Defective prodynorphin processing in mice lacking prohormone convertase PC2

Distinct Dibasic Cleavage Specificities of Neuropeptide-Producing Cathepsin L and Cathepsin V Cysteine Proteases Compared to PC1/3 and PC2 Serine Proteases

Neuropeptides, functioning as peptide neurotransmitters and hormones, are generated from proneuropeptide precursors by proteolytic processing at dibasic residue sites (i.e., KR, RK, KK, RR). The cysteine proteases cathepsin L and cathepsin V, combined with the serine proteases proprotein convertases 1 and 2 (PC1/3 and PC2), participate in proneuropeptide processing to generate active neuropeptides. To compare the dibasic cleavage properties of these proteases, this study conducted global, unbiased substrate profiling of these processing proteases using a diverse peptide library in multiplex substrate profiling by mass spectrometry (MSP-MS) assays. MSP-MS utilizes a library of 228 14-mer peptides designed to contain all possible protease cleavage sites, including the dibasic residue sites of KR, RK, KK, and RR. The comprehensive MSP-MS analyses demonstrated that cathepsin L and cathepsin V cleave at the N-terminal side and between the dibasic residues (e.g., ↓K↓R, ↓R↓K, and K↓K), with a preference for hydrophobic residues at the P2 position of the cleavage site. In contrast, the serine proteases PC1/3 and PC2 displayed cleavage at the C-terminal side of dibasic residues of a few peptide substrates. Further analyses with a series of dipeptide-AMC and tripeptide-AMC substrates containing variant dibasic sites with hydrophobic P2 residues indicated the preferences of cathepsin L and cathepsin V to cleave between dibasic residue sites with preferences for flanking hydrophobic residues at the P2 position consisting of Leu, Trp, Phe, and Tyr. Such hydrophobic amino acids reside in numerous proneuropeptides such as pro-NPY and proenkephalin that are known to be processed by cathepsin L. Notably, cathepsin L displayed the highest specific activity that was 10-, 64-, and 1268-fold greater than cathepsin V, PC1/3, and PC2, respectively. Peptide-AMC substrates with dibasic residues confirmed that PC1/3 and P2 cleaved almost exclusively at the C-terminal side of dibasic residues. These data demonstrate distinct dibasic cleavage site properties and a broad range of proteolytic activities of cathepsin L and cathepsin V, compared to PC1/3 and PC2, which participate in producing neuropeptides for cell-cell communication.

Mouse Models of Human Proprotein Convertase Insufficiency

The kexin-like proprotein convertases perform the initial proteolytic cleavages that ultimately generate a variety of different mature peptide and proteins, ranging from brain neuropeptides to endocrine peptide hormones, to structural proteins, among others. In this review, we present a general introduction to proprotein convertase structure and biochemistry, followed by a comprehensive discussion of each member of the kexin-like subfamily of proprotein convertases. We summarize current knowledge of human proprotein convertase insufficiency syndromes, including genome-wide analyses of convertase polymorphisms, and compare these to convertase null and mutant mouse models. These mouse models have illuminated our understanding of the roles specific convertases play in human disease and have led to the identification of convertase-specific substrates; for example, the identification of procorin as a specific PACE4 substrate in the heart. We also discuss the limitations of mouse null models in interpreting human disease, such as differential precursor cleavage due to species-specific sequence differences, and the challenges presented by functional redundancy among convertases in attempting to assign specific cleavages and/or physiological roles. However, in most cases, knockout mouse models have added substantively both to our knowledge of diseases caused by human proprotein convertase insufficiency and to our appreciation of their normal physiological roles, as clearly seen in the case of the furin, proprotein convertase 1/3, and proprotein convertase 5/6 mouse models. The creation of more sophisticated mouse models with tissue- or temporally-restricted expression of specific convertases will improve our understanding of human proprotein convertase insufficiency and potentially provide support for the emerging concept of therapeutic inhibition of convertases.

Targeted high-resolution quadrupole-Orbitrap mass spectrometry analyses reveal a significant reduction of tachykinin and opioid neuropeptides level in PC1 and PC2 mutant mouse spinal cords

Tachykinin and opioid neuropeptides play a fundamental role in pain transmission, modulation and inhibition. The proteolysis control of endogenous tachykinin and opioid neuropeptides has a significant impact on pain perception. The role of proprotein convertases (PCs) in the proteolysis of proneuropeptides was previously established but very few studies have shown the direct impact of PCs on the regulation of specific tachykinin and opioid peptides in the central nervous system. There is an increasing interest in the therapeutic targeting of PCs for the treatment of pain but it is imperative to assess the impact of PCs on the pronociceptive and the endogenous opioid systems. The objective of this study was to determine the relative concentration of targeted neuropeptides in the spinal cord of WT, PC1^-/+ and PC2^-/+ animals to establish the impact of a restricted PCs activity on the regulation of specific neuropeptides. The analysis of tachykinin and opioid neuropeptides were performed on a HPLC-MS/MS (High-Resolution Quadrupole-Orbitrap Mass Spectrometer). The results revealed a significant decrease of Dyn A (p<0.01), Leu-Enk (p<0.001), Met-Enk (p<0.001), Tach_58-71 (p<0.05), SP (p<0.01) and NKA (p<0.001) concentrations in both, PC1^-/+ and PC2^-/+ animals. Therefore, the modulation of PCs activity has an important impact on specific pronociceptive peptides (SP and NKA), but the results also shown that endogenous opioid system is hindered and consequently it will affect significantly the pain modulatory pathways. These observations may have insightful impact on future analgesic drug developments and therapeutic strategies.

The Emerging Role of Spinal Dynorphin in Chronic Pain: A Therapeutic Perspective

Notable findings point to the significance of the dynorphin peptide neurotransmitter in chronic pain. Spinal dynorphin neuropeptide levels are elevated during development of chronic pain and sustained during persistent chronic pain. Importantly, knockout of the dynorphin gene prevents development of chronic pain in mice, but acute nociception is unaffected. Intrathecal (IT) administration of opioid and nonopioid dynorphin peptides initiates allodynia through a nonopioid receptor mechanism; furthermore, antidynorphin antibodies administered by the IT route attenuate chronic pain. Thus, this review presents the compelling evidence in the field that supports the role of dynorphin in facilitating the development of a persistent pain state. These observations illustrate the importance of elucidating the control mechanisms responsible for the upregulation of spinal dynorphin in chronic pain. Also, spinal dynorphin regulation of downstream signaling molecules may be implicated in hyperpathic states. Therapeutic strategies to block the upregulation of spinal dynorphin may provide a nonaddictive approach to improve the devastating condition of chronic pain that occurs in numerous human diseases.

Reduced Vglut2/Slc17a6 Gene Expression Levels throughout the Mouse Subthalamic Nucleus Cause Cell Loss and Structural Disorganization Followed by Increased Motor Activity and Decreased Sugar Consumption

The subthalamic nucleus (STN) plays a central role in motor, cognitive, and affective behavior. Deep brain stimulation (DBS) of the STN is the most common surgical intervention for advanced Parkinson's disease (PD), and STN has lately gained attention as target for DBS in neuropsychiatric disorders, including obsessive compulsive disorder, eating disorders, and addiction. Animal studies using STN-DBS, lesioning, or inactivation of STN neurons have been used extensively alongside clinical studies to unravel the structural organization, circuitry, and function of the STN. Recent studies in rodent STN models have exposed different roles for STN neurons in reward-related functions. We have previously shown that the majority of STN neurons express the vesicular glutamate transporter 2 gene (Vglut2/Slc17a6) and that reduction of Vglut2 mRNA levels within the STN of mice [conditional knockout (cKO)] causes reduced postsynaptic activity and behavioral hyperlocomotion. The cKO mice showed less interest in fatty rewards, which motivated analysis of reward-response. The current results demonstrate decreased sugar consumption and strong rearing behavior, whereas biochemical analyses show altered dopaminergic and peptidergic activity in the striatum. The behavioral alterations were in fact correlated with opposite effects in the dorsal versus the ventral striatum. Significant cell loss and disorganization of the STN structure was identified, which likely accounts for the observed alterations. Rare genetic variants of the human VGLUT2 gene exist, and this study shows that reduced Vglut2/Slc17a6 gene expression levels exclusively within the STN of mice is sufficient to cause strong modifications in both the STN and the mesostriatal dopamine system.

