• the Korean Society for Brain and Neural Sciences


Short Communication

Exp Neurobiol 2019; 28(2): 172-182

Published online April 19, 2019

© The Korean Society for Brain and Neural Sciences

Farnesylation-defective Rheb Increases Axonal Length Independently of mTORC1 Activity in Embryonic Primary Neurons

Seunghyuk Choi1, Ali Sadra1, Jieun Kang1, Jae Ryun Ryu2, June Hoan Kim2, Woong Sun2*, and Sung-Oh Huh1*

1Department of Pharmacology, College of Medicine, Institute of Natural Medicine, Hallym University, Chuncheon 24252, Korea.

2Department of Anatomy, Korea University College of Medicine, Brain Korea 21 Plus, Seoul 02841, Korea.

Correspondence to: *To whom correspondence should be addressed.
Sung-Oh Huh, TEL: 82-33-248-2615, FAX: 82-33-248-2612
Woong Sun, TEL: 82-2-2286-1404, FAX: 82-2-929-5696

Received: February 22, 2019; Revised: March 27, 2019; Accepted: March 28, 2019

Rheb (Ras homolog enriched in the brain) is a small GTPase protein that plays an important role in cell signaling for development of the neocortex through modulation of mTORC1 (mammalian-target-of-rapamycin-complex-1) activity. mTORC1 is known to control various biological processes including axonal growth in forming complexes at the lysosomal membrane compartment. As such, anchoring of Rheb on the lysosomal membrane via the farnesylation of Rheb at its cysteine residue (C180) is required for its promotion of mTOR activity. To test the significance of Rheb farnesylation, we overexpressed a farnesylation mutant form of Rheb, Rheb C180S, in primary rat hippocampal neurons and also in mouse embryonic neurons using in utero electroporation. Interestingly, we found that Rheb C180S maintained promotional effect of axonal elongation similar to the wild-type Rheb in both test systems. On the other hand, Rheb C180S failed to exhibit the multiple axon-promoting effect which is found in wild-type Rheb. The levels of phospho-4EBP1, a downstream target of mTORC1, were surprisingly increased in Rheb C180S transfected neurons, despite the levels of phosphorylated mTOR being significantly decreased compared to control vector transfectants. A specific mTORC1 inhibitor, rapamycin, also could not completely abolish axon elongation characteristics of Rheb C180S in transfected cells. Our data suggests that Rheb in a non-membrane compartment can promote the axonal elongation via phosphorylation of 4EBP1 and through an mTORC1-independent pathway.

Graphical Abstract

Keywords: Axons, mTOR protein, Rheb protein, Protein farnesylation