A mouse with a loss-of-function mutation in the c-Cbl TKB domain shows perturbed thymocyte signaling without enhancing the activity of the ZAP-70 tyrosine kinase

Immune dysregulation associated with co-occurring germline CBL and SH2B3 variants

Background

CBL syndrome is a RASopathy caused by heterozygous germline mutations of the Casitas B-lineage lymphoma (CBL) gene. It is characterized by heterogeneous clinical phenotype, including developmental delay, facial dysmorphisms, cardiovascular malformations and an increased risk of cancer development, particularly juvenile myelomonocytic leukemia (JMML). Although the clinical phenotype has been progressively defined in recent years, immunological manifestations have not been well elucidated to date.

Methods

We studied the genetic, immunological, coagulative, and clinical profile of a family with CBL syndrome that came to our observation after the diagnosis of JMML, with homozygous CBL mutation, in one of the members.

Results

Variant analysis revealed the co-occurrence of CBL heterozygous mutation (c.1141 T > C) and SH2B3 mutation (c.1697G > A) in two other members. Patients carrying both mutations showed an ALPS-like phenotype characterized by lymphoproliferation, cytopenia, increased double-negative T-cells, impaired Fas-mediated lymphocyte apoptosis, altered cell death in PBMC and low TRECs expression. A coagulative work-up was also performed and showed the presence of subclinical coagulative alterations in patients carrying both mutations.

Conclusion

In the reported family, we described immune dysregulation, as part of the clinical spectrum of CBL mutation with the co-occurrence of SH2B3.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40246-022-00414-y.

Chimeric Antigen Receptor Designed to Prevent Ubiquitination and Downregulation Showed Durable Antitumor Efficacy

Clinical evidence suggests that poor persistence of chimeric antigen receptor-T cells (CAR-T) in patients limits therapeutic efficacy. Here, we designed a CAR with recyclable capability to promote in vivo persistence and to sustain antitumor activity. We showed that the engagement of tumor antigens induced rapid ubiquitination of CARs, causing CAR downmodulation followed by lysosomal degradation. Blocking CAR ubiquitination by mutating all lysines in the CAR cytoplasmic domain (CAR^KR) markedly repressed CAR downmodulation by inhibiting lysosomal degradation while enhancing recycling of internalized CARs back to the cell surface. Upon encountering tumor antigens, CAR^KR-T cells ameliorated the loss of surface CARs, which promoted their long-term killing capacity. Moreover, CAR^KR-T cells containing 4-1BB signaling domains displayed elevated endosomal 4-1BB signaling that enhanced oxidative phosphorylation and promoted memory T cell differentiation, leading to superior persistence in vivo. Collectively, our study provides a straightforward strategy to optimize CAR-T antitumor efficacy by redirecting CAR trafficking.

Feedback regulation of RTK signaling in development

Precise regulation of the amplitude and duration of receptor tyrosine kinase (RTK) signaling is critical for the execution of cellular programs and behaviors. Understanding these control mechanisms has important implications for the field of developmental biology, and in recent years, the question of how augmentation or attenuation of RTK signaling via feedback loops modulates development has become of increasing interest. RTK feedback regulation is also important for human disease research; for example, germline mutations in genes that encode RTK signaling pathway components cause numerous human congenital syndromes, and somatic alterations contribute to the pathogenesis of diseases such as cancers. In this review, we survey regulators of RTK signaling that tune receptor activity and intracellular transduction cascades, with a focus on the roles of these genes in the developing embryo. We detail the diverse inhibitory mechanisms utilized by negative feedback regulators that, when lost or perturbed, lead to aberrant increases in RTK signaling. We also discuss recent biochemical and genetic insights into positive regulators of RTK signaling and how these proteins function in tandem with negative regulators to guide embryonic development.

Regulation of T cell development by c-Cbl: essential role of Lck

A canonical pre-TCR/TCR signaling pathway critical for thymic T cell development involves sequential phosphorylation and signaling through Lck, Zap70, Lat and Slp76. However, we and others have previously reported that genomic deletion of c-Cbl (Cbl) partially or completely reverses the defects in thymic development in mice deficient in Zap70, Slp76, Lat or Vav1, indicating the presence of alternative pathways normally suppressed by Cbl. To further elucidate pre-TCR/TCR signaling pathways involved in thymic development, we characterized the effect of Cbl inactivation on developmental and signaling defects in mice deficient in proximal signaling molecules Lck and Zap70. Inactivation of Cbl partially reversed defective T cell development in Zap70 (-/-) mice and reversed defects in phosphorylation of Erk, Plc-γ1, Vav1 and Akt, in TCR-stimulated Cbl (-/-) Zap70 (-/-) thymocytes. Recent reports identified an essential role of Lck in associating with CD4 and CD8 coreceptors and mediating the requirement for MHC restriction in TCR recognition. Since TCR recognition has been shown to be MHC-restricted in Cbl (-/-) mice, it was of interest to determine whether the requirement for Lck remained unmodified by Cbl deletion. Indeed, in contrast to the effect of Cbl inactivation in partially or fully bypassing requirements for other TCR signaling components, inactivation of Cbl did not reverse either defective T cell development or defective phosphorylation of TCR signaling molecules in Lck (-/-) mice. Thus, Lck, which plays a unique role in enforcing MHC restriction, is essential for thymic development in presence or absence of Cbl, ensuring MHC restriction of T cells derived from either pathway.

