241 human active and 13 inactive phosphatases in total;
194 phosphatases have substrate data;
336 protein substrates;
83 non-protein substrates;
1215 dephosphorylation interactions;
299 KEGG pathways;
876 Reactome pathways;
last scientific update: 11 Mar, 2019
last maintenance update: 01 Sep, 2023
Cytoplasm Nucleus Cell membraneNote=Present and active in lipid rafts Palmitoylation is crucialfor proper trafficking
Function (UniProt annotation)
Non-receptor tyrosine-protein kinase that plays a rolein many biological processes including regulation of cell growthand survival, cell adhesion, integrin-mediated signaling,cytoskeletal remodeling, cell motility, immune response and axonguidance Inactive FYN is phosphorylated on its C-terminal tailwithin the catalytic domain Following activation by PKA, theprotein subsequently associates with PTK2/FAK1, allowing PTK2/FAK1phosphorylation, activation and targeting to focal adhesionsInvolved in the regulation of cell adhesion and motility throughphosphorylation of CTNNB1 (beta-catenin) and CTNND1 (delta-catenin) Regulates cytoskeletal remodeling by phosphorylatingseveral proteins including the actin regulator WAS and themicrotubule-associated proteins MAP2 and MAPT Promotes cellsurvival by phosphorylating AGAP2/PIKE-A and preventing itsapoptotic cleavage Participates in signal transduction pathwaysthat regulate the integrity of the glomerular slit diaphragm (anessential part of the glomerular filter of the kidney) byphosphorylating several slit diaphragm components including NPHS1,KIRREL1 and TRPC6 Plays a role in neural processes byphosphorylating DPYSL2, a multifunctional adapter protein withinthe central nervous system, ARHGAP32, a regulator for Rho familyGTPases implicated in various neural functions, and SNCA, a smallpre-synaptic protein Participates in the downstream signalingpathways that lead to T-cell differentiation and proliferationfollowing T-cell receptor (TCR) stimulation PhosphorylatesPTK2B/PYK2 in response to T-cell receptor activation Alsoparticipates in negative feedback regulation of TCR signalingthrough phosphorylation of PAG1, thereby promoting interactionbetween PAG1 and CSK and recruitment of CSK to lipid rafts CSKmaintains LCK and FYN in an inactive form Promotes CD28-inducedphosphorylation of VAV1
Catalytic Activity (UniProt annotation)
ATP + a [protein]-L-tyrosine = ADP + a[protein]-L-tyrosine phosphate
Sphingomyelin (SM) and its metabolic products are now known to have second messenger functions in a variety of cellular signaling pathways. Particularly, the sphingolipid metabolites, ceramide (Cer) and sphingosine-1-phosphate (S1P), have emerged as a new class of potent bioactive molecules. Ceramide can be generated de novo or by hydrolysis of membrane sphingomyelin by sphingomyelinase (SMase). Ceramide is subsequently metabolized by ceramidase to generate sphingosine (Sph) which in turn produces S1P through phosphorylation by sphingosine kinases 1 and 2 (SphK1, 2). Both ceramide and S1P regulate cellular responses to stress, with generally opposing effects. S1P functions as a growth and survival factor, acting as a ligand for a family of G protein-coupled receptors, whereas ceramide activates intrinsic and extrinsic apoptotic pathways through receptor-independent mechanisms.
Phospholipase D (PLD) is an essential enzyme responsible for the production of the lipid second messenger phosphatidic acid (PA), which is involved in fundamental cellular processes, including membrane trafficking, actin cytoskeleton remodeling, cell proliferation and cell survival. PLD activity can be stimulated by a large number of cell surface receptors and is elaborately regulated by intracellular factors, including protein kinase C isoforms, small GTPases of the ARF, Rho and Ras families and the phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PIP2). The PLD-produced PA activates signaling proteins and acts as a node within the membrane to which signaling proteins translocate. Several signaling proteins, including Raf-1 and mTOR, directly bind PA to mediate translocation or activation, respectively.
Axon guidance represents a key stage in the formation of neuronal network. Axons are guided by a variety of guidance factors, such as netrins, ephrins, Slits, and semaphorins. These guidance cues are read by growth cone receptors, and signal transduction pathways downstream of these receptors converge onto the Rho GTPases to elicit changes in cytoskeletal organization that determine which way the growth cone will turn.
The osteoclasts, multinucleared cells originating from the hematopoietic monocyte-macrophage lineage, are responsible for bone resorption. Osteoclastogenesis is mainly regulated by signaling pathways activated by RANK and immune receptors, whose ligands are expressed on the surface of osteoblasts. Signaling from RANK changes gene expression patterns through transcription factors like NFATc1 and characterizes the active osteoclast.
Cell-matrix adhesions play essential roles in important biological processes including cell motility, cell proliferation, cell differentiation, regulation of gene expression and cell survival. At the cell-extracellular matrix contact points, specialized structures are formed and termed focal adhesions, where bundles of actin filaments are anchored to transmembrane receptors of the integrin family through a multi-molecular complex of junctional plaque proteins. Some of the constituents of focal adhesions participate in the structural link between membrane receptors and the actin cytoskeleton, while others are signalling molecules, including different protein kinases and phosphatases, their substrates, and various adapter proteins. Integrin signaling is dependent upon the non-receptor tyrosine kinase activities of the FAK and src proteins as well as the adaptor protein functions of FAK, src and Shc to initiate downstream signaling events. These signalling events culminate in reorganization of the actin cytoskeleton; a prerequisite for changes in cell shape and motility, and gene expression. Similar morphological alterations and modulation of gene expression are initiated by the binding of growth factors to their respective receptors, emphasizing the considerable crosstalk between adhesion- and growth factor-mediated signalling.
Cell-cell adherens junctions (AJs), the most common type of intercellular adhesions, are important for maintaining tissue architecture and cell polarity and can limit cell movement and proliferation. At AJs, E-cadherin serves as an essential cell adhesion molecules (CAMs). The cytoplasmic tail binds beta-catenin, which in turn binds alpha-catenin. Alpha-catenin is associated with F-actin bundles directly and indirectly. The integrity of the cadherin-catenin complex is negatively regulated by phosphorylation of beta-catenin by receptor tyrosine kinases (RTKs) and cytoplasmic tyrosine kinases (Fer, Fyn, Yes, and Src), which leads to dissociation of the cadherin-catenin complex. Integrity of this complex is positively regulated by beta -catenin phosphorylation by casein kinase II, and dephosphorylation by protein tyrosine phosphatases. Changes in the phosphorylation state of beta-catenin affect cell-cell adhesion, cell migration and the level of signaling beta-catenin. Wnt signaling acts as a positive regulator of beta-catenin by inhibiting beta-catenin degradation, which stabilizes beta-catenin, and causes its accumulation. Cadherin may acts as a negative regulator of signaling beta-catenin as it binds beta-catenin at the cell surface and thereby sequesters it from the nucleus. Nectins also function as CAMs at AJs, but are more highly concentrated at AJs than E-cadherin. Nectins transduce signals through Cdc42 and Rac, which reorganize the actin cytoskeleton, regulate the formation of AJs, and strengthen cell-cell adhesion.
Platelets play a key and beneficial role for primary hemostasis on the disruption of the integrity of vessel wall. Platelet adhesion and activation at sites of vascular wall injury is initiated by adhesion to adhesive macromolecules, such as collagen and von Willebrand factor (vWF), or by soluble platelet agonists, such as ADP, thrombin, and thromboxane A2. Different receptors are stimulated by various agonists, almost converging in increasing intracellular Ca2+ concentration that stimulate platelet shape change and granule secretion and ultimately induce the inside-outsignaling process leading to activation of the ligand-binding function of integrin alpha IIb beta 3. Binding of alpha IIb beta 3 to its ligands, mainly fibrinogen, mediates platelet adhesion and aggregation and triggers outside-insignaling, resulting in platelet spreading, additional granule secretion, stabilization of platelet adhesion and aggregation, and clot retraction.
