RTK SLAP down: the emerging role of Src-like adaptor protein as a key player in receptor tyrosine kinase signaling

Adaptor molecules mediate negative regulation of macrophage inflammatory pathways: a closer look

Macrophages play a central role in initiating, maintaining, and terminating inflammation. For that, macrophages respond to various external stimuli in changing environments through signaling pathways that are tightly regulated and interconnected. This process involves, among others, autoregulatory loops that activate and deactivate macrophages through various cytokines, stimulants, and other chemical mediators. Adaptor proteins play an indispensable role in facilitating various inflammatory signals. These proteins are dynamic and flexible modulators of immune cell signaling and act as molecular bridges between cell surface receptors and intracellular effector molecules. They are involved in regulating physiological inflammation and also contribute significantly to the development of chronic inflammatory processes. This is at least partly due to their involvement in the activation and deactivation of macrophages, leading to changes in the macrophages' activation/phenotype. This review provides a comprehensive overview of the 20 adaptor molecules and proteins that act as negative regulators of inflammation in macrophages and effectively suppress inflammatory signaling pathways. We emphasize the functional role of adaptors in signal transduction in macrophages and their influence on the phenotypic transition of macrophages from pro-inflammatory M1-like states to anti-inflammatory M2-like phenotypes. This endeavor mainly aims at highlighting and orchestrating the intricate dynamics of adaptor molecules by elucidating the associated key roles along with respective domains and opening avenues for therapeutic and investigative purposes in clinical practice.

The EphB6 Receptor: Kinase-Dead but Very Much Alive

The Eph receptor tyrosine kinase member EphB6 is a pseudokinase, and similar to other pseudoenzymes has not attracted an equivalent amount of interest as its enzymatically-active counterparts. However, a greater appreciation for the role pseudoenzymes perform in expanding the repertoire of signals generated by signal transduction systems has fostered more interest in the field. EphB6 acts as a molecular switch that is capable of modulating the signal transduction output of Eph receptor clusters. Although the biological effects of EphB6 activity are well defined, the molecular mechanisms of EphB6 function remain enigmatic. In this review, we use a comparative approach to postulate how EphB6 acts as a scaffold to recruit adaptor proteins to an Eph receptor cluster and how this function is regulated. We suggest that the evolutionary repurposing of EphB6 into a kinase-independent molecular switch in mammals has involved repurposing the kinase activation loop into an SH3 domain-binding site. In addition, we suggest that EphB6 employs the same SAM domain linker and juxtamembrane domain allosteric regulatory mechanisms that are used in kinase-positive Eph receptors to regulate its scaffold function. As a result, although kinase-dead, EphB6 remains a strategically active component of Eph receptor signaling.

SLAP2 Adaptor Binding Disrupts c-CBL Autoinhibition to Activate Ubiquitin Ligase Function

CBL is a RING type E3 ubiquitin ligase that functions as a negative regulator of tyrosine kinase signaling and loss of CBL E3 function is implicated in several forms of leukemia. The Src-like adaptor proteins (SLAP/SLAP2) bind to CBL and are required for CBL-dependent downregulation of antigen receptor, cytokine receptor, and receptor tyrosine kinase signaling. Despite the established role of SLAP/SLAP2 in regulating CBL activity, the nature of the interaction and the mechanisms involved are not known. To understand the molecular basis of the interaction between SLAP/SLAP2 and CBL, we solved the crystal structure of CBL tyrosine kinase binding domain (TKBD) in complex with SLAP2. The carboxy-terminal region of SLAP2 adopts an α-helical structure which binds in a cleft between the 4H, EF-hand, and SH2 domains of the TKBD. This SLAP2 binding site is remote from the canonical TKBD phospho-tyrosine peptide binding site but overlaps with a region important for stabilizing CBL in its autoinhibited conformation. In addition, binding of SLAP2 to CBL in vitro activates the ubiquitin ligase function of autoinhibited CBL. Disruption of the CBL/SLAP2 interface through mutagenesis demonstrated a role for this protein-protein interaction in regulation of CBL E3 ligase activity in cells. Our results reveal that SLAP2 binding to a regulatory cleft of the TKBD provides an alternative mechanism for activation of CBL ubiquitin ligase function.

