Permeation of Molecules through Astroglial Connexin 43 Hemichannels Is Modulated by Cytokines with Parameters Depending on the Permeant Species.

Juan C Sáez,Christian Giaume,Fernando C Ortiz,Diego E Hernández,Aníbal A Vargas,Juan A Orellana

doi:10.3390/ijms21113970

Abstract

Recent studies indicate that connexin hemichannels do not act as freely permeable non-selective pores, but they select permeants in an isoform-specific manner with cooperative, competitive and saturable kinetics. The aim of this study was to investigate whether the treatment with a mixture of IL-1β plus TNF-α, a well-known pro-inflammatory condition that activates astroglial connexin 43 (Cx43) hemichannels, could alter their permeability to molecules. We found that IL-1β plus TNF-α left-shifted the dye uptake rate vs. dye concentration relationship for Etd and 2-NBDG, but the opposite took place for DAPI or YO-PRO-1, whereas no alterations were observed for Prd. The latter modifications were accompanied of changes in Kd (Etd, DAPI, YO-PRO-1 or 2-NBDG) and Hill coefficients (Etd and YO-PRO-1), but not in alterations of Vmax. We speculate that IL-1β plus TNF-α may distinctively affect the binding sites to permeants in astroglial Cx43 hemichannels rather than their number in the cell surface. Alternatively, IL-1β plus TNF-α could induce the production of endogenous permeants that may favor or compete for in the pore-lining residues of Cx43 hemichannels. Future studies shall elucidate whether the differential ionic/molecule permeation of Cx43 hemichannels in astrocytes could impact their communication with neurons in the normal and inflamed nervous system.

Highlights

The traditional notion of neurons acting as the unique functional unit at the synaptic cleft has been challenged with the discovery that intra and intercellular Ca2+ waves within and among astrocytes, respectively, underpin the regenerative transfer of physiological signals [1,2,3]
We found that major changes evoked by divalent cation-free solution (DCFS) and/or IL-1β/TNF-α on astroglial dye uptake (Figure 2I–L and Figure 3Q–S) remained almost unaltered for all dyes studied after normalizing the data to the amount of connexin 43 (Cx43) (Figure 4C)
The same stimulus greatly increased astroglial coupling to YO-PRO-1 from ~5 to ~14 cells (Figure 7G,H,Q,R). These findings indicate that IL-1β/TNF-α could alter the permeation of molecules via astroglial Cx43 gap junction channels depending on the permeant species

Summary

Introduction

The traditional notion of neurons acting as the unique functional unit at the synaptic cleft has been challenged with the discovery that intra and intercellular Ca2+ waves within and among astrocytes, respectively, underpin the regenerative (non-dissipative) transfer of physiological signals [1,2,3]. Despite astrocytes not being electrically silent cells [4], changes in intracellular free-Ca2+ concentration Equipped with these tools and accompanying the pre and postsynaptic neurons, astrocytes embrace the “tripartite synapse”—the centerpiece of the chemical synaptic transmission—where they sense neural function and react to it by the [Ca2+]i-dependent release of biomolecules that modulate neuronal activity called “gliotransmitters” [6]. The presence of functional and solitary hemichannels in “non-junctional” membranes may serve as a diffusional route for the release of relevant quantities of autocrine and paracrine signaling molecules (e.g., ATP, glutamate, D-serine, NAD+ and PGE2) and metabolites (i.e., GSH) as well as the influx of cell signals (i.e., Ca2+ and cADPR) and metabolites (i.e., glucose) [10]

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