Assembly of chimeric connexin-aequorin proteins into functional gap junction channels. Reporting intracellular and plasma membrane calcium environments

Using aequorin probes to measure Ca2+ in intracellular organelles

Aequorins are excellent tools for measuring intra-organellar Ca²⁺ and assessing its role in physiological and pathological functions. Here we review targeting strategies to express aequorins in various organelles. We address critical topics such as probe affinity tuning as well as normalization and calibration of the signal. We also focus on bioluminescent Ca²⁺ imaging in nucleus or mitochondria of living cells. Finally, recent advances with a new chimeric GFP-aequorin protein (GAP), which can be used either as luminescent or fluorescent Ca²⁺ probe, are presented. GAP is robustly expressed in transgenic flies and mice, where it has proven to be a suitable Ca²⁺ indicator for monitoring physiological Ca²⁺ signaling ex vivo and in vivo.

Cell communication across gap junctions: a historical perspective and current developments

Collaborative communication lies at the centre of multicellular life. Gap junctions (GJs) are surface membrane structures that allow direct communication between cells. They were discovered in the 1960s following the convergence of the detection of low-resistance electrical interactions between cells and anatomical studies of intercellular contact points. GJs purified from liver plasma membranes contained a 27 kDa protein constituent; it was later named Cx32 (connexin 32) after its full sequence was determined by recombinant technology. Identification of Cx43 in heart and later by a further GJ protein, Cx26 followed. Cxs have a tetraspan organization in the membrane and oligomerize during intracellular transit to the plasma membrane; these were shown to be hexameric hemichannels (connexons) that could interact end-to-end to generate GJs at areas of cell-to-cell contact. The structure of the GJ was confirmed and refined by a combination of biochemical and structural approaches. Progress continues towards obtaining higher atomic 3D resolution of the GJ channel. Today, there are 20 and 21 highly conserved members of the Cx family in the human and mouse genomes respectively. Model organisms such as Xenopus oocytes and zebra fish are increasingly used to relate structure to function. Proteins that form similar large pore membrane channels in cells called pannexins have also been identified in chordates. Innexins form GJs in prechordates; these two other proteins, although functionally similar, are very different in amino acid sequence to the Cxs. A time line tracing the historical progression of wide ranging research in GJ biology over 60 years is mapped out. The molecular basis of channel dysfunctions in disease is becoming evident and progress towards addressing Cx channel-dependent pathologies, especially in ischaemia and tissue repair, continues.

Methods to measure intracellular Ca(2+) fluxes with organelle-targeted aequorin-based probes

The photoprotein aequorin generates blue light upon binding of Ca(2+) ions. Together with its very low Ca(2+)-buffering capacity and the possibility to add specific targeting sequences, this property has rendered aequorin particularly suitable to monitor Ca(2+) concentrations in specific subcellular compartments. Recently, a new generation of genetically encoded Ca(2+) probes has been developed by fusing Ca(2+)-responsive elements with the green fluorescent protein (GFP). Aequorin has also been employed to this aim, resulting in an aequorin-GFP chimera with the Ca(2+) sensitivity of aequorin and the fluorescent properties of GFP. This setup has actually solved the major limitation of aequorin, for example, its poor ability to emit light, which rendered it inappropriate for the monitoring of Ca(2+) waves at the single-cell level by imaging. In spite of the numerous genetically encoded Ca(2+) indicators that are currently available, aequorin-based probes remain the method of election when an accurate quantification of Ca(2+) levels is required. Here, we describe currently available aequorin variants and their use for monitoring Ca(2+) waves in specific subcellular compartments. Among various applications, this method is relevant for the study of the alterations of Ca(2+) homeostasis that accompany oncogenesis, tumor progression, and response to therapy.

Inhibition of connexin 36 hemichannels by glucose contributes to the stimulation of insulin secretion

The existence of functional connexin36 (Cx36) hemichannels in β-cells was investigated in pancreatic islets of rat and wild-type (Cx36(+/+)), monoallelic (Cx36(+/-)), and biallelic (Cx36(-/-)) knockout mice. Hemichannel opening by KCl depolarization was studied by measuring ATP release and changes of intracellular ATP (ADP). Cx36(+/+) islets lost ATP after depolarization with 70 mM KCl at 5 mM glucose; ATP loss was prevented by 8 and 20 mM glucose or 50 μM mefloquine (connexin inhibitor). ATP content was higher in Cx36(-/-) than Cx36(+/+) islets and was not decreased by KCl depolarization; Cx36(+/-) islets showed values between that of control and homozygous islets. Five minimolar extracellular ATP increased ATP content and ATP/ADP ratio and induced a biphasic insulin secretion in depolarized Cx36(+/+) and Cx36(+/-) but not Cx36(-/-) islets. Cx36 hemichannels expressed in oocytes opened upon depolarization of membrane potential, and their activation was inhibited by mefloquine and glucose (IC₅₀ ∼8 mM). It is postulated that glucose-induced inhibition of Cx36 hemichannels in islet β-cells might avoid depolarization-induced ATP loss, allowing an optimum increase of the ATP/ADP ratio by sugar metabolism and a biphasic stimulation of insulin secretion. Gradual suppression of glucose-induced insulin release in Cx36(+/-) and Cx36(-/-) islets confirms that Cx36 gap junction channels are necessary for a full secretory stimulation and might account for the glucose intolerance observed in mice with defective Cx36 expression. Mefloquine targeting of Cx36 on both gap junctions and hemichannels also suppresses glucose-stimulated secretion. By contrast, glucose stimulation of insulin secretion requires Cx36 hemichannels' closure but keeping gap junction channels opened.

Using targeted variants of aequorin to measure Ca2+ levels in intracellular organelles

Aequorin is a Ca(2+)-sensitive photoprotein isolated from the jellyfish Aequorea victoria. It is an ideal probe for measuring Ca(2+) concentration ([Ca(2+)]) in intracellular organelles because it can be modified to include specific targeting sequences. On the binding of Ca(2+) to three high-affinity sites in aequorin, an irreversible reaction occurs in which the prosthetic group coelenterazine is released and a photon is emitted. This protocol presents procedures for expressing, targeting, and reconstituting aequorin in intact and permeabilized mammalian cells and describes how to use this photoprotein to measure intracellular [Ca(2+)] in various subcellular compartments.

The use of aequorin and its variants for Ca2+ measurements

Ca(2+)-sensitive photoproteins are ideal agents for measuring the Ca(2+) concentration ([Ca(2+)]) in intracellular organelles because they can be modified to include specific targeting sequences. Aequorin was the first Ca(2+)-sensitive photoprotein probe used to measure the [Ca(2+)] inside specific intracellular organelles in intact cells. Aequorin is a 22-kDa protein produced by the jellyfish Aequorea victoria. On the binding of Ca(2+) to three high-affinity sites in aequorin, an irreversible reaction occurs in which the prosthetic group is released and a photon is emitted. Aequorin has become widely used for intracellular Ca(2+) measurements because it offers many advantages: For example, it can be targeted with precision, functions over a wide range of [Ca(2+)], and shows low buffering capacity. In this article we describe the main characteristics of the aequorin probe and review the reasons why it is widely used to measure intracellular [Ca(2+)].

