Nitric Oxide and SOD1 Mutants: Possible Roles and Interactions in ALS

Nitric oxide (NO) is a gas sometimes found in the CNS. It is most commonly associated with nueroendocrinology, since even though it is produced in a neuron and oftentimes effects other neurons, it does not communicate via the traditional synaptic mechanism. Instead it diffuses through cells, effecting neurons that may not have a synaptic connection with the secreting cell, let alone be next to the secreting cell. Cellularly this novel messenger is produced by NO synthase, but may enter the CNS via simple diffusion from outside sources, among other ways.

Nitric oxide has been shown to be toxic to neurons in multiple ways. It has been associated with apoptosis and the production of compounds detrimental to the survivability of neurons. It's role in ALS is thought to be a given, yet the extent to which symptoms of ALS can be attributed to NO remains to be elucidated. The fact that NO in conjunction with mutant SOD1 can cause nearly all of the cellular insults witnessed in ALS lends some credence to it's possible role as a contributor to ALS symptoms.

Recent studies done by Maria Ros Ciriolo, et al., have linked increased NO levels and increased levels of mutant SOD to increased incidence of neuronal apoptosis. They noted that increased levels of wild type SOD seemed to protect the neuron from NO mediated apoptosis, and in wild type cells exhibiting a down regulation of SOD there was an increased incidence of apoptosis. Thus it may be concluded that the functions of SOD help to prevent NO mediated apoptosis in neurons. The fact that mutant SOD still exhibits the same superoxide scavenging ability of the wild type enzyme, supports the theory that the gain of function mutation in the enzyme is far more detrimental than the beneficial effects of the mutant's dismutase ability.

Recent work done by Anna Steiber, et al., at the University of Pennsylvania Medical Center has found aggregation of mutant SOD and ubiquitin in neuronal processes of ALS symptomatic cells. Their research has linked this aggregation to fragmentation of the Golgi apparatus, which appears to fragment distally and progressively fragment toward the soma. I believe that this aggregation may have facilitated the role of NO. I believe that these SOD mutant aggregations bring with them copper ions which may themselves interact with NO and/or promote increased free radical production in a localized area; these free radicals in turn reacting with NO. My theory is supported by experimental data showing that copper chelators protect neurons from NO mediated apoptosis. This protection supports the theory that the SOD mutants within cells contain reactive copper, else why would copper chelators have any effect on the neuron. This observed protection from copper related reactions in SOD mutant cells further supports the gain of function theory prevalent in ALS literature at this time.

The complete negative effects of NO on neurons may not be completely clear, but the products of it's reactions, the results of it's mere presence at certain levels within a cell, have known and established effects on cellular well being. NO induces apoptosis in several ways, one of which is activation of p53, a protein highly associated with apoptosis caused by DNA damage. In test cells p53 levels where measured, and it was found that heavy, phosphorylated p53 levels increased with NO treatment while the light, unphosphoralated p53 levels all but disappeared. It is thought then that p53 is activated by phosphoralation. Increased levels of p53 where associated with p21 ( a cell cycle regulatory protein) increases as well. These levels where measured by the cleavage product of p21, which followed its expression, and subsequent cleavage.

Nitric oxide reacts with free radicals to form peroxynitrite which has numerous detrimental effects. Peroxynitrite is an inhibitor of the mitochondrial transport chain. This type of inhibition will reduce the efficiency of mitochondria in producing ATP. Peroxynitrite will react with tyrosine molecules to generate nitrotyrosine. The formation of nitrotryrosine may promote degeneration of cytoskeletal elements, and has the potential to impair tyrosine trophic factor receptors. In addition peroxynitrite can be protonated to form peroxynitrous acid. Peroxynitrous acid may then decompose releasing a hydroxyl free radical. Hydroxyl and superoxide free radicals themselves may effect other biological molecules as well. They may damage certain cellular membranes, such as mitochondrial membranes, via lipid peroxidation and may also damage glutamate transporters, leading to further cellular insults.

The exact role of NO, and the extent of it's effects, as it pertains to ALS is still unclear. However, it has been shown that increased levels of NO in conjunction with free radicals and /or mutant SOD1 has a decidedly negative effect on neurons. I suspect that as research continues the role of NO in ALS and other neurodegenerative disorders will be greatly elucidated.