Circadian profiling of the transcriptome in immortalized rat SCN cells

Endogenous oscillations in gene expression are a prevalent feature of the circadian clock in the mammalian suprachiasmatic nucleus (SCN) and similar timekeeping systems in other organisms. To determine whether immortalized cells derived from the rat SCN (SCN2.2) retain these intrinsic rhythm-generating properties, oscillatory behavior of the SCN2.2 transcriptome was analyzed and compared with that found in the rat SCN in vivo using rat U34A Affymetrix GeneChips. In SCN2.2 cells, 116 unique genes and 46 ESTs or genes of unknown function exhibited circadian fluctuations with a 1.5-fold or greater difference in their mRNA abundance for two cycles. Many (35%) of these rhythmically regulated genes in SCN2.2 cells also exhibited circadian profiles of mRNA expression in the rat SCN in vivo. Functional analyses and cartography indicate that a diverse set of cellular pathways are strategically regulated by the circadian clock in SCN2.2 cells and that the largest categories of rhythmic genes are those involved in cellular and systems-level communication or in metabolic processes like cellular respiration, fatty acid recycling, and steroid synthesis. Because many of the same genes or nodes within these functional categories were rhythmically expressed in both SCN2.2 cells and the rat SCN, the circadian regulation of these pathways may be important in modulating input to or output from the SCN clock mechanism. In summary, global expression and circadian regulation of the SCN2.2 transcriptome retain many SCN-like properties, suggesting that genes displaying rhythmic profiles in both experimental models may be integral to their function as both circadian oscillators and pacemakers.