Interaction between nitric oxide signaling and gap junctions: effects on vascular function.

Nitric oxide signaling, through eNOS (or possibly nNOS), and gap junction communication are essential for normal vascular function. While each component controls specific aspects of vascular function, there is substantial evidence for cross-talk between nitric oxide signaling and the gap junction proteins (connexins), and more recently, protein-protein association between eNOS and connexins. This review will examine the evidence for interaction between these pathways in normal and diseased arteries, highlight the questions that remain about the mechanisms of their interaction, and explore the possible interaction between nitric oxide signaling and the newly discovered pannexin channels. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

The primary structure of NG2, a novel membrane-spanning proteoglycan.

The complete primary structure of the core protein of rat NG2, a large, chondroitin sulfate proteoglycan expressed on O2A progenitor cells, has been determined from cDNA clones. These cDNAs hybridize to an mRNA species of 8.9 kbp from rat neural cell lines. The total contiguous cDNA spans 8,071 nucleotides and contains an open reading frame for 2,325 amino acids. The predicted protein is an integral membrane protein with a large extracellular domain (2,224 amino acids), a single transmembrane domain (25 amino acids), and a short cytoplasmic tail (76 amino acids). Based on the deduced amino acid sequence and immunochemical analysis of proteolytic fragments of NG2, the extracellular region can be divided into three domains: an amino terminal cysteine-containing domain which is stabilized by intrachain disulfide bonds, a serine-glycine-containing domain to which chondroitin sulfate chains are attached, and another cysteine-containing domain. Four internal repeats, each consisting of 200 amino acids, are found in the extracellular domain of NG2. These repeats contain a short sequence that resembles the putative Ca(++)-binding region of the cadherins. The sequence of NG2 does not show significant homology with any other known proteins, suggesting that NG2 is a novel species of integral membrane proteoglycan.

The heavy chain of tetanus toxin can mediate the entry of cytotoxic gelonin into intact cells.

An artificial conjugate of the heavy chain of tetanus toxin linked by a disulphide bond to the impermeant ribosome-inactivating protein gelonin is cytotoxic to intact HT29 cells by inhibiting intracellular protein synthesis. Neither toxin nor gelonin alone has any significant effect. This shows that the heavy chain has the ability to mediate internalization of a protein to which it is bound by a disulphide bond. Thus the normal role of the tetanus toxin heavy chain may be to allow entry of the light chain into a cell.

Inactivation of platelet-derived growth factor-BB following modification by ADP-ribosyltransferase.

1. Arginine-specific ADP-ribosyltransferase (ART1) is expressed on the surface of a number of cell types, and catalyses the transfer of ADP-ribose from NAD(+) to target proteins. We investigated whether extracellular proteins such as growth factors may serve as substrates for this enzyme, with subsequent alteration in their biological activity. Experiments were performed with rat skeletal muscle membranes and V79 Chinese hamster lung fibroblasts with doxycycline-inducible expression of human ART. 2. From a panel of growth factors, platelet-derived growth factor-BB (PDGF-BB) was found to be the best substrate for ART1, whereas the structural homologue PDGF-AA was not a substrate. Under conditions of maximum labelling 5 mol ADP-ribose was incorporated per mol of PDGF-BB. 3. Purified (ADP-ribosyl)-PDGF-BB did not stimulate a mitogenic or chemotactic response in human pulmonary smooth muscle cells, and showed a reduced capacity to bind to PDGF receptors in competition binding experiments, when compared to unmodified PDGF-BB. 4. PDGF-dependent [(3)H-methyl]-thymidine incorporation was measured in the ART1-transfected fibroblast cell line at physiological concentrations of PDGF-BB, and without addition of extracellular NAD(+). Fibroblasts expressing human ART1 at the cell surface showed reduced mitogenic responses to PDGF-BB, but not to PDGF-AA. This loss of mitogenic response in cells expressing ART1 activity was reversed by the addition of agmatine (an ART1 substrate). 5. In conclusion, we propose that PDGF-BB-dependent signalling may be regulated by post-translational modification of the growth factor by ART1 at the cell surface. This has been demonstrated in membranes of rat skeletal muscle, and the reaction confirmed in ART1-transfected fibroblasts.

Cerebellar astroglial cells in primary culture: expression of different morphological appearances and different ability to take up [3H]D-aspartate and [3H]GABA.

In non-neuronal cultures of cells dissociated from postnatal rat cerebellum astrocytes, identified by the presence of the marker protein glial fibrillary acidic protein (GFAP), displayed two distinct morphological forms. One class was stellate in shape with radially distributed fine processes, while the other class was more varied in shape being polygonal or elongate. [3H]thymidine incorporation experiments revealed that cells of both morphologies were able to incorporate this nucleoside, suggesting the capacity for both cell types to undergo cell division. An autoradiographic study of the uptake of [3H]D-aspartate and [3H]GABA revealed that whilst the two classes of astrocytes took up the aspartate to apparently the same extent, only the stellate cells were found to be heavily labeled following incubation with [3H]GABA. A study of the cultures over a 12-day period showed that there was a disappearance of the stellate astrocytes. The time of disappearance was found to be dependent upon the initial plating density; the stellate morphology was apparent longer in lower density cultures. Time lapse studies suggested that one of the reasons for the disappearance of the stellate cells might be that in fact they underwent a change in shape following certain cell-cell interactions, but cell death also has to be considered as a further possibility. The relationships between the two classes of astroglial cells in these cultures is not yet clear. The possibilities are that they represent two different types of astrocytes, or just one type at different stages of differentiation, or maybe a combination of the two possibilities.

