Nutrient supply and nucleus pulposus cell function: effects of the transport properties of the cartilage endplate and potential implications for intradiscal biologic therapy

Abstract

Fourier transform infrared imaging Nutrient transport Nucleus pulposus cell Low back pain s u m m a r y Objective: Intradiscal biologic therapy is a promising strategy for managing intervertebral disc degeneration. However, these therapies require a rich nutrient supply, which may be limited by the transport properties of the cartilage endplate (CEP). This study investigated how fluctuations in CEP transport properties impact nutrient diffusion and disc cell survival and function. Design: Human CEP tissues harvested from six fresh cadaveric lumbar spines (38e66 years old) were placed at the open sides of diffusion chambers. Bovine nucleus pulposus (NP) cells cultured inside the chambers were nourished exclusively by nutrients diffusing through the CEP tissues. After 72 h in culture, depth-dependent NP cell viability and gene expression were measured, and related to CEP transport properties and biochemical composition determined using fluorescence recovery after photobleaching and Fourier transform infrared (FTIR) spectroscopy. Results: Solute diffusivity varied nearly 4-fold amongst the CEPs studied, and chambers with the least permeable CEPs appeared to have lower aggrecan, collagen-2, and matrix metalloproteinase-2 gene expression, as well as a significantly shorter viable distance from the CEP/nutrient interface. Increasing chamber cell density shortened the viable distance; however, this effect was lost for low-diffusivity CEPs, which suggests that these CEPs may not provide enough nutrient diffusion to satisfy cell demands. Solute diffusivity in the CEP was associated with biochemical composition: low-diffusivity CEPs had greater amounts of collagen and aggrecan, more mineral, and lower cross-link maturity. Conclusions: CEP transport properties dramatically affect NP cell survival/function. Degeneration-related CEP matrix changes could hinder the success of biologic therapies that require increased nutrient supply.

Matrix modification for enhancing the transport properties of the human cartilage endplate to improve disc nutrition

“…To determine if MMP-8 treatment improves nutrient diffusion through the CEP, we used diffusion chambers [17], which mimic the nutrient environment of the disc in vivo . Briefly, NP cells cultured inside the chamber obtain nutrients that diffuse through full-thickness CEP tissues at the open sides of the chamber (S1 Fig).…”

“…Proper disc nutrition involves nutrient and metabolite exchange between the nucleus pulposus (NP) cells and vertebral capillaries, and several factors can impair the normal patterns of solute exchange. For example, nutrients entering the disc and exiting metabolites must diffuse through the cartilage endplates (CEP), and diffusion could be hindered by age- or degeneration-related changes to the CEP matrix, including dehydration [14], mineralization [15–17], and fibrosis [14, 17, 18]. In addition to altering the mechanical functionality of the CEP [19], which is also physiologically important [20, 21], these matrix changes impact CEP biotransport functionality too.…”

“…Specifically, dehydration prevents solutes from diffusing freely within the CEP; and increased deposition of proteoglycan, collagen, and mineral limits the amount of pore space available to solutes [22, 23]. Indeed, we recently found that CEPs with higher amounts of collagen, aggrecan, and mineral hindered nutrient diffusion, thereby impairing NP cell survival and function [17]. This suggests that discs with deficient CEP transport properties may be poor candidates for biologic therapies that increase nutrient demands; it also motivates strategies for enhancing solute transport through the CEP to improve nutrition and cell survival.…”

“…Strategies for enhancing solute transport through the CEP are underexplored. Since solute-hindering CEPs had higher amounts of collagen and aggrecan [17], one potential strategy involves matrix modification to reduce collagen and aggrecan. To this end, several human and bacterial enzymes have activity for these matrix constituents, including collagenases, aggrecanases, and gelatinases.…”

Exosomes from normal cartilage endplate stem cells ameliorate intervertebral disc degeneration more effectively than exosomes from degenerated cartilage endplate stem cell by activating the AKT/autophagy pathway

“…Intervertebral disc degeneration (IVDD) is a common cause of lower back pain, limits activity [1] and is usually characterized by upregulation of matrix metalloproteinase (MMP) and proin ammatory cytokine expression, a reduction in the number of functional nucleus pulposus cells (NPCs) and anatomical and morphological changes [2,3]. The important mechanism by which IVDD occurs is through not only a reduction in the nutrient supply from cartilage endplates (CEP) to the inner layer of the annulus brosus and NPCs [4] but also weakened CEP-mediated regulation of IVDD-associated anabolism and catabolism [5], leading to NPCs senescence and apoptosis.…”