ROLE OF LEPTIN IN HIPPOCAMPAL SYNAPTOGENESIS

Thesis (Ph.D.), Program in Neuroscience, Washington State UniversityLeptin is a key regulator of food intake and energy homeostasis. In the brain, the hypothalamus is considered the primary site for leptin activity and its effects on feeding. However, leptin has been shown to affect hippocampal functioning and hippocampus-dependent behaviors such as spatial and emotion-related memory formation as well. Furthermore, intra-hippocampal leptin injection enhances memory retention and reduces depression like symptoms in animal models. Excitatory synaptic connections on hippocampal pyramidal neurons occur on small protrusions known as dendritic spines and abnormal hippocampal dendritic spine morphology and number are associated with cognitive and emotional disorders. In this dissertation, I explore the signaling mechanisms that mediate leptin's effects on synapse formation in the hippocampus. In chapter1, I show that a transient receptor potential channel C (TrpC) current activated by acute leptin treatment is important for leptin-induced filopodia formation. Filopodia are potential immature spines. Also, I reveal a signaling cascade by which leptin activates the TrpC current. Leptin increases membrane trafficking of TrpC channels via CaMKKβ, CaMKIγ, and β-Pix dependent activation of Rac1. In chapter2, I show for the first time that leptin induces formation of functional excitatory synapses/spines in hippocampal neurons. Additionally, leptin receptor (LepR) null db/db mice have lowered spine density in CA1 and CA3 regions of the hippocampus. I found that leptin acts through a cyclic AMP response-element binding protein (CREB)-dependent mechanism to induce these effects. CREB activity is required for glutamate synapse formation and is known to mediate the effects of neurotrophic factors on synapse formation. Activation of the LepR leads to CREB phosphorylation via the Mek/Erk pathway resulting in initiation of CREB-dependent transcription of microRNA-132 (miR132), a well-known CREB target. Increase in expression of miR132 and suppression of p250GAP expression via miR132 is required for leptin-induced synapse formation. In chapter3, I report a non-CREB dependent mechanism for leptin-induced synapse formation. Kruppel-like factor4 (KLF4) is a transcription factor first identified in the gut and skin epithelium. Leptin treatment increases KLF4 mRNA and protein expression in vitro. I found that KLF4 overexpression increases functional glutamate synapse/spine formation and KLF4 expression is required for leptin-induced synapse formation.Program in Neuroscience, Washington State Universit