MRGPRX2 as a novel therapeutic target in headache☆

As discussed above, mast cell degranulation has been proposed to contribute to migraine and given the expression of MRGPRB2 and MRGPRX2 on mast cells, as well as their activation by migraine-relevant neuropeptides, there is strong rationale to investigate these receptors as potential migraine therapeutic targets. Using MRGPRB2Cre; ROSA26tdTomato mice, it was demonstrated that 96 % of meningeal mast cells express MRGPRB2 and are in proximity to sensory nerve fibers, making them potentially responsive to factors released from meningeal afferents [ 80 ]. Release of neuropeptides such as SP, PACAP, CGRP, and vasoactive intestinal peptide (VIP) from sensory neurons within the meninges can both act on meningeal vasculature as well as activate dural mast cells via MRGPRX2/B2, causing degranulation and production of mediators, including interleukin-1 (IL-1), Tumor Necrosis Factor-alpha (TNF-α), interleukin-6 (IL-6), serotonin, nitric oxide (NO), and tryptase [ 7 ]. In turn, these mediators can sensitize and activate sensory nerve endings, leading to more neuropeptide release, thereby establishing a bidirectional communication loop between mast cells and sensory neurons that amplifies and sustains neurogenic inflammation ( Fig. 2 ; [ 81 ]). In particular, TNF-α release downstream of MRGPRB2 receptor activation acts on nearby sensory neurons to enhance TRPV1 channel responsiveness, thereby enhancing nociceptive transmission and contributing to headache pathophysiology. In parallel, released TNF-α acts as a positive autocrine feedback signal to produce the chemoattractant cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-8 (IL-8) from mast cells. GM-CSF and IL-8, as well as being growth factors for monocytes and neutrophil recruitment, are essential for inflammation and enhanced pain hypersensitivity [ 77 ]. In addition to direct neuropeptide-mediated activation, CRF released from the hypothalamus in response to stress can directly activate MRGPRB2 on dural mast cells, initiating degranulation and promoting meningeal vasodilation along with migraine-like behaviors [ 14 , 17 , 34 , 82 , 83 ]. In vitro, CRF induces Ca 2 + transients in HEK293 cells expressing MRGPRB2 and in human LAD2/peritoneal mast cells, effects that are blocked in MRGPRB2 KO cells but not by CRF1 or CRF2 receptor antagonists. Mast cells in response to CRF also release TNF-α, which in turn potentiates TRPV1 channels on trigeminal ganglion nociceptors, contributing to neuronal sensitization [ 84 ]. In experimental preclinical models of migraine induced by stress exposure, stressed mice demonstrated a significant increase in the number of spontaneously active TG neurons following 3-days of repeated stress [ 85 ]. This increase was not observed in MRGPRB2 KO mice. Further, male mice deficient of MRGPRB2 did not show stress-induced periorbital hypersensitivity following the repeated stress paradigm in male mice [ 85 ]. Together, these findings provide further evidence for the role of MRGPRB2 in stress-induced changes in migraine models. Historically, SP-mediated pain and neurogenic inflammation were attributed to the neurokinin-1 (NK-1) receptor, owing to its anatomical localization at synapses in the dorsal horn and its well-known role in nociceptive signaling [ 86 ]. However, human clinical trials have shown the ineffectiveness of peripheral NK-1 receptor antagonists in the treatment of pain and migraine [ 87 , 88 ]. Recently, it was found that the pro-inflammatory effects of SP are mediated through the activation of mast cells via MRGPRX2/B2 [ 77 ]. SP induces mast cell degranulation in human mast cell line LAD2 cells via MRGPRX2, leading to the release of IL-8, CCL2, CCL3, and CCL4, capable of sensitizing peripheral neurons [ 77 , [89] , [90] , [91] ]. It has also been shown that, in vivo, SP injection into the hindpaw of MRGPRB2 −/− mice resulted in a loss of CCL2 and CCL3 release, as well as failure to elicit the associated immune cell recruitment in the site of injury, an effect specific to mast cell-expressed MRGPRB2. SP has been extensively studied in preclinical models for its ability to promote neurogenic inflammation and modulate nociceptive transmission [ 77 ]. SP released from DRG sensory neurons binds to MRGPRX2 on mast cells, inducing the release of chymase, a