Restoration of Normal Cerebral Oxygen Consumption with Rapamycin Treatment in a Rat Model of Autism–Tuberous Sclerosis

Abstract

Tuberous sclerosis (TSC) is associated with autism spectrum disorders and has been linked to metabolic dysfunction and unrestrained signaling of the mammalian target of rapamycin (mTOR). Inhibition of mTOR by rapamycin can mitigate some of the phenotypic abnormalities associated with TSC and autism, but whether this is due to the mTOR-related function in energy metabolism remains to be elucidated. In young Eker rats, an animal model of TSC and autism, which harbors a germ line heterozygous Tsc2 mutation, we previously reported that cerebral oxygen consumption was pronouncedly elevated. Young (4 weeks) male control Long–Evans and Eker rats were divided into control and rapamycin-treated (20 mg/kg once daily for 2 days) animals. Cerebral regional blood flow (14C-iodoantipyrine) and O2 consumption (cryomicrospectrophotometry) were determined in isoflurane-anesthetized rats. We found significantly increased basal O2 consumption in the cortex (8.7 ± 1.5 ml O2/min/100 g Eker vs. 2.7 ± 0.2 control), hippocampus, pons and cerebellum. Regional cerebral blood flow and cerebral O2 extractions were also elevated in all brain regions. Rapamycin had no significant effect on O2 consumption in any brain region of the control rats, but significantly reduced consumption in the cortex (4.1 ± 0.3) and all other examined regions of the Eker rats. Phosphorylation of mTOR and S6K1 was similar in the two groups and equally reduced by rapamycin. Thus, a rapamycin-sensitive, mTOR-dependent but S6K1-independent, signal led to enhanced oxidative metabolism in the Eker brain. We found decreased Akt phosphorylation in Eker but not Long–Evans rat brains, suggesting that this may be related to the increased cerebral O2 consumption in the Eker rat. Our findings suggest that rapamycin targeting of Akt to restore normal cerebral metabolism could have therapeutic potential in tuberous sclerosis and autism.

Inhibition of p70 Ribosomal S6 Kinase (S6K1) Reduces Cortical Blood Flow in a Rat Model of Autism-Tuberous Sclerosis

“…As described previously (Chi et al, 2015 ; Weiss et al, 2007 , 2009 ), this study was conducted following the guidelines set forth by the US Public Health Service, as outlined in the Guide for the Care of Laboratory Animals (DHHS Publication No. 85-23, revised 1996), and received approval from our Institutional Animal Care and Use Committee.…”

“…As we described previously (Chi et al, 2015 ), the rats were anesthetized with a 2% isoflurane-air-oxygen mixture via tracheal tube and mechanical ventilation to maintain arterial PO 2 above 100 mm Hg and PCO 2 around 35 mmHg. A femoral artery and a femoral vein cannulation were performed.…”

“…As we previously described (Chi et al, 2015 ; Weiss et al, 2007 , 2009 ), for the determination of cerebral blood flow, 20 μCi of 14 C-iodoantipyrine was intravenously infused through the femoral venous catheter. As the isotope entered the venous circulation, the femoral arterial catheter was trimmed to about 20 mm in length.…”

“…Our preliminary investigations also indicated that the heightened cerebral metabolism and blood flow in the young Eker rats were not affected significantly by N -methyl-D-aspartate (NMDA) or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor blockade or stimulation (Weiss et al, 2007 , 2009 , 2012 ). In contrast rapamycin significantly reduced the basal elevated cerebral blood flow and metabolism in the Eker rat to the level of the control rats (Chi et al, 2015 ). Based on our earlier data, it appears that modifying the excitation/inhibition balance in synapses did not significantly contribute to the increased regional cerebral blood flow (rCBF) in Eker rats.…”

Insights into the structure and function of the hippocampus: implications for the pathophysiology and treatment of autism spectrum disorder

“…As discussed earlier, tuberous sclerosis complex models had increased hippocampal neuron volume, blood flow, and oxygen consumption. Correspondingly, the administration of rapamycin has been reported to lower cerebral blood flow and oxygen consumption in hippocampal regions of the Tsc2 mutant rat, potentially by downregulating Akt signals ( 146 ). This also accords with previous observations, which showed that pharmacological inhibition of mTORC suppressed granule cell hypertrophy in Pten mutant mice ( 258 ) and inhibited the PI3K/AKT/mTOR signaling pathway in VPA-induced rats ( 220 ).…”