Intracellular rupture, exocytosis and actin interaction of endocytic vacuoles in pancreatic acinar cells: initiating events in acute pancreatitis

The integration of single-cell and bulk RNA-seq atlas reveals ERS-mediated acinar cell damage in acute pancreatitis

BACKGROUND

Acute pancreatitis (AP) is a clinically common acute abdominal disease, whose pathogenesis remains unclear. The severe patients usually have multiple complications and lack specific drugs, leading to a high mortality and poor outcome. Acinar cells are recognized as the initial site of AP. However, there are no precise single-cell transcriptomic profiles to decipher the landscape of acinar cells during AP, which are the missing pieces of jigsaw we aimed to complete in this study.

METHODS

A single-cell sequencing dataset was used to identify the cell types in pancreas of AP mice and to depict the transcriptomic maps in acinar cells. The pathways' activities were evaluated by gene sets enrichment analysis (GSEA) and single-cell gene sets variation analysis (GSVA). Pseudotime analysis was performed to describe the development trajectories of acinar cells. We also constructed the protein-protein interaction (PPI) network and identified the hub genes. Another independent single-cell sequencing dataset of pancreas samples from AP mice and a bulk RNA sequencing dataset of peripheral blood samples from AP patients were also analyzed.

RESULTS

In this study, we identified genetic markers of each cell type in the pancreas of AP mice based on single-cell sequencing datasets and analyzed the transcription changes in acinar cells. We found that acinar cells featured acinar-ductal metaplasia (ADM), as well as increased endocytosis and vesicle transport activity during AP. Notably, the endoplasmic reticulum stress (ERS) and ER-associated degradation (ERAD) pathways activated by accumulation of unfolded/misfolded proteins in acinar cells could be pivotal for the development of AP.

CONCLUSION

We deciphered the distinct roadmap of acinar cells in the early stage of AP at single-cell level. ERS and ERAD pathways are crucially important for acinar homeostasis and the pathogenesis of AP.

Acute pancreatitis: pathogenesis and emerging therapies

Acute pancreatitis is a severe inflammatory disorder with limited treatment options. Improved understanding of disease mechanisms has led to new and potential therapies. Here we summarize what we view as some of the most promising new therapies for treating acute pancreatitis, emphasizing the rationale of specific treatments based on disease mechanisms. Targeted pharmacologic interventions are highlighted. We explore potential treatment benefits and risks concerning reducing acute injury, minimizing complications, and improving long-term outcomes. Mechanisms associated with acute pancreatitis initiation, perpetuation, and reconstitution are highlighted, along with potential therapeutic targets and how these relate to new treatments.

Autophagy, Acute Pancreatitis and the Metamorphoses of a Trypsinogen-Activating Organelle

Recent studies have highlighted the importance of autophagy and particularly non-canonical autophagy in the development and progression of acute pancreatitis (a frequent disease with considerable morbidity and significant mortality). An important early event in the development of acute pancreatitis is the intrapancreatic activation of trypsinogen, (i.e., formation of trypsin) leading to the autodigestion of the organ. Another prominent phenomenon associated with the initiation of this disease is vacuolisation and specifically the formation of giant endocytic vacuoles in pancreatic acinar cells. These organelles develop in acinar cells exposed to several inducers of acute pancreatitis (including taurolithocholic acid and high concentrations of secretagogues cholecystokinin and acetylcholine). Notably, early trypsinogen activation occurs in the endocytic vacuoles. These trypsinogen-activating organelles undergo activation, long-distance trafficking, and non-canonical autophagy. In this review, we will discuss the role of autophagy in acute pancreatitis and particularly focus on the recently discovered LAP-like non-canonical autophagy (LNCA) of endocytic vacuoles.

