Faculty Bibliography

Calcitonin receptor-like receptor (CLR) and the receptor activity-modifying protein 1 (RAMP1) comprise a receptor for calcitonin gene-related peptide (CGRP). Although CGRP induces endocytosis of CLR/RAMP1, little is known about post-endocytic sorting of these proteins. We observed that the duration of stimulation with CGRP markedly affected post-endocytic sorting of CLR/RAMP1. In HEK and SK-N-MC cells, transient stimulation (10(-7) M CGRP, 1 h), induced CLR/RAMP1 recycling with similar kinetics (2-6 h), demonstrated by labeling receptors in living cells with antibodies to extracellular epitopes. Recycling of CLR/RAMP1 correlated with resensitization of CGRP-induced increases in [Ca(2+)](i). Cycloheximide did not affect resensitization, but bafilomycin A(1), an inhibitor of vacuolar H(+)-ATPases, abolished resensitization. Recycling CLR and RAMP1 were detected in endosomes containing Rab4a and Rab11a, and expression of GTPase-defective Rab4aS22N and Rab11aS25N inhibited resensitization. After sustained stimulation (10(-7) M CGRP, >2 h), CLR/RAMP1 trafficked to lysosomes. RAMP1 was degraded approximately 4-fold more rapidly than CLR (RAMP1, 45% degradation, 5 h; CLR, 54% degradation, 16 h), determined by Western blotting. Inhibitors of lysosomal, but not proteasomal, proteases prevented degradation. Sustained stimulation did not induce detectable mono- or polyubiquitination of CLR or RAMP1, determined by immunoprecipitation and Western blotting. Moreover, a RAMP1 mutant lacking the only intracellular lysine (RAMP1K142R) internalized and was degraded normally. Thus, after transient stimulation with CGRP, CLR and RAMP1 traffic from endosomes to the plasma membrane, which mediates resensitization. After sustained stimulation, CLR and RAMP1 traffic from endosomes to lysosomes by ubiquitin-independent mechanisms, where they are degraded at different rates.

Somatostatin-receptor 1 (sst1) is an autoreceptor in the central nervous system that regulates the release of somatostatin. Sst1 is present intracellularly and at the cell surface. To investigate sst1 trafficking, rat sst1 tagged with epitope was expressed in rat insulinoma cells 1046-38 (RIN-1046-38) and tracked by antibody labeling. Confocal microscopic analysis revealed colocalization of intracellularly localized rat sst1-human simplex virus (HSV) with Rab5a-green fluorescent protein and Rab11a-green fluorescent protein, indicating the distribution of the receptor in endocytotic and recycling organelles. Somatostatin-14 induced internalization of cell surface receptors and reduction of binding sites on the cell surface. It also stimulated recruitment of intracellular sst1-HSV to the plasma membrane. Confocal analysis of sst1-HSV revealed that the receptor was initially transported within superficial vesicles. Prolonged stimulation of the cells with the peptide agonist induced intracellular accumulation of somatostatin-14. Because the number of cell surface binding sites did not change during prolonged stimulation, somatostatin-14 was internalized through a dynamic process of continuous endocytosis, recycling, and recruitment of intracellularly present sst1-HSV. Accumulated somatostatin-14 bypassed degradation via the endosomal-lysosomal route and was instead rapidly released as intact and biologically active somatostatin-14. Our results show for the first time that sst1 mediates a dynamic process of endocytosis, recycling, and re-endocytosis of its cognate ligand.

Mediators involved in the generation of symptoms in patients with irritable bowel syndrome (IBS) are poorly understood. Here we show that colonic biopsy samples from IBS patients release increased levels of proteolytic activity (arginine cleavage) compared to asymptomatic controls. This was dependent on the activation of NF-kappaB. In addition, increased proteolytic activity was measured in vivo, in colonic washes from IBS compared with control patients. Trypsin and tryptase expression and release were increased in colonic biopsies from IBS patients compared with control subjects. Biopsies from IBS patients (but not controls) released mediators that sensitized murine sensory neurons in culture. Sensitization was prevented by a serine protease inhibitor and was absent in neurons lacking functional protease-activated receptor-2 (PAR2). Supernatants from colonic biopsies of IBS patients, but not controls, also caused somatic and visceral hyperalgesia and allodynia in mice, when administered into the colon. These pronociceptive effects were inhibited by serine protease inhibitors and a PAR2 antagonist and were absent in PAR2-deficient mice. Our study establishes that proteases are released in IBS and that they can directly stimulate sensory neurons and generate hypersensitivity symptoms through the activation of PAR2.

