The migration of cells and cell extensions is essential throughout development and plays a central role in many human diseases including metastatic cancer. To better understand how cell extensions reach their target destination, we use specialized membrane extensions called muscle arms in C. elegans as a model system. Muscle arms harbor the post-synaptic element of the neuromuscular junction and extend from the body wall muscles to the nearest nerve cord in a stereotypical and regulated manner. Observation suggests that there is a passive embryonic phase of muscle arm development followed by an active phase of muscle arm extension during larval development. We have demonstrated that larval muscle arm extension is dependent on regulators of the actin-cytoskeleton, including UNC-60B/ADF/Cofilin. However, how the larval muscle arms reach the nerve cord remains poorly understood. To identify genes required for larval arm extension, we performed a forward genetic screen to isolate mutants with a muscle arm development-defective (Madd) phenotype. We screened 50,000 haploid genomes and isolated 25 mutants with penetrant and expressive Madd phenotypes, including
unc-60 mutants. We identified C39F7.2 as the gene mutated in one complementation group that we call
madd-2. A genomic
madd-2::GFP translational fusion transgene rescues the Madd phenotype of
madd-2 mutants and reveals
madd-2 expression in the head mesodermal cell, unidentified neurons, seam cells, vulval epithelium, rectal epithelium, vulval muscles, and body wall muscles (BWMs). Preliminary observations suggest that
madd-2::GFP is localized to the dense bodies of the BWMs and the muscle arm termini. Muscle-specific expression of a
madd-2 cDNA rescues the Madd phenotype of
madd-2 mutants, demonstrating a cell-autonomous role for
madd-2 in muscle arm extension.
madd-2 is the only gene in the C. elegans genome that encodes a member of the tripartate motif (TRIM) C-1 subfamily of proteins. MADD-2 shows a high degree of homology to Drosophila trim9, mouse spring and human
mid1. In humans, mutations in
mid1 give rise to a congenital disease called Opitz syndrome that is characterized by errant development of ventral midline structures derived from migratory neural crest cells. Our ongoing investigation into the mechanism by which
madd-2 regulates muscle arm extension will hopefully yield new insight into guided cell migrations and membrane extensions to the midline in both worms and by analogy, humans.