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

Endothelial TRPV4-Cx43 signalling complex regulates vasomotor tone in resistance arteries

S-nitrosylation of Cx43 gap junction channels critically regulates communication between smooth muscle cells and endothelial cells. This post-translational modification also induces the opening of undocked Cx43 hemichannels. However, its specific impact on vasomotor regulation remains unclear. Considering the role of endothelial TRPV4 channel activation in promoting vasodilatation through nitric oxide (NO) production, we investigated the direct modulation of endothelial Cx43 hemichannels by TRPV4 channel activation. Using the proximity ligation assay, we identified that Cx43 and TRPV4 are found in close proximity in the endothelium of resistance arteries. In primary endothelial cell (EC) cultures from resistance arteries, GSK 1016790A-induced TRPV4 activation enhances eNOS activity, increases NO production, and opens Cx43 hemichannels via direct S-nitrosylation. Notably, the elevated intracellular Ca2+ levels caused by TRPV4 activation were reduced by blocking Cx43 hemichannels. In ex vivo mesenteric arteries, inhibiting Cx43 hemichannels reduced endothelial hyperpolarization without affecting NO production in ECs, underscoring a critical role of TRPV4-Cx43 signalling in endothelial electrical behaviour. We perturbed the proximity of Cx43/TRPV4 by disrupting lipid rafts in ECs using β-cyclodextrin. Under these conditions, hemichannel activity, Ca2+ influx and endothelial hyperpolarization were blunted upon GSK stimulation. Intravital microscopy of mesenteric arterioles in vivo further demonstrated that inhibiting Cx43 hemichannel activity, NO production and disrupting endothelial integrity reduce TRPV4-induced relaxation. These findings underscore a new pivotal role of the Cx43 hemichannel associated with the TRPV4 signalling pathway in modulating endothelial electrical behaviour and vasomotor tone regulation. KEY POINTS: TRPV4-Cx43 interaction in endothelial cells: the study reveals a close proximity between Cx43 proteins and TRPV4 channels in endothelial cells of resistance arteries, establishing a functional interaction that is critical for vascular regulation. S-nitrosylation of Cx43 hemichannels: TRPV4 activation via GSK treatment induces S-nitrosylation of Cx43, facilitating the opening of Cx43 hemichannels. TRPV4-mediated calcium signalling: activation of TRPV4 leads to increased intracellular Ca2+ levels in endothelial cells, an effect that is mitigated by the inhibition of Cx43 hemichannels, indicating a regulatory feedback mechanism between these two channels. Endothelial hyperpolarization and vasomotor regulation: Blocking Cx43 hemichannels impairs endothelial hyperpolarization in mesenteric arteries, without affecting NO production, suggesting a role for Cx43 in modulating endothelial electrical behaviour and contributing to vasodilatation. In vivo role of Cx43 hemichannels in vasodilatation: intravital microscopy of mouse mesenteric arterioles demonstrated that inhibiting Cx43 hemichannel activity and disrupting endothelial integrity significantly impair TRPV4-induced vasodilatation, highlighting the crucial role of Cx43 in regulating endothelial function and vascular relaxation.

Endothelial TRPV4/Cx43 Signaling Complex Regulates Vasomotor Tone in Resistance Arteries

S-nitrosylation of Cx43 gap junction channels critically regulates communication between smooth muscle cells and endothelial cells. This posttranslational modification also induces the opening of undocked Cx43 hemichannels. However, its specific impact on vasomotor regulation remains unclear. Considering the role of endothelial TRPV4 channel activation in promoting vasodilation through nitric oxide (NO) production, we investigated the direct modulation of endothelial Cx43 hemichannels by TRPV4 channel activation. Using the proximity ligation assay, we identify that Cx43 and TRPV4 are found in close proximity in the endothelium of resistance arteries. In primary endothelial cell cultures from resistance arteries (ECs), GSK-induced TRPV4 activation enhances eNOS activity, increases NO production, and opens Cx43 hemichannels via direct S-nitrosylation. Notably, the elevated intracellular Ca2+ levels caused by TRPV4 activation were reduced by blocking Cx43 hemichannels. In ex vivo mesenteric arteries, inhibiting Cx43 hemichannels reduced endothelial hyperpolarization without affecting NO production in ECs, underscoring a critical role of TRPV4/Cx43 signaling in endothelial electrical behavior. We perturbed the proximity of Cx43/TRPV4 by disrupting lipid rafts in ECs using β-cyclodextrin. Under these conditions, hemichannel activity, Ca2+ influx, and endothelial hyperpolarization were blunted upon GSK stimulation. Intravital microscopy of mesenteric arterioles in vivo further demonstrated that inhibiting Cx43 hemichannels activity, NO production and disrupting endothelial integrity reduce TRPV4-induced relaxation. These findings underscore a new pivotal role of Cx43 hemichannel associated with TRPV4 signaling pathway in modulating endothelial electrical behavior and vasomotor tone regulation.

