Quantitatively Mapping the Assembly Pattern of EpCAM on Cell Membranes with Peptide Probes

Single-cell SERS imaging of dual cell membrane receptors expression influenced by extracellular matrix stiffness

Membrane receptors perform a diverse range of cellular functions, accounting for more than half of all drug targets. The mechanical microenvironment regulates cell behaviors and phenotype. However, conventional analysis methods of membrane receptors often ignore the effects of the extracellular matrix stiffness, failing to reveal the heterogeneity of cell membrane receptors expression. Herein, we developed an in-situ surface-enhanced Raman scattering (SERS) imaging method to visualize single-cell membrane receptors on substrates with different stiffness. Two SERS substrates, Au@4-mercaptobenzonitrile@Ag@Sgc8c and Au@4-pethynylaniline@Ag@SYL3c, were employed to specifically target protein tyrosine kinase-7 (PTK7) and epithelial cell adhesion molecule (EpCAM), respectively. The polyacrylamide (PA) gels with tunable stiffness (2.5-25 kPa) were constructed to mimic extracellular matrix. The simultaneous SERS imaging of dual membrane receptors on single cancer cells on substrates with different stiffness was achieved. Our findings reveal decreased expression of PTK7 and EpCAM on cells cultured on stiffer substrates and higher migration ability of the cells. The results elucidate the heterogeneity of membrane receptors expression of cells cultured on the substrates with different stiffness. This single-cell analysis method offers an in-situ platform for investigating the impacts of extracellular matrix stiffness on the expression of membrane receptors, providing insights into the role of cell membrane receptors in cancer metastasis.

Fluorescent dyes based on rhodamine derivatives for bioimaging and therapeutics: recent progress, challenges, and prospects

Since their inception, rhodamine dyes have been extensively applied in biotechnology as fluorescent markers or for the detection of biomolecules owing to their good optical physical properties. Accordingly, they have emerged as a powerful tool for the visualization of living systems. In addition to fluorescence bioimaging, the molecular design of rhodamine derivatives with disease therapeutic functions (e.g., cancer and bacterial infection) has recently attracted increased research attention, which is significantly important for the construction of molecular libraries for diagnostic and therapeutic integration. However, reviews focusing on integrated design strategies for rhodamine dye-based diagnosis and treatment and their wide application in disease treatment are extremely rare. In this review, first, a brief history of the development of rhodamine fluorescent dyes, the transformation of rhodamine fluorescent dyes from bioimaging to disease therapy, and the concept of optics-based diagnosis and treatment integration and its significance to human development are presented. Next, a systematic review of several excellent rhodamine-based derivatives for bioimaging, as well as for disease diagnosis and treatment, is presented. Finally, the challenges in practical integration of rhodamine-based diagnostic and treatment dyes and the future outlook of clinical translation are also discussed.

Baits-trap chip for accurate and ultrasensitive capture of living circulating tumor cells

Accurate analysis of living circulating tumor cells (CTCs) plays a crucial role in cancer diagnosis and prognosis evaluation. However, it is still challenging to develop a facile method for accurate, sensitive, and broad-spectrum isolation of living CTCs. Herein, inspired by the filopodia-extending behavior and clustered surface-biomarker of living CTCs, we present a unique baits-trap chip to achieve accurate and ultrasensitive capture of living CTCs from peripheral blood. The baits-trap chip is designed with the integration of nanocage (NCage) structure and branched aptamers. The NCage structure could "trap" the extended filopodia of living CTCs and resist the adhesion of filopodia-inhibited apoptotic cells, thus realizing the accurate capture (∼95% accuracy) of living CTCs independent of complex instruments. Using an in-situ rolling circle amplification (RCA) method, branched aptamers were easily modified onto the NCage structure, and served as "baits" to enhance the multi-interactions between CTC biomarker and chips, leading to ultrasensitive (99%) and reversible cell capture performance. The baits-trap chip successfully detects living CTCs in broad-spectrum cancer patients and achieves high diagnostic sensitivity (100%) and specificity (86%) of early prostate cancer. Therefore, our baits-trap chip provides a facile, accurate, and ultrasensitive strategy for living CTC isolation in clinical. STATEMENT OF SIGNIFICANCE: A unique baits-trap chip integrated with precise nanocage structure and branched aptamers was developed for the accurate and ultrasensitive capture of living CTCs. Compared with the current CTC isolation methods that are unable to distinguish CTC viability, the nanocage structure could not only "trap" the extended-filopodia of living CTCs, but also resist the adhesion of filopodia-inhibited apoptotic cells, thus realizing the accurate capture of living CTCs. Additionally, benefiting from the "baits-trap" synergistic effects generated by aptamer modification and nanocage structure, our chip achieved ultrasensitive, reversible capture of living CTCs. Moreover, this work provided a facile strategy for living CTC isolation from the blood of patients with early-stage and advanced cancer, exhibiting high consistency with the pathological diagnosis.

