STARS OF THE MASTER CIRCADIAN CLOCK: A CHARACTERIZATION OF GLIOGENESIS IN THE MOUSE SUPRACHIASMATIC NUCLEUS

In mammals, circadian rhythms are organised by the master pacemaker in the suprachiasmatic nucleus (SCN) of the hypothalamus. Astrocytes are active regulators of circadian rhythmicity, however, the developmental timing of their functional integration into the circuit remains poorly understood. Previous work has shown that in the mouse, molecular rhythms gain robustness and synchrony at a circuit level during late gestation, coinciding with the neurogenic-to-gliogenic transition. Thus, we hypothesize that astrogenesis may be critical for the functional maturation of the SCN. In this study, we characterized the temporal dynamics of SCN gliogenesis using birth-dating approaches, with a particular focus on astrocyte proliferation. Using EdU labelling in combination with cell-type specific markers we characterized numbers and distribution of newly born cells in the late gestation and early postnatal period within the mouse SCN. During that same period, we identified astrocyte-specific and astrocyte-enriched genes and characterized the expression patterns along the developmental trajectory. We found that genes related to astrocyte specification (Nfib, Nfix, Tgfb1) are sustained postnatally indicating continued proliferation. We also found that astrocytes show early maturation and integration in the SCN circuit, as shown by upregulation from the first postnatal week of genes necessary for GABA, glutamate and purinergic homeostasis (Slc12a2, Slc12a3, Slc6a11, Entp2d). Building on these insights, we used organotypic cultures to assess the robustness and synchrony of molecular rhythms when astrocyte proliferation or astrocyte functionality are selectively disrupted. Together, these results provide new insight into the importance of glial maturation for the functional assembly of the central biological clock.