Induction of integral membrane PAM expression in AtT-20 cells alters the storage and trafficking of POMC and PC1

Germline loss-of-function PAM variants are enriched in subjects with pituitary hypersecretion

Introduction

Pituitary adenomas (PAs) are common, usually benign tumors of the anterior pituitary gland which, for the most part, have no known genetic cause. PAs are associated with major clinical effects due to hormonal dysregulation and tumoral impingement on vital brain structures. PAM encodes a multifunctional protein responsible for the essential C-terminal amidation of secreted peptides.

Methods

Following the identification of a loss-of-function variant (p.Arg703Gln) in the peptidylglycine a-amidating monooxygenase (PAM) gene in a family with pituitary gigantism, we investigated 299 individuals with sporadic PAs and 17 familial isolated PA kindreds for PAM variants. Genetic screening was performed by germline and tumor sequencing and germline copy number variation (CNV) analysis.

Results

In germline DNA, we detected seven heterozygous, likely pathogenic missense, truncating, and regulatory SNVs. These SNVs were found in sporadic subjects with growth hormone excess (p.Gly552Arg and p.Phe759Ser), pediatric Cushing disease (c.-133T>C and p.His778fs), or different types of PAs (c.-361G>A, p.Ser539Trp, and p.Asp563Gly). The SNVs were functionally tested in vitro for protein expression and trafficking by Western blotting, splicing by minigene assays, and amidation activity in cell lysates and serum samples. These analyses confirmed a deleterious effect on protein expression and/or function. By interrogating 200,000 exomes from the UK Biobank, we confirmed a significant association of the PAM gene and rare PAM SNVs with diagnoses linked to pituitary gland hyperfunction.

Conclusion

The identification of PAM as a candidate gene associated with pituitary hypersecretion opens the possibility of developing novel therapeutics based on altering PAM function.

Germline loss-of-function PAM variants are enriched in subjects with pituitary hypersecretion

Pituitary adenomas (PAs) are common, usually benign tumors of the anterior pituitary gland which, for the most part, have no known genetic cause. PAs are associated with major clinical effects due to hormonal dysregulation and tumoral impingement on vital brain structures. Following the identification of a loss-of-function variant (p.Arg703Gln) in the PAM gene in a family with pituitary gigantism, we investigated 299 individuals with sporadic PAs and 17 familial isolated pituitary adenomas kindreds for PAM variants. PAM encodes a multifunctional protein responsible for the essential C-terminal amidation of secreted peptides. Genetic screening was performed by germline and tumor sequencing and germline copy number variation (CNV) analysis. No germline CNVs or somatic single nucleotide variants (SNVs) were identified. We detected seven likely pathogenic heterozygous missense, truncating, and regulatory SNVs. These SNVs were found in sporadic subjects with GH excess (p.Gly552Arg and p.Phe759Ser), pediatric Cushing disease (c.-133T>C and p.His778fs), or with different types of PAs (c.-361G>A, p.Ser539Trp, and p.Asp563Gly). The SNVs were functionally tested in vitro for protein expression and trafficking by Western blotting, for splicing by minigene assays, and for amidation activity in cell lysates and serum samples. These analyses confirmed a deleterious effect on protein expression and/or function. By interrogating 200,000 exomes from the UK Biobank, we confirmed a significant association of the PAM gene and rare PAM SNVs to diagnoses linked to pituitary gland hyperfunction. Identification of PAM as a candidate gene associated with pituitary hypersecretion opens the possibility of developing novel therapeutics based on altering PAM function.

PAM: diverse roles in neuroendocrine cells, cardiomyocytes, and green algae

Our understanding of the ways in which peptides are used for communication in the nervous and endocrine systems began with the identification of oxytocin, vasopressin, and insulin, each of which is stored in electron-dense granules, ready for release in response to an appropriate stimulus. For each of these peptides, entry of its newly synthesized precursor into the ER lumen is followed by transport through the secretory pathway, exposing the precursor to a sequence of environments and enzymes that produce the bioactive products stored in mature granules. A final step in the biosynthesis of many peptides is C-terminal amidation by peptidylglycine α-amidating monooxygenase (PAM), an ascorbate- and copper-dependent membrane enzyme that enters secretory granules along with its soluble substrates. Biochemical and cell biological studies elucidated the highly conserved mechanism for amidated peptide production and raised many questions about PAM trafficking and the effects of PAM on cytoskeletal organization and gene expression. Phylogenetic studies and the discovery of active PAM in the ciliary membranes of Chlamydomonas reinhardtii, a green alga lacking secretory granules, suggested that a PAM-like enzyme was present in the last eukaryotic common ancestor. While the catalytic features of human and C. reinhardtii PAM are strikingly similar, the trafficking of PAM in C. reinhardtii and neuroendocrine cells and secretion of its amidated products differ. A comparison of PAM function in neuroendocrine cells, atrial myocytes, and C. reinhardtii reveals multiple ways in which altered trafficking allows PAM to accomplish different tasks in different species and cell types.

Multiple roles for peptidylglycine α-amidating monooxygenase in the response to hypoxia

The biosynthesis of many of the peptides involved in homeostatic control requires peptidylglycine α-amidating monooxygenase (PAM), an ancient, highly conserved copper- and ascorbate-dependent enzyme. Using the production of amidated chromogranin A to monitor PAM function in tumor cells, physiologically relevant levels of hypoxia were shown to inhibit this monooxygenase. The ability of primary pituitary cells exposed to hypoxic conditions for 4 h to produce amidated chromogranin A was similarly inhibited. The affinity of the purified monooxygenase for oxygen (K_m  = 99 ± 19 μM) was consistent with this result. The ability of PAM to alter secretory pathway behavior under normoxic conditions required its monooxygenase activity. Under normoxic conditions, hypoxia-inducible factor 1a levels in dense cultures of corticotrope tumor cells expressing high levels of PAM exceeded those in control cells; expression of inactive monooxygenase did not have this effect. The effects of hypoxia on levels of two PAM-regulated genes (activating transcription factor 3 [Atf3] and FK506 binding protein 2 [Fkbp2]) differed in cells expressing high versus low levels of PAM. Putative hypoxia response elements occur in both human and mouse PAM, and hPAM has consistently been identified as one of the genes upregulated in response to hypoxia. Expression of PAM is also known to alter gene expression. A quarter of the genes consistently upregulated in response to hypoxia were downregulated following increased expression of PAM. Taken together, our data suggest roles for PAM and amidated peptide secretion in the coordination of tissue-specific responses to hypoxia.

Peptidylglycine α-amidating monooxygenase is required for atrial secretory granule formation

The discovery of atrial secretory granules and the natriuretic peptides stored in them identified the atrium as an endocrine organ. Although neither atrial nor brain natriuretic peptide (ANP, BNP) is amidated, the major membrane protein in atrial granules is peptidylglycine α-amidating monooxygenase (PAM), an enzyme essential for amidated peptide biosynthesis. Mice lacking cardiomyocyte PAM (Pam ^Myh6-cKO/cKO) are viable, but a gene dosage-dependent drop in atrial ANP and BNP content occurred. Ultrastructural analysis of adult Pam ^Myh6-cKO/cKO atria revealed a 13-fold drop in the number of secretory granules. When primary cultures of Pam ^{0-Cre-cKO/cKO} atrial myocytes (no Cre recombinase, PAM floxed) were transduced with Cre-GFP lentivirus, PAM protein levels dropped, followed by a decline in ANP precursor (proANP) levels. Expression of exogenous PAM in Pam ^Myh6-cKO/cKO atrial myocytes produced a dose-dependent rescue of proANP content; strikingly, this response did not require the monooxygenase activity of PAM. Unlike many prohormones, atrial proANP is stored intact. A threefold increase in the basal rate of proANP secretion by Pam ^Myh6-cKO/cKO myocytes was a major contributor to its reduced levels. While proANP secretion was increased following treatment of control cultures with drugs that block the activation of Golgi-localized Arf proteins and COPI vesicle formation, proANP secretion by Pam ^Myh6-cKO/cKO myocytes was unaffected. In cells lacking secretory granules, expression of exogenous PAM led to the accumulation of fluorescently tagged proANP in the cis-Golgi region. Our data indicate that COPI vesicle-mediated recycling of PAM from the cis-Golgi to the endoplasmic reticulum plays an essential role in the biogenesis of proANP containing atrial granules.

