Regulation of the kinase MARK from Rattus norvegicus (Brekenhout, 1769) by GSK3 ; Regulation der Kinase MARK aus Rattus norvegicus (Berkenhout, 1769) durch GSK3

MARK (MAP/Microtubule Affinity Regulating Kinase), a kinase family related to PAR-1, was first discovered because of the ability to phosphorylate the neuronal microtubule-associated protein (tau/MAP2/MAP4) at the KXGS motifs located in the repeat domain. The consequence of this phosphorylation is that MAPs detach from microtubules and microtubules become highly dynamic (Drewes et al., 1997). This kind of phosphorylation of tau is enhanced in the brain of Alzheimer patients (Augustinack et al., 2002). MARK2 isolated from mammalian brain is partly phosphorylated at both Thr208 and Ser212, which reside in the activation loop. Activation of MARK2 is achieved by phosphorylation at Thr208 by the activating kinase MARKK or LKB1 (Timm et al., 2003; Lizcano et al., 2004). It requires Ser212 to be present, but not phosphorylated, because this phosphorylation is inhibitory as judged by mutational analysis (Timm et al., 2003). In the present study, GSK3ß is identified as the inhibitory kinase that phosphorylates Ser212. This is shown by in vitro kinase assays as well as in cells. In N2a/F113 cells, overexpression of the constitutively active MARK2T208E together with the constitutively active GSK3ßS9A, leads to the complete loss of phosphorylation at the KXGS motifs of tau indicating that MARK2 activity is inhibited. To further characterize the influence of GSK3ß-MARK-Tau signaling cascade on the microtubule network, CHO cells are used as a cell model. Overexpression of MARK2 alone leads to phosphorylation and detachment of tau or other equivalent MAPs from the microtubules, leading to microtubule breakdown and eventually cell death. Overexpression of GSK3ß alone also leads to cell death probably due to apoptosis (Hetmann et al., 2000; Pap et al., 1998). In contrast, co-expression of GSK3ß with MARK2 inhibits the activity of MARK2 by phosphorylation of Ser212 and leads to a reduction of phosphorylation at the KXGS motifs of tau and rescue from microtubule break down and cell death. In PC12 cells, overexpression of MARK2 ...