InteractiveFly: GeneBrief

Bicaudal C: Biological Overview | References

BIOLOGICAL OVERVIEW

Bicaudal-C (Bic-C) encodes an RNA-binding protein required maternally for patterning the Drosophila embryo. A set of mRNAs have been identified that associate with Bic-C in ovarian ribonucleoprotein complexes. These mRNAs are enriched for mRNAs that function in oogenesis and in cytoskeletal regulation, and include Bic-C RNA itself. Bic-C binds specific segments of the Bic-C 5' untranslated region and negatively regulates its own expression by binding directly to NOT3/5, a component of the CCR4 core deadenylase complex, thereby promoting deadenylation. Bic-C overexpression induces premature cytoplasmic-streaming, a posterior-group phenotype, defects in Oskar and Kinesin heavy chain:βGal localization as well as dorsal-appendage defects. These phenotypes are largely reciprocal to those of Bic-C mutants, and they affect cellular processes that Bic-C-associated mRNAs are known, or predicted, to regulate. It is concluded that Bic-C regulates expression of specific germline mRNAs by controlling their poly(A)-tail length (Chicoine, 2007).

Precise coordination of translational control and mRNA localization regulates the temporal and spatial expression of proteins that define the dorsal/ventral and anterior/posterior axes of the Drosophila embryo . These axes are established during oogenesis through the activities of the TGF-α homolog Gurken (Grk) and subsequent posterior accumulation of Oskar (Osk). During oogenesis, osk and grk mRNAs are localized in particular regions of the oocyte cytoplasm, and their localization is dynamic, highly regulated, and essential for their developmental functions. Translation from both of these mRNAs is also under complex regulation (Chicoine, 2007 and references therein).

In wild-type oogenesis, rapid circular streaming of the oocyte cytoplasm begins in late stage 10 and continues until stage 12, when the nurse cells transfer their cytoplasm into the oocyte. Cytoplasmic streaming has been linked to osk localization, because its disruption prevents anterior to posterior translocation of injected osk mRNA in stage-10b to -11 oocytes. Furthermore, Kinesin-1 mutants blocked specifically in cytoplasmic streaming display an abnormal persistence of osk in the center of stage-10 oocytes. Oocytes produced by females homozygous for a hypomorphic orb allele (orb^mel) initiate cytoplasmic streaming prematurely. Because orb encodes an RNA-binding protein related to Xenopus cytoplasmic polyadenylation element binding protein (CPEB), this suggests that the timing of this process is regulated through one or more mRNA intermediates (Chicoine, 2007).

Stability and translational activity of maternally transcribed mRNAs are frequently regulated by cytoplasmic proteins that affect their polyadenylation state. In Xenopus oocytes, mos and cyclin B1 mRNAs undergo cytoplasmic elongation of their poly(A) tails at meiotic maturation, and this induces their translation. Cytoplasmic polyadenylation requires CPEB; CPSF (Cleavage and Polyadenylation Specificity Factor) and Symplekin, two factors also involved in nuclear polyadenylation; and Gld2, a cytoplasmic poly(A) polymerase (Barnard, 2004). Cytoplasmic poly(A)-tail elongation depends on phosphorylation of CPEB at meiotic maturation. Before maturation, the polyadenylation complex also contains PARN, a deadenylase whose activity appears to counterbalance Gld2-dependent poly(A)-tail elongation (Kim, 2006). CPEB phosphorylation leads to a remodeling of the mRNP, which has been proposed to result in the release of PARN from the complex, thus leading to polyadenylation and translational activation (Chicoine, 2007 and references therein).

Regulation of poly(A)-tail length also contributes to regulation of Drosophila mRNAs involved in axis patterning. Orb has been implicated in cytoplasmic polyadenylation of osk mRNA and accumulation of Osk protein at the posterior pole of the oocyte. There is no Drosophila PARN ortholog, and the CCR4-NOT complex, which contains the deadenylase CCR4, POP2, and four NOT proteins, is the major deadenylase in Drosophila (Temme, 2004). The CCR4-NOT deadenylation complex can be recruited to specific mRNA targets in Drosophila embryos, and in yeast, by RNA-binding proteins such as Smaug, Nanos, and PUF-family members, resulting in activated deadenylation (Semotok, 2005; Jeske, 2006; Zaessinger, 2006; Goldstrohm, 2006; Kadyrova, 2007). The mutant phenotypes of twin, the gene encoding CCR4, and measurements of cyclin A and B mRNA poly(A) tails in twin mutants (Morris, 2005; Zaessinger, 2006), suggest that regulated deadenylation also occurs in Drosophila oogenesis, but an activator of the CCR4 deadenylase complex in ovaries has not yet been identified (Chicoine, 2007).

