Giant viruses in the oceans: the 4th Algal Virus Workshop

Giant double-stranded DNA viruses (such as record breaking Acanthamoeba polyphaga Mimivirus), with particle sizes of 0.2 to 0.6 μm, genomes of 300 kbp to 1.200 kbp, and commensurate complex gene contents, constitute an evolutionary mystery. They challenge the common vision of viruses, traditionally seen as highly streamlined genomes optimally fitted to the smallest possible -filterable- package. Such giant viruses are now discovered in increasing numbers through the systematic sampling of ocean waters as well as freshwater aquatic environments, where they play a significant role in controlling phyto- and bacterio- plankton populations. The 4^th algal virus workshop showed that the study of these ecologically important viruses is now massively entering the genomic era, promising a better understanding of their diversity and, hopefully, some insights on their origin and the evolutionary forces that shaped their genomes.     

Polyamine Sharing between Tubulin Dimers Favours Microtubule Nucleation and Elongation via Facilitated Diffusion

We suggest for the first time that the action of multivalent cations on microtubule dynamics can result from facilitated diffusion of GTP-tubulin to the microtubule ends. Facilitated diffusion can promote microtubule assembly, because, upon encountering a growing nucleus or the microtubule wall, random GTP-tubulin sliding on their surfaces will increase the probability of association to the target sites (nucleation sites or MT ends). This is an original explanation for understanding the apparent discrepancy between the high rate of microtubule elongation and the low rate of tubulin association at the microtubule ends in the viscous cytoplasm. The mechanism of facilitated diffusion requires an attraction force between two tubulins, which can result from the sharing of multivalent counterions. Natural polyamines (putrescine, spermidine, and spermine) are present in all living cells and are potent agents to trigger tubulin self-attraction. By using an analytical model, we analyze the implication of facilitated diffusion mediated by polyamines on nucleation and elongation of microtubules. In vitro experiments using pure tubulin indicate that the promotion of microtubule assembly by polyamines is typical of facilitated diffusion. The results presented here show that polyamines can be of particular importance for the regulation of the microtubule network in vivo and provide the basis for further investigations into the effects of facilitated diffusion on cytoskeleton dynamics.     

Cellular prion protein coupling to TACE-dependent TNF-α shedding controls neurotransmitter catabolism in neuronal cells

Despite considerable efforts to unravel the role of cellular prion protein (PrP^C) in neuronal functions, the mechanisms by which PrP^C takes part in the homeostasis of a defined neuronal phenotype remain poorly characterized. By taking advantage of a neuroectodermal cell line (1C11) endowed with the capacity to differentiate into serotonergic (1C11^5-HT) or noradrenergic (1C11^NE) neurons, we assessed the contribution of PrP^C to bioaminergic cell functions. We established that in 1C11-derived neuronal cells antibody-mediated PrP^C ligation triggered tumor necrosis factor (TNF)-α release, through recruitement of the metalloproteinase TNF-α converting enzyme (TACE). TNF-α shed in response to PrP^C acts as a second message signal, eliciting serotonin (5-HT) or norepinephrine (NE) degradation in 1C11^5-HT or 1C11^NE cells, respectively. Our data thus introduced TNF-α as a PrP^C-dependent modulator of neuronal metabolism. Of note, we previously reported on a control of neurotransmitter catabolism by 5-HT_2B or α_1D autoreceptors in 1C11 bioaminergic neurons, via the same TACE/TNF-α pathway (Ann. N Y Acad. Sci. 1091, 123). Here, we show that combined stimulation of PrP^C and these two bioaminergic receptors add their effects on neurotransmitter degradation. Overall, these observations unveil a novel contribution of PrP^C to the control of neuronal functions and may have implications regarding dysfunction of the bioaminergic systems in prion diseases.     

A rigorous mathematical treatment for the excluded volume effect in Monte Carlo simulations of polymeric chains

In Monte Carlo simulations of polymeric chains, the chains are most often represented as spheres, or cylinders with flat ends. In this methodological paper, we adopt a representation of the chains as spherocylinders (continuous cylinders ending in semispheres). With such a representation the testing for chain overlap, which is the crucial step for the inclusion of the excluded volume effect in the simulations, can be defined in a rigorous geometrical framework. The treatment we then derive fulfills the following features: it allows a very simple, automatic, and exhaustive classification of all the possible configurations; and it provides a physical representation for steric hindrance effects more natural than the flat-ended cylinders. Notably, this representation avoids the introduction of artificial anisotropies in the treatments. This spherocylindrical representation is also well suited for several types of calculations that can be involved in elaborate Monte Carlo simulations.     

Hypoxia-induced autophagy drives colorectal cancer initiation and progression by activating the PRKC/PKC-EZR (ezrin) pathway

ABSTRACT

In solid tumors, cancer stem cells (CSCs) or tumor-initiating cells (TICs) are often found in hypoxic niches. Nevertheless, the influence of hypoxia on TICs is poorly understood. Using previously established, TIC-enrichedpatient-derived colorectal cancer (CRC) cultures, we show that hypoxia increases the self-renewal capacity of TICs while inducing proliferation arrest in their more differentiated counterpart cultures. Gene expression data revealed macroautophagy/autophagy as one of the major pathways induced by hypoxia in TICs. Interestingly, hypoxia-induced autophagy was found to induce phosphorylation of EZR (ezrin) at Thr567 residue, which could be reversed by knocking down ATG5, BNIP3, BNIP3L, or BECN1. Furthermore, we identified PRKCA/PKCα as a potential kinase involved in hypoxia-induced autophagy-mediated TIC self-renewal. Genetic targeting of autophagy or pharmacological inhibition of PRKC/PKC and EZR resulted in decreased tumor-initiating potential of TICs. In addition, we observed significantly reduced in vivo tumor initiation and growth after a stable knockdown of ATG5. Analysis of human CRC samples showed that p-EZR is often present in TICs located in the hypoxic and autophagic regions of the tumor. Altogether, our results establish the hypoxia-autophagy-PKC-EZR signaling axis as a novel regulatory mechanism of TIC self-renewal and CRC progression. Autophagy inhibition might thus represent a promising therapeutic strategy for cancer patients.