Molecular and Biological Aspects of Apoptosis - Execution: role of caspases

8 important questions on Molecular and Biological Aspects of Apoptosis - Execution: role of caspases

What are the general features of proteases?

  • Proteolysis is irreversible
  • Proteases are synthesized as precursors and can be activated by autokatalysis or other proteases
  • Proteases can regulate their activity by an amplificationloop resulting in a fast activation (cascade)
  • Proteases can be inhibited by protease inhibitors, increasing the threshold and prevent spontaneous activation.
  • Are very substrate specific

Activation of procaspase 8 by autocleavage

CD95/Fas ligands bring together procaspase 8 molecules, which due to close proximity are activated by autoproteolytical processing

What effect does FLIP have on the activity of procaspase 8?

  • Caspase-8 is normally involved in the activation of the RIPK1 and RIPK3 complex (necrosome).
  • High levels of FLIP inhibit caspase-8 resulting in cell survival.
  • FLIP also prevents the formation of the necrosome
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How do caspases disassemble a cell?

By cleaving a discrete set of proteins, using a variety of strategies:
  1. Inactivation of inhibitors of apoptosis
  2. Disassembly of cell structures
  3. Deregulation of protein activity

How does the activation of procaspase 1 induce inflammation?

  1. Signal 1: PAMPs/DAMPs bind to TLRs and activate NF-kB, which promotes the transcription of NLRP3, proIL-b and proIL-18.
  2. A stimulus will activate NLRP3 inflammasome by facilitating the oligomerization of NLRP3, ASC and procaspase-1.
  3. This complex catalyzes the conversion of procaspase-1 to caspase-1, which contributes to the production and secretion of mature IL-1B and IL-18.

Caspase-1 may also induce pyroptosis

The inflammasome activates caspase-1 which activates proteins gasdermin D.
When this is cleaved, it will form pores releasing inflammatory factors. The cell will swell and die in a necrotic way because water will enter the cell.

What are inhibitors of caspases?

XIAP: consists of 3 BIR domains (inhibiting caspases) and a RING domain (involved in ubiquitination, E3 ligase activity)
The regions can bind Smac/DIABLO to prevent inhibition of the caspase.

Interaction of caspase-3 and caspase-9 with XIAP

  • BIR domains can bind and inhibit caspase-9
  • When Smac/DIABLO is expressed it binds to the same region, making the caspase active.
  • The same for caspase-3 and caspase-7

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