Ribonucleoprotein condensation driven by retrotransposon LINE-1 sustains RNA integrity and translation in mouse spermatocytes

Abstract

Transposable elements (TE) are mobile DNA sequences whose excessive proliferation endangers the host. Although animals have evolved robust TE-targeting defenses, including Piwi- interacting (pi)RNAs, retrotransposon LINE-1 (L1) still thrives in humans and mice. To gain insights into L1 endurance, we characterized L1 Bodies (LBs) and ORF1p complexes in germ cells of piRNA-deficient Maelstrom null mice. We report that ORF1p interacts with TE RNAs, genic mRNAs, and stress granule proteins, consistent with earlier studies. We also show that ORF1p associates with the CCR4-NOT deadenylation complex and PRKRA, a Protein Kinase R factor. Despite ORF1p interactions with these negative regulators of RNA expression, the stability and translation of LB-localized mRNAs remain unchanged. To scrutinize these findings, we studied the effects of PRKRA on L1 in cultured cells and showed that it elevates ORF1p levels and L1 retrotransposition. These results suggest that ORF1p-driven condensates promote L1 propagation, without affecting the metabolism of endogenous RNAs.

Steady-state neuron-predominant LINE-1 encoded ORF1p protein and LINE-1 RNA increase with aging in the mouse and human brain

“…The presence of a significant number of overlapping ORF1p interactors in different non-neuronal human cells (HEK 53,58,59 , HeLa 60 , human breast and ovarian tumors 63 and hESCs 61 ) and mouse brain cells (our study), suggest conserved key interactors between both species and between cell types, with a subset of these proteins regulating RNA degradation and translation potentially relevant for the LINE-1 lifecycle itself. ORF1p interactors found in mouse spermatocytes 62 were also present in our analysis including CNOT10, CNOT11, PRKRA and FXR2 among others (TableS3). To unravel the physical interactions between the identified interactors of endogenous ORF1p within the mouse brain, we used the STRING database (Search Tool for Recurring Instances of Neighboring Genes, https://string-db.org/).…”

“…ORF1p interactors found in mouse spermatocytes 69 were also present in our analysis including CNOT10, CNOT11, PRKRA and FXR2 among others (Suppl_Table4). To unravel the physical interactions between the identified interactors of endogenous ORF1p within the mouse brain, we used the STRING database (Search Tool for Recurring Instances of Neighboring Genes, https://string-db.org/).…”

“…In cancer cell lines or mouse spermatocytes, ORF1p interacts with several “host” proteins, some if not most of which are related to the LINE-1 life cycle. However, a profile of endogenous ORF1p interactors in the mouse brain might inform on possible other and organ-specific functions besides its binding to the LINE-1 RNA in “cis” 69 . Among the total 424 potential interactors of endogenous ORF1p in the mouse brain, 38 partners had been previously identified by mass spectrometry in human cancers, cancerous cell lines and mouse spermatocytes 60,65–70 (Suppl_Table4).…”

“…Finally, we also observed proteins belonging to the GO term "neuronal cell body", corroborating with the neuron-specific presence of ORF1p in the brain. A comparative analysis with previous mass spectrometry studies 60,[65][66][67][68][69][70] aimed at identifying ORF1p interacting proteins unveiled significantly more common proteins than randomly expected (overrepresentation test; representation factor 2.6, p< 5.4e-08; Fig. 6C), including LARP1, STAU2, ATXN2, RALY, TARBP2 or DDX21 (for a full list see Suppl_Table4).…”

“…Finally, we also observed proteins belonging to the GO term "neuronal cell body", corroborating with the neuron-specific presence of ORF1p in the brain. A comparative analysis with previous mass spectrometry studies 53,[58][59][60][61][62][63] aimed at identifying ORF1p interacting proteins unveiled significantly more common proteins than expected by chance (overrepresentation test; representation factor 2.6, p< 5.4e-08; Figure 6C), including LARP1, STAU2, ATXN2, RALY, TARBP2 or DDX21 (for a full list see TableS3). The presence of a significant number of overlapping ORF1p interactors in different non-neuronal human cells (HEK 53,58,59 , HeLa 60 , human breast and ovarian tumors 63 and hESCs 61 ) and mouse brain cells (our study), suggest conserved key interactors between both species and between cell types, with a subset of these proteins regulating RNA degradation and translation potentially relevant for the LINE-1 lifecycle itself.…”