Expression of an Arabidopsis plasma membrane aquaporin in Dictyostelium results in hypoosmotic sensitivity and developmental abnormalities

Functional expression of Ca²⁺ dependent mammalian transmembrane gap junction protein Cx43 in slime mold Dictyostelium discoideum

In this paper, we expressed murine gap junction protein Cx43 in Dictyostelium discoideum by introducing the specific vector pDXA. In the first step, the successful expression of Cx43 and Cx43-eGFP was verified by (a) Western blot (anti-Cx43, anti-GFP), (b) fluorescence microscopy (eGFP-Cx43 co-expression, Cx43 immunostaining), and (c) flow cytometry analysis (eGFP-Cx43 co-expression). Although the fluorescence signals from cells expressing Cx43-eGFP detected by fluorescence microscopy seem relatively low, analysis by flow cytometry demonstrated that more than 60% of cells expressed Cx43-eGFP. In order to evaluate the function of expressed Cx43 in D. discoideum, we examined the hemi-channel function of Cx43. In this series of experiments, the passive uptake of carboxyfluorescein was monitored using flow cytometric analysis. A significant number of the transfected cells showed a prominent dye uptake in the absence of Ca(2+). The dye uptake by transfected cells in the presence of Ca(2+) was even lower than the non-specific dye uptake by non-transformed Ax3 orf+ cells, confirming that Cx43 expressed in D. discoideum retains its Ca(2+)-dependent, specific gating function. The expression of gap junction proteins expressed in slime molds opens a possibility to the biological significance of intercellular communications in development and maintenance of multicellular organisms.

Expression of StMYB1R-1, a novel potato single MYB-like domain transcription factor, increases drought tolerance

Potato (Solanum tuberosum) is relatively vulnerable to abiotic stress conditions such as drought, but the tolerance mechanisms for such stresses in potato are largely unknown. To identify stress-related factors in potato, we previously carried out a genetic screen of potato plants exposed to abiotic environmental stress conditions using reverse northern-blot analysis. A cDNA encoding a putative R1-type MYB-like transcription factor (StMYB1R-1) was identified as a putative stress-response gene. Here, the transcript levels of StMYB1R-1 were enhanced in response to several environmental stresses in addition to drought but were unaffected by biotic stresses. The results of intracellular targeting and quadruple 9-mer protein-binding microarray analysis indicated that StMYB1R-1 localizes to the nucleus and binds to the DNA sequence (G)/(A)GATAA. Overexpression of a StMYB1R-1 transgene in potato plants improved plant tolerance to drought stress while having no significant effects on other agricultural traits. Transgenic plants exhibited reduced rates of water loss and more rapid stomatal closing than wild-type plants under drought stress conditions. In addition, overexpression of StMYB1R-1 enhanced the expression of drought-regulated genes such as AtHB-7, RD28, ALDH22a1, and ERD1-like. Thus, the expression of StMYB1R-1 in potato enhanced drought tolerance via regulation of water loss. These results indicated that StMYB1R-1 functions as a transcription factor involved in the activation of drought-related genes.

Dictyostelium discoideum--a model for many reasons

The social amoeba or cellular slime mould Dictyostelium discoideum is a "professional" phagocyte that has long been recognized for its value as a biomedical model organism, particularly in studying the actomyosin cytoskeleton and chemotactic motility in non-muscle cells. The complete genome sequence of D. discoideum is known, it is genetically tractable, readily grown clonally as a eukaryotic microorganism and is highly accessible for biochemical, cell biological and physiological studies. These are the properties it shares with other microbial model organisms. However, Dictyostelium combines these with a unique life style, with motile unicellular and multicellular stages, and multiple cell types that offer for study an unparalleled variety of phenotypes and associated signalling pathways. These advantages have led to its recent emergence as a valuable model organism for studying the molecular pathogenesis and treatment of human disease, including a variety of infectious diseases caused by bacterial and fungal pathogens. Perhaps surprisingly, this organism, without neurons or brain, has begun to yield novel insights into the cytopathology of mitochondrial diseases as well as other genetic and idiopathic disorders affecting the central nervous system. Dictyostelium has also contributed significantly to our understanding of NDP kinase, as it was the Dictyostelium enzyme whose structure was first determined and related to enzymatic activity. The phenotypic richness and tractability of Dictyostelium should provide a fertile arena for future exploration of NDPK's cellular roles.

