Skp1 proteins are structural components of the synaptonemal complex in C. elegans.

The synaptonemal complex (SC) is a zipper-like protein assembly that links homologous chromosomes to regulate recombination and segregation during meiosis. The SC has been notoriously refractory to in vitro reconstitution, thus leaving its molecular organization largely unknown. Here, we report a moonlighting function of two paralogous S-phase kinase-associated protein 1 (Skp1)-related proteins (SKR-1 and SKR-2), well-known adaptors of the Skp1-Cul1-F-box (SCF) ubiquitin ligase, as the key missing components of the SC in Caenorhabditis elegans. SKR proteins repurpose their SCF-forming interfaces to dimerize and interact with meiosis-specific SC proteins, thereby driving synapsis independent of SCF activity. SKR-1 enables the formation of the long-sought-after soluble complex with previously identified SC proteins in vitro, which we propose it to represent a complete SC building block. Our findings demonstrate how a conserved cell cycle regulator has been co-opted to interact with rapidly evolving meiotic proteins to construct the SC and provide a foundation for understanding its structure and assembly mechanisms.

The Limitations and Potentials of Evaluating Economic Aspects of Community-Based Health Promotion: A Critical Review.

Community-based health promotion (CBHP) interventions are promising approaches to address public health problems; however, their economic evaluation presents unique challenges. This review aims to explore the opportunities and limitations of evaluating economic aspects of CBHP, focusing on the assessment of intervention costs and outcomes, and the consideration of political-level changes and health equity. A systematic search of the PubMed, Web of Science and PsycInfo databases identified 24 CBHP interventions, the majority of which targeted disadvantaged communities. Only five interventions included a detailed cost/resource assessment. Outcomes at the operational level were mainly quantitative, related to sociodemographics and environment or health status, while outcomes at the political level were often qualitative, related to public policy, capacity building or networks/collaboration. The study highlights the limitations of traditional health economic evaluation methods in capturing the complexity of CBHP interventions. It proposes the use of cost-consequence analysis (CCA) as a more comprehensive approach, offering a flexible and multifaceted assessment of costs and outcomes. However, challenges remain in the measurement and valuation of outcomes, equity considerations, intersectoral costs and attribution of effects. While CCA is a promising starting point, further research and methodological advancements are needed to refine its application and improve decision making in CBHP.

Segmentation of C. elegans germline nuclei.

Immunofluorescence microscopy is a widely adopted method for studying meiotic prophase in the nematode model organism, Caenorhabditis elegans . An in-depth examination of specific meiotic processes requires the quantitative analysis of immunofluorescence images, which often involves the segmentation of individual cells or nuclei. Here, we introduce our image analysis pipeline to automate significant portions of this task. This pipeline relies on the powerful deep learning model Cellpose 2.0 to segment cellular structures. To further improve the segmentation accuracy for germline nuclei stained for chromatin or synaptonemal complexes, we retrained the generalist Cellpose model and integrated our data processing pipeline into the easy-to-use Cell-ACDC image analysis software. Our pipeline thus makes deep learning-based segmentation of nuclei in the distal germline of C. elegans accessible for users without coding experience.

Pulsed stimulated Brillouin microscopy enables high-sensitivity mechanical imaging of live and fragile biological specimens.

Brillouin microscopy is an emerging optical elastography technique capable of assessing mechanical properties of biological samples in a three-dimensional, all-optical and noncontact fashion. The typically weak Brillouin scattering signal can be substantially enhanced via a stimulated Brillouin scattering (SBS) process; however, current implementations require high pump powers, which prohibit applications to photosensitive or live imaging of biological samples. Here we present a pulsed SBS scheme that takes advantage of the nonlinearity of the pump-probe interaction. In particular, we show that the required pump laser power can be decreased ~20-fold without affecting the signal levels or spectral precision. We demonstrate the low phototoxicity and high specificity of our pulsed SBS approach by imaging, with subcellular detail, sensitive single cells, zebrafish larvae, mouse embryos and adult Caenorhabditis elegans. Furthermore, our method permits observing the mechanics of organoids and C. elegans embryos over time, opening up further possibilities for the field of mechanobiology.

Multi-level stakeholders' perspectives on implementation and scaling up community-based health promotion in Germany.

