Focusing on glial cells to overcome an intractable disease, ALS
24 October 2008 (Volume 3 Issue 10)
Amyotrophic lateral sclerosis (ALS) is a devastating disease. Once ALS develops, the motor neurons that control the movement of muscles gradually start to die off, causing paralysis of the muscles in the hand and leg. The patient suffers from difficulty in using arms and legs, and in eating food and speaking. In about two to five years after the development of ALS, the muscles that control breathing are paralyzed, necessitating the support of a respirator. However, because the senses, memory, and cognitive functions remain normal, the patient is conscious of the progression of the disease. Unfortunately, no effective treatment has been found. So far, research into understanding ALS has focused mainly on motor neurons. However, Koji Yamanaka, Unit Leader, and colleagues at the Brain Science Institute has focused on cells neighboring the motor neurons, and have met with success in their discovery that the glial cells cause damage to the nerve cells, thus accelerating the progression of the disease. This discovery shows great promise in the development of new treatments to prevent the progression of ALS.
ALS, an incurable disease that exclusively destroys motor neurons
In the spring of 1939, Lou Gehrig, a Major League Baseball player for the New York Yankees in the US, was mired in a prolonged batting slump. His fans and team-mates were very surprised because he was a real slugger, who enjoyed many seasons with high batting averages; his batting record included 23 grand slam home runs, a Major League record, and a consecutive game-playing streak of 2130. He was called “Iron Horse,” but it was ALS that prevented him from continuing his playing streak. Lou Gehrig retired in June that year. Two years later he died young, at the age of 37 years.
In the US, ALS is known as ‘Lou Gehrig’s disease’ and is one of the neurodegenerative diseases caused by the gradual death of nerve cells. In Alzheimer’s disease, which is a well-known neurodegenerative disease, the patient develops dementia as a result of the gradual death of memory-related nerve cells. In ALS, in contrast, the patient becomes paralyzed because of the gradual death of the motor neurons in the brain and the spinal cord that control the muscles throughout the body.
There are about 6,000 patients with ALS and it is estimated that about 2,000 people may develop ALS every year in Japan. Patients with ALS develop the disease mostly at about 60 years of age, but young people can be affected, like Gehrig.
About 10% of patients with ALS develop the disease because they have inherited the causative genes, but no abnormal genes were found in the remaining 90%. “In other words, anybody can develop ALS,” says Yamanaka, who has worked as a neurologist and has treated patients with ALS.
Neurologists are the medical doctors who have been trained in the diagnosis of diseases of brain, spinal cord, and muscle, and their treatment with drugs. In fact, however, there are many other diseases that cannot be treated with drugs because the causes are unknown. “I faced a big dilemma in clinical practice, seriously thinking, ‘What can I provide for patients with ALS?’ So, I thought I would like to elucidate the cause of the neurodegenerative disease to develop new cures.”
Yamanaka trained and worked as a neurologist for four years. Then he devoted himself to basic research and started the study on ALS in 2001. Why did he select ALS as his subject of research? “I chose ALS because it is an incurable disease. ALS progresses quickly, and the symptoms of the patient worsen day by day. From the time the patient makes a clinical visit, he or she will be unable to walk within the first year, will be bed-ridden within the following year, and won’t be alive within three years from the first visit. I was greatly motivated by shocking experiences when I was responsible as a neurologist for treating patients with ALS.”
Focusing on cells surrounding motor neurons
In 1993 there was a discovery that greatly contributed to the development of research into ALS. It was found that the relatives of patients with ALS had mutant-type SOD1 genes, where SOD1 is an enzyme that is capable of detoxifying active oxygen. About 2% of patients with ALS are estimated to develop the disease because of mutant-type SOD1 genes.
Mutant-type SOD1 genes were then used to create a model in mice that developed ALS. The model mice served to provide the first effective method for investigating the details of the process by which ALS develops and progresses. How do mutant-type SOD1 genes cause ALS to develop? Model mice from which normal SOD1 genes had been removed were also created, but the mice did not develop ALS. This disproves the possibility that ALS develops as a result of the loss of function of SOD1 genes and hence a loss of the ability to detoxify active oxygen. On the basis of a subsequent study, researchers now think that the SOD1 proteins change shape because of the mutation of the gene, acquiring unknown toxic properties instead of functioning as enzymes, and the accumulation of such toxic proteins causes damage to motor neurons, leading to the development of ALS.
