Isotropic-Nematic transition of long thin hard spherocylinders confined in a quasi-two-dimensional planar geometry

We present computer simulations of long thin hard spherocylinders in a narrow planar slit. We observe a transition from the isotropic to a nematic phase with quasi-long-range orientational order upon increasing the density. This phase transition is intrinsically two dimensional and of the Kosterlitz-Thouless type. The effective two-dimensional density at which this transition occurs increases with plate separation. We qualitatively compare some of our results with experiments where microtubules are confined in a thin slit, which gave the original inspiration for this work.Comment: 8 pages, 10 figure

Dynamic instability of microtubules is regulated by force

Microtubules are long filamentous protein structures that randomly alternate between periods of elongation and shortening in a process termed dynamic instability. The average time a microtubule spends in an elongation phase, known as the catastrophe time, is regulated by the biochemical machinery of the cell throughout the cell cycle. In this light, observed changes in the catastrophe time near cellular boundaries (Brunner, D., and P. Nurse. 2000. Cell. 102:695–704; Komarova, Y.A., I.A. Vorobjev, and G.G. Borisy. 2002. J. Cell Sci. 115:3527–3539) may be attributed to regulatory effects of localized proteins. Here, we argue that the pushing force generated by a microtubule when growing against a cellular object may itself provide a regulatory mechanism of the catastrophe time. We observed an up to 20-fold, force-dependent decrease in the catastrophe time when microtubules grown from purified tubulin were polymerizing against microfabricated barriers. Comparison with catastrophe times for microtubules growing freely at different tubulin concentrations leads us to conclude that force reduces the catastrophe time only by limiting the rate of tubulin addition

General theory for the mechanics of confined microtubule asters

In cells, dynamic microtubules organize into asters or spindles to assist positioning of organelles. Two types of forces are suggested to contribute to the positioning process: (i) microtubule-growth based pushing forces ; and (ii) motor protein mediated pulling forces. In this paper, we present a general theory to account for aster positioning in a confinement of arbitrary shape. The theory takes account of microtubule nucleation, growth, catastrophe, slipping, as well as interaction with cortical force generators. We calculate microtubule distributions and forces acting on microtubule organizing centers in a sphere and in an ellipsoid. Positioning mechanisms based on both pushing forces and pulling forces can be distinguished in our theory for different parameter regimes or in different geometries. In addition, we investigate positioning of microtubule asters in the case of asymmetric distribution of motors. This analysis enables us to characterize situations relevant for Caenorrhabditis elegans embryos

Formation of helical membrane tubes around microtubules by single-headed kinesin KIF1A

The kinesin-3 motor KIF1A is in charge of vesicular transport in neuronal axons. Its single-headed form is known to be very inefficient due to the presence of a diffusive state in the mechanochemical cycle. However, recent theoretical studies have suggested that these motors could largely enhance force generation by working in teams. Here we test this prediction by challenging single-headed KIF1A to extract membrane tubes from giant vesicles along microtubule filaments in a minimal in vitro system. Remarkably, not only KIF1A motors are able to extract tubes but they feature a novel phenomenon: tubes are wound around microtubules forming tubular helices. This finding reveals an unforeseen combination of cooperative force generation and self-organized manoeuvreing capability, suggesting that the diffusive state may be a key ingredient for collective motor performance under demanding traffic conditions. Hence, we conclude that KIF1A is a genuinely cooperative motor, possibly explaining its specificity to axonal trafficking.Peer ReviewedPostprint (published version

Formation of helical membrane tubes around microtubules by single-headed kinesin KIF1A

