Secretory autophagy of lysozyme in Paneth cells

Secretion of antimicrobial proteins is an important host defense mechanism against bacteria, yet how secretory cells maintain function during bacterial invasion has been unclear. We discovered that Paneth cells, specialized secretory cells in the small intestine, react to bacterial invasion by rerouting a critical secreted antibacterial protein through a macroautophagy/autophagy-based secretion system termed secretory autophagy. Mice harboring a mutation in an essential autophagy gene, a mutation which is common in Crohn disease patients, cannot reroute their antimicrobial cargo during bacterial invasion and thus have compromised innate immunity. We showed that this alternative secretion system is triggered by both a cell-intrinsic mechanism, involving the ER stress response, and a cell-extrinsic mechanism, involving subepithelial innate immune cells. Our findings uncover a new role for secretory autophagy in host defense and suggest how a mutation in an autophagy gene can predispose individuals to Crohn disease.     

Secretory function of autophagy in innate immune cells

Eukaryotic cells utilize two main secretory pathways to transport proteins to the extracellular space. Proteins with a leader signal sequence often undergo co‐translational transport into the endoplasmic reticulum (ER), and then to the Golgi apparatus before they reach their destination. This pathway is called the conventional secretory pathway. Proteins without signal peptides can bypass this ER‐Golgi system and are secreted by a variety of mechanisms collectively called the unconventional secretory pathway. The molecular mechanisms of unconventional secretion are emerging. Autophagy is a conserved bulk degradation mechanism that regulates many intracellular functions. Recent evidence implicates autophagy in the secretory pathway. This review focuses on potential secretory roles of autophagy and how they could modulate the functions of innate immune cells that secrete a wide range of mediators in response to environmental and biological stimuli. We provide a brief overview of the secretory pathways, enumerate the potential mechanistic themes by which autophagy interacts with these pathways and describe their relevance in the context of innate immune cell function.     

Secretory autophagy holds the key to lysozyme secretion during bacterial infection of the intestine

In 2013, Dr. Lora Hooper and colleagues described the induction of antibacterial macroautophagy/autophagy in intestinal epithelial cells as a cytoprotective host defense mechanism against invading Salmonella enterica serovar Typhimurium (S. Typhimurium). Canonical autophagy functions in a primarily degradative capacity to safeguard cells and ensure survival during stress conditions, including pathogen infection. In contrast, secretory autophagy has emerged as an alternative nondegradative mechanism for cellular trafficking and unconventional protein secretion. More recently, a study by Bel et al. from Dr. Hooper's lab describes how intestinal Paneth cells exploit the endoplasmic reticulum (ER) stress response to release antibacterial lysozyme through secretory autophagy in response to S. Typhimurium infection.     

Secretory sphingomyelinase

Several physiologic and pathophysiologic processes in which sphingomyelinases (SMases) have been implicated may involve extracellular sphingomyelin (SM) hydrolysis. A candidate enzyme for these processes is a recently discovered SMase called secretory SMase, or S-SMase. S-SMase arises from the acid sphingomyelinase (ASM) gene via differential protein trafficking of a common protein precursor; this precursor can be targeted to either lysosomes or the Golgi secretory pathway. S-SMase is activated by physiologic levels of Zn2+, although the S-SMase from endothelial cells, which secrete abundant amounts of the enzyme, is partially Zn2+-independent. S-SMase functions best at acid pH but can hydrolyze certain physiologic substrates, such as atherogenic lipoproteins, at neutral pH. In endothelial cells, the secretion of S-SMase is regulated at the level of protein trafficking by inflammatory cytokines. Current work implicates a role for S-SMase in atherogenesis, and future work will be directed at understanding the potential roles of S-SMase in other processes, such as ceramide-mediated cell-signaling and the host inflammatory response.     

Secretory lysosomes

Regulated secretion of stored secretory products is important in many cell types. In contrast to professional secretory cells, which store their secretory products in specialized secretory granules, some secretory cells store their secretory proteins in a dual-function organelle, called a secretory lysosome. Functionally, secretory lysosomes are unusual in that they serve both as a degradative and as a secretory compartment. Recent work shows that cells with secretory lysosomes use new sorting and secretory pathways. The importance of these organelles is highlighted by several genetic diseases, in which immune function and pigmentation--two processes that normally involve secretory lysosomes--are impaired.     

Secretory granule formation

A deeper understanding of the regulated exocytic pathway, and for that matter the constitutive exocytic pathway, will depend on our ability to characterize the proteins in the vesicle membranes. Characterizing the protein composition of secretory granule membrane has proven to be a formidable task, and as far as I know, the work done to date has not told us a great deal about the mechanisms involved in sorting the contents of regulated secretory granules, or bringing about constitutive or regulated fusion with the plasma membrane. Without knowing a great deal more about the membranes, there seems to be little prospect of real further progress in understanding the key properties of the regulated exocytic pathway.     

