Classically, canonical autophagy has been considered a survival mechanism initiated in response to nutrient insufficiency. We now understand that autophagy functions in multiple scenarios where it is ...necessary to maintain homeostasis. Recent evidence has established that a variety of non-canonical functions for autophagy proteins are mechanistically and functionally distinct from autophagy. LC3-associated phagocytosis (LAP) is one such novel function for autophagy proteins and is a contributor to immune regulation and inflammatory responses across various cell and tissue types. Characterized by the conjugation of LC3 family proteins to phagosome membranes, LAP uses a portion of the canonical autophagy machinery, following ligation of surface receptors that recognize a variety of cargos including pathogens, dying cells, soluble ligands and protein aggregates. However, instead of affecting canonical autophagy, manipulation of the LAP pathway
alters immune activation and inflammatory responses. In this Cell Science at a Glance article and the accompanying poster, we detail the divergence of this distinctive mechanism from that of canonical autophagy by comparing and contrasting shared and unique components of each pathway.
The clearance of dead cells by efferocytosis Boada-Romero, Emilio; Martinez, Jennifer; Heckmann, Bradlee L ...
Nature reviews. Molecular cell biology,
07/2020, Letnik:
21, Številka:
7
Journal Article
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Multiple modes of cell death have been identified, each with a unique function and each induced in a setting-dependent manner. As billions of cells die during mammalian embryogenesis and daily in ...adult organisms, clearing dead cells and associated cellular debris is important in physiology. In this Review, we present an overview of the phagocytosis of dead and dying cells, a process known as efferocytosis. Efferocytosis is performed by macrophages and to a lesser extent by other 'professional' phagocytes (such as monocytes and dendritic cells) and 'non-professional' phagocytes, such as epithelial cells. Recent discoveries have shed light on this process and how it functions to maintain tissue homeostasis, tissue repair and organismal health. Here, we outline the mechanisms of efferocytosis, from the recognition of dying cells through to phagocytic engulfment and homeostatic resolution, and highlight the pathophysiological consequences that can arise when this process is abrogated.
LC3-Associated Phagocytosis and Inflammation Heckmann, Bradlee L.; Boada-Romero, Emilio; Cunha, Larissa D. ...
Journal of Molecular Biology/Journal of molecular biology,
11/2017, Letnik:
429, Številka:
23
Journal Article
Recenzirano
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LC3-associated phagocytosis (LAP) is a novel form of non-canonical autophagy where LC3 (microtubule-associated protein 1A/1B–light chain 3) is conjugated to phagosome membranes using a portion of the ...canonical autophagy machinery. The impact of LAP to immune regulation is best characterized in professional phagocytes, in particular macrophages, where LAP has instrumental roles in the clearance of extracellular particles including apoptotic cells and pathogens. Binding of dead cells via receptors present on the macrophage surface results in the translocation of the autophagy machinery to the phagosome and ultimately LC3 conjugation. These events promote a rapid form of phagocytosis that produces an “immunologically silent” clearance of the apoptotic cells. Consequences of LAP deficiency include a decreased capacity to clear dying cells and the establishment of a lupus-like autoimmune disease in mice. The ability of LAP to attenuate autoimmunity likely occurs through the dampening of pro-inflammatory signals upon engulfment of dying cells and prevention of autoantigen presentation to other immune cells. However, it remains unclear how LAP shapes both the activation and outcome of the immune response at the molecular level. Herein, we provide a detailed review of LAP and its known roles in the immune response and provide further speculation on the putative mechanisms by which LAP may regulate immune function, perhaps through the metabolic reprogramming and polarization of macrophages.
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•Comprehensive overview of LC3-associated phagocytosis (LAP).•LAP bridges the phagocytic and autophagic pathways.•Signaling to LAP and regulation of LAP activation by the canonical autophagy machinery.•LAP quenches inflammation and shapes the immune response towards anti-inflammation.•LAP promotes the immunosilent clearance of dying cells and prevents autoimmunity.
