Regulation of dendritic filopodial interactions by ZO-1 and implications for dendrite morphogenesis

Neuronal dendrites dynamically protrude many fine filopodia in the early stages of neuronal development and gradually establish complex structures. The importance of the dendritic filopodia in the formation of axo-dendritic connections is established, but their role in dendrite morphogenesis remains unknown. Using time-lapse imaging of cultured rat hippocampal neurons, we revealed here that many filopodia dynamically protruded from dendrites and transiently interacted with each other to form dendritic filopodia-filopodia contacts in the early stages of neuronal development. The MAGUK family member, Zonula Occludens-1 (ZO-1), which is known to be associated with the nectin and cadherin cell adhesion systems, was concentrated at these dendritic filopodia-filopodia contact sites and also at the tips of free dendritic filopodia. Overexpression of ZO-1 increased the formation of dendritic filopodia and their interactions, and induced abnormal dendrite morphology. Conversely, knockdown of ZO-1 decreased the formation of dendritic filopodia and their interactions, and induced abnormal dendrite morphology which was different from that induced by the overexpression of ZO-1. The components of the nectin and cadherin systems were co-localized with ZO-1 at the dendritic filopodia-filopodia contact sites, but not at the tips of free dendritic filopodia. Overexpression of ZO-1 increased the accumulation of these cell adhesive components at the dendritic filopodia-filopodia contact sites and stabilized their interactions, whereas knockdown of ZO-1 reduced their accumulation at the dendritic filopodia-filopodia contact sites. These results indicate that ZO-1 regulates dendritic filopodial dynamics, which is implicated in dendrite morphogenesis cooperatively with the nectin and cadherin systems in cultured neurons.