Isolation and purification of gap junction channels

Characterization of a novel MgtE homolog and its structural dynamics in membrane mimetics

Magnesium (Mg2+) is the most abundant divalent cation in the cell and is critical for numerous cellular processes. Despite its importance, the mechanisms of intracellular Mg2+ transport and its regulation are poorly understood. MgtE is the main Mg2+ transport system in almost half of bacterial species and is an ortholog of mammalian SLC41A1 transporters, which are implicated in neurodegenerative diseases and cancer. To date, only MgtE from Thermus thermophilus (MgtETT) has been extensively characterized, mostly in detergent micelles, and gating-related structural dynamics in biologically relevant membranes are scarce. The MgtE homolog from Bacillus firmus (MgtEBF) is unique since it lacks the entire Mg2+-sensing N-domain but has conserved structural motifs in the TM-domain for Mg2+ transport. In this work, we have successfully purified this novel homolog in a stable and functional form, and ColabFold structure prediction analysis suggests a homodimer. Further, microscale thermophoresis experiments show that MgtEBF binds Mg2+ and ATP, similar to MgtETT. Importantly, we show that, despite lacking the N-domain, MgtEBF mediates Mg2+ transport function in the presence of an inwardly directed Mg2+ gradient in reconstituted proteoliposomes. Furthermore, comparison of the organization and dynamics of Trp residues in the TM-domain of MgtEBF in membrane mimetics, in apo- and Mg2+-bound forms, suggests that the cytoplasmic binding of Mg2+ might involve modest gating-related conformational changes at the TM-domain. Overall, our results show that the gating-related structural dynamics (hydration dynamics, conformational heterogeneity) of the full-length MgtEBF is significantly changed in functionally pertinent membrane environment, emphasizing the importance of lipid-protein interactions in MgtE gating mechanisms.

Biochemical and biophysical characterization of a prokaryotic Mg2+ ion channel: Implications for cost-effective purification of membrane proteins

Although magnesium is the second most abundant cation present in the cell, the transport mechanism of Mg²⁺ across membranes is poorly understood. Importantly, the prokaryotic MgtE Mg²⁺ channel is related to mammalian SLC41A1 transporters and, therefore, biochemical and biophysical characterization of MgtE and its orthologs assumes significance. To date, the purification and structure determination of MgtE from Thermus thermophilus has been carried out using the widely used nonionic detergent, n-dodecyl-β-d-maltopyranoside (DDM). However, DDM is an expensive detergent and alternative methods to produce high-quality proteins in stable and functional form will be practically advantageous to carry out structural studies in a cost-effective manner. In this work, we have utilized 'dual-detergent strategy' to successfully purify MgtE channel in a stable and functional form by employing relatively inexpensive detergents (Triton X-100 and Anzergent 3-14) for membrane solubilization and subsequently changed to DDM during purification. Our results show that Triton X-100 and Anzergent 3-14 extract MgtE well and the quality of purified protein is comparable to DDM-extracted MgtE. Interestingly, addition of high concentration of salt and glycerol during solubilization does not significantly affect the quantity and quality of MgtE. Importantly, limited proteolysis assay, circular dichroism spectroscopy and ensemble tryptophan fluorescence strongly support the use of Triton X-100, in particular, as an inexpensive, alternative detergent for the purification of MgtE without compromising the structural integrity of the channel and Mg²⁺-induced gating-related conformational dynamics. Overall, these results are relevant for the cost-effective purification of stable and functional membrane proteins in general, and magnesium channels, in particular.

Design and Characterization of a Human Monoclonal Antibody that Modulates Mutant Connexin 26 Hemichannels Implicated in Deafness and Skin Disorders

Background: Mutations leading to changes in properties, regulation, or expression of connexin-made channels have been implicated in 28 distinct human hereditary diseases. Eight of these result from variants of connexin 26 (Cx26), a protein critically involved in cell-cell signaling in the inner ear and skin. Lack of non-toxic drugs with defined mechanisms of action poses a serious obstacle to therapeutic interventions for diseases caused by mutant connexins. In particular, molecules that specifically modulate connexin hemichannel function without affecting gap junction channels are considered of primary importance for the study of connexin hemichannel role in physiological as well as pathological conditions. Monoclonal antibodies developed in the last three decades have become the most important class of therapeutic biologicals. Recombinant methods permit rapid selection and improvement of monoclonal antibodies from libraries with large diversity.

Methods: By screening a combinatorial library of human single-chain fragment variable (scFv) antibodies expressed in phage, we identified a candidate that binds an extracellular epitope of Cx26. We characterized antibody action using a variety of biochemical and biophysical assays in HeLa cells, organotypic cultures of mouse cochlea and human keratinocyte-derived cells.

Results: We determined that the antibody is a remarkably efficient, non-toxic, and completely reversible inhibitor of hemichannels formed by connexin 26 and does not affect direct cell-cell communication via gap junction channels. Importantly, we also demonstrate that the antibody efficiently inhibits hyperative mutant Cx26 hemichannels implicated in autosomal dominant non-syndromic hearing impairment accompanied by keratitis and hystrix-like ichthyosis-deafness (KID/HID) syndrome. We solved the crystal structure of the antibody, identified residues that are critical for binding and used molecular dynamics to uncover its mechanism of action.

Conclusions: Although further studies will be necessary to validate the effect of the antibody in vivo, the methodology described here can be extended to select antibodies against hemichannels composed by other connexin isoforms and, consequently, to target other pathologies associated with hyperactive hemichannels. Our study highlights the potential of this approach and identifies connexins as therapeutic targets addressable by screening phage display libraries expressing human randomized antibodies.

