Many roads lead to CASM: Diverse stimuli of noncanonical autophagy share a unifying molecular mechanism

Abstract

Autophagy is a fundamental catabolic process coordinated by a network of autophagy-related (ATG) proteins. These ATG proteins also perform an important parallel role in "noncanonical" autophagy, a lysosome-associated signaling pathway with key functions in immunity, inflammation, cancer, and neurodegeneration. While the noncanonical autophagy pathway shares the common ATG machinery, it bears key mechanistic and functional distinctions, and is characterized by conjugation of ATG8 to single membranes (CASM). Here, we review the diverse, and still expanding, collection of stimuli and processes now known to harness the noncanonical autophagy pathway, including engulfment processes, drug treatments, TRPML1 and STING signaling, viral infection, and other pathogenic factors. We discuss the multiple associated routes to CASM and assess their shared and distinctive molecular features. By integrating these findings, we propose an updated and unifying mechanism for noncanonical autophagy, centered on ATG16L1 and V-ATPase.

Fig. 1.. Molecular features of autophagy-related pathways.

Schematic diagram highlighting the differences in molecular machinery and membrane target between canonical macroautophagy and noncanonical autophagy. BafA1, bafilomycin A1; Wm, wortmannin; IN-1, Vps34 inhibitor 1. Figure generated using Biorender.com

Fig. 2.. Diverse routes to noncanonical autophagy and CASM.

The diverse stimuli that are known to promote CASM on endolysosomal-related vesicles are shown. LANDO, LC3-associated endocytosis; LDELS, LC3-dependent EV loading and secretion. It is unclear whether ATG8 lipidation associated with LDELS or microautophagy constitutes a form of CASM. Figure generated using Biorender.com

Fig. 3.. Regulatory and unifying mechanisms of noncanonical autophagy activation.

An overview of the known mechanisms regulating noncanonical autophagy and CASM. Different stimuli converge to alter the ionic balance and pH of the target endolysosomal vesicle. LAP and LANDO depend on Vps34/Rubicon-mediated activation of NOX2 to induce ROS production, which consumes protons and raises pH. Multiple stimuli, including viroporins, ionophores, PFTs, lysosomotropic drugs, TRPML1 agonists, and STING agonists, all lead to alterations in luminal ion balances and pH independent of Rubicon and ROS. Endolysosomal pH perturbations are sensed through unknown mechanisms and lead to increased association of V0-V1 subunits of the V-ATPase complex. Increased V0-V1 association drives VAIL (V-ATPase–ATG16L1–induced LC3 lipidation) and the recruitment of the ATG16L1 complex through the C-terminal WD40 domain. The autophagy-related ubiquitin-like conjugation systems then target ATG8 proteins for lipidation to PE or PS on the endolysosomal membrane. Pathway inhibitors are in red text, and activators are in blue text. Figure generated using Biorender.com