Mechanical properties of pleural membrane

The pleural membrane is modeled as a planar collection of interconnected randomly oriented line elements. By assuming that the line elements follow the strain field of a continuum, a strain-energy function is formulated. From the strain-energy function, an explicit stress-strain equation for large deformations is derived. In the linear approximation of the stress-strain equation the shear modulus and the area modulus of the membrane are respectively found to be 2.4 and 2.8 times the tension at the reference state. The stress-strain equation for large deformations is used to predict the displacement field around a circular hole in pleura. Good agreement is found between these predictions and measurements made on ablated pleura from dog lungs. From these theoretical and experimental results the conclusion is drawn that the pleura has a significant role in carrying shear forces and maintaining the lung's shape.     

Is cytoskeletal tension a major determinant of cell deformability in adherent endothelial cells?

We tested the hypothesis that mechanical tension in thecytoskeleton (CSK) is a major determinant of cell deformability. To confirm that tension was present in adherent endothelial cells, weeither cut or detached them from their basal surface by a microneedle. After cutting or detachment, the cells rapidly retracted. This retraction was prevented, however, if the CSK actin lattice was disrupted by cytochalasin D (Cyto D). These results confirmed thatthere was preexisting CSK tension in these cells and that the actinlattice was a primary stress-bearing component of the CSK. Second, todetermine the extent to which that preexisting CSK tension could altercell deformability, we developed a stretchable cell culture membranesystem to impose a rapid mechanical distension (and presumably a rapidincrease in CSK tension) on adherent endothelial cells. Altered celldeformability was quantitated as the shear stiffness measured bymagnetic twisting cytometry. When membrane strain increased 2.5 or 5%,the cell stiffness increased 15 and 30%, respectively. Disruption ofactin lattice with Cyto D abolished this stretch-induced increase instiffness, demonstrating that the increased stiffness depended on theintegrity of the actin CSK. Permeabilizing the cells with saponin andwashing away ATP and Ca²⁺ did notinhibit the stretch-induced stiffening of the cell. These resultssuggest that the stretch-induced stiffening was primarily due to thedirect mechanical changes in the forces distending the CSK but not toATP- or Ca²⁺-dependent processes.Taken together, these results suggest preexisting CSK tension is amajor determinant of cell deformability in adherent endothelial cells.

     

Calcium efflux during germination ofBlastocladiella emersonii

Preloaded zoospores ofBlastocladiella emersonii were found to release large quantities of⁴⁵Ca²⁺ in the course of germination, whereas no calcium is released from nongerminating zoospores. Studies with inhibitors of germination and of calcium movement seem to support the existence of two temporally as well as developmentally distinct effluxes during this differentiative transition. The early efflux seems to be associated with the triggering of the process and begins prior to other known structural and physiological responses of the zoospores; the late efflux starts just prior to the appearance of the first germling cells and is probably related to the morphological progress of germination. The early efflux of calcium per se is not sufficient to elicit the process, since the calcium ionophore A23187 produces a comparable efflux, but induces germination only in the presence of noneffective cyclic AMP concentrations.     

Experimental tests of the cellular tensegrity hypothesis

The tensegrity model depicts the cytoskeleton (CSK) as a prestressed network of interconnected filaments. The prestress is generated by the CSK contractile apparatus and is partly balanced by traction at the cell-substrate interface and partly by CSK internal compression elements such as microtubules (MTs). A key feature of tensegrity is that the shear modulus (G) must increase in proportion with the prestress. Here we have tested that prediction as well as the idea that compression of MTs balance a portion of the cell prestress. Airway smooth muscle cells were studied. Traction microscopy was used to calculate traction. Because traction must be balanced by the stress within the cell, the prestress could be computed. Cell G was measured by oscillatory magnetic cytometry. The prestress was modulated using graded concentrations of contracting (histamine) or relaxing (isoproterenol) agonists and by disrupting MTs by colchicine. It was found that G increased in proportion with the prestress and that compression of MTs balanced a significant, but a relatively small fraction of the prestress. Taken together, these results do not disprove other models of cell deformability, nor they prove tensegrity. However, they do support a priori predictions of tensegrity. As such, it may not be necessary to invoke more complex mechanisms to explain these central features of cell deformability.     

