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Contains fulltext : 287797.pdf (Publisher’s version ) (Closed access) Sensory driven activity during early life is critical for setting up the proper connectivity of the sensory cortices. We ask here whether social play behavior, a particular form of social interaction that is highly abundant during postweaning development, is equally important for setting up connections in the developing prefrontal cortex (PFC). Young male rats were deprived from social play with peers during the period in life when social play behavior normally peaks [postnatal day 21-42] (SPD rats), followed by resocialization until adulthood. We recorded synaptic currents in layer 5 cells in slices from medial PFC of adult SPD and control rats and observed that inhibitory synaptic currents were reduced in SPD slices, while excitatory synaptic currents were unaffected. This was associated with a decrease in perisomatic inhibitory synapses from parvalbumin-positive GABAergic cells. In parallel experiments, adult SPD rats achieved more reversals in a probabilistic reversal learning (PRL) task, which depends on the integrity of the PFC, by using a more simplified cognitive strategy than controls. Interestingly, we observed that one daily hour of play during SPD partially rescued the behavioral performance in the PRL, but did not prevent the decrease in PFC inhibitory synaptic inputs. Our data demonstrate the importance of unrestricted social play for the development of inhibitory synapses in the PFC and cognitive skills in adulthood and show that specific synaptic alterations in the PFC can result in a complex behavioral outcome.SIGNIFICANCE STATEMENT This study addressed the question whether social play behavior in juvenile rats contributes to functional development of the prefrontal cortex (PFC). We found that rats that had been deprived from juvenile social play (social play deprivation - SPD) showed a reduction in inhibitory synapses in the PFC and a simplified strategy to solve a complex behavioral task in adulthood. Providing one daily hour of play during SPD partially rescued the cognitive skills in these rats, but did not prevent the reduction in PFC inhibitory synapses. Our results demonstrate a key role for unrestricted juvenile social play in PFC development and emphasize the complex relation between PFC circuit connectivity and cognitive function.

Antibiotics that use novel mechanisms are needed to combat antimicrobial resistance1–3. Teixobactin4 represents a new class of antibiotics with a unique chemical scaffold and lack of detectable resistance. Teixobactin targets lipid II, a precursor of peptidoglycan5. Here we unravel the mechanism of teixobactin at the atomic level using a combination of solid-state NMR, microscopy, in vivo assays and molecular dynamics simulations. The unique enduracididine C-terminal headgroup of teixobactin specifically binds to the pyrophosphate-sugar moiety of lipid II, whereas the N terminus coordinates the pyrophosphate of another lipid II molecule. This configuration favours the formation of a β-sheet of teixobactins bound to the target, creating a supramolecular fibrillar structure. Specific binding to the conserved pyrophosphate-sugar moiety accounts for the lack of resistance to teixobactin4. The supramolecular structure compromises membrane integrity. Atomic force microscopy and molecular dynamics simulations show that the supramolecular structure displaces phospholipids, thinning the membrane. The long hydrophobic tails of lipid II concentrated within the supramolecular structure apparently contribute to membrane disruption. Teixobactin hijacks lipid II to help destroy the membrane. Known membrane-acting antibiotics also damage human cells, producing undesirable side effects. Teixobactin damages only membranes that contain lipid II, which is absent in eukaryotes, elegantly resolving the toxicity problem. The two-pronged action against cell wall synthesis and cytoplasmic membrane produces a highly effective compound targeting the bacterial cell envelope. Structural knowledge of the mechanism of teixobactin will enable the rational design of improved drug candidates.

Microtubules are dynamic cytoskeletal filaments that can generate forces when polymerizing and depolymerizing. Proteins that follow growing or shortening microtubule ends and couple forces to cargo movement are important for a wide range of cellular processes. Quantifying these forces and the composition of protein complexes at dynamic microtubule ends is challenging and requires sophisticated instrumentation. Here, we present an experimental approach to estimate microtubule-generated forces through the extension of a fluorescent spring-shaped DNA origami molecule. Optical readout of the spring extension enables recording of force production simultaneously with single-molecule fluorescence of proteins getting recruited to the site of force generation. DNA nanosprings enable multiplexing of force measurements and only require a fluorescence microscope and basic laboratory equipment. We validate the performance of DNA nanosprings against results obtained using optical trapping. Finally, we demonstrate the use of the nanospring to study proteins that couple microtubule growth and shortening to force generation.

