Visualization of the Nucleolus Using 5' Ethynyl Uridine

Labeling of the nucleolus in Arabidopsis thaliana can be achieved by incorporation of 5'-ethynyl uridine (EU) into bulk RNA. Although EU does not selectively label the nucleolus, the abundance of ribosomal transcripts results in the predominant accumulation of the signal in the nucleolus. Ethynyl uridine has the advantage of being detected via Click-iT chemistry providing a specific signal and low background. While the protocol presented here employs fluorescent dye and allows visualization of the nucleolus by microscopy, this method can also be used for other downstream applications. Though we tested nucleolar labeling only in A. thaliana, in principle it can be applied to other plant species.

DeepFoci: Deep learning-based algorithm for fast automatic analysis of DNA double-strand break ionizing radiation-induced foci

DNA double-strand breaks (DSBs), marked by ionizing radiation-induced (repair) foci (IRIFs), are the most serious DNA lesions and are dangerous to human health. IRIF quantification based on confocal microscopy represents the most sensitive and gold-standard method in radiation biodosimetry and allows research on DSB induction and repair at the molecular and single-cell levels. In this study, we introduce DeepFoci - a deep learning-based fully automatic method for IRIF counting and morphometric analysis. DeepFoci is designed to work with 3D multichannel data (trained for 53BP1 and γH2AX) and uses U-Net for nucleus segmentation and IRIF detection, together with maximally stable extremal region-based IRIF segmentation. The proposed method was trained and tested on challenging datasets consisting of mixtures of nonirradiated and irradiated cells of different types and IRIF characteristics - permanent cell lines (NHDFs, U-87) and primary cell cultures prepared from tumors and adjacent normal tissues of head and neck cancer patients. The cells were dosed with 0.5-8 Gy γ-rays and fixed at multiple (0-24 h) postirradiation times. Under all circumstances, DeepFoci quantified the number of IRIFs with the highest accuracy among current advanced algorithms. Moreover, while the detection error of DeepFoci remained comparable to the variability between two experienced experts, the software maintained its sensitivity and fidelity across dramatically different IRIF counts per nucleus. In addition, information was extracted on IRIF 3D morphometric features and repair protein colocalization within IRIFs. This approach allowed multiparameter IRIF categorization of single- or multichannel data, thereby refining the analysis of DSB repair processes and classification of patient tumors, with the potential to identify specific cell subclones. The developed software improves IRIF quantification for various practical applications (radiotherapy monitoring, biodosimetry, etc.) and opens the door to advanced DSB focus analysis and, in turn, a better understanding of (radiation-induced) DNA damage and repair.

Mito-SinCe2 Approach to Analyze Mitochondrial Structure-Function Relationship in Single Cells

The cross talk between mitochondrial dynamic structure, determined primarily by mitochondrial fission and fusion events, and mitochondrial function of energetics, primarily ATP and ROS production, is widely appreciated. Understanding the mechanistic details of such cross talk between mitochondrial structure and function needs integrated quantitative analyses between mitochondrial dynamics and energetics. Here we describe our recently designed approach of mito-SinCe² that involves high resolution confocal microscopy of genetically expressed ratiometric fluorescent probes targeted to mitochondria, and its quantitative analyses. Mito-SinCe² analyses allows for quantitative analyses of mitochondrial structure-function relationship in single cells toward understanding the role of mitochondria and their heterogeneity in various physiological and pathological conditions.

Hypothermia-Associated Coagulopathy: A Comparison of Viscoelastic Monitoring, Platelet Function, and Real Time Live Confocal Microscopy at Low Blood Temperatures, an in vitro Experimental Study

Introduction

Hypothermia has notable effects on platelets, platelet function, fibrinogen, and coagulation factors. Common laboratory techniques cannot identify those effects, because blood samples are usually warmed to 37°C before analysis and do not fully reflect the in vivo situation. Multiple aspects of the pathophysiological changes in humoral and cellular coagulation remain obscure. This in vitro experimental study aimed to compare the measurements of thromboelastometry (TEM), multiple-electrode aggregometry (MEA) and Real Time Live Confocal Imaging for the purpose of identifying and characterizing hypothermia-associated coagulopathy.

Methods

Blood samples were drawn from 18 healthy volunteers and incubated for 30 min before being analyzed at the target temperatures (37, 32, 24, 18, and 13.7°C). At each temperature thromboelastometry and multiple-electrode aggregometry were measured. Real Time Live Confocal Imaging was performed at 4, 24, and 37°C. The images obtained by Real Time Live Confocal Imaging were compared with the functional results of thromboelastometry and multiple-electrode aggregometry.

Results

Thromboelastometry standard parameters were impaired at temperatures below baseline 37°C (ANOVA overall effect, p < 0.001): clotting time was prolonged by 27% at 13.7°C and by 60% at 18°C (p < 0.044); clot formation time was prolonged by 157% (p < 0.001). A reduction in platelet function with decreasing temperatures was observed (p < 0.001); the area under the curve at 13.7°C was reduced by 96% (ADP test), 92% (ASPI test), and 91% (TRAP test) of the baseline values. Temperature-associated changes in coagulation were visualized with Real Time Live Confocal Imaging. Molecular changes such as the temperature-associated decrease in the fibrin network are paralleled by cellular effects like the lesser activity of the platelets as a result of decreased temperature. The maximum clot firmness (MCF) in TEM only changed slightly within the temperature range tested.

Conclusion

The inhibitory effects of temperature on clot formation were visualized with Real Time Live Confocal Microscopy and compared with standard point-of-care testing. Inhibition of clotting factors and impaired platelet function are probably a result of hypothermia-induced impairment of thrombin. Measurement of MCF in TEM does not fully concur with Real Time Live Confocal Microscopy or MEA in hypothermia.

Combination of Dark-Field and Confocal Microscopy for the Optical Detection of Silver and Titanium Nanoparticles in Mammalian Cells

We describe here two optical microscopy techniques-dark-field confocal light scanning microscopy (DF-CLSM) and dark-field wide-field confocal microscopy (DF-WFCM), that can be used to study interaction between nanoparticles and cells in 3D space. Dark field microscopy can detect very small structures below the diffraction limit of conventional light microscopes, while a confocal setup provides vertical sectioning capabilities to render specimens in 3D. The use of DF-WFCM instead of DF-CLSM allows faster sample processing but yields lower resolution. We used a retinal pigment epithelial cell line ARPE-19 to illustrate different optical and lighting conditions necessary for optimal imaging of metal and metal oxide nanoparticles (TiO₂ and Ag). Our experimental setup primarily involved an E-800 Nikon and Nikon Ni upright microscopes and a Nikon Ti₂ microscope connected to a xenon light source along with special dark-field objectives. For confocal studies we used either Leica and Nikon inverted confocal microscopes. For microscopic analyses, ARPE-19 cells were fixed in situ in cultured chamber slides or collected from T-25 flasks and then fixed in suspension. At the lowest concentrations of TiO₂ or Ag tested (0.1-0.3 μg/mL), we were able to detect as few as 5-10 nanoparticles per cell due to intense light scattering by the particles. The degree of brightness detected indicated that the uptake of nanoparticles within ARPE-19 cells could be monitored using dark-field microscopy. Here we describe how to use wide-field microscopy to follow nanoparticle uptake by cells and how to assess some aspects of cellular health in in vitro cell cultures exposed to nanoparticles.

