Cullin-5 plays multiple roles in cell fate specification and synapse formation during Drosophila development

Peripheral Neuropathy and Decreased Locomotion of a RAB40B Mutation in Human and Model Animals

Rab40 proteins are an atypical subgroup of Rab GTPases containing a unique suppressor of the cytokine signaling (SOCS) domain that is recruited to assemble the CRL5 E3 ligase complex for proteolytic regulation in various biological processes. A nonsense mutation deleting the C-terminal SOCS box in the RAB40B gene was identified in a family with axonal peripheral neuropathy (Charcot-Marie-Tooth disease type 2), and pathogenicity of the mutation was assessed in model organisms of zebrafish and Drosophila. Compared to control fish, zebrafish larvae transformed by the human mutant hRAB40B-Y83X showed a defective swimming pattern of stalling with restricted localization and slower motility. We were consistently able to observe reduced labeling of synaptic markers along neuromuscular junctions of the transformed larvae. In addition to the neurodevelopmental phenotypes, compared to normal hRAB40B expression, we further examined ectopic expression of hRAB40B-Y83X in Drosophila to show a progressive decline of locomotion ability. Decreased ability of locomotion by ubiquitous expression of the human mutation was reproduced not with GAL4 drivers for neuron-specific expression but only when a pan-glial GAL4 driver was applied. Using the ectopic expression model of Drosophila, we identified a genetic interaction in which Cul5 down regulation exacerbated the defective motor performance, showing a consistent loss of SOCS box of the pathogenic RAB40B. Taken together, we could assess the possible gain-of-function of the human RAB40B mutation by comparing behavioral phenotypes in animal models; our results suggest that the mutant phenotypes may be associated with CRL5-mediated proteolytic regulation.

Socs36E limits STAT signaling via Cullin2 and a SOCS-box independent mechanism in the Drosophila egg chamber

The Suppressor of Cytokine Signaling (SOCS) proteins are critical, highly conserved feedback inhibitors of signal transduction cascades. The family of SOCS proteins is divided into two groups: ancestral and vertebrate-specific SOCS proteins. Vertebrate-specific SOCS proteins have been heavily studied as a result of their strong mutant phenotypes. However, the ancestral clade remains less studied, a potential result of genetic redundancies in mammals. Use of the genetically tractable organism Drosophila melanogaster enables in vivo assessment of signaling components and mechanisms with less concern about the functional redundancy observed in mammals. In this study, we investigated how the SOCS family member Suppressor of Cytokine Signaling at 36E (Socs36E) attenuates Janus Kinase/Signal Transducer and Activator of Transcription (Jak/STAT) activation during specification of motile border cells in Drosophila oogenesis. We found that Socs36E genetically interacts with the Cullin2 (Cul2) scaffolding protein. Like Socs36E, Cul2 is required to limit the number of motile cells in egg chambers. We demonstrated that loss of Cul2 in the follicle cells significantly increased nuclear STAT protein levels, which resulted in additional cells acquiring invasive properties. Further, reduction of Cul2 suppressed border cell migration defects that occur in a Stat92E-sensitized genetic background. Our data incorporated Cul2 into a previously described Jak/STAT-directed genetic regulatory network that is required to generate a discrete boundary between cell fates. We also found that Socs36E is able to attenuate STAT activity in the egg chamber when it does not have a functional SOCS box. Collectively, this work contributes mechanistic insight to a Jak/STAT regulatory genetic circuit, and suggests that Socs36E regulates Jak/STAT signaling via a Cul2-dependent mechanism, as well as by a Cullin-independent manner, in vivo.

Cullin-5 and cullin-2 play a role in the development of neuromuscular junction and the female germ line of Drosophila

Cullins confer substrate specificity to E3-ligases which are multi-protein complexes involved in ubiquitin-mediated protein degradation or modification. There are six cullin genes in Drosophila melanogaster. We have raised an antibody against Cul-5 and demonstrated that it expresses in neuronal and non-neuronal cells throughout development. In the embryonic tracheal system, Cul-5 is enriched at fusion sites together with E-Cadherin and Fasciclin III. Mutations of cul-5 do not affect tracheal development but do show defects in the organization of synaptic boutons at the larval neuromuscular junction where the protein is expressed in a subset of motoneuron terminals. Loss of function of another cullin gene 'cul-2' results in similar defects at the larval neuromuscular junction although cul-2;cul-5 double mutants do not show an enhanced phenotype. Both cul-2 and cul-5 mutants show similar aberrations in the development of female germ line. Our results suggest that both of these cullin proteins participate in similar developmental processes.

