Upregulated expression of substance P (SP) and NK1R in eczema and SP-induced mast cell accumulation

Substance P (SP) was reported to be associated with eczema and acts as a potent skin mast cell secretagogue. However, little is known of its expression in inflammatory cells in eczema and its ability in induction of mast cell accumulation. In the present study, we investigated expression of SP and neurokinin-1 receptor (NK1R) on peripheral blood leukocytes and mast cells from patients with eczema and influence of SP on mast cell accumulation by using flow cytometry analysis, trans-epithelial cell migration assay and mouse peritoneal model. The results showed that plasma SP and IL-17A levels in eczema patients were higher than that in healthy control subject. The percentages of SP+ and NK1R+ expression populations of monocytes, helper T cells, natural killer T cells and basophils in peripheral blood of eczema patients were markedly elevated. It was observed that not only absolute number of mast cells but also SP+ and NK1R+ mast cells are enhanced in the lesion skin of eczema. SP showed a potent chemoattractant action on mast cells as assessed by a mouse peritoneal model and a trans-endothelium cell migration assay. SP-induced mast cell accumulation appears a CD18/CD11a complex, l-selectin and ICAM-1-dependent event which can be blocked by a NK-1R antagonist RP67580. In conclusion, elevated expression of SP in patients with eczema and the ability of SP in induction of mast cell accumulation indicate strongly that SP is a potent proinflammatory mediator, which contributes to the pathogenesis of eczema. Inhibitors of SP and blockers of NK1R are likely useful agents for treatment of eczema.     

Metalloelastase in lungs and alveolar macrophages is modulated by extracellular substance P in mice

Metalloelastase (MMP-12), mainly produced by macrophages, has been shown to play a key role in the pathogenesis of emphysema in animal models. Chronic cigarette smoke increases pulmonary MMP-12, which is closely correlated with an elevation of pulmonary substance P (SP). Because alveolar macrophages (AMs) contain the neurokinin-1 receptor (NK1R), we tested whether SP was able to trigger the upregulation of MMP-12 synthesis in AMs by acting on the NK1R. AMs isolated from bronchoalveolar lavage cells in C3H/HeN mice were cultured with control medium or SP that was coupled without or with NK1R antagonists (CP-99,994 or aprepitant) for 24 h. We found that SP significantly increased the mRNA of MMP-12 and NK1R by 11-fold and 82%, respectively, in AMs (P<0.05), and these responses were abolished by NK1R antagonists with little change in the cells' viability. Because pulmonary SP is primarily released by bronchopulmonary C-fibers (PCFs), we further asked whether destruction of PCFs would reduce SP and MMP-12. Two groups of mice were pretreated with vehicle and neonatal capsaicin (NCAP) to degenerate PCFs, respectively. Our results show that NCAP treatment significantly decreased mRNA and protein levels of SP associated with a reduction NK1R and MMP-12 in the lungs and AMs. These findings suggest that SP has a modulatory effect on pulmonary MMP-12 by acting on NK1R to trigger MMP-12 syntheses in the AMs.     

Activation of neurokinin-1 receptors up-regulates substance P and neurokinin-1 receptor expression in murine pancreatic acinar cells

