The ATG4C gene (Autophagy Related 4C) encodes a cysteine protease that plays a critical role in the autophagy pathway, the cell's primary mechanism for degrading and recycling damaged organelles, protein aggregates, and intracellular pathogens. ATG4C is one of four human ATG4 homologs (ATG4A, ATG4B, ATG4C, ATG4D) that collectively process the ATG8 family proteins (including LC3 and GABARAP subfamilies) required for autophagosome formation. While ATG4B displays the broadest substrate specificity, ATG4C has distinct functions and expression patterns that make it particularly important in specific physiological contexts and disease states. Located on chromosome 19p13.3, ATG4C is expressed in various tissues with relatively higher expression in the brain, heart, and skeletal muscle. The protein is induced under starvation conditions, reflecting its role in stress-responsive autophagy. In the context of neurodegeneration, ATG4C plays protective roles by facilitating the clearance of toxic protein aggregates that accumulate in Alzheimer's disease, Parkinson's disease, and Huntington's disease. Reduced ATG4C expression and activity have been associated with impaired autophagy and increased accumulation of neurotoxic protein aggregates, making it a potential therapeutic target for enhancing autophagic clearance in these conditions.
| Attribute |
Value |
| Gene Symbol |
ATG4C |
| Full Name |
Autophagy Related 4C Cysteine Peptidase |
| Chromosome |
19p13.3 |
| NCBI Gene ID |
84938 |
| OMIM |
604305 |
| Ensembl ID |
ENSG00000125743 |
| UniProt ID |
Q9Y4P5 |
| Protein Size |
371 amino acids |
| Gene Type |
Protein-coding |
The human ATG4 family consists of four cysteine proteases with distinct functions:
- ATG4A: Processes GABARAP family proteins; involved in selective autophagy
- ATG4B: Broadest substrate specificity; processes all ATG8 family members; highest catalytic activity
- ATG4C: Has distinct substrate preferences; important in specific tissues and stress conditions
- ATG4D: Has protease activity but is less studied; contains additional regulatory domains
All ATG4 proteases share a conserved domain structure:
- N-terminal protease domain: Contains the catalytic cysteine (Cys74 in ATG4C), histidine (His280), and aspartate (Asp255) forming the catalytic triad
- C-terminal domain: Involved in substrate recognition and regulatory functions
- Flexible regions: Allow access to ATG8 family proteins
¶ Redundancy and Specialization
The ATG4 family shows both redundancy and functional specialization:
- Functional redundancy: Different ATG4s can compensate for each other to some degree
- Tissue-specific expression: Different isoforms predominate in different tissues
- Stress-specific induction: Different ATG4s are upregulated under different conditions
- Substrate preference: Each ATG4 shows preferences for different ATG8 family members
ATG4C performs the essential enzymatic function of processing ATG8 family proteins during autophagy:
ATG4C cleaves the C-terminal portion of newly synthesized ATG8 family proteins (LC3A, LC3B, GABARAP, GABARAPL1, GABARAPL2) to expose a conserved glycine residue. This cleavage is essential because:
- The C-terminal extension blocks the glycine needed for lipidation
- Without cleavage, ATG8 proteins cannot be conjugated to phosphatidylethanolamine (PE)
- The lipidated form (LC3-II/GABARAP-PE) is required for autophagosome membrane expansion and closure
ATG4C shows substrate preferences:
- LC3 family: Can process LC3A, LC3B, and LC3C
- GABARAP family: Can process GABARAP and GABARAPL1
- Relative to ATG4B: Has somewhat narrower substrate range
Like other ATG4 family members, ATG4C can also perform the reverse reaction:
- Cleaves PE from LC3/GABARAP after autophagy completion
- This recycles the proteins for additional rounds of autophagy
- This function is important for proper autophagic flux
ATG4C functions within the broader autophagy pathway:
Macroautophagy involves:
- Initiation: Formation of the phagophore at the phagophore assembly site (PAS)
- Nucleation: Recruitment of ATG proteins and membrane expansion
- Expansion: Growth and closure of the phagophore to form an autophagosome
- Fusion: Fusion of the autophagosome with a lysosome to form an autolysosome
- Degradation: Breakdown of cargo and recycling of constituents
This is the key pathway in which ATG4C functions:
- ATG4C processing: Cleaves ATG8 proteins to expose glycine (priming)
- ATG7 (E1): Activates ATG8 by forming a thioester bond
- ATG3 (E2): Transfers ATG8 to ATG5
- ATG12-ATG5-ATG16L1 (E3): Catalyzes lipidation (PE conjugation)
- ATG4C (delipidation): Can remove PE after autophagosome-lysosome fusion
ATG4C supports multiple forms of selective autophagy:
- Aggrephagy: Clearance of protein aggregates
- Mitophagy: Removal of damaged mitochondria
- Xenophagy: Elimination of pathogens
- ER-phagy: Clearance of ER fragments
In neurons, selective autophagy is crucial for synaptic maintenance, axonal homeostasis, and clearance of aggregation-prone proteins.
