As various jokes about "losing brain cells" illustrate, it has long been supposed that age-related cognitive decline is a result of neurodegeneration: the loss of neurons and synapses. This hypothesis is supported by high-resolution neuroanatomical investigations of the brains of patients with Alzheimer's disease and related dementias, which reveal that the level of cognitive decline and dementia is closely correlated with the extent of neuronal cell loss and synaptic degradation. Moreover, the neurodegeneration occurs selectively in brain regions associated with the functions lost in the dementias (Price et al., 1998).
This view of the neural basis for age-related cognitive decline has led to much productive research on the causes of neural cell death. Recent research developments, however, including provocative reports of neural regeneration in the adult brain (e.g., Gould et al., 1999b), are suggesting the need to expand the science in a new direction, toward understanding the determinants of change in neural health during the aging process. The underlying idea is that the causes of age-related cognitive decline, particularly in individuals who do not suffer from the dementias, may be found in dynamic processes that impair the health and functioning of living neurons. Although these mechanisms may eventually kill neurons, they may have deleterious cognitive effects even without causing neuronal loss, and the processes may begin well before old age. Little is known so far about mechanisms that maintain or impair neural health or that repair or restore unhealthy neurons.
RECENT SCIENTIFIC DEVELOPMENTS
One important line of recent evidence suggests that the mild age-related cognitive decline that occurs in nondemented individuals is different from that occurring in people with Alzheimer's disease and may not be due entirely or even primarily to neuronal loss. The progressive neuronal cell death that is observed in Alzheimer's disease is not characteristic of benign senescent memory loss that occurs frequently with increased aging (Morrison and Hof, 1997; Rapp and Gallagher, 1996; for further discussion and references, see Morrison, Appendix A). In particular, the entorhinal cortex, which displays massive neuronal loss in the brains of patients with advanced Alzheimer's disease, does not undergo any significant neuronal loss with aging in non-demented patients (Morrison, Appendix A). These findings support the hypothesis that the mild memory decline that occurs with age may be due to biochemical shifts in still-intact neural circuitry. Like neuronal loss, these disruptions may involve selective vulnerability in the entorhinal cortex and other brain regions, such as the hippocampus (West, 1993; Peters et al., 1998; Gomez-Isla et al., 1996; Morrison, Appendix A).
If cognitive decline can be attributed even in part to disruptions in the neural network other than cell loss, it is important to identify the responsible mechanisms. Recent research has identified various dynamic processes that occur in the adult brain, indicating that, in general, the brain undergoes more change than previously believed. Among the more dramatic kinds of change that have been observed are increased dendritic growth (Kolb and Whishaw, 1998) and neurogeneration in areas associated with higher cognitive functions (e.g., Gould et al., 1999b). For evidence of lasting changes in the brain as a result of life experiences, see Buonomano and Merzenich (1998), Greenough (1976), Elbert et al. (1995), and Pascual-Leone and Torres (1993). Genetic factors, such as the presence of the APOE-ε4 allele, well known to be associated with Alzheimer's disease, may also influence cognitive outcomes in non-demented older persons. In addition, neural health is likely to be affected by three cellular and molecular processes that contribute to neurodegeneration, in Alzheimer's disease and related dementias: apoptosis (programmed cell death), inflammation (acute phase injury), and the generation of free radicals (oxidative stress). The selective vulnerability of certain brain regions to neuronal loss in dementias is known to match closely with those regions in which these three processes are most active.
Especially intriguing is the fact that although these three processes are implicated in dementia, they are normally beneficial to cognitive functioning: each is involved throughout life in helping to maintain the integrity of healthy neural circuits. Thus, dysfunctions in these processes, even if not leading to cell loss, may provide mechanisms for cognitive decline.
The beneficial functions of apoptosis, inflammation, and free radical re-
lease can be briefly summarized as follows. Apoptosis is essential for brain plasticity, synaptic turnover, and selective removal of dysfunctional neurons and glia. A healthy apoptotic response mechanism is critical in the aging brain to allow for efficient adaptive remodeling of neural networks. Inflammation is critical in the acute phase response to provide basic "housekeeping" functions, including the removal of debris from dying cells and their exudates (e.g., amyloid). Reactive glia are the most critical components in maintaining neuronal homeostasis with increasing age. Free radicals released during inflammation by reactive glia are aimed at destroying foreign invaders in the brain and thus comprising a basic immune protection mechanism for brain functioning.
