Trypanosoma cruzi extracellular amastigotes and host cell signaling: more pieces to the puzzle

Abstract

Among the different infective stages that Trypanosoma cruzi employs to invade cells, extracellular amastigotes (EAs) have recently gained attention by our group. This is true primarily because these amastigotes are able to infect cultured cells and animals, establishing a sustainable infective cycle. EAs are thus an excellent means of adaptation and survival for T. cruzi, whose different infective stages each utilize unique mechanisms for attachment and penetration. Here we discuss some features of host cell invasion by EAs and the associated host cell signaling events that occur as part of the process.

Keywords: cell invasion; extracellular amastigotes; mevalonate kinase; protein kinase D; signaling.

FIGURE 1

Trypanosoma cruzi G strain amastigotes interact with microvilli on the surface of HeLa cells. Parasites (colored in red) imaged by field emission-scanning electron microscopy, attach to HeLa cell surface microvilli. Magnification bar = 5 μm.

FIGURE 2

Extracellular amastigotes (EAs) were incubated for 1 h with (A) Vero or (B) CHO cells transfected with a GFP tagged PKD (green) and stained with phalloidin-TRITC (actin) and DAPI (cells nuclei). Arrows show EA recruiting PKD at the invasion sites. Confocal microscopy followed by Huygens Surface Rendering (www.svi.nl). Magnification bars = 4 μm (A) and 5 μm (B).

FIGURE 3

Proposed model for EA signaling. EAs secrete factors such as P21 and MVK that aid in EA uptake by cells in culture. EAs also present specific glycoproteins (i.e., Ssp-4) important for attachment and penetration while other EA glycoproteins (i.e., amastins-Am) may negatively modulate amastigote invasion. From the cellular perspective, lipid rafts are essential to parasite internalization in both macrophages and epithelial cells. Furthermore, the mannose receptor of certain cells may recognize mannose residues expressed by EAs which also participate in EA uptake. Upon contact, EAs recruit PKD, actin (F-actin), and other actin-binding proteins (ABPs) such as cortactin (Cort), gelsolin, and α-actinin. Additionally, EAs likely activate Src-family kinase (SFK), PKC, and Rac1 signaling in host cells. The phosphatidylinositol 3-kinases (PI3k) pathway may also be involved in EA internalization. Cellular heparan sulfate and fibronectin are also important in the process of EA internalization (extracellular matrix components – ECMs). Red and green arrows: negative and positive modulation of the invasion, respectively; blue arrows: host protein activation; black arrows: recruitment. P, phosphorylation; n, nucleus; k, kinetoplast; MVK, mevalonate kinase; MR, mannose receptor. See text for more details.