Community-based health promotion has the potential to address existing health inequities, although such approaches are scarcely scaled up. For a successful scale up, various stakeholders at different levels and sectors need to be involved. The article's aims are to assess what kind of external support communities need for implementation and to identify facilitators and barriers for scaling up community-based health promotion. Two national digital workshops were conducted in Germany with stakeholders at the community level (n = 161) and with stakeholders at the federal and state levels (n = 84). Protocols were compiled and coded using qualitative content analysis. During the first workshop, we revealed 11 themes for external support needs ('Strategic approach', 'Define & compare indicators', 'International human resource', 'Tools & Aids', 'External conduction of the assessment', 'Involvement of people in difficult life situations', 'Overview of actors', 'Moderation', 'Obtain funding', 'Quality assurance/evaluation' and 'External support'). Eleven facilitators and barriers were identified for scaling up ('Assessment and evaluation', 'Intersectoral collaboration and partnerships', 'Communication', 'Characteristics of the program', 'Political and legal conditions', 'Political support', 'Local coordinator', 'Resources', 'Participation', 'Strategic planning/methods' and 'Intermediary organization'). The identified results provide practice-based evidence on support needed for scaling up, facilitators that promote scaling up and barriers that might hinder scaling up community-based health promotion in Germany. In a next step, this practice-based evidence needs to be systematically integrated with scientific-based evidence on key components for scaling up such approaches for the development of an effective scaling-up concept.

The major aim of this article is to describe how German stakeholders at different levels (local, federal and state) from various sectors (e.g. health, social or sports) were involved to develop a scaling-up concept for a community-based health promotion approach with a focus on health equity in the field of physical activity promotion. Different actors from science, policy and practice discussed facilitators and barriers for the scale up and what external support communities need to implement a community-based health promotion approach, with a focus on people in difficult life situations (e.g. individuals with social disadvantages). We identified 14 different kind of support needs and 11 facilitators and barriers for scaling up community-based health promotion approaches in the field of physical activity promotion. Various actors emphasized the need for improved collaboration and partnerships across several sectors as well as changes in current political and legal conditions. Furthermore, the role and some characteristics of an external organization to support a successful scaling-up process were also discussed among actors.

Population-based physical activity promotion with a focus on health equity: a review of reviews.

BACKGROUND: The extent to which people are physically active is dependent upon social gradients. Numerous studies have shown that especially people with social disadvantages do not meet the physical activity (PA) recommendations. A promising strategy to alleviate this issue are approaches that promote PA in the general population. In addition, several researchers have raised concerns that population-based health interventions may increase health inequities. The aim of the current review of reviews was therefore to identify successful population-based PA promotion approaches with a particular focus on health equity. METHODS: Six electronic databases were examined for systematic reviews on population-based PA promotion for the period 2015 to 2021. A reference list and grey literature search were also conducted. Two independent reviewers used inclusion/exclusion criteria to screen titles and abstracts of the potentially relevant literature and conducted a quality assessment for each identified review. All included reviews of population-based approaches for PA promotion with a focus on disadvantaged populations and/or health equity were narratively summarized. RESULTS: Our search resulted in 4,411 hits. After a systematic review process, six reviews met the inclusion criteria and were included after they were all rated as high quality. We identified that mass-media campaigns, point-of-decision prompts, environmental approaches, policy approaches, and community-based multi-component approaches can promote PA in the general population. Across populations with social disadvantages mass-media campaigns, point-of-decision prompts and policy approaches are likely to be effective as long as they are tailored. Regarding environmental approaches, the results are inconsistent. None of the reviews on community-based multi-component approaches provided evidence on health equity. CONCLUSION: There are several effective approaches to promote PA in the general population but evidence regarding health equity is still sparse. Future studies should therefore pay more attention to this missing focus. Furthermore, there is a lack of evidence regarding the type of tailoring and the long-term impact of population-based approaches to PA promotion. However, this requires appropriate funding programmes, complex study designs and evaluation methods.

Super-Resolution Microscopy of the Synaptonemal Complex within the Caenorhabditis elegans Germline.

