There is now compelling evidence that radial glial cells have the potential, not only to guide newly born neurons, but also to self-renew and to generate both neurons and astrocytes. Recent data has also shown that astrocytes increase the number of mature, functional synapses on central nervous system (CNS) neurons by sevenfold, demonstrating that CNS synapse numbers can be profoundly regulated by glia. Glial cells also play critical roles in regulating synaptic glutamate levels, CNS energy homeostasis, liberation of trophic factors, and form dynamic, complex synaptic networks with neurons. Nevertheless, the possibility of glial dysfunction in major psychiatric disorders has only recently received serious consideration due to the converging neuroimaging, postmortem morphometric and microarray studies, which have clearly revealed glial abnormalities in schizophrenia and mood disorders. To examine the effect of lithium (Li) on glia and neuron growth, we have established an astrocyte and neuronal primary culture system. We found that astrocytes, whose proliferation is increased by lithium, may indirectly (via liberation of factors from glial cells) regulate neuronal differentiation. Astrocytes may induce the pluripotent immature neuron to express an astrocytic phenotype. Next, we examined the alteration of cell signaling in astrocyte proliferation and neuronal differentiation to study the possible molecular mechanism of Li-induced action In addition, we examined whether Li affected growth of oligodendricytes, another type of glial cell. To investigate this more definitively, we began a series of in vitro and in vivo studies examining lithium's effects on oligodendrocytes. Chronic lithium treatment significantly increased the total number of oligodendrocytes in a dose-dependent manner, with a maximal effect observed with 1.0 mM lithium. To determine whether lithium affects BrdU incorporation, oligodendrocytes were treated with BrdU for 6 h in the absence or presence of lithium (1.0 mM). BrdU incorporation was determined by immunocytochemistry. BrdU-labeled cells were markedly increased by lithium treatment. O4 expression was examined by immunocytochemistry to further determine the cell phenotype of these BrdU-labeled cell. BrdU-positive cells were also O4+. Quantitatively, the percentage of BrdU-positive oligodendrocyte, as well as that of BrdU+O4+ cells were significantly increased by the lithium treatment. Our data demonstrate for the first time that chronic lithium exerts a major effect on oligodendrocytes, increasing their proliferation. These observations raise the possibility that lithium may serve to correct abnormalities in white matter tracts, thereby restoring the functioning of critical circuits mediating affective, cognitive and motor symptom. These mechanisms may provide a potential target for improved long-term therapeutics for severe neuropsychiatric disorders. Recent evidence suggested that ATP acting via ionotropic (P2X) purinergic receptors might be involved in signaling between glial cells and within glial-neuronal networks. The P2X7 receptor, known as the cytolytic P2Z receptor, has been implicated in signaling between neuron and astrocytes, and has recently been postulated to represent a candidate gene for recurrent mood disorders. P2X7 receptors have been proposed as mediators of inflammation, and a potential role in neurodegeneration has been suggested. The P2X7 receptor shares 35-40% homology with other P2X receptors. It has two hydrophobic membrane-spanning domains and an extracellular loop, and forms transmembrane ion channels. Under normal conditions, extracellular nucleotides are present in only low concentrations. However, activated immune cells, such as lymphocytes, macrophages, microgli, and platelets, and dying cells may release high concentrations of different nucleotide di- and tri-phosphates into the extracellular space. Under inflammatory conditions, P2X7 receptor activation stimulates the induction of multiple cytokine pathways that may co-ordinate inflammatory responses, and triggers massive transmembrane ion fluxes (particularly influx of Ca2+ and Na+, and efflux of K+) and the formation of non-selective plasma membrane pores that result in cell death. In contrast to their neuronal counterpart, the function of P2X receptors in CNS glial cells is largely unknown. By Westen blot protein analysis, immunocytochemistry and immunohistochemistry, we examined expression of P2X7 receptors in astrocytes in vivo and in vitro and examined effect of lithium (Li) on the expression of P2X7 in astrocytes. We found that P2X7 receptor is expressed in astrocytes. P2X7 positive cells can be GFAP- and S100beta- positive, suggesting a colocalization of astrocyte proteins and P2X7 receptor in CNS. Moreover, we also found that Li (0.5-1.0 mM) resulted in significantly decreased expression of P2X7 receptor in cultured astrocytes. Finally, we also found chronic (5-day) Li-treatment significantly blocked ATP-induced influx of Ca2+ in astrocytes. Our data demonstrated that P2X7 is expressed in astrocytes, and its expression levels can be regulated by chronic treatment of lithium at therapeutically relevant concentrations. Considering P2X7 receptor's role in the regulation of Ca2+ by ATP, this may be the molecular mechanism by which ATP-induced influx of Ca2+ can be blocked by lithium in astrocytes. Our data provide evidence showing that P2X7 receptor in astrocytes may be a therapeutic target for mood disorder treatments.
| Schloesser, Robert J; Manji, Husseini K; Martinowich, Keri (2009) Suppression of adult neurogenesis leads to an increased hypothalamo-pituitary-adrenal axis response. Neuroreport 20:553-7 |