非经典的蛋白质分泌途径

蛋白质分泌是细胞间进行信息传递的重要方式之一。一般的分泌蛋白在其N端都具有信号肽序列,可以通过经典的内质网-高尔基体途径进行运输,并最终分泌到细胞外。近来发现细胞内存在另一类分泌蛋白,它们能够分泌到细胞外发挥功能,但却没有典型的信号肽序列,被称为非经典分泌蛋白。越来越多的证据表明,这类蛋白质的分泌有其独特的机制。本综述了这类蛋白质的各种分泌机制及可能的生理意义。     

Secretory protein trafficking

Acute insulin secretion from stimulated pancreatic β-cells is derived from the intracellular pool of insulin secretory granules wherein insulin is packaged in a highly concentrated (and in some species, crystalline) state. Here we review experimental work, principally from our laboratory, on the question of biogenesis of mature secretory granules within the broader context of intracellular protein trafficking. Events occurring in the lumen of organelles at various stages of intracellular transport within the secretory pathway and events at the limiting membrane of newly forming secretory granules each contribute to formation of the insulin storage compartment comprising the readily releasable pool.     

Secretory transport in Plasmodium

The asexual blood stage of the human malaria parasite Plasmodium falciparum resides within the mature erythrocyte - a cell that has no intracellular organelles and few biosynthetic activities. However, Plasmodium, as on actively growing and dividing cell, has numerous requirements for the uptake o f nutrients and expulsion of waste. Hence, the parasite must extensively remodel the erythrocyte to facilitate its survival, not only by exporting numerous proteins, but also by providing the requisite machinery for their .trafficking. In this review, Heidi Elmendorf and Kastun Haldar propose a model for secretion in P. falciparum.     

分泌型溶酶体与疾病

传统意义的溶酶体被认为是细胞内消化途径的终点站,是含有多种水解酶和脂肪酶的细胞器,可以消化蛋白以及膜结构等.某些特殊类型的细胞中存在分泌型溶酶体,它既有胞内消化的功能,又有调节分泌功能.在调节溶酶体胞吐的蛋白质中,Rab27a蛋白起到了核心作用.相关基因特别是控制胞吐的基因突变,可造成各种免疫缺陷综合症.星型胶质细胞溶...     

Secretory carrier membrane proteins

Secretory carrier membrane proteins (SCAMPs) are a family of integral membrane proteins that play roles in mediating exocytosis in animal cells. However, relatively little is known about the subcellular localization, trafficking, and function of SCAMPs in plants. Several recent studies in plant cells indicate that plant SCAMPs share many similarities with their mammalian homologs although there are differences. In this review, we will first summarize and compare animal and plant SCAMPs in terms of their subcellular localization, trafficking, and possible functions. We will then present a phylogenetic analysis of plant and animal SCAMPs. Finally, we will present expression analysis on selective Arabidopsis SCAMPs in the hope of pointing to directions for functional characterization of plant SCAMPs in the future.     

肝细胞生成素——一种非经典分泌蛋白

肝细胞生成素(hepatopoietin ,HPO)是一种分泌蛋白.为了研究肝细胞生成素的分泌途径,利用SignalP软件分析了HPO的氨基酸序列,但HPO序列中没有经典分泌蛋白的信号肽.Western印迹实验证明,HPO能以双体形式从细胞中分泌出来.特异性体外阻断实验表明,布雷菲尔德菌素A(brefeldinA)和莫能菌素(monensin)都不能阻断HPO的分泌,说明HPO并不通过经典的内质网 高尔基体(ER -Golgi)途径分泌;优降糖(glyburide)对HPO的分泌没有抑制作用,说明HPO的分泌并不是由ABC1(ATP bindingcassette)转运子介导的;DNP和NH4Cl也不能刺激HPO的分泌,说明内体 溶酶体系统不参与HPO的分泌.上述结果表明,HPO是一种非经典分泌蛋白(non classicalsecretoryprotein) ,能以双体形式从细胞中分泌出来.但和已知的非经典分泌蛋白IL -1β不同,HPO的分泌并不是通过ABC1转运子介导的,内体 溶酶体系统也不参与其分泌.     

Tension-induced autophagy

Impairment of autophagy in patients and animal models severely affects mechanically strained tissues such as skeletal muscle, heart, lung and kidney, leading for example to muscle dystrophy, cardiomyopathy and renal injury. However, the reason for this high reliance on autophagy remained largely elusive. Recent work in our lab now provides a possible explanation. We identified chaperone-assisted selective autophagy (CASA) as a tension-induced autophagy pathway essential for mechanotransduction in mammalian cells.     

Bacterial autophagy

Autophagy is a vital process through which cellular material and dysfunctional organelles are degraded and recycled, and it is inhibited by the metabolic checkpoint kinase MTOR. Autophagy also targets intracellular bacteria (a process termed xenophagy) for lysosomal degradation, thereby playing a key role in innate immunity. In the past few years, the identification of molecules, such as CALCOCO2/NDP52, SQSTM1/p62 and ubiquitin, implicated in the specific targeting of intracellular bacteria, received considerable attention. However, it remains unclear how xenophagy is initiated, since this process commonly occurs in metabolically replete cells. In a recent study, we demonstrated that infection with Shigella and Salmonella triggered an early state of intracellular amino acid (AA) starvation causing MTOR dissociation from endomembranes, downregulation of MTOR activity and activation of the EIF2AK4/GCN2-EIF2S1/eIF2α/ATF3 signaling axis. We also observed that AA starvation was caused by host membrane damage, which appeared to be transient in the case of Salmonella and sustained in Shigella-infected cells, thus highlighting the existence of key timing disparities in xenophagy triggering, depending on the bacterial pathogen. Together, our findings demonstrate that xenophagy is only one arm of a more general metabolic switch geared toward AA starvation in bacteria-infected cells.