A new twist to adaptor proteins contributes to regulation of lymphocyte cell signaling

Myocardial ischemia/reperfusion injury: the role of adaptor proteins Crk

Recent studies have reported that the ischemia/reperfusion (I/R) myocardium may act as an immune system where an exaggerated inflammatory reaction initiates. With activation of the immune system, damage-associated molecular patterns migrate and adhere into the I/R region and, consequently, induce myocardial injury. Emerging data have indicated that the adaptor proteins Crk are thought to play essential roles in signaling during apoptosis and cell adhesion and migration. Accumulated data highlight that Crk proteins are potential immunotherapeutic targets in immune diseases. However, very few studies have determined the roles of Crk on myocardial I/R injury. This mini review will focus on the emerging roles of Crk adaptors during myocardial I/R injury.

Fibroblast Growth Requires CT10 Regulator of Kinase (Crk) and Crk-like (CrkL)

CT10 regulator of kinase (Crk) and Crk-like (CrkL) are the cellular counterparts of the viral oncogene v-Crk Elevated levels of Crk and CrkL have been observed in many human cancers; inhibition of Crk and CrkL expression reduced the tumor-forming potential of cancer cell lines. Despite a close relationship between the Crk family proteins and tumorigenesis, how Crk and CrkL contribute to cell growth is unclear. We ablated endogenous Crk and CrkL from cultured fibroblasts carrying floxed alleles of Crk and CrkL by transfection with synthetic Cre mRNA (synCre). Loss of Crk and CrkL induced by synCre transfection blocked cell proliferation and caused shrinkage of the cytoplasm and the nucleus, formation of adherens junctions, and reduced cell motility. Ablation of Crk or CrkL alone conferred a much more modest reduction in cell proliferation. Reintroduction of CrkI, CrkII, or CrkL individually rescued cell proliferation in the absence of the endogenous Crk and CrkL, suggesting that Crk and CrkL play overlapping functions in regulating fibroblast growth. Serum and basic FGF induced phosphorylation of Akt, MAP kinases, and S6 kinase and Fos expression in the absence of Crk and CrkL, suggesting that cells lacking Crk and CrkL are capable of initiating major signal transduction pathways in response to extracellular stimuli. Furthermore, cell cycle and cell death analyses demonstrated that fibroblasts lacking Crk and CrkL become arrested at the G₁-S transition and undergo a modest apoptosis. Taken together, our results suggest that Crk and CrkL play essential overlapping roles in fibroblast growth.

In vivo regulation of human CrkII by cyclophilin A and FK506-binding protein

Members of the Crk family of adaptor proteins are key players in signal transduction from a variety of cell surface receptors. CrkI and CrkII are two alternative-spliced forms of a single gene which possess an N-terminal SH2 domain and an SH3 domain that mediate interaction with other proteins. CrkII possesses an additional C-terminal linker region plus an extra SH3 domain, which does not interact with other proteins, but operates as regulatory moiety. Utilizing human Jurkat T cells, we demonstrate that CrkII-SH3N binding of C3G is inhibited by cyclosporin A (CsA) plus FK506 that inhibit the cyclophilin A (CypA) and FK506 binding protein (FKBP) peptidyl-prolyl cis-trans isomerases (PPIases; also termed immunophilins), respectively. Jurkat T cells were found to express ∼ 5-fold lower levels of CrkI protein compared to CrkII, but the efficiency of C3G binding by CrkI was ∼ 5-fold higher than that of CrkII, suggesting that the majority of cellular CrkII proteins adopt a conformation that is inaccessible for C3G. Treatment of Jurkat T cells with CsA plus FK506 led to a time-dependent conformational change in overexpressed human CrkII1-236 protein-containing FRET-based biosensor, supporting the accumulation of cis conformers of human CrkII1-236 in the presence of PPIase inhibitors. Our data suggest that the Gly(219)-Pro-Tyr motif in the human CrkII linker region serves as the recognition and isomerization site of PPIases, and raise the possibility that CsA and FK506 might interfere with selected effector T cell functions via a CrkII-, but not CrkI-dependent mechanisms.

