Protein micropatterning enables robust control of cell positioning on electron-microscopy substrates for cryogenic electron tomography (cryo-ET). However, the combination of regulated cell boundaries and the underlying electron-microscopy substrate (EM-grids) provides a poorly understood microenvironment for cell biology. Because substrate stiffness and morphology affect cellular behavior, we devised protocols to characterize the nanometer-scale details of the protein micropatterns on EM-grids by combining cryo-ET, atomic force microscopy, and scanning electron microscopy. Measuring force displacement characteristics of holey carbon EM-grids, we found that their effective spring constant is similar to physiological values expected from skin tissues. Despite their apparent smoothness at light-microscopy resolution, spatial boundaries of the protein micropatterns are irregular at nanometer scale. Our protein micropatterning workflow provides the means to steer both positioning and morphology of cell doublets to determine nanometer details of punctate adherens junctions. Our workflow serves as the foundation for studying the fundamental structural changes governing cell-cell signaling.
Nanodiscs have become a popular tool in structure determination of membrane proteins using cryogenic electron microscopy and single particle analysis. However, the structure determination of small membrane proteins remains challenging. When the embedded protein is in the same size range as the nanodisc, the nanodisc can significantly contribute to the alignment and classification during the structure determination process. In those cases, it is crucial to minimize the heterogeneity in the nanodisc preparations to assure maximum accuracy in the classification and alignment steps of single particle analysis. Here, we introduce a new in-silico method for the characterization of nanodisc samples that is based on analyzing the Feret diameter distribution of their particle projection as imaged in the electron microscope. We validated the method with comprehensive simulation studies and show that Feret signatures can detect subtle differences in nanodisc morphologies and composition that might otherwise go unnoticed. We used the method to identify a specific biochemical nanodisc preparation with low size variations, allowing us to obtain a structure of the 23-kDa single-span membrane protein Bcl-xL while embedded in a nanodisc. Feret signature analysis can steer experimental data collection strategies, allowing more efficient use of high-end data collection hardware, as well as image analysis investments in studies where nanodiscs significantly contribute to the total volume of the full molecular species.
We have now been in the grip of the COVID-19 pandemic for over two years with devastating consequences. The continual evolution of the virus has challenged the efficacy of many vaccines and treatment options based on immunotherapies are compromised by this viral escape. One treatment strategy that averts viral escape is the use of constructs based on its entry receptor Angiotensin-Converting Enzyme 2 (ACE2) acting as decoys. Here, we combined full-length human ACE2 with viral vectors commonly used for gene therapy to form nanoparticles that present ACE2 on their surface analogous to human cells. Using cell-based assays and direct, multiscale imaging including cryogenic cellular tomography, we show that these ACE2 nanoparticles are highly efficient in preventing entry of SARS-CoV-2, the virus causing COVID-19, in model cell systems as well as human respiratory tract ex-vivo cultures. Thus, ACE2 nanoparticles have high potential as the next generation therapeutics for addressing the immediate needs of the current pandemic and possible future outbreaks.
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