Final Report Summary - CALYX MMFF (Molecular mechanisms of the formation and early function of calyx of Held synapses in the auditory brainstem)
The overall objective of my postdoctoral project was to gain insight into the molecular cues that determine the formation of the large calyces of Held synapses in the auditory brainstem. Calyces of Held are large excitatory glutamatergic synapses in the auditory brainstem circuit of the superior olivary complex, and they rapidly relay information about sound arriving at the contralateral ear, and thus play a role in sound source localization. Despite their important function, the molecular mechanisms which guarantee the correct formation of these synapses during development are unknown. My postdoctoral work consisted of two main projects. Firstly, I seeked to determine what are the molecular mechanisms that guarantee that large calyx of Held synapses form on the contralateral side of the brain. Secondly, I looked for trophic factors that were responsible for making the Calyx of Held a large synapse.
To study the molecular mechanisms which guarantee the correct formation of the Calyx of Held on the contralateral side of the brain, I made use of a floxed Robo3 mouse developed by the laboratory of Pr. A. Chédotal, and on the availability of a Krox20-Cre mouse from the lab of Dr. P. Charnay, which expresses Cre recombinase in the auditory brainstem. Robo3Krox20-Cre mice have a strong pathfinding deficit on the level of the calyces of Held, i.e. calyces preferentially form on the ipsilateral and not the contralateral side of the brainstem. I therefore seeked to determine whether these abnormally formed calyceal synapses on the ipsilateral side of the brainstem had the same properties as the normally contralateral forming ones in wild-type mice. I have shown by patch clamping, anatomical and morphological studies that calyces from Robo3Krox20-Cre mice are abnormal in several aspects: excitatory postsynaptic currents are smaller, presynaptic vesicular release is more asynchronous, calyx size is smaller and the mono-innervation is lost in many cases. These effects were confirmed in near-adult mice. In addition, we are currently trying to determine what are the mechanisms responsible for these impairments, whether it is an unknown role of the robo3 protein in mature synapses, or else a developmental consequence of not having crossed the brain midline. Preliminary data suggests that these developmental abnormalities are the consequence of an absence of midline crossing of calyx of Held-generating axons prior to synapse formation. This project has been done in collaboration with the team of Alain Chédotal (Institut de la Vision, Paris) and a manuscript is in preparation (Michalski N., Babaï N., Renier N., Chédotal A. and Schneggenburger R., in preparation).
Full Summary is in attached document
To study the molecular mechanisms which guarantee the correct formation of the Calyx of Held on the contralateral side of the brain, I made use of a floxed Robo3 mouse developed by the laboratory of Pr. A. Chédotal, and on the availability of a Krox20-Cre mouse from the lab of Dr. P. Charnay, which expresses Cre recombinase in the auditory brainstem. Robo3Krox20-Cre mice have a strong pathfinding deficit on the level of the calyces of Held, i.e. calyces preferentially form on the ipsilateral and not the contralateral side of the brainstem. I therefore seeked to determine whether these abnormally formed calyceal synapses on the ipsilateral side of the brainstem had the same properties as the normally contralateral forming ones in wild-type mice. I have shown by patch clamping, anatomical and morphological studies that calyces from Robo3Krox20-Cre mice are abnormal in several aspects: excitatory postsynaptic currents are smaller, presynaptic vesicular release is more asynchronous, calyx size is smaller and the mono-innervation is lost in many cases. These effects were confirmed in near-adult mice. In addition, we are currently trying to determine what are the mechanisms responsible for these impairments, whether it is an unknown role of the robo3 protein in mature synapses, or else a developmental consequence of not having crossed the brain midline. Preliminary data suggests that these developmental abnormalities are the consequence of an absence of midline crossing of calyx of Held-generating axons prior to synapse formation. This project has been done in collaboration with the team of Alain Chédotal (Institut de la Vision, Paris) and a manuscript is in preparation (Michalski N., Babaï N., Renier N., Chédotal A. and Schneggenburger R., in preparation).
Full Summary is in attached document