Biology Reference
In-Depth Information
times to roughly 24 h (
Chiu, Vanselow, Kramer, & Edery, 2008; Fang,
Sathyanarayanan, & Sehgal, 2007; Kim & Edery, 2006; Kim, Ko, Yu,
Hardin, & Edery, 2007; Kivimae, Saez, & Young, 2008; Meissner, Kilman,
Lin, & Allada, 2008
). CRYPTOCHROME (CRY) is a cell-autonomous
blue light photoreceptor that mediates the entrainment of the core clock
by light, through a mechanism that involves the degradation of TIMELESS
upon stimulation by light (
Dubruille & Emery, 2008
).
Most cells harbor their own circadian machinery, however, these inde-
pendent clocks are usually aligned with the activity of a central clock that acts
on an organismal level. Central clocks are also referred to as pacemaker neu-
rons, such as the suprachiasmatic nuclei in humans and the LNs in
Drosophila
.
In particular, the adult
Drosophila
brain harbors
150 neurons that express
the molecular clock components and comprise the central pacemaker neural
networks. These neurons consist of three clusters of dorsal neurons (DN1,
DN2, and DN3), one cluster of the dorsal lateral neurons (LN
d
s), one of
large ventral lateral neurons (l-LN
v
s), one cluster of small ventral lateral neu-
rons (s-LN
v
s), and the lateral posterior neurons (LPN). A subset of the LNs
express a neuropeptide called PDF and are considered to represent the actual
pacemaker cells in the
Drosophila
brain (
Collins & Blau, 2007; Dubruille &
Emery, 2008; Nitabach & Taghert, 2008
). The s-LN
v
s appear to indirectly
project to the prothoracic gland by innervating neurosecretory neurons of
the lateral protocerebrum, PG-LP, which in turn directly project to the pro-
thoracic gland (
Siegmund & Korge, 2001
).
In
Drosophila
, circadian clocks are also found in peripheral tissues
(
Giebultowicz, 2001; Saunders, 1982
). Although there is evidence that
the central pacemaker clock can synchronize peripheral clocks through
secretion of humoral factors, emerging evidence supports the idea that
peripheral clocks can entrain their own rhythm independently (
Liu,
Lorenz, Yu, Hall, & Rosbash, 1988; Plautz, Kaneko, Hall, & Kay, 1997
).
While CRY is expressed in some of these peripheral clocks, it appears to
function in a photoreceptor-independent manner, suggesting fundamental
differences to oscillator mechanisms found in core clocks (
Krishnan et al.,
2001
). The presence of peripheral clocks independent from the central pace-
maker neurons has been reported in multiple
Drosophila
tissues, including the
fat body (
So et al., 2000; Xu, Zheng, & Sehgal, 2008
) and antennae
(
Krishnan et al., 2001
) where local clocks control metabolism and feeding
behavior in the adult fly. Other peripheral clocks in
Drosophila
seem to oper-
ate in the
Corpus allatum
(CA) and the PG, both structures of the central lar-
val endocrine system, the ring gland (RG) (
Allada & Chung, 2010; Emery,
