Loss of a Negative Regulator of mTORC1 Induces Aerobic Glycolysis and Altered Fiber Composition in Skeletal Muscle.

The conserved GATOR1 complex consisting of NPRL2-NPRL3-DEPDC5 inhibits mammalian target of rapamycin complex 1 (mTORC1) in response to amino acid insufficiency. Here, we show that loss of NPRL2 and GATOR1 function in skeletal muscle causes constitutive activation of mTORC1 signaling in the fed and fasted states. Muscle fibers of NPRL2 knockout animals are significantly larger and show altered fiber-type composition, with more fast-twitch glycolytic and fewer slow-twitch oxidative fibers. NPRL2 muscle knockout mice also have altered running behavior and enhanced glucose tolerance. Furthermore, loss of NPRL2 induces aerobic glycolysis and suppresses glucose entry into the TCA cycle. Such chronic activation of mTORC1 leads to compensatory increases in anaplerotic pathways to replenish TCA intermediates that are consumed for biosynthetic purposes. These phenotypes reveal a fundamental role for the GATOR1 complex in the homeostatic regulation of mitochondrial functions (biosynthesis versus ATP) to mediate carbohydrate utilization in muscle.

Systemic administration of 2-hydroxypropyl-ß-cyclodextrin to symptomatic Npc1-deficient mice slows cholesterol sequestration in the major organs and improves liver function.

In Niemann-Pick type C (NPC) disease, loss-of-function mutations in either NPC1 or NPC2 result in progressive accumulation of unesterified cholesterol (UC) and glycosphingolipids in all organs, leading to neurodegeneration, pulmonary dysfunction and sometimes liver failure. There is no cure for this disorder. Studies using primarily NPC mouse models have shown that systemic administration of 2-hydroxypropyl-ß-cyclodextrin (2HPßCD), starting in early neonatal life, diminishes UC accumulation in most organs, slows disease progression and extends lifespan. The key question now is whether delaying the start of 2HPßCD treatment until early adulthood, when the amount of entrapped UC throughout the body is markedly elevated, has any of the benefits found when treatment begins at 7 days of age. In the present study, Npc1(-/-) and Npc1(+/+) mice were given saline or 2HPßCD subcutaneously at 49, 56, 63 and 70 days of age, with measurements of organ weights, liver function tests and tissue cholesterol levels performed at 77 days. In Npc1(-/-) mice, treatment with 2HPßCD from 49 days reduced whole-liver cholesterol content at 77 days from 33.0 ± 1.0 to 9.1 ± 0.5 mg/organ. Comparable improvements were seen in other organs, such as the spleen, and in the animal as a whole. There was a transient increase in biliary cholesterol concentration in Npc1(-/-) mice after 2HPßCD. Plasma alanine aminotransferase and aspartate aminotransferase activities in 77-day-old 2HPßCD-treated Npc1(-/-) mice were reduced compared with saline-treated controls. The lifespan of Npc1(-/-) mice given 2HPßCD marginally exceeded that of the saline-treated controls (99 ± 1.1 vs 94 ± 1.4 days, respectively; P < 0.05). Thus, 2HPßCD is effective in mobilizing entrapped cholesterol in late-stage NPC disease leading to improved liver function.