Codon optimization of Caenorhabditis elegans GluCl ion channel genes for mammalian cells dramatically improves expression levels

Abstract

Organisms use synonymous codons in a highly non-random fashion. These codon usage biases sometimes frustrate attempts to express high levels of exogenous genes in hosts of widely divergent species. The Caenorhabditis elegans GluClα1 and GluClβ genes form a functional glutamate and ivermectin-gated chloride channel when expressed in Xenopus oocytes, but expression is weak in mammalian cells. We have constructed synthetic genes that retain the amino acid sequence of the wild-type GluCl channel proteins, but use codons that are optimal for mammalian cell expression. We have tagged the native and codon-optimized GluCl cDNAs with enhanced yellow fluorescent protein (EYFP, GluClα1 subunit) and enhanced cyan fluorescent protein (EFCP, GluClβ subunit), expressed the channels in E18 rat hippocampal neurons and measured the relative expression levels of the two genes with fluorescence microscopy as well as with electrophysiology. Codon optimization provides a 6- to 9-fold increase in expression, allowing the conclusions that the ivermectin-gated channel has an EC50 of 1.2 nM and a Hill coefficient of 1.9. We also confirm that the Y182F mutation in the codon-optimized β subunit results in a heteromeric channel that retains the response to ivermectin while reducing the response to 100 μM glutamate by 7-fold. The engineered GluCl channel is the first codon-optimized membrane protein expressed in mammalian cells and may be useful for selectively silencing specific neuronal populations in vivo.