Eileen F. Grady

Expression of AT2 receptors in the developing rat fetus.

Angiotensin II is known primarily for its effects on blood pressure and electrolyte homeostasis, but recent studies suggest that angiotensin II may play a role in the regulation of cellular growth. This study was undertaken to identify the angiotensin II receptor subtypes expressed during fetal and neonatal development and to characterize their cellular localization. Using an in situ receptor binding assay on sagittal frozen sections of fetal and neonatal rats, bound 125I-[Sar1,Ile8]-angiotensin II was visualized by film and emulsion autoradiography. Bound radioligand was detected by E11 (embryonic day 11) and maximal binding occurred by E19-21. Radioligand binding remained unaltered 30 min after birth, whereas a noticeable and stable decrease was observed 12 h postparturition. The highly abundant angiotensin II receptors were shown to be AT2 by the marked reduction in radioligand binding achieved with PD123177 (10(-7)M), a specific AT2 receptor antagonist, whereas DuP 753 (10(-5)M), an AT1 receptor antagonist, had little effect. Emulsion autoradiography showed radioligand binding in the undifferentiated mesenchyme of the submucosal layers of the intestine and stomach, connective tissue and choroid surrounding the retina, subdermal mesenchyme adjacent to developing cartilage, diaphragm, and tongue. Residual AT2 receptors were found on the dorsal subdermal region of the tongue 72 h after birth. AT1 receptors were detected in the placenta at E13 and in the aorta, kidney, lung, liver, and adrenal gland at E19-21, consistent with an adult distribution. The transient expression of AT2 receptors in the mesenchyme of the fetus suggests a role of angiotensin II in fetal development.

Protease-Activated Receptor 2 Sensitizes the Capsaicin Receptor Transient Receptor Potential Vanilloid Receptor 1 to Induce Hyperalgesia

Inflammatory proteases (mast cell tryptase and trypsins) cleave protease-activated receptor 2 (PAR2) on spinal afferent neurons and cause persistent inflammation and hyperalgesia by unknown mechanisms. We determined whether transient receptor potential vanilloid receptor 1 (TRPV1), a cation channel activated by capsaicin, protons, and noxious heat, mediates PAR2-induced hyperalgesia. PAR2 was coexpressed with TRPV1 in small- to medium-diameter neurons of the dorsal root ganglia (DRG), as determined by immunofluorescence. PAR2 agonists increased intracellular [Ca2+] ([Ca2+]i) in these neurons in culture, and PAR2-responsive neurons also responded to the TRPV1 agonist capsaicin, confirming coexpression of PAR2 and TRPV1. PAR2 agonists potentiated capsaicin-induced increases in [Ca2+]i in TRPV1-transfected human embryonic kidney (HEK) cells and DRG neurons and potentiated capsaicin-induced currents in DRG neurons. Inhibitors of phospholipase C and protein kinase C (PKC) suppressed PAR2-induced sensitization of TRPV1-mediated changes in [Ca2+]i and TRPV1 currents. Activation of PAR2 or PKC induced phosphorylation of TRPV1 in HEK cells, suggesting a direct regulation of the channel. Intraplantar injection of a PAR2 agonist caused persistent thermal hyperalgesia that was prevented by antagonism or deletion of TRPV1. Coinjection of nonhyperalgesic doses of PAR2 agonist and capsaicin induced hyperalgesia that was inhibited by deletion of TRPV1 or antagonism of PKC. PAR2 activation also potentiated capsaicin-induced release of substance P and calcitonin gene-related peptide from superfused segments of the dorsal horn of the spinal cord, where they mediate hyperalgesia. We have identified a novel mechanism by which proteases that activate PAR2 sensitize TRPV1 through PKC. Antagonism of PAR2, TRPV1, or PKC may abrogate protease-induced thermal hyperalgesia.

Protease‐activated receptor 2 sensitizes TRPV1 by protein kinase Cɛ‐ and A‐dependent mechanisms in rats and mice

