Parametric modeling of mechanical effects on circadian oscillators.

Circadian rhythms are archetypal examples of nonlinear oscillations. While these oscillations are usually attributed to circuits of biochemical interactions among clock genes and proteins, recent experimental studies reveal that they are also affected by the cell's mechanical environment. Here, we extend a standard biochemical model of circadian rhythmicity to include mechanical effects in a parametric manner. Using experimental observations to constrain the model, we suggest specific ways in which the mechanical signal might affect the clock. Additionally, a bifurcation analysis of the system predicts that these mechanical signals need to be within an optimal range for circadian oscillations to occur.

Piezo2 channel regulates RhoA and actin cytoskeleton to promote cell mechanobiological responses.

Actin polymerization and assembly into stress fibers (SFs) is central to many cellular processes. However, how SFs form in response to the mechanical interaction of cells with their environment is not fully understood. Here we have identified Piezo2 mechanosensitive cationic channel as a transducer of environmental physical cues into mechanobiological responses. Piezo2 is needed by brain metastatic cells from breast cancer (MDA-MB-231-BrM2) to probe their physical environment as they anchor and pull on their surroundings or when confronted with confined migration through narrow pores. Piezo2-mediated Ca2+ influx activates RhoA to control the formation and orientation of SFs and focal adhesions (FAs). A possible mechanism for the Piezo2-mediated activation of RhoA involves the recruitment of the Fyn kinase to the cell leading edge as well as calpain activation. Knockdown of Piezo2 in BrM2 cells alters SFs, FAs, and nuclear translocation of YAP; a phenotype rescued by overexpression of dominant-positive RhoA or its downstream effector, mDia1. Consequently, hallmarks of cancer invasion and metastasis related to RhoA, actin cytoskeleton, and/or force transmission, such as migration, extracellular matrix degradation, and Serpin B2 secretion, were reduced in cells lacking Piezo2.

Traction forces at the cytokinetic ring regulate cell division and polyploidy in the migrating zebrafish epicardium.

Epithelial repair and regeneration are driven by collective cell migration and division. Both cellular functions involve tightly controlled mechanical events, but how physical forces regulate cell division in migrating epithelia is largely unknown. Here we show that cells dividing in the migrating zebrafish epicardium exert large cell-extracellular matrix (ECM) forces during cytokinesis. These forces point towards the division axis and are exerted through focal adhesions that connect the cytokinetic ring to the underlying ECM. When subjected to high loading rates, these cytokinetic focal adhesions prevent closure of the contractile ring, leading to multi-nucleation through cytokinetic failure. By combining a clutch model with experiments on substrates of different rigidity, ECM composition and ligand density, we show that failed cytokinesis is triggered by adhesion reinforcement downstream of increased myosin density. The mechanical interaction between the cytokinetic ring and the ECM thus provides a mechanism for the regulation of cell division and polyploidy that may have implications in regeneration and cancer.

The Aspergillus nidulans syntaxin PepA(Pep12) is regulated by two Sec1/Munc-18 proteins to mediate fusion events at early endosomes, late endosomes and vacuoles.

Syntaxins are target-SNAREs that crucially contribute to determine membrane compartment identity. Three syntaxins, Tlg2p, Pep12p and Vam3p, organize the yeast endovacuolar system. Remarkably, filamentous fungi lack the equivalent of the yeast vacuolar syntaxin Vam3p, making unclear how these organisms regulate vacuole fusion. We show that the nearly essential Aspergillus nidulans syntaxin PepA(Pep12) , present in all endocytic compartments between early endosomes and vacuoles, shares features of Vam3p and Pep12p, and is capable of forming compositional equivalents of all known yeast endovacuolar SNARE bundles including that formed by yeast Vam3p for vacuolar fusion. Our data further indicate that regulation by two Sec1/Munc-18 proteins, Vps45 in early endosomes and Vps33 in early and late endosomes/vacuoles contributes to the wide domain of PepA(Pep12) action. The syntaxin TlgB(Tlg2) localizing to the TGN appears to mediate retrograde traffic connecting post-Golgi (sorting) endosomes with the TGN. TlgB(Tlg2) is dispensable for growth but becomes essential if the early Golgi syntaxin SedV(Sed5) is compromised, showing that the Golgi can function with a single syntaxin, SedV(Sed5) . Remarkably, its pattern of associations with endosomal SNAREs is consistent with SedV(Sed5) playing roles in retrograde pathway(s) connecting endocytic compartments downstream of the post-Golgi endosome with the Golgi, besides more conventional intra-Golgi roles.

