What's new in edition 62 August 2011 Gene sites new with this edition

The Interactive Fly was first released July/August 1996, with updates provided at approximately one month intervals, through September 1997 (edition 13). Updating quarterly started with edition 14. With edition 40, the Interactive Fly began to schedule updates three times a year: fall, winter and spring.

Gene sites new with this edition of the Interactive Fly:

Caskin

The multiprotein complexes that receive and transmit axon pathfinding cues during development are essential to circuit generation. Identified and characterized the Drosophila sterile α-motif (SAM) domain-containing protein Caskin, which shares homology with vertebrate Caskin, a CASK [calcium/calmodulin-(CaM)-activated serine-threonine kinase]-interacting protein. Drosophila caskin (ckn) is necessary for embryonic motor axon pathfinding and interacts genetically and physically with the leukocyte common antigen-related (Lar) receptor protein tyrosine phosphatase. In vivo and in vitro analyses of a panel of ckn loss-of-function alleles indicate that the N-terminal SAM domain of Ckn mediates its interaction with Lar. Like Caskin, Liprin-α is a neuronal adaptor protein that interacts with Lar via a SAM domain-mediated interaction. Evidence is presented that Lar does not bind Caskin and Liprin-α concurrently, suggesting they may assemble functionally distinct signaling complexes on Lar. Furthermore, a vertebrate Caskin homolog interacts with LAR family members, arguing that the role of ckn in Lar signal transduction is evolutionarily conserved. Last, several ckn mutants were characterized that retain Lar binding yet display guidance defects, implying the existence of additional Ckn binding partners. Indeed, the SH2/SH3 adaptor protein Dock was identified as a second Caskin-binding protein and it was found that Caskin binds Lar and Dock through distinct domains. Furthermore, whereas ckn has a nonredundant function in Lar-dependent signaling during motor axon targeting, ckn and dock have overlapping roles in axon outgrowth in the CNS. Together, these studies identify caskin as a neuronal adaptor protein required for axon growth and guidance (Weng, 2011).

Degringolade

Transcriptional cofactors are essential for proper embryonic development. One such cofactor in Drosophila, Degringolade (Dgrn), encodes a RING finger/E3 ubiquitin ligase. Dgrn and its mammalian ortholog RNF4 are SUMO-targeted ubiquitin ligases (STUbLs; see Model for SUMO-directed ubiquitination by the conserved STUbL family). STUbLs bind to SUMOylated proteins via their SUMO interaction motif (SIM) domains and facilitate substrate ubiquitylation. This study shows that Dgrn is a negative regulator of the repressor Hairy and its corepressor Groucho [Gro/transducin-like enhancer (TLE)] during embryonic segmentation and neurogenesis, as dgrn heterozygosity suppresses Hairy mutant phenotypes and embryonic lethality. Mechanistically Dgrn functions as a molecular selector: it targets Hairy for SUMO-independent ubiquitylation that inhibits the recruitment of its corepressor Gro, without affecting the recruitment of its other cofactors or the stability of Hairy. Concomitantly, Dgrn specifically targets SUMOylated Gro for sequestration and antagonizes Gro functions in vivo. These findings suggest that by targeting SUMOylated Gro, Dgrn serves as a molecular switch that regulates cofactor recruitment and function during development. As Gro/TLE proteins are conserved universal corepressors, this may be a general paradigm used to regulate the Gro/TLE corepressors in other developmental processes (Abed, 2011).

Dendritic arbor reduction 1

Dendrites and axons are two major neuronal compartments with differences that are critical for neuronal functions. To learn about the differential regulation of dendritic and axonal development, a genetic screen was conducted in Drosophila and the dendritic arbor reduction 1 (dar1) mutants were isolated that display defects in dendritic but not axonal growth. The dar1 gene encodes a novel transcription regulator in the Kruppel-like factor family. Neurons lacking dar1 function have severely reduced growth of microtubule- but not F-actin-based dendritic branches. In contrast, overexpression of Dar1 dramatically increased the growth of microtubule-based dendritic branches. These results suggest that Dar1 promotes dendrite growth in part by suppressing the expression of the microtubule-severing protein Spastin. This study thus uncovers a novel transcriptional program for microtubule regulation that preferentially controls dendrite growth (Ye, 2011).

