Rhythmic Serotonin N-Acetyltransferase mRNA Degradation Is Essential for the Maintenance of Its Circadian Oscillation

Abstract

Serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase [AANAT]) is the key enzyme in melatonin synthesis regulated by circadian rhythm. To date, our understanding of the oscillatory mechanism of melatonin has been limited to autoregulatory transcriptional and posttranslational regulations of AANAT mRNA. In this study, we identify three proteins from pineal glands that associate with cis-acting elements within species-specific AANAT 3′ untranslated regions to mediate mRNA degradation. These proteins include heterogeneous nuclear ribonucleoprotein R (hnRNP R), hnRNP Q, and hnRNP L. Their RNA-destabilizing function was determined by RNA interference and overexpression approaches. Expression patterns of these factors in pineal glands display robust circadian rhythm. The enhanced levels detected after midnight correlate with an abrupt decline in AANAT mRNA level. A mathematical model for the AANAT mRNA profile and its experimental evidence with rat pinealocytes indicates that rhythmic AANAT mRNA degradation mediated by hnRNP R, hnRNP Q, and hnRNP L is a key process in the regulation of its circadian oscillation.

ACKNOWLEDGMENTS

We thank G. Dreyfuss, D. C. Klein, K. Mikoshiba, C. M. Craft, and C. R. Astell for kindly providing the anti-hnRNP L antibody (4D11), anti-AANAT antibody, anti-SYNCRIP-N antibody, bovine AANAT cDNA 10-1B, and plasmid pRSETC9-15, respectively. We additionally thank J. B. Park for useful discussions on MALDI mass spectrometry, researcher J. E. Lee for performing the MALDI analysis, and Y. Kang, E. M. Hur, and K. S. Yi for critical reading of the manuscript.

This work was supported by the Brain Neurobiology Research Program, the National Research Laboratory Program, the Systems Bio-Dynamics Research Center of the Ministry of Science and Technology (MOST), and the Brain Korea 21 program of the Ministry of Education.