Vrille (gene)

Vrille (vri) is a bZIP transcription factor found on chromosome 2 in Drosophila melanogaster. Vrille mRNA and protein product (VRI) oscillate predictably on a 24-hour timescale and interact with other circadian clock genes to regulate circadian rhythms in Drosophila. It is also a regulator in embryogenesis; it is expressed in multiple cell types during multiple stages in development, coordinating embryonic dorsal/ventral polarity, wing-vein differentiation, and ensuring tracheal integrity.^[1][2] It is also active in the embryonic gut but the precise function there is unknown. Mutations in vri alter circadian period and cause circadian arrhythmicity and developmental defects in Drosophila.^[2]

Vrille
Identifiers
Organism	Drosophila melanogaster
Symbol	vri
UniProt	O18660
Search for Structures Swiss-model Domains InterPro

Discovery

Helene George and Regine Terracol discovered the first vrille alleles (vri¹ and vri²) in 1997 by EMS-mutagenesis assay and found their products to be transcription factors involved in embryogenesis.^[3] Justin Blau elucidated additional "vrille" implications in 1999 while screening for clock-controlled genes in Drosophila heads that responded to PER/TIM heterodimers.^[4]

Function

Circadian

Interlocked molecular feedback loops in Drosophila melanogaster. CLOCK/CYCLE heterodimer acts as transcriptional activator (positive element) for period (per) and timeless (tim) genes. The heterodimer of PER/TIM is phosphorylated in the cytoplasm in the presence of specific kinases, and the phosphorylated complex then acts as inhibitor for its own transcription (negative element). The VRI and PDP1 proteins regulate the levels of CLK/CYC complex, which in turn are regulated by CLK/CYC. Thus, CLK/CYC heterodimer appears to be an important component that connects the two loops and is important for sustaining molecular oscillations. The protein Cryptochrome (CRY) has been implicated in the light entrainment pathways of the Drosophila molecular clock.

The current model of the Drosophila circadian oscillator is composed of two interlocking transcription/translation negative feedback loops. One loop consists of per, tim, clk, cyc, and their mRNA and protein products. In this loop, the PER/TIM heterodimer is a repressor for the CLK/CYC complex while the CLK/CYC complex serves as an activator and transcription factor for per and tim. Vrille is activated by the CLK/CYC complex and encodes a repressor of CLK transcription.^[5][6] The second regulatory loop is composed of vri, pdp1, clk, and their mRNA and protein products. Vrille is activated by the CLK/CYC complex and encodes a repressor of CLK transcription. High levels of CLK/CYC activate the transcription of Vrille mRNA to increase protein production, leading to transcriptional repression of clk which ultimately reduces the concentration of CLK/CYC complexes.^[6] Lastly, PDP1 protein, a bZIP transcription factor, serves as an activator of clk transcription and peaks 3–6 hours after the VRI protein peak and, together with VRI, regulate daily rhythms in CLK protein.^[7]

Non-Circadian

Vrille acts as an enhancer of decapentaplegic (dpp) and easter, genes critical to the development of dorsal/ventral axis in the process of regional differentiation during Drosophila embryogenesis. Likewise, it can act as an enhancer of dpp mutations. Easter is involved in initiating a protease cascade that activates the dorsal gene, resulting in repression of dpp in the ventral portion of the embryo during early fly development. The mechanism by which vrille affects the dpp pathway is still unknown.^[8]

mRNA analysis and bioinformatic analysis have suggested that vrille is involved in the regulatory networks leading to cardiac senescence. In addition, overexpression of vri has led to heart dysfunctions, while a loss of function significantly reduces age-related cardiac dysfunction as a result of oxidative stress reduction ^[9]

Induction of vri in Drosophila eye precursor cells, the underdeveloped cells that will become cells of the eye, both reduces the number of ommatidia while simultaneously altering their internal structures. Overexpression of vri causes anti-proliferative effects in processes vital for limb generation, as well as abnormal phenotypes in salivary glands and internal organs. Lastly, it has been postulated that vri affects these processes by interacting with genes involved in the production of cytoskeleton although the exact process is still unknown^[10]

Homologs

The closest mammalian homolog to vri is the transcriptional regulator E4BP4 (NFIL3). Although the consensus binding site for vri has not yet been determined, its bZIP domain does share >85% homology with the protein sequence of E4BP4, and it does recognize the E4BP4 site.^[6] E4BP4 represses promoter activity in per1, per2, and osteoblasts, suggesting its role as a negative regulator complex of mammalian circadian clocks.^[11] It transcriptionally activates interleukin-3, and is implicated in apoptotic pathways.^[12] No other mammalian homologs have been discovered.