The bactericidal effect of an atmospheric-pressure plasma jet on Porphyromonas gingivalis biofilms on sandblasted and acid-etched titanium discs

Purpose

Direct application of atmospheric-pressure plasma jets (APPJs) has been established as an effective method of microbial decontamination. This study aimed to investigate the bactericidal effect of direct application of an APPJ using helium gas (He-APPJ) on Porphyromonas gingivalis biofilms on sandblasted and acid-etched (SLA) titanium discs.

Methods

On the SLA discs covered by P. gingivalis biofilms, an APPJ with helium (He) as a discharge gas was applied at 3 different time intervals (0, 3, and 5 minutes). To evaluate the effect of the plasma itself, the He gas-only group was used as the control group. The bactericidal effect of the He-APPJ was determined by the number of colony-forming units. Bacterial viability was observed by confocal laser scanning microscopy (CLSM), and bacterial morphology was examined by scanning electron microscopy (SEM).

Results

As the plasma treatment time increased, the amount of P. gingivalis decreased, and the difference was statistically significant. In the SEM images, compared to the control group, the bacterial biofilm structure on SLA discs treated by the He-APPJ for more than 3 minutes was destroyed. In addition, the CLSM images showed consistent results. Even in sites distant from the area of direct He-APPJ exposure, decontamination effects were observed in both SEM and CLSM images.

Conclusions

He-APPJ application was effective in removing P. gingivalis biofilm on SLA titanium discs in an in vitro experiment.

In vivo confocal microscopy indicates an inverse relationship between the sub-basal corneal plexus and the conjunctivalisation in patients with limbal stem cell deficiency

BACKGROUND/AIMS

Limbal stem cell deficiency (LSCD) is characterised by a marked decrease in limbal stem cells. It is classified primarily using subjective slit-lamp observations. In vivo confocal microscopy (IVCM) can non-invasively provide objective information on the condition of the limbal niche, the corneal epithelial basal cell density and the corneal sub-basal nerve plexus density (SND). We here used IVCM to evaluate changes in SND to improve LSCD classification.

METHODS

We evaluated and classified 38 patients (76 eyes, 44 with LSC and 32 control eyes) using the Rama, López-García and Deng (clinical and confocal) classifications and evaluated the concordance of the confocal and clinical classifications. We constructed a logistic regression model using multivariate analysis to correlate different degrees of conjunctivalisation with IVCM parameters and used receiver operating characteristic (ROC) curve analysis to establish the SND cut-off value with maximum diagnostic sensitivity and specificity.

RESULTS

The classification systems correlated moderately at best (kappa, 0.449). The corneal SND of cases (6469±6295 µm/mm²) was less (p<0.001) than in controls (20911±4142 µm/mm²). The SND, but not basal cell density, played a protective role against conjunctivalisation (OR, 0.069; 95% CI 0.008-0.619; p=0.01). An SND cut-off value of 17 215 µm/mm² yielded a sensitivity and specificity of 95.5% and 90.6%, respectively, for LSCD diagnosis.

CONCLUSION

The density of the corneal sub-basal nerve plexus was inversely related to conjunctivalisation in LSCD. Further studies are needed to verify this and to elucidate the directionality between these factors.

Dextran functionalization enhances nanoparticle-mediated siRNA delivery and silencing

Understanding the endocytosis and intracellular trafficking of short interfering RNA (siRNA) delivery vehicle complexes remains a critical bottleneck in designing siRNA delivery vehicles for highly active RNA interference (RNAi)-based therapeutics. In this study, we show that dextran functionalization of silica nanoparticles enhanced uptake and intracellular delivery of siRNAs in cultured cells. Using pharmacological inhibitors for endocytotic pathways, we determined that our complexes are endocytosed via a previously unreported mechanism for siRNA delivery in which dextran initiates scavenger receptor-mediated endocytosis through a clathrin/caveolin-independent process. Our findings suggest that siRNA delivery efficiency could be enhanced by incorporating dextran into existing delivery platforms to activate scavenger receptor activity across a variety of target cell types.

Amiodarone-induced multiorgan toxicity with ocular findings on confocal microscopy

Amiodarone is an antiarrhythmic medication that can adversely effect various organs including lungs, thyroid gland, liver, eyes, skin, and nerves. The risk of adverse effects increases with high doses and prolonged use. We report a 54-year-old female who presented with multiorgan toxicity after 8 months of low dose (200 mg/day) amiodarone treatment. The findings of confocal microscopy due to amiodarone-induced keratopathy are described. Amiodarone may cause multiorgan toxicity even at lower doses and for shorter treatment periods.

Centrobin-mediated regulation of the centrosomal protein 4.1-associated protein (CPAP) level limits centriole length during elongation stage

Microtubule-based centrioles in the centrosome mediate accurate bipolar cell division, spindle orientation, and primary cilia formation. Cellular checkpoints ensure that the centrioles duplicate only once in every cell cycle and achieve precise dimensions, dysregulation of which results in genetic instability and neuro- and ciliopathies. The normal cellular level of centrosomal protein 4.1-associated protein (CPAP), achieved by its degradation at mitosis, is considered as one of the major mechanisms that limits centriole growth at a predetermined length. Here we show that CPAP levels and centriole elongation are regulated by centrobin. Exogenous expression of centrobin causes abnormal elongation of centrioles due to massive accumulation of CPAP in the cell. Conversely, CPAP was undetectable in centrobin-depleted cells, suggesting that it undergoes degradation in the absence of centrobin. Only the reintroduction of full-length centrobin, but not its mutant form that lacks the CPAP binding site, could restore cellular CPAP levels in centrobin-depleted cells, indicating that persistence of CPAP requires its interaction with centrobin. Interestingly, inhibition of the proteasome in centrobin-depleted cells restored the cellular and centriolar CPAP expression, suggesting its ubiquitination and proteasome-mediated degradation when centrobin is absent. Intriguingly, however, centrobin-overexpressing cells also showed proteasome-independent accumulation of ubiquitinated CPAP and abnormal, ubiquitin-positive, elongated centrioles. Overall, our results show that centrobin interacts with ubiquitinated CPAP and prevents its degradation for normal centriole elongation function. Therefore, it appears that loss of centrobin expression destabilizes CPAP and triggers its degradation to restrict the centriole length during biogenesis.

CD36 binds oxidized low density lipoprotein (LDL) in a mechanism dependent upon fatty acid binding

The association of unesterified fatty acid (FA) with the scavenger receptor CD36 has been actively researched, with focuses on FA and oxidized low density lipoprotein (oxLDL) uptake. CD36 has been shown to bind FA, but this interaction has been poorly characterized to date. To gain new insights into the physiological relevance of binding of FA to CD36, we characterized FA binding to the ectodomain of CD36 by the biophysical method surface plasmon resonance. Five structurally distinct FAs (saturated, monounsaturated (cis and trans), polyunsaturated, and oxidized) were pulsed across surface plasmon resonance channels, generating association and dissociation binding curves. Except for the oxidized FA HODE, all FAs bound to CD36, with rapid association and dissociation kinetics similar to HSA. Next, to elucidate the role that each FA might play in CD36-mediated oxLDL uptake, we used a fluorescent oxLDL (Dii-oxLDL) live cell assay with confocal microscopy imaging. CD36-mediated uptake in serum-free medium was very low but greatly increased when serum was present. The addition of exogenous FA in serum-free medium increased oxLDL binding and uptake to levels found with serum and affected CD36 plasma membrane distribution. Binding/uptake of oxLDL was dependent upon the FA dose, except for docosahexaenoic acid, which exhibited binding to CD36 but did not activate the uptake of oxLDL. HODE also did not affect oxLDL uptake. High affinity FA binding to CD36 and the effects of each FA on oxLDL uptake have important implications for protein conformation, binding of other ligands, functional properties of CD36, and high plasma FA levels in obesity and type 2 diabetes.