During meiosis, homologous chromosomes must recognize and adhere to one another to allow for their correct segregation. One of the key events that secures the interaction of homologous chromosomes is the assembly of the synaptonemal complex (SC) in meiotic prophase I. Even though there is little sequence homology between protein components within the SC among different species, the general structure of the SC has been highly conserved during evolution. In electron micrographs, the SC appears as a tripartite, ladder-like structure composed of lateral elements or axes, transverse filaments, and a central element. However, precisely identifying the localization of individual components within the complex by electron microscopy to determine the molecular structure of the SC remains challenging. By contrast, fluorescence microscopy allows for the identification of individual protein components within the complex. However, since the SC is only ~100 nm wide, its substructure cannot be resolved by diffraction-limited conventional fluorescence microscopy. Thus, determining the molecular architecture of the SC requires super-resolution light microscopy techniques such as structured illumination microscopy (SIM), stimulated-emission depletion (STED) microscopy, or single-molecule localization microscopy (SMLM). To maintain the structure and interactions of individual components within the SC, it is important to observe the complex in an environment that is close to its native environment in the germ cells. Therefore, we demonstrate an immunohistochemistry and imaging protocol that enables the study of the substructure of the SC in intact, extruded Caenorhabditis elegans germline tissue with SMLM and STED microscopy. Directly fixing the tissue to the coverslip reduces the movement of the samples during imaging and minimizes aberrations in the sample to achieve the high resolution necessary to visualize the substructure of the SC in its biological context.

Development of a Framework for Scaling Up Community-Based Health Promotion: A Best Fit Framework Synthesis.

Community-based health promotion with a focus on people with social disadvantages is essential to address persistently existing health inequities. However, achieving an impact on public health requires scaling up such approaches beyond manifold funded pilot projects. The aim of this qualitative review is to provide an overview of scaling-up frameworks in health promotion and to identify key components for scaling up community-based health promotion. First, we conducted a systematic search for scaling-up frameworks for health promotion in PubMed, CINAHL, Scopus, Web of Science, PsycInfo, and SportDiscus. Based on the included frameworks, we created an a priori framework. Second, we searched for primary research studies in the same databases that reported scaling-up processes of community-based health promotion. We coded the data using the a priori framework. From 80 articles, a total of 12 frameworks were eligible, and 5 were included for data extraction. The analysis yielded 10 a priori defined key components: "innovation characteristics"; "clarify and coordinate roles and responsibilities"; "build up skills, knowledge, and capacity"; "mobilize and sustain resources"; "initiate and maintain regular communication"; "plan, conduct, and apply assessment, monitoring, and evaluation"; "develop political commitment and advocacy"; "build and foster collaboration"; "encourage participation and ownership"; and "plan and follow strategic approaches". We further identified 113 primary research studies; 10 were eligible. No new key components were found, but all a priori defined key components were supported by the studies. Ten key components for scaling up community-based health promotion represent the final framework. We further identified "encourage participation and ownership" as a crucial component regarding health equity.

Co-producing an action-oriented framework for community-based Physical Activity Promotion in Germany.

German National Recommendations for Physical Activity (PA) and PA Promotion recommend community-based approaches to promote PA at the local level with a focus on health equity. In addition, the German Federal Prevention Act addresses health equity and strengthens setting-based health promotion in communities. However, the implementation of both in the local context remains a challenge. This article describes Phase 1 of the KOMBINE project that aims to co-produce an action-oriented framework for community-based PA promotion focusing on structural change and health equity. (i) In a series of workshops, key stakeholders and researchers discussed facilitators, barriers and needs of community-based PA promotion focusing on health equity. (ii) The research team used an inductive approach to cluster all findings and to identify key components and then (iii) compared the key components with updated literature. (iv) Key components were discussed and incorporated into a gradually co-produced framework by the participants. The first result of the co-production process was a catalog of nine key components regarding PA-related health promotion in German communities. The comparison of key components with scientific evidence showed a high overlap. Finally, a six-phase action-oriented framework including key components for community-based PA promotion was co-produced. The six-phase action-oriented framework integrates practice-based and scientific evidence on PA-related health promotion and health equity. It represents a shared vision for the implementation of National Recommendations for PA and PA Promotion in Germany. The extent to which structural changes and health equity can be achieved is currently being investigated in pilot-studies.

This paper describes how the participants in the KOMBINE project were involved in an innovative approach to transfer the German National Recommendations for Physical Activity (PA) and PA Promotion into the local practice of communities. Scientists, politicians and community actors (e.g. mayors, heads of sports departments) discussed their knowledge and experiences of facilitators, barriers and needs to promote PA in communities, specifically for people in difficult life situations (e.g. individuals with social disadvantages). Based on the results, they jointly developed key components and an action-oriented framework to implement the National Recommendations for PA and PA Promotion in German communities.

Analysis of meiosis in Pristionchus pacificus reveals plasticity in homolog pairing and synapsis in the nematode lineage.

