Drosophila Ring Canal Growth Requires Src and Tec Kinases

The identification of ring canal proteins and insight into ring canal biogenesis have been aided by characterization of a class of mutants in which the flow of cytoLynn Cooley Department of Genetics Yale Medical School New Haven, Connecticut 06520-8005 plasm into the oocyte is impaired (Robinson and Cooley, 1996). The ultimate phenotype of these mutants is striking: egg chambers produce drastically undersized oocytes and the females are consequently sterile. Analysis One of the many ways cells can communicate with one another is through intercellular bridges that directly link of these mutants has revealed a pathway for the assembly of the actin cytoskeleton (Figure 2, left). Actin filathe cytoplasms of sister cells (Robinson and Cooley, 1996). The occurrence of intercellular cytoplasmic bridges ments accumulate coincident with HtsRC protein, followed by the addition of Kelch proteins. HtsRC and is best documented in the germline of both males and females in species ranging from fruit flies to humans. Kelch functions are not completely characterized; however, it is likely that they are required to maintain or Germline intercellular bridges facilitate the passage of cytoplasmic components between cells during developmodify the actin cytoskeleton. Before the accumulation of actin, an early indication of ring canal development ment. Intercellular bridges have also been found connecting somatic cells; however, the function of somatic is the appearance of proteins recognized by antibodies to phosphotyrosine (P-Y) by the end of the fourth mitobridges is not known, nor is the prevalence of somatic bridges established. sis. Later, as actin accumulates, P-Y staining of ring canals becomes quite prominent (Figure 1). During egg development in Drosophila, the maternal contribution to each oocyte is supplied largely by 15 A pathway parallel to the recruitment of actin has emerged from the study of the Src and Tec kinases nurse cells that are connected to each other and the oocyte by intercellular bridges called ring canals. The (Src64 and Tec29; Figure 2, right). Two papers published in Molecular Cell (Guarnieri et al., 1998; Roulier et al., nurse cells and oocyte of each egg chamber are sibling germline cells derived from a single precursor through 1998) show that mutations in Src64 and Tec29 dramatically reduce both P-Y staining at ring canals and the four rounds of mitosis. The mitotic cleavage furrows in these divisions mysteriously halt before daughter cells overall size of ring canals. Neither mutant affects the ring canal localization of actin, HtsRC, or Kelch; rather, are completely separated. At the conclusion of mitoses, the arrested cleavage furrows are transformed into ring the ability of the cytoskeleton to expand during ring canal growth appears to be affected. Src64 is present on canals by the addition of an actin cytoskeleton lining the tunnels between cells. Following the generation of the plasma membranes of nurse cells, perhaps including the ringcanals. In contrast, antibodies directed to Tec29 egg chambers within the germarium of fly ovaries, egg chambers undergo several days of growth, finally proprotein stained ring canals, and this localization pattern was abolished in Src64 mutant egg chambers. These ducing eggs almost half a millimeter long. During egg chamber growth, the diameter of ring canals increases data suggest that the key contributor to ring canal P-Y epitopes is Tec29, and that Src64 is required for Tec29 from less than 1 mm to about 10 mm (see Figure 1). The large size of both the germline cells and the ring canals, localization to ring canals. The two stories about Src64 and Tec29 had compleas well as the orderly progression of egg chamber stages within ovaries, permit detailed cytological analymentary beginnings. After a long search for mutations in Src64, M. Simon and his colleagues were surprised sis of ring canals during development.