RTP801 Is Elevated in Parkinson Brain Substantia Nigral Neurons and Mediates Death in Cellular Models of Parkinson's Disease by a Mechanism Involving Mammalian Target of Rapamycin Inactivation

Abstract

The molecules underlying neuron loss in Parkinson's disease (PD) are essentially unknown, and current therapies focus on diminishing symptoms rather than preventing neuron death. We identified RTP801 as a gene whose transcripts were highly induced in a cellular model of PD in which death of neuronal catecholaminergic PC12 cells was triggered by the PD mimetic 6-OHDA. Here, we find that RTP801 protein is also induced in this and additional cellular and animal PD models. To assess the relevance of these observations to PD, we used immunohistochemistry to compare RTP801 expression in postmortem brains from PD and control patients. For all PD brains examined, expression was highly elevated within neuromelanin-containing neurons of the substantia nigra but not in cerebellar neurons. Evaluation of the potential role of RTP801 induction in our cellular model revealed that RTP801 overexpression is sufficient to promote death but does not further elevate death caused by 6-OHDA. Furthermore, RTP801 induction is requisite for death in our cellular PD models and in 6-OHDA-treated cultured sympathetic neurons in that its knockdown by short hairpin RNAs (shRNAs) is protective. The mechanism by which 6-OHDA and RTP801 induce neuron death appears to involve repression of mammalian target of rapamycin (mTOR) kinase activity, and such death is inhibited by shRNAs targeting TSC2 (tuberous sclerosis complex), a protein with which RTP801 interacts to block mTOR activation. Our findings thus suggest that the elevation of RTP801 we detect in PD substantia nigral neurons may mediate their degeneration and death and that RTP801 and its signaling cascade may be novel potential therapeutic targets for the disease.

Nucleolar Disruption in Dopaminergic Neurons Leads to Oxidative Damage and Parkinsonism through Repression of Mammalian Target of Rapamycin Signaling

“…Attenuation of mTOR signaling has been reported in several neurodegenerative diseases, including Parkinson's disease (Inoki et al, 2005). Neurotoxins, such as 6-OHDA, MPTP, and rotenone, block the activation of mTOR signaling in neuronal cultures and in animals (Malagelada et al, 2006). In fact, we show here that mitochondrial damage induced by MPTP treatment results in a significant decrease of mTOR activity and inhibition of rRNA transcription.…”

RTP801 Is Induced in Parkinson's Disease and Mediates Neuron Death by Inhibiting Akt Phosphorylation/Activation

“…Our past studies have shown that the stress-responsive protein RTP801 is substantially induced in multiple cellular models of PD, in an animal model of PD and in dopaminergic neurons of patients with PD (Ryu et al, 2002;Malagelada et al, 2006). RTP801 over-expression is sufficient to promote neuron death and we have reported that this protein is required for neuron death in cellular models of PD .…”

“…RTP801 is induced by stresses including DNA damage, oxidative stress, hypoxia, ER stress and energy depletion (Ellisen et al, 2002;Shoshani et al, 2002;Wang et al, 2003;Sofer et al, 2005) that have been raised as causes of neuron degeneration in PD (Marras and Lang, 2008). RTP801 can be pro or antiapoptotic and in neuronal cells promotes death (Shoshani et al, 2002;Malagelada et al, 2006). We found that multiple PD mimetics (6-OHDA, MPPϩ and rotenone) induced RTP801 in neuronal cells and that it mediated death in each case .…”

“…6-OHDA was prepared before use in 10 mM stocks and diluted in medium to the indicated final concentrations. Neonatal rat superior cervical ganglion sympathetic neurons were cultured as described previously (Ryu et al, 2002;Malagelada et al, 2006). Treatments with 6-OHDA was performed at day 7 in vitro, and cell viability was assessed 24 h later.…”

Interleukin-6 influences stress-signalling by reducing the expression of the mTOR-inhibitor REDD1 in a STAT3-dependent manner

“…Increased REDD1 expression in neurodegenerative diseases such as Parkinson´s and Alzheimer´s disease leads to permanent inhibition of mTOR and thereby cellular death [22,23]. On the contrary, reduced REDD1 expression and constitutive mTOR signalling is found in cancer patients [16,18].…”