Author correction available at: https://doi.org/10.1038/s41467-019-11010-5The kinesin-3 motor KIF1A is in charge of vesicular transport in neuronal axons. Its single-headed form is known to be very inefficient due to the presence of a diffusive state in the mechanochemical cycle. However, recent theoretical studies have suggested that these motors could largely enhance force generation by working in teams. Here we test this prediction by challenging single-headed KIF1A to extract membrane tubes from giant vesicles along microtubule filaments in a minimal in vitro system. Remarkably, not only KIF1A motors are able to extract tubes but they feature a novel phenomenon: tubes are wound around microtubules forming tubular helices. This finding reveals an unforeseen combination of cooperative force generation and self-organized manoeuvreing capability, suggesting that the diffusive state may be a key ingredient for collective motor performance under demanding traffic conditions. Hence, we conclude that KIF1A is a genuinely cooperative motor, possibly explaining its specificity to axonal trafficking

Society and synthetic cells:A position paper by the Future Panel on Synthetic Life

The BaSyC consortium, whose acronym stands for building a synthetic cell, proposes to develop a synthetic cell from the bottom up. In the context of this joined effort, the Rathenau Instituut and Radboud University Nijmegen have organised the Future Panel on Synthetic Life, consisting of societal experts, to explore the social challenges, dilemmas, and possible societal impacts of synthetic cell research, and to advise how this research may contribute to a fair and sustainable future. The goal for the Future Panel is to create an initial agenda for future political, academic, and public debate on the synthetic cell.The profile of science and technology is two-sided. On the one hand, they act as drivers for problem-solving, progress, and emancipation, but techno-scientific innovation can also give rise to disruptive threats. Therefore, societal reflection should be timely and anticipatory. Rather than asking what risks and benefits are involved, the question will be how to engage society in such a way that synthetic cell research can become a joint endeavour, responsive to societal hopes and concerns. Consequently, the Future Panel aimed to:• map the social challenges and dilemmas in a society where a synthetic cell exists;• identify conditions under which synthetic cell technology can be considered beneficial for society; and• advice on how these conditions can be realised.To contribute to this, the Future Panel discussed the role and perspectives of key stakeholders (academia, government and governance, industry, and civil society), besides more specific issues like public responses, biosafety, biosecurity, and intellectual property rights during multiple online and offline meetings within a period of two years. This position paper summarises the most important points of conversation, shared insights, key challenges, dilemmas that were discussed during these meetings, resulting in four recommendations, as a starting point for further analysis and debate.Key challengesDuring the deliberations, the Future Panel encountered four overarching challenges.1. The novelty of synthetic cell research makes it challenging to devise amethodology capable of anticipating public concerns in a domain where overt public attitudes do not exist as of yet.Society and synthetic cells 132. As long as the existing power structures within the contexts that shape developments in science and technology are not explicitly addressed, the development of a synthetic cell will inevitably reproduce and may even strengthen existing power inequalities.3. In order to involve civil society and allow citizens to articulate their views and concerns, besides factual information, the synthetic cell has to be positioned in a proper context: how to develop a responsible narrative that allows the public to actively relate to these developments?4. Even though the BaSyC project is halfway, there are still many unknowns, even unknown unknowns. A key challenge is to connect social, ethical, and science perspectives, and dilemmas, ambitions, and uncertainties related to the building of a synthetic cell.DilemmasDuring the panel discussions, many reasons have arisen, from different perspectives, for involving the general public, governments, industry and NGOs in an anticipatory way. However, doing this reveals some fundamental dilemmas and tensions that should be addressed.1. The BaSyC project is curiosity-driven, aspiring to deepen our understanding of life. At the same time, our desire to know is driven by an impetus to control. How to practice synthetic cell research as a dialogue with nature rather than an appropriation and instrumentalisation of the living cell?2. Many aspects of synthetic cell research are yet unknown. How to allow space for the unknown while, at the same time, opt for an anticipatory and imaginative approach to take the future social and ethical implications and concerns into account?3. How to make research more inclusive by involving public, politics and policy in such a way that it is fostering and inspirational rather than detrimental for curiosity-driven experimentation and exploration?4. Curiosity-driven science requires a great deal of specialism and thrives on serendipity. How to achieve convergence in science, involving multiple stakeholders and taking into account societal expectations and concerns, without frustrating the process of discovery?5. Deliberation requires a dialogue across disciplines, languages, and levels of information. How to combine different vocabularies, perspectives, socio- cultural and time horizons in a meaningful way?6. Within science and technology, and in particular biotechnology, there has long been a discussion about how to deal with knowledge and intellectual property rights. Should life be considered patentable or should life be seen as a common heritage that belongs to everybody?7. How to deal with researchers who need to make their work openly accessible, and companies, incubators, and organisations that want to protect their invention?8. Within projects of four to five years, researchers are under pressure to focus on and deliver scientific publications, while at the same time being encouraged to actively reflect on and engage with the potential societal impact of their work. How to balance conflicting expectations related to different time horizons?RecommendationsThe Future Panel proposes four recommendations for fostering a socially responsible development of the synthetic cell:1. Ensure that the synthetic cell contributes to a fair and sustainable futureTo foster sustainable synthetic cells, we need co-constructed narratives that allow us to explore how synthetic cells may contribute to a sustainable future. It is not enough to stimulate techno-scientific innovation as such. Governments must simultaneously stimulate social innovation, and promote broad stakeholder involvement in synthetic cell research.2. Organise participation of civil society in synthetic cell researchIn order to ensure that synthetic cell research contributes to a fair and sustainable society, an inclusive and participatory process of reflection is required, open to public intelligence, and sensitive to societal expectations and concerns. This requires innovative methods to engage the wisdom of the crowd. Meetings with societal stakeholders should be organised on relevant issues at different moments of the project and should be designed as in-between spaces in which different meanings, interests, and societal values come together and are made explicit.3. Foster a socially responsive academic ecosystemRather than endorsing the status quo, synthetic cell research emphasises the importance of rethinking the university of the 21st century, where research and education must become more inclusive and interactive, bent on developing long- term partnerships with society: with industry and governmental organisations, but first and foremost with society at large. Societal reflection and interaction with society should be an integral part of academic research and education. Therefore, researchers must be empowered to engage with society in such a way that dialogue and interaction become an inherent part of their work, from design to publication.4. Design social governance experiments aimed at renewing the regulatory landscape for new biotechnologies, including the synthetic cellEnsuring that the synthetic cell may contribute to a more sustainable and socially equitable world requires an adequate social understanding of governance and regulatory systems. The current regulatory system is not prepared for that task. We need a new system, which does not reproduce previous polemics. Besides looking at risks, a more comprehensive regulatory regime would integrate questions concerning sustainability, human rights, ethics, and societal desirability. Governance experiments co-designed with societal actors are needed to gain insight into the contours of such a new regulatory landscape on synthetic biology or new biotechnologies, including the synthetic cell

