Running out of dopamine: treating Parkinson’s disease, from L-dopa to stem cells

Part 1: When the messenger runs out

1Margaret first noticed that her handwriting was shrinking. Then her right hand began to shake when it was resting on her knee, and getting up from a chair took a strange amount of effort. At 62, she was diagnosed with Parkinson’s disease, a condition that affects about ten million people worldwide. It usually begins after the age of 60 and becomes slowly worse over many years. There is no cure, but treatments can control the symptoms, often for a long time.

2The movement symptoms of Parkinson’s disease are caused by the death of neurones in a small region of the midbrain called the substantia nigra. These neurones release the neurotransmitter dopamine. Their axons run forward to a region deep inside the cerebrum called the striatum, part of which is known as the putamen. By the time symptoms appear, between 60 and 80 per cent of the dopamine neurones in the substantia nigra have usually already been lost.

3Dopamine released in the striatum helps the brain to select which movements to make and to start them smoothly. With too little dopamine, people find it hard to begin a movement, their muscles become stiff and their movements are slow and small. Many have a tremor when their muscles are at rest. Parkinson’s disease is not only a movement disorder, though. Loss of the sense of smell, disturbed sleep and depression often begin years before the tremor.

Messages across the gap

4When an action potential reaches the end of a dopamine neurone, voltage-gated calcium ion channels in the presynaptic membrane open and calcium ions diffuse into the terminal. This causes vesicles containing dopamine to move to the presynaptic membrane and fuse with it, releasing dopamine into the synaptic cleft by exocytosis. Dopamine diffuses across the cleft and binds to dopamine receptors on the postsynaptic membrane, changing the activity of the neurones in the striatum.

5Dopamine does not stay in the cleft for long. Most of it is taken back into the presynaptic neurone by transporter proteins and stored again in vesicles. Some is broken down by enzymes called monoamine oxidases, or MAOs, which are found inside neurones and the cells that surround them. Removing dopamine quickly means that each signal is short, so the postsynaptic neurones can respond to the next one. Drugs that block the dopamine transporter, such as cocaine, have the opposite effect and leave dopamine in the cleft for much longer.

6Loss of dopamine neurones can be seen using a PET scan. The patient is injected with F-DOPA, a form of L-dopa labelled with a radioactive isotope of fluorine. Dopamine neurones take up F-DOPA and convert it to dopamine, which they store in their terminals in the striatum. The scanner detects the radiation given off, and a computer builds up an image. In a healthy brain the striatum on each side shows a bright, comma-shaped area. In Parkinson’s disease the signal is much weaker, and the loss usually starts in the putamen.

7In the 1960s, doctors discovered that the symptoms could be reversed by L-dopa, the amino acid from which neurones make dopamine. Dopamine itself cannot be given as a drug because it cannot cross the blood–brain barrier, the layer of tightly joined cells lining the blood capillaries of the brain. L-dopa is carried across this barrier by transporter proteins. Inside the brain, an enzyme in the surviving dopamine neurones converts it to dopamine.

8The same enzyme is found in many other tissues, so most of a dose of L-dopa would be converted to dopamine before it reached the brain, causing nausea. It is therefore given with carbidopa, a drug that inhibits the enzyme but cannot cross the blood–brain barrier. After several years of treatment, many patients find that each dose wears off sooner, and some develop sudden, uncontrolled movements. As more neurones die, fewer are left to store dopamine and release it steadily.

9Other drugs work in different ways. MAO-B inhibitors such as selegiline block the enzyme that breaks down dopamine in the brain. Dopamine agonists are molecules with a shape similar enough to dopamine to bind to and activate dopamine receptors, even though they are not dopamine. Agonists last longer than L-dopa but can cause side effects such as sleepiness and, in a small number of people, compulsive gambling or shopping.

10Much of what is known about the disease came from an accident. In 1982, several young people in California injected a home-made synthetic drug that was contaminated with a chemical called MPTP. Within days they had severe symptoms of Parkinson’s disease. In the brain, MPTP is converted to a toxic substance that is taken up by the dopamine transporter, so it builds up in dopamine neurones and kills them. MPTP is now used to produce the disease in laboratory animals so that new treatments can be tested.

