Implications of a zinc uptake and transport model

While physicists regularly use mathematical equations to describe natural phenomena, mathematical modeling of biological systems is still not well established and is hampered by communication barriers between experimental and theoretical biologists. In a recent study we developed a mathematical model of zinc uptake and radial transport in Arabidopsis thaliana roots. By refraining from writing many equations in the main text and confining the derivation of formulas to a supplemental file, we attempted to reach both experimentalists and theoreticians likewise. Here, we give a short summary of our results on the accumulation pattern of zinc and the importance of transporter regulation, water flow and geometry. For a better understanding of the dynamics of adaptation to changes in external conditions, we plead for more detailed and frequent measurements. As a new aspect, we analyzed the effect of buffering. Simulations indicate that it dampens oscillations and may therefore play a key role in zinc homeostasis.     

Zinc uptake and radial transport in roots of Arabidopsis thaliana: a modelling approach to understand accumulation

Background and Aims

Zinc uptake in roots is believed to be mediated by ZIP (ZRT-, IRT-like proteins) transporters. Once inside the symplast, zinc is transported to the pericycle, where it exits by means of HMA (heavy metal ATPase) transporters. The combination of symplastic transport and spatial separation of influx and efflux produces a pattern in which zinc accumulates in the pericycle. Here, mathematical modelling was employed to study the importance of ZIP regulation, HMA abundance and symplastic transport in creation of the radial pattern of zinc in primary roots of Arabidopsis thaliana.

Methods

A comprehensive one-dimensional dynamic model of radial zinc transport in roots was developed and used to conduct simulations. The model accounts for the structure of the root consisting of symplast and apoplast and includes effects of water flow, diffusion and cross-membrane transport via transporters. It also incorporates the radial geometry and varying porosity of root tissues, as well as regulation of ZIP transporters.

Key Results

Steady-state patterns were calculated for various zinc concentrations in the medium, water influx and HMA abundance. The experimentally observed zinc gradient was reproduced very well. An increase of HMA or decrease in water influx led to loss of the gradient. The dynamic behaviour for a change in medium concentration and water influx was also simulated showing short adaptation times in the range of seconds to minutes. Slowing down regulation led to oscillations in expression levels, suggesting the need for rapid regulation and existence of buffering agents.

Conclusions

The model captures the experimental findings very well and confirms the hypothesis that low abundance of HMA4 produces a radial gradient in zinc concentration. Surprisingly, transpiration was found also to be a key parameter. The model suggests that ZIP regulation takes place on a comparable timescale as symplastic transport.     

Structure-function analysis of a novel member of the LIV-1 subfamily of zinc transporters, ZIP14

Here, we report the first investigation of a novel member of the LZT (LIV-1 subfamily of ZIP zinc Transporters) subfamily of zinc influx transporters. LZT subfamily sequences all contain a unique and highly conserved metalloprotease motif (HEXPHEXGD) in transmembrane domain V with both histidine residues essential for zinc transport by ZIP (Zrt-, Irt-like Proteins) transporters. We investigate here whether ZIP14 (SLC39A14), lacking the initial histidine in this motif, is still able to transport zinc. We demonstrate that this plasma membrane located glycosylated protein functions as a zinc influx transporter in a temperature-dependant manner.     

The Drosophila transmembrane protein Fear-of-intimacy controls glial cell migration

Development of complex organs depends on intensive cell-cell interactions, which help coordinate movements of many cell types. In a genetic screen aimed to identify genes controlling midline glia migration in the Drosophila nervous system, we have identified mutations in the gene kastchen. Here we show that during embryogenesis kastchen is also required for the normal migration of longitudinal and peripheral glial cells. During larval development, kastchen non-cell autonomously affects the migration of the subretinal glia into the eye disc. During embryonic development, kastchen not only affects glial cell migration but also controls the migration of muscle cells toward their attachment sites. In all cases, kastchen apparently functions in terminating or restricting cell migration. We identified the molecular nature of the gene by performing transgenic rescue experiments and by sequence analysis of mutant alleles. Kastchen corresponds to the recently described gene fear-of-intimacy (foi) that was identified in screen for genes affecting germ cell migration, suggesting that Foi-Kastchen is more generally involved in regulating cell migration. It encodes a member of an eight-transmembrane domain protein family of putative Zinc transporters or proteases. We determined the topology of the Foi protein by using antisera against luminal and intracellular domains of the protein and provide evidence that it does not act as a Zinc transporter. Genetic evidence suggests that one of the functions of foi may be associated with hedgehog signaling.     

