Electrical activity dwindles in cells long before movement issues become visible.
It’s an unsettling thought: You could be walking around for 20 years developing Parkinson’s disease and not even know it.
And once symptoms appear, it’s too late for a cure.
What if a therapy that treats the root causes of Parkinson’s, not just the symptoms, could be started earlier?
Researchers in the School of Medicine at The University of Texas Health Science Center at San Antonio are studying changes in Parkinson’s-affected cells at various stages of the disease, long before any symptoms are evident. They describe the changes in an April issue of the Journal of Neuroscience.
The hope of the research is twofold: 1) gain understandings that can be used to formulate a drug to arrest the disease at a halfway point, and 2) lengthen the time when patients with Parkinson’s can lead healthy, productive lives.
“For the first time we are getting a look at what’s going on in the time window before the disease visibly takes hold but while changes are occurring,” said study senior author Michael Beckstead, Ph.D., assistant professor of physiology and a member of the Barshop Institute for Aging and Longevity Studies at the UT Health Science Center.
Parkinson’s is marked by the degeneration and death of cells called dopamine neurons. These neurons are found in a brain structure called the substantia nigra. The Health Science Center researchers studied mice in which only these neurons are affected by a genetic mutation.
The MitoPark mouse, as it is called, is designed so that mitochondrial activity is hampered just in dopamine neurons of the substantia nigra. Mitochondria produce energy for our cells, and since these mice have impaired mitochondria, their dopamine neurons don’t make energy efficiently.
Mimics human Parkinson’s
At first, the mice are completely normal, but as weeks and months go by, the mutation causes their dopamine neurons to slowly become sick and die off. “It’s a progressive model in that these changes don’t take place overnight,” Dr. Beckstead said. “This makes it like the human disease, which is thought to be somewhere in the range of a 20-year process before symptoms become evident.”
In MitoPark mice, behavioral symptoms such as tremor start to manifest when the mice are about 20 weeks old. The UT Health Science Center study assessed functional status at time points before that — comparing dopamine neuron function at 6-10 weeks of age with function at 11-15 weeks of age and function at 16-plus weeks.
Timeline of decline
With these comparisons, the researchers constructed a timeline of functional decline in the dopamine neurons. They observed changes in three categories:
Smaller dopamine neurons
Reduced communication between the neurons
Impaired electrical activity of the neurons
“Pretty much everything we measured declined in these cells,” Dr. Beckstead said. “It was really remarkable how everything we studied changed. It was a general decline, and these changes were all occurring before the animals were symptomatic, before you could detect any sort of deficit in their movement.”
In older mice starting to display the abnormal movements of the disease, the scientists made another observation — heightened gene expression to increase the electrical activity in the dopamine neurons.
“This is a late occurrence in the disease process,” Dr. Beckstead said. “We believe the cells are trying to compensate for the declining electrical activity. That’s probably how humans are able to be free of symptoms for so long when they have Parkinson’s, even though 30 percent or more of their dopamine neurons have died out.”
The study results aren’t going to translate into a clinical therapy any time soon, but such findings offer the promise that one day the root cause of Parkinson’s may be understood and treated.
Current treatments for Parkinson’s disease are all symptomatic. They focus on improving the movement deficits and making the patients more comfortable.
“We don’t have any treatments right now that actually affect the disease process,” Dr. Beckstead said. “The reason we don’t have any is we don’t understand what’s going on in the early stages of this disease. Studies such as ours will help fill in those knowledge gaps.”
About this genetics research
Funding: Grants from the William and Ella Owens Medical Research Foundation, the National Institutes of Health and the U.S. Department of Veterans Affairs supported this work.
Source: Will Sansom – UT Health Science Center San Antonio Image Credit: The image is credited to Geoff B Hall and is in the public domain. Original Research:Abstract for “Dopaminergic Neurons Exhibit an Age-Dependent Decline in Electrophysiological Parameters in the MitoPark Mouse Model of Parkinson’s Disease” by Sarah Y. Branch, Cang Chen, Ramaswamy Sharma, James D. Lechleiter, Senlin Li, and Michael J. Beckstead in Journal of Neuroscience. Published online April 6 2016 doi:10.1523/JNEUROSCI.1395-15.2016
Dopaminergic Neurons Exhibit an Age-Dependent Decline in Electrophysiological Parameters in the MitoPark Mouse Model of Parkinson’s Disease
Dopaminergic neurons of the substantia nigra (SN) play a vital role in everyday tasks, such as reward-related behavior and voluntary movement, and excessive loss of these neurons is a primary hallmark of Parkinson’s disease (PD). Mitochondrial dysfunction has long been implicated in PD and many animal models induce parkinsonian features by disrupting mitochondrial function. MitoPark mice are a recently developed genetic model of PD that lacks the gene for mitochondrial transcription factor A specifically in dopaminergic neurons. This model mimics many distinct characteristics of PD including progressive and selective loss of SN dopamine neurons, motor deficits that are improved by L-DOPA, and development of inclusion bodies. Here, we used brain slice electrophysiology to construct a timeline of functional decline in SN dopaminergic neurons from MitoPark mice. Dopaminergic neurons from MitoPark mice exhibited decreased cell capacitance and increased input resistance that became more severe with age. Pacemaker firing regularity was disrupted in MitoPark mice and ion channel conductances associated with firing were decreased. Additionally, dopaminergic neurons from MitoPark mice showed a progressive decrease of endogenous dopamine levels, decreased dopamine release, and smaller D2 dopamine receptor-mediated outward currents. Interestingly, expression of ion channel subunits associated with impulse activity (Cav1.2, Cav1.3, HCN1, Nav1.2, and NavB3) was upregulated in older MitoPark mice. The results describe alterations in intrinsic and synaptic properties of dopaminergic neurons in MitoPark mice occurring at ages both before and concurrent with motor impairment. These findings may help inform future investigations into treatment targets for prodromal PD.
SIGNIFICANCE STATEMENT Parkinson’s disease (PD) is the second most diagnosed neurodegenerative disorder, and the classic motor symptoms of the disease are attributed to selective loss of dopaminergic neurons of the substantia nigra. The MitoPark mouse is a genetic model of PD that mimics many of the key characteristics of the disease and enables the study of progressive neurodegeneration in parkinsonism. Here we have identified functional deficits in the ion channel physiology of dopaminergic neurons from MitoPark mice that both precede and are concurrent with the time course of behavioral symptomatology. Because PD is a progressive disease with a long asymptomatic phase, identification of early functional adaptations could lay the groundwork to test therapeutic interventions that halt or reverse disease progression.
“Dopaminergic Neurons Exhibit an Age-Dependent Decline in Electrophysiological Parameters in the MitoPark Mouse Model of Parkinson’s Disease” by Sarah Y. Branch, Cang Chen, Ramaswamy Sharma, James D. Lechleiter, Senlin Li, and Michael J. Beckstead in Journal of Neuroscience. Published online April 6 2016 doi:10.1523/JNEUROSCI.1395-15.2016