Biology subjects

Müsken, M.

Publications and source records attributed to Müsken, M..

7 recordsLinked to original sources

bioRxiv2026

Ancestral phototrophic Rhizobiaceae evolved in association with algae, then plants

Rhizobiaceae serve as classical models for elucidating mutualistic plant-microbe interactions yet they represent a narrow phylogenetic subgroup of Alphaproteobacteria. Studying additional lineages of Rhizobiaceae, we observed broad associations with oxygenic phototrophs beyond land plants, including early branching clades of submerged plants, multicellular and unicellular algae, as well as cyanobacteria. In particular, bacteria of the genus Hoeflea were often affiliated with cyanobacteria or microbial algae, whereas Peteryoungia spp. colonized roots of submerged plants. While both genera were originally described as nonpigmented heterotrophs, our detailed genomic, biochemical and physiological analyses revealed that most strains actually contained genes for anoxygenic photosynthesis. Under oligotrophic, oxic growth conditions, each characterized representative expressed bacteriochlorophyll a-containing functional photosynthetic complexes. Photosynthesis genes shared the highest homology among phylogenetically closest relatives, displaying topologies congruent to cognate house-keeping gene phylogenies, and maintained highly conserved gene synteny across the chromosomes of different species. Together, this indicated a vertical inheritance and long ancestral history of aerobic anoxygenic photosynthesis in Rhizobiaceae rather than multiple recent horizontal transfers. Subsequent time-scale phylogenetic analysis suggested that the last common ancestor of Rhizobiaceae was an aquatic phototroph, with different lineages of Rhizobiaceae consecutively evolving in association with algae, land plants, then later legumes. While aquatic lineages maintained photosynthetic machinery till today, Rhizobia which developed symbioses with land plants either as mutualistic endosymbiosis within root nodules or as plant pathogens, concomitantly lost photosynthetic capability. Based on our results, multiple biotic interactions with diverse oxygenic phototrophs drove the early evolution of Rhizobiaceae.

evolutionary biology↗

bioRxiv2026

A rhizobial extracellular vesicle-conveyed auxin transporter mediates phytohormone mobilization and optimizes rhizobium-legume symbiosis

Bacterial extracellular vesicles (bEVs) are emerging as key players in interkingdom communication, yet their role in delivering functional proteins to host cells during symbiosis remains unexplored. This study shows that Sinorhizobium fredii HH103 packages a PIN-like auxin transporter, AuxT, into bEVs that traffic within the peribacteroid space of soybean nodules. AuxT is chromosomally encoded and constitutively expressed, genetically uncoupled from the flavonoid-inducible auxin biosynthesis machinery located on the symbiotic plasmid. Structural prediction reveals that AuxT adopts an eight-transmembrane-helix architecture with striking homology to plant PIN auxin exporters, despite negligible sequence identity. Molecular docking demonstrates that AuxT binds indole-3-acetic acid within a central cavity, with dimerization inducing ligand-specific conformational changes consistent with transport activity. The auxT mutant exhibits significant symbiotic defects including reduced shoot biomass, nodule number, and nodule mass that are fully restored by complementation. Critically, AuxT-enriched bEVs contain elevated auxin levels, and nodules colonized by the complemented strain accumulate more auxin specifically within the bEVs peribacteroid space, while bacteroids themselves show no auxin retention. We postulate that bEV-associated AuxT mediates localized auxin export into the symbiosome, modulating the host hormonal environment to optimize symbiotic development. This work reveals a previously unrecognized mechanism of interdomain hormonal modulation, where a bacterium uses a structurally convergent transporter and vesicular delivery to actively shape host physiology and to improve the symbiotic performance.

microbiology↗

bioRxiv2025

Discovery of a new fibronectin-binding surface protein of Streptococcus canis with serum opacification activity through Transposon Directed Insertion-Site Sequencing

