Cell-Cell Interaction During Drosophila Embryogenesis
69
on the expression of, a specific homeotic gene in a given cell (Bienz
1994, 1996). For example, the DPP-induced transcription of wingless
(wg) in anterior cells of parasegment 8 (which border the DPP-secreting
parasegment 7 cells) requires the activity of the homeotic gene abdA. In
analogy to the lab550 enhancer, it is likely that the DPP-dependent
enhancer of the wg gene requires a direct interaction with ABDA.
Homeotic proteins and signaling are also tightly linked to the development of segment-specific epidermal structures. The development of the
leg imaginal discs is repressed in abdominal segments by the homeotic
genes of the bithorax complex (Vachon et al. 1992). The signals involved in the generation of these disc primordia are also secreted and
transduced in the abdominal segments, but the expression of the homeotic proteins of the bithorax complex changes the way cells interpret
these signaling inputs in the posterior segments. A similar situation
occurs in the developing larva. Although the global coordinate systems
of the wing and the haltere are the same, VBX appears to interpret
signaling input differently in the haltere (Weatherbee et al. 1998; Halder
et al. 1998). Recently, genetic studies in C. elegans showed that the Hox
gene lin-39 is required during vulval induction to select the outcome of
RAS signaling (Maloof and Kenyon 1998). Therefore, the transcriptional targets of signaling pathways might be selectively activated or
repressed with the help of the homeotic protein present in a given cell.
Based on our findings and these general considerations, a major
focus of our future efforts will be centered around the elucidation of the
molecular mechanisms underlying the synergistic activation of the lab
enhancer by DPP signaling and by HOMIEXD, or, put more generally,
the synergy between signaling and HOX input. We will continue to
dissect the enhancer in vivo, and use biochemical methods as well as in
vivo systems to analyze possible molecular interactions between molecules of the DPP signaling pathway and homeotic proteins or their
cofactors. Experiments are underway which should allow us to determine the site(s) on lab550 through which the DPP signaling cascade
acts. In addition, we will try to identify the factor(s) which contribute to
the observed tissue specificity of the lab550 enhancer. We anticipate to
learn how specific nuclear responses to DPP are generated, what factors
are involved (signaling components, homeotic proteins, germ layer specific factors etc.), and how these factors interact on DPP-response elements. Since homeotic genes are region-specifically expressed in many
69
on the expression of, a specific homeotic gene in a given cell (Bienz
1994, 1996). For example, the DPP-induced transcription of wingless
(wg) in anterior cells of parasegment 8 (which border the DPP-secreting
parasegment 7 cells) requires the activity of the homeotic gene abdA. In
analogy to the lab550 enhancer, it is likely that the DPP-dependent
enhancer of the wg gene requires a direct interaction with ABDA.
Homeotic proteins and signaling are also tightly linked to the development of segment-specific epidermal structures. The development of the
leg imaginal discs is repressed in abdominal segments by the homeotic
genes of the bithorax complex (Vachon et al. 1992). The signals involved in the generation of these disc primordia are also secreted and
transduced in the abdominal segments, but the expression of the homeotic proteins of the bithorax complex changes the way cells interpret
these signaling inputs in the posterior segments. A similar situation
occurs in the developing larva. Although the global coordinate systems
of the wing and the haltere are the same, VBX appears to interpret
signaling input differently in the haltere (Weatherbee et al. 1998; Halder
et al. 1998). Recently, genetic studies in C. elegans showed that the Hox
gene lin-39 is required during vulval induction to select the outcome of
RAS signaling (Maloof and Kenyon 1998). Therefore, the transcriptional targets of signaling pathways might be selectively activated or
repressed with the help of the homeotic protein present in a given cell.
Based on our findings and these general considerations, a major
focus of our future efforts will be centered around the elucidation of the
molecular mechanisms underlying the synergistic activation of the lab
enhancer by DPP signaling and by HOMIEXD, or, put more generally,
the synergy between signaling and HOX input. We will continue to
dissect the enhancer in vivo, and use biochemical methods as well as in
vivo systems to analyze possible molecular interactions between molecules of the DPP signaling pathway and homeotic proteins or their
cofactors. Experiments are underway which should allow us to determine the site(s) on lab550 through which the DPP signaling cascade
acts. In addition, we will try to identify the factor(s) which contribute to
the observed tissue specificity of the lab550 enhancer. We anticipate to
learn how specific nuclear responses to DPP are generated, what factors
are involved (signaling components, homeotic proteins, germ layer specific factors etc.), and how these factors interact on DPP-response elements. Since homeotic genes are region-specifically expressed in many