Monoclonal antibodies identify micronemes and a new population of cytoplasmic granules cross-reacting with micronemes of cystozoites of Sarcocystis muris

Characterization of Neospora caninum protease, NcSUB1 (NC-P65), with rabbit anti-N54

NcSUB1 (formerly known as NC-p65) is the first molecularly described proteolytic enzyme of the intracellular protozoan parasite Neospora caninum. This report describes the characterization of a rabbit anti-N54, which is an antiserum generated against an internal fragment of NcSUB1 (amino acids 649-783). In immunofluorescence studies rabbit and-N54 labeled the apical end of the fixed parasite. By immuno-gold electron microscopy, the antibody bound primarily to the microneme organelles of the parasite. Analysis of secreted parasitic proteins indicated that a protein of molecular weight 65 kDa (reduced) or 55 kDa (nonreduced) was recognized bythe antibody. The same secreted proteins were affinity purified with rabbit anti-N54-coupled resins and were shown to contain major proteolytic activity by zymography. Thus, rabbit anti-N54 is the first antibody developed for N. caninum that binds to themicroneme proteins and recognizes a major secreted enzyme.

Mobilization of intracellular calcium stimulates microneme discharge in Toxoplasma gondii

Apicomplexan parasites, including Toxoplasma gondii, apically attach to their host cells before invasion. Recent studies have implicated the contents of micronemes, which are small secretory organelles confined to the apical region of the parasite, in the process of host cell attachment. Here, we demonstrate that microneme discharge is regulated by parasite cytoplasmic free Ca2+ and that the micronemal contents, including the MIC2 adhesin, are released through the extreme apical tip of the parasite. Microneme secretion was triggered by Ca2+ ionophores in both the presence and the absence of external Ca2+, while chelation of intracellular Ca2+ prevented release. Mobilization of intracellular calcium with thapsagargin or NH4Cl also triggered microneme secretion, indicating that intracellular calcium stores are sufficient to stimulate release. Following activation of secretion by the Ca2+ ionophore A23187, MIC2 initially occupied the apical surface of the parasite, but was then rapidly treadmilled to the posterior end and released into the culture supernatant. This capping and release of MIC2 by ionophore-stimulated tachyzoites mimics the redistribution of MIC2 that occurs during attachment and penetration of host cells, and both events are dependent on the actin-myosin cytoskeleton of the parasite. These studies indicate that microneme release is a stimulus-coupled secretion system responsible for releasing adhesins involved in cell attachment.

Combined preparative enzymatic synthesis of dTDP-6-deoxy-4-keto-D-glucose from dTDP and sucrose

dTDP-6-deoxy-4-keto-D-glucose (1), the common intermediate in the biosyntheses of the manifold deoxysugars, was synthesized on a gram-scale by the combination of sucrose synthase and dTDP-D-glucose 4,6-dehydratase in a fed batch, starting the reaction with dTDP. This process allowed a dTDP conversion with a 100% rate. An easy and efficient three-step purification with anion-exchange chromatography and gel filtration gave 1.1 g of 1 in an overall yield of 73%. This work realizes a first step for an economic access to activated deoxysugars.

In vitro biosynthesis and in vivo processing of the major microneme antigen of Sarcocystis muris cyst merozoites

The cDNA clone pSM/1.6 encoding the 26.5-kDa precursor molecule of the 16/17-kDa microneme antigen of Sarcocystis muris cyst merozoites was expressed in a cell-free translation/translocation system to study translocation of the protein across membranes. The antigen was found to be translocated across heterologous endoplasmic reticulum membranes. Translocation was accompanied by cleavage of a signal peptide to create a 23-kDa polypeptide that was completely protected from digestion with proteinase K. Pulse-chase analysis of [35S]-methionine-labeled S. muris cyst merozoites demonstrated that the 16/17-kDa antigen derived from a 23-kDa precursor molecule and that its processing occurred at between a few minutes and 2 h after biosynthesis. This leads to the conclusion that the native microneme antigen is secreted from the parasite cell via the endoplasmic reticulum. Sorting into micronemes might occur during transition through a Golgi-like structure, involving cleavage of the hydrophilic propeptide to create the mature 16/17-kDa protein.

