Dopamine D1 receptor activation induces tau phosphorylation via cdk5 and GSK3 signaling pathways

Abstract

Increasing evidence is demonstrating that drugs affecting dopamine levels in the brain induce cytoskeletal modifications. These evolving changes may impact neuronal synaptic plasticity as cytoskeletal constituents are involved in the maintenance of dendritic processes, and any alterations in their stability could influence major cellular compartments of neurons, such as dendrites, spines and synapses. Here, we describe a molecular chain of events that links dopamine D1 receptor activation to hyperphosphorylation of the microtubule-associated protein tau, which is normally involved in microtubules stabilization. We show, in SK-N-MC cells and rat striatal sections, that phosphorylation of tau at serines 199–202 and 214 appears to be mediated through activation of calcium-dependent intracellular mechanism, subsequent to D1 receptor-induced cAMP-dependent protein kinase A (PKA). We demonstrate, using pharmacological tools, that PKA activation causes increase of calcium levels, leading to cyclin-dependent kinase 5 activation by calpain proteolysis of p35 to p25 and glycogen synthase kinase 3β activation by its phosphorylation at tyrosine 216. The D2 receptor agonism or lowering cAMP levels has no effect in our experimental settings. Moreover, we do not observe any association between phosphorylated tau and cellular damage. These data unravel novel mechanisms of tau hyperphosphorylation during G-protein-coupled receptor activation and are the first to show that stimulation of D1 receptors could have a profound influence on the neuronal cytoskeletal constituent tau.

Introduction

Microtubule-associated proteins are cytoskeletal constituents involve in the maintenance of dendritic processes that are known to influence synaptic strength and neuronal plasticity. In mature brain neurons, tau is one of the key microtubule-associated proteins which has been reported to be under the influence of phosphorylation processes involving, for instance, cAMP-dependent protein kinase A (PKA), cyclin-dependent kinase 5 (cdk5) and glycogen synthase kinase 3 (GSK3) activities (Iqbal and Grundke-Iqbal, 2006, Mazanetz and Fischer, 2007). Normally, tau binds directly to microtubules and promotes their polymerization, but it is well-established that increasing tau phosphorylation negatively regulates microtubule-binding and leads to destabilization of the microtubule network, cytoskeletal dysfunction and modification of synaptic plasticity (Iqbal and Grundke-Iqbal, 2006, Mazanetz and Fischer, 2007). Recent studies have demonstrated that activation of dopamine receptors could change synaptic strength and plasticity through the regulation of PKA, cdk5 and GSK3 (Nishi et al., 2000, Cyr et al., 2003, Beaulieu et al., 2004, Beaulieu et al., 2007, Lebel et al., 2007). These observations support the interesting possibility that dopamine receptors might also control tau phosphorylation.

In the present study, therefore, we used selective dopamine receptor agonists and antagonists to analyze the involvement of D1 and D2 receptors in tau phosphorylation and their specific interactions with signaling pathways engage in the regulation of tau. We demonstrate, in SK-N-MC cells and rat striatal sections, that activation of D1, but not D2 receptors, augments tau phosphorylation. We observed, following D1-induced PKA activation, that tau was indirectly phosphorylated via heightened levels of intracellular calcium leading to activation of both cdk5 (through calpain proteolysis of p35 to p25) and GSK3β (via its phosphorylation at tyrosine 216). Our data reveal novel mechanisms of tau hyperphosphorylation during G-protein-coupled receptor activation and suggest a direct role of D1 dopamine signaling constituents in this process. These mechanisms may be well relevant to non-neurodegenerative conditions involving dopamine neurotransmission.