Time-of-Day- and Light-Dependent Expression of Ubiquitin Protein Ligase E3 Component N-Recognin 4 (UBR4) in the Suprachiasmatic Nucleus Circadian Clock

Abstract

Circadian rhythms of behavior and physiology are driven by the biological clock that operates endogenously but can also be entrained to the light-dark cycle of the environment. In mammals, the master circadian pacemaker is located in the suprachiasmatic nucleus (SCN), which is composed of individual cellular oscillators that are driven by a set of core clock genes interacting in transcriptional/translational feedback loops. Light signals can trigger molecular events in the SCN that ultimately impact on the phase of expression of core clock genes to reset the master pacemaker. While transcriptional regulation has received much attention in the field of circadian biology in the past, other mechanisms including targeted protein degradation likely contribute to the clock timing and entrainment process. In the present study, proteome-wide screens of the murine SCN led to the identification of ubiquitin protein ligase E3 component N-recognin 4 (UBR4), a novel E3 ubiquitin ligase component of the N-end rule pathway, as a time-of-day-dependent and light-inducible protein. The spatial and temporal expression pattern of UBR4 in the SCN was subsequently characterized by immunofluorescence microscopy. UBR4 is expressed across the entire rostrocaudal extent of the SCN in a time-of-day-dependent fashion. UBR4 is localized exclusively to arginine vasopressin (AVP)-expressing neurons of the SCN shell. Upon photic stimulation in the early subjective night, the number of UBR4-expressing cells within the SCN increases. This study is the first to identify a novel E3 ubiquitin ligase component, UBR4, in the murine SCN and to implicate the N-end rule degradation pathway as a potential player in regulating core clock mechanisms and photic entrainment.

Death of a Protein: The Role of E3 Ubiquitin Ligases in Circadian Rhythms of Mice and Flies

“…UBR4 and POE are broadly expressed in the murine and fly brains, including virtually all SCN neurons and the PDF-positive LN v s [170]. A prior study demonstrated that the abundance of UBR4 in the SCN is both rhythmic and light-inducible, suggesting that it may regulate clock-timing processes or photic entrainment [171]. However, ablating Ubr4 in all GABAergic neurons, and thus all SCN neurons, has no effect on free-running rhythms under DD or light-induced phase delays in mice [170].…”

“…In terms of N-recognins, only UBR1 and UBR4 have been implicated in the regulation of circadian rhythms [170][171][172]. In the case of UBR1, it was shown to induce the degradation of CONIDIAL SEPARATION 1 (CSP1), a Neurospora protein whose presence in the morning represses the transcription of evening-specific genes [172].…”

To Ub or not to Ub: Regulation of circadian clocks by ubiquitination and deubiquitination

“…Ubiquitin protein ligase E3 component N‐recognin 4 (UBR4) is an E3 ligase of the N‐end rule pathway with a potential role in PER2 ubiquitination and degradation. UBR4 protein peaks with a phase that coincides with the beginning of PER2 degradation in the SCN (early night) (Ling et al., 2014). More research will be needed to define the implication of this ubiquitin ligase in clock function.…”

UBR4 deficiency causes male sterility and testis abnormal in Drosophila

“…Up to now, research on UBR4 has mainly focused on the nervous system ( 5 – 7 ). Interestingly, the number of Drosophila pupae decreased after Ubr4 gene knockdown in our preliminary experiments, suggesting that Ubr4 may be related to reproductive capacity.…”