The textbook view of cell division terminates with the final separation of the two daughter cells in the process called abscission. However, in contrast to this classical view, a variety of cell types in multicellular organisms are connected through cytoplasmic bridges, which most often form by incomplete abscission or — more rarely — by local fusion of plasma membranes. In this review, we survey the distribution, function, and formation of cytoplasmic bridges across the eukaryotic tree of life. We find that cytoplasmic bridges are widespread, and were likely ancestrally present, in almost all lineages of eukaryotes with clonal multicellularity — including the five ‘complex multicellular’ lineages: animals, fungi, land plants, red algae, and brown algae. In animals, cytoplasmic bridges resulting from incomplete abscission are ubiquitous in the germline and common in pluripotent cell types. Although cytoplasmic bridges have been less studied than other structural mediators of multicellularity (such as adhesion proteins and extracellular matrix), we propose that they have played a pivotal role in the repeated evolution of eukaryotic clonal multicellularity — possibly by first performing a structural role and later by allowing exchange of nutrients and/or intercellular communication, which notably buffered cell–cell competition by averaging gene expression. Bridges were eventually lost from many animal tissues in concert with the evolution of spatial cell differentiation, cell motility within the organism, and other mechanisms for intercellular distribution of signals and metabolites. Finally, we discuss the molecular basis for the evolution of incomplete abscission and examine the alternative hypotheses of single or multiple origins.

Spondyloarthritis (SpA) patients suffer from joint inflammation resulting in tissue damage, characterized by the presence of numerous neutrophils in the synovium and synovial fluid (SF). As it is yet unclear to what extent neutrophils contribute to the pathogenesis of SpA, we set out to study SF neutrophils in more detail. We analyzed the functionality of SF neutrophils of 20 SpA patients and 7 disease controls, determining ROS production and degranulation in response to various stimuli. In addition, the effect of SF on neutrophil function was determined. Surprisingly, our data show that SF neutrophils in SpA patients have an inactive phenotype, despite the presence of many neutrophil-activating stimuli such as GM-CSF and TNF in SF. This was not due to exhaustion as SF neutrophils readily responded to stimulation. Therefore, this finding suggests that one or more inhibitors of neutrophil activation may be present in SF. Indeed, when blood neutrophils from healthy donors were activated in the presence of increasing concentrations of SF from SpA patients, degranulation and ROS production were dose-dependently inhibited. This effect was independent of diagnosis, gender, age, and medication in the patients from which the SF was isolated. Treatment of SF with the enzyme hyaluronidase strongly reduced the inhibitory effect of SF on neutrophil activation, indicating that hyaluronic acid that is present in SF may be an important factor in preventing SF neutrophil activation. This finding provides novel insights into the role of soluble factors in SF regulating neutrophil function and may lead to the development of novel therapeutics targeting neutrophil activation via hyaluronic acid or associated pathways.

The discovery that extracellular vesicles (EVs) can transfer functional extracellular RNAs (exRNAs) between cells opened new avenues into the study of EVs in health and disease. Growing interest in EV RNAs and other forms of exRNA has given rise to research programmes including but not limited to the Extracellular RNA Communication Consortium (ERCC) of the US National Institutes of Health. In 2017, the International Society for Extracellular Vesicles (ISEV) administered a survey focusing on EVs and exRNA to canvass-related views and perceived needs of the EV research community. Here, we report the results of this survey. Overall, respondents emphasized opportunities for technical developments, unraveling of molecular mechanisms and standardization of methodologies to increase understanding of the important roles of exRNAs in the broader context of EV science. In conclusion, although exRNA biology is a relatively recent emphasis in the EV field, it has driven considerable interest and resource commitment. The ISEV community looks forward to continuing developments in the science of exRNA and EVs, but without excluding other important molecular constituents of EVs.