Characterization of endoproteolytic processing of dynorphins by proprotein convertases using mouse spinal cord S9 fractions and mass spectrometry

Dynorphins are important neuropeptides with a central role in nociception and pain alleviation. Many mechanisms regulate endogenous dynorphin concentrations, including proteolysis. Proprotein convertases (PCs) are widely expressed in the central nervous system and specifically cleave at C-terminal of either a pair of basic amino acids, or a single basic residue. The proteolysis control of endogenous big dynorphin (BDyn) and dynorphin A (Dyn A) levels has a profound impact on pain perception and the role of PCs remain unclear. The objective of this study was to decipher the role of PC1 and PC2 in the proteolysis control of BDyn and Dyn A levels using cellular fractions of spinal cords from wild-type (WT), PC1(-/+) and PC2(-/+) animals and mass spectrometry. Our results clearly demonstrate that both PC1 and PC2 are involved in the proteolysis regulation of BDyn and Dyn A with a more important role for PC1. C-terminal processing of BDyn generates specific peptide fragments dynorphin 1-19, dynorphin 1-13, dynorphin 1-11 and dynorphin 1-7, and C-terminal processing of Dyn A generates dynorphin 1-13, dynorphin 1-11 and dynorphin 1-7, all these peptide fragments are associated with PC1 or PC2 processing. Moreover, the proteolysis of BDyn leads to the formation of Dyn A and Leu-Enk, two important opioid peptides. The rate of formation of both is significantly reduced in cellular fractions of spinal cord mutant mice. As a consequence, even the partial inhibition of PC1 or PC2 may impair the endogenous opioid system.

Prohormone convertase 2 (PC2) null mice have increased mu opioid receptor levels accompanied by altered morphine-induced antinociception, tolerance and dependence

Chronic morphine treatment increases the levels of prohormone convertase 2 (PC2) in brain regions involved in nociception, tolerance and dependence. Thus, we tested if PC2 null mice exhibit altered morphine-induced antinociception, tolerance and dependence. PC2 null mice and their wild-type controls were tested for baseline hot plate latency, injected with morphine (1.25-10mg/kg) and tested for antinociception 30min later. For tolerance studies, mice were tested in the hot plate test before and 30min following morphine (5mg/kg) on day 1. Mice then received an additional dose so that the final dose of morphine was 10mg/kg on this day. On days 2-4, mice received additional doses of morphine (20, 40 and 80mg/kg on days 1, 2, 3, and 4, respectively). On day 5, mice were tested in the hot plate test before and 30min following morphine (5mg/kg). For withdrawal studies, mice were treated with the escalating doses of morphine (10, 20, 40 and 80mg/kg) for 4days, implanted with a morphine pellet on day 5 and 3 days later injected with naloxone (1mg/kg) and signs of withdrawal were recorded. Morphine dose-dependently induced antinociception and the magnitude of this response was greater in PC2 null mice. Tolerance to morphine was observed in wild-type mice and this phenomenon was blunted in PC2 null mice. Withdrawal signs were also reduced in PC2 null mice. Immunohistochemical studies showed up-regulation of the mu opioid receptor (MOP) protein expression in the periaqueductal gray area, ventral tegmental area, lateral hypothalamus, medial hypothalamus, nucleus accumbens, and somatosensory cortex in PC2 null mice. Likewise, naloxone specific binding was increased in the brains of these mice compared to their wild-type controls. The results suggest that the PC2-derived peptides may play a functional role in morphine-induced antinociception, tolerance and dependence. Alternatively, lack of opioid peptides led to up-regulation of the MOP and altered morphine-induced antinociception, tolerance and dependence.

Genetics of the first seven proprotein convertase enzymes in health and disease

Members of the substilisin/kexin like proprotein convertase (PCSK) protease family cleave and convert immature pro-proteins into their biologically active forms. By cleaving for example prohormones, cytokines and cell membrane proteins, PCSKs participate in maintaining the homeostasis in a healthy human body. Conversely, erratic enzymatic function is thought to contribute to the pathogenesis of a wide variety of diseases, including obesity and hypercholestrolemia. The first characterized seven PCSK enzymes (PCSK1-2, FURIN, PCSK4-7) process their substrates at a motif made up of paired basic amino acid residues. This feature results in a variable degree of biochemical redundancy in vitro, and consequently, shared substrate molecules between the different PCSK enzymes. This redundancy has confounded our understanding of the specific biological functions of PCSKs. The physiological roles of these enzymes have been best illustrated by the phenotypes of genetically engineered mice and patients that carry mutations in the PCSK genes. Recent developments in genome-wide methodology have generated a large amount of novel information on the genetics of the first seven proprotein convertases. In this review we summarize the reported genetic alterations and their associated phenotypes.

Morphine treatment selectively regulates expression of rat pituitary POMC and the prohormone convertases PC1/3 and PC2

The prohormone convertases, PC1/3 and PC2 are thought to be responsible for the activation of many prohormones through processing including the endogenous opioid peptides. We propose that maintenance of hormonal homeostasis can be achieved, in part, via alterations in levels of these enzymes that control the ratio of active hormone to prohormone. In order to test the hypothesis that exogenous opioids regulate the endogenous opioid system and the enzymes responsible for their biosynthesis, we studied the effect of short-term morphine or naltrexone treatment on pituitary PC1/3 and PC2 as well as on the level of pro-opiomelanocortin (POMC), the precursor gene for the biosynthesis of the endogenous opioid peptide, β-endorphin. Using ribonuclease protection assays, we observed that morphine down-regulated and naltrexone up-regulated rat pituitary PC1/3 and PC2 mRNA. Immunofluorescence and Western blot analysis confirmed that the protein levels changed in parallel with the changes in mRNA levels and were accompanied by changes in the levels of phosphorylated cyclic-AMP response element binding protein. We propose that the alterations of the prohormone processing system may be a compensatory mechanism in response to an exogenous opioid ligand whereby the organism tries to restore its homeostatic hormonal milieu following exposure to the opioid, possibly by regulating the levels of multiple endogenous opioid peptides and other neuropeptides in concert.

Dynamic changes in proprotein convertase 2 activity in cortical neurons after ischemia/reperfusion and oxygen-glucose deprivation

In this study, a rat model of transient focal cerebral ischemia was established by performing 100 minutes of middle cerebral artery occlusion, and an in vitro model of experimental oxygen-glucose deprivation using cultured rat cortical neurons was established. Proprotein convertase 2 activity gradually decreased in the ischemic cortex with increasing duration of reperfusion. In cultured rat cortical neurons, the number of terminal deoxynucleotidyl transferase-mediated 2’-deoxyuridine 5’-triphosphate-biotin nick end labeling-positive neurons significantly increased and proprotein convertase 2 activity also decreased gradually with increasing duration of oxygen-glucose deprivation. These experimental findings indicate that proprotein convertase 2 activity decreases in ischemic rat cortex after reperfusion, as well as in cultured rat cortical neurons after oxygen-glucose deprivation. These changes in enzyme activity may play an important pathological role in brain injury.

