The role of activity and synaptic cell adhesion molecules of the neurexin family in the refinement of synapses between hippocampal neurons

Abstract

Nervous system development is characterized by the selective removal of superfluous synaptic contacts and the strengthening of synapses that are useful for circuit function and behaviour. Historically, neuronal activity was thought essential for the process of synapse refinement. However, recent work shows that synapses can form and mature in the absence of neuronal activity, suggesting that other factors may also modulate synapse refinement. In this thesis, I examine the role of activity and synaptic cell adhesion molecules (SAMs) in the modulation of synapse refinement in dissociated hippocampal neurons. To test the role of activity in synapse refinement, I sparsely transfected hippocampal neurons with tetanus toxin light chain (TeNT-LC), a protease that disrupts neurotransmitter release. To examine if SAMs function in synapse refinement, I designed shRNA and mutant constructs to perturb the function of neurexins, a family of presynaptic SAMs. I then performed time-lapse imaging of fluorescently labeled synapses and record how these manipulations effect the percentage of stable, eliminated, and newly formed synapses over 24 hours. Blockade of neurotransmission with TeNT-LC expression had no effect on synapse elimination rates. Interestingly, perturbation of neurexin function at synapses decreased the stability of synaptic contacts, causing synapses to be eliminated at an enhanced rate. The effect of neurexin-perturbation on synapse stability persisted even when tested during activity blockade, showing that neurexins are able to modulate synapse refinement independent of their effect on synaptic transmission. Our findings indicate that differential SAM expression by populations of afferent neurons may be important in establishing appropriate inputs onto postsynaptic neurons through activity-independent competitive mechanisms. Disruption of this function may have profound impacts on circuit development and function and explain why mutations in neurexins are associated with autism and other neurodevelopmental disorders.