Meiosis is conserved across eukaryotes yet varies in the details of its execution. Here we describe a new comparative model system for molecular analysis of meiosis, the nematode Pristionchus pacificus, a distant relative of the widely studied model organism Caenorhabditis elegans. P. pacificus shares many anatomical and other features that facilitate analysis of meiosis in C. elegans. However, while C. elegans has lost the meiosis-specific recombinase Dmc1 and evolved a recombination-independent mechanism to synapse its chromosomes, P. pacificus expresses both DMC-1 and RAD-51. We find that SPO-11 and DMC-1 are required for stable homolog pairing, synapsis, and crossover formation, while RAD-51 is dispensable for these key meiotic processes. RAD-51 and DMC-1 localize sequentially to chromosomes during meiotic prophase and show nonoverlapping functions. We also present a new genetic map for P. pacificus that reveals a crossover landscape very similar to that of C. elegans, despite marked divergence in the regulation of synapsis and crossing-over between these lineages.

Quantitative cytogenetics reveals molecular stoichiometry and longitudinal organization of meiotic chromosome axes and loops.

During meiosis, chromosomes adopt a specialized organization involving assembly of a cohesin-based axis along their lengths, with DNA loops emanating from this axis. We applied novel, quantitative, and widely applicable cytogenetic strategies to elucidate the molecular bases of this organization using Caenorhabditis elegans. Analyses of wild-type (WT) chromosomes and de novo circular minichromosomes revealed that meiosis-specific HORMA-domain proteins assemble into cohorts in defined numbers and co-organize the axis together with 2 functionally distinct cohesin complexes (REC-8 and COH-3/4) in defined stoichiometry. We further found that REC-8 cohesins, which load during S phase and mediate sister-chromatid cohesion, usually occur as individual complexes, supporting a model wherein sister cohesion is mediated locally by a single cohesin ring. REC-8 complexes are interspersed in an alternating pattern with cohorts of axis-organizing COH-3/4 complexes (averaging 3 per cohort), which are insufficient to confer cohesion but can bind to individual chromatids, suggesting a mechanism to enable formation of asymmetric sister-chromatid loops. Indeed, immunofluorescence/fluorescence in situ hybridization (immuno-FISH) assays demonstrate frequent asymmetry in genomic content between the loops formed on sister chromatids. We discuss how features of chromosome axis/loop architecture inferred from our data can help to explain enigmatic, yet essential, aspects of the meiotic program.

Identification of novel synaptonemal complex components in C. elegans.

The synaptonemal complex (SC) is a tripartite protein scaffold that forms between homologous chromosomes during meiosis. Although the SC is essential for stable homologue pairing and crossover recombination in diverse eukaryotes, it is unknown how individual components assemble into the highly conserved SC structure. Here we report the biochemical identification of two new SC components, SYP-5 and SYP-6, in Caenorhabditis elegans. SYP-5 and SYP-6 are paralogous to each other and play redundant roles in synapsis, providing an explanation for why these genes have evaded previous genetic screens. Superresolution microscopy reveals that they localize between the chromosome axes and span the width of the SC in a head-to-head manner, similar to the orientation of other known transverse filament proteins. Using genetic redundancy and structure-function analyses to truncate C-terminal tails of SYP-5/6, we provide evidence supporting the role of SC in both limiting and promoting crossover formation.

A compartmentalized signaling network mediates crossover control in meiosis.

During meiosis, each pair of homologous chromosomes typically undergoes at least one crossover (crossover assurance), but these exchanges are strictly limited in number and widely spaced along chromosomes (crossover interference). The molecular basis for this chromosome-wide regulation remains mysterious. A family of meiotic RING finger proteins has been implicated in crossover regulation across eukaryotes. Caenorhabditis elegans expresses four such proteins, of which one (ZHP-3) is known to be required for crossovers. Here we investigate the functions of ZHP-1, ZHP-2, and ZHP-4. We find that all four ZHP proteins, like their homologs in other species, localize to the synaptonemal complex, an unusual, liquid crystalline compartment that assembles between paired homologs. Together they promote accumulation of pro-crossover factors, including ZHP-3 and ZHP-4, at a single recombination intermediate, thereby patterning exchanges along paired chromosomes. These proteins also act at the top of a hierarchical, symmetry-breaking process that enables crossovers to direct accurate chromosome segregation.

Design and Evaluation of a Computer-Based 24-Hour Physical Activity Recall (cpar24) Instrument.

