Parallel worlds converge

The Nature of Disease in Plants by R.P. Scheffer Cambridge University Press, 1997. ￡45.00/$64.95 hbk (ix +325 pages) ISBN 0 521 48247 X.     

Proteasomal and autophagic pathways converge on lipid droplets

Apolipoprotein B (apoB) is the primary protein of very low-density lipoproteins (VLDL). We found that apoB accumulated on the surface of cytoplasmic lipid droplets (LDs) of hepatocytes when the proteasomal or autophagic processes were suppressed. ApoB associated with LDs was poly-ubiquitinated and surrounded by autophagic vacuoles. Moreover, proteasomal subunits were concentrated around LDs. Our data suggest that apoB that is destined to be degraded remains adhered to LDs until it is broken down by the proteasomal and autophagic pathways. We speculate that the LD surface serves as a platform to prevent hydrophobic apoB from forming aggregates, and that LDs may play a similar role for other aggregation-prone hydrophobic proteins.     

Cell polarity pathways converge and extend to regulate neural tube closure

Neural tube defects, such as spinabifida, craniorachischisis and anencephaly, are some of the most common birth defects in humans. Recent studies in mouse model systems suggest that craniorachischisis is associated with mutations in genes that regulate cell polarity. Using Xenopus as a model system, Wallingford and Harland have now shed light on the mechanism by which these pathways affect neural tube closure.     

Parallel quorum sensing systems converge to regulate virulence in Vibrio cholerae

The marine bacterium Vibrio harveyi possesses two quorum sensing systems (System 1 and System 2) that regulate bioluminescence. Although the Vibrio cholerae genome sequence reveals that a V. harveyi-like System 2 exists, it does not predict the existence of a V. harveyi-like System 1 or any obvious quorum sensing-controlled target genes. In this report we identify and characterize the genes encoding an additional V. cholerae autoinducer synthase and its cognate sensor. Analysis of double mutants indicates that a third as yet unidentified sensory circuit exists in V. cholerae. This quorum sensing apparatus is unusually complex, as it is composed of at least three parallel signaling channels. We show that in V. cholerae these communication systems converge to control virulence.     

Serotonergic pathways converge upon central melanocortin systems to regulate energy balance

Multiple lines of research provide compelling support for an important role for central serotonergic (5-hydroxytryptamine, 5-HT) and melanocortin pathways in the regulation of food intake and body weight. In this brief review, we outline data supporting a model in which serotonergic pathways affect energy balance, in part, by converging upon central melanocortin systems to stimulate the release of the endogenous melanocortin agonist, alpha-melanocyte stimulating hormone (alpha-MSH). Further, we review the neuroanatomical mapping of a downstream target of alpha-MSH, the melanocortin 4 receptor (MC4R), in the rodent brain. We propose that downstream activation of MC4R-expressing neurons substantially contributes to serotonin's effects on energy homeostasis.     

The Escherichia coli SRP and SecB targeting pathways converge at the translocon.

Two distinct protein targeting pathways can direct proteins to the Escherichia coli inner membrane. The Sec pathway involves the cytosolic chaperone SecB that binds to the mature region of pre-proteins. SecB targets the pre-protein to SecA that mediates pre-protein translocation through the SecYEG translocon. The SRP pathway is probably used primarily for the targeting and assembly of inner membrane proteins. It involves the signal recognition particle (SRP) that interacts with the hydrophobic targeting signal of nascent proteins. By using a protein cross-linking approach, we demonstrate here that the SRP pathway delivers nascent inner membrane proteins at the membrane. The SRP receptor FtsY, GTP and inner membranes are required for release of the nascent proteins from the SRP. Upon release of the SRP at the membrane, the targeted nascent proteins insert into a translocon that contains at least SecA, SecY and SecG. Hence, as appears to be the case for several other translocation systems, multiple targeting mechanisms deliver a variety of precursor proteins to a common membrane translocation complex of the E.coli inner membrane.     

