The Actin Binding Domain of βI-Spectrin Regulates the Morphological and Functional Dynamics of Dendritic Spines

Actin microfilaments regulate the size, shape and mobility of dendritic spines and are in turn regulated by actin binding proteins and small GTPases. The βI isoform of spectrin, a protein that links the actin cytoskeleton to membrane proteins, is present in spines. To understand its function, we expressed its actin-binding domain (ABD) in CA1 pyramidal neurons in hippocampal slice cultures. The ABD of βI-spectrin bundled actin in principal dendrites and was concentrated in dendritic spines, where it significantly increased the size of the spine head. These effects were not observed after expression of homologous ABDs of utrophin, dystrophin, and α-actinin. Treatment of slice cultures with latrunculin-B significantly decreased spine head size and decreased actin-GFP fluorescence in cells expressing the ABD of α-actinin, but not the ABD of βI-spectrin, suggesting that its presence inhibits actin depolymerization. We also observed an increase in the area of GFP-tagged PSD-95 in the spine head and an increase in the amplitude of mEPSCs at spines expressing the ABD of βI-spectrin. The effects of the βI-spectrin ABD on spine size and mEPSC amplitude were mimicked by expressing wild-type Rac3, a small GTPase that co-immunoprecipitates specifically with βI-spectrin in extracts of cultured cortical neurons. Spine size was normal in cells co-expressing a dominant negative Rac3 construct with the βI-spectrin ABD. We suggest that βI-spectrin is a synaptic protein that can modulate both the morphological and functional dynamics of dendritic spines, perhaps via interaction with actin and Rac3

A plasmid-borne Rap-Phr system of Bacillus subtilis can mediate cell-density controlled production of extracellular proteases.

Item does not contain fulltextBacillus subtilis uses two-component signal transduction systems to sense intra- and extracellular stimuli to adapt to fluctuating environmental situations. Regulator aspartate phosphatases (Raps) have important roles in these processes, as they can dephosphorylate certain response-regulators, and are themselves subject to cell-density-controlled inhibition by secreted Phr (phosphate regulator) peptides. Eleven chromosomal genes encode this family of phosphatases, but in addition, certain strains contain endogenous plasmids with genes for homologous Rap-Phr systems. Plasmid pTA1060 encodes Rap60 and its antagonistic signalling molecule Phr60. Strikingly, expression of Rap60 in B. subtilis 168 strongly repressed the production of proteolytic enzymes. In fact, the transcription of the aprE gene, encoding a major extracellular protease, was shown to be decreased upon Rap60 expression, whereas this effect could be antagonized by the extracellular addition of synthetic Phr60 pentapeptide. Finally, transcription studies suggest that Rap60 dephosphorylates a component of the phosphorelay and is coupled to aprE transcription by the transition-state regulator AbrB. In conclusion, these data show that endogenous plasmids contain functional Rap-Phr systems and for the first time, that Rap-Phr systems can mediate cell-density controlled production of secreted proteases. This quorum-sensing mechanism might enable B. subtilis to suppress protease production under conditions of low cell densities when nutrients are still available in sufficient amounts

Rac3 induces a molecular pathway triggering breast cancer cell aggressiveness: differences in MDA-MB-231 and MCF-7 breast cancer cell lines.

International audienceBACKGROUND: Rho GTPases are involved in cellular functions relevant to cancer. The roles of RhoA and Rac1 have already been established. However, the role of Rac3 in cancer aggressiveness is less well understood. METHODS: This work was conducted to analyze the implication of Rac3 in the aggressiveness of two breast cancer cell lines, MDA-MB-231 and MCF-7: both express Rac3, but MDA-MB-231 expresses more activated RhoA. The effect of Rac3 in cancer cells was also compared with its effect on the non-tumorigenic mammary epithelial cells MCF-10A. We analyzed the consequences of Rac3 depletion by anti-Rac3 siRNA. RESULTS: Firstly, we analyzed the effects of Rac3 depletion on the breast cancer cells' aggressiveness. In the invasive MDA-MB-231 cells, Rac3 inhibition caused a marked reduction of both invasion (40%) and cell adhesion to collagen (84%), accompanied by an increase in TNF-induced apoptosis (72%). This indicates that Rac3 is involved in the cancer cells' aggressiveness. Secondly, we investigated the effects of Rac3 inhibition on the expression and activation of related signaling molecules, including NF-κB and ERK. Cytokine secretion profiles were also analyzed. In the non-invasive MCF-7 line; Rac3 did not influence any of the parameters of aggressiveness. CONCLUSIONS: This discrepancy between the effects of Rac3 knockdown in the two cell lines could be explained as follows: in the MDA-MB-231 line, the Rac3-dependent aggressiveness of the cancer cells is due to the Rac3/ERK-2/NF-κB signaling pathway, which is responsible for MMP-9, interleukin-6, -8 and GRO secretion, as well as the resistance to TNF-induced apoptosis, whereas in the MCF-7 line, this pathway is not functional because of the low expression of NF-κB subunits in these cells. Rac3 may be a potent target for inhibiting aggressive breast cancer