Discovery of the first potent ROR1 degrader for the treatment of non-small cell lung cancer

ROR1 has been identified as a pseudokinase, functioning as an allosteric regulator in tumor progression. Aberrant overexpression of ROR1 has been observed in various malignancies, highlighting its potential as therapeutic target for cancer therapy. Modulation of ROR1 by proteolysis targeting chimera degrader instead of traditional inhibitor could offer great efficiency in blocking its kinase-independent regulatory function. Here, we report the first potent ROR1 degraders constructed by connecting the E3 ligand to a ROR1 binder. One representative compound 11d exhibited remarkable efficacy in depleting ROR1 protein with a DC50 value of 40.88 nM and Dmax of 93.7 %. Mechanistic investigations illuminated that compound 11d triggers ROR1 protein degradation in a ubiquitin proteasome system (UPS)-dependent manner. Additionally, compound 11d displayed a significantly enhanced ability to inhibit ROR1 signaling, induce apoptosis, and suppress proliferation in lung cell lines compared to the warhead ROR1 binder. These findings underscore the substantial potential of ROR1 degrader for the treatment of non-small cell lung cancer (NSCLC) cells.

The Therapeutic Significance of HER3 in Non-small Cell Lung Cancer (NSCLC): A Review Study

HER3 (Human Epidermal Growth Factor Receptor 3) is frequently overexpressed in various cancers, including non-small cell lung cancer (NSCLC), with a prevalence of 83% in primary tumors. Its involvement in tumorigenesis and resistance to targeted therapies makes HER3 a promising target for cancer treatment. Despite being initially considered "undruggable" due to its lack of catalytic activity, significant progress has been made in the development of anti-HER3 therapeutics. Monoclonal antibodies such as lumretuzumab, seribantumab, and patritumab have shown potential in targeting HER3 to overcome resistance to epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs). Additionally, antibody-drug conjugates (ADCs) like HER3-DXd (patritumab deruxtecan) are new drug candidates that have demonstrated selective delivery of cytotoxic chemicals to NSCLC cells by exploiting HER3's widespread expression, minimizing cytotoxicity. This review aims to evaluate the efficacy of current HER3 therapeutics in development and their therapeutic potential in NSCLC, incorporating evidence from clinical trials.

Unlocking the potential: advancements and future horizons in ROR1-targeted cancer therapies

While receptor tyrosine kinase-like orphan receptor 1 (ROR1) is typically expressed at low levels or absent in normal tissues, its expression is notably elevated in various malignant tumors and conditions, including chronic lymphocytic leukemia (CLL), breast cancer, ovarian cancer, melanoma, and lung adenocarcinoma. This distinctive feature positions ROR1 as an attractive target for tumor-specific treatments. Currently, several targeted drugs directed at ROR1 are undergoing clinical development, including monoclonal antibodies, antibody-drug conjugates (ADCs), and chimeric antigen receptor T-cell therapy (CAR-T). Additionally, there are four small molecule inhibitors designed to bind to ROR1, presenting promising avenues for the development of PROTAC degraders targeting ROR1. This review offers updated insights into ROR1's structural and functional characteristics, embryonic development implications, cell survival signaling pathways, and evolutionary targeting strategies, all of which have the potential to advance the treatment of malignant tumors.

Study of the Functions and Activities of Neuronal K-Cl Co-Transporter KCC2 Using Western Blotting

Potassium chloride cotransporters 2 (KCC2) is a member of the solute carrier family 12 (SLC12) of cation-chloride-cotransporters (CCCs), found exclusively in the neuron and is essential for the proper functioning of Cl- homeostasis and consequently functional GABAergic inhibition. Failure in proper regulation of KCC2 is deleterious and has been associated with the prevalence of several neurological diseases, including epilepsy. There has been considerable progress with regard to understanding the mechanisms involved in the regulation of KCC2, accredited to the development of techniques that enable researchers to study its functions and activities; either via direct (assessing kinase regulatory sites phosphorylation) or indirect (observing and monitoring GABA activity) investigations. Here, the protocol highlights how to investigate KCC2 phosphorylation at kinase regulatory sites - Thr906 and Thr1007- using western blotting technique. There are other classic methods used to directly measure KCC2 activity, such as rubidium ion and thallium ion uptake assay. Further techniques such as patch-clamp-electrophysiology are used to measure GABA activity; hence, indirectly reflecting activated and/or inactivated KCC2 as informed by the assessment of intracellular chloride ion homeostasis. A few of these additional techniques will be briefly discussed in this manuscript.

