Aging : Microarrays, Cellular Senescence, Proliferative Genes

by National Institute on Aging

(Page 5 of 15)

Microarrays in Action

All cells have the same complement of genes. The form and function of any given cell is determined by which of these genes are turned on and off. As a cell grows, matures and ages, the pattern of genes turned on and off changes. Detecting these changes was once a tedious process that involved testing one gene at a time. No longer.

Gene expression microarray technology, a potent scientific tool, is helping gerontologists rapidly clarify what genetic changes occur in cells as they get older. Also known as "gene chips," microarrays allow researchers to survey the expression of thousands of genes at once. The match-box size glass chips contain DNA that has been exposed to messenger RNA (mRNA), a nucleic acid that translates information contained in DNA into proteins. Each kind of mRNA binds to its corresponding DNA probe, and the amount of mRNA that binds to each gene's DNA on the chip is an indicator of the activity level of that gene.

Investigators using this technology have found relatively few changes in gene expression occur in aging tissues. In some studies, comparing tissue taken from young animals versus older animals, fewer than two of every 100 genes have shown major changes in activity over time. But these limited changes may have significant impact on the ability of aging tissues and organs to function properly. In time, microarrays might help gerontologists to more precisely characterize the genes involved in the aging of specific tissues or organs, and accelerate our understanding of its underlying mechanisms.

Cellular Senescence

During the process of cell division or mitosis, a cell's nucleus dissolves, and its chromosomes condense into visible thread-like structures that replicate. The resulting 92 chromosomes separate, migrating to opposite sides of the cell where new nuclei - each with 46 chromosomes - are formed. Once this occurs, the original cell, following the chromosomes' lead, pulls apart and forms two identical daughter cells. It is this process that allows us to grow from a single cell into 100 trillion cells, composing the organ systems that make our bodies.

Early in life, nearly all of the body's cells can divide. But this process doesn't go on indefinitely. Researchers have learned that cells have finite proliferative lifespans, at least when studied in test tubes - in vitro. After a certain number of divisions, they enter a state in which they no longer proliferate and DNA synthesis is blocked. For example, young human fibroblasts - structural cells that hold skin and other tissues together - divide about 50 times and then stop. This phenomenon is known as the Hayflick limit, after Leonard Hayflick, who with Paul Moorhead described it in 1961 while at the Wistar Institute in Philadelphia. At least four genes involved in this process have been identified. This special aspect of cellular senescence is known as replicative senescence.

However, we do not die because we run out of cells (even the oldest people have plenty of proliferating fibroblasts and other types of cells). In fact, most senescent cells are not dead or dying. They continue to respond to hormones and other outside stimuli, but can't proliferate. Evidence suggests they can continue to work at many levels for some time after they cease dividing. Senescence, however, can cause radical shifts in some important cellular functions. For instance, senescent cells are resistant to dying and, as a result, they occur more often in aging bodies. Cellular senescence also triggers important changes in gene expression. Normally, fibroblasts are responsible for creating an underlying structure, called the extracellular matrix, which controls the growth of other cells. But senescent fibroblasts secrete enzymes that actually degrade this matrix. Gerontologists suspect the breakdown of this structure may contribute to the increased risk of cancer as we age. So, cellular senescence may be critical early in life because it limits cell proliferation and helps suppress cancer. But as we get older, senescent cells might be harmful because changes in the genes they express might actually promote unregulated growth and tumor formation. This concept that genes, which have beneficial effects early in life, can also have detrimental effects later is known as antagonistic pleiotropy. Some gerontologists speculate that a better understanding of antagonistic pleiotropy might reveal much about what aging is, and how cellular senescence contributes to it.

But for now, many major questions about cellular senescence remain unanswered. Investigators, for example, are uncertain whether senescent cells accumulate in all tissues and organs with increasing age, thus contributing to the gradual loss of the body's capacity to heal wounds, maintain strong bones, and fend off infections. Accumulation of senescent cells, if it does occur, could, in turn, indirectly increase an individual's vulnerability to the diseases and disabilities often associated with aging. However, no feature of aging has yet been unequivocally explained by in vitro cellular senescence.

Proliferative Genes

Searching for explanations of proliferation and senescence, scientists have found certain genes that appear to trigger cell proliferation. One example of such a proliferative gene is c-fos, which encodes a short-lived protein that is thought to regulate the expression of other genes important in cell division.

Proliferative genes, such as c-fos and others of its kind, are countered by anti-proliferative genes, which seem to interfere with division. The first evidence of an anti-proliferative gene came from an eye tumor called retinoblastoma. When one of the genes from retinoblastoma cells - later called the RB gene - became inactive, the cells went on dividing indefinitely and produced a tumor. But when the RB gene product was activated, the cells stopped dividing. This gene's product, in other words, appeared to suppress proliferation. Another well characterized gene of this type is the p53 gene, which produces a protein that also limits cell proliferation. These genes are called tumor suppressor genes.

Limited proliferation is the norm in the world of human cells. In some cases, however, a cell somehow escapes this control mechanism and goes on dividing, becoming, in the terms of cell biology, immortal. And because immortal cells eventually form tumors, this is one area in which aging research and cancer research intersect. When tumor suppressor genes are inactivated, investigators theorize it turns on a complex process that leads to development of a tumor. So replicative senescence apparently has been retained through evolution as a defense against cancer.

Scientists are unraveling how the products of these genes promote and suppress cell proliferation. There are indications that a multi-layer control system is at work, involving a host of intricate mechanisms that interact to maintain a balance between the two kinds of genes. Some genes, for instance, appear to suppress or silence other genes. Mutations in these silencing genes have been shown to affect the lifespan of C. elegans and yeast. Many gerontologists are studying how silencing and other mechanisms such as telomere shortening influence replicative senescence.

About the Author

www.nia.nih.gov
NIA, one of the 27 Institutes and Centers of NIH, leads a broad scientific effort to understand the nature of aging and to extend the healthy, active years of life. In 1974, Congress granted authority to form NIA to provide leadership in aging research, training, health information dissemination, and other programs relevant to aging and older people.