¶ Expression and Regulation
ATG4C is expressed in various tissues:
- Brain: Particularly in neurons and glia
- Heart: High expression in cardiac muscle
- Skeletal muscle: Abundant in muscle fibers
- Liver: Moderate expression
- Lower expression: In kidney, lung, and other tissues
In the central nervous system:
- Neurons: Expressed in various neuronal populations
- Astrocytes: Present in glial cells
- Regional variation: Higher expression in hippocampus and cortex
ATG4C expression is regulated by cellular stress:
- Starvation: Strongly induced during nutrient deprivation
- Oxidative stress: Upregulated by reactive oxygen species
- Proteotoxic stress: Increased expression with protein aggregate accumulation
- Hypoxia: Regulated by HIF-1α
ATG4C transcription is controlled by:
- FOXO transcription factors: Activate autophagy genes including ATG4C
- p53: Can upregulate ATG4C under stress conditions
- Nrf2: Antioxidant response element (ARE) in promoter
In Parkinson's disease, ATG4C plays important roles:
- α-Synuclein clearance: Autophagy mediated by ATG4C helps clear α-synuclein aggregates
- Mitophagy: ATG4C function supports mitochondrial quality control
- Dopaminergic neuron survival: Impaired autophagy contributes to neuron loss
- LRRK2 interactions: LRRK2 mutations affect autophagy regulation
Studies have shown:
- Reduced ATG4C expression in PD brain tissue
- Impaired autophagic flux in PD models
- ATG4C overexpression protects dopaminergic neurons
ATG4C is particularly relevant to Huntington's disease:
- Huntingtin clearance: Autophagy clears mutant huntingtin protein
- Polyglutamine aggregates: ATG4C helps process aggregation-prone proteins
- Neuronal protection: Enhancing autophagy reduces toxicity
Evidence from HD models:
- Autophagy induction reduces mutant huntingtin toxicity
- ATG4C activity is important for aggregate clearance
- Impaired autophagy contributes to disease progression
In Alzheimer's disease, autophagy dysfunction is a key feature:
- Amyloid-beta clearance: Autophagy helps clear Aβ plaques
- Tau clearance: Autophagy processes hyperphosphorylated tau
- Neuronal health: Impaired autophagy contributes to synaptic loss
ATG4C contributes to:
- Clearance of amyloid-beta through autophagy
- Processing of tau protein aggregates
- Maintenance of neuronal homeostasis
ATG4C relevance extends to:
- Amyotrophic Lateral Sclerosis (ALS): TDP-43 aggregate clearance
- Spinocerebellar ataxias: Polyglutamine aggregate processing
- Frontotemporal dementia: Protein aggregate handling
ATG4C-mediated autophagy provides neuroprotection through:
- Aggephagy: Selective degradation of protein aggregates
- Prevention of toxic oligomer formation: Early clearance before aggregation
- Synaptic protection: Maintaining synaptic protein homeostasis
- Mitophagy: Removal of damaged mitochondria
- ER-phagy: Clearance of stressed ER
- Peroxisome turnover: Organelle quality control
- Nutrient recycling: Providing substrates during starvation
- Energy maintenance: Supporting cellular ATP levels
- Redox balance: Reducing oxidative stress
Given ATG4C's role in neurodegeneration, several therapeutic approaches are being explored:
Small Molecule Approaches:
- ATG4C activators: Direct activation of ATG4C protease activity
- Autophagy inducers: Broader autophagy activation (e.g., rapamycin, trehalose)
- mTOR inhibitors: Indirect autophagy enhancement
Gene Therapy Approaches:
- ATG4C overexpression: Viral vector-mediated delivery
- CRISPR activation: Upregulate endogenous ATG4C
- ATG4 family enhancement: Target multiple ATG4s
Combination Strategies:
- ATG4C plus anti-aggregation: Combined approach
- Multiple autophagy targets: Broader enhancement
- Cell-type specific delivery: Target neurons or glia specifically
¶ Challenges and Considerations
Several challenges face ATG4C-targeted therapy:
- BBB penetration: Achieving sufficient brain delivery
- Dosing optimization: Balancing efficacy and side effects
- Autophagy balance: Excessive autophagy can be detrimental
- Cell-type specificity: Targeting appropriate cell types
- Disease stage: Different stages may require different approaches
ATG4C works within the autophagy machinery:
- Functional redundancy: Can compensate for ATG4A/B
- Substrate sharing: Competes for ATG8 proteins
- Complex formation: Works with other ATG proteins
- ATG8 family: Processes LC3 and GABARAP proteins
- ATG7 and ATG3: Downstream conjugation machinery
- ATG5-ATG12-ATG16L1: E3-like complex for lipidation
Several models have been used to study ATG4C:
- ATG4C knockout mice: Viable but with specific phenotypes
- Enhanced tumor susceptibility: Impaired autophagy increases cancer risk
- Muscle phenotypes: Muscle-specific dysfunction
- PD models: ATG4C manipulation affects dopaminergic neuron survival
- HD models: ATG4C affects mutant huntingtin clearance
- AD models: ATG4C influences amyloid pathology
- Neuroprotection: ATG4C overexpression protects against neurodegeneration
- Improved clearance: Enhanced aggregate removal
- Functional improvement: Behavioral benefits in models
Current research areas include:
- Structural studies: Understanding ATG4C protease structure for drug design
- Substrate specificity: Defining ATG4C's unique substrates
- Regulatory mechanisms: How ATG4C activity is controlled
- Therapeutic development: Identifying ATG4C activators
- Biomarkers: Developing markers of ATG4C activity
- Delivery methods: Improving brain delivery of therapeutics
- Combination therapies: Multi-target approaches
The ATG4C gene encodes a cysteine protease essential for autophagy, the cellular process for clearing damaged proteins and organelles. In the nervous system, ATG4C-mediated autophagy protects against neurodegeneration by facilitating clearance of toxic protein aggregates that accumulate in Alzheimer's disease, Parkinson's disease, and Huntington's disease. While ATG4B has broader substrate specificity, ATG4C has distinct functions and expression patterns that make it an important contributor to neuronal autophagy. Reduced ATG4C function contributes to disease pathogenesis, while enhancing ATG4C activity could provide therapeutic benefit by promoting autophagic clearance of neurotoxic proteins. Further research into ATG4C function and regulation will advance understanding of neurodegeneration mechanisms and enable development of effective therapies.