RASSF1C oncogene elicits amoeboid invasion, cancer stemness, and extracellular vesicle release via a SRC/Rho axis.

Cell plasticity is a crucial hallmark leading to cancer metastasis. Upregulation of Rho/ROCK pathway drives actomyosin contractility, protrusive forces, and contributes to the occurrence of highly invasive amoeboid cells in tumors. Cancer stem cells are similarly associated with metastasis, but how these populations arise in tumors is not fully understood. Here, we show that the novel oncogene RASSF1C drives mesenchymal-to-amoeboid transition and stem cell attributes in breast cancer cells. Mechanistically, RASSF1C activates Rho/ROCK via SRC-mediated RhoGDI inhibition, resulting in generation of actomyosin contractility. Moreover, we demonstrate that RASSF1C-induced amoeboid cells display increased expression of cancer stem-like markers such as CD133, ALDH1, and Nanog, and are accompanied by higher invasive potential in vitro and in vivo. Further, RASSF1C-induced amoeboid cells employ extracellular vesicles to transfer the invasive phenotype to target cells and tissue. Importantly, the underlying RASSF1C-driven biological processes concur to explain clinical data: namely, methylation of the RASSF1C promoter correlates with better survival in early-stage breast cancer patients. Therefore, we propose the use of RASSF1 gene promoter methylation status as a biomarker for patient stratification.

MST2 kinase suppresses rDNA transcription in response to DNA damage by phosphorylating nucleolar histone H2B.

The heavily transcribed rDNA repeats that give rise to the ribosomal RNA are clustered in a unique chromatin structure, the nucleolus. Due to its highly repetitive nature and transcriptional activity, the nucleolus is considered a hotspot of genomic instability. Breaks in rDNA induce a transient transcriptional shut down to conserve energy and promote rDNA repair; however, how nucleolar chromatin is modified and impacts on rDNA repair is unknown. Here, we uncover that phosphorylation of serine 14 on histone H2B marks transcriptionally inactive nucleolar chromatin in response to DNA damage. We identified that the MST2 kinase localises at the nucleoli and targets phosphorylation of H2BS14p in an ATM-dependent manner. We show that establishment of H2BS14p is necessary for damage-induced rDNA transcriptional shut down and maintenance of genomic integrity. Ablation of MST2 kinase, or upstream activators, results in defective establishment of nucleolar H2BS14p, perturbed DNA damage repair, sensitisation to rDNA damage and increased cell lethality. We highlight the impact of chromatin regulation in the rDNA damage response and targeting of the nucleolus as an emerging cancer therapeutic approach.

Lack of involvement of CD63 and CD9 tetraspanins in the extracellular vesicle content delivery process.

Extracellular vesicles (EVs) are thought to mediate intercellular communication by transferring cargoes from donor to acceptor cells. The EV content-delivery process within acceptor cells is still poorly characterized and debated. CD63 and CD9, members of the tetraspanin family, are highly enriched within EV membranes and are respectively enriched within multivesicular bodies/endosomes and at the plasma membrane of the cells. CD63 and CD9 have been suspected to regulate the EV uptake and delivery process. Here we used two independent assays and different cell models (HeLa, MDA-MB-231 and HEK293T cells) to assess the putative role of CD63 and CD9 in the EV delivery process that includes uptake and cargo delivery. Our results suggest that neither CD63, nor CD9 are required for this function.

Composition and structure of synaptic ectosomes exporting antigen receptor linked to functional CD40 ligand from helper T cells.

Planar supported lipid bilayers (PSLB) presenting T cell receptor (TCR) ligands and ICAM-1 induce budding of extracellular microvesicles enriched in functional TCR, defined here as synaptic ectosomes (SE), from helper T cells. SE bind peptide-MHC directly exporting TCR into the synaptic cleft, but incorporation of other effectors is unknown. Here, we utilized bead supported lipid bilayers (BSLB) to capture SE from single immunological synapses (IS), determined SE composition by immunofluorescence flow cytometry and enriched SE for proteomic analysis by particle sorting. We demonstrate selective enrichment of CD40L and ICOS in SE in response to addition of CD40 and ICOSL, respectively, to SLB presenting TCR ligands and ICAM-1. SE are enriched in tetraspanins, BST-2, TCR signaling and ESCRT proteins. Super-resolution microscopy demonstrated that CD40L is present in microclusters within CD81 defined SE that are spatially segregated from TCR/ICOS/BST-2. CD40L+ SE retain the capacity to induce dendritic cell maturation and cytokine production.