Spatial and temporal regulation of the homeotic selector gene Antennapedia is required for the establishment of leg identity in Drosophila

Antennapedia is one of the homeotic selector genes required for specification of segment identity in Drosophila. Dominant mutations that ectopically express Antennapedia cause transformation of antenna to leg. Loss-of-function mutations cause partial transformation of leg to antenna. Here we examine the role of Antennapedia in the establishment of leg identity in light of recent advances in our understanding of antennal development. In Antennapedia mutant clones in the leg disc, Homothorax and Distal-less are coexpressed and act via spineless to transform proximal femur to antenna. Antennapedia is negatively regulated during leg development by Distal-less, spineless, and dachshund and this reduced Antennapedia expression is needed for the proper development of distal leg elements. These findings suggest that the temporal and spatial regulation of the homeotic selector gene Antennapedia in the leg disc is necessary for normal leg development in Drosophila.     

The pleiohomeotic gene is required for maintaining expression of genes functioning in ventral appendage formation in Drosophila melanogaster

Polycomb group (PcG) proteins are negative regulators that maintain the expression of homeotic genes and affect cell proliferation. Pleiohomeotic (Pho) is a unique PcG member with a DNA-binding zinc finger motif and was proposed to recruit other PcG proteins to form a complex. The pho null mutants exhibited several mutant phenotypes such as the transformation of antennae to mesothoracic legs. We examined the effects of pho on the identification of ventral appendages and proximo-distal axis formation during postembryogenesis. In the antennal disc of the pho mutant, Antennapedia (Antp), which is a selector gene in determining leg identity, was ectopically expressed. The homothorax (hth), dachshund (dac) and Distal-less (Dll) genes involved in proximo-distal axis formation were also abnormally expressed in both the antennal and leg discs of the pho mutant. The engrailed (en) gene, which affects the formation of the anterior-posterior axis, was also misexpressed in the anterior compartment of antennal and leg discs. These mutant phenotypes were enhanced in the mutant background of Posterior sex combs (Psc) and pleiohomeotic-like (phol), which are another PcG genes. These results suggest that pho functions in maintaining expression of genes involved in the formation of ventral appendages and the proximo-distal axis.     

Developmental regulation of glial cell phagocytic function during Drosophila embryogenesis

The proper removal of superfluous neurons through apoptosis and subsequent phagocytosis is essential for normal development of the central nervous system (CNS). During Drosophila embryogenesis, a large number of apoptotic neurons are efficiently engulfed and degraded by phagocytic glia. Here we demonstrate that glial proficiency to phagocytose relies on expression of phagocytic receptors for apoptotic cells, SIMU and DRPR. Moreover, we reveal that the phagocytic ability of embryonic glia is established as part of a developmental program responsible for glial cell fate determination and is not triggered by apoptosis per se. Explicitly, we provide evidence for a critical role of the major regulators of glial identity, gcm and repo, in controlling glial phagocytic function through regulation of SIMU and DRPR specific expression. Taken together, our study uncovers molecular mechanisms essential for establishment of embryonic glia as primary phagocytes during CNS development.     

Engrailed expression and body segmentation in the honeybee Apis mellifera

Summary Honeybee embryos were stained with a monoclonal antibody raised against the Drosophila engrailed protein. The antibody was found to label rows of nuclei in the transverse grooves that form the earliest external sign of metameric germ band organization. These grooves demarcate metameric units about seven cell rows wide, of which about three rows with reduced apical cell surfaces account for the grooves. The en stripes appear in the grooves as soon as these form and grow from one to about four cells in width and thus completely overlap the groove. During the rudimentary germ band retraction, the grooves shift slightly backwards relative to both the en stripes and the trachdeal pits. The spatio-temporal pattern by which the series of grooves and stripes arises is quite striking. Both become visible first in the gnathal and thoracic regions, then in the pregnathal parts of the head and in the abdomen. The stripes arise essentially in an antero-posterior sequence. In addition, the earliest stripes to form display a pattern of alternating intensities whereas the later stripes, those in the abdomen, arise with approximately equal strength. The latter trait was earlier observed in the grasshopper, while the former is known from Drosophila where, however, the strong stripes correspond to the weak stripes in the honeybee.     