Gap junctions

Gap junctions are essential to the function of multicellular animals, which require a high degree of coordination between cells. In vertebrates, gap junctions comprise connexins and currently 21 connexins are known in humans. The functions of gap junctions are highly diverse and include exchange of metabolites and electrical signals between cells, as well as functions, which are apparently unrelated to intercellular communication. Given the diversity of gap junction physiology, regulation of gap junction activity is complex. The structure of the various connexins is known to some extent; and structural rearrangements and intramolecular interactions are important for regulation of channel function. Intercellular coupling is further regulated by the number and activity of channels present in gap junctional plaques. The number of connexins in cell-cell channels is regulated by controlling transcription, translation, trafficking, and degradation; and all of these processes are under strict control. Once in the membrane, channel activity is determined by the conductive properties of the connexin involved, which can be regulated by voltage and chemical gating, as well as a large number of posttranslational modifications. The aim of the present article is to review our current knowledge on the structure, regulation, function, and pharmacology of gap junctions. This will be supported by examples of how different connexins and their regulation act in concert to achieve appropriate physiological control, and how disturbances of connexin function can lead to disease.

Human connexin30.2/31.3 (GJC3) does not form functional gap junction channels but causes enhanced ATP release in HeLa cells

Gap junctional intercellular communication has numerous functions, each of which meets the particular needs of organs, tissues, or groups of cells. Connexins (CXs) are homologous four-transmembrane-domain proteins that are the major components of gap junctions. CX30.2/CX31.3 (GJC3) is a relatively new member of the CX protein family. Until now, however, the functional characteristics of CX30.2/CX31.3 have been unclear. To elucidate the properties of CX30.2/CX31.3 channels, their subcellular localization in HeLa cells, their effectiveness in dye transfer, and function on channels were investigated. In the immunofluorescent assay, cells that express CX30.2/CX31.3-GFP exhibited continuous fluorescence along the apposed cell membranes, rather than punctated fluorescence in contacting membranes between two cells. Surprisingly, dyes that can be capable of being permeated by CX26 GJ, according to a scrape loading dye transfer assay in previous studies, are impermeated by CX30.2/CX31.3 GJ, suggesting a difference between the characteristics of CX30.2/CX31.3 GJ and CX26 GJ. Furthermore, a significant amount of ATP was released from the HeLa cells that stably expressed CX30.2/CX31.3, in a medium with low calcium ion concentration, suggesting a hemichannel-based function for CX30.2/CX31.3. Based on these findings, we suggest that CX30.2/CX31.3 shares functional properties with pannexin (hemi) channels rather than gap junction channels of other CXs.

A stochastic four-state model of contingent gating of gap junction channels containing two "fast" gates sensitive to transjunctional voltage

Connexins, a family of membrane proteins, form gap junction (GJ) channels that provide a direct pathway for electrical and metabolic signaling between cells. We developed a stochastic four-state model describing gating properties of homotypic and heterotypic GJ channels each composed of two hemichannels (connexons). GJ channel contain two "fast" gates (one per hemichannel) oriented opposite in respect to applied transjunctional voltage (V(j)). The model uses a formal scheme of peace-linear aggregate and accounts for voltage distribution inside the pore of the channel depending on the state, unitary conductances and gating properties of each hemichannel. We assume that each hemichannel can be in the open state with conductance gamma(h,o) and in the residual state with conductance gamma(h,res), and that both gamma(h,o) and gamma(h,res) rectifies. Gates can exhibit the same or different gating polarities. Gating of each hemichannel is determined by the fraction of V(j) that falls across the hemichannel, and takes into account contingent gating when gating of one hemichannel depends on the state of apposed hemichannel. At the single-channel level, the model revealed the relationship between unitary conductances of hemichannels and GJ channels and how this relationship is affected by gamma(h,o) and gamma(h,res) rectification. Simulation of junctions containing up to several thousands of homotypic or heterotypic GJs has been used to reproduce experimentally measured macroscopic junctional current and V(j)-dependent gating of GJs formed from different connexin isoforms. V(j)-gating was simulated by imitating several frequently used experimental protocols: 1), consecutive V(j) steps rising in amplitude, 2), slowly rising V(j) ramps, and 3), series of V(j) steps of high frequency. The model was used to predict V(j)-gating of heterotypic GJs from characteristics of corresponding homotypic channels. The model allowed us to identify the parameters of V(j)-gates under which small changes in the difference of holding potentials between cells forming heterotypic junctions effectively modulates cell-to-cell signaling from bidirectional to unidirectional. The proposed model can also be used to simulate gating properties of unapposed hemichannels.

Regulation of neuronal connexin-36 channels by pH

Neurotransmission through electrical synapses plays an important role in the spike synchrony among neurons and oscillation of neuronal networks. Indeed, electrical transmission has been implicated in the hypersynchronous electrical activity of epilepsy. We have investigated the influence of intracellular pH on the strength of electrical coupling mediated by connexin36 (Cx36), the principal gap junction protein in the electrical synapses of vertebrates. In striking contrast to other connexin isoforms, the activity of Cx36 channels decreases following alkalosis rather than acidosis when it is expressed in Xenopus oocytes and N2A cells. This uncoupling of Cx36 channels upon alkalinization occurred in the vertebrate orthologues analyzed (human, mouse, chicken, perch, and skate). While intracellular acidification caused a mild or moderate increase in the junctional conductance of virtually all these channels, the coupling of the skate Cx35 channel was partially blocked by acidosis. The mutational analysis suggests that the Cx36 channels may contain two gating mechanisms operating with opposing sensitivity to pH. One gate, the dominant mechanism, closes for alkalosis and it probably involves an interaction between the C- and N-terminal domains, while a secondary acid sensing gate only causes minor, albeit saturating, changes in coupling following acidosis and alkalosis. Thus, we conclude that neuronal Cx36 channels undergo unique regulation by pH(i) since their activity is inhibited by alkalosis rather than acidosis. These data provide a novel basis to define the relevance and consequences of the pH-dependent modulation of Cx36 synapses under physiological and pathological conditions.

Molecular basis of voltage dependence of connexin channels: An integrative appraisal

The importance of electrical and molecular signaling through connexin (Cx) channels is now widely recognized. The transfer of ions and other small molecules between adjacent cells is regulated by multiple stimuli, including voltage. Indeed, Cx channels typically exhibit complex voltage sensitivity. Most channels are sensitive to the voltage difference between the cell interiors (or transjunctional voltage, V(j)), while other channels are also sensitive to absolute inside-outside voltage (i.e., the membrane potential, V(m)). The first part of this review is focused on the description of the distinct forms of voltage sensitivity and the gating mechanisms that regulate hemichannel activity, both individually and as components of homotypic and heterotypic gap junctions. We then provide an up to date and precise picture of the molecular and structural aspects of how V(j) and V(m) are sensed, and how they, therefore, control channel opening and closing. Mutagenic strategies coupled with structural, biochemical and electrophysical studies are providing significant insights into how distinct forms of voltage dependence are brought about. The emerging picture indicates that Cx channels can undergo transitions between multiple conductance states driven by distinct voltage-gating mechanisms. Each hemichannel may contain a set of two V(j) gates, one fast and one slow, which mediate the transitions between the main open state to the residual state and to the fully closed state, respectively. Eventually, a V(m) gate regulates channel transitions between the open and closed states. Clusters of charged residues within separate domains of the Cx molecule have been identified as integral parts of the V(j) and V(m) sensors. The charges at the first positions of the amino terminal cytoplasmic domain determine the magnitude and polarity of the sensitivity to fast V(j)-gating, as well as contributing to the V(j)-rectifying properties of ion permeation. Additionally, important advances have been made in identifying the conformational rearrangements responsible for fast V(j)-gating transitions to the residual state in the Cx43 channel. These changes involve an intramolecular particle-receptor interaction between the carboxy terminal domain and the cytoplasmic loop.

