Detailed immunoautoradiographic mapping of enkephalinase (EC 3.4.24.11) in rat central nervous system: comparison with enkephalins and substance P

Endopeptidase-24.11, a cell-surface peptidase of central nervous system neurons, is expressed by Schwann cells in the pig peripheral nervous system

Endopeptidase-24.11 is a 90-kDa surface glycoprotein with the ability to hydrolyze a variety of biologically active peptides. Interest in this enzyme is based on the consensus that it may play a role in the termination of peptide signals in the central nervous system. In the present study, we have investigated the distribution of endopeptidase-24.11 in two nerves of the peripheral nervous system of newborn pigs: the sciatic, composed of a mixture of myelinated and nonmyelinated axons, and cervical sympathetic trunk in which greater than 99% of the axons are nonmyelinated. The endopeptidase was monitored enzymatically, as well as by immunoblotting and immunocytochemistry using mono- and polyclonal anti-endopeptidase antibodies. Endopeptidase-24.11 was detected in both the sciatic nerve and the cervical sympathetic trunk. Membrane preparations from sciatic nerve hydrolyzed 125I-insulin B-chain, and more than 50% of the activity was inhibited by phosphoramidon with an IC50 concentration of 3.2 nM. Moreover, a 90-kDa polypeptide was detected by immunoblotting of sciatic nerve membranes. The type of cells expressing the endopeptidase was determined by immunohistochemistry. In teased nerve preparations, these cells were identified morphologically as myelin- and non-myelin-forming Schwann cells. Endopeptidase-24.11 was also expressed by cultured Schwann cells from sciatic nerve and cervical sympathetic trunk maintained for 3 h in vitro. The presence of endopeptidase-24.11 on the Schwann cell surface raises the possibility of a potential role for the enzyme in nerve development and/or regeneration.

Chronic inhibition of enkephalinase induces changes in the antinociceptive and locomotor effects of the enkephalinase inhibitor acetorphan in rats

The enkephalinase inhibitor thiorphan was infused intracerebroventricularly in rats during 14 days (25 micrograms/5 microliters/hr), inducing an average inhibition of cerebral enkephalinase of about 65%. Animals were tested during the infusion for their response to acetorphan, a parenterally active derivative of thiorphan. When administered intravenously on day 8 of the infusion, acetorphan (5 mg/kg) significantly increased locomotion in chronic saline-infused rats but not in animals receiving thiorphan. Furthermore, when injected at the same dose on day 10, acetorphan did not modify the latency to jump, in the hot plate test, in thiorphan-treated rats, whereas it elicited a significant analgesia in chronic saline-treated controls. These data show that the effects induced by the administration of an enkephalinase inhibitor were diminished after a period of chronic inhibition of the enzyme, suggesting the development of tolerance.

Immunoautoradiographic localisation of enkephalinase (EC 3.4.24.11) in rat gastrointestinal tract

Enkephalinase (EC 3.4.24.11, membrane metalloendopeptidase) is a zinc peptidase expressed by neurons and a variety of epithelial cells, and responsible for the inactivation of enkephalins in brain. Its functions in the gastrointestinal (GI) tract are less well understood although enkephalinase inhibitors were reported to induce a constellation of antisecretory and motor responses. Its localisation in various segments of the rat GI tract was established autoradiographically using a 125I-labelled monoclonal antibody. All along the GI tract, the highest immunoreactivity was found in mucosal layers e.g., in intestinal villi, basal epithelial layers of the oesophagus or gastric cardia, muscularis mucosae of the stomach and large intestine. The immunoreactivity was also high in the stomach submucosae and moderate in the muscularis propria of the caecum. A faint patchy immunoreactivity was also observed in several other layers. This distribution suggests that the membrane peptidase is expressed by enterocytes and a variety of other cells. Its high expression in mucosal layers is consistent with its participation in protein digestion and also in the inactivation of endogenous peptides, particularly the enkephalins, acting at this level to control secretory mechanisms and hydroelectrolytic fluxes. Its presence in submucosal layers may account for some naloxone-reversible motor responses elicited by enkephalinase inhibitors.

