Promastigote-to-Amastigote Conversion in Leishmania spp.-A Molecular View

Abstract

A key factor in the successful infection of a mammalian host by Leishmania parasites is their conversion from extracellular motile promastigotes into intracellular amastigotes. We discuss the physical and chemical triggers that induce this conversion and the accompanying changes at the molecular level crucial for the survival of these intracellular parasites. Special emphasis is given to the reliance of these trypanosomatids on the post-transcriptional regulation of gene expression but also to the role played by protein kinases, chaperone proteins and proteolytic enzymes. Lastly, we offer a model to integrate the transduction of different stress signals for the induction of stage conversion.

Keywords: cell stress; heat shock proteins; post-transcriptional gene regulation; protein kinases; protein turnover.

Figure 1

Axenic stage conversion of L. donovani from promastigotes kept at 25 °C/pH 7.4. (A) to amastigotes at 37 °C/pH 5.5 (B) and chemically (radicicol)-induced amastigote-like forms (C). Samples were fixed and stained with DAPI (nuclei, blue) and anti-α-tubulin mAB (microtubuli, red). Fluorescence images were captured at a 100× magnification using an EVOS Autofluor microscope; overlays were implemented in Adobe Photoshop CS3. The bars represent 10 µm.

Figure 2

Schematic model of stage conversion through HSP90 quenching. Axenically grown promastigotes (A) as elongated, flagellated, highly proliferative cells are exposed to cell stress (increased temperature, acidified medium, iron depletion or ROS). This causes the sequestration of HSP90 and other foldosome components to denatured protein domains (B). The temperature-dependent shedding of HSP-containing exosomes and the phosphorylation of HSP90 by CK1.2 and MAPK1 adds to the reduction of free HSP90, which triggers conversion to the ovoid, non-motile, growth-impaired amastigote (C). Alternatively, the direct inhibition of HSP90 with radicicol or geldanamycin also depletes active HSP90 and causes a similar conversion to amastigote-like stages (D).