The primary function of Siglecs is to bind glycans containing sialic acids. These receptor-glycan interactions can be used in cell adhesion, cell signalling and others. The function of Siglecs is limited to their cellular distribution. For example, MAG is found only on
oligodendrocytes and
schwann cells whereas Sialoadhesin is localised to macrophages. Most Siglecs are short and do not extend far from the cell surface. This prevents most Siglecs from binding to other cells as mammalian cells are covered in sialic acid-containing glycans. This means that the majority of Siglecs only bind ligands on the surface of the same cell, so called
cis -ligands, as they are "swamped" by glycans on the same cell. One exception is Sialoadhesin which contains 16 C2-Ig domains, producing a long, extended protein allowing it to bind
trans-ligands, i.e. ligands found on other cells. Others, such as MAG, have also been shown to bind
trans-ligands.
Signalling The members of the siglec family are
paired receptors with opposing intracellular signaling functions. Due to their ITIM-containing cytoplasmic regions, most Siglecs interfere with cellular signalling, inhibiting immune cell activation. Once bound to their ligands, Siglecs recruit inhibitory proteins such as SHP phosphatases via their ITIM domains. The tyrosine contained within the ITIM is phosphorylated after ligand binding and acts as a docking site for
SH2 domain-containing proteins like SHP phosphatases. This leads to
de-phosphorylation of cellular proteins, down-regulating activating signalling pathways. Examples of negative signalling: • CD22 is found on B cells. B cells become active when the
B-cell receptor (BCR) binds to its cognate ligand. Once the BCR is bound to its ligand, the receptor auto-phosphorylates its cytoplasmic region (cytoplasmic tail). This leads to phosphorylation of the three ITIMs in CD22's cytoplasmic tail, leading to the recruitment of
SHP-1 which negatively regulates BCR-based cellular activation. This creates an activation threshold for B cell activation whereby transient activation of B cells is prevented. CD22 inhibition of BCR signalling was originally thought to be sialic acid-binding-independent, but evidence suggests α2,6 sialic acid ligands are required for inhibition. •
Siglec-7 is found on
Natural Killer cells (NK cells). Siglec-7 leads to cellular inactivation once bound to its sialic acid-containing cognate ligand and is found in high levels on NK cell surfaces. It is used in cell-cell contacts, binding to sialylated glycans on target cells leading to inhibition of NK cell-dependent killing of the target cell. Mammalian cells contain high levels of sialic acid and so when NK cells bind so called "self-cells", they are not activated and do not kill host cells. Siglec-14 contains an arginine residue in its transmembrane region. This binds to the ITAM-containing DAP10 and DAP12 proteins. When bound to its ligand, Siglec-14 leads to activation of cellular signalling pathways via the DAP10 and DAP12 proteins. Binding to these structures allows the macrophage to phagocytose these bacteria, clearing the system of pathogens. Siglec-7 is also used in binding to pathogens such as
Campylobacter jejuni. This occurs in a sialic acid-dependent manner and brings NK cells and
monocytes, on which Siglec-7 is expressed, into contact with these bacteria. The NK cell is then able to kill these foreign pathogens.
Knock-out studies Knock-out studies are often used to uncover the function proteins have within a cell. Mice are often used as they express
orthologous proteins of ours, or extremely similar homologues. Some examples of knock-out Siglecs include: • CD22: Walker & Smith conducted experiments with CD22 knock-outs and
deletion mutants to discern CD22's function. These mutant B cells did not infer any
autoimmune disease, but they did see an increased production of
autoantibodies due to the lack of BCR signalling inhibition, usually conducted by CD22. Autoantibodies are specific for self proteins and can harm the host. CD22 is normally up-regulated by lipopolysaccharide binding to
Toll-like receptors. The mutant B cells can not up-regulate the mutant protein and so become hyper-sensitive in the presence of lipopolysaccharide. This means that the B cells overproduce antibodies when antibodies would not normally have been produced. • MAG (Myelin-associated glycoprotein) is expressed on cells that form
myelin sheaths (schwann cells and oligodendrocytes) around
neurons. MAG binds to sialylated ligands on the neuron. Knock-out of MAG in the
peripheral nervous system leads to decreased myelination of neurons. Knock-out of MAG in the
central nervous system of mice does not appear to affect myelination, but the interaction between the myelin and the neuron does deteriorate with age. This leads to neurological defects as the
action potential can not pass so rapidly down the length of the
axon during neural stimulation. Removing the ligand for MAG, by knocking-out the GalNAc transferase gene required for ligand formation, has similar effects to that of the MAG knock-out mice ==Human/Primate Siglecs==