PERK-mediated expression of peptidylglycine α-amidating monooxygenase supports angiogenesis in glioblastoma

PKR-like kinase (PERK) plays a significant role in inducing angiogenesis in various cancer types including glioblastoma. By proteomics analysis of the conditioned medium from a glioblastoma cell line treated with a PERK inhibitor, we showed that peptidylglycine α-amidating monooxygenase (PAM) expression is regulated by PERK under hypoxic conditions. Moreover, PERK activation via CCT020312 (a PERK selective activator) increased the cleavage and thus the generation of PAM cleaved cytosolic domain (PAM sfCD) that acts as a signaling molecule from the cytoplasm to the nuclei. PERK was also found to interact with PAM, suggesting a possible involvement in the generation of PAM sfCD. Knockdown of PERK or PAM reduced the formation of tubes by HUVECs in vitro. Furthermore, in vivo data highlighted the importance of PAM in the growth of glioblastoma with reduction of PAM expression in engrafted tumor significantly increasing the survival in mice. In summary, our data revealed PAM as a potential target for antiangiogenic therapy in glioblastoma.

Ciliary and cytoskeletal functions of an ancient monooxygenase essential for bioactive amidated peptide synthesis

Many secreted peptides used for cell-cell communication require conversion of a C-terminal glycine to an amide for bioactivity. This reaction is catalyzed only by the integral membrane protein peptidylglycine α-amidating monooxygenase (PAM). PAM has been highly conserved and is found throughout the metazoa; PAM-like sequences are also present in choanoflagellates, filastereans, unicellular and colonial chlorophyte green algae, dinoflagellates and haptophytes. Recent studies have revealed that in addition to playing a key role in peptidergic signaling, PAM also regulates ciliogenesis in vertebrates, planaria and chlorophyte algae, and is required for the stability of actin-based microvilli. Here we briefly introduce the basic principles involved in ciliogenesis, the sequential reactions catalyzed by PAM and the trafficking of PAM through the secretory and endocytic pathways. We then discuss the multi-faceted roles this enzyme plays in the formation and maintenance of cytoskeleton-based cellular protrusions and propose models for how PAM protein and amidating activity might contribute to ciliogenesis. Finally, we consider why some ciliated organisms lack PAM, and discuss the potential ramifications of ciliary localized PAM for the endocrine features commonly observed in patients with ciliopathies.

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.

Changes in Corticotrope Gene Expression Upon Increased Expression of Peptidylglycine α-Amidating Monooxygenase

Throughout evolution, secretion has played an essential role in the ability of organisms and single cells to survive in the face of a changing environment. Peptidylglycine α-amidating monooxygenase (PAM) is an integral membrane monooxygenase, first identified for its role in the biosynthesis of neuroendocrine peptides released by the regulated secretory pathway. PAM was subsequently identified in Chlamydomonas reinhardtii, a unicellular green alga, where it plays an essential role in constitutive secretion and in ciliogenesis. Reduced expression of C. reinhardtii PAM resulted in significant changes in secretion and ciliogenesis. Hence, a screen was performed for transcripts and proteins whose expression responded to changes in PAM levels in a mammalian corticotrope tumor cell line. The goal was to identify genes not previously known to play a role in secretion. The screen identified transcription factors, peptidyl prolyl isomerases, endosomal/lysosomal proteins, and proteins involved in tissue-specific responses to glucose and amino acid availability that had not previously been recognized as relevant to the secretory pathway. Perhaps reflecting the dependence of PAM on molecular oxygen, many PAM-responsive genes are known to be hypoxia responsive. The data highlight the extent to which the performance of the secretory pathway may be integrated into a wide diversity of signaling pathways.

Microvillar and ciliary defects in zebrafish lacking an actin-binding bioactive peptide amidating enzyme

The assembly of membranous extensions such as microvilli and cilia in polarized cells is a tightly regulated, yet poorly understood, process. Peptidylglycine α-amidating monooxygenase (PAM), a membrane enzyme essential for the synthesis of amidated bioactive peptides, was recently identified in motile and non-motile (primary) cilia and has an essential role in ciliogenesis in Chlamydomonas, Schmidtea and mouse. In mammalian cells, changes in PAM levels alter secretion and organization of the actin cytoskeleton. Here we show that lack of Pam in zebrafish recapitulates the lethal edematous phenotype observed in Pam -/- mice and reveals additional defects. The pam -/- zebrafish embryos display an initial striking loss of microvilli and subsequently impaired ciliogenesis in the pronephros. In multiciliated mouse tracheal epithelial cells, vesicular PAM staining colocalizes with apical actin, below the microvilli. In PAM-deficient Chlamydomonas, the actin cytoskeleton is dramatically reorganized, and expression of an actin paralogue is upregulated. Biochemical assays reveal that the cytosolic PAM C-terminal domain interacts directly with filamentous actin but does not alter the rate of actin polymerization or disassembly. Our results point to a critical role for PAM in organizing the actin cytoskeleton during development, which could in turn impact both microvillus formation and ciliogenesis.

Anterior Pituitary Transcriptome Suggests Differences in ACTH Release in Tame and Aggressive Foxes

Domesticated species exhibit a suite of behavioral, endocrinological, and morphological changes referred to as "domestication syndrome." These changes may include a reduction in reactivity of the hypothalamic-pituitary-adrenal (HPA) axis and specifically reduced adrenocorticotropic hormone release from the anterior pituitary. To investigate the biological mechanisms targeted during domestication, we investigated gene expression in the pituitaries of experimentally domesticated foxes (Vulpes vulpes). RNA was sequenced from the anterior pituitary of six foxes selectively bred for tameness ("tame foxes") and six foxes selectively bred for aggression ("aggressive foxes"). Expression, splicing, and network differences identified between the two lines indicated the importance of genes related to regulation of exocytosis, specifically mediated by cAMP, organization of pseudopodia, and cell motility. These findings provide new insights into biological mechanisms that may have been targeted when these lines of foxes were selected for behavior and suggest new directions for research into HPA axis regulation and the biological underpinnings of domestication.

POMC Neurons: From Birth to Death

The hypothalamus is an evolutionarily conserved brain structure that regulates an organism's basic functions, such as homeostasis and reproduction. Several hypothalamic nuclei and neuronal circuits have been the focus of many studies seeking to understand their role in regulating these basic functions. Within the hypothalamic neuronal populations, the arcuate melanocortin system plays a major role in controlling homeostatic functions. The arcuate pro-opiomelanocortin (POMC) neurons in particular have been shown to be critical regulators of metabolism and reproduction because of their projections to several brain areas both in and outside of the hypothalamus, such as autonomic regions of the brain stem and spinal cord. Here, we review and discuss the current understanding of POMC neurons from their development and intracellular regulators to their physiological functions and pathological dysregulation.

A bioactive peptide amidating enzyme is required for ciliogenesis

The pathways controlling cilium biogenesis in different cell types have not been fully elucidated. We recently identified peptidylglycine α-amidating monooxygenase (PAM), an enzyme required for generating amidated bioactive signaling peptides, in Chlamydomonas and mammalian cilia. Here, we show that PAM is required for the normal assembly of motile and primary cilia in Chlamydomonas, planaria and mice. Chlamydomonas PAM knockdown lines failed to assemble cilia beyond the transition zone, had abnormal Golgi architecture and altered levels of cilia assembly components. Decreased PAM gene expression reduced motile ciliary density on the ventral surface of planaria and resulted in the appearance of cytosolic axonemes lacking a ciliary membrane. The architecture of primary cilia on neuroepithelial cells in Pam^-/- mouse embryos was also aberrant. Our data suggest that PAM activity and alterations in post-Golgi trafficking contribute to the observed ciliogenesis defects and provide an unanticipated, highly conserved link between PAM, amidation and ciliary assembly.

