Jos Jansen

98 Chapter 4 Here, we provide an update on COPI and COPII vesicles regarding steatogenesis and hypothesize that defects in CCDC115, TMEM199, and ATP6AP1 result in altered functioning of these vesicles resulting in the steatotic phenotype. The COPI Complex and SREBPs The COPI complex mediates retrograde protein transport within Golgi stacks and between cis -Golgi and ER.(54) Recently the structure of the COPI vesicle was determined with a resolution of 13 Å revealing a tight network of eight subunits recruited en bloc to form nascent vesicles.(55) These subunits are built on small GTPase Arf1 which is an activator of COPI assembly. The COPI complex is involved in the Sterol Regulatory Element-Binding Proteins (SREBPs) pathway, the main transcriptional regulator for hepatic lipogenesis (SREBP1) and for cholesterol metabolism (SREBP2).(56) SREBPs reside in the ER membrane as inactive precursors. Upon sterol-deficient conditions SREBPs translocate to the Golgi via SCAP, an escorting protein, and COPII vesicles. In the Golgi, specific proteases cleave SREBP to release a transcription factor domain which is translocated to the nucleus for lipogenic transcription. Recent data show that even under sterol rich conditions a fraction of the SCAP-SREBP complex escapes the ER and that COPI retrograde transport is needed for retrieval.(57) Inhibition of COPI complexes by siRNAs results in an increased activity of SREBPs and increased TG contents.(57) We hypothesize that retrograde COPI transport is affected by V-ATPase assembly factor deficiency resulting in increased activity of SREBPs, similar to COPI knockdown. Interestingly, loss-of-function mutations in COPA have recently been associated with a phenotype of autoimmune interstitial lung, joint, and kidney disease (MIM 616414) but steatosis has not been described.(58) Presence of the COPI complex, Arf1, GBF1, and ELMOD2 on the LD surface has been confirmed by proteomics and co-localization studies.(51, 52, 59- 61) GBF1 activates and ELMOD2 deactivates Arf1.(61, 62) Interestingly, GBF1 has been described to interact with LDAP ATGL, but data are not consistent. (63) , (64) Knockdown of COPI, Arf1, or GBF1 by siRNA results in increased cellular triglycerides levels and absent levels of LDAPs ATGL and Perilipin 2 on the LD surface.(59, 63-65) In line with these findings, knockdown of ELMOD2 results in increased LD-surface ATGL and decreased TG levels.(61)