Sleuthing biochemical evidence to elucidate unassigned electron density in a CBL-SLAP2 crystal complex

The Src-like adaptor proteins (SLAP/SLAP2) bind to CBL E3 ubiquitin ligase to downregulate antigen, cytokine and tyrosine kinase receptor signalling. In contrast to the phosphotyrosine-dependent binding of CBL substrates through its tyrosine kinase-binding domain (TKBD), CBL TKBD associates with the C-terminal tail of SLAP2 in a phospho-independent manner. To understand the distinct nature of this interaction, a purification protocol for SLAP2 in complex with CBL TKBD was established and the complex was crystallized. However, determination of the complex crystal structure was hindered by the apparent degradation of SLAP2 during the crystallization process, such that only the CBL TKBD residues could initially be modelled. Close examination of the CBL TKBD structure revealed a unique dimer interface that included two short segments of electron density of unknown origin. To elucidate which residues of SLAP2 to model into this unassigned density, a co-expression system was generated to test SLAP2 deletion mutants and define the minimal SLAP2 binding region. SLAP2 degradation products were also analysed by mass spectrometry. The model-building and map-generation features of the Phenix software package were employed, leading to successful modelling of the C-terminal tail of SLAP2 into the unassigned electron-density segments.

Harnessing the Power of Eph/ephrin Biosemiotics for Theranostic Applications

Comprehensive basic biological knowledge of the Eph/ephrin system in the physiologic setting is needed to facilitate an understanding of its role and the effects of pathological processes on its activity, thereby paving the way for development of prospective therapeutic targets. To this end, this review briefly addresses what is currently known and being investigated in order to highlight the gaps and possible avenues for further investigation to capitalize on their diverse potential.

FMS-like Tyrosine Kinase 3/FLT3: From Basic Science to Clinical Implications

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase that is expressed almost exclusively in the hematopoietic compartment. Its ligand, FLT3 ligand (FL), induces dimerization and activation of its intrinsic tyrosine kinase activity. Activation of FLT3 leads to its autophosphorylation and initiation of several signal transduction cascades. Signaling is initiated by the recruitment of signal transduction molecules to activated FLT3 through binding to specific phosphorylated tyrosine residues in the intracellular region of FLT3. Activation of FLT3 mediates cell survival, cell proliferation, and differentiation of hematopoietic progenitor cells. It acts in synergy with several other cytokines to promote its biological effects. Deregulated FLT3 activity has been implicated in several diseases, most prominently in acute myeloid leukemia where around one-third of patients carry an activating mutant of FLT3 which drives the disease and is correlated with poor prognosis. Overactivity of FLT3 has also been implicated in autoimmune diseases, such as rheumatoid arthritis. The observation that gain-of-function mutations of FLT3 can promote leukemogenesis has stimulated the development of inhibitors that target this receptor. Many of these are in clinical trials, and some have been approved for clinical use. However, problems with acquired resistance to these inhibitors are common and, furthermore, only a fraction of patients respond to these selective treatments. This review provides a summary of our current knowledge regarding structural and functional aspects of FLT3 signaling, both under normal and pathological conditions, and discusses challenges for the future regarding the use of targeted inhibition of these pathways for the treatment of patients.

Control of Tyrosine Kinase Signalling by Small Adaptors in Colorectal Cancer

Tyrosine kinases (TKs) phosphorylate proteins on tyrosine residues as an intracellular signalling mechanism to coordinate intestinal epithelial cell communication and fate decision. Deregulation of their activity is ultimately connected with carcinogenesis. In colorectal cancer (CRC), it is still unclear how aberrant TK activities contribute to tumour formation because TK-encoding genes are not frequently mutated in this cancer. In vertebrates, several TKs are under the control of small adaptor proteins with potential important physiopathological roles. For instance, they can exert tumour suppressor functions in human cancer by targeting several components of the oncogenic TK signalling cascades. Here, we review how the Src-like adaptor protein (SLAP) and the suppressor of cytokine signalling (SOCS) adaptor proteins regulate the SRC and the Janus kinase (JAK) oncogenic pathways, respectively, and how their loss of function in the intestinal epithelium may influence tumour formation. We also discuss the potential therapeutic value of these adaptors in CRC.

