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

Inactive metallopeptidase homologs: the secret lives of pseudopeptidases

Inactive enzyme homologs, or pseudoenzymes, are proteins, found within most enzyme families, that are incapable of performing catalysis. Rather than catalysis, they are involved in protein-protein interactions, sometimes regulating the activity of their active enzyme cousins, or scaffolding protein complexes. Pseudoenzymes found within metallopeptidase families likewise perform these functions. Pseudoenzymes within the M14 carboxypeptidase family interact with collagens within the extracellular space, while pseudopeptidase members of the M12 "a disintegrin and metalloprotease" (ADAM) family either discard their pseudopeptidase domains as unnecessary for their roles in sperm maturation or utilize surface loops to enable assembly of key complexes at neuronal synapses. Other metallopeptidase families contain pseudopeptidases involved in protein synthesis at the ribosome and protein import into organelles, sometimes using their pseudo-active sites for these interactions. Although the functions of these pseudopeptidases have been challenging to study, ongoing work is teasing out the secret lives of these proteins.

Non-cross-linking advanced glycation end products affect prohormone processing

Advanced glycation end products (AGEs) are non-enzymatic post-translational modifications of amino acids and are associated with diabetic complications. One proposed pathomechanism is the impaired processing of AGE-modified proteins or peptides including prohormones. Two approaches were applied to investigate whether substrate modification with AGEs affects the processing of substrates like prohormones to the active hormones. First, we employed solid-phase peptide synthesis to generate unmodified as well as AGE-modified protease substrates. Activity of proteases towards these substrates was quantified. Second, we tested the effect of AGE-modified proinsulin on the processing to insulin. Proteases showed the expected activity towards the unmodified peptide substrates containing arginine or lysine at the C-terminal cleavage site. Indeed, modification with Nε-carboxymethyllysine (CML) or methylglyoxal-hydroimidazolone 1 (MG-H1) affected all proteases tested. Cysteine cathepsins displayed a reduction in activity by ∼50% towards CML and MG-H1 modified substrates. The specific proteases trypsin, proprotein convertases subtilisin-kexins (PCSKs) type proteases, and carboxypeptidase E (CPE) were completely inactive towards modified substrates. Proinsulin incubation with methylglyoxal at physiological concentrations for 24 h resulted in the formation of MG-modified proinsulin. The formation of insulin was reduced by up to 80% in a concentration-dependent manner. Here, we demonstrate the inhibitory effect of substrate-AGE modifications on proteases. The finding that PCSKs and CPE, which are essential for prohormone processing, are inactive towards modified substrates could point to a yet unrecognized pathomechanism resulting from AGE modification relevant for the etiopathogenesis of diabetes and the development of obesity.

Quantitative Peptidomics: General Considerations

Peptidomics is the detection and identification of the peptides present in a sample, and quantitative peptidomics provides additional information about the amounts of these peptides. It is possible to perform absolute quantitation of peptide levels in which the biological sample is compared to synthetic standards of each peptide. More commonly, relative quantitation is performed to compare peptide levels between two or more samples. Relative quantitation can measure differences between all peptides that are detectable, which can exceed 1000 peptides in a complex sample. In this chapter, various techniques used for quantitative peptidomics are described along with discussion of the advantages and disadvantages of each approach. A guide to selecting the optimal quantitative approach is provided, based on the goals of the experiment and the resources that are available.

Recent advances in mass spectrometry analysis of neuropeptides

Due to their involvement in numerous biochemical pathways, neuropeptides have been the focus of many recent research studies. Unfortunately, classic analytical methods, such as western blots and enzyme-linked immunosorbent assays, are extremely limited in terms of global investigations, leading researchers to search for more advanced techniques capable of probing the entire neuropeptidome of an organism. With recent technological advances, mass spectrometry (MS) has provided methodology to gain global knowledge of a neuropeptidome on a spatial, temporal, and quantitative level. This review will cover key considerations for the analysis of neuropeptides by MS, including sample preparation strategies, instrumental advances for identification, structural characterization, and imaging; insightful functional studies; and newly developed absolute and relative quantitation strategies. While many discoveries have been made with MS, the methodology is still in its infancy. Many of the current challenges and areas that need development will also be highlighted in this review.

The G209R mutant mouse as a model for human PCSK1 polyendocrinopathy

PCSK1 encodes an enzyme required for prohormone maturation into bioactive peptides. A striking number of SNPs and rare mutations in PCSK1 are associated with a range of clinical phenotypes. Infants bearing two copies of a catalytically inactivating mutation, such as G209R, exhibit life-threatening chronic diarrhea and subsequently develop systemic endocrinopathies. Using CRISPR/Cas9 technology, we have engineered a mouse model bearing a G209R missense mutation in exon 6 of the murine Pcsk1 locus. Most pups homozygous for the G209R mutation succumbed by day 2, and surviving pups were severely dwarfed. In homozygous (but not heterozygous) pups, blood glucose levels were significantly lower, accompanied by elevated plasma insulin-like immunoreactivity and accumulation of large quantities of unprocessed proinsulin in the pancreas. Peptide hormone processing was also aberrant in G209R mouse pituitary, with mature ACTH levels markedly reduced in homozygotes, accompanied by a significant accumulation of POMC. We also observed a significant reduction in PC1/3 protein in the brains of G209R homozygous mice by Western blotting, while PC2 levels remained unaffected. Most likely due to the continued presence of PC2, pituitary and brain levels of α-MSH were not impaired. Analysis of intestinal cell types indicated a modest reduction of enteroendocrine cells in G209R homozygotes. We suggest that the G209R Pcsk1 mouse model recapitulates many of the dramatic neonatal deficiencies of human patients with this homozygous mutation.

Altered islet prohormone processing: A cause or consequence of diabetes?

Peptide hormones are first produced as larger precursor prohormones that require endoproteolytic cleavage to liberate the mature hormones. A structurally conserved but functionally distinct family of nine prohormone convertase enzymes (PCs) are responsible for cleavage of protein precursors of which PC1/3 and PC2 are known to be exclusive to neuroendocrine cells and responsible for prohormone cleavage. Differential expression of PCs within tissues define prohormone processing; whereas glucagon is the major product liberated from proglucagon via PC2 in pancreatic α-cells, proglucagon is preferentially processed by PC1/3 in intestinal L cells to produce glucagon-like peptides 1 and 2 (GLP-1, GLP-2). Beyond our understanding of processing of islet prohormones in healthy islets, there is convincing evidence that proinsulin, proIAPP, and proglucagon processing is altered during prediabetes and diabetes. There is predictive value of elevated circulating proinsulin or proinsulin : C-peptide ratio for progression to type 2 diabetes and elevated proinsulin or proinsulin : C-peptide is predictive for development of type 1 diabetes in at risk groups. After onset of diabetes, patients have elevated circulating proinsulin and proIAPP and proinsulin may be an autoantigen in type 1 diabetes. Further, preclinical studies reveal that α-cells have altered proglucagon processing during diabetes leading to increased GLP-1 production. We conclude that despite strong associative data, current evidence is inconclusive on the potential causal role of impaired prohormone processing in diabetes, and suggest that future work should focus on resolving the question of whether altered prohormone processing is a causal driver or merely a consequence of diabetes pathology.

