Immunolocalization of CRHSP28 in exocrine digestive glands and gastrointestinal tissues of the rat

Acute acinar pancreatitis blocks vesicle-associated membrane protein 8 (VAMP8)-dependent secretion, resulting in intracellular trypsin accumulation

Zymogen secretory granules in pancreatic acinar cells express two vesicle-associated membrane proteins (VAMP), VAMP2 and -8, each controlling 50% of stimulated secretion. Analysis of secretion kinetics identified a first phase (0-2 min) mediated by VAMP2 and second (2-10 min) and third phases (10-30 min) mediated by VAMP8. Induction of acinar pancreatitis by supramaximal cholecystokinin (CCK-8) stimulation inhibits VAMP8-mediated mid- and late-phase but not VAMP2-mediated early-phase secretion. Elevation of cAMP during supramaximal CCK-8 mitigates third-phase secretory inhibition and acinar damage caused by the accumulation of prematurely activated trypsin. VAMP8^-/- acini are resistant to secretory inhibition by supramaximal CCK-8, and despite a 4.5-fold increase in total cellular trypsinogen levels, are fully protected from intracellular trypsin accumulation and acinar damage. VAMP8-mediated secretion is dependent on expression of the early endosomal proteins Rab5, D52, and EEA1. Supramaximal CCK-8 (60 min) caused a 60% reduction in the expression of D52 followed by Rab5 and EEA1 in isolated acini and in in vivo The loss of D52 occurred as a consequence of its entry into autophagic vacuoles and was blocked by lysosomal cathepsin B and L inhibition. Accordingly, adenoviral overexpression of Rab5 or D52 enhanced secretion in response to supramaximal CCK-8 and prevented accumulation of activated trypsin. These data support that acute inhibition of VAMP8-mediated secretion during pancreatitis triggers intracellular trypsin accumulation and loss of the early endosomal compartment. Maintaining anterograde endosomal trafficking during pancreatitis maintains VAMP8-dependent secretion, thereby preventing accumulation of activated trypsin.

Vesicle associated membrane protein 8 (VAMP8)-mediated zymogen granule exocytosis is dependent on endosomal trafficking via the constitutive-like secretory pathway

Acinar cell zymogen granules (ZG) express 2 isoforms of the vesicle-associated membrane protein family (VAMP2 and -8) thought to regulate exocytosis. Expression of tetanus toxin to cleave VAMP2 in VAMP8 knock-out (-/-) acini confirmed that VAMP2 and -8 are the primary VAMPs for regulated exocytosis, each contributing ∼50% of the response. Analysis of VAMP8(-/-) acini indicated that although stimulated secretion was significantly reduced, a compensatory increase in constitutive secretion maintained total secretion equivalent to wild type (WT). Using a perifusion system to follow secretion over time revealed VAMP2 mediates an early rapid phase peaking and falling within 2-3 min, whereas VAMP8 controls a second prolonged phase that peaks at 4 min and slowly declines over 20 min to support the protracted secretory response. VAMP8(-/-) acini show increased expression of the endosomal proteins Ti-VAMP7 (2-fold) and Rab11a (4-fold) and their redistribution from endosomes to ZGs. Expression of GDP-trapped Rab11a-S25N inhibited secretion exclusively from the VAMP8 but not the VAMP2 pathway. VAMP8(-/-) acini also showed a >90% decrease in the early endosomal proteins Rab5/D52/EEA1, which control anterograde trafficking in the constitutive-like secretory pathway. In WT acini, short term (14-16 h) culture also results in a >90% decrease in Rab5/D52/EEA1 and a complete loss of the VAMP8 pathway, whereas VAMP2-secretion remains intact. Remarkably, rescue of Rab5/D52/EEA1 expression restored the VAMP8 pathway. Expressed D52 shows extensive colocalization with Rab11a and VAMP8 and partially copurifies with ZG fractions. These results indicate that robust trafficking within the constitutive-like secretory pathway is required for VAMP8- but not VAMP2-mediated ZG exocytosis.