Gating of Connexin Channels by transjunctional-voltage: Conformations and models of open and closed states

Voltage is an important physiologic regulator of channels formed by the connexin gene family. Connexins are unique among ion channels in that both plasma membrane inserted hemichannels (undocked hemichannels) and intercellular channels (aggregates of which form gap junctions) have important physiological roles. The hemichannel is the fundamental unit of gap junction voltage-gating. Each hemichannel displays two distinct voltage-gating mechanisms that are primarily sensitive to a voltage gradient formed along the length of the channel pore (the transjunctional voltage) rather than sensitivity to the absolute membrane potential (V_m or V_i-o). These transjunctional voltage dependent processes have been termed V_j- or fast-gating and loop- or slow-gating. Understanding the mechanism of voltage-gating, defined as the sequence of voltage-driven transitions that connect open and closed states, first and foremost requires atomic resolution models of the end states. Although ion channels formed by connexins were among the first to be characterized structurally by electron microscopy and x-ray diffraction in the early 1980's, subsequent progress has been slow. Much of the current understanding of the structure-function relations of connexin channels is based on two crystal structures of Cx26 gap junction channels. Refinement of crystal structure by all-atom molecular dynamics and incorporation of charge changing protein modifications has resulted in an atomic model of the open state that arguably corresponds to the physiologic open state. Obtaining validated atomic models of voltage-dependent closed states is more challenging, as there are currently no methods to solve protein structure while a stable voltage gradient is applied across the length of an oriented channel. It is widely believed that the best approach to solve the atomic structure of a voltage-gated closed ion channel is to apply different but complementary experimental and computational methods and to use the resulting information to derive a consensus atomic structure that is then subjected to rigorous validation. In this paper, we summarize our efforts to obtain and validate atomic models of the open and voltage-driven closed states of undocked connexin hemichannels. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

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

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

A cost-effective protocol for the over-expression and purification of fully-functional and more stable Erwinia chrysanthemi ligand-gated ion channel

The Erwinia chrysanthemi ligand-gated ion channel, ELIC, is considered an excellent structural and functional surrogate for the whole pentameric ligand-gated ion channel family. Despite its simplicity, ELIC is structurally capable of undergoing ligand-dependent activation and a concomitant desensitization process. To determine at the molecular level the structural changes underlying ELIC's function, it is desirable to produce large quantities of protein. This protein should be properly folded, fully-functional and amenable to structural determinations. In the current paper, we report a completely new protocol for the expression and purification of milligram quantities of fully-functional, more stable and crystallizable ELIC. The use of an autoinduction media and inexpensive detergents during ELIC extraction, in addition to the high-quality and large quantity of the purified channel, are the highlights of this improved biochemical protocol.

An improved method for the cost-effective expression and purification of large quantities of KcsA

KcsA, the bacterial K(+) channel from Streptomyces lividans, is the prototypical model system to study the functional and structural correlations of the pore domain of eukaryotic voltage-gated K(+) channels (Kv channels). It contains all the molecular elements responsible for ion conduction, activation, deactivation and inactivation gating [1]. KcsA's structural simplicity makes it highly amenable for structural studies. Therefore, it is methodological advantageous to produce large amounts of functional and properly folded KcsA in a cost-effective manner. In the present study, we show an optimized protocol for the over-expression and purification of large amounts of high-quality, fully functional and crystallizable KcsA using inexpensive detergents, which significantly lowered the cost of the purification process.

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.

Synaptopodin couples epithelial contractility to α-actinin-4-dependent junction maturation

A novel tension-sensitive junctional protein, synaptopodin, can relay biophysical input from cellular actomyosin contractility to induce biochemical changes at cell–cell contacts, resulting in structural reorganization of the junctional complex and epithelial barrier maturation.

The epithelial junction experiences mechanical force exerted by endogenous actomyosin activities and from interactions with neighboring cells. We hypothesize that tension generated at cell–cell adhesive contacts contributes to the maturation and assembly of the junctional complex. To test our hypothesis, we used a hydraulic apparatus that can apply mechanical force to intercellular junction in a confluent monolayer of cells. We found that mechanical force induces α-actinin-4 and actin accumulation at the cell junction in a time- and tension-dependent manner during junction development. Intercellular tension also induces α-actinin-4–dependent recruitment of vinculin to the cell junction. In addition, we have identified a tension-sensitive upstream regulator of α-actinin-4 as synaptopodin. Synaptopodin forms a complex containing α-actinin-4 and β-catenin and interacts with myosin II, indicating that it can physically link adhesion molecules to the cellular contractile apparatus. Synaptopodin depletion prevents junctional accumulation of α-actinin-4, vinculin, and actin. Knockdown of synaptopodin and α-actinin-4 decreases the strength of cell–cell adhesion, reduces the monolayer permeability barrier, and compromises cellular contractility. Our findings underscore the complexity of junction development and implicate a control process via tension-induced sequential incorporation of junctional components.

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

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

Permeation of calcium through purified connexin 26 hemichannels

BACKGROUND

Indirect evidence suggests that connexin hemichannels are permeable to Ca(2+), but direct demonstration is lacking.

RESULTS

Calcium moves into liposomes containing purified Cx26 in response to a concentration gradient.

CONCLUSION

Cx26 hemichannels are permeable to Ca(2+).

