The SYK tyrosine kinase: a crucial player in diverse biological functions

Abstract

Spleen tyrosine kinase (SYK) is known to have a crucial role in adaptive immune receptor signalling. However, recent reports indicate that SYK also mediates other, unexpectedly diverse biological functions, including cellular adhesion, innate immune recognition, osteoclast maturation, platelet activation and vascular development. SYK is activated by C-type lectins and integrins, and activates new targets, including the CARD9-BCL-10-MALT1 pathway and the NLRP3 inflammasome. Studies using Drosophila melanogaster suggest that there is an evolutionarily ancient origin of SYK-mediated signalling. Moreover, SYK has a crucial role in autoimmune diseases and haematological malignancies. This Review summarizes our current understanding of the diverse functions of SYK and how this is being translated for therapeutic purposes.

Figure 1. General mechanism of SYK-mediated signalling

a ∣ Recruitment of SYK or ZAP-70 to plasma membrane receptors through binding of the tandem SH2 domains of SYK/ZAP-70 to two phosphotyrosine residues in the receptor complex. The two phosphorylated tyrosines are either within a single ITAM motif or in two hemITAMs on two separate receptor peptide chains. The ITAM motifs are either present in receptor-associated transmembrane adaptors or in the cytoplasmic tail of the receptor chain itself. b ∣ General scheme of signal transduction through SYK. The signal transduction is mostly initiated by phosphorylation of ITAM tyrosines by SRC-family kinases. Recruitment of SYK to dually phosphorylated ITAMs triggers activation of SYK and its direct binding to members of the VAV and PLCγ families, the p85α subunit of PI3-kinases (PI3K), as well as the SLP76/SLP65 adaptors. These direct binding partners activate downstream signaling components which eventually trigger various cellular responses. The SYK-mediated signaling pathways are also regulated by several feedback-mechanisms such as the phosphorylation of ITAM tyrosines by SYK or the regulation of direct SYK binding partners by further downstream molecules. SYK activation by hemITAM-containing receptors likely proceeds through similar mechanisms.

Figure 2. Structural basis of SYK activation

a ∣ Three different states of SYK activation. SYK is autoinhibited in its resting state, due to the binding of interdomain A (“A”) and interdomain B (“B”) to the kinase domain, in particular, the C-terminal end. This autoinhibited conformation can be suspended by binding of the two SH2 domains to dually phosphorylated ITAMs or by phosphorylation of linker tyrosines in interdomain A or B. There is a logic “OR” relationship between those two activation mechanisms. b ∣ The domain structure of SYK with tyrosine residues shown to be sites of autophosphorylation. Proteins shown to bind to phosphorylated tyrosines are indicated above the tyrosine residues. p85α, the 85 kDa PI3K regulatory subunit. c ∣ Combined mechanism of SYK activation. Prior to activation, SYK is held in an autoinhibited conformation by interactions between interdomains A and B and the kinase domain. Initial phosphorylation of ITAM tyrosines by SRC-family kinases provides docking sites for the SYK SH2 domains inducing conformational changes and kinase activation. SYK activation results in autophoshorylation on multiple residues, leading to release from the ITAM and sustained SYK activation. SYK then recruits direct binding partners to its phosphorylated tyrosine residues, triggering downstream signaling. Phosphorylation of ITAM tyrosine by SYK provides a positive feedback loop. Amino acid numbers correspond to the sequence of mouse SYK. Phosphorylation of the linker tyrosines are highlighted in orange colour.

Figure 3. The role of SYK in integrin and PSGL1 signalling

a-c ∣ Mechanism of integrin-mediated SYK activation. a ∣ In the original, ITAM-independent model, integrins activate SYK without phosphorylated tyrosine binding to the SYK SH2 domains. The interaction is mediated by the intracellular domain of the integrin β-chain and the non-phosphorylated tyrosine-binding surface of the N-terminal SH2-domain of SYK. b ∣ In the ITAM-mediated model, integrin ligation triggers SRC-family-mediated phosphorylation of ITAM-bearing transmembrane adaptor molecules (DAP12 and FcRγ), leading to recruitment of SYK through its tandem SH2 domains. This signalling likely involves DAP12- and/or FcRγ-associated transmembrane receptors. c ∣ In the combined model, the non-phosphorylated tyrosine-binding surface of the SYK SH2 domains bind to the C-terminal end of the integrin β-chain while the two SH2 domains bind to the phosphorylated ITAM tyrosines. d ∣ SYK-mediated signalling by PSGL1. Ligation of leukocyte PSGL1 triggers phosphorylation of the ITAM tyrosines of ERM family proteins which recruit SYK via its tandem SH2 domains. A supposedly parallel mechanism under flow conditions leads to phosphorylation of the ITAM tyrosines of the DAP12 and FcRγ transmembrane adaptors by the FGR tyrosine kinase, leading to recruitment of SYK through its tandem SH2 domains. SYK activation triggers transcriptional changes and inside-out activation of LFA1.