Parvalbumin-expressing (PV+) basket cells are fast-spiking inhibitory interneurons that exert critical control over local circuit activity and oscillations. PV+ axons are often myelinated, but the electrical and metabolic roles of interneuron myelination remain poorly understood. Here, we developed viral constructs allowing cell type-specific investigation of mitochondria with genetically encoded fluorescent probes. Single-cell reconstructions revealed that mitochondria selectively cluster to myelinated segments of PV+ basket cells, confirmed by analyses of a high-resolution electron microscopy dataset. In contrast to the increased mitochondrial densities in excitatory axons cuprizone-induced demyelination abolished mitochondrial clustering in PV+ axons. Furthermore, with genetic deletion of myelin basic protein the mitochondrial clustering was still observed at internodes wrapped by noncompacted myelin, indicating that compaction is dispensable. Finally, two-photon imaging of action potential-evoked calcium (Ca2+) responses showed that interneuron myelination attenuates both the cytosolic and mitochondrial Ca2+ transients. These findings suggest that oligodendrocyte ensheathment of PV+ axons assembles mitochondria to branch selectively fine-tune metabolic demands.

While vaccination against HIV-1 has been so far unsuccessful, recently broadly neutralizing antibodies (bNAbs) against HIV-1 envelope glycoprotein were shown to induce long-term suppression in the absence of antiretroviral therapy in patients with antibody-sensitive viral reservoirs. The requirement of neutralizing antibodies indicates that the antibody mediated removal (clearance) of HIV-1 in itself is not efficient enough in these immune compromised patients. Here we present a novel, alternative approach that is independent of a functional immune system to clear HIV-1, by capturing the virus and redirecting it to non-target cells where it is internalized and degraded. We use bispecific antibodies with domains derived from small single chain Llama antibodies (VHHs). These bind with one domain to HIV-1 envelope proteins and with the other domain direct the virus to cells expressing epidermal growth factor receptor (EGFR), a receptor that is ubiquitously expressed in the body. We show that HIV envelope proteins, virus-like particles and HIV-1 viruses (representing HIV-1 subtypes A, B and C) are efficiently recruited to EGFR, internalized and degraded in the lysosomal pathway at low nM concentrations of bispecific VHHs. This directed degradation in non-target cells may provide a clearance platform for the removal of viruses and other unwanted agents from the circulation, including toxins, and may thus provide a novel method for curing.

Background The epidermal growth factor receptor (EGFR) is involved in various developmental processes, and alterations of its extracellular segment are associated with several types of cancers, in particular glioblastoma multiforme (GBM). The EGFR extracellular region is therefore a primary target for therapeutic agents, such as monoclonal antibodies and variable domains of heavy chain antibodies (VHH), also called nanobodies. Nanobodies have been previously shown to bind to EGFR, and to inhibit ligand-mediated EGFR activation. Results Here we present the X-ray crystal structures of the EgB4 nanobody, alone (to 1.48 Å resolution) and bound to the full extracellular EGFR-EGF complex in its active conformation (to 6.0 Å resolution). We show that EgB4 binds to a new epitope located on EGFR domains I and II, and we describe the molecular mechanism by which EgB4 plays a non-inhibitory role in EGFR signaling. Conclusion This work provides the structural basis for the application of EgB4 as a tool for research, for targeted therapy, or as a biomarker to locate EGFR-associated tumors, all without affecting EGFR activation.

Naked viruses can escape host cells before the induction of lysis via release in extracellular vesicles (EVs). These nanosized EVs cloak the secreted virus particles in a host-derived membrane, which alters virus-host interactions that affect infection efficiency and antiviral immunity. Currently, little is known about the viral and host factors regulating this form of virus release. Here, we assessed the role of the encephalomyocarditis virus (EMCV) Leader protein, a ‘viral security protein’ that subverts the host antiviral response. EV release upon infection with wildtype virus or a Leader-deficient mutant was characterized at the single particle level using high-resolution flow cytometry. Inactivation of the Leader abolished EV induction during infection and strongly reduced EV-enclosed virus release. We demonstrate that the Leader promotes the release of virions within EVs by stimulating a secretory arm of autophagy. This newly discovered role of the EMCV Leader adds to the variety of mechanisms via which this protein affects virus-host interactions. Moreover, these data provide first evidence for a crucial role of a non-structural viral protein in the non-lytic release of picornaviruses via packaging in EVs.

