T cell activation results in conformational changes in the Src family kinase Lck to induce its activation

Targeting SCD triggers lipotoxicity of cancer cells and enhances anti-tumor immunity in breast cancer brain metastasis mouse models

Breast cancer brain metastases (BCBM) are incurable, and new therapies are urgently needed. BCBM upregulates stearoyl-CoA desaturase (SCD), an enzyme that catalyzes the synthesis of monounsaturated fatty acids, suggesting a potential metabolic vulnerability. Here, we test the effect of a brain-penetrant, clinical-stage SCD inhibitor (SCDi) on breast cancer cells and mouse models of BCBM. We show that SCDi markedly reshapes the lipidome of breast cancer cells, resulting in endoplasmic reticulum stress, DNA damage, impaired DNA damage repair, and cytotoxicity. Importantly, SCDi alone or combined with a PARP inhibitor prolongs the survival of BCBM-bearing mice. Furthermore, pharmacological inhibition of SCD enhances antigen presentation by dendritic cells, increases interferon signaling, promotes the infiltration of cytotoxic T cells, and decreases the proportion of exhausted T cells and regulatory T cells (Tregs) in the tumor microenvironment (TME) in a syngeneic mouse model of BCBM. Additionally, SCDi reduces the engagement of immunosuppressive pathways, including the PD-1:PD-L1/PD-L2 and PVR/TIGIT axes in the TME. These findings suggest that SCD inhibition could be an effective strategy to both intrinsically reduce tumor growth and reprogram anti-tumor immunity in the brain microenvironment to treat BCBM.

Advancing precision therapy for colorectal cancer: Developing clinical indications for multi-target kinase inhibitor BPR1J481 using patient-derived xenograft models

The large and rapid increase in the incidence and mortality of colorectal cancer (CRC) demonstrates the urgent need for new drugs with higher efficacy to treat CRC. However, the lack of applicable and reliable preclinical models significantly hinders the progress of drug development. Patient-derived xenograft (PDX) models are currently considered reliable in vivo preclinical models for predicting drug efficacy in cancer patients. This study successfully uses the CRC PDX model to develop clinical indications for the new multi-target kinase inhibitor BPR1J481 and demonstrated the anti-cancer mechanism and competitive advantages of this drug candidate. The results demonstrate that BPR1J481 exhibits significant anticancer efficacy by inducing apoptosis in CRC PDX tumor tissues and corresponding PDX-derived CRC cells. Through kinase competitive binding and kinase activity assays, we discover that BPR1J481 effectively inhibits SRC kinase activity by directly binding to its active site. The reduction in SRC phosphorylation observed in CRC PDX tumor tissues and derived cells upon treatment with BPR1J481 further validates its inhibitory potential. Furthermore, the decrease in viable cells after SRC knockout and the poorer prognosis observed in patients with higher SRC expression, emphasizes the critical significance and clinical relevance of SRC in CRC. Additionally, BPR1J481 exhibits robust anti-angiogenic effects by suppressing VEGF- and PDGF-induced endothelial cell proliferation, migration, and capillary-like tube formation through inhibition of VEGFR2 and PDGFRβ phosphorylation. Remarkably, BPR1J481 appears to demonstrate greater efficacy against CRC compared to regorafenib. These findings highlight the therapeutic potential of BPR1J481 for patients with CRC.

Immune Rejection Mediated by prf1 and gzmb Affects the Colonization of Fat Greenling (Hexagrammos otakii) Spermatogonia in Heterotransplantation

Fish germ cell transplantation holds great potential for conserving endangered species, improving cultured fish breeds, and exploring reproductive techniques. However, low transplantation efficiency is a common issue in heterotransplantation. This study transplanted fat greenling (Hexagrammos otakii) spermatogonia into the testes of spotted sea bass (Lateolabrax maculatus) to investigate factors that might affect the colonization and fixation of heterologous transplanted germ cells. Results indicated that transplanted fat greenling spermatogonia cells were successfully detected in the early transplantation phase in spotted sea bass. Their numbers gradually decreased over time, and after 10 days post-transplantation, more than 90% of the transplanted cells underwent apoptosis. Transcriptome sequencing analysis of the testes of spotted sea bass and fat greenling spermatogonia on days 1 and 10 post-transplantation revealed that this apoptosis process involved many immune-related genes and their associated signaling pathways. Acute immune rejection marker genes prf1 and gzmb were detected in the spotted sea bass testes, while immune tolerance genes lck and zap-70 were expressed in the fat greenling spermatogonia. Additionally, differential expression of prf1 and gzmb genes was screened from spotted sea bass, with experimental evidence indicating that PRF1 and GZMB protein from spotted sea bass primarily induce apoptosis in transplanted fat greenling spermatogonia via the mitochondrial apoptosis pathway, at the protein level. This suggests that the difficulties in heterotransplantation are primarily related to acute immune rejection, with PRF1 and GZMB playing significant roles.

Optical sensing and control of T cell signaling pathways

T cells regulate adaptive immune responses through complex signaling pathways mediated by T cell receptor (TCR). The functional domains of the TCR are combined with specific antibodies for the development of chimeric antigen receptor (CAR) T cell therapy. In this review, we first overview current understanding on the T cell signaling pathways as well as traditional methods that have been widely used for the T cell study. These methods, however, are still limited to investigating dynamic molecular events with spatiotemporal resolutions. Therefore, genetically encoded biosensors and optogenetic tools have been developed to study dynamic T cell signaling pathways in live cells. We review these cutting-edge technologies that revealed dynamic and complex molecular mechanisms at each stage of T cell signaling pathways. They have been primarily applied to the study of dynamic molecular events in TCR signaling, and they will further aid in understanding the mechanisms of CAR activation and function. Therefore, genetically encoded biosensors and optogenetic tools offer powerful tools for enhancing our understanding of signaling mechanisms in T cells and CAR-T cells.

Combined Immunodeficiency Caused by a Novel Nonsense Mutation in LCK

Mutations affecting T-cell receptor (TCR) signaling typically cause combined immunodeficiency (CID) due to varying degrees of disturbed T-cell homeostasis and differentiation. Here, we describe two cousins with CID due to a novel nonsense mutation in LCK and investigate the effect of this novel nonsense mutation on TCR signaling, T-cell function, and differentiation. Patients underwent clinical, genetic, and immunological investigations. The effect was addressed in primary cells and LCK-deficient T-cell lines after expression of mutated LCK. RESULTS: Both patients primarily presented with infections in early infancy. The LCK mutation led to reduced expression of a truncated LCK protein lacking a substantial part of the kinase domain and two critical regulatory tyrosine residues. T cells were oligoclonal, and especially naïve CD4 and CD8 T-cell counts were reduced, but regulatory and memory including circulating follicular helper T cells were less severely affected. A diagnostic hallmark of this immunodeficiency is the reduced surface expression of CD4. Despite severely impaired TCR signaling mTOR activation was partially preserved in patients' T cells. LCK-deficient T-cell lines reconstituted with mutant LCK corroborated partially preserved signaling. Despite detectable differentiation of memory and effector T cells, their function was severely disturbed. NK cell cytotoxicity was unaffected. Residual TCR signaling in LCK deficiency allows for reduced, but detectable T-cell differentiation, while T-cell function is severely disturbed. Our findings expand the previous report on one single patient on the central role of LCK in human T-cell development and function.

