Barth D. Grant
Genes That Control Endocytic Traffic and Polarized Secretion in a Multicellular Animal
Endocytosis in C. elegans
Endocytosis is the vesicle-mediated cellular process by which membranes and extracellular macromolecules are internalized and sorted: a vital mechanism by which cells sense and interact with their environment. Endocytic trafficking is essential to many aspects of eukaryotic life, including nutrient uptake, membrane lipid and membrane protein recycling, antigen uptake by the immune system, synaptic vesicle recycling by the nervous system, and growth factor receptor regulation during development. Endocytosis impacts heavily upon the progression of a number of widespread disease processes such as viral infection, toxin entry, prion mediated Creutzfeld-Jacob syndrome, Alzheimer's disease, and atherosclerosis. Much remains to be learned about this important cellular pathway.
In order to study receptor-mediated endocytosis using the advanced genetics available in C. elegans, a simple nematode worm, we created a novel in vivo visual assay for the endocytosis of vitellogenin (yolk protein) by growing oocytes. Yolk uptake is one of the most dramatic examples of endocytosis known. C. elegans vitellogenins are homologous to avian vitellogenins and apoB-100, a component of low-density lipoprotein particles (LDL). Worm vitellogenins are expressed only in the adult hermaphrodite intestine, from which they are secreted into the body cavity (pseudocoelom) and taken up into membrane bound vesicles of the oocyte. We created strains expressing vitellogenin::gfp (YP170::GFP) that allows fluorescent visualization of VIT::GFP secretion and endocytosis in live animals.
We then used C. elegans to isolate a collection of receptor-mediated endocytosis mutants, the rme genes. They represent 12 proteins, including a member of the LDL-receptor family required for lipoprotein uptake by oocytes (RME-2), and three that function in endocytosis by all cells (RME-1, RME-6, and RME-8). We find that mammalian cells express homologs of all of these genes; significantly the yeast genome appears to lack homologs of these genes. Our results indicate that endocytosis proteins exist that are specific to multicellular organisms. We have begun to determine the specific functions of these new endocytosis proteins in C. elegans and mammalian cells.