Search

Imaging cellular structures and organelles in living cells by long time-lapse super-resolution microscopy is challenging, as it requires dense labeling, bright and highly photostable dyes, and non-toxic conditions. We introduce a set of high-density, environment-sensitive (HIDE) membrane probes, based on the membrane-permeable silicon-rhodamine dye HMSiR, that assemble in situ and enable long time-lapse, live-cell nanoscopy of discrete cellular structures and organelles with high spatiotemporal resolution. HIDE-enabled nanoscopy movies span tens of minutes, whereas movies obtained with labeled proteins span tens of seconds. Our data reveal 2D dynamics of the mitochondria, plasma membrane and filopodia, and the 2D and 3D dynamics of the endoplasmic reticulum, in living cells. HIDE probes also facilitate acquisition of live-cell, two-color, super-resolution images, expanding the utility of nanoscopy to visualize dynamic processes and structures in living cells.

Summary Super-resolution microscopy requires that subcellular structures are labeled with bright and photostable fluorophores, especially for live-cell imaging. Organic fluorophores may help here as they can yield more photons—by orders of magnitude—than fluorescent proteins. To achieve molecular specificity with organic fluorophores in live cells, self-labeling proteins are often used, with HaloTags and SNAP-tags being the most common. However, how these two different tagging systems compare with each other is unclear, especially for stimulated emission depletion (STED) microscopy, which is limited to a small repertoire of fluorophores in living cells. Herein, we compare the two labeling approaches in confocal and STED imaging using various proteins and two model systems. Strikingly, we find that the fluorescent signal can be up to 9-fold higher with HaloTags than with SNAP-tags when using far-red rhodamine derivatives. This result demonstrates that the labeling strategy matters and can greatly influence the duration of super-resolution imaging., Highlights • Systematic comparison of SNAP versus Halo tag labeling by confocal and STED microscopy • Target proteins, fluorophores, and model systems are compared • Large differences in Halo versus SNAP intensity with silicon rhodamine fluorophores • Guidelines for one- and two-color super-resolution imaging are provided, Self-labeling proteins leverage the superior photophysical properties of organic fluorophores and are the method of choice for live-cell nanoscopy. Comparing SNAP-tags and HaloTags, Erdmann et al. show that Halo tagging with silicon rhodamine fluorophores provides brighter labeling for confocal and STED nanoscopy.

The effect of charged versus neutral N- and C-termini on the stability of the collagen triple helix was examined. Thermal denaturation studies at different pH with collagen model peptides showed that an ammonium group at the N-terminus destabilizes the triple helix more than a carboxylate at the C-terminus. A neutral carboxylic acid stabilizes the triple helix more than an amido moiety at the C-terminus.

(4S)- and (4R)-configured aminoproline (Amp) residues were used as pH-responsive probes to tune the thermal stability of collagen triple helices in acidic and basic environments. The different steric and stereoelectronic properties of amino versus ammonium groups lead to a switch of the ring pucker of Amp upon changing the pH. The choice of the position of Amp within collagen model peptides (CMPs) as well as the absolute configuration at C(4) of the pH-responsive probe allows for tuning of the stability of Amp-containing collagen triple helices over a broad range. Comparative quantum chemical calculations on the steric and stereoelectronic effects of amino and ammonium groups versus fluorine, hydroxy, chlorine, and methyl substituents support the experimental findings. The research also shows that substitution of the naturally occurring hydroxy group in collagen by electron-withdrawing groups with a larger hydration shell than that of the hydroxy group is not tolerated.

We report a lipid-based strategy to visualize Golgi structure and dynamics at super-resolution in live cells. The method is based on two novel reagents: a trans-cyclooctene-containing ceramide lipid (Cer-TCO) and a highly reactive, tetrazine-tagged near-IR dye (SiR-Tz). These reagents assemble via an extremely rapid ‘tetrazine-click’ reaction into Cer-SiR, a highly photostable ‘vital dye’ that enables prolonged live cell imaging of the Golgi apparatus by 3D confocal and STED microscopy. Cer-SiR is non-toxic at concentrations as high as 2 μM and does not perturb the mobility of Golgi-resident enzymes or the traffic of cargo from the endoplasmic reticulum through the Golgi and to the plasma membrane.

Long time-lapse super-resolution imaging in live cells requires a labeling strategy that combines a bright, photostable fluorophore with a high-density localization probe. Lipids are ideal high-density localization probes, as they are >100 times more abundant than most membrane-bound proteins and simultaneously demark the boundaries of cellular organelles. Here, we describe Cer-SiR, a two-component, high-density lipid probe that is exceptionally photostable. Cer-SiR is generated in cells via a bioorthogonal reaction of two components: a ceramide lipid tagged with trans-cyclooctene (Cer-TCO) and a reactive, photostable Si-rhodamine dye (SiR-Tz). These components assemble within the Golgi apparatus of live cells to form Cer-SiR. Cer-SiR is benign to cellular function, localizes within the Golgi at a high density, and is sufficiently photostable to enable visualization of Golgi structure and dynamics by 3D confocal or long time-lapse STED microscopy.

The efficient synthesis of the tripeptidic building block Fmoc-Pro-Hyp(TBDPS)-Gly-OH and its application for the prepa- ration of collagen model peptides (CMPs) has been achieved. The silyl ether protecting group prevents undesired side reactions during the CMP synthesis thereby facilitating purification and allowing for selective deprotection of the hydroxyproline residue without affect- ing the solid-supported CMP.

NMR spectroscopic studies with a series of proline derivatives revealed that the polarity of the environment has a significant effect on the trans : cis isomer ratio of Xaa–Pro bonds. Computational studies showed that this effect is due to differences in the overall dipole moments of trans and cis conformers. Comparisons between the conformational properties of amide and ester derivatives revealed an intricate balance between polarity effects and n → π* interactions of adjacent carbonyl groups. The findings have important implications for protein folding and signaling as well as the performance of proline-based stereoselective catalysts., Chemical Science, 6 (12), ISSN:2041-6520, ISSN:2041-6539

The functionalizability and conformational properties of azidoproline (Azp)-containing collagen model peptides (CMPs) were studied. The results show that (4R)Azp has a similar stabilizing effect on the collagen triple helix as (4R)hydroxyproline and that functionalized CMPs are readily accessible by “click” chemistry. The resulting triazole-functionalized CMPs form stable triple helices, demonstrating that sterically demanding moieties in three symmetry-related positions in all strands are tolerated. The straightforward synthesis and facile functionalizability of the Azp-containing CMPs are intriguing for the development of functional collagen-based materials.

An 'azido gauche effect' determines the conformation of (4S)- and (4R)-azidoproline (Azp) derivatives and affects the s-cis:s-trans conformer ratio of Xaa-Azp bonds. The article summarizes our research on the conformational analysis of monomers as well as oligomers derived from (4S)Azp and (4R)Azp. We show that (4S)Azp and (4R)Azp can be used to tune the stability of the polyproline II (PPII) helix. In addition we demonstrate that Azpcontaining oligoprolines are attractive molecular scaffolds with a well-defined helical conformation that can be readily further functionalized using e.g. click chemistry.