The broad, long-term goals of this project are to elucidate and characterize NR4A nuclear receptor pathways regulating organ morphogenesis in the model organism Caenorhabditis elegans. The NR4A nuclear receptors are a conserved, functionally diverse group of transcription factors that regulate cell proliferation, cell differentiation, and apoptosis in a variety of developmental and physiological contexts. NR4A nuclear receptors are rapidly emerging as important factors in several human diseases including neurodegeneration, vascular disease, and cancer. However, much remains to be learned regarding the cellular mechanisms of NR4A function. In C. elegans, the NR4A ortholog, NHR-6, is required for proper morphogenesis of the spermatheca, a reproductive organ involved in ovulation and oocyte fertilization. Recent studies have demonstrated that NHR-6 is an important regulator of cell proliferation and cell differentiation during formation of this organ. Thus, NHR-6 appears to function as a critical, dualistic transcriptional regulator of organogenesis in C. elegans. NR4A is an atypical nuclear receptor and its activity in a variety of cellular processes is likely mediated through complex interactions with other signaling proteins. Recent genetic interaction studies indicate that NHR-6 functions with an Eph receptor tyrosine kinase signaling pathway during organogenesis of the spermatheca. Given the importance of NR4A in normal development and disease, the primary objective of this study is to uncover cellular mechanisms of NR4A function during cell proliferation and cell differentiation in an organismal and developmental context. This research project tests the hypothesis that NHR-6 functions to promote lineage-specific transcriptional programs regulating cell cycle progression and terminal differentiation. These specific hypotheses will be tested by the following specific aims: Aim 1 will utilize genetic analysis to place NHR-6 in a cell cycle regulatory pathway; Aim 2 will determine if NHR-6 functions in an Eph receptor signaling pathway regulating organ morphogenesis, and Aim 3 will utilize a tissue-specific microarray approach to identify genes that function downstream of NHR-6 and to identify potential NHR-6 signaling pathways. These studies will provide an opportunity to assemble a biologically relevant model of the gene regulatory activities of this important and poorly understood group of orphan nuclear receptor transcription factors.