Molecular and cellular basis of opioid drug addiction.
Why do we get addicted to drugs? Opioid drugs (like heroin and morphine) activate the same receptors and signaling pathways in the brain as activated by endogenous peptide neurotransmitterss like endorphins. But we rapidly develop tolerance and dependence to drugs, but far less so to endogenous opioid peptides. The few clues we have point towards differences in the regulatory events that follow receptor activation as the cause.
Intracellular trafficking of opioid receptors is such a critical regulatory event that might play a role in the development of drug addiction. Despite this significance, we know very little about how trafficking of receptors regulates their signaling, and vice versa. Research in the lab uses opioid receptors and related G protein-coupled receptors (GPCRs) as examples to investigate the molecular mechanisms that mediate trafficking of signaling receptors, the regulatory events that control these mechanisms, and the functional consequences of these regulatory events at the level of the neuron and the brain.
Differential regulation of opioid receptor endocytosis by addictive drugs: Activation of opioid and related GPCRs results in removal of activated receptors from the cell surface via endocytosis through clathrin-coated pits, leading to de-sensitization of the cell to the signal. Receptors were long thought to be passive ‘cargo’ or passengers in this endocytic process. Our recent data, however, suggest that receptors play a more active role in controlling their own destiny, by specifically regulating the kinetics of the subset of clathrin-coated pits that they use. In the case of opioid receptors, this ability is related to the activating drug. We are currently investigating the mechanisms and the drug-specific effects of this novel mode of opioid receptor regulation.
Sorting and recycling of GPCRs in the endosome Endocytosed receptors may either be returned (or recycled) to the cell surface, thus allowing the cell to respond to the signal again (i.e., be ‘re-sensitized’), or be targeted to the lysosome to be destroyed, leading to prolonged ‘de-sensitization’. Despite this clear significance to signaling, how different receptors are sorted from one another in the endosome and packaged. We have identified a novel role for an actin-based machinery in selectively directing specific signaling receptors to the recycling pathway, and are investigating the biochemical mechanism, regulation, and functional relevance of this machinery.
Trafficking of "single-use" receptors in the biosynthetic pathway In the case of receptors that are not recycled, after one round of signaling, the cell is re-sensitized only when new receptors are made and inserted on the plasma membrane. While there is evidence that such insertion of receptors is highly regulated in neurons, we know very little about these processes. We use the delta-opioid receptor, whose biosynthetic trafficking is regulated, as a model receptor to study the general principles, mechanisms, and functional consequences of regulated trafficking of newly synthesized signaling receptors.
Puthenveedu, M.A., Lauffer, B., Temkin, P., Vistein, R. Carlton, P., Thorn, K., Taunton, J., Weiner, O., Parton, R.G., and von Zastrow, M. Sequence-dependent sorting of recycling proteins by actin-stabilized endosomal microdomains. Cell. Nov 24;143(5): 761-73, 2010.
Yudowski, G.A.*, Puthenveedu, M.A.*, Henry, A., and von Zastrow, M. Cargo-mediated regulation of the Rab4-dependent recycling pathway. Mol. Biol. Cell. Jun; 20(11): 2774-84, 2009.
Yudowski, G.A.*, Puthenveedu, M.A.*, Leonoudakis, D., Panicker, S., Thorn, K., Nicoll, R.A., Beattie, E.C., and von Zastrow, M. Real-time imaging of discrete exocytic events mediating surface delivery of AMPA receptors. J. Neurosci. Oct 10; 27(41): 11112-11121, 2007.
Puthenveedu, M.A., and von Zastrow, M. Cargo regulates dynamics of coated pit subsets. Cell. Oct 6; 127(1): 113-124, 2006.
Yudowski, G.A., Puthenveedu, M.A., and von Zastrow, M. Distinct modes of regulated receptor insertion to the somatodendritic plasma membrane. Nat. Neurosci. May; 9(5): 622-627, 2006.