Caspases and Apoptotic Signaling

Caspases are cysteine proteases that cleave substrates after aspartate residues and serve as the central executioners of apoptosis. They are produced as inactive precursors and activated through a regulated cascade in which initiator caspases switch on executioner caspases, which then dismantle the cell.

Definition

Caspases are intracellular cysteine-aspartic proteases that, once activated by proteolytic processing or by recruitment to activation platforms, cleave a defined set of cellular substrates to carry out the controlled demolition of the cell during apoptosis.

Scope

This entry covers the caspase family, the distinction between initiator and executioner caspases, the mechanisms that activate them, the apoptosome and other activation platforms, and the substrate cleavage events that produce the apoptotic phenotype. It complements the entries on death receptors and the BCL-2 family. It is a mechanistic reference entry, not clinical guidance.

Core questions

• How are caspases kept inactive until needed and then switched on?
• What distinguishes initiator from executioner caspases?
• How do the extrinsic and intrinsic pathways converge on caspase activation?
• Which substrate cleavages produce the morphology of apoptosis?

Key concepts

• Cysteine-aspartic proteases
• Procaspases (zymogens)
• Initiator caspases (e.g., caspase-8, caspase-9)
• Executioner caspases (e.g., caspase-3, caspase-7)
• Apoptosome (Apaf-1/cytochrome c/caspase-9)
• Induced-proximity activation
• Substrate cleavage and the apoptotic phenotype

Mechanisms

Caspases exist as inactive zymogens. Initiator caspases are activated by recruitment to platforms that bring them into proximity, including the death-inducing signaling complex of the extrinsic pathway and the apoptosome of the intrinsic pathway, in which cytochrome c released from mitochondria nucleates an Apaf-1 complex that activates caspase-9 (Li et al., 1997; Hengartner, 2000). Active initiator caspases then cleave and activate executioner caspases such as caspase-3, which cleave a broad set of substrates to dismantle the cell, accounting for the characteristic biochemical and morphological features of apoptosis (Thornberry & Lazebnik, 1998; Earnshaw et al., 1999). This hierarchical cascade allows both the extrinsic and intrinsic pathways to converge on a shared execution machinery.

Clinical relevance

Because caspases are the effectors of apoptosis, their dysregulation contributes to diseases of insufficient cell death, such as cancer, and of excessive cell death, such as neurodegeneration, and they are a long-studied conceptual target for modulating apoptosis (Thornberry & Lazebnik, 1998). This entry describes mechanisms and is not a basis for individual treatment decisions.

Evidence & guidelines

The content reflects foundational reviews and primary work defining caspase structure, activation, and function (Thornberry & Lazebnik, 1998; Earnshaw et al., 1999; Li et al., 1997). It is mechanistic reference material, not clinical practice guidance.

History

The recognition that a conserved protease related to the nematode death gene CED-3 executes apoptosis established caspases as central death effectors. Subsequent work defined the initiator-executioner hierarchy, identified the substrates whose cleavage produces the apoptotic phenotype, and revealed the apoptosome as the activation platform linking mitochondrial cytochrome c release to caspase-9 (Thornberry & Lazebnik, 1998; Li et al., 1997).

Key figures

• Nancy A. Thornberry
• Yuri Lazebnik
• William C. Earnshaw
• Scott H. Kaufmann
• Xiaodong Wang

Seminal works

• thornberry-lazebnik-1998
• earnshaw-1999
• li-1997

Frequently asked questions

What is the difference between initiator and executioner caspases?

Initiator caspases are activated first at signaling platforms and then cleave executioner caspases, which carry out the bulk of substrate cleavage that dismantles the cell.

What is the apoptosome?

It is the activation platform of the intrinsic pathway, formed when cytochrome c released from mitochondria assembles with Apaf-1 to recruit and activate the initiator caspase-9.