Projects

Subcluster A		Subcluster B		Subcluster Z
Project	PI	Project	PI	Project	PI
A01	Klein	B01	Felmy	Z01	Conzelmann
A02	Guimera/Wurst	B02	Grothe/Siveke	Z02	Griesbeck
A03	Ninkovic/Götz	B03	Leibold/Koch	Z03	Trauner
A04	Kadow	B04	Benda
A05	Bareyre/ Kerschensteiner	B05	Wahl-Schott/ Biel
A06	Götz	B07	Mauss/Borst
A07	Scheuss/ Bonhoeffer	B09	Egger
A08	Hübener	B10	Biel
		B11	Konnerth/ Hartmann
		B12	Straka
		B13	Kunz/Grothe

Project Summaries

Subcluster A:

Project A01 (Klein):
Much has been learned in recent years about the molecular principles of axon guidance and synaptogenesis. However, little is known about the mechanisms that control the switch from repulsive guidance to synaptogenesis at the final target. We propose to study the bidirectional Eph/ephrin signaling system as a model to address this question in neurons of the somatosensory thalamic ventrobasal nucleus. A combination of transgenic mice expressing variants of green fluorescent protein and novel EphA4 knockin alleles will be used to investigate the requirement of Eph/ephrin signaling in axon pathfinding versus synaptogenesis.

Project A02 (Guimera/Wurst):
In the superior colliculus/tectum (SC) the topography of the world is represented in spatial maps of different sensory modalities, which are encoded in a common coordinate frame. Using MgntZ/tZ mice, in which inhibitory g-aminobutyric acid-ergic (GABAergic) neurons are missing in the SC, (i) the influence of the SC-intrinsic GABAergic signalling on the development of the alignment of these sensory maps and (ii) the molecular mechanisms by which the transcription factor Mgn exerts its function during the development of GABAergic neurons will be determined.

Project A03 (Ninkovic/Götz):
Still little is known about the molecular mechanisms instructing the crucial balance between the generation of excitatory glutamatergic and inhibitory GABAergic neurons in the brain. This project aims to determine the regulation of this neuronal subtype specification by examining 1) whether a single neural stem cell is capable to generate both glutamatergic and GABAergic neurons in the adult olfactory bulb, 2) address the role Ngn2 and Tbr2 in specification of the glutamatergic short axon cells in this system and 3) identify fate determinants acting up-stream of these determinants prior to the neuroblast stage.

Project A04 (Kadow):
Our aim is to understand the relationship of CO2 neuron number, neuronal processing in the brain, and the behavioral response to CO2. First, we will analyze the impact of CO2 neuron number reduction on the avoidance response by combining advanced genetic methods with different behavioral paradigms. Second, as olfactory input is processed by second order neurons (PNs) in the brain, we will investigate the effect of neuron number change on this processing center by using histological analysis. Third, to directly test the neuronal response of PNs, we will use electrophysiology and imaging. Given the strong repulsive response to CO2, and the availability of genetic methods, we focus our attention on the fruit fly as a model system.

Project A05 (Bareyre/Kerschensteiner):
Spinal cord injury often leads to pronounced motor and sensory deficits. Our previous work indicates that spontaneous recovery of motor function can be mediated by the formation of intraspinal detour circuits. We now want to study: (1) whether the formation of detour circuits is also possible in the somatosensory system and is thus a general blueprint for motor and sensory recovery, (2) if and how neuronal activity guides the stabilization or elimination of newly formed connections and (3) whether we can design training or activity paradigms that enhance the formation of detour circuits and improve functional recovery after CNS injury.

Project A06 (Götz):
Great progress has been made in identifying novel sources for neural repair, but still little is known how neurons integrate after injury into the remaining neural network. The visual cortex with its well-known layer-specific connections and highly ordered topographic map provides a well suited system to examine the layer specificity of the regenerated neurons as well as their function in regard to their synaptic input by the use of modified rabies virus retrograde tracing and their functional integration by the use of two-photon microscopy for structural and Ca-imaging. The long term aim of this project is to achieve an appropriate functional integration of regenerated neurons.

Project A07 (Scheuss/Bonhoeffer):
Sensory processing in cortical circuits relies on the specificity of neural connections. The majority of cortical synapses are excitatory and located on dendritic spines. Spines compartmentalize biochemical signals, contain protein complexes that regulate synaptic strength, and serve as substrate for synapse formation. In the visual cortex, we will investigate the hypotheses, (1) that spines of particular neural connections have specific properties, (2) that the specific placement of spine synapses within the anatomical meshwork of axons and dendrites prepares the cortical network for particular functions, and (3) that growth and retraction of specific spines rewires the circuit during experience dependent plasticity.

