Role of H+-ATPase-mediated acidification in sorting and release of the regulated secretory protein chromogranin A: evidence for a vesiculogenic function

Renal antiporter ClC-5 regulates collagen I/IV through the β-catenin pathway and lysosomal degradation

Mutations in Cl-/H+ antiporter ClC-5 cause Dent's disease type 1 (DD1), a rare tubulopathy that progresses to renal fibrosis and kidney failure. Here, we have used DD1 human cellular models and renal tissue from DD1 mice to unravel the role of ClC-5 in renal fibrosis. Our results in cell systems have shown that ClC-5 deletion causes an increase in collagen I (Col I) and IV (Col IV) intracellular levels by promoting their transcription through the β-catenin pathway and impairing their lysosomal-mediated degradation. Increased production of Col I/IV in ClC-5-depleted cells ends up in higher release to the extracellular medium, which may lead to renal fibrosis. Furthermore, our data have revealed that 3-mo-old mice lacking ClC-5 (Clcn5 +/- and Clcn5 -/- ) present higher renal collagen deposition and fibrosis than WT mice. Altogether, we describe a new regulatory mechanism for collagens' production and release by ClC-5, which is altered in DD1 and provides a better understanding of disease progression to renal fibrosis.

Forty years of the adrenal chromaffin cell through ISCCB meetings around the world

This historical review focuses on the evolution of the knowledge accumulated during the last two centuries on the biology of the adrenal medulla gland and its chromaffin cells (CCs). The review emerged in the context of a series of meetings that started on the Spanish island of Ibiza in 1982 with the name of the International Symposium on Chromaffin Cell Biology (ISCCB). Hence, the review is divided into two periods namely, before 1982 and from this year to 2022, when the 21st ISCCB meeting was just held in Hamburg, Germany. The first historical period extends back to 1852 when Albert Kölliker first described the fine structure and function of the adrenal medulla. Subsequently, the adrenal staining with chromate salts identified the CCs; this was followed by the establishment of the embryological origin of the adrenal medulla, and the identification of adrenaline-storing vesicles. By the end of the nineteenth century, the basic morphology, histochemistry, and embryology of the adrenal gland were known. The twentieth century began with breakthrough findings namely, the experiment of Elliott suggesting that adrenaline was the sympathetic neurotransmitter, the isolation of pure adrenaline, and the deciphering of its molecular structure and chemical synthesis in the laboratory. In the 1950s, Blaschko isolated the catecholamine-storing vesicles from adrenal medullary extracts. This switched the interest in CCs as models of sympathetic neurons with an explosion of studies concerning their functions, i.e., uptake of catecholamines by chromaffin vesicles through a specific coupled transport system; the identification of several vesicle components in addition to catecholamines including chromogranins, ATP, opioids, and other neuropeptides; the calcium-dependence of the release of catecholamines; the underlying mechanism of exocytosis of this release, as indicated by the co-release of proteins; the cross-talk between the adrenal cortex and the medulla; and the emission of neurite-like processes by CCs in culture, among other numerous findings. The 1980s began with the introduction of new high-resolution techniques such as patch-clamp, calcium probes, marine toxins-targeting ion channels and receptors, confocal microscopy, or amperometry. In this frame of technological advances at the Ibiza ISCCB meeting in 1982, 11 senior researchers in the field predicted a notable increase in our knowledge in the field of CCs and the adrenal medulla; this cumulative knowledge that occurred in the last 40 years of history of the CC is succinctly described in the second part of this historical review. It deals with cell excitability, ion channel currents, the exocytotic fusion pore, the handling of calcium ions by CCs, the kinetics of exocytosis and endocytosis, the exocytotic machinery, and the life cycle of secretory vesicles. These concepts together with studies on the dynamics of membrane fusion with super-resolution imaging techniques at the single-protein level were extensively reviewed by top scientists in the field at the 21st ISCCB meeting in Hamburg in the summer of 2022; this frontier topic is also briefly reviewed here. Many of the concepts arising from those studies contributed to our present understanding of synaptic transmission. This has been studied in physiological or pathophysiological conditions, in CCs from animal disease models. In conclusion, the lessons we have learned from CC biology as a peripheral model for brain and brain disease pertain more than ever to cutting-edge research in neurobiology. In the 22nd ISCCB meeting in Israel in 2024 that Uri Asheri is organizing, we will have the opportunity of seeing the progress of the questions posed in Ibiza, and on other questions that undoubtedly will arise.

Misrouting of glucagon and stathmin-2 towards lysosomal system of α-cells in glucagon hypersecretion of diabetes

Glucagon hypersecretion from the pancreatic α-cell is a characteristic sign of diabetes, which exacerbates fasting hyperglycemia. Thus, targeting glucagon secretion from α-cells may be a promising approach for combating hyperglucagonemia. We have recently identified stathmin-2 as an α-cell protein that regulates glucagon secretion by directing glucagon toward the endolysosomal system in αTC1-6 cells. We hypothesized that disruption of Stmn2-mediated trafficking of glucagon to the endolysosomes in diabetes contributes to hyperglucagonemia. In isolated islets from male mice treated with streptozotocin (STZ), glucagon secretion and cellular content were augmented, but cellular Stmn2 levels were reduced (p < .01), as measured by both ELISA and immunofluorescence intensity. Using confocal immunofluorescence microscopy, the colocalization of glucagon and Stmn2 in Lamp2A⁺ lysosomes was dramatically reduced (p < .001) in islets from diabetic mice, and the colocalization of Stmn2, but not glucagon, with the late endosome marker, Rab7, significantly (p < .01) increased. Further studies were conducted in αTC1-6 cells cultured in media containing high glucose (16.7 mM) for 2 weeks to mimic glucagon hypersecretion of diabetes. Surprisingly, treatment of αTC1-6 cells with the lysosomal inhibitor bafilomycin A1 reduced K⁺-induced glucagon secretion, suggesting that high glucose may induce glucagon secretion from another lysosomal compartment. Both glucagon and Stmn2 co-localized with Lamp1, which marks secretory lysosomes, in cells cultured in high glucose. We propose that, in addition to enhanced trafficking and secretion through the regulated secretory pathway, the hyperglucagonemia of diabetes may also be due to re-routing of glucagon from the degradative Lamp2A⁺ lysosome toward the secretory Lamp1⁺ lysosome.

Inside the Insulin Secretory Granule

The pancreatic β-cell is purpose-built for the production and secretion of insulin, the only hormone that can remove glucose from the bloodstream. Insulin is kept inside miniature membrane-bound storage compartments known as secretory granules (SGs), and these specialized organelles can readily fuse with the plasma membrane upon cellular stimulation to release insulin. Insulin is synthesized in the endoplasmic reticulum (ER) as a biologically inactive precursor, proinsulin, along with several other proteins that will also become members of the insulin SG. Their coordinated synthesis enables synchronized transit through the ER and Golgi apparatus for congregation at the trans-Golgi network, the initiating site of SG biogenesis. Here, proinsulin and its constituents enter the SG where conditions are optimized for proinsulin processing into insulin and subsequent insulin storage. A healthy β-cell is continually generating SGs to supply insulin in vast excess to what is secreted. Conversely, in type 2 diabetes (T2D), the inability of failing β-cells to secrete may be due to the limited biosynthesis of new insulin. Factors that drive the formation and maturation of SGs and thus the production of insulin are therefore critical for systemic glucose control. Here, we detail the formative hours of the insulin SG from the luminal perspective. We do this by mapping the journey of individual members of the SG as they contribute to its genesis.

