Caspase-3 (CASP3)

Introduction

Caspase 3 (Casp3) is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

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"]
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Caspase-3 (also known as CPP32, apopain, or YAMA) is a 32 kDa cysteine-aspartate protease encoded by the CASP3 gene on chromosome 4q35.1. [It is the principal executioner caspase of apoptosis — the major downstream effector protease that cleaves the majority of cellular substrates during programmed cell death. Caspase-3 is activated by initiator caspases (caspase-8 and caspase-9 and cleaves over 600 cellular substrates at specific aspartate residues, dismantling the cell in an orderly fashion (Asadi et al., 2022).

In the nervous system, caspase-3 plays dual roles: it is essential for normal brain development (mediating synaptic pruning, axon guidance, and neurogenesis) and is pathologically activated in virtually all neurodegenerative diseases. In alzheimers, caspase-3-mediated cleavage of tau]/proteins/tau generates toxic truncated fragments that accelerate neurofibrillary tangle formation and synaptic-dysfunction. In huntington-pathway, caspase cleavage of [huntingtin/proteins/[huntingtin) produces N-terminal fragments with enhanced aggregation propensity. The growing appreciation that caspase-3 can operate at sub-lethal levels to drive synaptic damage and protein toxicity — without full apoptosis — has repositioned it from a simple cell death executor to a central mediator of neurodegeneration (D’Amelio et al., 2010; Bhatt et al., 2023).

Structure and Activation

Zymogen Structure

Caspase-3 is synthesized as an inactive 32 kDa zymogen (procaspase-3) comprising:

1. Prodomain (residues 1–28): Short N-terminal peptide, lacking the CARD or DED recruitment domains found in initiator caspases

2. Large subunit (p17) (residues 29–175): Contains the catalytic Cys-163 residue

3. Small subunit (p12) (residues 176–277): Required for substrate recognition and dimer stabilization

Catalytic Mechanism

The active enzyme functions as a homodimer of heterodimers — a p17/p12 heterodimer pair forming a tetramer (p17₂p12₂) with two active sites. Each active site features:

• Catalytic dyad: Cys-163 (nucleophilic attack on the substrate peptide bond) and His-121 (stabilizes the oxyanion transition state)

• Substrate-binding groove: Recognizes the consensus sequence DEVD↓G (Asp-Glu-Val-Asp↓Gly), with strict preference for aspartate at the P1 position

• S4 subsite: Confers specificity for acidic residues at P4 (distinguishing caspase-3 from caspase-7, which has a more flexible S4 pocket) (Walsh et al., 2008)

Activation Pathways

Caspase-3 activation occurs through two principal pathways:

Both pathways converge on the cleavage of procaspase-3 at Asp-175 (between the large and small subunits), followed by autocatalytic removal of the prodomain at Asp-28. The activated enzyme then processes a cascade of downstream substrates.

Normal Function in the Nervous System

Developmental Roles

Caspase-3 is essential for normal brain development, acting at sub-lethal levels to shape neural circuits:

• Synaptic pruning: Local caspase-3 activation at synapses mediates activity-dependent synapse elimination during development, a process hijacked pathologically in neurodegeneration

• Axon guidance: Caspase-3 activity in growth cones mediates chemorepulsive responses

• Neurogenesis: Regulates neural progenitor proliferation in the ventricular zone

• Dendrite morphogenesis: Required for proper dendritic arbor development

Caspase-3 knockout mice are viable but display marked brain overgrowth (exencephaly) and disorganized neural architecture, confirming its developmental requirement (Bhatt et al., 2023).

Synaptic Plasticity

At sub-apoptotic levels, caspase-3 participates in:

• Long-term depression (LTD): Mediates AMPA receptor internalization at postsynaptic densities

• Homeostatic scaling: Adjusts synaptic strength in response to chronic activity changes

• Memory consolidation: Caspase-3 activity is transiently elevated during memory formation

Key Substrates in Neurodegeneration

tau protein

Caspase-3 cleaves tau[/proteins/tau at Asp-421, generating a C-terminally truncated fragment (Δ[tau](/proteins/tau at Asp-421, generating a C-terminally truncated fragment (Δtau) that:

• Aggregates more rapidly than full-length tau into neurofibrillary tangles (Gamblin et al., 2003)

• Accumulates at the postsynaptic density and impairs neuronal firing (Bhatt et al., 2023)

• Opens the mitochondrial permeability transition pore, causing sustained depolarization and bioenergetic failure (Pérez et al., 2023)

• Induces cellular senescence and disrupts axonal transport (Means et al., 2022)

Caspase-cleaved tau (detected by the TauC3 antibody at D421) is found in AD brains at early Braak stages, suggesting it is an early event in tangle pathology that precedes full-length tau aggregation (Rissman et al., 2004).

