AMPA receptor trafficking in the pathophysiology and treatment of mood disorders

点击翻译按钮获取中文摘要

Mood disorders are one of the leading causes of disability worldwide, ranking ahead of ischemic heart disease, cerebrovascular disease, cancers, and infectious diseases. Despite the devastating impact that mood disorders have on the lives of millions of individuals worldwide, little is known about their underlying etiology and neurobiology. Currently available treatments are insufficient for many patients suffering from mood disorders, and even for those patients who do respond to available antidepressants or mood stabilizers, there is a significant therapeutic lag before clinical benefits appear. Notably, to date, no drug has been developed specifically to treat bipolar disorder (BD) based on an understanding of the neurobiological basis of the illness or the mechanism of action of existing effective medications. Thus, the major challenge in BD research is to find a common, convergent, functional mechanism associated with BD in order to develop urgently needed and truly novel and effective therapeutics. In this study, we sought to identify this common, convergent system in mood disorders. In view of the growing body of data suggesting that severe mood disorders may be associated with impairments of synaptic plasticity in neurons, we undertook the present series of studies to determine if two clinically effective but structurally highly dissimilar antimania drugs -- lithium and valproate (VPA) -- regulate synaptic expression of alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptor, a key neurotransmitter receptor known to participate in synaptic plasticity. This AMPA-selective glutamate receptor subtype has been named GluR1. Chronic administration of lithium or valproate (at therapeutically relevant concentrations and durations) reduced rat hippocampal synaptosomal levels of GluR1 and GluR2. Lithium and valproate treatment also attenuated the phosphorylation of a specific site (S845) on a key molecule, protein kinase A (PKA), involved in mediating receptor-initiated intracellular events including receptor internalization. Sp-cAMP treatment reversed the attenuation of phosphorylation by lithium and valproate and also brought GluR1s back to the cell surface, suggesting that phosphorylation of GluRS845 is involved in the mechanism of GluR1 surface attenuation. In striking contrast, drugs, such as imipramine, which induce mania, increase the synaptic expression of GluR1 in vivo in the hippocampus. In order to develop a new potential drug that mimics the effect of mood stabilizers on GluR1 phosphorylation, TAT-peptides (TAT-p845 and TAT-SRC) were designed and synthesized. TAT peptide (YGRKKRRQRRR) linked to the functional peptide enables the functional peptide to pass through the blood-brain barrier and cell membrane, allowing it to get into cytosol or synapses of the neurons. A previous study has successfully utilized TAT peptide injection into animals to disrupt the interaction of the postsynaptic density protein PSD-95 with NMDA receptors in the brain and to provide a neuroprotective effect on a stroke animal model. TAT-p845 was able to inhibit the phosphorylation of AMPA receptors at the PKA site and down-regulate the surface expression of GluR1 in cultured hippocampal neurons, which is the same effect produced by lithium and valproate. Moreover, this TAT-p845 was able to pass the blood-brain barrier and inhibit the phosphorylation of GluR1 in the hippocampus in vivo, which again demonstrated its ability to induce the same effects as lithium and valproate. In addition, reduction of GluR1 phosphorylation at its PKA site by TAT-p845 was sufficient to attenuate synaptic GluR1/2 in hippocampal neurons in vivo. Intra-hippocampal infusion of AMPA-specific inhibitor GYKI54226, GluR1-specific TAT-p845 peptide, and GluR1-PDZ-specific TAT-TGL peptide were able to attenuate amphetamine-induced hyperactivity and/or amphetamine-induced conditioned-place preference in the mania animal model. These studies provide novel mechanisms for anti-manic effect through attenuation of AMPA receptor activity and suggest avenues for new drug development for mood disorders. TAT-p845, which attenuates AMPA receptor levels at synapses, may offer exciting possibilities as a new class of medicine with the potential for treatment of bipolar disorder. Considerable biochemical evidence suggests that the protein kinase C (PKC) signaling cascade may be an important pathway for the actions of anti-manic agents, and that excessive PKC activation can disrupt prefrontal cortical regulation of thinking and behavior. Currently, however, brain protein targets of PKCs anti-manic effects remain unclear. Based on a previous finding, we want to determine how PKC-mediated regulation of glutamate receptors plays an important role in the pathophysiology and treatment of mania. Here we showed that PKC activity was enhanced in the prefrontal cortex of animals treated with the psychostimulant amphetamine and the antidepressant imipramine. Phosphorylation of myristoylated alanine-rich C kinase substrate (MARCKS), a marker of PKC activity, was increased in the prefrontal cortex of psychostimulant-treated animals, as well as in sleep-deprived animals (another animal model of mania), but decreased in lithium-treated animals. The antidepressant imipramine, which shows promanic property on bipolar patients, also enhanced pMARCKS in prefrontal cortex in vivo. We further explored the functional targets of PKC in mania-associated behaviors. Neurogranin is a brain-specific, postsynaptically located PKC substrate. PKC phosphorylation of neurogranin was robustly increased by pro-manic manipulations and decreased by the anti-manic agent. PKC phosphorylation of the NMDA receptor site NR1S896 and the AMPA receptor site GluR1T840 was also enhanced in the prefrontal cortex of animals treated with antidepressant imipramine, as well as behaviorally sleep-deprived, in striking contrast to the reduced activity seen in lithium-treated animals. These results suggest that PKC may play an important role in regulating NMDA and AMPA receptor functions. The biochemical profile of the PKC pathway thus encompasses both pro- and anti-manic effects on behavior. These results suggest that PKC modulators or their intracellular targets may ultimately represent novel avenues for the development of new therapeutics for mood disorders.