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The cellular mechanisms allowing tissues to efficiently regenerate are not fully understood. In this issue of Developmental Cell, Cao et al. (2017) discover that during zebrafish heart regeneration, epicardial cells at the leading edge of regenerating tissue undergo endoreplication, possibly due to increased tissue tension, thereby boosting their regenerative capacity., (Copyright © 2017. Published by Elsevier Inc.)

The seminal observation that mechanical signals can elicit changes in biochemical signalling within cells, a process commonly termed mechanosensation and mechanotransduction, has revolutionized our understanding of the role of cell mechanics in various fundamental biological processes, such as cell motility, adhesion, proliferation and differentiation. In this Review, we will discuss how the interplay and feedback between mechanical and biochemical signals control tissue morphogenesis and cell fate specification in embryonic development.

An unresolved mystery in the field of spinal muscular atrophy (SMA) is why a reduction of the ubiquitously expressed Smn protein causes defects mostly in motoneurons. We addressed the possibility that this restricted vulnerability stems from elevated Smn expression in motoneurons. To explore this, we established an ex vivo zebrafish culture system of GFP-marked motoneurons to quantitatively measure Smn protein and smn mRNA levels as well as promoter activity in motoneurons versus other cell types. Importantly, we uncovered that Smn levels are elevated in motoneurons by means of transcriptional activation. In addition, we identified the ETS family transcription factor Etv5b to be responsible for increased smn transcription in motoneurons. Moreover, we established that the additional supply of Smn protein in motoneurons is necessary for proper axonogenesis in a cell-autonomous manner. These findings demonstrate the reliance of motoneurons on more Smn, thereby adding a novel piece of evidence for their increased vulnerability under SMA conditions.

The early C. elegans embryo is an attractive model system to investigate fundamental developmental processes. With the exception of mex-3 mRNA, maternally contributed mRNAs are thought to be distributed uniformly in the one-cell embryo. Here, we report and characterize the striking distribution of the mRNA encoding the novel protein ERA-1. We found that era-1 mRNA is enriched in the anterior of the one-cell embryo and present solely in anterior blastomeres thereafter. Although era-1 is not an essential gene, we uncovered that era-1 null mutant embryos are sensitive to slight impairment of embryonic polarity. We found that the asymmetric distribution of era-1 mRNA depends on anterior-posterior polarity cues and on the era-1 3'UTR. Similarly to the era-1 mRNA, the YFP-ERA-1 protein is enriched in anterior blastomeres. Interestingly, we found that the RNA-binding protein MEX-5 is required for era-1 mRNA asymmetry. Furthermore, we show that MEX-5, together with its partially redundant partner MEX-6, are needed to activate era-1 mRNA translation in anterior blastomeres. These findings lead us to propose that MEX-5/6-mediated regulation of era-1 mRNA contributes to robust embryonic development.

Regulation of centrosome and spindle positioning is crucial for spatial cell division control. The one-cell Caenorhabditis elegans embryo has proven attractive for dissecting the mechanisms underlying centrosome and spindle positioning in a metazoan organism. Previous work revealed that these processes rely on an evolutionarily conserved force generator complex located at the cell cortex. This complex anchors the motor protein dynein, thus allowing cortical pulling forces to be exerted on astral microtubules emanating from microtubule organizing centers (MTOCs). Here, we report that the clathrin heavy chain CHC-1 negatively regulates pulling forces acting on centrosomes during interphase and on spindle poles during mitosis in one-cell C. elegans embryos. We establish a similar role for the cytokinesis/apoptosis/RNA-binding protein CAR-1 and uncover that CAR-1 is needed to maintain proper levels of CHC-1. We demonstrate that CHC-1 is necessary for normal organization of the cortical acto-myosin network and for full cortical tension. Furthermore, we establish that the centrosome positioning phenotype of embryos depleted of CHC-1 is alleviated by stabilizing the acto-myosin network. Conversely, we demonstrate that slight perturbations of the acto-myosin network in otherwise wild-type embryos results in excess centrosome movements resembling those in chc-1(RNAi) embryos. We developed a 2D computational model to simulate cortical rigidity-dependent pulling forces, which recapitulates the experimental data and further demonstrates that excess centrosome movements are produced at medium cortical rigidity values. Overall, our findings lead us to propose that clathrin plays a critical role in centrosome positioning by promoting acto-myosin cortical tension., (© 2014. Published by The Company of Biologists Ltd.)

