The effect of polystyrene beads on cyclic 3',5'-adenosine monophosphate concentration in leukocytes.

After incubation with polystyrene latex beads for 5 min. the cyclic 3',5'-adenosine monophosphate (cyclic AMP) content of human peripheral blood leukocyte suspensions was increased severalfold. Preparations enriched in mononuclear cells and containing only 0-20% polymorphonuclear leukocytes (PMN) and no visible platelets exhibited a quantitatively similar response. Purified fractions of cells containing 85-90% PMN responded to polystyrene beads with a much smaller increase in cyclic AMP content. Phagocytosis of paraffin oil emulsion in the unfractionated mixed human leukocyte preparation was associated with little or no change in cyclic AMP levels. There was no change in cyclic AMP content of rabbit alveolar macrophages or guinea pig PMN during phagocytosis of polystyrene beads. All of these observations are consistent with the view that particle uptake per se does not increase cyclic AMP levels in phagocytic cells. It seems probable that the increase in cyclic AMP concentration that results when unfractionated human blood leukocytes are incubated with polystyrene beads occurs in cells other than PMN.

Signalling of DNA damage and cytokines across cell barriers exposed to nanoparticles depends on barrier thickness.

The use of nanoparticles in medicine is ever increasing, and it is important to understand their targeted and non-targeted effects. We have previously shown that nanoparticles can cause DNA damage to cells cultured below a cellular barrier without crossing this barrier. Here, we show that this indirect DNA damage depends on the thickness of the cellular barrier, and it is mediated by signalling through gap junction proteins following the generation of mitochondrial free radicals. Indirect damage was seen across both trophoblast and corneal barriers. Signalling, including cytokine release, occurred only across bilayer and multilayer barriers, but not across monolayer barriers. Indirect toxicity was also observed in mice and using ex vivo explants of the human placenta. If the importance of barrier thickness in signalling is a general feature for all types of barriers, our results may offer a principle with which to limit the adverse effects of nanoparticle exposure and offer new therapeutic approaches.

Assembly of heteromeric connexons in guinea-pig liver en route to the Golgi apparatus, plasma membrane and gap junctions.

Guinea-pig liver gap junctions are constructed from approximately equal amounts of connexins 26 and 32. The assembly of these connexins into connexon hemichannels and gap junctions was studied using antibodies specific to each connexin. Intracellular membranes were shown to contain low amounts of connexin 26 relative to connexin 32 in contrast to the equal connexin ratios detected in lateral plasma membranes and gap junctions. Assembly of gap junctions requires oligomerization of connexins into connexons that may be homomeric or heteromeric. Immunoprecipitation using antibodies to connexins 26 and 32 showed that liver gap junctions were heteromeric. A chemical cross-linking procedure showed that connexons solubilized from guinea-pig liver gap junctions were constructed of hexameric assemblies of connexin subunits. The intracellular site of oligomerization of connexins was investigated by velocity sedimentation in sucrose-detergent gradients. Oligomers of connexins 26 and 32 were extensively present in Golgi membranes and oligomeric intermediates, especially of connexin 26, were detected in the endoplasmic reticulum-Golgi intermediate subcellular fraction. Two intracellular trafficking pathways that may account for the delivery of connexin 26 to the plasma membrane and explain the heteromeric nature of liver gap junctions are discussed.

Multiple connexin expression in peripheral nerve, Schwann cells, and Schwannoma cells.

Myelinating Schwann cells express the gap junction protein, connexin (Cx)32, which is present at the nodes of Ranvier and Schmidt-Lantermann incisures (Bergoffen et al. [1993] Science (Wash. ) 262:2039-2042). Following peripheral nerve injury, other members of the connexin gene family are also expressed (Chandross et al. [1996a] Mol. Cell. Neurosci. 7:501-518). This study surveys the connexin(s) expressed by rat sciatic nerve, cultured Schwann cells, and a mouse Schwannoma (TR6 Bc1) cell line. Reverse transcriptase-polymerase chain reaction (RT-PCR) amplification revealed a constitutive expression of mRNA encoding Cx32 and 43 but not Cx26, 37, 40, 45, and 46 in sciatic nerve. Mitogenic stimulation of cultured Schwann cells expressing Cx32 also resulted in the appearance of Cx43 mRNA. Schwannoma cells expressed exclusively Cx43 mRNA. These results were confirmed by Northern blot analysis. Functional gap junctions in cultured Schwann and Schwannoma cells were shown by analysis of the intercellular transfer of Lucifer yellow, although the coupling between primary Schwann cells was weak or undetectable. Treatment of primary Schwann cells with mitogens resulted in extensive dye coupling. An immunohistochemical study of adult sciatic nerve sections demonstrated Cx32 immunoreactivity at the nodes of Ranvier and in Schwann cell bodies. Lower intensity staining of Cx43 along the myelin sheath and Schwann cell bodies was also observed. Indirect immunofluorescent studies of Schwann cells treated with mitogens showed characteristic punctate cell surface staining of Cx43; Cx32 staining was detected mainly intracellularly. These results lead to the conclusion that in addition to the expression of Cx32 by normal adult sciatic nerve, low amounts of Cx43 protein are also present. The implications of the expression of two connexins by Schwann cells in Charcot-Marie-Tooth X-linked disease, a demyelinating peripheral neuropathy, are discussed.

