Astrocytes become reactive upon injury and inflammation in the brain to alter their molecular profiles, morphologies and functions. Reactive astrocytes alter the expression of receptors which are responsible for their functions especially communications in neuron-glia and glia-glia. Among such receptors, the expression of P2Y1 receptors (P2Y1) is upregulated in many neurological diseases including epilepsy and Alzheimer's disease, in which neuronal hyperexcitability is commonly observed. We have previously shown that P2Y1 upregulation in astrocytes trigger neuronal hyperexcitability by enhancing neuron-astrocyte communications. However, the mechanism underlying the upregulation of P2Y1 in astrocytes remains unknown. We investigated the role of microglia in enhanced P2Y1R signaling in astrocytes during pathological conditions. To ask whether microglia play a role in P2Y1 upregulation in astrocytes, we depleted microglia by treatment with PLX5622 and found much larger Ca2+ elevation evoked by a P2Y1 agonist and more P2ry1 transcripts in astrocytes. Microglia depletion enhanced extracellular ATP level presumably through impairment of degradation of ATP. These findings suggest that microglia should has an important role to control P2Y1 receptor expression in astrocytes and negatively regulate neuron-astrocyte communication.
Microglia are the resident macrophages of the central nervous system and play complex roles in the milieu of diseases including the primary diseases of myelin. Although mitochondria are critical for cellular functions and survival in the nervous system, alterations in and the roles of mitochondrial dynamics and associated signaling in microglia are still poorly understood. In the present study, by combining immunohistochemistry and 3D ultrastructural analyses, we show that mitochondrial fission/fusion in reactive microglia is differentially regulated from that in monocyte-derived macrophages and the ramified microglia of normal white matter in myelin disease models. Mouse cerebral microglia in vitro demonstrated that stimulation of TLR4 with lipopolysaccharide, widely used to examine microglial reactions, caused the activation of the mitochondrial fission protein, dynamin-related protein 1 (Drp1) and enhanced production of reactive oxygen species (ROS). The increase in the ROS level activated 5' adenosine monophosphate-activated protein kinase (AMPK), and facilitated elongation of mitochondria along the microtubule tracks. These results suggest that the polymorphic regulation of mitochondrial fission and fusion in reactive microglia is mediated by distinct signaling under inflammatory conditions, and modulates microglial phenotypes through the production of ROS.
Abstract Metabotropic glutamate receptor 5 (mGluR5) in astrocytes is a key molecule for controlling synapse remodeling. Although mGluR5 is abundant in neonatal astrocytes, its level is gradually down‐regulated during development and is almost absent in the adult. However, in several pathological conditions, mGluR5 re‐emerges in adult astrocytes and contributes to disease pathogenesis by forming uncontrolled synapses. Thus, controlling mGluR5 expression in astrocyte is critical for several diseases, but the mechanism that regulates mGluR5 expression remains unknown. Here, we show that adenosine triphosphate (ATP)/adenosine‐mediated signals down‐regulate mGluR5 in astrocytes. First, in situ Ca 2+ imaging of astrocytes in acute cerebral slices from post‐natal day (P)7‐P28 mice showed that Ca 2+ responses evoked by (S)‐3,5‐dihydroxyphenylglycine (DHPG), a mGluR5 agonist, decreased during development, whereas those evoked by ATP or its metabolite, adenosine, increased. Second, ATP and adenosine suppressed expression of the mGluR5 gene, Grm5 , in cultured astrocytes. Third, the decrease in the DHPG‐evoked Ca 2+ responses was associated with down‐regulation of Grm5 . Interestingly, among several adenosine (P1) receptor and ATP (P2) receptor genes, only the adenosine A 2B receptor gene, Adora2b , was up‐regulated in the course of development. Indeed, we observed that down‐regulation of Grm5 was suppressed in Adora2b knockout astrocytes at P14 and in situ Ca 2+ imaging from Adora2b knockout mice indicated that the A 2B receptor inhibits mGluR5 expression in astrocytes. Furthermore, deletion of A 2B receptor increased the number of excitatory synapse in developmental stage. Taken together, the A 2B receptor is critical for down‐regulation of mGluR5 in astrocytes, which would contribute to terminate excess synaptogenesis during development.