These processes can become detrimental when they go out of balance, for example, shifting from "acute" responses to brain injuries to a "chronic" response pattern. Unbridled apoptosis, inflammation, and free radical release would quickly shift the balance from neural health to neural dysfunction and ultimately to rampant neurodegeneration. Such a shift might either be localized to small subsets of neurons or be widespread, affecting entire neural networks. The extent to which apoptosis, inflammation, and free radical release act as beneficial as opposed to detrimental events in the central nervous system would dictate whether the neural circuit is maintained in a healthy manner or is chronically disrupted, eventually leading to neurodegenerative changes. A hypothesis worthy of investigation is that progressive dysregulation of these processes with age is intimately involved with neural dysfunction and mild cognitive impairment relatively early in life, whereas chronic activity of these events over many years leads eventually to neuronal and synaptic deficits and to dementia.
This hypothesis is described in more detail by Cotman (Appendix B), who discusses the relevant evidence and proposes that although the initiation of acute events of the above-mentioned processes is beneficial for the maintenance of the neural circuitry, problems arise when "initiation" shifts to "propagation." For example, acute apoptosis can facilitate neuronal plasticity in the central nervous system, but chronic apoptosis can promote dysfunctional neurons (e.g., in those undergoing chronic caspase activation and managing to survive with compromised function). Chronic apoptosis would ultimately result in neurodegeneration, leading to major neural network abnormalities. Thus, in the early stages of chronic apoptosis, one would expect the promulgation of dysfunctional neurons and abnormally altered afferent/efferent profiles in the neural network. This would eventually proceed to neurodegenerative events, including neuronal cell loss, requiring robust plastic responses to maintain the integrity of the neural network. Likewise, the nurturing activities of reactive glia can shift from enhancing adaptive mechanisms in the brain to killing neurons (e.g., via free radical release) if acute phase responses become chronic ones.
Such hypotheses illustrate the critical need for investigating the determinants of change in neural health, including the ways in which apoptosis, inflammation, and free radical generation are regulated in the central nervous system over the life course. More specifically, studies are required to address how these processes might reach such an exaggerated level that neural circuits are damaged or neurons are killed rather than maintained. They should also investigate the life histories of the processes, which seem to begin by midlife (see Cotman, Appendix B). A better understanding of the role of homeostatic control of these three cellular processes in maintaining healthy neural functioning with age would also facilitate future attempts to replace lost neurons using differentiated stem cells, possibly including self-repopulation with a patient's own differentiated stem cells.
RESEARCH INITIATIVE ON NEURAL HEALTH
The NIA should undertake a major research initiative to build the scientific basis for promoting neural health in the aging brain.
Recent research opens the possibility that changes in neuronal health play a major role in cognitive function in older persons who are not suffering from Alzheimer's disease or related dementias. Indeed, cell regeneration may occur. Understanding the mechanisms affecting neural health through the life cycle can lead to health-maintaining interventions. Major advances are possible from research aimed at four goals: developing quantitative markers for neuronal health and neuronal dysfunction; identifying factors that affect neural health during the aging process; devising interventions for the maintenance of healthy neurons and the rescue and repair of dysfunctional neurons; and assessing the efficacy of intervention using quantitative biomarkers. This research will identify and evaluate biochemical, behavioral, and other interventions that can help maintain neural health and, by doing that, contribute to maintaining cognitive function in older people. The research initiative should emphasize four elements corresponding to these goals.
1. Developing quantitative functional and performance indicators that are indicative of the functional integrity of neurons with special emphasis on the aging brain.
Studies are needed to identify quantitative markers that can be monitored to assess the overall health of single neurons as well as the function of the neural network. These markers would complement indicators of cell death by providing indicators of degrees of neural health. Developing a catalogue of these markers will require molecular, biochemical, neuroanatomical, imaging, electrophysiological, and behavioral studies; the markers themselves may
require all these types of measurements. These markers can be utilized to test the efficacy of interventions aimed at maintaining and recovering neural health.
