Immobilization Strategies for Functional Complement Convertase Assembly at Lipid Membrane Interfaces

Challenges in diagnostic testing of nephritic factors

Nephritic factors (NeFs) are autoantibodies promoting the activity of the central enzymes of the complement cascade, an important first line of defense of our innate immune system. NeFs stabilize the complement convertase complexes and prevent their natural and regulator-mediated decay. They are mostly associated with rare complement-mediated kidney disorders, in particular with C3 glomerulopathy and related diseases. Although these autoantibodies were already described more than 50 years ago, measuring NeFs for diagnostic purposes remains difficult, and this also complicates our understanding of their clinical associations. In this review, we address the multifactorial challenges of NeF diagnostics. We describe the diseases NeFs are associated with, the heterogenic mechanisms of action of different NeF types, the different methods available in laboratories used for their detection, and efforts for standardization. Finally, we discuss the importance of proper NeF diagnostics for understanding the clinical impact of these autoantibodies in disease pathophysiology and for considering future complement-directed therapy.

Immobilization of arrestin-3 on different biosensor platforms for evaluating GPCR binding

G protein-coupled receptors (GPCRs) are a large and ubiquitous family of membrane receptors of great pharmacological interest. Cell-based assays are the primary tool for assessing GPCR interactions and activation but their design and intrinsic complexity limit their application. Biosensor-based assays that directly and specifically report GPCR-protein binding (e.g. arrestin or G protein) could provide a good alternative. We present an approach based on the stable immobilization of different arrestin-3 proteins (wild type, and two mutants, mutant X (arrestin-3 I386A) and mutant Y (arrestin-3 R393E)) via histidine tags on NTA(Ni2+)-coated sensors in a defined orientation. Using biolayer interferometry (BLI), surface plasmon resonance (SPR), and quartz crystal microbalance with dissipation (QCM-D), we were able to follow the interaction between the different arrestin-3 proteins and a representative GPCR, jumping spider rhodopsin-1 (JSR1), in a label-free manner in real-time. The interactions were quantified as binding affinity, association and dissociation rate constants. The combination of surface-based biosensing methods indicated that JSR1 showed the strongest binding to arrestin mutant Y. Taken together, this work introduces direct label-free, biosensor-based screening approaches that can be easily adapted for testing interactions of proteins and other compounds with different GPCRs.

Materials Nanoarchitectonics for Mechanical Tools in Chemical and Biological Sensing

In this Focus Review, nanoarchitectonic approaches for mechanical-action-based chemical and biological sensors are briefly discussed. In particular, recent examples of piezoelectric devices, such as quartz crystal microbalances (QCM and QCM-D) and a membrane-type surface stress sensor (MSS), are introduced. Sensors need well-designed nanostructured sensing materials for the sensitive and selective detection of specific targets. Nanoarchitectonic approaches for sensing materials, such as mesoporous materials, 2D materials, fullerene assemblies, supported lipid bilayers, and layer-by-layer assemblies, are highlighted. Based on these sensing approaches, examples of bioanalytical applications are presented for toxic gas detection, cell membrane interactions, label-free biomolecular assays, anticancer drug evaluation, complement activation-related multiprotein membrane attack complexes, and daily biodiagnosis, which are partially supported by data analysis, such as machine learning and principal component analysis.

Morphology and Physical Properties of Hydrophilic-Polymer-Modified Lipids in Supported Lipid Bilayers

Lipid molecules such as glycolipids that are modified with hydrophilic biopolymers participate in the biochemical reactions occurring on cell membranes. Their functions and efficiency are determined by the formation of microdomains and their physical properties. We investigated the morphology and properties of domains induced by the hydrophilic-polymer-modified lipid applying a polyethylene glycol (PEG)-modified lipid as a model modified lipid. We formed supported lipid bilayers (SLBs) using a 0-10 mol % range of PEG-modified lipid concentration ( C_PEG). We studied their morphology and fluidity by fluorescence microscopy, the fluorescence recovery after photobleaching method, and atomic force microscopy (AFM). Fluorescence images showed that domains rich in the PEG-modified lipid appeared and SLB fluidity decreased when C_PEG ≥ 5%. AFM topographies showed that clusters of the PEG-modified lipid appeared prior to domain formation and the PEG-lipid-rich domains were observed as depressions. Frequency-modulation AFM revealed a force-dependent appearance of the PEG-lipid-rich domain.