Regulation of bHLH-PAS protein subcellular localization during Drosophila embryogenesis

Expression and subcellular localization of the aryl hydrocarbon receptor nuclear translocator (ARNT) protein in mouse and chicken over developmental time

The aryl hydrocarbon receptor nuclear translocator (ARNT) is a basic-helix-loop-helix/Per- ARNT-Sim (bHLH/PAS) transcription factor that is involved in multiple signaling pathways. This study focuses on the tissue distribution and subcellular localization of ARNT during embryological development of the mouse and chicken. Two different species were chosen to determine the consistency of the ARNT staining pattern. Immunohistochemical techniques were used to stain sections of embryos over three developmental time points for each species. Mouse tissues evaluated from embryonic day 10.5, 12.5, and 15, exhibited predominant nuclear staining with little change in expression patterns over time. Chicken tissues evaluated from embryonic day 2, 4, and 10 also showed predominant nuclear staining within all cells and little change in expression over developmental time, as well as, low levels of cytoplasmic ARNT staining in some cells. Importantly, in all tissues, the level of ARNT staining within the nuclear compartment was greater than staining observed in the cytoplasm. Thus, the overall conclusions from these studies are that i) the predominant subcellular localization of ARNT protein is nuclear, and ii) that mouse and chicken appear to maintain ARNT protein expression in many cell types over developmental time. These data support vertebrate ARNT as a nuclear transcription factor and a model in which dimerization partners require nuclear localization for interaction.

Human Dermo-1 has attributes similar to twist in early bone development

Basic helix-loop-helix (bHLH) transcription factors are implicated in cell lineage determination and differentiation. Dermo-1 encodes a bHLH transcription factor that shares extensive homology with another bHLH transcription factor, Twist. We have cloned and characterized human Dermo-1 from two different bone cytoplasmic DNA (cDNA) libraries. Dermo-1 mRNA and protein expression were examined in human embryo and adult tissue sections. Dermo-1 is expressed in a subset of mesodermally and ectodermally derived tissues. We further examined expression of Dermo-1/Twist in human tissues and cell lines. In addition, we observed Dermo-1 expression in response to basic fibroblast growth factor in osteoblastic cell lines. To evaluate the functionality of the human Dermo-1 transcription factor in osteoblast metabolism, we made stable osteoblastic cell lines that over- and underexpress human Dermo-1. These cell lines were analyzed and compared with previously published data of similar cell lines transfected with Twist. Our results demonstrate that Dermo-1 caused changes similar to Twist in the osteogenic properties of osteoblastic cells, such as morphology, bone marker gene expression, and biochemical response to cytokines. However, Dermo-1 expression also has unique effects in regulating the mechanism of proliferation, on alkaline phosphatase enzyme activity, and in temporal expression patterns. We speculate that expression of Twist and Dermo-1 maintains cells in an osteoprogenitor or preosteoblast-like state, respectively, and prevents premature or ectopic osteoblast differentiation. Therefore, Twist and Dermo-1 must be sequentially downregulated in order to initiate the cascade of events responsible for osteogenic cell differentiation. These results indicate that, during osteoblast development, Dermo-1 may inhibit osteoblast maturation and maintain cells in a preosteoblast phenotype by utilizing mechanisms similar but not identical to those utilized by Twist.

The PAS superfamily: sensors of environmental and developmental signals

Over the past decade, PAS domains have been identified in dozens of signal transduction molecules and various forms have been found in animals, plants, and prokaryotes. In this review, we summarize this rapidly expanding research area by providing a detailed description of three signal transduction pathways that utilize PAS protein heterodimers to drive their transcriptional output. It is hoped that these model pathways can provide a framework for use in understanding the biology of the less well-understood members of this emerging superfamily, as well as of those to be characterized in the days to come. We use this review to develop the idea that most eukaryotic PAS proteins can be classified by functional similarities, as well as by predicted phylogenetic relationships. We focus on the alpha-class proteins, which often act as sensors of environmental signals, and the beta-class proteins, which typically act as broad-spectrum partners that target these heterodimers to their genomic targets.

Interaction between the bHLH-PAS protein Trachealess and the POU-domain protein Drifter, specifies tracheal cell fates

bHLH-PAS proteins represent a class of transcription factors involved in diverse biological activities. Previous experiments demonstrated that the PAS domain confers target specificity (Zelzer et al., 1997. Genes Dev. 11, 2079-2089). This suggested an association between the PAS domain and additional DNA-binding proteins, which is essential for the induction of specific target genes. A candidate for interaction with Trh is Drifter/Ventral veinless, a POU-domain protein. A dual requirement for Trh and Drifter was identified for the autoregulation of Trh and Drifter expression. Furthermore, ectopic expression of both Trh and Dfr (but not each one alone) triggered trh autoregulation in several embryonic tissues. A direct interaction between Drifter and Trh proteins, mediated by the PAS domain of Trh and the POU domain of Drifter, was demonstrated.

