The transporter GAT1 plays an important role in GABA-mediated carbon-nitrogen interactions in Arabidopsis

Glutamate derived γ-aminobutyric acid (GABA) is synthetized in the cytosol prior to delivery to the mitochondria where it is catabolized via the TCA cycle. GABA accumulates under various environmental conditions, but an increasing number of studies show its involvement at the crossroad between C and N metabolism. To assess the role of GABA in modulating cellular metabolism, we exposed seedlings of A. thaliana GABA transporter gat1 mutant to full nutrition medium and media deficient in C and N combined with feeding of different concentrations (0.5 and 1 mM) of exogenous GABA. GC-MS based metabolite profiling showed an expected effect of medium composition on the seedlings metabolism of mutant and wild type alike. That being said, a significant interaction between GAT1 deficiency and medium composition was determined with respect to magnitude of change in relative amino acid levels. The effect of exogenous GABA treatment on metabolism was contingent on both the medium and the genotype, leading for instance to a drop in asparagine under full nutrition and low C conditions and glucose under all tested media, but not to changes in GABA content. We additionally assessed the effect of GAT1 deficiency on the expression of glutamate metabolism related genes and genes involved in abiotic stress responses. These results suggest a role for GAT1 in GABA-mediated metabolic alterations in the context of the C-N equilibrium of plant cells.

Sodium and water disturbances in patients with Sheehan's syndrome

Sheehan's syndrome has been attributed to ischemic damage of the pituitary gland or hypothalamic-pituitary stalk during the peripartum period. Well-described clinical features of Sheehan's syndrome include hypothyroidism, adrenal insufficiency, hypogonadism, growth hormone deficiency, hypoprolactinemia, and different sodium and water disturbances. The occurrence of sodium and water disturbances associated with Sheehan's syndrome depends on the degree of pituitary damage, time of onset since the initial pituitary insult, and concurrent medical conditions that also may play a role in sodium and water balance. We present a patient with Sheehan's syndrome with severe chronic hyponatremia; discuss a potential problem in the patient's management; and review the literature for various sodium and water disturbances, including acute and chronic hyponatremia as well as overt and subclinical central diabetes insipidus. Although Sheehan's syndrome is more prevalent in developing countries, the increasingly large immigrant population within the United States warrants better awareness of this syndrome and its potential complicating sodium and water disturbances. Prompt diagnosis and an understanding of the pathogenic mechanisms of sodium and water disturbances associated with Sheehan's syndrome may avoid potential treatment-related complications.

A mechanical function of myosin II in cell motility

Myosin II mutant Dictyostelium amoebae crawl more slowly than wild-type cells. Thus, myosin II must contribute to amoeboid locomotion. We propose that contractile forces generated by myosin II help the cell's rear edge to detach from the substratum and retract, allowing the cell to continue forward. To test this hypothesis, we measured the speed of wild-type and myosin II null mutant Dictyostelium cells on surfaces of varying adhesivity. As substratum adhesivity increased, the speed of myosin II null mutant cells decreased substantially compared to wild-type cells, suggesting that the mutant is less able to retract from sticky surfaces. Furthermore, interference reflection microscopy revealed a myosin-II-dependent contraction in wild-type but not null mutant cells that is consistent with a balance of adhesive and contractile forces in retraction. Although myosin II null mutant cells have a defect in retraction, pseudopod extension does not cause the cells to become elongated on sticky surfaces. This suggests a mechanism, based possibly on cytoskeletal tension, for regulating cell shape in locomotion. The tension would result from the transmission of tractional forces through the cytoskeletal network, providing the myosin II null mutant with a limited means of retraction and cell division on a surface.