Research for a New Age

Where gerontologists once looked for a single, all-encompassing theory to explain aging-a single gene, for instance, or the decline of the immune system - they are now finding multiple processes, combining and interacting on many levels. Cells, proteins, tissues, and organ systems all are involved, and gerontologists are now able to discern more and more of the mechanisms by which they cause or react to aging. In fact, today, the biological picture of aging is emerging in much greater detail than ever before. And as more and more of the fundamental mechanisms of aging come to light, they promise to explain - and lead to cures for - the health problems that often accompany old age.

Along one corridor of the Gerontology Research center (GRC) in Baltimore, Maryland, are three small laboratories where dozens of round glass plates or petri dishes sit, like baking dishes being kept warm, in the incubators that line the walls. On the petri dishes are cells, slowly proliferating, immersed in a warm pink medium that promotes their growth. Like most cells, these will not go on proliferating indefinitely. After a certain number of divisions, they stop dividing, permanently, and enter a state called cellular senescence. And as the cells in the plates approach that point, something happens that intrigues gerontologists; the cells make less and less of certain proteins. A clue to aging? Scientists believe that one of these proteins, called heat shock protein or HSP, may indeed shed some light on what happens in human cells as they age. Produced in response to various kinds of stress-not just heat-HSP helps cells respond and adjust to outside challenges. Scientists at the GRC have found that HSP production falls not only in "aging" cells in laboratory culture but also in animals as they grow older. If the same is true in humans, it may help explain why older individuals are less able to respond to acute physical stress. The regulation of HSP production is only one of the cellular clues to aging that are beginning to emerge in laboratories from Baltimore to California. Research conducted or supported by NIA is uncovering some of the fundamental mechanisms of aging, in genes, in the biochemistry of cells, and in various critical organs.

How Do Genes Influence Life Span?

The simple observation that cats live longer than mice, and elephants longer than apes shows that genes are linked in some way to life span. But what genes are they and how do they influence aging? For the first time, answers to these questions are within reach as more and more studies pinpoint aging-related genes in laboratory animals.

A New Orleans researcher, for example, has identified 14 genes that seem to behave differently as a yeast cell grows. Regulating the expression of one of these genes has nearly doubled the yeast's life span (its number of divisions). Other genes may shorten life span. One such "death gene" has been identified in nematodes, tiny, 1-millimeter-long round-worms which, like yeast, are often used in genetic research.

These and other studies suggest that dozens, perhaps hundreds, of aging - and longevity - related genes exist. Galvanized by these findings, researchers are now searching intensely for evidence of how these genes influence aging.

One of the possibilities is that certain genes determine how many times a cell divides or proliferates and that the end of cell division, known as senescence, helps determine some aspects of aging. Most cells can divide only a certain number of times, a seemingly built-in barrier to unlimited growth. In longer-lived species, like humans, this limit is higher than in shorter-lived species. Human cells can proliferate more times than can, for example, mouse cells. This and other observations have led to speculation that the limit on cell division has something to do with life spans and aging.

Cellular senescence intrigues NIA supported researchers for another reason: while it may put a limit on life span, it may also prevent cancer. For, when this limit is removed as it is for some reason in cancer cells, the cells go on growing indefinitely. If cell senescence is indeed one of the fundamental mechanisms of aging, as some biologists speculate, then aging itself may be the other side of the cancer coin. The gradual deterioration of tissues and organs could be the by-product of a mechanism that prevents all cells from growing into tumors.

Whatever the "purpose" or end result of cell senescence, the genes that regulate it are the focus of intense study. Researchers have already isolated some genes that seem to promote cell proliferation-called oncogenes from the Latin root for cancer-and other genes that seem to stop proliferation, often referred to as tumor suppressor genes. Understanding why and how these genes are "turned on" or expressed may uncover new pathways for understanding both aging and cancer.

New technologies allow gerontologists to probe the molecular structures and functions of proteins, the substances most responsible for the day-to-day functioning of all living organisms. Some proteins hold secrets to the process of aging.

How Does Biochemistry Affect Aging?
When a gene is turned on, it produces a protein which may be an enzyme, an antibody, a hormone, or one of hundreds of other proteins that are essential to the body. Some of these, like heat shock protein, seems to be linked to the aging process and have sparked research on the biochemical aspects of aging.

