Wear and tear on joints can lead to inflammation, breakdown of cartilage and development of osteoarthritis. Scientists at UF Scripps Biomedical Research have found a possible new target to fight this painful cascade.
In a study published Thursday in the journal PLOS One, biochemist Patrick Griffin, Ph.D., and colleague Mi Ra Chang, Ph.D., describe a specific protein that manages activities within chondrocytes, a critical cell type that maintains healthy cartilage in joints.
As people age and stress their joints, their chondrocytes begin to fail. The UF Scripps team found that activating a specific protein in these cells called RORβ (beta) could restore multiple factors needed for smooth joints to healthier levels, helping to control inflammation.
Activating RORβ could thus present a useful new strategy to prevent or delay development of the degenerative joint disease osteoarthritis, said Griffin, a professor of molecular medicine and scientific director of UF Scripps Biomedical Research.
“People need an osteoarthritis medication that addresses the root cause of cartilage damage and depletion as there currently are no disease-modifying drugs for what is the No. 1 cause of disability in the United States,” Griffin said. “While our work is in the early stages, our study suggests that the nuclear receptor RORβ could present a novel therapeutic target to protect cartilage damage and perhaps turn on cartilage regeneration.”
RORβ, short for “retinoic acid receptor-related orphan receptor beta,” is a type of protein called a nuclear receptor. In our cells, genes switch between periods of activity and inactivity. When nuclear receptors bind to DNA, that activates the cell’s process of transcribing genes into proteins.
RORβ has been linked to development of the eye’s retina during fetal growth, and it can influence circadian rhythms by controlling clock genes. But its role in maintaining cartilage health was unclear.
Griffin has studied causes of bone diseases for many years. He zeroed in on RORβ for several reasons. While few studies have been focused on this receptor, some had shown correlation between the receptor’s activity and bone loss. So he and Chang set out to better understand it. Chang engineered cell lines to enable the studies.
“To our surprise, the gene program upregulated by increase in RORβ activity was supportive of the formation of chondrocytes, anti-inflammatory, and protective against cartilage degradation,” Chang said.
“This study suggests RORβ could be an attractive therapeutic target. However, there’s much more we need to unravel,” Griffin said. “Specifically, we want to understand more about the mechanism by which RORβ impacts chondrocytes and blunts the inflammatory signals that lead to cartilage destruction.”
Nuclear receptors (NRs) form a superfamily of sequence-specific transcription factors that regulate diverse biological processes including cell growth and differentiation, development, homeostasis and various organ functions in the adult by stimulating or repressing target gene expression (Gronemeyer and Laudet, 1995; Mangelsdorf et al., 1995).
NRs all share a common modular structure composed of several domains denoted A–F. Receptor dimerization, ligand binding, repression in the absence of ligand and ligand-dependent transactivation are mediated by the C-terminal region of NRs, termed the ligand-binding domain (LBD), by generating the proper interaction surfaces for multiple partners, including corepressors, coactivators and mediators.
The surface to which coactivators and mediators bind is assembled upon ligand binding and comprises H3, H4 and H12; it corresponds to the AF-2, the ligand-dependent transactivation function (reviewed in Renaud and Moras, 2000). In fact, ligand binding appears to trigger a switch in the LBD from a corepressor-binding to a coactivator-binding conformation (Glass and Rosenfeld, 2000; Renaud et al., 2000).
In addition to the ligand-dependent receptors, a vast number of structurally related gene products are described for which no ligands have yet been identified and, therefore, they are referred to as orphan nuclear receptors (Willy and Mangelsdorf, 1998; Giguère, 1999).
The retinoic acid-related orphan receptor β [RORβ; NR1F2 (Nuclear Receptors Nomenclature Committee, 1999)], also called retinoid Z receptor β (RZRβ), is an orphan member of family 1, which contains receptors such as RAR or TR. So far, three ROR isotypes, α, β and γ, have been described. Both RORα (NR1F1) and RORγ (NR1F3) are expressed in various tissues (Hirose et al., 1994; Matysiak-Scholze and Nehls, 1997; Koibuchi and Chin, 1998) and seem to be involved in cerebellum development, immune responses (Delerive et al., 2001), lymph node organogenesis and apoptosis during thymopoiesis (Kurebayashi et al., 2000), bone metabolism (Meyer et al., 2000) and adipocyte differenciation (Kurebayashi and Hirose, 1998).
In contrast, RORβ is expressed exclusively in areas of the central nervous system (CNS) that are involved in the processing of sensory information, including spinal cord, thalamus and cerebellar cortices, and also the three principal anatomical components of the mammalian timing system, the suprachiasmatic nuclei, the retina and the pineal gland (André et al., 1998).
Therefore, it seems that this orphan NR regulates genes whose products play important roles in the context of sensory input integration as well as in the context of the biological clock. RORβ knockout mice exhibit a behavioral phenotype with similarities to a phenotype described some 40 years ago for a spontaneous mouse mutation called vacillans (Sirlin, 1956).
These mice display a duck-like gait, transient male incapability to sexually reproduce and a severely disorganized retina that suffers from post-natal degeneration. Biochemical analyses indicated that RORβ can bind as a monomer to hormone response elements formed by the extented half-site sequence motif AnnTAGGTCA and activate reporter genes containing multiple copies of this half-site motif (Greiner et al., 1996).
However, in spite of the simplicity of the extented half-site sequence motif, no natural target gene regulated by RORβ could be identified up to now. In addition, RORβ is classified as an orphan receptor, and the lack of a putative ligand has complicated the identification of physiologically relevant targets further.
To understand better the role of RORβ in physiology, i.e. regulation of neuronal gene expression, the identification of specific ligands is of utmost importance. To gain insight into the geometry of the potential ligand-binding pocket (LBP) of RORβ and the nature of putative ligands, we concentrated on elucidating the crystal structure of this NR.
Here we present the 1.9 Å crystal structure of a complex between the LBD of the rat RORβ, a fortuitous ligand (stearate) and a peptide from the NR-interacting domain of the coactivator SRC-1 (Onate et al., 1995), a member of the p160 coactivator family (Torchia et al., 1998). The atomic level description of this orphan NR in the active conformation, stabilized by the combined action of the coactivator peptide and the pseudo-ligand, provides an accurate image of the LBP. This information greatly spurs the ability and the rationale for the design of isotype-specific agonists and antagonists that could be used to characterize and modulate the physiological functions of RORβ.
reference link :https://www.ncbi.nlm.nih.gov/pmc/articles/PMC125710/
More information: Mi Ra Chang et al, RORβ modulates a gene program that is protective against articular cartilage damage, PLOS ONE (2022). DOI: 10.1371/journal.pone.0268663