SIGNIFICANCE

Cx26 hemichannels may play a role in Ca(2+) influx into cells under conditions that lead to hemichannel activation, such as ischemic damage. Gap junction channels communicate the cytoplasms of two cells and are formed by head to head association of two hemichannels, one from each of the cells. Gap junction channels and hemichannels are permeable to ions and hydrophilic molecules of up to M(r) 1,000, including second messengers and metabolites. Intercellular Ca(2+) signaling can occur by movement of a number of second messengers, including Ca(2+), through gap junction channels, or by a paracrine pathway that involves activation of purinergic receptors in neighboring cells following ATP release through hemichannels. Understanding Ca(2+) permeation through Cx26 hemichannels is important to assess the role of gap junction channels and hemichannels in health and disease. In this context, it is possible that increased Ca(2+) influx through hemichannels under ischemic conditions contributes to cell damage. Previous studies suggest Ca(2+) permeation through hemichannels, based on indirect arguments. Here, we demonstrate for the first time hemichannel permeability to Ca(2+) by measuring Ca(2+) transport through purified Cx26 hemichannels reconstituted in liposomes. We trapped the low affinity Ca(2+)-sensitive fluorescent probe Fluo-5N into the liposomes and followed the increases in intraliposomal [Ca(2+)] in response to an imposed [Ca(2+)] gradient. We show that Ca(2+) does move through Cx26 hemichannels and that the permeability of the hemichannels to Ca(2+) is high, similar to that for Na(+). We suggest that hemichannels can be a significant pathway for Ca(2+) influx into cells under conditions such as ischemia.

Green fluorescent protein changes the conductance of connexin 43 (Cx43) hemichannels reconstituted in planar lipid bilayers

In mammalian tissues, connexin 43 (Cx43) is the most prominent member of the connexin family. In a single lipid bilayer, six connexin subunits assemble into a hemichannel (connexon). Direct communication of apposing cells is realized by two adjacent hemichannels, which can form gap junction channels. Here, we established an expression system in Pichia pastoris to recombinantly produce and purify Cx43 as well as Cx43 fused to green fluorescent protein (GFP). Proteins were isolated from crude cell membrane fractions via affinity chromatography. Cx43 and Cx43-GFP hemichannels were reconstituted in giant unilamellar vesicles as proven by fluorescence microscopy, and their electrophysiological behavior was analyzed on the single channel level by planar patch clamping. Cx43 and Cx43-GFP both showed an ohmic behavior and a voltage-dependent open probability. Cx43 hemichannels exhibited one major mean conductance of 224 ± 26 picosiemens (pS). In addition, a subconductance state at 124 ± 5 pS was identified. In contrast, the analysis of Cx43-GFP single channels revealed 10 distinct conductance states in the range of 15 to 250 pS, with a larger open probability at 0 mV as compared with Cx43, which suggests that intermolecular interactions between the GFP molecules alter the electrophysiology of the protein.

The biochemistry, ultrastructure, and subunit assembly mechanism of AMPA receptors

The AMPA-type ionotropic glutamate receptors (AMPA-Rs) are tetrameric ligand-gated ion channels that play crucial roles in synaptic transmission and plasticity. Our knowledge about the ultrastructure and subunit assembly mechanisms of intact AMPA-Rs was very limited. However, the new studies using single particle EM and X-ray crystallography are revealing important insights. For example, the tetrameric crystal structure of the GluA2cryst construct provided the atomic view of the intact receptor. In addition, the single particle EM structures of the subunit assembly intermediates revealed the conformational requirement for the dimer-to-tetramer transition during the maturation of AMPA-Rs. These new data in the field provide new models and interpretations. In the brain, the native AMPA-R complexes contain auxiliary subunits that influence subunit assembly, gating, and trafficking of the AMPA-Rs. Understanding the mechanisms of the auxiliary subunits will become increasingly important to precisely describe the function of AMPA-Rs in the brain. The AMPA-R proteomics studies continuously reveal a previously unexpected degree of molecular heterogeneity of the complex. Because the AMPA-Rs are important drug targets for treating various neurological and psychiatric diseases, it is likely that these new native complexes will require detailed mechanistic analysis in the future. The current ultrastructural data on the receptors and the receptor-expressing stable cell lines that were developed during the course of these studies are useful resources for high throughput drug screening and further drug designing. Moreover, we are getting closer to understanding the precise mechanisms of AMPA-R-mediated synaptic plasticity.

Analysis of four connexin26 mutant gap junctions and hemichannels reveals variations in hexamer stability

Connexin26 is a ubiquitous gap junction protein that serves critical homeostatic functions. Four single-site mutations found in the transmembrane helices (M1-M4) cause different types of dysfunctional channels: 1), Cx26T135A in M3 produces a closed channel; 2), Cx26M34A in M1 severely decreases channel activity; 3), Cx26P87L in M2 has been implicated in defective channel gating; and 4), Cx26V84L in M2, a nonsyndromic deafness mutant, retains normal dye coupling and electrophysiological properties but is deficient in IP(3) transfer. These mutations do not affect Cx26 trafficking in mammalian cells, and make normal-appearing channels in baculovirus-infected Sf9 membranes when imaged by negative stain electron microscopy. Upon dodecylmaltoside solubilization of the membrane fraction, Cx26M34A and Cx26V84L are stable as hexamers or dodecamers, but Cx26T135A and Cx26P87L oligomers are not. This instability is also found in Cx26T135A and Cx26P87L hemichannels isolated from mammalian cells. In this work, coexpression of both wild-type Cx26 and Cx26P87L in Sf9 cells rescued P87L hexamer stability. Similarly, in paired Xenopus oocytes, coexpression with wild-type restored function. In contrast, the stability of Cx26T135A hemichannels could not be rescued by coexpression with WT. Thus, T135 and P87 residues are in positions that are important for oligomer stability and can affect gap junction gating.

