Architectural roles of long noncoding RNAs in the intranuclear formation of functional paraspeckles.

The eukaryotic nucleus is highly compartmentalized, and this structural complexity allows the regulation of complex gene expression pathways. Some of the subnuclear structures called nuclear bodies are known to contain RNAs. Recently multiple noncoding RNAs (ncRNAs) have been identified as products from regions covering large portions of mammalian genomes. Several abundant ncRNAs were found to localize in nuclear bodies, suggesting new roles for ncRNAs in these nuclear bodies. Paraspeckle, one of these nuclear bodies, contains specific ncRNAs, termed MEN (Multiple Endocrine Neoplasia) epsilon/beta ncRNAs, and characteristic RNA-binding proteins. Paraspeckle is ubiquitously observed in cultured cell lines but is cell type-specific in mouse tissues. Paraspeckle reportedly plays an important role in the nuclear retention of inosine-containing mRNAs and is regulated under stress conditions. Intriguingly, MEN epsilon/beta ncRNAs are prerequisite for the formation of paraspeckles, indicating architectural roles for these ncRNAs and, presumably, significant roles in the nuclear retention of mRNAs as well. This review focuses on known aspects of the paraspeckle structure and its components, and we attempt here to construct a model of the ncRNA-dependent formation of nuclear body structures.