The intervertebral disc contains intrinsic circadian clocks that are regulated by age and cytokines and linked to degeneration

Abstract

ObjectivesThe circadian clocks are internal timing mechanisms that drive ∼24-hour rhythms in a tissue-specific manner. Many aspects of the physiology of the intervertebral disc (IVD) show clear diurnal rhythms. However, it is unknown whether IVD tissue contains functional circadian clocks and if so, how their dysregulation is implicated in IVD degeneration.MethodsClock gene dynamics in ex vivo IVD explants (from PER2:: luciferase (LUC) reporter mice) and human disc cells (transduced with lentivirus containing Per2::luc reporters) were monitored in real time by bioluminescence photon counting and imaging. Temporal gene expression changes were studied by RNAseq and quantitative reverse transcription (qRT)-PCR. IVD pathology was evaluated by histology in a mouse model with tissue-specific deletion of the core clock gene Bmal1.ResultsHere we show the existence of the circadian rhythm in mouse IVD tissue and human disc cells. This rhythm is dampened with ageing in mice and can be abolished by treatment with interleukin-1β but not tumour necrosis factor α. Time-series RNAseq revealed 607 genes with 24-hour patterns of expression representing several essential pathways in IVD physiology. Mice with conditional knockout of Bmal1 in their disc cells demonstrated age-related degeneration of IVDs.ConclusionsWe have established autonomous circadian clocks in mouse and human IVD cells which respond to age and cytokines, and control key pathways involved in the homeostasis of IVDs. Genetic disruption to the mouse IVD molecular clock predisposes to IVD degeneration. These results support the concept that disruptions to circadian rhythms may be a risk factor for degenerative IVD disease and low back pain.

RNA Sequencing Reveals a Role of TonEBP Transcription Factor in Regulation of Pro-inflammatory Genes in Response to Hyperosmolarity in Healthy Nucleus Pulposus Cells

“…ical loading of the healthy disc would activate an inflammatory program. It is, therefore, more likely that the acute nature of CCL2 induction may be tied to diurnal loading of the disc, considering that genes controlling circadian rhythm are essential for disc homeostasis (35,36). It is noteworthy that in other cell types, CCL2 promotes survival (37), proliferation (38), and phosphorylation of Akt, ERK, and STAT3 (39), molecules critical for NP function (22, 40 -43).…”

CircGLCE alleviates intervertebral disc degeneration by regulating apoptosis and matrix degradation through the targeting of miR-587/STAP1

“…2 The major underlying physiological process involved in lower back pain is intervertebral disc degeneration (IDD), which leads to progressive structural abnormality of the lumbar spine and accelerated ageing of the intervertebral discs. 3, 4 Anatomically, the gelatinous nucleus pulposus (NP) of the intervertebral disc is surrounded by annulus fibrosis (AF). 5 NP cells produce most of the major components of the extracellular matrix, including Collagen II and aggrecan, which are essential for the integrity of disc structure.…”

Circadian control of the secretory pathway is a central mechanism in tissue homeostasis

“…Disruption of the circadian clock leads to musculoskeletal abnormalities involving the ECM e.g. chondrocyte-specific disruption of the circadian clock resulted in progressive degeneration of the ECM in articular cartilage 26 and fibrosis in intervertebral disc 27 , and mice with a global knockout (KO) of Bmal1 28 or the ClockD19 mutation 29 develop thickened and calcified tendons with associated immobilization. These observations were indicative of circadian control of ECM homeostasis.…”