The Role of bI-Spectrin in Dendritic Spines two conditions. Spines expressing the ABD of bI-spectrin displayed significantly less variance in the shape of their heads than did spines expressing GFP alone (n = 5 neurons/group; * = p,0.001, Kruskal-Wallis test). There was no significant effect of blocking NMDARs and AMPARs on spontaneous morphing in any condition. doi:10.1371/journal.pone.0016197.g005 by over-expressing wild-type Rac3. Indeed, the size of dendritic spine heads in CA1 pyramidal cells transfected with Rac3 was comparable to spines in cells expressing the ABD of bI-spectrin. Overexpression of Rac3 also resulted in an increase in mEPSC amplitude. The mean amplitude of mEPSCs recorded from neurons expressing Rac3 was not significantly different from those expressing the ABD of bI-spectrin and both were larger than for mEPSCs in untransfected or GFP-transfected cells (Fig. 7B). We conclude that Rac3 can alter dendritic spine structure and function in hippocampal pyramidal cells. Discussion Actin is highly concentrated in dendritic spines and determines their morphology [52–54]. The actin cytoskeleton provides a scaffold to which a variety of transmembrane proteins and their intracellular signaling partners are anchored, often indirectly via actin binding proteins. The spectrin superfamily of proteins can form complexes via their actin binding domains, which interact with a number of other proteins, such as RACK-1, protein 4.1 and dynactin [25,55,56]. bI-spectrin has been immunocytochemically Figure 6. Expression of the ABD of bI-spectrin increases the size of the PSD. A, Merged images from CA1 pyramidal neurons in hippocampal slice cultures co-transfected with PSD-95 fused to GFP (green) and mCherry (red), with or without an unlabelled bI-spectrin ABD construct. The areas of the clusters of PSD-95 in the heads of spines expressing the ABD of bI-spectrin were larger than in cells expressing PSD-95 alone. Scale bar = 4 mm (left), 1 mm (right). B, Cumulative probability plot showing that the area of the PSD was significantly larger in spines expressing PSD-95-GFP together with the ABD of bI-spectrin than in spines expressing PSD-95-GFP alone (n = 400 spines/4 neurons/condition; p,0.05, K-S test). doi:10.1371/journal.pone.0016197.g006 PLoS ONE | www.plosone.org 10 January 2011 | Volume 6 | Issue 1 | e16197