NEUROFILAMENT-M MESSENGER-RNA IS EXPRESSED IN CONDUCTION SYSTEM MYOCYTES OF THE DEVELOPING AND ADULT-RABBIT HEART

We previously demonstrated that conduction-system myocytes of the rabb it heart express cytoskeletal proteins immunologically related to neur ofilaments. In order to determine more precisely the nature of these p roteins, we screened an expression cDNA library, prepared from the sin o-atrial node region of the rabbit heart, using a monoclonal antibody which reacts with the M subunit of neurofilaments, Sequence analysis o f the isolated cDNA clones shows high homology with rat and human neur ofilament M mRNAs. Northern blot analysis demonstrates hybridization w ith a transcript expressed in brain, with the size expected for neurof ilament 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 hybridi zation documents that in the adult rabbit the transcript accumulates i n neurons and is localized in myocytes of the sino-atrial and atrio-ve ntricular nodes and of the atrio-ventricular bundle and bundle branche s, but not in working atrial and ventricular myocytes, Developmental a nalysis was undertaken in order to determine the distribution of the n eurofilament M mRNA in the rabbit embryonic heart, In situ hybridizati on 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 pr otein, At subsequent stages of development neurofilament M mRNA is det ectable 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. (C) 1996 Academic Press Limited