Proteases that are released during inflammation and injury cleave protease‐activated receptor 2 (PAR2) on primary afferent neurons to cause neurogenic inflammation and hyperalgesia. PAR2‐induced thermal hyperalgesia depends on sensitization of transient receptor potential vanilloid receptor 1 (TRPV1), which is gated by capsaicin, protons and noxious heat. However, the signalling mechanisms by which PAR2 sensitizes TRPV1 are not fully characterized. Using immunofluorescence and confocal microscopy, we observed that PAR2 was colocalized with protein kinase (PK) Cɛ and PKA in a subset of dorsal root ganglia neurons in rats, and that PAR2 agonists promoted translocation of PKCɛ and PKA catalytic subunits from the cytosol to the plasma membrane of cultured neurons and HEK 293 cells. Subcellular fractionation and Western blotting confirmed this redistribution of kinases, which is indicative of activation. Although PAR2 couples to phospholipase Cβ, leading to stimulation of PKC, we also observed that PAR2 agonists increased cAMP generation in neurons and HEK 293 cells, which would activate PKA. PAR2 agonists enhanced capsaicin‐stimulated increases in [Ca2+]i and whole‐cell currents in HEK 293 cells, indicating TRPV1 sensitization. The combined intraplantar injection of non‐algesic doses of PAR2 agonist and capsaicin decreased the latency of paw withdrawal to radiant heat in mice, indicative of thermal hyperalgesia. Antagonists of PKCɛ and PKA prevented sensitization of TRPV1 Ca2+ signals and currents in HEK 293 cells, and suppressed thermal hyperalgesia in mice. Thus, PAR2 activates PKCɛ and PKA in sensory neurons, and thereby sensitizes TRPV1 to cause thermal hyperalgesia. These mechanisms may underlie inflammatory pain, where multiple proteases are generated and released.

Calcitonin receptor‐like receptor (CLR), receptor activity‐modifying protein 1 (RAMP1), and calcitonin gene‐related peptide (CGRP) immunoreactivity in the rat trigeminovascular system: Differences between peripheral and central CGRP receptor distribution

Calcitonin gene‐related peptide (CGRP) is a key mediator in primary headaches including migraine. Animal models of meningeal nociception demonstrate both peripheral and central CGRP effects; however, the target structures remain unclear. To study the distribution of CGRP receptors in the rat trigeminovascular system we used antibodies recognizing two components of the CGRP receptor, the calcitonin receptor‐like receptor (CLR) and the receptor activity‐modifying protein 1 (RAMP1). In the cranial dura mater, CLR and RAMP1 immunoreactivity (‐ir) was found within arterial blood vessels, mononuclear cells, and Schwann cells, but not sensory axons. In the trigeminal ganglion, besides Schwann and satellite cells, CLR‐ and RAMP1‐ir was found in subpopulations of CGRP‐ir neurons where colocalization of CGRP‐ and RAMP1‐ir was very rare (≈0.6%). CLR‐ and RAMP1‐ir was present on central, but not peripheral, axons. In the spinal trigeminal nucleus, CLR‐ and RAMP1‐ir was localized to “glomerular structures,” partly colocalized with CGRP‐ir. However, CLR‐ and RAMP1‐ir was lacking in central glia and neuronal cell bodies. We conclude that CGRP receptors are associated with structural targets of known CGRP effects (vasodilation, mast cell degranulation) and targets of unknown function (Schwann cells). In the spinal trigeminal nucleus, CGRP receptors are probably located on neuronal processes, including primary afferent endings, suggesting involvement in presynaptic regulation of nociceptive transmission. Thus, in the trigeminovascular system CGRP receptor localization suggests multiple targets for CGRP in the pathogenesis of primary headaches. J. Comp. Neurol. 507:1277–1299, 2008.

Trafficking of Proteinase-activated Receptor-2 and β-Arrestin-1 Tagged with Green Fluorescent Protein β-ARRESTIN-DEPENDENT ENDOCYTOSIS OF A PROTEINASE RECEPTOR

Proteases cleave proteinase-activated receptors (PARs) to expose N-terminal tethered ligands that bind and activate the cleaved receptors. The tethered ligand, once exposed, is always available to interact with its binding site. Thus, efficient mechanisms must prevent continuous activation, including receptor phosphorylation and uncoupling from G-proteins, receptor endocytosis, and lysosomal degradation. β-Arrestins mediate uncoupling and endocytosis of certain neurotransmitter receptors, which are activated in a reversible manner. However, the role of β-arrestins in trafficking of PARs, which are irreversibly activated, and the effects of proteases on the subcellular distribution of β-arrestins have not been examined. We studied trafficking of PAR2 and β-arrestin1 coupled to green fluorescent protein. Trypsin induced the following: (a) redistribution of β-arrestin1 from the cytosol to the plasma membrane, where it co-localized with PAR2; (b) internalization of β-arrestin1 and PAR2 into the same early endosomes; (c) redistribution of β-arrestin1 to the cytosol concurrent with PAR2 translocation to lysosomes; and (d) mobilization of PAR2 from the Golgi apparatus to the plasma membrane. Overexpression of a C-terminal fragment of β-arrestin-319–418, which interacts constitutively with clathrin but does not bind receptors, inhibited agonist-induced endocytosis of PAR2. Our results show that β-arrestins mediate endocytosis of PAR2 and support a role for β-arrestins in uncoupling of PARs.