Loss of Cbl-PI3K interaction in mice prevents significant bone loss following ovariectomy

Cbl and Cbl-b are E3 ubiquitin ligases and adaptor proteins, which perform regulatory roles in bone remodeling. Cbl-/- mice have delayed bone development due to decreased osteoclast migration. Cbl-b-/- mice are osteopenic due to increased bone resorbing activity of osteoclasts. Unique to Cbl, but not present in Cbl-b, is tyrosine 737 in the YEAM motif, which upon phosphorylation provides a binding site for the regulatory p85 subunit of PI3K. Substitution of tyrosine 737 with phenylalanine (Y737F, CblYF/YF mice) prevents Y737 phosphorylation and abrogates the Cbl-PI3K interaction. We have previously reported that CblYF/YF mice had increased bone volume due to defective bone resorption and increased bone formation. Here we show that the lumbar vertebra from CblYF/YF mice did not have significant bone loss following ovariectomy. Our data also suggests that abrogation of Cbl-PI3K interaction in mice results in the loss of coupling between bone resorption and formation, since ovariectomized CblYF/YF mice did not show significant changes in serum levels of c-terminal telopeptide (CTX), whereas the serum levels of pro-collagen type-1 amino-terminal pro-peptide (P1NP) were decreased. In contrast, following ovariectomy, Cbl-/- and Cbl-b-/- mice showed significant bone loss in the tibiae and L2 vertebrae, concomitant with increased serum CTX and P1NP levels. These data indicate that while lack of Cbl or Cbl-b distinctly affects bone remodeling, only the loss of Cbl-PI3K interaction protects mice from significant bone loss following ovariectomy.

Protein tyrosine kinase regulation by ubiquitination: critical roles of Cbl-family ubiquitin ligases

Protein tyrosine kinases (PTKs) coordinate a broad spectrum of cellular responses to extracellular stimuli and cell-cell interactions during development, tissue homeostasis, and responses to environmental challenges. Thus, an understanding of the regulatory mechanisms that ensure physiological PTK function and potential aberrations of these regulatory processes during diseases such as cancer are of broad interest in biology and medicine. Aside from the expected role of phospho-tyrosine phosphatases, recent studies have revealed a critical role of covalent modification of activated PTKs with ubiquitin as a critical mechanism of their negative regulation. Members of the Cbl protein family (Cbl, Cbl-b and Cbl-c in mammals) have emerged as dominant "activated PTK-selective" ubiquitin ligases. Structural, biochemical and cell biological studies have established that Cbl protein-dependent ubiquitination targets activated PTKs for degradation either by facilitating their endocytic sorting into lysosomes or by promoting their proteasomal degradation. This mechanism also targets PTK signaling intermediates that become associated with Cbl proteins in a PTK activation-dependent manner. Cellular and animal studies have established that the relatively broadly expressed mammalian Cbl family members Cbl and Cbl-b play key physiological roles, including their critical functions to prevent the transition of normal immune responses into autoimmune disease and as tumor suppressors; the latter function has received validation from human studies linking mutations in Cbl to human leukemia. These newer insights together with embryonic lethality seen in mice with a combined deletion of Cbl and Cbl-b genes suggest an unappreciated role of the Cbl family proteins, and by implication the ubiquitin-dependent control of activated PTKs, in stem/progenitor cell maintenance. Future studies of existing and emerging animal models and their various cell lineages should help test the broader implications of the evolutionarily-conserved Cbl family protein-mediated, ubiquitin-dependent, negative regulation of activated PTKs in physiology and disease.

Protein tyrosine kinase regulation by ubiquitination: critical roles of Cbl-family ubiquitin ligases

Protein tyrosine kinases (PTKs) coordinate a broad spectrum of cellular responses to extracellular stimuli and cell-cell interactions during development, tissue homeostasis, and responses to environmental challenges. Thus, an understanding of the regulatory mechanisms that ensure physiological PTK function and potential aberrations of these regulatory processes during diseases such as cancer are of broad interest in biology and medicine. Aside from the expected role of phospho-tyrosine phosphatases, recent studies have revealed a critical role of covalent modification of activated PTKs with ubiquitin as a critical mechanism of their negative regulation. Members of the Cbl protein family (Cbl, Cbl-b and Cbl-c in mammals) have emerged as dominant "activated PTK-selective" ubiquitin ligases. Structural, biochemical and cell biological studies have established that Cbl protein-dependent ubiquitination targets activated PTKs for degradation either by facilitating their endocytic sorting into lysosomes or by promoting their proteasomal degradation. This mechanism also targets PTK signaling intermediates that become associated with Cbl proteins in a PTK activation-dependent manner. Cellular and animal studies have established that the relatively broadly expressed mammalian Cbl family members Cbl and Cbl-b play key physiological roles, including their critical functions to prevent the transition of normal immune responses into autoimmune disease and as tumor suppressors; the latter function has received validation from human studies linking mutations in Cbl to human leukemia. These newer insights together with embryonic lethality seen in mice with a combined deletion of Cbl and Cbl-b genes suggest an unappreciated role of the Cbl family proteins, and by implication the ubiquitin-dependent control of activated PTKs, in stem/progenitor cell maintenance. Future studies of existing and emerging animal models and their various cell lineages should help test the broader implications of the evolutionarily-conserved Cbl family protein-mediated, ubiquitin-dependent, negative regulation of activated PTKs in physiology and disease.

Indispensable roles of mammalian Cbl family proteins as negative regulators of protein tyrosine kinase signaling: Insights from in vivo models

All higher eukaryotes utilize protein tyrosine kinases (PTKs) as molecular switches to control a variety of cellular signals. Notably, many PTKs have been identified as proto-oncogenes whose aberrant expression, mutations or co-option by pathogens can lead to human malignancies. Thus, it is obvious that PTK functions must be precisely regulated in order to maintain homeostasis of an organism. Investigations over the past fifteen years have revealed that members of the Cbl family proteins can serve as negative regulators of PTK signaling, and biochemical and cell biological studies have unraveled the mechanistic basis of this regulation. Yet, it is only recently that the field has begun to appreciate the real significance of this novel regulatory apparatus in shaping PTK-mediated signaling in organismic contexts and in human diseases. Here, we discuss recent progress in murine models that are beginning to provide insights into the critical roles of Cbl proteins in physiological pathways, with important implications in understanding how aberrations of Cbl proteins contribute to oncogenesis.

