Secretory granule targeting of atrial natriuretic peptide correlates with its calcium-mediated aggregation

Calcium-dependent subquantal peptide release from single docked lawn-resident vesicles of pituitary lactotrophs

Regulated exocytosis consists of the fusion between vesicles and the plasma membranes, leading to the formation of a narrow fusion pore through which secretions exit the vesicle lumen into the extracellular space. An increase in the cytosolic concentration of free Ca²⁺ ([Ca²⁺]_i) is considered the stimulus of this process. However, whether this mechanism can be preserved in a simplified system of membrane lawns with docked secretory vesicles, devoid of cellular components, is poorly understood. Here, we studied peptide discharge from individual secretory vesicles docked at the plasma membrane, prepared from primary endocrine pituitary cells (the lactotrophs), releasing hormone prolactin. To label secretory vesicles, we transfected lactotrophs to express the fluorescent atrial natriuretic peptide (ANP.emd), previously shown to be expressed in and released from prolactin-containing vesicles. We used stimulating solutions containing different [Ca²⁺] to evoke vesicle peptide discharge, which appeared similar in membrane lawns and in intact stimulated lactotrophs. All vesicles examined discharged peptides in a subquantal manner, either exhibiting a unitary or sequential time course. In the membrane lawns, the unitary vesicle peptide discharge was predominant and slightly slower than that recorded in intact cells, but with a shorter delay with respect to the stimulation onset. This study revealed directly that Ca²⁺ triggers peptide discharge from docked single vesicles in the membrane lawns with a half-maximal response of ∼8 µM [Ca²⁺], consistent with previous whole-cell patch-clamp studies in endocrine cells where the rapid component of exocytosis, interpreted to represent docked vesicles, was fully activated at <10 µM [Ca²⁺]. Interestingly, the sequential subquantal peptide vesicle discharge indicates that fluctuations between constricted and dilated fusion pore states are preserved in membrane lawns and that fusion pore regulation appears to be an autonomously controlled process.

Biochemistry of the Endocrine Heart

Simple Summary

Besides being a muscle and an electrochemically active organ, the heart is a true endocrine organ. As endocrine cells, cardiac myocytes possess all the needed chemical necessities for translation, post-translational modifications, and complex peptide proteolysis. In addition, intracellular granules in the cells contain not only peptides destined for secretion but also important granin molecules involved in maintaining a regulated secretory pathway. In this review, we highlight the biochemical phenotype of the endocrine heart, recapitulating that the cardiac myocytes are truly and fully capable endocrine cells.

Abstract

Production and release of natriuretic peptides and other vasoactive peptides are tightly regulated in mammalian physiology and involved in cardiovascular homeostasis. As endocrine cells, the cardiac myocytes seem to possess almost all known chemical necessities for translation, post-translational modifications, and complex peptide proteolysis. In several ways, intracellular granules in the cells contain not only peptides destined for secretion but also important granin molecules involved in maintaining a regulated secretory pathway. In this review, we will highlight the biochemical phenotype of the endocrine heart recapitulating that the cardiac myocytes are capable endocrine cells. Understanding the basal biochemistry of the endocrine heart in producing and secreting peptides to circulation could lead to new discoveries concerning known peptide products as well as hitherto unidentified cardiac peptide products. In perspective, studies on natriuretic peptides in the heart have shown that the post-translational phase of gene expression is not only relevant for human physiology but may prove implicated also in the development and, perhaps one day, cure of human cardiovascular disease.

Small disulfide loops in peptide hormones mediate self-aggregation and secretory granule sorting

Unlike constitutively secreted proteins, peptide hormones are stored in densely packed secretory granules, before regulated release upon stimulation. Secretory granules are formed at the TGN by self-aggregation of prohormones as functional amyloids. The nonapeptide hormone vasopressin, which forms a small disulfide loop, was shown to be responsible for granule formation of its precursor in the TGN as well as for toxic fibrillar aggregation of unfolded mutants in the ER. Several other hormone precursors also contain similar small disulfide loops suggesting their function as a general device to mediate aggregation for granule sorting. To test this hypothesis, we studied the capacity of small disulfide loops of different hormone precursors to mediate aggregation in the ER and the TGN. They indeed induced ER aggregation in Neuro-2a and COS-1 cells. Fused to a constitutively secreted reporter protein, they also promoted sorting into secretory granules, enhanced stimulated secretion, and increased Lubrol insolubility in AtT20 cells. These results support the hypothesis that small disulfide loops act as novel signals for sorting into secretory granules by self-aggregation.

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.

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.

Cytosolic dynamics of annexin A6 trigger feedback regulation of hypertrophy via atrial natriuretic peptide in cardiomyocytes

Malfunctions in regulatory pathways that control cell size are prominent in pathological cardiac hypertrophy. Here, we show annexin A6 (Anxa6) to be a crucial regulator of atrial natriuretic peptide (ANP)-mediated counterhypertrophic responses in cardiomyocytes. Adrenergic stimulation of H9c2 cardiomyocytes by phenylephrine (PE) increased the cell size with enhanced expression of biochemical markers of hypertrophy, concomitant with elevated expression and subcellular redistribution of Anxa6. Stable cell lines with controlled increase in Anxa6 levels were protected against PE-induced adverse changes, whereas Anxa6 knockdown augmented the hypertrophic responses. Strikingly, Anxa6 knockdown also abrogated PE-induced juxtanuclear accumulation of secretory granules (SG) containing ANP propeptides (pro-ANP), a signature of maladaptive hypertrophy having counteractive functions. Mechanistically, PE treatment prompted a dynamic association of Anxa6 with pro-ANP-SG, parallel to their participation in anterograde traffic, in an isoform-specific fashion. Moreover, Anxa6 mutants that failed to associate with pro-ANP hindered ANP-mediated protection against hypertrophy, which was rescued, at least partially, by WT Anxa6. Additionally, elevated intracellular calcium (Ca(2+)) stimulated Anxa6-pro-ANP colocalization and membrane association. It also rescued pro-ANP translocation in cells expressing an Anxa6 mutant (Anxa6(ΔC)). Furthermore, stable overexpression of Anxa6(T356D), a mutant with superior flexibility, provided enhanced protection against PE, compared with WT, presumably due to enhanced membrane-binding capacity. Together, the present study delivers a cooperative mechanism where Anxa6 potentiates ANP-dependent counterhypertrophic responses in cardiomyocytes by facilitating regulated traffic of pro-ANP.