Natural killer (NK) cells are lymphocytes of the innate immune system that are involved in early defenses against both allogeneic (nonself) cells and autologous cells undergoing various forms of stress, such as infection with viruses, bacteria, or parasites or malignant transformation. Although NK cells do not express classical antigen receptors of the immunoglobulin gene family, such as the antibodies produced by B cells or the T cell receptor expressed by T cells, they are equipped with various receptors whose engagement allows them to discriminate between target and nontarget cells. Activating receptors bind ligands on the target cell surface and trigger NK cell activation and target cell lysis. However Inhibitory receptors recognize MHC class I molecules (HLA) and inhibit killing by NK cells by overruling the actions of the activating receptors. This inhibitory signal is lost when the target cells do not express MHC class I and perhaps also in cells infected with virus, which might inhibit MHC class I exprssion or alter its conformation. The mechanism of NK cell killing is the same as that used by the cytotoxic T cells generated in an adaptive immune response; cytotoxic granules are released onto the surface of the bound target cell, and the effector proteins they contain penetrate the cell membrane and induce programmed cell death.
Activation of T lymphocytes is a key event for an efficient response of the immune system. It requires the involvement of the T-cell receptor (TCR) as well as costimulatory molecules such as CD28. Engagement of these receptors through the interaction with a foreign antigen associated with major histocompatibility complex molecules and CD28 counter-receptors B7.1/B7.2, respectively, results in a series of signaling cascades. These cascades comprise an array of protein-tyrosine kinases, phosphatases, GTP-binding proteins and adaptor proteins that regulate generic and specialised functions, leading to T-cell proliferation, cytokine production and differentiation into effector cells.
Fc epsilon RI-mediated signaling pathways in mast cells are initiated by the interaction of antigen (Ag) with IgE bound to the extracellular domain of the alpha chain of Fc epsilon RI. The activation pathways are regulated both positively and negatively by the interactions of numerous signaling molecules. Mast cells that are thus activated release preformed granules which contain biogenic amines (especially histamines) and proteoglycans (especially heparin). The activation of phospholipase A2 causes the release of membrane lipids followed by development of lipid mediators such as leukotrienes (LTC4, LTD4 and LTE4) and prostaglandins (especially PDG2). There is also secretion of cytokines, the most important of which are TNF-alpha, IL-4 and IL-5. These mediators and cytokines contribute to inflammatory responses.
Acetylcholine (ACh) is a neurotransmitter widely distributed in the central (and also peripheral, autonomic and enteric) nervous system (CNS). In the CNS, ACh facilitates many functions, such as learning, memory, attention and motor control. When released in the synaptic cleft, ACh binds to two distinct types of receptors: Ionotropic nicotinic acetylcholine receptors (nAChR) and metabotropic muscarinic acetylcholine receptors (mAChRs). The activation of nAChR by ACh leads to the rapid influx of Na+ and Ca2+ and subsequent cellular depolarization. Activation of mAChRs is relatively slow (milliseconds to seconds) and, depending on the subtypes present (M1-M5), they directly alter cellular homeostasis of phospholipase C, inositol trisphosphate, cAMP, and free calcium. In the cleft, ACh may also be hydrolyzed by acetylcholinesterase (AChE) into choline and acetate. The choline derived from ACh hydrolysis is recovered by a presynaptic high-affinity choline transporter (CHT).
Prion diseases, also termed transmissible spongiform encephalopathies (TSEs), are a group of fatal neurodegenerative diseases that affect humans and a number of other animal species. The etiology of these diseases is thought to be associated with the conversion of a normal protein, PrPC, into an infectious, pathogenic form, PrPSc. The conversion is induced by prion infections (for example, variant Creutzfeldt-Jakob disease (vCJD), iatrogenic CJD, Kuru), mutations (familial CJD, Gerstmann-Straussler-Scheinker syndrome, fatal familial insomnia (FFI)) or unknown factors (sporadic CJD (sCJD)), and is thought to occur after PrPC has reached the plasma membrane or is re-internalized for degradation. The PrPSc form shows greater protease resistance than PrPC and accumulates in affected individuals, often in the form of extracellular plaques. Pathways that may lead to neuronal death comprise oxidative stress, regulated activation of complement, ubiquitin-proteasome and endosomal-lysosomal systems, synaptic alterations and dendritic atrophy, corticosteroid response, and endoplasmic reticulum stress. In addition, the conformational transition could lead to the lost of a beneficial activity of the natively folded protein, PrPC.
Enteropathogenic E. coli (EPEC) and enterohemorrhagic E. coli (EHEC) are closely related pathogenic strains of Escherichia coli. The hallmark of EPEC/EHEC infections [DS:H00278 H00277] is induction of attaching and effacing (A/E) lesions that damage intestinal epithelial cells. The capacity to form A/E lesions is encoded mainly by the locus of enterocyte effacement (LEE) pathogenicity island. Tir, Map, EspF, EspG are known LEE-encoded effector proteins secreted via the type III secretion system, which is also LEE-encoded, into the host cell. EPEC and EHEC Tir's link the extracellular bacterium to the cell cytoskeleton. Map and EspF are involved in mitochondrion membrane permeabilization. EspG interacts with tubulins and stimulates microtubule destabilization. LEE-encoded adhesin or intimin (Eae) is exported via the general secretory pathway to the periplasm, where it is inserted into the outer membrane. In addition to Tir, two potential host cell-carried intimin receptors, beta1 integrin (ITGB1) and nucleolin (NCL), have so far been identified. The distinguishing feature of EHEC is the elaboration of Shiga-like toxin (Stx). Stx cleaves ribosomal RNA, thereby disrupting protein synthesis and killing the intoxicated epithelial or endothelial cells.
Measles virus (MV) is highly contagious virus that leads infant death worldwide. Humans are the unique natural reservoir for this virus. It causes severe immunosuppression favouring secondary bacterial infections. Several MV proteins have been suggested to disturb host immunity. After infection of host lymphoid cells via SLAM, MV inhibits cytokine response by direct interference with host signaling systems. Three proteins (P, V, and C) associate with Jak/STAT proteins in interferon-triggered pathway and other important proteins related to apoptosis. Interaction between MV and host brings about the shift towards a Th2 response by decreasing IL-12 production and induces lymphopenia by suppressing cell proliferation.
Myocarditis is a cardiac disease associated with inflammation and injury of the myocardium. It results from various etiologies, both noninfectious and infectious, but coxsackievirus B3 (CVB3) is still considered the dominant etiological agent. Myocarditis may be caused by direct cytopathic effects of virus, a pathologic immune response to persistent virus, or autoimmunity triggered by the viral infection. The virus enters the myocyte through internalization of the coxsackie-adenoviral receptor (CAR) and its coreceptor, decay-accelerating factor (DAF). Viral proteases cleave various proteins in the host cell. One example is viral protease 2A, which cleaves eukaryote initiation factor 4G (eIF4G) and the dystrophin protein, resulting in a complete shutdown of cap-dependent RNA translation and cytoskeletal destruction in infected cardiomyocytes, respectively. CVB3 also cleaves the member of the Bcl-2 family Bid, leading to apoptosis. CVB3 infection also induces the cleavage of cyclin D protein through a proteasome-dependent pathway, leading to the host cell-growth arrest. Viral infection and necrosis of myocytes may lead to the release of intracellular antigens, resulting in activation of self-reactive T cells. CVB infection is a significant cause of dilated cardiomyopathy (DCM) as well as myocarditis. Epidemiologically, myocarditis underlies a significant portion of patients with DCM.