Modular DNA-Origami-Based Nanoarrays Enhance Cell Binding Affinity through the "Lock-and-Key" Interaction

Surface proteins of cells are generally recognized through receptor-ligand interactions (RLIs) in disease diagnosis, but their nonuniform spatial distribution and higher-order structure lead to low binding affinity. Constructing nanotopologies that match the spatial distribution of membrane proteins to improve the binding affinity remains a challenge. Inspired by the multiantigen recognition of immune synapses, we developed modular DNA-origami-based nanoarrays with multivalent aptamers. By adjusting the valency and interspacing of the aptamers, we constructed specific nanotopology to match the spatial distribution of target protein clusters and avoid potential steric hindrance. We found that the nanoarrays significantly enhanced the binding affinity of target cells and synergistically recognized low-affinity antigen-specific cells. In addition, DNA nanoarrays used for the clinical detection of circulating tumor cells successfully verified their precise recognition ability and high-affinity RLIs. Such nanoarrays will further promote the potential application of DNA materials in clinical detection and even cell membrane engineering.

High glucose-induced glucagon resistance and membrane distribution of GCGR revealed by super-resolution imaging

The glucagon receptor (GCGR) is a member of the class B G protein-coupled receptor family. Many research works have been carried out on GCGR structure, glucagon signaling pathway, and GCGR antagonists. However, the expression and fine distribution of GCGR proteins in response to glucagon under high glucose remain unclear. Using direct stochastic optical reconstruction microscopy (dSTORM) imaging, nanoscale GCGR clusters were observed on HepG2 cell membranes, and high glucose promoted GCGR expression and the formation of more and larger clusters. Moreover, glucagon stimulation under high glucose did not inhibit GCGR levels as significantly as that under low glucose and did not increase the downstream cyclic 3,5'-adenosine monophosphate-protein kinase A (cAMP-PKA) signal, and there were still large-size clusters on the membranes, indicating that high glucose induced glucagon resistance. In addition, high glucose induced stronger glucagon resistance in hepatoma cells compared with hepatic cells. Our work will pave a way to further our understanding of the pathogenesis of diabetes and develop more effective drugs targeting GCGR.

Programming DNA Aptamer Arrays of Prescribed Spatial Features with Enhanced Bioavailability and Cell Growth Modulation

Epithelial cell adhesion molecules (EpCAMs) play pivotal roles in tumorigenesis in many cancer types, which is reported to reside within nano- to microscale membrane domains, forming small clusters. We propose that building multivalent ligands that spatially patch to EpCAM clusters may largely enhance their targeting capability. Herein, we assembled EpCAM aptamers into nanoscale arrays of prescribed valency and spatial arrangements by using a rectangular DNA pegboard. Our results revealed that EpCAM aptamer arrays exhibited significantly higher binding avidity to MCF-7 cells than free monovalent aptamers, which was affected by both valency and spatial arrangement of aptamers. Furthermore, EpCAM aptamer arrays showed improved tolerance against competing targets in an extracellular environment and potent bioavailability and targeting specificity in a xenograft tumor model in mice. This work may shed light on rationally designing multivalent ligand complexes of defined parameters with optimized binding avidity and targeting capability toward various applications in the biomedical fields.

