Signal Transduction
The word ‘sialic’ is derived from the Greek sialos, meaning saliva, where sialic acids were first discovered. They are ninecarbon sugars.
SIGLEC
The SIGLECs (sialic acid binding Ig-like lectin proteins) constitute another subgroup within the superfamily of immunoglobulins. Its members include sialoadhesin (SIGL1), CD22 (SIGL2), CD33 (SIGL3), MAG (SIGLET4), and SIGL5– 11. They are characterized by the presence of a single N-terminal Ig-like V-set domain which has the characteristics of an I-type lectin16 that binds sialic acid (see also below, page 392). Anywhere between 1 and 17 Ig-like C2-set domains lie between the sialic acid binding site and the plasma membrane (Figure 13.4). Each SIGLEC has a preference for a specific type of sialic acid and for a specific type of linkage to the subterminal sugar. The cytosolic tails vary in sequence and length but most have conserved tyrosine residues within immunoreceptor tyrosine-based inhibition motifs (ITIMs) that are implicated
FIG 13.4 Structure of CD22/SIGLEC. (a) The extracellular domain of CD22 is composed of six Ig-like C2-set domains with an N-terminal Ig-like V-set domain that binds sialic acid. The cytoplasmic domain contains four ITIM motifs with tyrosine phosphorylation sites. One of these is recognized by one of the two SH2 domains of the phosphatase SHP-1. (b) Detail of the binding of N-acetylneuraminic (a sialic acid, green sticks) to the Ig-like V-set domain (mouse). (c) Structure of sialic acid. (2hrl17).
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Signal Transduction to and from Adhesion Molecules
in signalling functions. SIGLEC adhesion molecules are present in myeloid cells (granulocytes, lymphocytes, monocytes), Schwann cells, and placental trophoblasts. They are not expressed in Drosophila or Caenorhabditis elegans, probably representing a later adaptation of an ancient protein binding
Ig domain (see Figure 13.4 and Table 13.1).
Of the many SIGLECs, CD22, a single-span membrane protein which recognizes sialic acid linked to galactose and is expressed on cells of B-cell lineage, has attracted much attention. This is because it is an inhibitory coreceptor that down-modulates signalling through the B cell receptor (BCR). It does this by setting a threshold that prevents overstimulation, important in the maintenance of tolerance to some antigens. In mice, disruption of the CD22 gene gives rise to a hyper-responsive BCR and the animals exhibit an augmented immune response.18
CD22 binds to sialic acid residues that are attached to the B cell itself (in cis), as well as those that are attached to other cells that present antigens (in trans). Binding in cis keeps CD22 away from the BCR and enhances signalling.19 When this is prevented, for instance through binding to sialic acid residues
of an adjacent antigen-presenting cell or by binding directly to the engaged antigen receptor, it inhibits BCR signalling. The reason for this is that the activated BCR recruits Lyn kinase which phosphorylates the adjacent tyrosine residues in the three ITIM motifs of CD22.20 This in turn engages the tyrosine
Table 13.1 Human Siglec adhesion molecules
Human Siglecs |
Ig-C2 domains |
Expression |
|
|
|
Sialoadhesin/hSiglec-1 |
17 |
macrophage |
|
|
|
CD22/Siglec-2 |
7 |
B cell |
|
|
|
CD33/Siglec-3, |
2 |
monocyte, myeloid progenitor |
|
|
|
MAG/hSiglec-4, |
5 |
oligodendrocyte, Schwann cell |
|
|
|
hSiglec-5 |
4 |
monocyte, neutrophil, B cell |
|
|
|
hSiglec-6 |
3 |
B cell, placental trophoblast |
|
|
|
hSiglec-7 |
3 |
monocyte, NK cell |
|
|
|
hSiglec-8 |
3 |
eosinophil, basophil, mast cell |
|
|
|
hSiglec-9 |
3 |
monocyte, neutrophil, NK cell |
|
|
|
hSiglec-10 |
5 |
monocyte, NK cell |
|
|
|
hSiglec-11 |
5 |
macrophage |
|
|
|
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Signal Transduction
phosphatase SHP-1 which prevents BCR signalling by dephosphorylation of its own ITAM motifs (see also page 517).21
Junctional adhesion molecules (JAMs)
The JAMs constitute a further subgroup of the immunoglobulin superfamily. Four members, JAMs 1–3 and JAML1, are expressed in humans. They are single-span membrane proteins with two extracellular Ig-like domains, of which the N-terminal is a V-set and the C-terminal an I-set domain. They possess a PDZ binding motif which interacts with the adaptor protein ZO-1 (zonula occludens-1) (Figure 13.5). They contribute to the architecture of
tight junctions in epithelial and endothelial cells and are also expressed on lymphocytes, megakaryocytes, platelets and red blood cells. Both homoand heterophilic interactions have been reported. Importantly, they play a role in leukocyte transendothelial migration (see page 500).
