Galectins that include tandem repeats of two homologous carbohydrate recognition domains (CDRs) in a single polypeptide chain with up to 70 amino acids are known as tandem-repeat Galectins. The tandem-repeat Galectin family consists of five members, and it is a novel class of Galectins: Gal-8 influences autoimmune diseases like rheumatoid arthritis and lupus erythematosus and regulates tumor progression; Gal-9 controls allergic inflammation and Th1/Th17-mediated autoimmunity and has prognostic value in certain tumor types, and Gal-12 is crucial to adipocyte physiology. Gal-4 is related to inflammatory bowel disease and functions as a pro-inflammatory or pro-apoptotic mediator.
Fig.1 Tandem-repeat Galectin and mode of self-association. (Ayona, et al., 2020)
Galectin-4 is a tandem-repeat Galectin, consisting of two CRDs joined by a junction. It has been demonstrated that Galectin-4 contributes to inflammatory bowel disease. Galectin-4 induces CD4+ T cells from colitis-affected animals to generate the inflammatory cytokine IL-6, which accelerates the development of colitis. Galectin-4 antibody treatment prevents the development of this condition in the intestines of mice experiencing mild intestinal inflammation. Galectin-4 has been demonstrated to have anticancer effects and suppress colorectal cancer, which is promoted by its knockdown. Inflammatory precursor lesions of colorectal cancer have been found to have lower levels of Galectin-4 expression.
A similar mRNA that encodes the novel Galectin Galectin-6 was discovered while researching the mouse colonic mRNA for Galectin-4. The connection region between the two CRDs present in Galectin-4 lacks a 24 amino acid extension, despite the two Galectins sharing 83% amino acid similarity and having two CRDs. Galectin-4 and Galectin-6 are only expressed in the epithelial cells of the adult and embryonic gastrointestinal tracts.
Galectin-8 is a tandem-repeat Galectin, which means that two CRDs are connected by just one junction. It can exist in two splice forms, each of which has a different junction length. There are a total of six potential isoforms, three with two CRDs and three with one. Galectin-8 is expressed in a variety of tumor cells as well as the liver, kidney, myocardium, lung, and brain. Galectin-8 causes cysteine-mediated apoptosis and promotes apoptosis in T cells via producing the death factor Fas ligand. Phosphatidylserine exposure in the extracellular bilayer, which typically takes place during apoptosis activation, can be caused by Di-Galectin-8. It could possibly be linked to a number of different potential disease processes.
The liver, small intestine, and thymus are the primary organs where Galectin-9 is expressed. Galectin-9 has been discovered in lipid rafts, much like Galectin-4, another double CRD galactoglucan. T cells are known to generate the powerful eosinophil chemotactic protein known as human Galectin-9. Additional research has revealed that the N- and C-terminal CRDs of Galectin-9 interact with ligands on the eosinophil membrane that are identical to one another or that are linked to one another. Galectin-9's crucial function in regulating the Th1 response in vivo. Galectin-9 is also produced by many types of tumor cells and may be crucial for tumor immunity by controlling immune cell survival, proliferation, and migration both inside the tumor cell itself and in the surrounding tissue.
Galectin-12 is a tandem Galectin with two CRDs; the N-terminal CRD has a traditional lectin fold with two -folds, whereas the C-terminal CRD is less like the classical CRD. The expression of Galectin-12, which is essential for adipocyte renewal, is mostly found in adipose tissue. When preadipocytes grow into adipocytes, their mRNA is markedly elevated. Adipocyte development was significantly hindered by RNA interference-mediated downregulation of endogenous Galectin-12 expression. Future research may show that Galectin-12 is crucial to the formation of adipose tissue.
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