: Clusters of absorbing disks on a reflecting wall: competition for diffusing particles.

Trapping of diffusing particles by a cluster of absorbing disks on the otherwise reflecting wall is a manifestly many-body problem because of the disk competition for the particles. By replacing the cluster with an effective uniformly absorbing spot, we derive a simple formula for the rate constant that characterizes the trapping. The formula shows how the rate constant depends on the size and shape of the cluster.

Long, saturated chains: tasty domains for kinases of insulin resistance.

The mechanistic basis of how cells respond to increased fatty acids (FAs) is murky but potentially involves receptor-mediated activation or inhibition by different FA classes. Holzer et al. (2011) recently propose in Cell that expansion of intracellular membrane microdomains induced by saturated FA recruit and activate c-Src for JNK activation.

Functional role for transporter isoforms in optimizing membrane transport.

Quantitative analysis of carrier parameters demonstrates that with decreasing substrate concentration the optimal strength of substrate-carrier interaction which maximizes the flux across the membrane increases and requires less fine-tuning than at higher concentrations of the substrate.

Insulin controls the spatial distribution of GLUT4 on the cell surface through regulation of its postfusion dispersal.

While the glucose transporter-4 (GLUT4) is fundamental to insulin-regulated glucose metabolism, its dynamic spatial organization in the plasma membrane (PM) is unclear. Here, using multicolor TIRF microscopy in transfected adipose cells, we demonstrate that insulin regulates not only the exocytosis of GLUT4 storage vesicles but also PM distribution of GLUT4 itself. In the basal state, domains (clusters) of GLUT4 molecules in PM are created by an exocytosis that retains GLUT4 at the fusion site.

A common trafficking route for GLUT4 in cardiomyocytes in response to insulin, contraction and energy-status signalling.

A new mouse model has been developed to study the localisation and trafficking of the glucose transporter GLUT4 in muscle. The mouse line has specific expression of a GFP and HA-epitope-tagged version of GLUT4 under the control of a muscle-specific promoter. The exofacial HA-tag has enabled fluorescent labelling of only the GLUT4 exposed at the external surface.

Insulin regulates fusion of GLUT4 vesicles independent of Exo70-mediated tethering.

Insulin regulates cellular glucose uptake by changing the amount of glucose transporter-4 (GLUT4) in the plasma membrane through stimulation of GLUT4 exocytosis. However, how the particular trafficking, tethering, and fusion steps are regulated by insulin is still debated. In a 3T3-L1 adipocyte cell line, the Exocyst complex and its Exo70 subunit were shown to critically affect GLUT4 exocytosis.

Cellular biophysics: bacterial endospore, membranes and random fluctuation.

Purposeful motion of biological processes can be driven by Brownian motion of macromolecular complexes with one-sided binding biasing movement in one direction: a Brownian ratchet, now proposed to explain membrane motion during a phagocytosis-like process in bacteria.

Insulin stimulates the halting, tethering, and fusion of mobile GLUT4 vesicles in rat adipose cells.

Glucose transport in adipose cells is regulated by changing the distribution of glucose transporter 4 (GLUT4) between the cell interior and the plasma membrane (PM). Insulin shifts this distribution by augmenting the rate of exocytosis of specialized GLUT4 vesicles. We applied time-lapse total internal reflection fluorescence microscopy to dissect intermediates of this GLUT4 translocation in rat adipose cells in primary culture.

Shape control through molecular segregation in giant surfactant aggregates.

Mixtures of cationic and anionic surfactants crystallized at various ratios in the absence of added salt form micrometer-sized colloids. Here, we propose and test a general mechanism explaining how this ratio controls the shape of the resulting colloidal structure, which can vary from nanodiscs to punctured planes; during cocrystallization, excess (nonstoichiometric) surfactant accumulates on edges or pores rather than being incorporated into crystalline bilayers. Molecular segregation then produces a sequence of shapes controlled by the initial mole ratio only.

Shape bistability of a membrane neck: a toggle switch to control vesicle content release.

Shape dynamics and permeability of a membrane neck connecting a vesicle and plasma membrane are considered. The neck is modeled by a lipid membrane tubule extended between two parallel axisymmetric rings. Within a range of lengths, defined by system geometry and mechanical properties of the membrane, the tubule has two stable shapes: catenoidal microtubule and cylindrical nanotubule.