Sorting and directed transport of membrane proteins during development of hippocampal neurons in culture

Hippocampal neurons in culture develop morphological polarity in a sequenti al pattern; axons form before dendrites. Molecular differences, particularl y those of membrane proteins, underlie the functional polarity of these dom ains, yet little is known about the temporal relationship between membrane protein polarization and morphological polarization. We took advantage of v iral expression systems to determine when during development the polarizati on of membrane proteins arises. All markers were unpolarized in neurons bef ore axonogenesis. In neurons with a morphologically distinguishable axon, e ven on the first day in culture, both axonal and dendritic proteins were po larized. The degree of polarization at these early stages was somewhat less than in mature cells and varied from cell to cell. The cellular mechanism responsible for the polarization of the dendritic marker protein transferri n receptor (TfR) in mature cells centers on directed transport to the dendr itic domain. To examine the relationship between cell surface polarization and transport, we assessed the selectivity of transport by live cell imagin g. TfR-green fluorescent protein-containing vesicles were already preferent ially transported into dendrites at 2 days, the earliest time point we coul d measure. The selectivity of transport also varied somewhat among cells, a nd the amount of TfR-green fluorescent protein fluorescence on intracellula r structures within the axon correlated with the amount of cell surface exp ression. This observation implies that selective microtubule-based transpor t is the primary mechanism that underlies the polarization of TfR on the ce ll surface. By 5 days in culture, the extent of polarization on the cell su rface and the selectivity of transport reached mature levels.