Proteomic screening of glutamatergic mouse brain synaptosomes isolated by fluorescence activated sorting

For decades, neuroscientists have used enriched preparations of synaptic particles called synaptosomes to study synapse function. However, the interpretation of corresponding data is problematic as synaptosome preparations contain multiple types of synapses and non-synaptic neuronal and glial contaminants. We established a novel Fluorescence Activated Synaptosome Sorting (FASS) method that substantially improves conventional synaptosome enrichment protocols and enables high-resolution biochemical analyses of specific synapse subpopulations. Employing knock-in mice with fluorescent glutamatergic synapses, we show that FASS isolates intact ultrapure synaptosomes composed of a resealed presynaptic terminal and a postsynaptic density as assessed by light and electron microscopy. FASS synaptosomes contain bona fide glutamatergic synapse proteins but are almost devoid of other synapse types and extrasynaptic or glial contaminants. We identified 163 enriched proteins in FASS samples, of which FXYD6 and Tpd52 were validated as new synaptic proteins. FASS purification thus enables high-resolution biochemical analyses of specific synapse subpopulations in health and disease.

Tumor protein D52 controls trafficking of an apical endolysosomal secretory pathway in pancreatic acinar cells

Zymogen granule (ZG) formation in acinar cells involves zymogen cargo sorting from trans-Golgi into immature secretory granules (ISGs). ISG maturation progresses by removal of lysosomal membrane and select content proteins, which enter endosomal intermediates prior to their apical exocytosis. Constitutive and stimulated secretion through this mechanism is termed the constitutive-like and minor-regulated pathways, respectively. However, the molecular components that control membrane trafficking within these endosomal compartments are largely unknown. We show that tumor protein D52 is highly expressed in endosomal compartments following pancreatic acinar cell stimulation and regulates apical exocytosis of an apically directed endolysosomal compartment. Secretion from the endolysosomal compartment was detected by cell-surface antigen labeling of lysosome-associated membrane protein LAMP1, which is absent from ZGs, and had incomplete overlap with surface labeling of synaptotagmin 1, a marker of ZG exocytosis. Although culturing (16-18 h) of isolated acinar cells is accompanied by a loss of secretory responsiveness, the levels of SNARE proteins necessary for ZG exocytosis were preserved. However, levels of endolysosomal proteins D52, EEA1, Rab5, and LAMP1 markedly decreased with culture. When D52 levels were restored by adenoviral delivery, the levels of these regulatory proteins and secretion of both LAMP1 (endolysosomal) and amylase was strongly enhanced. These secretory effects were absent in alanine and aspartate substitutions of serine 136, the major D52 phosphorylation site, and were inhibited by brefeldin A, which does not directly affect the ZG compartment. Our results indicate that D52 directly regulates apical endolysosomal secretion and are consistent with previous studies, suggesting that this pathway indirectly regulates ZG secretion of digestive enzymes.

A role for tumor protein TPD52 phosphorylation in endo-membrane trafficking during cytokinesis

Tumor protein D52 is expressed at high levels in exocrine cells containing large secretory granules where it regulates Ca(2+)-dependent protein secretion; however, D52 expression is also highly induced in multiple cancers. The present study investigated a role for the Ca(2+)-dependent phosphorylation of D52 at the single major phospho-acceptor site serine 136 on cell division. Ectopic expression of wild type D52 (D52wt) and the phosphomutants serine 136/alanine (S136A) or serine 136/glutamate (S136/E) resulted in significant multinucleation of cells. D52wt and S136/E each resulted in a greater than 2-fold increase in multinucleated cells compared to plasmid-transfected controls whereas the S136/A phospho-null mutant caused a 9-fold increase in multinucleation at 48h post-transfection. Electron microscopy revealed D52 expression induced a marked accumulation of vesicles along the mid-line between nuclei where the final stages of cell abscission normally occurs. Supporting this, D52wt strongly colocalized on vesicular structures containing the endosomal regulatory protein vesicle associated membrane protein 8 (VAMP 8) and this colocalization significantly increased with elevations in cellular Ca(2+). As VAMP 8 is known to be necessary for the endo-membrane fusion reactions that mediate the final stages of cytokinesis, these data indicate that D52 expression and phosphorylation at serine 136 play an important role in supporting the Ca(2+)-dependent membrane trafficking events necessary for cytokinesis in rapidly proliferating cancer cells.

