József Maléth

Translocation between PI(4,5)P2-poor and PI(4,5)P2-rich microdomains during store depletion determines STIM1 conformation and Orai1 gating

Summary The Orai1-STIM1 current undergoes slow Ca2+-dependent inactivation (SCDI) mediated by binding of SARAF to STIM1. Here, we report the use of SCDI by SARAF as a probe of the conformation and microdomain localization of the Orai1-STIM1 complex. We find that interaction of STIM1 with Orai1 C terminus and the STIM1 K-domain are required for interaction of SARAF with STIM1 and SCDI. STIM1-Orai1 must be in a PM/ER microdomain tethered by E-Syt1, stabilized by Septin4 and enriched in PI(4,5)P2 for STIM1-SARAF interaction. Targeting STIM1 to PI(4,5)P2 rich and poor microdomains reveals that SARAF-dependent SCDI is observed only when STIM1-Orai1 are within the PI(4,5)P2-rich microdomain. Notably, store depletion results in transient localization of STIM1-Orai1 in the PI(4,5)P2-poor microdomain, which then translocate to the PI(4,5)P2-rich domain. These findings reveal the role of PM/ER tethers in the regulation of Orai1 function and a new mode of regulation by PI(4,5)P2 involving translocation between PI(4,5)P2 microdomains.

Non-conjugated chenodeoxycholate induces severe mitochondrial damage and inhibits bicarbonate transport in pancreatic duct cells

We read the manuscripts by Lee and Muallem1 and Venglovecz et al 2 recently published in Gut with great interest. In both articles the authors highlighted the role of pancreatic ducts in maintaining the integrity of the pancreas. Venglovecz et al showed that a high concentration of the non-conjugated chenodeoxycholate (CDC) inhibits pancreatic ductal bicarbonate secretion; however, the mechanisms of the inhibition were not clarified. This is a follow-up study in which we show that this reduction of ductal bicarbonate secretion by CDC is evoked by inhibition of glycolytic and oxidative (caused by severe mitochondrial damage) metabolism with a consequent depletion of intracellular ATP levels. Physiologically, pancreatic ductal fluid and HCO3− secretion are necessary to wash out the digestive enzymes from the acinar cells into the duodenum. Under pathophysiological conditions toxic factors (such as bile acids and ethanol) involved in the pathogenesis of acute pancreatitis have dual effects on ductal HCO3− secretion.1 Low doses of CDC and ethanol were found to stimulate fluid and HCO3− secretion. However, these toxic agents in high concentrations inhibit the secretion. These data suggest that an elevation in pancreatic ductal fluid and HCO3− secretion may have protective roles. However, since under physiological conditions the pressure in the main pancreatic duct is higher than in the bile ducts, it is still controversial as to whether bile acids enter the pancreatic ductal tree. We have recently shown that a high concentration (1 mM) of the non-conjugated bile acid CDC has strong inhibitory effects on the activities of acid/base transporters (Na+/H+ exchanger (NHE), Na+/HCO3− cotransporter (NBC) …

Trypsin Reduces Pancreatic Ductal Bicarbonate Secretion by Inhibiting CFTR Cl− Channels and Luminal Anion Exchangers

BACKGROUND & AIMS The effects of trypsin on pancreatic ductal epithelial cells (PDECs) vary among species and depend on the localization of proteinase-activated receptor 2 (PAR-2). We compared PAR-2 localization in human and guinea-pig PDECs, and used isolated guinea pig ducts to study the effects of trypsin and a PAR-2 agonist on bicarbonate secretion. METHODS PAR-2 localization was analyzed by immunohistochemistry in guinea pig and human pancreatic tissue samples (from 15 patients with chronic pancreatitis and 15 without pancreatic disease). Functionally, guinea pig PDECs were studied by microperfusion of isolated ducts, measurements of intracellular pH and intracellular Ca(2+) concentration, and patch clamp analysis. The effect of pH on trypsinogen autoactivation was assessed using recombinant human cationic trypsinogen. RESULTS PAR-2 localized to the apical membrane of human and guinea pig PDECs. Trypsin increased intracellular Ca(2+) concentration and intracellular pH and inhibited secretion of bicarbonate by the luminal anion exchanger and the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. Autoactivation of human cationic trypsinogen accelerated when the pH was reduced from 8.5 to 6.0. PAR-2 expression was strongly down-regulated, at transcriptional and protein levels, in the ducts of patients with chronic pancreatitis, consistent with increased activity of intraductal trypsin. Importantly, in PAR-2 knockout mice, the effects of trypsin were markedly reduced. CONCLUSIONS Trypsin reduces pancreatic ductal bicarbonate secretion via PAR-2-dependent inhibition of the apical anion exchanger and the CFTR Cl(-) channel. This could contribute to the development of chronic pancreatitis by decreasing luminal pH and promoting premature activation of trypsinogen in the pancreatic ducts.

