An endothelial storage granule for tissue-type plasminogen activator

Area of Science:

• Endothelial cell biology
• Hemostasis and thrombosis
• Molecular cell biology

Background:

• Tissue-type plasminogen activator (tPA) and von Willebrand factor (vWf) are released from endothelial cells upon stimulation.
• Previous studies suggested differing secretion mechanisms for tPA and vWf, implying distinct storage compartments.

Purpose of the Study:

• To investigate the storage and secretion mechanisms of tPA in endothelial cells.
• To determine if tPA and vWf are stored in the same secretory granules.

Main Methods:

• Density gradient centrifugation of rat lung homogenates, endothelial cell lines, and primary HUVEC.
• Analysis of tPA and vWf distribution after thrombin stimulation.
• Double-immunofluorescence staining and confocal microscopy.
• Immunoelectronmicroscopy.

Main Results:

• A dense particle (density 1.11-1.12 g/ml) containing tPA activity was identified in rat lung, endothelial cells, and HUVEC.
• Stimulation led to decreased tPA in dense fractions and release into the medium.
• vWf distribution differed from tPA in sucrose gradients, and immunofluorescence showed distinct cellular localization.
• Immunoelectronmicroscopy revealed tPA in small vesicles distinct from Weibel-Palade bodies.

Conclusions:

• Endothelial cells store tPA in previously undescribed, small, dense vesicles (1.11-1.12 g/ml).
• These tPA-containing vesicles are morphologically and fractionally distinct from Weibel-Palade bodies, which store vWf.
• This finding elucidates the specialized storage and secretion pathways for tPA in endothelial cells.