titania is a zebrafish mutant which exhibits a markedly hypoplastic intestinal epithelium, smaller liver and pancreas. The genetic lesion in titania is a splice-site mutation in periodic tryptophan protein 2 homologue (pwp2h), a gene essential for the production of 18S rRNA and 40S ribosomal subunits. Using TEM, we observed that intestinal epithelial cells (IECs) in titania respond to impaired ribosome biogenesis by undergoing autophagy. Autophagy is a survival mechanism invoked during periods of cell starvation to generate nutrients by promoting the intracellular recycling of organelles. At 4-5dpf, the IECs in titania larvae accumulate abundant autophagosomes/autophagolysosomes and display increased conversion of LC3I to LC3II, a biochemical assay of increased autophagic activity.
It is known that autophagy may be activated by p53 and inhibited by the Tor pathway. While titania embryos exhibit elevated p53 levels at 4dpf, introducing titania onto a p53 mutant background does not alleviate autophagy. The process is also independent of the status of the Tor pathway, since titania larvae display elevated levels of pRPS6, an output of Tor activity, and inactivating tsc2 expression in titania, which negatively regulates Tor, does not moderate autophagy.
We inhibited autophagy in developing zebrafish by microinjecting 1-2 cell zebrafish embryos with an antisense morpholino oligonucleotide targeted to Atg5, an indispensable component of the initiation of autophagy machinery. While this had no effect on the lifespan of wildtype embryos, the average lifespan of titania mutants was reduced from 8 days to 5 days, thereby demonstrating that autophagy in this setting increases cell survival. This finding may be relevant to the ongoing development of therapeutic strategies aimed at killing cancer cells by targeting ribosome biogenesis. In some contexts, this approach could contribute resistance to such treatments by inducing autophagy.