Mechanosensitive Fluorescent Probes to Image Membrane Tension in Mitochondria, Endoplasmic Reticulum, and Lysosomes

Measuring forces inside cells is particularly challenging. With the development of quantitative microscopy, fluorophores which allow the measurement of forces became highly desirable. We have previously introduced a mechanosensitive flipper probe, which responds to the change of plasma membrane tension by changing its fluorescence lifetime and thus allows tension imaging by FLIM. Herein, we describe the design, synthesis, and evaluation of flipper probes that selectively label intracellular organelles, i.e., lysosomes, mitochondria, and the endoplasmic reticulum. The probes respond uniformly to osmotic shocks applied extracellularly, thus confirming sensitivity toward changes in membrane tension. At rest, different lifetimes found for different organelles relate to known differences in membrane organization rather than membrane tension and allow colabeling in the same cells. At the organelle scale, lifetime heterogeneity provides unprecedented insights on ER tubules and sheets, and nuclear membranes. Examples on endosomal trafficking or increase of tension at mitochondrial constriction sites outline the potential of intracellularly targeted fluorescent tension probes to address essential questions that were previously beyond reach.

Streptavidin interfacing as a general strategy to localize fluorescent membrane tension probes in cells

To image the mechanical properties of biological membranes, twisted push-pull mechanophores that respond to membrane tension by planarization in the ground state have been introduced recently. For their application in biological systems, these so-called fluorescent flippers will have to be localized to specific environments of cellular membranes. In this report, we explore streptavidin as a versatile connector between biotinylated flipper probes and biotinylated targets. Fluorescence spectroscopy and microscopy with LUVs and GUVs reveal the specific conditions needed for desthiobiotin-loaded streptavidin to deliver biotinylated flippers selectively to biotinylated membranes. Selectivity for biotinylated plasma membranes is also observed in HeLa cells, confirming the compatibility of this strategy with biological systems. Streptavidin interfacing does not affect the mechanosensitivity of the flipper probes, red shift in the excitation maximum and fluorescence lifetime increase with membrane order and tension, as demonstrated, inter alia, using FLIM.

Immobilizable fluorescent probes for monitoring the mitochondria microenvironment: a next step from the classic

Immobilizable probes represent a valuable trend.

A mitochondria-targeted two-photon fluorogenic probe for the dual-imaging of viscosity and H2O2 levels in Parkinson's disease models

Novel two-photon fluorogenic probe could simultaneously monitor changes in the mitochondrial viscosity and H₂O₂ levels using two different channels.

A two-photon ratiometric fluorescent probe for highly selective sensing of mitochondrial cysteine in live cells

We report herein a two-photon ratiometric fluorescent probe (DNEPI) for mitochondrial cysteine (Cys) detection on the basis of a merocyanine (compound 1) as the two-photon fluorophore and a 2,4-dinitrobenzensulfonyl (DNBS) unit as the biothiol reaction site. Upon reaction with Cys in DMSO/PBS (1/1, v/v), DNEPI showed a distinct ratiometric fluorescence emission characteristic (F_{583 nm}/F_{485 nm}) linearly proportional to Cys concentrations over the range of 2-10 μM, which was attribute to the enhanced intramolecular charge transfer (ICT) effect by cleavage of the sulfonic acid ester bond of DNEPI to release compound 1. More importantly, the probe could detect Cys with a fast response time (within 2 min) and the detection limit was quantitatively calculated as 0.29 μM. Furthermore, DNEPI not only exhibited high selectivity toward Cys over other similar biothiols, including homocysteine (Hcy) and glutathione (GSH), but also displayed significant mitochondrial-targeting ability, which were favorable for mitochondrial Cys-selective imaging. Subsequently, application of DNEPI to Cys imaging in live cells was successfully achieved by two-photon fluorescence microscopy, suggesting that the probe proposed here could be used to monitor mitochondrial Cys concentration changes in live cells with negligible interference from other biological thiols.

Conjugated Polyimine Dynamers as Phase-Sensitive Membrane Probes

In this report, dynamic polyimines are introduced as multifunctional sensors of lipid bilayer phases. Under mildly acidic conditions, self-condensation of push-pull amino formyl fluorenes into polyimines occurs in solid- or liquid-ordered phases but not in liquid-disordered phases of vesicular membranes. The obtained conjugated polymers are characterized by a progressive red shift of the absorption maxima, the appearance of exciton-coupled circular dichroism (CD) bands, and fluorescence quenching. These characteristics allow multiple modes of detection of membrane phases, which are known to change under membrane tension.