Short-term hemodynamic and neuroendocrine effects of pimobendan and benazapril in dogs with myxomatous mitral valve disease and congestive heart failure

BACKGROUND

Pimobendan and benazepril are frequently used with diuretics to treat dogs in congestive heart failure (CHF) caused by myxomatous mitral valve disease (MMVD).

AIM

To compare the short-term effects of pimobendan versus benazepril on pump function, heart size, and neuroendocrine profile in dogs with CHF caused by MMVD.

ANIMALS

Sixteen client-owned dogs.

MATERIAL AND METHODS

Seven-day prospective single-blinded study of dogs stabilized on furosemide monotherapy, randomized to pimobendan (0.4-0.6 mg/kg/day) or benazepril (0.25-1.0 mg/kg/day). Dogs had first-pass radionuclide angiocardiography, and heart size was measured by radiography and echocardiography. Circulating neuroendocrine hormones were measured.

RESULTS

Baseline variables did not differ between treatment groups. Greater decreases in the pimobendan than in the benazepril group were found for heart rate (P = .001), heart rate-normalized pulmonary transit time (P = .02), left atrial size (P = .03), and systolic and diastolic left ventricular diameters (P < .001 and P = .03, respectively) and volumes (P < .001 and P = .02, respectively), whereas ejection fraction increased more (P = .02) in the pimobendan group. Of the neuroendocrine hormones, only N-terminal proatrial natriuretic peptide (NT-ProANP) differed (P = .04) between groups. Within groups, plasma aldosterone increased (P = .01), and NT-proANP (P = .01) and NT-proB-type (P = .02) natriuretic peptide decreased in the pimobendan group, and NT-proANP (P = .02) and plasma vasopressin (P = .01) decreased in the benazepril group.

CONCLUSIONS AND CLINICAL IMPORTANCE

Pimobendan improves short-term cardiac function more than benazepril in dogs with CHF caused by MMVD. Pimobendan treatment enables the heart to work at smaller end-systolic and diastolic dimensions while maintaining adequate forward stroke volume. Some of the treatment responses found in neuroendocrine profile might have therapeutic relevance.

Prolactin and growth hormone aggregates in secretory granules: the need to understand the structure of the aggregate

Prolactin and GH form reversible aggregates in the trans-Golgi lumen that become the dense cores of secretory granules. Aggregation is an economical means of sorting, because self-association removes the hormones from other possible pathways. Secretory granules containing different aggregates show different behavior, such as the reduction in stimulated release of granules containing R183H-GH compared with release of those containing wild-type hormone. Aggregates may facilitate localization of membrane proteins necessary for transport and exocytosis of secretory granules, and therefore understanding their properties is important. Three types of self-association have been characterized: dimers of human GH that form with Zn(2+), low-affinity self-association of human prolactin caused by acidic pH and Zn(2+) with macromolecular crowding, and amyloid fibers of prolactin. The best candidate for the form in most granules may be low-affinity self-association because it occurs rapidly at Zn(2+) concentrations that are likely to be in granules and reverses rapidly in neutral pH. Amyloid may form in older granules. Determining differences between aggregates of wild type and those of R183H-GH should help to understand why granules containing the mutant behave differently from those containing wild-type hormone. If reversible aggregation of other hormones, including those that are proteolytically processed, is the crucial act in forming granules, rather than use of a sorting signal, then prohormones should form reversible aggregates in solution in conditions that resemble those of the trans-Golgi lumen, including macromolecular crowding.

New roles of carboxypeptidase E in endocrine and neural function and cancer

Carboxypeptidase E (CPE) or carboxypeptidase H was first discovered in 1982 as an enkephalin-convertase that cleaved a C-terminal basic residue from enkephalin precursors to generate enkephalin. Since then, CPE has been shown to be a multifunctional protein that subserves many essential nonenzymatic roles in the endocrine and nervous systems. Here, we review the phylogeny, structure, and function of CPE in hormone and neuropeptide sorting and vesicle transport for secretion, alternative splicing of the CPE transcript, and single nucleotide polymorphisms in humans. With this and the analysis of mutant and knockout mice, the data collectively support important roles for CPE in the modulation of metabolic and glucose homeostasis, bone remodeling, obesity, fertility, neuroprotection, stress, sexual behavior, mood and emotional responses, learning, and memory. Recently, a splice variant form of CPE has been found to be an inducer of tumor growth and metastasis and a prognostic biomarker for metastasis in endocrine and nonendocrine tumors.

B-type natriuretic peptide: from posttranslational processing to clinical measurement

BACKGROUND

Plasma cardiac natriuretic peptides and peptide fragments from their molecular precursors are markers of heart disease. Clinical studies have defined the current diagnostic utility of these markers, whereas biochemical elucidation of peptide structure and posttranslational processing has revealed new plasma peptide forms of potential clinical use.

CONTENT

Natriuretic propeptide structures undergo variable degrees of endo- and exoproteolytic cleavages as well as amino acid modifications, which leave the plasma phase of the peptides highly heterogeneous and dependent on cardiac pathophysiology and capacity. An ongoing characterization of the molecular heterogeneity may not only help us to appreciate the biosynthetic capacity of the endocrine heart but may also lead to the discovery of new and more disease-specific targets for future molecular diagnosis.

SUMMARY

Peptides derived from pro-atrial natriuretic peptide and pro-B-type natriuretic peptide are useful plasma markers in heart failure. New data have defined cardiac myocytes as competent endocrine cells in posttranslational processing and cellular secretion.

Biosynthesis of cardiac natriuretic peptides

Cardiac-derived peptide hormones were identified more than 25 years ago. An astonishing amount of clinical studies have established cardiac natriuretic peptides and their molecular precursors as useful markers of heart disease. In contrast to the clinical applications, the biogenesis of cardiac peptides has only been elucidated during the last decade. The cellular synthesis including amino acid modifications and proteolytic cleavages has proven considerably more complex than initially perceived. Consequently, the elimination phase of the peptide products in circulation is not yet well characterized. An ongoing characterization of the molecular heterogeneity will help appreciate the biosynthetic capacity of the endocrine heart and could introduce new diagnostic possibilities. Notably, different biosynthetic products may not be equal markers of the same pathophysiological processes. An inefficient post-translational prohormone maturation will also affect the biology of the cardiac natriuretic peptide system. This review aims at summarizing the myocardial synthesis of natriuretic peptides focusing on B-type natriuretic peptide, where new data has disclosed cardiac myocytes as highly competent endocrine cells. The structurally related atrial natriuretic peptide will be mentioned where appropriate, whereas C-type natriuretic peptide will not be considered as a cardiac peptide of relevance in mammalian physiology.