serine protease (mast cell protease 4 (MCPT4), chymase in mice). Chymase then activates protease-activated receptor 1 (PAR1) on adjacent sensory neurons which in turn sensitizes TRPV1 channels via PLC/PKC (phospho-protein kinase C), leading neuronal hyperexcitability, additional SP release, and sustained neurogenic inflammation [ 92 , 93 ]. Collectively, these studies provide an additional role for SP in addition to acting through its classical neuronal receptor NK1, where it can function as a key neuroimmune modulator that bridges sensory neurons and mast cells via the MRGPRX2/MRGPRB2 axis [ 93 ]. In addition to SP, PACAP is also able to activate MRGPRX2. This pathway is independent of canonical PACAP receptors (e.g., PAC 1 , VPAC 1 , VPAC 2 ; [ 94 ]). In experimental migraine models, dural injection of C48/80 and PACAP increased mechanical facial hypersensitivity, which was significantly reduced in in MRGPRB2 KO mice compared to wild type [ 80 ]. Dural PACAP resulted in approximately 30 % more degranulated mast cells identified on the dura in wild type mice using immunohistochemistry, an effect that was completely attenuated in MRGPRB2 KO mice [ 85 ]. Furthermore, stimulation of peritoneal mast cells with PACAP evoked a robust calcium influx in cells cultured from a humanized transgenic MRGPRX2 mouse line lacking the MRGPRB2 receptor, but this response was absent in control mice [ 80 ]. In vivo imaging studies further confirmed that PACAP activates mast cells within the dura mater. Using Mrgprb2-cre:GCaMP6 mice, PACAP application produced significant increases in Ca 2+ transients in dural mast cells, indicating direct mast cell activation and degranulation. In parallel, in vivo Pirt-GCaMP3 calcium imaging demonstrated that direct PACAP application to the dura increased Ca 2+ transients in trigeminal afferents, accompanied by a significant dilation of meningeal blood vessels. Both effects were absent in Mrgprb2-deficient mice, confirming that PACAP-induced mast cell activation, neuronal excitation, and dural vasodilation depend on MRGPRB2 signaling [ 85 ]. Together, these findings suggest that PACAP-induced activation of mast cells through MRGPRB2 establishes a neuroimmune interactions that enhances trigeminal neuron sensitization and may contribute to migraine pathophysiology. Consistent with these cellular findings, application of PACAP to the dura mater induced significantly greater migraine-like pain behavior in MRGPRX2-overexpressing mice than in control animals [ 80 ]. In addition, current evidence suggest that activation of meningeal mast cells by PACAP through MRGPRB2/MRGPRX2 induces the release of β-hexosaminidase and TNF-α, which contribute to the sensitization of trigeminal sensory neurons, lowering thresholds for mechanical/chemical stimuli [ 80 , 85 ]. It has been shown that PACAP-induced hypersensitivity is prevented by R-7050, a TNF-α receptor inhibitor, and SB366791, a TRPV1 channel antagonist, confirming that TRPV1 and TNF-α signaling act downstream of PACAP elevation [ 85 ]. TNF-α and TRPV1 were also shown to be involved in MRGPRB2-mediated mast cell degranulation in alcohol withdrawal-induced headache [ 84 ]. Longterm alcohol use leads to an increased number of mast cells and degranulation in peripheral tissue [ 95 ]. In vivo, headache-like behaviors provoked by alcohol withdrawal, increased grimace scores and reduced exploration in the open-field test, were all absent in MRGPRB2 KO mice, confirming the role of MRGPRB2 in alcohol withdrawal responses [ 84 ]. MRGPRB2 KO mice did not show alcohol withdrawal-induced increases in the total number of degranulated mast cells in the dura, nor an increase in activated TG neurons that is seen in wild-type mice in this model [ 84 ]. Wild-type mice demonstrated a significant increase in dural TNF-α levels with alcohol withdrawal, although this was not seen in the MRGPRB2 KO mice [ 70 ]. Furthermore, R-7050 attenuated alcohol withdrawal-induced migraine-like hypersensitivity and interestingly, migraine-like behaviors from alcohol withdrawal were also completely abolished with SB366791. As TNF-α is a known potentiator of TRPV1, these data provide evidence that MRGPRB2 activation of mast cells may produce downstream TRPV1 activation in alcohol withdrawal-induced headache, opening up another potential therapeutic avenue for MRGPRX2/B2.

