Selective Expression of Mutant SOD1 Associated with Familial ALS in Postnatal Motoneurons Does Not Cause Motoneuron Disease

Abstract & Commentary

Source: Lino MM, et al. Accumulation of SOD1 mutants in postnatal motoneurons does not cause motoneuron pathology or motoneuron disease. J Neurosci. 2002;22:4825-4832.

A major advance in studying the pathogenesis of amyotrophic lateral sclerosis (ALS) was the discovery that approximately 10% of the total number of ALS cases are familial and of these approximately 20% are attributable to point mutations in cytosolic Cu/Zn superoxide dismutase (SOD1). The process by which SOD1 leads to motor neuron degeneration remains unclear, but studies involving use of transgenic animals clearly point to a toxic gain of function model for the abnormal protein. Mice with targeted deletions of both SOD1 alleles do not develop spontaneous motoneuron loss. In contrast, transgenic mice overexpressing mutant SOD1 develop spontaneous motoneuron degeneration and represent an excellent animal model of the disease.

In prior studies, 3 familial ALS-associated SOD1 mutants—(G93A), (G85R), and (G37R)—were introduced into mice using a human SOD1 mini gene approach, leading to a ubiquitous high level of the human transgene in all cell types within the central nervous system. In these mice, mutant SOD1 accumulated at high levels in many and, possibly, all tissues and cell types. However, it seemed probable that the accumulation of mutant protein in motoneurons was the main cause of the disease, since pathology and dysfunction affected primarily motoneurons. The present study examined the effects of expressing the G93A and the G85R SOD1 mutations as transgenes using a Thy1 promotor that produces high constitutive expression if the transgene is in postnatal motoneurons, but not in astrocytes or microglia. Lino and colleagues made a number of notable observations. They noted that the mice did not develop motoneuron deficits or any evidence of neuromuscular denervation. Furthermore, there was no evidence of loss of motoneurons or any astrogliosis. The mice did not develop ubiquitin-stained protein deposits that are characteristic of mice with the G93A and G85R mutations. Lino et al demonstrated that the Thy1-SOD1 lines accumulated levels of SOD1 that were sufficient to cause disease in the other transgenic mouse models expressing SOD1 in all cell types.

Lino et al also carried out a further interesting experiment. They examined the effects of crossing their lines of transgenic mice that have expression only in motoneurons with the G93A mouse lines that have ubiquitous expression of the mutant SOD1. This should have led to 2-fold load of mutant SOD1 restricted to neurons in the double transgenic mice. If the motoneurons are the site at which mutant SOD1 affects disease onset and progression, then the double transgenic mice would be expected to have accelerated pathology and disease. Lino et al, however, observed no difference in the rate of clinical deficits or motoneuron loss in these double transgenic mice. This provides further evidence that the accumulation of mutant SOD1 in motoneurons is not necessary to induce motoneuron disease or motoneuron pathology.

Commentary

The present findings are extremely interesting because they demonstrate that the expression of mutant SOD1 within motoneurons is not sufficient to result in motoneuron disease. This suggests that the disease process requires an interaction with mutant SOD1 in other cell types. Lino et al have also demonstrated that the accumulation of intracellular deposits does not occur in these mice. This also suggests that this depends on an interaction with other cell types. Leading candidates would be expression in astrocytes and microglia. A possibility that astrocytes may play a role in disease pathogenesis has been suggested by observations that reduced glutamate uptake by the astrocytic glutamate transporter appears both in human ALS, as well in the transgenic mouse models. A prior study of expression of mutant SOD1 confined to astrocytes showed that expression only in this cell type also was insufficient to cause motoneuron pathology. Both astrocytosis as well as an increased number of microglia occurred in the transgenic mouse models of ALS. Augmenting chemokine receptor-4 signaling in microglia can lead to an excess release of tumor necrosis factor-alpha, as well as extracellular glutamate and motoneuron pathology. It appears possible that microglia may be the cells by which mutant SOD1 acts to initiate motoneuron pathology in the transgenic mouse models. This process may subsequently lead to mitochondrial pathology, the accumulation of deposits, a reduction in axonal transport, and cause paralysis and death. The fact that motoneurons have long axonal projections and appear to be particularly sensitive to excess glutamate may account for their vulnerability. Axonal transport also puts a high-energy load on neurons, and mitochondrial pathology is an early feature of the motoneurons in the transgenic mouse models of familial ALS. These findings provide further evidence that new therapeutic approaches targeting microglial activation might be particularly fruitful for the treatment of ALS. —M. Flint Beal

Dr. Beal, Professor and Chairman, Department of Neurology, Cornell University Medical College, New York, NY, is Editor of Neurology Alert.