Imaging Spatiotemporal Activities of ZAP-70 in Live T Cells Using a FRET-Based Biosensor

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.

Resource for FRET-Based Biosensor Optimization

After the development of Cameleon, the first fluorescence resonance energy transfer (FRET)-based calcium indicator, a variety of FRET-based genetically encoded biosensors (GEBs) have visualized numerous target players to monitor their cell physiological dynamics spatiotemporally. Many attempts have been made to optimize GEBs, which require labor-intensive effort, novel approaches, and precedents to develop more sensitive and versatile biosensors. However, researchers face considerable trial and error in upgrading biosensors because examples and methods of improving FRET-based GEBs are not well documented. In this review, we organize various optimization strategies after assembling the existing cases in which the non-fluorescent components of biosensors are upgraded. In addition, promising areas to which optimized biosensors can be applied are briefly discussed. Therefore, this review could serve as a resource for researchers attempting FRET-based GEB optimization.

Physics-Informed Machine Learning Improves Detection of Head Impacts

In this work we present a new physics-informed machine learning model that can be used to analyze kinematic data from an instrumented mouthguard and detect impacts to the head. Monitoring player impacts is vitally important to understanding and protecting from injuries like concussion. Typically, to analyze this data, a combination of video analysis and sensor data is used to ascertain the recorded events are true impacts and not false positives. In fact, due to the nature of using wearable devices in sports, false positives vastly outnumber the true positives. Yet, manual video analysis is time-consuming. This imbalance leads traditional machine learning approaches to exhibit poor performance in both detecting true positives and preventing false negatives. Here, we show that by simulating head impacts numerically using a standard Finite Element head-neck model, a large dataset of synthetic impacts can be created to augment the gathered, verified, impact data from mouthguards. This combined physics-informed machine learning impact detector reported improved performance on test datasets compared to traditional impact detectors with negative predictive value and positive predictive values of 88 and 87% respectively. Consequently, this model reported the best results to date for an impact detection algorithm for American football, achieving an F₁ score of 0.95. In addition, this physics-informed machine learning impact detector was able to accurately detect true and false impacts from a test dataset at a rate of 90% and 100% relative to a purely manual video analysis workflow. Saving over 12 h of manual video analysis for a modest dataset, at an overall accuracy of 92%, these results indicate that this model could be used in place of, or alongside, traditional video analysis to allow for larger scale and more efficient impact detection in sports such as American Football.

Signaling Dynamics of TSHR-Specific CAR-T Cells Revealed by FRET-Based Biosensors

Although most patients with thyroid cancers have good prognosis and long-term survival, some patients are refractory to traditional therapeutic approaches and face a high risk of mortality. CAR-T therapy provides an attractive strategy to treat these patients. Considering the limited expression in thyroid tissues, thyroid-stimulating hormone receptor (TSHR) has been considered as a promising candidate as CAR-T target. However, it is still a challenge to find the optimal CAR design for the treatment of thyroid cancers. Dynamic signaling cascade is initiated by CAR molecules during CAR-T cell activation. The development of FRET-based biosensors enables us to detect the signaling dynamics of key kinases during CAR-T cell activation with high spatiotemporal resolution. Here using the ZAP70 and ERK biosensors, we visualized the dynamics of ZAP70 and ERK activities in TSHR-specific CAR-T cells upon antigen stimulation. We first constructed several TSHR-targeting CARs for the treatment of advanced thyroid cancers. The TSHR CAR-T cells with CD28 or 4-1BB co-stimulatory signaling domains exhibited potent cytotoxicity in vitro. By FRET imaging, we observed rapid increase of ZAP70 and ERK activities in TSHR CAR-T cells upon target cell binding. Even though CD28-based CAR-T cells had similar ZAP70 activation dynamics as 4-1BB-based CAR-T cells, they displayed slightly enhanced ERK activation, which may contribute to their faster anti-tumor kinetics in vivo. These results demonstrated the efficacy of TSHR CAR-T cells to treat advanced thyroid cancers. Our study indicated the potential of applying FRET biosensors to optimize the design of CAR for effective CAR-T therapy.

