Investigating the Endoplasmic Reticulum-Golgi Intermediate Compartment (ERGIC) in rat hippocampal neurons

The plasticity of synapses, the communication sites between neurons, is crucial for learning and memory. It requires the modification of the strength of individual synapses independent of the several thousand other synapses of a neuron. Modifying the synaptic proteome through local protein synthesis immediately suggests how input specificity can be achieved. Although the machinery for local translation has been detected in dendrites, the processing of locally translated transmembrane proteins poses a special problem since newly synthesized membrane proteins require processing in the secretory pathway. The necessity of processing newly synthesized membrane proteins in the perinuclear somatic Golgi apparatus would lead to loss of input-specificity. Local dendritic Golgi-like processing could solve this conundrum. Small Golgi-like structures identified in proximal dendrites suggest that there is indeed the possibility of local secretory processing. The lack of robust detection of a full set of Golgi markers in distal dendrites suggests that the processing is not conventional. We are looking for transient structures with Golgi-like function in dendrites that could achieve local processing. We found that the ER-Golgi intermediate compartment (ERGIC) marker lectin ERGIC-53 can be detected in vesicular and tubular structures throughout the dendritic arbor. In the perinuclear region the ERGIC receives cargo from the ER and precedes the cis-Golgi cisternae but it can also bypass the Golgi.
 We investigated ERGIC morphology and distribution in cultured rat hippocampal neurons with immunocytochemistry and fluorescent in-situ hybridization (FISH) at different developmental stages and following chemically-induced synaptic plasticity. 

We observed a previously unreported ERGIC-53 positive structure with a prominent large tubular morphology (tERGIC) different from small tubules described so far in other cell types. In cultured preparations, tERGIC was only prevalent in neurons but not in glial cells. It was observed throughout development (DIV 1 - 35) in a fraction of neurons with a peak at DIV 11. In dendrites of older neurons tERGIC was located in the vicinity of synapses almost exclusively near axon-dendrite contact sites with an orientation in direction of the axon axis. Preliminary results suggest an influence of synaptic plasticity on the occurrence of tERGIC.