Significance Spatiotemporal coordination of cell growth underlies tissue development and disease. Mechanical feedback between cells has been proposed as a regulatory mechanism for growth control both in vivo and in cultured cells undergoing contact inhibition of proliferation. Evidence beyond theoretical and correlative observations falls short. In this study, we probe the impact of mechanical tissue perturbations on cell cycle progression by monitoring cell cycle dynamics of cells in tissues subject to acute changes in boundary conditions, as well as tissue stretching and compression. Taken together, we conclude that the ability of tissues to support cell cycle progression adapts to the available space through a memory-free control mechanism, which may coordinate proliferation patterns to maintain tissue homeostasis.

Filopodia are thin, spike-like cell surface protrusions containing bundles of parallel actin filaments. So far, filopodial dynamics has mainly been studied in the context of cell motility on coverslipadherent filopodia by using fluorescence and differential interference contrast (DIC) microscopy. In this study, we used an optical trap and interferometric particle tracking with nanometer precision to measure the three-dimensional dynamics of macrophage filopodia, which were not attached to flat surfaces. We found that filopodia act as cellular tentacles: a few seconds after binding to a particle, filopodia retract and pull the bound particle toward the cell. We observed F-actin-dependent stepwise retraction of filopodia with a mean step size of 36 nm, suggesting molecular motor activity during filopodial pulling. Remarkably, this intracellular stepping motion, which was measured at counteracting forces of up to 19 pN, was transmitted to the extracellular tracked particle via the filopodial F-actin bundle and the cell membrane. The pulling velocity depended strongly on the counteracting force and ranged between 600 nm/s at forces <1 pN and Ϸ40 nm/s at forces >15 pN. This result provides an explanation of the significant differences in filopodial retraction velocities previously reported in the literature. The measured filopodial retraction force-velocity relationship is in agreement with a model for force-dependent multiple motor kinetics.actin filaments ͉ interferometric three-dimensional particle tracking ͉ molecular motors ͉ nanomechanics ͉ optical trapping

Pharmacological modulation of cannabinoid type 2 receptor (CB2R) holds promise for the treatment of numerous conditions, including inflammatory diseases, autoimmune disorders, pain, and cancer. Despite the significance of this receptor, researchers lack reliable tools to address questions concerning the expression and complex mechanism of CB2R signaling, especially in cell-type and tissue-dependent context. Herein, we report for the first time a versatile ligand platform for the modular design of a collection of highly specific CB2R fluorescent probes, used successfully across applications, species and cell types. These include flow cytometry of endogenously expressing cells, real-time confocal microscopy of mouse splenocytes and human macrophages, as well as FRET-based kinetic and equilibrium binding assays. High CB2R specificity was demonstrated by competition experiments in living cells expressing CB2R at native levels. The probes were effectively applied to FACS analysis of microglial cells derived from a mouse model relevant to Alzheimer's disease and to the detection of CB2R in human breast cancer cells.

Epstein-Barr virus nuclear antigen 2 (EBNA2) is essential for viral transformation of B cells and transactivates cellular and viral target genes by binding RBPJ tethered to cognate promoter elements. EBNA2interacts with the DEAD-box protein DP103 (DDX20/Gemin3), which in turn is complexed to the survival motor neuron (SMN) protein. SMN is implicated in RNA processing, but a role in transcriptional regulation has also been suggested. Here, we show that DP103 and SMN are complexed in B cells and that SMN coactivates the viral LMP promoter in the presence of EBNA2 in reporter gene assays and in vivo. Subcellular localization studies revealed that nuclear gems and/or coiled bodies containing DP103 and SMN are targeted by EBNA2. Protein-protein interaction experiments demonstrated that DP103 binds to SMN exon 6 and that both EBNA2 and SMN interact with the C terminus of DP103. Furthermore, a DP103 binding-deficient SMN mutant was released from nuclear gems and/or coiled bodies and further enhanced coactivation. In addition, impaired transactivation of a DP103 binding-deficient EBNA2 mutant was rescued by overexpression of SMN. Testing different promoter constructs in luciferase assays showed that RBPJ is required but not sufficient for coactivation by EBNA2 and SMN. Overall, our data suggest that EBNA2 might target spliceosomal complexes by binding to DP103, thereby releasing SMN which subsequently exerts a coactivational function within the RNA-polymerase II transcription complex on the LMP1 promoter.The Epstein-Barr virus (EBV) causes infectious mononucleosis and is linked to the genesis of several human lymphoproliferative diseases (for a review, see reference 33). The EBVencoded nuclear antigen 2 (EBNA2) is a viral transactivator essential for EBV-induced transformation of resting human B lymphocytes, by promoting the expression of the transforming latent membrane proteins LMP1 and 2, the nuclear EBV Cp promoter-driven EBNA proteins, and the cellular genes CD23 and c-fgr (for review, see reference 15). EBNA2 does not bind directly to DNA but exerts its function by interacting with the cellular proteins RBPJ (CBF1) and, on the more complex LMP1 promoter, also Spi1 (PU.1), tethered to cognate response elements (12,17,20,45,46). Transcriptional activation is induced by binding of the C-terminal acidic domain (5) to components of the basal RNA polymerase II transcription machinery, such as RPA70, TAF40, TFIIB, and TFIIH (38, 39), and recruitment of the coactivators p300, CBP, and PCAF histone deacetylase (14, 41). In addition, by attracting the hSWI/SNF complex (42, 43) and targeting histone H1 (9, 34), EBNA2 likely promotes relief of nucleosome-mediated gene repression.We have recently shown that EBNA2 binds to DP103, a novel member of the DEAD-box family of putative RNA helicases (10). DP103 is a ubiquitously expressed 103-kDa phosphoprotein with an RNA-dependent ATPase activity; its other functions, in particular with regard to its interaction with EBNA2, remained unknown. While the work presented here was in prog...

Phagosome maturation is defined as the process by which phagosomes fuse sequentially with endosomes and lysosomes to acquire an acidic pH and hydrolases that degrade ingested particles. While the essential role of actin cytoskeleton remodeling during particle internalization is well established, its role during the later stages of phagosome maturation remains largely unknown. We have previously shown that purified mature phagosomes assemble F-actin at their membrane, and that the ezrin-radixin-moesin (ERM) proteins ezrin and moesin participate in this process. Moreover, we provided evidence that actin assembly on purified phagosomes stimulates their fusion with late endocytic compartments in vitro. In this study, we further investigated the role of ezrin in phagosome maturation. We engineered a structurally open form of ezrin and demonstrated that ezrin binds directly to the actin assembly promoting factor N-WASP (Neural WiskottAldrich Syndrome Protein) by its FERM domain. Using a cell-free system, we found that ezrin stimulates F-actin assembly on purified phagosomes by recruiting the N-WASP-Arp2/3 machinery. Accordingly, we showed that the down-regulation of ezrin activity in macrophages by a dominant-negative approach caused reduced F-actin accumulation on maturing phagosomes. Furthermore, using fluorescence and electron microscopy, we found that ezrin is required for the efficient fusion between phagosomes and lysosomes. Live-cell imaging analysis supported the notion that ezrin is necessary for the fusogenic process itself, promoting the transfer of the lysosome content into the phagosomal lumen.