Caspase-3 (CASP3)

Overview

flowchart TD
    CASPASE3["CASPASE3"] -->|"mediates"| Apoptosis["Apoptosis"]
    CASPASE3["CASPASE3"] -->|"regulates"| APOPTOSIS["APOPTOSIS"]
    CASPASE3["CASPASE3"] -->|"associated with"| Lymphoma["Lymphoma"]
    CASPASE3["CASPASE3"] -->|"inhibits"| Aging["Aging"]
    CASPASE3["CASPASE3"] -->|"inhibits"| Heart_Failure["Heart Failure"]
    CASPASE3["CASPASE3"] -->|"inhibits"| Cancer["Cancer"]
    CASPASE3["CASPASE3"] -->|"inhibits"| ALS["ALS"]
    CASPASE3["CASPASE3"] -->|"activates"| Cancer["Cancer"]
    CASPASE3["CASPASE3"] -->|"activates"| Als["Als"]
    CASPASE3["CASPASE3"] -->|"therapeutic target"| Als["Als"]
    CASPASE3["CASPASE3"] -->|"therapeutic target"| Neuroinflammation["Neuroinflammation"]
    CASPASE3["CASPASE3"] -->|"therapeutic target"| Alzheimer["Alzheimer"]
    CASPASE3["CASPASE3"] -->|"therapeutic target"| Aging["Aging"]
    CASPASE3["CASPASE3"] -->|"therapeutic target"| Inflammation["Inflammation"]
    style CASPASE3 fill:#4fc3f7,stroke:#333,color:#000

Caspase-3 is a member of the cysteine-aspartic protease family and serves as the primary executioner caspase in programmed cell death (apoptosis). It plays a crucial role in the systematic dismantling of cellular components during apoptosis, and its dysregulation is strongly implicated in the neuronal loss observed in neurodegenerative diseases including Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), and amyotrophic lateral sclerosis (ALS)^{1Caspases: guardians of the death sentence1997 · Cell · DOI 10.1016/S0092-8674(00)80508-5 · PMID 9390553Open reference2Caspase function in programmed cell death: evolution, regulation and orchestration1999 · Cell Death & Differentiation · DOI 10.1038/sj.cdd.4400596 · PMID 10578174Open reference3Caspase-3 activation as a key event in neuronal apoptosis2021 · Progress in Neurobiology · DOI 10.1016/j.pneurobio.2020.101951 · PMID 33217461Open reference}.

Structure and Activation

Enzyme Characteristics

Caspase-3 is synthesized as a proenzyme (procaspase-3, 32 kDa) that requires proteolytic cleavage for activation. The active enzyme consists of two heterodimers (p17/p12) forming a tetramer with two catalytic sites. The active site contains a catalytic cysteine residue that nucleophilically attacks the peptide bond of substrate proteins at specific aspartic acid residues.

Activation Pathways

Caspase-3 can be activated through two principal pathways:

1. Extrinsic Pathway: Death receptor ligation leads to activation of caspase-8, which then directly or indirectly (via caspase-6) activates caspase-3

2. Intrinsic Pathway: Mitochondrial outer membrane permeabilization (MOMP) releases cytochrome c, forming the apoptosome with Apaf-1 and procaspase-9, leading to caspase-3 activation

Physiological Functions

Apoptosis Execution

Caspase-3 is responsible for cleaving numerous substrate proteins that execute the apoptotic program:

• Structural Proteins: Lamin A/C, actin, tubulin

• DNA Repair Enzymes: PARP (poly ADP-ribose polymerase)

• Anti-apoptotic Proteins: Bcl-2, Bcl-xL

• Cell Cycle Proteins: p21-activated kinase, Wee1

• DNA Fragmentation Factor (DFF45): Leading to DNA fragmentation

Non-apoptotic Functions

Beyond apoptosis, caspase-3 participates in several physiological processes:

• Cell Proliferation: Cleavage of cell cycle regulators

• Differentiation: Processing of developmental transcription factors

• Synaptic Plasticity: Dendritic spine remodeling

• Immune Regulation: T-cell activation and proliferation

Role in Neurodegenerative Diseases

Alzheimer’s Disease

Caspase-3 activation is a hallmark of neuronal apoptosis in AD brain tissue. Multiple studies have demonstrated elevated caspase-3 levels and activity in AD brains, particularly in regions vulnerable to neurodegeneration such as the hippocampus and entorhinal cortex^{4Caspase-3 in Alzheimer's disease: from mechanism to therapy2020 · Journal of Alzheimer's Disease · DOI 10.3233/JAD-200362 · PMID 32651322Open reference5Caspase-3 and PARP cleavage in lymphocyte apoptosis in Alzheimer's disease2019 · Neurobiology of Aging · DOI 10.1016/j.neurobiolaging.2018.12.015 · PMID 30653937Open reference6Caspase-3 activation in sporadic and familial Alzheimer's disease lymphoblasts2018 · Journal of Cellular and Molecular Medicine · DOI 10.1111/jcmm.13630 · PMID 29624832Open reference}.

