Hemichannel-Mediated Inhibition in the Outer Retina

Functional analysis of gap junctions in ovarian granulosa cells: distinct role for connexin43 in early stages of folliculogenesis

Ovarian granulosa cells are coupled via gap junctions containing connexin43 (Cx43 or alpha-1 connexin). In the absence of Cx43, granulosa cells stop growing in an early preantral stage. However, the fact that granulosa cells of mature follicles express multiple connexins complicated interpretation of this finding. The present experiments were designed to clarify the role of Cx43 vs. these other connexins in the earliest stages of folliculogenesis. Dye injection experiments revealed that granulosa cells from Cx43 knockout follicles are not coupled, and this was confirmed by ionic current injections. Furthermore, electron microscopy revealed that gap junctions are extremely rare in mutant granulosa cells. In contrast, mutant granulosa cells were able to form gap junctions with wild-type granulosa cells in a dye preloading assay. It was concluded that mutant granulosa cells contain a population of connexons, composed of an unidentified connexin, that do not normally contribute to gap junctions. Therefore, although Cx43 is not the only gap junction protein present in granulosa cells of early preantral follicles, it is the only one that makes a significant contribution to intercellular coupling.

Oligodendrocyte progenitor enrichment in the connexin32 null-mutant mouse

Before the establishment of chemical synapses, neural progenitors are often coupled by connexin-mediated gap junctions providing a robust mechanism for cell-cell communication in developing brain. The present study was undertaken to determine whether alterations in junctional coupling also affect neural progenitor proliferation, survival, and differentiation in adult brain. We localized the connexin32 gap junction protein to a subset of NG2+ and platelet-derived growth factor alpha receptor+ early oligodendrocyte progenitors in the dentate gyrus of adult mice. In connexin32-deficient mice, we found an increase in the total number of proliferating nestin+ and NG2+ progenitors in the subgranular zone, hilus, and polymorphonuclear layer of the dentate gyrus in vivo and in the total number of nestin+ progenitors capable of clonogenic expansion in vitro. By bromodeoxyuridine labeling, lineage analysis, and terminal deoxynucleotidyl nick end labeling, we demonstrate that turnover of these cells is constitutively enhanced in the connexin32 knock-out dentate gyrus reflecting a dynamic defect in oligodendrogenesis in this population. Analyses of surviving bromodeoxyuridine-labeled cells at 1, 3, 7, and 28 d after injection demonstrate that this transient amplifying population fails to terminally differentiate and is deleted by an apoptotic-like mechanism within 3 d of labeling. These data provide empirical evidence to support the hypothesis that connexin expression influences adult progenitor number and specifically implicate connexin32-mediated signaling in the activation, survival, and differentiation of a subset of early oligodendrocyte progenitors in postnatal brain.

Connexin immunoreactivity in glial cells of the rat retina

The rat retina contains two types of macroglial cells, Müller cells, radial glial cells that are the principal macroglial cells of vertebrate retinas, and astrocytes associated with the surface vasculature. In addition to the often-described gap-junctional coupling between astrocytes, coupling also occurs between astrocytes and Müller cells. Immunohistochemistry and confocal microscopy were used to identify connexins in the retinas of pigmented rats. Several antibodies directed against connexin43 stained astrocytes, identified using antibodies directed against glial fibrillary acidic protein (GFAP). In addition, two connexin43 antibodies stained Müller cells, identified with antibodies directed against S100 or glutamine synthetase. Connexin30-immunoreactive puncta were confined to the vitreal surface of the retina and colocalized with GFAP-immunoreactive astrocyte processes. Connexin45 immunoreactivity was associated with both astrocytes and Müller cells. We conclude that retinal glial cells express multiple connexins, and the patterns of immunostaining that we observe in this study are consistent with the expression of connexins30, -43, and possibly -45 by astrocytes and the expression of connexins43 and -45 by Müller cells. As gap-junction channels may be formed by both homotypic and heterotypic hemichannels, and the hemichannels may themselves be homomeric or heteromeric, there exists a multitude of possible gap-junction channels that could underlie the homotypic coupling between retinal astrocytes and the heterotypic coupling between astrocytes and Müller cells.