Secretogranin III: a bridge between core hormone aggregates and the secretory granule membrane

Secretory granules in endocrine cells selectively store bioactive peptide hormones and amines, which are secreted in a regulated manner upon appropriate stimulation. In addition to bioactive substances, various proteins and lipids characteristic of secretory granules are likely recruited to a restricted space at the trans-Golgi Network (TGN), and the space then matures to the secretory granule. Although experimental findings so far have strongly suggested that aggregation- and receptor-mediated processes are essential for the formation of secretory granules, the putative link between these two processes remains to be clarified. Recently, secretogranin III (SgIII) has been identified as a specific binding protein for chromogranin A (CgA), a representative constituent of the core aggregate within secretory granules, and it was later revealed that SgIII can also bind to the cholesterol-rich membrane domain at the TGN. Based on its multifaceted binding properties, SgIII may act as a central player in the formation of cholesterol-rich membrane platforms. Upon these platforms, essential processes for secretory granule biogenesis coordinately occur; that is, selective recruitment of prohormones, processing and modifying of prohormones, and condensation of mature hormones as an aggregate. This review summarizes the findings and theoretical concepts on the issue to date and then focuses on the putative role of SgIII in secretory granule biogenesis in endocrine cells.

Functional and structural characterization of a dense core secretory granule sorting domain from the PC1/3 protease

Several peptide hormones are initially synthesized as inactive precursors. It is only on entry of these prohormones and their processing proteases into dense core secretory granules (DCSGs) that the precursors are cleaved to generate their active forms. Prohormone convertase (PC)1/3 is a processing protease that is targeted to DCSGs. The signal for targeting PC1/3 to DCSGs resides in its carboxy-terminal tail (PC1/3(617-753)), where 3 regions (PC1/3(617-625), PC1/3(665-682), and PC1/3(711-753)) are known to aid in sorting and membrane association. In this article, we have determined a high-resolution structure of the extreme carboxy-terminal sorting domain, PC1/3(711-753) in micelles by NMR spectroscopy. PC1/3(711-753) contains 2 alpha helices located between residues 722-728 and 738-750. Functional assays demonstrate that the second helix (PC1/3(738-750)) is necessary and sufficient to target a constitutively secreted protein to granules, and that L(745) anchors a hydrophobic patch that is critical for sorting. Also, we demonstrate that calcium binding by the second helix of PC1/3(711-753) promotes aggregation of the domain via the hydrophobic patch centered on L(745). These results provide a structure-function analysis of a DCSG-sorting domain, and reveal the importance of a hydrophobic patch and calcium binding in controlling the sorting of proteins containing alpha helices to DCSGs.

How peptide hormone vesicles are transported to the secretion site for exocytosis

Post-Golgi transport of peptide hormone-containing vesicles from the site of genesis at the trans-Golgi network to the release site at the plasma membrane is essential for activity-dependent hormone secretion to mediate various endocrinological functions. It is known that these vesicles are transported on microtubules to the proximity of the release site, and they are then loaded onto an actin/myosin system for distal transport through the actin cortex to just below the plasma membrane. The vesicles are then tethered to the plasma membrane, and a subpopulation of them are docked and primed to become the readily releasable pool. Cytoplasmic tails of vesicular transmembrane proteins, as well as many cytosolic proteins including adaptor proteins, motor proteins, and guanosine triphosphatases, are involved in vesicle budding, the anchoring of the vesicles, and the facilitation of movement along the transport systems. In addition, a set of cytosolic proteins is also necessary for tethering/docking of the vesicles to the plasma membrane. Many of these proteins have been identified from different types of (neuro)endocrine cells. Here, we summarize the proteins known to be involved in the mechanisms of sorting various cargo proteins into regulated secretory pathway hormone-containing vesicles, movement of these vesicles along microtubules and actin filaments, and their eventual tethering/docking to the plasma membrane for hormone secretion.

Sending proteins to dense core secretory granules: still a lot to sort out

The intracellular sorting of peptide hormone precursors to the dense core secretory granules (DCSGs) is essential for their bioactivation. Despite the fundamental importance of this cellular process, the nature of the sorting signals for entry of proteins into DCSGs remains a source of vigorous debate. This review highlights recent discoveries that are consistent with a model in which several protein domains, acting in a cell-specific fashion and at different steps in the sorting process, act in concert to regulate the entry of proteins into DCSGs.

Identification of a novel targeting sequence for regulated secretion in the serine protease inhibitor neuroserpin

Ns (neuroserpin) is a member of the serpin (serine protease inhibitor) gene family that is primarily expressed within the central nervous system. Its principal target protease is tPA (tissue plasminogen activator), which is thought to contribute to synaptic plasticity and to be secreted in a stimulus-dependent manner. In the present study, we demonstrate in primary neuronal cultures that Ns co-localizes in LDCVs (large dense core vesicles) with the regulated secretory protein chromogranin B. We also show that Ns secretion is regulated and can be specifically induced 4-fold by secretagogue treatment. A novel 13-amino-acid sorting signal located at the C-terminus of Ns is identified that is both necessary and sufficient to target Ns to the regulated secretion pathway. Its deletion renders Ns no longer responsive to secretagogue stimulation, whereas PAI-Ns [Ns (neuroserpin)-PAI-1 (plasminogen activator inhibitor-1) chimaera appending the last 13 residues of Ns sequence to the C-terminus of PAI-1] shifts PAI-1 secretion into a regulated secretory pathway.

Distribution of secretory pathway Ca2+ ATPase (SPCA1) in neuronal and glial cell cultures

1. Secretory pathway Ca(2+) ATPase type 1 (SPCA1) is a newly recognized Ca(2+)/Mn(2+)-transporting pump localized in membranes of the Golgi apparatus. 2. The expression level of SPCA1 in brain tissue is relatively high in comparison with other tissues. 3. With the aim to determine the expression of SPCA1 within the different types of neural cells, we investigated the distribution of SPCA1 in neuronal, astroglial, oligodendroglial, ependymal, and microglial cell cultures derived from rat brains. 4. Western Blot analysis with rabbit anti-SPCA1 antibodies revealed the presence of SPCA1 in homogenates derived from neuronal, astroglial, ependymal, and oligodendroglial, but not from microglial cells. 5. Cell cultures that gave rise to positive signal in the immunoblot analysis were also examined immunocytochemically. 6. Immunocytochemical double-labeling experiments with anti-SPCA1 serum in combination with antibodies against cell-type specific proteins showed a localization of the SPCA1signal within cells stained positively also for GFAP, alpha-tubulin or MBP. 7. These results definitely established the expression of SPCA1 in astroglial, ependymal, and oligodendroglial cells. 8. In addition, the evaluation of neuronal cultures for the presence of SPCA1 revealed an SPCA1-specific immunofluorescence signal in cells identified as neurons.

