Abstract
Oligodendrocytes are highly vulnerable to glutamate excitotoxicity, a central mechanism involved in tissue damage in Multiple Sclerosis (MS). Sustained activation of AMPA receptors in rat oligodendrocytes induces cytosolic calcium overload, mitochondrial depolarization, increase of reactive oxygen species, and activation of intracelular pathways resulting in apoptotic cell death. Although many signals driven by excitotoxicity have been identified, some of the key players are still under investigation. Casein kinase 2 (CK2) is a serine/threonine kinase, constitutively expressed in all eukaryotic tissues, involved in cell proliferation, malignant transformation and apoptosis. In this study, we identify CK2 as a critical regulator of oligodendrocytic death pathways and elucidate its role as a signal inductor following excitotoxic insults. We provide evidence that CK2 activity is up-regulated in AMPA-treated oligodendrocytes and CK2 inhibition significantly diminished AMPA receptor-induced oligodendroglial death. In addition, we analyzed mitogen-activated protein kinase (MAPK) signaling after excitotoxic insult. We observed that AMPA receptor activation induced a rapid increase in c-Jun N-terminal kinase (JNK) and p38 phosphorylation that was reduced after CK2 inhibition. Moreover, blocking their phosphorylation, we enhanced oligodendrocyte survival after excitotoxic insult. Finally, we observed that the tumor suppressor p53 is activated during AMPA receptor-induced cell death and, interestingly, down-regulated by JNK or CK2 inhibition. Together, these data indicate that the increase in CK2 activity induced by excitotoxic insults regulates MAPKs, triggers p53 activation and mediates subsequent oligodendroglial loss. Therefore, targeting CK2 may be a useful strategy to prevent oligodendrocyte death in MS and other diseases involving central nervous system (CNS) white matter.
Highlights
Glutamate excitotoxic cell death can occur in virtually all cells that express ionotropic glutamate receptors (GluRs) including oligodendrocytes (Yoshioka et al, 1995; Matute et al, 1997; McDonald et al, 1998) and it has been implicated in acute injury to the central nervous system (CNS) and in chronic neurodegenerative disorders (Lipton and Rosenberg, 1994; Choi, 1988; Lee et al, 1999)
As expected, when oligodendrocytes were pretreated with Casein Kinase 2 (CK2) activator spermine, AMPA-induced increase in CK2 activity was higher compared to that observed in cells treated only with agonist (Figure 1B)
Cells subjected to excitotoxic stimulus were loaded with the probe CM-DCFDA for ROS detection and we observed that ROS generation triggered by AMPA receptor activation was significantly reduced in the presence of CK2 inhibitor TBB (103.7 ± 8.1%; Figures 2D,E). These results indicate that pharmacological CK2 inhibition reduces both cell death as well as loss of mitochondrial membrane potential and acute generation of reactive oxygen species associated with AMPA-caused excitotoxicity in oligodendrocytes in vitro
Summary
Glutamate excitotoxic cell death can occur in virtually all cells that express ionotropic glutamate receptors (GluRs) including oligodendrocytes (Yoshioka et al, 1995; Matute et al, 1997; McDonald et al, 1998) and it has been implicated in acute injury to the central nervous system (CNS) and in chronic neurodegenerative disorders (Lipton and Rosenberg, 1994; Choi, 1988; Lee et al, 1999). In addition to its apoptotic function, a number of studies have suggested a pro-inflammatory role for CK2, including investigations using experimental autoimmune encephalomyelitis (EAE), a key animal model for MS. These studies established that the CD5-dependent CK2 signaling pathway represents a major signaling cascade initiated by CD5 that regulates the threshold of T cell activation and Th differentiation and impacts the outcome of EAE, so that mice deficient in CD5-CK2 signaling pathway are mostly resistant to EAE (Axtell et al, 2006; Sestero et al, 2012; Mier-Aguilar et al, 2016). CK2 pharmacological inhibition ameliorates EAE severity and relapse incidence (Ulges et al, 2016) as well as attenuates apoptosis and inflammatory cell infiltration after renal ischemia-reperfusion injury (Ka et al, 2015)
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