Signalling and crosstalk of Rho GTPases in mediating axon guidance

Axon extension during development of the nervous system is guided by many factors, but the signalling mechanisms responsible for triggering this extension remain mostly unknown. Here we have examined the role of Rho family small guanosine triphosphatases (GTPases) in mediating axon guidance by diffusible factors. Expression of either dominant-negative or constitutively active Cdc42 in cultured Xenopus laevis spinal neurons, at a concentration that does not substantially affect filopodial formation and neurite extension, abolishes the chemoattractive growth cone turning induced by a gradient of brain-derived neurotrophic factor that can activate Cdc42 and Rac in cultured neurons. Chemorepulsion induced by a gradient of lysophosphatidic acid is also abolished by the expression of dominant-negative RhoA. We also show that an asymmetry in Rho kinase or filopodial initiation across the growth cone is sufficient to trigger the turning response and that there is a crosstalk between the Cdc42 and RhoA pathways through their converging actions on the myosin activity essential for growth cone chemorepulsion.     

A cross talk between codon usage bias in human oncogenes

Background: Oncogenes are the genes that have the potential to induce cancer. The extent and origin of codon usage bias is an important indicator of the forces shaping genome evolution in living organisms. Results: We observed moderate correlations between gene expression as measured by CAI and GC content at any codon site. The findings of our results showed that there is a significant positive correlation (Spearman''s r= 0.45, P<0.01) between GC content at first and second codon position with that of third codon position. Further, striking negative correlation (r = -0.771, P < 0.01) between ENC with the GC3s values of each gene and positive correlation (r=0.644, P<0.01) in between CAI and ENC was also observed. Conclusions: The mutation pressure is the major determining factor in shaping the codon usage pattern of oncogenes rather than natural selection since its effects are present at all codon positions. The results revealed that codon usage bias determines the level of oncogene expression in human. Highly expressed oncogenes had rich GC contents with high degree of codon usage bias.     

Emerging evidence for cell-autonomous axon guidance

Current models of axon guidance within the central nervous system (CNS) involve the presentation of environmental cues to navigating growth cones. The surrounding and target tissues present a variety of ligands that either restrict or promote growth, thus providing pathfinding instructions to developing axons. Recent findings show that RGMb, a GPI anchored extracellular protein present on retinal ganglion cells, down-regulates Wnt3a signaling by lowering LRP5 levels at the membrane surface. When RGMb is phosphorylated by the extracellular tyrosine kinase VLK, phosphorylated RGMb (p-RGMb) is internalized and carries LRP5 towards intracellular compartments. In the eye, a dorsal-high ventral-low gradient of VLK generates a dorsal-low ventral-high gradient of LRP5 that modulates Wnt3a signaling. These molecules, which are all expressed by individual RGCs, generate Wnt-signal gradients along the dorso-ventral axis of the retina, resulting in differential axon growth which in turn regulates proper retino-tectal/collicular map formation. This pathway represents a regulatory mechanism whereby extracellular phosphorylation generates what may be the first example of a unique self-guiding mechanism that affects neuronal-target connections independent of paracrine signals from the surrounding target tissue.     

