Optical microwell array for large scale studies of single mitochondria metabolic responses

Active Particle Based Selective Transport and Release of Cell Organelles and Mechanical Probing of a Single Nucleus

Self-propelling micromotors are emerging as a promising microscale tool for single-cell analysis. The authors have recently shown that the field gradients necessary to manipulate matter via dielectrophoresis can be induced at the surface of a polarizable active ("self-propelling") metallo-dielectric Janus particle (JP) under an externally applied electric field, acting essentially as a mobile floating microelectrode. Here, the application of the mobile floating microelectrode to trap and transport cell organelles in a selective and releasable manner is successfully extended. This selectivity is driven by the different dielectrophoretic (DEP) potential wells on the JP surface that is controlled by the frequency of the electric field, along with the hydrodynamic shearing and size of the trapped organelles. Such selective and directed loading enables purification of targeted organelles of interest from a mixed biological sample while their dynamic release enables their harvesting for further analysis such as gene/RNA sequencing or proteomics. Moreover, the electro-deformation of the trapped nucleus is shown to be in correlation with the DEP force and hence, can act as a promising label-free biomechanical marker. Hence, the active carrier constitutes an important and novel ex vivo platform for manipulation and mechanical probing of subcellular components of potential for single cell analysis.

PDMS microwells for multi-parametric monitoring of single mitochondria on a large scale: a study of their individual membrane potential and endogenous NADH

Microwell arrays have been developed to monitor simultaneously, and on a large scale, multiple metabolic responses of single mitochondria. Wells of 50 to 1000 μm-diameter were prepared based on easy structuration of thin polydimethylsiloxane layers (PDMS; 100 μm thickness). Their surface treatment with oxygen plasma allowed the immobilization in situ and observation with time of populations of single isolated mitochondria. Their metabolic activities could be monitored individually by fluorescence microscopy under several activation/inhibition conditions. We measured the concomitant variations of two main metabolic parameters - the endogenous NADH level and the internal membrane potential difference Δψ owing to a cationic fluorescent probe (TMRM) - at energized, uncoupled and inhibited stages of the mitochondrial respiratory chain. Microwell arrays allowed analyses on large populations, and consequently statistical studies with a single organelle resolution. Thus, we observed rapid individual polarizations and depolarizations of mitochondria following their supply with the energetic substrate, while an averaged global polarization (increase of TMRM fluorescence within mitochondria) and NADH increase were detected for the whole population. In addition, statistical correlation studies show that the NADH content of all mitochondria tends toward a metabolic limit and that their polarization-depolarization ability is ubiquitous. These results demonstrate that PDMS microwell platforms provide an innovative approach to better characterize the individual metabolic status of isolated mitochondria, possibly as a function of their cell or organ origin or in different physio-pathological situations.

Nanohole Array-Directed Trapping of Mammalian Mitochondria Enabling Single Organelle Analysis

We present periodic nanohole arrays fabricated in free-standing metal-coated nitride films as a platform for trapping and analyzing single organelles. When a microliter-scale droplet containing mitochondria is dispensed above the nanohole array, the combination of evaporation and capillary flow directs individual mitochondria to the nanoholes. Mammalian mitochondria arrays were rapidly formed on chip using this technique without any surface modification steps, microfluidic interconnects, or external power sources. The trapped mitochondria were depolarized on chip using an ionophore with results showing that the organelle viability and behavior were preserved during the on-chip assembly process. Fluorescence signal related to mitochondrial membrane potential was obtained from single mitochondria trapped in individual nanoholes revealing statistical differences between the behavior of polarized vs depolarized mammalian mitochondria. This technique provides a fast and stable route for droplet-based directed localization of organelles-on-a-chip with minimal limitations and complexity, as well as promotes integration with other optical or electrochemical detection techniques.

Optical microwell arrays for large-scale studies of single mitochondria metabolic responses

Most of the methods dedicated to the monitoring of metabolic responses from isolated mitochondria are based on whole-population analyses. They rarely offer an individual resolution though fluorescence microscopy allows it, as demonstrated by numerous studies on single mitochondria activities in cells. Herein, we report on the preparation and use of microwell arrays for the entrapment and fluorescence microscopy of single isolated mitochondria. Highly dense arrays of 3 μm mean diameter wells were obtained by the chemical etching of optical fiber bundles (850 μm whole diameter). They were manipulated by a micro-positioner and placed in a chamber made of a biocompatible elastomer (polydimethylsiloxane or PDMS) and a glass coverslip, on the platform of an inverted microscope. The stable entrapment of individual mitochondria (extracted from Saccharomyces cerevisiae yeast strains, inter alia, expressing a green fluorescent protein) within the microwells was obtained by pretreating the optical bundles with an oxygen plasma and dipping the hydrophilic surface of the array in a concentrated solution of mitochondria. Based on the measurement of variations of the intrinsic NADH fluorescence of each mitochondrion in the array, their metabolic status was analyzed at different energetic respiratory stages: under resting state, following the addition of an energetic substrate to stimulate respiration (ethanol herein) and the addition of a respiratory inhibitor (antimycin A). Statistical analyses of mean variations of mitochondrial NADH in the population were subsequently achieved with a single organelle resolution.

Enzyme molecules in solitary confinement

Large arrays of homogeneous microwells each defining a femtoliter volume are a versatile platform for monitoring the substrate turnover of many individual enzyme molecules in parallel. The high degree of parallelization enables the analysis of a statistically representative enzyme population. Enclosing individual enzyme molecules in microwells does not require any surface immobilization step and enables the kinetic investigation of enzymes free in solution. This review describes various microwell array formats and explores their applications for the detection and investigation of single enzyme molecules. The development of new fabrication techniques and sensitive detection methods drives the field of single molecule enzymology. Here, we introduce recent progress in single enzyme molecule analysis in microwell arrays and discuss the challenges and opportunities.