Cbl enforces Vav1 dependence and a restricted pathway of T cell development

Extensive studies of pre-TCR- and TCR-dependent signaling have led to characterization of a pathway deemed essential for efficient T cell development, and comprised of a cascade of sequential events involving phosphorylation of Lck and ZAP-70, followed by phosphorylation of LAT and SLP-76, and subsequent additional downstream events. Of interest, however, reports from our lab as well as others have indicated that the requirements for ZAP-70, LAT, and SLP-76 are partially reversed by inactivation of c-Cbl (Cbl), an E3 ubiquitin ligase that targets multiple molecules for ubiquitination and degradation. Analysis of signaling events in these Cbl knockout models, including the recently reported analysis of SLP-76 transgenes defective in interaction with Vav1, suggested that activation of Vav1 might be a critical event in alternative pathways of T cell development. To extend the analysis of signaling requirements for thymic development, we have therefore assessed the effect of Cbl inactivation on the T cell developmental defects that occur in Vav1-deficient mice. The defects in Vav1-deficient thymic development, including a marked defect in DN3-DN4 transition, were completely reversed by Cbl inactivation, accompanied by enhanced phosphorylation of PLC-γ1 and ERKs in response to pre-TCR/TCR cross-linking of Vav1^-/-Cbl^-/- DP thymocytes. Taken together, these results suggest a substantially modified paradigm for pre-TCR/TCR signaling and T cell development. The observed consensus pathways of T cell development, including requirements for ZAP-70, LAT, SLP-76, and Vav1, appear to reflect the restriction by Cbl of an otherwise much broader set of molecular pathways capable of mediating T cell development.

Myeloid leukemia development in c-Cbl RING finger mutant mice is dependent on FLT3 signaling

Although myeloid leukemias are primarily caused by leukemic stem cells, the molecular basis of their transformation remains largely unknown. Here, by analyzing mice with a mutation in the RING finger domain of c-Cbl, we show that the E3 ubiquitin ligase activity of c-Cbl is required to restrict myeloid leukemia development. These mice develop a myeloproliferative disease which progresses to leukemia and involves hematopoietic progenitors that exhibit augmented FLT3 signaling. Suppressing this signaling through matings with FLT3 ligand knockout mice prevents leukemia development. We also observe enhanced c-Kit, Akt and Erk activity, and deregulated expression of leukemia-associated transcription factors in hematopoietic progenitors. The characterization of these perturbations provides direction for therapeutics that may aid the treatment of patients with c-Cbl mutations.

Rapidly fatal myeloproliferative disorders in mice with deletion of Casitas B-cell lymphoma (Cbl) and Cbl-b in hematopoietic stem cells

Casitas B-cell lymphoma (Cbl)-family E3 ubiquitin ligases are negative regulators of tyrosine kinase signaling. Recent work has revealed a critical role of Cbl in the maintenance of hematopoietic stem cell (HSC) homeostasis, and mutations in CBL have been identified in myeloid malignancies. Here we show that, in contrast to Cbl or Cbl-b single-deficient mice, concurrent loss of Cbl and Cbl-b in the HSC compartment leads to an early-onset lethal myeloproliferative disease in mice. Cbl, Cbl-b double-deficient bone marrow cells are hypersensitive to cytokines, and show altered biochemical response to thrombopoietin. Thus, Cbl and Cbl-b play redundant but essential roles in HSC regulation, whose breakdown leads to hematological abnormalities that phenocopy crucial aspects of mutant Cbl-driven human myeloid malignancies.

ZAP-70: an essential kinase in T-cell signaling

ZAP-70 is a cytoplasmic protein tyrosine kinase that plays a critical role in the events involved in initiating T-cell responses by the antigen receptor. Here we review the structure of ZAP-70, its regulation, its role in development and in disease. We also describe a model experimental system in which ZAP-70 function can be interrupted by a small chemical inhibitor.

SLAP, a regulator of immunoreceptor ubiquitination, signaling, and trafficking

Src-like adapter proteins (SLAP and SLAP-2) constitute a family of proteins that are expressed in a variety of cell types but are studied most extensively in lymphocytes. They have been shown to associate with proximal components of the T-cell receptor (TCR) and B-cell receptor (BCR) signaling complexes. An interaction of SLAP with c-Cbl leads to the ubiquitination and degradation of phosphorylated components of the TCR- and BCR-signaling complexes. The absence of this process in immature SLAP-deficient T and B cells leads to increased immunoreceptor levels due to decreased intracellular retention and degradation. We propose a model in which SLAP-dependent regulation of immunoreceptor levels allows for finer control of immunoreceptor signaling. Thus, SLAP functions to dampen immunoreceptor signaling, thereby influencing lymphocyte development and repertoire selection.

Tonic ubiquitylation controls T-cell receptor:CD3 complex expression during T-cell development

Expression of the T-cell receptor (TCR):CD3 complex is tightly regulated during T-cell development. The mechanism and physiological role of this regulation are unclear. Here, we show that the TCR:CD3 complex is constitutively ubiquitylated in immature double positive (DP) thymocytes, but not mature single positive (SP) thymocytes or splenic T cells. This steady state, tonic CD3 monoubiquitylation is mediated by the CD3varepsilon proline-rich sequence, Lck, c-Cbl, and SLAP, which collectively trigger the dynamin-dependent downmodulation, lysosomal sequestration and degradation of surface TCR:CD3 complexes. Blocking this tonic ubiquitylation by mutating all the lysines in the CD3 cytoplasmic tails significantly upregulates TCR levels on DP thymocytes. Mimicking monoubiquitylation by expression of a CD3zeta-monoubiquitin (monoUb) fusion molecule significantly reduces TCR levels on immature thymocytes. Moreover, modulating CD3 ubiquitylation alters immunological synapse (IS) formation and Erk phosphorylation, thereby shifting the signalling threshold for positive and negative selection, and regulatory T-cell development. Thus, tonic TCR:CD3 ubiquitylation results in precise regulation of TCR expression on immature T cells, which is required to maintain the fidelity of T-cell development.

