Enhancing the biocompatibility of rhodamine fluorescent probes by a neighbouring group effect

Fluorescence microscopy is an essential tool for understanding dynamic processes in living cells and organisms. However, many fluorescent probes for labelling cellular structures suffer from unspecific interactions and low cell permeability. Herein, we demonstrate that the neighbouring group effect which results from positioning an amide group next to a carboxyl group in the benzene ring of rhodamines dramatically increases cell permeability of the rhodamine-based probes through stabilizing a fluorophore in a hydrophobic spirolactone state. Based on this principle, we create probes targeting tubulin, actin and DNA. Their superb staining intensity, tuned toxicity and specificity allows long-term 3D confocal and STED nanoscopy with sub-30 nm resolution. Due to their unrestricted cell permeability and efficient accumulation on the target, the new probes produce high contrast images at low nanomolar concentrations. Superior performance is exemplified by resolving the real microtubule diameter of 23 nm and selective staining of the centrosome inside living cells for the first time

Direct visualization of amlodipine intervention into living cells by means of fluorescence microscopy

Amlodipine, a unique long-lasting calcium channel antagonist and antihypertensive drug, has weak fluorescence in aqueous solutions. In the current paper, we show that direct visualization of amlodipine in live cells is possible due to the enhanced emission in cellular environment. We examined the impact of pH, polarity and viscosity of the environment as well as protein binding on the spectral properties of amlodipine in vitro, and used quantum chemical calculations for assessing the mechanism of fluorescence quenching in aqueous solutions. The confocal fluorescence microscopy shows that the drug readily penetrates the plasma membrane and accumulates in the intracellular vesicles. Visible emission and photostability of amlodipine allow confocal time-lapse imaging and the drug uptake monitoring

Blinking fluorescent probes for tubulin nanoscopy in living and fixed cells

Here we report a small molecule tubulin probe for single-molecule localization microscopy (SMLM), stimulated emission depletion (STED) microscopy and MINFLUX nanoscopy, which can be used in living and fixed cells. We explored a series of taxane derivatives containing spontaneously blinking far-red dye hydroxymethyl silicon–rhodamine (HMSiR) and found that the linker length profoundly affects the probe permeability and off-targeting in living cells. The best performing probe, HMSiR-tubulin, is composed of cabazitaxel and the 6′-regioisomer of HMSiR bridged by a C6 linker. Microtubule diameter of ≤50 nm was routinely measured in SMLM experiments on living and fixed cells. HMSiR-tubulin allows a complementary use of different nanoscopy techniques for investigating microtubule functions and developing imaging methods. For the first time, we resolved the inner microtubule diameter of 16 ± 5 nm by optical nanoscopy and thereby demonstrated the utility of a self-blinking dye for MINFLUX imaging

Inositol pyrophosphates activate the vacuolar transport chaperone complex in yeast by disrupting a homotypic SPX domain interaction.

Many proteins involved in eukaryotic phosphate homeostasis are regulated by SPX domains. In yeast, the vacuolar transporter chaperone (VTC) complex contains two such domains, but mechanistic details of its regulation are not well understood. Here, we show at the atomic level how inositol pyrophosphates interact with SPX domains of subunits Vtc2 and Vtc3 to control the activity of the VTC complex. Vtc2 inhibits the catalytically active VTC subunit Vtc4 by homotypic SPX-SPX interactions via the conserved helix α1 and the previously undescribed helix α7. Binding of inositol pyrophosphates to Vtc2 abrogates this interaction, thus activating the VTC complex. Accordingly, VTC activation is also achieved by site-specific point mutations that disrupt the SPX-SPX interface. Structural data suggest that ligand binding induces reorientation of helix α1 and exposes the modifiable helix α7, which might facilitate its post-translational modification in vivo. The variable composition of these regions within the SPX domain family might contribute to the diversified SPX functions in eukaryotic phosphate homeostasis

Structure Analysis of Entamoeba histolytica DNMT2 (EhMeth)

In eukaryotes, DNA methylation is an important epigenetic modification that is generally involved in gene regulation. Methyltransferases (MTases) of the DNMT2 family have been shown to have a dual substrate specificity acting on DNA as well as on three specific tRNAs (tRNAAsp, tRNAVal, tRNAGly). Entamoeba histolytica is a major human pathogen, and expresses a single DNA MTase (EhMeth) that belongs to the DNMT2 family and shows high homology to the human enzyme as well as to the bacterial DNA MTase M.HhaI. The molecular basis for the recognition of the substrate tRNAs and discrimination of non-cognate tRNAs is unknown. Here we present the crystal structure of the cytosine-5-methyltransferase EhMeth at a resolution of 2.15 Å, in complex with its reaction product S-adenosyl-L-homocysteine, revealing all parts of a DNMT2 MTase, including the active site loop. Mobility shift assays show that in vitro the full length tRNA is required for stable complex formation with EhMeth

The use of Hoechst dyes for DNA staining and beyond.

