Biology Reference
In-Depth Information
In concert with systemic growth regulators such as insulin, the TOR
pathway acts cell-autonomously to coordinate growth according to the inter-
nal nutrient status (
Oldham & Hafen, 2003; Wullschleger, Loewith, & Hall,
2006
). The TOR complex is regulated by cellular levels of amino acids and
ATP and promotes cell growth by activating ribosome biogenesis and protein
synthesis through ribosomal S6 kinase (S6K). Cellular influx of amino acids
during feeding stimulates TOR activity and promotes growth. On the other
end, amino acid starvation causes developmental arrest and phenotypes
associatedwith disruption of TORactivity phenocopies amino acid deprived
animals (
Colombani et al., 2003; Geminard, Rulifson, & Leopold, 2009;
Oldham, Montagne, Radimerski, Thomas, & Hafen, 2000; Zhang,
Stallock, Ng, Reinhard, & Neufeld, 2000
). Crosstalk between the TOR
and insulin pathways exists through the tuberous sclerosis proteins (TSC1
and TSC2) that are regulated by PI3K/AKT and inhibits TOR. Thus,
TOR has a distinct role in sensing local amino acid concentrations, although
it also responds to systemic nutritional signals through its regulation by the
insulin pathway.
2.3. Influence of dietary amino acids, sugar, and fat on growth
and onset of maturation
The developmental time of
Drosophila
from embryo to adulthood takes
approximately nine days on a nutritionally rich balanced diet but can be pro-
longed depending on the quality and composition of the food. Amino acids
are a fundamental part of the diet required to drive developmental progres-
sion. Newly hatched larvae fed only on a sucrose diet die as first instar larvae
(L1) as they fail to grow (
Britton & Edgar, 1998; Colombani et al., 2003
).
The L1 growth arrest of sucrose fed larvae is completely reversible by adding
protein to media which allows continued growth, while adding fat or nucle-
otides precursors only allows the larva to develop to the second larval instar
(L2) or early L3 before arresting their development (
Britton & Edgar, 1998
).
Larvae fed on a low protein diet develop to adulthood, but with a negative
correlation between the amount of protein added to the food and the devel-
opmental time (
Fig. 2.1
B;
Layalle, Arquier, & Leopold, 2008
). This suggests
a tight connection between amino acid status, growth, and ecdysone release.
Developmental time and growth is, however, not only dependent on
sufficient protein but can also be affected by other dietary components such
as sugar. In a study by
Matzkin, Johnson, Paight, Bozinovic, and Markow
(2011)
Drosophila
larvae were fed on different diets with the same amount
of calories, but containing either a high sugar to protein ratio or a low sugar
