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Abstract
In Drosophila, the pattern of adult pigmentation is initiated during late pupal stages by the production of catecholamines DOPA and dopamine, which are converted to melanin. The pattern and degree of melanin deposition is controlled by the expression of genes such as ebony and yellow as well as by the enzymes involved in catecholamine biosynthesis. In this study, we show that the conserved TSC/TORC1 cell growth pathway controls catecholamine biosynthesis in Drosophila during pigmentation. We find that high levels of Rheb, an activator of the TORC1 complex, promote premature pigmentation in the mechanosensory bristles during pupal stages, and alter pigmentation in the cuticle of the adult fly. Disrupting either melanin synthesis by RNAi knockdown of melanogenic enzymes such as tyrosine hydroxylase (TH), or downregulating TORC1 activity by Raptor knockdown, suppresses the Rheb-dependent pigmentation phenotype in vivo. Increased Rheb activity drives pigmentation by increasing levels of TH in epidermal cells. Our findings indicate that control of pigmentation is linked to the cellular nutrient-sensing pathway by regulating levels of a critical enzyme in melanogenesis, providing further evidence that inappropriate activation of TORC1, a hallmark of the human tuberous sclerosis complex tumor syndrome disorder, can alter metabolic and differentiation pathways in unexpected ways.
Highlights
During development, organisms must coordinate growth, proliferation and differentiation
Rheb in turn is controlled by a heterodimeric complex composed of products of the tuberous sclerosis complex 1 and 2 genes (TSC1 and TSC2, or hamartin and tuberin, respectively) which act together as a GTPase-activating protein (GAP) to limit Rheb by maintaining it in a GDP bound state (Fig. 1A)
Chronic activation of the TORC1 complex is associated with human pathologies such as the Tuberous Sclerosis Complex, a tumor suppressor gene syndrome characterized by growth of benign tumors in multiple organs along with neurological manifestations resulting from inactivating mutations in either TSC1 or TSC2 genes [6]
Summary
Organisms must coordinate growth, proliferation and differentiation. The TORC1 complex is an evolutionarily conserved central node in coordination of cell growth by driving protein synthesis in response to growth factor signals and the availability of amino acids [1]. Rheb in turn is controlled by a heterodimeric complex composed of products of the tuberous sclerosis complex 1 and 2 genes (TSC1 and TSC2, or hamartin and tuberin, respectively) which act together as a GTPase-activating protein (GAP) to limit Rheb by maintaining it in a GDP bound state (Fig. 1A). Chronic activation of the TORC1 complex is associated with human pathologies such as the Tuberous Sclerosis Complex, a tumor suppressor gene syndrome characterized by growth of benign tumors in multiple organs along with neurological manifestations resulting from inactivating mutations in either TSC1 or TSC2 genes [6]. We show that chronic activation of TORC1 in the Drosophila pupal epidermis results in hyperpigmentation of mechanosensory bristles and adult cuticle due to increased levels of tyrosine hydroxylase
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