Direct formation of proteo-liposomes by in vitro synthesis and cellular cytosolic delivery with connexin-expressing liposomes

Liposomes are widely utilized in molecular biology and medicine as drug carriers. Here we report a new liposome-cell interaction through connexins. Connexin 43 (Cx43)-containing liposomes were prepared by using cell-free transcription/translation systems with plasmids encoding Cx43 in the presence of liposome. The expressed membrane protein, Cx43, was directly constituted to the liposome membrane upon in vitro synthesis, leading to pure membrane protein-containing liposomes. The hydrophilic dye calcein was efficiently transferred from Cx43-expressing liposomes to cultured cells (Cx43 expressing). The transfer is significantly blocked in the presence of gap junction inhibitor (18beta-glycyrrhetinic acid) and in the case of the other type of connexin (Cx32)-expressing cell. The results show that calcein entered the cell through connexin-mediated pathway. Cx43 liposomes containing a soluble NEMO-binding domain peptide suppressed the intracellular signaling cascade IL-1beta-induced NF-kappaB activation and cyclooxygenase-2 expression in Cx43-expressing cells, confirming effective peptide transfer into the cell. This is a new method for direct cytosolic delivery of hydrophilic molecules.

Projection structure of a N-terminal deletion mutant of connexin 26 channel with decreased central pore density

Gated gap junction channels are important cellular conduits for establishing and maintaining intercellular communication. The three-dimensional structure of a mutant human connexin 26 (Cx26M34A) by electron cryocrystallography revealed a plug-like density in the channel pore suggesting that physical blockage of the pore may be one mechanism of closure (Oshima et al. 2007, Proc Natl Acad Sci USA 104: 10034-10039). However, it remains to be determined what part of the sequence contributes to the plug. Here, we present the projection structure of an N-terminus deletion of Cx26M34A missing amino acids 2 to 7 (Cx26M34Adel2-7) crystallized in the same two-dimensional crystal form. A 10 A resolution projection map of Cx26M34Adel2-7 revealed that the plug density was dramatically reduced in comparison with that found in full-length Cx26 channel. The difference map between the deletion and full-length Cx26M34A channels strongly suggests that the N-terminus of connexin contributes to the plug for the physical closure of gap junction channels.

Change in permeant size selectivity by phosphorylation of connexin 43 gap-junctional hemichannels by PKC

Gap-junctional channels, permeable to large hydrophilic solutes of up to M(r) approximately 1,000, are responsible for cell-to-cell communication. Phosphorylation of connexin 43 (Cx43) by PKC abolishes the permeability of gap-junctional channels and hemichannels to large hydrophilic solutes, but not to small inorganic ions. Here, we report on a methodology to produce purified hemichannels of controlled subunit composition and apply it to the generation of hemichannels with variable number of PKC-phosphorylated subunits. The subunit composition was determined by luminescence resonance energy transfer. We show that all Cx43 subunits in the hemichannel hexamer have to be phosphorylated to abolish sucrose (M(r) 342) permeability. We also show that the hemichannel pores with all subunits phosphorylated by PKC have a sizable diameter, allowing for permeation of the small hydrophilic solute ethyleneglycol (M(r) 62). These results indicate that phosphorylation of Cx43 by PKC alters the hemichannel size selectivity and explain why PKC activity affects dye transfer between cells without consistent effects on electrical communication.

Negative growth control of osteosarcoma cell by Bowman-Birk protease inhibitor from soybean; involvement of connexin 43

Bowman-Birk protease inhibitor (BBI) from soybean acts as a potential chemopreventive agent in several types of tumors. However, the mechanism is still unclear. The present study was undertaken to estimate a mechanism of BBI-dependent negative growth control of human osteosarcoma cell (U2OS cell). BBI had negative growth control of the cells via induction of connexin (Cx) 43, a tumor suppressor gene in U2OS cells. This negative growth control by BBI was abrogated under down-regulation of Cx43 induced by a Cx43 antisense nucleotide treatment. It was also found that the BBI-dependent induction of Cx43 was due to elevation of Cx43 mRNA and stabilization of Cx43 protein. Especially, BBI-dependent inhibition of chymotrypsin-like activity in proteasome contributed to stabilization of Cx43 protein. These results suggest that a major negative growth effect of BBI is based on the restoration of Cx43 expression in U2OS cells.

Regulation of renal cell carcinoma cell proliferation, invasion and metastasis by connexin 32 gene

Gap junctions composed of connexin (Cx), a large protein family with a number of subtypes, are a main apparatus to maintain cellular homeostasis in many organs. Gap junctional intercellular communication (GJIC) is actively involved in all aspects of the cellular life cycle, ranging from cell growth to cell death. It is also known that the Cx gene acts as a tumor-suppressor due to the maintenance of cellular homeostasis via GJIC. In addition to this function, recent data show that the GJIC-independent function of Cx gene contributes to the tumor-suppressive effect of the gene with specificity to certain cells. With respect to the tumor-suppressive effects, Cx genes acts as tumor-suppressors in primary cancers, but the effects are still conflicting in invasive and metastatic cancers. We have previously reported that Cx32 is specifically downregulated in human renal cell carcinoma (RCC) cell lines as well as cancerous regions when compared to normal regions in kidneys. In recent studies, we have also reported that Cx32 suppresses growth, invasion and metastasis of RCC cells. In this minireview, we refer to a new aspect of Cx32-dependent functions against cell proliferation, invasion and metastasis in RCC cells, especially in a GJIC-independent manner.