Excitatory amino acid transporter 1

In the mammalian CNS, glial cells expressing excitatory amino acid transporters (EAATs) tightly regulate extracellular glutamate levels to control neurotransmission and protect neurons from excitotoxic damage. Dysregulated EAAT expression is associated with several CNS pathologies in humans, yet mechanisms of EAAT regulation and the importance of glutamate transport for CNS development and function in vivo remain incompletely understood. Drosophila is an advanced genetic model with only a single high-affinity glutamate transporter termed Eaat1. Eaat1 expression in CNS glia was found to be regulated by the glycosyltransferase Fringe, which promotes neuron-to-glia signaling through the Delta-Notch ligand-receptor pair during embryogenesis. Eaat1 loss-of-function mutations were made and it was found that homozygous larvae could not perform the rhythmic peristaltic contractions required for crawling. No evidence was found for excitotoxic cell death or overt defects in the development of neurons and glia, and the crawling defect could be induced by postembryonic inactivation of Eaat1. Eaat1 fully rescued locomotor activity when expressed in only a limited subpopulation of glial cells situated near potential glutamatergic synapses within the CNS neuropil. Eaat1 mutants had deficits in the frequency, amplitude, and kinetics of synaptic currents in motor neurons whose rhythmic patterns of activity may be regulated by glutamatergic neurotransmission among premotor interneurons; similar results were seen with pharmacological manipulations of glutamate transport. These findings indicate that Eaat1 expression is promoted by Fringe-mediated neuron-glial communication during development and suggest that Eaat1 plays an essential role in regulating CNS neural circuits that control locomotion in Drosophila (Stacey, 2010).

Katanin-60

Regulation of microtubule dynamics at the cell cortex is important for cell motility, morphogenesis and division. This study shows that the Drosophila katanin Dm-Kat60 functions to generate a dynamic cortical-microtubule interface in interphase cells. Dm-Kat60 concentrates at the cell cortex of S2 Drosophila cells during interphase, where it suppresses the polymerization of microtubule plus-ends, thereby preventing the formation of aberrantly dense cortical arrays. Dm-Kat60 also localizes at the leading edge of migratory D17 Drosophila cells and negatively regulates multiple parameters of their motility. Finally, in vitro, Dm-Kat60 severs and depolymerizes microtubules from their ends. On the basis of these data, it is proposed that Dm-Kat60 removes tubulin from microtubule lattice or microtubule ends that contact specific cortical sites to prevent stable and/or lateral attachments. The asymmetric distribution of such an activity could help generate regional variations in microtubule behaviours involved in cell migration (Zhang, 2011).

Kurtz

The non-visual β-arrestins are cytosolic proteins highly conserved across species that participate in a variety of signalling events, including plasma membrane receptor degradation, recycling, and signalling, and that can also act as scaffolding for kinases such as MAPK and Akt/PI3K. In Drosophila, there is only a single non-visual β-arrestin, encoded by kurtz, whose function is essential for neuronal activity. This study addressed the participation of Kurtz in signalling during the development of the imaginal discs, epithelial tissues requiring the activity of the Hedgehog, Wingless, EGFR, Notch, Insulin, and TGFβ pathways. Surprisingly, it was found that the complete elimination of kurtz by genetic techniques has no major consequences in imaginal cells. In contrast, the over-expression of Kurtz in the wing disc causes a phenotype identical to the loss of Hedgehog signalling and prevents the expression of Hedgehog targets in the corresponding wing discs. The mechanism by which Kurtz antagonises Hedgehog signalling is to promote Smoothened internalization and degradation in a clathrin- and proteosomal-dependent manner. Intriguingly, the effects of Kurtz on Smoothened are independent of Gprk2 activity and of the activation state of the receptor. These results suggest fundamental differences in the molecular mechanisms regulating receptor turnover and signalling in vertebrates and invertebrates, and they could provide important insights into divergent evolution of Hedgehog signalling in these organisms (Molnar, 2011).

