Improved ligand-binding- and signaling-competent human NK2R yields in yeast using a chimera with the rat NK2R C-terminus enable NK2R-G protein signaling platform

The tachykinin 2 receptor (NK2R) plays critical roles in gastrointestinal, respiratory and mental disorders and is a well-recognized target for therapeutic intervention. To date, therapeutics targeting NK2R have failed to meet regulatory agency approval due in large part to the limited characterization of the receptor-ligand interaction and downstream signaling. Herein, we report a protein engineering strategy to improve ligand-binding- and signaling-competent human NK2R that enables a yeast-based NK2R signaling platform by creating chimeras utilizing sequences from rat NK2R. We demonstrate that NK2R chimeras incorporating the rat NK2R C-terminus exhibited improved ligand-binding yields and downstream signaling in engineered yeast strains and mammalian cells, where observed yields were better than 4-fold over wild type. This work builds on our previous studies that suggest exchanging the C-termini of related and well-expressed family members may be a general protein engineering strategy to overcome limitations to ligand-binding and signaling-competent G protein-coupled receptor yields in yeast. We expect these efforts to result in NK2R drug candidates with better characterized signaling properties.

The immunomodulatory effects of tachykinins and their receptors

Tachykinins (TKs) are a family of neuropeptides mainly expressed by neuronal and non-neuronal cell types, especially immune cells. Expression of TKs receptors on immune cell surfaces, their involvement in immune-related disorders, and therefore, understanding their immunomodulatory roles have become of particular interest to researchers. In fact, the precise understanding of TKs intervention in the immune system would help to design novel therapeutic approaches for patients suffering from immune disorders. The present review summarizes studies on TKs function as modulators of the immune system by reviewing their roles in generation, activation, development, and migration of immune cells. Also, it discusses TKs involvement in three main cellular mechanisms including inflammation, apoptosis, and proliferation.

IGFs Potentiate TAC3-induced SLα Expression via Upregulation of TACR3 Expression in Grass Carp Pituitary Cells

In mammals, the tachykinin 3 (TAC3)/tachykinin receptor 3 (TACR3) systems have been confirmed to play an important role in the regulation of puberty onset. Using grass carp pituitary cells as the model, our recent study found that the TAC3 gene products could significantly induce somatolactin α (SLα) synthesis and secretion via TACR3 activation. In the present study, we seek to examine if pituitary TACR3 can serve as a regulatory target and contribute to TAC3 interactions with other SLα regulators. Firstly, grass carp TACR3 was cloned and tissue distribution showed that it could be highly detected in grass carp pituitary. Using HEK293 cells as the model, functional expression also revealed that grass carp TACR3 exhibited ligand binding selectivity and post-receptor signaling highly comparable to its mammalian counterpart. Using grass carp pituitary cells as the model, TACR3 mRNA expression could be stimulated by insulin-like growth factor (IGF)-I and -II via the IGF-I receptor coupled to phosphatidylinositol-3-kinase (PI₃K)/protein kinase B (Akt) and mitogen-activated protein kinase (MAPK) pathways. Interestingly, IGF-I/-II cotreatment could also significantly enhance TAC3-induced SLα mRNA expression and the potentiating effect was dependent on TACR3 expression and activation of adenylate cyclase (AC)/cAMP/protein kinase A (PKA), phospholipase C (PLC)/inositol 1,4,5-triphosphate (IP₃)/protein kinase C (PKC), and Ca²⁺/calmodulin (CaM)/calmodulin-dependent protein kinase II (CaMK-II) cascades. Besides, IGF-I-induced Akt phosphorylation but not MEK, extracellular signal-regulated kinase (ERK_1/2), and P₃₈MAPK phosphorylation was notably enhanced by TACR3 activation. These results, as a whole, suggest that the potentiating effect of IGFs on TAC3 gene products-induced SLα mRNA expression was mediated by TACR3 upregulation and functional crosstalk of post-receptor signaling in the pituitary.