Abstract P198: WNK-SPAK-NKCC1 Cascade Activation Contributes to Worsened Brain Damage in Mice With Hypertension Co-Morbidity after Ischemic Stroke

Objectives: The WNK-SPAK/OSR1 kinase complex plays an important role in renal salt handling and pathogenesis of hypertension by regulating ion transporters and channels. Hypertension is the most common risk factor for stroke and stroke patients with hypertension comorbidity have worsened outcome with an increased risk of dependency or death. However, the mechanisms underlying the worsened ischemic stroke pathophysiology with hypertension comorbidity remain poorly defined. In this study, we investigated roles of the WNK-SPAK-NKCC1 signaling pathway in ischemic brain damage in mice with hypertension comorbidity.

Methods: Hypertension was induced in C57BL/6j male mouse (12-15 weeks) by subcutaneous infusion of 1000 ng/kg/min angiotensin II (AngII, mini-osmotic pump) for two weeks. Permanent ischemic stroke was induced by permanent occlusion of the distal branches of the left middle cerebral artery (pd-MCAO). Brain tissues were harvested for immunoblot assessment of expression levels of NKCC1, SPAK/OSR1 or WNK1-4. Infarct volume and hemisphere swelling were determined by TTC staining, and behavioral deficits were analyzed by foot fault test, cylinder test and adhesive tape removal test.

Results: pd-MCAO stimulated expression of WNK proteins (isoforms 1, 2, 4), total and phosphorylated SPAK/OSR1 and NKCC1 proteins in ischemic brains of the AngII-infused hypertensive mice compared to normotensive saline controls. In parallel with the increased activation of WNK-SPAK-NKCC1 signaling, hypertensive mice displayed significantly larger infarct volume and hemispheric swelling at 24 h after pd-MCAO compared to normotensive controls. Moreover, hypertensive mice exhibited a slow recovery of neurological function after ischemic stroke compared to normotensive counterparts as assessed by sensory-motor sensitive tests.

Conclusions: These results suggest that activation of the WNK-SPAK-NKCC1 complex in hypertensive ischemic brains associates, at least in part, with the worsened brain damage and neurological deficits. Pharmacological inhibition of WNK-SPAK complex has therapeutic potentials for stroke therapy with hypertension comorbidity.

Subsecond accurate myelin water fraction reconstruction from FAST-T2 data with 3D UNET

PURPOSE

To develop a 3D UNET convolutional neural network for rapid extraction of myelin water fraction (MWF) maps from six-echo fast acquisition with spiral trajectory and T2 -prep data and to evaluate its accuracy in comparison with multilayer perceptron (MLP) network.

METHODS

The MWF maps were extracted from 138 patients with multiple sclerosis using an iterative three-pool nonlinear least-squares algorithm (NLLS) without and with spatial regularization (srNLLS), which were used as ground-truth labels to train, validate, and test UNET and MLP networks as a means to accelerate data fitting. Network testing was performed in 63 patients with multiple sclerosis and a numerically simulated brain phantom at SNR of 200, 100 and 50.

RESULTS

Simulations showed that UNET reduced the MWF mean absolute error by 30.1% to 56.4% and 16.8% to 53.6% over the whole brain and by 41.2% to 54.4% and 21.4% to 49.4% over the lesions for predicting srNLLS and NLLS MWF, respectively, compared to MLP, with better performance at lower SNRs. UNET also outperformed MLP for predicting srNLLS MWF in the in vivo multiple-sclerosis brain data, reducing mean absolute error over the whole brain by 61.9% and over the lesions by 67.5%. However, MLP yielded 41.1% and 51.7% lower mean absolute error for predicting in vivo NLLS MWF over the whole brain and the lesions, respectively, compared with UNET. The whole-brain MWF processing time using a GPU was 0.64 seconds for UNET and 0.74 seconds for MLP.

CONCLUSION

Subsecond whole-brain MWF extraction from fast acquisition with spiral trajectory and T2 -prep data using UNET is feasible and provides better accuracy than MLP for predicting MWF output of srNLLS algorithm.

