The Role of Sialic Acid-Binding Receptors (Siglecs) in the Immunomodulatory Effects of Trypanosoma cruzi Sialoglycoproteins on the Protective Immunity of the Host

Leishmania (Leishmania) amazonensis and Leishmania (Leishmania) infantum sialic acids enhance macrophage infection

Leishmaniases affect millions of people around the world, caused by Leishmania parasites. Leishmania are transmitted by female sandflies from Phlebotominae subfamily during their blood meals. In mammals, promastigotes are phagocytosed mainly by macrophages, differentiate into amastigotes and multiply. For entry and survival in macrophages, Leishmania uses virulence factors such as surface glycoconjugates. Sialic acids (Sias) are found in terminal portions of glycoconjugates and play important roles in human pathogens. The importance of Sias was explored only in L. (L.) donovani, associated with visceral leishmaniasis in Africa, Asia and Europe. Thus, the aim of this study was to characterize Sias of Leishmania (L.) amazonensis and Leishmania (L.) infantum, related to cutaneous and visceral leishmaniasis in South America, respectively. For that, we analyzed by HPLC-FLD the Sias of promastigotes of L. (L.) amazonensis LV79 and two L. (L.) infantum strains, and of L. (L.) amazonensis axenic amastigotes and amastigotes from paw lesions of infected mice. To evaluate Sias importance in promastigotes, we treated stationary phase parasites with sialidase and infected murine and human macrophages. We detected N-Acetylneuraminic Acid in promastigotes of all strains, with greater abundance in L. (L.) infantum. We identified N-Acetylneuraminic Acid and N-Glycolylneuraminic acid in amastigotes recovered from paw lesion, but only N-Acetylneuraminic Acid in axenic amastigotes. Promastigotes treated with sialidase infected less macrophages than parasites displaying total Sias. Our results demonstrate that Sias vary between Leishmania species and between L. (L.) amazonensis life stages and plays an important role in macrophage infection by L. (L.) amazonensis and L. (L.) infantum.

The Role of Sialic Acids in the Establishment of Infections by Pathogens, With Special Focus on Leishmania

Carbohydrates or glycans are ubiquitous components of the cell surface which play crucial biological and structural roles. Sialic acids (Sias) are nine-carbon atoms sugars usually present as terminal residues of glycoproteins and glycolipids on the cell surface or secreted. They have important roles in cellular communication and also in infection and survival of pathogens. More than 20 pathogens can synthesize or capture Sias from their hosts and incorporate them into their own glycoconjugates and derivatives. Sialylation of pathogens’ glycoconjugates may be crucial for survival inside the host for numerous reasons. The role of Sias in protozoa such as Trypanosoma and Leishmania was demonstrated in previous studies. This review highlights the importance of Sias in several pathogenic infections, focusing on Leishmania. We describe in detail the contributions of Sias, Siglecs (sialic acid binding Ig-like lectins) and Neuraminidase 1 (NEU 1) in the course of Leishmania infection. A detailed view on the structural and functional diversity of Leishmania-related Sias and host-cell receptors will be provided, as well as the results of functional studies performed with different Leishmania species.

The Putative TCP-1 Chaperonin Is an Important Player Involved in Sialic Acid-Dependent Host Cell Invasion by Toxoplasma gondii

Host cell invasion by Toxoplasma gondii is crucial for the survival and proliferation of parasite. The process of T. gondii tachyzoite invasion requires interaction between parasite proteins and receptors on the surface of host cells. Sialic acid is one of the important receptors for host cell invasion by T. gondii. However, the parasite-derived proteins interacting with sialic acid have not been well characterized. In this study, a novel protein named putative TCP-1 chaperonin (TGME49_318410) in T. gondii (TgTCP-1) was targeted and characterized. TgTCP-1 protein colocalized with MIC3 protein, which could be secreted from T. gondii tachyzoites, and this protein showed a specific binding activity to sialic acid, and DC and Vero cells in vitro. The binding of TgTCP-1 protein to DC and Vero cells were inhibited by either pre-incubation with free sialic acid or neuraminidase treatment of the cells. Moreover, a significant reduction of T. gondii invasion in Vero cells was observed after pre-incubation of the cells with recombinant TgTCP-1 protein. These results illustrated that TgTCP-1 is an important molecule involved in sialic acid-dependent host cell invasion by T. gondii.

Sialic acids: biomarkers in endocrinal cancers

Sialylations are post translational modification of proteins and lipids that play important role in recognition, signaling, immunological response and cell-cell interaction. Improper sialylations due to altered sialyl transferases, sialidases, gene structure and expression, sialic acid metabolism however lead to diseases and thus sialic acids form an important biomarker in disease. In the endocrinal biology such improper sialylations including altered expression of sialylated moieties have been shown to be associated with disorders. Cancer still remains to be the major cause of global death and the cancer of the endocrine organs suffer from the dearth of appropriate markers for disease prediction at the early stage and monitoring. This review is aimed at evaluating the role of sialic acids as markers in endocrinal disorders with special reference to cancer of the endocrine organs. The current study is summarized under the following headings of altered sialylations in endocrinal cancer of the (i) ovary (ii) pancreas (iii) thyroid (iv) adrenal and (v) pituitary gland. Studies in expression of sialic acid in testis cancer are limited. The future scope of this review remains in the targeting of endocrinal cancer by targeting altered sialylation which is a common expression associated with endocrinal cancer.