The synapse is essential for normal neuronal communication and synaptic abnormalities could underlie many neuronal pathologies leading to such diseases as epilepsy and autism. Recent reports suggest that the Wnt signaling pathway is essential for normal synaptic development and function. However the role of specific Wnt ligands and their downstream signaling molecules play in synapse formation and function remain unclear. PRICKLE1 (PK1) and PRICKLE2 (PK2) are downstream Wnt signaling molecules which are suggested to play essential roles in neuronal function. PK1 was recently shown to be mutated in three large families with epilepsy, and Pk2 interacts with post-synaptic density 95 and subunits of the NMDA receptor. Although it seems clear that PK1 and PK2 are critical for normal neuronal function, their role in synaptic function and animal behavior remain to be investigated.
In Aim1, we show that mutations in prickle (pk) genes are associated with seizures in humans, mice, and flies. We identified human epilepsy patients with heterozygous mutations in either PK1 or PK2. In overexpression assays in zebrafish, pk mutations resulted in aberrant pk function. A seizure phenotype was present in the Pk1-null mutant mouse, two Pk1 point mutant (missense and nonsense) mice, and a Pk2-null mutant mouse. Drosophila with pk mutations displayed seizures that were responsive to anti-epileptic medication, and homozygous mutant embryos showed neuronal defects.
In aim 2, we describe two families with ASD-specific mutations in the non-canonical Wnt gene PK2. These mutations reduced the co-localization of the human PK2 protein with PSD-95, another protein implicated in ASDs. Studying Pk2 function in mice, we found that disrupting Pk2 in mouse hippocampal neurons reduced dendrite branching, synapse number, and post-synaptic density size. Consistent with these findings, disrupting Pk2 decreased the frequency and size of spontaneous miniature synaptic currents. Interestingly, these phenotypes were rescued by wild-type human PK2, but not the ASD-associated PK2 mutants suggesting that these mutations cause a critical loss of PK2 function. Behavioral studies in Pk2-/- mice suggest that loss of Pk2 function lead to ASD-like behaviors. These studies provide new insight into the biological roles of PK2, its behavioral importance, and firmly link non-canonical Wnt signaling abnormalities and ASDs. Together, Aim1 and 2 show that the Pk proteins are critical regulators of normal neuronal function and suggest that Pk2 could be a link between epilepsy and autism.