Both endoplasmic reticulum and mitochondria are involved in disc cell apoptosis and intervertebral disc degeneration in rats

Intervertebral disc cell apoptosis occurs through either death receptor or mitochondrial pathway, but whether disc cell apoptosis is also mediated by the endoplasmic reticulum (ER) pathway remains unclear. The objective of this study was to investigate whether ER and mitochondria are co-involved in disc cell apoptosis and intervertebral disc degeneration (IVDD) in rats. Forty-eight rats were used for in vivo experiments. IVDD was characterized by X-ray and histomorphology examination, disc cell apoptosis was detected by TUNEL staining, and the co-involvement of ER and mitochondria in apoptosis was determined by immunohistochemical staining for GRP78, GADD153, caspase-12, and cytochrome C. Additional eight rats were used for annular cell isolation and culture. After sodium nitroprusside treatment, annular cell apoptosis was observed morphologically and quantified by flow cytometry; the expression of biomarkers of ER stress and mitochondrial dysfunction were analyzed by reverse transcriptase PCR (RT-PCR), fluorescence double labeling, and Western blot; and mitochondrial membrane potential was detected by 5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolcarbo cyanine iodide (JC-1) staining. Finally, NS3694 and Z-ATAD-FMK were employed to inhibit the formation of apoptosome complex and the activation of caspase-12, respectively, and apoptotic incidence and caspase-9 activity were assayed. We found that IVDD, induced by unbalanced dynamic and static forces in the rats, was accompanied by increased disc cell apoptosis and enhanced expression of GRP78, GADD153, caspase-12, and cytochrome C. Annular cell apoptosis induced by sodium nitroprusside was confirmed by morphologic observation and flow cytometry. With increased apoptosis, the expression of GRP78, GADD153, and caspase-12 upregulated, mitochondrial membrane potential decreased, and accumulation of cytochrome C in the cytosol manifested. Furthermore, NS3694 and Z-ATAD-FMK dramatically suppress annular cell apoptosis and caspase-9 activity. In conclusion, disc cell apoptosis mediated simultaneously by ER and mitochondria plays a potent role in IVDD.