Role of Ca2+ and calmodulin-dependent enzymes in the regulation of glycine transport in Müller glia.

Abstract

Glycine (Gly) is considered an obligatory co-agonist at NMDA receptors. Müller glia from the retina harbor functional NMDA receptors, as well as low and high affinity Gly transporters, the later identified as GLYT1. We here studied the regulation of Gly transport in primary cultures of Müller glia, as this process could contribute to the modulation of NMDA receptor activity at glutamatergic synapses in the retina. We demonstrate that neither glutamate stimulation nor the activation or inhibition of protein kinases A or C modify transport. In order to assess a function for Ca2+ and calmodulin (CaM)-dependent processes in the regulation of Gly transport, we explored the participation of Ca2+ concentration, CaM and Ca2+/CaM-dependent enzymes on Gly transporter activity. ATP and carbachol, known to induce Ca2+ waves in Müller cells, as well as caffeine-induced Ca2+ release from intracellular stores stimulated transport, whereas Ca2+ chelation by BAPTA-AM markedly reduced transport. CaM inhibitors W-7, ophiobolin A, R-24571 and trifluoperazine, induced a specific dose-dependent inhibition of transport. The inhibition of CaMKII by the autocamtide-2-related inhibitory peptide or by KN62 caused a decrease in transport which, in the case of KN62, was due to the abolition of the high affinity component, ascribed to GLYT1. Our results further suggest that Gly transport is under cytoskeletal control, as activation of calpain by major increases in [Ca2+]i induced by ionophores, as well as actin destabilization clearly inhibit uptake. We here demonstrate for the first time the participation of CaM, CaMKII and the actin cytoskeleton in the regulation of Gly transport in glia. Ca2+ waves are induced in Müller cells by distinct neuroactive compounds released by neurons and glia, hence the regulation of [Gly] by this system may be of physiological relevance in the control of retinal excitability.