Extracellular chloride signals collagen IV network assembly during basement membrane formation

Basement membranes are defining features of the cellular microenvironment; however, little is known regarding their assembly outside cells. We report that extracellular Cl⁻ ions signal the assembly of collagen IV networks outside cells by triggering a conformational switch within collagen IV noncollagenous 1 (NC1) domains. Depletion of Cl⁻ in cell culture perturbed collagen IV networks, disrupted matrix architecture, and repositioned basement membrane proteins. Phylogenetic evidence indicates this conformational switch is a fundamental mechanism of collagen IV network assembly throughout Metazoa. Using recombinant triple helical protomers, we prove that NC1 domains direct both protomer and network assembly and show in Drosophila that NC1 architecture is critical for incorporation into basement membranes. These discoveries provide an atomic-level understanding of the dynamic interactions between extracellular Cl⁻ and collagen IV assembly outside cells, a critical step in the assembly and organization of basement membranes that enable tissue architecture and function. Moreover, this provides a mechanistic framework for understanding the molecular pathobiology of NC1 domains.     

Extracellular signals and pancreatic beta-cell development: a brief review

Cell lineage development is a finely tuned process of proliferation and differentiation, survival and apoptosis, that is regulated by numerous extracellular signals. Here we review some of the extracellular signals--including insoluble cell-cell and extracellular matrix-cell interactions, as well as soluble factors--that appear critical for pancreatic beta-cell development. Knowledge of how these signals control the development of pancreatic endocrine stem/precursor cells into fully functional insulin-secreting beta cells is a platform for the restoration of beta-cell function and the cure therapy of type 1 diabetes.     

Plasma membrane of Synechocystis PCC 6803: a heterogeneous distribution of membrane proteins

Proteomic studies carried out previously on the plasma membrane of Synechocystis have identified several peripheral and integral proteins. The distribution of these proteins along the membrane still remains obscure. In this study, the distribution of proteins along the plasma membrane of Synechocystis was carried out using subfractions, the right-side-out (RSO) and inside-out (ISO) vesicles, fractionated from a pure and specific fraction of the plasma membrane. These subfractions were analyzed and quantified for several proteins by immunoblotting. It was found that the ISO fraction contained higher quantities of preD1, D1 and PsaD, the integral proteins of photosystem I and II known to be present also in the plasma membrane. Lower amounts of peripheral vesicle inducing protein Vipp1 and nitrate/nitrite binding protein NrtA were present in the ISO compared to the RSO fraction. On the contrary, the distribution of two integral transporter proteins, SbtA and PxcA, was found equal in both fractions. Our studies clearly establish that the plasma membrane of Synechocystis has a heterogeneous composition with respect to protein distribution. The accumulation of photosynthesis-associated proteins in the ISO fraction provides evidence that the discrete regions of the plasma membrane harbor sites for biogenesis of photosystems.     

Propagated electrical signals in plants

The present paper generalizes the new data and gives an account of the current notions about properties, mechanisms of generation, and functional role of propagated electrical signals in the form of action potentials and variation potentials in plants.     

Studies on mechano-perception in the Characeae: transduction of pressure signals into electrical signals

Mechano-perception by Chara cells was studied with an emphasis on the role of the nodal complex in transducing pressure signals into electrical signals. Three types of experimental material were used: (1) tandem internodal cells connected by a single layer of nodal cells; (2) single internodal cells, from which either apical or basal nodes were removed by ligation and cutting; (3) single internodes from which both nodes had been removed. Exposure to a hypertonic solution (sorbitol or sucrose) induced a depolarization at the node in 1 and 2. Depolarization did not occur at the ligated end of the cell in 2, or at all in 3. Addition of K+ increased the magnitude of the response, whilst it was significantly decreased by the divalent cations, Ca2+ and Mg2+. Electrical resistance decreased at the node during the depolarization, showing that a passive diffusion potential was responsible. I suggest that the change in the trans-nodal hydraulic pressure difference mechanically stretches the plasma membrane, and this induces the electrical depolarization.     

