Axon guidance and synaptogenesis...

The Drescher Lab studies axon guidance processes and the formation of neural circuits in the visual system. We are using the mouse and chick as a model system.

The retinotectal projection

The retinotectal projection is a well suited model system to investigate axon guidance processes and the formation of topographic maps. The principle of topographic projections is to faithfully transfer spatially organised information from one group of neurons, the projecting area, onto another group of neurons, the target area. In the retinotectal projection, retinal ganglion cell (RGC) axons grow into the tectum/SC in a non-topographic manner and initially overshoot their future termination zones (TZs). TZs are formed through interstitial branching, with branching of axons from nasal retina in the caudal tectum, and axons from temporal retina in rostral tectum.

The map is refined by arborisations and pruning of overshoot axon segments. The final map is a product of both activity-independent and activity-dependent processes. Key molecules in the first steps of the mapping process are members of the Eph family of receptor tyrosine kinases and the ephrins, while later correlated neural activity in form of 'retinal waves' drives a refinement of the initially crude map.

Modelling of map formation in the visual system

The wealth of information from various ephrinA and EphA mouse knockout analyses has opened up new possibilities to understand map formation in the visual system.

A collaboration of our group with those of I. Thompson (MRC Centre), D. Willshaw (Edinburgh University) and S. Eglen (Cambridge University) has now brought together expertise in experimental and modelling approaches as a new strategy to understand map formation.

The groups from Edinburgh and Cambridge provide expert knowledge in developing programmes to model the formation of topographic map (Willshaw (2006) in Development 133, p2705; Godfrey et al (2009) in PLoS Comput Biol. 5), while Thompsons and our group have good experience in relevant in vitro and in vivo molecular and functional approaches.

The data from mouse EphA and ephrinA knock-outs/-ins were instrumental to develop a range of new models which we are now using to make specific predictions on map formation if one or the other particular parameter is changed. Identified parameters of interest are the interplay between the EphA family and activity patterns, the form of EphA/ephrinA gradients, and the effects of changing locally ephrinA expression domains.

Our lab is currently performing various in vitro assays, and contributes to the generation of mutant mice with corresponding modifications affecting these parameters. We will analyse these mice anatomically (DiI tracings) and functional (optical imaging) to validate the modelling predictions. Overall, our target is a quantitative description of topographic map formation. This work is funded by a Wellcome Trust programme grant.

Integration of multiple guidance cues: Characterising the role of TrkB and p75NTR in map formation

Part of the mapping process in the visual system is controlled by ephrinA molecules which have a higher expression on nasal than temporal retinal axons. This differential expression mediates a repulsion of nasal axons from areas of the target area expressing high(er) amounts of EphA molecules i.e. the anterior tectum. Recently the neurotrophin receptors p75NTR and TrkB were identified as co-receptors for ephrinAs, which are GPI-anchored and therefore have no direct contact to the cytosol. The ligands for these receptors are proBDNF and BDNF. ProBDNF binds with high affinity to p75NTR, while the processed form, BDNF, binds with high affinity to TrkB.

The control of their processing is crucial, as the activation of either p75NTR or TrkB leads often to opposing biological effects. We have shown now (Neural Development 5, 30 (2010)) that proBDNF and its processed form BDNF control the branching of retinal axons antagonistically. Moreover, we have demonstrated that scavenging proneurotrophins by adding antibodies specific for the prodomain of proBNDF abolishes repellent ephrinA reverse signalling in the stripe assay. This indicates that retinal cells secrete proneurotrophins inducing the ephrinA/p75NTR interaction and enable repellent axon guidance. The antagonistic functions of proBDNF and BDNF raise the possibility that topographic branching is controlled by a local control of processing of proneurotrophins.

Currently we are working towards an understanding of the molecular mechanisms by which TrkB and p75NTR control local branching in vitro. In vivo approaches are directed towards a demonstration of the importance of the processing of proneurotrophins for map formation.

Cadherins in mouse visual circuit formation: an entry point towards untangling their roles in autism

A fundamental aim of brain research is to dissect out the molecular mechanisms underlying the specific patterns of synaptic connections between neurons. Even slight disturbances of these processes can result in major neuropsychiatric diseases such as autism spectrum disorders. A promising model system in which to elucidate the molecular basis of synaptic specificity is the visual system.

Visual information is integrated by synaptic circuits in the retina, and then relayed by retinal ganglion cells to their target nuclei in the brain such as the superior colliculus (SC). Both within the retina and the SC, the efficient processing of visual information depends on the formation of synapses between specific groups of neurons within multiple sublaminae.

We are currently testing the idea that synaptic specificity involves important roles of cell adhesion molecules of the cadherin family, some of which have been implicated in autism spectrum disorders in a recent genome wide association study (Nature 459, p528 (2009)).

In collaboration with R. Hindges lab, we are using in vivo approaches with shRNA constructs to inactivate selected cadherins in a spatially and temporarily controlled manner and study its consequences on synaptic specificity.