Rationale: Rat hippocampal neurons expressing MARCKS RNAi constructs exhibit a greatly reduced spine density, spine length, and spine head width (Fig.1B). Efficiency of the KD was assessed by ICC (Fig. 1A). MARCKS influences number of spines and spine morphology in a phosphorylation-dependent manner. Overexpression of MARCKS constructs that mimic constitutively phosphorylated MARCKS (S4D-MARCKS and Dbl-MARCKS) and therefore have reduced membrane-association than wildtype-MARCKS in neurons causes a reduced spine protrusion length and head width (Fig.2). Overexpression of a MARCKS construct that mimic constitutively phosphorylated MARCKS (S4D-MARCKS) causes a reduction in total F-actin in dendrites in comparison to control neurons (Fig. 3A,B).. Experimental description: We used a RNA interference (RNAi) approach to address the role of endogenous MARCKS in the maintenance of dendritic morphology. We first examined the pattern of MARCKS expression in 3-week-old cultures of rat hippocampus. Full-length bovine eGFP-tagged wild-type-MARCKS, and four MARCKS mutants were a gift from P. Blackshear (Spizz and Blackshear, 2001)