The role of neuroligin in synaptic remodeling

The neuroligin (NL) gene family codes for brain-specific postsynaptic cell adhesion molecules that play an important role in synaptic connectivity. The work presented here investigates the role of the NL family members in synaptic remodeling at both the synaptic and network level. This was accomplished by (a) determining the dynamics of neuroligin 1 (NL1) specific postsynaptic proteins in response to activity and (b) investigating the consequences of altering neuroligin 3 (NL3) expression levels or expression of an autism associated NL3 mutation on connectivity and activity in cultured hippocampal neurons.;The second part of my thesis shows how the expression of the autism-associated NL mutation R471C-NL3 affects synchrony in networks of rat hippocampal neurons. I show R471C-NL3 expression has a desynchronizing effect on activity patterns. I investigated the structural basis of this phenomenon using fractal dimension analysis of axon trajectories. R471C-NL3 cultures had higher lacunarity values, indicating a decrease in the complexity of axonal architecture. Transfection of R471C-NL3 into a subpopulation of cells in a network resulted in neuronal degeneration. This degeneration primarily affected the inhibitory population of neurons, as there were half as many glutamate decarboxylase (GAD) 65 expressing cells in R471C-NL3 cultures compared to wildtype NL3 and control cultures. Electrophysiological recordings showed a reduction of inhibitory activity in networks carrying the mutation.;Neuronal connectivity established during development and it can be modified in adulthood through new synapse formation and by potentiation/depression of synaptic efficacy. To investigate these processes, I used mature cultures of rat hippocampal neurons and focused on the proteins PSD-95, Shank, NL1, and actin. I found that high frequency activity altered the trajectory, but not velocity of PSD-95 and Shank clusters but reduced the speed and increased the number of NL1 clusters. Actin reorganized into puncta following activity and ∼50% of new puncta colocalized with NL1 clusters. Actin reorganization was enhanced by overexpression of NL1 and decreased by expression of a NL-1 mutant, NL1-R473C. These results report activity-dependent changes which may result in the formation of new postsynaptic sites and suggest NL1 modulates actin reorganization.