The GPVI receptor is a complex of the GPVI protein with Fc epsilon R1 gamma (FcR). The Src family kinases Fyn and Lyn constitutively associate with the GPVI-FcR complex in platelets and initiate platelet activation through phosphorylation of the immunoreceptor tyrosine-based activation motif (ITAM) in the FcR gamma chain, leading to binding and activation of the tyrosine kinase Syk. Downstream of Syk, a series of adapter molecules and effectors lead to platelet activation. The GPVI receptor signaling cascade is similar to that of T- and B-cell immune receptors, involving the formation of a signalosome composed of adapter and effector proteins. At the core of the T-cell receptor signalosome is the transmembrane adapter LAT and two cytosolic adapters SLP-76 and Gads. While LAT is essential for signalling to PLCgamma1 downstream of the T-cell receptor, the absence of LAT in platelets only impairs the activation of PLCgamma2, the response to collagen and GPVI receptor ligands remains sufficient to elicit a full aggregation response. In contrast, GPVI signalling is almost entirely abolished in the absence of SLP-76
ERBB2, also known as HER2 or NEU, is a receptor tyrosine kinase (RTK) belonging to the EGFR family. ERBB2 possesses an extracellular domain that does not bind any known ligand, contrary to other EGFR family members, a single transmembrane domain, and an intracellular domain consisting of an active kinase and a C-tail with multiple tyrosine phosphorylation sites. Inactive ERBB2 is associated with a chaperone heat shock protein 90 (HSP90) and its co-chaperone CDC37 (Xu et al. 2001, Citri et al. 2004, Xu et al. 2005). In addition, ERBB2 is associated with ERBB2IP (also known as ERBIN or LAP2), a protein responsible for proper localization of ERBB2. In epithelial cells, ERBB2IP restricts expression of ERBB2 to basolateral plasma membrane regions (Borg et al. 2000).\nERBB2 becomes activated by forming a heterodimer with another ligand-activated EGFR family member, either EGFR, ERBB3 or ERBB4, which is accompanied by dissociation of chaperoning proteins HSP90 and CDC37 (Citri et al. 2004), as well as ERBB2IP (Borg et al. 2000) from ERBB2. ERBB2 heterodimers function to promote cell proliferation, cell survival and differentiation, depending on the cellular context. ERBB2 can also be activated by homodimerization when it is overexpressed, in cancer for example. \nIn cells expressing both ERBB2 and EGFR, EGF stimulation of EGFR leads to formation of both ERBB2:EGFR heterodimers (Wada et al. 1990, Karunagaran et al. 1996) and EGFR homodimers. Heterodimers of ERBB2 and EGFR trans-autophosphorylate on twelve tyrosine residues, six in the C-tail of EGFR and six in the C-tail of ERBB2 - Y1023, Y1139, Y1196, Y1221, Y1222 and Y1248 (Margolis et al. 1989, Hazan et al. 1990,Walton et al. 1990, Helin et al. 1991, Ricci et al. 1995, Pinkas-Kramarski 1996). Phosphorylated tyrosine residues in the C-tail of EGFR and ERBB2 serve as docking sites for downstream signaling molecules. Three key signaling pathways activated by ERBB2:EGFR heterodimers are RAF/MAP kinase cascade, PI3K-induced AKT signaling, and signaling by phospholipase C gamma (PLCG1). Downregulation of EGFR signaling is mediated by ubiquitin ligase CBL, and is shown under Signaling by EGFR.\nIn cells expressing ERBB2 and ERBB3, ERBB3 activated by neuregulin NRG1 or NRG2 binding (Tzahar et al. 1994) forms a heterodimer with ERBB2 (Pinkas-Kramarski et al. 1996, Citri et al. 2004). ERBB3 is the only EGFR family member with no kinase activity, and can only function in heterodimers, with ERBB2 being its preferred heterodimerization partner. After heterodimerization, ERBB2 phosphorylates ten tyrosine residues in the C-tail of ERBB3, Y1054, Y1197, Y1199, Y1222, Y1224, Y1260, Y1262, Y1276, Y1289 and Y1328 (Prigent et al. 1994, Pinkas-Kramarski et al. 1996, Vijapurkar et al. 2003, Li et al. 2007) that subsequently serve as docking sites for downstream signaling molecules, resulting in activation of PI3K-induced AKT signaling and RAF/MAP kinase cascade. Signaling by ERBB3 is downregulated by the action of RNF41 ubiquitin ligase, also known as NRDP1. \nIn cells expressing ERBB2 and ERBB4, ligand stimulated ERBB4 can either homodimerize or form heterodimers with ERBB2 (Li et al. 2007), resulting in trans-autophosphorylation of ERBB2 and ERBB4 on C-tail tyrosine residues that will subsequently serve as docking sites for downstream signaling molecules, leading to activation of RAF/MAP kinase cascade and, in the case of ERBB4 CYT1 isoforms, PI3K-induced AKT signaling (Hazan et al. 1990, Cohen et al. 1996, Li et al. 2007, Kaushansky et al. 2008). Signaling by ERBB4 is downregulated by the action of WWP1 and ITCH ubiquitin ligases, and is shown under Signaling by ERBB4
Signaling by AKT is one of the key outcomes of receptor tyrosine kinase (RTK) activation. AKT is activated by the cellular second messenger PIP3, a phospholipid that is generated by PI3K. In ustimulated cells, PI3K class IA enzymes reside in the cytosol as inactive heterodimers composed of p85 regulatory subunit and p110 catalytic subunit. In this complex, p85 stabilizes p110 while inhibiting its catalytic activity. Upon binding of extracellular ligands to RTKs, receptors dimerize and undergo autophosphorylation. The regulatory subunit of PI3K, p85, is recruited to phosphorylated cytosolic RTK domains either directly or indirectly, through adaptor proteins, leading to a conformational change in the PI3K IA heterodimer that relieves inhibition of the p110 catalytic subunit. Activated PI3K IA phosphorylates PIP2, converting it to PIP3; this reaction is negatively regulated by PTEN phosphatase. PIP3 recruits AKT to the plasma membrane, allowing TORC2 to phosphorylate a conserved serine residue of AKT. Phosphorylation of this serine induces a conformation change in AKT, exposing a conserved threonine residue that is then phosphorylated by PDPK1 (PDK1). Phosphorylation of both the threonine and the serine residue is required to fully activate AKT. The active AKT then dissociates from PIP3 and phosphorylates a number of cytosolic and nuclear proteins that play important roles in cell survival and metabolism. For a recent review of AKT signaling, please refer to Manning and Cantley, 2007
Stem cell factor (SCF) is a growth factor with membrane bound and soluble forms. It is expressed by fibroblasts and endothelial cells throughout the body, promoting proliferation, migration, survival and differentiation of hematopoetic progenitors, melanocytes and germ cells.(Linnekin 1999, Ronnstrand 2004, Lennartsson and Ronnstrand 2006). The receptor for SCF is KIT, a tyrosine kinase receptor (RTK) closely related to the receptors for platelet derived growth factor receptor, colony stimulating factor 1 (Linnekin 1999) and Flt3 (Rosnet et al. 1991). Four isoforms of c-Kit have been identified in humans. Alternative splicing results in isoforms of KIT differing in the presence or absence of four residues (GNNK) in the extracellular region. This occurs due to the use of an alternate 5' splice donor site. These GNNK+ and GNNK- variants are co-expressed in most tissues; the GNNK- form predominates and was more strongly tyrosine-phosphorylated and more rapidly internalized (Ronnstrand 2004). There are also splice variants that arise from alternative usage of splice acceptor site resulting in the presence or absence of a serine residue (Crosier et al., 1993). Finally, there is an alternative shorter transcript of KIT expressed in postmeiotic germ cells in the testis which encodes a truncated KIT consisting only of the second part of the kinase domain and thus lackig the extracellular and transmembrane domains as well as the first part of the kinase domain (Rossi et al. 1991). Binding of SCF homodimers to KIT results in KIT homodimerization followed by activation of its intrinsic tyrosine kinase activity. KIT stimulation activates a wide array of signalling pathways including MAPK, PI3K and JAK/STAT (Reber et al. 2006, Ronnstrand 2004). Defects of KIT in humans are associated with different genetic diseases and also in several types of cancers like mast cell leukaemia, germ cell tumours, certain subtypes of malignant melanoma and gastrointestinal tumours
Nef interferes with cellular signal transduction pathways in a number of ways. Nef is associated with lipid rafts through its amino-terminal myristoylation and a proline-rich SH3-binding domain. These cholesterol-rich membrane microdomains appear to concentrate potent signaling mediators. Nef was found to complex with and activate serine/threonine protein kinase PAK-2, which may contribute to activation of infected cells. In vitro, HIV-infected T cells produce enhanced levels of interleukin-2 during activation. When expressed in macrophages, Nef intersects the CD40L signaling pathway inducing secretion of chemokines and other factors that attract resting T cells and promote their infection by HIV
Leukocyte extravasation is a rigorously controlled process that guides white cell movement from the vascular lumen to sites of tissue inflammation. The powerful adhesive interactions that are required for leukocytes to withstand local flow at the vessel wall is a multistep process mediated by different adhesion molecules. Platelets adhered to injured vessel walls form strong adhesive substrates for leukocytes. For instance, the initial tethering and rolling of leukocytes over the site of injury are mediated by reversible binding of selectins to their cognate cell-surface glycoconjugates.
