The circadian clock coordinates the daily rhythms of various cellular processes and behaviors in response to physiological and environmental changes. Circadian abnormalities have long been observed in patients with a variety of diseases, including the most well-known demyelinating disease—multiple sclerosis. However, very few studies have systematically assessed how the circadian clock changes in response to pathological conditions and its downstream impacts. Demyelination is a pathology in the central nervous system (CNS), characterized by the loss of myelin that is important for nerve conduction. It is accompanied by a repairing process, but repeated demyelination results in reduced efficiencies of remyelination. Therefore, more cellular and molecular mechanisms are needed to promote repair. In this study, we aimed to explore the implication of the circadian system in demyelination with two questions. The first one is how the circadian clock exposed to demyelinating lesions changes. The second one is how the circadian clock outside the lesions responds to demyelination.

To address the first question, we monitored circadian oscillations in ex vivo and in vivo models of demyelination by bioluminescence assay and quantitative real-time PCR (qRT-PCR). We observed a shortening of the circadian period in demyelinating areas. Using RNA-sequencing (RNA-seq) analysis and quantitative real-time PCR, we found that the circadian clock regulates Wnt inhibitors SFRP1 and SFRP5. They acted as signaling molecules to switch the neural stem cells (NSCs) in the subventricular zone (SVZ) into oligodendrocyte lineage cells to form new myelin. We identified that circadian transcription factor BMAL1 in the astrocytes of demyelinating lesions contributed to this oligodendrogenesis derived from the SVZ.

To address the second question, we developed an ex vivo co-cultured system to expose the SVZ to signals from the demyelinating lesions. Our results revealed that the circadian period of the SVZ was lengthened by demyelinating signals. Interestingly, SFRP1 and SFRP5 constituted part of these demyelinating signals. We further validated that the lengthened circadian period was caused by a reduced BMAL1 level in the SVZ. Cell lineage analysis revealed that deletion of Bmal1 in SVZ cells increased oligodendrogenesis and neurogenesis but did not affect progenitor cell proliferation. It highlighted that the reduced BMAL1 induced SVZ-derived oligodendrogenesis upon demyelination. By RNA-seq analysis, we identified Wnt signaling as a potential mechanism mediated by BMAL1 to affect NSC properties in the SVZ.

These results demonstrate that circadian clocks changed in demyelinating lesions and a demyelination-responsive brain region outside the lesions. Importantly, we revealed that an altered circadian clock positively affects both brain regions by affecting SVZ-derived oligodendrogenesis for myelin repair. These findings support the potential of chronotherapy to treat CNS diseases.