Modulation of strain-specific differences in gene expression by cannabinoid type 2 receptor deficiency.

Previous studies have shown that the skeletal consequences of cannabinoid receptor deficiency differ in different strains of mice. In order to explore the mechanisms responsible, we analysed global gene expression in bone from wild-type CD1 mice and littermates with targeted inactivation of the type 2 cannabinoid receptor (Cnr2 (-/-)) and compared the results with those obtained from a similar analysis of wild-type and Cnr2 (-/-) C57BL/6 mice. Trabecular bone volume was increased in Cnr2 (-/-) CD1 mice compared with wild-type littermates but decreased in Cnr2 (-/-) C57BL/6 mice. Microarray analysis identified 354 genes in which substantial differences in gene expression (>1.5-fold) were observed that were specifically affected by Cnr2 deficiency. Bioinformatic analysis of data from wild-type mice of each strain revealed Cnr2-dependent differences in expression of genes clustering within the gene ontology (GO) terms immune response (p < 0.0001), positive regulation of response to stimulus (p < 0.0001), nucleotide binding (p = 0.002), and ribonucleotide binding (p = 0.003). Bioinformatic analysis of data from Cnr2 (-/-) mice of each strain revealed associations between GO terms corresponding to the extracellular region (p = 0.002), the cell surface (p = 0.02), antigen binding (p = 0.03), external side of plasma membrane (p = 0.04), and regulation of the force of heart contraction (p = 0.04). We conclude that Cnr2 deficiency affects expression of a large number of genes in different strains of mice, and that these differences are likely to be responsible in part for the differences in skeletal phenotype that we and others have observed in mice with defective cannabinoid receptor signalling in different genetic backgrounds.