Single-cell profiling of D-2-hydroxyglutarate using surface-immobilized resazurin analogs

An enzymic l-2-hydroxyglutarate biosensor based on l-2-hydroxyglutarate dehydrogenase from Azoarcus olearius

l-2-Hydroxyglutarate (l-2-HG) is a critical signaling and immune metabolite but its excessive accumulation can lead to l-2-hydroxyglutaric aciduria, renal cancer, and other diseases. Development of efficient and high-throughput methods for selective l-2-HG detection is urgently required. In this study, l-2-HG dehydrogenase in Azoarcus olearius BH72 (AoL2HGDH) was screened from ten homologs and identified as an enzyme with high specificity and activity toward l-2-HG dehydrogenation. Then, an enzymatic assay-based l-2-HG-sensing fluorescent reporter, EaLHGFR which consists of AoL2HGDH and resazurin, was developed for the detection of l-2-HG. The response magnitude and limit of detection of EaLHGFR were systematically optimized using a single-factor screening strategy. The optimal biosensor EaLHGFR-2 exhibited a response magnitude of 2189.25 ± 26.89% and a limit of detection of 0.042 μM. It can accurately detect the concentration of l-2-HG in bacterial and cellular samples as well as human body fluids. Considering its desirable properties, EaLHGFR-2 may be a promising alternative for quantitation of l-2-HG in biological samples.

Non-Mass Spectrometric Targeted Single-Cell Metabolomics

Metabolic assays serve as pivotal tools in biomedical research, offering keen insights into cellular physiological and pathological states. While mass spectrometry (MS)-based metabolomics remains the gold standard for comprehensive, multiplexed analyses of cellular metabolites, innovative technologies are now emerging for the targeted, quantitative scrutiny of metabolites and metabolic pathways at the single-cell level. In this review, we elucidate an array of these advanced methodologies, spanning synthetic and surface chemistry techniques, imaging-based methods, and electrochemical approaches. We summarize the rationale, design principles, and practical applications for each method, and underscore the synergistic benefits of integrating single-cell metabolomics (scMet) with other single-cell omics technologies. Concluding, we identify prevailing challenges in the targeted scMet arena and offer a forward-looking commentary on future avenues and opportunities in this rapidly evolving field.

Single-Cell Analysis and Classification according to Multiplexed Proteins via Microdroplet-Based Self-Driven Magnetic Surface-Enhanced Raman Spectroscopy Platforms Assisted with Machine Learning Algorithms

A microdroplet-based surface-enhanced Raman spectroscopy (microdroplet SERS) platform was constructed to envelop individual cells in microdroplets, followed by the SERS detection of their extracellular vesicle-proteins (EV-proteins) via the in-drop immunoassays by use of immunomagnetic beads (iMBs) and immuno-SERS tags (iSERS tags). A unique phenomenon is found that iMBs can start a spontaneous reorientation on the probed cell surface based on the electrostatic force-driven interfacial aggregation effect, which leads EV-proteins and iSERS tags to be gathered from a liquid phase to a cell membrane interface and significantly improves SERS sensitivity to the single-cell analysis level due to the formation of numbers of SERS hotspots. Three EV-proteins from two breast cancer cell lines were collected and further analyzed by machine learning algorithmic tools, which will be helpful for a deeper understanding of breast cancer subtypes from the view of EV-proteins.

An Enzymatic Biosensor for the Detection of D-2-Hydroxyglutaric Acid in Serum and Urine

D-2-hydroxyglutaric acid (D2HG) is overproduced as a result of the D-2-hydroxyglutaric aciduria and relevant cancers, caused by gene mutation. Accurate analysis of D2HG could help rapid diagnosis of these diseases and allow for timely treatment. In this work, a D-2-hydroxyglutarate dehydrogenase from Ralstonia solanacearum (RsD2HGDH) is cloned and recombinantly expressed. This enzyme features the direct electron transfer to chemical electron mediators (such as methylene blue (MB)) in the absence of additional coenzymes. Therefore, NAD⁺, a natural electron acceptor for the commercial D2HGDH and usually known for being unstable and difficult for immobilization can be avoided in the preparation of biosensors. The RsD2HGDH and MB are co-immobilized on a two-dimensional material, Ti₃C₂ MXene, followed by drop-coating on the gold screen-printed electrode (AuSPE) to construct a compact and portable biosensor. The D2HG in samples can be catalyzed by RsD2HGDH, where the current change is measured by chronoamperometry at -0.23 V. The biosensor shows a D2HG detection range of 0.5 to 120 µM (R² = 0.9974) with a sensitivity of 22.26 μA mM^-1 cm^-2 and a detection limit of 0.1 µM (S/N = 3). The biosensor retains 72.52% performance of its incipient state after 30 days of storage. The samples of D2HG-containing fetal bovine serum and artificial urine were analyzed with the recovery of 99.56% to 106.83% and 97.30% to 102.47% further indicating the great application potential of our portable D2HG biosensor.