Neural Lazarillo

Metabolic homeostasis in metazoans is regulated by endocrine control of insulin/IGF signaling (IIS) activity. Stress and inflammatory signaling pathways -- such as Jun-N-terminal Kinase (JNK) signaling -- repress IIS, curtailing anabolic processes to promote stress tolerance and extend lifespan. While this interaction constitutes an adaptive response that allows managing energy resources under stress conditions, excessive JNK activity in adipose tissue of vertebrates has been found to cause insulin resistance, promoting type II diabetes. Thus, the interaction between JNK and IIS has to be tightly regulated to ensure proper metabolic adaptation to environmental challenges. This study identified a new regulatory mechanism by which JNK influences metabolism systemically. JNK signaling is required for metabolic homeostasis in flies and that this function is mediated by the Drosophila Lipocalin family member Neural Lazarillo (NLaz), a homologue of vertebrate Apolipoprotein D (ApoD) and Retinol Binding Protein 4 (RBP4). Lipocalins are emerging as central regulators of peripheral insulin sensitivity and have been implicated in metabolic diseases. NLaz is transcriptionally regulated by JNK signaling and is required for JNK-mediated stress and starvation tolerance. Loss of NLaz function reduces stress resistance and lifespan, while its over-expression represses growth, promotes stress tolerance and extends lifespan -- phenotypes that are consistent with reduced IIS activity. Accordingly, this study found that NLaz represses IIS activity in larvae and adult flies. The results show that JNK-NLaz signaling antagonizes IIS and is critical for metabolic adaptation of the organism to environmental challenges. The JNK pathway and Lipocalins are structurally and functionally conserved, suggesting that similar interactions represent an evolutionarily conserved system for the control of metabolic homeostasis (Hull-Thompson, 2009).

Neither inactivation nor afterpotential E

Many animals, including the fruit fly, are sensitive to small differences in ambient temperature. The ability of Drosophila larvae to choose their ideal temperature (18°C) over other comfortable temperatures (19° to 24°C) depends on a thermosensory signaling pathway that includes a heterotrimeric guanine nucleotide-binding protein (G protein), a phospholipase C, and the transient receptor potential TRPA1 channel. Mutation of the gene (ninaE) encoding a classical G protein-coupled receptor (GPCR), Drosophila rhodopsin, eliminates thermotactic discrimination in the comfortable temperature range. This role for rhodopsin in thermotaxis toward 18°C was light-independent. Introduction of mouse melanopsin restored normal thermotactic behavior in ninaE mutant larvae. It is proposed that rhodopsins represent a class of evolutionarily conserved GPCRs that are required for initiating thermosensory signaling cascades (Shen, 2011).

No mechanoreceptor potential C

The generation of coordinated body movements relies on sensory feedback from mechanosensitive proprioceptors. The proper function of NompC, a putative mechanosensitive TRP channel, is not only required for fly locomotion, but also crucial for larval crawling. Calcium imaging revealed that NompC is required for the activation of two subtypes of sensory neurons during peristaltic muscle contractions. Having isolated a full-length nompC cDNA with a protein coding sequence larger than previously predicted, nompC function was demonstrated by rescuing locomotion defects in nompC mutants. Antibodies against the novel C-terminus recognize NompC in chordotonal ciliary tips. Moreover, it was shown that the ankyrin repeats in NompC are required for proper localization and function of NompC in vivo and are required for association of NompC with microtubules. Taken together, these findings suggest that NompC mediates proprioception in locomotion and support its role as a mechanosensitive channel (Cheng, 2010).

