Type II brain sodium channel expression in non-neuronal cells: embryonic rat osteoblasts

Although voltage-sensitive sodium channels play a central role in electrogenesis in neurons, rat brain sodium channels are also present in some glial cells. To determine whether rat brain sodium channel alpha-subunit isotypes are expressed in other cell types, we examined osteoblasts within the embryonic day 17 (E17) vertebral column with in situ hybridization and immunocytochemical methods. For in situ hybridization studies, riboprobes hybridizing to isoform-specific sequences in the 3'-noncoding region of sodium channel mRNAs (NCI, NCII and NCIII) were utilized. Sodium channel mRNA I and III were not detectable in osteoblasts of the vertebra centrum or neural arches in E17 rats. In contrast, sodium channel mRNA II was moderately expressed by osteoblasts in the developing vertebral column of E17 rats. In immunocytochemical experiments, antipeptide antibodies directed against conserved and isotype-specific regions of the sodium channel alpha-subunit were used. Antibody SP20, which recognizes a conserved region of the sodium channel, intensely stains osteoblasts in both the vertebra centrum and neural arches. Antibody SP11-I, which recognizes sodium channel I, exhibited negligible-to-low levels of immunostaining in vertebral column osteoblasts. Osteoblasts reacted with antibody SP11-II, which recognizes sodium channel II, displayed moderate levels of immunostaining. Antibody SP32-III, which recognizes sodium channel III, displayed negligible levels of staining in osteoblasts within vertebra centrum and neural arches. These results demonstrate that osteoblasts in situ within E17 vertebral columns express sodium channel II mRNA and protein. Together with previous electrophysiological observations, the present results suggest that functional sodium channels are expressed in osteoblasts in vivo. These results extend the range of non-neuronal cells known to express rat brain sodium channels.