Cell cytoskeleton and tensegrity

The role of tensegrity architecture of the cytoskeleton in the mechanical behavior of living cells is examined by computational studies. Plane and spatial tensegrity models of the cytoskeleton are considered as well as their non-tensegrity counterparts. Local buckling including deep postbuckling response of the compressed microtubules of the cytoskeleton is considered. The tensioned microfilaments cannot sustain compression. Large deformation of the whole model is accounted and fully nonlinear analysis is performed. It is shown that in the case of local buckling of the microtubules non-tensegrity models exhibit qualitatively the same linear stiffening as their tensegrity counterparts. This result raises the question of experimental validation of the local buckling of microtubules. If the microtubules of real cells are not straight, then tensegrity (in a narrow sense) is not a necessary attribute of the cytoskeleton architecture. If the microtubules are straight then tensegrity is more likely to be the cytoskeletal architecture.

Tensegrity architecture explains linear stiffening and predicts softening of living cells

The problem of theoretical explanation of the experimentally observed linear stiffening of living cells is addressed. This explanation is based on Ingber's assumption that the cell cytoskeleton, which enjoys tensegrity architecture with compressed microtubules that provide tension to the microfilaments, affects the mechanical behavior of the living cell. Moreover, it is shown that the consideration of the extreme flexibility of microtubules and the unilateral response of microfilaments is crucial for the understanding of the living cell overall behavior. Formal nonlinear structural analysis of the cell cytoskeleton under external mechanical loads is performed. For this purpose, a general computer model for tensegrity assemblies with unilateral microfilaments and buckled microtubules is developed and applied to the theoretical analysis of the mechanical response of 2D and 3D examples of tensegrity cells mimicking the behavior of real living cells. Results of the computer simulations explain the experimentally observed cell stiffening. Moreover, the theoretical results predict the possible existence of a transient softening behavior of cells, a phenomenon, which has not been observed in experiments yet.

Color-coded duplex ultrasound compared to CT angiography for detection and quantification of carotid artery stenosis

OBJECTIVE

The purpose of this study was to compare findings on color-coded duplex ultrasound and CT angiography for grading internal carotid artery stenosis, and to investigate whether both these imaging modalities are necessary for an accurate diagnosis of carotid stenosis in the pre-surgical assessment of the internal carotid artery.

PATIENTS AND METHODS

We examined 92 internal carotid arteries from 46 randomly chosen patients with suspected carotid stenosis by color-coded duplex ultrasound and by three-dimensional CT angiography (CTA). This retrospective study adhered to international guidelines with gradings of mild, moderate, severe, and occlusive carotid disease.

RESULTS

The study demonstrated agreement between the degree of stenosis found on color-coded duplex ultrasound and that found on CTA in 78% of cases overall and in 79% of patients requiring surgical intervention. When compared to CTA, color-coded duplex ultrasound yielded a sensitivity of 78.9% and a specificity of 96.3%. Although findings on color-coded duplex ultrasound and CTA were comparable, disagreement affecting treatment decision occurred in 10:92 arteries.

CONCLUSION

CTA was not found to be beneficial for patients exhibiting mild stenosis on color-coded duplex ultrasound, as none of the mild groupings found by sonography were interpreted as severe or occluded by CTA. However, CTA may be an important adjunct to color-coded duplex ultrasound regarding the categories of moderate, severe and occluded when carotid endarterectomy is considered.

T lymphocyte surface antigen markers in osteoporosis

Two distinct syndromes of osteoporosis have been postulated: type I, which is characterized by accelerated bone loss occurring in women during the early postmenopausal period; and type II, an age-related process of bone loss affecting both men and women in and after the seventh decade. Recently there has been indirect evidence linking local products of the immune system with bone remodeling. We therefore studied peripheral blood profiles of specific lymphocyte phenotypes in women with type I osteoporosis and in older women and men with type II osteoporosis. The ratio of CD4-bearing (T helper) cells to CD8-bearing (T cytotoxic-suppressor) cells (CD4/CD8 ratio) was elevated in women with symptomatic type I disease. In addition there was a significant negative correlation (r = -0.62, P less than 0.001) between the CD4/CD8 ratio and the spinal bone mineral density measured by dual-photon absorptiometry. In contrast, older men with a history of fracture (hip or spine) had CD4/CD8 ratios similar to control men. The number of T cells bearing IL-2R or VLA-1 was not different between osteoporotic subjects and controls in either men or women. This study supports the concept that local products of the immune system may be directly or indirectly involved in the pathogenesis of type I osteoporosis.

Down-regulated insulin receptors in HepG2 cells have an altered intracellular itinerary

The delivery of insulin and the insulin receptor into an intracellular compartment may be important for eliciting some of the biologic responses of the cell to the hormone. Internalization of insulin-receptor complexes in cells from hyperinsulinemic type II diabetic patients is diminished, suggesting a possible role for this cellular process in insulin resistance. To examine whether hyperinsulinemia contributes to defective insulin-receptor processing in vitro, cultured hepatoma cells (HepG2) were incubated with high concentrations of (500 ng/ml) insulin from 1-3 days. Insulin induced a decrease in the number of total and surface insulin receptors within 24 hours; however, the hormone did not mediate a change in the number of intracellular receptors. The cellular itinerary of control and down-regulated receptors were then compared. Insulin mediated internalization of down-regulated receptors was impaired compared to control receptors; however, the down-regulated receptors that were internalized recycled back to the plasma membrane more efficiently. By covalently labeling the insulin receptor with the photoactive insulin derivative, 125I-NAPA-DP-insulin, it was demonstrated that the rates of receptor degradation of down-regulated and control receptors were similar. These results suggest that incubating HepG2 cells with high concentrations of insulin alters the cellular itinerary of the insulin receptor.