Acute pancreatitis (AP) has been associated with an up-regulation of substance P (SP) and neurokinin-1 receptor (NK1R) in the pancreas. Increased SP-NK1R interaction was suggested to be pro-inflammatory during AP. Previously, we showed that caerulein treatment increased SP/NK1R expression in mouse pancreatic acinar cells, but the effect of SP treatment was not evaluated. Pancreatic acinar cells were obtained from pancreas of male swiss mice (25–30 g). We measured mRNA expression of preprotachykinin-A (PPTA) and NK1R following treatment of SP (10⁻⁶M). SP treatment increased PPTA and NK1R expression in isolated pancreatic acinar cells, which was abolished by pretreatment of a selective NK1R antagonist, CP96,345. SP also time dependently increased protein expression of NK1R. Treatment of cells with a specific NK1R agonist, GR73,632, up-regulated SP protein levels in the cells. Using previously established concentrations, pre-treatment of pancreatic acinar cells with Gö6976 (10 nM), rottlerin (5 μM), PD98059 (30 μM), SP600125 (30 μM) or Bay11-7082 (30 μM) significantly inhibited up-regulation of SP and NK1R. These observations suggested that the PKC-ERK/JNK-NF-κB pathway is necessary for the modulation of expression levels. In comparison, pre-treatment of CP96,345 reversed gene expression in SP-induced cells, but not in caerulein-treated cells. Overall, the findings in this study suggested a possible auto-regulatory mechanism of SP/NK1R expression in mouse pancreatic acinar cells, via activation of NK1R. Elevated SP levels during AP might increase the occurrence of a positive feedback loop that contributes to abnormally high expression of SP and NK1R.     

N-terminal substance P fragments inhibit the spinally induced, NK 1 receptor mediated behavioural responses in mice.

N-terminal fragments of substance P (SP) were tested for antagonism against the aversive responses induced in mice by various tachykinin receptor agonists, somatostatin and bombesin. When co-administered with SP intrathecally, low doses (1.0-4.0 pmol) of SP (1-7) or SP (1-8) reduced the SP-induced behavioural responses of scratching, biting and licking. Aversive responses induced by two other neurokinin (NK) 1 receptor agonists, Septide and physalaemin, were also dose-dependently decreased by the simultaneous injection of small doses of SP (1-7) or SP (1-8). Aversive responses induced by 400 pmol of NK A were also significantly reduced by co-administration of SP (1-7) or SP (1-8). No significant effects of the N-terminal fragments were observed against the aversive responses elicited by NK A (300 pmol), eledoisin, NK B, somatostatin or bombesin. These results suggest that the behavioural antagonism produced by SP (1-7) and SP (1-8) may be limited to the NK 1 receptor at the spinal level in mice.     

Evaluation of serum level of substance P and tissue distribution of NK-1 receptor in colorectal cancer

Colorectal cancer (CRC) is known as the most common form of malignancies in the world and its occurrence is annually increasing. Due to the relatively high death rates in patients, finding better diagnostic and prognostic factors are required. Substance P (SP) belongs to the tachykinin family that acts as an immunomodulator by binding to the neurokinin-1 receptor (NK1R). The interaction of SP with NK1R might be involved in tumor cell proliferation, angiogenesis, and migration. Hence, this study was aimed to evaluate the serum SP level and tissue distribution of NK1Rs in CRC. Also, we assessed the relationship between tissue distribution of NK1R and some different tumor characteristics, including tumor size, and lymph node status. Recruiting 38 patients primarily diagnosed with CRC, the tissue distribution of NK1R was immunohistochemically evaluated in tumor tissues and their adjacent normal tissue. The serum level of SP was measured using an ELISA method in both cases and healthy control group. The SP value was significantly increased in the serum of patients in comparison with the healthy group (p?=?0.001). Tumor tissues expressed a higher number of NK1R than adjacent normal tissues (p?=?0.01) considering both the percentage of stained cells and intensity of staining. However, there was not any statistically significant relevance between NK1R distribution and tumor characteristics. The SP/NK1R system is involved in tumorigenesis of CRC, and might be suggested as a potent prognostic or diagnostic factor, or a new target in the treatment of CRC.

     

Capsaicin vanilloid receptor-1 mediates substance P release in experimental pancreatitis

