Lens fiber connexins, cx50 and cx46 (alpha3 and alpha8), belong to a small subset of connexins that can form functional hemichannels in nonjunctional membranes. Knockout of either cx50 or cx46 results in a cataract, so the properties of both connexins are likely essential for proper physiological functioning of the lens. Although portions of the sequences of these two connexins are nearly identical, their hemichannel properties are quite different. Cx50 hemichannels are much more sensitive to extracellular acidification than cx46 hemichannels and differ from cx46 hemichannels both in steady-state and kinetic properties. Comparison of the two branches of the cx50 hemichannel G-V curve with the junctional G-V curve suggests that cx50 gap junctions gate with positive relative polarity. The histidine-modifying reagent, diethyl pyrocarbonate, reversibly blocks cx50 hemichannel currents but not cx46 hemichannel currents. Because cx46 and cx50 have very similar amino acid sequences, one might expect that replacing the two histidines unique to the third transmembrane region of cx50 with the corresponding cx46 residues would produce mutants more closely resembling cx46. In fact this does not happen. Instead the mutant cx50H161N does not form detectable hemichannels but forms gap junctions indistinguishable from wild type. Cx50H176Q is oocyte lethal, and the double mutant, cx50H61N/H176Q, neither forms hemichannels nor kills oocytes.

Single site mutations in connexins have provided insights about the influence specific amino acids have on gap junction synthesis, assembly, trafficking, and functionality. We have discovered a single point mutation that eliminates functionality without interfering with gap junction formation. The mutation occurs at a threonine residue located near the cytoplasmic end of the third transmembrane helix. This threonine is strictly conserved among members of the ␣-and ␤-connexin subgroups but not the ␥-subgroup. In HeLa cells, connexin43 and connexin26 mutants are synthesized, traffic to the plasma membrane, and make gap junctions with the same overall appearance as wild type. We have isolated connexin26T135A gap junctions both from HeLa cells and baculovirus-infected insect Sf9 cells. By using cryoelectron microscopy and correlation averaging, difference images revealed a small but significant size change within the pore region and a slight rearrangement of the subunits between mutant and wild-type connexons expressed in Sf9 cells. Purified, detergent-solubilized mutant connexons contain both hexameric and partially disassembled structures, although wild-type connexons are almost all hexameric, suggesting that the three-dimensional mutant connexon is unstable. Mammalian cells expressing gap junction plaques composed of either connexin43T154A or connexin26T135A showed an absence of dye coupling. When expressed in Xenopus oocytes, these mutants, as well as a cysteine substitution mutant of connexin50 (connexin50T157C), failed to produce electrical coupling in homotypic and heteromeric pairings with wild type in a dominant-negative effect. This mutant may be useful as a tool for knocking down or knocking out connexin function in vitro or in vivo.Intercellular communication is a fundamental feature of all multicellular organisms. Gap junctions are one means by which cells communicate with each other and arise as tissue cells grow and abut each other. The morphologically distinctive cell-cell junctional areas allow the exchange of ions, nutrients, and small metabolites between neighboring cells. Gap junction structures are found throughout vertebrates and invertebrates, although the primary sequences of constituent proteins are different from each other even though electron micrographs and physiological assays indicate similar quaternary structure and functionality. Gap junctions are composed of two oligomeric channel structures called connexons, with each cell supplying one connexon that docks with the other at their extracellular surfaces. The connexin family of proteins has a very conserved protein folding topology with highly conserved transmembrane and extracellular primary sequences, but contains variable regions of the cytoplasmic loop and C terminus that confer the individual physiological properties to each connexin. Each connexon is made up of a cyclic arrangement of six protein monomers, called connexins (abbreviated as Cx 5 plus the molecular mass of the protein as predicted by the amino acid sequence, e.g. ...