c-Cbl promotes T cell receptor-induced thymocyte apoptosis by activating the phosphatidylinositol 3-kinase/Akt pathway

The ability of thymocytes to assess T cell receptor (TCR) signaling strength and initiate the appropriate downstream response is crucial for determining their fate. We have previously shown that a c-Cbl RING finger mutant knock-in mouse, in which the E3 ubiquitin ligase activity of c-Cbl is inactivated, is highly sensitive to TCR-induced death signals that cause thymic deletion. This high intensity signal involves the enhanced tyrosine phosphorylation of the mutant c-Cbl protein promoting a marked increase in the activation of Akt. Here we show that this high intensity signal in c-Cbl RING finger mutant thymocytes also promotes the enhanced induction of two mediators of TCR-directed thymocyte apoptosis, Nur77 and the pro-apoptotic Bcl-2 family member, Bim. In contrast, a knock-in mouse harboring a mutation at Tyr-737, the site in c-Cbl that activates phosphatidylinositol 3-kinase, shows reduced TCR-mediated responses including suppression of Akt activation, a reduced induction of Nur77 and Bim, and greater resistance to thymocyte death. These findings identify tyrosine-phosphorylated c-Cbl as a critical sensor of TCR signal strength that regulates the engagement of death-promoting signals.

Regulation of T-cell tolerance by calcium/NFAT signaling

Cells that escape negative selection in the thymus must be inactivated or eliminated in the periphery through a series of mechanisms that include the induction of anergy, dominant suppression by regulatory T cells, and peripheral deletion of self-reactive T cells. Calcium signaling plays a central role in the induction of anergy in T cells, which become functionally inactivated and incapable of proliferating and expressing cytokines following antigen re-encounter. Suboptimal stimulation of T cells results in the activation of a calcium/calcineurin/nuclear factor of activated T cells-dependent cell-intrinsic program of self-inactivation. The proteins encoded by those genes are required to impose a state of functional unresponsiveness through different mechanisms that include downregulation of T-cell receptor signaling and inhibition of cytokine transcription.

Organization of proximal signal initiation at the TCR:CD3 complex

The series of events leading to T-cell activation following antigen recognition has been extensively investigated. Although the exact mechanisms of ligand binding and transmission of this extracellular interaction into a productive intracellular signaling sequence remains incomplete, it has been known for many years that the immunoreceptor tyrosine activation motifs (ITAMs) of the T-cell receptor (TCR):CD3 complex are required for initiation of this signaling cascade because of the recruitment and activation of multiple protein tyrosine kinases, signaling intermediates, and adapter molecules. It however remains unclear why the TCR:CD3 complex requires 10 ITAMs, while many other ITAM-containing immune receptors, such as Fc receptors (FcRs) and the B cell receptor (BCR), contain far fewer ITAMs. We have recently demonstrated that various parameters of T cell development and activation are influenced by the number, as well as location and type, of ITAMs within the TCR:CD3 complex and hence propose that the TCR is capable of 'scalable signaling' that facilitates the initiation and orchestration of diverse T-cell functions. While many of the underlying mechanisms remain hypothetical, this review intends to amalgamate what we have learned from conventional biochemical analyses regarding initiation and diversification of T-cell signaling, with more recent evidence from molecular and fluorescent microscopic analyses, to propose a broader purpose for the TCR:CD3 ITAMs. Rather than simply signal initiation, individual ITAMs may also be responsible for the differential recruitment of signaling and regulatory molecules which ultimately affects T-cell development, activation and differentiation.

c-Cbl tyrosine kinase-binding domain mutant G306E abolishes the interaction of c-Cbl with CD38 and fails to promote retinoic acid-induced cell differentiation and G0 arrest

Retinoic acid (RA) causes HL-60 human myeloblastic leukemia cell myeloid differentiation that is dependent on MAPK signaling. The process is propelled by c-Cbl, which binds the CD38 receptor as part of a signaling complex generating MAPK signaling. Here we report that the capability of c-Cbl to do this is lost in the G306E tyrosine kinase-binding domain mutant. Unlike wild-type (WT) c-Cbl, the G306E mutant c-Cbl fails to propel RA-induced differentiation, and disrupts the normal association with CD38. The G306E mutant does, like WT c-Cbl, co-immunoprecipitate with Vav, Slp-76, and p38. But unlike WT c-Cbl, does not cause MAPK signaling. In contrast, the C381A Ring finger domain mutant functions like WT c-Cbl. It binds CD38 and is part of the same apparent c-Cbl/Slp-76/Vav/p38 signaling complex. The C381A mutant causes MAPK signaling and propels RA-induced differentiation. In addition to HL-60 cells and their WT or mutant c-Cbl stable transfectants, the c-Cbl/Vav/Slp-76 complex is also found in NB4 cells where c-Cbl was previously also found to bind CD38. The data are consistent with a model in which the G306E mutant c-Cbl forms a signaling complex that includes Slp-76, Vav, and p38; but does not drive MAPK signaling because it fails to bind the CD38 receptor. Without the G306E mutation the c-Cbl unites CD38 with the signaling complex and delivers a MAPK signal that drives RA-induced differentiation. The results demonstrate the importance of the Gly306 residue in the ability of c-Cbl to propel RA-induced differentiation.

