Regulation of autophagy by cytoplasmic p53

Multiple cellular stressors, including activation of the tumour suppressor p53, can stimulate autophagy. Here we show that deletion, depletion or inhibition of p53 can induce autophagy in human, mouse and nematode cells subjected to knockout, knockdown or pharmacological inhibition of p53. Enhanced autophagy improved the survival of p53-deficient cancer cells under conditions of hypoxia and nutrient depletion, allowing them to maintain high ATP levels. Inhibition of p53 led to autophagy in enucleated cells, and cytoplasmic, not nuclear, p53 was able to repress the enhanced autophagy of p53(-/-) cells. Many different inducers of autophagy (for example, starvation, rapamycin and toxins affecting the endoplasmic reticulum) stimulated proteasome-mediated degradation of p53 through a pathway relying on the E3 ubiquitin ligase HDM2. Inhibition of p53 degradation prevented the activation of autophagy in several cell lines, in response to several distinct stimuli. These results provide evidence of a key signalling pathway that links autophagy to the cancer-associated dysregulation of p53.     

Argonaute proteins: key players in RNA silencing

During the past decade, small non-coding RNAs have rapidly emerged as important contributors to gene regulation. To carry out their biological functions, these small RNAs require a unique class of proteins called Argonautes. The discovery and our comprehension of this highly conserved protein family is closely linked to the study of RNA-based gene silencing mechanisms. With their functional domains, Argonaute proteins can bind small non-coding RNAs and control protein synthesis, affect messenger RNA stability and even participate in the production of a new class of small RNAs, Piwi-interacting RNAs.     

Serotonin and epilepsy

In recent years, there has been increasing evidence that serotonergic neurotransmission modulates a wide variety of experimentally induced seizures. Generally, agents that elevate extracellular serotonin (5-HT) levels, such as 5-hydroxytryptophan and serotonin reuptake blockers, inhibit both focal and generalized seizures, although exceptions have been described, too. Conversely, depletion of brain 5-HT lowers the threshold to audiogenically, chemically and electrically evoked convulsions. Furthermore, it has been shown that several anti-epileptic drugs increase endogenous extracellular 5-HT concentration. 5-HT receptors are expressed in almost all networks involved in epilepsies. Currently, the role of at least 5-HT(1A), 5-HT(2C), 5-HT(3) and 5-HT(7) receptor subtypes in epileptogenesis and/or propagation has been described. Mutant mice lacking 5-HT(1A) or 5-HT(2C) receptors show increased seizure activity and/or lower threshold. In general, hyperpolarization of glutamatergic neurons by 5-HT(1A) receptors and depolarization of GABAergic neurons by 5-HT(2C) receptors as well as antagonists of 5-HT(3) and 5-HT(7) receptors decrease the excitability in most, but not all, networks involved in epilepsies. Imaging data and analysis of resected tissue of epileptic patients, and studies in animal models all provide evidence that endogenous 5-HT, the activity of its receptors, and pharmaceuticals with serotonin agonist and/or antagonist properties play a significant role in the pathogenesis of epilepsies.     

Two defined alleles of the LRR receptor kinase GmNARK in supernodulating soybean govern differing autoregulation of mycorrhization

Plants regulate the extent of nodulation and root colonization by arbuscular mycorrhizal fungi (AMF), a phenomenon named autoregulation of symbiosis. We tested AMF colonization in split roots of various soybean genotypes [ Glycine max (L.) Merr. cv. Bragg, Enrei, Harosoy and Williams], where precolonization of one side of the split-root system by the AMF Glomus mosseae resulted in reduced mycorrhization of the other. AMF precolonization failed to control secondary mycorrhization in the supernodulating Bragg nonsense mutant nts1007 (Q106*), indicating that the GmNARK gene (predicted to encode a leucine-rich repeats (LRR) receptor kinase related to CLAVATA1 in Arabidopsis ) is involved in autoregulation of the AMF symbiosis. Here, we tested whether the allelic En6500 nonsense supernodulating mutant ( GmNARK K606*, derived from cv. Enrei) and supernodulating mutants of cv. Williams ( Nod1-3 and Nod2-4 ) with yet-undefined genetic lesions exhibit a similar symbiotic phenotype in mycorrhizal split-root systems. Surprisingly, these supernodulating mutants retained their ability to autoregulate AMF. To examine possible differences between two allelic mutants, we determined levels of IAA, abscisic acid, coumestrol, daidzein and genistein in mycorrhizal and uninoculated control roots. Compared with wild-type plants, both mutants showed reduced IAA accumulation in mycorrhizal roots. Roots of cv. Enrei and En6500 exhibited high levels of isoflavonoids not seen in Bragg or nts1007 . Taken together, these findings showed that supernodulation mutants, despite a common nodulation phenotype, differ in their ability to autoregulate AMF root colonization. This suggests either that the GmNARK gene product of some mutants is still partially functional (Q106* vs. K606*) or that varietal differences reflected in altered physiological responses suppress the loss of function.     