A cell-based assay for CD63-containing extracellular vesicles

Extracellular vesicles (EVs) are thought to be important in cell-cell communication and have elicited extraordinary interest as potential biomarkers of disease. However, quantitative methods to enable elucidation of mechanisms underlying release are few. Here, we describe a cell-based assay for monitoring EV release using the EV-enriched tetraspanin CD63 fused to the small, ATP-independent reporter enzyme, Nanoluciferase. Release of CD63-containing EVs from stably expressing cell lines was monitored by comparing luciferase activity in culture media to that remaining in cells. HEK293, U2OS, U87 and SKMel28 cells released 0.3%-0.6% of total cellular CD63 in the form of EVs over 5 hrs, varying by cell line. To identify cellular machinery important for secretion of CD63-containing EVs, we performed a screen of biologically active chemicals in HEK293 cells. While a majority of compounds did not significantly affect EV release, treating cells with the plecomacrolides bafilomycin or concanamycin, known to inhibit the V-ATPase, dramatically increased EV release. Interestingly, alkalization of the endosomal lumen using weak bases had no effect, suggesting a pH-independent enhancement of EV release by V-ATPase inhibitors. The ability to quantify EVs in small samples will enable future detailed studies of release kinetics as well as further chemical and genetic screening to define pathways involved in EV secretion.

Buforin-1 blocks neuronal SNARE-mediated membrane fusion by inhibiting SNARE complex assembly

Assembly of neuronal SNARE protein complexes is essential for fusion of synaptic vesicles with the presynaptic plasma membrane, which releases neurotransmitters into the synaptic cleft and mediates neurotransmission. However, despite the potential of pharmacological regulation of this process for the treatment of various neurological disorders, only a few reagents, including botulinum neurotoxins, are currently available. Here, we report that buforin-1, an antimicrobial peptide from the Asian toad Bufo gargarizans, inhibits neuronal SNARE complex assembly, resulting in neuronal SNARE-mediated membrane fusion in vitro via its direct association with neuronal t-SNAREs syntaxin-1 and SNAP-25. Consistently, buforin-1 significantly inhibited neuronal-SNARE-mediated exocytosis in PC-12 cells. Thus, buforin-1 has potential for the treatment of neurological disorders caused by dysregulated neurotransmission.

The priming factor CAPS1 regulates dense-core vesicle acidification by interacting with rabconnectin3β/WDR7 in neuroendocrine cells

Vacuolar-type H⁺-ATPases (V-ATPases) contribute to pH regulation and play key roles in secretory and endocytic pathways. Dense-core vesicles (DCVs) in neuroendocrine cells are maintained at an acidic pH, which is part of the electrochemical driving force for neurotransmitter loading and is required for hormonal propeptide processing. Genetic loss of CAPS1 (aka calcium-dependent activator protein for secretion, CADPS), a vesicle-bound priming factor required for DCV exocytosis, dissipates the pH gradient across DCV membranes and reduces neurotransmitter loading. However, the basis for CAPS1 binding to DCVs and for its regulation of vesicle pH has not been determined. Here, MS analysis of CAPS1 immunoprecipitates from brain membrane fractions revealed that CAPS1 associates with a rabconnectin3 (Rbcn3) complex comprising Dmx-like 2 (DMXL2) and WD repeat domain 7 (WDR7) proteins. Using immunofluorescence microscopy, we found that Rbcn3α/DMXL2 and Rbcn3β/WDR7 colocalize with CAPS1 on DCVs in human neuroendocrine (BON) cells. The shRNA-mediated knockdown of Rbcn3β/WDR7 redistributed CAPS1 from DCVs to the cytosol, indicating that Rbcn3β/WDR7 is essential for optimal DCV localization of CAPS1. Moreover, cell-free experiments revealed direct binding of CAPS1 to Rbcn3β/WDR7, and cell assays indicated that Rbcn3β/WDR7 recruits soluble CAPS1 to membranes. As anticipated by the reported association of Rbcn3 with V-ATPase, we found that knocking down CAPS1, Rbcn3α, or Rbcn3β in neuroendocrine cells impaired rates of DCV reacidification. These findings reveal a basis for CAPS1 binding to DCVs and for CAPS1 regulation of V-ATPase activity via Rbcn3β/WDR7 interactions.

HID-1 controls formation of large dense core vesicles by influencing cargo sorting and trans-Golgi network acidification

The peripheral membrane protein HID-1 localizes to the trans-Golgi network, where it contributes to the formation of large dense core vesicles of neuroendocrine cells by influencing cargo sorting and trans-Golgi network acidification.

Large dense core vesicles (LDCVs) mediate the regulated release of neuropeptides and peptide hormones. They form at the trans-Golgi network (TGN), where their soluble content aggregates to form a dense core, but the mechanisms controlling biogenesis are still not completely understood. Recent studies have implicated the peripheral membrane protein HID-1 in neuropeptide sorting and insulin secretion. Using CRISPR/Cas9, we generated HID-1 KO rat neuroendocrine cells, and we show that the absence of HID-1 results in specific defects in peptide hormone and monoamine storage and regulated secretion. Loss of HID-1 causes a reduction in the number of LDCVs and affects their morphology and biochemical properties, due to impaired cargo sorting and dense core formation. HID-1 KO cells also exhibit defects in TGN acidification together with mislocalization of the Golgi-enriched vacuolar H⁺-ATPase subunit isoform a2. We propose that HID-1 influences early steps in LDCV formation by controlling dense core formation at the TGN.

Distinguishing signal from autofluorescence in cryogenic correlated light and electron microscopy of mammalian cells

In cryogenic correlated light and electron microscopy (cryo-CLEM), frozen targets of interest are identified and located on EM grids by fluorescence microscopy and then imaged at higher resolution by cryo-EM. Whilst working with these methods, we discovered that a variety of mammalian cells exhibit strong punctate autofluorescence when imaged under cryogenic conditions (80 K). Autofluorescence originated from multilamellar bodies (MLBs) and secretory granules. Here we describe a method to distinguish fluorescent protein tags from these autofluorescent sources based on the narrower emission spectrum of the former. The method is first tested on mitochondria and then applied to examine the ultrastructural variability of secretory granules within insulin-secreting pancreatic beta-cell-derived INS-1E cells.

The Parkinson's disease-linked protein TMEM230 is required for Rab8a-mediated secretory vesicle trafficking and retromer trafficking

TMEM230 is a newly identified Parkinson's disease (PD) gene encoding a transmembrane protein whose cellular and pathogenic roles remain largely unknown. Here, we demonstrate that loss of TMEM230 disrupts retromer cargo CI-M6PR (cation-independent mannose 6-phosphate receptor) trafficking and autophagic cargo degradation rates. TMEM230 depletion further inhibits extracellular secretion of the autophagic cargo p62 and immature lysosomal hydrolases in Golgi-derived vesicles leading to their intracellular accumulation, and is specifically mediated by loss of the small GTPase Rab8a. Importantly, PD-linked TMEM230 variants also induce retromer mislocalization, defective cargo trafficking, and impaired autophagy. Finally, we show that knockdown of another PD gene, LRRK2, which phosphorylates Rab8a, similarly impairs retromer trafficking, secretory autophagy and Golgi-derived vesicle secretion, thus demonstrating converging roles of two PD genes TMEM230 and LRRK2 on Rab8a function, and suggesting that retromer and secretory dysfunction play an important role in PD pathogenesis.