Mechanistic insights into HuR inhibitor MS-444 arresting embryonic development revealed by low-input RNA-seq and STORM

With improvements in the survival rate of patients with cancer, fertility maintenance has become a major concern in terms of cancer treatment for women of reproductive age. Thus, it is important to examine the impact on fertility of anticancer drugs that are used clinically or are undergoing trials. The HuR small-molecule inhibitor MS-444 has been used in many cancer treatment studies, but its reproductive toxicity in females is unknown. Here, we reported that MS-444 blocked the nucleocytoplasmic transport of Agbl2 mRNA by inhibiting HuR dimerization, resulting in the developmental arrest of 2-cell stage embryos in mouse. Combining analysis of low-input RNA-seq for MS-444-treated 2-cell embryos and mapping binding sites of RNA-binding protein, Agbl2 was predicted to be the target gene of MS-444. For further confirmation, RNAi experiment in wild-type zygotes showed that Agbl2 knockdown reduced the proportion of embryos successfully developed to the blastocyst stage: from 71% in controls to 23%. Furthermore, RNA-FISH and luciferase reporter analyses showed that MS-444 blocked the nucleocytoplasmic transport of Agbl2 mRNA and reduced its stability by inhibiting HuR dimerization. In addition, optimized stochastic optical reconstruction microscopy (STORM) imaging showed that MS-444 significantly reduced the HuR dimerization, and HuR mainly existed in cluster form in 2-cell stage embryos. In conclusion, this study provides clinical guidance for maintaining fertility during the treatment of cancer with MS-444 in women of reproductive age. And also, our research provides guidance for the application of STORM in nanometer scale studies of embryonic cells. HuR inhibitor MS-444 arrested embryonic development at 2-cell stage. Low-input RNA-seq revealed that Agbl2 was the target gene of MS-444. MS-444 blocked the nucleocytoplasmic transport of Agbl2 mRNA by inhibiting HuR dimerization and reduced the stability of Agbl2 mRNA. STORM with our optimized protocol showed that HuR tended to form elliptical and dense clusters in 2-cell stage embryos.

Mechanistic Insights into Membrane Protein Clustering Revealed by Visualizing EGFR Secretion

Most plasmalemmal proteins are organized into clusters to modulate various cellular functions. However, the machineries that regulate protein clustering remain largely unclear. Here, with EGFR as an example, we directly and in detail visualized the entire process of EGFR from synthesis to secretion onto the plasma membrane (PM) using a high-speed, high-resolution spinning-disk confocal microscope. First, colocalization imaging revealed that EGFR secretory vesicles underwent transport from the ER to the Golgi to the PM, eventually forming different distribution forms on the apical and basal membranes; that is, most EGFR formed larger clusters on the apical membrane than the basal membrane. A dynamic tracking image and further siRNA interference experiment confirmed that fusion of secretory vesicles with the plasma membrane led to EGFR clusters, and we showed that EGFR PM clustering may be intimately related to EGFR signaling and cell proliferation. Finally, we found that the size and origin of the secretory vesicles themselves may determine the difference in the distribution patterns of EGFR on the PM. More importantly, we showed that actin influenced the EGFR distribution by controlling the fusion of secretory vesicles with the PM. Collectively, a comprehensive understanding of the EGFR secretion process helps us to unravel the EGFR clustering process and elucidate the key factors determining the differences in the spatial distribution of EGFR PM, highlighting the correlation between EGFR secretion and its PM distribution pattern.

