Inositol 1,4,5-trisphosphate receptor/Ca(2+) channel modulatory role of chromogranins A and B

The surging role of Chromogranin A in cardiovascular homeostasis

Together with Chromogranin B and Secretogranins, Chromogranin A (CGA) is stored in secretory (chromaffin) granules of the diffuse neuroendocrine system and released with noradrenalin and adrenalin. Co-stored within the granule together with neuropeptideY, cardiac natriuretic peptide hormones, several prohormones and their proteolytic enzymes, CGA is a multifunctional protein and a major marker of the sympatho-adrenal neuroendocrine activity. Due to its partial processing to several biologically active peptides, CGA appears an important pro-hormone implicated in relevant modulatory actions on endocrine, cardiovascular, metabolic, and immune systems through both direct and indirect sympatho-adrenergic interactions. As a part of this scenario, we here illustrate the emerging role exerted by the full-length CGA and its three derived fragments, i.e., Vasostatin 1, catestatin and serpinin, in the control of circulatory homeostasis with particular emphasis on their cardio-vascular actions under both physiological and physio-pathological conditions. The Vasostatin 1- and catestatin-induced cardiodepressive influences are achieved through anti-beta-adrenergic-NO-cGMP signaling, while serpinin acts like beta1-adrenergic agonist through AD-cAMP-independent NO signaling. On the whole, these actions contribute to widen our knowledge regarding the sympatho-chromaffin control of the cardiovascular system and its highly integrated “whip-brake” networks.

Catecholamine storage vesicles: role of core protein genetic polymorphisms in hypertension

Hypertension is a complex trait with deranged autonomic control of the circulation. The sympathoadrenal system exerts minute-to-minute control over cardiac output and vascular tone. Catecholamine storage vesicles (or chromaffin granules) of the adrenal medulla contain remarkably high concentrations of chromogranins/secretogranins (or "granins"), catecholamines, neuropeptide Y, adenosine triphosphate (ATP), and Ca(2+). Within secretory granules, granins are co-stored with catecholamine neurotransmitters and co-released upon stimulation of the regulated secretory pathway. The principal granin family members, chromogranin A (CHGA), chromogranin B (CHGB), and secretogranin II (SCG2), may have evolved from shared ancestral exons by gene duplication. This article reviews human genetic variation at loci encoding the major granins and probes the effects of such polymorphisms on blood pressure, using twin pairs to probe heritability and individuals with the most extreme blood pressure values in the population to study hypertension.

Secretogranin III directs secretory vesicle biogenesis in mast cells in a manner dependent upon interaction with chromogranin A

Mast cells are granular immunocytes that reside in the body's barrier tissues. These cells orchestrate inflammatory responses. Proinflammatory mediators are stored in granular structures within the mast cell cytosol. Control of mast cell granule exocytosis is a major therapeutic goal for allergic and inflammatory diseases. However, the proteins that control granule biogenesis and abundance in mast cells have not been elucidated. In neuroendocrine cells, whose dense core granules are strikingly similar to mast cell granules, granin proteins regulate granulogenesis. Our studies suggest that the Secretogranin III (SgIII) protein is involved in secretory granule biogenesis in mast cells. SgIII is abundant in mast cells, and is organized into vesicular structures. Our results show that over-expression of SgIII in mast cells is sufficient to cause an expansion of a granular compartment in these cells. These novel granules store inflammatory mediators that are released in response to physiological stimuli, indicating that they function as bona fide secretory vesicles. In mast cells, as in neuroendocrine cells, we show that SgIII is complexed with Chromogranin A (CgA). CgA is granulogenic when complexed with SgIII. Our data show that a novel non-granulogenic truncation mutant of SgIII (1-210) lacks the ability to interact with CgA. Thus, in mast cells, a CgA-SgIII complex may play a key role in secretory granule biogenesis. SgIII function in mast cells is unlikely to be limited to its partnership with CgA, as our interaction trap analysis suggests that SgIII has multiple binding partners, including the mast cell ion channel TRPA1.

Naturally occurring human genetic variation in the 3'-untranslated region of the secretory protein chromogranin A is associated with autonomic blood pressure regulation and hypertension in a sex-dependent fashion

OBJECTIVES

We aimed to determine whether the common variation at the chromogranin A (CHGA) locus increases susceptibility to hypertension.

BACKGROUND

CHGA regulates catecholamine storage and release. Previously we systematically identified genetic variants across CHGA.

METHODS

We carried out dense genotyping across the CHGA locus in >1,000 individuals with the most extreme blood pressures (BPs) in the population, as well as twin pairs with autonomic phenotypes. We also characterized the function of a trait-associated 3'-untranslated region (3'-UTR) variant with transfected CHGA 3'-UTR/luciferase reporter plasmids.

