Abstract
Wnts are secreted signaling proteins that are essential for animal development and tissue maintenance. Wnt signaling plays deeply conserved roles in anterior-posterior axis patterning and commonly regulates growth, morphogenesis, and cell type specification during early development. In humans, abnormal Wnt pathway activity can lead to developmental defects and cancers. How Wnt expression is regulated and how Wnt proteins move between cells are fundamental questions. My research has used Caenorhabditis elegans as an experimentally tractable model system to investigate the regulation of Wnt signaling at two levels: control of Wnt ligand gene expression and Wnt protein transport. Due to essential lipid modifications Wnt proteins require specialized transport processes to navigate through the extracellular space to Wnt-receiving cells. Historically, Wnt proteins have been difficult to tag endogenously and visualize in vivo, but recent technological advancements have provided an opportunity to explore Wnt dispersal in living animals. C. elegans provides an excellent model organism for this research as it is amenable to genome engineering to fluorescently tag endogenous proteins, is suitable for in vivo imaging at all life stages, and has roles for several Wnt homologs in embryonic and larval development. My research focused on two posteriorly expressed Wnt homologs, egl-20 and cwn-1. These proteins are important for the development of multiple cell types and organ systems cells including body wall muscles, epidermal seam cells, several types of neurons, vulval precursor cells, and the somatic gonad. First, I investigated the upstream cis-regulatory elements that control egl-20/Wnt expression during early larval development. egl-20 has multiple roles in larval development, but its regulation has not previously been explored. Through this work, I identified a ~270 bp region, which was necessary and sufficient to drive egl-20 expression in specific posterior cells and contained consensus binding motifs for the posterior Hox transcription factor EGL-5. Using transgenic and endogenous reporters for egl-20 expression, I found that egl-5 is required for normal levels of egl-20, but predicted high-affinity EGL-5 binding sites are not essential. Second, I investigated the ligand transport mechanisms for another posteriorly expressed Wnt, CWN-1, which is capable of forming gradients by diffusion. Through this research, I characterized a previously unreported mode of long-distance CWN-1 transport in the axon-like projections of Canal-Associated Neurons (CANs). This mode of transport differed from EGL-20 transport in the DVB neurons, and EGL-20 misexpression experiments showed that the mode of Wnt transport depended on cell type rather than ligand. Together, this work contributes to elucidating the mechanisms of Wnt signaling at multiple levels, including the cis-regulatory control of Wnt expression and cell-type specific mechanisms of ligand transport.