Isolation and characterization of the hypoxia-inducible factor 1beta in Drosophila melanogaster

The hypoxia-inducible factor 1 (HIF-1), a heterodimer composed of alpha and beta subunits, plays an important role in the cellular response to O(2) deprivation. In this paper, Drosophila HIF-1beta (dHIF-1beta) homolog is cloned and characterized. Further, Northern analyses showed that dHIF-1alpha and dHIF-1beta expressed their highest level at an embryonic stage. From the pupal stage on, their expression was sharply reduced and maintained at a steady level. Anoxia treatment up-regulated the expression of the both alpha and beta subunits. Over-expression of dHIF-1alpha in transgenic embryos resulted in embryonic lethality, while over-expression of dHIF-1beta significantly prolonged fly recovery time from a 5-min anoxic stupor. The cloning and characterization dHIF-1beta reported in this paper provide a framework for further genetic dissection of the HIF-1 complex in its role in the cellular or tissue response to O(2) deprivation.

Remembrance of things PAS: regulation of development by bHLH-PAS proteins

Strange fits of passion I have knownellipsis (W Wordsworth, 'Strange fits of passion'.) bHLH-PAS proteins are regulators of developmental and physiological events that are well conserved between vertebrates and invertebrates. Recent studies using mouse knockouts of bHLH-PAS genes have provided novel insight into the roles of hypoxia inducible factors in controlling oxygen-regulated development and homeostasis, and the role of Single-minded-1 in regulating development and transcription in the hypothalamus. The Drosophila spineless and vertebrate Aryl hydrocarbon receptor bHLH-PAS orthologs both function in chemosensory processes, but in fundamentally different ways. Spineless controls antennal, limb, and sensory cell development, whereas the Aryl hydrocarbon receptor regulates the response to toxin metabolism. Structural analyses of the PAS domain provide insight into how this interaction domain can act as ligand-binding environmental sensor and signal transducer.

The spineless-aristapedia and tango bHLH-PAS proteins interact to control antennal and tarsal development in Drosophila

The Drosophila spineless (ss) gene encodes a basic-helix-loop-helix-PAS transcription factor that is required for proper specification of distal antennal identity, establishment of the tarsal regions of the legs, and normal bristle growth. ss is the closest known homolog of the mammalian aryl hydrocarbon receptor (Ahr), also known as the dioxin receptor. Dioxin and other aryl hydrocarbons bind to the PAS domain of Ahr, causing Ahr to translocate to the nucleus, where it dimerizes with another bHLH-PAS protein, the aryl hydrocarbon receptor nuclear translocator (Arnt). Ahr:Arnt heterodimers then activate transcription of target genes that encode enzymes involved in metabolizing aryl hydrocarbons. In this report, we present evidence that Ss functions as a heterodimer with the Drosophila ortholog of Arnt, Tango (Tgo). We show that the ss and tgo genes have a close functional relationship: loss-of-function alleles of tgo were recovered as dominant enhancers of a ss mutation, and tgo-mutant somatic clones show antennal, leg, and bristle defects almost identical to those caused by ss(−) mutations. The results of yeast two-hybrid assays indicate that the Ss and Tgo proteins interact directly, presumably by forming heterodimers. Coexpression of Ss and Tgo in Drosophila SL2 cells causes transcriptional activation of reporters containing mammalian Ahr:Arnt response elements, indicating that Ss:Tgo heterodimers are very similar to Ahr:Arnt heterodimers in DNA-binding specificity and transcriptional activation ability. During embryogenesis, Tgo is localized to the nucleus at sites of ss expression. This localization is lost in a ss null mutant, suggesting that Tgo requires heterodimerization for translocation to the nucleus. Ectopic expression of ss causes coincident ectopic nuclear localization of Tgo, independent of cell type or developmental stage. This suggests that the interaction of Ss and Tgo does not require additional signals, unlike the ligand-dependent interaction of Ahr and Arnt. Despite the very different biological roles of Ahr and Arnt in insects and mammals, the molecular mechanisms by which these proteins function appear to be largely conserved.

