Restoration of LDL receptor activity in mutant cells by intercellular junctional communication

Rescue of Notch signaling in cells incapable of GDP-L-fucose synthesis by gap junction transfer of GDP-L-fucose in Drosophila

Notch (N) is a transmembrane receptor that mediates cell-cell interactions to determine many cell-fate decisions. N contains EGF-like repeats, many of which have an O-fucose glycan modification that regulates N-ligand binding. This modification requires GDP-L-fucose as a donor of fucose. The GDP-L-fucose biosynthetic pathways are well understood, including the de novo pathway, which depends on GDP-mannose 4,6 dehydratase (Gmd) and GDP-4-keto-6-deoxy-D-mannose 3,5-epimerase/4-reductase (Gmer). However, the potential for intercellularly supplied GDP-L-fucose and the molecular basis of such transportation have not been explored in depth. To address these points, we studied the genetic effects of mutating Gmd and Gmer on fucose modifications in Drosophila. We found that these mutants functioned cell-nonautonomously, and that GDP-L-fucose was supplied intercellularly through gap junctions composed of Innexin-2. GDP-L-fucose was not supplied through body fluids from different isolated organs, indicating that the intercellular distribution of GDP-L-fucose is restricted within a given organ. Moreover, the gap junction-mediated supply of GDP-L-fucose was sufficient to support the fucosylation of N-glycans and the O-fucosylation of the N EGF-like repeats. Our results indicate that intercellular delivery is a metabolic pathway for nucleotide sugars in live animals under certain circumstances.

Connexins: key mediators of endocrine function

The appearance of multicellular organisms imposed the development of several mechanisms for cell-to-cell communication, whereby different types of cells coordinate their function. Some of these mechanisms depend on the intercellular diffusion of signal molecules in the extracellular spaces, whereas others require cell-to-cell contact. Among the latter mechanisms, those provided by the proteins of the connexin family are widespread in most tissues. Connexin signaling is achieved via direct exchanges of cytosolic molecules between adjacent cells at gap junctions, for cell-to-cell coupling, and possibly also involves the formation of membrane "hemi-channels," for the extracellular release of cytosolic signals, direct interactions between connexins and other cell proteins, and coordinated influence on the expression of multiple genes. Connexin signaling appears to be an obligatory attribute of all multicellular exocrine and endocrine glands. Specifically, the experimental evidence we review here points to a direct participation of the Cx36 isoform in the function of the insulin-producing β-cells of the endocrine pancreas, and of the Cx40 isoform in the function of the renin-producing juxtaglomerular epithelioid cells of the kidney cortex.

The role of connexins in human disease

Connexins are the building blocks of gap junctions. In forming a gap junction, six connexins oligomerize to form a hexameric torus called a connexon. The number of gap junctions in a cell ranges from a few to over 105 and imparts to interconnected cells a uniform phenotype. The crucial role that gap junctions play in normal physiology is reflected by the diverse spectrum of human diseases in which allele variants of different gap junction genes are implicated. In particular, mutations in GJB2 are a major cause of autosomal recessive non-syndromic deafness. This discovery has impacted medical practice and makes it incumbent on clinicians to familiarize themselves with the genetic advances that are rapidly occurring in our field.

Studies on scavenger receptor inhibitors. Part 1: synthesis and structure-activity relationships of novel derivatives of sulfatides

Scavenger receptors have been proven to be implicated in the formation of atherosclerotic lesions. A series of novel derivatives of sulfatides were synthesized, and their inhibitory activities against incorporation of DiI-acetyl-LDL into macrophages were evaluated in order to clarify the structure-activity relationships of sulfatides as a scavenger receptor inhibitor and find out novel inhibitors with synthetic easiness. The chemical modification of the substructures of sulfatides led to the establishment of the following structure-activity relationships; (1) the ceramide moiety can be replaced with another structure bearing two long chains, (2) the galactose moiety can be replaced with another structure or be deleted without a large decrease in the inhibitory activity, (3) the sulfate moiety was crucial, and it was the most preferable functional group for a potent inhibitory activity. The inhibitory activity of (S)-2-octadecanoylamino-2-tetradecylcarbamoyl)ethyl sulfate sodium salt (3a) against incorporation of DiI-acetyl-LDL into macrophages was proven to be based on the inhibition against the binding of acetyl-LDL to the surface of macrophages. We discovered novel scavenger receptor inhibitors with synthetic easiness, such as (S)-2-octadecanoylamino-2-(tetradecylcarbamoyl)ethyl sulfate sodium salt (3a) and 2-octadecanoylamino-1-(octadecanoylaminomethyl)ethyl sulfate sodium salt (13q).

