Effects of lithium on the expression and activity of the enzymes Phospholipase A2 and glycogen synthase kinase 3B and its relation to the phosphorylation state of Tau protein

Abstract

The therapeutic action of lithium occurs, in part, by inhibition of glutamatergic response via receptors N-methyl-d-aspartate (NMDA). The binding of glutamate or NMDA to these receptors in the synaptic membrane allows calcium influx and subsequent activation of cation-dependent enzymes, such as calcium-calmodulin protein kinase II (CaMKII), PKC, phospholipase C gamma and some subtypes of PLA2. Exposure of neurons in different brain regions to NMDA leads to activation of the cytosolic PLA2 (cPLA2) by influx of calcium in micromolar concentrations. The main product of cleavage of phosphatidylcholine by cPLA2 is arachidonic acid (AA), an important second-messenger cell signaling. AA regulates the conductance of ion channels, the release of neurotransmitters, the phosphorylation of substrates by the action of kinases and the synthesis of transcriptional factors, with effects on neuronal polarity, synaptic plasticity and long-term potentiation (LTP). Studies show that lithium inhibits the activity and expression of cPLA2, the latter effect observed in the transcriptional level (reduction in mRNA) and protein concentration. Other subtypes of PLA2, such as secretory PLA2 (sPLA2) and cytosolic PLA2 independent of calcium (iPLA2) are not affected by lithium. Lithium inhibits calcium-dependent PKC by reducing the expression of AP-2 (activating protein-2), which in turn regulates the transcription of cPLA2. If, on the one hand, the inhibition of cPLA2 by lithium is consistent with its therapeutic action in mood disorders, this effect may hinder the formation and consolidation of memory, which are processes dependent on the integrity of this pathway and the release of AA. In addition, by inhibiting cPLA2, lithium can, in theory, exacerbate the deficit in the activity of this enzyme is already observed in patients with AD. Glycogen synthase kinase 3² (GSK3²) is another important neuronal enzyme whose activity is inhibited by lithium. The GSK3² plays important roles in brain homeostasis, regulating the synthesis of glycogen under the influence of insulin cascade, remodeling of the cytoskeleton (synaptic plasticity, neurogenesis and phosphorylation of tau) and neuronal response to injury (apoptosis and gene regulation). Recent studies have shown the involvement of this enzyme in the pathophysiology of some neuropsychiatric disorders such as AD, schizophrenia and mood disorders. In bipolar disorder, the indirect inhibition of GSK3² by lithium, which leads to phosphorylation of the Ser9 residue (pGSK3²), was associated with therapeutic response. In AD, where the hyperactivity of GSK3² contributes to the production of Ab and the hyperphosphorylation of tau, inhibition of this enzyme by lithium is seen as a therapeutic approach that will mitigate the amiloidogênese and changes in the cytoskeleton. In conclusion, GSK3² and PLA2 enzymes are essential for neuronal homeostasis, both being involved in the pathophysiology of AD and mood disorders. However, we observed that the action of lithium may result in responses in two apparently antagonistic enzymes, considering the pathophysiological cascade of AD. Therefore it is necessary to clarify the effect of lithium in different subtypes of PLA2, and then discusses its implications in neurodegenerative and inflammatory processes. The study aims to investigate the effects of lithium on the activity of GSK3² and PLA2 subtypes in primary cultures of cortical and hippocampal neurons. Establish dose-response curves and treatment time for the expected inhibitory effects on enzymes above. To investigate downstream effects of the treatment of neuronal cultures with lithium, determining the concentrations of arachidonic acid, neurotrophins (BDNF, NGF), markers of apoptosis (Bcl-2, Bax and Bad), inflammatory cytokines (IL-1 and IL-6 ), kinases (PKA, AKT, CAMKII, PKC), tau phosphorylation (Ser 199, Ser 393, Ser 214, Ser 202, C terminus) and analysis of samples by microarray. (AU)