Membrane to cortex attachment determines different mechanical phenotypes in LGR5+ and LGR5- colorectal cancer cells.

Colorectal cancer (CRC) tumors are composed of heterogeneous and plastic cell populations, including a pool of cancer stem cells that express LGR5. Whether these distinct cell populations display different mechanical properties, and how these properties might contribute to metastasis is poorly understood. Using CRC patient derived organoids (PDOs), we find that compared to LGR5- cells, LGR5+ cancer stem cells are stiffer, adhere better to the extracellular matrix (ECM), move slower both as single cells and clusters, display higher nuclear YAP, show a higher survival rate in response to mechanical confinement, and form larger transendothelial gaps. These differences are largely explained by the downregulation of the membrane to cortex attachment proteins Ezrin/Radixin/Moesin (ERMs) in the LGR5+ cells. By analyzing single cell RNA-sequencing (scRNA-seq) expression patterns from a patient cohort, we show that this downregulation is a robust signature of colorectal tumors. Our results show that LGR5- cells display a mechanically dynamic phenotype suitable for dissemination from the primary tumor whereas LGR5+ cells display a mechanically stable and resilient phenotype suitable for extravasation and metastatic growth.

Rescue of Aspergillus nidulans severely debilitating null mutations in ESCRT-0, I, II and III genes by inactivation of a salt-tolerance pathway allows examination of ESCRT gene roles in pH signalling.

The Aspergillus pal pathway hijacks ESCRT proteins into ambient pH signalling complexes. We show that components of ESCRT-0, ESCRT-I, ESCRT-II and ESCRT-III are nearly essential for growth, precluding assessment of null mutants for pH signalling or trafficking. This severely debilitating effect is rescued by loss-of-function mutations in two cation tolerance genes, one of which, sltA, encodes a transcription factor whose inactivation promotes hypervacuolation. Exploiting a conditional expression sltA allele, we demonstrate that deletion of vps27 (ESCRT-0), vps23 (ESCRT-I), vps36 (ESCRT-II), or vps20 or vps32 (both ESCRT-III) leads to numerous small vacuoles, a phenotype also suppressed by SltA downregulation. This situation contrasts with normal vacuoles and vacuole-associated class E compartments seen in Saccharomyces cerevisiae ESCRT null mutants. Exploiting the suppressor phenotype of sltA(-) mutations, we establish that Vps23, Vps36, Vps20 and Vps32 are essential for pH signalling. Phosphatidylinositol 3-phosphate-recognising protein Vps27 (ESCRT-0) is not, consistent with normal pH signalling in rabB null mutants unable to recruit Vps34 kinase to early endosomes. In contrast to the lack of pH signalling in the absence of Vps20 or Vps32, detectable signalling occurs in the absence of ESCRT-III subunit Vps24. Our data support a model in which certain ESCRT proteins are recruited to the plasma membrane to mediate pH signalling.

Mechanical control of the mammalian circadian clock via YAP/TAZ and TEAD.

Autonomous circadian clocks exist in nearly every mammalian cell type. These cellular clocks are subjected to a multilayered regulation sensitive to the mechanochemical cell microenvironment. Whereas the biochemical signaling that controls the cellular circadian clock is increasingly well understood, mechanisms underlying regulation by mechanical cues are largely unknown. Here we show that the fibroblast circadian clock is mechanically regulated through YAP/TAZ nuclear levels. We use high-throughput analysis of single-cell circadian rhythms and apply controlled mechanical, biochemical, and genetic perturbations to study the expression of the clock gene Rev-erbα. We observe that Rev-erbα circadian oscillations are disrupted with YAP/TAZ nuclear translocation. By targeted mutations and overexpression of YAP/TAZ, we show that this mechanobiological regulation, which also impacts core components of the clock such as Bmal1 and Cry1, depends on the binding of YAP/TAZ to the transcriptional effector TEAD. This mechanism could explain the impairment of circadian rhythms observed when YAP/TAZ activity is upregulated, as in cancer and aging.

