Molecular regulation of the development of giant synaptic terminals
In the auditory system of birds and mammals large axosomatic synapses can be found: the giant synaptic terminals of Held. These terminals contain many (hundreds of) active zones and one can say they act as many synapses activated exactly simultaneously. The synaptic currents induced by these terminals are often big enough to individually cause an action potential in the postsynaptic neuron. The auditory nerve fibres, which are the axons of neurons in the cochlear ganglion (or spiral ganglion, in mammals) enter, in birds as well as in mammals, the auditory brainstem and bifurcate into two branches: one branch projects towards time-coding parts of the brainstem (birds: nucleus magnocellularis = NM; mammals: anteroventral cochlear nucleus = AVCN), the other branch connects to nuclei where sound intensity and spectral cues are predominantly coded (birds: nucleus angularis = NA; mammals: posteroventral cochlear nucleus and dorsal cochlear nucleus). The very same axon can thus form giant synapses in one target area while it terminates with regular synaptic endings (boutons) in a different part of the brain. It appears obvious, that properties of the target neurons induce the generation of giant synapses. Interestingly, axons from different origins (e.g. from the superior olivary complex in birds, or experimentally induced aberrant connections) do not form giant terminals on NM principal neurons. Therefore the interplay of signalling molecules on the surface of NM / AVCN neurons and the molecular properties of the auditory nerve axons must be crucial. Cell surface proteins with antero- and retrograde signalling properties that are candidates for this role are manifold.
In this project we try to ask fundamental cellbiological questions: which cell surface proteins known to play roles in genesis and differentiation of pre- and postsynapse are differentially expressed between NM and NA in development? Here we recently looked at the development of the postsynaptic organizer PSD-95 . We found that in birds the PSD-95 protein appears early and increase in PSD-95 levels, i.e. the size increase of the postsynaptic structures, seem to precede the presynaptic growth of the giant terminal. When also we analyze the synaptic physiology of the developing giant terminal to match the structural results with functional data.
In the future we want to further elucidate the role of postsynaptic organizers and transsynaptic signaling proteins in the size determination of synaptic connections using electrophysiological & pharmacological methods and molecular biology. We try to increase our understanding of molecular players involved in the synaptogenesis of giant terminals by exploiting the accessibility of the chicken embryo in ovo, which will allow targeted, temporally and spatially confined genetic manipulations like silencing the expression of identified signaling proteins.
- Histological sectioning and (immuno-)staining techniques with embryonal chicken skulls and brains
- Confocal laser-scanning microscopy and 3d-image analysis
- In-vitro electrophysiology of the developing giant terminals in acute brain slices
- In-ovo injection of tracer-dyes, in-ovo transfection: knock-down / silencing of signalling proteins
- Primary cell-culture of peripheral neurons from the cochlear ganglion and from central auditory brainstem neurons, coculture systems, pharmacological manipulations in vitro
- Luisa Fensky, Stefanie Kurth, Thomas Künzel – immunocytochemistry of synaptic proteins in developing giant terminals
- David Goyer, Anna Hilverling, Thomas Künzel – synaptic physiology of the developing giant terminals
- David Goyer, Stefanie Kurth – Development and characterization of a primary culture system of chicken cochlear ganglion neurons
David Goyer, Luise Fensky and Anna Hilverling are former members of the lab.
1. Goyer D, Fensky L, Hilverling AM, Kurth S and Kuenzel T (2015) Expression of the postsynaptic scaffold PSD-95 and development of synaptic physiology during giant terminal formation in the auditory brainstem of the chicken. European Journal of Neuroscience 41:1416-29. [doi: 10.1111/ejn.12902]