A SCARECROW-RETINOBLASTOMA protein network controls protective quiescence in the Arabidopsis root stem cell organizer.

Quiescent long-term somatic stem cells reside in plant and animal stem cell niches. Within the Arabidopsis root stem cell population, the Quiescent Centre (QC), which contains slowly dividing cells, maintains surrounding short-term stem cells and may act as a long-term reservoir for stem cells. The RETINOBLASTOMA-RELATED (RBR) protein cell-autonomously reinforces mitotic quiescence in the QC. RBR interacts with the stem cell transcription factor SCARECROW (SCR) through an LxCxE motif. Disruption of this interaction by point mutation in SCR or RBR promotes asymmetric divisions in the QC that renew short-term stem cells. Analysis of the in vivo role of quiescence in the root stem cell niche reveals that slow cycling within the QC is not needed for structural integrity of the niche but allows the growing root to cope with DNA damage.

A Bioorthogonal Chemical Reporter of Viral Infection.

Pathogen-selective labeling was achieved by using the novel gemcitabine metabolite analogue 2'-deoxy-2',2'-difluoro-5-ethynyluridine (dF-EdU) and click chemistry. Cells infected with Herpes Simplex Virus-1 (HSV-1), but not uninfected cells, exhibit nuclear staining upon the addition of dF-EdU and a fluorescent azide. The incorporation of the dF-EdU into DNA depends on its phosphorylation by a herpes virus thymidine kinase (TK). Crystallographic analyses revealed how dF-EdU is well accommodated in the active site of HSV-1 TK, but steric clashes prevent dF-EdU from binding human TK. These results provide the first example of pathogen-enzyme-dependent incorporation and labeling of bioorthogonal functional groups in human cells.

An azide-modified nucleoside for metabolic labeling of DNA.

Metabolic incorporation of azido nucleoside analogues into living cells can enable sensitive detection of DNA replication through copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) and strain-promoted azide-alkyne cycloaddition (SPAAC) "click" reactions. One major limitation to this approach is the poor chemical stability of nucleoside derivatives containing an aryl azide group. For example, 5-azido-2'-deoxyuridine (AdU) exhibits a 4 h half-life in water, and it gives little or no detectable labeling of cellular DNA. In contrast, the benzylic azide 5-(azidomethyl)-2'-deoxyuridine (AmdU) is stable in solution at 37 °C, and it gives robust labeling of cellular DNA upon addition of fluorescent alkyne derivatives. In addition to providing the first examples of metabolic incorporation into and imaging of azide groups in cellular DNA, these results highlight the general importance of assessing azide group stability in bioorthogonal chemical reporter strategies.

Dynamic metabolic labeling of DNA in vivo with arabinosyl nucleosides.

Commonly used metabolic labels for DNA, including 5-ethynyl-2'-deoxyuridine (EdU) and BrdU, are toxic antimetabolites that cause DNA instability, necrosis, and cell-cycle arrest. In addition to perturbing biological function, these properties can prevent metabolic labeling studies where subsequent tissue survival is needed. To bypass the metabolic pathways responsible for toxicity, while maintaining the ability to be metabolically incorporated into DNA, we synthesized and evaluated a small family of arabinofuranosyl-ethynyluracil derivatives. Among these, (2'S)-2'-deoxy-2'-fluoro-5-ethynyluridine (F-ara-EdU) exhibited selective DNA labeling, yet had a minimal impact on genome function in diverse tissue types. Metabolic incorporation of F-ara-EdU into DNA was readily detectable using copper(I)-catalyzed azide-alkyne "click" reactions with fluorescent azides. F-ara-EdU is less toxic than both BrdU and EdU, and it can be detected with greater sensitivity in experiments where long-term cell survival and/or deep-tissue imaging are desired. In contrast to previously reported 2'-arabino modified nucleosides and EdU, F-ara-EdU causes little or no cellular arrest or DNA synthesis inhibition. F-ara-EdU is therefore ideally suited for pulse-chase experiments aimed at "birth dating" DNA in vivo. As a demonstration, Zebrafish embryos were microinjected with F-ara-EdU at the one-cell stage and chased by BrdU at 10 h after fertilization. Following 3 d of development, complex patterns of quiescent/senescent cells containing only F-ara-EdU were observed in larvae along the dorsal side of the notochord and epithelia. Arabinosyl nucleoside derivatives therefore provide unique and effective means to introduce bioorthogonal functional groups into DNA for diverse applications in basic research, biotechnology, and drug discovery.

DNA template strand segregation in developing zebrafish.

Asymmetric inheritance of sister chromatids has long been predicted to be linked to discordant fates of daughter cells and even hypothesized to minimize accumulation of mutations in stem cells. Here, we use (2'S)-2'-deoxy-2'-fluoro-5-ethynyluridine (F-ara-EdU), bromodeoxyuridine (BrdU), and light sheet microscopy to track embryonic DNA in whole zebrafish. Larval development results in rapid depletion of older DNA template strands from stem cell niches in the retina, brain, and intestine. Prolonged label retention occurs in quiescent progenitors that resume replication in later development. High-resolution microscopy reveals no evidence of asymmetric template strand segregation in >100 daughter cell pairs, making it improbable that asymmetric DNA segregation prevents mutational burden according to the immortal strand hypothesis in developing zebrafish.