The roles of calcium and ATP in the physiology and pathology of the exocrine pancreas

This review deals with the roles of calcium ions and ATP in the control of the normal functions of the different cell types in the exocrine pancreas as well as the roles of these molecules in the pathophysiology of acute pancreatitis. Repetitive rises in the local cytosolic calcium ion concentration in the apical part of the acinar cells not only activate exocytosis but also, via an increase in the intramitochondrial calcium ion concentration, stimulate the ATP formation that is needed to fuel the energy-requiring secretion process. However, intracellular calcium overload, resulting in a global sustained elevation of the cytosolic calcium ion concentration, has the opposite effect of decreasing mitochondrial ATP production, and this initiates processes that lead to necrosis. In the last few years it has become possible to image calcium signaling events simultaneously in acinar, stellate, and immune cells in intact lobules of the exocrine pancreas. This has disclosed processes by which these cells interact with each other, particularly in relation to the initiation and development of acute pancreatitis. By unraveling the molecular mechanisms underlying this disease, several promising therapeutic intervention sites have been identified. This provides hope that we may soon be able to effectively treat this often fatal disease.

Pathophysiologic Mechanisms of Hypothermia-Induced Pancreatic Injury in a Rat Model of Body Surface Cooling

OBJECTIVE

The mechanisms underlying hypothermia-induced pancreatic injury are unclear. Thus, we investigated the pathophysiology of hypothermia-induced pancreatic injury.

METHODS

We created a normal circulatory model with body surface cooling in rats. We divided the rats into control (36°C-38°C), mild hypothermia (33°C-35°C), moderate hypothermia (30°C-32°C), and severe hypothermia (27°C-29°C) (n = 5 per group) groups. Then, we induced circulatory failure with a cooling model using high-dose inhalation anesthesia and divided the rats into control (36°C-38°C) and severe hypothermia (27°C-29°C) (n = 5 per group) groups. Serum samples were collected before the introduction of hypothermia. Serum and pancreatic tissue were collected after maintaining the target body temperature for 1 hour.

RESULTS

Hematoxylin and eosin staining of the pancreas revealed vacuoles and edema in the hypothermia group. Serum amylase (P = 0.056), lactic acid (P < 0.05), interleukin 1β (P < 0.05), interleukin 6 (P < 0.05), and tumor necrosis factor α (P = 0.13) levels were suppressed by hypothermia. The circulatory failure model exhibited pancreatic injury.

CONCLUSIONS

Hypothermia induced bilateral effects on the pancreas. Morphologically, hypothermia induced pancreatic injury based on characteristic pathology typified by vacuoles. Serologically, hypothermia induced protective effects on the pancreas by suppressing amylase and inflammatory cytokine levels.

A narrative review of acute pancreatitis and its diagnosis, pathogenetic mechanism, and management

Acute pancreatitis (AP) is an inflammatory disease that can progress to severe acute pancreatitis (SAP), which increases the risk of death. AP is characterized by inappropriate activation of trypsinogen, infiltration of inflammatory cells, and destruction of secretory cells. Other contributing factors may include calcium (Ca²⁺) overload, mitochondrial dysfunction, impaired autophagy, and endoplasmic reticulum (ER) stress. In addition, exosomes are also associated with pathophysiological processes of many human diseases and may play a biological role in AP. However, the pathogenic mechanism has not been fully elucidated and needs to be further explored to inform treatment. Recently, the treatment guidelines have changed; minimally invasive therapy is advocated more as the core multidisciplinary participation and "step-up" approach. The surgical procedures have gradually changed from open surgery to minimally invasive surgery that primarily includes percutaneous catheter drainage (PCD), endoscopy, small incision surgery, and video-assisted surgery. The current guidelines for the management of AP have been updated and revised in many aspects. The type of fluid to be used, the timing, volume, and speed of administration for fluid resuscitation has been controversial. In addition, the timing and role of nutritional support and prophylactic antibiotic therapy, as well as the timing of the surgical or endoscopic intervention, and the management of complications still have many uncertainties that could negatively impact the prognosis and patients' quality of life. Consequently, to inform clinicians about optimal treatment, we aimed to review recent advances in the understanding of the pathogenesis of AP and its diagnosis and management.

