Alexis M. Stranahan, Ph.D. - US grants

[unreadable] DESCRIPTION (provided by applicant): The brain continues to generate new neurons in the hippocampus throughout adulthood. The process of neurogenesis is influenced by environmental factors such as learning, stress, spatial complexity, and physical activity. The proposed research will explore the control of neurogenesis by wheel running activity, and investigate the temporal and physiological parameters of this phenomenon. In addition, the volitional aspect of exercise will be examined using forced treadmill and voluntary running wheel activity. The thymidine analog 5-bromodeoxyuridine (BrdU) is typically used in studies of cell proliferation and survival in the hippocampus. However, when studying environmental factors that elevate the circulatory system, the issue of availability causes difficulty in interpretation of results. To this end, immunolabeling for the endogenous markers Ki-67 and phospho-histone H3 will be applied to provide a measure of exercise-induced neurogenesis that is not affected by systemic elevation. Physical activity has been correlated with a lower incidence of dementia in older adults, and the data from studies of exercise-induced neurogenesis may one day be applied to the prevention or treatment of neurodegenerative disease. [unreadable] [unreadable]

DESCRIPTION (provided by applicant): Normal aging is typically accompanied by deficits in cognitive domains associated with the function of the hippocampus and temporal lobe. These functional deficits include the loss of spatial and episodic memory, recapitulating the symptoms of damage to the temporal structures. However, not all aged humans develop severe memory loss. The preservation of cognitive function in normal aging can be modeled in rodents using behavioral characterization on tasks that serve as a readout for hippocampal function. This model is useful in identifying mechanisms that are specific to aging, and separating those mechanisms from factors that determine successful or unsuccessful maintenance of cognitive function. A number of developmentally regulated proteins are emerging as mediators of central neuroplasticity in the adult. Among this group, reelin has been identified as a signaling glycoprotein that regulates neuronal positioning during development, and synaptic plasticity in the adult. Reelin localizes to the cytoplasm and extracellular matrix, where it competes with apolipoprotein E (APOE) for activation of the APOE receptor-2 (AP0ER2) and the very low density lipoprotein receptor (VLDLR). Even in the absence of gross developmental abnormalities, interference with reelin signaling through the AP0ER2 impairs hippocampal function at the behavioral and synaptic level. Although several studies now suggest a role for reelin signaling in learning in the adult brain, the contributions of reelin to neuroplasticity in the aging brain are still being elucidated. In our preliminary data, we observed a positive correlation between cognitive function, assessed using the water maze task, and levels of mRNA for the reelin intracellular signaling target, disabled-1 (DAB1) in aged rats. However, the functional significance of changes in reelin signaling with aging has yet to be determined. The studies in this proposal are designed to resolve the contributions of reelin signaling to age-related functional decline. By using both in situ hybridization and immunohistochemistry, we plan to identify the regional contributions to alterations in reelin signaling in the hippocampus and entorhinal cortex. After measuring the regional differences in mRNA and protein for reelin in behaviorally characterized aged rats, we plan to manipulate reelin levels to determine whether entorhinal cortical reelin expression is related to hippocampal learning and memory. The purpose of these studies is to identify novel activity-dependent mechanisms that govern cognitive decline.