Mast cell tryptase and proteinase‐activated receptor 2 induce hyperexcitability of guinea‐pig submucosal neurons

Mast cells that are in close proximity to autonomic and enteric nerves release several mediators that cause neuronal hyperexcitability. This study examined whether mast cell tryptase evokes acute and long‐term hyperexcitability in submucosal neurons from the guinea‐pig ileum by activating proteinase‐activated receptor 2 (PAR2) on these neurons. We detected the expression of PAR2 in the submucosal plexus using RT‐PCR. Most submucosal neurons displayed PAR2 immunoreactivity, including those colocalizing VIP. Brief (minutes) application of selective PAR2 agonists, including trypsin, the activating peptide SL‐NH2 and mast cell tryptase, evoked depolarizations of the submucosal neurons, as measured with intracellular recording techniques. The membrane potential returned to resting values following washout of agonists, but most neurons were hyperexcitable for the duration of recordings (> 30 min–hours) and exhibited an increased input resistance and amplitude of fast EPSPs. Trypsin, in the presence of soybean trypsin inhibitor, and the reverse sequence of the activating peptide (LR‐NH2) had no effect on neuronal membrane potential or long‐term excitability. Degranulation of mast cells in the presence of antagonists of established excitatory mast cell mediators (histamine, 5‐HT, prostaglandins) also caused depolarization, and following washout of antigen, long‐term excitation was observed. Mast cell degranulation resulted in the release of proteases, which desensitized neurons to other agonists of PAR2. Our results suggest that proteases from degranulated mast cells cleave PAR2 on submucosal neurons to cause acute and long‐term hyperexcitability. This signalling pathway between immune cells and neurons is a previously unrecognized mechanism that could contribute to chronic alterations in visceral function.

c-Cbl mediates ubiquitination, degradation, and down-regulation of human protease-activated receptor 2

Mechanisms that arrest G-protein-coupled receptor (GPCR) signaling prevent uncontrolled stimulation that could cause disease. Although uncoupling from heterotrimeric G-proteins, which transiently arrests signaling, is well described, little is known about the mechanisms that permanently arrest signaling. Here we reported on the mechanisms that terminate signaling by protease-activated receptor 2 (PAR2), which mediated the proinflammatory and nociceptive actions of proteases. Given its irreversible mechanism of proteolytic activation, PAR2 is a model to study the permanent arrest of GPCR signaling. By immunoprecipitation and immunoblotting, we observed that activated PAR2 was mono-ubiquitinated. Immunofluorescence indicated that activated PAR2 translocated from the plasma membrane to early endosomes and lysosomes where it was degraded, as determined by immunoblotting. Mutant PAR2 lacking intracellular lysine residues (PAR2Δ14K/R) was expressed at the plasma membrane and signaled normally but was not ubiquitinated. Activated PAR2 Δ14K/R internalized but was retained in early endosomes and avoided lysosomal degradation. Activation of wild type PAR2 stimulated tyrosine phosphorylation of the ubiquitin-protein isopeptide ligase c-Cbl and promoted its interaction with PAR2 at the plasma membrane and in endosomes in an Src-dependent manner. Dominant negative c-Cbl lacking the ring finger domain inhibited PAR2 ubiquitination and induced retention in early endosomes, thereby impeding lysosomal degradation. Although wild type PAR2 was degraded, and recovery of agonist responses required synthesis of new receptors, lysine mutation and dominant negative c-Cbl impeded receptor ubiquitination and degradation and allowed PAR2 to recycle and continue to signal. Thus, c-Cbl mediated ubiquitination and lysosomal degradation of PAR2 to irrevocably terminate signaling by this and perhaps other GPCRs.

Substance P mediates inflammatory oedema in acute pancreatitis via activation of the neurokinin-1 receptor in rats and mice.