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.

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.

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+)].

Cardiomyocyte FGF signaling is required for Cx43 phosphorylation and cardiac gap junction maintenance

Cardiac remodeling resulting from impairment of myocardial integrity leads to heart failure, through still incompletely understood mechanisms. The fibroblast growth factor (FGF) system has been implicated in tissue maintenance, but its role in the adult heart is not well defined. We hypothesized that the FGF system plays a role in the maintenance of cardiac homeostasis, and the impairment of cardiomyocyte FGF signaling leads to pathological cardiac remodeling. We showed that FGF signaling is required for connexin 43 (Cx43) localization at cell-cell contacts in isolated cardiomyocytes and COS7 cells. Lack of FGF signaling led to decreased Cx43 phosphorylation at serines 325/328/330 (S325/328/330), sites known to be important for assembly of gap junctions. Cx43 instability induced by FGF inhibition was restored by the Cx43 S325/328/330 phospho-mimetic mutant, suggesting FGF-dependent phosphorylation of these sites. Consistent with these in vitro findings, cardiomyocyte-specific inhibition of FGF signaling in adult mice demonstrated mislocalization of Cx43 at intercalated discs, whereas localization of N-cadherin and desmoplakin was not affected. This led to premature death resulting from impaired cardiac remodeling. We conclude that cardiomyocyte FGF signaling is essential for cardiomyocyte homeostasis through phosphorylation of Cx43 at S325/328/330 residues which are important for the maintenance of gap junction.

A high-throughput assay for connexin 43 (Cx43, GJA1) gap junctions using codon-optimized aequorin

Gap junctions (GJs) are intercellular channels which are composed of the connexin family of proteins that allow electrical and chemical communications and synchronization in tissue ensembles. Evidence suggests that pharmaceutical modulators of these channels may have therapeutic potential or carry undesired liability. In this report, we exogenously expressed human connexin 43 (Cx43, GJA1) and demonstrated functionality in a 96-well flow cytometry assay detecting intercellular transfer of the calcein dye. We have designed a 384-well high-throughput method for detecting the transfer of calcium between HeLa cells expressing Cx43. In this assay, donor cells coexpress Cx43 and the α1A adrenergic Gα-coupled receptor, while recipient cells coexpress Cx43 and the cytoplasmic version of the calcium-sensitive luminescent protein aequorin enhanced by codon optimization (cytoAeq). The two cell populations were mixed, dispensed to 384-well plates, and incubated for 3 h to allow the formation of GJs. Activation of α1A by epinephrine in donor cells led to dose-dependent calcium increases in recipient cells, which were detected by measuring the intensity of aequorin luminescence. The response was dependent on the expression of Cx43 and inhibited by the GJ blocker 18α-glycyrrhetinic acid, suggesting Cx43 GJ-mediated activity. In a parallel experiment with capsaicin and the TrpV1 ion channel in place of phenylephrine and α1A, a similar magnitude of difference in the maximal calcium response was detected in both donor and recipient cells, suggesting that calcium is likely the permeant ion through the GJ. This assay may pave the way for high-throughput screening of GJ modulators for drug discovery.

Synchronous intra-Golgi transport induces the release of Ca2+ from the Golgi apparatus

The mechanisms of secretory transport through the Golgi apparatus remain an issue of debate. The precise functional importance of calcium ions (Ca(2+)) for intra-Golgi transport has also been poorly studied. Here, using different approaches to measure free Ca(2+) concentrations in the cell cytosol ([Ca(2+)](cyt)) and inside the lumen of the Golgi apparatus ([Ca(2+)](GA)), we have revealed transient increases in [Ca(2+)](cyt) during the late phase of intra-Golgi transport that are concomitant with a decline in the maximal [Ca(2+)](GA) restoration ability. Thus, this redistribution of Ca(2+) from the Golgi apparatus into the cytosol during the movement of cargo through the Golgi apparatus appears to have a role in intra-Golgi transport, and mainly in the late Ca(2+)-dependent phase of SNARE-regulated fusion of Golgi compartments.

The use of aequorins to record and visualize Ca(2+) dynamics: from subcellular microdomains to whole organisms

In this chapter, we describe the practical aspects of measuring [Ca(2+)] transients that are generated in a particular cytoplasmic domain, or within a specific organelle or its periorganellar environment, using bioluminescent, genetically encoded and targeted Ca(2+) reporters, especially those based on apoaequorin. We also list examples of the organisms, tissues, and cells that have been transfected with apoaequorin or an apoaequorin-BRET complex, as well as of the organelles and subcellular domains that have been specifically targeted with these bioluminescent Ca(2+) reporters. In addition, we summarize the various techniques used to load the apoaequorin cofactor, coelenterazine, and its analogs into cells, tissues, and intact organisms, and we describe recent advances in the detection and imaging technologies that are currently being used to measure and visualize the luminescence generated by the aequorin-Ca(2+) reaction within these various cytoplasmic domains and subcellular compartments.

