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reduce average life span generally increase reproduction through activation
of insulin signaling. Of all the amino acids, addition of methionine (Met)
alone to a protein restricted diet increases fecundity as much as a non-
restricted diet, indicating that this essential amino acid plays a key role. Strik-
ingly, an extra copy of the initiator methionine tRNA (tRNA i Met ) also
accelerates pupariation ( Rideout, Marshall, & Grewal, 2012 ). Met may
therefore be a limiting factor for a nutritional checkpoint that couples dietary
uptake of amino acids with growth, developmental timing and reproduc-
tion. Another interesting observation is that leucine and isoleucine, two
other essential amino acids, are the only single amino acids that promotes
DILP secretion from the IPCs ( Geminard et al., 2009 ).
Studies unraveling the nutritional cues that affect the endocrine system
and onset of maturation in Drosophila may provide important insight that will
serve as a paradigm to understand the correlations between diet, obesity, dia-
betes, and timing of puberty in mammals. One important aspect is that
imbalanced diets such as carbohydrate overload affect developmental timing
in Drosophila , which may provide the framework for understanding how
obesity and diabetes influence pubertal timing. Studies on Drosophila have
established a key role for dietary proteins in affecting developmental timing.
Consistent with these observations, it has recently been found that dietary
intake of protein during early childhood is associated with timing of puberty
in humans ( Gunther, Karaolis-Danckert, Kroke, Remer, & Buyken, 2010 ).
How are amino acids sensed at the organismal level? In Drosophila the fat
body is a central relay that communicates dietary protein intake to peripheral
tissues and the endocrine system.
3. A NUTRITIONAL CHECKPOINT BASED ON CROSSTALK
BETWEEN DIFFERENT TISSUES
3.1. Coordinating nutrient uptake and systemic growth
Amino acids are more intimately linked to growth than sugar in Drosophila .
Although TOR cell-autonomously senses nutrients and regulates growth
according to amino acid levels, evidence suggests that growth is regulated
at the cellular level in response to circulating DILPs rather than levels of
amino acids ( Britton, Lockwood, Li, Cohen, & Edgar, 2002 ). Thus, when
larvae are deprived of dietary proteins, growth is suppressed due to a reduc-
tion of systemic insulin and not as a direct consequence of amino acid star-
vation at the cellular level. This implies the existence of a mechanism that
monitors amino acid levels and controls the secretion of DILPs. In Drosophila ,
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