Bic-C is required maternally for specifying anterior position during early Drosophila development and for oogenesis (Mohler, 1986; Schüpbach, 1991; Mahone, 1995). Females heterozygous for Bic-C mutations produce embryos of several different phenotypic classes, including bicaudal embryos that consist only of a mirror-image duplication of 2-4 posterior segments. Homozygous Bic-C females are sterile because the centripetal follicle cells fail to migrate over the anterior surface of the oocyte at stage 10. Most egg chambers degenerate shortly after this event. Bic-C protein contains five KH (hnRNP K homology) domains and a SAM (Sterile Alpha Motif) domain (Mahone, 1995). KH and SAM domains are RNA-binding motifs (Chmiel, 2006; Aviv, 2006; Johnson, 2006). The KH and SAM domains can also bind to domains of the same type (Smalla, 1999; Ramos, 2002; Kwan, 2006), and SAM domains can bind SH2 domains (Burd, 1994; Schultz, 1997). Bic-C binds RNA homopolymers in vitro, and a substitution mutation in its third KH domain (G296R) results in substantially decreased RNA-binding activity in vitro and a strong mutant phenotype in vivo (Saffman, 1998). No specific target RNA for Bic-C has heretofore been identified, although Osk accumulation is premature in homozygous Bic-C oocytes (Chicoine, 2007).

This study reports that Bic-C associates with Bic-C mRNA in an ovarian mRNP complex and in gel-shift experiments, and that it can repress its own expression in vivo. Bic-C was overexpressed in germline cells, and premature cytoplasmic streaming, abrogation of posterior osk localization, and dorsal-appendage defects, were observed. The latter phenotype was suppressed, and embryonic viability was increased, by mutations in twin. Furthermore, hiiragi [(hrg) which encodes poly(A) polymerase] and orb mutations are potent dominant enhancers of the Bic-C-overexpression phenotypes. Accordingly, a direct association is found between Bic-C and the NOT3/5 subunit of the CCR4-NOT deadenylation complex, and Bic-C is required for poly(A)-tail shortening of endogenous Bic-C mRNA during early stages of oogenesis. These data show that Bic-C negatively regulates target mRNAs, including Bic-C, by recruiting the CCR4-NOT deadenylation complex, thus identifying an ovarian activator of this complex. Moreover, the results provide direct evidence in support of the hypothesis that Bic-C and Orb act antagonistically to regulate poly(A)-tail lengths of specific mRNA targets essential for embryonic patterning (Chicoine, 2007).

Several lines of evidence indicate that Bic-C negatively regulates its own expression by binding to an element within its 5'UTR and recruiting the CCR4 deadenylase complex through a direct association with NOT3/5. Several other RNA-binding proteins, such as Nova-1, FMR1P, HuD, PABP, and Orb, bind specifically to their own mRNAs, and, in most cases, these interactions are autoregulatory (Schaeffer, 2001; Dredge, 2005). Posttranscriptional mechanisms of autoregulation may provide a means of 'fine tuning' levels of regulatory RNA-binding proteins with respect to their target mRNAs, creating the proper equilibrium between silenced and active targets (Chicoine, 2007).

Bic-C-mediated autoregulation is likely essential for development, since overexpression of Bic-C in the female germline induces premature cytoplasmic streaming, which, in turn, produces defects in pole-plasm assembly, posterior patterning, and dorsal-appendage formation. These phenotypes are largely reciprocal to those observed when Bic-C function is reduced, and they are suppressed by reduction of endogenous Bic-C activity. Although the nos::vp16 promoter used to drive UASP-containing transgenes does not recapitulate the normal transcriptional regulation of Bic-C, the level of Bic-C expression it supports is approximately equal to that of wild-type. No attempt was made to overexpress Bic-C from its own promoter, because it was anticipated that doing so in a noninducible manner would result in dominant female sterility resulting from the overexpression phenotypes that were observed. Germline expression of the UASP-Bic-C transgene restored fertility to Bic-C^YC33-homozygous females, albeit at a low frequency, possibly due to a lack of fine-tuned regulation, since overexpression phenotypes were observed. This demonstrates that the Bic-C protein produced from UASP-Bic-C is functional. Furthermore, a decrease was observed in the frequency and severity of dorsal-appendage defects induced by Bic-C overexpression through a concomitant reduction of endogenous Bic-C dosage. It is thus likely that the phenotypes observed upon Bic-C overexpression result from an increase in the wild-type function of Bic-C (Chicoine, 2007).

