Synenkephalin is coreleased with met-enkephalin from neuronal terminals in vitro

Identifying Receptors for Neuropeptides and Peptide Hormones: Challenges and Recent Progress

Intercellular signaling events mediated by neuropeptides and peptide hormones represent important targets for both basic science and drug discovery. For many bioactive peptides, the protein receptors that transmit information across the receiving cell membrane are not known, severely limiting these signaling pathways as potential therapeutic targets. Identifying the receptor(s) for a given peptide of interest is complicated by several factors. Most notably, cell-cell signaling peptides are generated through dynamic biosynthetic pathways, can act on many different families of receptor proteins, and can participate in complex ligand-receptor interactions that extend beyond a simple one-to-one archetype. Here, we discuss recent methodological advances to identify signaling partners for bioactive peptides. Recent efforts have centered on methods to identify candidate receptors via transcript expression, methods to match peptide-receptor pairs through high throughput screening, and methods to capture direct ligand-receptor interactions using chemical probes. Future applications of the receptor identification approaches discussed here, as well as technical advancements to address their limitations, promise to lead to a greater understanding of how cells communicate to deliver complex physiologies. Importantly, such advancements will likely provide novel targets for the treatment of human diseases within the central nervous and endocrine systems.

Diurnal variations of opioid peptides and synenkephalin in vitro release in the amygdala of kindled rats

Pentylenetetrazol (PTZ) kindling was induced in male Wistar rats (250-300 g) by daily intraperitoneal injections of 35 mg/kg of the convulsant agent. Immunoreactive (IR)-Met-enkephalin (IR-ME), IR-Leu-enkephalin (IR-LE), IR-heptapeptide (IR-HE), IR-octapeptide (IR-OC) and IR-synenkephalin (IR-Syn) in vitro release was measured from amygdala slices 24 h after the last stimulus, in groups of eight rats, every 4 h beginning at 08:00 h. Opioid peptides in vitro release displayed diurnal variations. IR-ME and IR-Syn showed maximal levels before the onset of darkness (16:00 h). IR-LE and IR-OC release was enhanced 4 h later (20:00 h), no changes were detected for IR-HE. These results show that endogenous opioid system (EOS) release displays diurnal variations. The peak for the analysed peptides was reached before and during the dark phase. It is suggested that EOS release enhancement in PTZ-kindled rats, seems to be due to a compensatory mechanism against the excitation induced by the blockade of the GABAergic transmission.

Synenkephalin processing in embryonic rat brain

Synenkephalin (proenkephalin 1-70) is produced and secreted as an intact molecule or as a part of precursors in the adult brain and adrenal medulla, respectively. However, it is cleaved to low molecular weight peptides in proliferating immune cells. Considering that the pre-proenkephalin gene is expressed in the embryonic rat brain during the cell proliferation stage, we studied the processing of synenkephalin in embryonic rat brains (E18) and compared it with the processing in adult rat brains. IR-synenkephalin was measured by RIA using a C-terminally directed antiserum. Adult rat brains contained higher concentrations of immunoreactive (IR)-synenkephalin (2,612 + 264) than embryonic rat brain (1,361 + 100) (results in fmol/mg proteins, n = 5). Gel filtration chromatography (Sephadex G-50) showed that in the extracts of adult rat brain, 50% of the IR-synenkephalin eluted in the position of the authentic peptide (8 kDa) and the rest of the immunoreactivity corresponded to partially processed peptides of 4.0 and 2.5 kDa. In embryonic rat brains synenkephalin was processed to intermediate peptides of 2.5, 1.7 and mainly to a low molecular weight peptide of 1.0 kDa. The concentration of this last peptide, which was further characterized by affinity column and HPLC, represented 45% of the total immunoreactivity. IR-met-enkephalin in embryonic rat brains (analyzed before and after enzymatic digestion with trypsin and carboxypeptidase B) corresponded principally to non-processed or partially processed products. However, these were cleaved to free met-enkephalin in adult rat brains.(ABSTRACT TRUNCATED AT 250 WORDS)

Deletions of the synenkephalin domain which do not alter cell-specific proteolytic processing or secretory targeting of human proenkephalin

