Biology Reference
In-Depth Information
discussed in an earlier section, PG cells are thought to gate eclosion timing,
and this gating in turn is linked to circadian-controlled outputs from the PG,
which is strongly supported by this study. It follows that exposing pupae to
light causes phase-shifting of
period
pulses, which in turn distorts the gating of
eclosion rhythms through a mechanism that is still unclear.
The above experiments used a
period-luciferase
transgene as a transcrip-
tional readout, but how do these transcript peaks relate to the nuclear oscil-
lations of PERIOD-TIMELESS heterodimers? After all, the circadian
circuit in insects is based on the nuclear translocation of PERIOD and
TIMELESS, which then act as negative regulators of
clock
and
cycle
(
Fig. 1.2
). When explanted, CNS-RGs were incubated under LD con-
ditions, PERIOD showed rhythmic nuclear localization, similar to what
has been observed previously (
Emery et al., 1997
), however, this was
not observed in constant darkness. In contrast, rhythmic accumulation of
nuclear PERIOD was evident in cultured LNs under DD conditions,
suggesting rhythmic accumulation of PERIOD in PG nuclei does not rely
on LN input alone.
Interestingly, previous work had shown that TIMELESS protein,
the heterodimeric partner of PERIOD, does exhibit nuclear rhythms in
the PG in constant darkness (
Myers et al., 2003
), raising the possibility that
PERIOD behaves differently from TIMELESS. To test this directly, the
authors examined the nuclear rhythm of TIMELESS in CNS-RGs or
RGs alone. Indeed, nuclear TIMELESS rhythms were maintained in PG
cells of CNS-RGs and even in RGs not attached to a brain, regardless of
whether the tissues were cultured in LD or DD environments. This data
suggest that TIMELESS behaves differently from its dimerization partner
PERIOD, since rhythmic accumulation of TIMELESS in PG nuclei is
not driven by signals originating from the CNS, and therefore appears to
be self-sustained. The difference between TIMELESS and PERIOD regu-
lation is intriguing, given their close functional relation, however the phys-
iological significance of this novel finding remains unclear. It is possible that
TIMELESS is also part of non-canonical circadian transcriptional complex
in PG cells where PERIOD is not required, in which case the dependency
on the CNS would reflect the presence of different TIMELESS protein
complexes.
Intracellular Ca
2
þ
signaling has long been known to play an important
role in ecdysteroidogenic signal transduction (
Henrich, 1995; Venkatesh &
Hasan, 1997
). Given the dependency of oscillatory PG components on the
CNS, Morioka and colleagues examined how Ca
2
þ
levels fluctuated in PG