Two ribbon synaptic units in rod photoreceptors of macaque, human, and cat

The rod photoreceptor's synaptic terminal (or spherule) uses an elaborate synaptic structure to signal absorption of one or more photons to its postsynaptic targets. This structure includes one or two synaptic ribbons inside the terminal and a pouch-like "invagination" outside the terminal, into which enter a widely variable number of incoming fibers and postsynaptic targets-central elements supplied by rod bipolar cells and lateral elements supplied by horizontal cells. Nonetheless, our three-dimensional reconstructions of this synaptic structure in foveal retina of macaque monkey and peripheral retina of human and cat reveal several features that are highly conserved across species and with eccentricity: 1). every spherule has one invagination; 2). with rare exceptions, every spherule has two ribbon synaptic units with these features: a). on the presynaptic side, each ribbon synaptic unit has a ribbon or part of a ribbon and one trough-shaped arciform density that demarcates its active zone; b). on the postsynaptic side, each ribbon synaptic unit has two apposed lateral elements and one or more central elements; 3). the volume of the extracellular space in the single invagination is small, approximately 0.1 microm(3); and 4). the largest distance from active zone to receptor regions on bipolar cells is small, less than approximately 1.5 microm. With such small dimensions, release of one quantum of transmitter can pulse glutamate to a concentration comparable to the EC(50) of the metabotropic glutamate receptors on the central elements associated with both synaptic units. We speculate that two ribbon synaptic units are required to sustain the high quantal release rate needed to signal a single photon.

Synchronization of motor neurons during locomotion in the neonatal rat: predictors and mechanisms

We describe here the robust synchronization of motor neurons at a millisecond time scale during locomotor activity in the neonatal rat. Action potential activity of motor neuron pairs was recorded extracellularly using tetrodes during locomotor activity in the in vitro neonatal rat spinal cord. Approximately 40% of motor neuron pairs recorded in the same spinal segment showed significant synchronization, with the duration of the central peak in cross-correlograms between motor neurons typically ranging between approximately 30 and 100 msec. The percentage of synchronized motor neuron pairs was considerably higher for pairs with similar locomotor-related activity and strong rhythmic modulation. We also found synchronization between the activities of different motor pools, even if located several segments apart. Such distant synchronization was abolished in the absence of chemical synapses, although local coupling between motor neurons persisted. On the other hand, both local and distant coupling between motor neurons were preserved after antagonism of gap junction coupling between motor neurons. These results demonstrate that motor neuron activity is strongly synchronized at a millisecond time scale during the production of locomotor activity in the neonatal rat. These results also demonstrate that chemical synaptic inputs, in addition to electrical synapses, contribute to this synchronization, suggesting the existence of multiple mechanisms underlying motor neuron synchronization in the neonatal rat. The fast synchronization described here might be involved in activity-dependent processes during development or in the coordination of individual motor neurons into a functional population underlying behavior.

Pharmacological enhancement of hemi-gap-junctional currents in Xenopus oocytes

Hemichannels formed by expressing connexin subunits in Xenopus oocytes provide a valuable tool for revealing the gating properties of intercellular gap junctions in electrically coupled cells. We used the two electrode voltage-clamp technique to demonstrate that activation of the time-dependent outward hemichannel currents brings into play a sodium current of similar time course and opposite polarity; the interaction between these opposing currents had not been explored previously. Using the endogenous connexin (Cx38) of Xenopus oocytes as a model system, we have shown that substituting choline for sodium in the bath solution eliminates the sodium current, thereby unmasking large hemichannel currents, and enabling pharmacological studies of agents that are known to modulate gap-junctional conductances. The cinchona alkaloid quinine also effectively blocked the inward current, and in addition, enhanced significantly the Cx38 hemichannel currents in a dose-dependent fashion; the Hill coefficient of 1.9 suggests that the binding of at least two molecules of quinine is required to produce the effect. Intracellular quinine had no effect on hemichannel currents, and experiments on the displacement of quinine suggest that binding is at an external site near or within the mouth of the hemichannel. Intracellular acidification suppressed the quinine-enhanced hemichannel currents, indicating that quinine does not block the proton binding site. We found that retinoic acid (RA) and carbenoxolone, agents that block gap-junctional channels in coupled neurons and other cell types, also suppressed Cx38 hemichannel currents with an IC(50) of approximately 2 and 34 microM for RA and carbenoxolone, respectively. Raising extracellular calcium to 3 mM suppressed both the hemichannel current and the inward sodium current. These results provide a foundation upon which to further characterize the gating of hemichannel currents mediated by connexins expressed in Xenopus oocytes.

