Apoptosis and cell death; Cytoskeleton and cell motility; Gene Expression; Molecular and Cellular Biology; Molecular genetics; Neurobiology; Neurodegenerative disorders; Neurology; Pathophysiology; Protein Folding; Signal Transduction; Toxicology; Transcription and Translation
My research is aimed at finding the cause and a cure for Parkinson’s disease.
Parkinson’s disease (PD) is defined by a characteristic set of locomotor symptoms (rest tremor, rigidity, bradykinesia and postural instability) that are believed to be caused by the selective loss of dopaminergic (DA) neurons in substantia nigra. The persistent difficulties in using animals to model this human disease suggest that human nigral dopaminergic neurons have certain vulnerabilities that are unique to our species.
One of our unique features is the large size of the human brain (1350 grams on average) relative to the body. A single nigral dopaminergic neuron in a rat brain (2 grams) has a massive axon arbor with a total length of 45 centimeters. Assuming that all mammalian species share a similar brain wiring plan, we can estimate (using the cube root of brain weight) that a single human nigral dopaminergic neuron may have an axon with gigantic arborization that totals 4 meters.
Another unique feature of our species is our strictly bipedal movement, which is affected by Parkinson’s disease, in contrast to the quadrupedal movement of almost all other mammalian species. The much more unstable bipedal movement may require more dopamine, which supports the neural computation necessary for movement.
The landmark discovery of human induced pluripotent stem cells (iPSC) made it possible to generate patient-specific human midbrain dopaminergic neurons to study Parkinson’s disease. A key problem for dopaminergic neurons is the duality of dopamine as a signal required for neural computation and a toxin as its oxidation produces free radicals. Our study using iPSC-derived midbrain dopaminergic neurons from PD patients with parkin mutations and normal subjects shows that parkin sustains this necessary duality by maintaining the precision of the signal while suppressing the toxicity. Mutations of parkin cause increased spontaneous release of dopamine and reduced dopamine uptake, thereby disrupting the precision of dopaminergic transmission. On the other hand, transcription of monoamine oxidase is greatly increased when parkin is mutated. This markedly increases dopamine oxidation and oxidative stress. These phenomena have not been seen in parkin knockout mice, suggesting the usefulness of parkin-deficient iPSC-derived midbrain DA neurons as a cellular model for Parkinson’s disease.
Currently, we are using iPS cells and induced DA neurons to expand our studies on parkin to idiopathic Parkinson’s disease. We are also utilizing the molecular targets identified in our studies to find small-molecule compounds that can mimic the beneficial functions of parkin. The availability of human midbrain DA neurons should significantly speed up the discovery of a cure for Parkinson’s disease.