RESULTS

CHGA was overexpressed in patients with hypertension, especially hypertensive men, and CHGA predicted catecholamines. In individuals with extreme BPs, CHGA genetic variants predicted BP, especially in men, with a peak association occurring in the 3'-UTR at C+87T, accounting for up to approximately 12/ approximately 9 mm Hg. The C+87T genotype predicted CHGA secretion in vivo, with the +87T allele (associated with lower BP) also diminishing plasma CHGA by approximately 10%. The C+87T 3'-UTR variant also predicted the BP response to environmental (cold) stress; the same allele (+87T) that diminished basal BP in the population also decreased the systolic BP response to stress by approximately 12 mm Hg, and the response was smaller in women (by approximately 6 mm Hg). In a chromaffin cell-transfected CHGA 3'-UTR/luciferase reporter plasmid, the +87T allele associated with lower BP also decreased reporter expression by approximately 30%. In cultured chromaffin cells, reducing endogenous CHGA expression by small interfering ribonucleic acid caused approximately two-thirds depletion of catecholamine storage vesicles.

CONCLUSIONS

Common variant C+87T in the CHGA 3'-UTR is a functional polymorphism causally associated with hypertension especially in men of the population, and we propose steps ("intermediate phenotypes") whereby in a sex-dependent fashion this genetic variant influences the ultimate disease trait. These observations suggest new molecular strategies to probe the pathophysiology, risk, and rational treatment of hypertension.

Identification of residues participating in the interaction between an intraluminal loop of inositol 1,4,5-trisphosphate receptor and a conserved N-terminal region of chromogranin B

The inositol 1,4,5-trisphosphate receptor (IP3R) is a membrane channel that conducts calcium ions from the intracellular calcium stores. Despite a wealth of information on the cytoplasmic regulation of the IP3R, little is known about its regulation on the luminal side of the calcium stores. Here, we report studies on the IP3R intraluminal loop L3-2 and a conserved N-terminal region of chromogranin B. The IP3R loop is an important part of the channel's pore-forming region, and the chromogranin peptide has been shown to competitively inhibit calcium signaling by IP3R. Using the NMR titration approach, we showed that a part of the L3-2 is involved in a specific interaction with the chromogranin B peptide. Further NMR resonance assignments revealed that the 14th-20th residues of L3-2 are the keys to the binding to the chromogranin B peptide. Through detailed analysis of the data, we suggest a mechanism of IP3R regulation by chromogranin B involving conformational exchanges of the L3-2 region. Our report presents the findings of the first study on the interaction between the luminal loop of the IP3 receptor and its regulator at residue-resolution. The approaches described here should help to guide further studies on the interactions between the IP3R and other luminal side regulators.

Pleiotropic effects of novel trans-acting loci influencing human sympathochromaffin secretion

Family studies have suggested a genetic contribution to variation in blood pressure, but the genes responsible have thus far eluded identification. The use of intermediate phenotypes associated with hypertension, such as chromogranin plasma concentrations, may assist the discovery of hypertension-predisposing loci. We measured the concentrations of four chromogranin A (CHGA) and B (CHGB) peptides in 742 individuals from 235 nuclear families. The CHGA- and CHGB-derived peptides displayed significant heritability and revealed significant genetic correlations, most strikingly observed between CHGA(361-372) (catestatin) and CHGB(439-451). A 5-cM microsatellite genome scan revealed significant and suggestive evidence for linkage on several chromosomes for three of the peptides. Subsequent bivariate linkage analysis for peptides CHGA(361-372) and CHGB(439-451), which showed evidence for convergent linkage peaks on chromosomes 2, 7, and 13, resulted in increased evidence for linkage to these regions, suggesting pleiotropic effects of these three loci on multiple chromogranin traits. Because CHGA itself is on chromosome 14q32, and CHGB itself is on chromosome 20pter-p12, the pleiotropic regions on chromosomes 2, 7, and 13 must represent trans-acting quantitative trait loci coordinately affecting CHGA/CHGB biosynthesis and/or exocytotic secretion, likely by regulating efferent sympathetic outflow, a conclusion consistent with the in vitro studies presented here of the dual control of both exocytosis and transcription of these peptides by secretory stimuli in chromaffin cells. The results suggest a new approach to heritable autonomic control of circulation and the genetic basis of cardiovascular diseases such as systemic hypertension.