Crystal structure of the human NK1 tachykinin receptor

The NK₁ tachykinin G-protein-coupled receptor (GPCR) binds substance P, the first neuropeptide to be discovered in mammals. Through activation of NK₁R, substance P modulates a wide variety of physiological and disease processes including nociception, inflammation, and depression. Human NK₁R (hNK₁R) modulators have shown promise in clinical trials for migraine, depression, and emesis. However, the only currently approved drugs targeting hNK₁R are inhibitors for chemotherapy-induced nausea and vomiting (CINV). To better understand the molecular basis of ligand recognition and selectivity, we solved the crystal structure of hNK₁R bound to the inhibitor L760735, a close analog of the drug aprepitant. Our crystal structure reveals the basis for antagonist interaction in the deep and narrow orthosteric pocket of the receptor. We used our structure as a template for computational docking and molecular-dynamics simulations to dissect the energetic importance of binding pocket interactions and model the binding of aprepitant. The structure of hNK₁R is a valuable tool in the further development of tachykinin receptor modulators for multiple clinical applications.

Direct activation of tachykinin receptors within baroreflex afferent pathway and neurocontrol of blood pressure regulation

AIM

Substance P (SP) causes vasodilation and blood pressure (BP) reduction. However, the involvement of tachykinin receptors (NKRs) within baroreflex afferent pathway in SP-mediated BP regulation is largely unknown.

METHODS

Under control and hypertensive condition, NKRs' expressions were evaluated in nodose (NG) and nucleus of tractus solitary (NTS) of male, female, and ovariectomized (OVX) rats; BP was recorded after microinjection of SP and NKRs agonists into NG; Baroreceptor sensitivity (BRS) was tested as well.

RESULTS

Immunostaining and immunoblotting data showed that NK1R and NK2R were estrogen-dependently expressed on myelinated and unmyelinated afferents in NG. A functional study showed that BP was reduced dose-dependently by SP microinjection, which was more dramatic in males and can be mimicked by NK1R and NK2R agonists. Notably, further BP elevation and BRS dysfunction were confirmed in desoxycorticosterone acetate (DOCA)-salt model in OVX compared with DOCA-salt model in intact female rats. Additionally, similar changes in NKRs' expression in NG were also detected using DOCA-salt and SHR. Compared with NG, inversed expression profiles of NKRs were also found in NTS with either gender.

CONCLUSION

The estrogen-dependent NKRs' expression in baroreflex afferent pathway participates at least partially in sexual-dimorphic and SP-mediated BP regulation under physiological and hypertensive conditions.

Neurokinin-3 Receptor Binding in Guinea Pig, Monkey, and Human Brain: In Vitro and in Vivo Imaging Using the Novel Radioligand, [18F]Lu AF10628

BACKGROUND

Previous autoradiography studies have suggested a marked interspecies variation in the neuroanatomical localization and expression levels of the neurokinin 3 receptor, with high density in the brain of rat, gerbil, and guinea pig, but at the time offered no conclusive evidence for its presence in the human brain. Hitherto available radioligands have displayed low affinity for the human neurokinin 3 receptor relative to the rodent homologue and may thus not be optimal for cross-species analyses of the expression of this protein.

METHODS

A novel neurokinin 3 receptor radioligand, [(18)F]Lu AF10628 ((S)-N-(cyclobutyl(3-fluorophenyl)methyl)-8-fluoro-2-((3-[(18)F]-fluoropropyl)amino)-3-methyl-1-oxo-1,2-dihydroisoquinoline-4-carboxamide), was synthesized and used for autoradiography studies in cryosections from guinea pig, monkey, and human brain as well as for positron emission tomography studies in guinea pig and monkey.

RESULTS

The results confirmed previous observations of interspecies variation in the neurokinin 3 receptor brain localization with more extensive distribution in guinea pig than in primate brain. In the human brain, specific binding to the neurokinin 3 receptor was highest in the amygdala and in the hypothalamus and very low in other regions examined. Positron emission tomography imaging showed a pattern consistent with that observed using autoradiography. The radioactivity was, however, found to accumulate in skull bone, which limits the use of this radioligand for in vivo quantification of neurokinin 3 receptor binding.

