Gap junctions play dynamic roles in cellular processes, however, there is a fundamental knowledge gap in understanding how gap junction proteins, the connexins, are regulated and gated based on a structure that has rigid and flexible domains. Connexin expression and function are highly regulated and the sequence of each isoform imparts specificity (permselectivity) to which molecules pass through the pore. The connexin hexamer (connexon or hemichannel) have three domains defined by the lipid bilayer. Two hemichannels pair at their extracellular domains to form an intercellular channel. The conserved transmembrane and extracellular domains are fairly rigid while the cytoplasmic domain is flexible. The sequence variability in the cytoplasmic domains, particularly in the C-terminus, allows for binding of partner proteins unique to each isoform. Within the context of this compartmentalized structure, our central hypothesis is that the monomer is tightly packed in its rigid domains, but flexibility in the cytoplasmic domains permit supra-molecular complexes to be formed in cells as well as binding of proteins controlling phosphorylation and gating. Connexin-opathies, hereditary human diseases, are often caused by mutations that often disrupt packing or partner interactions. For example, Cx26 mutations account for ~1/2 of cases of pre-lingual non-syndromic deafness in Caucasian populations but cases are found in populations across all continents. The proposed studies explore this hypothesis with three specific aims. (1) To investigate the stability of the transmembrane region of the Cx26 hexamer using mutations known to cause heredity deafness. These experiments will be correlated with ones probing channel function and structure. (2) To determine the 3D structure by cryo-electron microscopy (cryo-EM) and single particle reconstruction of Cx50 hemichannels. Cx50 intercellular channels serve critical functions in lens and its dysfunction leads to cataracts. It has extensive less ordered cytoplasmic domains typically not resolvable by crystallography. In this aim, single particle reconstruction is the best technique to obtain a structure of the large, full-length Cx50 hemichannel. (3) To create electron tomographic volumes of genetically labeled Cx43 intercellular channels and cytoskeletal and scaffolding proteins in situ to better understand the cytoplasmic architecture interacting with a gap junction. Cx43 contains binding domains for cytoskeletal components and the scaffolding protein, ZO-1. It is widespread through most organ systems with particularly important roles in vasculature and heart. The long-term goal is to obtain a more complete depiction of full-length connexins at the highest resolution obtainable. The approach is innovative because it uses a multi-resolution imaging strategy coordinated with biochemical and functional analyses of channels and hemichannels. The proposed research is significant because results will be useful in defining better drugs and other therapeutics that potentially ameliorate connexin-related diseases.

Public Health Relevance

Gap junction channels directly regulate cell-cell activities by passing metabolites, ions and signaling molecules. This project investigates the structure-function basis of intercellular communication between cells in tissues and organs by focusing on how gap junction intercellular membrane channels are put together from their constituent proteins, the connexins, and how specific parts of the connexin are involved in gating, docking or regulation of the channel.

National Institute of Health (NIH)
Research Project (R01)
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Special Emphasis Panel (ZRG1)
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Flicker, Paula F
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University of California San Diego
Schools of Medicine
La Jolla
United States
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Wang, Junjie; Ambrosi, Cinzia; Qiu, Feng et al. (2014) The membrane protein Pannexin1 forms two open-channel conformations depending on the mode of activation. Sci Signal 7:ra69
Ambrosi, Cinzia; Walker, Amy E; Depriest, Adam D et al. (2013) Analysis of trafficking, stability and function of human connexin 26 gap junction channels with deafness-causing mutations in the fourth transmembrane helix. PLoS One 8:e70916
Oshima, Atsunori; Tani, Kazutoshi; Toloue, Masoud M et al. (2011) Asymmetric configurations and N-terminal rearrangements in connexin26 gap junction channels. J Mol Biol 405:724-35
Ambrosi, Cinzia; Gassmann, Oliver; Pranskevich, Jennifer N et al. (2010) Pannexin1 and Pannexin2 channels show quaternary similarities to connexons and different oligomerization numbers from each other. J Biol Chem 285:24420-31
Ambrosi, Cinzia; Boassa, Daniela; Pranskevich, Jennifer et al. (2010) Analysis of four connexin26 mutant gap junctions and hemichannels reveals variations in hexamer stability. Biophys J 98:1809-19
Boassa, Daniela; Solan, Joell L; Papas, Adrian et al. (2010) Trafficking and recycling of the connexin43 gap junction protein during mitosis. Traffic 11:1471-86
Lichtenstein, Alexandra; Gaietta, Guido M; Deerinck, Thomas J et al. (2009) The cytoplasmic accumulations of the cataract-associated mutant, Connexin50P88S, are long-lived and form in the endoplasmic reticulum. Exp Eye Res 88:600-9
Gassmann, Oliver; Kreir, Mohamed; Ambrosi, Cinzia et al. (2009) The M34A mutant of Connexin26 reveals active conductance states in pore-suspending membranes. J Struct Biol 168:168-76
Sosinsky, Gina E; Crum, John; Jones, Ying Z et al. (2008) The combination of chemical fixation procedures with high pressure freezing and freeze substitution preserves highly labile tissue ultrastructure for electron tomography applications. J Struct Biol 161:359-71
Boassa, Daniela; Qiu, Feng; Dahl, Gerhard et al. (2008) Trafficking dynamics of glycosylated pannexin 1 proteins. Cell Commun Adhes 15:119-32

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