How Clocks and Hormones Act in Concert to Control the Timing of Insect Development - Current Topics in Developmental Biology - page 14

the rectal epithelium, oenocytes, the PG, and the salivary glands. EcR
shuttling was also observed in the lateral clock neurons, where EcR was
shown to co-localize in the nuclei with the circadian protein PERIOD
duringthenight,albeitmostofthePERsignalappearedtobecytoplasmic.
Other tissues, however, showed no circadian shuttling of EcR, including
the Malpighian tubules and spermatocytes.
Taken together, these findings suggest that hormones act as circadian
outputs to coordinate periodic changes in the physiology of the animal
( Fig. 1.1 B). Ecdysone is periodically released, while EcR is similarly
cycling in anticipation of maximum hormone levels. Recently, it was
shown that the circadian system itself is reorganized during metamorphosis
in response to ecdysone, suggesting that the endocrine system not only
responds to circadian cues, but also signals back to it ( Vafopoulou &
Steel, 2012b ).
EcRprotein exhibits circadian cycling in a range of tissues ( Vafopoulou&
Steel, 2006, 2012a ), and many nuclear receptors appear to underlie circadian
regulation at the transcript level. In particular, large-scale approaches exam-
ining all 49 mouse nuclear receptor genes revealed that roughly half of them
are rhythmically expressed and linked to basal metabolic functions ( Yang
et al., 2006 ). In addition, nuclear receptor-signaling pathways have been
shown to play a direct role in central and peripheral circadian clocks in mam-
mals. For instance, ROR a plays a direct role in the suprachiasmatic nuclei as
an activator of Bmal1 transcription, the vertebrate ortholog of the Drosophila
cycle ( Sato et al., 2004 ). In addition, the REV-ERB/retinoid-related
orphan receptor (ortholog of the insect E75 , an early ecdysone response gene)
regulates the expression of core clock genes and contributes to the robustness
of the clock mechanism ( Preitner et al., 2002; Solt et al., 2012 ). For a detailed
discussion of the link between nuclear receptors and circadian rhythms we
refer the reader to a review by Teboul, Guillaumond, Grechez-Cassiau,
and Delaunay (2008) .
8. OSCILLATIONS IN THE PROTHORACIC GLAND: INPUT
FROM THE BRAIN
In a recent study, Morioka, Matsumoto, and Ikeda (2012) demon-
strated that transcriptional rhythms of period in Drosophila PG cells are auton-
omous, but influenced by and dependent on the central nervous system
(CNS). To study circadian rhythms on a cellular level in prothoracic glands,
the authors used video imaging to follow the expression of a period -driven