The cellular Internet: on-line with connexins

Most cells communicate with their immediate neighbors through the exchange of cytosolic molecules such as ions, second messengers and small metabolites. This activity is made possible by clusters of intercellular channels called gap junctions, which connect adjacent cells. In terms of molecular architecture, intercellular channels consist of two channels, called connexons, which interact to span the plasma membranes of two adjacent cells and directly join the cytoplasm of one cell to another. Connexons are made of structural proteins named connexins, which compose a multigene family. Connexin channels participate in the regulation of signaling between developing and differentiated cell types, and recently there have been some unexpected findings. First, unique ionic- and size-selectivities are determined by each connexin; second, the establishment of intercellular communication is defined by the expression of compatible connexins; third, the discovery of connexin mutations associated with human diseases and the study of knockout mice have illustrated the vital role of cell-cell communication in a diverse array of tissue functions.

Connections with connexins: the molecular basis of direct intercellular signaling

Adjacent cells share ions, second messengers and small metabolites through intercellular channels which are present in gap junctions. This type of intercellular communication permits coordinated cellular activity, a critical feature for organ homeostasis during development and adult life of multicellular organisms. Intercellular channels are structurally more complex than other ion channels, because a complete cell-to-cell channel spans two plasma membranes and results from the association of two half channels, or connexons, contributed separately by each of the two participating cells. Each connexon, in turn, is a multimeric assembly of protein subunits. The structural proteins comprising these channels, collectively called connexins, are members of a highly related multigene family consisting of at least 13 members. Since the cloning of the first connexin in 1986, considerable progress has been made in our understanding of the complex molecular switches that control the formation and permeability of intercellular channels. Analysis of the mechanisms of channel assembly has revealed the selectivity of inter-connexin interactions and uncovered novel characteristics of the channel permeability and gating behavior. Structure/function studies have begun to provide a molecular understanding of the significance of connexin diversity and demonstrated the unique regulation of connexins by tyrosine kinases and oncogenes. Finally, mutations in two connexin genes have been linked to human diseases. The development of more specific approaches (dominant negative mutants, knockouts, transgenes) to study the functional role of connexins in organ homeostasis is providing a new perception about the significance of connexin diversity and the regulation of intercellular communication.

Selectivity of connexin-specific gap junctions does not correlate with channel conductance

Connexins form a variety of gap junction channels that vary in their developmental and tissue-specific levels of expression, modulation of gating by transjunctional voltage and posttranslational modification, and unitary channel conductance (gamma j). Despite a 10-fold variation in gamma j, whether connexin-specific channels possess distinct ionic and molecular permeabilities is presently unknown. A major assumption of the conventional model for a gap junction channel pore is that gamma j is determined primarily by pore diameter. Hence, molecular size permeability limits should increase and ionic selectivity should decrease with increasing channel gamma j (and pore diameter). Equimolar ion substitution of 120 mmol/L KCl for potassium glutamate was used to determine the unitary conductance ratios for rat connexin40 and connexin43, chicken connexin43 and connexin45, and human connexin37 channels functionally expressed in communication-deficient mouse neuroblastoma (N2A) cells. Comparison of experimental and predicted conductance ratios based on the aqueous mobilities of all ions according to the Goldman-Hodgkin-Katz current equation was used to determine relative anion-to-cation permeability ratios. Direct correlation of junctional conductance with dye transfer of two fluorescein-derivatives (2 mmol/L 6-carboxyfluorescein or 2',7'-dichlorofluorescein) was also performed. Both approaches revealed a range of selectivities and permeabilities for all five different connexins that was independent of channel conductance. These results are not consistent with the conventional simple aqueous pore model of a gap junction channel and suggest a new model for connexin channel conductance and permselectivity based on electrostatic interactions. Divergent conductance and permeability properties are features of other classes of ion channels (eg, Na+ and K+ channels), implying similar mechanisms for selectivity.

Characteristics of C6 glioma cells overexpressing a gap junction protein

1. C6 glioma cells transfected with connexin43 cDNA display a dramatic increase in the level of connexin43 mRNA and protein. 2. This overexpression of connexin43 is evident at the cellular level, as revealed with in situ hybridization and immunocytochemistry. Transfection with connexin43 cDNA also induced actin stress fibers in these glioma cells. 3. Although we observed up to a 50-fold increase in the level of connexin43 mRNA following transfection, virtually all of this mRNA was present in the polysomal fraction. 4. Overexpression of connexin43 mRNA did not appear to compete with other cellular mRNAs for access to the translational machinery. 5. It is likely that the reduced proliferation rate of the transfected cells, reported earlier, is due to enhanced connexin43 expression and intercellular coupling.

