Identification of key genes associated with the effect of osmotic stimuli on intervertebral discs using microarray analysis

The present study aimed to explore the effect of osmotic stimuli on intervertebral discs (IVDs) using microarray analysis. Gene expression dataset GSE1648 was downloaded from the Gene Expression Omnibus database. There were 11 IVD cell samples in this dataset, which included 4 hyperosmotic stimuli samples, 3 hypoosmotic stimuli samples and 4 isosmotic stimuli samples. The differentially expressed genes (DEGs) in hyperosmotic or hypoosmotic IVD cells (designated DEGs-hyper or DEGs-hypo) were identified, compared with isosmotic cells, using the limma package of R software. The Database for Annotation, Visualization and Integrated Discovery was used to perform a Gene Ontology (GO) term enrichment analysis for the DEG sets. Protein-protein interaction (PPI) network and microRNA (miRNA) gene-regulatory network data for the DEG sets were obtained using the Human Protein Reference Database (HPRD) and the TargetScan database, respectively, and these networks were constructed and visualized using Cytoscape software. There was a total of 43 DEGs in DEGs-hyper and 9 in DEGs-hypo. Analysis of DEGs-hyper revealed that 41 GO terms were significantly enriched. In total, 376 pairs and 382 nodes were involved in the PPI network, and 1,314 miRNA-gene pairs and 422 nodes were contained in the miRNA-gene-regulated network. The results of the present study indicated that potential target genes (including NCOA3, SOS1, XPO1, ZBTB18, EFNB2 and SOBP) may be involved in the effect of osmotic stimuli on IVD, and the biological processes of apoptosis and cell death may be associated with the effect of high osmolality on IVD disease. The potential targets identified in the present study are more reliable than those identified by previous studies.

Deficiency in the α1 subunit of Na+/K+-ATPase enhances the anti-proliferative effect of high osmolality in nucleus pulposus intervertebral disc cells

Intervertebral disc cells are constantly exposed to a hyperosmotic environment. Among cellular responses towards this stress is the inhibition of proliferation through the activation of p38 MAPK and p53. In an effort to further elucidate the biochemical pathways triggered by hyperosmotic stress, we assessed the high osmolality-induced transcriptional changes of bovine nucleus pulposus cells using whole-genome arrays. A 5- and a 24-h hyperosmotic treatment led to the differential expression of >100 and >200 genes, respectively, including nine genes encoding transporters (SLC4A11, SLC5A3, ATP1A1, SLC38A2, KCNK17, KCTD20, KCTD11, SLC7A5, and CLCA2). Differences in the transcriptional profile of these selected genes, as indicated by the microarrays experiments, were validated by qRT-PCR in 2D and 3D cell cultures, under hyperosmolar salt and sorbitol conditions, revealing the presence of a common triggering signal for osmotic adaptation. The key signaling molecules p38 MAPK and p53 were demonstrated to differently participate in the regulation of the aforementioned transporters. Finally, siRNA-mediated knocking-down of each one of the three transporters with the highest and sustained over-expression (i.e., SLC4A11, SLC5A3, and ATP1A1) had a distinct outcome on the transcriptional profile of the other transporters, on p38 MAPK and p53 phosphorylation and consequently on cell cycle progression. The inhibition of ATP1A1 had the most prominent effect on the transcription of the rest of the transporters and was found to enhance the anti-proliferative effect of hyperosmotic conditions through an increased G2/M cell cycle block, ascribing to this pump a central role in the osmoregulatory response of nucleus pulposus cells.