Gating properties of heterotypic gap junction channels formed of connexins 40, 43, and 45

Connexins (Cxs) 40, 43, and 45 are expressed in many different tissues, but most abundantly in the heart, blood vessels, and the nervous system. We examined formation and gating properties of heterotypic gap junction (GJ) channels assembled between cells expressing wild-type Cx40, Cx43, or Cx45 and their fusion forms tagged with color variants of green fluorescent protein. We show that these Cxs, with exception of Cxs 40 and 43, are compatible to form functional heterotypic GJ channels. Cx40 and Cx43 hemichannels are unable or effectively impaired in their ability to dock and/or assemble into junctional plaques. When cells expressing Cx45 contacted those expressing Cx40 or Cx43 they readily formed junctional plaques with cell-cell coupling characterized by asymmetric junctional conductance dependence on transjunctional voltage, V(j). Cx40/Cx45 heterotypic GJ channels preferentially exhibit V(j)-dependent gating transitions between open and residual states with a conductance of approximately 42 pS; transitions between fully open and closed states with conductance of approximately 52 pS in magnitude occur at substantially lower ( approximately 10-fold) frequency. Cx40/Cx45 junctions demonstrate electrical signal transfer asymmetry that can be modulated between unidirectional and bidirectional by small changes in the difference between holding potentials of the coupled cells. Furthermore, both fast and slow gating mechanisms of Cx40 exhibit a negative gating polarity.

Effect of charge substitutions at residue his-142 on voltage gating of connexin43 channels

Previous studies indicate that the carboxyl terminal of connexin43 (Cx43CT) is involved in fast transjunctional voltage gating. Separate studies support the notion of an intramolecular association between Cx43CT and a region of the cytoplasmic loop (amino acids 119-144; referred to as "L2"). Structural analysis of L2 shows two alpha-helical domains, each with a histidine residue in its sequence (H126 and H142). Here, we determined the effect of H142 replacement by lysine, alanine, and glutamate on the voltage gating of Cx43 channels. Mutation H142E led to a significant reduction in the frequency of occurrence of the residual state and a prolongation of dwell open time. Macroscopically, there was a large reduction in the fast component of voltage gating. These results resembled those observed for a mutant lacking the carboxyl terminal (CT) domain. NMR experiments showed that mutation H142E significantly decreased the Cx43CT-L2 interaction and disrupted the secondary structure of L2. Overall, our data support the hypothesis that fast voltage gating involves an intramolecular particle-receptor interaction between CT and L2. Some of the structural constrains of fast voltage gating may be shared with those involved in the chemical gating of Cx43.

Life cycle of connexins in health and disease

Evaluation of the human genome suggests that all members of the connexin family of gap-junction proteins have now been successfully identified. This large and diverse family of proteins facilitates a number of vital cellular functions coupled with their roles, which range from the intercellular propagation of electrical signals to the selective intercellular passage of small regulatory molecules. Importantly, the extent of gap-junctional intercellular communication is under the direct control of regulatory events associated with channel assembly and turnover, as the vast majority of connexins have remarkably short half-lives of only a few hours. Since most cell types express multiple members of the connexin family, compensatory mechanisms exist to salvage tissue function in cases when one connexin is mutated or lost. However, numerous studies of the last decade have revealed that mutations in connexin genes can also lead to severe and debilitating diseases. In many cases, single point mutations lead to dramatic effects on connexin trafficking, assembly and channel function. This review will assess the current understanding of wild-type and selected disease-linked mutant connexin transport through the secretory pathway, gap-junction assembly at the cell surface, internalization and degradation.

Gap junction channel gating

Over the last two decades, the view of gap junction (GJ) channel gating has changed from one with GJs having a single transjunctional voltage-sensitive (V(j)-sensitive) gating mechanism to one with each hemichannel of a formed GJ channel, as well as unapposed hemichannels, containing two, molecularly distinct gating mechanisms. These mechanisms are termed fast gating and slow or 'loop' gating. It appears that the fast gating mechanism is solely sensitive to V(j) and induces fast gating transitions between the open state and a particular substate, termed the residual conductance state. The slow gating mechanism is also sensitive to V(j), but there is evidence that this gate may mediate gating by transmembrane voltage (V(m)), intracellular Ca(2+) and pH, chemical uncouplers and GJ channel opening during de novo channel formation. A distinguishing feature of the slow gate is that the gating transitions appear to be slow, consisting of a series of transient substates en route to opening and closing. Published reports suggest that both sensorial and gating elements of the fast gating mechanism are formed by transmembrane and cytoplamic components of connexins among which the N terminus is most essential and which determines gating polarity. We propose that the gating element of the slow gating mechanism is located closer to the central region of the channel pore and serves as a 'common' gate linked to several sensing elements that are responsive to different factors and located in different regions of the channel.

Regulation of Connexin43 Protein Complexes by Intracellular Acidification

Ischemia-induced acidification of astrocytes or cardiac myocytes reduces intercellular communication by closing gap junction channels and subsequently internalizing gap junction proteins. To determine whether such coupling changes might be attributable to altered interactions between connexin43 (Cx43) and other proteins, we applied the nigericin/high K + method to vary intracellular pH (pHi) in cultured cortical astrocytes. Intracellular acidification was accompanied by internalization of Cx43 with retention of Cx43 scaffolding protein Zonula Occludens-1 (ZO-1) at cell surfaces, suggesting that ZO-1 and Cx43 dissociate at low pHi. Coimmunoprecipitation studies revealed decreased binding of ZO-1 and increased binding of c-Src to Cx43 at low pHi. Resonant mirror spectroscopy was used to quantify binding of the SH3 domain of c-Src and the PDZ domains of ZO-1 to the carboxyl terminal domain of Cx43 (Cx43CT). Data indicate that the c-Src/Cx43CT interaction is highly pH dependent whereas the ZO-1/Cx43CT interaction is not. Moreover, binding of c-Src to Cx43CT prevented and reversed ZO-1/Cx43CT binding. We hypothesize that increased affinity of c-Src for Cx43 at low pHi aids in separation of Cx43 from ZO-1 and that this may facilitate internalization of Cx43. These data suggest that protracted acidification may remodel protein-protein interactions involving Cx43 and thus provide an important protective mechanism to limit lesion spread after ischemic injury.

Gating and regulation of connexin 43 (Cx43) hemichannels

Connexin 43 (Cx43) nonjunctional or "unapposed" hemichannels can open under physiological or pathological conditions. We characterize hemichannels comprised of Cx43 or Cx43-EGFP (Cx43 with enhanced GFP fused to the C terminus) expressed in HeLa cells. Channel opening was induced at potentials greater than +60 mV. Open probability appeared to be very low. No comparable opening was detected in the parental, nontransfected HeLa cells. Conductance of fully open single hemichannels was approximately 220 pS, which is approximately double that of Cx43 cell-cell channels. Cx43 hemichannels exhibited two types of gating: fast transitions (<1 ms) between the fully open state and a substate of approximately 75 pS and slow transitions (>5 ms) between either open state and the fully closed state. Cx43-EGFP hemichannels exhibited only slow transitions (>5 ms) between closed and fully open states. These properties resemble those of the corresponding Cx43 and Cx43-EGFP cell-cell channels. Cx43 with EGFP on the N terminus (EGFP-Cx43) inserted into the surface and formed plaques but did not form hemichannels or cell-cell channels. Hemichannel blockers, 18beta-glycyrrhetinic acid or La3+, blocked depolarization-induced currents. Uptake of ethidium bromide (i) was faster in Cx43 and Cx43-EGFP than parental and EGFP-Cx43 cells, (ii) was directly correlated with Cx43-EGFP expression, (iii) was reduced by hemichannel blockers, and (iv) occurred at the same low rate in EGFP-Cx43 and parental cells. Although hemichannel opening was not detected electrophysiologically at the resting potential, infrequent or brief opening could account for ethidium bromide uptake. Opening of Cx43 hemichannels may mediate normal signaling or be deleterious.