Imaging Membrane Order and Dynamic Interactions in Living Cells with a DNA Zipper Probe

The cell membrane is a dynamic and heterogeneous structure composed of distinct sub-compartments. Within these compartments, preferential interactions occur among various lipids and proteins. Currently, it is still challenging to image these short-lived membrane complexes, especially in living cells. In this work, we present a DNA-based probe, termed "DNA Zipper", which allows the membrane order and pattern of transient interactions to be imaged in living cells using standard fluorescence microscopes. By fine-tuning the length and binding affinity of DNA duplex, these probes can precisely extend the duration of membrane lipid interactions via dynamic DNA hybridization. The correlation between membrane order and the activation of T-cell receptor signaling has also been studied. These programmable DNA probes function after a brief cell incubation, which can be easily adapted to study lipid interactions and membrane order during different membrane signaling events.

Integration of FRET and sequencing to engineer kinase biosensors from mammalian cell libraries

The limited sensitivity of Förster Resonance Energy Transfer (FRET) biosensors hinders their broader applications. Here, we develop an approach integrating high-throughput FRET sorting and next-generation sequencing (FRET-Seq) to identify sensitive biosensors with varying substrate sequences from large-scale libraries directly in mammalian cells, utilizing the design of self-activating FRET (saFRET) biosensor. The resulting biosensors of Fyn and ZAP70 kinases exhibit enhanced performance and enable the dynamic imaging of T-cell activation mediated by T cell receptor (TCR) or chimeric antigen receptor (CAR), revealing a highly organized ZAP70 subcellular activity pattern upon TCR but not CAR engagement. The ZAP70 biosensor elucidates the role of immunoreceptor tyrosine-based activation motif (ITAM) in affecting ZAP70 activation to regulate CAR functions. A saFRET biosensor-based high-throughput drug screening (saFRET-HTDS) assay further enables the identification of an FDA-approved cancer drug, Sunitinib, that can be repurposed to inhibit ZAP70 activity and autoimmune-disease-related T-cell activation.

Quantifying and visualising the nuances of cellular dynamics in vivo using intravital imaging

Intravital imaging is a powerful technology used to quantify and track dynamic changes in live cells and tissues within an intact environment. The ability to watch cell biology in real-time 'as it happens' has provided novel insight into tissue homeostasis, as well as disease initiation, progression and response to treatment. In this minireview, we highlight recent advances in the field of intravital microscopy, touching upon advances in awake versus anaesthesia-based approaches, as well as the integration of biosensors into intravital imaging. We also discuss current challenges that, in our opinion, need to be overcome to further advance the field of intravital imaging at the single-cell, subcellular and molecular resolution to reveal nuances of cell behaviour that can be targeted in complex disease settings.

Cellular Plasticity during Metastasis: New Insights Provided by Intravital Microscopy

Metastasis is a highly dynamic process during which cancer and microenvironmental cells undergo a cascade of events required for efficient dissemination throughout the body. During the metastatic cascade, tumor cells can change their state and behavior, a phenomenon commonly defined as cellular plasticity. To monitor cellular plasticity during metastasis, high-resolution intravital microscopy (IVM) techniques have been developed and allow us to visualize individual cells by repeated imaging in animal models. In this review, we summarize the latest technological advancements in the field of IVM and how they have been applied to monitor metastatic events. In particular, we highlight how longitudinal imaging in native tissues can provide new insights into the plastic physiological and developmental processes that are hijacked by cancer cells during metastasis.

Bioengineering tools for probing intracellular events in T lymphocytes

T lymphocytes are the central coordinator and executor of many immune functions. The activation and function of T lymphocytes are mediated through the engagement of cell surface receptors and regulated by a myriad of intracellular signaling network. Bioengineering tools, including imaging modalities and fluorescent probes, have been developed and employed to elucidate the cellular events throughout the functional lifespan of T cells. A better understanding of these events can broaden our knowledge in the immune systems biology, as well as accelerate the development of effective diagnostics and immunotherapies. Here we review the commonly used and recently developed techniques and probes for monitoring T lymphocyte intracellular events, following the order of intracellular events in T cells from activation, signaling, metabolism to apoptosis. The techniques introduced here can be broadly applied to other immune cells and cell systems. This article is categorized under: Immune System Diseases > Molecular and Cellular Physiology Immune System Diseases > Biomedical Engineering Infectious Diseases > Biomedical Engineering.

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.

