Purification and characterization of substance P endopeptidase activities in the rat spinal cord

Neuropeptides: metabolism to bioactive fragments and the pharmacology of their receptors

The proteolytic processing of neuropeptides has an important regulatory function and the peptide fragments resulting from the enzymatic degradation often exert essential physiological roles. The proteolytic processing generates, not only biologically inactive fragments, but also bioactive fragments that modulate or even counteract the response of their parent peptides. Frequently, these peptide fragments interact with receptors that are not recognized by the parent peptides. This review discusses tachykinins, opioid peptides, angiotensins, bradykinins, and neuropeptide Y that are present in the central nervous system and their processing to bioactive degradation products. These well-known neuropeptide systems have been selected since they provide illustrative examples that proteolytic degradation of parent peptides can lead to bioactive metabolites with different biological activities as compared to their parent peptides. For example, substance P, dynorphin A, angiotensin I and II, bradykinin, and neuropeptide Y are all degraded to bioactive fragments with pharmacological profiles that differ considerably from those of the parent peptides. The review discusses a selection of the large number of drug-like molecules that act as agonists or antagonists at receptors of neuropeptides. It focuses in particular on the efforts to identify selective drug-like agonists and antagonists mimicking the effects of the endogenous peptide fragments formed. As exemplified in this review, many common neuropeptides are degraded to a variety of smaller fragments but many of the fragments generated have not yet been examined in detail with regard to their potential biological activities. Since these bioactive fragments contain a small number of amino acid residues, they provide an ideal starting point for the development of drug-like substances with ability to mimic the effects of the degradation products. Thus, these substances could provide a rich source of new pharmaceuticals. However, as discussed herein relatively few examples have so far been disclosed of successful attempts to create bioavailable, drug-like agonists or antagonists, starting from the structure of endogenous peptide fragments and applying procedures relying on stepwise manipulations and simplifications of the peptide structures.

Proteolysis controls endogenous substance P levels

Substance P (SP) is a prototypical neuropeptide with roles in pain and inflammation. Numerous mechanisms regulate endogenous SP levels, including the differential expression of SP mRNA and the controlled secretion of SP from neurons. Proteolysis has long been suspected to regulate extracellular SP concentrations but data in support of this hypothesis is scarce. Here, we provide evidence that proteolysis controls SP levels in the spinal cord. Using peptidomics to detect and quantify endogenous SP fragments, we identify the primary SP cleavage site as the C-terminal side of the ninth residue of SP. If blocking this pathway increases SP levels, then proteolysis controls SP concentration. We performed a targeted chemical screen using spinal cord lysates as a proxy for the endogenous metabolic environment and identified GM6001 (galardin, ilomastat) as a potent inhibitor of the SP _1–9-producing activity present in the tissue. Administration of GM6001 to mice results in a greater-than-three-fold increase in the spinal cord levels of SP, which validates the hypothesis that proteolysis controls physiological SP levels.

Intrathecal substance P (1–7) prevents morphine-evoked spontaneous pain behavior via spinal NMDA-NO cascade

Previous research has shown that injection of high-dose of morphine into the spinal lumbar intrathecal (i.t.) space of rats elicits an excitatory behavioral syndrome indicative of severe vocalization and agitation. Substance P N-terminal fragments are known to inhibit nociceptive responses when injected i.t. into animals. In this study, we investigated the effect of i.t. substance P (1-7) on both the nociceptive response and the extracellular concentrations of glutamate and nitric oxide (NO) metabolites (nitrite/nitrate) evoked by high-dose i.t. morphine (500 nmol). The induced behavioral responses were attenuated dose-dependently by i.t. pretreatment with the substance P N-terminal fragment substance P (1-7) (100-400 pmol). The inhibitory effect of substance P (1-7) was reversed significantly by pretreatment with [d-Pro2, d-Phe7]substance P (1-7) (20 and 40 nmol), a d-isomer and antagonist of substance P (1-7). In vivo microdialysis analysis showed a significant elevation of extracellular glutamate and NO metabolites in the spinal cord after i.t. injection of high-dose morphine (500 nmol). Pretreatment with substance P (1-7) (400 pmol) produced a significant reduction on the elevated concentrations of glutamate and NO metabolites evoked by i.t. morphine. The reduced levels of glutamate and NO metabolites were significantly reversed by the substance P (1-7) antagonist (40 nmol). The present results suggest that i.t. substance P (1-7) may attenuate the excitatory behavior (vocalization and agitation) of high-dose i.t. morphine by inhibiting the presynaptic release of glutamate, and reducing NO production in the dorsal spinal cord.

