CTSB (Cathepsin B) encodes cathepsin B, a lysosomal cysteine protease belonging to the papain family. Cathepsin B is one of the most studied cathepsins due to its involvement in protein degradation, antigen processing, and more importantly, in the pathogenesis of Alzheimer's disease, Parkinson's disease, and other neurodegenerative disorders. Its dual endopeptidase and exopeptidase (carboxypeptidase) activities make it unique among cathepsins.
- Gene Symbol: CTSB
- Official Name: Cathepsin B
- Chromosomal Location: 8p23.1
- NCBI Gene ID: 1508
- Uniprot ID: P07846
¶ Protein Structure and Function
Cathepsin B is synthesized as a preproenzyme and undergoes processing:
¶ Processing and Activation
- Preprocathepsin B: N-terminal signal peptide directs to ER
- Procathepsin B: Signal peptide cleaved in ER, forms inactive zymogen
- Active cathepsin B: Cleaved in acidic lysosomes to form active enzyme
- Molecular weight: ~26 kDa (mature form)
- Active site: Cys29, His159, Asn175 (catalytic triad)
- pH optimum: Acidic (pH 5-6) for optimal activity
- Inhibitors: Cystatins, serpins, synthetic inhibitors
- Occluding loop: Gives carboxypeptidase activity
- Two-chain form: Heavy chain (25 kDa) + light chain (5 kDa)
- Stable in neutral pH: More stable than other cathepsins outside lysosomes
- Ubiquitous: Expressed in virtually all cell types
- Brain: High expression in:
Cathepsin B plays multiple roles in AD pathogenesis [1][2]:
- Aβ generation: Cathepsin B can cleave amyloid precursor protein (APP) at β- and γ-secretase sites, potentially generating Aβ peptides [3]
- Aβ degradation: Paradoxically, cathepsin B can also degrade Aβ42, reducing aggregation [4]
- AChE interaction: Cathepsin B may work with acetylcholinesterase to enhance Aβ aggregation [5]
- Pro-inflammatory signaling: Cathepsin B activates NLRP3 inflammasome in microglia [6]
- IL-1β processing: Converts pro-IL-1β to active IL-1β
- TNF-α processing: Generates active TNF-α
- Tau cleavage: Cathepsin B can cleave tau protein, generating toxic fragments [7]
- Lysosomal dysfunction: Impairs autophagy, affecting tau clearance
Cathepsin B is implicated in PD through multiple mechanisms [8]:
- Degradation: Can degrade α-synuclein aggregates [9]
- Generation of toxic fragments: Cleavage may generate aggregation-prone fragments
- Secretion: May promote cell-to-cell spread of α-synuclein pathology
- Mitochondrial dysfunction: Cathepsin B release from damaged lysosomes triggers apoptosis [10]
- ** oxidative stress**: Enhanced cathepsin B activity in PD brains
- LRRK2 mutations affect lysosomal function and cathepsin B activity [11]
- TDP-43 pathology: Cathepsin B may process TDP-43 fragments [12]
- Motor neuron degeneration: Lysosomal leakage and cathepsin B release
- Astrocyte reactivity: Increased cathepsin B in ALS astrocytes
- Mutant huntingtin affects lysosomal function, altering cathepsin B activity [13]
- May contribute to striatal neuron death
- Demyelination involves cathepsin B-mediated myelin protein degradation [14]
- Microglial cathepsin B in lesion formation
- CA-074: Selective cathepsin B inhibitor
- E-64: Broad cysteine protease inhibitor
- Novel brain-penetrant inhibitors: Being developed for neurodegenerative diseases
- Peripheral vs. central effects: Inhibitors must cross the blood-brain barrier
- Compensatory mechanisms: Other cathepsins may compensate
- Physiological functions: Complete inhibition may cause side effects
The role of cathepsin B in neurodegeneration is complex:
- Harmful: Inflammasome activation, Aβ generation, tau cleavage
- Protective: Aβ degradation, α-synuclein clearance
This dual role complicates therapeutic targeting.
Protein substrates → Endocytosis → Lysosome → Cathepsin B cleavage → Amino acids
Cathepsin B release → NLRP3 inflammasome → Caspase-1 → IL-1β, IL-18
- Cathepsin B essential for autophagosome-lysosome fusion
- Impairment leads to protein aggregate accumulation
- Cystatin C (CST3): Endogenous inhibitor
- APP: Substrate
- α-Synuclein: Substrate
- Tau: Substrate
- NLRP3: Activates inflammasome
- Cathepsin D: Synergistic proteolysis
- -1075A>C: Associated with AD risk
- -493G>A: May affect expression levels
- CTSB expression increases with age
- Upregulated in AD, PD, ALS brains
- CSF cathepsin B levels as potential biomarker
- Peripheral blood monocyte cathepsin B activity
- PET tracers for cathepsin B imaging (in development)
- Activity-based probes for visualization
Additional evidence sources: