Autophagy？

Autophagy Classifications and Molecular Mechanisms:

Autophagy is a cellular process that facilitates the degradation and recycling of damaged or dysfunctional cellular components and unnecessary proteins. It helps in maintaining cellular homeostasis and is linked with disease progression and aging. This process is highly complex, involving numerous different molecules and pathways. Here, I will briefly describe the primary classifications of autophagy and their associated molecular mechanisms:

Macroautophagy

• Initiation: At the initiation of autophagy, the cell forms a small membrane structure known as a phagophore or isolation membrane. This process involves complexes such as the ULK1 complex and the PI3K complex.
• Nucleation and Elongation: The phagophore then expands, forming a double-membrane structure called an autophagosome. This process involves ATG proteins and the lipidated form of LC3 (LC3-II).
• Fusion: The autophagosome then fuses with a lysosome to form an autolysosome, a process facilitated by molecules like SNARE proteins and LAMP1/2.
• Degradation and Recycling: Hydrolases within the lysosome then degrade the engulfed material, and the degraded materials are reused within the cell.

Microautophagy

• In this form of autophagy, the lysosomal membrane directly engulfs substrates. The lysosomal membrane has the capability to directly engulf proteins and organelles.

Chaperone-Mediated Autophagy (CMA)

• This is a selective form of autophagy where target proteins are recognized by a chaperone protein (like Hsc70) and then transported across the lysosomal membrane for degradation. This pathway is highly specific and involves a unique set of molecular interactions.

These three pathways represent the primary means through which cells efficiently process and recycle unwanted components, which is vital for maintaining healthy cellular function. Moreover, these pathways play roles in cellular stress responses, aging, and disease progression. As different types of autophagy occur via specific molecular mechanisms and pathways, researchers are working on developing new therapies targeting these processes for the treatment of various diseases.

Molecular mechanisms

1. Initiation (Induction Phase):

a. Nutrient Sensing and Upstream Signaling

• Cellular nutrient status is monitored by the mTOR (mammalian target of rapamycin) complex 1 (mTORC1). In nutrient-rich conditions, mTORC1 inhibits autophagy, while nutrient deprivation inactivates mTORC1, thereby activating autophagy.
• AMP-activated protein kinase (AMPK) can also induce autophagy by sensing low energy status in the cell and inhibiting mTORC1.

b. ULK1 Complex Formation

• The initiation process is regulated by the ULK1 complex, which consists of ULK1/2 (serine/threonine-protein kinase), ATG13, FIP200, and ATG101.
• Upon mTORC1 inactivation, the ULK1 complex becomes activated and translocates to the site of phagophore formation, initiating nucleation.

2. Nucleation and Elongation (Phagophore Formation):

The “nucleation” phase in autophagy refers to the initial stage in the autophagy process, where the formation of the autophagosome, a double-membraned structure, begins to take place.

a. PI3K Complex and Phagophore Nucleation

• The PI3K complex, consisting of VPS34 (class III PI3 kinase), Beclin-1, VPS15, and ATG14L, plays a central role in nucleation.
• This complex produces phosphatidylinositol 3-phosphate (PI3P), a signaling lipid essential for the recruitment of other ATG proteins and the expansion of the phagophore membrane.

b. ATG Proteins and LC3 Lipidation

• ATG proteins are crucial for elongation. Two ubiquitin-like conjugation systems are involved: the ATG12-ATG5-ATG16L1 complex and the LC3-II (Microtubule-associated protein 1A/1B-light chain 3) system.
• The LC3 protein is processed to LC3-I, which is then lipidated to form LC3-II, promoting membrane elongation. This lipidation involves ATG7 and ATG3 enzymes, and is facilitated by the ATG12-ATG5-ATG16L1 complex.

3. Autophagosome Formation:

a. Cargo Recognition

• Specific cargo (damaged organelles, proteins) is recognized and targeted by autophagy receptors such as p62/SQSTM1, which binds to LC3-II on the autophagosome membrane, facilitating cargo sequestration.

b. Membrane Closure

• The elongating membrane eventually closes to form a double-membraned vesicle known as an autophagosome, encapsulating the targeted cargo.

4. Fusion with Lysosome (Formation of Autolysosome):

a. Autophagosome Maturation

• The mature autophagosome moves along the microtubule network towards the lysosome, a process regulated by molecular motors and associated proteins.

b. Fusion with Lysosome

• Fusion with the lysosome involves SNARE proteins, the HOPS complex, and other molecules facilitating membrane fusion to form an autolysosome.

5. Degradation and Recycling:

a. Degradation of Contents

• Within the autolysosome, the acidic environment and lysosomal hydrolases degrade the encapsulated material into basic molecules (amino acids, fatty acids, etc.).

b. Recycling of Nutrients

• The degraded components are transported back into the cytosol for reuse, supporting cellular metabolism and biosynthesis.