The structure, regulation, and function of ZAP-70

The tyrosine ZAP-70 (zeta-associated protein of 70 kDa) kinase plays a critical role in activating many downstream signal transduction pathways in T cells following T-cell receptor (TCR) engagement. The importance of ZAP-70 is evidenced by the severe combined immunodeficiency that occurs in ZAP-70-deficient mice and humans. In this review, we describe recent analyses of the ZAP-70 crystal structure, revealing a complex regulatory mechanism of ZAP-70 activity, the differential requirements for ZAP-70 and spleen tyrosine kinase (SyK) in early T-cell development, as well as the role of ZAP-70 in chronic lymphocytic leukemia and autoimmunity. Thus, the critical importance of ZAP-70 in TCR signaling and its predominantly T-cell-restricted expression pattern make ZAP-70 an attractive drug target for the inhibition of pathological T-cell responses in disease.

Lymphocyte signaling: beyond knockouts

The analysis of lymphocyte signaling was greatly enhanced by the advent of gene targeting, which allows the selective inactivation of a single gene. Although this gene 'knockout' approach is often informative, in many cases, the phenotype resulting from gene ablation might not provide a complete picture of the function of the corresponding protein. If a protein has multiple functions within a single or several signaling pathways, or stabilizes other proteins in a complex, the phenotypic consequences of a gene knockout may manifest as a combination of several different perturbations. In these cases, gene targeting to 'knock in' subtle point mutations might provide more accurate insight into protein function. However, to be informative, such mutations must be carefully based on structural and biophysical data.

Differential impact of the CD45 juxtamembrane wedge on central and peripheral T cell receptor responses

The cooperative activity of protein tyrosine kinases and phosphatases plays a central role in regulation of T cell receptor (TCR) signal strength. Perturbing this balance, and thus the threshold for TCR signals, has profound impacts on T cell development and function. We previously generated mice containing a point mutation in the juxtamembrane wedge of the receptor-like protein tyrosine phosphatase CD45. Demonstrating the critical negative regulatory function of the wedge, the CD45 E613R (WEDGE) mutation led to a lymphoproliferative disorder (LPD) and a lupus-like autoimmune syndrome. Using genetic, cellular, and biochemical approaches, we now demonstrate that the CD45 wedge influences T cell development and function. Consistent with increased TCR signal strength, WEDGE mice have augmented positive selection and enhanced sensitivity to the CD4-mediated disease experimental autoimmune encephalitis (EAE). These correspond with hyperresponsive calcium and pERK responses to TCR stimulation in thymocytes, but surprisingly, not in peripheral T cells, where these responses are actually depressed. Together, the data support a role for the CD45 wedge in regulation of T cell responses in vivo and suggest that its effects depend on cellular context.

Dual role of Cbl links critical events in BCR endocytosis

Receptor endocytosis down-regulates ligand-induced signaling in a timely manner and depends on cytoskeletal remodeling. In B lymphocytes, internalization of B cell receptors (BCRs) is also critical to antigen presentation. However, the mechanisms underlying BCR endocytosis are not fully understood. Similarly, the molecular mechanisms linking endocytosis to cytoskeletal remodeling remain poorly defined. We used flow cytometry, pull-down assays, immunochemistry and fluorescence microscopy to investigate BCR internalization in the DT40 B cell line. We demonstrate that ablation of Cbl impacts BCR endocytosis and the underlying cytoskeletal dynamics. Specifically, we demonstrate that ligand-induced endocytosis is impaired in Cbl-/- cells and that the ubiquitin ligase activity is required for Cbl to promote endocytosis. We also show that phosphorylation of CrkII, activation of Rac downstream of CrkII and BCR capping require Cbl. Furthermore, we show that the association of Cbl and CrkII requires phosphorylation of Cbl, but not its ubiquitin ligase activity. Our data indicate that Cbl promotes BCR endocytosis and attenuates ligand-induced signaling by virtue of its ability to orchestrate receptor ubiquitylation and cytoskeletal dynamics.

The Cbl interactome and its functions

Cbl proteins are ubiquitin ligases and multifunctional adaptor proteins that are implicated in the regulation of signal transduction in various cell types and in response to different stimuli. Cbl-associated proteins can assemble together at a given time or space inside the cell, and such an interactome can form signal competent networks that control many physiological processes. Dysregulation of spatial or temporal constraints in the Cbl interactome results in the development of human pathologies such as immune diseases, diabetes and cancer.

Cbl-b is a negative regulator of inflammatory cytokines produced by IgE-activated mast cells

c-Cbl and Cbl-b E3 ubiquitin ligases are abundantly expressed in hemopoietic cells where they negatively regulate the activity and levels of many cell surface receptors and associated signaling molecules. By comparing bone marrow-derived mast cells from c-Cbl and Cbl-b-deficient mice it has recently been shown that Cbl-b is the dominant family member for negatively regulating signaling responses from high-affinity IgE receptors. In this study, we suggest that a possible reason for the greater enhancement of IgE receptor signaling in Cbl-b-deficient mice is the relatively higher levels of Cbl-b protein over c-Cbl in mast cells compared with other hemopoietic cells. We also directly compare mast cells from c-Cbl and Cbl-b-deficient mice and find that loss of Cbl-b, but not c-Cbl, increases cell growth, retards receptor internalization, and causes the sustained tyrosine phosphorylation of Syk and its substrates. However, loss of Cbl-b does not enhance the activation of ERK or Akt, nor does it promote a greater calcium response. Furthermore, loss of Cbl-b or c-Cbl does not increase levels of the Syk or Lyn protein tyrosine kinases. Most notable, however, is the extremely large increase in the production of proinflammatory cytokines TNF-alpha, IL-6, and MCP-1 by Cbl-b(-/-) mast cells compared with levels produced by c-Cbl(-/-) or wild-type cells. This marked induction, which appears to be restricted to these three cytokines, is dependent on IgE receptor activation and correlates with enhanced IkappaB kinase phosphorylation. Thus, Cbl-b functions as a potent negative regulator of cytokines that promote allergic and inflammatory reactions.

