Kristy L. Townsend, Ph.D. - US grants

Dissertation Research: Hormonal Correlates of Prehibernatory Fattening in Bats
Eric P. Widmaier and Kristy Townsend
Boston University

Obesity is associated with adverse health consequences in people and other animals. However, increased fat accumulation is also an important survival adaptation during certain stages of the life histories of many animals. For example, fat deposition is beneficial to successful pregnancies. Another example of adaptive fattening occurs in hibernating mammals. How seasonal animals deposit adequate fat stores to survive hibernation is currently unresolved, however; clarifying the mechanism of prehibernatory fattening is a key long-term goal of this research.
Leptin is an important hormone through which adipose tissue communicates with the brain in mammals. Leptin is produced by adipose cells in proportion to body fat. It acts within the brain by activating cell signaling molecules that result in increased metabolic rate and decreased appetite. During the prehibernatory period, when fat must be accumulated, the brain must somehow ignore the appetite-suppressing action of leptin. One way in which this could happen is if the cells of the brain which normally respond to leptin, fail to do because of reduced signaling molecules or leptin receptors. A second way in which the brain can be released from leptin inhibition is to reduce the amount of leptin secreted by adipose tissue. This project will test both possibilities by examining adipose tissue secretion of leptin in vitro in little brown bats before or during the prehibernatory fattening period. The activational state of the leptin gene will be determined using a sophisticated procedure called MALDI-TOF-mass spectrometry, which is capable of distinguishing the ratio of modified bases in DNA to non-modified bases (a marker of one key way in which genes are activated). In addition, the brains of the animals will be examined to test the hypothesis that active forms of the leptin receptor and the signaling molecules generated by the receptors are decreased in the prehibernatory period. Little brown bats are chosen as experimental subject animal for many reasons. They are extremely abundant and exist in very large, thriving maternity colonies and prehibernatory colonies, are of great importance ecologically, and are among the most well-understood hibernators in terms of their basic physiology and reproduction, providing substantial information on which to base this study.
Broader impacts of this research include: integrated training of one graduate student and several undergraduate students in field biology and molecular endocrinology; providing critical information about survival strategies in an ecologically important mammalian order; and possible relation of mechanisms of seasonal changes in leptin biology to abnormal leptin biology in obesity.

In order to maintain proper energy balance and metabolic health, the body must tightly regulate the processes that control energy intake (appetite, food intake, nutrient absorption) as well as energy expenditure (physical activity, basal metabolism, thermogenesis). An important aspect of regulating energy expenditure is the transfer of signals from the brain through peripheral nerves to activate lipolysis and thermogenesis in white and brown adipose tissues, respectively. Cold-stimulation is able to increase the sympathetic innervation and activation of adipose tissues and thus increase energy expenditure through lipolysis and thermogenesis. The exact mechanisms by which cold (or other stimuli that increase energy expenditure) are able to mediate peripheral nerve plasticity are currently under-investigated and largely unclear. In the current project, we provide new evidence that white adipose tissue (WAT) undergoes increases in neural innervation after cold exposure or exercise in mice (plasticity), and reductions in neural innervation with aging or obesity/diabetes in mice and humans (neuropathy). In addition, we have demonstrated that adipose-resident immune cells are able to secrete the neurotrophic factor Brain Derived Neurotrophic Factor (BDNF), which we believe stimulates sympathetic nerve branching, neurite outgrowth, and synapse formation in order to stimulate energy-expending processes in adipose depots. Indeed, in models of adipose neuropathy such as aging, BDNF levels are significantly decreased in WAT. BDNF is well-studied in the brain, but has not been investigated for adipose tissue neurotrophic activity. We have found that BDNF is expressed in immune cells of the stromovascular fraction (SVF) of WAT, and that the secretion of BDNF increases after cold or noradrenergic stimulation. Deletion of BDNF from the myeloid lineage results in a striking and specific lack of neural innervation of adipose depots, without affecting other nerves in the brain, spinal column or neuromuscular junction. As a result of this `genetic denervation' we observed that the knock-out (KO) animals undergo a shift in energy balance that leads to increased adipose mass and lower energy expenditure, including a lack of UCP1 induction in WAT after cold exposure. We specifically hypothesize polarized macrophages in adipose tissue SVF act similarly to microglia in the brain ? that is, they can be either immune cells that release nerve growth factors in response to injury or neuroplasticity needs, or they phagocytose neurites, leading to neuropathy. We have identified a population of macrophages we are calling cold-induced neuroimmune cells (CINCs) that we hypothesize secrete BDNF in response to cold/noradrenergic stimulation. In addition to investigating these mechanisms for adipose nerve plasticity and neuropathy, this project also seeks to better understand the types of nerves that innervate adipose as well as how proper innervation affects adipose tissue function, whole-body metabolism and the control of energy balance.