T1rho and T2 relaxation times of the normal adult knee meniscus at 3T: analysis of zonal differences

Abstract Background Prior studies describe histological and immunohistochemical differences in collagen and proteoglycan content in different meniscal zones. The aim of this study is to evaluate horizontal and vertical zonal differentiation of T1rho and T2 relaxation times of the entire meniscus from volunteers without symptom and imaging abnormality. Methods Twenty volunteers age between 19 and 38 who have no knee-related clinical symptoms, and no history of prior knee surgeries were enrolled in this study. Two T1rho mapping (b-FFE T1rho and SPGR T1rho) and T2 mapping images were acquired with a 3.0-T MR scanner. Each meniscus was divided manually into superficial and deep zones for horizontal zonal analysis. The anterior and posterior horns of each meniscus were divided manually into white, red-white and red zones for vertical zonal analysis. Zonal differences of average relaxation times among each zone, and both inter- and intra-observer reproducibility were statistically analyzed. Results In horizontal zonal analysis, T1rho relaxation times of the superficial zone tended to be higher than those of the deep zone, and this difference was statistically significant in the medial meniscal segments (84.3 ms vs 76.0 ms on b-FFE, p 0.74) or good (0.60–0.74) in all meniscal segments on both horizontal and vertical zonal analysis, except for inter-class correlation coefficients of the lateral meniscus on SPGR. Compared with SPGR T1rho images, b-FFE T1rho images demonstrated more significant zonal differentiation with higher inter- and intra-observer reproducibility. Conclusions There are zonal differences in T1rho and T2 relaxation times of the normal meniscus

Switching the mode of sucrose utilization by <it>Saccharomyces cerevisiae</it>

<p>Abstract</p> <p>Background</p> <p>Overflow metabolism is an undesirable characteristic of aerobic cultures of <it>Saccharomyces cerevisiae </it>during biomass-directed processes. It results from elevated sugar consumption rates that cause a high substrate conversion to ethanol and other bi-products, severely affecting cell physiology, bioprocess performance, and biomass yields. Fed-batch culture, where sucrose consumption rates are controlled by the external addition of sugar aiming at its low concentrations in the fermentor, is the classical bioprocessing alternative to prevent sugar fermentation by yeasts. However, fed-batch fermentations present drawbacks that could be overcome by simpler batch cultures at relatively high (e.g. 20 g/L) initial sugar concentrations. In this study, a <it>S. cerevisiae </it>strain lacking invertase activity was engineered to transport sucrose into the cells through a low-affinity and low-capacity sucrose-H⁺symport activity, and the growth kinetics and biomass yields on sucrose analyzed using simple batch cultures.</p> <p>Results</p> <p>We have deleted from the genome of a <it>S. cerevisiae </it>strain lacking invertase the high-affinity sucrose-H⁺symporter encoded by the <it>AGT1 </it>gene. This strain could still grow efficiently on sucrose due to a low-affinity and low-capacity sucrose-H⁺symport activity mediated by the <it>MALx1 </it>maltose permeases, and its further intracellular hydrolysis by cytoplasmic maltases. Although sucrose consumption by this engineered yeast strain was slower than with the parental yeast strain, the cells grew efficiently on sucrose due to an increased respiration of the carbon source. Consequently, this engineered yeast strain produced less ethanol and 1.5 to 2 times more biomass when cultivated in simple batch mode using 20 g/L sucrose as the carbon source.</p> <p>Conclusion</p> <p>Higher cell densities during batch cultures on 20 g/L sucrose were achieved by using a <it>S. cerevisiae </it>strain engineered in the sucrose uptake system. Such result was accomplished by effectively reducing sucrose uptake by the yeast cells, avoiding overflow metabolism, with the concomitant reduction in ethanol production. The use of this modified yeast strain in simpler batch culture mode can be a viable option to more complicated traditional sucrose-limited fed-batch cultures for biomass-directed processes of <it>S. cerevisiae</it>.</p