Single Cell RNA-Sequence Analyses Reveal Uniquely Expressed Genes and Heterogeneous Immune Cell Involvement in the Rat Model of Intervertebral Disc Degeneration

A longitudinal population-based study identifies THBS2 as a susceptibility gene for intervertebral disc degeneration

PURPOSE

Intervertebral disc degeneration (IDD) is a common degenerative disease associated with ageing. Additionally, IDD is recognized as one of the leading causes of low back pain and disability in the working-age population and is the first step in the process leading to degenerative spinal changes. However, the genetic factors and regulatory mechanisms of IDD remain unknown. Therefore, we selected eight single nucleotide polymorphisms of genes to reveal the progression of IDD in a 7-year longitudinal study of the general population in Japan.

METHODS

IDD was evaluated in the Wakayama Spine Study (WSS), which is a population-based cohort study. Overall, 574 participants from the general population cohort who underwent whole spine magnetic resonance imaging and provided clinical information were included in this longitudinal survey.

RESULTS

The progression of IDD was affected only by THBS2 at the lumbar region, T12-L1 (p = 0.0044) and L3-4 (p = 0.0045). The significant interaction between THBS2 and age with IDD negatively affected the thoracic spines and passively influenced both the thoracolumbar junction and thoracic spines. The higher progression per year of Pfirrmann's score was rapid in young people with age; however, this decelerated the IDD progression per year in different ages.

CONCLUSION

Our longitudinal study found the genes associated with IDD progression and that genetic factors' impact on IDD differs depending on disc level and age.

Advancing skeletal health and disease research with single-cell RNA sequencing

Orthopedic conditions have emerged as global health concerns, impacting approximately 1.7 billion individuals worldwide. However, the limited understanding of the underlying pathological processes at the cellular and molecular level has hindered the development of comprehensive treatment options for these disorders. The advent of single-cell RNA sequencing (scRNA-seq) technology has revolutionized biomedical research by enabling detailed examination of cellular and molecular diversity. Nevertheless, investigating mechanisms at the single-cell level in highly mineralized skeletal tissue poses technical challenges. In this comprehensive review, we present a streamlined approach to obtaining high-quality single cells from skeletal tissue and provide an overview of existing scRNA-seq technologies employed in skeletal studies along with practical bioinformatic analysis pipelines. By utilizing these methodologies, crucial insights into the developmental dynamics, maintenance of homeostasis, and pathological processes involved in spine, joint, bone, muscle, and tendon disorders have been uncovered. Specifically focusing on the joint diseases of degenerative disc disease, osteoarthritis, and rheumatoid arthritis using scRNA-seq has provided novel insights and a more nuanced comprehension. These findings have paved the way for discovering novel therapeutic targets that offer potential benefits to patients suffering from diverse skeletal disorders.

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.

TRPV4 differentially controls inflammatory cytokine networks during static and dynamic compression of the intervertebral disc

Background

The ion channel transient receptor potential vanilloid 4 (TRPV4) critically transduces mechanical forces in the IVD, and its inhibition can prevent IVD degeneration due to static overloading. However, it remains unknown whether different modes of loading signals through TRPV4 to regulate the expression of inflammatory cytokines. We hypothesized that TRPV4 signaling is essential during static and dynamic loading to mediate homeostasis and mechanotransduction.

Methods

Mouse functional spine units were isolated and either cyclically compressed for 5 days (1 Hz, 1 h, 10% strain) or statically compressed (24 h, 0.2 MPa). Conditioned media were monitored at 6 h, 24 h, 2 days, and 5 days, with and without TRPV4 inhibition. Effects of TRPV4 activation was also evaluated without loading. The media was analyzed for a panel of 44 cytokines using a microbead array and then a correlative network was constructed to explore the regulatory relationships during loading and TRPV4 inhibition. After the loading regimen, the IVDs were evaluated histologically for degeneration.

Results

Activation of TRPV4 led to an increase interleukin-6 (IL-6) family of cytokines (IL-6, IL-11, IL-16, and leukemia inhibitory factor [LIF]) and decreased the T-cell (CCL3, CCL4, CCL17, CCL20, CCL22, and CXCL10) and monocyte (CCL2 and CCL12) recruiting chemokines by the IVD. Dynamic and static loading each provoked unique chemokine correlation networks. The inhibition of TRPV4 during dynamic loading dysregulated the relationship between LIF and other cytokines, while the inhibition of TRPV4 during static loading disrupted the connectivity of IL-16 and VEGFA.

Conclusions

We demonstrated that TRPV4 critically mediates the cytokine production following dynamic and static loading. The activation of TRPV4 upregulated a diverse set of cytokines that may suppress the chemotaxis of T-cells and monocytes, implicating the role of TRPV4 in maintaining the immune privilege of healthy IVD.

Loss of function mutation in Ank causes aberrant mineralization and acquisition of osteoblast-like-phenotype by the cells of the intervertebral disc

Pathological mineralization of intervertebral disc is debilitating and painful and linked to disc degeneration in a subset of human patients. An adenosine triphosphate efflux transporter, progressive ankylosis (ANK) is a regulator of extracellular inorganic pyrophosphate levels and plays an important role in tissue mineralization. However, the function of ANK in intervertebral disc has not been fully explored. Herein we analyzed the spinal phenotype of Ank mutant mice (ank/ank) with attenuated ANK function. Micro-computed tomography and histological analysis showed that loss of ANK function results in the aberrant annulus fibrosus mineralization and peripheral disc fusions with cranial to caudal progression in the spine. Vertebrae in ank mice exhibit elevated cortical bone mass and increased tissue non-specific alkaline phosphatase-positive endplate chondrocytes with decreased subchondral endplate porosity. The acellular dystrophic mineral inclusions in the annulus fibrosus were localized adjacent to apoptotic cells and cells that acquired osteoblast-like phenotype. Fourier transform infrared spectral imaging showed that the apatite mineral in the outer annulus fibrosus had similar chemical composition to that of vertebral bone. Transcriptomic analysis of annulus fibrosus and nucleus pulposus tissues showed changes in several biological themes with a prominent dysregulation of BMAL1/CLOCK circadian regulation. The present study provides new insights into the role of ANK in the disc tissue compartments and highlights the importance of local inorganic pyrophosphate metabolism in inhibiting the mineralization of this important connective tissue.

Intervertebral disc degeneration-Current therapeutic options and challenges

Degeneration of the intervertebral disc (IVD) is a normal part of aging. Due to the spine's declining function and the development of pain, it may affect one's physical health, mental health, and socioeconomic status. Most of the intervertebral disc degeneration (IVDD) therapies today focus on the symptoms of low back pain rather than the underlying etiology or mechanical function of the disc. The deteriorated disc is typically not restored by conservative or surgical therapies that largely focus on correcting symptoms and structural abnormalities. To enhance the clinical outcome and the quality of life of a patient, several therapeutic modalities have been created. In this review, we discuss genetic and environmental causes of IVDD and describe promising modern endogenous and exogenous therapeutic approaches including their applicability and relevance to the degeneration process.