BACKGROUND: Widespread access to the Internet and an increasing number of Internet users offers the opportunity of using Web-based recalls to collect detailed physical activity data in epidemiologic studies. OBJECTIVE: The aim of this investigation was to evaluate the validity and reliability of a computer-based 24-hour physical activity recall (cpar24) instrument with respect to the recalled 24-h period. METHODS: A random sample of 67 German residents aged 22 to 70 years was instructed to wear an ActiGraph GT3X+ accelerometer for 3 days. Accelerometer counts per min were used to classify activities as sedentary (<100 counts per min), light (100-1951 counts per min), and moderate to vigorous (≥1952 counts per min). On day 3, participants were also requested to specify the type, intensity, timing, and context of all activities performed during day 2 using the cpar24. Using metabolic equivalent of task (MET), the cpar24 activities were classified as sedentary (<1.5 MET), light (1.5-2.9 MET), and moderate to vigorous (≥3.0 MET). The cpar24 was administered twice at a 3-h interval. The Spearman correlation coefficient (r) was used as primary measure of concurrent validity and test-retest reliability. RESULTS: As compared with accelerometry, the cpar24 underestimated light activity by -123 min (median difference, P difference <.001) and overestimated moderate to vigorous activity by 89 min (P difference <.001). By comparison, time spent sedentary assessed by the 2 methods was similar (median difference=+7 min, P difference=.39). There was modest agreement between the cpar24 and accelerometry regarding sedentary (r=.54), light (r=.46), and moderate to vigorous (r=.50) activities. Reliability analyses revealed modest to high intraclass correlation coefficients for sedentary (r=.75), light (r=.65), and moderate to vigorous (r=.92) activities and no statistically significant differences between replicate cpar24 measurements (median difference for sedentary activities=+10 min, for light activities=-5 min, for moderate to vigorous activities=0 min, all P difference ≥.60). CONCLUSION: These data show that the cpar24 is a valid and reproducible Web-based measure of physical activity in adults.

Superresolution microscopy reveals the three-dimensional organization of meiotic chromosome axes in intact Caenorhabditis elegans tissue.

When cells enter meiosis, their chromosomes reorganize as linear arrays of chromatin loops anchored to a central axis. Meiotic chromosome axes form a platform for the assembly of the synaptonemal complex (SC) and play central roles in other meiotic processes, including homologous pairing, recombination, and chromosome segregation. However, little is known about the 3D organization of components within the axes, which include cohesin complexes and additional meiosis-specific proteins. Here, we investigate the molecular organization of meiotic chromosome axes in Caenorhabditis elegans through STORM (stochastic optical reconstruction microscopy) and PALM (photo-activated localization microscopy) superresolution imaging of intact germ-line tissue. By tagging one axis protein (HIM-3) with a photoconvertible fluorescent protein, we established a spatial reference for other components, which were localized using antibodies against epitope tags inserted by CRISPR/Cas9 genome editing. Using 3D averaging, we determined the position of all known components within synapsed chromosome axes to high spatial precision in three dimensions. We find that meiosis-specific HORMA domain proteins span a gap between cohesin complexes and the central region of the SC, consistent with their essential roles in SC assembly. Our data further suggest that the two different meiotic cohesin complexes are distinctly arranged within the axes: Although cohesin complexes containing the kleisin REC-8 protrude above and below the plane defined by the SC, complexes containing COH-3 or -4 kleisins form a central core, which may physically separate sister chromatids. This organization may help to explain the role of the chromosome axes in promoting interhomolog repair of meiotic double-strand breaks by inhibiting intersister repair.

The synaptonemal complex has liquid crystalline properties and spatially regulates meiotic recombination factors.

The synaptonemal complex (SC) is a polymer that spans ~100 nm between paired homologous chromosomes during meiosis. Its striated, periodic appearance in electron micrographs led to the idea that transverse filaments within this structure 'crosslink' the axes of homologous chromosomes, stabilizing their pairing. SC proteins can also form polycomplexes, three-dimensional lattices that recapitulate the periodic structure of SCs but do not associate with chromosomes. Here we provide evidence that SCs and polycomplexes contain mobile subunits and that their assembly is promoted by weak hydrophobic interactions, indicative of a liquid crystalline phase. We further show that in the absence of recombination intermediates, polycomplexes recapitulate the dynamic localization of pro-crossover factors during meiotic progression, revealing how the SC might act as a conduit to regulate chromosome-wide crossover distribution. Properties unique to liquid crystals likely enable long-range signal transduction along meiotic chromosomes and underlie the rapid evolution of SC proteins.

