Neuroinflammation Leads to Region-Dependent Alterations in Astrocyte Gap Junction Communication and Hemichannel Activity

Abstract

Inflammation attenuates gap junction (GJ) communication in cultured astrocytes. Here we utilized a well-characterized model of experimental brain abscess as a tool to query effects of the CNS inflammatory milieu on astrocyte GJ communication and electrophysiological properties. Whole-cell patch-clamp recordings were performed on GFP-positive astrocytes in acute brain slices from GFAP-GFP mice at 3 or 7 days following S. aureus infection in the striatum. Astrocyte GJ communication was significantly attenuated in regions immediately surrounding the abscess margins and progressively increased to levels typical of uninfected brain with increasing distance from the abscess proper. Conversely, astrocytes bordering the abscess demonstrated hemichannel activity as evident by enhanced EtBr uptake that could be blocked by several pharmacological inhibitors including the connexin 43 (Cx43) mimetic peptide Gap26, carbenoxolone, the pannexin1 (Panx1) mimetic peptide 10Panx1, and probenecid. However, hemichannel opening was transient with astrocytic EtBr uptake observed near the abscess at day 3 but not day 7 post-infection. The region-dependent pattern of hemichannel activity at day 3 directly correlated with increases in Cx43, Cx30, Panx1, and glutamate transporter expression (GLT and GLAST) along the abscess margins. Changes in astrocyte resting membrane potential and input conductance correlated with the observed changes in GJ communication and hemichannel activity. Collectively, these findings indicate that astrocyte coupling and electrical properties are most dramatically affected near the primary inflammatory site and reveal an opposing relationship between the open states of GJ channels versus hemichannels during acute infection. This relationship may extend to other CNS diseases typified with an inflammatory component.

Connexin-based channels contribute to metabolic pathways in the oligodendroglial lineage

“…Moreover, they are triggered to open by some stimulation, such as inflammation, mechanical stress or ischemia (Batra et al, 2012;Johansen et al, 2011;Karpuk et al, 2011). In our study, however, we demonstrated for the first time that connexin-mediated hemichannels are responsible for glucose uptake in OPCs, implying a role for oligodendroglial connexin hemichannels under physiological conditions.…”

Astrocytes and microglia in acute cerebral injury underlying cerebral palsy associated with preterm birth

“…Both release of cytokines, such as interleukin-1β and TNF-α by activated microglia in vitro, and Staphylococcus aureus brain infection in mice, are associated with reduced gap junctional communication but paradoxically increased Cx43 hemichannel activity (78). Intriguingly, hemichannel activity was increased by cytokine exposure in vitro, through a p38 mitogen-activated protein kinase-dependent pathway, even though Cx43 levels were reduced in vitro (79), whereas bacterial infection was associated with induction of Cx43 (78). Enhanced hemichannel opening resolved by 7 d after bacterial infection (78), highlighting the importance of timing of exposure.…”

Role of Connexin Hemichannels in Neurodegeneration

“…Moreover, this transient hemichannel opening was blocked with the Cx43 mimetic peptide Gap26, carbenoxolone, the Panx1 mimetic peptide 10 panx1, and probenecid (Karpuk et al 2011), indicating the involvement of both Cx43 and Panx1 hemichannels in this response. In addition, astroglial gap junction coupling was significantly reduced in areas immediately surrounding the abscess margins, while regions far from abscess presented normal coupling (Karpuk et al 2011). These data are consistent with previous studies showing opposite regulation of gap junction channels versus hemichannels in astrocytes subjected to pro-inflammatory conditions (Froger et al 2010;Froger et al 2009;Orellana et al 2011b;Orellana et al 2010;Retamal et al 2007a).…”

“…Recently, hemichannels have been implicated in the pathogenesis of bacterial meningitis. Using a model of bacterial brain abscess, Karpuk and co-workers (Karpuk et al 2011) showed that a transient hemichannel activity is induced in astrocytes within close proximity to the abscess border, which dissipates with increasing distance from the inflammatory site. Moreover, this transient hemichannel opening was blocked with the Cx43 mimetic peptide Gap26, carbenoxolone, the Panx1 mimetic peptide 10 panx1, and probenecid (Karpuk et al 2011), indicating the involvement of both Cx43 and Panx1 hemichannels in this response.…”

“…Using a model of bacterial brain abscess, Karpuk and co-workers (Karpuk et al 2011) showed that a transient hemichannel activity is induced in astrocytes within close proximity to the abscess border, which dissipates with increasing distance from the inflammatory site. Moreover, this transient hemichannel opening was blocked with the Cx43 mimetic peptide Gap26, carbenoxolone, the Panx1 mimetic peptide 10 panx1, and probenecid (Karpuk et al 2011), indicating the involvement of both Cx43 and Panx1 hemichannels in this response. In addition, astroglial gap junction coupling was significantly reduced in areas immediately surrounding the abscess margins, while regions far from abscess presented normal coupling (Karpuk et al 2011).…”