The role of Wnt signalling in synaptic maintenance

The formation of functional synapses, points of contact between two nerve cells (neurons) is crucial for proper brain function. Synapses are very dynamic structures that grow, shrink and change shape through the entire life of the organism. A dialogue between the pre-synaptic neuron (neurotransmitter releasing cell) and its postsynaptic target (a neurotransmitter responding cell) is essential for the formation, growth and maintenance of synapses. Importantly, the formation of new synapses and the remodelling of old ones underlay important functions such as learning and memory. Moreover, recent studies suggest that synaptic loss precedes the death of neurons observed in several neurodegenerative diseases. However, little is known about how synapses are maintained or protected against injury.

Our laboratory has been studying the function of secreted factors, called Wnts, in the maintenance of synapses. We found that Wnt blockade induces the loss of mature synapses indicating that Wnts are critical for their maintenance. In this grant proposal, we will examine the molecular mechanism by which Wnts regulate synaptic maintenance and how this process influences neuronal connectivity and animal behaviour. Our results could contribute to the development of therapeutic strategies for the treatment of diseases in which synapses are compromised.

Understanding the molecular mechanisms that regulate the formation, maintenance and function of synapses has been central for developing therapeutic approaches for the treatment of neurological conditions, nerve and brain injury as well as neurodegenerative diseases.
In the last two decades a considerable progress has been made in the elucidation of the molecular mechanisms that control the formation of neuronal circuits. Our laboratory made a significant contribution to this field with the discovery that Wnt proteins regulate neuronal connectivity by inducing the formation of synapses in the central nervous system.
More recently we found that Wnt signaling is also required for the maintenance of mature synapses. We found that Wnt blockade by Dkk1, a Wnt antagonist that specifically blocks canonical Wnt signalling, induces the disassembly of mature synapses in cultured neurons. We also found that Wnt signalling specifically regulates excitatory synapses. In this grant proposal we are taking an interdisciplinary approach to elucidate the function of Wnt signalling in synaptic maintenance, its mechanism and its contribution to animal behaviour. We will examine:
1) The role of Wnt signalling on synaptic maintenance. The effect of loss of Wnt function or Wnt blockage on the maintenance of synapses will be examined in hippocampal cultures and in the brain. Inducible transgenic expressing Dkk1 and the conditional Wnt7b mutant mice, in which Wnt signalling is specifically affected in the adult hippocampus, will be analysed. A combination of immunofluorescence approaches, electron microscopy and live cell imaging techniques will be used to establish the cellular steps leading to the disassembly of synapses upon the loss of Wnt signalling.
2) The signalling transduction pathways involved in Wnt-mediated synaptic maintenance. For this purpose, we will specifically examine the role of the canonical Wnt signalling pathway using gain and loss of function approaches. These studies will contribute to further elucidate the mechanisms that govern synaptic assembly and disassembly.
3) The contribution of Wnt signalling in synaptic transmission and plasticity in mature neurons. We will specifically examine the function of Wnt signalling in the maintenance of the proper ratio of excitatory to inhibitory synapses, and of silent synapses. Furthermore, we will test whether Wnt signalling regulates LTP and LTD in the hippocampus.
4) The function of Wnt signalling on the behaviour of young adult mice by analysing inducible transgenic mice expressing Dkk1 and conditional Wnt7b mutant mice.