The tumor suppressor p53 is perhaps best known as a cellular watchdog that prevents cancer by initiating apoptosis in genetically damaged cells. But might this old faithful occasionally get a little overzealous? In the July 7 Neuron, Akira Sawa and colleagues at Johns Hopkins University in Baltimore, Maryland, show that nuclear p53 associates with mutant poly-Q expanded huntingtin protein (mHtt), leading to elevated levels of p53, mitochondrial depolarization, and cell death. The authors demonstrate that inhibiting p53 rescues neurons, and that deletion of the gene suppresses HD-like phenotypes in Drosophila or mice carrying mHtt. While activation of p53 in response to stress has been recognized in several neurodegenerative diseases including Alzheimer and Parkinson diseases (reviewed in Culmsee and Mattson, 2005), Sawa’s group shows for the first time direct connections among pathological nuclear events, mitochondrial dysfunction, and p53-mediated cell death.
Picking up on multiple clues that p53 might play a role in Huntington disease pathology—p53 associates with mHtt; neurons from mHtt knock-in mice have increased p53; and p53 regulates many mitochondrial genes and genes associated with oxidative stress—first author Byoung-Il Bae and colleagues measured p53 levels and activity in neuronal cells in vitro and in brain tissue from humans and transgenic mouse models of HD. The researchers found that expression of the huntingtin protein exon 1 fragment N63-148Q in PC12 cells caused increased nuclear p53 protein levels, and that the increase depended on the presence of the pathogenic polyQ repeats. They observed increases in p53 in mHtt-transgenic mouse brains, and in brains of people with Huntington disease, but only in regions affected by the disease. They went on to show that in neurons in culture, the mHtt and p53 proteins occur in a complex, and that mHtt augments p53-stimulated transcription. When Bae and colleagues transiently transfected PC12 cells with mHtt, they found increased levels of several p53-responsive proteins that associate with mitochondria and mediate apoptotic signals. These effects were not a general response to polyQ expansions, since the polyQ form of ataxin, which causes spinal cerebral ataxia, did not have similar effects.
The regulation of mitochondrial genes and genes for oxidative stress by p53 and their augmentation by mHtt suggested that the p53 might play a role in the mitochondrial dysfunction central to HD. In agreement with this idea, they found that the p53 inhibitor, pifithrin (see ARF related news story), blocked cyanide-stimulated mitochondrial depolarization in lymphoblasts from Huntington disease patients. In PC12 cells, pifithrin prevented mitochondrial depolarization and cell death caused by expression of mHtt. When given to mice, pifithrin reversed the impairment of mitochondrial complex IV activity in the striatum of transgenic mHtt animals. Finally, using another approach to inhibit p53, Bae et al. showed that genetic deletion of p53 prevented cell death after mHtt expression in primary cortical neurons. In contrast, mHtt nuclear and cytoplasmic aggregates were not changed by p53 deletion.
To conclusively nail down p53’s central place in HD neuropathology, the researchers showed that in both fruit flies and mice transgenic for mHtt, p53 deletion decreased the severity of HD-like phenotypes. In the fly HD model, animals progressively lose retinal photoreceptors, and deletion of p53 rescues these cells. In the mouse model, crossing p53 knockouts with mHtt-Tg mice ameliorated motor dysfunction by several measures. The abnormal escape reflex seen in the transgenic mice was normalized, as was increased rotational activity, an abnormal startle reflex in response to loud noise and deficits in rotarod performance.
“By presenting such a broad portfolio of consistent experimental results, the authors made a convincing case that p53 is involved in HD disease progression,” write Albert La Spada and Richard Morrison in a preview article accompanying the work. But they point out that many questions remain about the role of p53 in HD. How is p53 upregulated by mutant Htt protein, for example?
Although Sawa and colleagues showed that p53 levels were not enhanced by the polyQ protein ataxin, it is possible that p53 could turn out to be a key player in other polyglutamine repeat diseases. Beyond the glutamine expansion diseases, La Spada and Morrison point out that Sawa’s data indicate p53 might be involved in mitochondrial dysfunction in the absence of apoptosis, a process that could lead to synaptic degeneration that occurs in many neurodegenerative diseases. As they summarize, “Clearly, additional studies will be required to fully evaluate the role of p53 in HD and other neurological disorders, since other disease proteins may find the draw of p53’s dark side impossible to resist.”
Could p53 be a clinical target for neurodegenerative disease therapy? This idea has gained some currency as observations accumulate that p53 production is rapidly increased in neurons in response to a range of insults, including DNA damage, oxidative stress, metabolic compromise, cellular calcium overload, and amyloid-β (see ARF related news story and Culmsee and Mattson review). But the downside of systemic inhibition of a tumor suppressor may be considerable, as La Spada and Morrison point out. Alternatively, targeting the specific interaction of mHtt and p53 may yield a new therapeutic approach to Huntington disease.—Pat McCaffrey
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