The use of carbohydrate binding modules (CBMs) to monitor changes in fragmentation and cellulose fiber surface morphology during cellulase- and Swollenin-induced deconstruction of lignocellulosic substrates

Although the actions of many of the hydrolytic enzymes involved in cellulose hydrolysis are relatively well understood, the contributions that amorphogenesis-inducing proteins might contribute to cellulose deconstruction are still relatively undefined. Earlier work has shown that disruptive proteins, such as the non-hydrolytic non-oxidative protein Swollenin, can open up and disaggregate the less-ordered regions of lignocellulosic substrates. Within the cellulosic fraction, relatively disordered, amorphous regions known as dislocations are known to occur along the length of the fibers. It was postulated that Swollenin might act synergistically with hydrolytic enzymes to initiate biomass deconstruction within these dislocation regions. Carbohydrate binding modules (CBMs) that preferentially bind to cellulosic substructures were fluorescently labeled. They were imaged, using confocal microscopy, to assess the distribution of crystalline and amorphous cellulose at the fiber surface, as well as to track changes in surface morphology over the course of enzymatic hydrolysis and fiber fragmentation. Swollenin was shown to promote targeted disruption of the cellulosic structure at fiber dislocations.

Matrix confinement plays a pivotal role in regulating neutrophil-generated tractions, speed, and integrin utilization

Neutrophils are capable of switching from integrin-dependent motility on two-dimensional substrata to integrin-independent motion following entry into the confined three-dimensional matrix of an afflicted tissue. However, whether integrins still maintain a regulatory role for cell traction generation and cell locomotion under the physical confinement of the three-dimensional matrix is unknown, and this is challenging to deduce from motility studies alone. Using three-dimensional traction force microscopy and a double hydrogel sandwich system, we determined the three-dimensional spatiotemporal traction forces of motile neutrophils at unprecedented resolution and show, for the first time, that entry into a highly confined space (2.5D) is a sufficient trigger to convert to integrin-independent migration. We find that integrins exert a significant regulatory role in determining the magnitude and spatial distribution of tractions and cell speed on confined cells. We also find that 90% of neutrophil tractions are in the out-of-plane axis, and this may be a fundamental element of neutrophil traction force generation.

A kinase inhibitor screen reveals protein kinase C-dependent endocytic recycling of ErbB2 in breast cancer cells

ErbB2 overexpression drives oncogenesis in 20-30% cases of breast cancer. Oncogenic potential of ErbB2 is linked to inefficient endocytic traffic into lysosomes and preferential recycling. However, regulation of ErbB2 recycling is incompletely understood. We used a high-content immunofluorescence imaging-based kinase inhibitor screen on SKBR-3 breast cancer cells to identify kinases whose inhibition alters the clearance of cell surface ErbB2 induced by Hsp90 inhibitor 17-AAG. Less ErbB2 clearance was observed with broad-spectrum PKC inhibitor Ro 31-8220. A similar effect was observed with Go 6976, a selective inhibitor of classical Ca(2+)-dependent PKCs (α, β1, βII, and γ). PKC activation by PMA promoted surface ErbB2 clearance but without degradation, and ErbB2 was observed to move into a juxtanuclear compartment where it colocalized with PKC-α and PKC-δ together with the endocytic recycling regulator Arf6. PKC-α knockdown impaired the juxtanuclear localization of ErbB2. ErbB2 transit to the recycling compartment was also impaired upon PKC-δ knockdown. PMA-induced Erk phosphorylation was reduced by ErbB2 inhibitor lapatinib, as well as by knockdown of PKC-δ but not that of PKC-α. Our results suggest that activation of PKC-α and -δ mediates a novel positive feedback loop by promoting ErbB2 entry into the endocytic recycling compartment, consistent with reported positive roles for these PKCs in ErbB2-mediated tumorigenesis. As the endocytic recycling compartment/pericentrion has emerged as a PKC-dependent signaling hub for G-protein-coupled receptors, our findings raise the possibility that oncogenesis by ErbB2 involves previously unexplored PKC-dependent endosomal signaling.

HIV-1 protein Nef inhibits activity of ATP-binding cassette transporter A1 by targeting endoplasmic reticulum chaperone calnexin

HIV-infected patients are at increased risk of developing atherosclerosis, in part due to an altered high density lipoprotein profile exacerbated by down-modulation and impairment of ATP-binding cassette transporter A1 (ABCA1) activity by the HIV-1 protein Nef. However, the mechanisms of this Nef effect remain unknown. Here, we show that Nef interacts with an endoplasmic reticulum chaperone calnexin, which regulates folding and maturation of glycosylated proteins. Nef disrupted interaction between calnexin and ABCA1 but increased affinity and enhanced interaction of calnexin with HIV-1 gp160. The Nef mutant that did not bind to calnexin did not affect the calnexin-ABCA1 interaction. Interaction with calnexin was essential for functionality of ABCA1, as knockdown of calnexin blocked the ABCA1 exit from the endoplasmic reticulum, reduced ABCA1 abundance, and inhibited cholesterol efflux; the same effects were observed after Nef overexpression. However, the effects of calnexin knockdown and Nef on cholesterol efflux were not additive; in fact, the combined effect of these two factors together did not differ significantly from the effect of calnexin knockdown alone. Interestingly, gp160 and ABCA1 interacted with calnexin differently; although gp160 binding to calnexin was dependent on glycosylation, glycosylation was of little importance for the interaction between ABCA1 and calnexin. Thus, Nef regulates the activity of calnexin to stimulate its interaction with gp160 at the expense of ABCA1. This study identifies a mechanism for Nef-dependent inactivation of ABCA1 and dysregulation of cholesterol metabolism.

Motor-mediated cortical versus astral microtubule organization in lipid-monolayered droplets

The correct spatial organization of microtubules is of crucial importance for determining the internal architecture of eukaryotic cells. Microtubules are arranged in space by a multitude of biochemical activities and by spatial constraints imposed by the cell boundary. The principles underlying the establishment of distinct intracellular architectures are only poorly understood. Here, we studied the effect of spatial confinement on the self-organization of purified motors and microtubules that are encapsulated in lipid-monolayered droplets in oil, varying in diameter from 5-100 μm, which covers the size range of typical cell bodies. We found that droplet size alone had a major organizing influence. The presence of a microtubule-crosslinking motor protein decreased the number of accessible types of microtubule organizations. Depending on the degree of spatial confinement, the presence of the motor caused either the formation of a cortical array of bent microtubule bundles or the generation of single microtubule asters in the droplets. These are two of the most prominent forms of microtubule arrangements in plant and metazoan cells. Our results provide insights into the combined organizing influence of spatial constraints and cross-linking motor activities determining distinct microtubule architectures in a minimal biomimetic system. In the future, this simple lipid-monolayered droplet system characterized here can be expanded readily to include further biochemical activities or used as the starting point for the investigation of motor-mediated microtubule organization inside liposomes surrounded by a deformable lipid bilayer.

Reconstituted human polyclonal plasma-derived secretory-like IgM and IgA maintain the barrier function of epithelial cells infected with an enteropathogen

Intravenous administration of polyclonal and monoclonal antibodies has proven to be a clinically valid approach in the treatment, or at least relief, of many acute and chronic pathologies, such as infection, immunodeficiency, and a broad range of autoimmune conditions. Plasma-derived IgG or recombinant IgG are most frequently used for intravenous or subcutaneous administration, whereas a few IgM-based products are available as well. We have established recently that secretory-like IgA and IgM can be produced upon association of plasma-derived polymeric IgA and IgM with a recombinant secretory component. As a next step toward potential future mucosal administration, we sought to unravel the mechanisms by which these secretory Igs protect epithelial cells located at the interface between the environment and the inside of the body. By using polarized epithelial Caco-2 cell monolayers and Shigella flexneri as a model enteropathogen, we found that polyspecific plasma-derived SIgA and SIgM fulfill many protective functions, including dose-dependent recognition of the antigen via formation of aggregated immune complexes, reduction of bacterial infectivity, maintenance of epithelial cell integrity, and inhibition of proinflammatory cytokine/chemokine production by epithelial cells. In this in vitro model devoid of other cellular or molecular interfering partners, IgM and secretory IgM showed stronger bacterial neutralization than secretory IgA. Together, these data suggest that mucosally delivered antibody preparations may be most effective when combining both secretory-like IgA and IgM, which, together, play a crucial role in preserving several levels of epithelial cell integrity.

Phospholamban C-terminal residues are critical determinants of the structure and function of the calcium ATPase regulatory complex

To determine the structural and regulatory role of the C-terminal residues of phospholamban (PLB) in the membranes of living cells, we fused fluorescent protein tags to PLB and sarco/endoplasmic reticulum calcium ATPase (SERCA). Alanine substitution of PLB C-terminal residues significantly altered fluorescence resonance energy transfer (FRET) from PLB to PLB and SERCA to PLB, suggesting a change in quaternary conformation of PLB pentamer and SERCA-PLB regulatory complex. Val to Ala substitution at position 49 (V49A) had particularly large effects on PLB pentamer structure and PLB-SERCA regulatory complex conformation, increasing and decreasing probe separation distance, respectively. We also quantified a decrease in oligomerization affinity, an increase in binding affinity of V49A-PLB for SERCA, and a gain of inhibitory function as quantified by calcium-dependent ATPase activity. Notably, deletion of only a few C-terminal residues resulted in significant loss of PLB membrane anchoring and mislocalization to the cytoplasm and nucleus. C-terminal truncations also resulted in progressive loss of PLB-PLB FRET due to a decrease in the apparent affinity of PLB oligomerization. We quantified a similar decrease in the binding affinity of truncated PLB for SERCA and loss of inhibitory potency. However, despite decreased SERCA-PLB binding, intermolecular FRET for Val(49)-stop (V49X) truncation mutant was paradoxically increased as a result of an 11.3-Å decrease in the distance between donor and acceptor fluorophores. We conclude that PLB C-terminal residues are critical for localization, oligomerization, and regulatory function. In particular, the PLB C terminus is an important determinant of the quaternary structure of the SERCA regulatory complex.

The specific monomer/dimer equilibrium of the corticotropin-releasing factor receptor type 1 is established in the endoplasmic reticulum

G protein-coupled receptors (GPCRs) represent the most important drug targets. Although the smallest functional unit of a GPCR is a monomer, it became clear in the past decades that the vast majority of the receptors form dimers. Only very recently, however, data were presented that some receptors may in fact be expressed as a mixture of monomers and dimers and that the interaction of the receptor protomers is dynamic. To date, equilibrium measurements were restricted to the plasma membrane due to experimental limitations. We have addressed the question as to where this equilibrium is established for the corticotropin-releasing factor receptor type 1. By developing a novel approach to analyze single molecule fluorescence cross-correlation spectroscopy data for intracellular membrane compartments, we show that the corticotropin-releasing factor receptor type 1 has a specific monomer/dimer equilibrium that is already established in the endoplasmic reticulum (ER). It remains constant at the plasma membrane even following receptor activation. Moreover, we demonstrate for seven additional GPCRs that they are expressed in specific but substantially different monomer/dimer ratios. Although it is well known that proteins may dimerize in the ER in principle, our data show that the ER is also able to establish the specific monomer/dimer ratios of GPCRs, which sheds new light on the functions of this compartment.

Two dipolar α-helices within hormone-encoding regions of proglucagon are sorting signals to the regulated secretory pathway

Proglucagon is expressed in pancreatic α cells, intestinal L cells, and some hypothalamic and brainstem neurons. Tissue-specific processing of proglucagon yields three major peptide hormones as follows: glucagon in the α cells and glucagon-like peptides (GLP)-1 and -2 in the L cells and neurons. Efficient sorting and packaging into the secretory granules of the regulated secretory pathway in each cell type are required for nutrient-regulated secretion of these proglucagon-derived peptides. Our previous work suggested that proglucagon is directed into granules by intrinsic sorting signals after initial processing to glicentin and major proglucagon fragment (McGirr, R., Guizzetti, L., and Dhanvantari, S. (2013) J. Endocrinol. 217, 229-240), leading to the hypothesis that sorting signals may be present in multiple domains. In the present study, we show that the α-helices within glucagon and GLP-1, but not GLP-2, act as sorting signals by efficiently directing a heterologous secretory protein to the regulated secretory pathway. Biophysical characterization of these peptides revealed that glucagon and GLP-1 each encode a nonamphipathic, dipolar α-helix, whereas the helix in GLP-2 is not dipolar. Surprisingly, glicentin and major proglucagon fragment were sorted with different efficiencies, thus providing evidence that proglucagon is first sorted to granules prior to processing. In contrast to many other prohormones in which sorting is directed by ordered prodomains, the sorting determinants of proglucagon lie within the ordered hormone domains of glucagon and GLP-1, illustrating that each prohormone has its own sorting "signature."

Centrobin-centrosomal protein 4.1-associated protein (CPAP) interaction promotes CPAP localization to the centrioles during centriole duplication

Centriole duplication is the process by which two new daughter centrioles are generated from the proximal end of preexisting mother centrioles. Accurate centriole duplication is important for many cellular and physiological events, including cell division and ciliogenesis. Centrosomal protein 4.1-associated protein (CPAP), centrosomal protein of 152 kDa (CEP152), and centrobin are known to be essential for centriole duplication. However, the precise mechanism by which they contribute to centriole duplication is not known. In this study, we show that centrobin interacts with CEP152 and CPAP, and the centrobin-CPAP interaction is critical for centriole duplication. Although depletion of centrobin from cells did not have an effect on the centriolar levels of CEP152, it caused the disappearance of CPAP from both the preexisting and newly formed centrioles. Moreover, exogenous expression of the CPAP-binding fragment of centrobin also caused the disappearance of CPAP from both the preexisting and newly synthesized centrioles, possibly in a dominant negative manner, thereby inhibiting centriole duplication and the PLK4 overexpression-mediated centrosome amplification. Interestingly, exogenous overexpression of CPAP in the centrobin-depleted cells did not restore CPAP localization to the centrioles. However, restoration of centrobin expression in the centrobin-depleted cells led to the reappearance of centriolar CPAP. Hence, we conclude that centrobin-CPAP interaction is critical for the recruitment of CPAP to procentrioles to promote the elongation of daughter centrioles and for the persistence of CPAP on preexisting mother centrioles. Our study indicates that regulation of CPAP levels on the centrioles by centrobin is critical for preserving the normal size, shape, and number of centrioles in the cell.