Lipids implicated in the journey of a secretory granule: from biogenesis to fusion

The regulated secretory pathway begins with the formation of secretory granules by budding from the Golgi apparatus and ends by their fusion with the plasma membrane leading to the release of their content into the extracellular space, generally following a rise in cytosolic calcium. Generation of these membrane-bound transport carriers can be classified into three steps: (i) cargo sorting that segregates the cargo from resident proteins of the Golgi apparatus, (ii) membrane budding that encloses the cargo and depends on the creation of appropriate membrane curvature, and (iii) membrane fission events allowing the nascent carrier to separate from the donor membrane. These secretory vesicles then mature as they are actively transported along microtubules toward the cortical actin network at the cell periphery. The final stage known as regulated exocytosis involves the docking and the priming of the mature granules, necessary for merging of vesicular and plasma membranes, and the subsequent partial or total release of the secretory vesicle content. Here, we review the latest evidence detailing the functional roles played by lipids during secretory granule biogenesis, recruitment, and exocytosis steps. In this review, we highlight evidence supporting the notion that lipids play important functions in secretory vesicle biogenesis, maturation, recruitment, and membrane fusion steps. These effects include regulating various protein distribution and activity, but also directly modulating membrane topology. The challenges ahead to understand the pleiotropic functions of lipids in a secretory granule's journey are also discussed. This article is part of a mini review series on Chromaffin cells (ISCCB Meeting, 2015).

The V-ATPase membrane domain is a sensor of granular pH that controls the exocytotic machinery

Several studies have suggested that the V0 domain of the vacuolar-type H(+)-adenosine triphosphatase (V-ATPase) is directly implicated in secretory vesicle exocytosis through a role in membrane fusion. We report in this paper that there was a rapid decrease in neurotransmitter release after acute photoinactivation of the V0 a1-I subunit in neuronal pairs. Likewise, inactivation of the V0 a1-I subunit in chromaffin cells resulted in a decreased frequency and prolonged kinetics of amperometric spikes induced by depolarization, with shortening of the fusion pore open time. Dissipation of the granular pH gradient was associated with an inhibition of exocytosis and correlated with the V1-V0 association status in secretory granules. We thus conclude that V0 serves as a sensor of intragranular pH that controls exocytosis and synaptic transmission via the reversible dissociation of V1 at acidic pH. Hence, the V-ATPase membrane domain would allow the exocytotic machinery to discriminate fully loaded and acidified vesicles from vesicles undergoing neurotransmitter reloading.

Human diseases associated with form and function of the Golgi complex

The Golgi complex lies at the heart of the secretory pathway and is responsible for modifying proteins and lipids, as well as sorting newly synthesized molecules to their correct destination. As a consequence of these important roles, any changes in its proteome can negatively affect its function and in turn lead to disease. Recently, a number of proteins have been identified, which when either depleted or mutated, result in diseases that affect various organ systems. Here we describe how these proteins have been linked to the Golgi complex, and specifically how they affect either the morphology, membrane traffic or glycosylation ability of this organelle.

MicroRNA-22 and promoter motif polymorphisms at the Chga locus in genetic hypertension: functional and therapeutic implications for gene expression and the pathogenesis of hypertension

Hypertension is a common hereditary syndrome with unclear pathogenesis. Chromogranin A (Chga), which catalyzes formation and cargo storage of regulated secretory granules in neuroendocrine cells, contributes to blood pressure homeostasis centrally and peripherally. Elevated Chga occurs in spontaneously hypertensive rat (SHR) adrenal glands and plasma, but central expression is unexplored. In this report, we measured SHR and Wistar-Kyoto rat (control) Chga expression in central and peripheral nervous systems, and found Chga protein to be decreased in the SHR brainstem, yet increased in the adrenal and the plasma. By re-sequencing, we systematically identified five promoter, two coding and one 3'-untranslated region (3'-UTR) polymorphism at the SHR (versus WKY or BN) Chga locus. Using HXB/BXH recombinant inbred (RI) strain linkage and correlations, we demonstrated genetic determination of Chga expression in SHR, including a cis-quantitative trait loci (QTLs) (i.e. at the Chga locus), and such expression influenced biochemical determinants of blood pressure, including a cascade of catecholamine biosynthetic enzymes, catecholamines themselves and steroids. Luciferase reporter assays demonstrated that the 3'-UTR polymorphism (which disrupts a microRNA miR-22 motif) and promoter polymorphisms altered gene expression consistent with the decline in SHR central Chga expression. Coding region polymorphisms did not account for changes in Chga expression or function. Thus, we hypothesized that the 3'-UTR and promoter mutations lead to dysregulation (diminution) of Chga in brainstem cardiovascular control nuclei, ultimately contributing to the pathogenesis of hypertension in SHR. Accordingly, we demonstrated that in vivo administration of miR-22 antagomir to SHR causes substantial (∼18 mmHg) reductions in blood pressure, opening a novel therapeutic avenue for hypertension.

The regulated secretory pathway and human disease: insights from gene variants and single nucleotide polymorphisms

The regulated secretory pathway provides critical control of peptide, growth factor, and hormone release from neuroendocrine and endocrine cells, and neurons, maintaining physiological homeostasis. Propeptides and prohormones are packaged into dense core granules (DCGs), where they frequently undergo tissue-specific processing as the DCG matures. Proteins of the granin family are DCG components, and although their function is not fully understood, data suggest they are involved in DCG formation and regulated protein/peptide secretion, in addition to their role as precursors of bioactive peptides. Association of gene variation, including single nucleotide polymorphisms (SNPs), with neuropsychiatric, endocrine, and metabolic diseases, has implicated specific secreted proteins and peptides in disease pathogenesis. For example, a SNP at position 196 (G/A) of the human brain-derived neurotrophic factor gene dysregulates protein processing and secretion and leads to cognitive impairment. This suggests more generally that variants identified in genes encoding secreted growth factors, peptides, hormones, and proteins involved in DCG biogenesis, protein processing, and the secretory apparatus, could provide insight into the process of regulated secretion as well as disorders that result when it is impaired.

Vesicular Ca(2+) mediates granule motion and exocytosis

Secretory vesicles of chromaffin cells are acidic organelles that maintain an increasing pH gradient towards the cytosol (5.5 vs. 7.3) that is mediated by V-ATPase activity. This gradient is primarily responsible for the accumulation of large concentrations of amines and Ca(2+), although the mechanisms mediating Ca(2+) uptake and release from granules, and the physiological relevance of these processes, remain unclear. The presence of a vesicular matrix appears to create a bi-compartmentalised medium in which the major fractions of solutes, including catecholamines, nucleotides and Ca(2+), are strongly associated with vesicle proteins, particularly chromogranins. This association appears to be favoured at acidic pH values. It has been demonstrated that disrupting the pH gradient of secretory vesicles reduces their rate of exocytosis and promotes the leakage of vesicular amines and Ca(2+), dramatically increasing the movement of secretory vesicles and triggering exocytosis. In this short review, we will discuss the data available that highlights the importance of pH in regulating the association between chromogranins, vesicular amines and Ca(2+). We will also address the potential role of vesicular Ca(2+) in two major processes in secretory cells, vesicle movement and exocytosis.