Cellular and molecular imaging of CAR-T cell-based immunotherapy

Chimeric Antigen Receptor T cell (CAR-T) therapy has emerged as a transformative therapeutic strategy for hematological malignancies. However, its efficacy in treating solid tumors remains limited. An in-depth and comprehensive understanding of CAR-T cell signaling pathways and the ability to track CAR-T cell biodistribution and activation in real-time within the tumor microenvironment will be instrumental in designing the next generation of CAR-T cells for solid tumor therapy. This review summarizes the signaling network and the cellular and molecular imaging tools and platforms that are utilized in CAR-T cell-based immune therapies, covering both in vitro and in vivo studies. Firstly, we provide an overview of the existing understanding of the activation and cytotoxic mechanisms of CAR-T cells, compared to the mechanism of T cell receptor (TCR) signaling pathways. We further describe the commonly employed tools for live cell imaging, coupled with recent research progress, with a focus on genetically encoded fluorescent proteins (FPs) and biosensors. We then discuss the utility of diverse in vivo imaging modalities, including fluorescence and bioluminescence imaging, Magnetic Resonance Imaging (MRI), Positron Emission Tomography (PET), and photoacoustic (PA) imaging, for noninvasive monitoring of CAR-T cell dynamics within tumor tissues, thereby providing critical insights into therapy's strengths and weaknesses. Lastly, we discuss the current challenges and future directions of CAR-T cell therapy from the imaging perspective. We foresee that a comprehensive and integrative approach to CAR-T cell imaging will enable the development of more effective treatments for solid tumors in the future.

Impact of senolytic treatment on immunity, aging, and disease

Cellular senescence has been implicated in the pathophysiology of many age-related diseases. However, it also plays an important protective role in the context of tumor suppression and wound healing. Reducing senescence burden through treatment with senolytic drugs or the use of genetically targeted models of senescent cell elimination in animals has shown positive results in the context of mitigating disease and age-associated inflammation. Despite positive, albeit heterogenous, outcomes in clinical trials, very little is known about the short-term and long-term immunological consequences of using senolytics as a treatment for age-related conditions. Further, many studies examining cellular senescence and senolytic treatment have been demonstrated in non-infectious disease models. Several recent reports suggest that senescent cell elimination may have benefits in COVID-19 and influenza resolution and disease prognosis. In this review, we discuss the current clinical trials and pre-clinical studies that are exploring the impact of senolytics on cellular immunity. We propose that while eliminating senescent cells may have an acute beneficial impact on primary immune responses, immunological memory may be negatively impacted. Closer investigation of senolytics on immune function and memory generation would provide insight as to whether senolytics could be used to enhance the aging immune system and have potential to be used as therapeutics or prophylactics in populations that are severely and disproportionately affected by infections such as the elderly and immunocompromised.

High PPFIA1 expression promotes cancer survival by suppressing CD8+ T cells in breast cancer: drug discovery and machine learning approach

BACKGROUND

PTPRF-interacting protein alpha 1 (PPFIA1) plays an important role as a regulator of cell motility and tumor cell invasion and is frequently amplified in breast cancer. The aim of this study was to investigate the clinicopathologic features, survival, anticancer immunities and specific gene sets related to high PPFIA1 expression in patients with breast cancer. We verified the importance of PPFIA1 and survival rates using machine learning and identified drugs that can effectively reduce breast cancer cells with high PPFIA1 expression.

METHODS

This study analyzed clinicopathologic factors, survival rates, immune profiles and gene sets according to PPFIA1 expression in 3457 patients with breast cancer from the Kangbuk Samsung Medical Center cohort (456 cases), Molecular Taxonomy of Breast Cancer International Consortium (1904 cases) and The Cancer Genome Atlas (1097 cases). We applied gene set enrichment analysis (GSEA), in silico cytometry, pathway network analyses, in vitro drug screening, and gradient boosting machine (GBM) analysis.

RESULTS

High PPFIA1 expression in breast cancer was associated with worse prognosis, with reduced tumor-infiltrating lymphocytes, especially CD8+ T cells, and increased PD-L1 expression. In pathway network analysis, PPFIA1 was linked directly to the tyrosine-protein phosphatase pathway and indirectly to immune pathways. The importance of PPFIA1's association with survival in GBM analysis was higher than that of perineural and lymphovascular invasion. In in vitro drug screening, expression of PPFIA1 on mRNA level positively correlated with sensitivity of cell lines to erlotinib.

CONCLUSION

High PPFIA1 in patients with breast cancer is related to poor prognosis and decreased anticancer immune response, and erlotinib may be promising for development of therapeutic approaches in patients with tumors overexpressing PPFIA1.

A short hepatitis C virus NS5A peptide expression by AAV vector modulates human T cell activation and reduces vector immunogenicity

Viral vector-mediated gene therapies have the potential to treat many human diseases; however, host immune responses against the vector and/or the transgene pose a safety risk to the patients and can negatively impact product efficacy. Thus, novel strategies to reduce vector immunogenicity are critical for the advancement of these therapies. T cell activation (TCA) is required for the development of immune responses during gene therapy. We hypothesized that modulation of TCA by incorporating a novel viral immunomodulatory factor into a viral vector may reduce unwanted TCA and immune responses during gene therapy. To test this hypothesis, we identified an immunomodulatory domain of the hepatitis C virus (HCV) NS protein 5A (NS5A) protein and studied the effect of viral vectors expressing NS5A peptide on TCA. Lentiviral vector-mediated expression of a short 20-mer peptide derived from the NS5A protein in human T cells was sufficient to inhibit TCA. Synthetic 20-mer NS5A peptide also inhibited TCA in primary human T cells. Mechanistically, the NS5A protein interacted with Lck and inhibited proximal TCR signaling. Importantly, NS5A peptide expression did not cause global T cell signaling dysfunction as distal T cell signaling was not inhibited. Finally, recombinant adeno-associated virus (AAV) vector expressing the 20-mer NS5A peptide reduced both the recall antigen and the TCR-mediated activation of human T cells and did not cause global T cell signaling dysfunction. Together, these data suggest that expression of a 20-mer NS5A peptide by an AAV vector may reduce unwanted TCA and may contribute to lower vector immunogenicity during gene therapy.

