Brain macrophages inhibit gap junctional communication and downregulate connexin 43 expression in cultured astrocytes

Abstract

Astrocytes are typically interconnected by gap junction channels that allow, in vitro as well as in vivo, a high degree of intercellular communication between these glial cells. Using cocultures of astrocytes and neurons, we have demonstrated that gap junctional communication (GJC) and connexin 43 (Cx43) expression, the major junctional protein in astrocytes, are controlled by neuronal activity. Moreover, neuronal death downregulates these two parameters. Because in several brain pathologies neuronal loss is associated with an increase in brain macrophage (BM) density, we have now investigated whether coculture with BM affects astrocyte gap junctions. We report here that addition of BM for 24 h decreases the expression of GJC and Cx43 in astrocytes in a density-dependent manner. In contrast, Cx43 is not detected in BM and no heterotypic coupling is observed between the two cell types. A soluble factor does not seem to be involved in these inhibitions because they are not observed either in the presence of BM conditioned media or in the absence of direct contact between the two cell types by using inserts. These observations could have pathophysiological relevance as neuronal death, microglial proliferation and astrocytic reactions occur in brain injuries and pathologies. Because astrocyte interactions with BM and dying neurons both result in the downregulation of Cx43 expression and in the inhibition of GJC, a critical consequence on astrocytic phenotype in those situations could be the inhibition of gap junctions.

Role of connexin-based gap junction channels and hemichannels in ischemia-induced cell death in nervous tissue

“…A source of IL-1 could be reactive astrocytes as well as activated microglia. In agreement, gap junctional communication between astrocytes is reduced by activated micloglia in cultures [121,42]. These changes in connexin expression may represent reactive gliosis or be part of adaptive processes that reduce damage or provide tolerance to subsequent ischemic episodes (preconditioning).…”

“…A source of IL-1 could be reactive astrocytes as well as activated microglia. Gap junctional communication between astrocytes is reduced by activated microglia in cultures [121,42].…”

Cx43 Hemichannels and Gap Junction Channels in Astrocytes Are Regulated Oppositely by Proinflammatory Cytokines Released from Activated Microglia

“…Under normal conditions, cultured astrocytes are highly coupled through gap junction channels composed of connexin43 (Cx43) (Giaume et al, 1991). The inhibition of astrocytic gap junctional communication (GJC), proceeded by the addition of resting MG on cultured astrocytes, has suggested a functional interaction between astrocytic GJC and MG (Rouach et al, 2002b). Recently, soluble factors secreted by activated MG were identified as responsible for the stronger inhibition of astrocyte-astrocyte GJC (Faustmann et al, 2003;Même et al, 2006).…”

Connexin 43 stabilizes astrocytes in a stroke-like milieu to facilitate neuronal recovery

“…It has been reported that Cx43 could be protective by releasing ATP. Moreover, the inflammatory milieu could cause the opposite regulation of gap junctions and hemichannels [7][8][9] , which are two types of functional units composed of Cx43. Functionally increased hemichannels by hypoxia or proinflammatory factors can mediate the release of deleterious substances that could impair glial and neuronal survival [7,10] .…”