Connexin 32 as an anti-invasive and anti-metastatic gene in renal cell carcinoma

Cellular homeostasis in many organs is maintained via gap junctions composed of connexin (Cx), a large protein family with a number of isoforms. In fact, gap junctional intercellular communication (GJIC) is actively involved in all aspects of the cellular life cycle, ranging from cell growth to cell death. It has been well known that Cx gene acts as a tumor suppressor gene due to the maintenance of cellular homeostasis via GJIC. On the other hand, recent data show that GJIC-independent function for Cx gene contributes to tumor-suppressive effect of the gene with cell certain specificity. However, the mechanistic aspect of the GJIC-independent function remains largely unknown. In this review, we briefly summarize the tumor-suppressive effects of Cx genes, refer to a new aspect of Cx32 as an anti-invasive and anti-metastatic gene against renal cell carcinoma in a GJIC-independent function and establishment of a new cancer therapy based on the new function of Cx32.

Heteromeric, but not homomeric, connexin channels are selectively permeable to inositol phosphates

Previous work has shown that channels formed by both connexin (Cx)26 and Cx32 (heteromeric Cx26/Cx32 hemichannels) are selectively permeable to cAMP and cGMP. To further investigate differential connexin channel permeability among second messengers, and the influence of connexin channel composition on the selectivity, the permeability of inositol phosphates with one to four phosphate groups through homomeric Cx26, homomeric Cx32, and heteromeric Cx26/Cx32 channels was examined. Connexin channels were purified from transfected HeLa cells and from rat, mouse, and guinea pig livers, resulting in channels with a broad range of Cx26/Cx32 aggregate ratios. Permeability to inositol phosphates was assessed by flux through reconstituted channels. Surprisingly, myoinositol and all inositol phosphates tested were permeable through homomeric Cx32 and homomeric Cx26 channels. Even more surprising, heteromeric Cx26/Cx32 channels showed striking differences in permeability among inositol phosphates with three or four phosphate groups and among isomers of inositol triphosphate. Thus, heteromeric channels are selectively permeable among inositol phosphates, whereas the corresponding homomeric channels are not. There was no discernible difference in the permeability of channels with similar Cx26/Cx32 ratios purified from native and heterologous sources. The molecular selectivity of heteromeric channels among three inositol triphosphates could not be accounted for by simple connexin isoform stoichiometry distributions and therefore may depend on specific isoform radial arrangements within the hexameric channels. Dynamic regulation of channel composition in vivo may effectively and efficiently modulate intercellular signaling by inositol phosphates.

Isoelectric points and post-translational modifications of connexin26 and connexin32

The isoelectric points of the gap junction proteins connexin26 (Cx26) and connexin32 (Cx32) were determined by isoelectric focusing in free fluids. The isoelectric points were significantly more acidic than predicted from amino acid sequences and different from each other, allowing homomeric channels to be resolved separately. The isoelectric points of the homomeric channels bracketed the isoelectric points of heteromeric Cx26/Cx32 channels. For heteromeric channels, Cx26 and Cx32 were found in overlapping, pH-focused fractions, indicating quaternary structure was retained. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was used to identify post-translational modifications of Cx26 and Cx32 cytoplasmic domains, including the first reported post-translational modifications of Cx26. Suspected modifications were hydroxylation and/or phosphorylation near the amino terminus of both connexins, gamma-carboxyglutamate residues in the cytoplasmic loop of both connexins, phosphorylation in the carboxyl-terminal domain of Cx32, and palmitoylation at the carboxyl-terminus of Cx32. These modifications contribute to the measured acidic isoelectric points of Cx26 and Cx32, whereas their low molecular masses would not appreciably change connexin SDS-PAGE mobility. Most of these modifications have not previously been identified for connexins and may be instrumental in guiding and understanding novel aspects of channel trafficking and molecular mechanisms of channel regulation.

Structural organization of gap junction channels

Gap junctions were initially described morphologically, and identified as semi-crystalline arrays of channels linking two cells. This suggested that they may represent an amenable target for electron and X-ray crystallographic studies in much the same way that bacteriorhodopsin has. Over 30 years later, however, an atomic resolution structural solution of these unique intercellular pores is still lacking due to many challenges faced in obtaining high expression levels and purification of these structures. A variety of microscopic techniques, as well as NMR structure determination of fragments of the protein, have now provided clearer and correlated views of how these structures are assembled and function as intercellular conduits. As a complement to these structural approaches, a variety of mutagenic studies linking structure and function have now allowed molecular details to be superimposed on these lower resolution structures, so that a clearer image of pore architecture and its modes of regulation are beginning to emerge.

Connexin32 as a tumor suppressor gene in a metastatic renal cell carcinoma cell line

Connexin genes expressing gap junction proteins have tumor-suppressive effects on primary cancers with certain cell specificity, but the suppressive effects on metastatic cancers are still conflicting. In this study, we show that connexin32 (Cx32) has a strong tumor-suppressive effect on a human metastatic renal cell carcinoma cell line (Caki-1 cell). Cx32 expression in Caki-1 cells reduced in vitro malignant phenotypes of the cells such as anchorage independency and invasion capacity. Furthermore, the Cx32 expression drastically reduced the development of Caki-1 cells in nude mice. We also determined that Cx32 reduced the malignant phenotypes in Caki-1 cells mainly through the inactivation of Src signaling. Especially, Cx32-dependent inactivation of Src decreased the production of vascular epithelial growth factor (VEGF) via the suppression of signal transducers and activators of transcription 3 (Stat3) activation, and we confirmed this result using short interfering RNA. In nude mice, Cx32-transfected Caki-1 cells showed lower serum level of VEGF comparing mock transfectant, and the development of the cells in nude mice positively related to the VEGF level. These data suggest that Cx32 acts as a tumor suppressor gene in Caki-1 cells and that the tumor-suppressive effect partly depends on the inhibition of Src-Stat3-VEGF signal pathway.

