Formaldehyde fixation is detrimental to actin cables in glucose-depleted S. cerevisiae cells

Pleiotropy drives evolutionary repair of the responsiveness of polarized cell growth to environmental cues

The ability of cells to translate different extracellular cues into different intracellular responses is vital for their survival in unpredictable environments. In Saccharomyces cerevisiae, cell polarity is modulated in response to environmental signals which allows cells to adopt varying morphologies in different external conditions. The responsiveness of cell polarity to extracellular cues depends on the integration of the molecular network that regulates polarity establishment with networks that signal environmental changes. The coupling of molecular networks often leads to pleiotropic interactions that can make it difficult to determine whether the ability to respond to external signals emerges as an evolutionary response to environmental challenges or as a result of pleiotropic interactions between traits. Here, we study how the propensity of the polarity network of S. cerevisiae to evolve toward a state that is responsive to extracellular cues depends on the complexity of the environment. We show that the deletion of two genes, BEM3 and NRP1, disrupts the ability of the polarity network to respond to cues that signal the onset of the diauxic shift. By combining experimental evolution with whole-genome sequencing, we find that the restoration of the responsiveness to these cues correlates with mutations in genes involved in the sphingolipid synthesis pathway and that these mutations frequently settle in evolving populations irrespective of the complexity of the selective environment. We conclude that pleiotropic interactions make a significant contribution to the evolution of networks that are responsive to extracellular cues.

A Diaphanous and Enabled-dependent asymmetric actin cable array repositions nuclei during Drosophila oogenesis

Cells reposition their nuclei for diverse specialized functions through a wide variety of cytoskeletal mechanisms. During Drosophila oogenesis, 15 nurse cells connected by ring canals to each other and the oocyte contract, ‘dumping’ their cytoplasm into the oocyte. Prior to dumping, actin cables initiate from the nurse cell cortex and elongate toward their nuclei, pushing them away from ring canals to prevent obstruction. How the cable arrays reposition nuclei is unknown. We found that these arrays are asymmetric, with regional differences in actin cable growth rate dependent on the differential localization of the actin assembly factors Enabled and Diaphanous. Enabled mislocalization produces a uniform growth rate. In oocyte-contacting nurse cells with asymmetric cable arrays, nuclei move away from ring canals. With uniform arrays, these nuclei move toward the adjacent ring canal instead. This correlated with ring canal nuclear blockage and incomplete dumping. Our data suggest that nuclear repositioning relies on the regulated cortical localization of Diaphanous and Enabled to produce actin cable arrays with asymmetric growth that push nuclei away from ring canals, enabling successful oogenesis.

Staining of the Actin Cytoskeleton During Cell Division in Budding Yeast and Mammalian Cells

Observation of actin at the cortex in dividing cells can be accomplished using the fungal toxin phalloidin conjugated to fluorophores. Protocols for staining both budding yeast and cultured mammalian cells with fluorescent phalloidin are described. This technique can be combined with immunofluorescence to image actin filaments and other proteins involved in cell division simultaneously.

Mechanosensitive regulation of stanniocalcin-1 by zyxin and actin-myosin in human mesenchymal stromal cells

Stanniocalcin-1 (STC1) secreted by mesenchymal stromal cells (MSCs) has anti-inflammatory functions, reduces apoptosis, and aids in angiogenesis, both in vitro and in vivo. However, little is known about the molecular mechanisms of its regulation. Here, we show that STC1 secretion is increased only under specific cell-stress conditions. We find that this is due to a change in actin stress fibers and actin-myosin tension. Abolishment of stress fibers by blebbistatin and knockdown of the focal adhesion protein zyxin leads to an increase in STC1 secretion. To also study this connection in 3D, where few focal adhesions and actin stress fibers are present, STC1 expression was analyzed in 3D alginate hydrogels and 3D electrospun scaffolds. Indeed, STC1 secretion was increased in these low cellular tension 3D environments. Together, our data show that STC1 does not directly respond to cell stress, but that it is regulated through mechanotransduction. This research takes a step forward in the fundamental understanding of STC1 regulation and can have implications for cell-based regenerative medicine, where cell survival, anti-inflammatory factors, and angiogenesis are critical.

Shrinking Daughters: Rlm1-Dependent G1/S Checkpoint Maintains Saccharomyces cerevisiae Daughter Cell Size and Viability

The Rlm1 transcription factor is a target of the cell wall integrity pathway. We report that an rlm1Δ mutant grown on a nonfermentable carbon source at low osmolarity forms cell groups in which a mother cell is surrounded by smaller "satellite-daughter" cells. Mother cells in these groups progressed through repeated rounds of cell division with normal rates of bud growth and genetic stability; however, these cells underwent precocious START relative to wild-type mothers. Thus, once activated, Rlm1 delays the transition from G₁ to S, a mechanism we term the cell wall/START (CW/START) checkpoint. The rlm1Δ satellite-cell phenotype is suppressed by deletion of either SLT2, which encodes the kinase that activates Rlm1, or SWI4, which is also activated by Slt2; suggesting that Slt2 can have opposing roles in regulating the START transition. Consistent with an Rlm1-dependent CW/START checkpoint, rlm1Δ satellite daughters were unable to grow or divide further even after transfer to rich medium, but UV irradiation in G₁ could partially rescue rlm1Δ satellite daughters in the next division. Indeed, after cytokinesis, these satellite daughters shrank rapidly, displayed amorphous actin staining, and became more permeable. As a working hypothesis, we propose that duplication of an "actin-organizing center" in late G₁ may be required both to progress through START and to reestablish the actin cytoskeleton in daughter cells.

Extreme calorie restriction in yeast retentostats induces uniform non-quiescent growth arrest

Non-dividing Saccharomyces cerevisiae cultures are highly relevant for fundamental and applied studies. However, cultivation conditions in which non-dividing cells retain substantial metabolic activity are lacking. Unlike stationary-phase (SP) batch cultures, the current experimental paradigm for non-dividing yeast cultures, cultivation under extreme calorie restriction (ECR) in retentostat enables non-dividing yeast cells to retain substantial metabolic activity and to prevent rapid cellular deterioration. Distribution of F-actin structures and single-cell copy numbers of specific transcripts revealed that cultivation under ECR yields highly homogeneous cultures, in contrast to SP cultures that differentiate into quiescent and non-quiescent subpopulations. Combined with previous physiological studies, these results indicate that yeast cells subjected to ECR survive in an extended G₁ phase. This study demonstrates that yeast cells exposed to ECR differ from carbon-starved cells and offer a promising experimental model for studying non-dividing, metabolically active, and robust eukaryotic cells.