“In order to look for clues to alleviate damage to motor neurons and to slow the onset of ALS, mouse models of ALS were used in experiments. This achieved some results, but not amazing ones. Researchers began to think that they should focus on targets other than motor neurons. This was just when I started my study on ALS in 2001.”
Among the cell type other than neurons, astrocyte cells, most abundant glial cells, are known to support neurons by providing nutritional factors, whereas microglial cells, in turn, clear damaged or dead cells.
In a sense, focusing on glial cells to study ALS was challenging.
“It was well known that an increased number of glial cells are found in the spinal cord lesion of someone who has died from ALS. This used to be considered a secondary phenomenon arising from the death of motor neurons, because glial cells were not considered to be an important contribution to ALS.”
Why, then, did Yamanaka focus on the cells around motor neurons, such as glial cells? “Mutant-type SOD1 genes exist not only in motor neurons but also in all the cells in ALS model mice. Thus it is natural to think that mutant-type SOD1 genes in the cells around the motor neurons, for example in glial cells, are related in some way to ALS. The idea gave me an opportunity to start the study on ALS.”
Unexpected research findings
Yamanaka and colleagues began to create ALS model mice from which their mutant-type SOD1 genes had been removed from only certain kinds of cell groups in order to investigate how mutant-type SOD1 genes function in the cells around motor neurons. In 2003 the team successfully created model mice in which mutant-type SOD1 genes have been removed exclusively from the motor neurons.
“It was technically impossible to remove mutant-type SOD1 genes from all the motor neurons, but we succeeded in removing mutant-type SOD1 genes from 30–50% of them. However, we thought that the mice would not develop ALS because the causative genes had been removed in such large amounts. The experimental results were surprising: a delay was observed in the onset of ALS, but ALS progressed at the same speed.”
Many researchers have studied on ALS over the years, in the hope of finding an effective way to slow its progression, by selecting motor neurons as their target for treatment. The experimental results, however, suggest that focusing only on motor neurons in this way achieves only a delay in the time of onset of ALS, but provides no effective method of stopping ALS from progressing. “Patients visit a clinic after they notice the onset of ALS. Thus it is no use providing medical treatment to such patients to delay the time of onset of ALS.”
Glial cells were found to be key components in the progression of ALS
Figure 1: Effect of removing mutant-type SOD1 genes from astrocytes.
In ALS model mice, when mutant-type SOD1 genes were removed exclusively from their astrocytes they developed the disease almost at the same time, but showed a slower progression and doubling in the duration of illness, in comparison with ALS mice with mutant-type SOD1 genes in all of their cells.
Yamanaka and his colleagues also succeeded in creating model mice in which mutant-type SOD1 genes had been removed from the microglial cells. There was no change in the onset of ALS, but its progression was definitely slowed.
Furthermore, the experiment with the model mice in which mutant-type SOD1 genes had been removed from the astrocytes also showed a slower progression of the disease, with a doubling of the duration of the illness (Fig. 1).
This discovery by Yamanaka and his unit members suggests that motor neurons have the main role in the development of ALS, but there are two different leading players promoting the progression of ALS. In other words, microglial cells and astrocytes are closely related to the progression of ALS. “It is now considered that astrocytes activate the microglial cells, and the activated microglial cells, in turn, release toxic substances such as proteins that produce nitric oxide or inflammation, thus causing further damage to motor neurons and accelerating the progression of ALS”(Fig. 2 and Fig. 3).
