Mechanisms of nutritive endocytosis. II. The effect of charged agents on phagocytic recognition by digestive cells.

Abstract

ABSTRACT Phagocytosis by digestive cells of green hydra was affected by polycations and polyanions. Polycations probably act by reversing negative charges on surfaces of particles or digestive cells; polyanions by inducing dynamic changes in the morphology of digestive cell surfaces. The polycation, poly-L-lysine, at concentrations as low as 0.1 μg/ml, inhibited phagocytosis when presented concurrently with 5.7 μm latex spheres. Phagocytosis was also inhibited when latex spheres were pretreated with poly-L-lysine, washed and then presented to untreated digestive cells. In contrast, pretreatment of digestive cells with poly-L-lysine (1.0μg/ml) enhanced subsequent uptake of untreated latex spheres. Treatment with other agents, such as FeCl, and carbodiimide, that are presumed to reduce negative surface charge on the particle (the latter by covalent neutralization of surface carboxyl groups), also inhibited phagocytosis. The phagocytically active digestive cell surface bears a net positive surface charge since it showed an affinity for negatively but not positively charged ferritin. Unmodified latex spheres possessed a net negative surface-charge, as measured by electrophoresis of whole cells. In contrast, particles treated with polycations and other presumptive modifiers of surface charge were shown by electrophoresis to have reduced negative or, in some cases, net positive surfacecharge. Electrostatic attraction of particles for the digestive cell surface may play a major role in determining the extent of phagocytic uptake by digestive cells, and polycation inhibition of uptake may result from neutralization of negative charges on the particle surface. The polyanion, poly-L-glutamic acid, enhanced phagocytosis when presented concurrently with latex spheres at concentrations greater than 1.0 μg/ml. Pretreatment of digestive cells with poly-L-glutamic acid also enhanced uptake of latex spheres. Scanning electron microscopy showed that latex spheres presented concurrently with poly-L-glutamic acid rapidly (within 10 min) induced the formation of a dense array of microvilli on digestive cell surfaces, and that these microvilli participated in phagocytic enclosure of latex spheres. Polyanions, therefore, enhanced uptake by a mechanism unrelated to electrostatic forces; rather, by activating digestive cells to produce dynamic morphological alterations, they may reduce the resistance encountered by the enveloping plasma membrane to its movement over particles and increase the surface area available for interiorization. The influence of polyanions and polycations on phagocytosis of several other particles: freshly isolated symbionts, Chlorella spp.; heat-treated symbionts; and free-living algae, Chlorella vulgaris, were also examined, with results that were, in general, consistent with those described for latex spheres. Thus, the influence of charged agents on phagocytosis by digestive cells may be a general phenomenon.