The chemical cross talk between rice and barnyardgrass

The chemical cross talk between rice and barnyardgrass which is one of the most noxious weeds in rice cultivation was investigated. Allelopathic activity of rice was increased by the presence of barnyardgrass seedlings or barnyardgrass root exudates. Rice allelochemical, momilactone B, concentration in rice seedlings and momilactone B secretion level from rice were also increased by the presence of barnyardgrass seedlings or barnyardgrass root exudates. As momilactone B possesses strong growth inhibitory activity and acts as an allelochemical, barnyardgrass-induced rice allelopathy may be due to the increased momilactone B secretion. These results suggest that rice may respond to the presence of neighboring barnyardgrass by sensing the chemical components in barnyardgrass root exudates and increase allelopathic activity by elevated production and secretion levels of momilactone B. Thus, rice allelopathy may be one of the inducible defense mechanisms by chemical-mediated plant interaction between rice and barnyardgrass and the induced-allelopathy may provide a competitive advantage for rice through suppression of the growth of barnyardgrass.Key words: allelopathy, Echinochloa, chemical interaction, induced-allelopathy, momilactone, Oryza sativaThe chemical cross talk between host and symbiotic or parasitic plants is an essential process for the development of physical connections in symbiosis and parasitism.^1–3 Barnyardgrass is one of the most common and noxious weeds in rice paddy fields.⁴ Although barnyardgrass is adapted rice production system due to its similarity in growth habit, the reason why barnyardgrass so often invades into the rice paddy fields is unknown. There might be some special interactions between both plant species.Plants are able to accumulate phytoalexins around infection sites of pathogens soon after sensing elicitors of pathogen origin. This accumulation of phytoalexins can protect the plants from further pathogen infection.^5,6 Plants are also able to activate defense mechanisms against attacking herbivores by sensing volatile compounds, such as methacrolein and methyl jasmonate, released by herbivore-attacked plant cells. The volatile-sensed plants increase the production of phenolics, alkaloids, terpenes and defense proteins, which reduce herbivory attacks.^7,8 Therefore, plants are able to elevate the defense mechanisms against several biotic stress conditions by detection of various compounds.Allelopathy is the direct influence of organic chemicals released from plants on the growth and development of other plants.^9–11 Allelochemicals are such organic chemicals involved in the allelopathy.^12,13 Allelochemicals can provide a competitive advantage for host-plants through suppression of soil microorganism and inhibition of the growth of competing plant species because of their antibacterial, antifungal and growth inhibitory activities.^3,14,15Rice has been extensively studied with respect to its allelopathy as part of a strategy for sustainable weed management, such as breeding allelopathic rice strains. A large number of rice varieties were found to inhibit the growth of several plant species when these rice varieties were grown together with these plants under the field or/and laboratory conditions.^16–20 These findings suggest that rice may produce and release allelochemicals into the neighboring environments and may inhibit the growth of the neighboring plants by the allelochemicals.Potent allelochemical, momilactone B, was isolated from rice root exudates.²¹ Momilactone B inhibits the growth of typical rice weeds like barnyardgrass and Echinochloa colonum at concentrations greater than 1 µM and the toxicity of momilactone B to rice itself was very low.²² In addition, rice plants secrete momilactone B from the roots into the rhizosphere over their entire life cycle.²² The observations suggest rice allelopathy may be primarily dependant on the secretion levels of momilactone B from the rice seedlings.^22,23Allelopathic activity of rice exhibited 5.3- to 6.3-fold increases when rice and barnyardgrass seedlings were grown together. Root exudates of barnyardgrass seedlings also increased allelopathic activity and momilactone B concentration in rice seedlings. The increasing the exudate concentration increased the allelopathic activity and momilactone B concentration in rice.²⁴ Thus, the chemical components in barnyardgrass root exudates may affect gene expressions involved in momilactone B biosynthesis. However, effects of the barnyardgrass root exudates on the secretion level of mimilactone B from rice has not yet reported.Rice seedlings were incubated in the medium containing barnyardgrass root exudates for 10 d, and secretion level of momilactone B by rice was determined (Fig. 1). The root exudates increased the secretion level significantly at concentrations greater than 30 mg/L of barnyardgrass root exudates, and increasing the concentration increased the secretion level. At concentrations of 300 mg/L of the root exudates, the secretion level was 10-fold greater than that in control (0 mg of root exudate). There was no significant difference in the osmotic potential between the medium contained barnyardgrass root exudates and control medium (all about 10 mmol/kg), and pH value of the medium was maintained at 6.0 throughout the experiments.²⁵ These results suggest that unknown chemical components in the barnyardgrass root exudates may induce the secretion of momilactone B from rice. As momilactone B possesses strong phytotoxic and allelopathic activities,^21–23,25 the elevated production and secretion of momilactone B in rice may provide a competitive advantage for root establishment through local suppression of pathogens and inhibition of the growth of competing plant species including barnyardgrass. Thus, barnyardgrass-induced rice allelopathy may be caused by the chemical components in the barnyardgrass root exudates.Open in a separate windowFigure 1Effects of barnyardgrass root exudates on momilactone B secretion level in rice. Rice seedlings were incubated in the medium containing barnyardgrass root exudates for 10 d, and secretion level of momilactone B was determined as described by Kato-Noguchi.²⁴ The experiment was repeated six times with three assays for each determination. Different letters show significant difference (p < 0.01) according to Tukey''s HSD test.Although mechanisms of the exudation are not well understood, it is suggested that plants are able to secrete a wide variety of compounds from root cells by plasmalemma-derived exudation, endoplasmic-derived exudation and proton-pumping mechanisms.^3,15 Through the root exudation of compounds, plants are able to regulate the soil microbial community in their immediate vicinity, change the chemical and physical properties of the soil, and inhibit the growth of competing plant species.^3,14,15 The present research suggests that rice may be aware of the presence of neighboring barnyardgrass by detection of certain key in barnyardgrass root exudates, and this sensorial function may trigger a signal cascade resulting in increasing rice allelopathy through increasing production of momilactone B and secretion of momilactone B into the rhizosphere. Therefore, rice allelopathy may potentially be an inducible defense mechanism by chemical-mediated plant interactions between rice and barnyardgrass.     

