Diameter of the cell-to-cell junctional membrane channels as probed with neutral molecules

A structural and functional comparison of gap junction channels composed of connexins and innexins

Methods such as electron microscopy and electrophysiology led to the understanding that gap junctions were dense arrays of channels connecting the intracellular environments within almost all animal tissues. The characteristics of gap junctions were remarkably similar in preparations from phylogenetically diverse animals such as cnidarians and chordates. Although few studies directly compared them, minor differences were noted between gap junctions of vertebrates and invertebrates. For instance, a slightly wider gap was noted between cells of invertebrates and the spacing between invertebrate channels was generally greater. Connexins were identified as the structural component of vertebrate junctions in the 1980s and innexins as the structural component of pre-chordate junctions in the 1990s. Despite a lack of similarity in gene sequence, connexins and innexins are remarkably similar. Innexins and connexins have the same membrane topology and form intercellular channels that play a variety of tissue- and temporally specific roles. Both protein types oligomerize to form large aqueous channels that allow the passage of ions and small metabolites and are regulated by factors such as pH, calcium, and voltage. Much more is currently known about the structure, function, and structure-function relationships of connexins. However, the innexin field is expanding. Greater knowledge of innexin channels will permit more detailed comparisons with their connexin-based counterparts, and provide insight into the ubiquitous yet specific roles of gap junctions. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 522-547, 2017.

Distinct permeation profiles of the connexin 30 and 43 hemichannels

Connexin 43 (Cx43) hemichannels may form open channels in the plasma membrane when exposed to specific stimuli, e.g. reduced extracellular concentration of divalent cations, and allow passage of fluorescent molecules and presumably a range of smaller physiologically relevant molecules. However, the permeability profile of Cx43 hemichannels remains unresolved. Exposure of Cx43-expressing Xenopus laevis oocytes to divalent cation free solution induced a gadolinium-sensitive uptake of the fluorescent dye ethidium. In spite thereof, a range of biological molecules smaller than ethidium, such as glutamate, lactate, and glucose, did not permeate the pore whereas ATP did. In contrast, permeability of glutamate, glucose and ATP was observed in oocytes expressing Cx30. Exposure to divalent cation free solutions induced a robust membrane conductance in Cx30-expressing oocytes but none in Cx43-expressing oocytes. C-terminally truncated Cx43 (M257) displayed increased dye uptake and, unlike wild type Cx43 channels, conducted current. Neither Cx30 nor Cx43 acted as water channels in their hemichannel configuration. Our results demonstrate that connexin hemichannels have isoform-specific permeability profiles and that dye uptake cannot be equaled to permeability of smaller physiologically relevant molecules in given settings.

MicroRNAs transfer from human macrophages to hepato-carcinoma cells and inhibit proliferation

Recent research has indicated a new mode of intercellular communication facilitated by the movement of RNA between cells. There is evidence that RNA can transfer between cells in a multitude of ways, including in complex with proteins or lipids or in vesicles, including apoptotic bodies and exosomes. However, there remains little understanding of the function of nucleic acid transfer between human cells. In this article, we report that human macrophages transfer microRNAs (miRNAs) to hepato-carcinoma cells (HCCs) in a manner that required intercellular contact and involved gap junctions. Two specific miRNAs transferred efficiently between these cells--miR-142 and miR-223--and both were endogenously expressed in macrophages and not in HCCs. Transfer of these miRNAs influenced posttranscriptional regulation of proteins in HCCs, including decreased expression of reporter proteins and endogenously expressed stathmin-1 and insulin-like growth factor-1 receptor. Importantly, transfer of miRNAs from macrophages functionally inhibited proliferation of these cancerous cells. Thus, these data led us to propose that intercellular transfer of miRNA from immune cells could serve as a new defense against unwanted cell proliferation or tumor growth.

Oligomeric structure and functional characterization of Caenorhabditis elegans Innexin-6 gap junction protein

Innexin is the molecular component of invertebrate gap junctions. Here we successfully expressed and purified Caenorhabditis elegans innexin-6 (INX-6) gap junction channels and characterized the molecular dimensions and channel permeability using electron microscopy (EM) and microinjection of fluorescent dye tracers, respectively. Negative staining and thin-section EM of isolated INX-6 gap junction membranes revealed a loosely packed hexagonal lattice and a greater cross-sectional width than that of connexin26 and connexin43 (Cx43)-GFP. In gel filtration analysis, the elution profile of purified INX-6 channels in dodecyl maltoside solution exhibited a peak at ∼400 kDa that was shifted to ∼800 kDa in octyl glucose neopentyl glycol. We also obtained the class averages of purified INX-6 channels from these peak fractions by single particle analysis. The class average from the ∼800-kDa fraction showed features of the junction form with a longitudinal height of 220 Å, a channel diameter of 110 Å in the absence of detergent micelles, and an extracellular gap space of 60 Å, whereas the class averages from the ∼400-kDa fraction showed diameters of up to 140 Å in the presence of detergent micelles. These findings indicate that the purified INX-6 channels are predominantly hemichannels in dodecyl maltoside and docked junction channels in octyl glucose neopentyl glycol. Dye transfer experiments revealed that the INX-6-GFP-His channels are permeable to 3- and 10-kDa tracers, whereas no significant amounts of these tracers passed through the Cx43-GFP channels. Based on these findings, INX-6 channels have a larger overall structure and greater permeability than connexin channels.

Microscopic anatomy: normal structure

A peripheral nerve trunk is composed of nerve fascicles supported in a fibrous collagenous sheath and defined by concentric layers of cells (the perineurium) that separate the contents (the endoneurium) from its fibrous collagen support (the epineurium). In the endoneurium are myelinated and unmyelinated fibers that are axons combined with their supporting Schwann cells to provide physical and electrical connections with end-organs such as muscle fibers and sensory endings. Axons are tubular neuronal extensions with a cytoskeleton of neurotubules and tubulin along which organelles and proteins can travel between the neuronal cell body and the axon terminal. During development some axons enlarge and are covered by a chain of Schwann cells each associated with just one axon. As the axons grow in diameter, the Schwann cells wrap round them to produce a myelin sheath. This consists of many layers of compacted Schwann cell membrane plus some additional proteins. Adjacent myelin segments connect at highly specialized structures, the nodes of Ranvier. Myelin insulates the axon so that the nerve impulse can jump from one node to the next. The region adjacent to the node, the paranodal segment, is the site of myelin terminations on the axolemma. There are connections here between the Schwann cell and the axon via a complex chain of proteins. The Schwann cell cytoplasm in the adjacent segment, the juxtaparanode, contains most of the Schwann cell mitochondria. In addition to the node, continuity of myelin lamellae is broken at intervals along the internode by helical regions of decompaction known as Schmidt-Lanterman incisures; these are seen as paler conical segments in suitably stained microscopical preparations and provide a pathway between the adaxonal and abaxonal cytoplasm. Smaller axons without a myelin sheath conduct very much more slowly and have a more complex relationship with their supporting Schwann cells that has important implications for repair.