Rab23

The planar coordination of cellular polarization is an important, yet not well-understood aspect of animal development. In a screen for genes regulating planar cell polarization in Drosophila, Rab23, encoding a putative vesicular trafficking protein, was identified. Mutations in the Drosophila Rab23 ortholog result in abnormal trichome orientation and the formation of multiple hairs on the wing, leg, and abdomen. Rab23 is required for hexagonal packing of the wing cells. Rab23 is able to associate with the proximally accumulated Prickle protein, although Rab23 itself does not seem to display a polarized subcellular distribution in wing cells, and it appears to play a relatively subtle role in cortical polarization of the polarity proteins. The absence of Rab23 leads to increased actin accumulation in the subapical region of the pupal wing cells that fail to restrict prehair initiation to a single site. Rab23 acts as a dominant enhancer of the weak multiple hair phenotype exhibited by the core polarity mutations, whereas the Rab23 homozygous mutant phenotype is sensitive to the gene dose of the planar polarity effector genes. Together, these data suggest that Rab23 contributes to the mechanism that inhibits hair formation at positions outside of the distal vertex by activating the planar polarity effector system (Pataki, 2010).

Tarsal-less

A substantial proportion of eukaryotic transcripts are considered to be noncoding RNAs because they contain only short open reading frames (sORFs). Recent findings suggest, however, that some sORFs encode small bioactive peptides. This study shows that peptides of 11 to 32 amino acids encoded by the polished rice (pri) sORF gene (tarsal-less) control epidermal differentiation in Drosophila by modifying the transcription factor Shavenbaby (Svb). Pri peptides trigger the amino-terminal truncation of the Svb protein, which converts Svb from a repressor to an activator. These results demonstrate that during Drosophila embryogenesis, Pri sORF peptides provide a strict temporal control to the transcriptional program of epidermal morphogenesis (Kondo, 2010).

Wispy & Gld2

Cytoplasmic polyadenylation has an essential role in activating maternal mRNA translation during early development. In vertebrates, the reaction requires CPEB, an RNA-binding protein and the poly(A) polymerase GLD-2. GLD-2-type poly(A) polymerases form a family clearly distinguishable from canonical poly(A) polymerases (PAPs). In Drosophila, canonical PAP (see Hiiragi) is involved in cytoplasmic polyadenylation with Orb, the Drosophila CPEB, during mid-oogenesis. This study shows that the female germline GLD-2 is encoded by wispy. Wispy acts as a poly(A) polymerase in a tethering assay and in vivo for cytoplasmic polyadenylation of specific mRNA targets during late oogenesis and early embryogenesis. wispy function is required at the final stage of oogenesis for metaphase of meiosis I arrest and for progression beyond this stage. By contrast, canonical PAP acts with Orb for the earliest steps of oogenesis. Both Wispy and PAP interact with Orb genetically and physically in an ovarian complex. It is concluded that two distinct poly(A) polymerases have a role in cytoplasmic polyadenylation in the female germline, each of them being specifically required for different steps of oogenesis (Benoit, 2008).

The DNA of a developing sperm is normally inaccessible for transcription for part of spermatogenesis in many animals. In Drosophila melanogaster, many transcripts needed for late spermatid differentiation are synthesized in pre-meiotic spermatocytes, but are not translated until later stages. Thus, post-transcriptional control mechanisms are required to decouple transcription and translation during spermatogenesis. In the female germline, developing germ cells accomplish similar decoupling through poly(A) tail alterations to ensure that dormant transcripts are not prematurely translated: a transcript with a short poly(A) tail will remain untranslated, whereas elongating the poly(A) tail permits protein production. In Drosophila, the ovary-expressed cytoplasmic poly(A) polymerase WISPY is responsible for stage-specific poly(A) tail extension in the female germline. This study examined the possibility that a recently derived testis-expressed WISPY paralog, GLD2, plays a similar role in the Drosophila male germline. It was shown that knockdown of Gld2 transcripts causes male sterility, as GLD2-deficient males do not produce mature sperm. Spermatogenesis up to and including meiosis appears normal in the absence of GLD2, but post-meiotic spermatid development rapidly becomes abnormal. Nuclear bundling and F-actin assembly are defective in GLD2 knockdown testes and nuclei fail to undergo chromatin reorganization in elongated spermatids. GLD2 also affects the incorporation of protamines and the stability of dynamin and transition protein transcripts. The results indicate that GLD2 is an important regulator of late spermatogenesis and is the first example of a Gld-2 family member that plays a significant role specifically in male gametogenesis (Sartain, 2011).

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