Polarity of osteoblasts and osteoblast-like UMR-108 cells

Enveloped viruses, such as vesicular stomatitis virus (VSV) and Influenza virus, have been widely used in studying epithelial cell polarity. Viral particles of VSV-infected epithelial cells bud from the basolateral membrane, which is in contact with the internal milieu and the blood supply. Influenza-infected cells bud viral particles from the apical surface facing the external milieu. This feature can be utilized in labeling polarized membrane domains. We studied the polarity of mesenchymal osteoblasts using osteosarcoma cell line UMR-108 and endosteal osteoblasts in situ in bone tissue cultures. Immunofluorescence confocal microscopy revealed that the VSV glycoprotein (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 localized to the bone substrate-facing surface of the UMR-108 cells. Electron microscopy showed that in the cases where the cells were growing as a single layer, VSV particles were budding from the culture medium-facing surface, whereas Influenza viruses budded from the bone substrate-facing surface. When the cells overlapped, this polarity was lost. Cell surface biotinylation revealed that 55% of VSV G protein was biotinylated, whereas Influenza virus HA was only 22% biotinylated. These findings suggest that osteoblasts are polarized at some point of their life cycle. The bone-attaching plasma membrane of osteoblasts is apical, and the circulation or bone marrow-facing plasma membrane is basolateral in nature.