P-bodies and the miRNA pathway regulate translational repression ofbicoidmRNA duringDrosophila melanogasteroogenesis

Abstract

Embryonic axis patterning in Drosophila melanogaster is partly achieved by mRNAs that are maternally localized to the oocyte; the spatio-temporal regulation of these transcripts' stability and translation is a characteristic feature of oogenesis. While protein regulatory factors are necessary for the translational regulation of some maternal transcripts (e.g. oskar and gurken), small RNA pathways are also known to regulate mRNA stability and translation in eukaryotes. MicroRNAs (miRNAs) are small RNA regulators of gene expression, widely conserved throughout eukaryotic genomes and essential for animal development. The main D. melanogaster anterior determinant, bicoid, is maternally transcribed, but it is not translated until early embryogenesis. We investigated the possibility that its translational repression during oogenesis is mediated by miRNA activity. We found that the bicoid 3'UTR contains a highly conserved, predicted binding site for miR-305. Our studies reveal that miR-305 regulates the translation of a reporter gene containing the bicoid 3'UTR in cell culture, and that miR-305 only partially contributes to bicoid mRNA translational repression during oogenesis. We also found that Processing bodies (P-bodies) in the egg chamber may play a role in stabilizing bicoid and other maternal transcripts.Here, we offer insights into the possible role of P-bodies and the miRNA pathway in the translational repression of bicoid mRNA during oogenesis. INTRODUCTIONThe genes and signaling pathways that control embryonic axis patterning in Drosophila melanogaster have been studied for several decades. Valuable insight gained from this model system includes the importance of mRNA localization and the regulation of mRNA stability and translation for development. The D. melanogaster egg chamber develops as a multicellular structure containing the germline, one oocyte and 15 support cells (nurse cells), as well as somatic cells (follicle cells) (1).The oocyte nucleus is arrested in meiosis for the majority of egg chamber development, therefore gene products are provided by the adjacent nurse cells via intercellular ring canals; the microtubule (MT) network mediates most of this long-range transport.The main patterning determinants of D. melanogaster, gurken (grk), nanos (nos), oskar (osk) and bicoid (bcd), are localized as mRNAs to discrete compartments of the oocyte by mid-oogenesis (1).During oogenesis, gurken and nanos mRNAs localize to the dorsoventral and posterior compartments of the oocyte, respectively. When translated, their encoded proteins act to specify the dorsoventral and anteroposterior axes of the future embryo. The bcd mRNA is localized to the anterior of the oocyte during oogenesis and translated following egg activation, at which point the resulting Bicoid transcription factor activates anterior patterns of gene expression in the developing embryo (2). Protein factors that bind mRNA cis-elements are responsible for mediating the localization and translational control of several of these patterning transcr...

The microRNA, miR-18a, regulates NeuroD and photoreceptor differentiation in the retina of the zebrafish

“…This is consistent with the known role of NeuroD in governing differentiation in postmitotic photoreceptors (Taylor et al, 2015) and the functions of some miRNAs to effectively uncouple transcription and translation in order to ensure the correct spatiotemporal expression of proteins (Bao et al, 2016; McLaughlin, Smith, Catrina, & Bratu, 2018; Parchem et al, 2015). We propose that within the photoreceptor lineage, following cell cycle exit, fine-tuning of NeuroD protein levels by miR-18a regulates the spatiotemporal pattern of photoreceptor differentiation.…”

The MicroRNA, miR‐18a, Regulates NeuroD and Photoreceptor Differentiation in the Retina of Zebrafish

“…In conclusion, the data presented here demonstrate that during normal retinal development, miR‐18a regulates the timing of photoreceptor differentiation, and indicate that miR‐18a functions through post‐transcriptional regulation of NeuroD protein levels. This is consistent with the known role of NeuroD in governing differentiation in post‐mitotic photoreceptors (Taylor et al , ) and the functions of some miRNAs to effectively uncouple transcription and translation in order to ensure the correct spatiotemporal expression of proteins (Parchem et al , ; Bao et al , ; McLaughlin et al , ). We propose that within the photoreceptor lineage, following cell cycle exit, fine‐tuning of NeuroD protein levels by miR‐18a regulates the spatiotemporal pattern of photoreceptor differentiation.…”

Modulating the bicoid gradient in space and time

“…Consistent with this observation, the 3'UTR of bcd contains a highly-conserved motif for binding to miR305 (FlyBase; [69]) immediately upstream of the stability motif [32]. Moreover, miRNA 305 was shown to be one of the major inhibitor for Bcd translation in the oocyte [70] through binding of miRNA 305 to this highlyconserved motif.…”