The Cbl family proteins: ring leaders in regulation of cell signaling

The proto-oncogenic protein c-Cbl was discovered as the cellular form of v-Cbl, a retroviral transforming protein. This was followed over the years by important discoveries, which identified c-Cbl and other Cbl-family proteins as key players in several signaling pathways. c-Cbl has donned the role of a multivalent adaptor protein, capable of interacting with a plethora of proteins, and has been shown to positively influence certain biological processes. The identity of c-Cbl as an E3 ubiquitin ligase unveiled the existence of an important negative regulatory pathway involved in maintaining homeostasis in protein tyrosine kinase (PTK) signaling. Recent years have also seen the emergence of novel regulators of Cbl, which have provided further insights into the complexity of Cbl-influenced pathways. This review will endeavor to provide a summary of current studies focused on the effects of Cbl proteins on various biological processes and the mechanism of these effects. The major sections of the review are as follows: Structure and genomic organization of Cbl proteins; Phosphorylation of Cbl; Interactions of Cbl; Localization of Cbl; Mechanism of effects of Cbl: (a) Ubiquitylation-dependent events: This section elucidates the mechanism of Cbl-mediated downregulation of EGFR and details the PTK and non-PTKs targeted by Cbl. In addition, it addresses the functional requirements for E3 Ubiquitin ligase activity of Cbl and negative regulation of Cbl-mediated downregulation of PTKs, (b) Adaptor functions: This section discusses the mechanisms of adaptor functions of Cbl in mitogen-activated protein kinase (MAPK) activation, insulin signaling, regulation of Ras-related protein 1 (Rap1), PI-3' kinase signaling, and regulation of Rho-family GTPases and cytoskeleton; Biological functions: This section gives an account of the diverse biological functions of Cbl and includes the role of Cbl in transformation, T-cell signaling and thymus development, B-cell signaling, mast-cell degranulation, macrophage functions, bone development, neurite growth, platelet activation, muscle degeneration, and bacterial invasion; Conclusions and perspectives.

Alpha-MSH regulates protein ubiquitination in T cells

The neuropeptide alpha-melanocyte stimulating hormone (alpha-MSH) suppresses IFN-gamma + T cells from mice. We discovered, however, that despite this significant production by DTH-mediating effector CD4 supression of IFN-gamma production, alpha-MSH-treated effector T cells had the same level of IFN-gamma mRNA expression and intracellular IFN-gamma protein as untreated activated T cells. In order to explain why IFN-gamma production was suppressed in the face of unchanged mRNA and intracellular IFN-gamma levels, we looked for mechanisms that could increase the degradation of IFN-gamma within the alpha-MSH-treated T cells. Among the known pathways of post-translational intracellular protein modification, the ubiquitin-proteosome system was examined in alpha-MSH-treated T cells to see if a post-translational protein modification occurred to prevent IFN-gamma secretion from the cell. Immunoblots from alpha-MSH-treated T cells showed higher levels of protein ubiquitination when compared to untreated T cells. Resting T cells treated with alpha-MSH also demonstrated enhanced protein ubiquitination. We found that IFN-gamma is one of the ubiquitinated proteins in the alpha-MSH-treated activated T cells. Our results demonstrate that one of the mechanisms by which alpha-MSH regulates T cell activity is through mediating a change in the pattern of protein ubiquitination in T cells.

c-Cbl and Cbl-b ubiquitin ligases: substrate diversity and the negative regulation of signalling responses

The activation of signalling pathways by ligand engagement with transmembrane receptors is responsible for determining many aspects of cellular function and fate. While these outcomes are initially determined by the nature of the ligand and its receptor, it is also essential that intracellular enzymes, adaptor proteins and transcription factors are correctly assembled to convey the intended response. In recent years, it has become evident that proteins that regulate the amplitude and duration of these signalling responses are also critical in determining the function and fate of cells. Of these, the Cbl family of E3 ubiquitin ligases and adaptor proteins has emerged as key negative regulators of signals from many types of cell-surface receptors. The array of receptors and downstream signalling proteins that are regulated by Cbl proteins is diverse; however, in most cases, the receptors have a common link in that they either possess a tyrosine kinase domain or they form associations with cytoplasmic PTKs (protein tyrosine kinases). Thus Cbl proteins become involved in signalling responses at a time when PTKs are first activated and therefore provide an initial line of defence to ensure that signalling responses proceed at the desired intensity and duration.

Src-like adaptor protein down-regulates T cell receptor (TCR)-CD3 expression by targeting TCRzeta for degradation

Src-like adaptor protein (SLAP) down-regulates expression of the T cell receptor (TCR)–CD3 complex during a specific stage of thymocyte development when the TCR repertoire is selected. Consequently, SLAP−/− thymocytes display alterations in thymocyte development. Here, we have studied the mechanism of SLAP function. We demonstrate that SLAP-deficient thymocytes have increased TCRζ chain expression as a result of a defect in TCRζ degradation. Failure to degrade TCRζ leads to an increased pool of fully assembled TCR–CD3 complexes that are capable of recycling back to the cell surface. We also provide evidence that SLAP functions in a pathway that requires the phosphorylated TCRζ chain and the Src family kinase Lck, but not ZAP-70 (ζ-associated protein of 70 kD). These studies reveal a unique mechanism by which SLAP contributes to the regulation of TCR expression during a distinct stage of thymocyte development.

