Nuclear clustering in myotubes: a proposed role in acetylcholine receptor mRNA expression.

We investigated the functional relationship between nuclear topology, as expressed by degree and type of nuclear aggregation, and appearance of acetylcholine receptor (AChR) subunit mRNAs. Embryonic chick muscle cell cultures treated with the muscle activity blocking agents decamethonium (DCM), d-tubocurare (TBC), and tetrodotoxin (TTX) or co-cultured with cholinergic neurons were examined for the influence of muscle activity on nuclear aggregation and its effects on AChR alpha-, gamma-, and delta-subunit message expression. mRNA was measured by in situ hybridization and nuclei were visualized by bis-benzimide DNA staining. DCM and TBC treatments, as well as neuronal co-culture, resulted in increased nuclear clustering within myotubes and a per nucleus upregulation in mRNA expression relative to control for each subunit. The pattern of nuclear aggregation was treatment dependent, with more and larger aggregates found when myotubes were co-cultured with neurons. Moreover, as nuclear aggregates became larger: (1) nearly all nuclei within active aggregates expressed mRNA and (2) local accumulation (mRNA per unit area) was elevated relative to single nuclei, while per nucleus mRNA production decreased. To determine whether mRNA expression was transient and did not result in steady-state upregulation of AChR receptor protein, we performed a double labeling of surface AChRs with 125I-alpha-bungarotoxin (125I-alpha-BTX) concomitant to the in situ hybridization for mRNA quantification on TTX treated muscle cells. Surface receptor expression tracked mRNA expression forall types of nuclear topology observed, indicating that message levels are in fact reliable indicators of receptor population on the plasma membrane surface in myotubes. We propose that nuclear clustering is an organelle-level, accessory mechanism whereby cells concentrate relatively large amounts of AChR mRNA/protein in specific myotube regions.