Withdrawal of skeletal muscle cells from cell cycle progression triggers differentiation ofToxoplasma gondiitowards the bradyzoite stage

Abstract

SummaryToxoplasma gondii is a widespread intracellular parasite of mammals and birds and an important opportunistic pathogen of humans. Following primary infection, fast-replicating tachyzoites disseminate within the host and either are subsequently eliminated by the immune system or transform to latent bradyzoites which preferentially persist in brain and muscle tissues. The factors which determine the parasites' tissue distribution during chronic toxoplasmosis are unknown. Here we show that mouse skeletal muscle cells (SkMCs) after differentiation to mature, myosin heavy chainpositive, polynucleated myotubes, significantly restrict tachyzoite replication and facilitate expression of bradyzoite-specific antigens and tissue cyst formation. In contrast, proliferating mononuclear myoblasts and control fibroblasts enable vigorous T. gondii replication but do not sustain bradyzoite or tissue cyst formation. Bradyzoite formation correlates with upregulation of testis-specific Y-encoded-like protein-2 gene expression (Tspyl2) and p21Waf1/Cip1 as well as downregulation of cyclin B1 and absence of DNA synthesis, i.e. a cell cycle arrest of syncytial myotubes. Following infection with T. gondii, myotubes but not myoblasts or fibroblasts further upregulate the negative cell cycle regulator Tspyl2. Importantly, RNA interferencemediated knock-down of Tspyl2 abrogates differentiation of SkMCs to myotubes and enables T. gondii to replicate vigorously but abolishes bradyzoite-specific gene expression and tissue cyst formation. Together, these data indicate that Tspyl2-mediated host cell cycle withdrawal is a physiological trigger of Toxoplasma stage conversion in mature SkMCs. This finding might explain the preferred distribution of T. gondii tissue cysts in vivo.

Impact of the host on Toxoplasma stage differentiation

“…in myoblasts or in fibroblasts (Fig. 2) 39. This strongly suggests that the cellular microenvironment within differentiated myotubes may trigger a developmental pathway of T. gondii that leads to conversion to the persistent parasite stage.…”

“…Thus, the above findings indicate that host CDA-1 does suffice to trigger stage conversion in T. gondii . Importantly, CDA-1 has recently been proved to play a role in bradyzoite differentiation also in a more physiological environment of T. gondii stage conversion 39. First, these authors provided direct evidence that the host cell cycle impacts T. gondii development.…”

Reexamining Chronic Toxoplasma gondii Infection: Surprising Activity for a “Dormant” Parasite

“…Tissue cysts in muscle are an important source of infection of humans from the consumption of raw or undercooked meat making it among the most prevalent food borne infections in the USA [44]. Experimental studies on chronic toxoplasmosis in muscle are limited [45] although the recent development of specific skeletal muscle-based infection systems will undoubtedly accelerate this work [46–48]. …”

Activation of chronic toxoplasmosis by transportation stress in a mouse model

“…Besides the complex regulation network in parasites, host environments also play important roles in determining bradyzoite conversion [16]. Reactivation of chronic toxoplasmosis to acute infection has been thought to be associated with decreased immunity of the hosts [17].…”

“…Bradyzoites have crucial roles in the transmission and pathogenesis of Toxoplasma , however not much is known about their biology and how they are formed or reactivated [11, 15, 16]. It is widely accepted that the status of the hosts' immune function is a critical determinant for the outcome of T. gondii infection [17, 18].…”