Gap junction assembly: multiple connexin fluorophores identify complex trafficking pathways

The assembly of gap junction channels was studied using mammalian cells expressing connexin (Cx) 26, 32 and 43 in which the carboxyl terminus was fused to green, yellow or cyan fluorescent proteins (GFP, YFP, CFP). Intracellular targeting of Cx32-CFP and 43-GFP to gap junctions was disrupted by brefeldin A treatment and resulted in a severe loss of gap junctional intercellular communication reflected by low intercellular dye transfer. Cells expressing Cx43-GFP exposed to nocodazole showed normal targeting to gap junctions and dye transfer. Cx32 and 43 thus appear to be transported and assembled into gap junctions via the classical secretory pathway. In contrast, we found that assembly of Cx26-GFP into functional gap junctions was relatively unaffected by treatment of cells with brefeldin A, but was extremely sensitive to nocodazole treatment. Coexpression of Cx26-YFP and Cx32-CFP indicated a different intracellular distribution that was accentuated in the presence of brefeldin A, with the gap junctions in these cells constructed predominantly of Cx26-YFP. A site specific mutation in the first transmembrane domain that distinguished Cx32 from Cx26 (Cx32128L) resulted in the adoption of the trafficking properties of Cx26 as well as its unusual post-translational membrane integration characteristics. The results indicate that multiple intracellular connexin trafficking routes exist and provide a further mechanism for regulating the connexin composition of gap junctions and thus specificity in intercellular signalling.

Human genetic variations in the 5HT2A receptor: a single nucleotide polymorphism identified with altered response to clozapine

OBJECTIVE

to determine if the agonist serotonin and antagonists loxapine and clozapine have an altered potency for four allelic variants (T25N, I197V, A447V, and H452Y) of the human 5HT2A receptor when compared to the wild-type allele.

METHODS

The receptor or its variants are studied in an in-vitro functional assay system consisting of a Sf9 insect cell line that is stably transformed with the human wild-type and mutant alleles. This assay system measures release of calcium stores due to receptor activation by agonists and inhibition of this agonist stimulated response by antagonists.

RESULTS

Both loxapine and clozapine exhibit non-competitive antagonism of serotonin stimulation of the human 5HT2A receptor signal transduction system and loxapine is the more potent inhibitor. This study shows that the I197V allele requires a two-fold higher concentration of the atypical neuroleptic clozapine to inhibit serotonin stimulation compared to the wild-type receptor (P = 0.036). The I197V mutation does not affect the inhibition of serotonin stimulation by the typical neuroleptic loxapine nor does it alter the activation of the receptor by serotonin. It is also significant that the results of this study indicate that the T25N, A447V, and H452Y mutations in the human 5HT2A receptor do not significantly alter the response of the receptor to the agonist serotonin or the antagonists loxapine and clozapine.

Evidence for a role of the N-terminal domain in subcellular localization of the neuronal connexin36 (Cx36)

The expression and functional properties of connexin36 (Cx36) have been investigated in two neuroblastoma cell lines (Neuro2A, RT4-AC) and primary hippocampal neurons transfected with a Cx36-enhanced green fluorescent protein (EGFP) expression vector. Transfected cells express Cx36-EGFP mRNA, and Cx36-EGFP protein is localized in the perinuclear area and cell membrane. Upon differentiation of cell lines, Cx36-EGFP protein was detectable in processes with both axonal and dendritic characteristics. Small gap junction plaques were found between adjacent cells, and electrophysiological recordings demonstrated that the electrical properties of these gap junctions were virtually indistinguishable from those reported for native Cx36. Mutagenesis of Cx36 led to the identification of a structural element that interferes with normal protein localization. In contrast, site directed mutagenesis of putative protein phosphorylation motifs did not alter subcellular localization. This excludes phosphorylation/dephosphorylation as a major regulatory step in Cx36 protein transport.

Targeted gap junction protein constructs reveal connexin-specific differences in oligomerization

To define further the mechanisms of gap junction protein (connexin (Cx)) oligomerization without pharmacologic disruption, we have examined the transport and assembly of connexin constructs containing C-terminal di-lysine-based endoplasmic reticulum (ER) (HKKSL) or ER-Golgi intermediate compartment (AKKFF) targeting sequences. By immunofluorescence microscopy, Cx43-HKKSL transiently transfected into HeLa cells showed a predominantly ER localization, although Cx43-AKKFF was localized to the perinuclear region of the cell. Sucrose gradient analysis of Triton X-100-solubilized connexins showed that either Cx43-HKKSL or Cx43-AKKFF expressed alone by HeLa cells was maintained as an apparent monomer. In contrast to Cx43-HKKSL, Cx32-HKKSL was maintained in the ER as stable hexamers, consistent with the notion that Cx32 and Cx43 oligomerization occur in distinct intracellular compartments. Furthermore, Cx43-HKKSL and Cx43-AKKFF inhibited trafficking of Cx43 and Cx46 to the plasma membrane. The inhibitory effect was because of the formation of mixed oligomers between Cx43-HKKSL or Cx43-AKKF and wild type Cx43 or Cx46. Taken together, these results suggest that Cx43-HKKSL and Cx43-AKKFF recirculate through compartments where oligomerization occurs and may be maintained as apparent monomers by a putative Cx43-specific quality control mechanism.

Coupling asymmetry of heterotypic connexin 45/ connexin 43-EGFP gap junctions: properties of fast and slow gating mechanisms

Although fast and slow gating mechanisms have been described in gap junctions (GJs), their relative contributions to dependence on transjunctional voltage, V(j), is still unclear. We used cell lines expressing wild-type connexin 45 (Cx45) and connexin 43 fused with enhanced green fluorescent protein (Cx43-EGFP) to examine mechanisms of gating in homo- and heterotypic GJs formed of these connexins. Macroscopically Cx45/Cx45 channels show high sensitivity to V(j). Cx45 channels demonstrate two types of gating: fast transitions between open and residual states and slow transitions between open and completely closed states. Single-channel conductance of the Cx45 channel is approximately 32 pS for the open state and approximately 4 pS for the residual state. Cx45/Cx43-EGFP heterotypic junctions exhibit very asymmetrical V(j) gating with the maximum junctional conductance shifted to V(j) positive on the Cx45 side. Conductance of single Cx45/Cx43-EGFP channels is approximately 55 pS for the open state and approximately 4 pS for the residual state, values consistent with the simple-series connection of Cx45 and Cx43-EGFP hemichannels. At V(j) = 0, the slow gate of many Cx45 hemichannels is closed in both homotypic Cx45/Cx45 and heterotypic Cx45/Cx43-EGFP junctions. Fast and slow V(j) gates of both Cx45 and Cx43 hemichannels close for relative negativity at their cytoplasmic end. Coupling mediated by Cx45/Cx43-EGFP junctions can exhibit asymmetry that can be strongly modulated by small changes in difference of holding potentials. Cx45/Cx43 junctions are likely to be found in brain and heart and may mediate rectifying electrical transmission or modulatable chemical communication.

