Paraspeckle formation during the biogenesis of long non-coding RNAs.

Paraspeckles are unique subnuclear structures that are built around a specific long non-coding RNA (lncRNA), NEAT1, which is comprised of two isoforms (NEAT1_1 and NEAT1_2) that are produced by alternative 3'-end processing. NEAT1 lncRNAs are unusual RNA polymerase II transcripts that lack introns. The non-polyadenylated 3'-end of NEAT1_2 is non-canonically processed by RNase P. NEAT1_2 is an essential component for paraspeckle formation. Paraspeckles form during the NEAT1_2 lncRNA biogenesis process, which encompasses transcription from its own chromosome locus through lncRNA processing and accumulation. Recent RNAi analyses of 40 paraspeckle proteins (PSPs) identified four PSPs that are required for paraspeckle formation by mediating NEAT1 processing and accumulation. In particular, HNRNPK was shown to arrest CFIm-dependent NEAT1_1 polyadenylation, leading to NEAT1_2 synthesis. The other three PSPs were required for paraspeckle formation, but did not affect NEAT1_2 expression. This observation suggests that NEAT1_2 accumulation is necessary but not sufficient for paraspeckle formation. An additional step, presumably the bundling of NEAT1 ribonucleoprotein sub-complexes, may be required for construction of the intact paraspeckle structure. NEAT1 expression is likely regulated at transcriptional and post-transcriptional steps under certain stress conditions, suggesting roles for paraspeckles in the lncRNA-mediated regulation of gene expression, such as the nucleocytoplasmic transport of mRNA in response to certain stimuli.