Membrane peptidases in the peripheral nervous system of the pig: their localization by immunohistochemistry at light and electron microscopic levels

The presence and cellular localization of five membrane peptidases has been investigated in peripheral nerves, including those of the autonomic nervous system, in the pig. Endopeptidase-24.11 ("enkephalinase") peptidyl dipeptidase A, aminopeptidase N, aminopeptidase W and dipeptidyl peptidase IV were studied by both enzymic assays of membranes prepared from samples of nerve and by immunoperoxidase histochemistry at light and in two cases, endopeptidase-24.11 and aminopeptidase W, at electron microscopic levels. All five peptidases could be quantified by enzymic assay, though the activities were about 1% of those in renal microvilli and less than those of choroid plexus membranes. Endopeptidase-24.11 was associated with Schwann cell membranes in all types of nerve examined, including major nerves containing predominantly myelinated fibres as well as autonomic nerves, such as the vagus and splenic nerves and the sympathetic chain, staining being observed in membranes associated with myelinated and unmyelinated fibres. The Schwann cell location of endopeptidase-24.11 was confirmed by correlation with immunostaining for glial fibrillary acidic protein and by electron microscopy. This peptidase is known to have a wide repertoire of susceptible substrates among neuropeptides which was here shown to include vasoactive intestinal polypeptide (Km 268 microM, kcat 568 min-1), one of a number of neuropeptides present in peripheral nerve fibres. Three of the peptidases, peptidyl dipeptidase A, aminopeptidase N and dipeptidyl peptidase IV, were associated with microvessels of peripheral nerves. Aminopeptidase N was also observed in connective tissue elements, including the perineurium. Aminopeptidase W was unique among the five peptidases in having a neuronal localization. This was observed in unmyelinated and myelinated nerves and was supported by comparison with the pattern of staining observed for neurofilament protein and by electron microscopic immunoperoxidase staining. This observation was unexpected since aminopeptidase W has not been detected as a neuronal marker in the brain. Some possible roles for the membrane peptidases in peripheral nerves are discussed.

Immunocytochemical localization of endopeptidase-24.11 in the nucleus tractus solitarius of the rat brain

Recently, it has been hypothesized that the N-terminal portion of substance P (SP), SP(1-7), which results from the action of endopeptidase 24.11 (EC3.4.24.11), could be involved in mediating the depressor effects of baroreceptor afferent activation via its action on cells in the nucleus tractus solitarius (NTS). In this study, the binding of a monoclonal antibody to endopeptidase 24.11 was examined immunohistochemically at the level of the caudal medulla of the rat brain. By light microscopy, intense immunoreactivity was seen in the NTS, in fibers bordering the area postrema, and in the area postrema itself. After electron microscopy, endopeptidase 24.11-like immunoreactivity was seen to be associated with the cytoskeleton and plasma membrane in axons, dendrites and glial processes. Antigen was also associated with synaptic vesicles and plasma membranes in presynaptic terminals forming mainly axo-dendritic synapses typical of vagal afferent terminals involved in the baroreceptor reflex. Thus, endopeptidase 24.11 appears to be localized at sites where it could effectively process SP prior to its binding to postsynaptic receptors.

Fluorescent histochemical localization of neutral endopeptidase-24.11 (enkephalinase) in the rat brainstem

Characterization of the distribution of the peptide-degrading enzyme neutral endopeptidase-24.11 (E.C. 3.4.24.11; NEP; enkephalinase) in the rat brainstem was examined by means of a unique fluorescent histochemical method. Enzyme staining was completely blocked by three potent NEP inhibitors (thiorphan, phosphoramidon, and JHF-26) at a concentration of 50 nM, supporting the specificity of this method to visualize sites of NEP activity selectively. At all levels of the brainstem, NEP was localized to cell bodies, cell processes or terminal-like fields and was localized to more than 90 distinct nuclei or subnuclei. In the mesencephalon these included the central gray, cuneiform n., dorsal and lateral tegmental n., inferior colliculus, interpeduncular n., lateral and medial geniculate n., central linear raphe n., mesencephalic n. of the trigeminal nerve, mammillary nuclei, occulomotor n., red n., superior colliculus, ventral n. of the lateral lemniscus, substantia nigra-ventral tegmental area, and the zona incerta. In the pons, NEP staining was restricted to fewer regions or nuclei, including the dorsal and ventral cochlear n., facial n., motor trigeminal n., principal sensory trigeminal n., parabrachial nuclei, pontine n., the oral and caudal pontine reticular n., pontine olivary nuclei, several pontine tegmental nuclei, pontine raphe nuclei, and the trapezoid n. In the cerebellum, staining was localized largely to the granule cell layer of the cerebellar cortex. Scattered staining was observed in the molecular cell layer. The medulla contained extensive NEP staining localized to nuclei that included the ambiguous n., dorsal motor n. of the vagus, hypoglossal n., inferior olivary n., prepositus hypoglossus n., solitary tract n., nuclei of the spinal tract of the trigeminal n., and the lateral, medial, and superior vestibular nuclei. Nuclei of the medullary reticular formation that were also richly stained for NEP included the raphe magnus n., raphe obscurus n., raphe pallidus n., dorsal, lateral, and ventral reticular nuclei of the medulla, and the gigantocellular, lateral paragigantocellular, linear, paramedian and parvicellular reticular nuclei. The widespread distribution of NEP in the brainstem suggests the existence of a number of functional systems, including the pathways involved in the mechanisms of pain and analgesia, which are potential targets of NEP inhibitors. In most regions, the distribution of NEP closely overlapped with that reported for the enkephalins, and showed a more restricted overlap with the reported distribution of substance P.

