Variations on a theme: plant autophagy in comparison to yeast and mammals

Autophagy is an evolutionary conserved process of bulk degradation and nutrient sequestration that occurs in all eukaryotic cells. Yet, in recent years, autophagy has also been shown to play a role in the specific degradation of individual proteins or protein aggregates as well as of damaged organelles. The process was initially discovered in yeast and has also been very well studied in mammals and, to a lesser extent, in plants. In this review, we summarize what is known regarding the various functions of autopahgy in plants but also attempt to address some specific issues concerning plant autophagy, such as the insufficient knowledge regarding autophagy in various plant species other than Arabidopsis, the fact that some genes belonging to the core autophagy machinery in various organisms are still missing in plants, the existence of autophagy multigene families in plants and the possible operation of selective autophagy in plants, a study that is still in its infancy. In addition, we point to plant-specific autophagy processes, such as the participation of autophagy during development and germination of the seed, a unique plant organ. Throughout this review, we demonstrate that the use of innovative bioinformatic resources, together with recent biological discoveries (such as the ATG8-interacting motif), should pave the way to a more comprehensive understanding of the multiple functions of plant autophagy.     

Synthetic molecules: helping to unravel plant signal transduction

The application of small molecules has played a crucial role in identifying novel components involved in plant signalling. Compared to classic genetic approaches, small molecule screens offer notable advantages in dissecting plant biological processes, such as technical simplicity, low start-up costs, and most importantly, bypassing the problems of lethality and redundancy. To identify small molecules that target a biological process or protein of interest, robust and well-reasoned high-throughput screening approaches are essential. In this review, we present a series of principles and valuable approaches in small molecule screening in the plant model system Arabidopsis thaliana. We also provide an overview of small molecules that led to breakthroughs in uncovering phytohormone signalling pathways, endomembrane signalling cascades, novel growth regulators, and plant defence mechanisms. Meanwhile, the strategies to deciphering the mechanisms of these small molecules on Arabidopsis are highlighted. Moreover, the opportunities and challenges of small molecule applications in translational biology are discussed.     

Thrombopoietin signal transduction: studies from cell lines and primary cells

Thrombopoietin (TPO) and its receptor Mpl support all of the developmental step necessary for megakaryocytopoiesis. In the past few years, the signaling pathways utilized by this member of the cytokine receptor family have been extensively studied, especially JAK/STAT, Ras/MAP kinase, Shc, and other adapter molecules. Many if not most of the secondary signaling pathways activated by thrombopoietin have also been identified upon binding of other hematopoietic growth factors to their cognate receptors, making the study of Mpl signaling representative of the field in general. However, identifying unique molecules or combinations of signals that direct megakaryocyte development has been an elusive goal and has led some investigators to conclude that there is little specificity during Mpl signal transduction. In this article we review the data regarding Mpl signaling with particular attention to the methods employed and critical interpretation of the data generated. Future studies will have to focus on primary bone marrow cells and intact animal models rather than transformed cell lines. Furthermore, it is likely that a comprehensive, integrative analysis of the many pathways activated by ligand binding will be necessary to understand the physiology of cytokine signaling.     

Hedgehog signal transduction: from flies to vertebrates

The patterning and morphogenesis of multicellular organisms require a complex interplay of inductive signals which control proliferation, growth arrest, and differentiation of different cell types. A number of such signaling molecules have been identified in vertebrates and invertebrates. The molecular dissection of these pathways demonstrated that in vertebrates, mutations or abnormals function of these signaling pathways were often associated with developmental disorders and cancer formation. The Hedgehog (Hh) family of secreted proteins provides a perfect example of such signaling proteins. In the following review, we will not discuss in detail the role of Hh as a morphogen, but rather focus on its signal transduction pathway and its role in various human disorders.     

