AMPAR Cycling and Activity Dependent Regulation of Synaptic Function

Abstract

While most studies have examined how the activity regulated insertion and removal of neurotransmitter receptors control synaptic strength or properties, my studies focus on the less understood modulation of synaptic function by constitutive receptor trafficking. To this end, I have studied the mechanisms by which constitutive AMPA---type glutamate receptor (AMPAR) cycling influences surface receptor expression and synaptic plasticity. I then investigated how and whether constitutive cycling itself is regulated by activity, providing a previously unexplored means by which synaptic function can be regulated.;I first investigated the association of cycling AMPARs and the expression of synaptic plasticity in hippocampal neurons. The internalization of AMPARs following mGluR and NMDAR activation is additive, indicating that a different population of AMPARs are subjected to endocytosis following activation of the two signaling pathways. I found that NMDAR- but not mGluR- activation targets the internalization of cycling AMPARs. This suggests that modulation of the proportion of cycling and non-cycling AMPARs may influence the expression of synaptic depression in hippocampal neurons. Next, I examined whether the population of cycling AMPARs is subject to regulation via changes in chronic activity levels. Following a chronic decrease in excitation, GRIP protein expression was reduced. Decreased GRIP1 was associated with an increase in constitutive GluA2-AMPAR internalization. Furthermore, the increase in constitutive G1uA2 trafficking following decreased activity was linked to an increase in NMDA-induced G1uA2 endocytosis. These results provide initial evidence that baseline neuronal activity levels alter the expression of AMPAR associated proteins. This altered protein expression may act to modulate the expression of different forms of synaptic plasticity by regulating the population of cycling AMPARs.;To examine the activity-induced molecular events which regulate AMPAR cycling, I investigated retinal ganglion cells (RGC) which exhibit activity-dependent changes in AMPAR cycling and AMPAR regulation. I found that darkness (- Activity) decreases surface G1uA2 and changes the expression of three proteins associated with GluA2 trafficking: PICK1, Arc and GRIP. Darkness reduced GRIP levels and the ratio of GRIP found in complex with G1uA2. Darkness elevated PICK1 and Arc levels. These results support the model that correlated, activity-driven changes in multiple AMPAR trafficking proteins modulate G1uA2 cycling and consequently the synaptic AMPAR composition in RGCs.;Traditional views of receptor trafficking focus upon regulated insertion and removal as means to modulate synaptic strength and composition. My results provide strong evidence that the constitutive cycling of AMPARs plays a key role in these processes as well. Furthermore, these results provide insight into the role and regulation of neurotransmitter receptor trafficking, homeostatic plasticity and retinal adaptation.