Impact of del32-71-GH (exon 3 skipped GH) on intracellular GH distribution, secretion and cell viability: a quantitative confocal microscopy analysis

BACKGROUND

Familial isolated growth hormone deficiency (IGHD) is a disorder with about 5-30% of patients having affected relatives. Among those familial types, IGHD type II is an autosomal dominant form of short stature, associated in some families with mutations that result in missplicing to produce del32-71-GH, a GH peptide which cannot fold properly. The mechanism by which this mutant GH may alter the controlled secretory pathway and therefore suppress the secretion of the normal 22-kDa GH product of the normal allele is not known in detail. Previous studies have shown variance depending on cell type, transfection technique used, as well as on the method of analysis performed.

AIM

The aim of our study was to analyse and compare the subcellular distribution/localization of del32-71-GH or wild-type (wt)-GH (22-kDa GH), each stably transfected into AtT-20, a mouse pituitary cell line endogenously producing ACTH, employed as the internal control for secretion assessment.

METHODS

Colocalization of wt- and del32-71 mutant GH form was studied by quantitative confocal microscopy analysis. Using the immunofluorescent technique, cells were double stained for GH plus one of the following organelles: endoplasmic reticulum (ER anti-Grp94), Golgi (anti-betaCOP) or secretory granules (anti-Rab3a). In addition, GH secretion and cell viability were analysed in detail.

RESULTS/CONCLUSIONS

Our results show that in AtT-20 neuroendocrine cells, in comparison to the wt-GH, the del32-71-GH has a major impact on the secretory pathway not only affecting GH but also other peptides such as ACTH. The del32-71-GH is still present at the secretory vesicles' level, albeit in reduced quantity when compared to wt-GH but, importantly, was secretion-deficient. Furthermore, while focusing on cell viability an additional finding presented that the various splice site mutations, even though leading eventually to the same end product, namely del32-71-GH, have different and specific consequences on cell viability and proliferation rate.

Modulation of Secretory Granule-targeting Efficiency by Cis and Trans Compounding of Sorting Signals

Several protein domains acting through seemingly different mechanisms have been reported to have the capacity to target proteins to dense core secretory granules. Because proteins enter secretory granules with different efficiencies and because some of these proteins contain more than one granule-targeting motif, we have investigated whether compounding sorting signals could alter the efficiency of protein entry into secretory granules. In the current study we demonstrate that a paired basic cleavage site from human prorenin and an alpha-helix-containing secretory granule-sorting signal from the prohormone convertase PC1/3 can synergize to increase granule-sorting efficiency not only when located on the same protein, but also when located on distinct proteins that associate in the secretory pathway.

Sarcomeric genes involved in reverse remodeling of the heart during left ventricular assist device support

BACKGROUND

Left ventricular assist devices (LVADs) implanted in patients with severe congestive heart failure (CHF) as a bridge to transplantation have been shown to reverse chamber enlargement, regress cellular hypertrophy, and increase contractility. The purpose of this study was to gain a better understanding of the molecular changes associated with increased contractility after LVAD support.

METHODS

We took tissue sections from the left ventricular apex of 12 patients with CHF who were undergoing LVAD insertion (pre-LVAD) and from the LV free wall of those same patients before transplantation (post-LVAD). To control for sample-site differences, we obtained samples from the same regions in 7 patients with CHF who were undergoing transplantation without LVAD support and in 4 non- failing donor hearts. Gene expression was then probed on a custom DNA array containing 2,700 cardiac-enriched cDNA clones.

RESULTS

Calcium-handling genes were up-regulated by LVAD support, as previously reported. Sarcomeric genes were the other principle class of genes up-regulated by LVAD support, consistent with a possible restoration of sarcomere structure in reverse ventricular remodeling. However, a decrease in the fibrous component of the myocardium, also potentially involved in reverse remodeling, was not evident at the level of gene transcription because fibroblast markers were either unchanged or up-regulated. The remaining regulated genes did not fall into any defined functional class.

CONCLUSIONS

Changes in the regulation of sarcomeric, calcium-handling, and fibroblast genes during LVAD support indicate a cardiac molecular adaptation to mechanical unloading. These molecular changes may play a role in the observed increase in contractile function during reverse remodeling.

Peptidyl-Glycine α-Amidating Monooxygenase Targeting and Shaping of Atrial Secretory Vesicles

ANP (atrial natriuretic peptide) is widely recognized as an important vasorelaxant, diuretic, and cardioprotective hormone. Little is known, however, about how ANP-secretory vesicles form within the atrial myocytes. Secretory vesicles were visualized by fluorescence microscope imaging in live rat atrial myocytes expressing proANP–enhanced green fluorescent protein (EGFP), or N-terminal–mutated fusion proteins thought to suppress the calcium-dependent aggregation of proANP. Results showed the following: (1) aggregates of proANP and coexpressed proANP-EGFP recruited peptidylglycine α-amidating monooxygenase (PAM)-1, an abundant atrial integral vesicle membrane protein; (2) coexpressed N-terminal–mutated (Glu23,24→Gln23,24) and N-terminal–deleted proANP-EGFP inhibited recruitment of PAM-1 by up to 60%; (3) 4-phenyl-3-butenoic acid (PBA) (10 μmol/L), a pharmacological inhibitor of the lumenal peptidylglycine α-hydroxylating monooxygenase domain of PAM proteins, inhibited recruitment of endogenous PAM-1 and of coexpressed pro-EGFP–PAM-1; (4) PBA had no effect on exocytosis of the potassium inward rectifier KIR2.1; (5) PBA induced a deformation of the secretory vesicles but did not inhibit docking. These findings suggest that recruitment of PAM-1 to secretory vesicles depends on intact N-terminal proANP and on the lumenal domain of PAM-1. Conversely, PAM-1 participates in shaping the proANP-secretory vesicles. The full text of this article is available online at http://circres.ahajournals.org.

Is there structural specificity in the reversible protein aggregates that are stored in secretory granules?