Pancreatic oedema occurs early in the development of acute pancreatitis, and the overall extent of fluid loss correlates with disease severity. The tachykinin substance P (SP) is released from sensory nerves, binds to the neurokinin‐1 receptor (NK1‐R) on endothelial cells and induces plasma extravasation, oedema, and neutrophil infiltration, a process termed neurogenic inflammation. We sought to determine the importance of neurogenic mechanisms in acute pancreatitis. Pancreatic plasma extravasation was measured using the intravascular tracers Evans blue and Monastral blue after administration of specific NK1‐R agonists/antagonists in rats and NK1‐R(+/+)/(−/−) mice. The effects of NK1‐R genetic deletion/antagonism on pancreatic plasma extravasation, amylase, myeloperoxidase (MPO), and histology in cerulein‐induced pancreatitis were characterized. In rats, both SP and the NK1‐R selective agonist [Sar9 Met(O2)11]SP stimulated pancreatic plasma extravasation, and this response was blocked by the NK1‐R antagonist CP 96,345. Selective agonists of the NK‐2 or NK‐3 receptors had no effect. In rats, cerulein stimulated pancreatic plasma extravasation and serum amylase. These responses were blocked by the NK1‐R antagonist CP 96,345. In wildtype mice, SP induced plasma extravasation while SP had no effect in NK1‐R knockout mice. In NK1‐R knockout mice, the effects of cerulein on pancreatic plasma extravasation and hyperamylasemia were reduced by 60%, and pancreatic MPO by 75%, as compared to wildtype animals. Neurogenic mechanisms of inflammation are important in the development of inflammatory oedema in acute interstitial pancreatitis.

Substance P-induced trafficking of beta-arrestins. The role of beta-arrestins in endocytosis of the neurokinin-1 receptor.

Agonist-induced redistribution of G-protein-coupled receptors (GPCRs) and β-arrestins determines the subsequent cellular responsiveness to agonists and is important for signal transduction. We examined substance P (SP)-induced trafficking of β-arrestin1 and the neurokinin-1 receptor (NK1R) in KNRK cells in real time using green fluorescent protein. Green fluorescent protein did not alter function or localization of the NK1R or β-arrestin1. SP induced (a) striking and rapid (<1 min) translocation of β-arrestin1 from the cytosol to the plasma membrane, which preceded NK1R endocytosis; (b) redistribution of the NK1R and β-arrestin1 into the same endosomes containing SP and the transferrin receptor (2–10 min); (c) prolonged colocalization of the NK1R and β-arrestin1 in endosomes (>60 min); (d) gradual resumption of the steady state distribution of the NK1R at the plasma membrane and β-arrestin1 in the cytosol (4–6 h). SP stimulated a similar redistribution of immunoreactive β-arrestin1 and β-arrestin2. In contrast, SP did not affect Gαq/11distribution, which remained at the plasma membrane. Expression of the dominant negative β-arrestin319–418 inhibited SP-induced endocytosis of the NK1R. Thus, SP induces rapid translocation of β-arrestins to the plasma membrane, where they participate in NK1R endocytosis. β-Arrestins colocalize with the NK1R in endosomes until the NK1R recycles and β-arrestins return to the cytosol.

Acute ACE Inhibition Causes Plasma Extravasation in Mice That is Mediated by Bradykinin and Substance P

The use of angiotensin-converting enzyme (ACE) has been associated with the occurrence of adverse effects, including cough and angioneurotic edema. Accumulation of kinins has been suggested to play a major role in these adverse effects of ACE inhibitor, although conclusive evidence for such a role is lacking. We investigated whether ACE inhibition increases plasma extravasation in mice (Swiss, C57Bl/6J, and J129Sv/Ev strains) via inhibition of bradykinin metabolism and stimulation of neurogenic inflammatory mechanisms. Intravenous captopril and enalapril increased the extravasation of Evans blue dye in all tissues examined (trachea, stomach, duodenum, and pancreas). This effect was evident 15 minutes after drug administration. The particulate dye Monastral blue identified the sites of captopril-induced leakage in the microvasculature. Pretreatment with the bradykinin B2 receptor antagonist Hoe 140 or with the tachykinin NK1 receptor antagonist SR 140333 inhibited captopril-evoked increase in plasma extravasation. In mice in which the gene encoding the bradykinin B2 receptor was disrupted by gene targeting, neither bradykinin nor captopril increased plasma extravasation. Pretreatment with Hoe 140 did not reduce the hypotensive response induced by captopril. The present findings suggest that ACE inhibition increases kinin levels in tissues and/or plasma. These increased kinin levels increase microvascular leakage in mouse airways and digestive tract via the release of tachykinins from terminals of primary sensory neurons. Exaggerated kinin production and the subsequent stimulation of peptide release from sensory nerves may be involved in adverse effects of ACE inhibitors.