Xenopus laevis peripherin (XIF3) is expressed in radial glia and proliferating neural epithelial cells as well as in neurons

Neuronal intermediate filament (nIF) proteins form the most abundant compon ent of the axonal cytoskeleton. Thus, understanding their function and the regulation of their expression is essential for comprehending how axonal st ructure is regulated. Although most vertebrate nIF proteins are classified as type IV intermediate filament (IF) proteins, additional nIF proteins exi st in frogs (Xenopus laevis), cyprinid fishes, and mammals (called XIF3, pl asticin, and peripherin, respectively) that are classified as type III. Exp ression of a type III nIF protein is correlated strongly with the earliest phases of axonal outgrowth in fishes but less so in mammals. To understand better how the correlation between type III nIF protein expression and earl y phases of axonal outgrowth has changed during evolution, the authors exam ined XIE3 expression in Xenopus laevis. In Xenopus, the association between XIF3 expression and early axonal outgrowth was especially strong. For exam ple, during early axonal development, XIF3 expression preceded and was more abundant and widespread than that of any of the type TV nIF proteins. As a xons matured, neuronal expression of XIF3 gradually became more restricted while that of type IV nIF proteins increased. These results support the ide a that type III nIF proteins play a special role during early phases of axo nal outgrowth. In addition to finding XIF3 in neurons, the authors also une xpectedly found it in regions of the central nervous system that contain pr oliferating cells and radial glia. As a framework for interpreting variatio ns in nIF expression in different vertebrate species, the authors built phy logenetic trees to clarify relationships among vertebrate nIF proteins. The se trees supported the classification of XIF3, plasticin, and peripherin as orthologs (products of the same genetic locus, evolving separately only si nce the species lineages diverged). Thus, XIF3, plasticin, and peripherin p robably should be referred to as Xenopus, fish, and mammalian peripherin, r espectively. This finding argues that differences in expression of these th ree proteins in frogs, fishes, and mammals are the result of regulatory cha nges to the peripherin ancestral gene along each lineage. The expression of a peripherin ortholog in Xenopus glia may represent either an adaptation t hat arose since the divergence of Xenopus from mammals or, alternatively, a feature retained fi om an ancestral IF protein that was expressed original ly both in neurons and in glia. (C) 2000 Wiley-Liss, Inc.