Connexin47 Protein Phosphorylation and Stability in Oligodendrocytes Depend on Expression of Connexin43 Protein in Astrocytes

Abstract

Panglial networks are essential for normal physiology in the CNS, and the function of distinct connexins participating in these networks is not well understood. We generated Connexin32 (Cx32)-deficient mice with additional deletion of astrocytic Cx43 to explore the role of both connexins in panglial networks. Cx43/Cx32 double knock-out (dKO) mice revealed strong microglial activation in corpus callosum and cingulum along with severe astrogliosis and scar formation. In addition, most of the fine myelinated fibers projecting from the corpus callosum into the cortex were lost. Myelin loss was caused by a strong decrease of oligodendrocytes in the cingulum of Cx43/Cx32dKO mice. Immunoblot analyses using newly generated specific Cx47 antibodies revealed that oligodendrocytic Cx47 is phosphorylated in vivo depending on astrocytic Cx43 expression. In Cx43-deficient mice, Cx47 protein levels were strongly decreased, whereas Cx47 mRNA levels were not altered. Using Cx43G138R/Cx30KO mice, we show that Cx47 expression depends on the presence of astrocytic Cx43 protein and that its gap junctional channel function is not necessary for Cx47 stabilization. In consequence, Cx43/Cx32dKO mice additionally lack Cx47 expression and therefore cannot form oligodendrocytic gap junctions, which explains the phenotypic similarities to Cx32/Cx47dKO mice. Our findings provide strong evidence that phosphorylation and stability of oligodendrocytic Cx47 proteins is dependent on astrocytic Cx43 expression. These results further unravel the complexity of panglial networks and show that results of previous studies using astrocytic Cx43-deficient mice have to be reconsidered.

Connexin Hemichannels in Astrocytes: An Assessment of Controversies Regarding Their Functional Characteristics

“…Along these lines, a mouse strain with selective removal of the last five amino acids from Cx43 exhibited functional gap junctions but mislocalized voltage-gated Na + channels (Nav5.1); the misdirected Na + channels, rather than defective Cx43, were the direct cause of the observed lethal arrhythmias [89]. Astrocytic deletion of Cx43 caused strongly decreased oligodendrocytic Cx47 protein expression and phosphorylation [90] and significantly reduced basal ATP concentration [91]. Reduced ATP content could lead to a decline in ATP release on the basis of a diminished transmembrane gradient, completely independent of any changes in ATP-permeable Cx43 hemichannels at the cell membrane [91].…”

A start codon CMT1X mutation associated with transient encephalomyelitis causes complete loss of Cx32

“…Oligodendrocytes in CMT1X patients depending mainly on Cx47 for GJ connectivity could be more vulnerable to these stressors, as has been shown by EAE induction in Cx32 KO mice, which develop more demyelination and axonal loss than WT animals with EAE [36]. Oligodendrocyte-astrocyte connectivity is disrupted under inflammatory conditions as the main astrocytic partner of Cx47, Cx43, is downregulated [36] and Cx47 membrane expression depends on the interaction with Cx43 [37]. Downregulation of Cx43 was also shown to occur in acutely inflamed human brain [38].…”

Astrocytes induce proliferation of oligodendrocyte progenitor cells via connexin 47-mediated activation of the ERK/Id4 pathway

“…24,26 Cx47 expression relies on Cx43 in ASTs, and heterotypic GJs formed by ASTs and OLs are the major form of (O/A) GJs. 27 GJs allow ASTs in different regions to form 3-dimensional channel structures with other glial cells and to propagate intracellular calcium signaling, 28 among other important functions. Herrero-Gonz alez et al noted that calcium signaling through GJs can cause an increase in free calcium in neighboring cells, 29 and a study by Parys et al showed that calcium signaling can be transmitted from ASTs to OLs.…”