Engineered ribosomal RNA operon copy-number variants of E. coli reveal the evolutionary trade-offs shaping rRNA operon number

Ribosomal RNA (rrn) operons, characteristically present in several copies in bacterial genomes (7 in E. coli), play a central role in cellular physiology. We investigated the factors determining the optimal number of rrn operons in E. coli by constructing isogenic variants with 5–10 operons. We found that the total RNA and protein content, as well as the size of the cells reflected the number of rrn operons. While growth parameters showed only minor differences, competition experiments revealed a clear pattern: 7–8 copies were optimal under conditions of fluctuating, occasionally rich nutrient influx and lower numbers were favored in stable, nutrient-limited environments. We found that the advantages of quick adjustment to nutrient availability, rapid growth and economic regulation of ribosome number all contribute to the selection of the optimal rrn operon number. Our results suggest that the wt rrn operon number of E. coli reflects the natural, ‘feast and famine’ life-style of the bacterium, however, different copy numbers might be beneficial under different environmental conditions. Understanding the impact of the copy number of rrn operons on the fitness of the cell is an important step towards the creation of functional and robust genomes, the ultimate goal of synthetic biology.     

The Impact of Heterogeneous Thresholds on Social Contagion with Multiple Initiators

The threshold model is a simple but classic model of contagion spreading in complex social systems. To capture the complex nature of social influencing we investigate numerically and analytically the transition in the behavior of threshold-limited cascades in the presence of multiple initiators as the distribution of thresholds is varied between the two extreme cases of identical thresholds and a uniform distribution. We accomplish this by employing a truncated normal distribution of the nodes’ thresholds and observe a non-monotonic change in the cascade size as we vary the standard deviation. Further, for a sufficiently large spread in the threshold distribution, the tipping-point behavior of the social influencing process disappears and is replaced by a smooth crossover governed by the size of initiator set. We demonstrate that for a given size of the initiator set, there is a specific variance of the threshold distribution for which an opinion spreads optimally. Furthermore, in the case of synthetic graphs we show that the spread asymptotically becomes independent of the system size, and that global cascades can arise just by the addition of a single node to the initiator set.     

Gamma irradiation induced apoptotic changes in the chromatin structure of human erythroleukemia K562 cells

Exponentially growing human erythroleukemia K562 cells were synchronized by centrifugal elutriation prior to and after Co⁶⁰ γ-irradiation (4 Gy). Forward scatter flow cytometry used for size analysis revealed the increase of an early apoptotic cell population ranging from lower (0.05 C-value) to higher DNA content (∼1 C) as the cells progressed through the S phase. The increase in cellular DNA content expressed in C-values correlated with apoptotic chromatin changes manifested as many small apoptotic bodies in early S phase and larger but less numerous disintegrated apoptotic bodies in late S phase. Most significant changes after exposure to γ-irradiation took place in early S phase resulting in an increase of nuclear size by more than 50%. Cell fractions containing irradiated cells showed enhanced growth arrest at 2.4 C-value, which was accompanied by apoptosis. Apoptotic cell cycle arrest near to the G₁/G₀ checkpoint and apoptotic changes indicate that the radiation resistance of K562 cells is related to the bypass of the early stage of the p53 apoptotic pathway. Apoptotic changes in chromatin structure induced by γ-irradiation indicate that these injury-specific changes can be identified and distinguished from chromatin changes induced by UV radiation or heavy metals.     

Possible regulation of Toll-like receptor 4 by lysine acetylation through LPCAT2 activity in RAW264.7 cells

Inflammation is central to several diseases. TLR4 mediates inflammation by recognising and binding to bacterial lipopolysaccharides and interacting with other proteins in the TLR4 signalling pathway. Although there is extensive research on TLR4-mediated inflammation, there are gaps in understanding its mechanisms. Recently, TLR4 co-localised with LPCAT2, a lysophospholipid acetyltransferase. LPCAT2 is already known to influence lipopolysaccharide-induced inflammation; however, the mechanism of LPCAT2 influencing lipopolysaccharide-mediated inflammation is not understood.The present study combined computational analysis with biochemical analysis to investigate the influence of LPCAT2 on lysine acetylation in LPS-treated RAW264.7 cells.The results suggest for the first time that LPCAT2 influences lysine acetylation in LPS-treated RAW264.7 cells. Moreover, we detected acetylated lysine residues on TLR4. The present study lays a foundation for further research on the role of lysine acetylation on TLR4 signalling. Moreover, further research is required to characterise LPCAT2 as a protein acetyltransferase.