Connexin and pannexin mediated cell-cell communication

In this review, we briefly summarize what is known about the properties of the three families of gap junction proteins, connexins, innexins and pannexins, emphasizing their importance as intercellular channels that provide ionic and metabolic coupling and as non-junctional channels that can function as a paracrine signaling pathway. We discuss that two distinct groups of proteins form gap junctions in deuterostomes (connexins) and protostomes (innexins), and that channels formed of the deuterostome homologues of innexins (pannexins) differ from connexin channels in terms of important structural features and activation properties. These differences indicate that the two families of gap junction proteins serve distinct, complementary functions in deuterostomes. In several tissues, including the CNS, both connexins and pannexins are involved in intercellular communication, but have different roles. Connexins mainly contribute by forming the intercellular gap junction channels, which provide for junctional coupling and define the communication compartments in the CNS. We also provide new data supporting the concept that pannexins form the non-junctional channels that play paracrine roles by releasing ATP and, thus, modulating the range of the intercellular Ca(2+)-wave transmission between astrocytes in culture.

Molecular make-up of the Plasmodium parasitophorous vacuolar membrane

Plasmodium, the causative agent of malaria, is an obligate, intracellular, eukaryotic cell that invades, replicates, and differentiates within hepatocytes and erythrocytes. Inside a host cell, a second membrane delineates the developing pathogen in addition to the parasite plasma membrane, resulting in a distinct cellular compartment, termed parasitophorous vacuole (PV). The PV membrane (PVM) constitutes the parasite-host cell interface and is likely central to nutrient acquisition, host cell remodeling, waste disposal, environmental sensing, and protection from innate defense. Over the past two decades, a number of parasite-encoded PVM proteins have been identified. They include multigene families and protein complexes, such as early-transcribed membrane proteins (ETRAMPs) and the Plasmodium translocon for exported proteins (PTEX). Nearly all Plasmodium PVM proteins are restricted to this genus and display transient and stage-specific expression. Here, we provide an overview of the PVM proteins of Plasmodium blood and liver stages. Biochemical and experimental genetics data suggest that some PVM proteins are ideal targets for novel anti-malarial intervention strategies.

Role of adherens junction proteins in differential herpes simplex virus type 2 infectivity in communication-competent and -deficient cell lines

BACKGROUND

Gap junctional intercellular communication decreases with HSV-2 infection. To determine the importance of functional gap junctions for infectivity, we compared HSV-2 growth in communication-competent and -deficient cell lines.

METHODS

HSV-2 infectivity was tested in five cell lines: WB rat liver epithelial cells (communication-competent), WB-aB1 (communication-deficient), WB-a/32-10 (communication-rescued), HeLa (communication-deficient), and Cx43-transfected HeLa (communication-rescued) cells. HSV-2 growth curves and indirect immunofluorescence labeling of viral and cell proteins were performed in wild-type and mutant WB cells.

RESULTS

Although wild-type WB cells were highly permissive for HSV-2 infection, virus production was significantly attenuated in communication-deficient and -rescued mutant WB cells. HeLa exhibited no difference in infectivity between communication-competent and -deficient cell lines. Tight and adherens junction proteins, including zonula occludens-1 and nectin-1, were not different in the WB cell lines. However, E-cadherin levels were elevated and β-catenin was found to co-localize with glycoprotein E, a viral glycoprotein associated with cell-to-cell spread, in the mutant WB cells.

CONCLUSIONS

These results suggest that attenuated viral production in mutant WB cells is due to viral protein co-localization with adherens junction proteins rather than the loss or restoration of functional gap junctions.

Passage through vertebrate gap junctions of 17/18kDa molecules is primarily dependent upon molecular configuration

In fish, amphibians and mammals, gap junctions of some cells allow passage of elongate molecules as large as 18kDa, while excluding smaller, less elongate molecules. Fluorescently labeled Calmodulin (17kDa) and fluorescently labeled Troponin-C (18kDa), when microinjected into oocytes of Danio rerio, Xenopus laevis or Mus domestica, were able to transit the gap junctions between these oocytes and the granulosa cells which surrounded them. Co-microinjected with these Ca(2+)-binding proteins, Texas-red-labeled dextran (10kDa) remained in the microinjected cell. Osteocalcin (6kDa), also a Ca(2+)-binding protein, but with a wide "V" shape proved unable to transit these gap junctions. Calmodulin, but not Troponin-C, was able to transit gap junctions of gonadotropin treated WB cells in culture. We show evidence that molecules as large as 18kDa can pass through some vertebrate gap junctions, both homologous and heterologous, and that it is primarily molecular configuration which governs gap junctional permeability.

Antifertility effects of antigestogens

Since the discovery of the structure and function of steroids over 60 years ago, it has long been recognized that synthetic antagonists of the natural hormones would have potential therapeutic uses. Antagonists of mineralocorticoids, androgens and oestrogens, for example spironolactine, cyproterone, flutamide and tamoxifen, have already found a place in the management of hormone dependent conditions. In 1982, chemists at Roussel UCLAF announced that they had synthesized mifepristone (RU486) 17β-hydroxy-11(p-(dimethylamino)phenyl)-17-(1-propynyl) estra-411, 9-dien-3-one) a derivative of norethindrone which had potent antiprogestogenic as well as antiglucocorticoid activity. Although it was immediately realised that this compound would potentially have wide clinical application, its development in the last 10 years has been dominated by its abortifacient action. In the original clinical report by Herrman and colleagues it was shown that bleeding occurred when it was given to female volunteers in the second half of the menstrual cycle. In addition, complete abortion occurred in eight of 11 women who took the drug in the early weeks of pregnancy. These findings, which demonstrated that mifepristone could be used as the basis of a medical method of inducing abortion, were immediately made the focus of groups opposed to abortion on moral grounds. Experience over the last 10 years has confirmed the promise of these early studies and mifepristone, in combination with a suitable prostaglandin, is now licensed in France, UK and Sweden for use as a medical method of inducing abortion in early pregnancy.

Passage of 17kDa calmodulin through gap junctions of three vertebrate species

Gap junctions of some vertebrates are capable of passing the elongate molecule, calmodulin, with a molecular weight 8-17 times greater than the previously recognized size limits. Fluorescently labeled calmodulin (FCaM) (17.34 kDa) microinjected into oocytes of ovarian follicles from an amphibian, Xenopus laevis, and from two species of teleost fish, Danio rerio (Zebrafish) and Oryzias latipes (Medaka), is shown to transit their gap junctions and enter the surrounding epithelial cells. Passage of FCaM was terminated when follicles were first treated with 1 mM octanol, a molecule known to down-regulate gap junctions. There was no FCaM detected in the surrounding medium, nor did epithelial cells become fluorescent when follicles were incubated in medium containing dye. Calmodulin is well known to modulate many cytoplasmic reactions; thus, its passage through gap junctions opens possibilities of additional means by which cells may be supplied with this signaling molecule, and by which their supply may be regulated.

