Retinoid-related Orphan Receptors (RORs): Roles in Cellular Differentiation and Development

Abstract

Retinoid-related orphan receptors RORalpha, -beta, and -gamma are transcription factors belonging to the steroid hormone receptor superfamily. During embryonic development RORs are expressed in a spatial and temporal manner and are critical in the regulation of cellular differentiation and the development of several tissues. RORalpha plays a key role in the development of the cerebellum particularly in the regulation of the maturation and survival of Purkinje cells. In RORalpha-deficient mice, the reduced production of sonic hedgehog by these cells appears to be the major cause of the decreased proliferation of granule cell precursors and the observed cerebellar atrophy. RORalpha has been implicated in the regulation of a number of other physiological processes, including bone formation. RORbeta expression is largely restricted to several regions of the brain, the retina, and pineal gland. Mice deficient in RORbeta develop retinal degeneration that results in blindness. RORgamma is essential for lymph node organogenesis. In the intestine RORgamma is required for the formation of several other lymphoid tissues: Peyer's patches, cryptopatches, and isolated lymphoid follicles. RORgamma plays a key role in the generation of lymphoid tissue inducer (LTi) cells that are essential for the development of these lymphoid tissues. In addition, RORgamma is a critical regulator of thymopoiesis. It controls the differentiation of immature single-positive thymocytes into double-positive thymocytes and promotes the survival of double-positive thymocytes by inducing the expression of the anti-apoptotic gene Bcl-X(L). Interestingly, all three ROR receptors appear to play a role in the control of circadian rhythms. RORalpha positively regulates the expression of Bmal1, a transcription factor that is critical in the control of the circadian clock. This review intends to provide an overview of the current status of the functions RORs have in these biological processes.

Fig. (1)

Functions of RORα in the development of the cerebellum. A. RORα plays a critical role in the maturation of Purkinje cells. Immature Purkinje cells arise from RORα⁻ precursor cells. RORα becomes highly expressed in postmitotic Purkinje cells at E12.5 of mouse embryonic development. RORα is required for dendritogenesis to proceed and for the induction of a number of genes normally expressed in mature Purkinje cells. Dendritogenesis and the expression of several genes normally expressed in mature Purkinje cells are inhibited in RORα-deficient mice. B. RORα regulates expression of several genes, including Pcp2 and Shh, directly by binding to ROREs in their promoter regions. Interestingly, the composition of the co-activator complexes assembled by RORα appears to be distinct for each gene, suggesting that the promoter context plays a critical role. These co-activators include β-catenin (β-cat), SRC1, p300, CBP, and Tip60. Activation of β-catenin by Wnt signaling might influence the transcriptional activity of RORα. RORα may act downstream of Wnt and because Shh is an RORα target gene, RORα may function as a mediator between the Wnt and Shh signaling pathways. Shh released by Purkinje cells interacts with Patched (Ptch) receptors present on granule cell precursors and lead to the activation of GLI transcription factors. Subsequently, this results in the transcriptional activation of several growth regulatory genes, including PCNA and cyclins, and an induction of the proliferation of these cells. The reduced Shh expression in Purkinje cells from RORα-deficient mice is a major cause of the cerebellar atrophy observed in these mice (Boukhtouche et al., 2006; Gold et al., 2003; Gold et al., 2006; Goldowitz and Hamre, 1998; Hamilton et al., 1996; Herrup and Mullen, 1979; Kenney et al., 2003; Landis and Sidman, 1978; Steinmayr et al., 1998; Wallace, 1999).

Fig. (1)

Functions of RORα in the development of the cerebellum. A. RORα plays a critical role in the maturation of Purkinje cells. Immature Purkinje cells arise from RORα⁻ precursor cells. RORα becomes highly expressed in postmitotic Purkinje cells at E12.5 of mouse embryonic development. RORα is required for dendritogenesis to proceed and for the induction of a number of genes normally expressed in mature Purkinje cells. Dendritogenesis and the expression of several genes normally expressed in mature Purkinje cells are inhibited in RORα-deficient mice. B. RORα regulates expression of several genes, including Pcp2 and Shh, directly by binding to ROREs in their promoter regions. Interestingly, the composition of the co-activator complexes assembled by RORα appears to be distinct for each gene, suggesting that the promoter context plays a critical role. These co-activators include β-catenin (β-cat), SRC1, p300, CBP, and Tip60. Activation of β-catenin by Wnt signaling might influence the transcriptional activity of RORα. RORα may act downstream of Wnt and because Shh is an RORα target gene, RORα may function as a mediator between the Wnt and Shh signaling pathways. Shh released by Purkinje cells interacts with Patched (Ptch) receptors present on granule cell precursors and lead to the activation of GLI transcription factors. Subsequently, this results in the transcriptional activation of several growth regulatory genes, including PCNA and cyclins, and an induction of the proliferation of these cells. The reduced Shh expression in Purkinje cells from RORα-deficient mice is a major cause of the cerebellar atrophy observed in these mice (Boukhtouche et al., 2006; Gold et al., 2003; Gold et al., 2006; Goldowitz and Hamre, 1998; Hamilton et al., 1996; Herrup and Mullen, 1979; Kenney et al., 2003; Landis and Sidman, 1978; Steinmayr et al., 1998; Wallace, 1999).