Embryonic and tumorigenic pathways converge via Nodal signaling: role in melanoma aggressiveness

Bidirectional cellular communication is integral to both cancer progression and embryological development. In addition, aggressive tumor cells are phenotypically plastic, sharing many properties with embryonic cells. Owing to the similarities between these two types of cells, the developing zebrafish can be used as a biosensor for tumor-derived signals. Using this system, we show that aggressive melanoma cells secrete Nodal (a potent embryonic morphogen) and consequently can induce ectopic formation of the embryonic axis. We further show that Nodal is present in human metastatic tumors, but not in normal skin, and thus may be involved in melanoma pathogenesis. Inhibition of Nodal signaling reduces melanoma cell invasiveness, colony formation and tumorigenicity. Nodal inhibition also promotes the reversion of melanoma cells toward a melanocytic phenotype. These data suggest that Nodal signaling has a key role in melanoma cell plasticity and tumorigenicity, thereby providing a previously unknown molecular target for regulating tumor progression.     

Longevity pathways converge on autophagy genes to regulate life span in Caenorhabditis elegans

Aging is a multifactorial process with many mechanisms contributing to the decline. Mutations decreasing insulin/IGF-1 (insulin-like growth factor-1) or TOR (target of rapamycin) kinase-mediated signaling, mitochondrial activity and food intake each extend life span in divergent animal phyla. Understanding how these genetically distinct mechanisms interact to control longevity is a fundamental and fascinating problem in biology. Here we show that mutational inactivation of autophagy genes, which are involved in the degradation of aberrant, damaged cytoplasmic constituents accumulating in all aging cells, accelerates the rate at which the tissues age in the nematode Caenorhabditis elegans. According to our results Drosophila flies deficient in autophagy are also short-lived. We further demonstrate that reduced activity of autophagy genes suppresses life span extension in mutant nematodes with inherent dietary restriction, aberrant insulin/IGF-1 or TOR signaling, and lowered mitochondrial respiration. These findings suggest that the autophagy gene cascade functions downstream of and is inhibited by different longevity pathways in C. elegans, therefore, their effects converge on autophagy genes to slow down aging and lengthen life span. Thus, autophagy may act as a central regulatory mechanism of animal aging.     

Longevity pathways converge on autophagy genes to regulate life span in Caenorhabditis elegans

Aging is a multifactorial process with many mechanisms contributing to the decline. Mutations decreasing insulin/IGF-1 (insulin-like growth factor-1) or TOR (target of rapamycin) kinase-mediated signaling, mitochondrial activity and food intake each extend life span in divergent animal phyla. Understanding how these genetically distinct mechanisms interact to control longevity is a fundamental and fascinating problem in biology. Here we show that mutational inactivation of autophagy genes, which are involved in the degradation of aberrant, damaged cytoplasmic constituents accumulating in all aging cells, accelerates the rate at which the tissues age in the nematode Caenorhabditis elegans. According to our results Drosophila flies deficient in autophagy are also short-lived. We further demonstrate that reduced activity of autophagy genes suppresses life span extension in mutant nematodes with inherent dietary restriction, aberrant insulin/IGF-1 or TOR signaling, and lowered mitochondrial respiration. These findings suggest that the autophagy gene cascade functions downstream of and is inhibited by different longevity pathways in C. elegans, therefore, their effects converge on autophagy genes to slow down aging and lengthen life span. Thus, autophagy may act as a central regulatory mechanism of animal aging.     

Parallel protein-unfolding pathways revealed and mapped

Theoretical studies of protein folding suggest that multiple folding pathways should exist, but there is little experimental evidence to support this. Here we demonstrate changes in the flux between different transition states on parallel folding pathways, resulting in unprecedented upward curvature in the denaturant-dependent unfolding kinetics of a beta-sandwich protein. As denaturant concentration increases, the highly compact transition state of one pathway becomes destabilized and the dominant flux of protein molecules shifts toward another pathway with a less structured transition state. Furthermore, point mutations alter the relative accessibility of the pathways, allowing the structure of two transition states on separate, direct folding pathways to be mapped by systematic Phi-value analysis. It has been suggested that pathways with diffuse rather than localized transition states are evolutionarily selected to prevent misfolding, and indeed we find that the transition state favored at high concentrations of denaturant is more polarized than the physiologically relevant one.     