Infective endocarditis is a rare but severe disease in humans and dogs which can be caused by Streptococcus canis. The bacterial factors that mediate endothelial adherence remain poorly defined. To understand how S. canis can adhere to and invade the endocardium, we combined a high-throughput approach called transposon directed insertion-site sequencing (TraDIS) with a physiologically relevant endothelial infection model that incorporates venous-range shear stress to identify S. canis genes required for host cell interaction. A saturated transposon library of clinical strain IMT49926 was screened in a microfluidic infection assay, enabling genome-wide selection of mutants impaired in endothelial adhesion. Comparative analysis of input and non-adherent output pools revealed several candidate genes, including a fibronectin-binding LPXTG-anchored surface protein with high homology to streptococcal serum opacity factors (SOFs). Our findings identified ScSOF as a multifunctional surface protein that plays an important role in the infection potential of S. canis. It facilitates adhesion to endothelial cells, prevents endothelial wound closure, contributes to the streptococcal surface architecture, binds fibronectin, opacifies serum, and inhibits {beta}-haemolytic activity. ScSOF represents a strong candidate for future studies into pathogenesis, immune evasion, and (potentially) vaccine or therapeutic targeting in S. canis infections. Authors summaryStreptococcus canis is a bacterium commonly found in healthy dogs and cats, but it can sometimes cause serious infections in animals and humans, including infective endocarditis, a dangerous infection of the heart lining. To cause this disease, the bacteria must first attach to and invade the cells that line blood vessels. However, very little is known about how S. canis can do this. In this study, we used a genetic screening method that includes all the genes from S. canis to identify which bacterial genes are needed for attachment to human endothelial cells under conditions that mimic real blood flow. We discovered a previously uncharacterized surface protein, which we named ScSOF, that plays several important roles during infection. ScSOF helps the bacteria bind to fibronectin, a major host tissue protein, alters the bacterial surface structure, and causes serum opacification, a known marker of virulence in related streptococci. When ScSOF was removed, the bacteria were much less able to attach to endothelial cells, cause cell damage, or interfere with the healing of endothelial wounds. Our findings show that ScSOF is a key factor that enables S. canis to interact with host tissues and may contribute to heart valve infections.

microbiology↗

bioRxiv2024

Arp2/3 complex-dependent actin regulation protects the survival of tissue-resident mast cells

Actin network dynamics are pivotal in governing the motility and effector functions of immune cells. The Arp2/3 complex is a key regulator of actin filament branching, with mutations in its subunits being linked with human immunodeficiencies. While known for its role in phagocytosis and cell migration, our study uncovers a critical role of the Arp2/3 complex in safeguarding the tissue residency of mast cells (MCs), essential immune cells in allergies, venom detoxification and antigen-specific avoidance. Mechanistically, we show that MCs require Arp2/3-regulated actin filament assembly to resist their integrin-mediated mechano-coupling with their tissue niche. Arp2/3 complex depletion directs MCs into cell cycle arrest and death, which can be rescued by inhibiting their mechanical interactions with extracellular matrix. Our findings underscore the Arp2/3 complex as a mechano-protective element for maintaining MC survival and longevity in tissues, highlighting the importance of actin regulation in preserving the homeostasis of a tissue-resident immune cell population. One Sentence SummaryArp2/3 complex protects the tissue homeostasis of resident mast cell networks

immunology↗

bioRxiv2023

A rolling circle-replicating plasmid as an Inovirus phage satellite

Bacterial viruses (phages) are potent agents of lateral gene transfer and thus are important drivers of evolution. A group of mobile genetic elements (MGEs), referred to as phage satellites, exploit phages to disseminate their own genetic material. Here we isolated a novel member of the genus Inovirus, Shewanella phage Dolos, along with an autonomous rolling circle-replicating plasmid, pDolos. Dolos causes a chronic infection in its host Shewanella oneidensis by phage production with only minor effects on the host cell proliferation. When present, plasmid pDolos hijacks Dolos functions to be predominantly packaged into phage virions and released into the environment. pDolos can disseminate further genetic material encoding, e.g., resistances, fluorophores, and metabolically active proteins, to host cells sensitive to Dolos infection. Given the rather simple requirements of a plasmid for takeover of an inovirus, the wide distribution of phages of this group and the broad spectrum of rolling circle-replicating plasmids, we speculate that similar phage-satellite systems are common among bacteria.

