Receptor-Selective Diffusion Barrier Enhances Sensitivity of Astrocytic Processes to Metabotropic Glutamate Receptor Stimulation

Abstract

Metabotropic glutamate receptor (mGluR)-dependent calcium ion (Ca²+) signaling in astrocytic processes regulates synaptic transmission and local blood flow essential for brain function. However, because of difficulties in imaging astrocytic processes, the subcellular spatial organization of mGluR-dependent Ca²+ signaling is not well characterized and its regulatory mechanism remains unclear. Using genetically encoded Ca²+ indicators, we showed that despite global stimulation by an mGluR agonist, astrocyte processes intrinsically exhibited a marked enrichment of Ca²+ responses. Immunocytochemistry indicated that these polarized Ca²+ responses could be attributed to increased density of surface mGluR5 on processes relative to the soma. Single-particle tracking of surface mGluR5 dynamics revealed a membrane barrier that blocked the movement of mGluR5 between the processes and the soma. Overexpression of mGluR or expression of its carboxyl terminus enabled diffusion of mGluR5 between the soma and the processes, disrupting the polarization of mGluR5 and of mGluR-dependent Ca²+ signaling. Together, our results demonstrate an mGluR5-selective diffusion barrier between processes and soma that compartmentalized mGluR Ca²+ signaling in astrocytes and may allow control of synaptic and vascular activity in specific subcellular domains.

Chronic hypoxia and VEGF differentially modulate abundance and organization of myosin heavy chain isoforms in fetal and adult ovine arteries

“…This latter finding implies that both hypoxia and VEGF have significant and parallel effects on the abundances, trafficking, and localization of contractile proteins within arterial smooth muscle cells. The mechanisms mediating these effects are uncertain but could involve changes in cytoskeletal remodeling, microtubular transport, or protein-protein interactions as suggested in other cell types (3). Thus the present results follow a major trend in contemporary cell biology that is focused on the complex and unidentified mechanisms that govern subcellular protein distribution, trafficking, and localization.…”

Neuromodulation by Extracellular ATP and P2X Receptors in the CNS

“…Single-molecule imaging experiments now provide accurate and consistent values for P2X2, P2X4 and P2X7 receptor diffusion coefficients in the plasma membrane (0.027, 0.023 and 0.021 µm 2 /s, respectively) (Arizono et al, 2012; Richler et al, 2011; Toulme and Khakh, 2012). In the case of P2X2 and P2X4 receptors, activation by ATP causes the receptors to diffuse twice as fast in a cell- and subunit specific manner (Richler et al, 2011; Toulme and Khakh, 2012).…”

Morphological plasticity of astroglia: Understanding synaptic microenvironment

“…Although such evidence has been essential in advancing our understanding of astroglia‐neuron communication, it leaves unresolved some critical questions regarding molecular physiology of the synaptic microenvironment, and in particular, local astroglial protrusions, on the nanoscale. Single‐molecule tracking of mGluRs and glutamate transporters in cultured astroglia has provided first glimpses on what could be the dynamic molecular micro‐organization of astrocytes (Arizono et al, 2012; Murphy‐Royal et al, 2015; Shrivastava et al, 2013), and STED imaging in organised brain tissue (Tonnesen et al, 2014) has opened up the nanoscopic world of live astroglial architecture in situ . Clearly, further implementation of super‐resolution techniques in organised brain tissue should help to bridge the knowledge gap in our understanding of the molecular micro‐physiology of astroglia and its role in information processing of brain networks.…”

“…However, application of such methods to astroglia is only beginning to emerge. Single‐molecule tracking of mGluRs in cultured astroglia has provided some previously unattainable insights into the molecular dynamic organization of astrocytic compartments (Arizono et al, 2014, 2012; Shrivastava et al, 2013). Most recently, a similar method has unveiled strong, activity‐dependent mobility of glutamate transporters GLT‐1 on the surface of cultured astroglia (Murphy‐Royal et al, 2015), thus opening a new horizon in our understanding of astroglial microphysiology.…”