Influence of Genetic Background and Sex on Gene Expression in the Mouse (Mus musculus) Tail in a Model of Intervertebral Disc Injury

Gene Expression Changes Precede Elevated Mechanical Sensitivity in the Mouse Intervertebral Disc Injury Model

Background

Back pain after intervertebral disc (IVD) injury is a common clinical problem. Previous work examining early molecular changes post injury mainly used a candidate marker approach.

Methods

In this study, gene expression in the injured and intact mouse tail IVDs was determined with a nonbiased whole transcriptome approach and related to subsequent pain behavior. Mouse tail IVD injury was induced by a needle puncture. Whole murine transcriptome was determined by RNASeq. Transcriptomes of injured IVDs were compared with those of intact controls by bioinformatic methods. Mechanical allodynia was assessed by the Von Frey method.

Results

Among the 17,722 murine genes with meaningful expressions, 7242 genes were differentially expressed (P.adj ⟨ 0.01). Ontology study of upregulated genes revealed that leukocyte migration was the most enriched biological process, and network analysis showed that Tnfa had the most protein-protein interactions. The most enriched downregulated pathways were related to the pattern specification process. Mechanical allodynia persisted at the 4-week end point.

Conclusion

The RNASeq data revealed numerous early genes that participate in inflammation and repair processes post IVD injury. Mechanical allodynia followed these gene expression changes.

Sex-specific divergences in the types and timing of infiltrating immune cells during the intervertebral disc acute injury response and their associations with degeneration

OBJECTIVE

Inadequate repair of the intervertebral disc (IVD) contributes to low back pain. Infiltrating immune cells into damaged tissues are critical mediators of repair, yet little is known about the identities, roles, and temporal regulation following IVD injury. By analyzing transcripts of immune cell markers, histopathologic analysis, immunofluorescence, and flow cytometry, we aimed to define the temporal cascade of infiltrating immune cells and their associations with IVD degeneration.

METHODS

Caudal IVDs from 12-week-old C57BL6/J mice were injured and monitored for 42 days post-injury. Transcriptional markers identifying myeloid, B, and T immune cells, and angiogenic factors were measured from the IVDs every 2-3 days. Histopathologic degeneration of the IVD was measured throughout. Flow cytometry and immunofluorescence were used to identify and localize cells including B, T, natural killer T (NKT) cells, monocytes, neutrophils, macrophages, eosinophils, and dendritic cells.

RESULTS

The injured IVD revealed distinct phases of inflammation and proliferation. Robust temporal oscillation in the myeloid and T cell transcripts was observed in females. Cd3+ T cells were more abundant in females than in males. The Cd3+Cd4-Cd8- T cells that dominate the female cascade contain rare γδ T cells. Injury-mediated degeneration was prevalent in both sexes but more severe in males.

CONCLUSIONS

This study defines the coordinated infiltration of immune cells in the IVD following injury. We report the discovery of γδ T cells in the female IVD, and this was associated with less severe degeneration. γδ T cells have potent anti-inflammatory roles and may suppress degeneration following IVD injury.

Analysis of Infiltrating Immune Cells Following Intervertebral Disc Injury Reveals Recruitment of Gamma-Delta (γδ) T cells in Female Mice

Inadequate repair of injured intervertebral discs (IVD) leads to degeneration and contributes to low back pain. Infiltrating immune cells into damaged musculoskeletal tissues are critical mediators of repair, yet little is known about their identities, roles, and temporal regulation following IVD injury. By analyzing longitudinal changes in gene expression, tissue morphology, and the dynamics of infiltrating immune cells following injury, we characterize sex-specific differences in immune cell populations and identify the involvement of previously unreported immune cell types, γδ and NKT cells. Cd3+Cd4-Cd8- T cells are the largest infiltrating lymphocyte population with injury, and we identified the presence of γδ T cells in this population in female mice specifically, and NKT cells in males. Injury-mediated IVD degeneration was prevalent in both sexes, but more severe in males. Sex-specific degeneration may be associated with the differential immune response since γδ T cells have potent anti-inflammatory roles and may mediate IVD repair.

