Axon Guidance and Morphogenesis of the C. elegans Pharynx

Sammanfattning: The C. elegans pharynx is a single cell thick muscular tube that undergoes morphogenesis to attain it mature structure from a ball of 80 undifferentiated cells. This thesis is about understanding the mechanisms and the genetic pathways underlying the process of pharyngeogenesis. The mature pharynx consists of five different classes of cells (muscle, neuron, marginal, gland and epithelial cells) and is divided into four distinct regions: the anterior procorpus, the metacorpus, the isthmus and the terminal bulb that lies proximal to the pharyngeal-intestinal valve. The pha-2 and mnm-2 genes were cloned and characterized in the course of this thesis because of their role during the development of the pharynx. pha-2 encodes a homeodomain protein that shows homology to the vertebrate gene hex, and is essential for the proper differentiation and morphogenesis of the pm5 muscle cells that form the pharyngeal isthmus. During isthmus development, the pm5 cells elongate anteriorly while their nuclei remain sequestered to the posterior end resulting in a narrow, thin and nuclear free isthmus. In pha-2 mutants, the pm5 muscle cells elongate in both directions and have nuclei anchoring defect that results in thick, short and nucleated isthmuses. Additionally, it regulates other pharyngeal genes such as ceh-22 within the pm5 cells and is regulated by pha-4, a forkhead gene that functions as a pharyngeal identity gene. mnm-2 encodes a transcription factor that belongs to the Krüppel like factor (KLF) family of proteins. mnm-2 regulates the differentiation of two pharyngeal sister neurons, M2 and M3, and is necessary for the ability of M3 to guide the growth cone of M2. In addition to its role in M2 guidance function, mnm-2 is essential for proper the differentiation and function of the M3 neurons. To better understand how pharyngeal neurons establish their trajectories with in pharyngeal muscle folds, we studied the developmental genetics of the pharyngeal neuron NSM. We found out that similarly to the M2 neuron, NSM also uses both growth-cone dependent and growth-cone independent mechanism for its proper guidance. The different NSM processes require different genes and rely on different mechanisms to establish their trajectories. We have also studied a novel visible phenotype in C. elegans, namely the twisted pharynx. Our observation suggest that the regulation of the length of cytoskeletal filaments within pharyngeal muscle cells and/or remodeling of anchor points between this cytoskeleton and the extra-cellular matrix (ECM) during pharyngeal growth is the source of the twisting force. The novel mechanism of morphogenesis and axon guidance uncovered in this study help establish the C. elegans pharynx as a powerful model to study the development genetic of an organ.