Neurokinin-3 (NK(3)) receptors are prevalent within the substantia nigra (SN) and ventral tegmental area (VTA), where their activation can affect motor and motivational behaviors as well as cardiovascular function and stress responses. These actions are mediated, in part, by dopaminergic neurons in each region. To determine the relevant sites for activation of these receptors, we examined the electron microscopic localization of NK(3) receptors and tyrosine hydroxylase (TH), the catecholamine synthesizing enzyme in dopaminergic neurons in the SN and VTA of rat brain. In each region, immunogold-silver labeling for NK(3) receptors was detected in many somatodendritic profiles, some of which contained TH-immunoreactivity. NK(3)-immunogold particles were largely associated with endomembranes resembling smooth endoplasmic reticulum, and only occasionally located on the plasma membrane in TH-labeled dendrites. In comparison with these dendrites, non-TH immunoreactive dendrites contained significantly more total (VTA) and more plasmalemmal (VTA and SN) NK(3)-immunogold particles. In each region, NK(3) gold particles also were seen in axonal as well as glial profiles, some of which contacted TH-immunoreactive dendrites. The NK(3)-labeled axon terminals formed either symmetric or asymmetric, excitatory-type synapses, the latter of which were significantly more prevalent in the VTA, compared with SN. These results provide the first ultrastructural evidence indicating that NK(3) receptors are available in cytoplasmic reserve in dopaminergic neurons, but more immediately accessible at the plasmalemmal surface of non-dopaminergic dendrites in both the SN and VTA. The activation of these receptors, together with the NK(3) receptors in either the presynaptic axon terminals or glia may contribute to the diverse physiological effects of tachykinins in each region, and most prominently involving excitatory inputs to the VTA.

Exacerbated sensitivity to mechanical stimuli that are normally innocuous or mildly painful (mechanical allodynia and hyperalgesia) occurs during inflammation and underlies painful diseases. Proteases that are generated during inflammation and disease cleave protease-activated receptor 2 (PAR2) on afferent nerves to cause mechanical hyperalgesia in the skin and intestine by unknown mechanisms. We hypothesized that PAR2-mediated mechanical hyperalgesia requires sensitization of the ion channel transient receptor potential vanilloid 4 (TRPV4). Immunoreactive TRPV4 was coexpressed by rat dorsal root ganglia (DRG) neurons with PAR2, substance P (SP) and calcitonin gene-related peptide (CGRP), mediators of pain transmission. In PAR2-expressing cell lines that either naturally expressed TRPV4 (bronchial epithelial cells) or that were transfected to express TRPV4 (HEK cells), pretreatment with a PAR2 agonist enhanced Ca2+ and current responses to the TRPV4 agonists phorbol ester 4alpha-phorbol 12,13-didecanoate (4alphaPDD) and hypotonic solutions. PAR2-agonist similarly sensitized TRPV4 Ca2+ signals and currents in DRG neurons. Antagonists of phospholipase Cbeta and protein kinases A, C and D inhibited PAR2-induced sensitization of TRPV4 Ca2+ signals and currents. 4alphaPDD and hypotonic solutions stimulated SP and CGRP release from dorsal horn of rat spinal cord, and pretreatment with PAR2 agonist sensitized TRPV4-dependent peptide release. Intraplantar injection of PAR2 agonist caused mechanical hyperalgesia in mice and sensitized pain responses to the TRPV4 agonists 4alphaPDD and hypotonic solutions. Deletion of TRPV4 prevented PAR2 agonist-induced mechanical hyperalgesia and sensitization. This novel mechanism, by which PAR2 activates a second messenger to sensitize TRPV4-dependent release of nociceptive peptides and induce mechanical hyperalgesia, may underlie inflammatory hyperalgesia in diseases where proteases are activated and released.

