Cell membranes are complex multicomponent systems, which are highly heterogeneous in the lipid distribution and composition. To date, most molecular simulations have focussed on relatively simple lipid compositions, helping to inform our understanding of in vitro experimental studies. Here we describe on simulations of complex asymmetric plasma membrane model, which contains seven different lipids species including the glycolipid GM3 in the outer leaflet and the anionic lipid, phosphatidylinositol 4,5-bisphophate (PIP2), in the inner leaflet. Plasma membrane models consisting of 1500 lipids and resembling the in vivo composition were constructed and simulations were run for 5 µs. In these simulations the most striking feature was the formation of nano-clusters of GM3 within the outer leaflet. In simulations of protein interactions within a plasma membrane model, GM3, PIP2, and cholesterol all formed favorable interactions with the model α-helical protein. A larger scale simulation of a model plasma membrane containing 6000 lipid molecules revealed correlations between curvature of the bilayer surface and clustering of lipid molecules. In particular, the concave (when viewed from the extracellular side) regions of the bilayer surface were locally enriched in GM3. In summary, these simulations explore the nanoscale dynamics of model bilayers which mimic the in vivo lipid composition of mammalian plasma membranes, revealing emergent nanoscale membrane organization which may be coupled both to fluctuations in local membrane geometry and to interactions with proteins.

Lymph node (LN) stromal cells, particularly fibroblastic reticular cells (FRCs), provide critical structural support and regulate immunity, tolerance and transport properties of LNs. In many tumors, LN metastasis is predictive of poor prognosis. However, the stromal contribution to the evolving microenvironment of tumor draining LNs (TDLNs) remains poorly understood. Here we show that FRCs specifically of TDLNs proliferate in response to tumor-derived cues and that the network they form is remodeled. Comparative transcriptional analysis of non-draining and TDLN FRCs demonstrated reprogramming of key pathways including matrix remodeling, chemokine/cytokine signaling and immune functions including leukocyte recruitment, migration and activation. In particular, downregulation of FRC-derived CCL21 and IL-7 were accompanied by altered immune composition and aberrant localization. These data imply that stroma of TDLNs adapt on multiple levels, following exposure to tumor-derived factors, to exhibit features typically associated with immune suppression.

SummaryThe influenza virus is surrounded by an envelope composed of a lipid bilayer and integral membrane proteins. Understanding the structural dynamics of the membrane envelope provides biophysical insights into aspects of viral function, such as the wide-ranging survival times of the virion in different environments. We have combined experimental data from X-ray crystallography, nuclear magnetic resonance spectroscopy, cryo-electron microscopy, and lipidomics to build a model of the intact influenza A virion. This is the basis of microsecond-scale coarse-grained molecular dynamics simulations of the virion, providing simulations at different temperatures and with varying lipid compositions. The presence of the Forssman glycolipid alters a number of biophysical properties of the virion, resulting in reduced mobility of bilayer lipid and protein species. Reduced mobility in the virion membrane may confer physical robustness to changes in environmental conditions. Our simulations indicate that viral spike proteins do not aggregate and thus are competent for multivalent immunoglobulin G interactions.

Lipid molecules can bind to specific sites on integral membrane proteins, modulating their structure and function. We have undertaken coarse-grained simulations to calculate free energy profiles for glycolipids and phospholipids interacting with modulatory sites on the transmembrane helix dimer of the EGF receptor within a lipid bilayer environment. We identify lipid interaction sites at each end of the transmembrane domain and compute interaction free energy profiles for lipids with these sites. Interaction free energies ranged from ca. −40 to −4 kJ/mol for different lipid species. Those lipids (glycolipid GM3 and phosphoinositide PIP2) known to modulate EGFR function exhibit the strongest binding to interaction sites on the EGFR, and we are able to reproduce the preference for interaction with GM3 over other glycolipids suggested by experiment. Mutation of amino acid residues essential for EGFR function reduce the binding free energy of these key lipid species. The residues interacting with the lipids in the simulations are in agreement with those suggested by experimental (mutational) studies. This approach provides a generalizable tool for characterizing the interactions of lipids that bind to specific sites on integral membrane proteins.