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.

Transcription factor Creb3l1 regulates the synthesis of prohormone convertase enzyme PC1/3 in endocrine cells

Transcription factor cAMP responsive element-binding protein 3 like 1 (Creb3l1) is a non-classical endoplasmic reticulum stress molecule that is emerging as an important component for cellular homeostasis, particularly within cell types with high peptide secretory capabilities. We have previously shown that Creb3l1 serves an important role in body fluid homeostasis through its transcriptional control of the gene coding for antidiuretic hormone arginine vasopressin in the neuropeptide-rich magnocellular neurones of the supraoptic nucleus. In response to osmotic stimuli such as dehydration, vasopressin magnocellular neurones undergo remarkable transcriptome changes, including increased Creb3l1 expression, to ensure that the supply of vasopressin meets demand. To determine where else Creb3l1 fits into the secretory cell supply chain, we performed RNA-sequencing of Creb3l1 knockdown anterior pituitary mouse corticotroph cell line AtT20. The target chosen for further investigation was Pcsk1, which encodes proprotein convertase enzyme 1 (PC1/3). PC1/3 is crucial for processing of neuropeptides and peptide hormones such as pro-opiomelanocortin (POMC), proinsulin, proglucagon, vasopressin and oxytocin. Viral manipulations in supraoptic nuclei by over-expression of Creb3l1 increased Pcsk1, whereas Creb3l1 knockdown decreased Pcsk1 expression. In vitro promoter activity and binding studies showed that Creb3l1 was a transcription factor of the Pcsk1 gene binding directly to a G-box motif in the promoter. In the dehydrated rat anterior pituitary, Creb3l1 and Pcsk1 expression decreased in parallel compared to control, supporting our findings from manipulations in AtT20 cells and the supraoptic nucleus. No relationship was observed between Creb3l1 and Pcsk1 expression in the neurointermediate lobe of the pituitary, indicating a different mechanism of PC1/3 synthesis by these POMC-synthesising cells. Therefore, Creb3l1, by regulating the expression of Pcsk1, does not control the processing of POMC peptides in the intermediate lobe.

Effect of Protein Denaturation and Enzyme Inhibitors on Proteasomal-Mediated Production of Peptides in Human Embryonic Kidney Cells

Peptides produced by the proteasome have been proposed to function as signaling molecules that regulate a number of biological processes. In the current study, we used quantitative peptidomics to test whether conditions that affect protein stability, synthesis, or turnover cause changes in the levels of peptides in Human Embryonic Kidney 293T (HEK293T) cells. Mild heat shock (42 °C for 1 h) or treatment with the deubiquitinase inhibitor b-AP15 led to higher levels of ubiquitinated proteins but did not significantly increase the levels of intracellular peptides. Treatment with cycloheximide, an inhibitor of protein translation, did not substantially alter the levels of intracellular peptides identified herein. Cells treated with a combination of epoxomicin and bortezomib showed large increases in the levels of most peptides, relative to the levels in cells treated with either compound alone. Taken together with previous studies, these results support a mechanism in which the proteasome cleaves proteins into peptides that are readily detected in our assays (i.e., 6–37 amino acids) and then further degrades many of these peptides into smaller fragments.

Neuropeptide PEN and Its Receptor GPR83: Distribution, Signaling, and Regulation

Neuropeptides are chemical messengers that act to regulate a number of physiological processes, including feeding, reward, pain, and memory, among others. PEN is one of the most abundant hypothalamic neuropeptides; however, until recently, its target receptor remained unknown. In this Review, we summarize recent developments in research focusing on PEN and its receptor GPR83. We describe the studies leading to the deorphanization of GPR83 as the receptor for PEN. We also describe the signaling mediated by the PEN-GPR83 system, as well as the physiological roles in which PEN-GPR83 has been implicated. As studies have suggested a role for the PEN-GPR83 system in food intake and body weight regulation, as well as in drug addiction and reward disorders, a thorough understanding of this novel neuropeptide-receptor system will help identify novel therapeutic targets to treat pathophysiological conditions involving PEN-GPR83.

POMC: The Physiological Power of Hormone Processing

Pro-opiomelanocortin (POMC) is the archetypal polypeptide precursor of hormones and neuropeptides. In this review, we examine the variability in the individual peptides produced in different tissues and the impact of the simultaneous presence of their precursors or fragments. We also discuss the problems inherent in accurately measuring which of the precursors and their derived peptides are present in biological samples. We address how not being able to measure all the combinations of precursors and fragments quantitatively has affected our understanding of the pathophysiology associated with POMC processing. To understand how different ratios of peptides arise, we describe the role of the pro-hormone convertases (PCs) and their tissue specificities and consider the cellular processing pathways which enable regulated secretion of different peptides that play crucial roles in integrating a range of vital physiological functions. In the pituitary, correct processing of POMC peptides is essential to maintain the hypothalamic-pituitary-adrenal axis, and this processing can be disrupted in POMC-expressing tumors. In hypothalamic neurons expressing POMC, abnormalities in processing critically impact on the regulation of appetite, energy homeostasis, and body composition. More work is needed to understand whether expression of the POMC gene in a tissue equates to release of bioactive peptides. We suggest that this comprehensive view of POMC processing, with a focus on gaining a better understanding of the combination of peptides produced and their relative bioactivity, is a necessity for all involved in studying this fascinating physiological regulatory phenomenon.

Targeting the Recently Deorphanized Receptor GPR83 for the Treatment of Immunological, Neuroendocrine and Neuropsychiatric Disorders

G-protein coupled receptors (GPCRs) are a superfamily of receptors responsible for initiation of a myriad of intracellular signaling cascades. Currently, GPCRs represent approximately 34% of marketed pharmaceuticals, a large portion of which have no known endogenous ligand. These orphan GPCRs represent a large pool of novel targets for drug development. Very recently, the neuropeptide PEN, derived from the proteolytic processing of the precursor proSAAS, has been identified as a selective, high-affinity endogenous ligand for the orphan receptor, GPR83. GPR83 is highly expressed in the brain, spleen and thymus, indicating that this receptor may be a target to treat neurological and immune disorders. In the brain GPR83 is expressed in regions involved in the reward pathway, stress/anxiety responses, learning and memory and metabolism. However, the cell type specific expression of GPR83 in these regions has only recently begun to be characterized. In the immune system, GPR83 expression is regulated by Foxp3 in T-regulatory cells that are involved in autoimmune responses. Moreover, in the brain this receptor is regulated by interactions with other GPCRs, such as the recently deorphanized receptor, GPR171, and other hypothalamic receptors such as MC4R and GHSR. The following review will summarize the properties of GPR83 and highlight its known and potential significance in health and disease, as well as its promise as a novel target for drug development.

