UC San Diego

Animal primordial germ cells (PGCs) that become eggs and sperm pass their genomes onto the next generation. Little is known about how PGCs are protected from toxic chemicals. In this dissertation, I explore the function and activity of ATP binding cassette (ABC) transporters in PGCs of the sea urchin Strongylocentrotus purpuratus known as small micromeres (Smics). I found that Smics down-regulate the activity of two ABC transporter families that confer multidrug resistance to chemotherapeutic resistant cancers. Nascent Smic plasma membranes are formed in the 16-cell embryo, which have reduced ABCB activity. Other echinoid species, including Lytechinus pictus and Dendraster excentricus also produce Smics with reduced ABCB-mediated efflux activity, showing that this reduction in activity is conserved within euechinoids. ABC transporters may also play a developmental role in addition to a protective role. To understand why PGCs would remove protective transporters from their plasma membranes, I grew embryos in ABC transporter inhibitors. Smics in inhibitor-treated embryos display altered left/right coelomic pouch distributions in the pluteus larva. To confirm that ABC transporters efflux chemoattractants that serve to guide Smics along the left/right axis, I completed a study that is now the first characterization of PGC migration in the sea urchin embryo. In addition to passively riding on the elongating archenteron, the eight Smics are highly motile and move independently on the tip of the archenteron. Throughout gastrulation, the eight Smics produce cortical blebs and filopodia, hallmarks of migratory PGCs in other organisms. When the ABC transporter ABCB1a is overexpressed in the Smics, the production of blebs and filopodia is severely reduced and Smics do not migrate to the two coelomic pouches as fast as control cells or those cells expressing efflux null ABCB1a. Moreover, when ABCB1a is expressed in Smics, they become significantly more left fated. In summary, the reduction of ABC transporter activity, including ABCB1a activity in Smics, is necessary for their directed migration and morphogenesis, presumably through the detection of morphogenetic signals effluxed by ABC transporters from other cells.