Cellular interactions of Plasmodium liver stage with its host mammalian cell

The Plasmodium liver forms are bridgehead stages between the mosquito sporozoite stages and mammalian blood stages that instigate the malaria disease. In hepatocytes, Plasmodium achieves one of the fastest growth rates among eukaryotic cells. However, nothing is known about host hepatic cell interactions, e.g. nutrient scavenging and/or subversion of cellular functions necessary for Plasmodium development and replication. Plasmodium usually invades hepatocytes by establishing a parasitophorous vacuole wherein it undergoes multiple nuclear division cycles. We show that Plasmodium preferentially develops in the host juxtanuclear region. By comparison with the parasitophorous vacuole of other apicomplexan parasites which associate with diverse host organelles, the Plasmodium parasitophorous vacuole only forms an association with the host endoplasmic reticulum. Intrahepatic Plasmodium actively modifies the permeability of its vacuole to allow the transfer of a large variety of molecules from the host cytosol to the vacuolar space through open channels. In contrast with malaria blood stages, the pores within the parasitophorous vacuole membrane of the liver stage display a smaller size as they restrict the passage of solutes to less than 855Da. These pores are stably maintained during parasite karyokinesis until complete cellularisation. Host-derived cholesterol accumulated at the parasitophorous vacuole membrane may modulate the channel activity. These observations define the parasitophorous vacuole of the Plasmodium liver stage as a dynamic and highly permeable compartment that can ensure the sustained supply of host molecules to support parasite growth in the nutrient-rich environment of liver cells.

Primary cultures of chick osteocytes retain functional gap junctions between osteocytes and between osteocytes and osteoblasts

The inaccessibility of osteocytes due to their embedment in the calcified bone matrix in vivo has precluded direct demonstration that osteocytes use gap junctions as a means of intercellular communication. In this article, we report successfully isolating primary cultures of osteocytes from chick calvaria, and, using anti-connexin 43 immunocytochemistry, demonstrate gap junction distribution to be comparable to that found in vivo. Next, we demonstrate the functionality of the gap junctions by (1) dye coupling studies that showed the spread of microinjected Lucifer Yellow from osteoblast to osteocyte and between adjacent osteocytes and (2) analysis of fluorescence replacement after photobleaching (FRAP), in which photobleaching of cells loaded with a membrane-permeable dye resulted in rapid recovery of fluorescence into the photobleached osteocyte, within 5 min postbleaching. This FRAP effect did not occur when cells were treated with a gap junction blocker (18alpha-glycyrrhetinic acid), but replacement of fluorescence into the photobleached cell resumed when it was removed. These studies demonstrate that gap junctions are responsible for intercellular communication between adjacent osteocytes and between osteoblasts and osteocytes. This role is consistent with the ability of osteocytes to respond to and transmit signals over long distances while embedded in a calcified matrix.

Gap-junctional single-channel permeability for fluorescent tracers in mammalian cell cultures

We have developed a simple dye transfer method that allows quantification of the gap-junction permeability of small cultured cells. Fluorescent dyes (calcein and Lucifer yellow) were perfused into one cell of an isolated cell pair using a patch-type micropipette in the tight-seal whole cell configuration. Dye spreading into the neighboring cells was monitored using a low-light charge-coupled device camera. Permeation rates for calcein and Lucifer yellow were then estimated by fitting the time course of the fluorescence intensities in both cells. For curve fitting, we used a set of model equations derived from a compartment model of dye distribution. The permeation rates were correlated to the total ionic conductance of the gap junction measured immediately after the perfusion experiment. Assuming that dye permeation is through a unit-conductance channel, we were then able to calculate the single-channel permeance for each tracer dye. We have applied this technique to HeLa cells stably transfected with rat-Cx46 and Cx43, and to BICR/M1R(k) cells, a rat mammary tumor cell line that has very high dye coupling through endogenous Cx43 channels. Scatter plots of permeation rates versus junctional conductance did not show a strictly linear correlation of ionic versus dye permeance, as would have been expected for a simple pore. Instead, we found that the data scatter within a wide range of different single-channel permeances. In BICR/M1R(k) cells, the lower limiting single-channel permeance is 2.2 +/- 2.0 x 10(-12) mm3/s and the upper limit is 50 x 10(-12) mm3/s for calcein and 6.8 +/- 2.8 x 10(-12) mm3/s and 150 x 10(-12) mm3/s for Lucifer yellow, respectively. In HeLa-Cx43 transfectants we found 2.0 +/- 2.4 x 10(-12) mm3/s and 95 x 10(-12) mm3/s for calcein and 2.1 +/- 6.8 x 10(-12) mm3/s and 80 x 10(-12) mm3/s for Lucifer yellow, and in HeLa-Cx46 transfectants 1.7 +/- 0.3 x 10(-12) mm3/s and 120 x 10(-12) mm3/s for calcein and 1.3 +/- 1.1 x 10(-12) mm3/s and 34 x 10(-12) mm3/s for Lucifer yellow, respectively. This variability is most likely due to a yet unknown mechanism that differentially regulates single-channel permeability for larger molecules and for small inorganic ions.

Transforming growth factor-beta3 increases gap-junctional communication among folliculostellate cells to release basic fibroblast growth factor

Folliculostellate (FS) cells are known to communicate with each other and with endocrine cells via gap junctions in the anterior pituitary. We investigated whether TGFbeta3 and estradiol, known to regulate FS cell production and secretion of basic fibroblast growth factor (bFGF), increases gap junctional communication to alter bFGF secretion from FS cells. FS cells in monolayer cultures were treated with TGFbeta3 or vehicle alone for 24 h and then microinjected with Lucifer Yellow and high-molecular-weight Texas Red dextran. Ten minutes later the transfer of dye among adjacent cells was recorded with a digital microscope. TGFbeta3 increased the transfer of dye. The TGFbeta3-neutralizing antibody and the gap junction inhibitor octanol reduced the effect of TGFbeta3 on the transfer of dye. The TGFbeta3-induced transfer of dye was unaltered by simultaneous treatment with estradiol. The steroid alone also had no effect. TGFbeta3 increased total and phosphorylated levels of connexin 43. Estradiol treatment did not produce any significant changes on basal or TGFbeta3-induced increases in connexin 43 levels. The gap-junction inhibitor octanol reduced TGFbeta3-increased levels of bFGF in FS cells. Taken together, these results suggest that TGFbeta3 may act on FS cells to increase gap-junctional communication to maximize its effect on bFGF secretion.

