Rev-Erb beta (Rev-Erbò), also known as nuclear receptor subfamily 1 group D member 2 (NR1D2), is a member of the Rev-Erb protein family. Rev-Erbò, like Rev-Erbñ, belongs to the nuclear receptor superfamily of transcription factors and can modulate gene expression through binding to gene promoters. Together with Rev-Erbñ, Rev-Erbò functions as a major regulator of the circadian clock. These two proteins are partially redundant. Current research suggests that Rev-Erbò is less important in maintaining the circadian clock than Rev-Erbñ; knock-out studies of Rev-Erbñ result in significant circadian disruption but the same has not been found with Rev-Erbò. Rev-Erbò compensation for Rev-Erbñ varies across tissues, and further research is needed to elucidate the separate role of Rev-Erbò.
This gene is expressed in the central and peripheral nervous system, spleen, mandibular maxillary processes, and blood islands. Rev-Erbò plays a major role in the conduction of inductive signals to aid in controlling differentiating neurons.
Rev-Erbò was discovered in 1994, when B. Dumas et al. isolated its cDNA, naming the new receptor BD73. The name Rev-Erbò was coined a few months later in a paper by Eva Enmark, Tommi Kainu, Markku Tapio Pelto-Huikko, and Jan úke Gustafsson where they isolated Rev-Erb alpha cDNA in a rat brain.
A new isoform of Rev-Erbò, named Rev-Erbò 2, was discovered using rat cDNA a few months later in 1995 by N. Giambiagi and colleagues. They found it to beàidentical to Rev-Erbò 1, except that the Rev-Erbò 1 protein is 195 amino acids longer than Rev-Erbò 2. However, further research has indicated that the discovered Rev-Erbò 2 cDNA was likely a splice variant of the Nr1d2 gene that arose through alternative splicing and the use of a different polyadenylation site.
In mammals, the NR1D2 (nuclear receptor subfamily 1 group D member 2) gene encodes the protein Rev-Erbò. Unlike NR1D1, the strand opposite NR1D2 does not have any significant reading frames, and the gene is located on the forward strand of chromosome 3.àDespite their different locations, the NR1D1 and NR1D2 genes are highly homologous and are paralogs within the genome.àIn humans, the NR1D2 gene itself contains 10 exons which form 5 splice variants (NR1D2-201 - NR1D2-205), ranging from 5231 base pairs (NR1D2-201) to 600 base pairs (NR1D2-204). However, only NR1D2-201 produces a functional protein. In mammals, NR1D2 (Rev-Erbò) is expressed throughout the body and with high expression in several tissues, including the brain, liver, skeletal muscle, and adipose tissue.
Comparison of the human NR1D2 sequence with other species indicates a high level of conservation across animals, with 472 discovered orthologs, including in mice, chickens, lizards, and zebrafish.ÃÂ Similarly to NR1D1, this suggests NR1D2 was present in the most recent common animal ancestor. NR1D2 has only one paralog in humans, the NR1D1 gene, which is located on chromosome 17, but it is closely related to other members of the nuclear receptor family and is functionally related to other nuclear receptor genes, such as thyroid hormone receptor beta (THRB), peroxisome proliferator activated receptor delta (PPARD), and retinoic acid receptor beta (RARB).ÃÂ Linkage analysis reveals that NR1D2 and THRB are highly linked due to proximity on chromosome 3, and that they are both linked to RARB. Combined with the linkage between the NR1D1/THRA locus and the RARA gene, this suggests that these two gene clusters arose from a duplication event.
The human NR1D2 gene produces a protein product (REV-ERBò) of 579 amino acids. Rev-Erbò is similar to Rev-Erbñ in both its structure and mechanism of transcriptional repression. Like Rev-Erbñ, Rev-Erbò has 3 major functional domains which are common to nuclear receptor proteins, including a DNA-binding domain (DBD) and a ligand-binding domain (LBD) at the C-terminus, which are highly conserved in Rev-Erb orthologs, and a N-terminus domain which allows for activity modulation.
