Metabolic depletion of sphingolipids impairs ligand binding and signaling of human serotonin<SUB>1A</SUB> receptors

Sphingolipids are essential components of eukaryotic cell membranes and are thought to be involved in a variety of cellular functions. Mycotoxins such as fumonisins can disrupt sphingolipid metabolism, and treatment with fumonisins represents an efficient approach to modulate cellular sphingolipid levels. In this work, we modulated sphingolipid levels in CHO cells stably expressing the human serotonin1A receptor by metabolically inhibiting the biosynthesis of sphingolipids using fumonisin B1. Serotonin1A receptors belong to the family of seven-transmembrane domain receptors that couple to G-proteins and are implicated in the generation and modulation of various cognitive, behavioral, and developmental functions. We explored the function of serotonin1A receptors under sphingolipid-depleted conditions by monitoring ligand binding, G-protein coupling, and downstream signaling of the receptor. Importantly, our results show that the function of the serotonin1A receptor is impaired upon metabolic depletion of sphingolipids, although the membrane receptor level does not exhibit any reduction. Interestingly, we find that the replenishment of sphingolipids using sphingosine results in restoration of ligand binding of serotonin1A receptors, demonstrating that the loss of ligand binding due to metabolic depletion of sphingolipids is reversible. These novel results demonstrate that sphingolipids are necessary for ligand binding and downstream signaling of the human serotonin1A receptor. We discuss possible mechanisms of specific interaction of sphingolipids with the serotonin1A receptor that could involve the proposed "sphingolipidbinding domain" (SBD)