ADF/Cofilin Controls Synaptic Actin Dynamics and Regulates Synaptic Vesicle Mobilization and Exocytosis

Abstract

Actin is a regulator of synaptic vesicle mobilization and exocytosis, but little is known about the mechanisms that regulate actin at presynaptic terminals. Genetic data on LIMK1, a negative regulator of actin-depolymerizing proteins of the ADF/cofilin family, suggest a role for ADF/cofilin in presynaptic function. However, synapse physiology is fully preserved upon genetic ablation of ADF in mice, and n-cofilin mutant mice display defects in postsynaptic plasticity, but not in presynaptic function. One explanation for this phenomenon is overlapping functions of ADF and n-cofilin in presynaptic physiology. Here, we tested this hypothesis and genetically removed ADF together with n-cofilin from synapses. In double mutants for ADF and n-cofilin, synaptic actin dynamics was impaired and more severely affected than in single mutants. The resulting cytoskeletal defects heavily affected the organization, mobilization, and exocytosis of synaptic vesicles in hippocampal CA3-CA1 synapses. Our data for the first time identify overlapping functions for ADF and n-cofilin in presynaptic physiology and vesicle trafficking. We conclude that n-cofilin is a limiting factor in postsynaptic plasticity, a function which cannot be substituted by ADF. On the presynaptic side, the presence of either ADF or n-cofilin is sufficient to control actin remodeling during vesicle release.

ADF/cofilin: a crucial regulator of synapse physiology and behavior

“…In double mutants lacking ADF and cofilin1, the number of synaptic vesicles attached to the active zone was increased and consequently synaptic vesicle exocytosis was enhanced [24,25]. In line with a role of ADF/cofilin in vesicle exocytosis, inactivation of upstream regulators such as RhoB, ROCK2, PAK, LIMK1 or slingshot impaired vesicle exocytosis [30,44,50,51,63].…”

“…Interestingly, very similar to ADF/cofilin mutants, the number of docked vesicles and synaptic vesicle exocytosis was enhanced upon inactivation of profilin2 [39,64], an actin-binding protein that promotes actin polymerization by accelerating the ATP-ADP exchange on G-actin and by funneling G-actin to F-actin's plus ends [65]. This was an unexpected finding, since inactivation of ADF/cofilin and profilin2 resulted in strikingly different, almost contra-directional changes in synaptic actin dynamics: a shift towards F-actin under basal conditions and upon depolarization in ADF/cofilin mutants versus a block in depolarization-induced F-actin assembly in profilin2 mutants [24,39]. Together, these findings led to the suggestion that the presynaptic actin cytoskeleton needs to be highly dynamic to fulfill its function in synaptic vesicle exocytosis.…”

“…The most obvious explanation for this discrepancy is compensation of ADF inactivation in mice, which has been corroborated by elevated synaptic cofilin1 levels [18]. Indeed, compared to cofilin1 mutants, spine size was increased in the hippocampus and the striatum of double mutants lacking both ADF and cofilin1 [24,25], thereby demonstrating a role of ADF in spine morphology and overlapping, and redundant function for ADF and cofilin1 in dendritic spines. In line with these data, compound short hairpin RNA-mediated knockdown of cofilin1 and ADF, but not of cofilin1 alone, compromised spine enlargement during initial LTP in hippocampal slices, and this effect was rescued by the reexpression of cofilin1 [23].…”

“…Cofilin1 and ADF are broadly expressed in the adult brain [9,18], they are both present at excitatory synapses [18,19], and their synaptic function has been investigated in numerous recent studies. These studies established that (1) ADF/cofilin is critical for spine morphology [20][21][22][23], (2) ADF/cofilin controls functional and structural aspects of synaptic plasticity [4,[21][22][23], (3) cofilin1 and ADF have redundant and overlapping functions in neurotransmitter release [24,25], and (4) genetic inactivation of cofilin1 and ADF impairs learning and causes severe behavioral abnormalities [21,25,26]. Moreover, a number of regulatory pathways upstream of ADF/cofilin in excitatory synapses have been identified.…”

Membrane shaping by actin and myosin during regulated exocytosis

“…In nerve terminals, F-actin plays roles in synaptic vesicle mobilization, axonal vesicle trafficking and synaptic plasticity (Cingolani and Goda, 2008;Wolf et al, 2015) and endocytosis . High amounts of actin are found in synapses and dendritic spines were it is crucial for synapse function (Rust and Maritzen, 2015), the latter being formed by dendritic filopodia outgrowth (Fifkova, 1985;Landis et al, 1988;Matus et al, 1982)-an F-actin and myosin X dependent process (Kerber and Cheney, 2011;Plantman et al, 2013).…”