Neuronal Tsc1/2 complex controls autophagy through AMPK-dependent regulation of ULK1

Abstract

Tuberous sclerosis complex (TSC) is a disorder arising from mutation in the TSC1 or TSC2 gene, characterized by the development of hamartomas in various organs and neurological manifestations including epilepsy, intellectual disability and autism. TSC1/2 protein complex negatively regulates the mammalian target of rapamycin complex 1 (mTORC1) a master regulator of protein synthesis, cell growth and autophagy. Autophagy is a cellular quality-control process that sequesters cytosolic material in double membrane vesicles called autophagosomes and degrades it in autolysosomes. Previous studies in dividing cells have shown that mTORC1 blocks autophagy through inhibition of Unc-51-like-kinase1/2 (ULK1/2). Despite the fact that autophagy plays critical roles in neuronal homeostasis, little is known on the regulation of autophagy in neurons. Here we show that unlike in non-neuronal cells, Tsc2-deficient neurons have increased autolysosome accumulation and autophagic flux despite mTORC1-dependent inhibition of ULK1. Our data demonstrate that loss of Tsc2 results in autophagic activity via AMPK-dependent activation of ULK1. Thus, in Tsc2-knockdown neurons AMPK activation is the dominant regulator of autophagy. Notably, increased AMPK activity and autophagy activation are also found in the brains of Tsc1-conditional mouse models and in cortical tubers resected from TSC patients. Together, our findings indicate that neuronal Tsc1/2 complex activity is required for the coordinated regulation of autophagy by AMPK. By uncovering the autophagy dysfunction associated with Tsc2 loss in neurons, our work sheds light on a previously uncharacterized cellular mechanism that contributes to altered neuronal homeostasis in TSC disease.

Hydrogen sulfide reduces serum triglyceride by activating liver autophagy via the AMPK-mTOR pathway

“…required for the regulation of autophagy by AMPK. TSC2 could negatively regulate mTOR activity when complexed with TSC1 and thus inhibits activation of mTOR and the downstream pathway members (6,24).…”

Evidence Supporting a Lymphatic Endothelium Origin for Angiomyolipoma, a TSC2− Tumor Related to Lymphangioleiomyomatosis

“…However, in addition to the canonical pathway, data from a variety of experimental systems have indicate the existence of noncanonical functions for TSC1, 69,70 TSC2, 11,71e78 and Rheb. 79,80 Therefore, we assessed whether the LEC characteristics seen in TSC2 þ cells were the result of TSC2-mediated inhibition of the mTOR pathway.…”

AMPK in the central nervous system: physiological roles and pathological implications

“…[19][20][21][22][23][24] In addition, recent data showed that AMPK phosphorylates Beclin1 at Ser91/94, and this event is essential for the induction of autophagy in nutrient stress response. 25 An interesting study published by Sanchez et al sustains that AMPK also determines, at least in skeletal muscle, the FoxO3-dependent increase of autophagy-related proteins, such as LC3B, GABA-RAPL1, and Beclin1.…”

“…TSC is an autosomal dominant disorder characterized by hamartomas in multiple organs and neurological manifestations such as seizures, hyperactivity and aggression, intellectual disability, or learning problems. 20 TSC is caused by mutations in the TSC1 or TSC2 genes, leading to the disruption of TSC1-TSC2 intracellular protein complex and to the hyperactivation of mTORC1. 127 It was reported that TSC1/2 deficiency reflects in a reduced autophagy because of the inactivation of ULK1 operated by mTORC1 in non-neuronal cells.…”

“…Thus, despite the concomitant inhibitory effect of mTORC1 on ULK1 activity, AMPK seems to be the dominant modulator of autophagy in brain cells. 20 As a consequence, the accumulation of p62 and autolysosomes could contribute to the altered neuronal homeostasis in TSC disease.…”