Beta-cytoplasmic actin localization in vertebrate glomerular podocytes.

The unique cytoarchitecture of glomerular podocytes is conserved in vertebrate evolution. Actin filaments play a crucial role in the formation of the conserved cytoarchitecture, though several isoforms of cytoplasmic actin have been found in vertebrates. The present study examined the expression and subcellular distribution of the beta-cytoplasmic actin (beta-actin) isoform in the podocytes of six vertebrate species by means of immunohistochemical techniques to reveal whether the beta-actin isoform is involved in the formation of podocyte cytoarchitecture throughout vertebrates. beta-actin was predominantly localized at the foot processes in carp, turtle, quail, and rat podocytes in addition to actin filament condensations, which were found only in carp and rat podocytes. The actin filament condensations in rats were in direct contact with the basal plasma membrane, but those in carp were found at the cell body and separated from the basal plasma membrane. In contrast with the above four species, beta-actin was not detected in podocytes in two amphibians-newt and frog, although podocyte foot processes are actin-filament based cytoplasmic protrusions in these species as well as in other vertebrates. In conclusion, the beta-actin isoform is involved in the formation of the podocyte actin cytoskeleton in vertebrates except for amphibians. Several kinds of unconventional cytoplasmic actins other than beta- and gamma-cytoplasmic actins are known to be expressed in amphibians, making it highly likely that one of these isoforms, instead of beta-actin, constructs actin filaments in the foot processes of newt and frog podocytes.