We examined whether the capsaicin vanilloid receptor-1 (VR1) mediates substance P (SP) release from primary sensory neurons in experimental pancreatitis. Pancreatitis was achieved by 12 hourly injections of caerulein (50 microg/kg ip) in mice. One group received capsazepine (100 micromol/kg sc), a competitive VR1 antagonist, at 4-h intervals. Neurokinin-1 receptor (NK1R) internalization in acinar cells, used as an index of endogenous SP release, was assessed by immunocytochemical quantification of NK1R endocytosis. The severity of pancreatitis was assessed by measurements of serum amylase, pancreatic myeloperoxidase (MPO) activity, and histological grading. Caerulein administration caused significant elevations in serum amylase and pancreatic MPO activity, produced histological evidence of pancreatitis, and caused a dramatic increase in NK1R endocytosis. Capsazepine treatment significantly reduced the level of NK1R endocytosis, and this was associated with similar reductions in pancreatic MPO activity and histological severity of pancreatitis. These results demonstrate that repeated caerulein stimulation causes experimental pancreatitis that is mediated in part by stimulation of VR1 on primary sensory neurons, resulting in endogenous SP release.     

Immunocytochemical localization of substance P neurokinin-1 receptors in rat gingival tissue

The distributions of substance P (SP) and the neurokinin-1 receptor (NK1-R), the receptor preferentially activated by SP, were examined in rat gingiva by immunocytochemical methods with light and electron microscopy. SP-immunoreactive nerve fibers were located preferentially in the junctional epithelium (JE) but few in the other oral and oral sulcular epithelia. NK1-R immunoreactivity was found in the endothelial cells (capillaries and postcapillary venules underlying the JE). NK1-R-labeled and -unlabeled unmyelinated nerve fibers were located close to the blood vessels and partially or completely covered by a Schwann cell sheath. In the JE, labeled naked axons without Schwann cell sheaths were observed. Neutrophils and macrophages in the connective tissue underlying the JE and in the JE were also labeled with NK1-R. Furthermore, NK1-R was found in the JE cells. Basically, immunoreaction products for NK1-R were found throughout various cells (endothelial cells, neutrophils, and JE cells) at invaginations of the plasma membrane and in vesicular and granular structures that are probably endosomes and are found close to both the plasma membrane and the nucleus. This is a first report, demonstrating the presence of NK1-R in the gingival tissue in the normal nonstimulated condition. Furthermore, it is thought that SP may modulate the permeability of blood vessels beneath the JE, the production of antimicrobial agents in neutrophils, and the proliferation and endocytotic ability of JE cells through NK1-R.     

Akt‐mediated anti‐apoptotic effects of substance P in Anti‐Fas‐induced apoptosis of human tenocytes

Substance P (SP) and its receptor, the neurokinin‐1 receptor (NK‐1 R), are expressed by human tenocytes, and they are both up‐regulated in cases of tendinosis, a condition associated with excessive apoptosis. It is known that SP can phosphorylate/activate the protein kinase Akt, which has anti‐apoptotic effects. This mechanism has not been studied for tenocytes. The aims of this study were to investigate if Anti‐Fas treatment is a good apoptosis model for human tenocytes in vitro, if SP protects from Anti‐Fas‐induced apoptosis, and by which mechanisms SP mediates an anti‐apoptotic response. Anti‐Fas treatment resulted in a time‐ and dose‐dependent release of lactate dehydrogenase (LDH), i.e. induction of cell death, and SP dose‐dependently reduced the Anti‐Fas‐induced cell death through a NK‐1 R specific pathway. The same trend was seen for the TUNEL assay, i.e. SP reduced Anti‐Fas‐induced apoptosis via NK‐1 R. In addition, it was shown that SP reduces Anti‐Fas‐induced decrease in cell viability as shown with crystal violet assay. Protein analysis using Western blot confirmed that Anti‐Fas induces cleavage/activation of caspase‐3 and cleavage of PARP; both of which were inhibited by SP via NK‐1 R. Finally, SP treatment resulted in phosphorylation/activation of Akt as shown with Western blot, and it was confirmed that the anti‐apoptotic effect of SP was, at least partly, induced through the Akt‐dependent pathway. In conclusion, we show that SP reduces Anti‐Fas‐induced apoptosis in human tenocytes and that this anti‐apoptotic effect of SP is mediated through NK‐1 R and Akt‐specific pathways.     

