LTP induction translocates cortactin at distant synapses in wild-type, but not Fmr1-KO, mice

Summary Stabilization of long-term potentiation (LTP) depends on reorganization of the dendritic spine actin cytoskeleton. The present studies tested if this involves activity-driven effects on the actin-regulatory protein cortactin, and if such effects are disturbed in the Fmr1-knockout (KO) model of Fragile X Syndrome in which stabilization of both actin filaments and LTP is impaired. LTP induced by theta burst stimulation (TBS) in hippocampal slices from wild-type mice was associated with rapid, broadly distributed, and NMDA receptor-dependent decreases in synapse-associated cortactin. The reduction in cortactin content was blocked by blebbistatin while basal levels were reduced by nocodazole, indicating that cortactin’s movements into and away from synapses are respectively regulated by microtubule and actomyosin motors. These results further suggest that synapse-specific LTP influences cytoskeletal elements at distant connections. The rapid effects of TBS on synaptic cortactin content were absent in Fmr1-KOs as was evidence for activity-driven phosphorylation of the protein or its upstream kinase, ERK1/2. Phosphorylation regulates cortactin’s interactions with actin and co-precipitation of the two proteins was reduced in the KOs. We propose that, in the KOs, excessive basal phosphorylation of ERK1/2 disrupts its interactions with cortactin, thereby blocking the latter protein’s use of actomyosin transport systems. These impairments are predicted to compromise the response of the subsynaptic cytoskeleton to learning-related afferent activity, both locally and at distant sites. of each group’s respective controls ([value for each TBS slice minus the mean of the no-TBS group] / SD of the no-TBS group). TBS effects on cortactin+ PSDs were significantly smaller in the presence of APV as compared to those in normal aCSF (*p = 0.016). B, Representative antidromic population spikes, recorded in CA1 pyramidal cells following alvear stimulation, were unaffected by theta bursts (scale: 1mV/5ms). C, Bar graph shows quantification of cortactin+ PSDs following control antidromic or theta burst antidromic stimulation. Antidromic TBS elicited a non-significant (p = 0.22) increase in double-labeled counts in str. radiatum (Str. Rad.) with no change in number of cortactin+ PSDs in the str oriens, basal dendritic field (values expressed as a percent of yoked slices receiving antidromic con stimulation; n ≥ 5/group). D, Curve shows the cortactin immunofluorescence intensity distribution for cortactin/PSD95 double-labeled elements in field CA1b of WT slices receiving control stimulation. E, Bar graph, of the same results plotted in Figure 2D, shows the number of double-labeled elements with cortactin+ labeling intensities falling within the bottom 25% (weak labeling), middle 50% (medium labeling), and top 25% (dense labeling) of values from control slices. TBS caused a significant shift in the distribution of elements across these categories (2-way ANOVA: p = 0.0002), decreasing the intensely cortactin-immunolabeled population with a concomitant increase in the weakly immunolabeled population (*p < 0.05, **p < 0.01 vs respective control; n ≥ 21/group).