Reticulon 4 in chondrocytic cells: barosensitivity and intracellular localization

Members of the reticulon gene family are endoplasmic reticulum (ER)-related proteins expressed in various human tissues, but their molecular functions are not understood. The reticulon 4 subfamily consists of three members, reticulon 4/Nogo-A, -B and -C. Reticulon 4-A is under intense investigation because of its inhibitory effect on neurite outgrowth, and reticulon 4-B has been suggested to induce apoptosis. Reticulon 4-C, the shortest member of this subfamily, is the least characterized. Reticulons are presumably guided to endoplasmic reticulum by a putative N-terminal retention motif. In this study the expressions of reticulon 4 subtypes in human chondrosarcoma cell line and in primary bovine chondrocytes were analyzed on mRNA level. These cell types, exposed to strong mechanical forces in vivo, were subjected to high hydrostatic pressure and mechanical stretch to study the possible mechanosensitivity of reticulon 4 genes. In addition, a green fluorescent protein-tagged reticulon 4-C and a fusion protein with mutated endoplasmic reticulum retention signal were used to study the significance of the C-terminal translocation signal (the di-lysine motif). As the result, both cell types expressed the three main isoforms of reticulon 4 family. The steady-state level of reticulon 4-B mRNA was shown to be up-regulated by pressure, but not by mechanical stretch indicating transcriptional barosensitivity. The reticular distribution pattern of reticulon 4-C was observed indicating a close association with endoplasmic reticulum. Interestingly, this pattern was maintained despite of the disruption of the putative localization signal. This suggests the presence of another, yet unidentified endoplasmic reticulum retention mechanism.