Polarity of osteoblasts and osteoblast-like UMR-108 cells

Enveloped viruses, such as vesicular stomatitis virus (VSV) and Influenza v irus, have been widely used in studying epithelial cell polarity. Viral par ticles of VSV-infected epithelial cells bud from the basolateral membrane, which is in contact with the internal milieu and the blood supply. Influenz a-infected cells bud viral particles from the apical surface facing the ext ernal milieu. This feature can be utilized in labeling polarized membrane d omains, We studied the polarity of mesenchymal osteoblasts using osteosarco ma cell line UMR-108 and endosteal osteoblasts in situ in bone tissue cultu res. Immunofluorescence confocal microscopy revealed that the VSV glycoprot ein (VSV G) was targeted to the culture medium-facing surface. In endosteal osteoblasts, VSV G protein was found in the surface facing bone marrow and circulation, On the contrary, Influenza virus hemagglutinin (HA) was local ized to the bone substrate-facing surface of the UMR-108 cells. Electron mi croscopy showed that in the cases where the cells were growing as a single layer, VSV particles were budding from the culture medium-facing surface, w hereas Influenza viruses budded from the bone substrate-facing surface. Whe n the cells overlapped, this polarity was lost, Cell surface biotinylation revealed that 55% of VSV G protein was biotinylated, whereas influenza viru s HA was only 22% biotinylated, These findings suggest that osteoblasts are polarized at some point of their life cycle. The bone-attaching plasma mem brane of osteoblasts is apical, and the circulation or bone marrow-facing p lasma membrane is basolateral in nature.