Physical activity surveillance in the European Union: reliability and validity of the European Health Interview Survey-Physical Activity Questionnaire (EHIS-PAQ).

BACKGROUND: The current study examined the reliability and validity of the European Health Interview Survey-Physical Activity Questionnaire (EHIS-PAQ), a novel questionnaire for the surveillance of physical activity (PA) during work, transportation, leisure time, sports, health-enhancing and muscle-strengthening activities over a typical week. METHODS: Reliability was assessed by administering the 8-item questionnaire twice to a population-based sample of 123 participants aged 15-79 years at a 30-day interval. Concurrent (inter-method) validity was examined in 140 participants by comparisons with self-report (International Physical Activity Questionnaire-Long Form (IPAQ-LF), 7-day Physical Activity Record (PAR), and objective criterion measures (GT3X+ accelerometer, physical work capacity at 75% (PWC(75%)) from submaximal cycle ergometer test, hand grip strength). RESULTS: The EHIS-PAQ showed acceptable reliability, with a median intraclass correlation coefficient across PA domains of 0.55 (range 0.43-0.73). Compared to the GT3X+ (counts/minutes/day), the EHIS-PAQ underestimated moderate-to-vigorous PA (median difference -11.7, p-value = 0.054). Spearman correlation coefficients (ρ) for validity were moderate-to-strong (ρ's > 0.41) for work-related PA (IPAQ = 0.64, GT3X + =0.43, grip strength = 0.48), transportation-related PA (IPAQ = 0.62, GT3X + =0.43), walking (IPAQ = 0.58), and health-enhancing PA (IPAQ = 0.58, PAR = 0.64, GT3X + =0.44, PWC(75%) = 0.48), and fair-to-poor (ρ's < 0.41) for moderate-to-vigorous aerobic recreational and muscle-strengthening PA. CONCLUSIONS: The EHIS-PAQ showed good evidence for reliability and validity for the measurement of PA levels at work, during transportation and health-enhancing PA.

Healthy lifestyle behaviors and decreased risk of mortality in a large prospective study of U.S. women and men.

Adiposity, insufficient physical activity, cigarette smoking, and poor diet have all been related independently to increased chronic disease risk, but their joint impact on overall health remains unclear. In a cohort of 170,672 women and men aged 51-71 years at baseline in 1996/1997 and followed-up through 2009, we investigated the individual and joint impact of four low-risk lifestyle factors: abdominal leanness (waist circumference <88 cm in women and <102 cm in men); recommended physical activity level (30 min or more of moderate exercise at least 5 times per week or 20 min or more of vigorous exercise at least 3 times per week); long-term non-smoking (never-smoker or quit smoking more than 10 years ago); and healthy diet (Mediterranean diet score within the upper two sex-specific quintiles). During 2,126,089 person-years of follow-up, 20,903 participants died. In multivariate Cox models, statistically significant decreased risks of mortality were observed for the low-risk factors abdominal leanness (relative risk (RR) = 0.86; 95 % confidence interval (CI) = 0.83-0.89), physical activity (RR = 0.86; 95 % CI = 0.84-0.89), non-smoking (RR = 0.43; 95 % CI = 0.42-0.45), and healthy diet (RR = 0.86; 95 % CI = 0.83-0.88). The larger the number of low-risk lifestyle factors, the lower was the mortality risk. The RR comparing adherence to all versus none of the factors was 0.27 (95 % CI = 0.25-0.29). We estimate that 33 % (95 % CI = 30-35 %) of deaths in our cohort were premature and could have been avoided if all study participants had adhered to all low-risk factors.

Regulating contractility of the actomyosin cytoskeleton by pH.

The local interaction of F-actin with myosin-II motor filaments and crosslinking proteins is crucial for the force generation, dynamics, and reorganization of the intracellular cytoskeleton. By using a bottom-up approach, we are able to show that the contractility of reconstituted active actin systems is tightly controlled by the local pH. The pH-dependent intrinsic crossbridge strength of myosin-II is identified to account for a sharp transition of the actin/myosin-II activity from noncontractile to contractile by a change in pH of only 0.1. This pH-dependent contractility is a generic feature, which is observed in all studied crosslinked actin/myosin-II systems. The specific type and concentration of crosslinking protein allows one to sensitively adjust the range of pH where contraction occurs, which can recover the behavior found in Xenopus laevis oocyte extracts. Small variations in pH provide a mechanism of controlling the contractility of cytoskeletal structures, which can be expected to have broad implications in our understanding of cytoskeletal regulation.