Microelectrode measurements of low frequency electric field effects in cells and tissues

The average intensities of electric fields induced into tissue can be calculated if the morphology and conductivities of the tissue are known, and such values provide one estimate of dosage for a given field exposure level. However, the microanatomical structures of living tissue, which include gap junctions, tight junctions, highly charged cell coats, and extracellular matrices, as well as complex cell shapes, precludes a detailed characterization of the field and current distribution near the cells which are actually responding to the electric fields. This suggests that a more useful electric field dose metric may be one based on an induced physical effect on the cells. Electric fields have at least three distinct physical effects on cells: the normal plasma membrane potential will be altered; the ionic currents and ion distributions at the extracellular surface will be modified; and mechanical forces will be imposed at the cell surface. Each of these effects can, in principle, be measured through the application of specific microelectrode techniques. Here, the feasibility of using various intracellular and extracellular recording methods to obtain dosimetric values, as well as the contribution these measurements could make to our understanding of electric field interactions with biological tissue, are discussed.

Developmental changes in regulation of embryonic chick heart gap junctions

Embryonic chick myocyte pairs were isolated from ventricular tissue of 4-day, 14-day, and 18-day heart for the purpose of examining the relationship between macroscopic junctional conductance and transjunctional voltage during cardiac development. The double whole-cell patch-clamp technique was employed to directly measure junctional conductance over a transjunctional voltage range of +/- 100 mV. At all ages, the instantaneous junctional current (or conductance = current/voltage) varied linearly with respect to transjunctional voltage. This initial response was followed by a time- and voltage-dependent decline in junctional current to new steady-state values. For every experiment, the steady-state junctional conductance was normalized to the instantaneous value obtained at each potential and the data was pooled according to developmental age. The mean steady-state junctional conductance-voltage relationship for each age group was fit using a two-state Boltzmann distribution described previously for other voltage-dependent gap junctions. From this model, it was revealed that half-inactivation voltage for the transjunctional voltage-sensitive conductance shifted towards larger potentials by 10 mV, the equivalent gating charge increased by approximately 1 electron, and the minimal voltage-insensitive conductance exactly doubled (increased from 18 to 36%) between 4 and 18 days of development. Decay time constants were similar at all ages examined as rate increased with increasing transjunctional potential. This data provides the first direct experimental evidence for developmental changes in the regulation of intercellular communication within a given tissue. This information is consistent with the hypothesis that developmental expression of multiple gap junction proteins (connexins) may confer different regulatory mechanisms on intercellular communication pathways within a given cell or tissue.

Desensitization to follicle-stimulating hormone in cumulus cells is coincident with hormone induction of oocyte maturation in the rat follicle

The objective of the present studies was to assess whether hormone induction of oocyte maturation in isolated intact follicles may be linked to desensitization of follicle-stimulating hormone (FSH) in the oocyte-cumulus complex (OCC). Incubation of follicles with chorionic gonadotropin (hCG), FSH or epidermal growth factor (EGF) produced a marked inhibition of FSH-dependent cyclic AMP accumulation in OCC with a time-course coincident with the onset of germinal vesicle breakdown (GVBD). These effects were evident within 3 h for both hCG and FSH, but with EGF a reduced response to FSH was seen within 1 h of treatment followed by an increase in GVBD. In contrast, no inhibition of cyclic AMP accumulation was seen in response to cholera toxin, forskolin or LH in OCC derived from follicles incubated with hCG for 3 h. The time-course for induction of oocyte maturation by incubation of the intact follicle with hCG was also coincident with production of prostaglandin (PG) F2 alpha, an indirect marker of cyclooxygenase induction. No effect on metabolic coupling between the oocyte and cumulus cells was seen until 9 h after hCG treatment. Retinoic acid caused a marked decrease in metabolic coupling between the oocyte and cumulus cells but inhibited oocyte maturation both in denuded oocytes and OCC. Since FSH desensitization in OCC, the resumption of meiosis, and production of arachidonic acid-derived products were coincident, it is suggested that abrogation of FSH action in cumulus cells by the ovulatory surge of gonadotropins may initiate oocyte maturation.

Purification and characterization of a bovine acetyl low density lipoprotein receptor

The acetyl low density lipoprotein (LDL) receptor is expressed on macrophages and some endothelial cells and mediates macrophage-foam cell formation in culture. A 220-kDa acetyl LDL binding protein was partially purified from bovine liver membranes and was used to make a specific monoclonal antibody. The 220-kDa protein immunoprecipitated by this antibody retained binding activity, and the antibody was used to detect this protein in cells lining bovine liver sinusoids and on the surface of cultured bovine alveolar macrophages. In the human monocytic cell line THP-1, the expression of both acetyl LDL receptor activity and a 220-kDa acetyl LDL binding protein were dramatically induced in parallel after differentiation to a macrophage-like state induced by phorbol ester. The ligand specificity, tissue and cell-type specificity, and coinduction data indicated that this 220-kDa cell-surface binding protein is probably a receptor that mediates acetyl LDL endocytosis. The 220-kDa protein, which was purified 238,000-fold from bovine lung membranes to near homogeneity using monoclonal antibody affinity chromatography, is a trimer of 77-kDa subunits that contain asparagine-linked carbohydrate chains.