Membrane properties of an unusual intrinsically oscillating, wide-field teleost retinal amacrine cell

In the retina, amacrine cells modulate the transfer of information from bipolar to ganglion cells. The nature of the modulation depends on the synaptic input and the membrane properties of the cells. In the retina of white bass, we identified a class of bistratified, wide-field amacrine cell characterized by immunopositive labelling for GABA and calmodulin. In isolation, the cells presented resting membrane potentials averaging -69 mV although some cells settled at more depolarized values (-30 mV). Injection of depolarizing current pulses induced oscillatory membrane responses. When elicited from depolarized cells, the oscillations were short-lived (< 40 ms). For the most part, the oscillatory potentials of hyperpolarized cells remained unattenuated throughout the depolarizing pulse. The frequency of the oscillations increased logarithmically with mean membrane potential, ranging from 74 to 140 Hz. Cells exhibiting depolarized membrane potentials oscillated at twice that rate. When the membrane potential of these cells was hyperpolarized to -70 mV, the oscillations became unattenuated and slowed. We found the cells expressed voltage-gated sodium, potassium and calcium currents and calcium-dependent potassium currents. We demonstrate that the oscillatory potentials arose as a result of the interplay between calcium and potassium currents. The cells responded to local application of GABA and glycine, both of which modulate the oscillatory potentials. Glutamate and its analogues depolarized the cell and induced oscillatory potentials. Our results indicate that oscillatory responses of a type of wide-field amacrine cell are an intrinsic feature of the cell and not due to circuit properties.

Transport-mediated synapses in the retina

Most synapses rely on regulated exocytosis for determining the concentration of transmitter in the synaptic cleft. However, this mechanism may not be universal. Several synapses in the retina appear to use a synaptic machinery in which transmitter transporters play an essential role. Two types of transport-mediated synapses have been proposed. These synapses have been best observed in horizontal cells and cones of nonmammalian retinas. Horizontal cells use a transporter to mediate a bidirectional shuttle, whose balance point is set by ion concentrations and voltage. Nonmammalian cones combine exocytosis and the activity of a transporter. Because exocytosis is voltage independent over most of a cone's physiological voltage range, a voltage-dependent transporter determines the concentration of transmitter in the synaptic cleft. These two synapses may be models for transport-mediated synapses that operate in other parts of the brain.

Calcium regulation in photoreceptors

In this review we describe some of the remarkable and intricate mechanisms through which the calcium ion (Ca2+) contributes to detection, transduction and synaptic transfer of light stimuli in rod and cone photoreceptors. The function of Ca2+ is highly compartmentalized. In the outer segment, Ca2+ controls photoreceptor light adaptation by independently adjusting the gain of phototransduction at several stages in the transduction chain. In the inner segment and synaptic terminal, Ca2+ regulates cells' metabolism, glutamate release, cytoskeletal dynamics, gene expression and cell death. We discuss the mechanisms of Ca2+ entry, buffering, sequestration, release from internal stores and Ca2+ extrusion from both outer and inner segments, showing that these two compartments have little in common with respect to Ca2+ homeostasis. We also investigate the various roles played by Ca2+ as an integrator of intracellular signaling pathways, and emphasize the central role played by Ca2+ as a second messenger in neuromodulation of photoreceptor signaling by extracellular ligands such as dopamine, adenosine and somatostatin. Finally, we review the intimate link between dysfunction in photoreceptor Ca2+ homeostasis and pathologies leading to retinal dysfunction and blindness.

