Arotinoid Acid receptor Gap junction channels exhibit differ

Gap junction channels exhibit different types of gating properties. One alternative mechanism that should be mentioned is loop gating in which the extracellular loop of connexin assumes a configuration to form a narrow pathway for closure of unopposed hemichannels [53] and docked junction channels [54]. The properties were also evaluated by MD simulations with the X-ray structure of Cx26 [55]. To date, however, there is no evidence from high-resolution structures of gap junction channels to reconcile these models. In the structure of LRRC8A, the extracellular domain may act as a selectivity filter as the pore diameter is constricted by basic residue side-chains such as arginine, histidine, and lysine in E1 [15]. As multiple basic residues are not found in the corresponding region of Cx26, a high-resolution structure of an undocked gap junction hemichannel would be required to elucidate the mechanism of loop gating.
Conclusions Cryo-EM is an important technique for elucidating the structural and functional basis of gap junction channels. The atomic structure of Ce-INX6 suggests that gap junction channels may be amenable to single-particle cryo-EM due to their high symmetry and the large size of the molecular complex. The flexible cytoplasmic domains, including the N-terminus, are thought to be important modulators of channel function, but are not well resolved for connexin channels. A possible strategy to determine the flexibility is to use a monoclonal antibody or nanobody against the cytoplasmic domains, which is often used for crystallographic and cryo-EM studies of G-protein-coupled receptors [56,57]. This strategy may also be useful for labeling a particular subunit in a heteromeric subunit complex, as utilized for the Arotinoid Acid receptor and gamma-amino butyric acid receptors [58,59], as long as the subunit stoichiometry is constant. Using nanodiscs may be an effective method, as reconstituting the TRPV1 channel into lipid nanodiscs allowed for its detailed structural analysis in a more stable environment [60]. For fundamental comprehension of the structure and function of gap junction channels in the native environment, protein–lipid interactions should also be considered. Structures of undocked hemichannels of gap junction channels are required to investigate the docking mechanism of unopposed hemichannels in addition to understanding the loop gating mechanism. Single-particle cryo-EM will be a key and essential method for future research of gap junction channels.
Conflict of interest statement
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as:
Acknowledgements I am grateful to Yoshinori Fujiyoshi (Nagoya Univ.) and I. Martha Skerrett (SUNY Buffalo State) for critical reading and discussions. This work was supported by the Platform Project for Supporting Drug Discovery and Life Science Research (Basis for Supporting Innovative Drug Discovery and Life Science Research (BINDS)) from the Japan Agency for Medical Research and Development (AMED) under Grant Numbers JP18am0101074, JP18ae0101051, and Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (C) under Grant Number JP16K07266.
Introduction Direct intercellular communication through gap junction channels plays a key role in cellular homeostasis and in synchronizing physiological functions (Laird et al., 2015). In the brain, this is the case for glial cells and in particular for astrocytes that exhibit the highest expression level of gap junction proteins, named connexins (Cxs) (Ransom and Giaume, 2013). Two major Cxs are expressed in astrocytes of adult mouse brain: Cx43, that is already detected at birth, and Cx30, that starts to be detected after the second postnatal week (Nagy and Rash, 2000). Such high expression level allows important intercellular exchanges of ions, signaling molecules and metabolites that provide the basis for a glial networking (Giaume et al., 2010). A specific feature of astrocytes is that they are located at the interface between neurons and blood vessels: indeed, they contact the synapses where they contribute to the “tripartite synapse” (Araque et al., 1999) and the vascular system since astrocyte endfeet are associated to the brain blood barrier (BBB). In addition, astrocytes are coupled with oligodendrocytes and can form panglial networks with them (Griemsmann et al., 2015). Interestingly, gap junction proteins are detected nearby cortical excitatory synapses (Genoud et al., 2015) and Cx43 as well as Cx30 are highly expressed at contacts between astroglial endfeet that enwrap blood vessels (Ezan et al., 2012). Accordingly, gap junctional communication (GJC) is involved in the control of synaptic transmission and in the maintenance of BBB integrity. Indeed, in Double KO mice for Cx43 and Cx30, synaptic transmission and plasticity are impacted in the hippocampus (Pannasch et al., 2011) and the integrity of the BBB is weakened (Ezan et al., 2012). Also, it is established that GJC in astrocytes is controlled by a number of endogenous compounds such as neurotransmitters, cytokines and endogenous lipids (Ransom and Giaume, 2013) and depends on neuronal activity (Roux et al., 2011; Liu et al., 2013). In addition, the expression level of astroglial Cxs is modified in a number of brain pathologies and injuries although the functional status of gap junction channels is much less documented (see Giaume et al., 2010). Hence, it appears essential to develop techniques to monitor the level of GJC in astrocytes at adult stages and in pathological situations to investigate the correlation between changes in Cx expression and function.