Difference in Energy Metabolism of Annulus Fibrosus and Nucleus Pulposus Cells of the Intervertebral Disc

Abstract

Low back pain is associated with intervertebral disc degeneration. One of the main signs of degeneration is the inability to maintain extracellular matrix integrity. Extracellular matrix synthesis is closely related to production of adenosine triphosphate (i.e. energy) of the cells. The intervertebral disc is composed of two major anatomical regions: annulus fibrosus and nucleus pulposus, which are structurally and compositionally different, indicating that their cellular metabolisms may also be distinct. The objective of this study was to investigate energy metabolism of annulus fibrosus and nucleus pulposus cells with and without dynamic compression, and examine differences between the two cell types. Porcine annulus and nucleus tissues were harvested and enzymatically digested. Cells were isolated and embedded into agarose constructs. Dynamically loaded samples were subjected to a sinusoidal displacement at 2 Hz and 15% strain for 4 h. Energy metabolism of cells was analyzed by measuring adenosine triphosphate content and release, glucose consumption, and lactate/nitric oxide production. A comparison of those measurements between annulus and nucleus cells was conducted. Annulus and nucleus cells exhibited different metabolic pathways. Nucleus cells had higher adenosine triphosphate content with and without dynamic loading, while annulus cells had higher lactate production and glucose consumption. Compression increased adenosine triphosphate release from both cell types and increased energy production of annulus cells. Dynamic loading affected energy metabolism of intervertebral disc cells, with the effect being greater in annulus cells.

FIGURE 1

Schematic of the bioreactor system.

FIGURE 2

Comparison of ATP release, total ATP content, lactate production, glucose consumption, and NO production between the AF and NP control groups. Differences were only observed for total ATP content and glucose consumption, where total ATP content was higher for NP cells, and glucose consumption was higher for AF cells. *p < 0.05.

FIGURE 3

Comparison of ATP release, total ATP content, lactate production, glucose consumption, and NO production between the AF and NP loading groups. Differences were observed in all measurements, where NP had higher total ATP content and higher ATP release, and AF had higher glucose consumption, lactate production, and NO production. *p < 0.05.

FIGURE 4

Effects of dynamic loading on ATP release, total ATP content, lactate production, glucose consumption, and NO production of AF cells. Dynamic loading promoted ATP release, total ATP, lactate production, NO production, and glucose consumption. *p < 0.05.

FIGURE 5

Effects of dynamic loading on ATP release, total ATP content, lactate production, glucose consumption, and NO production of NP cells. Dynamic loading promoted ATP release and NO production. *p < 0.05.