Mirror-enhanced super-resolution microscopy

Axial excitation confinement beyond the diffraction limit is crucial to the development of next-generation,super-resolution microscopy.STimulated Emission Depletion(STED)nanoscopy offers lateral super-resolution using a donut-beam depletion,but its axial resolution is still over 500 nm.Total internal reflection fluorescence microscopy is widely used for single-molecule localization,but its ability to detect molecules is limited to within the evanescent field of~100 nm from the cell attachment surface.We find here that the axial thickness of the point spread function(PSF)during confocal excitation can be easily improved to 110 nm by replacing the microscopy slide with a mirror.The interference of the local electromagnetic field confined the confocal PSF to a 110-nm spot axially,which enables axial super-resolution with all laser-scanning microscopes.Axial sectioning can be obtained with wavelength modulation or by controlling the spacer between the mirror and the specimen.With no additional complexity,the mirror-assisted excitation confinement enhanced the axial resolution six-fold and the lateral resolution two-fold for STED,which together achieved 19-nm resolution to resolve the inner rim of a nuclear pore complex and to discriminate the contents of 120 nm viral filaments.The ability to increase the lateral resolution and decrease the thickness of an axial section using mirror-enhanced STED without increasing the laser power is of great importance for imaging biological specimens,which cannot tolerate high laser power.

Polarization modulation with optical lock-in detection reveals universal fluorescence anisotropy of subcellular structures in live cells

The orientation of fluorophores can reveal crucial information about the structure and dynamics of their associated subcellular organelles.Despite significant progress in super-resolution,fluorescence polarization microscopy remains limited to unique samples with relatively strong polarization modulation and not applicable to the weak polarization signals in samples due to the excessive background noise.Here we apply optical lock-in detection to amplify the weak polarization modulation with super-resolution.This novel technique,termed optical lock-in detection super-resolution dipole orientation mapping(OLID-SDOM),could achieve a maximum of 100 frames per second and rapid extraction of 2D orientation,and distinguish distance up to 50 nm,making it suitable for monitoring structural dynamics concerning orientation changes in vivo.OLID-SDOM was employed to explore the universal anisotropy of a large variety of GFP-tagged subcellular organelles,including mitochondria,lysosome,Golgi,endosome,etc.We found that OUF(Orientation Uniformity Factor)of OLID-SDOM can be specific for different subcellular organelles,indicating that the anisotropy was related to the function of the organelles,and OUF can potentially be an indicator to distinguish normal and abnormal cells(even cancer cells).Furthermore,dual-color super-resolution OLID-SDOM imaging of lysosomes and actins demonstrates its potential in studying dynamic molecular interactions.The subtle anisotropy changes of expanding and shrinking dendritic spines in live neurons were observed with real-time OLID-SDOM.Revealing previously unobservable fluorescence anisotropy in various samples and indicating their underlying dynamic molecular structural changes,OLID-SDOM expands the toolkit for live cell research.

Microscopy: looking into the mirror

Mirrors can create a virtual excitation source for optical microscopy,which can greatly enhance the spatiotemporal resolution of different fluorescence microscopy techniques,thus advancing toward longterm live cell imaging.In recent decades,many new discoveries have been obtained using novel optical microscopic techniques,such as confocal,multiphoton,super resolution,and light sheet microscopies,which have attracted intensive interest from biologists working in various fields.However,advances in live cell fluorescence microscopy are facing multiple challenges,such as low resolution,poor signal-to-background ratio(SBR),insufficient imaging speed,and phototoxicity.Interestingly,these grand challenges share a common solution:mirrors.When placing a reflective mirror after an objective,the beam can be reflected,and a“virtual”excitation source can be generated without additional cost.This conceptually simple approach provides an easy solution to the abovementioned challenges,such as greater signal,better contrast,improved optical section ability at relative low cost,and facilitating live cell imaging with improved spatial resolution at a high speed.

Developing biolmaging and quantitative methods to study 3D genome

The recent advances in chromosome configuration capture （3C）-based series molecular methods and optical super- resolution （SR） techniques offer powerful tools to investigate three dimensional （3D） genomic structure in prokaryotic and eukaryotic cell nucleus. In this review, we focus on the progress during the last decade in this exciting field. Here we at first introduce briefly genome organization at chromosome, domain and sub-domain level, respectively; then we provide a short introduction to various super-resolution microscopy techniques which can be employed to detect gcnome 3D structure. We also reviewed the progress of quantitative and visualization tools to evaluate and visualize chromatin interactions in 3D genome derived from Hi-C data. We end up with the discussion that imaging methods and 3C-based molecular methods are not mutually exclusive -- actually they arc complemental to each other and can be combined together to study 3D genome organization.