Discovery of ZLC491 as a Potent, Selective, and Orally Bioavailable CDK12/13 PROTAC Degrader

Selective degradation of cyclin-dependent kinases 12 and 13 (CDK12/13) emerges as a new potential therapeutic approach for triple-negative breast cancer (TNBC) and other human cancers. While several proteolysis-targeting chimera (PROTAC) degraders of CDK12/13 were reported, none are orally bioavailable. Here, we report the discovery of ZLC491 as a potent, selective, and orally bioavailable CDK12/13 PROTAC degrader. The compound effectively degraded CDK12 and CDK13 with DC50 values of 32 and 28 nM, respectively, in TNBC MDA-MB-231 cells. Global proteomic assessment and mechanistic studies revealed that ZLC491 selectively induced CDK12/13 degradation in a cereblon- and proteasome-dependent manner. Furthermore, the molecule efficiently suppressed transcription and expression of long genes, predominantly a subset of genes associated with DNA damage response, and significantly inhibited proliferation of multiple TNBC cell lines. Importantly, ZLC491 achieved an oral bioavailability of 46.8% in rats and demonstrated potent in vivo degradative effects on CDK12/13 in an MDA-MB-231 xenografted mouse model.

Discovery of the first selective and potent PROTAC degrader for the pseudokinase TRIB2

Pseudokinase TRIB2, a member of the CAMK Ser/Thr protein kinase family, regulates various cellular processes through phosphorylation-independent mechanisms. Dysregulation of TRIB2 has been implicated in promoting tumor growth, metastasis, and therapy resistance, making it a promising target for cancer treatment. In this study, we designed and synthesized a series of TRIB2 PROTAC degraders by conjugating a TRIB2 binder 1 with VHL or CRBN ligands via linkers of varying lengths and compositions. Among these compounds, 5k demonstrated potent TRIB2 degradation with a DC50 value of 16.84 nM (95 % CI: 13.66-20.64 nM) in prostate cancer PC3 cells. Mechanistic studies revealed that 5k directly interacted with TRIB2, selectively inducing its degradation through a CRBN-dependent ubiquitin-proteasomal pathway. Moreover, 5k outperformed the TRIB2 binder alone in inhibiting cell proliferation and inducing apoptosis, confirming that TRIB2 protein degradation could be a promising therapeutic strategy for TRIB2-associated cancers. Additionally, compound 5k also serves as an effective tool for probing TRIB2 biology.

374 Multi-omic Analysis Identifies a SPAK Kinase-regulated Ensemble of Choroid Plexus Ion Transport Proteins Relevant for Post-infectious Hydrocephalus

INTRODUCTION:

Postinfectious hydrocephalus (PIH) is the most common cause of acquired hydrocephalus worldwide. Treatment is typically through CSF diversion via ventriculoperitoneal shunting, which has high rates of morbidity and is often unavailable in developing countries.

METHODS:

SPAK knockout (KO) rats were generated using Crispr/CAS9. Control and SPAK KO male rats were implanted with an infusion catheter to introduce E. coli (genetically engineered +/-LPS) or LPS into the lateral ventricles over 72 hours. CSF secretion was measured by catheter placement into the lateral ventricle, and ventricular size by MRI. ChP were harvested for immunohistochemistry (IHC), western blot (WB), genomic/proteomic analysis, and single-cell RNA sequencing (scRNAseq). To investigate SPAK-associated ChP targets, we purified the SPAK-protein complex, identified physical interactors using liquid chromatography/dual mass spectroscopy (LC-MS/MS), and validated top candidates by WB/IHC.

RESULTS:

Robust bumetanide-sensitive, SPAK-dependent, CSF hypersecretion and ventriculomegaly was observed and dependent on the presence of LPS. LPS triggered significant up-regulation of activating SPAK phosphorylation (Thr233/185), and the bumetanide-sensitive cation-chloride cotransporter NKCC1 (Thr203/207/212). LC-MS/MS pull-down assay additionally revealed three other important SPAK-bound ion transporters, alpha-1 subunit of Na/K ATPase, K+ channel KCNJ13/Kir7.1, and Cl- channel CLIC6. All SPAK-bound targets co-localized with pSPAK at the ChP apical membrane in LPS conditions. This interaction, as well as CSF hypersecretion and ventriculomegaly, were abrogated in LPS-treated SPAK KO animals.

