Neuroendocrine Cells Revisited: A System for Studying Subcellular mRNA Compartmentalization

Abstract

A number of mRNAs are sorted to dendrites and, in some cases, to axons of nerve cells. The protein synthesizing capacity of dendrites is well documented. mRNAs targeted to this domain might, therefore, be locally translated. The physiological role of axonal transcripts is more enigmatic, since mammalian axons lack a protein synthesizing machinery. The peptide hormone genes encoding the vasopressin and oxytocin precursors are expressed in hypothalamic magnocellular neurons, and substantial amounts of their mRNAs are detectable in both types of neurites. In order to characterize the molecular determinants involved in subcellular vasopressin and oxytocin mRNA sorting, eukaryotic expression vector constructs have been injected into the cell nuclei of cultured neurons which do not express the peptide hormone genes endogenously. Subsequent in situ hybridization analyses revealed that vasopressin and oxytocin sense transcripts are targeted to dendrites while the corresponding anti-sense RNAs remained confined to the cell somata. The results indicate that the RNA nucleotide sequence or a secondary structure formed by that sequence is implicated in subcellular transport. Mapping of the targeting elements revealed two segments within vasopressin mRNA that are able to confer dendritic compartmentalization to α-tubulin transcripts which are normally restricted to the cell bodies. These segments span part of the coding region as well as the 3′-untranslated region. In contrast, localization of vasopressin and oxytocin transcripts to axons of cultured nerve cells has not been observed. Thus, factors directing mRNAs to this domain might be more cell-specific than those mediating dendritic targeting.