Edited by Dennis VoelkerTranscription factor tonicity-responsive enhancer-binding protein (TonEBP/NFAT5) is critical for osmo-adaptation and extracellular matrix homeostasis of nucleus pulposus (NP) cells in their hypertonic tissue niche. Recent studies implicate TonEBP signaling in inflammatory disease and rheumatoid arthritis pathogenesis. However, broader functions of TonEBP in the disc remain unknown. RNA sequencing was performed on NP cells with TonEBP knockdown under hypertonic conditions. 1140 TonEBP-dependent genes were identified and categorized using Ingenuity Pathway Analysis. Bioinformatic analysis showed enrichment of matrix homeostasis and cytokine/ chemokine signaling pathways. C-C motif chemokine ligand 2 (CCL2), interleukin 6 (IL6), tumor necrosis factor (TNF), and nitric oxide synthase 2 (NOS2) were studied further. Knockdown experiments showed that TonEBP was necessary to maintain expression levels of these genes. Gain-and loss-of-function experiments and site-directed mutagenesis demonstrated that TonEBP binding to a specific site in the CCL2 promoter is required for hypertonic inducibility. Despite inhibition by dominant-negative TonEBP, IL6 and NOS2 promoters were not hypertonicity-inducible. Whole-disc response to hypertonicity was studied in an ex vivo organ culture model, using wild-type and haploinsufficient TonEBP mice. Pro-inflammatory targets were induced by hypertonicity in discs from wild-type but not TonEBP-haploinsufficient mice. Mechanistically, NF-B activity increased with hypertonicity and was necessary for hypertonic induction of target genes IL6, TNF, and NOS2 but not CCL2. Although TonEBP maintains transcription of genes traditionally considered pro-inflammatory, it is important to note that some of these genes also serve anabolic and pro-survival roles. Therefore, in NP cells, this phenomenon may reflect a physiological adaptation to diurnal osmotic loading of the intervertebral disc.The intervertebral disc is well suited to fulfill its mechanical role in the human spine, where it permits flexion and rotation, and absorbs compressive loads (1). The matrix-rich nucleus pulposus (NP) 2 at the center of the disc gives the tissue its ability to resist compression through high osmotic swelling pressure (2-4), loss of which correlates with degeneration and back pain (5). Although the high fixed charge density of the aggrecan-rich matrix allows the nucleus pulposus its water-imbibing properties, it also results in a hypertonic environment for NP cells. Importantly, tonicity of the extracellular environment fluctuates widely with diurnal cycle: water is forced out of the disc during the day when the spine is loaded and imbibed during the unloaded phase at night (6).In mammalian cells, a key transcription factor TonEBP (NFAT5) is activated by elevated hypertonicity and promotes transcription of genes that produce or transport organic osmolytes (7). In addition, TonEBP controls transcription of several genes that are important for cell survival under hypertonic conditions independ...