Quantifying Protein-mRNA Interactions in Single Live Cells.

Specific binding proteins are crucial for the correct spatiotemporal expression of mRNA. To understand this process, a method is required to characterize RNA-protein interactions in single living cells with subcellular resolution. We combined endogenous single RNA and protein detection with two-photon fluorescence fluctuation analysis to measure the average number of proteins bound to mRNA at specific locations within live cells. We applied this to quantify the known binding of zipcode binding protein 1 (ZBP1) and ribosomes to ß-actin mRNA within subcellular compartments of primary fibroblasts and neurons. ZBP1-mRNA binding did not occur in nuclei, contrary to previous conclusions. ZBP1 interaction with ß-actin mRNA was enhanced perinuclearly in neurons compared to fibroblasts. Cytoplasmic ZBP1 and ribosome binding to the mRNA were anti-correlated depending on their location in the cell. These measurements support a mechanism whereby ZBP1 inhibits translation of localizing mRNA until its release from the mRNA peripherally, allowing ribosome binding.

A cycle of Zap70 kinase activation and release from the TCR amplifies and disperses antigenic stimuli.

Cell-surface-receptor pathways amplify weak, rare and local stimuli to induce cellular responses. This task is accomplished despite signaling components that segregate into nanometer-scale membrane domains. Here we describe a 'catch-and-release' mechanism that amplified and dispersed stimuli by releasing activated kinases from receptors lacking intrinsic catalytic activity. Specifically, we discovered a cycle of recruitment, activation and release for Zap70 kinases at phosphorylated T cell antigen receptors (TCRs). This turned the TCR into a 'catalytic unit' that amplified antigenic stimuli. Zap70 released from the TCR remained at the membrane, translocated, and phosphorylated spatially distinct substrates. The mechanisms described here are based on widely used protein domains and post-translational modifications; therefore, many membrane-associated pathways might employ similar mechanisms for signal amplification and dispersion.

Tagged actin mRNA dysregulation in IGF2BP1[Formula: see text] mice.

Gene expression is tightly regulated by RNA-binding proteins (RBPs) to facilitate cell survival, differentiation, and migration. Previous reports have shown the importance of the Insulin-like Growth Factor II mRNA-Binding Protein (IGF2BP1/IMP1/ZBP1) in regulating RNA fate, including localization, transport, and translation. Here, we generated and characterized a knockout mouse to study RBP regulation. We report that IGF2BP1 is essential for proper brain development and neonatal survival. Specifically, these mice display disorganization in the developing neocortex, and further investigation revealed a loss of cortical marginal cell density at E17.5. We also investigated migratory cell populations in the IGF2BP1[Formula: see text] mice, using BrdU labeling, and detected fewer mitotically active cells in the cortical plate. Since RNA localization is important for cellular migration and directionality, we investigated the regulation of ß-actin messenger RNA (mRNA), a well-characterized target with established roles in cell motility and development. To aid in our understanding of RBP and target mRNA regulation, we generated mice with endogenously labeled ß-actin mRNA (IGF2BP1[Formula: see text]; ß-actin-MS2[Formula: see text]). Using endogenously labeled ß-actin transcripts, we report IGF2BP1[Formula: see text] neurons have increased transcription rates and total ß-actin protein content. In addition, we found decreased transport and anchoring in knockout neurons. Overall, we present an important model for understanding RBP regulation of target mRNA.

International perspectives on the development, application, and evaluation of a multicancer early detection strategy.

The development and implementation of a multicancer early detection (MCED) test that is effective and affordable has the potential to change cancer care systems around the world. However, careful consideration is needed within the context of different health care settings (both low- and middle-income countries and high-income countries) to roll out an MCED test and promote equity in access.

ß-Actin mRNA compartmentalization enhances focal adhesion stability and directs cell migration.

