First person – Xiao Li

Xiao Li

How would you explain the main findings of your paper to non-scientific family and friends?

Aberrant lipid and cholesterol metabolism is closely linked to a wide variety of human diseases, such as obesity, diabetes, cardiovascular diseases, neurovegetative diseases and many types of cancers. The expression of the key enzymes required for lipid and cholesterol synthesis in our cells is controlled by a family of master transcription factors called sterol regulatory element-binding proteins (SREBPs). Accordingly, in the past three decades, researchers have made considerable progresses in understanding how SREBPs can sense the lipid and cholesterol levels in cells and turn on the expression of their target genes. However, exactly how the SREBP activities in the nucleus are turned off is still not fully understood. Based on studies using different experimental systems, several regulators have been proposed to modify SREBP at different serine or threonine residues. By collaborating with Drs Fajun Yang, Jeff Pessin and their colleagues at Albert Einstein College of Medicine, we previously reported that one of the regulators, cyclin-dependent kinase 8 (Cdk8), modifies and inhibits SREBP-dependent lipid biosynthesis in the fruit fly (Drosophila melanogaster) and mouse models (Zhao et al., 2012). Built on these discoveries, we used Drosophila as a model system and further validated the conserved role of Cdk8 in turning off SREBP activity in nucleus. Specifically, we mutated the Cdk8 modification target in SRBEP and found that the mutated SREBP can abolish the inhibitory effects of Cdk8 on SREBP protein stability and its activities in regulating SREBP target gene expression and lipid synthesis. Interestingly, we found that the transcriptional activities of wild-type SREBP are inhibited by starvation, but the mutated SREBP becomes resistant to the effects of starvation on SREBP target gene expression. These observations support the key role of Cdk8 in regulating SREBP activities in maintenance of intracellular lipid homeostasis during feeding and starvation.

What are the potential implications of these results for your field of research?

Our results provide in vivo evidence further supporting the notion that SREBP is negatively regulated by Cdk8 through direct phosphorylation of a conserved threonine residue. In addition, we found that SREBP can directly interact with Cdk8 or MED15 through exactly the same residues. Both Cdk8 and MED15 are known as the subunits of the Mediator complex, a very large and fascinating protein complex composed of ∼30 different subunits that are conserved in almost all eukaryotes. The Mediator complex is believed to be required in most RNA polymerase II-dependent transcription. Our results raise a question as to how the Cdk8 kinase module (CKM), composed of four subunits (Cdk8, Cyclin C, MED12 and MED13), acts in concert with the small Mediator complex containing MED15 in turning on and off SREBP-dependent gene transcription. Several subunits of the Mediator complex have been reported to be either mutated or amplified in cardiovascular diseases and different types of human cancers. This work may help us to understand how these mutations may impact the interplay between the CKM and the Mediator subunits by affecting the tight regulation of SREBP and other transcription factors in different diseases.

What are the main advantages and drawbacks of the experimental system you have used as it relates to the disease you are investigating?

Drosophila melanogaster has served as an excellent model organism to study fundamental questions in a wide variety of biological processes in the past century. The most attractive feature to me is the numerous sophisticated genetic tools created by generations of drosophilists. In addition, many regulatory mechanisms are conserved but simpler in Drosophila than in mammals or other vertebrates. Practically, flies can be maintained with relative low costs, and they have a short life cycle and well-charactered stereotypical morphological features. These features, together with the rich literature of genetic and genomic information of most genes, makes Drosophila an ideal experimental system to manipulate genes of interest and define the biological consequences more precisely. This is key to dissecting complex causal relationships of various factors involved in the many biological and pathological processes. One drawback of the Drosophila system is also related to its relative simplicity, and certain regulatory mechanisms and biological processes may be lacking or different in lower organisms such as Drosophila.

Related to our work on SREBP, for example, we have shown that the inhibitory effect of Cdk8 phosphorylation on SREBP is likely to be conserved during evolution. However, flies lack the key enzymes required for the synthesis of cholesterol. They need to obtain cholesterols from foods and convert cholesterols into sterol derivatives such as ecdysone. As a result, we can use Drosophila to study the role of SREBP to understand the regulation of mammalian SREBP-1C in controlling de novo lipogenesis, but not the role of mammalian SREBP-2 in regulation of cholesterogenesis. Therefore, it is necessary to take the advantages of the Drosophila system and, at the same time, to learn from other experimental systems and diverse organisms.

What has surprised you the most while conducting your research?

The most surprising result to me is to find that SREBP directly interacts with Cdk8 or MED15 through the exact same motif in the N-terminus of SREBP. We had expected that Cdk8 and MED15 might interact with different parts of SREBP proteins. Because SREBP binds to its target gene promoters as homodimers, it would be fascinating to figure out the dynamic process of how the CKM and the small Mediator complex that contains MED15 interact with the SREBP homodimers to turn on and turn off gene transcription. This dynamic process may play a key role in the tight regulation of SREBP target gene expression in the nucleus, and further investigation on this process may also serve as a model to understand the interplay between the CKM and the small Mediator complex in regulating other transcriptional factors.

What do you think is the most significant challenge impacting your research at this time and how will this be addressed over the next 10 years?

I think there are two major unsolved puzzles related to this work: the dynamic processes of how different transcriptional co-factors such as the Mediator complexes are recruited to the transcription start site in vivo, thereby regulating transcriptional initiation and elongation, and how these processes are turned off. It is unclear how Cdk8 interacts with SREBP at its N-terminus but phosphorylates a threonine residue near the C-terminus of SREBP. New techniques and approaches, such as super-resolution microscopy, cryo-electron microscopy and other advanced biophysical approaches, are needed to address this problem in the future. Another interesting direction is to understand how physiological perturbations such as feeding, starvation and different dietary components regulate the CKM and the Mediator complex. Perhaps, future studies in this direction may provide a new way to understand the general impacts of these physiological perturbations on gene expression, through the transcription machinery, instead of specific transcription factors.

What changes do you think could improve the professional lives of scientists?

Further improving the living support would always help scientists focus better on their professional efforts. I also think it is important for institutes and the scientific communities to promote a more diverse scientific environment, and enhance more open communications across the departmental and institutional boundaries. This is particularly important to junior scientists to broaden the perspectives on their research activities, training and career development.

What's next for you?

At the end of my PhD, I was intrigued to study spatial and temporal regulation of transcription, using Drosophila as a model system. To pursue this direction, I joined the Levine lab at Princeton University as a postdoctoral fellow. My current work focuses on addressing how different genomic elements regulate genome organization and gene transcription during development.