The clock transcription factor BMAL1 is a key regulator of extracellular matrix homeostasis and cell fate in the intervertebral disc

Abstract

The circadian clock in mammals temporally coordinates physiological and behavioural processes to anticipate daily rhythmic changes in their environment. Chronic disruption to circadian rhythms (e.g., through ageing or shift work) is thought to contribute to a multitude of diseases, including degeneration of the musculoskeletal system. The intervertebral disc (IVD) in the spine contains circadian clocks which control ~6% of the transcriptome in a rhythmic manner, including key genes involved in extracellular matrix (ECM) homeostasis. However, it remains largely unknown to what extent the local IVD molecular clock is required to drive rhythmic gene transcription and IVD physiology. In this work, we identified profound age-related changes of ECM microarchitecture and an endochondral ossification-like phenotype in the annulus fibrosus (AF) region of the IVD in the Col2a1-Bmal1 knockout mice. Circadian time series RNA-Seq of the whole IVD in Bmal1 knockout revealed loss of circadian patterns in gene expression, with an unexpected emergence of 12-hour ultradian rhythms, including FOXO transcription factors. Further RNA sequencing of the AF tissue identified region-specific changes in gene expression, evidencing a loss of AF phenotype markers and a dysregulation of ECM and FOXO pathways in Bmal1 knockout mice. Consistent with an up-regulation of FOXO1 mRNA and protein levels in Bmal1 knockout IVDs, inhibition of FOXO1 in AF cells suppressed their osteogenic differentiation. Collectively, these data highlight the importance of the local molecular clock mechanism in the maintenance of the cell fate and ECM homeostasis of the IVD. Further studies may identify potential new molecular targets for alleviating IVD degeneration.

Increased HIF-2α Activity in the Nucleus Pulposus Causes Intervertebral Disc Degeneration in the Aging Mouse Spine

“…Interestingly, there was another downregulated theme that contained the entity Rorα, RAR-related orphan receptor alpha, known to be a major circadian clock gene. We have shown that there is an interaction between BMAL1 and RORα and they modulate HIF-1α activity as well as ECM homeostasis and perturbation in circadian clock genes results in disc degeneration (51)(52)(53)(54). Notably, corroborating our current findings, HIF-2α FoxA2Cre mouse has shown upregulation in circadian clock pathways.…”

Fibroblast-like synoviocytes orchestrate daily rhythmic inflammation in arthritis

“…This is in line with our previous work showing that ablation of Bmal1 using the same Col6a1-cre driver in C57BL/6J mice results in joint stiffening 3 . Bmal1 ablation in other tissue contexts has also been shown to alter ECM organisation 62,63 . Together these findings reveal a role for FLS BMAL1 activity or wider circadian clock function in shaping joint ECM dynamics, and we speculate that altered collagen expression and/or resulting joint architecture contribute to the attenuated susceptibility to CIA observed here.…”

Single nuclei transcriptomics reveal the differentiation trajectories of periosteal skeletal/stem progenitor cells in bone regeneration

“…Bmal1 was previously shown to be a regulator of cartilage differentiation in bone development and homeostasis 36 . Bmal1 inactivation leads to chondrocyte apoptosis and disruption of signaling involved in cartilage formation, including TGFβ, Ihh, HIF1α and NFAT [37][38][39][40] . This suggests a role of the circadian clock genes as key regulators of chondrogenesis during bone repair.…”