Julija Filipovska

Opiates such as morphine are the choice analgesic in the treatment of chronic pain. However their long-term use is limited because of the development of tolerance and dependence. Due to its importance in therapy, different strategies have been considered for making opiates such as morphine more effective, while curbing its liability to be abused. One such strategy has been to use a combination of drugs to improve the effectiveness of morphine. In particular, delta opioid receptor ligands have been useful in enhancing morphine's potency. The underlying molecular basis for these observations is not understood. We propose the modulation of receptor function by physical association between mu and delta opioid receptors as a potential mechanism. In support of this hypothesis, we show that mu-delta interacting complexes exist in live cells and native membranes and that the occupancy of delta receptors (by antagonists) is sufficient to enhance mu opioid receptor binding and signaling activity. Furthermore, delta receptor antagonists enhance morphine-mediated intrathecal analgesia. Thus, heterodimeric associations between mu-delta opioid receptors can be used as a model for the development of novel combination therapies for the treatment of chronic pain and other pathologies.

Transcription elongation by RNA polymerase II (RNAPII) is negatively regulated by the human factors DRB-sensitivity inducing factor (DSIF) and negative elongation factor (NELF). A 66-kilodalton subunit of NELF (NELF-A) shows limited sequence similarity to hepatitis delta antigen (HDAg), the viral protein required for replication of hepatitis delta virus (HDV). The host RNAPII has been implicated in HDV replication, but the detailed mechanism and the role of HDAg in this process are not understood. We show that HDAg binds RNAPII directly and stimulates transcription by displacing NELF and promoting RNAPII elongation. These results suggest that HDAg may regulate RNAPII elongation during both cellular messenger RNA synthesis and HDV RNA replication.

RNA polymerase II is implicated in the RNA-templated RNA synthesis during replication of viroids and Hepatitis Delta Virus (HDV); however, neither the RNA template nor protein factor requirements for this process are well defined. We have developed an in vitro transcription system based on HeLa cell nuclear extract (NE), in which a segment of antigenomic RNA corresponding to the left-hand tip region of the HDV rod-like structure serves as a template for efficient and highly specific RNA synthesis. Accumulation of the unique RNA product is highly sensitive to alpha-amanitin in HeLa NE and only partially sensitive to this drug in NE from PMG cells that contain an allele of the alpha-amanitin-resistant subunit of pol II, strongly suggesting pol II involvement in this reaction. Detailed analysis of the RNA product revealed that it represents a chimeric molecule composed of a newly synthesized transcript covalently attached to the 5' half of the RNA template. Selection of the start site for transcription is remarkably specific and depends on the secondary structure of the RNA template, rather than on its primary sequence. Some features of this reaction resemble the RNA cleavage-extension process observed for pol II-arrested complexes in vitro. A possible involvement of the described reaction in HDV replication is discussed.