Dustin Wakeman from Rush University Medical Center in Chicago, IL has used human embryonic to produce large quantities of “dopaminergic neurons.” While the phrase dopaminergic neurons does not roll off the tongue, it simply refers to a special type of neuron that is found in the brain, but is particularly concentrated in a part of the brain responsible for voluntary movement.
To properly understand Parkinson’s disease (PD) we must have some understanding of the portions of the brain and the parts of it responsible for voluntary movements. The surface of the brain is divided into lobes: the frontal lobes in the front, the temporal lobes on the side, occipital lobes in the back of the brain, and two parietal lobes on either side on top. These lobes are covered with little folds and turns of gray matter. The grooves are called sulci and the folds of gray matter are called gyri. Collectively, the surface of the brain is known as the cerebral cortex. At the base of the cerebral cortex, underneath all those surface sulci and gyri are a cluster of neurons known as the basal ganglia. The basal ganglia are a large group of clusters of the neurons (nuclei) at the base of the cerebral cortex that controls movement, coordination, and affects voluntary movement.
The basal ganglia consists of several parts includes the caudate nucleus, putamen, globus pallidus, subthalamic nucleus, and substantia nigra. The caudate nucleus and the putamen compose the striatum, a mass of gray matter located in front of the thalamus that is important or movement. The substantia nigra generates the neurotransmitter dopamine for voluntary movement and reward behaviors. The globus pallidus lies next to the putamen, just inside of it. Just below the thalamus are the subthalamic nuclei. The thalamus is a mass of gray matter than sits on top of the brain stem and relays auditory, visual, movement, and sensory information to the cerebral cortex.
To review about neurons, these are the cells in the nervous system that can make and transmit nerve impulses to other neurons. They are the cells responsible for thinking, learning and memory, personality and so on. Neurons talk to each other by releasing small molecules called neurotransmitters. Neurotransmitters bind to receptors embedded in the cell surfaces of their neighbors and either stimulate those neighbor neurons to make a nerve impulse or inhibit their neighbors from making a nerve impulse. The difference depends on the receptor that the neurotransmitter binds. In the case of the basal ganglia, the neurotransmitter is dopamine. There are two receptors for dopamine: the D1 receptor, which if bound by dopamine, will inhibit the impulse on the receiving nerve, and the D2 receptor, which excites action within the receiving nerve cell.
Voluntary movement is the result of a pathway that begins in the substantia nigra. The substantia nigra has neurons that extend into the striatum and make connections with neurons in the putamen and caudate nuclei. The release of dopamine by these neurons causes excitatory and inhibitory results, since some of the dopamine binds D1 receptors and some binds D2 receptors. Therefore, the input to the striatum is excitatory and inhibitory. The neurons of the striatum connect with neurons in the globus pallidum, which connect to neurons in the thalamus. The thalamus sends signals to the cerebral cortex. The entire pathway requires an exquisite balance of excitatory and inhibitory signals. To see an animation of this pathway, see here.
In a healthy brain, within dopamine-making neurons, the amino acid tyrosine is converted to levodopa by the enzyme tyrosine hydroxylase. Levodopa is then converted to dopamine by the enzyme dopa decarboxylase. The dopamine is released into the space between connecting neurons (the synaptic cleft). The dopamine either binds receptors on the surface of the neighboring neuron or is degraded by enzymes that degrade unbound dopamine (monoamine oxidase or catechol-O-methyl transferase).
Parkinson’s disease, which is a movement disorder, results from changes in the basal ganglia and its associated structures. Basal ganglia disorders are typically due to neurotransmitter changes that affect the output of the striatum into the globus pallidus as well as into the thalamus and cerebral cortex as well. The brain of a patient with Parkinson’s disease shows a breakdown in the connection between those neurons in the substantia nigra and the putamen portion of the striatum. Symptoms of Parkinson’s disease appear after 60% – 80% of these cells become impaired or die. It is not necessarily the loss of the cells that causes the disease, but rather the decrease in dopamine. Dopamine decrease comes from a decrease in the number of substantia nigra neurons. Reduced dopamine causes abnormal activity in the putamen, which cause the primary features of the disease. Typically, symptoms appear after striatal dopamine levels have decreased by 20% – 50% of normal levels.
When substantia nigra projections to the putamen have been impaired, the globus pallidus interna and subthalamic nucleus begin to function abnormally. The result is that the brain is no longer able to sufficiently control motor function.
Postmortem examination of the brains of PD patients shows that the neurons of the substantia nigra are filled with Lewy Bodies. Lewy Bodies are inclusions in the cytoplasms of neurons that are composed of a mess of proteins, including ubiquitin and alpha-synuclein.
Implanting new dopaminergic neurons into the substantia nigra of PD lab animals can help reverse the symptoms of PD. Embryonic stem cells can also be differentiated into large quantities of dopaminergic neurons. Therefore, they are a natural place to look as a treatment for PD. Unfortunately, a widely-discussed experiment several years ago used dopaminergic neurons made from human embryonic stem cells to treat PD in rats, but while the rats showed a reversal of symptoms, they also developed robust brain tumors (Roy N et al., Nature Medicine 12, 1259-68; Nov 2006). Other experiments showed similar problems (Chiba S et al., Stem Cells 26, 2810-2820, 2008). Therefore, a new protocol is necessary.
In an abstract presented at the 2012 Society of Neuroscience Meeting in New Orleans, LA, Dustin Wakefield used a new protocol to generate dopaminergic midbrain neurons from human embryonic stem cells. He utilized a floor-plate induction strategy that was published in 2011 in Nature (Kriks et al., Nature 2011). When laboratory rats and mice were given PD by means of a drug called 6-hydroxy-dopamine, they showed all the symptoms of PD. However, transplantation of the dopaminergic neurons into the substantia nigra of these animals reversed the PD symptoms.
In order to show that this strategy could work for human patients, Wakefield sought to do a very expensive experiment – induce PD in non-human primates and then treat them with these same cells. Neurons derived from human embryonic stem cells and implanted in these monkeys that exhibited the symptoms of PD not only fully matured into the same type of dopamine-producing neurons destroyed by the PD, they caused no tumors or other adverse effects.
Challenges related to cell survival, maturation, and tumor risk have stymied early efforts to realize the potential of human embryonic stem cells to provide new treatments for Parkinson’s disease, which is characterized by tremors and movement problems caused by the loss of dopamine-producing neurons in the brain. Because tremors and other types of uncontrolled movements have been seen in human PD patients when they were implanted with new dopaminergic neurons, this problem has continued to haunt regenerative therapies for PD patients. However, Wakefield and his colleagues seem to have provided a treatment that is devoid of these other problems. Thus Wakefield has shown that he can transplanted these human cells and watch them fully mature in animals without causing problems.
“We ended up with excellent stem cell graft survival and the development of true, mature dopamine cells in the parkinsonian monkeys, with no evidence of tumors,” Wakeman said. “This is significant because rigorous testing of stem cells in parkinsonian monkeys is essential to providing data to justify a Phase 1 clinical trial in humans.”