The cullin protein family

Cullin proteins are molecular scaffolds that have crucial roles in the post-translational modification of cellular proteins involving ubiquitin. The mammalian cullin protein family comprises eight members (CUL1 to CUL7 and PARC), which are characterized by a cullin homology domain. CUL1 to CUL7 assemble multi-subunit Cullin-RING E3 ubiquitin ligase (CRL) complexes, the largest family of E3 ligases with more than 200 members. Although CUL7 and PARC are present only in chordates, other members of the cullin protein family are found in Drosophila melanogaster, Caenorhabditis elegans, Arabidopsis thaliana and yeast. A cullin protein tethers both a substrate-targeting unit, often through an adaptor protein, and the RING finger component in a CRL. The cullin-organized CRL thus positions a substrate close to the RING-bound E2 ubiquitin-conjugating enzyme, which catalyzes the transfer of ubiquitin to the substrate. In addition, conjugation of cullins with the ubiquitin-like molecule Nedd8 modulates activation of the corresponding CRL complex, probably through conformational regulation of the interactions between cullin's carboxy-terminal tail and CRL's RING subunit. Genetic studies in several model organisms have helped to unravel a multitude of physiological functions associated with cullin proteins and their respective CRLs. CRLs target numerous substrates and thus have an impact on a range of biological processes, including cell growth, development, signal transduction, transcriptional control, genomic integrity and tumor suppression. Moreover, mutations in CUL7 and CUL4B genes have been linked to hereditary human diseases.

Cullin5 reduces retinal cell death induced by glutamate toxicity

PURPOSE

To investigate the expression in the mouse retina and the function in a retinal cell of cullin5 that is known as a protein that is associated with ubiquitilation.

METHODS

The retinal sites of expressing cullin5 were determined by immunohistochemistry with a specific antibody for cullin5. Retinal cells were transfected with HA-tagged cullin5 using an adenovirus system, and retinal cells were exposed to L-glutamate with or without an over-expression of cullin5.

RESULTS

Cullin5 was expressed in the retina of C57BL/6N mice and the retinal ganglion cell layer (RGL) of retina was strongly immunostained. Further, cullin5 was localized in the retinal ganglion cell (RGC). An over-expression of cullin5 reduced the percentage of retinal cell deaths induced by L-glutamate.

CONCLUSIONS

The expression of cullin5 on retinal cells and reduction in the percentage of dead retinal cells induced by L-glutamate suggest that cullin5 has neuroprotective properties in retinal cells.

A modifier screen for Bazooka/PAR-3 interacting genes in the Drosophila embryo epithelium

Background

The development and homeostasis of multicellular organisms depends on sheets of epithelial cells. Bazooka (Baz; PAR-3) localizes to the apical circumference of epithelial cells and is a key hub in the protein interaction network regulating epithelial structure. We sought to identify additional proteins that function with Baz to regulate epithelial structure in the Drosophila embryo.

Methodology/Principal Findings

The baz zygotic mutant cuticle phenotype could be dominantly enhanced by loss of known interaction partners. To identify additional enhancers, we screened molecularly defined chromosome 2 and 3 deficiencies. 37 deficiencies acted as strong dominant enhancers. Using deficiency mapping, bioinformatics, and available single gene mutations, we identified 17 interacting genes encoding known and predicted polarity, cytoskeletal, transmembrane, trafficking and signaling proteins. For each gene, their loss of function enhanced adherens junction defects in zygotic baz mutants during early embryogenesis. To further evaluate involvement in epithelial polarity, we generated GFP fusion proteins for 15 of the genes which had not been found to localize to the apical domain previously. We found that GFP fusion proteins for Drosophila ASAP, Arf79F, CG11210, Septin 5 and Sds22 could be recruited to the apical circumference of epithelial cells. Nine of the other proteins showed various intracellular distributions, and one was not detected.

Conclusions/Significance

Our enhancer screen identified 17 genes that function with Baz to regulate epithelial structure in the Drosophila embryo. Our secondary localization screen indicated that some of the proteins may affect epithelial cell polarity by acting at the apical cell cortex while others may act through intracellular processes. For 13 of the 17 genes, this is the first report of a link to baz or the regulation of epithelial structure.