Functional role of the carboxyl terminal domain of human connexin 50 in gap junctional channels

Gap junction channels formed by connexin 50 (Cx50) are critical for maintenance of lens transparency. Because the C-terminus of Cx50 can be cleaved post-translationally, we hypothesized that channels formed by the truncated Cx50 exhibit altered properties or regulation. We used the dual whole-cell patch-clamp technique to investigate the macroscopic and single-channel properties of gap junctional channels formed by wild-type human Cx50 and a truncation mutant (Cx50A294stop) after transfection of N2A cells. Our results show that wild-type Cx50 formed functional gap junctional channels. The macroscopic Gjss-Vj relationship was well described by a Boltzmann equation with A of 0.10, V0 of 43.8 mV and Gjmin of 0.23. The single-channel conductance was 212 +/- 5 pS. Multiple long-lasting substates were observed with conductances ranging between 31 and 80 pS. Wild-type Cx50 gap junctional channels were reversibly blocked when pHi was reduced to 6.3. Truncating the C-terminus at amino acid 294 caused a loss of pHi sensitivity, but there were no significant changes in single-channel current amplitude or Gjss-Vj relationship. These results suggest that the C-terminus of human Cx50 is involved in pHi sensitivity, but has little influence over single-channel conductance, voltage dependence, or gating kinetics.

Multiple pathways in the trafficking and assembly of connexin 26, 32 and 43 into gap junction intercellular communication channels

The assembly of gap junctions was investigated in mammalian cells expressing connexin (Cx) 26, 32 and 43 fused to green, yellow or cyan fluorescent proteins (GFP, YFP, CFP). Targeting of Cx32-CFP and 43-GFP to gap junctions and gap junctional communication was inhibited in cells treated with Brefeldin A, a drug that disassembles the Golgi. However gap junctions constructed of Cx26-GFP were only minimally affected by Brefeldin A. Nocodazole, a microtubule disruptor, had little effect on the assembly of Cx43-GFP gap junctions, but perturbed assembly of Cx26-GFP gap junctions. Co-expression of Cx26-YFP and Cx32-CFP in cells treated with Brefeldin A resulted in assembly of gap junctions constructed of Cx26-YFP. Two amino acids that distinguish Cx26 from Cx32 in transmembrane domains were mutated in Cx32 to investigate underlying mechanisms determining trafficking routes to gap junctions. One mutation, Cx32I28L, conferred on it partial Cx26-like trafficking properties as well the post-translational membrane insertion characteristics of Cx26, suggesting that a key determinant regulating trafficking was present in the first transmembrane domain. The results provide a protein trafficking basis for specifying and regulating connexin composition of gap junctions and thus selectivity of intercellular signaling, with Cx32 and 43 trafficking through the secretory pathway and Cx26 also following an alternative pathway.

Assembly of gap junction channels: mechanism, effects of calmodulin antagonists and identification of connexin oligomerization determinants

The assembly of connexins (Cxs) into gap junction intercellular communication channels was studied. An in vitro cell-free synthesis system showed that formation of the hexameric connexon hemichannels involved dimeric and tetrameric connexin intermediates. Cx32 contains two putative cytoplasmic calmodulin-binding sites, and their role in gap junction channel assembly was investigated. The oligomerization of Cx32 into connexons was reversibly inhibited by a calmodulin-binding synthetic peptide, and by W7, a naphthalene sulfonamide calmodulin antagonist. Removing the calmodulin-binding site located at the carboxyl tail of Cx32 limited connexon formation and resulted in an accumulation of intermediate connexin oligomers. This truncation mutant, Cx32Delta215, when transiently expressed in COS-7 cells, accumulated intracellularly and had failed to target to gap junctions. Immunoprecipitation studies suggested that a C-terminal sequence of Cx32 incorporating the calmodulin-binding site was required for the formation of hetero-oligomers of Cx26 and Cx32 but not for Cx32 homomeric association. A chimera, Cx32TM3CFTR, in which the third transmembrane and proposed channel lining sequence of Cx32 was substituted by a transmembrane sequence of the cystic fibrosis transmembrane conductance regulator, did not oligomerize in vitro and it accumulated intracellularly when expressed in COS-7 cells. The results indicate that amino-acid sequences in the third transmembrane domain and a calmodulin-binding domain in the cytoplasmic tail of Cx32 are likely candidates for regulating connexin oligomerization.

Gating properties of gap junction channels assembled from connexin43 and connexin43 fused with green fluorescent protein

We used cell lines expressing wild-type connexin43 (Cx43) and Cx43 fused with enhanced green fluorescent protein (Cx43-EGFP) to examine mechanisms of gap junction channel gating. Previously it was suggested that each hemichannel in a cell-cell channel possesses two gates, a fast gate that closes channels to a nonzero conductance or residual state via fast (< approximately 2 ms) transitions and a slow gate that fully closes channels via slow transitions (> approximately 10 ms). Here we demonstrate that transjunctional voltage (V(j)) regulates both gates and that they are operating in series and in a contingent manner in which the state of one gate affects gating of the other. Cx43-EGFP channels lack fast V(j) gating to a residual state but show slow V(j) gating. Both Cx43 and Cx43-EGFP channels exhibit slow gating by chemical uncouplers such as CO(2) and alkanols. Chemical uncouplers do not induce obvious changes in Cx43-EGFP junctional plaques, indicating that uncoupling is not caused by dispersion or internalization of junctional plaques. Similarity of gating transitions during chemical gating and slow V(j) gating suggests that both gating mechanisms share common structural elements. Cx43/Cx43-EGFP heterotypic channels showed asymmetrical V(j) gating with fast transitions between open and residual states only when the Cx43 side was relatively negative. This result indicates that the fast V(j) gate of Cx43 hemichannels closes for relative negativity at its cytoplasmic end.

Immunoglobulin and cytokine expression in mixed lymphocyte cultures is reduced by disruption of gap junction intercellular communication

Connexins (Cx), the protein subunits assembled into gap junction intercellular communication channels, are expressed in primary lymphoid organs and by circulating leukocytes. Human tonsil-derived T and B lymphocytes express Cx40 and 43; circulating human T, B, and NK lymphocytes express Cx43 and directly transfer between each other a low molecular dye indicative that functional gap junctions exist. We now identify specific properties in the immune system underwritten by gap junctions. Mixed lymphocytes cultured in the presence of two reagents with independent inhibitory action on gap junction communication, a connexin mimetic peptide and 18-alpha-glycyrrhetinic acid, markedly reduced the secretion of IgM, IgG, and IgA. The secretion of these immunoglobulins by purified B cells was also reduced by the two classes of gap junction inhibitors. Complex temporal inhibitory effects on the expression of mRNA encoding interleukins, especially IL-10, were also observed. The results indicate that intercellular signaling across gap junctions is an important component of the mechanisms underlying metabolic cooperation in the immune system.