\r\n\r\nEndothelial cells are tightly connected through various proteins, which regulate the organization of the junctional complex and bind to cytoskeletal proteins or cytoplasmic interaction partners that allow the transfer of intracellular signals. An important role for these junctional proteins in governing the transendothelial migration of leukocytes under normal or inflammatory conditions has been established.
\r\n\r\nThis pathway describes some of the key interactions that assist in the process of platelet and leukocyte interaction with the endothelium, in response to injury
Cross-linking of FCGRs with IgG coated immune complexes results in tyrosine phosphorylation of the immuno tyrosine activation motif (ITAMs) of the rececptor by membrane-bound tyrosine kinases of the SRC family. The phosphorylated ITAM tyrosines serve as docking sites for Src homology 2 (SH2) domain-containing SYK kinase. Recruitment and activation of SYK is critical for FCGR-mediated signaling in phagocytosis, but the exact role of SYK in this process is unclear. Activated SYK then transmits downstream signals leading to actin polymerization and particle internalization
PECAM-1/CD31 is a member of the immunoglobulin superfamily (IgSF) and has been implicated to mediate the adhesion and trans-endothelial migration of T-lymphocytes into the vascular wall, T cell activation and angiogenesis. It has six Ig homology domains within its extracellularly and an ITIM motif within its cytoplasmic region. PECAM-1 mediates cellular interactions by both homophilic and heterophilic interactions. The cytoplasmic domain of PECAM-1 contains tyrosine residues which serves as docking sites for recruitment of cytosolic signaling molecules. Under conditions of platelet activation, PECAM-1 is phosphorylated by Src kinase members. The tyrosine residues 663 and 686 are required for recruitment of the SH2 domain containing PTPs
Signaling by PI3K/AKT is frequently constitutively activated in cancer via gain-of-function mutations in one of the two PI3K subunits - PI3KCA (encoding the catalytic subunit p110alpha) or PIK3R1 (encoding the regulatory subunit p85alpha). Gain-of-function mutations activate PI3K signaling by diverse mechanisms. Mutations affecting the helical domain of PIK3CA and mutations affecting nSH2 and iSH2 domains of PIK3R1 impair inhibitory interactions between these two subunits while preserving their association. Mutations in the catalytic domain of PIK3CA enable the kinase to achieve an active conformation. PI3K complexes with gain-of-function mutations therefore produce PIP3 and activate downstream AKT in the absence of growth factors (Huang et al. 2007, Zhao et al. 2005, Miled et al. 2007, Horn et al. 2008, Sun et al. 2010, Jaiswal et al. 2009, Zhao and Vogt 2010, Urick et al. 2011)
In response to receptor ligation, the tyrosine residues in DAP12's immunoreceptor tyrosine-based activation motif (ITAM) are phosphorylated by Src family kinases. These phosphotyrosines form the docking site for the protein tyrosine kinase SYK in myeloid cells and SYK and ZAP70 in NK cells. DAP12-bound SYK autophosphorylates and phosphorylates the scaffolding molecule LAT, recruiting the proximal signaling molecules phosphatidylinositol-3-OH kinase (PI3K), phospholipase-C gamma (PLC-gamma), GADS (GRB2-related adapter downstream of SHC), SLP76 (SH2 domain-containing leukocyte protein of 76 kDa), GRB2:SOS (Growth factor receptor-bound protein 2:Son of sevenless homolog 1) and VAV. All of these intermediate signalling molecules result in the recruitment and activation of kinases AKT, CBL (Casitas B-lineage lymphoma) and ERK (extracellular signal-regulated kinase), and rearrangement of the actin cytoskeleton (actin polymerization) finally leading to cellular activation. PLC-gamma generates the secondary messengers diacylglycerol (DAG) and inositol-1,4,5-trisphosphate (InsP3), leading to activation of protein kinase C (PKC) and calcium mobilization, respectively (Turnbull & Colonna 2007, Klesney-Tait et al. 2006)
During the development process cell migration and adhesion are the main forces involved in morphing the cells into critical anatomical structures. The ability of a cell to migrate to its correct destination depends heavily on signaling at the cell membrane. Erythropoietin producing hepatocellular carcinoma (EPH) receptors and their ligands, the ephrins (EPH receptors interacting proteins, EFNs), orchestrates the precise control necessary to guide a cell to its destination. They are expressed in all tissues of a developing embryo and are involved in multiple developmental processes such as axon guidance, cardiovascular and skeletal development and tissue patterning. In addition, EPH receptors and EFNs are expressed in developing and mature synapses in the nervous system, where they may have a role in regulating synaptic plasticity and long-term potentiation. Activation of EPHB receptors in neurons induces the rapid formation and enlargement of dendritic spines, as well as rapid synapse maturation (Dalva et al. 2007). On the other hand, EPHA4 activation leads to dendritic spine elimination (Murai et al. 2003, Fu et al. 2007).EPH receptors are the largest known family of receptor tyrosine kinases (RTKs), with fourteen total receptors divided into either A- or B-subclasses: EPHA (1-8 and 10) and EPHB (1-4 and 6). EPH receptors can have overlapping functions, and loss of one receptor can be partially compensated for by another EPH receptor that has similar expression pattern and ligand-binding specificities. EPH receptors have an N-terminal extracellular domain through which they bind to ephrin ligands, a short transmembrane domain, and an intracellular cytoplasmic signaling structure containing a canonical tyrosine kinase catalytic domain as well as other protein interaction sites. Ephrins are also sub-divided into an A-subclass (A1-A5), which are tethered to the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor, and a B-subclass (B1-B3), members of which have a transmembrane domain and a short, highly conserved cytoplasmic tail lacking endogenous catalytic activity. The interaction between EPH receptors and its ligands requires cell-cell interaction since both molecules are membrane-bound. Close contact between EPH receptors and EFNs is required for signaling to occur. EPH/EFN-initiated signaling occurs bi-directionally into either EPH- or EFN-expressing cells or axons. Signaling into the EPH receptor-expressing cell is referred as the forward signal and signaling into the EFN-expressing cell, the reverse signal. (Dalva et al. 2000, Grunwald et al. 2004, Davy & Robbins 2000, Cowan et al
The lipid raft resident adaptor molecules LAT1 and Non-T cell activation linker (NTAL), also known as linker for activation of B cells (LAB)/LAT2 are known participants in the regulation of mast cell calcium responses. Both LAT and NTAL are expressed and phosphorylated following engagement of FCERI on mast cells. NTAL is functionally and topographically different from LAT. There is a considerable debate on the role of NTAL in mast cell. Depending on the circumstances, NTAL appears to have a dual role as positive and negative regulator of MC responses elicited via FCERI. Studies conducted in bone marrow-derived mast cells (BMMCs) of mice lacking NTAL displayed enhanced FCERI-mediated tyrosine phosphorylation of several substrates, calcium response, degranulation, and cytokine production. This indicated that NTAL negatively regulates FCERI-mediated degranulation. However, in mice lacking both LAT and NTAL showed severe block in FCERI-mediated signaling than BMMCs deficient in LAT alone, suggesting that NTAL also shares a redundant function with LAT to play a positive role (Draberova et al. 2007, Orr & McVicar. 2011, Zhu et al. 2004, Volna et al. 2004)
