ABSTRACT
Macroautophagy/autophagy and inflammation are 2 intertwined processes vital for immune cells to perform their functions. Under resting conditions, autophagy acts as a brake to suppress inflammation in microglia. Upon signal stimulation, their fine-tuned interplay is pivotal for proper response to stress. How inflammatory signals remove this autophagy brake on inflammation remains unclear. In a recent study, we showed that the stress kinase MAPK14/p38α in microglia senses the inflammatory cue lipopolysaccharide (LPS), directly phosphorylates and inhibits ULK1, relieves the autophagic inhibition on the inflammatory machinery, and thus allows for a full immune response.
KEYWORDS: autophagy, inflammation, MAPK14/p38α, microglia, ULK1
Inflammation has long been a well-known symptom of many infectious diseases, but molecular and epidemiological research increasingly suggests that it is also intimately linked with a broad range of noninfectious diseases including neurodegeneration, obesity and diabetes, chronic autoimmune conditions, cancer, and aging. Autophagy is involved in the modulation of cell metabolism, host defense, and cell survival through its role in maintaining cellular homeostasis by the disposal of damaged organelles, and aggregated proteins, as well as invasive pathogens via a lysosomal degradation pathway. Autophagy and inflammation are 2 fundamental biological processes that are involved in both physiological and pathophysiological conditions.
Recent studies start to reveal the crosstalk between autophagy and inflammation. More and more studies indicate that the immune mediators have complex effects on autophagy. For example, it is well established that in general Th1 cytokines, including IFNG/IFN-γ, TNF/TNF-α, IL1, IL2, IL6 and TGFB/TGF-β, induce autophagy. In contrast, the classical Th2 cytokines, including IL4, IL10 and IL13, repress autophagy. Conversely, increasing evidence suggests that autophagy plays an important role in modulation of both innate and adaptive immunity through eliminating invading pathogens, regulating innate pathogen recognition, contributing to antigen presentation, and controlling B cell and T cell development and survival. Many steps in multiple immune signaling cascades are subject to regulation by autophagy, and autophagy dysfunction contributes to the pathogenesis of various inflammation-related disorders.
Under resting conditions, autophagy serves as a brake to suppress inflammation in microglia. Inflammatory signals such as LPS apparently can relieve such a brake. But mechanistically how LPS signals to enact on autophagy and remove the autophagy brake on inflammation remains unknown. The mitogen-activated protein kinase MAPK14/p38α is a serine/threonine kinase involved in the inflammatory process including the activation of inflammasomes in microglia. Inhibition of MAPK14 is currently being tested for several inflammatory diseases such as Crohn disease and rheumatoid arthritis. Notably, one of the early key initial events in autophagy is the formation of the phagophore (the precursor to the autophagosome), a unique double-membrane structure that engulfs the cytosolic cargo destined for degradation. This step is mediated by the serine/threonine protein kinase ULK1 (unc-51 like kinase 1), which functions in a complex with ATG13 (autophagy-related 13) to promote autophagy induction. Regulation of ULK1 by various upstream pathways, such as nutrient sensing by AMPK and MTOR, indicates that ULK1 functions as a signaling node and translates multiple cellular inputs into appropriate regulation of autophagosome formation.
Our recent work showed that MAPK14 plays a direct and essential role in relieving the inhibitory control by autophagy on inflammation in response to a stress signal. We discovered that MAPK14 and ULK1 interact with each other in microglia. LPS stimulation activates MAPK14 through binding of LPS to its receptor TLR4 on the microglia surface, which leads to MAPK14-dependent phosphorylation of ULK1 in cultured microglial cells and in animal brain. The phosphorylation on S757 by MAPK14 inhibits ULK1 kinase activity, disrupts its interaction with ATG13, a key partner in the autophagy initiation complex, and reduces the level and flux of autophagy. This autophagy inhibition induced by LPS through the MAPK14-ULK1 axis eases off the brake put on inflammation by autophagy. This response is necessary for LPS-induced full activation of the NLRP3 inflammasome and subsequent processing of pro-IL1B/interleukin-1β into IL1B by CASP1/caspase-1, and microglia morphological change in culture and in mouse brain (Figure 1). Thus, our work has identified a mechanism that functions to release the inhibitory effect of autophagy on inflammation upon the transmission of a stress signal and allows a rapid and full induction of the immune process during microglia activation. This mechanism may play an essential role in innate immune modulation in the central nervous system in response to various disease-causing insults.
Figure 1.
Release of the autophagy brake on inflammation through the MAPK14-ULK1 pathway in microglia. Under resting conditions, autophagy serves as a brake on the NLRP3 inflammasome in surveillance microglia, which is characterized by a small cell body with many ramified thin processes (left). Upon LPS binding to TLR4, activated MAPK14 phosphorylates ULK1, which inhibits ULK1 kinase activity and reduces autophagy in microglia. Autophagy inhibition results in releasing the brake on the NLRP3 inflammasome and leads to CASP1-dependent production of IL1B and microglia morphological changes (right).
It should be noted that how reduced autophagy activates inflammation warrants further investigation. There are several reports suggesting that aberrant accumulation of reactive oxygen species from damaged mitochondria and/or aggregated proteins and RNAs following autophagy inhibition can activate the inflammasome. However, it remains highly possible that autophagy machinery may regulate inflammatory machinery or the inflammasome directly. In support of this, ULK1 has been shown to prevent sustained innate immune signaling through phosphorylation of TMEM173/STING. It is widely accepted that a swift and proper immune response is good for battling against various insults, but a persistent and overactive inflammation causes irreversible damage. It is intriguing and challenging to know if the regulatory pedal for the autophagy brake on inflammation defined by our study plays a role re-setting the brake during the inflammation resolution phase.
During the past several years, experiments to curb inflammation with autophagy inducers in animal models of neurodegenerative diseases have produced contradictory results. More than a dozen clinical trials using immunosuppressive reagents for the treatment of neurodegenerative diseases including amyotrophic lateral sclerosis also yielded no positive results although the strategy was proven safe and well tolerated. These pioneer studies strongly suggest that the current strategy of a total blockade of inflammation provides no benefit for disease cure. A better understanding of the regulatory mechanism of beneficial and toxic inflammation transition during disease pathogenesis is clearly needed for developing more precise and effective immune-based approaches. It will be of great interest to test whether the MAPK14-ULK1 axis plays a role in fine-tuning or switching microglia activation status. A positive outcome will thus open new therapeutic avenues for inflammation-related diseases.
Funding Statement
This work was supported by the BrightFocus Foundation [grant number A2016501S]; National Institutes of Health [grant number NS079858], [grant number NS095269], [grant number AG023695], [grant number P50 AG025688 pilot].
Disclosure of potential conflicts of interest
The authors declare that no conflict of interest exists.
Acknowledgments
This work was supported by BrightFocus Foundation and NIH grants AG023695, NS079858, P50 AG025688 pilot, and NS095269 to Z. M.