Increased p53 Damages the Mitochondria in HD

HD Lighthouse Contributing Editor's Comment: Brilliant research by a team of Johns Hopkins scientists has led to a major advance in our knowledge about Huntington's Disease. This study links problems in the cell nucleus caused by the HD protein to mitochondrial dysfunction.

The huntingtin protein is a cytoplasmic protein (this is the fluid part of the cell outside of the nucleus). The mutated version fragments and goes into the nucleus of the cell where it doesn't belong. It was first noted that the protein forms aggregates which may or may not contribute to disease pathology. Later research has shown that the soluble version of the HD protein certainly causes damage, dysregulating the transcription of numerous genes in the nucleus that make proteins needed for cellular processes.

It is also known that HD causes mitochondrial problems as well. The mitochondria are the energy factories of the cell. Energy metabolism is compromised and there is also oxidative damage. But until now, researchers didn't know if this was a separate path of pathology or somehow linked to nuclear pathology.

The researchers were able to show that the presence of the HD protein increases the amount of p53 in the cell. They found this in striatal cell models of the disease, in the HD mice, and in the brains of people who had been affected by HD. The p53 gene encodes for a protein which is very important in suppressing cancer. It recognizes damaged DNA and interrupts the cell cycle while the DNA is repaired. If it can't be repaired, it will initiate apoptosis, programmed cell death. When the P53 gene is itself damaged, tumor suppression is severely reduced and many cancers are associated with the mutation or absence of p53. p53's role in the central nervous system is not yet fully understood but it is known to be a nuclear transcription factor which regulates a number of genes that affect the mitochondria.

Transcriptional activity by p53 is also increased. Two of the proteins known to be regulated by p53, Bax and Puma, are also increased. They are associated with the mitochondria and increased levels could cause the depolarization of mitochondrial membranes. In addition, mitochondrial abnormalities can produce elevated levels of calcium.

Alterations in the levels of calcium in the cell can lead to synaptic problems (which are now being shown by others to play a role in HD pathology) and to increased susceptibility to excitotoxins, a well supported theory about pathology. Further, calcium problems can elevate calpain which is a protease (an enzyme which breaks down proteins) which has already been found to cleave the HD protein in human brains. And remember, the fragmentation of the protein appears to be a key event in pathology.

It looks to me like the pieces of the puzzle which have been filling in since the gene was discovered are now connecting to each other. And this is specific to Huntington's Disease. p53 is not elevated in ataxia-1, another neurodegenerative disorder caused by expanded polyglutamines in a different gene, for example. And p53 isn't elevated in the cellebellum part of the brain which is not affected by Huntington's Disease. Interestingly, p53 is unrelated to the HD protein aggregates.

To see if they were right and p53 was responsible for significant HD pathology, the researchers knocked down p53 in three ways. One was with pifithrin-alpha, a small molecule that inhibits p53 (and which could be a potential treatment with further drug development), by genetically engineering fruitflies and mice that don't have p53, and through RNA interference. In every case, significant neuroprotection was achieved and the mitochondrial membranes didn't depolarize. The mice did not exhibit the behavioral abnormalities associated with the disease, such as an inhibited startle reflex, circling behavior, and poor performance on the rotarod test.

Could p53 be a target for therapy? The authors don't say but it has been discussed in other articles which I'll report on in later updates. p53 suppression is definitely being explored as a cancer therapy. Ironically, while P53 is a cancer preventative, once someone has cancer, it interacts with the chemotherapy and makes the death of normal cells more likely. The work done in cancer research may shorten the time to treatment for HD.

--Marsha L. Miller, Ph.D.
Posted to the HDL: 18 Jul 2005

Akira Sawa, M.D., Ph.D., Assistant Professor of Neurology, Johns Hopkins University

Molecular trigger for Huntington's disease found

Researchers have discovered a key regulatory molecule whose overactivation by the abnormal protein produced in Huntington's disease (HD) causes the central pathologies of the disease. The abnormal HD protein activates the regulatory protein called p53, which in turn switches on a host of other genes. This abnormal gene activation damages the cells' power plants, called the mitochondria, and kills brain cells.

The researchers also speculated that disturbances in p53 may also play a role in some forms of Parkinson's disease and amyotrophic lateral sclerosis, or Lou Gehrig's disease.

Ironically, p53 is the same regulatory protein that is inactivated in a large fraction of cancers. This inactivation allows abnormal cancer cells to escape the cell's protective "suicide program" that would normally kill them. The researchers theorize that the lower incidence of cancer in HD patients could be caused by the protective effect of overactive p53.

In the July 7, 2005, issue of Neuron, Akira Sawa and colleagues at Johns Hopkins University School of Medicine reported experiments ranging from molecular studies in cultured brain cells to analysis of the brains of human HD patients that demonstrated the central role of p53 in the pathologies of HD.

Their studies with cell cultures showed that the abnormal HD protein selectively binds to p53 and increases its level in cells. They noted that the brains of patients with HD also show substantial increases in the p53 protein, with the highest levels in cases with the most extensive pathology.

The researchers' experiments also revealed that this p53 increase causes an overactivation in the genes regulated by p53, which is called a "nuclear transcription factor" because it regulates the transcription of its target genes in cell nuclei.

In studies of cell cultures and of mice engineered to have HD, the researchers found that the p53 increase causes malfunctions in mitochondria. What's more, they found that this p53 increase induced by the abnormal HD protein greatly increases cell death.

The researchers also found effects of the abnormality in p53 in whole animals. They found that deleting p53 suppresses damage to neurons in the eye of fruit flies engineered to have the abnormal HD protein. And in mice with the abnormal protein, knocking out p53 corrects behavioral abnormalities that the mice otherwise display. These behaviors include abnormal reflexes such as an inhibited startle response to loud noise, which is also present in human HD patients.

"In summary, our study establishes a specific role for p53 in HD," concluded Sawa and colleagues. "As p53 is a nuclear transcription factor that regulates various mitochondrial genes and insofar as mitochondrial dysfunction appears important in HD, our findings provide a molecular mechanism linking disturbances of nuclei and mitochondria in HD."

The Abstract

We present evidence for a specific role of p53 in the mitochondria-associated cellular dysfunction and behavioral abnormalities of Huntington's disease (HD). Mutant huntingtin (mHtt) with expanded polyglutamine (polyQ) binds to p53 and upregulates levels of nuclear p53 as well as p53 transcriptional activity in neuronal cultures. The augmentation is specific, as it occurs with mHtt but not mutant ataxin-1 with expanded polyQ. p53 levels are also increased in the brains of mHtt transgenic (mHtt-Tg) mice and HD patients. Perturbation of p53 by pifithrin-alpha, RNA interference, or genetic deletion prevents mitochondrial membrane depolarization and cytotoxicity in HD cells, as well as the decreased respiratory complex IV activity of mHtt-Tg mice. Genetic deletion of p53 suppresses neurodegeneration in mHtt-Tg flies and neurobehavioral abnormalities of mHtt-Tg mice. Our findings suggest that p53 links nuclear and mitochondrial pathologies characteristic of HD.

Tracked on the Lighthouse:
p53
mitochondria
excitotoxicity
transcription

Source: Neuron. 2005 Jul 7;47(1):29-41

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