The inner ear contains heteromeric channels composed of cx26 and cx30 and deafness-related mutations in cx26 have a dominant negative effect on cx30

Cx26 and cx30 co-localize in tissues of the mammalian cochlea. Transfected HeLa cells were used to examine interactions between cx26 and cx30 and the effects on cx30 of four point mutations in cx26 that are associated with dominantly inherited hearing loss--W44S, G59A, D66H and R75W. When co-expressed, wtcx26 and wtcx30 trafficked to the same gap junction plaques. Cells transferred neurobiotin but not Lucifer Yellow, which passes freely through cx26 channels, suggesting cx30 affects the properties of cx26. G59A and D66H had a perinuclear localization when expressed alone but trafficked to the membrane when co-expressed with cx30. Co-expression of W44S, G59A or R75W with cx30, significantly reduced neurobiotin transfer in comparison with cells expressing cx30 only. These results indicate that cx26 and cx30 can oligomerize to form heteromeric connexons and demonstrate a dominant negative effect of some cx26 mutants on cx30. Immunogold labeling of thin sections of the cochlea showed both cx26 and cx30 distributed evenly on both sides of individual gap junction profiles. Immunoprecipitation of cochlear membrane proteins, isolated by procedures that preserve connexons, with either cx30 or cx26 antibodies precipitated both cx26 and cx30. Following co-injection of Lucifer Yellow and neurobiotin into individual supporting cells of the organ of Corti in cochlear slices, neurobiotin transferred to many cells, but Lucifer Yellow was retained in the injected cell. These observations are consistent with junctions composed of cx26/cx30 heteromeric connexons in the cochlea. The functional disruption caused by some cx26 mutations upon such heteromeric channels may underlie the non-syndromic nature of their effects on hearing.

The tight junction: a multifunctional complex

Multicellular organisms are separated from the external environment by a layer of epithelial cells whose integrity is maintained by intercellular junctional complexes composed of tight junctions, adherens junctions, and desmosomes, whereas gap junctions provide for intercellular communication. The aim of this review is to present an updated overview of recent developments in the area of tight junction biology. In a relatively short time, our knowledge of the tight junction has evolved from a relatively simple view of it being a permeability barrier in the paracellular space and a fence in the plane of the plasma membrane to one of it acting as a multicomponent, multifunctional complex that is involved in regulating numerous and diverse cell functions. A group of integral membrane proteins-occludin, claudins, and junction adhesion molecules-interact with an increasingly complex array of tight junction plaque proteins not only to regulate paracellular solute and water flux but also to integrate such diverse processes as gene transcription, tumor suppression, cell proliferation, and cell polarity.

Negative growth control of renal cell carcinoma cell by connexin 32: possible involvement of Her-2

Connexin (Cx) genes have negative growth effects on tumor cells with certain cell specificity. We have previously reported that Cx32 is specifically downregulated in human renal cell carcinoma cell (RCC) lines as well as cancerous regions of kidneys and that the Cx is expressed in the progenitor cells of the carcinoma. However, the precise role of Cx32 in growth control of RCC cells remains unknown. In this study, we examined whether Cx32 could act in growth control against a human RCC cell, Caki-2 cell. In order to estimate the cell growth control, we established Caki-2 cells that have stable expression of Cx32 genes. Cx32 expression in Caki-2 cells induced contact inhibition of growth and reduced anchorage-independent growth ability, but did not significantly affect lag phase growth rates. This growth control by Cx32 was dependent on the inhibition of the cell-cycle transition from G1 to S phase at high cell density, and the inhibition of the cell-cycle transition related to the suppression of Her-2 activation. Furthermore, the suppression of Cx32 expression in Caki-2 cells by short interfering RNA induced the activation of Her-2. These data suggest that Cx32 has negative growth control of Caki-2 cells, partly due to the inhibition of the Her-2 activation.

Regulation of purified and reconstituted connexin 43 hemichannels by protein kinase C-mediated phosphorylation of Serine 368

Indirect evidence suggests that the permeability of connexin 43 (Cx43) gap-junctional channels (connexons) to small organic molecules (M(r) < 1,000) is decreased by protein kinase C (PKC)-mediated phosphorylation of Ser-368. However, it is currently unknown whether this effect is produced directly by phosphorylation of this residue or whether cytoplasmic regulatory factors are required for the decrease in Cx43 gap-junctional channel permeability. Here we studied the effects of PKC-mediated phosphorylation on purified recombinant wild-type Cx43 and a PKC-unresponsive mutant (S368A). Our studies show that (a) PKC phosphorylates Ser-368, (b) the phosphorylation by PKC of purified and reconstituted connexons abolishes sucrose and Lucifer Yellow permeability, (c) the regulation of Cx43 by PKC is the direct result of phosphorylation of Ser-368 and does not involve intermediary regulatory factors, and (d) phosphorylation of Ser-368 produces a conformational change in purified Cx43 as demonstrated by changes in intrinsic Trp fluorescence and proteolytic digestion pattern. We conclude that phosphorylation of Ser-368 by PKC induces a conformational change of Cx43 that results in a decrease in connexon permeability.

