Identity Of Vesicular Transport Packets

The recruitment of postsynaptic proteins, such as PSD-95, NMDA, and AMPA receptors, to nascent synapses occurs separately and by distinct mechanisms. While PSD-95 accumulates from a cytosolic pool of molecules18-20. NMDA and AMPA receptors are present in clusters that travel along microtubules between bouts of exo- and endocytosis9,41,42. Despite this knowledge, it is unclear what constitutes these mobile clusters of glutamate receptors and which other proteins are associated with these clusters.

From the few studies addressing mobile NMDA receptors we know of a limited number of associated proteins. The NMDA receptors are associated with SAP102 during transport42,62. The NR2B subunit of the NMDA receptor is bound to the kinesin-like motor protein KIF17 via a large protein complex consisting of mLin-10 (Mint1), mLin-2 (CASK), and mLin-7 (MALS/Velis)41. However, it is unclear whether all NMDA receptor clusters are transported via KIF17, since transport velocities vary greatly and KIF17-purified vesicles are much smaller than the average size of NR1-labeled membrane structures in developing cortex42. One may then further enquire as to the nature of the NMDA receptor-containing membranous structures that are found within dendrites of neurons. These organelles are heterogeneous in size and shape; varying from 50 to 600nm in diameter, averaging around 200 nm42. They can be tubular in shape and are presumably of an endocytic nature, considering the high rates of exo- and endocytosis that is ongoing during development42,43. Indeed, the early endosomal marker EEA1 can be seen to be associated with these transport packets42. Similar tubulo-vesicular structures have been seen in the case of AMPA receptors and SAP-9776, suggesting that AMPA receptors are found in a similar early endosomal compartment.

Recent data show that large tubulovesicular structures of Golgi-derived origin are transported through neuronal processes to sites of contact82. These structures are so large that they can be seen by using phase contrast video microscopy. These structures are somehow tethered to neural cell adhesion molecule (NCAM) which is present at the plasma membrane82,83. This association is thought to occur via an interaction with spectrin. The fact that these mobile organelles are Golgi derived does not necessarily preclude the possibility that they are also undergoing exo/endocytosis. It remains to be determined whether these NCAM-associated, mobile organelles are indeed the same as the NMDA and AMPA receptor transport packets.

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