9-cis-13,14-Dihydroretinoic Acid Is an Endogenous Retinoid Acting as RXR Ligand in Mice

The retinoid X receptors (RXRs) are ligand-activated transcription factors which heterodimerize with a number of nuclear hormone receptors, thereby controlling a variety of (patho)-physiological processes. Although synthetic RXR ligands are developed for the treatment of various diseases, endogenous ligand(s) for these receptors have not been conclusively identified. We show here that mice lacking cellular retinol binding protein (Rbp1-/-) display memory deficits reflecting compromised RXR signaling. Using HPLC-MS and chemical synthesis we identified in Rbp1-/- mice reduced levels of 9-cis-13,14-dihydroretinoic acid (9CDHRA), which acts as an RXR ligand since it binds and transactivates RXR in various assays. 9CDHRA rescues the Rbp1-/- phenotype similarly to a synthetic RXR ligand and displays similar transcriptional activity in cultured human dendritic cells. High endogenous levels of 9CDHRA in mice indicate physiological relevance of these data and that 9CDHRA acts as an endogenous RXR ligand.

Daily nutrition, in addition to being a source of energy, contains micronutrients, a class of nutrients including vitamins which are essential for life and which act by orchestrating a vast number of developmental and physiological processes. During metabolism, micronutrients are frequently transformed into their bioactive forms. Nuclear hormone receptors are a family of proteins functioning as ligand-regulated transcription factors which can sense such bioactive molecules and translate those signals into transcriptional, adaptive responses. Retinoid X receptors occupy a central place in this signaling as they directly interact, and thereby control, activities of several nuclear hormone receptors. We report here the identification of a novel bioactive form of vitamin A, which is the first endogenous form of this vitamin capable to bind and activate retinoid X receptors. Accordingly, we show that this single molecule displays biological activity similar to synthetic agonists of retinoid X receptors and coordinates transcriptional activities of several nuclear receptor signaling pathways. Those findings may have immediate biomedical implications, as retinoid X receptors are implicated in the control of a number of physiological functions and their pathology.

Retinoid x receptor gamma is implicated in docosahexaenoic acid modulation of despair behaviors and working memory in mice

BACKGROUND

Omega-3 polyunsaturated fatty acids, including docosahexaenoic acid (DHA), have antidepressant and promnemonic functions. The mechanisms of such activities are still elusive and may involve retinoid X receptors (RXRs), transcription factors known to bind DHA in vitro.

METHODS

Promnemonic and antidespair activities of acute DHA treatment were tested in BALBcByJ mice using spontaneous alternation and forced swim test, respectively. The involvement of retinoid receptors in such DHA activities was investigated using RXR and/or retinoic acid receptor (RAR) agonists to mimic DHA activities or a synthetic pan-RXR antagonist to block them. Involvement of RXR isotypes was analyzed using the same tasks and delayed nonmatch to place for working memory in RXRγ knockout mice.

RESULTS

Docosahexaenoic acid decreased despair behavior and improved working memory in BALBcByJ mice. Such effects were suppressed by co-treatment with BR1211, a pan-RXR antagonist, whereas a pan-RXR agonist, UVI2108, mimicked DHA activities. Retinoic acid (RA), a natural ligand of RXRs, also reduced despair behavior and improved working memory and such activities did not require activation of RARs, as RA effects were abolished by co-treatment with BR1211 and they were not reproduced by TTNPB, a pan-RAR agonist. The RXRγ knockout mice displayed increased despair and deficits in working memory, which were insensitive to DHA and pan-RXR agonist treatments, whereas DHA or UVI2108 reversed these deficits in RXRγ heterozygous mice.

CONCLUSIONS

Our data suggest that RXRs are a converging point in mediating DHA and RA modulations of despair behavior and working memory and that RXRγ is the predominant RXR isotype in these regulations.