Blockade of the CD47-SIRPα axis improves lymphoma cell killing by myeloid effector cells, which is an important effector mechanism for CD20 antibodies in vivo. The approved CD20 antibodies rituximab, ofatumumab, and obinutuzumab are of human immunoglobulin G1 (IgG1) isotype. We investigated the impact of the variable regions of these 3 CD20 antibodies when expressed as human IgA2 isotype variants. All 3 IgA2 antibodies mediated antibody-dependent cellular phagocytosis (ADCP) by macrophages and antibody-dependent cellular cytotoxicity (ADCC) by polymorphonuclear cells. Both effector mechanisms were significantly enhanced in the presence of a CD47-blocking antibody or by glutaminyl cyclase inhibition to interfere with CD47-SIRPα interactions. Interestingly, an IgA2 variant of obinutuzumab (OBI-IgA2) was consistently more potent than an IgA2 variant of rituximab (RTX-IgA2) or an IgA2 variant of ofatumumab (OFA-IgA2) in triggering ADCC. Furthermore, we observed more effective direct tumor cell killing by OBI-IgA2 compared with RTX-IgA2 and OFA-IgA2, which was caspase independent and required a functional cytoskeleton. IgA2 variants of all 3 antibodies triggered complement-dependent cytotoxicity, with OBI-IgA2 being less effective than RTX-IgA2 and OFA-IgA2. When we investigated the therapeutic efficacy of the CD20 IgA2 antibodies in different in vivo models, OBI-IgA2 was therapeutically more effective than RTX-IgA2 or OFA-IgA2. In vivo efficacy required the presence of a functional IgA receptor on effector cells and was independent of complement activation or direct lymphoma cell killing. These data characterize the functional activities of human IgA2 antibodies against CD20, which were affected by the selection of the respective variable regions. OBI-IgA2 proved particularly effective in vitro and in vivo, which may be relevant in the context of CD47-SIRPα blockade.

Neutrophils are abundantly present in the synovium and synovial fluid of patients suffering from arthritis. Neutrophils can be activated by a multitude of stimuli and the current dogma states that this is a two-step process, consisting of a priming step followed by an activation step. Considering that neutrophil activation occurs in an inflammatory environment, where multiple stimuli are present, we argue that a two-step process is highly unlikely. Here, we indeed demonstrate that neutrophils require simultaneous ligation of two different receptors for efficient activation. We isolated human peripheral blood neutrophils and cultured them with various combinations of stimuli (GM-CSF, fMLF, TNF, and LPS). Next, we evaluated essential neutrophil functions, including degranulation and ROS production using flow cytometry, mediator release using ELISA, NETosis by a live cell imaging method, phagocytosis by imaging flow cytometry, and extracellular vesicle (EV) release quantified by high-resolution flow cytometry. Exposure of neutrophils to any combination of stimuli, but not to single stimuli, resulted in significant degranulation, and mediator and EV release. Furthermore, ROS production increased substantially by dual stimulation, yet appeared to be more dependent on the type of stimulation than on dual stimulation. Phagocytosis was induced to its maximum capacity by a single stimulus, while NETosis was not induced by any of the used physiological stimuli. Our data indicate that neutrophil activation is tightly regulated and requires activation by two simultaneous stimuli, which is largely independent of the combination of stimuli.