Antagonistic reactions of arginine and lysine against formaldehyde and their relation to cell proliferation,apoptosis, folate cycle and photosynthesis

¹H, ¹³C NMR, ESMS and MS/MS investigations proved that there is an antagonism in the spontaneous reaction of formaldehyde with L-lysine and L-arginine. L-Arginine can only be hydroxymethylated on the guanidino group in a very fast reaction forming mono-, di-, and trihydroxymethyl arginines (HMA). L-Lysine can be methylated on the -amino group forming mono-, di-, and trimethyl lysine on physiological pH. Hydroxymethyl arginines are relative stable, isolable products, and can also be formed in biological systems, especially in plants. Significant amounts of hydroxymethyl arginines were identified in the aqueous extract of lyophilized kohlrabi, which can be formed in photosynthesis during CO₂ fixation. ¹⁴C-Formaldehyde formed in a short-term (10, 30 sec) ¹⁴CO₂ fixation reaction in Zea mays L. (early maturity variety: Szegedi TC 277) was captured by L-arginine, which occurs in leaves in large amount. Formaldehyde formed during photosynthesis can react not only with the arginine, but with ribulose-1,5-diphosphate present in leaves. In model reactions formaldehyde can react with the ene diole group of ribulose-1,5-diphosphate in the absence of Rubisco enzyme, which is a similar reaction to the addition of formaldehyde to L-ascorbic acid. Hydroxymethyl arginines (HMA) are endogenous formaldehyde carrier molecules transferring the bound formaldehyde to thymidylate synthase enzyme system incorporating it into the folate cycle. HMA can also carry the bound formaldehyde to the cells especially to the tumorous cells (HT29 adenocarcinoma), and cause significant inhibition of cell proliferation and causes apoptosis.     

Effect of the cytoskeletal prestress on the mechanical impedance of cultured airway smooth muscle cells.

We investigated the effect of the cytoskeletal prestress (P) on the elastic and frictional properties of cultured human airway smooth muscle cells during oscillatory loading; P is preexisting tensile stress in the actin cytoskeleton generated by the cell contractile apparatus. We oscillated (0.1 Hz, 6 Pa peak to peak) small ferromagnetic beads bound to integrin receptors and computed the storage (elastic) modulus (G') and the loss (frictional) modulus (G") from the applied torque and the corresponding bead rotation. All measurements were done at baseline and after cells were treated with graded doses of either histamine (0.1, 1, 10 microM) or isoproterenol (0.01, 0.1, 1, 10 microM). Values for P for these concentrations were taken from a previous study (Wang et al., Am J Physiol Cell Physiol, in press). It was found that G' and G", as well as P, increased/decreased with increasing doses of histamine/isoproterenol. Both G' and G" exhibited linear dependences on P: G'(Pa) = 0.20P + 82 and G"(Pa) = 0.05P + 32. The dependence of G' on P is consistent with our previous findings and with the behavior of stress-supported structures. The dependence of G" on P is a novel finding. It could be attributed to a variety of mechanisms. Some of those mechanisms are discussed in detail. We concluded that, in addition to the central mechanisms by which stress-supported structures develop mechanical stresses, other mechanisms might need to be invoked to fully explain the observed dependence of the cell mechanical properties on the state of cell contractility.     

Phenobarbital Induction of Aldehyde Dehydrogenase Type 2 mRNA in Mouse Liver: A Candidate Region on Chromosome 7 for a Putative Regulatory Gene

Phenobarbital (PB) strongly induces in the liver the expression of many genes encoding detoxication enzymes, such as the aldehyde dehydrogenase type 2 in the mouse (Aldh2). With the aim of identifying genes involved in this response, we have undertaken an approach based on a genetic analysis in mice. In a previous report, the genetic analysis of both the C57BL/6J (B6) x DBA/2J (D2) F1 and the (F1 x F1) F2 led us to the hypothesis that Aldh2 responsiveness to PB was under the control of one major locus independent of the structural gene. In the present study, the genetic analysis of the inducibility by PB of Aldh2 in the backcross population B6D2F1 x D2 has allowed us to confirm the involvement of a major regulatory gene in this mechanism. By searching for genetic linkage between this locus and a series of microsatellites DNA markers, we obtained indicative evidence for a region on chromosome 7, which may carry this gene.