CLOCKWISE FROM CENTER FRONT: M. MARAL MOURADIAN, MD, WILLIAM DOW LOVETT PROFESSOR OF NEUROLOGY AND DIRECTOR OF THE CENTER FOR NEURODEGENERATIVE AND NEUROIMMUNOLOGIC DISEASES; EUNSUNG JUNN, PHD, ASSISTANT PROFESSOR; XIN ZHAO, MD, RESEARCH SPECIALIST; WON HEE JANG, PHD, VISITING RESEARCH SCHOLAR; BYEONG-SEON JEONG, PHD, POSTDOCTORAL FELLOW, ALL IN THE DEPARTMENT OF NEUROLOGY AT RWJMS.

Can we cure Parkinson’s disease? by M. Maral Mouradian

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urrently, there is no cure for Parkinson’s disease (PD) even though several available treatments help patients’ symptoms. Therefore, the search is on for disease modifying therapies that would stop, prevent or at least slow the progression of neuronal cell death in the brain. The discovery of multiple genes linked to PD and learning how these genes are involved in cell death are providing us with new targets for neuroprotective drug development. Several research projects at the Center for Neurodegenerative and Neuroimmunologic Diseases, Department of Neurology, RWJMS, are dedicated to this search. We are also utilizing knowledge gained about the molecular pathogenesis of PD in developing a biomarker of disease susceptibility. These research efforts are funded by the NIH, several foundations and the William Dow Lovett Endowment.

Parkinson’s disease (PD) is the second most common degenerative disorder of the brain after Alzheimer’s, affecting about one million individuals in the US. It is a progressive debilitating disease with significant societal and economic impact as it often affects people during their prime productive years. Clinically, PD manifests with slow movements, tremor, stiffness and balance difficulties, attributed to the insidious death of dopamine producing neurons in the brain. Although PD is the only neurodegenerative disease for which replacing the depleted neurotransmitter dopamine has had a huge impact on the quality of life and longevity of patients, this treatment strategy only ameliorates the symptoms while brain cells degenerate relentlessly.

As the disease advances, the currently available medications may bring about complications including abnormal involuntary movements and hallucinations. Moreover, additional symptoms emerge—such as cognitive impairment—that are obstinate to therapy. Therefore, finding a neuroprotective treatment that fundamentally alters the course or pace of the neurodegenerative process is an unmet need.

The goal of our research program is to elucidate the molecular cascades in PD that culminate in neuronal death and to identify therapeutically viable targets for new drug development in order to interdict the neurodegeneration and, hence, disease progression. Research in several laboratories, including ours, over the past decade has made clear that PD is not a single disease (Mouradian, Neurology 58:179, 2002). Rather, it is a clinical syndrome with diverse, underlying, initiating causes. The strongest evidence to date for this conclusion is the fact that mutations in at least five different genes cause PD. Some of these mutations are even found in patients with so called sporadic non-familial PD. Thus, although the end result is neuronal death in all cases, the early steps in the cascade are quite different depending on the etiology in a given case. Therefore, our laboratory is focused on understanding how some of these mutant genes kill neurons.

One of these PD-causing genes codes for the abundant brain protein alpha-synuclein. Using various experimental paradigms, we—and others—have shown that excess amounts of this protein are deleterious to dopaminergic neurons (Lee et al, Neurobiology of Aging 24:687, 2003; Tanaka et al, J. Biol. Chem 279:4625, 2004). In fact, individuals with multiple copies of this gene develop dominantly inherited PD. We have found that elevated levels of this protein cause neurons to generate more reactive oxygen species and, therefore, render them more vulnerable to oxidative insults, which is indeed the precarious state of dopaminergic neurons in PD (Junn et al, Neurosci. Lett. 320:146, 2002; Junn et al, J. Neurochemistry 78:374, 2001). Additionally, the salient property of alpha-synuclein that is central in the neuropathology of the disease is its strong tendency to aggregate, which is believed to be toxic. Many cellular conditions promote alpha-synuclein aggregation, including high concentration, mutations and interactions with other proteins (Lee et al, J. Biol. Chem. 279:6834, 2004). We have found that this process can be initiated by cross-linking alpha-synuclein through the enzymatic activity of tissue transglutaminase. The product of this initial enzymatic reaction can act as a nidus for subsequent polymerization of alpha-synuclein molecules. The importance of this observation lies in the fact that small molecule inhibitors of tissue transglutaminase can prevent alpha-synuclein aggregation (Junn et al, Proceedings of the National Academy of Sciences USA 100:2047, 2003). A grant awarded by the American Parkinson Disease Association allows us to test if mice that over-express alpha-synuclein and have motor behavioral deficits can perform better if given such inhibitors.