The Remedial Potential of Lycopene in Pancreatitis through Regulation of Autophagy

Autophagy is an evolutionarily conserved process that degrades damaged organelles and recycles macromolecules to support cell survival. However, in certain disease states, dysregulated autophagy can play an important role in cell death. In pancreatitis, the accumulation of autophagic vacuoles and damaged mitochondria and premature activation of trypsinogen are shown in pancreatic acinar cells (PACs), which are the hallmarks of impaired autophagy. Oxidative stress mediates inflammatory signaling and cytokine expression in PACs, and it also causes mitochondrial dysfunction and dysregulated autophagy. Thus, oxidative stress may be a mediator for autophagic impairment in pancreatitis. Lycopene is a natural pigment that contributes to the red color of fruits and vegetables. Due to its antioxidant activity, it inhibited oxidative stress-induced expression of cytokines in experimental models of acute pancreatitis. Lycopene reduces cell death through the activation of 5′-AMP-activated protein kinase-dependent autophagy in certain cells. Therefore, lycopene may ameliorate pancreatitis by preventing oxidative stress-induced impairment of autophagy and/or by directly activating autophagy in PACs.

LAP-like non-canonical autophagy and evolution of endocytic vacuoles in pancreatic acinar cells

Activation of trypsinogen (formation of trypsin) inside the pancreas is an early pathological event in the development of acute pancreatitis. In our previous studies we identified the activation of trypsinogen within endocytic vacuoles (EVs), cellular organelles that appear in pancreatic acinar cells treated with the inducers of acute pancreatitis. EVs are formed as a result of aberrant compound exocytosis and subsequent internalization of post-exocytic structures. These organelles can be up to 12 μm in diameter and can be actinated (i.e. coated with F-actin). Notably, EVs can undergo intracellular rupture and fusion with the plasma membrane, providing trypsin with access to cytoplasmic and extracellular targets. Unraveling the mechanisms involved in cellular processing of EVs is an interesting cell biological challenge with potential benefits for understanding acute pancreatitis. In this study we have investigated autophagy of EVs and discovered that it involves a non-canonical LC3-conjugation mechanism, reminiscent in its properties to LC3-associated phagocytosis (LAP); in both processes LC3 was recruited to single, outer organellar membranes. Trypsinogen activation peptide was observed in approximately 55% of LC3-coated EVs indicating the relevance of the described process to the early cellular events of acute pancreatitis. We also investigated relationships between actination and non-canonical autophagy of EVs and concluded that these processes represent sequential steps in the evolution of EVs. Our study expands the known roles of LAP and indicates that, in addition to its well-established functions in phagocytosis and macropinocytosis, LAP is also involved in the processing of post-exocytic organelles in exocrine secretory cells.

ABBREVIATIONS

AP: acute pancreatitis; CCK: cholecystokinin; CLEM: correlative light and electron microscopy; DPI: diphenyleneiodonium; EV: endocytic vacuole; LAP: LC3-associate phagocytosis; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; PACs: pancreatic acinar cells; PFA: paraformaldehyde; PtdIns3K: phosphatidylinositol 3-kinase; PtdIns3P: phosphatidylinositol 3-phosphate; Res: resveratrol; TAP: trypsinogen activation peptide; TEM: transmission electron microscopy; TLC-S: taurolithocholic acid 3-sulfate; TRD: Dextran Texas Red 3000 MW Neutral; ZGs: zymogen granules.

Knockout of the Mitochondrial Calcium Uniporter Strongly Suppresses Stimulus-Metabolism Coupling in Pancreatic Acinar Cells but Does Not Reduce Severity of Experimental Acute Pancreatitis

Acute pancreatitis is a frequent disease that lacks specific drug treatment. Unravelling the molecular mechanisms of acute pancreatitis is essential for the development of new therapeutics. Several inducers of acute pancreatitis trigger sustained Ca²⁺ increases in the cytosol and mitochondria of pancreatic acinar cells. The mitochondrial calcium uniporter (MCU) mediates mitochondrial Ca²⁺ uptake that regulates bioenergetics and plays an important role in cell survival, damage and death. Aberrant Ca²⁺ signaling and mitochondrial damage in pancreatic acinar cells have been implicated in the initiation of acute pancreatitis. The primary aim of this study was to assess the involvement of the MCU in experimental acute pancreatitis. We found that pancreatic acinar cells from MCU^-/- mice display dramatically reduced mitochondrial Ca²⁺ uptake. This is consistent with the drastic changes of stimulus-metabolism coupling, manifested by the reduction of mitochondrial NADH/FAD⁺ responses to cholecystokinin and in the decrease of cholecystokinin-stimulated oxygen consumption. However, in three experimental models of acute pancreatitis (induced by caerulein, taurolithocholic acid 3-sulfate or palmitoleic acid plus ethanol), MCU knockout failed to reduce the biochemical and histological changes characterizing the severity of local and systemic damage. A possible explanation of this surprising finding is the redundancy of damaging mechanisms activated by the inducers of acute pancreatitis.

