The Subcellular Localization of the Cystic Fibrosis Transmembrane Conductance Regulator in the Chicken Retina Suggests Multiple Roles in Retinal Function

Abstract

Protein function is influenced by multiple factors including cell type and subcellular localization. The cystic fibrosis transmembrane conductance regulator (CFTR) is well investigated in epithelial tissues, where life threatening symptoms stem from its dysfunction. A postsynaptic neuronal role was previously established by the Gleason lab where CFTR regulation of cytosolic Cl^– in retinal amacrine cells was shown. Other work from our lab showed that disruption of the synaptic vesicle cycle reduced action of CFTR, suggesting CFTR associates with synaptic vesicles. Here, we evaluate the hypothesis that CFTR localizes to the synapse with possible presynaptic function. To address this, the cellular and subcellular CFTR localization in mature chicken retina was examined using fluorescence light microscopy and immunogold-labeled transmission electron microscopy. CFTR labeling was detected throughout the retina, including photoreceptor outer segments and in the mitochondria rich region of the photoreceptor inner segment termed the ellipsoid. Synaptic labeling was found in both synaptic plexiform layers, pre-and post-synaptically. A subset of amacrine cells were strongly labeled and labeling was also found in Müller cells and in axons of the nerve fiber layer. Addressing whether the activity of CFTR plays a role in presynaptic function, amacrine cells were recorded using the whole cell voltage clamp method. Spontaneous postsynaptic quantal currents were recorded and found to increase in frequency upon pharmacological inhibition of CFTR suggesting that under normal circumstances, CFTR serves to limit the rate of spontaneous synaptic vesicle fusion. This work provides evidence CFTR might have multiple functions in the retina including synaptic transmission regulation.