Deletion of Protein Kinase C λ in POMC Neurons Predisposes to Diet-Induced Obesity

Effectors of the phosphoinositide 3-kinase (PI3K) signal transduction pathway contribute to the hypothalamic regulation of energy and glucose homeostasis in divergent ways. Here we show that central nervous system (CNS) action of the PI3K signaling intermediate atypical protein kinase C (aPKC) constrains food intake, weight gain, and glucose intolerance in both rats and mice. Pharmacological inhibition of CNS aPKC activity acutely increases food intake and worsens glucose tolerance in chow-fed rodents and causes excess weight gain during high-fat diet (HFD) feeding. Similarly, selective deletion of the aPKC isoform Pkc-λ in proopiomelanocortin (POMC) neurons disrupts leptin action, reduces melanocortin content in the paraventricular nucleus, and markedly increases susceptibility to obesity, glucose intolerance, and insulin resistance specifically in HFD-fed male mice. These data implicate aPKC as a novel regulator of energy and glucose homeostasis downstream of the leptin-PI3K pathway in POMC neurons.

O-Glycosylation of a Secretory Granule Membrane Enzyme Is Essential for Its Endocytic Trafficking

Peptidylglycine α-amidating monooxygenase (PAM) (EC 1.14.17.3) catalyzes peptide amidation, a crucial post-translational modification, through the sequential actions of its monooxygenase (peptidylglycine α-hydroxylating monooxygenase) and lyase (peptidyl-α-hydroxyglycine α-amidating lyase (PAL)) domains. Alternative splicing generates two different regions that connect the protease-resistant catalytic domains. Inclusion of exon 16 introduces a pair of Lys residues, providing a site for controlled endoproteolytic cleavage of PAM and the separation of soluble peptidylglycine α-hydroxylating monooxygenase from membrane-associated PAL. Exon 16 also includes two O-glycosylation sites. PAM-1 lacking both glycosylation sites (PAM-1/OSX; where OSX is O-glycan-depleted mutant of PAM-1) was stably expressed in AtT-20 corticotrope tumor cells. In PAM-1/OSX, a cleavage site for furin-like convertases was exposed, generating a shorter form of membrane-associated PAL. The endocytic trafficking of PAM-1/OSX differed dramatically from that of PAM-1. A soluble fragment of the cytosolic domain of PAM-1 was produced in the endocytic pathway and entered the nucleus; very little soluble fragment of the cytosolic domain was produced from PAM-1/OSX. Internalized PAM-1/OSX was rapidly degraded; unlike PAM-1, very little internalized PAM-1/OSX was detected in multivesicular bodies. Blue native PAGE analysis identified high molecular weight complexes containing PAM-1; the ability of PAM-1/OSX to form similar complexes was markedly diminished. By promoting the formation of high molecular weight complexes, O-glycans may facilitate the recycling of PAM-1 through the endocytic compartment.

Early eukaryotic origins for cilia-associated bioactive peptide-amidating activity

Ciliary axonemes and basal bodies were present in the last eukaryotic common ancestor and play crucial roles in sensing and responding to environmental cues. Peptidergic signaling, generally considered a metazoan innovation, is essential for organismal development and homeostasis. Peptidylglycine α-amidating monooxygenase (PAM) is crucial for the last step of bioactive peptide biosynthesis. However, identification of a complete PAM-like gene in green algal genomes suggests ancient evolutionary roots for bioactive peptide signaling. We demonstrate that the Chlamydomonas reinhardtii PAM gene encodes an active peptide-amidating enzyme (CrPAM) that shares key structural and functional features with the mammalian enzyme, indicating that components of the peptide biosynthetic pathway predate multicellularity. In addition to its secretory pathway localization, CrPAM localizes to cilia and tightly associates with the axonemal superstructure, revealing a new axonemal enzyme activity. This localization pattern is conserved in mammals, with PAM present in both motile and immotile sensory cilia. The conserved ciliary localization of PAM adds to the known signaling capabilities of the eukaryotic cilium and provides a potential mechanistic link between peptidergic signaling and endocrine abnormalities commonly observed in ciliopathies.

Optimizing production of Fc-amidated peptides by Chinese hamster ovary cells

BACKGROUND

Amidation of the carboxyl terminal of many peptides is essential for full biological potency, often increasing receptor binding and stability. The single enzyme responsible for this reaction is peptidylglycine α-amidating monooxygenase (PAM: EC 1.14.17.3), a copper- and ascorbate-dependent Type I membrane protein.

METHODS

To make large amounts of high molecular weight amidated product, Chinese hamster ovary (CHO) cells were engineered to express exogenous PAM. To vary access of the enzyme to its substrate, exogenous PAM was targeted to the endoplasmic reticulum, trans-Golgi network, endosomes and lysosomes or to the lumen of the secretory pathway.

RESULTS

PAM was equally active when targeted to each intracellular location and assayed in homogenates. Immunocytochemical analyses of CHO cells and a pituitary cell line demonstrated that targeting of exogenous PAM was partially successful. PAM substrates generated by expressing peptidylglycine substrates (glucagon-like peptide 1-Gly, peptide YY-Gly and neuromedin U-Gly) fused to the C-terminus of immunoglobulin Fc in CHO cell lines producing targeted PAM. The extent of amidation of the Fc-peptides was determined by mass spectrometry and amidation-specific enzyme immunoassays. Amidation was inhibited by copper chelation, but was not enhanced by the addition of additional copper or ascorbate.

CONCLUSIONS

Peptide amidation was increased over endogenous levels by exogenous PAM, and targeting PAM to the endoplasmic reticulum or trans-Golgi network increased peptide amidation compared to endogenous CHO PAM.

Secretion of Fc-amidated peptide fusion proteins by Chinese hamster ovary cells

Background

The therapeutic use of α-amidated peptides (e.g. calcitonin, glucagon-like peptide) has increased dramatically, but there are major impediments to wider use of such peptides. Larger peptides are expensive to synthesize, and short plasma half-lives frequently limit the clinical circumstances in which the peptides would be useful. Both problems are potentially solved by producing peptides as fusions with the Fc region of human immunoglobulin.

Methods

Glucagon-like peptide 1 (GLP1), peptide YY (PYY) and neuromedin U (NMU) were expressed and purified from stable CHO lines; since the α-amide group is essential for full biological potency of many peptides, Fc-fusion peptides were expressed in CHO lines stably expressing the α-amidating enzyme, peptidylglycine α-amidating monooxygenase (PAM: EC 1.14.17.3). Purified fusion proteins were analyzed intact and after HRV3C rhinovirus protease cleavage, at a site in the linker separating the Fc region from the peptide, by mass spectrometry and amide-specific immunoassays.

Results

The Fc fusions were expressed at 1–2.5 μg/mg cell protein and secreted at 5-20 % of cell content per hour, in a peptide-specific manner. CHO cells express measurable endogenous PAM activity, amidating 25 % of Fc-PYY and almost 90 % of Fc-GLP1. Expression of exogenous PAM increased the level of peptide amidation to 50 % of Fc-PYY and 95 % of Fc-NMU. The Fc-GLP1 fusions were 10,000-fold less active than synthetic GLP1 in a cell-receptor cyclic AMP-based assay, as expected since the amino terminal of GLP1 is essential for full biological activity. The Fc-PYY fusions were 100-fold less active than PYY-NH₂ but 10-fold more active than non-amidated PYY-Gly.

Conclusions

This type of approach can be used for the production of stabilized α-amidated peptides aimed at clinical trials.

AP-1A controls secretory granule biogenesis and trafficking of membrane secretory granule proteins

The adaptor protein 1A complex (AP-1A) transports cargo between the trans-Golgi network (TGN) and endosomes. In professional secretory cells, AP-1A also retrieves material from immature secretory granules (SGs). The role of AP-1A in SG biogenesis was explored using AtT-20 corticotrope tumor cells expressing reduced levels of the AP-1A μ1A subunit. A twofold reduction in μ1A resulted in a decrease in TGN cisternae and immature SGs and the appearance of regulated secretory pathway components in non-condensing SGs. Although basal secretion of endogenous SG proteins was unaffected, secretagogue-stimulated release was halved. The reduced μ1A levels interfered with the normal trafficking of carboxypeptidase D (CPD) and peptidylglycine α-amidating monooxygenase-1 (PAM-1), integral membrane enzymes that enter immature SGs. The non-condensing SGs contained POMC products and PAM-1, but not CPD. Based on metabolic labeling and secretion experiments, the cleavage of newly synthesized PAM-1 into PHM was unaltered, but PHM basal secretion was increased in sh-μ1A PAM-1 cells. Despite lacking a canonical AP-1A binding motif, yeast two-hybrid studies demonstrated an interaction between the PAM-1 cytosolic domain and AP-1A. Coimmunoprecipitation experiments with PAM-1 mutants revealed an influence of the luminal domains of PAM-1 on this interaction. Thus, AP-1A is crucial for normal SG biogenesis, function and composition.

