Organization of voltage gated ion channels in the neuronal membrane
A key element of cellular signaling is the spatiotemporal pattern of intracellular calcium changes. Within neurons intracellular calcium changes are transient and triggered to a large extend by the activation of voltage-gated calcium ion channels (VGCC). The opening of these channels is determined by the width and frequency of action potentials traveling along the neuronal membrane. Hence, the transmembrane calcium flux depends on activation time, kinetic properties of the channel and local density of VGCC in the membrane. Thus using fluorescent tagged subunits of VGCC and using localization microscopy techniques we investigate how dynamic are VGCC located in the membrane. First results on dendritic VGCC (L-type) show a confined mobility of VGCC in surface clusters where single channels exchange between such clusters (Di Biase et al. 2011).
The organization of presynaptic calcium channels (P/Q- and N-type) is partially dependent on the interaction with accessory channel subunits, particular the alpha2delta-subunits. Comparing the dynamic of alpha1- and alpha2delta-subunits we currently investigate how the surface interaction of these subunits modulates VGCC localisation and function.
Dynamic of adhesion molecules in the synapse
Adhesion molecules are seen as trans-synaptic anchors to accumulate and organize signaling proteins within the synapse as well as keep the apposition of the pre- and postsynaptic membrane. The time course of receptor accumulation and the critical number of proteins interacting with neuroligin1 are critical for postsynaptic assembly and AMPAR density (Mondin et al. 2011, Czondör et al. 2012).
Beside the synaptogenic function of the adhesion complex neurexin/neuroligin there are indications that this complex is also needed to maintain synaptic contacts. In particular, the alpha-neurexins are proposed to interact with presynaptic calcium channels (Cav 2.2, Cav 2.1; Missler et al. 2003) to ensure a certain probability of transmitter release. In collaboration with Prof. M. Missler we currently investigate the dynamic of alpha-neurexins in the axonal membrane and their interaction partners.
Monitoring neuronal network activity
In order to characterize the importance of molecular dynamics within neuronal networks we use multielectrode arrays. Here we have the ability to investigate neuronal networks in isolation without sensory input. This gives us the advantage to monitor activity and local molecular organization simultaneously to connect molecular dynamic with network output.
Removing the extracellular matrix (ECM), a structure that influence surface organization of signaling molecules as AMPAR (Frischknecht et al. 2009), does indeed interfere with network function and do show the feasibility of this approach. In current approaches we start to investigate how surface dynamic of VGCC subunits will tune network activity.
In parallel we currently develop methods to label and monitor single molecules in the complex cellular environment of a brain slice, in order to combine the molecular readout with the most common preparation to investigate neuronal plasticity on a cellular basis, the acute brain slice.