A Positive Feedback between Growth and Polarity Provides Directional Persistency and Flexibility to the Process of Tip Growth

Mechanical strategies supporting growth and size diversity in Filamentous Fungi

The stereotypical tip growth of filamentous fungi supports their lifestyles and functions. It relies on the polarized remodeling and expansion of a protective elastic cell wall (CW) driven by large cytoplasmic turgor pressure. Remarkably, hyphal filament diameters and cell elongation rates can vary extensively among different fungi. To date, however, how fungal cell mechanics may be adapted to support these morphological diversities while ensuring surface integrity remains unknown. Here, we combined super-resolution imaging and deflation assays to measure local CW thickness, elasticity and turgor in a set of fungal species spread on the evolutionary tree that spans a large range in cell size and growth speeds. While CW elasticity exhibited dispersed values, presumably reflecting differences in CW composition, both thickness and turgor scaled in dose-dependence with cell diameter and growth speeds. Notably, larger cells exhibited thinner lateral CWs, and faster cells thinner apical CWs. Counterintuitively, turgor pressure was also inversely scaled with cell diameter and tip growth speed, challenging the idea that turgor is the primary factor dictating tip elongation rates. We propose that fast-growing cells with rapid CW turnover have evolved strategies based on a less turgid cytoplasm and thin walls to safeguard surface integrity and survival.

RPG acts as a central determinant for infectosome formation and cellular polarization during intracellular rhizobial infections

Host-controlled intracellular accommodation of nitrogen-fixing bacteria is essential for the establishment of a functional Root Nodule Symbiosis (RNS). In many host plants, this occurs via transcellular tubular structures (infection threads - ITs) that extend across cell layers via polar tip-growth. Comparative phylogenomic studies have identified RPG (RHIZOBIUM-DIRECTED POLAR GROWTH) among the critical genetic determinants for bacterial infection. In Medicago truncatula, RPG is required for effective IT progression within root hairs but the cellular and molecular function of the encoded protein remains elusive. Here, we show that RPG resides in the protein complex formed by the core endosymbiotic components VAPYRIN (VPY) and LUMPY INFECTION (LIN) required for IT polar growth, co-localizes with both VPY and LIN in IT tip- and perinuclear-associated puncta of M. truncatula root hairs undergoing infection and is necessary for VPY recruitment into these structures. Fluorescence Lifetime Imaging Microscopy (FLIM) of phosphoinositide species during bacterial infection revealed that functional RPG is required to sustain strong membrane polarization at the advancing tip of the IT. In addition, loss of RPG functionality alters the cytoskeleton-mediated connectivity between the IT tip and the nucleus and affects the polar secretion of the cell wall modifying enzyme NODULE PECTATE LYASE (NPL). Our results integrate RPG into a core host machinery required to support symbiont accommodation, suggesting that its occurrence in plant host genomes is essential to co-opt a multimeric protein module committed to endosymbiosis to sustain IT-mediated bacterial infection.

Stress-induced cell depolarization through the MAP kinase-Cdc42 axis

General stress responses, which sense environmental or endogenous signals, aim at promoting cell survival and fitness during adverse conditions. In eukaryotes, mitogen-activated protein (MAP) kinase-driven cascades trigger a shift in the cell's gene expression program as a cellular adaptation to stress. Here, we review another aspect of activated MAP kinase cascades reported in fission yeast: the transient inhibition of cell polarity in response to oxidative stress. The phosphorylation by a stress-activated MAP kinase of regulators of the GTPase cell division cycle 42 (Cdc42) causes a transient inhibition of polarized cell growth. The formation of growth sites depends on limiting and essential polarity components. We summarize here some processes in which inhibition of Cdc42 may be a general mechanism to regulate polarized growth also under physiological conditions.