Elimination of Kalrn expression in POMC cells reduces anxiety-like behavior and contextual fear learning

Kalirin, a Rho GDP/GTP exchange factor for Rac1 and RhoG, is known to play an essential role in the formation and maintenance of excitatory synapses and in the secretion of neuropeptides. Mice unable to express any of the isoforms of Kalrn in cells that produce POMC at any time during development (POMC cells) exhibited reduced anxiety-like behavior and reduced acquisition of passive avoidance behavior, along with sex-specific alteration in the corticosterone response to restraint stress. Strikingly, lack of Kalrn expression in POMC cells closely mimicked the effects of global Kalrn knockout on anxiety-like behavior and passive avoidance conditioning without causing the other deficits noted in Kalrn knockout mice. Our data suggest that deficits in excitatory inputs onto POMC neurons are responsible for the behavioral phenotypes observed.

A histidine-rich linker region in peptidylglycine α-amidating monooxygenase has the properties of a pH sensor

Decreasing luminal pH is thought to play a role in the entry of newly synthesized and endocytosed membrane proteins into secretory granules. The two catalytic domains of peptidylglycine α-amidating monooxygenase (PAM), a type I integral membrane protein, catalyze the sequential reactions that convert peptidyl-Gly substrates into amidated products. We explored the hypothesis that a conserved His-rich cluster (His-Gly-His-His) in the linker region connecting its two catalytic domains senses pH and affects PAM trafficking by mutating these His residues to Ala (Ala-Gly-Ala-Ala; H3A). Purified recombinant wild-type and H3A linker peptides were examined using circular dichroism and tryptophan fluorescence; mutation of the His cluster largely eliminated its pH sensitivity. An enzymatically active PAM protein with the same mutations (PAM-1/H3A) was expressed in HEK293 cells and AtT-20 corticotrope tumor cells. Metabolic labeling followed by immunoprecipitation revealed more rapid loss of newly synthesized PAM-1/H3A than PAM-1; although release of newly synthesized monofunctional PHM/H3A was increased, release of soluble bifunctional PAM/H3A, a product of the endocytic pathway, was decreased. Surface biotinylation revealed rapid loss of PAM-1/H3A, with no detectable return of the mutant protein to secretory granules. Consistent with its altered endocytic trafficking, little PAM-1/H3A was subjected to regulated intramembrane proteolysis followed by release of a small nuclear-targeted cytosolic fragment. AtT-20 cells expressing PAM-1/H3A adopted the morphology of wild-type AtT-20 cells; secretory products no longer accumulated in the trans-Golgi network and secretory granule exocytosis was more responsive to secretagogue.

The many faces of the guanine-nucleotide exchange factor trio

Small Rho-GTPases are enzymes that are bound to GDP or GTP, which determines their inactive or active state, respectively. The exchange of GDP for GTP is catalyzed by so-called Rho-guanine nucleotide exchange factors (GEFs). Rho-GEFs are characterized by a Dbl-homology (DH) and adjacent Pleckstrin-homology (PH) domain that serves as enzymatic unit for the GDP/GTP exchange. Rho-GEFs show different GTPase specificities, meaning that a particular GEF can activate either multiple GTPases or only one specific GTPase. We recently reported that the Rho-GEF Trio, known to be able to exchange GTP on Rac1, RhoG and RhoA, regulates lamellipodia formation to mediate cell spreading and migration in a Rac1-dependent manner. In this commentary, we review the current knowledge of Trio in several aspects of cell biology.

Signaling from the secretory granule to the nucleus

Neurons and endocrine cells use a complex array of signaling molecules to communicate with each other and with various targets. The majority of these signaling molecules are stored in specialized organelles awaiting release on demand: 40-60 nm vesicles carry conventional or small molecule neurotransmitters, and 200-400 nm granules contain bioactive peptides. The supply of small molecule neurotransmitters is tightly regulated by local feedback of synthetic rates and transport processes at sites of release. The larger granules that contain bioactive peptides present the secretory cell with special challenges, as the peptide precursors are inserted into the lumen of the secretory pathway in the cell soma and undergo biosynthetic processing while being transported to distant sites for eventual secretion. One solution to this dilemma in information handling has been to employ proteolytic cleavage of secretory granule membrane proteins to produce cytosolic fragments that can signal to the nucleus, affecting gene expression. The use of regulated intramembrane proteolysis to signal from secretory granules to the nucleus is compared to its much better understood role in relaying information from the endoplasmic reticulum by SREBP and ATF6 and from the plasma membrane by cadherins, Notch and ErbB4.

Peptidylgycine α-amidating monooxygenase and copper: a gene-nutrient interaction critical to nervous system function

Peptidylgycine alpha-amidating monooxygenase (PAM), a highly conserved copper-dependent enzyme, is essential for the synthesis of all amidated neuropeptides. Biophysical studies revealed that the binding of copper to PAM affects its structure, and cell biological studies demonstrated that the endocytic trafficking of PAM was sensitive to copper. We review data indicating that genetic reduction of PAM expression and mild copper deficiency in mice cause similar alterations in several physiological functions known to be regulated by neuropeptides: thermal regulation, seizure sensitivity, and anxiety-like behavior.

New aspects of melanocortin signaling: a role for PRCP in α-MSH degradation

The role of the central melanocortin system in the regulation of energy metabolism has received much attention during the past decade since gene mutations of key components in melanocortin signaling cause monogenic forms of obesity in animals and humans. In the arcuate nucleus of the hypothalamus the prohormone proopiomelanocortin (POMC) is posttranslationally cleaved to produce α-melanocyte stimulating hormone (α-MSH), a peptide with anorexigenic effects upon activation of the melanocortin receptors (MCRs). α-MSH undergoes extensive post-translational processing and its in vivo activity is short lived due to rapid degradation. The enzymatic process that controls α-MSH inactivation is incompletely understood. Recent evidence suggests that prolyl carboxypeptidase (PRCP) is an enzyme responsible for α-MSH degradation. As for many key melanocortin peptides, gene mutation of PRCP causes a change in the metabolic phenotype of rodents. This review summarizes the current knowledge on the melanocortin system with particular focus on PRCP, a newly discovered component of the melanocortin system.

Haploinsufficiency in peptidylglycine alpha-amidating monooxygenase leads to altered synaptic transmission in the amygdala and impaired emotional responses

The mammalian amygdala expresses various neuropeptides whose signaling has been implicated in emotionality. Many neuropeptides require amidation for full activation by peptidylglycine α-amidating monooxygenase (PAM), a transmembrane vesicular cuproenzyme and regulator of the secretory pathway. Mice heterozygous for the Pam gene (PAM(+/-)) exhibit physiological and behavioral abnormalities related to specific peptidergic pathways. In the present study, we evaluated emotionality and examined molecular and cellular responses that characterize neurophysiological differences in the PAM(+/-) amygdala. PAM(+/-) mice presented with anxiety-like behaviors in the zero maze that were alleviated by diazepam. PAM(+/-) animals were deficient in short- and long-term contextual and cued fear conditioning and required higher shock intensities to establish fear-potentiated startle than their wild-type littermates. Immunohistochemical analysis of the amygdala revealed PAM expression in pyramidal neurons and local interneurons that synthesize GABA. We performed whole-cell recordings of pyramidal neurons in the PAM(+/-) amygdala to elucidate neurophysiological correlates of the fear behavioral phenotypes. Consistent with these observations, thalamic afferent synapses in the PAM(+/-) lateral nucleus were deficient in long-term potentiation. This deficit was apparent in the absence and presence of the GABA(A) receptor antagonist picrotoxin and was abolished when both GABA(A) and GABA(B) receptors were blocked. Both evoked and spontaneous excitatory signals were enhanced in the PAM(+/-) lateral nucleus. Phasic GABAergic signaling was also augmented in the PAM(+/-) amygdala, and this difference comprised activity-independent and -dependent components. These physiological findings represent perturbations in the PAM(+/-) amygdala that may underlie the aberrant emotional responses in the intact animal.