LOCALIZATION OF AQUAPORIN-7 IN HUMAN TESTIS AND EJACULATED SPERM: POSSIBLE INVOLVEMENT IN MAINTENANCE OF SPERM QUALITY

PURPOSE

We have previously reported the expression of aquaporin-7 (AQP7) in the rat testis and cloned human AQP7 from the testicular cDNA library. However, to our knowledge the spatial expression pattern and biological roles of AQP7 remain to be elucidated in humans.

MATERIALS AND METHODS

We investigated AQP7 expression in the human testis and ejaculated sperm from fertile men and from infertile patients.

RESULTS

AQP7 was expressed at the tail of spermatids and spermatozoa in the human testis. AQP7 protein was also detected at the middle piece and the anterior tale portion of ejaculated sperm. However, some infertile patients lacked AQP7 expression in ejaculated sperm, although all fertile men expressed AQP7 protein. The motility rate of AQP7 negative sperm was significantly lower than that of AQP7 positive sperm, while the sperm concentration was not different between AQP7 positive and negative subjects.

CONCLUSIONS

AQP7 shows a spatial expression pattern in the human testis. AQP7 may be involved in the maintenance of sperm motility. Furthermore, a lack of AQP7 expression in sperm may be an underlying mechanism of male infertility.

Hypertonic signal promotes stability of Dictyostelium spores via a PKA-independent pathway

Differentiation of Dictyostelium spores initiates with rapid encapsulation of prespore cells under the control of cAMP-dependent protein kinase (PKA), followed by further maturation processes involving cytoskeletal reorganization. Constitutive activation of PKA induces precocious formation of viable spores in development and confers the ability to encapsulate under specific submerged conditions. In this study, we show that the stability of these spores depends upon conditions of high osmotic strength during spore differentiation, indicating that a hypertonic signal is required in addition to PKA to induce maturation to stable spores. The formation of stable spores under hypertonic conditions requires high cell density, suggesting the involvement of additional cellular signaling.

Plant aquaporins

Aquaporins are ubiquitous membrane channel proteins that facilitate and regulate the permeation of water across biological membranes. Aquaporins are members of the MIP family and some of them seem to be also able to transport other molecules such as urea or glycerol. In the plant kingdom, a single plant expresses a considerably large number of MIP homologues. These homologues can be subdivided into four groups (PIP, TIP, NIP, SIP) with highly conserved amino acid sequences and intron positions in each group. Since their discovery, advancing knowledge of their structure led to an understanding of the basic features of the water transport mechanism. An optimal water balance is essential to the homeostasis of most organisms, and aquaporins may be one of the mechanisms involved under changing environmental and developmental conditions. In fact, this may be one reason for the abundance and diversity of aquaporins, in particular in plants. In addition, exposure to different types of stress alters water relations and thus, aquaporins may be involved in stress responses as well. The transcriptional and/or post-translational regulation of aquaporins would determine changes in membrane water permeability. Both phosphorylation and translocation to/from vesicles have been reported as post-translational mechanisms. However, translocation in plants has not yet been shown. Although significant advances have been achieved, complete understanding of aquaporin function and regulation remains elusive.

Real-time visualization of intracellular hydrodynamics in single living cells

Intracellular water concentrations in single living cells were visualized by nonlinear coherent anti-Stokes Raman scattering (CARS) microscopy. In combination with isotopic exchange measurements, CARS microscopy allowed the real-time observation of transient intracellular hydrodynamics at a high spatial resolution. Studies of the hydrodynamics in the microorganism Dictyostelium discoideum indicated the presence of a microscopic region near the plasma membrane where the mobility of water molecules is severely restricted. Modeling the transient hydrodynamics eventuated in the determination of cell-specific cytosolic diffusion and plasma membrane permeability constants. Our experiments demonstrate that CARS microscopy offers an invaluable tool for probing single-cell water dynamics.

