Mechanically regulated expression of a neural glutamate transporter in bone: a role for excitatory amino acids as osteotropic agents?

Without habitual exercise, bone is lost from the skeleton. Interactions between the effects of loading of bone and other osteotropic influences are thought to regulate bone mass. In an attempt to identify potential targets for therapeutic manipulation of bone mass, we used differential RNA display to investigate early changes in osteocyte gene expression following mechanical loading of rat bone in vivo. One gene found to be down-regulated by loading had high homology to a glutamate/ aspartate transporter (GLAST) previously identified only the mammalian CNS. RT-PCR analysis using primers targeted to the coding region of the published GLAST sequence amplified identical products from bone and brain (but not a range of other tissues). The amplicons were sequenced and found to be identical to the published CNS GLAST sequence. Northern analysis confirmed expression of GLAST mRNA in bone and brain, but not other tissues. In situ hybridization localized GLAST mRNA expression in rat bone to osteoblasts and osteocytes. A GLAST antibody localized high levels of protein expression to osteoblasts, and newly incorporated osteocytes. Interestingly, older osteocytes also expressed detectable levels of GLAST. Another neural glutamate transporter, GLT-1 was immunolocalized to the pericellular region of mononuclear bone marrow cells, while a further antibody to the EAAC-1 transporter failed to bind to bone cells. Five days after loading, GLAST protein expression was undetectable in osteocytes of loaded bone but present in control nonloaded sections, confirming the downregulation detected by differential display. On quiescent periosteal surfaces, GLAST expression was almost absent, while on surfaces where loading had induced cellular proliferation and bone formation, GLAST protein expression was elevated. In the CNS, the expression of glutamate transporters on neuronal membranes is associated with reuptake of released neurotransmitters at synapses, where they have a role in the termination of transmitter action. In this study, we describe for the first time, the expression of GLAST (and GLT-1) in bone, raising the possibility that excitatory amino acids may have a role in paracrine intercellular communication in bone. Manipulation of bone cell function by moderators of glutamate action could therefore provide novel treatments for bone diseases such as osteoporosis.