Chromatographic characterization of substance P endopeptidase in the rat brain reveals affected enzyme activity following heat stress

This paper describes a study of substance P endopeptidase (SPE)-like activity in various regions of the brain from male rats subjected to heat stress (HS). The enzyme activity was found to be affected in several brain areas including cerebellum, cerebral cortex, hippocampus, hypothalamus[sol ]thalamus and the spinal cord following HS. Significant increases in SPE activity were observed in, for example, hippocampus and the spinal cord. SPE-containing extracts from hippocampus were pooled and subsequently purified by size exclusion chromatography (using a Superdex 75 HR column) and by anion-exchange chromatography (using Resource Q column). The gel permeation chromatography separated the SPE-like activity into two fractions, one of which was suggested to be identical to neutral endopeptidase owing to its molecular size and inhibitory profile. The other active enzyme fraction behaved in conformity with SPE, previously identified in human cerebrospinal fluid. The activity of the purified fraction of these two enzymes was found to be increased (27%) in HS-treated animals.

Administration of the anabolic androgenic steroid nandrolone decanoate affects substance P endopeptidase-like activity in the rat brain

The effect of the anabolic androgenic steroid, nandrolone decanoate, on substance P endopeptidase-like activity was examined in adult male Sprague-Dawley rats. Nandrolone decanoate (15 mg/kg day) or oil vehicle (sterile arachidis oleum) were administered by intramuscular injections during 14 days. Substance P endopeptidase, a predominantly cytosolic enzyme, generates the bioactive N-terminal fragment substance P(1-7) from the enzyme substrate substance P. Nandrolone decanoate significantly reduced the substance P endopeptidase-like activity compared to control animals in hypothalamus (43% reduction), caudate putamen (44%), substantia nigra (32%) and the ventral tegmental area (27%). It was previously reported that both hypothalamus and caudate putamen contained significantly higher levels of substance P after nandrolone administration. The higher concentration of substance P in these regions could to an extent be attributed to the reduction in substance P endopeptidase-like activity. This result elucidates the important role of peptidase activity in the regulation of the substance P transmitter system. The present study provides additional support for the hypothesis that alterations in the substance P system in certain brain areas may contribute to some of the personality changes reported in connection with AAS abuse.

Substance P endopeptidase-like activity is altered in various regions of the rat central nervous system during morphine tolerance and withdrawal

In this study the level of a substance P endopeptidase (SPE)-like activity was measured in different regions of the rat central nervous system (CNS) after chronic administration of morphine. Male rats (200-220 g) were randomly divided into four groups. Two groups were injected (s.c.) with morphine (10 mg/kg) twice daily, whereas the other two received saline under identical conditions. After 8 days, when animals were completely tolerant to morphine, one of the morphine-treated groups and one group of saline-injected rats were given naloxone (s.c. 2 mg/kg). Withdrawal signs were observed and recorded. The enzyme activity was measured in extracts of the various CNS tissues by following the conversion of synthetic substance P (SP) to its N-terminal fragment SP(1-7) using a radioimmunoassay detecting this product. In discrete CNS areas including periaqueductal grey, spinal cord, substantia nigra and ventral tegmental area (VTA) a significant increase in enzyme activity was observed in the withdrawal group, while tolerant rats exhibited decreased SPE-like activity in the striatum (see Table 1). The enhanced enzyme activity during withdrawal is in agreement with our previous observation that the levels of SP(1-7) in rat brain are affected following naloxone precipitated withdrawal. In some tissues, including VTA, a correlation between the SPE-like activity and the intensity of the opioid abstinence was observed. Our result suggests that the elevated SPE-like activity is responsible for enhanced release of SP(1-7) in rats during morphine withdrawal, affirming a modulatory or regulative role of this enzyme in this state of opioid dependence.

Purification of substance P endopeptidase activity in the rat ventral tegemental area with the Akta-Purifier chromatographic system

The new chromatographic system Akta-Purifier 10 (Amersham-Pharmacia Biotech), scaled for preparative HPLC, was used for the purification of Substance P (SP) endopeptidase activity in the ventral tegemental area (VTA) of the rat brain. SP endopeptidase previously identified and purified from human cerebrospinal fluid has been found to degrade the neuroactive peptide SP in a specific pattern. In this study we have recovered SP endopeptidase from the rat VTA following a purification scheme involving homogenization (ultrasonication) and extraction of the excised tissue, size-exclusion chromatography (Superdex 75 HR), and ion-exchange chromatography (Resource Q). In this way we were able to achieve a purification factor of almost 7,500, based on specific activity. The obtained SP endopeptidase activity, was then subjected to characterization with regard to inhibition profile. The enzyme activity was monitored by following the conversion of SP to its N-terminal fragment SP(1-7) using a radioimmunoassay, specific for the heptapeptide product. On basis of inhibition profile it was possible to discern two different SP endopeptidase-like activities, one sensitive toward the protease inhibitor phosphoramidon (preparation A), and another non-sensitive to phosphoramidon or captopril (preparation B). The molecular masses of preparations A and B, as derived from sodium dodecyl sulfate-polyacrylamide gel electrophoresis, were found to be 90,000 and 76,000, respectively. Our data suggest that the purified phosphoramidon sensitive endopeptidase activity may be an enzyme that plays a major role in the conversion of SP to its bioactive fragment SP(1-7) in the rat VTA. This is likely to be identical to the previously known neutral endopeptidase (EC 3.4.24.11). However, this study also demonstrates the existence of a distinct endopeptidase activity with properties in agreement with rat spinal cord SP endopeptidase. In the context of previously shown altered levels of SP(1-7) in the VTA during morphine withdrawal both purified enzyme activities may turn out to be responsible.