Nephrin (NPHS1) is a member of the Super-IgG-Molecule family and is most prominently expressed in kidney podocytes. It is a major if not the most important structural component of the slit diaphragm, a modified adherens junction inbetween these cells. NPHS1 has an extracellular domain that contains eight distal IgG like domains and one proximal fibronectin type III domain, a transmembrane domain and a short intracellular domain. NPHS1 molecules show both homophilic and heterophilic interactions. Among heterophilic interaction partners, slit diaphragm proteins such as Kin of IRRE-like protein 1 (KIRREL, Nephrin-like protein 1, NEPH1), KIRREL3 (NEPH2) and KIRREL2 (NEPH3) were shown to stabilize the slit diaphragm structure. Intracellularly Podocin (NPHS2), CD2 associated protein (CD2AP) and adherins junction associated proteins like IQGAP, MAGI, CASK and spectrins all interact with NPHS1. Hence it seems to play a major role in organizing the molecular structure of the slit diaphragm itself and via its binding partners links it to the actin cytoskeleton. NPHS1 tyrosine phosphorylation by the Src kinase FYN initiates the PI3K-AKT signaling cascade, which seems to promote antiapoptotic signals
The neural cell adhesion molecule, NCAM, is a member of the immunoglobulin (Ig) superfamily and is involved in a variety of cellular processes of importance for the formation and maintenance of the nervous system. The role of NCAM in neural differentiation and synaptic plasticity is presumed to depend on the modulation of intracellular signal transduction cascades. NCAM based signaling complexes can initiate downstream intracellular signals by at least two mechanisms: (1) activation of FGFR and (2) formation of intracellular signaling complexes by direct interaction with cytoplasmic interaction partners such as Fyn and FAK. Tyrosine kinases Fyn and FAK interact with NCAM and undergo phosphorylation and this transiently activates the MAPK, ERK 1 and 2, cAMP response element binding protein (CREB) and transcription factors ELK and NFkB. CREB activates transcription of genes which are important for axonal growth, survival, and synaptic plasticity in neurons.NCAM1 mediated intracellular signal transduction is represented in the figure below. The Ig domains in NCAM1 are represented in orange ovals and Fn domains in green squares. The tyrosine residues susceptible to phosphorylation are represented in red circles and their positions are numbered. Phosphorylation is represented by red arrows and dephosphorylation by yellow. Ig, Immunoglobulin domain; Fn, Fibronectin domain; Fyn, Proto-oncogene tyrosine-protein kinase Fyn; FAK, focal adhesion kinase; RPTPalpha, Receptor-type tyrosine-protein phosphatase; Grb2, Growth factor receptor-bound protein 2; SOS, Son of sevenless homolog; Raf, RAF proto-oncogene serine/threonine-protein kinase; MEK, MAPK and ERK kinase; ERK, Extracellular signal-regulated kinase; MSK1, Mitogen and stress activated protein kinase 1; CREB, Cyclic AMP-responsive element-binding protein; CRE, cAMP response elements
In naive T cells, CD28 costimulation enhances cell cycle entry, potently stimulates expression of both the mitogenic lymphokine interleukin-2 (IL-2) and its receptor, and stimulates the activation of an antiapoptotic program. CD28 engages with one or both members of the B7 receptor family, B7.1 and B7.2. Upon ligand binding the tyrosines and proline-rich motifs present in the cytoplasmic tail of CD28 are phosphorylated by Lck or Fyn. Upon phosphorylation CD28 recruits and induces phosphorylation and activation of a more restricted set of intracellular signaling components that, together with those mobilized by the TCR, contribute to convert membrane-based biochemical and biophysical changes into gene activation events. Proteins like PI3K, Vav-1, Tec and Itk kinases, AKT, and the Dok-1 adaptor have been identified as elements of the CD28 signaling pathway by biochemical or genetic approaches or both
PI3Ks can be activated by a number of different receptors, including the TcR (T cell receptor), co-stimulatory receptors (CD28), cytokine receptors and chemokine receptors. However, the specific roles of PI3Ks downstream of these receptors vary. CD28 contains the YMNM consensus PI3K-binding motif, and PI3K recruitment by CD28 contributes to or complements TCR-dependent PI3K signaling. Activation of PI3K promotes PIP3 production at the plasma membrane and several potential target molecules for this phospholipid have been implicated in PI3K pathways downstream of the TcR and CD28. Of these targets, at least Vav and Akt have been implicated in CD28 costimulation of T cell activation. AKT/PKB connects PI3K to signaling pathways that promote cytokine transcription, survival, cell-cycle entry and growth
CD28 binds to several intracellular proteins including PI3 kinase, Grb-2, Gads and ITK. Grb-2 specifically co-operates with Vav-1 in the up-regulation of NFAT/AP-1 transcription. CD28 costimulation resulted in a prolonged and sustained phosphorylation and membrane localization of Vav1 in comparison to T-cell receptor activation alone. Tyrosine-phosphorylated Vav1 is an early point of integration between the signaling routes triggered by the T-cell receptor and CD28.Vav1 transduces TCR and co-stimulatory signals to multiple biochemical pathways and several cytoskeleton-dependent processes. The products of Vav1 activation, Rac1 and Cdc42, in turn activate the mitogen-activated protein kinases JNK and p38. Vav1 is also required for TCR-induced calcium flux, activation of the ERK MAP kinase pathway, activation of the NF-kB transcription factor, inside-out activation of the integrin LFA-1, TCR clustering, and polarisation of the T cell
CTLA4 is one of the best studied inhibitory receptors of the CD28 superfamily. CTLA4 inhibits Tcell activation by reducing IL2 production and IL2 expression, and by arresting T cells at the G1 phase of the cell cycle. CTLA-4 expressed by a T cell subpopulation exerts a dominant control on the proliferation of other T cells, which limits autoreactivity. CTLA4 also blocks CD28 signals by competing for the ligands B71 and B72 in the limited space between T cells and antigenpresenting cells. Though the mechanism is obscure, CTLA4 may also propagate inhibitory signals that actively counter those produced by CD28. CTLA4 can also function in a ligand-independent manner.?CTLA-4 regulates the activation of pathogenic T cells by directly modulating T cell receptor signaling (i.e. TCR-zeta chain phosphorylation) as well as downstream biochemical signals (i.e. ERK activation). The cytoplasmic region of CTLA4 contains a tyrosine motif YVKM and a proline rich region. After TCR stimulation, it undergoes tyrosine phosphorylation by src kinases, inducing surface retention
Multiple EPHB receptors contribute directly to dendritic spine development and morphogenesis. These are more broadly involved in post-synaptic development through activation of focal adhesion kinase (FAK) and Rho family GTPases and their GEFs. Dendritic spine morphogenesis is a vital part of the process of synapse formation and maturation during CNS development. Dendritic spine morphogenesis is characterized by filopodia shortening followed by the formation of mature mushroom-shaped spines (Moeller et al. 2006). EPHBs control neuronal morphology and motility by modulation of the actin cytoskeleton. EPHBs control dendritic filopodia motility, enabling synapse formation. EPHBs exert these effects through interacting with the guanine exchange factors (GEFs) such as intersectin and kalirin. The intersectin-CDC42-WASP-actin and kalirin-RAC-PAK-actin pathways have been proposed to regulate the EPHB receptor mediated morphogenesis and maturation of dendritic spines in cultured hippocampal and cortical neurons (Irie & Yamaguchi 2002, Penzes et al. 2003). EPHBs are also involved in the regulation of dendritic spine morphology through FAK which activates the RHOA-ROCK-LIMK-1 pathway to suppress cofilin activity and inhibit cofilin-mediated dendritic spine remodeling (Shi et al. 2009)
