PD is a neurodegenerative disease whose defining pathology is the selective degeneration of dopamine producing neurons in an area of the midbrain called the substantia nigra. The biggest mystery in PD research has centered around why these particular neurons degenerate. Research now offers a clue and a tantalizing drug target that may help in treating PD patients. A team led by Dr. James Surmeier, a professor at Northwestern University, has shown that a specific type of calcium channel expressed in substantia nigra dopaminergic neurons may render them more vulnerable to damage by oxidative stress (Guzman et al, 2010). By targeting this channel scientists may be able to develop an entirely new avenue of therapy for PD that would prevent the associated neurodegeneration. The best pharmacological treatment currently prescribed for Parkinson’s disease (PD), the dopamine precursor L-DOPA, was discovered over forty years ago. At best, the administration of L-DOPA can lead to a temporary reprieve from the debilitating motor symptoms associated with PD, but it does not affect disease progression and patients eventually develop a tolerance to it. L-DOPA is still the most widely prescribed medication for PD due to lack of a disease modifying therapy. Dr. Surmeier’s group now offers hope for a new therapy that could block the influx of calcium into dopaminergic (DA) neurons and prevent the progression of PD.
It had been shown previously by the same group that the activity of Cav1.3 (L-type) calcium channels, which allow calcium to enter the cytoplasm of the cell, was not necessary for the functioning of DA neurons, specifically for their pacemaking activity (Guzman et al, 2009). Furthermore, blocking these channels with a chemical called isradipine led to a rescue of DA neuron loss in a neurotoxin mouse model of PD (Chan et al, 2007). So what is it about these channels that is so bad for these highly active DA neurons?
To answer this question, the lead author of the paper, Jamie Guzman, created transgenic mice expressing the redox-sensitive version of GFP (roGFP) under the TH promoter. Upon getting oxidized, roGFP shifts it 490/400 nm excitation spectrum and can be differentiated from non-oxidized roGFP. Furthermore, the construct contained a matrix targeting sequence designed to localize the protein to the mitochondria, which have been centrally implicated in mechanisms of PD pathology. The team then used these mito-roGFP transgenic mice to measure the level of oxidation present in DA neurons from their brains.
Guzman et al found that substantia nigra (SN) DA neurons exhibit a much higher basal level of cellular oxidative stress than DA neurons from the neighboring ventral tegmental area (VTA) which lack these L-type calcium channels. This suggested that Ca2+ influx may somehow be responsible for increased levels of oxidative stress. Indeed when they blocked L-type channels with isradipine, the level of oxidative stress decreased dramatically in SN DA neurons. The research group went on to show that a curious phenomenon, mitochondrial uncoupling, occurs in SN DA neurons as a compensatory mechanism against increased levels of oxidative stress. By uncoupling the mitochondrial electron transport chain from the production of ATP, these neurons are able to reduce the production of toxic reactive oxygen species (ROS) which are a byproduct of the respiratory chain. The authors demonstrated that the uncoupling events, measured by fluctuations in the mitochondrial membrane potential, were dependent on Ca2+ influx and levels of ROS. Again, blocking L-type calcium channels decreased the incidence of these uncoupling events, presumably by reducing oxidative stress levels in the neurons. The data suggests that Ca2+ influx, oxidative stress, and mitochondrial uncoupling are intricately linked together.
The most interesting part of this study was the connection that the authors found between these processes and DJ-1, a protein implicated in Parkinson’s. People with a homozygous loss-of-function DJ-1 mutant genotype develop an early-onset form of PD. To test the role of DJ-1 Guzman et al used a mouse with the DJ-1 gene knocked out. DJ-1 knockout DA neurons exhibited very low levels of mitochondrial uncoupling and correspondingly higher levels of oxidative stress, suggesting DJ-1 might somehow be regulating the mitochondrial response to oxidative stress. Amazingly, blocking the L-type calcium channels completely rescued this oxidative stress effect. Earlier work had implicated DJ-1 in redox signaling pathways and upregulation of antioxidant proteins (Kahle et al, 2009). Thus the authors decided to check whether DJ-1 was exerting it’s effects at the gene expression level. They observed that DJ-1 knockouts had lower transcript levels of some of the key mitochondrial uncoupling proteins, but expression of antioxidant enzymes was unaffected. The authors concluded that a loss of DJ-1 functionality weakened the compensatory mechanisms in mitochondria, making the DA neuron much more vulnerable to oxidative stress in the SN. This may explain why people with homozygous DJ-1 mutations end up suffering from early-onset PD.
The key finding of this study was that the intracellular impairments resulting from a DJ-1 mutation can be reversed by using L-type calcium channel blockers such as isradipine. Isradipine belongs to a class of molecules known as dihydropyridines which are widely prescribed as a treatment for high blood pressure in humans. Perhaps most importantly, these drugs can cross the blood-brain barrier. Recent epidemiological studies support a decreased risk of developing PD in chronic users of dihydropyridines (Becker et al, 2008; Ritz et al, 2010). This offers a ray of hope for PD patients. In fact, clinical trials using isradipine in PD patients have already begun and are in Phase II, with Phase III trials expected to be launched in the next year.
The research by Dr. Surmeier’s group is remarkable in many ways in that it not only elucidated the mechanism underlying the function of a protein implicated in PD, but also identified a drug target and a potential drug candidate that is already in clinical trials. However a lot more needs to be understood about L-type calcium channels and their role in genetic as well as sporadic forms of PD. It may well be that blocking these channels only offers protection in DJ1 knockout and neurotoxin models of PD. Lets keep our fingers crossed that this is not the case.
A link to the Guzman et al study can be found here: http://www.nature.com/nature/journal/v468/n7324/full/nature09536.html