Multiple roles of ligand in transforming the dioxin receptor to an active basic helix-loop-helix/PAS transcription factor complex with the nuclear protein Arnt

The dioxin receptor is a ligand-activated transcription factor belonging to an emerging class of basic helix-loop-helix/PAS proteins which show interaction with the molecular chaperone hsp90 in their latent states and require heterodimerization with a general cofactor, Arnt, to form active DNA binding complexes. Upon binding of polycyclic aromatic hydrocarbons typified by dioxin, the dioxin receptor translocates from the cytoplasm to the nucleus to allow interaction with Arnt. Here we have bypassed the nuclear translocation step by creating a cell line which expresses a constitutively nuclear dioxin receptor, which we find remains in a latent form, demonstrating that ligand has functional roles beyond initiating nuclear import of the receptor. Treatment of the nuclear receptor with dioxin induces dimerization with Arnt to form an active transcription factor complex, while in stark contrast, treatment with the hsp90 ligand geldanamycin results in rapid degradation of the receptor. Inhibition of degradation by a proteasome inhibitor allowed geldanamycin to transform the nuclear dioxin receptor to a heterodimer with Arnt (DR-Arnt). Our results indicate that unchaperoned dioxin receptor is extremely labile and is consistent with a concerted nuclear mechanism for receptor activation whereby hsp90 is released from the ligand-bound dioxin receptor concomitant with Arnt dimerization. Strikingly, artificial transformation of the receptor by geldanamycin provided a DR-Arnt complex capable of binding DNA but incapable of stimulating transcription. Limited proteolysis of DR-Arnt heterodimers indicated different conformations for dioxin versus geldanamycin-transformed receptors. Our studies of intracellular dioxin receptor transformation indicate that ligand plays multiple mechanistic roles during receptor activation, being important for nuclear translocation, transformation to an Arnt heterodimer, and maintenance of a structural integrity key for transcriptional activation.

Drosophila center divider gene is expressed in CNS midline cells and encodes a developmentally regulated protein kinase orthologous to human TESK1

The Drosophila center divider gene (cdi) was isolated in an enhancer trap screen undertaken to identify genes involved in embryonic central nervous system (CNS) midline cell development. Three independent lines with P-element insertions at 91F were analyzed that all showed prominent beta-galactosidase expression in the CNS midline precursor cells and other cell types. Null mutations were created by imprecise P-element excision and shown to be larval lethal, although no severe CNS defects were observed in mutant embryos. The DNA surrounding the sites of insertion was cloned and found to contain a transcription unit that was dynamically expressed in a pattern corresponding to the enhancer trap line beta-galactosidase expression. Sequencing of cDNA clones revealed that the cdi gene encodes a 1140-amino acid protein that is an ortholog of the mammalian testis-specific TESK1 protein kinase. This serine/threonine kinase is distinct from other protein kinases because of sequence differences in the residues conferring substrate specificity. The unique sequence is conserved in Cdi, suggesting that Cdi/TESK1 represents a novel class of signaling proteins.

stumps, a Drosophila gene required for fibroblast growth factor (FGF)-directed migrations of tracheal and mesodermal cells

Fibroblast growth factors (FGFs) bind to FGF receptors, transmembrane tyrosine kinases that activate mitogenic, motogenic, and differentiative responses in different tissues. While there has been substantial progress in elucidating the Ras-MAP kinase pathway that mediates the differentiative responses, the signal transduction pathways that lead to directed cell migrations are not well defined. Here we describe a Drosophila gene called stumps that is required for FGF-dependent migrations of tracheal and mesodermal cells. These migrations are controlled by different FGF ligands and receptors, and they occur by different cellular mechanisms: the tracheal migrations occur as part of an epithelium whereas the mesodermal migrations are fibroblast-like. In the stumps mutant, tracheal cells fail to move out from the epithelial sacs, and only rudimentary tracheal branches form. Mesodermal cells fail in their dorsal migrations after gastrulation. The stumps mutation does not block all FGF signaling effects in these tissues: both random cell migrations and Ras-MAP kinase-mediated induction of FGF-specific effector genes occurred upon ectopic expression of the ligand or upon expression of a constitutively activated Ras protein in the migrating cells. The results suggest that stumps function promotes FGF-directed cell migrations, either by potentiating the FGF signaling process or by coupling the signal to the cellular machinery required for directed cell movement.

Generation of multiple antagonistic domains along the proximodistal axis during Drosophila leg development

homothorax (hth) is a Drosophila member of the Meis family of homeobox genes. hth function is required for the nuclear localization of the Hox cofactor Extradenticle (EXD). We show here that there is also a post-transcriptional control of HTH by exd: exd activity is required for the apparent stability of the HTH protein. In leg imaginal discs, hth expression is limited to the domain of exd function and this domain is complementary to the domain in which the Wingless (WG) and Decapentaplegic (DPP) signals are active. We demonstrate that WG and DPP act together through their targets Distal-less (Dll) and dachshund (dac) to restrict hth expression, and therefore EXD's nuclear localization, to the most proximal regions of the leg disc. Furthermore, there is a reciprocal repression exerted by HTH on these and other DPP and WG downstream targets that restricts their expression to non-hth-expressing cells. Thus, there exists in the leg disc a set of mutually antagonistic interactions between proximal cells, which we define as those that express hth, and distal cells, or those that do not express hth. In addition, we show that dac negatively regulates Dll. We suggest that these antagonistic relationships help to convert the WG and DPP activity gradients into discreet domains of gene expression along the proximodistal axis.