Lin28/LIN-28 is a conserved RNA-binding protein that promotes proliferation and pluripotency and can be oncogenic in mammals. Mammalian Lin28 and C. elegans LIN-28 have been shown to inhibit biogenesis of the conserved cellular differentiation-promoting microRNA let-7 by directly binding to unprocessed let-7 transcripts. Lin28/LIN-28 also bind and regulate many mRNAs in diverse cell types. However, the determinants and consequences of LIN-28-mRNA interactions are not well understood. Here, we report that C. elegans LIN-28 represses the expression of LIN-46, a downstream protein in the heterochronic pathway. We find that lin-28 and sequences within the lin-46 5' UTR are required to prevent LIN-46 expression at early larval stages. Moreover, we find that precocious LIN-46 expression caused by mutations in the lin-46 5' UTR is sufficient to cause precocious heterochronic defects similar to those of lin-28(lf) animals. Thus, our findings demonstrate the biological importance of the regulation of individual target mRNAs by LIN-28.

The let-7 gene encodes a highly conserved microRNA with critical functions integral to cell fate specification and developmental progression in diverse animals. In Caenorhabditis elegans, let-7 is a component of the heterochronic (developmental timing) gene regulatory network, and loss-of-function mutations of let-7 result in lethality during the larval to adult transition due to misregulation of the conserved let-7 target, lin-41. To date, no bilaterian animal lacking let-7 has been characterized. In this study, we identify a cohort of nematode species within the genus Caenorhabditis, closely related to C. elegans, that lack the let-7 microRNA, owing to absence of the let-7 gene. Using Caenorhabditis sulstoni as a representative let-7-lacking species to characterize normal larval development in the absence of let-7, we demonstrate that, except for the lack of let-7, the heterochronic gene network is otherwise functionally conserved. We also report that species lacking let-7 contain a group of divergent let-7 paralogs-also known as the let-7-family of microRNAs-that have apparently assumed the role of targeting the LIN-41 mRNA.

Autophagy delivers cytoplasmic material to the lysosome for degradation, and has been implicated in many cellular processes, including stress, infection, survival, and death. Although the regulation and role that autophagy plays in stress, infection, and survival is apparent, its involvement during cell death remains relatively unclear. In this thesis I summarize what is known about the roles autophagy can play in cell death, and the differences between the utilization of autophagy during nutrient deprivation and cell death. Utilizing Drosophila melanogaster as a model system, the roles autophagy plays in both of these contexts can be studied. The goal of this thesis is to provide a better understanding of the regulatory mechanisms that distinguish between autophagy as a survival mechanism and autophagy as a cell death mechanism. From my studies I was able to determine that microRNAs can regulate autophagy in vivo, and that the microRNA miR-14 controls autophagy specifically during the destruction of the larval salivary glands of Drosophila melanogaster. I found that miR-14 regulates autophagy through modulation of IP3 and calcium signaling, and this miR-14 control of IP3 and calcium signaling does not influence the induction of autophagy during nutrient deprivation. Therefore, this knowledge demonstrates how autophagy can be regulated to distinguish its use during cell survival and death providing insight into how autophagy can used to treat diseases.