EPH/Ephrin signaling is coupled to Rho family GTPases such as Rac, Rho and Cdc42 that connect bidirectional receptor-ligand interactions to changes in the actin cytoskeleton (Noren & Pasquale 2004, Groeger & Nobes 2007). RHOA regulates actin dynamics and is involved in EPHA-induced growth cone collapse. This is mediated by ephexins. Ephexin, a guanine nucleotide exchange factor for Rho GTPases, interacts with the EPHA kinase domain and its subsequent activation differentially affects Rho GTPases, such that RHOA is activated, whereas Cdc42 and Rac1 are inhibited. Activation of RHOA, and inhibition of Cdc42 and Rac, shifts actin cytoskeleton to increased contraction and reduced expansion leading to growth-cone collapse (Shamah et al. 2001, Sahin et al. 2005). The activation of EPH receptors in growing neurons typically, but not always, leads to a growth cone collapse response and retraction from an ephrin-expressing substrate (Poliakov et al. 2004, Pasquale 2005). EPHA-mediated repulsive responses prevent axons from growing into regions of excessive ephrin-A concentration, such as the posterior end of the superior colliculus (Pasquale 2005)
The interaction between ephrin (EFN) ligands and EPH receptors results not only in forward signaling through the EPH receptor, but also in 'reverse' signaling through the EFN ligand itself. Reverse signaling through EFNB is required for correct spine morphogenesis and proper path-finding of corpus callosum and dorsal retinal axons. The molecular mechanism by which EFNBs transduce a reverse signal involves phosphorylation of multiple, conserved tyrosines on the intracellular domain of B-type ephrins, facilitating binding of the SH2/SH3 domain adaptor protein GRB4 and subsequent cytoskeletal remodeling (Bruckner et al. 1997, Cowan & Henkemeyer 2001, Lu et al. 2001). The other mechanism of reverse signaling involves the C-terminus PSD-95/Dlg/ZO-1 (PDZ)-binding motif of EFNBs which recruits various PDZ domain containing proteins. Phosphorylation and PDZ-dependent reverse signaling by ephrin-B1 have each been proposed to play important roles in multiple contexts in development and disease (Bush & Soriano 2009)
Despite high-affinity multimeric interaction between EPHs and ephrins (EFNs), the cellular response to EPH-EFN engagement is usually repulsion between the two cells and signal termination. These repulsive responses induce an EPH receptor-expressing cell to retract from an ephrin-expressing cell after establishing initial contact. The repulsive responses mediated by EPH receptors in the growth cone at the leading edge of extending axons and in axonal collateral branches contribute to the formation of selective neuronal connections. It is unclear how high affinity trans-cellular interactions between EPHs and ephrins are broken to convert adhesion into repulsion. Two possible mechanisms have been proposed for the repulsion of EPH-EFN bearing cells: the first one involves regulated cleavage of ephrin ligands or EPH receptors by transmembrane proteases following cell-cell contact, while the second one is rapid endocytosis of whole EPH:EFN complexes during the retraction of the interacting cells or neuronal growth cones (Egea & Klein 2007, Janes et al. 2005). RAC also plays an essential role during growth cone collapse by promoting actin polymerization that drives membrane internalization by endocytosis (Marston et al. 2003)
Sema3A, a prototypical semaphorin, acts as a chemorepellent or a chemoattractant for axons by activating a receptor complex comprising neuropilin-1 as the ligand-binding subunit and plexin-A1 as the signal-transducing subunit. Sema3A inhibits cell migration by inhibiting integrin ligand-binding activity
CRMPs are a small family of plexinA-interacting cytosolic phosphoproteins identified as mediators of Sema3A signaling and neuronal differentiation. After Sema3A activation Plexin-A bound CRMP's undergo phosphorylation by Cdk5, GSK3beta and Fes kinases. Phosphorylation of CRMPs by these kinases blocks the ability of CRMP to bind to tubulin dimers, subsequently induces depolymerization of F-actin, and ultimately leads to growth cone collapse
The DCC family includes DCC and neogenin in vertebrates. DCC is required for netrin-induced axon attraction. DCC is a transmembrane protein lacking any identifiable catalytic activity. Protein tyrosine kinase 2/FAK and src family kinases bind constitutively to the cytoplasmic domain of DCC and their activation couples to downstream intracellular signaling complex that directs the organization of actin
Angiogenesis is the formation of new blood vessels from preexisting vasculature. One of the most important proangiogenic factors is vascular endothelial growth factor (VEGF). VEGF exerts its biologic effect through interaction with transmembrane tyrosine kinase receptors VEGFR, selectively expressed on vascular endothelial cells. VEGFA signaling through VEGFR2 is the major pathway that activates angiogenesis by inducing the proliferation, survival, sprouting and migration of endothelial cells (ECs), and also by increasing endothelial permeability (Lohela et al. 2009, Shibuya & Claesson-Welsh 2006, Claesson-Welsh & Welsh, 2013). The critical role of VEGFR2 in vascular development is highlighted by the fact that VEGFR2-/- mice die at E8.5-9.5 due to defective development of blood islands, endothelial cells and haematopoietic cells (Shalaby et al. 1995)
Dendritic cell-associated C-type lectin-2 (Dectin-2) family of C-type lectin receptors (CLRs) includes Dectin-2 (CLEC6A), blood dendritic antigen 2 (BDCA2/CLEC4C), macrophage C-type lectin (MCL/CLEC4D), Dendritic cell immunoreceptor (DCIR/CLEC4A) and macrophage inducible C-type lectin (Mincle/CLEC4E). These receptors possesses a single extracellular conserved C-type lectin domain (CTLD) with a short cytoplasmic tail that induces intracellular signalling indirectly by binding with the FCERG (High affinity immunoglobulin epsilon receptor subunit gamma) except for DCIR that has a longer cytoplasmic tail with an integral inhibitory signalling motif (Graham & Brown. 2009, Kerschera et al. 2013). CLEC6A (Dectin-2) binds to high mannose containing pathogen-associated molecular patterns (PAMPs) expressed by fungal hyphae, and CLEC4E (mincle) binds to alpha-mannaosyl PAMPs on fungal, mycobacterial and necrotic cell ligands. Both signaling pathways lead to Toll-like receptor (TLR)-independent production of cytokines such as tumor necrosis factor (TNF) and interleukin 6 (IL6). Similarities with Dectin-1 (CLC7A) signaling pathway suggests that both these CLRs couple SYK activation to NF-kB activation using a complex involving CARD9, BCL10 and MALT1 (Geijtenbeek & Gringhuis 2009)