Bic-C-overexpression phenotypes suggest that its targets include mRNAs involved in regulating the onset of rapid cytoplasmic streaming. Consistent with this, overexpression of Bic-C^G296R, a form with reduced RNA-binding activity (Saffman, 1998), did not affect cytoplasmic streaming. While the possiblility cannot be excluded that the G296R mutation abrogates other unknown functions of Bic-C, it is noteworthy that several mRNAs that coimmunoprecipated with Bic-C (par-1, Cp190, Cip4, Klp10A, RhoGAP18B, and CG17293) have proven or predicted roles in regulating the actin or tubulin cytoskeleton. It will be important to determine in future experiments whether Bic-C influences cytoplasmic streaming through a regulatory effect on one or more of these potential target mRNAs (Chicoine, 2007).

The results identify Bic-C as an activator of the CCR4 deadenylase complex. Recent data indicate that this complex can be targeted to mRNAs through interactions between different RNA-binding proteins and several of its subunits. PUF proteins interact with the POP2/CAF1 subunit of the complex (Goldstrohm, 2006; Kadyrova, 2007), as is also likely for Smaug (Semotok, 2005; Zaessinger, 2006), whereas Nanos binds the NOT4 subunit to recruit the complex to CyclinB 3'UTR (Kadyrova, 2007). Bic-C directly associates with the NOT3/5 subunit. It is speculated that the ability of different RNA-binding proteins to target different components of the CCR4 complex provides additional regulatory independence and diversity. An uncommon characteristic of activated deadenylation by Bic-C is that binding to the 5'UTR of the regulated mRNA is required, whereas recruitment of the deadenylation complex by other regulatory proteins occurs through 3'UTRs (Goldstrohm, 2006; Zaessinger, 2006; Kadyrova, 2007). Circularization of mRNAs through an association between poly(A)-binding protein and eukaryotic initiation factor 4G, which is part of the 5' cap-binding structure, places the 5' and 3'UTRs in close juxtaposition and enables them to function coordinately. Therefore, 3'UTR-binding proteins influence translation initiation (Mazunder, 2003). Conversely, this study demonstrates that Bic-C interacts with elements in the 5'UTR and influences poly(A)-tail length. Consistent with this, direct targeting of the yeast CCR4 deadenylation complex to a reporter mRNA results in its rapid decay, regardless of whether the targeting site is in the 3' or 5'UTR of the reporter (Finoux, 2006; Chicoine, 2007 and references therein).

orb mutants produce a premature cytoplasmic-streaming phenotype similar to that of Bic-C overexpression (Martin, 2003), reduction of orb activity suppresses Bic-C-mutant phenotypes (Castagnetti, 2003), and this study observed a remarkable enhancement of the Bic-C-overexpression phenotype in a heterozygous orb-mutant background. Because Bic-C overexpression disrupts posterior recruitment of pole-plasm components prior to any detectable effects on Orb levels or distribution, it is concluded that Bic-C and Orb directly regulate the expression of a common set of target mRNAs, rather than Bic-C operating solely through an effect on orb mRNA itself. Consistent with this, Bic-C and Orb proteins have been found in coimmunoprecipitation experiments to be in common mRNP complexes in ovaries (Castagnetti, 2003). Orb has a role in cytoplasmic polyadenylation of osk mRNA, and genetic interactions suggest that Orb achieves this function together with poly(A) polymerase. Taken together, these data support the model that regulation of the poly(A)-tail length of specific mRNAs results from concomitant polyadenylation and deadenylation regulated by specific RNA-binding proteins (Zaessinger, 2006). Consistent with this, in Xenopus oocytes, PARN deadenylase is present in the cytoplasmic polyadenylation complex and counteracts polyadenylation prior to meiotic maturation (Kim, 2006). Both deadenylation and polyadenylation depend on CPEB, the Orb homolog that interacts with both PARN deadenylase and Gld2 poly(A) polymerase (Barnard, 2004; Kim, 2006). A role in translational repression has not yet been described for Orb, but the observations that Bic-C is involved both in direct activation of deadenylation by CCR4, and also in poly(A)-tail length elongation in later oogenesis, suggests that it is central to poly(A)-tail length regulation and potentially responsible for a switch in the balance between deadenylation and polyadenylation. This switch appears to take place at mid-oogenesis, before stage 9, for Bic-C mRNA, but it could be timed differently for other mRNA targets. The transition from promoting deadenylation to promoting polyadenylation could depend on specific regulatory proteins bound to each mRNA and/or on posttranslational modifications to Bic-C itself (Chicoine, 2007).

