Binding and aggregation of pro-atrial natriuretic factor by calcium

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.

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.

Intracellular targeting of truncated secretory peptides in the mammalian heart and brain

Secretory polypeptides are vital for nervous system function, sleep, reproduction, growth, and metabolism. Ribosomes scanning the 5'-end of mRNA usually detect the first AUG site for initiating translation. The nascent propeptide chain is then directed via a signal-peptide into the endoplasmic reticulum, processed through the Golgi stacks, and packaged into secretory vesicles. By expressing prepropeptide-EGFP fusion proteins, we observed unusual destinations, mitochondria, nucleus, and cytoplasm, of neuropeptide Y (NPY), atrial natriuretic peptide, and growth hormone in living murine cardiac cells and hypothalamic slices. Subcellular expression was modulated by Zn++ or mutations of N-terminal prohormone sequences but was not due to overexpression in the trans-Golgi network. Mitochondrial targeting of NPY also occurred without the EGFP tag, was enhanced by site-directed mutagenesis of the first AUG initiation site, and abolished by mutation of the second AUG. Immunological methods indicated the presence of N-terminal truncated NPY in mitochondria. Imaging studies showed depolarization of NPY-containing mitochondria. P-SORT software correctly predicted the secondary intracellular destinations and suggested such destinations for many neuropeptides and peptide hormones known. Thus, mammalian cells may retarget secretory peptides from extracellular to intracellular sites by skipping the first translation-initiation codon and thereby alter mitochondrial function, gene expression, and secretion.

Differential aggregation properties of secretory proteins that are stored in exocrine secretory granules of the pancreas and parotid glands

Low-pH- and calcium-induced aggregation of regulated secretory proteins has been proposed to play a role in their retention and storage in secretory granules. However, this has not been tested for secretory proteins that are stored in the exocrine parotid secretory granules. Parotid granule matrix proteins were analyzed for aggregation in the presence or absence of calcium and in the pH range of 5.5 to 7.5. Amylase did not aggregate under these conditions, although <10% of parotid secretory protein (PSP) aggregated below pH 6.0. To test aggregation directly in isolated granules, rat parotid secretory granules were permeabilized with 0.1% saponin in the presence or absence of calcium and in the pH range of 5.0 to 8.4. In contrast to the low-pH-dependent retention of amylase in exocrine pancreatic granules, amylase was quantitatively released and most PSP was released from parotid granules under all conditions. Both proteins were completely released upon granule membrane solubilization. Thus neither amylase nor PSP show low-pH- or calcium-induced aggregation under physiological conditions in the exocrine parotid secretory granules.

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.

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.

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.

Analysis of constitutive and constitutive-like secretion in semi-intact pituitary cells

To study biosynthetic transport through the constitutive and regulated secretory pathways, we have designed a semi-intact mammalian cell system that restores the transport of secretory proteins from the trans-Golgi/trans-Golgi network (TGN) to the cell surface. The mouse pituitary AtT-20 cell line is a suitable model to biochemically analyze molecular sorting in the secretory pathway. The prohormone proopiomelanocortin is sulfated on N-linked carbohydrate chains in the trans-Golgi prior to proteolytic processing in the secretory granule. Radiolabeling with [35S]sulfate therefore provides a convenient tool to selectively follow molecular events in the regulated secretory pathway without interference from earlier steps. Likewise, transport through the constitutive secretory pathway may be monitored using sulfate-labeled glycosaminoglycan chains. We show that export from the TGN is efficiently reconstituted in cells made semi-intact with streptolysin O, and is dependent on temperature, ATP and GTP hydrolysis, and cytosol. Packaging of proopiomelanocortin into immature secretory granules also activates the proteolytic processing machinery which eventually converts the prohormone to its bioactive mature product, adrenocorticotropic hormone. In addition, a large fraction of incompletely processed proopiomelanocortin is secreted as the processing intermediates from immature secretory granules. This process of constitutive-like secretion can be clearly distinguished from direct constitutive secretion from the trans-Golgi network by kinetic and compositional criteria. Furthermore, we have found that specific inhibitors of different protein phosphatases and kinases are potent blockers of constitutive and constitutive-like secretion. This experimental model should provide a valuable system to elucidate the molecular mechanism regulating post-Golgi traffic during secretory granule biogenesis.

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

Atrial natriuretic peptide (ANP) is a 28-aa peptide hormone secreted predominantly from atrial cardiocytes. ANP is first synthesized in the form of a 126-aa precursor (proANP) which is targeted to dense core granules of the regulated secretory pathway. ProANP is stored until the cell receives a signal that triggers the processing and release of the mature peptide (regulated secretion). Various models have been proposed to explain the targeting of selected proteins to the regulated secretory pathway, including specific "sorting receptors" and calcium-mediated aggregation. As potential calcium binding regions had previously been reported in the profragment of ANP, the current study was undertaken in an effort to determine the relationship between the ability of ANP to enter the regulated secretory pathway and its calcium-mediated aggregation. Deletion and site-directed mutagenesis of selected regions of the prosegment demonstrates that acidic amino acids at positions 23 and 24 are critical for both regulated secretion of proANP from transfected AtT-20 cells and calcium-mediated aggregation of purified recombinant proANP in vitro. These results demonstrate that the ability of certain proteins to enter secretory granules is directly linked to their calcium-mediated aggregation.