Lysosome-Associated Membrane Proteins (LAMP) Maintain Pancreatic Acinar Cell Homeostasis: LAMP-2–Deficient Mice Develop Pancreatitis

Abstract

Background & Aims The pathogenic mechanism of pancreatitis is poorly understood. Recent evidence implicates defective autophagy in pancreatitis responses; however, the pathways mediating impaired autophagy in pancreas remain largely unknown. Here, we investigate the role of lysosome associated membrane proteins (LAMPs) in pancreatitis. Methods We analyzed changes in LAMPs in experimental models and human pancreatitis, and the underlying mechanisms: LAMP de-glycosylation and degradation. LAMP cleavage by cathepsin B (CatB) was analyzed by mass spectrometry. We used mice deficient in LAMP-2 to assess its role in pancreatitis. Results Pancreatic levels of LAMP-1 and LAMP-2 greatly decrease across various pancreatitis models and in human disease. Pancreatitis does not trigger LAMPs’ bulk de-glycosylation, but induces their degradation via CatB-mediated cleavage of LAMP molecule close to the boundary between luminal and transmembrane domains. LAMP-2 null mice spontaneously develop pancreatitis that begins with acinar cell vacuolization due to impaired autophagic flux, and progresses to severe pancreas damage characterized by trypsinogen activation, macrophage-driven inflammation, and acinar cell death. LAMP-2 deficiency causes a decrease in pancreatic digestive enzymes content, stimulates the basal and inhibits CCK-induced amylase secretion by acinar cells. The effects of LAMP-2 knockout and acute cerulein pancreatitis overlap, which corroborates the pathogenic role of LAMP decrease in experimental pancreatitis models. Conclusions The results indicate a critical role for LAMPs, particularly LAMP-2, in maintaining pancreatic acinar cell homeostasis, and provide evidence that defective lysosomal function, resulting in impaired autophagy, leads to pancreatitis. Mice with LAMP-2 deficiency present a novel genetic model of human pancreatitis caused by lysosomal/autophagic dysfunction.

New insights into the pathways initiating and driving pancreatitis

“…Complementing the findings in Atg knockouts, lysosomal dysfunction is reported [5 ■■ ] to cause impaired autophagy and spontaneous pancreatitis. This study elucidates the role of lysosome-associated membrane proteins (LAMPs), which are critical for maintaining both the structure and function of lysosomes.…”

“…Together, the recent findings [3,4,5 ■■ –7 ■■ ,8–10] reveal the essential role of autophagic/lysosomal pathways in maintaining pancreatic acinar cell homeostasis and secretory function. Further, the development of spontaneous pancreatitis in Atg5, Atg7, and LAMP-2 knockouts indicates that defects in these pathways play a pathogenic role not only in acute pancreatitis but also chronic pancreatitis.…”

“…Recent studies [3,4,5 ■■ ] show that vacuolization is a result of impaired autophagic flux, which also mediates another signature response of pancreatitis, the increased intra-acinar trypsin activity. Autophagy (macroautophagy) is the principal cellular pathway for degradation and recycling of organelles, lipids, and long-lived proteins; it starts with sequestration of the material destined for degradation into autophagosomes, which then fuse with lysosomes forming the autolysosomes where cargo is degraded by lysosomal hydrolases such as cathepsins [4].…”

“…The authors find dramatic decreases in pancreatic levels of LAMP-1 and LAMP-2 across various experimental models of nonalcoholic and alcoholic pancreatitis, because of LAMP degradation mediated by cathepsin B. LAMP-2 null mice gradually develop pancreatitis, starting with acinar cell vacuolization and progressing to severe pancreas damage characterized by trypsinogen activation, macrophage-driven inflammation, and acinar cell death. Moreover, amylase secretion, both basal and CCK-induced, is dysregulated in LAMP-2 deficient acinar cells [5 ■■ ]. …”

Cathepsin B Activity Initiates Apoptosis via Digestive Protease Activation in Pancreatic Acinar Cells and Experimental Pancreatitis

“…How they colocalize is another matter. Constitutive missorting of CTSB into the secretory pathway because of incomplete physiological mannose-6-phosphate receptor transport-induced (28) missorting in the initiating phase of pancreatitis (19), fusion of mature secretory vesicles with lysosomes (42), or colocalization via endocytosis from the luminal (22) or basolateral (43) cell surface as well as defective degradation of autophagolysosomes (21,44,45) have all been suggested to explain the subcellular redistribution of lysosomal enzyme activity into the secretory compartment and their colocalization with digestive zymogens. FIGURE 8.…”