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

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

Salinomycin inhibits Wnt signaling and selectively induces apoptosis in chronic lymphocytic leukemia cells

Salinomycin, an antibiotic potassium ionophore, has been reported recently to act as a selective breast cancer stem cell inhibitor, but the biochemical basis for its anticancer effects is not clear. The Wnt/β-catenin signal transduction pathway plays a central role in stem cell development, and its aberrant activation can cause cancer. In this study, we identified salinomycin as a potent inhibitor of the Wnt signaling cascade. In Wnt-transfected HEK293 cells, salinomycin blocked the phosphorylation of the Wnt coreceptor lipoprotein receptor related protein 6 (LRP6) and induced its degradation. Nigericin, another potassium ionophore with activity against cancer stem cells, exerted similar effects. In otherwise unmanipulated chronic lymphocytic leukemia cells with constitutive Wnt activation nanomolar concentrations of salinomycin down-regulated the expression of Wnt target genes such as LEF1, cyclin D1, and fibronectin, depressed LRP6 levels, and limited cell survival. Normal human peripheral blood lymphocytes resisted salinomycin toxicity. These results indicate that ionic changes induced by salinomycin and related drugs inhibit proximal Wnt signaling by interfering with LPR6 phosphorylation, and thus impair the survival of cells that depend on Wnt signaling at the plasma membrane.

Salinomycin inhibits Wnt signaling and selectively induces apoptosis in chronic lymphocytic leukemia cells

Salinomycin, an antibiotic potassium ionophore, has been reported recently to act as a selective breast cancer stem cell inhibitor, but the biochemical basis for its anticancer effects is not clear. The Wnt/β-catenin signal transduction pathway plays a central role in stem cell development, and its aberrant activation can cause cancer. In this study, we identified salinomycin as a potent inhibitor of the Wnt signaling cascade. In Wnt-transfected HEK293 cells, salinomycin blocked the phosphorylation of the Wnt coreceptor lipoprotein receptor related protein 6 (LRP6) and induced its degradation. Nigericin, another potassium ionophore with activity against cancer stem cells, exerted similar effects. In otherwise unmanipulated chronic lymphocytic leukemia cells with constitutive Wnt activation nanomolar concentrations of salinomycin down-regulated the expression of Wnt target genes such as LEF1, cyclin D1, and fibronectin, depressed LRP6 levels, and limited cell survival. Normal human peripheral blood lymphocytes resisted salinomycin toxicity. These results indicate that ionic changes induced by salinomycin and related drugs inhibit proximal Wnt signaling by interfering with LPR6 phosphorylation, and thus impair the survival of cells that depend on Wnt signaling at the plasma membrane.

On the role of intravesicular calcium in the motion and exocytosis of secretory organelles

Secretory vesicles of sympathetic neurons and chromaffin granules maintain a pH gradient towards the cytosol (5.5 vs. 7.2) promoted by the V-ATPase activity. This gradient of pH is mainly responsible for the accumulation of amines. The secretory vesicles contain large amounts of total Ca(2+), but the free intragranular [Ca(2+)], the mechanisms for Ca(2+) uptake and release from the granules and their physiological relevance regarding exocytosis are still matters of debate.We have recently shown that disruption of the pH gradient of secretory vesicles slowed down exocytosis. Fluorimetric measurements, using the dye Oregon green BAPTA-2, showed that the V-ATPase inhibitor bafilomycin A1 directly released Ca(2+) from freshly isolated vesicles. Accordingly, vesicle alkalinization released Ca(2+) from the granules to the cytosol, measured with fura-2 in intact chromaffin cells. Using TIRFM in cells overexpressing the EGFP-labeled synaptobrevin (VAMP2-EGFP) protein, we have then shown that the Ca(2+) released from the vesicles to the cytosol in the presence of bafilomycin, dramatically increased the granule motion of chromaffin- or PC12-derived granules, and triggered exocytosis (measured by amperometry).We conclude that the gradient of pH of secretory vesicles might be involved in the homeostatic regulation of the local cytosolic Ca(2+) around the vesicles and in two of the major functions of secretory cells, vesicle motion and exocytosis.1.

A common genetic variant in the 3'-UTR of vacuolar H+-ATPase ATP6V0A1 creates a micro-RNA motif to alter chromogranin A processing and hypertension risk

BACKGROUND

The catecholamine release-inhibitor catestatin and its precursor chromogranin A (CHGA) may constitute "intermediate phenotypes" in the analysis of genetic risk for cardiovascular disease such as hypertension. Previously, the vacuolar H(+)-ATPase subunit gene ATP6V0A1 was found within the confidence interval for linkage with catestatin secretion in a genome-wide study, and its 3'-UTR polymorphism T+3246C (rs938671) was associated with both catestatin processing from CHGA and population blood pressure. We explored the molecular mechanism of this effect by experiments with transfected chimeric photoproteins in chromaffin cells.

METHODS AND RESULTS

Placing the ATP6V0A1 3'-UTR downstream of a luciferase reporter, we found that the C (variant) allele decreased overall gene expression. The 3'-UTR effect was verified by coupled in vitro transcription/translation of the entire/intact human ATP6V0A1 mRNA. Chromaffin granule pH, monitored by fluorescence of CHGA/EGFP chimera during vesicular H(+)-ATPase inhibition by bafilomycin A1, was more easily perturbed during coexpression of the ATP6V0A1 3'-UTR C-allele than the T-allele. After bafilomycin A1 treatment, the ratio of CHGA precursor to its catestatin fragments in PC12 cells was substantially diminished, though the qualitative composition of such fragments was not affected (on immunoblot or matrix-assisted laser desorption ionization (MALDI) mass spectrometry). Bafilomycin A1 treatment also decreased exocytotic secretion from the regulated pathway, monitored by a CHGA chimera tagged with embryonic alkaline phosphatase. 3'-UTR T+3246C created a binding motif for micro-RNA hsa-miR-637; cotransfection of hsa-miR-637 precursor or antagomir/inhibitor oligonucleotides yielded the predicted changes in expression of luciferase reporter/ATP6V0A1-3'-UTR plasmids varying at T+3246C.

CONCLUSIONS

The results suggest a series of events whereby ATP6V0A1 3'-UTR variant T+3246C functioned: ATP6V0A1 expression probably was affected through differential micro-RNA effects, altering vacuolar pH and consequently CHGA processing and exocytotic secretion.

Proteomics of dense core secretory vesicles reveal distinct protein categories for secretion of neuroeffectors for cell-cell communication

Regulated secretion of neurotransmitters and neurohumoral factors from dense core secretory vesicles provides essential neuroeffectors for cell-cell communication in the nervous and endocrine systems. This study provides comprehensive proteomic characterization of the categories of proteins in chromaffin dense core secretory vesicles that participate in cell-cell communication from the adrenal medulla. Proteomic studies were conducted by nano-HPLC Chip MS/MS tandem mass spectrometry. Results demonstrate that these secretory vesicles contain proteins of distinct functional categories consisting of neuropeptides and neurohumoral factors, protease systems, neurotransmitter enzymes and transporters, receptors, enzymes for biochemical processes, reduction/oxidation regulation, ATPases, protein folding, lipid biochemistry, signal transduction, exocytosis, calcium regulation, as well as structural and cell adhesion proteins. The secretory vesicle proteomic data identified 371 proteins in the soluble fraction and 384 membrane proteins, for a total of 686 distinct secretory vesicle proteins. Notably, these proteomic analyses illustrate the presence of several neurological disease-related proteins in these secretory vesicles, including huntingtin interacting protein, cystatin C, ataxin 7, and prion protein. Overall, these findings demonstrate that multiple protein categories participate in dense core secretory vesicles for production, storage, and secretion of bioactive neuroeffectors for cell-cell communication in health and disease.

