Paraspinal muscle degeneration and regenerative potential in a Murine model of Lumbar Disc Injury

Abstract

Background Context: The association between low back pain and lumbar disc degeneration is not fully characterized. One potentially overlooked factor of this process is the lumbar multifidus, which plays a role in segmental stabilization and locomotion. Previous reports have shown multifidus degeneration to be associated with disc degeneration. The goal of this study was to develop a mouse model of advanced lumbar disc degeneration to recapitulate the pathology of the human multifidus in patients with lumbar disc degeneration and low back pain.Methods: C57BL/6 mice underwent a left anterolateral approach to the lumbar spine and disc puncture with a micro scalpel at L5/6 and L6/S1. Mice underwent behavioral analysis and functional gait testing. A subset of mice underwent 14T T2-weighted MRI to assess disc degeneration and paraspinal muscle quality. At 6 and 15 weeks, mice were sacrificed, and the multifidus muscles were harvested and divided into proximal and distal segments relative to disc injury. Histological analysis was performed to assess muscle degeneration, fiber type, and macrophage density. Fibroadipogenic progenitors (FAPs) were isolated for gene expression analysis with qPCR.Results: MRI demonstrated decreased intervertebral disc signal and paraspinal muscle atrophy at 6 weeks, with progressive degeneration and atrophy at 15 weeks. Disc injury resulted in delayed functional recovery and impaired gait. Histology demonstrated progressive multifidus fibrofatty degeneration between 6 and 15 weeks. CD68 + macrophage density was increased at 6 weeks but not 15 weeks. FAPs exhibited increased fibrotic and adipogenic gene expression at 6 weeks compared to sham with less of a difference in gene expression by 15 weeks. Conclusions:We have developed a mouse model of disc injury-mediated paraspinal muscle degeneration that recapitulates features of degenerative muscle pathology observed in patients with lumbar disc degeneration, and highlights the role of FAPs in mediating fibrofatty muscle degeneration after disc injury.

Intervertebral disc degeneration and how it leads to low back pain

“… 240 Rat and mouse multifidus resection models induce spinal instability and IVDD demonstrating the interdependence of the IVD and spinal muscles for spinal stability, flexibility and optimal weight bearing. 241 , 242 …”

“…Mouse and rat models of IVDD have been developed where the IVD is left intact but the supportive paraspinal muscles receive a discrete surgical lesion. 241 , 242 This destabilizes the spine and induces degenerative changes in the IVD and emphasizes the inter‐relationship between the IVD and other spinal components in normal spinal loading. Small animal models of IVDD are also amenable to quantitative assessment of pain using facial recognition methodology 275 , 276 , 277 , 278 , 279 , 280 , 281 , 282 , 283 although this has also now been applied to canine and ovine studies.…”

“…The SM/J poor healer mouse is an inbred small mouse strain that displays age dependent early‐onset, spontaneous disc degeneration characterized by a lack of cells in the NP, a clear increase in hypertrophic chondrocyte‐like cells in the CEPs, and clefts throughout the IVD 240 . Rat and mouse multifidus resection models induce spinal instability and IVDD demonstrating the interdependence of the IVD and spinal muscles for spinal stability, flexibility and optimal weight bearing 241,242 …”

“…However, small animal models of IVDD have other attributes relating mainly to gene manipulation effects on disc pathobiology 227,266–272 or the development of biomechanically induced whole animal models of IVDD. Mouse and rat models of IVDD have been developed where the IVD is left intact but the supportive paraspinal muscles receive a discrete surgical lesion 241,242 . This destabilizes the spine and induces degenerative changes in the IVD and emphasizes the inter‐relationship between the IVD and other spinal components in normal spinal loading.…”

“…240 Rat and mouse multifidus resection models induce spinal instability and IVDD demonstrating the interdependence of the IVD and spinal muscles for spinal stability, flexibility and optimal weight bearing. 241,242 A single level annular puncture model of IVDD, rat xenograft radiculopathy model 243 and rat multi-level models of IVDD have been developed and used to evaluate the properties of growth and differentiation factor-6 (GDF6) as a therapeutic agent to combat IVD degenerative processes. 244 and had a protective effect on IVD structure and morphology 32 weeks post induction of IVDD.…”