The microtubule plus-end localization of Aspergillus dynein is important for dynein-early-endosome interaction but not for dynein ATPase activation.

Cytoplasmic dynein in filamentous fungi accumulates at microtubule plus-ends near the hyphal tip, which is important for minus-end-directed transport of early endosomes. It was hypothesized that dynein is switched on at the plus-end by cargo association. Here, we show in Aspergillus nidulans that kinesin-1-dependent plus-end localization is not a prerequisite for dynein ATPase activation. First, the Walker A and Walker B mutations in the dynein heavy chain AAA1 domain implicated in blocking different steps of the ATPase cycle cause different effects on dynein localization to microtubules, arguing against the suggestion that ATPase is inactive before arriving at the plus-end. Second, dynein from ΔkinA (kinesin 1) mutant cells has normal ATPase activity despite the absence of dynein plus-end accumulation. In ΔkinA hyphae, dynein localizes along microtubules and does not colocalize with abnormally accumulated early endosomes at the hyphal tip. This is in contrast to the colocalization of dynein and early endosomes in the absence of NUDF/LIS1. However, the Walker B mutation allows dynein to colocalize with the hyphal-tip-accumulated early endosomes in the ΔkinA background. We suggest that the normal ability of dyenin to interact with microtubules as an active minus-end-directed motor demands kinesin-1-mediated plus-end accumulation for effective interactions with early endosomes.

Mechanical force application to the nucleus regulates nucleocytoplasmic transport.

Mechanical force controls fundamental cellular processes in health and disease, and increasing evidence shows that the nucleus both experiences and senses applied forces. Such forces can lead to the nuclear translocation of proteins, but whether force controls nucleocytoplasmic transport, and how, remains unknown. Here we show that nuclear forces differentially control passive and facilitated nucleocytoplasmic transport, setting the rules for the mechanosensitivity of shuttling proteins. We demonstrate that nuclear force increases permeability across nuclear pore complexes, with a dependence on molecular weight that is stronger for passive than for facilitated diffusion. Owing to this differential effect, force leads to the translocation of cargoes into or out of the nucleus within a given range of molecular weight and affinity for nuclear transport receptors. Further, we show that the mechanosensitivity of several transcriptional regulators can be both explained by this mechanism and engineered exogenously by introducing appropriate nuclear localization signals. Our work unveils a mechanism of mechanically induced signalling, probably operating in parallel with others, with potential applicability across signalling pathways.

Long-distance movement of Aspergillus nidulans early endosomes on microtubule tracks.

In fungal hyphal cells, intracellular membrane trafficking is constrained by the relatively long intracellular distances and the mode of growth, exclusively by apical extension. Endocytosis plays a key role in hyphal tip growth, which involves the coupling of secretory membrane delivery to the apical region with subapical compensatory endocytosis. However, the identity, dynamics and function of filamentous fungal endosomal compartments remain largely unexplored. Aspergillus nidulans RabA(Rab5) localizes to a population of endosomes that show long range bidirectional movement on microtubule (MT) tracks and are labelled with FM4-64 shortly after dye internalization. RabA(Rab5) membranes do not overlap with largely static mature endosomes/vacuoles. Impaired delivery of dynein to the MT plus ends or downregulation of cytoplasmic dynein using the dynein heavy chain nudA1(ts)mutation results in accumulation of RabA(Rab5) endosomal membranes in an abnormal NudA1 compartment at the tip, strongly supporting the existence in A. nidulans hyphal tips of a dynein loading region. We show that the SynA synaptobrevin endocytic recycling cargo traffics through this region, which strongly supports the contention that polarized hyphal growth involves the association of endocytic recycling with the plus ends of MTs located at the tip, near the endocytic internalization collar.

Characterization of Aspergillus nidulans DidB Did2, a non-essential component of the multivesicular body pathway.

