Neurofilament M mRNA is expressed in conduction system myocytes of the developing and adult rabbit heart

We previously demonstrated that conduction-system myocytes of the rabbit heart express cytoskeletal proteins immunologically related to neurofilaments. In order to determine more precisely the nature of these proteins, we screened an expression cDNA library, prepared from the sino-atrial node region of the rabbit heart, using a monoclonal antibody which reacts with the M subunit of neurofilaments, Sequence analysis of the isolated cDNA clones shows high homology with rat and human neurofilament M mRNAs. Northern blot analysis demonstrates hybridization with a transcript expressed in brain, with the size expected for neurofilament M mRNA. An mRNA species of the same size is also detectable in the Northern blot of the sino-atrial node region RNA. bl situ hybridization documents that in the adult rabbit the transcript accumulates in neurons and is localized in myocytes of the sino-atrial and atrio-ventricular nodes and of the atrio-ventricular bundle and bundle branches, but not in working atrial and ventricular myocytes, Developmental analysis was undertaken in order to determine the distribution of the neurofilament M mRNA in the rabbit embryonic heart, In situ hybridization shows that neurofilament M mRNA is detectable in a few ventricular myocytes in proximity to the atrioventricular groove after 9.5 days of embryonic development and it is accompanied by the presence of the protein, At subsequent stages of development neurofilament M mRNA is detectable in a number of cardiac myocytes, which are mainly localized at the atrio-ventricular junction and in the ventricular subendocardium and presumably correspond to myocytes of the heart conduction system.