Human brain gene expression profiles of the cathepsin V and cathepsin L cysteine proteases, with the PC1/3 and PC2 serine proteases, involved in neuropeptide production

Proteases are required to generate active peptide neurotransmitters, known as neuropeptides, from pro-neuropeptides. Model animal systems have recently illustrated roles for the cathepsin V (CTSV) and cathepsin L (CTSL) cysteine proteases, combined with the serine proteases PC1/3 (PCSK1) and PC2 (PCSK2), and exopeptidases in the production of neuropeptides. There is notable interest in the human-specific cathepsin V gene which is not present in rodent and other animal models used in prior studies of neuropeptide production. A gap in the field is knowledge of the human brain gene expression patterns of these neuropeptide-producing protease systems. Therefore, the goal of this study was to characterize the expression profiles of these pro-neuropeptide processing proteases in human brain. Quantitative gene expression microarray data for 169 human brain regions was obtained from the Allen Institute Human Brain Atlas resource, analyzed as log₂ of gene expression intensity normalized to the mean of human genes (21,245 genes) expressed in human brain. These proteases had log₂ values of 2–12, indicating expression levels above the average of all genes in the human brain, with varying expression levels among the 169 brain regions. CTSV and CTSL displayed moderate to high expression values of 1.9–8.6 and 7.1–10.6, respectively. Interestingly, CTSV and CTSL showed high expression in white matter composed of myelinated axons, consistent with the knowledge that neuropeptide production occurs in axons within transported neuropeptide secretory vesicles to nerve terminals. PCSK1 had a broad range of moderate to very high expression with log₂ of 2–12. PCSK2 had somewhat lower expression levels than PCSK1. The exopeptidase genes RNPEP, CTSH, and CPE each showed fairly even levels of expression throughout the brain, with CPE displaying high expression. The prevalence of these processing proteases throughout human brain regions, including areas rich in neuropeptides such as hypothalamus, is consistent with their roles for neuropeptide production. Further, proenkephalin and NPY precursors, substrates of CTSV and CTSL shown in prior model animal studies, were co-expressed with CTSV and CTSL. These data demonstrate that the human brain expresses the neuropeptide-producing cysteine and serine proteases, with exopeptidases, throughout a multitude of brain regions.

Neuropeptidome of the Hypothalamus and Pituitary Gland of Indicine × Taurine Heifers: Evidence of Differential Neuropeptide Processing in the Pituitary Gland before and after Puberty

Puberty in cattle is regulated by an endocrine axis, which includes a complex milieu of neuropeptides in the hypothalamus and pituitary gland. The neuropeptidome of hypothalamic-pituitary gland tissue of pre- (PRE) and postpubertal (POST) Bos indicus-influenced heifers was characterized, followed by quantitative analysis of 51 fertility-related neuropeptides in these tissues. Comparison of peptide abundances with gene expression levels allowed assessment of post-transcriptional peptide processing. On the basis of classical cleavage, 124 mature neuropeptides from 35 precursor proteins were detected in hypothalamus and pituitary gland tissues of three PRE and three POST Brangus heifers. An additional 19 peptides (cerebellins, PEN peptides) previously reported as neuropeptides that did not follow classical cleavage were also identified. In the pre-pubertal hypothalamus, a greater diversity of neuropeptides (25.8%) was identified relative to post-pubertal heifers, while in the pituitary gland, 38.6% more neuropeptides were detected in the post-pubertal heifers. Neuro-tissues of PRE and POST heifers revealed abundance differences ( p < 0.05) in peptides from protein precursors involved in packaging and processing (e.g., the granin family and ProSAAS) or neuron stimulation (PENK, CART, POMC, cerebellins). On their own, the transcriptome data of the precursor genes could not predict the neuropeptide profile in the exact same tissues in several cases. This provides further evidence of the importance of differential processing of the neuropeptide precursors in the pituitary before and after puberty.

Quantitative Peptidomics with Isotopic and Isobaric Tags

In differential peptidomics, peptide profiles are compared between biological samples and the resulting expression levels are correlated to a phenotype of interest. This, in turn, allows us insight into how peptides may affect the phenotype of interest. In quantitative differential peptidomics, both label-based and label-free techniques are often employed. Label-based techniques have several advantages over label-free methods, primarily that labels allow for various samples to be pooled prior to liquid chromatography-mass spectrometry (LC-MS) analysis, reducing between-run variation. Here, we detail a method for performing quantitative peptidomics using stable amine-binding isotopic and isobaric tags.

ProSAAS-derived peptides are regulated by cocaine and are required for sensitization to the locomotor effects of cocaine

To identify neuropeptides that are regulated by cocaine, we used a quantitative peptidomic technique to examine the relative levels of neuropeptides in several regions of mouse brain following daily intraperitoneal administration of 10 mg/kg cocaine or saline for 7 days. A total of 102 distinct peptides were identified in one or more of the following brain regions: nucleus accumbens, caudate putamen, frontal cortex, and ventral tegmental area. None of the peptides detected in the caudate putamen or frontal cortex were altered by cocaine administration. Three peptides in the nucleus accumbens and seven peptides in the ventral tegmental area were significantly decreased in cocaine-treated mice. Five of these ten peptides are derived from proSAAS, a secretory pathway protein and neuropeptide precursor. To investigate whether proSAAS peptides contribute to the physiological effects of psychostimulants, we examined acute responses to cocaine and amphetamine in the open field with wild-type (WT) and proSAAS knockout (KO) mice. Locomotion was stimulated more robustly in the WT compared to mutant mice for both psychostimulants. Behavioral sensitization to amphetamine was not maintained in proSAAS KO mice and these mutants failed to sensitize to cocaine. To determine whether the rewarding effects of cocaine were altered, mice were tested in conditioned place preference (CPP). Both WT and proSAAS KO mice showed dose-dependent CPP to cocaine that was not distinguished by genotype. Taken together, these results suggest that proSAAS-derived peptides contribute differentially to the behavioral sensitization to psychostimulants, while the rewarding effects of cocaine appear intact in mice lacking proSAAS.

