Michael Molumby and Salleh Ehaideb Will Present Their Research Wednesday, 9/10/14
Branching out by sticking together: gamma-protocadherins regulate dendrite arborization
The a-, b-, and g-Protocadherins (g-Pcdhs) are cadherin superfamily adhesion molecules encoded by clustered gene families. The 22 g-Pcdhs are combinatorially expressed in the brain, and play critical roles in synaptogenesis, dendrite arborization, and the survival of subsets of neurons. We have shown that the g-Pcdhs promiscuously form cis-tetramers that interact strictly homophilically in trans (Schreiner and Weiner, PNAS, 2010). The g-Pcdh cluster could thus generate 104-105distinct adhesive interfaces, providing CNS cells with molecular identities. We recently showed that the γ-Pcdhs promote cortical dendrite arborization by inhibiting a FAK/PKC signaling pathway (Garrett, et al., Neuron, 2012).
Here we provide further evidence for cortical dendrite arborization mediated by γ-Pcdhs homophilic matching. We demonstrate that Emx-Cre driven overexpression of a single γ-Pcdh isoform (A1 or C3) in the cortex increases dendrite arborization compared to control mice. We hypothesize that this increase is a result of increased matching of γ-Pcdhs tetramers in trans-interactions from the over expression of a single γ-Pcdhs isoform (γ-Pcdh-A1/C3) in the cortex. To further support our hypothesis, we are pursuing experiments to disrupt trans-interactions of g-Pcdhs in vitro and in vivo to examine the effect on dendrite arborization.
Elucidating the mechanism of epilepsy in flies
Prickle spiny-legs (Pksple) is one of two adult isoforms encoded by the prickle gene, and plays a role in the non-canonical WNT signaling/planar cell polarity (PCP) pathway in flies. We previously reported that pksple mutants are seizure-prone, and that mutations in prickle orthologues are associated with myoclonic (muscle jerk) seizures in both flies and humans. Using a seizure stimulation paradigm, we find that the pksple heterozygous flies have a lowered seizure threshold compared to control flies, with increases in spiking activity after electric shock, similar to what is observed for other seizure-prone flies. Such seizure activity can be ameliorated by treatment with valproic acid, a human anti-epileptic drug which has been shown to be effective in treating human patients with PRICKLE mutations. Since these patients also suffer from ataxia, or uncontrolled gait, we developed an assay to assess whether our pksple flies had similar phenotypes. Notably, both pksple homozygotes and heterozygotes exhibited a statistically significant loss of coordinated gate compared to controls, with loss of both functional pksple copies resulting in a more severe ataxia than loss of just one copy. These data further underscore the striking parallels between the prickle-associated myoclonic epilepsy syndromes observed in flies and humans.