The neuropeptide substance P is a critical mediator of burn-induced acute lung injury

The classical tachykinin substance P (SP) has numerous potent neuroimmunomodulatory effects on all kinds of airway functions. Belonging to a class of neuromediators targeting not only residential cells but also inflammatory cells, studying SP provides important information on the bidirectional linkage between how neural function affects inflammatory events and, in turn, how inflammatory responses alter neural activity. Therefore, this study aimed to investigate the effect of local burn injury on inducing distant organ pulmonary SP release and its relevance to lung injury. Our results show that burn injury in male BALB/c mice subjected to 30% total body surface area full thickness burn augments significant production of SP, preprotachykinin-A gene expression, which encodes for SP, and biological activity of SP-neurokinin-1 receptor (NK1R) signaling. Furthermore, the enhanced SP-NK1R response correlates with exacerbated lung damage after burn as evidenced by increased microvascular permeability, edema, and neutrophil accumulation. The development of heightened inflammation and lung damage was observed along with increased proinflammatory IL-1beta, TNF-alpha, and IL-6 mRNA and protein production after injury in lung. Chemokines MIP-2 and MIP-1alpha were markedly increased, suggesting the active role of SP-induced chemoattractants production in trafficking inflammatory cells. More importantly, administration of L703606, a specific NK1R antagonist, 1 h before burn injury significantly disrupted the SP-NK1R signaling and reversed pulmonary inflammation and injury. The present findings show for the first time the role of SP in contributing to exaggerated pulmonary inflammatory damage after burn injury via activation of NK1R signaling.     

The Hypothalamic–Pituitary–Adrenal Axis and Serotonin Metabolism in Individual Brain Nuclei of Mice with Genetic Disruption of the NK1 Receptor Exposed to Acute Stress

Mice lacking the substance P (SP) neurokinin-1 (NK1) receptor (NK1R?/?mice) were used to investigate whether SP affects serotonin (5-HT) function in the brain and to assess the effects of acute immobilisation stress on the hypothalamic–pituitary–adrenocortical (HPA) axis and 5-HT turnover in individual brain nuclei. Basal HPA activity and the expression of hypothalamic corticotropin-releasing hormone (CRH) in wild-type (WT)- and NK1R?/? mice were identical. Stress-induced increases in plasma ACTH concentration were considerably higher in NK1R?/? mice than in WT mice while corticosterone concentrations were equally elevated in both mouse lines. Acute stress did not alter the expression of CRH. In the dorsal raphe nucleus (DRN), basal 5-HT turnover was increased in NK1R?/? mice and a 15 min stress further magnified 5-HT utilisation in this region. In the frontoparietal cortex, medial prefrontal cortex, central nucleus of amygdala, and the hippocampal CA1 region, stress increased 5-HT and/or 5-hydroxyindoleacetic acid (5-HIAA) concentrations to a similar extent in WT and NK1R?/? mice. 5-HT turnover in the hypothalamic paraventricular nucleus was not affected by stress, but stress induced similar increases in 5-HT and 5-HIAA in the ventromedial and dorsomedial hypothalamic nuclei in WT and NK1R?/? mice. Our findings indicate that NK1 receptor activation suppresses ACTH release during acute stress but does not exert sustained inhibition of the HPA axis. Genetic deletion of the NK1 receptor accelerates 5-HT turnover in DRN under basal and stress conditions. No differences between the responses of serotonergic system to acute stress in WT and NK1R?/? mice occur in forebrain nuclei linked to the regulation of anxiety and neuroendocrine stress responses.     