Writing – Original Draft Preparation: S.N. and J.M.B.; Writing – Review & Editing: S.N., J.M.B, and G.D.; Supervision – G.D.

Compelling evidence from molecular, preclinical, and clinical studies has implicated MRGPRX2/B2 as a key receptor in mast cell-mediated effects, including neuroimmune interactions that may be involved in the pathophysiology of headache. Elucidating the role of MRGPRX2 in driving neurogenic inflammation could uncover novel mechanisms underlying headache disorders and pave the way for innovative therapies. Since MRGPRX2 can act as an alternative receptor for neuropeptides such as SP and PACAP, blocking this receptor may provide additional drug development opportunities to target the actions of these neuropeptides. The development of humanized MRGPRX2 mouse models provides an opportunity to study human-specific antagonists in relevant models before moving to the clinic. Targeting MRGPRX2 may effectively disrupt mast cell and trigeminovascular system-mediated neurogenic inflammation, offering therapeutic potential by blocking actions of a wide range of MRGPRX2 ligands that are implicated in migraine. The potential use of MRGPRX2 inhibitors in combination with established migraine treatments such as CGRP inhibitors, may provide enhanced relief for migraineurs not sufficiently treated by current therapeutics. The studies described above should provide sufficient evidence to justify further preclinical and clinical studies into MRGPRX2 antagonists as potential therapeutics for migraine and headache.

Many conditions, including mastocytosis, allergies, asthma and irritable bowel syndrome, in which mast cell activation is proposed to contribute to the pathophysiology, show a higher incidence of migraine [ [65] , [66] , [67] , [68] , [69] , [70] ]. Importantly, the overexpression of MRGPRX2 on mast cells has been identified in many of these conditions and is highly correlated to symptoms such as itch in many inflammatory conditions [ 71 , 72 ]. These findings have recently transitioned into clinical trials investigating MRGPRX2 antagonists in atopic dermatitis and chronic spontaneous urticaria [ [71] , [72] , [73] , [74] , [75] , [76] ] and should reveal further biological relevance for MRGPRX2 in these conditions in humans. Understanding the role of MRGPRX2 in these inflammatory conditions may provide a rationale for testing such inhibitors in other conditions such as migraine, where peripheral sensory neuron and mast cell interactions are also proposed to play a central role. The existence of mast cell-neuron communication pathways has been proposed to play a crucial role in inflammation-induced pain and headache [ 77 ]. The close proximity of mast cells to sensory nerve endings forms a functional homeostatic regulatory unit [ 77 ]. Dysregulation of this interaction has been identified as a significant component of neurogenic inflammation and pain. Green et al. [ 77 ] identified MRGPRB2 as a critical mediator of neuropeptide-induced immune cell recruitment. They showed that following hindpaw incision, both MRGPRB2 knockout (KO) mice and mast cell-depleted mice showed reduced thermal and mechanical hypersensitivity, emphasizing the central role of this receptor in post-incision behavioral responses. In addition, expression of activating transcription factor 3 (ATF3), a molecular marker of neuronal injury in DRG neurons, was reduced in MRGPRB2 KO mice, supporting the involvement of this receptor in mast cell-mediated neuronal activation and pain. In an endometriosis model of pain, it has been shown that binding of specific peptides to MRGPRX2 induces histamine release from mast cells through an IgE-independent pathway. The released histamine then acts on histamine H1 receptors (HRH1) expressed on DRG sensory neurons, leading to activation and sensitization of transient receptor potential vanilloid type 1 (TRPV1) ion channels. This occurs through direct interaction between HRH1 and the C-terminal domain of TRPV1, and indirect sensitization via HRH1-mediated activation of phospholipase A 2 and 12-lipoxygenase pathways. Both mechanisms enhance TRPV1 activity, resulting in increased calcium influx, neuronal hyperexcitability, and