The broad, long-term objective of the proposed research is to identify the underlying mechanisms for the increased susceptibility to age-related cognitive decline and dementia in obesity and diabetes. This objective will be addressed through a series of experiments in two mouse models of obesity arising from diet or genetic manipulation. The central hypothesis is that fat is a source of inflammatory cytokines that impair cognition by inducing local inflammation in the hippocampus, a brain region involved in learning and memory. While the brain is normally protected from systemic inflammation, the protective barrier between blood and brain is compromised in obesity, and the goal of these studies is to determine what blood-borne signal contributes to cognitive impairment in obesity and diabetes. These experiments are relevant to the goals of the National Institute of Diabetes, Digestive, and Kidney Diseases (NIDDK) because of the focus on inflammation in obesity and diabetes, and the implications of increased inflammation for cognition in individuals suffering from these debilitating conditions. The proposed studies will take place in the Physiology Department at the Medical College of Georgia, which has an institutional track record of excellence in diabetes and obesity research. The environment is extraordinarily supportive and conducive to the research career development plan described in the current application, which includes didactic and experimental approaches to further the candidate's goal of establishing an independent research program in neuroimmunology. The rationale for additional training under this award is twofold: first, the candidate has demonstrated excellence in neuroendocrinology, but has no experience with the methods and theories of immunology, therefore in order to pursue the studies in this proposal, additional training is required; and second, although the candidate currently holds an Assistant Professorship, the short duration of the candidate's postdoctoral training (2 years) means that additional time and training will be required in order to establish a productive research program in neuroimmunology. The candidate's immediate career goal is to establish an externally funded research program and demonstrate independence in publications. Over the long term, the candidate hopes to conduct research that identifies molecular mediators linking metabolism and immune function with cognition. By building a bridge between diabetes and obesity and the field of learning and memory research, it may be possible to develop personalized interventions to prevent increased rates of cognitive impairment and dementia in individuals with metabolic syndrome features.

DESCRIPTION (provided by applicant): Rates of depression and anxiety are greater among individuals with type 2 (insulin resistant) diabetes. Diabetes is associated with an increased prevalence of clinical depression. Changes in brain structure in diabetes occur within temporal lobe circuits that are also sensitive to stress-related mood disorders. Diabetics exhibit hippocampal atrophy, particularly in the dentate gyrus subfield, and hippocampal atrophy is associated with changes in memory and mood regulation in this population. Given the established role of hippocampal activity in appropriate termination of the hypothalamic-pituitary-adrenal axis (HPA axis) response to stress, it is likely that there is some mechanistic relationship between impairment of hippocampal function and HPA axis dysregulation in insulin resistant individuals. The studies in this proposal are designed to determine whether hippocampal atrophy disinhibits the HPA axis, initiating a cycle of glucocorticoid- mediated synaptic impairment in a genetic mouse model of obesity and insulin resistance. We will test this model using leptin receptor mutant mice (db/db mice), which are obese and diabetic and exhibit elevated levels of corticosterone, the primary glucocorticoid in rodents. Hippocampal brain-derived neurotrophic factor (BDNF) expression is significantly reduced and we have preliminary data demonstrating that negative feedback on the adrenocortical response to stress is impaired in db/db mice. We plan to manipulate BDNF expression using a lentivirus to determine the network consequences of alterations in hippocampal neurotrophic factor expression for HPA axis negative feedback. We will measure markers of neural activation in chemically identified neuronal populations to evaluate recruitment along the circuitry mediating HPA axis shutoff after restraint stress. These studies in leptin receptor deficient mice could elucidate mechanisms related to the comorbidity between diabetes and depression in human populations.

PROJECT SUMMARY Although some human studies suggest that obesity impairs memory and cognition, this finding is not consistent and other groups have failed to find similar associations. Because the ?apple-shaped? anatomical pattern that accompanies visceral obesity has different physiological consequences than the 'pear-shaped' distribution that reflects subcutaneous adiposity, the variable cognitive outcomes reported in obese humans may be due to the use of weight/height ratios to define obesity. Visceral obesity is accompanied by peripheral inflammation and impairs memory, while subcutaneous adiposity does not activate the innate immune system and may protect against obesity-induced cognitive dysfunction. Resident microglia in the central nervous system represent the first line of defense against inflammation, and microglial activation in impairs synaptic plasticity and cognition in other disease models. Microglia regulate neuronal plasticity via receptor-mediated signaling and physical interactions involving synaptic phagocytosis or ?stripping,? but neither of these processes has been investigated in obesity. We hypothesize that visceral and subcutaneous adiposity exert opposing effects on synaptic stripping, and that microglial interactions with neurons determine the cognitive consequences of obesity.