Mechanisms of polarized membrane trafficking in neurons -- focusing in on endosomes

Abstract

Neurons are polarized cells that have a complex and unique morphology: long processes (axons and dendrites) extending far from the cell body. In addition, the somatodendritic and axonal domains are further divided into specific subdomains, such as synapses (pre- and postsynaptic specializations), proximal and distal dendrites, axon initial segments, nodes of Ranvier, and axon growth cones. The striking asymmetry and complexity of neuronal cells are necessary for their function in receiving, processing and transferring electrical signals, with each domain playing a precise function in these processes. In order to establish and maintain distinct neuronal domains, mechanisms must exist for protein delivery to specific neuronal compartments, such that each compartment has the correct functional molecular composition. How polarized membrane domains are established and maintained is a long-standing question. Transmembrane proteins, such as receptors and adhesion molecules, can be transported to their proper membrane domains by several pathways. The biosynthetic secretory system delivers newly synthesized transmembrane proteins from the ER via the Golgi and trans-Golgi-network (TGN) to the plasma membrane. In addition, the endosomal system is critically involved in many instances in ensuring proper (re)targeting of membrane components because it can internalize and degrade mislocalized proteins, or recycle proteins from one domain to another. The endosomal system is thus crucial for establishing and maintaining neuronal polarity. In this review, we focus mainly on the intracellular compartments that serve as sorting stations for polarized transport, with particular emphasis on the emerging roles of endosomes.

Figure 1. Model of polarized trafficking in neurons

Membrane proteins can be sorted to axon vs. dendrites by three major pathways: 1) direct polarized delivery from the trans-golgi network (TGN), 2) non-polarized delivery followed by selective retrieval and retention, 3) indirect polarized delivery via endosomes (transcytosis). In the first pathway (1) axonal and somatodendritic proteins are sorted in TGN into axonally- and somatodendritically-targeted secretory vesicles. Those vesicles are transported and fuse with axonal and somatodendritic membranes, respectively. Some evidence, coming from epithelial cells studies, suggests that cargo, which exit TGN might enter endosomes without prior appearance on the plasma membrane, which would implicate the involvement of endosomes in secretory pathway. In the second pathway (2) axonal and somatodendritic cargo exits TGN into secretory vesicles, which can fuse with both somatodendritic and axonal membranes. After this initial non-polarized insertion, proper polarized distribution of proteins is achieved by subsequent endocytic removal of missorted proteins (presumably for degradation) and retention of the properly targeted proteins at the plasma membrane. In the third pathway (3) proteins coming out of TGN are sorted into somatodendritically-targeted secretory vesicles, and then inserted into somatodendritic membrane and subsequently endocytosed into axonally-targeted endosomal compartments, which finally fuse with axonal membrane.

Figure 2. Model of neuronal endomembrane system

Neurons contain an extensive somatic Golgi clustered near the nucleus which frequently extends substantially into the major apical dendrite. In addition to the somatic Golgi, distinct Golgi outposts can be found in more distal portions of major dendrites, especially at dendritic branch points. At the plasma membrane different membrane proteins are endocytosed into different populations of pre-early endosomal compartments (pre-EEs). Some of the pre-EEs contain EHD1 (shown in neurons) and/or APPL1 (not shown in neurons). Pre-EE mature and/or fuse into canonical early endosomal compartments containing such endosomal regulators as: EEA1, rab5, syntaxin13, and EHD1. Within those EEs different proteins might be sorted into different subdomains. At this point some proteins might be 1) directly recycled back to the plasma membrane or 2) directed to specialized recycling compartment (RE) that function in polarized sorting and recycling, or 3) targeted to late endosomes and subsequently to lysosomes where they are being degraded. In neurons there is also an additional early endosomal compartment containing neuron-enriched endosomal protein 21 (NEEP21). This compartment does not colocalize with rab5 and EEA1 but colocalizes with rab4 (in PC12 cells) and to certain extent with EHD1. Some cargoes (AMPAR and transferrin) reach the NEEP21-compartment after the canonical EE compartment and before the RE compartment on their recycling pathway to the somatodendritic plasma membrane. It is not known whether cargo can be endocytosed directly to NEEP21-positive EE, bypassing canonical EE. Furthermore the NEEP21-positive compartment is important on the L1/NgCAM transcytotic pathway to the axon. Another neuronal-specific endosomal regulator, GRASP1, is localized to rab4-positive early recycling endosomes mainly in the somatodendritic domain. Both NEEP21-positive and EEA1-positive compartments are present in somatodendritic domain but not in axon. Proteins that are endocytosed in the axon (as shown for neurotrophins) accumulate in the rab5-positive axonal early endosomes, which likely mature into rab7 late endosomal compartments, which travel retrogradely along the axon towards the cell soma. Recycling endosomal compartments positive for rab11, syntaxin13, and EHD1 are localized to both axons and dendrites and can travel bidirectionally along those processes presumably transporting cargo. Besides the endosomal compartments that are involved in protein sorting and long-range trafficking, there are endosomal compartments important for local protein recycling at the axon growth cone and later at the pre-and post-synaptic sites. Finally, work done in non-neuronal cells uncovered substantial cross-talk between secretory and endosomal pathways. Certain proteins can be transported from late endosomes back to TGN and at the same time proteins exiting TGN can enter endosomal compartments without being inserted into the plasma membrane.