Subpopulations of rat cerebellar astrocytes in primary culture: morphology, cell surface antigens and [3H]GABA transport.

Glial fibrillary acidic protein (GFAP)-positive astrocytes in preconfluent cultures derived from postnatal rat cerebellum have been previously shown to display two distinct morphologies, one stellate and the other irregularly epithelioid. The immunofluorescence studies described here showed that these cells also possess unique surface characteristics. In cultures derived from 8-day-old animals stellate cells bound the monoclonal antibody A2B5 whereas the epithelioid cells bound another monoclonal antibody against rat neural antigen-2 (RAN2). Some stellate cells derived from 2-day-old animals also bound tetanus toxin. The A2B5 labelling of the stellate cells made it possible to follow their fate in vitro. In confirmation of previous time-lapse studies, they underwent a shape transformation as confluence was approached, ultimately attaining a form resembling that of the epithelioid cells. Autoradiographic transport studies using two tritiated gamma-aminobutyric acid (GABA) analogues cis-1,3-aminocyclohexane carboxylic acid (ACHC) and beta-alanine revealed further differences between the two types of astrocytes. Whereas [3H]ACHC was taken up solely by the stellate cells [3H]beta-alanine was transported by both cell types. In other experiments in which various inhibitors of [3H]GABA transport were used ACHC virtually eliminated uptake into the stellate astrocyte, but had little effect on the epithelioid ones. The 'neuron-like' [3H]GABA transport process in the stellate astrocytes was confirmed in experiments comparing the effect of another compound which has been proposed as an astrocyte-selective GABA transport inhibitor, 4,5,6,7-tetrahydroisoxazolo-(4,5-C)pyridin-3-ol (THPO). No discrimination was found in its effect on the uptake of [3H]GABA into either neurons or stellate astrocytes. Further autoradiographic studies following the uptake of [3H]GABA by postnatal cerebellar slices showed that astrocytes in all layers of the cerebellar cortex and white matter transported [3H]GABA in contrast to the situation in culture where the amino acid is taken up predominantly by the stellate astrocytes. The possibility is discussed that the stellate astrocytes represent a population of cerebellar fibrous astrocytes whereas the identity of the epithelioid astrocytes is less certain.

Altered expression of the D1.1 ganglioside in the cerebellum of the Weaver mouse.

Expression of the D1.1 ganglioside was studied immunohistochemically in developing cerebella from normal and weaver mutant mice. In the normal cerebellum at postnatal day 7 (P7), D1.1 expression was restricted to the external granule-cell layer (EGL). At later ages, D1.1 disappeared as the developing granule neurons ceased mitosis and began migrating toward the internal granule-cell layer. In the weaver cerebellum, D1.1 was expressed in the EGL in apparently normal fashion at P7, but failed to disappear at later ages. As late as P35, D1.1 immunoreactivity was observed throughout the weaver cerebellar cortex. The relative amounts of D1.1 ganglioside in weaver and normal cerebella were compared by thin layer chromatography of total gangliosides, followed by overlay of the chromatogram with anti-D1.1 and 125I-labelled second antibody. Autoradiograms showed that at P12 and P35 the weaver tissue contains six- to tenfold more D1.1 than normal tissue. These findings suggest that one result of the weaver mutation is prolonged expression of D1.1. We speculate that the D1.1 ganglioside might be involved in adhesive interactions that regulate the timing of granule-cell migration from the EGL. The prolonged expression of D1.1 could be responsible, in part, for the failure of granule-cell migration in the weaver cerebellum.

The effect of diethylstilboestrol on the in vitro growth of human ectocervical cells.

The effect of chronic exposure to diethylstilboestrol (DES) over the concentration range 10(-4)-10(-6) M on the in vitro growth of cells derived from the squamo -columnar junction of the human cervix has been examined. DES over the dose range 10(-4)-2 X 10(-5) M inhibits cervical epithelial cell (HCE) growth in terms of colony formation and colony expansion. This effect involves both an effect on cell proliferation, as measured by cell number and mitotic index, and an induction of differentiation as measured by an increase in the number of PAS positive cells (glycogen accumulating cells) and in the number of cells with cornified envelopes. At lower doses of DES, 10(-5)-5 X 10(-6) M, the efficiency of colony formation and colony size are not inhibited but mitotic index and cell number are decreased and the fraction of differentiating cells is increased. The effects of DES on HCE cell growth can be modified to some extent by epidermal growth factor (EGF). EGF at 10 ng/ml, increases the fraction of proliferating cells, induces cellular hypertrophy, reduces stratification and the fraction of differentiating cells in HCE colonies. DES, at all concentrations other than 10(-6) M, in the presence of EGF inhibits cell proliferation but does not prevent the EGF induced cellular hypertrophy, reduction in stratification or reduction in the fraction of differentiating cells.