Hoechst dyes are among the most popular fluorophores used to stain DNA in living and fixed cells. Moreover, their high affinity and specificity towards DNA make Hoechst dyes excellent targeting moieties, which can be conjugated to various other molecules in order to tether them to DNA. The recent developments in the fields of microscopy and flow cytometry have sparked interest in such composite molecules, whose applications range from investigating nucleus microenvironment to drug delivery into tumours. Here we provide an overview of the properties of Hoechst dyes and discuss recent developments in Hoechst-based composite probes

Application of STED imaging for chromatin studies

Chromatin is the information center of a cell. It comprises proteins and nucleic acids that form a highly complex and dynamic structure within the nucleus. Its multiple organization levels span from micrometre to nanometre scale. For many years, the lower levels of chromatin organization have been beyond the resolution limit of fluorescent microscopy, thus impeding research on nucleus architecture, transcription, translation and DNA repair. Recent development in super-resolution fluorescence microscopy enables us to more easily observe objects at the nanometre scale and allows the study of complex cellular structures at unprecedented detail. This review focuses on the application of stimulated emission depletion microscopy for imaging two main components of the chromatin-DNA and the proteins interacting with it

Organelle acidification negatively regulates vacuole membrane fusion in vivo.

The V-ATPase is a proton pump consisting of a membrane-integral V0 sector and a peripheral V1 sector, which carries the ATPase activity. In vitro studies of yeast vacuole fusion and evidence from worms, flies, zebrafish and mice suggested that V0 interacts with the SNARE machinery for membrane fusion, that it promotes the induction of hemifusion and that this activity requires physical presence of V0 rather than its proton pump activity. A recent in vivo study in yeast has challenged these interpretations, concluding that fusion required solely lumenal acidification but not the V0 sector itself. Here, we identify the reasons for this discrepancy and reconcile it. We find that acute pharmacological or physiological inhibition of V-ATPase pump activity de-acidifies the vacuole lumen in living yeast cells within minutes. Time-lapse microscopy revealed that de-acidification induces vacuole fusion rather than inhibiting it. Cells expressing mutated V0 subunits that maintain vacuolar acidity were blocked in this fusion. Thus, proton pump activity of the V-ATPase negatively regulates vacuole fusion in vivo. Vacuole fusion in vivo does, however, require physical presence of a fusion-competent V0 sector

Inositol Pyrophosphate Specificity of the SPX-Dependent Polyphosphate Polymerase VTC.

The free energy of nucleotide hydrolysis depends on phosphate concentration. Cells regulate cytosolic phosphate levels by orchestrating phosphate acquisition and storage through inositol pyrophosphates (PP-InsP) and SPX domains. Here, we report the synthesis of the novel 5-PPP-InsP5 containing a triphosphate subunit. Using this and a series of synthetic PP-InsP, we examined the ligand specificity of the SPX domain in the PP-InsP-controlled yeast polyphosphate polymerase VTC. SPX decodes the relative positioning of the phosphoric anhydrides, their structure (diphosphate vs triphosphate), and the presence of other phosphates on the inositol ring. Despite the higher potency of 1,5-(PP)2-InsP4, 5-PP-InsP5 is the primary activator of VTC in cells, indicating that its higher concentration compensates for its lower potency. 1,5-(PP)2-InsP4 levels rise and could become relevant under stress conditions. Thus, SPX domains may integrate PP-InsP dependent signaling to adapt cytosolic phosphate concentrations to different metabolic situations

Efflux pump insensitive rhodamine–jasplakinolide conjugates for G- and F-actin imaging in living cells

The actin cytoskeleton is crucial for endocytosis, intracellular trafficking, cell shape maintenance and a wide range of other cellular functions. Recently introduced cell-permeable fluorescent actin probes, such as SiR-actin, suffer from poor membrane permeability and stain some cell populations inhomogeneously due to the active efflux by the plasma membrane pumps. We analyzed a series of new probes composed of jasplakinolide and modified rhodamine fluorophores and found that rhodamine positional isomerism has a profound effect on probe performance. The probes based on the 6'-carboxy-carbopyronine scaffold are considerably less susceptible to efflux and allow efficient staining without efflux pump inhibitors. They can be used for 2D and 3D fluorescence nanoscopy at high nanomolar concentrations without significant cytotoxicity. We show that jasplakinolide-based fluorescent probes bind not only to actin filaments, but also to G-actin, which enables imaging highly dynamic actin structures. We demonstrate an excellent performance of the new probes in multiple organisms and cell types: human cell lines, frog erythrocytes, fruit fly tissues and primary neurons