Histidine suppresses zinc modulation of connexin hemichannels

Aberrant Cx26 hemichannels and keratitis-ichthyosis-deafness syndrome: insights into syndromic hearing loss

Mutation of the GJB2 gene, which encodes the connexin 26 (Cx26) gap junction (GJ) protein, is the most common cause of hereditary, sensorineural hearing loss. Cx26 is not expressed in hair cells, but is widely expressed throughout the non-sensory epithelial cells of the cochlea. Most GJB2 mutations produce non-syndromic deafness, but a subset produces syndromic deafness in which profound hearing loss is accompanied by a diverse array of infectious and neoplastic cutaneous disorders that can be fatal. Although GJ channels, which are assembled by the docking of two, so-called hemichannels (HCs), have been the main focus of deafness-associated disease models, it is now evident that the HCs themselves can function in the absence of docking and contribute to signaling across the cell membrane as a novel class of ion channel. A notable feature of syndromic deafness mutants is that the HCs exhibit aberrant behaviors providing a plausible basis for disease that is associated with excessive or altered contributions of Cx26 HCs that, in turn, lead to compromised cell integrity. Here we discuss some of the aberrant Cx26 HC properties that have been described for mutants associated with keratitis-ichthyosis-deafness (KID) syndrome, a particularly severe Cx26-associated syndrome, which shed light on genotype-phenotype relationships and causes underlying cochlear dysfunction.

Altered inhibition of Cx26 hemichannels by pH and Zn2+ in the A40V mutation associated with keratitis-ichthyosis-deafness syndrome

Excessive opening of undocked Cx26 hemichannels in the plasma membrane is associated with disease pathogenesis in keratitis-ichthyosis-deafness (KID) syndrome. Thus far, excessive opening of KID mutant hemichannels has been attributed, almost solely, to aberrant inhibition by extracellular Ca(2+). This study presents two new possible contributing factors, pH and Zn(2+). Plasma pH levels and micromolar concentrations of Zn(2+) inhibit WT Cx26 hemichannels. However, A40V KID mutant hemichannels show substantially reduced inhibition by these factors. Using excised patches, acidification was shown to be effective from either side of the membrane, suggesting a protonation site accessible to H(+) flux through the pore. Sensitivity to pH was not dependent on extracellular aminosulfonate pH buffers. Single channel recordings showed that acidification did not affect unitary conductance or block the hemichannel but rather promoted gating to the closed state with transitions characteristic of the intrinsic loop gating mechanism. Examination of two nearby KID mutants in the E1 domain, G45E and D50N, showed no changes in modulation by pH or Zn(2+). N-bromo-succinimide, but not thiol-specific reagents, attenuated both pH and Zn(2+) responses. Individually mutating each of the five His residues in WT Cx26 did not reveal a key His residue that conferred sensitivity to pH or Zn(2+). From these data and the crystal structure of Cx26 that suggests that Ala-40 contributes to an intrasubunit hydrophobic core, the principal effect of the A40V mutation is probably a perturbation in structure that affects loop gating, thereby affecting multiple factors that act to close Cx26 hemichannels via this gating mechanism.

Motifs in the permeation pathway of connexin channels mediate voltage and Ca (2+) sensing

Connexin channels mediate electrical coupling, intercellular molecular signaling, and extracellular release of signaling molecules. Connexin proteins assemble intracellularly as hexamers to form plasma membrane hemichannels. The docking of two hemichannels in apposed cells forms a gap junction channel that allows direct electrical and selective cytoplasmic communication between adjacent cells. Hemichannels and junctional channels are gated by voltage, but extracellular Ca (2+) also gates unpaired plasma membrane hemichannels. Unlike other ion channels, connexin channels do not contain discrete voltage- or Ca (2+)-sensing modules linked to a separate pore-forming module. All studies to date indicate that voltage and Ca (2+) sensing are predominantly mediated by motifs that lie within or are exposed to the pore lumen. The sensors appear to be integral components of the gates, imposing an obligatory structural linkage between sensing and gating not commonly present in other ion channels, in which the sensors are semi-independent domains distinct from the pore. Because of this, the structural and electrostatic features that define connexin channel gating also define pore permeability properties, and vice versa; analysis/mutagenesis of gating and of permeability properties are linked. This offers unique challenges and opportunities for elucidating mechanisms of ligand and voltage-driven gating.

Gap junctions

Gap junctions are essential to the function of multicellular animals, which require a high degree of coordination between cells. In vertebrates, gap junctions comprise connexins and currently 21 connexins are known in humans. The functions of gap junctions are highly diverse and include exchange of metabolites and electrical signals between cells, as well as functions, which are apparently unrelated to intercellular communication. Given the diversity of gap junction physiology, regulation of gap junction activity is complex. The structure of the various connexins is known to some extent; and structural rearrangements and intramolecular interactions are important for regulation of channel function. Intercellular coupling is further regulated by the number and activity of channels present in gap junctional plaques. The number of connexins in cell-cell channels is regulated by controlling transcription, translation, trafficking, and degradation; and all of these processes are under strict control. Once in the membrane, channel activity is determined by the conductive properties of the connexin involved, which can be regulated by voltage and chemical gating, as well as a large number of posttranslational modifications. The aim of the present article is to review our current knowledge on the structure, regulation, function, and pharmacology of gap junctions. This will be supported by examples of how different connexins and their regulation act in concert to achieve appropriate physiological control, and how disturbances of connexin function can lead to disease.

Zinc modulation of hemi-gap-junction channel currents in retinal horizontal cells

Hemi-gap-junction (HGJ) channels of retinal horizontal cells (HCs) function as transmembrane ion channels that are modulated by voltage and calcium. As an endogenous retinal neuromodulator, zinc, which is coreleased with glutamate at photoreceptor synapses, plays an important role in shaping visual signals by acting on postsynaptic HCs in vivo. To understand more fully the regulation and function of HC HGJ channels, we examined the effect of Zn(2+) on HGJ channel currents in bass retinal HCs. Hemichannel currents elicited by depolarization in Ca(2+)-free medium and in 1 mM Ca(2+) medium were significantly inhibited by extracellular Zn(2+). The inhibition by Zn(2+) of hemichannel currents was dose dependent with a half-maximum inhibitory concentration of 37 microM. Compared with other divalent cations, Zn(2+) exhibited higher inhibitory potency, with the order being Zn(2+) > Cd(2+) approximately Co(2+) > Ca(2+) > Ba(2+) > Mg(2+). Zn(2+) and Ca(2+) were found to modulate HGJ channels independently in additivity experiments. Modification of histidine residues with N-bromosuccinimide suppressed the inhibitory action of Zn(2+), whereas modification of cysteine residues had no significant effect on Zn(2+) inhibition. Taken together, these results suggest that zinc acts on HGJ channels in a calcium-independent way and that histidine residues on the extracellular domain of HGJ channels mediate the inhibitory action of zinc.