Dynorphin--still an extraordinarily potent opioid peptide

This issue of Molecular Pharmacology is dedicated to Dr. Avram Goldstein, the journal's founding editor and one of the leaders in the development of modern pharmacology. This article focuses on his contributions to the discovery of the dynorphins and evidence that members of this family of opioid peptides are endogenous agonists for the kappa opioid receptor. In his original publication describing the purification and sequencing of dynorphin A, Avram described this peptide as "extraordinarily potent" ("dyn" from the Greek, dynamis = power and "orphin" for endogenous morphine peptide). The name originally referred to its high affinity and great potency in the bioassay that was used to follow its activity during purification, but the name has come to have a second meaning: studies of its physiologic function in brain continue to provide powerful insights to the molecular mechanisms controlling mood disorders and drug addiction. During the 30 years since its discovery, we have learned that the dynorphin peptides are released in brain during stress exposure. After they are released, they activate kappa opioid receptors distributed throughout the brain and spinal cord, where they trigger cellular responses resulting in different stress responses: analgesia, dysphoria-like behaviors, anxiety-like responses, and increased addiction behaviors in experimental animals. Avram predicted that a detailed molecular analysis of opiate drug actions would someday lead to better treatments for drug addiction, and he would be gratified to know that subsequent studies enabled by his discovery of the dynorphins resulted in insights that hold great promise for new treatments for addiction and depressive disorders.

The biology and therapeutic targeting of the proprotein convertases

The mammalian proprotein convertases constitute a family of nine secretory serine proteases that are related to bacterial subtilisin and yeast kexin. Seven of these (proprotein convertase 1 (PC1), PC2, furin, PC4, PC5, paired basic amino acid cleaving enzyme 4 (PACE4) and PC7) activate cellular and pathogenic precursor proteins by cleavage at single or paired basic residues, whereas subtilisin kexin isozyme 1 (SKI-1) and proprotein convertase subtilisin kexin 9 (PCSK9) regulate cholesterol and/or lipid homeostasis via cleavage at non-basic residues or through induced degradation of receptors. Proprotein convertases are now considered to be attractive targets for the development of powerful novel therapeutics. In this Review, we summarize the physiological functions and pathological implications of the proprotein convertases, and discuss proposed strategies to control some of their activities, including their therapeutic application and validation in selected disease states.

Secretogranin II; a protein increased in the myocardium and circulation in heart failure with cardioprotective properties

BACKGROUND

Several beneficial effects have been demonstrated for secretogranin II (SgII) in non-cardiac tissue. As cardiac production of chromogranin A and B, two related proteins, is increased in heart failure (HF), we hypothesized that SgII could play a role in cardiovascular pathophysiology.

METHODOLOGY/PRINCIPAL FINDINGS

SgII production was characterized in a post-myocardial infarction heart failure (HF) mouse model, functional properties explored in experimental models, and circulating levels measured in mice and patients with stable HF of moderate severity. SgII mRNA levels were 10.5 fold upregulated in the left ventricle (LV) of animals with myocardial infarction and HF (p<0.001 vs. sham-operated animals). SgII protein levels were also increased in the LV, but not in other organs investigated. SgII was produced in several cell types in the myocardium and cardiomyocyte synthesis of SgII was potently induced by transforming growth factor-β and norepinephrine stimulation in vitro. Processing of SgII to shorter peptides was enhanced in the failing myocardium due to increased levels of the proteases PC1/3 and PC2 and circulating SgII levels were increased in mice with HF. Examining a pathophysiological role of SgII in the initial phase of post-infarction HF, the SgII fragment secretoneurin reduced myocardial ischemia-reperfusion injury and cardiomyocyte apoptosis by 30% and rapidly increased cardiomyocyte Erk1/2 and Stat3 phosphorylation. SgII levels were also higher in patients with stable, chronic HF compared to age- and gender-matched control subjects: median 0.16 (Q1-3 0.14-0.18) vs. 0.12 (0.10-0.14) nmol/L, p<0.001.

CONCLUSIONS

We demonstrate increased myocardial SgII production and processing in the LV in animals with myocardial infarction and HF, which could be beneficial as the SgII fragment secretoneurin protects from ischemia-reperfusion injury and cardiomyocyte apoptosis. Circulating SgII levels are also increased in patients with chronic, stable HF and may represent a new cardiac biomarker.

The dynorphin/κ-opioid receptor system and its role in psychiatric disorders

The dynorphin/κ-opioid receptor system has been implicated in the pathogenesis and pathophysiology of several psychiatric disorders. In the present review, we present evidence indicating a key role for this system in modulating neurotransmission in brain circuits that subserve mood, motivation, and cognitive function. We overview the pharmacology, signaling, post-translational, post-transcriptional, transcriptional, epigenetic and cis regulation of the dynorphin/κ-opioid receptor system, and critically review functional neuroanatomical, neurochemical, and pharmacological evidence, suggesting that alterations in this system may contribute to affective disorders, drug addiction, and schizophrenia. We also overview the dynorphin/κ-opioid receptor system in the genetics of psychiatric disorders and discuss implications of the reviewed material for therapeutics development.

Investigating endogenous peptides and peptidases using peptidomics

Rather than simply being protein degradation products, peptides have proven to be important bioactive molecules. Bioactive peptides act as hormones, neurotransmitters, and antimicrobial agents in vivo. The dysregulation of bioactive peptide signaling is also known to be involved in disease, and targeting peptide hormone pathways has been a successful strategy in the development of novel therapeutics. The importance of bioactive peptides in biology has spurred research to elucidate the function and regulation of these molecules. Classical methods for peptide analysis have relied on targeted immunoassays, but certain scientific questions necessitated a broader and more detailed view of the peptidome--all the peptides in a cell, tissue, or organism. In this review we discuss how peptidomics has emerged to fill this need through the application of advanced liquid chromatography--tandem mass spectrometry (LC-MS/MS) methods that provide unique insights into peptide activity and regulation.

Cardiovascular effects of melanocortins

Melanocortins (MSH's) are three structurally related peptides derived from proopiomelanocortin. They regulate several physiologic functions including energy metabolism, appetite, and inflammation. Recent work in rodents has also identified important effects of MSH's, particularly γ-MSH, on sodium metabolism and blood pressure regulation. Normal rats and mice respond to a high sodium diet with an increase in the plasma concentration of γ-MSH, and remain normotensive, while those with genetic or pharmacologic γ-MSH deficiency become hypertensive on a high sodium diet. This hypertension is corrected by exogenous administration of the peptide. Mice lacking the γ-MSH receptor (the melanocortin 3 receptor, Mc3r) also become hypertensive on a high sodium diet but remain so when administered γ-MSH, and infusions of physiologic levels of the peptide stimulate urinary sodium excretion in normal rats and mice, but not in mice with deletion of Mc3r. The salt-sensitive hypertension in rodents with impaired γ-MSH signaling appears due to stimulation of noradrenergic activity, since plasma noradrenaline is increased and the hypertension is rapidly corrected with infusion of the α-adrenoceptor antagonist phentolamine. In contrast to the antihypertensive property of physiologic levels of γ-MSH, intravenous or intracerebroventricular injections of high levels of the peptide raise blood pressure. This occurs in mice lacking Mc3r, indicating an interaction with some other central receptor. Finally, the salt-sensitive hypertension in rodents with disruption of γ-MSH signaling is accompanied by insulin resistance, an observation which offers a new window into the study of the association of salt-sensitive hypertension with insulin resistance and type II diabetes.

Genetic deficiency for proprotein convertase subtilisin/kexin type 2 in mice is associated with decreased adiposity and protection from dietary fat-induced body weight gain

BACKGROUND

Proprotein convertase subtilisin/xexin type 2 (PCSK2) is an endoproteinase responsible for proteolytic activation of a number of precursors to active neuropeptides and peptide hormones, known to influence glucose homeostasis, food intake and ultimately body mass. In this study, we examined the consequences of PCSK2 deficiency on these phenotypic traits.

