Evaluation of RNAs present in synaptodendrosomes: dendritic, glial, and neuronal cell body contribution.

"Synaptodendrosomes" are subcellular fractions that contain pinched-off axon terminals and dendrites. These fractions are a potential source of RNAs that are localized in dendrites. However, these fractions may also contain RNAs that are seen in situ in neuronal cell bodies and glia. To evaluate whether synaptodendrosomes could be used as a source of dendritic RNA, we studied the RNA content of this fraction as compared with RNA isolated from total forebrain and a cell body-enriched fraction. RNA was analyzed by gel electrophoresis, oligo-dT chromatography, and northern blot hybridization. RNA from synaptodendrosomes contained a greater proportion of low-molecular-weight nonpolyadenylated RNAs. RNAs known to be present in dendrites (mRNA for the alpha subunit of the calcium/calmodulin-dependent protein kinase II and the polymerase III transcript BC1) were detected in synaptodendrosomes; RNAs that are restricted to neuronal and glial cell bodies (mRNAs for the 68-kDa neurofilament protein, 43-kDa growth-associated protein, beta-tubulin, and beta-actin) were present only at low levels. However, the mRNA for glial fibrillary acidic protein (seen in situ in glial cell bodies and processes) was present at high levels in the synaptodendrosomes. These results support and extend previous studies indicating that a limited subset of mRNAs is present in neuronal and astrocyte processes and reveal that both of these types of mRNAs are present in synaptodendrosomes. Thus, synaptodendrosomes may be useful as a source of dendritic RNAs, but it will be necessary to develop strategies to subtract mRNAs present in astroglial processes.