Acting on actin: the electric motility assay

We have developed a novel technique which allows one to direct the two dimensional motion of actin filaments on a myosin coated sheet using a weak electric field parallel to the plane of motion. The filament velocity can be increased or decreased, and even reversed, as a function of orientation and strength of the field. PMMA (poly(methylmethacrylate)) gratings, which act as rails for actin, allow one for the first time to explore three quadrants of the force velocity diagram. We discuss effective friction, duty ratio and stall force at different myosin densities. A discontinuity in the velocity force relationship suggests the existence of dynamical phase transition. Received: 2 March 1998 / Accepted: 23 March 1998     

Gaits of monkeys and horses: A methodological critique

In a previous paper it was shown that the traditional methodology used to analyze quadrupedal gaits is inadequate when applied to the gaits of monkeys. The traditional methods were modified for use with the monkey sample (Prost, '65). Was the modification a general improvement of the traditional approach or was it dictated by peculiarities of monkey locomotion? The traditional analysis receives its support primarily from its usefulness in handling horse gaits. If it can be shown that the modified methods improve the analysis for horses, then it can be claimed that the system is not a special case designed uniquely for use with monkeys. Motion pictures were taken of an American Saddle Bred, five-gaited horse performing several classic gaits. The films were analyzed using the modified methods developed to analyze monkey gaits. The analysis was superior to the traditional approach. All of the organizing principles which structure horse gaits were found to be identical to those which structure monkey gaits.     

Adrenal and liver metabolites of 18-hydroxy-11-deoxycorticosterone in the rat. Identification of reduced compounds (18-OH-TH-DOC)

The presence of reduced metabolites of 18-hydroxy-11-deoxycorticosterone has been investigated in the adrenals of 23 day-old and adult rats and in the liver of adult rats. By thin-layer chromatography a fraction of the adrenal steroid extract migrating like tetrahydro-corticosterone has been isolated. By gas chromatography-mass spectrometry several isomers of 3,18,21-trihydroxy-pregnan-20-one (18-OH-TH-DOC) have been separated in this fraction and identified by comparison with authentic samples which have been chemically and enzymatically synthesized. The major tetrahydrogenated metabolite in the adult and prepuberal rat adrenals is 3β,18,21-trihydroxy-5α-pregnan-20-one (18-OH-TH-DOC II). The 3α,18,21-trihydroxy-5β-pregnan-20-one has been found only in the prepuberal rat adrenal. A third tetrahydrogenated isomer has been tentatively identified as 3α,18,21-trihydroxy-5α-pregnan-20-one. Quantitative measurements by mass fragmentography show that adrenal reductase activity on 18-hydroxy-11-deoxycorticosterone is higher than on corticosterone. The 18-OH-TH-DOC II has been identified in the liver of adult male rat.     

Influence of protein malnutrition followed by a balanced refeeding on the synthesis level of pancreatic hydrolases in growing rats

The intake of 5% gluten + casein (4:1, W:W) (PM) (E group) diet compared to 20% (16:4, W:W) (control group) caused an overall decreased synthesis. With the duration of PM, Tg2 and Chtg1 synthesis remained steady, while lipase synthesis decreased and amylase synthesis increased. At the beginning of refeeding (two days), in E group Tg2 and amylase synthesis was increased with rebound effect compared to controls, then decreased, Chtg1 synthesis was higher than control synthesis and remained constant, lipase synthesis was increased and reached the control value only after 9 days.     

Compressive Stress Inhibits Proliferation in Tumor Spheroids through a Volume Limitation

In most instances, the growth of solid tumors occurs in constrained environments and requires a competition for space. A mechanical crosstalk can arise from this competition. In this article, we dissect the biomechanical sequence caused by a controlled compressive stress on multicellular spheroids (MCSs) used as a tumor model system. On timescales of minutes, we show that a compressive stress causes a reduction of the MCS volume, linked to a reduction of the cell volume in the core of the MCS. On timescales of hours, we observe a reversible induction of the proliferation inhibitor, p27^Kip1, from the center to the periphery of the spheroid. On timescales of days, we observe that cells are blocked in the cell cycle at the late G1 checkpoint, the restriction point. We show that the effect of pressure on the proliferation can be antagonized by silencing p27^Kip1. Finally, we quantify a clear correlation between the pressure-induced volume change and the growth rate of the spheroid. The compression-induced proliferation arrest that we studied is conserved for five cell lines, and is completely reversible. It demonstrates a generic crosstalk between mechanical stresses and the key players of cell cycle regulation. Our results suggest a role of volume change in the sensitivity to pressure, and that p27^Kip1 is strongly influenced by this change.