InteractiveFly: GeneBrief

jing interacting gene regulatory 1: Biological Overview | References

Intragenic microRNAs (miRNAs), located mostly in the introns of protein-coding genes, are often co-expressed with their host mRNAs. However, their functional interaction in development is largely unknown. This study shows that in Drosophila, miR-92a and miR-92b are embedded in the intron and 3'UTR of jigr1, respectively, and co-expressed with some jigr1 isoforms. miR-92a and miR-92b were highly expressed in neuroblasts of larval brain where Jigr1 expression was low. Genetic deletion of both miR-92a and miR-92b demonstrated an essential cell-autonomous role for these miRNAs in maintaining neuroblast self-renewal through inhibiting premature differentiation. miR-92a and miR-92b directly targeted jigr1 in vivo and some phenotypes due to the absence of these miRNAs were partially rescued by reducing the level of jigr1. These results reveal a novel function of the miR-92 family in Drosophila neuroblasts and provide another example that local negative feedback regulation of host genes by intragenic miRNAs is essential for animal development (Yuva-Aydemir, 2015).

MicroRNAs (miRNAs) are short (~21-23 nt) noncoding RNAs that regulate gene expression post-transcriptionally in many physiological and pathological processes. In the canonical miRNA biogenesis pathway, a long primary transcript (pri-miRNA) is generated by RNA polymerase II and cleaved by a nuclear complex formed by Drosha and DGCR8. Some pri-miRNAs produce miRNAs only (intergenic miRNAs) while others contain miRNAs in the intronic regions of protein-coding 'host' genes (intragenic miRNAs). Many intronic miRNAs and host gene mRNAs are likely co-expressed but others may not be. Few cases have been experimentally confirmed, and the functional significance of such a genomic arrangement is largely unknown (Yuva-Aydemir, 2015).

This study used the differentiation of Drosophila neuroblasts as a model system to examine the expression and function of specific miRNAs. Drosophila neuroblasts form during embryonic development and enter a proliferative quiescent state at the end of embryogenesis. In the early larval stage, neuroblasts reenter the cell cycle and undergo a series of proliferative symmetric and self-renewing asymmetric cell divisions to maintain the progenitor pool and generate diverse cell types. In each asymmetric cell division, neuroblasts divide to generate a neuroblast cell and a ganglion mother cell, which divides only once to generate two neurons or one neuron and one glial cell. The balance between self-renewal and differentiation is critical for normal development, but the mechanisms are incompletely understood (Yuva-Aydemir, 2015).

This study shows that the gene encoding jing-interacting gene regulatory 1 (jigr1), a putative DNA-binding protein containing MADF domain with unknown function (Sun, 2006), hosts miR-92a in the intron and miR-92b in the 3'UTR. The functional significance of this intragenic miRNA-host gene interaction was uncovered through genetic knockout of both miR-92a and miR-92b. During larval development, miR-92 family limits jigr1 expression in neuroblasts and is essential for maintenance of a neuroblast pool. It is proposed that this genomic arrangement and local feed-back regulatory loop are essential for animal development to ensure the generation of the proper number of neuronal and glial cells (Yuva-Aydemir, 2015).

This paper reports an unusual genomic arrangement in which miR-92a and miR-92b are embedded in the intron and 3'UTR of the host gene jigr1, respectively. In neuroblasts, miR-92a and miR-92b were highly expressed as a single transcriptional unit also containing jigr1 coding region. Genetic analysis in Drosophila showed that downregulation of jigr1 by intragenic miR-92a and miR-92b is required for neuroblast self-renewal, providing an example of the functional significance of miRNA-host gene interactions in animal development (Yuva-Aydemir, 2015).

