BMC Neuroscience BioMed Central Research article Open Access Regulation of prokineticin 2 expression by light and the circadian clock Michelle Y Cheng1, Eric L Bittman2, Samer Hattar3 and Qun-Yong Zhou*1 Address: 1Department of Pharmacology, University of California, Irvine, CA, USA, 2Department of Biology, University of Massachusetts, Amherst, MA, USA and 3Departments of Biology and Neuroscience, Johns Hopkins University, Baltimore, MD, USA Email: Michelle Y Cheng - [email protected]; Eric L Bittman - [email protected]; Samer Hattar - [email protected]; Qun- Yong Zhou* - [email protected] * Corresponding author Published: 11 March 2005 Received: 17 November 2004 Accepted: 11 March 2005 BMC Neuroscience 2005, 6:17 doi:10.1186/1471-2202-6-17 This article is available from: http://www.biomedcentral.com/1471-2202/6/17 © 2005 Cheng et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Background: The suprachiasmatic nucleus (SCN) contains the master circadian clock that regulates daily rhythms of many physiological and behavioural processes in mammals. Previously we have shown that prokineticin 2 (PK2) is a clock-controlled gene that may function as a critical SCN output molecule responsible for circadian locomotor rhythms. As light is the principal zeitgeber that entrains the circadian oscillator, and PK2 expression is responsive to nocturnal light pulses, we further investigated the effects of light on the molecular rhythm of PK2 in the SCN. In particular, we examined how PK2 responds to shifts of light/dark cycles and changes in photoperiod. We also investigated which photoreceptors are responsible for the light-induced PK2 expression in the SCN. To determine whether light requires an intact functional circadian pacemaker to regulate PK2, we examined PK2 expression in cryptochrome1,2-deficient (Cry1-/-Cry2-/-) mice that lack functional circadian clock under normal light/dark cycles and constant darkness. Results: Upon abrupt shifts of the light/dark cycle, PK2 expression exhibits transients in response to phase advances but rapidly entrains to phase delays. Photoperiod studies indicate that PK2 responds differentially to changes in light period. Although the phase of PK2 expression expands as the light period increases, decreasing light period does not further condense the phase of PK2 expression. Genetic knockout studies revealed that functional melanopsin and rod-cone photoreceptive systems are required for the light-inducibility of PK2. In Cry1-/-Cry2-/- mice that lack a functional circadian clock, a low amplitude PK2 rhythm is detected under light/dark conditions, but not in constant darkness. This suggests that light can directly regulate PK2 expression in the SCN. Conclusion: These data demonstrate that the molecular rhythm of PK2 in the SCN is regulated by both the circadian clock and light. PK2 is predominantly controlled by the endogenous circadian clock, while light plays a modulatory role. The Cry1-/-Cry2-/- mice studies reveal a light-driven PK2 rhythm, indicating that light can induce PK2 expression independent of the circadian oscillator. The light inducibility of PK2 suggests that in addition to its role in clock-driven rhythms of locomotor behaviour, PK2 may also participate in the photic entrainment of circadian locomotor rhythms. Page 1 of 11 (page number not for citation purposes) BMC Neuroscience 2005, 6:17 http://www.biomedcentral.com/1471-2202/6/17 Background lation of the rhythm of PK2 expression in the SCN. In Light is the principal zeitgeber that entrains circadian particular, we investigated the photoreceptive mecha- rhythms of physiology and behaviour [1,2]. The major nisms responsible for the light-induced PK2 expression in light input pathway to the suprachiasmatic nucleus (SCN) the SCN. Utilizing Cry1-/-Cry2-/- mice, we also deter- is the retinohypothalamic tract [3], which arises from a mined whether light can drive PK2 expression in the SCN population of retinal ganglion cells [4]. Recent studies independent of a functional circadian clock. have demonstrated that melanopsin-containing retinal ganglion cells, rods, and cones all convey photic informa- Results tion to the SCN, and mice lacking these photoreceptive PK2 responds differentially to the delay and advance of systems cannot be entrained by light [5-11]. Excellent light/dark cycles progress has been made in the understanding of circadian We first examined the effects of abrupt shifts of light/dark photic entrainment [12-15]. This includes light-induced cycles on PK2 mRNA rhythm in the SCN. Animals were transcriptional activation of core clock genes in the SCN, first entrained for two weeks under 12 hour light: 12 hour such as Per1 and Per2, as well as immediate-early gene c- dark (LD), then subjected to either a 6 hour delay (6hrD) fos. Exposure to light pulses at night induces expression of shift or 6 hour advance (6hrA) shift of light/dark cycles. these genes in the SCN, and this light induction mecha- We measured PK2 mRNA