Cell wall dynamics stabilize tip growth in a filamentous fungus

Hyphal tip growth allows filamentous fungi to colonize space, reproduce, or infect. It features remarkable morphogenetic plasticity including unusually fast elongation rates, tip turning, branching, or bulging. These shape changes are all driven from the expansion of a protective cell wall (CW) secreted from apical pools of exocytic vesicles. How CW secretion, remodeling, and deformation are modulated in concert to support rapid tip growth and morphogenesis while ensuring surface integrity remains poorly understood. We implemented subresolution imaging to map the dynamics of CW thickness and secretory vesicles in Aspergillus nidulans. We found that tip growth is associated with balanced rates of CW secretion and expansion, which limit temporal fluctuations in CW thickness, elongation speed, and vesicle amount, to less than 10% to 20%. Affecting this balance through modulations of growth or trafficking yield to near-immediate changes in CW thickness, mechanics, and shape. We developed a model with mechanical feedback that accounts for steady states of hyphal growth as well as rapid adaptation of CW mechanics and vesicle recruitment to different perturbations. These data provide unprecedented details on how CW dynamics emerges from material secretion and expansion, to stabilize fungal tip growth as well as promote its morphogenetic plasticity.

The Cdc42 GAP Rga6 promotes monopolar outgrowth of spores

The molecular mechanisms underlying the establishment of the monopolar growth of fission yeast spores have been less characterized. Here, we report that the Cdc42 GTPase-activating protein (GAP) Rga6 is required for promoting monopolar growth during spore germination. The absence of Rga6 increases the number of spores that grow in a bipolar fashion. Rga6 decorates the non-growing cortical region, binds phosphatidylinositol 4,5-bisphosphate, and colocalizes with the phosphatidylinositol 4,5-bisphosphate-binding protein Opy1. Overexpression of Opy1 diminishes the cortical localization of Rga6. The characteristic localization of Rga6 on the cell cortex depends on the C-terminal PBR region of Rga6. Moreover, engineered chimera composed of the Rga6 C-terminal PBR region fused to the GAP domain of Rga3 or Rga4 are sufficient to rescue the spore growth phenotype caused by the absence of Rga6. Hence, our work establishes a paradigm in which the lipid composition of the plasma membrane directs polarized cell growth by specifying the cortical localization of a GAP protein.

Mechanobiology of the cell wall – insights from tip-growing plant and fungal cells

The cell wall (CW) is a thin and rigid layer encasing the membrane of all plant and fungal cells. It ensures mechanical integrity by bearing mechanical stresses derived from large cytoplasmic turgor pressure, contacts with growing neighbors or growth within restricted spaces. The CW is made of polysaccharides and proteins, but is dynamic in nature, changing composition and geometry during growth, reproduction or infection. Such continuous and often rapid remodeling entails risks of enhanced stress and consequent damages or fractures, raising the question of how the CW detects and measures surface mechanical stress and how it strengthens to ensure surface integrity? Although early studies in model fungal and plant cells have identified homeostatic pathways required for CW integrity, recent methodologies are now allowing the measurement of pressure and local mechanical properties of CWs in live cells, as well as addressing how forces and stresses can be detected at the CW surface, fostering the emergence of the field of CW mechanobiology. Here, using tip-growing cells of plants and fungi as case study models, we review recent progress on CW mechanosensation and mechanical regulation, and their implications for the control of cell growth, morphogenesis and survival.

Chemotropism and Cell-Cell Fusion in Fungi

Fungi exhibit an enormous variety of morphologies, including yeast colonies, hyphal mycelia, and elaborate fruiting bodies. This diversity arises through a combination of polar growth, cell division, and cell fusion. Because fungal cells are nonmotile and surrounded by a protective cell wall that is essential for cell integrity, potential fusion partners must grow toward each other until they touch and then degrade the intervening cell walls without impacting cell integrity. Here, we review recent progress on understanding how fungi overcome these challenges. Extracellular chemoattractants, including small peptide pheromones, mediate communication between potential fusion partners, promoting the local activation of core cell polarity regulators to orient polar growth and cell wall degradation. However, in crowded environments, pheromone gradients can be complex and potentially confusing, raising the question of how cells can effectively find their partners. Recent findings suggest that the cell polarity circuit exhibits searching behavior that can respond to pheromone cues through a remarkably flexible and effective strategy called exploratory polarization.