STUDY DESIGN

Weight gain with age under diets of different fat contents was monitored. White adipose tissue (WAT) and muscle masses were evaluated. Plasma levels of triglycerides, leptin, ghrelin, insulin and proglucagon-derived peptides were measured as well as leptin and acetyl coenzyme-α carboxylase (ACCα) mRNA levels in adipose tissue.

RESULTS

Compared with their Pcsk2 (+/+) littermates, Pcsk2 (-/-) mice weighed significantly less as weanlings and as adults. As adults, they carried noticeably less fat mass, with similar lean muscle mass: their plasma leptin level and adipose tissue leptin mRNA level were accordingly lower. PCSK2 deficiency did not affect food intake or the level of the orexigenic hormone ghrelin. However, PCSK2 deficiency resulted in decreased plasma triglycerides and reduced ACCα mRNA levels in WAT. Interestingly, unlike their Pcsk2 (+/+) littermates, Pcsk2 (-/-) were resistant to enhanced body weight gain when fed a high-fat diet. Consistent with a role of PCSK2 in body mass gain, diet-induced or genetically obese mice were found to contain significantly higher levels of PCSK2 mRNA in their brain and stomach than their lean counterparts.

CONCLUSION

Collectively, these results suggest that PCSK2 contributes to increase in body mass through the various regulatory peptides generated through its action. It represents a potential target in the prevention and treatment of obesity.

Analysis of peptides in prohormone convertase 1/3 null mouse brain using quantitative peptidomics

Neuropeptides are produced from larger precursors by limited proteolysis, first by endopeptidases and then by carboxypeptidases. Major endopeptidases required for these cleavages include prohormone convertase (PC) 1/3 and PC2. In this study, quantitative peptidomics analysis was used to characterize the specific role PC1/3 plays in this process. Peptides isolated from hypothalamus, amygdala, and striatum of PC1/3 null mice were compared with those from heterozygous and wild-type mice. Extracts were labeled with stable isotopic tags and fractionated by HPLC, after which relative peptide levels were determined using tandem mass spectrometry. In total, 92 peptides were found, of which 35 were known neuropeptides or related peptides derived from 15 distinct secretory pathway proteins: 7B2, chromogranin A and B, cocaine- and amphetamine-regulated transcript, procholecystokinin, proenkephalin, promelanin concentrating hormone, proneurotensin, propituitary adenylate cyclase-activating peptide, proSAAS, prosomatosatin, provasoactive intestinal peptide, provasopressin, secretogranin III, and VGF. Among the peptides derived from these proteins, approximately 1/3 were decreased in the PC1/3 null mice relative to wild-type mice, approximately 1/3 showed no change, and approximately 1/3 increased in PC1/3 null. Cleavage sites were analyzed in peptides that showed no change or that decreased in PC1/3 mice, and these results were compared with peptides that showed no change or decreased in previous peptidomic studies with PC2 null mice. Analysis of these sites showed that while PC1/3 and PC2 have overlapping substrate preferences, there are particular cleavage site residues that distinguish peptides preferred by each PC.

Neuropeptidomic analysis establishes a major role for prohormone convertase-2 in neuropeptide biosynthesis

Prohormone convertase 2 (PC2) functions in the generation of neuropeptides from their precursors. A quantitative peptidomics approach was used to evaluate the role of PC2 in the processing of peptides in a variety of brain regions. Altogether, 115 neuropeptides or other peptides derived from secretory pathway proteins were identified. These peptides arise from 28 distinct secretory pathway proteins, including proenkephalin, proopiomelanocortin, prodynorphin, protachykinin A and B, procholecystokinin, and many others. Forty one of the peptides found in wild-type (WT) mice were not detectable in any of the brain regions of PC2 knockout mice, and another 24 peptides were present at levels ranging from 20% to 79% of WT levels. Most of the other peptides were not substantially affected by the mutation, with levels ranging from 80% to 120% of WT levels, and only three peptides were found to increase in one or more brain regions of PC2 knockout mice. Taken together, these results are consistent with a broad role for PC2 in neuropeptide processing, but with functional redundancy for many of the cleavages. Comparison of the cleavage sites affected by the absence of PC2 confirms previous suggestions that sequences with a Trp, Tyr, and/or Pro in the P1' or P2' position are preferentially cleaved by PC2 and not by other enzymes present in the secretory pathway.

Cathepsin L participates in dynorphin production in brain cortex, illustrated by protease gene knockout and expression

Dynorphin opioid neuropeptides mediate neurotransmission for analgesia and behavioral functions. Dynorphin A, dynorphin B, and alpha-neoendorphin are generated from prodynorphin by proteolytic processing. This study demonstrates the significant role of the cysteine protease cathepsin L for producing dynorphins. Cathepsin L knockout mouse brains showed extensive decreases in dynorphin A, dynorphin B, and alpha-neoendorphin that were reduced by 75%, 83%, and 90%, respectively, compared to controls. Moreover, cathepsin L in brain cortical neurons was colocalized with dynorphins in secretory vesicles, the primary site of neuropeptide production. Cellular coexpression of cathepsin L with prodynorphin in PC12 cells resulted in increased production of dynorphins A and B. Comparative studies of PC1/3 and PC2 convertases showed that PC1/3 knockout mouse brains had a modest decrease in dynorphin A, and PC2 knockout mice showed a minor decrease in alpha-neoendorphin. Overall, these results demonstrate a prominent role for cathepsin L, jointly with PC1/3 and PC2, for production of dynorphins in brain.

Pro-protein convertase-2/carboxypeptidase-E mediated neuropeptide processing of RGC-5 cell after in vitro ischemia

OBJECTIVE

To observe the change of the neuropeptide pro-protein processing system in the ischemic retina ganglion cell-5 (RGC-5) cells, pro-protein convertase-2 (PC2), carboxypeptidase-E (CPE) and preproneuropeptide Y (preproNPY) protein levels in the ischemic RGC-5 cells and conditioned medium were analyzed.

METHODS

The RGC-5 cell was differentiated in 0.1 mumol/L staurosporine for 24 h and then stressed by different doses of oxygen and glucose deprivation (OGD). The acute or chronic OGD-induced cell death rates were obtained by using PI or TUNEL staining. The protein expression levels were determined by using the Western blot method and PC2 activity analysis.

RESULTS

The ischemia caused substantial cell death in an OGD dose-dependent manner. In the cells, proPC2 and preproNPY protein levels gradually increased whereas proCPE gradually decreased. After OGD, PC2 activity was decreased. In the conditioned medium, proPC2 and PC2 proteins gradually decreased whereas proCPE, CPE, and preproNPY proteins gradually increased.

CONCLUSION

These results demonstrated that OGD inhibited the neuropeptide pro-protein processing system by reducing PC2 activity and the maturation of proPC2. The aggregation of the pro-proteins and the increase of the active CPE excision adversely exacerbated the cell injury. The pro-protein processing system might play a critical role in the ischemic stress of RGC-5 cells.