In addition to using our knowledge about the biology of alpha-synuclein to develop new therapeutic strategies, we are also testing the potential for developing a biomarker for PD. We postulated that a measure of increased alpha-synuclein expression in a given individual can be an index of susceptibility to the disease. For example, in a family with an inherited form of PD due to a known mutation in the alpha-synuclein gene, the age range when symptoms begin is decades apart. An important variable in such a genetically well-defined population could be the level of expression of their alpha-synuclein gene. One way to regulate gene expression is through DNA methylation. This modification renders DNA less active transcriptionally and, therefore, less protein is produced. Thus, individuals with sparse DNA methylation at critical sites could have higher levels of alpha-synuclein in their neurons and might develop disease symptoms at an earlier age. With a grant from the Michael J. Fox Foundation for Parkinson’s Research, we are developing a test of DNA methylation as a predictor of disease risk.

Parkin is another gene that causes PD when mutated. This gene product is believed to be involved in the processing of other proteins for degradation, lest damaged proteins would accumulate in the cell and cause harm. Our findings suggest that Parkin is also involved in forming large cellular inclusions where damaged proteins are stored away from the vital parts of the cell as a protective strategy (Junn et al, J. Biol. Chem. 277:47870, 2002). A grant from the Parkinson Disease Foundation has helped support our studies with parkin.

Another gene linked to inherited PD is DJ-1. One of the properties of this multi-functional protein is its ability to quench reactive oxygen species. Therefore, discovery of this genetic link was quickly followed by speculations that the impaired anti-oxidant function is key in the loss-of-function mutation in DJ-1. However, we found that the anti-oxidant ability of DJ-1 is quite modest compared to its robust cell protective capability. Instead, our research indicated that DJ-1 blocks a specific cell suicide pathway by sequestering the death protein Daxx in the nucleus and preventing it from activating its effector molecular apoptosis signal regulating kinase 1 (ASK-1) (Junn et al, Proceedings of the National Academy of Sciences USA 102:9691, 2005). A PD causing mutation leaves DJ-1 unable to protect cells from this cell death cascade. Furthermore, DJ-1 counteracts Daxx in repressing the transcription of genes that could undermine cell survival. A grant from the National Institutes of Health is currently funding further studies in our laboratory using genetically engineered mice with altered expression of DJ-1 and ASK-1.

Despite the varied ways by which brain cells degenerate, the final steps appear to be common among the different forms of PD. Therefore, neuroprotective strategies that target early steps in the process have the advantage of providing disease specific therapies without tampering with the later steps that not only lack specificity but can potentially lead to untoward consequences. Our studies of these mechanisms, some of which are outlined above, allow the identification of such therapeutic targets. Loss of function mutations such as parkin and DJ-1 would need to be addressed by replacing the missing function, while gain of function mutations such as alpha-synuclein need to be inhibited at an early stage as described above. The implication of these discoveries is that such targeted therapies may have to be tailored for each PD type based on its underlying etiology and that no single treatment approach would benefit all patients with the clinical syndrome we still call Parkinson’s disease. Alternatively, since multiple parallel pathways keep an appropriate balance between cellular life and death, pharmacological manipulations of such parallel pathways may still impact the disease by boosting survival mechanisms. Our research program is focused on these goals.

M. Maral Mouradian, MD, is the William Dow Lovett Professor of Neurology and Director of the Center for Neurodegenerative and Neuroimmunologic Diseases in the Department of Neurology at RWJMS. She also holds joint professorships in the Department of Neuroscience and Cell Biology and in the Department of Molecular Biology, Microbiology and Immunology. After obtaining her medical education and neurology training, Dr. Mouradian joined the NIH, where she obtained further training in clinical research on Parkinson’s disease as well as in molecular biology under the tutelage of the Nobel Laureate Marshall Nirenberg. During the last 13 years of her NIH tenure, Dr. Mouradian directed the Genetic Pharmacology Unit of NINDS. In her current position at RWJMS, she carries out basic and translational research, evaluates and treats patients with Parkinson’s disease, and conducts clinical research. Dr. Mouradian is a member of the Scientific Advisory Board of the American Parkinson Disease Association, an Editorial Board member of Neurology, and an associate editor of Pharmacology and Therapeutics. She is the recipient of the NIH Award of Merit and a member of the Alpha Omega Alpha honor medical society.