Mechanism by Which MC Controls Harmful Algal Blooms Revealed by Cell Morphology of Aureococcus anophagefferens

On the basis of field experience, a bloom does not continue after treatment with modified clay (MC), even though the residual harmful algal bloom (HAB) biomass accounts for 20–30% of the initial cells. This interesting phenomenon indicates that, in addition to causing flocculation, MC can inhibit the growth of residual cells. Here, from a cell morphology perspective, Aureococcus anophagefferens was used as a model organism to explore this scientific issue and clarify the mechanism by which MC mitigates harmful algal blooms (HABs). The results showed that, at an ~70% removal efficiency, neutral clay (NC) could not effectively inhibit the growth of residual cells, although it caused various forms of damage to residual cells, such as cell deformation, cell breakage, decreased extracellular polysaccharides (EPS), increased cell membrane permeability, and increased cytoplasmic granularity, due to physical collisions. After modification, some physical and chemical properties of the clay particle surface were changed; for example, the surface electrical properties changed from negative to positive, lamellar spacing increased, hardness decreased, adhesion chains increased, adhesion improved, and the number of absorption sites increased, enhancing the occurrence of chemical and electrochemical effects and physical collisions with residual cells, leading to severe cell deformation and chemical cell breakage. Thus, MC effectively inhibited the growth of residual cells and controlled HABs.

Decoding cellular deformation from pseudo-simultaneously observed Rho GTPase activities

The inability to simultaneously observe all of the important Rho GTPases (Cdc42, Rac1, and RhoA) has prevented us from obtaining evidence of their coordinated regulation during cell deformation. Here, we propose Motion-Triggered Average (MTA), an algorithm that converts individually observed GTPases into pseudo-simultaneous observations. Using the time series obtained by MTA and mathematical model, we succeeded for the first time in decoding the cell edge velocity from the three GTPase activities to provide clear numerical evidence for coordinated cell edge regulation by the three GTPases. We found that the characteristics of the obtained activities were consistent with those of previous studies, and that GTPase activities and their derivatives were involved in edge regulation. Our approach provides an effective strategy for using single-molecule observations to elucidate problems hampered by the lack of simultaneous observations.

Growth Inhibition and Oxidative Stress Caused by 4,4'-dibromodiphenyl Ether (BDE-15) on a Marine Diatom Phaeodactylum Tricornutum

In this work, Phaeodactylum tricornutum was used to investigate the toxicity of 4,4'-dibromodiphenyl ether (BDE-15). Results showed that BDE-15 inhibited the photosynthetic activity and growth of P. tricornutum significantly with 24, 48, 72, and 96 h EC50 values of 1.03, 0.44, 0.41, and 0.42 mg L-1, respectively, indicating that it was a highly toxic substance. Moreover, BDE-15 could cause cell deformation, and a series of physiological, biochemical, and molecular changes in the cells. Under the exposure of BDE-15, contents of chlorophyll a and soluble protein decreased significantly, but reactive oxygen species (ROS) were accumulated in the algal cells, which may cause or intensify the peroxidation of membrane lipids. For alleviating the toxicity of excessive ROS, activities of antioxidant enzymes increased dramatically when this diatom was exposed to BDE-15. Thus, it is concluded that the overproduction of ROS may be regarded as one of the major factors in BDE-15 toxicity.

A mechano-osmotic feedback couples cell volume to the rate of cell deformation

Mechanics has been a central focus of physical biology in the past decade. In comparison, the osmotic and electric properties of cells are less understood. Here we show that a parameter central to both the physics and the physiology of the cell, its volume, depends on a mechano-osmotic coupling. We found that cells change their volume depending on the rate at which they change shape, when they spread, migrate or are externally deformed. Cells undergo slow deformation at constant volume, while fast deformation leads to volume loss. We propose a mechano-sensitive pump and leak model to explain this phenomenon. Our model and experiments suggest that volume modulation depends on the state of the actin cortex and the coupling of ion fluxes to membrane tension. This mechano-osmotic coupling defines a membrane tension homeostasis module constantly at work in cells, causing volume fluctuations associated with fast cell shape changes, with potential consequences on cellular physiology.

Cyst formation in proximal renal tubules caused by dysfunction of the microtubule minus-end regulator CAMSAP3

Epithelial cells organize an ordered array of non-centrosomal microtubules, the minus ends of which are regulated by CAMSAP3. The role of these microtubules in epithelial functions, however, is poorly understood. Here, we show that the kidneys of mice in which Camsap3 is mutated develop cysts at the proximal convoluted tubules (PCTs). PCTs were severely dilated in the mutant kidneys, and they also exhibited enhanced cell proliferation. In these PCTs, epithelial cells became flattened along with perturbation of microtubule arrays as well as of certain subcellular structures such as interdigitating basal processes. Furthermore, YAP and PIEZO1, which are known as mechanosensitive regulators for cell shaping and proliferation, were activated in these mutant PCT cells. These observations suggest that CAMSAP3-mediated microtubule networks are important for maintaining the proper mechanical properties of PCT cells, and its loss triggers cell deformation and proliferation via activation of mechanosensors, resulting in the dilation of PCTs.

Direct Observation of Conversion From Walled Cells to Wall-Deficient L-Form and Vice Versa in Escherichia coli Indicates the Essentiality of the Outer Membrane for Proliferation of L-Form Cells

Gram-negative bacteria such as Escherichia coli are surrounded by an outer membrane, which encloses a peptidoglycan layer. Even if thinner than in many Gram-positive bacteria, the peptidoglycan in E. coli allows cells to withstand turgor pressure in hypotonic medium. In hypertonic medium, E. coli treated with a cell wall synthesis inhibitor such as penicillin G form wall-deficient cells. These so-called L-form cells grow well under anaerobic conditions (i.e., in the absence of oxidative stress), becoming deformed and dividing as L-form. Upon removal of the inhibitor, they return to the walled rod-shaped state. Recently, the outer membrane was reported to provide rigidity to Gram-negative bacteria and to strengthen wall-deficient cells. However, it remains unclear why L-form cells need the outer membrane for growth. Using a microfluidic system, we found that, upon treatment with the outer membrane-disrupting drugs polymyxin B and polymyxin B nonapeptide or with the outer membrane synthesis inhibitor CHIR-090, the cells lysed during cell deformation and division, indicating that the outer membrane was important even in hypertonic medium. L-form cells could return to rod-shaped when trapped in a narrow space, but not in a wide space, likely due to insufficient physical force. Outer membrane rigidity could be compromised by lack of outer membrane proteins; Lpp, OmpA, or Pal. Deletion of lpp caused cells to lyse during cell deformation and cell division. In contrast, ompA and pal mutants could be deformed and return to small oval cells even when less physical force was exerted. These results strongly suggest that wall-deficient E. coli cells require a rigid outer membrane to survive, but not too rigid to prevent them from changing cell shape.