CONCLUSION:

Multi-omic analysis identified a SPAK-regulated ensemble of ion transport proteins relevant for PIH at the ChP apical membrane. Given the recent development of SPAK kinase inhibitors, our findings uncover a targetable mechanism for the pharmacological treatment of PIH and potentially other forms of inflammation-associated acquired hydrocephalus.

Multi-Echo Complex Quantitative Susceptibility Mapping and Quantitative Blood Oxygen Level-Dependent Magnitude (mcQSM + qBOLD or mcQQ) for Oxygen Extraction Fraction (OEF) Mapping

Oxygen extraction fraction (OEF), the fraction of oxygen that tissue extracts from blood, is an essential biomarker used to directly assess tissue viability and function in neurologic disorders. In ischemic stroke, for example, increased OEF can indicate the presence of penumbra-tissue with low perfusion yet intact cellular integrity-making it a primary therapeutic target. However, practical OEF mapping methods are not currently available in clinical settings, owing to the impractical data acquisitions in positron emission tomography (PET) and the limitations of existing MRI techniques. Recently, a novel MRI-based OEF mapping technique, termed QQ, was proposed. It shows high potential for clinical use by utilizing a routine sequence and removing the need for impractical multiple gas inhalations. However, QQ relies on the assumptions of Gaussian noise in susceptibility and multi-echo gradient echo (mGRE) magnitude signals for OEF estimation. This assumption is unreliable in low signal-to-noise ratio (SNR) regions like disease-related lesions, risking inaccurate OEF estimation and potentially impacting clinical decisions. Addressing this, our study presents a novel multi-echo complex QQ (mcQQ) that models realistic Gaussian noise in mGRE complex signals. We implemented mcQQ using a deep learning framework (mcQQ-NET) and compared it with the existing QQ-NET in simulations, ischemic stroke patients, and healthy subjects, using identical training and testing datasets and schemes. In simulations, mcQQ-NET provided more accurate OEF than QQ-NET. In the subacute stroke patients, mcQQ-NET showed a lower average OEF ratio in lesions relative to unaffected contralateral normal tissue than QQ-NET. In the healthy subjects, mcQQ-NET provided uniform OEF maps, similar to QQ-NET, but without unrealistically high OEF outliers in areas of low SNR, such as SNR ≤ 15 (dB). Therefore, mcQQ-NET improves OEF accuracy by more accurately reflecting realistic Gaussian noise in complex mGRE signals. Its enhanced sensitivity to OEF abnormalities, based on more realistic biophysics modeling, suggests that mcQQ-NET has potential for investigating tissue variability in neurologic disorders.

Role of 1,25-dihydroxyvitamin D3 in alleviating hepatic steatosis: Targeting M1 macrophage polarization in non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD), a critical global health concern, continues to challenge medical researchers with limited treatment options. This letter examines on the study by Luo et al, demonstrating that vitamin D 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] improves hepatic steatosis in NAFLD by inhibiting M1 macrophage polarization via the vitamin D receptor-peroxisome proliferator-activated receptor gamma signaling pathway. This letter critically appraises these findings, comparing them to similar studies, and discusses their potential implications for treating NAFLD. Furthermore, we highlight future directions, including dose optimization and mechanistic studies.

Developmentally regulated KCC2 phosphorylation is essential for dynamic GABA-mediated inhibition and survival

Despite its importance for γ-aminobutyric acid (GABA) inhibition and involvement in neurodevelopmental disease, the regulatory mechanisms of the K+/Cl- cotransporter KCC2 (encoded by SLC12A5) during maturation of the central nervous system (CNS) are not entirely understood. Here, we applied quantitative phosphoproteomics to systematically map sites of KCC2 phosphorylation during CNS development in the mouse. KCC2 phosphorylation at Thr906 and Thr1007, which inhibits KCC2 activity, underwent dephosphorylation in parallel with the GABA excitatory-inhibitory sequence in vivo. Knockin mice expressing the homozygous phosphomimetic KCC2 mutations T906E/T1007E (Kcc2E/E ), which prevented the normal developmentally regulated dephosphorylation of these sites, exhibited early postnatal death from respiratory arrest and a marked absence of cervical spinal neuron respiratory discharges. Kcc2E/E mice also displayed disrupted lumbar spinal neuron locomotor rhythmogenesis and touch-evoked status epilepticus associated with markedly impaired KCC2-dependent Cl- extrusion. These data identify a previously unknown phosphorylation-dependent KCC2 regulatory mechanism during CNS development that is essential for dynamic GABA-mediated inhibition and survival.

