Biphasic effect of linoleic acid on connexin 46 hemichannels

Abstract

Connexins form hemichannels at undocked plasma membranes and gap-junction channels (GJCs) at intercellular contacting zones. Under physiological conditions, hemichannels have low open probabilities, but their activation under pathological conditions, such as ischemia, induces and/or accelerates cell death. Connexin 46 (Cx46) is a major connexin of the lens, and mutations of this connexin induce cataracts. Here, we report the effects of linoleic acid (LA) on the electrical properties of Cx46 GJCs and hemichannels expressed in Xenopus laevis oocytes. LA has a biphasic effect, increasing hemichannel current at 0.1 μM and decreasing it at concentrations of 100 μM or higher. The effects of extracellular and microinjected LA conjugated to coenzyme A (LA-CoA) suggest that the current activation site is accessible from the intracellular but not extracellular compartment, whereas the current inhibitory site is either located in a region of the hemichannel pore inaccessible to intracellular LA-CoA, or requires crossing of LA through an organelle membrane. Experiments with other fatty acids demonstrated that the block of hemichannels depends on the presence of a hydrogenated double bond at position 9 and is directly proportional to the number of double bonds. Experiments in paired oocytes expressing Cx46 showed that LA does not affect GJCs. The block by unsaturated fatty acids reported here opens the possibility that increases in the concentration of these lipids in the lens induce cataract formation by blocking Cx46 hemichannels.

Fatty Acid Regulation of Voltage- and Ligand-Gated Ion Channel Function

“…This dual mechanism was also observed in the muscle rNa(V)1.4 channel isoform, which appears to depend on the depolarizing potential: AA, but not its metabolites, increased the channel current evoked by a −30 or −40 mV depolarization of the membrane potential, but significantly decreased it by a depolarization over −10 mV (Gu et al, 2009). It was also reported that cis -unsaturated FFA with a double bond at position 9 have a biphasic effect on connexin 46 hemichannels: current activation at low FFA concentration and current inhibition at higher concentrations, the effect being directly proportional to the number of double bonds (Retamar et al, 2011). The authors ruled out the possibility that the biphasic effect was mediated by changes in the biophysical properties of the plasma membrane.…”

“…The authors ruled out the possibility that the biphasic effect was mediated by changes in the biophysical properties of the plasma membrane. They concluded that activation and inhibition current mechanisms involve different sites of action (Retamar et al, 2011). …”

Connexin channel and its role in diabetic retinopathy

“…However, the activation or opening of these channels is involved in autocrine/paracrine signaling, which provides a pathway for release of ATP (Cotrina et al, 1998), glutamate (Ye et al, 2003), NAD + (Bruzzone et al, 2001) and prostaglandins (Cherian et al, 2005), and mediate various aspects of cell signaling and function. Hemichannel activity is regulated by posttranslational modifications of connexin subunits, e.g., S-nitrosylation, phosphorylation and dephosphorylation (Contreras et al, 2003; Retamal et al, 2006), membrane physiochemical conditions, e.g., membrane voltage, pH, mechanical stimulation and concentrations of monovalent and divalent cations (Burra et al, 2010; Schalper et al, 2010; Siller-Jackson et al, 2008) and by extracellular signaling molecules, e.g., ATP, FGF1/2 and lipophilic molecules (Garre et al, 2010; Kar et al, 2012; Retamal et al, 2011; Schalper et al, 2012). In addition to connexin hemichannels, pannexins also form channels with similar molecular stoichiometry and permeability properties as connexin hemichannels.…”

Linoleic acid permeabilizes gastric epithelial cells by increasing connexin 43 levels in the cell membrane via a GPR40- and Akt-dependent mechanism

“…However, knowledge about the regulation of HCs (or GJCs) by FFAs in the epithelium of the gastrointestinal tract is very limited and is incompletely understood in other cell types [21-24]. …”

“…Linoleic acid (LA) has been shown to modulate the functional state of Cx46 HCs [21]. Moreover, in HeLa cells transfected with different Cxs, our group reported that LA increases Cx26, Cx43 and Cx45 HC activity, and this effect is associated with elevated intracellular free Ca 2+ concentration and activation of the PI3K/Akt-dependent pathway [24].…”