Secretion stimulates intramembrane proteolysis of a secretory granule membrane enzyme

Regulated intramembrane proteolysis, a highly conserved process employed by diverse regulatory pathways, can release soluble fragments that directly or indirectly modulate gene expression. In this study we used pharmacological tools to identify peptidylglycine α-amidating monooxygenase (PAM), a type I secretory granule membrane protein, as a γ-secretase substrate. PAM, an essential enzyme, catalyzes the final step in the synthesis of the majority of neuropeptides that control metabolic homeostasis. Mass spectroscopy was most consistent with the presence of multiple closely spaced NH(2) termini, suggesting that cleavage occurred near the middle of the PAM transmembrane domain. The luminal domains of PAM must undergo a series of prohormone convertase or α-secretase-mediated cleavages before the remaining transmembrane domain/cytosolic domain fragment can undergo a γ-secretase-like cleavage. Cleavage by γ-secretase generates a soluble fragment of the cytosolic domain (sf-CD) that is known to localize to the nucleus. Although PAM sf-CD is unstable in AtT-20 corticotroph tumor cells, it is readily detected in primary rat anterior pituitary cells. PAM isoform expression, which is tissue-specific and developmentally regulated, affects the efficiency with which sf-CD is produced. sf-CD levels are also modulated by the phosphorylation status of the cytosolic domain and by the ability of the cytosolic domain to interact with cytosolic proteins. sf-CD is produced by primary rat anterior pituitary cells in response to secretogogue, suggesting that sf-CD acts as a signaling molecule relaying information about secretion from the secretory granule to the nucleus.

Signaling from the secretory granule to the nucleus: Uhmk1 and PAM

Neurons and endocrine cells package peptides in secretory granules (large dense-core vesicles) for storage and stimulated release. Studies of peptidylglycine alpha-amidating monooxygenase (PAM), an essential secretory granule membrane enzyme, revealed a pathway that can relay information from secretory granules to the nucleus, resulting in alterations in gene expression. The cytosolic domain (CD) of PAM, a type 1 membrane enzyme essential for the production of amidated peptides, is basally phosphorylated by U2AF homology motif kinase 1 (Uhmk1) and other Ser/Thr kinases. Proopiomelanocortin processing in AtT-20 corticotrope tumor cells was increased when Uhmk1 expression was reduced. Uhmk1 was concentrated in the nucleus, but cycled rapidly between nucleus and cytosol. Endoproteolytic cleavage of PAM releases a soluble CD fragment that localizes to the nucleus. Localization of PAM-CD to the nucleus was decreased when PAM-CD with phosphomimetic mutations was examined and when active Uhmk1 was simultaneously overexpressed. Membrane-tethering Uhmk1 did not eliminate its ability to exclude PAM-CD from the nucleus, suggesting that cytosolic Uhmk1 could cause this response. Microarray analysis demonstrated the ability of PAM to increase expression of a small subset of genes, including aquaporin 1 (Aqp1) in AtT-20 cells. Aqp1 mRNA levels were higher in wild-type mice than in mice heterozygous for PAM, indicating that a similar relationship occurs in vivo. Expression of PAM-CD also increased Aqp1 levels whereas expression of Uhmk1 diminished Aqp1 expression. The outlines of a pathway that ties secretory granule metabolism to the transcriptome are thus apparent.

Interactions of peptide amidation and copper: novel biomarkers and mechanisms of neural dysfunction

Mammalian genomes encode only a small number of cuproenzymes. The many genes involved in coordinating copper uptake, distribution, storage and efflux make gene/nutrient interactions especially important for these cuproenzymes. Copper deficiency and copper excess both disrupt neural function. Using mice heterozygous for peptidylglycine alpha-amidating monooxygenase (PAM), a cuproenzyme essential for the synthesis of many neuropeptides, we identified alterations in anxiety-like behavior, thermoregulation and seizure sensitivity. Dietary copper supplementation reversed a subset of these deficits. Wildtype mice maintained on a marginally copper-deficient diet exhibited some of the same deficits observed in PAM(+/-) mice and displayed alterations in PAM metabolism. Altered copper homeostasis in PAM(+/-) mice suggested a role for PAM in the cell type specific regulation of copper metabolism. Physiological functions sensitive to genetic limitations of PAM that are reversed by supplemental copper and mimicked by copper deficiency may serve as indicators of marginal copper deficiency.

Secretory granule to the nucleus: role of a multiply phosphorylated intrinsically unstructured domain

Intrinsically unstructured domains occur in one-third of all proteins and are characterized by conformational flexibility, protease sensitivity, and the occurrence of multiple phosphorylation. They provide large interfaces for diverse protein-protein interactions. Peptidylglycine alpha-amidating monooxygenase (PAM), an enzyme essential for neuropeptide biosynthesis, is a secretory granule membrane protein. As one of the few proteins spanning the granule membrane, PAM is a candidate to relay information about the status of the granule pool and conditions in the granule lumen. Here, we show that the PAM cytosolic domain is unstructured. Mass spectroscopy and two-dimensional gel electrophoresis demonstrated phosphorylation at 10-12 sites in the cytosolic domain. Stimulation of exocytosis resulted in coupled phosphorylation and dephosphorylation of specific sites and in the endoproteolytic release of a soluble, proteasome-sensitive cytosolic domain fragment. Analysis of granule-rich tissues, such as pituitary and heart, showed that a similar fragment was generated endogenously and translocated to the nucleus. This multiply phosphorylated unstructured domain may act as a signaling molecule that relays information from secretory granules to both cytosol and nucleus.

Reversal of physiological deficits caused by diminished levels of peptidylglycine alpha-amidating monooxygenase by dietary copper

Amidated peptides are critically involved in many physiological functions. Genetic deletion of peptidylglycine alpha-amidating monooxygenase (PAM), the only enzyme that can synthesize these peptides, is embryonically lethal. The goal of the present study was the identification of physiological functions impaired by haploinsufficiency of PAM. Regulation of the hypothalamic-pituitary-thyroid axis and body temperature, functions requiring contributions from multiple amidated peptides, were selected for evaluation. Based on serum T(4) and pituitary TSH-beta mRNA levels, mice heterozygous for PAM (PAM(+/-)) were euthyroid at baseline. Feedback within the hypothalamic-pituitary-thyroid axis was impaired in PAM(+/-) mice made hypothyroid using a low iodine/propylthiouracil diet. Despite their normal endocrine response to cold, PAM(+/-) mice were unable to maintain body temperature as well as wild-type littermates when kept in a 4 C environment. When provided with additional dietary copper, PAM(+/-) mice maintained body temperature as well as wild-type mice. Pharmacological activation of vasoconstriction or shivering also allowed PAM(+/-) mice to maintain body temperature. Cold-induced vasoconstriction was deficient in PAM(+/-) mice. This deficit was eliminated in PAM(+/-) mice receiving a diet with supplemental copper. These results suggest that dietary deficiency of copper, coupled with genetic deficits in PAM, could result in physiological deficits in humans.

Calnuc plays a role in dynamic distribution of Galphai but not Gbeta subunits and modulates ACTH secretion in AtT-20 neuroendocrine secretory cells

In AtT-20 cells ACTH secretion is regulated by both Ca²⁺ and G proteins. We previously demonstrated that calnuc, an EF-hand Ca²⁺ binding protein which regulates Alzheimer's β-amyloid precursor protein (APP) biogenesis, binds both Ca²⁺ as well as Gα subunits. Here we investigate calnuc's role in G protein-mediated regulation of ACTH secretion in AtT-20 neuroendocrine secretory cells stably overexpressing calnuc-GFP. Similar to endogenous calnuc, calnuc-GFP is mainly found in the Golgi, on the plasma membrane (PM), and associated with regulated secretion granules (RSG). By deconvolution immunofluorescence, calnuc-GFP partially colocalizes with Gαi1/2 and Gαi3 at the PM and on RSG. Cytosolic calnuc(ΔSS)-CFP with the signal sequence deleted also partially colocalizes with RSG and partially cosediments with Gαi1/2 in fractions enriched in RSG. Overexpression of calnuc-GFP specifically increases the distribution of Gαi1/2 on the PM whereas the distribution of Gβ subunits and synaptobrevin 2 (Vamp 2) is unchanged. Overexpression of calnuc-GFP or cytosolic calnuc(ΔSS)-CFP enhances ACTH secretion two-fold triggered by mastoparan or GTPγS but does not significantly affect glycosaminoglycan (GAG) chain secretion along the constitutive pathway or basal secretion of ACTH. Calnuc's facilitating effects on ACTH secretion are decreased after introducing anti-Gαi1/2, Gαi3, Gβ or calnuc IgG into permeabilized cells but not when Gα12 or preimmune IgG is introduced. The results suggest that calnuc binds to Gα subunits on the Golgi and on RSG and that overexpression of calnuc causes redistribution of Gαi subunits to the PM and RSG, indicating that calnuc plays a role in dynamic distribution of only Gα but not Gβ subunits. Thus calnuc may connect G protein signaling and calcium signaling during regulated secretion.