Various cancer types exhibit characteristic and recurrent aneuploidy patterns. The origins of these cancer type-specific karyotypes are still unknown, partly because introducing or eliminating specific chromosomes in human cells still poses a challenge. Here, we describe a novel strategy to induce mis-segregation of specific chromosomes in different human cell types. We employed Tet repressor or nuclease-dead Cas9 to link a microtubule minus-end-directed kinesin (Kinesin14VIb) from Physcomitrella patens to integrated Tet operon repeats and chromosome-specific endogenous repeats, respectively. By live- and fixed-cell imaging, we observed poleward movement of the targeted loci during (pro)metaphase. Kinesin14VIb-mediated pulling forces on the targeted chromosome were counteracted by forces from kinetochore-attached microtubules. This tug-of-war resulted in chromosome-specific segregation errors during anaphase and revealed that spindle forces can heavily stretch chromosomal arms. By single-cell whole-genome sequencing, we established that kinesin-induced targeted mis-segregations predominantly result in chromosomal arm aneuploidies after a single cell division. Our kinesin-based strategy opens the possibility to investigate the immediate cellular responses to specific aneuploidies in different cell types; an important step toward understanding how tissue-specific aneuploidy patterns evolve.

Microtubules are one of the major types of cytoskeletal filaments in cells. They are very dynamic polymers composed of αβ-tubulin dimers arranged longitudinally in head-to-tail fashion as well as laterally to assemble 13-protofilament hollow cylindrical tubes. The incorporation of GTP-bound αβ-tubulin dimers generates a fast growing plus end exposing β-tubulin and a slow growing minus end exposing α-tubulin. In cells, microtubules are assembled de novo from a template, called γ-TuRC, which interacts with α-tubulin. Microtubules can either remain capped by γ-TuRC and anchored to the microtubule-organizing centers (MTOCs) or be released if they are cut by microtubule severing enzymes like katanin. The release of microtubules from MTOC generates free minus ends, which are then stabilized by minus-end binding proteins called CAMSAPs. However, the plus ends remain very dynamic and undergo transitions from growth to shrinkage, termed “catastrophes”, and the opposite transitions termed “rescues”. Numerous microtubule regulatory proteins act at the plus ends, minus ends and the microtubule shafts connecting the two ends to control the organization and density of cellular microtubule networks. In this thesis, we focused on each of these aspects and explored the dynamic life of microtubules by reconstituting these processes in vitro using purified proteins. We first focused on the birth and growth of microtubules. We reconstituted microtubule nucleation using purified γ-TuRC and microtubule regulatory proteins and showed that CDK5RAP2, CLASP2 and chTOG promoted microtubule nucleation from γ-TuRC. We discovered that CAMSAPs can bind to γ-TuRC-capped microtubule minus ends and displace γ-TuRC from these ends, generating free and stable microtubule minus ends. Furthermore, we found out that CDK5RAP2, but not CLASP2 or chTOG, can inhibit CAMSAP binding and microtubule release. We propose that the destiny of a microtubule depends on the type of protein complex that activates its nucleation. We then described a mechanism for stabilization of microtubule lattice by TRIM46, a neuronal protein, which can bundle parallel microtubules and promote microtubule rescues within these bundles. We also revealed that Ankyrin-G, a scaffold protein, can recruit TRIM46-stabilized microtubule bundles to the axonal membrane to drive the assembly of the axon initial segment in neurons. We also uncovered a new role of CLASP2 as a microtubule repair factor participating in microtubule maintenance. We demonstrated that CLASP2, an anti-catastrophe factor, can promote complete repair of damaged microtubule lattices by inhibiting microtubule depolymerization and promoting tube closure at the damage sites, causing lattice renewal. Finally, we described a three-protein module involving katanin, CAMSAPs, and WDR47 that can regulate microtubule polymer mass and minus-end stability. We showed that katanin can cut and amplify CAMSAP2/3-stabilized microtubule minus ends. WDR47 can inhibit the binding of katanin to CAMSAP2/3-stabilized minus ends and protect them from severing. The presence of WDR47 shifts the balance from microtubule amplification to minus-end growth regulation. To conclude, we obtained mechanistic insights into the regulation of microtubule nucleation, minus-end dynamics, lattice stabilization and maintenance, microtubule number and the interplay between microtubule regulatory proteins. These insights will help to understand how microtubule arrays are organized in cells.

