Dynamic relationships of conventional and neuron-specific endosomal regulators in dendrites
The unique functions and morphology of neurons require a specialized endosomal system that recycles, sorts, and targets proteins. The neuronal endosome is unique in that it is polarized into somatodendritic and axonal domains, has neuronal-specifi c regulators, and is modulated by generic regulators that confer neuronal-specifi c properties. NEEP21, a neuronal-specifi c early endosomal regulator, and EEA1, a generic early endosomal regulator, are polarized to somatodendritic domain and bind syntaxin13 (stx13), which occupies early/recycling endosomes in somatodendritic and axonal domains. NEEP21 and stx13 regulate AMPA receptor recycling; NEEP21 regulates traffi cking of L1 cell adhesion molecule. It is not known how these endosomal regulators function together to transport cargo in neurons. Due to the dynamic nature of the endosomal system, live imaging is required to understand the relationships and dynamics of different compartments. We carried out live imaging of GFP/RFPtagged regulators in cultured hippocampal neurons. NEEP21 occupies stationary (containing L1) and dynamic (devoid of L1) round compartments. EEA1-containing round and static compartments do not signifi cantly colocalize with NEEP21, but often are adjacent to it. Stx13 resides in dynamic, elongated carriers and in round stationary compartments where it partially colocalizes with EEA1 and NEEP21. We interfered with function of stx13 using expression of dominantnegative construct and found that it affects L1 traffi cking.