Selective defect in antigen-induced TCR internalization at the immune synapse of CD8 T cells bearing the ZAP-70(Y292F) mutation

Cbl proteins have been implicated in ligand-induced TCR/CD3 down-modulation, but underlying mechanisms are unclear. We analyzed the effect of mutation of a cbl-binding site on ZAP-70 (ZAP-Y292F) on dynamics, internalization, and degradation of the TCR/CD3 complex in response to distinct stimuli. Naive CD8 T cells expressing the P14 transgenic TCR from ZAP-Y292F mice were selectively affected in TCR/CD3 down-modulation in response to antigenic stimulation, whereas neither anti-CD3 Ab-, and PMA-induced TCR down-modulation, nor constitutive receptor endocytosis/cycling were impaired. We further established that the defect in TCR/CD3 down-modulation in response to Ag was paralleled by an impaired TCR/CD3 internalization and CD3zeta degradation. Analysis of T/APC conjugates revealed that delayed redistribution of TCR at the T/APC contact zone was paralleled by a delay in TCR internalization in the synaptic zone in ZAP-Y292F compared with ZAP-wild-type T cells. Cbl recruitment to the synapse was also retarded in ZAP-Y292F T cells, although F-actin and LFA-1 redistribution was similar for both cell types. This study identifies a step involving ZAP-70/cbl interaction that is critical for rapid internalization of the TCR/CD3 complex at the CD8 T cell/APC synapse.

Loss of c-Cbl RING finger function results in high-intensity TCR signaling and thymic deletion

Signaling from the T-cell receptor (TCR) in thymocytes is negatively regulated by the RING finger-type ubiquitin ligase c-Cbl. To further investigate this regulation, we generated mice with a loss-of-function mutation in the c-Cbl RING finger domain. These mice exhibit complete thymic deletion by young adulthood, which is not caused by a developmental block, lack of progenitors or peripheral T-cell activation. Rather, this phenotype correlates with greatly increased expression of the CD5 and CD69 activation markers and increased sensitivity to anti-CD3-induced cell death. Thymic loss contrasts the normal fate of the c-Cbl-/- thymus, even though thymocytes from both mutant mice show equivalent enhancement in proximal TCR signaling, Erk activation and calcium mobilization. Remarkably, only the RING finger mutant thymocytes show prominent TCR-directed activation of Akt. We show that the mutant c-Cbl protein itself is essential for activating this pathway by recruiting the p85 regulatory subunit of PI 3-kinase. This study provides a unique model for analyzing high-intensity TCR signals that cause thymocyte deletion and highlights multiple roles of c-Cbl in regulating this process.

Intramolecular regulatory switch in ZAP-70: analogy with receptor tyrosine kinases

ZAP-70, a Syk family cytoplasmic protein tyrosine kinase (PTK), is required to couple the activated T-cell antigen receptor (TCR) to downstream signaling pathways. It contains two tandem SH2 domains that bind to phosphorylated TCR subunits and a C-terminal catalytic domain. The region connecting the SH2 domains with the kinase domain, termed interdomain B, has previously been shown to have striking regulatory effects on ZAP-70 function, presumed to be due to the recruitment of key substrates. Paradoxically, deletion of interdomain B preserves ZAP-70 function. Recent structural studies of several receptor tyrosine kinases (RTKs) revealed that their juxtamembrane regions negatively regulate their catalytic activities. In EphB2 and several other RTKs, this autoinhibition depends upon interaction between the kinase domain and tyrosine residues within the juxtamembrane region. Autoinhibition is released when these tyrosines become phosphorylated following receptor stimulation. Sequence homology suggested analogous regulation for ZAP-70. Based on mutagenesis analysis of ZAP-70 interdomain B, we find that this region downregulates ZAP-70 catalytic activity in a similar manner as the juxtamembrane region of EphB2. Similar regulation was also noted for the related Syk kinase. These findings suggest that a general autoinhibitory mechanism employed by RTKs is also used by some cytoplasmic tyrosine kinases.

Negative regulation of PTK signalling by Cbl proteins

Signalling pathways that are activated by ligands binding to cell surface receptors are responsible for determining many aspects of cellular function and fate. Although this outcome is primarily determined by the nature of the ligand and its receptor, it is also essential that the array of intracellular enzymes, adaptor proteins and transcription factors are correctly assembled to convey the intended response. In recent years it has become apparent that proteins which regulate the amplitude and duration of these responses can also affect cell function and fate. The Cbl family of E3 ubiquitin ligases and adaptor proteins have now emerged as key negative regulators of signals from many surface receptors. Although the array of these receptors is diverse, they have a common link in that they either possess a tyrosine kinase domain or they form associations with cytoplasmic protein tyrosine kinases (PTKs). Thus Cbl proteins become involved in signaling responses at a time when PTKs are first activated and therefore provide an initial line of defense to ensure signaling responses proceed at the desired intensity and kinetics.

Adaptors and linkers in T and B cells

By mediating non-covalent protein-protein interactions, adaptors organize and assemble the multimolecular signalling complexes that coordinate intracellular programs leading to the activation and differentiation of lymphocytes. The co-ordinated interaction between adaptor and effector molecules is required for the propagation and dynamic modification of externally applied signals. Recent advances have been made regarding our understanding of how adaptors regulate signalling within lymphocytes. An unexpected function has been revealed for the well-known adaptor protein LAT in pre-B-cell receptor signalling. In addition, the adaptors BCAP, Bcl10, CARMA1 and Malt1 seem to regulate the development of particular B-cell subsets. In contrast to Shc, c-Cbl and LAT, which are involved in early signalling events, BCAP, Bcl10, CARMA1 and Malt1 seem to act more distally, by controlling NF-kappaB activation. Additional transmembrane adaptors, such as NTAL/LAB and LIME, have been identified and partially characterized. Finally, an involvement of the cytosolic adaptors ADAP, SKAP-55 and Cbl-b in the regulation of lymphocyte adhesion and migration has been demonstrated.

