Dying by fire: noncanonical functions of autophagy proteins in neuroinflammation and neurodegeneration

Abstract

Neuroinflammation and neurodegeneration are key components in the establishment and progression of neurodegenerative diseases including Alzheimer's Disease (AD). Over the past decade increasing evidence is emerging for the use of components of the canonical autophagy machinery in pathways that are characterized by LC3 lipidation yet are distinct from traditional macro-autophagy. One such pathway that utilizes components of the autophagy machinery to target LC3 to endosomes, a process termed LC3-associated endocytosis (LANDO), has recently been identified and regulates neuroinflammation. Abrogation of LANDO in microglia cells results in a propensity for elevated neuroinflammatory cytokine production. Using the well-established 5xFAD model of AD to interrogate neuroinflammatory regulation, impairment of LANDO through deletion of a key upstream regulator Rubicon or other downstream autophagy components, exacerbated disease onset and severity, while deletion of microglial autophagy alone had no measurable effect. Mice presented with robust deposition of the neurotoxic AD protein β-amyloid (Aβ), microglial activation and inflammatory cytokine production, tau phosphorylation, and aggressive neurodegeneration culminating in severe memory impairment. LANDO-deficiency impaired recycling of receptors that recognize Aβ, including TLR4 and TREM2. LANDO-deficiency alone through deletion of the WD-domain of the autophagy protein ATG16L, revealed a role for LANDO in the spontaneous establishment of age-associated AD. LANDO-deficient mice aged to 2 years presented with advanced AD-like disease and pathology correlative to that observed in human AD patients. Together, these studies illustrate an important role for microglial LANDO in regulating CNS immune activation and protection against neurodegeneration. New evidence is emerging that demonstrates a putative linkage between pathways such as LANDO and cell death regulation via apoptosis and possibly necroptosis. Herein, we provide a review of the use of the autophagy machinery in non-canonical mechanisms that alter immune regulation and could have significant impact in furthering our understanding of not only CNS diseases like AD, but likely beyond.

Keywords: Alzheimer’s disease; LC3-associated endocytosis; aging; autophagy; inflammation; microglia; neurodegeneration; neuroinflammation.

Figure 1

LANDO sufficiency or deficiency in microglia. (A) LANDO-sufficient microglia internalize β-amyloid through a clathrin-associated endocytic process and the Rubicon containing PI3-kinase complex in LANDO initiates the downstream processing and lipidation of LC3 to the endosome. The LC3 ligation machinery is shared between canonical autophagy and LANDO, consisting of ATG5, ATG12, ATG16L, ATG7 and ATG4 as reviewed previously (Heckmann et al., 2017; Heckmann and Green, 2019), and no unique components that alter the function of the complex in LANDO compared to autophagy have yet to be identified. The Rab GTPases including Rab7 and Rab5 are required for endocytic trafficking and are present on LC3⁺ endosomes. LC3 conjugation facilitates the trafficking of the β-amyloid containing endosome towards recycling or degradation, by mechanisms which have not been fully elucidated. LANDO promotes the recycling of β-amyloid receptors including TREM2, TLR4, and CD36. LANDO suppresses inflammation following β-amyloid stimulation by currently unknown mechanisms, although putative hypotheses linking LANDO to inflammatory suppression include restriction of NLRP3 inflammasome assembly, decreases in signaling time of activated receptors by facilitating vesicle trafficking, and possible interactions between the LANDO machinery and inflammatory cytokines. B) LANDO-deficient microglia through deletion of Rubicon or ATG16L WD-domain fail to lipidate LC3 on endosomes and subsequently there is an impairment in the recycling of β-amyloid receptors leading to extracellular accumulation of β-amyloid, as continued uptake and degradation is decreased due to reduced receptor recycling. Consequently, there is an increase in inflammatory cytokine production from LANDO-deficient cells. ATG: Autophagy related genes; CD36: CLUSTER of differentiation 36; LANDO: LC3-associated endocytosis; LC3: microtubule-associated protein 1A/1B-light chain 3; NLRP3: NOD-, LRR- and pyrin domain-containing protein 3; Rab: Rab GTPase family; TLR4: Toll-like receptor 4; TREM2: triggering receptor expressed on myeloid cells 2.