Functional ryanodine receptors are expressed by human microglia and THP-1 cells: Their possible involvement in modulation of neurotoxicity

Ryanodine receptors (RyRs) are intracellular Ca(2+) channels that mediate the release of calcium from internal stores and therefore play an important role in Ca(2+) signaling and homeostasis. Three RyR isoforms have been described thus far, and various areas of brain are known to express each of them. It is well established that neurons can express different RyR isoforms, but it is not known whether microglial cells do so. In the present study we showed that cultured human microglia from both fetal and adult brain specimens express mRNA for RyR1 and RyR2, whereas RyR3 mRNA can be detected only in fetal microglial cells. Calcium spectrofluorometry showed that high levels of the RyR agonist 4-chloro-m-cresol (4-CmC, 1-5 mM) induced elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)) in both types of cultured human microglial cells. This effect was attenuated by the RyR antagonist 1,1'-diheptyl-4,4'-bipyridinium dibromide (DHBP, 10 microM). Neurotoxicity of conditioned medium from human microglia and THP-1 monocytic cells stimulated with a combination of interferon-gamma (IFN-gamma) with either lipopolysaccharide (LPS) or alpha-synuclein was diminished by DHBP. It was also diminished by 4-CmC at concentrations approximately 100-fold lower than those used to stimulate intracellular Ca(2+) release. These data indicate that human microglial cells express functional RyRs and that selective RyR ligands exert antineurotoxic action on this cell type. Therefore, RyR ligands may represent a novel class of compounds that have utility in reducing microglial-mediated inflammation, which is believed to contribute to the pathogenesis of a number of neurodegenerative disorders including Alzheimer's disease and Parkinson's disease.