Obese yeast: triglyceride lipolysis is functionally conserved from mammals to yeast

Storage and degradation of triglycerides are essential processes to ensure energy homeostasis and availability of precursors for membrane lipid synthesis. Recent evidence suggests that an emerging class of enzymes containing a conserved patatin domain are centrally important players in lipid degradation. Here we describe the identification and characterization of a major triglyceride lipase of the adipose triglyceride lipase/Brummer family, Tgl4, in the yeast Saccharomyces cerevisiae. Elimination of Tgl4 in a tgl3 background led to fat yeast, rendering growing cells unable to degrade triglycerides. Tgl4 and Tgl3 lipases localized to lipid droplets, independent of each other. Serine 315 in the GXSXG lipase active site consensus sequence of the patatin domain of Tgl4 is essential for catalytic activity. Mouse adipose triglyceride lipase (which also contains a patatin domain but is otherwise highly divergent in primary structure from any yeast protein) localized to lipid droplets when expressed in yeast, and significantly restored triglyceride breakdown in tgl4 mutants in vivo. Our data identify yeast Tgl4 as a functional ortholog of mammalian adipose triglyceride lipase.     

From yeast to mammals: Recent advances in genetic control of homologous recombination

Misregulation of DNA repair is associated with genetic instability and tumorigenesis. To preserve the integrity of the genome, eukaryotic cells have evolved extremely intricate mechanisms for repairing DNA damage. One type of DNA lesion is a double-strand break (DSB), which is highly toxic when unrepaired. Repair of DSBs can occur through multiple mechanisms. Aside from religating the DNA ends, a homologous template can be used for repair in a process called homologous recombination (HR). One key step in committing to HR is the formation of Rad51 filaments, which perform the homology search and strand invasion steps. In S. cerevisiae, Srs2 is a key regulator of Rad51 filament formation and disassembly. In this review, we highlight potential candidates of Srs2 orthologues in human cells, and we discuss recent advances in understanding how Srs2's so-called “anti-recombinase” activity is regulated.     

Selenium and signal transduction: roads to cell death and anti-tumour activity

Accumulated evidence from prospective studies, intervention trials and studies on animal models of cancer have suggested a strong inverse correlation between selenium intake and cancer incidence. Several putative mechanisms have been suggested to mediate the chemopreventive activities of selenium: of these, the inhibition of cellular proliferation and the induction of apoptosis are particularly attractive. The mitogen activated protein kinase (MAPK) pathways are known to be important regulators of cell death and our recent work has focused on the involvement of these pathways in selenium-induced apoptosis in primary cultures of oral cancers and corresponding normal mucosa derived from biopsy material. Using this system, the oral carcinoma cells were found to have enhanced sensitivity to apoptosis when treated with certain selenium compounds compared to normal oral mucosa. Induction of Fas ligand was associated with selenium-induced apoptosis. Signal transduction studies suggests that selenium induces several changes in the MAPK signalling pathways but functional intervention/inhibitor studies indicate that activation of the JNK pathway seems to be most important.     

Cycling of auxin-binding protein through the plant cell: pathways in auxin signal transduction.

The auxin receptor literature contains a glaring discrepancy that invites explanation. While some physiological experiments suggest that active auxin receptors are sited inside the cell, others point to action at the cell surface. Furthermore, although the major auxin-binding protein (ABP) of maize (Zea mays) coleoptiles is found in the lumen of the endoplasmic reticulum (ER), exogenous ABP can mediate auxin-dependent changes in the plasma membrane potential of protoplasts. How can an ER protein mediate changes in cell potential? To resolve this dilemma, I propose that ABP cycles through the cell. In response to auxin, ABP is released from the ER and follows a secretory pathway to the cell surface. After secretion, ABP would bind sites on the cell surface and become subject to endocytosis, cycling back to the ER. Elevated auxin would accelerate the cycling of ABP between the ER and the cell surface. If cell wall precursors interacted with ABP during their progression through the secretory pathway, this would provide a mechanism for regulating cell wall synthesis. At the cell surface ABP would regulate an enzyme responsible for maintaining membrane potential. Both of these responses are components of auxin-regulated growth. This hypothesis does not exclude other mechanisms of signal transduction, particularly in gene regulation.     

Actin in the endocytic pathway: from yeast to mammals

Genetic analysis of endocytosis in yeast early pointed to the essential role of actin in the uptake step. Efforts to identify the machinery involved demonstrated the important contribution of Arp2/3 and the myosins-I. Analysis of the process using live-cell fluorescence microscopy and electron microscopy have recently contributed to refine molecular models explaining clathrin and actin-dependent endocytic uptake. Increasing evidence now also indicates that actin plays important roles in post-internalization events along the endocytic pathway in yeast, including transport of vesicles, motility of endosomes and vacuole fusion. This review describes the present knowledge state on the roles of actin in endocytosis in yeast and points to similarities and differences with analogous processes in mammals.