ATG14 and STX18: gatekeepers of lipid droplet degradation and the implications for disease modulation

ABSTRACT

Lipophagy, a form of autophagy specific to the degradation of lipid droplets (LDs), plays an important role in the maintenance of cellular homeostasis and metabolic processes. A recent study has identified ATG14 (autophagy related 14) as a molecule that targets LDs and marks them for degradation via lipophagy; a process that is inhibited by the binding of STX18 (syntaxin 18) to ATG14 in mammalian cells. The exact mechanism of regulation of lipophagy, and subsequently of cellular LD levels, is still under investigation; however, dysregulation of this process has been linked to a number of disease phenotypes. An imbalance of lipid levels can result in a wide variety of conditions depending on the cell/tissue type in which they occur. In cells of the retinal pigment epithelium, lipid accumulation can result in dry age-related macular degeneration, in hepatocytes it can result in nonalcoholic fatty liver diseases and in neural cells it can result in the pathogenesis of neurodegenerative conditions such as Alzheimer and Parkinson diseases. Based upon its wide range of implications in diseases, modulation of lipophagy is currently being further investigated for its potential as a treatment for a variety of conditions ranging from viral infection to developmental illnesses.

KEYWORDS:

• ATG14
• lipid droplets
• lipid metabolism-associated diseases
• lipophagy
• STX18

Lipophagy, a specific form of autophagy that targets lipid droplets (LDs) for degradation, plays a crucial role in cellular lipid homeostasis and energy metabolism [1]. Lipophagy ensures the dynamic turnover of lipid droplets, facilitating the release of fatty acids for beta-oxidation and energy production, particularly under stress conditions. Given the role of lipids in cellular function and their dysregulation in numerous diseases, lipophagy has emerged as a critical area of research. A recent study by Yuan et al. [2] has uncovered the interactions between the core-autophagy protein ATG14 (autophagy related 14) and STX18 (syntaxin18), focusing on their combined role in lipid droplet metabolism via lipophagy. ATG14, associated with autophagy initiation as a component of the class III phosphatidylinositol 3-kinase complex I, has been identified as a novel receptor targeting LDs directly for degradation. ATG14’s affinity for LDs is enhanced upon treatment with oleic acid, indicating a dynamic mechanism influenced by LD biogenesis. The study also uncovers that ATG14 can directly interact with Atg8-family proteins through an LC3-interacting region/LIR motif. Furthermore, ATG14 functions as a receptor for STX18, which acts as a negative regulator of lipophagy by binding to ATG14 and preventing the formation of the class III phosphatidylinositol 3-kinase complex I. Knockdown of STX18 activates lipophagy dependent on ATG14, demonstrating the role of STX18 as an inhibitor of lipophagy. The coronavirus M protein induces lipophagy by binding STX18 and preventing STX18-ATG14 interaction, facilitating RSAD2/Viperin degradation and subsequent virus proliferation. Discoveries about the regulatory components of lipophagy provide a basis for which medical researchers may look for new drug targets in disease contexts where lipophagy is implicated. Due to the importance of lipid droplet turnover in maintaining cellular homeostasis, dysregulation of lipophagy is implicated in the pathologies of several common human medical conditions that currently lack effective treatment options such as macular degeneration, nonalcoholic fatty liver disease (NAFLD), and Alzheimer disease (AD).

Dry age-related macular degeneration (AMD) is the leading cause of vision loss in the developed world and is characterized in part by dysregulation of lipid metabolism via lipophagy [3]. Lipid droplets play a crucial role in the pathology of AMD, particularly in the formation and accumulation of extracellular lipid deposits termed “drusen”, which is a hallmark feature of the disease. The retinal pigment epithelium (RPE) is a cellular monolayer of the macula that provides myriad support for the photoreceptors of the retina, including constant breakdown and recycling of photoreceptor outer segments. Photoreceptor segments are lipid-rich and the RPE must efficiently metabolize lipids at high volumes to not only dispose of waste but also to ensure reutilization of nutrients. The dynamic relationship between the RPE and photoreceptors is fundamental to the upkeep of photoreceptor viability [4,5]. Dysregulation of lipid metabolism results in the accumulation of drusen and “reticular pseudodrusen.” When the RPE fails to effectively degrade lipid droplets via lipophagy, extracellular, lipid-rich deposits accumulate on both sides of the RPE monolayer, significantly affecting the ability of the RPE to provide support for the photoreceptors apically, and to receive nutrients from the choroid basally. Lipophagy has emerged as a potential therapeutic target in AMD treatment; by inducing lipophagy in the RPE, it may be possible to degrade lipid droplets and reduce the accumulation of drusen [6]. A growing understanding of the pathways governing lipophagy induction has great potential to reveal novel drug targets. Research targeting lipophagy to treat dry AMD by interaction with known regulators, such as STX18 or ATG14 as elucidated by mechanistic lipophagy studies, has great therapeutic potential and promise.

