TMEM41B is a novel regulator of autophagy and lipid mobilization.

Autophagy maintains cellular homeostasis by targeting damaged organelles, pathogens, or misfolded protein aggregates for lysosomal degradation. The autophagic process is initiated by the formation of autophagosomes, which can selectively enclose cargo via autophagy cargo receptors. A machinery of well-characterized autophagy-related proteins orchestrates the biogenesis of autophagosomes; however, the origin of the required membranes is incompletely understood. Here, we have applied sensitized pooled CRISPR screens and identify the uncharacterized transmembrane protein TMEM41B as a novel regulator of autophagy. In the absence of TMEM41B, autophagosome biogenesis is stalled, LC3 accumulates at WIPI2- and DFCP1-positive isolation membranes, and lysosomal flux of autophagy cargo receptors and intracellular bacteria is impaired. In addition to defective autophagy, TMEM41B knockout cells display significantly enlarged lipid droplets and reduced mobilization and ß-oxidation of fatty acids. Immunostaining and interaction proteomics data suggest that TMEM41B localizes to the endoplasmic reticulum (ER). Taken together, we propose that TMEM41B is a novel ER-localized regulator of autophagosome biogenesis and lipid mobilization.

X-ray structure of the mouse serotonin 5-HT3 receptor.

Neurotransmitter-gated ion channels of the Cys-loop receptor family mediate fast neurotransmission throughout the nervous system. The molecular processes of neurotransmitter binding, subsequent opening of the ion channel and ion permeation remain poorly understood. Here we present the X-ray structure of a mammalian Cys-loop receptor, the mouse serotonin 5-HT3 receptor, at 3.5 Å resolution. The structure of the proteolysed receptor, made up of two fragments and comprising part of the intracellular domain, was determined in complex with stabilizing nanobodies. The extracellular domain reveals the detailed anatomy of the neurotransmitter binding site capped by a nanobody. The membrane domain delimits an aqueous pore with a 4.6 Å constriction. In the intracellular domain, a bundle of five intracellular helices creates a closed vestibule where lateral portals are obstructed by loops. This 5-HT3 receptor structure, revealing part of the intracellular domain, expands the structural basis for understanding the operating mechanism of mammalian Cys-loop receptors.

Thermal unfolding of a mammalian pentameric ligand-gated ion channel proceeds at consecutive, distinct steps.

Pentameric ligand-gated ion channels (LGICs) play an important role in fast synaptic signal transduction. Binding of agonists to the ß-sheet-structured extracellular domain opens an ion channel in the transmembrane α-helical region of the LGIC. How the structurally distinct and distant domains are functionally coupled for such central transmembrane signaling processes remains an open question. To obtain detailed information about the stability of and the coupling between these different functional domains, we analyzed the thermal unfolding of a homopentameric LGIC, the 5-hydroxytryptamine receptor (ligand binding, secondary structure, accessibility of Trp and Cys residues, and aggregation), in plasma membranes as well as during detergent extraction, purification, and reconstitution into artificial lipid bilayers. We found a large loss in thermostability correlating with the loss of the lipid bilayer during membrane solubilization and purification. Thermal unfolding of the 5-hydroxytryptamine receptor occurred in consecutive steps at distinct protein locations. A loss of ligand binding was detected first, followed by formation of different transient low oligomeric states of receptor pentamers, followed by partial unfolding of helical parts of the protein, which finally lead to the formation receptor aggregates. Structural destabilization of the receptor in detergents could be partially reversed by reconstituting the receptor into lipid bilayers. Our results are important because they quantify the stability of LGICs during detergent extraction and purification and can be used to create stabilized receptor proteins for structural and functional studies.

Large scale expression and purification of the mouse 5-HT3 receptor.

Receptors of the Cys-loop family are central to neurotransmission and primary therapeutic targets. In order to decipher their gating and modulation mechanisms, structural data is essential. However, structural studies require large amounts of pure, functional receptors. Here, we present the expression and purification of the mouse serotonin 5-HT3 receptor to high purity and homogeneity levels. Inducible expression in human embryonic kidney 293 cells in suspension cultures with orbital shaking resulted in yields of 6-8mg receptor per liter of culture. Affinity purification using a strep tag provided pure protein in active form. Further deglycosylation and removal of the purification tag led to a pentameric receptor after size-exclusion chromatography, at the milligram scale. This material is suitable for crystallography, as demonstrated by X-ray diffraction of receptor crystals at low resolution.

Systematic characterization of extracellular vesicle sorting domains and quantification at the single molecule - single vesicle level by fluorescence correlation spectroscopy and single particle imaging.

Extracellular vesicles (EV) convey biological information by transmitting macromolecules between cells and tissues and are of great promise as pharmaceutical nanocarriers, and as therapeutic per se. Strategies for customizing the EV surface and cargo are being developed to enable their tracking, visualization, loading with pharmaceutical agents and decoration of the surface with tissue targeting ligands. While much progress has been made in the engineering of EVs, an exhaustive comparative analysis of the most commonly exploited EV-associated proteins, as well as a quantification at the molecular level are lacking. Here, we selected 12 EV-related proteins based on MS-proteomics data for comparative quantification of their EV engineering potential. All proteins were expressed with fluorescent protein (FP) tags in EV-producing cells; both parent cells as well as the recovered vesicles were characterized biochemically and biophysically. Using Fluorescence Correlation Spectroscopy (FCS) we quantified the number of FP-tagged molecules per vesicle. We observed different loading efficiencies and specificities for the different proteins into EVs. For the candidates showing the highest loading efficiency in terms of engineering, the molecular levels in the vesicles did not exceed ca 40-60 fluorescent proteins per vesicle upon transient overexpression in the cells. Some of the GFP-tagged EV reporters showed quenched fluorescence and were either non-vesicular, despite co-purification with EVs, or comprised a significant fraction of truncated GFP. The co-expression of each target protein with CD63 was further quantified by widefield and confocal imaging of single vesicles after double transfection of parent cells. In summary, we provide a quantitative comparison for the most commonly used sorting proteins for bioengineering of EVs and introduce a set of biophysical techniques for straightforward quantitative and qualitative characterization of fluorescent EVs to link single vesicle analysis with single molecule quantification.

