Recent Advances in Molecular Basis of Adhesion of Acanthamoeba to Human Tissues

As the understanding of the molecular basis of adhesion of Acanthamoeba to the human tissue plays a vital role in the disease pathogenesis such as AK, GAE, and wound infections, researchers have probed the adhesion molecules in vast details. Human leukocytic a integrins, CDl-а and CD11-b are known to help tight adherence prior to transendothelial migration and the ensuing entry into the blood vessel. Recently, with the help of bioinformatics computational tools molecules like a cell surface a integrin homolog on Acanthamoeba spp. termed integrin-а FG-GAP repeat-containing protein has been identified [152]. This amebal protein has been proposed for its interactions with the endothelium to initiate the wwadhesion in

TABLE 23.1 Novel hypothetical proteins that have structural homology with human cell adhesion proteins. The template-based models were developed by SWISS model database (data not shown) on submitting the Acanthamoeba proteins sequences, which were found to be resembling human cell adhesion (second column)

mediating interendothelial passage to cross the BBB. We investigated the ameba genome and proteome databases to explore the amebal counterparts of human adhesion molecules to understand the interaction of Acanthamoeba trophozoites with human tissues like endothelium, basement membranes, and connective tissue matrix.

Use of bioinformatics computational tools in providing evidence of novel adhesion molecules in Acanthamoeba spp. Of various methods used in elucidating the existence of adhesion molecules in Acanthamoeba spp., bioinformatics computational tools can be of immense help in exploring adhesion molecules. A list of known adhesion molecules expressed by human leukocytes and tissues can be searched for similar molecules in Acanthamoeba genome and proteome databases. Additionally, the level of expression of the molecules by Acanthamoeba trophozoites can also be examined by the use of the transcriptomics databases. Recently, the use of these bioinformatic computational tools and three-dimensional structural homology have helped discover novel voltage-gated ion channels [153,154], cell surface GPCRs [155], and hinted toward novel adhesion molecules [152].

A list of Acanthamoeba hypothetical and putative proteins (Table 23.1), the details of which are available from AmoebaDB.org NCBI and UniprotKB, are selected and explored at SWISS model database for structural homology with the human leukocyte adhesion proteins. The results show that some proteins previously designated as hypothetical and putative proteins turned out to have structural homology with human adhesion proteins selectins, integrins, and actins (Table 23.1). These adhesion proteins are well known in human leukocytes to promote adhesion with the endothelium and cell matrix. The proteins that were found to have homology w'ith parallel proteins in Acanthamoeba include molecules like selectins and integrins is shown in (Figure 23.5). This method of discovery of adhesion molecules is expected to open the investigations into these previously reported proteins with unknown functions. This line of research can help establish the significance of the discovered adhesion molecules in Acanthamoeba adhesion to human cells like corneal epithelium, endothelium lining the BBB, cerebral neurons, and squamous epithelium of skin.

FIGURE 23.5 Methodology that uncovered selectins- and integrin-like proteins in Acanthamoeba spp. (a and b). The amino acid sequence of ACA1_369840 was submitted to SWISS model database that identified it to be an E-selectin (a). Similarly, ACA1J88330 was identified as lntegrinAlpha-2 Beta-3 (b). Both these proteins in humans are established adhesion molecules.

Future Perspectives

Acanthamoeba is a genus of free-living ameba (FLA), which amount to nearly 25 recognized species. Of these, A. castellanii has the potential to cause amebic keratitis (AK), granulomatous amebic encephalitis (GAE), and cutaneous acanthamoebiasis in human populations, especially individuals with suppressed immune functions. Deciphering the role of a diverse list of Acanthamoeba adhesion molecules in detail through the use of contemporary imaging and computational tools is vital for the elucidation of its pathogenesis and subsequent development of novel approaches for its control and prevention. Direct visualization of Acanthamoeba trophozoites, with adhesion molecules tagged with fluorophore, in animal models with induced AK and GAE helps understand the factual role of the adhesion molecules in the disease process. Bioinformatics computational tools are promising in providing clues toward adhesion molecules of interests. The future use of miRNA and siRNA to knockout the genes discovered by computational methods will be instrumental for validating their significance in the biology of Acanthamoeba spp.