Cloning, expression, and characterization of a cold-adapted lipase gene from an antarctic deep-sea psychrotrophic bacterium, Psychrobacter sp 7195

A psychrotrophic strain 7195 showing extracellular lipolytic activity towards tributyrin was isolated from deep-sea sediment of Prydz Bay and identified as a Psychrobacter species. By screening a genomic DNA library of Psychrobacter sp. 7195, an open reading frame of 954 bp coding for a lipase gene, lipA1, was identified, cloned, and sequenced. The deduced LipA1 consisted of 317 amino acids with a molecular mass of 35,210 kDa. It had one consensus motif, G-N-S-M-G (GXSXG), containing the putative active-site serine, which was conserved in other cold-adapted lipolytic enzymes. The recombinant LipA1 was purified by column chromatography with DEAE Sepharose CL-4B, and Sephadex G-75, and preparative polyacrylamide gel electrophoresis, in sequence. The purified enzyme showed highest activity at 30 degrees C, and was unstable at temperatures higher than 30 degrees C, indicating that it was a typical cold-adapted enzyme. The optimal pH for activity was 9.0, and the enzyme was stable between pH 7.0-10.0 after 24 h incubation at 4 degrees C. The addition of Ca2+ and Mg2+ enhanced the enzyme activity of LipA1, whereas the Cd2, Zn2+, Co2+, Fe3+, Hg2+, Fe2+, Rb2+, and EDTA strongly inhibited the activity. The LipA1 was activated by various detergents, such as Triton X-100, Tween 80, Tween 40, Span 60, Span 40, CHAPS, and SDS, and showed better resistance towards them. Substrate specificity analysis showed that there was a preference for trimyristin and p-nitrophenyl myristate (C14 acyl groups).

Hereditary causes of hypertension due to increased sodium transport

PURPOSE OF REVIEW

Hypertension, commonly known as high blood pressure, is a widespread health condition affecting a large number of individuals across the globe. Although lifestyle choices and environmental factors are known to have a significant impact on its development, there is growing recognition of the influence of genetic factors in the pathogenesis of hypertension. This review specifically focuses on the hereditary causes of hypertension that are associated with increased sodium transport through the thiazide-sensitive NaCl cotransporter (NCC) or amiloride-sensitive epithelial sodium channel (ENaC), crucial mechanisms involved in regulating blood pressure in the kidneys. By examining genetic mutations and signaling molecules linked to the dysregulation of sodium transport, this review aims to deepen our understanding of the hereditary causes of hypertension and shed light on potential therapeutic targets.

RECENT FINDINGS

Liddle syndrome (LS) is a genetic disorder that typically manifests early in life and is characterized by hypertension, hypokalemic metabolic alkalosis, hyporeninemia, and suppressed aldosterone secretion. This condition is primarily caused by gain-of-function mutations in ENaC. In contrast, Pseudohypoaldosteronism type II (PHAII) is marked by hyperkalemia and hypertension, alongside other clinical features such as hyperchloremia, metabolic acidosis, and suppressed plasma renin levels. PHAII results from overactivations of NCC, brought about by gain-of-function mutations in its upstream signaling molecules, including WNK1 (with no lysine (K) 1), WNK4, Kelch-like 3 (KLHL3), and cullin3 (CUL3).

SUMMARY

NCC and ENaC are integral components, and their malfunctions lead to disorders like LS and PHAII, hereditary causes of hypertension. Current treatments for LS involve ENaC blockers (e.g., triamterene and amiloride) in conjunction with low-sodium diets, effectively normalizing blood pressure and potassium levels. In PHAII, thiazide diuretics, which inhibit NCC, are the mainstay treatment, albeit with some limitations and potential side effects. Ongoing research in developing alternative treatments, including small molecules targeting key regulators, holds promise for more effective and tailored hypertension solutions.