Development of a Novel Anti-EpCAM Monoclonal Antibody for Various Applications

The epithelial cell adhesion molecule (EpCAM) is a cell surface glycoprotein, which is widely expressed on normal and cancer cells. EpCAM is involved in cell adhesion, proliferation, survival, stemness, and tumorigenesis. Therefore, EpCAM is thought to be a promising target for cancer diagnosis and therapy. In this study, we established anti-EpCAM monoclonal antibodies (mAbs) using the Cell-Based Immunization and Screening (CBIS) method. We characterized them using flow cytometry, Western blotting, and immunohistochemistry. One of the established recombinant anti-EpCAM mAbs, recEpMab-37 (mouse IgG₁, kappa), reacted with EpCAM-overexpressed Chinese hamster ovary-K1 cells (CHO/EpCAM) or a colorectal carcinoma cell line (Caco-2). In contrast, recEpMab-37 did not react with EpCAM-knocked out Caco-2 cells. The K_D of recEpMab-37 for CHO/EpCAM and Caco-2 was 2.0 × 10⁻⁸ M and 3.2 × 10⁻⁸ M, respectively. We observed that EpCAM amino acids between 144 to 164 are involved in recEpMab-37 binding. In Western blot analysis, recEpMab-37 detected the EpCAM of CHO/EpCAM and Caco-2 cells. Furthermore, recEpMab-37 could stain formalin-fixed paraffin-embedded colorectal carcinoma tissues by immunohistochemistry. Taken together, recEpMab-37, established by the CBIS method, is useful for detecting EpCAM in various applications.

Directing Multivalent Aptamer-Receptor Binding on the Cell Surface with Programmable Atom-Like Nanoparticles

Multivalent interactions of biomolecules play pivotal roles in physiological and pathological settings. Whereas the directionality of the interactions is crucial, the state-of-the-art synthetic multivalent ligand-receptor systems generally lack programmable approaches for orthogonal directionality. Here, we report the design of programmable atom-like nanoparticles (aptPANs) to direct multivalent aptamer-receptor binding on the cell interface. The positions of the aptamer motifs can be prescribed on tetrahedral DNA frameworks to realize atom-like orthogonal valence and direction, enabling the construction of multivalent molecules with fixed aptamer copy numbers but different directionality. These directional-yet-flexible aptPAN molecules exhibit the adaptability to the receptor distribution on cell surfaces. We demonstrate the high-affinity tumor cell binding with a linear aptPAN oligomer (≈13-fold improved compared to free aptamers), which leads to ≈50 % suppression of cell growth.

Interferometrical single-molecule localization based on dynamic PSF engineering

We present a method for interferometric single-molecule localization based on dynamic point spread function (PSF) engineering. By using two galvo mirrors, a hexagonal PSF is constructed and the fluorescent signal under different illumination patterns could be acquired simultaneously. This method was evaluated using simulation, fluorescent nanosphere imaging, and single-molecule imaging. The study indicates a twofold improvement in localization precision while maintaining the same photon budget. This strategy, we believe, is a cost-effective way to increase the resolution of single-molecule localization microscopy.

A multidrug-resistant P-glycoprotein assembly revealed by tariquidar-probe's super-resolution imaging

As an efflux pump, P-glycoproteins (P-gps) are over-expressed in many cancer cell types to confer them with multi-drug resistance. Many studies have focused on elucidating their molecular structure or protein expression; however, the relationship between the molecular assembly and dysfunction remains unclear. Super-resolution microscope is an excellent imaging tool to reveal the molecular biological details, but its high-quality imaging often suffers from the labeling method currently available. In this work, by exploiting its specificity and small size, tariquidar (specific inhibitor of P-gp) was modified by TAMRA to form a small chemical probe of P-gp. By direct stochastic optical reconstruction microscopic (dSTORM) imaging, tariquidar-TAMRA was first revealed to possess a higher labeling superiority and high binding specificity. Then, with the application of tariquidar-TAMRA labeling, we found that P-gps accumulate into larger and denser clusters on cancer cells and drug-resistant cells than on normal cells and drug-sensitive cells, indicating that P-gps can facilitate the pumping efficiency by aggregating together to form functional platforms. Moreover, these specific distribution patterns might serve as potential biomarkers for tumor and drug therapy screening.

