Influence of extracellular osmolarity and mechanical stimulation on gene expression of intervertebral disc cells

Abstract

Intervertebral discs (IVD) have a higher extracellular osmolarity than most other tissues; moreover their osmolarity changes by around 25% during each diurnal cycle. In this study, changes in aggrecan, collagen I and collagen II expression of IVD cells were examined after exposure to osmotic environment alterations or mechanical stimulation under different osmotic conditions. Human and bovine IVD cells seeded in three-dimensional (3D) collagen type I matrices were cultured under hypo-osmotic (300 mOsm), iso-osmotic (400 mOsm), or hyperosmotic (500 mOsm) conditions. Osmolarity-induced changes in gene expression of IVD cells were measured after 5 days. Loadinduced changes in gene expression under the different osmotic conditions were measured after application of hydrostatic pressure (0.25 MPa, 0.1 Hz, 30 min) or cyclic strain (4%, 1 Hz, 24 h). The results showed that IVD cells respond strongly to changes in the osmotic environment by altering mRNA expression. Human cells cultured over 5 days increased expression of aggrecan and collagen II in both nucleus and annulus cells under increasing osmolarity. In contrast, collagen I expression was inhibited at high osmolarity in both cell types. Mechanically induced alterations in gene expression appear to have only modest effects on matrix protein expression, but the same stimulus partly resulted in an inhibition or stimulation of gene expression, depending on the osmotic conditions. This study showed that the osmotic environment does not only have an appreciable effect on gene expression but also affects responses to mechanical stimuli. This suggests that the osmotic conditions cannot be ignored when examining physiological and pathological behavior of IVD cells.

The effects of dynamic loading on the intervertebral disc

“…Cell volume changes could affect various membrane ion transporters and alter cell metabolism, resulting in a different biosynthetic activity of the cell [35,116]. Furthermore, studies have shown that osmolality and pH directly affect IVD cell metabolism [12,18,27,37,115,129].…”

“…When NP cells were cultured in various osmotic conditions and exposed to the same IHP, hypo-and hyper-osmotic condition resulted in an anabolic and catabolic response, respectively [129]. This may imply that the initial volume of the cells affects their response to mechanical stimulus.…”

Effect of varying osmotic conditions on the response of bovine nucleus pulposus cells to growth factors and the activation of the ERK and Akt pathways

“…300 mOsm/kg H 2 O are hypo-osmotic for nucleus pulposus cells in vivo and characterize degenerated discs. 17 Extracellular osmolality of 400 mOsm/kg H 2 O, although high for cells deriving from the majority of the tissues, is considered as normal for disc cells, which are surrounded by a cation-attracting extracellular matrix. 6 500 mOsm/kg H 2 O represent hyper-osmotic conditions for disc cells, being the outcome of fluid loss due to daily activities.…”

Recent advances in annular pathobiology provide insights into rim-lesion mediated intervertebral disc degeneration and potential new approaches to annular repair strategies

“…These include the presence of growth factors, changes in local biomechanical stresses and in the osmotic environment of the disc cell [44,47,72,98,143,144,157]. Stimulation of endogenous disc cell populations by administered growth factors and other biomimetics in slow delivery carriers within bioscaffolds aim to address some of these questions [53,85].…”