The Role of Crk Adaptor Proteins in T-Cell Adhesion and Migration

Crk adaptor proteins are key players in signal transduction from a variety of cell surface receptors. They are involved in early steps of lymphocyte activation through their SH2-mediated transient interaction with signal transducing effector molecules, such as Cbl, ZAP-70, CasL, and STAT5. In addition, they constitutively associate, via their SH3 domain, with effector molecules, such as C3G, that mediate cell adhesion and regulate lymphocyte extravasation and recruitment to sites of inflammation. Recent studies demonstrated that the conformation and function of CrkII is subjected to a regulation by immunophilins, which also affect CrkII-dependent T-cell adhesion to fibronectin and migration toward chemokines. This article addresses mechanisms that regulate CrkII conformation and function, in general, and emphasizes the role of Crk proteins in receptor-coupled signaling pathways that control T-lymphocyte adhesion and migration to inflammatory sites.

Contribution of Crk adaptor proteins to host cell and bacteria interactions

The Crk adaptor family of proteins comprises the alternatively spliced CrkI and CrkII isoforms, as well as the paralog Crk-like (CrkL) protein, which is encoded by a different gene. Initially thought to be involved in signaling during apoptosis and cell adhesion, this ubiquitously expressed family of proteins is now known to play essential roles in integrating signals from a wide range of stimuli. In this review, we describe the structure and function of the different Crk proteins. We then focus on the emerging roles of Crk adaptors during Enterobacteriaceae pathogenesis, with special emphasis on the important human pathogens Salmonella, Shigella, Yersinia, and enteropathogenic Escherichia coli. Throughout, we remark on opportunities for future research into this intriguing family of proteins.

CRKL overexpression suppresses in vitro proliferation, invasion and migration of murine hepatocarcinoma Hca-P cells

The signal adaptor CRK family protein play important roles in cancer cell progression, proliferation, migration and invasion. Previously, we showed that CRK was involved in lymphatic metastatic potential of murine hepatocarcinoma cells. In current work, as a member of CRK family, chicken tumour virus number 10 regulator of kinase-like protein (CRKL) was revealed to be associated with malignant behaviors of Hca-P, a murine HCC cell with lymph node metastatic (LNM) rate of ∼25%. CRKL overexpression in Hca-P by a constructed eukaryotic expression vector of pcDNA3.1/V5-HisB-CRKL significantly ameliorated its malignant biological properties. CCK-8 and soft agar colony formation assays indicated CRKL overexpression significantly inhibits the cell proliferation and colony formation abilities of Hca-P. Additionally, transwell assays indicated that the Hca-P cell migration and invasion capacities were apparently reduced following CRKL overexpression. As Hca-P is an ideal hepatocarcinoma cell model with low (initial) LNM potential, CRKL is shown to act as a potential suppressor and to provide new insight for both the malignant behaviors of hepatocarcinoma cells and lymphatic metastasis mechanism of hepatocarcinoma.

Adaptor regulation of LFA-1 signaling in T lymphocyte migration: Potential druggable targets for immunotherapies?

The integrin lymphocyte function associated antigen-1 (LFA-1) plays a key role in leukocyte trafficking and in adaptive immune responses through interactions with adhesive ligands, such as ICAM-1. Specific blockade of these interactions has validated LFA-1 as a therapeutic target in many chronic inflammatory diseases, however LFA-1 antagonists have not been clinically successful due to the development of a general immunosuppression, causing fatal side effects. Growing evidence has now established that LFA-1 mediates an array of intracellular signaling pathways by triggering a number of downstream molecules. In this context, a class of multimodular domain-containing proteins capable of recruiting two or more effector molecules, collectively known as "adaptor proteins," has emerged as important mediators in LFA-1 signal transduction. Here, we provide an overview of the adaptor proteins involved in the intracellular signaling cascades by which LFA-1 regulates T-cell motility and immune responses. The complexity of the LFA-1-associated signaling delineated in this review suggests that it may be an important and challenging focus for future research, enabling the identification of "tunable" targets for the development of immunotherapies.

