LONDON Using mice to study the pathology of Alzheimer¿s disease has not always been as illuminating as researchers would like. Even though several different transgenic mice now exist, each bearing mutations in a gene encoding a protein known to play a role in Alzheimer¿s disease, it has often been difficult to link the changes seen in these animals¿ brains with what happens in humans affected by the disease.
Not any more. A team of scientists in Switzerland has now shown that such mice have fewer neurons in exactly the same area of the brain that suffers loss of neurons in Alzheimer¿s disease in people. The discovery should speed the search for compounds that slow the progress of Alzheimer¿s disease, and will help researchers gain a better understanding of exactly what causes the death of nerve cells in this condition.
About two out of three cases of senile dementia are caused by Alzheimer¿s disease. Key symptoms include increasing memory loss, poor concentration and aggressive and agitated behavior. Eventually, the sufferer becomes totally dependent on others.
While about 90 percent of cases of Alzheimer¿s disease are sporadic, the remainder are clustered in families. Genetic studies of these families have made it possible to link the appearance of the disease to mutations in specific genes. Researchers hope that, because the changes in the brain are similar in sporadic and familial cases of the disease, study of these mutations will tell them more about what causes Alzheimer¿s and how its progression might be halted.
One of the mutations identified is in the gene encoding the amyloid precursor protein (APP). This fits with the knowledge that deposits of beta-amyloid, one of the breakdown products of APP, are found in the brains of people who died of Alzheimer¿s disease.
The Swiss team decided to look for loss of neurons in transgenic mice that express mutant APP, a strain known as APP23. These mice develop amyloid plaques in their brains as they age, and the plaques resemble those seen in the brains of people who have died of Alzheimer¿s disease.
Mathias Jucker, assistant professor of neurobiology in the department of neuropathology at the Institute for Pathology at the University of Basel, in Switzerland, told BioWorld International the mice ¿are a model for cerebral amyloidosis, which is only one aspect of Alzheimer¿s disease, but this means they are a very good model to study the significance of amyloid for the disease process.¿
Counting The Neurons
The researchers first estimated the amount of amyloid plaques in the mice¿s brains, and found that the transgenic mice had up to 30 percent of their brain volume occupied by amyloid plaques. They then used a technique called unbiased stereology to estimate the number of neurons in different parts of the animals¿ brains.
¿This technique involves systematically sampling through the brain and counting the number of neurons in small, defined tissue volumes,¿ Jucker said. ¿You can then multiply up, according to the total volume of the entire structure, and this gives you a very accurate estimate of the total number of neurons.¿
The method is called ¿unbiased,¿ he added, because the estimate is arrived at without making any assumptions about factors such as the degree to which the brain might have shrunk during preservation.
Jucker, with Michael Calhoun at the Institute for Pathology, and with their colleagues Matthias Staufenbiel and Bernd Sommer from the nervous system research department at Novartis Pharma Ltd., also of Basel, report the results in a letter in the Oct. 22 issue of Nature, titled ¿Neuron loss in APP transgenic mice.¿
¿When we estimated neuron loss, we found that it varied from zero to 25 percent,¿ Jucker told BioWorld Inter national. ¿We purposely investigated animals with a lot of plaques, and animals with only a few plaques. We got a beautiful correlation, plotted it and found that with increasing load of amyloid, you see increasing loss of neurons. In addition, we showed that neurons around the plaques appeared to be dying.¿
The correlation was found in the CA1 area of the hippocampus. Neuron loss also was observed in subregions of the neocortex parts of the brain that have been shown to suffer selective loss of neurons in people who suffered from Alzheimer¿s disease.
Biotech Companies On The Right Track
¿Our data show clearly that the process of amyloid plaque formation leads to neuron loss in these mice,¿ Jucker added. ¿This suggests that the approach being taken by many biotechnology companies, of trying to develop compounds that will inhibit or reduce amyloid formation, is a reasonable one.¿ Also, there is now a model that makes it possible to study the death of neurons caused by amyloid, he said.
Commenting on the finding, George Martin, professor of pathology and director of the Alzheimer¿s Disease Research Center at the University of Washington, in Seattle, said Jucker¿s and colleagues¿ findings are ¿most encouraging to those of us who are trying to develop mouse models of aspects of the pathology of dementias of the Alzheimer type.¿
Researchers have, he added, been ¿puzzled by the apparent failure to document loss of neurons in relevant portions of the brain in those models which are designed to produce deposits of beta-amyloid peptides. But the work of this outstanding group of Basel investigators now gives us such evidence.¿ The finding should help to expedite research to test compounds that may influence the load of beta-amyloid deposits in the brain, Martin said.
Again using unbiased stereology, Jucker, Calhoun and their colleagues already have confirmed that the buildup of amyloid plaques is also associated with loss of synapses and cholinergic changes, two other hallmarks of Alzheimer¿s disease.
The next questions for Jucker and colleagues have to do with what the findings mean for cognition, and how the mice¿s behavior may be linked to neuron loss, synaptic deficit and cholinergic alterations. ¿That is something we are working on, but this study will not be an easy one,¿ he said.