Of particular interest to gerontologists are the molecules that damage cells and the enzymes that prevent or repair that damage. Prime culprits in cell damage are oxygen free radicals. These products of normal metabolism are highly reactive oxygen molecules that enter readily into chemical combinations that can damage cell membranes, proteins, and DNA. The body's defense against free radicals include anti-oxidants, which neutralize these oxygen radicals, rendering them harmless. The anti-oxidants include enzymes such as superoxide dismutase (SOD) and catalase, as well as the vitamins A,C, and E. A number of studies suggest that anti-oxidants affect the progress of some diseases more common in middle and old age, including atherosclerosis, cancer, cataracts, Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease). In a major discovery, the link between antioxidants and disease has been traced back to a particular gene. An NIA-supported researcher in Boston found that this gene, which contains defects in people with a familial form of ALS, directs the production of the anti-oxidant SOD.

Proteins are also involved in DNA repair, another critical cellular process that may throw light on both aging and disease. DNA undergoes constant damage from a variety of agents-free radicals, toxic substances, and ultraviolet light, for example. Most such damage is repaired by certain enzymes that chew away the damaged sections and others that replace them.

This repair process is not 100-percent efficient, however. Some damage goes unrepaired and gradually accumulates. Over a lifetime this may lead-so the theory goes- to malfunctioning genes, proteins, cells, and ultimately the deterioration of the entire organism.

Support for this theory comes from the finding that DNA repair is rare or non-existent in people with Cockayne's syndrome, a disease whose victims have symptoms similar to the signs of aging.

Researchers focusing on DNA repair have found that cells repair active genes more quickly than others, and that the transcribed strand of DNA is repaired before the non-transcribed strand. As the intricacies of this process are gradually unraveled, they are expected to point the way to methods of enhancing DNA repair, thus perhaps alleviating or preventing the diseases with which it is associated.

Hormones In a small study in 1989, a Milwaukee researcher gave injections of synthetic human growth hormone to a few men over age 60 who had lower-than-average levels of this substance. The effect of the injections was impressive. The men's bodies became leaner, their muscles stronger, and their skin thicker. When the injections stopped, these signs of youth vanished after a few months. Encouraged by these results, biologists are now scrutinizing a range of substances-human growth hormone, estrogen, testosterone, insulin-like growth factor, and others- to see if they can prevent the loss of bone and muscle that often causes frailty in older people. Known collectively as growth or trophic factors, these naturally occurring substances often decline as we grow older. Replacing them could be the key to making old age healthier for millions of people and exploring their side effects, which could be harmful. And in the laboratory, a Stanford researcher has found that certain muscle cells can be genetically modified and injected into mouse muscle where they produce and secrete growth hormone into the blood circulation system.

The Immune System
We inhabit a world of microscopic particles, friendly and unfriendly, all struggling to survive and reproduce. Some find the human body an ideal place to live. Bacteria, viruses, fungi, and parasites try to get in; a complex network of defenses-the immune system - keeps them out or, if they do gain entry, attacks and destroys them.

The immune system has long intrigued gerontologists because it seems to become weaker with age, making older people more prone to influenza, pneumonia, and other infections. In fact, the decline in function in the immune system-actually a set of interacting organs, cells, and substances- has been proposed by some as the single most important event in the aging process, a sort of pacemaker or regulator of aging.

Made up of the thymus, lymphatic organs, spleen, bone marrow, and other tissues and organs, the immune system produces a multiplicity of specialized cells, such as B-lymphocytes, which produce antibodies, and T-lymphocytes, which attack foreign cells. T-cells are key suspects in the aging process. Pinpointed as a major element in immune system decline, they are the focus of much current research on the aging immune system.

What researchers have learned so far is that T-cells generally do not decline in number as we age, but that their functions do diminish. T-cell products, particularly one group of substances called interleukins, are found in different levels in young and old animals. Interleukin-2, for example, declines with age, and experiments with older animals have shown that injections of interleukin-2 can boost their immune response.

Hormonal control of immune system functions has recently become another key area of research. One of the hormones that intrigues immunologists is DHEA (short for dehydroepiandrosterone) which begins to decline in humans at about age 30. Experiments in Salt Lake City show that giving DHEAS (DHEA sulfate) and its active form, DHEA, to laboratory mice dramatically boosts their immune response. Depressed levels of this hormone have been linked to cardiovascular disease in men, some cancers, trauma, and stress.

Investigators at NIA and around the country are looking at the human body's ability to respond to DHEA, the interleukins, and other substances, because response mechanisms seem also to alter with age. The ultimate goal, once these mechanisms are understood, is to identify what can be done to augment, alter, or maintain the immune response to enhance health as people grow older.

