Our experimental findings suggest that pADPr binding to Drosophila
hnRNP A1 homolog Hrp38 regulats stem cell maintenance and oocyte polarity. Earlier research demonstrated that Hrp38 regulates alternative splicing (AS) (19
), while hnRNP A1 was reported to control translation initiation through an IRES-mediated mechanism by binding to the 5′UTR of regulated genes (35
). Here we show that pADPr association with Hrp38 controls DE-cadherin translation through an IRES–mediated mechanism. Since Hrp38 binds to the 5′UTR of DE-cadherin and is associated with the DE-cadherin
5′UTR-luciferase transcript, it seems very likely that Hrp38 is an IRES transacting factor for promoting DE-cadherin translation. In mammals, post-translational modification of hnRNP A1 has been shown to regulate IRES-dependent translation. For instance, phosphorylation of hnRNP A1 can promote the binding of hnRNP A1 to IRES of c-Myc
in myeloma cells (37
). Similarly, our results suggest that pADPr modification of hnRNPs negatively regulats IRES-mediated translation by inhibiting the binding of hnRNPs to IRES.
The inhibitory function of pADPr binding to hnRNPs likely occurs in the nucleus for two reasons. First, Hrp38 is predominantly a nuclear protein that binds to the 5′UTR of DE-cadherin presumably in the nucleus. Second, pADPr metabolism is mainly localized in the nucleus. This inference is consistent with a currently accepted model suggesting that IRES transacting factors, such as hnRNP A1, are associated with IRES-containing mRNA in the nucleus from the beginning (38
). However, since recent research has shown that pADPr can regulate microRNA-mediated translation in the cytoplasm (39
), it is entirely plausible that poly(ADP-ribosy)lation also functions in the cytoplasm to inhibit IRES-mediated translation.
Our finding that pADPr modification of Hrp38 regulates DE-cadherin expression in the ovary reveals how DE-cadherin expression is regulated during oocyte localization. The localization of the oocyte in the posterior end occurs during the transition from region 2b to region 3 in the germarium (10
). We found that Hrp38 is also expressed in the oocyte and its surrounding follicle cells in region 2b of the germarium (). We also found that the hrp38
mutant has a lower expression of DE-cadherin
at the interface between the oocyte and surrounding posterior cells, implying that the hrp38
gene is required for upregulating DE-cadherin expression for oocyte localization. Therefore, we suggest that Hrp38 is required for upregulation of DE-cadherin expression in both the oocyte and surrounding follicle cells by promoting translation (). Basically, the expression of Hrp38 in the oocyte and surrounding follicle cells in region 2b helps increase the translation of DE-cadherin by binding to the 5′ UTR of DE-cadherin
mRNA. The newly produced DE-cadherin assists with anchoring the oocyte in the posterior pole. In mid-body follicle cells and nurse cells, Hrp38 association with pADPr can downregulate DE-cadherin expression (). Since expression of the DE-cadherin transgene in the germline was sufficient to rescue oocyte mislocalization in the Parg
mutant double clones (), our finding supports the notion that DE-Cadherin
germline mutant clones alone aren’t sufficient to cause oocyte mislocalization (40
). In summary, we have identified a novel pathway that regulates stem cell maintenance and oocyte polarity via pADPr modification of an hnRNP protein (Hrp38) in Drosophila
(see Supplementary Discussion
for broader implications of these findings).
Diagram illustrating how Hrp38 modification by pADPr controls maintenance of GSC and oocyte localization