Real-time visualization of intracellular hydrodynamics in single living cells

Intracellular water concentrations in single living cells were visualized by nonlinear coherent anti-Stokes Raman scattering (CARS) microscopy. In combination with isotopic exchange measurements, CARS microscopy allowed the real-time observation of transient intracellular hydrodynamics at a high spatial resolution. Studies of the hydrodynamics in the microorganism Dictyostelium discoideum indicated the presence of a microscopic region near the plasma membrane where the mobility of water molecules is severely restricted. Modeling the transient hydrodynamics eventuated in the determination of cell-specific cytosolic diffusion and plasma membrane permeability constants. Our experiments demonstrate that CARS microscopy offers an invaluable tool for probing single-cell water dynamics.

Loss of a member of the aquaporin gene family, aqpA affects spore dormancy in Dictyostelium

We isolated and characterized a gene from Dictyostelium discoideum, which encodes a protein of 279 amino acids (30.6kDa) containing six transmembrane domains with two highly conserved motifs of asparagine-proline-alanine (NPA) found in the aquaporin family of water-channel proteins, although the second motif of the protein has been modified into NPV (asparagine-proline-valine). The deduced amino acid sequence of the gene, which we have named aqpA, is 39% identical to D. discoideum WacA, 26% identical to human Aqp5, 26% identical to Oryza sativa PIP2a, 25% identical to yeast Aqy1 and 24% identical to E.coli AqpZ. Southern analyses indicated that aqpA is present as a single copy in the genome. Northern blot analysis showed that the developmentally regulated 1kb mRNA transcript first appears at the tight mound stage (12h), and is abundant in fingers (16h) and late culminants (20h). In-situ hybridization of slugs revealed that aqpA mRNA accumulated in cells of the prespore region but not in those of the prestalk region. Disruption of aqpA by homologous recombination did not significantly affect growth or developmental morphogenesis. Although mutant spores were viable, when assayed soon after encapsulation, they became permeable to propidium iodide and lost viability after a week on the top of a fruiting body. Thus, AqpA is essential to maintain spore dormancy perhaps through the regulation of water flow.

Xenopus oocytes as an expression system for plant transporters

The Xenopus oocyte provides a powerful system for the expression and characterisation of plant membrane proteins. Many different types of plant membrane proteins have been expressed and characterised using this system. As there are already several general reviews on the methodology for oocyte expression of channel proteins, we have summarised the particular advantages and disadvantages of using the system for the characterisation of plant cotransporter proteins. As an example of how the system can be used to identify transporters, we describe evidence for a low affinity nitrate transporter in oocytes injected with poly(A) RNA extracted from nitrate-induced barley roots. Furthermore, we describe evidence that the expression of some transporters in oocytes can modify the properties of endogenous membrane proteins. We conclude that although care must be taken in the interpretation of results and in choosing appropriate controls for experiments, oocyte expression is an excellent tool which will have an important role in characterising plant membrane proteins.

Plasma membrane intrinsic proteins from maize cluster in two sequence subgroups with differential aquaporin activity

The transport of water through membranes is regulated in part by aquaporins or water channel proteins. These proteins are members of the larger family of major intrinsic proteins (MIPs). Plant aquaporins are categorized as either tonoplast intrinsic proteins (TIPs) or plasma membrane intrinsic proteins (PIPs). Sequence analysis shows that PIPs form several subclasses. We report on the characterization of three maize (Zea mays) PIPs belonging to the PIP1 and PIP2 subfamilies (ZmPIP1a, ZmPIP1b, and ZmPIP2a). The ZmPIP2a clone has normal aquaporin activity in Xenopus laevis oocytes. ZmPIP1a and ZmPIP1b have no activity, and a review of the literature shows that most PIP1 proteins identified in other plants have no or very low activity in oocytes. Arabidopsis PIP1 proteins are the only exception. Control experiments show that this lack of activity of maize PIP1 proteins is not caused by their failure to arrive at the plasma membrane of the oocytes. ZmPIP1b also does not appear to facilitate the transport of any of the small solutes tried (glycerol, choline, ethanol, urea, and amino acids). These results are discussed in relationship to the function and regulation of the PIP family of aquaporins.