Precise Encoding of Triple-Bond Raman Scattering of Single Polymer Nanoparticles for Multiplexed Imaging Application

Stimulated Raman scattering (SRS) microscopy in combination with innovative tagging strategies offers great potential as a universal high-throughput biomedical imaging tool. Here, we report rationally tailored small molecular monomers containing triple-bond units with large Raman scattering cross-sections, which can be polymerized at the nanoscale for enhancement of SRS contrast with smaller but brighter optical nanotags with artificial fingerprint output. From this, a class of triple-bond rich polymer nanoparticles (NPs) was engineered by regulating the relative dosages of three chemically different triple-bond monomers in co-polymerization. The bonding strategy allowed for 15 spectrally distinguishable triple-bond combinations. These accurately structured nano molecular aggregates, rather than long-chain macromolecules, could establish a universal method for generating small-sized biological SRS imaging tags with high sensitivity for high-throughput multi-color biomedical imaging.

Cell surface-localized imaging and sensing

Systematically dissecting the molecular basis of the cell surface as well as its related biological activities is considered as one of the most cutting-edge fields in fundamental sciences. The advent of various advanced cell imaging techniques allows us to gain a glimpse of how the cell surface is structured and coordinated with other cellular components to respond to intracellular signals and environmental stimuli. Nowadays, cell surface-related studies have entered a new era featured by a redirected aim of not just understanding but artificially manipulating/remodeling the cell surface properties. To meet this goal, biologists and chemists are intensely engaged in developing more maneuverable cell surface labeling strategies by exploiting the cell's intrinsic biosynthetic machinery or direct chemical/physical binding methods for imaging, sensing, and biomedical applications. In this review, we summarize the recent advances that focus on the visualization of various cell surface structures/dynamics and accurate monitoring of the microenvironment of the cell surface. Future challenges and opportunities in these fields are discussed, and the importance of cell surface-based studies is highlighted.

Mechanistic Insights into Trop2 Clustering on Lung Cancer Cell Membranes Revealed by Super-resolution Imaging

The transmembrane glycoprotein Trop2 plays important roles in various types of human cancers, especially lung cancer. Although it has been found to form clusters on cancer cell membranes, the factors that affect its clustering are not yet fully understood. Here, using direct stochastic optical reconstruction microscopy (dSTORM), we found that Trop2 generated more, larger, and denser clusters on apical cell membranes than on basal membranes and that the differences might be related to the different membrane structures. Moreover, dual-color dSTORM imaging revealed significant colocalization of Trop2 and lipid rafts, and methyl-β-cyclodextrin disruption dramatically impaired the formation of Trop2 clusters, indicating a key role of lipid rafts in Trop2 clustering. Additionally, depolymerization of the actin cytoskeleton decreased Trop2 cluster numbers and areas, revealing that actin can stabilize the clusters. More importantly, stimulation of Trop2 in cancer cells hardly changed the cluster morphology, suggesting that Trop2 is activated and forms clusters in cancer cells. Altogether, our work links the spatial organization of Trop2 to different membrane structures and Trop activation and uncovers the essential roles of lipid rafts and actin in Trop2 cluster maintenance.

Application of an inhibitor-based probe to reveal the distribution of membrane PSMA in dSTORM imaging

Relying on an inhibitor-based probe, we reveal the clustered distribution of membrane PSMA by dSTORM imaging and uncover its potential interaction with folate receptor. This inhibitor-based strategy realizes more accurate labeling than antibody labeling, which would make it a powerful tool in the field of dSTORM imaging.