A Cysteine Residue within the Kinase Domain of Zap70 Regulates Lck Activity and Proximal TCR Signaling

Alterations in both the expression and function of the non-receptor tyrosine kinase Zap70 are associated with numerous human diseases including immunodeficiency, autoimmunity, and leukemia. Zap70 propagates the TCR signal by phosphorylating two important adaptor molecules, LAT and SLP76, which orchestrate the assembly of the signaling complex, leading to the activation of PLCγ1 and further downstream pathways. These events are crucial to drive T-cell development and T-cell activation. Recently, it has been proposed that C564, located in the kinase domain of Zap70, is palmitoylated. A non-palmitoylable C564R Zap70 mutant, which has been reported in a patient suffering from immunodeficiency, is incapable of propagating TCR signaling and activating T cells. The lack of palmitoylation was suggested as the cause of this human disease. Here, we confirm that Zap70^C564R is signaling defective, but surprisingly, the defective Zap70 function does not appear to be due to a loss in palmitoylation. We engineered a C564A mutant of Zap70 which, similarly to Zap70^C564R, is non-palmitoylatable. However, this mutant was capable of propagating TCR signaling. Moreover, Zap70^C564A enhanced the activity of Lck and increased its proximity to the TCR. Accordingly, Zap70-deficient P116 T cells expressing Zap70^C564A displayed the hyperphosphorylation of TCR-ζ and Zap70 (Y319), two well-known Lck substrates. Collectively, these data indicate that C564 is important for the regulation of Lck activity and proximal TCR signaling, but not for the palmitoylation of Zap70.

The Multiple Roles of the Cytosolic Adapter Proteins ADAP, SKAP1 and SKAP2 for TCR/CD3 -Mediated Signaling Events

T cells are the key players of the adaptive immune response. They coordinate the activation of other immune cells and kill malignant and virus-infected cells. For full activation T cells require at least two signals. Signal 1 is induced after recognition of MHC/peptide complexes presented on antigen presenting cells (APCs) by the clonotypic TCR (T-cell receptor)/CD3 complex whereas Signal 2 is mediated via the co-stimulatory receptor CD28, which binds to CD80/CD86 molecules that are present on APCs. These signaling events control the activation, proliferation and differentiation of T cells. In addition, triggering of the TCR/CD3 complex induces the activation of the integrin LFA-1 (leukocyte function associated antigen 1) leading to increased ligand binding (affinity regulation) and LFA-1 clustering (avidity regulation). This process is termed "inside-out signaling". Subsequently, ligand bound LFA-1 transmits a signal into the T cells ("outside-in signaling") which enhances T-cell interaction with APCs (adhesion), T-cell activation and T-cell proliferation. After triggering of signal transducing receptors, adapter proteins organize the proper processing of membrane proximal and intracellular signals as well as the activation of downstream effector molecules. Adapter proteins are molecules that lack enzymatic or transcriptional activity and are composed of protein-protein and protein-lipid interacting domains/motifs. They organize and assemble macromolecular complexes (signalosomes) in space and time. Here, we review recent findings regarding three cytosolic adapter proteins, ADAP (Adhesion and Degranulation-promoting Adapter Protein), SKAP1 and SKAP2 (Src Kinase Associated Protein 1 and 2) with respect to their role in TCR/CD3-mediated activation, proliferation and integrin regulation.

Identification and Characterization of the Amphioxus Lck and Its Associated Tyrosine Phosphorylation-Dependent Inhibitory LRR Receptor

Amphioxus (e.g., Branchiostoma belcheri, Bb) has recently emerged as a new model for studying the origin and evolution of vertebrate immunity. Mammalian lymphocyte-specific tyrosine kinase (Lck) plays crucial roles in T cell activation, differentiation and homeostasis, and is reported to phosphorylate both the ITIM and ITSM of PD-1 to induce the recruitment of phosphatases and thus the inhibitory function of PD-1. Here, we identified and cloned the amphioxus homolog of human Lck. By generating and using an antibody against BbLck, we found that BbLck is expressed in the amphioxus gut and gill. Through overexpression of BbLck in Jurkat T cells, we found that upon TCR stimulation, BbLck was subjected to tyrosine phosphorylation and could partially rescue Lck-dependent tyrosine phosphorylation in Lck-knockdown T cells. Mass spectrometric analysis of BbLck immunoprecipitates from immunostimulants-treated amphioxus, revealed a BbLck-associated membrane-bound receptor LRR (BbLcLRR). By overexpressing BbLcLRR in Jurkat T cells, we demonstrated that BbLcLRR was tyrosine phosphorylated upon TCR stimulation, which was inhibited by Lck knockdown and was rescued by overexpression of BbLck. By mutating single tyrosine to phenylalanine (Y-F), we identified three tyrosine residues (Y539, Y655, and Y690) (3Y) of BbLcLRR as the major Lck phosphorylation sites. Reporter gene assays showed that overexpression of BbLcLRR but not the BbLcLRR-3YF mutant inhibited TCR-induced NF-κB activation. In Lck-knockdown T cells, the decline of TCR-induced IL-2 production was reversed by overexpression of BbLck, and this reversion was inhibited by co-expression of BbLcLRR but not the BbLcLRR-3YF mutant. Sequence analysis showed that the three tyrosine-containing sequences were conserved with the tyrosine-based inhibition motifs (ITIMs) or ITIM-like motifs. And TCR stimulation induced the association of BbLcLRR with tyrosine phosphatases SHIP1 and to a lesser extent with SHP1/2. Moreover, overexpression of wild-type BbLcLRR but not its 3YF mutant inhibited TCR-induced tyrosine phosphorylation of multiple signaling proteins probably via recruiting SHIP1. Thus, we identified a novel immunoreceptor BbLcLRR, which is phosphorylated by Lck and then exerts a phosphorylation-dependent inhibitory role in TCR-mediated T-cell activation, implying a mechanism for the maintenance of self-tolerance and homeostasis of amphioxus immune system and the evolutionary conservatism of Lck-regulated inhibitory receptor pathway.

Cooperative Interaction of Nck and Lck Orchestrates Optimal TCR Signaling

The T cell antigen receptor (TCR) is expressed on T cells, which orchestrate adaptive immune responses. It is composed of the ligand-binding clonotypic TCRαβ heterodimer and the non-covalently bound invariant signal-transducing CD3 complex. Among the CD3 subunits, the CD3ε cytoplasmic tail contains binding motifs for the Src family kinase, Lck, and the adaptor protein, Nck. Lck binds to a receptor kinase (RK) motif and Nck binds to a proline-rich sequence (PRS). Both motifs only become accessible upon ligand binding to the TCR and facilitate the recruitment of Lck and Nck independently of phosphorylation of the TCR. Mutations in each of these motifs cause defects in TCR signaling and T cell activation. Here, we investigated the role of Nck in proximal TCR signaling by silencing both Nck isoforms, Nck1 and Nck2. In the absence of Nck, TCR phosphorylation, ZAP70 recruitment, and ZAP70 phosphorylation was impaired. Mechanistically, this is explained by loss of Lck recruitment to the stimulated TCR in cells lacking Nck. Hence, our data uncover a previously unknown cooperative interaction between Lck and Nck to promote optimal TCR signaling.

