The information stored in DNA is transcribed into mRNAs, which are then translated into the proteins that make up the structure of the cell as well as influence its activity. This is known as the central dogma of molecular biology. In order to make sure that this process happens efficiently and properly, the cell has many ways of monitoring the progress and products of each step. One such mechanism degrades RNAs and is called nonsense-mediated decay (NMD) after its well-studied role in degrading abnormal mRNAs. However, it is estimated that 10-30% of normal human mRNAs are degraded by nonsense-mediated decay.1,2,3 Interestingly, previous research from the Bradley Lab (Public Health and Basic Sciences Divisions) found that NMD is compromised in cells overexpressing a gene known to cause facioscapulohumeral muscular dystrophy (FSHD).4 In addition to this, another study found that NMD efficiency and the levels of an essential NMD factor, UPF1, appear to decrease during the course of muscle cell development, called myogenesis.5 Given these observations, scientists in the Bradley Lab wondered whether the decrease in UPF1 and NMD efficiency was a cause or consequence of myogenesis. In a recent study published in Molecular Cell, they reported that expression of the NMD factor UPF1 does indeed control normal muscle cell development and through a surprising mechanism. They found that UPF1 controls the protein levels but not the mRNA of the master myogenic factor MYOD.
The scientists began by artificially altering the expression level of UPF1 to see if this altered muscle cell differentiation, or the molecular maturation of cells. To do this, they first "knocked-down" the expression of UPF1 in myoblasts (young muscle cells) by introducing small interfering RNAs (siRNAs) against UPF1 mRNAs. They found that cells where UPF1 expression was knocked-down expressed markers of muscle cell maturation much faster than wild-type myoblasts. Given this, they next performed experiments to test whether excess UPF1 would cause the opposite outcome: slowed myogenesis. They created myoblast cells where they could over-express UPF1 when the chemical doxycycline was added to the growth media. They found that when these cells were treated with doxycycline, they took longer to express markers of muscle cell differentiation. Thus, their results demonstrate that UPF1 slows myogenesis.
Image provided by Dr. Qing Feng (Bradley Lab)
UPF1 is an essential regulator of RNA so the scientists hypothesized that UPF1 slows myogenesis by degrading the mRNAs of factors that promote myogenesis, such as MYOD. If that were true, they expected to find that knocking down UPF1 stabilized MYOD mRNAs and also that the excess MYOD protein appeared after the accumulation of MYOD mRNAs. To their surprise however, they did not find either prediction to be true. They measured MYOD mRNA stability in cells where they had knocked down UPF1 and did not find that the mRNAs were abnormally stable compared to control-treated cells. Secondly, the appearance of excess MYOD protein in cells where UPF1 had been knocked down preceded the appearance of excess MYOD mRNA by 15-18 hours. Therefore, UPF1 regulates MYOD protein levels independently of regulating its mRNA.
The timing of MYOD protein stabilization when UPF1 was knocked down suggested that UPF1 directly stabilized MYOD protein. Additionally, previous studies had reported that UPF1 physically interacts with components of the ubiquitin-proteasome degradation machinery and that a part of the protein appears structurally similar to the essential RING domain of E3 ubiquitin ligases, enzymes that mark proteins with ubiquitins, which often targets them for degradation by the proteasome. Given these clues, scientists in the Bradley Lab wanted to test whether the RING-like domain of UPF1 was required for its activity in regulating MYOD protein. They mutated amino acids in the RING domain of UPF1 and first tested whether these mutations affected its role in RNA surveillance. They confirmed that three known RNA substrates of UPF1 in muscle cells were unaffected by the mutations. Given this, they next tested how the mutations to UPF1 affected its role in stabilizing MYOD protein. They found that MYOD protein was marked with ubiquitin, a mark that often precedes protein degradation, in cells with wild-type UPF1 but that these marks were dramatically reduced in cells expressing the RING-mutant UPF1. These results and others strongly suggest that UPF1 is involved in stabilizing MYOD protein, but not its mRNA, during muscle cell differentiation.
Overall, this research uncovers that a single protein has distinct roles in mRNA and protein quality control. A connection between protein and mRNA stability has also been found in the ribosome-quality control complex, which degrades abnormal peptides but can also induce ribosome disassembly and degradation of the associated mRNA. Discovering such connections between molecular quality control machinery is not only exciting because it advances our understanding of normal cell function but also because it has implications for understanding mutations associated with disease. Indeed, downregulation of UPF1 was found in facioscapulohumeral muscular dystrophy and mutations to the UPF1 gene are found in several different cancers. Thus, understanding all of the functions carried out by core RNA processing factors such as UPF1 may improve our understanding and enable better treatment of certain diseases.
Feng Q, Jagannathan S, Bradley RK. 2017. "The RNA Surveillance Factor UPF1 Represses Myogenesis via Its E3 Ubiquitin Ligase Activity." Molecular Cell. 67, 239-251.
This research was supported by the Ellison Medical Foundation, the National Institutes of Health, and the FSH society.
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