Characterization of the cellular function of TBC1D7, independent of- and as the third component of the TSC complex

• Applicant: Dr. Sandra Schrötter
• Subject Area: Cell Biology
• Term: from 2018 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 403153940

Tuberous sclerosis complex (TSC) is caused by mutations in TSC1 or TSC2, which form a functional complex (the TSC complex). Major clinical features of TSC are benign tumor growth of the skin, heart, lung and brain together with seizures and TSC-associated neuropsychiatric disorders (e.g. autism spectrum disorder) [Henske et al. 2016]. Together with the TBC1D7 protein, the TSC1 and TSC2 proteins form a negative regulatory complex upstream of mTORC1, a master regulator of anabolic cell growth and proliferation. TBC1D7 is a ubiquitous core component of the TSC complex, important for the complex´s stability and GAP activity toward Rheb [Dibble et al. 2012]. Germline loss of function mutations in the TBC1D7 gene have not been identified in TSC patients so far, but loss of TBC1D7 has been associated with a hereditary megalencephaly syndrome accompanied with elevated mTORC1 signaling, but distinct from TSC [Capo-Chichi et al. 2013]. Over the last 10 years, the Manning lab has made important discoveries in our understanding of the TSC complex components TSC1, TSC2 and TBC1D7. Besides its role as a component of the TSC complex, TBC1D7 was also found to be present in a pool of free protein in the cell cytoplasm. The exact function of either protein population remains unknown.The main goal of this fellowship is to define the molecular functions of TBC1D7 and how alterations in these functions contribute to cellular phenotypes underlying the diverse clinical manifestations of TSC. I believe that understanding the role of TBC1D7 both within the TSC complex and as a free entity, which increases upon loss of the TSC genes, will provide insights into new pathways and processes that become aberrantly regulated in TSC patients. I will use various cellular biological and biochemical approaches to study the function of TBC1D7 in particular in the brain in vitro and in vivo. The three specific aims of this fellowship are (1) to define new interaction partners of TBC1D7 within the brain, (2) to characterize a whole body Tbc1d7 knock-out mouse and (3) to decipher the role of TBC1D7 in cellular metabolism. In my first months in the Manning lab I generated Tbc1d7 knock-out mice using the CRISPR/Cas9 technique. These mice are viable and show no gross abnormalities, and they will be an essential tool to the project to gain a deeper understanding of the role of TBC1D7 in a physiological context. Furthermore, preliminary results from the lab suggest that TBC1D7 interacts with Rab17 and Citron, as well as a role for TBC1D7 in the regulation of creatine metabolism. I will use this fellowship to further explore these findings and analyze interaction interactions with other proteins in more detail. In this proposal, I will focus on gaining a better comprehension of the relationship between new pathways (involving the free TBC1D7 pool) and the TSC complex, fundamental knowledge that is essential before exploring the therapeutic implications in TSC.

• DFG Programme: Research Fellowships
• International Connection: USA
• Host: Professor Dr. Brendan Manning