Treatment with a substance P receptor antagonist is neuroprotective in the intrastriatal 6-hydroxydopamine model of early Parkinson's disease

Neuroinflammation and blood brain barrier (BBB) dysfunction have been implicated in the pathogenesis of Parkinson's disease (PD). The neuropeptide substance P (SP) is an important mediator of both neuroinflammation and BBB dysfunction through its NK1 receptor in a process known as neurogenic inflammation. Increased SP content has previously been reported following 6-OHDA treatment in vitro, with the levels of SP correlating with cell death. The present study used an in vivo 6-OHDA lesion model to determine if dopaminergic degeneration was associated with increased SP in the substantia nigra and whether this degeneration could be prevented by using a SP, NK1 receptor antagonist. Unilateral, intrastriatal 6-OHDA lesions were induced and SP (10 μg/2 μL) or the NK1 receptor antagonists, N-acetyl-L-tryptophan (2 μL at 50 nM) or L-333,060 (2 μL at 100 nM), administered immediately after the neurotoxin. Nigral SP content was then determined using immunohistochemical and ELISA methods, neuroinflammation and barrier integrity was assessed using Iba-1, ED-1, GFAP and albumin immunohistochemistry, while dopaminergic cell loss was assessed with tyrosine hydroxylase immunohistochemistry. Motor function in all animals was assessed using the rotarod task. Intrastriatal 6-OHDA lesioning produced an early and sustained increase in ipsilateral nigral SP content, along with a breakdown of the BBB and activation of microglia and astrocytes. Further exacerbation of SP levels accelerated disease progression, whereas NK1 receptor antagonist treatment protected dopaminergic neurons, preserved barrier integrity, reduced neuroinflammation and significantly improved motor function. We propose that neurogenic inflammation contributes to dopaminergic degeneration in early experimental PD and demonstrate that an NK1 receptor antagonist may represent a novel neuroprotective therapy.     

Temporal increase in the reactivity of pulmonary vasculature to substance P in chronically hypoxic rats

We previously demonstrated that the pulmonary vascular response to substance P (SP) increased in chronically hypoxic rats. This study explored the temporal increase in reactivity of the pulmonary vascular response to SP and its underlying mechanisms. First, young female Wistar rats were exposed to sea level (SL) or simulated high altitude (HA) for 15 h/day for 3 days, 1 wk, 2 wk, and 4 wk. Lungs were isolated and perfused with 4% bovine serum albumin in Krebs-Henseleit buffer solution. SP (1.5 x 10(-4) M) induced significant increases in pulmonary arterial pressure (P(pa)), venous pressure (P(v)), capillary pressure (P(c)), arterial resistance (R(a)), and filtration coefficient (K(fc)) in SL lungs. Increases in P(pa) and R(a) were significantly augmented in HA lungs, with a temporal increase trend peaking at 2 wk of HA exposure. The selective neurokinin (NK) type 1 (NK1) receptor antagonist SR-14033 significantly attenuated SP-induced increases in P(pa), P(v), P(c), R(a), and K(fc) in SL lungs. In lungs exposed to HA for 2 wk, SR-14033 suppressed the effect of SP on P(pa). Also, chronic hypoxia induced significant increases in NK1 receptors and NK1 receptor mRNA, with a temporal trend. We conclude that chronic hypoxia temporally augments SP-induced vascular responses, which are closely associated with increases in NK1 receptors and gene expression.     

The role of substance P in the marginal division of the neostriatum in learning and memory is mediated through the neurokinin 1 receptor in rats

Substance P (SP) is a neuropeptide that plays an important role in inflammation, respiration, pain, aggression, anxiety, and learning and memory mainly through its high affinity neurokinin 1 receptor (NK1R). The marginal division (MrD) is a pan-shaped subdivision in the caudomedial margin of the neostriatum in the mammalian brain and is known to be involved in learning and memory. We studied the expression of SP, NK1R and NK1R mRNA in the rat striatum by immunohistochemistry, immunofluorescence and in situ hybridization, and found that the levels of SP, NK1R protein and NK1R mRNA were high in the cell bodies, fibers and terminals of neurons in the neostriatum, especially in the MrD. Knocking down NK1R activity in the MrD by using an antisense oligonucleotide against NK1R mRNA inhibited learning and memory in a Y-maze behavioral test. Our results show that NK1R mediates the role of SP in the MrD in learning and memory.     

