Weibel-Palade body membrane proteins exhibit differential trafficking after exocytosis in endothelial cells

Weibel-Palade bodies, the secretory granules of endothelial cells, possess two different membrane proteins. However, P-selectin is seen only in Weibel -Palade bodies in HUVECs, whereas CD63 is also seen in late endosomes/lysos omes. Since P-selectin is targeted to lysosomes in heterologous expression studies, we have determined whether a lysosomal targeting signal also opera tes within HUVECs. We have also examined the trafficking of CD63 to its two different intracellular locations. By following antibodies bound at the pl asma membrane during stimulation, we have discovered that while half of the P-selectin recycles to the WPBs, 50% is rapidly delivered to a lamp-1-posi tive compartment. Thus, the lysosomal targeting signal of this protein also operates in HUVECs. CD63 is found constitutively at the cell surface of HU VECs and most of it is delivered to the late endosomes/lysosomes after inte rnalisation. However, stimulation causes both a rise in the CD63 plasma mem brane level and in the amount that recycles to the WPBs. Our data strongly suggest that the CD63 that originates in the WPB preferentially recycles to the granule rather than being delivered to the late endosome/lysosome, and that there are, therefore, two separate pools of this protein within HUVEC s. Our findings indicate that although P-selectin and CD63 are both targete d to the same compartments from the PM, the kinetics and the ratio of their targeting to Weibel-Palade bodies versus lysosomes are very different.