The effect of extracellular pH on matrix turnover by cells of the bovine nucleus pulposus

Abstract

IntroductionNucleus pulposus cells are solely responsible for the production and maintenance of the extracellular matrix of the nucleus throughout life [31]. The cells have no direct contact with each other and rely on cell-matrix interactions and signals from the environment to maintain the function of the tissue. The cells synthesize all the matrix components [10,37,38,50] and also produce degradative enzymes including metalloproteinases (MMPs) and their inhibitors (tissue inhibitors of metalloproteinases or TIMPs), involved in matrix turnover [7,45,53]. The fine balance between synthesis of components and the breakdown of tissue determines the composition of the matrix; tissue integrity is thus dependent on the activity of the nucleus pulposus cells [53]. In vivo [20,43] and in vitro [14] environmental factors such as mechanical stress [30,34] and nutrient levels have been found to affect matrix composition, presumably by affecting macromolecule biosyntheAbstract It has long been known that very acidic conditions can be found in degenerate discs. The effect of these acid conditions on matrix turnover are, however, unknown. This study aimed to examine the effect of acidity on production of matrix components and on agents which break down the matrix in order to gain insight into the effect of pathological values of pH on matrix turnover. Cells were isolated from the nucleus of bovine discs and from bovine articular cartilage, embedded in alginate beads and cultured at pH levels maintained within the ranges seen in normal and pathological discs: pH 7.4-pH 6.3 for 48 h. Rates of sulphated glycosaminoglycan (GAG) and protein synthesis were measured, as well as rates of production of some agents involved in matrix breakdown, i.e. total and activated matrix metalloproteinases (MMPs) and their inhibitors (TIMPs). The results showed that acid conditions had a profound effect on cell matrix turnover; at pH 6.4, total production of most species measured was inhibited by more than 50% compared to production at pH 7.2; production of sulphated GAGs and of TIMP-1 fell by >90%. However production of active metalloproteinases by disc cells was relatively insensitive to pH, with activity at pH 6.3 not statistically different from that at pH 7.2. These findings indicate that exposure to acid conditions appears particularly deleterious for the disc matrix, as it inhibits the disc cells from synthesising functionally important molecules such as the sulphated GAGs but does not prevent the production of agents able to degrade matrix components. The low values of pH seen in some degenerate discs are thus likely to be involved in breakdown of the disc matrix.

Dynamic regulation of plasma matrix metalloproteinases in human diabetic ketoacidosis

“…Acidic pH directly stimulates the induction and release of cellular MMPs (32,33), and a lower pH is associated with markedly more MMP-8 and MMP-9 catalytic activity (33)(34)(35)(36). Thus, DKA patients by virtue of their blood's acidic pH are particularly susceptible to the deleterious actions of MMP-8 and MMP-9.…”

“…DKA correction with intravenous fluid and exogenous insulin administration normalizes the blood pH. While MMP-8 activity would decrease with increasing pH to normal values (34,36), MMP-9 still maintains much of its catalytic activity (33,35). In contrast, MMP-2 is noted to have decreased activity in acidic pH conditions (37), perhaps resulting in further reduced MMP-2 functional capacity.…”

Tissue engineering and the intervertebral disc: the challenges

“…In addition the presence of the scaffold can influence the recipient tissue reaction after implantation. For example the polylactic/glycolic acid systems used currently for tissue engineering purposes release acid as they degrade and the lowered pH can affect cell synthesis [88].…”

Computational Study of Culture Conditions and Nutrient Supply in Cartilage Tissue Engineering

“…Most modeling studies consider only one metabolite, most often oxygen. It has been shown, however, that GAG synthesis is affected by nutrient availability (25), pH (26), and oxygen (27)(28)(29)(30)(31)(32)(33)(34). Complicated interactions exist between chondrocyte energy balance and matrix metabolism, and the precise regulation mechanisms remain largely unknown (28,(35)(36)(37)(38)(39)(40).…”