The interplay between clathrin-coated vesicles and cell signalling

Internalization of cargo proteins and lipids at the cell surface occurs in both a constitutive and signal-regulated manner through clathrin-mediated and other endocytic pathways. Clathrin-coated vesicle formation is a principal uptake route in response to signalling events. Protein-lipid and protein-protein interactions control both the targeting of signalling molecules and their binding partners to membrane compartments and the assembly of clathrin coats. An emerging aspect of membrane trafficking research is now addressing how signalling cascades and vesicle coat assembly and subsequently disassembly are integrated.     

Mitochondria as sensors and regulators of calcium signalling

During the past two decades calcium (Ca(2+)) accumulation in energized mitochondria has emerged as a biological process of utmost physiological relevance. Mitochondrial Ca(2+) uptake was shown to control intracellular Ca(2+) signalling, cell metabolism, cell survival and other cell-type specific functions by buffering cytosolic Ca(2+) levels and regulating mitochondrial effectors. Recently, the identity of mitochondrial Ca(2+) transporters has been revealed, opening new perspectives for investigation and molecular intervention.     

Polarity, protrusion-retraction dynamics and their interplay during keratinocyte cell migration.

Keratinocyte migration on a two-dimensional substrate can be split into four distinct phases: cell extension, attachment, contraction, and detachment. It is preceded by polarization of the cell which leads to a functional asymmetry observable by the formation of a leading lamella. In this work variation of fibronectin coating concentrations and competitive inhibition with RGD peptides are used to investigate the dependency of polarization, migration, lamella dynamics, and ruffling on substrate adhesiveness. Looking at migrating human epidermal keratinocytes with a well-defined polarity we find that a fibronectin-coating concentration of 10 microg/cm(2) stimulates migration and ruffling speed twofold, whereas protrusion speed increases only by 20% (compared to 2.5 microg/cm(2) fibronectin). Nonpolar cells show a constant migration and ruffling speed independent of the amount of fibronectin. In contrast protrusion speeds of polar and nonpolar cells are equal. Treatment of cells on 10 microg/cm(2) fibronectin with 1 mg/ml GRGDS reduces the characteristic migration, protrusion, and ruffling speed of polar cells which corresponds to lowering the effective coating concentration to under 5 microg/cm(2). The probability of being polarized (quantified by a polarity index) increases with increasing fibronectin concentration. However, addition of soluble RGD on 10 microg/cm(2) fibronectin does not simply reduce the polarity index like one would expect from the corresponding changes in the other motility parameters, but it remains unchanged.     

Molecular interplay between mechanical and humoral signalling in cardiac hypertrophy

Heart failure is a major clinical problem, only partly mitigated by current pharmacological therapy. An early marker of heart failure is hypertrophic remodelling of the heart, which represents a compensatory mechanism for the mechanical stress imposed by haemodynamic overload, but can eventually affect cardiac function. Recently, using genetically modified animals, have we started to identify the molecular components that elaborate the mechanical stimulus leading to cardiac hypertrophy, with its beneficial and detrimental effects. Characterization of the relative roles of the molecules implicated in the signalling pathways involved in the hypertrophic process might allow us to control the hypertrophic response to haemodynamic overload, directing it to more favourable outcomes.     

The interplay between light and jasmonate signalling during defence and development

During their evolution, plants have acquired diverse capabilities to sense their environment and modify their growth and development as required. The versatile utilization of solar radiation for photosynthesis as well as a signal to coordinate developmental responses to the environment is an excellent example of such a capability. Specific light quality inputs are converted to developmental outputs mainly through hormonal signalling pathways. Accordingly, extensive interactions between light and the signalling pathways of every known plant hormone have been uncovered in recent years. One such interaction that has received recent attention and forms the focus of this review occurs between light and the signalling pathway of the jasmonate hormone with roles in regulating plant defence and development. Here the recent research that revealed new mechanistic insights into how plants might integrate light and jasmonate signals to modify their growth and development, especially when defending themselves from either pests, pathogens, or encroaching neighbours, is discussed.     

The regulatory interplay between membrane-integrated sensors and transport proteins in bacteria

Bacteria sense environmental stimuli and transduce this information to cytoplasmic components of the signal transduction machinery to cope with and to adapt to ever changing conditions. Hence, bacteria are equipped with numerous membrane-integrated proteins responsible for sensing such as histidine kinases, chemoreceptors and ToxR-like proteins. There is increasing evidence that sensors employ transport proteins as co-sensors. Transport proteins are well-suited information carriers as they bind low-molecular-weight molecules in the external medium and transport them into the cytoplasm, allowing them to provide dynamic information on the metabolic flux. This review explores the sensing capabilities of secondary permeases, primary ABC-transporters, and soluble substrate-binding proteins. Employing transporters as co-sensors seems to be a sophisticated and probably widely distributed mechanism.     

Tyrosine kinases and their interactions with signalling proteins.

