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Schneider, 1972 ). They respond to fatty acid loading to form LDs, but they are not
primary adipocyte cultures. The fatty acid concentration and treatment duration used
in the screens are optimized to allow additional up- and downregulations of LD stor-
age by the RNAi manipulations, but might not reflect the physiological condition
in vivo . Another limitation for cell-based screens is the lack of capacity to reflect
tissue-tissue communication, as obesity is a complex problem involving multiple
tissues in vivo . Thus, systematic in vivo validations are required in follow-up studies
for the cell-based screens. Taking these limitations for the cell-based screens into
account, it is highly significant that both the S2 and Kc 167 cell LD screens pinpointed
the COPI complex as a novel regulator for LD biology. Fifty-four genes among the
227 genes identified in the S2 cell LD screen were also identified in the Kc 167 cell LD
screen (among 526 genes). This is despite the difficulty to directly compare the hit
lists obtained, since different dsRNA libraries, cell types, and different phenotypical
readouts were used for these two screens.
Conversely, the organism-level screens on a genome-wide scale are limited to a
single genetic manipulation at limited time points, and biochemical readouts feasible
in a high-throughput manner ( Pospisilik et al., 2010 ). The whole fly triacylglyceride
measur ement used in the genome-wide RNAi screen is a complex readout involving
multiple tissues. Thus, sorting out the complex tissue interplay and cellular mecha-
nisms leading to organism triacylglyceride level changes could be a long and chal-
lenging task.
4.2 LD DYNAMICS IN DIFFERENT DROSOPHILA
DEVELOPMENTAL STAGES
Drosophila is one of the fastest developing multicellular model organisms and has
contributed a great deal to our understanding of metabolism and energy homeostasis
( Kuhnlein, 2011 ). During embryonic development, which lasts about 24 h, the em-
bryos are solely dependent on the energy source deposited maternally within the
eggs, mainly in the form of LDs and yolk proteins. During the first few hours of em-
bryonic development (including the stages of syncytial blastoderm, cellularization,
and the beginning of gastrulation), LDs within the embryo undergo developmental
stage-dependent bidirectional transports at the periphery of the embryo ( Welte,
Gross, Postner, Block, & Wieschaus, 1998 ). Because LDs give rise to the visually
turbid characteristic of the embryo under light microscopy, such gross movement
of LDs with the progress of development can be examined in live embryos under
a standard light microscope. Extensive genetic and biophysical studies have charac-
terized that individual LDs indeed undergo bidirectional transports carried by molec-
ular motors along polarized microtubule tracks ( Shubeita et al., 2008; Welte et al.,
1998 ). Even though a large quantity of LDs swamps the embryos, the movement tra-
jectory of individual droplets can be precisely tracked with differential interference
contrast microscopy. Furthermore, it is also feasible to “trap” an individual LD in
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