Ehsan Azimi

Redefining the concept of protease-activated receptors: cathepsin S evokes itch via activation of Mrgprs

Sensory neurons expressing Mas-related G protein coupled receptors (Mrgprs) mediate histamine-independent itch. We show that the cysteine protease cathepsin S activates MrgprC11 and evokes receptor-dependent scratching in mice. In contrast to its activation of conventional protease-activated receptors, cathepsin S mediated activation of MrgprC11 did not involve the generation of a tethered ligand. We demonstrate further that different cysteine proteases selectively activate specific mouse and human Mrgpr family members. This expansion of our understanding by which proteases interact with GPCRs redefines the concept of what constitutes a protease-activated receptor. The findings also implicate proteases as ligands to members of this orphan receptor family while providing new insights into how cysteine proteases contribute to itch.

Dual action of neurokinin-1 antagonists on Mas-related GPCRs.

The challenge of translating findings from animal models to the clinic is well known. An example of this challenge is the striking effectiveness of neurokinin-1 receptor (NK-1R) antagonists in mouse models of inflammation coupled with their equally striking failure in clinical investigations in humans. Here, we provide an explanation for this dichotomy: Mas-related GPCRs (Mrgprs) mediate some aspects of inflammation that had been considered mediated by NK-1R. In support of this explanation, we show that conventional NK-1R antagonists have off-target activity on the mouse receptor MrgprB2 but not on the homologous human receptor MRGPRX2. An unrelated tripeptide NK-1R antagonist has dual activity on MRGPRX2. This tripeptide both suppresses itch in mice and inhibits degranulation from the LAD-2 human mast cell line elicited by basic secretagogue activation of MRGPRX2. Antagonists of Mrgprs may fill the void left by the failure of NK-1R antagonists.

Substance P activates Mas-related G protein–coupled receptors to induce itch

Background Substance P (SP) is linked to itch and inflammation through activation of receptors on mast cells and sensory neurons. There is increasing evidence that SP functions through Mas‐related G protein–coupled receptors (Mrgprs) in addition to its conventional receptor, neurokinin‐1. Objective Because Mrgprs mediate some aspects of inflammation that had been considered mediated by neurokinin‐1 receptor (NK‐1R), we sought to determine whether itch induced by SP can also be mediated by Mrgprs. Methods Genetic and pharmacologic approaches were used to evaluate the contribution of Mrgprs to SP‐induced scratching behavior and activation of cultured dorsal root ganglion neurons from mice. Results SP‐induced scratching behavior and activation of cultured dorsal root ganglion neurons was dependent on Mrgprs rather than NK‐1R. Conclusion We deduce that SP activates MrgprA1 on sensory neurons rather than NK‐1R to induce itch.

Involvement of nitric oxide in serotonin‐induced scratching in mice

Serotonin is a pruritogenic substance in humans and animals, but the mechanisms of action through which serotonin induces itch response are not yet understood.

Altered manifestations of skin disease at sites affected by neurological deficit

The contribution of the nervous system to inflammation in general and inflammatory skin disease in particular has been underappreciated. It is now apparent that an intact neural component is required for the conventional clinical manifestations of many inflammatory skin diseases.

Peripheral Mechanisms of Itch.

A multitude of exogenous environmental stimuli and endogenous molecular and cellular components interface directly or indirectly with the free nerve endings of sensory nerves in the skin. Environmental stimuli include substances derived from the microbiome and materials, such as allergens, that otherwise come in contact with the skin. Endogenous stimuli include components of or mediators derived from the epidermal barrier, keratinocytes, mast cells, and additional resident and skin-homing immune cells. The sensation of itch is ultimately provoked by mediators that interact with and activate pruriceptors on the sensory nerve fibers. These peripheral fibers convey signals from the skin to the dorsal root and trigeminal ganglia and on to the spinal cord and brain where central processing of the itch sensation occurs. A discussion of the nature and sources of itch stimuli and receptors in the periphery form the basis of this chapter. The development of drugs that target these processes is in the process of revolutionizing therapeutic approaches to itch.