60 YEARS OF POMC: Biosynthesis, trafficking, and secretion of pro-opiomelanocortin-derived peptides

Pro-opiomelanocortin (POMC) is a prohormone that encodes multiple smaller peptide hormones within its structure. These peptide hormones can be generated by cleavage of POMC at basic residue cleavage sites by prohormone-converting enzymes in the regulated secretory pathway (RSP) of POMC-synthesizing endocrine cells and neurons. The peptides are stored inside the cells in dense-core secretory granules until released in a stimulus-dependent manner. The complexity of the regulation of the biosynthesis, trafficking, and secretion of POMC and its peptides reflects an impressive level of control over many factors involved in the ultimate role of POMC-expressing cells, that is, to produce a range of different biologically active peptide hormones ready for action when signaled by the body. From the discovery of POMC as the precursor to adrenocorticotropic hormone (ACTH) and β-lipotropin in the late 1970s to our current knowledge, the understanding of POMC physiology remains a monumental body of work that has provided insight into many aspects of molecular endocrinology. In this article, we describe the intracellular trafficking of POMC in endocrine cells, its sorting into dense-core secretory granules and transport of these granules to the RSP. Additionally, we review the enzymes involved in the maturation of POMC to its various peptides and the mechanisms involved in the differential processing of POMC in different cell types. Finally, we highlight studies pertaining to the regulation of ACTH secretion in the anterior and intermediate pituitary and POMC neurons of the hypothalamus.

Identification of GPR83 as the receptor for the neuroendocrine peptide PEN

PEN is an abundant peptide in the brain that has been implicated in the regulation of feeding. We identified a receptor for PEN in mouse hypothalamus and Neuro2A cells. PEN bound to and activated GPR83, a G protein (heterotrimeric guanine nucleotide)-binding protein)-coupled receptor (GPCR). Reduction of GPR83 expression in mouse brain and Neuro2A cells reduced PEN binding and signaling, consistent with GPR83 functioning as the major receptor for PEN. In some brain regions, GPR83 colocalized with GPR171, a GPCR that binds the neuropeptide bigLEN, another neuropeptide that is involved in feeding and is generated from the same precursor protein as is PEN. Coexpression of these two receptors in cell lines altered the signaling properties of each receptor, suggesting a functional interaction. Our data established PEN as a neuropeptide that binds GPR83 and suggested that these two ligand-receptor systems-PEN-GPR83 and bigLEN-GPR171-may be functionally coupled in the regulation of feeding.

Neuropeptidomics: Mass Spectrometry-Based Identification and Quantitation of Neuropeptides

Neuropeptides produced from prohormones by selective action of endopeptidases are vital signaling molecules, playing a critical role in a variety of physiological processes, such as addiction, depression, pain, and circadian rhythms. Neuropeptides bind to post-synaptic receptors and elicit cellular effects like classical neurotransmitters. While each neuropeptide could have its own biological function, mass spectrometry (MS) allows for the identification of the precise molecular forms of each peptide without a priori knowledge of the peptide identity and for the quantitation of neuropeptides in different conditions of the samples. MS-based neuropeptidomics approaches have been applied to various animal models and conditions to characterize and quantify novel neuropeptides, as well as known neuropeptides, advancing our understanding of nervous system function over the past decade. Here, we will present an overview of neuropeptides and MS-based neuropeptidomic strategies for the identification and quantitation of neuropeptides.

PCSK1 Variants and Human Obesity

PCSK1, encoding prohormone convertase 1/3 (PC1/3), was one of the first genes linked to monogenic early-onset obesity. PC1/3 is a protease involved in the biosynthetic processing of a variety of neuropeptides and prohormones in endocrine tissues. PC1/3 activity is essential for the activating cleavage of many peptide hormone precursors implicated in the regulation of food ingestion, glucose homeostasis, and energy homeostasis, for example, proopiomelanocortin, proinsulin, proglucagon, and proghrelin. A large number of genome-wide association studies in a variety of different populations have now firmly established a link between three PCSK1 polymorphisms frequent in the population and increased risk of obesity. Human subjects with PC1/3 deficiency, a rare autosomal-recessive disorder caused by the presence of loss-of-function mutations in both alleles, are obese and display a complex set of endocrinopathies. Increasing numbers of genetic diagnoses of infants with persistent diarrhea has recently led to the finding of many novel PCSK1 mutations. PCSK1-deficient infants experience severe intestinal malabsorption during the first years of life, requiring controlled nutrition; these children then become hyperphagic, with associated obesity. The biochemical characterization of novel loss-of-function PCSK1 mutations has resulted in the discovery of new pathological mechanisms affecting the cell biology of the endocrine cell beyond simple loss of enzyme activity, for example, dominant-negative effects of certain mutants on wild-type PC1/3 protein, and activation of the cellular unfolded protein response by endoplasmic reticulum-retained mutants. A better understanding of these molecular and cellular pathologies may illuminate possible treatments for the complex endocrinopathy of PCSK1 deficiency, including obesity.

Limitations of Mass Spectrometry-Based Peptidomic Approaches

Mass spectrometry-based peptidomic approaches are powerful techniques to detect and identify the peptide content of biological samples. The present study investigated the limitations of peptidomic approaches using trimethylammonium butyrate isotopic tags to quantify relative peptide levels and Mascot searches to identify peptides. Data were combined from previous studies on human cell lines or mouse tissues. The combined databases contain 2155 unique peptides ranging in mass from 444 to 8765 Da, with the vast majority between 1 and 3 kDa. The amino acid composition of the identified peptides generally reflected the frequency in the Eukaryotic proteome with the exception of Cys, which was not present in any of the identified peptides in the free-SH form but was detected at low frequency as a disulfide with Cys residues, a disulfide with glutathione, or as S-cyanocysteine. To test if the low detection rate of peptides smaller than 500 Da, larger than 3 kDa, or containing Cys was a limitation of the peptidomics procedure, tryptic peptides of known proteins were processed for peptidomics using the same approach used for human cell lines and mouse tissues. The identified tryptic peptides ranged from 516 to 2418 Da, whereas the theoretical digest ranged from 217 to 7559 Da. Peptides with Cys were rarely detected and, if present, the Cys was usually modified S-cyanocysteine. Additionally, peptides with mono- and di-iodo Tyr and His were identified. Taken together, there are limitations of peptidomic techniques, and awareness of these limitations is important to properly use and interpret results. Graphical Abstract ᅟ.