Reduced cul-5 activity causes aberrant follicular morphogenesis and germ cell loss in Drosophila oogenesis

Drosophila oogenesis is especially well suited for studying stem cell biology, cellular differentiation, and morphogenesis. The small modifier protein ubiquitin regulates many cellular pathways. Ubiquitin is conjugated to target proteins by a diverse class of enzymes called ubiquitin E3 ligases. Here we characterize the requirement of Cul-5, a key component of a subgroup of Cullin-RING-type ubiquitin E3 ligases, in Drosophila oogenesis. We find that reduced cul-5 activity causes the formation of aberrant follicles that are characterized by excess germ cells. We show that germ line cells overproliferate in cul-5 mutant females, causing the formation of abnormally large germ line cysts. Also, the follicular epithelium that normally encapsulates single germ line cysts develops aberrantly in cul-5 mutant, leading to defects in cyst formation. We additionally found that Cul-5 is required for germ cell maintenance, as germ cells are depleted in a substantial fraction of cul-5 mutant ovaries. All of these cul-5 phenotypes are strongly enhanced by reduced activity of gustavus (gus), which encodes a substrate receptor of Cul-5-based ubiquitin E3 ligases. Taken together, our results implicate Cul-5/Gus ubiquitin E3 ligases in ovarian tissue morphogenesis, germ cell proliferation and maintenance of the ovarian germ cell population.

Granulocytic differentiation of HL-60 promyelocytic leukemia cells is associated with increased expression of Cul5

The human HL-60 promyelocytic leukemia cell line has been widely used as a model for studying granulocytic differentiation. All-trans retinoic acid (ATRA) treatment of HL-60 cells promotes granulocytic differentiation and is effective as differentiation therapy for patients with acute promyelocytic leukemia. The identification of genes that are transcriptionally regulated by ATRA has provided insight into granulocytic differentiation and differentiation therapy. The Asb-2 (ankyrin repeat SOCS box 2) gene has previously been identified as a transcriptional target in ATRA-treated HL-60 cells. The ASB-2 protein forms an E3 ubiquitin ligase complex with the proteins, Cul5, regulator of cullin 2 (ROC2), and elongin B and C. The purpose of this study was to determine if there is increased expression of Cul5 during granulocytic differentiation of HL-60 cells. To induce granulocytic differentiation, HL-60 cells were treated for 5 d with ATRA and differentiation was confirmed by examining superoxide anion production, nuclear morphology, and changes in the expression of CD11b, CD13, and CD15. Quantitative real-time RT-PCR was used to measure Cul5 mRNA expression and also the expression of other components of the E3 ubiquitin ligase (ASB-2, ROC2, elongin B and C). Granulocytic differentiation of HL-60 cells was associated with a 1.6-, 1.7-, and 23-fold statistically significant (P <or= 0.05) increase in mRNA expression for Cul5, ROC2, and ASB-2, respectively. No significant change was found in elongin B and C mRNA expression. Using Western blot analysis, the expression of Cul5 protein was increased 6.5-fold with granulocytic differentiation of the HL-60 cells. Increased expression of multiple components of the Cul5-containing E3 ubiquitin ligase complex with ATRA treatment of HL-60 cells indicates that this complex may play an important role in granulocytic differentiation.

Identifying determinants of cullin binding specificity among the three functionally different Drosophila melanogaster Roc proteins via domain swapping

BACKGROUND

Cullin-dependent E3 ubiquitin ligases (CDL) are key regulators of protein destruction that participate in a wide range of cell biological processes. The Roc subunit of CDL contains an evolutionarily conserved RING domain that binds ubiquitin charged E2 and is essential for ubiquitylation. Drosophila melanogaster contains three highly related Roc proteins: Roc1a and Roc2, which are conserved in vertebrates, and Roc1b, which is specific to Drosophila. Our previous genetic data analyzing Roc1a and Roc1b mutants suggested that Roc proteins are functionally distinct, but the molecular basis for this distinction is not known.

METHODOLOGY/PRINCIPAL FINDINGS

Using co-immunoprecipitation studies we show that Drosophila Roc proteins bind specific Cullins: Roc1a binds Cul1-4, Roc1b binds Cul3, and Roc2 binds Cul5. Through domain swapping experiments, we demonstrate that Cullin binding specificity is strongly influenced by the Roc NH(2)-terminal domain, which forms an inter-molecular beta sheet with the Cullin. Substitution of the Roc1a RING domain with that of Roc1b results in a protein with similar Cullin binding properties to Roc1a that is active as an E3 ligase but cannot complement Roc1a mutant lethality, indicating that the identity of the RING domain can be an important determinant of CDL function. In contrast, the converse chimeric protein with a substitution of the Roc1b RING domain with that of Roc1a can rescue the male sterility of Roc1b mutants, but only when expressed from the endogenous Roc1b promoter. We also identified mutations of Roc2 and Cul5 and show that they cause no overt developmental phenotype, consistent with our finding that Roc2 and Cul5 proteins are exclusive binding partners, which others have observed in human cells as well.

