The laboratory of Erik P. Pioro, MD, PhD, in The Lerner Research Institute focuses on identifying molecular pathways leading to degeneration of motor neurons by studying two mouse models of motor neuron disease and ALS. These include the wobbler mouse, a spontaneous mutant closely resembling human ALS in which the responsible gene has just been identified, and a transgenic mouse overexpressing a mutated copper-zinc superoxide dismutase (SOD1) gene, as found in some patients with hereditary ALS.
As in ALS, protein breakdown appears to be abnormal in wobbler mouse spinal cord and brain, resulting in accumulation of potentially damaging intraneuronal inclusions. Figure 1 shows wobbler motor neurons in the cortex (arrows) with abnormal aggregates which contain ubiquitin-tagged protein. Identifying the proteins which accumulate and the mechanisms by which this happens may lead to novel treatments to prevent motor neuron degeneration. Efforts are underway in Dr. Pioro's lab to determine whether the wobbler gene product is directly responsible for forming these inclusions and how the accumulation of ubiquitinated proteins may be detrimental to the motor neurons. By identifying whether mechanisms involved in removing the accumulated proteins are defective would help to direct novel therapies for patients with ALS and other neurodegenerative diseases in which similar protein inclusions form, including Alzheimer's disease and Parkinson's disease.
The immune system has recently been implicated in the progression of ALS. Using SOD1 transgenic mice (mice genetically engineered to have an abnormal mutation of the human SOD1 gene), Dr. Pioro and Richard Ransohoff, MD, (Department of Neurosciences) are seeking to determine whether microglia, a key component of the CNS immune system, contribute to motor neuron de6+generation. Microglia are kept under control by the fractalkine receptor (CX3CR1) which if genetically removed, appears to allow them to be activated. Although this activation can sometimes be helpful, as in fighting an infection, it can also be harmful if excessive. By replacing CX3CR1 with a green fluorescent label in the SOD1 transgenic mice, the microglia can be clearly seen under a microscope.
Figure 2 shows normal small microglia (colored green) in the spinal cord of healthy mice (A), more numerous and larger microglia in the transgenic mouse with ALS (B) and very numerous and more activated appearing microglia in the ALS mouse in which the receptor has been completely removed. Drs. Pioro and Ransohoff have found that activation of microglia in SOD1 ALS mice results in increased motor neuron death and more rapid disease progression. Interestingly, this is most marked in male mice, similar to how ALS tends to affect males more frequently and more severely than females. Microglia in human ALS may also be causing motor neuron degeneration because CX3CR1 may not work normally in some individuals allowing their activation. Experiments are planned to characterize gene pathways involved in this microglia-mediated neuronal death so that novel treatments can be found to block it and slow ALS progression.
Another area of study involving the wobbler mouse includes retarding disease progression by incorporating a gene mutation (slow wallerian degeneration, WLDS) known to delay axonal degeneration and identifying the gene pathways responsible for such disease slowing. Wobbler mice expressing the WLDS gene (WLDS positive) retain a significant amount of muscle strength and bulk, and have less brain and spinal cord loss compared to wobbler mice without the WLDS gene (WLDS negative). As shown in Figure 3, wobbler mice negative for the WLDS gene have more degenerating spinal cord motor neurons (arrows in top left) and have a dramatic decline in grip strength (bottom) whereas those positive for the WLDS gene have fewer degenerating neurons (arrow in top right) and retain grip strength, even with some later recovery (bottom). Future studies will test whether muscle weakness in wobbler mice can be delayed or reduced after administration of the WLDS gene directly into the nerves or muscles of mice once disease has begun. Validating the effectiveness of this gene therapy in wobbler mice with ALS is necessary before proceeding to human clinical trials.