Lycopersicon esculentum lectin staining of mouse retina by perfusion
Glycobiology is proving complicated to study, partly due to the sheer possible combinations of carbohydrates and their different conformations, and the fact that there is no blueprint for carbohydrates such as DNA provides for proteins. Lectins are not only worthy of being researched in their own right, but also useful tools for wider glycobiology research.
The term ‘Lectin’ is a more recent term to describe proteins and glycoproteins that specifically bind to carbohydrates can agglutinate cells and/or precipitate polysaccharides or glycoconjugates. ‘Lectins’ comes from the Latin verb ‘legere’ which means ‘to pick out, select or choose’, as lectins are selective and specific for their preferred carbohydrates to bind. Since the discovery of plant lectins, a wide range of lectins have been identified, many with vital functions, such as the Siglec family of lectins and their role in immune response regulation.
They were first described as far back as 1888 when an extract from the Caster oil plant agglutinated red blood cells. They were originally classed as haemagglutinins, or phytoagglutinins due to their plan origin – so many lectins still bear the name ‘agglutinin’. The ABO blood groupings resulted from noting that lectins did not agglutinate red blood cells from all samples, later shown to be inhibited by sugars, and was key evidence that cell surfaces had carbohydrates present.
Lectins have now been isolated from a variety of plants, and are being used as tools to further our knowledge of the glycocalyx, and there are a few lectins of animal origin, such as the Roman snail, Helix pomatia being purified for research use. Lectins usually take their name from the Latin name or common name, from the species they were isolated from and may be abbreviated to their initials – Lectin isolated from Horse gram plants is Dolichos biflorus agglutinin, or DBA. Triticum vulgaris agglutinin is also known as Wheatgerm agglutinin, and generally shortened to WGA.
The binding of lectins does not modify the carbohydrate structure that they bind to, so they are different from the glycosidases and glycosyl transferase enzymes, although modified versions of these enzymes are starting to be commercialised and may be developed further in future to complement data from lectin binding.
The nominal specificity of a lectin is usually expressed as the simple monosaccharide that best inhibits its effect, however this is an over simplification of the lectin’s binding. Although the simple sugars can inhibit binding, the actual structure recognised can be much more complex and also spatially reliant. Sometimes it is possible to find combinations of lectins that can separate different oligosaccahrides, even though there are identical sugar compositions. For specific binding preferences of lectins, micro array data may be available.
For agglutination, lectins must have more than one carbohydrate binding site; lectins are often made up of several subunits, these can sometimes be dissociated to produce isolectins, as in the case of Griffonia simplicifolia I, isolectin B4 that has a narrower binding preference that it’s parent lectin.
Lectins can be used for a wide range of applications in much the same scope as primary antibodies, however for many applications such as immunohistochemistry, western blotting, flow cytometry and ELISA etc, the use of a labelled lectin is preferred, as there is no ubiquitous secondary antibody that can be applied – they must be a specific anti-lectin as the structure of lectins as a group is widely varied.
Lectins can be used for affinity purification of glycosylated proteins, as fungi, bacteria and viruses use a much broader range of carbohydrates than eukaryotic cells. These present potential targets, such as for vaccine production or separation of fucosylated antibodies for therapeutic antibodies. Glycoproteins of interest can be passed into a column of agarose bound lectin, washed, then eluted using a competing solution of the relevant carbohydrate. These columns can be reused and refreshed a number of times.
Lectins have a place in neurobiology studies as there are a number of lectins that can be used as anterograde and/or retrograde tracers.
Systems such as the lectin based GlycoSense™ Array Kit allow for fast throughput simple glycan profiling without the need for mass spectrometry or chromatography, expensive and time-consuming techniques commonly used to assess samples due to the complexity of glycobiology.
Lectins are wonderful tools for observing glycosylation changes associated with cell behaviour, development and disease. Some lectins can also induce mitogenesis. Concanavalin A is known to trigger mitosis in T cells, most likely cross linking the TCR via it’s carbohydrates and bypassing the need for interaction with the MHC. Erythrina cristagalli lectin coated plates can be used for enrichment of NK cells.
In nature, plant lectins seem to offer a defence against some insect pests or invasive organisms offering potential benefit to agriculture.
Recombinant lectins are also becoming available, manufactured to fulfil needs in biopharmaceutical, diagnostic, research, cosmetic and food industry requirements. These lectins tend to be named after their sugar specificity rather than the species of origin. GlycoSelect is a company specialising in producing recombinant lectins.
Select Examples of Lectin Applications
A variety of lectins have also proved useful for coronavirus research permitting study of the viral envelope glycans and their role in viral entry to cells.