Aging in humans is often, but not always, accompanied by a marked decrease in the ability of the aged person to perform cognitive (thought-related) functions. It is the exception that particularly interests James Herndon, a researcher at Yerkes Primate Research Center. Some people age well, and have no difficulty with short-term memory or tasks that require divided attention; this is what modern medical science calls "successful aging." Herndon wants to learn why some individuals age successfully and others do not. He believes that changes in the ways cells communicate with each other may have a significant impact on the aging process.
In the human body, millions of cells must act in a coordinated fashion so that the body can function properly. This means that cells must communicate with each other. In the body, much of this communication occurs through small signaling molecules that cells secrete into the fluid environment around them. A cell can produce molecules that signal its closest neighboring cells or cells within the same tissue. Cells can even produce signaling molecules that communicate and signal other cells that are very far away. This last type of signaling, called endocrine signaling, interests Herndon particularly as it relates to the process of aging. Herndon recently received a Shannon Award from the National Institutes of Health to study how these endocrine molecules and their patterns of secretion change as the process of aging occurs.
Herndon's research supported by the Shannon Award will explore the multidimensional nature of aging. While much work has been done to document changes in brain size and changes in the number and types of cells in the brain with age, very little work has focused on other parameters that seem also to change with age. The central hypothesis of Herndon's research is that changes in the ability to perform thought-related tasks vary along with changes in neuroendocrine function; in other words, the ability of the brain (neuro) to carry out long-distance cell communciation (endocrine) may relate to whether aging in a particular individual is successful or unsuccessful. Herndon predicts that measurements of neuroendocrine function will be excellent biomarkers of aging. The most immediate goal of this work is to evaluate the usefulness of endocrine variables, which can be measured in simple blood tests, as markers for aging.
Most often, endocrine functions are described in association with reproductive behavior and cycles, and Herndon's early research at Emory focused on the seasonal reproductive changes in the rhesus monkey. Herndon takes full advantage of this expertise in reproductive endocrine signaling as he begins to explore endocrine function as it relates to aging.
Recently, studies have provided evidence that endocrine substances can influence learning, memory and cognition (the ability to think) in aging humans. More basic science research indicates that a number of steroidal substances (one type of signal molecule that cells secrete for endocrine communication) improve memory performance in mice. Several observational studies have been carried out using humans, and these studies suggest a correlation between endocrine function and aging. Unfortunately, studies in humans are only observational, and it is impossible to obtain specific data in a controlled environment in these studies. This is where Herndon and the rhesus monkey enter the picture.
"The rhesus monkey is the right system for this research," said Herndon. He explained that first of all, multiple characteristics such as behavior, neurology and physiology can be closely monitored in the same individual animal. Additionally, there are significant similarities in the age-related changes in cognitive ability and short-term memory loss between the rhesus monkey and humans. Also, the monkeys show the same variability in aging as humans; some are extremely successful and others are not.
Herndon's study also takes advantage of an ongoing aging study in rhesus monkeys at Yerkes and Boston University, so no additional animals will be needed for the study, and start-up costs will be minimal. By combining his research with the ongoing study, Herndon also achieves the interdisciplinary collaboration necessary for investigating a complex phenomenon such as aging.
According to Herndon, biomedical research on aging, both basic and applied, is faced with the overwhelming long-term goal to "describe, understand and ameliorate the cognitive decline which has been associated with the prolongation of human life." It is a daunting task that may never be completed, but studies such as Herndon's are the first step in the process -- describing in a scientific way the changes that take place as individuals age.
-- Michelle Arduengo