Qing-Jun Meng

In vitro synchronisation is widely used to study circadian clocks in cells, but whether cultured cells recapitulate tissue-level rhythmic outputs remains unclear. Articular cartilage provides a useful model to address this because chondrocytes are the only resident cell type. Here, we compared circadian time series transcriptomes between primary mouse chondrocytes synchronised by heat shock, dexamethasone, or osmotic stress and in vivo cartilage tissue. All three stimuli robustly synchronised core clock gene rhythms but produced distinct circadian phases and markedly different rhythmic transcriptomes, depending on the synchronizer. Heat shock, dexamethasone, and osmotic stress yielded 5255, 2008, and 879 transcripts classified as rhythmic, respectively, in primary chondrocytes, yet only 64 genes were shared across the three in vitro datasets, and only 15 were shared when in vivo cartilage transcriptome was included. Pairwise comparisons between synchronizers revealed some statistically enriched overlaps, but only marginally above chance, and shared genes showed limited conservation of circadian phase. Functional enrichment analysis also revealed stimulus-dependent rhythmic programmes with modest pathway-level overlap. These findings indicate that circadian output in cultured cells is shaped by the synchronising cue and cellular microenvironment. We also present BodyClocks.org, an interactive resource implementing this comparative framework across a curated collection of circadian transcriptomic datasets.

The suprachiasmatic nucleus (SCN) synchronises circadian rhythmicity (~24 h) across the body. The SCN cell-autonomous clock is modelled qualitatively as a transcriptional-translational feedback loop (TTFL), with heteromeric complexes of transcriptional activator and repressor proteins driving cyclical gene expression. How these proteins really behave within the SCN, individually and in relation to each other, is poorly understood. Imaging SCN slices from a novel array of knock-in reporter mice, we quantify the dynamic behaviours of combined repressors PERIOD2 (PER2) and CRYPTOCHROME1 (CRY1), and activator BMAL1. We reveal a spectrum of protein-specific intracellular and spatiotemporal behaviours that run counter to the qualitative TTFL model. We also show that PER and CRY1 exert independent actions on TTFL oscillations, and that their individual stabilities play a critical role in SCN circadian dynamics. These results reveal a rich and unanticipated complexity in the dynamic behaviours and functions of endogenous circadian proteins, prompting re-appraisal of current transcriptional-translational feedback loop models of the suprachiasmatic nucleus.

Collagen deposition in fibroblasts, the primary collagen-producing cells, is regulated by both macrophages and the circadian rhythm, although how these regulatory processes interact with one another is unknown. Here, we reveal that macrophage-fibroblast interaction enhances collagen deposition which requires a functional circadian rhythm. This response is also dependent on macrophage polarisation status, where M0 (naïve) and M1 (pro-inflammatory) macrophages require direct cell-cell contact; whereas M2 (anti-inflammatory) macrophages utilise an additional mechanism through secreted soluble factors that strengthen circadian rhythms and significantly increase collagen deposition in fibroblasts, independent of cell-cell contact. Using mass spectrometry proteomics analysis, we identified PDGFA as a key factor in induction of circadian amplitude in fibroblasts. Crucially, macrophages lacking PDGFA showed diminished ability to modulate fibroblast circadian rhythms and collagen production. Further, in fibroblasts with impaired circadian rhythms, neither macrophages nor macrophage-secreted factors are capable of inducing collagen deposition. These results confirmed that the collagen secretory pathway is also under circadian clock control in lung fibroblasts, and demonstrated that macrophages influence collagen deposition via circadian clock mediated mechanisms. In conclusion, our findings highlight M2-derived PDGFA as a key macrophage-derived factor in extracellular matrix remodelling.

Circadian medicine, the delivery of therapeutic interventions based on an individual’s daily rhythms, has shown improved efficacy and reduced side-effects for various treatments. Rheumatoid arthritis and other inflammatory diseases are characterized by diurnal changes in cytokines, leading to inflammatory flares, with peak disease activity in the early morning. Using a combination of synthetic biology and tissue engineering, we developed circadian-based gene circuits, termed “chronogenetics”, that express a prescribed transgene downstream of the core clock gene promoter, Period2 (Per2). Gene circuits were transduced into induced pluripotent stem cells that were tissue-engineered into cartilage constructs. Our anti-inflammatory chronogenetic constructs produced therapeutic concentrations of interleukin-1 receptor antagonist in vitro. Once implanted in vivo, the constructs expressed circadian rhythms and entrained to daily light cycles, producing daily increases in biologic drug at the peak of Per2 expression. This approach represents the development of a cell-based chronogenetic therapy for various applications in circadian medicine.