Gap junctions in the inner ear: comparison of distribution patterns in different vertebrates and assessement of connexin composition in mammals

The distribution and size of gap junctions (GJ) in the sensory epithelia of the inner ear have been examined in a reptile (gecko), birds (chicken and owl), and mammals (mouse, guinea pig, gerbil, and bat), and the connexin composition of GJs in the mammalian inner ear has been assessed. Freeze fracture revealed a common pattern of GJ distribution in auditory and vestibular sensory epithelia in the different vertebrate classes. In all these tissues, GJs are numerous, often occupying more than 25% of the plasma membrane area of supporting cells and sometimes composed of more than 100,000 channels. Screening for 12 members of the connexin family in the mammalian inner ear by RT-PCR, Western blotting, and immunohistochemistry revealed four connexin isotypes, cx26, cx30, cx31, and cx43, in the cochlea and three, cx26, cx30, and cx43, in the vestibular organs. With antibodies characterised for their specificity, cx26 and cx30 colocalised in supporting cells of the organ of Corti, in the basal cell region of the stria vascularis, and in type 1 fibrocytes of the spiral ligament. No other connexin was detected in these regions. Cx31 was localised among type 2 fibrocytes below the spiral prominence, a region where cx30 was not expressed and cx26 expression appeared to be low. Cx43 was detected only in the region of "tension fibrocytes" lining the inner aspect of the otic capsule. This suggests separate functional compartments in the cochlea. In addition to cx26 and cx30, cx43 was detected in supporting cells of the vestibular sensory epithelia. Where cx26 and cx30 were colocalised, double immunogold labelling of thin sections showed both cx26 and cx30 evenly distributed in individual GJ plaques, a pattern consistent with the presence of heteromeric connexons. Coimmunoprecipitation of cochlear membrane proteins solubilised with a procedure that preserves the oligomeric structure of connexons confirmed the presence of heteromeric cx26/cx30 connexons. Heteromeric cx26/cx30 connexons may be unique to the inner ear, which could be one factor underlying the non-syndromic character of the deafness caused by mutations in cx26.

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Down-regulation of connexin 32 gene expression through DNA methylation in a human renal cell carcinoma cell

BACKGROUND

We have recently reported that connexin (Cx) 32 is down-regulated in a human renal cell carcinoma (RCC) cell (Caki-2 cell).

HYPOTHESIS

We postulated that the down-regulation of Cx32 gene in the RCC cell is due to hypermethylation of its promoter region.

METHODS

We estimated methylation status in the promoter region of Cx32 gene in the RCC cell by DNA digestion with methylation-sensitive restriction enzyme and PCR, and methylation-specific PCR (MSP). We also checked the recovery of Cx32 gene expression in the RCC cell treated with a DNA methyltranferase inhibitor, 5-Aza-2'-deoxycytidine (5-Aza-CdR).

RESULTS

Treatment with 5-Aza-CdR resulted in induction of Cx32 expression in the RCC cell. Hypermethylation of the Cx32 promoter region in the RCC cell was confirmed by DNA digestion with methylation-sensitive restriction enzyme and PCR, and MSP.

CONCLUSION

We suggest that hypermethylation in the promoter region is a mechanism for the Cx32 gene repression in the RCC cell.

Expression, solubilization, and biochemical characterization of the tight junction transmembrane protein claudin-4

The tight junction tetraspan protein claudin-4 creates a charge-selective pore in the paracellular pathway across epithelia. The structure of the pore is unknown, but is presumed to result from transcellular adhesive contacts between claudin's extracellular loops. Here we report the expression of claudin-4 by baculovirus infection of Sf9 cells and describe the biochemical analysis suggesting it has a hexameric quaternary configuration. We show the detergent perfluoro-octanoic acid is able to maintain oligomeric claudin species. Sucrose velocity centrifugation and laser light scattering are also used to investigate the oligomeric state of claudin-4. In contrast to proteins of similar topology, such as gap junction family connexins, the oligomeric state of claudins appears more dynamic. These data suggest the structural organization of claudins in tight junction pores is unique.

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.

Isolation and characterization of gap junctions from tissue culture cells

The purification of membrane proteins in a form and amount suitable for structural or biochemical studies still remains a great challenge. Gap junctions have long been studied using electron microscopy and X-ray diffraction. However, only a limited number of proteins in the connexin family have been amenable to protein or membrane purification techniques. Molecular biology techniques for expressing large gap junctions in tissue culture cells combined with improvements in electron crystallography have shown great promise for determining the channel structure to better than 10 A resolution. Here, we have isolated two-dimensional (2D) gap junction crystals from HeLa Cx26 transfectants. This isoform has never been isolated in large fractions from tissues. We characterize these preparations by SDS-PAGE, Western blotting, negative stain electron microscopy and atomic force microscopy. In our preparations, the Cx26 is easily detected in the Western blots and we have increased expression levels so that connexin bands are visible on SDS-PAGE gels. Preliminary assessment of the samples by electron cryo-microscopy shows that these 2D crystals diffract to at least 22 A. Atomic force microscopy of these Cx26 gap junctions show exquisite surface modulation at the extracellular surface in force dissected gap junctions. We also applied our protocol to cell lines such as NRK cells that express endogenous Cx43 and NRK and HeLa cell lines transfected with exogenous connexins. While the gap junction membrane channels are recognizable in negatively stained electron micrographs, these lattices are disordered and the gap junction plaques are smaller. SDS-PAGE and Western blotting revealed expression of connexins, but at a lower level than with our HeLa Cx26 transfectants. Therefore, the purity and morphology of the gap junction plaques depends the size and abundance of the gap junctions in the cell line itself.