ACTH: cellular peptide hormone synthesis and secretory pathways

Adrenocorticotrophic hormone (ACTH) is derived from the prohormone, pro-opiomelanocortin (POMC). This precursor undergoes proteolytic cleavage to yield a number of different peptides which vary depending on the tissue. In the anterior pituitary, POMC is processed to ACTH by the prohormone convertase, PC1 and packaged in secretory granules ready for stimulated secretion. In response to stress, corticotrophin releasing hormone (CRH), stimulates release of ACTH from the pituitary cell which in turn causes release of glucocorticoids from the adrenal gland. In tissues, such as the hypothalamus and skin, ACTH is further processed intracellularly to alpha melanocyte stimulating hormone (alphaMSH) which has distinct roles in these tissues. The prohormone, POMC, is itself released from cells and found in the human circulation at concentrations greater than ACTH. While much is known about the tightly regulated synthesis of POMC, there is still a lot to learn about the mechanisms for differentiating secretion of POMC, and the POMC-derived peptides. Understanding what happens to the POMC released from cells will provide new insights into its function.

Major role of cathepsin L for producing the peptide hormones ACTH, beta-endorphin, and alpha-MSH, illustrated by protease gene knockout and expression

The pituitary hormones adrenocorticotropic hormone (ACTH), beta-endorphin, and alpha-melanocyte stimulating hormone (alpha-MSH) are synthesized by proteolytic processing of their common proopiomelanocortin (POMC) precursor. Key findings from this study show that cathepsin L functions as a major proteolytic enzyme for the production of POMC-derived peptide hormones in secretory vesicles. Specifically, cathepsin L knock-out mice showed major decreases in ACTH, beta-endorphin, and alpha-MSH that were reduced to 23, 18, and 7% of wild-type controls (100%) in pituitary. These decreased peptide levels were accompanied by increased levels of POMC consistent with proteolysis of POMC by cathepsin L. Immunofluorescence microscopy showed colocalization of cathepsin L with beta-endorphin and alpha-MSH in the intermediate pituitary and with ACTH in the anterior pituitary. In contrast, cathepsin L was only partially colocalized with the lysosomal marker Lamp-1 in pituitary, consistent with its extralysosomal function in secretory vesicles. Expression of cathepsin L in pituitary AtT-20 cells resulted in increased ACTH and beta-endorphin in the regulated secretory pathway. Furthermore, treatment of AtT-20 cells with CLIK-148, a specific inhibitor of cathepsin L, resulted in reduced production of ACTH and accumulation of POMC. These findings demonstrate a prominent role for cathepsin L in the production of ACTH, beta-endorphin, and alpha-MSH peptide hormones in the regulated secretory pathway.

Aftiphilin and gamma-synergin are required for secretagogue sensitivity of Weibel-Palade bodies in endothelial cells

Formation of secretory organelles requires the coupling of cargo selection to targeting into the correct exocytic pathway. Although the assembly of regulated secretory granules is driven in part by selective aggregation and retention of content, we recently reported that adaptor protein-1 (AP-1) recruitment of clathrin is essential to the initial formation of Weibel-Palade bodies (WPBs) at the trans-Golgi network. A selective co-aggregation process might include recruitment of components required for targeting to the regulated secretory pathway. However, we find that acquisition of the regulated secretory phenotype by WPBs in endothelial cells is coupled to but can be separated from formation of the distinctive granule core by ablation of the AP-1 effectors aftiphilin and gamma-synergin. Their depletion by small interfering RNA leads to WPBs that fail to respond to secretagogue and release their content in an unregulated manner. We find that these non-responsive WPBs have density, markers of maturation, and highly multimerized von Willebrand factor similar to those of wild-type granules. Thus, by also recruiting aftiphilin/gamma-synergin in addition to clathrin, AP-1 coordinates formation of WPBs with their acquisition of a regulated secretory phenotype.

Characterization of carboxypeptidase A6, an extracellular matrix peptidase

Carboxypeptidase A6 (CPA6) is a member of the M14 metallocarboxypeptidase family that is highly expressed in the adult mouse olfactory bulb and broadly expressed in embryonic brain and other tissues. A disruption in the human CPA6 gene is linked to Duane syndrome, a defect in the abducens nerve/lateral rectus muscle connection. In this study the cellular distribution, processing, and substrate specificity of human CPA6 were investigated. The 50-kDa pro-CPA6 is routed through the constitutive secretory pathway, processed by furin or a furin-like enzyme into the 37-kDa active form, and secreted into the extracellular matrix. CPA6 cleaves the C-terminal residue from a range of substrates, including small synthetic substrates, larger peptides, and proteins. CPA6 has a preference for large hydrophobic C-terminal amino acids as well as histidine. Peptides with a penultimate glycine or proline are very poorly cleaved. Several neuropeptides were found to be processed by CPA6, including Met- and Leu-enkephalin, angiotensin I, and neurotensin. Whereas CPA6 converts enkephalin and neurotensin into forms known to be inactive toward their receptors, CPA6 converts inactive angiotensin I into the biologically active angiotensin II. Taken together, these data suggest a role for CPA6 in the regulation of neuropeptides in the extracellular environment within the olfactory bulb and other parts of the brain.

Trafficking of a secretory granule membrane protein is sensitive to copper

We explored the effect of copper availability on the synthesis and trafficking of peptidylglycine alpha-amidating monooxygenase (PAM), an essential cuproenzyme whose catalytic domains function in the lumen of peptide-containing secretory granules. Corticotrope tumor cell lines expressing integral membrane and soluble forms of PAM were depleted of copper using bathocuproinedisulfonic acid or loaded with copper by incubation with CuCl(2). Depleting cellular copper stimulates basal secretion of soluble enzyme produced by endoproteolytic cleavage of PAM in secretory granules and transit of membrane PAM though the endocytic pathway and back into secretory granules. Unlike many cuproenzymes, lack of copper does not lead to instability of PAM. Copper loading decreases cleavage of PAM in secretory granules, secretion of soluble enzyme, and the return of internalized PAM to secretory granules. The trafficking and stability of the soluble, luminal domain of PAM and truncated membrane PAM lacking a cytosolic domain are not affected by copper availability. Taken together, our data demonstrate a role for copper-sensitive cytosolic machinery in directing endocytosed membrane PAM back to secretory granules or to a degradative pathway. The response of PAM to lack of copper suggests that it facilitates copper homeostasis.

Deletion of peptide amidation enzymatic activity leads to edema and embryonic lethality in the mouse

Peptidylglycine alpha-amidating monooxygenase (PAM) catalyzes the COOH-terminal amidation of peptide hormones. We previously had found high expression of PAM in several regions of the developing rodent. To determine the function of PAM during mouse embryogenesis, we produced a null mutant of the PAM gene. Homozygous mutants die in utero between e14.5 and e15.5 with severe edema that is likely due to cardiovascular deficits. These defects include thinning of the aorta and carotid arteries and are very similar to those of the recently characterized adrenomedullin (AM) gene KO despite the presence of elevated immunoreactive AM in PAM KO embryos. No peptide amidation activity was detected in PAM mutant embryos, and there was no moderation of the AM-like phenotype that could be expected if any alternative peptide amidation mechanism exists in the mouse. Despite the proposed contribution of amidated peptides to neuronal cell proliferation, no alteration in neuroblast proliferation was observed in homozygous mutant embryos prior to lethality. Mice heterozygous for the mutant PAM allele develop normally and express wildtype levels of several amidated peptides despite having one half the wildtype levels of PAM activity and PAM protein. Nonetheless, both an increase in adiposity and a mild glucose intolerance developed in aged (>10 months) heterozygous mice compared to littermate controls. Ablation of PAM thus demonstrates an essential function for this gene during mouse development, while alterations in PAM activity in the adult may underlie more subtle physiologic effects.

