Inhibition of autophagy in microglia and macrophages exacerbates innate immune responses and worsens brain injury outcomes

Abstract

Excessive and prolonged neuroinflammation following traumatic brain injury (TBI) contributes to long-term tissue damage and poor functional outcomes. However, the mechanisms contributing to exacerbated inflammatory responses after brain injury remain poorly understood. Our previous work showed that macroautophagy/autophagy flux is inhibited in neurons following TBI in mice and contributes to neuronal cell death. In the present study, we demonstrate that autophagy is also inhibited in activated microglia and infiltrating macrophages, and that this potentiates injury-induced neuroinflammatory responses. Macrophage/microglia-specific knockout of the essential autophagy gene Becn1 led to overall increase in neuroinflammation after TBI. In particular, we observed excessive activation of the innate immune responses, including both the type-I interferon and inflammasome pathways. Defects in microglial and macrophage autophagy following injury were associated with decreased phagocytic clearance of danger/damage-associated molecular patterns (DAMP) responsible for activation of the cellular innate immune responses. Our data also demonstrated a role for precision autophagy in targeting and degradation of innate immune pathways components, such as the NLRP3 inflammasome. Finally, inhibition of microglial/macrophage autophagy led to increased neurodegeneration and worse long-term cognitive outcomes after TBI. Conversely, increasing autophagy by treatment with rapamycin decreased inflammation and improved outcomes in wild-type mice after TBI. Overall, our work demonstrates that inhibition of autophagy in microglia and infiltrating macrophages contributes to excessive neuroinflammation following brain injury and in the long term may prevent resolution of inflammation and tissue regeneration. Abbreviations: Becn1 /BECN1, beclin 1, autophagy related; CCI, controlled cortical impact; Cybb /CYBB/NOX2: cytochrome b-245, beta polypeptide; DAMP, danger/damage-associated molecular patterns; Il1b /IL1B/Il-1β, interleukin 1 beta; LAP, LC3-associated phagocytosis; Map1lc3b /MAP1LC3/LC3, microtubule-associated protein 1 light chain 3 beta; Mefv /MEFV/TRIM20: Mediterranean fever; Nos2 /NOS2/iNOS: nitric oxide synthase 2, inducible; Nlrp3 /NLRP3, NLR family, pyrin domain containing 3; Sqstm1 /SQSTM1/p62, sequestosome 1; TBI, traumatic brain injury; Tnf /TNF/TNF-α, tumor necrosis factor; Ulk1 /ULK1, unc-51 like kinase 1.

Sepsis‐associated encephalopathy: Autophagy and miRNAs regulate microglial activation

“…In recent years, researchers have begun to focus on the effect of autophagy on the polarization of microglia. A study further confirmed that autophagy is inhibited in activated microglia and that this change promotes brain injury-induced neuroinflammatory responses (Hegdekar et al, 2023). Inhibition of autophagy attenuates microglial phagocytosis, resulting in the accumulation of damaged neurons and an inflammatory response (Beccari et al, 2023).…”

High-dimensional proteomic analysis for pathophysiological classification of Traumatic Brain Injury

“…Raised SQSTM1 in acute TBI was associated with less axonal injury, as measured on DTI, supporting a role for post-injury autophagy in mitigating axonal injury. Indeed, experimental TBI studies have shown that impaired autophagy by microglial and macrophages exacerbates worse post-injury neuroinflammation, through reduced clearance of DAMPs and proinflammatory signals, such as the NLRP3 inflammasome, and worsens outcomes 9 .…”

Autophagy may protect the brain against prolonged consequences of headache attacks: A narrative/hypothesis review

“…74 Autophagy in microglia may project beyond such a backup role, becoming an important element of their immune functions. 75 So far, autophagy in the CNS has been studied in neurons, mainly in their axons, to show its role in neuronal transmission. 11,18,19,76 As we stated above, mechanisms of autophagy in microglia are less known-in fact, it is not clear whether microglial autophagy occurs with the involvement of the main proteins of canonical autophagy; 77 therefore, it is difficult to anticipate the consequences of blocking MA in microglia in the context of overall cellular autophagic response.…”

“…Phagocytosis, a primary mechanism to eliminate pathogens, has much in common with autophagy and when phagocytosis fails, autophagy can take its role acting as a backup system to fight invaders 74 . Autophagy in microglia may project beyond such a backup role, becoming an important element of their immune functions 75 . So far, autophagy in the CNS has been studied in neurons, mainly in their axons, to show its role in neuronal transmission 11,18,19,76 .…”