In hepatocytes, lipophagy plays a key role in the breakdown of lipids and is crucial in the maintenance of proper homeostasis and metabolism. When dysregulated, lipophagy becomes a key component in the pathophysiologies of many liver disorders such as NAFLDs [7]. NAFLDs are among the most common hepatic disorders and are oftentimes related to type 1 and type 2 diabetes and obesity and can present with a broad spectrum of physical liver changes including fat buildup and cirrhosis. On a cellular level, NAFLD is associated with an increase in triglycerides in the hepatocytes which can occur when lipophagy is functioning improperly. In a study by Lin et al. it was found that knockdown of the IRGM gene, which codes for a protein associated with autophagy initiation, inhibits autophagy within hepatocytes leading to an increase in lipid droplets [8]. Lipophagy within hepatocytes can be impaired in a number of ways leading to short-term and long-term inhibition. In vitro studies primarily highlight the role of genes in lipophagy-linked NAFLDs, identifying several key genes including ones associated with autophagosome formation, trafficking to the vacuole, and protease inhibitors among others [7]. In vivo, high-fat diets are a trigger for this impairment and are linked to defects in autophagosome-lysosome fusion as well as overactivity of MTOR, an inhibitor of lipophagy, resulting in an overall decreased flux and increased accumulation of LDs [9]. Given the association of impaired LD turnover with liver-related illnesses such as NAFLDs, lipophagy regulation is being targeted as a potential treatment for hepatic illnesses.

Beyond hepatocytes, abnormal LD accumulation in neurons and glial cells is a hallmark in the pathogenesis of neurodegenerative disorders. Two separate studies from Hamilton et al. and Moreau et al. show that, in AD, an increase in LD leads to impairment of the regenerative capabilities of neuronal stem cells, and impairment of lipophagy in microglia results in inflammatory cascades, reducing the protective capabilities of neural tissues against AD progression [10,11]. Similarly, in the context of Parkinson disease (PD), aggregation of the protein SNCA/alpha-synuclein, a key pathological marker of PD, is linked to increased accumulation of LDs as SNCA clumps accumulate on the surface of LDs. Studies have further shown that SNCA is able to inhibit the function of the enzyme PLD (phospholipase D), resulting in the accumulation of LDs, further highlighting the link between LD accumulation, induced by dysregulated lipophagy, and PD pathogenesis [12]. Even though the exact role of lipophagy in neurodegeneration remains unclear, evidence has shown that increased LD accumulation in cells of the central nervous system plays a key role in a number of pathologies including AD, PD, multiple sclerosis, Huntington disease, and more [13]. Lipophagy is being targeted as a potential mechanism for treatment of these disorders with further studies into the exact role of lipophagy and the mechanisms of its regulation in the central nervous system being carried out in the hope of a better understanding of neurodegenerative disease pathology, progression, and treatment.

Recent exploration of lipophagy has elucidated mechanisms underscoring its pivotal role in cellular lipid management and energy homeostasis. Beyond the ATG14-STX18 interaction, other proteins and signaling pathways directly regulate this process. The participation of MAP1LC3/LC3 and its lipidation status in the sequestration of lipid droplets, and the involvement of RAB proteins in targeting lipid droplets for lysosomal degradation, reveal the complexity and specificity of lipophagy machinery [14,15]. These discoveries unveil potential targets for therapeutic interventions for diseases where lipid metabolism is dysregulated. The pursuit of these promising therapeutic targets is not without challenges, including limitations in assaying for lipophagy specifically. Traditional assays for autophagy, such as monitoring LC3-II levels or autophagosome formation, do not specifically reflect lipophagic activity. Moreover, the dynamic and transient nature of lipid droplets coupled with the lack of specific markers for lipophagic vesicles complicates the accurate measurement of lipophagy flux. In their study, Yuan et al. relied on a diversity of colocalization and immunofluorescence assays to visualize lipid droplet interactions, as well as a tandem reporter assay to monitor the autophagic degradation of lipid droplets [2]. Developing more specific and sensitive assays for lipophagy, perhaps through the identification of lipophagy-specific markers, is crucial for advancing our understanding and manipulation of this process. The interplay between lipophagy and other lipid metabolic pathways, such as beta-oxidation, remains to be explored. Understanding how lipophagy is coordinated with these pathways will be essential for developing targeted therapies that can modulate lipid metabolism while limiting unintended effects. As we improve our methodological approaches and further our mechanistic insights, the full therapeutic potential of targeting lipophagy becomes increasingly promising.

Abbreviations

AD	=	Alzheimer disease
AMD	=	age-related macular degeneration
LDs	=	lipid droplets
NAFLD	=	nonalcoholic fatty liver disease
PD	=	Parkinson disease
RPE	=	retinal pigment epithelium

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the National Institute of General Medical Sciences [GM131919].