Upregulation of Connexin 43 Expression Via C-Jun N-Terminal Kinase Signaling in Prion Disease

Abstract

Prion infection leads to neuronal cell death, glial cell activation, and the accumulation of misfolded prion proteins. However, the altered cellular environments in animals with prion diseases are poorly understood. In the central nervous system, cells connect the cytoplasm of adjacent cells via connexin (Cx)-assembled gap junction channels to allow the direct exchange of small molecules, including ions, neurotransmitters, and signaling molecules, which regulate the activities of the connected cells. Here, we investigate the role of Cx43 in the pathogenesis of prion diseases. Upregulated Cx43 expression, which was dependent on c-Jun N-Terminal Kinase (JNK)/c-Jun signaling cascades, was found in prion-affected brain tissues and hippocampal neuronal cells. Scrapie infection-induced Cx43 formed aggregated plaques within the cytoplasmic compartments at the cell-cell interfaces. The ethidium bromide (EtBr) uptake assay and scrape-loading dye transfer assay demonstrated that increased Cx43 has functional consequences for the activity of Cx43 hemichannels. Interestingly, blockade of PrPSc accumulation reduced Cx43 expression through the inhibition of JNK signaling, indicating that PrPSc accumulation may be directly involved in JNK activation-mediated Cx43 upregulation. Overall, our findings describe a scrapie infection-mediated novel regulatory signaling pathway of Cx43 expression and may suggest a role for Cx43 in the pathogenesis of prion diseases.

RhoA/ROCK Regulates Prion Pathogenesis by Controlling Connexin 43 Activity

“…Moreover, in a Parkinson's disease model, Cx43 immunoreactivity was elevated in astrocytes, glial cells and the methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP)-lesioned striatum [30]. We also demonstrated previously that the level of Cx43 was upregulated in prion disease models, but the exact mechanism of such overexpression has not been elucidated [33]. These results clearly indicate the involvement of Cx43 in the pathophysiology of various tissue conditions.…”

“…A recent study demonstrated that overexpression of the disease-associated mutants of PrP C (P102L and MΔ8) impaired neuronal differentiation because of the failure to inactivate RhoA/ROCK and reduced the coimmunoprecipitation of RhoA and p190RhoGAP in PC12 cells [17,18]. In addition, scrapie infection upregulated the expression of Cx43 through the JNK signaling pathway in both in vitro and in vivo models of prion disease [33].…”

“…We first investigated the effect of scrapie infection on RhoA activity and the RhoA-ROCK-LIMK-cofilin pathway. As shown in Figure 1, compared to levels in control hippocampal neuronal cells, increased RhoA-guanosine triphosphate (GTP) levels were found in scrapie-infected mouse hippocampal neuronal cells ( Figure 1A), which were used as an in vitro model of prion replication after infection with either of two mouse-derived scrapie strains (22L or 139A) [33,36]. Moreover, scrapie infection induced the phosphorylation of LIMK1/2 at Thr508 and Thr505 (P-LIMK1/2) and cofilin at Ser3 (P-cofilin) with a reduction in the phosphorylation of RhoA at Ser188 (P-RhoA) ( Figure 1B).…”

“…Previous studies have reported that scrapie infection upregulates Cx43 expression through the c-Jun N-terminal kinase (JNK) signaling pathway in the brains of mice and hippocampal neuronal cells [33]. Furthermore, RhoA was previously identified as a Cx43-interacting protein by proteomic analysis [35].…”

“…Gap junction alpha-1 protein (GJA1), known as Cx43, forms intercellular channels that connect adjacent cells to permit the exchange of low molecular weight molecules, such as adenosine triphosphate, glutamate, and Ca 2+ ions, to maintain homeostasis [23][24][25]. Cx43 is widely expressed in the CNS, especially neurons, microglia, astrocytes, and oligodendrocytes [26][27][28], and has been shown to be involved in various neurodegenerative diseases, including Alzheimer's disease [29], Parkinson's disease [30], multiple sclerosis [31], epilepsy [32] and prion diseases [33]. In addition, a report demonstrated that Cx43 channel permeability is controlled by RhoA/ROCK activity and F-actin formation [34].…”