Fe2+-Mediated Activation of BKCa Channels by Rapid Photolysis of CORM-S1 Releasing CO and Fe2

Plight of CORMs: The unreliability of four commercially available CO-releasing molecules, CORM-2, CORM-3, CORM-A1, and CORM-401, in studying CO biology

Carbon monoxide (CO) is an endogenously produced gaseous signaling molecule with demonstrated pharmacological effects. In studying CO biology, three delivery forms have been used: CO gas, CO in solution, and CO donors of various types. Among the CO donors, four carbonyl complexes with either a transition metal ion or borane (BH3) (termed CO-releasing molecules or CORMs) have played the most prominent roles appearing in over 650 publications. These are CORM-2, CORM-3, CORM-A1, and CORM-401. Intriguingly, there have been unique biology findings that were only observed with these CORMs, but not CO gas; yet these properties were often attributed to CO, raising puzzling questions as to why CO source would make such a fundamental difference in terms of CO biology. Recent years have seen a large number of reports of chemical reactivity (e.g., catalase-like activity, reaction with thiol, and reduction of NAD(P)+) and demonstrated CO-independent biological activity for these four CORMs. Further, CORM-A1 releases CO in an idiosyncratic fashion; CO release from CORM-401 is strongly influenced or even dependent on reaction with an oxidant and/or a nucleophile; CORM-2 mostly releases CO2, not CO, after a water-gas shift reaction except in the presence of a strong nucleophile; and CORM-3 does not release CO except in the presence of a strong nucleophile. All these beg the question as to what constitutes an appropriate CO donor for studying CO biology. This review critically summarizes literature findings related to these aspects, with the aim of helping result interpretation when using these CORMs and development of essential criteria for an appropriate donor for studying CO biology.

Switching on Cytotoxicity of Water-Soluble Diiron Organometallics by UV Irradiation

The diiron compounds [Fe₂Cp₂(CO)₂(μ-CO)(μ-CSEt)]CF₃SO₃, [1]CF₃SO₃, K[Fe₂Cp₂(CO)₃(CNCH₂CO₂)], K[2], [Fe₂Cp₂(CO)₂(μ-CO)(μ-CNMe₂)]NO₃, [3]NO₃, [Fe₂Cp₂(CO)₂(PTA){μ-CNMe(Xyl)}]CF₃SO₃, [4]CF₃SO₃, and [Fe₂Cp₂(CO)(μ-CO){μ-η:¹η³-C(4-C₆H₄CO₂H)CHCNMe₂}]CF₃SO₃, [5]CF₃SO₃, containing a bridging carbyne, isocyanoacetate, or vinyliminium ligand, were investigated for their photoinduced cytotoxicity. Specifically, the novel water-soluble compounds K[2], [3]NO₃, and [4]CF₃SO₃ were synthesized and characterized by elemental analysis and IR and multinuclear NMR spectroscopy. Stereochemical aspects concerning [4]CF₃SO₃ were elucidated by ¹H NOESY NMR and single-crystal X-ray diffraction. Cell proliferation studies on human skin cancer (A431) and nontumoral embryonic kidney (HEK293) cells, with and without a 10-min exposure to low-power UV light (350 nm), highlighted the performance of the aminocarbyne [3]NO₃, nicknamed NIRAC (Nitrate-Iron-Aminocarbyne), which is substantially nontoxic in the dark but shows a marked photoinduced cytotoxicity. Spectroscopic (IR, UV-vis, NMR) measurements and the myoglobin assay indicated that the release of one carbon monoxide ligand represents the first step of the photoactivation process of NIRAC, followed by an extensive disassembly of the organometallic scaffold.