The ER/PM microdomain, PI(4,5)P2 and the regulation of STIM1–Orai1 channel function

All forms of cell signaling occur in discreet cellular microdomains in which the ER is the main participant and include microdomains formed by the ER with lysosomes, endosomes, the nucleus, mitochondria and the plasma membrane. In the microdomains the two opposing organelles transfer and exchange constituents including lipids and ions. As is the case for other forms of signaling pathways, many components of the receptor-evoked Ca(2+) signal are clustered at the ER/PM microdomain, including the Orai1-STIM1 complex. This review discusses recent advances in understanding the molecular components that tether the ER and plasma membrane to form the ER/PM microdomains in which PI(4,5)P2 is enriched, and how dynamic targeting of the Orai1-STIM1 complex to PI(4,5)P2-poor and PI(4,5)P2-rich microdomains controls the activity of Orai1 and its regulation by Ca(2+) that is mediated by SARAF.

Central role of mitochondrial injury in the pathogenesis of acute pancreatitis

Acute pancreatitis is an inflammatory disease with no specific treatment. One of the main reasons behind the lack of specific therapy is that the pathogenesis of acute pancreatitis is poorly understood. During the development of acute pancreatitis, the disease‐inducing factors can damage both cell types of the exocrine pancreas, namely the acinar and ductal cells. Because damage of either of the cell types can contribute to the inflammation, it is crucial to find common intracellular mechanisms that can be targeted by pharmacological therapies. Despite the many differences, recent studies revealed that the most common factors that induce pancreatitis cause mitochondrial damage with the consequent breakdown of bioenergetics, that is, ATP depletion in both cell types. In this review, we summarize our knowledge of mitochondrial function and damage within both pancreatic acinar and ductal cells. We also suggest that colloidal ATP delivery systems for pancreatic energy supply may be able to protect acinar and ductal cells from cellular damage in the early phase of the disease. An effective energy delivery system combined with the prevention of further mitochondrial damage may, for the first time, open up the possibility of pharmacological therapy for acute pancreatitis, leading to reduced disease severity and mortality.

Pancreatic Ductal Bicarbonate Secretion: Challenge of the Acinar Acid Load

Acinar and ductal cells of the exocrine pancreas form a close functional unit. Although most studies contain data either on acinar or ductal cells, an increasing number of evidence highlights the importance of the pancreatic acinar-ductal functional unit. One of the best examples for this functional unit is the regulation of luminal pH by both cell types. Protons co-released during exocytosis from acini cause significant acidosis, whereas, bicarbonate secreted by ductal cells cause alkalization in the lumen. This suggests that the first and probably one of the most important role of bicarbonate secretion by pancreatic ductal cells is not only to neutralize the acid chyme entering into the duodenum from the stomach, but to neutralize acidic content secreted by acinar cells. To accomplish this role, it is more than likely that ductal cells have physiological sensing mechanisms which would allow them to regulate luminal pH. To date, four different classes of acid-sensing ion channels have been identified in the gastrointestinal tract (transient receptor potential ion channels, two-pore domain potassium channel, ionotropic purinoceptor and acid-sensing ion channel), however, none of these have been studied in pancreatic ductal cells. In this mini-review, we summarize our current knowledge of these channels and urge scientists to characterize ductal acid-sensing mechanisms and also to investigate the challenge of the acinar acid load on ductal cells.

The role of pancreatic ductal secretion in protection against acute pancreatitis in mice

Objectives:A common potentially fatal disease of the pancreas is acute pancreatitis, for which there is no treatment. Most studies of this disorder focus on the damage to acinar cells since they are assumed to be the primary target of multiple stressors affecting the pancreas. However, increasing evidence suggests that the ducts may also have a crucial role in induction of the disease. To test this hypothesis, we sought to determine the specific role of the duct in the induction of acute pancreatitis using well-established disease models and mice with deletion of the Na+/H+ exchanger regulatory factor-1 that have selectively impaired ductal function. Design:Randomized animal study. Setting:Animal research laboratory. Subjects:Wild-type and Na+/H+ exchanger regulatory factor-1 knockout mice. Interventions:Acute necrotizing pancreatitis was induced by i.p. administration of cerulein or by intraductal administration of sodium taurocholate. The pancreatic expression of Na+/H+ exchanger regulatory factor-1 and cystic fibrosis transmembrane conductance regulator (a key player in the control of ductal secretion) was analyzed by immunohistochemistry. In vivo pancreatic ductal secretion was studied in anesthetized mice. Functions of pancreatic acinar and ductal cells as well as inflammatory cells were analyzed in vitro. Measurements and Main Results:Deletion of Na+/H+ exchanger regulatory factor-1 resulted in gross mislocalization of cystic fibrosis transmembrane conductance regulator, causing marked reduction in pancreatic ductal fluid and bicarbonate secretion. Importantly, deletion of Na+/H+ exchanger regulatory factor-1 had no deleterious effect on functions of acinar and inflammatory cells. Deletion of Na+/H+ exchanger regulatory factor-1, which specifically impaired ductal function, increased the severity of acute pancreatitis in the two mouse models tested. Conclusions:Our findings provide the first direct evidence for the crucial role of ductal secretion in protecting the pancreas from acute pancreatitis and strongly suggest that improved ductal function should be an important modality in prevention and treatment of the disease.

