Lecture abstracts will be updated soon.
| title | Dissecting the cell cycle oscillator |
| speaker | James Ferrell |
| abstract | We have been studying the systems-level design principles of the Cdc2-APC system, the network of proteins responsible for driving cell cycle oscillations. The system is driven by alternating periods of cyclin accumulation (during interphase) and destruction (during M-phase). The switch between accumulation and destruction is carried out by a negative feedback loop. In addition, there are multiple positive feedback loops: active Cdc2 activates its own activator Cdc25, and active Cdc2 inactivates its inhibitors Wee1 and Myt1. We have investigated the roles of the positive and negative feedback loops individually, using biochemically-tractable Xenopus egg extracts as our experimental system and computational modeling to guide our experiments. |
| reference |
|
| title | BDNF and Hippocampal Synaptic Plasticity |
| speaker | Bai Lu |
| abstract | While traditionally viewed as secretory growth factors involved in long-term survival and differentiation of neurons, neurotrophins have now been recognized as important factors that regulate synaptic transmission and plasticity, synapse development, and learning and memory. We have previously demonstrated that brain-derived neurotrophic factor (BDNF) acutely facilitates hippocampal long-term potentiation (LTP), a cellular model for learning and memory. A prevailing phenomenon is that BDNF preferentially modulates active synapses. To address how diffusible molecules such as BDNF elicit local and synapse-specific modulation, we have carried out a number of studies on the cell biology of BDNF and its receptor TrkB. One mechanism is the activity-dependent control of the trafficking of the TrkB receptors. We demonstrated that high frequency neuronal activity facilitates the insertion of TrkB on cell surface. The endocytosis of the BDNF-TrkB complex, which is critical for certain functions of BDNF, is also regulated by neuronal activity and BDNF-induced tyrosine kinase phosphorylation of the TrkB itself. Recently, we have demonstrated that neuronal activity and consequent elevation of intracellular cAMP concentration control TrkB translocation into the dendritic spines as well as TrkB tyrosine kinase activity. Thus, activity control of TrkB receptors is mediated by multiple mechanisms. The second way by which the synapse-specific BDNF regulation could be achieved is the activity-dependent secretion of BDNF locally at synapses. Our recent studies showed that synaptic targeting and activity-dependent secretion of BDNF may be critical for human memory and hippocampal function. In the human BDNF gene, there is a single nucleotide polimorphism (SNP) that convert a valine (val) to a methionin (met) in the 5' pro-region of the BDNF protein. Human subjects with met allele exhibit lower neuronal activity and abnormal hippocampal activation, as well as poorer episodic memory. We showed that in transfected hippocampal cultures met-BDNF cannot be correctly targeted to the synapses and cannot undergo activity-dependent secretion. Recently studies indicate that pro-neurotrophins could serve as signalling molecules by interacting with p75 neurotrophin receptor (p75NTR). Interestingly, pro-neurotrophins often elicit biological effects that are opposite to those induced by mature neurotrophins. Thus, protealytic cleavage of pro-neurotrophins, either intracellularly or extracellularly, represents a new mechanism that controls the direction of neurotrophin regulation. In a recent study, we demonstrated that tPA, by activating the extracellular protease plasmin, converts the precursor proBDNF to mature BDNF, and that such conversion is critical for L-LTP expression in the hippocampus 11. There is also evidence that secreted proBDNF, if not cleaved, binds p75NTR on the dendritic spines of CA1 pyramidal neurons, leading to an enhancement of NMDA-dependent LTD. Thus, cleavage of proBDNF (or not) by tPA/plasmin system may serve as an important mechanism for bi-directional control of hippocampal plasticity: uncleaved proBDNF enhances LTD through p75NTR, while mature BDNF facilitates LTP though TrkB. Taken together, our studies provide new insights into the molecular mechanisms for the synaptic functions of neurotrophins. |
| reference |
|
| title | Distinc Dynamics of transient and sustained ERK activation |
| speaker | Shinya Kuroda |
| abstract | To elucidate how epidermal growth factor (EGF) and nerve growth factor (NGF) specifically encode their distinct physical properties into transient, and transient and sustained extracellular signal-regulated kinase (ERK) activation respectively, we developed a kinetic simulation model of ERK signalling networks by constraining in silico dynamics on the basis of in vivo dynamics in PC12 cells. Measured in vivo dynamics can be consistently reproduced in dose- and temporal-dependent manners in growth factors in silico. This model allowed us to predict in silico and validate in vivo that transient ERK activation depends on rapid increases of EGF and NGF but not on their final concentrations, whereas sustained ERK activation depends on the final concentration of NGF but not on the temporal rate of increase. This system of ERK dynamics is produced by Ras and Rap1 dynamics, which capture the rapid temporal rate of growth factors and the final concentration of NGF, respectively. These results indicate that the Ras and Rap1 systems capture the temporal rate and concentration of growth factors, and encode these distinct physical properties into transient and sustained ERK activation, respectively. |
