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.