Enkephalinase (EC 3.4.24.11) inhibitors: protection of endogenous ANF against inactivation and potential therapeutic applications

Atrial natriuretic factor (ANF) is a cardiac hormone exerting potent cardiovascular and renal effects but its poor intestinal absorption and rapid inactivation have prevented so far its therapeutic utilisation. However inhibition of endogenous ANF metabolism progressively emerges as a novel therapeutic approach in cardiovascular and renal disorders. The critical role played by enkephalinase (membrane metalloendopeptidase, EC 3.4.24.11) in ANF inactivation was deduced from the effects of inhibitors. These compounds not only protect partially exogenous ANF from hydrolysis by some tissue preparations in vitro but also, in vivo, they increase the half-life of the exogenous hormone in plasma and, even more markedly, its recovery in intact form in kidney, a major target organ. In addition, enkephalinase inhibitors increase by two- to three-fold the circulating level of endogenous ANF, even when the latter is already markedly elevated, such as in patients with chronic heart failure. Finally, enkephalinase inhibitors induce a series of ANF-like responses such as natriuresis, diuresis or increase in cGMP excretion which are attributable to the hormone. These pharmacological observations, as well as preliminary clinical trials, suggest that enkephalinase inhibitors may represent a novel class of therapeutic agents with potential applications in congestive heart failure, essential hypertension and various sodium-retaining states.

Isolation and identification of a peptide from rat brain which inhibits [3H]TCP binding

1. A stereospecific radioreceptor binding assay for the phencyclidine analogue [3H]TCP was utilized to screen for inhibitors of binding in extracts of rat brain. 2. Extracts were prepared from rat cortex and hippocampus by methods employing aqueous acid or acidified methanol. Samples were fractionated by reversed phase-HPLC (RP-HPLC) and tested for activity in the radioreceptor assay. Three zones of activity were detected. The most active fraction was further purified by high performance-size exclusion chromatography. 3. Size exclusion chromatography revealed two zones of activity, corresponding to mol. wts of 4000-8000 Da and 1000-2000 Da. Final purification of the lower molecular weight material was achieved by RP-HPLC. 4. Two well-separated peaks were shown to be homogeneous. Their amino acid sequences were determined by automated Edman degradation and data base searching identified these two peaks as the undecapeptide Substance P and its oxidized counterpart (Substance P sulfoxide). 5. Comparative HPLC of synthetic Substance P, or its sulfoxide, as well as spectral analysis confirmed the identity of the isolated peptides. 6. Synthetic Substance P inhibits specific [3H]TCP binding in the radioreceptor assay.

Changes in levels of the tripeptide Tyr-Gly-Gly as an index of enkephalin release in the spinal cord: effects of noxious stimuli and parenterally-active peptidase inhibitors

The tripeptide Tyr-Gly-Gly (YGG), representing the product of enkephalin hydrolysis by enkephalinase (EC 3.4.24.11), was characterized and its levels measured in spinal cord perfusates of halothane-anaesthetized rats. During noxious pinching of the muzzle, which is known to trigger enkephalin release, YGG levels were enhanced more markedly and for longer than were those of [Met5]enkephalin (YGGFM), in the same samples. By contrast, neither YGG nor YGGFM levels were affected by pinching the tail. Treatment with carbaphethiol, a parenterally-active aminopeptidase inhibitor, markedly increased YGG levels and lengthened the duration of the increase produced by pinching the muzzle. Treatment with acetorphan, a parenterally-active enkephalinase inhibitor, given alone or in combination with carbaphethiol, completely prevented the rise in YGG triggered by noxious stimulation. By contrast, [Met5]enkephalin levels in the perfusates were increased by the combined administration of the two peptidase inhibitors but these levels were not further enhanced by noxious stimulation. Thus, spinal cord YGG appears to be formed under the influence of enkephalinase and to constitute a sensitive index of enkephalin release.