Microsporidia are a fascinating group of single-celled eukaryotes that infect a wide range of animal species, including humans. These divergent fungal pathogens employ a unique harpoon-like apparatus called the polar tube to invade host cells. The polar tube—nearly 50 microns long but just 100 nm in diameter—is fired out of the microsporidian spore over the course of just a few hundred milliseconds. Once fired, the polar tube is thought to pierce the plasma membrane of a target cell and act as a conduit for the injection of the parasite cell into the host cell interior, which initiates infection. The polar tube architecture and its association with neighboring organelles within the parasite cell remain poorly understood. In addition, microsporidia are highly diverged from other extant eukaryotes, lack mitochondria, and represent a minimal eukaryotic cell. Here, we will present our latest work using cryoelectron tomography to investigate the structural cell biology of dormant spores from the human-infecting microsporidian species, Encephalitozoon intestinalis . Segmentation and subtomogram averaging of the polar tube a protein-based skeleton formed from regularly spaced protein filaments, unrelated to any known cytoskeletal proteins. Combining cryoelectron tomography with cellular modeling, we propose a model for the three-dimensional organization polar tube and an associated membranous organelle, called the polaroplast. Our results reveal the ultrastructure of the microsporidian cell and the polar tube invasion apparatus in situ, shedding light on the infection mechanism employed by this neglected human pathogen.
Usmani et al. (Sun,) studied this question.