Phospholipase C-dependent Ca2+-sensing pathways leading to endocytosis and inhibition of the plasma membrane vacuolar H+-ATPase in osteoclasts

In osteoclasts, elevation of extracellular Ca2+ is an endogenous signal that inhibits bone resorption. We recently found that an elevation of extracellular Ca2+ decreased proton extrusion through the plasma membrane vacuolar H+-ATPase (V-ATPase) rapidly. In this study we investigated mechanisms underlying this early Ca2+-sensing response, particularly in reference to the activity of the plasma membrane V-ATPase and to membrane retrieval. Whole cell clamp recordings allowed us to measure the V-ATPase currents and the cell capacitance ( Cm) simultaneously. Cm is a measure of cell surface. Extracellular Ca2+ (2.5–40 mM) decreased Cm and the V-ATPase current simultaneously. The decreased Cm, together with the enhanced uptake of a lipophilic dye (FM1–43), indicated that Ca2+ facilitated endocytosis. The endocytosis was blocked by dynamin inhibitors (dynasore and dynamin-inhibitory peptide), by small interfering RNA (siRNA) targeting for dynanmin-2 and also by bafilomycin A1, a blocker of V-ATPases. The extracellular Ca2+-induced endocytosis and inhibition of the V-ATPase current were diminished by a phospholipase C inhibitor (U73122) and siRNA targeting for phospholipase C γ2 subunit. Holding the cytosolic Ca2+ at either high (0.5–5 μM) or low levels or inhibiting calmodulin by an inhibitor (W7) or an antibody (anti-CaM) decreased the stimulated endocytosis and the inhibition of the V-ATPase current. These data suggest that extracellular Ca2+ facilitated dynamin- and V-ATPase-dependent endocytosis in association with an inhibition of the plasma membrane V-ATPase. Phospholipase C, cytosolic Ca2+, and calmodulin were involved in the signaling pathways. Membrane retrieval and the plasma membrane V-ATPase activity may cooperate during the early phase of Ca2+-sensing response in osteoclasts.

Neuropeptidomic components generated by proteomic functions in secretory vesicles for cell-cell communication

Diverse neuropeptides participate in cell-cell communication to coordinate neuronal and endocrine regulation of physiological processes in health and disease. Neuropeptides are short peptides ranging in length from ~3 to 40 amino acid residues that are involved in biological functions of pain, stress, obesity, hypertension, mental disorders, cancer, and numerous health conditions. The unique neuropeptide sequences define their specific biological actions. Significantly, this review article discusses how the neuropeptide field is at the crest of expanding knowledge gained from mass-spectrometry-based neuropeptidomic studies, combined with proteomic analyses for understanding the biosynthesis of neuropeptidomes. The ongoing expansion in neuropeptide diversity lies in the unbiased and global mass-spectrometry-based approaches for identification and quantitation of peptides. Current mass spectrometry technology allows definition of neuropeptide amino acid sequence structures, profiling of multiple neuropeptides in normal and disease conditions, and quantitative peptide measures in biomarker applications to monitor therapeutic drug efficacies. Complementary proteomic studies of neuropeptide secretory vesicles provide valuable insight into the protein processes utilized for neuropeptide production, storage, and secretion. Furthermore, ongoing research in developing new computational tools will facilitate advancements in mass-spectrometry-based identification of small peptides. Knowledge of the entire repertoire of neuropeptides that regulate physiological systems will provide novel insight into regulatory mechanisms in health, disease, and therapeutics.

V-ATPase-mediated granular acidification is regulated by the V-ATPase accessory subunit Ac45 in POMC-producing cells

The regulation of the V-ATPase, the proton pump mediating intraorganellar acidification, is still elusive. We find that excess of the neuroendocrine V-ATPase accessory subunit Ac45 reduces the intragranular pH and consequently disturbs prohormone convertase activation and prohormone processing. Thus, Ac45 represents the first V-ATPase regulator.

The vacuolar (H⁺)-ATPase (V-ATPase) is an important proton pump, and multiple critical cell-biological processes depend on the proton gradient provided by the pump. Yet, the mechanism underlying the control of the V-ATPase is still elusive but has been hypothesized to involve an accessory subunit of the pump. Here we studied as a candidate V-ATPase regulator the neuroendocrine V-ATPase accessory subunit Ac45. We transgenically manipulated the expression levels of the Ac45 protein specifically in Xenopus intermediate pituitary melanotrope cells and analyzed in detail the functioning of the transgenic cells. We found in the transgenic melanotrope cells the following: i) significantly increased granular acidification; ii) reduced sensitivity for a V-ATPase-specific inhibitor; iii) enhanced early processing of proopiomelanocortin (POMC) by prohormone convertase PC1; iv) reduced, neutral pH–dependent cleavage of the PC2 chaperone 7B2; v) reduced 7B2-proPC2 dissociation and consequently reduced proPC2 maturation; vi) decreased levels of mature PC2 and consequently reduced late POMC processing. Together, our results show that the V-ATPase accessory subunit Ac45 represents the first regulator of the proton pump and controls V-ATPase-mediated granular acidification that is necessary for efficient prohormone processing.

Pro-hormone secretogranin II regulates dense core secretory granule biogenesis in catecholaminergic cells

Processes underlying the formation of dense core secretory granules (DCGs) of neuroendocrine cells are poorly understood. Here, we present evidence that DCG biogenesis is dependent on the secretory protein secretogranin (Sg) II, a member of the granin family of pro-hormone cargo of DCGs in neuroendocrine cells. Depletion of SgII expression in PC12 cells leads to a decrease in both the number and size of DCGs and impairs DCG trafficking of other regulated hormones. Expression of SgII fusion proteins in a secretory-deficient PC12 variant rescues a regulated secretory pathway. SgII-containing dense core vesicles share morphological and physical properties with bona fide DCGs, are competent for regulated exocytosis, and maintain an acidic luminal pH through the V-type H(+)-translocating ATPase. The granulogenic activity of SgII requires a pH gradient along this secretory pathway. We conclude that SgII is a critical factor for the regulation of DCG biogenesis in neuroendocrine cells, mediating the formation of functional DCGs via its pH-dependent aggregation at the trans-Golgi network.

Vacuolar ATPase regulates surfactant secretion in rat alveolar type II cells by modulating lamellar body calcium

Lung surfactant reduces surface tension and maintains the stability of alveoli. How surfactant is released from alveolar epithelial type II cells is not fully understood. Vacuolar ATPase (V-ATPase) is the enzyme responsible for pumping H(+) into lamellar bodies and is required for the processing of surfactant proteins and the packaging of surfactant lipids. However, its role in lung surfactant secretion is unknown. Proteomic analysis revealed that vacuolar ATPase (V-ATPase) dominated the alveolar type II cell lipid raft proteome. Western blotting confirmed the association of V-ATPase a1 and B1/2 subunits with lipid rafts and their enrichment in lamellar bodies. The dissipation of lamellar body pH gradient by Bafilomycin A1 (Baf A1), an inhibitor of V-ATPase, increased surfactant secretion. Baf A1-stimulated secretion was blocked by the intracellular Ca(2+) chelator, BAPTA-AM, the protein kinase C (PKC) inhibitor, staurosporine, and the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), KN-62. Baf A1 induced Ca(2+) release from isolated lamellar bodies. Thapsigargin reduced the Baf A1-induced secretion, indicating cross-talk between lamellar body and endoplasmic reticulum Ca(2+) pools. Stimulation of type II cells with surfactant secretagogues dissipated the pH gradient across lamellar bodies and disassembled the V-ATPase complex, indicating the physiological relevance of the V-ATPase-mediated surfactant secretion. Finally, silencing of V-ATPase a1 and B2 subunits decreased stimulated surfactant secretion, indicating that these subunits were crucial for surfactant secretion. We conclude that V-ATPase regulates surfactant secretion via an increased Ca(2+) mobilization from lamellar bodies and endoplasmic reticulum, and the activation of PKC and CaMKII. Our finding revealed a previously unrealized role of V-ATPase in surfactant secretion.

