Biology subjects

Tachon, S.

Publications and source records attributed to Tachon, S..

5 recordsLinked to original sources

bioRxiv2026

Rapid repurposing of microvillar content drives a flagellate-to-amoeboid switch in the closest relative of animals

Animal cells extensively remodel their cytoskeleton during differentiation and can notably switch between two major motility modes: flagellum-based swimming and actin-based crawling. We previously showed that choanoflagellates, the closest living relatives of animals and classically viewed as obligate flagellated swimmers, can retract their collar complex and adopt an amoeboid form within seconds under spatial confinement, independently of regulated gene expression. Here, using live imaging, ultrastructural expansion microscopy, and cryo-electron tomography in Salpingoeca rosetta, we identify rapid, cell-wide cytoskeletal remodeling as the ultrastructural basis of this switch. Unconfined choanoflagellates lack a detectable actin cortex but display an apical flagellum and cortical microtubules, with F-actin being largely restricted to microvilli. Confinement triggers calcium release from intracellular stores, which induces microvillar retraction and absorption of microvillar material into the cell body, including actin, ezrin-radixin-moesin 1, and plasma membrane. Remodeling of the internalized F-actin and repurposing of associated proteins supports de novo actin cortex formation, which is necessary for amoeboid motility. In parallel, cortical microtubules are disassembled, and the reabsorbed microvillar plasma membrane increases the surface area of the cell body, allowing the cell to flatten under confinement. Cryo-electron tomography reveals stepwise actin reorganization from internalized microvillar bundles to a cortical contractile meshwork combining bundles and scattered filaments. This work reveals considerable ultrastructural plasticity in the cytoskeletal architecture of choanoflagellates and supports an ancestral role for microvilli as reservoirs of membrane and cytoskeleton to potentiate cell phenotypic transitions.

evolutionary biology↗

bioRxiv2025

Nitrate-responsive Mycobacterial Intracytoplasmic Membranes dampen Inflammation during Mycobacterium tuberculosis Infection

Subcellular compartmentalization of metabolic processes is a main feature of prokaryotic and eukaryotic architecture. Environmental bacteria generate intracytoplasmic membranes (ICMs) as a crucial strategy to adapt their metabolism to environmental changes. While pathogenic intracellular bacteria also perceive various stressful stimuli during host interactions, the subsequent re-organization of their internal architecture has not been explored. Using cryo-electron tomography, we show that Mycobacterium tuberculosis (Mtb), a major human pathogen, is able to form ICMs outside and inside host cells in a strain-dependent manner. We characterize these Mycobacterial intracytoplasmic Membranes (MIMs) as nitrate-induced structures involved in regulation of metabolism. Furthermore, we uncover that MIM formation during macrophage infection correlates with the ability of Mtb to dampen cellular inflammatory responses. Our findings reveal a previously uncharacterized cytoplasmic structure in Mtb and link it to a functional mechanism that enables the bacterium to adapt to its intracellular niche.

microbiology↗

bioRxiv2024

TYPE THREE SECRETION SYSTEM-INDUCED MECHANOPORATION OF VACUOLAR MEMBRANES

SO_SCPLOWUMMARYC_SCPLOWEndomembrane breaching is a crucial strategy employed by intracellular pathogens enclosed within vacuoles to access the nutrient-rich cytosol for intracellular replication. While bacteria use various mechanisms to compromise host membranes, the specific processes and factors involved are often unknown. Shigella flexneri, a major human pathogen, accesses the cytosol relying on the Type Three Secretion System (T3SS) and secreted effectors. Using in-cell correlative light and electron microscopy, we tracked the sequential steps of Shigella host cell entry. Moreover, we captured the T3SS, which projects a needle from the bacterial surface, in the process of puncturing holes in the vacuolar membrane. This initial puncture ensures disruption of the vacuolar membrane. Together this introduces the concept of mechanoporation via a bacterial secretion system as a crucial process for bacterial pathogen-induced membrane damage. GO_SCPLOWRAPHICALC_SCPLOW AO_SCPLOWBSTRACTC_SCPLOW O_FIG O_LINKSMALLFIG WIDTH=199 HEIGHT=200 SRC="FIGDIR/small/613840v1_ufig1.gif" ALT="Figure 1"> View larger version (54K): org.highwire.dtl.DTLVardef@1669028org.highwire.dtl.DTLVardef@8259cborg.highwire.dtl.DTLVardef@1d522caorg.highwire.dtl.DTLVardef@1df0f2d_HPS_FORMAT_FIGEXP M_FIG C_FIG

microbiology↗

bioRxiv2024

Biogenesis of DNA-carrying extracellular vesicles by the dominant human gut methanogenic archaeon

Extracellular vesicles (EVs) are membrane-bound particles secreted by cells from all domains of life and implicated in a variety of important processes, from intercellular communication to pathogenesis. Here, we characterize EVs produced by the dominant human gut methanogen, Methanobrevibacter smithii, which, unlike most archaea, contains a peptidoglycan cell wall. Using quantitative proteomics, we show that M. smithii EVs are enriched in various proteins responsible for chromatin structure, including histones, and DNA repair. Consistently, the M. smithii EVs carry DNA, with fragments covering the entire cellular chromosome. Notably, the EVs are strongly enriched in extrachromosomal circular DNA (eccDNA) molecules which originate from excision of a 2.9-kb chromosomal fragment and a proviral genome. The eccDNA encodes two of the key methanogenesis enzymes and could boost their expression inside the cells through the gene dosage effect. Furthermore, four of the top ten most abundant EV proteins are implicated in methanogenesis. Cryo-electron tomography (Cryo-ET) suggests that EVs are formed by budding from the cell membrane and are trapped under the cell wall prior to liberation through local disruptions in the cell wall. Collectively, our results reveal parallels with EV biogenesis in bacteria and suggest that M. smithii EVs facilitate the export of both cellular and viral DNA as well as key metabolic proteins in the gut environment, with potential impact on methane production.

microbiology↗

bioRxiv2023

Diving into bacterial dormancy: emergence of osmotically stable wall-less forms in an aquatic environment

Bacteria can respond to environmental stresses by entering a dormant state, called viable but non-culturable (VBNC) state, in which they no longer grow in routine culture media. VBNC pathogens pose thus a significant risk for human and animal health as they are not detected by standard growth-based techniques and can "wake up" back into a vegetative and virulent state. Although hundreds of species were reported to become VBNC in response to different stresses, the molecular mechanisms governing this phenotypic switch remain largely elusive. Here, we characterized the VBNC state transition process in the Gram-positive pathogen Listeria monocytogenes in response to nutritional deprivation. By combining fluorescence microscopy, cryo-electron tomography and analytical biochemistry, we found that starvation in mineral water drives L. monocytogenes into a VBNC state via a mechanism of cell wall (CW) shedding that generates osmotically stable CW-deficient (CWD) coccoid forms. This phenomenon occurs in multiple L. monocytogenes strains and in other Listeria species, suggesting it may be a stress-adapting process transversal to the Listeria genus. Transcriptomic and gene-targeted approaches revealed the stress response regulator SigB and the autolysin NamA as major moderators of CW loss and VBNC state transition. Finally, we show that this CWD dormant state is transient as VBNC Listeria revert back to a walled, vegetative and virulent state after passage in embryonated eggs. Our findings provide unprecedented detail on the mechanisms governing the transition to a VBNC state, and reveal that dormant CWD bacterial forms can naturally arise in aquatic environments without osmotic stabilization. This may represent an alternative strategy for bacterial survival in oligotrophic conditions, which can potentially generate public health-threatening reservoirs of undetectable pathogens.

microbiology↗