There are several steps that must occur for secretory granules to form: (1) Secretory proteins that make up the dense cores of the granules must be concentrated; (2) membrane proteins necessary for granule function must accumulate in the correct location; and (3) inappropriate membrane proteins and excess membrane must be removed. Reversible aggregation of secretory granule proteins provides a mechanism for concentrating and sorting these proteins. There is specificity in the way secretory granule proteins are treated in cells that make granules. The specificity has been shown in some cases to occur after the aggregation process, so that granules containing different aggregates function differently. An explanation could be that a property of the aggregate, such as a surface motif, might influence the accumulation of membrane proteins necessary for granule function. Such a conclusion implies that the aggregates are not amorphous but have structure. Use of NMR spectroscopy to investigate changes in the environment of amino acid residues in secretory granule proteins as they form oligomers by using 15N relaxation times might provide a means to determine which residues are specifically involved in aggregation.

Manipulating the reversible aggregation of protein hormones in secretory granules: potential impact on biopharmaceutical development

Neuroendocrine cells and other secretory cell types are able to store secretory proteins in a concentrated form for extended periods until the release of large quantities of protein is triggered. The proteins are stored in dense core secretory granules. The dense cores of these granules are made up of large, insoluble aggregates that form by self-association. These aggregates solubilise rapidly into monomeric proteins in their native conformations when released from the cells by exocytosis of secretory granules. Formation of aggregates is an early event in secretory granule formation in at least some cell types. The function of secretory granules containing protein aggregates varies, depending upon the contents. This may occur because recognition of an aspect, such as a surface motif, of the aggregate facilitates correct assembly of the membrane proteins necessary for transport and exocytosis of the granules. Understanding the principles necessary for aggregation of protein hormones may help in the formulation of proteins for clinical use. Formation of aggregates of human prolactin has been investigated both in cells and in solution. In cells, the aggregation of human prolactin requires a mildly acidic pH, and is slowed in the presence of a membrane-permeable chelator of zinc. In solution, the aggregation of human prolactin at mildly acidic pH and physiological concentrations of Zn(2+) resembles that which occurs in cells if the reaction is performed with macromolecular crowding, which will mimic the conditions in cells. The factors causing protein aggregation and the extent to which aggregation plays a role in secretory granule formation are likely to vary with the protein and cell type. Further understanding of the principles involved in forming these aggregates that readily disassociate may enhance the ability to formulate protein preparations. Knowledge of the exact residues involved in the protein : protein interfaces in the aggregates of secretory granule proteins may lead to the ability to use small molecules to interfere with self-association and to regulate the storage of secretory granule proteins.

Progastrin Is Directed to the Regulated Secretory Pathway by Synergistically Acting Basic and Acidic Motifs

Bioactivation of prohormones occurs in the granules of the regulated secretory pathway of endocrine cells, which release hormones in response to external stimulation. How secretory granules are formed and how the cargo is selected is still unclear, but it has been shown for several prohormones and processing enzymes that domains within the prohormone structure can act as "sorting signals" for this pathway. The domains mediate interactions with other proteins or with the membrane or facilitate aggregation of the (pro)peptides. We have now searched for domains in progastrin that are active in sorting the prohormone into secretory granules. Truncation studies showed that the N-terminal 30 residues of progastrin are dispensable, whereas the last 49 residues are sufficient for correct biosynthesis of bioactive gastrin. Thus, further N-terminal truncation abolished gastrin expression. C-terminal truncation of 8 residues resulted in an increase in basal secretion as did point mutations in the dibasic processing sites of progastrin. These mutants, however, still responded to secretagogues, suggesting a residual sorting capacity to the regulated pathway. Amino acid substitutions in an acidic, polyglutamate motif within gastrin-17, the main bioactive, cellular gastrin form, did not alter secretion per se, but when these residues were substituted in C-terminally truncated mutants, double mutants increased in basal secretion and did not respond to secretagogue stimulation. This implies that the mutants are constitutively secreted. Our data suggest that the dibasic processing sites constitute the most important sorting domain of progastrin, and these sites act in synergy with the acidic domain.

A gonadotropin-releasing hormone insensitive, thapsigargin-sensitive Ca2+ store reduces basal gonadotropin exocytosis and gene expression: comparison with agonist-sensitive Ca2+ stores

We examined whether distinct Ca2+ stores differentially control basal and gonadotropin (GTH-II)-releasing hormone (GnRH)-evoked GTH-II release, long-term GTH-II secretion and contents, and GTH-II-beta mRNA expression in goldfish. Thapsigargin (Tg)-sensitive Ca2+ stores mediated neither caffeine-evoked GTH-II release, nor salmon (s)GnRH- and chicken (c)GnRH-II-stimulated secretion; the latter responses were previously shown to involve ryanodine (Ry)-sensitive Ca2+ stores. Surprisingly, Tg decreased basal GTH-II release. This response was attenuated by prior exposure to sGnRH and caffeine, but was insensitive to the phosphatase inhibitor okadaic acid, the inhibitor of constitutive release brefeldin A and cGnRH-II. GTH-II-beta mRNA expression was decreased at 24 h by 2 microm Tg, and by inhibiting (10 microm Ry) and stimulating (1 nm Ry) Ry receptors. Transient increases in GTH-II-beta mRNA were observed at 2 h and 12 h following 10 microm and 1 nm Ry treatment, respectively. Effects of Tg, Ry and GnRH on long-term GTH-II secretion, contents and apparent production differed from one another, and these changes were not well correlated with changes in GTH-II-beta mRNA expression. Our data show that GTH-II secretion, storage and transcription can be independently controlled by distinct Ca2+ stores.

In vitro aggregation of the regulated secretory protein chromogranin A

Aggregation chaperones, consisting of secretory proteins that contain a hexa-histidine epitope tag, enhance the calcium-induced aggregation of regulated secretory proteins and their sorting to secretory granules. The goal of this study was to gain a better understanding of this unusual aggregation mechanism. Hexa-histidine-epitope-tagged secreted alkaline phosphatase, an aggregation chaperone, enhanced the in vitro aggregation of chromogranin A in the presence of calcium, but not in the presence of magnesium or other divalent cations. As an exception, chromogranin was completely aggregated by zinc, even in the absence of the aggregation chaperone. In addition, fluorescence spectroscopy of the aggregation reaction mixture showed an increase in fluorescence intensity consistent with the formation of protein aggregates. The calcium-induced aggregation of chromogranin A was completely inhibited by 0.2% Triton X-100, suggesting that it involves hydrophobic interactions. In contrast, the detergent did not affect chaperone-enhanced aggregation, suggesting that this aggregation does not depend on hydrophobic interactions. EDTA-treated chaperone did not enhance chromogranin A aggregation, indicating that divalent cations are necessary for chaperone action. Although the structure of the aggregation chaperone was not important, the size of the chaperone was. Thus the free His-hexapeptide could not substitute for the aggregation chaperone. Based on these results, we propose that the hexa-histidine tag, in the context of a polypeptide, acts as a divalent cation-dependent nucleation site for chromogranin A aggregation.