MAL2 selectively regulates polymeric IgA receptor delivery from the Golgi to the plasma membrane in WIF-B cells

Myelin and lymphocyte protein 2 (MAL2) has been identified as a hepatic transcytotic regulator that mediates delivery from basolateral endosomes to the subapical compartment (SAC). However, overexpression of polymeric immunoglobulin A-receptor (pIgA-R) in polarized, hepatic WIF-B cells led to the dramatic redistribution of MAL2 into the Golgi and all the transcytotic intermediates occupied by the receptor. Although overexpressed hemagglutinin and dipeptidylpeptidase IV (DPPIV) distributed to the same compartments, MAL2 distributions did not change indicating the effect is selective. Cycloheximide treatment led to decreased pIgA-R and MAL2 intracellular staining, first in the Golgi then the SAC, suggesting they were apically delivered and that MAL2 was mediating the process. This was tested in Clone 9 cells (that lack endogenous MAL2). When expressed alone, pIgA-R was restricted to the Golgi whereas when coexpressed with MAL2, it distributed to the surface, was internalized and delivered to MAL2-positive puncta. In contrast, DPPIV distributions were independent of MAL2. Surface delivery of newly synthesized pIgA-R, but not DPPIV, was enhanced greater than ninefold by MAL2 coexpression. In WIF-B cells where MAL2 expression was knocked down, pIgA-R, but not DPPIV, was retained in the Golgi and its basolateral delivery was impaired. Thus, in addition to its role in transcytosis, MAL2 also regulates pIgA-R delivery from the Golgi to the plasma membrane.

Tumor protein D52 expression and Ca2+-dependent phosphorylation modulates lysosomal membrane protein trafficking to the plasma membrane

Tumor protein D52 (also known as CRHSP-28) is highly expressed in multiple cancers and tumor-derived cell lines; however, it is normally abundant in secretory epithelia throughout the digestive system, where it has been implicated in Ca(2+)-dependent digestive enzyme secretion (41). Here we demonstrate, using site-specific mutations, that Ca(2+)-sensitive phosphorylation at serine 136 modulates the accumulation of D52 at the plasma membrane within 2 min of cell stimulation. When expressed in Chinese hamster ovary CHO-K1 cells, D52 colocalized with adaptor protein AP-3, Rab27A, vesicle-associated membrane protein VAMP7, and lysosomal-associated membrane protein LAMP1, all of which are present in lysosome-like secretory organelles. Overexpression of D52 resulted in a marked accumulation of LAMP1 on the plasma membrane that was further enhanced following elevation of cellular Ca(2+). Strikingly, mutation of serine 136 to alanine abolished the Ca(2+)-stimulated accumulation of LAMP1 at the plasma membrane whereas phosphomimetic mutants constitutively induced LAMP1 plasma membrane accumulation independent of elevated Ca(2+). Identical results were obtained for endogenous D52 in normal rat kidney and HeLA cells, where both LAMP1 and D52 rapidly accumulated on the plasma membrane in response to elevated cellular Ca(2+). Finally, D52 induced the uptake of LAMP1 antibodies from the cell surface in accordance with both the level of D52 expression and phosphorylation at serine 136 demonstrating that D52 altered the plasma membrane recycling of LAMP1-associated secretory vesicles. These findings implicate both D52 expression and Ca(2+)-dependent phosphorylation at serine 136 in lysosomal membrane trafficking to and from the plasma membrane providing a novel Ca(2+)-sensitive pathway modulating the lysosome-like secretory pathway.

Identification of calcineurin regulated phosphorylation sites on CRHSP-24

CRHSP-24 is a prominently regulated phosphoprotein in pancreatic acinar cells where it is the major substrate for the serine/threonine protein phosphatase, calcineurin, in response to secretagogues. We now identify the four regulated sites of CRHSP-24 phosphorylation as serines 30, 32, 41, and 52 and show that Ser(30) and Ser(32) are directly dephosphorylated by calcineurin. Coordinate phosphorylation/dephosphorylation of these four serines explains the multiple phosphorylated isoforms of CRHSP-24 present in acinar cells and provides a molecular framework to study CRHSP-24 regulation by secretagogues and growth factor-induced kinases and phosphatases in vivo.