Intraventricular administration of substance p increases the dendritic arborisation and the synaptic surfaces of Purkinje cells in rat's cerebellum

Substance P was infused in the lateral ventricles of twenty Lewis rats for twenty days. On the twentieth day the animals were sacrificed and the cerebellar cortex was processed for electron microscopy. The ultrastructural morphometric analysis revealed that the Purkinje cell dendritic arborisation and the number of the synapses between the parallel fibres and the Purkinje cell dendritic spines were much higher than in control animals. Numerous unattached spines of the secondary and tertiary dendritic branches of the Purkinje cells were also seen in the molecular layer either free or surrounded by astrocytic sheath. The increased number of synapses between the Purkinje cell dendrites and the parallel fibres in the animals, which received substance P intraventricularly, in correlation to control animals, supports a neurotrophine-like activity of the substance P in the mammalian cerebellum, enforcing the pre-programmed capability of the Purkinje cells to develop new synaptic surfaces.

Purification of substance P endopeptidase (SPE) activity in human spinal cord and subsequent comparative studies with SPE in cerebrospinal fluid and with chymotrypsin

An enzyme activity capable of hydrolysing the neuroactive undecapeptide substance P (SP) between its Phe7-Phe8 residues was purified from the membrane-bound fraction of human spinal cords. The enzyme preparation yielded was compared with a previously described SP-hydrolysing enzyme from human cerebrospinal fluid (CSF) with regard to inhibition profile, protein chemical properties and kinetics. In addition, the results were compared with those of bovine pancreatic chymotrypsin (a serine protease that cleaves the carboxy-terminal side preferentially at hydrophobic amino acids). The SP peptidase activity was extracted from human spinal cords with 1% Triton X-100 in 20 mM Tris-HCI pH 7.8. After ion exchange chromatography (DEAE-Sepharose) where the enzyme activity was separated from other proteins by gradient elution, the pooled enzyme fraction was further purified by molecular sieving (Sephadex G-50). The enzyme activity was finally recovered by HPLC molecular sieving (Superdex 75 HR 10/30) using a new preparative system, AKTA-purifier, controlled by UNICORN software version 2.20.

Major metabolites of substance P degraded by spinal synaptic membranes antagonize the behavioral response to substance P in rats

Substance P (SP) was degraded by synaptic membranes of rat spinal cord. Cleavage products were separated by reversed phase high performance liquid chromatography and identified by amino acid composition analyses. Major products of SP were phenylalanine, SP(1-4), SP(1-6), SP(1-7), SP(10-11), and SP(8-9). Both the degradation of SP and the accumulation of the major cleavage products were strongly inhibited by a metal chelator, o-phenanthroline, and also by specific inhibitors of endopeptidase-24.11, thiorphan, and phosphoramidon. Thus, endopeptidase-24.11 plays a major role in SP degradation in the rat spinal cord. N-Terminal fragments, SP(1-7) and SP(1-4), detected after incubation with spinal synaptic membranes were examined in vivo for antagonism against the scratching, biting, and licking response induced by intrathecal (IT) injection of SP (3.0 nmol) in rats. When IT coadministered with SP, SP(1-7) and SP(1-4) produced a significant inhibition of behavioral response to SP with ED50 of 135.0 pmol and 6.2 nmol, respectively. These results suggest that the degradation of SP in the spinal cord is not only responsible for inactivation of parent peptide, but may also lead to the formation of N-terminal SP-fragments which are shown to display a novel physiological function.

Alteration in the brain content of substance P (1-7) during withdrawal in morphine-dependent rats

Previous studies have shown that substance P (SP) may modulate the abstinence reaction to opioid withdrawal. Its N-terminal fragment SP1-7 may inhibit the intensity of the withdrawal reactions in morphine dependent mice. This study was designed to determine whether the endogenous concentrations of the SP1-7 fragment in the brain are affected during naloxone-precipitated withdrawal in the male rat. The amounts of the peptide was assessed by a specific radioimmunoassay in extracts of discrete brain regions (including the cerebral cortex, hippocampus, hypothalamus, nucleus accumbens, striatum, substantia nigra, ventral tegmental area and the spinal cord) during morphine tolerance and withdrawal. The results indicated that the concentrations of SP1-7 were significantly elevated in the ventral tegmental area both in morphine tolerant rats and during naloxone-precipitated withdrawal. During morphine withdrawal significant increases in the peptide concentration were also observed in the hypothalamus and the spinal cord. It was concluded that the enhanced content of SP1-7 may also indicate the involvement of the SP system during opioid withdrawal in the rat. The enhanced production of SP1-7 may reflect an increased release and/or metabolism of SP, which, in turn, counteracts the withdrawal.