A novel human connexin gene (GJA11) was cloned from a genomic library. The open reading frame encoded a hypothetical protein of 294 amino acid residues with a predicted molecular mass of 31,933, hence referred to as connexin-31.9 (Cx31.9) or ␣11 connexin. A clone in GenBank TM containing the Cx31.9 gene localized to chromosome 17q21.2. Northern analysis of Cx31.9 showed a major 4.4-kilobase transcript, which was expressed at varying levels in all tissues analyzed. Two monoclonal antibodies generated against different domains of Cx31.9 recognized a 30 -33-kDa protein from cells overexpressing Cx31.9. Immunofluorescence of overexpressing cells indicated the presence of Cx31.9 between adjacent cells, consistent with its localization to gap junctions. Double voltage clamp analyses of Cx31.9-overexpressing cells, and of paired Xenopus oocytes injected with Cx31.9 cRNA, demonstrated junctional currents indicative of gap junction channel formation. In contrast to previously characterized connexins, Cx31.9 showed no voltage-dependent gating within a physiologically relevant range. Cx31.9 was detected in human tissues by immunoblot analysis, and immunofluorescence localized Cx31.9 expression to vascular smooth muscle cells. Furthermore, it was demonstrated that Cx31.9 interacted with ZO-1. Thus, Cx31.9 represents a novel connexin gene that in vivo generates a protein with unique voltage gating properties.Most cells in vertebrate tissues are coupled by gap junction channels, which allow the transfer of low molecular mass substances (Ͻ1 kDa) between cells. The ability of cells to communicate in this manner is thought to be important for coordinating tissue growth, development, and physiological activities. Connexins are the sole proteins required for the formation of gap junctions (1). Six connexin monomers form a hemichannel (connexon) on the cell surface, which can interact with a connexon from a neighboring cell, thus forming a channel linking the cytoplasm of the two cells.A number of different connexin isoforms are found in vertebrates. The connexins characterized to date have distinct channel permeabilities and gating properties (2, 3), as well as distinct cell and tissue expression patterns, which is presumed to orchestrate a spatially and temporally regulated diffusion of small molecules between cells. This has recently been demonstrated by studies of genetically altered mice, in which the expression of Cx43 1 was replaced by expression of Cx40 or Cx32. These mice differed functionally and morphologically from wild type mice, demonstrating that one connexin isoform cannot fully substitute for another (4). Regulation of the diffusion of small molecules can be further modulated by combining different connexin isoforms in each hemichannel (heteromeric gap junctions) and by combining different hemichannels (heterotypic gap junctions), thus creating channels with highly specific permeability and gating properties.All connexin isoforms are presumed to have a similar topology, which has been deduced from limited proteolys...

KCNQ1 (Kv7.1 or KvLQT1) encodes the alpha-subunit of a voltage-gated potassium channel found in tissues including heart, brain, epithelia and smooth muscle. Tissue-specific characteristics of KCNQ1 current are diverse, due to modification by ancillary subunits. In heart, KCNQ1 associates with KCNE1 (MinK), producing a slowly activating voltage-dependent channel. In epithelia, KCNQ1 co-assembles with KCNE3 (Mirp2) producing a constitutively open channel. Chromanol 293B is a selective KCNQ1 blocker. We studied drug binding and frequency dependence of 293B on KCNQ1 and ancillary subunits expressed in Xenopus oocytes. Ancillary subunits altered 293B potency up to 100-fold (IC 50 for KCNQ1 = 65.4 ± 1.7 μM; KCNQ1/KCNE1 = 15.1 ± 3.3 μM; KCNQ1/KCNE3 = 0.54 ± 0.18 μM). Block of KCNQ1 and KCNQ1/KCNE3 was time independent, but 293B altered KCNQ1/KCNE1 activation. We therefore studied frequency-dependent block of KCNQ1/KCNE1. Repetitive rapid stimulation increased KCNQ1/KCNE1 current biphasically, and 293B abolished the slow component. KCNQ1/KCNE3[V72T] activates slowly with a KCNQ1/KCNE1-like phenotype, but retains the high affinity binding of KCNQ1/KCNE3, demonstrating that subunit-mediated changes in gating can be dissociated from subunit-mediated changes in affinity. This study demonstrates the KCNQ1 pharmacology is significantly altered by ancillary subunits. The response of KCNQ1 to specific blockers will therefore be critically dependent on the electrical stimulation pattern of the target organ. Furthermore, the dissociation between gating and overall affinity suggests that mutations in ancillary subunits can potentially strongly alter drug sensitivity without obvious functional changes in gating behaviour, giving rise to unexpected side-effects such as a predisposition to acquired long QT syndrome.

Cerebral edema in ischemic stroke can lead to increased intracranial pressure, reduced cerebral blood flow and neuronal death. Unfortunately, current therapies for cerebral edema are either ineffective or highly invasive. During the development of cytotoxic and subsequent ionic cerebral edema water enters the brain by moving across an intact blood brain barrier and through aquaporin-4 (AQP4) at astrocyte endfeet. Using AQP4-expressing cells, we screened small molecule libraries for inhibitors that reduce AQP4-mediated water permeability. Additional functional assays were used to validate AQP4 inhibition and identified a promising structural series for medicinal chemistry. These efforts improved potency and revealed a compound we designated AER-270, N-[3,5-bis(trifluoromethyl)phenyl]-5-chloro-2-hydroxybenzamide. AER-270 and a prodrug with enhanced solubility, AER-271 2-{[3,5-Bis(trifluoromethyl)