This overview provides a more detailed insight into the molecular mechanisms driving macroautophagy, highlighting the key complexes, molecules, and processes involved at each stage. It is a coordinated and regulated process essential for cellular homeostasis and health.

Autophagy and Macrophages

As an important component of the innate immune system, macrophages are involved in defending cells from invading pathogens, clearing cellular debris, and regulating inflammatory responses. During the past two decades, accumulated evidence has revealed the intrinsic connection between autophagy and macrophage function. This part focuses on the role of autophagy, both as nonselective and selective forms, in the regulation of the inflammatory and phagocytotic functions of macrophages.

Growing evidence indicates that macrophages serve as a critical link between autophagy and immunity. Autophagy plays a key role in the cellular development of monocytes, which in turn affects the differentiation of macrophages. Activating autophagy facilitates the recycling of cellular components and ATP, essential elements that macrophages need for their energy metabolism, particularly when they are activated. Moreover, autophagy and phagocytosis in macrophages share a number of genes during the process, including Beclin1, Vps34, and Atg5.

Toll-Like Receptors (TLRs): Key Players in Autophagy and Immunity

Toll-Like Receptors (TLRs) are type 1 integral transmembrane proteins characterized by their horseshoe-shaped structure, crucial for recognizing pathogens. Activation of TLRs triggers a cascade of events that lead to the recruitment of specific adaptor proteins, such as myeloid differentiation factor 88 (MyD88) and TIR domain-containing adapter-inducing interferon-beta (TRIF).

Numerous studies have shown that various TLRs, including TLR1, 2, 3, 4, 5, and 7, play a role in initiating the formation of autophagosomes during immune responses. Mechanistic research has unveiled that both MyD88 and TRIF engage with Beclin 1 as part of the TLR signaling complex, facilitating autophagy by blocking the interaction between Beclin 1 and Bcl-2. Additionally, TRAF6, a pivotal ubiquitin E3 ligase in the TLR signaling pathway, interacts with Beclin 1 to regulate its lysine (K) 63-linked ubiquitination, thus inducing autophagy.

Importantly, TLR-induced autophagy can also be selective in nature. For instance, upon treating macrophages with Escherichia coli or lipopolysaccharide (LPS), TLR4 initiates autophagy to selectively target aggresome-like induced structures (ALIS), aided by the protein p62.

In summary, while TLRs are instrumental in autophagosome formation during pathogen invasion, the precise roles of TLR-induced autophagy in regulating macrophage function are still not fully understood.

NOD-Like Receptors (NLRs): Intricate Interplay with Autophagy and Immunity

NOD-Like Receptors (NLRs) serve as vital components of internal surveillance mechanisms that detect intracellular pathogens. Among these, NOD1 and NOD2 are particularly notable for their ability to recognize bacterial peptidoglycan and initiate pro-inflammatory responses. Research indicates that both NOD1 and NOD2 can induce autophagy by interacting with and recruiting ATG16L1. Interestingly, genetic variations in both NOD2 and ATG16L1 have been linked to Crohn’s disease, highlighting the complex relationship between these factors in biological processes and human diseases.

Further studies have unveiled that ATG16L1 moderates the cytokine responses induced by NOD1 and NOD2 via RIP2 activation, but this effect is independent of autophagosome formation. Therefore, the autophagy process triggered by NOD2 is distinct from pathways involving RIP2 or NF-κB.

This accumulating evidence points to a close interaction between Pattern Recognition Receptors (PRRs) and autophagy in the regulation of macrophage function. Typically, PRRs are the first to signal during pathogen recognition and primarily regulate autophagy at this initial stage. For instance, Beclin 1 directly interacts with MyD88 and TRIF. Inhibiting autophagy appears to diminish PRR-associated biological functions, such as dectin-1-induced vesicle-mediated protein secretion, TLR-mediated bacterial clearance, and IFN-α secretion.

In summary, while the role of PRRs in regulating autophagy is becoming clearer, the exact mechanisms and implications for macrophage anti-pathogenic functions still require more in-depth investigation.

The Complex Interplay Between Cytokines and Autophagy in Macrophages

Cytokines are specialized secretory proteins chiefly produced by macrophages and lymphocytes. They play a pivotal role in orchestrating effective immune responses by modulating the inflammatory microenvironment. Although autophagy deficiency has been implicated in a range of inflammatory diseases—such as inflammatory bowel disease (IBD), systemic lupus erythematosus (SLE), and arthritis—its role in cytokine production remains ambiguous.

