Characteristics and regulation of glycine transport in Bergmann glia.

In the vertebrate CNS, glycine acts as an inhibitory neurotransmitter and as the obligatory coagonist of glutamate at N-methyl-D-aspartate receptors. These roles depend on extracellular glycine levels, regulated by Na+/Cl--dependent transporters GLYT1, present mainly in glial cells, and GLYT2, predominantly neuronal. In Bergmann glia, GLYT1 mediates both, glycine uptake and efflux, which, in turn, influences excitatory neurotransmission at Purkinje cell synapses. The biochemical properties of GLYTs and their regulation by signaling pathways in these cells are largely unknown. We characterized Gly uptake in confluent primary cultures of Bergmann glia from chick cerebellum. Transport was found to be energy- and Na+-dependent, and was resolved into a high (Km=25 microM) and a low affinity (Km=1.1 mM) components identified as GLYT1 and transport System A, respectively. Results show that high affinity transport by GLYT1 is regulated by calcium from intracellular stores, calmodulin, and myosin light chain kinase through an actin cytoskeleton-mediated action.