The RNA-binding protein bicaudal C regulates polycystin 2 in the kidney by antagonizing miR-17 activity

The RNA-binding protein Bicaudal C is an important regulator of embryonic development in C. elegans, Drosophila and Xenopus. In mouse, bicaudal C (Bicc1) mutants are characterized by the formation of fluid-filled cysts in the kidney and by expansion of epithelial ducts in liver and pancreas. This phenotype is reminiscent of human forms of polycystic kidney disease (PKD). This study provides data that Bicc1 functions by modulating the expression of polycystin 2 (Pkd2), a member of the transient receptor potential (TRP) superfamily. Molecular analyses demonstrate that Bicc1 acts as a post-transcriptional regulator upstream of Pkd2. It regulates the stability of Pkd2 mRNA and its translation efficiency. Bicc1 antagonized the repressive activity of the miR-17 microRNA family on the 3'UTR of Pkd2 mRNA. This was substantiated in Xenopus, in which the pronephric defects of bicc1 knockdowns were rescued by reducing miR-17 activity. At the cellular level, Bicc1 protein is localized to cytoplasmic foci that are positive for the P-body markers GW182 and HEDLs. Based on these data, it is proposed that the kidney phenotype in Bicc1^-/- mutant mice is caused by dysregulation of a microRNA-based translational control mechanism (Tran, 2010).

Loss of Polycystin-1 inhibits Bicc1 expression during mouse development

Bicc1 is a mouse homologue of Drosophila Bicaudal-C (dBic-C), which encodes an RNA-binding protein. Orthologs of dBic-C have been identified in many species, from C. elegans to humans. Bicc1-mutant mice exhibit a cystic phenotype in the kidney that is very similar to human polycystic kidney disease. Even though many studies have explored the gene characteristics and its functions in multiple species, the developmental profile of the Bicc1 gene product (Bicc1) in mammal has not yet been completely characterized. To this end, a polyclonal antibody was generated against Bicc1, and its spatial and temporal expression patterns were examined during mouse embryogenesis and organogenesis. The results demonstrated that Bicc1 starts to be expressed in the neural tube as early as embryonic day (E) 8.5 and is widely expressed in epithelial derivatives including the gut and hepatic cells at E10.5, and the pulmonary bronchi at E11.5. In mouse kidney development, Bicc1 appears in the early ureteric bud and mesonephric tubules at E11.5 and is also expressed in the metanephros at the same stage. During postnatal kidney development, Bicc1 expression gradually expands from the cortical to the medullary and papillary regions, and it is highly expressed in the proximal tubules. In addition, this study discovered that loss of the Pkd1 gene product, polycystin-1 (PC1), whose mutation causes human autosomal dominant polycystic kidney disease (ADPKD), downregulates Bicc1 expression in vitro and in vivo. These findings demonstrate that Bicc1 is developmentally regulated and reveal a new molecular link between Bicc1 and Pkd1 (Lian, 2014).

Search PubMed for articles about Drosophila Bicaudal C

Aviv, T., et al. (2006). Sequence-specific recognition of RNA hairpins by the SAM domain of Vts1p. Nat. Struct. Mol. Biol. 13: 168-176. PubMed citation: 16429151

Barnard, D. C., et al. (2004). Symplekin and xGLD-2 are required for CPEB-mediated cytoplasmic polyadenylation. Cell 119: 641-651. PubMed citation: 15550246

Burd, C. G. and Dreyfuss, G. (1994), Conserved structures and diversity of functions of RNA-binding proteins. Science 265: 615-621. PubMed citation: 8036511

Castagnetti, S. and Ephrussi, A. (2003). Orb and a long poly(A) tail are required for efficient oskar translation at the posterior pole of the Drosophila oocyte. Development 130: 835-843. PubMed citation: 12538512

Chicoine, J., Benoit, P., Gamberi, C., Paliouras, M., Simonelig, M. and Lasko, P. (2007). Bicaudal-C recruits CCR4-NOT deadenylase to target mRNAs and regulates oogenesis, cytoskeletal organization, and its own expression. Dev. Cell 13(5): 691-704. PubMed citation: 17981137

Chmiel, N. H., Rio, D. C. and Doudna, J. A. (2006). Distinct contributions of KH domains to substrate binding affinity of Drosophila P-element somatic inhibitor protein. RNA 12: 283-291. PubMed citation: 16428607

Dredge, B. K., Stefani, G., Engelhard, C. C. and Darnell, R. B. (2005). Nova autoregulation reveals dual functions in neuronal splicing. EMBO J. 24(8): 1608-20. PubMed ID: 15933722

Goldstrohm, A. C., et al. (2006). PUF proteins bind Pop2p to regulate messenger RNAs. Nat. Struct. Mol. Biol. 13: 533-539. PubMed citation: 16715093