A nanocompartment system (Synthosome) designed for biotechnological applications.

A nanocompartment system based on two deletion mutants of the large channel protein FhuA (FhuA Delta1-129; FhuA Delta1-160) and an ABA triblock copolymer (PMOXA-PDMS-PMOXA) has been developed for putative biotechnological applications. FhuA is ideally suited for applications in biotechnology due to its monomeric structure, large pore diameter (39-46 A elliptical cross-section) that ensures rapid compound flux, and solved crystallographic structure. Two areas of application were targeted as proof of principle: (A) selective product recovery in nanocompartments and (B) enzymatic conversion in nanocompartments. Selective recovery of negatively charged compounds has been achieved on the example of sulforhodamine B by using positively charged polylysine molecules as trap inside the nanocompartment. Conversion in nanocompartments has been achieved by 3,3',5,5'-tetramethylbenzidine oxidation employing horseradish peroxidase (HRP).

Asymmetric ABC-triblock copolymer membranes induce a directed insertion of membrane proteins.

Asymmetric molecules and materials provide an important basis for the organization and function of biological systems. It is well known that, for example, the inner and outer leaflets of biological membranes are strictly asymmetric with respect to lipid composition and distribution. This plays a crucial role for many membrane-related processes like carrier-mediated transport or insertion and orientation of integral membrane proteins. Most artificial membrane systems are, however, symmetric with respect to their midplane and membrane proteins are incorporated with random orientation. Here we describe a new approach to induce a directed insertion of membrane proteins into asymmetric membranes formed by amphiphilic ABC triblock copolymers with two chemically different water-soluble blocks A and C. In a comparative study we have reconstituted His-tag labeled Aquaporin 0 in lipid, ABA block copolymer, and ABC block copolymer vesicles. Immunolabeling, colorimetric, and fluorescence studies clearly show that a preferential orientation of the protein is only observed in the asymmetric ABC triblock copolymer membranes.

Disease modeling and phenotypic drug screening for diabetic cardiomyopathy using human induced pluripotent stem cells.

Diabetic cardiomyopathy is a complication of type 2 diabetes, with known contributions of lifestyle and genetics. We develop environmentally and genetically driven in vitro models of the condition using human-induced-pluripotent-stem-cell-derived cardiomyocytes. First, we mimic diabetic clinical chemistry to induce a phenotypic surrogate of diabetic cardiomyopathy, observing structural and functional disarray. Next, we consider genetic effects by deriving cardiomyocytes from two diabetic patients with variable disease progression. The cardiomyopathic phenotype is recapitulated in the patient-specific cells basally, with a severity dependent on their original clinical status. These models are incorporated into successive levels of a screening platform, identifying drugs that preserve cardiomyocyte phenotype in vitro during diabetic stress. In this work, we present a patient-specific induced pluripotent stem cell (iPSC) model of a complex metabolic condition, showing the power of this technique for discovery and testing of therapeutic strategies for a disease with ever-increasing clinical significance.

Peptide nanoparticles serve as a powerful platform for the immunogenic display of poorly antigenic actin determinants.

The role of actin in transcription and RNA processing is now widely accepted but the form of nuclear actin remains enigmatic. Monomeric, oligomeric or polymeric forms of actin seem to be involved in nuclear functions. Moreover, uncommon forms of actin such as the "lower dimer" have been observed in vitro. Antibodies have been pivotal in revealing the presence and distribution of different forms of actin in different cellular locations. Because of its high degree of conservation, actin is a poor immunogen and only few specific actin antibodies are available. To unravel the mystery of less common forms of actin, in particular those in the nucleus, we chose to tailor monoclonal antibodies to recognize distinct forms of actin. To increase the immune response, we used a new approach based on peptide nanoparticles, which are designed to mimic an icosahedral virus capsid and allow the repetitive, ordered display of a specific epitope on their surface. Actin sequences representing the highly conserved "hydrophobic loop," which is buried in the filamentous actin filament, were grafted onto the surface of nanoparticles by genetic engineering. After immunization with "loop nanoparticles," a number of monoclonal antibodies were established that bind to the hydrophobic loop both in vitro and in situ. Immunofluorescence studies on cells revealed that filamentous actin filaments were only labeled once the epitope had been exposed. Our studies indicate that self-assembling peptide nanoparticles represent a versatile platform that can easily be customized to present antigenic determinants in repetitive, ordered arrays and elicit an immune response against poor antigens.

Virus-assisted loading of polymer nanocontainer.

We present a DNA-containing polymeric nanocontainer using the self-assembled superstructure of amphiphilic block copolymers in aqueous solutions. To demonstrate that DNA translocation is possible across a completely synthetic block copolymer membrane, we have used a phage transfection strategy as a DNA-transfer model system. For this purpose the bacterial channel forming protein LamB was reconstituted in ABA-triblock copolymer vesicles. The outer membrane protein LamB is a specific transporter for maltodextrins but also serves as a receptor for lambda phage to trigger the ejection of lambda phage DNA. We demonstrate that the functionality of the LamB protein is fully preserved despite the artificial surrounding. This leads to a type of polymeric vehicle for DNA that could be useful for gene therapy.