Single-locus polymorphism in a heterogeneous two-deme model

Environmental heterogeneity has long been considered a likely explanation for the high levels of genetic variation found in most natural populations: selection in a spatially heterogeneous environment can maintain more variation. While this theoretical result has been extensively studied in models with limited parameters (e.g., two alleles, fixed gene flow, and particular selection schemes), the effect of spatial heterogeneity is poorly understood for models with a wider range of parameters (e.g., multiple alleles, different levels of gene flow, and more general selection schemes). We have compared the volume of fitness space that maintains variation in a single-deme model to the volume in a two-deme model for multiple alleles, random selection schemes, and various levels of migration. Furthermore, equilibrium allele-frequency vectors were examined to see if particular patterns of variation are more prevalent than first expected. The two-deme model maintains variation for substantially larger volumes of fitness space with lower heterozygote fitness than the single-deme model. This result implies that selection schemes in the two-deme model can have a wider range of fitness patterns while still maintaining variation. The equilibrium allele-frequency patterns emerging from the two-deme model are more variable and strongly influenced by gene flow.     

Extracellular noise-induced stochastic synchronization in heterogeneous quorum sensing network

Quorum sensing is a bacterial mechanism used to synchronize the coordinated response of a microbial population. Because quorum sensing in Gram-negative bacteria depends on release and detection of a diffusible signaling molecule (autoinducer) among a multicellular group, it is considered a simple form of cell-cell communication for the purposes of mathematical analysis. Stochastic equation systems have provided a common approach to model biochemical or biophysical processes. Recently, the effect of noise to synchronize a specific homogeneous quorum sensing network was successfully modeled using a stochastic equation system with fixed parameters. The question remains of how to model quorum sensing networks in a general setting. To address this question, we first set a stochastic equation system as a general model for a heterogeneous quorum sensing network. Then, using two relevant biophysical characteristics of Gram-negative bacteria (the permeability of the cell membrane to the autoinducer and the symmetry of autoinducer diffusion) we construct the solution of the stochastic equation system at an abstract level. The solution indicates that stable synchronization of a quorum sensing network is robustly induced by an environment with a heterogenous distribution of extracellular and intracellular noise. The synchronization is independent of the initial state of the system and is solely the result of the connectivity of the cell network established through the effects of extracellular noise.     

On the electrical conductivity of rhodopsin solutions

Measurements were made on the conductivity of digitonin extracts of frog rhodopsin with and without previous light exposure. The light-dark difference in conductivity is observed at low concentrations of rhodopsin and detergent.     

Fusion-fission reorganization of membrane: a developing membrane model for thrombocytogenesis in megakaryocytes

Freeze-fracture and thin-section electron microscopy indicate that a sequence of fusion-fission leads to reorganization of membranes and the demarcation of platelets within the cytoplasm of megakaryocyte. Invagination of the megakaryocyte plasma membrane leads to the formation of tubular structures within the cytoplasm of megakaryocytes. Fusion of these tubular membranes in the plane of their long axes is followed by fission in the perpendicular plane. This results in the formation of two flat membranes, forming plasma membranes of two adjacent platelets. A similar fusion-fission reorganization of membranes could mediate a wide variety of other biologic phenomena. These observations also indicate that megakaryocytes are located in the subendothelial compartment of the marrow with their projections penetrating the endothelium and reaching the lumen. This direct contact with the circulation may serve as a means of receiving information as to the requirements of the body for platelet production.     

Extracellular electrical activity from the photoreceptors of midge

The ontogeny of photosensitivity has been studied in a holometabolous insect, the midgeChironomus ramosus. The life cycle of midges shifts from an aquatic environment to a non-aquatic environment. Extracellular electrical activity of photoreceptor organs was recorded at larval and adult stages. We found an increase in photosensitivity as the larva metamorphosed to the adult stage. This is the first report of changes in photosensitivity during the development of any insect described in an ecological context.     

Sharing signals: connecting lung epithelial cells with gap junction channels

Gap junction channels enable the direct flow of signaling molecules and metabolites between cells. Alveolar epithelial cells show great variability in the expression of gap junction proteins (connexins) as a function of cell phenotype and cell state. Differential connexin expression and control by alveolar epithelial cells have the potential to enable these cells to regulate the extent of intercellular coupling in response to cell stress and to regulate surfactant secretion. However, defining the precise signals transmitted through gap junction channels and the cross talk between gap junctions and other signaling pathways has proven difficult. Insights from what is known about roles for gap junctions in other systems in the context of the connexin expression pattern by lung cells can be used to predict potential roles for gap junctional communication between alveolar epithelial cells.     

An immunization model for a heterogeneous population

A model for the spread of an infectious disease for which recovery gives temporary or permanent immunity is analysed. A heterogeneous population is divided into homogeneous groups in which immunization of newborns and of susceptibles of all ages is possible. Births and deaths occur at equal rates in each group and all newborns are susceptible. The threshold criterion obtained for the nonlinear differential equation model can be used to determine the immunization rates which will cause the disease to die out.