RhoA GTPase and F-actin dynamically regulate the permeability of Cx43-made channels in rat cardiac myocytes

Gap junctions are clusters of transmembrane channels allowing a passive diffusion of ions and small molecules between adjacent cells. Connexin43, the main channel-forming protein expressed in ventricular myocytes, can associate with zonula occludens-1, a scaffolding protein linked to the actin cytoskeleton and to signal transduction molecules. The possible influence of Rho GTPases, major regulators of cellular junctions and of the actin cytoskeleton, in the modulation of gap junctional intercellular communication (GJIC) was examined. The activation of RhoA by cytoxic necrotizing factor 1 markedly enhanced GJIC, whereas its specific inhibition by the Clostridium botulinum C3 exoenzyme significantly reduced it. RhoA activity affects GJIC without major cellular redistribution of junctional plaques or changes in the Cx43 phosphorylation pattern. As these GTPases frequently act via the cortical cytoskeleton, the importance of F-actin in the modulation of GJIC was investigated by means of agents interfering with actin polymerization. Cytoskeleton stabilization by phalloidin slowed down the kinetics of channel rundown in the absence of ATP, whereas its disruption by cytochalasin D rapidly and markedly reduced GJIC despite ATP presence. Cytoskeleton stabilization by phalloidin markedly reduced the consequences of RhoA activation or inactivation. This mechanism appears to be the first described capable to both up- or down-regulate GJIC through RhoA activation or, conversely, inhibition. The inhibition of Rho downstream kinase effectors had no effect on GJIC. The present results provide further insight into the gating and regulation of junctional channels and identify a new downstream target for the small G-protein RhoA.     

Drosophila MFAP1 is required for pre-mRNA processing and G2/M progression

The mammalian spliceosome has mainly been studied using proteomics. The isolation and comparison of different splicing intermediates has revealed the dynamic association of more than 200 splicing factors with the spliceosome, relatively few of which have been studied in detail. Here, we report the characterization of the Drosophila homologue of microfibril-associated protein 1 (dMFAP1), a previously uncharacterized protein found in some human spliceosomal fractions ( Jurica, M. S., and Moore, M. J. (2003) Mol. Cell 12, 5-14 ). We show that dMFAP1 binds directly to the Drosophila homologue of Prp38p (dPrp38), a tri-small nuclear ribonucleoprotein component ( Xie, J., Beickman, K., Otte, E., and Rymond, B. C. (1998) EMBO J. 17, 2938-2946 ), and is required for pre-mRNA processing. dMFAP1, like dPrp38, is essential for viability, and our in vivo data show that cells with reduced levels of dMFAP1 or dPrp38 proliferate more slowly than normal cells and undergo apoptosis. Consistent with this, double-stranded RNA-mediated depletion of dPrp38 or dMFAP1 causes cells to arrest in G(2)/M, and this is paralleled by a reduction in mRNA levels of the mitotic phosphatase string/cdc25. Interestingly double-stranded RNA-mediated depletion of a wide range of core splicing factors elicits a similar phenotype, suggesting that the observed G(2)/M arrest might be a general consequence of interfering with spliceosome function.     

Drosophila retinal pigment cell death is regulated in a position-dependent manner by a cell memory gene

The stereotyped organization of the Drosophila compound eye depends on the elimination by apoptosis of about 25% of the inter-ommatidial pigment cell precursors (IOCs) during metamorphosis. This program of cell death is under antagonistic effects of the Notch and the EGFR pathways. In addition, uncharacterized positional cues may underlie death versus survival choices among IOCs. Our results provide new genetic evidences that cell death is regulated in a position- dependent manner in the eye. We show that mutations in Trithorax-like (Trl) and lola-like/batman specifically block IOC death during eye morphogenesis. These genes share characteristics of both Polycomb-Group and trithorax-Group genes, in that they are required for chromatin-mediated repression and activation of Hox genes. However, Trl function in triggering IOC death is independent from a function in repressing Hox gene expression during eye development. Analysis of mosaic ommatidiae containing Trl mutant cells revealed that Trl function for IOC death is required in cone cells. Strikingly, cell death suppression in Trl mutants depends on the position of IOCs. Our results further support a model whereby death of IOCs on the oblique sides of ommatidiae requires Trl-dependent reduction of a survival signal, or an increase of a death signal, emanating from cone cells. Trl does not have the same effect on horizontal IOCs whose survival seems to involve additional topological constraints.     

Vascularization in tissue engineering

Tissue engineering has been an active field of research for several decades now. However, the amount of clinical applications in the field of tissue engineering is still limited. One of the current limitations of tissue engineering is its inability to provide sufficient blood supply in the initial phase after implantation. Insufficient vascularization can lead to improper cell integration or cell death in tissue-engineered constructs. This review will discuss the advantages and limitations of recent strategies aimed at enhancing the vascularization of tissue-engineered constructs. We will illustrate that combining the efforts of different research lines might be necessary to obtain optimal results in the field.     