CONCLUSION

Species differences in the brain distribution of neurokinin 3 receptors should be considered when using animal models for predicting human neurokinin 3 receptor pharmacology. For positron emission tomography imaging of brain neurokinin 3 receptors, additional work is required to develop a radioligand with more favorable in vivo properties.

Colonic smooth muscle cells and colonic motility patterns as a target for irritable bowel syndrome therapy: mechanisms of action of otilonium bromide

Otilonium bromide (OB) is a spasmolytic compound of the family of quaternary ammonium derivatives and has been successfully used in the treatment of patients with irritable bowel syndrome (IBS) due to its specific pharmacodynamic effects on motility patterns in the human colon and the contractility of colonic smooth muscle cells. This article examines how. OB inhibits the main patterns of human sigmoid motility in vitro, which are spontaneous rhythmic phasic contractions, smooth muscle tone, contractions induced by stimulation of excitatory motor neurons and contractions induced by direct effect of excitatory neurotransmitters. It does this mainly by blocking calcium influx through L-type calcium channels and interfering with mobilization of cellular calcium required for smooth muscle contraction, thereby limiting excessive intestinal contractility and abdominal cramping. OB also inhibits T-type calcium channels and muscarinic responses. Finally, OB inhibits tachykinin receptors on smooth muscle and primary afferent neurons which may have the joint effect of reducing motility and abdominal pain. All these mechanisms mediate the therapeutic effects of OB in patients with IBS and might be useful in patients with other spastic colonic motility disorders such as diverticular disease.

Genetic association of the tachykinin receptor 1 TACR1 gene in bipolar disorder, attention deficit hyperactivity disorder, and the alcohol dependence syndrome

Single nucleotide polymorphisms (SNPs) in the tachykinin receptor 1 gene (TACR1) are nominally associated with bipolar affective disorder (BPAD) in a genome-wide association study and in several case-control samples of BPAD, alcohol dependence syndrome (ADS) and attention-deficit hyperactivity disorder (ADHD). Eighteen TACR1 SNPs were associated with BPAD in a sample (506 subjects) from University College London (UCL1), the most significant being rs3771829, previously associated with ADHD. To further elucidate the role of TACR1 in affective disorders, rs3771829 was genotyped in a second BPAD sample of 593 subjects (UCL2), in 997 subjects with ADS, and a subsample of 143 individuals diagnosed with BPAD and comorbid alcohol dependence (BPALC). rs3771829 was associated with BPAD (UCL1 and UCL2 combined: P = 2.0 × 10(-3)), ADS (P = 2.0 × 10(-3)) and BPALC (P = 6.0 × 10(-4)) compared with controls screened for the absence of mental illness and alcohol dependence. DNA sequencing in selected cases of BPAD and ADHD who had inherited TACR1-susceptibility haplotypes identified 19 SNPs in the promoter region, 5' UTR, exons, intron/exon junctions and 3' UTR of TACR1 that could increase vulnerability to BPAD, ADS, ADHD, and BPALC. Alternative splicing of TACR1 excludes intron 4 and exon 5, giving rise to two variants of the neurokinin 1 receptor (NK1R) that differ in binding affinity of substance P by 10-fold. A mutation in intron four, rs1106854, was associated with BPAD, although a regulatory role for rs1106854 is unclear. The association with TACR1 and BPAD, ADS, and ADHD suggests a shared molecular pathophysiology between these affective disorders.

