Toxoplasma gondii down modulates cadherin expression in skeletal muscle cells inhibiting myogenesis

Abstract

Background: Toxoplasma gondii belongs to a large and diverse group of obligate intracellular parasitic protozoa. Primary culture of mice skeletal muscle cells (SkMC) was employed as a model for experimental toxoplasmosis studies. The myogenesis of SkMC was reproduced in vitro and the ability of T. gondii tachyzoite forms to infect myoblasts and myotubes and its influence on SkMC myogenesis were analyzed.

Results: In this study we show that, after 24 h of interaction, myoblasts (61%) were more infected with T. gondii than myotubes (38%) and inhibition of myogenesis was about 75%. The role of adhesion molecules such as cadherin in this event was investigated. First, we demonstrate that cadherin localization was restricted to the contact areas between myocytes/myocytes and myocytes/myotubes during the myogenesis process. Immunofluorescence and immunoblotting analysis of parasite-host cell interaction showed a 54% reduction in cadherin expression at 24 h of infection. Concomitantly, a reduction in M-cadherin mRNA levels was observed after 3 and 24 h of T. gondii-host cell interaction.

Conclusions: These data suggest that T. gondii is able to down regulate M-cadherin expression, leading to molecular modifications in the host cell surface that interfere with membrane fusion and consequently affect the myogenesis process.

Figure 1

Percentage of T. gondii infected SkMC after 24 h of interaction. (A) Percentage of myoblasts (61%) and myotubes (38%) infected with T. gondii after 24 h of interaction. Student's T-test (*) p ≤ 0.05. (B) Details of SkMC cultures profile observed by fluorescence microscopy with phaloidin-TRITC labeling showing actin filaments in red; nuclei of the cells and the parasites labeled with DAPI, in blue. Infected cultures present myoblasts containing several parasites (thick arrow) and young myotubes with 2 nuclei without parasites (thin arrows). Bars, 20 μm

Figure 2

Quantitative analysis of myotube formation percentage during myogenesis in T. gondii infected cultures. (A) In uninfected cultures, after 3 days, the percentage of myotubes was 19.5% while in infected cultures, after 24 h of interaction, this percentage decreased to 2.5%. Note the 75% reduction in the formation of myotubes in infected cultures. Student's T-test (*) p = 0.0025. (B) Differential interference contrast (DIC) image showing influence of the infection by T. gondii (24 h of interaction) on SkMC myogenesis. Parasite (thick arrows) and unfused myocytes (thin arrows).

Figure 3

Cadherin localization in primary SkMC cultures. Indirect immunofluorescence assays showing: (A) 2-day-old myoblasts under multiplication and differentiation. Cadherin (in green) is strongly marked in every cell with high concentrations in edges near the membrane and points of cell-cell contact (arrows). (B) apparently, the existence of a single newly internalized parasite (inset) did not lead to any change in the profile of cadherin expression and distribution in host cells (arrow). Nuclei of cells and parasites are labeled with DAPI, in blue. Bars, 20 μm

Figure 4

Cadherin distribution in SkMC after 24 h of T. gondii interaction. Confocal Microscopy analysis showing: (A) In 3-day-old SkMC cultures, after differentiation, myoblasts present intense cadherin labeling at the contact points (arrows). (B and C) In myoblasts after 24 h of interaction with T. gondii (thick arrow), cadherin (thin arrow) becomes disorganized forming aggregates at different sites, around and inside the parasitophorous vacuole (for detail, see inset). (D) Infected myoblasts after 24 h of interaction with T. gondii have little or no labeling for cadherin at points of cell-cell contact (thick arrow). Note that only uninfected cells show strong cadherin expression (thin arrow). Nuclei of cells and parasites labeled with DAPI, in blue. Bars, 20 μm

Figure 5

Cadherin profile in differentiated cultures after 24 h of T. gondii interaction. (A and inset) Mature (arrowhead) and young myotubes in fusion process with myoblasts (arrows) can be observed by phase contrast microscopy. (B and inset) By fluorescence microscopy, cadherin (in green) appears distributed throughout the myotubes, being more concentrated at the cell membrane during adhesion, while mature myotubes alone show more intense labeling at the extremities. (C) Interferential microscopy shows the adhesion of uninfected myoblasts (arrowhead) with a mature infected myotube (thick arrows). (D) Confocal microscopy analysis shows that infected myoblasts do not reveal cadherin labeling and more infected myotubes present weaker cadherin labeling (arrow). Observe that despite the weak labeling, in infected myotubes cadherin molecules appear to migrate to the point of contact with uninfected myoblasts (arrowhead). (E) Merge. Bars, 20 μm

Figure 6

Western blot analysis of cadherin protein expression. (A) Percent index variance analysis of the western blot showing cadherin expression: (C1) 2-day old uninfected cultures, (C2) 3-day old uninfected SkMC (control), (I) cultures after 24 h of interaction with T. gondii tachyzoites, and (P) parasites alone (confirming the absence of synthesis of cadherin by T. gondii tachyzoites). Quantitative analysis revealed only 10% reduction in the expression of cadherin between normal cultures, reaching values of more than 50% reduction in T. gondii infected SkMC after 24 h. Results are representative of three independent experiments. Student's T-test (*) p ≤ 0.05.

Figure 7

Profile of M-cadherin mRNA expression by SkMC experimentally infected with T. gondii. (A) The arbitrary values presented in the graph are based on the densytometric analysis of the PCR gel image shown in panel B, corresponding to 3, 12 and 24 h of infection. Light bars indicate uninfected control cells and black bars indicate the infected cells. (B) Polyacrylamide, silver stained gels for visualization of the amplified M-cadherin and GAPDH mRNAs (from top to bottom, respectively). Lanes 1, 3 and 5 show the profiles of negative controls and lanes 2, 4 and 6 the profiles of infected cells (3, 12 and 24 h, respectively). NC, negative PCR control. Molecular size markers are indicated to the left. Student's T-test (*) p ≤ 0.05.