30 years of dynorphins--new insights on their functions in neuropsychiatric diseases

Since the first description of their opioid properties three decades ago, dynorphins have increasingly been thought to play a regulatory role in numerous functional pathways of the brain. Dynorphins are members of the opioid peptide family and preferentially bind to kappa opioid receptors. In line with their localization in the hippocampus, amygdala, hypothalamus, striatum and spinal cord, their functions are related to learning and memory, emotional control, stress response and pain. Pathophysiological mechanisms that may involve dynorphins/kappa opioid receptors include epilepsy, addiction, depression and schizophrenia. Most of these functions were proposed in the 1980s and 1990s following histochemical, pharmacological and electrophysiological experiments using kappa receptor-specific or general opioid receptor agonists and antagonists in animal models. However, at that time, we had little information on the functional relevance of endogenous dynorphins. This was mainly due to the complexity of the opioid system. Besides actions of peptides from all three classical opioid precursors (proenkephalin, prodynorphin, proopiomelanocortin) on the three classical opioid receptors (delta, mu and kappa), dynorphins were also shown to exert non-opioid effects mainly through direct effects on NMDA receptors. Moreover, discrepancies between the distribution of opioid receptor binding sites and dynorphin immunoreactivity contributed to the difficulties in interpretation. In recent years, the generation of prodynorphin- and opioid receptor-deficient mice has provided the tools to investigate open questions on network effects of endogenous dynorphins. This article examines the physiological, pathophysiological and pharmacological implications of dynorphins in the light of new insights in part obtained from genetically modified animals.

PC2/CPE-mediated pro-protein processing in tumor cells and its differentiated cells or tissues

Pro-protein convertase-2 (PC2) and carboxypeptidase-E (CPE) proteins are two major members of the pro-protein convertases that involve in the maturation of protein precursor. By using PC2 activity, immunocytochemistry (ICC) and Western blot method, PC2, CPE and preproNPY protein expression levels were compared among mature retina tissue, RGC-5 cells and its differentiated cells, or brain cortex tissue, NS20Y tumor cells and its differentiated cells, or mature breast tissue, breast tumor cell RM1 and breast adenocarcinoma tissue. The experimental results indicated that the differentiated cells or tissues had higher or highest PC2 activity. In the comparative experiments, more PC2 protein expression in the mature tissues and more CPE and preproNPY protein expression in the tumor cells or tumor tissue were observed, but no expression of preproNPY protein was observed in the mature tissues. Compared with NS20Y or RGC-5 undifferentiated cells, its differentiated cells showed less proPC2, more proCPE and more preproNPY protein expressions. The results demonstrated that the mature tissues showed stronger PC2/CPE-mediated pro-protein processing ability than the tumor cells or tissue. The results also showed that the artificial differentiation of RGC-5 or NS20Y cells was different from maturation of its corresponding normal tissue.

Defective neuropeptide processing and ischemic brain injury: a study on proprotein convertase 2 and its substrate neuropeptide in ischemic brains

Using a focal cerebral ischemia model in rats, brain ischemia-induced changes in expression levels of mRNA and protein, and activities of proprotein convertase 2 (PC2) in the cortex were examined. In situ hybridization analyses revealed a transient upregulation of the mRNA level for PC2 at an early reperfusion hour, at which the level of PC2 protein was also high as determined by immunocytochemistry and western blotting. When enzymatic activities of PC2 were analyzed using a synthetic substrate, a significant decrease was observed at early reperfusion hours at which levels of PC2 protein were still high. Also decreased at these reperfusion hours were tissue levels of dynorphin-A(1-8) (DYN-A(1-8)), a PC2 substrate, as determined by radioimmunoassay. Further examination of PC2 protein biosynthesis by metabolic labeling in cultured neuronal cells showed that in ischemic cells, the proteolytic processing of PC2 was greatly attenuated. Finally, in mice, an intracerebroventricular administration of synthetic DYN-A(1-8) significantly reduced the extent of ischemic brain injury. In mice those lack an active PC2, exacerbated brain injury was observed after an otherwise non-lethal focal ischemia. We conclude that brain ischemia attenuates PC2 and PC2-mediated neuropeptide processing. This attenuation may play a role in the pathology of ischemic brain injury.

Differential regulation of prohormone convertase 1/3, prohormone convertase 2 and phosphorylated cyclic-AMP-response element binding protein by short-term and long-term morphine treatment: implications for understanding the "switch" to opiate addiction

Drug addiction is a state of altered brain reward and self-regulation mediated by both neurotransmitter and hormonal systems. Although an organism's internal system attempts to maintain homeostasis when challenged by exogenous opiates and other drugs of abuse, it eventually fails, resulting in the transition from drug use to drug abuse. We propose that the attempted maintenance of hormonal homeostasis is achieved, in part, through alterations in levels of processing enzymes that control the ratio of active hormone to pro-hormone. Two pro-hormone convertases, PC1/3 and PC2 are believed to be responsible for the activation of many neurohormones and expression of these enzymes is dependent on the presence of a cyclic-AMP response element (CRE) in their promoters. Therefore, we studied the effects of short-term (24-h) and long-term (7-day) morphine treatment on the expression of hypothalamic PC1/3 and PC2 and levels of phosphorylated cyclic-AMP-response element binding protein (P-CREB). While short-term morphine exposure down-regulated, long-term morphine exposure up-regulated P-CREB, PC1/3 and PC2 protein levels in the rat hypothalamus as determined by Western blot analysis. Quantitative immunofluorescence studies confirmed these regulatory actions of morphine in the paraventricular and dorsomedial nucleus of the hypothalamus. Specific radioimmunoassays demonstrated that the increase in PC1/3 and PC2 levels following long-term morphine led to increased TRH biosynthesis as evidence by increased TRH/5.4 kDa C-terminal proTRH-derived peptide ratios in the median eminence. Promoter activity experiments in rat somatomammotrope GH3 cells containing the mu-opioid receptor demonstrated that the CRE(s) in the promoter of PC1/3 and PC2 is required for morphine-induced regulation of PC1/3 and PC2. Our data suggest that the regulation of the prohormone processing system by morphine may lead to alterations in the levels of multiple bioactive hormones and may be a compensatory mechanism whereby the organism tries to restore its homeostatic hormonal milieu. The down-regulation of PC1/3, PC2 and P-CREB by short-term morphine and up-regulation by long-term morphine treatment may be a signal mediating the switch from drug use to drug abuse.

Proteases for processing proneuropeptides into peptide neurotransmitters and hormones

Peptide neurotransmitters and peptide hormones, collectively known as neuropeptides, are required for cell-cell communication in neurotransmission and for regulation of endocrine functions. Neuropeptides are synthesized from protein precursors (termed proneuropeptides or prohormones) that require proteolytic processing primarily within secretory vesicles that store and secrete the mature neuropeptides to control target cellular and organ systems. This review describes interdisciplinary strategies that have elucidated two primary protease pathways for prohormone processing consisting of the cysteine protease pathway mediated by secretory vesicle cathepsin L and the well-known subtilisin-like proprotein convertase pathway that together support neuropeptide biosynthesis. Importantly, this review discusses important areas of current and future biomedical neuropeptide research with respect to biological regulation, inhibitors, structural features of proneuropeptide and protease interactions, and peptidomics combined with proteomics for systems biological approaches. Future studies that gain in-depth understanding of protease mechanisms for generating active neuropeptides will be instrumental for translational research to develop pharmacological strategies for regulation of neuropeptide functions. Pharmacological applications for neuropeptide research may provide valuable therapeutics in health and disease.

Optimization of neuropeptide extraction from the mouse hypothalamus

Sample preparation for neuropeptidomic studies is a critical issue since protein degradation can produce high levels of peptides that obscure the endogenous neuropeptides. We compared different extraction conditions for the recovery of neuropeptides and the formation of protein breakdown fragments from mouse hypothalami. Sonication and heating in water (70 degrees C for 20 min) followed by cold acid and centrifugation enabled the efficient extraction of many neuropeptides without the formation of protein degradation fragments seen with hot acid extractions. The hot water/cold acid extraction procedure resulted in the reproducible recovery of many hypothalamic peptides, including several novel peptides.

