Biomedical Engineering Reference
In-Depth Information
Guévremont et al., 2004; Ochieng et al., 1994). The single CRD is mainly described to have
an increased affinity for different carbohydrates such as
N
-acetyllactosamine, the
glycoprotein asialofetuin or glycans presented on endothelial cells but to have less biological
activity as it looses the ability to form oligomers. This reveals the possible regulatory
function of galectin-3 cleavage (Dam et al., 2005; Dumic et al., 2006; Ochieng et al., 1998a;
Shekhar et al., 2004). In terms of this regulation it is suggested that the single galectin-3-CRD
binds with high affinity to glycans on cell surfaces thereby blocking these interaction
partners for full-length galectin-3 binding. After this blockage the full-length protein cannot
perform its physiological functions anymore. In this way galectin-3 cleavage could act as
down-regulation of galectin-3 function (John et al., 2003; Shekhar et al., 2004).
2.3.4 Galectin-8: Several isoforms of a tandem-repeat galectin
The specific properties of galectin-8 are also implied in its structure and the different
isoforms arising from it. At least 6 different isoforms are described so far of which some
only consist of the N-terminal CRD with an extension and others consist of both CRDs
linked by different hinge domains (Bidon et al., 2001; Delgado et al., 2011; Zick et al., 2004).
The two galectin-8 CRDs show approximately 35% sequence similarity but reveal different
fine specificity for glycan structures. Therefore galectin-8 can act as “hetero-bifunctional
crosslinking agent” (Zick et al., 2004). The length and structure of the linker domain has
direct influence on the biological function (Levy et al., 2006). Moreover the linker domain
regulates susceptibility to protease cleavage. It was for example shown that a long linker can
be cleaved by thrombin while shorter linker variants are not substrate for this protease
(Nishi et al., 2006).
3. Glycan binding assays for galectins
As galectins play a fundamental role in cell adhesion, cell signalling, inflammation, tumor
progression etc. there is an enormous interest in the evaluation of galectin-glycan
interactions regulating those functions.
3.1 Comparison of different common assays
Various assay set-ups have been designed to analyse the binding behaviour of different
galectins to specific glycan structures. Binding assays can be subdivided regarding the
presentation of the different binding partners: 1) the glycan structure is immobilised, 2) the
galectin is immobilised and 3) both binding partners are soluble.
The chosen assay format influences the data generated as each assay set-up has its own
advantages and disadvantages (Rapoport et al., 2008):
Assays in which one of the binding partners is immobilised raise the problem that the
amount of this ligand is not completely known. Moreover it is possible that side interactions
with the surface occur or that the conformation and flexibility of the bound partner differ
slightly from its soluble parameters. The natural oligomerisation of galectins is blocked after
immobilisation. Beside this the presentation of the immobilised binding partner is
multivalent which influences the binding (Sörme et al., 2004). This can be useful for the
glycan structures, as they are multivalently presented in nature as well, but not for galectins.
Examples for studies with immobilised glycans or glycoproteins are glycan arrays, ELISA
