Synapse Structure and Function

Axons are thin and extend enormous distances from the cell body creating a resource issue for vesicles and organelles, including mitochondria.  By cryo-preserving hippocampal neurons on EM grids and imaging axons and synaptic varicosities using cryo-electron tomography, we provide some of the highest resolution structures yet available of organelle distribution in synaptic varicosities.  The image at left is from a cyro-preserved hippocampal neuron after 10 days in culture.  The thin axon is filled with microtubules (in blue) and endoplasmic reticulum (in yellow) and other organelles, while the varicosity also includes mitochondria (Mito), vesicles (purple) and a multi-vesicular body (MVB) .  

Synaptic spines (small compartments extending from dendrites as see in the right figure) are distinct morphological structures decorating dendrites of all excitatory synapses in the mammalian brain.  They are chemically isolated compartments that decode incoming signals, compute a reaction probability and store information.  We believe the problem of information storage is partially solved by having these short term chemical transformations alter the structure of the spine via reorganization of the actin cytoskeleton.

1. What molecules couple changes in spine chemistry to structural changes?

2. What mechanism provides structural stability to spines given ongoing spontaneous  activity?

The post-synaptic density is a macromolecular assembly of receptors, signaling molecules, and scaffold/cytoskeletal proteins closely adherent to the post-synaptic plasma membrane (top of the cartoon on the right).  We use immunogold labeling and stain and cryo-electron tomography to determine how proteins are recruited to and organized within the postsynaptic density.  Every PSD is unique and despite its importance in regulating synaptic communication there are major gaps in understanding the fundamental properties of the PSD:

1. How is the PSD assembled?

2. What proteins are central for assembly and which are along for the ride?

3. How is the PSD remodeled to support synaptic plasticity?

We discovered that at least half of isolated PSDs carry a significant fraction of plasma membrane fragments (PSD-PMs).  With the Levental lab, we analyzed the developmental profile and lipidomic composition of these membranes.  Their lipid composition resembles raft like domains that help organize receptors and  membrane associated proteins.  We also showed increased PSD-95 palmitoylation correlates with the appearance of these membrane fragments. We are interested in:

1. What is the mechanism of attachment of membrane fragments to the PSD?

2. Do the membrane fragments play a role in organizing the overlying plasma membrane.