Transient Increases in Intracellular Calcium Result in Prolonged Site-selective Increases in Tau Phosphorylation through a Glycogen Synthase Kinase 3β-dependent Pathway

Abstract

Calcium is a universal intracellular signaling molecule. Through variations in both the amplitude and frequency of intracellular calcium increases, the same calcium ion can elicit different responses. In this report, we investigated the effect of a calcium transient, lasting 2-5 min, on alterations in the phosphorylation state of the cytoskeletal protein, tau. Transient increases in calcium result in a prolonged (1-4 h) ϳ60% increase in tau phosphorylation at the Tau-1 epitope. These increases in tau phosphorylation appear to be more dependent upon the duration of the increase in intracellular calcium and less on the amplitude. The calcium-induced increases in tau phosphorylation are not dependent upon protein synthesis,norareproteinkinaseCorcalcium/calmodulindependent protein kinase II involved in the response. However, the calcium-induced increase in tau phosphorylation was inhibited by lithium, a noncompetitive inhibitor of glycogen synthase kinase-3␤ (GSK-3␤), and by the tyrosine kinase inhibitor, genistein. Furthermore, transient increases in calcium resulted in a prolonged increase in GSK-3␤ tyrosine phosphorylation concomitant with the increase in tau phosphorylation. Therefore, this study is the first to indicate that transient increases in intracellular calcium result in increased tyrosine phosphorylation and activation of GSK-3␤ which subsequently results in a sustained increase in the phosphorylation state of tau.Calcium is an important intracellular messenger for neuronal signaling pathways. Cytoplasmic calcium concentrations fluctuate when extracellular calcium enters the cell and/or when calcium is released from intracellular stores, and both temporal and spatial aspects of the calcium signal are important in regulating cellular responses (1, 2). The regulation of specific protein kinases and phosphatases by different calcium signals is likely to play a role in numerous physiological and pathological processes. For example, recent studies have demonstrated that increases in intracellular calcium differentially altered phosphorylation of the microtubule-associated protein, MAP-2, in neonatal and adult rat hippocampal slices (3). These findings suggest that mature and immature neurons respond differently to calcium signals and this in turn results in specific phospho-dependent changes in the cytoskeleton. Considering these previous findings, it is likely that the phosphorylation and function of other cytoskeletal proteins are modulated by specific calcium signals.Tau, a family of microtubule-associated proteins produced by alternative splicing of a single gene, is a prominent phosphoprotein of the neuronal cytoskeleton. The classical function of tau is to promote microtubule nucleation and assembly, as well as to stabilize microtubules (for review, see Ref. 4). However, tau also appears to be involved in other cellular functions, including signal transduction (5-7) and organelle transport (8, 9). It has been well documented that increasing the phosphorylation state of tau greatly reduces its ability ...

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