Unlabelled: Increased oxidative stress is associated with various diseases and aging, while adaptation to heat stress is an important determinant of survival and contributes to longevity. However, the impact of oxidative stress on heat resistance remains largely unclear., Aim: In this study we investigated how oxidative stress impinges on heat stress responses., Results: We report that hydrogen-peroxide (H(2)O(2)) pretreatment inhibits both acquired thermotolerance and heat-induced Hsp70 expression in mammalian cells, as well as acquired thermotolerance in the nematode Caenorhabditis elegans, via RNA interference. Moreover, we demonstrate that elimination of RNA interference by silencing key enzymes in microRNA biogenesis, dcr-1 or pash-1, restores the diminished intrinsic thermotolerance of aged and H(2)O(2)-elimination compromised (catalase-2 and peroxiredoxin-2 deficient) worms., Innovation and Conclusion: These results uncover a novel post-transcriptional element in the regulation of heat stress adaptation under oxidative conditions that may have implications in disease susceptibility and aging.

Motivation: Signaling pathways control a large variety of cellular processes. However, currently, even within the same database signaling pathways are often curated at different levels of detail. This makes comparative and cross-talk analyses difficult., Results: We present SignaLink, a database containing eight major signaling pathways from Caenorhabditis elegans, Drosophila melanogaster and humans. Based on 170 review and approximately 800 research articles, we have compiled pathways with semi-automatic searches and uniform, well-documented curation rules. We found that in humans any two of the eight pathways can cross-talk. We quantified the possible tissue- and cancer-specific activity of cross-talks and found pathway-specific expression profiles. In addition, we identified 327 proteins relevant for drug target discovery., Conclusions: We provide a novel resource for comparative and cross-talk analyses of signaling pathways. The identified multi-pathway and tissue-specific cross-talks contribute to the understanding of the signaling complexity in health and disease, and underscore its importance in network-based drug target selection., Availability: http://SignaLink.org.

Molecular chaperones and the heat shock response guard and modulate protein conformation, protect proteins from misfolding and aggregation, and maintain signalling and organellar networks. Overnutrition and the metabolic syndrome represent a pro-aging condition, and dietary restriction is the most robust environmental intervention that induces longevity from yeast to mammals. In recent years a considerable effort has been made to elucidate the signaling pathways involved in metabolic signaling. Here we review the current understanding on the connection between metabolic stress, dietary restriction and the heat shock response and highlight results showing chaperone induction as a promising therapeutic strategy to promote healthy aging and to prevent metabolic disorders.

The network concept is increasingly used for the description of complex systems. Here, we summarize key aspects of the evolvability and robustness of the hierarchical network set of macromolecules, cells, organisms and ecosystems. Listing the costs and benefits of cooperation as a necessary behaviour to build this network hierarchy, we outline the major hypothesis of the paper: the emergence of hierarchical complexity needs cooperation leading to the ageing (i.e. gradual deterioration) of the constituent networks. A stable environment develops cooperation leading to over-optimization, and forming an 'always-old' network, which accumulates damage, and dies in an apoptosis-like process. A rapidly changing environment develops competition forming a 'forever-young' network, which may suffer an occasional over-perturbation exhausting system resources, and causing death in a necrosis-like process. Giving a number of examples we demonstrate how cooperation evokes the gradual accumulation of damage typical to ageing. Finally, we show how various forms of cooperation and consequent ageing emerge as key elements in all major steps of evolution from the formation of protocells to the establishment of the globalized, modern human society.