Protein kinase CK2 impairs spatial memory formation through differential cross talk with PI-3 kinase signaling: activation of Akt and inactivation of SGK1

Abstract

Casein kinase II (CK2) is a multifunctional serine/threonine protein kinase that is associated with the development of neuritogenesis and synaptic plasticity. The phosphoinositide 3-kinase (PI-3K)/Akt pathway is implicated in long-term memory formation. In addition, serum- and glucocorticoid-inducible kinase 1 (SGK1) is another downstream target of PI-3K signaling that was shown to play an important role in spatial memory formation. Whether CK2 may also affect memory formation and whether CK2 interacts with Akt and SGK1 during this process is unknown. In the present study, we found that water maze training significantly decreased CK2 activity in the rat hippocampal CA1 area but not in the dentate gyrus (DG) area. Transfection of the dominant negative mutant of CK2, CK2alphaA(156), to the CA1 area, but not to the DG area, decreased CK2 activity but enhanced spatial memory formation. Meanwhile, it increased SGK1 phosphorylation at Ser422, decreased Akt phosphorylation at Ser473, and increased cAMP response element-binding protein phosphorylation at Ser133. Transfection of the constitutively active SGK1, SGKS422D, enhanced whereas transfection of the wild-type Akt impaired spatial memory formation. Also, administration of the protein phosphatase 2A inhibitor, fostriecin, reversed the memory-impairing effect of CK2alphaWT. It also reversed the effect of CK2alphaWT in decreasing SGK1 phosphorylation. Akt Ser473 phosphorylation was moderately increased by CK2alphaWT and fostriecin treatment, but AktS473A mutant transfection reversed the memory-impairing effect of CK2alphaWT. These results together suggest that CK2 impairs spatial memory formation through differential cross talk with PI-3 kinase signaling by activation of Akt and inactivation of SGK1 through protein phosphatase 2A.

Figure 1.

Effects of CK2 on spatial memory formation and phosphorylation of SGK and Akt. A, CK2 activity is markedly decreased in CA1 area, but not DG area, after water maze training (n = 5 each group). B, Overexpression of CK2αA¹⁵⁶ in CA1 neurons enhanced spatial memory formation (left; n = 7–8 each group), but overexpression of CK2αA¹⁵⁶ in DG neurons did not markedly affect spatial memory performance (right; n = 6 each group). C, Immunohistochemistry showing the transfection and expression of CK2αA¹⁵⁶ in individual CA1 neurons and DG neurons. Arrow indicates the area of transfection. Scale bars: top, 400 μm; bottom, 20 μm. D, Western blot showing that CK2αA¹⁵⁶ transfection increased SGK1 phosphorylation at Ser422 (left), decreased Akt phosphorylation at Ser473 (middle), and increased CREB phosphorylation at Ser133 (right) in CA1 neurons. Data are mean ± SEM. *p < 0.05; **p < 0.01; ***p = 0.001.

Figure 2.

SGK1 enhanced whereas Akt impaired spatial memory formation. A, Transfection of SGKS422D to CA1 area enhanced, whereas transfection of SGKS422A impaired spatial memory formation (n = 7–8 for each group). B, Transfection of wild-type Akt to CA1 area impaired, but transfection of AktS473A enhanced spatial memory formation (n = 9–10 each group). Data are mean ± SEM. p < 0.01.

Figure 3.

Inhibition of PP2A and transfection of AktS473A both reversed the memory-impairing effect of CK2α. A, CK2α transfection decreased SGK1 phosphorylation at Ser422, but this effect is reversed by PP2A inhibition through FST administration (n = 7–8 each group). *p < 0.05. A, Compared with control group; b, compared with CK2αWT group. B, CK2α overexpression moderately increased Akt phosphorylation at Ser473, and this effect is slightly potentiated by FST injection. C, A representative illustration showing needle placement and dye distribution in the CA1 area. Scale bar, 400 μm. D, Both PP2A inhibition (by FST injection) and AktS473A transfection reversed the memory-impairing effect of CK2αWT (n = 8–11 each group). FST, Fostriecin. Data are mean ± SEM. Both p < 0.01.

Figure 4.

A schematic diagram showing the signaling cascade from CK2 to Akt and SGK1 involved in spatial memory formation. On stimulation, CK2 phosphorylates Akt directly, whereas overexpression of Akt impaired spatial memory formation through phosphorylation of substrate proteins. Conversely, CK2 activates PP2A, but PP2A inactivates SGK1 through dephosphorylation of SGK1. SGK1 facilitates spatial memory through phosphorylation of substrate proteins, such as CREB. Thus, CK2 impairs spatial memory formation through activation of Akt and inactivation of SGK1.