Downloaded from http://cshperspectives.cshlp.org/ on September 24, 2021 - Published by Cold Spring Harbor Laboratory Press Src-family and Syk Kinases in Activating and Inhibitory Pathways in Innate Immune Cells: Signaling Cross Talk Clifford A. Lowell Department of Laboratory Medicine, University of California, San Francisco, California 94143 Correspondence: [email protected] The response of innate immune cells to growth factors, immune complexes, extracellular matrix proteins, cytokines, pathogens, cellular damage, and many other stimuli is regulated by a complex net of intracellular signal transduction pathways. The majority of these path- ways are either initiated or modulated by Src-family or Syk tyrosine kinases present in innate cells. The Src-family kinases modulate the broadest range of signaling responses, including regulating immunoreceptors, C-type lectins, integrins, G-protein-coupled recep- tors, and many others. Src-family kinases also modulate the activity of other kinases, includ- ing the Tec-family members as well as FAK and Pyk2. Syk kinase is required for initiation of signaling involving receptors that utilize immunoreceptor tyrosine activation (ITAM) domains. This article reviews the major activating and inhibitory signaling pathways regu- lated by these cytoplasmic tyrosine kinases, illuminating the many examples of signaling cross talk between pathways. nnate immune cells, including macrophages, important roles in innate cells. They are not dis- Idendritic cells, granulocytes, and mast cells, cussed in detail in this article, but are reviewed function as the first line of defense against elsewhere in articles on the subject. pathogens. These cells use a dizzying array of There are eight members of the Src family; cell-surface receptors, which are connected to innate immune cells primarily express Hck, an equally complicated intracellular signal trans- Fgr, Lyn, and to a lesser extent, Src (Lowell duction network, to sense pathogen molecules 2004). The Syk-ZAP70 family has only two and then orchestrate the appropriate immune members and only Syk is found in innate cells. response. Among the intracellular signaling mol- Most innate cell types express the same spec- ecules that are most crucial for innate immune trum of kinases with some specific cellular dif- cells are the cytoplasmic tyrosine kinases. Two ferences. For example, mast cells express a majorkinasefamiliesthat operateintheproximal broader range of Src-family kinases than macro- intracellular signaling pathways in innate cells are phages or dendritic cells (Colgan and Hankel the Src-family kinases and the Syk-ZAP70 family. 2010). In general, Src-family and Syk kinases A third familyof kinases, the Tek family, also have tend to operate together in signaling pathways, Editors: Lawrence E. Samelson and Andrey S. Shaw Additional Perspectives on Immunoreceptor Signaling available at www.cshperspectives.org Copyright # 2010 Cold Spring Harbor Laboratory Press; all rights reserved. Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a002352 1 Downloaded from http://cshperspectives.cshlp.org/ on September 24, 2021 - Published by Cold Spring Harbor Laboratory Press C.A. Lowell with the Src-family being “upstream” or acti- of interaction and cross regulation. Together, vated first in response to pathogen detection. these pathways impinge on downstream factors, These enzymes then communicate downstream such as MAPK kinases, which have broad effects to Tec-family members. The Tec-family kinases on gene transcription; the Rac/Rho pathway to expressed in innate cells include Btk, Bmx, and modulate cytoskeletal function; the inositol Tec (Koprulu and Ellmeier 2009; Tohyama and trisphosphate (IP3), and diacylglycerol pathway Yamamura 2009). Additionally, Src-family kin- (DAG), which regulates Ca2þ entry into cells ases activate yet another family of PTKs, the and activation of various isoforms of protein FAK/Pyk2 tyrosine kinases, which play a major kinase C (PKC). Overall, the outline of the pro- role in integrin signaling (Hauck et al. 2000). totypical immunoreceptor pathway as described Though primarily studied in activating path- here is similar in both innate and adaptive im- ways, Src-family and Syk kinases also activate mune cells (Smith-Garvin et al. 2009; Kurosaki inhibitory signaling pathways (Nimmerjahn and et al. 2010). Ravetch 2008). In many situations, inhibitory signaling often overrides the activating signal. New Concepts in Immunoreceptor Signaling: Pathways can also be initiated at different times CARD9 and Receptor Diversity or rates. Finally, to add even more complexity, activating and inhibitory pathways often inter- Recent and exciting developments in the immu- act indirectly, for example, through the produc- noreceptor paradigm include recent progress in tion of cytokines and growth factors and not delineating how this pathway is connected to through direct intracellular biochemical inter- NF-kB and the demonstration that many innate actions; Hence the term signaling “cross talk,” immune receptors utilize the “immunoreceptor which now appears commonly in the literature pathway” even though they lack ITAMs and (O’Neill 2008; Ivashkiv 2009; Page et al. 2009). therefore are not technically immunoreceptors. The adapter protein CARD9, which con- tains a caspase-recruitment domain (CARD), OVERVIEW OF ACTIVATING PATHWAYS has now been shown to be the link between a variety of ITAM-containing receptors involved Classical Immunoreceptor Pathways in recognition of fungal and probably other In the prototypical immunoreceptor pathway, pathogen structures and NF-kB (Fig. 2) (Gross engagement of the receptor leads to activation et al. 2006; Gross et al. 2009). CARD9 is closely of Src-family kinases, which in turn phosphor- related to the lymphocyte protein CARMA-1, ylate immunoreceptor tyrosine-based activa- which forms a complex with the adapter pro- tion motifs (ITAMs) present on either the teins Bcl-10 and MALT1, and thus links the T- receptor or associated subunits (Fig. 1). This and B-cell receptors to the NF-kBpathway leads to recruitment of Syk, by binding of the (Rawlings et al. 2006). CARD9 forms the same Syk SH2 protein domain to the phospho-ITAM complex in innate cells. In lymphoid cells, residues, and activation of Syk allowing it CARMA-1 is activated by PKC isoforms (PKCu to phosphorylate downstream substrates. One in T-cells and PKCb in B-cells), which phos- of the enzymes activated downstream is phos- phorylate CARMA-1, resulting in a conforma- phoinositide3-kinase(PI3-kinase), which gener- tional change that allows it to interact with ates membrane-associated phosphatidylinositol IKK, leading to IkB turnover and NF-kB activa- (3,4,5)-triphosphate (PIP3). The FAK/Pyk2 tion. In myeloid cells, it remains unclear if kinases are activated directly via the Src-family CARD9 activation is directly downstream of kinases, where they contribute to downstream PKC activation (Hara and Saito 2009). Never- responses involving cell adhesion and migra- theless, it is clear that in the absence of tion. Though usually depicted as a linear signal- CARD9, receptors involved in fungal pathogen ing pathway with Src-kinases at the top and recognition (Dectin-1, Dectin-2) are unable to FAK/Pyk2 at the bottom, there are many points activate NF-kB, and more importantly, the 2 Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a002352 Downloaded from http://cshperspectives.cshlp.org/ on September 24, 2021 - Published by Cold Spring Harbor Laboratory Press Cytoplasmic Tyrosine Kinases in Innate Immune Cells Classical immunoreceptors Receptors co-opting ITAM signaling FcRγ Receptors DAP12 Receptors FcεR, FcγRs TREMs Dual Receptors Dectin-2 MDL-1 Hemi-ITAM Receptors Integrins Mincle CD200R3 Dectin-1 IL-3R DCAR Siglec-H CLEC2 PSGL-1 γ PIR-A MAIR-II CLEC9A IFN R SIRP-β RANK FcRγ DAP12 DAP12 FcRγ +– + – PIP3 PIP2 SLP-76SSLLLP-7P-P P Y Y P P Y Y P Y P SOS PI3-KPPI3-K P Y Y P Y Y Syk Syk Syk Syk P P LAT/NTAL Y Y P P Y Y P Y γ P Y Y Y Y SFK SFK PLCPLC Vav Tec Ks Fak/Pyk2 CARD9 Ras DAG IP3 RasGRP RhoRho GTPase WASPW NF-kB pathway PKC 2+ MAPKs Ca Actin polymerization Cytokine secretion Cytokine secretion NFAT proliferation gene transcription Migration degranulation Figure 1. Cytoplasmic tyrosine kinases in the activating signaling pathways utilizing ITAM-containing adapters. Examples of immunoreceptors, hemi-ITAM C-type lectin receptors, and nonimmunoreceptors that utilize ITAM-signaling adapters and the cytoplasmic tyrosine kinases (indicated as shaded molecules) discussed in this article are shown. “Classical immunoreceptors” refers to those signaling molecules that are directly coupled to ITAMadapters FcRg (group shown on the left) or DAP12 (group shown on the right) through transmembrane charged residues, shown as “þ” and “2” in the figure. These immunoreceptors consist of immunoglobulin superfamily-containing proteins (such as the Fc receptors, PIR-A, or the TREMs) or the C-type lectin receptors (Dectin-2, Mincle, or MDL-1). In some cases, receptors may utilize either signaling adapter (see Hamerman et al. 2009). Examples of the C-type lectin receptors that have ITAM-like sequences as imbedded domains within their cytoplasmic tails are Dectin-1, CLEC2, and CLEC9A. The sequences with the ITAM domain of these receptors differs in that the membrane distal tyrosine resides in a YxxxL sequence (or YxxxI for mouse), as opposed to the traditional YxxL ITAM sequence found around the proximal tyrosine. As a result, the membrane distal tyro- sine is dispensable for signaling, leading to the designation of these receptors as “hemi-ITAM” molecules (Ker- rigan and Brown 2010). For a more complete list of FcRg- and DAP12-associated receptors, see Lanier (2009), Graham and Brown (2009), and Kanazawa (2007). Not shown is the human FcgRIIA receptor, which is unique among the Fc receptors for having an ITAMsequence directly imbedded in its cytoplasmic tail (see Nimmerjahn and Ravetch 2008). Tothe right are shown examples of receptorsthat link to orco-opt the ITAMpathway, with the best example being the leukocyte integrins (see Abram and Lowell 2009).