Transcription factors (TFs) are DNA binding proteins that control the expression of genes. The regulation of transcription is a complex process that involves binding of TFs to specific sequences, recruitment of cofactors and chromatin remodelers, assembly of the pre-initiation complex and ultimately the recruitment of RNA polymerase II. Increasing evidence suggests that TFs are highly dynamic and interact only transiently with DNA. Single molecule microscopy techniques are powerful approaches for visualizing and tracking individual TF molecules as they diffuse in the nucleus and interact with DNA. In this work, we employ multifocus microscopy and highly inclined and laminated optical sheet microscopy to track TF dynamics in response to perturbations in labile zinc inside cells. We sought to define whether zinc-dependent TFs sense changes in the labile zinc pool by determining whether their dynamics and DNA binding can be modulated by zinc. While it is widely appreciated that TFs need zinc to bind DNA, whether zinc occupancy and hence TF function are sensitive to changes in cellular zinc remain open questions. We utilized fluorescently tagged versions of the glucocorticoid receptor (GR), with two C4 zinc finger domains, and CCCTC-binding factor (CTCF), with eleven C2H2 zinc finger domains. We found that the biophysical dynamics of both TFs are susceptible to changes in zinc, but in subtly different ways. These results indicate that at least some transcription factors are sensitive to zinc dynamics, revealing a potential new layer of transcriptional regulation.

The regulation of transcription is a complex process that involves binding of transcription factors (TFs) to specific sequences, recruitment of cofactors and chromatin remodelers, assembly of the pre-initiation complex and recruitment of RNA polymerase II. Increasing evidence suggests that TFs are highly dynamic and interact only transiently with DNA. Single molecule microscopy techniques are powerful approaches for tracking individual TF molecules as they diffuse in the nucleus and interact with DNA. Here we employ multifocus microscopy and highly inclined laminated optical sheet microscopy to track TF dynamics in response to perturbations in labile zinc inside cells. We sought to define whether zinc-dependent TFs sense changes in the labile zinc pool by determining whether their dynamics and DNA binding can be modulated by zinc. We used fluorescently tagged versions of the glucocorticoid receptor (GR), with two C4 zinc finger domains, and CCCTC-binding factor (CTCF), with eleven C2H2 zinc finger domains. We found that GR was largely insensitive to perturbations of zinc, whereas CTCF was significantly affected by zinc depletion and its dwell time was affected by zinc elevation. These results indicate that at least some transcription factors are sensitive to zinc dynamics, revealing a potential new layer of transcriptional regulation.

Abstract Single molecule RNA fluorescence in situ hybridization (smFISH) has become the standard tool for high spatial resolution analysis of gene expression in the context of tissue organization. This article describes protocols to perform smFISH on whole-mount mouse embryonic organs, where tissue organization can be compared to RNA expression by co-immunostaining of known protein markers. An enzymatic labeling strategy is also introduced to produce low-cost smFISH probes. Important considerations and practical guidelines for imaging smFISH samples using fluorescence confocal microscopy are described. Finally, a suite of custom-written ImageJ macros is included with detailed instructions to enable semi-automated smFISH image analysis of both 2D and 3D images. © 2018 by John Wiley & Sons, Inc.

Individual carbocyanine dye molecules in a sub-monolayer spread have been imaged with near-field scanning optical microscopy. Molecules can be repeatedly detected and spatially localized (to approximately lambda/50 where lambda is the wavelength of light) with a sensitivity of at least 0.005 molecules/(Hz)(1/2) and the orientation of each molecular dipole can be determined. This information is exploited to map the electric field distribution in the near-field aperture with molecular spatial resolution.

Commentary: A paper of many firsts: the first single molecule microscopy; the first extended observations of single molecules under ambient conditions; the first localization of single molecules to near-molecular precision ( 15 nm), the first determination of the dipole axes of single fluorescent molecules; and the first near-molecular resolution optical microscopy, when a single fluorescent molecule was used to map the evanescent electric field components in the vicinity of a 100 nm diameter near-field aperture. Although eventually supplanted by simpler far-field methods, this paper ushered in the era of single molecule imaging and biophysics, and inspired the concept that would eventually lead to PALM. Even today, near-field single molecule detection lives on in the “zero mode waveguide” sequencing approach promoted by Pacific Biosciences.

The resolution of a microscope is determined by the diffraction limit in classical microscopy, whereby objects that are separated by half a wavelength can no longer be visually separated. To go below the diffraction limit required several tricks and discoveries. In his Nobel Lecture, E. Betzig describes the developments that have led to modern super high-resolution microscopy.

