Biology subjects

Safran, S.

Publications and source records attributed to Safran, S..

3 recordsLinked to original sources

bioRxiv2021

Physical theory of biological noise buffering by multi-component phase separation

Maintaining homeostasis is a fundamental characteristic of living systems. In cells, this is contributed to by assembly of biochemically-distinct organelles, many of whom are not membrane-bound, but form by the physical process of liquid-liquid phase separation (LLPS). By analogy with LLPS in binary solutions, cellular LLPS was hypothesized to contribute to homeostasis by facilitating "concentration buffering", which renders the local protein concentration within the organelle robust to global variations in the average, cellular concentration (e.g. due to expression noise). Interestingly, concentration buffering was experimentally measured in vivo, in a simple organelle with a single solute, while it was observed not to be obeyed in one with several solutes. Here, we formulate theoretically and solve analytically a physical model of LLPS in a ternary solution of two solutes (A and B) that interact both homotypically (A-A attractions) and heterotypically (A-B attractions). Our physical theory predicts how the equilibrium concentrations in LLPS are related to expression noise and thus generalizes the concept of concentration buffering to multi-component systems. This allows us to reconcile the seemingly contradictory experimental observations. Furthermore, we predict that incremental changes of the homotypic and heterotypic interactions among the molecules that undergo LLPS, such as those that are caused by mutations in the genes encoding the proteins, may increase the efficiency of concentration buffering of a given system. Thus, we hypothesize that evolution may optimize concentration buffering as an efficient mechanism to maintain LLPS homeostasis, and suggest experimental approaches to test this in different systems. SignificanceThe stochastic nature of transcription/translation implies that the concentrations of cellular proteins are "noisy" and not constant in time or across cell populations. Liquid-liquid phase separation (LLPS), can reduce or "buffer" this noise by maintaining well-defined concentrations, even in the presence of concentration distributions. However, this idea was recently challenged experimentally in multicomponent systems. Our physical theory of LLPS in ternary systems (solutes A and B in a solvent), predicts their LLPS properties as a function of the A-A (homotypic) and A-B (heterotypic) interaction strengths. We show how buffering can be effective if the noise distribution aligns with the tie-lines of the phase diagram and suggest that evolution may optimize concentration buffering by selecting mutations that lead to this.

biophysics↗

bioRxiv2020

Mesoscale phase separation of chromatin in the nucleus

Intact-organism imaging of Drosophila larvae reveals and quantifies chromatin-aqueous phase separation. The chromatin can be organized near the lamina layer of the nuclear envelope, conventionally fill the nucleus, be organized centrally, or as a wetting droplet. These transitions are controlled by changes in nuclear volume and the interaction of chromatin with the lamina (part of the nuclear envelope) at the nuclear periphery. Using a simple polymeric model that includes the key features of chromatin self-attraction and its binding to the lamina, we demonstrate theoretically that it is the competition of these two effects that determines the mode of chromatin distribution. The qualitative trends as well as the compositional profiles obtained in our simulations compare well with the observed intact-organism imaging and quantification. Since the simulations contain only a small number of physical variables we can identify the generic mechanisms underlying the changes in the observed phase separations.

biophysics↗

bioRxiv2020

Live imaging of chromatin distribution in muscle nuclei reveals novel principles of nuclear architecture and chromatin compartmentalization

Packaging of the chromatin within the nucleus serves as an important factor in the regulation of transcriptional output. However, information on chromatin architecture on nuclear scale in fully differentiated cells, under physiological conditions and in live organisms, is largely unavailable. Here, we imaged nuclei and chromatin in muscle fibers of live, intact Drosophila larvae. In contrast to the common view that chromatin is distributed throughout the nuclear volume, we show that the entire chromatin, including active and repressed regions, forms a peripheral layer underneath the nuclear lamina, leaving a chromatin-devoid compartment at the nucleus center. Importantly, visualization of nuclear compartmentalization required imaging of un-fixed nuclei embedded within their intrinsic tissue environment, with preserved nuclear volume. Upon fixation of similar muscle nuclei, we observed an average of three-fold reduction in nuclear volume caused by dehydration and evidenced by nuclear flattening. In these conditions, the peripheral chromatin layer was not detected anymore, demonstrating the importance of preserving native biophysical tissue environment. We further show that nuclear compartmentalization is sensitive to the levels of lamin C, since over-expression of lamin C-GFP in muscle nuclei resulted in detachment of the peripheral chromatin layer from the lamina and its collapse into the nuclear center. Computer simulations of chromatin distribution recapitulated the peripheral chromatin organization observed experimentally, when binding of lamina associated domains (LADs) was incorporated with chromatin self-attractive interactions. Reducing the number of LADs led to collapse of the chromatin, similarly to our observations following lamin C over-expression. Taken together, our findings reveal a novel mode of mesoscale organization of chromatin within the nucleus in a live organism, in which the chromatin forms a peripheral layer separated from the nuclear interior. This architecture may be essential for robust transcriptional regulation in fully differentiated cells.

cell biology↗