Investigation into the effects and mechanisms of rapamycin treatment in two mouse models of Complex I-deficient neurological pathology
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Treatments to stop or reverse the debilitating progression of early- and late-onset neurological diseases remain undiscovered. Collective evidence suggests that inhibition of the mechanistic target of rapamycin (mTOR) signaling pathway is effective at reducing markers of pathology in experimental models of age-related and developmental neurological diseases. These include, but are not limited to, models for Alzheimer's disease, Parkinson's disease, Huntington's disease, Fragile X syndrome, Tuberous Sclerosis Complex and Leigh syndrome. How regulation of mTOR activity and its downstream effectors interact with underlying neural mechanisms of disease has been a topic of considerable debate. The studies presented here investigate the potential therapeutic effects of the mTOR inhibitor rapamycin in two different mouse models for neurological disease: the NDUFS4 knockout (NDUFS4 KO) mouse for Leigh Syndrome and the MPTP mouse model for Parkinson's disease. In these models, mitochondrial electron transport chain complex I activity is reduced but results in distinct patterns of neuronal pathology. Our major objectives were (1) to identify previously unreported effects of rapamycin treatment in these models; and (2) to identify the potential cellular mechanisms that mediate these effects. First, we demonstrate that daily treatment with high dose rapamycin effectively extends the short lifespan of NDUFS4 KO mice. Rapamycin treatment also resulted in prolonged healthspan in KO mice, as indicated by the offset of neurological damage, maintenance of weight and body fat, and the improvement of deleterious behavioral phenotypes. Systematic testing of potential mechanisms mediating these effects led us to favor a model in which rapamycin induces a metabolic shift in NDUFS4 KO brains toward amino acid catabolism and away from glycolysis, thus alleviating the buildup of glycolytic intermediates. Following this set of discoveries, we expanded our findings to test whether dietary rapamycin delivered at higher doses than previously used could improve lifespan and abnormal weight phenotypes in NDUFS4 KO mice, similar to what was found for high dose injections. Dietary rapamycin doses were tested at 42 ppm, 126 ppm and 378 ppm, corresponding to 3-fold, 9-fold and 27-fold increases from the standard 14 ppm dosage used by the NIH Interventions Testing Program, respectively. As a result of these treatments, NDUFS4 KO lifespan was significantly extended, with successively higher dosages correlating with increased survival. We also found that dietary rapamycin at 126 and 378 ppm had significant effects on body weight and fat mass in male and female wild-type mice. Finally, we conducted a pilot study investigating the effectiveness of high dose rapamycin treatment in treating the Parkinson's disease-like pathology of mice exposed to the toxic drug MPTP. Our results show that rapamycin treatment partially reduced neuron degeneration in the substantia nigra resulting from MPTP exposure, consistent with previous reports. In addition, MPTP mice showed evidence for hyperactive mTOR signaling compared to control mice, which could be potently reduced by rapamycin treatment. No significant changes in body weight or fast mass were found as a result of MPTP exposure, or as a result of an interaction between MPTP and rapamycin. When accounting for different age cohorts, middle aged mice that had been exposed to MPTP performed better on a rotarod task after receiving rapamycin treatment. Our young cohort, however, did not show any differences in performance between treatment groups. Thus, we believe that MPTP induces age-dependent phenotypes that may have been overlooked in previous studies utilizing young mice. Thus far, comparison of these studies suggests that rapamycin treatment has both overlapping and distinct effects that contribute to attenuation of neural pathology of NDUFS4 and MPTP mouse models.
- Psychology