Molecular determinants of membrane potential dependence in vertebrate gap junction channels

The conductance, g(j), of many gap junctions depends on voltage between the coupled cells (transjunctional voltage, V(j)) with little effect of the absolute membrane potential (V(m)) in the two cells; others show combined V(j) and V(m) dependence. We examined the molecular determinants of V(m) dependence by using rat connexin 43 expressed in paired Xenopus oocytes. These junctions have, in addition to V(j) dependence, V(m) dependence such that equal depolarization of both cells decreases g(j). The dependence of g(j) on V(m) was abolished by truncation of the C-terminal domain (CT) at residue 242 but not at 257. There are two charged residues between 242 and 257. In full-length Cx43, mutations neutralizing either one of these charges, Arg243Gln and Asp245Gln, decreased and increased V(m) dependence, respectively, suggesting that these residues are part of the V(m) sensor. Mutating both residues together abolished V(m) dependence, although there is no net change in charge. The neutralizing mutations, together or separately, had no effect on V(j) dependence. Thus, the voltage sensors must differ. However, V(j) gating was somewhat modulated by V(m), and V(m) gating was reduced when the V(j) gate was closed. These data suggest that the two forms of voltage dependence are mediated by separate but interacting domains.

Targeting motifs and functional parameters governing the assembly of connexins into gap junctions

To study the assembly of gap junctions, connexin--green-fluorescent-protein (Cx--GFP) chimeras were expressed in COS-7 and HeLa cells. Cx26-- and Cx32--GFP were targeted to gap junctions where they formed functional channels that transferred Lucifer Yellow. A series of Cx32--GFP chimeras, truncated from the C-terminal cytoplasmic tail, were studied to identify amino acid sequences governing targeting from intracellular assembly sites to the gap junction. Extensive truncation of Cx32 resulted in failure to integrate into membranes. Truncation of Cx32 to residue 207, corresponding to removal of most of the 78 amino acids on the cytoplasmic C-terminal tail, led to arrest in the endoplasmic reticulum and incomplete oligomerization. However, truncation to amino acid 219 did not impair Cx oligomerization and connexon hemichannels were targeted to the plasma membrane. It was concluded that a crucial gap-junction targeting sequence resides between amino acid residues 207 and 219 on the cytoplasmic C-terminal tail of Cx32. Studies of a Cx32E208K mutation identified this as one of the key amino acids dictating targeting to the gap junction, although oligomerization of this site-specific mutation into hexameric hemichannels was relatively unimpaired. The studies show that expression of these Cx--GFP constructs in mammalian cells allowed an analysis of amino acid residues involved in gap-junction assembly.

Intercellular calcium waves in HeLa cells expressing GFP-labeled connexin 43, 32, or 26

This study was undertaken to obtain direct evidence for the involvement of gap junctions in the propagation of intercellular Ca(2+) waves. Gap junction-deficient HeLa cells were transfected with plasmids encoding for green fluorescent protein (GFP) fused to the cytoplasmic carboxyl termini of connexin 43 (Cx43), 32 (Cx32), or 26 (Cx26). The subsequently expressed GFP-labeled gap junctions rendered the cells dye- and electrically coupled and were detected at the plasma membranes at points of contact between adjacent cells. To correlate the distribution of gap junctions with the changes in [Ca(2+)](i) associated with Ca(2+) waves and the distribution of the endoplasmic reticulum (ER), cells were loaded with fluorescent Ca(2+)-sensitive (fluo-3 and fura-2) and ER membrane (ER-Tracker) dyes. Digital high-speed microscopy was used to collect a series of image slices from which the three-dimensional distribution of the gap junctions and ER were reconstructed. Subsequently, intercellular Ca(2+) waves were induced in these cells by mechanical stimulation with or without extracellular apyrase, an ATP-degrading enzyme. In untransfected HeLa cells and in the absence of apyrase, cell-to-cell propagating [Ca(2+)](i) changes were characterized by initiating Ca(2+) puffs associated with the perinuclear ER. By contrast, in Cx-GFP-transfected cells and in the presence of apyrase, [Ca(2+)](i) changes were propagated without initiating perinuclear Ca(2+) puffs and were communicated between cells at the sites of the Cx-GFP gap junctions. The efficiency of Cx expression determined the extent of Ca(2+) wave propagation. These results demonstrate that intercellular Ca(2+) waves may be propagated simultaneously via an extracellular pathway and an intracellular pathway through gap junctions and that one form of communication may mask the other.

Developmental expression and assembly of connexins into homomeric and heteromeric gap junction hemichannels in the mouse mammary gland

During the development of the mammary gland, duct-lining epithelial cells progress through a program of expansive proliferation, followed by a terminal differentiation that allows for the biosynthesis and secretion of milk during lactation. The role of gap junction proteins, connexins, in the development and function of this secretory epithelium was investigated. Connexins, Cx26 and Cx32, were differentially expressed throughout pregnancy and lactation in alveolar cells. Cx26 poly-(A)(+) RNA and protein levels increased from early pregnancy, whereas Cx32 was detectable only during lactation. At this time, immunolocalization of connexins by confocal microscopy and immunogold labeling of high-pressure frozen freeze-substituted tissue showed that both connexins colocalized to the same junctional plaque. Analysis of gap junction hemichannels (connexons) isolated from lactating mammary gland plasma membranes by a rate-density centrifugation procedure, followed by immunoprecipitation and by size-exclusion chromatography, showed that Cx26 and Cx32 were organized as homomeric and heteromeric connexons. Structural diversity in the assembly of gap junction hemichannels demonstrated between pregnant and lactating mammary gland may account for differences in ionic and molecular signaling that may physiologically influence the onset and/or maintenance of the secretory phenotype of alveolar epithelial cells.

Involvement of gap junctional communication in myogenesis

Cell-to-cell communication plays important roles in development and in tissue morphogenesis. Gap junctional intercellular communication (GJIC) has been implicated in embryonic development of various tissues and provides a pathway to exchange ions, secondary messengers, and metabolites through the intercellular gap junction channels. Although GJIC is absent in adult skeletal muscles, the formation of skeletal muscles involves a sequence of complex events including cell-cell interaction processes where myogenic cells closely adhere to each other. Much experimental evidence has shown that myogenic precursors and developing muscle fibers can directly communicate through junctional channels. This review summarizes current knowledge on the GJIC and developmental events involved in the formation of skeletal muscle fibers and describes recent progress in the investigation of the role of GJIC in myogenesis: evidence of gap junctions in somitic and myotomal tissue as well as in developing muscle fibers in situ, GJIC between perfusion myoblasts in culture, and involvement of GJIC in cytodifferentiation of skeletal muscle cells and in myoblast fusion. A model of intercellular signaling is proposed where GJIC participates to coordinate a multicellular population of interacting myogenic precursors to allow commitment to the skeletal muscle fate.

Intercellular communication in the immune system: differential expression of connexin40 and 43, and perturbation of gap junction channel functions in peripheral blood and tonsil human lymphocyte subpopulations

The distribution and function of connexins (integral membrane proteins assembled into gap junction intercellular communication channels) were studied in human lymphocyte subpopulations. The expression of mRNA encoding connexins in peripheral blood and tonsil-derived T, B and natural killer (NK) lymphocytes was examined. Connexin43 (Cx43) mRNA was expressed in peripheral blood and tonsil lymphocytes, but Cx40 mRNA expression was confined to tonsil-derived T and B lymphocytes; Cx26, Cx32, Cx37 and Cx45 were not detected by reverse transcription-polymerase chain reaction (RT-PCR). Western blot analysis also demonstrated the presence of Cx40 and Cx43 proteins in T and B lymphocytes in a manner coincidental to the mRNA detection. Stimulation in vitro of T and B lymphocytes with phytohaemagglutinin (PHA) and lipopolysaccharide (LPS), respectively, increased Cx40 and Cx43 protein expression. Flow cytometric analysis, using antibodies to extracellular loop amino acid sequences of connexins, confirmed the surface expression of connexins in all lymphocyte subpopulations. Assembly of connexins into gap junctions providing direct intercellular channels linking attached lymphocytes was demonstrated by using a dye transfer technique. The exchange of dye between lymphocytes was inhibited by a connexin extracellular loop mimetic peptide and alpha-glycyrrhetinic acid, two reagents that restrict intercellular communication across gap junctions. Dye coupling occurred between homologous and heterologous co-cultures of T and B lymphocytes, and was not influenced by their stimulation with PHA and LPS. The connexin mimetic peptide caused a significant decrease in the in vitro synthesis of immunoglobulin M (IgM) by T- and B-lymphocyte co-cultured populations in the presence or absence of stimulation by PHA. The results identify connexins as important cell surface components that modulate immune processes.

