Feature

Death of a Brain Cell

Is a tumour-killing molecule called p53 also a killer of brain cells?

Paul E Hughes

Since its discovery in 1979 many scientists have been interested in a molecule called p53, mainly because of its possible role in cancer biology. Normally, the p53 molecule functions to kill cells that have the potential to become cancerous. It therefore acts in all cells of the body as an important natural defence mechanism against the development of cancer.

For this reason, the gene that codes for p53 (i.e. that directs the synthesis of the p53 molecule by the cell) has been called a tumour-suppressor gene, since p53 suppresses tumour formation. However, in up to 50% of human cancers, p53 appears to be mutated (or damaged) in cancer cells. The damaged form of p53 is unable to perform its usual function of killing cancerous cells, so it has been suggested that damage or mutation of p53 within a cell may be an important initiating event in cancer.

This has important implications for the treatment of many different types of cancers where p53 is damaged, because it may be possible to replace the damaged p53 with active p53 thus restoring its normal effects. This might then halt the cancer. Recent studies have shown promising effects of such p53 gene therapy for lung cancer.

Brain Cell Death

Neurons, or brain cells, are the basic unit of the nervous system. Their correct functioning allows our brains to work properly. However, in many brain diseases, such as Alzheimer's, Parkinson's and Huntington's disease, and after injuries to the brain, such as a stroke or a traumatic brain injury, neurons in particular parts of the brain die by unknown mechanisms. The negative consequences of the diseases or injuries could be prevented if we could discover just how brain cells die in these situations.

The first thing we have learned from our research is that brain cell death, no matter how it is induced, probably involves common pathways. Furthermore, it appears that neurons can die by at least two very distinct mechanisms. The first is by a process called "necrosis". Necrotic death is associated with cell swelling because the cell takes in too much water. This eventually leads to the cell rupturing, which often induces an immune response. This type of neuronal death is found at the focal point of a stroke. It is a rapid event and these neurons die within minutes or hours of the stroke.

In contrast, we and others have shown that neurons may also die by a process called "apoptosis". Apoptosis is characterised by shrinkage of the cell and its nucleus, fragmentation of the nucleus and the appearance of apoptotic bodies, non-random fragmentation of DNA and the lack of any immune reaction. While the beginning of apoptosis is usually delayed (hours to days) with respect to necrosis, once initiated, neuronal death via apoptosis occurs rapidly.

Apoptosis is a more subtle way for the cell to die compared with necrosis but it requires energy from the cell to succeed. For this reason, apoptosis is an active form of cell death while necrosis occurs passively, not requiring cellular energy. Apoptosis has therefore been termed cell "suicide" to show that the cell actually directs how it dies. In contrast, necrosis has been compared to "accidental" death, where the cell has no say in its demise.

It is now believed that a significant percentage of delayed neuronal death after a stroke may occur by apoptosis. Apoptotic-like neuronal death can be found in the "penumbral" regions which do not include, but surround, the central necrotic focal point where the stroke has occurred. Because of its delayed appearance, therapeutic intervention with drugs to block apoptosis might be possible because the "window of opportunity" for treatment is long -- hours up to even days after the injury.

Further, it is believed that neuronal death in neurodegenerative diseases such as Alzheimer's, Parkinson's and Huntington's may also involve apoptosis, suggesting that treatment of these currently incurable diseases might be possible with a greater understanding of the mechanisms which control neuronal apoptosis.

Suicide Proteins

One important discovery that has been made recently regarding apoptosis is that new proteins need to be synthesized by the dying neuron for its successful "suicide". If protein synthesis is prevented by inhibitor drugs, then the neuron will not die. Although it is not feasible to block a patient's protein synthesis globally with these drugs following a brain injury or over the prolonged period of treatment of a chronic neurodegenerative disease, it is feasible to identify those proteins that are newly made by the cell and which are necessary for its death.

For this reason the hunt has begun for these so called "cell death" proteins, since by selectively preventing their production, neuronal death might be prevented. It is at this point that p53 emerges again.

While it had been known for some time that p53 could function very effectively to kill tumour cells by the mechanism of apoptosis, it was not known whether p53 would kill normal cells. Experiments went on to show that overproduction of p53 alone within a variety of normal cells would lead the cell to die by apoptosis, a very interesting finding. The connection between neurons and p53 however was not be made until we and others discovered that p53 was expressed in neurons in response to brain injury. It didn't require a huge leap of faith for us to suggest that p53 might be one of the so-called "killer proteins" whose new expression in injured neurons lead to their death.

A few years earlier, researchers had genetically engineered a mouse that lacked the p53 gene; every other gene was normal except for the deleted p53 gene. As might have been expected, these mice died early in life from spontaneous tumours throughout their bodies, further highlighting the tumour-suppressor role that p53 plays in cancer.

More recently however, the availability of these mice has allowed researchers to directly examine the involvement of p53 in neuronal death. What these studies show is that brain cells grown in culture, that are not able to make p53 when they are injured, do not undergo apoptosis and do not die. Furthermore when mice that lack the p53 have a stroke, they also have less brain injury than similar mice that have had the same stroke but whose neurons do make p53.

It has now also been shown that overproduction of p53 in neurons produced using similar viral infection techniques as mentioned above also causes neurons to die by apoptosis. Taken together with our results showing that p53 is made by injured or dying neurons, these findings strongly support a role for p53 in neuronal apoptosis and related brain injury.

While it appears likely from our work and that of others that p53 production plays a major role in neuronal apoptosis (which is one form of brain cell death), further research is being conducted to determine the importance of apoptosis and p53 expression in different neurodegenerative diseases. If p53 is found to be involved in these diseases, then further work must identify ways to selectively inhibit its expression.

Possible complications that may arise with inhibition of neuronal p53 expression might be related to the associated loss of tumour-suppressor function. Therefore it will be necessary to fully investigate the potential benefits and pitfalls associated with inhibition of p53 function in neurodegenerative disorders such as Alzheimer's, Parkinsons's and Huntington's or following a traumatic brain injury or stroke.

Dr Paul E Hughes works in the Research Centre for Developmental Medicine and Biology at the University of Auckland's School of Medicine.