Amyloid Precursor Protein (APP

Caspase-3 cleaves app-protein at Asp-664 (in the cytoplasmic domain), releasing:

• C31 peptide: A cytotoxic 31-amino acid C-terminal fragment that amplifies apoptotic signaling

• Jcasp peptide: An N-terminal fragment of the app intracellular domain

Amyloid-Beta-induced synapse loss in hippocampal-ca1 correlates with localized caspase-3 activation and app cleavage at D664 (Bhatt et al., 2020).

Huntingtin

Caspase-3 cleaves huntingtin at Asp-513 and Asp-530, generating N-terminal fragments that:

• Form cytoplasmic and nuclear aggregates at greatly increased rates compared to full-length huntingtin

• Are more toxic to neuronal and non-neuronal cells

• However, caspase-6 cleavage at Asp-586 appears more critical for huntington-pathway pathogenesis — mice expressing caspase-6-resistant mutant huntingtin are protected from striatal neurodegeneration (Wellington et al., 2000; Graham et al., 2006)

Other Substrates

Role in Neurodegenerative Diseases

Alzheimer’s Disease

Caspase-3 activation is a consistent finding in AD brain tissue and models:

• Elevated active caspase-3 in hippocampal and cortical neurons from early disease stages

• amyloid-beta oligomers activate caspase-3 through the intrinsic pathway via mitochondrial-dysfunction

• Caspase-3-cleaved tau (TauC3) is an early marker of tau pathology] that precedes PHF-tau formation

• Sub-lethal caspase-3 activation drives dendritic spine loss and synaptic-plasticity-deficits without overt cell death

• Links the amyloid cascade to tau hyperphosphorylation] via tau cleavage (Rissman et al., 2004)

Parkinson’s Disease

• Activated caspase-3 is detected in dopaminergic-neurons-snpc of the substantia-nigra in PD patients

• Mitochondrial complex I deficiency and oxidative-stress trigger caspase-9/caspase-3 cascade

• Note: alpha-synuclein is not a direct caspase-3 substrate (lacks consensus cleavage sites); instead, caspase-1 cleaves α-synuclein to promote aggregation

• Dopaminergic toxins (MPTP, 6-OHDA, rotenone) activate caspase-3 in PD models

Huntington’s Disease

• Caspase-3 and caspase-6 activation observed in striatal medium-spiny-neurons

• N-terminal huntingtin-protein fragments generated by caspase cleavage seed polyglutamine-aggregation

• Caspase-6-resistant mutant huntingtin prevents neurodegeneration, while caspase-3-resistant mutant does not fully protect, highlighting the hierarchical importance of different cleavage sites (Graham et al., 2006)

ALS

• Activated caspase-3 in motor-neurons of als patients and SOD1 mutant mice

• sod1-protein aggregates trigger ER stress and the mitochondrial apoptotic pathway

• tdp-43 pathology activates caspase-3, which may further cleave tdp-43 to generate cytoplasmic aggregates

Therapeutic Targeting

Caspase Inhibitors

Several caspase-3 inhibitors have been explored in preclinical neurodegenerative disease models:

• z-DEVD-fmk: Irreversible caspase-3 inhibitor; reduces amyloid-beta-induced neurotoxicity and tau cleavage in vitro

• Q-VD-OPh: Broad-spectrum caspase inhibitor with good blood-brain-barrier penetration; protects neurons in stroke and AD models

• Minocycline: Tetracycline antibiotic that indirectly inhibits caspase-3 activation; tested in ALS and PD clinical trials with mixed results

Targeting Tau Cleavage

Blocking caspase-3-mediated tau cleavage at D421 is being explored as a disease-modifying strategy:

• Anti-TauC3 immunotherapy targets the neoepitope exposed by caspase cleavage

• tau-protein mutations that prevent D421 cleavage reduce aggregation and toxicity in animal models

Challenges

• Systemic caspase-3 inhibition may impair normal developmental and homeostatic apoptosis

• Sub-lethal caspase activity is required for synaptic plasticity and immune function

• Selective targeting of pathological caspase-3 activation while preserving physiological roles remains a major challenge

Background

The study of Caspase 3 (Casp3) has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.

Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.