CaMKIIβ association with F-actin in developing cortical neurons

Abstract

Calcium/calmodulin-dependent protein kinase II (CaMKII) is a serine/threonine kinase that is best known for its role in synaptic plasticity and memory .. Although multiple roles of CaMKII have been identified in the hippocampus, its role in the developing cerebral cortex is less well understood. Immunostaining showed Ca~KII~, but not CaMKIIa was expressed in embryonic day 18 (E 18) cortical neurons at 4 days in vitro (DIV) and localized to a F-actin rich cytoskeletal structure we termed "micro spike". Further characterization of micro spikes revealed that micro spikes were composed of bundled actin, and were stable over time. Besides CaMKII~, several actin binding proteins, such as Arp3, cortactiti"and ~1-integrin were also colocalized in microspikes. Fluorescence recovery after photo bleaching (FRAP) analyses showed different dynamics of actin and CaMKII~ in microspikes compared to dendrite spines. The colocalization of CaMKII~ and F-actin in microspikes was dependent on the F-actin binding domain and the oligomerization domain. FRAP analyses confirmed the association of CaMKIIP with F-actin in microspikes was via the F-actin binding domain. This association was altered by the co-expression of CaMKIIa. FRAP analyses with stabilized F-actin using jasplakinolide or cytochalasin-D further indicated CaMKIIP, but not CaMKIIa, had a strong interaction with stable F-actin. Inhibiting calmodulin binding on CaMKII using a CaMKII inhibitor, KN93, dissociated CaMKIIP from stable F-actin. Increasing CaMKIIP activity with KCl or an active form of CaMKIIP, CaMKIIPT287D, also dissociated CaMKIIP from stable F-actin. A calmodulin binding mutant, CaMKIIPA303R, or a kinase dead mutant, CaMKIIPK43R, however, did not recover differently from wildtype CaMKIIp. The differential binding of CaMKIIP with F-actin shown in FRAP analyses correlated with CaMKIIP enrichment in microspikes and the prominence of microspikes. While overexpressed CaMKIIP increased the number of cells with microspikes, knockdown of CaMKIIP with shRNA reduced it. Taken together, these data suggested that CaMKIIP is associated with F-actin in cortical neurons, and this association is regulated by CaMKIIa and calcium signals · contributing to the stability of micro spikes.