Research indicates that the loss of Atg7, a critical gene for autophagy, leads to increased production of IL-1β and pyroptosis following P. aeruginosa infection. However, levels of other important cytokines like IL-6 and TNF-α are not affected. Further experiments have shown that administration of 3-methyladenine (3-MA), an autophagy inhibitor, ameliorates the symptoms of sepsis, as well as the production of IL-6 and TNF-α in a lethal murine sepsis model. Moreover, mice with an Atg5 gene deletion specifically in macrophages (Atg5^fl/fl lysM−Cre+) showed significantly increased levels of IL-1α, IL-12, and CXCL1 in lung tissue after M. tuberculosis infection, yet no impact was observed on the universal pro-inflammatory cytokines like IFN-γ, TNF-α, and IL-6.

This data suggests that the impact of autophagy on cytokine production is not the result of broad inflammatory stimulation. Rather, autophagy appears to influence cytokine production through more intricate, targeted mechanisms. As such, the role of autophagy in the regulation of macrophage function—and, by extension, in the inflammatory response—warrants further in-depth investigation to uncover these nuanced interactions.

The NF-κB Pathway and its Interplay with Autophagy

NF-κB is a pivotal transcription factor responsible for regulating a broad spectrum of genes associated with the inflammatory response. In macrophages, both pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) are capable of activating the NF-κB pathway. This activation leads to enhanced cell survival, proliferation, inflammatory responses, and angiogenesis. It accomplishes this through the production of various cytokines like IL-6 and TNF-α, chemokines such as MCP-1 and IL-18, cell cycle regulators including Bcl-2L1 and Cyclin, as well as adhesion molecules like ICAM-1 and VCAM-1.

Autophagy has been demonstrated to influence the NF-κB pathway by regulating the degradation of key components like NF-κB-inducing kinase (NIK) and IκB kinase (IKK), particularly under conditions where Hsp90 is inhibited. Emerging research confirms that alterations in the NF-κB pathway can modulate autophagy levels. For example, a study by Mojavaheri-Mergny et al. found that activation of the NF-κB pathway suppressed TNF-α-induced autophagy in various cancer cell lines. Subsequent studies identified a NF-κB binding site in the promoter region of BECN1, and showed that the NF-κB family member p65/RelA could upregulate BECN1 mRNA expression to activate autophagy.

Recent findings also indicate that the NF-κB factor, Relish, regulates autophagy by modulating ATG1 expression, which facilitates salivary gland degradation in Drosophila. Additionally, the NF-κB pathway itself relies on autophagy for the degradation of kinases involved in its own activation, thereby serving as a regulatory mechanism to limit excessive inflammatory responses.

The Inflammasome Pathway and its Relationship with Autophagy

In addition to the NF-κB pathway, inflammasome formation serves as another critical mechanism for regulating innate immunity in macrophages. The activation of inflammasomes facilitates the maturation of proinflammatory cytokines like IL-1β and IL-18 through the action of the proteolytic enzyme, caspase-1. Just like NF-κB, inflammasomes can be triggered by a diverse array of stimuli, such as pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs).

Structurally, inflammasomes are localized in the cytoplasm and are assembled from multiple protein components. These include a cytosolic sensor protein, which could be either a nucleotide-binding oligomerization domain and leucine-rich repeat-containing protein (NLR) or an AIM2-like receptor (ALR). Additionally, they contain an adaptor protein known as apoptosis-associated speck-like protein (ASC), which includes a caspase-recruitment domain (CARD), and an effector molecule, pro-caspase-1.

ASC is unique in that it contains both an N-terminal Pyrin domain (PYD) and a CARD. To date, several types of inflammasomes have been identified, including but not limited to NLRP1, NLRP3, AIM2, and NLRC4. The NLRP3 inflammasome, for instance, is composed of NLRP3, ASC, and pro-caspase-1.

In 2008, Tatsuya Saitoh and colleagues highlighted the relationship between autophagy and inflammasomes. They discovered that macrophages deficient in Atg16L1 had increased production of IL-1β and IL-18, but not of LPS-induced IL-6 or TNF-α. This led to further research to understand the underlying mechanisms.

One significant finding is the role of mitochondrial-derived DAMPs (Damage-Associated Molecular Patterns) as triggers for inflammasome activation. Autophagy is capable of removing these DAMPs, effectively inhibiting inflammasome activation. When autophagy is disrupted, there’s an exaggerated activation of the NLRP3 inflammasome in response to various stimuli in macrophages. This exaggerated response is accompanied by an increase in ROS (Reactive Oxygen Species)-producing mitochondria.

Furthermore, released mitochondrial DNA (mtDNA) has been identified as another trigger for NLRP3 inflammasome activation. Autophagy plays a crucial role in monitoring and efficiently clearing dysfunctional mitochondria, thus providing another layer of regulation in inflammasome activation.

Role of P62 in Autophagy and Inflammasome Pathways

P62 serves as a crucial receptor in the selective autophagic clearance of both inflammasomes and dysfunctional mitochondria during inflammasome activation. It is specifically recruited through K63 (Lys63)-linked polyubiquitination of ASC, which in turn triggers the formation of ASC-targeted autophagosomes for degradation.

