Biology subjects

Cassani, D. A. D.

Publications and source records attributed to Cassani, D. A. D..

2 recordsLinked to original sources

bioRxiv2024

Cryo-electron tomography reveals the nanoscale architecture of the actin cortex in cellular blebs

In animal cells, cellular deformations driving cytokinesis, migration, and epithelial constriction are driven by contractile tension in the actomyosin cortex, a thin network of actin and myosin underlying the plasma membrane. Cortical tension results from myosin-generated forces and as such, cortical myosin organization and dynamics have received significant attention. However, recent studies highlight that alongside myosin motor activity, the organization of the cortical actin network is a key regulator of tension. Yet, very little is known about the structural arrangement of cortical actin filaments. This is mostly due to the small thickness and high density of the cortex, which makes the visualization of cortical actin filaments challenging. Here, we use cryo-electron tomography (cryo-ET) to unveil the structural organization of cortical actin. As a model, we use isolated cellular blebs, which assemble an actin cortex comparable to the cortex of entire cells, but are small enough to be amenable to cryo-ET. We find that the bleb actin cortex is mostly composed of short and straight actin filaments. We then characterize cortex structural parameters, including the density of potential cross-linking and membrane attachment points. Our study unveils the nanoscale three-dimensional organization of the cortical actin network in cellular blebs. As such, it provides a quantitative framework for models of cortical tension generation, and will help understanding the nanoscale basis of cell surface contractions.

biophysics↗

bioRxiv2020

Cell surface fluctuations regulate early embryonic lineage sorting

In development, lineage segregation of multiple lineages must be coordinated in time and space. An important example is the mammalian inner cell mass (ICM), in which the primitive endoderm (PrE, founder of the yolk sac) physically segregates from the epiblast (EPI, founder of the foetus). The physical mechanisms that determine this spatial segregation between EPI and PrE are still poorly understood. Here, we identify an asymmetry in cell-cell affinity, a mechanical property thought to play a significant role in tissue sorting in other systems, between EPI and PrE precursors (pEPI and pPrE). However, a computational model of cell sorting indicated that these differences alone appeared insufficient to explain the spatial segregation. We also observed significantly greater surface fluctuations in pPrE compared to pEPI. Including the enhanced surface fluctuation in pPrE in our simulation led to robust cell sorting. We identify phospho-ERM regulated membrane tension as an important mediator of the increased surface fluctuations in pPrE. Using aggregates of engineered cell lines with different surface fluctuation levels cells with higher surface fluctuations were consistently excluded to the outside of the aggregate. These cells behaved similarly when incorporated in the embryo. Surface fluctuations-driven segregation is reminiscent of activity-induced phase separation, a sorting phenomenon in colloidal physics. Together, our experiments and model identify dynamic cell surface fluctuations, in addition to static mechanical properties, as a key factor for orchestrating the correct spatial positioning of the founder embryonic lineages.

developmental biology↗