Physical properties of the cytoplasm modulate the rates of microtubule polymerization and depolymerization

The cytoplasm is a crowded, visco-elastic environment whose physical properties change according to physiological or developmental states. How the physical properties of the cytoplasm impact cellular functions in vivo remains poorly understood. Here, we probe the effects of cytoplasmic concentration on microtubules by applying osmotic shifts to fission yeast, moss, and mammalian cells. We show that the rates of both microtubule polymerization and depolymerization scale linearly and inversely with cytoplasmic concentration; an increase in cytoplasmic concentration decreases the rates of microtubule polymerization and depolymerization proportionally, whereas a decrease in cytoplasmic concentration leads to the opposite. Numerous lines of evidence indicate that these effects are due to changes in cytoplasmic viscosity rather than cellular stress responses or macromolecular crowding per se. We reconstituted these effects on microtubules in vitro by tuning viscosity. Our findings indicate that, even in normal conditions, the viscosity of the cytoplasm modulates the reactions that underlie microtubule dynamic behaviors.

Fission yeast polycystin Pkd2p promotes cell size expansion and antagonizes the Hippo-related SIN pathway

Polycystins are conserved mechanosensitive channels whose mutations lead to the common human renal disorder autosomal dominant polycystic kidney disease (ADPKD). Previously, we discovered that the plasma membrane-localized fission yeast polycystin homolog Pkd2p is an essential protein required for cytokinesis; however, its role remains unclear. Here, we isolated a novel temperature-sensitive pkd2 mutant, pkd2-B42. Among the strong growth defects of this mutant, the most striking was that many mutant cells often lost a significant portion of their volume in just 5 min followed by a gradual recovery, a process that we termed 'deflation'. Unlike cell lysis, deflation did not result in plasma membrane rupture and occurred independently of cell cycle progression. The tip extension of pkd2-B42 cells was 80% slower than that of wild-type cells, and their turgor pressure was 50% lower. Both pkd2-B42 and the hypomorphic depletion mutant pkd2-81KD partially rescued mutants of the septation initiation network (SIN), a yeast Hippo-related signaling pathway, by preventing cell lysis, enhancing septum formation and doubling the number of Sid2p and Mob1p molecules at the spindle pole bodies. We conclude that Pkd2p promotes cell size expansion during interphase by regulating turgor pressure and antagonizes the SIN during cytokinesis. This article has an associated First Person interview with the first author of the paper.

Cells under pressure: how yeast cells respond to mechanical forces

In their natural habitats, unicellular fungal microbes are exposed to a myriad of mechanical cues such as shear forces from fluid flow, osmotic changes, and contact forces arising from microbial expansion in confined niches. While the rigidity of the cell wall is critical to withstand such external forces and balance high internal turgor pressure, it poses mechanical challenges during physiological processes such as cell growth, division, and mating that require cell wall remodeling. Thus, even organisms as simple as yeast have evolved complex signaling networks to sense and respond to intrinsic and extrinsic mechanical forces. In this review, we summarize the type and origin of mechanical forces experienced by unicellular yeast and discuss how these forces reorganize cell polarity and how pathogenic fungi exploit polarized assemblies to track weak spots in host tissues for successful penetration. We then describe mechanisms of force-sensing by conserved sets of mechanosensors. Finally, we elaborate downstream mechanotransduction mechanisms that orchestrate appropriate cellular responses, leading to improved mechanical fitness.

Stress-dependent inhibition of polarized cell growth through unbalancing the GEF/GAP regulation of Cdc42

Cdc42 GTPase rules cell polarity and growth in fission yeast. It is negatively and positively regulated by GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs), respectively. Active Cdc42-GTP localizes to the poles, where it associates with numerous proteins constituting the polarity module. However, little is known about its downregulation. We describe here that oxidative stress causes Sty1-kinase-dependent Cdc42 inactivation at cell poles. Both the amount of active Cdc42 at tips and cell length inversely correlate with Sty1 activity, explaining the elongated morphology of Δsty1 cells. We have created stress-blinded cell poles either by eliminating two Cdc42 GAPs or through the constitutive tethering of Gef1 to cell tips, and we biochemically demonstrate that the GAPs Rga3/6 and the GEF Gef1 are direct substrates of Sty1. We propose that phosphorylation of Rga3/6 and Gef1 mediates the Sty1-dependent inhibition of Cdc42 at cell tips, halting polarized growth during stress adaptation.

