Paraspeckle

West et al. (2016)[7] suggest that the Neat1 is folded and bound to paraspeckle core proteins to first form units, which are bridged together by FUS proteins to form the ordered paraspeckle sphere.[8]

Paraspeckles are organised into core-shell spheroidal structures; seven proteins on a scaffold of lncRNA Neat1 (the 23kb isoform termed NEAT1_2 or NEAT1v2). [9] In 2016, West et al. proposed the currently accepted model for Paraspeckles. This was based on their current findings using super-resolution microscopy.[10] Their models state that the Neat1_2 scaffold folds into a V-shaped unit. Many of these units then are assembled into a core-shell spheroid by FUS proteins. Core proteins SFPQ, NONO and PSPC1 tightly associate to the assembled structure. Finally, the shell forms, composed of partially co-localised TDP43 proteins. Due to the integral nature of Neat1 to paraspeckles assembly, assembly is thought to occur in close proximity to Neat1 transcription sites.[9]

It has been noted that Paraspeckles have a great deal of commonality both in features and structures with cytoplasmic stress granules, another type of membrane-less organelle. This conclusion arose from the fact that both contain common component proteins[11], become more abundant with stress, seem to function through sequestering other proteins and both have distinct core or shell regions with predictable localised molecules[11].

Paraspeckles are dynamic structures that are altered in response to changes in cellular metabolic activity. They are transcription-dependent.[2] All five of the proposed protein components have RNA recognition motifs (RRMs)[3] and, in the absence of RNA polymerase II transcription, the Paraspeckle disappears and all of its associated proponents form a crescent shaped perinucleolar cap in the nucleolus. This phenomenon is demonstrated during the cell cycle. In the cell cycle, Paraspeckles are present during interphase and during all of mitosis except for telophase because, when the two daughter nuclei are formed, there is no RNA Pol II transcription so the protein components instead form a perinucleolar cap. The localization patterns were also duplicated in experiments using transcription inhibiting drugs.[4]

The function of the paraspeckle nuclear domain, as a whole, is still not well understood. It has been postulated that the activity of p54nrb, a protein component, is dependent on its localization.[4] It is therefore possible that the paraspeckle's role is to provide ordered localization of its component proteins and to thereby help direct their activity. It has also been suggested that the paraspeckle contributes to transcriptional regulation.[12] Neither of these hypotheses, however, is universally accepted and therefore insight into the paraspeckle's larger role must be derived from the function of its protein components (PSP1, p54nrb, PSP2 and possibly CFI(m)68 and PSF).

The function of PSP1, the protein whose localization pattern led to the discovery of the paraspeckle,[2] is not well understood. Myojin et al. speculated that PSP1, which is highly concentrated in the testis, may regulate the germ cells' early mRNA processing and assist in chromatin remodeling and nuclear shaping during spermatogenesis.[13] PSP1 also forms a dimer with the second protein component: p54nrb. P54nrb has reported involvement in numerous nuclear events including "transcriptional regulation, splicing, DNA unwinding, nuclear retention of hyperedited dsRNA, viral RNA processing, control and cell proliferation, and circadian rhythm maintenance".[4] The final confirmed component, PSP2, is involved in RNA splicing and coactivates hormone receptors.[3]

Later studies have led to the identification of two additional proteins that are likely components of the paraspeckle. In 2004 Dettwiler et al. revealed CFI(m)68 as a possible component of the paraspeckle.[14] CFI(m)68 has been implicated with the preliminary step in pre-mRNA 3' end splicing. Fox et al.'s 2005 article also contains evidence of a possible fifth protein component of the paraspeckle: PSF.[4] PSF can bind both RNA and DNA and interacts with pre-mRNA splicing proteins that work in conjunction with proteins like CFI(m)68.[13] It can dimerize with p54nrb. Furthermore, it colocalizes with PSP1 both in the paraspeckle and, if in the presence of transcription inhibiting drugs, in the same perinucleolar cap.[4] If PSF is in fact part of the paraspeckle that would help further substantiate an assertion by Myojin, et al. that paraspeckle components may participate in pre-mRNA splicing.

In 2005 a new role for paraspeckles in a novel method for controlling gene expression was reported by Prasanth et al.[15] In this study, a nuclear enriched non-coding RNA (termed CTNRNA) was identified that specifically localised to paraspeckles in the nuclei of several cell types. The group found that the RNA was retained in the nucleus at paraspeckles, and was associated with the paraspeckle proteins P54nrb and PSP1, likely through direct interactions between the proteins and motifs in the 3' untranslated (3' UTR) region of the RNA. The CTN non-coding RNA is a longer transcript produced from a gene that also encodes the membrane protein MCAT2, a cationic amino acid transporter. When cells became stressed, the nuclear non-coding RNA levels were reduced, coupled with an increase in cytoplasmic signal for the MCAT2 mRNA and protein. This led the authors to speculate that the paraspeckles were effectively a storage site for the spliced and processed CTN RNA, that were able to release the RNA in a functional protein-encoding form when the cell received a signal. This shorter form was then free to be transported to the cytoplasm and used as a template for protein production. This 'Rapid Release Nuclear Retention mechanism' is thought to save the cell 25 minutes in the production of the mCAT2 protein, as the RNA has already been transcribed, processed and spliced whilst it is being held in the paraspeckles.

While paraspeckles are originally defined as nuclear bodies containing paraspeckle proteins, the field is re-defining these nuclear bodies as structures containing the long noncoding RNA Neat1_2.[16] Neat1_2 is expressed in a particular cell types in mouse tissues, and thus paraspeckles are not ubiquitous nuclear bodies in living animals.[17].

• The Nuclear Compartments:Paraspeckle page on the Nuclear Protein Database, written by Dr. Archa Fox and Dr. Wendy Bickmore, provides a factsheet and links to information on paraspeckle components.