Calcium/calmodulin-dependent phosphorylation of tumor protein D52 on serine residue 136 may be mediated by CAMK2delta6

Tumor protein D52 is expressed at relatively high levels in cells within the gastrointestinal tract that undergo classical exocytosis and is overexpressed in several cancers. Current evidence supports a role for D52 in the regulation of vesicular trafficking. D52 function(s) are regulated by calcium-dependent phosphorylation; however, the intracellular mechanisms that mediate this process are not well characterized. The goal of this study was to identify the calcium-dependent phosphorylation site(s) in D52 and to characterize the protein kinase(s) that mediate this phosphorylation. Using mass spectrometry and site-directed mutagenesis, we identified a single amino acid residue, S(136), that undergoes increased phosphorylation upon elevation of intracellular Ca(2+) concentration. A phosphospecific antibody (pS(136)) was produced and used to characterize D52 kinase activity in gastric mucosal, colonic T84, and HEK293 cells. By using D52 as a substrate, a protein kinase with a molecular weight (M(r)) of approximately 50 kDa was identified with "in gel" assays. This kinase comigrated with rat brain calcium/calmodulin-dependent protein kinase (CAMK2)alpha cross-reacted with pan-specific CAMK2 antibodies as well as with anti-active CAMK2 (pT(286/287)) antibody when activated. Carbachol-stimulated phosphorylation of S(136) was inhibited by the CAMK2 inhibitor KN93 (IC(50) 38 microM) and by the calmodulin antagonist W7 (IC(50) 3.3 nM). A previously uncharacterized CAMK2 isoform, CAMK2delta6, which has the same domain structure and M(r) as CAM2alpha, was identified in gastric mucosa by RT-PCR. The cloned, expressed protein comigrated with D52 kinase and colocalized with D52 protein in T84 and HEK293 cells. These findings support a role for CAMK2delta6 in the mediation of D52 phosphorylation.

Secretagogue-induced translocation of CRHSP-28 within an early apical endosomal compartment in acinar cells

Ca(2+)-regulated heat-stable protein (CRHSP-28) is a member of the TPD52 protein family that has been shown to regulate Ca(2+)-dependent secretory activity in pancreatic acinar cells. Immunofluorescence microscopy of isolated lobules demonstrated that CRHSP-28 is localized to a supranuclear apical compartment in acini and accumulates immediately below the apical membrane within 2 min of CCK octapeptide (CCK-8) stimulation. Dual-immunofluorescence microscopy demonstrated an endosomal localization of CRHSP-28 that strongly overlapped with early endosomal antigen-1 (EEA-1) on vesicular structures throughout the apical cytoplasm but showed only minimal overlap with the transferrin receptor, which is present in basolaterally derived endosomes. Significant overlapping of CRHSP-28 with the trans-Golgi network marker-38 was also noted in supranuclear regions of acini. Interestingly, treatment of lobules with brefeldin A reversibly disrupted the vesicular localization of CRHSP-28 and EEA-1 within the apical cytoplasm. The CCK-8-induced accumulation of CRHSP-28 in subapical regions of acini was not altered by inhibition of apical endocytosis with the actin filament-disrupting agent latrunculin B. Immunoelectron microscopy confirmed that CRHSP-28 is associated with the limiting membrane of irregularly shaped vesicular structures of low electron density in the apical cytoplasm that are positive for EEA-1 staining. Sparse, but significant, CRHSP-28 immunoreactivity was also observed along the limiting membrane of zymogen granules. Consistent with immunofluorescence data, CRHSP-28 was found to accumulate in clusters on endosomes and positioned between zymogen granules below the cell apex on CCK-8 stimulation. These data indicate that CRHSP-28 is present within endocytic and exocytic compartments of acinar cells and is acutely regulated by secretagogue stimulation.

CaM kinase II regulation of CRHSP-28 phosphorylation in cultured mucosal T84 cells

Ca(2+)-regulated heat-stable protein of 28 kDa (CRHSP-28; a member of the tumor protein D52 family) is highly expressed in exocrine glands and was shown to regulate digestive enzyme secretion from pancreatic acinar cells. We found CRHSP-28 highly expressed in cultured mucosal secretory T84 cells, consistent with an important regulatory role in apical membrane trafficking. Stimulation of cells with carbachol (CCh) induced rapid, concentration-dependent phosphorylation of CRHSP-28 on at least two serine residues. Isoelectric focusing and immunoblotting were used to characterize cellular mechanisms governing CRHSP-28 phosphorylation. Phosphorylation depends on elevated cellular Ca2+, being maximally induced by ionomycin and thapsigargin and fully inhibited by BAPTAAM. In vitro phosphorylation of recombinant CRHSP-28 was 10-fold greater by casein kinase II (CKII) than Ca2+/calmodulin-dependent protein kinase II (CaMKII). However, phosphopeptide mapping studies demonstrated that CaMKII induced an identical phosphopeptide profile to endogenous CRHSP-28 immunoprecipitated from T84 cells. Although calmodulin antagonists had no effect on CCh-stimulated phosphorylation, disruption of actin filaments by cytochalasin D inhibited phosphorylation by 50%. Confocal microscopy indicated that CRHSP-28 is expressed in perinuclear regions of cells and accumulates immediately below the apical membrane of polarized monolayers following CCh stimulation. CaMKII was also localized to the subapical cytoplasm and was clearly displaced following actin filament disruption. These data suggest that CRHSP-28 phosphorylation is regulated by a CaMKII-like enzyme and likely involves a translocation of the protein within the apical cytoplasm of epithelial cells.

