Multi-dimensional Fluorescence Live-Cell Imaging for Glucosome Dynamics in Living Human Cells

Fluorescence live-cell imaging that has contributed to our understanding of cell biology is now at the frontline of studying quantitative biochemistry in a cell. Particularly, technological advancements of fluorescence live-cell imaging and associated strategies in recent years have allowed us to discover various subcellular macromolecular assemblies in living human cells. Here we describe how real-time dynamics of a multienzyme metabolic assembly, the "glucosome," that is responsible for regulating glucose flux at subcellular levels, has been investigated in both 2- and 3-dimensional space of single human cells. We envision that such multi-dimensional fluorescence live-cell imaging will continue to revolutionize our understanding of how intracellular metabolic pathways and their network are functionally orchestrated at single-cell levels.

Estrogen-related Receptor β Reduces the Subnuclear Mobility of Estrogen Receptor α and Suppresses Estrogen-dependent Cellular Function

Estrogen-related receptor (ERR) is a member of the nuclear receptor superfamily that has strong homology with estrogen receptor (ER) α. ERR has three subtypes (α, β, and γ) expressed in estrogen-sensitive organs, including ovary, breast, and brain. No endogenous ligands of ERRs have been identified, but these receptors share a common DNA element with ERα and control estrogen-mediated gene transcription. Recent evidence suggests a role of ERRs in estrogen-related pathophysiology, but the detailed mechanisms of ERR functions in estrogen-related tissues are unclear. Using live-cell imaging with fluorescent protein labeling, we found that only ERRβ among the ERRs exhibits a punctate intranuclear pattern overlapping with ERα following 17β-estradiol (E2)-stimulation. Fluorescence recovery after photobleaching showed significant reduction of the mobility of ligand-activated ERα with co-expression of ERRβ. Fluorescence resonance energy transfer revealed that ERRβ directly interacts with ERα. The N-terminal domain of ERRβ was identified as the region that interacts with ERα. We also found a correlation between punctate cluster formation of ERα and interaction between the receptors. Expression of ERRβ significantly repressed ERα-mediated transactivity, whereas that of other ERR subtypes had no effect on the transactivity of ERα. Consistent with this finding, E2-stimulated proliferation of MCF-7 breast carcinoma cells and bcl-2 expression was significantly inhibited by expression of ERRβ. These results provide strong evidence for a suppressive effect of ERRβ on estrogen signaling through reduction of the intranuclear mobility of ERα. The findings further suggest a unique inhibitory role for ERRβ in estrogen-dependent cellular function such as cancer cell proliferation.

Single-cell analysis of G-protein signal transduction

The growing use of fluorescent biosensors to directly probe the spatiotemporal dynamics of biochemical processes in living cells has revolutionized the study of intracellular signaling. In this review, we summarize recent developments in the use of biosensors to illuminate the molecular details of G-protein-coupled receptor (GPCR) signaling pathways, which have long served as the model for our understanding of signal transduction, while also offering our perspectives on the future of this exciting field. Specifically, we highlight several ways in which biosensor-based single-cell analyses are being used to unravel many of the enduring mysteries that surround these diverse signaling pathways.

Multi-step loading of human minichromosome maintenance proteins in live human cells

Once-per-cell cycle replication is regulated through the assembly onto chromatin of multisubunit protein complexes that license DNA for a further round of replication. Licensing consists of the loading of the hexameric MCM2-7 complex onto chromatin during G1 phase and is dependent on the licensing factor Cdt1. In vitro experiments have suggested a two-step binding mode for minichromosome maintenance (MCM) proteins, with transient initial interactions converted to stable chromatin loading. Here, we assess MCM loading in live human cells using an in vivo licensing assay on the basis of fluorescence recovery after photobleaching of GFP-tagged MCM protein subunits through the cell cycle. We show that, in telophase, MCM2 and MCM4 maintain transient interactions with chromatin, exhibiting kinetics similar to Cdt1. These are converted to stable interactions from early G1 phase. The immobile fraction of MCM2 and MCM4 increases during G1 phase, suggestive of reiterative licensing. In late G1 phase, a large fraction of MCM proteins are loaded onto chromatin, with maximal licensing observed just prior to S phase onset. Fluorescence loss in photobleaching experiments show subnuclear concentrations of MCM-chromatin interactions that differ as G1 phase progresses and do not colocalize with sites of DNA synthesis in S phase.