Inactivation of c-Cbl reverses neonatal lethality and T cell developmental arrest of SLP-76-deficient mice

c-Cbl is an adaptor protein that negatively regulates signal transduction events involved in thymic-positive selection. To further characterize the function of c-Cbl in T cell development, we analyzed the effect of c-Cbl inactivation in mice deficient in the scaffolding molecule SLP-76. SLP-76–deficient mice show a high frequency of neonatal lethality; and in surviving mice, T cell development is blocked at the DN3 stage. Inactivation of c-cbl completely reversed the neonatal lethality seen in SLP-76–deficient mice and partially reversed the T cell development arrest in these mice. SLP-76^−/− Cbl^−/− mice exhibited marked expansion of polarized T helper type (Th)1 and Th2 cell peripheral CD4⁺ T cells, lymphoid infiltrates of parenchymal organs, and premature death. This rescue of T cell development is T cell receptor dependent because it does not occur in recombination activating gene 2^−/− SLP-76^−/− Cbl^−/− triple knockout mice. Analysis of the signal transduction properties of SLP-76^−/− Cbl^−/− T cells reveals a novel SLP-76– and linker for activation of T cells–independent pathway of extracellular signal–regulated kinase activation, which is normally down-regulated by c-Cbl.

Ubiquitin ligases in T cell activation and autoimmunity

Ubiquitination-mediated protein modifications are increasingly recognized as key regulatory events in many basic cell biology processes. A key class of enzymes called ubiquitin ligases, which has been shown to play a crucial role in the ubiquitination process, can positively or negatively regulate T cell responses. This review summarizes the recent advances defining the roles of several ubiquitin ligases in T cell activation and autoimmunity.

Critical roles for transcription factor GATA-3 in thymocyte development

The transcription factor GATA-3 is expressed at every stage of thymic development, but its role in thymocyte differentiation is unknown. The fact that RAG chimeric animals lacking GATA-3 cannot generate early thymocytes from common lymphoid progenitors has thus far precluded investigation of the function of GATA-3 in the thymus. To address this, we generated mice deficient in GATA-3 at early and late stages of thymic differentiation. Our studies revealed that GATA-3 is involved in beta selection and is indispensable for single-positive CD4 thymocyte development. Thus, our data demonstrate that the coordinated and regulated expression of GATA-3 at each stage of thymic development is critical for the generation of mature T cells.

Rapid microtubule-dependent induction of neurite-like extensions in NIH 3T3 fibroblasts by inhibition of ROCK and Cbl

A number of key cellular functions, such as morphological differentiation and cell motility, are closely associated with changes in cytoskeletal dynamics. Many of the principal signaling components involved in actin cytoskeletal dynamics have been identified, and these have been shown to be critically involved in cell motility. In contrast, signaling to microtubules remains relatively uncharacterized, and the importance of signaling pathways in modulation of microtubule dynamics has so far not been established clearly. We report here that the Rho-effector ROCK and the multiadaptor proto-oncoprotein Cbl can profoundly affect the microtubule cytoskeleton. Simultaneous inhibition of these two signaling molecules induces a dramatic rearrangement of the microtubule cytoskeleton into microtubule bundles. The formation of these microtubule bundles, which does not involve signaling by Rac, Cdc42, Crk, phosphatidylinositol 3-kinase, and Abl, is sufficient to induce distinct neurite-like extensions in NIH 3T3 fibroblasts, even in the absence of microfilaments. This novel microtubule-dependent function that promotes neurite-like extensions is not dependent on net changes in microtubule polymerization or stabilization, but rather involves selective elongation and reorganization of microtubules into long bundles.

c-Cbl negatively regulates platelet activation by glycoprotein VI

BACKGROUND

The adapter protein c-Cbl has emerged as having a potential role in negative regulation of immune receptor signaling. The major platelet-signaling receptor for collagen, glycoprotein VI (GpVI), is associated with the Fc receptor (FcR) gamma-chain, and signals through a similar pathway to immune receptors. c-Cbl is tyrosine-phosphorylated in response to stimulation of GpVI, whereas phosphorylation of c-Cbl in thrombin-activated platelets is dependent on fibrinogen binding to the integrin GpIIb/IIIa.

OBJECTIVE

To investigate the role of c-Cbl in platelet signaling.

METHODS

Murine platelets lacking functional c-Cbl or Src family kinases were analyzed.

RESULTS

Phosphorylation of c-Cbl through GpVI is reduced in murine platelets deficient in the Src-family kinases Fyn and Lyn, demonstrating that they lie upstream of c-Cbl phosphorylation. Phosphorylation of several proteins of the GpVI-signaling pathway, including the FcR gamma-chain, Syk and phospholipase Cgamma2 (PLCgamma2), is increased in the absence of c-Cbl. In line with this, aggregation is potentiated in response to the GpVI-specific collagen-related peptide (CRP) after a slight delay. A delay in potentiation is also seen in response to stimulation by thrombin.

CONCLUSIONS

These observations demonstrate that c-Cbl negatively regulates platelet responses to GpVI agonists and to thrombin, with the latter effect possibly being mediated downstream of GpIIb/IIIa. c-Cbl may play a physiological role in helping to prevent unwanted platelet activation in vivo.

Cbl-mediated ubiquitinylation and negative regulation of Vav

The Cbl ubiquitin ligase has emerged as a negative regulator of receptor and non-receptor tyrosine kinases. Cbl is known to associate with the proto-oncogene product Vav, a hematopoietic-restricted Rac guanine nucleotide exchange factor, but the consequences of this interaction remain to be elucidated. Using immortalized T cell lines from Cbl(+/+) and Cbl(-/-) mice, and transfection analyses in 293T cells, we demonstrate that Vav undergoes Cbl-dependent ubiquitinylation under conditions that promote Cbl and Vav phosphorylation. Interaction with Cbl also induced the loss of phosphorylated Vav. In addition, we show that an activated Vav mutant (Vav-Y174F) is more sensitive to Cbl-dependent ubiquitinylation. We demonstrate that the Cbl-dependent ubiquitinylation of Vav requires Cbl/Vav association through phosphorylated Tyr-700 on Cbl, and also requires an intact Cbl RING finger domain. Finally, using transfection analyses in the Jurkat T cell line, we show that Cbl, but not its ubiquitin ligase mutant, can inhibit Vav-dependent signaling. Thus, our findings strongly support the role of Cbl, via its ubiquitin ligase activity, as a negative regulator of activated Vav.