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Complement factor C4a does not activate protease-activated receptor 1 (PAR1) or PAR4 on human platelets.

Xu Han1, Maria de la Fuente1, Marvin T Nieman1

  • 1Department of Pharmacology Case Western Reserve University Cleveland OH USA.

Research and Practice in Thrombosis and Haemostasis
|February 4, 2021
PubMed
Summary

Complement factor C4a does not activate protease-activated receptors (PAR1 and PAR4) on human platelets. This suggests cell-specific regulation of protease signaling, as C4a activation may require a cofactor on microvascular cells.

Keywords:
PAR1PAR4complement factor C4aplatelet aggregationplatelets

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Area of Science:

  • Biochemistry
  • Hematology
  • Immunology

Background:

  • Protease-activated receptors (PAR1 and PAR4) mediate thrombin signaling in human platelets.
  • While thrombin typically activates PARs via a tethered ligand, other proteases can also elicit signaling.
  • Complement factor C4a was recently identified as an endogenous, non-tethered agonist for PAR1 and PAR4 on microvascular cells.

Purpose of the Study:

  • To investigate the activation of PAR1 and PAR4 by complement factor C4a on human platelets.

Main Methods:

  • Platelet-rich plasma from healthy donors was stimulated with C4a to measure platelet aggregation.
  • HEK293 cells expressing PAR1 or PAR4 were used to assess C4a-induced Gαq signaling via calcium mobilization.

Main Results:

  • Complement factor C4a did not induce platelet aggregation through PAR1 or PAR4.
  • No PAR1- or PAR4-mediated calcium mobilization was observed in HEK293 cells upon C4a stimulation.

Conclusions:

  • Complement factor C4a does not activate PAR1 or PAR4 on human platelets.
  • The activation of PAR1 and PAR4 by C4a on microvascular cells likely necessitates a cofactor.
  • These findings highlight cell-type-specific regulation of protease signaling pathways.