Cell-Cell Interaction During Drosophila Embryogenesis
67
bryonic lethal mutations. Helped by mutant phenotypes observed in the
absence of these essential DPP receptors, we subsequently identified a
novel nuclear component (SCHNURRI), which is essential in most cells
for the generation of an appropriate nuclear response to DPP (Grieder et
al. 1995). The schnurri gene encodes a long protein of more than 2500
amino acids containing eight zinc fingers; it presumably assists other
factors in the generation of appropriate transcriptional changes induced
by DPP in responding cells. Although we have invested a considerable
effort in the genetic and biochemical analysis of schnurri, we do not
know with certainty whether SCHNURRI binds directly to cis-acting
elements of DPP-responsive genes or plays a more indirect role in DPP
signaling.
At the same time as we and others reported the identification of the
cell surface receptors for DPP, Bill Gelbart's group (Harvard) reported
the identification of two genes, mad and medea, which when mutated,
modify the response of cells to DPP (Raftery et al. 1995; Sekelsky et al.
1995). Subsequently, the biochemical characterization of the vertebrate
homologues of MAD (invertebrate and vertebrate proteins are now
collectively referred to as SMADS) showed that they encode a class of
proteins that function as intracellular signaling effectors for the TGFp
superfamily of secreted polypeptides.
The two essential DPP receptors PUT and TKV, and the signal
transducers MAD and MEDEA appear to constitute the core components of the DPP signaling pathway and are required in all cells to
transmit the DPP signal. How is it possible then that cells respond
differently to the reception of the DPP molecule if they use the same
signal transduction components? The transcriptional responses to DPP
in the developing midgut are extremely diverse and depend on the germ
layer provenance of a given cell as well as on the parasegmental origin
of cells within the visceral mesoderm. For example, visceral mesoderm
cells of parasegment 7 induce the transcription of the homeotic gene
Ubx and the dpp gene itself as a response to DPP signaling, parasegment
8 cells induce wg transcription, whereas parasegment 6 cells induce
neither of these genes. Endodermal cells induce expression of the homeotic gene labial (lab) upon stimulation with DPP.
67
bryonic lethal mutations. Helped by mutant phenotypes observed in the
absence of these essential DPP receptors, we subsequently identified a
novel nuclear component (SCHNURRI), which is essential in most cells
for the generation of an appropriate nuclear response to DPP (Grieder et
al. 1995). The schnurri gene encodes a long protein of more than 2500
amino acids containing eight zinc fingers; it presumably assists other
factors in the generation of appropriate transcriptional changes induced
by DPP in responding cells. Although we have invested a considerable
effort in the genetic and biochemical analysis of schnurri, we do not
know with certainty whether SCHNURRI binds directly to cis-acting
elements of DPP-responsive genes or plays a more indirect role in DPP
signaling.
At the same time as we and others reported the identification of the
cell surface receptors for DPP, Bill Gelbart's group (Harvard) reported
the identification of two genes, mad and medea, which when mutated,
modify the response of cells to DPP (Raftery et al. 1995; Sekelsky et al.
1995). Subsequently, the biochemical characterization of the vertebrate
homologues of MAD (invertebrate and vertebrate proteins are now
collectively referred to as SMADS) showed that they encode a class of
proteins that function as intracellular signaling effectors for the TGFp
superfamily of secreted polypeptides.
The two essential DPP receptors PUT and TKV, and the signal
transducers MAD and MEDEA appear to constitute the core components of the DPP signaling pathway and are required in all cells to
transmit the DPP signal. How is it possible then that cells respond
differently to the reception of the DPP molecule if they use the same
signal transduction components? The transcriptional responses to DPP
in the developing midgut are extremely diverse and depend on the germ
layer provenance of a given cell as well as on the parasegmental origin
of cells within the visceral mesoderm. For example, visceral mesoderm
cells of parasegment 7 induce the transcription of the homeotic gene
Ubx and the dpp gene itself as a response to DPP signaling, parasegment
8 cells induce wg transcription, whereas parasegment 6 cells induce
neither of these genes. Endodermal cells induce expression of the homeotic gene labial (lab) upon stimulation with DPP.