Suprachiasmatic nuclei development: A characterization of transcription factors and the influence of retinal innervation and VIP signaling

The suprachiasmatic nuclei: SCN) are highly specialized neural structures with an essential behavioral function; creating the rhythm of the mammalian central clock and entraining that internal clock to the external world. The nuclei each consist of approximately 10,000 neurons, each capable of creating near 24 h rhythms, organized into a highly structured network. While the molecular clockwork underlying the rhythm within neurons and network properties have been well studied, how the nuclei are initially specified and how the network develops is poorly understood. Herein, we seek to elucidate the genes and mechanisms involved in the specification and development of SCN neurons, the SCN network, and circadian function. We first identified genes expressed relatively discretely with the SCN. Using these genes we provided a detailed analysis of transcription factor: TF) and developmental-gene expression within the SCN from neurogenesis through to adulthood in mice (Mus musculus). Through this analysis we identified a genetically distinct neuroepithelium from which SCN neurons are derived and described a gene cascade through which SCN neurons progress as they become postmitotic. In addition, we observed changes in patterns of TF expression through development indicating maturation of nuclei both prenatally and postnatally. We investigated the contribution of critical circadian components in shaping SCN development by monitoring the localization of TF expression in mouse models that lacked either Atoh7, necessary for retinal ganglion cell development, or functional VIP peptide or VIP receptor 2: VPAC2, Vipr2). We found that maturation of TF expression patterns within the SCN occurred independent of retinal innervation and VIP signaling, suggesting that localizations may reflect intrinsic differences in subsets of neurons within the nuclei rather than induced changes. Finally, we began to define specific TFs necessary for SCN development using a Cre/loxP system to temporally localize TF deletion. We found that the well-conserved TF, Six3, is necessary for the initial formation and specification of SCN neurons, but not involved postmitotically in onset or localization of TF or peptide expression. This work begins to reveal aspects of the development of circadian function, by providing a characterization of SCN anatomical development and the first descriptions of TFs necessary for specification

Development, maturation, and necessity of transcription factors in the mouse suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) of the hypothalamus is the master mammalian circadian clock. The SCN is highly specialized as it is responsible for generating a near 24-hour rhythm, integrating external cues, and translating the rhythm throughout the body. Currently, our understanding of the developmental origin and genetic program involved in the proper specification and maturation of the SCN is limited. Herein, we provide a detailed analysis of transcription factor (TF) and developmental-gene expression in the SCN from neurogenesis to adulthood in mice (Mus musculus). TF expression within the postmitotic SCN was not static but rather showed specific temporal and spatial changes during pre- and postnatal development. In addition, we found both global and regional patterns of TF expression extending into the adult. We found the SCN is derived from a distinct region of the neuroepithelium expressing a combination of developmental genes: Six3, Six6, Fzd5, and transient Rx, allowing us to pinpoint the origin of this region within the broader developing telencephalon/diencephalon. We tested the necessity of two TFs in SCN development, RORα and Six3, which were expressed during SCN development, persisted into adulthood, and showed diurnal rhythmicity. Loss of RORα function had no effect on SCN peptide expression or localization. In marked contrast, the conditional deletion of Six3 from early neural progenitors completely eliminated the formation of the SCN. Our results provide the first description of the involvement of TFs in the specification and maturation of a neural population necessary for circadian behavior