Mechanisms for storage of prolactin and growth hormone in secretory granules

There are three steps in the formation of secretory granules: aggregation of proteins to form the dense cores of granules, accumulation of appropriate membrane proteins necessary for function of the granules, and removal of extraneous membrane and inappropriate proteins by small vesicles. Formation of protein aggregates may be the initial step in this process, which is not well understood. Assays of aggregation of human prolactin and growth hormone in neuroendocrine cells indicate that acidic intracellular compartments are necessary, and Zn2+ and Cu2+ may facilitate aggregation through low affinity binding sites. There is more than one way to make proteins aggregate in solution; precipitates of human prolactin formed in "crowded" conditions most closely resemble what is likely to occur in cells. Understanding the properties of aggregates formed in cells may be important, as there are several examples of granules with different contents that function differently; human R183H-growth hormone, a mutant that causes autosomal dominant isolated growth hormone deficiency, also appears to be an example. Recognition of surface motifs on aggregates of proteins may be important to localize correctly membrane proteins necessary for function, an explanation for the means by which granule content may influence function.

Aggregation of human wild-type and H27A-prolactin in cells and in solution: roles of Zn(2+), Cu(2+), and pH

Aggregation of hormones is an important step in the formation of secretory granules that results in concentration of hormones. In transfected AtT20 cells, but not COS cells, Lubrol-insoluble aggregates of human prolactin (PRL) accumulated within 30 min after synthesis. Aggregation in AtT20 cells was reduced by incubation with 30 microM chloroquine, which neutralizes intracellular compartments, and was slowed by incubation with diethyldithiocarbamate, which chelates Cu(2+) and Zn(2+). H27A-PRL aggregated in AtT20 cells as well as wild-type PRL, indicating that a high affinity Zn(2+)-binding site is not necessary. In solution, purified recombinant human PRL was precipitated by 20 microM Cu(2+) or Zn(2+). In solution without polyethylene glycol there was no precipitation with acidic pH alone, precipitation with Zn(2+) was most effective at neutral pH, and the ratio of Zn(2+) to PRL was greater than 1 in the precipitate. In solution with polyethylene glycol, precipitation occurred with acidic pH, precipitation with Zn(2+) occurred effectively at acidic pH, and the ratio of Zn(2+) to PRL was less than 1. The aggregates obtained in polyethylene glycol are therefore better models for aggregates in cells. Unlike human PRL, aggregation of rat PRL has been shown to occur at neutral pH in cells and in solution, and therefore these two similar proteins form aggregates that are the cores of secretory granules in ways that are not completely identical.

Function-specific calcium stores selectively regulate growth hormone secretion, storage, and mRNA level

Ca(+) stores may regulate multiple components of the secretory pathway. We examined the roles of biochemically independent intracellular Ca(2+) stores on acute and long-term growth hormone (GH) release, storage, and mRNA levels in goldfish somatotropes. Thapsigargin-evoked intracellular Ca(2+) concentration ([Ca(2+)](i)) signal amplitude was similar to the Ca(2+)-mobilizing agonist gonadotropin-releasing hormone, but thapsigargin (2 microM) did not acutely increase GH release, suggesting uncoupling between [Ca(2+)](i) and exocytosis. However, 2 microM thapsigargin affected long-term secretory function. Thapsigargin-treated cells displayed a steady secretion of GH (2, 12, and 24 h), which decreased GH content (12 and 24 h), but not GH mRNA/production (24 h). In contrast to the results with thapsigargin, activating the ryanodine (Ry) receptor (RyR) with 1 nM Ry transiently increased GH release (2 h). Prolonged activation of RyR (24 h) reduced GH release, contents and apparent production, without changing GH mRNA levels. Inhibiting RyR with 10 microM Ry increased GH mRNA levels, production, and storage (2 h). Increasing [Ca(2+)](i) independently of Ca(2+) stores with the use of 30 mM KCl decreased GH mRNA. Collectively, these results suggest that parts of the secretory pathway can be controlled independently by function-specific Ca(2+) stores.

Acid prohormone sequence determines size, shape, and docking of secretory vesicles in atrial myocytes

How vesicles are born in the trans-Golgi network and reach their docking sites at the plasma membrane is still largely unknown and is investigated in the present study on live, primary cultured atrial cardiomyocytes. Secretory vesicles (n=422) are visualized by expressing fusion proteins of proatrial natriuretic peptide (proANP) and green fluorescent protein. Myocytes expressing fusion proteins with intact proANP display two populations of fluorescent vesicles with apparent diameters of 120 and 175 nm, moving at a top velocity of 0.3 microm/s. The number of docked vesicles is significantly correlated with the number of mobile vesicles (r=0.71, P<0.0005). The deletion of the acidic N-terminal proANP[1-44] or point mutations (glu(23,24)-->gln(23,24)) change size and shape-but not velocity-of the vesicles, and, strikingly, abolish their docking at the plasma membrane. The shapes thus change from spheres to larger, irregular floppy bags or vesicle trains. Deletion of the C-terminal proANP[45-127], where the ANP and its disulfide bond reside, does not change size, shape, docking, or velocity of the mobile vesicles. The N-terminal acid calcium-binding sequence of proANP is known to cause protein aggregation at the high calcium concentration prevailing in the trans-Golgi network. Therefore, these results indicate that amino acid residues favoring cargo aggregation are critically important in shaping the secretory vesicles and determining their fate-docking or not docking-at the plasma membrane. The full text of this article is available at http://www.circresaha.org.

