Differential entrainment susceptibility of central and peripheral circadian clocks affecting energy metabolism in Pitx3-deficient Aphakia mice

Abstract

The hypothalamic suprachiasmatic nucleus (SCN) is a master regulator of circadian rhythmicity by synchronizing metabolic activity with daily light-dark cycles. The synchronizing function of the central molecular clock, consisting of self-regulated genes encoding transcriptional activators and repressors, is critically dependent on lightdependent stimuli reaching the SCN directly from the retina. In turn, the SCN translates this information to entrain molecular clocks in peripheral organs, which are essential to adapt metabolic activity to circadian variation. Here, we investigated the relative importance of the input pathway relying light information to the SCN for the function of central and peripheral clocks and for the circadian regulation of energy metabolism. We used Pitx3-deficient Aphakia (Pitx3Ak) mice, characterized by a congenital defect that severely disrupts eye development. Expression of clock genes was determined by RT-PCR, and energy metabolism was assessed by indirect calorimetry. Electroretinogram recordings showed that Pitx3Ak mice were completely unresponsive to light. Indirect calorimetry revealed that, contrary to control mice, Pitx3Ak lacked cyclic day-night oscillations in oxygen consumption, energy expenditure, CO2 production and respiratory exchange ratio. Expression levels of clock genes in the SCN and the liver of control mice showed night-day oscillations, but in Pitx3Ak mice they were reduced and showed no significant day-night variation. Time-restricted-feeding (TRF) allowing Pitx3Ak mice access to food only during the night resulted in the partial restoration of circadian rhythmicity in energy metabolism and expression of liver clock genes, but no change was observed in the expression of SCN clock genes. Our data show that congenital eye defects preventing the relay of light-dependent information to the SCN result in permanent defects in the expression of clock genes in the SCN. In addition, our results indicate that clock genes in peripheral tissues retain an autonomous capacity to be entrained by external stimuli in the absence of SCN-dependent activity.