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The Journal of Neuroscience, August 1, 2007, 27(31):8314-8323; doi:10.1523/JNEUROSCI.1972-07.2007

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Neurobiology of Disease
An Endogenous Serine/Threonine Protein Phosphatase Inhibitor, G-Substrate, Reduces Vulnerability in Models of Parkinson's Disease

Chee Yeun Chung,^1,2,4 James B. Koprich,^1,2,4 Shogo Endo,³ and Ole Isacson^1,2,4

¹Neuroregeneration Laboratories, Harvard Medical School, McLean Hospital, Belmont, Massachusetts 02478, ²Harvard Center for Neurodegeneration and Repair, Boston, Massachusetts 02114, ³Okinawa Institute of Science and Technology, Okinawa 904-2234, Japan, and ⁴Udall Parkinson's Disease Research Center of Excellence, McLean Hospital and Harvard University, Belmont, Massachusetts 02478

Correspondence should be addressed to Dr. Ole Isacson, Neuroregeneration Laboratories, Harvard Medical School, McLean Hospital, 115 Mill Street, Belmont, MA 02478. Email: isacson{at}hms.harvard.edu

Relative neuronal vulnerability is a universal yet poorly understood feature of neurodegenerative diseases. In Parkinson's disease, dopaminergic (DA) neurons in the substantia nigra (SN) (A9) are particularly vulnerable, whereas adjacent DA neurons within the ventral tegmental area (A10) are essentially spared. Our previous laser capture microdissection and microarray study (Chung et al., 2005) demonstrated that molecular differences between these DA neurons may underlie their differential vulnerability. Here we show that G-substrate, an endogenous inhibitor of Ser/Thr protein phosphatases, exhibits higher expression in A10 compared with A9 DA neurons in both rodent and human midbrain. Overexpression of G-substrate protected dopaminergic BE(2)-M17 cells against toxins, including 6-OHDA and MG-132 (carbobenzoxy-L-leucyl- L-leucyl-L-leucinal), whereas RNA interference (RNAi)-mediated knockdown of endogenous G-substrate increased their vulnerability to these toxins. G-substrate reduced 6-OHDA-mediated protein phosphatase 2A (PP2A) activation in vitro and increased phosphorylated levels of PP2A targets including Akt, glycogen synthase kinase 3ß, and extracellular signal-regulated kinase 2 but not p38. RNAi to Akt diminished the protective effect of G-substrate against 6-OHDA. In vivo, lentiviral delivery of G-substrate to the rat SN increased baseline levels of phosphorylated Akt and protected A9 DA neurons from 6-OHDA-induced toxicity. These results suggest that inherent differences in the levels of G-substrate contribute to the differential vulnerability of DA neurons and that enhancing G-substrate levels may be a neuroprotective strategy for the vulnerable A9 (SN) DA neurons in Parkinson's disease.

Key words: protein phosphatase 2A; G-substrate; Akt; differential vulnerability; selective vulnerability; A9; A10; 6-hydroxydopamine; neuroprotection; Parkinson's disease; substantia nigra; ventral tegmental area

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Received May 1, 2007; revised June 17, 2007; accepted June 18, 2007.

Correspondence should be addressed to Dr. Ole Isacson, Neuroregeneration Laboratories, Harvard Medical School, McLean Hospital, 115 Mill Street, Belmont, MA 02478. Email: isacson{at}hms.harvard.edu

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