Tight junctional localization of claudin-16 is regulated by syntaxin 8 in renal tubular epithelial cells

Claudin-16 (CLDN16) regulates the paracellular reabsorption of Mg(2+) in the thick ascending limb of Henle's loop. However, the mechanism regulating the tight junctional localization of CLDN16 remains unknown. In yeast two-hybrid systems, we found that CLDN16 bound to syntaxin 8 (STX8), a target soluble N-ethylmaleimide-sensitive factor attachment protein receptor. We have examined the effect of STX8 on the localization and function of CLDN16 using Madin-Darby canine kidney cells expressing FLAG-tagged CLDN16. A pulldown assay showed that the carboxyl cytoplasmic region of human CLDN16 bound to STX8. CLDN16 was localized in the thick ascending limb, whereas STX8 was widely distributed throughout the rat kidney. An association between CLDN16 and STX8 was observed in rat renal homogenates and Madin-Darby canine kidney cells. STX8 siRNA decreased the cell surface localization of CLDN16 and transepithelial electrical resistance and permeability to Mg(2+) but increased the co-localization of CLDN16 with early endosome and lysosome markers. Dephosphorylation of CLDN16 by protein kinase A inhibitors and S217A mutant, a dephosphorylated form, decreased the association with STX8 and the cell surface localization of CLDN16. Recycling assays indicated that STX8 siRNA decreased the trafficking of CLDN16 to the plasma membrane without affecting endocytosis. Dominant negative Rab11 and recycling inhibitor primaquine decreased the cell surface localization of CLDN16, which was similar to that in STX8 siRNA-transfected cells. These results suggest that STX8 mediates the recycling of CLDN16 and constitutes an important component of the CLDN16 trafficking machinery in the kidney.

A novel MitoTimer reporter gene for mitochondrial content, structure, stress, and damage in vivo

Mitochondrial dysfunction plays important roles in many diseases, but there is no satisfactory method to assess mitochondrial health in vivo. Here, we engineered a MitoTimer reporter gene from the existing Timer reporter gene. MitoTimer encodes a mitochondria-targeted green fluorescent protein when newly synthesized, which shifts irreversibly to red fluorescence when oxidized. Confocal microscopy confirmed targeting of the MitoTimer protein to mitochondria in cultured cells, Caenorhabditis elegans touch receptor neurons, Drosophila melanogaster heart and indirect flight muscle, and mouse skeletal muscle. A ratiometric algorithm revealed that conditions that cause mitochondrial stress led to a significant shift toward red fluorescence as well as accumulation of pure red fluorescent puncta of damaged mitochondria targeted for mitophagy. Long term voluntary exercise resulted in a significant fluorescence shift toward green, in mice and D. melanogaster, as well as significantly improved structure and increased content in mouse FDB muscle. In contrast, high-fat feeding in mice resulted in a significant shift toward red fluorescence and accumulation of pure red puncta in skeletal muscle, which were completely ameliorated by voluntary wheel running. Hence, MitoTimer allows for robust analysis of multiple parameters of mitochondrial health under both physiological and pathological conditions and will be highly useful for future research of mitochondrial health in multiple disciplines in vivo.

Claudin-3 and claudin-5 protein folding and assembly into the tight junction are controlled by non-conserved residues in the transmembrane 3 (TM3) and extracellular loop 2 (ECL2) segments

The mechanism of tight junction (TJ) assembly and the structure of claudins (Cldn) that form the TJ strands are unclear. This limits the molecular understanding of paracellular barriers and strategies for drug delivery across tissue barriers. Cldn3 and Cldn5 are both common in the blood-brain barrier but form TJ strands with different ultrastructures. To identify the molecular determinants of folding and assembly of these classic claudins, Cldn3/Cldn5 chimeric mutants were generated and analyzed by cellular reconstitution of TJ strands, live cell confocal imaging, and freeze-fracture electron microscopy. A comprehensive screening was performed on the basis of the rescue of mutants deficient for strand formation. Cldn3/Cldn5 residues in transmembrane segment 3, TM3 (Ala-127/Cys-128, Ser-136/Cys-137, Ser-138/Phe-139), and the transition of TM3 to extracellular loop 2, ECL2 (Thr-141/Ile-142) and ECL2 (Asn-148/Asp-149, Leu-150/Thr-151, Arg-157/Tyr-158), were identified to be involved in claudin folding and/or assembly. Blue native PAGE and FRET assays revealed 1% n-dodecyl β-d-maltoside-resistant cis-dimerization for Cldn5 but not for Cldn3. This homophilic interaction was found to be stabilized by residues in TM3. The resulting subtype-specific cis-dimer is suggested to be a subunit of polymeric TJ strands and contributes to the specific ultrastructure of the TJ detected by freeze-fracture electron microscopy. In particular, the Cldn5-like exoplasmic face-associated and particle-type strands were found to be related to cis-dimerization. These results provide new insight into the mechanisms of paracellular barrier formation by demonstrating that defined non-conserved residues in TM3 and ECL2 of classic claudins contribute to the formation of TJ strands with differing ultrastructures.

Phosphorylation of amyloid precursor protein at threonine 668 is essential for its copper-responsive trafficking in SH-SY5Y neuroblastoma cells

Amyloid precursor protein (APP) undergoes post-translational modification, including O- and N-glycosylation, ubiquitination, and phosphorylation as it traffics through the secretory pathway. We have previously reported that copper promotes a change in the cellular localization of APP. We now report that copper increases the phosphorylation of endogenous APP at threonine 668 (Thr-668) in SH-SY5Y neuronal cells. The level of APPT668-p (detected using a phospho-site-specific antibody) exhibited a copper-dependent increase. Using confocal microscopy imaging we demonstrate that the phospho-deficient mutant, Thr-668 to alanine (T668A), does not exhibit detectable copper-responsive APP trafficking. In contrast, mutating a serine to an alanine at residue 655 does not affect copper-responsive trafficking. We further investigated the importance of the Thr-668 residue in copper-responsive trafficking by treating SH-SY5Y cells with inhibitors for glycogen synthase kinase 3-β (GSK3β) and cyclin-dependent kinases (Cdk), the main kinases that phosphorylate APP at Thr-668 in neurons. Our results show that the GSK3β kinase inhibitors LiCl, SB 216763, and SB 415286 prevent copper-responsive APP trafficking. In contrast, the Cdk inhibitors Purvalanol A and B had no significant effect on copper-responsive trafficking in SH-SY5Y cells. In cultured primary hippocampal neurons, copper promoted APP re-localization to the axon, and this effect was inhibited by the addition of LiCl, indicating that a lithium-sensitive kinase(s) is involved in copper-responsive trafficking in hippocampal neurons. This is consistent with APP axonal transport to the synapse, where APP is involved in a number of functions. We conclude that copper promotes APP trafficking by promoting a GSK3β-dependent phosphorylation in SH-SY5Y cells.