Understanding the Logistics for the Distribution of Heme in Cells

Heme is essential for the survival of virtually all living systems-from bacteria, fungi, and yeast, through plants to animals. No eukaryote has been identified that can survive without heme. There are thousands of different proteins that require heme in order to function properly, and these are responsible for processes such as oxygen transport, electron transfer, oxidative stress response, respiration, and catalysis. Further to this, in the past few years, heme has been shown to have an important regulatory role in cells, in processes such as transcription, regulation of the circadian clock, and the gating of ion channels. To act in a regulatory capacity, heme needs to move from its place of synthesis (in mitochondria) to other locations in cells. But while there is detailed information on how the heme lifecycle begins (heme synthesis), and how it ends (heme degradation), what happens in between is largely a mystery. Here we summarize recent information on the quantification of heme in cells, and we present a discussion of a mechanistic framework that could meet the logistical challenge of heme distribution.

Diffraction-Unlimited Photomanipulation at the Plasma Membrane via Specifically Targeted Upconversion Nanoparticles

Engineered UCNP are used to trigger rapid photoconversion of the fluorescent protein Dendra2 with nanoscopic precision and over longer distances in mammalian cells. By exploiting the synergy of high-level thulium doping with core-shell design and elevated excitation intensities, intense UCNP emission is achieved, allowing fast photoconversion of Dendra2 with <10 nm resolution. A tailored biocompatible surface coating and functionalization with a derivate of green fluorescent protein (GFP) for recognition of antiGFP nanobodies are developed. Highly specific targeting of UCNP to fusion proteins of antiGFP on the surface of mammalian cells is demonstrated. UCNP bound to extracellular Dendra2 enable rapid photoconversion selectively in molecular proximity and thus unambiguous detection of cytokine receptor dimerization in the plasma membrane and in endosomes. Remarkably, UCNPs are also suited for manipulating intracellular Dendra2 across the plasma membrane. This study thus establishes UCNP-controlled photomanipulation with nanoscale precision, opening exciting opportunities for bioanalytical applications in cell biology.

Monitoring of compound resting membrane potentials of cell cultures with ratiometric genetically encoded voltage indicators

The cellular resting membrane potential (V_m) not only determines electrical responsiveness of excitable cells but also plays pivotal roles in non-excitable cells, mediating membrane transport, cell-cycle progression, and tumorigenesis. Studying these processes requires estimation of V_m, ideally over long periods of time. Here, we introduce two ratiometric genetically encoded V_m indicators, rArc and rASAP, and imaging and analysis procedures for measuring differences in average resting V_m between cell groups. We investigated the influence of ectopic expression of K⁺ channels and their disease-causing mutations involved in Andersen-Tawil (Kir2.1) and Temple-Baraitser (K_V10.1) syndrome on median resting V_m of HEK293T cells. Real-time long-term monitoring of V_m changes allowed to estimate a 40–50 min latency from induction of transcription to functional Kir2.1 channels in HEK293T cells. The presented methodology is readily implemented with standard fluorescence microscopes and offers deeper insights into the role of the resting V_m in health and disease.

Rühl et al. report the generation of ratiometric genetically encoded voltage indicators (GEVIs) and establish that they can be used in high-throughput automated imaging to measure compound membrane potential (Vm) in mammalian cells. This method is implementable with standard fluorescence microscopes and has the potential to offer insights into the role of the resting Vm in health and disease.

Click, release, and fluoresce: In-vivo generation of CO with concomitant synthesis of a fluorescent reporter

Delivering a therapeutically active gaseous molecule represents very unique challenges in terms of both precise dosing and concentration assessment. To overcome these obstacles, there have been recent reports of using prodrug approaches for the in-vitro and in-vivo generation of carbon monoxide (CO), which is an endogenous signaling molecule with validated therapeutic efficacy in a range of animal models. Some key components of these approaches include the use of a hydrophobicity-driven Diels-Alder reaction under physiological conditions followed by a cheletropic reaction of the corresponding norbornadien-7-one intermediate, leading to extrusion of CO. With proper design, the same approach also leads to the formation of a fluorescent reporter, allowing for quantitative assessment of the amount of CO released. All these allow for a strategy of "click, release, and fluoresce" in delivering a precise dose of carbon monoxide with the ability to "self-report" delivery quantity and efficiency. This strategy has also been further refined to construct a CO delivery platform with additional functionalities such as bioorthogonal labeling, targeting, triggered release, and simultaneously delivery of more than one payload. This review highlights recent developments in this area.