Dysregulation of dynorphins in Alzheimer disease

The opioid peptides dynorphins may be involved in pathogenesis of Alzheimer disease (AD) by inducing neurodegeneration or cognitive impairment. To test this hypothesis, the dynorphin system was analyzed in postmortem samples from AD and control subjects, and subjects with Parkinson or cerebro-vascular diseases for comparison. Dynorphin A, dynorphin B and related neuropeptide nociceptin were determined in the Brodmann area 7 by radioimmunoassay. The precursor protein prodynorphin, processing convertase PC2 and the neuroendocrine pro7B2 and 7B2 proteins required for PC2 maturation were analyzed by Western blot. AD subjects displayed robustly elevated levels of dynorphin A and no differences in dynorphin B and nociceptin compared to controls. Subjects with Parkinson or cerebro-vascular diseases did not differ from controls with respect to any of the three peptides. PC2 levels were also increased, whereas, those of prodynorphin and pro7B2/7B2 were not changed in AD. Dynorphin A levels correlated with the neuritic plaque density. These results along with the known non-opioid ability of dynorphin A to induce neurodegeneration suggest a role for this neuropeptide in AD neuropathology.

Unique neuronal functions of cathepsin L and cathepsin B in secretory vesicles: biosynthesis of peptides in neurotransmission and neurodegenerative disease

Proteases are required for the production of peptide neurotransmitters and toxic peptides in neurodegenerative diseases. Unique roles of the cysteine proteases cathepsin L and cathepsin B in secretory vesicles for the production of biologically active peptides have been demonstrated in recent studies. Secretory vesicle cathepsin L participates in the proteolytic conversion of proenkephalin into the active enkephalin, an opioid peptide neurotransmitter that mediates pain relief. Moreover, recent findings provide evidence that cathepsin B in regulated secretory vesicles participates in the production of toxic beta-amyloid peptides that are known to accumulate extracellularly in Alzheimer's disease brains. The neurobiological functions of cathepsins L and B demonstrate that these secretory vesicle cysteine proteases produce biologically active peptides. These results demonstrate newly identified roles for cathepsins L and B in neurosecretory vesicles in the production of biologically active peptides.

Increased stress-induced analgesia in mice lacking the proneuropeptide convertase PC2

Many neuropeptides involved in pain perception are generated by endoproteolytic cleavages of their precursor proteins by the proprotein convertases PC1 and PC2. To investigate the role of PC2 in nociception and analgesia, we tested wild-type and PC2-null mice for their responses to mechanical and thermal nociceptive stimuli, before and after a short swim in cold or warm water. Basal responses and responses after a cold swim were similar between the two groups. However, after a short forced swim in warm water, PC2-null mice were significantly less responsive to the stimuli than wild-type mice, an indication of increased opioid-mediated stress-induced analgesia. The enhanced analgesia in PC2-null mice may be caused by an accumulation of opioid precursor processing intermediates with potent analgesic effects, or by loss of anti-opioid peptides.

Proprotein convertases: lessons from knockouts

The physiological role of the subtilisin/kexin-like proprotein convertases (PCs) in rodents has been examined through the use of knockout mice. This review will summarize the major in vivo defects that result from the disruption of the expression of their genes. This includes abnormal embryonic development, hormonal disorder, infertility, and/or modified lipid/sterol metabolism. Members of the PC family play a central role in the processing of various protein precursors ranging from hormones and growth factors to bacterial toxins and viral glycoproteins. Proteolysis occurring at basic residues is mediated by the basic amino acid-specific proprotein convertases, namely: PC1/3, PC2, furin, PACE4, PC4, PC5/6, and PC7. In contrast, proteolysis at nonbasic residues is performed by the subtilisin/kexin-like isozyme-1 (SKI-1/S1P) and the newly identified neural apoptosis-regulated convertase-1 (PCSK9/NARC-1). In addition to their requirement for many physiological processes, these enzymes are also involved in various pathologies such as cancer, obesity, diabetes, lipid disorders, infectious diseases, atherosclerosis and neurodegenerative diseases.

Prodynorphin storage and processing in axon terminals and dendrites

The classical view postulates that neuropeptide precursors in neurons are processed into mature neuropeptides in the somatic trans-Golgi network (TGN) and in secretory vesicles during axonal transport. Here we show that prodynorphin (PDYN), precursor to dynorphin opioid peptides, is predominantly located in axon terminals and dendrites in hippocampal and striatal neurons. The molar content of unprocessed PDYN was much greater than that of dynorphin peptides in axon terminals of PDYN-containing neurons projecting to the CA3 region of the hippocampus and in the striatal projections to the ventral tegmental area. Electron microscopy showed coexistence of PDYN and dynorphins in the same axon terminals with occasional codistribution in individual dense core vesicles. Thus, the precursor protein is apparently stored at presynaptic sites. In comparison with the hippocampus and striatum, PDYN and dynorphins were more equally distributed between neuronal somata and processes in the amygdala and cerebral cortex, suggesting regional differences in the regulation of trafficking and processing of the precursor protein. Potassium-induced depolarization activated PDYN processing and secretion of opioid peptides in neuronal cultures and in a model cell line. Regulation of PDYN storage and processing at synapses by neuronal activity or extracellular stimuli may provide a local mechanism for regulation of synaptic transmission.

The role of prohormone convertase-2 in hypothalamic neuropeptide processing: a quantitative neuropeptidomic study

Prohormone convertase (PC) 1/3 and 2 are involved in the generation of neuropeptides from their precursors. A quantitative peptidomic approach was used to explore the role PC2 plays in the processing of hypothalamic peptides. In this approach, extracts from mice lacking PC2 activity and from wild-type littermates were labeled with isotopic tags, combined, fractionated on a reverse phase HPLC column, and analyzed by electrospray ionization mass spectrometry. Altogether, 53 neuropeptides or other peptides derived from secretory pathway proteins were identified and sequenced using tandem mass spectrometry. These peptides arise from 21 distinct proteins: proenkephalin, proopiomelanocortin, prodynorphin, protachykinin A and B, procholecystokinin, promelanin-concentrating hormone, proneurotensin, proneuropeptide Y, provasopressin, pronociceptin/orphanin, prothyrotropin-releasing hormone, cocaine- and amphetamine-regulated transcript, chromogranin A and B, secretogranin II, prohormone convertase 1 and 2, propeptidyl-amidating monooxygenase, and proteins designated proSAAS and VGF. Approximately one third of the peptides found in wild-type mice were not detectable in PC2 knock-out mice, and another third were present at levels ranging from 25 to 75% of wild-type levels. Comparison of the cleavage sites suggests that sequences with a Trp, Tyr and/or Pro in the P1' or P2' position, or a basic residue in the P3 position, are preferentially cleaved by PC2 and not by other enzymes present in the secretory pathway.

Implication of the proprotein convertase NARC-1/PCSK9 in the development of the nervous system

Neural apoptosis-regulated convertase-1/proprotein convertase subtilisin-kexin like-9 (NARC-1/PCSK9) is a proprotein convertase recently described to play a major role in cholesterol homeostasis through enhanced degradation of the low-density lipoprotein receptor (LDLR) and possibly in neural development. Herein, we investigated the potential involvement of this proteinase in the development of the CNS using mouse embryonal pluripotent P19 cells and the zebrafish as models. Time course quantitative RT-PCR analyses were performed following retinoic acid (RA)-induced neuroectodermal differentiation of P19 cells. Accordingly, the mRNA levels of NARC-1/PCSK9 peaked at day 2 of differentiation and fell off thereafter. In contrast, the expression of the proprotein convertases subtilisin kexin isozyme 1/site 1 protease and Furin was unaffected by RA, whereas that of PC5/6 and PC2 increased within and/or after the first 4 days of the differentiation period respectively. This pattern was not affected by the cholesterogenic transcription factor sterol regulatory element-binding protein-2, which normally up-regulates NARC-1/PCSK9 mRNA levels in liver. Furthermore, in P19 cells, RA treatment did not affect the protein level of the endogenous LDLR. This agrees with the unique expression pattern of NARC-1/PCSK9 in the rodent CNS, including the cerebellum, where the LDLR is not significantly expressed. Whole-mount in situ hybridization revealed that the pattern of expression of zebrafish NARC-1/PCSK9 is similar to that of mouse both in the CNS and periphery. Specific knockdown of zebrafish NARC-1/PCSK9 mRNA resulted in a general disorganization of cerebellar neurons and loss of hindbrain-midbrain boundaries, leading to embryonic death at approximately 96 h after fertilization. These data support a novel role for NARC-1/PCSK9 in CNS development, distinct from that in cholesterogenic organs such as liver.