Gene Expression Profiles Perturbed by Injury to the Mouse Intervertebral Disc

OBJECTIVES

Back pain subsequent to intervertebral disc injury is a common clinical problem. Previous work examining early molecular changes post injury mainly used a candidate marker approach. In this study, gene expression in the injured and intact mouse tail intervertebral discs was determined with a nonbiased whole transcriptome approach.

DESIGN

Mouse tail intervertebral disc injury was induced by a needle puncture. Whole murine transcriptome was determined by RNASeq. Transcriptomes of injured intervertebral discs were compared with those of intact controls by bioinformatic methods.

RESULTS

Among the 18,078 murine genes examined, 592 genes were differentially expressed ( P.adj < 0.01). Novel genes upregulated in injured compared with intact intervertebral discs included Chl1, Lum , etc. Ontology study of upregulated genes revealed that leukocyte migration was the most enriched biological process, and network analysis showed that Tnfa had the most protein-protein interactions. Novel downregulated genes in the injured intervertebral discs included 4833412C05Rik , Myoc , etc . The most enriched downregulated pathways were related to cytoskeletal organization.

CONCLUSIONS

Novel genes highly regulated after disc injury were identified with an unbiased approach; they may serve as biomarkers of injury and response to treatments in future experiments. Enriched biological pathways and molecules with high numbers of connections may be targets for treatments after injury.

Tnfa , Il6 , Cxcl1 , and Adam8 Genes Are the Early Markers After Mouse Tail Intervertebral Disc Injury

OBJECTIVES

The early molecular events after intervertebral disc injury remain unclear. In this study, we aimed to compare inflammatory markers from 1 day to 4 wks after injury to have a comprehensive understanding of the intervertebral disc response to injury.

DESIGN

Mouse tail intervertebral disc injury was induced by a needle puncture. Inflammatory marker gene expression and morphological changes were recorded at 1 day, 1 wk, and 4 wks after injury.

RESULTS

Tnfa , Il6 , and Cxcl1 gene expression peaked at day 1 post-needle puncture of the mouse intervertebral disc, Adam8 gene expression peaked at 1-wk time point, while Tipe2 gene expression was upregulated at week 4 postinjury. F4/80 positive cells, likely to be macrophages, are present as early as day 1 in the injured intervertebral discs and consistently present at week 4 postinjury. Loss of Safranin O staining and increased histological scores of the injured intervertebral discs are consistent with progressive degeneration after injury.

CONCLUSIONS

Inflammatory cytokines including Tnfa precede Tipe2 , suggesting that Tipe2 is likely induced by Tnfa . Upregulation of Adam8 and Cxcl1 gene expression persisted at week 4, suggesting that they play a role in the transition to chronic phase of intervertebral disc degeneration.

Expression of Human Interleukin 8 in Mice Alters Their Natural Behaviors

Objective

To examine the effects of human interleukin (IL) 8 expression on mouse behavior.

Methods

A mouse line expressing human IL8 in the intervertebral discs (IVD) and cartilaginous tissues (hIL8⁺ ) was generated. Mouse spontaneous behaviors, including locomotion, climbing, rearing, grooming, eating, drinking, and immobility were recorded with a fully automatic, non-invasive platform.

Results

Distance traveled by the hIL8⁺ mice declined with age compared with control littermates, and male hIL8⁺ mice traveled a shorter distance than male controls and females of either genotype (p <0.05). The hIL8⁺ mice also spent less time in locomotion than control mice (p <0.01), and male hIL8⁺ mice spent the least amount of time and had lowest count in locomotion compared with the other 3 groups at 12 weeks of age or greater (p <0.05). The hIL8⁺ mice spent less time climbing than controls, and male mice spent less time climbing than female mice of the same genotype (p <0.01). The hIL8⁺ mice spent more time eating and less time drinking than controls, and all mice spent less time eating and more time drinking with increasing age. Finally, hIL8⁺ mice spent more time immobile than controls, and male hIL8⁺ mice spent more time immobile than any other group (p <0.05).

Conclusion

The hIL8⁺ mice, especially hIL8⁺ males, showed reduced ambulation and climbing. Mice showed age-related decrease in eating and increase in drinking and grooming time that was also influenced by expression of hIL8. These changes in natural behaviors in control mice are consistent with functional decline with age. Effects of hIL8 superimposed on the natural aging process could involve systemic (e.g., on the brain) and local (e.g., in the spine and joint tissues) mechanisms. Future exploration of these mechanisms might be productive.

