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It has been proposed that cell growth and autophagy are coordinated in response to cellular nutrient status, but the relationship between them is not fully understood. Here, we have characterized the fly mutants of Autophagy-specific gene 1 (ATG1), an autophagy-regulating kinase, and found that ATG1 is a negative regulator of the target of rapamycin (TOR)/S6 kinase (S6K) pathway. Our Drosophila studies have shown that ATG1 inhibits TOR/S6K-dependent cell growth and development by interfering with S6K activation. Consistently, overexpression of ATG1 in mammalian cells also markedly inhibits S6K in a kinase activity-dependent manner, and short interfering RNA-mediated knockdown of ATG1 induces ectopic activation of S6K and S6 phosphorylation. Moreover, we demonstrated that ATG1 specifically inhibits S6K activity by blocking phosphorylation of S6K at Thr 389. Taken together, our genetic and biochemical results strongly indicate crosstalk between autophagy and cell growth regulation.

DJ-1 is linked to an early-onset autosomal recessive Parkinson's disease (PD) characterized primarily by selective loss of dopaminergic (DA) neurons, which results in motor disturbances. However, our understanding on how mutations in DJ-1 are related to PD is unclear. Here, we isolated the DJ-1 orthologue, DJ-1beta, in Drosophila and characterized its expression and loss-of-function mutants. We observed its strongest expression in the adult stage of development and ubiquitous expression in the larval brain. Our homozygous mutants showed severe defects in locomotor ability without loss of DA neurons, consistent with the previous mice DJ-1 mutant studies ([Goldberg, M.S., Pisani, A., Haburcak, M., Vortherms, T.A., Kitada, T., Costa, C., Tong, Y., Martella, G., Tscherter, A., Martins, A., et al., 2005. Nigrostriatal dopaminergic deficits and hypokinesia caused by inactivation of the familial Parkinsonism-linked gene DJ-1. Neuron 45, 489-496.]; [Kim, R.H., Smith, P.D., Aleyasin, H., Hayley, S., Mount, M.P., Pownall, S., Wakeham, A., You-Ten, A.J., Kalia, S.K., Horne, P., Westaway, D., Lozano, A.M., Anisman, H., Park, D.S., Mak, T.W., 2005. Hypersensitivity of DJ-1-deficient mice to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and oxidative stress. Proc. Natl. Acad. Sci. USA 102, 5215-5220.]; [Chen, L., Cagniard, B., Mathews, T., Jones, S., Koh, H.C., Ding, Y., Carvey, P.M., Ling, Z., Kang, U.J., Zhuang, X., 2005. Age-dependent motor deficits and dopaminergic dysfunction in DJ-1 null mice. J. Biol. Chem. 280, 21418-21426.]). The locomotor activity of DJ-1beta mutants was further decreased by paraquat-induced oxidative stress. Moreover, we found that Drosophila DJ-1 is prominently localized in mitochondria, suggesting that DJ-1 functions as a protector against oxidative stress in mitochondria.

Parkin, an E3 ubiquitin ligase, has been found to be responsible for autosomal recessive juvenile parkinsonism characterized primarily by selective loss of dopaminergic neurons with subsequent defects in movements. However, the molecular mechanisms underlying this neuron loss remain elusive. Here, we characterized Drosophila parkin loss-of-function mutants, which exhibit shrinkage of dopaminergic neurons with decreased tyrosine hydroxylase level and impaired locomotion. The behavioral defect of parkin mutant flies was partially restored by administering L-DOPA, and the dopamine level in the brains of parkin mutant flies was highly decreased. Intriguingly, we found that c-Jun N-terminal kinase (JNK) is strongly activated in the dopaminergic neurons of parkin mutants and that impaired dopaminergic neuron phenotypes are dependent on the activation of the JNK signaling pathway. In consistent with this, our epistatic analysis and mammalian cell studies showed that Parkin inhibits the JNK signaling pathway in an E3 activity-dependent manner. These results suggest that loss of Parkin function up-regulates the JNK signaling pathway, which may contribute to the vulnerability of dopaminergic neurons in Drosophila parkin mutants and perhaps autosomal recessive juvenile parkinsonism patients.