Identification and analysis of Eimeria nieschulzi gametocyte genes reveal splicing events of gam genes and conserved motifs in the wall-forming proteins within the genus Eimeria (Coccidia, Apicomplexa).

The genus Eimeria (Apicomplexa, Coccidia) provides a wide range of different species with different hosts to study common and variable features within the genus and its species. A common characteristic of all known Eimeria species is the oocyst, the infectious stage where its life cycle starts and ends. In our study, we utilized Eimeria nieschulzi as a model organism. This rat-specific parasite has complex oocyst morphology and can be transfected and even cultivated in vitro up to the oocyst stage. We wanted to elucidate how the known oocyst wall-forming proteins are preserved in this rodent Eimeria species compared to other Eimeria. In newly obtained genomics data, we were able to identify different gametocyte genes that are orthologous to already known gam genes involved in the oocyst wall formation of avian Eimeria species. These genes appeared putatively as single exon genes, but cDNA analysis showed alternative splicing events in the transcripts. The analysis of the translated sequence revealed different conserved motifs but also dissimilar regions in GAM proteins, as well as polymorphic regions. The occurrence of an underrepresented gam56 gene version suggests the existence of a second distinct E. nieschulzi genotype within the E. nieschulzi Landers isolate that we maintain.

Development of Eimeria nieschulzi (Coccidia, Apicomplexa) Gamonts and Oocysts in Primary Fetal Rat Cells.

The in vitro production of gametocytes and oocysts of the apicomplexan parasite genus Eimeria is still a challenge in coccidiosis research. Until today, an in vitro development of gametocytes or oocysts had only been shown in some Eimeria species. For several mammalian Eimeria species, partial developments could be achieved in different cell types, but a development up to gametocytes or oocysts is still lacking. This study compares several permanent cell lines with primary fetal cells of the black rat (Rattus norvegicus) concerning the qualitative in vitro development of the rat parasite Eimeria nieschulzi. With the help of transgenic parasites, the developmental progress was documented. The selected Eimeria nieschulzi strain constitutively expresses the yellow fluorescent protein and a macrogamont specific upregulated red tandem dimer tomato. In the majority of all investigated host cells the development stopped at the second merozoite stage. In a mixed culture of cells derived from inner fetal organs the development of schizont generations I-IV, macrogamonts, and oocysts were observed in crypt-like organoid structures. Microgamonts and microgametes could not be observed and oocysts did not sporulate under air supply. By immunohistology, we could confirm that wild-type E. nieschulzi stages can be found in the crypts of the small intestine. The results of this study may be helpful for characterization of native host cells and for development of an in vitro cultivation system for Eimeria species.

Chimeric fluorescent reporter as a tool for generation of transgenic Eimeria (Apicomplexa, Coccidia) strains with stage specific reporter gene expression.

Progress in transfection of Eimeria sporozoites leads to transformed oocysts, however the output of mutants after passages in the host animals is low. Further enrichment of transgenic oocysts was dependent on fluorescent activated cell sorting and could not be achieved by drug selection. In this study, we fused the Toxoplasma gondii DHFR-TSm2m3 pyrimethamine resistance gene with the yellow fluorescent protein (YFP) encoding sequence to provide continuous pyrimethamine resistance and fluorescence in the Eimeria parasite from a single transcript. The permanent YFP signal of transgenic parasites allows differentiating transgenic parasites from wild type parasites throughout the entire life cycle. The output of transformed oocysts increased up to more than 30% after initial transfection and completion of the life cycle in the host animal. Within three passages under pyrimethamine treatment, a strain with 100% transformed sporulated oocysts of the parasite could be isolated. This new method provides the potential to produce and monitor transgenic Eimeria strains without additional fluorescence activated cell sorting (FACS). The chimeric fluorescent reporter can be utilized as a continuous internal control for plasmids containing stage specific promoter. By this means we utilized an Eimeria tenella gamogony gene specific regulatory sequence to confer macrogamont specific tandem dimer tomato (tdtomato) reporter gene expression in Eimeria nieschulzi.