Expression and Analysis of Flow-regulated Ion Channels in Xenopus Oocytes

A method for assessing ionic and molecular permeation in connexin hemichannels

Connexin hemichannels are permeable to both atomic ions and small molecules. Yet, they have different selectivity for ions and signaling molecules critical for biological functions. Activity of connexin hemichannels in living cells is commonly evaluated by methods that include electrophysiology and fluorescence-based approaches. Although less common, luminescence and radioactivity-based uptake/release assays have been also successfully used to determine selectivity and permeability to different molecules. The current methods, however, have important technical and quantitative limitations that make them unsuitable for simultaneously evaluating ionic and molecular permeability using different stimuli that control channel gating (e.g., voltage or extracellular Ca²⁺). To address this, we have recently designed a novel methodology that combines two-electrode voltage clamp (TEVC) and dye uptake assays in translucent Xenopus oocytes. This method allows for the evaluation of molecular transport kinetics in connexin hemichannels, and its utility can also be extended to other large pore channels, such as those formed by pannexin and CALHM. In this article, we describe step by step the protocol to perform the TEVC/Dye uptake assay.

Pore-lining residues of MEC-4 and MEC-10 channel subunits tune the Caenorhabditis elegans degenerin channel's response to shear stress

The Caenorhabditis elegans MEC-4/MEC-10 channel mediates the worm's response to gentle body touch and is activated by laminar shear stress (LSS) when expressed in Xenopus oocytes. Substitutions at multiple sites within the second transmembrane domain (TM2) of MEC-4 or MEC-10 abolish the gentle touch response in worms, but the roles of these residues in mechanosensing are unclear. The present study therefore examined the role of specific MEC-4 and MEC-10 TM2 residues in the channel's response to LSS. We found that introducing mutations within the TM2 of MEC-4 or MEC-10 not only altered channel activity, but also affected the channel's response to LSS. This response was enhanced by Cys substitutions at selected MEC-4 sites (Phe⁷¹⁵, Gly⁷¹⁶, Gln⁷¹⁸, and Leu⁷¹⁹) between the degenerin and the putative amiloride-binding sites in this subunit. In contrast, the LSS response was largely blunted in MEC-10 variants bearing single Cys substitutions in the regions preceding and following the amiloride-binding site (Gly⁶⁷⁷-Leu⁶⁸¹), as well as with four MEC-10 touch-deficient mutations that introduced charged residues into the TM2 domain. An enhanced response to LSS was observed with a MEC-10 mutation in the putative selectivity filter. Overall, MEC-4 or MEC-10 mutants that altered the channel's LSS response are primarily clustered between the degenerin site and the selectivity filter, a region that probably forms the narrowest portion of the channel pore. Our results suggest that pore-lining residues of MEC-4 and MEC-10 have important yet different roles in tuning the channel's response to mechanical forces.

Boron Uptake Assay in Xenopus laevis Oocytes

Boron (B) is essential for plant growth and taken up by plant roots as boric acid. Under B limitation, B uptake and translocation in plants are dependent on the boric acid channels located in the plasma membrane. Xenopus leavis oocyte is a reliable heterologous expression system to characterize transport activities of boric acid channels and related major intrinsic proteins (aquaporins). Here, we outline the protocols for expression of boric acid channels and boric acid uptake assay in Xenopus leavis oocytes.