Project A08 (Hübener):
Overall aim of the project is the characterization of the cellular mechanisms of plasticity in mouse visual cortex using chronic, in vivo two photon imaging of neuronal structure and function. We will induce retinal lesions and follow ensuing changes in receptive field organization of neurons expressing genetically encoded calcium indicators. We will also study the sprouting of GFP labeled intracortical axons, thought to underlie the functional changes observed after retinal lesions. The structural remodeling of thalamic input fibers to the visual cortex will be imaged after monocular deprivation and recovery from deprivation.

 

Subcluster B:

Project B01 (Felmy):
The quantitative analysis of neuronal micro- and macrocircuits is of fundamental importance to our understanding of brain function. In this project we aim to quantify the biophysical, anatomical and synaptic input parameters of neurons in the medial superior olive of adult Mongolian gerbils. These cells generate the neuronal output of an auditory brainstem circuit that processes sensory information used for low frequency sound source localization. To obtain quantitative estimates of membrane physiological and structural parameters of these cells we will combine in vitro patch-clamp recordings, Ca2+-imaging and optical simulations with quantitative neuroanatomy. Our investigation focuses on basal synaptic transmission and synaptic short-term-plasticity as well as mechanisms of dendritic integration.

Project B02 (Grothe/Siveke):
There is increasing evidence that inhibitory connections represent essential components in neural circuits that process precise temporal information, e.g. in audition. However, the importance of the timing of the inhibition itself is controversial. We want to study the role and the temporal nature of inhibition in different neural circuits in the mammalian auditory system in vivo by (1) directly measuring its timing using patch-clamprecordings, (2) experimentally driving or manipulating firing of inhibitory neurons using light-actuated molecules, and (3) means of genetic manipulations.

Project B03 (Leibold/Koch):
Neurons receive information about the functional relevance of their outputs via local signals that are indirectly related to the objectives on the systems level. Mechanisms underlying this functional adjustment are usually based on local teacher signals. Here we investigate functional adjustment using the frequency dependence of interaural-time-difference sensitivity in the medial superior olive. We will characterize biophysical parameters which are organized in tonotopic gradients and their dynamic adaptation. Modelling will reveal the physiological function of these gradients and predict local mechanisms for this adjustment.

Project B04 (Benda):
Representing a stimulus by a population of neurons makes activity correlations between neurons available as an additional coding dimension. We want to explore how sensory stimuli can be represented by populations of independent neurons. Specifically we will investigate how spike-frequency adaptation together with neural noise influence possible synchrony codes. Further we will analyze decoding strategies of a target neuron for extracting specific aspects of the sensory stimulus. Our general results will be discussed in the context of the electrosensory system of weakly electric fish and the auditory system of grasshoppers.

Project B05 (Wahl-Schott/Biel):
Retinal network activity crucially depends on Ca2+ influx through presynaptic Cav1.4 L-type Ca2+ channels. The fundamental role of these proteins for retinal network activity is highlighted by the fact that dysfunction or loss of calcium channel activity causes night blindness. Currently, there is only very little information on the mechanisms by which intracellular domains regulate Cav1.4 channels. Moreover, it is unclear to which extent this modulation affects fine tuning of the retinal network. In this proposal we want to address these key issues. We will use both in vitro and in vivo approaches to define which effects Cav1.4 channel regulators exert on retinal network activity. Moreover, we will perform yeast two hybrid screens to identify novel Cav1.4 channel regulators and determine their retinal network properties.

Project B07 (Mauss/Borst):
Motion vision is one of the most fundamental tasks of all visual systems. The Reichardt model accurately describes, at an algorithmic level, this process in flies. However, due to the small size of columnar neurons, its neural implementation remains elusive. We aim at identifying the neurons in Drosophila performing such computations and at elucidating their connectivity. We will combine visual stimulation in vivo with genetic Calcium indicators, 2-Photon microscopy, genetic blockers of neuronal function and whole cell recording. The underlying circuitry will be reconstructed by serial block face scanning electron microscopy allowing us to build a biophysically realistic model equivalent to the Reichardt detector.