Altered expression of connexin-43 and impaired capacity of gap junctional intercellular communication in prostate cancer cells

Connexin-43 (Cx43) expression in prostate cancer (PCa) cells and the potency of gap junctional intercellular communication (GJIC) in the cells were investigated, with an attempt to elucidate the reason why the so-called "bystander effect" mediated by thymidine kinase (TK) suicide gene therapy on PCa cells is not of significance and to explore the role of GJIC in PCa carcinogenesis. mRNA and protein expression of Cx43 in a PCa cell line PC-3m was detected by reverse-transcription polymerase chain reaction (RT-PCR) and strapt-avidin-biotin-enzyme complex (SABC) immunohistochemical staining, and inherent GJIC of PC-3m cells was assayed by scrape-loading and dye transfer (SLDT) assay. The expression of Cx43 in human normal and malignant prostate tissues was determined by SABC immunohistochemistry as well. It was found that Cx43 mRNA and protein expression in PC-3m cells was slightly reduced as compared with positive controls and the location of Cx43 protein was aberrant in cytoplasm rather than on membrane. Assessment of paraffin sections demonstrated that the expression of Cx43 protein in PCa cells was abnormally located and markedly diminished as compared with normal prostatic epithelial ones, displaying a negative correlation to the pathological grade (chi2=4.025, P<0.05). Additionally, capacity of inherent GJIC in PC-3m cells was disrupted, which was semi-quantified as (+) or (-). It was indicated that both down-regulated expression of Cx43 mRNA and aberrant location of Cx43 protein participated in the mechanisms leading to deficient GJIC in PC-3m cells. Lack of efficient GJIC is a molecular event, which may contribute not only to limited extent of "bystander effect", but also to initiation and progression of prostatic neoplasm.

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.

The gap junction cellular internet: connexin hemichannels enter the signalling limelight

Cxs (connexins), the protein subunits forming gap junction intercellular communication channels, are transported to the plasma membrane after oligomerizing into hexameric assemblies called connexin hemichannels (CxHcs) or connexons, which dock head-to-head with partner hexameric channels positioned on neighbouring cells. The double membrane channel or gap junction generated directly couples the cytoplasms of interacting cells and underpins the integration and co-ordination of cellular metabolism, signalling and functions, such as secretion or contraction in cell assemblies. In contrast, CxHcs prior to forming gap junctions provide a pathway for the release from cells of ATP, glutamate, NAD+ and prostaglandin E2, which act as paracrine messengers. ATP activates purinergic receptors on neighbouring cells and forms the basis of intercellular Ca2+ signal propagation, complementing that occuring more directly via gap junctions. CxHcs open in response to various types of external changes, including mechanical, shear, ionic and ischaemic stress. In addition, CxHcs are influenced by intracellular signals, such as membrane potential, phosphorylation and redox status, which translate external stresses to CxHc responses. Also, recent studies demonstrate that cytoplasmic Ca2+ changes in the physiological range act to trigger CxHc opening, indicating their involvement under normal non-pathological conditions. CxHcs not only respond to cytoplasmic Ca2+, but also determine cytoplasmic Ca2+, as they are large conductance channels, suggesting a prominent role in cellular Ca2+ homoeostasis and signalling. The functions of gap-junction channels and CxHcs have been difficult to separate, but synthetic peptides that mimic short sequences in the Cx subunit are emerging as promising tools to determine the role of CxHcs in physiology and pathology.

Microdomains of intracellular Ca2+: molecular determinants and functional consequences

Calcium ions are ubiquitous and versatile signaling molecules, capable of decoding a variety of extracellular stimuli (hormones, neurotransmitters, growth factors, etc.) into markedly different intracellular actions, ranging from contraction to secretion, from proliferation to cell death. The key to this pleiotropic role is the complex spatiotemporal organization of the [Ca(2+)] rise evoked by extracellular agonists, which allows selected effectors to be recruited and specific actions to be initiated. In this review, we discuss the structural and functional bases that generate the subcellular heterogeneity in cellular Ca(2+) levels at rest and under stimulation. This complex choreography requires the concerted action of many different players; the central role is, of course, that of the calcium ion, with the main supporting characters being all the entities responsible for moving Ca(2+) between different compartments, while the cellular architecture provides a determining framework within which all the players have their exits and their entrances. In particular, we concentrate on the molecular mechanisms that lead to the generation of cytoplasmic Ca(2+) microdomains, focusing on their different subcellular location, mechanism of generation, and functional role.

