New details on the fine structure of the rhoptry of Toxoplasma gondii

Abstract

Rhoptries are organelles that have important, complex roles in Apicomplexa biology. During Toxoplasma gondii infection, these organelles take part in several essential and complex processes that include host cell entry and parasite development. Using different electron microscopy techniques, we characterized the fine morphology of the rhoptries of two of the most important life stages of T. gondii: the tachyzoite and the bradyzoite forms. The observed tachyzoite and bradyzoite rhoptries had delimited regions characterized by a dark and electron-dense neck, an amorphous and less electron-dense bulb, and a region of intermediate electron density, which connects the bulb to the neck. Metal replicas of frozen-fractured tachyzoites showed intramembranous particles of different densities and sizes on the fractured faces of rhoptry membranes. Both in tachyzoites and bradyzoites, the intramembranous particles were arranged in distinctive parallel arrays that decorated most part of these organelles. Tubulo-vesicular subcompartments and free particles within the rhoptry lumen were observed on freeze-fractured replicas. Cryo-fixed, deep-etched samples showed several pore-like structures localized in the bulb portion. No obvious evidence was found of a possible connection between rhoptries and micronemes.

Structural and functional dissection of Toxoplasma gondii armadillo repeats only protein

“…Here, we show that T. gondii ACβ is recruited to the rhoptry surface through interactions with ARO and that, together with AIP, they represent the first identified markers of a third sub-compartment separating the rhoptry bulb and neck (Lemgruber et al, 2010). AIP is a prerequisite for the targeting of ACβ to the rhoptry.…”

“…1F). Interestingly, it has been reported in previous transmission electron microscopy (TEM) studies that there are three sub-compartments of rhoptries delimited by a dark and electrondense neck, an amorphous and less electron-dense bulb, and a region of intermediate electron density, which connects the bulb to the neck (Lemgruber et al, 2010). Taken together, these results demonstrate that ARO interacts with ACβ and that, together with AIP, these two ARO partners constitute the first known markers restricted to the surface of an intermediate sub-compartment of the rhoptries.…”

“…Based on combined biochemical and genetic evidence, we have established that ACβ interacts with AIP, which in turn binds directly or indirectly to ARO. Both proteins are the first known markers of a morphologically defined intermediate compartment of the rhoptries that separates the neck from the bulb (Lemgruber et al, 2010). It is unclear at this point what restricts the two proteins to this subcompartment.…”

Dynamics and 3D organization of secretory organelles of Toxoplasma gondii

“…On the other hand, it is clear from observation of random sections and from previous publications (Vivier and Petitprez, 1972;Monteiro et al, 2001;Lemgruber et al, 2010) that rhoptries and micronemes are not randomly distributed in the tachyzoite. To investigate further, we employed a new method, using a FIB-dual beam scanning electron microscope, to obtain serial sections for 3D reconstruction.…”

“…There are two recognized compartments in rhoptries: a bulb with a honeycombed appearance and a tapered neck oriented towards the conoidal channel. As observed by freezefracture replicas, the rhoptries present a linear distribution of inner-membrane particles along the peduncle (Lemgruber et al, 2010). The contents of the rhoptries are both lipidic, giving the organelle its shape (Besteiro et al, 2008), and proteic: RONs and ROPs.…”