The non-canonical initiation factor DENR promotes translation reinitiation on mRNAs harbouring upstream open reading frames (uORFs). Moreover, DENR depletion shortens circadian period in mouse fibroblasts, suggesting involvement of uORF usage and reinitiation in clock regulation. To identify DENR-regulated translation events transcriptome-wide and, in particular, specific core clock transcripts affected by this mechanism, we have used ribosome profiling in DENR-deficient NIH3T3 cells. We uncovered 240 transcripts with altered translation rate, and used linear regression analysis to extract 5' UTR features predictive of DENR dependence. Among core clock genes, we identified Clock as a DENR target. Using Clock 5' UTR mutants, we mapped the specific uORF through which DENR acts to regulate CLOCK protein biosynthesis. Notably, these experiments revealed an alternative downstream start codon, likely representing the bona fide CLOCK N-terminus. Our findings provide insights into uORF-mediated translational regulation that can regulate the mammalian circadian clock and gene expression at large. regression analysis to characterise 5' UTR features associated with DENR-dependent translational regulation and validate targets identified from our translatome-wide approaches. Finally, we show that DENR regulates efficient CLOCK biosynthesis involving a strong uORF and a so-far unannotated CDS initiation codon.
MATERIALS AND METHODSCell culture.NIH3T3 and HEK293FT cells were cultured under standard conditions (DMEM; 10% FCS, 1% penicillin/streptomycin, all from Invitrogen; 37 o C; 5% CO 2 ). Lentiviral particle production in HEK293FT cells using envelope pMD2.G and packaging psPAX2 plasmids, and viral transduction of NIH3T3 cells, were performed following published protocols (10), with puromycin selection at 5 µg/ml for 4 days.
Cloning and plasmids.For the generation of lentiviral shRNA expression vectors, two different sequences targeting Denr were cloned into pLKO.1puro backbone vector (Addgene no. 10878 (11)): shRNA1: GTACCACAGAAGGTCACGATA, corresponding to clone TRCN0000308443 of the TRC shRNA Library from the Broad Institute; and shRNA2: GTGCCAAGTTAGATGCGGATT, corresponding to clone TRCN0000098826. pLKO.1puro constructs containing Gfp, and Scramble (Addgene no. 1864) shRNAs served as controls.For the generation of dual luciferase (Firefly/Renilla) reporter plasmids, fragments containing the 5' UTR and first 5-14 codons of the selected DENR targets/controls were amplified by PCR from genomic DNA or cDNA, cloned into the BamHI site of the prLV1 dual luciferase reporter plasmid (12), and validated by sequencing. The following primers were used for PCR: Etaa1, forward (for): aaaggatccGACTTGCAA AGATGGCGCTGCAC, reverse (rev): tttggatccactagtGTCCT TCAGCTGCATTCACATTT; Map2k5, for: aaaggatccGGTT CCGGAGTAACAGCGGTCTAAC, rev: tttggatccactagtGGC CAGCCACAGCATTACAGGTTAA; Lrrc28, for: aaaggatcc GCCGCTGAGTGCCGGTCAGCGGGC, rev: tttggatccactag tGATTTCGGATGCCATGACTGAACA; Klhdc8a, for: aaag gatccGTCTCCGACCCTGTAGACACTGCAG, rev: tttggatc cactagtA...