Mechanisms of Cx43 and Cx26 transport to the plasma membrane and gap junction regeneration

Previous reports have suggested that Cx26 exhibits unique intracellular transport pathways en route to the cell surface compared with other members of the connexin family. To directly examine and compare nascent and steady-state delivery of Cx43 and Cx26 to the plasma membrane and gap junction biogenesis we expressed fluorescent-protein-tagged Cx43 and Cx26 in BICR-M1Rk and NRK cells. Static and time-lapse imaging revealed that both connexins were routed through the Golgi apparatus prior to being transported to the cell surface, a process inhibited in the presence of brefeldin A (BFA) or the expression of a dominant-negative form of Sar1 GTPase. During recovery from BFA, time-lapse imaging of nascent connexin Golgi-to-plasma membrane delivery revealed many dynamic post-Golgi carriers (PGCs) originating from the distal side of the Golgi apparatus consisting of heterogeneous vesicles and long, tubular-like extensions. Vesicles and tubular extensions were also observed in HBL-100 cells expressing a human, disease-linked, Golgi-localized Cx26 mutant, D66H-GFP. A diffuse cell surface rim of fluorescent-protein-tagged wild-type connexins was observed prior to the appearance of punctate gap junctions, which suggests that random fusion of PGCs occurred with the plasma membrane followed by lateral diffusion of connexins into clusters. Fluorescence recovery after photobleaching studies revealed that Cx26-YFP was more mobile within gap junction plaques compared with Cx43-GFP. Intriguingly, Cx43-GFP delivery and gap junction regeneration was inhibited by BFA and nocodazole, whereas Cx26-GFP delivery was prevented by BFA but not nocodazole. Collectively, these studies suggest that during gap junction biogenesis two phylogenetically distinct members of the connexin family, Cx43 and Cx26, share common secretory pathways, types of transport intermediates and turnover dynamics but differ in their microtubule-dependence and mobility within the plasma membrane, which might reflect differences in binding to protein scaffolds.

Trafficking pathways of Cx49-GFP in living mammalian cells

In the present study we examined the trafficking pathways of connexin49 (Cx49) fused to green fluorescent protein (GFP) in polar and non-polar cell lines. The Cx49 gene was isolated from ovine lens by RT-PCR. Cx49 cDNA was fused to GFP and the hybrid cDNA was transfected into several cell lines. After transfection of Cx49-GFP cDNA into HeLa cells, it was shown using the double whole-cell patch-clamp technique that the expressed fusion protein was still able to form conducting gap junction channels. Synthesis, assembly, and turnover of the Cx49-GFP hybrid protein were investigated using a pulse-chase protocol. A major 78-kDa protein band corresponding to Cx49-GFP could be detected with a turnover of 16-20 h and a half-life time of 10 h. The trafficking pathways of Cx49-GFP were monitored by confocal laser microscopy. Fusion proteins were localized in subcellular compartments, including the endoplasmic reticulum (ER), the ER-Golgi intermediate compartment, the Golgi apparatus, and the trans-Golgi network, as well as vesicles traveling towards the plasma membrane. Time-dependent sequential localization of Cx49-GFP in the ER and then the Golgi apparatus supports the notion of a slow turnover of Cx49-GFP compared to other connexins analyzed so far. Gap junction plaques resembling the usual punctuate distribution pattern could be demonstrated for COS-1 and MDCK cells. Basolateral distribution of Cx49-GFP was observed in polar MDCK cells, indicating specific sorting behavior of Cx49 in polarized cells. Together, this report describes the first characterization of biosynthesis and trafficking of lens Cx49.

Examining intracellular organelle function using fluorescent probes: from animalcules to quantum dots

Fluorescence microscopy imaging has become one of the most useful techniques to assess the activity of individual cells, subcellular trafficking of signals to and between organelles, and to appreciate how organelle function is regulated. The past 2 decades have seen a tremendous advance in the rational design and development in the nature and selectivity of probes to serve as reporters of the intracellular environment in live cells. These probes range from small organic fluorescent molecules to fluorescent biomolecules and photoproteins ingeniously engineered to follow signaling traffic, sense ionic and nonionic second messengers, and report various kinase activities. These probes, together with recent advances in imaging technology, have enabled significantly enhanced spatial and temporal resolution. This review summarizes some of these developments and their applications to assess intracellular organelle function.

Roles of Met-34, Cys-64, and Arg-75 in the assembly of human connexin 26. Implication for key amino acid residues for channel formation and function

Connexins form a family of membrane proteins that assemble into communication channels and directly connect the cytoplasms of adjoining cells. Malfunctioning of connexin channels often cause disease, such as the mutations M34T and R75W in human connexin 26, which are associated with hereditary deafness. Another residue known to be essential for normal channel activity in the connexin is Cys-64. To obtain structural and functional insights of connexin 26, we studied the roles of these three residues by expressing mutant connexins in insect Sf9 and HeLa cells. The M34T and M34A mutants both formed gap junction plaques, but dye transfer assays showed that the M34A mutant had a significantly reduced permeability, suggesting that for proper channel function a side chain of adequate size is required at this position. We propose that Met-34 is located in the innermost helix of the channel, where it ensures a fully open channel structure via interactions with other transmembrane helices. Gap junction channels formed by the R75W and R75D mutants dissociated upon solubilization in dodecyl maltoside, whereas the R75A mutant remained hexameric. All gap junctions formed by Arg-75 mutants also showed only negligible activity in dye transfer experiments. These results suggest that residue Arg-75 plays a role in subunit interactions needed to retain a functional and stable connexin hexamer. The C64S mutant was suggested to be defective in oligomerization and/or protein folding even in the presence of wild-type connexin.