Recent progress has been made in identifying signal transduction pathways controlled by receptor protein-tyrosine kinases. The receptors for nerve growth factor and hepatocyte growth factor have been identified as the Trk and Met tyrosine kinases. The stimulation of intracellular signal transduction pathways by activated receptors appears to involve the association of SH2-containing cytoplasmic signalling proteins with autophosphorylated receptors.     

SUMOylation and cell signalling

SUMOylation is a highly transient post-translational protein modification. Attachment of SUMO to target proteins occurs via a number of specific activating and ligating enzymes that form the SUMO-substrate complex, and other SUMO-specific proteases that cleave the covalent bond, thus leaving both SUMO and target protein free for the next round of modification. SUMO modification has major effects on numerous aspects of substrate function, including subcellular localisation, regulation of their target genes, and interactions with other molecules. The modified SUMO-protein complex is a very transient state, and it thus facilitates rapid response and actions by the cell, when needed. Like phosphorylation, acetylation and ubiquitination, SUMOylation has been associated with a number of cellular processes. In addition to its nuclear role, important sides of mitochondrial activity, stress response signalling and the decision of cells to undergo senescence or apoptosis, have now been shown to involve the SUMO pathway. With ever increasing numbers of reports linking SUMO to human disease, like neurodegeneration and cancer metastasis, it is highly likely that novel and equally important functions of components of the SUMOylation process in cell signalling pathways will be elucidated in the near future.     

Deciphering cell wall integrity signalling in Aspergillus fumigatus: identification and functional characterization of cell wall stress sensors and relevant Rho GTPases

The fungal cell wall, a conserved and highly dynamic structure, is essential for virulence and viability of fungal pathogens and is an important antifungal drug target. The cell wall integrity (CWI) signalling pathway regulates shape and biosynthesis of the cell wall. In this work we identified, localized and functionally characterized four putative CWI stress sensors of Aspergillus fumigatus, an airborne opportunistic human pathogen and the cause of invasive aspergillosis. We show that Wsc1 is specifically required for resistance to echinocandin antifungals. MidA is specifically required for elevated temperature tolerance and resistance to the cell wall perturbing agents congo red and calcofluor white. Wsc1, Wsc3 and MidA additionally have overlapping functions and are redundantly required for radial growth and conidiation. We have also analysed the roles of three Rho GTPases that have been implicated in CWI signalling in other fungi. We show that Rho1 is essential and that conditional downregulation of rho1 or deletion of rho2 or rho4 results in severely impaired CWI. Our data indicate significant functional differences between the CWI stress sensors of yeasts and moulds.     

Host cell interactions and signalling with Neisseria gonorrhoeae

Neisseria gonorrhoeae is a highly adapted human pathogen that utilises multiple adhesins to interact with a variety of host cell receptors. Recently, substantial progress has been made in unravelling the signalling events induced by N. gonorrhoae that can lead to cytoskeletal reorganisation, invasion or phagocytic uptake, intraphagosomal accommodation, nuclear signalling, cytokine/chemokine release and apoptosis.     

Mammalian cell cycle checkpoints: signalling pathways and their organization in space and time

The major mission of the cell division cycle is a faithful and complete duplication of the genome followed by an equal partitioning of chromosomes to subsequent cell generations. In this review, we discuss the advances in our understanding of how mammalian cells control the fidelity of these fundamental processes when exposed to diverse genotoxic insults. We focus on the most recent insights into the molecular pathways that link the sites of DNA lesions with the cell cycle machinery in specific phases of the cell cycle. We also highlight the potential of a new technology allowing direct visualization of molecular interactions and redistribution of checkpoint proteins in live cell nuclei, and document the emerging significance of live-cell imaging for elucidation of the spatio-temporal organization of the DNA damage response network.     

DIF signalling and cell fate

The DIFs are a family of secreted chlorinated molecules that control cell fate during development of Dictyostelium cells in culture and probably during normal development too. They induce stalk cell differentiation and suppress spore cell formation. The biosynthetic and inactivation pathways of DIF-1 (the major bioactivity) have been worked out. DIF-1 is probably synthesised in prespore cells and inactivated in prestalk cells, by dechlorination. Thus, each cell type tends to alter DIF-1 level so as to favour differentiation of the other cell type. This relationship leads to a model for cell-type proportioning during normal development.     

Calcium signalling and cell proliferation

The orderly sequence of events that constitutes the cell cycle is carefully regulated. A part of this regulation depends upon the ubiquitous calcium signalling system. Many growth factors utilize the messenger inositol trisphosphate (InsP₃) to set up prolonged calcium signals, often organized in an oscillatory pattern. These repetitive calcium spikes require both the entry of external calcium and its release from internal stores. One function of this calcium signal is to activate the immediate early genes responsible for inducing resting cells (G₀) to re-enter the cell cycle. It may also promote the initiation of DNA synthesis at the G₁/S transition. Finally, calcium contributes to the completion of the cell cycle by stimulating events at mitosis. The role of calcium in cell proliferation is highlighted by the increasing number of anticancer therapies and immunosuppressant drugs directed towards this calcium signalling pathway.