Biochemical and cell biological properties of the human prohormone convertase 1/3 Ser357Gly mutation: a PC1/3 hypermorph

Satiety and appetite signaling are accomplished by circulating peptide hormones. These peptide hormones require processing from larger precursors to become bioactive, often by the proprotein convertase 1/3 (PC1/3). Several subcellular maturation steps are necessary for PC1/3 to achieve its optimal enzymatic activity. Certain PC1/3 variants found in the general population slightly attenuate its enzymatic activity and are associated with obesity and diabetes. However, mutations that increase PC1/3 activity and/or affect its specificity could also have physiological consequences. We here present data showing that the known human Ser357Gly PC1/3 mutant (PC1/3(S357G)) represents a PC1/3 hypermorph. Conditioned media from human embryonic kidney-293 cells transfected with PC1/3(WT) and PC1/3(S357G) were collected and enzymatic activity characterized. PC1/3(S357G) exhibited a lower calcium dependence; a higher pH optimum (neutral); and a higher resistance to peptide inhibitors than the wild-type enzyme. PC1/3(S357G) exhibited increased cleavage to the C-terminally truncated form, and kinetic parameters of the full-length and truncated mutant enzymes were also altered. Lastly, the S357G mutation broadened the specificity of the enzyme; we detected PC2-like specificity on the substrate proCART, the precursor of the cocaine- and amphetamine regulated transcript neuropeptide known to be associated with obesity. The production of another anorexigenic peptide normally synthesized only by PC2, αMSH, was increased when proopiomelanocortin was coexpressed with PC1/3(S357G). Considering the aberrant enzymatic profile of PC1/3(S357G), we hypothesize that this enzyme possesses unusual processing activity that may significantly change the profile of circulating peptide hormones.

ProSAAS-derived peptides are differentially processed and sorted in mouse brain and AtT-20 cells

ProSAAS is the precursor for some of the most abundant peptides found in mouse brain and other tissues, including peptides named SAAS, PEN, and LEN. Both SAAS and LEN are found in big and little forms due to differential processing. Initial processing of proSAAS is mediated by furin (and/or furin-like enzymes) and carboxypeptidase D, while the smaller forms are generated by secretory granule prohormone convertases and carboxypeptidase E. In mouse hypothalamus, PEN and big LEN colocalize with neuropeptide Y. In the present study, little LEN and SAAS were detected in mouse hypothalamus but not in cell bodies of neuropeptide Y-expressing neurons. PEN and big LEN show substantial colocalization in hypothalamus, but big LEN and little LEN do not. An antiserum to SAAS that detects both big and little forms of this peptide did not show substantial colocalization with PEN or big LEN. To further study this, the AtT-20 cells mouse pituitary corticotrophic cell line was transfected with rat proSAAS and the distribution of peptides examined. As found in mouse hypothalamus, only some of the proSAAS-derived peptides colocalized with each other in AtT-20 cells. The two sites within proSAAS that are known to be efficiently cleaved by furin were altered by site-directed mutagenesis to convert the P4 Arg into Lys; this change converts the sequences from furin consensus sites into prohormone convertase consensus sites. Upon expression of the mutated form of proSAAS in AtT-20 cells, there was significantly more colocalization of proSAAS-derived peptides PEN and SAAS. Taken together, these results indicate that proSAAS is initially cleaved in the Golgi or trans-Golgi network by furin and/or furin-like enzymes and the resulting fragments are sorted into distinct vesicles and further processed by additional enzymes into the mature peptides.

Applying mass spectrometry-based qualitative proteomics to human amygdaloid complex

The amygdaloid complex is a key brain structure involved in the expression of behaviors and emotions such as learning, fear, and anxiety. Brain diseases including depression, epilepsy, autism, schizophrenia, and Alzheimer's disease, have been associated with amygdala dysfunction. For several decades, neuroanatomical, neurophysiological, volumetric, and cognitive approaches have been the gold standard techniques employed to characterize the amygdala functionality. However, little attention has been focused specifically on the molecular composition of the human amygdala from the perspective of proteomics. We have performed a global proteome analysis employing protein and peptide fractionation methods followed by nano-liquid chromatography tandem mass spectrometry (nanoLC-MS/MS), detecting expression of at least 1820 protein species in human amygdala, corresponding to 1814 proteins which represent a nine-fold increase in proteome coverage with respect to previous proteomic profiling of the rat amygdala. Gene ontology analysis were used to determine biological process represented in human amygdala highlighting molecule transport, nucleotide binding, and oxidoreductase and GTPase activities. Bioinformatic analyses have revealed that nearly 4% of identified proteins have been previously associated to neurodegenerative syndromes, and 26% of amygdaloid proteins were also found to be present in cerebrospinal fluid (CSF). In particular, a subset of amygdaloid proteins was mainly involved in axon guidance, synaptic vesicle release, L1CAM interactome, and signaling pathways transduced by NGF and NCAM1. Taken together, our data contributes to the repertoire of the human brain proteome, serving as a reference library to provide basic information for understanding the neurobiology of the human amygdala.

Comparing label-free quantitative peptidomics approaches to characterize diurnal variation of peptides in the rat suprachiasmatic nucleus

Mammalian circadian rhythm is maintained by the suprachiasmatic nucleus (SCN) via an intricate set of neuropeptides and other signaling molecules. In this work, peptidomic analyses from two times of day were examined to characterize variation in SCN peptides using three different label-free quantitation approaches: spectral count, spectra index and SIEVE. Of the 448 identified peptides, 207 peptides were analyzed by two label-free methods, spectral count and spectral index. There were 24 peptides with significant (adjusted p-value < 0.01) differential peptide abundances between daytime and nighttime, including multiple peptides derived from secretogranin II, cocaine and amphetamine regulated transcript, and proprotein convertase subtilisin/kexin type 1 inhibitor. Interestingly, more peptides were analyzable and had significantly different abundances between the two time points using the spectral count and spectral index methods than with a prior analysis using the SIEVE method with the same data. The results of this study reveal the importance of using the appropriate data analysis approaches for label-free relative quantitation of peptides. The detection of significant changes in so rich a set of neuropeptides reflects the dynamic nature of the SCN and the number of influences such as feeding behavior on circadian rhythm. Using spectral count and spectral index, peptide level changes are correlated to time of day, suggesting their key role in circadian function.

Severe obesity and diabetes insipidus in a patient with PCSK1 deficiency

Non-synonymous mutations affecting both alleles of PCSK1 (proprotein convertase 1/3) are associated with obesity and impaired prohormone processing. We report a proband who was compound heterozygous for a maternally inherited frameshift mutation and a paternally inherited 474kb deletion that encompasses PCSK1, representing a novel genetic mechanism underlying this phenotype. Although pro-vasopressin is not a known physiological substrate of PCSK1, the development of central diabetes insipidus in this proband suggests that PCSK1 deficiency can be associated with impaired osmoregulation.