Additional sex combs-like 1 belongs to the enhancer of trithorax and polycomb group and genetically interacts with Cbx2 in mice

The Additional sex combs (Asx) gene of Drosophila behaves genetically as an enhancer of trithorax and polycomb (ETP) in displaying bidirectional homeotic phenotypes, suggesting that is required for maintenance of both activation and silencing of Hox genes. There are three murine homologs of Asx called Additional sex combs-like1, 2, and 3. Asxl1 is required for normal adult hematopoiesis; however, its embryonic function is unknown. We used a targeted mouse mutant line Asxl1^tm1Bc to determine if Asxl1 is required to silence and activate Hox genes in mice during axial patterning. The mutant embryos exhibit simultaneous anterior and posterior transformations of the axial skeleton, consistent with a role for Asxl1 in activation and silencing of Hox genes. Transformations of the axial skeleton are enhanced in compound mutant embryos for the polycomb group gene M33/Cbx2. Hoxa4, Hoxa7, and Hoxc8 are derepressed in Asxl1^tm1Bc mutants in the antero-posterior axis, but Hoxc8 expression is reduced in the brain of mutants, consistent with Asxl1 being required both for activation and repression of Hox genes. We discuss the genetic and molecular definition of ETPs, and suggest that the function of Asxl1 depends on its cellular context.     

Barentsz is essential for the posterior localization of oskar mRNA and colocalizes with it to the posterior pole

The localization of Oskar at the posterior pole of the Drosophila oocyte induces the assembly of the pole plasm and therefore defines where the abdomen and germ cells form in the embryo. This localization is achieved by the targeting of oskar mRNA to the posterior and the localized activation of its translation. oskar mRNA seems likely to be actively transported along microtubules, since its localization requires both an intact microtubule cytoskeleton and the plus end-directed motor kinesin I, but nothing is known about how the RNA is coupled to the motor. Here, we describe barentsz, a novel gene required for the localization of oskar mRNA. In contrast to all other mutations that disrupt this process, barentsz-null mutants completely block the posterior localization of oskar mRNA without affecting bicoid and gurken mRNA localization, the organization of the microtubules, or subsequent steps in pole plasm assembly. Surprisingly, most mutant embryos still form an abdomen, indicating that oskar mRNA localization is partially redundant with the translational control. Barentsz protein colocalizes to the posterior with oskar mRNA, and this localization is oskar mRNA dependent. Thus, Barentsz is essential for the posterior localization of oskar mRNA and behaves as a specific component of the oskar RNA transport complex.     

Drosophila Anillin is unequally required during asymmetric cell divisions of the PNS

During Drosophila embryogenesis, timely and orderly asymmetric cell divisions ensure the correct number of each cell type that make up the sensory organs of the larval PNS. We report a role of scraps, Drosophila Anillin, during these divisions. Anillin, a constitutive member of the contractile ring is essential for cytokinesis in Drosophila and vertebrates. During embryogenesis we find that zygotically transcribed scraps is required specifically for the unequal cell divisions, those in which cytokinesis occurs in an “off-centred” manner, of the pIIb and pIIIb neuronal precursor cells, but not the equal cell divisions of the lineage related precursor cells. Complementation and genetic rescue studies demonstrate this effect results from zygotic scraps and leads to polyploidy, ectopic mitosis, and loss of the neuronal precursor daughter cells. The net result of which is the formation of incomplete sense organs and embryonic lethality.     

Function and regulation of Alx4 in limb development: complex genetic interactions with Gli3 and Shh

The role of the aristaless-related homeobox gene Alx4 in antero-posterior (AP-) patterning of the developing vertebrate limb has remained somewhat elusive. Polydactyly of Alx4 mutant mice is known to be accompanied by ectopic anterior expression of genes like Shh, Fgf4 and 5'Hoxd. We reported previously that polydactyly in Alx4 mutant mice requires SHH signaling, but we now show that in early Alx4-/- limb buds the anterior ectopic expression of Fgf4 and Hoxd13, and therefore disruption of AP-patterning, occurs independently of SHH signaling. To better understand how Alx4 functions in the pathways that regulate AP-patterning, we also studied genomic regulatory sequences that are capable of directing expression of a reporter gene in a pattern corresponding to endogenous Alx4 expression in anterior limb bud mesenchyme. We observed, as expected for authentic Alx4 expression, expansion of reporter construct expression in a Shh-/- background. Total lack of reporter expression in a Gli3-/- background confirms the existence of Gli3-dependent and -independent Alx4 expression in the limb bud. Apparently, these two modules of Alx4 expression are linked to dissimilar functions.     

