Per Niklas Hedde

A photoactivatable marker protein for pulse-chase imaging with superresolution

IrisFP is a photoactivatable fluorescent protein that combines irreversible photoconversion from a green- to a red-emitting form with reversible photoswitching between a fluorescent and a nonfluorescent state in both forms. Here we introduce a monomeric variant, mIrisFP, and demonstrate how its multiple photoactivation modes can be used for pulse-chase experiments combined with subdiffraction-resolution imaging in living cells by using dual-color photoactivation localization microscopy (PALM).

Online image analysis software for photoactivation localization microscopy

data fitting is required. The localization procedure resembles the one used for the global positioning system (GPS). We assessed the performance of our analysis routine by taking images (Supplementary Figs. 3,4) with our home-built total internal reflection fluorescence (TIRF) microscope (Supplementary Fig. 5). Live-cell imaging will benefit the most from our real-time data analysis, and the substantial resolution improvement was evident from comparing the standard TIRF microscopy and PALM images of a live HeLa cell (Fig. 1a–c and Supplementary Fig. 3), which show cytoskeletal structures composed of actin filaments (F-actin) cross-linked by α-actinin. We labeled F-actin by transient transfection with a plasmid encoding a fusion construct of the actin-binding peptide Lifeact8 with d2EosFP9,10. EosFP and its variants are green-to-red photoconvertible fluorescent proteins excellently suited for PALM, using 561-nm light for excitation and 405-nm light for photoconversion. To evaluate our Livepalm analysis software, we analyzed the individual PALM frames by both the full least-squares fit and the fluoroBancroft algorithms. Foremost, a real-time PALM routine should feature an excellent localization precision. Indeed, mere visual inspection did not reveal noticeable resolution differences between the least-squares fit (Fig. 1b) and the fluoroBancroft (Fig. 1c) algorithms. To compare the performance, we calculated the distributions of fluorophores around the mean contour line. Gaussian least-squares fitting (Fig. 1d) and the fluoroBancroft algorithm (Fig. 1e) yielded distributions with full widths at half maximum Online image analysis software for photoactivation localization microscopy

LXRs link metabolism to inflammation through Abca1-dependent regulation of membrane composition and TLR signaling

The liver X receptors (LXRs) are transcriptional regulators of lipid homeostasis that also have potent anti-inflammatory effects. The molecular basis for their anti-inflammatory effects is incompletely understood, but has been proposed to involve the indirect tethering of LXRs to inflammatory gene promoters. Here we demonstrate that the ability of LXRs to repress inflammatory gene expression in cells and mice derives primarily from their ability to regulate lipid metabolism through transcriptional activation and can occur in the absence of SUMOylation. Moreover, we identify the putative lipid transporter Abca1 as a critical mediator of LXRs anti-inflammatory effects. Activation of LXR inhibits signaling from TLRs 2, 4 and 9 to their downstream NF-κB and MAPK effectors through Abca1-dependent changes in membrane lipid organization that disrupt the recruitment of MyD88 and TRAF6. These data suggest that a common mechanism-direct transcriptional activation-underlies the dual biological functions of LXRs in metabolism and inflammation. DOI: http://dx.doi.org/10.7554/eLife.08009.001

Ultra-fast, high-precision image analysis for localization-based super resolution microscopy

Localization-based super resolution microscopy holds superior performances in live cell imaging, but its widespread use is thus far mainly hindered by the slow image analysis speed. Here we show a powerful image analysis method based on the combination of the maximum likelihood algorithm and a Graphics Processing Unit (GPU). Results indicate that our method is fast enough for real-time processing of experimental images even from fast EMCCD cameras working at full frame rate without compromising localization precision or field of view. This newly developed method is also capable of revealing movements from the images immediately after data acquisition, which is of great benefit to live cell imaging.

Stimulated emission depletion-based raster image correlation spectroscopy reveals biomolecular dynamics in live cells

Raster image correlation spectroscopy is a powerful tool to study fast molecular dynamics such as protein diffusion or receptor-ligand interactions inside living cells and tissues. By analysing spatio-temporal correlations of fluorescence intensity fluctuations from raster-scanned microscopy images, molecular motions can be revealed in a spatially resolved manner. Because of the diffraction-limited optical resolution, however, conventional raster image correlation spectroscopy can only distinguish larger regions of interest and requires low fluorophore concentrations in the nanomolar range. Here, to overcome these limitations, we combine raster image correlation spectroscopy with stimulated emission depletion microscopy. With imaging experiments on model membranes and live cells, we show that stimulated emission depletion-raster image correlation spectroscopy offers an enhanced multiplexing capability because of the enhanced spatial resolution as well as access to 10-100 times higher fluorophore concentrations.