Dynamic trafficking and delivery of connexons to the plasma membrane and accretion to gap junctions in living cells

Certain membrane channels including acetylcholine receptors, gap junction (GJ) channels, and aquaporins arrange into large clusters in the plasma membrane (PM). However, how these channels are recruited to the clusters is unknown. To address this question, we have investigated delivery of GJ channel subunits (connexons) assembled from green fluorescent protein (GFP)-tagged connexin 43 (Cx43) to the PM and GJs in living cells. Fluorescence-photobleaching of distinct areas of Cx43-GFP GJs demonstrated that newly synthesized channels were accrued to the outer margins of channel clusters. Time-lapse microscopy further revealed that connexons were delivered in vesicular carriers traveling along microtubules from the Golgi to the PM. Routing and insertion of connexons occurred predominantly into the nonjunctional PM. These PM connexons can move laterally as shown by photo-bleaching and thus, can reach the margins of channel clusters. There, the apposing PMs are close enough to allow connexons to dock into complete GJ channels. When connexon delivery to the PM was inhibited by brefeldin A, or nocodazole pretreatment, the PM pool initially enabled connexon accrual to the clusters but further accrual was inhibited upon depletion. Taken together, our results indicate that GJ channel clusters grow by accretion at their outer margins from connexon subunits that were delivered to the nonjunctional PM, and explain how connexons in the PM can function in intra-/extracellular signaling before GJ channel formation and direct cell-cell communication.

GABAergic modulation of primary gustatory afferent synaptic efficacy

Modulation of synaptic transmission at the primary sensory afferent synapse is well documented for the somatosensory and olfactory systems. The present study was undertaken to test whether GABA impacts on transmission of gustatory information at the primary afferent synapse. In goldfish, the vagal gustatory input terminates in a laminated structure, the vagal lobes, whose sensory layers are homologous to the mammalian nucleus of the solitary tract. We relied on immunoreactivity for the GABA-transporter, GAT-1, to determine the distribution of GABAergic synapses in the vagal lobe. Immunocytochemistry showed dense, punctate GAT-1 immunoreactivity coincident with the layers of termination of primary afferent fibers. The laminar nature and polarized dendritic structure of the vagal lobe make it amenable to an in vitro slice preparation to study early synaptic events in the transmission of gustatory input. Electrical stimulation of the gustatory nerves in vitro produces synaptic field potentials (fEPSPs) predominantly mediated by ionotropic glutamate receptors. Bath application of either the GABA(A) receptor agonist muscimol or the GABA(B) receptor agonist baclofen caused a nearly complete suppression of the primary fEPSP. Coapplication of the appropriate GABA(A) or GABA(B) receptor antagonist bicuculline or CGP-55845 significantly reversed the effects of the agonists. These data indicate that GABAergic terminals situated in proximity to primary gustatory afferent terminals can modulate primary afferent input via both GABA(A) and GABA(B) receptors. The mechanism of action of GABA(B) receptors suggests a presynaptic locus of action for that receptor.

Conformational changes in surface structures of isolated connexin 26 gap junctions

Gap junction channels mediate communication between adjacent cells. Using atomic force microscopy (AFM), we have imaged conformational changes of the cytoplasmic and extracellular surfaces of native connexin 26 gap junction plaques. The cytoplasmic domains of the gap junction surface, imaged at submolecular resolution, form a hexameric pore protruding from the membrane bilayer. Exhibiting an intrinsic flexibility, these cytoplasmic domains, comprising the C-terminal connexin end, reversibly collapse by increasing the forces applied to the AFM stylus. The extracellular connexon surface was imaged after dissection of the gap junction with the AFM stylus. Upon injection of Ca(2+) into the buffer solution, the extracellular channel entrance reduced its diameter from 1.5 to 0.6 nm, a conformational change that is fully reversible and specific among the divalent cations tested. Ca(2+) had a profound effect on the cytoplasmic surface also, inducing the formation of microdomains. Consequently, the plaque height increased by 0.6 nm to 18 nm. This suggests that calcium ions induce conformational changes affecting the structure of both the hemichannels and the intact channels forming cell-cell contacts.

