UC San Diego

Circadian rhythms coordinate the body to Earth’s day and night cycles, relying on sunlight to induce expression of circadian genes which signal downstream and synchronize other tissues. Studies have found that both the positioning of a cellular nucleus, as well as the chromatin organization inside the nucleus, are dynamic to spatially facilitate fluctuating cell functions. Because circadian cycles are rhythmically activating and deactivating genetic material, the connection between spatial organization of the nucleus and circadian rhythms should be further explored. To study chromatin reorganization and nuclear positioning in the context of circadian rhythms, we used fluorescence and electron microscopy for time-dependent imaging of nuclei of both live mammalian cells and fixed mouse tissue. We measured time-dependent nuclear rotation rates of mouse embryonic fibroblasts (MEFs), human epithelial kidney cells (HEK293Ts), and primary mouse neurons. We used electron microscopy to compare chromatin organization in mouse neurons of the suprachiasmatic nucleus (SCN) between morning, afternoon, and night. Chromatin density analysis of SCN neurons revealed varying condensation patterns of heterochromatin and euchromatin within the nucleus, these data serving as a time-dependent contextual map for distribution of genetic material. In conclusion, these data supply further understanding of how nuclear reorganization may connect to circadian activity. Ultimately, structural data from studies like this can be used together with proteomic and epigenetic data to gain insight into circadian function and how pathologies arise from dysregulation.