Receptor-rich intracellular membrane vesicles transporting asialotransferrin and insulin in liver.

A wide range of receptors are located at the blood sinusoidal aspect of the hepatocyte plasma membrane. Many circulating ligands that bind to receptors on the cell surfaces are interiorized along two pathways. Asialoglycoproteins are transferred from the plasma membrane to lysosomes and degraded, whereas immunoglobulin A and bile acids are transported across the hepatocyte interior and released into bile. Asialotransferrin type 3 (ref. 6) follows a further pathway termed diacytosis. After binding to the asialoglycoprotein receptor, asialotransferrin is endocytosed and then returned to blood with a proportion of its carbohydrate side chains resialylated. We now describe in liver the properties of intracellular asialotransferrin-enclosing vesicles (diacytosomes) and show that they differ from Golgi, lysosome and plasma membrane fractions. Furthermore, we show that the asialoglycoprotein binding sites are located on the cytoplasmic (outer) surface of diacytosomes.

Synthesis and assembly of connexins in vitro into homomeric and heteromeric functional gap junction hemichannels.

The biogenesis of connexins and their assembly into functional gap junction hemichannels (connexons) was studied with the use of a cell-free transcription/translation system. Velocity sedimentation on sucrose gradients showed that a small proportion of connexin (Cx) 26 and Cx32 that were co-translationally translocated into microsomes were oligomers of Cx26 and Cx32. Chemical cross-linking studies showed that these corresponded to hexameric connexons. Reconstitution of connexons synthesized in vitro into liposomes induced permeability properties consistent with the view that open gap junction hemichannels were produced. By using an immunoprecipitation approach, a simultaneous translation of Cx26 and Cx32 incorporated into microsomes resulted in homomeric connexons. However, supplementation of the translation system in vitro with liver Golgi membranes produced heteromeric connexons constructed of Cx32 and Cx26, and also resulted in an increased oligomerization especially of Cx32. All of the connexins analysed were inserted co-translationally into canine pancreatic microsomal membranes. In addition, Cx26 and Cx43, but not Cx32, were also inserted into microsomal membranes post-translationally. Analysis of various connexin constructs in which the cytoplasmic carboxy tails were transposed, the cytoplasmic tail of Cx43 was truncated or a reporter protein, aequorin, was attached to the C-terminus showed that tail length was not the major determinant of the post-translational membrane insertion of connexins.

Analysis of gap junction assembly using mutated connexins detected in Charcot-Marie-Tooth X-linked disease.

The assembly of gap junction intercellular communication channels was studied by analysis of the molecular basis of the dysfunction of connexin 32 mutations associated with the X-linked form of Charcot-Marie-Tooth disease in which peripheral nervous transmission is impaired. A cell-free translation system showed that six recombinant connexin 32 mutated proteins-four point mutations at the cytoplasmic amino terminus, one at the membrane aspect of the cytoplasmic carboxyl terminus, and a deletion in the intracellular loop-were inserted into microsomal membranes and oligomerised into connexon hemichannels with varying efficiencies. The functionality of the connexons was determined by the ability of HeLa cells expressing the respective connexin cDNAs to transfer Lucifer yellow. The intracellular trafficking properties of the mutated connexins were determined by immunocytochemistry. The results show a relationship between intracellular interruption of connexin trafficking, the efficiency of intercellular communication, and the severity of the disease phenotype. Intracellular retention was explained either by deficiencies in the ability of connexins to oligomerise or by mutational changes at two targeting motifs. The results point to dominance of two specific targeting motifs: one at the amino terminus and one at the membrane aspect of the cytoplasmically located carboxyl tail. An intracellular loop deletion of six amino acids, associated with a mild phenotype, showed partial oligomerisation and low intercellular dye transfer compared with wild-type connexin 32. The results show that modifications in trafficking and assembly of gap junction channels emerge as a major feature of Charcot-Marie-Tooth X-linked disease.