Early trypsin activation develops independently of autophagy in caerulein-induced pancreatitis in mice

Premature intrapancreatic trypsinogen activation is widely regarded as an initiating event for acute pancreatitis. Previous studies have alternatively implicated secretory vesicles, endosomes, lysosomes, or autophagosomes/autophagolysosomes as the primary site of trypsinogen activation, from which a cell-damaging proteolytic cascade originates. To identify the subcellular compartment of initial trypsinogen activation we performed a time-resolution analysis of the first 12 h of caerulein-induced pancreatitis in transgenic light chain 3 (LC3)-GFP autophagy reporter mice. Intrapancreatic trypsin activity increased within 60 min and serum amylase within 2 h, but fluorescent autophagosome formation only by 4 h of pancreatitis in parallel with a shift from cytosolic LC3-I to membranous LC3-II on Western blots. At 60 min, activated trypsin in heavier subcellular fractions was co-distributed with cathepsin B, but not with the autophagy markers LC3 or autophagy protein 16 (ATG16). Supramaximal caerulein stimulation of primary pancreatic acini derived from LC3-GFP mice revealed that trypsinogen activation is independent of autophagolysosome formation already during the first 15 min of exposure to caerulein. Co-localization studies (with GFP-LC3 autophagosomes versus Ile-Pro-Arg-AMC trypsin activity and immunogold-labelling of lysosomal-associated membrane protein 2 [LAMP-2] versus trypsinogen activation peptide [TAP]) indicated active trypsin in autophagolysosomes only at the later timepoints. In conclusion, during the initiating phase of caerulein-induced pancreatitis, premature protease activation develops independently of autophagolysosome formation and in vesicles arising from the secretory pathway. However, autophagy is likely to regulate overall intracellular trypsin activity during the later stages of this disease.

Roles of Autophagy and Pancreatic Secretory Trypsin Inhibitor in Trypsinogen Activation in Acute Pancreatitis

The focus of the review is on roles of autophagy and pancreatic secretory trypsin inhibitor (PSTI), an endogenous trypsin inhibitor, in trypsinogen activation in acute pancreatitis. Acute pancreatitis is a disease in which tissues in and around the pancreas are autodigested by pancreatic digestive enzymes. This reaction is triggered by the intrapancreatic activation of trypsinogen. Autophagy causes trypsinogen and cathepsin B, a trypsinogen activator, to colocalize within the autolysosomes. Consequently, if the resultant trypsin activity exceeds the inhibitory activity of PSTI, the pancreatic digestive enzymes are activated, and they cause autodigestion of the acinar cells. Thus, autophagy and PSTI play important roles in the development and suppression of acute pancreatitis, respectively.

A New Role for the Spleen: Aggravation of the Systemic Inflammatory Response in Rats with Severe Acute Pancreatitis

Little is known about the role of the spleen in mediating systemic inflammatory responses in severe acute pancreatitis (SAP). We investigated the role played by the spleen in rats after SAP induction. Splenectomy was performed at designated time points after SAP induction. Pancreatic tissue and serum samples were collected and subjected to histologic, immunohistochemical, and immunologic analyses. After SAP induction, the splenic immune response was enhanced during SAP progression, as shown by the increased diameter of the splenic periarterial lymphatic sheath and the thickness of the splenic marginal zone. Rats with splenectomy developed acute pancreatitis more slowly than rats without splenectomy. In addition, pancreatic tissues of rats with splenectomy contained lower levels of serum amylase, tumor necrosis factor-α, and IL-6 and exhibited less acinar cell death, leukocyte infiltration, and interstitial edema than those of rats without splenectomy. Compared with splenectomy alone, cotreatment with splenectomy and the administration of splenic cells originating from a rat with SAP 12 hours after induction increased systemic inflammation in SAP rats. Splenic factors exacerbated SAP-associated liver and lung injury and accentuated intestinal mucosal barrier dysfunction. Splenectomy altered the serum cytokine profile in rats with SAP. In a rat model of SAP, the spleen exacerbated the systematic inflammatory responses and injury to multiple organs, indicating a new role for the spleen in SAP.

