Leptin Induces Hippocampal Synaptogenesis via CREB-Regulated MicroRNA-132 Suppression of p250GAP

Leptin acts in the hippocampus to enhance cognition and reduce depression and anxiety. Cognitive and emotional disorders are associated with abnormal hippocampal dendritic spine formation and synaptogenesis. Although leptin has been shown to induce synaptogenesis in the hypothalamus, its effects on hippocampal synaptogenesis and the mechanism(s) involved are not well understood. Here we show that leptin receptors (LepRs) are critical for hippocampal dendritic spine formation in vivo because db/db mice lacking the long form of the leptin receptor (LepRb) have reduced spine density on CA1 and CA3 neurons. Leptin promotes the formation of mature spines and functional glutamate synapses on hippocampal pyramidal neurons in both dissociated and slice cultures. These effects are blocked by short hairpin RNAs specifically targeting the LepRb and are absent in cultures from db/db mice. Activation of the LepR leads to cAMP response element–binding protein (CREB) phosphorylation and initiation of CREB-dependent transcription via the MAPK kinase/Erk pathway. Furthermore, both Mek/Erk and CREB activation are required for leptin-induced synaptogenesis. Leptin also increases expression of microRNA-132 (miR132), a well-known CREB target, which is also required for leptin-induced synaptogenesis. Last, leptin suppresses the expression of p250GAP, a miR132 target, and this suppression is obligatory for leptin's effects as is the downstream target of p250GAP, Rac1. LepRs appear to be critical in vivo as db/db mice have lowered hippocampal miR132 levels and elevated p250GAP expression. In conclusion, we identify a novel signaling pathway by which leptin increases synaptogenesis through inducing CREB transcription and increasing microRNA-mediated suppression of p250GAP activity, thus removing a known inhibitor of Rac1-stimulated synaptogenesis.

Estradiol Increases Pre- and Post-Synaptic Proteins in the CA1 Region of the Hippocampus in Female Rhesus Macaques (Macaca mulatta)

The role of estrogen (E) in promoting learning and memory in females has been well studied in both rodent and primate models. In female rats, E increases dendritic spine number, synaptogenesis, and synaptic proteins in the CA1 region of the hippocampus, an area of the brain that mediates learning and memory. In the present study, we used radioimmunocytochemistry to examine whether E and progesterone were capable of modulating the levels of pre- and postsynaptic proteins in the CA1 region of the female nonhuman primate hippocampus. It was found that E increased syntaxin, synaptophysin (presynaptic), and spinophilin (postsynaptic) levels in the stratum oriens and radiatum of the CA1 region, whereas combined E and progesterone treatment decreased these synaptic proteins to the levels found in untreated, spayed controls. Furthermore, progesterone treatment alone significantly increased synaptophysin levels in the stratum oriens and radiatum of the CA1 region. The levels of these synaptic proteins were unaltered by hormone treatment in the dentate gyrus, suggesting that this steroid-induced plasticity is hippocampal region specific. As these synaptic proteins are important components of the synaptic apparatus and reliable markers of synaptogenesis, it appears that E-induced increases in cognitive function of higher order mammals may be mediated in part by the effect of E on hippocampal synaptogenesis and synaptic plasticity.