Detection of surface forces by the cell-wall mechanosensor Wsc1 in yeast

Surface receptors of animal cells, such as integrins, promote mechanosensation by forming clusters as signaling hubs that transduce tensile forces. Walled cells of plants and fungi also feature surface sensors, with long extracellular domains that are embedded in their cell walls (CWs) and are thought to detect injuries and promote repair. How these sensors probe surface forces remains unknown. By studying the conserved CW sensor Wsc1 in fission yeast, we uncovered the formation of micrometer-sized clusters at sites of force application onto the CW. Clusters assembled within minutes of CW compression, in dose dependence with mechanical stress and disassembled upon relaxation. Our data support that Wsc1 accumulates to sites of enhanced mechanical stress through reduced lateral diffusivity, mediated by the binding of its extracellular WSC domain to CW polysaccharides, independent of canonical polarity, trafficking, and downstream CW regulatory pathways. Wsc1 may represent an autonomous module to detect and transduce local surface forces onto the CW.

The Multiple Functions of Rho GTPases in Fission Yeasts

The Rho family of GTPases represents highly conserved molecular switches involved in a plethora of physiological processes. Fission yeast Schizosaccharomyces pombe has become a fundamental model organism to study the functions of Rho GTPases over the past few decades. In recent years, another fission yeast species, Schizosaccharomyces japonicus, has come into focus offering insight into evolutionary changes within the genus. Both fission yeasts contain only six Rho-type GTPases that are spatiotemporally controlled by multiple guanine-nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs), and whose intricate regulation in response to external cues is starting to be uncovered. In the present review, we will outline and discuss the current knowledge and recent advances on how the fission yeasts Rho family GTPases regulate essential physiological processes such as morphogenesis and polarity, cellular integrity, cytokinesis and cellular differentiation.

Coordinating cell polarization and morphogenesis through mechanical feedback

Many cellular processes require cell polarization to be maintained as the cell changes shape, grows or moves. Without feedback mechanisms relaying information about cell shape to the polarity molecular machinery, the coordination between cell polarization and morphogenesis, movement or growth would not be possible. Here we theoretically and computationally study the role of a genetically-encoded mechanical feedback (in the Cell Wall Integrity pathway) as a potential coordination mechanism between cell morphogenesis and polarity during budding yeast mating projection growth. We developed a coarse-grained continuum description of the coupled dynamics of cell polarization and morphogenesis as well as 3D stochastic simulations of the molecular polarization machinery in the evolving cell shape. Both theoretical approaches show that in the absence of mechanical feedback (or in the presence of weak feedback), cell polarity cannot be maintained at the projection tip during growth, with the polarization cap wandering off the projection tip, arresting morphogenesis. In contrast, for mechanical feedback strengths above a threshold, cells can robustly maintain cell polarization at the tip and simultaneously sustain mating projection growth. These results indicate that the mechanical feedback encoded in the Cell Wall Integrity pathway can provide important positional information to the molecular machinery in the cell, thereby enabling the coordination of cell polarization and morphogenesis.

An image analysis method to survey the dynamics of polar protein abundance in the regulation of tip growth

Tip growth is critical for the lifestyle of many walled cells. In yeast and fungi, this process is typically associated with the polarized deposition of conserved tip factors, including landmarks, Rho GTPases, cytoskeleton regulators, and membrane and cell wall remodelers. Because tip growth speeds may vary extensively between life cycles or species, we asked whether the local amount of specific polar elements could determine or limit tip growth speeds. Using the model fission yeast, we developed a quantitative image analysis pipeline to dynamically correlate single tip elongation speeds and polar protein abundance in large data sets. We found that polarity landmarks are typically diluted by growth. In contrast, tip growth speed is positively correlated with the local amount of factors related to actin, secretion or cell wall remodeling, but, surprisingly, exhibits long saturation plateaus above certain concentrations of those factors. Similar saturation observed for Spitzenkörper components in much faster growing fungal hyphae suggests that elements independent of canonical surface remodelers may limit single tip growth. This work provides standardized methods and resources to decipher the complex mechanisms that control cell growth.This article has an associated First Person interview with Sarah Taheraly, joint first author of the paper.