BACKGROUND & AIMS/OBJECTIVE:Intestinal mast cell infiltration may participate to abdominal pain in irritable bowel syndrome (IBS) patients. However, the underlying mechanisms remain unknown. We assessed the effect of mast cell mediators released from the colonic mucosa of IBS patients on the activation of rat sensory neurons in vitro. METHODS:Colonic mast cell infiltration and mediator release were assessed with quantitative immunofluorescence and immunoenzymatic assays. The effect of mucosal mediators was tested on mesenteric sensory nerve firing and Ca(2+) mobilization in dorsal root ganglia in rats. RESULTS:Mediators from IBS patients, but not controls, markedly enhanced the firing of mesenteric nerves (14.7 +/- 3.2 imp/sec vs 2.8 +/- 1.5 imp/sec; P < .05) and stimulated mobilization of Ca(2+) in dorsal root ganglia neurons (29% +/- 4% vs 11% +/- 4%; P < .05). On average, 64% of dorsal root ganglia responsive to mediators were capsaicin-sensitive, known to mediate nociception. Histamine and tryptase were mainly localized to mucosal mast cells. IBS-dependent nerve firing and Ca(2+) mobilization were correlated with the area of the colonic lamina propria occupied by mast cells (r = 0.74; P < .01, and r = 0.78; P < .01, respectively). IBS-dependent excitation of dorsal root ganglia was inhibited by histamine H(1) receptor blockade and serine protease inactivation (inhibition of 51.7%; P < .05 and 74.5%; P < .05; respectively). CONCLUSIONS:Mucosal mast cell mediators from IBS patients excite rat nociceptive visceral sensory nerves. These results provide new insights into the mechanism underlying visceral hypersensitivity in IBS.

Prokineticins (PKs), consisting of PK1 and PK2, are a pair of newly identified regulatory peptides. Two closely related G-protein coupled receptors, PKR1 and PKR2, mediate the signaling of PKs. PKs/PKRs participate in the regulation of diverse biological processes, ranging from development to adult physiology. A number of studies have indicated the involvement of PKs/PKRs in nociception. Here we show that PK2 is a sensitizer for nociception. Intraplantar injection of recombinant PK2 resulted in a strong and localized hyperalgesia with reduced thresholds to nociceptive stimuli. PK2 mobilizes calcium in dissociated dorsal root ganglion (DRG) neurons. Mice lacking the PK2 gene displayed strong reduction in nociception induced by thermal and chemical stimuli, including capsaicin. However, PK2 mutant mice showed no difference in inflammatory response to capsaicin. As the majority of PK2-responsive DRG neurons also expressed transient receptor potential vanilloid (TRPV1) and exhibited sensitivity to capsaicin, TRPV1 is likely a significant downstream molecule of PK2 signaling. Taken together, these results reveal that PK2 sensitize nociception without affecting inflammation.

Serine proteinases like thrombin can signal to cells by the cleavage/activation of proteinase-activated receptors (PARs). Although thrombin is a recognized physiological activator of PAR(1) and PAR(4), the endogenous enzymes responsible for activating PAR(2) in settings other than the gastrointestinal system, where trypsin can activate PAR(2), are unknown. We tested the hypothesis that the human tissue kallikrein (hK) family of proteinases regulates PAR signaling by using the following: 1) a high pressure liquid chromatography (HPLC)-mass spectral analysis of the cleavage products yielded upon incubation of hK5, -6, and -14 with synthetic PAR N-terminal peptide sequences representing the cleavage/activation motifs of PAR(1), PAR(2), and PAR(4); 2) PAR-dependent calcium signaling responses in cells expressing PAR(1), PAR(2), and PAR(4) and in human platelets; 3) a vascular ring vasorelaxation assay; and 4) a PAR(4)-dependent rat and human platelet aggregation assay. We found that hK5, -6, and -14 all yielded PAR peptide cleavage sequences consistent with either receptor activation or inactivation/disarming. Furthermore, hK14 was able to activate PAR(1), PAR(2), and PAR(4) and to disarm/inhibit PAR(1). Although hK5 and -6 were also able to activate PAR(2), they failed to cause PAR(4)-dependent aggregation of rat and human platelets, although hK14 did. Furthermore, the relative potencies and maximum effects of hK14 and -6 to activate PAR(2)-mediated calcium signaling differed. Our data indicate that in physiological settings, hKs may represent important endogenous regulators of the PARs and that different hKs can have differential actions on PAR(1), PAR(2), and PAR(4).