Quantitation of endogenous peptides using mass spectrometry based methods

The mass spectrometry-based 'omics' sub-discipline that focuses on comprehensive, often exploratory, analyses of endogenous peptides involved in cell-to-cell communication is oftentimes referred to as peptidomics. Although the progress in bioanalytical technology development for peptide discovery has been tremendous, perhaps the largest advances have involved robust quantitative mass spectrometric approaches and data mining algorithms. These efforts have accelerated the discovery and validation of biomarkers, functionally important posttranslational modifications, and unexpected molecular interactions, information that aids drug development. In this article we outline the current approaches used in quantitative peptidomics and the technical challenges that stimulate new advances in the field, while also reviewing the newest literature on functional characterizations of endogenous peptides using quantitative mass spectrometry.

The multifaceted proprotein convertases: their unique, redundant, complementary, and opposite functions

The secretory proprotein convertase (PC) family comprises nine members: PC1/3, PC2, furin, PC4, PC5/6, PACE4, PC7, SKI-1/S1P, and PCSK9. The first seven PCs cleave their substrates at single or paired basic residues, and SKI-1/S1P cleaves its substrates at non-basic residues in the Golgi. PCSK9 cleaves itself once, and the secreted inactive protease escorts specific receptors for lysosomal degradation. It regulates the levels of circulating LDL cholesterol and is considered a major therapeutic target in phase III clinical trials. In vivo, PCs exhibit unique and often essential functions during development and/or in adulthood, but certain convertases also exhibit complementary, redundant, or opposite functions.

Recent advances in quantitative neuroproteomics

The field of proteomics is undergoing rapid development in a number of different areas including improvements in mass spectrometric platforms, peptide identification algorithms and bioinformatics. In particular, new and/or improved approaches have established robust methods that not only allow for in-depth and accurate peptide and protein identification and modification, but also allow for sensitive measurement of relative or absolute quantitation. These methods are beginning to be applied to the area of neuroproteomics, but the central nervous system poses many specific challenges in terms of quantitative proteomics, given the large number of different neuronal cell types that are intermixed and that exhibit distinct patterns of gene and protein expression. This review highlights the recent advances that have been made in quantitative neuroproteomics, with a focus on work published over the last five years that applies emerging methods to normal brain function as well as to various neuropsychiatric disorders including schizophrenia and drug addiction as well as of neurodegenerative diseases including Parkinson's disease and Alzheimer's disease. While older methods such as two-dimensional polyacrylamide electrophoresis continued to be used, a variety of more in-depth MS-based approaches including both label (ICAT, iTRAQ, TMT, SILAC, SILAM), label-free (label-free, MRM, SWATH) and absolute quantification methods, are rapidly being applied to neurobiological investigations of normal and diseased brain tissue as well as of cerebrospinal fluid (CSF). While the biological implications of many of these studies remain to be clearly established, that there is a clear need for standardization of experimental design and data analysis, and that the analysis of protein changes in specific neuronal cell types in the central nervous system remains a serious challenge, it appears that the quality and depth of the more recent quantitative proteomics studies is beginning to shed light on a number of aspects of neuroscience that relates to normal brain function as well as of the changes in protein expression and regulation that occurs in neuropsychiatric and neurodegenerative disorders.

Quantitative peptidomics of Purkinje cell degeneration mice

Cytosolic carboxypeptidase 1 (CCP1) is a metallopeptidase that removes C-terminal and side-chain glutamates from tubulin. The Purkinje cell degeneration (pcd) mouse lacks CCP1 due to a mutation. Previously, elevated levels of peptides derived from cytosolic and mitochondrial proteins were found in adult pcd mouse brain, raising the possibility that CCP1 functions in the degradation of intracellular peptides. To test this hypothesis, we used a quantitative peptidomics technique to compare peptide levels in wild-type and pcd mice, examining adult heart, spleen, and brain, and presymptomatic 3 week-old amygdala and cerebellum. Contrary to adult mouse brain, young pcd brain and adult heart and spleen did not show a large increase in levels of intracellular peptides. Unexpectedly, levels of peptides derived from secretory pathway proteins were altered in adult pcd mouse brain. The pattern of changes for the intracellular and secretory pathway peptides in pcd mice was generally similar to the pattern observed in mice lacking primary cilia. Collectively, these results suggest that intracellular peptide accumulation in adult pcd mouse brain is a secondary effect and is not due to a role of CCP1 in peptide turnover.

Analysis of peptides secreted from cultured mouse brain tissue

Peptides represent a major class of cell-cell signaling molecules. Most peptidomic studies have focused on peptides present in brain or other tissues. For a peptide to function in intercellular signaling, it must be secreted. The present study was undertaken to identify the major peptides secreted from mouse brain slices that were cultured in oxygenated buffer for 3-4h. Approximately 75% of the peptides identified in extracts of cultured slices matched the previously reported peptide content of heat-inactivated mouse brain tissue, whereas only 2% matched the peptide content of unheated brain tissue; the latter showed a large number of postmortem changes. As found with extracts of heat-inactivated mouse brain, the extracts of cultured brain slices represented secretory pathway peptides as well as peptides derived from intracellular proteins such as those present in the cytosol and mitochondria. A subset of the peptides detected in the extracts of the cultured slices was detected in the culture media. The vast majority of secreted peptides arose from intracellular proteins and not secretory pathway proteins. The peptide RVD-hemopressin, a CB1 cannabinoid receptor agonist, was detected in culture media, which is consistent with a role for RVD-hemopressin as a non-classical neuropeptide. Taken together with previous studies, the present results show that short-term culture of mouse brain slices is an appropriate system to study peptide secretion, especially the non-conventional pathway(s) by which peptides produced from intracellular proteins are secreted. This article is part of a Special Issue entitled: An Updated Secretome.

Relative quantitation of neuropeptides over a thousand-fold concentration range

Neuropeptides are essential cell-to-cell signaling molecules that influence diverse regulatory and behavioral functions within biological systems. Differing in their amino acid sequences and post-translational modifications, hundreds of neuropeptides are produced via a series of enzymatic processing steps, and their levels vary with location, time, and physiological condition. Due to their wide range of endogenous concentrations and inherent chemical complexity, using mass spectrometry (MS) to accurately quantify changes in peptide levels can be challenging. Here we evaluate three different MS systems for their ability to accurately measure neuropeptide levels: capillary liquid chromatography-electrospray ionization-ion trap (CapLC-ESI-IT) MS, ultraperformance liquid chromatography-electrospray ionization-quadrupole-time-of-flight (UPLC-LC-ESI-Q-TOF) MS, and matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) MS. Specifically, eight sample mixtures composed of five neuropeptide standards, with four technical replicates of each, were labeled with H(4)/D(4)-succinic anhydride, followed by relative peptide quantitation using the three MS platforms. For these samples, the CapLC-ESI-IT MS platform offered the most robust ability to accurately quantify peptides over a concentration range of 1200-fold, although it required larger sample sizes than the other two platforms. Both the UPLC-ESI-Q-TOF MS and the MALDI-TOF MS systems had lower limits of quantification, with the MALDI-TOF having the lowest. By implementing several data acquisition schemes and optimizing the data analysis approaches, we were able to accurately quantify peptides over a three orders of magnitude concentration range using either the UPLC or MALDI-TOF platforms. Overall these results increase our understanding of both the capabilities and limits of using MS-based approaches to measure peptides.

