MOV10 RNA helicase is a potent inhibitor of retrotransposition in cells.
However, unlike MOV10, these other helicases do not strongly inhibit retrotransposition, an activity dependent upon intact helicase domains.Therefore, the host has evolved defense mechanisms to protect against retrotransposition, an arsenal we are only beginning to understand.With homologs in other vertebrates, insects, and plants, MOV10 may represent an ancient and innate form of immunity against both infective viruses and endogenous retroelements.
Affiliation: McKusick-Nathans Institute for Genetic Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA. email@example.com
MOV10 protein, a putative RNA helicase and component of the RNA-induced silencing complex (RISC), inhibits retrovirus replication. We show that MOV10 also severely restricts human LINE1 (L1), Alu, and SVA retrotransposons. MOV10 associates with the L1 ribonucleoprotein particle, along with other RNA helicases including DDX5, DHX9, DDX17, DDX21, and DDX39A. However, unlike MOV10, these other helicases do not strongly inhibit retrotransposition, an activity dependent upon intact helicase domains. MOV10 association with retrotransposons is further supported by its colocalization with L1 ORF1 protein in stress granules, by cytoplasmic structures associated with RNA silencing, and by the ability of MOV10 to reduce endogenous and ectopic L1 expression. The majority of the human genome is repetitive DNA, most of which is the detritus of millions of years of accumulated retrotransposition. Retrotransposons remain active mutagens, and their insertion can disrupt gene function. Therefore, the host has evolved defense mechanisms to protect against retrotransposition, an arsenal we are only beginning to understand. With homologs in other vertebrates, insects, and plants, MOV10 may represent an ancient and innate form of immunity against both infective viruses and endogenous retroelements.
- RNA Helicases/chemistry/metabolism*
- Adenosine Triphosphate/metabolism
- Amino Acid Motifs
- Cell Line
- Cytoplasmic Granules/metabolism
- Gene Expression
- Mutagenesis, Insertional
- Protein Binding
- Protein Transport
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pgen-1002941-g003: Evidence from cell culture retrotransposition assays that MOV10 inhibits insertion of non-LTR retrotransposons.(A) The reporter construct 99 PUR RPS EGFP was cotransfected in 293T cells with empty vector (pcDNA3) or constructs expressing V5-MOV10 or other epitope-tagged L1 RNP-associated helicases. Constructs are numbered and named at the bottom of Panel C. Five days later, percentages of EGFP-positive cells (ie. cells with a retrotransposition event) were determined by flow cytometry. Each construct pair was tested in quadruplicate (n = 4), and results are normalized to pcDNA3 empty vector control (#2). An L1 mutant, 99 PUR JM111 EGFP (JM111), was used as negative control for retrotransposition and FACs gating (#1). V5-DDX17 (isoform (iso)2, #7), is generated from an abbreviated transcript variant spanning only residues 549 to 729 of the C-terminal region of full-length DDX17 (#6). (B) Same as (A) in HeLa-HA cells. (C) To assess transfection efficiency and cell toxicity, helicase constructs were cotransfected with CEP-EGFP and fluorescent cells were assayed 4 days later. Results are from 293T cells. (D) Expression of MOV10 inhibits retrotransposition in a dose-dependent manner. Decreasing microgram amounts of MOV10-expessing plasmid, mixed with empty vector to normalize DNA concentrations, were cotransfected with the L1-reporter construct, 99 PUR RPS EGFP, and assayed for retrotransposition at 5 days. Western blot analysis of cytoplasmic lysates (below) shows that even very small amounts of exogenous MOV10 protein can inhibit retrotransposition. High-molecular weight aggregates of overexpressed MOV10 protein are also visible on overexposed gels (marked by *). (E) Loss of endogenous MOV10 expression increases retrotransposition. Stable Hela and 293T cell lines were established by infection with lentiviral particles expressing shRNAs against MOV10, GAPDH or empty vector, followed by selection with puromycin. These cell lines were then tested for retrotransposition competency of 99 PUR RPS EGFP. Results are normalized to empty vector control. Bottom panels: Western blot showing that shE1 and shE2 decrease endogenous MOV10 protein levels by about 90 percent in 293T cells, and loading control blot showing β-tubulin. (F) Overexpression of MOV10 decreases Alu and SVA retrotransposition in HeLa-HA cells. As described in Dewannieux et al. , a Ya5 Alu is cloned in a plasmid containing the 7SL pol III enhancer and neoTET cassette interrupted by a Tetrahymena self-splicing intron. Upon transcription, the intron is spliced out. When this construct is co-expressed with L1 ORF2 alone (#3 and 4) or full-length L1-RP (#5 and 6), Alu RNAs are reverse transcribed along with the neo gene and integrated into the genome to confer neomycin resistance. Following 15 days of treatment with neomycin, resistant colonies were stained and counted. Either empty vector (#1, 3, 5, 7, and 9) or V5-MOV10 plasmid (#2, 4, 6, 8 and 10) was included in the reactions. Retrotransposition data for SVA.2 and L1-RP containing the mneoI antibiotic-selection cassette  are also shown (#7–10). Colony counts are not normalized. To the right are representative T75 flasks with Giemsa-stained Alu retrotransposition-positive colonies in the absence (left) or presence (right) of MOV10. (G) The SVA EGFP retrotransposition assay  was performed in 293T cells with SVA.2 EGFP cotransfected with constructs expressing L1 ORF1 and ORF2 separately, in the presence or absence of MOV10. (H) Mutations in helicase motifs I–VI impede anti-retrotransposition activity of MOV10. Top panel: MOV10-V5-His6 wild-type and mutant proteins  were tested for effect on retrotransposition of 99 PUR RPS EGFP. Results are normalized to empty vector control (#2). Middle panel: mutant and wild-type MOV10-V5-His6 proteins are expressed at similar levels in 293T cells. Bottom panel: mutant and wild-type MOV10-V5-HIS6 proteins all efficiently co-IP with L1 RNPs expressed from pc-L1-1FH.
Cognizant of the ability of MOV10 to inhibit retroviral infection, and of the related MOV10L protein to suppress transcription of L1s and IAP retrotransposons in male germ cells of knock-out (KO) mice , we wished to determine if MOV10 inhibits genomic insertion of retrotransposons in cells. For this purpose, we employed widely-used cell culture L1 retrotransposition assays originally developed in our lab , . An EGFP reporter cassette, interrupted by an intron in opposite transcriptional orientation and inserted into the 3′ UTR of L1-RP (construct 99 PUR RPS EGFP), is expressed only when the L1 transcript is spliced, reverse transcribed, its cDNA inserted in the genome, and the EGFP reporter gene expressed from its own SV40 promoter. We cotransfected construct 99 PUR RPS EGFP , together with empty (pcDNA3) or tagged MOV10 vectors, in 293T and HeLa cells, and 5 days post-transfection assayed for fluorescent cells by flow cytometry. Coexpressed MOV10 caused a precipitous decrease, over 95%, in the number of retrotransposition-positive 293T cells relative to empty vector control (Figure 3A, compare bar #2 and 3). In HeLa cells, MOV10 also caused a significant, but less dramatic, decline in retrotransposition when assayed by EGFP reporter (80%; Figure 3B, #2 and 3). However, by using the sensitive mneoI-reporter assay , a more pronounced loss of retrotransposition (95%) was seen in HeLa cells (Figure 3F, #9 and 10). Ectopically-expressed MOV10 inhibits L1 retrotransposition in a dose-dependent manner, and its effects remain significant at protein levels undetectable by Western blotting (Figure 3D).