Dendrite labeled
Yet, whether the entire ER-to-Golgi secretory pathway or just late-stage post-Golgi vesicles contribute to dendritic secretory trafficking remains poorly understood. Moreover, isolated dendrites retain the capacity to incorporate sugars ( Torre and Steward, 1996) and translate membrane protein-encoding mRNAs ( Kacharmina et al., 2000), consistent with Golgi-like function. Such receptors may derive from dendritic membrane compartments immunopositive for secretory proteins (Pierce et al., 2000, 2001). For example, trans-Golgi network-derived vesicles undergo calcium-evoked dendritic exocytosis ( Maletic-Savatic and Malinow, 1998), and newly inserted glycine receptors and glutamate receptors appear rapidly on the surface of dendrites ( Passafaro et al., 2001 Rosenberg et al., 2001). Recent studies suggest the presence of local dendritic secretory capacity. Although well established in model eukaryotic cells, it is unclear whether this canonical organization applies to neurons, given their immense size and surface area and their unique morphology and signaling requirements. Subsequently, post-Golgi carriers deliver their contents to the plasma membrane ( Presley et al., 1997 Hirschberg et al., 1998). On emerging from ER exit sites, secretory carriers traffic centripetally to a perinuclear Golgi apparatus, in which protein processing and sorting occurs. The ER extends throughout the cell, as do specialized ER exit sites, where coat protein complex II-coated vesicles containing nascent cargo bud off en route to the Golgi complex ( Kuehn and Schekman, 1997 Stephens et al., 2000 Antonny and Schekman, 2001 Baumann and Walz, 2001). In non-neuronal cells, the organelles of the secretory pathway have a highly conserved spatial organization ( Hirschberg et al., 1998 Hong, 1998 Lippincott-Schwartz et al., 2000 Antonny and Schekman, 2001). Despite a likely central role in neuronal morphogenesis and membrane trafficking, very little is known about the distribution, function, and regulation of secretory organelles or secretory transport pathways in neurons.
DENDRITE LABELED SERIES
Both the addition of membrane and the localization of membrane constituents are dependent on the secretory pathway, the series of intracellular organelles consisting of the endoplasmic reticulum (ER), ER-Golgi intermediate compartment, and cis-, medial-, and trans-Golgi, which are specialized for the synthesis, targeting, and delivery of new membrane lipids and proteins ( Nelson and Yeaman, 2001 Stephens and Pepperkok, 2001 Storrie and Nilsson, 2002). The addition of plasma membrane that accompanies polarization and process outgrowth is a fundamental task of the developing neuron ( Futerman and Banker, 1996 Lecuit and Pilot, 2003).
![dendrite labeled dendrite labeled](https://i.pinimg.com/200x150/d6/55/ba/d655baf856a715ff64571c69228962f6.jpg)
This distributed dendritic Golgi represents an organization of the secretory pathway unique among mammalian cells. Dendritic Golgi outposts, which appear developmentally during the phase of process outgrowth, are involved in the trafficking of both integral membrane proteins and the secreted neuronal growth factor BDNF. We find that ER-to-Golgi trafficking involves highly mobile vesicular carriers that traffic in both the anterograde and retrograde directions throughout the dendritic arbor. Using time-lapse imaging of green fluorescent protein-tagged cargo proteins and compartment markers, we show that organelles of the secretory pathway, including ER, ER exit sites, and Golgi, are present and engage in trafficking in neuronal dendrites. Here, we demonstrate that two organizations of the secretory pathway exist in neurons: one requiring processing of membrane and lipids in the Golgi complex of the cell body and one in which endoplasmic reticulum (ER)-to-Golgi trafficking is localized to dendrites. Organelles of the neuronal secretory pathway are critical for the addition of membrane that accompanies neuronal development, as well as for the proper localization of plasma membrane proteins necessary for polarity, synaptic transmission, and plasticity.