Establishment and characterization of an osteocyte-like cell line, MLO-Y4

Much attention has focused on osteoblast and osteoclast biology, but little research has been performed on the cell that composes 90% of all bone cells-the osteocyte. Osteocyte function has been difficult to study because these cells are embedded in mineralized tissue and are difficult to obtain in reasonable numbers and purity. Establishment of an osteocyte cell line makes it possible to study osteocyte function more readily and easily by the application of protein chemistry and molecular biology. We have established a cell line that appears to have the properties of primary osteocytes [1]. The cell line is designated MLO-Y4 (for murine longbone osteocyte) and was established from transgenic mice created using the osteocalcin promoter: driving the large T antigen. This cell line, compared to osteoblasts, expresses long dendritic processes and produces small amounts of alkaline phosphatase and collagen type 1, large amounts of osteocalcin, and very large amounts of connexin 43, a gap junction protein, but similar amounts of osteopontin and CD44. Interestingly, this cell line does not express osteoblast-specific factor 2. This cell line also has functional gap junctions transmitting through the dendritic processes. On exposure to fluid flow, an increase in prostaglandin production occurs concomittant with a dramatic increase in functional gap junctions. Monoclonal antibodies have been generated using these cells, which appear to recognize osteocyte-specific antigens. This cell line and these antibodies should prove to be useful tools for examining osteocyte function.