Biology subjects

Alisafaei, F.

Publications and source records attributed to Alisafaei, F..

4 recordsLinked to original sources

bioRxiv2022

Lipid droplets are intracellular mechanical stressors in fatty hepatocytes

Matrix stiffening and external mechanical stress have been linked to disease and cancer development in multiple tissues, including the liver, where cirrhosis (which increases stiffness markedly) is the major risk factor for hepatocellular carcinoma. Patients with non-alcoholic fatty liver disease and lipid-droplet-filled hepatocytes, however, can develop cancer in non-cirrhotic, relatively soft tissue. Here, we show that lipid droplets are intracellular mechanical stressors with similar effects to tissue stiffening, including nuclear deformation, chromatin condensation, and hepatocyte dedifferentiation. Mathematical modelling of lipid droplets as inclusions that have only mechanical interactions with other cellular components generated results consistent with our experiments. These data show that lipid droplets are intracellular sources of mechanical stress and suggest that nuclear membrane tension integrates cell responses to combined internal and external stresses. Significance StatementDeformation of the nucleus as a result of extracellular sources of stress, including increased substrate stiffness, constricted migration, and compression, has been well documented to lead to increased nuclear rupture, changes in gene expression, and accumulation of DNA damage. Lipid droplet accumulation in hepatocytes provides a unique scenario to investigate potential intracellular mechanical stresses and sources of nuclear deformation. Our results show that lipid droplets are significant mechanical elements in the cell, deforming the nucleus in a way that promotes hepatocyte dedifferentiation and resisting cytoskeletal contraction and alignment.

cell biology↗

bioRxiv2022

Vimentin Intermediate Filaments Can Enhance or Abate Active Cellular Forces in a Microenvironmental Stiffness-Dependent Manner

The mechanical properties of cells are largely determined by the cytoskeleton, which is a complex network of interconnected biopolymers consisting of actin filaments, microtubules, and intermediate filaments. While disruption of the actin filament and microtubule networks is known to decrease and increase cell-generated forces, respectively, the effect of intermediate filaments on cellular forces is not well understood. Using a combination of theoretical modeling and experiments, we show that disruption of vimentin intermediate filaments can either increase or decrease cell-generated forces, depending on microenvironment stiffness, reconciling seemingly opposite results in the literature. On the one hand, vimentin is involved in the transmission of actomyosin-based tensile forces to the matrix and therefore enhances traction forces. On the other hand, vimentin reinforces microtubules and their stability under compression, thus promoting the role of microtubules in suppressing cellular traction forces. We show that the competition between these two opposing effects of vimentin is regulated by the microenvironment stiffness. For low matrix stiffness, the force-transmitting role of vimentin dominates over their microtubule-reinforcing role and therefore vimentin increases traction forces. At high matrix stiffness, vimentin decreases traction forces as the microtubule-reinforcing role of vimentin becomes more important with increasing matrix stiffness. Our theory reconciles seemingly disparate experimental observations on the role of vimentin in active cellular forces and provides a unified description of stiffness-dependent chemo-mechanical regulation of cell contractility by vimentin. SignificanceVimentin is a marker of the epithelial to mesenchymal transition which takes place during important biological processes including embryogenesis, metastasis, tumorigenesis, fibrosis, and wound healing. While the roles of the actin and microtubule networks in the transmission of cellular forces to the extracellular matrix are known, it is not clear how vimentin intermediate filaments impact cellular forces. Here, we show that vimentin impacts cellular forces in a matrix stiffness-dependent manner. Disruption of vimentin in cells on soft matrices reduces cellular forces, while it increases cellular forces in cells on stiff matrices. Given that cellular forces are central to both physiological and pathological processes, our study has broad implications for understanding the effect of vimentin on cellular forces in different microenvironments.

biophysics↗

bioRxiv2022

Tension anisotropy drives phenotypic transitions of cells via two-way cell-ECM feedback

Mechanical factors such as stress in the extracellular environment are known to affect phenotypic commitment of cells. However, the stress fields experienced by cells in tissues are multiaxial, and the ways that cells integrate this multiaxial information are largely unknown. Here, we report that the anisotropy of these stress fields is a critical factor triggering phenotypic transition in fibroblast cells, outweighing the previously reported role of stress amplitude. Using a combined experimental and computational approach, we discovered a self-reinforcing mechanism in which cellular protrusions interact with collagen fibers to develop tension anisotropy, which in turn stabilizes protrusions and amplifies their contractile forces. Disruption of this self-reinforcing process, either by reducing tension anisotropy or by inhibiting contractile protrusions, prevented phenotypic conversion of fibroblasts to contractile myofibroblasts.

biophysics↗

bioRxiv2021

An ex vivo culture model of kidney podocyte injury reveals mechanosensitive, synaptopodin-templating, sarcomere-like structures.

Chronic kidney diseases are widespread and incurable. The biophysical mechanisms underlying them are unclear, in part because material systems for reconstituting the microenvironment of the relevant kidney cells are limited. A critical question is how kidney podocytes (glomerular epithelial cells) regenerate the foot processes of the filtration apparatus following injury. Recently identified sarcomere-like structures (SLSs) with periodically spaced myosin IIA (a contractile protein) and synaptopodin (an actin-associated protein) appear in injured podocytes in vivo. We hypothesized that SLSs template synaptopodin in the initial stages of recovery, and tested this hypothesis by developing an ex vivo culture system that models both kidney physiology and pathophysiology. SLSs were observed in vitro for the first time as podocytes migrated out of harvested kidney glomeruli onto micropatterns of physiologically relevant proteins. SLSs emerged over two days, and cells formed foot process-like extensions from these periodically spaced proteins. SLS distributions and morphology were sensitive to actomyosin inhibitors, substrate stiffness, and extracellular matrix proteins associated with pathology. These results indicate a role for mechanobiological factors in podocyte recovery from injury, and suggest SLSs as a target for therapeutic intervention.

cell biology↗