ESCRT-III heteropolymers mediate membrane protein cargo sorting into multivesicular endosomes for subsequent vacuolar degradation. We studied the localization of largely uncharacterized Aspergillus nidulans ESCRT-III using its key structural component Vps32 and the 'associated' component DidB(Did2). Vps32-GFP localizes to motile early endosomes as reported, but predominates in aggregates often associated with vacuoles due to inability to dissociate from endosomes. DidB(Did)(2) regulating Vps4 (the ATPase disassembling ESCRT-III) is not essential. Consistent with this accessory role, didB Delta is unable to block the MVB sorting of the glutamate transporter AgtA, but increases its steady-state level and mislocalizes a fraction of the permease to the plasma membrane under conditions promoting its vacuolar targeting. didB Delta exacerbates the dominant-negative growth defect resulting from Vps32-GFP over-expression. A proportion of DidB-GFP is detectable in early endosomes colocalizing with RabA(Rab5) and accumulating in nudA1 tips, suggesting that ESCRT-III assembles on endosomes from the early steps of the endocytic pathway.

Wall mechanics and exocytosis define the shape of growth domains in fission yeast.

The amazing structural variety of cells is matched only by their functional diversity, and reflects the complex interplay between biochemical and mechanical regulation. How both regulatory layers generate specifically shaped cellular domains is not fully understood. Here, we report how cell growth domains are shaped in fission yeast. Based on quantitative analysis of cell wall expansion and elasticity, we develop a model for how mechanics and cell wall assembly interact and use it to look for factors underpinning growth domain morphogenesis. Surprisingly, we find that neither the global cell shape regulators Cdc42-Scd1-Scd2 nor the major cell wall synthesis regulators Bgs1-Bgs4-Rgf1 are reliable predictors of growth domain geometry. Instead, their geometry can be defined by cell wall mechanics and the cortical localization pattern of the exocytic factors Sec6-Syb1-Exo70. Forceful re-directioning of exocytic vesicle fusion to broader cortical areas induces proportional shape changes to growth domains, demonstrating that both features are causally linked.

Dynamics of cell shape inheritance in fission yeast.

Every cell has a characteristic shape key to its fate and function. That shape is not only the product of genetic design and of the physical and biochemical environment, but it is also subject to inheritance. However, the nature and contribution of cell shape inheritance to morphogenetic control is mostly ignored. Here, we investigate morphogenetic inheritance in the cylindrically-shaped fission yeast Schizosaccharomyces pombe. Focusing on sixteen different 'curved' mutants--a class of mutants which often fail to grow axially straight--we quantitatively characterize their dynamics of cell shape inheritance throughout generations. We show that mutants of similar machineries display similar dynamics of cell shape inheritance, and exploit this feature to show that persistent axial cell growth in S. pombe is secured by multiple, separable molecular pathways. Finally, we find that one of those pathways corresponds to the swc2-swr1-vps71 SWR1/SRCAP chromatin remodelling complex, which acts additively to the known mal3-tip1-mto1-mto2 microtubule and tea1-tea2-tea4-pom1 polarity machineries.

Endosomal maturation by Rab conversion in Aspergillus nidulans is coupled to dynein-mediated basipetal movement.

We exploit the ease with which highly motile early endosomes are distinguished from static late endosomes in order to study Aspergillus nidulans endosomal traffic. RabS(Rab7) mediates homotypic fusion of late endosomes/vacuoles in a homotypic fusion- and vacuole protein sorting/Vps41-dependent manner. Progression across the endocytic pathway involves endosomal maturation because the end products of the pathway in the absence of RabS(Rab7) are minivacuoles that are competent in multivesicular body sorting and cargo degradation but retain early endosomal features, such as the ability to undergo long-distance movement and propensity to accumulate in the tip region if dynein function is impaired. Without RabS(Rab7), early endosomal Rab5s-RabA and RabB-reach minivacuoles, in agreement with the view that Rab7 homologues facilitate the release of Rab5 homologues from endosomes. RabS(Rab7) is recruited to membranes already at the stage of late endosomes still lacking vacuolar morphology, but the transition between early and late endosomes is sharp, as only in a minor proportion of examples are RabA/RabB and RabS(Rab7) detectable in the same-frequently the less motile-structures. This early-to-late endosome/vacuole transition is coupled to dynein-dependent movement away from the tip, resembling the periphery-to-center traffic of endosomes accompanying mammalian cell endosomal maturation. Genetic studies establish that endosomal maturation is essential, whereas homotypic vacuolar fusion is not.