heightened pain sensitivity [ 46 ]. Given that TRPV1 channels are key components of trigeminovascular nociceptive transmission, and that activation of dural mast cells by SP, CGRP, and histamine is implicated in migraine, together with the expression of functional MRGPRX2/B2, these findings suggest the existence of a conserved mast cell-TRPV1 axis operating in both peripheral and meningeal pain [ 24 , 25 , 78 , 79 ]. This provides a mechanistic basis for investigating MRGPRX2/B2 as a potential therapeutic target in migraine pathophysiology ( Fig. 2 ). Fig. 2 MRGPRX2-mediated interactions between mast cells and neurons contribute to migraine pathophysiology. The release of neuropeptides, including SP and PACAP from meningeal nociceptors activate dural mast cells via MRGPRX2/B2. This activation initiates degranulation of mast cells and the production of mediators, including histamine and TNF-α. These mediators can then act on meningeal vasculature and can activate meningeal nociceptors, resulting in further neuropeptide release, sustaining the loop of neurogenic inflammation in migraine. Fig. 2 MRGPRX2-mediated interactions between mast cells and neurons contribute to migraine pathophysiology. The release of neuropeptides, including SP and PACAP from meningeal nociceptors activate dural mast cells via MRGPRX2/B2. This activation initiates degranulation of mast cells and the production of mediators, including histamine and TNF-α. These mediators can then act on meningeal vasculature and can activate meningeal nociceptors, resulting in further neuropeptide release, sustaining the loop of neurogenic inflammation in migraine.

MRGPRX2 may play a fundamental role in a range of disease though neurogenic inflammation induction. The use of MRGPRX2 inhibitors to block MRGPRX2 preventing mast cell activation and subsequent degranulation has already shown great promise preclinically offering a new therapeutic approach for many disorders, including atopic dermatitis [ 45 , 96 , 97 ]. An increased number of activated mast cells [ 98 , 99 ], along with elevated expression of MRGPRX2 [ 100 ] is seen in lesional skin of AD. Elevated levels of MRGPRX2 agonists, including SP, are also seen to be elevated in serum of AD patients [ [101] , [102] , [103] ]. Due to the role of SP in migraine- and headache-related neurogenic inflammation, findings from the use of MRGPRX2 in AD may help to establish a rationale for potential as a migraine therapeutic [ 104 ]. MRGPRX2 inhibitors such as Celastrol, derived from Tripterygium Wilfordii roots, and small molecule MRGPRX2 antagonists, KHM-45, GE1111, and C9, are currently being investigated as potential AD therapeutics and may reveal insights for migraine research ( Fig. 1 ). These candidates act by directly inhibiting MRGPRX2-mediated mast cell activation [ [105] , [106] , [107] , [108] , [109] ]. Celastrol and GE1111, both significantly reduce expression of IL-6 and IL-1B through targeting of the MRGPRX2 [ 105 , 107 ]. This reduction in IL-6 expression would be an invaluable asset for a migraine therapeutic due to the role of IL-6 in migraine and the increased serum levels of IL-6 [ 110 ]. Similarly, KHM-45 reduced β-hexosaminidase release from mouse peritoneal mast cells [ 106 , 111 , 112 ]. This reduction in mast cell degranulation was seen to be dose-dependent following activation of mast cells with C48/80. C9 and Paeoniflorin, two further MRGPRX2 inhibitors, have successfully shown to significantly reduce SP or C48/80-induced degranulation of mast cells, respectively [ 108 , 113 , 114 ]. C9 inhibited degranulation, β-arrestin recruitment and MRGPRX2 internalization in response to SP [ 108 ]. Many of these compounds can be tested in preclinical migraine models, helping to justify further exploration as potential therapeutics for this disorder. Preclinical success of several MRGPRX2 inhibitors in targeting mast cell activation has resulted in ongoing clinical trials for AD and chronic spontaneous urticaria patients (AD, NCT07150845 and NCT06144424 [ 73 , 74 ]; CSU NCT06873516 and NCT06077773 [ 75 , 76 ] that will reveal further insight into MRGPRX2 mediated diseases. EP262 and related EP9907 act as highly