Gap junctions, homeostasis, and injury

Gap junctions (Gj) play an important role in the communication between cells of many tissues. They are composed of channels that permit the passage of ions and low molecular weight metabolites between adjacent cells, without exposure to the extracellular environment. These pathways are formed by the interaction between two hemichannels on the surface of opposing cells. These hemichannels are formed by the association of six identical subunits, named connexins (Cx), which are integral membrane proteins. Cell coupling via Gj is dependent on the specific pattern of Cx gene expression. This pattern of gene expression is altered during several pathological conditions resulting in changes of cell coupling. The regulation of Cx gene expression is affected at different levels from transcription to post translational processes during injury. In addition, Gj cellular communication is regulated by gating mechanisms. The alteration of Gj communication during injury could be rationalized by two opposite theories. One hypothesis proposes that the alteration of Gj communication attenuates the spread of toxic metabolites from the injured area to healthy organ regions. The alternative proposition is that a reduction of cellular communication reduces the loss of important cellular metabolisms, such as ATP and glucose.

Observing structure, function and assembly of single proteins by AFM

Single molecule experiments provide insight into the individuality of biological macromolecules, their unique function, reaction pathways, trajectories and molecular interactions. The exceptional signal-to-noise ratio of the atomic force microscope allows individual proteins to be imaged under physiologically relevant conditions at a lateral resolution of 0.5-1nm and a vertical resolution of 0.1-0.2nm. Recently, it has become possible to observe single molecule events using this technique. This capability is reviewed on various water-soluble and membrane proteins. Examples of the observation of function, variability, and assembly of single proteins are discussed. Statistical analysis is important to extend conclusions derived from single molecule experiments to protein species. Such approaches allow the classification of protein conformations and movements. Recent developments of probe microscopy techniques allow simultaneous measurement of multiple signals on individual macromolecules, and greatly extend the range of experiments possible for probing biological systems at the molecular level. Biologists exploring molecular mechanisms will benefit from a burgeoning of scanning probe microscopes and of their future combination with molecular biological experiments.

Voltage opens unopposed gap junction hemichannels formed by a connexin 32 mutant associated with X-linked Charcot-Marie-Tooth disease

The X-linked form of Charcot-Marie-Tooth disease (CMTX) is an inherited peripheral neuropathy that arises in patients with mutations in the gene encoding the gap junction protein connexin 32 (Cx32), which is expressed by Schwann cells. We recently showed that Cx32 containing the CMTX-associated mutation, Ser-85-Cys (S85C), forms functional cell-cell channels in paired Xenopus oocytes. Here, we describe that this mutant connexin also shows increased opening of hemichannels in nonjunctional surface membrane. Open hemichannels may damage the cells through loss of ionic gradients and small metabolites and increased influx of Ca(2+), and provide a mechanism by which this and other mutant forms of Cx32 may damage cells in which they are expressed. Evidence for open hemichannels includes: (i) oocytes expressing the Cx32(S85C) mutant show greatly increased conductance at inside positive potentials, significantly larger than in oocytes expressing wild-type Cx32 (Cx32WT); and (ii) the induced currents are similar to those previously described for several other connexin hemichannels, and exhibit slowly developing increases with increasing levels of positivity and reversible reduction when intracellular pH is decreased or extracellular Ca(2+) concentration is increased. Although increased currents are seen, oocytes expressing Cx32(S85C) have lower levels of the protein in the surface and in total homogenates than do oocytes expressing Cx32WT; thus, under the conditions examined here, hemichannels in the surface membrane formed of the Cx32(S85C) mutant have a higher open probability than hemichannels formed of Cx32WT. This increase in functional hemichannels may damage Schwann cells and ultimately lead to loss of function in peripheral nerves of patients harboring this mutation.

