Localization of inositol 1,4,5-trisphosphate receptors in mouse retinal ganglion cells

Inositol 1,4,5‐trisphosphate receptors (IP3R) are ligand‐gated intracellular Ca2+channels that mediate release of Ca2+ from intracellular stores into the cytosol on activation by second messenger IP3. Similarly, IP3R mediated changes in cytosolic Ca2+ concentrations control neuronal functions ranging from synaptic transmission to differentiation and apoptosis. IP3R‐generated cytosolic Ca2+ transients also control intracellular Ca2+ release and subsequent retinal ganglion cell (RGC) physiology and pathophysiology. The distribution of IP3R isotypes in primary adult mouse RGC cultures was determined to identify molecular substrates of IP3R mediated signaling in these neurons. Immunocytochemical labeling of IP3Rs in retinal sections and cultured RGCs was carried out using isoform specific antibodies and was detected with fluorescence microscopy. RGCs were identified by the use of morphologic criteria and RGC‐specific immunocytochemical markers, neurofilament 68 kDa, Thy 1.1, and Thy 1.2. RGC morphology and immunoreactivity to neurofilament 68 kDa and Thy 1.1 or Thy 1.2 were identified in both RGC primary cultures and tissue cryosections. RGCs showed localization on intracellular membranes with a differential distribution of IP3R isoforms 1, 2, and 3. IP3R Types 1 and 3 were detected intracellularly throughout the cell whereas Type 2 was expressed predominantly in soma. Expression of all three IP3Rs by RGCs indicates that all IP3R types potentially play a role in Ca2+ homeostasis and Ca2+ signaling in these cells. Differential localization of IP3 receptor subtypes in combination with biophysical properties of IP3R types may be an important molecular mechanism by which RGCs organize their cytosolic Ca2+ signals. © 2006 Wiley‐Liss, Inc.