Immunophilins control T lymphocyte adhesion and migration by regulating CrkII binding to C3G

Crk adaptor proteins are key players in signal transduction from a variety of cell surface receptors. CrkI and CrkII, the two alternative spliced forms of CRK, possess an N-terminal Src homology 2 domain, followed by a Src homology 3 (SH3) domain, whereas CrkII possesses in addition a C-terminal linker region plus a SH3 domain, which operate as regulatory moieties. In this study, we investigated the ability of immunophilins, which function as peptidyl-prolyl isomerases, to regulate Crk proteins in human T lymphocytes. We found that endogenous CrkII, but not CrkI, associates with the immunophilins, cyclophilin A, and 12-kDa FK506-binding protein, in resting human Jurkat T cells. In addition, cyclophilin A increased Crk SH3 domain-binding guanine-nucleotide releasing factor (C3G) binding to CrkII, whereas inhibitors of immunophilins, such as cyclosporine A (CsA) and FK506, inhibited CrkII, but not CrkI association with C3G. Expression in Jurkat T cells of phosphorylation indicator of Crk chimeric unit plasmid, a plasmid encoding the human CrkII1-236 sandwiched between cyan fluorescent protein and yellow fluorescent protein, demonstrated a basal level of fluorescence resonance energy transfer, which increased in response to cell treatment with CsA and FK506, reflecting increased trans-to-cis conversion of CrkII. Crk-C3G complexes are known to play an important role in integrin-mediated cell adhesion and migration. We found that overexpression of CrkI or CrkII increased adhesion and migration of Jurkat T cells. However, immunophilin inhibitors suppressed the ability of CrkII- but not CrkI-overexpressing cells to adhere to fibronectin-coated surfaces and migrate toward the stromal cell-derived factor 1α chemokine. The present data demonstrate that immunophilins regulate CrkII, but not CrkI activity in T cells and suggest that CsA and FK506 inhibit selected effector T cell functions via a CrkII-dependent mechanism.

Molecular dynamics of the proline switch and its role in Crk signaling

The Crk adaptor proteins play a central role as a molecular timer for the formation of protein complexes including various growth and differentiation factors. The loss of regulation of Crk results in many kinds of cancers. A self-regulatory mechanism for Crk was recently proposed, which involves domain–domain rearrangement. It is initiated by a cis–trans isomerization of a specific proline residue (Pro238 in chicken Crk II) and can be accelerated by Cyclophilin A. To understand how the proline switch controls the autoinhibition at the molecular level, we performed large-scale molecular dynamics and metadynamics simulations in the context of short peptides and multidomain constructs of chicken Crk II. We found that the equilibrium and kinetic properties of the macrostates are regulated not only by the local environments of specified prolines but also by the global organization of multiple domains. We observe the two macrostates (cis closed/autoinhibited and trans open/uninhibited) consistent with NMR experiments and predict barriers. We also propose an intermediate state, the trans closed state, which interestingly was reported to be a prevalent state in human Crk II. The existence of this macrostate suggests that the rate of switching off the autoinhibition by Cyp A may be limited by the relaxation rate of this intermediate state.