Programmed cell death (PCD) plays critical roles in development, homeostasis, and both control and progression of a plethora of diseases, including cancer and neurodegenerative pathologies. Besides ...classical apoptosis, several different forms of PCD have now been recognized, including necroptosis. The way a cell dies determines the reaction of the surrounding environment, and immune activation in response to cell death proceeds in a manner dependent on which death pathways are activated. Apoptosis and necroptosis are major mechanisms of cell death that typically result in opposing immune responses. Apoptotic death usually leads to immunologically silent responses whereas necroptotic death releases molecules that promote inflammation, a process referred to as necroinflammation. Diseases of the nervous system, in particular neurodegenerative diseases, are characterized by neuronal death and progressive neuroinflammation. The mechanisms of neuronal death are not well defined and significant cross-talk between pathways has been suggested. Moreover, it has been proposed that the dying of neurons is a catalyst for activating immune cells in the brain and sustaining inflammatory output. In the current review we discuss the effects of apoptotis and necroptosis on inflammatory immune activation, and evaluate the roles of each cell death pathway in a variety of pathologies with specific focus on neurodegeneration. A putative model is proposed for the regulation of neuronal death and neuroinflammation that features a role for both the apoptotic and necroptotic pathways in disease establishment and progression.
Administration of glucocorticoids is an effective strategy for treating many inflammatory and autoimmune diseases. However, glucocorticoid treatment can have adverse effects on bone, leading to ...glucocorticoid‐induced osteoporosis (GIO), the most common form of secondary osteoporosis. Although the pathogenesis of GIO has been studied for decades, over the past ten years the autophagy machinery has been implicated as a novel mechanism. Autophagy in osteoblasts, osteocytes, and osteoclasts plays a critical role in the maintenance of bone homeostasis. Herein, we specifically discuss how osteoblast autophagy responds to glucocorticoids and its role in the development of GIO.
Although the pathogenesis of glucocorticoid‐induced osteoporosis (GIO) has been studied for decades, over the past ten years the autophagy machinery has been implicated as a novel mechanism. Autophagy in osteoblasts, osteocytes, and osteoclasts plays a critical role in the maintenance of bone homeostasis. Herein, we specifically discuss how osteoblast autophagy responds to glucocorticoids and its role in the development of GIO.
The expression of some proteins in the autophagy pathway declines with age, which may impact neurodegeneration in diseases, including Alzheimer’s Disease. We have identified a novel non-canonical ...function of several autophagy proteins in the conjugation of LC3 to Rab5+, clathrin+ endosomes containing β-amyloid in a process of LC3-associated endocytosis (LANDO). We found that LANDO in microglia is a critical regulator of immune-mediated aggregate removal and microglial activation in a murine model of AD. Mice lacking LANDO but not canonical autophagy in the myeloid compartment or specifically in microglia have a robust increase in pro-inflammatory cytokine production in the hippocampus and increased levels of neurotoxic β-amyloid. This inflammation and β-amyloid deposition were associated with reactive microgliosis and tau hyperphosphorylation. LANDO-deficient AD mice displayed accelerated neurodegeneration, impaired neuronal signaling, and memory deficits. Our data support a protective role for LANDO in microglia in neurodegenerative pathologies resulting from β-amyloid deposition.
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•LC3-associated endocytosis (LANDO) requires Rubicon and ATG5, but not FIP200•LANDO is required for recycling of Aβ receptors including TREM2 in microglia.•LANDO confers protection against Aβ deposition and murine Alzheimer’s disease (AD)•Microglial LANDO protects against neuronal loss and memory impairment in murine AD
An LC3-associated endocytosis pathway involved in the recycling of amyloid receptors is essential for the clearance of amyloid aggregates by microglia in a model of AD
The discovery of adipose triglyceride lipase (ATGL) and its coactivator comparative gene identification-58 (CGI-58) provided a major paradigm shift in the understanding of intracellular lipolysis in ...both adipocytes and nonadipocyte cells. The subsequent discovery of G0/G1 switch gene 2 (G0S2) as a potent endogenous inhibitor of ATGL revealed a unique mechanism governing lipolysis and fatty acid (FA) availability. G0S2 is highly conserved in vertebrates, and exhibits cyclical expression pattern between adipose tissue and liver that is critical to lipid flux and energy homeostasis in these two tissues. Biochemical and cell biological studies have demonstrated that a direct interaction with ATGL mediates G0S2's inhibitory effects on lipolysis and lipid droplet degradation. In this review we examine evidence obtained from recent in vitro and in vivo studies that lends support to the proof-of-principle concept that G0S2 functions as a master regulator of tissue-specific balance of TG storage vs. mobilization, partitioning of metabolic fuels between adipose and liver, and the whole-body adaptive energy response. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.
•G0/G1 switch gene 2 (G0S2) is a specific inhibitor of intracellular lipolysis mediated by adipose triglyceride lipase (ATGL).•The review covers expression and function of G0S2 in adipose tissue and liver as a regulator of lipid and energy metabolism.•Targeting G0S2-ATGL interaction should be considered in the future.