Retrieval and reuse of pituitary secretory granule proteins

The pituitary contains professional secretory cells, devoting a large fraction of their energy to the synthesis of hormones that are stored for secretion in response to a complex mixture of inputs. Ba2+, a substitute for Ca2+, and phorbol ester, a mimic for diacylglycerol, have a synergistic effect on exocytosis. By using these secretagogues, we developed a paradigm in which phorbol ester potentiation of Ba2+-evoked exocytosis produces a robust secretory response in multiple pituitary cell types. Because cells subjected to this stimulatory paradigm remain healthy despite their greatly reduced hormone content, we used this paradigm to study the fate of granule membrane proteins. We examined the turnover of peptidylglycine alpha-amidating monooxygenase (PAM), a membrane enzyme involved in the final maturation of many peptides, and VAMP2, a vesicle soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE). The stability of recently synthesized PAM was increased by sustained exocytosis. Biotinylation studies established that the appearance of integral membrane PAM at the plasma membrane was stimulated along with hormone secretion. PAM biotinylated on the cell surface undergoes cleavage to yield soluble peptidylglycine-alpha-hydroxylating monooxygenase that can then be secreted in a regulated fashion. Consistent with a kiss-and-run or cavicapture mode of secretion (Taraska, J. W., Perrais, D., Ohara-Imaizumi, M., Nagamatsu, S., and Almers, W. (2003) Proc. Natl. Acad. Sci. U. S. A. 100, 2070-2075), biotinylated prolactin was also retained by the cells and later released in response to secretagogues. Thus, pituitary cells can retrieve and reuse components of the machinery involved in the final stages of exocytosis (the SNAREs) as well as soluble and membrane granule proteins.

Peptidylglycine-alpha-amidating monooxygenase activity and protein are lower in copper-deficient rats and suckling copper-deficient mice

Peptidylglycine-alpha-amidating monooxygenase (PAM) is a copper-dependent enzyme involved in peptide posttranslational activation. Dietary Cu deficiency (Cu-) was studied to determine if lower PAM activity was due to reduction in protein or cofactor limitation. PAM activity was lower in cardiac atria of Cu- rats than Cu-adequate (Cu+) rats and there was a 50% equivalent reduction in PAM protein. No reduction in Cu- rat midbrain PAM protein was detected although PAM activity was reduced 40%. In 12-day-old (P12) mouse pups derived from dams that began Cu deficiency on day 7 of gestation, there was a parallel reduction in brain PAM activity and protein of 40-50%. PAM mRNA levels assessed in atria and brains from Cu+ and Cu- rats and mice were not altered by dietary treatment, suggesting a posttranscriptional mechanism for lower PAM protein when Cu is limiting in the cell, perhaps due to enhanced apoprotein turnover.

Cdk5 and Trio modulate endocrine cell exocytosis

Hormone secretion by pituitary cells is decreased by roscovitine, an inhibitor of cyclin-dependent kinase 5 (Cdk5). Roscovitine treatment reorganizes cortical actin and ultrastructural analysis demonstrates that roscovitine limits the ability of secretory granules to approach the plasma membrane or one another. Trio, a multifunctional RhoGEF expressed in pituitary cells, interacts with peptidylglycine α-amidating monooxygenase, a secretory granule membrane protein known to affect the actin cytoskeleton. Roscovitine inhibits the ability of Trio to activate Rac, and peptides corresponding to the Cdk5 consensus sites in Trio are phosphorylated by Cdk5. Together, these data suggest that control of the cortical actin cytoskeleton, long known to modulate hormone exocytosis and subsequent endocytosis, involves Cdk5-mediated activation of Trio.

Menkes protein contributes to the function of peptidylglycine alpha-amidating monooxygenase

Menkes protein (ATP7A) is a P-type ATPase involved in copper uptake and homeostasis. Disturbed copper homeostasis occurs in patients with Menkes disease, an X-linked disorder characterized by mental retardation, neurodegeneration, connective tissue disorders, and early childhood death. Mutations in ATP7A result in malfunction of copper-requiring enzymes, such as tyrosinase and copper/zinc superoxide dismutase. The first step of the two-step amidation reaction carried out by peptidylglycine alpha-amidating monooxygenase (PAM) also requires copper. We used tissue from wild-type rats and mice and an ATP7A-specific antibody to determine that ATP7A is expressed at high levels in tissues expressing high levels of PAM. ATP7A is largely localized to the trans Golgi network in pituitary endocrine cells. The Atp7a mouse, bearing a mutation in the Atp7a gene, is an excellent model system for examining the consequences of ATP7A malfunction. Despite normal levels of PAM protein, levels of several amidated peptides were reduced in pituitary and brain extracts of Atp7a mice, demonstrating that PAM function is compromised when ATP7A is inactive. Based on these results, we conclude that a reduction in the ability of PAM to produce bioactive end-products involved in neuronal growth and development could contribute to many of the biological effects associated with Menkes disease.

Access of a membrane protein to secretory granules is facilitated by phosphorylation

Peptidylglycine alpha-amidating monooxygenase (PAM), an integral membrane protein essential for the biosynthesis of amidated peptides, was used to assess the role of cytosolic acidic clusters in trafficking to regulated secretory granules. Casein kinase II phosphorylates Ser(949) and Thr(946) of PAM, generating a short, cytosolic acidic cluster. P-CIP2, a protein kinase identified by its ability to interact with several juxtamembrane determinants in the PAM cytosolic domain, also phosphorylates Ser(949). Antibody specific for phospho-Ser(949)-PAM-CD demonstrates that a small fraction of the PAM-1 localized to the perinuclear region bears this modification. Pituitary cell lines expressing PAM-1 mutants that mimic (TS/DD) or prevent (TS/AA) phosphorylation at these sites were studied. PAM-1 TS/AA yields a lumenal monooxygenase domain that enters secretory granules inefficiently and is rapidly degraded. In contrast, PAM-1 TS/DD is routed to regulated secretory granules more efficiently than wild-type PAM-1 and monooxygenase release is more responsive to secretagogue. Furthermore, this acidic cluster affects exit of internalized PAM-antibody complexes from late endosomes; internalized PAM-1 TS/DD accumulates in a late endocytic compartment instead of the trans-Golgi network. The increased ability of solubilized PAM-1 TS/DD to aggregate at neutral pH may play an important role in its altered trafficking.

Response of an integral granule membrane protein to changes in pH

A key feature of the regulated secretory pathway in neuroendocrine cells is lumenal pH, which decreases between trans-Golgi network and mature secretory granules. Because peptidylglycine alpha-amidating monooxygenase (PAM) is one of the few membrane-spanning proteins concentrated in secretory granules and is a known effector of regulated secretion, we examined its sensitivity to pH. Based on antibody binding experiments, the noncatalytic linker regions between the two enzymatic domains of PAM show pH-dependent conformational changes; these changes occur in the presence or absence of a transmembrane domain. Integral membrane PAM-1 solubilized from rat anterior pituitary or from transfected AtT-20 cells aggregates reversibly at pH 5.5 while retaining enzyme activity. Over 35% of the PAM-1 in anterior pituitary extracts aggregates at pH 5.5, whereas only about 5% aggregates at pH 7.5. PAM-1 recovered from secretory granules and endosomes is highly responsive to low pH-induced aggregation, whereas PAM-1 recovered from a light, intracellular recycling compartment is not. Mutagenesis studies indicate that a transmembrane domain is necessary but not sufficient for low pH-induced aggregation and reveal a short lumenal, juxtamembrane segment that also contributes to pH-dependent aggregation. Taken together, these results demonstrate that several properties of membrane PAM serve as indicators of granule pH in neuroendocrine cells.

