Nature Communications publishes Dr. Sara Hegazi's research that transcends cell type or species

Dr. Sara Hegazi and her colleagues from Cheng and Levine Labs new paper was published by Nature Communications: UBR4/POE facilitates secretory trafficking to maintain circadian clock synchrony

https://rdcu.be/cKEyY

https://doi.org/10.1038/s41467-022-29244-1

When asked about the research on Ubr4/poe, Dr. Hegazi said:

"When I first took on the Ubr4/poe project, I was simply excited by the prospects of discovering how this gene works within living cells. At the time, there was quite a bit of evidence pointing to important physiological roles of Ubr4 and its homologs. However, in many cases, the underlying molecular mechanism was a mystery.

During my Ph.D., I was curious and determined to find out how this large protein of ~600 kDa may function in the cell to influence physiology and behavior, and if this function is conserved across species. I was lucky to have two brilliant mentors, Dr. Mary Cheng and Dr. Joel Levine, to co-supervise me on this extremely challenging yet exciting project. I used the circadian system to conduct my investigations simultaneously in mice and flies.

For this project, I asked if Ubr4/poe impacts circadian behavior, and if so, what is the underlying cellular and molecular basis? When exposed to shifts in the day:night cycle, a paradigm that mimics travel across multiple time zones, mice with a deletion of the Ubr4 gene quickly synchronized their sleep:wake schedule to the new cycle, i.e., they were unusually resistant to jetlag. In flies, poe was essential in maintaining a sense of biological time, and without it they became arrhythmic. Over the next few years, my goal was to understand how these defects are produced. I tested many hypotheses but, to my disappointment, none could explain the specific set of behaviors exhibited by our mutant models.

This was until, one day, we noticed an unusual pattern of neuropeptide expression in the clock of Ubr4-deficient mice. The total levels of neuropeptides was not different compared to wild-type mice, but their distribution was – instead of localizing in the neuronal projections, the neuropeptides were largely restricted to the cell body. I later found a similar pattern in the clock neurons of poe-deficient flies.

This finding turned out to be the missing piece of our Ubr4/poe puzzle. Through a range of techniques, we ultimately showed that Ubr4 is required for the proper trafficking and secretion of neuropeptides, providing an explanation for the aberrant behavior of our Ubr4/poe-deficient models. The Ubr4/poe story, however, does not end here.

I believe that our findings can be extended beyond the circadian system, given that protein trafficking is a fundamental process in all eukaryotic cells. It would be interesting to see if altered protein trafficking contributes to the physiological disruptions and/or diseases that have previously been linked to Ubr4 and its homologs in various species, including humans."

Prof. Mary Cheng explained that:

"The life of a protein begins with its synthesis and ends with its degradation.  In between, a protein needs to be transported or trafficked to its intended destination within, or outside of, the cell where it can carry out its function. UBR4 belongs to a family of proteins known as E3 ligases, which control the degradation of other proteins. In 2012, Harrod Ling, a Master's student in the Cheng lab, conducted the first experiments to unravel the functions of UBR4 in the master circadian clock of mice. The genetic tools that were available to us at the time did not allow us to reach definitive conclusions regarding UBR4's role in circadian rhythms. In 2014, Sara Hegazi started her PhD under the joint supervision of myself and Joel. Using a new and more powerful mouse model, as well as genetic models in flies, Sara determined that deleting the ubr4 gene (in flies, the gene is called poe or "purity of essence")  compromised the ability of the circadian clock to maintain robust rhythms or synchronize to the day-night cycle.  However, it took a few more years before Sara was able to identify the underlying reason for these defects.  She, and we, were operating on the assumption that the defects were related to the degradative functions of UBR4, following conventional wisdom. Sadly, our hypothesis that UBR4 may be required for the degradation of clock proteins did not prove to be fruitful. The project took a sharp U-turn when, in 2017, Sara carried out a simple experiment looking at neuropeptide expression in the mouse clock. She was shocked to find that neuropeptide trafficking was impaired in our ubr4 knockout mice.  At that point, it dawned on us that, perhaps, UBR4 was regulating the circadian clock through non-degradative mechanisms. Sara worked feverishly for the next 3 years to prove that UBR4 is needed for efficient trafficking, and eventual secretion, of neuropeptides in clock neurons. More importantly, Sara's data suggest that this is likely a universal function of UBR4 that transcends cell type or species."    

"Several years ago I had the good fortune to be a committee member for one of Mary’s students when they made a discovery. Working to

gether Mary and her student, Harrod Ling discovered that UBR4 affected circadian rhythms in mice. Soon after that, Sara Hegazi, a very talented UTM Biology undergraduate became interested in working on this same story. Mary and I agreed to co-supervise Sara on this project. Flies have a very similar gene to UBR4, but in flies it is called purity of essence or just POE. Sara learned about mice in Mary’s lab and flies in my lab. She did a great job characterizing how UBR4 and POE contribute to neuronal function. I am proud of her and honored to be associated with Mary’s original discovery. Stay tuned….there’s lots to learn about UBR4 and its contributions to clocks in the brain and more." said Prof. Joel Levine