- Wnt signalling in the regulation of structural and functional plasticity
- Role of Frizzled receptors in synaptic assembly and function
- Role of Wnt signalling in synaptic stability in the adult brain
- Dysfunction of Wnt signalling and synaptic loss in neurodegenerative diseases
Neuronal networks are generated gradually by a complex pattern of axon guidance, target selection and synapse formation. Great progress has been made in the discovery of molecules that regulate synapse formation. Importantly, new studies revealed that the loss of synapses is an early step in the degeneration of neurons in diseases such as Alzheimer’s and Parkinson’s. However, little is known about how signalling molecules modulate the assembly and maintenance of synapses. Understanding the mechanisms that control the formation, growth and maintenance of synapses during development and in the adult CNS will provide critical information for developing therapeutic strategies for neurodevelopmental disorders and neurodegenerative diseases where synapses are compromised.
My laboratory is studying the molecular mechanisms that regulate synapse formation, maintenance and function in the vertebrate nervous system. We are particularly interested in the hippocampus and the striatum, which are implicated in learning and memory and movement and reward, respectively.
We use a multidisciplinary approach that combines cellular and molecular biology techniques, with the latest technology of live cell imaging, electrophysiology and behavioural analyses. Our lab was the first to demonstrate that Wnt proteins regulate the formation of neuronal circuits. We found that specific Wnts promote the formation of synapses in the central and peripheral nervous system by stimulating the assembly of presynaptic sites and the clustering of postsynaptic receptors. Currently, we are investigating the mechanisms by which Wnt signalling regulates the formation of central synapses, in particular the formation and growth of dendritic spines in the hippocampus and how these structural changes impinge on synaptic plasticity.
In addition, we are interested in understanding the mechanisms by which Wnt signalling promotes the maintenance of synapses in the adult brain and how dysfunction of Wnt signalling contributes to synaptic disassembly and dysfunction in neurodegenerative diseases such as Alzheimer’s and Parkinson’s Disease.
Wnt signalling regulation of dendritic spine growth and synaptic strength
We found that Wnt7a specifically promotes the formation of excitatory synapses by stimulating the formation and growth of dendritic spines. Using loss and gain of function studies both in dissociated cells and using mice deficient in Wnt signalling, we demonstrated that Wnt signalling regulates synaptic strength through the Ca2+ calmodulin kinase II (CaMKII), a key protein in synaptic plasticity and memory. Our findings demonstrate a link between Wnt signalling and synaptic plasticity (Ciani et al, PNAS, 2011).
We are currently investigating how Wnts locally modulate CaMKII activity and how this in turn results in short and long term changes in structural and synaptic plasticity. We are also investigating the receptors for Wnts involved in this process. We use state of the art cell imaging techniques in dissociated neurons and brain slices from transgenic mice, in which Wnt signalling is affected.
Role of Frizzled receptors in synaptic assembly and function
We have recently found that the Wnt receptor Frizzled-5 (Fz5) localizes to synapses in the hippocampus. Importantly, Fz5 functions as a receptor for the synaptogenic factor Wnt7a. The trafficking of Fz5 to the plasma membrane and to the synapse is regulated by neuronal activity. For instance, high frequency stimulation (HFS), which induces long-term plasticity (LTP), increases the trafficking of Fz5 receptor to the synapse. This mobilization is regulated by Wnt proteins (Sahores et al, Development, 2010). Importantly, inhibition of Wnts completely blocks the ability of HFS to promote the formation of synapses. We are currently examining the molecular mechanisms by which Wnts mediate the activity dependent synaptic localization of Fz receptors and how this process influences synaptic assembly and growth.
Wnt signalling in synaptic stability in the adult brain
We recently found that Wnts are required for the stability of synapses in mature hippocampal neurons. We have shown that blockade of Wnts with the secreted Wnt antagonist Dickkopf-1 (Dkk1) results in the rapid loss and shrinkage of synapses in mature hippocampal neurons (Purro et al, J. Neurosci, 2012).
Using a combination of cell imaging techniques and time-lapse microscopy of GFP-labelled presynaptic sites, we showed that Dkk1 promotes synaptic loss by inducing the dispersal of synaptic components. Our studies have demonstrated that Wnt signalling is not only required for the formation but also the maintenance of synapses. One of the projects in the lab aims to characterize the contribution of Wnt signalling to the structural and functional integrity of synapses in the adult CNS. We are using a combination of cultured neurons, brain slices and transgenic mouse models to study the function of Wnt signalling in the adult brain.
Dysfunction of Wnt signalling and synaptic loss in neurodegenerative diseases
Recent studies have demonstrated that synaptic instability or loss is correlated with the cognitive decline observed in neurodegenerative diseases such as Alzheimer’s. Many studies suggest that Amyloid-ß induces both synapse dysfunction and loss. However, the molecular mechanisms are poorly understood. We recently found that Dkk1 contributes to Amyloid-ß induced synaptic loss (Purro et al, J. Neurosci, 2012).
Previous studies have demonstrated that Dkk1 is elevated in biopsies of AD patients and in animal models of AD. We recently found that Amyloid-ß rapidly increases the level of Dkk1 mRNAs. Importantly, we discovered that blockade of Dkk1 using specific neutralizing antibodies completely suppresses the toxic effect of Amyloid-ß on synapses in brain slices. These exciting findings identify Dkk1 as a potential therapeutic target for the treatment of AD.