selective, non-competitive inverse agonists that can inhibit MRGPRX2 activation ( Fig. 1 ; [ 55 ]). Using MRGPRX2 knock-in mice, EP262 demonstrated a dose-dependent inhibition of cortistatin 14-induced mast cell degranulation and inhibited MRGPRX2 activation in several human mast cell lines and effectively blocked the release of inflammatory cytokines and tryptase [ 55 ], supporting robust target engagement. Remarkably, pretreatment with EP262 inhibited degranulation ex vivo in intact human skin tissue and led to a dose-dependent reduction in agonist-induced histamine release. Although the phase 2 clinical trial for EP262 in chronic spontaneous urticaria was terminated due to in vivo preclinical toxicology findings, preclinical data still highlights its potential [ 115 ]. These proof-of-concept findings strengthen the rationale for targeting MRGPRX2 in disorders characterized by neurogenic inflammation, including migraine, in which histamine levels are elevated in patient plasma [ 23 ], and support continued investigation of MRGPRX2 inhibition using optimized antagonists with improved safety profiles. EVO756 is a potent, highly selective, small-molecule antagonist shown to reduce MRGPRX2-mediated mast cell degranulation induced by SP, C48/80, cortistatin-14 and LL-34 in LAD2 cells, ROSA cells, primary human skin cells, as well as inhibited MRGPRX2 activation in human DRG sensory neurons following treatment with icatibant, a known ligand of MRGPRX2 [ 116 , 117 ]. These findings present EVO756 as an effective antagonist and the role of EVO756 in other neuroinflammatory conditions, including migraine, should be investigated due to the role of neurogenic inflammation and mast cell activation ( Fig. 1 ). Many compounds may show promise for the use in migraine and headache inhibiting MRGPRX2 and downstream effects. QWF is a tripeptide antagonist that directly inhibits MRGPRX2 and competitively inhibits SP binding to this receptor ( Fig. 1 ). QWF inhibits SP-induced mast cell activation and ameliorates SP-induced pain. Since QWF can also have inhibitory effects on the NK-1 and MRGPRA1, it is not fully clear whether these effects are entirely due to actions on MRGPRX2 [ 94 ]. Resveratrol, a polyphenol compound found in grapes and red wine, suppresses nuclear factor-kappa B (NF-κB) signaling downstream of MRGPRX2 through activation of the Nrf2/HO-1 pathway, which is one of the indirect mechanisms of MRGPRX2 inhibition [ 94 ]. The Nrf2/HO-1 pathway has been shown to have a protective effect in migraine, and animal models activating this pathway reduced migraine-like behavioral signs in mice [ 118 ], demonstrating its potential as a migraine therapeutic. Osthole, a coumarin derivative extracted from Cnidium monnieri (L.) Cusson, inhibits the increase in intracellular Ca 2+ concentration stimulated by ligands such as SP and other neuropeptides, and suppresses mast cell activation through MRGPRX2 [ 94 ]. Notably, recent in vivo studies further demonstrate that osthole acts in an MRGPRB2-dependent manner to reduce post-stroke neuroinflammation, a condition associated with migraine [ 119 , 120 ]. In addition, dexamethasone has been shown to inhibit mast cell activation by suppressing downstream Gi protein–coupled and MRGPRX2-mediated signaling pathways [ 94 ]. PSB-172656, 3-ethyl-7,8-difluoro-2-isopropylbenzo [4,5] imidazo [1,2-a] pyrimidin-4(1H)-one, is another small-molecule antagonist of MRGPRX2 that exhibits metabolic stability, low cytotoxicity, and competitive blockade of MRGPRX2 activation induced by a diverse range of agonists ( Fig. 1 ). This antagonist blocks MRGPRX2-mediated Gαq and Gαi dissociation, in addition to β-arrestin-2 recruitment, both of which may play a role in the neurogenic inflammation seen in migraine. Notably, PSB-172656 is selective for MRGPRX2 versus all other MRGPRX subtypes [ 121 ]. The described antagonists and compounds, as well as those described above including EP262 and EVO957 that can inhibit MRGPRX2 directly or indirectly, represent a unique set of tools that may be used as a treatment for inflammatory diseases, chronic pain, and headache. However, the benefits and side effects of each of these compounds need to be evaluated.

