STAGE-SPECIFIC BINDING OF LEISHMANIA-DONOVANI TO THE SAND FLY VECTOR MIDGUT IS REGULATED BY CONFORMATIONAL-CHANGES IN THE ABUNDANT SURFACE LIPOPHOSPHOGLYCAN

The life cycle of Leishmania parasites within the sand fly vector incl udes the development of extracellular promastigotes from a noninfectiv e, procyclic stage into an infective, metacyclic stage that is uniquel y adapted for transmission by the fly and survival in the vertebrate h ost. These adaptations were explored in the context of the structure a nd function of the abundant surface lipophosphoglycan (LPG) on Leishma nia donovani promastigotes. During metacyclogenesis, the salient struc tural feature oft, donavani LPG is conserved, involving expression of a phosphoglycan chain made up of unsubstituted disaccharide-phosphate repeats. Two important developmental modifications were also observed. First, the size of the molecule is substantially increased because of a twofold increase in the number of phosphorylated disaccharide repea t units expressed. Second, there is a concomitant decrease in the pres entation of terminally exposed sugars. This later property was indicat ed by the reduced accessibility of terminal galactose residues to gala ctose oxidase and the loss of binding by the lectins, peanut agglutini n, and concanavalin A, to metacyclic LPG in vivo and in vitro. The los s of lectin binding was not due to downregulation of the capping oligo saccharides as the same P-linked galactose or cu-linked mannose-termin ating oligosaccharides were present in both procyclic and metacyclic p romastigotes. The capping sugars on procyclic LPG were found to mediat e procyclic attachment to the sand fly midgut, whereas these same suga rs on metacyclic LPG failed to mediate metacyclic binding. And whereas intact metacyclic LPG did not inhibit procyclic attachment, depolymer ized LPG inhibited as well as procyclic LPG, demonstrating that the li gands are normally buried. The masking of the terminal sugars is attri buted to folding and clustering of the extended phosphoglycan chains, which form densely distributed particulate structures visible on fract ure-flip preparations of the metacyclic surface. The exposure and subs equent masking of the terminal capping sugars explains the stage speci ficity of promastigote attachment to and release from the vector midgu t, which are key events in the development of transmissible infections in the fly.