Sequential delivery of synaptic GluA1- and GluA4-containing AMPA receptors (AMPARs) by SAP97 anchored protein complexes in classical conditioning

Multiple signaling pathways are involved in AMPAR trafficking to synapses during synaptic plasticity and learning. The mechanisms for how these pathways are coordinated in parallel but maintain their functional specificity involves subcellular compartmentalization of kinase function by scaffolding proteins, but how this is accomplished is not well understood. Here, we focused on characterizing the molecular machinery that functions in the sequential synaptic delivery of GluA1- and GluA4-containing AMPARs using an in vitro model of eyeblink classical conditioning. We show that conditioning induces the interaction of selective protein complexes with the key structural protein SAP97, which tightly regulates the synaptic delivery of GluA1 and GluA4 AMPAR subunits. The results demonstrate that in the early stages of conditioning the initial activation of PKA stimulates the formation of a SAP97-AKAP/PKA-GluA1 protein complex leading to synaptic delivery of GluA1-containing AMPARs through a SAP97-PSD95 interaction. This is followed shortly thereafter by generation of a SAP97-KSR1/PKC-GluA4 complex for GluA4 AMPAR subunit delivery again through a SAP97-PSD95 interaction. These data suggest that SAP97 forms the molecular backbone of a protein scaffold critical for delivery of AMPARs to the PSD during conditioning. Together, the findings reveal a cooperative interaction of multiple scaffolding proteins for appropriately timed delivery of subunit-specific AMPARs to synapses and support a sequential two-stage model of AMPAR synaptic delivery during classical conditioning.

In vivo confocal microscopy in chloroquine-induced keratopathy

In vivo confocal microscopy is becoming a mandatory examination to study corneal abnormalities such as drug deposits in systemic disease. A female diagnosed with fibromyalgia on systemic chloroquine for 9 months presented for an ophthalmic examination. Confocal microscopy was performed using the Confoscan 4 (Nidek Co. Ltd., Gamagori, Japan) and multiple highly reflective deposits in the epithelial basal cells were found, that were consistent with choloquine. Deposits were also present in the wing cell layer. In the anterior stroma these deposits were rare. Atypically shaped and branched nerves were also present in the anterior stroma. Corneal deposits of chloroquine can be evaluated by confocal microscopy. Confocal microscopy provides information on corneal metabolism and physiology. Chloroquine keratopathy can affect the anterior stroma in addition to the epithelium.

Sorting of β1-adrenergic receptors is mediated by pathways that are either dependent on or independent of type I PDZ, protein kinase A (PKA), and SAP97

The β1-adrenergic receptor (β1-AR) is a target for treatment of major cardiovascular diseases, such as heart failure and hypertension. Recycling of agonist-internalized β1-AR is dependent on type I PSD-95/DLG/ZO1 (PDZ) in the C-tail of the β1-AR and on protein kinase A (PKA) activity (Gardner, L. A., Naren, A. P., and Bahouth, S. W. (2007) J. Biol. Chem. 282, 5085-5099). We explored the effects of point mutations in the PDZ and in the activity of PKA on recycling of the β1-AR and its binding to the PDZ-binding protein SAP97. These studies indicated that β1-AR recycling was inhibited by PKA inhibitors and by mutations in the PDZ that interfered with SAP97 binding. The trafficking effects of short sequences differing in PDZ and SAP97 binding were examined using chimeric mutant β1-AR. β1-AR chimera containing the type I PDZ of the β2-adrenergic receptor that does not bind to SAP97 failed to recycle except when serine 312 was mutated to aspartic acid. β1-AR chimera with type I PDZ sequences from the C-tails of aquaporin-2 or GluR1 recycled in a SAP97- and PKA-dependent manner. Non-PDZ β1-AR chimera derived from μ-opioid, dopamine 1, or GluR2 receptors promoted rapid recycling of chimeric β1-AR in a SAP97- and PKA-independent manner. Moreover, the nature of the residue at position -3 in the PDZ regulated whether the β1-AR was internalized alone or in complex with SAP97. These results indicate that divergent pathways were involved in trafficking the β1-AR and provide a roadmap for its trafficking via type I PDZs versus non-PDZs.

Glu-44 in the amino-terminal α-helix of yeast vacuolar ATPase E subunit (Vma4p) has a role for VoV1 assembly

The proton (H(+)) pumping vacuolar-type ATPase (V-ATPase) is a rotary enzyme that plays a pivotal role in forming intracellular acidic compartments in eukaryotic cells. In Saccharomyces cerevisiae, the membrane extrinsic catalytic V1 and the transmembrane proton-pumping Vo complexes have been shown to reversibly dissociate upon removal of glucose from the medium. However, the basis of this disassembly is largely unknown. In the earlier study, we have found that the amino-terminal α-helical domain between Lys-33 and Lys-83 of yeast E subunit (Vma4p) in the peripheral stalk of the V1 complex has a role in glucose-dependent VoV1 assembly. Results of alanine-scanning mutagenesis within the domain revealed that the Vma4p Glu-44 is a key residue in VoV1 disassembly. Biochemical analysis on Vma4p Glu-44 to Ala, Asn, Asp, and Gln substitutions indicated that Glu-44 has a role in V-ATPase catalysis. These results suggest that Glu-44 is one of the key functional residues for subunit interaction in the V-ATPase stalk complex that allows both efficient rotation catalysis and assembly.

[Ca2+]i elevation and oxidative stress induce KCNQ1 protein translocation from the cytosol to the cell surface and increase slow delayed rectifier (IKs) in cardiac myocytes

Our goals are to simultaneously determine the three-dimensional distribution patterns of KCNQ1 and KCNE1 in cardiac myocytes and to study the mechanism and functional implications for variations in KCNQ1/KCNE1 colocalization in myocytes. We monitored the distribution patterns of KCNQ1, KCNE1, and markers for subcellular compartments/organelles using immunofluorescence/confocal microscopy and confirmed the findings in ventricular myocytes by directly observing fluorescently tagged KCNQ1-GFP and KCNE1-dsRed expressed in these cells. We also monitored the effects of stress on KCNQ1-GFP and endoplasmic reticulum (ER) remodeling during live cell imaging. The data showed that 1) KCNE1 maintained a stable cell surface localization, whereas KCNQ1 exhibited variations in the cytosolic compartment (striations versus vesicles) and the degree of presence on the cell surface; 2) the degree of cell surface KCNQ1/KCNE1 colocalization was positively correlated with slow delayed rectifier (IKs) current density; 3) KCNQ1 and calnexin (an ER marker) shared a cytosolic compartment; and 4) in response to stress ([Ca(2+)]i elevation, oxidative overload, or AT1R stimulation), KCNQ1 exited the cytosolic compartment and trafficked to the cell periphery in vesicles. This was accompanied by partial ER fragmentation. We conclude that the cellular milieu regulates KCNQ1 distribution in cardiac myocytes and that stressful conditions can increase IKs by inducing KCNQ1 movement to the cell surface. This represents a hitherto unrecognized mechanism by which IKs fulfills its function as a repolarization reserve in ventricular myocytes.

