Novel Roles of Transcription Factor TonEBP in Nucleus Pulposus Cells of the Intervertebral Disc
The intervertebral discs are essential for upright posture and flexibility of the spine. The inner nucleus pulposus (NP) of these discs distributes loads axial applied to the spine. This mechanical function depends on the abundant extracellular matrix of the NP, which is rich in proteoglycans and collagens that attract and hold water. Thus, resident NP cells must withstand an environment of high osmolarity, which fluctuates with diurnal water changes. The transcription factor Tonicity enhancer binding protein (TonEBP/NFAT5) is known to promote transcription of genes involved in transport and synthesis of non-ionic osmolytes to prevent apoptosis in NP and other cells exposed to hyperosmotic challenges. However, understanding of the broader role of TonEBP in the disc is limited. There is evidence that TonEBP may also play a role in inflammatory conditions, such as intervertebral disc degeneration (IDD). In severe IDD cases, matrix deterioration contributes to loss of biomechanical function and subsequent pain. While many factors contribute to pathology of IDD, inflammatory cytokines, chemokines, and downstream matrix catabolism, cellular senescence, and apoptosis play an important role. In immune and other cell types, TonEBP is activated by non-osmotic inflammatory stimuli and promotes production of cytokines and inflammatory disease progression. We combined unbiased RNA-sequencing, molecular methods, and animal models to better elucidate the role of TonEBP in the healthy and degenerative disc environments. In NP cells, analysis of sequencing results and ex vivo organ culture disc models revealed that several genes associated with inflammation were induced by transient hyperosmolarity in a TonEBP-dependent manner. For a subset of genes, osmotic induction also required activity of NF-κB, but we were unable to detect binding between Rel family members TonEBP and p65. Treatment with a cytokine commonly increased in IDD, TNF-α, activated TonEBP via increased nuclear abundance. This activation of TonEBP had no effect on its ability to modulate osmo-regulatory targets but did promote the pro-inflammatory response to TNF-α. Evaluating the role of TonEBP in NP tissue development, we were surprised to find that gross morphology and cellularity of the tissue did not depend on TonEBP activity up to E18.5. Together, the results herein provide a more complete understanding of the roles of TonEBP under various stimuli including regulation of inflammation-related genes under hyperosmotic and inflammatory conditions.
Johnson, Zariel I, "Novel Roles of Transcription Factor TonEBP in Nucleus Pulposus Cells of the Intervertebral Disc" (2017). ETD Collection for Thomas Jefferson University. AAI10827021.