Additionally, the AIM2 inflammasome is associated with tripartite motif11 (TRIM11), an E3 ubiquitin ligase, further facilitating P62’s role in autophagic degradation. Given that inflammasomes are complex platforms made up of multiple proteins, autophagy becomes instrumental in their degradation.

Regulatory Interactions Between Autophagy and Inflammasomes

Collectively, autophagy acts as a regulatory mechanism for the inflammasome pathway by mediating the degradation of either the activators or the components of inflammasomes themselves. In a compelling twist, research by Zhenyu Zhong and colleagues has shown that NF-κB can induce anti-inflammatory effects by promoting p62 expression, thereby accelerating mitophagy and reducing inflammasome activation.

These insights point to a complex regulatory network that exists between autophagy and inflammasome pathways, further underscoring the intricate balance that governs immune responses and inflammatory regulation.

Macrophage Polarization and Its Significance

Macrophages display a high degree of heterogeneity, adapting their phenotypes in response to different tissue microenvironments. Broadly, macrophages can be categorized into two main phenotypes: M1 (classically activated) and M2 (alternatively activated).

M1 Phenotype

The M1 phenotype is induced primarily by Interferon-γ and lipopolysaccharide (LPS). Macrophages with this phenotype have increased production of pro-inflammatory cytokines and play a critical role in cellular immunity.

M2 Phenotype

Conversely, the M2 phenotype is activated by cytokines such as IL-4 or IL-13. Macrophages adopting the M2 phenotype are typically involved in tissue repair and promote humoral immunity.

This dual polarization capability of macrophages underlines their flexibility and critical role in both pro-inflammatory responses and tissue repair mechanisms.

The Complex Interplay Between Autophagy and Macrophage Polarization

Autophagy plays a critical role in regulating macrophage polarization through various signaling pathways, including the NF-κB pathway and the mTOR pathway.

NF-κB Pathway

Although NF-κB activation is commonly associated with M1 polarization, it can drive macrophages to either the M1 or M2 phenotype, particularly within tumor microenvironments. In 2013, a study by Chih-Peng Chang and colleagues demonstrated that TLR2 signaling induces cytosolic ubiquitination of NF-κB p65, leading to its degradation via p62-mediated autophagy. When autophagy was inhibited, the rescued NF-κB activity shifted macrophages towards the M2 phenotype.

mTOR Pathway

mTOR is another key regulator of both autophagy and macrophage polarization. Activation of the mTOR pathway has been shown to induce macrophage polarization. For instance, Rapamycin, an autophagy inducer that works by inhibiting the mTOR pathway, stimulates macrophages towards the M1 phenotype. Conversely, silencing TSC2 (tuberous sclerosis 2) had the opposite effect, indicating the nuanced relationship between mTOR and macrophage polarization.

Beyond the NF-κB and mTOR pathways, emerging evidence suggests other mechanisms also contribute to the interplay between autophagy and macrophage polarization. CCL2 and IL-6, for example, are strong inducers of autophagy in macrophages and can steer macrophages towards the M2 phenotype. Additionally, Sorafenib, a multi-kinase inhibitor, has been shown to induce autophagy while suppressing macrophage activation by inhibiting the expression of macrophage surface antigens.

In summary, the regulation of macrophage polarization by autophagy is a complex process influenced by multiple pathways and factors, warranting further investigation for potential therapeutic applications.

Role of Enzymes iNOS and Arginase 1 in Macrophage Polarization and Autophagy

Inducible nitrogen oxidase (iNOS) and Arginase 1 serve as markers for macrophage polarization into M1 and M2 phenotypes, respectively. M1 macrophages express iNOS and convert arginine into nitric oxide (NO) and citrulline. On the other hand, M2 macrophages utilize Arginase 1 to hydrolyze arginine into ornithine and urea.

Relationship Between Autophagy and Enzyme Expression

Research indicates that autophagy could be linked to the expression of iNOS and Arginase 1. Overexpression of miR-326, for instance, inhibits iNOS expression while promoting autophagy. Additionally, glucocorticoids have been shown to suppress antimicrobial autophagy, simultaneously enhancing iNOS expression and NO production. In LPS-stimulated microglia, activation of autophagy suppresses iNOS expression. Recombinant human arginase, interestingly, has been shown to induce autophagy.

Unresolved Questions and Future Directions

While it is clear that autophagy plays a role in macrophage polarization, the precise mechanisms remain elusive. Open questions include whether autophagy genes directly modulate signaling pathways for macrophage polarization or if they act by regulating the degradation of key proteins involved in this process.

In summary, understanding the complex interplay between autophagy and macrophage polarization, particularly the role of iNOS and Arginase 1, could have significant implications for the development of targeted therapies for inflammatory conditions. Further research is needed to elucidate the exact mechanisms.