Mechanical force-induced morphology changes in a human fungal pathogen

Background

The initial step of a number of human or plant fungal infections requires active penetration of host tissue. For example, active penetration of intestinal epithelia by Candida albicans is critical for dissemination from the gut into the bloodstream. However, little is known about how this fungal pathogen copes with resistive forces upon host cell invasion.

Results

In the present study, we have used PDMS micro-fabrication to probe the ability of filamentous C. albicans cells to penetrate and grow invasively in substrates of different stiffness. We show that there is a threshold for penetration that corresponds to a stiffness of ~ 200 kPa and that invasive growth within a stiff substrate is characterized by dramatic filament buckling, along with a stiffness-dependent decrease in extension rate. We observed a striking alteration in cell morphology, i.e., reduced cell compartment length and increased diameter during invasive growth, that is not due to depolarization of active Cdc42, but rather occurs at a substantial distance from the site of growth as a result of mechanical compression.

Conclusions

Our data reveal that in response to this compression, active Cdc42 levels are increased at the apex, whereas active Rho1 becomes depolarized, similar to that observed in membrane protrusions. Our results show that cell growth and morphology are altered during invasive growth, suggesting stiffness dictates the host cells that C. albicans can penetrate.

Secretory Vesicle Clustering in Fungal Filamentous Cells Does Not Require Directional Growth

During symmetry breaking, the highly conserved Rho GTPase Cdc42 becomes stabilized at a defined site via an amplification process. However, little is known about how a new polarity site is established in an already asymmetric cell-a critical process in a changing environment. The human fungal pathogen Candida albicans switches from budding to filamentous growth in response to external cues, a transition controlled by Cdc42. Here, we have used optogenetic manipulation of cell polarity to reset growth in asymmetric filamentous C. albicans cells. We show that increasing the level of active Cdc42 on the plasma membrane results in disruption of the exocyst subunit Sec3 localization and a striking de novo clustering of secretory vesicles. This new cluster of secretory vesicles is highly dynamic, moving by hops and jumps, until a new growth site is established. Our results reveal that secretory vesicle clustering can occur in the absence of directional growth.

Fission Yeast Polarization: Modeling Cdc42 Oscillations, Symmetry Breaking, and Zones of Activation and Inhibition

Cells polarize for growth, motion, or mating through regulation of membrane-bound small GTPases between active GTP-bound and inactive GDP-bound forms. Activators (GEFs, GTP exchange factors) and inhibitors (GAPs, GTPase activating proteins) provide positive and negative feedbacks. We show that a reaction-diffusion model on a curved surface accounts for key features of polarization of model organism fission yeast. The model implements Cdc42 membrane diffusion using measured values for diffusion coefficients and dissociation rates and assumes a limiting GEF pool (proteins Gef1 and Scd1), as in prior models for budding yeast. The model includes two types of GAPs, one representing tip-localized GAPs, such as Rga3; and one representing side-localized GAPs, such as Rga4 and Rga6, that we assume switch between fast and slow diffusing states. After adjustment of unknown rate constants, the model reproduces active Cdc42 zones at cell tips and the pattern of GEF and GAP localization at cell tips and sides. The model reproduces observed tip-to-tip oscillations with periods of the order of several minutes, as well as asymmetric to symmetric oscillations transitions (corresponding to NETO "new end take off"), assuming the limiting GEF amount increases with cell size.

