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Platelet transfusion is a very common lifesaving medical procedure. Not widely known is the fact that platelets, unlike other blood cells, rapidly leave the circulation if refrigerated prior to transfusion. This peculiarity requires blood services to store platelets at room temperature, limiting platelet supplies for clinical needs. Here, we describe the mechanism of this clearance system, a longstanding mystery. Chilling platelets clusters their von Willebrand (vWf) receptors, eliciting recognition of mouse and human platelets by hepatic macrophage complement type 3 (CR3) receptors. CR3-expressing but not CR3-deficient mice exposed to cold rapidly decrease platelet counts. Cooling primes platelets for activation. We propose that platelets are thermosensors, primed at peripheral sites where most injuries occurred throughout evolution. Clearance prevents pathologic thrombosis by primed platelets. Chilled platelets bind vWf and function normally in vitro and ex vivo after transfusion into CR3-deficient mice. Therefore, GPIb modification might permit cold platelet storage.

This year, the Lasker Foundation confers its Clinical Medical Research Award on Akira Endo for his isolation from fungi of statins, potent inhibitors of cholesterol synthesis in the liver. The introduction of statins to clinical practice has markedly reduced morbidity and mortality from atherosclerotic cardiovascular disease.

Gelsolin, an 82 kDa actin-binding protein, has potent actin filament-severing activity in vitro. To investigate the in vivo function of gelsolin, transgenic gelsolinnull ( Gsn − ) mice were generated and found to have normal embryonic development and longevity. However, platelet shape changes are decreased in Gsn − mice, causing prolonged bleeding times. Neutrophil migration in vivo into peritoneal exudates and in vitro is delayed. Gsn − dermal fibroblasts have excessive actin stress fibers and migrate more slowly than wildtype fibroblasts, but have increased contractility in vitro. These observations establish the requirement of gelsolin for rapid motile responses in cell types involved in stress responses such as hemostasis, inflammation, and wound healing. Neither gelsolin nor other proteins with similar actin filament-severing activity are expressed in early embryonic cells, indicating that this mechanism of actin filament dynamics is not essential for motility during early embryogenesis.

The high affinity receptor that binds the Fc domain of immunoglobulin G (IgG) subclasses 1 and 3 (Fc gamma RI) mediates important immune defense functions by inducing cell surface changes on human leukocytes. In this article, we document direct high affinity binding of Fc gamma RI to the actin filament cross-linking protein, actin-binding protein (ABP). In the absence of IgG, all Fc gamma RI molecules in undifferentiated cells of myeloid line U937 bound to ABP over a 9-fold range of Fc gamma RI expression induced by human IFN-gamma. Binding of IgG to U937 cells constitutively expressing Fc gamma RI or to COS cells genetically transfected to express Fc gamma RI rapidly decreased the avidity of Fc gamma RI for ABP. This finding suggests the existence of a pathway communicating a signal between a functional IgG receptor and intracellular components involved in the effector responses to Fc gamma RI-ligand interaction.

Cells respond to diverse external stimuli by polymerizing cytoplasmic actin, and recent evidence indicates that GTPases can specify where this polymerization takes place. Actin assembly in stimulated blood platelets occurs where sequestered monomers add onto the fast-growing (barbed) ends of actin filaments (F-actin), which are capped in the resting cells. We report that D3 and D4 polyphosphoinositides, Pl(4)P, Pl(4,5)P2, Pl(3,4)P2, and Pl(3,4,5)P3, uncap F-actin in resting permeabilized platelets. The thrombin receptor-activating peptide (TRAP), GTP, and GTP gamma S, but not GDP beta S, also uncap F-actin in permeabilized platelets. GDP beta S inhibits TRAP-induced F-actin uncapping, and Pl(4,5)P2 overcomes this inhibition. Constitutively active mutant Rac, but not Rho, activates uncapping of F-actin. Pl(4,5)P2-binding peptides derived from gelsolin inhibit F-actin uncapping by TRAP, Rac, and GTP gamma S. TRAP and Rac induce rapid Pl(4,5)P2 synthesis in permeabilized platelets. The findings establish a signaling pathway for actin assembly involving Rac in which the final message is phosphoinositide-mediated F-actin uncapping.

Polymorphonuclear (PMN) leukocytes, highly motile ameboid cells vital for mammalian defense against infection, acquire a distinct polarized morphology during locomotion and phagocytosis. An organelle-excluding pseudopod extends in the direction of movement and surrounds objects during phagocytosis. The anterior pseudopod contains a three-dimensional network of actin filaments. Actin-binding protein (ABP) and myosin cause the crosslinking and contraction, respectively, of actin filaments in vitro. We used indirect immunofluorescence to study the redistribution of myosin and ABP molecules in rabbit PMN leukocytes during locomotion and phagocytosis. In unpolarized PMN leukocytes, ABP and myosin had a diffuse distribution with some predilection for the cortex. In polarized PMN leukocytes crawling toward yeast particles, myosin and ABP staining concentrated in the anterior pseudopod. In PMN leukocytes fixed during phagocytosis of the yeast particles, antimyosin and anti-ABP staining concentrated strikingly in the distal portions of the pseudopod embracing the yeasts. Staining for catalase, a cytoplasmic protein in PMN leukocytes, for lactoferrin, a protein of specific granules, and for myeloperoxidase, a protein of azurophilic granules, was not concentrated in pseudopods. Taken together with available morphologic and biochemical information, these findings are consistent with a mechanism wherein interactions of actin, ABP and myosin redistribute cortical cytoplasm into pseudopods involved in locomotion and phagocytosis.