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

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.

Astroglial Hemichannels and Pannexons: The Hidden Link between Maternal Inflammation and Neurological Disorders

“…The latter agrees with the fact that both cytokines directly: (i) enhance ion currents mediated by astrocytic Cx43 hemichannels [152] and (ii) modulate the uptake of cationic molecules via these channels [172]. Relevantly, the cytokine-mediated cell influx of small molecules is dependent on the properties of the permeant species (e.g., ethidium, 2-NBDG, DAPI) [172]. Altogether this evidence suggests that maternal exposure to LPS modifies the in utero fetal brain environment, and thus, it shapes the function of astroglial hemichannels and pannexons in the offspring.…”

“…Accordingly, prenatal LPS-induced activation of Cx43 hemichannels and Panx1 channels is mitigated by the inhibition of IL-1β/TNF-α signaling and accompanied by high autocrine production of astroglial IL-1β/TNF-α [170]. The latter agrees with the fact that both cytokines directly: (i) enhance ion currents mediated by astrocytic Cx43 hemichannels [152] and (ii) modulate the uptake of cationic molecules via these channels [172]. Relevantly, the cytokine-mediated cell influx of small molecules is dependent on the properties of the permeant species (e.g., ethidium, 2-NBDG, DAPI) [172].…”

The Polyamine Spermine Potentiates the Propagation of Negatively Charged Molecules through the Astrocytic Syncytium

“…In addition, there are large pores, such as connexin [ 1 , 64 , 99 , 100 ] and pannexin hemichannels [ 2 ], which can potentially take up the fluorescent molecules from the extracellular space, particularly if astrocytes are in a low metabolic stage or during gliosis [ 99 , 101 ]. To minimize this uptake, we prevented activation and opening of large pores, such as connexin and pannexin hemichannels, P2X receptors, and others by (i) keeping concentrations of extracellular divalent cations at high physiological millimolar concentrations (block of Cx43 HCs), (ii) avoiding metabolic deficiency, and (iii) avoiding the use of any agonists which open pannexin hemichannels and P2X receptors.…”

Astroglial Connexins in Neurodegenerative Diseases

“…However, during reactive gliosis hemichannel permeability is dysregulated in a series of stress-associated conditions, such as inflammation (Orellana et al, 2009;De Bock et al, 2017), ischemia, oxidative stress (Ramachandran et al, 2007), or increased intracellular free Ca 2+ concentration ([Ca 2+ ] i ) (De Vuyst et al, 2009). A recent study further revealed that the permeability of Cx43 hemichannels in astrocytes is modulated by cytokines and relies on the permeant species characters (Sáez et al, 2020). Regions crucial for hemichannel activation regulation was also highlighted.…”