Pituitary adenylyl cyclase-activating peptides and alpha-amidation in olfactory neurogenesis and neuronal survival in vitro

We investigated the role of amidated neuropeptides, and specifically pituitary adenylyl cyclase-activating polypeptide (PACAP), in olfactory neurogenesis and olfactory receptor neuronal survival. Using both immunohistochemistry and in situ hybridization, we find that both peptidylglycine alpha-amidating monooxygenase (PAM), the enzyme responsible for amidation and therefore activation of all amidated neuropeptides, and amidated PACAP are expressed in developing and adult olfactory epithelium. Amidated PACAP is highly expressed in proliferative basal cells and in immature olfactory neurons. The PACAP-specific receptor PAC(1) receptor is also expressed in this population, establishing that these cells can be PACAP responsive. Experiments were conducted to determine whether amidated neuropeptides, such as PACAP38, might function in olfactory neurogenesis and neuronal survival. Addition of PACAP38 to olfactory cultures increased the number of neurons to >250% of control and stimulated neuronal proliferation and survival. In primary olfactory cultures, pharmacologically decreased PAM activity, as well as neutralization of PACAP38, caused neuron-specific loss that was reversed by PACAP38. Mottled (Brindled) mice, which lack a functional ATP7A copper transporter and serve as a model for Menkes disease, provided an in vivo partial loss-of-function PAM knock-out. These mice had decreased amidated PACAP production and concomitant decreased numbers of olfactory receptor neurons. These data establish amidated peptides and specifically PACAP as having important roles in proliferation in the olfactory system and suggest that a similar function exists in vivo.

Pituitary adenylyl cyclase-activating peptides and alpha-amidation in olfactory neurogenesis and neuronal survival in vitro

We investigated the role of amidated neuropeptides, and specifically pituitary adenylyl cyclase-activating polypeptide (PACAP), in olfactory neurogenesis and olfactory receptor neuronal survival. Using both immunohistochemistry and in situ hybridization, we find that both peptidylglycine alpha-amidating monooxygenase (PAM), the enzyme responsible for amidation and therefore activation of all amidated neuropeptides, and amidated PACAP are expressed in developing and adult olfactory epithelium. Amidated PACAP is highly expressed in proliferative basal cells and in immature olfactory neurons. The PACAP-specific receptor PAC(1) receptor is also expressed in this population, establishing that these cells can be PACAP responsive. Experiments were conducted to determine whether amidated neuropeptides, such as PACAP38, might function in olfactory neurogenesis and neuronal survival. Addition of PACAP38 to olfactory cultures increased the number of neurons to >250% of control and stimulated neuronal proliferation and survival. In primary olfactory cultures, pharmacologically decreased PAM activity, as well as neutralization of PACAP38, caused neuron-specific loss that was reversed by PACAP38. Mottled (Brindled) mice, which lack a functional ATP7A copper transporter and serve as a model for Menkes disease, provided an in vivo partial loss-of-function PAM knock-out. These mice had decreased amidated PACAP production and concomitant decreased numbers of olfactory receptor neurons. These data establish amidated peptides and specifically PACAP as having important roles in proliferation in the olfactory system and suggest that a similar function exists in vivo.

Rab8b and its interacting partner TRIP8b are involved in regulated secretion in AtT20 cells

Rab proteins are a family of small GTPases that regulate intracellular vesicle traffic. Rab8b, because of its homology with Rab8, has been suggested to function in vesicle transport to the plasma membrane. Using the yeast two-hybrid system, we identified a Rab8b interacting clone, termed TRIP8b, from a rat brain cDNA library. The gene encodes a 66-kDa protein with homology to the peroxisomal targeting signal 1 receptor. The interaction between Rab8b and TRIP8b was further verified by in vitro binding assays and co-immunoprecipitation studies. Additional experiments with Rab8b mutants demonstrated that Rab8b requires a guanine nucleotide but not prenylation for its interaction with TRIP8b. Western immunoblot analysis showed that TRIP8b was primarily expressed in brain. Subcellular fractionation of AtT20 cells revealed that TRIP8b was present in both cytosolic and membrane fractions. To investigate the function of Rab8b and TRIP8b in secretion, we examined the release of ACTH from AtT20 cells. Results from stable cell lines expressing Rab8b or TRIP8b indicated that both proteins had a stimulatory effect on cAMP-induced secretion of ACTH. In summary, these data suggest that Rab8b and TRIP8b interact with each other and are involved in the regulated secretory pathway in AtT20 cells.

Signaling mediated by the cytosolic domain of peptidylglycine alpha-amidating monooxygenase

The luminal domains of membrane peptidylglycine alpha-amidating monooxygenase (PAM) are essential for peptide alpha-amidation, and the cytosolic domain (CD) is essential for trafficking. Overexpression of membrane PAM in corticotrope tumor cells reorganizes the actin cytoskeleton, shifts endogenous adrenocorticotropic hormone (ACTH) from mature granules localized at the tips of processes to the TGN region, and blocks regulated secretion. PAM-CD interactor proteins include a protein kinase that phosphorylates PAM (P-CIP2) and Kalirin, a Rho family GDP/GTP exchange factor. We engineered a PAM protein unable to interact with either P-CIP2 or Kalirin (PAM-1/K919R), along with PAM proteins able to interact with Kalirin but not with P-CIP2. AtT-20 cells expressing PAM-1/K919R produce fully active membrane enzyme but still exhibit regulated secretion, with ACTH-containing granules localized to process tips. Immunoelectron microscopy demonstrates accumulation of PAM and ACTH in tubular structures at the trans side of the Golgi in AtT-20 cells expressing PAM-1 but not in AtT-20 cells expressing PAM-1/K919R. The ability of PAM to interact with P-CIP2 is critical to its ability to block exit from the Golgi and affect regulated secretion. Consistent with this, mutation of its P-CIP2 phosphorylation site alters the ability of PAM to affect regulated secretion.

Targeting of membrane proteins to the regulated secretory pathway in anterior pituitary endocrine cells

Unlike the neuroendocrine cell lines widely used to study trafficking of soluble and membrane proteins to secretory granules, the endocrine cells of the anterior pituitary are highly specialized for the production of mature secretory granules. Therefore, we investigated the trafficking of three membrane proteins in primary anterior pituitary endocrine cells. Peptidylglycine alpha-amidating monooxygenase (PAM), an integral membrane protein essential to the production of many bioactive peptides, is cleaved and enters the regulated secretory pathway even when expressed at levels 40-fold higher than endogenous levels. Myc-TMD/CD, a membrane protein lacking the lumenal, catalytic domains of PAM, is still stored in granules. Secretory granules are not the default pathway for all membrane proteins, because Tac accumulates on the surface of pituitary endocrine cells. Overexpression of PAM is accompanied by a diminution in its endoproteolytic cleavage and in its BaCl(2)-stimulated release from mature granules. Because internalized PAM/PAM-antibody complexes are returned to secretory granules, the endocytic machinery of the pituitary endocrine cells is not saturated. As in corticotrope tumor cells, expression of PAM or Myc-TMD/CD alters the organization of the actin cytoskeleton. PAM-mediated alterations in the cytoskeleton may limit maturation of PAM and storage in mature granules.

Cell type-specific metabolism of peptidylglycine alpha-amidating monooxygenase in anterior pituitary

Peptidylglycine alpha-amidating monooxygenase (PAM) is a bifunctional enzyme expressed in each major anterior pituitary cell type. We used primary cultures of adult male rat anterior pituitary to examine PAM expression, processing, and secretion in the different pituitary cell types and to compare these patterns to those observed in transfected AtT-20 corticotrope tumor cells. Immunostaining and subcellular fractionation identified PAM in pituitary secretory granules and additional vesicular compartments; in contrast, in AtT-20 cells, transfected PAM was primarily localized to the trans-Golgi network. PAM expression was highest in gonadotropes, with moderate levels in somatotropes and thyrotropes and lower levels in corticotropes and lactotropes. Under basal conditions, less than 1% of the cell content of monooxygenase activity was secreted per h, a rate comparable to the basal rate of release of individual pituitary hormones. General secretagogues stimulated PAM secretion 3- to 5-fold. Stimulation with specific hypothalamic releasing hormones demonstrated that different pituitary cell types secrete characteristic sets of PAM proteins. Gonadotropes and thyrotropes release primarily monofunctional monooxygenase. Somatotropes secrete primarily bifunctional PAM, whereas corticotropes secrete a mixture of mono- and bifunctional proteins. As observed in transfected AtT-20 cells, pituitary cells rapidly internalize the PAM/PAM-antibody complex from the cell surface. The distinctly different steady-state localizations of endogenous PAM in primary pituitary cells and transfected PAM in AtT-20 cell lines may simply reflect the increased storage capacity of primary pituitary cells.