in the SCN of these animals to nism has been suggested as a critical pathway for the reset- examine how quickly the PK2 mRNA rhythm re-entrains ting of circadian clock in response to changes in light/dark to the shifted light/dark cycles. Under LD, PK2 mRNA conditions [16-19]. Intercellular signalling mechanisms peaks during the day and remains low or undetectable between SCN neurons are also important in circadian during the night. During the first cycle of the delayed shift photic entrainment, as mice with mutation in a neuropep- (6hrD), the PK2 mRNA rhythm responds quickly: the ris- tide receptor for VIP (Vasoactive Intestinal Peptide) and ing phase of PK2 expression adjusts rapidly to the delayed PACAP (Pituitary Adenylate Cyclase Activating Peptide) light/dark cycles, while the falling phase still resembles are unable to sustain normal circadian behaviour and that of the unshifted light/dark cycles (Figure 1A). In con- exhibit loss of sensitivity to light [20]. trast, the PK2 mRNA rhythm responds very little to a 6 hour advance shift (6hrA). During the first cycle of the In addition to the effect of light on circadian entrainment, advance shift, the PK2 oscillation pattern remains similar light also has a direct effect on physiology and behaviour, to that of the unshifted LD (Figure 1B). These changes in generally termed as "masking" [21,22]. For instance, light PK2 expression during 6hrD or 6hrA shift indicate that the pulses given at night acutely suppress the locomotor endogenous circadian clock exerts dominant control over behaviour of nocturnal rodents [21,22], and this can the PK2 rhythm, as PK2 expression cannot respond imme- occur without functional clockwork [23-27]. Masking diately and completely to the shifts of light/dark cycles. may account for the maintenance under normal light/ dark conditions of wheel-running rhythms in crypto- As it normally takes about 1–2 days for locomotor chrome-deficient (Cry1-/-Cry2-/-) mice, which are behav- rhythms to stably entrain to phase delays and about 5–6 iourally arrhythmic under constant darkness. The days to entrain to phase advances [30,31], we next exam- contribution of masking to normal locomotor activity ined the timecourse of shifts of the PK2 rhythm to 6 hour rhythms is unclear, as is the participation of the SCN in phase advances and delays. Consistent with the animal's masking effects of light. Vitaterna et al (1999) first locomotor behaviour, the PK2 mRNA rhythm reaches sta- observed a light-driven Per2 rhythm in the SCN in Cry1-/- ble phase within 2 days of 6hrD shift (Figure 1C). In con- Cry2-/- mice, and have suggested that the light-driven trast, only the rise of PK2 reaches stable phase within 2 molecular rhythm in the SCN may be related to the pres- days of 6hrA shift, while the fall of PK2 takes longer (Fig- ervation of their locomotor rhythm [25]. ure 1D). Thus, we further examined whether the PK2 rhythm is stably entrained after 6 days of 6hrA shift. As We previously found that prokineticin 2 (PK2) is a first expected, the PK2 rhythm is completely entrained to 6hrA order clock-controlled gene, whose expression in the SCN shift after 6 days (Figure 1D). Together, the differential is regulated by CLOCK and BMAL1 acting on the E-boxes responses of PK2 rhythm to a 6hrD or 6hrA shift indicate in the gene's promoter [28]. We have also demonstrated that the endogenous circadian clock predominantly con- that PK2 may function as a SCN output molecule that trols PK2 rhythm, as circadian oscillators typically show transmits circadian locomotor rhythm via activation of a rapid phase delays but advance with transients [31,32]. G protein-coupled receptor [28,29]. Interestingly, we also The entrainment patterns of PK2 during phase shifts are observed that PK2 expression is rapidly induced by light consistent with behavioural studies in animals and pulses administered at night [28], a characteristic that is human subjects [30,31]. usually seen with core clockwork genes but not clock-con- trolled genes. Here we further investigated the light regu- Page 2 of 11 (page number not for citation purposes) BMC Neuroscience 2005, 6:17 http://www.biomedcentral.com/1471-2202/6/17 A LD B LD 6hrD 6hrA LD 3000 LD 3000 6hrD LD 2500 2500 LD 6hrA 2000 2000 1500 1500 1000 1000 PK2 mRNA (dpm/mg) 500 PK2 mRNA (dpm/mg) 500 0 0 LD 1 4 7 10 13 16 19 22 1 4 7 10 13 16 19 22 LD 1 4 7 10 13 16 19 22 1 4 7 10 13 16 19 22 6hrD 13 16 19 22 1 4 7 10 13 16 6hrA 13 16 19 22 1 4 7 10 13 16 C LD D LD 6hrD 6hrA LD 3000 LD 3000 LD LD 6hrA 6hrD 6hrA+2LD 2500 6hrD+2LD 2500 6hrA+6LD 2000 2000 1500 1500 1000 1000 PK2 mRNA (dpm/mg) PK2 mRNA (dpm/mg) 500 500 0 0 LD 1 4 7 10 13 16 19 22 1 4 7 10 13 16 19 22 LD 1 4 7 10 13 16 19 22 1 4 7 10 13 16 19 22 6hrD 13 16 19 22 1 4 7 10 13 16 6hrA 13 16 19 22 1 4 7 10 13 16 TemporalFigure 1 profiles of PK2 mRNA in the SCN in response to abrupt shifts of light/dark cycles Temporal profiles of PK2 mRNA in the SCN in response to abrupt shifts of light/dark cycles.