Aquaporin Trafficking in Plant Cells: An Emerging Membrane‐Protein Model

Aquaporins (AQPs) are channel proteins that facilitate the transport of water and small solutes across biological membranes. In plants, AQPs exhibit a high multiplicity of isoforms in relation to a high diversity of sub‐cellular localizations, at the plasma membrane (PM) and in various intracellular compartments. Some members also exhibit a dual localization in distinct cell compartments, whereas others show polarized or domain‐specific expression at the PM or tonoplast, respectively. A diversity of mechanisms controlling the routing of newly synthesized AQPs towards their destination membranes and involving diacidic motifs, phosphorylation or tetramer assembly is being uncovered. Recent approaches using single particle tracking, fluorescence correlation spectroscopy and fluorescence recovery after photobleaching have, in combination with pharmacological interference, stressed the peculiarities of AQP sub‐cellular dynamics in environmentally challenging conditions. A role for clathrin and sterol‐rich domains in cell surface dynamics and endocytosis of PM AQPs was uncovered. These recent advances provide deep insights into the cellular mechanisms of water transport regulation in plants. They also point to AQPs as an emerging model for studying the sub‐cellular dynamics of plant membrane proteins .     

Trafficking of the IKs‐Complex in MDCK Cells: Site of Subunit Assembly and Determinants of Polarized Localization

The voltage‐gated potassium channel K_V7.1 is regulated by non‐pore forming regulatory KCNE β‐subunits. Together with KCNE1, it forms the slowly activating delayed rectifier potassium current I_Ks. However, where the subunits assemble and which of the subunits determines localization of the I_Ks‐complex has not been unequivocally resolved yet. We employed trafficking‐deficient K_V7.1 and KCNE1 mutants to investigate I_Ks trafficking using the polarized Madin‐Darby Canine Kidney cell line. We find that the assembly happens early in the secretory pathway but provide three lines of evidence that it takes place in a post‐endoplasmic reticulum compartment. We demonstrate that K_V7.1 targets the I_Ks‐complex to the basolateral membrane, but that KCNE1 can redirect the complex to the apical membrane upon mutation of critical K_V7.1 basolateral targeting signals. Our data provide a possible explanation to the fact that K_V7.1 can be localized apically or basolaterally in different epithelial tissues and offer a solution to divergent literature results regarding the effect of KCNE subunits on the subcellular localization of K_V7.1/KCNE complexes .     

Cryo‐Immunoelectron Microscopy of Adherent Cells Improved by the Use of Electrospun Cell Culture Substrates

Electrospun nanofibres are an excellent cell culture substrate, enabling the fast and non‐disruptive harvest and transfer of adherent cells for microscopical and biochemical analyses. Metabolic activity and cellular structures are maintained during the only half a minute‐long harvest and transfer process. We show here that such samples can be optimally processed by means of cryofixation combined either with freeze‐substitution, sample rehydration and cryosection‐immunolabelling or with freeze‐fracture replica‐immunolabelling. Moreover, electrospun fibre substrates are equally suitable for complementary approaches, such as biochemistry, fluorescence microscopy and cytochemistry.     

A trs20 Mutation That Mimics an SEDT‐Causing Mutation Blocks Selective and Non‐Selective Autophagy: A Model for TRAPP III Organization

TRAPP is a multisubunit complex that functions in membrane traffic. Mutations in the mammalian TRAPP protein C2 are linked to the skeletal disorder spondyloepiphyseal dysplasia tarda (SEDT) that is thought to arise from an inability to secrete procollagen from the endoplasmic reticulum. Here, we show that C2 binds to the SNARE protein Syntaxin 5 and this interaction is weakened by an SEDT‐causing missense mutation (D47Y). Interestingly, the equivalent mutation (D46Y) in the yeast C2 homolog Trs20p does not block anterograde traffic but did affect endocytosis. The trs20D46Y mutation interfered with the interaction between Trs20p and Trs85p (TRAPP III‐specific subunit), Trs120p and Trs130p (TRAPP II‐specific subunits). Size exclusion chromatography suggested that this yeast mutation destabilized the TRAPP III complex that is involved in autophagy. We further show that this mutation blocks both the selective cytosol‐to‐vacuole (cvt) pathway as well as non‐selective autophagy. We demonstrate that the apparent molecular size of the TRAPP III complex is dependent upon membranes, and that the presence of TRAPP III is dependent upon Atg9p. Finally, we demonstrate that lipidated Bet3p is enriched in TRAPP III and that lipidation increases the efficiency of autophagy. Our study suggests that Trs20p acts as an adaptor for Trs85p and Trs120p and reveals complexities in TRAPP III assembly and function. The implications of C2D47Y in SEDT are discussed .     