Directed cell motility is at the basis of biological phenomena such as development, wound healing, and metastasis. It has been shown that substrate attachments mediate motility by coupling the cell's cytoskeleton with force generation. However, it has been unclear how the persistence of cell directionality is facilitated. We show that mRNA localization plays an important role in this process, but the mechanism of action is still unknown. In this study, we show that the zipcode-binding protein 1 transports ß-actin mRNA to the focal adhesion compartment, where it dwells for minutes, suggesting a means for associating its localization with motility through the formation of stable connections between adhesions and newly synthesized actin filaments. In order to demonstrate this, we developed an approach for assessing the functional consequences of ß-actin mRNA and protein localization by tethering the mRNA to a specific location-in this case, the focal adhesion complex. This approach will have a significant impact on cell biology because it is now possible to forcibly direct any mRNA and its cognate protein to specific locations in the cell. This will reveal the importance of localized protein translation on various cellular processes.

mRNA on the move: the road to its biological destiny.

Cells have evolved to regulate the asymmetric distribution of specific mRNA targets to institute spatial and temporal control over gene expression. Over the last few decades, evidence has mounted as to the importance of localization elements in the mRNA sequence and their respective RNA-binding proteins. Live imaging methodologies have shown mechanistic details of this phenomenon. In this minireview, we focus on the advanced biochemical and cell imaging techniques used to tweeze out the finer aspects of mechanisms of mRNA movement.

i-shaped antibody engineering enables conformational tuning of biotherapeutic receptor agonists.

The ability to leverage antibodies to agonize disease relevant biological pathways has tremendous potential for clinical investigation. Yet while antibodies have been successful as antagonists, immune mediators, and targeting agents, they are not readily effective at recapitulating the biology of natural ligands. Among the important determinants of antibody agonist activity is the geometry of target receptor engagement. Here, we describe an engineering approach inspired by a naturally occurring Fab-Fab homotypic interaction that constrains IgG in a unique i-shaped conformation. i-shaped antibody (iAb) engineering enables potent intrinsic agonism of five tumor necrosis factor receptor superfamily (TNFRSF) targets. When applied to bispecific antibodies against the heterodimeric IL-2 receptor pair, constrained bispecific IgG formats recapitulate IL-2 agonist activity. iAb engineering provides a tool to tune agonist antibody function and this work provides a framework for the development of intrinsic antibody agonists with the potential for generalization across broad receptor classes.

Septins organize endoplasmic reticulum-plasma membrane junctions for STIM1-ORAI1 calcium signalling.

ORAI1 Ca2+ channels in the plasma membrane (PM) are gated by STIM1 at endoplasmic reticulum (ER)-PM junctions to effect store-dependent Ca2+ entry into cells, but little is known about how local STIM-ORAI signalling at junctions is coordinated with overall cellular architecture. Filamentous septins can specify cytoskeletal rearrangements and have been found recently to modulate STIM-ORAI signalling. Here we show by super-resolution imaging of ORAI1, STIM1, and septin 4 in living cells that septins facilitate Ca2+ signalling indirectly. Septin 4 does not colocalize preferentially with ORAI1 in resting or stimulated cells, assemble stably at ER-PM junctions, or specify a boundary that directs or confines ORAI1 to junctions. Rather, ORAI1 is recruited to junctions solely through interaction with STIM proteins, while septins regulate the number of ER-PM junctions and enhance STIM1-ORAI1 interactions within junctions. Thus septins communicate with STIM1 and ORAI1 through protein or lipid intermediaries, and are favorably positioned to coordinate Ca2+ signalling with rearrangements in cellular architecture.

Mapping translation 'hot-spots' in live cells by tracking single molecules of mRNA and ribosomes.

Messenger RNA localization is important for cell motility by local protein translation. However, while single mRNAs can be imaged and their movements tracked in single cells, it has not yet been possible to determine whether these mRNAs are actively translating. Therefore, we imaged single ß-actin mRNAs tagged with MS2 stem loops colocalizing with labeled ribosomes to determine when polysomes formed. A dataset of tracking information consisting of thousands of trajectories per cell demonstrated that mRNAs co-moving with ribosomes have significantly different diffusion properties from non-translating mRNAs that were exposed to translation inhibitors. These data indicate that ribosome load changes mRNA movement and therefore highly translating mRNAs move slower. Importantly, ß-actin mRNA near focal adhesions exhibited sub-diffusive corralled movement characteristic of increased translation. This method can identify where ribosomes become engaged for local protein production and how spatial regulation of mRNA-protein interactions mediates cell directionality.