Development of small molecule inhibitor-based fluorescent probes for highly specific super-resolution imaging

To ensure the ultimate high-quality imaging of super-resolution fluorescence microscopy with increasingly high resolution, it is significant to use small specific fluorescent probes. Compared with the common biological fluorescent labeling technology, because of small size, strong specificity, abundance and special binding sites, single-targeted small-molecule inhibitors (SMIs) can link with organic dyes to form small fluorescent probes for various biomolecules. Herein, to confirm the feasibility of the SMI-probes, epidermal growth factor (EGF) receptor (EGFR)-targeted tyrosine kinase inhibitor Gefitinib was selected for modification with the fluorescent dye to form Gefitinib-probe. Then, the labeling superiority of Gefitinib-probe was revealed by comparing the direct stochastic optical reconstruction microscopy (dSTORM) images of EGFR labeled with different probes. Additionally, a high co-localization of fluorescent points from Gefitinib-probe and EGF-probe labeling indicated a high specificity of Gefitinib-probe to EGFR. Finally, higher co-localization of EGFR and HER3 labeled with the probe pair containing Gefitinib-probe than with the antibody-probe pair suggested that Gefitinib-probe with a cytoplasmic binding site benefited dual-color imaging. These results indicate that the SMI-probes are able to serve as versatile labeling tools for high-quality super-resolution imaging.

Anti‑EpCAM monoclonal antibody exerts antitumor activity against oral squamous cell carcinomas

The epithelial cell adhesion molecule (EpCAM) is a calcium-independent, homophilic, intercellular adhesion factor classified as a transmembrane glycoprotein. In addition to cell adhesion, EpCAM also contributes to cell signaling, differentiation, proliferation, and migration. EpCAM is an essential factor in the carcinogenesis of numerous human cancers. In the present study, we developed and validated an anti-EpCAM monoclonal antibody (mAb), EpMab-16 (IgG_2a, kappa), by immunizing mice with EpCAM-overexpressing CHO-K1 cells. EpMab-16 specifically reacted with endogenous EpCAM in oral squamous cell carcinoma (OSCC) cell lines in flow cytometry and Western blot analyses. It exhibited a plasma membrane-like stain pattern in OSCC tissues upon immunohistochemical analysis. The K_D for EpMab-16 in SAS and HSC-2 OSCC cells were assessed via flow cytometry at 1.1×10⁻⁸ and 1.9×10⁻⁸ M, respectively, suggesting moderate binding affinity of EpMab-16 for EpCAM. We then assessed whether the EpMab-16 induced antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) against OSCC cell lines, and antitumor capacity in a murine xenograft model. In vitro experiments revealed strong ADCC and CDC inducement against OSCC cells treated with EpMab-16. In vivo experiments on OSCC xenografts revealed that EpMab-16 treatment significantly reduced tumor growth compared with the control mouse IgG. These data indicated that EpMab-16 could be a promising treatment option for EpCAM-expressing OSCCs.

Aptamer AS1411 utilized for super-resolution imaging of nucleolin

Nucleolin (NCL) is a multifunctional protein that mainly localizes in the nucleolus and also distributes in the nucleoplasm, cytoplasm and cell membrane. Most studies focus on its biofunctions in cell activities and diseases, however, its detailed distribution and organization pattern in situ remains obscure. Moreover, antibodies were commonly used to investigate NCL in cells. It is worth noting that antibody labeling of intracellular proteins needs detergents to permeabilize the membrane, which could disrupt the membrane structure and proteins. The emergence of aptamer AS1411 provides us a good choice to recognize the NCL without permeabilization owing to its superior cellular uptake and enhanced stability. Therefore, we applied aptamer AS1411 to super-resolution imaging to visualize the distribution of NCL at a nanometer level. Aptamer achieved a better recognition of intracellular NCL and displayed the detailed structure of NCL in different parts of cells. Significantly, cytoplasmic and membrane NCL have higher expression and larger clusters in cancer cells than that in normal cells. Our work presented a detailed organization of NCL in cells and revealed the distribution differences between cancer cells and normal cells, which promote the understanding of its functions in physiology and pathology.