Septin 6 regulates the cytoarchitecture of neurons through localization at dendritic branch points and bases of protrusions

Septins, a conserved family of GTP-binding proteins with a conserved role in cytokinesis, are present in eukaryotes ranging from yeast to mammals. Septins are also highly expressed in neurons, which are post-mitotic cells. Septin6 (SEPT6) forms SEPT2/6/7 complexes in vivo. In this study, we produced a very specific SEPT6 antibody. Immunocytochemisty (ICC) of dissociated hippocampal cultures revealed that SEPT6 was highly expressed in neurons. Developmentally, the expression of SEPT6 was very low until stage 3 (axonal outgrowth). Significant expression of SEPT6 began at stage 4 (outgrowth of dendrites). At this stage, SEPT6 clusters were positioned at the branch points of developing dendrites. In maturing and mature neurons (stage 5), SEPT6 clusters were positioned at the base of filopodia and spines, and pre-synaptic boutons. Detergent extraction experiments also indicated that SEPT6 is not a post-synaptic density (PSD) protein. Throughout morphologic development of neurons, SEPT6 always formed tiny rings (external diameter, ∼0.5 μm), which appear to be clusters at low magnification. When a Sept6 RNAi vector was introduced at the early developmental stage (DIV 2), a significant reduction in dendritic length and branch number was evident. Taken together, our results indicate that SEPT6 begins to be expressed at the stage of dendritic outgrowth and regulates the cytoarchitecture.