Searching for gold beyond mitosis: Mining intracellular membrane traffic in Aspergillus nidulans.

The genetically tractable filamentous ascomycete fungus Aspergillus nidulans has been successfully exploited to gain major insight into the eukaryotic cell cycle. More recently, its amenability to in vivo multidimensional microscopy has fueled a potentially gilded second age of A. nidulans cell biology studies. This review specifically deals with studies on intracellular membrane traffic in A. nidulans. The cellular logistics are subordinated to the needs imposed by the polarized mode of growth of the multinucleated hyphal tip cells, whereas membrane traffic is adapted to the large intracellular distances. Recent work illustrates the usefulness of this fungus for morphological and biochemical studies on endosome and Golgi maturation, and on the role of microtubule-dependent motors in the long-distance movement of endosomes. The fungus is ideally suited for genetic studies on the secretory pathway, as mutations impairing secretion reduce apical extension rates, resulting in phenotypes detectable by visual inspection of colonies.

The p25 subunit of the dynactin complex is required for dynein-early endosome interaction.

Cytoplasmic dynein transports various cellular cargoes including early endosomes, but how dynein is linked to early endosomes is unclear. We find that the Aspergillus nidulans orthologue of the p25 subunit of dynactin is critical for dynein-mediated early endosome movement but not for dynein-mediated nuclear distribution. In the absence of NUDF/LIS1, p25 deletion abolished the localization of dynein-dynactin to the hyphal tip where early endosomes abnormally accumulate but did not prevent dynein-dynactin localization to microtubule plus ends. Within the dynactin complex, p25 locates at the pointed end of the Arp1 filament with Arp11 and p62, and our data suggest that Arp11 but not p62 is important for p25-dynactin association. Loss of either Arp1 or p25 significantly weakened the physical interaction between dynein and early endosomes, although loss of p25 did not apparently affect the integrity of the Arp1 filament. These results indicate that p25, in conjunction with the rest of the dynactin complex, is important for dynein-early endosome interaction.

Aspergillus RabB Rab5 integrates acquisition of degradative identity with the long distance movement of early endosomes.

Aspergillus nidulans early endosomes display characteristic long-distance bidirectional motility. Simultaneous dual-channel acquisition showed that the two Rab5 paralogues RabB and RabA colocalize in these early endosomes and also in larger, immotile mature endosomes. However, RabB-GTP is the sole recruiter to endosomes of Vps34 PI3K (phosphatidylinositol-3-kinase) and the phosphatidylinositol-3-phosphate [PI(3)P] effector AnVps19 and rabB Delta, leading to thermosensitivity prevents multivesicular body sorting of endocytic cargo. Thus, RabB is the sole mediator of degradative endosomal identity. Importantly, rabB Delta, unlike rabA Delta, prevents early endosome movement. As affinity experiments and pulldowns showed that RabB-GTP recruits AnVps45, RabB coordinates PI(3)P-dependent endosome-to-vacuole traffic with incoming traffic from the Golgi and with long-distance endosomal motility. However, the finding that Anvps45 Delta, unlike rabB Delta, severely impairs growth indicates that AnVps45 plays RabB-independent functions. Affinity chromatography showed that the CORVET complex is a RabB and, to a lesser extent, a RabA effector, in agreement with GST pulldown assays of AnVps8. rabB Delta leads to smaller vacuoles, suggesting that it impairs homotypic vacuolar fusion, which would agree with the sequential maturation of endosomal CORVET into HOPS proposed for Saccharomyces cerevisiae. rabB Delta and rabA Delta mutations are synthetically lethal, demonstrating that Rab5-mediated establishment of endosomal identity is essential for A. nidulans.