Drosophila Dscam is an axon guidance receptor exhibiting extraordinary molecular diversity

A Drosophila homolog of human Down syndrome cell adhesion molecule (DSCAM), an immunoglobulin superfamily member, was isolated by its affinity to Dock, an SH3/SH2 adaptor protein required for axon guidance. Dscam binds directly to both Dock's SH2 and SH3 domains. Genetic studies revealed that Dscam, Dock and Pak, a serine/threonine kinase, act together to direct pathfinding of Bolwig's nerve, containing a subclass of sensory axons, to an intermediate target in the embryo. Dscam also is required for the formation of axon pathways in the embryonic central nervous system. cDNA and genomic analyses reveal the existence of multiple forms of Dscam with a conserved architecture containing variable Ig and transmembrane domains. Alternative splicing can potentially generate more than 38,000 Dscam isoforms. This molecular diversity may contribute to the specificity of neuronal connectivity.     

Neurobiology: New connections between integrins and axon guidance

Axon guidance molecules such as netrins, semaphorins, Slits and ephrins provide the cues required for accurate patterning of axonal projections in the nervous system. Recent reports have described multiple paradigms by which these molecules interact with integrin adhesion receptors in and outside the neuronal tissues.     

A chemoattractant role for NT-3 in proprioceptive axon guidance

Neurotrophin-3 (NT-3) is required for proprioceptive neuron survival. Deletion of the proapoptotic gene Bax in NT-3 knockout mice rescues these neurons and allows for examination of their axon growth in the absence of NT-3 signaling. TrkC-positive peripheral and central axons from dorsal root ganglia follow proper trajectories and arrive in close proximity to their targets but fail to innervate them. Peripherally, muscle spindles are absent and TrkC-positive axons do not enter their target muscles. Centrally, proprioceptive axons branch in ectopic regions of the spinal cord, even crossing the midline. In vitro assays reveal chemoattractant effects of NT-3 on dorsal root ganglion axons. Our results show that survival factor NT-3 acts as a short-distance axon guidance molecule for muscle sensory afferents as they approach their proper targets.     

Dynamic properties of a delayed protein cross talk model

In this paper we investigate how the inclusion of time delay alters the dynamical properties of the Jacob-Monod model, describing the control of the beta-galactosidase synthesis by the lac repressor protein in E. coli. The consequences of a time delay on the dynamics of this system are analysed using Hopf's theorem and Lyapunov-Andronov's theory applied to the original mathematical model and to an approximated version. Our analytical calculations predict that time delay acts as a key bifurcation parameter. This is confirmed by numerical simulations. A critical value of time delay, which depends on the values of the model parameters, is analytically established. Around this critical value, the properties of the system change drastically, allowing under certain conditions the emergence of stable limit cycles, that is self-sustained oscillations. In addition, the features of the end product repression play an essential role in the characterisation of these limit cycles: if cooperativity is considered in the end product repression, time delay higher than the mentioned critical value induce differentiated responses during the oscillations, provoking cycles of all-or-nothing response in the concentration of the species.     

Molecular cross talk between epithelial cells and pathogenic microorganisms

The conference brought together epithelial cell biologists and molecular microbiologists and emphasized that these seemingly diverse disciplines are intricately intertwined. The model systems discussed throughout the meeting emphasized the novel approaches available to address key issues and begin to understand the molecular details of responses triggered at the microbial-epithelial interface. For example, co-crystallization of native ligand-receptor complexes as well as biologically or chemically altered forms of these complexes will allow fine details of receptor-ligand interactions to be determined. This approach is critical in development of new generation antimicrobial agents. Furthermore, transfection techniques that allow receptor expression in model epithelia, development of representative animal model systems, and development of transgenic mouse strains will aid in dissecting microbial-epithelial interactions and will provide further advances in studies on pathogenesis and tissue and host tropism. We are only beginning to uncover the nature of the bidirectional regulatory signals that occur between microbes and hosts. We know little about how these signals relate to the disease state, to microbial virulence, or to immune function. Clearly the cross talk between cell biologists and microbiologists is an important step in unraveling the events occurring between microbes and eukaryotic cells.     

