Biochemical characterization of aminopeptidase N2 from Toxoplasma gondii

Infection of Mouse Neural Progenitor Cells by Toxoplasma gondii Reduces Proliferation, Migration, and Neuronal Differentiation in Vitro

Congenital toxoplasmosis constitutes a major cause of pre- and postnatal complications. Fetal infection with Toxoplasma gondii influences development and can lead to microcephaly, encephalitis, and neurologic abnormalities. Systematic studies concerning the effects of neural progenitor cell infection with T. gondii are unavailable. Cortical intermediate progenitor cells cultivated as neurospheres obtained from E16.5 Swiss Webster mice were infected with T. gondii (ME49 strain) tachyzoites to mimic the developing mouse cerebral cortex in vitro. Infection was associated with decreased cell proliferation, detected by Ki-67 staining at 48 and 72 hours after infection in floating neurospheres, and reduced cellularity at 96 hours. Transient decreases in the expression of the neurogenesis-related transcription factors T-box brain protein 1, mouse atonal homolog protein 1, and hairy and enhancer of split protein 1 were found in infected cultures, while the level of transcription factor SOX-2 remained unaltered. Neurogenic potential, assessed in plated neurospheres, was impaired in infected cultures, as indicated by decreased late neuronal marker neurofilament heavy chain immunoreactivity. Infected cultures exhibited decreased overall migration rates at 48 and 120 hours. These findings indicate that T. gondii infection of neural progenitor cells may lead to reduced neurogenesis due to an imbalance in cell proliferation alongside an altered migratory profile. If translated to the in vivo situation, these data could explain, in part, cortical malformations in congenitally infected individuals.

Biochemical and cellular characterisation of the Plasmodium falciparum M1 alanyl aminopeptidase (PfM1AAP) and M17 leucyl aminopeptidase (PfM17LAP)

The Plasmodium falciparum M1 alanyl aminopeptidase and M17 leucyl aminopeptidase, PfM1AAP and PfM17LAP, are potential targets for novel anti-malarial drug development. Inhibitors of these aminopeptidases have been shown to kill malaria parasites in culture and reduce parasite growth in murine models. The two enzymes may function in the terminal stages of haemoglobin digestion, providing free amino acids for protein synthesis by the rapidly growing intra-erythrocytic parasites. Here we have performed a comparative cellular and biochemical characterisation of the two enzymes. Cell fractionation and immunolocalisation studies reveal that both enzymes are associated with the soluble cytosolic fraction of the parasite, with no evidence that they are present within other compartments, such as the digestive vacuole (DV). Enzyme kinetic studies show that the optimal pH of both enzymes is in the neutral range (pH 7.0-8.0), although PfM1AAP also possesses some activity (< 20%) at the lower pH range of 5.0-5.5. The data supports the proposal that PfM1AAP and PfM17LAP function in the cytoplasm of the parasite, likely in the degradation of haemoglobin-derived peptides generated in the DV and transported to the cytosol.

X-ray crystal structure and specificity of the Toxoplasma gondii ME49 TgAPN2

Toxoplasmosis is a parasitic disease caused by infection with Toxoplasma gondii that currently has few therapeutic options. The M1 aminopeptidase enzymes have been shown to be attractive targets for anti-parasitic agents and/or vaccine candidates, suggesting potential to re-purpose inhibitors between parasite M1 aminopeptidase targets. The M1 aminopeptidase TgAPN2 has been suggested to be a potential new drug target for toxoplasmosis. Here we investigate the structure and function of TgAPN2, a homologue of the antimalarial drug target PfA-M1, and evaluate the capacity to use inhibitors that target PfA-M1 against TgAPN2. The results show that despite a similar overall fold, the TgAPN2 has a unique substrate specificity and inhibition profile. Sequence and structure differences are investigated and show how comparative structure-activity relationships may provide a route to obtaining potent inhibitors of TgAPN2.

Localization and enzyme kinetics of aminopeptidase N3 from Toxoplasma gondii

Aminopeptidase N is an important metalloenzyme from the M1 zinc metallopeptidase family, which is present in numerous apicomplexan parasites, including Plasmodium, Eimeria, and Cryptosporidium. Aminopeptidase N is a potential drug target, and hence, its properties have been widely investigated. In the current study, the cellular localization and enzyme characteristics of Toxoplasma gondii aminopeptidase N3 (TgAPN3) were evaluated in vitro. Cellular localization analysis revealed that TgAPN3 and GRA protein were co-located in the organelle and parasitophorous vacuole of T. gondii. The secretion assay showed that TgAPN3 could be co-secreted from the tachyzoites with GRA protein. A functional recombinant Toxoplasma aminopeptidase N3 (rTgAPN3) was produced in Escherichia coli. The enzyme activity was first determined using a fluorogenic H-Ala-MCA substrate. Some activity of rTgAPN3 was observed between pH 3.0 and 8.0, with a peak at pH 7.0. The activity was significantly enhanced in the presence of Co²⁺ ions. Substrate specificity of rTgAPN3 was then evaluated. The enzyme showed a preference for substrates containing N-terminal Ala residues, followed by Tyr and Cys. The rTgAPN3 activity was significantly inhibited by bestatin and phebestatin. In general, TgAPN3 was a structurally conserved member of the M1 family, although it also displayed unique biochemical characteristics. These results lay the foundation for a functional study of TgAPN3 and constitute its putative identification as a drug target.