Lpcat3-dependent production of arachidonoyl phospholipids is a key determinant of triglyceride secretion

The role of specific phospholipids (PLs) in lipid transport has been difficult to assess due to an inability to selectively manipulate membrane composition in vivo. Here we show that the phospholipid remodeling enzyme lysophosphatidylcholine acyltransferase 3 (Lpcat3) is a critical determinant of triglyceride (TG) secretion due to its unique ability to catalyze the incorporation of arachidonate into membranes. Mice lacking Lpcat3 in the intestine fail to thrive during weaning and exhibit enterocyte lipid accumulation and reduced plasma TGs. Mice lacking Lpcat3 in the liver show reduced plasma TGs, hepatosteatosis, and secrete lipid-poor very low-density lipoprotein (VLDL) lacking arachidonoyl PLs. Mechanistic studies indicate that Lpcat3 activity impacts membrane lipid mobility in living cells, suggesting a biophysical basis for the requirement of arachidonoyl PLs in lipidating lipoprotein particles. These data identify Lpcat3 as a key factor in lipoprotein production and illustrate how manipulation of membrane composition can be used as a regulatory mechanism to control metabolic pathways. DOI: http://dx.doi.org/10.7554/eLife.06557.001

Intestinal Phospholipid Remodeling Is Required for Dietary-Lipid Uptake and Survival on a High-Fat Diet.

Phospholipids are important determinants of membrane biophysical properties, but the impact of membrane acyl chain composition on dietary-lipid absorption is unknown. Here we demonstrate that the LXR-responsive phospholipid-remodeling enzyme Lpcat3 modulates intestinal fatty acid and cholesterol absorption and is required for survival on a high-fat diet. Mice lacking Lpcat3 in the intestine thrive on carbohydrate-based chow but lose body weight rapidly and become moribund on a triglyceride-rich diet. Lpcat3-dependent incorporation of polyunsaturated fatty acids into phospholipids is required for the efficient transport of dietary lipids into enterocytes. Furthermore, loss of Lpcat3 amplifies the production of gut hormones, including GLP-1 and oleoylethanolamide, in response to high-fat feeding, contributing to the paradoxical cessation of food intake in the setting of starvation. These results reveal that membrane phospholipid composition is a gating factor in passive lipid absorption and implicate LXR-Lpcat3 signaling in a gut-brain feedback loop that couples absorption to food intake.

Localization and Dynamics of Glucocorticoid Receptor at the Plasma Membrane of Activated Mast Cells

In addition to their actions in the cell nucleus, glucocorticoids exhibit rapid non-nuclear responses that are mechanistically not well understood. To explain these effects, the localization of a glucocorticoid receptor (GR) expressed in mast cells as a GFP fusion was analyzed after activation of the cells on allergenic lipid arrays. These arrays were produced on glass slides by dip-pen nanolithography (DPN) and total internal reflection (TIRF) microscopy was used to visualize the GR. A rapid glucocorticoid-independent and -dependent recruitment of the GR-GFP to the plasma cell membrane was observed following contact of the cells with the allergenic array. In addition, the mobility of the GR at the membrane was monitored by fluorescence recovery after photobleaching (FRAP) and shown to follow binding kinetics demonstrating interactions of the receptor with membrane-bound factors. Furthermore the recruitment of the GR to the cell membrane was shown to result in a glucocorticoid-mediated increase in Erk phosphorylation. This is evidenced by findings that destruction of the membrane composition of the mast cells by cholesterol depletion impairs the membrane localization of the GR and subsequent glucocorticoid-mediated enhancement of Erk phosphorylation. These results demonstrate a membrane localization and function of the GR in mast cell signaling.

Dual Color Photoactivation Localization Microscopy of Cardiomyopathy-associated Desmin Mutants

Background: Heterozygous DES mutations affect filament formation leading to skeletal and cardiomyopathies. Results: Our results reveal different extent of filament formation defects by various desmin mutants under heterozygous conditions. Conclusion: Analysis of interaction and co-localization of mutant and wild-type desmin proves the co-existence of heterogeneous filaments in living cells. Significance: These results might be of relevance for the understanding of filament formation defects. Mutations in the DES gene coding for the intermediate filament protein desmin may cause skeletal and cardiac myopathies, which are frequently characterized by cytoplasmic aggregates of desmin and associated proteins at the cellular level. By atomic force microscopy, we demonstrated filament formation defects of desmin mutants, associated with arrhythmogenic right ventricular cardiomyopathy. To understand the pathogenesis of this disease, it is essential to analyze desmin filament structures under conditions in which both healthy and mutant desmin are expressed at equimolar levels mimicking an in vivo situation. Here, we applied dual color photoactivation localization microscopy using photoactivatable fluorescent proteins genetically fused to desmin and characterized the heterozygous status in living cells lacking endogenous desmin. In addition, we applied fluorescence resonance energy transfer to unravel short distance structural patterns of desmin mutants in filaments. For the first time, we present consistent high resolution data on the structural effects of five heterozygous desmin mutations on filament formation in vitro and in living cells. Our results may contribute to the molecular understanding of the pathological filament formation defects of heterozygous DES mutations in cardiomyopathies.

Fast and efficient molecule detection in localization-based super-resolution microscopy by parallel adaptive histogram equalization.

In localization-based super-resolution microscopy, individual fluorescent markers are stochastically photoactivated and subsequently localized within a series of camera frames, yielding a final image with a resolution far beyond the diffraction limit. Yet, before localization can be performed, the subregions within the frames where the individual molecules are present have to be identified-oftentimes in the presence of high background. In this work, we address the importance of reliable molecule identification for the quality of the final reconstructed super-resolution image. We present a fast and robust algorithm (a-livePALM) that vastly improves the molecule detection efficiency while minimizing false assignments that can lead to image artifacts.