Secretory and endocytic pathways converge in a dynamic endosomal system in a primitive protozoan

Leishmania are a group of primitive eukaryotic trypanosomatid protozoa that are apically polarized with a flagellum at their anterior end. Surrounding the base of the flagellum is the flagellar reservoir that constitutes the site for endocytosis and exocytosis in these organisms. In the present study, we define a novel multivesicular tubular compartment involved in the intracellular trafficking of macromolecules in Leishmania . This dynamic structure appears to subtend the flagellar reservoir and extends towards the posterior end of the cell. Functional domains of several surface-expressed proteins, such as the gp63 glycosyl phosphatidyl inositol anchor and the 3'nucleotidase/nuclease transmembrane domain were fused to green fluorescent protein. These chimeric proteins were found to traffic through the secretory pathway and, while reaching their intended destinations, also accumulated within the intracellular tubular compartment. Using various compounds that are efficient fluid-phase markers used to track endocytosis in higher eukaryotes, we showed that this tubular compartment constitutes an important station in the endocytic pathway of these cells. Based on our functional observations of its role in the trafficking of expressed proteins and endocytosed markers, this compartment appears to have properties similar to endosomes of higher eukaryotes.     

Multiple Ca2+ signaling pathways converge on CaM kinase in PC12 cells.

The role of multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) in mediating various Ca2+ signaling pathways was examined in PC12 cells. Conversion of the kinase to a Ca(2+)-independent form was used to monitor which neurotransmitters activate the enzyme in situ. CaM kinase responds to Ca2+ influx elicited by ligand-gated Ca2+ channels for ATP and acetylcholine. It also responds to Ca2+ mobilization of IP3-sensitive stores elicited by phospholipase C-linked receptors for ATP and acetylcholine as well as by caffeine. CaM kinase mediates the actions of many neurotransmitters and Ca2+ signaling pathways.     

Opposing activities of the Ras and Hippo pathways converge on regulation of YAP protein turnover

Cancer genomes accumulate numerous genetic and epigenetic modifications. Yet, human cellular transformation can be accomplished by a few genetically defined elements. These elements activate key pathways required to support replicative immortality and anchorage independent growth, a predictor of tumorigenesis in vivo. Here, we provide evidence that the Hippo tumor suppressor pathway is a key barrier to Ras‐mediated cellular transformation. The Hippo pathway targets YAP1 for degradation via the βTrCP‐SCF ubiquitin ligase complex. In contrast, the Ras pathway acts oppositely, to promote YAP1 stability through downregulation of the ubiquitin ligase complex substrate recognition factors SOCS5/6. Depletion of SOCS5/6 or upregulation of YAP1 can bypass the requirement for oncogenic Ras in anchorage independent growth in vitro and tumor formation in vivo. Through the YAP1 target, Amphiregulin, Ras activates the endogenous EGFR pathway, which is required for transformation. Thus, the oncogenic activity of Ras^V12 depends on its ability to counteract Hippo pathway activity, creating a positive feedback loop, which depends on stabilization of YAP1.     

Import pathways of chloroplast interior proteins and the outer-membrane protein OEP14 converge at Toc75

Most chloroplast outer-membrane proteins are synthesized at their mature size without cleavable targeting signals. Their insertion into the outer membrane is insensitive to thermolysin pretreatment of chloroplasts and does not require ATP. It has therefore been assumed that insertion of outer-membrane proteins proceeds through a different pathway from import into the interior of chloroplasts, which requires a thermolysin-sensitive translocon complex and ATP. Here, we show that a model outer-membrane protein, OEP14, competed with the import of a chloroplast interior protein, indicating that the two import pathways partially overlapped. Cross-linking studies showed that, during insertion, OEP14 was associated with Toc75, a thermolysin-resistant component of the outer-membrane protein-conducting channel that mediates the import of interior-targeted precursor proteins. Whereas almost no OEP14 inserted into protein-free liposomes, OEP14 inserted into proteoliposomes containing reconstituted Toc75 with a high efficiency. Taken together, our data indicate that Toc75 mediates OEP14 insertion, and therefore plays a dual role in the targeting of proteins to the outer envelope membrane and interior of chloroplasts.