Protein synthesis within dendrites: ionic and neurotransmitter modulation of synthesis of particular polypeptides characterized by gel electrophoresis.
This study evaluates whether physiological variables differentially affect the local synthesis of protein constituents of synapses in subcellular fractions containing pinched-off dendrites (synaptodendrosomes). Synaptodendrosomes were pulse-labeled in a medium containing 35S-methionine with 3 or 25 mM KCl and in the presence or absence of 0.5 mM EGTA or 10 microM glutamate. Synaptodendrosomes were then subfractionated to prepare synaptic plasma membranes and synaptic junctional complexes. The protein constituents of the synaptic plasma membrane and synaptic junctional complex fractions that were locally synthesized were identified using SDS-PAGE and two-dimensional gel electrophoresis and the extent of labeling of individual bands was analyzed using a Phosphorimager. Analysis of incorporation into individual bands resolved by SDS-PAGE revealed that depolarizing conditions (25 mM KCl) increased the extent of labeling of different bands to a different extent (ranging from 10-70% increases in labeling). Addition of 0.5 mM EGTA decreased the extent of labeling of the same group of bands in both 3 mM KCl and 25 mM KCl conditions. Addition of 10 microM glutamate reduced incorporation especially in the synaptodendrosomes incubated in 25 mM KCl. Two-dimensional gel electrophoresis analyses revealed that the labeled spots that showed differential labeling under the different conditions did not correspond to the most prominent Coomassie-stained spots. These results indicate that the proteins that are synthesized in synaptodendrosomes and regulated by physiological variables are not amongst the more abundant protein constituents of the fractions. Taken together, these results are consistent with the idea that protein synthesis within dendrites may be regulated by synaptic activity.