1‐Stearoyl‐2‐docosahexaenoyl‐phosphatidic acid interacts with and activates Praja‐1, the E3 ubiquitin ligase acting on the serotonin transporter in the brain

Abstract

Edited by Sandro SonninoSerotonin transporter (SERT) is involved in serotonergic system regulation and in the pathophysiology/therapeutics of serotonin-/SERT-related diseases such as obsessive-compulsive disorder, depression, autism, and schizophrenia. We recently revealed that diacylglycerol (DG) kinase (DGK) d induces ubiquitination/degradation of SERT in a DGK activity-dependent manner through Praja-1 E3 ubiquitin-protein ligase. However, it is still unclear how Praja-1 activity is regulated by DGKd. Here, we reveal that 1-stearoyl-2-docosahexaenoyl (18:0/22:6)-phosphatidic acid (PA) and 18:0/22:6-DG are simultaneously decreased and accumulated, respectively, in the DGKd-knockout mouse brain, indicating that DGKd selectively phosphorylates 18:0/22:6-DG to generate 18:0/22:6-PA. Moreover, we find that 18:0/22:6-PA selectively binds to Praja-1 and enhances its activity. These results strongly suggest that 18:0/ 22:6-PA generated by DGKd activates Praja-1 to degrade SERT in the brain.Keywords: diacylglycerol kinase; docosahexaenoic acid; E3 ubiquitinprotein ligase; obsessive-compulsive disorder; Praja-1; serotonin transporter Serotonin/5-hydroxytryptamine (5-HT) is known to be involved in anxiety, depression, compulsiveness, and impulsivity [1]. The serotonin/5-HT transporter (SERT) reuptakes 5-HT from the synaptic cleft into the presynaptic neuron for recycling and metabolic degradation [2,3]. Selective serotonin reuptake (SERT) inhibitors (SSRIs) are used for the treatment of obsessive-compulsive disorder (OCD) [2,3] and major depressive disorder [4]. Several studies using SERTdeficient rodents have demonstrated that SERT is implicated in neurodevelopmental disorders, such as depression, anxiety, and autism [5]. Therefore, hyperfunction of SERT, which reduces the amount of 5-HT in the synaptic cleft, causes various psychiatric disorders mentioned above. However, the regulatory mechanisms of SERT function are still obscure.Diacylglycerol (DG) kinase (DGK), which consists of ten isozymes (a-j), is a lipid-metabolizing enzyme that phosphorylates DG to generate phosphatidic acid (PA) [6][7][8][9][10]. DG and PA are established lipid second messengers that regulate a wide variety of physiological and pathological events.

New Era of Diacylglycerol Kinase, Phosphatidic Acid and Phosphatidic Acid-Binding Protein

“…DGKδ interacted with SERT [ 40 , 123 ], melanoma antigen gene-D1 (MAGE-D1) [ 40 ], and Praja-1 E3 ubiquitin-protein ligase [ 40 ], which ubiquitinates SERT [ 124 ], and induced SERT degradation in a DGK activity-dependent manner [ 40 ]. It is noteworthy that only the level of 1-stearoyl-2-docosahexaenoyl (18:0/22:6)-PA was decreased in the DGKδ-KO mouse brain [ 66 ], suggesting that DGKδ generates 18:0/22:6-PA in the brain. Intriguingly, 18:0/22:6-PA selectively bound to Praja-1 and enhanced its activity (see Table 1 ) [ 66 ].…”

“…It is noteworthy that only the level of 1-stearoyl-2-docosahexaenoyl (18:0/22:6)-PA was decreased in the DGKδ-KO mouse brain [ 66 ], suggesting that DGKδ generates 18:0/22:6-PA in the brain. Intriguingly, 18:0/22:6-PA selectively bound to Praja-1 and enhanced its activity (see Table 1 ) [ 66 ]. These results indicate that DGKδ generates distinct PA species in different tissue/cells and/or in response to different stimuli.…”

“…It is possible that the general screening with 16:0/18:1-PA and PA mixtures missed some PABPs that are selective for minor PA species. Therefore, we recently started a comprehensive screening for PABPs in the skeletal muscle and brain using several minor PA species, including 16:0/16:0-PA, which is generated by DGKδ in myoblast cells [ 76 ] and DGKζ in neuronal cells [ 109 ], and 18:0/22:6-PA, which is produced by DGKδ in the brain [ 66 ]. As a result, we found several new PABPs that have different selectivities to PA species.…”

“…Ubiquitinated SERT is quickly degraded in proteasomes and, consequently, the serotonergic system, especially the level of serotonin in the synaptic cleft, is upregulated. Praja-1 was recently found to strongly interact with 18:0/22:6-PA but not 16:0/16:0-PA, 16:0/18:1-PA, 18:1/18:1-PA, 18:0/20:4-PA, 18:0/18:0-PA or PS [ 66 ] ( Table 1 ). Moreover, it is noteworthy that the E3 ubiquitin-protein ligase activity of Praja-1 is selectively enhanced by the DHA-containing PA, 18:0/22:6-PA, which is generated by DGKδ in the brain [ 66 ].…”

“…It was reported that DGKα and DGKγ activated PKCζ [ 63 , 64 ] and β2-chimaerin [ 65 ], respectively, probably via PA production. DGKδ was recently revealed to activate Praja-1, the E3 ubiquitin ligase acting on the serotonin transporter in the brain, through PA production [ 40 , 66 ]. It was demonstrated that creatine kinase-muscle type (CKM) functionally associated with DGKδ and was activated by PA [ 67 , 68 ].…”