Thyroid hormones selectively regulate the posttranslational processing of prothyrotropin-releasing hormone in the paraventricular nucleus of the hypothalamus

Over the last few years, our laboratory has demonstrated that different physiological conditions or stressors affect the posttranslational processing of hypophysiotropic and nonhypophysiotropic proTRH and, consequently, the output of TRH and other proTRH-derived peptides. These alterations in proTRH processing are generally associated with parallel changes in the levels of two members of the family of prohormone convertases 1/3 and 2 (PC1/3 and PC2). An important regulator of proTRH is thyroid hormone, which is the peripheral end product of the hypothalamic (TRH)-pituitary (TSH)-thyroid (T3/4) (HPT) axis. In this study we investigated the effect of thyroid status on the processing of proTRH inside and outside the HPT axis. Our data showed that high levels of thyroid hormone down-regulated PC1/3 and PC2 and TRH synthesis, which led to an accumulation of intermediate forms of proTRH processing. Conversely, low levels of thyroid hormone up-regulated proTRH synthesis and PC1/3 and PC2 levels. Control of the activity of PCs and proTRH processing occurred specifically in the paraventricular nucleus, whereas no change due to thyroid status was found in the lateral hypothalamus or preoptic area. The posttranslational regulation of proTRH processing in the paraventricular nucleus by thyroid status is a novel aspect of the regulation of the HPT axis, which may have important implications for the pathophysiology of hypo- and hyperthyroidism.

Protease pathways in peptide neurotransmission and neurodegenerative diseases

1. Recent research demonstrates the critical importance of neuroproteases for the production of peptide neurotransmitters, and for the production of toxic peptides in major neurodegenerative diseases that include Alzheimer's (AD) and Huntington's diseases. This review describes the strategies utilized to identify the appropriate proteases responsible for producing active peptides for neurotransmission, with application of such approaches for defining protease mechanisms in neurodegenerative diseases. 2. Integration of multidisciplinary approaches in neurobiology, biochemistry, chemistry, proteomics, molecular biology, and genetics has been utilized for neuroprotease studies. These investigations have identified secretory vesicle cathepsin L for the production of the enkephalin opioid peptide neurotransmitter and other neuropeptides. Furthermore, new results using these strategies have identified secretory vesicle cathepsin B for the production of beta-amyloid (Abeta) in the major regulated secretory pathway that provides activity-dependent secretion of Abeta peptides, which accumulate in AD. 3. CNS neuroproteases that participate in peptide neurotransmission and in neurodegenerative diseases represent new candidate drug targets that may be explored in future research for the development of novel therapeutic agents for neurological conditions.

Mu opioid receptor A118G polymorphism in association with striatal opioid neuropeptide gene expression in heroin abusers

Mu opioid receptors are critical for heroin dependence, and A118G SNP of the mu opioid receptor gene (OPRM1) has been linked with heroin abuse. In our population of European Caucasians (n = 118), approximately 90% of 118G allelic carriers were heroin users. Postmortem brain analyses showed the OPRM1 genotype associated with transcription, translation, and processing of the human striatal opioid neuropeptide system. Whereas down-regulation of preproenkephalin and preprodynorphin genes was evident in all heroin users, the effects were exaggerated in 118G subjects and were most prominent for preproenkephalin in the nucleus accumbens shell. Reduced opioid neuropeptide transcription was accompanied by increased dynorphin and enkephalin peptide concentrations exclusively in 118G heroin subjects, suggesting that the peptide processing is associated with the OPRM1 genotype. Abnormal gene expression related to peptide convertase and ubiquitin/proteosome regulation was also evident in heroin users. Taken together, alterations in opioid neuropeptide systems might underlie enhanced opiate abuse vulnerability apparent in 118G individuals.

Neuroproteases in Peptide Neurotransmission and Neurodegenerative Diseases

The nervous system represents a key area for development of novel therapeutic agents for the treatment of neurological and neurodegenerative diseases. Recent research has demonstrated the critical importance of neuroproteases for the production of specific peptide neurotransmitters and for the production of toxic peptides in major neurodegenerative diseases that include Alzheimer, Huntington, and Parkinson diseases. This review illustrates the successful criteria that have allowed identification of proteases responsible for converting protein precursors into active peptide neurotransmitters, consisting of dual cysteine protease and subtilisin-like protease pathways in neuroendocrine cells. These peptide neurotransmitters are critical regulators of neurologic conditions, including analgesia and cognition, and numerous behaviors. Importantly, protease pathways also represent prominent mechanisms in neurodegenerative diseases, especially Alzheimer, Huntington, and Parkinson diseases. Recent studies have identified secretory vesicle cathepsin B as a novel beta-secretase for production of the neurotoxic beta-amyloid (Abeta) peptide of Alzheimer disease. Moreover, inhibition of cathepsin B reduces Abeta peptide levels in brain. These neuroproteases potentially represent new drug targets that should be explored in future pharmaceutical research endeavors for drug discovery.

Embryonic gene expression and pro-protein processing of proSAAS during rodent development

In vitro assays have demonstrated that peptides derived from the recently-identified proSAAS precursor inhibit prohormone convertase 1 (PC1) suggesting that this novel peptide may function as an endogenous inhibitor of PC1. To further understand the role of proSAAS in vivo, we have investigated the expression of proSAAS mRNA and processing of proSAAS during pre- and early postnatal rodent development. In situ hybridization showed that, by embryonic day 12.5 (e12.5) in the rat, proSAAS mRNA was present in essentially all differentiating neurons in the mantle layer of the myelencephalon, metencephalon, diencephalon, spinal cord and several sympathetic ganglia. During later stages of prenatal development, widespread proSAAS expression continues in post-mitotic neurons of both the CNS and PNS and begins in endocrine cells of the anterior and intermediate pituitary. Although proSAAS expression overlaps with PC1 in several regions, its overall expression pattern is significantly more extensive, suggesting that proSAAS may be multifunctional during development. Processed forms of proSAAS are present by at least mid-gestation with marked accumulation of two C-terminal forms, comprising the PC1 inhibitory fragment of proSAAS.

Cathepsin L and Arg/Lys aminopeptidase: a distinct prohormone processing pathway for the biosynthesis of peptide neurotransmitters and hormones

Peptide neurotransmitters and hormones are synthesized as protein precursors that require proteolytic processing to generate smaller, biologically active peptides that are secreted to mediate neurotransmission and hormone actions. Neuropeptides within their precursors are typically flanked by pairs of basic residues, as well as by monobasic residues. In this review, evidence for secretory vesicle cathepsin L and Arg/Lys aminopeptidase as a distinct proteolytic pathway for processing the prohormone proenkephalin is presented. Cleavage of prohormone processing sites by secretory vesicle cathepsin L occurs at the NH2-terminal side of dibasic residues, as well as between the dibasic residues, resulting in peptide intermediates with Arg or Lys extensions at their NH2-termini. A subsequent Arg/Lys aminopeptidase step is then required to remove NH2-terminal basic residues to generate the final enkephalin neuropeptide. The cathepsin L and Arg/Lys aminopeptidase prohormone processing pathway is distinct from the proteolytic pathway mediated by the subtilisin-like prohormone convertases 1/3 and 2 (PC1/3 and PC2) with carboxypeptidase E/H. Differences in specific cleavage sites at paired basic residue sites distinguish these two pathways. These two proteolytic pathways demonstrate the increasing complexity of regulatory mechanisms for the production of peptide neurotransmitters and hormones.