Nearly half of the miRNAs in mammals and Drosophila are located within protein-coding genes. Most of these intragenic miRNAs are co-expressed with their host genes, but both positive and negative feedback regulation of host gene expression and function by miRNAs remains largely unknown. Most intragenic miRNAs are located in the introns of their host genes and are processed by the mirtron pathway, bypassing the microprocessor complex (Okamura, 2007). On the other hand, miRNAs are rarely located in the 3'UTR of a protein-coding gene, and the effect of this organization on host gene expression or miRNA processing is not clear. The exonic miR-198, which is located in the 3'UTR of the gene encoding human folistatin like 1, is processed from a single transcript with its host gene in a mutually exclusive way. In contrast, direct regulation of some jigr1 isoforms by miR-92a and miR-92b largely accounts for the observed complementary expression domains of jigr1 and these miRNAs, although the possibility cannot be completely rule out that other mechanisms may also contribute to jigr1 repression (Yuva-Aydemir, 2015).

Through a genetic knockout of both miR-92a and miR-92b in Drosophila, which has not been done so far in any other model organism, this study discovered a novel function for miR-92a and miR-92b in neuroblast self-renewal. The findings are consistent with results obtained in mammals. However, unlike Drosophila, human and mouse miR-92a and miR-92b genes are not intragenic. Mouse miR-92a genes are located in two clusters: miR-17-92 and miR-106a-303. In developing mouse neocortex, the miR-17-92 cluster promotes neural stem cell expansion and regulates the transition to intermediate progenitors through repression of Pten by miR-19 and Tbr2 by miR-92a (Bian, 2013). Similarly, acute loss and gain of miR-92b function in mouse cortex showed that miR-92b restricts the generation of intermediate progenitor cells by suppressing Tbr2. Moreover, miR-92b maintains asymmetric division of neural stem cells by restricting Tis21 expression in mouse neocortex. However, phenotypes caused by loss of both miR-92a and miR-92b in mammals have not been reported yet. This study found that miR-92a and miR-92b work in concert to restrict Jigr1 expression in Drosophila larval neuroblasts and thereby maintain the neuroblast pool (Yuva-Aydemir, 2015 and references therein).

Jigr1, a putative MADF-domain-containing transcription factor of unknown function expressed ubiquitously in the larval central nervous system, is expressed at low levels in neural progenitor cells. The findings suggest that the progressive loss of neuroblasts in miR-92-/- brains is due to premature differentiation of these cells resulted from cell-autonomous effect of loss of miR-92, as shown by ectopic expression of nuclear Prospero, decreased BrdU uptake, and reduced cell size. Upregulation of Jigr1 seems to play a role in nuclear Prospero expression in miR-92-/- neuroblasts, since reducing Jigr1 expression eliminated this phenotype. Moreover, overexpression of Jigr1 in neuroblasts on a wild type background leads to ectopic Prospero expression and a premature differentiation phenotype. In summary, these results reveal a local regulatory loop in which miR-92a and miR-92b are expressed in the jigr1 transcription unit and also work in concert to prevent premature differentiation of neuroblasts by limiting expression of the host gene (Yuva-Aydemir, 2015).

Glial and neuronal functions of the Drosophila homolog of the human SWI/SNF gene ATR-X (DATR-X) and the jing zinc-finger gene specify the lateral positioning of longitudinal glia and axons

Neuronal-glial communication is essential for constructing the orthogonal axon scaffold in the developing Drosophila central nervous system (CNS). Longitudinal glia (LG) guide extending commissural and longitudinal axons while pioneer and commissural neurons maintain glial survival and positioning. However, the transcriptional regulatory mechanisms controlling these processes are not known. The midline function of the jing C2H2-type zinc finger transcription factor has been shown to be only partially required for axon scaffold formation in the Drosophila CNS. A screen was performed for gain-of-function enhancers of jing gain-of-function in the eye; the Drosophila homolog DATR-X (also termed XNP) of the disease gene of human alpha-thalassemia/mental retardation X-linked (ATR-X) was identified, as well as other genes with potential roles in gene expression, translation, synaptic transmission and cell cycle. jing and DATR-X reporter genes are expressed in both CNS neurons and glia including the longitudinal glia. Co-expression of jing and DATR-X in embryonic neurons synergistically affects longitudinal connective formation. During embryogenesis, jing and DATR-X have autonomous and non-autonomous roles in the lateral positioning of LG, neurons and longitudinal axons as shown by cell-specific knock-down of gene expression. jing and DATR-X are also required autonomously for glial survival. jing and DATR-X mutations show synergistic effects during longitudinal axon formation, suggesting they are functionally related. These observations support a model in which downstream gene expression, controlled by a potential DATR-X-Jing complex, facilitates cellular positioning and axon guidance, ultimately allowing for proper connectivity in the developing Drosophila CNS (Sun, 2006).

