NR1H4 and major depressive disorder: The bile acid nuclear receptor FXR is an essential component of the liver-gut-brain axis that has also been shown to play a vital role in diabetes and depression (Bao et al., 2021; Caron et al., 2013; Duran-Sandoval et al., 2005; Huang et al., 2015; Watanabe et al., 2004b; Yan et al., 2021).