Selective fluorescence labeling of lipids in living cells.

Click chemistry in vivo: Three phosphatidic acid derivatives with alkyne groups in their fatty acid chains were synthesized and incorporated into mammalian cell membranes. Copper(I)-catalyzed and strain-promoted azide-alkyne cycloaddition reactions were used for their visualization (see schematic representation and fluorescence microscopic image).

A Bioorthogonal Chemical Reporter of Viral Infection.

Pathogen-selective labeling was achieved by using the novel gemcitabine metabolite analogue 2'-deoxy-2',2'-difluoro-5-ethynyluridine (dF-EdU) and click chemistry. Cells infected with Herpes Simplex Virus-1 (HSV-1), but not uninfected cells, exhibit nuclear staining upon the addition of dF-EdU and a fluorescent azide. The incorporation of the dF-EdU into DNA depends on its phosphorylation by a herpes virus thymidine kinase (TK). Crystallographic analyses revealed how dF-EdU is well accommodated in the active site of HSV-1 TK, but steric clashes prevent dF-EdU from binding human TK. These results provide the first example of pathogen-enzyme-dependent incorporation and labeling of bioorthogonal functional groups in human cells.

Labeling lipids for imaging in fixed cells.

INTRODUCTION: Fluorescently tagged lipid-binding domains have become a popular tool to image lipids that are involved in intracellular signaling processes. The readout usually involves the translocation of the lipid-binding domain from the cytosol or nucleosol to the membrane of interest, or vice versa. Unfortunately, this method seems to work predominantly for lipids in the plasma membrane, whereas lipids such as phosphatidylinositol 4,5-bisphosphate (PIP(2)) are not recognized in the membranes of the endoplasmic reticulum or the Golgi. Very recently, we developed an alternative way of localizing a lipid of interest by fluorescent labeling of minimally modified lipid derivatives using a single specific chemical reaction. This protocol describes how to directly label lipids in fixed cells for lipid location analyses.

Transfection of cells with DNA encoding a visible fluorescent protein-tagged lipid-binding domain.

INTRODUCTION: Visualization of a certain lipid species can be achieved by producing a fusion protein between a lipid-binding domain and a visible fluorescent protein (VFP). After a DNA construct for a VFP-tagged lipid-binding domain has been prepared, the desired cell line is transfected with the DNA and visualized using fluorescence microscopy, as described here. The DNA encoding a VFP-tagged lipid-binding domain is isolated from Escherichia coli, and the cells to be transfected are grown on glass-bottom dishes or on coverslips. We routinely perform transfection with commercially available reagents, although, depending on the cell line, the calcium phosphate precipitation method may provide an economic alternative.

Labeling lipids for imaging in live cells.

INTRODUCTION: Fluorescently tagged lipid-binding domains have become a popular tool to image lipids that are involved in intracellular signaling processes. The readout usually involves the translocation of the lipid-binding domain from the cytosol or nucleosol to the membrane of interest, or vice versa. Unfortunately, this method seems to work predominantly for lipids in the plasma membrane, whereas lipids such as phosphatidylinositol 4,5-bisphosphate (PIP(2)) are not recognized in the membranes of the endoplasmic reticulum or the Golgi. Very recently, we developed an alternative way of localizing a lipid of interest by fluorescent labeling of minimally modified lipid derivatives using a single specific chemical reaction. For lipid location analyses, the method is used in fixed cells. However, for studying lipid dynamics, specific labeling in living cells is also possible. This protocol describes how to directly label lipids for imaging in living cells.

Imaging lipids in living cells.

INTRODUCTION: The investigation of lipids in living cells is one of the underdeveloped areas in cell biology. Although it is possible to analyze the global lipid composition of a cell type, fractionation of the various types of membranes from cells is extraordinarily difficult, mainly because most membranes appear to be in contact with each other. Therefore, we know the lipid components, but we have a difficult time finding out their exact position, how dynamically they change location, and how rapidly they are metabolized. Imaging lipids in cells seems to be the obvious solution to the problem. The most common way to image molecules is by the artificial addition of a fluorescent tag. The use of fluorescent proteins has become the mainstay of protein imaging, but this method is, of course, not suitable for small molecules such as lipids. Unfortunately, the fluorescent tag is usually as large as the lipid and is therefore likely to have a severe influence on lipid location and metabolism. To circumvent this problem, two solutions have been developed--namely, the use of fluorescently labeled proteins that specifically recognize lipids and a chemical method to introduce the fluorescent tag inside the cell. This article describes procedures necessary to image lipids by fluorescently tagged lipid-binding domains and by labeling lipid derivatives in fixed and living cells.