TRIP8b Is Required for Maximal Expression of HCN1 in the Mouse Retina

Yuan Pan,Sajag Bhattarai,Dane M Chetkovich,Modestos Modestou,Sheila A Baker,Arlene V Drack,Erica Lucy Fletcher

doi:10.1371/journal.pone.0085850

Abstract

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are cation-selective channels present in retina, brain and heart. The activity of HCN channels contributes to signal integration, cell excitability and pacemaker activity. HCN1 channels expressed in photoreceptors participate in keeping light responses transient and are required for normal mesopic vision. The subcellular localization of HCN1 varies among cell types. In photoreceptors HCN1 is concentrated in the inner segments while in other retinal neurons, HCN1 is evenly distributed though the cell. This is in contrast to hippocampal neurons where HCN1 is concentrated in a subset of dendrites. A key regulator of HCN1 trafficking and activity is tetratricopeptide repeat-containing Rab8b interacting protein (TRIP8b). Multiple splice isoforms of TRIP8b are expressed throughout the brain and can differentially regulate the surface expression and activity of HCN1. The purpose of the present study was to determine which isoforms of TRIP8b are expressed in the retina and to test if loss of TRIP8b alters HCN1 expression or trafficking. We found that TRIP8b colocalizes with HCN1 in multiple retina neurons and all major splice isoforms of TRIP8b are expressed in the retina. Photoreceptors express three different isoforms. In TRIP8b knockout mice, the ability of HCN1 to traffic to the surface of retinal neurons is unaffected. However, there is a large decrease in the total amount of HCN1. We conclude that TRIP8b in the retina is needed to achieve maximal expression of HCN1.

Highlights

Hyperpolarization-activated current (Ih) was discovered in photoreceptors where absorption of light triggers a signal transduction cascade that leads to closure of cyclic nucleotide gated channels which in turn causes the cell to hyperpolarize
Multiple tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b) Isoforms are Present in the Retina TRIP8b is an accessory subunit of HCN1 channels that influences its trafficking and activity in various neurons [37], but its expression in the retina has not been investigated
TRIP8b is present in the inner segment (IS) and synapses of the outer plexiform layer (OPL) but excluded from outer segments – the same distribution as HCN1 (Figure 1B, C)

Summary

Introduction

Hyperpolarization-activated current (Ih) was discovered in photoreceptors where absorption of light triggers a signal transduction cascade that leads to closure of cyclic nucleotide gated channels which in turn causes the cell to hyperpolarize. HCN channels can serve as pacemakers, maintain resting membrane potential, shape synaptic output or modulate integration of dendritic signaling [7,8]. Rod photoreceptors express HCN1 which is concentrated in the inner segments and to a lesser degree in the plasma membrane surrounding the nuclei (cell soma) and in the presynaptic terminals; HCN1 is excluded from the photosensitive outer segment compartment [10,11,12,13]. HCN1 is expressed in multiple inner retina neurons, seemingly less abundantly, and is found in multiple cellular compartments (dendrites, soma, axons and presynaptic terminals) [10,11,12,13,14,15,16,17,18,19,20]. The best understood regulator of HCN1 subcellular localization is tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b)

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