Neuronal ribosomal protein function regulates Drosophila growth and development
Stimulation of ribosome biogenesis is a conserved mechanism of growth control. Studies, mostly in yeast and cell culture, have shown how ribosome synthesis controls cell growth. However, less is know about how ribosome synthesis promotes body growth and development. We have been studying this issue by studying the Minutes. These are a class of dominant ribosomal protein (rp/+) mutants that exhibit a characteristic delay in larval development – a phenotype classically thought to due to an overall reduction in ribosome numbers and protein synthesis. However, when we examined three Minutes (rpS13/+, rpS26/+ and rpL38/+) we saw little or no change in either global ribosome numbers or in protein synthesis rates, using a puromycin labelling assay, when compared to wild-type controls, as shown in the first box of data characterizing rpS13 mutants. Instead, as discussed below, we found evidence of a cell type-specific function for one RP (S13) in the control of development.
Termination of the larval period is controlled by a neuroendocrine circuit that leads to a pulse of secretion of the steroid hormone ecdysone from the prothoracic gland (PG) in response to signals from specific CNS neurons. We found that rpS13/+ animals had a delayed ecdysone pulse as seen by delayed expression of the ‘Halloween’ genes, spooky and phantom, which are required for ecdysone synthesis in the PG. Ecdysone feeding also partially reversed the delay in development therefore we postulated that these effects might reflect a specific role for S13 in the CNS-PG neuroendocrine circuit. To test this, we used the GAL4/UAS system to see if tissue selective expression of S13 could rescue the delayed development seen in rps13/+ animals, expression of S13 in the PG (using P0206-Gal4) had no effect. However, we found that expression of S13 in neurons (using either elav-Gal4, or nSyb-Gal4) could rescue the delay in development in rpS13/+ animals by ~40%. Body overgrowth was also reduced by neuronal re-expression of RpS13. However, we found no obvious defects in protein synthesis in the brain or in brain size of rpS13/+ animals. Furthermore, we discovered that expression of S13 in serotonergic (5-HT) neurons alone (Trh-Gal4) lead to the same rescue in developmental timing as pan-neuronal S13 expression, while re-expression in other neuronal subtypes had mostly negligible effects. Three pairs of 5-HT innervate the PG to control ecdysone release, and S13 expression in these neurons (using the R29H01-Gal4) driver also partially rescued the delay in development in rpS13/+ animals. While axonal projection growth of these neurons do not appear to be affected, electrical activation of rpS13/+ animals with TRPA1 channel expression partially rescues adult eclosion delay. Our model suggests that RpS13 is required for proper synaptic activity in the serotonergic neurons that signal to the prothoracic gland to produce ecdysone. A reduction in RpS13 in these neurons thus leads to delayed ecdysone production and delay in development.