Drivers of Morphogenesis: Curvature Sensor Self-Assembly at the Membrane.

This review examines the relationships between membrane chemistry, curvature-sensing proteins, and cellular morphogenesis. Curvature-sensing proteins are often orders of magnitude smaller than the membrane curvatures they localize to. How are nanometer-scale proteins used to sense micrometer-scale membrane features? Here, we trace the journey of curvature-sensing proteins as they engage with lipid membranes through a combination of electrostatic and hydrophobic interactions. We discuss how curvature sensing hinges on membrane features like lipid charge, packing, and the directionality of membrane curvature. Once bound to the membrane, many curvature sensors undergo self-assembly (i.e., they oligomerize or form higher-order assemblies that are key for initiating and regulating cell shape transformations). Central to these discussions are the micrometer-scale curvature-sensing proteins' septins. By discussing recent literature surrounding septin membrane association, assembly, and their many functions in morphogenesis with support from other well-studied curvature sensors, we aim to synthesize possible mechanisms underlining cell shape sensing.

Comparative analysis of the syncytiotrophoblast in placenta tissue and trophoblast organoids using snRNA sequencing.

The outer surface of chorionic villi in the human placenta consists of a single multinucleated cell called the syncytiotrophoblast (STB). The unique cellular ultrastructure of the STB presents challenges in deciphering its gene expression signature at the single-cell level, as the STB contains billions of nuclei in a single cell. There are many gaps in understanding the molecular mechanisms and developmental trajectories involved in STB formation and differentiation. To identify the underlying control of the STB, we performed comparative single nucleus (SN) and single cell (SC) RNA sequencing on placental tissue and tissue-derived trophoblast organoids (TOs). We found that SN was essential to capture the STB population from both tissue and TOs. Differential gene expression and pseudotime analysis of TO-derived STB identified three distinct nuclear subtypes reminiscent of those recently identified in vivo . These included a juvenile nuclear population that exhibited both CTB and STB marker expression, a population enriched in genes involved in oxygen sensing, and a fully differentiated subtype. Notably, suspension culture conditions of TOs that restore the native orientation of the STB (STB out ) showed elevated expression of canonical STB markers and pregnancy hormones, along with a greater proportion of the terminally differentiated mature STB subtype, compared to those cultivated with an inverted STB polarity (STB in ). Gene regulatory analysis identified novel markers of STB differentiation conserved in tissue and TOs, including the chromatin remodeler RYBP, that exhibited STB-specific RNA and protein expression. Finally, we compared STB gene expression signatures amongst first trimester tissue, full-term tissue, and TOs, identifying many commonalities but also notable variability across each sample type. This indicates that STB gene expression is responsive to its environmental context. Our findings emphasize the utility of TOs to accurately model STB differentiation and the distinct nuclear subtypes observed in vivo , offering a versatile platform for unraveling the molecular mechanisms governing STB functions in placental biology and disease.

Tools for live-cell imaging of cytoskeletal and nuclear behavior in the unconventional yeast, Aureobasidium pullulans.

Aureobasidium pullulans is a ubiquitous fungus with a wide variety of morphologies and growth modes including "typical" single-budding yeast, and interestingly, larger multinucleate yeast than can make multiple buds in a single cell cycle. The study of A. pullulans promises to uncover novel cell biology, but currently tools are lacking to achieve this goal. Here, we describe initial components of a cell biology toolkit for A. pullulans, which is used to express and image fluorescent probes for nuclei as well as components of the cytoskeleton. These tools allowed live-cell imaging of the multinucleate and multibudding cycles, revealing highly synchronous mitoses in multinucleate yeast that occur in a semiopen manner with an intact but permeable nuclear envelope. These findings open the door to using this ubiquitous polyextremotolerant fungus as a model for evolutionary cell biology.

Intrinsically disordered sequences can tune fungal growth and the cell cycle for specific temperatures.

Temperature can impact every reaction essential to a cell. For organisms that cannot regulate their own temperature, adapting to temperatures that fluctuate unpredictably and on variable timescales is a major challenge. Extremes in the magnitude and frequency of temperature changes are increasing across the planet, raising questions as to how the biosphere will respond. To examine mechanisms of adaptation to temperature, we collected wild isolates from different climates of the fungus Ashbya gossypii, which has a compact genome of only ∼4,600 genes. We found control of the nuclear division cycle and polarized morphogenesis, both critical processes for fungal growth, were temperature sensitive and varied among the isolates. The phenotypes were associated with naturally varying sequences within the glutamine-rich region (QRR) IDR of an RNA-binding protein called Whi3. This protein regulates both nuclear division and polarized growth via its ability to form biomolecular condensates. In cells and in cell-free reconstitution assays, we found that temperature tunes the properties of Whi3-based condensates. Exchanging Whi3 sequences between isolates was sufficient to rescue temperature-sensitive phenotypes, and specifically, a heptad repeat sequence within the QRR confers temperature-sensitive behavior. Together, these data demonstrate that sequence variation in the size and composition of an IDR can promote cell adaptation to growth at specific temperature ranges. These data demonstrate the power of IDRs as tuning knobs for rapid adaptation to environmental fluctuations.