Priming as a strategy to overcome detrimental pH effects on cells for intervertebral disc regeneration

Abstract

Disc disease is characterised by degeneration of the nucleus pulposus (NP), the central gelatinous tissue of the intervertebral disc (IVD). As degeneration progresses, the microenvironment of the IVD becomes more hostile (i.e. decrease in oxygen, glucose and pH), providing a significant challenge for regeneration using cell-based therapies. Tissue engineering strategies such as priming cells or micro tissues with growth factors prior to implantation may overcome some of these issues by providing a pre-formed protective niche composed of extracellular matrix. The present study investigated the effect of priming on bone-marrow-derived stem cells (BMSCs) and articular chondrocytes (ACs) using transforming growth factor β3 (TGF-β3), cultured at different pH levels (pH 7.1, 6.8 and 6.5) representative of the in vivo disc microenvironment. Low pH was found to have a detrimental effect on both cell viability and matrix accumulation, which could be mitigated by priming cells using TGF-β3. Investigating the activation of the transmembrane acid-sensing ion channels (ASIC-1 and -3) showed an increased expression of ASIC-1 in BMSCs and ASIC-3 in ACs at lower pH levels post-priming. Metabolic activity in terms of lactic acid production was also found to be affected significantly by priming, whereas oxygen and glucose consumptions did not change considerably. Overall, the study demonstrated that cells could be equipped to sustain the harsh environment of the IVD and promote accumulation of NP-like matrix through priming. Such an approach may open new avenues to engineer tissues capable of sustaining challenging microenvironments such as those found in the IVD.

Consolidating and re‐evaluating the human disc nutrient microenvironment

“…In addition to assessing cell functionality under limited nutrition and acidity, studies have used these adjusted culture concentrations as a priming strategy to support survival and enhanced extracellular matrix (ECM) deposition under harsher microenvironments. 45 , 46 , 47 These studies are preliminary in vitro investigations and in vivo studies must be performed to further consolidate the efficacy of priming. Another method is preconditioning to improve cell differentiation into native phenotypes within the diseased microenvironment.…”

Dynamic loading leads to increased metabolic activity and spatial redistribution of viable cell density in nucleus pulposus tissue

“… 64 A promising approach to overcome the sensitivity issue is cellular priming, where cells are cultured in conditions that mimic the NP milieu. 65 Additionally, many biomaterials are currently being investigated for their potential use to protect the injected cells and regenerate the IVD. 66 Nevertheless, we demonstrate that dynamic loading can also affect cell metabolism and that physiological‐like loading as well as appropriate nutrient milieus must be applied in IVD organ and explant cultures when cellular therapies are explored.…”

“…Additionally, a gradient formation in cell viability with lower viability in the core was observed in the stem cell group after 21 days 64 . A promising approach to overcome the sensitivity issue is cellular priming, where cells are cultured in conditions that mimic the NP milieu 65 . Additionally, many biomaterials are currently being investigated for their potential use to protect the injected cells and regenerate the IVD 66 .…”

Editorial – Disc Biology Special Issue

“…Panebianco et al (2020) reviewed the state of the art in injectable biomaterial cell carriers, highlighting the important need for next-generation biomaterials that can provide both significant initial biomechanical restoration and a functional niche permitting biological function of delivered cells. Gansau and Buckley (2021) studied the important challenge of injected cell survival in the acidic microenvironment of the degenerated IVD. They showed that priming bone-marrow-derived stem cells (BMSCs) and articular chondrocytes (ACs) with transforming growth factor (TGF)-β3 mitigated the detrimental effects of low pH on cell viability and matrix accumulation.…”