Key Points
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Adaptor proteins are key intermediates in signal cascades downstream of several receptors. SLP76 (SH2-domain-containing leukocyte protein of 76 kDa) regulates signals from immunoreceptors, such as T-cell receptors, the high-affinity receptor for IgG (FcγRI), the low-affinity receptor for IgG (FcγRIII) and glycoprotein VI (GPVI), whereas SLP65 mediates B-cell-receptor signalling.
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In addition to immunoreceptors, SLP76 is also required for signalling by integrins in neutrophils and platelets. There are, however, key differences between SLP76 function in immunoreceptor versus integrin signalling.
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Translocation of SLP76 and SLP65 to the cell membrane is crucial for regulating the formation of multimolecular signalling complexes that link receptor signals to cellular responses. The mechanism(s) by which SLP76 and SLP65 are localized to the plasma membrane seem to differ.
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Recent studies have indicated that SLP76, SLP65 and other adaptors might function by enhancing cooperative binding of other signalling intermediates to form stable signalling complexes.
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The absence of SLP76 has profound effects on T-cell development as well as platelet, neutrophil and mast-cell function, whereas loss of SLP65 undermines B-cell development. In addition, absence of SLP76 results in mixture of blood and lymphatic vasculature.
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Future studies of SLP76 and SLP65 should focus on determining their spatial and temporal localization, specifically in comparison with other signalling intermediates. Lineage- and temporal-specific regulation of SLP76 and SLP65 expression will be required to understand their roles in mature T- and B-cell development and in the function of other haematopoietic lineages in vivo.
Abstract
SLP76 and SLP65 are adaptor proteins that lack intrinsic enzymatic activity but contain multiple protein-binding domains. These proteins are essential for signalling downstream of integrins and receptors that contain immunoreceptor tyrosine-based activation motifs. The absence of these adaptor proteins profoundly affects various lineages in the haematopoietic compartment and severely compromises vascular development, highlighting their importance as regulators of signalling cascades. In this Review, we discuss the role of SLP76 and SLP65 in several signalling pathways in haematopoietic cells, with an emphasis on recent studies that provide insight into their mechanisms of action.
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Acknowledgements
We thank M. Kahn, Division of Cardiology, Department of Medicine, University of Pennsylvania, for thoughtful discussions.
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Glossary
- Guanine-nucleotide-exchange factor
-
Proteins that activate low-molecular-mass GTPases, such as RHO-family GTPases and RAS, by stimulating the dissociation of GDP, and thereby promoting formation of the active GTP-bound state of these GTPases.
- Immunoreceptor tyrosine-based activation motif
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(ITAM). A sequence that is present in the cytoplasmic domains of the invariant chains of various cell-surface immune receptors, such as T- and B-cell receptors, the receptor for IgE (FcεR) and natural-killer-cell activating receptors. Following phosphorylation of their tyrosine residue, ITAMs function as docking sites for SRC homology 2-domain-containing tyrosine kinases and adaptor molecules, thereby facilitating intracellular-signalling cascades.
- Lipid rafts
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An area of the plasma membrane that is rich in cholesterol, glycosphingolipids, several signalling proteins — such as SRC-family kinases, RAS and LAT (linker for activation of T cells) — and glycosylphosphatidylinositol-anchored proteins. These domains are also known as glycolipid-enriched membrane domains, detergent-insoluble glycolipid-enriched membranes and detergent-resistant membranes.
- Live-cell confocal imaging
-
A technique whereby cells expressing fluorescently tagged molecules can be visualized in real time. This technique offers temporal and spatial resolution (∼6 μm) of tagged proteins. Colocalization of multiple tagged proteins by confocal microscopy indicates the potential for direct or indirect interactions, but requires confirmation by more sensitive techniques, such as fluorescence resonance energy transfer.
- Fluorescence resonance energy transfer
-
A technique that is used to measure protein–protein interactions either by microscopy or flow cytometry. Proteins fused to cyan, yellow or red fluorescent proteins are expressed and assessed for interaction by measuring the energy transfer between fluorophores, which can only occur if proteins physically interact.
- Reactive oxygen intermediates
-
(ROIs). Toxic free-radicals that are produced in neutrophils by univalent reduction of oxygen through NADPH oxidase to superoxide anions. Superoxide anions can subsequently leave the cell through anion channels and can be converted to hydrogen peroxide by superoxide dismutases. Both superoxide and hydrogen peroxide can be used to generate other types of ROI.
- Allelic exclusion
-
In theory, every B cell has the potential to produce two immunoglobulin heavy chains and two immunoglobulin light chains, and every T cell can generate two different T-cell receptors (TCRs). In practice, however, a B cell produces only one immunoglobulin heavy chain and one immunoglobulin light chain and each T cell only one TCR α-chain and one TCR β-chain. The process by which the production of two different chains is prevented is known as allelic exclusion.
- β-selection
-
Successful rearrangement and expression of the T-cell receptor (TCR) β-chain, which can then pair with the pre-TCR α-chain and support signal transduction from the pre-TCR.
- Cre-loxP approach
-
A site-specific recombination system. Two short DNA sequences (loxP sites) are engineered to flank the target DNA. Expression of Cre recombinase leads to excision of the intervening sequence. Depending on the type of promoter, Cre can be expressed at specific times during development or in specific sets of cells.
- Anastomosis
-
The site of connection between two separate spaces, such that they intercommunicate. This can arise during normal embryonic development, during pathological processes or artificially through surgical means.
- B1 cells
-
Peripheral B cells exist as two populations, CD5+ B1 cells or CD5− B1 cells. Peritoneal B1a cells are thought to have a role not only in adaptive immunity but also innate immunity.
- T-cell-independent antigens
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Antigens, such as polysaccharides and lipids, that directly activate B cells without the requirement for T-cell help.
- Acute lymphoblastic leukaemia
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The most common cancer in children that results from oncogenic transformation in a lymphocyte precursor population. Malignant cells can have either a B- or T-cell phenotype.
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Koretzky, G., Abtahian, F. & Silverman, M. SLP76 and SLP65: complex regulation of signalling in lymphocytes and beyond. Nat Rev Immunol 6, 67–78 (2006). https://doi.org/10.1038/nri1750
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DOI: https://doi.org/10.1038/nri1750
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