Regulation of regional expression in rat brain PC2 by thyroid hormone/characterization of novel negative thyroid hormone response elements in the PC2 promoter

The prohormone convertases (PCs) PC1 and PC2 are involved in the tissue-specific endoproteolytic processing of neuropeptide precursors within the secretory pathway. We previously showed that changes in thyroid status altered pituitary PC2 mRNA and that this regulation was due to triiodothyronine-dependent interaction of the thyroid hormone receptor (TR) with negative thyroid hormone response elements (nTREs) contained in a large proximal region of the human PC2 promoter. In the current study, we examined the in vivo regulation of brain PC2 mRNA by thyroid status and found that 6-n-propyl-2-thiouracil-induced hypothyroidism stimulated, whereas thyroxine-induced hyperthyroidism suppressed, PC2 mRNA levels in the rat hypothalamus and cerebral cortex. To address the mechanism of T3 regulation of the PC2 gene, we used human PC2 (hPC2) promoter constructs transiently transfected into GH3 cells and found that triiodothyronine negatively and 9-cis-retinoic acid positively regulated hPC2 promoter activity. EMSAs, using purified TRalpha1 and retinoid X receptor-beta (RXRbeta) proteins demonstrated that TRalpha bound the distal putative nTRE-containing oligonucleotide in the PC2 promoter, and RXR bound to both nTRE-containing oligonucleotides. EMSAs with oligonucleotides containing deletion mutations of the nTREs demonstrated that the binding to TR and RXR separately is reduced, but specific binding to TR and RXR together persists even with deletion of each putative nTRE. We conclude that there are two novel TRE-like sequences in the hPC2 promoter and that these regions act in concert in a unique manner to facilitate the effects of thyroid hormone and 9-cis-retinoic acid on PC2.

Pathobiology of dynorphins in trauma and disease

Dynorphins, endogenous opioid neuropeptides derived from the prodynorphin gene, are involved in a variety of normative physiologic functions including antinociception and neuroendocrine signaling, and may be protective to neurons and oligodendroglia via their opioid receptor-mediated effects. However, under experimental or pathophysiological conditions in which dynorphin levels are substantially elevated, these peptides are excitotoxic largely through actions at glutamate receptors. Because the excitotoxic actions of dynorphins require supraphysiological concentrations or prolonged tissue exposure, there has likely been little evolutionary pressure to ameliorate the maladaptive, non-opioid receptor mediated consequences of dynorphins. Thus, dynorphins can have protective and/or proapoptotic actions in neurons and glia, and the net effect may depend upon the distribution of receptors in a particular region and the amount of dynorphin released. Increased prodynorphin gene expression is observed in several disease states and disruptions in dynorphin processing can accompany pathophysiological situations. Aberrant processing may contribute to the net negative effects of dysregulated dynorphin production by tilting the balance towards dynorphin derivatives that are toxic to neurons and/or oligodendroglia. Evidence outlined in this review suggests that a variety of CNS pathologies alter dynorphin biogenesis. Such alterations are likely maladaptive and contribute to secondary injury and the pathogenesis of disease.

Furin and prohormone convertase 1/3 are major convertases in the processing of mouse pro-growth hormone-releasing hormone

We investigated the proteolytic processing of mouse pro-GHRH [84 amino acids (aa)] by furin, PC1/3, PC2, and PC5/6A. We created six point mutations in the N- and C-terminal cleavage sites, RXXR decreased and RXRXXR decreased, respectively. The following results were obtained after transient transfection/cotransfection and metabolic pulse-chase labeling studies in several neuroendocrine cells. 1) Furin was the most efficient convertase in cleaving the N-terminal RXXR/RXRR site to generate intermediate I, 12-84aa, whereas PC1/3 was the most potent in processing the C-terminal RXRXXR site to yield mature GHRH, 12-53aa. 2) Both PC1/3 and PC5/6A also processed the N-terminal site but less efficiently than furin. 3) PC2 was much weaker in cleaving the C-terminal site relative to PC1/3 to generate mature GHRH. 4) The Q10R mutant was significantly more susceptible to furin cleavage at the N-terminal site than the wild-type pro-GHRH. And 5) the N- and C-terminal P1 Arg residues, R11 and R54, respectively, were essential for mature GHRH production. We also showed localization of the GHRH immunoreactive peptides in Golgi and secretory granules in neuroendocrine cells by an immunofluorescence assay. We conclude that the efficient production of mature GHRH from pro-GHRH is a stepwise process mediated predominantly by furin at the N-terminal cleavage site followed by PC1/3 at the C terminus.

Gamma-MSH, sodium metabolism, and salt-sensitive hypertension

Alpha-, beta-, and gamma-melanocyte stimulating hormones (MSHs) are melanotropin peptides that are derived from the ACTH/beta-endorphin prohormone proopiomelanocortin (POMC). They have been highly conserved through evolutionary development, although their functions in mammals have remained obscure. The identification in the last decade of a family of five membrane-spanning melanocortin receptors (MC-Rs), for which the melanotropins are the natural ligands, has permitted the characterization of a number of important actions of these peptides, although the physiological function(s) of gamma-MSH have remained elusive. Much evidence indicates that gamma-MSH stimulates sympathetic outflow and raises blood pressure through a central mechanism. However, this review focuses on newer cardiovascular and renal actions of the peptide, acting in most cases through the MC3-R. In rodents, a high-sodium diet (HSD) increases the pituitary abundance of POMC mRNA and of gamma-MSH content and results in a doubling of plasma gamma-MSH concentration. The peptide is natriuretic and acts through renal MC3-Rs, which are also upregulated by the HSD. Thus the system appears designed to participate in the integrated response to dietary sodium excess. Genetic or pharmacologic induction of gamma-MSH deficiency results in marked salt-sensitive hypertension that is corrected by the administration of the peptide, probably through a central site of action. Deletion of the MC3-R also produces salt-sensitive hypertension, which, however, is not corrected by infusion of the hormone. These observations in aggregate suggest the operation of a hormonal system important in blood pressure control and in the regulation of sodium excretion. The relationship of these two actions to each other and the significance of this system in humans are important questions for future research.

Silencing of SPC2 expression using an engineered delta ribozyme in the mouse betaTC-3 endocrine cell line

Endoproteolytic processing is carried out by subtilase-like pro-protein convertases in mammalian cells. In order to understand the distinct roles of a member of this family (SPC2), gene silencing in cultured cells is an ideal approach. Previous studies showed limited success in either the degree of inhibition obtained or the stability of the cell lines. Here we demonstrate the high potential of delta ribozyme as a post-transcriptional gene silencing tool in cultured cells. We used an expression vector based on the RNA polymerase III promoter to establish betaTC-3 stable cell lines expressing the chimeric tRNA(Val)-delta ribozyme transcript targeting SPC2 mRNA. Northern and Western blot hybridizations showed a specific reduction of SPC2 mRNA and protein. Validation of processing effects was tested by measuring the levels of dynorphin A-(1-8), which are present in betaTC-3 cells as a result of the unique cleavage of dynorphin A-(1-17) by SPC2. Moreover, a differential proteomic analysis confirmed these results and allowed identification of secretogranin II as a potential substrate of SPC2. The development of efficient, specific, and durable silencing tools, such as described in the present work, will be of great importance in elucidating the functions of the subtilase-like pro-protein convertases in regard to peptide processing and derived cellular events.