Morphological similarities after compression trauma of bovine and human intervertebral discs: Do disc cells have a chance of surviving?

Alterations of bovine nucleus pulposus cells with aging

Aging is one of the major etiological factors driving intervertebral disc (IVD) degeneration, the main cause of low back pain. The nucleus pulposus (NP) includes a heterogeneous cell population, which is still poorly characterized. Here, we aimed to uncover main alterations in NP cells with aging. For that, bovine coccygeal discs from young (12 months) and old (10-16 years old) animals were dissected and primary NP cells were isolated. Gene expression and proteomics of fresh NP cells were performed. NP cells were labelled with propidium iodide and analysed by flow cytometry for the expression of CD29, CD44, CD45, CD146, GD2, Tie2, CD34 and Stro-1. Morphological cell features were also dissected by imaging flow cytometry. Elder NP cells (up-regulated bIL-6 and bMMP1 gene expression) presented lower percentages of CD29+, CD44+, CD45+ and Tie2+ cells compared with young NP cells (upregulated bIL-8, bCOL2A1 and bACAN gene expression), while GD2, CD146, Stro-1 and CD34 expression were maintained with age. NP cellulome showed an upregulation of proteins related to endoplasmic reticulum (ER) and melanosome independently of age, whereas proteins upregulated in elder NP cells were also associated with glycosylation and disulfide bonds. Flow cytometry analysis of NP cells disclosed the existence of 4 subpopulations with distinct auto-fluorescence and size with different dynamics along aging. Regarding cell morphology, aging increases NP cell area, diameter and vesicles. These results contribute to a better understanding of NP cells aging and highlighting potential anti-aging targets that can help to mitigate age-related disc disease.

Programmed NP Cell Death Induced by Mitochondrial ROS in a One-Strike Loading Disc Degeneration Organ Culture Model

Increasing evidence has indicated that mitochondrial reactive oxygen species (ROS) play critical roles in mechanical stress-induced lumbar degenerative disc disease (DDD). However, the detailed underlying pathological mechanism needs further investigation. In this study, we utilized a one-strike loading disc degeneration organ culture model to explore the responses of intervertebral discs (IVDs) to mechanical stress. IVDs were subjected to a strain of 40% of the disc height for one second and then cultured under physiological loading. Mitoquinone mesylate (MitoQ) or other inhibitors were injected into the IVDs. IVDs subjected to only physiological loading culture were used as controls. Mitochondrial membrane potential was significantly depressed immediately after mechanical stress (P < 0.01). The percentage of ROS-positive cells significantly increased in the first 12 hours after mechanical stress and then declined to a low level by 48 hours. Pretreatment with MitoQ or rotenone significantly decreased the proportion of ROS-positive cells (P < 0.01). Nucleus pulposus (NP) cell viability was sharply reduced at 12 hours after mechanical stress and reached a stable status by 48 hours. While the levels of necroptosis- and apoptosis-related markers were significantly increased at 12 hours after mechanical stress, no significant changes were observed at day 7. Pretreatment with MitoQ increased NP cell viability and alleviated the marker changes by 12 hours after mechanical stress. Elevated mitochondrial ROS levels were also related to extracellular matrix (ECM) degeneration signs, including catabolic marker upregulation, anabolic marker downregulation, increased glycosaminoglycan (GAG) loss, IVD dynamic compressive stiffness reduction, and morphological degradation changes at the early time points after mechanical stress. Pretreatment with MitoQ alleviated some of these degenerative changes by 12 hours after mechanical stress. These changes were eliminated by day 7. Taken together, our findings demonstrate that mitochondrial ROS act as important regulators of programmed NP cell death and ECM degeneration in IVDs at early time points after mechanical stress.

Morphological changes in the human cervical intervertebral disc post trauma: response to fracture-type and degeneration grade over time

PURPOSE

In the first 24 h post-intervertebral disc (IVD) trauma, up to 75 % cell death has been reported. In addition, burst fractures cause post-traumatic disc degeneration by elevated pro-apoptotic and pro-inflammatory gene transcription. Moreover, some patients have pre-trauma degenerative disc disease. The aim of the study was to assess histological changes and cell-death over a time period of up to 1 year caused by mechanical and structural factors.

METHODS

116 anterior portions of IVDs of the cervical spine were studied histologically by light microscopy and ultrastructurally by transmission electron microscopy (TEM). The group was investigated with regard to three main parameters: fracture mechanism (compressive vs. tensile/shear loads), degeneration grade (low vs. high) and endplate fracture (with vs. without). Disc architecture (e.g. ruptures) was studied histologically. Cell morphology was examined ultrastructurally to quantify cell-death, healthy and balloon cells. According to ultrastructural observations, two time-groups (up to 6 days vs. later) were established. Statistical analyses were carried out within and between time-groups.

RESULTS

Histological changes were obvious in the annulus fibrosus where ruptures with haematoma were replaced by granulation tissue. Significant differences in cell-death were seen in the first few days due to different loads. In contrast to the more degenerated segments, low degenerated ones revealed significantly less cell death with time post-trauma. Interestingly, no difference was found between groups after the sixth day. Cell-death (mean 44 % for all investigated groups) remained high after day 6 post-trauma.

CONCLUSION

IVDs retrieved from low grade degenerated segments revealed a significant recovery, with less cell-death and a partially restored disc matrix, although cell-death remained high. Long-term clinical studies of stabilized segments arising from different fracture mechanisms are required.