To identify signals that direct the proteolytic processing and regulated secretion of human proenkephalin (hPE), we have transfected the hPE gene or minigene constructs into pituitary tumor cells, either rat GH4Cl cells or mouse AtT-20 cells. Cells transfected with either the hPE gene or minigene contained similar levels of methionine-enkephalin (ME)-containing peptides and hPE mRNA. In the GH4Cl clones, ME was present predominantly in high-molecular-mass forms (5-25 kDa). In contrast, the AtT-20 clones contained almost exclusively free ME and low-molecular-mass forms (< 5 kDa), with very little high-molecular-mass species present. Thus, among pituitary cells, corticotroph-derived cells appear better equipped to process hPE than lactotroph-derived cells. Despite limited proteolytic processing, GH4Cl clones secreted large amounts of unprocessed (> 20 kDa) hPE into the medium, making up to 10% of endogenous rat prolactin secretion. Both precursor and processed forms of ME were cosecreted acutely (< 1 h) with rat prolactin, and release of both polypeptides was stimulated up to 12-fold by secretagogues. Thus, complete proteolytic processing was not required for accurate targeting of hPE to the regulated secretory pathway. When transfected with constructs bearing deletions of amino-terminal amino acids 2-43 or 2-67, i.e., part or nearly all of the synenkephalin moiety, GH4Cl cells handled the modified protein much like cells expressing the complete protein. They did not process the modified hPE extensively, but the protein was correctly targeted to the regulated secretory pathway. AtT-20 cells transfected with truncated hPE cDNA constructs expressed and processed the protein as efficiently as cells expressing unmodified hPE and expressed predominantly low-molecular-mass forms of ME. Therefore, the structural features required for correct targeting and processing are not present in the cysteine-rich amino-terminal third of the prohormone. It is interesting that the deletions did not include the SHLL peptide motif in synenkephalin, a motif that has been proposed as a sorting signal.

Ontogeny of the proenkephalin system in the rat corpus striatum: its relationship to dopaminergic innervation and transient compartmental expression

The earliest detection of the proenkephalin gene was on embryonic day 16 in neuronal cell bodies in the ventrolateral portion of the caudal neostriatum. This expression was identified by both immunocytochemistry for synenkephalin, the nonopioid N-terminus of proenkephalin (1-70), and preproenkephalin in situ hybridization with a complementary DNA probe. Two developmental gradients of preproenkephalin expression and synenkephalin immunoreactivity were observed: (i) a ventrolateral to dorsomedial and caudal to rostral gradient in the rostral caudate-putamen; and (ii) a ventromedial to dorsolateral and rostral to caudal gradient in the caudal caudate-putamen. Ventrolateral to dorsomedial and caudal to rostral developmental gradients of synenkephalin fiber immunoreactivity were also identified in the globus pallidus. Methionine enkephalin immunoreactivity was not consistently detectable until postnatal day 10 and 15 in the rostral and caudal globus pallidus, respectively. A transient patchy distribution of increased preproenkephalin expression from embryonic day 20 through postnatal day 5 occurred. These patches and a subcallosal streak were found to overlap partially with areas of increased tyrosine hydroxylase immunoreactivity by adjacent section analyses. The earliest detection of tyrosine hydroxylase immunoreactivity was found to coincide with that of proenkephalin on embryonic day 16, but in differing regions of the corpus striatum. Tyrosine hydroxylase immunoreactivity in the rostral caudate-putamen preceded, while in the caudal caudate-putamen it followed first expression of the proenkephalin gene. Early proenkephalin expression, by both synenkephalin immunocytochemistry and preproenkephalin in situ hybridization, was also detected in the central nucleus of the amygdala on embryonic day 16 immediately ventral to the area of expression in the caudate-putamen. Preproenkephalin expression in the olfactory tubercle and nucleus accumbens first appeared on embryonic day 20 and expression proceeded in a lateral to dorsomedial gradient continuous with the ventral part of the rostral caudal-putamen. Relatively late detection of methionine enkephalin immunoreactivity in comparison to synenkephalin possibly indicates a developmental delay in the complete enzymatic processing of the proenkephalin precursor. Differing gradients in the ontogeny of preproenkephalin expression in the rostral vs the caudal caudate-putamen suggest possible anatomical and developmental differences of these two regions. Also, transient compartmentalization of preproenkephalin expression and differences in dopaminergic innervation as detected by tyrosine hydroxylase immunoreactivity were further support for the existence of two subsets of proenkephalinergic neurons in the caudate-putamen. Contemporaneous development of preproenkephalin expression and synenkephalin immunoreactivity in the central nucleus of the amygdala with the ventral part of the caudal caudate-putamen also suggested developmental homology.(ABSTRACT TRUNCATED AT 400 WORDS)