Lighting up gap junction channels in a flash

Gap junction intercellular communication channels permit the exchange of small regulatory molecules and ions between neighbouring cells and coordinate cellular activity in diverse tissue and organ systems. These channels have short half-lives and complex assembly and degradation pathways. Much of the recent work elucidating gap junction biogenesis has featured the use of connexins (Cx), the constituent proteins of gap junctions, tagged with reporter proteins such as Green Fluorescent Protein (GFP) and has illuminated the dynamics of channel assembly in live cells by high-resolution time-lapse microscopy. With some studies, however, there are potential short-comings associated with the GFP chimeric protein technologies. A recent report by Gaietta et al., has highlighted the use of recombinant proteins with tetracysteine tags attached to the carboxyl terminus of Cx43, which differentially labels 'old' and 'new' connexins thus opening up new avenues for studying temporal and spatial localisation of proteins and in situ trafficking events.

Selective effects of calcium chelators on anterograde and retrograde protein transport in the cell

Calcium plays a regulatory role in several aspects of protein trafficking in the cell. Both vesicle fusion and vesicle formation can be inhibited by the addition of calcium chelators. Because the effects of calcium chelators have been studied predominantly in cell-free systems, it is not clear exactly which transport steps in the secretory pathway are sensitive to calcium levels. In this regard, we have studied the effects of calcium chelators on both anterograde and retrograde protein transport in whole cells. Using both cytochemical and biochemical analyses, we find that the anterograde-directed exit of vesicular stomatitis virus G protein and the retrograde-directed exit of Shiga toxin from the Golgi apparatus are both inhibited by calcium chelation. The exit of vesicular stomatitis virus G from a pre-Golgi compartment and the exit of Shiga toxin from an endosomal compartment are sensitive to the membrane-permeant calcium chelator 1,2-bis(2-amino phenoxy)ethane-N,N,N',N'-tetraacetic acid-tetrakis (acetoxymethyl ester) (BAPTA-AM). By contrast, endoplasmic reticulum exit and endocytic internalization from the plasma membrane are not affected by BAPTA. Together, our data show that some, but not all, trafficking steps in the cell may be regulated by calcium. These studies provide a framework for a more detailed analysis of the role of calcium as a regulatory agent during protein transport.

Post-translational integration and oligomerization of connexin 26 in plasma membranes and evidence of formation of membrane pores: implications for the assembly of gap junctions

Gap-junction channels provide a widespread intercellular signalling mechanism. They are constructed of a family of connexin membrane proteins that thread across the membrane four times and oligomerize to generate hexameric gap-junction hemichannels. Using an in vitro cell-free transcription/translation system, we demonstrate that connexin (Cx) 26, one of the smallest connexins, is integrated directly in a post-translational manner into plasma membranes. Protein-cleavage studies of Cx26 integrated into plasma membranes indicate a similar native transmembrane topography to that of Cx26 integrated co-translationally into microsomes. Cx26 integrated post-translationally into plasma membranes oligomerizes and, when incorporated into liposomes, provides permeability to ascorbic acid, suggesting that gap-junction hemichannels are generated. The results provide the basis of a novel alternative mechanism for spontaneous assembly in plasma membranes of Cx26 gap-junction hemichannels that occurs independently of the conventional biogenesis of gap junctions involving connexin trafficking and oligomerization via membrane components of the secretory pathway.

Chemical approaches to the investigation of cellular systems

Biochemistry in the context of a living cell or organism is complicated by many variables such as supramolecular organization, cytoplasmic viscosity, and substrate heterogeneity. While these variables are easily excluded or avoided in reconstituted systems, they must be dealt with in cellular environments. New developments have allowed researchers to begin probing the inner workings of the cell to gain new insight into cell function and metabolism. Advances in cellular imaging and in small molecule-controlled gene expression, signal transduction and cell surface modification are discussed in this review. These techniques have permitted the study of molecular components within the context of living cells.

Endothelium-derived hyperpolarizing factor: is there a novel chemical mediator?

