Biomedical subjects
Publications and source records attributed to A Jacinto.
Recent studies show that the underlying amnioserosa works actively to help bring together the two epithelial sheets and close the embryonic hole during dorsal closure in fruitfly development.
One of the principal functions of any epithelium in the embryonic or adult organism is to act as a self-sealing barrier layer. From the earliest stages of development, embryonic epithelia are required to close naturally occurring holes and to fuse wherever two free edges are brought together, and at the simplest level that is precisely what the epidermis must do to repair itself wherever it is damaged. Parallels can be drawn between the artificially triggered epithelial movements of wound repair and the naturally occurring epithelial movements that shape the embryo during morphogenesis. Recent in vitro and in vivo wound-healing studies and analysis of paradigm morphogenetic movements in genetically tractable embryos, like those of Drosophila and Caenorhabditis elegans, have begun to identify both the signals that initiate these movements and the cytoskeletal machinery that drives motility. We are also gaining insight into the nature of the brakes and stop signals, and the mechanisms by which the confronting epithelial sheets knit together to form a seam.
BACKGROUND: The adhesion of two epithelial sheets is a fundamental process that occurs throughout embryogenesis and during wound repair. Sealing of the dorsal epidermis along the midline of the Drosophila embryo provides a genetically tractable model to analyse the closure of such holes. Several studies indicate that the actin cytoskeleton plays a critical role in dorsal closure. Although many components of the signalling cascade directing this process have been identified, the precise cell-biological events upon which these signals act remain poorly described. RESULTS: By confocal imaging of living fly embryos expressing green fluorescent protein (GFP)-tagged actin, we found that dorsal closure relies on the activity of dynamic filopodia and lamellipodia that extend from front-row cells to actively zipper the epithelial sheets together. As these epithelial fronts approach one another, we observed long, thin filopodia, apparently 'sampling' cells on the opposing face. When the assembly of these actin-based protrusions was blocked (by interfering with the activities of Cdc42 and Jun N-terminal kinase signalling), the adhesion and fusion of opposing epithelial cells was prevented and their ability to 'sense' correct partners was also blocked, leading to segment misalignment along the midline seam. CONCLUSIONS: Dynamic, actin-based protrusions (filopodia and lamellae) are critical, both in the mechanics of epithelial adhesion during dorsal closure and in the correct 'matching' of opposing cells along the fusion seam.
Members of the Hedgehog (Hh) family of secreted proteins have been identified recently as key signaling molecules that regulate a variety of inductive interactions central to the development of both Drosophila and vertebrates. Despite their widespread importance, the way in which Hh signals are transduced inside the cell remains poorly understood. The best candidate for a transcription factor that mediates Hh signaling in Drosophila is the product of the cubitus interruptus (ci) gene, a zinc finger protein that exhibits significant homology to protein products of the vertebrate GLI gene family. Here, we show that elevated levels of Ci are sufficient to activate patched (ptc) and other hh target genes, even in the absence of hh activity. We also show that Ci can function as a transcriptional activator in yeast and demonstrate that the zinc finger domain of the protein is sufficient for its target specificity. Finally, we identify sequences in the promoter region of the ptc gene, a primary target of Hh signaling, that are identical to the consensus-binding sequence of the GLI protein and are required for reporter gene expression in response to Hh activity. Taken together, our results strongly support the role for Ci as the transcriptional activator that mediates hh signaling.
BACKGROUND: The Drosophila segment polarity gene hedgehog encodes a member of a family of secreted proteins that are involved in a variety of patterning processes, in both vertebrates and invertebrates. Some of these processes depend upon short-range or contact-dependent interactions, whereas others seem to involve long-range signalling. Two different models have been proposed to account for the execution of these contrasting processes by the same proteins: one postulates that Hedgehog acts exclusively over short distances, its long-range influences being effected through regulation of other signalling factors; the second postulates that different aspects of Hedgehog activity are mediated by distinct forms of the protein that are generated by autoproteolysis. RESULTS: We have investigated these models by mutating the hedgehog coding region such that only the amino-terminal or carboxy-terminal half of the protein is secreted. Deletion of the carboxy-terminal portion has little effect on the signalling activity of the protein, whereas abolishing the secretion of the amino-terminal half leads to a complete loss of signalling. In addition, we find that increases in the level of expression within the normal hedgehog transcriptional domain of either the wild-type protein or the carboxy-terminal-deleted form expand the range of activity to a limited extent, but have only minor effects on cell identity. CONCLUSIONS: In Drosophila, all of the signalling activity of Hedgehog resides in the amino-terminal portion of the protein, the secretion of which is essential for its function. The range of Hedgehog is limited by the close association of the amino-terminal peptide with the cell surface but can be extended by elevating the level of its expression.
Two different ubiquitin::79-amino-acid (aa) extension protein-encoding fusion genes were isolated from a Lupinus albus nuclear DNA library and sequenced. Both genes have 465-nucleotide open reading frames encoding a single ubiquitin (Ub) monomer fused in frame to a 79-aa extension (Ext) protein. The deduced aa sequences of the encoded Ub are identical to the aa sequences of Ub from other plants. The encoded Ext proteins are putative ribosomal proteins, highly basic, differing by 2 aa from each other, and have a high degree of similarity to Ext proteins from other plants.