Molecular and histological characteristics of bovine caudal nucleus pulposus by combined changes in hydrostatic and osmotic pressures in vitro

Intervertebral disc degeneration is ubiquitous among aging patients, and altered matrix homeostasis is one of the key features of this condition. Physicochemical stresses have a significant impact on matrix homeostasis as they lead to progressive degeneration and may be associated with spinal pain and dysfunction. Thus, it is important to understand the cellular and matrix characteristics of nucleus pulposus in response to these stresses, which include hydrostatic and osmotic pressures during alternate loading conditions. We hypothesized that a combination of changes in hydrostatic pressure and in osmotic pressure that mimic normal, daily spinal stress would stimulate anabolic function, whereas a non‐realistic combination of those stresses would stimulate catabolic function in nucleus pulposus cells. We examined the effects of these combined stresses, represented by 12 systematic conditions, on the metabolic activities of enzymatically isolated bovine caudal nucleus pulposus in vitro. We measured the gene expression of extracellular matrix (ECM) molecules and proliferating cell nuclear antigen (PCNA) and evaluated the quality of the matrix and the capability of cell proliferation immunohistologically. Combined cyclic hydrostatic pressure at 0.5 MPa, 0.5 Hz, and high osmotic pressure at 450 mOsm upregulated the aggrecan core protein and collagen type‐II gene expression significantly (p < 0.05), and showed trends of upregulation of chondroitin sulfate N‐acetylgalactosaminyltransferase 1, matrix metalloproteinase‐13, and PCNA. ECM, however, contained empty spaces at a high osmotic pressure with and without hydrostatic pressure. Since ECM has highly specialized physicochemical properties, homeostasis should involve not only phenotypic cellular behavior but also turnover of ECM. © 2018 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 37:466–476, 2019.