2. Identifying factors that affect neural health during aging.
Research under this element of the neural health initiative would examine the extent to which changes in brain function are due to neural dysfunction other than neural loss. These studies would examine hypotheses regarding mechanisms that underlie neural dysfunction, especially those affecting brain function and neural health through gene expression and through such homeostatic processes as apoptosis, inflammation, and oxidative activity, which may have either beneficial or detrimental effects.
Research under this element of the initiative would also examine factors in life experience that may affect neural health. These studies would focus on such factors as past cognitive training, occupational experience, mental activity across the life span, and other life history and cultural factors that may affect neural health during the aging process, thus linking the neural health initiative to the initiative on cognition in context (see Chapter 3). This element of the initiative would seek to specify the life course of processes affecting neural health in aging. It would support research to develop and test models of the processes that affect neural health in aging and that link neural health to cognitive perfornance.
3. Devising interventions for the maintenance of healthy neurons and the rescue and repair of neural networks.
Studies are needed to develop strategies for maintaining healthy neurons and recovering dysfunctional neurons. These strategies may involve interventions at the molecular, cellular, and behavioral levels. Studies may involve such diverse interventions as transplantation of differentiated and genetically engineered stem cells; gene delivery by transgenic and viral vector mediated approaches; the administration of drugs and nutritional supplements, especially those predicted to be beneficial from epidemiological studies; and training and other behavioral and cognitive interventions. Government support is particularly necessary for trials aimed at clinically testing the efficacy of non-prescription items, such as over-the-counter drugs and nutritional supplements, that have anti-inflammatory or antioxidant properties. Some of these have been associated with reduced incidence of dementia in epidemiological studies (Breitner, 1996; Morris et al., 1998), but the private sector is unlikely to support clinical trials of inexpensive, freely available therapeutics for which there is no patent protection. The NIA might support some such studies in
collaboration with the National Center for Complementary and Alternative Medicine.
4. Assessing the efficacy of intervention strategies using quantitative, functional, and performance indicators.
Research under this element of the neural health initiative would employ a variety of methods to evaluate the success of intervention strategies. These methods will rely heavily on the quantitative markers for neural health identified in the first element of the research initiative. For evaluation purposes, it is imperative to combine molecular, biochemical, imaging, neuroanatomical, electrophysiological, and behavioral assays. Thus, the effects of intervention may be monitored by:
measuring global and region-specific changes in brain volume and neural activity with such techniques as structural or functional magnetic resonance imaging, positron emission tomography, and magnetoencephalography,
conducting high-resolution ultra-structural analyses (e.g., with confocal and electron microscopy or magnetic resonance microscopy),
electrophysiological techniques, including single-unit recording, ensemble recording, and evoked potential studies,
behavioral assays, including existing and improved cognitive tests designed to be sensitive to the integrity of specific brain regions targeted for repair,
gene expression profiling using chip technology, and
biochemical and molecular markers of apoptosis, inflammation, and oxidative stress.
The neural health initiative will require much more intensive involvement of behavioral scientists than has been typical in past neuroscience research. Behavioral scientists will be needed to develop some of the quantitative markers used to measure neural health and assess the effectiveness of interventions. They will also be needed to develop and test hypotheses about the ways life experience may affect neural health. Some useful behavioral research under this initiative can go on in disciplinary fashion, but some will require true integration of behavioral science and neuroscience. For example, the most fruitful hypotheses about how certain life experiences might protect against cognitive decline are likely to postulate causal mechanisms that involve experiential influences on biochemical processes in the brain. And in order to evaluate some molecular-level and cellular-level interventions, it will
be necessary to identify or create behavioral measures of the functions the interventions are intended to improve. The issue of measure development is discussed in some detail in Chapter 4.
The research initiative will also depend on the availability of appropriate infrastructure. For instance, easy access to aged transgenic and naïve animals in the scientific research community is critical, as is the development of standardized behavioral testing paradigms for mice and rats. Emphasis in this latter area has traditionally been placed on attempting to develop behavioral protocols for mice and rats that can be related to human cognition. It is also worth considering ways to murinize behavioral testing paradigms for humans, that is, to use types of cognitive tests with aging humans that can tap specific functions that have also been studied in rodent models (for a recent example, see Kahana et al., 1999). These issues of interdisciplinary collaboration and infrastructure are discussed in more detail in Chapter 5.