Structural insights into redox-active cysteine residues of the Src family kinases

The Src Family Kinases (SFKs) are pivotal regulators of cellular signal transduction and highly sought-after targets in drug discovery. Their actions within cells are controlled by alterations in protein phosphorylation that switch the SFKs from autoinhibited to active states. The SFKs are also well recognized to contain redox-active cysteine residues where oxidation of certain residues directly contribute to kinase function. To more completely understand the factors that influence cysteine oxidation within the SFKs, a review is presented of the local structural environments surrounding SFK cysteine residues compared to their quantified oxidation in vivo from the Oximouse database. Generally, cysteine local structure and degree of redox sensitivity vary with respect to sequence conservation. Cysteine residues found in conserved positions are more mildly redox-active as they are found in hydrophobic environments and not fully exposed to solvent. Non-conserved redox-active cysteines are generally the most reactive with direct solvent access and/or in hydrophilic environments. Results from this analysis motivate future efforts to conduct comprehensive proteome-wide analysis of redox-sensitivity, conservation, and local structural environments of proteins containing reactive cysteine residues.

Sprouty2 positively regulates T cell function and airway inflammation through regulation of CSK and LCK kinases

The function of Sprouty2 (Spry2) in T cells is unknown. Using 2 different (inducible and T cell-targeted) knockout mouse strains, we found that Spry2 positively regulated extracellular signal-regulated kinase 1/2 (ERK1/2) signaling by modulating the activity of LCK. Spry2-/- CD4+ T cells were unable to activate LCK, proliferate, differentiate into T helper cells, or produce cytokines. Spry2 deficiency abrogated type 2 inflammation and airway hyperreactivity in a murine model of asthma. Spry2 expression was higher in blood and airway CD4+ T cells from patients with asthma, and Spry2 knockdown impaired human T cell proliferation and cytokine production. Spry2 deficiency up-regulated the lipid raft protein caveolin-1, enhanced its interaction with CSK, and increased CSK interaction with LCK, culminating in augmented inhibitory phosphorylation of LCK. Knockdown of CSK or dislodgment of caveolin-1-bound CSK restored ERK1/2 activation in Spry2-/- T cells, suggesting an essential role for Spry2 in LCK activation and T cell function.

Research Advances in the Molecular Functions and Relevant Diseases of TAOKs, Novel STE20 Kinase Family Members

As serine/threonine-protein kinases, Thousand and One Kinases(TAOKs) are members of the GCKlike superfamily, one of two well-known branches of the Ste20 kinase family. Within the last two decades, three functionally similar kinases, namely TAOK1-3, were identified. TAOKs are involved in many molecular and cellular events. Scholars widely believe that TAOKs act as kinases upstream of the MAPK cascade and as factors that interact with MST family kinases, the cytoskeleton, and apoptosis-associated proteins. Therefore, TAOKs are thought to function in tumorigenesis. Additionally, TAOKs participate in signal transduction induced by Notch, TCR, and IL-17. Recent studies found that TAOKs play roles in a series of diseases and conditions, such as the central nervous system dysfunction, herpes viral infection, immune system imbalance, urogenital system malformation during development, cardiovascular events, and childhood obesity. Therefore, inhibitory chemicals targeting TAOKs may be of great significance as potential drugs for these diseases.

The Activity and Stability of p56Lck and TCR Signaling Do Not Depend on the Co-Chaperone Cdc37

Lymphocyte-specific protein tyrosine kinase (Lck) is a pivotal tyrosine kinase involved in T cell receptor (TCR) signaling. Because of its importance, the activity of Lck is regulated at different levels including phosphorylation of tyrosine residues, protein-protein interactions, and localization. It has been proposed that the co-chaperone Cdc37, which assists the chaperone heat shock protein 90 (Hsp90) in the folding of client proteins, is also involved in the regulation of the activity/stability of Lck. Nevertheless, the available experimental data do not clearly support this conclusion. Thus, we assessed whether or not Cdc37 regulates Lck. We performed experiments in which the expression of Cdc37 was either augmented or suppressed in Jurkat T cells. The results of our experiments indicated that neither the overexpression nor the suppression of Cdc37 affected Lck stability and activity. Moreover, TCR signaling proceeded normally in T cells in which Cdc37 expression was either augmented or suppressed. Finally, we demonstrated that also under stress conditions Cdc37 was dispensable for the regulation of Lck activity/stability. In conclusion, our data do not support the idea that Lck is a Cdc37 client.

Tyrosine 192 within the SH2 domain of the Src-protein tyrosine kinase p56Lck regulates T-cell activation independently of Lck/CD45 interactions

BACKGROUND

Upon engagement of the T-cell receptor (TCR), the Src-family protein tyrosine kinase p56Lck phosphorylates components of the TCR (e.g. the TCRζ chains), thereby initiating T-cell activation. The enzymatic activity of Lck is primarily regulated via reversible and dynamic phosphorylation of two tyrosine residues, Y394 and Y505. Lck possesses an additional highly conserved tyrosine Y192, located within the SH2 domain, whose role in T-cell activation is not fully understood.

METHODS

Knock-in mice expressing a phospho-mimetic (Y192E) form of Lck were generated. Cellular and biochemical characterization was performed to elucidate the function of Y192 in primary T cells. HEK 293T and Jurkat T cells were used for in vitro studies.

RESULTS

Co-immunoprecipitation studies and biochemical analyses using T cells from Lck^Y192E knock-in mice revealed a diminished binding of Lck^Y192E to CD45 and a concomitant hyperphosphorylation of Y505, thus corroborating previous data obtained in Jurkat T cells. Surprisingly however, in vitro kinase assays showed that Lck^Y192E possesses a normal enzymatic activity in human and murine T cells. FLIM/FRET measurements employing an Lck^Y192E biosensor further indicated that the steady state conformation of the Lck^Y192E mutant is similar to Lck^wt. These data suggest that Y192 might regulate Lck functions also independently from the Lck/CD45-association. Indeed, when Lck^Y192E was expressed in CD45^-/-/Csk^-/- non-T cells (HEK 293T cells), phosphorylation of Y505 was similar to Lck^wt, but Lck^Y192E still failed to optimally phosphorylate and activate the Lck downstream substrate ZAP70. Furthermore, Lck^Y19E was recruited less to CD3 after TCR stimulation.

CONCLUSIONS

Taken together, phosphorylation of Y192 regulates Lck functions in T cells at least twofold, by preventing Lck association to CD45 and by modulating ligand-induced recruitment of Lck to the TCR.

MAJOR FINDINGS

Our data change the current view on the function of Y192 and suggest that Y192 also regulates Lck activity in a manner independent of Y505 phosphorylation. Video Abstract.