11In most patients, though, the cause is not known. A few cases are inherited, but most involve a combination of many genes and the environment. What almost all patients share is a protein called alpha-synuclein, which is normally found in presynaptic terminals. In Parkinson’s disease, alpha-synuclein folds into an abnormal shape. The misfolded molecules stick together into clumps inside neurones and can cause normal alpha-synuclein molecules to misfold as well. The clumps seem to spread slowly from one neurone to the next, which may explain why the disease gradually affects more regions of the brain.

12"Every drug we use today does the same basic job," says Professor Daniel Osei, a neurologist who runs an illustrative cell therapy programme. "It tops up dopamine signalling for a few hours. None of them replaces the neurones that have died, and none of them slows the disease. That is why so many groups are now trying to put new dopamine neurones back into the brain." He estimates that a successful cell therapy could reduce, or even replace, daily tablets for many patients.

Part 2: Replacing the lost neurones

13The idea is not new. In the 1980s and 1990s, surgeons transplanted tissue from the brains of aborted fetuses into the putamen of people with Parkinson’s disease. The results varied. A few patients improved so much that they stopped taking L-dopa for more than ten years, while others gained little or developed uncontrolled movements. Each operation needed tissue from several fetuses, which raised serious ethical concerns and made the treatment impossible to offer widely.

14Stem cells could provide an unlimited supply. Embryonic stem cells are pluripotent, so they can divide repeatedly and can develop into any type of cell in the body. Induced pluripotent stem cells, or iPS cells, are made from adult cells such as skin cells by introducing genes for a small number of transcription factors. By adding a sequence of signalling molecules over several weeks, researchers can make either type of stem cell develop into immature dopamine neurones.

15Cells grown from another person’s stem cells have different antigens on their surface from the patient’s own cells. Patients who receive them must take immunosuppressant drugs for a year or more, which increases the risk of infection. iPS cells made from the patient’s own skin would avoid this problem, but producing and testing a separate batch of cells for every patient takes many months and is very expensive. Some groups are therefore building banks of iPS cells from donors whose antigens match those of large numbers of patients.

16New treatments are first tested on rats in which the dopamine neurones on one side of the brain have been destroyed with a toxin. When these rats are given a drug that increases dopamine release, they turn repeatedly in circles towards one side. In an illustrative study, rats that received transplanted neurones into the damaged striatum made a mean of 2 turns per minute after 16 weeks, compared with 8 turns per minute in rats that had surgery without cells.

17In an illustrative early human trial, 12 patients had immature dopamine neurones injected into the putamen on both sides of the brain. After 18 months, PET scans showed that F-DOPA uptake in the putamen had increased. Movement symptoms were scored by doctors when patients had not taken their medication for 12 hours, using a scale on which a higher score means more severe symptoms. The mean score fell from 46 to 34. No tumours were found.

18Tumours are the main safety concern. Any pluripotent cells left in the transplant could keep dividing, so the cells are sorted carefully before surgery to remove them. There are other limits too. Some patients from the fetal tissue trials were found, many years later, to have clumps of alpha-synuclein inside their transplanted neurones. New neurones also do nothing for the loss of smell, sleep problems or depression, which are caused by damage to other parts of the nervous system.

19For some patients, a different technique called deep brain stimulation is already used. Thin electrodes are placed in a region of the brain that helps to control movement, and a battery-powered device under the skin of the chest sends a constant series of electrical pulses along them. This can greatly reduce tremor and stiffness and lower the dose of L-dopa needed, although, like the drugs, it does not replace the lost neurones or stop the disease from progressing.

20"The best transplant in the world will not help if the disease simply destroys the new cells," says Professor Osei. "In the long run we need to stop alpha-synuclein spreading. But if replacement cells can give a patient like Margaret ten good years without her symptoms getting worse, that would already be a huge step forward. The next trials will tell us how long the new neurones survive and whether the benefit lasts." He expects the first results from larger trials within a few years.