Danielle Smith
Hepatocytes, polarized epithelial cells of the liver, play a crucial role in maintaining systemic glucose and lipid homeostasis in the face of an ever-changing nutritional environment. Hepatocytes are both incredibly metabolically flexible and also have an efficient intracellular trafficking system that allows them to transport cargo across several apical and basolateral domains with high fidelity. Due to this necessity for efficient endocytosis and secretion, the family of Rab GTPases, principal regulators of the intracellular trafficking network, play a particularly important role in hepatocytes. However, despite their importance in vesicle trafficking, the physiological roles of many Rabs in animals have yet to be characterized. Using a combination of mouse models and cell culture studies, my thesis work has defined the role for the Golgi-localized family member Rab30 in liver physiology during fasting. I have uncovered that Rab30 expression is specifically induced by fasting and not by other dietary interventions in the mouse liver. Fasting-induced Rab30 expression is regulated by a master transcriptional regulator of lipid metabolism in the liver called Pparα. Additionally, Rab30 expression is further amplified in liver-specific carnitine palmitoyltransferase 2 knockout mice (Cpt2L-/-) that lack the ability to oxidize fatty acids and have fasting-induced hepatic steatosis, serum dyslipidemia and a hyper-induction of Pparα transcriptional activity, leading us to the hypothesis that Rab30 contributes to lipid homeostasis. Live-cell super-resolution imaging and biochemical in vivo proximity labeling demonstrate that Rab30-marked vesicles are highly dynamic and interact with proteins at the Golgi apparatus and throughout the secretory pathway. While analysis of liver-specific knockouts of Rab30 reveals its dispensability in the fasting response, analysis of Rab30;Cpt2 double knockout mice, which have a potentiated Pparα response and enable us to amplify the effect of loss of Rab30 in the liver, reveals a retention of proteins within the livers of fasted mice, a reversal of serum dyslipidemia observed in Cpt2L-/- mice, and a corresponding decrease in serum apolipoprotein A4 levels. Together, these data suggest a role for Rab30 in the sorting of secreted proteins and lipoproteins to influence hepatic and circulating triglyceride levels during fasting, particularly during times of excessive lipid burden.
Questions & Answers
Why did you choose Johns Hopkins for your work?
Johns Hopkins has a history of research excellence and has great resources and funding in the basic sciences.
What does receiving this award mean to you personally and professionally? Do you have any connection with the particular award you received?
I am honored to have received the Claude & Barbara Migeon Research Award, which recognizes outstanding basic research performed as a trainee at Johns Hopkins. While there is no direct clinical relevance that can be attributed to my research, basic science is important to informing the molecular pathways underlying both normal physiology and disease states. Professionally, receiving this award motivates me to continue investigating fundamental biological problems in physiology.
What contributed to your project’s success?
We have generated many tools (mouse models, viral vectors for in vivo overexpression, cell lines) and used different techniques (electron microscopy, super-resolution live-cell imaging, protein interaction assays) that allowed us to glimpse the role of Rab30 at both the physiological and cellular level. I was lucky to have my mentor, Dr. Michael Wolfgang, for numerous discussions and for his creativity and guidance.
What thoughts do you have about Young Investigators’ Day itself, as a celebration of the roles students and fellows play in research at Johns Hopkins?
I am extremely thankful for Young Investigators’ Day for granting trainees like me a forum for presenting our research. We are very passionate about our research, and this day allows us to share what we have been working hard on with the rest of the Johns Hopkins scientific community.
What has been your best/most memorable experience while at Johns Hopkins?
Being a part of Dr. Wolfgang’s lab allowed me to immerse myself in many different areas of physiology and metabolism outside of my main dissertation project. I have had incredible opportunities that allowed me to externally collaborate with other labs and learn about the physiology of different organisms. The lab culture fosters scientific creativity, and I have learned so much from Dr. Wolfgang and other lab members.
What are your plans over the next year or so? Graduating, looking for faculty positions, etc.?
I am graduating from my Ph.D. program and will be continuing on to an academic postdoctoral position.
Tell me something interesting about yourself that makes you unique. Do you have any special hobbies, interests or life experiences?
I enjoy being active in my free time, especially spending time outdoors on trails, running, and playing in a Baltimore City field hockey league.