Calmodulin transit via gap junctions is reduced in the absence of an electric field

Gap junctional transport of Calmodulin (CaM) from epithelial cells to insect oocytes is enhanced by alignment of the molecules via an electric field. It has recently been shown that CaM is needed for uptake of vitellogenins, is produced in the epithelial cells and reaches oocytes via gap junctions. For CaM to transit the gap junctions something must align these elongated molecules with the lumina of the gap junctions. This might be accomplished by the electric field that exists at the membrane of any cell with an Em of >0 mV. Fluorescently labeled CaM was injected into oocytes. At t=0, the epithelial cell/oocyte "fluorescence" ratio showed epithelial cells to be 24%+/-1.5% as bright as the injected oocyte. In follicles which maintained an electric field for one hour the epithelial cell/oocyte fluorescence ratio had risen to 79%+/-1.4%, while for follicles in which the field was cancelled by holding Em at 0 mV the ratio was only 45%+/-1.7%. After termination of the holding current follicles regained their original Em and their original electric field. At the end of a second hour of incubation the ratio had risen to 76%+/-1.2%, very close to what was observed in the untreated control follicles.

Polyvalent cations constitute the voltage gating particle in human connexin37 hemichannels

Connexins oligomerize to form intercellular channels that gate in response to voltage and chemical agents such as divalent cations. Historically, these are believed to be two independent processes. Here, data for human connexin37 (hCx37) hemichannels indicate that voltage gating can be explained as block/unblock without the necessity for an independent voltage gate. hCx37 hemichannels closed at negative potentials and opened in a time-dependent fashion at positive potentials. In the absence of polyvalent cations, however, the channels were open at relatively negative potentials, passing current linearly with respect to voltage. Current at negative potentials could be inhibited in a concentration-dependent manner by the addition of polyvalent cations to the bathing solution. Inhibition could be explained as voltage-dependent block of hCx37, with the field acting directly on polyvalent cations, driving them through the pore to an intracellular site. At positive potentials, in the presence of polyvalent cations, the field favored polyvalent efflux from the intracellular blocking site, allowing current flow. The rate of appearance of current depended on the species and valence of the polyvalent cation in the bathing solution. The rate of current decay upon repolarization depended on the concentration of polyvalent cations in the bathing solution, consistent with deactivation by polyvalent block, and was rapid (time constants of tens of milliseconds), implying a high local concentration of polyvalents in or near the channel pore. Sustained depolarization slowed deactivation in a flux-dependent, voltage- and time-independent fashion. The model for hCx37 voltage gating as polyvalent block/unblock can be expanded to account for observations in the literature regarding hCx37 gap junction channel behavior.

Gap junction communication between cells aggregated on a cellulose-coated polystyrene: influence of connexin 43 phosphorylation

The appropriate functioning of tissues and organ systems depends on intercellular communication such as gap junctions formed by connexin (Cx) protein channels between adjacent cells. We have previously shown that Swiss 3T3 cells aggregated on hydrophilic cellulose substratum Cuprophan (CU) establish short linear gap junctions composed of Cx 43 in cell surface plaques. This phenomenon seems to depend on the high intracellular cyclic AMP (cAMP) concentration triggered by attachment of the cells to CU. We have now used a cellulose-coated polystyrene inducing the same cell behaviour to analyse the gap junction communication between aggregated cells. The transfer of the dye Lucifer Yellow (LY) between cells showed that cells aggregated on cellulose substratum rapidly (within 90 min) establish functional gap junctions. Inhibitors of cAMP protein kinase (PKI) or protein kinase C (GF109203X) both inhibited the diffusion of LY between neighbouring cells. Western blot analysis showed that this change in permeability was correlated with a decrease in Cx 43 phosphorylation. Thus, cellulose substrata seem to induce cell-cell communication through Cx 43 phosphorylation modulated by PKA and PKC. To understand the mechanisms by which a substratum regulates gap junctional communication is critically important for the emerging fields of tissue engineering and biohybrid devices.

A transient diffusion model yields unitary gap junctional permeabilities from images of cell-to-cell fluorescent dye transfer between Xenopus oocytes

As ubiquitous conduits for intercellular transport and communication, gap junctional pores have been the subject of numerous investigations aimed at elucidating the molecular mechanisms underlying permeability and selectivity. Dye transfer studies provide a broadly useful means of detecting coupling and assessing these properties. However, given evidence for selective permeability of gap junctions and some anomalous correlations between junctional electrical conductance and dye permeability by passive diffusion, the need exists to give such studies a more quantitative basis. This article develops a detailed diffusion model describing experiments (reported separately) involving transport of fluorescent dye from a "donor" region to an "acceptor" region within a pair of Xenopus oocytes coupled by gap junctions. Analysis of transport within a single oocyte is used to determine the diffusion and binding characteristics of the cellular cytoplasm. Subsequent double-cell calculations then yield the intercellular junction permeability, which is translated into a single-channel permeability using concomitant measurements of intercellular conductance, and known single-channel conductances of gap junctions made up of specific connexins, to count channels. The preceding strategy, combined with use of a graded size series of Alexa dyes, permits a determination of absolute values of gap junctional permeability as a function of dye size and connexin type. Interpretation of the results in terms of pore theory suggests significant levels of dye-pore affinity consistent with the expected order of magnitude of typical (e.g., van der Waals) intermolecular attractions.

Biophysical properties of connexin-45 gap junction hemichannels studied in vertebrate cells

Human HeLa cells transfected with mouse Cx45 and rat RIN cells transfected with chicken Cx45 were used to study the electrical and permeability properties of Cx45 gap junction hemichannels. With no extracellular Ca(2+), whole-cell recording revealed currents arising from hemichannels in both transfected cell lines. Multichannel currents showed a time-dependent activation or deactivation sensitive to voltage, V(m). These currents did not occur in non-transfected cells. The hemichannel currents were inhibited by raising extracellular Ca(2+) or by acidification with CO(2). The unitary conductance exhibited V(m) dependence (i.e., gamma(hc,main) increased/decreased with hyperpolarization/depolarization). Extrapolation to V(m) = 0 mV led to a gamma(hc,main) of 57 pS, roughly twice the conductance of an intact Cx45 gap junction channel. The open channel probability, P(o), was V(m)-dependent, declining at negative V(m) (P(o) < 0.11, V(m) < -50 mV), and increasing at positive V(m) (P(o) approximately 0.76, V(m) > 50 mV). Moreover, Cx45 nonjunctional hemichannels appeared to mediate lucifer yellow (LY) and propidium iodide (PI) dye uptake from the external solution when extracellular Ca(2+) level was reduced. Dye uptake was directly proportional to the number of functioning hemichannels. No significant dye uptake was detected in non-transfected cells. Cx45 transfected HeLa and RIN cells also allowed dye to leak out when preloaded with LY and then incubated in Ca(2+)-free external solution, whereas little or no dye leakage was observed when these cells were incubated with 2 mM external Ca(2+). Intact Cx45 gap junction channels allowed passage of either LY or PI dye, but their respective flux rates were different. Comparison of LY diffusion through Cx45 hemichannels and intact gap junction channels revealed that the former is more permeable, suggesting that gap junction channel pores exhibit more allosterical restriction to the dye molecules than the unopposed hemichannel. The data demonstrate the opening of Cx45 nonjunctional hemichannels in vertebrate cells when the external Ca(2+) concentration is reduced.