SRC-like adaptor protein negatively regulates Wnt signaling in intrahepatic cholangiocarcinoma

Currently, the molecular mechanisms underlying intrahepatic cholangiocarcinoma (IHCC) are poorly understood. In the present study, the focus was primarily on SRC-like adaptor protein (SLAP), an adaptor protein, which is aberrantly expressed in various cancer types. To the best of our knowledge, the present study was the first to demonstrate that SLAP was decreased in IHCC tissues and cells, compared with controls. Further study indicated that SLAP overexpression suppressed IHCC cell proliferation and induced cell cycle arrest, indicating the tumor suppressor role of SLAP in IHCC progression. To demonstrate the effects of SLAP on Wnt signaling, the β-catenin/T cell factor transcription reporter assay was conducted. Compared with the negative adenovirus vector control (Ad-NC), overexpression of SLAP reduced TOPflash activity, and no changes in FOPflash activity were identified. Furthermore, the expression levels of Wnt target genes, including β-catenin, c-Myc, cluster of differentiation 44, Slug, Vimentin and matrix metallopeptidase-9, were reduced in RBE and Huh28 cells overexpressing SLAP. Additionally, the effects of SLAP on IHCC cell invasion and migration were determined. Compared with the Ad-NC control, the migration and invasion capacity was reduced following overexpression of SLAP in RBE and Huh28 cells. In summary, reduced SLAP expression may enhance IHCC malignant progression by activating Wnt signaling.

Membrane translocation of Bruton kinase in multiple myeloma cells is associated with osteoclastogenic phenotype in bone metastatic lesions

Using bone biopsy samples, we examined whether osteolytic cytokine profile is changed in situ in bone samples of metastatic multiple myeloma, and whether this creates an environment of lysis within the bone to which it has spread. This also produces the clinical features of increased circulating plasma calcium, and deleterious effects on the kidney.Using multiple myeloma biopsy and cell extracts from bone metastatic lesions, Bruton kinase, a tyrosine kinase, was demonstrated to be translocated to the membrane. Several transcription factors were upregulated included activin A, inflammatory transcription activator like such as nuclear factor kappa B, and specific bone lytic factor such as receptor activator of nuclear factor kappa-B ligand that is known to drive osteoclastogenesis as opposed to a osteogenic environment. The transcript for Bruton kinase was also elevated in its expression.Cytokines that support osteolytic activity such as a proliferation-inducing ligand, RANTES (regulated on activation, normal T cell expressed and secreted), interleukin-8, and activin A were upregulated. Tartrate resistant acid phosphatase (TRAP)-positive osteoclastic enzymatic activity was significantly elevated in the bone microenvironment in metastatic multiple myeloma. Several tyrosine kinase inhibitors, including inhibitors for Bruton kinase such as ibrutinib have been developed. The results of the present study provide evidence that multiple myeloma possess signal transduction mechanisms to support a bone lytic environment.The results provide a preliminary molecular basis to design specific inhibitors for management of bone metastasis of multiple myeloma.

Skb5, an SH3 adaptor protein, regulates Pmk1 MAPK signaling by controlling the intracellular localization of Mkh1 MAPKKK

The MAPK cascade is a highly conserved signaling module composed of MAPK/MAPKK/MAPKKK. MAPKKK Mkh1 is an initiating kinase in Pmk1 MAPK signaling, which regulates cell integrity in fission yeast. Our genetic screen for regulators of Pmk1 signaling identified Skb5 (Shk1 kinase binding protein 5), an SH3 domain-containing adaptor protein. Here, we showed that Skb5 serves as an inhibitor of Pmk1 MAPK signaling activation by downregulating Mkh1 localization to cell tips via its interaction with the SH3 domain. Consistently, the Mkh13PA mutant protein, with impaired Skb5 binding, remained in the cell tips, even when Skb5 was overproduced. Intriguingly, Skb5 needs Mkh1 to localize to the growing ends as Mkh1 deletion and disruption of Mkh1 binding impairs Skb5 localization. Deletion of Pck2, an upstream activator of Mkh1, impaired the cell tip localization of Mkh1 and Skb5 as well as Mkh1/Skb5 interaction. Interestingly, both Pck2 and Mkh1 localized to the cell tips at the G1/S phase, which coincided with Pmk1 MAPK activation. Altogether, Mkh1 localization to cell tips is important for transmitting upstream signaling to Pmk1 and Skb5 spatially regulates this process.

The Emerging and Diverse Roles of Src-Like Adaptor Proteins in Health and Disease

Although Src-like adaptor proteins (SLAP-1 and SLAP-2) were mainly studied in lymphocytes, where they act as negative regulators and provide fine control of receptor signaling, recently, several other functions of these proteins were discovered. In addition to the well-characterized immunoregulatory functions, SLAP proteins appear to have an essential role in the pathogenesis of type I hypersensitivity, osteoporosis, and numerous malignant diseases. Both adaptor proteins are expressed in a wide variety of tissues, where they have mostly inhibitory effects on multiple intracellular signaling pathways. In this review, we summarize the diverse effects of SLAP proteins.