The adaptor protein Crk in immune response

The adaptor proteins Crk (CT10 (chicken tumor virus number 10) regulator of kinase), including CrkI, CrkII and Crk-like, are important signal molecules that regulate a variety of cellular processes. Considerable progress has been made in understanding the roles of the Crk family proteins in signal transduction, with a focus on cellular transformation and differentiation. However, since Crk was identified in 1988, very few studies have addressed how Crk regulates the immune response. Recent work demonstrates that Crk proteins function as critical signal molecules in regulating immune cell functions. Emerging data on the roles of Crk in activation and inhibitory immunoreceptor signaling suggest that Crk proteins are potential immunotherapeutic targets in cancer and infectious diseases. The aim of this review is to summarize recent key findings regarding the role of Crk in immune responses mediated by T, B and natural killer (NK) cells. In particular, the roles of Crk in NK cell functions are discussed.

Dynamic protein pathway activation mapping of adipose-derived stem cell differentiation implicates novel regulators of adipocyte differentiation

Next to embryonic stem cell research, adult stem cell research is providing a promising alternative for enhanced tissue regeneration and transplantation. The key biochemical networks controlling the differentiation processes regulating stem cell biology remain largely disputed and or undefined, contributing to a lack of knowledge of the principle phosphoregulatory events propagating signal transduction. To effectively monitor these events relative to adipocyte differentiation, this study utilized a high throughput reverse phase protein microarray platform and characterized adult adipose-derived stem cell (ASC) differentiation through the monitoring of ∼100 phosphospecific endpoints with 33 distinct time points examined across 14 days. This kinetic-based analysis showed time ordered signal transduction ultimately implicating pathways correlated with adipogenic differentiation. To further validate the causal significance of these network activations, pharmacological targeting was implemented to include the chemical inhibitors MAPK inhibitor PD169316, rapamycin, and HNMPA-(AM)3 yielding partial or complete disruption of adipocytic differentiation, as noted by a decrease or lack of lipid formation within the mature adipocytes. Based on this analysis, v-crk sarcoma virus CT10 oncogene homolog (CRKII) and c-abl oncogene 1, non-receptor tyrosine kinase (c-ABL) were implicated as novel key regulators of adipocyte differentiation, with v-akt murine thymoma viral oncogene (AKT), mammalian target of rapamycin (mTOR), and SMAD family member (SMAD) pathways being implicated as secondary regulators. This dynamic molecular profiling provides a novel insight into the signaling architecture of mesenchymal stem cell differentiation and may be useful in the development of therapeutic modulators for clinical applications; in addition to advancing the collective understanding of key cellular processes, ultimately contributing to more confident stem cell manipulation.

Effector-triggered immunity signaling: from gene-for-gene pathways to protein-protein interaction networks

In its simplicity and testability, Flor's gene-for-gene hypothesis has been a powerful driver in plant immunity research for decades. Once the molecular underpinnings of gene-for-gene resistance had come into sharper focus, there was a reassessment of Flor's hypothesis and a name change to effector-triggered immunity. As implied by the name change and exemplified by pioneering studies, plant immunity is increasingly described in terms of protein rather than genetic interactions. This progress leads to a reinterpretation of old concepts of pathogen recognition and resistance signaling and, of course, opens up new questions. Here, we provide a brief historical overview of resistance gene function and how a new focus on protein interactions can lead to a deeper understanding of the logic of plant innate immunity signaling.

Domain organization differences explain Bcr-Abl's preference for CrkL over CrkII

CrkL is a key signaling protein that mediates the leukemogenic activity of Bcr-Abl. CrkL is thought to adopt a structure that is similar to that of its CrkII homolog. The two proteins share high sequence identity and indistinguishable ligand binding preferences; yet they have distinct physiological roles. Here we show that the structures of CrkL and phosphorylated CrkL are drastically different than the corresponding structures of CrkII. As a result, the binding activities of the SH2 and SH3 domains in the two proteins are regulated in a distinct manner and to a different extent. The different structural architecture of CrkL and CrkII may account for their distinct functional roles. The data show that CrkL forms a constitutive complex with Abl thus explaining the strong preference of Bcr-Abl for CrkL. The results also highlight how the structural organization of the modular domains in adaptor proteins can control signaling outcome.