Fig. (2)

RORγt is essential for the development of secondary lymphoid tisues. A. Lymphoid tissue inducer (LTi) cells are derived from hematopoietic stem cells and are essential in the development of lymph nodes, Peyer’s patches, and cryptopatches. The isolated lymphoid follicles (ILFs) are thought to be derived from cryptopatches after the colonization of the intestine by bacteria. RORγt is required for the generation and/or survival of LTi cells. The absence of these lymphoid tissues in RORγ-deficient mice is due to a deficiency of LTi cells in these mice. B. Role of RORγt and LTi cells in lymph node development. The transcription factors Id2 and RORγt are both essential for the generation of LTi cells (CD4⁺CD3⁻CD45⁺IL-7Rα⁺RORγt⁺Id2⁺). LTi cells are recruited to lymph node anlagen through their interaction with mesenchymal organizer cells. Interactions between these two cell types are key in the recruitment recruitment of LTi cell from the circulation to the lymph node anlagen. Activation of the RANK signaling pathway enhances the expression of lymphotoxins (LT) in LTi cells and promotes lymph node development. Binding of LTα1β2 on LTi cells to the LTβR on mesenchymal organizer cells plays a key role in lymph node development. The latter results in the activation of NF-κB pathways and induction of VCAM-1, ICAM-1, and MAdCAM-1 in mesenchymal organizer cells and their subsequent binding to integrins on LTi cells further promote their interaction and the recruitment of additional LTi cells. In addition, induction of various chemokines in mesenchymal organizer cells, including CXCL13 and CCL19, interact with their corresponding receptors on LTi cells thereby promoting their interaction and allow the recruitment of monocytes, T and B lymphocytes. (Cupedo et al., 2002; Eberl, 2005; Eberl and Littman, 2003; Eberl et al., 2004; Jetten, 2004; Jetten et al., 2001; Kurebayashi et al., 2000; Lipp and Muller, 2004; Sun et al., 2000; Yokota et al., 1999).

Fig. (2)

RORγt is essential for the development of secondary lymphoid tisues. A. Lymphoid tissue inducer (LTi) cells are derived from hematopoietic stem cells and are essential in the development of lymph nodes, Peyer’s patches, and cryptopatches. The isolated lymphoid follicles (ILFs) are thought to be derived from cryptopatches after the colonization of the intestine by bacteria. RORγt is required for the generation and/or survival of LTi cells. The absence of these lymphoid tissues in RORγ-deficient mice is due to a deficiency of LTi cells in these mice. B. Role of RORγt and LTi cells in lymph node development. The transcription factors Id2 and RORγt are both essential for the generation of LTi cells (CD4⁺CD3⁻CD45⁺IL-7Rα⁺RORγt⁺Id2⁺). LTi cells are recruited to lymph node anlagen through their interaction with mesenchymal organizer cells. Interactions between these two cell types are key in the recruitment recruitment of LTi cell from the circulation to the lymph node anlagen. Activation of the RANK signaling pathway enhances the expression of lymphotoxins (LT) in LTi cells and promotes lymph node development. Binding of LTα1β2 on LTi cells to the LTβR on mesenchymal organizer cells plays a key role in lymph node development. The latter results in the activation of NF-κB pathways and induction of VCAM-1, ICAM-1, and MAdCAM-1 in mesenchymal organizer cells and their subsequent binding to integrins on LTi cells further promote their interaction and the recruitment of additional LTi cells. In addition, induction of various chemokines in mesenchymal organizer cells, including CXCL13 and CCL19, interact with their corresponding receptors on LTi cells thereby promoting their interaction and allow the recruitment of monocytes, T and B lymphocytes. (Cupedo et al., 2002; Eberl, 2005; Eberl and Littman, 2003; Eberl et al., 2004; Jetten, 2004; Jetten et al., 2001; Kurebayashi et al., 2000; Lipp and Muller, 2004; Sun et al., 2000; Yokota et al., 1999).

Fig. (3)

RORγ exhibits multiple functions in thymopoiesis. CD4⁻CD8⁻CD25⁻CD44⁺ (DN1) cells hematopoietic precursor cells, differentiate via DN2 and DN3 into CD4⁻CD8⁻CD25⁻CD44⁻ (DN4) cells. The DN4 cells then give rise to immature single positive (ISP) cells (CD3⁻CD4⁻CD8^low). The ISP cells subsequently differentiate into CD3⁺CD4⁺CD8⁺, DP thymocytes. Expression of RORγt, as well as Bcl-X_L, LEF, and TCF-1, are induced during the ISP-DP transition and again down-regulated during the differentiation of DP into SP cells. RORγt promotes the differentiation of ISP into DP cells and is a positive regulator of Bcl-X_L expression. The latter stabilizes the cdk inhibitor p27, which subsequently inhibits cdk2 activity and increases the lifespan of DP thymocytes. Lack of RORγt expression inhibits the ISP to DP transition. In addition, expression of Bcl-X_L in DP thymocytes is reduced resulting in increased apoptosis, reduced the lifespan of DP thymocytes and consequently impaired TCRα rearrangements (Eberl and Littman, 2003; Guo et al., 2002; He, 2002; He et al., 2000; He et al., 1998; Jetten et al., 2001; Jetten and Ueda, 2002; Kurebayashi et al., 2000; Sun et al., 2000).