How synapses adhere
Different classes of adhesion molecules exist at synapses and are believed to mediate their match making, maintenance of their close apposition, and the spatially defined anchorage of pre- and postsynaptic apparatuses. Synaptic adhesion is considered to be highly redundant and, so far, not a single phenotype of synaptic de-adhesion has been reported (>>>). We consider it pivotal to demonstrate the functional relevance of adhesion factors before elucidating the detail of their developmental dynamics. Therefore, we are in the process of determining which adhesion receptors are required to maintain the neuromuscular junction (NMJ) in flies. At the embryonic NMJ, basement membrane envelopes neuronal terminals attached to muscles. Deleting the laminin-A gene leads to basement membrane detachment and significant reduction of neuromuscular adhesion (>>>). By removing laminin-A together with cell-cell adhesion molecules (e.g. various cadherins, N-CAM, L1, several other IG superfamily CAMs), singly or in combinations, we aim to determine which combinations ablate adhesion completely. This is the first time that systematic combinatorial genetics is being used to crack the synaptic adhesion code. To gain a better understanding of the principal organisation of the Drosophila NMJ and its adhesion factors (>>>), we complement our genetic strategies with attempts to establish EM tomography for this synapse. Unravelling the adhesion code will provide opportunities to determine how specific adhesion proteins control the function of distinct neurons. Such findings will have a major impact on understanding principals of brain development and function.
How axons grow
Neurons extend their axons via the directed movement of growth cones at their tips. This requires coordinated dynamics of the axon’s internal cytoskeleton, yet the regulation of these dynamics is poorly understood (>>>). Capitalising on the power of fly genetics and its history in the field of axonal growth (>>>), we determine precisely how the neuronal cytoskeleton is regulated. A breakthrough in our work has been the development of a worldwide unique, novel fly growth cone model based on embryonic primary neurons, which shows striking similarities to growth cone models for vertebrates. We use this model in parallel to in vivo analyses.
Part of our research is focussed on Short stop (Shot), which is a spectraplakin that links actin filaments and microtubules, and is essential for both, actin-related pathfinding processes and microtubule-related axon extension. Using knock-down of the close mouse homologue ACF7 in mouse primary cortical neurons, we could show that Shot function in axon growth is evolutionary conserved, suggesting high translatable potential of our work on Shot (>>>).
To explore the cytoskeletal context of Shot function, we also study F-actin dynamics in Drosophila growth cones, using the formation and length regulation of filopodia as an efficient readout. Such work offers new opportunities to study the various F-actin regulators and integrate insights within the same cellular model (>>>). As one key outcome so far we find that Arp2/3 together with the formin Daam act as the prime nucleators in these cells, and combined absence of their activities abolishes most F-actin and all filopodia.
As a future prospect of our work on growth cones, we expect to be able to apply our improved insights on cytoskeletal regulation to our studies also of NMJ formation and link it to adhesion factor function in that context.
Future prospects
Together these innovative basic research studies are uncovering crucial aspects of how neurons form and differentiate, and will provide novel opportunities for applied research on neuronal regeneration or migration processes in human diseases and wound healing.
