Patterning of Drosophila leg sensory organs through combinatorial signaling by hedgehog, decapentaplegic and wingless.

During development, global patterning events initiate signal transduction cascades which gradually establish an array of individual cell fates. Many of the genes which pattern Drosophila are expressed throughout development and specify diverse cell types by creating unique local environments which establish the expression of locally acting genes. This process is exemplified by the patterning of leg microchaete rows. hairy (h) is expressed in a spatially restricted manner in the leg imaginal disc and functions to position adult leg bristle rows by negatively regulating the proneural gene achaete, which specifies sensory cell fates. While much is known about the events that partition the leg imaginal disc and about sensory cell differentiation, the mechanisms that refine early patterning events to the level of individual cell fate specification are not well understood. We have investigated the regulation of h expression along the dorsal/ventral (D/V) axis of the leg adjacent to the anterior/posterior (A/P) compartment boundary and have found that it requires input from both D/V and A/P patterning mechanisms. Expression of the D/V axis h stripe (D/V-h) is controlled by dorsal- and ventral-specific enhancer elements which are targets of Decapentaplegic (Dpp) and Wingless (Wg) signaling, respectively, but which are also dependent on Hedgehog (Hh) signaling for activation. D/V-h expression is lost in smoothened mutant clones and is specifically activated by exogenously supplied Cubitus interruptus (Ci). D/V-h expression is also lost in clones deficient for Dpp and Wg signaling, but ectopic activation of D/V-h by Dpp and Wg is limited to the A/P compartment boundary where endogenous levels of full-length Ci are high. We propose that D/V-h expression is regulated in a non-linear pathway in which Ci plays a dual role. In addition to serving as an upstream activator of Dpp and Wg, Ci acts combinatorially with them to activate D/V-h expression.

Drosophila cubitus interruptus forms a negative feedback loop with patched and regulates expression of Hedgehog target genes.

The Drosophila segment polarity gene cubitus interruptus (ci) encodes a zinc finger protein that is required for the proper patterning of segments and imaginal discs. Epistasis analysis indicates that ci functions in the Hedgehog (Hh) signal transduction pathway and is required to maintain wingless expression in the embryo. In this paper, the role of the Ci protein in the Hh signaling pathway is examined in more detail. Our results show that ectopic expression of ci in imaginal discs and the embryo activates the expression of Hh target genes. One of these target genes, patched, forms a negative feedback loop with ci that is regulated by Hh signal transduction. Activation is also achieved using the Ci zinc finger domain fused to a heterologous transactivation domain. Conversely, repression of Hh target genes occurs in animals expressing the Ci zinc finger domain fused to a repression domain. To examine Ci function in more detail, regions of the Ci protein that are responsible for its ability to transactivate and its subcellular distribution have been identified.

The spatial organization of epidermal structures: hairy establishes the geometrical pattern of Drosophila leg bristles by delimiting the domains of achaete expression.

The spatial organization of Drosophila melanogaster epidermal structures in embryos and adults constitutes a classic model system for understanding how the two dimensional arrangement of particular cell types is generated. For example, the legs of the Drosophila melanogaster adult are covered with bristles, which in most segments are arranged in longitudinal rows. Here we elucidate the key roles of two regulatory genes, hairy and achaete, in setting up this periodic bristle pattern. We show that achaete is expressed during pupal leg development in a dynamic pattern which changes, by approximately 6 hours after puparium formation, into narrow longitudinal stripes of 3-4 cells in width, each of which represents a field of cells (proneural field) from which bristle precursor cells are selected. This pattern of gene expression foreshadows the adult bristle pattern and is established in part through the function of the hairy gene, which also functions in patterning other adult sense organs. In pupal legs, hairy is expressed in four longitudinal stripes, located between every other pair of achaete stripes. We show that in the absence of hairy function achaete expression expands into the interstripe regions that normally express hairy, fusing the two achaete stripes and resulting in extra-wide stripes of achaete expression. This misexpression of achaete, in turn, alters the fields of potential bristle precursor cells which leads to the misalignment of bristle rows in the adult. This function of hairy in patterning achaete expression is distinct from that in the wing in which hairy suppresses late expression of achaete but has no effect on the initial patterning of achaete expression. Thus, the leg bristle pattern is apparently regulated at two levels: a global regulation of the hairy and achaete expression patterns which partitions the leg epidermis into striped zones (this study) and a local regulation (inferred from other studies on the selection of neural precursor cells) that involves refinement steps which may control the alignment and spacing of bristle cells within these zones.

Cloning and characterization of the segment polarity gene cubitus interruptus Dominant of Drosophila.

The segment polarity mutation, cubitus interruptus Dominant (ciD), of Drosophila melanogaster causes defects in the posterior half of every embryonic segment. We cloned sequences from the ciD region on the proximal fourth chromosome by "tagging" the gene with the transposable element P. Genetic and molecular evidence indicates that the P-element insertions, which all occurred within the same restriction fragment, are in 5'-regulatory regions of the ciD gene within 3 kb of the first exon of its transcript. The putative ciD transcript was identified on the basis of its absence in homozygous ciD embryos. Its spatial pattern of expression during development is unusual in that, unlike most other segmentation genes, it exhibits uniform expression throughout cellular blastoderm and gastrulation and does not resolve into a periodic pattern until the end of the fast phase of germ-band elongation when it is present in 15 broad segmentally repeating stripes along the anterior-posterior axis of the embryo. Registration of the ciD stripes of expression relative to the stripes of other segment polarity genes shows that ciD is expressed in the anterior three-quarters of every segment. This registration does not correlate with the pattern defects observed in ciD mutants. Sequence analysis indicates that the protein encoded by the ciD transcript contains a domain of five tandem amino acid repeats that have sequence similarity to the zinc-finger repeats of the Xenopus transcription factor TFIIIA and that share the highest degree of identity with the human zinc-finger protein GLI, which has been found to be amplified in several human glioblastomas.