STATIC AND DYNAMIC LENGTHS OF NEUTROPHIL MICROVILLI

Containing most of the L-selectin and P-selectin glycoprotein ligand-1 (PSGL-1) on their tips, microvilli are believed to promote the initia l arrest of neutrophils on endothelium. At the rolling stage following arrest, the lifetimes of the involved molecular bonds depend on the p ulling force imposed by the sheer stress of blood flow. With two diffe rent methods, electron microscopy and micropipette manipulation, we ha ve obtained two comparable neutrophil microvillus lengths, both approx imate to 0.3 mu m in average. We have found also that, under a pulling force, a microvillus can be extended (microvillus extension) or a lon g thin membrane cylinder (a tether) can be formed from it (tether form ation). If the force is less than or equal to 34 pN (+/- 3 pN), the le ngth of the microvillus will be extended; if the force is >61 pN (+/- 5 pN), a tether will be formed from the microvillus at a constant velo city, which depends linearly on the force. When the force is between 3 4 pN and 61 pN (transition zone), the degree of association between me mbrane and cytoskeleton in individual microvilli will dictate whether microvillus extension or tether formation occurs. When a microvillus i s extended, it acts like a spring with a spring constant of approximat e to 43 pN/mu m. In contrast to a rigid or nonextendible microvillus, both microvillus extension and tether formation can decrease the pulli ng force imposed on the adhesive bonds, and thus prolonging the persis tence of the bonds at high physiological sheer stresses.