oskar RNA plays multiple noncoding roles to support oogenesis and maintain integrity of the germline/soma distinction

Abstract

The Drosophila oskar (osk) mRNA is unusual in that it has both coding and noncoding functions. As an mRNA, osk encodes a protein required for embryonic patterning and germ cell formation. Independent of that function, the absence of osk mRNA disrupts formation of the karyosome and blocks progression through oogenesis. Here we show that loss of osk mRNA also affects the distribution of regulatory proteins, relaxing their association with large RNPs within the germline, and allowing them to accumulate in the somatic follicle cells. This and other noncoding functions of the osk mRNA are mediated by multiple sequence elements with distinct roles. One role, provided by numerous binding sites in two distinct regions of the osk 3 ′ UTR, is to sequester the translational regulator Bruno (Bru), which itself controls translation of osk mRNA. This defines a novel regulatory circuit, with Bru restricting the activity of osk, and osk in turn restricting the activity of Bru. Other functional elements, which do not bind Bru and are positioned close to the 3 ′ end of the RNA, act in the oocyte and are essential. Despite the different roles played by the different types of elements contributing to RNA function, mutation of any leads to accumulation of the germline regulatory factors in the follicle cells.

CncRNAs: RNAs with both coding and non-coding roles in development

“…The comparison of egg-laying by females lacking endogenous osk RNA with those expressing osk transgenes that disrupt some or all the BREs showed that the mutation of any BRE reduces egg-laying to some extent, with the strongest effect observed when all BREs are lost. The reduction in egg-laying appears to be due to disruption of the non-coding function of osk RNA (Kanke et al, 2015).…”

“…A more recent study showed, remarkably, that the expression of a transgenic 191-nucleotide segment of the osk 3′UTR is sufficient to rescue the oogenesis arrest of osk RNA null oocytes, as long as the RNA is fused to a stem-loop structure that promotes its dyneindependent import into the oocyte (Kanke et al, 2015). It was further shown that mutations in the osk 3′UTR that abolish binding of the translational regulator Bruno (Bru; Aret -FlyBase) to Bru response elements (BREs) in osk also affect egg laying (Kanke et al, 2015).…”

“…It was further shown that mutations in the osk 3′UTR that abolish binding of the translational regulator Bruno (Bru; Aret -FlyBase) to Bru response elements (BREs) in osk also affect egg laying (Kanke et al, 2015). The comparison of egg-laying by females lacking endogenous osk RNA with those expressing osk transgenes that disrupt some or all the BREs showed that the mutation of any BRE reduces egg-laying to some extent, with the strongest effect observed when all BREs are lost.…”

“…Evidence that the osk 3′UTR acts via sequestration was provided by mutations affecting Bru, which binds to Osk; mis-sense mutations in bru that reduce Bru levels suppress the osk null mutant phenotype, and this effect is even stronger with bru nonsense mutant alleles. However, mutations in regions of osk mRNA that are not involved in Bru binding also affect egg-laying, indicating that additional unknown factors contribute to the noncoding functions of osk (Kanke et al, 2015).…”

“…SRA RNA co-activation of MYOD, for example, is mediated via interactions between SRA RNA and the p68/p72 RNA helicases (Caretti et al, 2006). A sequestration and scaffolding function has also been proposed for osk 3′UTR sequences during early oogenesis, via their binding to Bru and some unknown factors (Kanke et al, 2015). In Xenopus eggs, vegt RNA forms aggregates that colocalize with cytokeratin filaments, and the depletion of vegt destabilizes the cytokeratin network and disrupts the anchoring of vegetal RNAs (Heasman et al, 2001;Kloc et al, 2005), suggesting that vegt RNA also plays a scaffolding or sequestering role.…”