不同模型对拟合大兴安岭林火发生与气象因素关系的适用性

选用符合林火发生数据结构的Poisson和零膨胀Poisson(ZIP)模型对大兴安岭林区1980—2005年间林火发生与气象因素关系进行建模分析,并与普通最小二乘回归(ordinary least squares,OLS)方法的结果进行了对比分析.结果表明:OLS模型对研究区域林火发生与气象因素关系的拟合结果较差(R2=0.215);Poisson和ZIP模型的拟合效果较好,具有较好的火灾次数预测能力,且ZIP模型的预测能力高于Poisson模型.运用AIC和Vuong检验方法对Poisson和ZIP模型的拟合水平进行进一步检验,表明ZIP模型的数据拟合度优于Poisson模型.     

Expression and regulation of SLC39A family zinc transporters in the developing mouse intestine

Several ZIP genes (SLC39A family of metal transporters) play roles in zinc homeostasis. Herein, the temporal and spatial patterns of expression of the mouse ZIP1, 3, 4, and 5 genes in the developing intestine and the effects of maternal dietary zinc deficiency on these patterns of expression were examined. ZIP1 and ZIP3 genes, conserved members of the ZIP subfamily II, were found to be coexpressed during development. Expression of these genes was detected on day 14 of gestation in smooth muscle and the pseudostratified endoderm. By 5 days post-partum, prominent expression became restricted to muscle and connective stroma. In contrast, expression of ZIP4 and ZIP5 genes, members of the ZIP subfamily called LIV-1, coincided with epithelial morphogenesis. ZIP5 expression was detected on d16 of gestation and localized to the basolateral membranes of the single-layered epithelium. ZIP4 expression was detected on d18 of gestation and localized to the apical membrane of villus epithelial cells. When dams were fed a zinc-deficient diet beginning at parturition, ZIP4 expression in the nursing neonate was greatly induced. In contrast, neonatal ZIP5 expression remained unchanged, but this protein was removed from the basolateral membrane of the enterocyte. These responses to dietary zinc deficiency mimic those found in the adult intestine. These studies reveal cell-type-specific expression of ZIP genes during development of the intestine, and suggest that the mouse intestine can elicit an adaptive response to dietary zinc availability at birth.     

Fagus sylvatica trunk epicormics in relation to primary and secondary growth

Background and Aims

European beech epicormics have received far less attention than epicormics of other species, especially sessile oak. However, previous work on beech has demonstrated that there is a negative effect of radial growth on trunk sprouting, while more recent investigations on sessile oak proved a strong positive influence of the presence of epicormics. The aims of this study were, first, to make a general quantification of the epicormics present along beech stems and, secondly, to test the effects of both radial growth and epicormic frequency on sprouting.

Methods

In order to test the effect of radial growth, ten forked individuals were sampled, with a dominant and a dominated fork of almost equal length for every individual. To test the effects of primary growth and epicormic frequency, on the last 17 annual shoots of each fork arm, the number of axillary buds, shoot length, ring width profiles, epicormic shoots and other epicormics were carefully recorded.

Key Results

The distribution of annual shoot length, radial growth profiles and parallel frequencies of all epicormics are presented. The latter frequencies were parallel to the annual shoot lengths, nearly equivalent for both arms of each tree, and radial growth profiles included very narrow rings in the lowest annual shoots and even missing rings in the dominated arms alone. The location of the latent buds and the epicormics was mainly at branch base, while epicormic shoots, bud clusters and spheroblasts were present mainly in the lowest annual shoots investigated. Using a zero-inflated mixed model, sprouting was shown to depend positively on epicormic frequency and negatively on radial growth.

Conclusions

Support for a trade-off between cambial activity and sprouting is put forward. Sprouting mainly depends on the frequency of epicormics. Between- and within-tree variability of the epicormic composition in a given species may thus have fundamental and applied implications.