A mis-expression study of factors affecting Drosophila PNS cell identity

Drosophila PNS sense organs arise from single sensory organ precursor (SOP) cells through a series of asymmetric divisions. In a mis-expression screen for factors affecting PNS development, we identified string and dappled as being important for the proper formation of adult external sensory (ES) organs. string is a G2 regulator. dappled has no described function but is implicated in tumorigenesis. The mis-expression effect from string was analysed using timed over expression during adult ES-organ and, for comparison, embryonic Chordotonal (Ch) organ formation. Surprisingly, string mis-expression prior to SOP division gave the greatest effect in both systems. In adult ES-organs, this lead to cell fate transformations producing structural cells, whilst in the embryo organs were lost, hence differences within the lineages exist. Mis-expression of dappled, lead to loss and duplications of entire organs in both systems, potentially affecting SOP specification, in addition to affecting neuronal guidance.     

Mitochondrial involvement in cell death of non-mammalian eukaryotes

Although mitochondria are essential organelles for long-term survival of eukaryotic cells, recent discoveries in biochemistry and genetics have advanced our understanding of the requirements for mitochondria in cell death. Much of what we understand about cell death is based on the identification of conserved cell death genes in Drosophila melanogaster and Caenorhabditis elegans. However, the role of mitochondria in cell death in these models has been much less clear. Considering the active role that mitochondria play in apoptosis in mammalian cells, the mitochondrial contribution to cell death in non-mammalian systems has been an area of active investigation. In this article, we review the current research on this topic in three non-mammalian models, C. elegans, Drosophila, and Saccharomyces cerevisiae. In addition, we discuss how non-mammalian models have provided important insight into the mechanisms of human disease as they relate to the mitochondrial pathway of cell death. The unique perspective derived from each of these model systems provides a more complete understanding of mitochondria in programmed cell death. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.     

klumpfuss regulates cell death in the Drosophila retina

Programmed cell death (PCD) plays a central role in the sculpting and maturation of developing epithelia. In adult tissue, PCD plays a further role in the prevention of malignancy though removal of damaged cells. Here, we report that mutations in klumpfuss result in an excess of support cells during maturation of the developing Drosophila pupal retina. These ectopic cells are the result of a partial and specific failure of apoptotic death during normal cell fate selection. klumpfuss is required and differentially expressed in the cells that choose the life or death cell fate. We also provide genetic and biochemical evidence that klumpfuss regulates this process through down-regulation of the Epidermal Growth Factor Receptor/dRas1 signaling pathway. Based on its sequence Klumpfuss is an EGR-class nuclear factor, and our results suggest a mechanism by which mutations in EGR-class factors such as Wilms' Tumor Suppressor-1 may result in oncogenic events such as pediatric kidney tumors.     

Discrete functions of rictor and raptor in cell growth regulation in Drosophila

The TOR signaling pathway regulates cell growth and metabolism in response to various nutrient signals by forming complexes with either rictor or raptor. To distinguish the physiological roles of the complexes formed by the two different TOR partners, we compared the in vivo functions of rictor and raptor in Drosophila. In rictor-null mutants, Akt-induced tissue hyperplasia was reduced and Akt-Ser-505 phosphorylation was decreased. Furthermore, FOXO-dependent apoptosis, which is inhibited by Akt, was augmented in the rictor-null background, indicating that rictor is essential for the Akt-FOXO signaling module. We found that neither S6K-dependent cell growth nor S6K-Thr-398 phosphorylation was affected in rictor-null mutants. However, the knockdown of another TOR binding partner, raptor, decreased S6K-Thr-398 phosphorylation and inhibited S6K-induced cell overgrowth. Collectively, our findings strongly support that the association of TOR with rictor or raptor plays pivotal roles in TOR-mediated cell apoptosis and growth control by differentially regulating Akt- and S6K-dependent signaling pathways, respectively.