Cell-Cell Interaction During Drosophila Embryogenesis
73
5.2.3 Isolation of a Novel Component Required for FGF Signaling
We have isolated three mutants in which tracheal cells are normally
specified but completely fail to migrate. The mutation in one strain
occurred in the btl gene; a second mutant strain carried a deletion which
removed the bnllocus (unpublished observations). The mutation in the
third strain was located genetically with the aid of a deficiency "kit" to
chromosomal region 88 on the third chromosome. In collaboration with
the group of Maria Leptin (Cologne), we have recently identified the
gene corresponding to the mutated locus in this strain and shown that it
encodes a novel protein containing two ankyrin repeats and a coiled-coil
region (Vincent et al. 1998). The gene is transcribed specifically in two
tissues, the trachea and the mesoderm; in both tissues, expression starts
very early, during or just after cell determination. Interestingly, both
tissues rearrange through extensive cell migrations requiring FGF signaling (through the FGF receptors btl in the trachea and heartless (htl)
in the mesoderm; Beiman et al. 1996; Gisselbrecht et al. 1996). Inspection of the mesodermal phenotype in embryos of our mutant showed that
in addition to tracheal cells, mesodermal cells completely failed to
migrate. Using a large number of antibodies as markers, we found that
both in mesodermal and tracheal cells of the mutants, the FGF receptors
htl and btl, respectively, fail to activate the RAS-MAP kinase pathway
(through which the FGF receptor signal is propagated; Reichman-Fried
et al. 1994). Genetic epistasis experiments demonstrated that the novel
gene acts downstream of the activated receptor but upstream of ras.
Based on these findings we named the gene Downstream of FGFR (dof).
Our studies strongly suggest that the novel DOF protein links the activated FGF receptors to the RASIMAPK signaling cascade and possibly
to other cytoplasmic components. A protein with a similar function,
FRS2/SNT, has recently been identified in vertebrate cells (Kouhara et
al. 1997); its genetic requirement for FGF signaling remains to be
demonstrated. The possible function of DOF will be discussed further at
the end of the chapter in the context of all other components which have
been shown to be required for directed migration.
73
5.2.3 Isolation of a Novel Component Required for FGF Signaling
We have isolated three mutants in which tracheal cells are normally
specified but completely fail to migrate. The mutation in one strain
occurred in the btl gene; a second mutant strain carried a deletion which
removed the bnllocus (unpublished observations). The mutation in the
third strain was located genetically with the aid of a deficiency "kit" to
chromosomal region 88 on the third chromosome. In collaboration with
the group of Maria Leptin (Cologne), we have recently identified the
gene corresponding to the mutated locus in this strain and shown that it
encodes a novel protein containing two ankyrin repeats and a coiled-coil
region (Vincent et al. 1998). The gene is transcribed specifically in two
tissues, the trachea and the mesoderm; in both tissues, expression starts
very early, during or just after cell determination. Interestingly, both
tissues rearrange through extensive cell migrations requiring FGF signaling (through the FGF receptors btl in the trachea and heartless (htl)
in the mesoderm; Beiman et al. 1996; Gisselbrecht et al. 1996). Inspection of the mesodermal phenotype in embryos of our mutant showed that
in addition to tracheal cells, mesodermal cells completely failed to
migrate. Using a large number of antibodies as markers, we found that
both in mesodermal and tracheal cells of the mutants, the FGF receptors
htl and btl, respectively, fail to activate the RAS-MAP kinase pathway
(through which the FGF receptor signal is propagated; Reichman-Fried
et al. 1994). Genetic epistasis experiments demonstrated that the novel
gene acts downstream of the activated receptor but upstream of ras.
Based on these findings we named the gene Downstream of FGFR (dof).
Our studies strongly suggest that the novel DOF protein links the activated FGF receptors to the RASIMAPK signaling cascade and possibly
to other cytoplasmic components. A protein with a similar function,
FRS2/SNT, has recently been identified in vertebrate cells (Kouhara et
al. 1997); its genetic requirement for FGF signaling remains to be
demonstrated. The possible function of DOF will be discussed further at
the end of the chapter in the context of all other components which have
been shown to be required for directed migration.