Current antidepressant treatments are inadequate for many individuals, and when they are effective, they require several weeks of administration before a therapeutic effect can be observed. Improving the treatment of depression is challenging. Recently, the two-pore domain potassium channel TREK-1 has been identified as a new target in depression, and its antagonists might become effective antidepressants. In mice, deletion of the TREK-1 gene results in a depression-resistant phenotype that mimics antidepressant treatments. Here, we validate in mice the antidepressant effects of spadin, a secreted peptide derived from the propeptide generated by the maturation of the neurotensin receptor 3 (NTSR3/Sortilin) and acting through TREK-1 inhibition. NTSR3/Sortilin interacted with the TREK-1 channel, as shown by immunoprecipitation of TREK-1 and NTSR3/Sortilin from COS-7 cells and cortical neurons co-expressing both proteins. TREK-1 and NTSR3/Sortilin were colocalized in mouse cortical neurons. Spadin bound specifically to TREK-1 with an affinity of 10 nM. Electrophysiological studies showed that spadin efficiently blocked the TREK-1 activity in COS-7 cells, cultured hippocampal pyramidal neurons, and CA3 hippocampal neurons in brain slices. Spadin also induced in vivo an increase of the 5-HT neuron firing rate in the Dorsal Raphe Nucleus. In five behavioral tests predicting an antidepressant response, spadin-treated mice showed a resistance to depression as found in TREK-1 deficient mice. More importantly, an intravenous 4-d treatment with spadin not only induced a strong antidepressant effect but also enhanced hippocampal phosphorylation of CREB protein and neurogenesis, considered to be key markers of antidepressant action after chronic treatment with selective serotonin reuptake inhibitors. This work also shows the development of a reliable method for dosing the propeptide in serum of mice by using AlphaScreen technology. These findings point out spadin as a putative antidepressant of new generation with a rapid onset of action. Spadin can be regarded as the first natural antidepressant peptide identified. It corresponds to a new concept to address the treatment of depression. Depression is the most common of psychiatric illnesses, with prevalence estimates ranging from 5% to 20% within the general population. The design of effective treatments for this disorder is a challenging process, and the use of antidepressants has an overall low clinical efficacy as full remission only occurs in one-third of the patients. Moreover, the time between initial treatment and beneficial effects is relatively protracted. These limitations confirm the need to find new biological targets and drugs for the treatment of depression. We recently identified a conserved mouse potassium channel protein called TREK-1 (KCNK2) as a new target for treating depression. Here, we demonstrate that spadin, a natural peptide derived from a propeptide released in blood, is able to block the TREK-1 channel activity and has an antidepressant effect in mouse models of depression. We showed that spadin is an efficient antidepressant in mice that acts much faster (4 d versus several weeks) than fluoxetine, the most commonly used antidepressant. Our results with spadin in mice highlight the potential for novel and more efficacious treatments for depression in humans.

Arno is a 47-kDa human protein recently identified as a guanine nucleotide exchange factor for ADP ribosylation factor 1 (ARF1) with a central Sec7 domain responsible for the exchange activity and a carboxyl-terminal pleckstrin homology (PH) domain (Chardin, P., Paris, S., Antonny, B., Robineau, S., Béraud-Dufour, S., Jackson, C. L., and Chabre, M. (1996)Nature 384, 481–484). Binding of the PH domain to phosphatidylinositol 4,5-bisphosphate (PIP₂) greatly enhances Arno-mediated activation of myristoylated ARF1. We show here that in the absence of phospholipids, Arno promotes nucleotide exchange on [Δ17]ARF1, a soluble mutant of ARF1 lacking the first 17 amino acids. This reaction is unaffected by PIP₂, which suggests that the PIP₂-PH domain interaction does not directly regulate the catalytic activity of Arno but rather serves to recruit Arno to membranes. Arno catalyzes the release of GDP more efficiently than that of GTP from [Δ17]ARF1, and a stable complex between Arno Sec7 domain and nucleotide-free [Δ17]ARF1 can be isolated. In contrast to [Δ17]ARF1, full-length unmyristoylated ARF1 is not readily activated by Arno in solution. Its activation requires the presence of phospholipids and a reduction of ionic strength and Mg²⁺ concentration. PIP₂ is strongly stimulatory, indicating that binding of Arno to phospholipids is involved, but in addition, electrostatic interactions between phospholipids and the amino-terminal portion of unmyristoylated ARF1_GDP seem to be important. We conclude that efficient activation of full-length ARF1 by Arno requires a membrane surface and two distinct protein-phospholipid interactions: one between the PH domain of Arno and PIP₂, and the other between amino-terminal cationic residues of ARF1 and anionic phospholipids. The latter interaction is normally induced by insertion of the amino-terminal myristate into the bilayer but can also be artificially facilitated by decreasing Mg²⁺ and salt concentrations.

The small G protein Rap1 regulates diverse cellular processes such as integrin activation, cell adhesion, cell–cell junction formation and cell polarity. It is crucial to identify Rap1 effectors to better understand the signalling pathways controlling these processes. Krev interaction trapped 1 (Krit1), a protein with FERM (band four‐point‐one/ezrin/radixin/moesin) domain, was identified as a Rap1 partner in a yeast two‐hybrid screen, but this interaction was not confirmed in subsequent studies. As the evidence suggests a role for Krit1 in Rap1‐dependent pathways, we readdressed this question. In the present study, we demonstrate by biochemical assays that Krit1 interacts with Rap1A, preferentially its GTP‐bound form. We show that, like other FERM proteins, Krit1 adopts two conformations: a closed conformation in which its N‐terminal NPAY motif interacts with its C‐terminus and an opened conformation bound to integrin cytoplasmic domain associated protein (ICAP)‐1, a negative regulator of focal adhesion assembly. We show that a ternary complex can form in vitro between Krit1, Rap1 and ICAP‐1 and that Rap1 binds the Krit1 FERM domain in both closed and opened conformations. Unlike ICAP‐1, Rap1 does not open Krit1. Using sedimentation assays, we show that Krit1 binds in vitro to microtubules through its N‐ and C‐termini and that Rap1 and ICAP‐1 inhibit Krit1 binding to microtubules. Consistently, YFP‐Krit1 localizes on cyan fluorescent protein‐labelled microtubules in baby hamster kidney cells and is delocalized from microtubules upon coexpression with activated Rap1V12. Finally, we show that Krit1 binds to phosphatidylinositol 4,5‐P₂‐containing liposomes and that Rap1 enhances this binding. Based on these results, we propose a model in which Krit1 would be delivered by microtubules to the plasma membrane where it would be captured by Rap1 and ICAP‐1.