Gary Wayman, PhD - US grants

DESCRIPTION (PROVIDED BY APPLICANT): Major Depressive Disorder (MDD) is a recurrent mental illness that afflicts approximately 1 in 6 Americans at least once during their lifetime. While the causes are likely to be multi-factoral, the onset of MDD correlates with structural defects in the hippocampus, including alterations in dendritic morphology. The levels of neurotrophic factors like Brain Derived Neurotrophic Factor (BDNF), which stimulates dendritic growth, are decreased in both humans suffering from MDD and in animal models of MDD. However, while the decrease in BDNF levels is known to correlate with alterations in dendritic morphology, the underlying signaling mechanisms by which BDNF stimulates dendritic growth are unknown. Identifying these pathways is critical to understand how dendritic development and synaptogenesis occurs normally and how it is altered during bouts of MDD. Our long-range goal is to identify the intracellular signals by which neurotrophic factors control dendritic development and to determine how these signals are regulated in both normal and depressed brains. The objective of this application is to determine the signaling pathways by which one critical neurotrophic factor, BDNF, controls dendritic morphogenesis, synaptogenesis and the maintenance of synaptic structures. Our central hypothesis is that BDNF stimulates dendritic development via a coordinated response comprised of genomic regulation of both critical protein-coding genes and non-coding microRNAs whose expression are altered during depression. We have formulated this hypothesis on the basis of our published and unpublished data, which shows that an increase CREB dependent regulation of both protein coding genes and non-coding microRNAs are essential for BDNF dependent dendritic growth and synaptogenesis. The combination of these events not only activates pathways that promote dendritic growth but also inhibits pathways that suppress dendritic growth. We will test our central hypothesis with the following Specific Aims: 1. Determine the requirement of CREB regulated protein coding genes in BDNF-stimulated dendritic growth and synaptogenesis. 2. Determine the requirement for CREB regulated micro-RNAs in BDNF dependent dendritic growth and synaptogenesis. Determine the role of protein coding genes and micro-RNAs in vivo during normal development and during periods of altered BDNF expression. We will use biochemical, genetic, cell biological, behavioral and imaging approaches to address these specific aims. The rationale that underlies the proposed research is that once the critical signaling molecules linking BDNF to dendritic and spine remodeling become known they can be targeted for the treatment and prevention of MDD. These studies are innovative because they apply the expertise of the PI in signal transduction to a critical and under examined problem: to identify the molecular mechanisms underlying structural changes occurring during depression. We have enlisted the help of our consultant Jaak Panksepp, who has extensive experience in studying models of depression. Additionally, these results are expected to have a broad impact on the field as abnormalities in dendritic arborization and spinogenesis are common to numerous forms of mental retardation including Fragile X, Down's, and Rett syndromes. PUBLIC HEALTH RELEVANCE: The proposed research is relevant to public health because Major Depressive Disorder (MDD) is a debilitating mental illness that affects 1 in 6 Americans at some point in their lives. Unfortunately the currently available antidepressants only help a subset (50-70%) of patients with MDD. Onset of MDD is associated with a decrease in the levels of Brain Derived Neurotrophic Factor (BDNF). However, while the decrease in BDNF levels is known to correlate with alterations in dendritic morphology, the underlying signaling mechanisms by which BDNF stimulates dendritic growth are unknown. By understanding these processes, we are laying the foundation for the development of new therapies, such as genetic screens, gene therapies, and targeted drug design, to treat MDD.

Leptin is a critical neurotrophic factor during development. Its receptors (LepRs) are found throughout the brain, including in the hippocampus. Leptin deficiency is also associated with cognitive and emotional impairment, behaviors impacted by hippocampal function. Intriguingly, leptin levels rise during a critical developmental period when hippocampal synaptogenesis is occurring. We have demonstrated that leptin induces GABAergic synaptogenesis and controls Cl- homeostasis to promote an excitatory effect of GABA during this critical period. In contrast, a lack of leptin shifts the excitation/inhibition balance so that GABA is more inhibitory, and reduces GABAergic synaptogenesis. Excessive leptin during development (hyperleptinemia) prolongs the excitatory action of GABA and increases GABA receptor expression, suggesting that it may have long-term effects on hippocampal function. Intriguingly, leptin levels are elevated in children with early onset autism spectrum disorders (ASD) and Rett syndrome, a disease showing ?autistic-like? behaviors. Maternal obesity, which affects 1 in 5 pregnancies, is also associated with hyperleptinemia in humans, and also heightens the risk of ASD and other neuropsychiatric disorders in children. One potential mechanism by which maternal obesity, and the associated hyperleptinemia, could impact the likelihood of a child or an adult developing emotional and cognitive disorders is through alterations in the development, maintenance, function or plasticity of GABAergic connections. However, the effects of hyperleptinemia and maternal obesity on the development and function of GABA synapses is not known. Understanding how maternal obesity alters the developmental effects of leptin and the formation of critical hippocampal synaptic connections in vivo is an essential first step to understanding the mechanisms by which maternal obesity impacts hippocampal function later in life. Our central hypothesis is that leptin plays a key role in regulating GABAergic synaptic development and plasticity and that pathological hyperleptinemia alters this process through changes in the expression and membrane localization of key components of GABAergic synapses and regulators of Cl- homeostasis. We will test our central hypothesis with three specific aims. 1) Determine how leptin alters Cl- homeostasis and stimulates GABAergic synaptogenesis in vivo 2) Determine whether developmental leptin impacts GABAergic synaptic function and plasticity. 3) Determine if maternal obesity and associated hyperleptinemia alters GABAergic synaptogenesis, Cl- homeostasis and GABAergic synaptic function and plasticity. While we have focused on the hippocampus, this knowledge is expected to have broad impact, as it should also be applicable to leptin-induced synapse formation in other brain regions, including pathways critical for the control of food intake and energy homeostasis. This research therefore should have implications for both mental health disorders, such as mood, cognitive disorders and metabolic disorders such as obesity.