Claudins
The claudins form a large group of adhesion molecules (21 members in human, designated Cldn-1–21). They have a molecular mass of 23 kDa, span the membrane four times, and have short cytosolic N- and C-terminal domains. They are members of an even larger family of adhesion molecules, the PMP22/ EMP/MP20/claudin proteins, all having the same membrane topology and a characteristic signature motif in the first extracellular loop (Figure 13.5).
FIG 13.5 Tight junction proteins: JAM, occludin, and claudin.
(a) Diagram illustrating a tight junction as present in endothelial and epithelial tissues. (b) The junctions are composed of three types of protein. JAM is a member of the immunoglobulin family of adhesion molecules, but claudin and occludin are not. Both of these span the membrane four times and have very short extracellular domains. Their interaction ensures close apposition of the two membranes. There are many variants of the claudins. The combination expressed in tight junctions determines the accessibility of the paracellular space. They all bind to ZO-1 (or ZO-2, ZO-3) which connects the junctional proteins to the actin cytoskeleton. (c) Domain architecture of junctional proteins. Cyan indicates PDZ or PDZ binding motif.
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Signal Transduction to and from Adhesion Molecules
The claudins contribute to the selectivity of paracellular transport (meaning transport between, not through, cells). Thus, they are the principle barrier-forming proteins of the tight junction and their expression pattern determines the effectiveness of barriers imposed by epithelial sheets. Epithelia may have high resistance, maintaining steep ionic gradients (such as in the distal nephron and urinary bladder) or they may be leaky, of low resistance, allowing the movement of large volumes of iso-osmotic fluid (as occurs in much of the gastrointestinal tract).22,23 The C-terminus contains
a PDZ motif which binds homologous motifs in adaptor proteins such as ZO-1, 2, and 3.24
Occludin
Occludin is a four-transmembrane-spanning protein of 60 kDa having a short N-terminal and a much longer C-terminal region both exposed in the cytosol. Only a single gene exists. It is a component of tight junctions in endothelial and epithelial cells (Figure 13.5). Its typical apical localization occurs through interaction with the ZO adaptor proteins, which are linked to
the actin cytoskeleton, but unlike the claudins it does not act as a regulator of paracellular transport.
Integrins
The integrins are the most dynamic and versatile of the adhesion molecules. They are composed of two subunits ( and ), linked non-covalently. There are at least 18 - and 8 -subunits and these can form at least 25 different integrin heterodimers (Figure 13.6). Depending on the particular combination, they may bind to either ICAMs, VCAM-1, or MadCAM (on mucosal cells). They
may also bind to components of the extracellular matrix such as collagen, fibronectin, laminin, or vitronectin and to the blood proteins fibrinogen, or von Willebrand factor.
The subunits of the integrins ( and ) are transmembrane glycoproteins consisting of a head, a stalk, and a short (40–60 amino acid) cytoplasmic region. Integrin 4, which is specialized to connect to the keratin cytoskeleton in hemidesmosomes (complexes at epithelial junctions with the extracellular matrix), has a more extended cytoplasmic region. The stalk is composed of a number of highly conserved domains as depicted in Figure 13.7). As will become evident, the positioning of the ‘hybrid domain’ within the stalk region of the-integrin subunit has a role in regulating the affinity for ligand. The head regions of -integrin subunits all have a 1 domain (also referred to as the I-like domain), while the head regions of -integrin subunits vary according to integrin type. They all contain a -propeller structure, but some have an additional I domain; (these integrins are indicated with an asterisk in Figure 13.6). Two examples of the different -subunits are presented in Figure 13.7
Claudins 3 and 4 act as receptors for the
bacterial endotoxin CPE produced by Clostridium perfringens. This causes a profound change in the structure of the
tight junctions of the enterocytes, followed by an increased paracellular leakage of water. The failure to reabsorb water in the colon results in diarrhoea.
Tight junctions are also referred to as zonula occludens (hence ZO). The Latin word occludens comes from occludere, meaning to lock up. Claudin is derived from the Latin claudere, meaning to close.
383