Humboldt-Universität zu Berlin - Mathematisch-Naturwissen­schaft­liche Fakultät - International Research Training Group 1740


Project D3: Cell signalling on hierarchical molecular networks

Research Team: G. Antunes (USP), M. Falcke (HUB), A. Roque (USP)

Outline: Signalling by chemical messengers is used for communication between and inside living cells, and for control of cell responses to environmental changes. The network motif appears on all structural levels. The communicating cells itself are nodes of a cell network, neurons form a web of dendrites, the molecular components of the signalling pathways (the sequence of biochemical reactions transmitting the information) form an interaction network, and on the lowest structural level individual molecules form networks as clusters of (sometimes identi-cal) elements. Within pathways, these clusters may represent an individual functional ele-ment. That causes a network hierarchy. This project deals with such a hierarchy of molecular networks both as the interaction network of signaling pathways as well as clusters of identical molecules.

Research in Germany: Signalling by Ca2+ ions is one of the initial step of long-term forms of synaptic plasticity in neurons. Puffs are the elemental event of Ca2+ signaling. The term describes the stochastic release of Ca2+ from intracellular storage compartments through small clusters of Ca2+ channels (1-15 IP3 receptor Ca2+ channels). The dynamics of these clusters has been the subject of research for about 15 years by now. However, recently new experimental results posed new questions. It has been discovered, that a specific cluster may produce rather short release events as well as another type of events lasting almost an order of magnitude longer. Duration and amplitude distributions of both event types have been measured. At the same time, the channels in the cluster exhibit coupled gating, i.e. closing of a channel increases the closing probability of other open channels. Long event duration and coupled gating appear intuitively incompatible. The research in Germany would like to address this topic by stochastic modelling of IP3 receptor Ca2+ channel clusters.

Research in Brazil: Long-term depression (LTD) and long-term potentiation (LTP) of granule-Purkinje cell synapses are long-lasting alterations in the efficacy of the synaptic transmission induced by elevations of the intracellular Ca2+ concentration [Ca2+]. It is generally accepted that LTP induction requires a moderate rise of [Ca2+], while LTD is induced by large changes in the [Ca2+]. The occurrence of LTP involves the activity of phosphatases, including the Ca2+-regulated phosphatase calcineurin, and the increase in the number of AMPA receptors in the postsynaptic membrane. LTD requires the removal of AMPA receptors from the postsynaptic membrane and the activation of a feedback loop formed by the Ca2+-dependent protein kinase C (PKC), the mitogen-activated protein kinase (MAPK), and the cytosolic phospholipase A2 (cPLA2). Recently, a stochastic kinetic model developed to simulate LTD induced by single Ca2+ pulses demonstrated that, in single synapses, LTD is probabilistic and bistable. This model has been expanded to include the signaling network involved with LTP. Its preliminary results indicate that both LTP and LTD in single granule-Purkinje cell synapses occur in a stochastic manner when stimulated by single Ca2+ pulses. However, in intact animals, the induction of LTP and LTD requires complex Ca2+ signals that arise primarily from the activation of IP3 receptors following synaptic stimulation. Thus, the research in Brazil intends to develop a stochastic model of the Purkinje cells Ca2+ dynamics containing the activation of IP3 receptors as the source of Ca2+ in single synapses. The stochastic model of IP3 receptor Ca2+ channel clusters that will be developed in Germany will be used in this model to verify the different patterns of Ca2+ changes obtained with protocols of LTP and LTD induction and the factors that regulate these Ca2+ signals. In a next stage, the model of Ca2+ dynamics will be incorporated in the stochastic model of the signaling network of LTP and LTD to address the role of complex patterns of Ca2+ signals in the induction of both forms of long-term synaptic plasticity.