Jeske, M., et al. (2006). Rapid ATP-dependent deadenylation of nanos mRNA in a cell-free system from Drosophila embryos. J. Biol. Chem. 281: 25124-25133. PubMed citation: 16793774

Johnson, P. E. and Donaldson, L. W. (2006). RNA recognition by the Vts1p SAM domain. Nat. Struct. Mol. Biol. 13: 177-178. PubMed citation: 16429155

Kadyrova, L. Y. et al. (2007). Translational control of Cyclin B mRNA by Nanos in the Drosophila germline. Development 134: 1519-1527. PubMed citation: 17360772

Kim, J. H. and Richter, J. D. (2006). Opposing polymerase-deadenylase activities regulate cytoplasmic polyadenylation. Mol. Cell 24: 173-183. PubMed citation: 17052452

Kwan, J. J. et al. (2006). Saccharomyces cerevisiae Ste50 binds the MAPKKK Ste11 through a head-to-tail SAM domain interaction. J. Mol. Biol. 356: 142-154. PubMed citation: 16337230

Lian, P., Li, A., Li, Y., Liu, H., Liang, D., Hu, B., Lin, D., Jiang, T., Moeckel, G., Qin, D. and Wu, G. (2014). Loss of Polycystin-1 inhibits Bicc1 expression during mouse development. PLoS One 9: e88816. PubMed ID: 24594709

Mahone, M., Saffman, E. E. and Lasko, P. (1995). Localized Bicaudal-C RNA encodes a protein containing a KH domain, the RNA binding motif of FMR1. EMBO J. 14: 2043-2055. PubMed citation: 7538070

Martin, S. G., et al. (2003). The identification of novel genes required for Drosophila anteroposterior axis formation in a germline clone screen using GFP-Staufen. Development 130: 4201-4215. PubMed citation: 12874138

Mazunder, B., Seshadri, V. and Fox, P. L. (2003). Translational control by the 3' UTR: the ends justify the means. Trends Biochem. Sci. 28: 91-98. PubMed citation: 12575997

Mohler, J. and Wieschaus, E. F. (1986). Dominant maternal-effect mutations of Drosophila melanogaster causing the production of double-abdomen embryos. Genetics 112: 803-822. PubMed citation: 3082713

Morris, J. Z., et al. (2005). twin, a CCR4 homolog, regulates cyclin poly(A) tail length to permit Drosophila oogenesis. Development 132: 1165-1174. PubMed citation: 15703281

Ramos, A., et al. (2002). Role of dimerization in KH/RNA complexes: the example of Nova KH3. Biochemistry 41: 4193-4201. PubMed citation: 11914064

Saffman, E. E., et al. (1998). Premature translation of oskar in oocytes lacking the RNA-binding protein Bicaudal-C. Mol. Cell. Biol. 18: 4855-4862. PubMed citation: 9671494

Schaeffer, C., et al. (2001). The fragile X mental retardation protein binds specifically to its mRNA via a purine quartet motif. EMBO J. 20: 4803-4813. PubMed citation: 11532944

Schultz, J., et al. (1997), SAM as a protein interaction domain involved in developmental regulation. Protein Sci. 6: 249-253. PubMed citation: 9007998

Schüpbach, T. and Wieschaus, E. (1991). Female sterile mutations on the second chromosome of Drosophila melanogaster. Genetics 129: 1119-1136. PubMed citation: 1783295

Semotok, J. L., et al. (2005). Smaug recruits the CCR4/POP2/NOT deadenylase complex to trigger maternal transcript localization in the early Drosophila embryo. Curr. Biol. 15(4): 284-94. PubMed ID: 15723788

Smalla, M. et al. (1999). Solution structure of the receptor tyrosine kinase EphB2 SAM domain and identification of two distinct homotypic interaction sites. Protein Sci. 8: 1954-1961. PubMed citation: 10548040

Temme, C., Zaessinger, S., Meyer, S., Simonelig, M. and Wahle, E. (2004). A complex containing the CCR4 and CAF1 proteins is involved in mRNA deadenylation in Drosophila. EMBO J. 23: 2862-2871. PubMed ID: 15215893

Tran, U., et al. (2010). The RNA-binding protein bicaudal C regulates polycystin 2 in the kidney by antagonizing miR-17 activity. Development 137(7): 1107-16. PubMed ID: 20215348

Zaessinger, S., Busseau, I. and Simonelig, M. (2006). Oskar allows nanos mRNA translation in Drosophila embryos by preventing its deadenylation by Smaug/CCR4. Development 133(22): 4573-83. PubMed ID: 17050620

date revised: 5 August 2021

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