Evidence for the involvement of nitric oxide in cholestasis-induced itch associated response in mice

Cholestasis is a major systemic disorder associated with distressing pruritus (itch). Nitric oxide (NO) is a neurotransmitter, assumed to be involved in pruritus. Based on over-production of NO in cholestatic liver diseases, this project aimed to investigate involvement of NO in cholestasis-related itch in mice. To achieve this, cholestasis was induced by bile duct ligation (BDL). Our results showed that BDL mice elicited significant itch on fifth and seventh day after the procedure. This scratching behavior was inhibited by intraperitoneal (IP) treatment of mice with non-selective NOS inhibitor N-nitro-l-arginine methyl ester (l-NAME; 3mg/kg) and inducible NOS (iNOS) inhibitor aminoguanidine (AG; 100mg/kg). The inhibitory effects of l-NAME and AG were reversed by pretreatment with l-arginine (100mg/kg). Administration of l-NAME, AG and l-arginine per se, in BDL and SHAM mice did not produce scratching behaviors. In addition, intradermal injection of l-arginine at dose of 300 nmol/site significantly increased itch in BDL mice. Furthermore, nitrite levels in skin and serum of BDL animals significantly increased after 7 d of operation and administration of NOS inhibitors decreased this enhancement. l-arginine injection reversed the effects of NOS inhibitors on reduction of nitrite levels in the skin and serum of BDL mice. Finally, cutaneous iNOS expression increased in BDL mice 7 d after surgery. Taken together, our study showed for the first time that BDL, as a model of acute cholestasis in rodents, induces NO over-production by activating NOS enzymes, especially iNOS, which contribute to pruritus.

Development of resistance to serotonin-induced itch in bile duct ligated mice

Cholestatic itch can be severe and significantly impair the quality of life of patients. The serotonin system is implicated in cholestatic itch; however, the pruritogenic properties of serotonin have not been evaluated in cholestatic mice. Here, we investigated the serotonin‐induced itch in cholestatic mice which was induced by bile duct ligation (BDL). Serotonin, sertraline or saline were administered intradermally to the rostral back area in BDL and sham operated (SHAM) mice, and the scratching behaviour was videotaped for 1 hour. Bile duct ligated mice had significantly increased scratching responses to saline injection on the seventh day after surgery. Additionally, serotonin or sertraline significantly induced scratching behaviour in BDL mice compared to saline at day 7 after surgery, while it did not induce itch at day 5. The scratching behaviour induced by serotonin or sertraline was significantly less in BDL mice compared to SHAM mice. Likewise, the locomotor activity of BDL or SHAM mice was not significantly different from unoperated (UNOP) mice on the fifth and seventh day, suggesting that the scratching behaviour was not affected by motor dysfunctions. Our data suggest that despite the potentiation of evoked itch, a resistance to serotonin‐induced itch is developed in cholestatic mice.

Mas-Related G-Protein Coupled Receptors and Cowhage-Induced Itch

Cowhage is routinely used in human studies of histamine-independent itch. As such, it is of interest to understand its mechanism of action. The active component of cowhage is a cysteine protease called mucunain. As serine proteases can activate members of the protease-activated receptor (PAR) family to induce itch and pain, the mechanism of action of mucunain and cathepsin S, a human cysteine protease, were likewise thought to be through activation of PARs. The demonstration that cathepsin S activates members of the Mas-related G-protein coupled receptor family to induce itch led us to evaluate the activity of mucunain on these receptors. We find that mucunain activates the human receptors MRPGRX1 and MRGPRX2 and induces degranulation of human mast cells. These findings indicate that Mrgprs may be involved in itch induced by cowhage.

MRGPRX2, atopic dermatitis, and red man syndrome

Vancoymycin causes red man syndrome, an itchy erythematous eruption involving the face, neck, and upper torso. Atopic dermatitis also manifests itch and erythema, and staphylococcus &dgr;-toxin contributes to this process. The antibiotic and toxin each provoke mast cell degranulation but the mechanism had not been understood. We have determined that these compounds evoke degranulation via interaction with the same receptor, MRGPRX2, on mast cells. A receptor antagonist inhibits this process. Antagonists of this receptor may have therapeutic potential.