Effect of the long-acting tachykinin NK(1) receptor antagonist MEN 11467 on tracheal mucus secretion in allergic ferrets

1. We investigated the effect of MEN 11467 ((1R,2S)-2-N[1(H)indol-3-yl-carbonyl]-1-N-[N(alpha)(p-tolylacetyl)-N(alpha)(methyl)-D-3-(2-naphthyl)alanyl]diaminocyclohexane) on tachykinin-induced mucus secretion in ferret trachea in vitro and determined its effect on secretion by tracheae from allergic ferrets in response to allergen challenge. 2. Repeated administration of [Sar(9),Met(O(2))(11)]-substance P ([Sar(9)]SP, 1 microM) maintained mucus output above control values for at least 1.75 h. MEN 11467 inhibited secretion in a concentration-dependent manner with maximal inhibition at 10 microM and an approximate IC(50) of 0.3 microM. Inhibition by MEN 11467 (0.1--10 microM) was maintained, to varying degree, for at least 1.75 h after washout in the continued presence of [Sar(9)]SP. 3. In electrically stimulated tracheae, tachykininergic neural secretion was virtually abolished by 1 microM MEN 11467. 4. In tracheae from ovalbumin-sensitised animals, repeated administration of ovalbumin maintained mucus output above controls for 1.5 h. MEN 11467 inhibited ovalbumin-induced secretion in a concentration-dependent manner, with complete inhibition at 1 microM. Inhibition by MEN 11467 (1 and 10 microM) was maintained, to varying degree, after drug washout for the 1.5 h of ovalbumin stimulation. 5. MEN 11467 1 microM did not affect secretion induced by either acetylcholine or histamine, whereas 10 microM MEN 11467 did inhibit agonist-induced secretion. 6. We conclude that, in ferret trachea in vitro, MEN 11467 at concentrations of 0.1--1 microM is a long acting and selective inhibitor of tachykininergic-induced mucus secretion, and may have therapeutic potential for bronchial hypersecretion associated with allergic conditions, for example in asthma.

mu-Opioid receptors often colocalize with the substance P receptor (NK1) in the trigeminal dorsal horn

Substance P (SP) is a peptide that is present in unmyelinated primary afferents to the dorsal horn and is released in response to painful or noxious stimuli. Opiates active at the mu-opiate receptor (MOR) produce antinociception, in part, through modulation of responses to SP. MOR ligands may either inhibit the release of SP or reduce the excitatory responses of second-order neurons to SP. We examined potential functional sites for interactions between SP and MOR with dual electron microscopic immmunocytochemical localization of the SP receptor (NK1) and MOR in rat trigeminal dorsal horn. We also examined the relationship between SP-containing profiles and NK1-bearing profiles. We found that 56% of SP-immunoreactive terminals contact NK1 dendrites, whereas 34% of NK1-immunoreactive dendrites receive SP afferents. This result indicates that there is not a significant mismatch between sites of SP release and available NK1 receptors, although receptive neurons may contain receptors at sites distant from the peptide release site. With regard to opioid receptors, we found that many MOR-immunoreactive dendrites also contain NK1 (32%), whereas a smaller proportion of NK1-immunoreactive dendrites contain MOR (17%). Few NK1 dendrites (2%) were contacted by MOR-immunoreactive afferents. These results provide the first direct evidence that MORs are on the same neurons as NK1 receptors, suggesting that MOR ligands directly modulate SP-induced nociceptive responses primarily at postsynaptic sites, rather than through inhibition of SP release from primary afferents. This colocalization of NK1 and MORs has significant implications for the development of pain therapies targeted at these nociceptive neurons.

A new pharyngitis model using capsaicin in rats

1. Application of capsaicin solution onto the rat pharyngeal mucosa caused a well-reproducible increase in vascular permeability in the pharynx. 2. Capsaicin-induced pharyngeal inflammation was unaffected by a histamine H1 blocker and non-steroidal anti-inflammatory agents, whereas dexamethasone was effective in its inhibition. 3. FK224, a dual antagonist of tachykinin NK1 and NK2 receptors, and FK888, a selective antagonist of NK1 receptor, significantly inhibited capsaicin-induced plasma exudation in the pharynx. 4. In capsaicinized animals, the application of capsaicin solution in the pharyngeal mucosa did not induce pharyngitis. 5. These results suggest that the mechanism of the capsaicin-induced pharyngitis primarily involves tachykinins.