Wnt proteins are a large and diverse family of secreted signalling factors that play key
roles in the development of the nervous system, including control of neuronal proliferation and differentiation, axon guidance, dendritogenesis and synaptogenesis.
Despite recent advances in our understanding of Wnt function at synapses, key
questions remain unanswered. For example the role of Wnt signalling in central
postsynaptic development remains unclear, as does the specificity of Wnts for
regulating different sub-types of synapse. The aim of this thesis was to investigate the
role of Wnts in regulating the formation and function of central glutamatergic and
GABAergic synapses in the rodent hippocampus, using complementary cell biological
and electrophysiological approaches.
I find that Wnt7a specifically promotes the formation of excitatory glutamatergic
synapses in cultured hippocampal neurons, with no effect on inhibitory GABAergic
synapses. Furthermore, specific postsynaptic activation of Wnt signalling results in
increased dendritic spine size, increased clustering of the postsynaptic protein PSD-95
and increased presynaptic innervation of dendritic spines. In contrast, GABAergic
synapses are unaffected by Dishevelled-1 expression.
I also find that endogenous Wnt signalling regulates excitatory synaptic function. Acute
blockade of endogenous Wnt signalling using the Wnt antagonists sFRP1, 2 and 3
results in a decrease in mEPSC frequency and evoked release probability at
glutamatergic synapses, with no effect on GABAergic synapses. A similar decrease in
evoked release probability is observed at glutamatergic Schaffer collateral-CA1
synapses in hippocampal slices from Wnt7a-/-; Dvl1-/- double knockout mice. Finally,
I demonstrate that a prolonged reduction in glutamatergic release probability caused by
chronic Wnt signalling blockade elicits a homeostatic increase in glutamatergic synapse
number that acts to maintain normal levels of excitatory signalling.
In conclusion, the work presented in this thesis significantly advances our
understanding of the role of Wnts at central synapses. Wnt signalling regulates multiple
processes throughout the lifetime of an excitatory glutamatergic synapse. Wnt7a
promotes the formation of excitatory synapses through the co-ordinated clustering of
pre- and postsynaptic proteins. Postsynaptic Wnt signalling can directly regulate excitatory postsynaptic formation at central synapses, and can also signal back to the
presynaptic side. Endogenous Wnt signalling plays a role in maintaining normal levels
of glutamate release, and chronic perturbation of this signalling results in compensatory
changes in synapse density