Key mechanisms linking caspase-3 to AD pathogenesis:

• Amyloid-β Induces Caspase-3: Aβ peptides directly or indirectly activate caspase-3 through mitochondrial dysfunction and oxidative stress

• Tau Cleavage: Caspase-3 cleaves tau protein at multiple sites, generating fragments that may promote aggregation and spread of tau pathology

• Synaptic Loss: Caspase-3-mediated cleavage of synaptic proteins contributes to synaptic dysfunction

• PARP Cleavage: Excessive PARP cleavage depletes cellular energy reserves, accelerating neuronal death

Parkinson’s Disease

In PD, caspase-3 activation is observed in dopaminergic neurons of the substantia nigra pars compacta. The characteristic feature is the presence of Lewy bodies (α-synuclein aggregates) which can be cleaved by caspase-3, generating more aggregation-prone fragments^{7Caspase-3 activation in Parkinson's disease: mechanisms and therapeutic implications2019 · Neurology · DOI 10.1212/WNL.0000000000007502 · PMID 30926697Open reference8Caspase-3 cleavage of α-synuclein and dopaminergic neuron death2006 · Journal of Biological Chemistry · DOI 10.1074/jbc.M605835200 · PMID 17056598Open reference}.

Mechanisms include:

• α-Synuclein Cleavage: Caspase-3 cleavage of α-synuclein produces fragments that accelerate aggregation

• Mitochondrial Dysfunction: PD-associated mitochondrial toxins activate caspase-3

• Neuroinflammation: Microglial activation leads to increased caspase-3 in neurons

Huntington’s Disease

Caspase-3 cleaves the huntingtin (HTT) protein at multiple positions, generating fragments that are more toxic than the full-length protein. This creates a feed-forward loop where caspase-3 activation produces toxic fragments that further promote neurodegeneration^{9Caspase-3 cleavage of huntingtin protein and Huntington's disease2019 · Brain Research · DOI 10.1016/j.brainres.2019.04.035 · PMID 31054921Open reference}.

Amyotrophic Lateral Sclerosis

Caspase-3 activation contributes to the progressive loss of motor neurons in ALS. Studies show elevated caspase-3 in spinal cord motor neurons and peripheral blood mononuclear cells of ALS patients^{10Caspase-3 gene expression in peripheral blood mononuclear cells in ALS2020 · Annals of Neurology · DOI 10.1002/ana.25804 · PMID 32363608Open reference2Caspase function in programmed cell death: evolution, regulation and orchestration1999 · Cell Death & Differentiation · DOI 10.1038/sj.cdd.4400596 · PMID 10578174Open reference}0.

Traumatic Brain Injury

Caspase-3 activation following traumatic brain injury (TBI) contributes to secondary neuronal loss. The caspase-3 cleavage of tau generates pathogenic fragments that may contribute to chronic neurodegeneration post-TBI^{2Caspase function in programmed cell death: evolution, regulation and orchestration1999 · Cell Death & Differentiation · DOI 10.1038/sj.cdd.4400596 · PMID 10578174Open reference}1.

Therapeutic Implications

Caspase Inhibitors

The development of caspase-3 inhibitors represents a promising therapeutic strategy for neuroprotection. Several approaches have been explored^{2Caspase function in programmed cell death: evolution, regulation and orchestration1999 · Cell Death & Differentiation · DOI 10.1038/sj.cdd.4400596 · PMID 10578174Open reference}2^{2Caspase function in programmed cell death: evolution, regulation and orchestration1999 · Cell Death & Differentiation · DOI 10.1038/sj.cdd.4400596 · PMID 10578174Open reference}3:

1. Peptide Inhibitors: Broad-spectrum and selective caspase inhibitors

2. Small Molecule Inhibitors: DEVD-based and non-peptidic compounds

3. Viral Vector Delivery: Gene therapy approaches delivering caspase-3 inhibitory proteins

Challenges

Caspase-3 inhibition as a therapeutic strategy faces significant challenges:

• Timing: Caspase activation occurs relatively late in the cell death cascade

• Side Effects: Complete inhibition may impair essential physiological functions

• Blood-Brain Barrier: CNS penetration remains a significant hurdle

• Cell Type Specificity: Need to target neurons specifically without affecting other cell types

Promising Approaches

• Temporal Control: Using inducible expression systems to limit inhibition duration

• Substrate-Specific Inhibitors: Targeting specific cleavage events rather than catalytic activity

• Upstream Modulation: Targeting activators rather than the executioner itself

• Combination Therapy: Caspase inhibition combined with other neuroprotective strategies

Interaction with Other Proteases

Caspase-3 does not act in isolation but interacts with a network of proteolytic enzymes:

• Caspase-6: Acts upstream and activates caspase-3; cleaves tau

• Caspase-7: Has overlapping substrate specificity

• Caspase-9: Primary activator in the intrinsic pathway

• Calpains: Calcium-activated proteases that can activate caspase-3

• Cathepsins: Lysosomal proteases that may initiate apoptosis

Biomarker Potential

Caspase-3 activation products have been investigated as potential biomarkers for neurodegenerative diseases:

• Caspase-cleaved Cytokeratin 18: Detected in blood of AD patients

• Caspase-3 Activity in Lymphocytes: Elevated in AD and PD

• Soluble Caspase-3: Increased in CSF of neurodegenerative disease patients

Cross-Linkages

Caspase-3 intersects with multiple neurodegenerative mechanisms:

• Mitochondrial Dysfunction: Both cause and consequence of MOMP

• Neuroinflammation: Caspase-3 in glial cells affects inflammatory responses

• Protein Aggregation: Cleavage of aggregation-prone proteins

• DNA Damage: PARP cleavage following DNA damage

See Also

• Apoptosis Pathway

• Alzheimer’s Disease

• Parkinson’s Disease

• Caspase Family

• PARP

External Links

• CASP3 Gene - NCBI

• Caspase Database - CASPEP

• PDB - Caspase-3