Proenkephalin system in human polymorphonuclear cells. Production and release of a novel 1.0-kD peptide derived from synenkephalin

In the hematopoietic system a pluripotent stem cell generates precursors for lymphoid and myeloid lineages. Proenkephalin-derived peptides were previously detected in differentiated lymphoid cells. We have studied whether the proenkephalin system is expressed in a typical differentiated cell of the myeloid lineage, the neutrophil. Human peripheral polymorphonuclear cells contain and release proenkephalin-derived peptides. The opioid portion of proenkephalin (met-enkephalin-containing peptides) was incompletely processed, resulting in the absence of low molecular weight products. The nonopioid synenkephalin (proenkephalin 1-70) molecule was completely processed to a 1.0-kD peptide derived from the COOH-terminal. This molecule was characterized in neutrophils by biochemical and immunocytochemical methods. The chemotactic peptide FMLP and the calcium ionophore A23187 induced the release of the proenkephalin-derived peptides, and this effect was potentiated by cytochalasin B. The materials secreted were similar to those present in the cell, although in the supernatant a higher proportion corresponded to more processed products. The 1.0-kD peptide was detected in human, bovine, and rat neutrophils, but the chromatographic pattern of synenkephalin-derived peptides suggests a differential posttranslational processing among species. These findings demonstrate the existence of the proenkephalin system in human neutrophils and the production and release of a novel 1.0-kD peptide derived from the synenkephalin molecule. The presence of opioid peptides in neutrophils suggests their participation in the inflammatory process, including a local analgesic effect.

Detection of synenkephalin, the amino-terminal portion of proenkephalin, by antisera directed against its carboxyl terminus

Synenkephalin (SYN), the nonopioid amino-terminal portion of proenkephalin (PRO), is stable and well conserved in mammals and therefore a promising marker for PRO systems. We immunized rabbits with synthetic [Tyr63]SYN(63-70)-octapeptide, coupled by glutaraldehyde to bovine serum albumin. In radioimmunoassay (RIA) using antiserum no. 681, [Tyr63]SYN(63-70)-octapeptide as standard, and 125I-[Tyr63]SYN(63-70)-octapeptide as tracer, the IC50 was approximately 51 fmol/100-microliters sample at equilibrium or 12 fmol/100 microliters in disequilibrium, and the sensitivity was approximately 3 fmol/100 microliters. Cross-reactivity of the assay was 100% with [Cys63]SYN(63-70)-octapeptide and with bovine adrenal 8.6-kilodalton peptide digested with trypsin and carboxypeptidase B, but less than 0.1% with transforming growth factor-alpha, less than or equal to 2 x 10(-6) with Leu-Leu-Ala [SYN(68-70)-tripeptide], and much less than 10(-6) with all other peptides tested. Therefore in RIA this antiserum is specific for the free carboxyl terminus of SYN. Because the peptide detected after enzyme digestion is the complete SYN(63-70)-octapeptide, we refer to the RIA as an assay for SYN(63-70). Tissue extracts were made in 1 M acetic acid, dried, reconstituted in Tris-CaCl2, and digested sequentially with trypsin plus carboxypeptidase B. Extracts from bovine corpus striatum gave SYN(63-70) RIA dilution curves parallel to the standard curve both before and after digestion. Digestion increased the amount of immunoreactive SYN(63-70) in striatum by a factor of 1.5-2.0. The ratio of total immunoreactive [Met5]enkephalin to total immunoreactive SYN(63-70) (after sequential digestion) was approximately 6:1. At least 90% of the immunoreactive SYN(63-70) in extracts of bovine caudate nucleus eluted from Sephadex G-100 with an apparent molecular weight equal to that of bovine PRO(1-77). Using the new RIA we were able to detect and characterize SYN processing for the first time in extracts of whole rat brain, human globus pallidus, and human pheochromocytoma. Results in these tissues were similar to those in cattle, in that most stored SYN had been processed to a free carboxyl terminus. Since the C-terminal octapeptide of SYN is practically identical in all known mammalian PRO, antiserum no. 681 should be useful for detecting, measuring, and purifying SYN from various mammals, including human beings.