Skin cancer risk varies substantially across the body, yet how this relates to the mutations found in normal skin is unknown. Here we mapped mutant clones in skin from high and low risk sites. The density of mutations varied by location. The prevalence of NOTCH1 and FAT1 mutations in forearm, trunk and leg skin was similar to that in keratinocyte cancers. Most mutations were caused by ultraviolet (UV) light, but mutational signature analysis suggested differences in DNA repair processes between sites. 11 mutant genes were under positive selection, with TP53 preferentially selected in the head and FAT1 in the leg. Fine scale mapping revealed 10% of clones had copy number alterations. Analysis of hair follicles showed mutations in the upper follicle resembled adjacent skin, but the lower follicle was sparsely mutated. Normal skin is dense patchwork of mutant clones arising from competitive selection that varies by location. Statement of significance Mapping mutant clones across the body reveals normal skin is a dense patchwork of mutant cells. The variation in cancer risk between sites substantially exceeds that in mutant clone density. More generally, mutant genes cannot be assigned as cancer drivers until their prevalence in normal tissue is known.

Lung adenocarcinoma accounts for ∼40% of lung cancers, the leading cause of cancer-related death worldwide, and current therapies provide only limited survival benefit. Approximately half of lung adenocarcinomas harbor mutations in TP53 (p53), making these mutants appealing targets for lung cancer therapy. As mutant p53 remains untargetable, mutant p53-dependent phenotypes represent alternative targeting opportunities, but the prevalence and therapeutic relevance of such effects (gain of function and dominant-negative activity) in lung adenocarcinoma are unclear. Through transcriptional and functional analysis of murine Kras G12D -p53 null , -p53 R172H (conformational), and -p53 R270H (contact) mutant lung tumors, we identified genotype-independent and genotype-dependent therapeutic sensitivities. Unexpectedly, we found that wild-type p53 exerts a dominant tumor-suppressive effect on mutant tumors, as all genotypes were similarly sensitive to its restoration in vivo. These data show that the potential of p53 targeted therapies is comparable across all p53-deficient genotypes and may explain the high incidence of p53 loss of heterozygosity in mutant tumors. In contrast, mutant p53 gain of function and their associated vulnerabilities can vary according to mutation type. Notably, we identified a p53 R270H -specific sensitivity to simvastatin in lung tumors, and the transcriptional signature that underlies this sensitivity was also present in human lung tumors, indicating that this therapeutic approach may be clinically relevant.

Glycolipids are key components of mammalian cell membranes, influencing a diverse range of cellular functions. For example, a number of receptor tyrosine kinases, including the epidermal growth factor receptor (EGFR), are allosterically regulated by the glycolipid monosialodihexosylganglioside (GM3). Recent advances in molecular dynamics methods, especially the development of coarse-grained models, have enabled simulations of increasingly complex models of cell membranes. We demonstrate these methodological developments via a case study of a coarse-grained model for the ganglioside GM3. This glycolipid is included in simulations of a mixed lipid bilayer model reflecting the compositional complexity of a mammalian cell membrane. The resultant membrane model is used to simulate the interactions of GM3 with the transmembrane domain of the EGFR.

Cancer-causing missense mutations in the 3418 amino acid BRCA2 breast and ovarian cancer suppressor protein frequently affect a short (∼340 residue) segment in its carboxyl-terminal domain (DBD). Here, we identify a shared molecular mechanism underlying their pathogenicity. Pathogenic BRCA2 missense mutations cluster in the DBD’s helical domain (HD) and OB1-fold motifs, which engage the partner protein DSS1. Pathogenic - but not benign – DBD mutations weaken or abolish DSS1-BRCA2 assembly, provoking mutant BRCA2 oligomers that are excluded from the cell nucleus, and disable DNA repair by homologous DNA recombination (HDR). DSS1 inhibits the intracellular oligomerization of wildtype, but not mutant, forms of BRCA2. Remarkably, DSS1 expression corrects defective HDR in cells bearing pathogenic BRCA2 missense mutants with weakened, but not absent, DSS1 binding. Our findings identify a DSS1-mediated intracellular protein assembly mechanism that is disrupted by cancer-causing BRCA2 missense mutations, and suggest an approach for its therapeutic correction.