H. Zieler et al., Plasmodium gallinaceum ookinetes adhere specifically to the midgut epithelium of Aedes aegypti by interaction with a carbohydrate ligand, J EXP BIOL, 202(5), 1999, pp. 485-495
During the course of its development in the mosquito and transmission to a
new vertebrate host, the malaria parasite must interact with the mosquito m
idgut and invade the gut epithelium. To investigate how the parasite recogn
izes the midgut before invasion, we have developed an in vitro adhesion ass
ay based on combining fluorescently labelled ookinetes with isolated midgut
epithelia from blood-fed mosquitoes, Using this assay, we found that Plasm
odium gallinaceum ookinetes readily adhered to midguts of Aedes aegypti, mi
micking the natural recognition of the epithelium by the parasite, This int
eraction is specific: the ookinetes preferentially adhered to the lumen (mi
crovillar) side of the gut epithelium and did not bind to other mosquito ti
ssues, Conversely, the binding was not due to a non-specific adhesive prope
rty of the midguts, because a variety of other cell types, including untran
sformed P. gallinaceum zygotes or macrogametes, did not show similar bindin
g to the midguts, High concentrations of glycosylated (fetuin, orosomucoid,
ovalbumin) or non-glycosylated (bovine serum albumin) proteins, added as n
on-specific competitors, failed to compete with the ookinetes in binding as
says. We also found that the adhesion of ookinetes to the midgut surface is
necessary for sporogonic development of the parasite in the mosquito. Anti
bodies and other reagents that blocked adhesion in vitro also reduced oocys
t formation when these reagents were combined with mature ookinetes and fed
to mosquitoes. Chemical modification of the midguts with sodium periodate
at pH5.5 destroyed adhesion, indicating that the ookinete binds to a carboh
ydrate ligand on the surface of the midgut, The ligand is sensitive to peri
odate concentrations of less than 1 mmol l(-1), suggesting that it may cont
ain sialic-acid-like sugars. Furthermore, free N-acetylneuraminic acid comp
eted with the ookinetes in binding assays, while other monosaccharides had
no effect. However, in agreement with the current belief that adult insects
do not contain sialic acids, we were unable to detect any sialic acids in
mosquito midguts using the most sensitive HPLC-based fluorometric assay cur
rently available, We postulate that a specific carbohydrate group is used b
y the ookinete to recognize the midgut epithelium and to attach to its surf
ace. This is the first receptor-ligand interaction demonstrated for the ook
inete stage of a malaria parasite. Further characterization of the midgut l
igand and its parasite counterpart may lead to novel strategies of blocking
oocyst development in the mosquito.