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

Abstract

Background Nucleus pulposus (NP) cell density is orchestrated by an interplay between nutrient supply and metabolite accumulation. Physiological loading is essential for tissue homeostasis. However, dynamic loading is also believed to increase metabolic activity and could thereby interfere with cell density regulation and regenerative strategies. The aim of this study was to determine whether dynamic loading could reduce the NP cell density by interacting with its energy metabolism. Methods Bovine NP explants were cultured in a novel NP bioreactor with and without dynamic loading in milieus mimicking the pathophysiological or physiological NP environment. The extracellular content was evaluated biochemically and by Alcian Blue staining. Metabolic activity was determined by measuring glucose and lactate in tissue and medium supernatants. A lactate‐dehydrogenase staining was performed to determine the viable cell density (VCD) in the peripheral and core regions of the NP. Results The histological appearance and tissue composition of NP explants did not change in any of the groups. Glucose levels in the tissue reached critical values for cell survival (≤0.5 mM) in all groups. Lactate released into the medium was increased in the dynamically loaded compared to the unloaded groups. While the VCD was unchanged on Day 2 in all regions, it was significantly reduced in the dynamically loaded groups on Day 7 ( p ≤ 0.01) in the NP core, which led to a gradient formation of VCD in the group with degenerated NP milieu and dynamic loading ( p ≤ 0.05). Conclusion It was demonstrated that dynamic loading in a nutrient deprived environment similar to that during IVD degeneration can increase cell metabolism to the extent that it was associated with changes in cell viability leading to a new equilibrium in the NP core. This should be considered for cell injections and therapies that lead to cell proliferation for treatment of IVD degeneration.

Repetitive strikes loading organ culture model to investigate the biological and biomechanical responses of the intervertebral disc

“…proposed that dynamic loading acts as an additional stressor, leading to a decrease in nucleus pulposus (NP) cell density. 33 and high (0.1-1.2 MPa). They observed that cell viability remained above 80% at low and medium loads, while both nucleus pulposus (NP) and annulus fibrosus (AF) cell viability were impacted by high loads.…”

“…13,31,32 Salzer et al observed an increase in the metabolic activity of NP tissue with dynamic loading, coupled with a notable reduction in living cell density by the seventh day. 33 Kurakawa et al reported diminished cell density and viability in nucleus pulposus cells subjected to dynamic compression loading of rat intervertebral discs at 1.5 Hz. 34 collagen in loading conditions involving 1.3 MPa, 5.0 Hz, and 8 h. 35 Cui identified new epitope peptides formed by proteoglycan and collagen cleavage during IVD degeneration in one strike model, serving as promising biomarkers of early disc degeneration.…”

“…The IVD organ culture bioreactor has long been used in basic and translational research for its advantages of precise control in biochemical and biomechanical aspects and is an indispensable tool for bridging in vitro and in vivo models 13,31,32 . Salzer et al observed an increase in the metabolic activity of NP tissue with dynamic loading, coupled with a notable reduction in living cell density by the seventh day 33 . Kurakawa et al reported diminished cell density and viability in nucleus pulposus cells subjected to dynamic compression loading of rat intervertebral discs at 1.5 Hz 34 .…”

“…Conversely, dynamic loading had no discernible impact on cell viability in the disc 31 . Salzer et al proposed that dynamic loading acts as an additional stressor, leading to a decrease in nucleus pulposus (NP) cell density 33 . Rosenzweig et al assessed cell viability in an intervertebral disc bioreactor under three loading conditions: low (0.1–0.3 MPa), medium (0.1–0.3 MPa), and high (0.1–1.2 MPa).…”