Transduction of cell survival signals by connexin-43 hemichannels

Bisphosphonates, drugs used widely in the treatment of bone diseases, prevent osteoblast and osteocyte apoptosis by a mechanism involving extracellular signal-regulated kinase (ERK) activation. We report herein that hexameric connexin (Cx)-43 hemichannels, but not gap junctions, are the essential transducers of the ERK-activating/anti-apoptotic effects of bisphosphonates. Transfection of Cx-43, but not other Cxs, into Cx-43 naive cells confers de novo responsiveness to the drugs. The signal-transducing property of Cx-43 requires the pore forming as well as the C-terminal domains of the protein, the activation of both Src and ERK kinases, and the SH2 and SH3 domains of Src. This evidence adds Cx-43 to the list of transmembrane proteins capable of transducing survival signals in response to extracellular cues and raises the possibility that it may serve in this capacity for endogenously produced molecules or even other drugs.

Localization of the AMPA subunit GluR2 in the outer plexiform layer of goldfish retina

L-glutamate, the photoreceptor neurotransmitter, depolarizes horizontal cells and OFF bipolar cells by ionotropic AMPA-glutamate receptors. The AMPA-receptor subunit (GluR4) is localized to dendrites of OFF bipolar cells in goldfish retina. Here, we used immunohistochemical techniques to identify AMPA-receptor subunits on horizontal cell dendrites. A monoclonal antibody against rat GluR2, with high sequence homology to the recently cloned goldfish GluR2a receptor, was used for light- and electron-microscopical immunocytochemistry. Light- and dark-adapted retinas were analyzed, with no major difference in results. GluR2-immunoreactivity (IR) was restricted to a narrow band in the outer plexiform layer, in which it appeared as bright dome-shaped structures amidst numerous puncta. At the ultrastructural level, GluR2-IR was found in horizontal cell dendrites that invaginated cones and rods. Dendrites of OFF bipolar cells were not labeled. GluR2-IR was present mostly in horizontal cell dendrites that were the lateral elements of the triad, rather than in dendrites that were the central elements. In light-adapted retinas, GluR2-IR was found in many horizontal cell spinules. GluR2-IR was observed, on occasion, in a mixed rod/cone (Mb) ON bipolar cell process that innervated rod spherules. Verification of the Mb ON bipolar cell was made by protein kinase C and metabotropic mGluR1alpha immunolabeling. The presence of GluR2-IR in lateral elements suggests that lateral horizontal cell dendrites are postsynaptic to cones rather than only sites of feedback inhibition. All horizontal cell types express the GluR2 subunit, uniquely differentiating themselves from OFF bipolar cells that express the GluR4 subunit. This differentiation most likely has a major influence on the glutamate pharmacology and response kinetics of these cell types to glutamate.

Mouse horizontal cells do not express connexin26 or connexin36

Gap junctions between neurons function as electrical synapses, and are present in all layers of mammalian and teleost retina. These synapses are largest and most prominent between horizontal cells where they function to increase the receptive field of a single neuron beyond the width of its dendrites. Receptive field size and the extent of gap junctional coupling between horizontal cells is regulated by ambient light levels and may mediate light/dark adaptation. Furthermore, teleost horizontal cell gap junction hemichannels may facilitate a mechanism of feedback inhibition between horizontal cells and cone photoreceptors. As a prelude to using mouse genetic models to study horizontal cell gap junctions and hemichannels, we sought to determine the connexin complement of mouse horizontal cells. Cx36, Cx37, Cx43, Cx45 and Cx57 mRNA could be detected in mouse retina by RT-PCR. Microscopy was used to further examine the distribution of Cx26 and Cx36. Cx26 immunofluorescence and a beta-gal reporter under regulatory control of the Cx36 promoter did not colocalize with a horizontal cell marker, indicating that these genes are not expressed by horizontal cells. The identity of the connexin(s) forming electrical synapses between mouse horizontal cells and the connexin that may form hemichannels in the horizontal cell telodendria remains unknown.