The cysteine-rich protein (CRP) family is a subgroup of LIM domain proteins. 
CRP1, which cross-links actin filaments to make actin bundles, is the only CRP 
family member expressed in the CNS with little known about its function in nerve 
cells. Here, we report that CRP1 colocalizes with actin in the filopodia of 
growth cones in cultured rat hippocampal neurons. Knockdown of CRP1 expression 
by short hairpin RNA interference results in inhibition of filopodia formation 
and dendritic growth in neurons. Overexpression of CRP1 increases filopodia 
formation and neurite branching, which require its actin-bundling activity. 
Expression of CRP1 with a constitutively active form of Cdc42, a GTPase involved 
in filopodia formation, increases filopodia formation in COS-7 cells, suggesting 
cooperation between the two proteins. Moreover, we demonstrate that neuronal 
activity upregulates CRP1 expression in hippocampal neurons via Ca²⁺ influx 
after depolarization. Ca²⁺/calmodulin-dependent protein kinase IV (CaMKIV) and 
cAMP response element binding protein mediate the Ca²⁺-induced upregulation of 
CRP1 expression. Furthermore, CRP1 is required for the dendritic growth induced 
by Ca²⁺ influx or CaMKIV. Together, these data are the first to demonstrate a 
role for CRP1 in dendritic growth.