Project B09 (Egger):
Our aim is to elucidate the role of oscillations for integration of sensory inputs, using the paradigm of olfactory perception. We plan to investigate oscillatory activity in the mitral cell – granule cell network of the olfactory bulb with respect to the interaction between several glomerular inputs, the induction of long-term plasticity and the role of molecular components such as T-type calcium channels, TRP channels and NMDA receptors. We will use largescale two-photon calcium imaging of the bulbar neuronal network in a semi-intact nose-brain preparation of the rodent, complemented by imaging and single-cell recordings in acute slices.

Project B10 (Biel):
Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are widely expressed in the CNS. While the electrophysiological properties of these channels have been investigated in quite detail, the functional roles and the modulation of the channels in the context of neuronal circuits have been only poorly defined so far. Therefore, the first aim of our proposal is to identify and characterize regulatory proteins of HCN channels in different brain regions. Secondly, we will analyze two genetic mouse models (HCN2/4-FEA) to define the relevance of cAMP-dependent regulation of HCN channels in vivo. Finally, we will generate two mouse lines (HCN2-1/4 Switch mice) to study the correlation between the distinct biophysical properties of HCN channels and their functional roles in neuronal networks.

Project B11 (Konnerth/Hartmann):
The aim of this project is the analysis of mGluR1/TRPC3-mediated slow excitatory postsynaptic potentials (sEPSPs) in cerebellar Purkinje neurons. In mouse cerebellar slices we will study basic features of this novel postsynaptic channel, in particular Ca2+ signaling mechanisms and the intracellular regulation. Using newly created genetic mouse models, two-photon imaging and patch-clamp-recordings in vivo, we will determine the dynamics of sEPSPs in the intact cerebellum and then identify their role for Purkinje neuron signaling in response to sensory stimulation.

Project B12 (Straka):
The aim of this project is to unravel the respective roles of central pattern generator-derived spinal locomotor efference copies and locomotion-related visuo-vestibular sensory signals for gaze stabilization. Different developmental stages of Xenopus tadpoles will be employed that allow elucidating the mechanisms of adaptive plasticity of this vertebrate motor behavior. The main focus concerns the ontogeny and plasticity of gaze-stabilizing neuronal circuits, the mutual interaction of intrinsic locomotor and sensory feed-back signals and the synaptic integration at single cell levels in spinal, visuo-vestibular and extra-ocular neuronal elements.

Project B13 (Kunz/Grothe):
Glycinergic neurons in the medial nucleus of the trapezoid body (MNTB) target several auditory nuclei involved in precise spatio-temporal auditory processing. MNTB neurons as well as their source of inputs show many specializations for fast and high-fidelity processing (e.g. the calyx of Held) indicating that timing of the provided inhibition itself matters. However, many crucial aspects of transmission in the MNTB itself are unclear e.g. absolute delays and the question of contingent failures in action potential firing). We want to solve these questions in vitro and develop methods that bridge to MNTB functions in vivo.

Subcluster Z:

Project Z01 (Conzelmann):
A rabies virus (RV)-based system for identification of neurons that are monosynaptically connected to a single cell has been developed recently. The system employs defective, pseudotyped RV allowing for infection of defined cells, and transient complementation in situ to support a single step of transsynaptic crossing to presynaptic cells. This technology should enable a more detailed understanding of neuronal connectivity and plasticity than has previously been possible. This central project is producing pseudotyped RV for tracing of synaptic connections and developing novel vectors for manipulation of activity and function of synaptically connected cells and for advanced readout.

Project Z02 (Griesbeck):
Central to the study of neuronal networks is the simultaneous measurement of activity at many locations. In spite of recent advances in loading neuronal tissues with synthetic calcium dyes cell specificity or subcellular targeting can so far not be achieved. Genetically encoded indicators of cellular calcium dynamics offer the advantage that they can be expressed in specific subpopulations of neurons within transgenic mice, are generated in situ inside cells without the need for cofactors and do not leak out of cells even during longer observation periods. We will further improve performance of these sensors, generate color variants with orange or red emission and express them in neurons of transgenic mice.

Project Z03 (Trauner):
Many important receptors in neural circuits are now characterized with atomic resolution and are thus subject to rational functional manipulation. We have recently engineered a lightgated glutamate receptor (LiGluR) by attaching a photoswitch to a kainate receptor. We now propose to extend this concept to NMDA receptors and AMPA receptors. In addition, we propose to generate orthogonal receptors that can be genetically targeted, do not respond to endogenous glutamate at physiological levels, but can be activated with a high-affinity artificial ligand.