Every country has borders and – no less important – border crossings where goods and people can enter and leave. In the world of the cell, the cellular organelles act like little independent states: each conducting its specialized, vital processes within its borders while at the same time ensuring the flow of information and materials from one to the other. The recent discovery of a new “border crossing” between two organelles that were not even known to have “diplomatic relations” sheds new light on the ways that the organelles work together in a “federated state.”

The trans-border business of organelles can be conducted through couriers – secreted proteins that go from one to the other. But some types of passage require physical contact – to move materials from one to the other, for example. This takes place at specific contact points located around the border of the organelle. The first such contact point was identified over three decades ago, and new ones are still being discovered.

 

Dr. Maya Schuldiner of the Molecular Genetics Department says that every new discovery of this sort adds to the growing picture of the interrelations between organelles, the movement of information and materials within the cell, and obstacles that can cause disease.

In her postdoctoral research at the University of California, San Francisco, in the group of Prof. Jonathan Weissman, Schuldiner had worked with the team in Prof. Peter Walter’s lab that discovered a contact point between two of the cell’s main organelles. It connected the mitochondria – the cell’s power plants – with the endoplasmic reticulum in which, in addition to its function in readying proteins for secretion outside the cell, fats are produced. Together, these two organelles take up close to a third of the cell’s volume. They cling together at their contact points through “zipper proteins,” allowing fat molecules from the endoplasmic reticulum to pass into the mitochondrion, which uses them to build membranes.

 

The scientists surmised that if there were damage to the zipper proteins, the passage of fats, and thus the membrane structure of the mitochondria, would be impaired. But to their surprise, no such effect ensued. The conclusion was that the mitochondria had an as-yet-undiscovered contact point for obtaining fat.

Dr. Yael Elbaz-Alon, a postdoctoral fellow in Schuldiner’s lab, took up the challenge of finding that unknown contact point. Her starting theory was that if the flow of material (in this case fat) to the mitochondria remains constant when one of two available sources dries up, crossing points for the other one must double in order to keep up with demand. So the scientists looked at yeast cells – which have a somewhat manageable 6,200 proteins – and disabled these proteins one by one, looking for any case in which the known contact point, highlighted with a fluorescent marker for visibility, would double its activity. The lab’s robotic system enabled the researchers to conduct this test for all the proteins automatically.
 

 

 

Although checking for OGG1, alone, could give a good indication of cancer risk, Livneh and his team continued searching for further DNA repair mechanisms in hopes of improving the test and narrowing the margin of error. Around a year ago, they discovered a second factor, called MPG, which is linked to the tendency to develop lung cancer. Surprisingly, it was high levels of MPG activity, rather than low ones, which were associated with increased risk. The researchers think that a balance between OGG1, MPG and other DNA repair enzymes is critical; imbalances between them may lead to inefficient repair efforts.

Now, in a study that was recently published in Cancer Prevention Research, Livneh, Paz-Elizur and Drs. Ziv Sevilya and Yael Leitner-Dagan, and Dalia Elinger; in collaboration with Prof. Gad Rennert, Dr. Mila Pinchev and Hedy Rennert of the Technion School of Medicine and Carmel Medical Center; Dr. Ran Kremer of Rambam Medical Center; Prof. Laurence Freedman of Sheba Medical Center; and Prof. Edna Schechtman of Ben-Gurion University of the Negev, have found that a third DNA repair enzyme, called APE1, is also strongly tied to lung cancer risk. Higher risk came with reduced APE1 activity, although, interestingly enough, it has been known to be overexpressed in certain established cancers. The scientists think that APE1 may play a dual role in cancer: In healthy cells it acts weakly, allowing the accumulation of mutations that can speed up the development of cancer; while in cells that have already become cancerous, increased APE1 activity grants an advantage, enabling faster DNA replication and proliferation.

The researchers developed a method of weighting the levels of all three biomolecules in the form of a “DNA repair score,” along with the history of smoking, to determine the total risk. Checking these factors in 100 lung cancer patients and comparing them with those of healthy people, they found that people with a low DNA repair score have a 10-20-fold increase in their risk of getting lung cancer. While larger-scale clinical trials are needed to confirm the efficacy of the so-called OMA (OGG1-MPG-APE1) DNA repair score – a personalized measure of DNA repair activity – Livneh is optimistic that it will become a powerful tool for assessing cancer risk and directing individuals at risk to seek early detection though proactive CT scans. In addition, a study is planned to search for novel drugs that will improve DNA repair as a strategy to reduce the risk of lung cancer and perhaps other types of cancer.

 

Prof. Zvi Livneh’s research is supported by the Y. Leon Benoziyo Institute for Molecular Medicine, which he heads; the Leona M. and Harry B. Helmsley Charitable Trust; the David M. Polen Charitable Trust; Dana and Yossie Hollander, Israel ; Mike and Valeria Rosenbloom through the Mike Rosenbloom Foundation; and the Sergio Lombroso Award for Cancer Research. Prof. Livneh is the incumbent of the Maxwell Ellis Professorial Chair of Biomedical Research.

Anybody who has ever seen how plastic bottles or wastepaper are compressed before being recycled is familiar with the brute-force equipment that crushes these materials in preparation for the recycling process. In contrast, the molecular machinery that prepares proteins for being recycled in living cells is subtle and sophisticated – and, according to a new Weizmann Institute study, much more versatile than previously thought.