CONCLUSIONS

The Drosophila Roc proteins are highly similar, but have diverged during evolution to bind a distinct set of Cullins and to utilize RING domains that have overlapping, but not identical, function in vivo.

XRab40 and XCullin5 form a ubiquitin ligase complex essential for the noncanonical Wnt pathway

Rab GTPases are key regulators of intracellular membrane trafficking. We sought to elucidate the roles of Rab GTPases in Xenopus gastrulation, and found that a Xenopus homolog of Rab40 (XRab40) is required for normal gastrulation. XRab40 is localized at the Golgi apparatus and interacts with ElonginB/C and Cullin5 to form a ubiquitin ligase. XRab40/XCullin5 functions cooperatively and regulates the ubiquitination and localization of Rap2 GTPase. Furthermore, XRab40/XCullin5 regulates the membrane localization of Dishevelled (Dsh), a key signaling molecule in the Wnt pathway, through Rap2 and its effector Misshapen/Nck-interacting kinase (XMINK). XMINK interacts with Dsh, and is translocated to the plasma membrane by Wnt activation. We propose a novel signaling cascade consisting of XRab40/XCullin5, Rap2 and XMINK, which plays a crucial role in the regulation of the noncanonical Wnt pathway.

C. elegans RBX-2-CUL-5- and RBX-1-CUL-2-based complexes are redundant for oogenesis and activation of the MAP kinase MPK-1

Cul5-based complex is a member of ECS (Elongin B/C-Cul2/Cul5-SOCS-box protein) ubiquitin ligase family. The cellular function of the Cul5-based complex is poorly understood. In this study, we found that oocyte septum formation and egg production did not occur in either cul-5- or rbx-2-depleted cul-2 homozygotes, although control cul-2 homozygotes laid approximately 50 eggs. These phenotypes are reminiscent of those caused by the MAP kinase mpk-1 depletion. In fact, activation of MPK-1 was significantly inhibited in cul-5-depleted cul-2 mutant and cul-2-depleted cul-5 mutant. Yeast two-hybrid analysis and RNAi-knockdown experiments suggest that oocyte maturation from pachytene exit and MPK-1 activation are redundantly controlled by the RBX-2-CUL-5- and RBX-1-CUL-2-based complexes.

Cullin 5 gene expression in the rat cerebral cortex and hippocampus following traumatic brain injury (TBI)

Cullin-5 (Cul-5), a member of the cullin gene family of scaffold proteins of E3 ubiquitin-ligase complexes, has a role in proteolysis and cell cycle regulation. We recently demonstrated that cul-5 mRNA is ubiquitously expressed in the central nervous system. The present study used quantitative real time polymerase chain reaction and western blotting to measure changes in cul-5 mRNA and Cul-5 protein expression, respectively, in the injured CNS in response to traumatic brain injury (TBI). cul-5 mRNA levels were significantly decreased in the ipsilateral rat cerebral cortex on Days 1 and 7, but not on Day 3 following TBI. In the ipsilateral hippocampus, cul-5 mRNA was significantly reduced on Day 1 after TBI. Cul-5 protein levels were significantly decreased in the ipsilateral rat cerebral cortex on Days 1-7 post-TBI while levels were significantly lower in the ipsilateral hippocampus on Days 3-7 post-TBI. Since Cul-5 is ubiquitously expressed in eukaryotic cells and is linked to proteasome-mediated protein degradation, it may have a role in CNS cell fate determination under conditions of traumatic stress.

Drosophila homolog of APP-BP1 (dAPP-BP1) interacts antagonistically with APPL during Drosophila development

beta-Amyloid precursor protein binding protein 1 (APP-BP1) was previously identified based on its binding to the carboxyl terminal of beta-amyloid precursor protein. In this report, we have discovered that a mutation of dAPP-BP1 (Drosophila ortholog of APP-BP1) hinders tissue development, causes apoptosis in imaginal disc cells, and blocks the NEDD8 conjugation pathway. We show that dAPP-BP1 specifically binds the intracellular domain of APP-like protein (APPL). The dAPP-BP1 mutation partially suppresses the abnormal macrochaete phenotype of Appl(d), while overexpression of dAPP-BP1 causes abnormal macrochaetes. When APPL is overexpressed, the normal bristle pattern in the fly thorax is disturbed and apoptosis is induced in wing imaginal discs. APPL overexpression phenotypes are enhanced by reducing the level of dAPP-BP1. APPL overexpression is shown to inhibit the NEDD8 conjugation pathway. APPL-induced apoptosis is rescued by overexpression of dAPP-BP1. Our data suggest that APPL and dAPP-BP1 interact antagonistically during Drosophila development.