External signal-mediated polarized growth in fungi

As the majority of fungi are nonmotile, polarized growth in response to an external signal enables them to search for nutrients and mating partners, and hence is crucial for survival and proliferation. Although the mechanisms underlying polarization in response to external signals has commonalities with polarization during mitotic division, during budding, and fission growth, the importance of diverse feedback loops regulating external signal-mediated polarized growth is likely to be distinct and uniquely adapted to a dynamic environment. Here, we highlight recent advances in our understanding of the mechanisms that are crucial for polarity in response to external signals in fungi, with particular focus on the roles of membrane traffic, small GTPases, and lipids, as well as the interplay between cell shape and cell growth.

Decoupling of Rates of Protein Synthesis from Cell Expansion Leads to Supergrowth

Cell growth is a complex process in which cells synthesize cellular components while they increase in size. It is generally assumed that the rate of biosynthesis must somehow be coordinated with the rate of growth in order to maintain intracellular concentrations. However, little is known about potential feedback mechanisms that could achieve proteome homeostasis or the consequences when this homeostasis is perturbed. Here, we identify conditions in which fission yeast cells are prevented from volume expansion but nevertheless continue to synthesize biomass, leading to general accumulation of proteins and increased cytoplasmic density. Upon removal of these perturbations, this biomass accumulation drove cells to undergo a multi-generational period of "supergrowth" wherein rapid volume growth outpaced biosynthesis, returning proteome concentrations back to normal within hours. These findings demonstrate a mechanism for global proteome homeostasis based on modulation of volume growth and dilution.

Systematic mapping of cell wall mechanics in the regulation of cell morphogenesis

Walled cells of plants, fungi, and bacteria come with a large range of shapes and sizes, which are ultimately dictated by the mechanics of their cell wall. This stiff and thin polymeric layer encases the plasma membrane and protects the cells mechanically by opposing large turgor pressure derived mechanical stresses. To date, however, we still lack a quantitative understanding for how local and/or global mechanical properties of the wall support cell morphogenesis. Here, we combine subresolution imaging and laser-mediated wall relaxation to quantitate subcellular values of wall thickness (h) and bulk elastic moduli (Y) in large populations of live mutant cells and in conditions affecting cell diameter in the rod-shaped model fission yeast. We find that lateral wall stiffness, defined by the surface modulus, σ = hY, robustly scales with cell diameter. This scaling is valid across tens of mutants spanning various functions-within the population of individual isogenic strains, along single misshaped cells, and even across the fission yeasts clade. Dynamic modulations of cell diameter by chemical and/or mechanical means suggest that the cell wall can rapidly adapt its surface mechanics, rendering stretched wall portions stiffer than unstretched ones. Size-dependent wall stiffening constrains diameter definition and limits size variations; it may also provide an efficient means to keep elastic strains in the wall below failure strains, potentially promoting cell survival. This quantitative set of data impacts our current understanding of the mechanics of cell walls and its contribution to morphogenesis.

Functional interaction between Cdc42 and the stress MAPK signaling pathway during the regulation of fission yeast polarized growth

Cell polarization can be defined as the generation and maintenance of directional cellular organization. The spatial distribution and protein or lipid composition of the cell are not symmetric but organized in specialized domains which allow cells to grow and acquire a certain shape that is closely linked to their physiological function. The establishment and maintenance of polarized growth requires the coordination of diverse processes including cytoskeletal dynamics, membrane trafficking, and signaling cascade regulation. Some of the major players involved in the selection and maintenance of sites for polarized growth are Rho GTPases, which recognize the polarization site and transmit the signal to regulatory proteins of the cytoskeleton. Additionally, cytoskeletal organization, polarized secretion, and endocytosis are controlled by signaling pathways including those mediated by mitogen-activated protein kinases (MAPKs). Rho GTPases and the MAPK signaling pathways are strongly conserved from yeast to mammals, suggesting that the basic mechanisms of polarized growth have been maintained throughout evolution. For this reason, the study of how polarized growth is established and regulated in simple organisms such as the fission yeast Schizosaccharomyces pombe has contributed to broaden our knowledge about these processes in multicellular organisms. We review here the function of the Cdc42 GTPase and the stress activated MAPK (SAPK) signaling pathways during fission yeast polarized growth, and discuss the relevance of the crosstalk between both pathways.