Structural basis for regulation of the Crk signaling protein by a proline switch

Proline switches, controlled by cis–trans isomerization, have emerged as a particularly effective regulatory mechanism in a wide range of biological processes. Here we report the structures of both the cis and trans conformers of a proline switch in Crk signaling protein. Proline isomerization toggles Crk between two conformations: an autoinhibitory, stabilized by the intramolecular association of two tandem SH3 domains in the cis form, and an uninhibited, activated conformation promoted by the trans form. In addition to acting as a structural switch the heterogeneous proline recruits cyclophilin A, which accelerates the interconversion rate between the isomers thereby regulating the kinetics of Crk activation. The data provide atomic insight into the mechanisms that underpin the functionality of this binary switch and elucidate its remarkable efficiency. The results also reveal novel SH3 binding surfaces highlighting the binding versatility and expanding the non-canonical ligand repertoire of this important signaling domain.

Mechanism of the calcium-induced trans-cis isomerization of a non-prolyl peptide bond in Clostridium histolyticum collagenase

cis peptide bonds in proteins are often rate-limiting steps in protein folding or conformational change and are frequently stabilized by metal ions. In the collagen-binding domain of Clostridium histolyticum collagenase, the binding of calcium ions triggers the formation of a cis peptide bond. We present free energy simulations of the formation of this cis peptide bond using a combined quantum mechanics/molecular mechanics approach together with adaptive umbrella sampling. From these simulations, we have determined that the calcium ions not only stabilize the cis peptide bond thermodynamically but also catalyze its formation; the free energy barrier to the formation of the cis peptide bond decreases from 21.4 kcal/mol in the absence of calcium ions to 10.3 kcal/mol in their presence. Two principal factors contribute to this reduction in the energy barrier. The calcium ions electrostatically stabilize the lone pair on the nitrogen atom that forms during the isomerization. In addition, their attraction to acidic amino acid side chains and formation of a hydrogen bond network constrain the peptide backbone in a way that makes it easier for the nitrogen to pyramidalize. Factors that explain the observed cooperativity of calcium binding are discussed.

Function, regulation and pathological roles of the Gab/DOS docking proteins

Since their discovery a little more than a decade ago, the docking proteins of the Gab/DOS family have emerged as important signalling elements in metazoans. Gab/DOS proteins integrate and amplify signals from a wide variety of sources including growth factor, cytokine and antigen receptors as well as cell adhesion molecules. They also contribute to signal diversification by channelling the information from activated receptors into signalling pathways with distinct biological functions. Recent approaches in protein biochemistry and systems biology have revealed that Gab proteins are subject to complex regulation by feed-forward and feedback phosphorylation events as well as protein-protein interactions. Thus, Gab/DOS docking proteins are at the centre of entire signalling subsystems and fulfil an important if not essential role in many physiological processes. Furthermore, aberrant signalling by Gab proteins has been increasingly linked to human diseases from various forms of neoplasia to Alzheimer's disease.

In this review, we provide a detailed overview of the structure, effector functions, regulation and evolution of the Gab/DOS family. We also summarize recent findings implicating Gab proteins, in particular the Gab2 isoform, in leukaemia, solid tumours and other human diseases.

Crk and CrkL adaptor proteins: networks for physiological and pathological signaling

The Crk adaptor proteins (Crk and CrkL) constitute an integral part of a network of essential signal transduction pathways in humans and other organisms that act as major convergence points in tyrosine kinase signaling. Crk proteins integrate signals from a wide variety of sources, including growth factors, extracellular matrix molecules, bacterial pathogens, and apoptotic cells. Mounting evidence indicates that dysregulation of Crk proteins is associated with human diseases, including cancer and susceptibility to pathogen infections. Recent structural work has identified new and unusual insights into the regulation of Crk proteins, providing a rationale for how Crk can sense diverse signals and produce a myriad of biological responses.