ER and Golgi trafficking in axons, dendrites, and glial processes

Abstract

Both neurons and glia in mammalian brains are highly ramified. Neurons form complex neural networks using axons and dendrites. Axons are long with few branches and form pre-synaptic boutons that connect to target neurons and effector tissues. Dendrites are shorter, highly branched, and form post-synaptic boutons. Astrocyte processes contact synapses and blood vessels in order to regulate neuronal activity and blood flow, respectively. Oligodendrocyte processes extend toward axons to make myelin sheaths. Microglia processes dynamically survey their environments. Here, we describe the local secretory system (ER and Golgi) in neuronal and glial processes. We focus on Golgi outpost functions in acentrosomal microtubule nucleation, cargo trafficking, and protein glycosylation. Thus, satellite ER and Golgi are critical for local structure and function in neurons and glia.

Figure 1.. Secretory pathways and satellite organelles in neuronal dendrites.

Three potential pathways for dendritic cargo transport. 1) Classical secretion: Protein translation occurs in the neuronal cell body on ribosomes associated with the ER. These proteins exit the cell body ER, then merge with ER-Golgi intermediate compartments (ERGICs), are further processed through the cell body Golgi, and finally are trafficked via vesicles to dendrites or elsewhere in the neuron. 2) Local dendritic translation: Proteins may be locally translated off dendritic ER then trafficked to dendritic Golgi outposts, where they can be modified. After leaving the Golgi outpost in post-Golgi vesicles, these proteins can be further transported along dendrites or targeted for fusion with dendritic plasma membrane or synaptic spines. 3) Golgi bypass: Cargo translated off cell body ER can bypass cell body Golgi and be directly transported to dendrites, where they may be modified by Golgi outposts and subsequently transported in post-Golgi vesicles destined for synapses or the plasma membrane.

Figure 2.. Glial cells have Golgi outposts.

In oligodendrocytes, Golgi outposts are present in processes and in the myelin sheath. They are found along both radial microtubules that contact axons and along lamellar microtubules that spiral around the myelin sheath. These Golgi outposts use TPPP to nucleate or form new microtubules. In microglia, Golgi outposts also function to nucleate new microtubules and to establish branched processes. In astrocytes, endfeet that contact blood vessels contain Golgi outposts by EM, but their function remains unclear.