Paraspeckles are small, irregularly sized and unevenly distributed ribonucleoprotein (RNP) bodies found in the interchromatin space of mammalian cell nuclei. They are found between larger nuclear speckles and chromatin. They have an irregular sausage like shape. They are approximately 0.5-1 micrometer (one millionth of a meter) in diameter.
Depending on the cell type, paraspeckles number between 5 and 20 foci per nucleus. For example, NIH3T3 cells contain 5-10 foci per nucleus. Paraspeckles were first documented in HeLa cells. They exist in both primary and transformed human cells. Paraspeckles were discovered in the year 2002 while studying the localization pattern of PSP1.
The core paraspeckle proteins are members of the DBHS (Drosophila melanogaster behavior, human splicing) family. Paraspeckles contain p54nrb (an RNA recognition domain containing protein) and other RNA-binding proteins like PSP1. They are RNA protein structures. Paraspeckles are not found in human embryonic stem cells (hESC).
These subpopulation specific nuclear bodies are formed by the interaction between a long non-protein-coding RNA species and members of the DBHS family of proteins. Their name is derived from their distribution in the nucleus. “Para” is the short form of parallel and “Spekles” refers to the splicing speckles found near them.
Paraspeckles are dynamic bodies. They alter in response to changes in cellular metabolic activity. Paraspeckles are transcription dependent. Blocking the reinitiation of Polymerase 2 transcription at the end of mitosis with DRB prevents paraspeckle formation. It recommences after removal of DRB. This shows that paraspeckle formation is dependent on RNA Polymerase 2 transcription.
Paraspeckles play a role in regulating the expression of certain genes in differentiated cells by nuclear retention of RNA. Research scientists have also suggested that paraspeckles contribute to transcriptional regulation. Some experts believe that they provide ordered localization of their component proteins and thereby help direct their activity.
Paraspeckles may have a role in controlling gene expression during viral infection. Research scientists led by Shinichi Nakagawa analyzed gene expression in different mouse tissues. The study was conducted in The Rockefeller University. These scientists suggest that paraspeckles play a major role in providing relief from stress.
Paraspeckles are relatively new class of subnuclear bodies. As on today, there are no examples of diseases caused by either absence or presence of these bodies. Research is going on in this area. Paraspeckles may be a paradigm for a class of subnuclear bodies formed around long non-coding RNA.