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Related Experiment Video

Updated: May 5, 2026

Identification of Intracellular Signaling Events Induced in Viable Cells by Interaction with Neighboring Cells Undergoing Apoptotic Cell Death
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Modeling a snap-action, variable-delay switch controlling extrinsic cell death.

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  • 1Department of Systems Biology, Harvard Medical School, Boston, Massachusetts, USA.

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Extrinsic apoptosis uses a variable-delay, snap-action switch controlled by initiator caspases. This switch, modeled mathematically, determines cell life or death following tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) exposure.

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Area of Science:

  • Cellular biology
  • Molecular mechanisms of apoptosis
  • Mathematical modeling of biological systems

Background:

  • Extrinsic apoptosis involves a delay before effector caspases activate, leading to cell death.
  • This process is triggered by death ligands like tumor necrosis factor (TNF) and TNF-related apoptosis-inducing ligand (TRAIL).
  • The precise quantitative mechanisms governing this variable-delay, snap-action switch remain incompletely understood.

Purpose of the Study:

  • To quantitatively understand the mechanisms controlling the extrinsic apoptosis switch.
  • To investigate the roles of initiator and effector caspase activities in determining cell fate.
  • To identify key molecular interactions critical for the switch's snap-action control.

Main Methods:

  • Construction of a mathematical model based on mass-action principles of known reaction pathways.
  • Training the model against experimental data from live-cell imaging, flow cytometry, and immunoblotting.
  • Perturbing cellular systems via protein depletion and overexpression to validate model predictions.

Main Results:

  • The trained mathematical model accurately reproduces cellular responses to TRAIL in both normal and perturbed cells.
  • Initiator caspase-8 activity and downstream reaction rates dictate the delay period before effector caspase activation.
  • Mitochondrial membrane permeabilization, Smac relocalization, and Bcl-2 family interactions are crucial for snap-action control.

Conclusions:

  • The extrinsic apoptosis pathway functions as a precise variable-delay, snap-action switch.
  • Initiator caspase activity and mitochondrial pathway engagement are key determinants of apoptosis timing and execution.
  • Interactions among Bcl-2 family proteins, Smac, and XIAP are critical for the switch's rapid and decisive action.