| reference |
|
| title | Molecular mechanisms of neurite growth: Co-ordinated roles of the cytoskeleton, cell adhesion molecules, and membrane microenvironment |
| speaker | Hiroyuki Kamiguchi |
| abstract | During development, neurons send out neurites that differentiate into axons or dendrites. The axons elongate along the correct path to reach their appropriate targets. The tip of neurites, called the growth cone, expresses a variety of functional molecules including the cytoskeleton (actin filaments and microtubules) and cell adhesion molecules (CAMs). Actin filaments move in a backward direction and generate traction force that can be transmitted to the extracellular substrate via CAMs. The growth cone internalizes and recycles CAMs in a region-specific manner to polarize its adhesivity. Furthermore, CAM-associated signals could be influenced by the lipid microenvironment in the cell membrane. In this seminar, I will demonstrate the importance of dynamic and co-ordinated functions of these molecules in neurite growth. |
| reference |
|
| title | Calcium signaling in growth cone induced by diffusible guidance molecules |
| speaker | Kyonsoo Hong |
| abstract | Growth cone navigation is an essential process in establishing neural connections in the developing nervous system. Nerve growth cones are guided to specific targets by substrate-bound or diffusible guidance molecules. Most known guidance molecules have bi-functional roles, serving either as attractants or repellents, depending on the developmental stage or the specific classes of neurons they encounter. Using the nerve growth cone turning assay, combined with whole-cell recordings and Ca2+ imaging of Xenopus spinal neuronal growth cones, our studies seek to understand the molecular mechanisms underlying the bi-functional roles of these guidance molecules. Our studies have found that Ca2+ influx through voltage-dependent Ca2+ channels and Ca2+-induced Ca2+ release from internal Ca2+ stores tightly regulate intracellular Ca2+ levels which, in turn, mediate the polarity of growth cone turning in response to the diffusible guidance molecules netrin-1 and Sema3A. However, different classes of Ca2+ channels respond to each of these distinct guidance signals, suggesting that coordinated regulation of guidance molecules and Ca2+ channels account for specific neuronal guidance during development. |
| reference |
|
| title | Multiple modulations of neuronal functions by brain-derived neurotrophic factor (BDNF) |
| speaker | Masami Kojima |
| abstract | Some genes and chromosomal domains have been associated with risks for the illness, although no single gene or genes have been defined as causal for this complicated brain dysfunction. The gene for brain-derived neurotrophic factor (BDNF) has been suggested to be implicated in the risk for schizophrenia. Individual human has one or more different sequential variations (alleles) that are determined in part by past mutations and their permanent incorporation in the genome. This is also the case for BDNF gene. One frequent and non-conservative polymorphism for BDNF is a single nucleotide polymorphism (SNP) found in its pro-domain whose biological functional role remains to be fully understood. In the rodent brain, BDNF facilitate long-term potentiation in hippocampus and cortex, by enhancing synaptic transmission and vesicle docking. Therefore, we have looked for subtle cognitive and physiological differences between persons with different BDNF alleles and the underlying molecular mechanisms using several measures. Here I present our recent studies which demonstrate that different polymorphisms in the BDNF gene alter the distribution of its product, activity-dependent secretion manner, biological activity to decide neuronal survival or death, electrophysiological property to elicit synaptic transmission, and cognitive performance on tests of human learning and memory using an array of different tools. This integrative approach is of the most importance as it suggests that a polymorphism in neuronal genes modulates cognitive performance and clinical expression of brain dysfunction in human. In this lecture, I also talk about a novel mechanism of BDNF signaling through its receptor TrkB on cell surface. One idea for specific signal-function coupling is that different signaling pathways may be transduced in different subcellular compartments. More specifically, it has been proposed that cholesterol/sphingolipid-rich micro-domains called lipid rafts make a specialized signaling platform in the plasma membrane, and therefore can transduce signals different from those in the non-raft membrane. Here we show that BDNF rapidly recruits full-length TrkB (TrkB-FL) receptor into cholesterol-rich lipid rafts from non-raft regions of neuronal plasma membranes. Translocation of TrkB-FL was blocked by Trk inhibitors, suggesting a role of TrkB tyrosine kinase in the translocation. Disruption of lipid rafts by depleting cholesterol from cell surface blocked the ligand-induced translocation. Moreover, disruption of lipid rafts prevented potentiating effects of BDNF on transmitter release in cultured neurons and synaptic response to tetanus in hippocampal slices. In contrast, lipid rafts are not required for BDNF regulation of neuronal survival. Therefore, ligand-induced TrkB translocation into lipid rafts may represent a signaling mechanism selective for synaptic modulation by BDNF in the CNS. |
| reference |
|
SNSS2004
SNSS2004