Cellular microtransport processes: intercellular, intracellular, and aggregate behavior

Ionic and molecular transfer among cells occurs by a variety of transport processes operative at different length scales. Cell membrane permeability and electrical conductance derive from channel proteins producing pores at the molecular (ultrastructural) scale. Intracellular mobility involves the dynamics of motion through the complex ultrastructure of the cytoplasm. These phenomena unite in the larger-scale (microscopic) process of gross intercellular transfer. When such movement occurs among sufficiently many cells, it in turn begins to reflect their average collective (macroscopic) behavior as bulk tissue. This article surveys selected aspects of intercellular and intracellular transport, with emphasis on detailed mechanistic theory, experimental probes of cellular permeability, and systematic transcendence from small to large length scales.

A series of biotinylated tracers distinguishes three types of gap junction in retina

Gap junctions serve many important roles in various tissues, but their abundance and diversity in neurons is only beginning to be understood. The tracer Neurobiotin has revealed many different networks interconnected by gap junctions in retina. We compared the relative permeabilities of five different retinal gap junctions by measuring their permeabilities to a series of structurally related tracers. When large tracers were injected, the staining of coupled cells fell off more rapidly in some networks than others relative to Neurobiotin controls. Three distinctly different permeability profiles were found, suggesting that multiple neuronal connexin types were present. The most permeant to large molecules were gap junctions from A-type horizontal cells. The permeability of gap junctions of two types of amacrine cell were not distinguishable from those from B-type horizontal cells. The lowest permeability was found for gap junctions between cone bipolar cells and the AII amacrine cells to which they are coupled. Because only a single neural connexin type has been identified in retina, our results suggest more types remain to be found. To determine whether the unitary permeability of channels is altered by channel modulators, we reduced permeability with octanol and a cAMP analog. Although net permeability was substantially diminished, the proportion by which it declined was constant across tracer size. This suggests that these agents act only to close channels rather than alter individual channel permeabilities. This tracer series can therefore be used to contrast permeability properties of gap junctions in intact circuits, even at the level of individual channels.

The first extracellular loop domain is a major determinant of charge selectivity in connexin46 channels

Intercellular channels formed of members of the gene family of connexins (Cxs) vary from being substantially cation selective to being anion selective. We took advantage of the ability of Cx46 to function as an unopposed hemichannel to examine the basis of Cx charge selectivity. Previously we showed Cx46 hemichannels to be large pores that predominantly conduct cations and inwardly rectify in symmetric salts, properties suggesting selectivity is influenced by fixed negative charges located toward the extracellular end of the pore. Here we demonstrate that high ionic strength solutions applied to the extracellular, but not the intracellular, side of Cx46 hemichannels substantially reduce the ratio of cation to anion permeability. Substitution of the first extracellular loop (E1) domain of Cx32, an anion-preferring Cx, reduces conductance, converts Cx46 from cation to anion preferring, and changes the I-V relation form inwardly to outwardly rectifying. These data suggest that fixed negative charges influencing selectivity in Cx46 are located in E1 and are substantially reduced and/or are replaced with positive charges from the Cx32 E1 sequence. Extending studies to Cx46 cell-cell channels, we show that they maintain a strong preference for cations, have a conductance nearly that expected by the series addition of hemichannels, but lack rectification in symmetric salts. These properties are consistent with preservation of the fixed charge region in E1 of hemichannels, which upon docking, become symmetrically placed near the center of the cell-cell channel pore. Furthermore, heterotypic cell-cell channels formed by pairing Cx46 with Cx32 or Cx43 rectify in symmetric salts in accordance with the differences in the charges we ascribed to E1. These data are consistent with charged residues in E1 facing the channel lumen and playing an important role in determining Cx channel conductance and selectivity.

Immunolocalization of connexin 26 in the developing mouse cochlea

Gap junctions play a pivotal role in embryonic development by forming specialized regions of cell-cell communication. In this study, we demonstrate the temporal-spatial distribution of connexin 26 in the embryonic and early postnatal mouse cochlea. Our results show localization of this gap junction protein to specific cochlear structures, including the inner and outer sulcus cells, the supporting cells of the inner hair cells, the mesenchyme derived portion of the stria vascularis, and the cells of the spiral ligament that interface with the basal cells of the stria vascularis. This suggests that this gap junction protein of served patterns of connexin 26 distribution is important for the differentiation and development of these structures (e.g., the role of the inner sulcus cells in producing the tectorial membrane).

Different ionic selectivities for connexins 26 and 32 produce rectifying gap junction channels

The functional diversity of gap junction intercellular channels arising from the large number of connexin isoforms is significantly increased by heterotypic interactions between members of this family. This is particularly evident in the rectifying behavior of Cx26/Cx32 heterotypic channels (. Proc. Natl. Acad. Sci. USA. 88:8410-8414). The channel properties responsible for producing the rectifying current observed for Cx26/Cx32 heterotypic gap junction channels were determined in transfected mouse neuroblastoma 2A (N2A) cells. Transfectants revealed maximum unitary conductances (gamma(j)) of 135 pS for Cx26 and 53 pS for Cx32 homotypic channels in 120 mM KCl. Anionic substitution of glutamate for Cl indicated that Cx26 channels favored cations by 2.6:1, whereas Cx32 channels were relatively nonselective with respect to charge. In Cx26/Cx32 heterotypic cell pairs, the macroscopic fast rectification of the current-voltage relationship was fully explained at the single-channel level by a rectifying gamma(j) that increased by a factor of 2.9 as the transjunctional voltage (V(j)) changed from -100 to +100 mV with the Cx26 cell as the positive pole. A model of electrodiffusion of ions through the gap junction pore based on Nernst-Planck equations for ion concentrations and the Poisson equation for the electrical potential within the junction is developed. Selectivity characteristics are ascribed to each hemichannel based on either pore features (treated as uniform along the length of the hemichannel) or entrance effects unique to each connexin. Both analytical GHK approximations and full numerical solutions predict rectifying characteristics for Cx32/Cx26 heterotypic channels, although not to the full extent seen empirically. The model predicts that asymmetries in the conductance/permeability properties of the hemichannels (also cast as Donnan potentials) will produce either an accumulation or a depletion of ions within the channel, depending on voltage polarity, that will result in rectification.