Na+/Ca2+ exchangers regulate the migration and proliferation of human gastric myofibroblasts

Gastrointestinal myofibroblasts are contractile, electrically nonexcitable, transitional cells that play a role in extracellular matrix production, in ulcer healing, and in pathophysiological conditions they contribute to chronic inflammation and tumor development. Na+/Ca2+ exchangers (NCX) are known to have a crucial role in Ca2+ homeostasis of contractile cells, however, no information is available concerning the role of NCX in the proliferation and migration of gastrointestinal myofibroblasts. In this study, our aim was to investigate the role of NCX in the Ca2+ homeostasis, migration, and proliferation of human gastrointestinal myofibroblasts, focusing on human gastric myofibroblasts (HGMs). We used microfluorometric measurements to investigate the intracellular Ca2+ and Na+ concentrations, PCR analysis and immunostaining to show the presence of the NCX, patch clamp for measuring NCX activity, and proliferation and migration assays to investigate the functional role of the exchanger. We showed that 53.0±8.1% of the HGMs present Ca2+ oscillations, which depend on extracellular Ca2+ and Na+, and can be inhibited by NCX inhibitors. NCX1, NCX2, and NCX3 were expressed at both mRNA and protein levels in HGMs, and they contribute to the intracellular Ca2+ and Na+ homeostasis as well, regardless of the oscillatory activity. NCX inhibitors significantly blocked the basal and insulin-like growth factor II-stimulated migration and proliferation rates of HGMs. In conclusion, we showed that NCX plays a pivotal role in regulating the Ca2+ homeostasis, migration, and proliferation of HGMs. The inhibition of NCX activity may be a potential therapeutic target in hyperproliferative gastric diseases.

CFTR: A New Horizon in the Pathomechanism and Treatment of Pancreatitis

Cystic fibrosis transmembrane conductance regulator (CFTR) is an ion channel that conducts chloride and bicarbonate ions across epithelial cell membranes. Mutations in the CFTR gene diminish the ion channel function and lead to impaired epithelial fluid transport in multiple organs such as the lung and the pancreas resulting in cystic fibrosis. Heterozygous carriers of CFTR mutations do not develop cystic fibrosis but exhibit increased risk for pancreatitis and associated pancreatic damage characterized by elevated mucus levels, fibrosis, and cyst formation. Importantly, recent studies demonstrated that pancreatitis causing insults, such as alcohol, smoking, or bile acids, strongly inhibit CFTR function. Furthermore, human studies showed reduced levels of CFTR expression and function in all forms of pancreatitis. These findings indicate that impairment of CFTR is critical in the development of pancreatitis; therefore, correcting CFTR function could be the first specific therapy in pancreatitis. In this review, we summarize recent advances in the field and discuss new possibilities for the treatment of pancreatitis.

Spatiotemporally limited BDNF and GDNF overexpression rescues motoneurons destined to die and induces elongative axon growth

Axonal injury close to cell bodies of motoneurons induces the death of the vast majority of affected cells. Neurotrophic factors, such as brain derived neurotrophic factor (BDNF) and glial cell derived neurotrophic factor (GDNF), delivered close to the damaged motor pool in a non-regulated manner induce good survival of injured motoneurons and sprouting of their axons but fail to induce functional reinnervation. To avoid these drawbacks of high levels of neurotrophic expression, we devised an ex vivo gene therapy system to induce transient expression of BDNF/GDNF in transfected rat adipose tissue-derived stem cells (rASCs) which were grafted around the reimplanted ventral root, embedded in collagen gel. Strong BDNF/GDNF expression was induced in vitro in the first days after transfection with a significant decline in expression 10-14 days following transfection. Numerous axons of injured motoneurons were able to enter the reimplanted root following reimplantation and BDNF or GDNF treatment (192±17 SEM vs 187±12 SEM, respectively) and produce morphological and functional reinnervation. Treatment with a combined cell population (BDNF+GDNF-transfected rASCs) induced slightly improved reinnervation (247±24 SEM). In contrast, only few motoneurons regenerated their axons in control animals (63±4 SEM) which received untransfected cells. The axons of surviving motoneurons showed elongative growth typical of regenerative axons, without aberrant growth or coil formation of sprouting axons. These findings provide evidence that damaged motoneurons require limited and spatially directed amounts of BDNF and GDNF to support their survival and regeneration. Moreover, neurotrophic support appears to be needed only for a critical period of time not longer than for two weeks after injury.