Comparative sorting of neuroendocrine secretory proteins: a search for common ground in a mosaic of sorting models and mechanisms

Endocrine, neuroendocrine and exocrine cells store regulated secretory proteins in secretory granules, while constitutive and constitutive-like secretory proteins are secreted directly without storage. Sorting of secretory proteins takes place in the trans-Golgi network (sorting for entry) or immature secretory granules (sorting by retention). The relative contribution of these sorting steps and the sorting signals and mechanisms involved in each step has been the subject of intense studies and debate in recent years. New evidence now suggests that: (1) two proteins with structurally similar sorting signals can use different sorting mechanisms; (2) one protein with multiple sorting signals can be sorted differently in different cell types; and (3) one cell type can recognize different sorting signals and use different sorting mechanisms. The latter finding suggests that sorting must be a regulated event. While the current image of sorting is complex, recent findings are pointing to common features that form a mosaic of related sorting mechanisms.

Basic mechanisms of secretion: sorting into the regulated secretory pathway

Targeting proteins to their correct cellular location is crucial for their biological function. In neuroendocrine cells, proteins can be secreted by either the constitutive or the regulated secretory pathways but the mechanism(s) whereby proteins are sorted into either pathway is unclear. In this review we discuss the possibility that sorting is either an active process occurring at the level of the trans-Golgi network, or that sorting occurs passively in the immature granules. The possible involvement of protein-lipid interactions in the sorting process is also raised.

Key words: lipid rafts, regulated secretory pathway, secretion, sorting receptors, sorting signals, trans-Golgi network.

Function- and agonist-specific Ca2+signalling: The requirement for and mechanism of spatial and temporal complexity in Ca2+signals

Calcium signals have been implicated in the regulation of many diverse cellular processes. The problem of how information from extracellular signals is delivered with specificity and fidelity using fluctuations in cytosolic Ca2+concentration remains unresolved. The capacity of cells to generate Ca2+signals of sufficient spatial and temporal complexity is the primary constraint on their ability to effectively encode information through Ca2+. Over the past decade, a large body of literature has dealt with some basic features of Ca2+-handling in cells, as well as the multiplicity and functional diversity of intracellular Ca2+stores and extracellular Ca2+influx pathways. In principle, physiologists now have the necessary information to attack the problem of function- and agonist-specificity in Ca2+signal transduction. This review explores the data indicating that Ca2+release from diverse sources, including many types of intracellular stores, generates Ca2+signals with sufficient complexity to regulate the vast number of cellular functions that have been reported as Ca2+-dependent. Some examples where such complexity may relate to neuroendocrine regulation of hormone secretion/synthesis are discussed. We show that the functional and spatial heterogeneity of Ca2+stores generates Ca2+signals with sufficient spatiotemporal complexity to simultaneously control multiple Ca2+-dependent cellular functions in neuroendocrine systems.Key words: signal coding, IP3receptor, ryanodine receptor, endoplasmic reticulum, Golgi, secretory granules, mitochondria, exocytosis.

N- and C-terminal domains direct cell type-specific sorting of chromogranin A to secretory granules

Chromogranins are a family of regulated secretory proteins that are stored in secretory granules in endocrine and neuroendocrine cells and released in response to extracellular stimulation (regulated secretion). A conserved N-terminal disulfide bond is necessary for sorting of chromogranins in neuroendocrine PC12 cells. Surprisingly, this disulfide bond is not necessary for sorting of chromogranins in endocrine GH4C1 cells. To investigate the sorting mechanism in GH4C1 cells, we made several mutant forms removing highly conserved N- and C-terminal regions of bovine chromogranin A. Removing the conserved N-terminal disulfide bond and the conserved C-terminal dimerization and tetramerization domain did not affect the sorting of chromogranin A to the regulated secretory pathway. In contrast, removing the C-terminal 90 amino acids of chromogranin A caused rerouting to the constitutive secretory pathway and impaired aggregation properties as compared with wild-type chromogranin A. Since this mutant was sorted to the regulated secretory pathway in PC12 cells, these results demonstrate that chromogranins contain independent N- and C-terminal sorting domains that function in a cell type-specific manner. Moreover, this is the first evidence that low pH/calcium-induced aggregation is necessary for sorting of a chromogranin to the regulated secretory pathway of endocrine cells.

Disruption of disulfide bonds exhibits differential effects on trafficking of regulated secretory proteins

For several secretory proteins, it has been hypothesized that disulfide-bonded loop structures are required for sorting to secretory granules. To explore this hypothesis, we employed dithiothreitol (DTT) treatment in live pancreatic islets, as well as in PC-12 and GH(4)C(1) cells. In islets, disulfide reduction in the distal secretory pathway did not increase constitutive or constitutive-like secretion of proinsulin (or insulin). In PC-12 cells, DTT treatment caused a dramatic increase in unstimulated secretion of newly synthesized chromogranin B (CgB), presumably as a consequence of reducing the single conserved chromogranin disulfide bond (E. Chanat, U. Weiss, W. B. Huttner, and S. A. Tooze. EMBO J. 12: 2159-2168, 1993). However, in GH(4)C(1) cells that also synthesize CgB endogenously, DTT treatment reduced newly synthesized prolactin and blocked its export, whereas newly synthesized CgB was routed normally to secretory granules. Moreover, on transient expression in GH(4)C(1) cells, CgA and a CgA mutant lacking the conserved disulfide bond showed comparable multimeric aggregation properties and targeting to secretory granules, as measured by stimulated secretion assays. Thus the conformational perturbation of regulated secretory proteins caused by disulfide disruption leads to consequences in protein trafficking that are both protein and cell type dependent.

Ca2+-induced deprotonation of peptide hormones inside secretory vesicles in preparation for release

The acidic environment inside secretory vesicles ensures that neuropeptides and peptide hormones are packaged in a concentrated condensed form. Although this is optimal for storage, decondensation limits release. Thus, it would be advantageous to alter the physical state of peptides in preparation for exocytosis. Here, we report that depolarization of the plasma membrane rapidly increases enhanced green fluorescent protein (EGFP)-tagged hormone fluorescence inside secretory vesicles. This effect requires Ca2+ influx and persists when exocytosis is inhibited by N-ethylmaleimide. Peptide deprotonation appears to produce this response, because it is not seen when the vesicle pH gradient is collapsed or when a pH-insensitive GFP variant is used. These data demonstrate that Ca2+ evokes alkalinization of the inside of secretory vesicles before exocytosis. Thus, Ca2+ influx into the cytoplasm alters the physical state of intravesicular contents in preparation for release.

