The ATG4 family: Molecular gatekeepers of autophagic flux and emerging targets for disease intervention.

Abstract

Autophagy represents an evolutionarily conserved intracellular degradation pathway. Aberrant activation or impaired autophagic flux contributes substantially to the pathogenesis and progression of numerous human diseases, such as tumors and neurological disorders. As the only cysteine proteases within the core autophagy machinery, autophagy-related protein 4 (ATG4) family proteins directly drive the processing and delipidation cycle of microtubule-associated protein 1 light chain 3 (LC3) and gamma-aminobutyric acid receptor-associated protein (GABARAP). In this review, we systematically summarize research advances concerning the four mammalian ATG4 isoforms and elaborate on their evolutionary conservation, structural basis, and conformational activation mechanisms. Beyond their canonical functions in cleaving autophagy-related protein 8 (ATG8) precursors and mediating delipidation, we also cover emerging non-canonical activities of the ATG4 family. Particular attention is paid to isoform-specific and context-dependent roles exerted by ATG4 members across diverse diseases. Cumulative evidence identifies ATG4B as the most extensively investigated and functionally central isoform, which participates in cytoprotective autophagy, therapeutic resistance, neuronal injury, inflammatory regulation, and tumor progression. In comparison, ATG4A, ATG4C, and ATG4D may exert compensatory or independent functions depending on tissue context, disease stage, and stress stimuli. Furthermore, this review classifies small-molecule inhibitors and activators targeting ATG4 and discusses their potential applications in combination therapies for reversing drug resistance. Finally, we point out deficiencies in current research regarding subtype functional differentiation, structural analysis, and drug development, and propose reference solutions to provide a theoretical basis for basic research and targeted clinical treatment of autophagy-related diseases.