Targeting two-pore domain K(+) channels TREK-1 and TASK-3 for the treatment of depression: a new therapeutic concept

Abstract

Depression is a disease that is particularly frequent, affecting up to 20% of the population in Western countries. The origins of this pathology involve multiple genes as well as environmental and developmental factors leading to a disorder that remains difficult to treat. Several therapies for depression have been developed and these mainly target monoamine neurotransmitters. However, these treatments are not only associated with numerous adverse effects, but they are also ineffective for more than one-third of patients. Therefore, the need to develop new concepts to treat depression is crucial. Recently, studies using knockout mouse models have provided evidence for a crucial role of two members of the two-pore domain potassium channel (K2P ) family, tandem P-domain weak inward rectifying K(+) (TWIK)-related K(+) channel 1 (TREK-1) and TWIK-related acid-sensitive K(+) channel 3 (TASK-3) in the pathophysiology of depression. It is believed that TREK-1 and TASK-3 antagonists could lead to the development of new antidepressants. Herein, we describe the discovery of spadin, a natural peptide released from the maturation of the neurotensin receptor-3 (also known as sortilin), which specifically blocks the activity of the TREK-1 channel and displays particular antidepressant properties, with a rapid onset of action and the absence of adverse effects. The development of such molecules may open a new era in the field of psychiatry.

Figure 1

Two-pore domain K⁺ (K_2P) channels. (A) Structure of K_2P channels: these channels are constituted by four transmembrane segments (M1 to M4) and two pore domains (P1 and P2) that constitute the wall of the channel pore. (B) K_2P channel family: these channels are classified into six different subfamilies as symbolized by the branch colour code.

Figure 2

Pathologies where TREK-1 and TASK-3 are involved. Green and red arrows indicate that channels have to be respectively opened or closed for exerting a positive influence on the physiopathological process.

Figure 3

Effects of the deletion of TREK-1 channels or spadin in animal models of depression. (A) FST: mice were introduced into a beaker half-filled with 21–23°C water for 6 min. and the immobility time was measured during the last 4 min. Saline, mice injected with NaCl 0.9%, TREK1 (kcnk2)^−/−, TREK-1 knock-out mice, Fluox, mice injected i.p. with fluoxetine (3 mg·kg⁻¹), spadin (10⁻⁶, 10⁻⁷ and 10⁻⁸ M), mice injected i.v. at 10 μg·kg⁻¹, 1.0 and 0.1 μg·kg⁻¹, respectively. (B) NSF: mice were deprived of food for 24 h. Then, a food pellet was placed in a white circle at the centre of a brightly lit area (45 × 45 × 20 cm), and the mouse was placed in a corner of the box. The time needed to go and eat the food pellet was measured. In the chronic treatment, fluoxetine (Fluox) was given through the drinking water (80 mg·mL⁻¹) for 21 days. For the subchronic treatment, saline (0.9% NaCl) and Fluox (3 mg·kg⁻¹) were administered by i.p. injections (once a day) and spadin (10 μg·kg⁻¹) was administered by i.v. injections (once a day). ***P < 0.001.

Figure 4

Schematic involvement of TREK-1/sortilin complex in the 5-HT transmission. Red arrows represent the natural pathway of synthesis, secretion re-uptake of 5-HT (red circles). Black bars represent the pathway that slows down or inhibits the release of 5-HT. Black arrows represent pathways that are beneficial for 5-HT release. TREK-1/sortilin complex is beneficial when spadin is bound. AP, action potential; SERT, 5-HT transporter.

Figure 5

Sortilin and spadin. (A) Structure of the sortilin receptor: sortilin is constituted by a unique transmembrane segment, a short cytoplasmic C-terminal moiety and a large external N-terminal sequence. (B) Sortilin secretion pathway: Sortilin is processed in the Golgi network where the enzyme named furin releases the last 44 amino acids (aa) from the N-terminus; this released peptide is called propeptide (PE). (C) Spadin sequence designed from the PE sequence.

Figure 6

Spadin, a specific TREK-1 channel blocker. (A) Binding experiments with [¹²⁵I]-spadin performed on COS-7 cells transfected with the TREK-1 channel. Unlike neurotensine (NT), unlabelled spadin was able to compete with radiolabelled spadin. (B, C and D) All the experiments were performed in the whole-cell configuration of the patch-clamp protocol and in the presence of a cocktail of potassium channel inhibitors (K⁺ blockers: 3 mM 4-aminopyridine, 10 mM tetraethylamonium, 10 μM glibenclamide, 100 nM apamin and 50 nM charybdotoxin). aa indicates that currents were recorded in the presence of 10 μM arachidonic acid. As previously described (Patel et al., ; Mazella et al., 2010), the amplitude of the basal whole-cell TREK-1 current (control) was currently low. It is the reason why aa was applied before spadin to first activate the TREK-1 channel and then observed the blocking of these channels by spadin. (B) Effects of spadin on the TREK-1 channel current expressed in transfected COS-7 cells. Inset. Dose–response curves of the percentage of current inhibition measured at 0 mV as function of spadin concentration, current values were obtained in the presence of arachidonic acid. IC₅₀ = 70.7 nM. (C and D) Effects of spadin on neurons from hippocampus slices. Currents increased by aa application were inhibited by spadin on the wild-type neurons (C), aa had no effect on currents generated by neurons from TREK-1 knock-out mice; consequently, spadin had no effect on these currents (D).