Emerging Kinase Therapeutic Targets in Pancreatic Ductal Adenocarcinoma and Pancreatic Cancer Desmoplasia

Kinase drug discovery represents an active area of therapeutic research, with previous pharmaceutical success improving patient outcomes across a wide variety of human diseases. In pancreatic ductal adenocarcinoma (PDAC), innovative pharmaceutical strategies such as kinase targeting have been unable to appreciably increase patient survival. This may be due, in part, to unchecked desmoplastic reactions to pancreatic tumors. Desmoplastic stroma enhances tumor development and progression while simultaneously restricting drug delivery to the tumor cells it protects. Emerging evidence indicates that many of the pathologic fibrotic processes directly or indirectly supporting desmoplasia may be driven by targetable protein tyrosine kinases such as Fyn-related kinase (FRK); B lymphoid kinase (BLK); hemopoietic cell kinase (HCK); ABL proto-oncogene 2 kinase (ABL2); discoidin domain receptor 1 kinase (DDR1); Lck/Yes-related novel kinase (LYN); ephrin receptor A8 kinase (EPHA8); FYN proto-oncogene kinase (FYN); lymphocyte cell-specific kinase (LCK); tec protein kinase (TEC). Herein, we review literature related to these kinases and posit signaling networks, mechanisms, and biochemical relationships by which this group may contribute to PDAC tumor growth and desmoplasia.

High expression levels of DEF6 predicts a poor prognosis for patients with clear cell renal cell carcinoma

Clear cell renal cell carcinoma (ccRCC) is one of the most common types of malignant tumors and early detection contributes to a better prognosis. Finding new biomarkers for the diagnosis or treatment remains meaningful. DEF6 guanine nucleotide exchange factor (DEF6) is upregulated in ccRCC compared to normal controls, but the relationship between DEF6 expression and prognosis in ccRCC is unclear. Moreover, the potential biological functions of DEF6 in ccRCC remains unclear. In the present study, the Cancer Genome Atlas (TCGA), Gene Expression Omnibus (GEO), TISIDB and the clinical database of the Peking University First Hospital were used to analyze DEF6 expression in ccRCC. Immunohistochemistry (IHC), western blotting and reverse transcription-quantitative PCR were used to examine the DEF6 protein and mRNA expression levels in cell lines and clinical samples. Subsequently, the Kaplan-Meier method and Cox regression analyses were used to determine the impact of DEF6 expression on the overall survival of patients alongside other clinical variables in both the TCGA database and the present clinical database. The results showed that both DEF6 mRNA and protein expression levels were upregulated in ccRCC compared to normal controls. The Kaplan-Meier survival analysis showed that patients with high DEF6 expression had poor prognoses from both the TCGA database and the present clinical database. Univariate survival analysis and multivariate survival analysis revealed that DEF6 could be an independent prognostic factor for ccRCC. Additionally, bioinformatics analysis indicated that differentially expressed genes related to DEF6 expression influenced ccRCC by regulating the tumor immune microenvironment. In conclusion, overexpression of DEF6 is significantly correlated with a poor prognosis for patients with ccRCC and DEF6 may influence the biological processes involved with ccRCC by regulating the immune microenvironment.

Noncanonical binding of Lck to CD3ε promotes TCR signaling and CAR function

Initiation of T cell antigen receptor (TCR) signaling involves phosphorylation of CD3 cytoplasmic tails by the tyrosine kinase Lck. How Lck is recruited to the TCR to initiate signaling is not well known. We report a previously unknown binding motif in the CD3ε cytoplasmic tail that interacts in a noncanonical mode with the Lck SH3 domain: the receptor kinase (RK) motif. The RK motif is accessible only upon TCR ligation, demonstrating how ligand binding leads to Lck recruitment. Binding of the Lck SH3 domain to the exposed RK motif resulted in local augmentation of Lck activity, CD3 phosphorylation, T cell activation and thymocyte development. Introducing the RK motif into a well-characterized 41BB-based chimeric antigen receptor enhanced its antitumor function in vitro and in vivo. Our findings underscore how a better understanding of the functioning of the TCR might promote rational improvement of chimeric antigen receptor design for the treatment of cancer.

An Integrative Network Modeling Approach to T CD4 Cell Activation

The adaptive immune response is initiated by the interaction of the T cell antigen receptor/CD3 complex (TCR) with a cognate peptide bound to a MHC molecule. This interaction, along with the activity of co-stimulatory molecules and cytokines in the microenvironment, enables cells to proliferate and produce soluble factors that stimulate other branches of the immune response for inactivation of infectious agents. The intracellular activation signals are reinforced, amplified and diversified by a complex network of biochemical interactions, and includes the activity of molecules that modulate the activation process and stimulate the metabolic changes necessary for fulfilling the cell energy demands. We present an approach to the analysis of the main early signaling events of T cell activation by proposing a concise 46-node hybrid Boolean model of the main steps of TCR and CD28 downstream signaling, encompassing the activity of the anergy factor Ndrg1, modulation of activation by CTLA-4, and the activity of the nutrient sensor AMPK as intrinsic players of the activation process. The model generates stable states that reflect the overcoming of activation signals and induction of anergy by the expression of Ndrg1 in the absence of co-stimulation. The model also includes the induction of CTLA-4 upon activation and its competition with CD28 for binding to the co-stimulatory CD80/86 molecules, leading to stable states that reflect the activation arrest. Furthermore, the model integrates the activity of AMPK to the general pathways driving differentiation to functional cell subsets (Th1, Th2, Th17, and Treg). Thus, the network topology incorporates basic mechanism associated to activation, regulation and induction of effector cell phenotypes. The model puts forth a conceptual framework for the integration of functionally relevant processes in the analysis of the T CD4 cell function.

Conformational States Control Lck Switching between Free and Confined Diffusion Modes in T Cells

T cell receptor phosphorylation by Lck is an essential step in T cell activation. It is known that the conformational states of Lck control enzymatic activity; however, the underlying principles of how Lck finds its substrate over the plasma membrane remain elusive. Here, single-particle tracking is paired with photoactivatable localization microscopy to observe the diffusive modes of Lck in the plasma membrane. Individual Lck molecules switched between free and confined diffusion in both resting and stimulated T cells. Lck mutants locked in the open conformation were more confined than Lck mutants in the closed conformation. Further confinement of kinase-dead versions of Lck suggests that Lck confinement was not caused by phosphorylated substrates. Our data support a model in which confined diffusion of open Lck results in high local phosphorylation rates, and inactive, closed Lck diffuses freely to enable long-range distribution over the plasma membrane.

Imaging of the immune system - towards a subcellular and molecular understanding

Immune responses involve many types of leukocytes that traffic to the site of injury, recognize the insult and respond appropriately. Imaging of the immune system involves a set of methods and analytical tools that are used to visualize immune responses at the cellular and molecular level as they occur in real time. We will review recent and emerging technological advances in optical imaging, and their application to understanding the molecular and cellular responses of neutrophils, macrophages and lymphocytes. Optical live-cell imaging provides deep mechanistic insights at the molecular, cellular, tissue and organism levels. Live-cell imaging can capture quantitative information in real time at subcellular resolution with minimal phototoxicity and repeatedly in the same living cells or in accessible tissues of the living organism. Advanced FRET probes allow tracking signaling events in live cells. Light-sheet microscopy allows for deeper tissue penetration in optically clear samples, enriching our understanding of the higher-level organization of the immune response. Super-resolution microscopy offers insights into compartmentalized signaling at a resolution beyond the diffraction limit, approaching single-molecule resolution. This Review provides a current perspective on live-cell imaging in vitro and in vivo with a focus on the assessment of the immune system.