Proapoptotic Bax and Bak proteins form stable protein-permeable pores of tunable size

The Bcl-2 proapoptotic proteins Bax and Bak mediate the permeabilization of the mitochondrial outer membrane during apoptosis. Current models consider that Bax and Bak form pores at the mitochondrial outer membrane that are responsible for the release of cytochrome c and other larger mitochondrial apoptotic factors (i.e. Smac/DIABLO, AIF, and endoglycosidase G). However, the properties and nature of Bax/Bak apoptotic pores remain enigmatic. Here, we performed a detailed analysis of the membrane permeabilizing activity of Bax and Bak at the single vesicle level. We directly visualized that cBid-activated Bax and BakΔC21 can form membrane pores large enough to release not only cytochrome c, but also allophycocyanine, a protein of 104 kDa. Interestingly, the size of Bax and BakΔC21 pores is not constant, as typically observed in purely proteinaceous channels, but evolves with time and depends on protein concentration. We found that Bax and BakΔC21 formed long-lived pores, whose areas changed with the amount of Bax/BakΔC21 but not with cardiolipin concentration. Altogether, our results demonstrate that Bax and BakΔC21 follow similar mechanisms of membrane permeabilization characterized by the formation of protein-permeable pores of dynamic size, in agreement with the proteolipidic nature of these apoptotic pores.

LMBD1 protein serves as a specific adaptor for insulin receptor internalization

Energy homeostasis is crucial for maintaining normally functioning cells; disturbances in this balance often cause various diseases. The limb region 1 (LMBR1) domain containing 1 gene (lmbrd1) encodes the LMBD1 protein that possesses 9 putative transmembrane domains. LMBD1 has been suggested to be involved in the lysosome in aiding the export of cobalamin. In this study, we determined that LMBD1 plays a regulatory role in the plasma membrane. A micro-positron emission tomography analysis showed that a single-allele knock-out of lmbrd1 increased the (18)F-fluorodeoxyglucose uptake in murine hearts. In addition, the knockdown of lmbrd1 resulted in an up-regulated signaling of the insulin receptor (IR) and its downstream signaling molecule, Akt. Confocal and live total internal reflection fluorescence microscopy showed that LMBD1 co-localized and co-internalized with clathrin and the IR, but not with the transferrin receptor. The results of the mutation analysis and phenotypic rescue experiments indicate that LMBD1 interacts with adaptor protein-2 and is involved in the unique clathrin-mediated endocytosis of the IR. LMBD1 selectively interacts with the IR. The knockdown of lmbrd1 attenuated IR endocytosis, resulting in the perturbation of the IR recycling pathway and consequential enhancement of the IR signaling cascade. In summary, LMBD1 plays an imperative role in mediating and regulating the endocytosis of the IR.

Increased expression of reticulon 3 in neurons leads to reduced axonal transport of β site amyloid precursor protein-cleaving enzyme 1

BACE1 is the sole enzyme responsible for cleaving amyloid precursor protein at the β-secretase site, and this cleavage initiates the generation of β-amyloid peptide (Aβ). Because amyloid precursor protein is predominantly expressed by neurons and deposition of Aβ aggregates in the human brain is highly correlated with the Aβ released at axonal terminals, we focused our investigation of BACE1 localization on the neuritic region. We show that BACE1 was not only enriched in the late Golgi, trans-Golgi network, and early endosomes but also in both axons and dendrites. BACE1 was colocalized with the presynaptic vesicle marker synaptophysin, indicating the presence of BACE1 in synapses. Because the excessive release of Aβ from synapses is attributable to an increase in amyloid deposition, we further explored whether the presence of BACE1 in synapses was regulated by reticulon 3 (RTN3), a protein identified previously as a negative regulator of BACE1. We found that RTN3 is not only localized in the endoplasmic reticulum but also in neuritic regions where no endoplasmic reticulum-shaping proteins are detected, implicating additional functions of RTN3 in neurons. Coexpression of RTN3 with BACE1 in cultured neurons was sufficient to reduce colocalization of BACE1 with synaptophysin. This reduction correlated with decreased anterograde transport of BACE1 in axons in response to overexpressed RTN3. Our results in this study suggest that altered RTN3 levels can impact the axonal transport of BACE1 and demonstrate that reducing axonal transport of BACE1 in axons is a viable strategy for decreasing BACE1 in axonal terminals and, perhaps, reducing amyloid deposition.

FAD synthesis and degradation in the nucleus create a local flavin cofactor pool

FAD is a redox cofactor ensuring the activity of many flavoenzymes mainly located in mitochondria but also relevant for nuclear redox activities. The last enzyme in the metabolic pathway producing FAD is FAD synthase (EC 2.7.7.2), a protein known to be localized both in cytosol and in mitochondria. FAD degradation to riboflavin occurs via still poorly characterized enzymes, possibly belonging to the NUDIX hydrolase family. By confocal microscopy and immunoblotting experiments, we demonstrate here the existence of FAD synthase in the nucleus of different experimental rat models. HPLC experiments demonstrated that isolated rat liver nuclei contain ∼300 pmol of FAD·mg(-1) protein, which was mainly protein-bound FAD. A mean FAD synthesis rate of 18.1 pmol·min(-1)·mg(-1) protein was estimated by both HPLC and continuous coupled enzymatic spectrophotometric assays. Rat liver nuclei were also shown to be endowed with a FAD pyrophosphatase that hydrolyzes FAD with an optimum at alkaline pH and is significantly inhibited by adenylate-containing nucleotides. The coordinate activity of these FAD forming and degrading enzymes provides a potential mechanism by which a dynamic pool of flavin cofactor is created in the nucleus. These data, which significantly add to the biochemical comprehension of flavin metabolism and its subcellular compartmentation, may also provide the basis for a more detailed comprehension of the role of flavin homeostasis in biologically and clinically relevant epigenetic events.

Adaptor proteins MiD49 and MiD51 can act independently of Mff and Fis1 in Drp1 recruitment and are specific for mitochondrial fission

Drp1 (dynamin-related protein 1) is recruited to both mitochondrial and peroxisomal membranes to execute fission. Fis1 and Mff are Drp1 receptor/effector proteins of mitochondria and peroxisomes. Recently, MiD49 and MiD51 were also shown to recruit Drp1 to the mitochondrial surface; however, different reports have ascribed opposing roles in fission and fusion. Here, we show that MiD49 or MiD51 overexpression blocked fission by acting in a dominant-negative manner by sequestering Drp1 specifically at mitochondria, causing unopposed fusion events at mitochondria along with elongation of peroxisomes. Mitochondrial elongation caused by MiD49/51 overexpression required the action of fusion mediators mitofusins 1 and 2. Furthermore, at low level overexpression when MiD49 and MiD51 form discrete foci at mitochondria, mitochondrial fission events still occurred. Unlike Fis1 and Mff, MiD49 and MiD51 were not targeted to the peroxisomal surface, suggesting that they specifically act to facilitate Drp1-directed fission at mitochondria. Moreover, when MiD49 or MiD51 was targeted to the surface of peroxisomes or lysosomes, Drp1 was specifically recruited to these organelles. Moreover, the Drp1 recruitment activity of MiD49/51 appeared stronger than that of Mff or Fis1. We conclude that MiD49 and MiD51 can act independently of Mff and Fis1 in Drp1 recruitment and suggest that they provide specificity to the division of mitochondria.