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.

Quantitative peptidomics of mice lacking peptide-processing enzymes

Peptidomics is defined as the analysis of peptides present in a tissue extract, usually using mass spectrometry-based approaches. Unlike radioimmunoassay-based detection techniques, peptidomics measures the precise form of each peptide, including post-translational modifications, and can readily distinguish between longer and shorter forms of the same peptide. Also, peptidomics is not limited to known peptides and can detect hundreds of peptides in a single experiment. Quantitative peptidomics enables comparisons between two or more groups of samples and is perfect for studies examining the effect of gene knockouts on tissue levels of peptides. We describe the method for quantitative peptidomics using isotopic labels based on trimethylammonium butyrate, which can be synthesized in five different isotopic forms; this permits multivariate analysis of five different groups of tissue extracts in a single liquid chromatography/mass spectrometry run.

Disruption of proprotein convertase 1/3 (PC1/3) expression in mice causes innate immune defects and uncontrolled cytokine secretion

The proprotein convertase 1/3 is expressed in the regulated secretory pathway of neural and endocrine cells. Its major function is in the post-translational processing and activation of precursor proteins. The PC1/3 knock-out (KO) mouse model has allowed us to elucidate its physiological functions in studies focused primarily on neuroendocrine tissues. However, PC1/3 is also expressed in cells of the immune system, mainly in macrophages. The present study explores the effects of innate immune challenge in the PC1/3 KO mouse. PC1/3 KO mice have an enlarged spleen with marked disorganization of the marginal zone and red pulp. Immunohistochemical studies using various markers demonstrate a depletion of dendritic cells in PC1/3 KO spleens. When challenged with lipopolysaccharide, PC1/3 KO mice are more susceptible to septic shock than wild-type controls or other PC KO mice, such as PC2 and PC7 null mice. Plasma levels of proinflammatory cytokines (IL-6, IL-1β, and TNF-α) were very significantly elevated in PC1/3 KO mice, consistent with a hypercytokinemia, i.e. indicative of a major systemic uncontrolled inflammatory response or cytokine storm. Peritoneal macrophages isolated from PC1/3 KO mice also demonstrate elevated cytokine secretion when treated with LPS. Electron micrographs show morphological features indicating a prolonged activation of these cells following LPS stimulation. We also present evidence that the proinflammatory T(h)1 pathway is dominant in the PC1/3 KO mouse model. We conclude that aside from its important role in neuroendocrine functions PC1/3 also has an important role in the regulation of the innate immune system, most likely through the regulation of cytokine secretion in macrophages.

Heterozygous mutations causing partial prohormone convertase 1 deficiency contribute to human obesity

Null mutations in the PCSK1 gene, encoding the proprotein convertase 1/3 (PC1/3), cause recessive monogenic early onset obesity. Frequent coding variants that modestly impair PC1/3 function mildly increase the risk for common obesity. The aim of this study was to determine the contribution of rare functional PCSK1 mutations to obesity. PCSK1 exons were sequenced in 845 nonconsanguineous extremely obese Europeans. Eight novel nonsynonymous PCSK1 mutations were identified, all heterozygous. Seven mutations had a deleterious effect on either the maturation or the enzymatic activity of PC1/3 in cell lines. Of interest, five of these novel mutations, one of the previously described frequent variants (N221D), and the mutation found in an obese mouse model (N222D), affect residues at or near the structural calcium binding site Ca-1. The prevalence of the newly identified mutations was assessed in 6,233 obese and 6,274 lean European adults and children, which showed that carriers of any of these mutations causing partial PCSK1 deficiency had an 8.7-fold higher risk to be obese than wild-type carriers. These results provide the first evidence of an increased risk of obesity in heterozygous carriers of mutations in the PCSK1 gene. Furthermore, mutations causing partial PCSK1 deficiency are present in 0.83% of extreme obesity phenotypes.

Profiling protease activities by dynamic proteomics workflows

Proteases play prominent roles in many physiological processes and the pathogenesis of various diseases, which makes them interesting drug targets. To fully understand the functional role of proteases in these processes, it is necessary to characterize the target specificity of the enzymes, identify endogenous substrates and cleavage products as well as protease activators and inhibitors. The complexity of these proteolytic networks presents a considerable analytic challenge. To comprehensively characterize these systems, quantitative methods that capture the spatial and temporal distributions of the network members are needed. Recently, activity-based workflows have come to the forefront to tackle the dynamic aspects of proteolytic processing networks in vitro, ex vivo and in vivo. In this review, we will discuss how mass spectrometry-based approaches can be used to gain new insights into protease biology by determining substrate specificities, profiling the activity-states of proteases, monitoring proteolysis in vivo, measuring reaction kinetics and defining in vitro and in vivo proteolytic events. In addition, examples of future aspects of protease research that go beyond mass spectrometry-based applications are given.

ProSAAS-derived peptides are colocalized with neuropeptide Y and function as neuropeptides in the regulation of food intake

ProSAAS is the precursor of a number of peptides that have been proposed to function as neuropeptides. Because proSAAS mRNA is highly expressed in the arcuate nucleus of the hypothalamus, we examined the cellular localization of several proSAAS-derived peptides in the mouse hypothalamus and found that they generally colocalized with neuropeptide Y (NPY), but not α-melanocyte stimulating hormone. However, unlike proNPY mRNA, which is upregulated by food deprivation in the mediobasal hypothalamus, neither proSAAS mRNA nor proSAAS-derived peptides were significantly altered by 1–2 days of food deprivation in wild-type mice. Furthermore, while proSAAS mRNA levels in the mediobasal hypothalamus were significantly lower in Cpe^fat/fat mice as compared to wild-type littermates, proNPY mRNA levels in the mediobasal hypothalamus and in other subregions of the hypothalamus were not significantly different between wild-type and Cpe^fat/fat mice. Intracerebroventricular injections of antibodies to two proSAAS-derived peptides (big LEN and PEN) significantly reduced food intake in fasted mice, while injections of antibodies to two other proSAAS-derived peptides (little LEN and little SAAS) did not. Whole-cell patch clamp recordings of parvocellular neurons in the hypothalamic paraventricular nucleus, a target of arcuate NPY projections, showed that big LEN produced a rapid and reversible inhibition of synaptic glutamate release that was spike independent and abolished by blocking postsynaptic G protein activity, suggesting the involvement of a postsynaptic G protein-coupled receptor and the release of a retrograde synaptic messenger. Taken together with previous studies, these findings support a role for proSAAS-derived peptides such as big LEN as neuropeptides regulating food intake.

