Role of Purα in targeting mRNA to sites of translation in hippocampal neuronal dendrites

Using genetic inactivation in the mouse, PURA, encoding Pur alpha, is demonstrated to be essential for developmentally-timed dendrite formation in the cerebellum and hippocampus. Comparison of RNA species bound by Pur alpha prompts the hypothesis that Pur alpha functions with non-coding RNA in transport of certain mRNA molecules to sites of translation in dendrites. Pur alpha binds to human BC200 RNA, implicated in dendritic targeting, and this has homologies to 7SL RNA, implicated in compartmentalized translation. Results using hippocampal rat neurons in situ show that Pur alpha binds to BC1 RNA, implicated in dendritic targeting as a mouse counterpart of BC200, and to mRNA molecules translated in dendrites; Pur alpha is specifically located in dendrites, where it is colocalized with Map2, but not in axons, where it fails to colocalize with Ankyrin G. Pur alpha and Staufen are colocalized at dendritic sites of mRNA translation. Microtubule disruptors inhibit Pur alpha dendritic targeting and allow its mislocalization to axons. Using mouse brain, double-RNA immunoprecipitation places Pur alpha together with Staufen or FMRP on BC1 RNA and specific mRNA species in vivo. These results help define a mechanism by which Pur alpha targets specific mRNA molecules to sites of dendritic translation.

Poly(A)-binding protein is associated with neuronal BC1 and BC200 ribonucleoprotein particles

BC1 RNA and BC200 RNA are two non-homologous, small non-messenger RNAs (snmRNAs) that were generated, evolutionarily, quite recently by retroposition. This process endowed the RNA polymerase III transcripts with central adenosine-rich regions. Both RNAs are expressed almost exclusively in neurons, where they are transported into dendritic processes as ribonucleoprotein particles (RNPs). Here, we demonstrate with a variety of experimental approaches that poly(A)-binding protein (PABP1), a regulator of translation initiation, binds to both RNAs in vitro and in vivo. We identified the association of PABP with BC200 RNA in a tri-hybrid screen and confirmed this binding in electrophoretic mobility-shift assays and via anti-PABP immunoprecipitation of BC1 and BC200 RNAs from crude extracts, immunodepleted extracts, partially purified RNPs and cells transfected with naked RNA. Furthermore, PABP immunoreactivity was localized to neuronal dendrites. Competition experiments using variants of BC1 and BC200 RNAs demonstrated that the central adenosine-rich region of both RNAs mediates binding to PABP. These findings lend support to the hypothesis that the BC1 and BC200 RNPs are involved in protein translation in neuronal dendrites.

The single-stranded DNA- and RNA-binding proteins pur alpha and pur beta link BC1 RNA to microtubules through binding to the dendrite-targeting RNA motifs

Neural BC1 RNA is distributed in neuronal dendrites as RNA-protein complexes (BC1 RNPs) containing Translin. In this study, we demonstrated that the single-stranded DNA- and RNA-binding protein pur alpha and its isoform, pur beta, which have been implicated in control of DNA replication and transcription, linked BC1 RNA to microtubules (MTs). The binding site was within the 5' proximal region of BC1 RNA containing putative dendrite-targeting RNA motifs rich in G and U residues, suggesting that in the cytoplasm of neurons, these nuclear factors are involved in the BC1 RNA transport along dendritic MTs. The pur proteins were not components of BC1 RNP but appeared to associate with MTs in brain cells. Therefore, it is suggested that they may transiently interact with the RNP during transport. In this respect, the interaction of pur proteins with BC1 RNA could be regulated by the Translin present within the RNP, because the binding mode of these two classes of proteins (pur proteins and Translin) to the dendrite-targeting RNA motifs was mutually exclusive. As the motifs are well conserved in microtubule-associated protein 2a/b mRNA as well, the pur proteins may also play a role(s) in the dendritic transport of a subset of mRNAs.

Neural BC1 RNA associates with pur alpha, a single-stranded DNA and RNA binding protein, which is involved in the transcription of the BC1 RNA gene

BC1 RNA is preferentially expressed in neural cells by RNA polymerase III (Pol III) and forms ribonucleoprotein particles (RNP) in the somatodendritic domain of neurons. Our previous studies have suggested that, in the nucleus, BC1 RNA forms an RNP containing a nuclear protein(s) that participates in the transcription of the BC1 RNA gene. In this study, we have shown that newly synthesized BC1 RNA in purified brain nuclear extracts is immunoprecipitated by an antibody against Pur alpha. Pur alpha is a protein that binds single-stranded DNA and RNA and is known to regulate transcription of Pol II system. Although BC1 RNA is transcribed by Pol III, the BC1 RNA gene has two putative Pur alpha binding sites, which Pur alpha specifically recognizes. Point mutations within these sites reduced transcriptional activity in vitro. Furthermore, transcription was inhibited by depletion of Pur alpha from the nuclear extracts, either by the coexistence of its binding region of BC1 RNA or by the antibody that was able to precipitate the nuclear BC1 RNP. These observations suggest that BC1 RNA associates with Pur alpha which is involved in the transcription of the BC1 RNA gene.

The specific endoribonuclease activity of small nuclear and cytoplasmic alpha-RNPs

For the first time small nuclear ribonucleoprotein particles (alpha-RNP) tightly bound to chromatin as well as cytoplasmic alpha-RNP are shown to possess strong and regulated endonuclease activity specific for mRNAs and hnRNAs. The enzymatic nature of this activity is confirmed, and the optimal conditions detected. This RNase activity is controlled by the action of a differentiating stimulus, dimethylsulfoxide, in human K562 cells. Small alpha-RNP involvement in the coordinated control of stability of pre-messenger RNA and messenger RNA molecules is suggested.