Gene silencing in nicotine dependence

Drug dependence affects the brain reward circuitry through still largely unknown processes of molecular and cellular adaptation. Growing evidence indicates that small RNA pathways (microRNA and small interfering RNA) modulate drug-dependent behaviours. However, little is known about which genes are targeted by such pathways.

The EU-funded CELNIC (Role of gene silencing pathways in C. elegans nicotine dependence) project investigated the role of genetic components in nicotine addiction. The project used Caenorhabditis elegans as a well-established model for nicotine dependence, showing acute response, adaptation, withdrawal and sensitisation.

CELNIC identified genes and proteins that are differentially regulated upon nicotine treatment. Scientists used large-scale proteomic and genomic approaches to identify the potential candidates. Stable isotope labelling with amino acids in cell culture (SILAC), a mass spectrometry quantitative proteomic technique, allowed identification of up- or downregulated proteins.

Project members found that 35 proteins were downregulated and 101 proteins were upregulated upon prolonged nicotine treatment. These proteins were involved in various biological processes, such as growth, reproduction or stress responses. Some of these proteins were involved in small RNA interference machinery.

On the genetic level, the activity of 373 genes was affected by low or high doses of nicotine. CELNIC found 120 genes responded to low-dose treatment and 352 genes were affected by higher doses of nicotine. Out of these, 99 genes were common between the two treatment groups. Comparison of mass spectrometry and microarray data identified genetic factors differentially regulated both at the transcriptional and translational levels after prolonged nicotine treatment.

Most importantly, CELNIC revealed the mechanisms by which the microRNA machinery regulates and is being regulated by acetylcholine receptors to generate nicotine-dependent behaviours. The next step would be to study the role of the differentially regulated genes and proteins in mouse models or human cell lines. Finding the role of the gene-silencing pathways in nicotine dependence will have therapeutic applications that will reduce drug dependence relapses.