CD209 (also called as DC-SIGN (DC-specific intracellular adhesion molecule-3-grabbing non-integrin)) is a type II transmembrane C-type lectin receptor preferentially expressed on dendritic cells (DCs). CD209 functions as a pattern recognition receptor (PRR) that recognises several microorganisms and pathogens, contributing to generation of pathogen-tailored immune responses (Gringhuis & Geijtenbeek 2010, den Dunnen et al. 2009, Svajger et al. 2010). CD209 interacts with different mannose-expressing pathogens such as Mycobacterium tuberculosis and HIV-1 (Gringhuis et al. 2007, Geijtenbeek et al. 2000a). It also acts as an adhesion receptor that interacts with ICAM2 (intracellular adhesion molecule-2) on endothelial cells and ICAM3 on T cells (Geijtenbeek et al. 2000b,c). \nCD209 functions not only as an independent PRR, but is also implicated in the modulation of Toll-like receptor (TLR) signaling at the level of the transcription factor NF-kB (Gringhuis et al. 2009). CLEC7A (Dectin-1) and CD209 (DC-SIGN) signalling modulates Toll-like receptor (TLR) signalling through the kinase RAF1 that is independent of the SYK pathway but integrated with it at the level of NF-kB activation. The activation of RAF1 by CLEC7A or CD209 does not lead to activation of extracellular signal-regulated kinase 1 (ERK1)/2 or Mitogen-activated protein kinase kinase 1 (MEK1)/2 but leads to the phosphorylation and subsequent acetylation of RELA (p65). RELA phosphorylated on S276 not only positively regulates the activity of p65 through acetylation of p65, but also represses RELB activity by sequestering active RELB into inactive p65-RELB dimers that do not bind DNA (Gringhuis et al. 2007, Svajger et al. 2010, Jacque et al. 2005). RAF1-dependent signaling pathway is crucial in dectin-1 mediated immunity as it modulates both the canonical (promoting p65 phosphorylation and acetylation) and non-canonical (forming inactive p65-RELB dimers) NK-kB activation
The RAS-RAF-MEK-ERK pathway regulates processes such as proliferation, differentiation, survival, senescence and cell motility in response to growth factors, hormones and cytokines, among others. Binding of these stimuli to receptors in the plasma membrane promotes the GEF-mediated activation of RAS at the plasma membrane and initiates the three-tiered kinase cascade of the conventional MAPK cascades. GTP-bound RAS recruits RAF (the MAPK kinase kinase), and promotes its dimerization and activation (reviewed in Cseh et al, 2014; Roskoski, 2010; McKay and Morrison, 2007; Wellbrock et al, 2004). Activated RAF phosphorylates the MAPK kinase proteins MEK1 and MEK2 (also known as MAP2K1 and MAP2K2), which in turn phophorylate the proline-directed kinases ERK1 and 2 (also known as MAPK3 and MAPK1) (reviewed in Roskoski, 2012a, b; Kryiakis and Avruch, 2012). Activated ERK proteins may undergo dimerization and have identified targets in both the nucleus and the cytosol; consistent with this, a proportion of activated ERK protein relocalizes to the nucleus in response to stimuli (reviewed in Roskoski 2012b; Turjanski et al, 2007; Plotnikov et al, 2010; Cargnello et al, 2011). Although initially seen as a linear cascade originating at the plasma membrane and culminating in the nucleus, the RAS/RAF MAPK cascade is now also known to be activated from various intracellular location. Temporal and spatial specificity of the cascade is achieved in part through the interaction of pathway components with numerous scaffolding proteins (reviewed in McKay and Morrison, 2007; Brown and Sacks, 2009). The importance of the RAS/RAF MAPK cascade is highlighted by the fact that components of this pathway are mutated with high frequency in a large number of human cancers. Activating mutations in RAS are found in approximately one third of human cancers, while ~8% of tumors express an activated form of BRAF (Roberts and Der, 2007; Davies et al, 2002; Cantwell-Dorris et al, 2011)
Phosphatidylinositol-5-phosphate (PI5P) may modulate PI3K/AKT signaling in several ways. PI5P is used as a substrate for production of phosphatidylinositol-4,5-bisphosphate, PI(4,5)P2 (Rameh et al. 1997, Clarke et al. 2008, Clarke et al. 2010, Clarke and Irvine 2013, Clarke et al. 2015), which serves as a substrate for activated PI3K, resulting in the production of PIP3 (Mandelker et al. 2009, Burke et al. 2011). The majority of PI(4,5)P2 in the cell, however, is produced from the phosphatidylinositol-4-phosphate (PI4P) substrate (Zhang et al. 1997, Di Paolo et al. 2002, Oude Weernink et al. 2004, Halstead et al. 2006, Oude Weernink et al. 2007). PIP3 is necessary for the activating phosphorylation of AKT. AKT1 can be deactivated by the protein phosphatase 2A (PP2A) complex that contains a regulatory subunit B56-beta (PPP2R5B) or B56-gamma (PPP2R5C). PI5P inhibits AKT1 dephosphorylation by PP2A through an unknown mechanism (Ramel et al. 2009). Increased PI5P levels correlate with inhibitory phosphorylation(s) of the PP2A complex. MAPK1 (ERK2) and MAPK3 (ERK1) are involved in inhibitory phosphorylation of PP2A, in a process that involves IER3 (IEX-1) (Letourneux et al. 2006, Rocher et al. 2007). It is uncertain, however, whether PI5P is in any way involved in ERK-mediated phosphorylation of PP2A or if it regulates another PP2A kinase
Initiation of platelet adhesion is the first step in the formation of the platelet plug. Circulating platelets are arrested and subsequently activated by exposed collagen and vWF. It is not entirely clear which type of collagen is responsible for adhesion and activation; collagen types I and III are abundant in vascular epithelia but several other types incluing IV are present (Farndale 2006). Several collagen binding proteins are expressed on platelets, including integrin alpha2 beta1, GPVI, and GPIV. Integrin alpha2 beta1, known on leukocytes as VLA-2, is the major platelet collagen receptor (Kunicki et al. 1988). It requires Mg2+ to interact with collagen and may require initiation mediated by the activation of integrin alphaIIb beta3 (van de Walle 2007). Binding occurs via the alpha2 subunit I domain to a collagen motif with the sequence Gly-Phe-Hyp-Gly-Glu-Arg (Emsley 2000). Binding of collagen to alpha2 beta1 generates intracellular signals that contribute to platelet activation. These facilitate the engagement of the lower-affinity collagen receptor, GPVI (Keely 1996), the key receptor involved in collagen-induced platelet activation. The GPVI receptor is a complex of the GPVI protein with a dimer of Fc epsilon R1 gamma (FceRI gamma). The Src family kinases Fyn and Lyn constitutively associate with the GPVI:FceRIgamma complex in platelets and initiate platelet activation through phosphorylation of the immunoreceptor tyrosine-based activation motif (ITAM) in FceRI gamma, leading to binding and activation of the tyrosine kinase Syk. Downstream of Syk, a series of adapter molecules and effectors lead to platelet activation. vWF protein is a polymeric structure of variable size. It is secreted in two directions, by the endothelium basolaterally and into the bloodstream. Shear-induced aggregation is achieved when vWF binds via its A1 domain to GPIb (part of GPIb-IX-V), and via its A3 domain mediating collagen binding to the subendothelium. The interaction between vWF and GPIb is regulated by shear force; an increase in the shear stress results in a corresponding increase in the affinity of vWF for GPIb
Reelin (RELN) is an extracellular, multifunctional signal glycoprotein that controls not only the positioning of neurons in the developing brain, but also their growth, maturation, and synaptic activity in the adult brain (Stranahan et al. 2013). Abnormal Reelin expression in the brain is implicated in a number of neuropsychiatric disorders including autism, schizophrenia, bipolar disorder and Alzheimer's disease (Folsom & Fatemi 2013)
In mouse brain, Fyn activation downstream of Bdnf-induced Ntrk2 (TrkB) signaling results in increased protein levels of AMPA receptor subunits Gria2 (GluR2), Gria3 (GluR3) and Gria1 (GluR1) without change in mRNA levels (Narisawa-Saito et al. 1999).
