Autophagy Modulators in Coronavirus Diseases: A Double Strike in Viral Burden and Inflammation

Abstract

Coronaviruses are the etiologic agents of several diseases. Coronaviruses of critical medical importance are characterized by highly inflammatory pathophysiology, involving severe pulmonary impairment and infection of multiple cell types within the body. Here, we discuss the interplay between coronaviruses and autophagy regarding virus life cycle, cell resistance, and inflammation, highlighting distinct mechanisms by which autophagy restrains inflammatory responses, especially those involved in coronavirus pathogenesis. We also address different autophagy modulators available and the rationale for drug repurposing as an attractive adjunctive therapy. We focused on pharmaceuticals being tested in clinical trials with distinct mechanisms but with autophagy as a common target. These autophagy modulators act in cell resistance to virus infection and immunomodulation, providing a double-strike to prevent or treat severe disease development and death from coronaviruses diseases.

Keywords: autophagy; coronaviral infection; inflammation; tissue damage; viral replication.

Figure 1

The autophagic flux and its components. Diverse complexes tightly regulate the autophagic pathway. Two kinase complexes are involved in phagophore formation. The ULK1 complex activates the class III PI3K that performs the phospholipid 3-phosphatidyl inositol (PI3) phosphorylation, generating PI3P, which is crucial for phagophore nucleation. Both complexes can be oppositely modulated by distinct kinases, like AMPK, associated with activation, and mTOR complex, associated with inhibition, depending on the amino acid residue targeted for phosphorylation by each kinase. The pharmaceuticals metformin and rapamycin drive autophagosome formation after activating AMPK and inhibition of mTOR complex, respectively. The kinase Akt (or protein kinase B-PKB) is upstream from the mTOR complex, activated by various stimuli. Once a phagophore is formed, it is elongated by different proteins, especially LC3. Pro-LC3 is converted to LC3-I by ATG4. ATG7, ATG3 and the complex formed by ATG12-ATG5-ATG16L1 are crucial for LC3-II formation and binding to the phagophore lipid phosphatidylethanolamine (PE). LC3-II promotes phagophore maturation and closure. The mature autophagosomes are fused with lysosomes and degraded by acid proteases, leading to macromolecules and organelles recycling. Pharmaceuticals that induce lysosome biogenesis, like niclosamide, resveratrol, and valinomycin, promote autophagosomes degradation. Chloroquine is an alkalinizing lysosomotropic agent, inhibiting acid proteases activity and autolysosome degradative function, leading to autophagosome accumulation.

Figure 2

Opposing effects of intracellular Ca⁺² in autophagy modulation. Calcium ions can inhibit or induce autophagy, depending on its levels and interactive proteins. Calcium release from the ER is mediated by IP3R, which links PLC activity to autophagy. IP3 is generated from PIP2 by PLC, which is dependent on free inositol levels. PLC activity also depends on cAMP levels that keep Epac and Rap2b activated. Rap2b-mediated activation of calpain drives Beclin-1 degradation, restraining phagophore formation. IP3R also recruits Beclin-1, which further restrains Beclin-1 availability from forming the class III PI3K complex. Thus, pharmaceuticals like verapamil and clonidine that lead to reduced cAMP levels are associated with autophagy induction (A). On the other hand, high levels of intracellular Ca⁺² lead to calmodulin activation and subsequent AMPK-induced autophagy after phosphorylation of ULK-1 complex and class III PI3K (B).

Figure 3

Autophagy can restrain intracellular viral burden and secretion of pro-inflammatory cytokines by infected host cells: Autophagy modulators that increase lysosome biogenesis and fusion with autophagosomes lead to degradation of viral particles, organelles, and macromolecules, like NLRP3, a known cytoplasmic PRR. Mitochondrial ROS also promotes both cytoplasmic, such as RLRs, and endosomal, such as TLR7 and TLR3, PRRs signaling, and mitophagy-mediated restriction of ROS can further restrain intracellular inflammatory pathways (from activated PRRs). Thus, niclosamide, resveratrol, and curcumin potentially drive increased resistance and disease tolerance, after autophagy induction, towards coronavirus diseases, reducing the intracellular viral burden and inflammatory mediators associated with tissue damage. Additionally, resveratrol and curcumin might also contribute to disease tolerance through its anti-oxidant activities, restraining mitochondrial ROS, a critical second messenger in PRRs signaling (Forman et al., 2010).