Migraine, tension-type headache, and cluster headache, are among the most common and debilitating types of primary headache disorders [ 1 ]. Headache affects people across the lifespan, across socioeconomic and racial groups, and is more prevalent in women than in men [ [2] , [3] , [4] ]. Migraine is the second leading cause of global disability, impacting approximately 15 % of the population, and is the most common neurological disorder [ 5 ]. Typically, migraine presents as unilateral, throbbing pain of moderate to severe intensity, often aggravated by physical activity and the attacks last between four and 72 h. Other symptoms include nausea, vomiting, photophobia, and phonophobia, all of which contribute to the significant burden of disease and help distinguish migraine from other types of headache [ 6 ]. Cluster headache is characterized by recurrent, severe, unilateral headaches that are typically localized to the orbital, supraorbital, or temporal areas, with attacks lasting from 15 to 180 min. Nasal congestion or rhinorrhea, lacrimation, forehead and facial sweating, and periorbital swelling are associated with autonomic symptoms [ 6 ]. The trigemino-parasympathetic reflex is thought to play a vital role in the pathophysiological mechanisms of cluster headache, contributing to the characteristic autonomic symptoms [ 7 ]. Tension-type headache is the most common primary headache disorder and it is characterized by mild to moderate pain of variable duration, typically bilateral, with a pressing or tightening quality [ 6 ]. The main category of therapeutics for headache disorders that has been the standard for acute use for decades is the triptans, drugs that as a class act at serotonin 5-HT 1 B and 5-HT 1 D receptors. Based on preclinical and clinical evidence, treatments such as anti-calcitonin gene-related peptide (CGRP) monoclonal antibodies and CGRP receptor antagonists have been developed more recently. However, existing treatments, including triptans, CGRP monoclonal antibodies, and gepants, fail to relieve symptoms in 30–50 % of migraine patients [ 8 ]. In addition, these treatments may also cause significant side effects, such as fatigue, weight changes, constipation, and cardiovascular issues, which contribute to low treatment adherence [ 8 ]. These limitations underscore the need to identify additional mechanisms and novel therapeutic targets that contribute to migraine pathophysiology. Migraine pathophysiology involves both peripheral and central mechanisms. In the periphery, a major mechanistic hypothesis has focused on the dura mater that is part of the meninges comprising nociceptors, blood vessels, and immune cells [ 9 ]. Trigeminal neurons that innervate the dura are thought to play a central role in mediating the pathophysiology of migraine headaches [ 10 , 11 ]. Trigeminal sensory neurons innervate the face, jaw, anterior part of the head, as well as cerebral vessels, with their cell bodies located in the trigeminal ganglion (TG; [ 9 ], [ 12 ]). The afferent fibers of the TG project to the dura mater, where they interact within a complex microenvironment and neurovascular network [ 13 ]. Activation of trigeminal afferents results in the release of vasoactive neuropeptides. Various factors, including chronic stress, dietary and metabolic factors, sleep disturbances, hormonal fluctuations, and cortical spreading depression, have been shown to increase trigeminal excitability and may promote the release of neuropeptides including CGRP, substance P (SP), and pituitary adenylate cyclase-activating polypeptide (PACAP; [ [14] , [15] , [16] ]). Upon release from nerve endings, these neuropeptides can instigate neurogenic inflammation, a process consisting of increase blood flow, increase vascular permeability, and immune cell recruitment [ 17 ]. Plasma levels of PACAP and CGRP have been shown to be elevated during migraine attacks [ [18] , [19] , [20] , [21] , [22] , [23] ] and infusion of CGRP or PACAP alone is sufficient to trigger migraine-like attacks in people with migraine [ 24 , 25 ]. The involvement of the immune system and the role of neuroinflammation in the pathophysiology of primary headaches has been debated for decades. Emerging evidence supports the contribution of proinflammatory cytokines and immune cells to headache development [ 26 ]. The existence of a systemic proinflammatory state, along with alterations in immunological parameters such as elevated levels of interleukins and other cytokines, represents one of the most consistent findings across primary headache disorders, particularly migraine [ 20 , [27] , [28] , [29] ]. However, their specific role in headache pathophysiology remains unclear. Most neuroimmunological findings are related to