Acetylcholinesterase activity in Clytia hemisphaerica (Cnidaria)

Cholinesterase activity is known in representatives of all living organisms phyla but the origin of the cholinergic system as known in bilaterian animals is still undeciphered. In particular the implication of cholinesterases in the nervous system of non-bilaterian Metazoa is not well known. We thus chose to investigate this activity in the Clytia hemisphaerica (Cnidaria) medusa. In toto histochemical staining revealed an acetylcholinesterase activity in the tentacle bulbs but not in the nervous system. Sequences homologous to acetylcholinesterase were searched within Clytia ESTs and compared to other sequences found in public databases.     

Multifaceted roles of human elafin and secretory leukocyte proteinase inhibitor (SLPI), two serine protease inhibitors of the chelonianin family

Elafin and SLPI are low-molecular weight proteins that were first identified as protease inhibitors in mucous fluids including lung secretions, where they help control excessive proteolysis due to neutrophil serine proteases (elastase, proteinase 3 and cathepsin G). Elafin and SLPI are structurally related in that both have a fold with a four-disulfide core or whey acidic protein (WAP) domain responsible for inhibiting proteases. Elafin is derived from a precursor, trappin-2 or pre-elafin, by proteolysis. Trappin-2, which is itself a protease inhibitor, has a unique N-terminal domain that enables it to become cross-linked to extracellular matrix proteins by transglutaminase(s). SLPI and elafin/trappin-2 are attractive candidates as therapeutic molecules for inhibiting neutrophil serine proteases in inflammatory lung diseases. Hence, they have become the WAP proteins most studied over the last decade. This review focuses on recent findings revealing that SLPI and elafin/trappin-2 have many biological functions as diverse as anti-bacterial, anti-fungal, anti-viral, anti-inflammatory and immuno-modulatory functions, in addition to their well-recognized role as protease inhibitors.     

Hyaluronidase activity is modulated by complexing with various polyelectrolytes including hyaluronan.

Hyaluronidase (HAase) plays an important role in the control of the size and concentration of hyaluronan (HA) chains, whose biological properties strongly depend on their length. Our previous studies of HA hydrolysis catalyzed by testicular HAase demonstrated that, whilst the substrate-dependence curve has a Michaelis-Menten shape with a 0.15 mol L(-1) ionic strength, at low ionic strength (5 mmol L(-1)), a strong decrease in the initial hydrolysis rate is observed at high substrate concentrations; the HA concentration for which the initial rate is maximum increases when the HAase concentration is increased. After examination of various hypotheses, we suggested that this could be explained by the ability of HA to form non-specific complexes with HAase, which thus becomes unable to catalyze HA hydrolysis. In order to verify this hypothesis, we first showed from turbidimetric measurements that HAase, like albumin, is able to form electrostatic complexes with HA. Albumin then was used as a non-catalytic protein able to compete with HAase for the formation of non-specific complexes with HA, allowing HAase to be free and catalytically active. The kinetic results showed that the HA-HAase non-specific complex inhibits HAase catalytic activity towards HA. Depending on the albumin concentration with respect to the HAase and HA concentrations, albumin can either remove this inhibition or induce another type of inhibition. Finally, the extent of such non-specific interactions between polyelectrolytes and proteins in HAase inhibition or activation, in particular under in vivo conditions, is discussed.     

Protofibril assemblies of the arctic, Dutch, and Flemish mutants of the Alzheimer's Abeta1-40 peptide

Using a coarse-grained model of the Aβ peptide, we analyze the Arctic (E22G), Dutch (E22Q), and Flemish (A21G) familial Alzheimer's disease (FAD) mutants for any changes in the stability of amyloid assemblies with respect to the wild-type (WT) sequence. Based on a structural reference state of two protofilaments aligned to create the “agitated” protofibril as determined by solid-state NMR, we determine free energy trends for Aβ assemblies for the WT and FAD familial sequences. We find that the structural characteristics and oligomer size of the critical nucleus vary dramatically among the hereditary mutants. The Arctic mutant's disorder in the turn region introduces new stabilizing interactions that better align the two protofilaments, yielding a well-defined protofibril axis at relatively small oligomer sizes with respect to WT. By contrast, the critical nucleus for the Flemish mutant is beyond the 20 chains characterized in this study, thereby showing a strong shift in the equilibrium toward monomers with respect to larger protofibril assemblies. The Dutch mutant forms more ordered protofilaments than WT, but exhibits greater disorder in protofibril structure that includes an alternative polymorph of the WT fibril. An important conclusion of this work is that the Dutch mutant does not support the agitated protofibril assembly. We discuss the implications of the structural ensembles and free energy profiles for the FAD mutants in regards to interpretation of the kinetics of fibril assembly using chromatography and dye-binding experiments.