The ATP-Releasing Maxi-Cl Channel: Its Identity, Molecular Partners and Physiological/Pathophysiological Implications

Abstract

The Maxi-Cl phenotype accounts for the majority (app. 60%) of reports on the large-conductance maxi-anion channels (MACs) and has been detected in almost every type of cell, including placenta, endothelium, lymphocyte, cardiac myocyte, neuron, and glial cells, and in cells originating from humans to frogs. A unitary conductance of 300-400 pS, linear current-to-voltage relationship, relatively high anion-to-cation selectivity, bell-shaped voltage dependency, and sensitivity to extracellular gadolinium are biophysical and pharmacological hallmarks of the Maxi-Cl channel. Its identification as a complex with SLCO2A1 as a core pore-forming component and two auxiliary regulatory proteins, annexin A2 and S100A10 (p11), explains the activation mechanism as Tyr23 dephosphorylation at ANXA2 in parallel with calcium binding at S100A10. In the resting state, SLCO2A1 functions as a prostaglandin transporter whereas upon activation it turns to an anion channel. As an efficient pathway for chloride, Maxi-Cl is implicated in a number of physiologically and pathophysiologically important processes, such as cell volume regulation, fluid secretion, apoptosis, and charge transfer. Maxi-Cl is permeable for ATP and other small signaling molecules serving as an electrogenic pathway in cell-to-cell signal transduction. Mutations at the SLCO2A1 gene cause inherited bone and gut pathologies and malignancies, signifying the Maxi-Cl channel as a perspective pharmacological target.

Keywords: ANXA2; ATP release; Maxi-Cl; S100A10; SLCO2A1; biophysical properties; diseases; glutamate release; maxi-anion channels; purinergic signaling.

Figure 1

Maxi-Cl activation in mammary C127 cells at the macroscopic current and single-channel levels. (A) Whole-cell currents activated in response to hypoosmotic stress; pipette ATP was omitted to favor dephosphorylation and phloretin was used to block residual VSOR currents. (B) Consecutive activation of single Maxi-Cl channels upon patch excision into an artificial intracellular solution. (C) Maxi-Cl activation in the on-cell mode in response to pharmacological inhibition of the tyrosine kinases. Adopted from [54,62].

Figure 2

Maxi-Cl voltage dependence and permeability to physiologically important organic anions. (A) Six consecutive traces of the current response to the ramp pulse applied to KML melanoma cells. Inset shows the open-channel probability obtained by averaging 47 traces recorded from the same patch. Experimental conditions were the same as reported previously [54]. (B) Current-to-voltage relationship of the Maxi-Cl excised from H9C2 cells into the normal Ringer solution (open symbols) and after replacing the bath with a solution containing 135 mM GSH and 11 mM Cl (filled blue symbols); the curve reverses at –34.8 ± 0.9 mV, yielding the P_GSH/P_Cl = 0.20 ± 0.01. Experimental conditions were the same as reported previously [54,63,71]. (C) The current responses to the ramp pulse in normal Ringer solution (black) and after replacing the bath with a solution containing only 100 mM MgATP (blue) in neonatal cardiac myocytes. The small inward current was carried by MgATP^2–. (D) Maxi-Cl permeability to short chain fatty acids and small signaling molecules evaluated in C127 cells and neonatal rat cardiomyocytes. Adopted from [13,57,67].

Figure 3

Geometry of the Maxi-Cl channel pore probed with ion permeability and nonelectrolyte partitioning. (A) Permeability to organic anions of different sizes (red symbols and curves) and relative channel conductance in the presence of nonelectrolytes (ethylene glycol, glycerol, and polyethylene glycols 200–4000) applied from the intracellular (magenta symbols and lines) and extracellular (blue symbols and lines) side of the membrane patch. The red dashed and solid lines are approximations of permeability using the excluded area theory without or with taking into account the friction force yielding the pore radii of 0.55 nm and 0.75 nm, respectively. The vertical dashed lines in magenta and blue mark the radii of the inner and outer vestibules, respectively. Adopted from [67,72]. (B) Maxi-Cl pore geometry pictured using estimates in (A). The molecules of ATP and GSH were drawn using Molecular Modeling Pro software (Norgwyn Montgomery Software Inc.) and are shown approximately in scale.

Figure 4

Steps in molecular identification of Maxi-Cl. The scheme was modelled after [35,113]. See text for details.

Figure 5

Regulatory components of Maxi-Cl and its bimodal function as a channel and a transporter. (A) Steps in identification of ANXA2 and S100A10 (p11) as regulatory components of Maxi-Cl. The scheme was modelled after [91]. See text for details. (B) Function of SLCO2A1 as a prostaglandin transporter (PGT) in the resting state. The background molecular model was adopted from [113]. Disease-related mutations are indicated. The cylinder with two hypothetical gates (hatched) denotes a path for PGE2 uptake. (C) Function of SLCO2A1 as a Maxi-Cl channel in the activated state. The sizes of two vestibules and of the selectivity filter are same as in Figure 3B. ANXA2 and p11 (S100A10) are known to form a heterotetramer, which supposedly binds to the intracellular surface of the lipid matrix and interacts with the channel core. Ca²⁺ binding at p11 and dephosphorylation at the Tyr23 residue of ANXA2 (by RPTPζ and some other tyrosine phosphatases) are prerequisite events for channel activation. The radii of PGE2 and ATP are calculated as geometric means of three dimensions using Molecular Modeling Pro software (Norgwyn Montgomery Software Inc., North Wales, PA, USA); the molecules are shown bigger compared to the pore, not in scale. For in-scale relationship see Figure 3B.