Bodian's silver method reveals molecular variation in the evolution of neurofilament proteins

The recent demonstration that Bodian's silver method specifically stains mammalian neurofilament subunits (NFs), but not other intermediate filament proteins (IFs), provides a specific marker for the identification of neurofilament polypeptides. We have applied the Bodian stain to SDS-PAGE separated polypeptides in nervous tissues from 9 species, representing neuronal evolution in 4 major phyla: chordata, mollusca, arthropoda and annelida. Every species tested except the arthropod showed intense silver staining of a set of polypeptides, each subsequently identified as NFs by immunomethods. These results demonstrate that the affinity of NFs for Bodian's silver stain is conserved during the evolution of nervous systems in a diverse spectrum of animals. Further, considerable variation in the molecular weight of NF subunits was found among the 6 vertebrates studied. This variation suggests that the molecular weight of NFs has not been conserved during evolution, a quality which appears to be unusual for a structural protein.