microbiology↗

bioRxiv2022

Transcriptome dynamics of Pseudomonas aeruginosa during transition from replication-uncoupled to -coupled growth

In bacteria, either chromosome duplication is coupled to cell division with only one replication round per cell cycle or DNA is replicated faster than the cells divide thus both processes are uncoupled. Here, we show that the opportunistic pathogen Pseudomonas aeruginosa switches from fast uncoupled to sustained coupled growth when cultivated under standard laboratory conditions. The transition was characterized by fast-paced, sequential changes in transcriptional activity along the ori-ter axis of the chromosome reflecting adaptation to the metabolic needs during both growth phases. Quorum sensing (QS) activity was highest at the onset of the coupled growth phase during which only a quarter of the cells keeps replicating. RNA sequencing of subpopulations of these cultures sorted based on their DNA content, revealed a strong gene dosage effect as well as specific expression patterns for replicating and non-replicating cells. Expression of flagella and mexE, involved in multi drug efflux was restricted to cells that did not replicate, while those that did showed a high activity of the cell division locus and recombination genes. A possible role of QS in the formation of these subpopulations upon switching to coupled growth could be a subject of further research. Significance statementThe coordination of gene expression with the cell cycle has so far been studied only in a handful of bacteria, the bottleneck being the need for synchronized cultures. Here, we determined replication-associated effects on transcription by comparing Pseudomonas aeruginosa cultures that differ in their growth mode and number of replicating chromosomes. We further show that cell cycle-specific gene regulation can be principally identified by RNA sequencing of subpopulations from cultures that replicate only once per cell division and that are sorted according to their DNA content. Our approach opens the possibility to study asynchronously growing bacteria from a wide phylogenetic range and thereby enhance our understanding of the evolution of cell-cycle control on the transcriptional level.

microbiology↗

bioRxiv2020

Loss of Hem1 disrupts macrophage function and impacts on migration, phagocytosis and integrin-mediated adhesion

The hematopoietic-specific protein 1 (Hem1) comprises an essential subunit of the WAVE Regulatory Complex (WRC) in immune cells. WRC has a fundamental role in Arp2/3 complex activation and the protrusion of branched actin networks in motile cells. Hem1 deficiency leads to suppression of the entire WRC in immune cells. Defective WRC function in macrophages results in loss of lamellipodia and migration defects. Moreover, phagocytosis, commonly accompanied by lamellipodium protrusion during cup formation, is altered in Hem1 null cells concerning frequency and efficacy. When analyzing cell spreading, adhesion and podosome formation, we found that Hem1 null cells are capable, in principle, of podosome formation and consequently, do not show any quantitative differences in extracellular matrix degradation. Their adhesive behavior, however, was significantly altered. Specifically, adhesion as well as de-adhesion of Hem1 null cells was strongly compromised, likely contributing to the observed reduced efficiency of phagocytosis. In line with this, phosphorylation of the prominent adhesion component paxillin was diminished. Non-hematopoietic somatic cells disrupted in expression for both Hem1 and its ubiquitous orthologue Nck-associated protein 1 (Nap1) or the essential WRC components Sra-1/PIR121 did not only confirm defective paxillin phosphorylation, but also revealed that paxillin turnover in focal adhesions is accelerated in the absence of WRC. Finally, adhesion assays using platelets lacking functional WRC as model system unmasked radically decreased IIb{beta}3 integrin activation. Our results thus demonstrate that WRC-driven actin networks impact on integrin-dependent processes controlling formation and dismantling of different types of cell-substratum adhesion. One sentence summaryInterference of Hem1 function in mice and cells uncovers a hitherto unrecognized role in integrin-mediated cell adhesion that is crucial for macrophage function and connects to recently discovered immunodeficiencies in patients carrying Hem1 mutations.

cell biology↗