Genetic intersection of human leukocyte antigen-DP/DQ and hepatitis B virus-related liver disease: Insights from a multi-clustering study

Hepatitis B virus infection remains a significant global health challenge, particularly in endemic regions like Vietnam. This article examines the groundbreaking study by Nguyen et al, which investigates the relationship between human leukocyte antigen-DP/DQ polymorphisms and hepatitis B virus-related liver disease progression. Through advanced multi-clustering analysis, the study reveals that the A-A-A haplotype (rs2856718-rs3077-rs9277535) provides protection against disease progression, while the G-G-G haplotype correlates with increased hepatocellular carcinoma susceptibility. The integration of machine learning approaches with genetic data offers promising avenues for refined disease prediction and personalized therapeutic strategies. This article discusses the implications for expanding study populations, implementing longitudinal cohort studies, and leveraging artificial intelligence for improved patient outcomes.

Commentaries: Lecanemab: pioneering the way as the first approved drug for Alzheimer's disease treatment

Alzheimer's disease (AD) is a progressive neurodegenerative disorder marked by cognitive decline and neuronal abnormalities. Current therapies address symptoms without altering disease progression. Lecanemab, an anti-amyloid antibody, binds to amyloid-beta (Aβ) protofibrils. Phase II trials revealed dose-dependent amyloid clearance and reduced clinical decline. Phase III trials demonstrated cognitive benefits with potential adverse events. Full FDA approval was granted for lecanemab due to its ability to eliminate toxic brain amyloids. However, longer trials are needed to assess its efficacy and safety. While lecanemab marks a significant advancement, further breakthroughs are essential for effective AD treatment.

Institute of Biomedical and Clinical Sciences, Medical School, Faculty of Health and Life Sciences, University of Exeter, Exeter, UK, Zhang J. Oral Paclovid Significantly Reduces Hospitalization and Mortality in Non-hospitalized Elderly Patients with COVID-19

The development of antiviral drugs against coronavirus has proceeded at an unprecedented pace. In December 2021, the US FDA has successively approved two oral drugs against coronavirus pneumonia (COVID-19), namely PAXLOVID™ (PF-07321332) developed by Pfizer and Molnupiravir developed by Merck by targeting papain-like protease (PLpro) and RNA-dependent RNA polymerase (RdRp), respectively. PLpro and RdRp are important for the normal life cycle of coronaviruses. Inhibition of their activities could impair the synthesis of viral RNA and aid therapeutic treatments. However, little is known about the real-world effectiveness of oral antivirals against the severe acute respiratory syndrome (SARS)-CoV-2 omicron (B.1.1.529) variant. Recent studies have shown that Paxlovid can lead to a marked reduction hospitalization or death among unvaccinated outpatients with early COVID-19, whereas it was found no significant benefit for patients aged 64 and younger. In comparison, Molnupiravir did not reduce the risk of hospitalization or death after infection in high-risk groups who had been vaccinated against the coronaviruses, but only accelerated their recovery.

O-Cyanobenzaldehydes Irreversibly Modify Both Buried and Exposed Lysine Residues in Live Cells

Lysine residue represents an attractive site for covalent drug development due to its high abundance (5.6%) and critical functions. However, very few lysines have been characterized to be accessible to covalent ligands and perturb the protein functions, owing to their protonation state and adjacent steric hindrance. Herein, we report a new lysine bioconjugation chemistry, O-cyanobenzaldehyde (CNBA), that enables selective modification of the lysine ε-amine to form iso-indolinones under physiological conditions. Activity-based proteome profiling enabled the mapping of 3451 lysine residues and 85 endogenous kinases in live cells, highlighting its potential for modifying hyper-reactive lysines within the proteome or buried catalytic lysines within the kinome. Further protein crystallography and mass spectrometry confirmed that K271_ABL1 and K162_AURKA are covalently targetable sites in kinases. Leveraging a structure-based drug design, we incorporated CNBA into the core structure of Nutlin-3 to irreversibly inhibit the MDM2-p53 interaction by targeting an exposed lysine K94 on the surface of murine double minute 2. Importantly, we have demonstrated the potential application of CNBA as a lysine-recognized bioconjugation agent for developing new antibody-drug conjugates. The results collectively validate CNBA as a new selective and efficient modifying agent with broad applications for both buried and exposed lysine residues in live cells.