The Weizmann scientists, led by Dr. Michal Sharon, have revealed dynamic adjustments that take place in this machinery as it labels proteins intended for recycling. Understanding exactly how the recycling occurs is essential because mistakes in this mechanism are responsible for a host of widespread diseases. In fact, so central is the recycling machinery to the life of the cell that in 2004, the Nobel Prize in Chemistry was awarded to scientists who had clarified the function of ubiquitin, a small molecular tag that marks proteins slated for destruction by this machinery.

In virtually all the cells of our body, proteins that are worn, damaged or simply no longer needed are constantly being dismantled and reassembled for future use. Among the cellular structures that regulate this process is a large molecular complex called the signalosome. Itself consisting of eight proteins, it releases a command that ultimately results in tagging the targeted protein with ubiquitin.

Until now, the signalosome was thought to maintain a fairly permanent structure. But the new Weizmann study has shown that, in fact, this large complex is highly dynamic: Each of its eight subunits can assume different shapes; furthermore, the subunits can rearrange themselves into various compositions. In addition, the entire signalosome can move to different parts of the cell, depending on the task at hand.

Working with living human cells and using several technologies, including advanced mass spectroscopy, the researchers clarified how the signalosome operates in a crisis as it responds to DNA damage inflicted by ultraviolet radiation. The researchers found that the signalosome undergoes several adjustments. It rushes from the cytoplasm to the nucleus, where the damage has occurred; the more extensive the damage, the more massive its migration to the nucleus. Moreover, the signalosome’s subunits assume different shapes compared with the complex remaining in the cytoplasm. These findings reveal that the signalosome can adjust itself to match the cell’s changing needs.

The study, reported recently in Molecular and Cellular Biology, was performed by Sharon, and Drs. Maria G. Füzesi-Levi and Gili Ben-Nisan in the Institute’s Biological Chemistry Department, together with Dr. Yishai Levin of the Nancy and Stephen Grand Israel National Center for Personalized Medicine on the Weizmann campus, Dr. Elisabetta Bianchi of the Pasteur Institute in France, and Dr. Houjiang Zhou, Dr. Michael J. Deery and Prof. Kathryn S. Lilley of the University of Cambridge.
 

The study may in the future shed new light on molecular mechanisms involved in the cell’s repair process following DNA damage; errors in this process may ultimately lead to cancer.

On a more basic level, the new findings may have broad implications for our understanding of how humans and other living organisms function on the molecular level. They suggest that not only the signalosome but also other large cellular machines might be much more dynamic and versatile than currently believed.

 

Dr. Michal Sharon’s research is supported by the European Research Council; the Sergio Lombroso Award for Cancer Research; and Karen Siem, UK. Dr. Sharon is the incumbent of the Elaine Blond Career Development Chair in Perpetuity.
 

 

In studying the role of circadian rhythm in the accumulation of lipids in the liver, postdoctoral fellow Yaarit Adamovich and the team in the lab of Dr. Gad Asher of the Weizmann Institute’s Biological Chemistry Department, together with scientists from Dr. Xianlin Han’s lab in the Sanford-Burnham Medical Research Institute, Orlando, US, quantified hundreds of different lipids present in the mouse liver. They discovered that a certain group of lipids, namely the triglycerides (TAG), exhibit circadian behavior, with levels peaking about eight hours after sunrise. The scientists were astonished to find, however, that daily fluctuations in this group of lipids persist even in mice lacking a functional biological clock, albeit with levels cresting at a completely different time – 12 hours later than the natural schedule. “These results came as a complete surprise: One would expect that if the inherent clock mechanism is ‘dead,’ TAG could not accumulate in a time-dependent fashion,” says Adamovich. So what was making the fluctuating lipid levels “tick” if not the clocks? “One thing that came to mind was that, since food is a major source of lipids – particularly TAG – the eating habits of these mice might play a role.” Usually, mice consume 20% of their food during the day and 80% at night. However, in mice lacking a functional clock, the team noted that they ingest food constantly throughout the day. This observation excluded the possibility that food is responsible for the fluctuating patterns seen in TAG levels in these mice. 

When the scientists proceeded to check the effect of an imposed feeding regimen upon wild type mice, however, they were in for another surprise: After they provided the same amount of food – but restricted 100% of the feeding to nighttime hours – the team observed a dramatic 50% decrease in overall liver TAG levels. 

These results suggest that the time at which TAG accumulation occurs, as well as its levels, are determined by the clocks together with timing of meals. The details of the mechanism that drives the actual fluctuating behavior are yet to be discovered. 

Asher: “The striking outcome of restricted nighttime feeding – lowering liver TAG levels in the very short time period of 10 days in the mice – is of clinical importance. Hyperlipidemia and hypertriglyceridemia are common diseases characterized by abnormally elevated levels of lipids in blood and liver cells, which lead to fatty liver and other metabolic diseases. Yet no currently available drugs have been shown to change lipid accumulation as efficiently and drastically as simply adjusting meal time – not to mention the possible side effects that may be associated with such drugs.” Of course, mice are nocturnal animals, so in order to construe these results for humans, the timetable would need to be reversed. 

Time is a crucial element in all biological systems, so these findings are likely to impact biological research in general: Circadian clock mechanisms function even in cultured cells, so research results could vary depending on the time at which samples are analyzed, or, with animals, their feeding regimen might significantly affect the experimental outcomes. In other words, when it comes to designing experiments, scientists should be aware that “timing is everything.”