Receptor trafficking

Trafficking is an active process in which proteins are re-located from one region of a cell to another. For soluble proteins, such as many enzymes, movement around a cell is achieved by simple diffusion. However, integral membrane proteins cannot diffuse so easily in this way, as they are embedded within membrane structures. Mechanisms are thus needed that can transport proteins from the site of synthesis to the final functional location.

The distance to be moved may only be short. For most cells, proteins need only be transported from the Golgi apparatus to the adjacent cell surface. But in neurons, the functional location of proteins at axon terminals or distal regions of the dendrites may be 100s of microns or even millimeters away from the cell body, where they may be made. In addition, once in position, surface expressed receptors in both pre-synaptic and post-synaptic regions are often in dynamic equilibrium with intracellular pools of receptors, so that rapid changes in the surface populations can be mediated. These rapid changes in receptor populations is fundamental to the functioning of neurons and gives rise to many of the mechanisms that underlie synaptic plasticity.

From soma to dendrite

Distance travelling

There are a number of possible mechanisms that could be used to transport membrane proteins significant distances from the site of synthesis. Proteins may be made and processed in the ER and Golgi, moved to the cell surface and transported through the plasma membrane to the final site of action. Alternatively, proteins contained in membrane vesicles could be linked to the microtubule networks that exist inside the cells, transported to their target site and then exocytosed to the cell surface. A third possibility is that it is the mRNA that is transported so that the protein can be synthesised near to it's site of action. While there is evidence for all three mechanisms to be active in neurons, by far the most common transport mechanism is via microtubules.

What are microtubules?

Growth of microtubules. Tubulin dimers of alpha- and beta-tubulin are added to either end of a growing tubule. Growth is faster in the (+) direction.

Microtubules are dynamic assemblies of heteromeric dimers made up of alpha- and beta-tubulin that form transport networks throughout all cells. They grow by addition of tubulin dimers to either end forming a hollow 'tube' that can span 10's of microns in length. The speed of growth is always faster at one end of the tubule, the 'plus' end, that determines the direction of growth. In neuronal dendrites and axons, the plus end is normally distal to the cell body i.e. the microtubule grows away from the soma of the neuron.

Microtubules form cellular transport 'highways'

  The transport of vesicles along the microtubule networks in neuronal processes is illustrated here by the transport of GluA2-containing AMPA receptors. The GluA2 subunit binds to the 5th PDZ domain of GRIP, which itself interacts with a Kinesin motor protein, KIF-5. This protein transports the whole complex along the microtubule, using a 'hand-over-hand' mechanism.

Microtubules are found at high densities in neuronal dendrites and axons, where long distance shipping of proteins is required. Transport along these 'highways' is an active process, involving molecular motor proteins which 'walk' along the surface of the microtubules in opposite directions. These motor proteins in turn bind to membrane bound receptors, either directly or indirectly. The membranes containing the receptors form vesicles and thus the vesicles can be transported long distances.

The transport of vesicles along the microtubule networks in neuronal processes is illustrated here by the transport of GluA2-containing AMPA receptors. The GluA2 subunit binds to the 5th PDZ domain of GRIP, which itself interacts with a Kinesin motor protein, KIF-5. This protein transports the whole complex along the microtubule, using a 'hand-over-hand' mechanism.

The tubulin dimers contain binding domains for a whole host of molecular motor proteins. Kinesins and dyeins transport proteins and vesicles in opposite directions; kinesins move towards the (+) end, away from the cell body and down the dendrite; dyeins move towards the (-) end, back towards the cell body. In addition, not all the (+) ends of microtubules in neuronal processes are distal to the soma - some point towards the cell body. Thus transport is possible in both directions.

The transport of receptors along the microtubules is highly regulated, with different receptors and even different subunits of the same receptor complex able to bind to different partners and different motor proteins. For instance, while the GluA2 subunit of the AMPA receptor binds to GRIP and the kinesin KIF-5, the GluA1 subunit binds to SAP-97 and Myosin VI/KIF-1. In addition, GRIP also binds liprin-alpha that in turn binds to KIF-1. The NMDA receptor complex can be transported by KIF-17 via an interaction with the mLin complex but also by KIF-1B via PSD-95. The subunits involved in these interactions are not as yet known.

Differential transport of AMPA receptor subunits

Work here at the MRC Centre has begun to investigate the transport of AMPA receptor subunits in real time using the expression of GluA1 and GluA2 subunits that have been tagged with GFP. Using photobleaching protocols, we have demonstrated that the transport of GluA1 is significantly faster than that of GluA2 (Perestenko & Henley, 2003). Both subunits show bi-directional transport in dendrites, although movement down the dendrite is markedly faster than retrograde transport back up towards the cell body. In additions, the GluA1 subunit can enter neuronal spines easily, while the movement of GluA2 into spines is much more restricted.