Functional expression and biochemical characterization of an epitope-tagged connexin37

To study the gap junction protein connexin37 (Cx37), we stably transfected cell lines with constructs of human Cx37 containing the epitope tag FLAG (DYKDDDDK). A Cx37 construct containing the FLAG moiety at the carboxyl terminus (Cx37F) was expressed in BWEM cells, and did not substantially alter the levels of endogenous Cx43 in these cells. Immunostaining showed that Cx37F colocalized with Cx43 at cell-cell contacts. Pulse-chase metabolic labeling and immunoprecipitation with anti-FLAG antibodies indicated that Cx37F was synthesized as a protein that ran at 35.9 +/- 0.9 kDa on reducing SDS-PAGE but chased into a slower migrating band at 38.0 +/- 1.0 kDa. This shift in mobility was due to phosphorylation on serine residues, based on [(32)P]-metabolic labeling, immunoprecipitation, and phosphoamino acid analyses. The transition to the phosphoCx37F correlated with a loss of solubility in 1% Triton X-100. Based on the [(35)S]-methionine pulse-chase experiments, the half-life of the labeled Cx37F was approximately 3 h, which is within the range reported for other connexins. Analysis of dye injection experiments indicated that dye transfer was reduced in Cx37-transfected cells in comparison to parental BWEM cells, suggesting that formation of heteromeric Cx37-Cx43 channels reduced the molecular permeability of communication between these cells. Moreover, the similarities of previously demonstrated kinetic details and modification of Cx43 to our new data regarding Cx37 provide evidence for a commonality in processing and assembly steps of these two connexins.

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

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

Properties of connexin26 gap junctional proteins derived from mutations associated with non-syndromal heriditary deafness

Three point mutations of the connexin26 (GJB2) gene associated with hereditary deafness were studied using in vitro expression systems. Mutation M34T results in an amino acid substitution in the first transmembrane domain of the connexin protein, W77R is located in the second transmembrane domain and W44C is in the first extracellular loop. Wild-type and mutated connexin vectors were constructed and transfected into communication-deficient HeLa cells to obtain transient expression of the connexin proteins. Intercellular coupling was subsequently assessed by examining transfer of Lucifer yellow between cells. All three mutations resulted in impaired intercellular coupling. The mechanistic reasons for the functional inadequacies of the mutated proteins were investigated. First, intracellular trafficking and targeting of the expressed connexins were determined by immunohistochemistry. Mutation W77R was inefficiently targeted to the plasma membrane and retained in intracellular stores whereas the other two were targeted to the plasma membrane. Oligomerization assays showed that connexins M34T and W77R failed to assemble efficiently into hexameric gap junction hemichannels, but the W44C mutation did so. A cell-free translation system showed that the mutated proteins were inserted into microsomal membranes but the mutations have different effects on the post-translational properties of the expressed proteins. The results point to the conclusion that mutations in the transmembrane domains of connexin proteins influence gap junction assembly.

Calcium imaging with chemiluminescence

This review updates the imaging of free cytosolic calcium with the chemiluminescent aequorins. Basic principles of chemiluminescence are discussed and the biochemistry of aequorins is briefly described. The review provides practical tips on handling and microinjecting aequorins and describes available ultra low light imaging systems. It is argued that aequorin-based calcium imaging is the method of choice for exploratory studies, since it is extremely sensitive, can detect a broad range of calcium concentrations, and allows for continuous recording during long periods of time. However, fluorescent methods are needed to attain high spatial resolution.

Molecular dissection of transjunctional voltage dependence in the connexin-32 and connexin-43 junctions

Most gap junction channels are sensitive to the voltage difference between the two cellular interiors, termed the transjunctional voltage (V(j)). In several junctions, the conductance transitions induced by V(j) show more than one kinetic component. To elucidate the structural basis of the fast and slow components that characterize the V(j )dependence of connexin-32 (Cx32) and connexin-43 (Cx43) junctions, we created deletions of both connexins, where most of the carboxy-terminal (CT) domain was removed. The wild-type and "tailless" mutants were expressed in paired Xenopus oocytes, and the macroscopic gating properties were analyzed using the dual voltage clamp technique. Truncation of the CT domain of Cx32 and Cx43 abolished the fast mechanism of conductance transitions and induced novel gating properties largely attributable to the slow mechanism of gating. The formation of hybrid junctions comprising wild-type and truncated hemichannels allowed us to infer that the fast and slow components of gating reside in each hemichannel and that both gates close at a negative V(j) on the cytoplasmic side. Thus we conclude that the two kinetic components of V(j)-sensitive conductance are a result of the action of two different gating mechanisms. They constitute separate structures in the Cx32 and Cx43 molecules, the CT domain being an integral part of fast V(j) gating.

Gap junctions containing alpha8-connexin (MP70) in the adult mammalian lens epithelium suggests a re-evaluation of its role in the lens

A missense mutation in one of the three lens connexins, alpha8-connexin, has been recently shown to be the genetic basis of the zonular pulverant lens cataract. This connexin had been considered to be expressed only in lens fibre cells. The present studies show that alpha8-connexin is also expressed in the lens epithelial cell layer. For this study, the distribution of gap junctions in the adult bovine lens has been investigated by confocal immunofluorescence microscopy using antibodies against alpha8-connexin (MP70) and alpha1-connexin (Cx43). In addition to the anticipated localisation of alpha8-connexin to the broad faces of lens fibre cells as reported in other species, alpha8-connexin was also found colocalized with alpha1-connexin at plaques in the lateral epithelial-epithelial plasma membranes of the bovine lens. These data suggest that mixed alpha8-connexin/alpha1-connexin plaques are between epithelial cells at their apico-lateral plasma membranes, rather than between epithelial and fibre cells. Indeed, freeze fracture analyses of the epithelial-fibre cell interface failed to reveal gap junctions connecting the epithelium and the underlying fibre cells. Importantly, microdissection and subsequent immunoblotting of lens epithelium samples confirmed the immunolocalisation results. The data suggest mature mammalian lens epithelial cells could form either heteromeric, heterotypic and/or mixed homomeric-homotypic gap junctional complexes with unique physiological properties, an important point when considering the role of epithelial cell connexins in cataractogenesis.

