Faculty Bibliography

Several lines of evidence suggest that sensory nerves of the carotid body have an efferent function in addition to their afferent function of conducting chemoreceptive impulses to the brain. However, it has been difficult to document the release of substances from sensory nerve terminals on glomus cells and to determine whether such an efferent function plays a role in chemoreception. By comparison, the phenomenon of neurogenic inflammation has been relatively easy to study in rats and guinea pigs and has proven to be an informative model system for analyzing efferent actions of sensory nerves. The main characteristic of neurogenic inflammation is plasma leakage. Chemical irritants that activate unmyelinated sensory nerves cause plasma leakage in the skin, respiratory tract, and other organs by triggering the release of substances from sensory nerve fibers. Substance P, which is synthesized and released by some sensory neurons, appears to be the main active mediator, although other tachykinins, calcitonin gene-related peptide, and perhaps other peptides may also participate. Neurogenic inflammation results from the action of substance P on NK1 receptors, as demonstrated by selective NK1 receptor agonists and antagonists. The NK1 receptors involved in plasma leakage are located on the endothelial cells of postcapillary venules and collecting venules. Within seconds of the activation of NK1 receptors by substance P, gaps form in the endothelium of target vessels. The endothelial gaps are transient, and the leak normally ends in a few minutes. However, the magnitude of the response can increase in pathological conditions such as Mycoplasma pulmonis infection in rats, which results in a chronic inflammatory disease of the respiratory tract. The infected airway mucosa becomes abnormally vascular as a result of angiogenesis, and the endothelial cells of the newly formed vessels express increased numbers of NK1 receptors and thus are abnormally sensitive to substance P. Studies of neurogenic inflammation not only have helped to understand the efferent actions of sensory nerves but also have given insight into the mechanism and consequences of inflammatory changes in endothelial cells and in the plasma leakage that follows.

The aim of this investigation was to purify aminopeptidase M (APM) from the muscle layer of the small intestine, to compare it with APM of the mucosa and kidney, and to examine the degradation of gastrointestinal neural and hormonal peptides by muscle APM. APM was purified from the muscle and mucosa of the pig small intestine by DEAE-Sepharose and immuno-affinity chromatography. The specific activity of APM from muscle, mucosa, and kidney was 3900, 3000, and 3800 nmol/min per mg protein, respectively (substrate [Leu5]enkephalin). Muscle and mucosa APM contained four protein bands with apparent molecular weights of 150, 110, 73, and 52 kDa. Kidney APM contained three protein bands of 150, 110, and 56 kDa. The 150, 110, and 52/56 kDa bands cross-rected with an APM antiserum. APM from each tissue degraded [Leu5]enkephalin and [Met5]enkephalin, but not cholecystokinin-8, gastrin releasing peptide-10, somatostatin-14, substance P, and vasoactive intestinal peptide. The enzymes were identically inhibited by APM antiserum, amastatin, bestatin, actinonin, and 1, 10 phenanthroline. Non-mucosal APM may degrade enkephalins and terminate their biological actions.

In the digestive system, substance P is an excitatory transmitter to muscle, a putative excitatory neuro-neuronal transmitter, a vasodilator, and a mediator in inflammatory processes. Many of the biological effects of substance P are mediated by a high-affinity interaction with the tachykinin receptor neurokinin-1. The aim of the present study was to identify the sites of expression of this receptor in the rat stomach and intestine by immunohistochemistry with a polyclonal antiserum raised to the intracellular C-terminal portion of the rat neurokinin-1 receptor. Neurokinin-1 receptor immunoreactivity is present in a large population of enteric neurons. The relative density of these neurons along the gut is colon > ileum > stomach. In the intestine, stained neurons have a smooth cell body with processes that can be followed within and between plexuses, and make close approaches to other neuronal cells, but do not appear to project outside the plexuses, suggesting that they are interneurons. In the stomach, neurokinin-1 receptor-immunoreactive neurons are infrequent and have a poorly defined and irregular shape. Neurokinin-1 receptor immunoreactivity is also localized to numerous non-neuronal cells in the inner portion of the circular muscle layer of the small intestine, which have the appearance of small dark smooth muscle cells or interstitial cells of Cajal. These cells are postulated to form a "stretch-sensitive" system with the deep muscular plexus and thus constitute an important site of regulation of muscle activity. Double labeling immunofluorescence was used to simultaneously localize neurokinin-1 receptor and substance P/tachykinin immunoreactivities. These experiments demonstrate that in the enteric plexuses, substance P/tachykinin-immunoreactive varicose fibers encircle the cell bodies of most neurokinin-1 receptor-containing neurons, and in the inner portion of the circular muscle layer of the small intestine they lie close to neurokinin-1 receptor-immunoreactive non-neuronal cells. In addition, some enteric neurons express both neurokinin-1 receptor and substance P/tachykinin immunoreactivities. The present study provides strong evidence that the neurokinin-1 receptor is the tachykinin receptor mediating the actions of substance P on enteric neurons and smooth muscle.