Natural variation within the neuronal nicotinic acetylcholine receptor cluster on human chromosome 15q24: influence on heritable autonomic traits in twin pairs

Nicotinic acetylcholine receptors (nAChRs) are combinations of subunits arranged as pentamers encircling a central cation channel. At least nine alpha and four beta subunits are expressed in the central and peripheral nervous systems; their presence in autonomic ganglia, the adrenal medulla, and central nervous system, with accompanying responses elicited by nicotinic agonists, point to their involvement in cardiovascular homeostasis. nAChRs formed by alpha3, alpha5, and beta4 subunits may regulate blood pressure (BP) by mediating release of catestatin, the endogenous nicotinic antagonist fragment of chromogranin A (CHGA) and potent inhibitor of catecholamine secretion. Genes encoding these subunits (CHRNA3, CHRNA5, and CHRNB4) are clustered on human chromosome 15q24. Because variation in this cluster may alter autonomic regulation of BP, we sequenced approximately 15 kilobase pairs in 15q24 containing their coding and 5'- and 3'-untranslated regions in 80 individuals. We identified 63 variants: 25 in coding regions of CHRNA3, CHRNA5, and CHRNB4 and 48 noncoding single-nucleotide polymorphisms (SNPs). Haplotype frequencies varied across ethnic populations. We assessed the contribution of six SNPs in the putative catestatin binding region of CHRNA3 and CHRNB4 to autonomic traits. In twins, catestatin and BP were heritable. CHRNA3 SNPs and haplotypes containing K95K (G285A) associated with circulating plasma catestatin, epinephrine levels, as well as systolic BP, suggesting altered coupling of the nAChRs to BP. Studies of chromaffin cells in vitro reveal that nicotinic agonist stimulation releases catecholamines and CHGA, a process augmented by overexpression of CHRNA3 and blocked by catestatin. These cellular events suggest a homeostatic mechanism underlying the pleiotropic actions of CHRNA3 genetic variation on autonomic function observed in twins.

Inhibitors of the V0 subunit of the vacuolar H+-ATPase prevent segregation of lysosomal- and secretory-pathway proteins

The vacuolar H(+)-ATPase (V-ATPase) establishes pH gradients along secretory and endocytic pathways. Progressive acidification is essential for proteolytic processing of prohormones and aggregation of soluble content proteins. The V-ATPase V(0) subunit is thought to have a separate role in budding and fusion events. Prolonged treatment of professional secretory cells with selective V-ATPase inhibitors (bafilomycin A1, concanamycin A) was used to investigate its role in secretory-granule biogenesis. As expected, these inhibitors eliminated regulated secretion and blocked prohormone processing. Drug treatment caused the formation of large, mixed organelles, with components of immature granules and lysosomes and some markers of autophagy. Markers of the trans-Golgi network and earlier secretory pathway were unaffected. Ammonium chloride and methylamine treatment blocked acidification to a similar extent as the V-ATPase inhibitors without producing mixed organelles. Newly synthesized granule content proteins appeared in mixed organelles, whereas mature secretory granules were spared. Following concanamycin treatment, selected membrane proteins enter tubulovesicular structures budding into the interior of mixed organelles. shRNA-mediated knockdown of the proteolipid subunit of V(0) also caused vesiculation of immature granules. Thus, V-ATPase has a role in protein sorting in immature granules that is distinct from its role in acidification.

Loss-of-function mutations in ATP6V0A2 impair vesicular trafficking, tropoelastin secretion and cell survival

Autosomal recessive cutis laxa type 2 (ARCL2), a syndrome of growth and developmental delay and redundant, inelastic skin, is caused by mutations in the a2 subunit of the vesicular ATPase H+-pump (ATP6V0A2). The goal of this study was to define the disease mechanisms that lead to connective tissue lesions in ARCL2. In a new cohort of 17 patients, DNA sequencing of ATP6V0A2 detected either homozygous or compound heterozygous mutations. Considerable allelic and phenotypic heterogeneity was observed, with a missense mutation of a moderately conserved residue p.P87L leading to unusually mild disease. Abnormal N- and/or mucin type O-glycosylation was observed in all patients tested. Premature stop codon mutations led to decreased ATP6V0A2 mRNA levels by destabilizing the mutant mRNA via the nonsense-mediated decay pathway. Loss of ATP6V0A2 either by siRNA knockdown or in ARCL2 cells resulted in distended Golgi cisternae, accumulation of abnormal lysosomes and multivesicular bodies. Immunostaining of ARCL2 cells showed the accumulation of tropoelastin (TE) in the Golgi and in large, abnormal intracellular and extracellular aggregates. Pulse-chase studies confirmed impaired secretion and increased intracellular retention of TE, and insoluble elastin assays showed significantly reduced extracellular deposition of mature elastin. Fibrillin-1 microfibril assembly and secreted lysyl oxidase activity were normal in ARCL2 cells. TUNEL staining demonstrated increased rates of apoptosis in ARCL2 cell cultures. We conclude that loss-of-function mutations in ATP6V0A2 lead to TE aggregation in the Golgi, impaired clearance of TE aggregates and increased apoptosis of elastogenic cells.

Neutralization of endomembrane compartments in epithelial MDCK cells affects proteoglycan synthesis in the apical secretory pathway

PGs (proteoglycans) are proteins acquiring long, linear and sulfated GAG (glycosaminoglycan) chains during Golgi passage. In MDCK cells (Madin–Darby canine kidney cells), most of the CS (chondroitin sulfate) PGs are secreted apically, whereas most of the HS (heparan sulfate) PGs are secreted basolaterally. The apical and basolateral secretory routes differ in their GAG synthesis, since a protein core that traverses both routes acquires shorter chains, but more sulfate, in the basolateral pathway than in the apical counterpart [Tveit, Dick, Skibeli and Prydz (2005) J. Biol. Chem. 280, 29596–29603]. Golgi cisternae and the trans-Golgi network have slightly acidic lumens. We therefore investigated how neutralization of endomembrane compartments with the vacuolar H+-ATPase inhibitor Baf A1 (bafilomycin A1) affected GAG synthesis and PG sorting in MDCK cells. Baf A1 induced a slight reduction in basolateral secretion of macromolecules, which was compensated by an apical increase. More dramatic changes occurred to PG synthesis in the apical pathway on neutralization. The difference in apical and basolateral PG sulfation levels observed for control cells was abolished, due to enhanced sulfation of apical CS-GAGs. In addition, a large fraction of apical HS-GAGs was elongated to longer chain lengths. The differential sensitivity of the apical and basolateral secretory pathways to Baf A1 indicates that the apical pathway is more acidic than the basolateral counterpart in untreated MDCK cells. Neutralization gave an apical GAG output that was more similar to that of the basolateral pathway, suggesting that neutralization made the luminal environments of the two pathways more similar.