The TCR is an allosterically regulated macromolecular machinery changing its conformation while working

The αβ T-cell receptor (TCR) is a multiprotein complex controlling the activation of T cells. Although the structure of the complete TCR is not known, cumulative evidence supports that the TCR cycles between different conformational states that are promoted either by thermal motion or by force. These structural transitions determine whether the TCR engages intracellular effectors or not, regulating TCR phosphorylation and signaling. As for other membrane receptors, ligand binding selects and stabilizes the TCR in active conformations, and/or switches the TCR to activating states that were not visited before ligand engagement. Here we review the main models of TCR allostery, that is, ligand binding at TCRαβ changes the structure at CD3 and ζ. (a) The ITAM and proline-rich sequence exposure model, in which the TCR's cytoplasmic tails shield each other and ligand binding exposes them for phosphorylation. (b) The membrane-ITAM model, in which the CD3ε and ζ tails are sequestered inside the membrane and again ligand binding exposes them. (c) The mechanosensor model in which ligand binding exerts force on the TCR, inducing structural changes that allow signaling. Since these models are complementary rather than competing, we propose a unified model that aims to incorporate all existing data.

A Fyn biosensor reveals pulsatile, spatially localized kinase activity and signaling crosstalk in live mammalian cells

Cell behavior is controlled through spatio-temporally localized protein activity. Despite unique and often contradictory roles played by Src-family-kinases (SFKs) in regulating cell physiology, activity patterns of individual SFKs have remained elusive. Here, we report a biosensor for specifically visualizing active conformation of SFK-Fyn in live cells. We deployed combinatorial library screening to isolate a binding-protein (F29) targeting activated Fyn. Nuclear-magnetic-resonance (NMR) analysis provides the structural basis of F29 specificity for Fyn over homologous SFKs. Using F29, we engineered a sensitive, minimally-perturbing fluorescence-resonance-energy-transfer (FRET) biosensor (FynSensor) that reveals cellular Fyn activity to be spatially localized, pulsatile and sensitive to adhesion/integrin signaling. Strikingly, growth factor stimulation further enhanced Fyn activity in pre-activated intracellular zones. However, inhibition of focal-adhesion-kinase activity not only attenuates Fyn activity, but abolishes growth-factor modulation. FynSensor imaging uncovers spatially organized, sensitized signaling clusters, direct crosstalk between integrin and growth-factor-signaling, and clarifies how compartmentalized Src-kinase activity may drive cell fate.

CD45 functions as a signaling gatekeeper in T cells

T cells require the protein tyrosine phosphatase CD45 to detect and respond to antigen because it activates the Src family kinase Lck, which phosphorylates the T cell antigen receptor (TCR) complex. CD45 activates Lck by opposing the negative regulatory kinase Csk. Paradoxically, CD45 has also been implicated in suppressing TCR signaling by dephosphorylating the same signaling motifs within the TCR complex upon which Lck acts. We sought to reconcile these observations using chemical and genetic perturbations of the Csk/CD45 regulatory axis incorporated with computational analyses. Specifically, we titrated the activities of Csk and CD45 and assessed their influence on Lck activation, TCR-associated ζ-chain phosphorylation, and more downstream signaling events. Acute inhibition of Csk revealed that CD45 suppressed ζ-chain phosphorylation and was necessary for a regulatable pool of active Lck, thereby interconnecting the activating and suppressive roles of CD45 that tune antigen discrimination. CD45 suppressed signaling events that were antigen independent or induced by low-affinity antigen but not those initiated by high-affinity antigen. Together, our findings reveal that CD45 acts as a signaling "gatekeeper," enabling graded signaling outputs while filtering weak or spurious signaling events.

Beyond TCR Signaling: Emerging Functions of Lck in Cancer and Immunotherapy

In recent years, the lymphocyte-specific protein tyrosine kinase (Lck) has emerged as one of the key molecules regulating T-cell functions. Studies using Lck knock-out mice or Lck-deficient T-cell lines have shown that Lck regulates the initiation of TCR signaling, T-cell development, and T-cell homeostasis. Because of the crucial role of Lck in T-cell responses, strategies have been employed to redirect Lck activity to improve the efficacy of chimeric antigen receptors (CARs) and to potentiate T-cell responses in cancer immunotherapy. In addition to the well-studied role of Lck in T cells, evidence has been accumulated suggesting that Lck is also expressed in the brain and in tumor cells, where it actively takes part in signaling processes regulating cellular functions like proliferation, survival and memory. Therefore, Lck has emerged as a novel druggable target molecule for the treatment of cancer and neuronal diseases. In this review, we will focus on these new functions of Lck.

Biophysical basis underlying dynamic Lck activation visualized by ZapLck FRET biosensor

Lck plays crucial roles in TCR signaling. We developed a new and sensitive FRET biosensor (ZapLck) to visualize Lck kinase activity with high spatiotemporal resolutions in live cells. ZapLck revealed that 62% of Lck signal was preactivated in T-cells. In Lck-deficient JCam T-cells, Lck preactivation was abolished, which can be restored to 51% by reconstitution with wild-type Lck (LckWT) but not a putatively inactive mutant LckY394F. LckWT also showed a stronger basal Lck-Lck interaction and a slower diffusion rate than LckY394F. Interestingly, aggregation of TCR receptors by antibodies in JCam cells led to a strong activation of reconstituted LckY394F similar to LckWT. Both activated LckY394F and LckWT diffused more slowly and displayed increased Lck-Lck interaction at a similar level. Therefore, these results suggest that a phosphorylatable Y394 is necessary for the basal-level interaction and preactivation of LckWT, while antibody-induced TCR aggregation can trigger the full activation of LckY394F.

TAOK3 Regulates Canonical TCR Signaling by Preventing Early SHP-1-Mediated Inactivation of LCK

Activation of LCK is required for canonical TCR signaling leading to T cell responses. LCK activation also initiates a negative feedback loop mediated by the phosphatase SHP-1 that turns off TCR signaling. In this article, we report that the thousand-and-one amino acid kinase 3 (TAOK3) is a key regulator of this feedback. TAOK3 is a serine/threonine kinase expressed in many different cell types including T cells. TAOK3-deficient human T cells had impaired LCK-dependent TCR signaling resulting in a defect in IL-2 response to canonical TCR signaling but not to bacterial superantigens, which use an LCK-independent pathway. This impairment was associated with enhanced interaction of LCK with SHP-1 after TCR engagement and rapid termination of TCR signals, a defect corrected by TAOK3 reconstitution. Thus, TAOK3 is a positive regulator of TCR signaling by preventing premature SHP-1-mediated inactivation of LCK. This mechanism may also regulate signaling by other Src family kinase-dependent receptors.