1. Endothelium-derived hyperpolarization (EDH) has been reported in many vessels and an extensive literature suggests that a novel, non-nitric oxide and non-prostanoid, endothelium-derived factor(s) may be synthesized in endothelial cells. 2. The endothelium-dependent hyperpolarizing factor, or EDHF, is synthesized by the putative EDHF synthase and mediates its cellular effects by either, directly or indirectly, opening K channels on vascular smooth muscle cells or, via hyperpolarization of the endothelial cell, by facilitating electrical coupling between the endothelial and the vascular smooth muscle cell. 3. The question of the chemical identity of EDHF has received considerable attention; however, no consensus has been reached. Tissue and species heterogeneity exists that may imply there are multiple EDHF. Leading candidate molecules for EDHF include an arachidonic acid product, possibly an epoxygenase product, or an endogenous cannabinoid, or simply an increase in extracellular K+. 4. An increasing body of evidence suggests that EDH, notably in the resistance vasculature, may be mediated via electrical coupling through myoendothelial gap junctions and the existence of electrical coupling may negate the need to hypothesize the existence of a true endothelium-derived chemical mediator. 5. In this paper we review the evidence that supports and refutes the existence of a novel EDHF versus a hyperpolarization event mediated solely by myoendothelial gap junctions.

A dominant negative mutation of neuronal connexin 36 that blocks intercellular permeability

Rat connexin 36 (Cx36) was mutated by substituting serine for cysteine at residue 231 (C231S) and the mutant's effect on the subcellular localization of wild-type Cx36 and the intercellular permeability that it confers was determined in human HeLa and rat PC12 cells. Cells transfected with the mutant or wild-type Cx36 cDNA expressed the expected 36 kDa protein and Cx36 immunoreactivity. Co-immunoprecipitation experiments with monkey COS-7 cells transiently transfected with both mutant and wild-type Cx36 cDNAs demonstrated that the mutant protein bound to the wild-type. Double immunofluorescence microscopy of stably transfected HeLa cells demonstrated that mutant Cx36 blocked the transport of the wild-type Cx36 to the cell membrane, primarily by trapping it in the endoplasmic reticulum around the nucleus. Coexpression of the mutant Cx36 with the wild-type protein abolished the ability of the latter to permit dye transfer in both HeLa and PC12 cells. The findings are the first demonstration of a mutation of Cx36 that inhibits wild-type Cx36 function in mammalian cells.

Multimeric connexin interactions prior to the trans-Golgi network

Cells that express multiple connexins have the capacity to form heteromeric (mixed) gap junction hemichannels. We used a dominant negative connexin construct, consisting of bacterial β-galactosidase fused to the C terminus of connexin43 (Cx43/β-gal), to examine connexin compatibility in NIH 3T3 cells. Cx43/β-gal is retained in a perinuclear compartment and inhibits Cx43 transport to the cell surface. The intracellular connexin pool induced by Cx43/β-gal colocalized with a medial Golgi apparatus marker and was readily disassembled by treatment with brefeldin A. This was unexpected, since previous studies indicated that Cx43 assembly into hexameric hemichannels occurs in the trans-Golgi network (TGN) and is sensitive to brefeldin A. Further analysis by sucrose gradient fractionation showed that Cx43 and Cx43/β-gal were assembled into a subhexameric complex. Cx43/β-gal also specifically interacted with Cx46, but not Cx32, consistent with the ability of Cx43/β-gal to simultaneously inhibit multiple connexins. We confirmed that interactions between Cx43/β-gal and Cx46 reflect the ability of Cx43 and Cx46 to form heteromeric complexes, using HeLa and alveolar epithelial cells, which express both connexins. In contrast, ROS osteoblastic cells, which differentially sort Cx43 and Cx46, did not form Cx43/Cx46 heteromers. Thus, cells have the capacity to regulate whether or not compatible connexins intermix.

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.

C-Terminal and N-Terminal Fusions of Aequorin with Small Peptides in Immunoassay Development

Aequorin fusion proteins have been used extensively in intracellular Ca2+ measurements and in the development of binding assays. Gene fusions to aequorin for production of fusion proteins have been so far limited to its N-terminus, as previous studies have indicated that aequorin loses its activity upon modification of its C-terminus. To further investigate this, two model peptides, an octapeptide (DTLDDDDL), and leu-enkephalin (TGGFL), an opioid peptide, were fused to the C-terminus of a cysteine-free mutant of aequorin through genetic engineering. The octapeptide was also fused to the N-terminus of the aequorin-leu-enkephalin fusion protein, which enables its affinity purification. Contrary to reports of earlier studies, we found that aequorin retains its bioluminescence activity after modification of the C-terminus. The half-life of light emission and the calibration curves obtained with the fusion proteins were comparable to those of the cysteine-free mutant of aequorin. Dose-response curves for the octapeptide were generated using two aequorin-octapeptide fusion proteins with the octapeptide fused to the N-terminus in one case, and to the C-terminus in the other. Similar detection limits for the octapeptide were obtained using both fusion proteins. The C-terminal fusion system has advantages in cases where antibodies recognize only the C-terminus of the peptide, as well as in cases where the functionality of the peptide lies in its C-terminus. The purification is also simplified as the affinity tag can be engineered at one terminus and the peptide of interest at the other.