Yamanaka and his colleagues demonstrated, for the first time ever, the importance of microglial cells and astrocytes as a target of treatment for stopping ALS from progressing. However, they used ALS model mice into which mutant-type SOD1 genes had been introduced, but only 2% of patients with ALS develop the disease as a result of mutations in SOD1 genes. Their final goal is to develop a new cure that will be effective for almost all hereditary ALS types as well as non-hereditary types. “Pathological changes are observed in astrocyte or microglial cells in patients with ALS who have developed non-hereditary disease. In these patients with ALS, it is considered that glial cells have a toxic effect on the motor neurons and stimulate the progression of the disease. To confirm this, we are planning to create model mice that develop ALS resulting from causes other than mutant-type SOD1 genes.”
Figure 2: Spinal cord lesion of an ALS model mouse.
Motor neurons (blue) are surrounded by activated microglial cells (red) and astrocytes (green).
Working to overcome ALS
In February 2008, Yamanaka released his recent findings to the press, saying that two types of glial cell are effective as targets of treatment for ALS, and received a great response. He received emotional requests from patients and their relatives asking for the earliest development of effective cures. For example, he received an autograph letter from a patient suffering from paralysis in his hands as a result of ALS. Some patients said that they would be willing to serve as a subject for experiments.
Now that we understand that the next targets are astrocyte or microglial cells, the next step will be to determine the molecular mechanisms occurring in the abnormal astrocyte or microglial cells that are producing the progression of ALS, and to identify the target molecules for treatment.
Yamanaka asserts, “Besides SOD1 genes, there should be a definite difference between ALS model mice and normal mice, for example, in terms of the types and amounts of genes or molecules that act in the astrocyte and microglial cells, or the places in which these cells are functioning. By investigating these factors, we should be able to elucidate the mechanism that promotes the progression of ALS and identify the target molecules for treatment.”
Figure 3: Glial cells accelerate the progression of ALS.
The research findings now revealed that astrocyte cells with mutant-type SOD1 genes activate microglial cells; in turn, the activated microglial cells release proteins that produce nitric oxide or others (cytokines) that cause inflammation, thus causing damage to motor neurons and accelerating the progress of ALS. There is a possibility that the motor neurons that have a mutant-type SOD gene cause the activation of microglial cells, or that astrocytes directly produce toxic effects on motor neurons.
Once these target molecules are known, their functions can be controlled so that glial cells will return to their normal state, which will enable the development of cures that can slow the progression of ALS.
The study by Yamanaka has raised expectations for regenerative treatment for ALS. It has been considered that if the treatment of ALS is targeted only to motor neurons, regenerative methods in which cells are transplanted to restore function cannot be effective because of the low rate at which transplanted cells form a correct network system and because of the low rate of extension of axons that transmit instructions from motor neurons to muscles.
“An axon extends its tip only by 1 mm a day. Some axons have a length of 1 m, which means that up to 1,000 days (about three years) are necessary for an axon to reach its final length. This is too long: by that time ALS will have progressed to its terminal stage.”
If glial cells are closely related to the progression of ALS, as Yamanaka has indicated, glial cells, if successfully transplanted, should have a therapeutic effect. “Glial cells can immediately start to function at the place where they are transplanted, and stop ALS from progressing.”
Contributing to a new perspective on Alzheimer’s disease
Figure 4: Members of the Yamanaka Research Unit.
Many researchers with various backgrounds in fields such as medicine, biochemistry, and embryology have joined together to examine the cause of ALS from a diversity of viewpoints.
Alzheimer’s and Parkinson’s diseases are also classified as neurodegenerative diseases that gradually destroy nerve cells. “Most current studies on neurodegenerative diseases other than ALS also focused exclusively on nerve cells. Our research findings on ALS have shown that toxic substances released from abnormal glial cells can destroy nerve cells, and this fact is significantly affecting research trends in other neurodegenerative diseases. A therapeutic agent that makes glial cells return to normal will possibly be very effective in treating not only ALS but also other neurodegenerative diseases such as Alzheimer’s or Parkinson’s.”
When can a therapeutic agent against ALS be developed? “First of all, our mission is to elucidate the detailed mechanism of ALS. Then we will proceed with a study of ALS so that a therapeutic agent against ALS can be verified in a clinical test in ten years.”
Yamanaka and members of his unit are making a steady progress on their basic studies towards overcoming ALS, based on compelling requests from patients with the disease.