Tracking EBV-encoded RNAs (EBERs) from the nucleus to the excreted exosomes of B-lymphocytes

Epstein-Barr virus-encoded RNAs (EBER1 and EBER2) are two highly abundant, non-protein coding RNAs consistently expressed in all EBV infected cells, but their function remains poorly understood. Conventional in situ hybridization studies have indicated that these RNAs are present exclusively in the nucleus. We have recently demonstrated that EBERs can be excreted from infected cells via exosomes. However, the details of the steps involved in their excretion remain unknown. In this study, we aimed to directly track the journey of EBERs from the nucleus to the excretory exosomes of EBV immortalized B-lymphocytes. Using a combination of molecular and novel immuno-gold labelled electron microscopy (EM) based techniques, we demonstrate the presence of EBERs, not only in the nucleus, but also in the cytoplasm of EBV infected B cell lines. EBERs were also seen in exosomes shed from infected cells along with the EBER binding protein La. Our results show, for the first time, that at least a proportion of EBERs are transported from the nucleus to the cytoplasm where they appear to be loaded into multi-vesicular bodies for eventual excretion via exosomes.