Role of gap junctions in endothelium-derived hyperpolarizing factor responses and mechanisms of K(+)-relaxation

We have examined the effects of ouabain (1 mM), the gap junction inhibitors, 18 alpha-glycyrrhetinic acid (100 microM), N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2, 4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR141716A; 10 microM) and palmitoleic acid (50 microM), and clotrimazole (10 microM) against endothelium-derived hyperpolarizing factor (EDHF)-mediated and K(+)-induced vasorelaxations in the rat mesentery. In the presence of indomethacin (10 microM) and 300-microM N(G)nitro-L-arginine methyl ester (L-NAME), carbachol caused EDHF-mediated relaxations (R(max)=85.3+/-4.0%). In the presence of ouabain, these responses were substantially reduced (R(max)=11.0+/-2.3%). 18 alpha-glycyrrhetinic acid, SR141716A, palmitoleic acid and clotrimazole also significantly inhibited these EDHF-mediated responses. K(+) caused vasorelaxation of preparations perfused with K(+)-free buffer (R(max)=73.7+/-2.4%), which were reduced by 10-microM indomethacin (R(max)=56.4+/-6.2%). K(+) vasorelaxation was essentially abolished by endothelial denudation. Both ouabain and 18 alpha-glycyrrhetinic acid opposed K(+) relaxations, however, neither SR141716A, clotrimazole nor palmitoleic acid had any effect. Direct cell-cell coupling via gap junctions was attenuated by ouabain, clotrimazole and palmitoleic acid. We conclude that: (i) that gap junctional communication plays a major role in EDHF-mediated relaxations, (ii) that K(+)-vasorelaxation is endothelium-dependent (thus, K(+) is unlikely to represent an EDHF), and (iii) that the inhibitory actions of ouabain and clotrimazole on gap junctions might contribute towards their effects against EDHF.

Biosynthesis and structural composition of gap junction intercellular membrane channels

Gap junction channels assemble as dodecameric complexes, in which a hexameric connexon (hemichannel) in one plasma membrane docks end-to-end with a connexon in the membrane of a closely apposed cell to provide direct cell-to-cell communication. Synthesis, assembly, and trafficking of the gap junction channel subunit proteins referred to as connexins, largely appear to follow the general secretory pathway for membrane proteins. The connexin subunits can assemble into homo-, as well as distinct hetero-oligomeric connexons. Assembly appears to be based on specific signals located within the connexin polypeptides. Plaque formation by the clustering of gap junction channels in the plane of the membrane, as well as channel degradation are poorly understood processes that are topics of current research. Recently, we tagged connexins with the autofluorescent reporter green fluorescent protein (GFP), and its cyan (CFP), and yellow (YFP) color variants and combined this reporter technology with single, and dual-color, high resolution deconvolution microscopy, computational volume rendering, and time-lapse microscopy to examine the detailed organization, structural composition, and dynamics of gap junctions in live cells. This technology provided for the first time a realistic, three-dimensional impression of gap junctions as they appear in the plasma membranes of adjoining cells, and revealed an excitingly detailed structural organization of gap junctions never seen before in live cells. Here, I summarize recent progress in areas encompassing the synthesis, assembly and structural composition of gap junctions with a special emphasis on the recent results we obtained using cell-free translation/ membrane-protein translocation, and autofluorescent reporters in combination with live-cell deconvolution microscopy.

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.

Bioluminescence detection of proteolytic bond cleavage by using recombinant aequorin

Detection of proteolytic bond cleavage was achieved by taking advantage of the bioluminescence emission generated by the photoprotein aequorin. A genetically engineered HIV-1 protease substrate was coupled with a cysteine-free mutant of aequorin by employing the polymerase chain reaction to produce a fusion protein that incorporates an optimum natural protease cleavage site. The fusion protein was immobilized on a solid phase and employed as the substrate for the HIV-1 protease. Proteolytic bond cleavage was detected by a decrease in the bioluminescence generated by the aequorin fusion protein on the solid phase. A dose-response curve for HIV-1 protease was constructed by relating the decrease in bioluminescence signal with varying amounts of the protease. The system was also used to evaluate two competitive and one noncompetitive inhibitor of the HIV-1 protease. Among the advantages of this assay is that by using recombinant methods a complete bioluminescently labeled protease recognition site can be designed and produced. The assay yields very sensitive detection limits, which are inherent to bioluminescence-based methods. An application of this system may be in the high-throughput screening of biopharmaceutical drugs that are potential inhibitors of a target protease.

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.