Cross-linkers at growing microtubule ends generate forces that drive actin transport

The actin and microtubule cytoskeletons form active networks in the cell that can contract and remodel, resulting in vital cellular processes such as cell division and motility. Motor proteins play an important role in generating the forces required for these processes, but more recently the concept of passive cross-linkers being able to generate forces has emerged. So far, these passive cross-linkers have been studied in the context of separate actin and microtubule systems. Here, we show that cross-linkers also allow actin and microtubules to exert forces on each other. More specifically, we study single actin filaments that are cross-linked to growing microtubule ends, using in vitro reconstitution, computer simulations, and a minimal theoretical model. We show that microtubules can transport actin filaments over large (micrometer-range) distances and find that this transport results from two antagonistic forces arising from the binding of cross-linkers to the overlap between the actin and microtubule filaments. The cross-linkers attempt to maximize the overlap between the actin and the tip of the growing microtubules, creating an affinity-driven forward condensation force, and simultaneously create a competing friction force along the microtubule lattice. We predict and verify experimentally how the average transport time depends on the actin filament length and the microtubule growth velocity, confirming the competition between a forward condensation force and a backward friction force. In addition, we theoretically predict and experimentally verify that the condensation force is of the order of 0.1 pN. Thus, our results reveal an active mechanism for local actin remodeling by growing microtubules that relies on passive cross-linkers