A Stretch of Negatively Charged Amino Acids of Linker for Activation of T-Cell Adaptor Has a Dual Role in T-Cell Antigen Receptor Intracellular Signaling

The adaptor protein linker for activation of T cells (LAT) has an essential role transducing activatory intracellular signals coming from the TCR/CD3 complex. Previous reports have shown that upon T-cell activation, LAT interacts with the tyrosine kinase Lck, leading to the inhibition of its kinase activity. LAT-Lck interaction seemed to depend on a stretch of negatively charged amino acids in LAT. Here, we have substituted this segment of LAT between amino acids 113 and 126 with a non-charged segment and expressed the mutant LAT (LAT-NIL) in J.CaM2 cells in order to analyze TCR signaling. Substitution of this segment in LAT prevented the activation-induced interaction with Lck. Moreover, cells expressing this mutant form of LAT showed a statistically significant increase of proximal intracellular signals such as phosphorylation of LAT in tyrosine residues 171 and 191, and also enhanced ZAP70 phosphorylation approaching borderline statistical significance (p = 0.051). Nevertheless, downstream signals such as Ca²⁺ influx or MAPK pathways were partially inhibited. Overall, our data reveal that LAT-Lck interaction constitutes a key element regulating proximal intracellular signals coming from the TCR/CD3 complex.

Selected signalling proteins recruited to the T-cell receptor-CD3 complex

The T-cell receptor (TCR)-CD3 complex, expressed on T cells, determines the outcome of a T-cell response. It consists of the TCR-αβ heterodimer and the non-covalently associated signalling dimers of CD3εγ, CD3εδ and CD3ζζ. TCR-αβ binds specifically to a cognate peptide antigen bound to an MHC molecule, whereas the CD3 subunits transmit the signal into the cytosol to activate signalling events. Recruitment of proteins to specialized localizations is one mechanism to regulate activation and termination of signalling. In the last 25 years a large number of signalling molecules recruited to the TCR-CD3 complex upon antigen binding to TCR-αβ have been described. Here, we review knowledge about five of those interaction partners: Lck, ZAP-70, Nck, WASP and Numb. Some of these proteins have been targeted in the development of immunomodulatory drugs aiming to treat patients with autoimmune diseases and organ transplants.

TCR Signaling: Mechanisms of Initiation and Propagation

The mechanisms by which a T cell detects antigen using its T cell antigen receptor (TCR) are crucial to our understanding of immunity and the harnessing of T cells therapeutically. A hallmark of the T cell response is the ability of T cells to quantitatively respond to antigenic ligands derived from pathogens while remaining inert to similar ligands derived from host tissues. Recent studies have revealed exciting properties of the TCR and the behaviors of its signaling effectors that are used to detect and discriminate between antigens. Here we highlight these recent findings, focusing on the proximal TCR signaling molecules Zap70, Lck, and LAT, to provide mechanistic models and insights into the exquisite sensitivity and specificity of the TCR.

Encoding optical control in LCK kinase to quantitatively investigate its activity in live cells

LCK is a tyrosine kinase essential for initiating T-cell antigen receptor (TCR) signaling. A complete understanding of LCK function is constrained by a paucity of methods to quantitatively study its function within live cells. To address this limitation, we generated LCK*, in which a key active site lysine is replaced by a photo-caged equivalent, using genetic code expansion. This enabled fine temporal and spatial control over kinase activity, allowing us to quantify phosphorylation kinetics in situ using biochemical and imaging approaches. We find that auto-phosphorylation of the LCK active site loop is indispensable for its catalytic activity and that LCK can stimulate its own activation by adopting a more open conformation, which can be modulated by point mutations. We then show that CD4 and CD8, the T cell coreceptors, can enhance LCK activity, helping to explain their effect in physiological TCR signaling. Our approach also provides general insights into SRC-family kinase dynamics.

Src-family kinases negatively regulate NFAT signaling in resting human T cells

T cell signaling is required for activation of both natural and therapeutic T cells including chimeric antigen receptor (CAR) T cells. Identification of novel factors and pathways regulating T cell signaling may aid in development of effective T cell therapies. In resting human T cells, the majority of Src-family of tyrosine kinases (SFKs) are inactive due to phosphorylation of a conserved carboxy-terminal tyrosine residue. Recently, a pool of enzymatically active SFKs has been identified in resting T cells; however, the significance of these is incompletely understood. Here, we characterized the role of active SFKs in resting human T cells. Pharmacologic inhibition of active SFKs enhanced distal TCR signaling as measured by IL-2 release and CD25 surface expression following TCR-independent activation. Mechanistically, inhibition of the active pool of SFKs induced nuclear translocation of NFAT1, and enhanced NFAT1-dependent signaling in resting T cells. The negative regulation of NFAT1 signaling was in part mediated by the Src-kinase Lck as human T cells lacking Lck had increased levels of nuclear NFAT1 and demonstrated enhanced NFAT1-dependent gene expression. Inhibition of active SFKs in resting primary human T cells also increased nuclear NFAT1 and enhanced NFAT1-dependent signaling. Finally, the calcineurin inhibitor FK506 and Cyclosporin A reversed the effect of SFKs inhibition on NFAT1. Together, these data identified a novel role of SFKs in preventing aberrant NFAT1 activation in resting T cells, and suggest that maintaining this pool of active SFKs in therapeutic T cells may increase the efficacy of T cell therapies.

A Phosphosite within the SH2 Domain of Lck Regulates Its Activation by CD45

The Src Family kinase Lck sets a critical threshold for T cell activation because it phosphorylates the TCR complex and the Zap70 kinase. How a T cell controls the abundance of active Lck molecules remains poorly understood. We have identified an unappreciated role for a phosphosite, Y192, within the Lck SH2 domain that profoundly affects the amount of active Lck in cells. Notably, mutation of Y192 blocks critical TCR-proximal signaling events and impairs thymocyte development in retrogenic mice. We determined that these defects are caused by hyperphosphorylation of the inhibitory C-terminal tail of Lck. Our findings reveal that modification of Y192 inhibits the ability of CD45 to associate with Lck in cells and dephosphorylate the C-terminal tail of Lck, which prevents its adoption of an active open conformation. These results suggest a negative feedback loop that responds to signaling events that tune active Lck amounts and TCR sensitivity.

