The Drosophila dysfusion Basic Helix-Loop-Helix (bHLH)–PAS Gene Controls Tracheal Fusion and Levels of the Trachealess bHLH-PAS Protein

Abstract

The development of the mature insect trachea requires a complex series of cellular events, including tracheal cell specification, cell migration, tubule branching, and tubule fusion. Here we describe the identification of the Drosophila melanogaster dysfusion gene, which encodes a novel basic helix-loop-helix (bHLH)-PAS protein conserved between Caenorhabditis elegans, insects, and humans, and controls tracheal fusion events. The Dysfusion protein functions as a heterodimer with the Tango bHLH-PAS protein in vivo to form a putative DNA-binding complex. The dysfusion gene is expressed in a variety of embryonic cell types, including trachealfusion, leading-edge, foregut atrium cells, nervous system, hindgut, and anal pad cells. RNAi experiments indicate that dysfusion is required for dorsal branch, lateral trunk, and ganglionic branch fusion but not for fusion of the dorsal trunk. The escargot gene, which is also expressed in fusion cells and is required for tracheal fusion, precedes dysfusion expression. Analysis of escargot mutants indicates a complex pattern of dysfusion regulation, such that dysfusion expression is dependent on escargot in the dorsal and ganglionic branches but not the dorsal trunk. Early in tracheal development, the Trachealess bHLH-PAS protein is present at uniformly high levels in all tracheal cells, but since the levels of Dysfusion rise in wild-type fusion cells, the levels of Trachealess in fusion cells decline. The downregulation of Trachealess is dependent on dysfusion function. These results suggest the possibility that competitive interactions between basic helix-loop-helix-PAS proteins (Dysfusion, Trachealess, and possibly Similar) may be important for the proper development of the trachea.The insect tracheal system consists of an intricately branched system of tubules that provide oxygen throughout the animal. The formation of the trachea consists of a series of developmental events, and its analysis provides an excellent model system for studying the morphogenesis of other branched structures, such as the vertebrate lung airways, circulatory system, kidney ducts, and excretory epithelia (9, 27, 38). The trachea is derived from an array of segmentally repeated clusters of precursor cells. After the tracheal precursor cells divide and invaginate, they extend branches, and the branches from neighboring segments fuse to form the mature tracheal tree. The fusion process is mediated by a distinct fusion cell residing on each branch (41). Branching and fusion are complex cellular processes and pose a number of developmental questions. How are fusion cells specified during tracheal development? What are the short-range and long-range factors that guide tracheal branches to their fusion partners? What is the nature of the adhesive and contact-guidance interactions that mediate fusion and allow the formation of adherens junctions that seal intercellular junctions? How is the cytoskeleton rearranged to allow the tracheal lumen to extend throughout the branch?The tracheal primordia extend bra...

Regulation of Drosophila Hypoxia-inducible Factor (HIF) Activity in SL2 Cells

“…2d) in nuclear extracts with enhanced concentration in hypoxic samples (lanes 2 and 4) and, for overexpressed Tango protein, a more or less constitutively stable ϳ83-kDa band that also predominates in nuclear extracts (lanes 1-4). In vivo, Tango protein specifically translocates from the cytosol to the nucleus only in the presence of an ␣-like partner protein such as Sim or Trh (54,58,59). To find it within the nuclei of normoxic SL2 cells (lanes 1 and 3) suggests the presence of a Tango partner protein other than endogenous Sima, Sim, or Trh (see above).…”

“…Therefore, Tango can form functional heterodimers with five partners: Sima, Sim, Trh, Ss, and Dys. This growing list of Tango heterodimers with close or even overlapping DNA specificities (40, 58, 60 -62) begs the question as to how, in the face of potential competition for Tango (36,54,59) and/or binding site interference (59) by coexpressed Tango partners, the signaling pathways mediated by each ␣-like bHLH/PAS protein are maintained and controlled.…”

“…These include the neurogenic factor Sim (Single-minded) (55) and, most notably, Trh (Trachealess), the master regulator that drives early tracheal development (56,57). Ss (Spineless), the Drosophila aryl hydrocarbon receptor homolog, and Dys (Dysfusion), another tracheogenetic bHLH/PAS factor, are two additional partner proteins for Tango (58,59). Therefore, Tango can form functional heterodimers with five partners: Sima, Sim, Trh, Ss, and Dys.…”

Tube fusion: Making connections in branched tubular networks

“…An indirect Esg target gene is the bHLH-PAS transcription factor Dysfusion (Dys; [40,41]). Dys is required for FC-specific gene expression and for downregulation of trh (and consequently btl) in FCs [40][41][42]. Although misexpression of Dys is sufficient to induce ectopic branch fusions, these ectopic connections do not form lumina, suggesting that Dys target genes mediate early steps of the fusion program, but are not sufficient for lumen formation [41].…”

“…Esg is required for upregulating E-Cadherin (E-Cad) expression and for repressing TC genes in FCs [21,39]. An indirect Esg target gene is the bHLH-PAS transcription factor Dysfusion (Dys; [40,41]). Dys is required for FC-specific gene expression and for downregulation of trh (and consequently btl) in FCs [40][41][42].…”