A novel type of myosin implicated in signalling by rho family GTPases.

A novel widely expressed type of myosin (fifth unconventional myosin from rat: myr 5) from rat tissues, defining a ninth class of myosins, was identified. The predicted amino acid sequence of myr 5 exhibits several features not found previously in myosins. The myosin head domain contains a unique N-terminal extension and an insertion of 120 amino acids at a postulated myosin-actin contact site. Nevertheless, myr 5 is able to bind actin filaments in an ATP-regulated manner. The head domain is followed by four putative light chain binding sites. The tail domain of myr 5 contains a region which coordinates two atoms of zinc followed by a region that stimulates GTP hydrolysis of members of the ras-related rho subfamily of small G-proteins. Myr 5 therefore provides the first direct link between rho GTPases which have been implicated in the regulation of actin organization and the actin cytoskeleton. It is also the first unconventional myosin for which a tail binding partner(s), namely members of the rho family, has been identified.     

Retinal axon guidance: novel mechanisms for steering

Axons from the retina traverse different molecular territories as they navigate to the tectum. A single territory might span only a few cell diameters and harbour multiple guidance cues, many of which are beginning to be characterized. Also present in the pathway are 'modulators' that influence a growth cone's response to a coincident signal but do not guide growth directly. An emerging principle is that the growth cone, itself, changes molecularly as it journeys through the visual pathway. Growing retinal axons contain mRNAs, ubiquitinating and apoptotic enzymes, translation and degradation machinery. Guidance cues can trigger rapid and local synthesis, degradation and endocytosis of proteins, providing a fast and flexible way for growth cones to respond to cues in their microenvironment and to alter their responsiveness. The data raise the idea that the localized synthesis and downregulation of proteins might help to steer retinal axon growth and, further, might contribute to the changing character of a growth cone as it ages.     

Presenilin-dependent receptor processing is required for axon guidance

The Alzheimer's disease-linked gene presenilin is required for intramembrane proteolysis of amyloid-β precursor protein, contributing to the pathogenesis of neurodegeneration that is characterized by loss of neuronal connections, but the role of Presenilin in establishing neuronal connections is less clear. Through a forward genetic screen in mice for recessive genes affecting motor neurons, we identified the Columbus allele, which disrupts motor axon projections from the spinal cord. We mapped this mutation to the Presenilin-1 gene. Motor neurons and commissural interneurons in Columbus mutants lacking Presenilin-1 acquire an inappropriate attraction to Netrin produced by the floor plate because of an accumulation of DCC receptor fragments within the membrane that are insensitive to Slit/Robo silencing. Our findings reveal that Presenilin-dependent DCC receptor processing coordinates the interplay between Netrin/DCC and Slit/Robo signaling. Thus, Presenilin is a key neural circuit builder that gates the spatiotemporal pattern of guidance signaling, thereby ensuring neural projections occur with high fidelity.     

Opposite effects of rho family GTPases on engulfment of apoptotic cells by macrophages

The efficient engulfment of apoptotic cells by professional or nonprofessional phagocytes is critical to maintain mammalian homeostasis. To identify molecules involved in the engulfment of apoptotic cells, we established a retrovirus-based expression cloning system coupled with the engulfment assay. By screening a cDNA library of a mouse macrophage cell line, we identified two small GTPase family members (RhoG and Rab5) that enhanced the engulfment of apoptotic cells. By examining other small GTPase family members, we found that Rac1 enhanced the engulfment of apoptotic cells, whereas RhoA inhibited the process. Accordingly, the expression of a dominant-negative form of RhoG or Rac1 in primary macrophage cultures severely reduced the ability of the macrophages to engulf apoptotic cells, and a dominant-negative form of RhoA enhanced the process. These results indicated that the efficient engulfment of apoptotic cells requires the concerted action of small GTPase family members. We demonstrated previously that NIH3T3 cells expressing the alphav beta3 integrin efficiently engulf apoptotic cells in the presence of milk fat globule epidermal growth factor 8 via a phosphatidylserine-dependent mechanism. The dominant-negative form of RhoG or Rac1 inhibited this process, which suggested RhoG and Rac1 are also involved in the integrin-mediated engulfment.