On the road to replication

How similar are the molecular mechanisms of yeast and metazoan genome replication initiation?

DNA replication start sites are licensed for replication when two hexameric ring-shaped motors of the replicative helicase are loaded as an inactive double hexamer around duplex DNA. Activation requires untwisting of the double helix and ejection of one DNA strand from the central channel of each helicase ring. The process of replication initiation is best understood in yeast, thanks to reconstitution with purified yeast proteins, which allowed systematic structural analysis of the replication initiation process. Orthologs of most yeast replication factors have been identified in higher eukaryotes; however, reconstitution of metazoan replication initiation is still in its infancy, with double hexamer loading but not activation having been achieved. Nonetheless, artificial intelligence-driven structure prediction and cryo-EM studies on native complexes, combined with cell-based and cell-free approaches, are starting to provide insights into metazoan replication initiation mechanisms. Here, we describe the emerging picture.

Recent advances in understanding DNA replication: cell type-specific adaptation of the DNA replication program

DNA replication is an essential process occurring prior to cell division. Cell division coupled to proliferation ensures the growth and renewal of a large variety of specialized cell types generated during embryonic development. Changes in the DNA replication program occur during development. Embryonic undifferentiated cells show a high replication rate and fast proliferation, whereas more differentiated cells are characterized by reduced DNA synthesis and a low proliferation rate. Hence, the DNA replication program must adapt to the specific features of cells committed to different fates. Recent findings on DNA synthesis regulation in different cell types open new perspectives for developing efficient and more adapted therapies to treat various diseases such as genetic diseases and cancer. This review will put the emphasis on recent progress made in this field.

Medicine, neurosciences and interdisciplinarity

In terms of fundamental, universal scientific progress in the area of “hard sciences,” the past century was the one of relativity, quantum mechanics, and solid state/surface physics, whereas the 21st century prospects a focus on genomics/life/brain sciences, computer sciences/artificial intelligence, climate, and extraplanetary research. Among these, neurosciences represent a meta-plane, where new tools and insights provide key contributions to revolutionizing our current understandings. Increasingly, we move toward a quantitative description of various neural phenomena as well as to a new understanding of the nervous system and brain organization/functioning in particular. All this is strongly influencing the way we see the world. From this process, finally yet importantly, we derive large benefits for the cure of diseases as well as the management of an aging society.

The eukaryotic CMG helicase pumpjack and integration into the replisome

The eukaryotic replisome is α multiprotein machine that contains DNA polymerases, sliding clamps, helicase, and primase along with several factors that participate in cell cycle and checkpoint control. The detailed structure of the 11-subunit CMG helicase (Cdc45/Mcm2-7/GINS) has been solved recently by cryoEM single-particle 3D reconstruction and reveals pumpjack motions that imply an unexpected mechanism of DNA translocation. CMG is also the organizing center of the replisome. Recent in vitro reconstitution of leading and lagging strand DNA synthesis has enabled structural analysis of the replisome. By building the replisome in stages from pure proteins, single-particle EM studies have identified the overall architecture of the eukaryotic replisome. Suprisingly leading and lagging strand polymerases bind to opposite faces of the CMG helicase, unlike the long-held view that DNA polymerases are located in back of the helicase to act on the unwound strands.

Expression profiling of colorectal cancer cells reveals inhibition of DNA replication licensing by extracellular calcium

Colorectal cancer is one of the most common cancers in industrialised societies. Epidemiological studies, animal experiments, and randomized clinical trials have shown that dietary factors can influence all stages of colorectal carcinogenesis, from initiation through promotion to progression. Calcium is one of the factors with a chemoprophylactic effect in colorectal cancer. The aim of this study was to understand the molecular mechanisms of the anti-tumorigenic effects of extracellular calcium ([Ca²⁺]_o) in colon cancer cells.

Gene expression microarray analysis of colon cancer cells treated for 1, 4, and 24 h with 2 mM [Ca²⁺]_o identified significant changes in expression of 1571 probe sets (ANOVA, p < 10^− 5). The main biological processes affected by [Ca²⁺]_o were DNA replication, cell division, and regulation of transcription. All factors involved in DNA replication-licensing were significantly downregulated by [Ca²⁺]_o. Furthermore, we show that the calcium-sensing receptor (CaSR), a G protein-coupled receptor is a mediator involved in this process. To test whether these results were physiologically relevant, we fed mice with a standard diet containing low (0.04%), intermediate (0.1%), or high (0.9%) levels of dietary calcium. The main molecules regulating replication licensing were inhibited also in vivo, in the colon of mice fed high calcium diet.

We show that among the mechanisms behind the chemopreventive effect of [Ca²⁺]_o is inhibition of replication licensing, a process often deregulated in neoplastic transformation. Our data suggest that dietary calcium is effective in preventing replicative stress, one of the main drivers of cancer and this process is mediated by the calcium-sensing receptor.

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Extracellular calcium inhibits cell division, DNA replication, in colon cancer cells.

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Extracellular calcium inhibits replication licensing in vitro and in vivo.

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The calcium-sensing receptor is a critical mediator of this process.

On the road to replication

The 2016 Louis‐Jeantet Prize for Medicine winner John Diffley tells the story of his path to characterise and understand DNA replication.