Circadian clock regulation of the glycogen synthase ( <i>gsn</i> ) gene by WCC is critical for rhythmic glycogen metabolism in <i>Neurospora crassa</i>

Abstract

Circadian clocks generate rhythms in cellular functions, including metabolism, to align biological processes with the 24-hour environment. Disruption of this alignment by shift work alters glucose homeostasis. Glucose homeostasis depends on signaling and allosteric control; however, the molecular mechanisms linking the clock to glucose homeostasis remain largely unknown. We investigated the molecular links between the clock and glycogen metabolism, a conserved glucose homeostatic process, in <i>Neurospora crassa</i> We find that glycogen synthase (<i>gsn</i>) mRNA, glycogen phosphorylase (<i>gpn</i>) mRNA, and glycogen levels, accumulate with a daily rhythm controlled by the circadian clock. Because the synthase and phosphorylase are critical to homeostasis, their roles in generating glycogen rhythms were investigated. We demonstrate that while <i>gsn</i> was necessary for glycogen production, constitutive <i>gsn</i> expression resulted in high and arrhythmic glycogen levels, and deletion of <i>gpn</i> abolished <i>gsn</i> mRNA rhythms and rhythmic glycogen accumulation. Furthermore, we show that <i>gsn</i> promoter activity is rhythmic and is directly controlled by core clock component white collar complex (WCC). We also discovered that WCC-regulated transcription factors, VOS-1 and CSP-1, modulate the phase and amplitude of rhythmic <i>gsn</i> mRNA, and these changes are similarly reflected in glycogen oscillations. Together, these data indicate the importance of clock-regulated <i>gsn</i> transcription over signaling or allosteric control of glycogen rhythms, a mechanism that is potentially conserved in mammals and critical to metabolic homeostasis.