Direct isolation and analysis of endogenous transjunctional ADP from Cx43 transfected C6 glioma cells

Gap junctional communication has been implicated in numerous cellular processes. However, the repertoire of specific transjunctional substances which mediate these processes remains relatively unexplored. A few selected secondary messengers have been identified, at least indirectly (e.g., cAMP and IP3) and phenotypic complementation experiments have indicated that gap junctions enable communicating cells to distribute nucleotide pools as a shared resource. The latter would include high energy compounds such as ADP and ATP, allowing cells to share energy resources. We have utilized a nonbiased process to directly capture, identify, and quantify transjunctional compounds from C6 glioma cells, the transformed phenotype of which has been ameliorated by transfection with connexin43 (Cx43). This technique involves the direct isolation, identification, and quantitation of radioactive transjunctional molecules that travel from metabolically labeled "donor" cells to "receiver" cells. This report demonstrates that ADP and/or ATP represents over 6% of the transjunctional material derived from glucose in Cx43-transfected C6 glioma cells. Furthermore, equilibration of these high energy metabolites among first order neighbors is shown to occur in less than 20 min of communication.

A quantitative analysis of connexin-specific permeability differences of gap junctions expressed in HeLa transfectants and Xenopus oocytes

Gap junctions provide direct intercellular communication by linking adjacent cells with aqueous pores permeable to molecules up to 1 kDa in molecular mass and 8–14 A in diameter. The identification of over a dozen connexins in the mammalian gap junction family has stimulated interest in the functional significance of this diversity, including the possibility of selectivity for permeants as seen in other channel classes. Here we present a quantitative comparison of channel permeabilities of different connexins expressed in both HeLa transfectants (rat Cx26, rat Cx32 and mouse Cx45) and Xenopus oocytes (rat Cx26 and rat Cx32). In HeLa cells, we examined permeability to two fluorescent molecules: Lucifer Yellow (LY: anionic, MW 457) and 4′,6-diamidino-2-phenylindole, dihydrochloride (DAPI, cationic, MW 350). A comparison of the kinetics of fluorescent dye transfer showed Cx32, Cx26 and Cx45 to have progressively decreasing permeabilities to LY, but increasing permeabilities to DAPI. This pattern was inconsistent with selection based on physical size of the probe, nor could it be accounted for by the differences between clones in the electrical conductance of the monolayers. In Xenopus oocytes, where electrical and dye coupling could be assessed in the same cells, Cx32 coupled oocytes showed an estimated 6-fold greater permeability to LY than those coupled by Cx26, a comparable result to that seen in HeLa cells, where an approximately 9-fold difference was seen. The oocyte system also allowed an examination of Cx32/Cx26 heterotypic gap junction that proved to have a permeability intermediate between the two homotypic forms. Thus, independent of the expression system, it appears that connexins show differential permeabilities that cannot be predicted based on size considerations, but must depend on other features of the probe, such as charge.

Estimation of the pore size of the large-conductance mechanosensitive ion channel of Escherichia coli

The open channel diameter of Escherichia coli recombinant large-conductance mechanosensitive ion channels (MscL) was estimated using the model of Hille (Hille, B. 1968. Pharmacological modifications of the sodium channels of frog nerve. J. Gen. Physiol. 51:199-219) that relates the pore size to conductance. Based on the MscL conductance of 3.8 nS, and assumed pore lengths, a channel diameter of 34 to 46 A was calculated. To estimate the pore size experimentally, the effect of large organic ions on the conductance of MscL was examined. Poly-L-lysines (PLLs) with a diameter of 37 A or larger significantly reduced channel conductance, whereas spermine (approximately 15 A), PLL19 (approximately 25 A) and 1,1'-bis-(3-(1'-methyl-(4,4'-bipyridinium)-1-yl)-propyl)-4,4'-b ipyridinium (approximately 30 A) had no effect. The smaller organic ions putrescine, cadaverine, spermine, and succinate all permeated the channel. We conclude that the open pore diameter of the MscL is approximately 40 A, indicating that the MscL has one of the largest channel pores yet described. This channel diameter is consistent with the proposed homohexameric model of the MscL.

Connexin43 immunoreactivity in the catfish retina

We used antibodies directed against rat heart connexin43 (Cx43) to perform immunoblot and immunohistochemical (IHC) analyses of the catfish retina. The antibodies recognized a retinal protein which co-migrated with mouse brain Cx43. IHC staining resulted in punctate labeling of the external limiting membrane and the outer nuclear layer. Although infrequent, labeling was also observed in the inner nuclear layer. These results suggest that a Cx43 isoform may be present in Muller glial cells and neurons of the distal catfish retina.

Monovalent ion selectivity sequences of the rat connexin43 gap junction channel

The relative permeability sequences of the rat connexin 43 (rCx43) gap junction channel to seven cations and chloride were examined by double whole cell patch clamp recording of single gap junction channel currents in rCx43 transfected neuroblastoma 2A (N2A) cell pairs. The measured maximal single channel slope conductances (gammaj, in pS) of the junctional current-voltage relationships in 115 mM XCI were RbC1 (103) > or = CsC1 (102) > KC1 (97) > NaC1 (79) > or = LiC1 (78) > TMAC1 (65) > TEAC1 (53) and for 115 mM KY were KBr (105) > KC1 (97) > Kacetate (77) > Kglutamate (61). The single channel conductance- aqueous mobility relationships for the test cations and anions were linear. However, the predicted minimum anionic and cationic conductances of these plots did not accurately predict the rCx43 channel conductance in 115 mM KC1. Instead, the conductance of the rCx43 channel in 115 mM KC1 was accurately predicted from cationic and anionic conductance-mobility plots by applying a mobility scaling factor Dx/Do, which depends upon the relative radii of the permeant ions to an estimated pore radius. Relative permeabilities were determined for all of the monovalent catious and anions tested from asymmetric salt reversal potential measurements and the Goldman-Hodgkin-Katz voltage equation. These experiments estimate the relative chloride to potassium permeability to be 0.13. The relationship between the relative cation permeability and hydrated radius was modeled using the hydrodynamic equation assuming a pore radius of 6.3 +/- 0.4 A. Our data quantitatively demonstrate that the rCx43 gap junction channel is permeable to monovalent atomic and organic cations and anions and the relative permeability sequences are consistent with an Eisenman sequence II or I, respectively. These predictions about the rCx43 channel pore provide a useful basis for future investigations into the structural determinants of the conductance and permeability properties of the connexin channel pore.