The Brain In a 1990 study at Pennsylvania State University, 37 volunteers, all in their late sixties, sat at a table and took at test which measured certain aspects of mental ability. Their task was to look at changing patterns-pictures of a triangle, for instance, that rotates its position within a circle-and predict the changes (e.g., the fourth picture should show the triangle at the bottom of the circle). At the end of the training session, a test showed that the mental exercise has strengthened their recognition, memory, and other mental skills. This initial improvement did not surprise the researchers; other studies had also shown that training could improve mental skills. But the improvement was short lived. In this study, the investigators wanted to learn what repeated training over time could accomplish, so they asked the volunteers to return 2 years later and again, 4 years after that. In these later phases of the study, the findings broke new ground: Most of the volunteers, then in their seventies, did better on the final tests than they had when they were 7 years younger. The repeated, periodic training sessions demonstrated that at least under study conditions, mental abilities can be maintained and improved. This is just one of the studies that is changing our ideas of what happens - or doesn't happen - in the brain as people grow older. More and more evidence supports the idea that aging is not inevitably linked to declining mental abilities; while changes do occur in the brain, they are not necessarily reflected in how an individual functions. Moreover it is possible that at least some part of the changes in the brain are due not to aging per se, but to lifestyle or disease. The mechanisms underlying changes in the brain, in both normal aging and in disease, are the focus of intense study. Especially intriguing to neuroscientists is the loss of brain cells (neurons). This occurs in normal aging but also in dementia and may be related to problems with cognition, hearing, vision, sleep disturbance, or other problems that sometimes-but not always-accompany aging. The kind of problem depends on the part of the brain affected by neuron loss. Why groups of neurons in certain parts of the brain die is a key question and it has led researchers in several directions. One possibility is that exposure to poisons from either inside or outside the body damages the brain cells. Another is that neuron death is linked in some way to other body systems and regulatory mechanisms; hormones or immune-system substances, for example, could trigger changes in brain cells. The interactions among hormones, the immune system, and the central nervous system-an area known as psychoneuroimmunology-constitute another group of mechanisms of special interest to gerontologists. NIA-supported researchers have shown that psycho-social stresses affect various components of the immune response and because older adults often face stress-in the form of social isolation, financial worries, or physical frailty, for instance-it may be an important factor in disorders associated with aging. Investigators studying psychoneuroimmunology are searching for the underlying mechanisms and for the factors that make people vulnerable to stress. Their ultimate goal is to develop interventions to prevent or halt the disease processes related to these factors.

Cellular biologists are probing the fundamental mechanisms of health and disease in aging. In immulogy laboratories such as this one, researchers are probing the reasons that T-cells and other components of the immune system function less efficiently with age.

Cognitive Changes
Data from the Baltimore Longitudinal Study of Aging show that mental skills may decline in certain categories, such as short-term memory, but not in all. The quality of reasoning and problem-solving remains much the same as we age, according to this long-term study, although the speed of mental processes may slow. Whether cognitive functions slow down because of neuron loss or because the aging brain makes less effective use of neurons is one of the key questions neuroscientists ask.

Memory impairment, a common complaint of older adults, is also a major component of the cognitive dysfunctions seen in dementia. Researchers are learning that what we call by the single term, memory, can actually be broken down into various components (e.g., retrospective and prospective, verbal and non-verbal memory). The components particularly vulnerable to aging can thus be identified and linked to specific brain regions and neural mechanisms. The splitting of memory into component parts will help in detecting memory problems early and in finding ways to prevent or ameliorate such problems.

Exciting to many neuroscientists is the possibility that we may be able to prevent mental decline by keeping our brains active. The "use it or lose it" hypothesis is still far from proven, but some studies suggest that the brain can maintain its functions through exercise, just as the body can. Studies of rats show that when they live in challenging and stimulating environments, the cerebral cortex - the part of the brain that receives and integrates information - becomes thicker. What's more, the stimulated rats have larger neurons, more glia (support cells that nourish neurons), and connections among more neurons, suggesting enhanced activity and greater communication among neurons.

Sleep and the Biological Clock Sleep disorders affect about half of everyone age 65 and over who lives at home, according to the National Commission on Sleep Disorders Research. Now, researchers are beginning to understand the age-related changes in the nervous system that underlie these disorders. Many of the changes in sleep may be related to changes in the body's biological clock, the circadian pacemaker, which is located in the part of the brain known as the suprachiasmatic nucleus. The internal pacemaker gradually speeds up with advancing age in association with a tendency to fall asleep and wake earlier and possibly with other changes in sleep patterns. Researchers at Northwestern University have discovered an age-related loss of neurons in the suprachiasmatic nucleus, which may be linked to disturbances of the biological clock. Neuroscientists are exploring not only the causes, but also treatments, of sleep disorders. Light has a powerful effect on the circadian clock, and carefully timed exposures to bright lights have been used to reset the clock and correct the changes in circadian timing found in older people. Exercise has also been found to play an important role in circadian rhythms. Preliminary studies at the University of Washington suggest that in older men, endurance training can shift circadian rhythms and sleep measures, bringing them closer to those observed in healthy young individuals.