BDNF-mediated activation of NTRK2 increases phosphorylation of voltage gated sodium channels by FYN, resulting in decrease of sodium currents (Ahn et al. 2007).
FYN activation downstream of NTRK2 is implicated in olygodendrocyte myelination and contributes to BDNF-induced activation of ERK1/2 (MAPK3/1) through an unknown mechanism (Peckham et al. 2015).
Besides acting downstream of NTRK2, FYN and other SRC kinases, activated by other receptors such as GPCRs, may phosphorylate NTRK2 and enhance its catalytic activity (Rajagopal and Chao 2006, Huang and McNamara 2010)
DOCK3-mediated activation of RAC1 downstream of BDNF-induced signaling by NTRK2 (TRKB) plays a role in axonal growth and regeneration. DOCK3 can be recruited to the plasma membrane to activate RAC1 by binding to NTRK-associated FYN (Namekata et al. 2010). Alternatively, DOCK3 can, upon poorly elucidated RHOG activation by the BDNF:NTRK2 complex, bind to the RHOG:GTP complex and activate RAC1 in an ELMO1-dependent manner (Namekata et al. 2012)
Cbl is an E3 ubiquitin-protein ligase that negatively regulates signaling pathways by targeting proteins for ubiquitination and proteasomal degradation (Rao et al. 2002). Cbl negatively regulates PI3K via this mechanism (Dufour et al. 2008). The binding of Cbl to the p85 subunit of PI3K is mediated at least in part by tyrosine phosphorylation at Y731 (Dufour et al. 2008). Fyn and the related kinases Hck and Lyn are known to be associated with Cbl (Anderson et al. 1997, Hunter et al. 1999). Fyn is proven capable of Cbl Y731 phosphorylation (Hunter et al. 1999).The association of Fyn and Cbl has been described as constitutive (Hunter et al. 1999). CBL further associates with the p85 subunit of PI3K (Hartley et al. 1995, Anderson et al. 1997, Hunter et al. 1997), this also described as constitutive and mediated by the SH3 domain of p85. Binding of the SH2 domain of p85 to a specific phosphorylation site in Cbl is postulated to explain the the increase in Cbl/p85 association seen in activated cells (Panchamoorthy et al 1996) which negatively regulates PI3K activity (Fang et al. 2001). The negative effect of increased Cbl-PI3K interaction is mediated by Y731 of Cbl. Cbl binding increases PI3K ubiquitination and proteasome degradation (Dufour et al. 2008).Cbl is constitutively associated with Grb in resting hematopoietic cells (Anderson et al. 1997, Odai et al. 1995, Park et al. 1998, Panchamoorthy et al. 1996). Both the SH2 and SH3 domains of Grb2 are involved. Cbl has 2 distinct C-terminal domains, proximal and distal. The proximal domain binds Grb2 in resting and stimulated cells, and in stimulated cells also binds Shc. The distal domain binds the adaptor protein CRKL. Tyrosine phosphorylation of Cbl in response to IL-3 releases the SH3 domain of Grb2 which then is free to bind other molecules (Park et al. 1998). Cbl is tyrosine phosphorylated in response to many cytokines including IL-3, IL-2 (Gesbert et al. 1998) and IL-4 (Ueno et al. 1998)
Mature B cells express IgM and IgD immunoglobulins which are complexed with Ig-alpha (CD79A, MB-1) and Ig-beta (CD79B, B29) to form the B cell receptor (BCR) (Fu et al. 1974, Fu et al. 1975, Kunkel et al. 1975, Van Noesal et al. 1992, Sanchez et al. 1993, reviewed in Brezski and Monroe 2008). Binding of antigen to the immunoglobulin activates phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) in the cytoplasmic tails of Ig-alpha and Ig-beta by Src family tyrosine kinases, including LYN, FYN, and BLK (Nel et al. 1984, Yamanashi et al. 1991, Flaswinkel and Reth 1994, Saouaf et al. 1994, Hata et al. 1994, Saouaf et al. 1995, reviewed in Gauld and Cambier 2004, reviewed in Harwood and Batista 2010). The protein kinase SYK may also be involved in phosphorylating the ITAMs.The protein kinase SYK binds the phosphorylated immunoreceptor tyrosine-activated motifs (ITAMs) on the cytoplasmic tails of Ig-alpha (CD79A, MB-1) and Ig-beta (CD79B, B29) (Wienands et al. 1995, Rowley et al. 1995, Tsang et al. 2008). The binding causes the activation and autophosphorylation of SYK (Law et al. 1994, Irish et al. 2006, Baldock et al. 2008, Tsang et al. 2008, reviewed in Bradshaw 2010).Activated SYK and other kinases phosphorylate BLNK (SLP-65, BASH) and BCAP. LYN and FYN phosphorylate CD19. Phosphorylated BLNK, BCAP, and CD19 serve as scaffolds which recruit effectors to the plasma membrane and assemble large complexes, the signalosomes. BCAP and CD19 recruit phosphoinositol 3-kinase (PI3K). BLNK recruits phospholipase C gamma (predominantly PLC-gamma2 in B cells, Coggeshall et al. 1992), NCK, BAM32, BTK, VAV1, and SHC. The effectors are phosphorylated by SYK and other kinases.Phosphorylated BCAP recruits PI3K, which is phosphorylated by a SYK-dependent mechanism (Kuwahara et al. 1996) and produces phosphatidylinositol-3,4,5-trisphosphate (PIP3). Phosphorylated CD19 likewise recruits PIP3K. PIP3 recruits BAM32 (Marshall et al. 2000) and BTK (de Weers et al. 1994, Baba et al. 2001) to the plasma membrane via their PH domains. PIP3 also recruits and activates PLC-gamma1 and PLC-gamma2 (Bae et al. 1998). BTK binds phosphorylated BLNK via its SH2 domain (Baba et al. 2001). BTK phosphorylates PLC-gamma2 (Rodriguez et al. 2001), which activates phospholipase activity (Carter et al. 1991, Roifman and Wang 1992, Kim et al. 2004, Sekiya et al. 2004). Phosphorylated BLNK recruits PLC-gamma, VAV, GRB2, and NCK (Fu and Chan 1997, Fu et al. 1998, Chiu et al. 2002).PLC-gamma hydrolyzes phosphatidylinositol-4,5-bisphosphate to yield inositol-1,4,5-trisphosphate (IP3) and diacylglycerol (Carter et al. 1991, Kim et al. 2004). IP3 binds receptors on the endoplasmic reticulum and causes release of Ca2+ ions from the ER into the cytosol. The depletion of calcium from the ER in turn activates STIM1 to interact with ORAI and TRPC1 channels (and possibly other TRP channels) in the plasma membrane, resulting in an influx of extracellular calcium ions (Mori et al. 2002, Muik et al. 2008, Luik et al. 2008, Park et al. 2009)