Ionic CD3-Lck interaction regulates the initiation of T-cell receptor signaling

Antigen-triggered T-cell receptor (TCR) phosphorylation is the first signaling event in T cells to elicit adaptive immunity against invading pathogens and tumor cells. Despite its physiological importance, the underlying mechanism of TCR phosphorylation remains elusive. Here, we report a key mechanism regulating the initiation of TCR phosphorylation. The major TCR kinase Lck shows high selectivity on the four CD3 signaling proteins of TCR. CD3ε is the only CD3 chain that can efficiently interact with Lck, mainly through the ionic interactions between CD3ε basic residue-rich sequence (BRS) and acidic residues in the Unique domain of Lck. We applied a TCR reconstitution system to explicitly study the initiation of TCR phosphorylation. The ionic CD3ε-Lck interaction controls the phosphorylation level of the whole TCR upon antigen stimulation. CD3ε BRS is sequestered in the membrane, and antigen stimulation can unlock this motif. Dynamic opening of CD3ε BRS and its subsequent recruitment of Lck thus can serve as an important switch of the initiation of TCR phosphorylation.

Aurora-A shines on T cell activation through the regulation of Lck

Different protein kinases control signaling emanating from the T cell receptor (TCR) during antigen-specific T cell activation. Mitotic kinases, e.g. Aurora-A, have been widely studied in the context of mitosis due to their role during microtubule (MT) nucleation, becoming critical regulators of cell cycle progression. We have recently described a specific role for Aurora-A kinase in antigenic T cell activation. Blockade of Aurora-A in T cells severely disrupts the dynamics of MTs and CD3ζ-bearing signaling vesicles during T cell activation. Furthermore, Aurora-A deletion impairs the activation of signaling molecules downstream of the TCR. Targeting Aurora-A disturbs the activation of Lck, which is one of the first signals that drive T cell activation in an antigen-dependent manner. This work describes possible models of regulation of Lck by Aurora-A during T cell activation. We also discuss possible roles for Aurora-A in other systems similar to the IS, and its putative functions in cell polarization.

New Insights into How Trafficking Regulates T Cell Receptor Signaling

There is emerging evidence that exocytosis plays an important role in regulating T cell receptor (TCR) signaling. The trafficking molecules involved in lytic granule (LG) secretion in cytotoxic T lymphocytes (CTL) have been well-studied due to the immune disorder known as familial hemophagocytic lymphohistiocytosis (FHLH). However, the knowledge of trafficking machineries regulating the exocytosis of receptors and signaling molecules remains quite limited. In this review, we summarize the reported trafficking molecules involved in the transport of the TCR and downstream signaling molecules to the cell surface. By combining this information with the known knowledge of LG exocytosis and general exocytic trafficking machinery, we attempt to draw a more complete picture of how the TCR signaling network and exocytic trafficking matrix are interconnected to facilitate T cell activation. This also highlights how membrane compartmentalization facilitates the spatiotemporal organization of cellular responses that are essential for immune functions.

Lipids Regulate Lck Protein Activity through Their Interactions with the Lck Src Homology 2 Domain

Lymphocyte-specific protein-tyrosine kinase (Lck) plays an essential role in T cell receptor (TCR) signaling and T cell development, but its activation mechanism is not fully understood. To explore the possibility that plasma membrane (PM) lipids control TCR signaling activities of Lck, we measured the membrane binding properties of its regulatory Src homology 2 (SH2) and Src homology 3 domains. The Lck SH2 domain binds anionic PM lipids with high affinity but with low specificity. Electrostatic potential calculation, NMR analysis, and mutational studies identified the lipid-binding site of the Lck SH2 domain that includes surface-exposed basic, aromatic, and hydrophobic residues but not the phospho-Tyr binding pocket. Mutation of lipid binding residues greatly reduced the interaction of Lck with the ζ chain in the activated TCR signaling complex and its overall TCR signaling activities. These results suggest that PM lipids, including phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate, modulate interaction of Lck with its binding partners in the TCR signaling complex and its TCR signaling activities in a spatiotemporally specific manner via its SH2 domain.

Aurora A drives early signalling and vesicle dynamics during T-cell activation

Aurora A is a serine/threonine kinase that contributes to the progression of mitosis by inducing microtubule nucleation. Here we have identified an unexpected role for Aurora A kinase in antigen-driven T-cell activation. We find that Aurora A is phosphorylated at the immunological synapse (IS) during TCR-driven cell contact. Inhibition of Aurora A with pharmacological agents or genetic deletion in human or mouse T cells severely disrupts the dynamics of microtubules and CD3ζ-bearing vesicles at the IS. The absence of Aurora A activity also impairs the activation of early signalling molecules downstream of the TCR and the expression of IL-2, CD25 and CD69. Aurora A inhibition causes delocalized clustering of Lck at the IS and decreases phosphorylation levels of tyrosine kinase Lck, thus indicating Aurora A is required for maintaining Lck active. These findings implicate Aurora A in the propagation of the TCR activation signal.

Aurora A is a protein kinase that contributes to the progression of mitosis by stimulating microtubule nucleation. Here the authors show that Aurora A also functions during T cell activation by maintaining TCR signaling through Lck activation.

Distinct Mechanisms Regulate Lck Spatial Organization in Activated T Cells

Phosphorylation of the T cell receptor (TCR) by the kinase Lck is the first detectable signaling event upon antigen engagement. The distribution of Lck within the plasma membrane, its conformational state, kinase activity, and protein–protein interactions all contribute to determine how efficiently Lck phosphorylates the engaged TCR. Here, we used cross-correlation raster image correlation spectroscopy and photoactivated localization microscopy to identify two mechanisms of Lck clustering: an intrinsic mechanism of Lck clustering induced by locking Lck in its open conformation and an extrinsic mechanism of clustering controlled by the phosphorylation of tyrosine 192, which regulates the affinity of Lck SH2 domain. Both mechanisms of clustering were differently affected by the absence of the kinase Zap70 or the adaptor Lat. We further observed that the adaptor TSAd bound to and promoted the diffusion of Lck when it is phosphorylated on tyrosine 192. Our data suggest that while Lck open conformation drives aggregation and clustering, the spatial organization of Lck is further controlled by signaling events downstream of TCR phosphorylation.

FRAP, FLIM, and FRET: Detection and analysis of cellular dynamics on a molecular scale using fluorescence microscopy

The combination of fluorescent-probe technology plus modern optical microscopes allows investigators to monitor dynamic events in living cells with exquisite temporal and spatial resolution. Fluorescence recovery after photobleaching (FRAP), for example, has long been used to monitor molecular dynamics both within cells and on cellular surfaces. Although bound by the diffraction limit imposed on all optical microscopes, the combination of digital cameras and the application of fluorescence intensity information on large-pixel arrays have allowed such dynamic information to be monitored and quantified. Fluorescence lifetime imaging microscopy (FLIM), on the other hand, utilizes the information from an ensemble of fluorophores to probe changes in the local environment. Using either fluorescence-intensity or lifetime approaches, fluorescence resonance energy transfer (FRET) microscopy provides information about molecular interactions, with Ångstrom resolution. In this review, we summarize the theoretical framework underlying these methods and illustrate their utility in addressing important problems in reproductive and developmental systems.