RP 67580, a potent and selective substance P non-peptide antagonist]

The pharmacological properties of 7,7-Diphenyl-2 [1-imino-2 (2-methoxy-phenyl)-ethyl] perhydroisoindol-4-one (3 aR, 7 aR) or RP67580 are described. This compound, derived from a novel chemical family, is a potent and selective substance P (SP) antagonist, in vitro and in vivo. In vitro, it inhibited in a competitive manner (IC50 = 10 nM) 3H-SP binding in rat brain (NK1 receptors). It did not interact with the two other tachykinin receptor sites (NK2 and NK3) nor the other receptor sites tested. Moreover, RP67580 competitively antagonized the contractile activity of SP on guinea-pig ileum (pA2 = 7.16); in contrast, it was inactive in rabbit pulmonary artery and in rat portal vein tissues which contain NK2 and NK3 receptors, respectively. In vivo, in the rat, RP67580 inhibited the plasmatic extravasation induced by administration of SP (ED50 = 0.04 mg/kg i.v.) as well as that induced by antidromic stimulation of a peripheral sensory nerve (ED50 = 0.15 mg/kg i.v.). In mice and rats, RP67580, like morphine, potently blocked the nociceptive effects of phenylbenzoquinone and formalin; its antinociceptive effect does not involve opiate receptors since it was not reversed by naloxone. These results indicate that RP67580 is a particularly valuable tool for investigating the physiological and pathological role of SP.     

Basophil depletion downregulates Schistosoma mansoni egg-induced granuloma formation

Granuloma formation around parasite eggs during schistosomal infection is considered to be controlled by Th2 cytokines. However, it is still controversial which cell populations are responsible for the host Th2 cytokine-dependent granuloma formation. Basophils have recently attracted attention because of their ability to produce large amounts of IL-4. Therefore, we investigated whether basophils play an essential role in the induction of granuloma formation induced by Schistosoma mansoni eggs. Together with our previous observation that basophil numbers increased markedly in the spleen at 7 weeks postinfection, immunohistochemical staining using anti-mMCP8 monoclonal antibody (mAb) showed basophil infiltration in the granulomatous lesions formed around parasite eggs. To examine the roles of basophils more directly, we treated mice with anti-CD200R3 mAb to deplete basophils. Depletion of basophils resulted in a reduction of basophil number with concomitant downregulation of egg granuloma formation at 7 weeks postinfection. Moreover, we observed a significant reduction in the size of egg granulomas formed in basophil-depleted mice in the pulmonary granuloma model. Taken together, these findings indicated that basophils are essential for S. mansoni egg-induced granuloma formation, and this may serve as a novel therapeutic target in ameliorating the pathology of schistosomiasis.     

Modulation of basal and stress-induced amygdaloid substance P release by the potent and selective NK1 receptor antagonist L-822429

It has been shown that anxiety and stress responses are modulated by substance P (SP) released within the amygdala. However, there is an important gap in our knowledge concerning the mechanisms regulating extracellular SP in this brain region. To study a possible self-regulating role of SP, we used a selective neurokinin-1 (NK1) receptor antagonist to investigate whether blockade of NK1 receptors results in altered basal and/or stress-evoked SP release in the medial amygdala (MeA), a critical brain area for a functional involvement of SP transmission in enhanced anxiety responses induced by stressor exposure. In vitro binding and functional receptor assays revealed that L-822429 represents a potent and selective rat NK1 receptor antagonist. Intra-amygdaloid administration of L-822429 via inverse microdialysis enhanced basal, but attenuated swim stress-induced SP release, while the low-affinity enantiomer of L-822429 had no effect. Using light and electron microscopy, synaptic contacts between SP-containing fibres and dendrites expressing NK1 receptors was demonstrated in the medial amygdala. Our findings suggest self-regulatory capacity of SP-mediated neurotransmission that differs in the effect on basal and stress-induced release of SP. Under basal conditions endogenous SP can serve as a signal that tonically inhibits its own release via a NK1 receptor-mediated negative feedback action, while under stress conditions SP release is further facilitated by activation of NK1 receptors, likely leading to high local levels of SP and activation of receptors to which SP binds with lower affinity.     