The amino terminal regions of proBNP and proANP oligomerise through leucine zipper-like coiled-coil motifs

We provide the first report of unique leucine zipper-like coiled-coil sequence motifs at the amino terminus (N-) of proBrain Natriuretic Peptide (proBNP) and proAtrial Natriuretic Peptide (proANP). These motifs were highly conserved across most of the known natriuretic peptide sequences from different species. Consistent with computer based modelling predictions, size exclusion (SE) chromatography analysis confirmed human and ovine N-BNP, N-ANP and human proBNP in plasma extracts to elute as high molecular weight oligomers. Synthetic model peptides corresponding to the proposed leucine zipper-like coiled-coil regions of proBNP, proANP and their related N-terminal peptides were shown to be sufficient to induce oligomerisation when assessed on size exclusion HPLC. To our knowledge, this is the first report of circulating peptides that oligomerise through leucine zipper-like coiled-coil motifs, and adds a new dimension to the field of vasoactive peptide research.

Ca2+-induced deprotonation of peptide hormones inside secretory vesicles in preparation for release

The acidic environment inside secretory vesicles ensures that neuropeptides and peptide hormones are packaged in a concentrated condensed form. Although this is optimal for storage, decondensation limits release. Thus, it would be advantageous to alter the physical state of peptides in preparation for exocytosis. Here, we report that depolarization of the plasma membrane rapidly increases enhanced green fluorescent protein (EGFP)-tagged hormone fluorescence inside secretory vesicles. This effect requires Ca2+ influx and persists when exocytosis is inhibited by N-ethylmaleimide. Peptide deprotonation appears to produce this response, because it is not seen when the vesicle pH gradient is collapsed or when a pH-insensitive GFP variant is used. These data demonstrate that Ca2+ evokes alkalinization of the inside of secretory vesicles before exocytosis. Thus, Ca2+ influx into the cytoplasm alters the physical state of intravesicular contents in preparation for release.

The mammalian calcium-binding protein, nucleobindin (CALNUC), is a Golgi resident protein

We have identified CALNUC, an EF-hand, Ca2+-binding protein, as a Golgi resident protein. CALNUC corresponds to a previously identified EF-hand/calcium-binding protein known as nucleobindin. CALNUC interacts with Galphai3 subunits in the yeast two-hybrid system and in GST-CALNUC pull-down assays. Analysis of deletion mutants demonstrated that the EF-hand and intervening acidic regions are the site of CALNUC's interaction with Galphai3. CALNUC is found in both cytosolic and membrane fractions. The membrane pool is tightly associated with the luminal surface of Golgi membranes. CALNUC is widely expressed, as it is detected by immunofluorescence in the Golgi region of all tissues and cell lines examined. By immunoelectron microscopy, CALNUC is localized to cis-Golgi cisternae and the cis-Golgi network (CGN). CALNUC is the major Ca2+-binding protein detected by 45Ca2+-binding assay on Golgi fractions. The properties of CALNUC and its high homology to calreticulin suggest that it may play a key role in calcium homeostasis in the CGN and cis-Golgi cisternae.

Sorting and storage during secretory granule biogenesis: looking backward and looking forward

Secretory granules are specialized intracellular organelles that serve as a storage pool for selected secretory products. The exocytosis of secretory granules is markedly amplified under physiologically stimulated conditions. While granules have been recognized as post-Golgi carriers for almost 40 years, the molecular mechanisms involved in their formation from the trans-Golgi network are only beginning to be defined. This review summarizes and evaluates current information about how secretory proteins are thought to be sorted for the regulated secretory pathway and how these activities are positioned with respect to other post-Golgi sorting events that must occur in parallel. In the first half of the review, the emerging role of immature secretory granules in protein sorting is highlighted. The second half of the review summarizes what is known about the composition of granule membranes. The numerous similarities and relatively limited differences identified between granule membranes and other vesicular carriers that convey products to and from the plasmalemma, serve as a basis for examining how granule membrane composition might be established and how its unique functions interface with general post-Golgi membrane traffic. Studies of granule formation in vitro offer additional new insights, but also important challenges for future efforts to understand how regulated secretory pathways are constructed and maintained.

High-resolution calcium mapping of the endoplasmic reticulum-Golgi-exocytic membrane system. Electron energy loss imaging analysis of quick frozen-freeze dried PC12 cells

The calcium pools segregated within the endoplasmic reticulum, Golgi complex, exocytic, and other organelles are believed to participate in the regulation of a variety of cell functions. Until now, however, the precise intracellular distribution of the element had not been established. Here, we report about the first high-resolution calcium mapping obtained in neurosecretory PC12 cells by the imaging mode of the electron energy loss spectroscopy technique. The preparation procedure used included quick freezing of cell monolayers, followed by freeze-drying, fixation with OSO4 vapors, resin embedding, and cutting of very thin sections. Conventional electron microscopy and high-resolution immunocytochemistry revealed a high degree of structural preservation, a condition in which inorganic elements are expected to maintain their native distribution. Within these cells, calcium signals of nucleus, cytosol, and most mitochondria remained below detection, whereas in other organelles specific patterns were identified. In the endoplasmic reticulum, the distribution was heterogeneous with strongly positive cisternae (more often the nuclear envelope and stacks of parallel elements that are frequent in quick frozen preparations) lying in the proximity of or even in direct continuity with other, apparently negative cisternae. The Golgi complexes were labeled strongly and uniformly in all cisternae and part of their vesicles, with no appreciable differences along the cis-trans axis. Weaker or negative signals were recorded from the trans-Golgi network elements and from scattered vesicles, whereas in contrast secretion granules were strongly positive for calcium. These results are discussed in relation to the existing knowledge about the mechanisms of calcium transport in the variations organelles, and about the processes and functions regulated by organelle lumenal calcium in eukaryotic cells.

Structural analysis of natriuretic peptide receptor-C by truncation and site-directed mutagenesis

Natriuretic peptide receptor-C (NPR-C) has a unique structure consisting of pre-existing covalent homodimers, but it is not known whether each subunit has ligand-binding activity or whether the dimeric structure is necessary for binding activity. To answer this question, a number of C-terminally truncated mutants were designed, subcloned into the mammalian expression vector pcDNA3 and expressed by transient transfection in COS-1 cells. Truncation at position 461, which eliminates the residue Cys469 that is involved in disulphide-linked dimerization, produced a soluble and monomeric form of NPR-C, as determined by gel filtration on Superose 12. Binding assays of the gel-filtration fractions clearly demonstrated that even monomeric NPR-C contains a high-affinity binding site for natriuretic peptides. Site-directed mutagenesis of the invariant residues (Asp407-Arg408 and Asp411-Phe412) in a region highly conserved among various species established that these invariant residues are essential for ligand-binding activity.