Trafficking, assembly, and function of a connexin43-green fluorescent protein chimera in live mammalian cells

To examine the trafficking, assembly, and turnover of connexin43 (Cx43) in living cells, we used an enhanced red-shifted mutant of green fluorescent protein (GFP) to construct a Cx43-GFP chimera. When cDNA encoding Cx43-GFP was transfected into communication-competent normal rat kidney cells, Cx43-negative Madin-Darby canine kidney (MDCK) cells, or communication-deficient Neuro2A or HeLa cells, the fusion protein of predicted length was expressed, transported, and assembled into gap junctions that exhibited the classical pentalaminar profile. Dye transfer studies showed that Cx43-GFP formed functional gap junction channels when transfected into otherwise communication-deficient HeLa or Neuro2A cells. Live imaging of Cx43-GFP in MDCK cells revealed that many gap junction plaques remained relatively immobile, whereas others coalesced laterally within the plasma membrane. Time-lapse imaging of live MDCK cells also revealed that Cx43-GFP was transported via highly mobile transport intermediates that could be divided into two size classes of <0.5 microm and 0.5-1.5 microm. In some cases, the larger intracellular Cx43-GFP transport intermediates were observed to form from the internalization of gap junctions, whereas the smaller transport intermediates may represent other routes of trafficking to or from the plasma membrane. The localization of Cx43-GFP in two transport compartments suggests that the dynamic formation and turnover of connexins may involve at least two distinct pathways.

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

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

Intracellular trafficking pathways in the assembly of connexins into gap junctions

Trafficking pathways underlying the assembly of connexins into gap junctions were examined using living COS-7 cells expressing a range of connexin-aequorin (Cx-Aeq) chimeras. By measuring the chemiluminescence of the aequorin fusion partner, the translocation of oligomerized connexins from intracellular stores to the plasma membrane was shown to occur at different rates that depended on the connexin isoform. Treatment of COS-7 cells expressing Cx32-Aeq and Cx43-Aeq with brefeldin A inhibited the movement of these chimera to the plasma membrane by 84 +/- 4 and 88 +/- 4%, respectively. Nocodazole treatment of the cells expressing Cx32-Aeq and Cx43-Aeq produced 29 +/- 16 and 4 +/- 7% inhibition, respectively. In contrast, the transport of Cx26 to the plasma membrane, studied using a construct (Cx26/43T-Aeq) in which the short cytoplasmic carboxyl-terminal tail of Cx26 was replaced with the extended carboxyl terminus of Cx43, was inhibited 89 +/- 5% by nocodazole and was minimally affected by exposure of cells to brefeldin A (17 +/-11%). The transfer of Lucifer yellow across gap junctions between cells expressing wild-type Cx32, Cx43, and the corresponding Cx32-Aeq and Cx43-Aeq chimeras was reduced by nocodazole treatment and abolished by brefeldin A treatment. However, the extent of dye coupling between cells expressing wild-type Cx26 or the Cx26/43T-Aeq chimeras was not significantly affected by brefeldin A treatment, but after nocodazole treatment, transfer of dye to neighboring cells was greatly reduced. These contrasting effects of brefeldin A and nocodazole on the trafficking properties and intercellular dye transfer are interpreted to suggest that two pathways contribute to the routing of connexins to the gap junction.

Trafficking pathways leading to the formation of gap junctions

This chapter reports the mechanisms resulting in the assembly of gap junction intercellular communication channels. The connexin channel protein subunits are required to oligomerize into hexameric hemichannels (connexons) that may be homoor heteromeric in composition. Pairing of connexons in contacting cells leads to the formation of a gap junction unit. Subcellular fractionation studies using guinea-pig liver showed that oligomerization of connexins was complete on entry into Golgi, and that connexons showed heteromeric properties. The low ratio of connexin26 (Cx26; beta 2) relative to Cx32 (beta 1) in endomembranes compared to the approximately equal ratios found in plasma membranes and gap junctions suggest that Cx26 takes a non-classical route to the plasma membrane. Cultured cells, expressing connexin-aequorin chimeras, also provided evidence that Cx26 takes a more rapid non-classical route to the plasma membrane, because brefeldin A, a drug that disrupts the Golgi, had minimal effects on trafficking of Cx26 to the plasma membrane in contrast to its disruption of Cx32 trafficking. Finally, a cell-free approach for studying synthesis of connexons provided further evidence that Cx26 showed membrane insertion properties compatible with a more direct intracellular route to gap junctions. The presence of dual gap junction assembly pathways can explain many of the differential properties exhibited by connexins in cells.

Connexin-aequorin chimerae report cytoplasmic calcium environments along trafficking pathways leading to gap junction biogenesis in living COS-7 cells

The cytoplasmic calcium environments along membrane trafficking pathways leading to gap junction intercellular communication channels at the plasma membrane were studied. Connexins, the constitutive proteins of gap junctions, were fused at their carboxyl terminus to the calcium-sensitive photoprotein aequorin. The cellular location of the chimeric proteins was determined by immunolocalization and subcellular fractionation. The generation of functional gap junctions by the connexin chimerae was monitored by the ability of the cells to exchange small dyes. Although aequorin fused to connexin-26 was nonfunctional, its ability to report Ca2+ and to form functional gap junctions was rescued by replacement of its cytoplasmic carboxyl tail with that of connexin-43. In COS-7 cells expressing these connexin-aequorin chimerae, calcium levels below the plasma membrane were higher (approximately 5 microM) than those in the cytoplasm (approximately 100 nM); gap junctions were able to transfer dyes under these conditions. Cytoplasmic levels of free calcium surrounding the ERGIC/Golgi reported by connexin-43 chimera (approximately 420 nM) were twice those measured by connexin-32 chimera (approximately 200 nM); both chimerae measured calcium levels substantially higher than those reported by a connexin-26 chimera (approximately 130 nM). Dispersion of the ERGIC and Golgi complex by brefeldin A led to a marked reduction in calcium levels. The results show that the various connexin chimerae were located in spatially different subcellular stores and that the ERGIC/Golgi regions of the cell maintain heterogeneous cytoplasmic domains of calcium. The implications of the subplasma-membrane Ca2+ levels on the gating of gap junctions are discussed.

High micromolar Ca2+ beneath the plasma membrane in stimulated neutrophils

Ca2+ near the inner face of the plasma membrane, as reported by the membrane associated fluorescent Ca2+ probe FFP-18, was higher than the bulk cytosolic free Ca2+ concentration both in resting neutrophils and in response to f-met-leu-phe. Influx caused Ca2+ close to the plasma membrane to rise more rapidly than the bulk cytosolic free Ca2+ and to reach a peak concentration of at least 30 microM. This zone of high Ca2+ was localised to just beneath the plasma membrane and did not extend more than 0.1 micron into the cell, as it was undetected by the bulk cytosolic free Ca2+ probes magfura2 and fura2. From these data, reconstruction of the distribution of Ca2+ within the neutrophil showed that the high Ca2+ signal at the cell cortex rapidly subsided to give a uniform free Ca2+ across the cell.

Rapid determination of gap junction formation using HeLa cells microinjected with cDNAs encoding wild-type and chimeric connexins

A procedure for rapidly determining the functionality of gap junctions constructed of recombinant connexins in communication-deficient HeLa cells is described. Nuclear microinjection of cDNA encoding wild-type connexins (Cx) 26, 32, 43, and a range of connexin-aequorin (Cx-Aeq) chimerase resulted in generation of gap junction intercellular communication channels. Expression of recombinant protein was detected in > 95% of cells 18-72 h following nuclear microinjection, and the functionality of the channels generated was determined according to their ability to transfer the fluorescent dye tracers Lucifer yellow and propidium iodide. The dye transfer results obtained correlated closely with other published studies using stably transfected cells and yet are obtained as rapidly as 18 h following microinjection of cDNA. Expression of a truncated form of Cx43 (Cx43 delta 244) by this new method indicated diminished intercellular transfer of both dyes and supports a channel-gating mechanism that postulates interaction between the carboxyl tail and the intracellular loop.