The centrosome is the main microtubule organizing center (MTOC) in animal cells. It consists of a pair of centrioles surrounded by pericentriolar material (PCM) that nucleates and anchors microtubules. The binding of PCM to centrioles, PCM self-assembly and PCM transport by the microtubule-based motor dynein are the prerequisites for centrosome assembly, but the mechanisms of PCM self-assembly in interphase cells are poorly understood. Moreover, the relative importance of different molecular pathways of PCM assembly varies between different cell systems and different phases of the cell cycle. Previous studies have mostly focused on mitotic cells, but systematic investigations have not been performed in interphase cells. In addition, γ-tubulin ring complex (γ-TuRC) as template for microtubule assembly largely determines the function of MTOC, but the relative importance of different centrosomal components in microtubule nucleation and maintenance of microtubule networks has not been systematically investigated. Moreover, it is unclear whether γ-TuRC can nucleate microtubules and maintain microtubule arrays in mammalian cells in the absence of γ-TuRC adaptors. In this thesis, we address these questions. In addition to the PCM, the other major component of the centrosome are centrioles, microtubule-based organelles whose formation and elongation are regulated by a large number of specific proteins. In vitro reconstitution experiments showed that two centriole biogenesis factors, CPAP and CP110, cooperatively regulate microtubule plus-end growth in vitro, but the importance of their interaction for centriole formation has not been examined on cells. In this thesis, we investigated the role of the CPAP-CP110 interaction in regulating centriole elongation. In Chapter 2, we reported that when centrioles are lost due to either depletion or inhibition of PLK4, self-clustering of PCM proteins is sufficient to form a compact acentriolar MTOC (caMTOC) and thus organize a dense radial microtubule array in interphase cells. We showed that such self-clustering of PCM is dynein-dependent and requires pericentrin, CDK5RAP2, ninein and γ-tubulin, but not CEP192, NEDD1 and CEP152. We found that the formation of caMTOC is sensitive to the non-centrosomal MTOC pathways such as AKAP450-dependent PCM recruitment to the Golgi apparatus and CAMSAP2-mediated stabilization of free microtubule minus ends. We also reproduced the findings of our cellular experiments using computer simulations. In Chapter 3, we describe tools for comprehensive and systematic functional analysis of the complex regulatory pathways of microtubule nucleation and anchoring at mammalian centrosomes using a cellular model that simultaneously lacks multiple PCM components. We found that in cells lacking the non-centrosomal microtubule minus-end stabilization pathways, different γ-TuRC adaptors exert additive effects on microtubule nucleation and attachment at the centrosome, and knocking them out in interphase cells prevents γ-TuRC from generating a dense microtubule array, even though all positive regulators of microtubule dynamics, are still present. In Chapter 4, we examined the importance of CPAP-CP110 interaction in cells by replacing wild-type CPAP with CPAP mutants that cannot bind CP110. Our data indicate that centrioles, albeit shorter ones, could still form in the presence of the CPAP mutant, demonstrating that the CPAP-CP110 interaction is not essential for centriole formation, but promotes centriole elongation.

Microtubules are cytoskeletal polymers that separate chromosomes during mitosis and serve as rails for intracellular transport and organelle positioning. Manipulation of microtubules is widely used in cell and developmental biology, but tools for precise subcellular spatiotemporal control of microtubules are currently lacking. Here, we describe a light-activated system for localized recruitment of the microtubule-severing enzyme katanin. This system, named opto-katanin, uses targeted illumination with blue light to induce rapid, localized, and reversible microtubule depolymerization. This tool allows precise clearing of a subcellular region of microtubules while preserving the rest of the microtubule network, demonstrating that regulation of katanin recruitment to microtubules is sufficient to control its severing activity. The tool is not toxic in the absence of blue light and can be used to disassemble both dynamic and stable microtubules in primary neurons as well as in dividing cells. We show that opto-katanin can be used to locally block vesicle transport and to clarify the dependence of organelle morphology and dynamics on microtubules. Specifically, our data indicate that microtubules are not required for the maintenance of the Golgi stacks or the tubules of the endoplasmic reticulum but are needed for the formation of new membrane tubules. Finally, we demonstrate that this tool can be applied to study the contribution of microtubules to cell mechanics by showing that microtubule bundles can exert forces constricting the nucleus.