The enhancer screen used the third chromosome collection of EP lines. Of 591 third-chromosome EP lines screened, 7 were found to repeatedly interact genetically when coexpressed with jing in the eye under control of GMR-Gal4. Eye-specific expression of one copy of a UAS-jing transgene has no visible effects on ommatidial formation as compared with eyes heterozygous for GMR-Gal4. However, coexpression of jing with EP(3)0635, which controls expression of the Drosophila homolog of human ATR-X (DATR-X), disrupted ommatidial formation. Eye-specific expression of EP(3)0635 alone had no effect. jing also interacted with 4 other EP lines, including EP(3)3145, EP(3)3705, EP(3)3354, and EP(3)0473, resulting in rougher eyes. A loss in pigmentation occurred in the case of EP(3)3354. These lines had no effect when expressed alone in the eye (Sun, 2006).

Eye-specific coexpression of EP(3)3354 with EP(3)3145 and EP(3)0635 resulted in glossy eyes with severely reduced pigmentation representing synergistic interactions. EP(3)3354 was designated as jing interacting gene regulatory 1 (JIGR1), given its potential role in regulating gene expression (Brody, 2002). The EP element in JIGR1 lies upstream of the transcript CG17383, which was identified in a differential head cDNA screen (Brody, 2002). JIGR1 contains an MADF domain shown in the Adf-1 transcription activator to bind DNA specifically to several developmentally regulated Drosophila gene promoters. JIGR1 also interacted with EP(3)3705 resulting in a reduced eye size (Sun, 2006).

Search PubMed for articles about Drosophila Jigr1

Bian, S., Hong, J., Li, Q., Schebelle, L., Pollock, A., Knauss, J. L., Garg, V. and Sun, T. (2013). MicroRNA cluster miR-17-92 regulates neural stem cell expansion and transition to intermediate progenitors in the developing mouse neocortex. Cell Rep 3: 1398-1406. PubMed ID: 23623502

Brody, T., Stivers, C., Nagle, J. and Odenwald, W. F. (2002). Identification of novel Drosophila neural precursor genes using a differential embryonic head cDNA screen. Mech Dev 113: 41-59. PubMed ID: 11900973

Okamura, K., Hagen, J. W., Duan, H., Tyler, D. M. and Lai, E. C. (2007). The mirtron pathway generates microRNA-class regulatory RNAs in Drosophila. Cell 130: 89-100. PubMed ID: 17599402

Sun, X., Morozova, T. and Sonnenfeld, M. (2006). Glial and neuronal functions of the Drosophila homolog of the human SWI/SNF gene ATR-X (DATR-X) and the jing zinc-finger gene specify the lateral positioning of longitudinal glia and axons. Genetics 173: 1397-1415. PubMed ID: 16648585

Yuva-Aydemir, Y., Xu, X.L., Aydemir, O., Gascon, E., Sayin, S., Zhou, W., Hong, Y. and Gao, F.B. (2015). Downregulation of the host gene jigr1 by miR-92 is essential for neuroblast self-renewal in Drosophila. PLoS Genet 11: e1005264. PubMed ID: 26000445

date revised: 6 August 2015

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