During animal and human development, the single-cell embryo divides to generate all of the cell types found in the adult. For this process to occur correctly, the genetic information stored in the genome of every cell must be expressed in the right cells and at the right time, and when these events do not occur correctly developmental anomalies and birth defects can occur. While much is known about the spatial control of gene expression in metazoans, less is known about the temporal control of gene expression and how events are coordinated in time during animal development. In the nematode worm C. elegans, a pathway of genes and microRNAs called the ?heterochronic pathway? has been characterized that controls the timing of developmental events during larval life. The terminal component of this pathway is the zinc finger transcription factor LIN-29, which regulates the important developmental transition from late larval life to adulthood (L/A transition) in the epidermis and other worm tissues. Few direct targets of LIN-29 are known: the best characterized is a gene expressed at the L/A transition which encodes a cuticle collagen, the major component of the outer covering of the worm secreted by the epidermal cells. To understand the temporal regulation of developmental transitions LIN-29 regulated genes were identified by temporally mis- expressing LIN-29 at an earlier developmental time and analyzing gene expression. This RNA-Seq analysis identified 232 genes upregulated by LIN-29, many of which encode cuticle collagens (33/232), and/or show a normal peak of expression before the L4/A transition (63/232). This analysis also identified 352 genes down-regulated upon inappropriate early expression of LIN-29. The most significantly overrepresented category of genes on the down-regulated list are a large group involved in fatty acid metabolism that are expressed in the worm intestine, and whose gene products are part of a protein-protein interaction network conserved in humans. In addition, temporal mis-expression of LIN-29 during adulthood leads to a significant shortening of lifespan. These results and others suggest that LIN-29 regulates a signal that moves from the epidermis to the intestine, the reception of which results in the transcriptional down-regulation of a set of fatty acid metabolic genes which shifts the metabolism of the worm from that meeting the needs of the larva (rapid growth) to that meeting the needs of the adult (long life, vitellogenesis). Experiments to test this hypothesis will be performed, and to specifically identify the nature of the signal(s) and the transcription factor(s) mediating this inter-tissue communication. The accomplishment of the work proposed here will have a significant impact on our understanding of temporal regulation of gene expression in a model metazoan, and shed new light on the control of metabolism during developmental transitions. Because homologs of LIN-29 and its putative targets are conserved in humans, the knowledge gained will have relevance to our understanding of normal human development, health and aging.
Birth defects affect 1 in every 33 babies born in the US and are due to normal processes of development unfolding incorrectly. To understand human development better, we are investigating the early development of an animal model system, the roundworm C. elegans, which has powerful genetic and molecular tools available for learning about normal animal development. We will determine the mechanisms by which important metabolic genes are turned on at the appropriate time during development of this organism, and this work will be relevant to our further understanding of the link between metabolism and developmental timing in all animals and in humans.