Arginine induced acute pancreatitis alters the actin cytoskeleton and increases heat shock protein expression in rat pancreatic acinar cells

Arginine induced acute pancreatitis was evaluated as a novel and distinct form of experimental pancreatitis with particular attention to the actin cytoskeleton and expression of heat shock or stress proteins. Arginine induced a dose related necrotising pancreatitis in rats, as shown by histological evaluation, and an increase in serum amylase. Severe pancreatitis induced by 4.5 g/kg arginine was accompanied by dramatic changes in the actin cytoskeleton, as visualised with rhodamine phallodin. Intermediate filaments were also disrupted, as visualised by cytokeratin 8/18 immunocytochemistry. Arginine pancreatitis was accompanied by a stress response with a large increase in the small heat shock protein HSP27, as well as HSP70, peaking at 24 hours and localised to acinar cells. There was a lower increase in HSP60 and HSP90 and no effect on GRP78. HSP27 was also shifted to phosphorylated forms during pancreatitis. A lower dose of arginine (3.0 g/kg) induced less pancreatitis but a larger increase in HSP70 and HSP27 expression and phosphorylation of HSP27. Thus HSP expression can be overwhelmed by severe damage. The present work in conjunction with earlier work on caerulein induced pancreatitis indicates that changes in the actin cytoskeleton are an early component in experimental pancreatitis.

Syncollin is differentially expressed in rat proximal small intestine and regulated by feeding behavior

Gradients of gene expression are maintained along the proximal-distal axis of the mammalian small intestine despite a continuously regenerating epithelium. To study the molecular mechanisms responsible for this phenomenon, we utilized a subtractive hybridization strategy to isolate genes differentially expressed in the duodenum but not ileum. We isolated and sequenced 15 clones. The clones were fragments of genes encoding lipases, proteases, and an esterase. A novel clone was characterized and subsequently shown to encode syncollin, a secretory granule protein that binds to syntaxin in a calcium-sensitive manner. RT-PCR and S1 nuclease protection assay were used to clarify the 5'-end of syncollin. Syncollin was expressed in the rat pancreas, spleen, duodenum, and colon. In situ hybridization localized syncollin expression in the pancreas to acinar cells and in the duodenum to villus epithelial cells.

CEP-1347 inhibits caerulein-induced rat pancreatic JNK activation and ameliorates caerulein pancreatitis

Pancreatic caerulein-induced activation of c-Jun NH(2)-terminal kinase (JNK) has been reported, and JNK has been proposed as a mediator during induction of hyperstimulated pancreatitis. CEP-1347 has recently been described as a specific JNK inhibitor. We tested whether CEP-1347 inhibits caerulein-induced pancreatic JNK activation in isolated acini and in vivo. CEP-1347 dose dependently inhibited acinar caerulein-induced JNK activation with nearly complete inhibition at 2 microM but had no effect on digestive enzyme release. For in vivo studies, rats were pretreated with CEP-1347 before caerulein hyperstimulation. For assessment of JNK activation and histological alterations, animals were killed 30 min or 2 and 4 h after caerulein hyperstimulation, respectively. Pancreatic wet weight, serum enzyme levels, and pancreatic activity of p38 and extracellular signal-regulated kinase (ERK) were also determined. Caerulein hyperstimulation strongly activated JNK, p38, and ERK. CEP-1347 pretreatment dose dependently reduced caerulein-induced pancreatic JNK activation without p38 or ERK inhibition. JNK inhibition also reduced pancreatic edema formation and reduced histological severity of pancreatitis. Thus we show that CEP-1347 inhibits JNK activation in vivo and ameliorates caerulein-induced pancreatitis.