Acute Pancreatitis: A Multifaceted Set of Organelle and Cellular Interactions

Acute pancreatitis is an inflammatory disorder of the exocrine pancreas associated with tissue injury and necrosis. The disease can be mild, involving only the pancreas, and resolve spontaneously within days or severe, with systemic inflammatory response syndrome-associated extrapancreatic organ failure and even death. Importantly, there are no therapeutic agents currently in use that can alter the course of the disease. This article emphasizes emerging findings that stressors (environmental and genetic) that cause acute pancreatitis initially cause injury to organelles of the acinar cell (endoplasmic reticulum, mitochondria, and endolysosomal-autophagy system), and that disorders in the functions of the organelles lead to inappropriate intracellular activation of trypsinogen and inflammatory pathways. We also review emerging work on the role of damage-associated molecular patterns in mediating the local and systemic inflammatory response in addition to known cytokines and chemokine pathways. In the review, we provide considerations for correction of organelle functions in acute pancreatitis to create a discussion for clinical trial treatment and design options.

Genetics, Cell Biology, and Pathophysiology of Pancreatitis

Since the discovery of the first trypsinogen mutation in families with hereditary pancreatitis, pancreatic genetics has made rapid progress. The identification of mutations in genes involved in the digestive protease-antiprotease pathway has lent additional support to the notion that pancreatitis is a disease of autodigestion. Clinical and experimental observations have provided compelling evidence that premature intrapancreatic activation of digestive proteases is critical in pancreatitis onset. However, disease course and severity are mostly governed by inflammatory cells that drive local and systemic immune responses. In this article, we review the genetics, cell biology, and immunology of pancreatitis with a focus on protease activation pathways and other early events.

Mechanisms of Pancreatic Injury Induced by Basic Amino Acids Differ Between L-Arginine, L-Ornithine, and L-Histidine

Pancreatic acinar cells require high rates of amino acid uptake for digestive enzyme synthesis, but excessive concentrations can trigger acute pancreatitis (AP) by mechanisms that are not well understood. We have used three basic natural amino acids L-arginine, L-ornithine, and L-histidine to determine mechanisms of amino acid-induced pancreatic injury and whether these are common to all three amino acids. Caffeine markedly inhibited necrotic cell death pathway activation in isolated pancreatic acinar cells induced by L-arginine, but not L-ornithine, whereas caffeine accelerated L-histidine-induced cell death. Both necroptosis inhibitors of RIPK1 and RIPK3 and a necroptosis activator/apoptosis inhibitor z-VAD increased cell death caused by L-histidine, but not L-arginine or L-ornithine. Cyclophilin D knock-out (Ppif-/-) significantly attenuated cell death induced by L-histidine, but not L-arginine, or L-ornithine. Allosteric modulators of calcium-sensing receptor (CaSR) and G-protein coupled receptor class C group 6 member A (GPRC6A) had inhibitory effects on cell death induced by L-arginine but not L-ornithine or L-histidine. We developed a novel amino acid-induced AP murine model with high doses of L-histidine and confirmed AP severity was significantly reduced in Ppif-/- vs. wild type mice. In L-arginine-induced AP neither Ppif-/-, caffeine, or allosteric modulators of CaSR or GPRC6A reduced pancreatic damage, even though CaSR inhibition with NPS-2143 significantly reduced pancreatic and systemic injury in caerulein-induced AP. These findings demonstrate marked differences in the mechanisms of pancreatic injury induced by different basic amino acids and suggest the lack of effect of treatments on L-arginine-induced AP may be due to conversion to L-ornithine in the urea cycle.