Zebrafish mutants calamity and catastrophe define critical pathways of gene-nutrient interactions in developmental copper metabolism

Nutrient availability is an important environmental variable during development that has significant effects on the metabolism, health, and viability of an organism. To understand these interactions for the nutrient copper, we used a chemical genetic screen for zebrafish mutants sensitive to developmental copper deficiency. In this screen, we isolated two mutants that define subtleties of copper metabolism. The first contains a viable hypomorphic allele of atp7a and results in a loss of pigmentation when exposed to mild nutritional copper deficiency. This mutant displays incompletely penetrant skeletal defects affected by developmental copper availability. The second carries an inactivating mutation in the vacuolar ATPase that causes punctate melanocytes and embryonic lethality. This mutant, catastrophe, is sensitive to copper deprivation revealing overlap between ion metabolic pathways. Together, the two mutants illustrate the utility of chemical genetic screens in zebrafish to elucidate the interaction of nutrient availability and genetic polymorphisms in cellular metabolism.

Copper is an essential nutrient required for multiple biologic functions. Proper uptake, transport, and excretion of copper are critical for use of this metal while reducing its inherent toxicity. While several key proteins involved in this process have been identified, there are still gaps in our understanding of copper metabolism—particularly during early development. We have used zebrafish, a genetically useful animal model system, to study genetic interactions with copper deficiency during development. We treated mutant embryonic zebrafish with a chelator that reduces the level of available copper and screened for mutants that displayed a copper deficient phenotype only in the presence of the chelator. We identified and characterized two mutants that advance our understanding of copper metabolism during the early periods of development, as well as show an interaction between copper metabolism and another fundamental pathway—that of proton transport. Our results expand our knowledge of copper metabolism and illustrate the power of this type of genetic screen in zebrafish to elucidate mechanisms of nutrient metabolism.

Role of the vesicular chloride transporter ClC-3 in neuroendocrine tissue

ClC-3 is an intracellular chloride transport protein known to reside on endosomes and synaptic vesicles. The endogenous protein has been notoriously difficult to detect in immunohistological experiments because of the lack of reliable antibodies. Using newly generated antibodies, we now examine its expression pattern at the cellular and subcellular level. In all tissues examined, immunostaining indicated that ClC-3 is a vesicular protein, with a prominent expression in endocrine cells like adrenal chromaffin cells and pancreatic islet cells. In line with a possible function of ClC-3 in regulating vesicle trafficking or exocytosis in those secretory cells, capacitance measurements and amperometry indicated that exocytosis of large dense-core vesicles (LDCVs) was decreased in chromaffin cells from ClC-3 knock-out mice. However, immunohistochemistry complemented with subcellular fractionation showed that ClC-3 is not detectable on LDCVs of endocrine cells, but localizes to endosomes and synaptic-like microvesicles in both adrenal chromaffin and pancreatic beta cells. This observation points to an indirect influence of ClC-3 on LDCV exocytosis in chromaffin cells, possibly by affecting an intracellular trafficking step.

Accessory subunit Ac45 controls the V-ATPase in the regulated secretory pathway

The vacuolar (H(+))-ATPase (V-ATPase) is crucial for multiple processes within the eukaryotic cell, including membrane transport and neurotransmitter secretion. How the V-ATPase is regulated, e.g. by an accessory subunit, remains elusive. Here we explored the role of the neuroendocrine V-ATPase accessory subunit Ac45 via its transgenic expression specifically in the Xenopus intermediate pituitary melanotrope cell model. The Ac45-transgene product did not affect the levels of the prohormone proopiomelanocortin nor of V-ATPase subunits, but rather caused an accumulation of the V-ATPase at the plasma membrane. Furthermore, a higher abundance of secretory granules, protrusions of the plasma membrane and an increased Ca(2+)-dependent secretion efficiency were observed in the Ac45-transgenic cells. We conclude that in neuroendocrine cells Ac45 guides the V-ATPase through the secretory pathway, thereby regulating the V-ATPase-mediated process of Ca(2+)-dependent peptide secretion.

Heritability and genome-wide linkage in US and australian twins identify novel genomic regions controlling chromogranin a: implications for secretion and blood pressure

BACKGROUND

Chromogranin A (CHGA) triggers catecholamine secretory granule biogenesis, and its catestatin fragment inhibits catecholamine release. We approached catestatin heritability using twin pairs, coupled with genome-wide linkage, in a series of twin and sibling pairs from 2 continents.

METHODS AND RESULTS

Hypertensive patients had elevated CHGA coupled with reduction in catestatin, suggesting diminished conversion of precursor to catestatin. Heritability for catestatin in twins was 44% to 60%. Six hundred fifteen nuclear families yielded 870 sib pairs for linkage, with significant logarithm of odds peaks on chromosomes 4p, 4q, and 17q. Because acidification of catecholamine secretory vesicles determines CHGA trafficking and processing to catestatin, we genotyped at positional candidate ATP6N1, bracketed by peak linkage markers on chromosome 17q, encoding a subunit of vesicular H(+)-translocating ATPase. The minor allele diminished CHGA secretion and processing to catestatin. The ATP6N1 variant also influenced blood pressure in 1178 individuals with the most extreme blood pressure values in the population. In chromaffin cells, inhibition of H(+)-ATPase diverted CHGA from regulated to constitutive secretory pathways.

CONCLUSIONS

We established heritability of catestatin in twins from 2 continents. Linkage identified 3 regions contributing to catestatin, likely novel determinants of sympathochromaffin exocytosis. At 1 such positional candidate (ATP6N1), variation influenced CHGA secretion and processing to catestatin, confirming the mechanism of a novel trans-QTL for sympathochromaffin activity and blood pressure.

Intravesicular calcium release mediates the motion and exocytosis of secretory organelles: a study with adrenal chromaffin cells

Secretory vesicles of sympathetic neurons and chromaffin granules maintain a pH gradient toward the cytosol (pH 5.5 versus 7.2) promoted by the V-ATPase activity. This gradient of pH is also responsible for the accumulation of amines and Ca2+ because their transporters use H+ as the counter ion. We have recently shown that alkalinization of secretory vesicles slowed down exocytosis, whereas acidification caused the opposite effect. In this paper, we measure the alkalinization of vesicular pH, caused by the V-ATPase inhibitor bafilomycin A1, by total internal reflection fluorescence microscopy in cells overexpressing the enhanced green fluorescent protein-labeled synaptobrevin (VAMP2-EGFP) protein. The disruption of the vesicular gradient of pH caused the leak of Ca2+, measured with fura-2. Fluorimetric measurements, using the dye Oregon green BAPTA-2, showed that bafilomycin directly released Ca2+ from freshly isolated vesicles. The Ca2+ released from vesicles to the cytosol dramatically increased the granule motion of chromaffin- or PC12-derived granules and triggered exocytosis (measured by amperometry). We conclude that the gradient of pH of secretory vesicles might be involved in the homeostatic regulation of cytosolic Ca2+ and in two of the major functions of secretory cells, vesicle motion and exocytosis.

The trans-Golgi proteins SCLIP and SCG10 interact with chromogranin A to regulate neuroendocrine secretion

Secretion of proteins and peptides from eukaryotic cells takes place by both constitutive and regulated pathways. Regulated secretion may involve interplay of proteins that are currently unknown. Recent studies suggest an important role of chromogranin A (CHGA) in the regulated secretory pathway in neuroendocrine cells, but the mechanism by which CHGA enters the regulated pathway, or even triggers the formation of the pathway, remains unclear. In this study, we used a transcriptome/proteome-wide approach, to discover binding partners for CHGA, by employing a phage display cDNA library method. Several proteins within or adjacent to the secretory pathway were initially detected as binding partners of recombinant human CHGA. We then focused on the trans-Golgi protein SCLIP (STMN3) and its stathmin paralog SCG10 (STMN2) for functional study. Co-immunoprecipitation experiments confirmed the interaction of each of these two proteins with CHGA in vitro. SCLIP and SCG10 were colocalized to the Golgi apparatus of chromaffin cells in vivo and shared localization with CHGA as it transited the Golgi. Downregulation of either SCLIP or SCG10 by synthetic siRNAs virtually abolished chromaffin cell secretion of a transfected CHGA-EAP chimera (expressing CHGA fused to an enzymatic reporter, and trafficked to the regulated pathway). SCLIP siRNA also decreased the level of secretion of endogenous CHGA and SCG2, as well as transfected human growth hormone, while SCG10 siRNA decreased the level of regulated secretion of endogenous CHGB. Moreover, a dominant negative mutant of SCG10 (Cys 22,Cys 24-->Ala 22,Ala 24) significantly blocked secretion of the transfected CHGA-EAP chimera. A decrease in the buoyant density of chromaffin granules was observed after downregulation of SCG10 by siRNA, suggesting participation of these stathmins in granule formation or maturation. We conclude that SCLIP and SCG10 interact with CHGA, share partial colocalization in the Golgi apparatus, and may be necessary for typical transmitter storage and release from chromaffin cells.

