Neural BC1 RNA in mouse skeletal muscle is a denervation-induced RNA whose expression is developmentally regulated

Coordinated and conserved expression of alphoid repeat and alphoid repeat-tagged coding sequences

We have found an alpha-like simple-sequence DNA repeat that is differentially expressed during early embryogenesis in both chick and zebrafish. Before and during the primitive streak stage, transcripts of the alphoid repeat sequence were ubiquitously expressed throughout zebrafish and chick embryos. After headfold formation, expression was limited to the cardiac neural crest, the head, and the heart. Two types of alphoid repeat sequence transcripts were identified: alphoid repeat RNA and alphoid repeat-tagged mRNA (ESalphaT). Several of the ESalphaTs were identified by (1) searching expressed sequence tag databases, (2) arbitrary rapid amplification of cDNA ends (RACE), and (3) screening embryonic cDNA libraries. The alphoid element was located in the 3' untranslated region of one ESalphaT that was obtained by RACE. The ESalphaT sequences encoded a variety of different types of proteins, but all were expressed within tissues that were positive for the alphoid repeat RNA. The presence of two types of coordinately expressed alphoid-like repeat transcripts in maternal RNA with subsequent restriction to the head and heart, and the conservation of these features in disparate vertebrate embryos, suggest that the alphoid repeat sequence may serve as a control element in the gene regulation network.

Mouse muscle denervation increases expression of an alpha7 nicotinic receptor with unusual pharmacology

Neuronal nicotinic alpha7 subunits have been found in chick and rat skeletal muscle during development and denervation. In the present study, reverse transcriptase-polymerase chain reaction was used to detect alpha7 subunit mRNA in denervated mouse muscle. To determine whether the alpha7 subunit forms functional nicotinic acetylcholine receptors (nAChRs) in muscle, choline was used to induce a membrane depolarization because choline has been considered a specific agonist of alpha7-containing (alpha7*) nAChRs. We found, however, that choline (3-10 mM) also weakly activates muscle nAChRs. After inhibiting muscle nAChRs with a specific muscle nAChR inhibitor, alpha-conotoxin GI (alphaCTxGI), choline was used to activate the alpha7* nAChRs on muscle selectively. Four weeks after denervation, rapid application of choline (10 mM) elicited a substantial depolarization in the presence of alphaCTxGI (0.1 microM). This component of the depolarization was never present in denervated muscles obtained from mutant mice lacking the alpha7 subunit (i.e. alpha7-null mice). The depolarization component that is resistant to alphaCTxGI was antagonized by pancuronium (3-10 microM) and by a 4-oxystilbene derivative (F3, 0.1-0.5 microM) at concentrations considered highly specific for alpha7* nAChRs. Another selective alpha7 antagonist, methyllycaconitine (0.05-5 microM), did not strongly inhibit this choline-induced depolarization. Furthermore, the choline-sensitive nAChRs showed little desensitization over 10 s of application with choline (10-30 mM). These results indicate that functional alpha7* nAChRs are significantly present on denervated muscle, and that these receptors display unusual functional and pharmacological characteristics.

The dendritic translocation of translin protein in the form of BC1 RNA protein particles in developing rat hippocampal neurons in primary culture

Neural BC1 RNA is distributed in neuronal dendrites as ribonucleoprotein particles (RNP). Our previous studies indicated the presence of Translin in BC1 RNPs, which is a translational repressor and links a subset of mRNAs to microtubules. In this study, we confirmed that Translin associates with BC1 RNP and we used immunocytochemical methods to examine the subcellular distribution of Translin in developing hippocampal cells in primary cultures. Translin was detected in both the nuclei and cytoplasm of neurons, whereas in glial cells it was localized in the nuclei. Consistent with the reported developmental time course of BC1 RNA expression and dendritic delivery the translocation of Translin to the neuronal dendrites appeared to correlate with neuronal development and differentiation events such as the onset of synaptogenesis in culture. These observations suggest that BC1 RNP or Translin itself may be relevant to the dendritic translation of mRNAs in response to transsynaptic activity.

An E-box sequence acts as a transcriptional activator for BC1 RNA expression by RNA polymerase III in the brain

BC1 RNA is a small cytoplasmic RNA that is transcribed by RNA polymerase III (Pol III) in the rodent nervous system. In addition to essential intragenic promoter elements for Pol III, the BC1 RNA gene has five E-box sequences (CANNTG) in its 5' flanking region. Deletion analysis using an in vitro transcription system revealed that the region containing the E2 site (CAATTG) was necessary for effective transcription of BC1 RNA. A construct with point mutations within the E2 site showed reduced transcriptional activity. Furthermore, DNaseT I protection and gel retardation assays demonstrated that the E2 site was recognized specifically by a brain nuclear protein(s). These results suggest that the upstream E-box sequence and its binding protein may be involved in the regulation by Pol III of preferential BC1 RNA expression in the brain.

Mutational analysis reveals that an array of GCAAG/CTTGC motifs between sprit promoter sequences for RNA polymerase III is essential for neural BC1 RNA transcription

BC1 RNA is expressed from an identifier (ID) sequence by RNA polymerase III (Pol III) and occurs in neural cells as a ribonucleoprotein particle (BC1 RNP). On the BC1 RNA gene, between the Pol III promoter A and B boxes, there is a region which contains short inverted repeats, including three GCAAG/CTTGC motifs. We found that a nuclear protein binds specifically to this region and, using an in vitro transcription system, demonstrated that point mutations within these motifs markedly inhibit BC1 RNA transcription. These results suggest that the GCAAG/CTTGC motif region and its binding protein may play a role in the transcription of BC1 RNA. Moreover, we demonstrated that transcription is repressed by a concomitant molar excess of BC1 RNA and that the BC1 RNA transcribed by this system forms an RNP with nuclear protein(s), suggesting some interaction of BC1 RNA with transcription factor(s).