Sensitive FRET Biosensor Reveals Fyn Kinase Regulation by Submembrane Localization

Fyn kinase plays crucial roles in hematology and T cell signaling; however, there are currently limited tools to visualize the dynamic Fyn activity in live cells. Here we developed and characterized a highly sensitive Fyn biosensor based on fluorescence resonance energy transfer (FRET) to monitor Fyn kinase activity in live cells. Our results show that Fyn kinase activity can be induced in both mouse embryonic fibroblasts (MEFs) and T cells by ligand engagement. Two different motifs were further introduced to target the biosensor at the cellular membrane microdomains in MEFs, revealing that the Fyn-tagged biosensor had 70% greater response to growth factor stimulation than the Lyn-tagged version. This suggests that the plasma membrane microdomains can be categorized into different functional subdomains. Further experiments show that while the membrane accessibility is necessary for Fyn activation, the localization of Fyn outside of its microdomains causes its hyperactivity, indicating that membrane microdomains provide a suppressive microenvironment for Fyn regulation in MEFs. Interestingly, a relatively high Fyn activity can be observed at perinuclear regions, further supporting the notion that the membrane microenvironment has a significant impact on the local molecular functions. Our work hence highlights a novel Fyn FRET biosensor for live cell imaging and its application in revealing an intricate submembrane regulation of Fyn in live MEFs.

Molecular mobility and activity in an intravital imaging setting - implications for cancer progression and targeting

Molecular mobility, localisation and spatiotemporal activity are at the core of cell biological processes and deregulation of these dynamic events can underpin disease development and progression. Recent advances in intravital imaging techniques in mice are providing new avenues to study real-time molecular behaviour in intact tissues within a live organism and to gain exciting insights into the intricate regulation of live cell biology at the microscale level. The monitoring of fluorescently labelled proteins and agents can be combined with autofluorescent properties of the microenvironment to provide a comprehensive snapshot of in vivo cell biology. In this Review, we summarise recent intravital microscopy approaches in mice, in processes ranging from normal development and homeostasis to disease progression and treatment in cancer, where we emphasise the utility of intravital imaging to observe dynamic and transient events in vivo We also highlight the recent integration of advanced subcellular imaging techniques into the intravital imaging pipeline, which can provide in-depth biological information beyond the single-cell level. We conclude with an outlook of ongoing developments in intravital microscopy towards imaging in humans, as well as provide an overview of the challenges the intravital imaging community currently faces and outline potential ways for overcoming these hurdles.

Ca2+ Microdomains in T-Lymphocytes

Early Ca²⁺ signaling is characterized by occurrence of Ca²⁺ microdomains formed by opening of single or clusters of Ca²⁺ channels, thereby initiating first signaling and subsequently activating global Ca²⁺ signaling mechanisms. However, only few data are available focusing on the first seconds and minutes of Ca²⁺ microdomain formation and related signaling pathways in activated T-lymphocytes. In this review, we condense current knowledge on Ca²⁺ microdomain formation in T-lymphocytes and early Ca²⁺ signaling, function of Ca²⁺ microdomains, and microdomain organization. Interestingly, considering the first seconds of T cell activation, a triphasic Ca²⁺ signal is becoming apparent: (i) initial Ca²⁺ microdomains occurring in the first second of T cell activation, (ii) amplification of Ca²⁺ microdomains by recruitment of further channels in the next 5-10 s, and (iii) a transition to global Ca²⁺ increase. Apparently, the second messenger nicotinic acid adenine dinucleotide phosphate is the first second messenger involved in initiation of Ca²⁺ microdomains. Ryanodine receptors type 1 act as initial Ca²⁺ release channels in CD4⁺ T-lymphocytes. Regarding the temporal correlation of Ca²⁺ microdomains with other molecular events of T cell activation, T cell receptor-dependent microdomain organization of signaling molecules Grb2 and Src homology [SH2] domain-containing leukocyte protein of 65 kDa was observed within the first 20 s. In addition, fast cytoskeletal changes are initiated. Furthermore, the involvement of additional Ca²⁺ channels and organelles, such as the Ca²⁺ buffering mitochondria, is discussed. Future research developments will comprise analysis of the causal relation between these temporally coordinated signaling events. Taken together, high-resolution Ca²⁺ imaging techniques applied to T cell activation in the past years paved the way to detailed molecular understanding of initial Ca²⁺ signaling mechanisms in non-excitable cells.