Stimulation-induced differential redistributions of clathrin and clathrin-coated vesicles in axons compared to soma/dendrites

Abstract

Clathrin-mediated endocytosis plays an important role in the recycling of synaptic vesicle in presynaptic terminals, and in the recycling of transmitter receptors in neuronal soma/dendrites. The present study uses electron microscopy (EM) and immunogold EM to document the different categories of clathrin-coated vesicles (CCV) and pits (CCP) in axons compared to soma/dendrites, and the depolarization-induced redistribution of clathrin in these two polarized compartments of the neuron. The size of CCVs in presynaptic terminals (~ 40 nm; similar to the size of synaptic vesicles) is considerably smaller than the size of CCVs in soma/dendrites (~ 90 nm). Furthermore, neuronal stimulation induces an increase in the number of CCV/CCP in presynaptic terminals, but a decrease in soma/dendrites. Immunogold labeling of clathrin revealed that in presynaptic terminals under resting conditions, the majority of clathrin molecules are unassembled and concentrated outside of synaptic vesicle clusters. Upon depolarization with high K+, label for clathrin became scattered among de-clustered synaptic vesicles and moved closer to the presynaptic active zone. In contrast to axons, clathrin-labeled CCVs and CCPs were prominent in soma/dendrites under resting conditions, and became inconspicuous upon depolarization with high K+. Thus, EM examination suggests that the regulation and mechanism of clathrin-mediated endocytosis differ between axon and dendrite, and that clathrin redistributes differently in these two neuronal compartments upon depolarization.

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“…In contrast to soma, the smaller structures dominated in neurites ( Fig. 1L, M ), in line with earlier studies reporting a diameter of 40 nm for CCVs in axon terminals [ 35 ], suggesting that the ratio of CCPs to CCVs was lower in neurites than in soma.…”

“…The smaller vesicles observed in Fig. 1A presumably represent clathrin-coated vesicles (CCVs), in line with previous findings reporting diameters of CCVs in soma to be around 90 nm [ 35 ]. Note that the labeling by primary and secondary antibodies results in apparently larger structures due to the size of the immuno-complex.…”

“…Note that the labeling by primary and secondary antibodies results in apparently larger structures due to the size of the immuno-complex. Even larger clathrin-coated structures, up to 600 nm, some of which potentially correspond to clathrin patches on a multivesicular body [ 35 ] were also found. In soma, 30%of APP-CT staining colocalized with clathrin and, interestingly, as much as 85%of the clathrin colocalized with APP-CT ( Fig.…”

“…Interestingly, the ratio of CCPs to CCVs was higher in soma than in neurites. This finding may be related to a higher turnover rate in presynapses, since clathrin-mediated endocytosis is tightly connected to the synaptic vesicle cycle in presynapses, while it is important for recycling of receptors in soma/dendrites [ 35 ].…”

“…Interestingly, Aβ 42 colocalized to a high extent with clathrin in some presynapses but to a low extent in others. This may be related to the fact that the majority of clathrin molecules are unassembled and concentrated outside of synaptic vesicle clusters under resting conditions and become scattered among de-clustered synaptic vesicles and move closer to the active zone upon depolarization [ 35 ]. Thus, the difference in distribution of Aβ 42 in relation to clathrin may reflect differences between various types of synapses, for instance depending on whether they are resting or active synapses, reflecting different mechanisms of endocytosis.…”