Analysis of regulated secretion using PC12 cells

The catecholamine-secreting PC12 cell line derived from the rat adrenal medulla has long been considered a model system for neurosecretion and neuronal differentiation. PC12 cells contain a large number of secretory granules (otherwise known as large dense-core vesicles) for storage of small molecules, processing enzymes, neuropeptides, and peptide hormones. Secretory granule exocytosis in PC12 cells is tightly regulated by calcium and occurs in response to a secretagogue. This unit provides protocols for maintenance and transfection of PC12 cells. Several secretion assays are described to measure the release of secretory granule cargo molecules by detection of radioactive catecholamine, or by immunochemical or chemiluminescence detection of transfected regulated secretory proteins.

Sorting of the neuroendocrine secretory protein Secretogranin II into the regulated secretory pathway: role of N- and C-terminal alpha-helical domains

Secretogranin II (SgII) belongs to the granin family of prohormones widely distributed in dense-core secretory granules (DCGs) of endocrine, neuroendocrine, and neuronal cells, including sympathoadrenal chromaffin cells. The mechanisms by which secretory proteins, and granins in particular, are sorted into the regulated secretory pathway are unsettled. We designed a strategy based on novel chimeric forms of human SgII fused to fluorescent (green fluorescent protein) or chemiluminescent (embryonic alkaline phosphatase) reporters to identify trafficking determinants mediating DCG targeting of SgII in sympathoadrenal cells. Three-dimensional deconvolution fluorescence microscopy and secretagogue-stimulated release studies demonstrate that SgII chimeras are correctly targeted to DCGs and released by exocytosis in PC12 and primary chromaffin cells. Results from a Golgi-retained mutant form of SgII suggest that sorting of SgII into DCGs depends on a saturable sorting machinery at the trans-Golgi/trans-Golgi network. Truncation analyses reveal the presence of DCG-targeting signals within both the N- and C-terminal regions of SgII, with the putative alpha-helix-containing SgII-(25-41) and SgII-(334-348) acting as sufficient, independent sorting domains. This study defines sequence features of SgII mediating vesicular targeting in sympathoadrenal cells and suggests a mechanism by which discrete domains of the molecule function in sorting, perhaps by virtue of a particular arrangement in tertiary structure and/or interaction with a specific component of the DCG membrane.

Chromogranin A polymorphisms are associated with hypertensive renal disease

Chromogranin A is released together with epinephrine and norepinephrine from catecholaminergic cells. Specific endopeptidases cleave chromogranin A into biologically active peptide fragments, including catestatin, which inhibits catecholamine release. Previous studies have suggested that a deficit in this sympathetic "braking" system might be an early event in the pathogenesis of human hypertension. Whether chromogranin A (CHGA) polymorphisms predict end-organ complications of hypertension, such as end-stage renal disease, is unknown. Among blacks, we studied common genetic variants spanning the CHGA locus in 2 independent case-control studies of hypertensive ESRD. Two haplotypes were significantly more frequent among subjects with hypertensive ESRD: 1) in the promoter (5') region, G-462A-->T-415C-->C-89A, haplotype ATC (adjusted odds ratio = 2.65; P = 0.037), and 2) at the 3'-end, C11825T (3'-UTR, C+87T)-->G12602C, haplotype TC (adjusted odds ratio = 2.73, P = 0.0196). Circulating levels of catestatin were lower among those with hypertensive ESRD than controls, an unexpected finding given that peptide levels are usually elevated in ESRD because of reduced renal elimination. We found that the 3'-UTR + 87T variant decreased reporter gene expression, providing a possible mechanistic explanation for diminished catestatin. In summary, common variants in chromogranin A associate with the risk of hypertensive ESRD in blacks.

Origins of the regulated secretory pathway

Modes of transport of soluble (or luminal) secretory proteins synthesized in the endoplasmic reticulum (ER) could be divided into two groups. The socalled constitutive secretory pathway (CSP) is common to all eukaryotic cells, constantly delivering constitutive soluble secretory proteins (CSSPs) linked to the rate of protein synthesis but largely independent of external stimuli. In regulated secretion, protein is sorted from the Golgi into storage/secretory granules (SGs) whose contents are released when stimuli trigger their final fusion with the plasma membrane (Hannah et al. 1999).

Biogenesis of the secretory granule: chromogranin A coiled-coil structure results in unusual physical properties and suggests a mechanism for granule core condensation

The secretory pro-hormone chromogranin A (CHGA) is densely packed into storage granules along with catecholamines, playing a catalytic role in granule biogenesis. 3-Dimensional structural data on CHGA are lacking. We found a superfamily structural homology for CHGA in the tropomyosin family of alpha-helical coiled-coils, even in mid-molecule regions where primary sequence identity is only modest. The assignment was confirmed by an independent algorithm, suggesting approximately 6-7 such domains spanning CHGA. We provide additional physiochemical evidence (chromatographic, spectral, microscopic) consistent with this unusual structure. Alpha-helical secondary structure (at up to approximately 45%) was confirmed by circular dichroism. CHGA molecular mass was estimated by MALDI-TOF mass spectrometry at approximately 50 kDa and by denaturing gel filtration at approximately 50-61 kDa, while its native Stokes radius was approximately 84.8 A, as compared to an expected approximately 30 A; the increase gave rise to an apparent native molecular weight of approximately 578 kDa, also consistent with the extended conformation of a coiled-coil. Small-angle X-ray scattering (SAXS) on CHGA in solution best fit an elongated cylindrical conformation in the monodisperse region with a radius of gyration of the rod cross-section (Rt) of approximately 52 A, compatible with a coiled-coil in the hydrated, aqueous state, or a multimeric coiled-coil. Electron microscopy with negative staining revealed an extended, filamentous CHGA structure with a diameter of approximately 94 +/- 4.5 A. Extended, coiled-coil conformation is likely to permit protein "packing" in the secretory granule at approximately 50% higher density than a globular/spherical conformation. Natural allelic variation in the catestatin region was predicted to disrupt the coiled-coil. Chromaffin granule ultrastructure revealed a approximately 108 +/- 6.3 A periodicity of electron density, suggesting nucleation of a binding complex by the CHGA core. Inhibition of CHGA expression, by siRNA, disrupted regulated secretory protein traffic by approximately 65%, while targeted ablation of the CHGA gene in the mouse reduced chromaffin granule cotransmitter concentrations by approximately 40-80%. These results suggest new roles for secretory protein tertiary structure in hormone and transmitter storage, with implications for secretory cargo condensation (or dense core "packing" structure) within the regulated pathway.