Figure 4. SYK-dependent innate pathogen- and damage-recognition pathways

a ∣ Innate immune receptors coupled to SYK or SYK-related molecules sense foreign pathogens and tissue damage. Fungal pathogens and M. tuberculosis engage CLEC7A (Dectin-1), CLEC6A (Dectin-2) and CLEC4E (Mincle); Dengue virus activates CLEC5A; necrotic cells activate the mammalian CLEC9A and CLEC4E, and the Drosophila Draper receptors. The ligand-binding chains of those receptors either have a hemITAM or a complete ITAM, or associate with the ITAM-bearing transmembrane adaptors FcRγ or DAP12. All of these receptors activate SYK-family kinases (SYK or SHARK) by SH2-mediated recruitment to phosphorylated tyrosines. The kinase responsible for phosphorylation of the Draper ITAM is the Drosophila SRC-family kinase SRC42A. b ∣ Mechanism of pro-inflammatory responses triggered by recognition of fungal pathogens by C-type lectins. CLEC7A binds SYK directly through an intracellular hemITAM, whereas CLEC6A and CLEC4E engage SYK through the ITAM-containing FcRγ adaptor. Pathogens trigger SYK-mediated phagocytosis, cytokine and chemokine production, and ROS generation. SYK-mediated NF-κB signalling is controlled by the CARD9–BCL10–MALT1 complex. Additionally, SYK is required for IL-1β processing by the NLRP3 inflammasome, which is activated by SYK-dependent ROS production, potassium ion (K⁺) efflux and presumably additional unknown danger signals (“?”).

Figure 5. Non-immune functions of SYK

a ∣ The role of DAP12 and FcRγ in osteoclasts. In addition to a RANK–RANK ligand interaction, osteoblasts also express yet unidentified ligands that engage a putative FcR-associated receptor (most likely OSCAR or PIR-A) on the osteoclast surface. Osteoclasts also express putative DAP12-associated receptors (most likely TREM2) which bind to yet unidentified ligands that are not expressed by osteoblasts. Both FcRγ and DAP12 promote osteoclast development and function through ITAM-mediated SYK activation and the PLCγ2-Calcineurin-NFATc1 pathway. Inset: Mechanism of osteoclast development. Osteoclasts develop from myeloid progenitors re-programmed by osteoblast-derived RANK ligand, leading to biochemical differentiation. Pre-osteoclasts then fuse to generate mature multi-nucleated osteoclasts and resorb the bone surface. αVβ3 integrins are required for sealing the resorption area. b ∣ SYK-mediated signaling in platelets. Collagen activates the FcRγ-associated GpVI receptor on platelets and triggers Syk activation by an ITAM-mediated manner. The snake venom rhodocytin, the endogenous podoplanin and possible other platelet agonists activate the CLEC2 receptor which recruits SYK to phosphorylated tyrosine in the CLEC2 hemITAMs at a 2:1 stoichiometry. Fibrinogen engages the αIIbβ3 integrin which associates with the ITAM-containing FcγRIIA receptor. This complex likely activates SYK through both conventional ITAM-mediated SYK activation pathways and by binding of the integrin β-chain to the N-terminal SH2-domain of SYK in a phosphotyrosine-independent manner. All three cases of SYK activation lead to platelet activation through PLCγ2. c ∣ The role of SYK in separation of blood and lymphatic vessels. Lymphatic vessels express podoplanin which trigger platelet activation and aggregation. This and possible other mechanisms (“?”) lead to closing off any blood-lymphatic shunts. The mechanism of podoplanin-mediated platelet activation involves the CLEC2 receptor which activates SYK through hemITAM phosphorylation, as well as SLP76 and PLCγ2 acting downstream of SYK.

Figure 6. The Role of SYK in malignancies of the B-cell lineage

SYK is required for various processes of B-cell development, including transition from pro-B to pre-B cells, proliferation and survival of pre-B cells, as well as the survival of mature B-cells. When other transforming factors are present, loss of SYK in pro-B cells may contribute to development of pro-B cell ALL, possibly by blocking further differentiation. Deregulated or constitutive activation of SYK is able to transform pre-B cells to pre-B cell ALL. In addition, pre-BCR-induced SYK activation is required for transformation of pre-B cells to pre-B cell ALL by other transforming factors (e. g. c-MYC). BCR-mediated tonic activation of SYK, as well as other SYK-dependent but BCR-independent signals are required for survival of mature B-cell lymphomas and B-lineage CLLs.