Freeze-induced shrinkage of individual cells and cell-to-cell propagation of intracellular ice in cell chains from salivary glands

The formation of intracellular ice (IIF), usually a lethal event to be avoided when cryopreserving cells, should, however, be enforced during the cryosurgical destruction of tumour cells. IIF has been investigated so far only in single cells in suspension. Because cells in tissues cannot be successfully cryopreserved, in contrast to single cells in suspension, the mechanism of IIF in tissues may depend on factors that facilitate IIF. We studied IIF in cell strands from salivary glands, which represent a simple form of a tissue. Their cells are connected by channels responsible for intercellular communication. A substantial fraction of cell dehydration during freezing occurs before cells are encapsulated by ice, and the degree of this pre-ice-front shrinkage appears to influence IIF. In strands with coupled cells IIF spread from one cell to adjacent cells in a sequential manner with short delays (200-300 ms), suggesting cell-to-cell propagation via intercellular channels. In strands pretreated with decoupling agents (dinitrophenol, heptanol), sequential IIF was absent. Instead, formation of ice was random, with longer and variable delays between consecutive darkenings indicating IIF. Results suggest that the mechanism of IIF spread, and consequently the degree of cryodamage in tissue, can be influenced by the presence of intercellular channels (gap junctions).

Properties and regulation of gap junctional hemichannels in the plasma membranes of cultured cells

During the assembly of gap junctions, a hemichannel in the plasma membrane of one cell is thought to align and dock with another in an apposed membrane to form a cell-to-cell channel. We report here on the existence and properties of nonjunctional, plasma membrane connexin43 (Cx43) hemichannels. The opening of the hemichannels was demonstrated by the cellular uptake of 5(6)-carboxyfluorescein from the culture medium when extracellular calcium levels were reduced. Dye uptake exhibited properties similar to those of gap junction channels. For example, using different dyes, the levels of uptake were correlated with molecular size: 5(6)-carboxyfluorescein (approximately 32%), 7-hydroxycoumarin-3-carboxylic acid (approximately 24%), fura-2 (approximately 11%), and fluorescein-dextran (approximately 0.4%). Octanol and heptanol also reduced dye uptake by approximately 50%. Detailed analysis of one clone of Novikoff cells transfected with a Cx43 antisense expression vector revealed a reduction in dye uptake levels according to uptake assays and a corresponding decrease in intercellular dye transfer rates in microinjection experiments. In addition, a more limited decrease in membrane resistance upon reduction of extracellular calcium was detected in electrophysiological studies of antisense transfectants, in contrast to control cells. Studies of dye uptake in HeLa cells also demonstrated a large increase following transfection with Cx43. Together these observations indicate that Cx43 is responsible for the hemichannel function in these cultured cells. Similar dye uptake results were obtained with normal rat kidney (NRK) cells, which express Cx43. Dye uptake can be dramatically inhibited by 12-O-tetradeconylphorbol-13-acetate-activated protein kinase C in these cell systems and by a temperature-sensitive tyrosine protein kinase, pp60v-src in LA25-NRK cells. We conclude that Cx43 hemichannels are found in the plasma membrane, where they are regulated by multiple signaling pathways, and likely represent an important stage in gap junction assembly.

Size and selectivity of gap junction channels formed from different connexins

Gap junction channels have long been viewed as static structures containing a large-diameter, aqueous pore. This pore has a high permeability to hydrophilic molecules of approximately 900 daltons in molecular weight and a weak ionic selectivity. The evidence leading to these conclusions is reviewed in the context of more recent observations primarily coming from unitary channel recordings from transfected connexin channels expressed in communication-deficient cell lines. What is emerging is a more diverse view of connexin-specific gap junction channel structure and function where electrical conductance, ionic selectivity, and dye permeability vary by one full order of magnitude or more. furthermore, the often held contention that channel conductance and ionic or molecular selectivity are inversely proportional is refuted by recent evidence from five distinct connexin channels. The molecular basis for this diversity of channel function remains to be identified for the connexin family of gap junction proteins.

eat-5 and unc-7 represent a multigene family in Caenorhabditis elegans involved in cell-cell coupling

The Drosophila melanogaster genes Passover and l(1)ogre and the Caenorhabditis elegans gene unc-7 define a gene family whose function is not known. We have isolated and characterized the C. elegans gene eat-5, which is required for synchronized pharyngeal muscle contractions, and find that it is a new member of this family. Simultaneous electrical and video recordings reveal that in eat-5 mutants, action potentials of muscles in the anterior and posterior pharynx are unsynchronized. Injection of carboxyfluorescein into muscles of the posterior pharynx demonstrates that all pharyngeal muscles are dye-coupled in wild-type animals; in eat-5 mutants, however, muscles of the anterior pharynx are no longer dye-coupled to posterior pharyngeal muscles. We show that a gene fusion of eat-5 to the green fluorescent protein is expressed in pharyngeal muscles. unc-7 and eat-5 are two of at least sixteen members of this family in C. elegans as determined by database searches and PCR-based screens. The amino acid sequences of five of these members in C. elegans have been deduced from cDNA sequences. Polypeptides of the family are predicted to have four transmembrane domains with cytoplasmic amino and carboxyl termini. We have constructed fusions of one of these polypeptides with beta-galactosidase and with green fluorescent protein. The fusion proteins appear to be localized in a punctate pattern at or near plasma membranes. We speculate that this gene family is required for the formation of gap junctions.

Two structural configurations of the skeletal muscle calcium release channel

Here we present the determination of the three-dimensional structure of the skeletal muscle Ca2+-release channel in an open state using electron cryomicroscopy and angular reconstitution. In contrast to our reconstruction of the channel in its closed state, the density map of the channel driven towards its open state, by the presence of Ca2+ and ryanodine, features a central opening in the transmembrane region-the likely passageway for Ca2+ ions from the sarcoplasmic reticulum to the cytosol. The opening of the channel is associated with a 4 degree rotation of its transmembrane region with respect to its cytoplasmic region, and with significant mass translocations within the entire cytoplasmic region of the channel tetramer.

Inhibition of glycosylation induces formation of open connexin-43 cell-to-cell channels and phosphorylation and triton X-100 insolubility of connexin-43

We transfected the cDNA for the cell-to-cell channel protein connexin-43 (Cx43) into Morris hepatoma H5123 cells, which express little Cx43 and lack gap junctional communication (open cell-to-cell channels). We found that cells overexpressing Cx43 nonetheless lacked open cell-to-cell channels, but that inhibition of glycosylation by tunicamycin induced open channels in these cells. Tunicamycin also induced biochemical changes in Cx43 protein; the level increased, and a considerable fraction became phosphorylated and Triton X-100 insoluble, in contrast to untreated cells where Cx43 was non-phosphorylated and Triton X-100 soluble. Although tunicamycin caused the formation of open channels, channels were not found aggregated into gap junctional plaques, as they are when they have been induced by elevation of intracellular cAMP. The results suggest that although Cx43 itself is not glycosylated, other glycosylated proteins influence Cx43 posttranslational modification and the formation of Cx43 cell-to-cell channels.