Cell-free synthesis for analyzing the membrane integration, oligomerization, and assembly characteristics of gap junction connexins

For gap junction channels to function, their subunit proteins, referred to as connexins, have to be synthesized and inserted into the cell membrane in their native configuration. Like other transmembrane proteins, connexins are synthesized and inserted cotranslationally into the endoplasmic reticulum membrane. Membrane insertion is followed by their assembly and transport to the plasma membrane. Finally, the end-to-end pairing of two half-channels, referred to as connexons, each provided by one of two neighboring cells, and clustering of the channels into larger plaques complete the gap junction channel formation. Gap junction channel formation is further complicated by the potential assembly of homo- as well as heterooligomeric connexons, and the pairing of identical or different connexons into homo- and heterotypic gap junction channels. In this article, I describe the cell-free synthesis approach that we have used to study the biosynthesis of connexins and gap junction channels. Special emphasis is placed on the synthesis of full-length, membrane-integrated connexins, assembly into gap junction connexons, homo- as well as heterooligomerization, and characterization of connexin-specific assembly signals.

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.

Dynamics of gap junctions observed in living cells with connexin43-GFP chimeric protein

To study the aggregation of cell-to-cell channels into gap junctions at individual cell-cell contacts, we transfected cells with an expression vector for a chimeric protein composed of the cell-to-cell channel protein connexin43 and a green fluorescent protein. The chimeric channel protein was visualized in the fluorescence microscope and was found to form gap junctions at the cell-cell contacts just like wild-type connexin43. Cells expressing the chimeric protein had functional cell-to-cell channels. Using timelapse videomicroscopy on live cells we observed individual gap junctions over long periods and recorded the time course of aggregation of the chimeric channel protein into gap junctions at newly formed cell-cell contacts. We found that individual small gap junctions were very dynamic, moving about or becoming assembled and disassembled in the course of minutes. Larger gap junctions were more stable than small punctate ones. In control condition, stable new gap junctions were not formed during observation times of 30 min or longer. But at elevated levels of cyclic adenosine monophosphate, the chimeric channel protein began aggregating at new junctions 5-10 minutes after cell-cell contact and continued to concentrate there for at least one hour. Also already established junctions grew in size. The fluorescent chimeric channel protein will be an excellent tool to investigate the regulation of trafficking of connexin from and to the membrane and the mechanism of connexin channel aggregation into gap junctions.

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.

The endothelial component of cannabinoid-induced relaxation in rabbit mesenteric artery depends on gap junctional communication

1. We have shown that the endocannabinoid anandamide and its stable analogue methanandamide relax rings of rabbit superior mesenteric artery through endothelium-dependent and -independent mechanisms that are unaffected by blockade of NO synthase and cyclooxygenase. 2. The endothelium-dependent component of the responses was attenuated by the gap junction inhibitor 18alpha-glycyrrhetinic acid (18alpha-GA; 50 microM), and a synthetic connexin-mimetic peptide homologous to the extracellular Gap 27 sequence of connexin 43 (43Gap 27, SRPTEKTIFII; 300 microM). By contrast, the corresponding connexin 40 peptide (40Gap 27, SRPTEKNVFIV) was inactive. 3. The cannabinoid CB1 receptor antagonist SR141716A (10 microM) also attenuated endothelium-dependent relaxations but this inhibition was not observed with the CB1 receptor antagonist LY320135 (10 microM). Furthermore, SR141716A mimicked the effects of 43Gap 27 peptide in blocking Lucifer Yellow dye transfer between coupled COS-7 cells (a monkey fibroblast cell line), whereas LY320135 was without effect, thus suggesting that the action of SR141716A was directly attributable to effects on gap junctions. 4. The endothelium-dependent component of cannabinoid-induced relaxation was also attenuated by AM404 (10 microM), an inhibitor of the high-affinity anandamide transporter, which was without effect on dye transfer. 5. Taken together, the findings suggest that cannabinoids derived from arachidonic acid gain access to the endothelial cytosol via a transporter mechanism and subsequently stimulate relaxation by promoting diffusion of an to adjacent smooth muscle cells via gap junctions. 6. Relaxations of endothelium-denuded preparations to anandamide and methanandamide were unaffected by 43Gap 27 peptide, 18alpha-GA, SR141716A, AM404 and indomethacin and their genesis remains to be established.

Measurement of intracellular calcium

To a certain extent, all cellular, physiological, and pathological phenomena that occur in cells are accompanied by ionic changes. The development of techniques allowing the measurement of such ion activities has contributed substantially to our understanding of normal and abnormal cellular function. Digital video microscopy, confocal laser scanning microscopy, and more recently multiphoton microscopy have allowed the precise spatial analysis of intracellular ion activity at the subcellular level in addition to measurement of its concentration. It is well known that Ca2+ regulates numerous physiological cellular phenomena as a second messenger as well as triggering pathological events such as cell injury and death. A number of methods have been developed to measure intracellular Ca2+. In this review, we summarize the advantages and pitfalls of a variety of Ca2+ indicators used in both optical and nonoptical techniques employed for measuring intracellular Ca2+ concentration.

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.

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.