Genetic interaction of Per- and Dec-genes in the mammalian circadian clock

Abstract

Many behavioural and physiological processes in mammals display circadian (24 hour) rhythms controlled by an internal timekeeping system the circadian clock. The pacemaker of the circadian clock of mammals is located in the suprachiasmatic nuclei (SCN) of the hypothalamus and synchronizes peripheral oscillators in a hierarchical manner to the external light/dark (LD) cycles by humoral and neuronal pathways. The molecular timekeeping machinery consists of a network of transcriptional translational feedback loops (TTL). The mammalian core TTL includes CLOCK/(NPAS2) and BMAL1/(ARNTL) which together activate the transcription of E box controlled clock genes such as Period (Per1 3), Cryptochrome (Cry1,2) and Dec (1,2). PER (1,2) and CRY (1,2) heterodimerize and translocate back to the nucleus where they repress CLOCK/BMAL1 mediated transcription. The basic helix loop helix transcription factors DEC1 (BHLHE40) and DEC2 (BHLHE41) can interact with the core TTL by binding to CLOCK/BMAL1 complexes or to E-box elements, forming an accessory feedback mechanism. In Drosophila, the DEC ortholog CWO shows synergistic interaction to PER. This promoted, us to analyze PER(1,2) DEC interactions in the murine circadian system. We generated Per(1,2)/Dec double and triple mutant mice and measured circadian locomotor behaviour and clock gene expression in the SCN. Our wheel running data suggest synergistic Per(1,2) Dec interactions in photic entrainment with an advanced activity onset indicative of impaired sleep behaviour. Under free run, the Per1 Dec interactions remain synergistic whereas the Per2 Dec interaction becomes antagonistic together with a partial rescue of the Per2 phenotype. This rescue is seen at multiple levels including period length and rhythmicity of behaviour as well as clock gene expression in the SCN. The molecular data suggest a bimodular regulatory function of Per(1,2) Dec on E box controlled clock genes in the SCN, moreover Per1 Dec bimodularity is time of day dependent. For the first time, we show that DECs together with PER1 activate the transcription of Bmal1 in the SCN. Investigations of photic phase delay response suggest a model for phase delay resetting with an essential role of Per2/PER2 and a minor role of Per1/PER1. Together, our results show interactions of Per(1,2) and Dec(1,2) feedback in the mammalian pacemaker with different modes of interactivities under entrained and free run conditions together with a compensation effect. These data postulate a conservation of Per Dec (Cwo) interactions between vertebrate and invertebrate circadian clocks.