Interaction between basic residues of Epstein-Barr virus EBNA1 protein and cellular chromatin mediates viral plasmid maintenance

The Epstein-Barr virus (EBV) genome is episomally maintained in latently infected cells. The viral protein EBNA1 is a bridging molecule that tethers EBV episomes to host mitotic chromosomes as well as to interphase chromatin. EBNA1 localizes to cellular chromosomes (chromatin) via its chromosome binding domains (CBDs), which are rich in glycine and arginine residues. However, the molecular mechanism by which the CBDs of EBNA1 attach to cellular chromatin is still under debate. Mutation analyses revealed that stepwise substitution of arginine residues within the CBD1 (amino acids 40-54) and CBD2 (amino acids 328-377) regions with alanines progressively impaired chromosome binding activity of EBNA1. The complete arginine-to-alanine substitutions within the CBD1 and -2 regions abolished the ability of EBNA1 to stably maintain EBV-derived oriP plasmids in dividing cells. Importantly, replacing the same arginines with lysines had minimal effect, if any, on chromosome binding of EBNA1 as well as on its ability to stably maintain oriP plasmids. Furthermore, a glycine-arginine-rich peptide derived from the CBD1 region bound to reconstituted nucleosome core particles in vitro, as did a glycine-lysine rich peptide, whereas a glycine-alanine rich peptide did not. These results support the idea that the chromosome binding of EBNA1 is mediated by electrostatic interactions between the basic amino acids within the CBDs and negatively charged cellular chromatin.

Altered cortical GABAA receptor composition, physiology, and endocytosis in a mouse model of a human genetic absence epilepsy syndrome

Patients with generalized epilepsy exhibit cerebral cortical disinhibition. Likewise, mutations in the inhibitory ligand-gated ion channels, GABAA receptors (GABAARs), cause generalized epilepsy syndromes in humans. Recently, we demonstrated that heterozygous knock-out (Hetα1KO) of the human epilepsy gene, the GABAAR α1 subunit, produced absence epilepsy in mice. Here, we determined the effects of Hetα1KO on the expression and physiology of GABAARs in the mouse cortex. We found that Hetα1KO caused modest reductions in the total and surface expression of the β2 subunit but did not alter β1 or β3 subunit expression, results consistent with a small reduction of GABAARs. Cortices partially compensated for Hetα1KO by increasing the fraction of residual α1 subunit on the cell surface and by increasing total and surface expression of α3, but not α2, subunits. Co-immunoprecipitation experiments revealed that Hetα1KO increased the fraction of α1 subunits, and decreased the fraction of α3 subunits, that associated in hybrid α1α3βγ receptors. Patch clamp electrophysiology studies showed that Hetα1KO layer VI cortical neurons exhibited reduced inhibitory postsynaptic current peak amplitudes, prolonged current rise and decay times, and altered responses to benzodiazepine agonists. Finally, application of inhibitors of dynamin-mediated endocytosis revealed that Hetα1KO reduced base-line GABAAR endocytosis, an effect that probably contributes to the observed changes in GABAAR expression. These findings demonstrate that Hetα1KO exerts two principle disinhibitory effects on cortical GABAAR-mediated inhibitory neurotransmission: 1) a modest reduction of GABAAR number and 2) a partial compensation with GABAAR isoforms that possess physiological properties different from those of the otherwise predominant α1βγ GABAARs.

The DNAJB6 and DNAJB8 protein chaperones prevent intracellular aggregation of polyglutamine peptides

Fragments of proteins containing an expanded polyglutamine (polyQ) tract are thought to initiate aggregation and toxicity in at least nine neurodegenerative diseases, including Huntington's disease. Because proteasomes appear unable to digest the polyQ tract, which can initiate intracellular protein aggregation, preventing polyQ peptide aggregation by chaperones should greatly improve polyQ clearance and prevent aggregate formation. Here we expressed polyQ peptides in cells and show that their intracellular aggregation is prevented by DNAJB6 and DNAJB8, members of the DNAJ (Hsp40) chaperone family. In contrast, HSPA/Hsp70 and DNAJB1, also members of the DNAJ chaperone family, did not prevent peptide-initiated aggregation. Intriguingly, DNAJB6 and DNAJB8 also affected the soluble levels of polyQ peptides, indicating that DNAJB6 and DNAJB8 inhibit polyQ peptide aggregation directly. Together with recent data showing that purified DNAJB6 can suppress fibrillation of polyQ peptides far more efficiently than polyQ expanded protein fragments in vitro, we conclude that the mechanism of DNAJB6 and DNAJB8 is suppression of polyQ protein aggregation by directly binding the polyQ tract.

Auxiliary KChIP4a suppresses A-type K+ current through endoplasmic reticulum (ER) retention and promoting closed-state inactivation of Kv4 channels

In the brain and heart, auxiliary Kv channel-interacting proteins (KChIPs) co-assemble with pore-forming Kv4 α-subunits to form a native K(+) channel complex and regulate the expression and gating properties of Kv4 currents. Among the KChIP1-4 members, KChIP4a exhibits a unique N terminus that is known to suppress Kv4 function, but the underlying mechanism of Kv4 inhibition remains unknown. Using a combination of confocal imaging, surface biotinylation, and electrophysiological recordings, we identified a novel endoplasmic reticulum (ER) retention motif, consisting of six hydrophobic and aliphatic residues, 12-17 (LIVIVL), within the KChIP4a N-terminal KID, that functions to reduce surface expression of Kv4-KChIP complexes. This ER retention capacity is transferable and depends on its flanking location. In addition, adjacent to the ER retention motif, the residues 19-21 (VKL motif) directly promote closed-state inactivation of Kv4.3, thus leading to an inhibition of channel current. Taken together, our findings demonstrate that KChIP4a suppresses A-type Kv4 current via ER retention and enhancement of Kv4 closed-state inactivation.

The effects of blunt trauma and cataract surgery on corneal endothelial cell density

Purpose:

This study was designed to investigate the effects of trauma and cataract surgery on corneal endothelial cell density (ECD) in patients with a traumatic cataract due to blunt trauma without globe laceration.

Materials and Methods:

In this prospective study, 31 subjects with traumatic cataract (traumatic cataract group) and 30 subjects with a senile cataract (control group) were enrolled. The subjects with traumatic cataract were subdivided into two groups: uncomplicated surgery subgroup (n = 19) in which subjects underwent standard phacoemulsification with intraocular lens implantation and complicated surgery subgroup (n = 12) in which subjects underwent cataract surgery other than standard phacoemulsification. The ECD of the traumatic cataract group and the control group was compared preoperatively and at 3 months or later postoperatively. A P value less than 0.05 was considered statistically significant.

Results:

The ECD in the eyes with traumatic cataract was 13.1% lower than that for healthy eyes preoperatively (P = 0.043). Postsurgical ECD decreased by 16.7% in complicated surgery subgroup and 11.9% in uncomplicated surgery subgroup (P = 0.049) after 3 months postoperatively. The ECD decreased by 10.8% in the control group (P = 0.489).

Conclusions:

Patients with cataracts due to blunt trauma had a decreased endothelial cell count, which was significantly aggravated by cataract surgery. The loss of corneal endothelium cells due to surgery depends on the surgical approach.