Double-stranded (ds) RNA can induce sequence-specific inhibition of gene function in several organisms. However, both the mechanism and the physiological role of the interference process remain mysterious. In order to study the interference process, we have selected C. elegans mutants resistant to dsRNA-mediated interference (RNAi). Two loci, rde-1 and rde-4, are defined by mutants strongly resistant to RNAi but with no obvious defects in growth or development. We show that rde-1 is a member of the piwi/sting/argonaute/zwille/eIF2C gene family conserved from plants to vertebrates. Interestingly, several, but not all, RNAi-deficient strains exhibit mobilization of the endogenous transposons. We discuss implications for the mechanism of RNAi and the possibility that one natural function of RNAi is transposon silencing.

X-linked mental retardation (XLMR) is a complex human disease that causes intellectual disability1. Causal mutations have been found in approximately 90 X-linked genes2; however, molecular and biological functions of many of these genetically defined XLMR genes remain unknown. PHF8 (PHD Finger 8) is a JmjC domain-containing protein and its mutations have been found in patients with XLMR and craniofacial deformities. Here we provide multiple lines of evidence establishing PHF8 as the first mono-methyl histone H4 lysine 20 (H4K20me1) demethylase, with additional activities towards histone H3K9me1 and me2. PHF8 is located around the transcription start sites (TSS) of ~7,000 refseq genes and in gene bodies and intergenic regions (non-TSS). PHF8 depletion resulted in up-regulation of H4K20me1 and H3K9me1 at the TSS and H3K9me2 in the non-TSS sites, respectively, demonstrating differential substrate specificities at different target locations. PHF8 positively regulates gene expression, which is dependent on its H3K4me3-binding PHD and catalytic domains. Importantly, patient mutations significantly compromised PHF8 catalytic function. PHF8 regulates cell survival in the zebrafish developing brain and jaw development, thus providing a potentially relevant biological context for understanding the clinical symptoms associated with PHF8 patients. Lastly, genetic and molecular evidence supports a model whereby PHF8 regulates zebrafish neuronal cell survival and jaw development in part by directly regulating the expression of the homeodomain transcription factor MSX1/MSXB, which functions downstream of multiple signaling and developmental pathways3. Our findings suggest that an imbalance of histone methylation dynamics plays a critical role in XLMR.

The synthesis and destruction of cyclin B drives mitosis in eukaryotic cells. Cell cycle progression is also regulated at the level of cyclin B translation. In cycling extracts from Xenopus embryos, progression into M phase requires the polyadenylation-induced translation of cyclin B1 mRNA. Polyadenylation is mediated by the phosphorylation of CPEB by Aurora, a kinase whose activity oscillates with the cell cycle. Exit from M phase seems to require deadenylation and subsequent translational silencing of cyclin B1 mRNA by Maskin, a CPEB and eIF4E binding factor, whose expression is cell cycle regulated. These observations suggest that regulated cyclin B1 mRNA translation is essential for the embryonic cell cycle. Mammalian cells also display a cell cycle-dependent cytoplasmic polyadenylation, suggesting that translational control by polyadenylation might be a general feature of mitosis in animal cells.

Progesterone stimulation of Xenopus oocyte maturation requires the cytoplasmic polyadenylation-induced translation of mos and cyclin B mRNAs. One cis element that drives polyadenylation is the CPE, which is bound by the protein CPEB. Polyadenylation is stimulated by Aurora A (Eg2)-catalyzed CPEB serine 174 phosphorylation, which occurs soon after oocytes are exposed to progesterone. Here, we show that insulin also stimulates Aurora A-catalyzed CPEB S174 phosphorylation, cytoplasmic polyadenylation, translation, and oocyte maturation. However, these insulin-induced events are uniquely controlled by PI3 kinase and PKC-, which act upstream of Aurora A. The intersection of the progesterone and insulin signaling pathways occurs at glycogen synthase kinase 3 (GSK-3), which regulates the activity of Aurora A. GSK-3 and Aurora A interact in vivo, and overexpressed GSK-3 inhibits Aurora A-catalyzed CPEB phosphorylation. In vitro, GSK-3 phosphorylates Aurora A on S290/291, the result of which is an autophosphorylation of serine 349. GSK-3 phosphorylated Aurora A, or Aurora A proteins with S290/291D or S349D mutations, have reduced or no capacity to phosphorylate CPEB. Conversely, Aurora A proteins with S290/291A or S349A mutations are constitutively active. These results suggest that the progesterone and insulin stimulate maturation by inhibiting GSK-3, which allows Aurora A activation and CPEB-mediated translation.[Keywords: Insulin; GSK-3; Xenopus oocytes; Aurora A; cytoplasmic polyadenylation; CPEB] Fully grown Xenopus oocytes arrested at the end of prophase I are stimulated to re-enter into the meiotic divisions (oocyte maturation) by progesterone. Although the initial signaling event that is propagated by progesterone is unclear, it involves an immediate but transient decrease in cyclic AMP (cAMP; Sadler and Maller 1989; for review, see Ferrell 1999) and the activation of Aurora A (Eg2), a member of the Aurora family of protein kinases (Andresson and Ruderman 1998). The most proximal known substrate of Aurora A is CPEB, a sequence-specific RNA binding protein that stimulates cytoplasmic polyadenylation and translational activation (Hake and Richter 1994;Mendez et al. 2000a). CPEB interacts with the cytoplasmic polyadenylation element (CPE), a cis element present in the 3Ј untranslated regions (UTRs) of several mRNAs including those that encode mos and cyclin B. The translation of mos mRNA is necessary to induce the MAP kinase cascade that indirectly activates M-phase promoting factor (MPF), a heterodimer of cyclin B and cdc2. MPF is responsible for many manifestations of oocyte maturation such as germinal vesicle breakdown (GVBD). Aurora phosphorylation of CPEB serine 174 enhances the association of CPEB with CPSF (cleavage and polyadenylation specificity factor), possibly helping to stabilize this group of proteins on the AAUAAA hexanucleotide, a second cis element essential for polyadenylation (Mendez et al. 2000a,b). CPSF is probably responsible for recruiting poly(A) polymerase to the end of the mRNA.Polyadenylat...