Inorganic
polyphosphates (
PolyP) are widespread molecules that have been shown to play a role in
metal detoxification and
heavy-metal tolerance. In the present report, we investigated the functional role of spherites as
PolyP-
metal binding stores in epithelial cells of the midgut of Anticarsia gemmatalis, a lepidopteran pest of soybean.
PolyP stores were detected by
DAPI staining and indirect immunohistochemistry as vesicles distributed in columnar cells and around goblet cell cavities. These
PolyP vesicles were identified as spherites by their elemental profile in cell lysates that were partially modulated by P- or V-
ATPases.
PolyP levels along the midgut were detected using a recombinant
exopolyphosphatase assay. When
copper was added in the diet of larva,
copper detection in spherites by X-ray microanalysis correlated with an increase in the relative phosphorous X-ray signal and with an increase in
PolyP levels in epithelia cell lysate. Transmission electron microscopy of chemically fixed or cryofixed and freeze substituted tissues confirmed a preferential localization of spherites around the goblet cell cavity. Taken together, these results suggest that spherites store high levels of
PolyP that are modulated during
metal uptake and detoxification. The similarity between
PolyP granules and spherites herein described also suggest that
PolyP is one of the main phosphorous source of spherites found in different
biological models. This suggests physiological roles played by spherites in the midgut of arthropods and mechanisms involved in
heavy metal resistance among different insect genera.