Mechanisms of leukocyte activation in the circulation

Receptor cleavage reduces the fluid shear response in neutrophils of the spontaneously hypertensive rat

Physiological fluid shear stress evokes pseudopod retraction in normal leukocytes by a mechanism that involves the formyl peptide receptor (FPR) as mechanosensor. In hypertensives, such as the spontaneously hypertensive rat (SHR), leukocytes lack the normal fluid shear response. The increased activity of matrix metalloproteinases (MMPs, including MMP-9) in SHR plasma is associated with cleavage of several cell membrane receptors. We hypothesize that the attenuated fluid shear response in leukocytes (neutrophils) of the SHR is due to extracellular proteolytic cleavage of the FPR. We show that suspended SHR neutrophils in whole blood sheared in a cone-and-plate device or individual neutrophils adherent to a glass surface and subject to fluid shear exhibited reduced pseudopod retractions compared with neutrophils of control Wistar-Kyoto (WKY) rats. SHR neutrophils and naïve Wistar rat neutrophils exposed to SHR plasma also exhibited impaired fluid shear responses as shown by their inability to project pseudopods with fluid shear. Labeling of extracellular FPR revealed that the FPR density in SHR neutrophils is on average 27% reduced compared with those of the WKY rats. Exposure of Wistar rat neutrophils to the gelatinase MMP-9 (final concentration 5 nM) led to attenuation of fluid shear response and decrease in extracellular FPR density. Chronic treatment of the SHR with a broad-acting MMP inhibitor, doxycycline, significantly improved the fluid shear response and increased the FPR extracellular density of SHR neutrophils. These results suggest that proteolytic cleavage of the FPR may interfere with normal fluid shear-induced pseudopod retractions in SHR neutrophils.