The Inner Annulus Fibrosus Encroaches on the Nucleus Pulposus in the Injured Mouse Tail Intervertebral Disc

OBJECTIVE

The aim was to identify the source of cells within the center of the abnormal fibrocartilage tissue of the degenerative intervertebral disc after injury.

DESIGN

Cross-breeding of mice with an inducible type II promoter collagen construct (Col2CreER) to Rosa26-TdTomato mice has been shown to result in Cre-recombinase activity and Tomato expression in inner annulus fibrosus cells after tamoxifen injection. To investigate the role of the inner annulus fibrosus in the intervertebral disc injury response, tail intervertebral discs of Col2CreER/tdTomato mice were punctured with a needle and examined 1-4 wks after injury. N-cadherin was examined by immunostaining.

RESULTS

After the injury, the fibrocartilage in the degenerative intervertebral disc consisted of residual diseased nucleus pulposus cells and encroaching inner annulus fibrosus cells. The residual nucleus pulposus cells had lost their epithelial cell-like morphology and instead became oval shaped, with reduced adhesion to neighboring nucleus pulposus cells. This change in cellular morphology coincided with a loss of N-cadherin, which contributes to maintenance of healthy nucleus pulposus cell morphology. As expected, injured tail intervertebral discs showed reduced compressive properties as determined by biomechanical assessments.

CONCLUSIONS

The cellular composition of the degenerative intervertebral disc has been defined here, which is an important step in developing future treatments.

Elevated inflammatory gene expression in intervertebral disc tissues in mice with ADAM8 inactivated

We found ADAM8 enzymatic activity elevated in degenerative human intervertebral disc (IVD). Here, we examined the discs in ADAM8-inactivation mice that carry a mutation preventing self-activation of the enzyme. Surprisingly, elevated gene expression for inflammatory markers (Cxcl1, IL6) was observed in injured discs of ADAM8 mutant mice, along with elevated expression of type 2 collagen gene (Col2a1), compared with wild type controls. Injured annulus fibrosus of mutant and wild type mice contained a higher proportion of large collagen fibers compared with intact discs, as documented by microscopic examination under circular polarized light. In the intact IVDs, Adam8EQ mouse AF contained lower proportion of yellow (intermediate) fiber than WT mice. This suggests that ADAM8 may regulate inflammation and collagen fiber assembly. The seemingly contradictory findings of elevated inflammatory markers in mutant mice and excessive ADAM8 activity in human degenerative discs suggest that ADAM8 may interact with other enzymatic and pro-inflammatory processes needed for tissue maintenance and repair. As a future therapeutic intervention to retard intervertebral disc degeneration, partial inhibition of ADAM8 proteolysis may be more desirable than complete inactivation of this enzyme.

TNFAIP8 family gene expressions in the mouse tail intervertebral disc injury model

INTRODUCTION

The TNF-α-induced protein-8 (TNFAIP8, also known as TIPE) family of molecules comprises four members: TNFAIP8 and TIPEs1-3. Since the first description of these proteins, their roles in fine-tuning inflammation and in directing leukocyte migration have been described in several organ systems. However, their relationship with intervertebral disc (IVD) is unknown.

MATERIALS AND METHODS

Here, we describe the expression of TNFAIP8 family genes in the nucleus pulposus (NP) and annulus fibrosus (AF) of the normal adult murine IVD. We further describe the expression of these genes in the injured male and female murine IVD.

RESULTS

Tnfaip8 gene expression was decreased, and Tipe1 gene expression was essentially unchanged, in response to injury. Tipe2 and Tipe3 gene expression was markedly elevated in response to IVD injury, along with those encoding known inflammatory markers (ie, Tnfa, Il6, Cxcl1, and Adam8). Additionally, sex-related differences were also observed for some of these genes in intact and injured mouse IVDs. Future studies include examining tissue distribution of TNFAIP8 family proteins and identifying cells that produce them. In addition, examining mice that are deficient in TNFAIP8 molecules, in relation to gene expression, tissue morphology and mouse behavior, may further delineate the roles of these molecules in IVD inflammation and degeneration.