The crustacean Daphnia magna co-opts its vri ortholog for activating the doublesex sex-determination gene, a task accomplished by tra in flies. It recognizes the dsx1 enhancer sequence CGTTATA:ACATTGTTAT.^[13]

References

1. Szuplewski S, Fraisse-Véron I, George H, Terracol R (June 2010). "vrille is required to ensure tracheal integrity in Drosophila embryo". Development, Growth & Differentiation. 52 (5): 409–18. doi:10.1111/j.1440-169X.2010.01186.x. PMID 20507356.
2. Brody, Thomas. "Vrille1". The Interactive Fly. Society for Developmental Biology. Retrieved 12 April 2017.
3. George H, Terracol R (August 1997). "The vrille gene of Drosophila is a maternal enhancer of decapentaplegic and encodes a new member of the bZIP family of transcription factors". Genetics. 146 (4): 1345–63. doi:10.1093/genetics/146.4.1345. PMC 1208080. PMID 9258679.
4. Blau J, Young MW (December 1999). "Cycling vrille expression is required for a functional Drosophila clock". Cell. 99 (6): 661–71. doi:10.1016/S0092-8674(00)81554-8. PMID 10612401.
5. Ishida N, Kaneko M, Allada R (August 1999). "Biological clocks". Proceedings of the National Academy of Sciences of the United States of America. 96 (16): 8819–20. Bibcode:1999PNAS...96.8819I. doi:10.1073/pnas.96.16.8819. PMC 33693. PMID 10430850.
6. Glossop NR, Houl JH, Zheng H, Ng FS, Dudek SM, Hardin PE (January 2003). "VRILLE feeds back to control circadian transcription of Clock in the Drosophila circadian oscillator". Neuron. 37 (2): 249–61. doi:10.1016/S0896-6273(03)00002-3. PMID 12546820.
7. Cyran SA, Buchsbaum AM, Reddy KL, Lin MC, Glossop NR, Hardin PE, et al. (February 2003). "vrille, Pdp1, and dClock form a second feedback loop in the Drosophila circadian clock". Cell. 112 (3): 329–41. doi:10.1016/S0092-8674(03)00074-6. PMID 12581523.
8. Skripka FA (August 1975). "[Orthopedic and sanatorium-health resort therapy of patients with infantile cerebral palsy]". Ortopediia Travmatologiia I Protezirovanie. 146 (8): 36–8. PMID 1208080.
9. Monnier V, Iché-Torres M, Rera M, Contremoulins V, Guichard C, Lalevée N, et al. (29 November 2012). "dJun and Vri/dNFIL3 are major regulators of cardiac aging in Drosophila". PLOS Genetics. 8 (11): e1003081. doi:10.1371/journal.pgen.1003081. PMC 3510041. PMID 23209438.
10. Szuplewski S, Kottler B, Terracol R (August 2003). "The Drosophila bZIP transcription factor Vrille is involved in hair and cell growth". Development. 130 (16): 3651–62. doi:10.1242/dev.00588. PMID 12835382.
11. Ohno T, Onishi Y, Ishida N (March 2007). "The negative transcription factor E4BP4 is associated with circadian clock protein PERIOD2". Biochemical and Biophysical Research Communications. 354 (4): 1010–5. doi:10.1016/j.bbrc.2007.01.084. PMID 17274955.
12. "E4BP4(human)". PhosphoSitePlus. Cell Signaling Technology. Retrieved 12 April 2017.
13. Mohamad Ishak NS, Nong QD, Matsuura T, Kato Y, Watanabe H (November 2017). "Co-option of the bZIP transcription factor Vrille as the activator of Doublesex1 in environmental sex determination of the crustacean Daphnia magna". PLOS Genetics. 13 (11): e1006953. doi:10.1371/journal.pgen.1006953. PMC 5667737. PMID 29095827.