paul janmey

Mammalian cells are highly complex, structured, active soft materials. The cytoskeleton is largely responsible for activity and mechanical response of whole cells. The cell nucleus typically occupies a substantial fraction of the cell volume and is itself an active material with a complex structure. We here report on measurements of viscoelastic properties as well as non-equilibrium fluctuations of both, suspended cells and isolated cell nuclei, using optical traps. Combining manipulation experiments with confocal microscopy, we also observe how drug interference affects mechanical response and activity.

408a Tuesday, February 13, 2024 individuals with obesity, where gravity significantly contributes to these forces. From a physics perspective, these forces, primarily compressive, can impact the cells within adipose tissues directly by deforming them and/or indirectly by altering the tissue characteristics, particularly its mechanical properties. This study employs rheology to investigate how mechanical compressions affect the mechanics of various adipose tissues, including subcutaneous and visceral types, collected from male mice on normal or high-fat diets. We first observed the relaxation of normal stress under compression, signifying adaptations and changes in tissue structure upon compression. Notably, we also observed a substantial 10-fold increase in the shear modulus of the tissues under compression that is comparable to the forces experienced when individuals sit. Additionally, this compression-induced …

The nucleus is generally considered to be the stiffest part of the cell with an apparent Young's modulus of~ 10 kPa, but cells deform their nuclei as they move through much softer matrices and apply< kPa stresses. Metabolically active nuclei were produced from cells by a centrifugation process that enucleates the cell, leaving behind a cytoplast and producing a nucleus that is wrapped with a plasma membrane and a few hundred nanometers of cytosol (a karyoplast) but no discernible cytoskeleton, endoplasmic reticulum, ribosomes or other large organelles. Karyoplasts have a wrinkled nuclear lamina, as expected since the nuclei transform from flattened structure to spheres as they are centrifuged out of the cell spread on a substrate. If the nucleus is modeled as an elastic object, force-indentation relations lead to an apparent Young's modulus of 5-10 kPa when measured at low indentations that increases with …

Polyacrylamide microparticles incorporated within a fibrous polymer matrix have been found useful for mimicking the native properties of soft tissues. These microparticles can be incorporated into hydrogels to create biomaterials that more closely resemble tissues. Previous work has shown that compression of tissues increases shear elastic modulus. This is theorized to be caused by volume-conserving materials interwoven between the fibers of the tissues. In contrast, increasing compression of fibrin networks alone decreases the shear elastic modulus. This compression softening of hydrogels is unfavored since a model biomaterial would closely resemble the properties of tissues. To replicate the stiffening of tissues after compression, polyacrylamide microparticles were incorporated into fibrin hydrogels. These microparticles simulate the cells and other materials found inside tissues. Soft and stiff polyacrylamide …

Background Microbial biofilms, as a hallmark of cystic fibrosis (CF) lung disease and other chronic infections, remain a desirable target for antimicrobial therapy. These biopolymer-based viscoelastic structures protect pathogenic organisms from immune responses and antibiotics. Consequently, treatments directed at disrupting biofilms represent a promising strategy for combating biofilm-associated infections. In CF patients, the viscoelasticity of biofilms is determined mainly by their polymicrobial nature and species-specific traits, such as Pseudomonas aeruginosa filamentous (Pf) bacteriophages. Therefore, we examined the impact of microbicidal ceragenins (CSAs) supported by mucolytic agents–DNase I and poly-aspartic acid (pASP), on the viability and viscoelasticity of mono- and bispecies biofilms formed by Pf-positive and Pf-negative P. aeruginosa strains co-cultured with Staphylococcus aureus or Candida albicans. Methods The in vitro antimicrobial activity of ceragenins against P. aeruginosa in mono- and dual-species cultures was assessed by determining minimum inhibitory concentration (MIC) and minimum bactericidal/fungicidal concentration (MBC/MFC). Inhibition of P. aeruginosa mono- and dual-species biofilms formation by ceragenins alone and in combination with DNase I or poly-aspartic acid (pASP) was estimated by the crystal violet assay. Additionally, the viability of the biofilms was measured by colony-forming unit (CFU) counting. Finally, the biofilms’ viscoelastic properties characterized by shear storage (G’) and loss moduli (G”), were analyzed with a rotational rheometer. Results Our results demonstrated that …

The structure and dynamics of the cell nucleus regulate nearly every facet of the cell. Changes in nuclear shape limit cell motility and gene expression. Although the nucleus is generally seen as the stiffest organelle in the cell, cells can nevertheless deform the nucleus to large strains by small mechanical stresses. Here, we show that the mechanical response of the cell nucleus exhibits active fluidization that is driven by the BRG 1 motor of the SWI/SNF/BAF chromatin-remodeling complex. Atomic force microscopy measurements show that the nucleus alters stiffness in response to the cell substrate stiffness, which is retained after the nucleus is isolated and that the work of nuclear compression is mostly dissipated rather than elastically stored. Inhibiting BRG 1 stiffens the nucleus and eliminates dissipation and nuclear remodeling both in isolated nuclei and in intact cells. These findings demonstrate a novel link between nuclear motor activity and global nuclear mechanics.

Rationale Mutations in COL4A3,4, or 5 cause Alport syndrome and contribute to other glomerular diseases. Potential mechanisms include abnormal GBM structure, chemistry, elasticity, or ER stress. The causes of initial podocyte injury are not fully defined in collagen IV nephropathies. Objectives We characterized renal function, structural and biophysical properties of glomerular capillaries and podocytes, and renal gene expression patterns in Col4a3 KO mice (BL6) over the 8-month course of disease. To test for a role for ER stress, we treated Col4a3 KO mice with TUDCA. Findings Col4a3 KO mice develop proteinuria and reduced renal function at 4 to 5 months, but their glomerular capillaries become abnormally deformable, and podocytes are dislodged into the urine with volume loading at 3 months. Glomeruli become maximally deformable at 4 months with loss of 40% of their podocytes, and then stiffen progressively with progression of renal disease. In parallel, kidneys become increasingly fibrotic with proteinuria and inflammatory infiltrates as renal failure progresses. Bulk renal gene expression changes from 2 and 4 months where it shows increased expression of genes related to matrix organization, cytokine signaling, and cell injury to reduction in genes coding for differentiated renal tubular and metabolic proteins. Treatment of mice with TUDCA minimized glomerular injury, loss of renal function, and increased expression of genes that inhibit MAP, tyrosine kinase, and TGF Beta. Conclusion Renal disease in Col4a3 KO mice progresses in two phases, before overt proteinuria with early podocyte injury and loss, and later with overt …

The mechanical integrities of intracellular biopolymer networks which constitute the cytoskeleton and extracellular scaffolds like the extracellular matrix (ECM) are crucial for dynamic cellular processes. Both of these polymer network structures enclose liquids in considerable volumes and can thus be considered as a biphasic material which comprise complex biopolymer networks and viscous liquids. While the experimental and theoretical characterization of the viscoelastic properties of polymer networks is well advanced, comparatively little is known about the contribution of fluid motion to their rheological properties. However, especially for compressive deformations in the nonlinear regime which reflects physiological conditions well, network stress and fluid pressure are tightly coupled. Accordingly, the stress field in such biphasic materials is originated in elastic stresses from the deformed network but also in …