Abstract

Suprachiasmatic nuclei (SCN) neurons contain an intracellular molecular circadian clock and the Cryptochromes (CRY1/2), key transcriptional repressors of this molecular apparatus, are subject to post-translational modification through ubiquitination and targeting for proteosomal degradation by the ubiquitin E3 ligase complex. Loss-of-function point mutations in a component of this ligase complex, Fbxl3, delay CRY1/2 degradation, reduce circadian rhythm strength, and lengthen the circadian period by ∼2.5 h. The molecular clock drives circadian changes in the membrane properties of SCN neurons, but it is unclear how alterations in CRY1/2 stability affect SCN neurophysiology. Here we use male and female Afterhours mice which carry the circadian period lengthening loss-of-function Fbxl3Afh mutation and perform patch-clamp recordings from SCN brain slices across the projected day/night cycle. We find that the daily rhythm in membrane excitability in the ventral SCN (vSCN) was enhanced in amplitude and delayed in timing in Fbxl3Afh/Afh mice. At night, vSCN cells from Fbxl3Afh/Afh mice were more hyperpolarized, receiving more GABAergic input than their Fbxl3+/+ counterparts. Unexpectedly, the progression to daytime hyperexcited states was slowed by Afh mutation, whereas the decline to hypoexcited states was accelerated. In long-term bioluminescence recordings, GABAA receptor blockade desynchronized the Fbxl3+/+ but not the Fbxl3Afh/Afh vSCN neuronal network. Further, a neurochemical mimic of the light input pathway evoked larger shifts in molecular clock rhythms in Fbxl3Afh/Afh compared with Fbxl3+/+ SCN slices. These results reveal unanticipated consequences of delaying CRY degradation, indicating that the Afh mutation prolongs nighttime hyperpolarized states of vSCN cells through increased GABAergic synaptic transmission.SIGNIFICANCE STATEMENT The intracellular molecular clock drives changes in SCN neuronal excitability, but it is unclear how mutations affecting post-translational modification of molecular clock proteins influence the temporal expression of SCN neuronal state or intercellular communication within the SCN network. Here we show for the first time, that a mutation that prolongs the stability of key components of the intracellular clock, the cryptochrome proteins, unexpectedly increases in the expression of hypoexcited neuronal state in the ventral SCN at night and enhances hyperpolarization of ventral SCN neurons at this time. This is accompanied by increased GABAergic signaling and by enhanced responsiveness to a neurochemical mimic of the light input pathway to the SCN. Therefore, post-translational modification shapes SCN neuronal state and network properties.

Highlights

  • Post-translational modification plays key roles in shaping neuronal activity and function

  • We show for the first time, that a mutation that prolongs the stability of key components of the intracellular clock, the cryptochrome proteins, unexpectedly increases in the expression of hypoexcited neuronal state in the ventral Suprachiasmatic nuclei (SCN) at night and enhances hyperpolarization of ventral SCN neurons at this time

  • Regional and phase-related reductions in the activity of Fbxl3 Afh/Afh SCN neurons To determine how the Afh mutation influences the electrical states of SCN neurons, we performed in vitro whole-cell currentclamp recordings from dorsal and ventral cells in brain slices prepared from adult Fbxl3Afh/Afh and Fbxl3ϩ/ϩ littermate mice that had been housed under 12 h LD conditions (Fig. 1E shows delineation of dorsal SCN (dSCN) and ventral SCN (vSCN) subregions)

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Summary

Introduction

Post-translational modification plays key roles in shaping neuronal activity and function A key exemplar of this is the brain’s master circadian pacemaker of the hypothalamic suprachiasmatic nuclei (SCN). Several thousands of neurons contain an intracellular circadian clock in which transcriptional/translational feedback loops (TTFLs) provide a molecular representation of the external 24 h cycle

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