Presynaptic Deletion of GIT Proteins Results in Increased Synaptic Strength at a Mammalian Central Synapse

Abstract

A cytomatrix of proteins at the presynaptic active zone (CAZ) controls the strength and speed of neurotransmitter release at synapses in response to action potentials. However, the functional role of many CAZ proteins and their respective isoforms remains unresolved. Here, we demonstrate that presynaptic deletion of the two G-protein-coupled receptor kinase-interacting proteins (GITs), GIT1 and GIT2, at the mouse calyx of Held leads to a large increase in AP-evoked release with no change in the readily releasable pool size. Selective presynaptic GIT1 ablation identified a GIT1 specific role in regulating release probability that was largely responsible for increased synaptic strength. Increased synaptic strength was not due to changes in voltage gated calcium channel currents or activation kinetics. Quantitative electron microscopy revealed unaltered ultrastructural parameters. Thus, our data uncover distinct roles for GIT1 and GIT2 in regulating neurotransmitter release strength, with GIT1 as a specific regulator of presynaptic release probability.

Superpriming of synaptic vesicles as a common basis for intersynapse variability and modulation of synaptic strength

“…Several recent studies described perturbations, either pharmacological or genetic, which change the synaptic release probability in the absence of changes in Ca 2+ current, [Ca 2+ ] i dynamics, and SV pool sizes (8,(49)(50)(51). Whereas changes in the latter parameters are well understood and have been recognized as strong modulators of transmitter release, the mechanisms underlying differences in "intrinsic" release readiness have received less attention.…”

“…This, together with the considerations above regarding the molecules involved, suggests an attractive scenario for the case that a sequential reaction scheme applies (51): Release-competent SRP vesicles with a fully functional release apparatus undergo a process of positional priming, which moves them closer to Ca 2+ channels by interaction with the active zone through Rim, Rab3, and Rim binding protein (58,59), turning them into FRP vesicles. This process depends on an intact cytoskeleton (33) ] i and by interactions of Munc13 with the abovementioned active zone proteins, possibly including GIT (49). In the scenario of a parallel model, these interactions would provide a dynamically changing number of sites for superpriming.…”

Expanding functions of GIT Arf GTPase-activating proteins, PIX Rho guanine nucleotide exchange factors and GIT–PIX complexes

“…In Drosophila, dGIT interacts with stoned B at the periphery of the synaptic active zone, and dgit mutant flies have defective synaptic vesicle recycling (Podufall et al, 2014). In the mouse calyx of Held synapse, presynaptic GIT1, or GIT1 together with GIT2, constrains the probability of synaptic vesicle release without altering the readily releasable pool of vesicles; however, GIT2 alone has no significant effect (Montesinos et al, 2015). In adrenal chromaffin cells, neuroendocrine vesicle exocytosis requires both GIT1 and β-PIX, which regulate Arf6 activity to control phospholipase D that promotes vesicle fusion with the membrane (Meyer et al, 2006;Momboisse et al, 2009).…”

Functional analysis of rare variants found in schizophrenia implicates a critical role for GIT1–PAK3 signaling in neuroplasticity

“…GIT1 is highly expressed in the human and rodent central nervous system (CNS) (13) (Supplemental Figure 1) where it can be found both presynaptically (14–17) and postsynaptically (15, 16) in both excitatory (16) and inhibitory (16, 18) synapses. At synapses, GIT1 is reported to regulate pre-synaptic vesicle recycling and release probability (17, 19), to promote dendritic spine growth and synapse formation (20), and to regulate AMPA and GABA A receptor synaptic localization (15, 18). Consistent with the importance of these multiple roles of GIT1 at synapses, whole body GIT1 knockout ( GIT1 −/− ) mice have deficits in behavioral models of learning and memory, including fear conditioning, novel object recognition, operant conditioning, and spatial learning (21–23).…”