The role of pyrimidine synthesis and proteoglycans in Caenorhabditis elegans pharyngeal morphogenesis

C. elegans is a model system widely used to study developmental processes in multicellular organisms. The nematode has several organs whose developments are the object of intense scrutiny. One of these organs is the pharynx, which rhythmically pumps to ingest and crush its food. The pharynx undergoes dramatic morphological changes as the worm develops before assuming its final complex, bilobed morphology. Little is known about the genetic or cellular basis controlling pharyngeal morphology.; We have isolated a mutant, pyr-1(cu8), exhibiting defects in pharyngeal isthmus morphology and other pleiotropic defects. pyr-1 encodes C. elegans CAD, which carries out the rate limiting step in de novo pyrimidine synthesis. One use for pyrimidines is to provide UDP-sugar precursors for proteoglycan synthesis. We screened the pharynxes of mutants in proteoglycan synthesis genes and found they exhibit the same isthmus elongation defect observed in pyr-1(cu8) , suggesting that proteoglycans are required for pharyngeal development. Biochemical analysis indicates that pyr-1(cu8) mutants exhibit decreased heparan sulfate levels and genetic analysis showed the function of wild type heparan sulfate polymerase enzyme is affected by the pyr-1(cu8) mutation. Together, these data strongly support the hypothesis that PYR-1 supplies pyrimidines for heparan sulfate synthesis.; By immunohistochemistry, we have localized proteoglycans to the pharyngeal lumen as well as the nerve ring surrounding the pharyngeal isthmus. In addition, heparan sulfate core proteins accumulate in the pharyngeal basement membrane providing several avenues by which pharyngeal morphology might be affected by proteoglycans. We examined the pharynxes of several mutants in heparan sulfate core proteins and found that both UNC-52/perlecan and SDN-1/syndecan mutants exhibit short, thick pharyngeal isthmuses. We have further identified the same pharyngeal cytoskeletal defects in pyr-1 and heparan sulfate polymerase mutants and find that these defects are similar to cytoskeletal abnormalities reported for patients suffering from hereditary multiple exotoses, a condition resulting from mutations in heparan sulfate polymerase genes. Our data support a model whereby heparan sulfate proteoglycans in the nerve ring and basement membrane affect isthmus elongation and pharyngeal actin organization respectively.