Cleary, J. H. | bioRxiv Science

bioRxiv2026

Genome-Wide CRISPRi Screening Identifies XPO5 as a Regulator of B Cell Mutation and Fitness

The B cell receptor (BCR) is the defining factor of B lymphocyte identity and function, allowing for a robust adaptive immune response through antigen recognition. Strict regulation of BCR surface density dictates proper B cell signaling, immune regulation, and the prevention of malignancy, yet the factors regulating this density remain undefined. Here, we performed a genome-wide CRISPR interference (CRISPRi) screen in Ramos B cells, which undergo constitutive somatic hypermutation (SHM) and identified Exportin-5 (XPO5) as a central regulator of BCR surface expression. XPO5 depleted cells exhibited an accelerated loss of surface BCR with no change in transcript levels, suggesting a potential post-transcriptional regulatory mechanism. Further analysis revealed XPO5 depletion led to an accumulation of non-functional BCR light chain sequences driven by an increase in AID signature mutations, implicating XPO5 in balancing mutagenesis and repair during somatic hypermutation (SHM). Transcriptomic and small RNA sequencing revealed a global reduction in miRNA levels and enrichment of target gene sets indicative of cell cycle arrest and increased DNA damage response. These data suggest that XPO5 plays a multi-faceted regulatory role in B cells via a miRNA-mediated control, supporting both proliferation and regulating DNA repair thresholds to maintain B cell receptor expression and functionality. SignificancePrecise regulation of B cell receptor (BCR) density is essential for immune function and preventing malignancy. Through a genome-wide CRISPR interference (CRISPRi) screen, we identified Exportin-5 (XPO5) as a critical regulator of BCR surface expression. We show that XPO5 is essential to maintain the miRNA landscape that supports DNA repair during somatic hypermutation. Loss of XPO5 destabilizes this mutational balance, driving the accumulation of non-functional BCR sequences. This study uncovers a novel connection between miRNA nuclear export and the preservation of B cell identity and genomic fidelity, highlighting the multi-faceted regulatory role of XPO5.

cell biology↗

bioRxiv2025

Endothelial Trauma Depends on Surface Charge and Extracellular Calcium Levels

We tested the hypothesis that the ubiquitous store-operated Ca2+ entry (SOCE) pathway contributes to histone-induced endothelial Ca2+ events. We also considered an alternate hypothesis: cationic electrostatic interactions between histones and negatively charged phospholipids deform endothelial membranes and thereby allow extracellular Ca2+ entry. A role for SOCE in histone responses was ruled out by genetic ablation of the ORAI1/2/3 channel trio; yet, histone effects were blocked by application of the multivalent cation gadolinium Gd3+. Using live cell video microscopy of endothelial cells labeled with membrane dye FM1-43, we recorded plasma membrane movements including vesiculation, blebbing, and ruffling of lamellipodia over 60 minutes following histone exposure. These cell membrane theatrics were markedly different from the uniform pattern of exocytosis and subsequent blebbing produced by calcium overload with ionomycin. The membrane permeabilization produced by histones, and not ionomycin, was transient and a subset of cells recovered membrane integrity within 1 hour. Removal of extracellular Ca2+ prevented histone-induced intracellular Ca2+ overload while surprisingly exacerbating plasma membrane deformation. Conversely, decreasing the density of the negative charge surface by adding calcium or or increasing extracellular Ca2+ levels effectively screened common membrane phospholipids from interactions with labeled histones and prevented endothelial damage in cells exposed to histones. Collectively these results indicate that low extracellular Ca2+ levels enhance interactions between histones and endothelial cell membrane phospholipids to increase cytotoxicity. Importantly, this supports the concept of aggressive Ca2+ repletion during resuscitation to prevent hypocalcemia, stabilize endothelial cell membranes and improve cardiovascular recovery from shock. SignificanceIn acute critical illness, the rapid collapse of vascular endothelial functions drives aberrant blood clotting and organ failure through mechanisms that are not understood. Emerging evidence that early administration of donor plasma improves survival of trauma patients has transformed the massive transfusion protocols used in surgical settings, but the sodium citrate included in transfused blood products to prevent coagulation often produces significant and severe hypocalcemia. Here, we demonstrate that cytotoxic trauma factors that are elevated in the blood during resuscitation interact electrostatically with endothelial cell phospholipids, and that low Ca2+ exacerbates toxicity by increasing this interaction. Using high speed video imaging, we demonstrate fast endothelial cell membrane movements in response to injury, including protrusion and ruffling of lamellipodia, release and reuptake of extracellular vesicles, and blebbing. These findings provide important insights into the nature of shock-induced endotheliopathy and highlight the potential cardiovascular risk associated with chelation-induced hypocalcemia during resuscitation.

physiology↗