Comparison of synenkephalin and methionine enkephalin immunocytochemistry in rat brain

Immunocytochemistry using an antiserum to the C-terminal octapeptide of synenkephalin, proenkephalin(63-70), was performed throughout the rat brain and revealed numerous immunopositive fibers and some cell bodies. The morphology and distribution of synenkephalin immunoreactivity was extremely similar to that of a commercial methionine enkephalin (Met-ENK) antiserum. Colchicine pretreatment allowed the immunostaining of cell bodies not otherwise possible without pretreatment, but did not affect the distribution of immunoreactive fibers. Using 6 microns serial sections, we were able to colocalize synenkephalin and Met-ENK immunoreactivities in gigantocellular neurons of the medullary reticular formation. Preabsorption of the antiserum with [Tyr63]proenkephalin(63-70) octapeptide (YEESHLLA) completely eliminated immunoreactivity in the rat brain, while preabsorption with all other peptides used had no detectable effect. We conclude that our antiserum to synenkephalin is specific for enkephalinergic cell bodies, fibers and terminals. The synenkephalin antiserum used in these studies may have advantages over other antisera utilized for immunocytochemical detection of proenkephalin gene expression.

Identification of human synenkephalin-like immunoreactivity in phaeochromocytoma tissue using a novel carboxy-terminal radioimmunoassay

An antiserum raised against the synthetic tyrosinylated carboxy-terminal sequence of synenkephalin (Tyr-Glu-Glu-Ser-His-Leu-Leu-Ala) has been used to chromatographically characterize the human synenkephalin-like immunoreactivity extracted from 3 adrenal medullary phaeochromocytomas. Gel filtration chromatography identified in each tumor a single peak of 8 kDa which on subsequent ion-exchange chromatography had the elution characteristics of an acidic polypeptide. These results are compatible with the human synenkephalin sequence predicted from cDNA studies, and indicate that this is the authentic peptide.

Electron microscopic localization of N-terminal proenkephalin (synenkephalin) immunostaining in the guinea pig organ of Corti

Using a pre-embedding immunoelectron microscopic technique, anti-synenkephalin immunostaining has been demonstrated within efferent varicosities (originating from the brainstem) of the inner spiral bundle and the tunnel spiral bundle of the organ of Corti. Axodendritic synapses were observed between the anti-synenkephalin immunostained varicosities and auditory dendrites connected to inner hair cells. No anti-synenkephalin immunostaining was found in any efferents at the outer hair cell level. We suggest that this immunolocalization of synenkephalin in the organ of Corti allows a better differentiation of the cochlear efferent systems on a neurochemical basis. The whole lateral system, or at least a significant part of it, could be referred to as the 'enkephalin-containing efferent system'.

Evidence for synenkephalin-like immunoreactivity in pontobulbar monoaminergic neurons of the cat

Using indirect immunofluorescence, evidence that enkephalin- and synenkephalin-like-immunoreactivities are colocalized within numerous monoaminergic neurons of the cat pontobulbar formation is presented. The colocalization concerns most catecholaminergic cell bodies in the locus coeruleus region and numerous serotoninergic cell bodies in the raphe nuclei. Synenkephalin is the 1-70 N-terminal region of the bovine adrenal medulla proenkephalin. Therefore, the proenkephalin (or a related) system seems to represent the biosynthetic pathway for the enkephalins immunodetected within these monoaminergic neurons.