Regulation of cellular invasion and matrix metalloproteinase activity in HepG2 cell by connexin 26 transfection

We previously reported that connexin (Cx) 26 expression is involved in negative growth control of HepG2 cells established from a human hepatoma. We also found that induction of E-cadherin and subsequent formation of a cell adhesion complex were induced in HepG2 cells by Cx 26 expression. To examine the exact role of Cx 26-induced E-cadherin junctions in regulating appearance of malignant phenotypes of HepG2 cells, we expressed a Cx 26 antisense oligodeoxynucleotide (AS-ODN) in an established HepG2 cell clone that has stable expression of Cx 26 genes. We investigated changes in the expression of E-cadherin, the localization of beta-catenin, and some malignant phenotypes of HepG2 clone after the suppression of Cx 26 expression by AS-ODN treatment. The AS-ODN treatment prevented the expression of Cx 26 and E-cadherin, and the localization of beta-catenin was changed from cytoplasmic membrane to the cytoplasm. In parallel, a morphological change from a monolayer of polygonal cells to multilayered colonies was induced by the treatment, indicating a change of a malignant phenotype of HepG2 cells. The activity of matrix metalloproteinase 9 (MMP-9) was elevated by the AS-ODN treatment. A concomitant increase in invasiveness of the Cx 26-expressing cells by the treatment was also observed in an in vitro assay with Matrigel matrix. These results suggest that the induction of E-cadherin and formation of the cell adhesion complex by Cx 26 expression contribute to the reversal of some malignant phenotypes of HepG2 cells. Furthermore, the Cx 26-dependent expression of E-cadherin leads to reduction of the invasiveness of the cells through suppression of MMP-9 activity.

Imaging molecular structure and physiological function of gap junctions and hemijunctions by multimodal atomic force microscopy

Gap junctions are specialized plasma membrane structures that join neighboring cells via specialized intercellular ion channels (hemichannels) and provide a direct pathway for cell-cell communication. They presumably mediate regulation of growth, transmission of developmental signals, coordination of muscle contraction, and maintenance of metabolic homeostasis. Hemichannels are also present in the non-junctional regions of the cell plasma membrane and they provide a direct pathway for communication between the cytoplasm and the extracellular region. Recent studies suggest that gap junctional communication is much more complex than previously anticipated, in terms of both its structure as well as its activity. While the mechanism of gap junction activity is being studied extensively, their quaternary structure, assembly, and conformational changes underlying gating of their activity as well as their physiological role are poorly understood because, due to their complex structure, these junctions are less amenable to existing techniques for high-resolution three-dimensional structure-function analyses. Atomic Force Microscopy (AFM) images molecular structure of biological specimens in an aqueous environment, allows on-line perturbations, and can be coupled with electrophysiological, biochemical, and other microscopic techniques. The present review examines the potential of AFM application for the study of the molecular structure of hydrated, native gap junctions and hemijunctions as well as their physiological functions. Special attention is paid to new, complementary, or provocative findings from AFM studies of both vertebrate and invertebrate gap junctions and hemijunctions.

Connexins/connexons. Cell-free expression

With a few exceptions, all secretory and plasma membrane proteins studied to date are synthesized in the endoplasmic reticulum (ER) membrane. Then, they are transported by successive vesicle budding and fusion from the ER through the Golgi stacks to the plasma membrane following the general intracellular transport route referred to as secretory pathway (originally reviewed in 1). Gap junction connexins have been shown to follow this pathway.

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.

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.

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.

Expression, two-dimensional crystallization, and electron cryo-crystallography of recombinant gap junction membrane channels

We used electron cryo-microscopy and image analysis to examine frozen-hydrated, two-dimensional (2D) crystals of a recombinant, 30-kDa C-terminal truncation mutant of the cardiac gap junction channel formed by 43-kDa alpha(1) connexin. To our knowledge this is the first example of a structural analysis of a membrane protein that has been accomplished using microgram amounts of starting material. The recombinant alpha(1) connexin was expressed in a stably transfected line of baby hamster kidney cells and spontaneously assembled gap junction plaques. Detergent treatment with Tween 20 and 1,2-diheptanoyl-sn-phosphocholine resulted in well-ordered 2D crystals. A three-dimensional density (3D) map with an in-plane resolution of approximately 7.5 A revealed that each hexameric connexon was formed by 24 closely packed rods of density, consistent with an alpha-helical conformation for the four transmembrane domains of each connexin subunit. In the extracellular gap the aqueous channel was bounded by a continuous wall of protein that formed a tight electrical and chemical seal to exclude exchange of substances with the extracellular milieu.

Electron cryo-crystallography of a recombinant cardiac gap junction channel

Gap junctions in the heart play an important functional role by electrically coupling cells, thereby organizing the pattern of current flow to allow co-ordinated muscle contraction. Cardiac gap junctions are therefore intimately involved in normal conduction as well as the genesis of potentially lethal arrhythmias. We recently utilized electron cryo-microscopy and image analysis to examine frozen-hydrated 2D crystals of a recombinant, C-terminal truncated form of connexin 43 (Cx43; alpha 1), the principal cardiac gap junction protein. The projection map at 7 A resolution revealed that each 30 kDa connexin subunit has a transmembrane alpha-helix that lines the aqueous pore and a second alpha-helix in close contact with the membrane lipids. The distribution of densities allowed us to propose a model in which the two apposing connexons that form the channel are staggered by approximately 30 degrees. We are now recording images of tilted, frozen-hydrated 2D crystals, and a preliminary 3D map has been computed at an in-plane resolution of approximately 7.5 A and a vertical resolution of approximately 25 A. As predicted by our model, the two apposing connexons that form the channel are staggered with respect to each other for certain connexin molecular boundaries within the hexamer. Within the membrane interior each connexin subunit displays four rods of density, which are consistent with an alpha-helical conformation for the four transmembrane domains. Preliminary studies of BHK hamster cells that express the truncated Cx43 designated alpha 1 Cx263T demonstrate that oleamide, a sleep inducing lipid, blocks in vivo dye transfer, suggesting that oleamide causes closure of alpha 1 Cx263T channels. The comparison of the 3D structures in the presence and absence of oleamide may provide an opportunity to explore the conformational changes that are associated with oleamide-induced blockage of dye transfer. The structural details revealed by our analysis will be essential for delineating the molecular basis for intercellular current flow in the heart, as well as the general molecular design and functional properties of this important class of channel proteins.