Glycogen Synthase Kinase-3.BETA.2 Has Lower Phosphorylation Activity to Tau than Glycogen Synthase Kinase-3.BETA.1

Abstract

Glycogen synthase kinase-3b (GSK-3b) is a serine/threonine kinase that is involved in the regulation of many signaling pathways in vertebrates.1-3) GSK-3b is expressed in two splice variants. GSK-3b1 (the short form of GSK-3b) ubiquitously distributes in organs. In contrast, GSK-3b2 (the long form of GSK-3b), whose structural difference is the only 13 amino acids insert at the C-terminal side of the catalytic site, is present in central nervous system. 4,5) Although, so far, many researches on GSK-3b are concerned with GSK-3b1 because of its high level of expression in various tissues, there is little research attention to the minor form, GSK-3b2. At present, the substrate preferences and physiological significances of the two variants remains unclear.GSK-3b is known to phosphorylate the microtubule-associated protein tau at Alzhemier's disease (AD)-relevant epitopes in vivo and in vitro.6,7) The hyperphosphorylated tau is known to the major component of neurofibrillary tangles observed in AD and over-activation of GSK-3b has been implicated in the aberrant phosphorylation of tau in AD. 8,9) In addition, GSK-3b phosphorylates the intracellular domain of amyloid precursor protein (APP), which is closely implicated in AD.10-12) However, since the above reports deal with GSK-3b1, the physiological significance of the neuron specific isoform GSK-3b2 yet remains to be dissolved.In this study, we investigated whether the phosphorylation activities of GSK-3b1 and -3b2 to tau and APP are different in cells. We found that the phosphoryaltion level of tau at AD-relevant epitope (Ser396) by GSK-3b2 is considerably lower than that by GSK-3b1, while the phosphorylation levels of APP by the two variants are almost equivalent. Furthermore, in vitro analyses revealed that recombinant GSK3b2 has lower phosphorylation activity to tau than GSK-3b1, although the phosphorylation activities of the two variants to a synthetic peptide substrate, pGS-2, are almost equivalent. These results indicate that tau is an unfavorable substrate for GSK-3b2. Moreover, we found that, by the deletion analyses of C-terminal tail (CT), the phosphorylation activity of GSK3b2DCT to tau is significantly reduced as compared with that of GSK-3b1DCT. These results suggest that the lower phosphorylation activity of GSK-3b2 to tau is attributed to its unique higher-order structure of CT constructed by the 13 amino acids insertion. Furthermore, these observations may imply that changes in the balance of GSK-3b2/-3b1 in neurons underlie tau hyperphosphorylation in AD. Our data provide new insights into the physiological significances of the two variants in the regulation of tau phosphorylation in central nervous system and into the mechanism of the onset of AD via dysregulation of the GSK-3b2/-3b1 balance.

Functional Implications of Glycogen Synthase Kinase‐3‐Mediated Tau Phosphorylation

“…For example, Ser396 in tau is phosphorylated by both splice variants, whereas Ser199 is a significant target of GSK-3 β 1, but not of GSK-3 β 2 activity. It has recently been suggested that the interaction of GSK-3 β 2 with tau is weaker than that of GSK-3 β 1 [76]. Taken together, these results suggest that, not only are there differential activities of GSK-3 α and GSK-3 β towards tau, there are also partially overlapping, but distinct, tau phosphorylation sites recognised by each of the two isoforms of GSK-3 β [74–77].…”

“…There is also a difference in the kinetics and sites of tau phosphorylation induced by the two GSK-3 β isoforms, with GSK-3 β 2 appearing to phosphorylate tau more slowly than GSK-3 β 1 and on different, with some overlapping, tau residues, even under conditions in which other GSK-3 β substrates, such as amyloid precursor protein, are phosphorylated equally [75, 76]. For example, Ser396 in tau is phosphorylated by both splice variants, whereas Ser199 is a significant target of GSK-3 β 1, but not of GSK-3 β 2 activity.…”

GSK-3β, a pivotal kinase in Alzheimer disease

“…After neuronal cell death, intracellular NFTs are released into the extracellular space (Dickson et al, 1992). Interestingly, growing evidence indicates that hyperphosphorylated tau activates GSK-3β through an increase in oxidative stress, neuroinflammation, and apoptosis (Saeki et al, 2011). In addition, GSK-3β impairs lysosomal acidification, a process that entails an inadequate clearance of non-functional proteins (Avrahami et al, 2013).…”

Roles of Glycogen Synthase Kinase 3 in Alzheimer’s Disease

“…GSK-3, strikingly different behavior to other protein kinases, is active in the resting state [34]. The most well defined mechanism of GSK-3 regulation is the phosphorylation of Serine 9 (Ser9) in GSK-3 and Ser21 in the GSK-3 isoform [35].…”