Peptidomic approaches to study proteolytic activity

Peptidomics, the analysis of the peptide content of cells or tissues, can be used to study proteases in several ways. First, nearly all of the peptides detected in cells and tissues are proteolytic fragments of proteins. Analysis of the peptides therefore provides information regarding the proteolytic activities that occurred to generate the observed peptides. The use of quantitative peptidomic approaches allows the comparison of relative peptide levels in two or more different samples, which enables studies examining the consequences of increasing proteolytic activity (by enzyme activation or overexpression) or reducing proteolytic activity (by inhibition, knock down, or knock out). Quantitative peptidomics can also be used to directly test the cleavage specificity of purified proteases. For this, peptides are purified from the tissue or cell line of interest, incubated in the presence of various amounts of protease or in the absence of protease, and then analyzed by the quantitative peptidomics approach. This reveals which peptides are preferred substrates, which are products, and which are not cleaved. Collectively, these studies complement conventional approaches to study proteolytic activity and allow for a more complete understanding of an enzyme's substrate specificity. This unit describes the use of quantitative peptidomics in the analysis of the biological peptidome as well as in the in vitro analysis of peptidase activity.

On the cutting edge of proprotein convertase pharmacology: from molecular concepts to clinical applications

There is increasing interest in the therapeutic targeting of proteases for the treatment of important diseases. Additionally new protein-based therapeutic strategies have the potential to widen the available treatments against these pathologies. In the last decade, accumulated evidence has confirmed that the family of proteases known as proprotein convertases (PCs) are potential targets for viral infections, osteoarthritis, cancer and cardiovascular disease, among others. Nevertheless, there are still many unanswered questions about the relevance of targeting PCs in a therapeutic context, especially regarding the anticipated secondary effects of treatment, considering the observed embryonic lethality of some PC knockout mice. In this review, the benefits of PCs as pharmacological targets will be discussed, with focus on concepts and strategies, as well as on the state of advancement of actual and future inhibitors.

Direct cellular peptidomics of hypothalamic neurons

The chemical complexity of cell-to-cell communication has emerged as a fundamental challenge to understanding brain systems. This is certainly true for the hypothalamus, where neuropeptide signals are heterogeneous, localized and dynamic. Thus far, most hypothalamic peptidomic studies have centered on the entire structure; however, recent advances in collection strategies and analytical technologies have enabled direct, high-resolution peptidomic profiles focused on two regions of interest, the suprachiasmatic and supraoptic nuclei, including their sub-regions and individual cells. Suites of peptides now can be identified and probed for function. High spatial and analytical sensitivities reveal that discrete hypothalamic nuclei have distinct peptidomic signatures. Peptidomic discovery not only reveals unanticipated complexity, but also peptides previously unknown that act as key circuit components. Analysis of tissue releasates identifies peptides secreted into the extracellular environment and available for transmitting intercellular signals. Direct sampling techniques define peptide-releasate profiles in spatial, temporal and event-dependent patterns. These approaches are providing remarkable new insights into the complexity of neuropeptidergic cell-to-cell signaling central to neuroendocrine physiology.

Cysteine Cathepsins in the secretory vesicle produce active peptides: Cathepsin L generates peptide neurotransmitters and cathepsin B produces beta-amyloid of Alzheimer's disease

Recent new findings indicate significant biological roles of cysteine cathepsin proteases in secretory vesicles for production of biologically active peptides. Notably, cathepsin L in secretory vesicles functions as a key protease for proteolytic processing of proneuropeptides (and prohormones) into active neuropeptides that are released to mediate cell-cell communication in the nervous system for neurotransmission. Moreover, cathepsin B in secretory vesicles has been recently identified as a β-secretase for production of neurotoxic β- amyloid (Aβ) peptides that accumulate in Alzheimer's disease (AD), participating as a notable factor in the severe memory loss in AD. These secretory vesicle functions of cathepsins L and B for production of biologically active peptides contrast with the well-known role of cathepsin proteases in lysosomes for the degradation of proteins to result in their inactivation. The unique secretory vesicle proteome indicates proteins of distinct functional categories that provide the intravesicular environment for support of cysteine cathepsin functions. Features of the secretory vesicle protein systems insure optimized intravesicular conditions that support the proteolytic activity of cathepsins. These new findings of recently discovered biological roles of cathepsins L and B indicate their significance in human health and disease. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Unique biological function of cathepsin L in secretory vesicles for biosynthesis of neuropeptides

Neuropeptides are essential for cell-cell communication in the nervous and neuroendocrine systems. Production of active neuropeptides requires proteolytic processing of proneuropeptide precursors in secretory vesicles that produce, store, and release neuropeptides that regulate physiological functions. This review describes recent findings indicating the prominent role of cathepsin L in secretory vesicles for production of neuropeptides from their protein precursors. The role of cathepsin L in neuropeptide production was discovered using the strategy of activity-based probes for proenkephalin-cleaving activity for identification of the enzyme protein by mass spectrometry. The novel role of cathepsin L in secretory vesicles for neuropeptide production has been demonstrated in vivo by cathepsin L gene knockout studies, cathepsin L gene expression in neuroendocrine cells, and notably, cathepsin L localization in neuropeptide-containing secretory vesicles. Cathepsin L is involved in producing opioid neuropeptides consisting of enkephalin, β-endorphin, and dynorphin, as well as in generating the POMC-derived peptide hormones ACTH and α-MSH. In addition, NPY, CCK, and catestatin neuropeptides utilize cathepsin L for their biosynthesis. The neuropeptide-synthesizing functions of cathepsin L represent its unique activity in secretory vesicles, which contrasts with its role in lysosomes. Interesting evaluations of protease gene knockout studies in mice that lack cathepsin L compared to those lacking PC1/3 and PC2 (PC, prohormone convertase) indicate the key role of cathepsin L in neuropeptide production. Therefore, dual cathepsin L and prohormone convertase protease pathways participate in neuropeptide production. Significantly, the recent new findings indicate cathepsin L as a novel 'proprotein convertase' for production of neuropeptides that mediate cell-cell communication in health and disease.