Unraveling the structural organization of neurons can provide fundamental insights into brain function. However, visualizing neurite morphology in vivo remains difficult due to the high density and complexity of neural packing in the nervous system. Detailed analysis of neural morphology requires distinction of closely neighboring, highly intricate cellular structures such as neurites with high contrast. Green-to-red photoconvertible fluorescent proteins have become powerful tools to optically highlight molecular and cellular structures for developmental and cell biological studies. Yet, selective labeling of single cells of interest in vivo has been precluded due to inefficient photoconversion when using high intensity, pulsed, near-infrared laser sources that are commonly applied for achieving axially confined two-photon (2P) fluorescence excitation. Here we describe a novel optical mechanism, "confined primed conversion," which employs continuous dual-wave illumination to achieve confined green-to-red photoconversion of single cells in live zebrafish embryos. Confined primed conversion exhibits wide applicability and this chapter specifically elaborates on employing this imaging modality to analyze neural morphology of optically targeted single neurons in the developing zebrafish brain.

The physical manifestation of learning and memory formation in the brain can be expressed by strengthening or weakening of synaptic connections through morphological changes. Local actin remodeling underlies some forms of plasticity and may be facilitated by local β-actin synthesis, but dynamic information is lacking. In this work, we use single-molecule in situ hybridization to demonstrate that dendritic β-actin messenger RNA (mRNA) and ribosomes are in a masked, neuron-specific form. Chemically induced long-term potentiation prompts transient mRNA unmasking, which depends on factors active during synaptic activity. Ribosomes and single β-actin mRNA motility increase after stimulation, indicative of release from complexes. Hence, the single-molecule assays we developed allow for the quantification of activity-induced unmasking and availability for active translation. Further, our work demonstrates that β-actin mRNA and ribosomes are in a masked state that is alleviated by stimulation.

Axonal transport of synaptic vesicle proteins is required to maintain neurons' ability to communicate via synaptic transmission. Neurotransmitter-containing synaptic vesicles are assembled at synaptic terminals via highly regulated endocytosis of membrane proteins. These synaptic vesicle membrane proteins are synthesized in the cell body and transported to synapses in carrier vesicles that make their way down axons via microtubule-based transport utilizing kinesin molecular motors. Identifying the cargos that each kinesin motor protein carries from the cell bodies to the synapse is key to understanding both diseases caused by motor protein dysfunction and how synaptic vesicles are assembled. However, obtaining a bulk sample of axonal transport complexes from central nervous system (CNS) neurons to use for identification of their contents has posed a challenge to researchers. To obtain axonal carrier vesicles from primary cultured neurons, we fabricated a microfluidic chip designed to physically isolate axons from dendrites and cell bodies and developed a method to remove bulk axonal samples and label their contents. Synaptic vesicle protein carrier vesicles in these samples were labeled with antibodies to the synaptic vesicle proteins p38, SV2A, and VAMP2, and the anterograde axonal transport motor KIF1A, after which antibody overlap was evaluated using single-organelle TIRF microscopy. This work confirms a previously discovered association between KIF1A and p38 and shows that KIF1A also transports SV2A- and VAMP2-containing carrier vesicles.

The representation of magnitude information enables humans and animal species alike to successfully interact with the external environment. However, how various types of magnitudes are processed by single neurons to guide goal-directed behavior remains elusive. Here, we recorded single-cell activity from the dorsolateral prefrontal (PFC), dorsal premotor (PMd) and cingulate motor (CMA) cortices in monkeys discriminating discrete numerical (numerosity), continuous spatial (line length) and basic sensory (spatial frequency) stimuli. We found that almost exclusively PFC neurons represented the different magnitude types during sample presentation and working memory periods. The frequency of magnitude-selective cells in PMd and CMA did not exceed chance level. The proportion of PFC neurons selectively tuned to each of the three magnitude types were comparable. Magnitude coding was mainly dissociated at the single-neuron level, with individual neurons representing only one of the three tested magnitude types. Neuronal magnitude discriminability, coding strength and temporal evolution were comparable between magnitude types encoded by PFC neuron populations. Our data highlight the importance of PFC neurons in representing various magnitude categories. Such magnitude representations are based on largely distributed coding by single neurons that are anatomically intermingled within the same cortical area.

Probing the architecture, mechanism, and dynamics of genome folding is fundamental to our understanding of genome function in homeostasis and disease. Most chromosome conformation capture studies dissect the genome architecture with population- and time-averaged snapshots and thus have limited capabilities to reveal 3D nuclear organization and dynamics at the single-cell level. Here, we discuss emerging imaging techniques ranging from light microscopy to electron microscopy that enable investigation of genome folding and dynamics at high spatial and temporal resolution. Results from these studies complement genomic data, unveiling principles underlying the spatial arrangement of the genome and its potential functional links to diverse biological activities in the nucleus.