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  • Introduction We have recently identified

    2019-10-09

    Introduction We have recently identified that caseinkinase 2 (CK2) inhibition protects white matter (WM) from ischemic injury. We focused our research on CK2 because upregulation of CK2 activity is associated with many diseases, including ischemic injury [1,[6], [7], [8]]. This review highlights and contrasts the role of CK2 in WM and gray matter (GM) portions of the brain following ischemia together with their molecular mechanisms. Finally, we incorporate findings from cancer research to explore whether some of those identified mechanisms of CK2 activity can be extrapolated to stroke research. CK2 is not a conventional protein kinase (PK). CK2 is composed of two catalytic α-subunits (α and α’) and two regulatory β-subunits [[2], [3], [4], [5]]. First, despite the initial lack of substrates following its discovery, CK2 was subsequently shown to phosphorylate numerous substrates, including other PKs, thus acting as a “master regulator” [10,11]. Second, unlike other PKs, the catalytic activity of CK2 is not regulated by second messengers or phosphorylation [11,12] and it was found to be constitutively active. However, CK2 was recently proposed to be activated via a polymerization/depolymerization mechanism [[13], [14], [15]] together with reactive oxygen species (ROS) [[16], [17], [18]].
    CK2 and WM ischemia Transient CK2 inhibition is a novel and effective therapeutic strategy to protect WM against ischemic stroke. We recently reported that a brief application of the CK2 inhibitor CX-4945 preserves mouse optic nerve (MON) axon function following ischemic injury (Fig. 1A–C) [1]. This is the first report that a PK inhibitor protects young, aging, and old WM from ischemic injury (Fig. 1D–F) [1]. Improving MON function following ischemia is an important endpoint, as axon injury is an independent risk factor and burden for adverse outcomes following a stroke, even in intravenous thrombolysis patients [19]. To investigate the mechanisms by which CK2 inhibition protects WM from ischemic injury, we investigated the impact of CX-4945 on the cellular components of MONs. WM is composed of astrocytes, oligodendrocytes, microglia, and axons [20]. First, we showed that the CK2α subunit is expressed in glial cell compartments. We evaluated the expression and localization of CK2 in MONs using immunohistochemistry and glial cell-specific Reboxetine mesylate and in conjunction with confocal imaging to support a biological basis for CK2 inhibitor action in MONs. The expression of CK2α subunit co-localized with GFAP (+) astrocyte nuclei and some processes and also with NF-200 (+) axons. Almost all Olig2 (+) oligodendrocytes were strongly immunoreactive for CK2α. Immunolabeling was also evident on PLP (+) myelin. This extensive expression of CK2α in glial cells in addition to axons implicate them as cellular targets of CX-4945 [1]. Expectedly, CK2 inhibition protected axons and oligodendrocytes from ischemic injury. We assessed the impact of CK2 inhibition on nuclear morphology, oligodendrocytes, and the axon cytoskeleton, which are critical elements that show widespread injury after ischemia [21,22]. CK2 inhibition preserved APC (+) oligodendrocytes and SMI-31 (+) axonal labeling (Fig. 2A). Whether CK2 inhibition directly or indirectly protects axon function and/or structure remains unresolved. We suggest that oligodendrocyte death predisposes axons to injury and improper conduction because oligodendrocytes not only provide the structural and electrical framework for fast and synchronized axonal conduction [23], but they also support axons metabolically [24]. Therefore, oligodendrocyte death will lead to disruption of axonal myelin and nodes of Ranvier to alter axonal conduction, while interruption of metabolic support to the axon will increase the strain on already-decreased levels of ATP in the face of decreased oxygen levels during ischemic stress [25,27]. Note that axons are independent of neuronal cell bodies in terms of energy supply and are completely dependent upon local energy production. We previously reported that in WM, post-injury protection correlated with conservation of mitochondrial integrity during ischemic injury [29,30,41,42]. Therefore, we postulated that the protective effects of CK2 inhibition in promoting axon function recovery after ischemia correlate with axonal mitochondrial preservation. Indeed, when we monitored mitochondrial fluorescence in MONs isolated from Thy-1 mito CFP mice [28] during control conditions and during oxygen glucose deprivation (OGD) with or without CX-4945 treatment, we confirmed this hypothesis. Mitochondria in control MONs displayed short tubular morphology (Fig. 2B, left panel). After OGD, there was a dramatic loss of mitochondrial CFP (+) fluorescence (Fig. 2B, middle panel). The remaining mitochondria exhibited a small punctate morphology that is typical of ischemia-induced mitochondrial fission [31]. Pretreatment of MONs with CX-4945 effectively attenuated the loss of mitochondrial fluorescence and preserved mitochondrial morphology (Fig. 2B, right panel, and 2C). These findings confirmed and expanded our previous reports that approaches conferring post-ischemic protection to WM function correlate with preserved mitochondrial integrity [29,30].