Amyotrophic lateral sclerosis (ALS) is a catastrophic degenerative disease of motor neurons that inexorably leads to progressive weakness and death. Genetic depletion of vascular endothelial growth factor (VEGF) leads to selective degeneration of motor neurons, providing a direct link between VEGF and ALS. Furthermore, VEGF attenuates the phenotype of ALS mice expressing the copper-zinc superoxide dismutase (SOD)-1 mutation. Posttranscriptional regulation plays a critical role in VEGF expression, particularly under conditions of cellular stress, allowing an adaptive response to promote cell survival. We recently showed that mutant SOD1 disrupts VEGF posttranscriptional regulation, leading to a significant decline in RNA and protein expression. This finding supports the toxic gain of function hypothesis for SOD1-associated ALS. These initial observations, funded by an exploratory R21 grant and recently published (Lu et al., 2007), have led to three important findings that begin to delineate the mechanism for this toxic effect. First, we discovered that mutant SOD1 is capable of directly binding AU-rich elements (ARE) in the 3'untranslated region (3'UTR) of VEGF. These cis elements are critical for posttranscriptional regulation of VEGF through interaction with cellular factors. Second, we have found that mutant but not wild-type SOD1 associates with HuR, a major regulator of VEGF mRNA stability via the ARE. Third, mutant SOD1 leads to cytoplasmic translocation and posttranslational modification of HuR. Based on these findings, we hypothesize in this proposal that mutant SOD1 exerts its negative effect on VEGF mRNA processing by disrupting the normal regulatory function of HuR. We propose three specific aims: 1) To investigate the novel RNA binding property of mutant SOD1 and its negative effect on HuR. 2) To investigate whether mutant SOD1, through its protein-protein or protein-RNA interaction, disrupts the RNA stabilizing function of HuR, or increases the association of VEGF mRNA with translational silencers or RNA destabilizers. 3) To investigate how mutant SOD1 induces HuR translocation to the cytoplasm and its post-translational modification. The long term objective of this proposal is: a) to determine how mutant SOD1 disrupts VEGF posttranscriptional regulation and b) the impact of this novel function on the ALS phenotype.
Amyotrophic lateral sclerosis is a relentless disease of motor neurons that leads to progressive paralysis of the muscles and ultimately death. There is no cure for this disease or any mitigating therapy. This proposal will address a novel area of growth factor regulation that may contribute to the cause of this disease, and thus may ultimately lead to novel therapies.
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