Much like Rev-Erbñ, Rev-Erbò can bind to two classes of DNA response elements via its DBD, which contains two C4-type zinc fingers. These two classes include a DNA sequence commonly referred to as RORE due to its interaction with the transcriptional activator Retinoic Acid Receptor-related Orphan Receptor (ROR) and a direct repeat 2 element of RORE known as RevDR2. The Rev-Erb proteins are unique from other nuclear receptors in that they do not have a helix in the C-terminal that is necessary for coactivator recruitment and activation by nuclear receptors via their LBD. Instead, the Rev-Erbs can repress transcription as a monomer through competitive binding at single RORE elements by preventing the binding of constitutive transcription activator ROR or as a homodimer through binding to RevDR2 sites. The Rev-Erb homodimer is required for its interaction with Nuclear Receptor Co-Repressor (NCoR), or more weakly, with Silencing Mediator of Retinoid and Thyroid Receptors (SMRT). The interaction with NCoR is stabilized by interaction with heme, which binds the to the Rev-Erb ligand-binding pocket. Rev-Erbò undergoes a conformational change when complexed with heme, as its structure shows that helices 3,7, and 11 move to enlarge the ligand binding pocket in order to accommodate heme. The repression by Rev-Erb proteins also requires interaction of class I histone deacetylase 3 (HDAC3) with NCoR, which results in gene repression via histone deacetylation.
Rev-Erbò binds to genomic Rev-Erbñ-binding sites that have a diurnal profile identical or similar to Rev-Erbñ. This protein also helps maintain clock and metabolic gene regulation and protects system functioning when Rev-Erbñ is missing. Rev-Erbò compensates for loss of function from metabolic distress in the case that Rev-Erbñ is lost. The liver and metabolic processes can still run when Rev-Erbñ is missing and Rev-Erbò is present. Losing both Rev-Erbñ and Rev-Erbò causes cells to become arrhythmic.
When Rev-Erbò is missing, there can be significant change in performance of metabolic activity with drastic effects. For example:
Rev-Erbò plays a role in blocking the trans-activation of retinoic acid-related orphan receptor-ñ (RORñ). RORñ is involved in the regulation of lipoprotein cholesterol, lipid homeostasis, and inflammation. Rev-Erbò and RORñ are both expressed in similar tissues, such as skeletal muscle. They have similar expression patterns, target genes, and cognate sequences within the skeletal muscle. Rev-Erbò causes several genes assisting in lipid absorption to decrease expression. Rev-Erbò controls lipid and energy homoeostasis in skeletal muscle. Rev-Erbò may be useful in therapeutic treatments of dyslipidemia and regulating muscle growth.
Rev-Erbò is also a circadian regulated gene; its mRNA displays rhythmic expression in vivo and in serum-synchronized cell cultures. However, it is currently unknown to what extent Rev-Erbò contributes to oscillations of the core circadian clock. However it has been shown that heme suppresses hepatic gluconeogenic gene expression and glucose output through the related Rev-Erbñ receptor which mediates gene repression. Hence, the Rev-Erbñ receptor detects heme and thereby coordinates the cellular clock, glucose homeostasis, and energy metabolism.
Rev-Erbò plays a role in skeletal muscle mitochondrial biogenesis. Originally Rev-Erbò was thought to be functionally redundant of Rev-Erbñ but recent findings prove that there are subtle differences. Rev-Erbò ligands may be used in the treatment of metabolic disorders, like metabolic syndrome. It has control of skeletal muscle metabolism and energy that can be beneficial in treatment options.
Rev-Erbò gene contributes to the downstream regulation of clock output genes by generating specific KO mutants. It is still unknown all of the functions Rev-Erbò has in the core circadian clock and exactly how it differs from Rev-Erbñ.