Abstract
Photoreceptor-specific nuclear receptor (PNR/NR2E3) and Tailless homolog (TLX/NR2E1) are human orthologs of the NR2E group, a subgroup of phylogenetically related members of the nuclear receptor (NR) superfamily of transcription factors. We assessed the ability of these NRs to form heterodimers with other members of the human NRs representing all major subgroups. The TLX ligand-binding domain (LBD) did not appear to form homodimers or interact directly with any other NR tested. The PNR LBD was able to form homodimers, but also exhibited robust interactions with the LBDs of peroxisome proliferator-activated receptor-γ (PPARγ)/NR1C3 and thyroid hormone receptor b (TRb) TRβ/NR1A2. The binding of PNR to PPARγ was specific for this paralog, as no interaction was observed with the LBDs of PPARα/NR1C1 or PPARδ/NR1C2. In support of these findings, PPARγ and PNR were found to be co-expressed in human retinal tissue extracts and could be co-immunoprecipitated as a native complex. Selected sequence variants in the PNR LBD associated with human retinopathies, or a mutation in the dimerization region of PPARγ LBD associated with familial partial lipodystrophy type 3, were found to disrupt PNR/PPARγ complex formation. Wild-type PNR, but not a PNR309G mutant, was able to repress PPARγ-mediated transcription in reporter assays. In summary, our results reveal novel heterodimer interactions in the NR superfamily, suggesting previously unknown functional interactions of PNR with PPARγ and TRβ that have potential importance in retinal development and disease.
Highlights
Nuclear receptors (NRs) are a superfamily of gene regulators controlling a range of physiological processes and developmental pathways.[1]
Novel photoreceptor-specific NR (PNR)/PPARγ heterodimers J Fulton et al PNR and TLX regulate gene expression through recruitment of corepressor complexes,[22,23,24] and recently, we demonstrated that these NRs interact directly with BCL11A/B proteins,[25] cofactors for the COUPTF/NR2F subfamily that function in globin gene switching and neurogenesis.[26,27]
We assessed the abilities of PNR and TLX ligand-binding domain (LBD) to form homo- or heterodimers in yeast two-hybrid experiments, using a panel of human NR LBDs expressed as AAD fusion proteins.[25]
Summary
Nuclear receptors (NRs) are a superfamily of gene regulators controlling a range of physiological processes and developmental pathways.[1]. Replacement of L375 with alanine strongly reduced the heterodimerization with PPARγ LBD in two-hybrid assays (Figures 2a and e), consistent with the location of this leucine at the proposed interface of PNR LBD homodimers.[29] The loss of PPARγ LBD interactions suggests that similar molecular surfaces are involved in homodimeric and heterodimeric complexes of PNR, whereas the inability to bind the corepressor motif may be due to perturbation of the cofactor binding site, or failure of the mutant constructs to form LBD dimers in the assay.
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