Summary NEAT1 RNA, a highly abundant 4 kb ncRNA, is retained in nuclei in ~10–20 large foci that we show is completely coincident with paraspeckles, nuclear domains implicated in mRNA nuclear retention. Depletion of NEAT1 RNA via RNAi eradicates paraspeckles, suggesting it controls sequestration of the paraspeckle proteins, PSP1 and p54, factors linked to A-I editing. Unlike over-expression of PSP1, NEAT1 over-expression increases paraspeckle number, and paraspeckles emanate exclusively from the NEAT1 transcription site. The PSP-1 RNA binding domain is required for its co-localization with NEAT1 RNA in paraspeckles, and biochemical analyses supports that NEAT1 RNA binds with paraspeckle proteins. Unlike other nuclear retained RNAs, NEAT1 RNA is not A-I edited, consistent with a structural role in paraspeckles. Collectively results demonstrate that NEAT1 functions as an essential structural determinant of paraspeckles, providing a precedent for a ncRNA as the foundation of a nuclear domain.

BackgroundNoncoding RNA species play a diverse set of roles in the eukaryotic cell. While much recent attention has focused on smaller RNA species, larger noncoding transcripts are also thought to be highly abundant in mammalian cells. To search for large noncoding RNAs that might control gene expression or mRNA metabolism, we used Affymetrix expression arrays to identify polyadenylated RNA transcripts displaying nuclear enrichment.ResultsThis screen identified no more than three transcripts; XIST, and two unique noncoding nuclear enriched abundant transcripts (NEAT) RNAs strikingly located less than 70 kb apart on human chromosome 11: NEAT1, a noncoding RNA from the locus encoding for TncRNA, and NEAT2 (also known as MALAT-1). While the two NEAT transcripts share no significant homology with each other, each is conserved within the mammalian lineage, suggesting significant function for these noncoding RNAs. NEAT2 is extraordinarily well conserved for a noncoding RNA, more so than even XIST. Bioinformatic analyses of publicly available mouse transcriptome data support our findings from human cells as they confirm that the murine homologs of these noncoding RNAs are also nuclear enriched. RNA FISH analyses suggest that these noncoding RNAs function in mRNA metabolism as they demonstrate an intimate association of these RNA species with SC35 nuclear speckles in both human and mouse cells. These studies show that one of these transcripts, NEAT1 localizes to the periphery of such domains, whereas the neighboring transcript, NEAT2, is part of the long-sought polyadenylated component of nuclear speckles.ConclusionOur genome-wide screens in two mammalian species reveal no more than three abundant large non-coding polyadenylated RNAs in the nucleus; the canonical large noncoding RNA XIST and NEAT1 and NEAT2. The function of these noncoding RNAs in mRNA metabolism is suggested by their high levels of conservation and their intimate association with SC35 splicing domains in multiple mammalian species.

Monoallelic expression with random choice between the maternal and paternal alleles defines an unusual class of genes comprising X-inactivated genes and a few autosomal gene families. Using a genome-wide approach, we assessed allele-specific transcription of about 4000 human genes in clonal cell lines and found that more than 300 were subject to random monoallelic expression. For a majority of monoallelic genes, we also observed some clonal lines displaying biallelic expression. Clonal cell lines reflect an independent choice to express the maternal, the paternal, or both alleles for each of these genes. This can lead to differences in expressed protein sequence and to differences in levels of gene expression. Unexpectedly widespread monoallelic expression suggests a mechanism that generates diversity in individual cells and their clonal descendants.