Neisseria meningitidis colonization of the brain endothelium and cerebrospinal fluid invasion

The brain and meningeal spaces are protected from bacterial invasion by the blood–brain barrier, formed by specialized endothelial cells and tight intercellular junctional complexes. However, once in the bloodstream, Neisseria meningitidis crosses this barrier in about 60% of the cases. This highlights the particular efficacy with which N. meningitidis targets the brain vascular cell wall. The first step of central nervous system invasion is the direct interaction between bacteria and endothelial cells. This step is mediated by the type IV pili, which induce a remodelling of the endothelial monolayer, leading to the opening of the intercellular space. In this review, strategies used by the bacteria to survive in the bloodstream, to colonize the brain vasculature and to cross the blood–brain barrier will be discussed.     

Investigating genetic discrimination in Australia: opportunities and challenges in the early stages

Genetic discrimination, defined as the differential treatment of individuals or their relatives on the basis of actual or presumed genetic differences, is an emerging issue of interest in academic, clinical, social and legal contexts. While its potential significance has been discussed widely, verified empirical data are scarce. Genetic discrimination is a complex phenomenon to describe and investigate, as evidenced by the recent Australian Law Reform Commission inquiry in Australia. The authors research project, which commenced in 2002, aims to document the multiple perspectives and experiences regarding genetic discrimination in Australia and inform future policy development and law reform. Data are being collected from consumers, employers, insurers and the legal system. Attempted verification of alleged accounts of genetic discrimination will be a novel feature of the research. This paper overviews the early stages of the research, including conceptual challenges and their methodological implications.     

A role for the miR396/GRF network in specification of organ type during flower development,as supported by ectopic expression of Populus trichocarpa miR396c in transgenic tobacco

The MIR396 family, composed of ath‐miR396a and ath‐miR396b in Arabidopsis, is conserved among plant species and is known to target the Growth‐Regulating Factor (GRF) gene family. ath‐miR396 overexpressors or grf mutants are characterised by small and narrow leaves and show embryogenic defects such as cotyledon fusion. Heterologous expression of ath‐miR396a has been reported in tobacco and resulted in reduction of the expression of three NtGRF genes. In this study, the precursor of the Populus trichocarpa ptc‐miR396c, with a mature sequence identical to ath‐miR396b, was expressed under control of the CaMV35S promoter in tobacco. Typical phenotypes of GRF down‐regulation were observed, including cotyledon fusion and lack of shoot apical meristem (SAM). At later stage of growth, transgenic plants had delayed development and altered specification of organ type during flower development. The third and fourth whorls of floral organs were modified into stigmatoid anthers and fasciated carpels, respectively. Several NtGRF genes containing a miR396 binding site were found to be down‐regulated, and the cleavage of their corresponding mRNA at the miR396 binding site was confirmed for two of them using RACE‐PCR analysis. The data obtained agree with the functional conservation of the miR396 family in plants and suggest a role for the miR396/GRF network in determination of floral organ specification.     

Low‐molecular weight carbohydrates modulate dormancy and are required for post‐germination growth in turions of Spirodela polyrhiza

The aquatic duckweed Spirodela polyrhiza propagates itself vegetatively by forming turions – bud‐like perennation organs – in the autumn, which spend the winter on the bottom of ponds and then germinate in the following spring and proliferate on the water surface. Newly formed turions usually require a period of cold after‐ripening and light to germinate effectively, but an ample supply of exogenous sugar can lead to germination even in the dark and independent of after‐ripening. The results of the present study indicate that the availability of readily metabolised carbohydrates is a determining factor for turion germination. Freshly harvested turions do not contain soluble, low‐molecular weight carbohydrates at a level sufficient to allow germination to take place, but after‐ripened turions do. Augmentation of the soluble carbohydrate content during after‐ripening derives from gradual breakdown of reserve starch of the turions. The long time required for any germination to be observed in turions incubated in darkness and the limited frequency of germination in the dark (about 50% of turion population), even with an ample external sugar, supply emphasise that both after‐ripening and light are essential for ensuring rapid germination and subsequent frond proliferation at an ecologically appropriate time. The carbohydrate supply required for rapid proliferation of the fronds produced at germination is provided by the rapid light‐induced breakdown of turion reserve starch.