Irsogladine inhibits ionomycin-induced decrease in intercellular communication in cultured rabbit gastric epithelial cells

Effects of irsogladine on ionomycin-induced decrease in intercellular communication and increase in intracellular concentration of Ca2+ ([Ca2+]i) were investigated in cultured rabbit gastric epithelial cells. Ionomycin (10(-7)-10(-6) M) transiently and concentration-dependently inhibited intercellular communication concomitantly with the elevation of [Ca2+]i in the presence and absence of extracellular Ca2+. Irsogladine (10(-5) M), which has been shown to facilitate intercellular communication, suppressed the ionomycin-induced elevation of [Ca2+]i and decrease in intercellular communication. The suppression of the ionomycin effects by irsogladine was independent of extracellular Ca2+. TMB-8 [8-(diethylamino)octyl-3,4,5-trimethoxy-benzoate hydrochloride] (10(-6) M) also suppressed the ionomycin-induced elevation of [Ca2+]i and decrease in intercellular communication. These results indicate that the ionomycin-induced decrease in intercellular communication may be due to Ca2+ mobilization from intracellular stores. Inhibitory effects of irsogladine and TMB-8 on the ionomycin-induced decrease in intercellular communication may be produced by suppressing Ca2+ mobilization.

Clustering of Cx43 cell-to-cell channels into gap junction plaques: regulation by cAMP and microfilaments

Cell-to-cell channels are often seen clustered at cell-cell contacts into the so-called gap junction plaques. The mechanism of this clustering is unknown. We show that the clustering of cell-to-cell channels composed of connexin43 is induced by elevation of cyclic AMP. The cAMP-induced clustering is enhanced by inhibition of glycosylation and abolished by disruption of microfilaments. Channel clustering thus seems to be regulated by cAMP and glycosylation and to involve microfilaments.

Quantitative junctional permeability measurements using the confocal microscope

This paper describes the use of a photobleach method and confocal microscopy to compare the cell-to-cell transfer rate of 5,6 carboxyfluorescein in dissociated embryonic chick lens cells with those in the anterior epithelium of the whole embryonic chick lens. The average cell-to-cell transfer rates obtained were 7.9 x 10(-3) sec-1 in the dissociated cells and 2.6 x 10(-3) sec-1 in the anterior epithelium in an intact lens.

Expression of a dominant negative inhibitor of intercellular communication in the early Xenopus embryo causes delamination and extrusion of cells

A chimeric construct, termed 3243H7, composed of fused portions of the rat gap junction proteins connexin32 (Cx32) and connexin43 (Cx43) has been shown to have selective dominant inhibitory activity when tested in the Xenopus oocyte pair system. Co-injection of mRNA coding for 3243H7 together with mRNAs coding for Cx32 or Cx43 completely blocked the development of channel conductances, while the construct was ineffective at blocking intercellular channel assembly when coinjected with rat connexin37 (Cx37). Injection of 3243H7 into the right anterodorsal blastomere of 8-cell-stage Xenopus embryos resulted in disadhesion and delamination of the resultant clone of cells evident by embryonic stage 8; a substantial number, although not all, of the progeny of the injected cell were eliminated from the embryo by stage 12. A second construct, 3243H8, differing from 3243H7 in the relative position of the middle splice, had no dominant negative activity in the oocyte pair assay, nor any detectable effects on Xenopus development, even when injected at four-fold higher concentrations. The 3243H7-induced embryonic defects could be rescued by coinjection of Cx37 with 3243H7. A blastomere reaggregation assay was used to demonstrate that a depression of dye-transfer could be detected in 3243H7-injected cells as early as stage 7; Lucifer yellow injections into single cells also demonstrated that injection of 3243H7 resulted in a block of intercellular communication. These experiments indicate that maintenance of embryonic cell adhesion with concomitant positional information requires gap junction-mediated intercellular communication.

Stages leading to and following fusion of sperm and egg plasma membranes

The site of gamete interaction of electrophysiologically recorded Lytechinus variegatus eggs, fixed with osmium tetroxide (OsO4) and/or glutaraldehyde (GTA) at varying intervals after the onset of the increase in membrane conductance induced by an attached sperm, has been examined by high-voltage and conventional transmission electron microscopy. Although GTA and a GTA-OsO4 mixture induced different electrical responses, specimens prepared with the two fixatives were ultrastructurally similar. In specimens observed within 5 s of the change in conductance, the acrosomal process projected through the vitelline layer and abutted the egg plasma membrane. A conspicuous layer of bindin surrounded the acrosomal process and connected the sperm to the egg's vitelline layer. In a fortuitous specimen fixed within 4 s following the change in conductance, the area of contact between the gamete plasma membranes possessed a trilaminar structure that separated the egg's and sperm's cytoplasms. The morphology of this area of contact was consistent with previously proposed intermediates of membrane fusion. Five to six seconds after the change in conductance, the sperm was connected to the egg via a narrow cytoplasmic bridge that consisted of the former acrosomal process and a projection of the egg cortex. The region of the bridge midway between the fused gametes was encircled by dense material that marked the site of sperm-egg fusion. Gamete interactions in which the activation potential was recorded (unclamped egg) were comparable in time and ultrastructure to events taking place in voltage-clamped eggs except for one major difference. Intact cortical granules (one to three) were observed beneath the tip of the incorporating sperm in unclamped eggs fixed following the onset of the activation potential, whereas all cortical granules dehisced in clamped eggs.

The survival of transected axonal segments of cultured Aplysia neurons is prolonged by contact with intact nerve cells

Axonal segments transected from their cell body in vivo commonly undergo degeneration within 3-4 days (Wallerian degeneration). In lower vertebrates and invertebrates, however, some transected axonal segments survive for long periods ranging between 30 and 200 days. To circumvent the technical complications of studying the mechanisms underlying long-term survival of transected axons in vivo, we developed an in vitro system. We found previously that isolated axonal segments of cultured Aplysia neurons preserved their morphological integrity for an average duration of 7.6 days (range 2-14 days) and maintain their passive and excitable membrane properties. This survival occurred in the absence of de novo protein synthesis. In the present study we examined the influence of homologous neurons on the survival of transected axonal segments. We found that the average survival time of transected axons was doubled when co-cultured in physical contact with intact homologous neurons (average 15.3 days, range 2-27 days). During this period, the transected axons extended neurites, maintained normal passive and excitable membrane properties, formed electrotonic junctions with the intact neurons and maintained normal free intracellular Ca2+ levels. Consistent with these observations, electron micrographs of the transected axon revealed that the cytoskeletal elements of the axon appeared normal even 20 days after transection. In contrast, the mitochondria and smooth endoplasmic reticulum appeared damaged. As the prolonged survival was conditional on physical contact between the transected axon and the surrounding intact neurons, we suggest that the prolongation of survival time is promoted by the direct transfer of material from the intact neurons to the transected axon. However, co-culture of transected axons with homologous neurons did not fully mimic in vivo conditions, in which transected axons can survive for several months.