Tryptophanemia is controlled by a tryptophan-sensing mechanism ubiquitinating tryptophan 2,3-dioxygenase

Maintaining stable tryptophan levels is required to control neuronal and immune activity. We report that tryptophan homeostasis is largely controlled by the stability of tryptophan 2,3-dioxygenase (TDO), the hepatic enzyme responsible for tryptophan catabolism. High tryptophan levels stabilize the active tetrameric conformation of TDO through binding noncatalytic exosites, resulting in rapid catabolism of tryptophan. In low tryptophan, the lack of tryptophan binding in the exosites destabilizes the tetramer into inactive monomers and dimers and unmasks a four–amino acid degron that triggers TDO polyubiquitination by SKP1-CUL1-F-box complexes, resulting in proteasome-mediated degradation of TDO and rapid interruption of tryptophan catabolism. The nonmetabolizable analog alpha-methyl-tryptophan stabilizes tetrameric TDO and thereby stably reduces tryptophanemia. Our results uncover a mechanism allowing a rapid adaptation of tryptophan catabolism to ensure quick degradation of excess tryptophan while preventing further catabolism below physiological levels. This ensures a tight control of tryptophanemia as required for both neurological and immune homeostasis.

Blood levels of essential amino acids are remarkably constant despite large variations in diet supply, but the mechanisms ensuring amino acid homeostasis remain poorly understood (1). Systemic homeostasis is particularly important for tryptophan given its key roles as a neurotransmitter precursor and a regulator of immune responses (2–5). In humans, tryptophanemia is stably maintained around 60 ± 15 µM (mean ± SD) (6). Tryptophan catabolism involves dioxygenation leading to the production of kynurenine and derivatives (7, 8). This first and rate-limiting step can be catalyzed by two enzymes: TDO and indoleamine 2,3-dioxygenase (IDO1). Despite functional homology, these two enzymes differ in sequence, structure, expression, and physiological role. TDO (gene name TDO2) is a tetrameric enzyme expressed in the liver and responsible for degradation of excess dietary tryptophan (7, 9, 10). IDO1 is monomeric, only expressed in immune and inflammatory sites and mostly involved in immunoregulation (7, 11–13). Tryptophan catabolism by IDO1 can locally suppress T lymphocyte responses by depleting tryptophan and producing kynurenine. This immunosuppressive effect is exploited by tumors to resist immune rejection, and IDO1 inhibitors have been developed for cancer immunotherapy (3, 14). While IDO1 activity produces detectable levels of kynurenine in the blood, TDO does not as the kynurenine produced by TDO undergoes further degradation in the liver along the kynurenine pathway, leading to NAD and/or quinolinic acid (8). However, TDO activity is needed to control tryptophanemia. TDO-knockout (TDO-KO) mice and TDO-deficient humans have plasmatic tryptophan concentrations eight- to ninefold higher than wild-type mice or healthy humans (9, 15). As a result, TDO-KO mice better reject tumors and have higher levels of serotonin and other tryptophan metabolites in the brain, resulting in anxiolytic modulation and increased neurogenesis (9, 16). TDO is also expressed in some human tumors and may contribute to tumoral immune resistance (10, 16–18).