Monoclonal antibodies identify a subset of dense granules in Cryptosporidium parvum zoites and gamonts

Two monoclonal antibodies raised against purified oocysts and excysted sporozoites of Cryptosporidium parvum identified antigens located in the anterior half of sporozoites by indirect immunofluorescence microscopic assay. The monoclonal antibodies also reacted with Triton-X-100-insoluble antigens of asexual and sexual stage parasites developing in epithelial cells in vitro and identified a 110 kilodalton antigen on immunoblots of sodium dodecyl sulfate-extracted oocysts. Immunoblotting reactivity was abolished by prior treatment of blotted antigen with periodic acid suggesting that the monoclonal antibodies recognize a carbohydrate or carbohydrate-dependent epitope(s). By immunoelectron microscopy, the antibodies reacted with a family of small, electron-dense granules located predominantly in the central region of merozoites and also with a population of cytoplasmic inclusions in macrogamonts. In addition, the monoclonal antibodies prominently labeled the parasitophorous vacuole membrane of all intracellular stages examined suggesting that the corresponding antigen(s) may be exocytosed from the granules to become associated with Triton X-100-insoluble components of the vacuolar membrane or cytoskeleton.

In vitro and in vivo immunolabeling of sporozoites, schizonts, and sexual stages of Eimeria acervulina and E. tenella by a species- and stage-cross-reactive monoclonal antibody

A cross-reactive monoclonal antibody (mAb), designated 1205, was used to study redistribution, parasitophorous vacuole (PV) incorporation, and in situ antigen production during the intracellular parasite development of Eimeria acervulina and E. tenella. Western-blot analysis of sporozoite preparations showed that the mAb recognized antigenic bands at 55 and 80 kDa. Indirect immunofluorescent antibody (IFA) labeling of sporozoites produced an internal dot pattern. Immunogold electron microscopy (IM) showed labeling of dense granules within sporozoites. The IFA pattern changed to a general-internal label in immature schizonts followed by a surface-tip pattern in mature merozoites both in vitro and in vivo. IM of the asexual stages revealed the same labeling pattern for the in vivo development of both species, and labeling of rhoptries was seen. In vitro, the PV membrane together with amorphous material within the PV was labeled by IFA during schizont development for E. tenella. No IM labeling of either the PV membrane or material within the PV was observed. Sexual stages seen in vivo for both species had the general-internal IFA pattern.

Characterization of cDNA clones encoding a major microneme antigen of Sarcocystis muris (Apicomplexa) cyst merozoites

Two monoclonal antibodies directed against a microneme antigen of Sarcocystis muris cyst merozoites (16/17 kDa band doublet) were used to isolate cDNA clones from a lambda ZAP expression library. Restriction analysis revealed that the inserts were highly similar, with sizes ranging between 1.8 and 2.3 kb. In addition, a full-length cDNA insert of 2.6 kb was obtained by hybridization screening. On Northern blots, a single mRNA species of 2.7 kb was detected by a cDNA-derived probe. Southern blot hybridization suggests that the gene is present as a single copy. The nucleotide sequence of the full-length clone contains a single reading frame with a coding capacity of 26.5 kDa. The hypothetical polypeptide consists of a putative N-terminal signal peptide followed by a hydrophilic domain of unknown function, and the mature protein sequence. After purifying the 16/17 kDa antigen from cyst merozoites, a partial N-terminal amino acid sequence was obtained. Thus, the identity of the cDNA sequence was confirmed. The deduced sequence of the mature protein is predominantly hydrophilic and rich in cysteine (8.7%). Database searching suggested weak homologies of the hypothetical polypeptide to plasma kallikrein, tenascin and blood coagulation factors.

A new antigen of Cryptosporidium parvum micronemes possessing epitopes cross-reactive with macrogamete granules

Three monoclonal antibodies (mAbs) raised against purified excysted oocysts and sporozoites of Cryptosporidium parvum reacted in an immunofluorescent assay with antigens located at the apical pole of the sporozoites. On Western blots of oocyst extract, an antigen of 100 kDa was recognized by the three mAbs. Periodic acid treatment abolished this reactivity, suggesting that the corresponding epitopes are carbohydrates. As determined by immunoelectron microscopy, the three mAbs reacted with micronemes in sporozoites and merozoites and recognized a heterogeneous population of granules in the developing macrogametes. Gold particles were also detected in the parasitophorous vacuole of the macrogametes and sporulating oocysts. The detection of a microneme antigen in a family of macrogamete granules has not previously been reported in coccidia. The significance of this observation and the nature of these macrogamete inclusions are discussed.