Chapter 4 Axonal transport in retinal ganglion cells

Axonal transport of synaptic vesicle proteins in the rat optic nerve

The optic nerve, as a part of the central nervous system (CNS), has been used to study axonal transport for decades. The present study has concentrated on the axonal transport of synaptic vesicle proteins in the optic nerve, using the "stop-flow/nerve crush" method. After blocking fast axonal transport, distinct accumulations of synaptic vesicle proteins developed during the first hour after crush-operation and marked increases were observed up to 8 h postoperative. Semiquantitative analysis, using cytofluorimetric scanning (CFS) of immunoincubated sections, revealed that the ratio between distal accumulations (organelles in retrograde transport) and proximal accumulations (organelles in anterograde transport) was much higher (up to 80-90%) for the transmembrane proteins than that for surface adsorbed proteins (only 10-20%). The pattern of axonal transport in the optic nerve was comparable to that in the sciatic nerve. However, clathrin and Rab3a immunoreactivities were accumulated in much lower amounts than that in the sciatic nerve. Most synaptic vesicle proteins were colocalized in the axons proximal to the crush. A differential distribution of synaptobrevin I and II, however, was observed in the optic nerve axons; synaptobrevin I was present in large-sized axons, while synaptobrevin II immunoreactivity was present in most axons, including the large ones. The two isoforms were, thus, partially colocalized. The results demonstrate that (1) cytofluorimetric scanning techniques could be successfully used to study axonal transport not only in peripheral nerves, but also in the CNS; (2) synaptic vesicles are transported with fast axonal transport in this nerve; and (3) some differences were noted compared with the sciatic nerve, especially for Rab3a and clathrin.

Effect of light-adaptation on the binding of 48-kDa protein (S-antigen) to photoreceptor cell membranes

During the process of light-adaptation, a part of retinal S-antigen ("48-kDa protein") is bound to the photoreceptor cell membranes. This fraction can be isolated by first extracting the soluble S-antigen with isotonic buffer and subsequently extracting the bound S-antigen with detergent. In this way we found that light-adaptation to 250 lx or more induces a maximum binding of 62% of total S-antigen within 2 minutes in rat retina in vivo. At low light intensity (50 lx) this process lasts 15 minutes, while at 5 lx only 30% of S-antigen is bound. Presumably the number of available phosphorylated (bleached) rhodopsin molecules is the limiting factor in time and quantity. Dark-adaptation causes an initial rapid release of S-antigen during the first 5 minutes, but it takes more than 2 hours to reach the minimum level of about 10% bound S-antigen. The rates of binding of S-antigen in the light and of release of S-antigen in the dark are compared to other phenomena of light and dark adaptation.