A dynamic pool of calcium in catecholamine storage vesicles. Exploration in living cells by a novel vesicle-targeted chromogranin A-aequorin chimeric photoprotein

Chromaffin vesicles contain very high concentration of Ca2+ (approximately 20-40 mM total), compared with approximately 100 nM in the cytosol. Aequorin, a jellyfish photoprotein with Ca(2+)-dependent luminescence, measures [Ca2+] in specific subcellular compartments wherein proteins with organelle-specific trafficking domains are fused in-frame to aequorin. Because of the presence of vesicular trafficking domain within CgA we engineered sorting of an expressed human CgA-Aequorin fusion protein (hCgA-Aeq) into the vesicle compartment as confirmed by sucrose density gradients and confocal immunofluorescent co-localization studies. hCgA-Aeq and cytoplasmic aequorin (Cyto-Aeq) luminescence displayed linear functions of [Ca2+] in vitro, over >5 log10 orders of magnitude (r > 0.99), and down to at least 10(-7) M sensitivity. Calibrating the pH dependence of hCgA-Aeq luminescence allowed estimation of [Ca2+]ves at granule interior pH (approximately 5.5). In the cytoplasm, Cyto-Aeq accurately determined [Ca2+]cyto under both basal ([Ca2+]cyto = 130 +/- 35 nM) and exocytosis-stimulated conditions, confirmed by an independent reference technique (Indo-1 fluorescence). The hCgA-Aeq chimera determined vesicular free [Ca2+]ves = 1.4 +/- 0.3 microM under basal conditions indicating that >99% of granule total Ca2+ is in a "bound" state. The basal free [Ca2+]ves/[Ca2+]cyto ratio was thus approximately 10.8-fold, indicating active, dynamic Ca2+ uptake from cytosol into the granules. Stimulation of exocytotic secretion revealed prompt, dynamic increases in both [Ca2+](ves) and [Ca2+]cyto, and an exponential relation between the two (y = 0.99 x e(1.53x), r = 0.99), reflecting a persistent [Ca2+]ves/[Ca2+]cyto gradient, even during sharp increments of both values. Studies with inhibitors of Ca2+ translocation (Ca(2+)-ATPase), Na+/Ca(+)-exchange, Na+/H(+)-exchange, and vesicle acidification (H(+)-translocating ATPase), documented a role for these four ion transporter classes in accumulation of Ca2+ inside the vesicles.

Genome-wide linkage analysis of chromogranin B expression in the CEPH pedigrees: implications for exocytotic sympathochromaffin secretion in humans

Chromogranin B (CgB), a major member of the chromogranin/secretogranin family of catecholamine storage vesicle secretory proteins, plays both intracellular (vesiculogenic) and extracellular (prohormone) roles in the neuroendocrine system, and its biosynthesis and release are under the control of efferent sympathetic nerve traffic ("stimulus-transcription coupling"). To explore the role of heredity in control of CgB, we conducted a genome-wide linkage analysis of CgB release in 12 extended CEPH (Centre d'Etude du Polymorphisme Humain) pedigrees. Region-specific radioimmunoassays were used to measure five CgB fragments in plasma: CgB1-16, CgB312-331, CgB439-451, CgB568-577, and CgB647-657. Substantial heritability, as measured by h2r, was observed for three of the fragment concentrations, CgB312-331, CgB439-451, and CgB568-577, which yielded h2r estimates ranging from 0.378 (P = 0.002) to 0.910 (P < 0.0000001). Variance-component genome-wide linkage analysis with 654 microsatellite markers at 5 cM spacing identified a major quantitative trait locus for CgB312-331 on chromosome 11q24-q25 with a maximum multipoint LOD score of 5.84. Significant allelic associations between markers in the region and CgB levels were also observed. Although the 2-LOD confidence interval for linkage did not include the CgB locus itself, known trans-activators of the CgB gene promoter, or prohormone cleaving proteases, examination of positional candidate loci within this region yielded novel and plausible physiological candidates for further exploration. Allelic variation in this region may thus influence effects of sympathetic outflow on target organs in humans.

Both rare and common polymorphisms contribute functional variation at CHGA, a regulator of catecholamine physiology

The chromogranin/secretogranin proteins are costored and coreleased with catecholamines from secretory vesicles in chromaffin cells and noradrenergic neurons. Chromogranin A (CHGA) regulates catecholamine storage and release through intracellular (vesiculogenic) and extracellular (catecholamine release-inhibitory) mechanisms. CHGA is a candidate gene for autonomic dysfunction syndromes, including intermediate phenotypes that contribute to human hypertension. Here, we show a surprising pattern of CHGA variants that alter the expression and function of this gene, both in vivo and in vitro. Functional variants include both common alleles that quantitatively alter gene expression and rare alleles that qualitatively change the encoded product to alter the signaling potency of CHGA-derived catecholamine release-inhibitory catestatin peptides.