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Characterizing Modulators of Protease-Activated Receptors with a Calcium Mobilization Assay Using a Plate Reader
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Characterizing Modulators of Protease-Activated Receptors with a Calcium Mobilization Assay Using a Plate Reader

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Regulation of plasmin-induced protease-activated receptor 4 activation in platelets

Yingying Mao1, Jianguo Jin, James L Daniel

  • 1Department of Physiology, Temple University School of Medicine, Philadelphia, PA, USA.

Platelets
|May 14, 2009
PubMed

Insights

Murine platelets are more sensitive to plasmin activation than human platelets due to differences in the PAR4 receptor sequence. Interestingly, PAR3 inhibits, rather than co-activates, plasmin-induced PAR4 signaling in mice.

Area of Science:

  • Biochemistry
  • Hematology
  • Molecular Biology

Background:

  • Plasmin, a key extracellular protease, activates platelets via Protease-Activated Receptors (PARs).
  • Platelet activation sensitivity to plasmin differs between species, with murine platelets showing higher responsiveness.
  • The role of PAR3 in plasmin-induced platelet activation remains incompletely understood.

Purpose of the Study:

  • To elucidate the molecular mechanisms underlying species-specific differences in plasmin-induced platelet activation.
  • To investigate the role of PAR3 as a potential co-receptor or modulator in plasmin-PAR4 signaling.
  • To compare the functional consequences of PAR4 sequence variations between murine and human platelets.

Main Methods:

  • Site-directed mutagenesis to interchange tethered ligand sequences of murine PAR4 (mPAR4) and human PAR4 (hPAR4).
  • Calcium mobilization assays in COS7 cells expressing different PAR4 variants (hPAR4, mPAR4) and co-expressing PAR3.
  • Platelet aggregation and calcium mobilization assays using wild-type and PAR3 null murine platelets.

Main Results:

  • Murine platelets exhibit significantly higher aggregation and calcium mobilization in response to lower plasmin concentrations compared to human platelets.
  • Swapping tethered ligand sequences between mPAR4 and hPAR4 reversed the species-specific differences in plasmin-induced calcium mobilization.
  • Co-expression of mPAR3 with mPAR4 reduced plasmin-induced calcium mobilization, and PAR3 null murine platelets showed enhanced responses, indicating an inhibitory role for mPAR3.

Conclusions:

  • Murine platelets demonstrate heightened sensitivity to plasmin activation primarily due to variations in the mPAR4 receptor sequence compared to hPAR4.
  • PAR3 acts as an inhibitor of plasmin-induced PAR4 activation in murine platelets, contrasting with its co-receptor role in thrombin signaling.
  • These findings reveal novel species-specific regulatory mechanisms in platelet activation by plasmin.

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