migraine, with considerably fewer data on cluster headache and tension-type headache. Among the immune cell populations, mast cells have attracted particular attention in migraine. Mast cells, a type of granulocyte in the innate immune system, are thought to play a pivotal role in migraine by acting as a bridge between the immune and nervous systems, responding to triggers such as stress and neuropeptides to initiate or exacerbate the headache phase of attacks [ 30 , 31 ]. Within the cranial meninges, mast cells are abundant, particularly around blood vessels and in close proximity to meningeal nociceptors [ 32 ]. Dural mast cells, in response to neuropeptides such as SP, CGRP, and PACAP, can degranulate and release bioactive mediators such as histamine, prostaglandin E2 (PGE2), prostacyclin (PGI2), tryptase, heparin, serotonin, chemokines and cytokines. These mediators may contribute to vascular permeability, plasma protein extravasation, and vasodilation, processes associated with neurogenic inflammation [ [32] , [33] , [34] , [35] ]. These processes enhance nociceptive signaling along trigeminal nerve fibers, contributing to migraine headache perception [ [36] , [37] , [38] ]. The involvement of mast cells in migraine pathogenesis has been proposed through studies using their activators. Stimulation of mast cell degranulation with compound 48/80 (C48/80), a potent histamine-releasing agent, induces vasodilation, tissue edema, and nociceptive responses that resemble the clinical manifestations of migraine [ 39 ]. Over the years, additional evidence has shown that plasma histamine levels may increase during migraine attacks. Moreover, histamine infusion in patients with migraine has been shown to induce severe migraine attacks, supporting the role of mast cells in migraine mechanisms [ 40 ]. Despite this close link, the use of anti-histamines as a treatment for migraines has not been successful [ 23 ]. One major physiological trigger of mast cell activation in migraine is stress. Stress activates the hypothalamic–pituitary–adrenal (HPA) axis and the release of corticotropin-releasing factor (CRF). Experimental studies have shown that restraint stress leads to degranulation in approximately 70 % of dural mast cells in rats [ 31 ]. Clinically, significantly elevated levels of CRF have been detected in the cerebrospinal fluid (CSF) of patients with chronic migraine and medication-overuse headache (MOH;[ 41 ]). These findings highlight how stress-induced mast cell activation contributes to elevated levels of bioactive mediators, as demonstrated in experimental models where such degranulation leads to increased mast cell protease release [ 31 ]. Tryptase is a serine protease capable of stimulating trigeminal nociceptors through cleavage and activation of the protease-activated receptor 2 (PAR2; [ 42 ]). Experimental studies have shown that antibodies targeting PAR2 prevent cutaneous allodynia induced by supradural administration of CGRP or a transient receptor potential ankyrin 1 (TRPA1) agonist. These findings suggest that PAR2 contributes to both CGRP-dependent and CGRP-independent mechanisms involved in migraine-like pain signaling and may serve as a molecular link between resident immune cells and meningeal nociceptor activation [ [42] , [43] , [44] ]. Given the potential importance of mast cells in migraine pain, it is crucial to understand the upstream regulators of the mast cell–nociceptor axis. Recent discoveries have identified the Mas-related G protein–coupled receptors (MRGPRs), MRGPRB2 in rodents and its human ortholog MRGPRX2, as key mediators of mast cell–neuron crosstalk and potent drivers of neurogenic inflammation [ 45 ]. MRGPRX2 has been implicated in a range of conditions, including inflammation and pain [ 46 ]. Understanding how this receptor contributes to mast cell activation in the meninges may uncover novel mechanisms underlying migraine pathophysiology. Such insights could help identify new therapeutic targets, particularly since current migraine treatments fail to provide adequate relief for many patients. This narrative review focuses on MRGPRX2 receptors as a novel therapeutic target in migraine.

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Gregory Dussor reports financial support was provided by National Institutes of Health. Gregory Dussor reports a relationship with Delphian Therapeutics that includes: equity or stocks. Gregory Dussor reports a relationship with Acadia Pharmaceuticals Inc that includes: equity or stocks. Gregory Dussor reports a relationship with PARMedics that includes: equity or stocks. Gregory Dussor reports a relationship with Evommune Inc that includes: funding grants. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.