T cell homing into extravascular sites requires penetration across the subendothelial basal lamina, a specialized nonfibrillar connective tissue structure that anchors endothelial cells to parenchymal surfaces. Herein, we show that normal human T cells express gelatinases A and B, two matrix metalloproteinases active against the major basal lamina constituents, collagen types IV and V. Expression is confirmed at both the mRNA and protein levels. Gelatinase B is expressed constitutively, whereas gelatinases A and B expression is induced by T cell activation. In vitro migration of resting T cells across a basal lamina equivalent is mediated by gelatinase B, because it is specifically blocked by GM6001, a hydroxamic acid inhibitor of matrix metalloproteinases. Inhibition of T cell homing by interference with gelatinase function may represent a useful approach to the treatment of T cell-mediated autoimmune diseases.

Many of the actions of the neuropeptide substance P (SP) that are mediated by the neurokinin 1 receptor (NK1-R) desensitize and resensitize, which may be associated with NK1-R endocytosis and recycling. We delineated this endocytic pathway in transfected cells by confocal microscopy using cyanine 3-SP and NK1-R antibodies. SP and the NK1-R were internalized into the same clathrin immunoreactive vesicles, and then sorted into different compartments. The NK1-R was colocalized with a marker of early endosomes, but not with markers of late endosomes or lysosomes. We quantified the NK1-R at the cell surface by incubating cells with an antibody to an extracellular epitope. After exposure to SP, there was a loss and subsequent recovery of surface NK1-R. The loss was prevented by hypertonic sucrose and potassium depletion, inhibitors of clathrin-mediated endocytosis. Recovery was independent of new protein synthesis because it was unaffected by cycloheximide. Recovery required endosomal acidification because it was prevented by an H(+)-ATPase inhibitor. The fate of internalized 125I-SP was examined by chromatography. SP was intact at the cell surface and in early endosomes, but slowly degraded in perinuclear vesicles. We conclude that SP induces clathrin-dependent internalization of the NK1-R. The SP/NK1-R complex dissociates in acidified endosomes. SP is degraded, whereas the NK1-R recycles to the cell surface.

Endocytosis of the gastrin releasing peptide receptor (GRP-R) may regulate cellular responses to GRP. We observed endocytosis in transfected epithelial cells by confocal microscopy using cyanine 3-GRP (cyanine 3.18-labeled gastrin releasing peptide) and GRP-R antibodies. At 4 degrees C, cy3-GRP and GRP-R were confined to the plasma membrane. After 5 min at 37 degrees C, ligand and receptor were internalized into early endosomes with fluorescein isothiocyanate-transferrin. After 10 min, cy3-GRP and GRP-R were in perinuclear vesicles, and at 60 min cy3-GRP was in large, central vesicles, while GRP-R was at the cell surface. We quantified surface GRP-R using an antibody to an extracellular epitope and an 125I-labeled secondary antibody. After exposure to GRP, there was a loss and subsequent recovery of surface GRP-R. Recovery was unaffected by cycloheximide, and thus independent of new protein synthesis, but was attenuated by acidotropic agents, and therefore required endosomal acidification. Internalization of 125I-GRP, assessed using an acid wash, was maximal after 10-20 min, and was clathrin-mediated since it was inhibited by hyperosmolar sucrose and phenylarsine oxide. Thus, GRP and its receptor are rapidly internalized into early endosomes and then dissociate in an acidified compartment. GRP is probably degraded whereas the GRP-R recycles.

Substance P (SP) degradation by the cell-surface enzyme neutral endopeptidase (NEP), and endocytosis of the NK-1 receptor (NK-1R) are potential mechanisms that limit the responsiveness of cells to SP. These mechanisms were studied in epithelial cells (KNRK) transfected with cDNA encoding an epitope labeled Flag-NK-1R or NEP. In cells expressing NK-1R alone, specific saturable ¹²⁵I-Sp binding at 37°C was 28% ± 3% of total counts. Co-expression of NEP with the NK-1R reduced binding to 4 ± 0.3% of total counts. Addition of the NEP inhibitor thiorphan restored binding to control levels. In cells expressing NK-1R alone, SP (1 nM) stimulated an increase in [Ca²⁺](i) of 163 ± 16 nM. Go-expression of NEP reduced the [Ca²⁺](i) response to 52 ± 13 nM, and this response was restored to control levels by thiorphan. Antibodies to the NK-1R and rhodamine labeled SP were used to observe endocytosis in KNRK cells expressing the NK-1R alone. When cells were incubated with SP for 60 min at 4°C, NK-1R and rhodamine SP were confined to the plasma membrane. After 3 and 10 min at 37°C, there was a reduction in cell-surface staining and NK-1R and rhodamine-SP were localized in intracellular vesicles. Binding experiments with ¹²⁵I-Sp confirmed the rapid internalization of peptide. Therefore, the NK-1R and SP are internalized within minutes of binding. Thus, ligand degradation and receptor endocytosis limit the cellular response to SP.

We labeled substance P (SP), neurokinin A (NKA), and [Lys0]gastrin-releasing peptide-27 (GRP) with cyanine 3.18 (cy3). Cy3-peptides purified by HPLC were fully active, determined by [Ca2+]i mobilization. Binding was specific because it was abolished by unlabeled peptides and receptor antagonists. Transfected cells yielded a log-fold greater cy3 intensity than control cells by FACS. Confocal microscopy of transfected cells and cultured enteric neurons showed that cy3-SP bound to surface receptors and was internalized. Internalization was observed in living cells by capture of sequential images. Recovery of surface binding sites was monitored by flow cytometry using cy3-SP. Thus, cy3 neuropeptides can be used to isolate and study receptor-bearing cells.