Accumulation in fetal muscle and localization to the neuromuscular junction of cAMP-dependent protein kinase A regulatory and catalytic subunits RIα and Cα

Abstract

Using probes specific for cAMP-dependent protein kinase, we have analyzed by in situ hybridization the patterns of expression of regulatory and catalytic subunits in mouse embryos and in adult muscle. RIα transcripts are distributed in muscle fibers exactly as acetylcholinesterase, showing that this RNA is localized at the neuromuscular junction. The transcript levels increase upon denervation of the muscle, but the RNA remains localized, indicating a regulation pattern similar to that of the ε subunit of nicotinic acetylcholine receptor. RIα transcripts have accumulated in the muscle by day 12 of mouse embryogenesis, and localization is established by day 14, at about the time of formation of junctions. This localization is maintained throughout development and in the adult. Immunocytochemical analysis has demonstrated that RIα protein is also localized. In addition, RIα recruits Cα protein to the junction, providing at this site the potential for local responsiveness to cAMP. PKA could be implicated in the establishment and/or maintenance of the unique pattern of gene expression occurring at the junction, or in the modulation of synaptic activity via protein phosphorylation. Embryonic skeletal muscle shows a high level of Cα transcripts and protein throughout the fiber: the transcripts are already present by day 12 of embryogenesis, and their elevated level is maintained only through fetal life. In the adult, the Cα hybridization signal of muscle is weak and homogeneous.