Substance P upregulates cyclooxygenase-2 and prostaglandin E metabolite by activating ERK1/2 and NF-kappaB in a mouse model of burn-induced remote acute lung injury

Acute lung injury (ALI) is a major cause of mortality in burn patients, even without direct inhalational injury. Identification of early mediators that instigate ALI after burn and of the molecular mechanisms by which they work are of high importance but remain poorly understood. We previously reported that an endogenous neuropeptide, substance P (SP), via binding neurokinin-1 receptor (NK1R), heightens remote ALI early after severe local burn. In this study, we examined the downstream signaling pathway following SP-NK1R coupling that leads to remote ALI after burn. A 30% total body surface area full-thickness burn was induced in male BALB/c wild-type (WT) mice, preprotachykinin-A (PPT-A) gene-deficient mice, which encode for SP, and PPT-A(-/-) mice challenged with exogenous SP. Local burn injury induced excessive SP-NK1R signaling, which activated ERK1/2 and NF-κB, leading to significant upregulation of cyclooxygenase (COX)-2, PGE metabolite, and remote ALI. Notably, lung COX-2 levels were abrogated in burn-injured WT mice by L703606, PD98059, and Bay 11-7082, which are specific NK1R, MEK-1, and NF-κB antagonists, respectively. Additionally, burn-injured PPT-A(-/-) mice showed suppressed lung COX-2 levels, whereas PPT-A(-/-) mice injected with SP showed augmented COX-2 levels postburn, and administration of PD98059 and Bay 11-7082 to burn-injured PPT-A(-/-) mice injected with SP abolished the COX-2 levels. Furthermore, treatment with parecoxib, a selective COX-2 inhibitor, attenuated proinflammatory cytokines, chemokines, and ALI in burn-injured WT mice and PPT-A(-/-) mice injected with SP. To our knowledge, we show for the first time that SP-NK1R signaling markedly elevates COX-2 activity via ERK1/2 and NF-κB, leading to remote ALI after burn.     

Recycling and resensitization of the neurokinin 1 receptor. Influence of agonist concentration and Rab GTPases

Substance P (SP) induces endocytosis and recycling of the neurokinin 1 receptor (NK1R) in endothelial cells and spinal neurons at sites of inflammation and pain, and it is thus important to understand the mechanism and function of receptor trafficking. We investigated how the SP concentration affects NK1R trafficking and determined the role of Rab GTPases in trafficking. NK1R trafficking was markedly influenced by the SP concentration. High SP (10 nM) induced translocation of the NK1R and beta-arrestin 1 to perinuclear sorting endosomes containing Rab5a, where NK1R remained for >60 min. Low SP (1 nM) induced translocation of the NK1R to early endosomes located immediately beneath the plasma membrane that also contained Rab5a and beta-arrestin 1, followed by rapid recycling of the NK1R. Overexpression of Rab5a promoted NK1R translocation to perinuclear sorting endosomes, whereas the GTP binding-deficient mutant Rab5aS34N caused retention of the NK1R in superficial early endosomes. NK1R translocated from superficial early endosomes to recycling endosomes containing Rab4a and Rab11a, and Rab11aS25N inhibited NK1R recycling. Rapid NK1R recycling coincided with resensitization of SP-induced Ca2+ mobilization and with the return of surface SP binding sites. Resensitization was minimally affected by inhibition of vacuolar H(+)-ATPase and phosphatases but was markedly suppressed by disruption of Rab4a and Rab11a. Thus, whereas beta-arrestins mediate NK1R endocytosis, Rab5a regulates translocation between early and sorting endosomes, and Rab4a and Rab11a regulate trafficking through recycling endosomes. We have thus identified a new function of Rab5a as a control protein for directing concentration-dependent trafficking of the NK1R into different intracellular compartments and obtained evidence that Rab4a and Rab11a contribute to G-protein-coupled receptor recycling from early endosomes.