Art - Cleveland (J)

ALS is a progressive neurodegenerative disease occurring in middle to late life and characterized by the loss of large motor neurons of the spinal cord, brain stem and motor cortex. Dysfunction and death of these motor neurons causes muscle weakness and atrophy leading to paralysis and death in 3–5 years1. Approximately 10% of ALS cases are inherited in an autosomal dominant fashion. An estimated 15–20% of these familial ALS (FALS) patients have missense mutations in the gene encoding SOD1 (refs 2, 3), a metalloenzyme that catalyzes the formation of hydrogen peroxide through dismutation of superoxide anions4. SOD1 is thought to protect against cellular damage induced by oxygen radicals5,6. The mechanism(s) through which mutations in SOD1 lead to lateonset motor neuron degeneration remains unidentified. It was initially proposed that SOD1-linked FALS resulted from free radical damage due to decreased SOD1 activity3,7. However, in vitro, different FALS-linked SOD1 mutants have different enzymatic activities, and at least some FALS mutations (for example, glycine changed to arginine at amino acid 37, or SOD1G37R) retain full specific activity8. In addition, neither age of onset nor rapidity of progression correlate with SOD1 activity levels9, and SOD1 null mice live to adulthood and do not develop motor neuron disease10. Furthermore, transgenic mice expressing three different FALS mutants show progressive motor neuron disease despite elevated11,12 or unchanged13,14 levels of SOD1 activity, demonstrating that the FALS mutations in SOD1 act through the gain of a novel, cytotoxic property, rather than increased or decreased SOD1 activity. The mechanism of this cytotoxicity is currently unknown, although SOD1G85R mice expressing high levels of wild-type SOD1 or deleted in the endogenous SOD1 alleles have shown that (at least for this mutant) toxicity is independent of SOD1 activity and the wild-type SOD1 polypeptide15. Disruption of the neurofilament-L (NF-L) gene in SOD1 mutant mice to eliminate assembly and accumulation of axonal neurofilaments and/or subunits has shown that neurofilaments are one determinant of the selectivity of motor neuron toxicity mediated by SOD1 mutants16. Moreover, aberrant perikaryal and axonal aggregates of neurofilaments are common hallmarks of both sporadic17,18 and SOD1-mediated19–21 ALS, and the largecaliber, neurofilament-rich axons are most at risk in mice14 and in human disease22. A plausible hypothesis for how neurofilaments may participate in motor neuron disease mechanism and selectivity arises from the known neurofilament-dependent slowing of slow axonal transport as neurofilaments accumulate to become the most abundant structural component of large-caliber axons23. Because biosynthesis of macromolecules is almost completely restricted to the cell bodies, transport is especially crucial to these large-caliber axons. In humans, motor neurons can extend for more than a meter in length, yielding a cell volume about 5000 times that typical for an animal cell, almost all of which is in the axon24. Overexpression of the largest human neurofilament subunit NF-H in mice results in perikaryal and axonal neurofilamentous accumulations accompanied by fine tremors, forelimb and muscle atrophy25 and a slowing of slow axonal transport26. When combined, these findings suggest that an important aspect of toxicity may arise from SOD1-mediated damage to the transport machinery or its cargoes. This in turn would promote motor axon degeneration through the simple mechanism of axonal strangulation. Axonal transport has two components: transport of vesicles and mitochondria by kinesin and related proteins (fast transport) and movement of the major structural components of the neuron, many enzymes and other cytoplasmic proteins (slow transport). Slow transport can be divided into two components based on rate of movement: SCa (~0.5 mm/day), containing neurofilament proteins, tubulin and actin, and SCb (~1–2 mm/day), containing tubulin, actin and other cytoplasmic proteins27. The mechanism of slow transport (and the motor(s) that power it) is unknown. To determine whether axonal transport defects may be Slowing of axonal transport is a very early event in the toxicity of ALS-linked SOD1 mutants to motor neurons