Intracellular triglyceride (TG) hydrolysis or lipolysis is catalyzed by the key intracellular triglyceride hydrolase, adipose triglyceride lipase (ATGL). The G0/G1 Switch Gene 2 (G0S2) was recently ...identified as the major selective inhibitor of ATGL and its hydrolase function. Since G0S2 levels are dynamically linked and rapidly responsive to nutrient status or metabolic requirements, the identification of its regulation at the protein level is of significant value. Earlier evidence from our laboratory demonstrated that G0S2 is a short-lived protein degraded through the proteasomal pathway. However, little is currently known regarding the underlying mechanisms. In the current study we find that 1) protein degradation is initiated by K48-linked polyubiquitination of the lysine- 25 in G0S2; and 2) G0S2 protein is stabilized in response to ATGL expression and TG accumulation. Mutation of lysine-25 of G0S2 abolished ubiquitination and increased protein stability. More importantly, G0S2 was stabilized via different mechanisms in the presence of ATGL vs. in response to fatty acid (FA)-induced TG accumulation. Furthermore, G0S2 protein but not mRNA levels were reduced in the adipose tissue of ATGL-deficient mice, corroborating the involvement of ATGL in the stabilization of G0S2. Taken together our data illustrate for the first time a crucial multifaceted mechanism for the stabilization of G0S2 at the protein level.
Recent biochemical and cell-based studies identified G0/G1 switch gene 2 (G0S2) as an inhibitor of adipose triglyceride lipase (ATGL), a key mediator of intracellular triacylglycerol (TG) ...mobilization. Here, we show that upon fasting, G0S2 protein expression exhibits an increase in liver and a decrease in adipose tissue. Global knockout of G0S2 in mice enhanced adipose lipolysis and attenuated gain of body weight and adiposity. More strikingly, G0S2 knockout mice displayed a drastic decrease in hepatic TG content and were resistant to high-fat diet (HFD)-induced liver steatosis, both of which were reproduced by liver-specific G0S2 knockdown. Mice with hepatic G0S2 knockdown also showed increased ketogenesis, accelerated gluconeogenesis, and decelerated glycogenolysis. Conversely, overexpression of G0S2 inhibited fatty acid oxidation in mouse primary hepatocytes and caused sustained steatosis in liver accompanied by deficient TG clearance during the fasting-refeeding transition. In response to HFD, there was a profound increase in hepatic G0S2 expression in the fed state. Global and hepatic ablation of G0S2 both led to improved insulin sensitivity in HFD-fed mice. Our findings implicate a physiological role for G0S2 in the control of adaptive energy response to fasting and as a contributor to obesity-associated liver steatosis.
BACKGROUND AND PURPOSE The class III PI3K inhibitor, 3‐methyladenine (3‐MA), is commonly used to selectively block autophagy. Recent findings suggest a strong relationship between autophagy and lipid ...turnover. Here, we explore the effect of 3‐MA on adipocyte lipolysis.
EXPERIMENTAL APPROACH Assays were performed in 3T3‐L1 cells. Cells were treated with 3‐MA and wortmannin, a pan PI3K and autophagy inhibitor. Pharmacological and genetic manipulation of endogenous autophagic and lipolytic pathways was used to ascertain the contribution of 3‐MA to the observed effects on lipolysis.
KEY RESULTS 3T3‐L1 cells that were exposed to 3‐MA showed a consistent increase in lipolysis, approximately 50% over basal levels. The effect of 3‐MA was not secondary to autophagic inhibition as treatment of 3T3‐L1 cells with wortmannin yielded no such changes. Dosing and time course experiments showed that 3‐MA's ability to activate lipolysis was more sensitive than its inhibitory effect on autophagy. Knockdown of adipose triglyceride lipase (ATGL) negated the stimulatory effect of 3‐MA by >90%, indicating that 3‐MA enhanced ATGL‐dependent hydrolysis of triacylglycerols. Additionally, the lipolytic effect of 3‐MA was dependent on the activation of PKA and 3‐MA induced a rapid and potent elevation of intracellular cAMP levels in adipocytes.
CONCLUSIONS AND IMPLICATIONS Cumulatively, we show that 3‐MA potently modulated a cellular mechanism and its underlying signalling pathways not associated with autophagy. Furthermore, we describe a novel stimulatory effect on a major signalling pathway. Our findings provide valuable information to studies employing 3‐MA as a specific inhibitor for PI3K and autophagy.
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