Biology subjects

Lee, D. S. W.

Publications and source records attributed to Lee, D. S. W..

5 recordsLinked to original sources

bioRxiv2024

Cell surface crowding is a tunable biophysical barrier to cell-cell fusion

Cell-cell fusion is fundamental to developmental processes such as muscle formation, as well as to viral infections that cause pathological syncytia. An essential step in fusion is close membrane apposition, but cell membranes are crowded with proteins, glycoproteins, and glycolipids, all of which must be cleared before a fusion pore can be nucleated. Here, we find that cell surface crowding drastically reduces fusogenicity in multiple systems, independent of the method for driving fusion. We estimate that cell surface crowding presents an energetic barrier to membrane apposition on the scale of [~]100kBT, greater than that of bare membrane fusion. We show that increasing cell surface crowding reduces fusion efficiency of PEG-mediated and fusogen-mediated cell-cell fusion, as well as synthetic membranes under force. Interestingly, we find that differentiating myoblasts naturally decrease cell surface crowding prior to fusion. Cell surface crowding presents an underappreciated biophysical barrier that may be tuned developmentally and could be targeted externally to control tissue-specific cell-cell fusion.

biophysics↗

bioRxiv2022

A rich get richer effect governs intracellular condensate size distributions

Phase separation of biomolecules into condensates has emerged as a ubiquitous mechanism for intracellular organization and impacts many intracellular processes, including reaction pathways through clustering of enzymes and their intermediates. Precise and rapid spatiotemporal control of reactions by condensates requires tuning of their sizes. However, the physical processes that govern the distribution of condensate sizes remain unclear. Here, we utilize a combination of synthetic and native condensates to probe the underlying physical mechanisms determining condensate size. We find that both native nuclear speckles and FUS condensates formed with the synthetic Corelet system obey an exponential size distribution, which can be recapitulated in Monte Carlo simulations of fast nucleation followed by coalescence. By contrast, pathological aggregation of cytoplasmic Huntingtin polyQ protein exhibits a power-law size distribution, with an exponent of -1.41 {+/-} 0.02. These distinct behaviors reflect the relative importance of nucleation and coalescence kinetics: introducing continuous condensate nucleation into the Monte Carlo coarsening simulations gives rise to polyQ-like power-law behavior. We demonstrate that the emergence of power-law distributions under continuous nucleation reflects a "rich get richer" effect, whose extent may play a general role in the determination of condensate size distributions.

biophysics↗

bioRxiv2022

Viscoelastic RNA entanglement and advective flow underlie nucleolar form and function

The nucleolus facilitates transcription, processing, and assembly of ribosomal RNA (rRNA), the most abundant RNA in cells. Nucleolar function is facilitated by its multiphase liquid properties, but nucleolar fluidity and its connection to ribosome biogenesis remain unclear. Here, we used quantitative imaging, mathematical modeling, and pulse-chase nucleotide labelling to map nucleolar rRNA dynamics. Inconsistent with a purely diffusive process, rRNA steadily expands away from the transcriptional sites, moving in a slow (~1[A]/s), radially-directed fashion. This motion reflects the viscoelastic properties of a highly concentrated gel of entangled rRNA, whose constant polymerization drives steady outward flow. We propose a new viscoelastic rRNA release model, where nucleolar rRNA cleavage and processing reduce entanglement, fluidizing the nucleolar periphery to facilitate release of mature pre-ribosomal particles.

biophysics↗

bioRxiv2020

Mechanical frustration of phase separation in the cell nucleus by chromatin

Liquid-liquid phase separation is a fundamental mechanism underlying subcellular organization. Motivated by the striking observation that optogenetically-generated droplets in the nucleus display suppressed coarsening dynamics, we study the impact of chromatin mechanics on droplet phase separation. We combine theory and simulation to show that crosslinked chromatin can mechanically suppress droplets coalescence and ripening, as well as quantitatively control their number, size, and placement. Our results highlight the role of the subcellular mechanical environment on condensate regulation.

biophysics↗

bioRxiv2020

Chromatin Mechanics Dictates Subdiffusion and Coarsening Dynamics of Embedded Condensates

DNA is organized into chromatin, a complex polymeric material which stores information and controls gene expression. An emerging mechanism for biological organization, particularly within the crowded nucleus, is biomolecular phase separation into condensed droplets of protein and nucleic acids. However, the way in which chromatin impacts the dynamics of phase separation and condensate formation is poorly understood. Here, we utilize a powerful optogenetic strategy to examine the interplay of droplet coarsening with the surrounding viscoelastic chromatin network. We demonstrate that droplet growth dynamics are directly inhibited by the chromatin-dense environment, which gives rise to an anomalously slow coarsening exponent, {beta}[~]0.12, contrasting with the classical prediction of {beta}[~] 1/3. Using scaling arguments and simulations, we show how this arrested growth can arise due to subdiffusion of individual condensates, predicting {beta}[~]/3, where is the diffusion exponent. Tracking the fluctuating motion of condensates within chromatin reveals a subdiffusive exponent, [~]0.5, which explains the anomalous coarsening behavior and is also consistent with Rouse-like dynamics arising from the entangled chromatin. Our findings have implications for the biophysical regulation of the size and shape of biomolecular condensates, and suggest that condensate emulsions can be used to probe the viscoelastic mechanical environment within living cells.

biophysics↗